MadSci Network: Engineering

Re: I dont understand how Anhydrous Ammonia is made.How do they 'compress'it?

Date: Tue May 25 21:22:39 1999
Posted By: Edward Peterson, Staff, Chemical Engineering, S&B Engineers and Constructors
Area of science: Engineering
ID: 927512324.Eg

I guess you picked, or were maybe assigned, a harder topic than you 
needed.  Never, fear, a chemist is here.

Ammonia is made many different ways.  Our bodies make it every day as they 
digest proteins in burgers and pizza.  It is converted to a water soluble 
compound named uric acid, named after urine.  How interesting!  Yuck.

In industry, ammonia is made using pressure, temperature and very often, 
catalysts.  Ammonia is made from hydrogen and nitrogen.  Hydrogen is a 
very light gas that can be made from water by passing an electrical 
current through water.  The remainder is Oxygen.  Hydrogen can react with 
oxygen very easily to form water.  Because you can make water from 
hydrogen and oxygen, then convert the separate gases back to water, the 
reaction is called reversible.  The reaction to make ammonia is also 
reversible.  Nitrogen is a gas that does not react with other chemicals 
very easily.  It makes up about four fifths of the air we breathe.  
HOwever, at very high tempeature and pressure, it will convert to ammonia.

There really is no difference between anhydrous ammonia and pure ammonia.  
Anhydrous simply means free of water.

Why is the reaction that makes ammonia from hydrogen and nitrogen done at 
high temperature?  Nitrogen is very unreactive at low temperature.  It is 
made of two nitrogen atoms that stick together very tightly.  As 
temperature is increased, the nitrogen atoms move farther and farther 
apart.  When the temperature is high enough, they can move far enough 
apart that when a hydrogen molecule (also made of two hydrogen atoms) gets 
close enough, one nitrogen atom quickly combines with the two hydrogen 
atoms.  That makes NH2, which by the laws of chemistry, is very, very 
reactive.  It wants another hydrogen atom very badly to balance out its 
quota of electrons.  Therefore, once the first molecule or atom of 
hydrogen gets close, the NH2 becomes NH3, which is very stable.

Every reaction that occurs on this earth has a reaction rate.  The 
conversion of hydrogen and nitrogen to ammonia has one rate at any 
temperature, and the reverse reaction, the conversion of ammonia to 
hydrogen and nitrogen, has another rate.  For making ammonia, finding the 
temperature when the forward rate is high and the reverse rate is low 
makes the most ammonia.  Unfortunately, the rate is relatively slow at 
normal pressures and high temperatures.  At high pressures, the reaction 
makes more ammonia. This behavior with gaseous reactions is known as 
LeChatlier's principle, that says when the pressure is increased, the 
reaction will favor products that tend to decrease the pressure.  Because 
four molocules of gas form two when hydrogen and nitrogen form ammonia, 
forming ammonia at higher pressures makes more ammonia and leaves less of 
the reactants behind.

Catalysts, like metallic iron, tend to speed up the reaction of nitrogen 
and hydrogen to form ammonia, so that lower tempeatures and pressures can 
be used to make just as much.  This lowers the cost of the reaction and 
lets the manufacturer sell it for less.

Compressing ammonia or any other gas is usually done by using a piston in 
a tube.  An ordinary bicycle pump is used to take air that is around and 
compress it to higher pressure.  The same principle is used to compress 
ammonia.  The ammonia gas is let into a chamber or tube that has the 
piston in it.  The piston is pulled all the way back to make the chamber 
as big as possible.  When the ammonia gas fills the tube at its beginning 
pressure, the piston is pushed forward, squeezing the gas in the chamber 
to a small fraction of its original volume.  That squeezing increases the 
pressure of the gas.  That is how gases are compressed, like ammonia.

If you need more information, you can email me directly 
( or you might try to talk to someone at MW Kellogg 
Corporation, in Houston, Texas.  They are one of the largest manufacturers 
of ammonia production plants in the entire world.

Good Luck

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