Membrane and Desalination Technologies - TCE Moodle Website

Membrane Technology: Past, Present and Future 17
Feed
Retentate
Vapor
Membrane
Condenser
Fig. 1.9. Schematics of pervaporation process.
Condensed
Permeate
As early as 1906, L. Kahlenberg reported a qualitative study on the separation of a mixture
of a hydrocarbon and an alcohol through a rubber membrane (58). In 1917, Kober introduced
the term “PV” in his report about the selective permeation of water from aqueous solutions of
albumin and toluene through collodion (cellulose nitrate) films (59). Between 1958 and 1962,
Binning and coworkers at American Oil established the principles of PV and proposed the
potential of this technology for the separation of a liquid-liquid mixture into a vapor mixture
using a nonporous polymeric film (60). However, the low permeation flow rate through
homogenous dense films hindered the large-scale industrial applications of PV. Commercialization
of PV systems was not economically viable until the fabrication techniques of
asymmetric membranes and composite membranes were invented. The real breakthrough
was achieved in Gesellchaft fur Trenntechnik mbH (GFT) of Germany by developing a
polyvinyl alcohol and polyacrylonitrile composite membrane to dehydrate alcohol solutions
and the first pilot plant was installed in 1982 (61), which heralded a surge of commercial
applications of PV for dehydration of alcohol and other solvents. The other important
application of PV is the removal of small amounts of organic compounds from contaminated
water. These two main applications of PV take advantage of the different polarities of water
and organic solvents to achieve easy and economical separations. In addition, the utilization
of this process were extended to other industrial fields in the following 10 years, including
pollution control, solvent recovery and organic–organic separations (62).
Compared with the traditional distillation process, where the vapor–liquid equilibrium
determines the separation characteristic, PV provides a potentially more efficient approach to
separate a given mixture. The energy consumption of PV is mainly involved in the vaporization
of the permeate stream so it is attractive when the concentration of the permeating
species is low. In general, PV systems typically use plate-and-frame modules and spiralwound
modules which consist of flat membranes made from glassy, rubbery or ionic polymers
(63). Most PV membranes are composites formed by solution-coating the selective layer
onto a microporous support. For dehydration of organic solvents, several excellent membrane
materials are available such as crosslinked poly(vinyl alcohol) coated on a microporous
polyacrylonitrile support, Chitosan and Nafion. For separating volatile organic compounds