ExothermicReaction
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Venting gas through a tube?
I'm setting up to do a Sandenmeyer reaction which will produce some NO2. I'd rather not let it out in the room. I have the ability to run a small
house up and out the side of my building. Would this be an ok way to vent the gas? I've got a 2L Flask Fitted with a Claisen to accomadate an addition
funnel (for dopping the acid) and a reflux condenser with a gas outlet adapter. My question is : If I attach a 5 foot tube to the outlet and run it
outside, will the pressure differential created by the generation of the NO2 be enough to drive the fumes through the tube, or might I need to add
some sort of pump to the mix?(a small aquarium pump)
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YT2095
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you`ll have to work out the gas capacity of the hose length x dia, and the work out how much will be evolved.
if it greater than the CC capacity of the hose then it will come out the other end, if it`s less you`ll end up the a hose full instead.
\"In a world full of wonders mankind has managed to invent boredom\" - Death
Twinkies don\'t have a shelf life. They have a half-life! -Caine (a friend of mine)
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Klute
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Why don't you neutralise the gas with a wash bottle? Much more practical. But two in series to be sure of getting rid of all of it. Check for
yourself, but I would think a base would neutralise the NOx formed. Put some phenolphtaleine into the solution to be sure it stays basic. And then
keep that basic solution in a jug for each time you need to neutralize acidic gases, adding some solid NaOH/KOH now and then to compensate.
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Magpie
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The hose will vent the gas out of the building. The question is really: will it vent it fast enough to prevent an excessive pressure build up in the
flask.
To determine that pressure you will need to know the maximum gas generation rate (liters/min), the hose ID, and the ID of the
smallest opening in the hose/flask route of the gas. Provide those values and I will calculate the internal flask pressure for you.
[Edited on by Magpie]
The single most important condition for a successful synthesis is good mixing - Nicodem
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ExothermicReaction
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YT : Even if its greater, won't I still have a hose-full to deal with when the reaction is done? This is why I was thinking it might be good to get
some sort of pump to evacuate the hose.
Klute : I considered that, but I wan't sure that bubbling the gas through an NaOH solution would clean it enough that I'd be comfortable letting the
result out into my room. Then I thought of getting a small fish-tank to bubble into and rigging a home-depot-style stove-hood fan onto the tank and
piping the air out the windo with a slinky dryer hose. It was just as this course started getting complicated that I realized that I have a small
opening to the outside currently occupied by a garden-hose pipe and not quite sealed off, so it seems that it would be simpler to just let it outside.
I could run the hose into a small dish of NaOH solution outside if I were concerned that the vapors would be harmful to anyone once outside, but I
doubt this is the case with the amounts that I expect will be produced.
Magpie : The hose is 5/16 ID connected to a 24/40 gas outlet adapter on my reflux condenser with a 1/4 in ID. Its about 6 feet to the great outdoors.
If needed, I can probably get this down to 4 if I rearrange my bench. I'm not sure how to calculate how much gas will be produced, but I don't think
its very large. I will be reacting a mole of NaNO2 and a mole of GABA with a little over a mole of HCl. The reports I've read indicate that as long as
dropping is done very slowly (like one drop every 5 to 10 secs at first) and the solution is kept cool in an ice bath, it should be possible to get
through the whole reaction without seeing any brown gas inside the flask. Not sure how this translates to liters/min except that we're probably
talking mL's/min rather than liters. If anything, my concern is that there won't be enough gas generated to push the column of NO2 out the hose.
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Magpie
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I will make a calculation based on an assumed maximum generation rate and get back to you.
The fact that you have a dropping funnel (w/stopcock I assume) means that you can keep this under good control, so I'm thinking you should not have a
problem as long as the reaction is slow and steady. However, if you get an exotherm, and things take off ( a runaway) all bets are off. I usually
leave one port unsecured, ie., just closed with a greased glass stopper. Then if everything goes to hell it will hopefully just blow out the stopper
instead of blowing up the flask. 
When your reaction is finished you will indeed have a system filled with the gas you generated. I'm designing for this myself for chlorine
generation. I like Organikum's suggestion of blowing it out with air using an aquarium pump at the end of the reaction. I just bought a very small
one for $10. No need for a big one; there's no hurry.
The single most important condition for a successful synthesis is good mixing - Nicodem
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ExothermicReaction
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As I write this I am sitting here watching the reaction take place. I've been dropping the HCl at 1 drop every 10 seconds for about 40 minutes now.
Maybe this is a bit conservative, but I've got time. I taped a digital thermometer probe to the outside of the flask just above the water line in the
ice bath and its holding steady at just under 10C. So far, the only difficulty has been the sensitivity of the stop-cock. The difference between one
drop every 10 seconds and one drop every 5 seconds is so small that i just need to apply some pressure to see the change. I'm planning on cutting a
small notch on the inside of the stopcock with an exacto knife. This should give a little more resolution at the border.
There is a perceptible amount of NO2 inside the flask, but just barely. Its just a slight yellow haze which appeared in the first few minutes and has
stayed the same until now. There is also a narrow ring of foam or froth around the flask where the liquid meets the air. It sort of looks like the
kind of froth you sometimes see in stagnant pools of water or bays around cities. I am assuming that this is the NO2 being born.
Will post on how this all comes out. As you can probably tell, i am fairly new to this level of experimentation (at least in this field) so a good
outcome here will be a pretty exciting.
[Edited on 29-11-2007 by ExothermicReaction]
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Magpie
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Glad the reaction and venting went well for you.
Here's the results of my quick and rough calculations using air as fluid. I used the orifice equation in Crane's Technical Paper 410, and the table
in Appendix B, "Flow of Air Through Schedule 40 Pipe," (same publication) for the hose pressure drop.
Assumptions:
1. Escaping gas is air.
2. Escaping gas temperature =70 F (21C), and ambient pressure = 14.7 psia.
3. Orifice ID = 1/4", orifice enclosure ID = 5/16".
4. Hose ID = 5/16", hose length = 6 feet.
For a pressure drop of 0.23 psi (6.4"H2O) the gas flow through the orifice = 3.5 cfm = 99 liters/min.
For this same flow the pressure drop in the 6' hose was 0.83 psi = 23.0"H2O.
Therefore, to pass air at a peak rate of 99 lters/min, the flask internal pressure would only be 0.23 + 0.83 = 1.06 psi. Surely the flask, condenser,
adaptor, and hose could easily take this pressure. The connections would just have to be secure.
Edit: Recalculated the hose pressure drop using longhand calculation rather than the table. This seems more reasonable.
[Edited on by Magpie]
The single most important condition for a successful synthesis is good mixing - Nicodem
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YT2095
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| Quote: | Originally posted by ExothermicReaction
YT : Even if its greater, won't I still have a hose-full to deal with when the reaction is done? This is why I was thinking it might be good to get
some sort of pump to evacuate the hose.
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sure you could purge it with air, if the presence of air isn`t going to bork up your rxn. failing that you could clamp the pipe off and take it
outside.
\"In a world full of wonders mankind has managed to invent boredom\" - Death
Twinkies don\'t have a shelf life. They have a half-life! -Caine (a friend of mine)
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ExothermicReaction
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Magpie : Thanks so much! Thats very cool of you to take the time! This will help a lot as I seek to scale things up.
YT : First reaction is done and the only visible trace of gas left at the end was a bit that escaped up into the addition funnel just after the last
of the HCl dropped, creating an opening through the valve. I stoppered the top of the addition funnel and after letting it settle down, I very quickly
removed the funnel and submerged it in water and rinsed it out. I felt the amount was of N02 was small enough that this would be ok. I never smelled a
bit of the gas while rinsing so I think this was a good decision.
This morning the flask is still there, vented to the outide, and there are thousands of tiny columns of bubbles rising to the surface. Its slowed down
a bit since last night, but the reaction is still chugging along nicely. It should take about 24 hours to complete. Hmm..altough I haven't seen anyone
mention it, I'm wondering if, in the future I could accelerate the reaction by bringing it to a light reflux after things initially cool down. Or
maybe there is some reason that this one just can't be (or shouldn't be) rushed. Will look into this.
Thanks again guys.
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chemrox
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I have generated a lot of hydrogen halide gasses that I ran up and out with a hose through the window and down and out with a hose down the drain. I
didn't use wash bottles because i didn't want the pressure build up. Since I ws running condensers the down the drain approach worked quite well as
the gas was carried with the water current and partially dissolved in the water along the way. If I were running a Gatterman I'd do it in a hood or
glove box with an exit hose to the outside on a day with moderate winds. I like having a respirator with a cartridge that will handle HX, HCN and
organic vapors. If there's any doubt as to the integrity of the system the respirator goes on.
"When you let the dumbasses vote you end up with populism followed by autocracy and getting back is a bitch." Plato (sort of)
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ExothermicReaction
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Yeah, the setup seems to be working quite well and I do have a couple serious respirators sitting close by just in case. The gas outlet came in handy
again tonight as I am now distilling the result of my dcm extraction of last nights reaction. I hooked the tub to the vacuum adapter on my takeoff and
the dcm vapors are all outside.
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