Studying Of Revamping Schemes For Capacity Expansion Evaporation System Debottleneke Of Lutianhua Urea Plant With The Capacity Of 1620t/d

There are several hundreds of urea plant with all scales in China, including about 30 plants with the capacity of 500kt/a or above. Output of urea has reached 60% of the total output of nitrogen fertilizers. As China joining into WTO and the continue increasing of price of civil raw materials such as natural gas, the competition became more and more violent among urea manufacturers. Facing serious situations, LTH Co Ltd. decides to expand capacity of CO2 stripping urea plant from 1620t/d to 2000t/d to reduce per cost. This paper debottlenecked the evaporation system and promised the whole schemes succeeded.By being detail analyzed the thermo properties of the aqueous urea solution in evaporator, mechanism, react rate and react conditions about the appearance of buret, the hydrolysis of urea and the decomposition of urea were presented clearly. The result shows the amounts of byproduct appear is positive to the temperature of evaporator and the hold on time of urea solution. A new film-rising evaporator with high coefficient and short hold on time was selected based on the properties of urea solution and the evaporator available.Phase diagram of equilibrium of Ur-H2O system was analyzed. The result shows because there is a crystal zone of saturated urea solution, one-step evaporation is impossible for low concentration urea solution concentrating directly to melt urea. So, two-step operation is needed. Based on these studies, the suitable operation conditions were determined. That is : temperature of 130 , pressure of32kPa(abs)(240mmHg), urea concentration of 95%(wt) in first step of evaporation; temperature of 140, pressure of 3.4kPa(abs)(25.5mmHg), urea concentration of 99.7%(wt) in second step of evaporation.By being analyzed the heat transfer of the film rising heater, math models were determined, which including enthalpy math model of gas-liquid phase for the heat transfer calculation of the heater, coefficient math model for condensate film heat transfer, coefficient math model for urea solution film heat transfer and so on.The calculation value and the actual value were compared of old facility by using these determined math models. The result showed the math models were reliable. Then, these math models were used in the design of new evaporator, and the revamp scheme is created.