Comparison of nonhomogeneous and homogeneous mass transfer in reverse osmosis membrane processes

Abstract

Reverse osmosis (RO) membranes are pressure driven, diffusion controlled processes. Current diffusion controlled solute mass transfer models assume a homogeneous membrane surface. This study evaluated mass transfer processes assuming mass transport is not a homogeneous process, which is dependent on the thickness variation of the membrane’s active layer. Three-dimensional ridge and valley active layer morphologies were created numerically using Gaussian random vectors. A nonhomogeneous solution diffusion model (NHDM) was then developed to account for surface variation through the active layer. NHDM was further modified by incorporating concentration polarization (CP) effects. A comparison of the NHDM and the NHDMCP with the commonly accepted homogeneous solution diffusion model (HSDM) using pilot-scale brackish water RO operating data indicated that the NHDM is more accurate, when the solute concentration in the feed stream is low, while NHDMCP appears to predict better with a high solute feed concentration.

Keywords:

• Membrane active-layer
• Reverse osmosis
• Mass transfer coefficients
• Nonhomogeneous
• Gaussian random vector
• Concentration polarization

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 help 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 and Ms. Katherine Gussie of the City of Sarasota's Public Works and Utilities Division (Sarasota, FL) for providing full-scale and pilot-scale reverse osmosis membrane process operations data that was relied upon for model development and validation. The contributions of UCF graduate students Mr. Jaya Tharamapalan and Mrs. Rebecca Wilder were significant; their efforts strengthened the results and their assistance was greatly appreciated.