3.4 UREA SYNTHESIS PLANTS

Like the NH3 synthesis plants also urea synthesis plants have developed in recent years reaching ever greater capacities while the trend has been towards relatively low pressure synthesis processes.

Now plants utilizing centrifugal compressors only vary in size from a minimum of 600 to a maximum of 1800 MTPD; up to the early 70's CO2 was compressed by a combination of centrifugal and reciprocating compressors.

The processes that have become most common can be divided into two categories requiring pressures of 150 to 160 bars and 230 to 250 bars respectively.

Urea is obtained by synthesis of liquid NH3 and gaseous CO2 through the known chemical reactions

ZNH₃ + CO₂ ^→_← NH₄CO₂NH₂ (exothermic reaction)

NH₄CO₂NH₂  ^→_← NH²CONH₂ + H₂0 (exothermic reaction)

In practice, with the first reaction carbamate is formed and then dehydrated to form urea.

In terms of machines Centrifugal compressors were used that compress almost completely pure CO2 from atmospheric pressure to the synthesis pressure. Compression normally takes place in two casings (150-60 bar synthesis) or in three casings (230-250 bar synthesis).

A train is made up of a low pressure compressor with two compression stages (2MCL compressors) and of one or two high pressure compressors (2BCL and BCL compressors) (see figure. 3.9).

These machines have to achieve very high pressure ratios consequently with great differences in flow rates between the beginning and end of compression (the pressure ratios are 150-160 or 230-250 respectively).

High gas density even at not particularly high pressures calls for special attention in the mechanical sizing of the rotors.

This is peculiar to urea synthesis as in ammonia synthesis, for instance, the problem does not exist on account of the lesser molecular weight of the gas.

Another difference is that CO2 is corrosive, particulary  if wet;

 its corrosiveness is related to pressure and temperature and therefore also the type of material employed has to vary along the compression path. Although carbon steel is normally used in the first compressor casing all parts in contact with the gas in following casings have to be stainless steel.

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