Reducing Concentration Polarization With Sinusoidal Spacers: Computational Modeling and Experimental Verification

A key to maximizing flux and decreasing fouling in reverse osmosis (RO) processes is to decrease concentration polarization, which is currently partially accomplished by mesh feed spacers. Even though mesh spacers can be designed to create turbulence, thus decreasing concentration polarization, the presence of the mesh spacer increases the longitudinal pressure drop. Moreover, investigations have revealed that dead zones are created around the mesh spacer leading to membrane fouling; evidence for this is that membrane fouling often occurs in a pattern defined by the spacer.    

To improve upon the current mesh feed spacer we developed spacers with a geometry that causes sinusoidal flow patterns. These have two main advantages: unobstructed flow and adaptable geometry. First, this design allows unobstructed flow inside the membrane channel so as to minimize longitudinal pressure drop and dead zones. Second, the frequency and amplitude of the sinusoid (along with the width and height of the flow channels) can be tuned, thus changing the crossflow velocity independent of the total module flow rate. Faster crossflow velocity is achieved with higher amplitude and frequency of the sinusoids.

Analysis of the designs was accomplished using computational fluid dynamics (CFD) modeling via COMSOL Multiphysics software. The influence of spacer geometry (e.g. amplitude, frequency, and cross-sectional shape), applied pressure, feed velocity, and concentration were investigated to determine how concentration polarization could be reduced while maintaining low longitudinal pressure drop. Modeling data were compared and verified with bench scale experimental data to prove the accuracy of the model.

Modeling data matched very well with experimental data, suggesting the modeling results were reliable. More tortuous sinusoidal channels induced secondary flow in both longitudinal and cross-sectional directions. The secondary flow increased mass transfer in the channel, thus reducing concentration polarization and enhancing flux.

The ultimate goal of this work is to show that these novel membrane spacers can decrease energy use and fouling in RO, thus enabling more sustainable implementation of desalination. The methods may also enable water reuse and other RO processes that deal with high-fouling waters.