Siloxane
Disiloxane
Hexamethyldisiloxane, a di - siloxane
Octamethyltrisiloxane a tri- siloxane
Decamethylcyclopentasiloxane a cyclic siloxane
Siloxanes are chemical compounds having the general formula R 3 Si - [O- SiR 2 ] n -O- SiR3 wherein R can be hydrogen atoms or alkyl groups. In contrast to the silane, the silicon atoms are not to each other but connected by exactly an oxygen atom having its adjacent silicon atom Si -O-Si. Siloxanes with R = CH3 called polydimethylsiloxane. Oligomers or polymeric organosiloxane ( siloxanes with R ≠ H ) are called silicones. They have long Si -O- backbones and are viscous or permanently elastic, depending on molecular weight and networking. If they are liquid, they are also known as silicone oils.
Use
Siloxanes, mostly as polydimethylsiloxane, find application in cosmetics, deodorants, soaps and detergents use. Special recognition and significance of higher molecular weight siloxanes, called silicones.
In the industry, see siloxanes ( silicon oils ) as defoamers application, but also during freeze -drying as a cooling medium.
Industrial production
Siloxanes can be prepared in various ways. The authors of the Winnacker- Küchler work: Chemical Engineering lead as the most important production methods to follow methods:
- Continuous hydrolysis of dimethyldichlorosilane with elimination of hydrogen chloride
- Continuous reaction of dimethyl dichlorosilane with methane elimination of chloromethane
- By reaction of chlorinated silanes with alkoxides with elimination of water and an organic chlorine compound
Siloxanes in sewage and landfill gas
Siloxanes may be present in sewage or landfill gas at low concentrations (in the range 10 mg of silicon per standard cubic meter ). When the sewage Siloxaneintrag done by municipal or industrial wastewater. Landfills one goes more out of a silicone-containing waste decomposition into volatile siloxanes.
Siloxanes are from the combustion of sewage and landfill gas into a problem since from the raw gas (eg CHP ) solid silica (sand ) is formed, which leads to wear of the moving parts of the equipment. White - gray deposits were deposited on machine parts and are transported via the oil to all moving parts of the machine, where they lead to abrasion. Affected parts are esp. valves, cylinder heads, turbine blades, pipes.
Before combustion in gas engines siloxanes are today usually (typically at temperatures below -25 ° C ) is at least partially removed by adsorption on activated carbon, absorption or cryogenic cooling. In addition to these techniques, there are in principle several additional ways of Siloxanabscheidung, including adsorption by silica, alumina or catalytic materials, biofilters and gas permeation. A typical limit of most gas engine manufacturer for silicon is 5 mg of silicon per standard cubic meter of methane.
Activated carbon adsorption
Activated carbon reduces the Siloxankonzentrationen in the raw gas quite complete. Values below 0.1 mg of silicon per standard cubic meter are typical. Likewise, most other volatile hydrocarbons such as BTEX are removed ( which is not necessarily required). In fact, the use of activated carbon in the landfill gas to a relatively rapid exhaustion of the load capacitance results, since not only siloxanes, but also a variety of volatile hydrocarbons adsorbed. Therefore Aktivkohladsorption often, a drying step (for example, condensation at 5 ° C) upstream of which reduces the relative humidity of the gas after re-heating and to remove a majority of the hydrophilic substances in the raw gas in advance track. A regeneration of the activated carbon is usually not, the spent activated carbon is replaced with a fresh batch. Major cost of an activated carbon -based Siloxanreinigung is esp. the necessary replacement of the activated carbon. However, commercial adsorption systems are offered that regenerate the adsorbent.
Absorption
A wide range of different washing liquids was studied to either physically or chemically absorbing the siloxanes of the crude gas stream. Chemical absorption (the destruction of the siloxane molecule ) is carried out in principle at low or high pH values. Since alkaline washing liquids, in conjunction with the carbon dioxide in the biogas to carbonate formation, but only acids as absorbents in question. In addition to the strength of the acid, an increased temperature for the Siloxanabsorption is conducive. Dealing with hot acids is possible, however, provides some security challenge dar. Physical Siloxanabsorption been tested mainly with water, organic solvents and mineral oil. Siloxanes are i.d.R. hydrophobic, such that water adsorption (pH 7) causes no significant depletion. However, the use of acidic water sump of the absorber as the washing liquid can be useful. Experience with the use of mineral oil show a relatively low cleaning efficiency and problems arose with registered in the gas engine oil vapor.
Cryogenic cooling
The effectiveness of the cleaning performance of the low-temperature cooling system depends on the cooling temperature. The higher the Siloxanbelastung in the raw gas, the better the relative deposition of silicon at the chosen cooling temperature. In particular, the volatile Hexadimethylsiloxan (L2 ), which is present especially in landfill gas at higher concentrations, but can be even at temperatures around -40 ° C did not significantly condense. Sewage gas, however, contains much higher levels of D4 and D5. To reduce the total silicon concentration, low temperature cooling is at sewage therefore more effective than landfill gas. Connected to the temperature reduction is also a condensation of many other substances ( esp. water). Since separating the resulting acidic condensate is contacted with the raw gas, is also presumed a certain absorption of siloxanes in the condensate.
Siloxanes in sewage
Also in membrane filtration siloxanes are undesirable because they intercalate into the pores of the membrane and lead to fouling, which is to be removed by backwashing and use of chemicals is limited.
Designation
M group: ( CH3) 3SiO0.5, D group: (CH 3 ) 2 SiO, T group: ( CH3) SiO1.5