Advanced CFD Modeling of Mineral Scaling of RO Membranes

In membrane separation processes, solute rejection by the membrane leads to an increased salt concentration near the membrane surface. In reverse osmosis membrane desalination this concentration polarization effect, results in salt concentrations that may exceed saturation, thereby leading to mineral scale build-up on the membrane surface. This mineral scale leads to decreased membrane module performance and shorter membrane lifetime. To better understand the effects of concentration polarization, a 3-D numerical model was developed to describe fluid dynamics and salt solute mass transfer within the feed flow channel of spiral-wound RO modules. This model includes the effect of spacer filaments that are used to maintain the opening of the feed channel, but result in partially obstructed fluid flow patterns. These complex hydrodynamic conditions reduce the rate of solute mass transfer away from the membrane surface in regions surrounding the spacer filaments and lead to much higher levels of concentration polarization. In order to observe the effect of these regions of higher concentration polarization on overall mineral scaling behavior, the CFD results were coupled with an appropriate model for mineral scaling on the membrane surface. This scaling model was based on single crystal growth kinetics obtained from direct observation of membrane scaling phenomena in a plate-and-frame RO module coupled with a stochastic model for crystal nucleation on the membrane. The potential use of the present modeling approach will be discussed with respect to exploring advanced RO module design configurations (including the design of feed spacer filaments) for the prevention of mineral scaling.