Higher pressures favour the forward reaction ( since there are 4 molecules of gas on the left-hand side, for every 2 molecules on the right).
So the pressure is set as high as possible to give the best %yield, without making the plant too expensive to build so, only 200 atmospheres operating pressure is used.
The forward reaction is exothermic, which means that increasing the temperature will actually move the equilibrium the wrong way (away from ammonia and toward N2 and H2. So the yield of ammonia would be greatter at lower temperatures.
The trouble is, lower temperatures mean a lower rate of reaction. So what they do is increase the temperature anyway, to get a much faster rate of reaction.
The 450 oC is a compromise between maximum yield and speed of reaction. It's better to wait just 20 seconds for a 10% yield than to wait 60 seconds for a 20% yield.
The ammonia is formed as a gas but, as it cools in the condenser it liquefies and is removerd.
The unused hydrogen (H2) and nitrogen (N2) are recycled so nothing is wasted.
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