Membrane and Desalination Technologies - TCE Moodle Website

8 A.G. (Tony) Fane et al.
Conc.
water
Lower Higher
water
water
Conc.
Semipermeable
Membrane
Osmosis
Lower
Conc.
Semipermeable
Membrane
diffusion, which retains almost all ionic species. RO membranes are generally categorized
into asymmetric membranes and thin-film, composite membranes. An asymmetric RO
membrane contains one polymer material and has a thin, permselective skin layer supported
by a more porous sublayer in structure. The dense skin layer (from 0.1 to 1 mm) determines
the flux and selectivity while the porous sublayer has little effect on the membrane separation
properties. Asymmetric membranes are most commonly formed by a phase inversion (polymer
precipitation) process based on the Loeb–Sourirajan technique. Since the invention of
the asymmetric cellulose acetate RO membranes in 1962, significant improvements were
achieved in the following 20 years and cellulosic polymers (CA, cellulose triacetate, etc.) and
linear aromatic polyamide membranes are found to be the most widely used examples of
asymmetric membranes (17–19). A thin-film composite membrane is made of two or more
polymer materials and consists of a thin polymer barrier layer formed on one or more porous
support layers (different from the surface layer) in structure. The extremely thin surface layer
(in the order of 0.1 mm or less) allows high water fluxes. The most important thin-film,
composite membranes are made from cross-linked aromatic polyamide on a polymer (e.g.,
polysulfone or aryl-alkyl polyetherurea) support layer by the interfacial polymerization
method (17, 19–20). Comparison between the asymmetric membranes and the thin-film
composite membranes in terms of the characteristics which are relevant to their applications
in seawater and brackish water desalination is given in Table 1.2 (21).
A typical efficient RO process is designed to achieve higher water flux with relatively
lower energy expenditure. In addition to the membrane materials, the packaging of membranes
is also a very important factor in the feasibility of the RO process. The commercially
available membrane modules are plate-and-frame, tubular, spiral-wound and hollow fiber
elements. RO systems can be arranged either in single-pass or in multi-pass configurations
(22). A typical single-pass RO system is depicted in Fig. 1.7. Limitation in product recovery
(defined as product flow divided by feed flow) is governed by the concentration of the feed,
Osmotic
Pressure
Higher
Conc.
Equilibrium
Fig. 1.5. Osmosis and reverse osmosis.
Applied Pressure > Osmotic Pressure
water
Lower Higher
water
Conc. Conc.
water
Semipermeable
Membrane
Reverse Osmosis