Abstract:
Biogas released from landfills and sewage treatment plants is freed of siloxane contaminants by passing the biogas through a bed containing activated alumina, which absorbs the siloxanes. When the activated alumina becomes saturated with siloxanes, the absorption capability of the activated alumina can be recovered by passing a regeneration gas through the bed of activated alumina. A system containing two or more beds of activated alumina can use one bed to remove siloxanes from biogas while one or more of the other beds are being regenerated.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to processes for removing silicon-containing contaminants from gases. In particular, the present invention relates to processes for removing siloxanes from biogases released from landfills and sewage treatment plants. 
   2. Discussion of the Background 
   Landfills and sewage treatment plants contain siloxanes from many sources. 
   One source is the semiconductor industry, which produces siloxanes as by-products of reactions involving silicon compound gases. Because siloxanes have detrimental effects on semiconductor products, siloxanes are removed from semiconductor process gases by processes such as adsorption onto diatomaceous earth, silica gel, molecular sieves and activated alumina. 
   The personal care industry uses volatile methyl siloxanes in products such as deodorants, tooth-pastes, skin care preparations, hair conditioners and anti-perspirants. 
   The cleaning industry finds many applications for siloxanes. In dry cleaning siloxanes are used as a more environmentally friendly solvent than traditional chlorofluorocarbons. In the electronics industry, siloxanes are used to clean circuitry. 
   Siloxane-containing waste from industrial and domestic sources is discharged into landfill sites and sewage treatment plants, along with a variety of biological organic matter. 
   The organic matter in the waste decomposes to produce biogas containing various volatile organic compounds, such as methane. The biogas can be used to fuel various combustion engines. 
   However, the biogas from landfill sites and sewage treatment plants is contaminated with siloxanes. When an engine burns siloxane-contaminated biogas, the siloxane forms precipitates of silicon dioxide. The precipitates are deposited on engine parts such as turbine blades, cylinders, heat exchangers and emission control equipment. The deposits increase the abrasion of engine surfaces, leading to a loss of engine efficiency and premature engine failure. The deposits also poison catalytic converters. 
   Previous attempts at removing siloxane contaminants from biogas have used adsorbents such as activated charcoal, molecular sieves and silica gel. 
   Improved techniques for removing siloxanes from biogas are needed. 
   SUMMARY OF THE INVENTION 
   The present invention provides a siloxane removal system for removing siloxanes from biogas emitted from landfills and sewage treatment plants. The biogas is passed through a bed containing activated alumina. Siloxane contaminants in the biogas are adsorbed on the activated alumina. When the activated alumina becomes saturated with siloxanes and loses its ability to remove siloxanes from biogas, the activated alumina is regenerated by passing a regeneration gas through the activated alumina to remove the adsorbed siloxanes. In systems containing two or more beds of activated alumina, one bed can be used to remove siloxanes from biogas while one or more of the other beds are being regenerated. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The preferred embodiments of this invention will be described in detail with reference to the following figures. 
       FIG. 1  is a schematic of a single bed siloxane removal system. 
       FIG. 2  is a schematic of a single bed siloxane removal system. 
       FIG. 3  is a schematic of a single bed siloxane removal system. 
       FIG. 4  is a schematic of a single bed siloxane removal system. 
       FIG. 5  is a schematic of a single bed siloxane removal system. 
       FIG. 6  is a schematic of a dual bed siloxane removal system. 
       FIG. 7  is a schematic of a dual bed siloxane removal system. 
       FIG. 8  is a schematic of a dual bed siloxane removal system. 
       FIG. 9  is a schematic of a dual bed siloxane removal system. 
       FIG. 10  is a schematic of a dual bed siloxane removal system. 
       FIG. 11  is a schematic of a dual bed siloxane removal system. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   The present invention removes siloxane contaminates from biogas by passing the biogas through a bed containing activated alumina. The siloxanes are adsorbed onto the activated alumina, and a purified biogas leaves the bed. When the bed of activated alumina becomes saturated with siloxanes, the flow of biogas through the bed is stopped. The bed is then regenerated by passing a regeneration gas through the activated alumina and removing adsorbed siloxanes from the activated alumina. 
   The term “biogas” as used herein refers to a gas produced by the decomposition of organic matter. As used herein, a “biogas” can be obtained from both a landfill and a sewage treatment plant. If necessary, a pump can be used to extract and move biogas from a landfill or sewage treatment plant. 
   The siloxane removal system of the present invention can be made of any suitable material. To minimize corrosion when the biogas contains H 2 S, the system can be made with 316L stainless steel. 
   The siloxane removal system can include one or more beds containing activated alumina. In addition to removing siloxanes, the activated alumina can act as a dessicant and remove water from the biogas. Various other biogas constituents (e.g., carbon dioxide) can also be adsorbed on the activated alumina and removed from the biogas. 
   The beds can include activated alumina mixed with other materials. For example, the beds can include a mixture of activated alumina and silica (e.g., the product sold under the trademark SORBEAD™). 
   The activated alumina can be in the form of powders, beads and pellets. Activated alumina powder can include particles having diameters in the range of from 0.0003 inches to 1.5 inches, preferably from 0.01 inches to 1 inch, more preferably from 0.1 inches to 0.5 inches. 
   The regeneration gas can be any gas that does not react with activated alumina. Preferably, the regeneration gas is air or nitrogen. Preferably, the regeneration gas is dry and essentially free of water. For example, the regeneration gas can be clean dry air as defined by ISO Standard 8573.1 and Quality Class 1.2.1. In embodiments, the regeneration gas can be biogas. Because biogas is not suitable for release into the environment, biogas used as regeneration gas should be vented to a flare or introduced back into the source of the biogas. 
   To speed regeneration, the activated alumina can be heated during regeneration. The heating of the activated alumina can be accomplished using regeneration gas that is heated before coming into contact with the activated alumina. The heating of the activated alumina can also be accomplished using electrical heaters in contact with the activated alumina. The activated alumina can be heated to a temperature in the range of from 100° F. to 250° F., preferably from 150° F. to 225° F. 
   A “gas mover”, such as a blower, a compressor or a vacuum pump, can be used to move the regeneration gas through a bed of activated alumina. The blower and the compressor push the regeneration gas through the activated alumina. The vacuum pump pulls the regeneration gas through the activated alumina. In embodiments, two or more gas movers can be used to move the regeneration gas through the activated alumina. 
   Biogas from which siloxanes have been removed can be burned in various combustion engines without forming harmful silicon dioxide precipitates. 
   Having generally described the present invention, reference is now made to the following examples, which are provided for purposes of illustration only and are not intended to be limiting. 
   EXAMPLES 
   Example 1 
   Single Bed Siloxane Removal System 
     FIG. 1  illustrates the passage of siloxane-contaminated biogas through a single bed containing activated alumina. In  FIG. 1 , siloxane removal system  1  includes a single chamber  100  containing a bed comprising activated alumina. Chamber  100  includes a biogas input  102 , a biogas output  104 , a regeneration gas input  106  and a regeneration gas output  108 . Biogas from biogas source  10  enters chamber  100  via a biogas input  102 . If the pressure differential between biogas source  10  and chamber  100  does not provide a sufficient flow of biogas, the biogas can be forced from biogas source  10  into the chamber  100  by techniques well known in the art using a gas mover (not shown), such as a blower, a compressor or a vacuum pump. In chamber  100 , siloxane contaminants in the biogas are adsorbed onto the activated alumina. Purified biogas then leaves chamber  100  via biogas output  104 . The purified biogas can be immediately burned in biogas burner  150 . Alternatively, the purified gas can be collected in a storage tank (not shown) for later use. 
   In embodiments, at biogas input  102  the biogas can have a pressure in a range of from 2 psig to 5000 psig, more preferably from 10 psig to 1000 psig; a temperature in a range of from 34° F. to 125° F., more preferably from 50° F. to 100° F.; and a relative humidity at 75° F. in a range of from 10% to 100% RH, more preferably from 15% to 50% RH. 
   When the activated alumina bed in chamber  100  has become saturated with siloxane, the activated alumina must be regenerated. To regenerate a siloxane-saturated bed of activated alumina, the bed is first isolated from the biogas source  10 . Preferably the bed is then brought to atmospheric pressure conditions. 
   Regeneration can be accomplished in a variety of ways in which a regeneration gas is forced through the bed of siloxane-saturated activated alumina using a gas mover. In embodiments of the present invention, the gas mover is one or more of a blower, a compressor and a vacuum pump. 
     FIG. 2  illustrates embodiments in which the single bed system of  FIG. 1  is regenerated by first disconnecting chamber  100  from biogas source  10  and then connecting chamber  100  to regeneration gas source  110 . Regeneration gas from regeneration source  110  is pushed by blower  120  into chamber  100  via regeneration gas input  106 . The siloxane concentration gradient between the siloxane-saturated activated alumina in chamber  100  and the regeneration gas releases adsorbed siloxanes to the regeneration gas. Preferably, the regeneration gas is blown through chamber  100  until all of the siloxane is removed from the activated alumina. Siloxane-contaminated regeneration gas leaves chamber  100  via regeneration gas output  108 . 
   In embodiments of the present invention, the flow of the regeneration gas is nominally parallel to the flow of the biogas in the bed of activated alumina. In other embodiments the flow of the regeneration gas can be countercurrent to the flow of biogas in the bed of activated alumina. 
   In embodiments, at regeneration gas input  106  the regeneration gas can have a relative humidity at 75° F. in a range of from 0% to 10% RH, preferably from 0% to 5% RH. 
   In embodiments (not shown), biogas input  102  and regeneration gas input  106  can be the same. In other words, chamber  100  can have a single input for both the biogas and the regeneration gas. 
   In other embodiments (not shown), biogas output  104  and regeneration gas output  108  can be the same. In other words, chamber  100  can have a single output for both the biogas and the regeneration gas. 
     FIG. 3  illustrates a refinement of the embodiments of  FIG. 2 . In  FIG. 3  the regeneration gas is pushed by blower  120  through a regeneration gas heater  130  before entering chamber  100  via regeneration gas input  106 . The regeneration gas heater  130  heats the regeneration gas. Upon entering chamber  100 , the heated regeneration gas heats the activated alumina and causes adsorbed siloxanes to desorb from the activated alumina into the regeneration gas. Preferably the heating is continued until all of the siloxane is removed from the activated alumina. Siloxane-contaminated regeneration gas leaves chamber  100  via regeneration gas output  108 . If biogas is used as the heated regeneration gas, then the biogas exiting chamber  100  can be fed back (not shown) into biogas source  10  for reuse as a regeneration gas or for purification. 
     FIG. 4  illustrates embodiments in which the single bed system of  FIG. 1  is regenerated by first isolating chamber  100  from biogas source  10  and then connecting chamber  100  to regeneration gas source  110 . Regeneration gas from regeneration source  110  is pulled by vacuum pump  140  into chamber  100  via regeneration gas input  106 . The siloxane concentration gradient between the siloxane-saturated activated alumina in chamber  100  and the regeneration gas releases adsorbed siloxanes to the regeneration gas. Preferably, the regeneration gas is pumped through the chamber  100  until all of the siloxane is removed from the activated alumina. Siloxane-contaminated regeneration gas leaves chamber  100  via regeneration gas output  108  and enters vacuum pump  140 . 
     FIG. 5  illustrates a refinement of the embodiments of  FIG. 4 . In  FIG. 5  the regeneration gas is pulled by vacuum pump  140  through a regeneration gas heater  130  before entering chamber  100  via regeneration gas input  106 . The regeneration gas heater  130  heats the regeneration gas. Upon entering chamber  100 , the heated regeneration gas heats the activated alumina and causes adsorbed siloxanes to desorb from the activated alumina into the regeneration gas. Preferably the heating is continued until all of the siloxane is removed from the activated alumina. Siloxane-contaminated regeneration gas leaves chamber  100  via regeneration gas output  108  and enters vacuum pump  140 . 
   In the embodiments of  FIGS. 2-5 , after passing through chamber  100  the regeneration gas can be vented to the atmosphere or recycled (not shown) back to the regeneration gas source  110 . 
   Using single bed siloxane removal system  1 , siloxane removal and activated alumina regeneration alternate in chamber  100  to provide an intermittent flow of biogas from which siloxanes have been removed. 
   Example 2 
   Dual Bed Siloxane Removal System 
   Greater efficiency in removing siloxanes from biogas can be achieved using siloxane removal systems including two or more beds containing activated alumina, because one bed can be used to remove siloxanes from biogas while one or more of the other beds are being regenerated. 
     FIG. 6  illustrates a two bed siloxane removal system. In  FIG. 6  siloxane removal system  2  includes a chamber  100  and a chamber  200 , where each chamber contains a bed comprising activated alumina. Chamber  100  includes a biogas input  102 , a biogas output  104 , a regeneration gas input  106  and a regeneration gas output  108 . Chamber  200  includes a biogas input  202 , a biogas output  204 , a regeneration gas input  206  and a regeneration gas output  208 . Biogas input  102  is connected to biogas input  202 . Biogas output  104  is connected to biogas output  204 . 
   Initially chamber  200  remains in stand-by mode, while biogas from biogas source  10  enters chamber  100  via a biogas input  102 . If necessary, the biogas can be forced from biogas source  10  into the chamber  100  by techniques well known in the art using a gas mover (not shown), such as a blower or a vacuum pump. In chamber  100 , siloxane contaminants in the biogas are adsorbed onto the activated alumina. Purified biogas then leaves chamber  100  via biogas output  104 . The purified biogas can be immediately burned in biogas burner  150 . Alternatively, the purified gas can be collected in a storage tank (not shown) for later use. 
   Eventually the activated alumina in chamber  100  becomes saturated with siloxanes and needs to be regenerated. 
     FIGS. 7-10  illustrate the regeneration of chamber  100  in embodiments that correspond to the single bed embodiments illustrated in  FIGS. 2-5 , respectively. In the embodiments of  FIGS. 7-10 , while chamber  100  is being regenerated the flow of biogas from biogas source  10  is redirected. The biogas is diverted from chamber  100  to chamber  200  (i.e. from biogas input  102  to biogas input  202 ) for removal of siloxane contaminants. At the same time the source of purified biogas to biogas burner  150  is switched from biogas output  104  of chamber  100  and to biogas output  204  of chamber  200 . Means of redirecting gas flows are well known in the art and include, e.g., valves. In  FIGS. 7-10 , while activated alumina is being regenerated in chamber  100 , siloxanes are being removed from biogas in chamber  200 . 
   In  FIG. 7 , regeneration of the activated alumina in chamber  100  by removal of adsorbed siloxanes is accomplished by blowing regeneration gas through chamber  100  using blower  120 . In  FIG. 8 , the regeneration gas from blower  120  is heated by regeneration gas heater  130  before entering chamber  100 . 
   In  FIG. 9 , regeneration of the activated alumina in chamber  100  is accomplished by pulling regeneration gas through chamber  100  using vacuum pump  140 . In  FIG. 10 , the regeneration gas pulled by vacuum pump  140  is heated by regeneration gas heater  130  before entering chamber  100 . 
   After the activated alumina in chamber  100  is regenerated, chamber  100  can be reconnected to biogas source  10  to remove siloxanes from biogas. At the same time chamber  200  can be disconnected from biogas source  10  and regenerated using processes analogous to those described above. In embodiments, regeneration gas can be pushed through chamber  200  using blower  220  or can be pulled through chamber  200  using vacuum pump  140 . In these embodiments, the regeneration gas can be heated by regeneration gas heater  230  before entering chamber  200 . 
   In other embodiments, as illustrated in  FIG. 11 , the regeneration system formed by blower  120 , regeneration gas heater  130  and vacuum pump  140  used to regenerate the activated alumina in chamber  100  can be used to regenerate the activated alumina in chamber  200 . In particular, regeneration gas can be pushed through chamber  200  using blower  120  or can be pulled through chamber  200  used vacuum pump  140 . In these other embodiments, the regeneration gas can be heated by regeneration gas heater  130  before entering chamber  200 . 
   In still further embodiments, not shown but well within the skill in the art, three or more beds of activated alumina can be regenerated using a single regeneration system. 
   In even still further embodiments, not shown but well within the skill in the art, a bed of activated alumina can be regenerated using the regeneration gas from two or more regeneration systems. 
   In some cases activated alumina that has been heated during regeneration will have to be cooled before the regenerated activated alumina can again be used to remove siloxanes from biogas. The cooling can be accomplished by a variety of methods known in the art. For example, the hot activated alumina can be cooled by pushing with blower  120  (or blower  220 ), or pulling with vacuum pump  140 , unheated regeneration gas through the activated alumina and venting the cooling gas to the atmosphere. A gas having a different composition than the regeneration gas (not shown) can be used as a cooling gas to cool the activated alumina. Cooling can also be accomplished by convection cooling, preferably with the regenerated activated alumina at atmospheric pressure. 
   Using dual bed siloxane removal system  2 , siloxane removal and activated alumina regeneration can be carried out cyclically in chambers  100  and  200  to provide a continuous flow of biogas from which siloxanes have been removed. 
   While the present invention has been described with respect to specific embodiments, it is not confined to the specific details set forth, but includes various changes and modifications that may suggest themselves to those skilled in the art, all falling within the scope of the invention as defined by the following claims. The disclosure herein of a range of values is a disclosure of every numerical value within that range.