Computational fluid dynamics modelling of flow and permeation for pressure-driven membrane processes

While models of membrane systems with varying degrees of complexity have been developed since the early 1960s, reports of the use of computational fluid dynamics to provide in-depth insights into the separation phenomenon have only recently appeared in the literature. This paper describes the validation and application of a computational fluid dynamics model of pressure-driven membrane processes involving selective removal of components in the feed channel and their transfer to the permeate channel. The effects of changes in rejection, wall permeation rates and solution properties on velocity and concentration profiles are presented for empty channels and channels with eddy promoters. For high polarisation applications typically encountered in conventional ultrafiltration applications, the results demonstrate the need for very fine meshes near the membrane wall, together with high order numerical schemes and accurate modelling of rejection and physical property variations in order to obtain accurate and reliable predictions of the polarisation and flow phenomenon.

The ability to completely model the flow and concentration phenomena in three-dimensional membrane modules is still not possible. Modern CFD packages have simplified the creation of complex geometries. Their solution algorithms permit efficient and accurate solving by the knowledgeable user of the flow, continuity, momentum and concentration equations without the necessity for simplifying assumptions that were once essential. However, the use of these packages is not straightforward and the appropriate method for handling the complexities inherent in membrane systems still requires much fundamental research. This paper describes an enhancement of the existing CFD package CFX for the modelling of membrane systems that comprises a model for the selective removal and transferal of components between the feed and permeate streams. The CFD model has been validated against known semi-analytical solutions and shown to correctly reproduce the data. The modular nature of this validated CFD model makes it an extremely powerful tool in the design and optimization of membrane systems.