Abstract:
A method of using a triple-effect absorption system to recover methane from landfill gas contaminated with CO 2  and trace contaminates such as chlorinated hydrocarbons and aromatics involves processing the landfill gas with three absorbers and a flash system. One absorber uses a solvent to absorb the trace contaminants from the landfill gas, the second absorber in conjunction with the flash system extracts CO 2  from the gas, and just a first portion of that CO 2  is used for stripping the trace contaminates from the solvent in the third absorber. The rest of the extracted CO 2  is vented to atmosphere to prevent dampening the combustion of the trace contaminants absorbed by the first portion of CO 2 .

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The subject invention generally pertains to processing landfill gas and more specifically to an absorption system for recovering methane gas. 
     2. Description of Related Art 
     Decomposing garbage buried in a landfill can generate landfill gas that can be extracted and processed to provide relatively clean methane gas. Processing plants have been developed for extracting the methane and disposing of its contaminants in a responsible manner. In order to attain the full value of the methane, processing plants need to remove the contaminants efficiently and thoroughly. Otherwise, unacceptably high concentrations of the contaminants can remain in the methane, thereby decreasing its value. In some cases, certain contaminants may accumulate in the processing plant itself, which can reduce the plant&#39;s operating efficiency. 
     Current gas processing plants are more efficient at removing some impurities than others, so there is still a need for a better method of recovering clean methane gas from landfills. 
     SUMMARY OF THE INVENTION 
     To provide a more efficient method of recovering methane gas from a landfill, it is an object of some embodiments of the invention to use one landfill gas impurity to help extract another impurity from a solvent that is used for purifying contaminated methane. 
     Another object of some embodiments, is to use CO 2  to strip trace contaminants from a solvent that absorbed the contaminants from a landfill gas. Using CO 2  as a stripping agent can help prevent the accumulation of solid sulfur, which is notorious for occluding various heat exchangers and other fluid passageways. 
     Another object of some embodiments, is to is extract CO 2  from one portion of solvent, and then use that CO 2  to strip trace contaminants from another portion of solvent. 
     Another object of some embodiments, is to extract CO 2  from a landfill gas and use just a minimal amount of the CO 2  as a stripping agent but only a minimal amount of it so as not create an unnecessarily high solvent loss or fuel gas requirements. 
     Another object of some embodiments is to extract CO 2  from a landfill gas and use only a minimal portion of the CO 2  to strip trace contaminants from a solvent so that the trace contaminants can be readily burned to heat the minimal portion of CO 2  to at least 1400° F. 
     Another object of some embodiments is to extract CO 2  from a landfill gas, vent a first portion of the CO 2  directly to atmosphere, and use only the remaining portion of the CO 2  to strip trace contaminants from a solvent so that the trace contaminants can be readily burned to heat the remaining portion of CO 2  to at least 1400° F. 
     Another object of some embodiments is to extract CO 2  from a landfill gas and heat a portion of the CO 2  to enhance its ability to absorb trace contaminants from a solvent. 
     Another object of some embodiments is to extract CO 2  from a landfill gas and heat a portion of the CO 2  to enhance its ability to absorb trace contaminants from a solvent, wherein the heat is generated by compressing the landfill gas. 
     Another object of some embodiments is to maintain the pressure in a first absorber nearly equal to or at least within 10% of the pressure in a second absorber so that landfill gas can be readily conveyed from one absorber to the other. 
     Another object of some embodiments is to maintain a first absorber and second absorber at a much greater pressure (preferably over 5 times as great) than a third absorber so that contaminants can be readily absorbed from the landfill gas in the first and second absorbers, and relatively low pressure CO 2  can be used to absorb trace contaminants from a solvent. 
     Another object of some embodiments is to convey solvent through two absorbers at substantially the same flow rate so as to balance the flow between the two. 
     Another object of some embodiments is to circulate solvent through one absorber at a flow rate that is much greater (preferably at least 10 times as great) than the flow rate of solvent through another absorber so that the flow rates are appropriate for removing CO 2  and trace contaminants from a landfill gas. 
     Another object of some embodiments is to responsibly exhaust CO 2  at a temperature of at least 1400° F. to ensure complete combustion of the trace contaminants such as chlorinated hydrocarbons and aromatics. 
     Another object of some embodiments is to control the flow of liquid solvent through a series of three flash tanks in such a way as to create a liquid gas barrier between adjacent flash tanks and to avoid starving a downstream solvent pump. 
     Another object of some embodiments is to install a control valve and a solvent pump between two absorbers, wherein the valve is controlled to maintain a predetermined liquid level in one of the absorbers, and the pump in conjunction with the valve helps maintain a substantial pressure differential between the two absorbers. 
     One or more of these and/or other objects of the invention are provided by a method of using a triple-effect absorption system that includes three absorbers and a flash system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  is a schematic diagram of a triple-effect absorption system that illustrates a method of processing landfill gas to recover methane. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to  FIG. 1 , a triple-effect absorption system  10  includes a first absorber  12 , a second absorber  14 , a third absorber  16 , plus a flash system  18  that work together to recover relatively clean methane gas  20  from a landfill  22 . Landfill  22  is a large field of buried garbage with a series of wells  24  that tap a landfill gas  30  generated by the decomposing garbage. Landfill gas  30  may be comprised of methane contaminated with various impurities such as CO 2  (carbon dioxide), chlorinated hydrocarbons, H 2 S (hydrogen sulfide), aromatics and water. Each impurity&#39;s concentration may vary from its intitial level in the landfill down to zero as gas  30  is progressively processed through system  10 . 
     To recover and separate the methane from its contaminants, a solvent  32  having an affinity for contaminants is circulated through absorbers  12 ,  14  and  16 . In first absorber  12 , solvent  32  absorbs trace contaminants of chlorinated hydrocarbons, aromatics and water from landfill gas  30 . In second absorber  14 , solvent  32  absorbs CO 2  from gas  30 . And in third absorber  16 , CO 2  absorbs trace contaminants from solvent  32 . Solvent  32  represents any chemical that can absorb and subsequently release one or more impurities that can contaminate methane gas. Examples of solvent  32  include, but are not limited to, SELEXOL (registered trademark of Union Carbide Chemicals &amp; Plastics Technology Corporation of The Dow Chemical Company) and DEPG (diethylpropylene glycol). System  10  has two charges of solvent  32 . A first portion  32   a  of solvent  32  circulates between absorbers  12  and  16 , and a second, much larger portion  32   b  of solvent  32  circulates between absorber  14  and flash system  18 . 
     In operation, a blower  34  draws landfill gas  30  up from within wells  24  into a collection tank  36 . Blower  34  operates at an absolute suction pressure of about 310-inch water head (subatmospheric pressure) and a discharge pressure of about 3 psig. A cooler  38  reduces the temperature of gas  30  from about 160° F. to about 100° F. A screw compressor  40  takes the temperature and pressure of gas  30  to about 230° F. and 85 psig. A cooler  42  reduces the temperature of gas  30  to about 110° F. A reciprocating compressor  44  increases the pressure of gas  30  to about 450 psig. A solvent heat exchanger  46 , a CO 2  heat exchanger  48 , and a methane heat exchanger  50  each extracts waste heat from compressed gas  30  to enhance the effectiveness of system  10 . A conventional sulfur treater  52  can be used to help extract at least some hydrogen sulfide from gas  30 . 
     Gas  30  enters a lower gas inlet  54  of absorber  12  at about 75° F. and 450 psig, travels upward through absorber  12 , and exits through an upper gas outlet  56  of absorber  12  at about 450 psig. As gas  30  travels through first absorber  12 , first solvent portion  32   a  travels downward in intimate contact with gas  30  to absorb trace contaminants from gas  30 . With some of the trace contaminants removed, gas  30  enters a lower gas inlet  58  of second absorber  14  at about 125° F. and 450 psig. Gas  30  leaving absorber  12  is comprised of about 42% CO 2 . 
     To remove the CO 2  from gas  30 , the gas travels upward from lower gas inlet  58  to an upper gas outlet  60  to release the CO 2  to second solvent portion  32   b , which travels downward in intimate, CO 2 -absorbing contact with gas  30 . With most of the CO 2  now removed from gas  30 , the gas is conveyed to a supply line  62  where the treated gas  20  can be delivered to wherever it may be needed. Prior to reaching supply line  62 , however, gas  20  leaving second absorber  14  first passes through heat exchanger  50  to precool gas  30  that is about to enter lower gas inlet  54  of first absorber  12 . Precooling gas  30  prior to it entering first absorber  12  promotes the absorption of trace contaminants into the high CO 2  gas stream. 
     Second solvent portion  32   b , which absorbs. CO 2  from gas  30  in second absorber  14 , travels downward from an upper liquid inlet  64  to collect just above a lower liquid outlet  66 . The second solvent portion  32   b  is at about 50 to 55° F. A control valve  68  in a solvent line  70  (second solvent line) responds to a liquid level sensor  72  to maintain a predetermined head of liquid solvent  32   b  at the bottom of second absorber  14 . Valve  68  controllably releases solvent  32   b  at about 450 psig in second absorber  14  to first flash tank  76  at about 250 psig. The lower pressure in first flash tank  76  causes CO 2  to be released from the second solvent portion  32   b . Compressor  74  returns this CO 2  along with some methane to a gas line  78  to mix with gas  30  from first absorber  12 . Together, gas line  78  and compressor  74  feed second absorber  14  with gas  30  that is about 45% CO 2 . 
     The second solvent portion  32   b  pools at the bottom of first flash tank  76 . A control valve  80  (first control valve) responsive to a liquid level sensor  82  controls the liquid level in first flash tank  76  and controllably feeds second solvent portion  32   b  into a second flash tank  84 , which is slightly above atmospheric pressure. The pressure drop from flash tank  76  to flash tank  84  causes more CO 2  to escape from the second solvent portion  32   b . That CO 2  is surplus, as it is not needed for stripping trace contaminants from the first solvent portion  32   a  in third absorber  16 , thus that portion of the CO 2  can be directly vented to atmosphere via a vent line  86 . If that CO 2  were not vented to atmosphere but instead directed into third absorber  16 , the surplus CO 2  would create an unnecessary incineration load on an incinerator  88 , which will be explained later. 
     Another control valve  90  (second control valve) responsive to a liquid level sensor  92  in a downstream third flash vessel  94  controls the liquid level in third flash tank  94  and controllably feeds the second solvent portion  32   b  into third flash tank  94 . A compressor  96  maintains third flash tank  94  at about a 4 to 5 psia (negative gage pressure of about −9 to −10 psig), which cause additional CO 2  to escape from the second solvent portion  32   b . This additional CO 2  is later used in third absorber  16  to remove the trace contaminants from first solvent portion  32   a . A pump  98  draws the liquid second portion  32   b  of solvent  32  from the bottom of flash tank  94  and returns it to upper liquid inlet  60  of second absorber  14  to drive the solvent cycle of second absorber  14  and flash system  18 . 
     To strip the trace contaminants from the first portion  32   a  of solvent  32 , compressor  96  draws CO 2  from third flash tank  94 , and a CO 2  line  100  and heat exchanger  48  convey the CO 2  into a lower gas inlet  102  of third absorber  16 . Vent line  86  represent a first flow path, and CO 2  line  100  represents a second flow path for the CO 2 . With two flow paths, only a minimal amount of CO 2  is used for stripping trace contaminants from first portion  32   a  of solvent  32  in third absorber  16 , and surplus CO 2  can be vented directly to atmosphere. 
     In some cases, heat exchanger  48  heats the CO 2  before the CO 2  enters third absorber  16 . Once inside third absorber  16 , the CO 2  travels upward to an upper gas outlet  104 . At the same time, the first solvent portion  32   a  with absorbed trace contaminants travels from an upper liquid inlet  106  in third absorber  16  down to a lower liquid outlet  108 . As this first solvent portion  32   a  and the CO 2  travel in intimate contact with each other inside third absorber  16 , the CO 2  strips the contaminants from the first solvent portion  32   a.    
     The resulting relatively uncontaminated first solvent portion  32   a  collects at the bottom of third absorber  16 . A pump  110  returns the clean first solvent portion  32   a  to an upper gas inlet  112  of absorber  12  so that the first solvent portion  32   a  can absorb additional trace contaminants from the incoming landfill gas  30 . 
     To maintain first solvent portion  32   a  at a certain liquid level at the bottom of first absorber  12 , a control valve  114  in a first solvent line  116  responds to a liquid level sensor  118 , thereby controlling the delivery of first solvent portion  32   a  to third absorber  16  and maintaining a predetermined pressure differential between absorbers  12  and  16 . The pressure differential is about 450 psig and it is that pressure that forces first solvent portion  32   a  to upper liquid inlet  106  of third absorber  16 . 
     Before entering third absorber  16 , first solvent portion  32   a  is heated by gas  30  within heat exchanger  46 . Heating first solvent portion  32   a  enables the CO 2  in third absorber  16  to more readily strip the trace contaminants from the first solvent portion  32   a , thus less CO 2  is needed for absorbing the contaminants. 
     After absorbing the trace contaminants from first solvent portion  32   a , the CO 2  and trace contaminants exhaust out through an upper gas outlet  120  of third absorber  16  and enter incinerator  88 . Using the trace contaminants and treated gas  20  as fuel, incinerator  88  heats the CO 2  (from CO 2  line  100 ) to at least 1400° F. before exhausting the CO 2  and the resulting combustion products to atmosphere  124 . By venting a portion of the CO 2  through vent line  86 , as opposed to directing all of the CO 2  into third absorber  16 , less energy is needed to heat the contaminated CO 2  to 1400° F., thus the trace contaminants can provide all or at least most of the necessary combustion energy. 
     To effectively strip the CO 2  from the second solvent portion  32   b  and supply third absorber  16  with a sufficient amount of CO 2  to thoroughly strip the first solvent portion  32   a  of its absorbed trace contaminants yet limit the amount of CO 2  delivered to third absorber  16  so as not to extinguish or dampen the combustion within incinerator  88 , the relative fluid flow rates, temperatures and pressures of system  10  need to be properly balanced. In a currently preferred embodiment, for example, the pressure in first absorber  12  is nearly equal to or at least within 10% of the pressure in second absorber  14 , the pressure in first absorber  12  and second absorber  14  are much greater than and preferably over 5 times as great as the pressure in third absorber  16 , the flow rate of solvent  32  in first absorber  12  and third absorber  16  are substantially equal or at least within 10% of each other, the flow rate of solvent  32  through second absorber  14  is much greater than and preferably at least 10 times as great as the flow rate of solvent through first absorber  12 , and the flow rate of solvent  32  through second absorber  14  is much greater than and preferably at least 10 times as great as the flow rate of solvent through third absorber  16 . In some cases, the first solvent portion  32   a  flows at about 10 gpm, and the second solvent portion  32   b  flows at about 210 gpm. 
     The pressure inside first absorber  12  is approximately 450 psig, thus the pressure of gas  30  inside first absorber  12  and the pressure of solvent  30  inside first absorber  12  are also at about 450 psig. The pressure inside second absorber  14  is approximately 450 psig, thus the pressure of gas  30  inside second absorber  14  and the pressure of solvent  30  inside second absorber  14  are also at about 450 psig. The pressure inside third absorber  16  is near zero psig, thus the pressure of gas  30  inside third absorber  16  and the pressure of solvent  30  inside third absorber  16  are also at about zero psig. 
     A refrigerated or otherwise cooled heat exchanger  122  can be added to cool the second solvent portion  32   b  circulated through second absorber  14 . Such cooling increases the second portion&#39;s ability to absorb CO 2  inside second absorber  14 . In a currently preferred embodiment, the second solvent portion  32   b  entering second absorber  14  is naturally cooled to a temperature of about 40 to 50° F. As for the other heat exchangers of system  10 , the heat supplied to heat exchangers  46 ,  48  and  50  would otherwise be wasted heat created directly or indirectly by compressors  34 ,  40  and/or  44 . It should be noted that any one or more of heat exchangers  38 ,  42 ,  46 ,  48 ,  50 , and  122  may be optionally omitted. 
     Although the invention is described with reference to a preferred embodiment, it should be appreciated by those of ordinary skill in the art that various modifications are well within the scope of the invention. Therefore, the scope of the invention is to be determined by reference to the following claims.