This invention relates to urea synthesis from ammonia and carbon dioxide, and in particular to a new method of processing the urea reactor effluent solution taken from the reactor.
Urea is conventionally synthesized by reacting ammonia and carbon dioxide in a reactor at elevated temperature and pressure to form ammonium carbamate, which, in turn, is reacted to urea and water. The first reaction of carbamate formation is very rapid and practically complete. The second reaction of urea and water formation from ammonium carbamate is slow and incomplete. In the presence of excess ammonia, i.e. above the stoichiometric amount required to form ammonium carbamate, the conversion of ammonium carbamate to urea and water is promoted. In the presence of excess water in the reactor, i.e. above the stoichiometric amount formed with the urea from ammonium carbamate, the conversion of ammonium carbamate to urea and water is hindered.
Ammonia and carbon dioxide are generally fed to the urea synthesis reactor either separately, or as an aqueous ammoniacal solution containing ammonium carbamate and/or carbonate formed by reaction of ammonia and carbon dioxide, or in a combination of separate fluid ammonia and fluid carbon dioxide with a stream of an aqueous ammoniacal solution of carbamate and/or carbonate. Generally the overall NH.sub.3 to CO.sub.2 molar ratio in the urea synthesis reactor is maintained between about 2.5 and 6 to one at a temperature from about 330.degree. F. to about 400.degree. F. and at a pressure from about 1,800 PSIG to about 6,000 PSIG.
The conversion of ammonium carbamate to urea in the urea synthesis reactor thus attained is generally in the range from about 50% to about 75%. At completion of reaction in the urea synthesis reactor, the reactor fluid is let down in pressure for the purpose of separating the aqueous urea product solution from the unconverted ammonium carbamate and from excess ammonia, both generally present in the reactor effluent. Due to the rapid adiabatic flashing of some NH.sub.3, CO.sub.2 and H.sub.2 O from the solution after pressure reduction, the reactor effluent is cooled to about 100 to 150 degrees F below the reactor temperature. The separation of the urea product is further attained by heating the reactor effluent after adiabatic flashing at reduced pressure in a heat exchanger, generally known as decomposer. As a consequence of the heating, excess ammonia with some water vapor is driven off from the aqueous urea product solution, and the unconverted ammonium carbamate is decomposed back to ammonia and carbon dioxide gas, and the gases are expelled from the aqueous urea solution with some water vapor.
Generally the decomposer off gas containing ammonia, carbon dioxide and water vapor is condensed in a water cooled heat exchanger, and the resulting aqueous ammoniacal solution of ammonium carbamate thus formed by condensation is recycled back to the urea synthesis reactor for recovery of ammonia and carbon dioxide. In such a recirculation process, unless excess ammonia is separated before carbamate decomposition in said decomposer, an excessive amount of water vapor will result in the decomposer off gas. Consequently, an insufficient amount of water will remain available for evaporation, within the limits allowed by the internal water balance of the whole synthesis-decomposition-absorption-recirculation system, in the subsequent second stage decomposition and absorption section that usually follows the first decomposition and absorption stage for more complete recovery of unconverted reactants. Ultimately, it will not be possible to condense all the second stage decomposer off gas due to the above-described reduction in the amount of water available for condensation in the second stage condenser. If water from an external source is added to the second stage condenser for the purpose of reducing the loss of unabsorbed off gas, this amount of excess water, eventually recycled to the reactor, hinders the conversion of carbamate to urea.
In the process described in U.S. Pat. No. 3,886,210 the decomposer off gas containing NH.sub.3, Carbon Dioxide and Water (stream 11 of FIG. 1.B of U.S. Pat. No. 3,886,210) is condensed in indirect heat exchange with the urea product solution being heated for the purpose of decomposing carbamate in heat exchanger 7 of said FIG. 1.B. In such a process, unless excess ammonia is separated from the reactor effluent solution at a temperature that is lower than the adiabatic flash temperature of the reactor effluent solution after let down in pressure, an excessive amount of carbon dioxide will be present in said stream of excess ammonia separated from the reactor effluent solution after let down in pressure from the reactor. As a consequence, in the subsequent step the decomposer off gas will be depleted of valuable carbon dioxide, which is the main source of heat required for exchange with the urea product solution.
In the process described in U.S. Pat. No. 3,527,799 excess ammonia is adiabatically flashed off (stream 11 of Fig. in U.S. Pat. No. 3,527,799) in Separator 10 from the residual urea effluent stream 12 before decomposition of carbamate in decomposer 13 of said figure. In said process the decomposer off gas stream 19 is condensed in heat exchanger 26 in indirect heat exchange and heat recovery with stream 25 from which carbamate is decomposed. In this process there is the drawback that too much water vapor and carbon dioxide are present in stream 11, thus depleting stream 19 of valuable water and carbon dioxide and reducing the efficiency of the heat recovery in heat exchanger 26. Moreover, the second stage off gas stream 35 will be depleted of the equivalent amount of excess water evaporated in gaseous stream 11, thus preventing total condensation and recovery of second stage decomposer off gas in condenser 36. If additional water from an external source is added to condenser 36 via stream 44, such excess water shall be recycled to the reactor 4 via streams 27, 30, 45, 52 and 6, and shall cause a reduction in conversion of carbamate to urea in said rector 4.
The process of the present invention provides a means to overcome both of said problems, namely, an excessive content of water vapor and an excessive content of carbon dioxide in the stream of excess ammonia flashed off from the reactor effluent after reduction in pressure.