Preparation And Characterization Of Pebax/SBS Composite Membrane For CH₄/N₂ Separation

Some coal-bed methane(CBM)with low concentration of methane(CH4)can not be applied directly due to its predominant impurity of nitrogen(N2).Such CBM is usually discharged to atmosphere or burned because lack of cost effective enrichment technology,causing environmental pollution and wasting energy resource.Therefore,the enrichment of CH4 especially separating CH4 from N2 plays a key technology role for the utilization of the coal-bed gas.Due to their close kinetic diameters and their similarity in polarity,separation of CH4 and N2 by dense polymer membrane is a cost-effective method.However,it exits a contradiction between permeability and selectivity for polymer membranes,as the so called Robeson’s upper bound.Block copolymers are one of the potential gas separation membrane materials that may overcome Roberson’s upper bound due to combination the merits of high selectivity from glassy polymers and high permeability from rubbery polymers.For the purpose of improve the CH4/N2 separation performance,block copolymer membranes based on Pebax were prepared by solution-casting method for CH4/N2 separation for the first time,and the membrane structure,thermal behavior,morphology,single gas of CH4/N2 separation and CH4/N2 mixed gas(CH4/N2=10:90 vol%)separation properties were studied in this thesis.Firstly,we prepared Pebax2533(PE 80wt%,PA 20wt%),Pebax3533(PE 70wt%,PA 30wt%),and Pebax4033(PE 53wt%,PA 47wt%)gas separation membranes with different composition of polyamide(PA)and polyether(PE)by the solvent of N,N-dimethylacetamide(DMAC).The membranes were characterized by FT-IR,DSC,AFM for their structures,thermal property,and morphology.The single gase properties for CH4/N2 separation were measured.Results indicated that Pebax membranes were prepared with different forms of semi-crystalline polyamide(PA)dispersed in the amorphous polyether(PE)matrix.It is found from DSC results that the crystallinity of semi-crystalline PA increased with the increasing of PA content(from Pebax2533 to Pebax4033),so as the melting point of PA.Since the segments of PA blocks and PE blocks in Pebax polymers have different behavior for CH4 and N2 gas transportation,the CH4/N2 separation performance varies among these three Pebax membranes.Single gas permeability tests showed that the CH4 and N2 permeability increased with the increase of temperature but with the decrease of pressure.The CH4/N2 selectivity showed an opposite trend to gas permeability.Among three Pebax membranes,Pebax 2533 membrane showed the best gas permeability(67.4 Barrer for CH4 and 24.2 Barrer for N2)at 1.0atm and 55oC.Secondly,the Pebax/SBS blend membranes were fabricated,for the first time,by the solution casting method with different amount of SBS1301 adding into Pebax2533 matrix.The Pebax/SBS composite membranes were characterized by FT-IR,TG,SEM,AFM,DSC,XRD for their structures,thermal property,and morphology,followed by the single gas measurement of CH4/N2 separation properties.Results indicated that the thermal stability of Pebax/SBS composite membranes was slightly changed compared with the pristine Pebax membrane.Importantly,the formation of PA crystalline structure in Pebax polymer was interfered by the addition of SBS resulting the lower crystallinity in PA segments of Pebax polymer and the increased the gas permeability.It was found that the Pebax/SBS(10wt%)exhibited the best permeation behavior with CH4 permeability of 106.2 Barrer at 55oC,which was enhanced about 60% compared with the pristine Pebax membrane and even higher than that of SBS.Single gas permeability tests showed that the CH4 and N2 permeability of the blend membranes increased with the increase of temperature but with the decrease of pressure for Pebax/SBS membranes.Furthermore,in order to investigate gas property relevant to the applications of CH4/N2 separation process and to provide some guidelines for potential industry users,the separation performance of CH4 and N2 mixed gas(CH4/N2=10:90 vol%)was measured for Pebax2533 and SBS/Pebax(10wt%)membranes.It is demonstrated that,with the change of temperature,the 10% volume fraction of CH4 in CH4/N2 mixture increased to 21.7% and 26.4% for Pebax2533 and Pebax/SBS(10wt%)membranes,respectively.With the change of pressure,the 10% volume fraction of CH4 in CH4/N2 mixture increased to 22.9% and 27.1% for Pebax2533 and Pebax/SBS(10wt%)membranes,respectively.The results indicated that the Pebax polymer could be a potential material for the applications of CH4 enrichment in coal-bed gas sources where the CH4 concentration is low.