(653a) Proper Accounting of Mass Transfer Resistances in Forward Osmosis: Improving the Accuracy of Model Predictions of Structural Parameter

Proper Accounting of Mass Transfer Resistances in
Forward Osmosis: 

Improving the Accuracy of Model Predictions of
Structural Parameter

Ngoc Bui, Lawrence
Livermore National Laboratory, Livermore, CA

Jason Arena, Jeffrey
R. McCutcheon, University of Connecticut, Chemical and Biomolecular
Engineering Department, Storrs, CT

Forward
Osmosis (FO) and pressure retarded osmosis (PRO) have recently been revitalized
as a sustainable and versatile membrane-based separation technology platform
for water and power production, respectively. To design membranes for various
FO processes, it is important to understand critical structure-performance
relationships, especially with respect to mass transfer. This work demonstrates
a more accurate method for calculating structural parameter (S) of asymmetric
osmotic membranes using experimental data and a theoretical flux model which
encapsulates all significant boundary layer phenomena. External boundary layer
effects on the porous side of the membrane have been neglected in many current
models. In these models, external concentration polarization (ECP) effects get
combined with the internal concentration polarization (ICP), resulting in
inflated S values. In this study, we proposed a new flux model in which ECP
effects are accounted for so that S can be more accurately measured. This model
considered the in-series resistances for solute transport based on intrinsic
properties of the membrane as well as boundary layers at membrane surfaces and
within the support layer. The results indicate that ICP is less severe than
previously predicted and that cross-flow velocity, temperature and
concentration of the draw and the feed solutions impact both external and
internal concentration polarization. Our calculations also surprisingly show
that changes in cross-flow velocity impact internal concentration polarization
due to induced mixing within the support layer. Furthermore, new definitions of
membrane reflection coefficient and total resistance to solute transport emerge
from this work. Also, we
suggest that it is critical to consider the ?residence time? of solutes in the
vicinity of the selective layer in determining the membrane selectivity.