Zeul Mos
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Ammonia Generator idea
Hello everyone,
Recently I got pretty interesting in the birch reduction, but mostly in ammonia. I almost ran a birch reduction but I ran out of drying agent so too
little ammonia for me.
While thinking about it I came up with an idea to make it but I would like to know some opinions.
My idea is to put reactants that generate ammonia in a container/tank that can take up to a few hundreds psi and let them react. Due to high pressure
ammonia should become liquid but there will be impurities like water and other products depending on the reagents used to make ammonia.
I was thinking that one could dry the ammonia using some drying agent (sodium or lithium metal maybe).
After the water is eliminated, one could filter the ammonia to get rid of the solid impurities (NaNO3, CaNO3, NaCl, etc.).
I think that while doing the filtration, the chemist should have a very good fumehood be fast working so that ammonia loses due to evaporation could
be minimal.
What do you think? First off, would it work? If no, why? If yes, is there something to be improved and what?
Thanks in advance.
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Etaoin Shrdlu
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You want to filter liquified ammonia? Isn't condensing going to be easier? You're going to have to keep it at a low temperature when working no matter
how you prepare it initially.
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woelen
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I do not see this working. It looks highly impractical to me. As soon as you open up the pressurized reaction chamber, most of the ammonia will
evaporate violently. In the reaction chanber you will have a nasty high pressure mud, made of water, solid reactants and ammonia (mixed with the
water).
The only practical method is making NH3-gas, leading this through a drying agent (e.g. solid prilled NaOH) and then condensing this in a cold trap
(cooled with dry ice). Making NH3-gas in fairly large amounts and at a reasonable rate is not that difficult. A good option is mixing NH4Cl and NaOH
and initiating the reaction with a small amount of water.
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Chemetix
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Gas on demand
Getting ammonia from the old Ammonium salt/Hydroxide reaction is one thing, getting a steady stream of gas from this is another.
I had tried using urea and sodium hydroxide but found it generates too much gas once the reaction reaches a critical temperature, the reaction runs to
completion with sodium carbonate as the main byproduct.
What if this was in a tank?
So long as there wasn't too much water initially, just enough to solvate some of the two reagents, maybe some hydroxide in excess to ensure the water
remains saturated and drives the ammonia out as a gas, then the tank should pressurise and become ammonia-in-a-can.
Let's give it a go.
The old soda-stream tank with a stainless steel fitting welded onto it and a stainless high pressure valve seemed to do it.
I tried a 1/4 mol test and there was a pressurisation with ammonia. It just didn't go to completion and it didn't give nearly enough gas as I thought
it would. Based on test tube reactions that is.
But, I did use more water than I needed and there was no excess of hydroxide; rinsing solid reactants from the thread took more water than I really
wanted. I think that's where the ammonia went.
But test one; success!
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Melgar
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Interesting, although you should maybe check to see whether you have sodium carbonate like you thought, or sodium cyanate. I would expect sodium
cyanate, since that's what you get when you heat alkali hydroxides and carbonates with urea. (In addition to ammonia gas, of course) Really though,
you don't need any sodium hydroxide at all. All you have to do is heat urea to its decomposition temperature, and it will form cyanuric acid and
ammonia gas. This happens at about 175C. Cyanuric acid is very thermally stable, but if you get it hot enough, it will eventually form isocyanic
acid, which may or may not be useful to you. I believe cyanide salts can be made by heating alkali hydroxides or carbonates with cyanuric acid and
carbon, though of course, you'd have to double-check your references before trying that.
Really though, all signs point towards you actually having sodium cyanate, since the water didn't get used and you didn't get as much ammonia as you
thought you would. If this is the case, then the sodium hydroxide is entirely optional, and only serves to make the reaction happen at a lower
temperature, and of course, making it more prone to thermal runaway as you seem to have witnessed.
edit: this probably deserves to have its own thread, since this one seems to have been spawned by the fever dreams of a tweaker.
[Edited on 5/22/17 by Melgar]
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Chemetix
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Based on test tube reactions the ppt fizzles away with acids, carbonate as far as that test indicates. Cyanates would be good to prepare, many fun
rxns to be had. Hydantoins anybody?
Urea by itself and water tends to give ammonia and carbon dioxide but only when the water is INXS, any less and it gives the carbamate in a
reversible reaction. If there is sodium around the carbonate should ppt and the ammonia left in gaseous phase. Which is what I have been banking on
with this scheme.
Maybe there are cyanates forming as you indicate Melgar, I might have to double check with some test reactions and quantitative probing.
(edit) And as for the origin of the thread... well, better to use an already established thread than have the admins decide it should have gone there
in the first place. I kinda hate the creation of new threads based on minuscule differences from another established one or the lack of research to
see that their post already has a dedicated thread.
There is the cringe factor with tweaker chem though...
[Edited on 22-5-2017 by Chemetix]
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Melgar
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Cyanates also bubble when you add acid to them. I forgot what the kinetics of the reaction are, though. If you don't believe me regarding urea though,
here's a link:
https://en.m.wikipedia.org/wiki/Cyanuric_acid#Synthesis
If you heat urea with no water at all, then not only is your ammonia anhydrous and free of CO2, but it results in cyanuric acid as essentially the
only product besides ammonia. I will warn that cyanuric acid is about as much fun to get out of glassware as anhydrous calcium sulfate though. Best to
start with something you're not to attached to, unless you have a really hot torch and good ventilation.
Still, if urea is cheaper for you than NaOH, then so what if you're not using all the nitrogen?
edit: Acid + alkali cyanate + water = ammonium salt of acid + alkali salt of acid + CO2. A test for cyanate would be to first acidify, then add NaOH
when it finishes dissolving/bubbling. If the salt was originally a cyanate, then the NaOH would release ammonia.
[Edited on 5/22/17 by Melgar]
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Chemetix
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I chose urea because of the lack of byproducts, because the industrial on demand ammonia generation process relies on the heating of aqueous urea.
More accurately, heating urea in the presence of water. From what I have observed the only by products have been biuret. My text books tell me heating
biuret generates cyanates and isocyanates at higher temperatures. My urea boiler has run for days and no indication of the low solubility cyanates
present. https://en.wikipedia.org/wiki/Sodium_cyanate
BTW, your link points to cyanuric acid which is a different beast to cyanate such as the sodium cyanate reaction you proposed for testing.
Urea and water at around 110C generates NH3 and CO2 somewhat slowly, if the water is in too low a quantity the CO2 and the NH3 form the volatile and
unstable ammonium carbamate, which sublimes then decomposes in air. Under pressure this can be suppressed but the amount of heating required is
higher. With sodium hydroxide you can form sodium carbonate which can then react with urea and go on to form the cyanate. But when the hydroxide is in
excess, ammonia is liberated along with the complete absorption of the carbon dioxide as sodium carbonate. This drives the reaction strongly to the
right and explains what I saw in the test tube as a rapid decomposition of the urea and large volumes of ammonia generated after heating.
It was this behavior that suggests a fire extinguisher type reaction in a tank might be possible.
The problem is urea tends to undergo a varied and complicated set of reaction pathways under all sorts of conditions. And ammonia dissolves
extraordinarily well in water.
I think that explains where a good portion of the 8g of NH3 ended up in the test.
I'll run your test for cyanates, I'd like to see if if there is a significant quantity ending up down this reaction pathway.
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Melgar
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Actually, I was suggesting that you simply heat urea to generate your ammonia, with no sodium salt or water. The link that I pasted was just to show
that the reaction (thermal decomposition of urea) would yield only cyanuric acid and anhydrous ammonia as its products. Since cyanuric acid is a very
stable solid, and ammonia is a gas, and there is no need for any water at all if all you're doing to urea is thermally decomposing it, I'd posit that
this would be a cleaner, easier way of charging a tank with dry ammonia. You're probably thinking that you need a strong base to keep the reaction
from reversing itself, since ammonia is a base and cyanuric acid is an acid, but in the absence of water, I'm fairly certain they don't react at all.
Cyanuric acid is a very weak acid, and is rarely even in its acidic tautomer.
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Chemetix
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I guess that would work, but cleaning for reuse? Do you know of anything that cleans the stuff. I worked with a chlorine generator that used the old
HCl and TCCA reaction once and it was a mess to clean.
I've come this far with the Ostwald process, I might just consider benchtop Haber as well.
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Melgar
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If you heat cyanuric acid hot enough, it will turn into isocyanic acid, which is a gas. This is at perhaps 500C, so certainly doable. The other
option is to charge your tank from the generating tank, by cooling the storage tank and heating the generation tank. As long as you can get all the
air out of your tanks, then in a situation like this, eventually a pressure would be reached at which ammonia would be liquid at the temperature on
the storage tank side, and the ammonia would condense there. Just using a tank that can be heated to cyanuric acid's decomposition temperature seems
like an easier solution though.
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