Optimization Of Demethanation System And Cold Box System In Ethylene Plant

As the basic raw material for the petrochemical industry, the level of ethyleneproduction determines the level of development of a country's petrochemical industry.The ethylene industry started early in China and developed rapidly, the current annualproduction of ethylene in China has ranked second in the world. However, with thelarge-scale tendency of domestic ethylene plants, energy loss problem becameincreasingly prominent, Chinese ethylene energy consumption is much higher thanthe average in the world. Therefore, it is a trend to improve the energy efficiency ofethylene plants, it has a very important significance.In this paper, an ethylene plant of front-end depropanization and front-endhydrogenation is researched, including the pre-demethanizer, demethanizer,deethanizer and cold box systems. The distillation systemic integration and theoptimization of cold box systems will be conducted.In the ethylene plant, potential for heat coupling exists between thepre-demethanizer, demethanizer and deethanizer, in order to improve the energyefficiency and reduce equipment cost, the pre-demethanizer, demethanizer anddeethanizer will be optimized in the way of energy integration and energy coupling.MultiFrac model in the aspen plus software is used to simulate the process, and theresult is that: the number of the heat exchangers changes from the original eight totwo; the number of the distillation columns changes from the original three to one, sothat the investment cost of the system is greatly reduced; the number of the originalprocess streams drops from twelve to eight, so the design of heat exchanger networksbecomes simple; the most important thing is that the power of the dividing wallcolumn is3.681MW less than the original columns, and it has very large economicbenefits.The paper uses pinch design method and exergy analysis method to optimize theheat exchanger network. Pinch design method and exergy analysis method can alwayschange the temperature level of the hot and cold utilities in the exergy grandcomposite curve to reduce the shaft work in the whole system by avoiding the designof heat exchanger network. This paper introduces the "two-step" method to optimizethe heat exchanger network and cold box systems in the ethylene plant. First, thepockets in the exergy grand composite curve are optimized, a lower quality cold utility is used to exchange heat with the hot process streams, the cold process streamsare used to generate a higher quality cold utility, and it is determined by the minimumtemperature difference; then the exergy grand composite curve without pockets areoptimized. By using the mathematical programming of PSO and Matlab software,different levels of cold utilities are calculated with the target of minimum shaft work.With this "two-step" method, heat exchanger network can be optimized with theoptimal levels of cold utilities. The result is that: when the level of the cold utilities is6, the reduction of the shaft work is1.4828MW, when the level of the cold utilities is7, the reduction of the shaft work is2.3248MW, when the level of the cold utilities is8, the reduction of the shaft work is2.7274MW.