Technical characteristics of total-recycle processes

Technical characteristics of total-recycle processes A, Base scheme (Fig. 1.32) 

The reactants, purihed to remove carbon monoxide and moisture, are compressed sq arately and then introduced into the reactor, where the carbamate is ra dly formed. Tte reaaor normaily operates adiabadcally, and the temperature must stringently controlled due to severe corrosion by ammonium carbamate solutions above 20(K.  

The reactor effiuent, consisting of urea, ammonium carbamate and unconverted reactants, is subjected, by altering the operating conditions, to a decomposition that converts part of the ammonium carbamate to carbon Uoxide and nmonia. The resisting gaseous product is compressed and condensed. This leads to renewed formation of carbamate which is recycled to the reactor in aqueous s ution, while the excess ammonia is mixed with fresh ammonia. The entire operation is repeated to decompose aO the carbamate. The final solution obtained contains 72 to 76 per cent weight of urea, and the fmal purity desired can be obtained by a finishing treatment. 

If the biuret content has to be in the range of 0.7 to 0.9 per cent (standard fertilizer), several evaporadon operations are first conducted under vacuum with limited residence rirng, followed by centrifuging, and terminating in spraying of the product in a prilling tower. Lower biuret contents (02 to OJ per cent) require fractionated crystallization followed by granuladon. 

The Stamicarbon 1 process is a conventional total-recycle technique. Its special feature consists in introducing small amounts of oxygen into the reaction medium to minimize corrosion. 

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