Modeling mass transfer using surface morphology in full-scale reverse osmosis membrane processes

Abstract

The influence of surface characteristics on membrane process performance is considered significant and is not well understood. Current mass transport models generally assume constant mass transfer coefficients (MTCs) based on a homogeneous flat surface. This study evaluated membrane mass transfer by incorporating surface morphology into a diffusion-based model assuming that the MTCs are dependent on the thickness variation of the membrane’s active layer. Concentration polarization is also affected by this nonuniform surface property and was incorporated into the model. A simulation was performed using parameters from a full-scale 4.5 million gallon per day brackish water reverse osmosis membrane process. The process was simulated by modeling one thousand uniform slices of the membrane channel and the permeate water quality was determined locally through a finite difference approach. It was determined that solute mass transport is controlled by diffusion in the nonhomogeneous thinner regions (membrane valleys) of the active layer. This nonuniform surface affected the concentration polarization layer, where more solutes tended to accumulate within the valleys than on the ridges. Prediction of the permeate total dissolved solids concentration was accurate, ranging between 5 and 15%, as measured as an average percent difference between predicted and actual values.

Keywords:

• Membrane active-layer
• Reverse osmosis
• Mass transfer coefficients
• Concentration polarization
• Mathematical models

Acknowledgments

The research was funded, in part, by UCF’s Research Foundation through a grant provided by the Jones Edmunds Research Fund (Project 05-1620-0002 – RF1047820), as well as Funding Agreement 16208081 with the city of Sarasota, Florida. Any opinions, findings, and conclusions expressed in this material are those of the authors and do not necessarily reflect the views of UCF (Orlando, FL), its Research Foundation, Jones Edmunds Associates, Inc. (Gainesville, FL) or the city of Sarasota, Florida. The mention of trade names or commercial products does not constitute endorsement or recommendation. The authors acknowledge the contribution of Hydranautics (Oceanside, CA) in their providing UCF with AFM membrane images, without which this study would not have been made possible. The authors would like to thank Mr. Javier Vargas, Mr. Peter Perez, Ms. Katherine Gussie, and Mr. Gerald Boyce of the City of Sarasota’s Public Works and Utilities Division (Sarasota, FL) for providing full-scale reverse osmosis membrane process operations data that was relied upon for model development and validation.