This document summarizes a 3D CFD analysis of hydrodynamics and concentration polarization in an industrial reverse osmosis feed channel. The analysis compared a spacer-filled channel model to an open channel model. Results showed that spacers increase velocity magnitude and pressure drop but significantly reduce concentration polarization along the membrane by interrupting boundary layer formation. Ongoing work is focusing on varying feed velocities and validating the models with industrial data.

1. Three-Dimensional CFD Analysis of Hydrodynamics and Concentration
Polarization in an Industrial RO Feed Channel
ABSTRACT
Eric Pearce, Hien Nguyen, Nhu Nguyen, Wei Hua Wu, Dr. Mingheng
Department of Chemical & Materials Engineering
Figure B: Pressure contour at top surface in spacer-filled channel and open channel.
Figure A: Velocity magnitude and streamline in spacer-filled channel and open channel.
Figure C: Water flux through top surface of spacer-filled channel and open channel.
Figure D: Concentration on top and bottom surfaces of spacer-filled channel and open channel.
Figure E: Concentration along membrane in spacer-filled channel and open channel.
Figure F: Mass transfer coefficient along membrane in spacer-filled channel and open channel.
MODEL DEVELOPMENT
Diffusion-Convection Equation of salt:
INTRODUCTION
Diagram of a spiral wound membrane
CONCLUSIONS
Diagram of a pressure vessel and a RO train [1]
RESULTS AND DISCUSSION
 Uniform velocity field is interrupted by spacers which lead to
flexural streamlines. The open channel has straight streamlines.
The velocity magnitude in spacer-filled channel is higher than
the one in open channel even if they have the same inlet aver-
age velocity (Figure A).
 Pressure drops for both spacer-filled and open channel models
from inlet to outlet. The spacer leads to a higher pressure drop
(Figure B).
 Fluxes in both channels are roughly constant due to the small
computational domain. However, the gradient is very different.
The flux reduces significantly near the filaments in spacer-filled
channel. It reduces along the open channel (Figure C).
 In the open channel, salt concentration increases gradually
along the feed flow direction due to concentration polarization.
In spacer-filled channel, salt in concentrated in regions near
spacer filaments where flow is relatively stagnant (Figure D).
 Concentration polarization increases pumping pressure require-
ments and leads to increased scale build up. This in turn results
in increased energy costs and more frequent down time for
back flushing.
 A mass transfer boundary layer develops whenever a laminar
fluid flow boundary layer is developing, using the Chilton-
Coburn analogy. Inside this boundary layer, the primary mecha-
nism of mass transport is diffusion, which can result in mem-
brane concentration polarization reducing the flux of the solvent
through the membrane.
 By having spacers between the membrane, laminar boundary
layer development is interrupted as well as thinned by localized
areas of high velocity. As the thickness of the mass boundary
layer is inversely proportional to velocity, flux through the mem-
brane is increased.
 This work shows the necessity of spacer inclusion in the devel-
opment of the concentration polarization theories in commercial
spiral wound membranes.
The open channel shows a predictable concentration polarization
along the direction of flow, as has been validated by previous litera-
ture. The spacer-filled channel shows a repeating pattern of mass
transfer, as the laminar boundary layer formation along the mem-
brane surfaces were interrupted by the zig-zag spacer placement.
This work shows that concentration polarization theory based on
open channel cannot be used to describe the behavior in commer-
cial spiral wound RO. Ongoing work focuses on varying feed veloci-
ties to investigate its effect on pressure drop and mass transfer as
well as validation of model using industrial data.
Hydrodynamics and mass transfer characteristics of a commer-
cial 28 mil Reverse Osmosis (RO) feed channel were investigated
using fully-coupled three-dimensional (3D) computational fluid dy-
namics (CFD). Two models of a three unit cell RO feed channel -
the open channel and the spacer-filled channel were solved by
COMSOL Multiphysics. By comparing velocity fields, pressure
drops and salt concentrations between the two models, it is be-
lieved that the significant effect of the feed spacers in the RO mem-
brane systems plays an important role and cannot be ignored in
process modeling. The presence of spacers increases the average
mass transfer coefficient and helps reduce the development of con-
centration polarization significantly, at the cost of a higher pressure
drop.
 Spacer geometry is based on 28 mil FilmTecTM
BW 30-400 mem-
brane. Filament geometry is based on literature [2]. The spacer-
filled channel has a porosity of 0.91.
 The fluid mechanics are fully coupled with mass transfer of salt.
Navier-Stokes Equation for laminar flow:
 The computational domain consists of three unit cells. The re-
sults of a spacer-filled channel and an open channel were com-
pared to demonstrate the difference in concentration polariza-
tion and pressure drop.
 For the spacer-filled channel, “periodic steady-state” velocity
profile is obtained and used as the inlet condition. For the open
channel, fully developed laminar flow velocity profile is used.
REFERENCES
[1] Sydney Water Corporation, "How does reverse osmosis work,"
www.youtube.com/watch?v=aVdWqbpbv_Y, New South Wales,
Australia, 2013.
[2] Sz.S. Bucs, A.I. Radu, V. Lavric, J.S. Vrouwenvelder, C. Pi-
cioreanu. Effect of different commercial feed spacers on biofouling
of reverse osmosis membrane systems: A numerical study. Desali-
nation, 343, 26-37, 2014.