1 Applications 2 Mass transfer
2.1 Solution-diffusion model 2.2 Hydrodynamic model
3 Membrane operations 4 Membrane shapes and flow geometries 5 Membrane performance and governing equations 6 Membrane separation processes 7 Pore size and selectivity 8 See also 9 Notes 10 References
Venous-arterial extracorporeal membrane oxygenation scheme
Membrane separation processes operate without heating and therefore use less energy than conventional thermal separation processes such as distillation, sublimation or crystallization. The separation process is purely physical and both fractions (permeate and retentate) can be used. Cold separation using membrane technology is widely used in the food technology, biotechnology and pharmaceutical 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 liquids or solutes which form isomorphic 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 (worldwide approximately 7 million cubic metres annually), filtrations in the food industry, the recovery of organic vapours such as petro-chemical vapour recovery and the electrolysis for chlorine production. In waste water treatment, membrane technology is becoming increasingly important. With the help of ultra/microfiltration 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 and as artificial lung for bubble-free supply of oxygen in the blood. 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 fuel cells 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.
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
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).
Pressure driven operations
microfiltration ultrafiltration nanofiltration reverse osmosis
Concentration driven operations
dialysis pervaporation forward osmosis artificial lung gas separation
Operations in an electric potential gradient
electrodialysis membrane electrolysis e.g. chloralkali process electrodeionization electrofiltration fuel cell
Operations in a temperature gradient
Membrane shapes and flow geometries
There are two main flow configurations of membrane processes: cross-flow (or) tangential 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. Generally, dead-end filtration is used for feasibility studies on a laboratory scale. 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 batch-type process, where the filtering solution is loaded (or slowly fed) into the 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 higher driving forces. Membrane fouling and particle retention in a feed solution also builds up a concentration gradients and particle back flow (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 sheets/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 the 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 dense skin separation layers, and a 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.
Spiral wound membrane module
Separation of air into oxygen and nitrogen through a membrane
Disc tube module is using a cross-flow geometry, and consists of a pressure tube and hydraulic discs, which are held by a central tension rod, and membrane cushions that lie between two discs. 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 selectivity (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:
= Q =
displaystyle frac dV_ p dt =Q= frac Delta p mu Aleft( frac 1 R_ m +R right)
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 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:
displaystyle S= frac C_ p C_ f
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:
displaystyle L_ p = frac J Delta p
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
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, 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.
Ranges of membrane based separations
Pore size and selectivity
The pore distribution of a fictitious ultrafiltration membrane with the nominal pore size and the D90
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, with units in 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. Filter membranes are divided into four classes according to pore size:
Pore size Molecular mass Process Filtration Removal of
> 0.1 µm > 5000 kDa microfiltration < 2 bar larger bacteria, yeast, particles
100-2 nm 5-5000 kDa ultrafiltration 1-10 bar bacteria, macromolecules, proteins, larger viruses
2-1 nm 0.1-5 kDa nanofiltration 3-20 bar viruses, 2- valent ions
< 1 nm < 100 Da reverse osmosis 10-80 bar salts, small organic molecules
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 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.
Nominal pore size micro-organism ATCC root number
0.1 µm Acholeplasma laidlawii 23206
0.45 µm Serratia marcescens 14756
0.65 µm Lactobacillus brevis
To determine the pore diameter, 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 cylindrical or concatenated 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 (uranium enrichment) in nuclear engineering or industrial gases like nitrogen can be recovered (gas separation). Ideally, even 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. See also
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