【作者】 李勇；

【导师】 党洁修； 茅启昌；

【作者基本信息】 四川大学 ， 化学工程， 2003， 硕士

【摘要】 我国大大小小的尿素生产装置已达几百套，其中生产能力在500kt／a及以上的大型装置有30套左右，尿素总产量已占氮肥总量的60％。随着我国加入世贸组织和国内天然气等原材料价格的持续上涨，尿素行业的竞争将日趋激烈。面对市场的巨大挑战和压力，改造尿素装置瓶颈、扩大生产能力，从而降低产品单耗成了我们急需解决的问题。 本文结合500kt／a尿素装置扩能改造任务，重点对其瓶颈工序蒸发系统进行分析研究和设计。 通过对尿素溶液在蒸发过程中热敏性质的详细分析，阐明了缩二脲生成、尿素水解反应和尿素分解反应等副反应的机理、反应速度和反应条件，发现副反应产物的生成量与蒸发温度和尿液的停留时间成递增关系。针对此热敏性质，并结合各种蒸发加热器型式的特性，选择了传热系数大、停留时间短的升膜式蒸发器作为蒸发尿液的加热器。 通过对Ur-H₂O体系平衡相图的分析发现，由于要穿越饱和溶液的结晶区，尿液从低浓度进料一次浓缩到熔融尿素是不可能的。且也不符合为减少副反应的影响对操作温度条件的要求。因此，确定尿液的蒸发，采用两段真空蒸发流程。进而由Ur-H₂O体系的平衡相图选择了适合尿液蒸发的两段真空蒸发工艺参数：一段蒸发工艺参数，温度130℃、压力32kPa（绝）（240mmHg），尿素浓度95％（质量％）；二段蒸发工艺参数，温度140℃、压力3.4kPa（绝）（25.5mmHg），尿素浓度为99.7%（质量%）。 通过对升膜式加热器传热过程的细致分析，从而确定了计算加热器传热过程的汽一液相热负荷计算模型、蒸汽冷凝液膜给热系数模型、尿液液膜给热系数模型和蒸发加热器计算方法。 在对比原设计负荷（1620此）的计算值和装置实际能力的基础上，检验了数学模型的可靠性，从而将选择的数学模型用于2000肉负荷的改造计算中，确定了蒸发加热器的改造方案。更多还原

【Abstract】 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.更多还原