Author Topic: A basic chem/physics question  (Read 908 times)

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chemisfun

  • Guest
A basic chem/physics question
« on: July 19, 2003, 03:05:00 PM »
The basic rxn

NaOH  +   NH4CL   ->   h2O  +  NH3  +  NaCL

The gases etc are all pumped through a pre-cooler before a drying column and then into a condenser.

A temperature sensor in the pre-cooler indicates the temp to be around -10 to -15 degrees Celsius.

My question is at this temperature will all the h2O vapor be taken out while passing through the pre-cooler???

Aurelius

  • Guest
Precooler
« Reply #1 on: July 19, 2003, 04:11:00 PM »
you shouldn't try to condense the water, you want to dry your NH3 gas (remove the water vapor) using a drying tube and passing the gas mixture through it. 

Then pass it through the condensor.


Rhodium

  • Guest
All the water will not be removed by just...
« Reply #2 on: July 20, 2003, 01:49:00 AM »
All the water will not be removed by just passing the gas through the pre-cooler, even though much of it may condense if it is narrow/turbulent. Absorption by a drying agent is required.

chemisfun

  • Guest
I agree fully...
« Reply #3 on: July 20, 2003, 01:31:00 PM »
after the pre-cooler the next stop is through a kilo of NaOH in a drying column. Does anybody think the order matters?? ie drying column 1st and then pre-cooler or vice-versa??

thanks alot.

Rhodium

  • Guest
Myself, I have a problem following orders... ;-)
« Reply #4 on: July 20, 2003, 08:13:00 PM »
The drying column will generate heat as the NaOH absorbs water, so putting that after the pre-cooler sounds like a bad idea...

Aurelius

  • Guest
Order
« Reply #5 on: July 21, 2003, 01:59:00 AM »
you should first pass it through calcium chloride tube (or anhydrous Mg sulfate)  Calcium chloride *might* form a complex with the ammonia (somebody verify or refute this statement with certainty) so Mg sulfate may be better.  Don't use a pre-cooler/condesor for the water as MUCH of your ammonia will dissolve in the water and be trapped at that low temperature.  I think you might lose to much ammonia with water sitting in its path.  Then condense the ammonia.


Rhodium

  • Guest
drying agent for ammonia
« Reply #6 on: July 21, 2003, 02:38:00 AM »
I can verify that CaCl2 will complex with the ammonia and therefore it will be unsuitable. I don't think MgSO4 is up to the job either as its drying action is not fast enough to work with gases.

The drying agent of choice for gaseous amines is KOH flakes, but if that is unavailable, NaOH pellets works good too.

Aurelius

  • Guest
Khedjahlalabama
« Reply #7 on: July 21, 2003, 07:54:00 AM »
Or however you spell the dang method for ammonia distillation/titration for Nitrogen content, is the only time I've had to collect ammonia (tanks for anhydrous)- and it doesn't need to be dry, so I couldn't remember.  Rhodium, thanks for the clarification of my statement.


ragnaroekk

  • Guest
KOH or NaOH plus CaO
« Reply #8 on: July 21, 2003, 09:20:00 AM »
KOH or NaOH plus CaO (lime, quicklime, calx, burnt lime)is the advised drying mixture for ammonia

CaCl reacts and MgSO4 sucks ammonia up together with the water. (Same for methylamine, btw). Drying tubes are better longer than thicker, also they should provide a significant slowdown in gas velocity for to make uptake of water easier. So the free volume in a filled drying tube has to be more as in the tube before and after it. Dont press the drying agents in the tube but take a bigger (longer) one.

There is no condensing of water at all wanted! It would suck up ammonia in huge amounts at once. To produce as less water as possible (low temps in reaction and slow motion) and to adsorb the rest is the way to go. So go direct in a sufficient drying tube which may be mounted to the precooler. This results in a compact design and a temperature gradient which makes sense.

The final condensor should be made of metal for the heattransfer properties. NO COPPER! Black iron does fine here, is asscheap and easy to work on as it can be epoxied and has by no way to be welded. (choosing a epoxy usable at these low temperatures is understood)
Surface area is whats needed so rough surfaces beat polished ones. After the condensor a tube should run to near the bottom of the metal made, precooled receiving vessel so after some ammonia was made the outlet is below the surface of liquid ammonia. This way the production can get speeded up significantly after a slow starting phase. Also a astonishing small condensor  sufficices in this setup where the receiving flask is also utilized for liquidification purposes.

Starting to cool the parts in time, say several hours in advance is a very good idea. A high humidity on the working space sucks up unavoidable ammoniasmell. Better inside a garage and boiling some water beneath as outsides in dry air which takes the smell over miles.

Methylamine is a cake in comparism to ammonia. What problematic is seen in a compound having a boiling point of -6,5°C is hard to understand. A look over the fence to the Birchers might be a good idea - anhydrous methylamine gas is not hard at all, neither to make, to store and not to use.


chemisfun

  • Guest
very bad advice mon ami.
« Reply #9 on: July 21, 2003, 01:50:00 PM »
Aurelius In swim's younger days he used CaCl2, ala MrClean, and it lead to a situation swim did not need. Without going into details it rates up there with the time swim experienced the dreaded SUCKBACK.....  yeh we can laugh at this shit now ha ha ha.

Basically the CaCl2 'clumps' together to the stage where it forms a blockage.

Thanks Rhodium and bees.