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. 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 solutions , colloids from a 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.
Pressure Driven Membrane Processes: Modeling & Analysis by Sergey P. Agashichev - ebavohoton.tk
The tangential flow on the front creates a shear stress that cracks the filter cake and reduces the fouling. 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.
What is a Pressure Membrane – Pressure-Driven Membrane Definition
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.
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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. 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. 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.
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: . R m can be interpreted as a membrane resistance to the solvent water permeation.
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: . Hydraulic permeability is defined as the inverse of resistance and is represented by the equation: .
Pressure Driven Membrane Processes
The solute sieving coefficient and hydraulic permeability allow the quick assessment of the synthetic membrane performance. Membrane separation processes have a very important role in the separation industry. Nevertheless, they were not considered technically important until the mids. 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.
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All processes except electro dialysis 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 CO 2 from natural gas, separating N 2 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. 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. Anthocyanins are naturally occurring polyphenolic compounds and give many flowers, fruits and vegetable their orange, red, purple and blue colors. Besides their color attributes, anthocyanins have received much attention in recent years due to the growing evidence of their antioxidant capacity and health benefits on humans.
However, these compounds usually occur in low concentrations in mixtures of complex matrices, and therefore large-scale harvesting is needed to obtain sufficient amounts for their practical usage. Effective fractionation or separation technologies are therefore essential for the screening and production of these bioactive compounds.
Membrane fractionation is based on the molecular or particle sizes pressure-driven processes , on their charge electrically driven processes or are dependent on both size and charge.