Research On Ethylene Recovery Reformation Scheme In Ethylene Compression Cycle Refrigeration System

Ethylene is one of the most productive chemical products widely used in various fields in the world.In butyl rubber plant,ethylene provides isothermal cooling capacity of-76~-110?as a refrigerant.In order to achieve the refrigeration temperature required by butyl rubber unit,the refrigeration system of ethylene compressor needs to operate under negative pressure.Because the system can not be absolutely tight,trace air will enter the ethylene refrigeration system.Non-condensable gas in ethylene refrigeration system must be discharged regularly to ensure the safety of production.Large amounts of ethylene will enter the torch system together with the non-condensable gas,leading to a huge waste of resources.It is necessary to recover ethylene from the exhaust gas.At present,the cryogenic recovery effect is not good.The recovery rate of ethylene is 56.7%,and the molar fraction of ethylene in torch gas is still as high as 94.7%.In view of the above problems,the recovery process of membrane separation and distillation was designed and optimized in this paper,and proposes the distillation-membrane separation coupling method to recover ethylene,further improve the ethylene recovery rate and reduce the ethylene in the torch gas.In Aspen HYSYS simulation software,the recycling refrigeration and cryogenic separation process of ethylene compressor is established.The simulation results show that the error between simulation value and actual value of main process parameters is less than 2%,which prove the accuracy of PR state equation and operation unit model.The simulation processes of ethylene recovery by membrane separation,distillation and distillation-membrane separation coupling were established and optimized.The optimum operating conditions for ethylene recovery by membrane separation are as follows:intake pressure 1540 kPa,temperature 30?,membrane area 4.6 m~2,ethylene recovery 93.5%,ethylene molar fraction 73.1%in torch gas.The optimum operating conditions for recovering ethylene by distillation are as follows:the height of packing layer is 1.5 m,the intake pressure is 1740 kPa,the top temperature of tower is-100?,the recovery rate of ethylene is 99.7%,and the molar fraction of ethylene in torch-free gas is 9.8%.The optimum operating conditions for the coupling recovery of ethylene by distillation-membrane separation are as follows:the height of packing layer is 1.5 m,the intake pressure is 1740 kPa,the top temperature of tower is-100?,the recovery rate of ethylene is 99.9%,and the molar fraction of ethylene in torch-free gas is 4.6%.Compared with the membrane separation method,the membrane area of the distillation-membrane separation coupling method is reduced by 4.5 m~2.Compared with the distillation method,the top temperature of the tower can be increased by 15?in order to achieve the same effect of ethylene recovery.Compared with the four methods of recovering ethylene,the coupling of distillation and membrane separation is the best one.The design of ethylene recovery by distillation-membrane separation coupling method was carried out.The economic benefit of the recovery unit was 2.088 million yuan per year,and the investment return period was 1.3 months.