Granulated Urea Production from Natural Gas (Self-Stripping Process)

The process under analysis comprises four major sections: (1) Steam Reforming; (2) Ammonia Synthesis; (3) Urea Reaction; and (4) Urea Concentration.

Steam Reforming. The natural gas is first desulfurized then mixed with steam for primary steam reforming. In a tubular fired heater filled with a Ni-based catalyst, the methane reacts with steam, generating crude syngas, which is fed to the Autothermal Reformer (ATR), to convert remaining methane into CO and H2.

Ammonia Synthesis. The outlet from the ATR is fed to a two-stage set of high and low temperature shift reactors, where carbon monoxide is converted into CO2 and H2. The shifted gas is then cooled, generating steam and condensing present water, that is removed in a knockout drum. Next, CO2 is removed from the gas stream by a MDEA wash. The gas stream, free from CO2, is heated and fed to the Methanator, where residual carbon oxides are converted to methane in the presence of catalysts. After being cooled and dried in molecular sieves, impurities are removed from the syngas and used as fuel. The purified syngas is then fed to an intercooled horizontal ammonia converter. The cooled down reactor outlet is then condensed.

Urea Reaction. CO2 and a liquid mixture of ammonia and carbamate are fed to a reactor, yielding ammonia carbamate and urea. The reactor effluent, is fed to a stripper where excess NH3 strips out the carbamate. The urea solution is directed to decomposers for the removal of residual carbamate and carbon dioxide. After decomposition of carbamate and evaporation of ammonia, a purified urea solution is obtained.

Urea Concentration. The urea-water mixture is concentrated in an evaporator, forming a urea melt. The vapor obtained is condensed and treated. The urea melt is sprayed from in the granulator into a fluid bed, where solid granules are formed. The granules are cooled and screened. The onsize granules are cooled again and sent to the Packing area, where they are packed in bags before being stored. Offspec materials are recycled to the granulator.