Principles of Membrane Technology (Pressure-driven processes)
A membrane can be defined as an interface between two phases. In membrane separation processes, this interface is usually a physical barrier that is permeable to some of the species present in one of the streams. In order to carry out the process, a driving force is necessary, in this case pressure. There are in essence four over lapping pressure-driven, cross flow membrane technologies:
- Reverse Osmosis (RO)
- Nanofiltration (NF)
- Ultrafiltration (UF)
- Microfiltration (MF)
The nomenclature used in conjunction with membranes tends to vary depending on the industry and the use to which the separated streams are put. The underlined terms are preferred. Some other definitions are:
Flux: Volumetric flow of liquid through the membrane per unit area (litres/m2.hr)
SP (%): Solute Passage. The ratio of concentration of solute in the permeate stream to the retentate stream
Rejection (%): (100-SP)
π (O.P.): Osmotic Pressure
RO
This uses a tight membrane that retains most dissolved species. The resistance to flow across the membrane is mainly the osmotic pressures of the solution being processed.
NF
Similar to RO, but allows small inorganic molecules to pass through the membrane. Osmotic pressure is still the major resistance to solvent flow.
UF
Membranes are porous but surface porosity is low at <5% of the surface area. Fractionates dissolved molecules on the basis of size.
MF
Membranes are truly porous with surface porosity >50% in some cases.
