Simulation And Transformation Of Difluoromethane Distillation Process And Study On Performance Of New Fixed Valve Tray

HFCs is a general term for hydrofluorocarbons,and most HFCs is colorless liquid or gas.Its boiling point and surface tension decrease with the increasing of fluorine's quantity.HFCs is easily hydrolyzed when exposed to water,acids,bases or hot condition,leading its chemical stability lower than that of CFCs.As a new environmentally friendly refrigerant,HFCs is an ideal substitute for Freon.The purpose of this study is to simulate the separation process of difluoromethane in a domestic chemical industry with an annual output of 100,000 tons.The factory uses three columns,including an absorption column,a degassing column,a distillation column,to separate difluoromethane.The target product difluoromethane is obtained from the top of the distillation column.In order to increase recovery rate of difluoromethane and reduce energy consumption in seperation process,this simulation modifies the factary's process into a four-colum separation process and optimizates the operating condition of distillation column,such as feeding position,operating pressure and reflux ratio.The optimal operating conditions can obtain by studing the influence of those matters on separation effect.The results of Aspen Plus simulation show the steam consumption in the distillation column is reduced by 28%;the condensed water consumption is reduced by 25%;the difluoromethane yield of the distillation column is increased by 16% and its mass content reaches to 99.995%.The internal parts of fluorine distillation column generally use random packing or structured packing,but its mass transfer performance is inefficient and the operation stability is inflexible.Based on above reasons,this paper applies the new fixed valve tray to difluoromethane distillation column.It has the advantages of high mass transfer efficiency and flexibility,which can,meanwhile,reduce the internals cost.The experiment was performed in a plexiglass column with 600-mm diameter.The air-water system was used to test the hydrodynamic performance of the new fixed valve tray,like dry-plate pressure drop,wet-plate pressure drop,leakage rate,entrainment rate and clear liquid height.Four sets of parallel experiments were completed in order to study the variation law with the ratio of new fixed valve on plate.The experimental results show that new fixed valve tray can effectively reduce entrainment rate and leakage rate.Its dry/wet-plate pressure drop are almost the same as that of round fixed valve tray.Its higher clear liquid height is beneficial for gas-liquid transfer.By comparing the hydrodynamic data of four sets of trays,we can obtain that when the ratio of new fixed valve on plate is around 30~40%,the entrainment and leakage rate keep in low level and clear liquid height is moderate.Considering both economic cost and separation effect,the optimal ratio of new fixed valve is in the range of 30%~40%.The hydrodynamic performance of the new fixed valve tray,such as entrainment rate,leakage rate and clear liquid height,is superior to that of the round fixed valve tray,indicating it is an efficient tray with comprehensive performance.