Tangential flow filtration
The tangential flow filtration , including tangential flow , cross-flow or cross-flow filtration called, is a method for filtering liquids. It is used in the chemical , food and pharmaceutical industries . In contrast to cake filtration , which also belongs to the group of surface filtration , tangential flow filtration largely avoids the formation of a filter cake .
In nature, cross-flow filtration occurs in plankton- filtering animals, e.g. B. in fish in the gill trap , and in the removal of urinary substances from the blood via the glomerulum of the kidneys .
Diafiltration is a special form of tangential flow filtration .
principle
In cross-flow filtration, the suspension to be filtered is pumped parallel to a membrane or filter medium at a speed of about 2.5 to 3 m / s and the permeate (also called filtrate) is drawn off transversely to the direction of flow. The shear forces that occur on the filter surface due to the turbulent flow can be varied depending on the volume flow .
The high speed largely prevents a filter cake (cover layer or fouling ) from building up on the membrane from the solid particles to be separated. It would increase the filtration resistance and thus the pressure loss across the filter, which would be associated with higher expenditure on equipment and energy.
While the solids to be separated out are obtained as filter cakes with conventional filters , in cross-flow filtration the solids can only be concentrated to such an extent that the suspension can still be pumped; the filtrate is free of solids in both cases. The part of the liquid flow which does not pass through the membrane is called the retentate .
Construction methods
In contrast to static filtration techniques , tangential flow filtration is able to clarify liquids with relatively high levels of turbidity .
Hollow fibers (capillary membrane or hollow filaments) are particularly suitable for this , the performance of which is further enhanced by the pinch effect . A conventional hollow fiber has an inside diameter of approx. 1.5 mm (3.0 mm to 0.1 µm possible) and a pore size of 200 to 5 nm (2000 nm to 1.0 nm possible). Depending on the application, hundreds to thousands of capillaries are combined in modules and potted (hollow fiber modules). With the help of a circulation pump, the unfiltered product is circulated through the capillaries until the sediment in the retentate is so concentrated that emptying and cleaning is necessary.
Areas of application
Preferred areas of application are areas in which a high volume of particles or large concentration gradients must be expected. The permanent outflow of the retentate results in an equilibrium value which can be maintained for a long time if the concentration is not continued.
Tangential flow filtration is of great importance in the
- Beverage filtration and
- Dialysis ,
but also in all other areas of membrane technology:
economics
Characteristic features of tangential flow filtration are the extensive elimination of filter aids , i. H. their procurement, storage, handling and disposal, as well as the rapid, labor-intensive and quality-saving processing.
However, the tangential flow filtration has been compared with other filtration techniques, a low energy efficiency , a large part of the promotion of the feed ( Feed invested) energy is lost through namely the retentate. That is why more and more use is made of dead-end filtration in places where tangential flow filtration can be dispensed with.
literature
- Munir Cheryan: Handbook of Ultrafiltration . B. Behr's Verlag GmbH & Co, 1990, ISBN 3-925673-87-3 .
- Rautenbach, Robert: Membrane process basics of module and system design . Springer-Verlag, 1997, ISBN 3-540-61573-3 .
- Staude, Eberhard: Membranes and membrane processes . VCH Verlagsgesellschaft mbH, 1992, ISBN 3-527-28041-3 .