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Commercial Production of Ammonia

Ammonia Production with the Haber- Bosch Process

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Nearly all modern commercial production of ammonia is based on the Haber-Bosch synthesis process. The process reaction is given by the equation:                               N2 + 3 H2 < == > 2 NH+ Heat     
 
This reaction happens in a special high pressure reactor in the presence of a special catalyst, usually a porous iron oxide. The reaction is exothermic which means that energy is released.

The equation shows that the nitrogen, hydrogen and the ammonia exist in  equilirium  which is determined by the conditions existing in the reactor. Typically for ammonia synthesis these condions are:

  • Pressure - about 150 atmospheres
  • Temperatures 370 - 500 oC

Definition: Under equilibrium conditions the proportion of reactants and the product of a chemcal reaction are balanced and detrmined by the existing physical contions such as pressure, temperature and concentrations. 

Factors influencing the rate of ammonia production (rate of reaction rx):  

  • Temperature - because the reaction is exothermic, lowering the temperature in the reactor will increase the yield of ammonia. But this also slows down the reaction therefore for the reason of efficiency in commercial production the temperature is kept as high as possible.
  • Pressure - increasing the pressure will increase the yield of ammonia but there is a limit in pressure for safety reasons.

Click here to read about equilibrium conditions for ammonia reaction

The Process (Haber-Bosch):

In large scale commercial ammonia production plants the feed stock which makes up the reactants are water, methane and air. Through a series of chemical processes the feedstock is converted to nitrogen, hydrogen and carbon dioxide. After removal of the waste carbon dioxide, the remaining mixture of nitrogen and hydrogen is called "synthesis gas". The synthesis gas enters the ammonia reactor, where it undergoes the synthesis reaction shown above.

The synthesis reactor consists of a number of beds containing the catalyst through which the reactants pass and are converted to ammonia under equilibrium conditions. Because the reaction is exothermic, heat is removed by coolers (heat exchangers). This ensures that the maximum amount of ammonia is converted.

At the end of the reactor a stream of mixed gas (ammonia + nitrogen + hydrogen) is removed and cooled. The cooled ammonia condenses and is separated from the other gas which is then returned to the reactor for reprocessing. This is a continous process - at the one end ammonia is continuously removed and this is balanced by new "synthesis gas" which is continously added to the reactor.

Click here to see a flow diagram for ammonia synthesis