operate on different energy gradients leading to separation of pure water (Schrotter
et al. 2010); specific removal of contaminants such as heavy metals (Qudais and
Moussa 2004), dyes (Karisma et al. 2017), and microorganisms; production of sterile
and safe drinking water both as point-of-use and point-of-source devices; and
splitting of salts to produce parent acid and base (Reig et al. 2016). Membrane
processes, using porous membranes, operate on physical or physicochemical mechanisms and are mostly rate-governed ambient temperature operation without phase
change. Being modular in nature capacity addition is simple, and the economics is
not sensitive to capacity.
8.2 Membranes and Membrane Configuration
Membranes can be understood as physical barriers which can selectively allow
permeation of particular species, under an appropriate gradient, when in contact
with a solution. The separation mechanism can be physical, physicochemical, or
chemical in nature independently or severally. The membrane materials can be
natural or synthetic products, organic or inorganic. The commercial membrane
processes use mostly organic membranes made up of synthetic polymer materials
and operate under pressure, concentration, or electrical potential gradient.
8.2.1 Membrane Preparation
Membranes can be in any state, solid, liquid, or gas. As of now, the gaseous
membranes are not known, while the liquid membranes are used in small scale,
high value separations (K. K. Bhatluri et al. 2014). All the commercial membranes
used otherwise are solid matrices. Depending on the mechanism of separation, the
membranes used can be porous or nonporous.
Membranes can be prepared using different techniques involving phase inversion, stretching, sintering, track etching, and electrospinning (Zare and Kargari
2018). Phase inversion technique involves the dissolution of the polymer in a
solvent and precipitating the same by the release of the solvent from the matrix.
Any of the techniques such as evaporation-induced phase separation, non-solventinduced phase separation, vapor-induced phase separation, and thermally induced
phase separation (Ulbricht 2006) can be used depending on the nature of solvent. In
evaporation-induced phase separation, the polymer is dissolved in a volatile solvent
and cast as a film. The solvent is then allowed to evaporate under controlled
conditions. The solvent is withdrawn from the cast sheet by immersing it in a
non-solvent medium in the case of non-solvent-induced phase separation. In
vapor-induced phase separation, non-solvent vapor is kept in contact with the cast
film, allowing the solvent to saturate the non-solvent present in the vapor phase and
enabling precipitation of the membrane. The thermal energy enables evaporation of
250
A. Kapoor et al.
et al. 2010); specific removal of contaminants such as heavy metals (Qudais and
Moussa 2004), dyes (Karisma et al. 2017), and microorganisms; production of sterile
and safe drinking water both as point-of-use and point-of-source devices; and
splitting of salts to produce parent acid and base (Reig et al. 2016). Membrane
processes, using porous membranes, operate on physical or physicochemical mechanisms and are mostly rate-governed ambient temperature operation without phase
change. Being modular in nature capacity addition is simple, and the economics is
not sensitive to capacity.
8.2 Membranes and Membrane Configuration
Membranes can be understood as physical barriers which can selectively allow
permeation of particular species, under an appropriate gradient, when in contact
with a solution. The separation mechanism can be physical, physicochemical, or
chemical in nature independently or severally. The membrane materials can be
natural or synthetic products, organic or inorganic. The commercial membrane
processes use mostly organic membranes made up of synthetic polymer materials
and operate under pressure, concentration, or electrical potential gradient.
8.2.1 Membrane Preparation
Membranes can be in any state, solid, liquid, or gas. As of now, the gaseous
membranes are not known, while the liquid membranes are used in small scale,
high value separations (K. K. Bhatluri et al. 2014). All the commercial membranes
used otherwise are solid matrices. Depending on the mechanism of separation, the
membranes used can be porous or nonporous.
Membranes can be prepared using different techniques involving phase inversion, stretching, sintering, track etching, and electrospinning (Zare and Kargari
2018). Phase inversion technique involves the dissolution of the polymer in a
solvent and precipitating the same by the release of the solvent from the matrix.
Any of the techniques such as evaporation-induced phase separation, non-solventinduced phase separation, vapor-induced phase separation, and thermally induced
phase separation (Ulbricht 2006) can be used depending on the nature of solvent. In
evaporation-induced phase separation, the polymer is dissolved in a volatile solvent
and cast as a film. The solvent is then allowed to evaporate under controlled
conditions. The solvent is withdrawn from the cast sheet by immersing it in a
non-solvent medium in the case of non-solvent-induced phase separation. In
vapor-induced phase separation, non-solvent vapor is kept in contact with the cast
film, allowing the solvent to saturate the non-solvent present in the vapor phase and
enabling precipitation of the membrane. The thermal energy enables evaporation of
250
A. Kapoor et al.