Membrane and Desalination Technologies - TCE Moodle Website

16 A.G. (Tony) Fane et al.
membranes made by Loeb and Sourirajan in 1962. For GS purposes, this membrane invention
was modified while maintaining the integrity between the thin dense selective layer and the
porous layer. The asymmetric structure was first made into flat sheets and eventually
developed into hollow fibers (51). The Loeb-Sourirajan technique not only laid a foundation
for the development of RO, NF, UF, MF membranes for liquid separation, but also paved the
way for the commercialization of GS membranes.
In the 1980s, membrane technology in GS made a large stride forward. The revolutionary
composite membranes were invented by Henis and Tripodi who coated a thin layer of
silicone-rubber on the asymmetric substrate to achieve high hydrogen/carbon monoxide
selectivity with a low cost module system (52). This major breakthrough led to the success
of the first large-scale membrane GS system, Prism 1 , which was installed by the Monsanto
Co. in 1980 for hydrogen recovery from the purge gas in an ammonia plant (50). Monsanto’s
success has stimulated intensive research in novel membranes and separation processes.
Membrane engineers in Permea introduced Lewis acid:base complex dopes to fabricate
asymmetric hollow fibers with a graded density skin (53, 54). Dow produced systems to
separate nitrogen from air, and Cynara and Separex developed systems to separate carbon
dioxide from natural gas. In 1990, Pinnau and Koros invented defect-free ultra-thin asymmetric
membranes by a dry/wet phase inversion method, which enhanced gas permeation
through the membrane tremendously to meet the need for large-scale industrial applications
(55). Later intensive studies mainly focused on improving the fabrication of high
performance asymmetric hollow fibers. One of the examples is that of Chung et al. who
had successfully modified Permea’s technology to produce 6FDA-polyimide membranes for
air separation (56).
A simple GS membrane process setup is not significantly different from that used for
solid–liquid separations except for the different phases in the streams. A gas mixture is fed
into the membrane module at high pressure. One component diffuses faster through the
membrane and is enriched in the permeate stream, the rest of the gas is concentrated in
the residue stream. For a more complete separation, recycling of some of the permeate or
residue stream may be required.
2.7. Pervaporation
PV is a relatively new membrane process used for the separation of liquid mixtures. In PV,
a liquid mixture is brought into contact with one side of a membrane and the permeate is
removed as low pressure vapor from the other side (Fig. 1.9). Transport through the
membrane is induced by the vapor pressure difference between the feed solution and
the permeate vapor (57). Separation is achieved based on the sorption and diffusion differences
between the feed components, which are mainly controlled by the complex interactions
between the feed components, the membrane materials, and the permeate. PV has certain
common elements with both RO and GS. Nevertheless, it is different from RO, UF, or MF, as
PV involves a change of the permeating species from liquid to vapor phase, and the driving
force of this process is provided by lowering the chemical potential of the permeate stream.