Page 362 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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

Membranes, Synthetic, Applications 297

adsorbed molecules create a hindrance to transport of brane. The above mentioned steps then occur in reverse
smaller nonadsorbed species through the void space in the order at the downstream membrane face (Fig. 7D). Since
pores. These membranes have reasonable transport prop- the permeation process is controlled by the diffusion of
erties and can be attractive if the desired separation cannot atomic hydrogen, the flux is proportional to the differ-
be achieved by conventional methods. Pilot-scale mem- ence of the square root of pressures of hydrogen (Sievert’s
brane modules using surface selective flow for hydrogen law). Palladium alloys are often preferred, because pure
enrichment have recently been tested (Anand, Langsam, palladium tends to become brittle after repeated cycles of
Rao, and Sircar, 1997). hydrogen adsorption and desorption. These membranes
are typically used as membrane reactors, which combine
some reaction leading to generation of hydrogen along
2. “Complex” Sorption-Diffusion Membranes
with hydrogen separation in a single unit. For certain
These membranes are similar to the “simple” sorption- chemical reactions, e.g., propane dehydrogenation, nat-
diffusion membranes, but involve some additional phe- ural gas steam reforming, these membrane reactors show
nomena as well as simple penetrant dissolution and diffu- good transport properties as well as temperature resistance
sion. Two types can be identified: (i) facilitated transport (Ma, 1999). However, there are still considerable difficul-
for various gas types, and (ii) palladium and related alloys ties in preparing these membranes for economic operation
for hydrogen. on a large scale.
Facilitated transport membranes involve a reversible
complexation reaction in addition to simple penetrant dis-
3. Ion-Conducting Membranes
solution and diffusion. The penetrant sorbs into the mem-
brane and diffuses down the conventional concentration Organic polymeric and ceramic ion conducting materials
gradient, or it can react with complexation agent or car- can be used in formulating membranes for some specialty
rier agent and diffuse down a concentration gradient of a gasseparationapplication.Themostimportantoftheseare
carrier–gas complex (Fig. 7D). The later transport mech- solid oxides and proton exchange types (Fig. 7E and 7F).
anism is not accessible to other penetrants that do not The solid oxide materials are permeable to oxygen ions
react with complexation agent. Transmembrane chem- and can be further divided into two classes: mixed ionic
ical potential difference, is of course, still the driving electronic conductors and purely oxygen ion conductors.
force for permeation. These membranes are highly se- The mixed ionic electronic conductors are capable of con-
lective and can potentially achieve high permeabilities at ducting both oxygen ions and electrons. These mixed
low concentration driving force (Way and Noble, 1992; ion-conducting materials are being studied being for pro-
Cussler, 1994). These membranes are configured either as cesseswhereoxygenoroxygenionsarerequired.Theoxy-
an immobilized liquid film, a solvent swollen polymer, gen permeation process through oxygen ionic conducting
or a solid polymer film containing reactive functional membranes involves three mass transfer steps: electro-
groups. The main disadvantage of these membranes is the chemical surface reactions at the two gas-membrane in-
potential lack of stability: the membranes can dry out or terfaces and oxygen ion transport through the bulk oxide.
the carrier species can be lost. Until the issues relating to These materials are mostly oxides called perovskite and
stability are resolved, facilitated transport membranes are have the generic formula ABO 3 , where A is a large cation
unlikely to be used for large-scale gas separations. Besides with a 12-fold coordination and B is a smaller cation with
gas separations such carrier facilitated membrane can be a sixfold coordination with oxygen ions. When the ions
used in liquid separtions or ion fractionation, but similar take a mixed-valence state, the partial substitution of the
instabilities have plagued these cases as well until recently A site by other metal cations with lower valences can
(Ho, 2000). usually cause the formation of oxygen vacancies and a
Palladium-based membranes are highly selective to hy- change in the valence state of the B ions in order to main-
drogen (Ma, 1999; Wood, 1968) that can also be inter- tain charge neutrality (Ma, 1999). Oxygen ions (created by
preted in terms of a “complex sorption-diffusion” mecha- electrochemical reduction reaction on the surface) migrate
nism. In this case, permeation of hydrogen through Pd via oxygen vacancies in the bulk of the membranes and
membranes involves the dissociative adsorption of hy- then form molecular oxygen at the downstream interface
drogen onto the surface. A palladium hydride is believed by a surface oxidation reaction. These membranes have
to form with partial covalent bonds (something between exceptionally high selectivity and high fluxes compared
true chemical binding and interstitial alloys) (Glasstone, to polymeric membranes, and typically operate at high
◦
1950). This initial step is followed by the transition of temperature (700 C). Despite their expected high cost,
atomic hydrogen from the surface into the bulk of the these so-called mixed ionic electronic conductors (MIEC)
metal, followed by atomic diffusion through the mem- (Nigara, Mizusaki, and Ishigame, 1995; Balachandran,