Sciencemadness Discussion Board

Ostwald reactor from platinized silica wool

Heptylene - 14-7-2018 at 13:58

I started a thread recently on making platinized silica wool from e-cigarette wick and platinum metal. The goal was to use the catalyst in a reactor based on the Ostwald process to make nitric acid (or nitrate/nitrites). Since the original thread was about the catalyst and specifically the silica support I used, I'm creating this new thread to report my progress in making the reactor.

The main idea behind the reactor is to pass a mixture of ammonia and excess air in a tube containing a finely dividied platinum catalyst preheated to red heat. If the ammonia air mixuture is right andthe flowrate is appropriate and a bunch of other things work out, you should obtain a mixture of nitrogen oxides and air which can be dissolved in water to make nitric acid.

I just did a pilot run with a source of 2.5 moles of ammonia gas to gauge how well the catalyst work, and get a feel for the different parameters that are important.

Experimental
The setup I used can be seen in the attachments w/ close ups of the different parts. The first part in the setup is the vacuum pump, the output of which supplies the air for the reaction. The input has a needle valve to control the air flowrate. The air goes into the three neck flask containing NH4NO3 (235 g of 85 % ammonium nitrate fertilizer in this run). The addition funnel contained 100 g of NaOH in water (solution density 1.53 g/ml).

To start the reactor, NaOH solution was dripped at a rate of about 1 drop/second and the catalyst tube was heated to red heat with a blowtorch. The air flowrate was the adjusted to maintain continuous oxidation without additional heat. About 10 large bubbles per second were coming through in the absorption flask.

Immediately a brown gas was seen forming inside the tube and white smoke appeared in the suckback trap. A tube at the end of the setup leading outside to vent excess gases was also spewing white smoke for the whole run. After the reaction is started, no much else to besides enjoying the show. At one point the plastic tube coming out of the reactor tube melted from the hot exhaust gases. The air flowrate was decreased to mitigate the reaction.

Results
At the end of the run, I tested the pH of the solution in the absorption flask: strongly basic! Obviously a lot of ammonia passed through the reactor without being oxidized, even though enough heat was generated to keep the catalyst red hot.

Discussion
A major problem with this run was that too much NH3 passed through the catalyst without being oxidized. This in turn was due to a too low air flowrate (not enough to oxidize all the NH3) or too high NH3 flowrate. Unfortunately high flowrates tended to push the catalyst towards the end of the tube, I'll have to somehow hold it in place next time. And at high flowrates the hot gases didn't have to time to tool down enough before reaching the plastic tubes, which melted it releasing NOx and NH3 everywhere.

The generator worked relatively well, stirring was surprisingly not an issue and the "waste" leftover is actually mostly sodium nitrate which will be useful. But I think too much ammonia is produced too quickly even at the slowest addition rate my funnel allows. (Quick calculation: adding 1 drop NaOH = 0.05 ml <=> ca. 1 mmol <=> 25 ml pure NH3 released.)

I haven't yet decided how to cool the exhaust gases from the catalyst tube. A fan would be practical but might not be sufficient.

This post is getting long, I might add other remarks later.

Anyway if you have ideas for improvement or experiences with those kind of reactors...

EDIT: pictures and legends: the setup, the red hot catalyst and the plastic tube that melted from the hot gasses.

overview.jpg - 258kBhot catalyst 2.jpg - 143kB tube failure.jpg - 139kB

[Edited on 15-7-2018 by Heptylene]

JJay - 14-7-2018 at 14:15

I assume you're using a halogenated plastic for the hoses. I have been meaning to try cooling a quartz tube by wrapping copper tubing around it and running water through it. I've read that it's much easier to wrap copper tubing if you pack it with table salt.

Heptylene - 14-7-2018 at 16:37

What do you mean by halogenated plastic? PTFE? The tubes are soft PVC, which is not really chemically resistant but its all I can find easily. I guess PTFE would actually be a significant improvement over PVC, especially in terms of temperature resistance.

VSEPR_VOID - 14-7-2018 at 18:17

you should pack the excess space in the reaction tube with glass wool or calcium chloride.

[Edited on 15-7-2018 by VSEPR_VOID]

unionised - 15-7-2018 at 01:32

Quote: Originally posted by VSEPR_VOID  
you should pack the excess space in the reaction tube with glass wool or calcium chloride.

[Edited on 15-7-2018 by VSEPR_VOID]


Why?

Heptylene - 15-7-2018 at 01:53

Quote: Originally posted by unionised  
Quote: Originally posted by VSEPR_VOID  
you should pack the excess space in the reaction tube with glass wool or calcium chloride.

[Edited on 15-7-2018 by VSEPR_VOID]


Why?


The catalyst mesh has some resistance to gas flow and is pushed along the length of the tube if the flowrate is too high.

Actually packing the whole tube with silica wool might make the pressure in the system to high to be practical (pressure difference scales with length of packing). Maybe packing with small rocks? Calcium chloride is not suitable because some water is generated by the reaction which condenses further. The CaCl2 would turn to a solution and contaminate everything. I was thinking of hooking the catalyst to the entrace of the tube with some titanium wire, I'll see if this work.

@YamYamMan: Yes the flowrate was likely too high. I'll likely make more catalyst, I just need more silica wool. Oxygen is out of the question, its infinitely more expensive than air. Unless I happen to find a working oxygen concentrator on the street.

Ah also a problem I forgot to mention: the catalyst tends to cool down if the flow is unstable, and if it cools down too much the reaction doesn't start again. I'll probably add a nichrome wire heating element around the reactor at some point.

unionised - 15-7-2018 at 02:08

Wet CaCl2 will react with nitric acid to form Cl2 and NOCl/ NO2Cl etc.

Silica/ glass wool would be an idea to keep the catalyst in place.
I'm not sure, but I think it might help with cooling the gas after the reaction.

Heptylene - 15-7-2018 at 02:58

Quote: Originally posted by unionised  
Wet CaCl2 will react with nitric acid to form Cl2 and NOCl/ NO2Cl etc.

Silica/ glass wool would be an idea to keep the catalyst in place.
I'm not sure, but I think it might help with cooling the gas after the reaction.


Yes actually I could just tie the catalyst in place with some of that silica thread! Probably even better than titanium, I don't know if titanium holds up to 1000 °C nitric acid vapor

WGTR - 15-7-2018 at 16:40

I have an unfinished project or two floating around the forum that is related to making nitric acid/nitrates through various means, even though I have no problems obtaining any chemicals that I need from sources like Fisher or Sigma, etc.

If I were to tackle the Ostwald process, what kind of budget for supplies and equipment would interest people here, who might want to duplicate the procedure? Starting almost from scratch I think $100 would be a stretch, but perhaps $200 would do it. This would include buying a variac or PID controller, some firebrick, platinum, etc. A lot of things could be reused for other projects. I don't have time to do things like this solely for my own amusement, being way too busy these days. However, I might be willing to set some time aside if I thought the procedure would be useful to a number of people here.

Anyway Heptylene, even though I haven't been commenting on the thread, I've been following your efforts with enjoyment. A few suggestions that I'd like to throw out there that might be useful:

1. The quartz tube is a bit short after the catalyst. There's not much room for cooling the exit gasses before it hits your plastic tubing. Quartz tubing of this size is rather inexpensive by the foot, perhaps just get a longer piece.

2. To hold the catalyst in place, perhaps push it into place by using a smaller diameter length of quartz tubing inside the larger reaction tube. The idea is that the smaller tube would prevent the catalyst from getting blown down the larger tube during operation.

3. Preheat the gasses electrically with something like this: https://www.sciencemadness.org/whisper/viewthread.php?tid=55... It allows you to experiment with various fuel-air mixtures, without worrying about the self-sustainability of the reaction at the catalyst. The catalyst could be located just outside of the furnace, where it would be heated by the pre-heated gasses.

4. Minding the flammability limits of ammonia in air, mix up a batch of ammonia-air in a bag using known quantities of both, instead of trying to generate and use it directly. This removes several variables and makes the results more meaningful and duplicable. Placing the bag inside of a container like this http://www.sciencemadness.org/talk/viewthread.php?tid=71282&... makes flow control very easy. The compressed air filling the container is separated from the gas mixture by the plastic bag, and the air flow going into the container can be measured easily with a simple rotameter. Certainly a larger container can be used, once you figure out the best fuel-air ratio.

5. Given a long enough quartz tube, drill through the sides of a plastic storage container, such that the reaction tube will fit through it horizontally after it exits the tube furnace. Maybe drill the holes oversized a bit so that a cloth or paper towel can be wrapped tightly around the tube where it enters/exits the container to plug up the opening. Fill up the container with water for cooling. The paper towels should remain continually moist.

Thanks for posting your efforts. I know it takes a lot of time, not just for the lab work, but also for posting and editing.

JJay - 15-7-2018 at 17:33

Quote: Originally posted by Heptylene  
What do you mean by halogenated plastic? PTFE? The tubes are soft PVC, which is not really chemically resistant but its all I can find easily. I guess PTFE would actually be a significant improvement over PVC, especially in terms of temperature resistance.


PVC is a halogenated plastic... it can withstand some nitric acid, although it is not completely impervious to it. PTFE would be better, of course.

Heptylene - 16-7-2018 at 14:14

@WGTR That's some really nice advice! I'll adress each point in order:
1. I'll probably don't buy a longer tube as those are pretty expensive. From a quick search on eBay on the order of 50 $ for 1 meter length. I already have three 50 cm tubes 18 mm OD, I might be able to connect them somehow. Ironically it would be pretty hard to fuse two tube together...

2. I'll see if I can do that with some extra silica wick holding the catalyst (think like a kite in the wind inside the tube). If that doesn't work I'll try your idea, it looks robust.

3. Pre-heating is a good idea, it would probably save on the quantity of catalyst needed. Heating the catalyst itself is definitely planned as re-lighting it with a blowtorch is impractical. And any ammonia that passes over when the catalyst is cold will neutralize the product... Pre-heating would just be adding some more wire so no big deal.

4. The gas bag idea is nice but seems impractical for large quantities (Maybe with a large garbage bag inside a 55 gallon drum?). Chemetix used thermal decomposition of urea as a mean to produce ammonia (+ CO2) on demand (although I recall the rate of production of ammonia was a bit too low to be practical). But a gas bag is surely the easiest way to measure exactly how much ammonia goes through the reactor so I might give it a shot at some point.

5. Excellent suggestion, I'm definitely going to try that! Actually I just thought of something along the same lines: Instead of used a towel dipping in water maybe an ultrasonic mister could deposit water on the tube continuously. Then again a towel is cheaper that an ultrasonic mister.

@JJay Oops my bad, PVC is indeed halogenated! For some reason I thought the chlorine was removed during the polymerization of vinyl chloride...

VSEPR_VOID - 17-7-2018 at 04:42

Is there a better material for coating in platinum than glass wool? What about activated carbon as a substrate?

WGTR - 17-7-2018 at 16:00

I'm going to post these references here, since I think they will be useful to the thread.

The Gauze Catalyst in Ammonia Oxidation

G. A. Perley, and M. W. Varrell
Ind. Eng. Chem., 1929, 21 (3), pp 222–223
DOI: 10.1021/ie50231a007
Publication Date: March 1929

Attachment: perley1929.pdf (301kB)
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The following reference is referred to by the previous one:

Temperature Control in Ammonia Oxidation

G. A. Perley, R. P. Smith
pp 258–260
Publication Date: March 1925 (Article)
DOI: 10.1021/ie50183a011

Attachment: perley1925.pdf (426kB)
This file has been downloaded 222 times

[Edited on 7-18-2018 by WGTR]

WGTR - 18-7-2018 at 06:17

Quote: Originally posted by VSEPR_VOID  
Is there a better material for coating in platinum than glass wool? What about activated carbon as a substrate?


I think that even graphite would be attacked by oxygen at 800-1000C, so activated carbon probably wouldn't work well in my thinking.

The trick is also to pick a substrate that will effect the decomposition of ammonia and/or nitric oxide much more slowly than the platinum catalyzes the oxidation of ammonia to nitric oxide. Even platinum catalyzes the decomposition of nitric oxide, but more slowly, so if the reaction zone is thin and the reactant flow rates and temperature are optimal, then 90-100% efficiency can be obtained. Silica doesn't seem to have an effect on ammonia or nitric oxide at high temperatures, so it's a good choice for tubing, and as a catalyst support.

Heptylene, I find it interesting that so many people on eBay sell quartz tubing in such impractically short lengths. It's like they think everyone wants 18" long tubing or something. I suppose it saves on shipping costs. Anyway, there are other suppliers online that will sell it by the foot for pretty reasonable costs, but there's always that shipping charge that makes it impractical to just buy a single stick.

One that I see is Mountain Glass (if you're in the US). https://www.mountainglass.com/Quartz-Tube-12mm-x-2mm-4ft
Shipping will add probably $10 to that. Technical Glass also looks good, but they have a $50 minimum. http://www.technicalglass.com/product_pages/fused_quartz_tub...

I priced some platinum gauze. For small quantities that requires some pretty crazy money. The price scales more attractively if I wanted a full square meter of it (If I wanted to mortgage the house, ha ha!). I'm thinking that it may be more practical to make our own, since we're only talking about a screen sized to fit into a 10mm tube.

I used a 110 mesh polyester stencil (from my screen printing days) as a template, and weaved a small screen using 40 AWG copper wire. I had to do it under a microscope just to see that the wire went through the right holes. The result looked like a metal screen, so I guess the idea would work. The question would be how to remove the polyester mesh from the screen, but I suspect that boiling sodium hydroxide solution would dissolve it. Anyway, the cheapest place that I could find 3 mil platinum/10% rhodium wire is from Omega (https://www.omega.com/pptst/SPPL.html). The wire is sold by the inch, and I'm thinking that 1 foot might be enough. That would come out to about $25 + $13 (!!!!!) shipping. Surepure Chemicals offers 5 mil pure platinum wire in 5 foot lengths for $85 + $15 shipping, so about $100. I'm currently undecided on which way I'd go with that.

If the flowmeter is compatible with ammonia (would probably have to be stainless instead of brass fittings), then it would be possible to just store the ammonia separately in a free-standing bag without the secondary containment vessel, and meter out the ammonia directly.

Even if you want to generate ammonia directly, I still think it's important to use a storage bag as a buffer, much like a windchest stores air from the blowers for a pipe organ. For example, a plastic bag is weighted slightly with a rigid piece of cardboard and a magazine or something. One rubber tube enters the bag from the gas generator, and then another tube goes from the bag to the flowmeter, with a flow that can be adjusted periodically with the knob on the flowmeter. That way the reaction in the quartz tube is decoupled from the reaction conditions in the ammonia generator at a given instant. You don't need the pressure in the system to be high, just a fraction of a psi.

I just discovered that Walmart sells 20 gallon Ziploc bags. Those are massive. Something that size could store 2 cubic feet easily. If the seal is as gas-tight as the smaller 1 gallon bags, then these might be a good choice as storage or as a buffer.

WGTR - 18-7-2018 at 16:13

I decided to post some resources for silica/ceramic cloth, thread, and bulk fiber below. Maybe they will be useful to someone.

-------------------------------------------------------------------------------

From www.ceramicfiberonline.com:

Ceramic Fiber: Bulk alumina-silica fiber, 25 lbs per box, rated to 1260C, price: $49

-------------------------------------------------------------------------------

From Armil CFS:

Silica Cloth: 96% silica, 0.036" x 36" x 36", Price: $40

-------------------------------------------------------------------------------

From Vitcas:

Silica Fiber Sewing Thread: Good to 1400C, Price: £80.00

Silica Fiber Cloth: 0.7mm x 1m square, good to 1000C, Price: £12.99

wg48 - 19-7-2018 at 04:01

Quote: Originally posted by WGTR  

The trick is also to pick a substrate that will effect the decomposition of ammonia and/or nitric oxide much more slowly than the platinum catalyzes the oxidation of ammonia to nitric oxide. Even platinum catalyzes the decomposition of nitric oxide, but more slowly, so if the reaction zone is thin and the reactant flow rates and temperature are optimal, then 90-100% efficiency can be obtained.


I am surprised that a single thin catalytic mesh with 80 threads per inch (so the wirers are about 1/160in dia) can achieve 90% efficiency with a residence time of only about 1ms.

For example the length of a the catalyst of catalytic converter for a car is say 12in long or 1000 times the thickness of a 80 thread mesh. I guess its due to different flow velocity and catalyst efficiency.

[Edited on 19-7-2018 by wg48]

WGTR - 22-7-2018 at 18:49

This is currently unfinished, and I'm kind of in a hurry right at the moment, but I've uploaded a picture of the condensor. It is intended to be mounted vertically on a board. The left side is the bottom, and the right side faces upward.

IMG_4460.JPG - 2.9MB

Here's the parts list:

From Home Depot:

1. (1x) 3/4" PVC pipe, 2 ft. Part # 22075 Price $1.55 ea.
2. (2x) 3/4" slip x 1/2" FIP x 3/4" slip PVC tee. Part # C402-101 Price $1.14 ea.
3. (1x) 3/8" all-thread, 2 ft. Part # 802167 Price$1.97 ea
4. (1x) 3/4" slip x 3/4" FIP PVC adaptor. Part # C435-007 Price $0.74 ea.
5. (1x) 1/2" barb x 3/4" MIP adaptor, nylon. Part # 800229 Price $2.24 ea.

From Lowe's:

1. (2x) 3/8" barb x 1/2" MIP, nylon. Part # 877100 Price $1.89 ea.
2. (2x) 3/4" split ring pipe hanger. Part # 302049 Price $1.67 ea.
3. (1x) 1/2" split ring pipe hanger. Part # 302044 Price $1.68 ea.
3. (3x) 3/8" ceiling bracket. Part # 301689 Price $2.24 ea.
4. (1x) 5/8" OD x 1/2" ID clear tubing 10 ft. Part # 814319 Price $4.59 ea.
5. (1x) 1/2" OD x 3/8" ID c;ear tubing 10 ft. Part # 879263 Price $4.18 ea.
6. (1x) 3/4" OD x 5/8" ID clear tubing 10 ft. Part # 814320 Price $7.99 ea.
7. (1x) 3-pack 0.956" rubber diverter. Part # 891223 Price $1.49 ea.

From Napa Auto Parts:

1. 5/8" -> 9/16" fuel injection hose clamp. Part # 7051227 Price $0.99 ea.
2. 1/2" -> 7/16" fuel injection hose clamp. Part # 7051225 Price $0.99 ea.

I'm still waiting on the 8mm x 6mm x 4' quartz tube, but it will slide all the way through the PVC pipe and out either end. The bottom end has a plastic hose barb connector that the quartz tube will slide through. The short piece of clear tubing on the end of the barb is intended to provide a water-tight seal. If you look closely you can see that there are two pieces of clear tubing, a smaller piece inside of the larger one. A 5/8" fuel injection clamp is there to provide a seal between the clear tubing and the quartz tube.

Cool water will flow in through the bottom barb fitting, and out the top one. The condensor is intended to be completely open on the top. The water flow rate will have to be adjusted such that gravity is sufficient to keep water from spilling out the top of the condensor. I discovered that the hardware store also sells a fitting that adapts from a barb fitting to a standard garden hose fitting. Interesting.

The condensor will be positioned into place using a pair of 3/4" split ring pipe hangers. These things have a threaded 3/8" hole in the bottom that accepts a length of 3/8" all-thread. The other end of the all-thread screws into a ceiling bracket, that I am going to mount on a 2 X 4. If one drills out a hole through the wood under the bracket, then it is possible to screw a standard length of all-thread through the board (maybe a 6" long piece?). This means that you can adjust the precise stand-off of the pipe hangers without cutting each piece of all-thread to an exact length. I'll show this later.

The 2' piece of all-thread shown at the bottom of the picture shows how the hanger and the bracket fits together, although I'll be using shorter pieces of all-thread.

The "hot end" of the quartz tube will fit into the tube furnace that I referred to earlier. It's about 18" long. Right now the condensor is about 2' long. I may cut it and shorten it a bit later, so that a 4' long piece of quartz is long enough.

IMG_4461.JPG - 766kB

This is a rubber diverter that I picked up in the store. It's made to fit inside of a garden hose fitting to pick up debris and stuff. I think the screen is stainless steel, but I'm not sure yet. I'm planning to cut it carefully to squeeze tightly inside of the quartz tube, and provide a support for the catalyst. Once up to temperature the screen may anneal and fall out of the tube, meaning that I'll have to think about this a bit. Maybe a thin quartz rod placed into the tube would support the screen.

Heptylene - 24-7-2018 at 03:10

@WGTR That cooling design is really neat! Looking forward to seeing it when its finished.

RogueRose - 24-7-2018 at 09:43

I didn't read the OPs last couple posts, so IDK if he figured out a way to keep the catalyst from being blown to the back of the tube. I was thinking that you might want to try finding a gravel quarry/pit, gravel dump or gravel driveway (if you have a limestone area) where there is often solid white rocks incorperated into the grey limestone. These are quartz rocks, some are beautiful and you can see the crystal structure in them and I've found ones 3"x4"x2" in size. They break nicely and could be filed down with something like a dremel (IDK if a metal file would work). They may be heavy enough to hold the catalyst in place, you could put a few in line and it would allow air flow around them as well as not contaminate your product with anything that isn't already there.

I know I used to be able to pick up a bucket full of solid quartz rock when they would dump a new load on the driveway. I have some samples I'll post pics of if you aren't sure what I'm talking about.

quartz.jpg - 1.5MB

[Edited on 7-24-2018 by RogueRose]

WGTR - 24-7-2018 at 10:03

Thanks Heptylene! Glancing back over my previous post, I'd like to clarify that I posted a parts list in case I GHBAB (got hit by a bus). I wouldn't suggest anyone buy those exact parts until I get everything assembled, as some of the parts may change.

I just put in an order for several sizes of quartz tubing. The idea is that a metal screen will be mounted inside of a tube to support the catalyst, then a smaller tube will telescope into the larger one to keep the screen positioned correctly. Another bit of tubing could be on the other side of the screen, allowing gravity to clamp the screen in between the two pieces of tubing inside the larger tube.

I have some 6mm x 8mm and 4mm x 6mm borosilicate tubing. It just so happens that the smaller tube telescopes smoothly into the larger one, with just a bit of extra clearance. I talked with the glass supplier and was informed that I just got lucky with those results. Their tubing has a tolerance on the OD of +/- 2%, and a tolerance on the wall thickness of +/- 10%. It may telescope that way, or it may not.

For that reason I bought a number of different sizes to play with:

1. 4mm x 6.35mm
2. 5mm x 7mm
3. 5mm x 7.25mm
4. 7mm x 9mm
5. 7.5mm x 9.75mm

Possible pairings:
1. Tubes 1 and 4
2. Tubes 2 and 4
3. Tubes 2 and 5
4. Tubes 3 and 5

Thanks for the idea RogueRose. I'm familiar with the type of rock that you're referring to, and have seen them in driveways, etc. If one is choosy, it's possible to find them with large facets and pretty transparent. It's a possible option. It would be best to get the purest crystalline samples that you can scrape up, so that there aren't other minerals present that could decompose the product. It would probably be best to leach them in nitric acid beforehand to make sure they are chemically "clean".


[Edited on 7-24-2018 by WGTR]

RogueRose - 24-7-2018 at 14:10

Quote: Originally posted by WGTR  

Thanks for the idea RogueRose. I'm familiar with the type of rock that you're referring to, and have seen them in driveways, etc. If one is choosy, it's possible to find them with large facets and pretty transparent. It's a possible option. It would be best to get the purest crystalline samples that you can scrape up, so that there aren't other minerals present that could decompose the product. It would probably be best to leach them in nitric acid beforehand to make sure they are chemically "clean".


[Edited on 7-24-2018 by WGTR]


I agree about them being pure. These came out of a muddy driveway and haven't been washed yet so they aren't as bright as they could have been. O have a nice collection of pure white crystal rocks that look like they were cut almost like diamonds (sharp angles and flat sides). I threw in some of the mixed ones with the limestone, just to show comparison.

I actually found that these are easily found at night with a good (focused) flashlight as the white REALLY stands out on the ultra pure ones, and your field of view is limited to the beam, so you aren't over-stimulated by all the stones surrounding you all the time while hunting. These can be heated with MAPP gas * oxy and no fumes come off and no oxide forms. They can also be dunked in water and only rarely do they split (probably an internal flaw or stress point).

They are actually pretty interesting for being a common rock found in driveways!

Chemetix - 24-7-2018 at 16:31

Smashed up brick or tiles have a great surface area and chemically neutral, It worked with cobalt, platinum oxide would also work.

WGTR - 26-7-2018 at 20:07

I attached some more reading for the thread's enjoyment. The figures show yields of N2 and NO for various temperatures over a ceria catalyst, and also the ability of ceria to catalyze the oxidation of NO to NO2. I happen to have about 200g of CeO2 that I picked up on eBay for cheap some time ago.

Characterization of Ceria’s Interaction with NOx and NH3
Li Zhang, John Pierce, Victor L. Leung, Di Wang, and William S. Epling
J. Phys. Chem. C, 2013, 117 (16), pp 8282–8289
DOI: 10.1021/jp401442e
Publication Date (Web): April 1, 2013

Attachment: zhang2013.pdf (2MB)
This file has been downloaded 213 times

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Here's some general, but interesting information:

Oxidation of Ammonia
Gray B. Taylor
Ind. Eng. Chem., 1927, 19 (11), pp 1250–1252
DOI: 10.1021/ie50215a017
Publication Date: November 1927

Attachment: taylor1927.pdf (454kB)
This file has been downloaded 209 times

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Page 150 describes the preparation of CuO/CeO2 catalyst pellets by using acetic acid as a binder. Interesting idea...

Selective Catalytic Oxidation of Ammonia to Nitrogen on CuO-CeO 2 Bimetallic Oxide Catalysts
Hung, Aerosol and Air Quality Research,
Vol. 6, No. 2, pp. 150-169, 2006

Attachment: 4_AAQR-06-06-OA-0004_150-169.pdf (744kB)
This file has been downloaded 210 times

[Edited on 7-27-2018 by WGTR]

WGTR - 27-7-2018 at 08:05

Quote: Originally posted by Chemetix  
Smashed up brick or tiles have a great surface area and chemically neutral, It worked with cobalt, platinum oxide would also work.


I don't doubt you for an instant. From observing your fascinating results, and also from a lot of reading, I've seen that a lot of different things can be used to oxidize ammonia to nitric oxide, so long as certain reaction conditions are minded.

From my understanding platinum is so popular in industry for several reasons. When mixed with a bit of rhodium, platinum mesh happens to be a very convenient and durable material to use under the temperatures and conditions of the reaction. Once started, the reactor can run for several months at least, before the catalyst loses about 5% of its weight and needs to be recycled. It the reactor cooled down or stopped for whatever reason, the nitric acid doesn't go all ommm nommm nommm nomm on the catalyst, since it's well, platinum. Also, it seems to be the most efficient and stable material that delivers predictable results.

In spite of its high initial cost, in industry the platinum expense is insignificant. Considering the amount of platinum that is consumed for every pound of acid produced and the cost of the ammonia, I remember reading that even a 1% increase in oxidation efficiency easily pays for the cost of the platinum that is consumed. In our case we're just making bits of acid for the lab, and a single liter of acid may cost quite a bit when shipping is considered. Maybe a 50 lb. bag of urea would costs $20 or less. At that point it doesn't matter if efficiency is 90%, 80%, or even 50%, and a lot of different catalysts and oxidation conditions can be used.

What I'm attempting to do is develop a process that is directly repeatable by others and can be well-documented. In other words, I'm trying to use materials that I can either post the part numbers for, or demonstrate how to synthesize the various materials needed from scratch. An example of that would be when I was demonstrating how to make solar cells, and showed how to synthesize the anatase form of titania from the pottery grade rutile form, rather than trying to source the difficult-to-find anatase form directly. If I use ceria as a catalyst, I'd like to react the oxide to a soluble salt, precipitate and calcinate it, and then form it into beads of a certain size. That way my ceria is the same as everyone else's. Hopefully I have enough time to do that.

WGTR - 30-7-2018 at 03:03

It looks like my quartz tubing will arrive Wednesday, so I'll start picking things up again later this week. I'm going to be working late for most of the week, and might be a bit scarce for a while.

My ceria samples seem to be dissolving well, so I should have enough to try out some different catalyst designs.

WGTR - 4-8-2018 at 23:20

Here's an update:

The quartz tubing arrived, and I selected the 7mm x 9mm tube for the reaction tube. I had thought of holding the catalyst in place with a smaller piece of quartz tubing, but I think I will instead borrow an idea from Heptylene, and use some silica wick.

I checked around town today at some "Vape" shops, but it seems that no one locally carries silica wick. eBay it is, then. I can get several feet for a few bucks including shipping. The idea is to make a loose knot at the end of the wick and fluff it out (or something) to support the catalyst directly on the end of the wick. The rest of the wick will be kept clean (no catalytic activity wanted there) and will be threaded through the reaction tube all the way through the hot zone to the inlet port for the ammonia/air mixture. The wick could be tucked under the tubing just a bit, to clamp it in place.

I put together the condenser and got some deionized water running through it. I made a short video demonstrating its operation. I decided to try a horizontal configuration instead of a vertical one.

Attachment: Condensor.MOV (2.3MB)
This file has been downloaded 542 times

Water comes in through the 3/8" barb fitting at the right rear side and flows left through the PVC tube to the copper fittings. There is a short copper transition from 3/4" to 1/2" tubing. At the left end of this 1/2" piece of tubing the inside diameter is necked down with a length of copper wire, formed into a ring that fits inside the tube. This wittles down the clearance between the quartz reaction tube and the copper pipe to about 1/32". This leaves just enough clearance for water to flow through, keeping the copper piping cool. Any water that squeezes through here goes through the 3/4" "drain" at the bottom of the copper tee fitting. It may be difficult to see from the video, but there is a bit of cotton towel stuffed inside the open end of the copper tubing, wrapped around the quartz tube. As the water squeezes through the small 1/32" clearance in the 1/2" coupling, it tends to travel further along the tube towards the open end, where it can possibly drip everywhere. The bit of cotton towel merely interrupts this flow of water and strongly encourages it to go down the drain instead.

There is another plastic 3/8" barb fitting pointing up from the top of the tube. This has a short piece of tubing connected to it. This serves two purposes:

1. It allows air to bleed completely from the top of the condenser.
2. It allows one to measure pressure at the end of the condenser, and by extension, maintain a given water flow rate through the condenser. The pressure in the video is about 6" of water.

I have a tube furnace already assembled from a previous project that I'll use to preheat the gas flow. Some of the next things that I plan to do are:

1. Remove the reaction tube from the condenser and hook up the inlet to dry N2 and adjust the flow for a rate that is determined by the tube diameter and some existing ammonia oxidation literature. Flow rate can be measured by water displacement vs. time.
2. Insert the tube into the furnace, such that the end of the tube protrudes slightly from the furnace. Outlet temperature can be measured with a thermocouple, and power input for a given flow rate and preheat temperature can be determined.
3. Insert the tube fully through the furnace, and through the condenser. With the furnace active and cooling water flowing, measure the N2 temperature as it leaves the condenser, to determine if heat rejection is sufficient.

If anyone's interested, I intend to document things better as I go. I just sort-of threw everything together right now, on a brain-storming session. Perhaps I can rebuild the condenser from scratch, showing how everything is assembled. All of it's parts come from either Lowe's or Home Depot, and the hose clamps come from Napa.


WGTR - 8-8-2018 at 21:12

I inserted the quartz tube into the furnace and ran some brief tests today.

I calibrated a flow of nitrogen to be 1L/min by measuring water displacement from an inverted beaker vs. time. The 9mm quartz tube was inserted into the furnace just enough so that the hot end barely protruded from the brick. A thermocouple junction was inserted about 2" into the tube to measure nitrogen temperature as it exited the furnace. The nitrogen flow was connected to the inlet of the quartz tube, and the connection secured with a fuel injection hose clamp.

After 30 minutes of running the furnace at 200-250W, a temperature of 520°C was measured by the thermocouple. This temperature seemed to be stable.

IMG_4471.JPG - 1.6MB

Power was removed from the furnace, and the tube allowed to cool down. Then, the tube was inserted fully through the furnace so that the condenser could be installed. Deionized water of about 2" of water pressure (measured at the top nylon barb fitting) was used to provide cooling. Water came in from the right rear end of the condenser, and left through the copper drain on the bottom left. The waste water was just run to a floor drain for this test.

IMG_4473.JPG - 1.6MB IMG_4475.JPG - 1.7MB

With a flow of 1L/min of nitrogen and the furnace operating at 200W, the temperature of the quartz tube after the condenser never deviated from 23.8°C, during the 10 minutes that measurements were taken. With this type of gas flow rate the condenser is more than adequate for the job.

Here's a photo of the cover removed from the tube furnace:

IMG_4477.JPG - 2.1MB

Heptylene - 10-8-2018 at 09:23

Wow! Impressive work WGTR! I must say that furnace look just perfect for heating the reaction tube.

Do you have any concern that the quartz tube might shatter from the high temperature differential (about 500 °C difference between the furnace and the cooling water)? I know quartz/fused silica possess very low thermal expansion coefficients and I heard that a red hot quartz piece can be plunged in water without shattering, although I haven't tested this myself.

At any rate I am blown away by your design and the care and precision with which you report your results!

WGTR - 10-8-2018 at 12:39

Thanks for the compliments Heptylene. I don't think there is any problem with that temperature differential on the tubing. The temperature doesn't drop from 500°C to 25°C all at once; there is a transition that occurs once the tubing leaves the furnace. I discovered that I can measure temperature at various points in the tubing by merely inserting the temperature probe after everything is already installed and operational, like this:

IMG_4480.JPG - 2.1MB

The probe is inserted from the condenser end, to protect the wire insulation. For some reason the thermocouple insulation seems to hold up to the output temperature of the furnace, at least for the few seconds needed to take a measurement. I noticed that there is a considerable temperature gradient from the furnace output to the condenser input.

After thinking about it for a bit, I'd like to go back and measure the temperature at regular intervals inside the tubing, so that I can get an idea of how quickly the furnace heats up the air, and how quickly it cools down again in the condenser. I could plot tube temperature vs. position. I'll have to strip some more insulation off the thermocouple wire for that, so that I can push the junction further into the furnace. Do you think that kind of data would be useful?

Here's another data point after further testing:

Flow rate, air: 3.3L/min
Input power, furnace: 250W
Temperature at furnace output: 526.5°C
Temperature at condenser output: 31.8°C
Condenser water temperature, input: 25.7°C
Water pressure: 4"

IMG_4482.JPG - 1.5MB

I think that I have some quartz wool in another lab, so I'll borrow a page from your book, and try soaking some of it in my hexachloroplatinic acid. Some quick back-of-the-envelope calculations seem to show that a 3-4% ammonia solution should give a 3% concentration in air if air is bubbled through it at room temperature. I should get some results out of that, I would think.

Heptylene - 10-8-2018 at 15:41

If measuring the temperature of the gradient is not too much of a hassle I would be interested in the results. I will probably use a similar heating design for my reaction tube (wrapping nichrome wire coil/sleeve around the silica tube with some rock wool insulation) but I don't know how long the heating sleeve should be.

WGTR - 10-8-2018 at 21:51

I added graduated marks to the side of the condenser and furnace for reference, and photographed the setup with a tape measure for clarity.

IMG_4486.JPG - 1.3MB

My thermocouple could only reach three feet into the tube, and the tube is four feet long. This means that I was able to reach only halfway into the furnace with the thermocouple junction. I could go back later and gather some more data, but I'll have to remove the condenser and slide the quartz tube back through the furnace a foot or so to be able to measure temperatures in the first half of the furnace. Anyway, you can still see the temperature at the halfway point in the furnace, and that it is still rising until about 2/3 of the way through the furnace, then it starts to drop.

With 150W to furnace:
150W_Furnace_Power.jpg - 139kB

With 250W to furnace:
250W_Furnace_Power.jpg - 180kB

I figure that with 200W input, the temperature would max out around 500C.

Right at the 26 inch mark the temperature drops noticeably. This is because there is a cloth ring that is stuffed around the reaction tube at this location. Its purpose is to divert water into the drain that is flowing along the reaction tube, and it remains wet constantly. The temperature is going to vary a bit within the tube, and the gas flow may or may not be a different temperature than the walls of the tubing. The thermocouple junction is generally touching the tubing, not directly sampling the air temperature. This also explains why the tube seems to be getting hotter as it exists the bottom end of the condenser; the gas flow is about 40C as it exits the condenser, and there is no water flowing on the tubing right at that point to cool down the walls of the tubing. The temperature drops off near the end of the furnace. I think this can be attributed to air leakage from the furnace.

I set the air flow on this test to 4L/min, as this seems to approximate the conditions recorded in industry, considering the size of the tubing. It's a starting point anyway.

If a 10% ammonia gas feed is utilized, then a 300C preheat will probably be all that is needed. If ammonia feeds drop to 1-3%, then it may be necessary to raise the preheat temperature to 500-600C.

[Edited on 8-11-2018 by WGTR]

Heptylene - 11-8-2018 at 07:00

Thank you for the measurements, I might get away with using a 10-20 cm furnace then, although my tube is 18 mm outside diameter so extrapolating like is probably not quite correct.

WGTR - 11-8-2018 at 14:04

OK, I'm going to go back and make another set of measurements with a slower flow rate. In the references uploaded in this post for some reason I had understood that the flow rates mentioned there were for ammonia only, not the total ammonia/air mixture. Not sure why I thought that.

I was re-reading the references again as a sanity check, because I'm getting ready to order a flow meter. It's time consuming to calculate flow rate iteratively by measuring air displacement over water. If it's sized correctly it will be more accurate, as these meters are most accurate the closer you are to their maximum capacity. Anyway, the measurements that I took at 4 lpm were for a flow rate that is around 3 times too much under the most optimistic conditions.

A 7mm ID tube (9mm OD) should correspond to an internal area of 0.385 cm2. For a flow rate of 0.636 lpm/cm2, this corresponds to a total flow rate of 0.385*0.636 = 0.245 lpm.

For the fastest flow rate of 3.51 lpm/cm2, this corresponds to a total flow rate of 0.385*3.51 = 1.35 lpm.

Realistically, I think that I'll keep the flow rate around 0.5 to 1.0 lpm. The condenser seems to handle a 1 lpm flow rate easily, and the air exiting the condenser under that condition was practically room temperature.

I'm considering bubbling air though an ammonia solution in water to formulate the correct gas feed. If I'm understanding this correctly, a roughly 2.5-3.0% solution of ammonia in water will give a partial pressure of ammonia that is equal to that of water at 20C. The idea is that as air is bubbled through the ammonia solution, the ammonia concentration in the solution will hopefully remain the same. This would provide a consistent gas composition over several hours if the solution temperature can be maintained.

I pulled some data from the Wikipedia pages (not exactly an authoritative source, I know) to create a chart. I looked at the curve for ammonia, and extrapolated the 2.5% data point.



ammonia and water vapor pressure at 20C.jpg - 55kB


The idea is that air would be dried with a desiccant and measured with a flow meter, then bubbled through the ammonia solution. If desired, the ammonia/air mixture could be dried with calcium oxide before sending it to the furnace.

Assuming 24.5 liters of gas per mole at 25C and 1 atm, that corresponds to 0.0408 moles per liter. With a 2.25% ammonia concentration in air, this gives 0.000918 moles per liter of gas feed. If the yield is 100%, this gives 3.47g of nitric acid per hour at 1 lpm gas flow. This also would consume 0.980g of ammonia during the same time. Feel free to check my math.

[Edited on 8-11-2018 by WGTR]

WGTR - 12-8-2018 at 18:06

I managed to get a full temperature profile of the furnace and the condenser together, but not at the exact same time. The first set of readings was taken of the furnace itself without the condenser installed, after the furnace operated for 1.5 hours. It was necessary to remove the condenser for these measurements, as the temperature probe was not long enough to fit all the way through the furnace with the condenser installed. The next set of measurements was taken after 2.5 hours, after re-installing the condenser. Part of the furnace was re-profiled, in addition to the full length of the condenser.

150W_with_1_lpm_air.png - 80kB

These measurements were rather difficult and time consuming to do correctly, but as can be seen, the temperatures along the two curves still do not match up exactly. This is because even after 1.5 hours the furnace continued to heat up during the following hour.

From looking at all three charts, I think it is apparent where the hottest zone is located in the furnace, and also that the condenser parameters are adequate. After completing this one full temperature profile I don't think it is necessary to record other ones, even when other parameters such as air flow or furnace wattage change. It should be sufficient to measure the temperature at one location in the middle of the hot zone.

Unfortunately, I think it is necessary to add a PID controller to the furnace. I already have one so this is no problem for me. It is an added expense for others, though. At the same time it simplifies control of the furnace. I am currently providing power via an adjustable DC power supply, and most people do not have one of these high power units. In contrast a PID controller with an SSR can run directly from the AC outlet.

I currently have a flow meter on order for the gas supply. I am getting one of these for development purposes, but I don't think it is necessary for everyone to get one of these. It is possible to measure air flow by measuring how fast the air can push water out of an inverted beaker. If it pushes out 1 liter in one minute, then that is 1 lpm air flow.

One additional point is that I managed to center the thermocouple junction within the tube, so it's measuring actual air temperature, and there isn't that characteristic "hump" right there at the end of the condenser that was seen in previous charts.

That is all for now, probably until next weekend.

[Edited on 8-13-2018 by WGTR]

WGTR - 12-8-2018 at 18:42

Quote: Originally posted by Heptylene  
Thank you for the measurements, I might get away with using a 10-20 cm furnace then, although my tube is 18 mm outside diameter so extrapolating like is probably not quite correct.


Keep in mind that the internal area of the tube increases approximately twice as fast as its external surface area. Heat transfer becomes more difficult for a given length of tubing the larger its diameter becomes. The inverse is also true, internal area decreases twice as fast as the circumference, when going to smaller tubing. I know that 48" tubing carries a hefty shipping charge because of the length. That's unfortunate. Off the top of my head I think I paid $30 or $40 for shipping, so I bought $50 worth of quartz just to make it worthwhile. If you want to stick with 18" long tubing, I'd suggest going to a smaller diameter, maybe 4 or 5mm ID.

If you look over my chart with the full temperature profile, you can get an idea how fast the temperature comes up, and how quickly heat is rejected to the condenser. That is for 7mm ID tubing (9mm OD). Hopefully this will get you started.

Referring to the graduations marked on the condenser (in the picture), the quartz tube is submerged fully in water from 3" to the 23" position. From 23" to the 26" position, water trickles along the underside of the tubing until it gets diverted to the copper drain by a piece of paper towel wrapped around the tube.

[Edited on 8-13-2018 by WGTR]

WGTR - 14-8-2018 at 07:56

It may be possible to use borosilicate tubing instead of quartz, if that is a more available option to some people. Quartz will of course, be the best option for thermal shock resistance and temperature rating, but I don't think that the temperature is yet too high to use boro if it's being preheated to around 400-500°C. That is for a straight piece of tubing, nothing fancy. I wouldn't go significantly higher in temperature though, hence the use of a PID controller.

Whenever I've seen someone use preheating for an Ostwald process, the heat always seems to be concentrated in a small area of the tube. Naturally, in that case it will be necessary to use quartz since the localized temperatures need to be high to get enough preheating. At the same time, if the heating is spread over a long length of the tubing, then this may not be an issue. With an 18" long furnace, I noticed that the preheating was gradual enough that even when operating around 500°C, the interior of the furnace wasn't even glowing. In fact, it was necessary to turn off the lights and get acclimated to the dark to even see a faint, dark red glow.

I happen to have a piece of 9mm borosilicate laying around, the same size as my quartz tubing. It's not a full length piece, but at 3.5' it's long enough to bring up to temperature and check for suitability.

Chemetix - 14-8-2018 at 17:58

Just a heads up with boro, it goes soggy very quickly as an oxidiser chamber. The catalyst will get to 700C easily. Boro starts getting a bit mushy at this stage. Quartz, aluminosilicate, or ceramic are going to be the materials of choice.

I'm appreciating your ever precise building style. I am anticipating your first couple of runs.


[Edited on 15-8-2018 by Chemetix]

WGTR - 15-8-2018 at 08:59

Quote: Originally posted by Chemetix  
Just a heads up with boro, it goes soggy very quickly as an oxidiser chamber. The catalyst will get to 700C easily. Boro starts getting a bit mushy at this stage. Quartz, aluminosilicate, or ceramic are going to be the materials of choice.

I'm appreciating your ever precise building style. I am anticipating your first couple of runs.


[Edited on 15-8-2018 by Chemetix]


Thanks for the compliments there, you and VSEPR_VOID. It’s much easier for me to pursue something like this when I know that others are interested in it as well.

I think that your words of caution are well-placed about the use of borosilicate tubing for this type of application, and they are probably prophetic. I don’t plan on trying out the boro right away since I have other points of investigation that are more pressing, and already have a good tube made of quartz to play with. However, I have a number of different ideas floating around back there in my head (too many to try and list), and it seems slightly possible with the right combination of variables that boro might actually work. If it did work it would be marginal for sure, but it would be cool, because I can buy 4’ borosilicate tubing on the way into work every morning if I wanted to, with no shipping charges. The same store doesn’t sell quartz.

Here’s my thought process on this: I’m not thinking of using a 10% ammonia feed. Referring to a chart from this reference https://www.sciencemadness.org/whisper/files.php?pid=525953&...

conversion_efficiency.jpg - 117kB

selectivity to nitric oxide over nitrogen gas begins to be noticeable around 400C with a ceria catalyst. At 500C it’s at 70-80% already. This is with only a 0.05% (500ppm) ammonia feed, so in this case I don’t think the temperature rise is very measurable during that oxidation. Using such a diluted feed would be impractical in our case, since with a 3% feed I think that yields would be around 3g per hour of acid if everything goes more or less correctly. It would be interesting to see how high I can get the ammonia feed % without getting much temperature rise from the oxidation.

Diluted ammonia feeds also give diluted nitric oxide products. These oxidize much more slowly to nitrogen dioxide as the dilution increases. This might seem to be a bug, but really I think it’s a feature. With a 7mm tube ID and, let’s say, a 600ml/min flow rate, that should translate to a flow rate of 10” per second through the tube. I’m already flowing more air than that through the condenser, and the condenser is still overkill. It cools down the air almost immediately. Anyway, the dilute nitric oxide would simply not have enough time to oxidize to a noticeable degree in the condenser due to the matter of seconds spent in the tube.

The output gasses could then be bubbled through a very cold (below freezing) nitrate salt solution in order to remove most moisture from the gasses. Some nitric oxide may be lost as nitrate in this solution, but the idea is that the losses would be very minimal. Silica gel can be used to both remove moisture, and also catalyze the oxidation of nitric oxide to nitrogen dioxide. The former reaction is much faster than the latter, however, and the overall effect is a matter of residence time. Also, moist silica gel does not catalyze nitric oxide oxidation. The first stage of silca gel would be located in a short tube to remove any residual moisture not captured by the chilled salt solution. Gasses would pass very quickly through this. Following a void space for oxidation, the following stage of silica gel would serve to catalyze the oxidation of the dry nitric oxide, and also to capture the resulting nitrogen dioxide. Gasses would pass very slowly through this catalyst bed. Practically I plan to use indicating silica gel for moisture adsorption, and non-indicating for nitrogen dioxide adsorption (it is it’s own indicator).

To measure yield, before and after weights of the second silica gel stage will be taken, and yields will be based on the weight of dry nitrogen dioxide.

WGTR - 19-8-2018 at 22:41

I was hoping to find an ammonia concentration in solution that would allow me to maintain a constant concentration as the solution was evaporated by the air that I would be bubbling through it. Due to excessive brain flatulence (how else can I describe it?) I completely misinterpreted my carefully plotted ammonia chart, and thought that ammonia and water would form an azeotrope at low concentration. Technically it may form one at 0.001% or something, but the main point is that I need way more ammonia than this, on the order of a few percent concentration in air.

Since I can’t rely on the solution remaining at the same concentration during the course of the experiment, I had to decide just how much variation in concentration that I could tolerate during the course of the experiment. Given a certain experiment duration, this in turn would determine how much ammonia solution that I need of a given starting concentration.

This problem requires a bit of calculus, something that I haven’t done in quite a while. I enjoy math, but find it time-consuming, and it makes my head hurt. Also, many times math does not enjoy my company so much, and will fight me by giving stupid answers and mistakes. Hopefully this time will be better. Check my math please! Anyway, although my brain cells were crying, I dusted them off and forced them to wake up.

I’m planning to start with an air flow of 0.6L/min, bubbling through the ammonia solution. It may change a bit one way or the other, but this seems like a good starting point. I also figured to start the experiment with a 4% ammonia concentration in air, and could tolerate a drop down to 3% during the course of the experiment.

Using data pulled from this chart the corresponding values of ammonia concentration in air were plugged in:

For a 4.5% solution (by mass) of ammonia in water:

\[\frac{4.5\%\, NH_3\, sol'n\, mass}{1} * \frac{4.0\%\, NH_3\, vol\, air}{4.5\%\, NH_3\, sol'n\, mass} *\frac{1}{100\% \, NH_3\, vol\, air}*\frac{1mol}{24.5L} *\frac{17.034g\, NH_3}{1mol}* \frac{0.600L}{1min} *\frac{17.034g}{17.034g + 18.06g}*\frac{100\%}{50\%} = \frac{0.0162g\, NH_3}{min}\]

For a 3.5% solution (by mass) of ammonia in water:

\[\frac{3.5\%\, NH_3\, sol'n\, mass}{1} * \frac{3.0\%\, NH_3\, vol\, air}{3.5\%\, NH_3\, sol'n\, mass} *\frac{1}{100\% \, NH_3\, vol\, air}*\frac{1mol}{24.5L} *\frac{17.034g\, NH_3}{1mol}* \frac{0.600L}{1min} *\frac{17.034g}{17.034g + 18.06g}*\frac{100\%}{50\%} = \frac{0.0122g\, NH_3}{min}\]

So at the start of the experiment, 0.0162g of ammonia is stripped from the solution every minute. At the end of the experiment, the ammonia removal rate has dropped to 0.0122g per minute, since the solution concentration has decreased. These values are assuming that I can maintain a constant solution temperature of 20°C. Estimating the slope to a straight line equation:

\[\frac{-\left ( 0.0162 -0.0122 \right )x}{t}+0.0162 = \frac{-0.00400x}{t}+0.0162 \,,\: \: t = total\, time\, in\, minutes\]

Let’s say that we want the experiment to run for 240 minutes, to give time for experimental conditions to reach steady state. We want to be able to get good yield data, and want to minimize the amount of time that the experiment is either ramping up or down. We want to know how much ammonia will be used during the course of the experiment, so we can calculate how much 4.5% solution we need:

\[\frac{-0.00400x}{240}+0.0162 = -0.000016667x+0.0162\]
\[\int_{0}^{240}\left ( -0.000016667x+0.0162\right )dx\]
\[\left | \left [ -0.000008333x^2+0.0162x + C\right ]_{0}^{240}\right |\]
\[\left | -0.000008333\left ( 0\right )^2+0.0162\left ( 0 \right ) + C + 0.000008333\left ( 240 \right )^2-0.0162\left ( 240 \right ) - C \right |\]
\[\left | 0.480-3.89 \right |= 3.41g\,NH_3\]

For a sanity check, we take the starting value for the concentration and plug in the rate for the full 240 minutes: \[0.0162 * 240 = 3.89g\,NH_3\]
Good, the absolute value is greater than our answer. Now plugging in the minimum rate for 240 minutes: \[0.0122 * 240 = 2.93g\,NH_3\]
This also looks good, as the absolute value is less than our answer, and the final answer is reasonably between these two limits. From there we want to determine how much ammonia solution that we need.
\[\frac{3.41g\,NH_3}{1} * \frac{100\%\, NH_3\,sol'n}{\left ( 4.5\%-3.5\%\right )\,NH_3}=341g\,NH_3\, sol'n\]

Checking out this number:
\[\left ( \frac{341g}{1} * \frac{4.5\%}{100\%}\right )- \left ( \frac{341g}{1} * \frac{3.5\%}{100\%}\right ) = 3.41g\]

This is a useful online LaTex equation editor:
https://www.codecogs.com/latex/eqneditor.php

[Edited on 8-20-2018 by WGTR]

Platinum Loaded Ceramic Beads

WGTR - 24-8-2018 at 18:22

I made some ceramic beads and infused them with platinum.

20ml of deionized water was warmed gently in a 25ml glass beaker on a hot plate, and stirred vigorously using a magnetic stir bar. A glass cover was placed over the beaker to keep evaporation to a minimum. 2.00g of bentonite clay was carefully added to the beaker under heavy stirring. Eventually, all of the clumps dissolved into a brown suspension, and the solution was allowed to stir for an additional 30 minutes. Then, 18.00g of alumina was stirred in as well, and 30 more minutes were allowed for the bentonite and alumina to thoroughly mix together, and for the bentonite to fully hydrate.

The resulting suspension had low viscosity, and much extra water. The clay was de-watered by carefully pouring the contents of the beaker onto absorbent paper towels. After standing for several minutes excess water floated to the top of the clay and ran off, or was soaked into the towels. A metal spatula was then used to scrape the clay together from the towels.

With much care, small pieces of the clay were rolled by hand into round beads and pills. The clay seemed plastic enough to form, so long as the moisture remained sufficient. A bit too much moisture caused the clay to become very gooey and soft, and too little caused the clay to become stiff and non-cohesive. The right amount gave a firm clay that could still be gently shaped.

The resulting clay bodies seemed porous enough to dry rapidly, and they seemed dry under warm air after only an hour. Still, they were allowed to dry in ambient room air overnight before firing. In their green state the clay seemed fairly strong and could be easily handled, but no attempt was made to try to test the strength by crushing samples.

The beads were placed in a small kiln (linked in signature) and ramped slowly up to 1200°C over a period of about 3 hours. One half hour was spent at 100°C to make sure that everything was dry, then the kiln was ramped to 1200°C. The kiln spent 20 minutes soaking between 1200°C and 1210°C. The kiln was allowed to cool down naturally to 1000°C, and then the lid was opened partially. Cool-down was rapid at this point.


IMG_4493.JPG - 2.1MB

WGTR - 24-8-2018 at 18:28

The sintered clay samples were quite strong after firing. One piece was dropped from a height of 1 foot, and it just bounced on the table top. They all gave a characteristic sharp “tap” when dropped into a glass dish.

Previously prepared 5mM hexachloroplatinic acid in ethanol was used to prepare the catalyst. I prepared this solution some years ago, and if I labeled it correctly, there is only 10mg of platinum in this 100ml of ethanol solution. If I labeled it incorrectly, then it’s about 100mg of platinum in 100ml of ethanol. I found the reference that I used to prepare the solution, and it specified 5mM. Anyway, it is less than $5 of platinum in the entire container.

One ceramic bead was introduced carefully to a single drop of water. Once touched, the water was immediately adsorbed. The beads appeared to be finely porous, though mechanically strong. Generally, the ceramic beads were gripped with stainless steel tweezers, and heated with a heat gun to make sure they were dry. They were cooled down briefly in cool air and then dipped into the ethanol solution. Vigorous bubbling was evident right when the beads were submerged into the ethanol. The bubbles appeared to be trapped air, as they ascended completely to the top of the ethanol without being reabsorbed.

In the following video, you can see how the bead that is wet with ethanol solution is dried with a heat gun. Upon further heating, the chloroplatinic acid decomposes and platinum appears on the bead.

Attachment: Pt_loaded_ceramic_support.mpg (5.9MB)
This file has been downloaded 423 times

Old video replaced with new one

[Edited on 8-25-2018 by WGTR]

WGTR - 24-8-2018 at 18:40

Once the ceramic bead was heated to the decomposition point of the platinum salt, the bead was cooled down and reintroduced back into the ethanol solution, and this cycle was completed four times in totality. The following video shows the fourth cycle in progress. Note how dark the bead is by this point. Practically none of the ethanol solution has been used, and it’s probably less than a few cents of platinum for each bead.

Attachment: Pt_loaded_ceramic_support_4th_cycle.mpg (4.9MB)
This file has been downloaded 442 times

Can you see the difference between coated and uncoated samples?

IMG_4497.JPG - 1.1MB

If you look closely you can see that there are some small pinholes that are not covered well with platinum. Looking under high mag with an optical microscope, it appears that those small areas are relatively non-porous. The ethanol solution was simply not able to soak into those areas very well.

Video file updated


[Edited on 8-25-2018 by WGTR]

WGTR - 24-8-2018 at 18:55

The last video for tonight briefly shows how the beads bubble when initially placed under the ethanol solution. This bead was a bit warm before being submerged.

Attachment: Pt_loading.mpg (5.8MB)
This file has been downloaded 449 times

Video updated

[Edited on 8-25-2018 by WGTR]

WGTR - 25-8-2018 at 10:46

I just updated the videos in the last few posts. I wasn't too happy with the previous ones. From now on, I plan to use a normal DVD recorder instead of the phone. Too many complications trying to get good video off the phone to a Linux machine.

WGTR - 25-8-2018 at 18:02

I decided to measure the platinum loading % on a ceramic bead, just so i could present a measurable result that is hopefully duplicable by others. I picked out a fresh ceramic ball from my recently fired batch, and weighed it on a microbalance. This was a bit tricky, as I was unsure whether or not the ceramic was picking up room moisture, and if so, how quickly. One additional complication is that the ceramic bead is heated as part of the loading procedure, and heat will cause the microbalance to drift slightly during the measurement. Finally I decided on a procedure that involved setting the bead out in ambient conditions for a fixed amount of time between each weighing cycle, and to make several mass measurements and zero out the balance each time until a stable reading was obtained. The following results were obtained for one bead:

-bare ceramic bead: 0.310291g
-after 1st coat: 0.310301g
-after 2nd coat: 0.310392g
-after 3rd coat: 0.310451g
-after 4th coat: 0.310520g

I should mention, that these measurements were obtained after each successive coating, after the bead was heated enough to decompose the Pt salt. According to these measurements there are only 229 micrograms of platinum loaded onto the bead. That equates to roughly $0.006 of platinum on the single bead at a loading of 0.074%.

Heptylene - 26-8-2018 at 02:37

Incredible! That's a nice catalyst support, I'm guessing the beads fit in the reaction tube right? They seem quite porous from their bubbling in water.

How did you get access to a microbalance? I can't imagine those are common outside very specialized research labs. The best I've seen was at my uni had 10 micrograms resolution.

The coating with platinum by thermal decomposition seems to be a better method than that by reduction with hydrazine which I've been using. The problem with hydrazine reduction is that most of the platinum just forms a suspension in the solution (which I recycle of course) and doesn't stick to the substrate. I'll try thermal decomposition if I need to make more catalyst.

Btw I haven't been able to reply to your U2U, I'm having problems sending messages. I hope this will be fixed soon.



WGTR - 26-8-2018 at 07:43

Quote: Originally posted by Heptylene  
Incredible! That's a nice catalyst support, I'm guessing the beads fit in the reaction tube right? They seem quite porous from their bubbling in water.

How did you get access to a microbalance? I can't imagine those are common outside very specialized research labs. The best I've seen was at my uni had 10 micrograms resolution.

The coating with platinum by thermal decomposition seems to be a better method than that by reduction with hydrazine which I've been using. The problem with hydrazine reduction is that most of the platinum just forms a suspension in the solution (which I recycle of course) and doesn't stick to the substrate. I'll try thermal decomposition if I need to make more catalyst.

Btw I haven't been able to reply to your U2U, I'm having problems sending messages. I hope this will be fixed soon.


Yes, the beads just roll right into the tube...and like bad chili, come rolling right out the other end. But I also have some pill-shaped pieces of ceramic that are heavy enough to hold the catalyst in place. At least this arrangement seems to work at room temperature, with 0.6 liters per minute air flow. I'm hoping that will be sufficient.

I do work in a lab, one that I won't name publicly to respect both its and my respective privacies. But yes, I do have access to a microbalance, as well as an XRAY system, SEM, high res optical microscopes, etc. It's enough to get by. But this is nothing compared to a well-funded public university, who has government grants full of crazy money. A local one has equipment that is so far advanced and expensive that I could only dream of it.

The solution in the video isn't water actually, but ethanol. Hexachloroplatinic acid is soluble enough in it to dissolve completely at these minute concentrations. I originally made the solution so that I could dip coat a platinum coating onto a glass slide, back when I was making counter electrodes for dye-sensitized solar cells (see link in signature). The original journal article that I was following actually specified 100% isopropanol, but the ethanol was more readily available to me at the time. After thinking about it overnight, I really do need to verify the platinum concentration in the solution that I was using. There's little usefulness to the amateur community in taking such care to weigh samples if we don't know how much platinum was in the solution to begin with, right? Like you say, not everybody has a microbalance. Presumably if we know the starting concentration of the platinum solution, and exactly how the ceramic beads were made (mentioned in a previous post), then it should be possible for anyone to get a similar Pt loading that I did. Being able to weigh the results would just be an added bonus. While I'm thinking about it, the bentonite and aluminum oxide are all pottery grade, and came from a local pottery store.

Even if I was confident that my bottle label was correct, over the years some of the platinum has precipitated out as a very fine powder in the ethanol, presumably reduced very slowly at room temperature by the ethanol. This precipitate is quite dense, and sits contently at the bottom of the container. I suppose that I could take 1 ml of the Pt solution and evaporate it in a weighing bottle, reduce it to metallic Pt, dry it, and see what kind of results that I get. If I have to, I can keep evaporating several ml's of solution in the same bottle, until I get enough to weigh reliably.

I'm starting to get a bit impatient (probably like everyone else), and am thinking of running some ammonia vapor through the reactor today, just to do a qualitative test to answer some basic questions, like "Is the catalyst active enough?", "Is it too active?", "Is the catalyst geometry sufficient?", and most importantly, "how does pH paper respond to the gases coming from the condenser?". I'm not planning on taking any real measurements at this point.

I was honestly not expecting the platinum to be that visible on the ceramic. That was a bit of a surprise. In regards to the bubbling observed when inserting the ceramic beads into the ethanol, I'm a bit undecided on what the gas is. I initially decided that it was trapped air, and it may be. As I mentioned before, however, the beads were still warm from being heated by the heat gun. When the beads are at room temperature they still bubble, but very slowly and not as much, I think. While it's possible that the faster bubbling when the beads are warm is due to a change in surface tension of the alcohol at the localized higher temperatures, it's also possible that the gases contain carbon dioxide from the decomposition of ethanol. When the beads are very warm, it's conceivable that platinum metal internal to the bead is quite hot, and catalyzes the decomposition of ethanol with the air that's trapped internally. Some water or acetic acid or something would be produced as by-products. If you look carefully at the video, you can see that something is floating off of the bead other than just gas, maybe some product is dissolving into the alcohol. It's not something I intend to investigate carefully, anyway.

[Edited on 8-26-2018 by WGTR]

WGTR - 26-8-2018 at 17:44

I checked the mass of platinum per ml for my Pt/ethanol solution, and came up with some reasonable results. I tared the measuring vial at 2.230,588g. 0.5ml of the solution was evaporated in the vial, and then the remaining Pt salt was decomposed with heat to leave a black residue in the vial. After cooling, the total mass of 2.231,227g was obtained.

\[\frac{1\, mol\, Pt}{195.08g\, Pt} *\frac{\left ( 2.231227g-2.230588g\right )}{1}*\frac{1}{0.5\, ml}=\frac{0.000,006,551\, mol}{1\, ml}\]

or, 0.000,655 mol/100ml.

This is neither the expected 0.005 mol/100 ml or 0.000,5 mol/100 ml, but the results are fairly close to the latter. Some measurement error is likely in all the different steps that I performed from synthesis to final mass measurement. Over the years, it's possible that some alcohol evaporated from the bottle. Anyway I think that the original label reads correctly, "5 milli-molarity H2PtCl6 in 100ml ethyl alcohol". The measurements are giving roughly 1mg Pt per ml ethanol.

In reality, any rough concentration of platinum in alcohol similar to 0.5-1mg per ml should give you results similar to what I got. You can buy 1 grain of platinum on eBay for less than $7 including shipping. This is about 67mg of Pt, enough to make between 50 and 100ml of solution. I used ethanol, but isopropanol should work also as the solvent.
,
Edit: You know, I just checked the molar mass of PtCl2, and it is about 266 g/mol. Platinum is about 195 g/mol. The difference in molar weights is about the same as the error that I'm seeing in the measurements. So maybe my measurements are only a few % off, and I'm just not heating the platinum salt hot enough to achieve pure platinum, but rather platinum (II) chloride. The next task is to check the catalyst for activity.


[Edited on 8-27-2018 by WGTR]

Tonight, I joined the Ostwald club...

WGTR - 28-8-2018 at 19:30

I’ll have wait for the weekend to say much due to time constraints. This was just a qualitative go/no-go test. However, I bubbled 150ml/min of air through a 2-3% solution of ammonia using one of those fish tank air stones, with 300W input to the furnace. Three catalyst beads were used, placed in between two bare ceramic pills to hold them in place. I started with 600ml/min but found that it was way too much air flow; ammonia was getting through and forming a cloud of ammonium nitrate after the condenser. It’s not a surprise, really. The catalyst beads block most of the tube, and I was calculating air flow based on the old figures related to platinum mesh.

I had to bump up the furnace temperature a bit. Unfortunately Chemetix is right, borosilicate isn’t going to work in this application. In room light the furnace was glowing dull orange; in the dark it was more of a bright orange. I’ll have to measure the exact temperature when I have the chance, but it’s probably around 800°C.

I put a bit of water in a 2l vacuum flask and bubbled the brown gas through that. There was still some faint yellow coloration in the flask, but a lot of it seemed to be absorbing into the water. The clear tubing leading to the flask was noticeably colored light brown/yellow.

After a couple of hours I ended up with about 50ml of acid, but I need to titrate it. The pH measured at 1, which doesn’t mean a whole lot. I’ll dilute a bit of it out 10:1 and recheck the pH as a quick test. I’d be surprised if I had more than 0.5g of acid in there, with all of the different flow rates, temperatures, and everything else that I was trying. I originally calculated my anticipated yield based on a 600ml/min flow rate and a 3-4% ammonia concentration in air. With a slower flow rate, inefficient gas absorption, and some acid left in the condenser, well, I’m not expecting too much.

I looked at the catalyst beads after the experiment, and they look just like the day they were born, no apparent change. The air flow didn't push them down the tube; they stayed put. There was no staining or damage on the quartz tubing that I could see. The clear PVC tubing seemed to hold up to the cool exit gases and acid vapor, but it remains to be seen how long it will last.

For now, some pictures...

WGTR - 29-8-2018 at 16:31

Furnace glowing:


IMG_4507.JPG - 1.2MB

Catalyst beads after furnace cool-down:


IMG_4510.JPG - 2.7MB

Jar full of NO2:

IMG_4511.JPG - 1.4MB

[Edited on 8-30-2018 by WGTR]

WGTR - 30-8-2018 at 17:28

I've let the reactor run for a few hours, and it looks like right now I'm getting around 0.5-1 gram of acid per hour. Those numbers resulted from some quick tests. If those numbers hold during later tests, then conversion % of ammonia to nitric oxide is not bad. I really need to let it run a full day to get a better idea, as a certain amount of acid gets held up in the condenser and tubing. Once the concentration of acid gets high in those areas, the acid starts breaking down back to nitrogen dioxide, and that gas gets pushed closer towards my absorption system. Right now I'm getting about 0.5 grams per hour absorbed into a 250ml graduated cylinder. I'm probably losing quite a bit of nitric oxide from there, as my absorption system isn't even close to being large enough.

It's not a phenomenal amount of acid production by any means, but I wonder what people here really hope for. A liter of acid per month? Per week? Or...per day? For me, I have a large 4 liter beaker that had stubborn carbonate stains all over it. I cleaned it with some diluted acid from an earlier run and the whole beaker shined right up. But I don't have any real needs for large amounts of nitric.

Right now I'm using about 300W in the furnace, which is too much $. I'm considering rebuilding the furnace to be a bit tighter and smaller, and will hopefully get the power down to about 1/3 what it is now.

PVC Tubing and 800°C Ceramic Beads have Compatibility Issues

WGTR - 30-8-2018 at 19:41

That is an important lesson that I learned tonight. This may seem obvious, of course, but sometimes the tired or the stupid need to be reminded of such things.

Tired and in a hurry, I wanted to leave the lab quickly. Usually I will pull the air tube out of the acid in the graduated cylinder, and then switch the gas feed to dry nitrogen for 15 minutes, to let the reactor, condenser, tubing, etc., dry out completely. During this time I'll run cooling water through the condenser, because the reactor is still very hot, and the nitrogen is carrying that heat through the tube to the condenser. I'll lift the top off the furnace so it will cool down faster. Once things are fairly cool, I turn off the nitrogen flow, shut off the water supply, and take off.

This time I decided to do things a little quicker, and shut off the air flow immediately. This way, no extra heat would be carried from the furnace to the condenser, and I could shut off the water supply right away. The first problem was that I didn't pull the tube out of the graduated cylinder (that was full of acid) first. Second, instead of shutting off the air valve to the reactor, I simply popped the hose off the fitting. This allowed water to rise abruptly through the tubing in the graduated cylinder, and the sudden change in pressure ejected the catalyst right out the front of the reactor, right into the blissfully unaware PVC tubing.

Sooooo anyway....I have a bit of a cleanup job to do. Aside from the melted tubing there is a layer of dark soot inside the reactor that I have to clean out somehow. Maybe I can burn it out. Also, the catalyst beads are fairly well-covered in burnt PVC.

I could burn all the crud off the beads in the kiln and probably reuse them, but then again I could take this opportunity to made a new catalyst support with a bit better geometry. The round beads are easy to make and they do work, but I'd like to get much better air flow rates than what I'm currently getting with the spherical geometry. I have been entertaining the idea of making a ceramic disk that is carefully drilled full of 0.010" holes, and then loading that with platinum. I'm hoping with that, I'll have significantly better porosity and higher allowable flow rates, before ammonia starts slipping past the catalyst unconverted.


WGTR - 3-9-2018 at 06:43

I have a couple of hours of video from the last batch of experiments, but have to edit it down before posting anything. Also, there is some more video that I'd like to shoot using what I've learned from this latest go-around.

I made another catalyst support, but instead of making beads this time, I made a ceramic screen. Using the same alumina clay composition as last time, I pressed some clay into a 0.25" hole drilled through aluminum plate. The plate was used as a mold. The clay dried in the mold will practically no noticeable shrinkage. When dry, small 0.031" holes were drilled in the clay using a diamond burr bit. I used a small high speed drill press made for small work like this, but it was still very hard. Some of the hole walls are only 7 mils thick, but they did not break when drilling or handling. A previous one that I tried to make using a handheld Dremel tool was a mess. I simply was unable to hold the tool steady enough.

After successfully (!) drilling all the holes, I had to find a way to remove the fragile piece of dried clay from the mold without breaking it. It was too tight a fit to be pressed out without destroying the piece. Next time around I could make a 2 piece mold and simply take it out that way, but this time, I decided to dissolve the mold with HCl. The clay would also break apart in this solution, so I first made it waterproof by melting microcrystalline wax into the clay body. The wax soaked in like a sponge.

After this wax treatment the mold was dissolved away, and the clay piece survived with minimal cosmetic damage. The clay was then fired to 1200C, which at the same time burned away all the wax. After cooling down the ceramic screen, I accidentally gave it a drop test, and it simply bounced on the tabletop. I thought for sure it would break, but it didn't.



IMG_4515.JPG - 1MB

I used the same firing schedule that I did for the beads, and due to the very thin wall features, I think I overfired the piece a little bit. It wasn't quite as porous as the beads, but still soaked up enough platinum solution to give a good coating.


IMG_4517.JPG - 669kB

wg48 - 3-9-2018 at 08:16

That’s impressive work given its 0.25in dia.

Heptylene - 5-9-2018 at 08:41

I find it impressive how dark the catalyst is colored given the small amount of platinum that is present.
After your success with the catalyst beads, I'm sure this new catalyst will work great!

I ran a quick test this afternoon at a much reduced flowrate compared to my previous attempt to see if air cooling would be sufficient to cool the tube.
I bubbled air through about 10 % commercial ammonia solution at an air flowrate of 0.6 liters per minute. The resulting gas mixture, which should consist of about 10 % ammonia by volume, was passed through the tube (18 mm OD, 16 mm ID) where the catalyst is located. I used a blowtorch to heat the catalyst to red heat. The gasses coming out of the tube were brown from nitrogen oxides and after a few minutes the tube was still cool to touch about 15 cm downstream from the catalyst. The gasses in the tube move at about 5 cm per second.

A sample of the gas was collected in flask with some water and was found to be acidic, showing that a large portion of the ammonia was converted.

Btw this 0.6 LPM flowrate is just what my cheap aquarium air pump outputs. Assuming a 100 % conversion, this should consume a few moles of ammonia per day.

So my next step will be building a heating element for the tube to replace the blowtorch. Using a cheap lights dimmer seems to be the most cost effective way to regulate the power going through the element. Those can be bought on ebay for $2 apparently.

WGTR - 5-9-2018 at 13:57

Quote: Originally posted by Heptylene  
So my next step will be building a heating element for the tube to replace the blowtorch. Using a cheap lights dimmer seems to be the most cost effective way to regulate the power going through the element. Those can be bought on ebay for $2 apparently.


I think you have an excellent idea. I recently bought both 24 and 16 gauge Kanthal heating wire from Temco Industrial. I originally used 24 gauge wire for the tube furnace. While it works fine, it's rather flimsy when wound into a 1" diameter. I was thinking of migrating to the heavier 16 gauge wire for the added structural stability and durability. The catch is that the 16 gauge wire has very low resistance, about 0.324Ω per foot. I figured that with a few feet I could get to 1Ω, and then use a 12V transformer for power. This would work for sure, but would require a 150W+ transformer. If I also wanted to use a dimmer, it would have to be one rated for inductive loads. Those types are several times more expensive than the usual ones that are used to control simple resistive loads like light bulbs (or heating elements).

After thinking about it some more, it would be so much easier to just drive the heating element directly from 120VAC through a generic dimmer, and just deal with the flimsy, higher resistance, wire. I'm being too much of a perfectionist; the current wire selection is not a problem, really. It's not like the wire is being over-worked, or is going to fail any time soon, and you can see from the pictures that it holds its shape fairly well.

Quote: Originally posted by Heptylene  
I find it impressive how dark the catalyst is colored given the small amount of platinum that is present.
After your success with the catalyst beads, I'm sure this new catalyst will work great!


I'm hopeful. I've got the open area of the catalyst up to about 40% of the tubing ID. That's still less than the 60% or so obtained in industry when using platinum/rhodium wire mesh, but it's better than the 10-20% that I was getting before with the bead-shaped geometry. Hopefully this translates into a higher maximum flow rate through the reactor, before ammonia starts passing through unconverted. Fingers crossed...

Quote: Originally posted by Heptylene  
I ran a quick test this afternoon at a much reduced flowrate compared to my previous attempt to see if air cooling would be sufficient to cool the tube.
I bubbled air through about 10 % commercial ammonia solution at an air flowrate of 0.6 liters per minute. The resulting gas mixture, which should consist of about 10 % ammonia by volume, was passed through the tube (18 mm OD, 16 mm ID) where the catalyst is located. I used a blowtorch to heat the catalyst to red heat. The gasses coming out of the tube were brown from nitrogen oxides and after a few minutes the tube was still cool to touch about 15 cm downstream from the catalyst. The gasses in the tube move at about 5 cm per second.

A sample of the gas was collected in flask with some water and was found to be acidic, showing that a large portion of the ammonia was converted.

Btw this 0.6 LPM flowrate is just what my cheap aquarium air pump outputs. Assuming a 100 % conversion, this should consume a few moles of ammonia per day.


That is very cool. So I suppose that you're not having any trouble with the tubing melting anymore? Are you noticing any ammonia getting past the catalyst at that flow rate? It seems that moisture condensing on the tubing is normal, but the tell-tale evidence of ammonia is if there is a fog of ammonium nitrate at the output. Air cooling would certainly be preferable over water cooling if for no other reason than simplicity. I may try that as well, although with a condenser I can manipulate the temperature at will for testing purposes.

I did notice from my temperature measurements that both my furnace and condenser are considerably larger than they need to be. I'm going to try shortening the furnace by half, and the condenser down to about 20cm (8") or so. Hopefully that would allow a tubing length of 2' or so, allowing much lower shipping costs, etc.


[Edited on 9-5-2018 by WGTR]

Wow

WGTR - 5-9-2018 at 19:17

I went from running a maximum of 150ml/min of air with three round catalyst beads, to 600ml/min with one honeycomb bead! And still no magic ammonia smoke in sight! I'm pleasantly--no, ecstatically--surprised!

I stopped at 600ml/min, primarily because I didn't want to blow the catalyst down the tube. I need to think about a good way to fix it into place. I'm reluctant to use the pill shaped pieces of ceramic that I have, because I don't want to obstruct the gas flow through the center of the catalyst. Perhaps if I have an appropriately-sized piece of quartz tubing that will fit around the edge of the catalyst, I can use a short piece to hold it into place that way.

I need to find some of that 10% janitorial ammonia. I think ACE carries it. Right now I'm using about 3% concentrated ammonia, and don't want to use my ACS graded ammonia to bring it up to 9-10%...too expensive that way.

[Edited on 9-6-2018 by WGTR]

The Blue Color of Dinitrogen Trioxide

WGTR - 9-9-2018 at 10:28

No, the solution isn't gravity-defying, for some reason the picture just loaded sideways.


IMG_4536(1).JPG - 533kB


One thing that I’ve noticed when running my Ostwald reactor, is that a lot of moisture is produced:

4 NH3 + 5 O2 → 4 NO + 6 H2O [1]

That’s to be expected with so much hydrogen being present in the ammonia. This may seem desirable; after all, we need water to produce the acid. One problem is that more water is produced in the reaction than we really need for this overall reaction:

4 NO2 + O2 + 2 H2O → 4 HNO3 [2]

Basically we have three times more water than needed.

A second problem is that the overall reaction to nitric acid doesn’t occur in one step. First nitric oxide is oxidized to nitrogen dioxide in air:

2 NO + O2 → 2 NO2 [3]

This is a relatively slow reaction, taking seconds, minutes, or days depending on concentration. At extremely dilute concentrations this reaction may effectively never complete. The reaction with water is somewhat complicated, but is

3 NO2 + H2O → 2 HNO3 + NO [4]

overall under the conditions of bubbling the dilute gases through warm water or concentrated nitric acid. This reaction is reversible, and by bubbling nitric oxide through concentrated acid, nitrogen dioxide can be reformed and stripped away, diluting the acid with water. Thus it is important to ensure that ample time is given for [2] to occur in order to obtain the maximum concentration of nitric acid. Nitrous acid is an intermediate that forms during the overall reaction of [4]:

2 NO2 + H2O → HNO3 + HNO2 [5]

Nitrous acid can either disproportionate and form nitric oxide and more nitric acid:

3 HNO2 → HNO3 + 2 NO + H2O [6]

or under very cold conditions can form the anhydrous dinitrogen trioxide:

2HNO2→ N2O3 + H2O [7]

The same compound can be formed directly from partially oxidized nitric oxide below 0°C:

NO + NO2 ⇌ N2O3 [8]

The chemistry of nitrogen oxides and water is quite complicated, and I’m not doing it justice. The book “Absorption of Nitrous Gases” is in the Sciencemadness library, and dedicates a generous number of pages to the topic.

Anyway, armed with the previous information, I plan to make some changes to the reactor design, and borrow some ideas from Heptylene in the process. Recently I ran some clear PVC tubing from the condenser output to a container filled with ice, with the intention of lowering the vapor pressure of water enough to remove most of it from the product gases. This kind of thing has to be done quickly after the ammonia oxidation reaction takes place, so that most of the NO does not have enough time to oxidize to NO2. NO does not react with water by itself, while NO2 does. Without removing moisture with this ice bath condenser, moisture gradually reacts in [5] downstream. One byproduct of this reaction is the generation of heat, which in turn causes the acid product to vaporize. This mist gets carried along, where it deposits as a thin film on pretty much everything downstream.

Even with my overly-long condenser, I was still able to remove almost all of the moisture just in a couple loops of PVC tubing submerged in ice, without losing much NO. The gases in the downstream NO oxidation chamber remained clear and brown, with no cloudiness or traces of moisture showing up on the glass walls. Any NO that was already oxidized before hitting the ice bath was simply trapped in the tubing as dilute nitric acid.

The resultant NO2 was then led into a “cold finger”, basically just a test tube that was partially submerged in liquid nitrogen. I happen to have large quantities of liquid nitrogen just sitting around so it’s handy to me, but dry ice, a freezing salt mixture, or running a tube through a cold freezer might work just as well. The result was a frozen mixture of N2O4 and N2O3, a rather pale greenish solid. Letting it warm up somewhere below 0°C allowed a rather pale-green liquid to gather at the bottom of the test tube. Slowly adding a few drops of water (this is exothermic, so this has to be done gradually, and with cooling, to keep the mixture around 0°C), caused the formation of two layers, one clear, and another one deep blue. The deep blue layer is quite dense and is primarily N2O3, whereas the upper layer is concentrated nitric, up to 60-70%, with possibly some N2O4 still dissolved in it. It is not possible to obtain greater than 60-70% nitric by only adding water to cold N2O4, because of the equilibrium that is established between nitric acid and N2O3. If 100% acid is desired, oxygen gas can be introduced to the mixture, and the blue N2O3 layer will slowly disappear as it oxidizes and forms more nitric acid. If red fuming acid is desired, less water is added than theoretically required, while adding oxygen.

Getting back to my planned design changes, I think that Heptylene has a good idea, and that a water condenser isn’t needed on the quartz reactor tube itself. The tube cools down below 300°C in air before even 15-20 cm or so. Kapton tape is usable up to 300°C, and I think that it’s possible to join the quartz tube to a length of borosilicate or soda-lime tubing while the gases are still fairly hot, by wrapping the joint with Kapton. The advantage to this is that even borosilicate tubing is pretty easy to shape over a propane torch, and can be shaped into a “U” or something that can be submerged in ice. An additional advantage is that I would feel more comfortable turning my back to something like this while it’s running, because I wouldn’t have to worry about the drain backing up in the condenser and running water out under the door into the hallway, if you know what I mean. Finally, it would allow a pretty short tubing length after the reactor, allowing minimal loss of NO to oxidation in the ice bath. Further design details need to be worked out in the lab.

Morgan - 9-9-2018 at 21:44

Some tibits in the link maybe of interest .. perfecting the industrial process.
"Under the strongly oxidizing conditions of the Ostwald process, considerable quantities of volatile platinum oxides, primarily platinum dioxide, are formed which are carried off by the gas flow and thus can lead to high precious metal losses during the course of typical process campaigns lasting several months. In the 1960s it was found that palladium gauzes installed beneath the platinum gauzes (gas stream from above) can catch, i.e. reclaim, a large proportion of the platinum. Initially, palladium-gold alloys were used for the catchment gauzes, in particular PdAu80/20. However, since the 1980s the alloy PdNi95/5 has been used almost exclusively. The catchment of platinum is presumably achieved via an exchange reaction between the platinum dioxide in the gas phase and metallic palladium resulting in the formation of palladium oxide. The value of the palladium lost in this way is normally considerably less than the value of the reclaimed platinum."
http://www.chemgapedia.de/vsengine/vlu/vsc/en/ch/25/heraeus/...

Heptylene - 10-9-2018 at 14:11

@WGTR
Kapton tape! I knew that some gas chromatography columns (which are heated in an oven) were made of polyimide, but I had no idea that there existed polyimide tape! Since I have 3 pieces of quartz tube (18x500 mm) I could always join them if needed in the future. Or join a borosilicate condenser for that matter.

I should take a look at that book on nitrous gasses, looks interesting and directly relevant to what we're doing.

@Morgan
I had always thought of the catalyst as something that would not deteriorate. Let's hope this loss of platinum will not be too significant for an amateur reactor. Depositing additional platinum won't be expensive given the small quantities needed, but might be time-consuming if it has to be done too often.

Chemetix - 10-9-2018 at 19:18

"Basically we have three times more water than needed.

A second problem is that the overall reaction to nitric acid doesn’t occur in one step. First nitric oxide is oxidized to nitrogen dioxide in air: "

This was what I found to be the limit of the concentration available from a simple NH3 oxidation, I was getting a crude value of 8M from the first condensation. The answer I suspect is a long length of black irrigation tube 30 or 60 meters long in a coil, and about 25mm dia for long residence times for the oxidation to NO2 and the subsequent reaction with water to re-emit NO in a iterative sequence towards full absorption.

As for concentrating the acid once made, does a distillation from calcium nitrate seem a reasonable way to break the azeotrope with water? Seems a little easier than working with semi cryogenic oxidations with oxygen and N2O3 with water. Although the chemistry has always fascinated me since reading about N2O4 as a rocket oxidant, The image of blue N2O3 is really amazing.

WGTR - 10-9-2018 at 21:56

Quote: Originally posted by Chemetix  
This was what I found to be the limit of the concentration available from a simple NH3 oxidation, I was getting a crude value of 8M from the first condensation. The answer I suspect is a long length of black irrigation tube 30 or 60 meters long in a coil, and about 25mm dia for long residence times for the oxidation to NO2 and the subsequent reaction with water to re-emit NO in a iterative sequence towards full absorption.

As for concentrating the acid once made, does a distillation from calcium nitrate seem a reasonable way to break the azeotrope with water? Seems a little easier than working with semi cryogenic oxidations with oxygen and N2O3 with water. Although the chemistry has always fascinated me since reading about N2O4 as a rocket oxidant, The image of blue N2O3 is really amazing.


The idea of using irrigation tubing is an interesting one. I wonder if it would hold up OK to the acid? I think that for it to work right, pure water would have to be added at one end, and the nitrous gases would have to be piped in at the other, such that both were flowing in opposing directions. That way the most concentrated nitrogen dioxide meets the most concentrated acid just before it leaves the tube, and the weakest acid interacts with the weakest leftover gas concentration. That's how it's done in industry, and should give the highest acid concentration.

It sounds like you were getting about 50% acid, if my brain is working correctly at this time of the night. That's not bad at all. I'm not prepared to say much about distillation with nitrate salts, as I've never tried it.

While I'm thinking about it, I'd like to mention some thoughts on running the reactor continuously, rather than loading fresh ammonia solution as a batch process. I'm currently bubbling air through ammonia of a specific concentration and temperature in order to obtain a specific ammonia/air mixture. This seems to work fine over a period of several hours. Ammonia is gradually stripped out of solution as time goes on however. In order to replenish what is removed, I'm thinking of an ammonia generator with urea and sodium hydroxide, with the ammonia output mixed directly with the air feed. Of course, under these conditions the ammonia/air mixture is unknown, as it proves difficult to control the exact parameters of the ammonia generator. What I suggest, is then taking this gas mixture and bubbling it through the ammonia solution as is already being done. The ammonia solution then acts as an ammonia reservoir. The generator can run for a few hours, during which the ammonia solution slowly increases in concentration as the gases bubble through it. When the generator is turned off, the ammonia concentration slowly decreases as the air strips it from solution and carries it to the reactor. Momentary surges or interruptions in ammonia production would have no affect on the operation of the reactor with this type of setup. An added benefit is that with the generated ammonia being mixed directly with air before bubbling it through ammonia solution, suck-back is prevented.

Quote: Originally posted by Heptylene  
@WGTR
Kapton tape! I knew that some gas chromatography columns (which are heated in an oven) were made of polyimide, but I had no idea that there existed polyimide tape! Since I have 3 pieces of quartz tube (18x500 mm) I could always join them if needed in the future. Or join a borosilicate condenser for that matter.

I should take a look at that book on nitrous gasses, looks interesting and directly relevant to what we're doing.

@Morgan
I had always thought of the catalyst as something that would not deteriorate. Let's hope this loss of platinum will not be too significant for an amateur reactor. Depositing additional platinum won't be expensive given the small quantities needed, but might be time-consuming if it has to be done too often.


Yes, Kapton is like some kind of miracle tape! Here, I made some changes to the reactor design this evening. The furnace was cut in half (9" long now), and the water condenser was removed and replaced with some borosilicate tubing in an ice bath. The quartz reactor tube is joined to the borosilicate tubing with Kapton tape. The boro was wound into a spiral condenser by hand (easy with a propane torch). Power input to the furnace was cut from 300W to 150W, saving energy. It could probably be less than half of this if I insulated things better. I keep the furnace running at bright red. Right now the catalyst is the honeycomb type, with a 300ml/min air flow rate, and an ammonia solution of 6% concentration. The tubing has 7mm ID, the same as before. I believe it is possible for someone to use a 12" long quartz tube in this application, meaning that it should not be so expensive for people to obtain this.


IMG_4537.JPG - 1.9MB

I looked at my earlier temperature profile measurements to determine how much quartz tubing length I needed outside of the furnace before the Kapton joint. Right now I have it at about 4" of spacing, and the joint stays barely warm to the touch. The Kapton seems to be holding up OK.

Unfortunately, I didn't realize that I've been working on this project for a month and a half now. I'm getting very behind on some other stuff, and need to move on after the end of this weekend. Are there any questions about my current setup, requests for better pictures, etc? I'm hoping that there is enough information in our posts at this point, between you, Chemetix, others, and myself, for someone else to put something together and get it working.

Here's some home-made copper nitrate:



IMG_4538.JPG - 1MB

Chemetix - 10-9-2018 at 22:25

The Ammonia generator has been one of those things that roll around in my mind as I come up with a better way of doing it. You are getting close to my idea of running an absorption tower and stripping tower with some sort of reservoir of fluid to keep the fluctuation of concentrations to a minimum.

Two towers with a vigreaux style mixing surface running vertically with a fluid pumped from the bottom of the first tower to the top of the second tower and back via the reservoir to the first tower again in a closed loop.

Gas from an ammonia generator gets directed into the bottom of the first tower and and absorbed. Air gets pumped into the bottom of the second tower where a constant stream of dilute ammonia exits the top of the second tower to be fed into the oxidiser.

This was going be a way of bypassing the CO2 from the urea decomposition reaction and still keep the reactor running fairly constantly even when a urea batch gets changed. But any ammonia generating reaction could feed the scrubbing column.

Morgan - 11-9-2018 at 06:24

I was wondering if you etched the inside of a quartz tube with bifluoride if you could get the platinum catalyst to stick to it, as a way to increase the output or yields?

As an aside, it might be fun to construct a Döbereiner lighter with the platinized ceramic or a lighter using the ceramic beads and fine platinum wire ignited using methanol vapor.

"Humphry’s cousin Edmund Davy, working at the Cork Institution, was then carrying out a series of researches on the chemistry of platinum. In the course of this he found that platinum sulphate could be reduced by alcohol to platinum in finely divided form. The platinum powder, Davy observed, reacted strongly with alcohol vapour at room temperature, remaining white hot until all the alcohol was consumed. “This mode of igniting metal”, he remarked, “seems to be quite a new fact in the history of chemistry; but the means of keeping it in a state of ignition is only another illustration of the facts previously pointed out by Sir H Davy” (2).

"On 3rd August Döbereiner produced an even more striking version of his experiment. Instead of the previous static arrangement, he directed a fine jet of hydrogen at the platinum from a distance of 4 cm, so that it was mixed with air before reaching its target. This had the effect of making the platinum immediately white hot and igniting the hydrogen jet. More excited letters were dispatched, commenting that “this experiment is most surprising and amazes every observer when one tells him that it is the result of a dynamic interaction between two types of matter, one of which is the lightest and the other the heaviest of all known bodies” (4).

"The warmth of this appraisal becomes all the greater when one considers Berzelius’ earlier low opinion of Döbereiner. In July 1821 he had commented to Gaspard de la Rive, professor of chemistry in Geneva, “I do not know whether he (Thomas Thomson) or Döbereiner in Germany is the worst chemist in existence at the moment” (12). On 30th October 1823, however de la Rive found himself grudgingly writing to Berzelius: “We have had nothing new here since Döbereiner’s experiment with his platinum; in view of the reputation of the said Döbereiner we were sceptical of it; it is, nonetheless, true” (13).

"By 1828 some 20,000 Döbereiner lighters were in use in England and Germany alone, and it eventually found its way into most European countries. In spite of the invention of the safety match in 1848 by one of his former students, R. C. Böttger, the Döbereiner lighter was still in use at the beginning of the First World War. Part of its attraction lay in the scope it offered to the imaginative decorator: Döbereiner himself suggested that one could “embellish it with two alchemical symbols, namely the lion and the snake, and so arrange it that the snake takes the place of the capillary tube for the stream of hydrogen and the open jaws of the lion sitting opposite the snake hold the platinum” (15).

The Pivotal Role of Platinum in the Discovery of CatalysisThe Pioneering Work of Johann Wolfgang DÖbereiner During the 1820s
https://www.technology.matthey.com/article/30/3/141-146/

Heptylene - 14-9-2018 at 01:35

I just ordered some nichrome wire and a very cheap light dimmer on ebay. This should become a high temperature pre-heating sleeve for the reaction tube.

I've tried to make more catalyst by the method you used WGTR: pyrolysis of hexachloroplatinic acid. However I only have potassium hexachloroplatinate, which doesn't decompose below about 500 °C from what I've seen. This is not a careful measurement because I don't have a high temperature furnace nor a high temperature thermometer. Rather I dipped a piece of silica in a solution of the salt and placed it inside a beaker on my hotplate. I oberseved almost no decomposition at the max temperature of my hotplate: 500 °C.

In a propane flame the decomposition is much more complete (dark deposit of Pt), but this has the side-effect of weakening the silica fiber and making it brittle.

So I'm currently making some hexachloroplatinic acid, which should be easier to decompose.

@Chemetix I'm not sure I understand the setup you propose for generating the ammonia/air mixture. Are the two columns on top of each other or are they separate? Could you provide a drawing?

WGTR - 14-9-2018 at 06:22

Yeah, the ammonium salt has the advantage that there's no leftover residue other than platinum itself. It's worth the extra effort to make it, and a 100ml bottle of the weak solution should last you forever. I used ethyl alcohol as a solvent. I don't know how much effect that has on how easy the salt is to reduce later since I didn't try using water as a solvent, and my catalyst support was also quite absorbent.

I did find some high silica content wool in the lab, but I personally wasn't too happy with it. It felt like fiberglass, and after being heated with a propane torch became very brittle and friable. The stuff just broke into pieces just the right size to make hands and lungs itchy. :( If you don't mind me asking, what country are you located in?

If you can't get some insulative firebrick, then perhaps some ceramic wool would work. You could ask Chemetix what he was using. I think you'll need some kind of insulation, although you could compensate by using lots of power in the heater.

Chemetix, I'm not sure that I understand your idea either, but it's probably because I'm either making or not making certain assumptions about it. Could you describe the theory behind how it should work?

For the benefit of people reading, be careful handling platinum salts. The cautions about platinum salts being strongly sensitizing for people are no joke. Gloves are recommended. This stuff can sensitize people to various skin ailments at any time in their lives. Certain people are genetically predisposed towards skin sensitivities. I've known people who lived a good part of their lives with no problems, and then got some cleaning chemicals under a ring one day, developed a rash on the entire hand, and now randomly battle severe skin rashes than can only be mitigated with strong topical steroids, years later. Just practice good lab hygiene and one should be fine.


Heptylene - 14-9-2018 at 12:25

Yes the silica has a tendency to make very fine fibrous particles, I think much like those of asbestos. Those fibers are pretty damaging to the skin. I tried to break down a piece of silica wick into separate strands and my fingers had a "fibrous" sensation at the end, like I had a lot of very small cuts. Now I always handle silica with gloves and outside for this reason. Once the catalyst is installed this should not be a problem.

The heating element I'm going to build will be about 2 meters of 0.25 mm (30 AWG) nichrome wire coiled around the quartz tube over a distance of 10 cm (about 33 turns). Around this I'll wrap some ceramic fiber cloth (ebay) for insulation. I must say deciding which wire gauge to use was pretty difficult. Not too fragile, but not too conductive either. 2 meters should draw about 4-5 amps at 240 V, hence the need for a dimmer. And/or I might make the heating element longer. I'm hoping I can get high enough temperatures without making this prohibitively expensive in electricity.

I've read about platinum being toxic by ingestion (side effect of chemotherapy and such), but after doing a bit of research it seems I had underestimated what chloroplatinate salts can do even upon skin contact. I don't usually wear gloves, but maybe I should with this.

I live in Switzerland. A bit of a nanny state chemistry-wise, but a nice place nevertheless. WGTR I assume you are in the US?

I finished the hexachloroplatinic acid btw. I'm now to proud owner of an expensive orange solution containing 0.49 g Pt/100 ml water.
The acid decomposes much more readily than the potassium salt. Heating a piece of silica wick soaked in the solution to 500 °C on my hotplate, the decomposition is evident by the appearence of a black deposit of platinum on the silica.

Chemetix - 14-9-2018 at 16:38

I'll get a schematic up describing the scrubbing tower idea, but first, a note with fibers. They were my first attempt and they just melt into a lump. They lose their surface area almost instantly and they don't hang onto the metals very well. I hope you have more success but I'm not optimistic.

Chemetix - 15-9-2018 at 01:02

The ammonia scrubbing tower idea needed a bit of a diagram it seemed. So, have a look at the schematic and tell me what you think.

Ammonia stripper .png - 42kB

Heptylene - 15-9-2018 at 02:37

Thanks for the schematic, the absorption and stripping towers make sense now. So what happens is the tower to the right removes most of the ammonia from the solution dripping downward, and this "stripped" solution is replenished by absorption of the ammonia coming from the gas generator through the left tower, correct?

And I'm guessing the absorption would be very efficient given the counter-flow of ammonia gas and dilute ammonia. Really nice design, although probably a whole project on its own.

I'd like to start from 25 % ammonia solution because I could get 20 liters easily. As stupid as it sounds, I can't find a cheap source of urea where I live. Local gardening shops don't have it it in large bags it seems, or they sell it for some ridiculous price for a small quantity.

Now the problem with ammonia solution is that bubbling air through it lowers the concentration progressively, and therefore changes to the composition of the ammonia/air feed to the reactor. In fact for solutions of ammonia below 10 %, the vapor pressure of ammonia above the solution is almost proportional to the concentration of the solution (ammonia data page). So what I'd like to find a way to strip the solution while keeping a constant gas feed composition.

One way to do this would be to simply drip ammonia solution inside a hot kettle at a controlled rate (peristaltic pump) and with a controlled air flowrate removing the vapor from the kettle. The gas coming out of the kettle would have a constant composition but would also contain a lot of water. Furthermore this might tend to condense later before entering the reactor. Just my thoughts.

Morgan - 15-9-2018 at 07:08

Quote: Originally posted by WGTR  
Yeah, the ammonium salt has the advantage that there's no leftover residue other than platinum itself. It's worth the extra effort to make it, and a 100ml bottle of the weak solution should last you forever. I used ethyl alcohol as a solvent. I don't know how much effect that has on how easy the salt is to reduce later since I didn't try using water as a solvent, and my catalyst support was also quite absorbent.

I did find some high silica content wool in the lab, but I personally wasn't too happy with it. It felt like fiberglass, and after being heated with a propane torch became very brittle and friable. The stuff just broke into pieces just the right size to make hands and lungs itchy. :( If you don't mind me asking, what country are you located in?

If you can't get some insulative firebrick, then perhaps some ceramic wool would work. You could ask Chemetix what he was using. I think you'll need some kind of insulation, although you could compensate by using lots of power in the heater.

Chemetix, I'm not sure that I understand your idea either, but it's probably because I'm either making or not making certain assumptions about it. Could you describe the theory behind how it should work?

For the benefit of people reading, be careful handling platinum salts. The cautions about platinum salts being strongly sensitizing for people are no joke. Gloves are recommended. This stuff can sensitize people to various skin ailments at any time in their lives. Certain people are genetically predisposed towards skin sensitivities. I've known people who lived a good part of their lives with no problems, and then got some cleaning chemicals under a ring one day, developed a rash on the entire hand, and now randomly battle severe skin rashes than can only be mitigated with strong topical steroids, years later. Just practice good lab hygiene and one should be fine.



Platinosis from Wiki
"Halogeno-platinum compounds are among the most potent respiratory and skin sensitisers known, therefore it is vital that exposure via the skin and by breathing contaminated air is carefully controlled."

"In practice, the compounds mainly responsible for platinum sensitisation are typically the soluble, ionic, platinum-chloro compounds such as ammonium hexachloroplatinate and tetrachloroplatinate, and hexachloroplatinic acid. Other ionic halogeno compounds are also sensitisers, the order of allergenicity being Cl > Br > I."

WGTR - 15-9-2018 at 10:46

Quote: Originally posted by Heptylene  
The resulting gas mixture, which should consist of about 10 % ammonia by volume, was passed through the tube (18 mm OD, 16 mm ID) where the catalyst is located.


Can you measure the inside of the tube, and make sure that it's 16mm? 16mm should be 0.630", and 5/8" is 0.625". If you have a 5/8" bit, can you see if it slides (care)fully through the quartz tube? Sorry for the cryptic question. I'm going to be in the lab this weekend, and I may try "something".

Quote: Originally posted by Heptylene  
I live in Switzerland. A bit of a nanny state chemistry-wise, but a nice place nevertheless. WGTR I assume you are in the US?


Yes, in Texas.

Congratulations on your proud new ownership of some hexachloroplatinic acid. Yours is about 5 times more concentrated than mine. A little bit goes a long way.

Chemetix, what did you use to draw that illustration? It looks pretty neat.

I'm thinking of an ammonia generator that is probably not very fancy by comparison: An ammonia solution drips slowly into a stoppered bottle that is partially full of dry calcium oxide. The quicklime removes the water and produces dry ammonia gas, which is then led out through a glass tube into a second bottle that is empty. The air supply is also led into this second bottle (or "T" fitting, etc.) where the two gas feeds mix together (they don't have to mix particularly well at all). A third tube exits from this second bottle, and is led into a third bottle where the gases are bubbled with an air stone through an ammonia solution. The gases that leave the third bottle can be led into a fourth bottle, which can either be empty or full of calcium oxide. After enough air has been bubbled through the ammonia solution, I've noticed that water droplets can start building up inside the tube that is leaving the bottle. If enough water gets in the wrong place, it's possible for a water droplet to get pushed into the hot reactor. The fourth bottle would simply collect these droplets, and keep them from proceeding further.

The first bottle produces dry ammonia gas under its own vapor pressure. The second bottle is only for mixing of the air and ammonia. The third bottle absorbs the ammonia produced in the first bottle, and releases a fixed ammonia/air mixture that is dependent upon its own ammonia solution concentration. The fourth bottle is a water trap and/or a drying train.

Basically (no pun intended), the reactor can be run for several hours without the ammonia generator running. Only when the concentration in the third bottle starts dropping off, is it necessary to "recharge" it by turning on the ammonia generator.

Chemetix - 15-9-2018 at 16:01

"Chemetix, what did you use to draw that illustration? It looks pretty neat."

Good ol' MS paint. It's quick, light and easy to use.

"The first bottle produces dry ammonia gas under its own vapor pressure. The second bottle is only for mixing of the air and ammonia. The third bottle absorbs the ammonia produced in the first bottle, and releases a fixed ammonia/air mixture that is dependent upon its own ammonia solution concentration. The fourth bottle is a water trap and/or a drying train."

Such a simple solution really, I like it. I think the third bottle just needs a hydrometer to show the density of the solution in real time. My set up could ultimately change the flow rate down the ammonia stripping tower to keep the output of ammonia pretty constant. But it is semi industrial in terms of the amount of variables that need controlling and the amount of setup. But it would be a nice way of absorbing ammonia formed by a low pressure Haber system. N2 and H2 cycling around once the ammonia has been stripped out.

I think this would be an amazing achievement to have a bench top Haber to nitric acid system working.

Heptylene - 16-9-2018 at 14:45

@Chemetix: I like the idea of an hydrometer, I'll keep that in mind when building the ammonia generator.

@WGTR: I don't have a caliper to measure the ID of the tube, but using a ruler I can say its at least close to 16 mm. Not 15 nor 17 mm. Why do you need a precise measurement of the ID?

The setup you propose would be useful to regulate the composition of the gas feed. One thing I don't like about it is the use of calcium oxide, which is consummed and has to be bought, or regenerated by heating Ca(OH)2 in a furnace. This could get expensive in the long run. But for testing purposes this should be a pretty stable source of gas.

The same idea could be used if, for instance, the gas generator is a 25 % ammonia solution.
I had originally intended to dilute 20 liters of 25 % ammonia to 50 liters of (approx.) 10 % solution and use that directly. But this would be wasteful as the solution would become too dilute at some point, wasting the remaining ammonia unless there was a way to concentrate it. If "too dilute" is a concentration below say 5%, then half the ammonia would go to waste. Fractional distillation could be used,but this would be time-consuming.

Instead ammonia gas could be stripped from the 25 % solution (call it "A") with air and absorbed through 10 % solution (call it "B") and fed into the reactor. If solutions A and B have about the same volumes, solution A should be progressively depleted and solution B should be enriched in ammonia and then be depelted too.

I made a drawing of the concentration curves vs time for both solutions (A and B). This is scenario 1. The drawing isn't pretty but should get the point across. The curves are only intended to show the general trend.

If instead one uses a 10 % solution only, the depletion should be much more rapid (Scenario 2 , solution B*).

I hope this makes sense, its late and my mind a bit foggy.
Ammonia solution stripping-absorption.png - 69kB

WGTR - 17-9-2018 at 11:46

I like the idea of a hydrometer as well; I hadn't thought of that yet. It's certainly much more practical than monitoring it with other means, and should be very robust. If one wanted to, it would seem possible to make one with a float switch of sorts, that would turn on or turn off the ammonia generator automatically as needed.

OK, I agree that using calcium oxide is an unnecessary added materials cost. Looking at your illustration, one way to improve it would be to add heat to solution "A". This will lower the partial pressure of air, and raise the partial pressures of ammonia and water. Referring to Wikipedia's ammonia data page, the ammonia vapor pressure of a 10% solution at 20°C is roughly the same as a 3% solution at 50°C.

A 3% solution at 50°C has roughly equal amounts of ammonia and water vapor. Since ammonia and water are roughly equal in molecular weight, if condensed at a cold enough temperature would give a roughly 50% ammonia solution (not accounting for a density of less than 1), way more than 10%.

The idea is, that a hydrometer could be used with some kind of float switch, that would turn on a heater for solution "A". The ammonia and water vapor would be absorbed in solution "B" at 20°C until the density was low enough, at which point the heater would turn off. Every now and then some solution would have to be removed from solution "B" and put into solution "A" to account for water migrating from "A" to "B".

I'm asking about the tubing ID because I'm entertaining the idea of making a catalyst support for you. I want to make sure that I don't make it unnecessarily small, but I want to account for thermal expansion of the ceramic as well, since silica doesn't expand much when heated. I had a rough weekend and didn't have time to try anything yesterday, but I have some new ideas for making the support, and may be able to shape the green clay with a CNC mill.

Heptylene - 21-9-2018 at 12:13

A float switch sounds good. I assume you're thinking of something similar to those used in dehumidifiers to turn off the compressor when the water tank is full. A peristaltic pump could be used to transfer liquids easily from one container to the others automatically. Those pumps are easy to control with an Arduino or similar board.

I truly appreciate your proposition (or at least considering the possibility) to make a catalyst support for me, but for now the catalyst doesn't appear to be the an issue. I will of course test the catalyst more thoroughly once I've built a heater, as I've only had qualitative results up to now. Maybe platinum loss by evaporation will turn out to be a problem (I hope not). In the meantime I received my cheap thermocouple probe which should be useful to do measurements of the temperature inside the heater.

WGTR - 23-9-2018 at 08:40

Yes, something like that. The assembly would have to float, since the solution level may not be constant. The height of the hydrometer would have to be considered, relative to the solution level. It may be a challenge for corrosion resistance if the switch resides inside the bottle with the ammonia, but this could probably be figured out.

Maybe with some electronics it would be possible to sense these levels optically, or with some Hall Effect sensors. This could allow the sensors to be located outside of the bottle. In any case, I'd suggest putting the float in a tube that is connected at the top and bottom of the bottle, isolating the float from the main solution. This is because air is being bubbled through the solution, and this can artificially decrease the density of the solution if it's being bubbled around the float.

Heptylene - 30-9-2018 at 12:09

Quick update:
I got my nichrome wire in the mail yesterday (30 AWG/0.25 mm diam.). My original idea was wrapping the wire directly onto the quartz tube, but the wire doesn't stay in place on its own. In fact when released, the diameter of the wound coil approximately doubles. I might use a larger diameter wire (0.5-1 mm) that holds its shape more easily. This would also allow the use of lower voltages across the coil. Annealing the wire in place on a smaller tube and sliding the formed coil on the larger tube could work.

Regarding the catalyst, I recently had access to a stereo microscope and took a picture of silica thread with and without platinum deposit at 30x magnification. I thought this could be interesting to share.

The catalyst was prepared by dipping some silica thread in aq. H2PtCl6 (0.49 g Pt/100 ml) and heating the thread to 500 °C with a heat gun. This was repeated 3-4 times.

The two red marks are 4 mm apart to give a sense of scale.

Left: without platinum, right: with platinum.
silica wo platinum.JPG - 110kB Silica w platinum.JPG - 157kB

You can see that the coating seems uniform. In fact I've observed some wicking of the Pt solution to the edges of the strand which we can't see here. When heated to dryness, the Pt solution refluxes and deposits more solid at the ends and surface of the strand than in the middle. Also to note is how fine the silica fibers are. Much finer than a human hair.

I also bought some silica sleeve to insulate the heating element, but it turns out to be too tight to slide over the coil, so I have yet another source of silica for the catalyst. The strands can be more easily separated than those of silica wick.

silica sleeve 20 mm.JPG - 230kB silica gauze separated strands.JPG - 217kB silica gauge.JPG - 216kB.

Heptylene - 9-10-2018 at 07:53

I finished the heating element today. I wound about 5 meters of 0.25 mm diam. nichrome wire into a 4 mm coil, stretched it a bit and wound the coil around the silica reaction tube (about 8 turns I think).

The nichrome wire on each end was secured to the tube using some silica sleeve. The wire goes under the sleeves and the sleeves themselves are tightened on the tube with some wire. Then silica thread (6 mm diam.) was wound around the silica tube between each turn of nichrome coil to provide electrical insulation. To provide some thermal insulation and rigidity, mica sheet (obtained from an old heat gun) was wrapped around the heating element and secured with 2 knots of nichrome wire.

I plan to wrap some rockwool/kaowool around the whole thing to provide additional thermal insulation.

I made a side-view diagram of the heating element:
heating element diagram.png - 21kB

The nichrome 4 mm coil:
4 mm coil.JPG - 190kB

The finished element (but still lacking sufficient insulation):
Heating element coil.JPG - 182kBHeating element 1.JPG - 257kB

I briefly tested the heating element by connecting it directly to the mains (240 V). I'll be getting a multimeter shortly, but from what I've calculated it should have a resistance on the order of 100-150 Ω, giving a dissipated power of about 400-550 W. The heating element was glowing orange, although it was a bit too hot to be run for any length of time. A dimmer will be used to reduce the power slightly. I'm hoping the direct contact between the heating element and the quartz will not be an issue. Otherwise I will have to build a proper tube furnace like yours WGTR.

I might end up doing that anyway since a tube furnace would be handy to have for other projects (acetaldehyde from ethanol, anyhdrous element chlorides such as AlCl3, SiCl4, etc.)

I also followed up on the catalyst support I propsed in my previous post. I placed some silica fabric at the bottom of an evaporating dish and soaked it with 0.8 g of aq. H2PtCl6 solution (about 4 mg of Pt). The dish was heated on the hotplate to 500 °C for 20 minutes. The Pt deposited uniformly on the surface and was capable of decomposing 12 % hydrogen peroxide almost instantly. I'll upload a video on youtube at some point. I think this silica fabric will be the final choice for catalyst support. Its easy to work with and easy to coat with platinum. Much more practical than silica wick in fact.

Silica fabric soaked with Pt solution:
Silica wool imbibed with Pt.JPG - 204kB

Silica fabric after heating:
Finished Pt on silica gauze.JPG - 177kB

[Edited on 9-10-2018 by Heptylene]

WGTR - 9-10-2018 at 09:29

Good job on the setup, and thanks for posting the pictures. It's still amazing how well such a small amount of platinum coats the support. As the catalyst "ages" it seems to lighten up a bit, but part of this is the platinum surface oxidizing, I think. Hopefully it will work well for you.

Update: Tube furnace finished

Heptylene - 31-12-2018 at 07:04

I recently completed my tube furnace for the Ostwald reactor, so here are some picture:

overview.JPG - 252kB view 2.JPG - 247kB
view 3.JPG - 258kB hot2.JPG - 245kB

When the core of the furnace is at 800 °C, the tube is at about 50 °C on at the outlet, with 0.6 LPM of air running through it.

The two metal angles serve to hold the connectors for the nichrome wire. The conductors are in open air, which makes this thing very unsafe. I'll probably build a better (non-deathtrap) furnace at some point. The metal box is a 3$ dimmer from ebay rated for 4000 W. I would trust that thing with 4000 W though. The nichrome element resistance is about 130 ohms. At 240 V that should be about 450 W of power. On the picture it was running with the potentiometer about halfway, whatever that tells you. I have no way of measuring the duty cycle of the dimmer so I don't know the power consumption. I know full power is too much for the element to handle.

The temperature inside reached about 800 °C. Then the glass sleeve of my thermocouple started to melt and fuse to the silica tube.

Anyway that's it for now.

[Edited on 31-12-2018 by Heptylene]

[Edited on 31-12-2018 by Heptylene]

WGTR - 1-1-2019 at 06:55

This looks beautiful in its simplicity, yet effectiveness. A simply fan blowing on the output end of the tube could cool it down even further. What are you using to clamp the two ends of the furnace tube?

Heptylene - 1-1-2019 at 14:57

Thank you WGTR!:)

The tube is held at both ends using plumbing pipe clamps. The clamps initially had a rubber pad which I removed and replaced with a few layers of regular paper. This is to allow the tube to slip when it expands due to heating up. The clamps are not tightened much, to avoid breaking the tube. The clamps themselves have a threaded base that screws on threaded rod (M8). Overall I'm satisfied with what the furnace can do, but there is a major design flaw: if the silica tube breaks for whatever reason, I cannot remove it, I'd have to break the heating element apart.

When I finished the furnace I did a quick test and tried to pass some dilute (3% ± ish) ammonia/air mixture over the heated Pt catalyst and got some brown nitrogen oxides, with no visible ammonium nitrate residue. I still have to figure out what to do with the gases. Maybe dissolution in NaOH, as I don't really need nitric acid.

I'll also make some acetaldehyde eventually, by passing ethanol vapor over hot copper wool.

But I've been busy recently with college and other projects (namely a 150 kV power supply for x-ray experiments). It's been months since I did any proper chemistry!

P.S. There is a typo in my last post regarding the dimmer rated for 4000W: I meant to say "I wouldn't trust that thing with 4000 W."

Counter-current gas absorption tower!

Heptylene - 1-4-2020 at 13:49

I've been working on and off on my Ostwald reactor over the past year, and have improved the gas absorption system. Previously I used a simple glass tube leading into a (100 ml) graduated cylinder filled with water. This hardly worked at all and a lot of nitrogen oxides escaped into my workspace, which is both dangerous and wasteful. Indeed I haven't produced any usable quantity of nitric acid using this method.

The new system is a counter-current gas absorption tower, inspired by systems used at the pilot or industrial scale. See for instance this system by chemglass and a general illustration of the process:

chemglass 20 L gas scrubber.jpeg - 44kB packed column concept.jpg - 60kB

I am currently testing this new system. So far, absorption seems to work well, and only a little NOx can be detected by smell at the outlet, but a little still goes through so I've added a simple NaOH scrubber at the outlet (inverted glass funnel in a beaker, classic type).

The heart of the system is a 500 mm long column, 30 mm O.D. filled with 10 mm PTFE raschig rings. Water is continuously passed over the packing from top to bottom. The gasses (nitrogen oxides here) are introduced at the bottom and escape at the top. This ensures intimate contact between gas and liquid. The liquid is pumped using a polypropylene centrifugal pump (similar to Iwaki-type pumps, 50 $ on aliexpress) at a rate of a few liters per minute. The liquid is sprayed on top of the packing using a homemade nozzle with 1 mm holes. The liquid at the bottom of the column is collected in a chromatography reservoir and withdrawn from the bottom of said reservoir by the pump. The pump is placed below the reservoir as it is not self-priming: Liquid must be present for it to start pumping, it produces no air suction when dry. In fact, the pump loses a lot of head pressures when even bubbles are present in the pumped liquid. An advantage of placing the pump at the very bottom is the ability to purge virtually all the liquid simply by turning a stopcock ("product outlet valve" in the pictures). This will make testing easy, no need to disassemble anything to get a small sample or recover all the product. The sep funnel is unrelated to this. It is simply a way to collect the condensate that comes out of the furnace so it doesn't stay in the hoses.

For the packing I used 10 mm PTFE raschig rings because the 6 mm raschig rings I already had (for distillation) did not let enough liquid pass through, which caused column flooding. I had some PTFE tubing on hand so I chopped it into pieces (10 mm long) so I didn't have to buy a whole kilo of glass packing from my supplier. I think 8 mm raschig rings would pack better, the 10 mm one tends to make channels, which isn't good for liquid-gas exchange.

A quick note about the connections from the hose to the quartz tube furnace: I replaced the rubber stoppers with GL-32 to ground glass adapters. The GL-32 thread can be used as a compression fitting with PTFE/silicone joints and a cap (red) and form an air tight seal with the quartz tube. There is no problem with thermal expansion and chemical resistance either. This provides my tube furnace with two male ground glass joints, to which I simply attach classic hose barb adapters, or in this case a distillation receiver and a sep funnel.
I have also used this system for liquid connection at the bottom of the reservoir, GL-18 thread this time.

The next step is to test the system for an extended period of time and titrate samples of the liquid over time. I actually got a burette for this specific purpose. I also want to play with the temperature of the furnace and the ammonia/air feed composition. It seems at high temperatures almost no NOx are produced, which I attribute to over-oxidation of ammonia to dinitrogen. I don't have a probe to measure the furnace temperature yet, so I'll need to get that first. So far, I've been running with air and measured the temperature with the furnace open (around 800 °C). I can only assume the temperature is close when running on ammonia/air, but it's likely to be higher.

The column could definitely be larger in diameter and longer. I think 50 mm by 1 meter would be good but I couldn't find a column this large. Ideally I would like a 100 mm diameter column with duran flanges, but apparatus at this scale is getting very expensive. Possibly a heat exchanger for the liquid, as it's getting warm under operation (maybe 30 °C?). Time will tell if this is an issue.

Below are some picture of the apparatus (with legends). Zoom in to see the text.

entire setup - annotated.jpg - 2.6MB pump and product outlet.jpg - 834kB

reservoir.jpg - 2MB entire setup.jpg - 2.2MB brown gasses.jpg - 1.5MB

WGTR - 1-4-2020 at 18:45

Thanks for posting this Heptylene; it's looking good.

Do you know what your air-flow rate is into the reactor? Also, what is the concentration of your ammonia solution? What is the column packing for? Is it mainly to increase contact time between the gas and acid solution, or to trap acid mists that are formed, or both?

It looks like the absorption column is fairly narrow. I wonder if the packing is necessary. One side effect is that it decreases the volume of the column, which also decreases the amount of time that the gasses spend in the column. As you probably know, each time the NO2 reacts with water, the NO that results needs even more time to oxidize since it becomes more diluted as time goes on. Maybe this isn't a significant problem in your case though, since the concentration of NO2 from the Ostwald Process is much greater than say the Birkeland Reactor, and the NO tends to re-oxidize more quickly.

I was using a cooled condensing coil right at the output of my reactor to trap any leftover ammonia, and to remove as much moisture as possible. I tried minimizing the gas residence time between the reactor output and cooling coil, so as to minimize loss of acid yield due to absorption of NO that was already oxided to NO2 in that short amount of time. In any case, if the acid concentration gets too high in the condenser it begins to break down again anyway, so the loss of NO is limited in the condenser in this way. Some ammonium nitrate will still get trapped there. The only real loss is the overoxidation to N2 as you say.

You might get better results with a bit larger oxidation chamber before you hit the gas absorption tower. It might be useful to add a second oxidation chamber after the absorption column, just to see how much NO gas is getting past the column.

800C is probably too hot, especially since you have a long catalyst bed. You might even get better results if you only use a small amount of platinum-loaded wool that is packed in between uncoated plugs of quartz wool. The reaction is very fast on platinum, and the residence times are very short for optimal efficiency.

Good job!

Heptylene - 2-4-2020 at 05:01

Thank you WGTR!

The flowrate is approx. 0.5-1 liter per minute of air coming out of an aquarium pump. I want to get a better pump at some point and I'll need to get a flowmeter as well. Ammonia solution of roughly 8 % was used, which should give around 5-8 kPa of ammonia vapor pressure (so about 5-8 % ammonia in air). This wiki page contains ammonia vapor pressure for different concentrations and temperatures, very handy to know your gas composition (at least an upper bound of the ammonia content).

The packing is to ensure a high gas/liquid contact area. I tried without packing and the water simply runs down the walls of the column, which I suspect is not as good. With about 27 mm ID, the column has a volume of around 286 cm2. I estimate the packing efficiency is about 50 %, and the packing itself (10 mm OD, 8 mm ID, 10 mm long PTFE rings) occupies 36 % of the volume of same size cylinders. So in total about 18 % of column volume is taken up by the packing, so a corresponding decrease in residence time of the gasses.

The column alone has an internal surface area of 424 cm2. The packing has a surface area of about 6.2 cm2 per ring. At 50% packing efficiency, that's 182 rings in my column, or 1130 cm2 in addition to the column itself, so 1554 cm2 total for packed column. These are just estimates of course, but the loss of column volume is negligible compared to gained surface area.

I must say I haven't worked on the gas feed composition and temperature at all in the past year. The catalyst is about 3 cm long silica wool as before, but there is definitely some improvements to be made, I doubt I have stumbled accross the best catalyst by chance. A bigger oxidation chamber could indeed help. I think a long coil of transparent tubing would provide a nice way to monitor the oxidation of NO to NO2 (think plug-flow reactor).

Good suggestion to try and collect some of the column output gasses. I'll probably get a 2 liter sample in a glass bottle and let it sit with a little CaCl2 at the bottom to see if any brown color appears over time. It would be nice to have a way to measure the gas composition at various points in the reactor. Maybe using a spectrophotometer (I don't have one)? Or a GC (more expensive though). Maybe by making colorimetric standards of pure NO2 in glass ampoules for a visual inspection? Say 100, 10, 1, 0.1 % etc?

EDIT: I don't intend the column to work as an acid mist catcher. I think this would have to be dealt with separatly at the column gas output

[Edited on 2-4-2020 by Heptylene]

WGTR - 5-4-2020 at 17:53

Well, I put in a small order for some glassware...it should all arrive in a week or so. I have to say, "Yay for no more weird Texas glassware restrictions!" After all these years I finally ordered some of my own real glassware (stuff with actual ground glass joints). Yeah, I could have ordered this stuff anyway without a permit, but I was too honest.

I'd like to try out some variations of what you're doing with gas absorption. I'm planning to go straight from the reactor output to a hennion cold finger. I'm thinking to cut the tip off the cold finger, and fit it into a gas washing bottle that is full of ice water. The way that I'm envisioning it, is that the outer tube of the cold finger will contain the gas as it leaves the inner tube. This should allow me to limit the headspace and residence time of the gas in the wash bottle. There are a number of different variations of this that I could try, as I have a diamond saw and some propane torches to play with, and can do some crude glasswork.

After the cold water wash the gas will go to a vacuum takeoff adapter that has a center stem, like what you have. I could have bought one with a long stem but those are more expensive and harder to find. I'll just push a hose onto the stem to lengthen it like you did. Instead of using a separation funnel I'll just use a 1 liter round bottom flask, as I think that will find more use on my other projects.

I plan to stack a 200mm West condenser on top of a 105 degree adapter that is sitting on a 50mL round bottom flask. The gas will come in the side joint of the adapter. On top of the condenser I'll put a 1 liter chromatography reservoir, and then finally a 250mL pressure equalizing addition funnel. The reservoir is mainly to see how much NO is getting past the first condenser. If I need another one, then I'll probably get a small graham condenser to stack in between the reservoir and the addition funnel. The idea is to keep stacking reservoirs and condensers until this becomes impractical.

I only plan to pass the water down the column once. This means that it will be dripping very, very, slowly from the addition funnel. I'll probably stuff a PTFE washer or something down into the top of the condenser, to try and wick the water all the way around the inner tube so it hopefully runs down the condenser in a thin sheet. The glass will have to be very clean for this to work, but after all, we are making nitric acid here. That should clean the condenser very well I think.

I noticed that it looks like you're recirculating your acid water through the column. I don't know if that's the most efficient way to get concentrated acid; as an idea it might work better to use a slower water flow and only pass it through once.

Heptylene - 6-4-2020 at 05:24

You're going to have a hard time doing glasswork with a propane-only torch, I speak from experience, at least with borosilicate. You'll need some an oxy-fuel torch.

Quote: Originally posted by WGTR  
The idea is to keep stacking reservoirs and condensers until this becomes impractical.


I think this is the way to go here. I ran a quick test last week and the ouput gasses of my column are still slighly brown. I might get a longer column at some point, maybe 1 meter long.

Your idea of running the water in a film down the condenser is interesting, but I think cleanliness will be a big factor. Piranha solution could help with cleaning.

Have you considered using a very long coiled plastic tubing as a column? Say a 10 meter coil going downwards, 50 cm diameter, 5 cm pitch? With a 10 mm ID, you could feed water almost dropwise at the top and have the nitrous gasses coming the opposite direction from the bottom. With a slow enough flowrate of water and gas, both could flow freely (no "column flooding").

WGTR - 6-4-2020 at 13:17

I forgot to mention earlier that you could probably keep using the aquarium pump if you add some ballast in the form of plastic bags. The pump fills up one bag, which is used to fill up another bag through another length of tubing. The second bag then supplies air to the reactor. The idea is that the two bags and the restriction between them (the tubing) would serve to smooth out the pulsations from the pump. But of course an oil-less air compressor and a regulator would work better. If it uses oil, some sort of trap is probably needed. I've noticed before that my ammonia solution slowly turns yellow over time, probably from an oily house air supply.

Working borosilicate with straight propane is no fun, for sure. I can just barely do it if the glass isn't too large, however. I also have a small butane torch that puts out a narrow flame that provides some extra boost. The combination of the two is usually enough for me to fuse tubing together, if just barely. I do have a welding torch, but got rid of my bottles years ago, and haven't been able to justify getting new ones.

I have considered running tubing as a column as you say. PTFE tubing might work well for something like this, if water will wet it well enough. However, I think glass tubing would work better in that water should wet it better. I can buy it locally, wind it into a coil, and fuse together several lengths if necessary. At this point though, I still think that the reaction of NO2 with water is already fast, and occurs almost immediately. A gas washing bottle with pure water would probably be enough to absorb it all in one pass. The problem is this:

\[3NO_2 \:+ H_2O \leftarrow \rightarrow 2HNO_3 + NO\]

One-third of the nitrogen is converted to nitric oxide, and needs additional time to re-oxidize before it can be absorbed. This oxidation process is normally the limiting factor as it is rather slow, and becomes slower as more of the nitric oxide becomes diluted with the remaining air. It is not practically possible to absorb 100% of nitrogen as NO2 into water alone because that last 0.0001% would never actually oxidize due to the slowness of the reaction (very low concentrations of NO are used in some breathing treatments after all). Having said that, two or three full absorptions into water (after re-oxidation) should be enough to get almost all of the nitrogen oxides absorbed for practical purposes.

Here's the kicker: The equation shown above reaches some equilibrium under normal conditions. If you take the concentrated acid and bubble nitric oxide through it, the acid concentration will readily decrease and become quite diluted. Concentrated NO2 can produce more concentrated acid. As the concentration of NO2 drops, the concentration of acid that it can produce is less and less. At the top of the absorption column the water should be almost pure for maximum NO2 absorption. As the water flows down the column in counterflow with the gasses, it naturally gets more and more concentrated with acid. At the bottom of the column should be the most concentrated acid that you can get without some additional process.

Additionally, if the nitric oxide is not given sufficient time to oxidize it will strip NO2 out the top of the absorption column, especially if you're recirculating column acid.

The good news is that if you have an addition funnel that you can put at the top of the column, you may be able to get more concentrated acid and better absorption at the same time, without having to recirculate your column acid. With the amount of air flow that you have going through the reactor and your ammonia concentration, you may only need a drop of water falling onto the top of the column packing once every minute or so to get best results. This is of course after the packing is already wetted completely and has reached steady-state (i.e. one drop on top of the column = one drop falling into the receiving flask). Some adjustment would be required on the water flow, I think. It should be the maximum amount possible without causing significant dilution of the acid as it falls into the receiving flask. If a noticeable amount of nitric oxide is still escaping the column at that point, then additional oxidation and absorption is required. I look forward to trying this myself as soon as my glassware arrives.

One last thing is that keeping the column cold does noticeably improve the achievable acid concentration. I could pull some info from my references if needed.

Heptylene - 20-4-2020 at 01:58

I had never thought about that equilibrium. I have a peristaltic pump which I could use to feed water in the column to test this. I have 6 mm glass packing available as I mentionned, so I might try to use it instead of 10 mm PTFE. Provided I can clean it well enough it should wet very well.

Sadly the quartz tube in my furnace just broke. I knew using quartz as the main body would cause issues, and I'm surprised it lasted as long as it did. The next one will use an alumina tube and some refractory mortar probably. At any rate this will take some time to fix, so I won't be able to continue for at least a few months.

Just a few unformatted thoughts:
- Maybe we could use H2O2 instead of water to absorb the NO2. I haven't tried anything, but I think H2O2 would work well. I think we could use dilute peroxide, which is available without restrictions.
- I wonder if there is a way to use NO2 to oxidize SO2 into SO3, or even oxidize SO2(aq) into H2SO4 directly?

Refinery - 30-4-2020 at 01:26

I was thinking of following:

- Ammonia generator, lead to bubbler filled with either saturated ammonia solution or other liquid to monitor flow rate,
- Air pump
- They are led into flask filled with CaCl2 to dry and mix both gases
- They are led into 30-50mm D quartz glass tube furnace, fitted with perforated ceramic plate coated with Platinum as described in this
- A long, in order of tens of meters of tube, resistant to all forms of nitric fumes, for NO to react into NO2
- A final bubbler to collect NO2 to form HNO3, and a condenser for exhaust.

Methods to control flow rate:

- Either by heat, or by dropping funnel using substrate that will gasify at controlled rate (NH4CL + NaOH?) - Urea and NaOH will generate ammonia gas at controlled rate based on my recent test
- Air-rich mixture could be preferable to minimize any ammonia residue, resulting in ammonium nitrate formation, though when HNO3 is distilled pure, sulfuric acid would anyways react with it and no losses should result

_

If a reaction is supposed to be made as a test/curiosity, the cost of the reagents used is not a factor. If any usable quantity of HNO3 is the goal, the reagents would need to be sourced in bulk. Any OTC lab stuff or any H2O2 I've come up to is prohibitively expensive. (In my personal pov, if the synthesis to make anything other than curiosity exceeds the cost of buying the material, I prefer to just buy it, unless there are complications(e.g restrictions). I understand that developing a functional Ostwald tabletop process is driven by the concept of manufacturing usable quantities of HNO3, in the magnitude of liters at minimum.