Abstract:
A method of removing siloxanes from a gas that contains siloxanes and water, the method comprising: (a) expanding the gas to cool the gas and freeze at least some of the water in the gas; and (b) removing the siloxanes and frozen water from the expanded and cooled gas. The method may also include compressing the gas prior to expanding it. The step of expanding the gas may include expanding it through a turbine. The method may also include using an energy input mechanism to drive one or both of the compressor or turbine. The ice and siloxanes may be removed from the gas with a cyclonic separator.

Description:
BACKGROUND 
       [0001]    The present invention relates to a system and method for removing water and siloxanes from a gas. 
       SUMMARY 
       [0002]    In one embodiment, the invention provides a method of removing water and siloxanes from a gas, the method comprising: (a) expanding the gas to cool the gas and freeze at least some of the water in the gas; and (b) removing the siloxanes and frozen water from the expanded and cooled gas. The method may also include compressing the gas prior to expanding it. The step of expanding the gas may include expanding it through a turbine. The method may also include using an energy input mechanism to drive one or both of the compressor or turbine. The ice and siloxanes may be removed from the gas with a cyclonic separator. 
         [0003]    In another embodiment, the invention provides a system for removing siloxanes and water from a gas, the system comprising: means for expanding and cooling the compressed gas to freeze water in the gas; and a separator configured to remove ice and siloxanes from the expanded and cooled gas. 
         [0004]    Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIG. 1  is a schematic illustration of a first embodiment of a fuel conditioner of the present invention. 
           [0006]      FIG. 2  is a schematic illustration of a second embodiment of a fuel conditioner of the present invention. 
           [0007]      FIG. 3  is a schematic illustration of an optional fuel booster. 
           [0008]      FIG. 4  is a schematic illustration of a microturbine engine generator system for use with the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0009]    Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless specified or limited otherwise, the terms “mounted,” “connected,” “supported,” and “coupled” and variations thereof are used broadly and encompass both direct and indirect mountings, connections, supports, and couplings. Further, “connected” and “coupled” are not restricted to physical or mechanical connections or couplings. Also, although the illustrated embodiments include specific gas pressure and temperature data, such data is specific to the illustrated embodiments and should not be regarded as limiting the scope of the invention except to the extent specified in the claims. 
         [0010]      FIG. 1  illustrates a fuel conditioning system  10  that receives gas from a fuel source  15 , removes water and impurities from the gas, and delivers the gas to a fuel consuming device  20 . The system  10  includes a scrubber  25 , a compressor  30 , an energy input mechanism  35 , a turbine  40  coupled for rotation with the compressor  30  by way of a shaft  45 , an aftercooler  50 , an airflow mechanism  55 , an economizer  60 , a moisture separator  65 , and a solids separator  70 . 
         [0011]    The fuel source  15  may be, for example, a waste water treatment facility, landfill, or other site from which gas is extracted. The impurities in the gas may be, for example, siloxanes or other contaminants that would cause pollution or damage to a combustion chamber and associated moving parts if not removed from the gas. The fuel consuming device  20  may be, for example, a flare that burns the gas to reduce the amount of unburned hydrocarbons that are released into the environment. Alternatively, the fuel consuming device may be an engine that uses the gas fuel for doing work. Examples of such engines include reciprocating engines, microturbine engines, and larger gas turbine engines. Examples of work done by such engines include production of electricity, driving chillers, refrigerators, or compressors, cogeneration of hot water, and raising, lowering, or otherwise moving objects. 
         [0012]    In a typical waste water treatment facility or landfill, gas is extracted from the site at about 0 psig and 100° F., which is the pressure and temperature at which certain reactions take place in a gas digester at the facility. In some cases, the temperature and pressure of the gas will vary depending on the type of reaction taking place. The gas is fed into the scrubber  25 , which removes water droplets that are entrained in the gas. The gas is 100% saturated (e.g., at its dewpoint) at the outlet of the scrubber  25 . The saturated gas flows into the compressor  30 , in which the pressure of the gas is raised to about 15 psig and the temperature is raised to 179° F. In this regard, the compressor  30  energizes the gas prior to the gas entering the turbine  40 . In other embodiments, the gas can be energized by different means, such as a blower. 
         [0013]    The inherent inefficiencies of the compressor-turbine assembly require additional work to make the fuel conditioning system  10  function. This additional energy is provided by the energy input mechanism  35 , which may also be used to start the process. The illustrated energy input mechanism  35  includes a Pelton wheel  75  mounted for rotation with the compressor  30  (e.g., on the shaft  45 ), an electric motor  80 , an oil compressor  85 , and a variable frequency drive control system  90 . The variable frequency drive control system  90  senses a parameter within the fuel conditioning system  10  and adjusts the speed of the motor  80  to maintain the parameter within a desired range. The measured parameter may be, for example, the pressure, temperature, or volumetric flow of the gas at the inlet or outlet of the turbine  40 , or some other parameter that is indicative (i.e., from which can be calculated or inferred) the temperature of the gas. The motor  80  drives the oil compressor  85 , which in turn causes a flow of oil to impinge upon the Pelton wheel  75  to cause rotation of the Pelton wheel  75  and compressor  30 . In the illustrated embodiment, the control system  90  controls the motor  80  to maintain a turbine outlet temperature of about −20° F. 
         [0014]    In alternative embodiments, the compressor  30  and turbine  40  may not be coupled for rotation together and the energy input mechanism  35  may only drive rotation of one of them. For example, if the energy input mechanism  35  drives rotation of the compressor  30  only, the energy in the compressed gas will cause rotation of the turbine  40 . In other embodiments, a pre-compressor (driven by an energy input mechanism) may be positioned upstream of the compressor  30  to provide sufficient energy to the flow of gas to drive rotation of the compressor/turbine assembly, in which case the compressor/turbine assembly may be free-spinning. In other embodiments, the energy input mechanism  35  may include an electric motor  80  directly driving the compressor  30  or driving the compressor  30  through a magnetic coupling. The energy input mechanism  35  may take on many other forms in other embodiments, provided that the energy input mechanism  35  provides energy to perform work. 
         [0015]    From the compressor  30 , the gas flows through the aftercooler  50 , which in the illustrated embodiment utilizes a flow of air to cool the compressed gas. The flow of air is supplied by the airflow mechanism  55 . In the illustrated embodiment, the airflow mechanism  55  includes a motor  95 , a fan  100 , and a variable frequency drive control system  105 . The variable frequency drive control system  105  controls the speed of operation of the motor  95  and fan  100  to maintain another parameter within a desired range. In the illustrated embodiment, for example, the variable frequency drive control system  105  attempts to maintain the gas temperature at the outlet of the economizer  60  at around 40° F. There will be some pressure drop in the gas as it flows through the aftercooler  50 , and the pressure in the illustrated embodiment will be around 15.01 psig at the aftercooler outlet. The temperature of the gas upon exiting the aftercooler is about 83° F. In other embodiments, a temperature-controlled mixing valve can be used in place of the variable frequency drive control system  105 . 
         [0016]    Then the gas flows through the economizer  60 , which in the illustrated embodiment is a counterflow heat exchanger that cools the gas about to enter the turbine  40  (the “inflowing gas”) while warming the gas leaving the solids separator  70  (the “outflowing gas”). The economizer  60  may be, for example, a plate-fin heat exchanger that permits heat to flow from the relatively hot inflowing gas to the relatively cold outflowing gas without mixing the gas flows. As mentioned above, the airflow mechanism  55  is controlled to create a gas temperature of about 40° F at the outlet of the economizer  60 . A slight pressure drop across the economizer  60  will drop the gas pressure to around 14.72 psig. 
         [0017]    In alternative embodiments, the aftercooler  50  or the economizer  60  or both may be replaced with a refrigeration system that cools the gas temperature to the temperatures described above. 
         [0018]    Prior to flowing into the turbine  40 , the gas flows through the moisture separator  65 . The moisture separator  65  removes any droplets of water that have formed within the gas as a result of condensation during the reduction of the gas temperature through the aftercooler  50  and economizer  60 . Because the gas temperature has been maintained above the freezing temperature of water (such temperature referred to herein as “freezing” for simplicity) to this point, there should not be significant ice or frost buildup within the aftercooler  50  and economizer  60 . The aftercooler  50  and economizer  60  are helpful, however, in reducing the gas temperature to slightly above freezing so that the temperature reduction that results from expansion through the turbine  40  drops the gas temperature well below freezing. 
         [0019]    For embodiments in which a relatively large turbine  40  is used, the pressure of the gas may be reduced in an optional expander prior to the gas entering the turbine  40 , such that the gas pressure is within a range that matches the turbine size. Examples of relatively large turbines for this application include the Garrett Corporation models GT1241 and GT1544, which are sized for small displacement applications, including motorcycles. These relatively large turbines are suitable for a pressure drop of about 7 to 15 psig as contemplated in the embodiments of  FIGS. 1 and 2 . Relatively small turbines for this application, such as those used in dental equipment, may be more appropriate for high pressure applications. 
         [0020]    The gas next flows through the turbine  40 , which rotates with the compressor  30  under the influence of the energy input mechanism  35 . As the gas expands through the rotating turbine  40 , its temperature drops to about −20° F. and its pressure drops to about 0.76 psig. This causes remaining water in the gas to condense and freeze, which results in a flow of gas and ice at the outlet of the turbine  40 . Conventional heat exchangers rely on contact between air and large cooling surfaces to transfer heat. When gas having moisture content is passed through such conventional heat exchangers and the temperature is dropped below freezing, such conventional heat exchangers are prone to freezing up and becoming fouled with ice because of such contact, reducing the effectiveness of the heat exchanger. The expanding turbine of the present invention cools through expansion of the gas, not heat transfer across surfaces, which greatly reduces the incidence of ice fouling. Additionally, the turbine  40  in the illustrated embodiment rotates at a rate of between about 40,000 and 100,000 or higher rpm, depending on the size of the turbine  40 , and such high rate of rotation naturally sheds most ice that may form. To further inhibit the formation of ice in the illustrated turbine  40 , the temperature of oil lubricating the turbine  40  bearings can be adjusted to maintain warmer turbine blade temperatures and keep the material of the turbine blade at temperature above the temperature of the gas flowing through the turbine  40 . In alternative embodiments, the turbine  40  may be replaced with an air motor, a gear pump, a vane pump, a nozzle (e.g., a Joule-Thompson valve), or another mechanism for indirectly cooling the gas through expansion without substantial contact of the gas on the mechanism. 
         [0021]    The gas and ice flows into the solids separator  70 , in which the ice is separated from the gas. As the vapor pressure and temperature of the gas drops in the turbine  40 , siloxanes nucleate around the water and ice. Siloxanes are thus removed with the ice in the separator  70 . The gas flowing out of the solids separator  70  (i.e., the above-mentioned outflowing gas) is therefore dry and clean, is still at a temperature of about −20° F., and is at a pressure of about 0.45 psig (owing to a pressure drop through the solids separator  70 ). In the illustrated embodiment, the solids separator  70  includes two separators  110  so that if one separator  110  is fouled with ice, a valve  115  may be actuated to direct the flow to the other separator  110  while the fouled separator  110  is thawed. In one embodiment, the solids separator  70  takes the form of a cyclonic separator, and in other embodiments it may be a coalescer filter or a low-velocity plenum. 
         [0022]    The outflowing gas then flows through the economizer  60  to pre-cool the inflowing gas. This increases the temperature of the outflowing gas to about 23° F., and decreases the gas pressure to about 0.30 psig. Raising the outflowing gas temperature through the economizer  60  ensures that the gas will be above its dewpoint, thereby creating dewpoint suppression. Although the outflowing gas should be completely dry upon leaving the separator  70 , the dewpoint suppression reduces the likelihood that any remaining water will condense in the gas while it is being consumed in the fuel consuming device  20 . From the economizer  60 , the gas flows into the fuel consuming device  20 , or is directed back (via a valve  120 ) to mix with and cool the wet, dirty gas as it flows into the scrubber  25 . 
         [0023]      FIG. 2  schematically illustrates an alternative construction  125  of the fuel conditioning system, in which like components are identified with the same reference numerals used in  FIG. 1 . In this embodiment, there is no aftercooler  50 . The gas flowing out of the scrubber  25  is first run through the economizer  60  to reduce its temperature to about 40° F. A bypass valve  130  controls the amount of gas flowing into the cold side of the economizer  60  to ensure that the gas flowing out of the economizer is kept above freezing. Condensed water within the gas is then removed in the moisture separator  65 . Then the gas flows through the turbine  40 , in which the gas pressure is reduced to −7.5 psig and the gas temperature is reduced to −20° F. Then the gas flows through the solids separator  70  to remove ice and siloxanes. The gas then flows through the economizer  60 , where its temperature is raised to about 40° F. Finally, the gas flows through the compressor  30 , where the gas pressure is raised to about −1 psig and the gas temperature is raised to about 102° F. The compressor  30  is driven by an energy input mechanism  35  similar to the first embodiment. 
         [0024]    With reference to  FIG. 3 , some fuel consuming devices  20 , such as microturbine engine generators, operate most efficiently if the fuel gas is provided at elevated pressures (e.g., around 90 psig). Should the fuel consuming device  20  require relatively high-pressure fuel gas, an optional compressor or gas booster assembly  135  may be used to raise the gas pressure upstream of the fuel consuming device  20 . The illustrated optional compressor assembly  135  includes a compressor  140  driven by a motor  145  and a variable frequency drive control system  150  that is referenced to a parameter (e.g., gas pressure) of the gas entering the compressor assembly  135 . In the compressor  140 , the gas pressure is raised to about 90 psig and the gas temperature is raised to about 200° F. Also within the compressor assembly  135  is an aftercooler  155  that reduces the gas temperature to about 100° F. A fan  160  powered by a motor  165  blows air across the aftercooler  155  to facilitate heat transfer. In other embodiments, the gas booster  135  can be positioned upstream of the fuel conditioning system  10  such that relatively high pressure gas enters the system  10 . In such embodiments the boosted gas may provide sufficient energy to drive the compressor  30  and turbine  40  in the fuel conditioning system  10 , which would obviate the energy input mechanism  35 . For that matter, in a closed system, positioning the gas booster  135  downstream of the fuel conditioning system  10  may augment the expansion ratio across the turbine  40  of the fuel conditioning system  10 , and this may also obviate the energy input mechanism  35 . 
         [0025]      FIG. 4  schematically illustrates one type of fuel consuming device  20  that may be used in conjunction with either of the fuel conditioning systems  10 ,  125  described above and illustrated in  FIGS. 1 and 2 . The fuel consuming device in  FIG. 3  is a microturbine engine generator  170 , which is useful in distributed power applications, and can even be mounted on skids and moved between job sites. Microturbine engine generators usually generate 2 MW of power or less, and are therefore relatively small when compared to power generators in power plants that are on the grid. 
         [0026]    The illustrated microturbine engine generator  170  includes a compressor  175 , a recuperator  180 , a combustor  185 , a power turbine  190 , and an electric power generator  195 . Air is compressed in the compressor  175  and delivered to a cool side of the recuperator  180 . The recuperator  180  may be, for example, a counterflow plate-fin type heat exchanger. The compressed air is preheated within the recuperator  180  and mixed with a gaseous fuel from a fuel supply (e.g., one of the fuel conditioning systems  10 ,  125  described above and illustrated in  FIGS. 1 and 2 ) to create a combustible mixture. It is advantageous in a microturbine engine generator  170  to raise the pressure of fuel gas used in the combustible mixture to 90 psig, and the temperature to about 100° F. For such applications, the above-mentioned compressor assembly  135  may be positioned downstream of the fuel conditioning system  10 ,  125  and upstream of the microturbine engine generator  170 . 
         [0027]    The combustible mixture is combusted in the combustor  185  to create products of combustion. The products of combustion are then permitted to expand through the power turbine  190  to impart rotational energy to the power turbine  190 . Rotation of the power turbine  190  drives operation of the electric generator  195  through an optional gearbox  200  to produce electrical power at a useful frequency. In other embodiments, the power electronics may be used in place of the gearbox to condition the electrical signal into a useful frequency. In the illustrated mnicroturbine  170 , the power turbine  190  and compressor  175  are coupled for rotation together via a shaft  205 , so rotation of the power turbine  190  also drives rotation of the compressor  175 . In other embodiments, the power turbine  190  may only drive the power generator  195 , and an additional gasifier turbine may be used to drive the compressor  175 . In such embodiments, the products of combustion are expanded through both the power turbine  190  and the gasifier turbine. Prior to exhausting the products of combustion from the microturbine engine  170 , they flow into a hot side of the recuperator  180  to preheat the inflowing compressed air. Any remaining heat in the products of combustion is used for some other useful purpose (e.g., heating water) in a final heat exchanger  210  before the products of combustion are exhausted. 
         [0028]    Various features and advantages of the invention are set forth in the following claims.