The Study On The Mixed Refrigerant Process For Ethylene Cryogenic Separation

Ethylene is the basic chemical raw material in the petrochemical engineering, makinggreatly effect on the economic development. Its production marks the level of thedevelopment of the petrochemical industry in a country. In recent years, the ethylene industryhas been developing rapidly in our country. The production has been increased obviously; andthe technology has greatly improved. However, the efficiency of the refrigeration systems isstill some low. Currently, the cascade refrigeration and the mixture refrigeration are mainforms for the cryogenic systems. Binary and ternary refrigerants mixtures are mainly used inthe mixture refrigeration system. This paper focuses on analyzing and optimizing thecryogenic systems.According to the feature of the refrigeration demand of ethylene plant, the cracking gascooling-separation process was divided into variable-temperature cooling processes and theconstant-temperature cooling processes，corresponding to the precooling process of thecracking gas and condensing processes of a overhead condenser of towers, respectively. Then,the heat exchange composite curve of the original refrigeration system(ethylene-propylenecascade refrigeration systems) was obtained, which indicated that the heat exchangetemperature difference between the cold and hot streams was large in the precooling processof the cracking gas, because of the temperature glide of the cooling process of the crackinggas. Thus the heat exchange losses can be reduced by a suitable mixed refrigerant (MR)system. As a result, the mixture refrigeration and cascade refrigeration combining system wasproposed. In order to analyze the performance of the combinatorial refrigeration system, itwas modeled by Aspen Plus, a commercial software in chemical engineering. Moreover, toget the optimal solution, the genetic algorithm was used. System variables include the mixedrefrigerant composition, the outlet pressure of compressor, the pressure at the outlet of thethrottle, etc. Eventually, the optimal solution was obtained, of which the compression powerwas21.9%lower than that of the pure refrigerant system.Compared with pure refrigerant systems, mixed refrigerant systems are better forvariable-temperature cooling processes. In which good heat transfer characteristic curvefitting can be gotten by adjusting the proportion of refrigerants mixture under some pressure.mixed refrigerant systems reduce the heat transfer loss and the irreversible exergy loss, andreduce costs. In order to simulate the detailed heat transfer processes. This paper calculatesthe boiling heat transfer coefficient of MR, which combines and solves two correlations, Chenand Shah’s, which are widely used. By calculating the heat transfer process, we can obtain that the boiling heat transfer coefficient of the ternary refrigerants mixture increases with thedryness.After that, this paper analyzed the heat transfer process of the overhead condenser for theethylene distillation in a actual plant, employing ternary refrigerants mixture system.Several operation problems were pointed out, including the excessive refrigerant liquidquantity carried out from heat exchanger and the excessively high pressures of ethylenedistillation column. In order to ensure the operation efficiency, stability and safety, theethylene distillation tower should increase the heat transfer area of the overhead condenser.