Gases - Fundamentals of Industrial Chemistry: Pharmaceuticals, Polymers, and Business - page 23

its isomers to separate p-xylene. In the context of this discussion, zeolites
can be used to separate carbon monoxide and hydrogen. Zeolites look like
powder, but sometimes are extruded into cylindrical pellets.
A third separation technique relies on membranes. The syn-gas feed
stream is passed over a semi-permeable membrane. A membrane is a layer
of material that selectively allows one component to permeate through.
Hydrogen selectively permeates the membrane and the retentate is enriched
in carbon monoxide [11]. There are many materials used for membranes,
including various polymers. Imagine a long plastic tube (think of a garden
hose) that is made with a polymer that selectively allows hydrogen to perme-
ate. We feed a pressurized mixture of hydrogen and carbon monoxide through
the tube. Hydrogen passes through the tube walls as the gas travels through
the tube. The gas exiting the end of the tube is enriched in carbon monoxide.
A membrane module might consist of thousands of these tubes in the form
of hollow fibers. The permeation rate through a membrane varies based on
molecule size and solubility in the membrane material. Typically, smaller,
more soluble materials permeate at a faster rate. Different plastics have been
studied for this use with the optimum ones being those that allow one gas to
selectively permeate through the plastic walls. The size of the molecule and
the solubility of the gas in the plastic are factors in permeation rates with
smaller soluble gases permeating faster. If you have ever stored a plastic
soda bottle for a long period of time, you might have noticed that the soda
loses its fizz. That is because the carbon dioxide has permeated through the
plastic.
There are many types of materials that can be used for membranes useful
for separating carbon monoxide from hydrogen. Material cost, chemical and
thermal stability, and overall durability are important factors in the selection
process. Hydrogen permeability and selectivity through the membrane are
critical. Membrane materials have been reviewed [12] and can be categorized
as metallic (pure metals or alloys), inorganics (including oxides, zeolites,
glasses, and ceramics), porous carbons, purely organic polymers, and hybrids
or composites.
Metallic membranes are often palladium alloys. They require the presence
of specific catalytic surfaces to dissociate hydrogen on the raw feed stream
side and reassociate the protons and electrons on the product side. Hydrogen
selectivity is typically very high in these systems, but poisoning (rendering
inactive) the metal surface with contaminants such as hydrogen sulfide is
a major issue. Hydrogen can also cause metal embrittlement. Ceramics are
also used as membranes. They are often based upon silica or coated silica
and the separation principle is based upon molecular size as is the case
with zeolites. Indeed, there are membranes based upon synthetic zeolites.
Carbon-based membranes have been demonstrated but some forms suffer