Compressors, centrifugal Recycle compression

Compression of synthesis gas. The synthesis gas composed by hydrogen and nitrogen is compressed to required pressures, usually 10-30 MPa, by piston-type or centrifugal compressors. During ammonia synthesis, single-pass conversion is only 10%-20%, and therefore, most of the synthesis gas must be recycled, compressed and returned to the synthesis loop again. 

Figure 75. Centrifugal compressor for make-up and recycle gas compression of an ammonia plant (courtesy of Uhde)...

The disadvantage of this layout is that all impurities present in the synthesis gas or introduced by the make-up and recycle compressors are passed directly to the synthesis converter. Therefore, this layout is used mainly in cases where catalyst poisons are removed very efficiently from the synthesis gas, and where non-lubricated compressors such as centrifugal compressors are used for makeup gas compression and recirculation. This loop layout has been used in many commercial processes. Some early examples are the Fauser process [406], the Fauser Montecatini process [407] and the Casale process [408] (both with ejector recycle), and the modified Claude process [384] (in the original Claude process a series of converters with intermediate cooling and product recovery was used this is the only process which has ever been used commercially without... 

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