Filtration (separation process)
The filtration (including filtration , filtering or filtering called) is a method for separation or purification of substances, mostly of a suspension or an aerosol . Filtration is one of the mechanical separation processes that are based exclusively on a physical basis.
A filtered liquid is called a filtrate . Pressure drop is the pressure drop that occurs during filtration, which is also referred to as filter resistance . With surface filtration, the filtered solids form a layer on the filter medium, the filter cake . There is no uniform term for a filtered gas, the term clean gas is often used. In membrane technology , the terms retentate (retained phase ) and permeate ( phase penetrating the filter ) are common.
The word (from Italian filtrare , “through siehen ”) originally means “to let run through felt ” (in Germanic * felt “felt”), the finest method that was used before the invention of paper and which is still used in some today Represents the state of the art.
The mixture to be separated runs through a filter which, for. B. consists of paper or textile fabric or metal , or by a container in which there is a filling of a filter material . All filter materials provide resistance to all particles in the mixture to be separated.
Contrary to popular belief, not only are particles retained that are larger than the pore size of the filter - this sieve effect is just one of many mechanisms. Other mechanisms are particle inertia , diffusion effects , electrostatics or blocking effects . Therefore, particles that are much smaller than the pore size of the filter are also deposited . Especially in the field of gas filtration, filters have a range of particle sizes called a filter gap , in which particles are only inadequately separated - on the other hand, significantly larger particles (due to inertia and blocking effect) and also significantly smaller particles (due to diffusion) are completely retained.
Depending on the filter process used, after a certain period of time the retained particles either form a layer, the filter cake , or the pores of the filter material are reduced in size by the deposition of the retained substances. After building up a sufficiently thick filter cake complete is usually separation of the particles given, but also increases the flow resistance significantly the filter to. Depending on the design of the filter, the filter cake or the absorbed solids must be removed from time to time (e.g. by shaking , backwashing or a pressure pulse against the direction of flow), or the filter must be replaced (with air filters often long before a filter cake is built up). In the filtration of liquids, the periodic interruptions of the filtration operation due to cleaning or backwashing processes are referred to as dead-end filtration ; it is necessary in many filter systems. In the tangential flow is not required, but is used for economic reasons also frequently.
Many different filter systems have been developed for industrial applications. In general, in the solid / liquid separation is distinguished in clear filtration and cut filtration . In the former, the solids are separated from the liquid and the liquid is cleaned. With the latter, solids contained in the liquid are separated off.
Filtration devices are used in a variety of ways both in households, laboratories , test facilities and in industry. A typical household application is paper vacuum cleaner filter bags. A filter device in the chemical laboratory is e.g. B. the nutsche , a porcelain or glass funnel with a flat sieve bottom, is placed on the filter paper. The liquid to be filtered is poured in and the air is pumped out of the suction bottle, which is located under the suction filter and is tightly connected to it. This significantly speeds up the filtering process because the filtrate (liquid or gas) is sucked through the filter. Another method is suction in a glass sintered crucible , a frit .
Depending on the required filtrate quality, the particle size and the volume to be cleaned, from which undissolved material is to be separated, this is done in technology with different devices. For purification of larger amounts of process water or drinking water from surface water , the undissolved components are often filtered out in several stages: the coarser components in the raw water are first computing systems separated, followed by screening machines for the removal of impurities average particle size; the final fine cleaning can be carried out using filters with filter materials , precoat filters or fine filters with membranes , as required. If, on the other hand, the liquid phase is to be separated from sludge , then filter presses are suitable.
Solid particles are often removed from gases with surface filters . In addition to surface filters, electrostatic precipitators are also used to clean large quantities of gas, such as the exhaust gases that are produced during combustion in large boilers in power plants with fossil fuels . With these, the solid particles are separated on electrostatically charged plates, so strictly speaking, they are not filters in the classic sense.
Solid-liquid separation process
The basis of the filtration of liquids are different filter designs, especially in technical applications. A basic distinction is made between static filtration and dynamic filtration . Both processes can be used either in discontinuous set operation / batch process / batch process or partially, e.g. B. in screening machines with simultaneous filter and backwashing processes, work continuously .
With static filtration, which was already used in antiquity, different pressures occur between the raw medium and the pure medium and enable the required separation effect. The pressure difference is either below atmospheric pressure ( suction filtration ) or above ( pressure filtration ).
Primarily the static filters include precoat filters , but designs are also used that have dynamic filtration properties. Precoat filters enable a particularly extensive removal of small undissolved particles by means of a filter material applied to a portafilter (e.g. made of diatomite , activated carbon or powdered ion exchangers ). In addition, with this system, depending on the precoat used, for example AOX , traces of oil or smells and flavors can be removed from a liquid by adsorption .
Sterile filtration can be carried out with both dynamic and static filtration. It is only decisive that the pore openings of the filter medium or the filter are smaller than the dimensions of the germs .
With normal high-speed filtration , the filter speed is 5–20 m / h. With long-term filtration, on the other hand, which is sometimes used in drinking water purification, the filter speed is only 0.1-0.2 m / h. The long dwell time in the area of the filter material (gravel bed) enables bacterial and chemical reactions such as e.g. B. the oxidation of dissolved iron (II) - or manganese compounds to undissolved and thus filterable iron (III) oxide hydrate .
A more modern filtration process is membrane filtration , in which dynamic filtration takes place. The basis is tangential flow filtration , also known as cross-flow filtration . With this type of filtration, the filtrate is not drawn off in the direction of the flow, but across it. With this technique it is possible to select the size range up to which the particles are filtered off. Depending on the dimensions of the solids that are to be separated, particle sizes are referred to as:
- 0.5-0.1 µm (500-100 nm) from microfiltration
- 0.1-0.01 µm (100-10 nm) from ultrafiltration
- 0.01-0.001 µm (10-1 nm) from nanofiltration
- 1.0-0.1 nm from reverse osmosis .
In the case of the latter, however, undissolved particles are no longer separated, since here the dimension of the molecules has already been reached and only truly dissolved particles are retained.
- VDI guideline 3677 sheet 1: Filtering separators - surface filters . In: Association of German Engineers (Ed.): VDI / DIN manual for keeping the air clean - Volume 6: Exhaust gas cleaning - Dust technology . July 1997.
- Heinz GO Becker, Werner Berger, Günter Domschke, Egon Fanghänel, Jürgen Faust, Mechthild Fischer, Fritjof Gentz, Karl Gewald, Reiner Gluch, Roland Mayer, Klaus Müller, Dietrich Pavel, Hermann Schmidt, Karl Schollberg, Klaus Schwetlick, Erika Seiler, Günter Zeppenfeld: Organikum. 19th edition. Johann Ambrosius Barth Verlag, 1993, ISBN 3-335-00343-8 , pp. 30-32.
- Harald Anlauf, in: Mechanical solid / liquid separation. Chemie Ingenieur Technik, 2003, vol. 75, no. 19th
- Rolf Gimbel; in: Influence of the filter grain structure on the behavior of depth filters. 1982, gwf-wasser- / abwasser, Vol. 123, Issue 5, pp. 220-228.
- L. Plaisier, in: Newer developments in powder resin precoat filtration for condensate cleaning in BWR nuclear power plants. 1991, VGB Kraftwerkstechnik, vol. 71, issue 12, pp. 1127–1129.
- filtration. ( Memento of the original from March 4, 2016 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. (PDF; 170 kB) at: tci.uni-hannover.de
- For the dimensions of the particle sizes in micro and ultrafiltration, slightly different size ranges are given depending on the detection source