Concn. of soln. of urea obtd. from carbon di:oxide and ammonia - by stepwise decomposition of carbamate, and evapn. using heat from condensn. of gas from decomposition step

摘要

In concentrating a urea soln. by evapn. of water from the soln., where (i) a urea soln. contg. carbamate and free NH3 is prepd. from CO2 and an excess of NH3 in a synthesis zone at 125-350 bar and the corresp. temp., (ii) part of the carbamate is decomposed in a 1st decomposition step at not above the synthesis pressure, with addn. of heat, and the gas mixt. obtd. is (partly) condensed in a 1st condensation zone, and the condensate is recycled to the synthesis zone, (iii) a further part of the carbamate still present is decomposed in 1 or more further decomposition steps, maintaining a pressure of 4-40 bar and applying heat in the first of these further steps, and carrying out heat-exchange in the 1st condensation zone between the condensed gas mixt. and (part of) the stripped urea synthesis soln. reduced in pressure in the first of these further decomposition steps, and (iv) the remaining urea-contg. soln. is conc. by evapn., obtg. the heat (partly) by condensation of the gas mixt. obtd. in the first of the further decomposition steps, the novelty is that a gas stream of constant size is fed from the first of the further decomposition steps to the evapn. step, the heat requirement of the evapn. step is supplied (partly) by condensation of (part of) this gas stream, and if necessary the temp. of the urea soln. withdrawn is regulated by condensation of a low pressure stream.