Zhongwei Ding

A new model for mass transfer in direct contact membrane distillation

The mass transfer process in direct contact membrane distillation (DCMD) for three kinds of membranes was measured. Water fluxes at different temperatures and the membrane distillation coefficients (MDC) for each membrane were obtained directly from experimental data. The fact that the MDC values of membranes with larger pore size increase with temperature indicates that Poiseuille flow plays an important role in the process of mass transfer through the membrane. Based on this conclusion, a three-parameter model, named the Knudsen diffusion-molecular diffusion-Poiseuille flow transition (KMPT) model, was developed to predict MDC and water flux for membrane distillation. The parameters of the KMPT model for each membrane employed in this study, by which MDC at various temperatures can be determined, were evaluated by a nonlinear regression. The values of MDC and water fluxes for each membrane predicted by KMPT model agree well with that obtained directly from the experiment results. A large contribution of Poiseuille flow to mass transfer was observed and can be attributed to the distribution of large pores in the membranes. The KMPT model also provides a method for estimation of the effect pore size using the ratio of the MDCs; the ratio of the Poiseuille flow to molecular diffusion MDC provides the best estimation.

A Method to Obtain Gas-PDMS Membrane Interaction Parameters for UNIQUAC Model

Abstract The recovery or capture of one or more components from gas mixture by membrane separation has become a research focus in recent years. This study investigates the gas-membrane solution equilibrium, for which Henrys law is not applicable if the gas phase is a mixture. This problem can be solved by using UNIQUAC model to calculate the activity coefficient of gas dissolved in the membrane. A method was proposed in this study to obtain the gas-membrane interaction parameter for UNIQUAC model. By the experiments of gas permeation through polydimethylsiloxane PDMS membrane, the solubility coefficients of some gases (N 2 , CO 2 , CH 4 ) were measured. Through non-linear fitting UNIQUAC model to the experimental results from this study and in literature (H 2 , O 2 , C 3 H 8 ), the gas-membrane interaction parameters for these gases were obtained. Based on these parameters, the activity coefficients of the dissolved gas were calculated by UNIQUAC model, and their values agree well with the experimental data. These results confirm the feasibility and effectiveness of the proposed method, which makes it possible to better predict gas-membrane solution equilibrium.

Modeling spiral-wound membrane modules with applications for gas/vapor permeation

Abstract Gas/vapor permeation has become a common method for VOC (volatile organic compound) recovery and CO 2 capture. The spiral-wound membrane module, which has high packing density, has been commonly used in gas/vapor permeation. In this study, the model that suits the spiral-wound PDMS (polydimethylsiloxane) membrane is obtained by using an integral transform from an N–S equation (Navier–Stokes) and a mass transfer differential equation. The predicted results indicate that the concentration polarization and spiral-wound membrane structure influence can be neglected. The mass transfer based on the MS–UNIQUAC (MS–U) model is also proposed and compared to that based on the Fick–Henry (F–H) model. The comparison results show that the mass transfer based on the F–H model can only be used in a gas mixture without condensable gas; the MS–U model should be used for a gas mixture that includes condensable gas. The influence of permeate flux and feed-side composition in the spiral-wound membrane at different operation conditions is also investigated using the MS–U model.