Study On Experiment And Theory Of PDMS/PS Hollow Fiber Composite Membrane For Hydrogen Recovery From Catalytic Cracking Dry Gas

The hydrogen of catalytic cracking dry gas is in very vast scale. Separation andrecovery of hydrogen using membrane technology is one of the most importantsubjects about energy and environment. PDMS/PS hollow fiber compositemembranes, which are the "resistance type" membranes, are the very suitable forrecovery of hydrogen from catalytic cracking dry gas. The preparation andapplication of PDMS/PS composite membranes and the mass transfer mechanism ofhydrogen membrane separation process are both very important. In this study,hydrogen recovery process from the catalytic cracking dry gas was investigated.Firstly, the polysulfone membrane is chose as the substrate layer, and PDMS is asthe coating layer material in order to cover the shortage in the dense layer of PS. Thedense layer of PS plays an important role in hydrogen separation process, but thePDMS layer only cover the shortage in PS dense layer. The PDMS/PS hollow fibercomposite membranes were prepared by dip-coating method. Some penetratingbehaviors of hydrogen through the composite membranes were discussed. Thestructure and properties of silicone rubber, polysulfone membrane and hollow fibercomposite membranes was analyzed by SEM, IR and TG/DTA methods.Secondly, the effects of polysulfone substrate layer and the silicone rubber coatinglayer on preparation process of composite membranes were investigated. The effectsof some factors, which are different structures of polysulfone substrate layer, thetemperature of heat treatment, the time of heat treatment, the number of coating andthe PDMS, catalyst and cross-linker concentration, on gas separation performance ofcomposite membranes also were discussed. The suitable formula and operationconditions for preparing the composite membrane were found, and the separationresult of the PS membrane whose cut molecular weight is 20000 was better. Theeffects of feed pressure, feed flow rate, the partial pressure of hydrogen in permeateand operating temperature on the gas separation performance also investigated, andthe hydrogen permeance and the separation factor could be to 16.12GPU and morethan 40, respectively.Then, a resistances-in-series model based on the Henis-Tripodi's model and thetheory of the boundary layer for gas separation in composite membranes waspresented, in which the resistance of the boundary layer and the resistance ofmembrane have been taken into account at the first time. The composite membranestructure parameters were calculated according to the resistance-in-series model. Theeffect of boundary layer resistance on gas separation properties is investigated. Thenthe influences of feed flow rate, feed pressure, and the partial pressure of hydrogen inpermeate gas on gas permeability and selectivity of hollow fiber compositemembranes have been considered and analyzed. In our study, the effect of thecomposite membrane structure parameters such as the dense layer porosity of PSsubstrate, the thickness of dense layer, the average pore size of defects in dense layerand the coating layer thickness on gas separation properties are also discussed.Lastly, for PDMS/PS hollow fiber membrane, a countercurrent-flow design modelwas established which can describe hydrogen separation process. This mathematicalmodel was based on the gas linear flow rate and could be applied in more gasseparation processes, which was better than the model in most references. Thesimulation using the model is performed by changing the operating conditions and theoptimum operating factors and the required membrane area are investigated. Then thedesign proposal of this technique for industrial application was brought forward.