Removing siloxanes from a gas stream using a mineral based adsorption media Number:7,393,381 from the United States Patent and Trademark Office (PTO) owispatent A plurality of different layers of filter media are used to remove siloxanes from a gas stream. Based on an analysis of the specific gas stream to be filtered, a filter media having an average pore size enabling the preferential removal of a specific class of contaminants is selected for each different class of contaminants. The layers are arranged in sequential order such that contaminants having a higher molecular weight are preferentially removed by the first layers. Collectively, the layers define a segmented activity gradient that enables each class of contaminants present in the gas stream to be preferentially removed in a different layer, preventing removal competition between different classes of contaminants. Preferable adsorption media exhibit a relatively narrow range of pore sizes. Both inorganic adsorption media and carbon-based adsorption media exhibiting a relatively narrow range of pore sizes can be used.<H adsorption, siloxanes, Removing, siloxa, 7393381.html, Patent, pto, 7393381

Title: Removing siloxanes from a gas stream using a mineral based adsorption media

Abstract: A plurality of different layers of filter media are used to remove siloxanes from a gas stream. Based on an analysis of the specific gas stream to be filtered, a filter media having an average pore size enabling the preferential removal of a specific class of contaminants is selected for each different class of contaminants. The layers are arranged in sequential order such that contaminants having a higher molecular weight are preferentially removed by the first layers. Collectively, the layers define a segmented activity gradient that enables each class of contaminants present in the gas stream to be preferentially removed in a different layer, preventing removal competition between different classes of contaminants. Preferable adsorption media exhibit a relatively narrow range of pore sizes. Both inorganic adsorption media and carbon-based adsorption media exhibiting a relatively narrow range of pore sizes can be used.

Patent Number: 7,393,381 Issued on 07/01/2008 to Tower, et al.

Inventors: Tower; Paul M. (Snohomish, WA), Wetzel; Jeffrey V. (Lake Stevens, WA)
Assignee: Applied Filter Technology, Inc. (Snohomish, WA)

Appl. No.: 11/079,459

Filed: March 8, 2005

Related U.S. Patent Documents


Application Number	Filing Date	Patent Number	Issue Date
10871920	Jun., 2004	7264648
60550343	Mar., 2004
60479592	Jun., 2003

Current U.S. Class: 95/8 ; 95/11; 95/141; 95/147; 95/148; 95/901; 95/903; 96/111; 96/121; 96/131; 96/143; 96/146; 96/154

Current International Class: B01D 53/02 (20060101)

Field of Search: 95/1,8,11,90,141,143,147,148,901,903 96/108,111,121,131,143,144,146,154

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Other References

Tower, Paul, "Removal of Siloxanes from Landfill Gas by SAG.TM. Polymorphous Porous Graphite Treatment Systems" Paper presented at SWANA 26th Landfill Gas Symposium Mar. 27, 2003. cited by other .
Press Release Malcolm Pirnie, Engineers, wins "Best New Environmental Technology, Category E" ACEC 2003 Engineering Excellence Awards for SAG.TM. System installed at BCUA, Little Ferry, NJ, Jan. 10, 2003. cited by other .
Liang, Kit Y., P.E., Ramon, Li, P.E., Pirnie, Malcolm, "Removing Siloxanes: Solution to Combustion Equipment Problems" Paper presented at WEFTEC02 by Malcolm Pirnie Engineers and Bergen County Utility Authority, New Jersey, Oct. 2002. cited by other .
Glus, Peter H., Liang, Kit Y., P.E., Ramon, Li, P.E., Pope, Richard J., P.E., "Recent Advances in the Removal of Volatile Methylsiloxanes from Biogas at Sewage Treatment Plants and Landfills" Paper presented at the Annual Air and Waste Management (AWMA) 2001 Conference in Orlando, Florida. http://www.appliedfiltertechnology.com/page1252.asp. cited by other .
Gary, Daniel, Acosta, Glenn, Kilgore, John, Min, Seong, Adams, Greg, Lost Angeles County Sanitation Districts Research Project to Remove Siloxanes from Digester Gas Paper presented at the California Water Pollution Controls Conference in Palm Springs, CA, Apr. 2001 http://www.appliedfiltertechnology.com/page1253.asp. cited by other .
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Liang, Kit Y., P.E., Ramon, Li, P.E., Tudman, Scott, Schneider,Robert, J., P.E., Sheehan, Jerome F., P.E., Anderson, Eric, P.E., Pilot Testing Case Study: Pilot Testing Case Study: "Removal of Volatile Methylsiloxanes from Anaerobic Digester Gas Fired Engines" Paper No. 960. Paper presented at the Annual Air and Waste Management (AWMA) 1999 Conference in St. Louis, Missouri http://www.appliedfiltertechnology.com/page1257.asp. cited by other .
*Tower, Paul, Principal Applied Filter Technology. "New Technology For Removal of Siloxanes in Digester Gas Results in Lower Maintenance Costs and Air Quality Benefits in Power Generation Equipment." WEFTEC 78.sup.th Annual Technical Exhibition and Conference. Oct. 11-15, 2003. 9pp. cited by other .
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Primary Examiner: Greene; Jason M
Attorney, Agent or Firm: Anderson; Ronald M.

Parent Case Text

RELATED APPLICATIONS

This application is based on prior provisional application Ser. No. 60/550,343, filed on Mar. 8, 2004, the benefit of the filing date of which is hereby claimed under 35 U.S.C. .sctn. 119(e). The present application is further a continuation in part application of prior conventional application Ser. No. 10/871,920, filed on Jun. 18, 2004, issued as U.S. Pat. No. 7,264,648 B1 on Sep. 4, 2007, which itself is based on prior provisional application Ser. No. 60/479,592, filed on Jun. 19, 2003, the benefits of the filing dates of which are hereby claimed under 35 U.S.C. .sctn. 119(e) and 35 U.S.C. .sctn. 120.

Claims

The invention in which an exclusive right is claimed is defined by the following:

1. A method of removing siloxanes from a gas stream, comprising the steps of: (a) providing a plurality of different grades of mineral-based adsorbent media, wherein each different grade is characterized as having a different average pore size; (b) analyzing the gas stream to determine the contaminants present in the gas stream; (c) for each different contaminant, selecting the grade of mineral-based adsorbent media whose pore size will enable the preferential removal of that contaminant; (d) using the different grades of mineral-based adsorbent media selected to produce a multi-layer filter bed; and (e) passing the gas stream through the multi-layer filter bed to remove the contaminants, the different layers minimizing removal competition among the contaminants, thereby enhancing the removal of the siloxanes from the gas stream.

2. The method of claim 1, wherein a plurality of the different grades of the mineral-based adsorbent media are characterized such that at least 50% of the pores in that grade fall within a range that spans less than about 50 nm.

3. The method of claim 1, wherein a plurality of the different grades of the mineral-based adsorbent media are characterized such that at least 50% of the pores in that grade fall within a range that spans less than about 10 nm.

4. The method of claim 1, wherein a plurality of the different grades of the mineral-based adsorbent media are characterized such that at least 50% of the pores in that grade fall within a range that spans less than about 1 nm.

5. The method of claim 1, wherein the different grades of the mineral-based adsorbent media are produced from silica sand.

6. The method of claim 1, wherein the mineral-based adsorbent media comprise at least one of a synthetic silica gel and a synthetic zeolite.

7. The method of claim 1, wherein the mineral-based adsorbent media comprise at least one of an activated silica, an activated silica gel, a silicate acid condensation product, a silicate acid condensation based polymer, and a silicate acid condensation based resin.

8. The method of claim 1, wherein the step of selecting the grade of mineral-based adsorbent media whose pore size will enable the preferential removal of a specific contaminant comprises the step of using a computer model that correlates the pore size of each grade of filter media to a specific contaminant.

9. The method of claim 8, wherein the computer model is based on both theoretical data and empirical data.

10. The method of claim 1, wherein the step of using the different grades of mineral-based adsorbent media selected to produce a multi-layer filter bed comprises the step of configuring the multi-layer filter bed to achieve a segmented activity gradient.

11. The method of claim 1, wherein the step of using the different grades of mineral-based adsorbent media selected to produce a multi-layer filter bed comprises the step of configuring the multi-layer filter bed such that the different grades of mineral-based adsorbent media are arranged in a sequence, so that the gas stream to be filtered will pass through a mineral-based adsorbent media having a largest average pore size first, and the gas stream to be filtered will pass through a mineral-based adsorbent media having a smallest average pore size last.

12. The method of claim 1, wherein the step of using the multi-layer filter bed to remove the contaminants comprises the step of passing the gas stream through the multi-layer filter at a flow rate substantially lower than employed in conventional carbon filter beds.

13. The method of claim 1, further comprising the step of regenerating the mineral-based adsorbent media using a hot inert gas.

14. The method of claim 1, wherein the step of using the different grades of mineral-based adsorbent media selected to produce a multi-layer filter bed further comprises the step of incorporating at least one layer of a carbon-based adsorbent in the multi-layer filter bed.

15. The method of claim 14, wherein the carbon-based adsorbent comprises an adsorbent exhibiting a relatively narrow range of pore sizes.

16. The method of claim 14, wherein the carbon-based adsorbent comprises an adsorbent exhibiting a relatively wide range of pore sizes.

17. The method of claim 1, further comprising the step of color coding the different grades of mineral-based adsorbent media, to facilitate distinguishing one grade from another.

18. A method of removing siloxanes and other contaminants from a gas stream, comprising the steps of: (a) providing a plurality of different grades of adsorbent media, wherein each different grade is characterized as exhibiting a relatively narrow range of pore sizes; (b) analyzing the gas stream to determine the siloxanes and other contaminants present in the gas stream; (c) organizing the contaminants into different classes based on molecular weights of the contaminants; (d) for each different class of contaminant, selecting the grade of adsorbent media whose pore size will enable a preferential removal of that class; (e) using the different grades of adsorbent media selected to produce a multi-layer filter bed; and (f) passing the gas stream through the multi-layer filter bed to remove the siloxanes and the other contaminants, different layers of the multi-layer filter bed minimizing removal competition among the different classes of contaminants, thereby enhancing the removal of the siloxanes from the gas stream.

19. The method of claim 18, wherein the step of using the different grades of adsorbent media selected to produce the multi-layered filter bed comprises the step of using at least one mineral-based adsorbent media.

20. The method of claim 18, wherein the step of using the different grades of adsorbent media selected to produce the multi-layered filter bed comprises the step of using at least one carbon-based adsorbent media.

21. A method of removing siloxanes and other contaminants from a gas stream, comprising the steps of: (a) providing a plurality of different grades of mineral-based adsorbent media, wherein each different grade is characterized as exhibiting a relatively narrow range of pore sizes; (b) analyzing the gas stream to determine the siloxanes and other contaminants present in the gas stream; (c) organizing the contaminants into different classes based on molecular weights of the contaminants; (d) selecting the grade of mineral-based adsorbent media whose pore size will enable a preferential removal of the siloxanes; (e) using the grade of mineral-based adsorbent media selected to produce a filter bed; and (f) passing the gas stream through the filter bed to remove the siloxanes.

22. A multi-layer filter bed for removing contaminants of different molecular weights from a gas stream, comprising: (a) a first layer of adsorption media, wherein a majority of pores in the first layer of the adsorption media fall within a range that spans less than about 10 nm and which preferentially remove contaminants having greater molecular weights; and (b) a second layer of adsorption media, wherein a majority of pores in the second layer of adsorption media fall within a range that spans less than about 10 nm and which preferentially remove contaminants having smaller molecular weights, the first layer being disposed relatively closer to an inlet than the second layer, the second layer being disposed relatively closer to an outlet than the first layer, and an average pore size of the first layer being generally larger than an average pore size of the second layer, wherein one of the first layer and the second layer comprises carbon-based filter media, and the other of the first layer and the second layer comprises mineral-based filter media.

23. The multi-layer filter bed of claim 22, wherein the second layer of adsorption media comprises the mineral-based adsorption media.

24. The multi-layer filter bed of claim 22, wherein the second layer of adsorption media comprises the carbon-based adsorption media.

25. The multi-layer filter bed of claim 22, further comprising an intermediate layer of adsorption media disposed between the first layer of adsorption media and the second layer of adsorption media, wherein a majority of pores in the intermediate layer of adsorption media fall within a range that spans less than about 10 nm, and which preferentially remove contaminants having intermediate molecular weights.

26. The multi-layer filter bed of claim 22, wherein the mineral-based adsorption media comprises at least one of a synthetic silica gel, a synthetic zeolite, an activated silica, an activated silica gel, a silicate acid condensation product, a silicate acid condensation based polymer, and a silicate acid condensation based resin.

27. A system for removing contaminants of different molecular weights from a gas stream, comprising: (a) a multi-layer mineral-based adsorbent media filter bed for removing contaminants of different molecular weights from a gas stream, the multi-layer mineral-based adsorbent media filter bed comprising: (i) a first layer of adsorption media, wherein a majority of pores in the first layer of the adsorption media fall within a range that spans less than about 10 nm and which preferentially remove contaminants having greater molecular weights; and (ii) a second layer of adsorption media, wherein a majority of pores in the second layer of adsorption media fall within a range that spans less than about 10 nm and which preferentially remove contaminants having smaller molecular weights, the first layer being disposed relatively closer to an inlet than the second layer, the second layer being disposed relatively closer to an outlet than the first layer, and an average pore size of the first layer being generally larger than an average pore size of the second layer, wherein one of the first layer and the second layer comprises a carbon-based filter media, and the other of the first layer and the second layer comprises a mineral-based filter media; and (b) a hot inert gas generator configured to regenerate the multi-layer mineral-based adsorbent media filter bed.

Description

FIELD OF THE INVENTION

The present invention generally relates to removing trace siloxane contaminants from a gas stream, and more specifically, to employing a filtration system including a plurality of filter beds customized to a gas stream composition, wherein the plurality of filter beds remove the siloxanes both by adsorption and by acting as a molecular sieve.

BACKGROUND OF THE INVENTION

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 (CH.sub.3) 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.

SUMMARY OF THE INVENTION

The present invention is a method for removing siloxanes from a gas stream using a mineral-based adsorbent media (i.e., an inorganic-based adsorbent media). The mineral-based adsorbent media can be used alone, or in conjunction with other adsorbent media, particularly carbon based adsorbent media. A key aspect of the present invention is the recognition that certain commercially available mineral-based adsorbent media exhibits a relatively narrow range of pore that vary by grade. Thus a plurality of different grades of mineral-based absorbent media can be purchased and tested to identify the narrow range of pore sizes exhibited by each different grade. A gas stream can be analyzed to determine the types of siloxanes and other contaminants present to enable selection of an inorganic-based absorbent media having a range of pore sizes corresponding to the particular contaminants to be removed. The source and composition of the mineral-based absorbent media exhibiting a relatively narrow range of pore sizes is discussed in greater detail below. The term "HOX media" is used hereinafter to refer to an inorganic-based absorbent media exhibiting a relatively narrow range of pore sizes.

In one aspect of the present invention, the HOX media are arranged in a filter including a plurality of different layers. In embodiments where the HOX media are used without a carbon based adsorbent media, each layer in the filter bed includes a grade of HOX media having well defined physical properties, such as average pore size and total pore volume. The grade of HOX media in each layer is specifically selected so that the properties associated with that grade preferentially removes a specific class of contaminants from the gas stream. The layers are arranged in sequential order such that contaminants having a higher molecular weight (or molecular size) are preferentially removed by the first layers. Collectively, the layers define a segmented activity gradient that enables each class of contaminants present in the gas stream to be preferentially removed in a different layer, preventing removal competition between different classes of contaminants. Should there be just one class of contaminants in a process gas (such as a fuel gas), then a single layer of specifically selected HOX media would be employed.

When combined with the use of a carbon based absorption media, the HOX media enables siloxanes to be efficiently removed from process gas containing oxygenated organics. In such a system, the HOX media are used to remove the oxygenated organics from the gas stream, and the carbon-adsorbent media are used to remove siloxanes from the gas stream. The HOX media can also be used without carbon adsorbent media, such that the HOX media removes both the oxygenated organics as well as the siloxanes. Different grades of HOX media are employed, such that at least one grade of the HOX media whose pore sizes and other physical properties are particularly well-suited to remove oxygenated organics are utilized, as well as at least one grade of HOX media whose pore sizes and other physical properties are particularly well-suited to remove siloxanes.

HOX media that are generally based on both silica gel and zeolites can be employed, depending on the specific characteristics of the gas stream to be processed. Silica gel is a porous, amorphous form of silica (SiO.sub.2). Due to its unique internal structure, silica gel is radically different than other SiO.sub.2-based materials, and includes a vast network of interconnected microscopic pores. Conventional silica gel exhibits a relatively broad range of pore sizes, having larger pores than zeolites, with a wide range of diameters (typically between about 5 .ANG. and 3000 .ANG.). Thus, conventional (i.e., not specially modified) silica gel is unusable as HOX media. However, the silica gel industry is a mature industry, and several different chemical manufacturers have learned how to manipulate process techniques to achieve silica gel-related materials that do exhibit a relatively narrow range of pore sizes. These types of modified or enhanced silica gels are therefore also encompassed by the term "HOX media," as used herein. Zeolites are a class of aluminum silicates characterized by relatively small pore sizes that do not vary substantially. Natural and synthetic zeolites are known, and their characteristic tightly controlled pore sizes has lead to zeolites being referred to as molecular sieves. Thus, zeolites, both natural and synthetic, are encompassed by the term "HOX media," as used herein.

The plurality of different grades of HOX media available make it possible to achieve a layered filter bed including an inorganic adsorbent media having different properties, each layer of the filter bed having been selected to preferentially remove a particular class of contaminants from a gas stream. Such mineral-based adsorbent media encompassed by the term "HOX media" are referred to variously when marketed, as enhanced silica gels, modified silica gels, custom silica gels, synthetic silica gels, activated silica, activated silica gel, silicate acid condensation (SAC), synthetic SACs, desiccants, and silicic acid adsorbent. Note that not all products referred to by one of those terms will exhibit a relatively narrow range of pore sizes, and porosity tests or manufacturer's data will need to be consulted to determine whether such a product exhibits a relatively narrow range of pore sizes (hence being usable as HOX media) or exhibits a relatively broad range of pore sizes. HOX media are generally available in powders, granules, spheres, irregular granules, pellets and other shapes.

In one aspect of the invention, samples of available grades of HOX media are analyzed to determine the average pore size and pore volume of each grade. Particularly preferred HOX media, referred to as SAC type gels, are produced from high purity silica sand and sulfuric acid. Preferred HOX media will have an average pore size such that at least 50% of the pores vary within a relatively narrow range, preferably varying plus or minus about 25 nm from an average value, and more preferably varying plus or minus about 5 nm from an average value, and most preferably varying plus or minus about 0.5 nm from an average value. Particularly for siloxane removal, the narrowest range is preferred.

A model is developed, which correlates the pore sizes of available grades of HOX media to classes of contaminants, based on the molecular weight of the contaminant. The model will enable a specific one of the different grades of HOX media to be selected based on the molecular weight of the contaminant. Empirical data can be used to improve the model. A sample of the gas to be treated is taken, and the contaminants to be removed will be identified. For each different class of contaminant, a specific grade of filter media having a pore size expected to preferentially remove that class of contaminant will be identified. The concentration of the contaminant will determine the volume of the selected grade of HOX media required. A multilayer filter bed is constructed using each different grade of HOX media identified by the model, such that HOX media selected to preferentially remove the larger-sized contaminants are disposed closest to the gas inlet, while HOX media selected to preferentially remove the smaller-sized contaminants are disposed closest to the gas outlet.

A particularly preferred embodiment of the present invention combines the use of HOX media with carbon adsorbent media, where both the mineral-based adsorbent media and the carbon-adsorbent media exhibit well-defined pore sizes, and so that the pore sizes of the adsorbent media (inorganic and carbon) are particularly selected based on the composition of the gas stream to be treated. A layered filter bed is provided, wherein each layer includes adsorbent media whose pore sizes are particularly well adapted to removing a specific contaminant or class of contaminants from the gas stream.

Systems in accord with the present invention can include pre-treatment units, post-treatment units, and filter media regenerators. The use of hot inert gas processes or microwave heaters represent particularly preferred regeneration techniques.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a flowchart which illustrates the overall sequence of steps utilized to design and implement a siloxane removal system in accord with the present invention;

FIG. 2A schematically illustrates a prior art homogenous filter bed in which a plurality of contaminants compete for removal in the filter bed;

FIG. 2B schematically illustrates a filter bed including three sub-gradient layers; each of which exhibits an average pore size specifically selected to favor the removal of a specific class of contaminants, and which collectively define a segmented activity gradient;

FIG. 3 is a block diagram schematically illustrating the elements present in a basic segmented activity gradient siloxane removal system in accord with the present invention;

FIG. 4A schematically illustrates a vertically oriented filter canister including a plurality of sub-gradient layers;

FIG. 4B schematically illustrates a horizontally oriented filter canister including a plurality of sub-gradient layers;

FIGS. 4C and 4D schematically illustrate a radially oriented filter canister including a plurality of sub-gradient layers, in which the gas to be filtered moves from an annular outer volume to an annular inner volume;

FIGS. 4E and 4F schematically illustrate a radially oriented filter canister including a plurality of sub-gradient layers, in which the gas to be filtered moves from an annular inner volume to an annular outer volume;

FIG. 5A is a block diagram schematically illustrating the elements present in a segmented activity gradient siloxane removal system which includes a filter media regeneration unit in accord with the present invention;

FIG. 5B is a block diagram of a pneumatic transfer system configured to transfer filter media from a filter canister to a microwave heater for regeneration in a batch process;

FIG. 5C is a block diagram of a pneumatic transfer system configured to transfer filter media from a filter canister to a microwave heater for regeneration in a continuous process;

FIG. 5D schematically illustrates a moving bed filter canister in fluid communication with a filter media regeneration unit; and

FIG. 6 is a block diagram schematically illustrating the elements present in a segmented activity gradient siloxane removal system which includes pre-treatment and post-treatment processing units.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Overview

In the present invention, a filter media available with a range of properties (such as pore size) that can be tightly controlled is used to filter a gas stream to remove siloxanes. An inorganic filter media, generally similar to silica gel, and a carbon-based polymorphous graphite, both available with tightly controlled properties (including a relatively narrow range of pore sizes), are utilized in various embodiments of the present invention. It should be understood however, that the present invention encompasses the use of the mineral-based adsorbent media alone, as well as the use of the mineral-based adsorbent media with conventional carbon adsorbents.

Preferably, siloxanes are removed from a gas stream using a plurality of different grades of adsorbent media, each grade of adsorbent media having been specifically selected to preferentially remove a single contaminant (or a class of contaminants). Generally, the pore size of the adsorbent media will be a criteria employed to select one grade of adsorbent media over another.

With respect to the drawing figures, each Figure generally refers to a grade or layer of adsorbent material, without specifying whether that grade or layer is implemented using a mineral-based adsorbent media exhibiting a relatively wide range of pore sizes, a mineral-based adsorbent media exhibiting a relatively narrow range of pore sizes (HOX media), a carbon based adsorbent media exhibiting a relatively wide range of pore sizes, or a carbon-based adsorbent media exhibiting a relatively narrow range of pore sizes. It should be understood that the present invention is directed to the following embodiments, although embodiments utilizing adsorbent media exhibiting a relatively narrow range of pore sizes are particularly preferred: the use of a mineral-based adsorbent media exhibiting a relatively wide range of pore sizes in combination with a carbon-based adsorbent media exhibiting a relatively wide range of pore sizes, to remove siloxanes from a gas stream the use of a mineral-based adsorbent media exhibiting a relatively wide range of pore sizes (HOX media) in combination with a carbon-based adsorbent media exhibiting a relatively narrow range of pore sizes, to remove siloxanes from a gas stream the use of a mineral-based adsorbent media exhibiting a relatively narrow range of pore sizes (HOX media) to remove siloxanes from a gas stream the use of a mineral-based adsorbent media exhibiting a relatively narrow range of pore sizes (HOX media) in combination with a carbon-based adsorbent media exhibiting a relatively wide range of pore sizes, to remove siloxanes from a gas stream the use of a mineral-based adsorbent media exhibiting a relatively narrow range of pore sizes (HOX media) in combination with a carbon-based adsorbent media exhibiting a relatively narrow range of pore sizes, to remove siloxanes from a gas stream.

Before discussing the present invention in detail, it will be useful to review various types of inorganic and carbon-based adsorbents that can be utilized, alone or in combination, in accord with the present invention.

Mineral-Based Adsorbent Media Exhibiting a Relatively Wide Range of Pore Sizes

Silica gel is one of the most widely recognized non-carbon based adsorbents. Silica gel is a granular (or beaded), porous form of silica made synthetically from sodium silicate. Despite the name, silica gel is a solid. It is usually distributed in the form of beads. It was invented at Johns Hopkins University, Baltimore, Md. in the earlier 1900s. Its high porosity, around 800 m.sup.2/g, enables it to readily adsorb water (or other chemical species), making it useful as a desiccant (drying agent). Once saturated, the gel can be regenerated by heating the gel until the adsorbed species are driven off. Silica gel is non-toxic, non-flammable and chemically un-reactive. Silica gel will not normally attack or corrode other materials, and with the exception of strong alkalis and hydrofluoric acid, silica gel is itself resistant to attack. The strong adsorption capability of silica gel may cause drying effects on human tissue. Even when saturated with adsorbed chemical species, silica gel still has the appearance of a dry product, its shape remaining unchanged.

The reason silica gel is such an effective adsorbent is that it includes an internal network of interconnecting microscopic pores, yielding a typical surface area of 700-800 m.sup.2/g. Stated another way, the internal surface area of a teaspoon full of silica gel is roughly equivalent to the area of a football field. Molecules are adsorbed or desorbed by these micro-capillaries until vapor pressure equilibrium is achieved with the surrounding fluid. In general, the pore size exhibited in traditional silica gel varies over a relatively wide range. As opposed to zeolites (micro porous crystalline solids with well-defined structures, generally exhibiting a relatively narrow range of relatively small pore sizes), inorganic silica gels have larger pores with a wide range of diameters--typically between about 5 .ANG. and 3000 .ANG.--and generally are not useful for separation of molecules solely dependent on their size. Silica gels maintain their structure when activated. Activation frees the large internal surface area and pore volume, enabling physical adsorption and capillary condensation.

Clay desiccant is fairly common in commercial and industrial use. While not as efficient and adsorbent as silica gel, clay desiccant is relatively inexpensive. Clay desiccant is generally produced from Montmorillonite clay, composed primarily of magnesium aluminum silicate, a naturally occurring mineral. After mining, it is purified, reduced to granules, and subjected to a controlled dehydration process to increase its sorbent porosity. It recharges easily and does not swell as it adsorbs water vapor. It works well at low and room temperatures, but has a rather low ceiling temperature. At 120.degree. F., it will begin to desorb or shed the moisture it has adsorbed. This characteristic is an important consideration for storage of the material in hot areas. While clay desiccant is not a particularly preferred mineral-based adsorbent media in the context of the present invention, it may be useful when used in addition to the more preferred silica gels.

Mineral-based adsorbent media exhibiting a relatively wide range of pore sizes can be used to remove siloxanes from a gas stream, because some of the pores will likely be sufficiently large to accommodate the siloxanes. A more efficient system, requiring less inorganic adsorbent, can be achieved if a carbon-based adsorbent media exhibiting a relatively narrow range of pore sizes is also employed, where the pore sizes of the carbon-based adsorbent media have been selected to correspond to the size of the siloxanes to be removed. The mineral-based adsorbent media exhibiting a relatively wide range of pore sizes can be used to prevent other species, such as oxygenated organics, from fouling the carbon-based adsorbent media selected to remove the siloxanes.

Dolomite is a magnesia-rich, sedimentary rock resembling limestone, primarily comprising calcium magnesium carbonate, CaMg(CO.sub.3).sub.2. It is commonly crystalline and is white, gray, brown, or reddish in color, with a vitreous to pearly luster. The magnesium is sometimes replaced in part by iron or manganese. Calcined dolomite is dolomite that has been heated to drive off some of the carbon, thereby increasing the percentage of magnesium by weight and imparting a network of pores. Calcined dolomite can be used in combination with the other adsorbent media described herein.

Mineral Adsorbent Exhibiting a Relatively Narrow Range of Pore Sizes (HOX Media)

As noted above, some common silica gels exhibit a relatively wide range of pore sizes, such that those silica gels are generally not useful for selectively adsorbing chemical species as a function of pore size. However, the silica gel industry has matured sufficiently such that variations in manufacturing processes can enable the production of silica gels exhibiting a relatively narrow range of pore sizes. Manufacturers are currently able to provide silica gels, and other mineral (or inorganic)-based adsorbent media, exhibiting precisely controlled pore sizes. Manufacturing of silica gels can also be carried out so as to enable precise control of surface areas, and to facilitate surface treatment modifications that achieve silica gels appropriate for specific process applications. As noted above, inorganic adsorbent media exhibiting a relatively narrow range of pore sizes are collectively referred to herein as HOX media.

In general, silica gels synthesized with an average pore size of about 20 .ANG. are known as "narrow pore" silica gels, whereas silica gels with an average pore size of about 110 .ANG. and beyond are called "wide pore" silica gels.

In addition to specially processed silica gel, HOX media can be implemented based on: activated silica (AS), silicate acid condensation (SAC), such as synthetic silica gel, and SAC type resins and polymers. These adsorbents can be used alone or in combination with other adsorbents described herein.

In particular, SAC type gels are amenable to being produced with tailored properties, such as exhibiting a relatively narrow range of pore sizes. Manufacturers are able to produce SAC type gels with very distinct and well controlled pore sizes, dramatically unlike the pore structure of conventional activated carbons or conventional silica gel. By empirically measuring the pore size of each grade of SAC type gel filter media, it has been determined that more than 48 distinctly different types of HOX media exhibiting a relatively narrow range of pore sizes are readily available, each with a unique and narrow range of pore sizes. This determination has enabled an improved method for removing siloxane contaminants from gases to be achieved by constructing a single deep layer or several-layered filter bed, wherein the media in each layer has been selected, based on the pore sizes of the media, to selectively favor the removal of one class of the contaminants entrained in the gas stream. Where multiple layers are implemented, the layers are arranged in sequence such that the layer with the largest pore sizes is closest to the gas inlet, and the layer with the smallest pore sizes is closest to the gas outlet.

Two chemical companies that can provide HOX media (including SAC type gels) exhibiting a relatively narrow range of pore sizes are Grace Davison of Columbia, Md., and the Qingdao Haiyang Chemical Company of Qingdao, China.

Another useful class of materials for implementing HOX media are zeolite molecular sieves, which are crystalline, highly porous aluminosilicates. Zeolites are characterized by a three-dimensional pore system, with pores of precisely defined diameter. Generally, they contain silicon, aluminum, and oxygen in their framework and cations, and water and/or other molecules within their pores. Many occur naturally as minerals and are extensively mined in many parts of the world. Others are synthetic and are made commercially for specific uses, or produced by research scientists trying to understand more about their chemistry. The corresponding crystallographic structure is formed by tetrahedras of (AlO.sub.4) and (SiO.sub.4). These tetrahedras are the basic building blocks for various zeolite structures, such as zeolites A and X, the most common commercial adsorbents. Synthetic zeolites are available. For example, the Qingdao Haiyang Chemical Company of Qingdao, China offers synthetic zeolites that can be beneficially utilized in accord with the present invention.

Zeolites and silica gels function on the basis of the physics of adsorption. Adsorption occurs due to van der Waals interactions and capillary condensation at high humidity. Due to the presence of alumina, zeolites exhibit a negatively charged framework, which is counter-balanced by positive cations, resulting in a strong electrostatic field on the internal surface. These cations can be exchanged to fine-tune the pore size or the adsorption characteristics. For instance, the sodium form of zeolite A has a pore opening of approximately 4 .ANG.ngstrom (4.times.10-.sup.10 m) and is referred to as a 4 A molecular sieve. If the sodium ion is exchanged with the larger potassium ion, the pore opening is reduced to approximately 3 .ANG.ngstrom (i.e., providing a 3 A molecular sieve). On ion exchange with calcium, one calcium ion replaces two sodium ions. Thus, the pore opening increases to approximately 5 .ANG.ngstrom (providing a 5 A molecular sieve). Ion exchange with other cations is sometimes used for particular separation purposes.

The adsorption force of SAC type gels is less than the adsorption force for zeolites, resulting in a lower adsorption capacity at low concentrations of adsorbates. However, SAC type gels are available in a wider variety of different pore sizes than are zeolites. Thus, it is more likely that an SAC type gel can be found with a range of pore sizes corresponding to a contaminant to be removed from a gas stream. If a zeolite or synthetic zeolite is available having a pore size that can be used to remove a particular contaminant from a gas stream, then zeolites can also be beneficially employed as an adsorbent in accord with the present invention.

Particularly with respect to HOX media available from the Qingdao Haiyang Chemical Company, useful mineral adsorption media comprise aluminum, calcium, hydrogen, magnesium, manganese, oxygen, potassium, iron, silicon, and sodium. The HOX media varies in color from opaque, off-white, grey, tan, and other colors or combinations of these colors, depending on the grade. The HOX media available as irregular granules or spheres and primarily comprise silicon, hydrogen, oxygen, and sodium (or potassium) with smaller varying amounts of other elements, such as aluminum (as an oxide), are similar to silica gel. The HOX media comprising smaller amounts of silica and greater varying amounts of other elements are more similar to naturally occurring zeolites or diatomites.

With respect to manufactured HOX media, manufactures can readily vary the particular physio-chemical properties of the HOX media, in a manner not easily accomplished with carbon-based media. Of particular benefit is the ability to vary the range of pore sizes in the HOX media to accommodate removal of a broader spectrum of contaminants. The ability to manufacture such HOX media in a wide variety of shapes is also a benefit. Still another benefit is that empirical tests data indicate that the HOX media can adsorb and store a greater volume by weight of contaminants than can carbon or graphite-based media (most likely due to the huge internal volume of HOX media based on silica gel). Since HOX media are not graphite-based, they are not subject to ignition, an important factor to consider in fuel gas processing. Further, some types of HOX media are more easily regenerated thermally, in place, for reuse, providing an additional economy over graphite-based media. Because the pore structure of the HOX media is available in a wide variety of sizes (and manufacturers have the process knowledge required to selectively control pore size), many different contaminants can be adsorbed by selecting HOX media having an appropriate corresponding pore size.

Carbon Adsorbent Media Exhibiting a Relatively Wide Range of Pore Sizes

Commercially available activated carbon, manufactured from wood, sawdust, bituminous coal, rice hulls, lignite, peat, or petroleum residues, generally exhibits a range of pore sizes varying widely in an individual batch, including pores as small as about 0.7 nm to as large as about 10,000 nm. For filtration based on a homogenous bed of activated carbon, some variation in pore sizes is actually a benefit, because the homogenous filter bed will include pores able to facilitate the removal of molecules of widely disparate sizes. While pore sizes in excess of about 500 nm are generally too large to facilitate adsorption of gas molecules, the wide variation of pores sizes from about 0.7 nm to about 500 nm enable activated carbons to serve as a good adsorbent material for a wide variety of materials. Homogeneous activated carbon filter beds can successfully remove a wide range of contaminants from fluids; however, certain compounds, such as siloxanes in parts per million concentratio