Urea production with bi-pressurized synthesis

A method of modifying a pre-existing urea recycle plant so as to provide a modified plant, said pre-existing plant comprising, placed in series, a HP reactor adapted to be operated at a high pressure of 16 to 40 MPa so as to obtain a urea synthesis solution, a MP recirculation section adapted to be operated at a medium pressure of 1 to 5 MPa, and an LP recirculation section adapted to be operated at a low pressure of 0.1 to 1 MPa, said recirculation sections adapted to decompose ammonium carbamate and to recirculate a liquid condensate comprising non-reacted carbon dioxide and ammonia, and liquid components comprising ammonium carbamate, back to the reactor, the method comprising:

• adding a HP stripper downstream of the reactor and upstream of the MP recirculation section, said stripper being adapted so as to direct liquid obtained therefrom to either or both of the MP and LP recirculation sections;
• adding a HP condenser, whereby the HP condenser and the HP stripper are mutually arranged to subject gas obtained from the HP stripper to condensation in said HP condenser so as to form a liquid condensate and remaining vapors;
• arranging said HP condenser in connection with a compression unit positioned downstream of the condenser and upstream of the reactor
• wherein said compression unit is configured so as to increase the pressure of liquid condensate, obtained from the condenser, to substantially the same pressure under which the reactor is operated, whereby said compression unit is configured to utilize mechanical energy recovered from decompression of liquid downstream of the stripper and upstream of the recovery section.

A process for the preparation of urea in a urea production plant, said plant comprising a synthesis section operating under a high pressure between 12 and 40 MPa and a recovery section operating under a pressure below 1 MPa, said synthesis section comprising, connected so as to be capable of forming a urea synthesis loop, a urea reactor, a stripper, a condenser and optionally a scrubber, the process comprising the steps of:

• a) reacting ammonia and CO2 under urea forming conditions in the urea reactor to obtain a urea synthesis solution;
• b) stripping the urea synthesis solution in the stripper to form a stripped urea solution and a stripper vapor comprising ammonia, CO2 and water;
• c) condensing the stripper vapor obtained in b) in the condenser to form an ammonium carbamate solution;
• d) recirculating said ammonium carbamate solution to the reactor;
• e) subjecting the stripped urea solution to recovery of ammonia, and optionally CO2, in the recovery section to obtain a urea product solution and a recovery vapor comprising the ammonia and the optional CO2;
• f) recycling the recovery vapor to the urea synthesis section;
• the process further comprising operating the urea reactor under a pressure at least 2 MPa higher pressure than the pressure of the stripper and the condenser, and increasing the pressure of the ammonium carbamate solution upon recirculation to the reactor, wherein energy utilized in increasing said pressure comprises mechanical energy recovered from a decompression unit positioned downstream of the stripper and upstream of the recovery section.