Ben Bettens

Transport of Binary Mixtures in Pervaporation through a Microporous Silica Membrane: Shortcomings of Fickian Models

Abstract This study explores the applicability of the adsorption‐diffusion mechanism to describe the transport of binary “methanol‐water” and “ethanol‐water” mixtures in pervaporation through a commercial microporous silica membrane. Two different adsorption‐diffusion models are considered: one based on Ficks diffusion equation and another based on the Maxwell‐Stefan formulation. Basic models (Fick) assume concentration independent parameters; more complex models (Maxwell‐Stefan) incorporate flux coupling and other non‐idealities. The influence of feed temperature (40°C–90°C) on permeation flux was analysed in terms of activation energy for flux, permeability and diffusion, and heat of adsorption and vaporization. Also the occurrence of coupling effects was studied by determining the effect of feed composition (entire composition range) on permeation flux, permeability and selectivity. Adsorption‐diffusion models based on Ficks diffusion equation can be used to describe coupling effects if they are modified with concentration dependent diffusion and/or sorption coefficients. They are incapable of describing drag effects by water on alcohols. These drag effects should be modeled through models based on the Maxwell‐Stefan theory.

Maxwell-Stefan in mass fractions for numerical simulation of the pervaporation process

Abstract Membrane processes are becoming more and more important in industry. Therefore, modelling of these processes is also gaining interest, as this offers opportunities to predict process performance, membrane properties, efficiencies and economical aspects. With this information, industry can be helped to improve existing separation processes, or switch to more profitable alternatives. The choice of the transport model is important to describe membrane transport correctly. For processes with a solution-diffusion transport mechanism, like pervaporation, the Maxwell-Stefan equations have proven to be capable of describing multicomponent transport. In this model, the membrane is considered to be part of the system, and interactions between all system components are accounted for. Because for the generalized Maxwell-Stefan equations unknown information about the membrane is necessary, like the molar mass, a conversion to mass fractions is performed in this paper. This conversion has consequences for several parameters and these are discussed in this paper. From an economical point of view, pervaporation is a possible alternative to energy consuming processes, like distillation. If executed solely or in a hybrid process, it can reduce the process energy consumption. As an example of the capabilities of the conversion, a pervaporation case study is simulated. The structure of this simulation program is briefly discussed, and an example is elaborated that shows the applicability of the conversion.