A membrane is a thin, film-like structure that separates two fluids. It acts as a selective barrier, allowing some particles or chemicals to pass through, but not others. In some cases, especially in anatomy, membrane may refer to a thin film that is primarily a separating structure rather than a selective barrier.
The concept of a membrane has been known since the eighteenth century, but was used little outside of the laboratory until the end of World War II. Drinking water supplies in Europe had been compromised by the war and membrane filters were used to test for water safety. However, due to the lack of reliability, slow operation, reduced selectivity and elevated costs, membranes were not widely exploited. The first use of membranes on a large scale was with microfiltration and ultra-filtration technologies. Since the 1980’s, these separation processes, along with electrodialysis, are employed in large plants and, today, a number of experienced companies serve the market.
A membrane is a layer of material which serves as a selective barrier between two phases and is impermeable to specific particles, molecules, or substances when exposed to the action of a driving force. Some components are allowed passage by the membrane into a permeate stream, whereas others are retained by it and accumulate in the retentate stream.
Membranes can be of various thickness, with homogeneous or heterogeneous structure. Membrane can also be classified according to their pore diameter. According to IUPAC, there are three different types of pore size classifications: microporous (dp < 2 nm), mesoporous (2 nm < dp < 50 nm) and macroporous (dp > 50 nm). Membranes can be neutral or charged, and particles transport can be active or passive. The latter can be facilitated by pressure, concentration, chemical or electrical gradients of the membrane process. Membranes can be generally classified into synthetic membranes and biological membranes.
- Less energy-intensive, since they do not require major phase changes
- Do not demand adsorbents or solvents, which may be expensive or difficult to handle
- Equipment simplicity and modularity, which facilitates the incorporation of more efficient membranes
Membranes are used with pressure as the driving processes in membrane filtration of solutes and in reverse osmosis. In dialysis and pervaporation the chemical potential along a concentration gradient is the driving force. Also pertraction as a membrane assisted extraction process relies on the gradient in chemical potential.
However, their overwhelming success in biological systems is not matched by their application. The main reasons for this are named
- Fouling -- the decrease of function with use
- Prohibitive cost per membrane area
- Lack of solvent resistant materials
- Scale up risks
- "Membranes on Polyolefins Plants Vent Recovery, Improvement Economics Program". by Intratec, ISBN 978-0615678917, Q3 2012.
- Zydney, Andrew L.; Zeman, Leos J. (1996). Microfiltration and ultrafiltration: principles and applications. New York: CRC. ISBN 0-8247-9735-3.
- Macroporous Materials Containing Three Dimensional Periodic Structures
- Mulder, Marcel (1996). Basic principles of membrane technology (2 ed.). Kluwer Academic: Springer. ISBN 0-7923-4248-8.
- Chmiel, Horst (2006). Bioprozesstechnik : Einführung in die Bioverfahrenstechnik (2nd ed.). München: Elsevier, Spektrum Akad. Verl. p. 279. ISBN 3827416078.