Argon Recovery from Ammonia Purge Gas

Influence of Pressure and Other Variables of the Synthesis Loop 

The gas enters the converter (a) at 295 °C and is subsequently heated in the internal heat exchanger to 390 °C before it enters the first catalyst layer. The outlet gas from the first layer then passes through the aforementioned heat exchanger and enters the second bed, after which the gas leaves the converter with 469 °C and passes a waste heat boiler generating 125 bar steam. The inlet gas of the second vessel, which accommodates the third catalyst bed, has a temperature of 401 °C and the outlet enters a further waste heat boiler generating 125 bar steam. 

Waste-Heat Boilers for High-Pressure Steam Generation 

After the secondary reformer of steam reforming plants the gas has to be brought down from around 1000 °C to about 350 °C for the HT shift. In earlier-generation plants two boilers were usually installed in series, with a bypass around the second to control the inlet temperature for the HTS. Common practice for a long time was to use a water-tube design. A famous example is the Kellogg bayonet-tube boiler, applied in more than 100 plants. Because of size limitations two parallel units were installed. For sufficient natural water circulation these boilers needed a steam drum at a rather high elevation and a considerable number of downcomers (feed water) and risers (steam/water mixture). 

In contrast, fire-tube boilers are much better suited for natural circulation and the steam drum can sit in piggyback-fashion, right on top of the boiler. This makes it possible to provide each boiler with its own separate steam drum, which allows greater flexibility in the plot plan. But it took some time before this boiler type was accepted in ammonia plants as the stress pattern is more complex and less predictable than in water-tube boilers. 

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