Abstract:
A method for recovering methane gas from a landfill involves the use of a main absorber, a flash system, an ancillary absorber and a polishing absorber. The main absorber uses a main current of solvent for absorbing most of the carbon dioxide from raw landfill gas. The flash system removes much of the carbon dioxide from the solvent exiting the main absorber. A portion of the solvent downstream of the flash system is diverted to the ancillary absorber in which a current of air removes additional carbon dioxide from that portion of solvent. From the ancillary absorber, the diverted portion of solvent flows through the polishing absorber to remove additional carbon dioxide from the main current of solvent that was previously treated in the main absorber. To increase the energy content of the processed gas, in some examples, propane is added to the final outgoing gas stream.

Description:
FIELD OF THE DISCLOSURE 
     The subject invention generally pertains to processing landfill gas and more specifically to an absorption system and method for recovering and purifying methane gas. 
     BACKGROUND 
     Decomposing garbage buried in a landfill can generate landfill gas that can be extracted and processed to provide methane gas of varying degrees of purity and energy content. Processing plants have been developed for recovering and purifying methane gas, but there continues to be a need for better systems and methods. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of an example absorption system and method. 
         FIG. 2  a schematic view of an example absorption system connectable to the system shown in  FIG. 1 . 
         FIG. 3  is a schematic view of another example absorption system connectable to the system shown in  FIG. 1 . 
         FIG. 4  is a schematic view of yet another example absorption system and method. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows an example absorption system  11  for improving the gas purifying operation of various methane gas processing systems. Examples of such gas processing systems include, but are not limited to, a triple-effect absorption system  10   a , shown in FIG.  2 , and an absorption system  10   b , shown in  FIG. 3 . To understand the purpose and function of absorption system  11 , the structure and operation of systems  10   a  and  10   b  will be explained first. 
     Referring to  FIG. 2 , triple-effect absorption system  10   a  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), air, hydrocarbons, H 2 S (hydrogen sulfide), aromatics and water. Each impurity&#39;s concentration may vary from its initial level in the landfill down to zero as gas  30  is progressively processed through system  10   a.    
     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 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   a  has two charges of solvent  32 . A first portion  32   a  of solvent  32  circulates between absorbers  12  and  16 , and a second 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 20 to 60 inch water vacuum (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 . 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 mol % CO 2 . It should be noted that the term, “mol %,” as used throughout this patent, means molar percent, which is the ratio of the moles of a substance in a mixture to the moles of the mixture with the ratio being multiplied by a hundred, i.e., mol % represents the number of moles of a substance in a mixture as a percentage of the total number of moles in the mixture. The term, “concentration,” as used throughout this patent, is expressed in terms of mol %. 
     To remove 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  is available for further processing. 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 some 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 mol % 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 vented directly, or through a carbon filter, 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 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 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   a  need to be properly balanced. In some examples, 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 five 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 ten 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 ten 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. 
     In some examples, a refrigerated or otherwise cooled heat exchanger  122  is 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 some examples, 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   a , 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. 
     In the example shown in  FIG. 3 , absorption system  10   b  is created by eliminating several components of system  10   a . The eliminated items include absorbers  12  and  14  and their associated components (e.g., items  46 ,  48 ,  88 ,  110 ,  114 ,  116  and  118 . Remaining portions of absorption system  10   b , shown in  FIG. 2 , are retained to operate in a manner similar to that of system  10   a , wherein supply line  62  makes treated gas  20  available for further processing. 
     Absorption system  11 , of  FIG. 1 , can be added to systems  10   a  and  10   b  to improve the quality of methane gas  20 . In some examples, gas  20  has a concentration of carbon dioxide of about 2 mol % (or slightly less or more), and system  11  can improve that to provide methane gas  20   a  with a carbon dioxide concentration of less than 1 mol % and perhaps as low as 0.6 to 0.8 mol %. In some examples, system  11  further improves the quality of gas  20   a  by injecting a gas with a higher energy content than that of methane. In some cases, for example, propane gas  126  with an energy content of about 2,500 BTU/scf is injected into a discharge line  128  to mix with methane gas  20   a . While pure methane has an energy content of about 1,010 BTU/scf, methane gas  20   a  might have an energy content of less than 950 BTU/scf due to gas  20   a  having various contaminants, such as nitrogen and some carbon dioxide. Thus, system  11  minimizing the concentration of carbon dioxide in gas  20   a  and, in some examples, adding propane  126  provides high quality methane  20   b  having significantly less than 2 mol % of carbon dioxide and an energy content greater than 950 BTU/scf and in some cases greater than 970 BTU/scf. 
     In the example shown in  FIG. 1 , system  11  comprises an ancillary absorber  130 , a polishing absorber  132 , one or more pumps  134  pumping a portion  148  of solvent  32  (portion of solvent  148 ) through absorbers  130  and  132 , an air supply  136  (e.g., a blower, fan, compressor, etc.) forcing a current of air  138  through ancillary absorber  130 , a line  140  conveying the portion of solvent  148  from one absorption system (e.g., system  10   a  or  10   b ) to system  11 , a return line  142  for injecting the portion of solvent  148  back into the main absorption system (e.g., system  10   a  or  10   b ), and discharge line  128  for conveying gas  20   a  from polishing absorber  132 . In some examples, ancillary absorber  130  and/or  132  includes or is associated with means for controlling the flow of solvent through absorber  130  and/or  132 . Examples of such means include, but are not limited to, controlling the operation of one or more pumps  134  and/or the use of various flow control elements such as those used in system  10   a  of  FIG. 2  (e.g., control valves  68 ,  80 ,  90 ,  114 ; and liquid level sensors  72 ,  82 ,  92  and  118 ). Item  144  schematically represents an optional source of propane  126  for injection into gas  20   a  to produce gas  20   b , wherein gas  20   b  has a higher energy content than that of gas  20   a.    
     Connecting system  11  of  FIG. 1  to system  10   a  of  FIG. 2  or system  10   b  of  FIG. 3  provides a combined absorption system comprising a main absorber (e.g., absorber  14 ), ancillary absorber  130 , polishing absorber  132 , flash system  18 , and lines  140  and  142  connecting system  11  to system  10   a  or  10   b . In the operation of combined systems  11  and  10   a  or  11  and  10   b , a current of gas  146  (comprising gas  30 ) flows up through main absorber  14  from inlet  58  to outlet  60 . From outlet  60 , the current of gas  146  flows sequentially through line  62  to polishing absorber  132 , up through polishing absorber  132 , and out through discharge line  128  to be used or sold. 
     To remove carbon dioxide from gas  30 , a main current of solvent  150  (comprising solvent  32 ) flows through main absorber  14  while in intimate contact with the current of gas  146 . After the main current of solvent  150  absorbs carbon dioxide from current of gas  146 , the main current of solvent  150  flows through flash system  18 , which removes carbon dioxide from the main current of solvent  150 . While pump  98  pumps most of the current of solvent  150  from the bottom of flash system  18  to inlet  64  of main absorber  14 , pump  134  pumps a lesser portion of solvent  148  through line  140  to ancillary absorber  130  ( FIG. 1 ). The portion of solvent  148  flows through ancillary absorber  130  in intimate contact with the current of air  138 . 
     As the current of air  138  and the portion of solvent  148  flow through ancillary absorber  130 , the current of air  138  extracts carbon dioxide from the portion of solvent  148 . After air  138  removes carbon dioxide from the portion of solvent  148 , air  138  is vented to atmosphere via a line  124 , and a line  152  conveys the portion of solvent  148  to polishing absorber  132 . As the portion of solvent  148  flows through polishing absorber  132 , the current of gas  146  from line  62  flows up through polishing absorber  132  in intimate contact with the portion of solvent  148 , whereby the portion of solvent  148  absorbs carbon dioxide from the current of gas  146 . The current of gas  146  now becomes gas  20   a  and, in some examples, ultimately becomes gas  20   b  in cases where propane  126  is added to gas  20   a . Gas  20   a  or  20   b  can be sold or used as needed. 
     As for the portion of solvent  148  after having flowed through polishing absorber  132 , line  142  injects the portion of solvent  148  back into a main solvent loop  154 , wherein main solvent loop  154  comprises main absorber  14 , line  70 , flash system  18 , and a return line  156 . In some examples, line  142  injects the portion of solvent  148  at a point between main absorber  14  and flash system  18  (e.g., at or downstream of absorber  14  and at or upstream of flash system  18  with respect to solvent flow). Once injected in main solvent loop  154 , in some examples, the portion of solvent  148  becomes part of the main current of solvent  150 . 
     In some examples, as shown in  FIG. 4 , an absorption system  158  includes the combination of a main absorber  14 ′ and a polishing absorber  132 ′ that share a common outer shell  160  (i.e., absorbers  14 ′ and  132 ′ are combined in a single vessel). In this example, line  78  conveys gas  30  to an inlet  58 ′ of main absorber  14 ′. From inlet  58 ′, a current of gas  146  flows up through main absorber  14 ′, through an area of transition  162  between main absorber  14 ′ and polishing absorber  132 ′, through polishing absorber  138 ′, and out through discharge line  128 . 
     To remove carbon dioxide from gas  146 , a main current of solvent  150  flows through main absorber  14 ′ while being in intimate contact with the current of gas  146 . After the main current of solvent  150  absorbs carbon dioxide from the current of gas  146 , the main current of solvent  150  flows through flash system  18 , which removes carbon dioxide from the main current of solvent  150 . Pump  98  pumps most of the main current of solvent  150  from the bottom of flash system  18  to an inlet  164  at the area of transition  162  between absorbers  14 ′ and  132 ′. At least one pump  134  pumps a lesser portion of solvent  148  through line  140  to ancillary absorber  130 . The portion of solvent  148  flows through ancillary absorber  130  in intimate contact with the current of air  138 , basically in the manner as shown in  FIG. 1 . 
     As the current of air  138  and the portion of solvent  148  flow through ancillary absorber  130 , the current of air  138  extracts carbon dioxide from the portion of solvent  148 . After air  138  removes carbon dioxide from the portion of solvent  148 , air  138  is vented to atmosphere via line  124 , and a line  166  conveys the portion of solvent  148  to polishing absorber  132 ′. As the portion of solvent  148  flows downward through polishing absorber  132 ′, the current of gas  146  from within main absorber  14 ′ flows up through polishing absorber  132 ′ in intimate contact with the portion of solvent  148 , whereby the portion of solvent  148  absorbs carbon dioxide from the current of gas  146 . The current of gas  146  now becomes gas  20   a  and, in some examples, ultimately becomes gas  20   b  in cases where propane  126  is added to gas  20   a . Gas  20   a  or  20   b  can be sold or used as needed. 
     The portion of solvent  148  after having flowed down through polishing absorber  132 ′, the portion of solvent  148  passes through area of transition  162  to mix with and become part of main current of solvent  150 , wherein the main current of solvent  150 , including portion  148 , flows down through main absorber  14 ′. In this example, system  158  includes a main solvent loop  168  comprising main absorber  14 ′, line  70 , flash system  18 , and a return line  170 . 
     As for various methods pertaining to the examples illustrated in  FIGS. 1-4 , arrow  146  in  FIG. 3  provides at least one example of conveying gas through a main absorber. Arrow  146  of  FIGS. 1 and 3  provides at least one example of conveying substantially all of the gas from the main absorber through a polishing absorber. Arrow  150  of  FIG. 3  provides at least one example of conveying at a main mass flow rate a main current of solvent through the main absorber, thereby exposing the gas to the main current of solvent. An arrow  172  of  FIG. 3  provides at least one example of the main current of solvent extracting carbon dioxide from the gas. Arrow  148  of  FIG. 1  provides at least one example of conveying at a polishing mass flow rate a polishing current of solvent through the polishing absorber, thereby exposing the gas to the polishing current of solvent. An arrow  174  of  FIG. 1  provides at least one example of the polishing current of solvent extracting additional carbon dioxide from the gas. It has been discovered that, in some examples, it appears that having the solvent&#39;s main mass flow rate through the main absorber be at least three times greater than the solvent&#39;s polishing mass flow rate in the polishing absorber provides surprisingly good results. In some examples, as shown in  FIG. 4 , the solvent&#39;s mass flow rate pertaining to arrow  150  is at least three times greater than the solvent&#39;s mass flow rate pertaining to arrow  148 . In some examples, as shown in  FIGS. 1 and 3 , the solvent&#39;s mass flow rate pertaining to arrow  150  ( FIG. 3 ) is at least three times greater than the solvent&#39;s mass flow rate pertaining to arrow  148  ( FIG. 1 ).  FIG. 4  showing absorbers  14 ′ and  132 ′ as a single vessel provides at least one example illustrating housing the main absorber and the polishing absorber within a common outer shell. Transition area  162  between absorbers  14 ′ and  132 ′ provides at least one example illustrating the main absorber and the polishing absorber defining an area of transition therebetween. In  FIG. 4 , the merging of arrows  148  and  150  provides at least one example illustrating the polishing current of solvent joining and becoming part of the main current of solvent at the area of transition. In  FIG. 1 , arrow  148  provides at least one example illustrating conveying an ancillary current of solvent through an ancillary absorber. In  FIG. 1 , an arrow  176  provides at least one example illustrating the ancillary current of solvent flowing from the ancillary absorber to the polishing absorber. In  FIG. 1 , arrows  148  and  176  provides at least one example illustrating the ancillary current of solvent flowing from the ancillary absorber becoming the polishing current of solvent flowing through the polishing absorber. In  FIG. 1 , arrow  138  with reference to arrow  148  provides at least one example illustrating conveying a current of air through the ancillary absorber, thereby exposing the ancillary current of solvent to the current of air. An arrow  178  of  FIG. 1  provides at least one example illustrating the current of air extracting carbon dioxide from the ancillary current of solvent flowing through the ancillary absorber. Arrow  126  of  FIG. 1  provides at least one example illustrating adding propane to the gas after the polishing current of solvent extracts additional carbon dioxide from the gas, wherein arrow  174  provides at least one example illustrating extracting additional carbon dioxide from the gas. 
     In  FIG. 3 , arrow  180  provide at least one example illustrating circulating a main current of solvent through a main solvent loop. Arrows  146  and  150  of  FIG. 3  provides at least one example illustrating exposing the gas to the main current of solvent, thereby reducing the concentration of carbon dioxide in the gas, wherein arrow  172  provides at least one example illustrating reducing the concentration of carbon dioxide in the gas. Arrow  182 , shown in  FIGS. 2-4 , provides at least one example illustrating diverting a portion of solvent from the main solvent loop. Arrows  146 ,  148  and  174  of  FIG. 1  provide at least one example illustrating exposing the gas to the portion of solvent, thereby further reducing the concentration of carbon dioxide in the gas. Arrows  184 ,  186  and/or  188  of  FIG. 3  provide at least one example illustrating decreasing the concentration of carbon dioxide in the main current of solvent to a lower level (e.g. to about 3 mol % carbon dioxide at a point between flash vessel  94  and pump  98 ), wherein arrows  184 ,  186  and  188  represent carbon dioxide leaving the main current of solvent. Arrow  178  of  FIG. 1  provides at least one example illustrating decreasing the concentration of carbon dioxide in the portion of solvent to less than the lower level (e.g., to about 0.5 mole % carbon dioxide or even less than that, which in either case, is less than 3 mol % carbon dioxide). Arrow  174  of  FIG. 1  provides at least one example illustrating increasing the concentration of carbon dioxide in the portion of solvent to an upper level (e.g., 0.6 to 5 mol % carbon dioxide). Arrow  172  of  FIG. 3  provides at least one example illustrating increasing the concentration of carbon dioxide in the main current of solvent to greater than the upper level (e.g., to 42 mol % carbon dioxide). Arrows  150  and  180  of  FIG. 3  provide at least one example illustrating the main solvent loop passing through the main absorber and the flash system. Arrows  176  and  148  of  FIG. 1  provide at least one example illustrating the portion of solvent flowing through the ancillary absorber and the polishing absorber. Arrows  150 ,  146  and  172  of  FIG. 3  provide at least one example illustrating exposing the gas to the main current of solvent and reducing the concentration of carbon dioxide in the gas flowing through the main absorber. Arrows  146 ,  148  and  174  of  FIG. 1  provide at least one example illustrating exposing the gas to the portion of solvent and further reducing the concentration of carbon dioxide in the gas flowing through the polishing absorber. Arrow  188  of  FIG. 3  provides at least one example illustrating decreasing the concentration of carbon dioxide in the main current of solvent to the lower level and doing so within the flash system. Arrow  178  of  FIG. 1  provides at least one example illustrating decreasing the concentration of carbon dioxide in the portion of solvent to less than the lower level and doing so within the ancillary absorber. Arrow  174  of  FIG. 1  provides at least one example illustrating increasing the concentration of carbon dioxide in the portion of solvent to the upper level and doing so within the polishing absorber. Arrow  172  of  FIG. 3  provides at least one example illustrating increasing the concentration of carbon dioxide in the main current of solvent to greater than the upper level and doing so within the main absorber. Arrow  142  of  FIG. 3  provides at least one example illustrating that after diverting the portion of solvent from the main solvent loop (arrow  182  illustrates diverting the portion), injecting the portion of solvent back into the main solvent loop at a point between the main absorber and the flash system. An arrow  190  of  FIG. 4  provides at least one example illustrating that after diverting the portion of solvent from the main solvent loop (e.g., arrow  182  illustrates diverting the portion), injecting the portion of solvent back into the main solvent loop at a point (e.g., transition area  162 ) between the polishing absorber (e.g., absorber  132 ′) and the main absorber (e.g., absorber  14 ′). Arrow  142  of  FIG. 3  illustrates that after diverting the portion of solvent from the main solvent loop (e.g., arrow  182  illustrates diverting the portion), injecting the portion of solvent back into the main solvent loop at a point (e.g., such injecting being illustrated by arrow  142  of  FIG. 3 ) where the portion of solvent has a concentration of carbon dioxide that is closer to the upper lever (e.g., 2 to 6 mol % carbon dioxide) than to the lower level (e.g., 1 to 3.5 mol % carbon dioxide). Arrows  138 ,  148  and  178  of  FIG. 1  provide at least one example of decreasing the concentration of carbon dioxide in the portion of solvent to less than the lower level and doing so by conveying a current of air in intimate contact with the portion of solvent. 
     Arrows  148  and  182  and line  140  of  FIGS. 1 and 3  provide at least one example illustrating diverting (arrow  182 ) a portion of solvent from the main solvent loop to create an offshoot solvent path (line  140 ) conveying an ancillary current of solvent (arrow  148 ) and a polishing current of solvent (arrow  148 ), the ancillary current of solvent flowing through an ancillary absorber, the polishing current of solvent flowing through a polishing absorber. Arrow  192  and line  62  of  FIGS. 1 and 3  provide at least one example illustrating a pipe conveying the gas from the main shell to the polishing shell. Arrow  142  of  FIG. 3  provides at least one example illustrating injecting the portion of solvent back into the main solvent loop at a point downstream of the main absorber and upstream of the flash system. 
     Additional points worth noting are as follows. Each of the various absorbers mentioned herein (e.g., main absorber, ancillary absorber, polishing absorber) do not necessarily have to be a single vessel but, in some examples, can actually be a group or set of absorber vessels. For instance, in some examples, a main absorber comprises two or more main absorber vessels connected in series or parallel flow relationship with each other. In examples where two absorbers are incorporated within a single vessel, e.g., absorbers  14 ′ and  132 ′ of  FIG. 4 , a transition area (e.g., area  162 ) can serve as both a fluid inlet for one absorber and a fluid outlet for the other absorber. For example, area  162  serves as a gas inlet for polishing absorber  132 ′ and a gas outlet for main absorber  14 ′. Likewise, area  162  serves as a solvent inlet for main absorber  14 ′ and a solvent outlet for polishing absorber  132 ′. The term, “main solvent loop” means the fluid path along which the solvent circulates through a main absorber and a flash system. The terms, “after” and “following” refer to a flow stream&#39;s molecules&#39; experience and not the overall stream&#39;s experience. For example, a stream of solvent might flow continuously through two vessels connected in series flow relationship; however, individual molecules in the solvent stream flow through the vessels sequentially, i.e., the molecules flow through one vessel “after” the other, or the molecules flow through a downstream vessel “following” their flowing through an upstream vessel. 
     Although certain example methods, apparatus and articles of manufacture have been described herein, the scope of the coverage of this patent is not limited thereto. On the contrary, this patent covers all methods, apparatus and articles of manufacture fairly falling within the scope of the appended claims either literally or under the doctrine of equivalents.