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corresponding to the template; good accessibility to the specific cavities; and a high
number and strength of interactions between complementary functional groups of the
template and the polymer [89].
Several research papers on the development of chirally imprinted membranes have
focused on different strategies. An alternative method for the production of imprinted
polymers was developed by Yoshikawa et al. [90]. They preformed noncrosslinked
polymers, which served simultaneously as functional recognition elements for the tem-
plates and as supporting matrices for the future binding sites. Imprinted membranes are
obtained by simple casting of the template-containing polymer solutions using cross-
linking polymerization reactions, avoiding the unfavourable reaction of thermal and/or
photochemical stress on templates and pre-polymerization complexes. A variety of chiral
and achiral polymers have been evaluated for alternative molecular imprinting. Examples
include blends of peptide-grafted polystyrene-polyacrylonitrile [90], cellulose acetate
[91], carboxylated polysulfones [92] and polyamides [93,94].
12.4
Integrated Membrane Processes
The possibility of integrating various membrane operations in the same production line or
in combination with conventional separation units allows, in many cases, better perform-
ance in terms of product quality, plant compactness, environmental impact and energy use.
These characteristics are fundamental to the development of a green process. In this
section, some important examples of integrated membrane processes will be presented,
beginning with water desalination.
12.4.1 Integrated Membrane Processes for Water Desalination
'Desalination' refers to processes that remove salts and other minerals from water in order
to produce fresh water suitable for human consumption or for irrigation in countries where
the availability of water is scarce.
The principal desalination technologies are divided into thermal and membrane. With
the former, it is possible to separate water from salts by evaporation and condensation,
while with the latter, as already explained in Section 12.1, a semipermeable membrane is
used. The main technologies used in thermal desalination are divided into multistage flash
(MSF) and multi-effect distillation (MED). The main membrane desalination technologies
are reverse osmosis, nanofiltration and electrodialysis.
Membrane processes are already recognized as the best technology for water
desalination, since thermal desalination techniques are about 10 times less energy efficient.
The production capacities of membrane and thermal systems for water desalination are
shared equally, with reverse osmosis dominating the membrane processes and MSF the
thermal processes. When the number of plants is taken into consideration, a different
picture is obtained: membrane-based systems are the most widely used, with their
installation representing about 80% of the worldwide total [95,96].
Saudi Arabia is theworld leader in desalination processes, with about the 26%of the global
production capacity. The USA is second with 17%, mainly achieved using membrane
processes and particularly reverse osmosis integrated with nanofiltration. The desalination
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