Membrane technology
Encyclopedia
The membrane technology covers all process engineering
measures for the transport of substances between two fractions with the help of permeable
membrane
s. That means in general mechanical separation process for separation of gaseous or liquid streams using membrane technology.
, Sublimation or crystallization
). This separation process is purely physical and, thanks to its gentle separation, the use of both fractions (permeate
and retentate) is possible. Therefore, the cold separation by membrane processes has been established particularly in the food technology
, the biotechnology
and pharmacy
industrie. Furthermore, with the help of membrane separations realizeable that with thermal processes are not possible. For example, because azeotropic
s or isomorphic
s crystallization making a separation by distillation or recrystallization
impossible. Depending on the type of membrane, the selective separation of certain individual substances or substance mixtures is possible. Important technical applications include drinking water by reverse osmosis
(worldwide approximately 7 million cubic meters annually), filtrations in the food industry
, the recovery of organic vapors such as gasoline vapor recovery and the electrolysis
for chlorine production. But also in wastewater treatment, the membrane technology is becoming increasingly important. With the help of UF and MF (Ultra-/Mikrofiltration) it is possible to remove particles, colloids and macromolecules, so that wastewater can be disinfected in this way. This is needed if wastewater is discharged into sensitive outfalls, or in swimming lakes.
About half of the market has applications in medicine. As an artificial kidney to remove toxic substances by hemodialysis
and as artificial lung
for bubble-free supply of oxygen in the blood
. Also the importance of membrane technology is growing in the field of environmental protection (NanoMemPro IPPC Database
). Even in modern energy recovery techniques membranes are increasingly used, for example in the fuel cell
or the osmotic power plant.
. The component that need to be transported must be first dissolved in the membrane. This principle is more important for dense membranes without real Pore
s such as those used for reverse osmosis and in a fuel cell. During the filtration
process is formed on the membrane a boundary layer
. This concentration gradient is created by molecule
s which can not pass through the membrane. This effect is referred as concentration polarization. It occurs during the filtration and leads to a reduced transmembrane flow (flux
). The concentration polarization is in principle reversible by cleaning the membrane and the initial flux can be almost restored. Also the use of a tangential flow to the membrane (cross-flow filtration) minimizes concentration polarization.
. This requires the size of the Pore
s to be smaller than the diameter of the to separate components. Membranes, which function according to this principle are used mainly in micro- and ultrafiltration. They are used to separate macromolecule
s from solutions, colloid
s from a dispersion
or remove bacteria. During this process the not passing particles or molecules are forming on the membrane a more or less a pulpy mass (filter cake
). This hampered by the blockage of the membrane the filtration. By the so-called cross-flow method (cross-flow filtration
) this can be reduced. Here, the liquid to be filtered flows along the front of the membrane and is separated by the pressure difference between the front and back of the fractions into retentate (the flowing concentrate) and permeate (filtrate). This creates a shear stress that cracks the filter cake and lower the formation of fouling
.
-type process, where the filtering solution is loaded (or slowly fed) into membrane device, which then allows passage of some particles subject to the driving force. The main disadvantage of a dead end filtration is the extensive membrane fouling
and concentration polarization. The fouling is usually induced faster at the higher driving forces. Membrane fouling and particle retention in a feed solution also builds up a concentration gradients and particle backflow (concentration polarization). The tangential flow devices are more cost and labor intensive, but they are less susceptible to fouling due to the sweeping effects and high shear rates of the passing flow. The most commonly used synthetic membrane devices (modules) are flat plates, spiral wounds, and hollow fibers.
Flat plates are usually constructed as circular thin flat membrane surfaces to be used in dead-end geometry modules. Spiral wounds are constructed from similar flat membranes but in a form of a “pocket” containing two membrane sheets separated by a highly porous support plate. Several such pockets are then wound around a tube to create a tangential flow geometry and to reduce membrane fouling. Hollow fiber
modules consist of an assembly of self-supporting fibers with a dense skin separation layers, and more open matrix helping to withstand pressure gradients and maintain structural integrity. The hollow fiber modules can contain up to 10,000 fibers ranging from 200 to 2500 μm in diameter; The main advantage of hollow fiber modules is very large surface area within an enclosed volume, increasing the efficiency of the separation process.
(rejection
) properties for certain particles; it has to resist fouling
and to have high mechanical stability. It also needs to be reproducible and to have low manufacturing costs. The main modeling equation for the dead-end filtration at constant pressure drop
is represented by Darcy’s law:
where Vp and Q are the volume of the permeate and its volumetric flow rate respectively (proportional to same characteristics of the feed flow), μ is dynamic viscosity of permeating fluid, A is membrane area, Rm and R are the respective resistances of membrane and growing deposit of the foulants. Rm can be interpreted as a membrane resistance to the solvent (water) permeation. This resistance is a membrane intrinsic property and expected to be fairly constant and independent of the driving force, Δp. R is related to the type of membrane foulant, its concentration in the filtering solution, and the nature of foulant-membrane interactions. Darcy’s law allows to calculate the membrane area for a targeted separation at given conditions. The solute
sieving coefficient is defined by the equation:
where Cf and Cp are the solute concentrations in feed and permeate respectively. Hydraulic permeability is defined as the inverse of resistance and is represented by the equation:
where J is the permeate flux
which is the volumetric flow rate per unit of membrane area. The solute sieving coefficient and hydraulic permeability allow the quick assessment of the synthetic membrane performance.
, ultrafiltration
, nanofiltration
, reverse osmosis
, electrolysis
, dialysis
, electrodialysis
, gas separation
, vapor permeation, pervaporation
, membrane distillation
, and membrane contactors. All processes except for pervaporation involve no phase change. All processes except (electro)dialysis are pressure driven. Microfltration and ultrafiltration is widely used in food and beverage processing (beer microfiltration, apple juice ultrafiltration), biotechnological applications and pharmaceutical industry (antibiotic
production, protein purification), water purification and wastewater treatment
, microelectronics industry, and others. Nanofiltration and reverse osmosis membranes are mainly used for water purification purposes. Dense membranes are utilized for gas separations (removal of CO2 from natural gas, separating N2 from air, organic vapor removal from air or nitrogen stream) and sometimes in membrane distillation. The later process helps in separating of azeotropic compositions reducing the costs of distillation processes.
Process engineering
Process engineering focuses on the design, operation, control, and optimization of chemical, physical, and biological processes through the aid of systematic computer-based methods...
measures for the transport of substances between two fractions with the help of permeable
Semipermeable membrane
A semipermeable membrane, also termed a selectively permeable membrane, a partially permeable membrane or a differentially permeable membrane, is a membrane that will allow certain molecules or ions to pass through it by diffusion and occasionally specialized "facilitated diffusion".The rate of...
membrane
Membrane (selective barrier)
A membrane is a layer of material which serves as a selective barrier between two phases and remains impermeable to specific particles, molecules, or substances when exposed to the action of a driving force...
s. That means in general mechanical separation process for separation of gaseous or liquid streams using membrane technology.
Applications
The particular advantage of membrane separation processes is that it operate without heating and thus are energetically usually lower than conventional thermal separation processes (distillationDistillation
Distillation is a method of separating mixtures based on differences in volatilities of components in a boiling liquid mixture. Distillation is a unit operation, or a physical separation process, and not a chemical reaction....
, Sublimation or crystallization
Crystallization
Crystallization is the process of formation of solid crystals precipitating from a solution, melt or more rarely deposited directly from a gas. Crystallization is also a chemical solid–liquid separation technique, in which mass transfer of a solute from the liquid solution to a pure solid...
). This separation process is purely physical and, thanks to its gentle separation, the use of both fractions (permeate
Permeation
Permeation, in physics and engineering, is the penetration of a permeate through a solid, and is related to a material's intrinsic permeability...
and retentate) is possible. Therefore, the cold separation by membrane processes has been established particularly in the food technology
Food technology
Food technology, is a branch of food science which deals with the actual production processes to make foods.-Early history of food technology:...
, the biotechnology
Biotechnology
Biotechnology is a field of applied biology that involves the use of living organisms and bioprocesses in engineering, technology, medicine and other fields requiring bioproducts. Biotechnology also utilizes these products for manufacturing purpose...
and pharmacy
Pharmacy
Pharmacy is the health profession that links the health sciences with the chemical sciences and it is charged with ensuring the safe and effective use of pharmaceutical drugs...
industrie. Furthermore, with the help of membrane separations realizeable that with thermal processes are not possible. For example, because azeotropic
Azeotrope
An azeotrope is a mixture of two or more liquids in such a ratio that its composition cannot be changed by simple distillation. This occurs because, when an azeotrope is boiled, the resulting vapor has the same ratio of constituents as the original mixture....
s or isomorphic
Isomorphism (crystallography)
In crystallography crystals are described as isomorphous if they are closely similar in shape. Historically crystal shape was defined by measuring the angles between crystal faces with a goniometer...
s crystallization making a separation by distillation or recrystallization
Recrystallization (chemistry)
-Chemistry:In chemistry, recrystallization is a procedure for purifying compounds. The most typical situation is that a desired "compound A" is contaminated by a small amount of "impurity B". There are various methods of purification that may be attempted , which includes recrystallization...
impossible. Depending on the type of membrane, the selective separation of certain individual substances or substance mixtures is possible. Important technical applications include drinking water by reverse osmosis
Reverse osmosis
Reverse osmosis is a membrane technical filtration method that removes many types of large molecules and ions from solutions by applying pressure to the solution when it is on one side of a selective membrane. The result is that the solute is retained on the pressurized side of the membrane and...
(worldwide approximately 7 million cubic meters annually), filtrations in the food industry
Food industry
The food production is a complex, global collective of diverse businesses that together supply much of the food energy consumed by the world population...
, the recovery of organic vapors such as gasoline vapor recovery and the electrolysis
Chloralkali process
The chloralkali process is an industrial process for the electrolysis of sodium chloride solution . Depending on the method several products beside hydrogen can be produced. If the products are separated, chlorine and sodium hydroxide are the products; by mixing, sodium hypochlorite or sodium...
for chlorine production. But also in wastewater treatment, the membrane technology is becoming increasingly important. With the help of UF and MF (Ultra-/Mikrofiltration) it is possible to remove particles, colloids and macromolecules, so that wastewater can be disinfected in this way. This is needed if wastewater is discharged into sensitive outfalls, or in swimming lakes.
About half of the market has applications in medicine. As an artificial kidney to remove toxic substances by hemodialysis
Dialysis
In medicine, dialysis is a process for removing waste and excess water from the blood, and is primarily used to provide an artificial replacement for lost kidney function in people with renal failure...
and as artificial lung
Extracorporeal membrane oxygenation
In intensive care medicine, extracorporeal membrane oxygenation is an extracorporeal technique of providing both cardiac and respiratory support oxygen to patients whose heart and lungs are so severely diseased or damaged that they can no longer serve their function...
for bubble-free supply of oxygen in the blood
Blood
Blood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....
. Also the importance of membrane technology is growing in the field of environmental protection (NanoMemPro IPPC Database
NanoMemPro IPPC Database
The NanoMemPro IPPC database focus the operations where membranes are introduced as Best Available Techniques in the industrial areas addressed by the IPPC Directive....
). Even in modern energy recovery techniques membranes are increasingly used, for example in the fuel cell
Fuel cell
A fuel cell is a device that converts the chemical energy from a fuel into electricity through a chemical reaction with oxygen or another oxidizing agent. Hydrogen is the most common fuel, but hydrocarbons such as natural gas and alcohols like methanol are sometimes used...
or the osmotic power plant.
Mass transfer
For the mass transfer at the membrane, two basic models can be distinguished: the solution-diffusion model and the hydrodynamic model. In real membranes, these two transport mechanisms certainly occur side by side, especially during the ultrafiltration.Solution-diffusion model
The transport is done only by diffusionDiffusion
Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
. The component that need to be transported must be first dissolved in the membrane. This principle is more important for dense membranes without real Pore
Pore
- Animal biology and microbiology :* Sweat pore, an anatomical structure of the skin of humans used for secretion of sweat* Canal pore, an anatomical structure that is part of the lateral line sense system of some aquatic organisms...
s such as those used for reverse osmosis and in a fuel cell. During the filtration
Filtration
Filtration is commonly the mechanical or physical operation which is used for the separation of solids from fluids by interposing a medium through which only the fluid can pass...
process is formed on the membrane a boundary layer
Boundary layer
In physics and fluid mechanics, a boundary layer is that layer of fluid in the immediate vicinity of a bounding surface where effects of viscosity of the fluid are considered in detail. In the Earth's atmosphere, the planetary boundary layer is the air layer near the ground affected by diurnal...
. This concentration gradient is created by molecule
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
s which can not pass through the membrane. This effect is referred as concentration polarization. It occurs during the filtration and leads to a reduced transmembrane flow (flux
Flux
In the various subfields of physics, there exist two common usages of the term flux, both with rigorous mathematical frameworks.* In the study of transport phenomena , flux is defined as flow per unit area, where flow is the movement of some quantity per time...
). The concentration polarization is in principle reversible by cleaning the membrane and the initial flux can be almost restored. Also the use of a tangential flow to the membrane (cross-flow filtration) minimizes concentration polarization.
Hydrodynamic model
Transport through pores - in the simplest case of pure transport convectivelyConvection
Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....
. This requires the size of the Pore
Pore
- Animal biology and microbiology :* Sweat pore, an anatomical structure of the skin of humans used for secretion of sweat* Canal pore, an anatomical structure that is part of the lateral line sense system of some aquatic organisms...
s to be smaller than the diameter of the to separate components. Membranes, which function according to this principle are used mainly in micro- and ultrafiltration. They are used to separate macromolecule
Macromolecule
A macromolecule is a very large molecule commonly created by some form of polymerization. In biochemistry, the term is applied to the four conventional biopolymers , as well as non-polymeric molecules with large molecular mass such as macrocycles...
s from solutions, colloid
Colloid
A colloid is a substance microscopically dispersed evenly throughout another substance.A colloidal system consists of two separate phases: a dispersed phase and a continuous phase . A colloidal system may be solid, liquid, or gaseous.Many familiar substances are colloids, as shown in the chart below...
s from a dispersion
Dispersion (chemistry)
A dispersion is a system in which particles are dispersed in a continuous phase of a different composition . See also emulsion. A dispersion is classified in a number of different ways, including how large the particles are in relation to the particles of the continuous phase, whether or not...
or remove bacteria. During this process the not passing particles or molecules are forming on the membrane a more or less a pulpy mass (filter cake
Filter cake
A filter cake is formed by the substances that are retained on a filter. The filter cake grows in the course of filtration, becomes "thicker" as particulate matter is being retained. With increasing layer thickness the flow resistance of the filter cake increases. After a certain time of use the...
). This hampered by the blockage of the membrane the filtration. By the so-called cross-flow method (cross-flow filtration
Cross-flow filtration
In chemical engineering, biochemical engineering and protein purification, crossflow filtration is a type of filtration...
) this can be reduced. Here, the liquid to be filtered flows along the front of the membrane and is separated by the pressure difference between the front and back of the fractions into retentate (the flowing concentrate) and permeate (filtrate). This creates a shear stress that cracks the filter cake and lower the formation of fouling
Membrane fouling
Membrane fouling is a process where solute or particles deposit onto a membrane surface or into membrane pores in a way that degrades the membrane's performance. It is a major obstacle to the widespread use of this technology. Membrane fouling can cause severe flux decline and affect the quality of...
.
Membrane operationes
According to driving force of the operation it is possible to distinguish:- pressure driven operations
- microfiltrationMicrofiltrationMicrofiltration is a membrane technical filtration process which removes contaminants from a fluid by passage through a microporous membrane. A typical microfiltration membrane pore size range is 0.1 to 10 micrometres...
- ultrafiltrationUltrafiltrationUltrafiltration is a variety of membrane filtration in which hydrostatic pressure forces a liquid against a semipermeable membrane. Suspended solids and solutes of high molecular weight are retained, while water and low molecular weight solutes pass through the membrane...
- nanofiltrationNanofiltrationNanofiltration is a relatively recent membrane filtration process used most often with low total dissolved solids water such as surface water and fresh groundwater, with the purpose of softening and removal of disinfection by-product precursors such as natural organic matter and synthetic organic...
- reverse osmosisReverse osmosisReverse osmosis is a membrane technical filtration method that removes many types of large molecules and ions from solutions by applying pressure to the solution when it is on one side of a selective membrane. The result is that the solute is retained on the pressurized side of the membrane and...
- gas separationGas separationGas mixtures can be effectively separated by synthetic membranes. For other methods see adsorption, absorption, cryogenic distillation.Membranes are employed in:* separation of nitrogen or oxygen out of air...
- pervaporationPervaporationPervaporation is a membrane technical method for the separation of mixtures of liquids by partial vaporization through a non-porous or porous membrane.-Theory:...
- microfiltration
- concentration driven operations
- dialysisDialysisIn medicine, dialysis is a process for removing waste and excess water from the blood, and is primarily used to provide an artificial replacement for lost kidney function in people with renal failure...
- osmosisOsmosisOsmosis is the movement of solvent molecules through a selectively permeable membrane into a region of higher solute concentration, aiming to equalize the solute concentrations on the two sides...
- forward osmosisForward osmosisForward osmosis is an osmotic process that, like reverse osmosis, uses a semi-permeable membrane to effect separation of water from dissolved solutes...
- dialysis
- operations in electric potential gradient
- electrodialysisElectrodialysisElectrodialysis is used to transport salt ions from one solution through ion-exchange membranes to another solution under the influence of an applied electric potential difference. This is done in a configuration called an electrodialysis cell...
- membrane electrolysis
- electrophoresisElectrophoresisElectrophoresis, also called cataphoresis, is the motion of dispersed particles relative to a fluid under the influence of a spatially uniform electric field. This electrokinetic phenomenon was observed for the first time in 1807 by Reuss , who noticed that the application of a constant electric...
- electrodialysis
- operations in temperature gradient
- membrane distillation
Membrane shapes and flow geometries
There are two main flow configurations of membrane processes: cross-flow and dead-end filtrations. In cross-flow filtration the feed flow is tangential to the surface of membrane, retentate is removed from the same side further downstream, whereas the permeate flow is tracked on the other side. In dead-end filtration the direction of the fluid flow is normal to the membrane surface. Both flow geometries offer some advantages and disadvantages. The dead-end membranes are relatively easy to fabricate which reduces the cost of the separation process. The dead-end membrane separation process is easy to implement and the process is usually cheaper than cross-flow membrane filtration. The dead-end filtration process is usually a batchBatch
Batch may refer to:Food and drink*Batch , an alcoholic fruit beverage*Batch loaf, a type of bread popular in Ireland*A dialect term for a bread roll used in Nuneaton and Coventry, England*Small batch, bourbon whiskey blended from selected barrels...
-type process, where the filtering solution is loaded (or slowly fed) into membrane device, which then allows passage of some particles subject to the driving force. The main disadvantage of a dead end filtration is the extensive membrane fouling
Fouling
Fouling refers to the accumulation of unwanted material on solid surfaces, most often in an aquatic environment. The fouling material can consist of either living organisms or a non-living substance...
and concentration polarization. The fouling is usually induced faster at the higher driving forces. Membrane fouling and particle retention in a feed solution also builds up a concentration gradients and particle backflow (concentration polarization). The tangential flow devices are more cost and labor intensive, but they are less susceptible to fouling due to the sweeping effects and high shear rates of the passing flow. The most commonly used synthetic membrane devices (modules) are flat plates, spiral wounds, and hollow fibers.
Flat plates are usually constructed as circular thin flat membrane surfaces to be used in dead-end geometry modules. Spiral wounds are constructed from similar flat membranes but in a form of a “pocket” containing two membrane sheets separated by a highly porous support plate. Several such pockets are then wound around a tube to create a tangential flow geometry and to reduce membrane fouling. Hollow fiber
Fiber
Fiber is a class of materials that are continuous filaments or are in discrete elongated pieces, similar to lengths of thread.They are very important in the biology of both plants and animals, for holding tissues together....
modules consist of an assembly of self-supporting fibers with a dense skin separation layers, and more open matrix helping to withstand pressure gradients and maintain structural integrity. The hollow fiber modules can contain up to 10,000 fibers ranging from 200 to 2500 μm in diameter; The main advantage of hollow fiber modules is very large surface area within an enclosed volume, increasing the efficiency of the separation process.
Membrane performance and governing equations
The selection of synthetic membranes for a targeted separation process is usually based on few requirements. Membranes have to provide enough mass transfer area to process large amounts of feed stream. The selected membrane has to have high selectivitySelectivity
Selectivity may refer to:* Selectivity , in radio transmission* Binding selectivity, in pharmacology* Functional selectivity, in pharmacology* Socioemotional selectivity theory, in social psychology...
(rejection
Rejection
The word "rejection" was first used in 1415. The original meaning was "to throw" or "to throw back".Rejection may mean:* Social rejection, in psychology, an interpersonal situation that occurs when a person or group of people exclude an individual from a social relationship* Transplant rejection,...
) properties for certain particles; it has to resist fouling
Fouling
Fouling refers to the accumulation of unwanted material on solid surfaces, most often in an aquatic environment. The fouling material can consist of either living organisms or a non-living substance...
and to have high mechanical stability. It also needs to be reproducible and to have low manufacturing costs. The main modeling equation for the dead-end filtration at constant pressure drop
Pressure drop
Pressure drop is a term used to describe the decrease in pressure from one point in a pipe or tube to another point downstream. "Pressure drop" is the result of frictional forces on the fluid as it flows through the tube. The frictional forces are caused by a resistance to flow. The main...
is represented by Darcy’s law:
where Vp and Q are the volume of the permeate and its volumetric flow rate respectively (proportional to same characteristics of the feed flow), μ is dynamic viscosity of permeating fluid, A is membrane area, Rm and R are the respective resistances of membrane and growing deposit of the foulants. Rm can be interpreted as a membrane resistance to the solvent (water) permeation. This resistance is a membrane intrinsic property and expected to be fairly constant and independent of the driving force, Δp. R is related to the type of membrane foulant, its concentration in the filtering solution, and the nature of foulant-membrane interactions. Darcy’s law allows to calculate the membrane area for a targeted separation at given conditions. The solute
Solution
In chemistry, a solution is a homogeneous mixture composed of only one phase. In such a mixture, a solute is dissolved in another substance, known as a solvent. The solvent does the dissolving.- Types of solutions :...
sieving coefficient is defined by the equation:
where Cf and Cp are the solute concentrations in feed and permeate respectively. Hydraulic permeability is defined as the inverse of resistance and is represented by the equation:
where J is the permeate flux
Flux
In the various subfields of physics, there exist two common usages of the term flux, both with rigorous mathematical frameworks.* In the study of transport phenomena , flux is defined as flow per unit area, where flow is the movement of some quantity per time...
which is the volumetric flow rate per unit of membrane area. The solute sieving coefficient and hydraulic permeability allow the quick assessment of the synthetic membrane performance.
Membrane separation processes
Membrane separation processes have very important role in separation industry. Nevertheless, they were not considered technically important until mid-1970. Membrane separation processes differ based on separation mechanisms and size of the separated particles. The widely used membrane processes include microfiltrationMicrofiltration
Microfiltration is a membrane technical filtration process which removes contaminants from a fluid by passage through a microporous membrane. A typical microfiltration membrane pore size range is 0.1 to 10 micrometres...
, ultrafiltration
Ultrafiltration
Ultrafiltration is a variety of membrane filtration in which hydrostatic pressure forces a liquid against a semipermeable membrane. Suspended solids and solutes of high molecular weight are retained, while water and low molecular weight solutes pass through the membrane...
, nanofiltration
Nanofiltration
Nanofiltration is a relatively recent membrane filtration process used most often with low total dissolved solids water such as surface water and fresh groundwater, with the purpose of softening and removal of disinfection by-product precursors such as natural organic matter and synthetic organic...
, reverse osmosis
Reverse osmosis
Reverse osmosis is a membrane technical filtration method that removes many types of large molecules and ions from solutions by applying pressure to the solution when it is on one side of a selective membrane. The result is that the solute is retained on the pressurized side of the membrane and...
, electrolysis
Electrolysis
In chemistry and manufacturing, electrolysis is a method of using a direct electric current to drive an otherwise non-spontaneous chemical reaction...
, dialysis
Dialysis
In medicine, dialysis is a process for removing waste and excess water from the blood, and is primarily used to provide an artificial replacement for lost kidney function in people with renal failure...
, electrodialysis
Electrodialysis
Electrodialysis is used to transport salt ions from one solution through ion-exchange membranes to another solution under the influence of an applied electric potential difference. This is done in a configuration called an electrodialysis cell...
, gas separation
Gas separation
Gas mixtures can be effectively separated by synthetic membranes. For other methods see adsorption, absorption, cryogenic distillation.Membranes are employed in:* separation of nitrogen or oxygen out of air...
, vapor permeation, pervaporation
Pervaporation
Pervaporation is a membrane technical method for the separation of mixtures of liquids by partial vaporization through a non-porous or porous membrane.-Theory:...
, membrane distillation
Distillation
Distillation is a method of separating mixtures based on differences in volatilities of components in a boiling liquid mixture. Distillation is a unit operation, or a physical separation process, and not a chemical reaction....
, and membrane contactors. All processes except for pervaporation involve no phase change. All processes except (electro)dialysis are pressure driven. Microfltration and ultrafiltration is widely used in food and beverage processing (beer microfiltration, apple juice ultrafiltration), biotechnological applications and pharmaceutical industry (antibiotic
Antibiotic
An antibacterial is a compound or substance that kills or slows down the growth of bacteria.The term is often used synonymously with the term antibiotic; today, however, with increased knowledge of the causative agents of various infectious diseases, antibiotic has come to denote a broader range of...
production, protein purification), water purification and wastewater treatment
Wastewater Treatment
Wastewater treatment may refer to:* Sewage treatment* Industrial wastewater treatment...
, microelectronics industry, and others. Nanofiltration and reverse osmosis membranes are mainly used for water purification purposes. Dense membranes are utilized for gas separations (removal of CO2 from natural gas, separating N2 from air, organic vapor removal from air or nitrogen stream) and sometimes in membrane distillation. The later process helps in separating of azeotropic compositions reducing the costs of distillation processes.