Cross-Flow-Filtration

The cross-flow filtration, also known as tangential flow or cross-flow filtration, is a method for the filtration of liquids. It is used in the chemical, in the food and in the pharmaceutical industry. Cross-flow filtration is also available at planktonfiltrierenden animals, such as fish ( gill - gut: gill Reuse ).

Principle

In cross-flow filtration, the suspension to be filtered at a high speed of about 2.5 to 3 m / s in parallel is pumped to a membrane or a filter medium, and the permeate is withdrawn transverse to the flow direction. The shear forces acting on the filter surface due to the turbulent flow can be varied as a function of the volume flow. The high speed is avoided that a filter cake ( top layer or fouling) can build from the separated solid particles on the membrane. A filter cake would thus increase the filtration resistance and the pressure drop across the filter, which is associated with a higher apparatus and energy. While in ordinary filters, the deposited solids are recovered as a filter cake, the solids can be concentrated in the cross-flow filtration only to the extent that the suspension is still pumpable. The filtrate free of solids in both cases.

Construction

Unlike static filtration techniques cross- flow filtration systems are able to clarify liquids with relatively high Trubstoffgehalten. Particularly suitable are hollow fibers ( capillary or hollow fibers ), and in which performance is enhanced by the so-called pinch effect. A conventional hollow fiber has an inner diameter of about 1.5 mm (3.0 mm to 0.1 micron is possible), and a pore size of 200 to 5 nm ( 2 microns to about 1.0 nm are possible ). Depending on the application are summarized hundreds to thousands of capillaries in modules and cast ( hollow fiber modules). By means of a circulation pump, the unfiltered product is as long as circulating through the capillaries to the sediment particles are concentrated in the retentate that a draining and cleaning is required.

Areas of application

Preferred areas are areas in which to be reckoned with high particle volume or large concentration gradient. Through the permanent outflow of the retentate arises in not further concentration an equilibrium value, which can be kept for long. A major significance of the cross-flow filtration in the beverage filtration and dialysis, but also in all other areas of membrane technology:

• Microfiltration
• Ultrafiltration
• Nanofiltration
• Gas separation
• Pervaporation
• Reverse osmosis

A special form of cross-flow filtration represents the diafiltration

Economy

Characteristic features of the cross-flow filtration technology are the substantial elimination of filter aids, ie, their procurement, storage, handling and disposal, rapid, labor-extensive and the quality gentle handling.

The cross-flow filtration is very energy consuming. A great deal of money invested in the promotion of the feed energy is lost through the retentate. Therefore, at the points where it can be waived, increasingly resorted to the dead-end filtration.