乙烯深冷分离中混合工质制冷系统研究

发布时间：2018-09-05 19:17

【摘要】：乙烯是石油化工生产有机原料的基础，乙烯产量是衡量一个国家工业发展水平的重要标志。近年来我国乙烯工业得到飞速的发展和进步，但目前仍然存在效率较低、能耗过大等问题，我国的乙烯工业仍有较大的发展空间。目前在乙烯装置常使用的深冷系统有复叠制冷系统和混合冷剂制冷系统，其中混合冷剂制冷系统主要有二元制冷和三元制冷，本文将重点对乙烯装置中的深冷系统进行分析和优化。本文首先分析某200kt/a乙烯装置中的用冷需求，将乙烯装置的深冷分离过程分为变温冷却过程和定温冷凝过程，变温冷却过程指的是裂解气的预冷过程，定温冷凝过程指的是乙烯装置中各塔塔顶冷凝器中的换热；再分析乙烯-丙烯复叠制冷系统的换热集成曲线，可知在裂解气的预冷阶段，冷热物流换热温差较大。故提出一种组合制冷系统，它集成了纯工质复叠制冷和混合冷剂制冷，在各塔塔顶冷凝器中的换热采用纯工质复叠制冷系统，多元混合冷剂制冷系统为乙烯深冷分离的变温换热过程提供冷量。并用AspenPlus软件对混合冷剂系统进行建模，采用遗传算法优化，得到了节能21.9%的优化结果。混合冷剂制冷系统相比于纯工质制冷系统更适用于大温程变温降温过程，在一定的蒸发压力下，通过调整混合冷剂的配比，可以使冷热物流曲线匹配得更好，，尽可能减小换热温差，减小换热过程的不可逆损失；还可以简化设备，节省投资。针对混合制冷剂换热过程复杂的特点，采用广泛应用的Chen公式和Shah公式计算混合冷剂的沸腾换热系数和冷凝换热系数，并将两组公式联立迭代求解计算乙烯塔顶冷凝器中乙烯蒸气与三元冷剂的换热过程。分析结果可知，混合制冷剂沸腾换热在干度较小时换热系数小，在干度较大时换热系数大。混合制冷剂的强制对流换热系数在流动沸腾换热系数占主导作用。某乙烯装置的深冷系统采用了三元冷剂制冷系统。本文针对该装置目前运行存在的乙烯塔压过高等问题进行分析，并提出串并联方式增设乙烯塔顶板翅式冷凝器的改进方案，较好的解决了现有的问题，能使乙烯装置高效稳定安全的运行。
[Abstract]:Ethylene is the basis of petrochemical production of organic raw materials, ethylene production is an important indicator of a country's industrial development level. In recent years, China's ethylene industry has made rapid development and progress, but there are still some problems such as low efficiency and excessive energy consumption. At present, the cryogenic systems commonly used in ethylene plants include cascade refrigeration systems and mixed refrigerant refrigeration systems, among which the mixed refrigerant refrigeration systems mainly consist of binary refrigeration and ternary refrigeration. This paper will focus on the analysis and optimization of cryogenic system in ethylene plant. In this paper, the cold requirement of a 200kt/a ethylene plant is analyzed firstly. The cryogenic separation process of the ethylene plant is divided into variable temperature cooling process and constant temperature condensation process. The variable temperature cooling process refers to the precooling process of cracking gas. The process of constant temperature condensation refers to the heat transfer in the top condenser of each tower in ethylene plant, and the analysis of the heat transfer integration curve of ethylene propylene cascade refrigeration system shows that in the precooling stage of cracking gas, the difference of heat exchange temperature between cold and hot material flow is large. Therefore, a combined refrigeration system is proposed, which integrates pure refrigerant cascade refrigeration and mixed refrigerant refrigeration, and uses pure working medium cascade refrigeration system for heat transfer in the top condenser of each tower. The multi-component refrigerant system provides cooling capacity for the variable temperature heat transfer process of ethylene cryogenic separation. The mixed coolant system was modeled by AspenPlus software and optimized by genetic algorithm, and the result of energy saving was 21.9%. Compared with the pure refrigerant refrigeration system, the mixed refrigerant refrigeration system is more suitable for the process of changing temperature and cooling temperature at a large temperature range. Under a certain evaporation pressure, by adjusting the ratio of the mixed refrigerant, the cold and heat flow curves can be matched better. It can reduce the difference of heat transfer temperature, reduce the irreversible loss of heat transfer process, simplify the equipment and save the investment. In view of the complex heat transfer process of mixed refrigerant, the boiling heat transfer coefficient and condensation heat transfer coefficient of mixed refrigerant are calculated by widely used Chen formula and Shah formula. The heat transfer process of ethylene vapor and ternary refrigerant in the top condenser of ethylene tower is calculated by simultaneous iteration of two sets of formulas. The results show that the boiling heat transfer coefficient of mixed refrigerant is smaller when the dryness is small and the heat transfer coefficient is larger when the dryness is larger. The forced convection heat transfer coefficient of mixed refrigerant plays a dominant role in the flow boiling heat transfer coefficient. A ternary refrigerant refrigeration system is used in the cryogenic system of a ethylene plant. This paper analyzes the problems existing in the operation of the plant, such as the high pressure of the ethylene tower, and puts forward an improved scheme of adding the plate-fin condenser on the top of the ethylene tower in series-parallel mode, which solves the existing problems well. Can make ethylene plant efficient, stable and safe operation.
【学位授予单位】：华南理工大学
【学位级别】：硕士
【学位授予年份】：2014
【分类号】：TB657

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