Carbamide Synthesis Column - Drawings DBE

1. Manufacturing Technology

The synthesis of carbamide from ammonia and carbon dioxide proceeds according to the total reaction

2NH3 + CO2 → CO (NH2) 2 + H2O

and consists of the step of forming ammonium carbamate:

2NH3 + CO2 → NH4-COO-NH2

and ammonium carbamate dehydration:

NH4–COO–NH2 → CO(NH2)2 + Н2O

Based on the goals and tasks set, we choose the Montecatini process plan. The scheme (Figure 1) is based on the process of complete liquid recycling of NH3 and C02 in the form of aqueous solutions of ammonium salts, developed by the Italian company Montecatini.

Liquid ammonia and gaseous carbon dioxide containing not less than 97% C02 and not more than 1 part per million sulphur impurities are used as initial components for the urea synthesis process. Before entering the synthesis column, liquid ammonia is passed through a steam preheater. Compression of carbon dioxide up to the pressure at which synthesis is carried out is carried out in the five-stage compressor.

The molar ratio NH3: C02 when fed to the column is 3:1. The pressure in the column is maintained at 260 at, and the temperature is 190 ° C. The process temperature limits are due to the need to reduce corrosion of stainless steel, which is used for lining the inner surfaces of the column, as well as for the manufacture of fittings, etc.

The melt exiting the column is throttled into a first stage evaporator operating at a pressure of about 20 at., in which ammonium carbamate partially decomposes into NH3 and C02 when heated. The vapor-liquid mixture then enters the first stage separator, where the gases are separated from the liquid and absorbed by the aqueous ammonium salt solution supplied by the pump from the second stage separator.

From the evaporator of the first stage, the urea solution containing residual ammonium carbamate and uncoated ammonia enters the evaporator and separator of the second stage, operating at a pressure close to atmospheric. In these devices, the final decomposition of carbamate and the distillation of NH3 and CO2 are carried out. The necessary amount of water is supplied to the separator to absorb the gases. The resulting aqueous ammonium salt solution is directed, as already mentioned, for uptake of NH3 and C02 into the first stage separator. A highly concentrated aqueous solution of ammonium salts from the first stage separator is pumped into the synthesis column. Thus, ammonia and carbon dioxide not converted to urea are recycled.

An aqueous urea solution substantially free of unreacted products flows from the first stage evaporator to a receptacle in which a low level of the solution is specially maintained, which provides a minimum residence time for the urea hot solution (to avoid the formation of a biuret). From this buffer tank, the solution is sent to a vacuum evaporator of a special design, from where a highly concentrated float enters the granulation tower. The finished granular product (pellet size mainly 1-2 mm) is cooled after leaving the tower and sent to packing or storage. The commercial product contains 46% nitrogen, 0.6% moisture and up to 1% biuret. The Montecatini method is characterized by very low operating costs.