Membrane and Desalination Technologies - TCE Moodle Website

Desalination of Seawater by Reverse Osmosis 565
membrane helps in better combating biofouling and raising the effective plant availability in
view of the higher resistance of the spiral-wound configuration to mechanical blocking and of
the PA polymer to biodegradation. Particularly in seawater desalination, higher RO process
efficiency and recovery rates are realized by the development of the very high-rejection TFC
membranes, which are resistant to compaction at high applied pressures and enable brine
recovery in two-stage systems. Consequently, operation and project cost have decreased
significantly due to the application of TFC PA membranes.
2.3. Membrane Filtration Theory
2.3.1. Membrane Transport
Although the transport in the solution circulating in the space between the membranes is
important, it is the ion and water transport in the membranes that determine the performance
of membrane process. Theories used to characterize transport in the skin layer of membranes
have been reviewed (10–16). Mechanistic and mathematical models proposed can be divided
into three types: irreversible thermodynamics models; nonporous or homogeneous membrane
models (such as the solution-diffusion [SD], SD-imperfection, and extended SD models); and
pore models (such as the finely porous, preferential sorption capillary flow [PSCF] and
surface force-pore flow models [SFPF]). Some of these descriptions rely on relatively simple
concepts, whereas others are more complex and require sophisticated solution techniques.
Models that adequately describe the performance of RO membranes are very important
because these are needed in the design of RO processes. Models that predict separation
characteristics also minimize the number of experiments that must be performed to describe a
particular system. The transport models focus on the top thin skin of asymmetric membranes
or the top thin skin layer of composite membranes because these determine fluxes and
selectivities of most membranes (17). Also, most of the membrane models assume equilibrium
(or near equilibrium) or steady-state conditions in the membrane.
2.3.2. Irreversible Thermodynamics Models
Irreversible thermodynamics lead to the following expressions for solvent and solute flow:
Jw ¼ Lp ðDP DPÞ; (6Þ
Js ¼ Csð1 sÞJw þ DeDC: (7Þ
DP and DP are transmembrane pressure and osmotic pressure across membrane, respectively;
Cs and DC are average concentration of solute and solute concentration difference
across membrane, respectively; D is the diffusivity and e is the porosity of the membrane;
Lp is a coefficient; and s is a measure of the solute–water coupling within the membrane and
may often be treated as 1. The theory then characterizes the membrane by the parameters Lp
and De, which may be measured by experiments other than RO and then used to quantify the
performance of the membrane. A particular strength of this approach is its extension to
multicomponent systems in which it can be used to predict membrane behavior. However, it
does not elucidate the actual transfer mechanisms within the membrane.