The Urea plant is a single stream plant and utilises a Mitsui Toatsu total recycle- C process. The plant has a capacity of 1,140 metric tonnes per day. The urea plant boasts of an upgraded water recovery system. Barometric condenser water is recycled in a close-loop cooling tower. Recycled water is used for sealing gland packing of various pumps. Water that accumulates due to the blow down from the tower is reused as process water in the manufacturing process.
Urea Plant is designed by Toyo Engineering Corporation, Japan using Mitsui Toatsu Total Recycle – C Process having liquid ammonia and carbon dioxide as feedstock. It has the following sections:
4. Drying and prilling
The Urea synthesis reaction occurs in two stages. In the first reaction ammonia reacts with CO2 to form ammonium carbamate. In the second reaction carbamate dehydrolysis to give urea and water. The first reaction goes to completion and is exothermic whereas the second reaction is endothermic and governed by equilibrium conditions. The urea converter bottom temperature is 182 deg.C and the top temperature is 198 deg.C. Typical CO2 conversion achieved in the reactor is 68%. Process fluid going out of reactor has Urea, excess ammonia, unconverted carbamate and water.
The reactor effluent mixture is progressively decomposed in three stages. The first stage (High Pressure Decomposer) operates at 17.5 kg/cm2(A). The second stage (Low Pressure Decomposer) operates at 3.5 kg/cm2(A) and the last stage (Gas Separator) operates at 1.3kg/cm2(A)/atmospheric. As the process stream flows through these decomposing stages excess ammonia is stripped off and unconverted carbamate is decomposed to yield ammonia and carbon dioxide which are again stripped off. Thus the process stream gets increasingly concentrated with Urea. Steam is used to supply the energy requirement in the decomposers. With the exit of the gas separatorthe Urea solution is concentrated to 72-75%.
Urea solution coming from the Gas separator is fed to the crystallizer. In the crystallizer the solution is further concentrated by evaporating water under vacuum. Urea crystals formed are separated by centrifuging and the mother liquor is recycled back to the crystallizer. Water vapour emerging out of crystallizer is condensed into barometric condenser and circulating water is cooled in a dedicated cooling tower.
Drying and Prilling
The urea crystals are dried in a fluidizing dryer using hot air and pneumatically conveyed to the top of the prilling tower. The crystals are separated from the air at the top of prilling tower by cyclones, re-melted by using steam and fed to the vibro priller basket. The urea prills are formed while descending down the prilling tower and these are cooled by the rising air stream supplied by the blowers installed at the bottom of the CFD belt. On the CFD belt prills are cooled to temperature of 40 deg.C. and sent to Bagging.
In this section the excess ammonia present in the reactor and unconverted ammonium carbamate solution which is decomposed in the decomposers is recovered in respective condensers of decomposers and recycled back to the reactor. The excess ammonia from the reactor is again condensed in ammonia condensers and recycled back to the reactor.