Page 363 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

P1: GLQ Final Pages
Encyclopedia of Physical Science and Technology EN009K-419 July 19, 2001 20:57

298 Membranes, Synthetic, Applications

Kleefisch, Kobylinski et al., 1996; Balachandran, Dusek, For fuel cells, the assembly consists of an ion-conduc-
Maiya et al., 1997) are being considered for nonelectro- ting film sandwiched between two platinum based elec-
chemical processes such as the production of synthesis gas trodes. Hydrogen fuel is typically supplied to the anode,
frommethane.Inthiscase,asoxygenionsemergefromthe while the oxidant is supplied to the cathode. Hydrogen
downstream side of the membrane and react with methane is dissociated at the anode, catalyzed by the platinum,
to form syngas, the electrons that are released can diffuse to yield electrons and hydrogen ions. The hydrogen ions
back through the membrane to maintain electrical neutral- migrate through the proton exchange membrane while
ity. In addition, there is work to pursue methane oxidative electrons travel to the cathode through an external circuit.
coupling to produce ethylene and propylene directly from The protons and electrons react with oxygen at the
methane. Other problems that need to be resolved include cathode to produce water and heat. The driving force for
difficulties in proper sealing of the membranes as well as the reaction manifests itself in the voltage that drives the
high sensitivity of membranesto the temperaturegradients electrons through the external circuit (Singh, 1999).
that can result in membrane cracking (Bessarabov, 1999). The biggest advantages of fuel cells over conventional
Nevertheless, these are interesting and exciting additions automotive energy production is the efficiency (twice as
to the membrane spectrum. high internal combustion engines) and near zero emis-
Unlike the mixed ion conductors, solid oxides that can sions. There are, however, still a number of technical
only conduct oxygen ions and not electrons have appli- hurdles that need to be overcome before this process is
cations involving electrons flow through an external cir- commercialized; these hurdles include how the fuel may
cuit to produce power in fuel cells (Fig. 7F). Fuel cells safely be supplied and how the cost of the catalyst can be
are electrochemical devices that directly convert avail- minimized.
able chemical free energy in a fuel by oxidizing the fuel,
typically hydrogen, methanol, or some other hydrocarbon C. Strategies to Deal with Gas Separation
into electrical energy. One type of fuel cell uses oxygen- Membranes Shortcomings
conducting materials (Lin, Wang, and Han, 1994). Here
oxygen ionizes to form oxygen ions and the oxygen ions While concentration polarization and fouling are the main
diffuse through the membrane to react with a hydrocar- challenges facing membranes for liquid separations, gas
bon on the other side to form CO 2 and H 2 O. As a result, separation systems are limited more generally by lack of
electrons flow back through the external circuit to main- durability and adequate selectivity. Therefore, a generic
tain electrical neutrality, thus providing electrical power. technical challenge typical of most potential applications
To provide adequate oxygen fluxes, high temperatures are of gas separation membranes includes finding ways to
◦
required (>650 C). achievehigherpermselectivitywithatleastequivalentpro-
A second type of fuel cell is based on the proton- ductivity. Maintaining these properties in the presence of
exchange membranes described below (Heitner-Wirguin, complex and aggressive feeds is the second challenge that
1996). Unlike the solid oxide membranes, proton ex- must be balanced against cost in all cases. The relative
change membranes offer the opportunity to operate at importance of each of these requirements varies with the
lower temperatures than the solid oxides. Proton exchange application. Of these requirements, selectivity (or sepa-
membranes (Fig. 7E) are the mirror image of the oxygen ration efficiency) and permeation rate (or productivity)
ion conducting solid oxide membranes described earlier are clearly the most basic. The higher the selectivity, the
(not the MIEC), since they only conduct protons and not more efficient the process, the lower the driving force
electrons. These can be polymeric or inorganic, and the (pressure ratio) required to achieve a given separation,
most popular of these is Nafion, a perfluorinated sulfonic and therefore the lower the operating cost of the mem-
acid polymer. Other sulfonic acid containing materials are brane system. The higher the flux, the smaller the required
also under study. Addition of water to these sulfonated membrane area and, therefore the lower the capital cost of
polymers causes the hydrogen ions on the SO 3 H groups the membrane system.
to become mobile. It is proposed that proton conductivity The preceding discussion of gas separation membrane
in these materials is a result of two different mechanisms types illustrates the large number of options available. A
(Pivovar, Wang, and Cussler, 1999). In one mechanism the correspondingly large number of potential opportunities
protons add on to one side of a water molecule and hop off for gas separation membranes exist, but economics ulti-
the other side to a different water molecule, and so on. The mately must dictate which membrane approach, if any,
other mechanism is somewhat like the facilitated transport should be used in each application. Moreover, the key re-
mechanismdescribedearlier.Specifically,theprotoncom- quirements of durability, productivity, and separation effi-
bines with a solvent molecule to yield a complex and then ciency must be balanced against cost in all cases. The cur-
the complex diffuses through the membrane. rentspectrumofapplicationsofgasseparationmembranes