Transport Modeling and CFD Simulation of Membrane Gas Separation Materials and Modules.

摘要

Gas separation using polymer membranes has become a commercially attractive area during the last fifty years. It has wide application including nitrogen production from air, carbon dioxide and water removal from natural gas, and organic vapor removal from air or nitrogen. Theoretically there are two basic parameters to characterize the performance of the polymer membranes: permeability and selectivity. Both high permeability coefficient and selectivity are desired in order to achieve savings on both capital and operating costs. However there is a trade-off relationship between permeability and selectivity. The ability to tune the transport properties of polymer materials through changes in primary and secondary chain architecture appears to be limited by the existence of an "upper bound". Theoretical transport models and correlations have been proposed to provide guidance on structure-property relationships and the location of the upper bound. In the first part of this dissertation a novel model is developed to predict the gas transport properties. The non-equilibrium lattice fluid theory (NELF) predicts the existence of an upper bound for solubility selectivity. The theory provides very good a priori predictions of solubility and solubility selectivity. Furthermore the temperature dependence of solubility and solubility selectivity is predicted by the analysis. The NELF theoretical analysis also can be used to predict diffusivity and diffusivity selectivity in combination with transition state theory (TST). The diffusivity upper bound is investigated using model parameters to indicate the effect of properties of both gas pairs and polymeric materials. The temperature dependence of diffusivity and diffusivity selectivity is investigated. Finally gas permeability is successfully predicted and the existence of a permeability upper bound tested with the new model. Besides the influence of temperature on the permeability and permeability selectivity is evaluated with the model. Membrane modules are widely used for large scale gas separation process. Spacers are a critical component of membrane modules. Spacers provide mechanical support, create a uniform flow channel, enhance mass transfer coefficient and mitigate concentration polarization. However spacers generate higher pressure drop on both sides of channel and which increases energy input and operation cost. In the second part of this dissertation computational fluid dynamics (CFD) is used to investigate the flow within a spacer-filled channel. Three dimensional simulations were performed to visualize flow within the channel and evaluate pressure drop as a function of flow rate. The simulation method is validated by comparing results with experimental measurements for nitrogen flows. Simulations are also performed to investigate the effect of spacer geometry on velocity field and associated pressure drop. The influence of spacer geometry on the membrane module performance is examined and results show a promising effect of spacer geometry on membrane separation performance and associated module pressure drop. The effect of asymmetrical spacer design also is investigated. In contrast to a symmetrical spacer, asymmetrical spacers consist of two filaments of different diameter aligned asymmetrically to the nominal flow direction. Simulation results indicate the asymmetrical design can reduce pressure drop dramatically. Finally the multiphysics simulation is performed to study a combination of fluid flow and mass transfer process in a triple-layer spacer configuration. Effect of the density of thinner spacer adjacent to the membrane on the module performance is studied.