Physical Aging Behaviors Of Polymer Membranes At High Pressure Gas Atmosphere

Natural gas has the advantage of clean and low carbon emissions,but raw natural gas needs to be purified before entering the transportation pipeline.If the natural gas in the pipeline contains too much carbon dioxide,it will reduce the calorific value of natural gas and corrode the pipeline.Membrane technology is a very promising separation method that can separate carbon dioxide from natural gas and consume less energy during operation.However,the effect of membrane separation is limited by the performance of the membrane material,and the gas flux will decrease with time.This phenomenon is known as physical aging.At the same time,the presence of CO2 causes the polymer film to swell,so that the permeability coefficient of all gases increases,and the flux of the low permeability gas increases more,thus destroying the selectivity of the gas film.In industrial applications,the membrane material is exposed to high pressure for a long time,and high-pressure CO2 will aggravate the swelling phenomenon.In order to investigate the physical aging behavior of polymer membrane materials in industrial environments,this paper selected high permeable polymer of intrinsic microporosity(PIM-1)and low permeable polysulfone(PSF)and monitored the changes of the gas permeability under constant C02 pressure of 10 atm and 23.5 atm,respectively.PIM-1 has large free volumes.In the early stage of aging,physical aging dominates and the gas permeability coefficient decreases.In the middle of aging,the physical aging rate is weakened,and CO2 swelling dominates,leading to an increase in the permeability coefficient.In the late stage of aging,the gas permeability coefficient shows only a small increase in a long time,and the gas mass transfer performance tends to be stable.In the initial state,the low-flux polymer of PSF has a small free volume and a slow aging rate.In the initial stage,the CO2 swelling effect is dominant,and the gas permeability coefficient of the polymer film increases.In the middle of aging,the inside of the low-flux polymer film generates excessive free volume due to the swelling of the previous C02,the driving force for aging becomes large,the aging effect is dominant,and the gas permeability coefficient is decreased.In the late stage of aging,the rate of physical aging slows down and gradually stabilizes,CO2 swelling and physical aging reach equilibrium,and the gas permeability coefficient of the polymer film increases slightly.The greater the CO2 pressure,the more pronounced the swelling effect.Whether it is high FFV(fraction free volume)PIM-1 or low FFV PSF,the polymer membrane gas permeability at 23.5 atm is greater than 10 atm during the late aging period.In addition,when aging in a high-pressure CO2 environment,the greater the pressure of CO2,the smaller the variation of the permeability coefficient of the five different gases.The FFV value calculated by molecular simulation proves that since FFV of PIM-1 is larger than that of PSF,PIM-1 has a large driving force in the initial stage of aging,and physical aging is dominant.However,due to the high solubility of CO2 in the polymer,the swelling effect can not be ignored.Therefore,the gas permeability coefficients of PIM-1 and PSF will increase at different times.