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Author: Subject: Synthesis of Sulfur Trioxide and Oleum: The persulfate method
DJF90
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Thanks gc, I now have something to look forward to reading next week. It's a shame I can't read german
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 Quote: Originally posted by garage chemist In the meantime I have succeeded in making SO3 from NaHSO4. It worked extremely well, I got 23,8g SO3 from 100g pool pH-minus. It's simply a matter of enough heat. From 680-880°C, plain sodium pyrosulfate (from NaHSO4 at 480°C) gives off all its SO3. And the best part: not a bit of it is decomposed to SO2. Because decomposition of SO3 requires a catalyst, like for example iron compounds, which are lacking here. I just have to write a documentation. Oleum has now become a simple OTC preparation. [Edited on 22-3-2008 by garage chemist]

EDIT: I found the writeup, very impressive! I have 400g of NaHSO4 and look forward to doing this as a project.

garage chemist
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Yes, of course, it's in Prepublication- has been for a long time.

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Pulverulescent
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Quote:
 Quote: Originally posted by garage chemist In the meantime I have succeeded in making SO3 from NaHSO4. It worked extremely well, I got 23,8g SO3 from 100g pool pH-minus.

I have 400g of NaHSO4 and look forward to doing this as a project.

Without wishing to detract, in any way, from what must have been a pretty sustained effort, garage chemist, IMO, a yield of 23.8g from 100g looks somewhat skimpy for the amount of work expended.

If ADP gets a corresponding yield from his 400g he'll end up with less than 100g of product for his pains.

And I know this could be the maximum obtainable under the conditions.

I've seen a patent fairly recently which stated "The electrolysis is stopped at the point concentration of ~98%, since proceeding past this point would result in the production of oleum."

It's not quite verbatim, and the patentee could, let's face it, be mistaken, for all anyone knows, but if he's right, it could be another grail, if that's not a contradiction.

'One slight problem, I can't for the life of me, find it, or remember
which friggin' site I found it on.

It's frustrating, but I'm still looking.

If anyone else has come across something similar, I'd be most
interested?

I can't actually set up any kind of electrolysis at the moment, but might be able to have a go in a few weeks.

In the meantime, I want to look at the, so far, fugitive patent to see how authoritative it looks as I only skimmed through it the one time I saw the blasted thing.

(sigh)Someone else must have seen it!

P
alphacheese
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 Quote: Originally posted by garage chemist Yes, of course, it's in Prepublication- has been for a long time.
Forgive my ignorance... but where exactly is the write up? A link maybe?
garage chemist
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Go to prepublication, press Ctrl + F, enter hydrogen, there it is.

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alphacheese
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Ahh, fein dank. I didn't exactly know what the prepublication was. Found it.
Hennig Brand
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I have been off and on working on building a small electric furnace for the last several weeks so that I could produce SO3 among other things. The furnace was made from kiln bricks (2.5 inches thick), angle iron (2" X 2"), an old clothes dryer heating element (ca. 15 gauge nichrome wire) and an infinite switch from an old kitchen range for temperature control. The infinite switch was rated for 240V, but seems to work fine at 120V. It is the current rating, in this case, which is of greater importance I think. I have a decent digital thermocouple meter but the best probe I have has insulation rated to only 500C. I have ordered a probe that is rated for up to 1250C, but I don't have it yet. The 4000-6000W at 240V dryer heating element was reduced in length slightly (cut) and was run at 120V instead of 240V. The current to the element was measured as about 9.5A when the infinite switch was in the on state. This means the furnace element is putting out about 1140W (P=IV). With the infinite switch turned to the maximum setting and the cover on the furnace an internal air temperature of 500 Celsius could be reached in about 2 minutes. I didn't actually time it, but the temperature rises very quickly. Until I have the higher temperature thermocouple probe I won't know exactly how high the temperature can go. I used the following video as a guide to build my furnace, but I also read David J. Gingery's book, "LI'L BERTHA, A Compact Electric Resistance Shop Furnace", which had in it a lot of useful information.

Below are pictures of a trial run. I did two trial runs and led the SO3 into a Florence flask which contained what was left in the bottom of a bottle of drain cleaner sulfuric (91% H2SO4). I obviously still need to do some work on the collection part of the apparatus, as some SO3 was escaping and water was undoubtedly getting in and being absorbed, but the furnace works wonderfully and produces SO3 from NaHSO4 with ease. I have read enough to know that I need a moisture protection tube with anhydrous calcium chloride or some other suitable desiccant in it.

The quartz tube that a glass blower made for me is 30mm id. I had the side arm made rather long so as to avoid troublesome connections and also to act as an air condenser. About 130-140g of sodium bisulfate (pool pH down) was put into the tube for each run. It was just a trial run as I said before, but after the first run the 91% sulfuric acid in the flask was ~94% and after the second ~97% as measured by titration. I didn't measure the volume at the start because I was mostly focused on the furnace for the initial runs, but the final volume of 97% sulfuric acid was 166mL. This is likely not the best use of SO3, since the acid could be brought up to at least 95% reasonable efficiently by distilling off water. I also have a much bigger quartz tube on the way. I am trying for a 50mm tube, with a little greater length also, which would allow me to make about 3 times as much SO3 per run.

Air temperature in the furnace was kept between 300 and 400C for about an hour where it seemed mostly water came over. Once the air temperature in the kiln got up over 500C it was not long until much more dense white fumes appeared and the drops coming out of the side arm made hissing sounds that could be heard from 50 feet away when they hit the lawn below. The drops instantly carbonized the lawn they fell on. This is when I moved the collection flask into place and began collecting SO3.

[Edited on 29-7-2014 by Hennig Brand]

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Zyklon-A
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Nice! Did you calculate the % yields?
Surly they could have been increased by using a ground glass joint to a RBF rather then a Florence flask.
I doubt there'd be any pressure buildup, as the SO3 would dissolve quickly into the sulfuric acid.

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Hennig Brand
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I didn't have it all thought through when I gave the specifications to the glass blower for the quartz tube. I think the larger tube I am about to have made will have the same design though, just a larger reaction tube. I think the long side arm/air condenser was a good addition and that by the time the vapor makes it to the collection flask it will have cooled enough that a Teflon bushing could be used to connect the side arm to the flask and the vent line to the flask. According to "Small Scale Synthesis of Laboratory Reagents", by Leonid Lerner, the maximum distillate temperature is about 340C, even when the melted reaction mixture is at 820C. I may do something similar to what I did here, http://www.sciencemadness.org/talk/viewthread.php?tid=15676&... , but with a moisture protection tube connected to the vent line.

It is the non-condensable gases (air), in the system, that would be most responsible for building up pressure if the system was sealed. Maybe it could be sealed once it was up and running, but I would feel a lot better about it being vented especially given the fact that the SO3 and sulfuric acid are pretty dangerous materials to have an accident with.

I actually didn't measure the sulfuric acid volume started with, which is a real shame. It would have taken me 10 or 15 minutes to go get a graduated cylinder and I was mostly just keen to test out the furnace and associated equipment. It turned out that the apparatus works really well and it is actually a trivial matter to produce SO3 with a bit of equipment by this method. I wish I had measured the sulfuric acid volume at the start because it would have told us how much SO3 was absorbed as well as how much water was absorbed. I suppose I could always do another run in the same way.

[Edited on 29-7-2014 by Hennig Brand]

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garage chemist
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Well Done, Henning Brand. It's nice to see that my work is still an inspiration to members.
For your next batch, try to condense the SO3 directly instead of absorbing it in H2SO4. Do not use a CaCl2 tube, the SO3 vapors will violently react with every bit of moisture that it already contains. Instead, tightly stopper the neck of the flask with glass wool stuffed around the tube from the condenser. This prevents circulation of moist air into the receiver. Cool the receiver with ice, immersing the flask in it.
Hennig Brand
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Thanks Garage Chemist, I will definitely use your suggestions on my next batch.

Edit:

Ok, I ran another batch. I used the suggestion above and put some glass wool around the end of the side arm/air condenser so that it fit reasonably snuggly in the neck of the Florence flask. I left it a little looser than I probably should have, mostly because I was a little nervous of sealing the system off too tightly. I also used ice around the Florence flask as suggested and didn't add the concentrated sulfuric acid until after all the SO3 had been collected.

Included are a few pictures that I thought were interesting. The charge of sodium bisulfate containing feed used was 130g, which came in a 4kg pail as pH down for swimming pools. After SO3 generation had gotten quite slow, and increasing the furnace temperature no longer seemed to help, the flask was removed and covered tightly with a polyethylene plastic bag. It only took me a few minutes to go and come back with 100mL of sulfuric acid, but in that time the SO3 vapors had turned the polyethylene cover completely black. The flask holding the SO3 was left in the ice bath, while I went for sulfuric acid, so the vapor pressure would have been quite low. When the 100mL of sulfuric acid was poured into the flask holding the SO3, the SO3 dissolved very quickly (approximately a few seconds) and the flask got hot to the point of being almost uncomfortable to hold.

Yield

About one gram of acid was measured out for each titration using scales accurate to hundredths of a gram. A sodium hydroxide solution was first standardized against a known sample of HCl. Phenolphthalein was used. Only one titration was performed for the sulfuric acid before SO3 addition and one for after. Performing more than one titration for each and taking an average could have increased accuracy, but I was very careful. As stated before the scales used were only accurate to hundredths of a gram and the amount of acid used for each titration was about a gram. Ideally much more accurate scales should be used, but a larger quantity of acid could be used per titration as well.

Volumes of acid were measured with a 250mL graduated cylinder. The sulfuric acid density table from Perry's was used.

Pre SO3 Addition: Titrated as 94.4% H2SO4
0.944(100mL)(1.832g/mL) = 172.94g H2SO4
(1-0.944)(100mL)(1.832g/mL) = 10.26g H2O

Post SO3 Addition: Titrated as 97.2% H2SO4
0.972(114mL)(1.8364g/mL) = 203.49g H2SO4
(1-0.972)(114mL)(1.8364g/mL) = 5.86g H2O

H2SO4 added = 203.49g - 172.94g = 30.55g
SO3 added = 30.55g (80.066g/mol / 98.079g/mol) = 24.94g
H2O added = 30.55g - 24.94g - (10.26 - 5.86) = 1.21g

Theoretical Yield

According to the MSDS for the pH minus product used its composition is 91.5-94.7% sodium bisulfate and 4.8-8% sodium sulfate.

For 130g of Feed (assuming 91.5% NaHSO4)
0.915 * 130g / 120.06g/mol (80.066g/mol)(1 SO3 / 2 NaHSO4) = 39.67g SO3

For 130g of Feed (assuming 94.7% NaHSO4)
0.947 * 130g / 120.06g/mol (80.066g/mol)(1 SO3 / 2 NaHSO4) = 41.05g SO3

Percentage Yield of SO3

Assuming 91.5% NaHSO4 Feed = 24.94g / 39.67g * 100% = 62.9%

Assuming 94.7% NaHSO4 Feed = 24.94g / 41.05g * 100% = 60.8%

There is a purity versus quantity trade-off here. If collection had been started earlier more SO3 would have been collected, but the product would have contained a larger proportion of water. It looks as though the glass wool did a great job of keeping the water out of the collection flask. I still noticed a bit of SO3 escaping from the flask, through the glass wool, but the wool could have been packed much more tightly and a lot more ice could have been used around the receiver as well (I was low on ice).

[Edited on 31-7-2014 by Hennig Brand]

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Hennig Brand
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I have a few more pictures which I thought might be appreciated by someone trying to follow the experiment.

Once the furnace cooled down the quartz tube was removed and the sodium sulfate residue/plug was easily removed by turning the tube upside down and gently tapping it on a soft surface like wood. Sometimes the sodium sulfate plug sticks a bit, but pouring a little water into the tube and letting it soak down past and around the plug frees the plug and allows it to be simply dumped out by inverting the tube.

Here are a couple pictures I found on my phone from when I was building the furnace. The first picture shows the frame just after the welding was completed. The second picture was taken during the middle of heating element groove filing and grinding. A metal file and a rotary cutting bit and cordless drill were used to make the element grooves in the kiln brick. The third picture shows the infinite switch that is used. It is the old fashioned type dial controller for the burner on an electric cook stove. This infinite switch was for one of the large burners on an old stove.

The kiln bricks were expensive for me, mostly because I live in a lower populated area of the country and anything remotely specialized often needs to be ordered. The box of 12 kiln bricks was about $40 and because they came from halfway across the country shipping was about$35. The steel was purchased from a scrap yard, as new steel, for about \$10. I removed about 8 infinite switches from old stoves and about 6 dryer elements from old clothes dryers, which I was allowed to take free of charge from the same scrap yard. The element will need to be replaced from time to time, especially if operating the furnace at very high temperatures (>1000C for instance), but they can easily be found free (old clothes dryers) and with this design replacement is a fairly simple procedure.

[Edited on 27-7-2014 by Hennig Brand]

"A risk-free world is a very dull world, one from which we are apt to learn little of consequence." -Geerat Vermeij
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 Sciencemadness Discussion Board » Special topics » Prepublication » Synthesis of Sulfur Trioxide and Oleum: The persulfate method Select A Forum Fundamentals   » Chemistry in General   » Organic Chemistry   » Reagents and Apparatus Acquisition   » Beginnings   » Miscellaneous Special topics   » Technochemistry   » Energetic Materials   » Biochemistry   » Computational Models and Techniques   » Prepublication   » References Non-chemistry   » Forum Matters   » Legal and Societal Issues   » Whimsy   » Detritus   » The Moderators' Lounge