Patent Application
20130137567
The invention relates generally to the field of biogas processing, and more particularly, to the reuse of media used to remove siloxanes from biogas.
To position Applied Filter Technology as the leader of biogas cleaning technology, a careful review of current technologies has been made regarding siloxane remediation. Adsorbent media is clearly the best solution since other developed technologies, such as membranes and acid-base catalysis have higher total costs. There are many types of adsorbent media, however, and regeneration can be effected by various techniques.
The available adsorbent media ranges from activated carbon to zeolites to polyamide resins. Polyamide resins are expensive and have poor temperature stability (below 160C.), although they are currently successfully used for this purpose by PpTek, (J. Hayward; Fuel cleaning for gas fired engines, U.K. Patent 2,440,123, Jul. 19, 2006). Not only do zeolites have a higher price per adsorptive capacity (Finocchio et al., Purification of biogases from siloxanes by adsorption: On the regenerability of activated carbon sorbents; Energy & Fuels, 23(8):4156-4159, 2009.), they are destroyed by siloxanes, reducing their life (Parker et al., Unexpected destructive dealumination of zeolite beta by silylation; Journal of Physical Chemistry C, 114(8):8459-8, May 13, 2010).
Activated carbon also experiences a loss in its ability to adsorb siloxanes (Finocchio et al., supra.), however, due to its attractive cost and since it has been successfully regenerated, (Mark Rawson; Removal of siloxane and H2S from biogas using microwave energy; Draft final report, Sacramento Municipal Utility District, 6201 S Street, Sacramento, Calif. 95817, 2011. Public Interest Energy Research Program, (PIER)) it appears to be the most likely candidate for a regenerable system.
Considering the technique for regeneration, generally the process is performed using a temperature-swing adsorption (TSA) technique. Indeed, simply using pressure-swing adsorption or pressure-vacuum-swing adsorption does not yield a significant enough amount of media regeneration for a variety of reasons. However, temperature-swing adsorption can be implemented in a variety of ways, such as heating the media using hot gas which is what PpTek (refer to Hayward, supra) or Domnick Hunter (a division of Parker Hannifin Corp., Gateshead, U.K.) uses, or heating the media using microwaves (C.Y. Cha; Process for microwave air purification, U.S. Pat. No. 6,207,023, Mar. 27, 2001). Other permutations of these ideas exist (Paul Tower et al.; Regenerable purification system for removal of siloxanes and volatile organic carbons, U.S. Pat. No. 7,410,524, Aug. 12, 2008).
Both of these techniques are fraught with problems. PpTek and Domnick Hunter need large volumes of gas at a very low specific heat, such as air or exhaust gas, to heat a refractory substance which has a high specific heat The very large volume of gas required necessitates equipment which can process the resulting large, dilute volume of gas. On the other hand, microwaves have poor penetration so the device described in U.S. Pat. No. 6,207,023 goes through great lengths to move the media into a small regeneration chamber. The utility of this process is the smaller amount of gas produced, which can then be more easily destroyed catalytically.
A search using the search terms “carbon media heating electric” yields one similar patent, Carbon fiber composite molecular sieve electrically regenerable air filter media (Wilson et al.; U.S. Pat. No. 5,827,355, Oct. 27, 1998). This patent discusses the technique of electrical heating of carbon fibers, but we recognize that most other carbon media can be regenerated by passing an electric current through it. It also primarily address malodorous substances and focuses this technology on its employment in air, not biogas. Also, the media in this patent is a block of carbon fibers. Such a solid block is quite easily heated with an electric current.
Another relevant patent is Gas separation device based on electrical swing adsorption (Judkins et al.; U.S. Pat. No. 5,972,077, Oct. 26, 1999). However, this patent discusses the technique of electrical heating of carbon fibers, concerns itself with the remediation of hydrogen sulfide and carbon dioxide, not siloxanes, and focuses its employment for natural gas, not biogas.
Herein we propose yet another technique for heating the media. It requires no movement of the media to another location, yet heats the media evenly and the small volume of exhaust gas generated can be catalytically destroyed. Notably, the present invention recognizes that any carbon media can be regenerated by passing an electric current through it.
As is usual with adsorbent media, a pressure vessel is employed to contain the media. In this case, however, the vessel contains a special insert, typically configured as a rectangular cylinder, so that the media, in the insert, is held in a generally rectangular (e.g., square) shape, see
Two opposing faces of the insert are conductive, while the rest of the insert is manufactured from a suitable dielectric material. Such a dielectric material would be non-conductive, resistant to melting or deformation at the regeneration temperature within the range of 200-400° C., and have no components which could contaminate the gas for a specific application. Suitable dielectric materials could include a fluoropolymer material (e.g., TEFLON, etc.), a vitreous material (e.g., glass) or a ceramic material. The meaning of ‘suitable’ in this context means at least that a selected dielectric material is not so brittle that it would be easily damaged by vibration or handling that would be expected during ordinary use.
Unlike the solid block media of U.S. Pat. No. 5,827,355, a distinguishing feature of this invention is that we employ granular or pelletized media. Because granular or pelletized media is not as easily heated as is a solid block, we include another device as described below. The advantage to using granulated or pelletized media is that the large volume of media, sometimes on the order of tons, can be moved in and out of a vessel with a vacuum truck, which would be impossible with a solid block.
During normal operation, no heating current is passed through the media, and the media is used in a typical way, as depicted in
An important feature of this design is that media loading can be calculated from the relative capacitance change of the media. Such a capacitance-measuring device, 16, is also depicted in
During this maneuver, heating current is passed through the media so that it begins to release the siloxanes or any other contaminant contained within it. This is effected by turning on the power source 13, until the capacitance measured by device 16 is low enough to indicate that the media has been regenerated.
During media regeneration as well, granular or pelletized media may crumble, oxidize, or in some other way lose electrical contact with the electrodes. This drives the net resistance of the system up and therefore the current downwards, which leads to a decrease in heating. This effect has been observed using the first prototype. To prevent this effect from happening, a vibrator 17 is attached to the exterior of the insert or to a vessel within which the insert is retained during use. This vibrator operates during system regeneration, and its operation is controlled by the control system.
Since the media is held in a square shape, the electric field through the media is homogeneous throughout. This ensures even heating and complete regeneration.
We recognize that non-carbon media which is conductive such as that invented by H. Shigemitsu (Polyamide resin composition excellent in plate adhesion, U.S. Pat. No. 4562221, Dec. 31, 1985) can also be used for this purpose.
We also recognize that the media can be used to clean any number of contaminants, such as volatile organic hydrocarbons.
We recognize that the media may not return to its original adsorptive capacity due to polymerization of the captured siloxanes. However, a control system can track the changes in capacitance and predict when the media may eventually require replacement.
It will be understood that the present invention is not limited to the method or detail of construction, fabrication, material, application or use described and illustrated herein. Indeed, any suitable variation of fabrication, use, or application is contemplated as an alternative embodiment, and thus is within the spirit and scope of the invention.
It is further intended that any other embodiments of the present invention that result from any changes in application or method of use or operation, configuration, method of manufacture, shape, size, or material, which are not specified within the detailed written description or illustrations contained herein yet would be understood by one skilled in the art, are within the scope of the present invention.
Preferably, although not exclusively, those of skill in the art will appreciate that the invented method, system and apparatus described and illustrated herein may be implemented in a combination of the three, for purposes of low cost and flexibility.
Accordingly, while the present invention has been shown and described with reference to the foregoing embodiments of the invented apparatus, it will be apparent to those skilled in the art that other changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined in the appended claims.
This application claims the benefit of priority to U.S. Provisional Application number 61/491,104, filed on May 27, 2011 and entitled A DEVICE TO CLEAN SILOXANES FROM BIOGAS, WHICH IS REGENERABLE BY WAY OF ELECTRIC SWING ADSORPTION, the contents of which are hereby incorporated herein in their entirety by this reference.
| Number | Date | Country | |
|---|---|---|---|
| 61491104 | May 2011 | US |
