Siloxanes

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Siloxanes
Disiloxane.png

Disiloxane

Hexamethyldisiloxane.svg

Hexamethyldisiloxane ,
a di-siloxane

Octamethyltrisiloxane Structural Formula V.1.svg

Octamethyltrisiloxane ,
a tri-siloxane

Structure of decamethylcyclopentasiloxane

Decamethylcyclopentasiloxane ,
a cyclic siloxane

Siloxanes are chemical compounds with the general formula R 3 Si− [O − SiR 2 ] n −O − SiR 3 , where R can be hydrogen atoms or alkyl groups . In contrast to the silanes , the silicon atoms are not linked to one another, but to their neighboring silicon atom by exactly one oxygen atom : Si – O – Si. Siloxanes with R = CH 3 are called polydimethylsiloxanes . Oligomeric or polymeric organosiloxanes (siloxanes with R ≠ H) are called silicones . They have long Si − O main chains and, depending on their molar mass and network, are viscous or permanently elastic. If they are liquid, they are also called silicone oils .

use

Siloxanes, mostly as polydimethylsiloxanes , are used in cleaning agents, cosmetics , deodorants , soaps and detergents , among other things . The hydrophobic properties are used in the area of ​​building waterproofing . In industry, siloxanes ( silicone oils ) are used as defoamers , but also as coolants in freeze-drying .

High molecular weight siloxanes, the so-called silicones, are particularly well known and important .

Industrial manufacture

Siloxanes can be made in a number of ways. The authors of the work Winnacker-Küchler: Chemische Technik list the following methods as the most important manufacturing methods:

Siloxanes in sewage and landfill gas

Siloxanes can be found in low concentrations (in the range of 10 mg silicon per standard cubic meter) in sewage or landfill gas . In the case of sewage gas, the siloxane is introduced through municipal or industrial wastewater. In landfills, it is more likely that silicon-containing waste will break down into volatile siloxanes.

Siloxanes become a problem in the incineration of sewage and landfill gas, as solid silicon dioxide (sand) is produced from the raw gas (e.g. in the CHP ) , which leads to the wear and tear of the moving parts of the systems. White-gray deposits settle on machine parts and are transported via the oil to all moving parts of the machine, where they lead to abrasion. Affected parts are v. a. Valves , cylinder heads , turbine blades , pipelines .

Before they are burned in gas engines , siloxanes are usually at least partially removed by adsorption on activated carbon , absorption or low-temperature cooling (typically at temperatures below −25 ° C). In addition to these techniques, there are also other methods of siloxane separation, including adsorption using silica gel , aluminum oxide or catalytic materials, biofilters and gas permeation . A typical limit value for silicon for most gas engine manufacturers is 5 mg silicon per standard cubic meter of methane.

Activated carbon adsorption

Activated carbon reduces the siloxane concentration in the raw gas quite completely. Values ​​below 0.1 mg silicon per standard cubic meter are typical. It also removes most of the other volatile hydrocarbons such as BTEX (which is not essential). In fact, the use of activated carbon with landfill gas leads to a relatively quick exhaustion of the loading capacity, since not only siloxanes but also a large number of volatile hydrocarbons are adsorbed. Therefore, activated carbon adsorption is often preceded by a drying step (e.g. condensation at 5 ° C), which reduces the relative humidity after the gas has been reheated and is intended to remove a large part of the hydrophilic trace substances in the raw gas beforehand. As a rule, the activated carbon is not regenerated, the used activated carbon is replaced with a fresh batch. The main cost point of an activated carbon-based siloxane cleaning is v. a. the necessary replacement of the activated carbon. However, commercial adsorption systems are also available that regenerate the adsorbent.

absorption

A wide range of different scrubbing liquids have been studied to either physically or chemically absorb siloxanes from the raw gas stream. Chemical absorption (i.e. the destruction of the siloxane molecule) takes place in principle at low or high pH values. Since basic washing liquids in connection with the carbon dioxide in the biogas lead to the formation of carbonate, only acids can be used as absorbents. In addition to the strength of the acid, an elevated temperature is also conducive to siloxane absorption. Dealing with hot acids is possible, but poses a certain safety challenge. Physical siloxane absorption was tested primarily with water, organic solvents and mineral oil. Siloxanes are i. d. Usually hydrophobic, so that water adsorption (pH 7) does not cause any significant depletion. The use of the acidic sump water of the absorber as washing liquid can, however, be useful. Experience with the use of mineral oil shows a relatively poor cleaning performance and resulted in problems with oil vapor introduced into the gas engine.

Low temperature cooling

The effectiveness of the cleaning performance of cryogenic cooling depends on the cooling temperature. The higher the siloxane load in the raw gas, the better the relative silicon deposition at the selected cooling temperature. Above all, the highly volatile hexamethyldisiloxane (L2), which is present in higher concentrations, especially in landfill gases, cannot, however, condense out significantly even at temperatures around −40 ° C. Sewage gas, on the other hand, contains significantly higher proportions of D4 and D5. In order to reduce the total silicon concentration, cryogenic cooling is therefore more effective with sewage gas than with landfill gas. The lowering of temperature is also associated with the condensation of many other substances (above all water). Since the resulting acidic condensate comes into contact with the raw gas, a certain absorption of the siloxanes into the condensate can also be assumed.

Side effects of siloxanes

Wastewater: Siloxanes are undesirable in membrane filtration , as they become embedded in the pores of the membrane and lead to fouling , which can only be removed to a limited extent by backwashing and the use of chemicals.

Electromechanical contacts ( switches , buttons , relays ) that are under increased stress can be glazed by siloxanes. The arc breaks down the molecules of the siloxanes above 1000 ° C in such a way that SiO 2 is formed and the contact is isolated. In this type of failure, the contamination of the environment also plays a role, since, for example, siloxanes bound in silicone oils reach migration speeds of 30… 50 mm per 1000 h, with the mobility decreasing with longer molecules. Therefore, cleaning and care products that contain siloxanes can have an indirect effect on nearby contacts and cause failure.

designation

M group: (CH 3 ) 3 SiO 0.5 , D group: (CH 3 ) 2 SiO, T group: (CH 3 ) SiO 1.5

Cyclic siloxanes Linear siloxanes
D 3 : hexamethylcyclotrisiloxane MM or L 2 : hexamethyldisiloxane
D 4 : octamethylcyclotetrasiloxane MDM or L 3 : octamethyltrisiloxane
D 5 : decamethylcyclopentasiloxane MD 2 M or L 4 : decamethyltetrasiloxane
D 6 : dodecamethylcyclohexasiloxane MD n M or PDMS: polydimethylsiloxane

literature

See also

Web links

Individual evidence

  1. ↑ Entry on siloxanes . In: IUPAC Compendium of Chemical Terminology (the “Gold Book”) . doi : 10.1351 / goldbook.S05671 .
  2. ↑ Entry on silicones . In: IUPAC Compendium of Chemical Terminology (the “Gold Book”) . doi : 10.1351 / goldbook.S05670 .
  3. ↑ Jürgen Weber; Volker Hafkesbrink (Ed.): Building sealing in the renovation of old buildings - procedure and legal approach . 4th edition. Springer Fachmedien, Wiesbaden 2016, ISBN 978-3-658-07843-0 , 10.2 "Injection substances and their mode of action", section "Siloxanes - silicone microemulsions" .
  4. ↑ W. Kaiser, R. Riedle: Silicones . In: H. Harnisch, R. Steiner, K. Winnacker (eds.): Winnacker-Küchler: Chemische Technologie , Organische Technologie I, I, 4th edition, Vol. 6. Carl Hanser Verlag, Munich 1982, p. 830– 834
  5. ↑ M. Ajhar, M. Travesset, S. Yüce, T. Melin: Siloxane removal from landfill and digester gas - A technology overview . In: Bioresource Technology , 101, 2010, pp. 2913-2923
  6. ↑ a b c D. Rossol, K.-G. Schmelz, R. Hohmann: Siloxanes in digester gas. In: KA - Abwasser Abfall 8.8 2003.
  7. ↑ EP Wheless, Jeffrey: Siloxanes in Landfill and Digester Gas Update SWANA. 27th Landfill Gas Conference, March 22-25 2004.
  8. ↑ M. Schweigkofler, R. Niessner: Removal of siloxanes in biogases . In: Journal of Haradous Materials , 83, 2001, p. 183-1196.
  9. ↑ S. Rasi, J. Lantela, A. Veijanen, J. Rintala: Landfill gas upgrading with countercurrent water wash. In: Waste Management 28, 2008, pp. 1528–1534.
  10. ↑ P. Martin, E. Ellersdorfer, A. Zemann: Effects of volatile siloxanes in waste water and sewage gas on internal combustion engines . In: Korrespondenz Abwasser , 43, 5, 1996
  11. ^ S. Wilhelm: water treatment chemistry and chemical process engineering . 7th edition. Springer Verlag, 2003, ISBN 978-3-540-25163-7 , p. 126/127.
  12. ↑ Eduard Vinaricky (Ed.): Electrical contacts, materials and applications - fundamentals, technologies, test methods . 3. Edition. Springer-Verlag, Berlin Heidelberg 2016, ISBN 978-3-642-45426-4 , 4.4 "Influence of silicon-containing substances".