Retrofit And Optimization Of Propane Dehydrogenation Cryogenic Liquefaction Process With Hydrogen Membrane Separation

Propane dehydrogenation Process is an important new source of propylene.The reaction effluents contain considerable permanent gases(mainly hydrogen),resulting in huge energy consumption of the traditional cryogenic liquefaction technology.Specifically,in the cryogenic liquefaction process in high pressure propane dehydrogenation process,a large amount of hydrogen is compressed with the reaction effluent,resulting in huge compression energy consumption.Hydrogen accumulation leads the cryogenic liquefaction process in low pressure propane dehydrogenation technology to be carried out at about-95°C,which needs large cryogenic load.In this study,hydrogen membrane separation is proposed to transform the cryogenic liquefaction process in propane dehydrogenation technology,membrane separation is used to removal most hydrogen in the reaction products.It can improve the C3partial pressure and reduce the amount of subsequent liquefaction treatment to ameliorate the cryogenic liquefaction process,in order to improve the economic benefits of the liquefaction process.Taking propane dehydrogenation device with an annual output of 350 kt/a propylene as an example,HYSYS software is used to simulate and optimize the membrane separation transformation process of the cryogenic liquefaction process in high and low pressure propane dehydrogenation technology to determine the operating parameters with the greatest economic benefits.For the cryogenic liquefaction process of high pressure STAR technology,cascade compression can be achieved by embedding membrane units in the cooling process.A hydrogen membrane separation unit is embedded after the shallow cooling unit to remove a large amount of hydrogen and achieve cascade compression.The optimum shallow cooling pressure,temperature and membrane area are 2.40 MPa,-24°C and 2680 m~2,respectively.Total compression power consumption is reduced from 7040 kW to 5830 kW,saving 6.0×10~6 CNY/a utility cost;hydrogen concentration is increased from 82.8 mol%to 99.0 mol%,with the recovery rate over 85%.Considering energy saving and hydrogen output,the annual gross profit can be increased by 8.70×10~7 CNY.Based on the high water flux of hydrogen separation membrane,the membrane separation unit is embedded after the precooling unit to realize deep dehydration while reducing energy consumption by step compression and increasing byproduct hydrogen.The optimum precooling pressure,temperature and membrane area are 3.00 MPa,5°C and 3000 m~2,respectively.The water content in the residual gas is 0.9 ppmw,omitting drying tower and 190kW compression energy consumption of desorption gas;the compression energy consumption is reduced by 310 kW by cascade compression,saving 2.8×10~6 CNY/a utility cost.The hydrogen concentration is increased to95.9 mol%,with the recovery rate of 83.9%.The annual gross profit can be increased by by9.04×10~7 million CNY.For the cryogenic liquefaction process of low pressure propane dehydrogenation technology,the Oleflex technology was taken as an example to be revamped by hydrogen membrane separation.The cooling capacity of propane vaporization in the original liquefaction process can be used to liquefy light hydrocarbons in shallow cooling,cooperating with hydrogen removal by membrane separation to reduce cryogenic capacity.For single membrane separation/shallow cooling revamped process,the optimum pre-cooling pressure,temperature and membrane area of single-stage membrane separation/shallow cooling revamped process are 2.70 MPa,5°C and 15 000 m~2.The cryogenic load is 90 kW,which is96.4%lower than before,and the concentration of hydrogen is 97.8 mol,which is 5.1 mol%higher than before.However,there are some problems,such as low production of high concentration hydrogen and large loss of carbon three.On this basis,two-stage membrane separation/shallow cooling revamped process was designed.The optimal precooling pressure,first and second-stage membrane area are 2.10 MPa,7500 m~2 and 15 000 m~2,respectively.The cryogenic load is 1280 kW,48.3%lower than that of the original process.The product and circulating hydrogen concentration are 99.0 mol%,96.5 mol,which increased by 7.3mol%,3.8 mol%,respectively.Total hydrogen recovery rate reached 98.6%,and the gross profit increased by 1.47×10~7 million CNY.The above modification results show that the cryogenic liquefaction process in propane dehydrogenation process can be effectively improved by hydrogen membrane separation modification,and the by-product hydrogen concentration increases at the same time,thereby improving economic benefits.