Method of recovering unreacted ammonium carbamate in urea synthesis

In a process for recovering unreacted ammonium carbamate contained in the effluent obtained by reacting at urea synthesis temperatures and pressures carbon dioxide with ammonia in stoichiometric excess, wherein said effluent is subjected to a first ammonium carbamate decomposition stage at a gauge pressure of from 40 to 120 kg/cm.sup.2, then to a second ammonium carbamate decomposition stage at a gauge pressure of from 10 to 25 kg/cm.sup.2 and finally to a third ammonium carbamate decomposition stage at a gauge pressure of below 5 kg/cm.sup.2, thereby forming an off-gas in each stage and wherein the off-gases from at least said first and second stages are absorbed in an absorbent selected from the group consisting of water, aqueous ammonia, aqueous urea, aqueous ammonium carbamate and an aqueous solution containing urea and ammonium carbamate, the resultant recovered ammonium carbamate solution being recirculated to the urea synthesis, the improvement which comprises absorbing said off-gas from said second decomposition stage in said absorbent to form an ammonium carbamate containing absorbate and unabsorbed gaseous ammonia;

• cooling said unabsorbed gaseous ammonia, to form recovered liquid ammonia;
• absorbing in said ammonium carbamate containing absorbate pressurized to the pressure substantially equal to that of said first decomposition stage substantially all of said-off-gas from said decomposition stage at a temperature of from 120.degree.to 180.degree.C while removing the heat of absorption by indirect heat exchange with a mixture of make-up liquid ammonia and said recovered liquid ammonia pressurized to said urea synthesis pressure, whereby said recovered ammonium carbamate solution is formed and said mixture of said make-up liquid ammonia and said recovered liquid ammonia are heated to a temperature of from 100.degree. to 160.degree.C;
• reacting said mixture of said make-up liquid ammonia and said recovered liquid ammonia and said recovered ammonium carbamate solution with make-up carbon dioxide in a heat recovery zone to form a reaction mixture containing ammonium carbamate while concurrently removing from said zone the surplus heat of reaction by indirect heat exchange;
• and introducing said formed reaction mixture containing ammonium carbamate to the urea synthesis.