Industrial Application: The Haber Process

The Haber process is the industrial synthesis of ammonia gas from nitrogen and hydrogen gas, reacted in a fixed stoichiometric ratio by volume at a selected optimum temperature and pressure: $N_2(g)+3H_2(g)\rightleftharpoons 2NH_3(g)+\text{Heat}$. The forward reaction proceeds with a decrease in the number of gas moles ($\Delta n=-2$) and is exothermic; iron, containing a small quantity of molybdenum as a promoter, is used as the catalyst. Applying Le Chatelier's principle (Section 12.8.1) to choose practical operating conditions: (a) Effect of temperature -- since ammonia formation is exothermic, a LOWER temperature would favour a higher equilibrium yield of NH3, but at low temperature the reaction rate is too slow to be useful, taking a long time to approach equilibrium at all; at very high temperature the reaction is fast, but the equilibrium shifts back toward N2 and H2, decomposing much of the ammonia formed. The optimum temperature used industrially is about 773 K, balancing an acceptable rate against an acceptable yield. (b) Effect of pressure -- since the forward reaction reduces the number of gas moles, high pressure favours it and increases NH3 yield; but at very high pressures the iron catalyst becomes less efficient and the engineering cost of containing the gas rises sharply. The optimum pressure used industrially is about 250 atm. Nitrogen (from air) and hydrogen (from natural gas) are fed in continuously in a 1:3 ratio by volume; after passing over the catalyst, the gas mixture is …