Study On Membrane-based Polypropylene Tail Gas Recovery Process For Improving Propylene Recovery Efficiency

In the production of polypropylene,the exhaust gas from the polymerization reactor and degas bin contain a large amount of propylene.Currently,the coupled compression condensation/membrane separation process is mainly used for recovery,but the recovery rate is not high,only 70～80%.The reasons for this are,first,the low loading density of commercial rolled membrane modules and their excessive footprint make it difficult to achieve high propylene recovery rates;second,the presence of large amounts of propane in the recovered propylene leads to the accumulation of propane in the reactor at concentrations exceeding the15.00 mol%limit,which significantly reduces both the activity of the polymerization catalyst and the volumetric efficiency of the reactor.To further improve the propylene recovery,a modification of the coupled compression condensation/membrane separation process was required.To improve the loading density of membrane modules,hollow fiber membranes with higher loading density need to be developed,but the double skin structure of hollow fiber membranes leads to low gas permeability.To improve the gas permeability of hollow fiber composite membranes,it is necessary to break the check and balance between coating ultra-thinness and pore permeability effect.In this work,the PDMS/PEI hollow fiber composite membranes with high gas permeability were prepared by optimizing the PDMS coating process through a synergistic strategy of"size matching"and"media plugging",and suppressing the pore permeation effect while ultra-thinning the selected layer.The PDMS/PEI hollow fiber composite membranes with high gas permeability were produced,and square meter gas separation membrane modules were fabricated.The PDMS/PEI hollow fiber composite membrane achieves a selectivity of 12 and a permeation rate of more than 936 GPU,which is the same as that of commercial membrane modules,and the filling density of hollow fiber membrane modules reaches 785 m~2/m~3,which is 2.5 times higher than that of commercial rolled membrane modules,and the space occupation ratio is reduced by 70%,which effectively improves the space utilization.Based on the performance of the prepared hollow fiber composite membranes,the propylene recovery process was designed for the tail gas of a 300,000 ton per year polypropylene plant.The hollow fiber membrane module replaces the rolled membrane module,and the propylene recovery rate is increased from 80.0%to 92.2%in the same footprint.With economic efficiency as the optimization target,the optimal membrane area is 220 m~2,the membrane module footprint is reduced by 45%,the propylene recovery rate reaches 83.0%,and the net profit is 35.01 million CNY/year.To further improve the propylene recovery rate,a compression condensation/secondary PDMS membrane recovery process was proposed.The optimal membrane area ratio(2nd/1st)is 0.5,and the first and second PM membrane areas are250 m~2 and 125 m~2,respectively,at which time the propylene recovery rate reaches 88.0%and the net profit is 45.43 million CNY/year.To address the problem of propylene recovery leading to propane accumulation,which makes the polypropylene reactor operation unstable,a compression condensation/PDMS membrane/ZIF-8 membrane(CCPM/ZM)coupling process was proposed,and a ZIF-8membrane was introduced to reduce the propane concentration in the circulating flow unit.The ZIF-8 membrane was preferentially selected by data with a C3H6/C3H8 selectivity of 32 and a propylene permeation rate of 80 GPU.three recovery flows were designed according to the different positions of the embedded ZIF-8 membrane,CCPM/ZM-1(after PM-1),CCPM/ZM-2(before PM-1),and CCPM/ZM-3(liquid phase outlet of split tank V-1),all of which can reduce the propane concentration in the reactor is reduced to below 3.00 mol%,effectively reducing the accumulation of propane and achieving stable production.With the economic return as the optimization target,the best process is CCPM/ZM-2,the optimal ZM membrane area and PM membrane area are 400 m~2 and 400 m~2 respectively,the propane concentration in the reactor is only 1.04 mol%,the propylene recovery rate reaches 91.5%,and the net profit is81.59 million CNY/year.