(443a) Phase Selective Membrane Based Separators for Portable Direct Methanol Fuel Cell Systems

Direct methanol fuel cell (DMFC) systems convert
methanol into electricity at high thermodynamic efficiencies. They provide high
energy density and the possibility for an instant refill of methanol. To enable
an autonomous operation of a DMFC system, the gas has to be separated from the
gas/liquid stream at both, anode and cathode exit. In the same way, the
methanol solution of the anode loop has to be recycled and the product water of
the cathode has to be recovered to compensate for the water losses in the
anodic loop [1]. In the macroscopic domain, passive separation systems often
use gravity to separate different phases of fluids. Such separation systems
provide excellent separation properties as long as they are not subjected to
mobile or portable requirements. In the domain of small energy systems, such as
portable DMFC systems for off-grid power supply, orientation dependent
separation units cannot be applied. New perspectives and approaches are
provided by the domain of microfluidic systems because the surface forces
occurring in such systems dominate over gravity forces. Capillary pressure may
be used to hold back or extract a certain phase.

This contribution gives an insight into the removal of
gases or liquids from gas/liquid streams by the combination of milli/micro structured channels and micro structured membranes
for DMFC systems (figure 1). To achieve a complete separation of gases from a
gas/liquid stream, a combination of hydrophilic walls of the supporting channel
and a hydrophobic surface of the membrane is used. For the removal of liquid
from a gas/liquid stream, a hydrophilic membrane and a hydrophobic main channel
is applied. The aim of this work is to determine the limits of multi-phase
separation by gas permeation and by liquid permeation using micro structured
membranes and supporting channels. For systems that are open to the environment,
the influence of environmental parameters like temperature and relative
humidity is studied in experiment and model. The effect of system parameters
like pressure, temperature, volume flow rates and orientation of the device on the
separation and the recycling efficiency is investigated experimentally for both
kinds of separation units. It is shown experimentally that, compared to other
parameters, the orientation of the separation unit has only a minor effect on
the separation and the recycling efficiency. Model based analysis gives a
further insight into geometry based restrictions for the separation and
recycling efficiency and clarifies the observed experimental results and
effects.

Figure  SEQ Figure \* ARABIC 1: Position and design of phase selective separators in a portable DMFC system

References:

[1]  
Zenith, F., Krewer, U.,
2010. Modelling, dynamics and control of a portable

DMFC system. Journal of Process Control 20 (5),
630?642.