Urea Manufacturing Method And Urea Manufacturing Apparatus

Abstract

Provided are urea manufacturing method and apparatus, which can increase the conversion ratio into urea and to reduce the consumption of steam. The temperature of the reactor is increased by introducing the entire amount of raw material ammonia and introducing a portion of the decomposed gas from the stripper into the reactor. The raw material ammonia is preferably heated using the steam condensate generated in the purification step, and/or the steam generated by the heat of condensation of the decomposed gas and the unreacted substances in the condensation step. The heating temperature is preferably between 70 and 140° C.

What is claimed is: 
     
         1 . A urea manufacturing method comprising:
 an ammonia introduction step of introducing a substantially entire amount of raw material ammonia into a reactor;   a synthesis step of reacting carbon dioxide and ammonia under a condition of excessive ammonia in the reactor, thereby providing a synthesis mixture containing urea, ammonium carbamate, water, unreacted ammonia, and unreacted carbon dioxide;   a decomposition step of decomposing the ammonium carbamate by heating the synthesis mixture and stripping using at least a portion of raw material carbon dioxide as an auxiliary agent, thereby providing a decomposed gas containing ammonia and carbon dioxide, and a urea synthesis solution containing ammonia, carbon dioxide, water, and urea;   a purification step of separating water and an unreacted substances including ammonia and carbon dioxide from the urea synthesis solution, thereby providing a purified urea solution and recovering the separated water and unreacted substances;   a decomposed gas introduction step of introducing between 5 and 20 wt % a portion of the decomposed gas into the reactor, using an ejector which uses at least a portion of the raw material ammonia as a driving fluid;   a condensation step of condensing the rest of the decomposed gas and at least a portion of the water and unreacted substances recovered in the purification step in a condenser, thereby providing a condensate and uncondensed gas, separately;   an off gas returning step for returning the uncondensed gas obtained in the condenser to the purification system; and   a condensate introduction step of introducing the condensate to the reactor using another ejector for introducing the condensate to the reactor which uses the rest of the raw material ammonia as a driving fluid;   wherein the decomposed gas introduced into the reactor in the decomposed gas introduction step is condensed to increase the temperature of the reactor.   
     
     
         2 . The method according to  claim 1 , wherein the raw material ammonia is heated in the ammonia introduction step using a steam condensate generated in the purification step and/or a steam generated by heat of condensation in the condensation step. 
     
     
         3 . The method according to  claim 1 , wherein the raw material ammonia is heated up to from 70 to 140° C. in the ammonia introduction step. 
     
     
         4 . The method according to  claim 1 , wherein the condenser is a shell and tube condenser, and wherein the decomposed gas from the stripper and the unreacted substances from the purification system are introduced to the shell side of the condenser. 
     
     
         5 . The method according to  claim 1 , wherein the condenser is a bubble column vertical condensation reactor. 
     
     
         6 . The method according to  claim 1 , further comprising:
 a carbon dioxide introduction step of introducing a part of raw material carbon dioxide directly into the reactor and the rest of the raw material carbon dioxide into the stripper, using a carbon dioxide introduction line that is directly connected to the reactor.   
     
     
         7 . The method according to  claim 1 , wherein the pressure of the urea synthesis solution obtained in the decomposition step is reduced using a control valve to between 15 and 20 bar to obtain a gas-liquid mixture, and the obtained gas-liquid mixture is introduced into the purification step. 
     
     
         8 . The method according to  claim 1 , wherein the molar ratio of the NH 3  component to the CO 2  component (N/C) in the synthesis step is between 3.0 and 4.0. 
     
     
         9 . The method according to  claim 1 , wherein the molar ratio of the NH 3  component to the CO 2  component (N/C) of the condensate obtained in the condensation step is between 2.5 and 3.5.