Siloxanes are chemically stable compounds used in many consumer and industrial products, ranging from cosmetics to adhesives. Siloxanes can enhance product flow capability, texture, adhesion, uniformity, and flavor. In consumer products, they are used as a volatile dispersant agent for other organic chemical additives. Siloxanes are also used in many different manufacturing processes, such as the production of silicon based semiconductors.
Siloxanes are saturated silicon-oxygen hydrides formed from atoms of carbon (C), Hydrogen (H), Oxygen (O) and Silicon (Si), and which include at least one chain of alternating silicon and oxygen atoms (—Si—O—Si—). Most siloxanes volatilize rapidly during anaerobic digestion, and also in many manufacturing processes. Common siloxanes are known as volatile methyl siloxanes (VMS), which can be linear molecule or rings structures (cyclomethicones). In cyclical VMS, each Si atom has two methyl-groups (CH3) attached to it. Some of the more common siloxanes include the linear hexamethyldisiloxane (“MM”) and octamethyltrisiloxane (“MDM”). Some of the more common cyclical siloxanes are hexamethylcycloclotrisiloxane (referred to as “D3”), octamethylcyclotetrasiloxane (referred to as “D4”) decamethylcyclopentasiloxane (referred to as “D5”), and dodecamethylcyclohexasiloxane (referred to as “D6”). “D” is used to represent the repeated dimethyl-silicon-oxygen group in a ring structure and is followed by either an ordinal or a subscript indicating the number of D groups that are present.
Although siloxanes are stable and non toxic, their presence in a gas stream is often undesirable. When present in a fuel gas, such as methane rich gas from a landfill or a digester, siloxanes will be carried throughout the process/treatment facility as a constituent of the methane gas, and under certain temperature and pressure conditions, the siloxanes will cause undesirable silica deposits to form in process-related equipment. For example, siloxanes are known to be present in trace amounts in biogas produced in Waste Water Treatment Plants (WWTP) and landfills. Such biogas is often used as an alternate fuel to run engines that power equipment or produce electrical power. When the biogas is burned as a fuel, the siloxanes cause silica deposits to form in the engines, and such deposits can significantly increase maintenance costs. The silica deposits form on hot engine components, such as cylinder heads. Abrasive silica particles can also become entrained in the engine oil, increasing wear on bearings. The result of silica being introduced into internal combustion engines is a significant increase in engine wear, causing more frequent engine rebuilding and concomitant downtime.
In electrical power generation employing emission catalysts, siloxanes can form a silica film on the catalyst surface, rapidly and significantly reducing the catalyst's activity. This form of damage (or “poisoning”) is irreversible, meaning that the catalysts, which are often quite costly, must be replaced.
Siloxanes can also be unintentionally introduced in an industrial process. For example, siloxanes are formed during electronic semiconductor fabrication processes, and can contaminate process gas streams (such as silane) used in the production process. The presence of such siloxanes can lead to an increase in the rejection rate of silicon wafers. In industrial emission control processes, silica deposits (as noted above) can foul solvent recovery equipment and thermal oxidation equipment.
Siloxanes are also often present in gas distribution environments, where methane and/or natural gas is compressed and injected into pipelines for distribution. Siloxanes are sometimes added to compressor oils to increase lubricity and to the pipelines themselves during pigging operations. When the siloxane contaminated gas is combusted as a fuel (such as for heat), silica deposits foul the combustion equipment in a manner similar to the fouling of internal combustion engines described above.
Siloxanes have been found to cause problems such as those noted above when present in concentrations as low as 50 ppbv (parts per billion by volume), which is at or near the state-of-the-art detection limit for most siloxanes. Siloxanes are damaging at such low levels because the negative impact of silica deposition is cumulative. Homogeneous activated carbon filters have been successfully employed to remove some siloxanes; however, the performance of such filters in removing siloxanes from a gas stream is inadequate, clearly leaving room for improvement. Accordingly, it would be desirable to provide a better method and apparatus to effectively remove siloxanes from a gas stream.
As noted above, biogas often includes one of more constituents that complicate the removal of siloxanes from the gas, or whose presence is also undesirable. For example, halogenated organic species (such as chlorinated solvents and chlorofluorocarbons) are also found in biogas. When these halogenated species are burned along with the methane in internal combustion engines, hydrochloric acid is formed, which causes increased corrosion of metal parts. Halogenated species are also poisons to emission catalysts used to control nitrogen oxides (NOx) and carbon monoxide (CO). The presence of heavy organics (such as benzene, toluene, and xylene) in sufficiently high concentrations can adversely affect the removal of siloxanes and halogenated organics.
Clearly, it would be desirable to remove the chlorinated organics, and heavy organics, as well as the siloxanes, from gas streams. One prior art approach uses activated carbon for this purpose. While activated carbon can effectively remove all three of the offending species, breakthrough of these species can occur rapidly, whereupon the media must be replaced. Moreover, the capacity for typical activated carbon comprising bituminous coal-based carbons, coconut shell carbons, or wood-based carbons is limited, requiring their frequent replacement. It would therefore be desirable to provide a more effective system and method for removing siloxanes, chlorinated organics, and heavy organics from gas streams.
In addition, landfill gas is often grossly contaminated with a number of undesirable volatile organic species at high concentrations, some of which render the removal of siloxanes using a carbon filtration media particularly problematical. Among the problematic volatile species often found in landfill gas are oxygenated organics. Oxygenated organics contain one or more oxygen atoms; and include acetates, alcohols, aldehydes, esters, ethers, formates, furals, furans, glycols, ketones, oxides, and other substances. Oxygenated organics can hamper the ability of adsorbent media to remove siloxanes due to several factors. First, oxygenated organics can function as solvents to strip siloxanes already picked up by an adsorbent. Very high concentrations of oxygenated organics can condense in adsorbent media beds and essentially flush siloxanes out of the adsorbent media bed. Second, heavier molecular weight oxygenated organics, such as acetates, can physically displace siloxanes already picked up or captured by adsorbent media, causing the siloxanes to be released back into the gas flow. Removal of siloxanes from a gas stream using a carbon based adsorbent in the presence of oxygenated organics is particularly challenging. It would therefore be desirable to provide a more effective system and method for removing siloxanes from gas streams in which oxygenated organics are present.