NAMS 2002 Workshop - ICOM 2008

Membrane Contactors – 4
Friday July 18, 4:00 PM-4:30 PM, O’ahu/Waialua
Effect of Spacer, Baffled and Modified Hollow Fiber Geometries in the
Membrane Distillation Process
M. Teoh (Speaker), National University of Singapore, Singapore
S. Bonyadi, National University of Singapore, Singapore
T. Chung, National University of Singapore, Singapore - chencts@nus.edu.sg
M. Gryta, Szczecin University of Technology, Szczecin, Poland
For about three decades, Membrane distillation (MD) has been considered as a
possible alternative for the conventional desalination technologies such as multistage
flash vaporization (MSFV) and reverse osmosis (RO). However, MD has
gained little acceptance and yet to be implemented in industry for several
reasons: barrier of suitable MD membrane and module design, membrane pore
wetting, low permeate flow rate & water flux (i.e., productivity) as well as
uncertain energetic and economic costs. In this respect, opportunities therefore
beckon membrane researchers to improve the permeate flux to bring MD closer
to commercialization. Based on the MD mechanism, the obtained flux in MD
depends both on the membrane permeation properties as well as the flow
geometry in the membrane modules. A good flow geometry maintaining
turbulence among the fibers can minimize the undesirable temperature
polarization which leads to a lower driving force across the membrane and
consequently a lower obtained flux. Therefore, research on the flux enhancement
in MD can be divided into two large areas: (1) the fabrication of highly permeable
membranes and (2) designing optimized membrane modules.
Because of the effect of temperature polarisation or temperature drop crossing
membrane, heat transfer across the boundary layer from the bulk to the
membrane surface often limits the rate of flux transfer in MD. Thus to improve
mass transfer of flux in MD, researchers must minimise this phenomenon. An
alternative approach to the flux enhancement in MD application lies in the
modification of module design by spacers/baffles/turbulence promoters and
modified hollow fiber geometries. By modeling the transport phenomena, the
application of baffles increase the feed-side heat-transfer coefficient, which
correspond to ~20% flux enhancements. Besides, it was observed that using
spacers among the fibers may prevent the fibers from sticking together hence
efficiently increase the effective membrane area ~33%. The un-straight geometry
of the hollow fibers (braided and twisted) may act as a static mixer for the shellside
flow which can increase the associated heat-transfer coefficients and led to
flux enhancements as high as 36% without inserting an external turbulent
promoter. In overall, greater flux enhancements with modified hollow fiber
membranes modules were achieved at higher feed temperatures.