(287a) The Importance of Mass Transfer Limitations in the Application of Forward Osmosis (Keynote Talk)

Abstract

The growth in world
population, an increase in demand for resources, a changing global climate, and
pollution of available water resources are all exerting unprecedented demands
on fresh water supplies around the world. Furthermore, rising energy costs and
global warming, both linked to use of fossil-fuels, are driving the need for
low energy, more sustainable forms of water reclamation. Forward Osmosis (FO)
has attracted growing interest recently as a highly promising technology for
desalination and water reuse. The basis of FO is osmosis itself, a natural and
spontaneously occurring process. It has attracted wide interest for its
potential applications in a number of areas [1] including desalination [2],
wastewater treatment [3-4], and food processing [5]. A related membrane
process, known as pressure retarded osmosis (PRO), has also been used for
osmotic power generation [6].

The FO process offers
some advantages over conventional pressure-driven membrane processes such as (i)
potentially lower energy consumption and (ii) minimal disturbance to sensitive components/species
in the feed solution and it is the former that we address herein. It is widely
recognized that the water flux in an FO process is severely limited by internal
concentration polarization (ICP) which is a term for describing the phenomenon by
which there is either dilution of the high-osmotic-pressure draw solution (DS)
or undesirable concentration of the feed solution (FS) inside the FO support
structure [see ref 1 for more details]. The former occurs if the support faces
the draw solution; the latter if the support faces the feed.

The overall aim of
the analysis is to ask whether even an ideal FO membrane of very large A and K
will be superior to existing RO systems, and if so under what circumstances. 
Initially we have ourselves with three specific questions:

(1)   What is the relationship
between the flux, ratio of osmotic pressures and mass transfer coefficients?

(2)   Can external mass transfer
coefficients be neglected?

(3)   Can explicit equations be obtained
if J/K < 1.0?

Two answers to the
third question will be covered in the presentation. Regarding the first
question a sample result is presented in Figure 1.  This is based upon the
widely used equations found in [1] and elsewhere.  These do not include a full
allowance for mass transfer in the feed and draw solution boundary layer mass
transfer coefficients, k_f and k_d.  The importance of both
K and the osmotic pressure ratio is apparent.

Figure 1 Predicted fluxes for various values of (π_draw/π_feed)
and mass transfer coefficient K (as defined in Ref 1) for active layer facing
draw solution. (Draw by Gaoqi Zhang)

As the mass transfer
coefficient of support has improved in practice (made thinner and with great
porosity) allowance for k_f and k_d becomes essential. 
Under circumstances in which the membrane parameters A and K become large (i.e.
a tendency towards an ideal FO membrane), it can be shown that the flux tends
to

)  (1)

which for very large
K would simplify to:

)  (2)

Considerations of
limitations due to scaling coupled with equation (2) suggest that even if membranes
with high A, were available FO would be unattractive for high salinity feeds. 
This is certainly not contradicted by recent experimental results [7].  The
main conclusion is that in the area of water treatment application of FO should
be directed at low salinity feed waters.

Less Common Symbols
Used

A  Overall water
permeability coefficient (m/s Pa)

K  Membrane
support mass transfer coefficient (m/s)

pi ratio =
π_d/π_f where d refers to draw and f to feed

[1] Tang CY, She Q , Lay WCL, Wang R,Field RW and Fane AG Modeling
double-skinned FO embranes Desalination 283 (2011) 178-186

[2] J.R. McCutcheon, R.L. McGinnis, M. Elimelech, Desalination by
ammonia-carbon dioxide forward osmosis: Influence of draw and feed solution concentrations
on process performance, Journal of Membrane Science 278 (2006) 114?123.

[3] A. Achilli, T.Y. Cath, E.A. Marchand, A.E. Childress, The
forward osmosis membrane bioreactor: a low fouling alternative to MBR
processes, Desalination 238 (2009) 10?21.

[4] W.C.L. Lay, Y. Liu, A.G. Fane, Impacts of salinity on the
performance of high retention membrane bioreactors for water reclamation: a
review, Water Research 44 (2010) 21?40.

[5] M.I. Dova, K.B. Petrotos, H.N. Lazarides, On the direct osmotic
concentration of liquid foods. Part I: Impact of process parameters on process
performance, Journal of Food Engineering 78 (2007) 422?430.

[6] S. Loeb, Large-scale power production by pressure-retarded
osmosis, using river water and sea water passing through spiral modules,
Desalination 143 (2002) 115?122.

[7] Y. Xu, X. Peng, C.Y. Tang, Q.S. Fu, S. Nie, Effect of draw
solution concentration and operating conditions on forward osmosis and pressure
retarded osmosis performance in a spiral wound module, Journal of Membrane
Science 348 (2010) 298?309.