Feeding sequence of the urea synthesis tower
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What are the differences in the sequence for initial startup and short-term shutdown feeding of the urea synthesis tower (aqueous solution process)? (Please also analyze the reasons)
Initial operation: At this stage, ammonia has already been introduced into the tower; the tower is filled with gaseous ammonia at 8.0 MPa, with no water present. Since the conversion of methylammonium to urea takes place in the liquid phase, a small amount of water must be added first in order to lower the melting point of methylammonium. After that, CO2 is introduced, which allows liquid methylammonium to be formed immediately within the tower. Starting up after a short shutdown: ammonia and CO2 are used first, with methylamine being added last, as water has already been introduced during the flushing of the three materials in the urine tower after the short shutdown. The water-to-carbon ratio in the urine tower is already very high; delaying the addition of liquid A can prevent the CO2 conversion rate from dropping too much.