Preparation,Optimization Of PEBA2533-based Mixed Matrix Membranes And Their Separation Of C₂H₄/C₃H₆

Ethylene（C₂H₄）and propylene（C₃H₆）are two kinds of important chemical feedstock.The product of preparing ethylene or propylene in industry is a mixture containing both C₂H₄and C₃H₆,however,the high purity of C₂H₄ and C₃H₆ is essential to their industrial applications.It is meaningful to explore the separation and purification process of C₂H₄ and C₃H₆ from the product.The traditional separation processes of C₂H₄ and C₃H₆ from the products are cryogenic distillation or intercooling propane absorption processes.The energy consumption of the cryogenic distillation process is high,which accounts for about 2/3 of the entire production process.The energy consumption of the intercooled propane absorption process is not reduced because plenty of propane circulates in this process.In the context of carbon neutrality and carbon peaking,it is particularly important to save energy and reduce consumption in this separation process.Membrane separation technology is widely used for gas separation because of its energy saving,environmental and efficient,however,few membranes have been reported for the separation of C₂H₄/C₃H₆ mixture.In this thesis,PEBA2533 was used as the matrix,which were optimized by PEG rich in ether groups.A series of mixed matrix membranes for separation of C₃H₆/C₂H₄ were prepared with zeolite Na Y,Na A,ZNa Y（Z=Ag,Cu,Ca,Ce）and polar Ui O-67 as additives,respectively.FESEM,EDX,DSC,ATR-FTIR and AFM were used to characterize the microstructure and properties of prepared membranes and additives.Grand Canonical Monte Carlo（GCMC）was also used to simulate the adsorption process of C₂H₄ and C₃H₆ mixture on UIO-67 crystals.The effect of membranes on C₂H₄/C₃H₆ separation performance in different operating conditions was investigated in detail.The relationship between the microstructure of the membranes and C₂H₄/C₃H₆ separation performance was also explored.The main conclusions are as follows:（1）The polar zeolite Na Y or Na A was introduced into PEBA2533 to prepare mixed matrix membranes.Both the polarity and the pore size of zeolite crystals affect their adsorption performance to C₃H₆ molecules.Na Y is more effective compared with Na A for C₃H₆/C₂H₄ adsorption selectivity and Na Y zeolite improves the separation performance of membrane more significantly as an additive.Na Y crystals are dispersed uniformly in membranes when Na Y content is not more than 6 wt.%.Agglomeration of Na Y crystals occurs and non-selective interfacial voids appear when the Na Y content is greater than 6 wt.%.Membrane PEBA2533/Na Y-6 with 6 wt.%crystals showed the best separation performance.An increase in membrane thickness leads to an increase in C₃H₆/C₂H₄ selectivity and a decrease in permeability.The membrane with the best performance is the thickness of 65μm,if both C₃H₆/C₂H₄ selectivity and permeability are taken into consideration.The operation temperature and pressure also affect C₃H₆/C₂H₄ separation.Lower temperature and lower pressure favor the C₃H₆ permselectivity,while higher temperature and higher pressure favors the permeability of the gas molecules.（2）PEG rich in ether groups was introduced into PEBA2533 to form PEBA2533/PEG membranes.The molecular weight of PEG affects the microstructure and separation performance of the membranes.In the investigated range of 10–40 wt.%,PEG is homogeneously blended with PEBA2533 when the molecular weight of PEG（M_W）is≤600,however,the granular and rod-like PEO crystals appear in the blend membranes,and the compatibility of the two becomes poor when M_W of PEG is>600.The C₃H₆/C₂H₄ selectivity on PEBA2533/PEG membranes increases firstly and then decreases with the increase of PEG molecular weight,and the membrane with PEG whose M_Wequals 600 shows the maximum C₃H₆/C₂H₄ selectivity.Increasing the content of PEG600 in the blend membranes,the hydrophilicity and fractional free volume of the membranes increase.When their hydrophilicity increases,the membranes show bigger capacity to dissolve C₃H₆;When their free volume increases,the membranes are more favorable for C₂H₄molecules to diffuse.Under the cooperation of the hydrophilicity and free volume of the membranes,the blend membrane with PEG600 content of 15 wt.%possesses the best gas separation performance.（3）The polarity of Y zeolite crystals was modified by exchanging Na⁺in Na Y with Ag⁺,Cu²⁺,Ca²⁺and Ce³⁺,respectively.The zeolite crystals obtained were introduced into PEBA2533/PEG600 to prepare mixed matrix membranes.It was found that the mixed matrix membranes prepared with the Y zeolite containing Cu²⁺,which has the smallest radius and the strongest polarization ability,have the best separation performance.An increase of Cu²⁺exchange degree causes a decrease in the crystallinity of Cu Na Y zeolite,which does not affect the microstructure of the mixed matrix membranes.However,as an additive,an increase of Cu²⁺exchange degree in Cu Na Y zeolite promotes the permeability of C₃H₆ and C₂H₄,and the C₃H₆/C₂H₄ selectivity increases firstly and then decreases with the increasing of the Cu²⁺exchange degree.The C₃H₆/C₂H₄ selectivity is the highest when the Cu²⁺exchange degree is73.77%.Cu Na Y crystals are uniformly dispersed in PEBA2533/PEG600 without voids between Cu Na Y crystals and PEBA2533/PEG600 matrix,when the content of Cu Na Y crystals in the membranes does not exceed 6 wt.%.Cu Na Y crystals agglomerate and interfacial voids are formed when the content of Cu Na Y crystals are greater than 6 wt.%.The mixed matrix membrane with Cu Na Y content of 6 wt.%and Cu²⁺exchange degree of 73.77%shows the best separation performance.（4）PEBA2533/PEG600/Ui O-67 mixed matrix membranes were prepared with homemade polar Ui O-67 crystals as additives.HAc as an acid modulator adjusts the Ui O-67crystal morphology to be regular octahedron,and triethylamine（TEA）as a base modulator regulates the particle size of Ui O-67 crystals.The monodisperse Ui O-67 crystals with a particle size of about 180 nm were synthesized with the regulation of both acid and base modulators.Both GCMC simulation adsorption of C₃H₆ and C₂H₄ and the pure gas adsorption demonstrated that the Ui O-67 crystals have the property of preferential adsorption of C₃H₆from C₃H₆/C₂H₄ mixture,and the low temperature is more favorable to the preferential adsorption of C₃H₆.Hydrogen bonds can be formed between Ui O-67 crystals and PEBA2533/PEG600 matrix,which leads to the uniform distribution of Ui O-67 crystals in the matrix and the well combination of the two even if the Ui O-67 content is up to 20 wt.%.Further increase Ui O-67 content to 24 wt.%,the crystals disperses unevenly in the matrix,some regions accommdate more Ui O-67 crystals and the crystals do not agglomerate.The mixed matrix membrane with 20 wt.%Ui O-67 crystals shows the best C₃H₆/C₂H₄separation performance in all membranes we prepared.At-35℃ and 0.2 MPa,C₃H₆/C₂H₄selectivity increases from 6 to 70.3 and the C₃H₆ permeability increases from 390 to 889Barrer when the concentration of C₃H₆ in the feed increases from 10%to 80%.When the concentration of C₃H₆ in the feed is enhanced to 90%,the separation temperature is increased simultaneously to-25℃ to prevent the liquefaction of C₃H₆,under this conditions,C₃H₆/C₂H₄ selectivity is 193 and the C₃H₆ permeability is 1200 Barrer.The performance of all the membranes we prepared remain stable during the 250 h separation process.