Molecular Simulation And Experimental Study On Propylene Dehumidification Through PVA-PAA Blend Membrane

Gas membrane separation technology, exhibiting advantages of high selectivity, low energy consumption, simple process, low investment, has been proved both technically and economically better than traditional technologies of gas separation such as adsorption, absorption, and cryogenic distillation. Propylene is an important organic material in petrochemical industry, and it should undergo dehumidification to remove water before being processed into polypropylene, which is the main usage of propylene. The propylene/water gas feed was used as model system in the current study with the main purpose of developing a membrane and exploring modification method to enhance membrane separation performance.Thin film composite hollow fiber membrane was used in this study. Polysulfone (PS) was selected as porous hollow fibers while poly(vinyl alcohol) (PVA) was chosen as selective skin. In the current study, poly(acrylic acid) (PAA) was blended with PVA to improve the membrane dehumidification performance. Molecular simulation and experimental study methods were both employed to investigate the influence of different blend proportions of PVA to PAA on the structural morphology and separation performance of membranes.Molecular simulation is an effective method for researching and predicting microstructure and transport properties of membranes. In the present study, the properties of polymer membranes were analyzed by molecular simulation. It was shown that with the increasing content of PAA, the membrane density increased and the attractive force between polymer chains became stronger, fractional free volume of membranes became smaller and glass transition temperature became higher. The adsorption situation and diffusion coefficients of propylene and water in membranes were calculated. It was observed that more PAA was contained in the membrane, more small molecules were adsorbed. And the diffusion coefficients were smaller in higher PAA content membrane. The simulation results showed blend modification was helpful to enhance affinity between water and membranes, and it also could help to improve the separation factor. Because the permeation rate was influenced by adsorption and diffusion processes, there might be an appropriate value of blend proportion of PVA to PAA. Through experimental study, PVA/PS and PVA-PAA/PS hollow fiber composite membranes were prepared and the performances were evaluated. With the increase of PAA content in the membranes, the adsorption quantity of gas feed in membranes became larger. The permeation rate of gas feed firstly increased, and then fell down with increase of PAA content in the membranes. It was noted that the maximum value appeared at the membrane in which the proportion of PVA to PAA was 1:1.2. The membranes were characterized by SEM, FT-IR, XRD and DSC. It showed that the interaction energy between PVA and PAA polymer chains was strong. And the crystallinity degree of blend membranes decreased which was helpful to enhance membrane separation performance. The glass transition temperature obtained from DSC was consistent well with simulation conclusions. Effect of operating conditions on membrane separation performance was also investigated. It was found that the permeation rate was higher when the feed humidity increased or the operating temperature declined.The results showed that PVA-PAA blend membranes had better performance than PVA control membrane. The calculating results agreed well with experimental ones. It indicated that molecular simulation technique had high value in analyzing microscopic properties of membranes, exploring affinity between gas and membranes, and predicting permeation rate and separation factor.