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Topic: Ammonium Chloride to Ammonia + HCl  (Read 16576 times)

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Offline curiouscat

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Ammonium Chloride to Ammonia + HCl
« on: April 23, 2015, 01:40:02 AM »
Is it practically feasible to process NH4Cl industrial scale to yield NH3 & HCl. Is this reaction quantitative? To get reasonably fast rates how high do I need to heat it.

Context: I have a waste stream containing NH4Cl dissolved in water. Trying to think how to reprocess it.

I could send it to a multi effect evaporator. Then get solid NH4Cl & heat it. Upstream process has use for both NH3 (aq.) & HCl (32%).

Or could I somehow just heat the aq. solution & get NH3 + HCl without bothering to ever go to the solid NH4Cl?

Once I get the mixed gases how do I separately dissolve them in water?

Comments? 

Offline Hunter2

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Re: Ammonium Chloride to Ammonia + HCl
« Reply #1 on: April 23, 2015, 01:47:51 AM »
I think this will not work, ammonia and hydrochloric gas immediately form ammonium chloride by re-sublimation.  Experiment: open a bottle of each component and stay them together you will see white smoke development.

Offline Enthalpy

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Re: Ammonium Chloride to Ammonia + HCl
« Reply #2 on: April 23, 2015, 03:25:11 AM »
Instead of heating, could you displace NH3 from NH4Cl brine? With a strong nonvolatile base like CaO maybe? That would be half of the work.

Then, reprocess CaCl2 by molten electrolysis? I'm not quite sure...

Maybe HCl can be displaced from CaCl2 by H2SO4, and then you dump CaSO4, but this may not make economical sense.

What would the electrolysis of aqueous NH3Cl lead to?

Offline curiouscat

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Re: Ammonium Chloride to Ammonia + HCl
« Reply #3 on: April 23, 2015, 04:49:43 AM »
Instead of heating, could you displace NH3 from NH4Cl brine? With a strong nonvolatile base like CaO maybe? That would be half of the work.

Then, reprocess CaCl2 by molten electrolysis? I'm not quite sure...

Maybe HCl can be displaced from CaCl2 by H2SO4, and then you dump CaSO4, but this may not make economical sense.

What would the electrolysis of aqueous NH3Cl lead to?

Thanks @Enthalpy. Those ideas would work in principle.

But at that point I might as well use NaOH / Ca(OH)2 in the upstream process directly.

Basically, I have an upstream reaction that can be carried out with either NaOH or NH3 or Ca(OH)2. More prefrably either of the first two.

Using NaOH is the default but that generates a large amount of NaCl (aq.) getting rid of which is a nuisance.

I was wondering if using NH3 might let us reprocess & recycle. The key challenge is how to do this:

NH4Cl  :rarrow: NH3 + HCl

Offline curiouscat

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Re: Ammonium Chloride to Ammonia + HCl
« Reply #4 on: April 23, 2015, 05:14:06 AM »
I think this will not work, ammonia and hydrochloric gas immediately form ammonium chloride by re-sublimation.  Experiment: open a bottle of each component and stay them together you will see white smoke development.

Yes, but that's at room Temp. I'm suspecting (hoping?) that at elevated T the equilibrium will favor the gases.

Then it'd be a question of separating them while still hot. Not easy by any means, but is there a fundamental chemistry problem?

Offline Enthalpy

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Re: Ammonium Chloride to Ammonia + HCl
« Reply #5 on: April 23, 2015, 07:15:23 PM »
I have misued CPropepShell (meant to compute chemical equilibria in a rocket engine), mixing ionized helium (to bring neutral heat) with NH4Cl at 1atm, and:
- The lowest temperature to get an answer is 792K.
- Then, all Cl makes HCl, but N and H make N2 and H2, only 1/1000 make NH3.
- At 2375K, atomic Cl climbs to 0.01* of HCl, Cl2 is trace, the rest is essentially the same.
- At 2830K, atomic Cl climbs to 0.1* of HCl, the rest is the same.
- At 3400K, atomic Cl is 0.5* HCl, NH3 stays as traces

Tried again at 500bar: the smallest T for an answer is 912K, then
- NH3 moles are half as abundent as N2 moles.

These are strict equilibria, which make sense in a hot flame, not necessarily at lower temperatures where some reaction paths aren't allowed. He is in small mount here, no big equilibrium shift.

Do I remember that NH3 is synthesized at a lower temperature with a catalyst (and big pressure) to favour it over N2 and H2? At least CPropepShell predicts very little of it. Logically, NH3 is destroyed at a lower temperature than the salt.

I still don't grasp how you could separate HCl from NH3 even if this latter did form. It would require some form of chemical separation, wouldn't it?

But what looks credible is to recycle HCl and maybe H2, release N2. Just heat at 1atm does that. Upon cooling, all the Cl2 fraction will form HCl, N2 and the remaining H2 won't react. You plant would buy NH3 and destroy it cleanly, cycle HCl.

Or make ammonium chlorophosphate, sell it as a fertilizer?

Offline curiouscat

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Re: Ammonium Chloride to Ammonia + HCl
« Reply #6 on: April 24, 2015, 01:24:57 PM »

Or make ammonium chlorophosphate, sell it as a fertilizer?

If it comes to that then might as well sell Ammonium Chloride itself. :)

Offline curiouscat

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Re: Ammonium Chloride to Ammonia + HCl
« Reply #7 on: April 24, 2015, 01:26:50 PM »
I still don't grasp how you could separate HCl from NH3 even if this latter did form. It would require some form of chemical separation, wouldn't it?

Agreed. I don't know of a good way yet. I'm just brainstorming for now.

Offline unsu

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Re: Ammonium Chloride to Ammonia + HCl
« Reply #8 on: April 24, 2015, 06:52:56 PM »
In the Solvay process, NH4Cl is treated with Ca(OH)2 to recover NH3. CaCl2 is a waste, it is either dumped into the sea or used in winter road clearence.

Offline Enthalpy

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Re: Ammonium Chloride to Ammonia + HCl
« Reply #9 on: April 26, 2015, 03:15:15 PM »
To my opinion, and unless someone has an astounding idea, the proper way is to decompose NH4Cl at enough heat and atmospheric pressure. Dissolution in water separates HCl, the N2 and H2 left over can be burnt together if no better use. Clean, cheap.

Use successive water-gas contacts so the gas meets clean water in the last step, leaving well washed N2 and H2.

Offline curiouscat

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Re: Ammonium Chloride to Ammonia + HCl
« Reply #10 on: April 26, 2015, 05:36:59 PM »
To my opinion, and unless someone has an astounding idea, the proper way is to decompose NH4Cl at enough heat and atmospheric pressure. Dissolution in water separates HCl, the N2 and H2 left over can be burnt together if no better use. Clean, cheap.

It will be cheaper for me to just sell the NH4Cl I think. Unless I can recycle the NH3 the idea is an economic dud.


Offline curiouscat

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Re: Ammonium Chloride to Ammonia + HCl
« Reply #11 on: April 26, 2015, 05:46:33 PM »
I have misued CPropepShell (meant to compute chemical equilibria in a rocket engine), mixing ionized helium (to bring neutral heat) with NH4Cl at 1atm, and:
- The lowest temperature to get an answer is 792K.


I don't understand what you wrote here. What  is the equilibrium prediction of your code at say, 200 C or 300 C.

To me that seems the interesting region.

Offline Enthalpy

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Re: Ammonium Chloride to Ammonia + HCl
« Reply #12 on: April 27, 2015, 10:16:30 AM »
CPropepShell failed to compute an equilibrium below 500°C. Equilibria of solid+gas products are difficult to compute, and 300C isn't the normal use of this software.

Offline Enthalpy

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Re: Ammonium Chloride to Ammonia + HCl
« Reply #13 on: April 29, 2015, 10:26:17 AM »
More in detail: Propep's aim is to compute equilibria in rocket combustion chambers (and nozzles if supposing they shift during the expansion). Because of 3000-4000K, it neglects all chemical structure, all reaction details, and needs only to know the atomic composition and heat of formation of the propellants. Then it checks which of its known combustion products can apply (including OH, gaseous MgO, Cl*, even C2) and solves all equilibria.

As a sidenote, it was programmed in Fortran for I dont' know what computer, ported much later to Ms-dos and never serviced since. Several graphical user interfaces were added, none is perfect nor really easy, one package being CPropepShell. Rpa (Rocket Propellant Analysis) was written recently and is far better than Propep plus interface, except that it lacks flexibility for my horrible tinkering.

You get it: what you ask at 200 or 300°C is cute lace, what Propep does is sheet stamping. Propep answers the best thermochemical equilibrium even if no reaction path leads to it. Sometimes it also fails to converge.

With ammonium chloride at 1atm and +200°C I get no answer, at +300°C I get the molar fractions
HCL                  3.3650e-001
H2                   4.9050e-001
NH3                  9.5031e-003
N2                   1.6350e-001
and at +274°C, NH3 climbs to 1.54e-2,
but I suspect Propep ignores NH4Cl as a combustion products.

Maybe an other equilibrium software suits the needs of chemistry better.

Offline curiouscat

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Re: Ammonium Chloride to Ammonia + HCl
« Reply #14 on: April 29, 2015, 10:32:54 AM »
Quote
Maybe an other equilibrium software suits the needs of chemistry better.

Indeed. I'd love recommendations.

I guess I could always do it the old school way. With NIST data & by hand.

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