Modified Haber Process

[Ollie Snowie]

Somebody please move this. I'm still a newbie so I'd better post all my new threads in the water cooler.

Several ideas have struck me as making the haber process more accessible to the home laboratory:

1. I have heard (in another thread here) that ammonium nitrate will absorb ammonia even at atmospheric pressure. So how about putting a mixture of ammonium nitrate and iron filings on shelves in a heat and pressure resistant chamber, pumping in the nitrogen and hydrogen, heating it to a moderate temperature (nowhere near decomp. of ammonium nitrate) and leaving it until the pressure has dropped to almost normal, signifying that the reaction has almost finished. Then drive the ammonia off the ammonium nitrate with gentle heat or low pressure.

2. I presume the reason why ammonium nitrate absorbs ammonia is because it is an acidic salt (weak base with strong acid) where the acid ion has all its valences filled up with alkali ions (-NO2 and -NH4 have equal and opposite charges). So the ammonia is attracted to the ammonium nitrate but cannot bond with it. If this is so, you could use a more stable alternative (e.g. ammonium chloride) and run the reaction at a higher temperature, more like what they use in industry (~400*C).

3. As for the high pressure, I would use a centrifuge pump, because it is simple, has no rubbing parts, and needs no moveable gaskets (which wear out). You could then fill a large container with nitrogen and hydrogen, pump water in, leave it for a few hours, and then run off the dilute ammonia solution. As the ammonia is formed, it would dissolve in the water and be effectively removed from the gas phase where it can easily decompose again.

4. The previous method would not be very effective because the ammonia comes out as a dilute solution, so I came up with this idea:
Use centrifuge pumps to pump a dense liquid which nitrogen and hydrogen are not very soluble in (e.g. mercury) into the bottom of a pressure resistant cylinder which has a hole in the bottom and two holes in the top. This pushes all the air out of the cylinder. Then put two one way valves into the top two holes, on going in, the other going out. Attach the one going in to a balloon filled with nitrogen and hydrogen and let the liquid out of the cylinder, drawing the gasses into the cylinder. Then switch the pump on again and force the gasses out of the cylinder, through the outgoing one way valve, and into the catalyst chamber, which is filled with iron filings and ammonium nitrate. Keep this cycle going until the pressure in high enough for your liking, let the ammonia form overnight, and then use it to wake yourself up in the morning. :)

Please point out any mistakes - I'm sure there is something to stop it working. I'm rather tired. :)

[Ollie Snowie]

Mods - Sorry about this-I can’t edit.

I was so tired when I posted that that I didn't think of the problem of oxygen contamination (Good thing I didn't fall asleep and start dreaming of it ;)) I must have thought something like this - fractional distillation of air=easy no problem, I can reach 160 psi with my trusty football pump! Anyway – I’ll stop blabbering now:

I was thinking about this modified Haber process (AGAIN) and I thought of how to solve this problem of oxygen contamination as well as reducing the required pressure. I would pass air slowly over hot charcoal so nearly all the O2 gets removed as CO2, and use the heat produced to heat another chamber full of charcoal. Steam would be passed over the 2nd lot of charcoal to produce a gas containing hydrogen and carbon monoxide. These two gases would be mixed in the right proportions so as to have 3H2 to 1N2 and then sparked (Passing them over a hot catalyst for oxidation such as platinum would probably be safer). Any remaining oxygen would oxidise the CO in preference to the H2 or N2 (I think). Then the gases would be pumped into a pressure and carbonic acid resistant bottle (pop bottle is fine) at whatever pressure the bottle is happy taking. The concentration of NH3 at 10 atmospheres is 2.04% (from Chambers Encyclopaedia) but it didn’t say at what temperature this was or whether it was with a catalyst. A small amount of water would be introduced into the reaction chamber and the reaction would start:

CO2 + H2O --> H2CO3

3H2 + N2 --> 2NH3

2NH3 + 2H2O --> 2NH4OH

2NH4OH + H2CO3 --> (NH4)2CO3 + 2H2O

CO is left unchanged. I think that a lot of nitrogen would be left over as well as CO. CO could be burned to provide extra heat for the first stage and to make it less toxic but it would probably also need a catalyst because of all that N2. Is it REALLY worth it?

Then the product would be reacted with lime to make ammonia and CaCO3. If you really wanted to make a little, almost self contained, ammonia plant, you could make the lime from the CaCO3 again and feed the resulting CO2 into another type of plant and then turn it into charcoal for the first stage, but that would be a little obsessive!

Questions:

I think that the ammonia formed would react with carbonic acid to make ammonium carbonate, not with carbon dioxide to make urea. Am I right?

Is there anything else wrong with this?

Am I re-inventing the wheel?

[EDIT - Spelling, additions and making it make sense]

[vulture]

Steam would be passed over the 2nd lot of charcoal to produce a gas containing hydrogen and carbon monoxide

This needs very high pressure and a temperature of 900C. Good luck constructing a reactor that'll withstand that.

Any remaining oxygen would oxidise the CO in preference to the H2 or N2 (I think)

Nope. The oxygen will combine explosively with any H2 at the temperatures you're working with. Furthermore, the spark or platinum will assure ignition of any H2/O2 mixtures!

Then the gases would be pumped into a pressure and carbonic acid resistant bottle (pop bottle is fine) at whatever pressure the bottle is happy taking.

CO2 is soluble in water, but not that good. I suggest using a CaCO3 solution (as this can be recycled, whereas NaOH cannot).

2NH4OH + H2CO3 --> (NH4)2CO3 + 2H2O

The equilibrium is strongly to the left here.

Ammonia preparation is dirt cheap on industrial scale, but this does not mean you can get away with simple apparatus. Pressure, temperature and molar ratio control is vital to the yield.

Your ammonia will be bloody expensive compared to industrial.

[Ollie Snowie]

Originally posted by vulture
This needs very high pressure and a temperature of 900C. Good luck constructing a reactor that'll withstand that.

-I know that this is true for methane + steam, but I thought that carbon + steam could be done at 1atm. Oh well, I hate buying chems and I'm quite obsessive about it. I don't think anyone else here is quite THAT obsessive. I'd electrolyse water! My motto - Electricity is cheap. :)

Originally posted by vulture
Nope. The oxygen will combine explosively with any H2 at the temperatures you're working with. Furthermore, the spark or platinum will assure ignition of any H2/O2 mixtures!

-There won't be much oxygen so the rection will be very diluted. That's why I thought I might need a catalyst.

Originally posted by vulture
CO2 is soluble in water, but not that good. I suggest using a CaCO3 solution (as this can be recycled, whereas NaOH cannot).

-When I thought "carbonic acid", fizzy drinks sprung to mind, so I couldn't help mentioning the pop bottle. This would still be under pressure, so a considerable amount of CO2 would dissolve. Anyway, The low solubility of CO2 would not matter much here, because the carbonic acid would be neutralized by the ammonia as it formed, bringing it away from the equilibrium as ammonium carbonate. That was the whole point of having the CO2 in there (apart from it being pointless to remove).

Since when was limestone, CaCO3 soluble in water?! And where did the NaOH come into it? I don't get what you mean about using CaCO3 solution (if it exists) instead of CO2. Perhaps I wasn't clear enough on my previous posts.

Originally posted by vulture
The equilibrium is strongly to the left here.

-What? An acid/base neutralization recation favouring the acid/base mixture to the salt? I know it will ionize to -NH4 and -CO3 and both of these are gases in solution, but this is a salt from a weak base and a weak acid, so not much will ionize. I'm probably wrong on this one.

I don't suppose anyone else here wants to synthesise their own ammonia via Haber and I only wanted it for nitric acid. I suppose I'll just have to use the arc process to do it. I wasn't seriously thinking of a home ammonia plant at this stage but this process could still be developed. I still have a chemistry project to do at school and don't know what field to do it in yet. ;)