Membrane and Desalination Technologies - TCE Moodle Website

52 J. Ren and R. Wang
pervaporation possess integrally asymmetric structure or composite structure. For some
commercial gas separation membranes (N2/H2, CO2/CH4, etc.), a thin layer is coated on the
top surface of the integrally asymmetric membranes to block the defects on the active skin
layer of the original membranes, which cannot contribute to the intrinsic separation properties
of the membranes. However, for other gas separations such as O2/N2, volatile organic
compounds (VOCs)/N2, pervaporation and vapor permeation, etc., pervaporation and vapor
permeation, the composite coating mainly acts as the selective layer, which contributes to the
intrinsic separation properties of the membranes, and the original microporous membranes
only provide the mechanical strength.
2.3.2. Reverse Osmosis and Nanofiltration
Generally, polymeric reverse osmosis membranes have a nonporous structure on the
membrane surface, but consist of a polymer network in which solutes can be dissolved. The
transport process is controlled by solution and diffusion, which retains almost all ionic species.
Nanofiltration membranes can be divided into porous membranes and nonporous membranes
with a swollen network structure. Though the porous and network swollen nanofiltration
membranes may have the same separation performance, their transport mechanisms are
different. For a porous membrane, uncharged solutes will be separated by a sieving mechanism,
whereas for a nonporous membrane, solution-diffusion mechanism will determine the transport
phenomena in the swollen network. Compared with reverse osmosis, nanofiltration membranes
have a high flux and relative low retention of monovalent ionic species.
Reverse osmosis membranes only permit some small molecules such as water to transport
and reject other species such as monovalent ions Na þ ,K þ et al. But nanofiltration is mainly
for the separation of bivalent ions such as Ca 2þ ,Mg 2þ , etc., or molecules with the molecular
weight in the range of 200–5,000.
Reverse osmosis and nanofiltration membranes are both pressure driving processes. The
commercial reverse osmosis and nanofiltration membranes are normally prepared using the
same materials. Currently, the thin-film composite polyamide membranes via interfacial
polymerization are the main productions of reverse osmosis and nanofiltration by using
slightly different formation conditions to produce a more or less open polymer structure.
2.3.3. Ultrafiltration and Microfiltration
Ultrafiltration and microfiltration membranes are porous structures, they have a distinct,
permanent porous network through which transport occurs. The retain species are usually
several orders of magnitude larger than that of the permeated ones. The flow in the membrane
pores can be described as Pouiseille flow. Compared with nonporous membranes, ultrafiltration
and microfiltration membranes cannot reveal intrinsic properties of the polymeric
materials and the intrinsic selectivity for the transport species.
2.3.4. Filter
Filters are usually limited to the structures that separate particulate suspensions larger than
1–10 mm. From Fig. 2.3, the dividing line between different membrane processes cannot be
distinguished precisely and they are overlapped partially sometimes.