Introduction
1.1. Background
1.1.2. Classification of membrane separation processes
Asymmetric membranes consist of a very dense top layer or skin of thickness 0.1 – 0.5µm supported by a porous sub-layer with a thickness of about 50 -150µm. The top layer and its substructure may be formed in a single operation or separately. In composite membranes, the layers are usually made from different polymers; each layer of which can be optimized independently. The separation properties and permeation rates of the membrane are determined exclusively by the top layer; the sub-layer functions as a mechanical support.
The advantages of the higher fluxes provided by such membranes are so great that almost all commercial processes use such membranes.
There are two types of liquid membranes; supported liquid membranes and unsupported liquid membranes. Supported liquid membranes have a microporous structure which is filled with the liquid membrane phase; the microporous structure provides the mechanical strength and the liquid-filled pores the selective separation barrier. The microporous substructure should have a high porosity and a pore size small enough to support the liquid membrane phase sufficiently under hydrostatic pressure. Unsupported liquid membranes are composed of thin films of liquid stabilized by a surfactant in an emulsion type mixture. Figure 1.1 shows membrane classes according to their morphology. On the basis of mechanism of action, membranes are classified as adsorptive or diffusive, ion- exchange, osmotic or nonselective (inert) membranes [1].
purification and concentration of water-soluble solutes or water dispersible materials in industries such as pharmaceutical, chemical processing, food processing, oil industries, wastewater treatment plant, etc. Other processes, which are still in experimental stage, are yet to get into industrial use.
Membrane separation processes may be classified and categorized by a number of criteria. It is a filtration technique in which a membrane acts as a selective barrier between two phases [1]. As a result of a driving force across the membrane, components are transported towards the membrane surface, where some components pass the membrane and others are retained at the membrane surface. Membrane processes are available for numerous applications, each with its own driving force and separation characteristics. Table 1.1 provides further information on these processes, such as membrane type, method of separation and range of applications [3, 4].
Pressure driven membrane separation processes differ mainly in the pore size of their membranes, which makes a particular membrane effective for the removal of a specific range of impurities. Reverse osmosis has the smallest membrane pore size and therefore is used to remove all ionic species. Nanofiltration is sometimes referred to as ‘loose reverse osmosis’ as it can remove divalent ions and low molecular weight contaminants while allowing monovalent ions to pass through. Ultrafiltration is used for removal of macromolecules such as proteins and small colloids, but not ionic species. Microfiltration is used to remove particulates, bacteria, and other larger colloids only. Table 1.2 reports a summary of the main characteristics of the pressure driven processes in terms of pressure, pore size and removable components.
In pressure driven membrane systems the pressure of the feed solution permits passage of the major portion of the solution through a semi-permeable membrane. The portion of the feed solution that passes through the membrane is called permeate, or filtrate.
The portion of the feed solution that does not pass through the membrane is called concentrate or retentate. A simple schematic representation of a membrane process is shown in Figure 1.2.
Figure 1.2: Schematic representation of a membrane process.
Table 1.1: Typical membrane separation processes: Operating principles, driving force and applications [3, 4].
Separation process
Membrane type
Driving force
Separation mechanism
Range of application
Size range (nm) Micro
filtration
Symmetric and
asymmetric microporou s membrane
Pressure difference (ΔP)
Sieving
mechanism as a function of pore size and
adsorption
Sterile filtration clarification
100-10000
Ultra filtration
Asymmetric microporou s membrane
Pressure difference (ΔP)
Sieving mechanism
Separation of macromolecu lar solutions
10-100
Nano filtration
Asymmetric
‘skin type’
membrane
Pressure difference (ΔP)
Solution diffusion mechanism
Separation of divalent ions from
solutions
0.5-5
Retentate
Feed Permeate
Reverse osmosis
Asymmetric
‘skin type’
membrane
Pressure difference (ΔP)
Solution diffusion mechanism
Separation of salts and micro solutes from
solutions
<1
Dialysis Symmetric microporou s
Concentrat ion
difference (ΔC)
Diffusion Separation of salts and micro solutes from macro moleculr solutions
<1
Electro dialysis
Cation and anion exchange membrane
Electric potential difference (ΔE)
Selective transport of ions or molecules according to electric charge
Desalting of ionic
solutions
<1
Supported liquid membrane
Microporou s
membranes supporting adsorbed organic liquid
Concentrat ion
difference (ΔC)
Solution diffusion via carrier
Separation and
concentration of metal ions and
biological species
<1
Membrane distillation
Microporou s membrane
Temperatu re
difference (ΔT)
Vapour transport into hydrophobic membrane
Ultra pure water
concentration of solutions
1-10
Pervaporation Asymmetric membrane
Concentrat ion
difference (ΔC)
Solution diffusion mechanism
Separation of organics
<1
The UF membrane is considerably more porous i.e. its nominal pore size is larger compared to the reverse osmosis (RO) membrane. As a result, most soluble species including inorganic salts pass through the membrane with the water; but colloids, suspended solids and high molecular weight organic molecules (e.g. BSA) do not pass through the
stream. The porous nature of the UF membrane allows the process to be operated with high fluxes at relatively low pressures (e.g. 1 – 10 bars). This is possible because the osmotic pressure of colloids and high molecular weight organics is extremely low. The degree and quantity of the separation are a result of the pore size of the membrane and the molecular structure, size, shape and flexibility of the colloids and organic molecules. Pore sizes ranging from 0.001–0.01 μm allow separation from solution of molecules with a molecular weight between 500 and 3,00,000.
Table 1.2: Characteristics of the pressure driven membrane processes [1]
Membrane processes
Transmembrane pressure (bar)
Pore size (nm)
Removable components Microfiltration 1 - 2 100 - 1000 Suspended solids,
bacteria
Ultrafiltration 2 - 10 1 - 100 Macromolecules,
viruses, proteins
Nanofiltration 10 - 30 0.5 - 5 Micropollutants,
bivalent ions
Reverse osmosis 35 - 100 < 1 Monovalent ions,
hardness