Membrane technology

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Membrane technology covers all engineering approaches for the transport of substances between two fractions with the help of
permeable Permeability, permeable, and semipermeable may refer to: Chemistry *Semipermeable membrane, a membrane which will allow certain molecules or ions to pass through it by diffusion *Vascular permeability, the movement of fluids and molecules betwee ...

membrane Image:Schematic size.jpg, up150px, Schematic of size-based membrane exclusion A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Biological membr ...
s. In general, mechanical separation processes for separating gaseous or liquid streams use membrane technology.

# Applications

Membrane separation processes operate without heating and therefore use less energy than conventional thermal separation processes such as
distillation Distillation, or classical distillation, is the process of separating the components or substances from a liquid mixture by using selective boiling and condensation. Dry distillation is the heating of solid materials to produce gaseous prod ...
, sublimation or
crystallization Crystallization or crystallisation is the process by which a solid forms, where the atoms or molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five car ...

. The separation process is purely physical and both fractions (
permeate In physics and engineering, permeation (also called imbuing) is the penetration of a permeate (such as a liquid, gas, or vapor) through a solid. It is directly related to the concentration gradient of the permeate, a material's intrinsic permeabil ...
and retentate) can be used. Cold separation using membrane technology is widely used in the food technology,
biotechnology Biotechnology is a broad area of biology, involving the use of living systems and organisms to develop or make products. Depending on the tools and applications, it often overlaps with related scientific fields. In the late 20th and early 21st c ...
and
pharmaceutical A medication (also referred to as medicine, pharmaceutical drug, medicinal drug or simply drug) is a drug File:Aspirine macro shot.jpg, Uncoated aspirin Tablet (pharmacy), tablets, consisting of about 90% acetylsalicylic acid, along with a ...
industries. Furthermore, using membranes enables separations to take place that would be impossible using thermal separation methods. For example, it is impossible to separate the constituents of
azeotropic An azeotrope () or a constant boiling point mixture is a mixture of two or more liquids whose proportions cannot be altered or changed by simple distillation.Moore, Walter J. ''Physical Chemistry'', 3rd e Prentice-Hall 1962, pp. 140–142 This ha ...
liquids or solutes which form
isomorphic In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). I ...
crystals by distillation or recrystallization but such separations can be achieved using membrane technology. Depending on the type of membrane, the selective separation of certain individual substances or substance mixtures is possible. Important technical applications include the production of drinking water by
reverse osmosis Reverse osmosis (RO) is a water purification process that uses a partially permeable membrane to separate ions, unwanted molecules and larger particles from drinking water. In reverse osmosis, an applied pressure is used to overcome osmotic press ...

. The largest RO desalination plant is in Sorek, Israel, and has an output of a day. Other uses include filtrations in the
food industry The food industry is a complex, global network of diverse businesses that supplies most of the food consumed by the World population, world's population. The term food industries covers a series of industrial activities directed at the producti ...
, the recovery of organic vapours such as petro-chemical
vapour In physics, a vapor (American English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of varieties of the English language native to the United States. Currently, America ...
recovery and the
electrolysis In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds composed of atoms ...
for chlorine production. In
waste water Wastewater is any water Water is an Inorganic compound, inorganic, Transparency and translucency, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosph ...

treatment, membrane technology is becoming increasingly important. With the help of
ultra Ultra was the designation adopted by United Kingdom, British military intelligence in June 1941 for wartime signals intelligence obtained by breaking high-level encrypted enemy radio and teleprinter communications at the Government Communicatio ...

/
microfiltration Microfiltration is a type of filtration Filtration is a physical, biological or chemical operation that separates solid matter and fluid from a mixture with a filter medium that has a complex structure through which only the fluid can pass. So ...
it is possible to remove particles, colloids and macromolecules, so that waste-water can be disinfected in this way. This is needed if waste-water is discharged into sensitive waters especially those designated for contact water-sports and recreation. About half of the market is in medical applications such as use in artificial kidneys to remove toxic substances by
hemodialysis Hemodialysis, also spelled haemodialysis, or simply dialysis, is a process of purifying the blood of a person whose kidney The kidneys are two reddish-brown bean-shaped organs An organ is a group of tissues with similar functions. Plant ...

and as artificial lung for bubble-free supply of oxygen in the
blood Blood is a body fluid Body fluids, bodily fluids, or biofluids are liquid A liquid is a nearly incompressible fluid In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an applied shear s ...

. The importance of membrane technology is growing in the field of environmental protection (
NanoMemPro IPPC DatabaseThe NanoMemPro IPPC database focus the operations where membranes are introduced as Best Available Techniques in the industrial areas addressed by the IPPC Directive. The Integrated Pollution Prevention and Control (IPPC) Directive was adopted by th ...
). Even in modern
energy recovery Energy recovery includes any technique or method of minimizing the input of energy to an overall system by the energy transfer, exchange of energy from one sub-system of the overall system with another. The energy can be in any form in either subs ...
techniques membranes are increasingly used, for example in
fuel cell A fuel cell is an electrochemical cell An electrochemical cell is a device capable of either generating electrical energy from chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of ...

s and in osmotic power plants.

# Mass transfer

Two basic models can be distinguished for mass transfer through the membrane: *the ''solution-diffusion model'' and *the ''hydrodynamic model''. In real membranes, these two transport mechanisms certainly occur side by side, especially during ultra-filtration.

## Solution-diffusion model

In the solution-diffusion model, transport occurs only by diffusion. The component that needs to be transported must first be dissolved in the membrane. The general approach of the solution-diffusion model is to assume that the chemical potential of the feed and permeate fluids are in equilibrium with the adjacent membrane surfaces such that appropriate expressions for the chemical potential in the fluid and membrane phases can be equated at the solution-membrane interface. This principle is more important for ''dense'' membranes without natural wiktionary:pore, pores such as those used for reverse osmosis and in fuel cells. During the filtration process a boundary layer forms on the membrane. This concentration gradient is created by molecules which cannot pass through the membrane. The effect is referred as concentration polarization and, occurring during the filtration, leads to a reduced trans-membrane flow (flux). Concentration polarization is, in principle, reversible by cleaning the membrane which results in the initial flux being almost totally restored. Using a tangential flow to the membrane (cross-flow filtration) can also minimize concentration polarization.

## Hydrodynamic model

Transport through pores – in the simplest case – is done Convection, convectively. This requires the size of the pores to be smaller than the diameter of the two separate components. Membranes which function according to this principle are used mainly in micro- and ultrafiltration. They are used to separate macromolecules from solution (chemistry), solutions, colloids from a dispersion (chemistry), dispersion or remove bacteria. During this process the retained particles or molecules form a pulpy mass (filter cake) on the membrane, and this blockage of the membrane hampers the filtration. This blockage can be reduced by the use of the cross-flow method (cross-flow filtration). 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 membrane into retentate (the flowing concentrate) on the front and permeate (filtrate) on the back. The tangential flow on the front creates a shear stress that cracks the filter cake and reduces the Membrane fouling, fouling.

# Membrane operations

According to the driving force of the operation it is possible to distinguish: *Pressure driven operations **
microfiltration Microfiltration is a type of filtration Filtration is a physical, biological or chemical operation that separates solid matter and fluid from a mixture with a filter medium that has a complex structure through which only the fluid can pass. So ...
**ultrafiltration **nanofiltration **
reverse osmosis Reverse osmosis (RO) is a water purification process that uses a partially permeable membrane to separate ions, unwanted molecules and larger particles from drinking water. In reverse osmosis, an applied pressure is used to overcome osmotic press ...

**gas separation *Concentration driven operations **dialysis **pervaporation **forward osmosis **extracorporeal membrane oxygenation, artificial lung *Operations in an electric potential gradient **electrodialysis **membrane electrolysis e.g. chloralkali process **electrodeionization **electrofiltration **
fuel cell A fuel cell is an electrochemical cell An electrochemical cell is a device capable of either generating electrical energy from chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of ...

*Operations in a temperature gradient **membrane distillation

# 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 Binding selectivity, selectivity (wikt:reject, 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:Osada, Y., Nakagawa, T., ''Membrane Science and Technology'', New York: Marcel Dekker, Inc,1992. $\frac=Q=\frac\ A\left\left( \frac \right\right)$ where Vp and Q are the volume of the permeate and its volumetric Volumetric flow rate, 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 is 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 for calculation of the membrane area for a targeted separation at given conditions. The solute sieving coefficient is defined by the equation: $S=\frac$ 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: $L_p=\frac$ 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.

# Membrane separation processes

Membrane separation processes have a very important role in the separation industry. Nevertheless, they were not considered technically important until the mid-1970s. Membrane separation processes differ based on separation mechanisms and size of the separated particles. The widely used membrane processes include
microfiltration Microfiltration is a type of filtration Filtration is a physical, biological or chemical operation that separates solid matter and fluid from a mixture with a filter medium that has a complex structure through which only the fluid can pass. So ...
, ultrafiltration, nanofiltration,
reverse osmosis Reverse osmosis (RO) is a water purification process that uses a partially permeable membrane to separate ions, unwanted molecules and larger particles from drinking water. In reverse osmosis, an applied pressure is used to overcome osmotic press ...

, electrolysis, dialysis, electrodialysis, gas separation, vapor permeation, pervaporation, membrane
distillation Distillation, or classical distillation, is the process of separating the components or substances from a liquid mixture by using selective boiling and condensation. Dry distillation is the heating of solid materials to produce gaseous prod ...
, and membrane contactors.Pinnau, I., Freeman, B.D., ''Membrane Formation and Modification'', ACS, 1999. All processes except for pervaporation involve no phase change. All processes except electrodialysis are pressure driven. Microfiltration 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, the 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 a nitrogen stream) and sometimes in membrane distillation. The later process helps in the separation of azeotropic compositions reducing the costs of distillation processes.

# Pore size and selectivity

The pore sizes of technical membranes are specified differently depending on the manufacturer. One common distinction is by ''nominal pore size''. It describes the maximum pore size distribution and gives only vague information about the retention capacity of a membrane. The exclusion limit or "cut-off" of the membrane is usually specified in the form of ''NMWC'' (nominal molecular weight cut-off, or ''MWCO'', Molecular Weight Cut Off, molecular weight cut off, with units in Dalton (unit), Dalton). It is defined as the minimum molecular weight of a globular molecule that is retained to 90% by the membrane. The cut-off, depending on the method, can by converted to so-called ''D90'', which is then expressed in a metric unit. In practice the MWCO of the membrane should be at least 20% lower than the molecular weight of the molecule that is to be separated. Using track etched mica membranes Beck and Schultz demonstrated that hindered diffusion of molecules in pores can be described by the Renkin equation. Filter membranes are divided into four classes according to pore size: The form and shape of the membrane pores are highly dependent on the manufacturing process and are often difficult to specify. Therefore, for characterization, test filtrations are carried out and the pore diameter refers to the diameter of the smallest particles which could not pass through the membrane. The rejection can be determined in various ways and provides an indirect measurement of the pore size. One possibility is the filtration of macromolecules (often dextrans, dextran, polyethylene glycol or albumin), another is measurement of the cut-off by gel permeation chromatography. These methods are used mainly to measure membranes for ultrafiltration applications. Another testing method is the filtration of particles with defined size and their measurement with a particle sizer or by laser induced breakdown spectroscopy (LIBS). A vivid characterization is to measure the rejection of dextran blue or other colored molecules. The retention of bacteriophage and bacteria, the so-called "bacteriachallenge test", can also provide information about the pore size. To determine the pore diameter, physics, physical methods such as porosimetry (mercury, liquid-liquid porosimetry and Bubble Point Test) are also used, but a certain form of the pores (such as Cylinder (geometry), cylindrical or concatenated sphere, spherical holes) is assumed. Such methods are used for membranes whose pore geometry does not match the ideal, and we get "nominal" pore diameter, which characterizes the membrane, but does not necessarily reflect its actual filtration behavior and selectivity. The selectivity is highly dependent on the separation process, the composition of the membrane and its electrochemical properties in addition to the pore size. With high selectivity, isotopes can be enriched Gaseous diffusion, (uranium enrichment) in nuclear engineering or industrial gases like nitrogen can be recovered (gas separation). Ideally, even Racemic mixture, racemics can be enriched with a suitable membrane. When choosing membranes selectivity has priority over a high permeability, as low flows can easily be offset by increasing the filter surface with a modular structure. In gas phase filtration different deposition mechanisms are operative, so that particles having sizes below the pore size of the membrane can be retained as well.