Patent Publication Number: US-2011077447-A1

Title: Method and apparatus for the removal of a sorbate component from a process stream

Description:
The present invention provides a method of, and apparatus for, removing a sorbate component from a process stream comprising one or more sorbate components. 
     Natural gas is a useful fuel source, as well as being a source of various hydrocarbon compounds. It is often desirable to liquefy natural gas in a liquefied natural gas (LNG) plant at or near the source of a natural gas stream for a number of reasons. As an example, natural gas can be stored and transported over long distances more readily as a liquid than in gaseous form because it occupies a small volume and does not need to be stored at high pressure. 
     Usually, natural gas, comprising predominantly methane, enters an LNG plant at elevated pressures and is pre-treated to produce a purified feed stream suitable for liquefaction at cryogenic temperatures. The purified gas is processed through a plurality of cooling stages using heat exchangers to progressively reduce its temperature until liquefaction is achieved. The liquid natural gas is then further cooled and expanded to final atmospheric pressure suitable for storage and transportation. 
     In addition to methane, natural gas usually includes some heavier hydrocarbons and impurities, including but not limited to carbon dioxide, sulphur, hydrogen sulphide and other sulphur compounds, nitrogen, helium, water and other non-hydrocarbon acid gases, ethane, propane, butanes, C 5 + hydrocarbons and aromatic hydrocarbons. These and any other common or known heavier hydrocarbons and impurities either prevent or hinder the usual known methods of liquefying the methane, especially the most efficient methods of liquefying methane. Most if not all known or proposed methods of liquefying hydrocarbons, especially liquefying natural gas, are based on reducing as far as possible the levels of at least most of the heavier hydrocarbons and impurities prior to the liquefying process. 
     Hydrocarbons heavier than methane and usually ethane are typically condensed and recovered as natural gas liquids (NGLs) from a natural gas stream. The NGLs are usually fractionated to yield valuable hydrocarbon products, either as product streams per se or for use in liquefaction, for example as a component of a refrigerant. 
     Meanwhile, methane recovered from the NGL recovery is usually recompressed for use or reuse either in the liquefaction, such as a fuel gas, or being recombined with the main methane stream being liquefied, or it can be provided as a separate stream. 
     Acid gasses such as carbon dioxide and sulphur compounds, hydrogen sulphide and other sulphur compounds, such as the oxides of sulphur, are normally removed from the natural gas by an initial acid gas treatment step, in which the natural gas stream is exposed to a solvent, which reacts with the acid gasses to dissolve them and extract them from the stream. The solvent extraction reaction is reversible, allowing the solvent to be regenerated by heating. 
     The utility of the extraction of acid gasses is not limited to the treatment of a natural gas stream. Similar processes can be used to remove acid gasses contained in a flue gas stream, such as the flue gas from the combustion of hydrocarbon fuels in gas turbines and boilers commonly used in LNG and power plants. The current cost of flue gas CO 2  capture only, excluding transportation and storage is estimated around $60/tonne CO 2 . This makes flue gas capture uneconomic and prohibitive for most current and planned plants. 
     However, acid gas treatment is increasing in importance as the impact on climate change of the rising concentration of greenhouse gasses, such as carbon dioxide, in the atmosphere is understood. Solvent extraction provides one way of capturing carbon dioxide from a gaseous stream to prevent its release into the atmosphere. 
     For instance, U.S. Pat. No. 6,782,714 discloses an LNG plant including a carbon dioxide recovery apparatus for natural gas. The sorbent, which can be an amine solution, is used to absorb and remove carbon dioxide and hydrogen sulphide. The amine solution containing the absorbed carbon dioxide is heated in a regeneration tower to release the carbon dioxide and regenerate the sorbent solution. The heat for the regeneration process is supplied by low pressure steam from a steam turbine. The low pressure steam is generated in a boiler by burning fuel, such as natural gas. 
     A disadvantage of the solvent extraction process is the energy required to regeneration the sorbent, and release the acid gas, such as carbon dioxide. For instance, the solvent extraction of carbon dioxide using an aqueous amine solution as the solvent, requires 1.0 to 2.0 MJ per kg of CO 2  to regenerate the solvent. 
     In order to regenerate the loaded sorbent, boilers are conventionally used to provide the heat required, normally in the form of steam. The boilers operate by combusting a hydrocarbon fuel and normally generate steam from the heat of the reaction. The burning of the hydrocarbon fuel required by the boilers generates additional carbon dioxide, which should also be removed from the flue gas stream of the boiler. This in turn requires the acid gas treatment of the boiler flue gas stream, utilising additional solvent, which must also be regenerated by heating, using yet more hydrocarbon fuel and generating additional carbon dioxide, increasing the financial costs of the acid gas treatment to isolate and capture the carbon dioxide. 
     In a first aspect, the present invention provides a method for the removal of a sorbate component from a process stream comprising one or more sorbate components, said method comprising at least the steps of: 
     (a) providing a process stream in the form of a hydrocarbon stream comprising one or more sorbate components;
 
(b) treating the process stream with a sorbent to capture one or more of the one or more sorbate components and to provide a treated process stream that is diminished in sorbate component content and a loaded sorbent comprising the sorbent and one or more sorbate components; and
 
(c) regenerating a loaded sorbent to provide a sorbent component and one or more sorbate component streams, according to a regeneration method comprising:
         collecting solar energy from the sun in a concentrated solar power system to provide captured solar thermal energy; and   using at least a part of the captured solar thermal energy to heat the loaded sorbent to provide the sorbent component and one or more sorbate component streams.       

     In a further aspect, the present invention provides an apparatus for removing a sorbate component from a process stream comprising one or more sorbate components, said apparatus comprising at least:
     a process stream inlet line connected to a source of a hydrocarbon stream comprising one or more sorbate components;   a treating unit connected to the process stream inlet line arranged to treat the process stream with a sorbent and to provide a treated process stream and a loaded sorbent; and   a loaded sorbent regeneration apparatus comprising:   a concentrated solar power system to capture solar thermal energy; and
       a sorbent heat exchanger connected to the concentrated solar power system, to generate a sorbent component and an sorbate component stream from the loaded sorbent using at least part of the captured solar thermal energy.   
       

     As used herein, the term “sorbent” means any solid or liquid substance which can reversibly absorb, adsorb and/or capture one or more substances, the latter called “sorbates”. The term “sorbate” as used herein refers to the solid, liquid or gaseous substance (to be) absorbed, adsorbed and/or captured by the sorbent. Thus, a sorbate component as used hereinafter may be in the form of an absorbate component or an adsorbate component. Any difference between absorbate or adsorbate is for the purpose of the present invention not of essence, as long as loaded sorbent is or can be regenerated using heat. Often, the sorbate components to be removed from the process stream are considered to be contaminants in the process stream. 
     The term “loaded sorbent” is used herein to describe the sorbate-containing sorbent, and is synonymous with the terms “rich sorbent” and “fat sorbent”. The term “loaded sorbent” encompasses both partially- and fully-loaded sorbents. 
    
    
     
       Embodiments and examples of the present invention will now be described by way of example only with reference to the accompanying drawings. 
         FIG. 1  is a diagrammatic scheme for a first apparatus and method comprising regenerating a loaded sorbent using Concentrated Solar Power. 
         FIG. 2  is a diagrammatic scheme for a second apparatus and method comprising regenerating a loaded sorbent using CSP. 
         FIG. 3  is a diagrammatic scheme for a third apparatus and method comprising regenerating a loaded sorbent using CSP. 
     
    
    
     For the purpose of this description, a single reference number will be assigned to a line as well as a stream carried in that line. 
     A concentrated solar power (CSP) system collects light from the sun and captures this solar thermal energy by heating a first thermal transfer fluid stream to provide a heated first thermal transfer fluid stream. It is presently proposed to employ CSP in a method of regenerating a loaded sorbent to provide a sorbent component and an sorbate component. The method may comprise at least the steps of:
         providing a loaded sorbent comprising a sorbent and one or more sorbate components;   providing a concentrated solar power system;   collecting solar energy from the sun in the concentrated solar power system to provide captured solar thermal energy; and   using at least a part of the captured solar thermal energy to heat the loaded sorbent to provide a sorbent component and one or more sorbate component streams.       

     A suitable loaded sorbent regeneration apparatus may comprise:
         a concentrated solar power system to capture solar thermal energy; and   a sorbent heat exchanger to generate a sorbent component and an sorbate component stream using at least part of the captured solar thermal energy.       

     The sorbent regenerated using CSP may be a solid entrapping a liquid or gas, such as a zeolite or metal oxide framework (MOF) entrapping water, or a liquid carrying a dissolved gas, such as a solvent comprising dissolved carbon dioxide produced by acid gas removal in the production of Liquefied Natural Gas (LNG). 
     By using CSP to help to regenerate the loaded sorbent, the quantities of hydrocarbon fuel which must be burned, e.g. in a boiler, to provide steam to heat the loaded sorbent can be reduced, and in some cases the boilers can be dispensed with entirely. By supplementing the heat of combustion (e.g. in a boiler) with captured solar thermal energy, hydrocarbon fuel costs can be reduced, and the carbon dioxide emissions associated with the manufacturing plant lowered. 
     In addition, the method and apparatus described herein can provide peak shaving of the fuel requirements of a plant. During daylight hours, the thermal energy provided by the CSP system can be used to supplement, and in some instances entirely replace the heat generated by boilers or from the flue gas of gas turbines conventionally used to regenerate the loaded sorbent. 
     Such a method and apparatus can be used to regenerate solid sorbents such as zeolites and metal oxide frameworks, and liquid sorbents such as those used in the solvent extraction of acid gases including, but not limited to carbon dioxide, oxides of sulphur and hydrogen sulphide. The sorbents can be used in the removal of acid gases from a hydrocarbon stream, such as a natural gas stream for a LNG plant, or a flue gas stream. 
     The method and apparatus of the present invention may advantageously addresses the problem of the prohibitive energy requirements of acid gas loaded solvent regeneration by utilising Concentrated Solar Power (CSP) to provide the thermal energy required to separate the acid gas sorbate from the solvent sorbent. CSP thermal energy production does not result in the generation of carbon dioxide. The present invention is therefore assists in the provision of a zero-emissions CO 2  plant. 
     A concentrated solar power system typically comprises one or more concentrators for concentrating solar radiation, and one or more receivers on which the solar radiation may be concentrated and to capture solar thermal energy. The solar energy may for instance be reflected onto the one or more receivers, with the one or more concentrators, to heat one or more first thermal transfer fluid streams in the one or more receivers to provide one or more heated first thermal transfer fluid streams. 
     The first thermal transfer fluid is preferably selected from the group consisting of: H 2 O, liquid sodium, molten salt, natural or synthetic oil and air. More preferably it comprises H 2 O. A heated first thermal transfer fluid such as steam may have a pressure of 100 bar and a temperature of about 375° C. 
     The heated first thermal transfer fluid streams may be heat exchanged with the loaded sorbent, e.g. in the sorbent heat exchanger, to heat the loaded sorbent. After the heat exchanging, the first thermal transfer fluid stream may be provided to the receiver to be re-heated. In addition to the first thermal transfer fluid stream, the sorbent component and the sorbate component stream may thus be provided. 
     Alternatively, at least one of the heated first thermal transfer fluid streams may be heat exchanged against one or more second thermal transfer fluid streams, e.g. in an additionally provided first heat exchanger, to provide at least one first thermal transfer fluid stream and one or more heated second thermal transfer fluid streams. The heated second thermal transfer fluid stream may then be further heat exchanged, e.g. in the sorbent heat exchanger, against the loaded sorbent to provide the second thermal transfer fluid stream the sorbent and the sorbate stream. Subsequently, the second thermal transfer fluid stream may be passed back to to the first heat exchanger for re-heating. In particular embodiments, the sorbent stream itself may function as the second thermal transfer fluid stream, in which case the heated second thermal transfer fluid stream may be the heated sorbent stream. 
       FIG. 1  shows a first embodiment of the invention. A CSP system  10  is shown which comprises one or more first thermal transfer fluid streams  12  in a first thermal transfer fluid circuit  40  which captures solar thermal energy to heat the one or more first thermal transfer fluid streams  12  to provide one or more heated first thermal transfer fluid streams  42 . For simplicity, only a single first thermal transfer fluid stream  12  and heated first thermal transfer fluid stream  42  is shown in  FIG. 1  and these streams will be referred to in the singular for the discussion of  FIG. 1 . However, the method and apparatus disclosed herein encompasses the use of a plurality of thermal transfer fluid streams and heated first thermal transfer fluid streams. 
     The CSP system  10  concentrates and collects direct solar radiation to provide medium to high temperature heat. A CSP system may contain three main elements: one or more concentrators, one or more receivers and one or more first thermal transfer fluid circuits. The one or more concentrators reflect and concentrate light from the sun onto the one or more receivers. The one or more receivers receive the reflected and concentrated sunlight and heat the first thermal transfer fluid in the first thermal transfer fluid circuit  40 . In  FIG. 1 , the one or more concentrators and one or more receivers are represented by the unit CSP. 
     Parabolic trough CSP systems use trough-shaped mirrors as the concentrators to reflect and concentrate sunlight onto one or more receivers in the form of tubes. A first thermal transfer fluid  40  can be heated in the receiver tubes to about 500° C. The heated first thermal transfer fluid  42  can be heat exchanged against the loaded sorbent directly. For example, direct solar steam can be generated at a pressure of 100 bar and a temperature of about 375° C. Alternatively, the heated first thermal transfer fluid  42  can be heat exchanged with a second or further thermal transfer fluid (not shown) which can then be used to regenerate the loaded sorbent. 
     As an alternative to parabolic trough concentrators, a linear Fresnel reflector array of concentrators can be used. This is a line focus system similar to parabolic trough systems in which solar radiation is concentrated on an elevated inverted linear receiver using an array of nearly flat reflectors. The receiver contains the first thermal transfer fluid stream  12  which is heated to provide the heated first thermal transfer fluid stream. The use of linear concentrators provides a lower-cost alternative to parabolic trough concentrators and provides a number of advantages over parabolic systems such as lower structural support and concentrator costs, fixed fluid joints, a receiver separated from the concentrators and long focal lengths allowing the use of conventional glass. 
     Central receiver (solar tower) CSP systems use a circular array of large individually tracking plain mirrors (heliostats) as the one or more concentrators to concentrate sunlight onto a central receiver mounted on top of a tower. The first thermal transfer fluid stream  12  is passed to the central receiver where it is heated to provide the heated first thermal transfer fluid stream  42 . Such systems can provide high conversion efficiencies. If pressurised gas or air is used as the thermal transfer fluid, temperatures of about 1000° C. or more may be achieved. In a similar manner to the parabolic trough CSP systems, the solar thermal energy can be captured directly in the first thermal transfer fluid or used to heat a second or further thermal transfer fluid, which can be subsequently heat exchanged against the loaded sorbent to release the sorbate and regenerate the sorbent. 
     Parabolic dish CSP systems are smaller units which use dish shaped concentrators to reflect and concentrate sunlight into a receiver situated at the focal point of the dish. The concentrated radiation is absorbed into the receiver and can heat the first thermal transfer fluid to temperatures of about 750° C. 
     The heated first thermal transfer fluid stream  42  is passed to a sorbent heat exchanger  50  where it heats a loaded sorbent  200  to regenerate the sorbent component  210 . In the example of  FIG. 1 , a sorbent heat exchanger  50  for a solid sorbent component  210  is shown. The solid sorbent component  210  is preferably selected from the group comprising zeolites and metal oxide frameworks. The sorbate component may be a liquid such as water or an organic solvent. 
     When it is required to regenerate the loaded sorbent  200 , first valve  48  is opened allowing the heated first thermal transfer stream  42  to flow through the sorbent heat exchanger  50 . The heated first thermal transfer stream  42  heats the loaded sorbent  200  to a temperature sufficient to release the captured sorbate. The released sorbate exits the sorbent heat exchanger  50  as sorbate component stream  242 . Once the sorbate has been removed to regenerate the sorbent  210 , first valve  48  can be closed and the operation of the sorbent component  210  to capture the one or more sorbate components resumed. 
     Operation of the sorbent  210  to capture the one or more sorbate components may be carried out in the sorbent heat exchanger  50 , or in another process vessel. Operation of the sorbent  210  in the sorbent heat exchanger  50  is preferred, as this does not require the removal of the loaded sorbent from a separate sorbate extraction vessel and transfer of the loaded sorbent to the sorbent heat exchanger  50  for regeneration. 
     In operation, a process stream  212  comprising one or more sorbate components is provided to the sorbent heat exchanger  50  via a process stream inlet line. The process stream inlet line may be connected to a source of a hydrocarbon stream comprising one or more sorbate components. 
     The sorbent heat exchanger  50  is operating as an sorbate extraction vessel. The process stream flows through the solid sorbent  210 , such as a packed bed of zeolite or MOF sorbent, and the one or more sorbate components are captured by the sorbent  210  to provide a treated process stream  214 . The treated process stream  214  is diminished in sorbate component content compared to process stream  212 . 
     A thermal storage system  60  can be provided in the first thermal transfer fluid circuit  40  to store captured thermal energy at those times when the heated first thermal transfer fluid  42  is not required to regenerate the loaded sorbent  200 . During the daytime when the CSP system  10  can capture solar thermal energy to produce heated first thermal transfer fluid stream  42 , a portion of the heated first thermal transfer fluid stream  42  may be passed to thermal storage system  60  via first junction  46 , which can be a first shunt valve, along heated first thermal transfer fluid storage stream  44 . Thermal storage unit  60  functions as a heat exchanger to remove and store heat from the heated first thermal transfer fluid storage stream  44  to provide a first thermal transfer fluid storage stream  62 , which is returned to first thermal transfer fluid stream  12  via second junction  64 , which can be a second shunt valve. 
     When the stored heat in the thermal storage system  60  is required by the CSP system  10 , for instance at night when the CSP system  10  cannot capture solar thermal energy to produce a flow of heated first thermal transfer fluid stream  42 , or during cloudy daylight periods when the intensity of the sunlight is reduced, second junction  64  can direct a part of first thermal transfer fluid stream  12  to the thermal storage system  60  along first thermal transfer fluid storage stream  62 . First thermal transfer fluid storage stream  62  is heated in the thermal storage system  60  to provide heated first thermal transfer fluid storage stream  44 , which can returned to heated first thermal transfer fluid stream  42  via first junction  46 . The heated first thermal transfer fluid stream  42  can then be heat exchanged with the loaded sorbent  200  in the sorbent heat exchanger  50 . In this way, the thermal storage system  60  can release captured solar thermal energy to regenerate a loaded sorbent  200  even when there is insufficient sunlight available to CSP system  10 . 
     The thermal storage system  60  may be a molten-salt storage system. Inside the thermal storage system  60  the thermal energy from the heated first thermal transfer fluid storage stream  44  can be passed to a cold salt stream from a cold salt storage tank to generate a hot salt stream. The hot salt stream is passed to a hot salt storage tank where it can be stored until the thermal energy of the hot salt is required. For example, a sixteen hour molten-salt storage system can allow CSP systems to be run on a 24 hour basis in Summertime when there is sufficient daytime sunlight to provide a store of captured thermal energy. 
       FIG. 2  illustrates a second embodiment of the invention. The embodiment of  FIG. 2  shows a liquid sorbent system. In this embodiment, a second thermal transfer fluid circuit  140 , which is a different circuit from the first thermal transfer fluid circuit  40  which first receives the captured solar thermal energy, provides the heat to regenerate the loaded sorbent  200 . 
     In particular, the second thermal transfer circuit  140  comprises one or more second thermal transfer fluid streams  112  which are provided with the captured solar thermal energy by heat exchange with the one or more heated first thermal transfer fluid streams  42  in a first heat exchanger  150 . The second thermal transfer fluid stream  112  can be the same or different from the first thermal transfer fluid stream, and selected from the group comprising: H 2 O, liquid sodium, molten salt, natural or synthetic oil and air. H 2 O is a preferred second thermal transfer fluid. 
     As discussed in relation to  FIG. 1 , the CSP system can additionally comprise one or more concentrators and one or more receivers, and may be of the parabolic trough, linear Fresnel reflector array, central receiver or parabolic dish types. 
     The heated first thermal transfer fluid stream  42  is passed to a first heat exchanger  150 , where it is heat exchanged against the second thermal transfer fluid stream  112  to provide the first thermal transfer fluid stream  12  in the first thermal transfer fluid circuit  40  and a heated second thermal transfer fluid stream  142  in the second thermal transfer fluid circuit  140 . The first heat exchanger  150  can be any heat exchanger known in the art, such as a plate and fin heat exchanger or a shell and tube heat exchanger. Shell and tube heat exchangers, and more particularly kettle heat exchangers are preferred. Although only a single first heat exchanger  150  is shown in  FIG. 2 , the method and apparatus disclosed herein encompasses the possibility of a plurality of heat exchangers, in series and/or in parallel. 
     In a further embodiment not shown in  FIG. 2 , the captured solar thermal energy may be transferred between one of more further thermal transfer circuits, prior to the regeneration of the loaded sorbent  200 . Each further thermal transfer circuit may comprise further heat exchangers, further thermal transfer fluid streams and heated further thermal transfer fluid streams. 
     For instance, where a third thermal transfer circuit is present, the heated second thermal transfer fluid stream  142  would be heat exchanged against a third thermal transfer fluid in a second heat exchanger to provide the second thermal transfer fluid stream  112  in the second thermal transfer fluid circuit  140  and a heated third thermal transfer fluid in a third thermal transfer fluid circuit. The heated third thermal transfer fluid stream could then be heat exchanged with the loaded sorbent  200  in the sorbent heat exchanger to regenerate the sorbent. The third and further thermal transfer fluids may be the same or different to the first or second thermal transfer fluids. 
     Returning to  FIG. 2 , the heated second thermal transfer fluid stream  142 , which may be a steam stream, is passed to the sorbent heat exchanger  50   a  where it is heat exchanged against loaded sorbent  200  to regenerate the loaded sorbent  200  to provide a (regenerated) sorbent stream  238 , an sorbate component stream  242  and second thermal transfer fluid stream  112 . 
     The loaded sorbent  200  is produced in solvent extraction reactor  220 , such as an absorber tower. Process stream  212  comprising one or more sorbate components is passed via the process stream inlet line to the solvent extraction reactor  220  where it is intimately contacted with liquid sorbent, thereby allowing the liquid sorbent to extract the one or more sorbate components from the process stream  212 . In one preferred aspect, the one or more sorbate components comprise carbon dioxide. A number of chemical solvents are useful as the sorbent such as primary, secondary and/or tertiary amines derived from alkanolamines, especially amines are derived from ethanolamine, especially monoethanolamine (MEA), diethanolamine (DEA), triethanolamine (TEA), diisopropanolamine (DIPA) and methyldiethanolamine (MDEA) or mixtures thereof; diglycolamines and sterically hindered amines. 
     For example, the carbon dioxide can be captured in the solvent extraction reactor  220  by an acid-base reaction to yield a soluble carbonate salt according to the reactions: 
       2RNH 2 +CO 2 →RNH 3   + +RNH—CO 2  and/or RNH 2 +CO 2 +H 2 O→RNH 3   + +HCO 3   − 
 
     These solvent extraction reactions are reversible, allowing the aqueous amine solvent to be regenerated by heating in the sorbent heat exchanger  50   a.    
     Preferred sterically hindered amines can be a metal sulphonate, metal phosphonate, metal phosphate, metal sulfamate, metal phosphoramidate or metal carboxylate of at least one hindered secondary or tertiary amine, wherein the metal sulphonate, phosphonate, phosphate, sulfamate or phosphoramidate is attached to the amine nitrogen through a group containing at least one chain carbon, and the metal carboxylate is attached to the amine nitrogen through an alkylene group containing two or more chain carbons. Such sterically hindered amines are disclosed WO 2007/021531. 
     The sorbents discussed can be present as a liquid sorbent mixture in which the sorbent is dissolved in a solvent or mixed with a liquid, the solvent selected from water or a physical solvent or mixtures thereof. 
     Physical solvents which are suitable in the method described herein are cyclo-tetramethylenesulfone and its derivatives, aliphatic acid amides, N-methylpyrrolidone, N-alkylated pyrrolidones and the corresponding piperidones, methanol, ethanol and mixtures of dialkylethers of polyethylene glycols or mixtures thereof. 
     The sorbent may preferably comprise one or more amines selected from the group: monoethanolamine (MEA), diethanolamine (DEA), diglycolamine (DGA), methyl-diethanolamine (MDEA), triethanolamine (TEA), N,N′-di(hydroxyalkyl)piperazine, N,N,N′,N′-tetrakis(hydroxyl-alkyl)-1,6-hexanediamine and tertiary alkylamine sulfonic acid compound. This is particularly useful if the process stream  212  comprises CO 2 . 
     MEA is an especially preferred amine, due to its ability to absorb a relatively high percentage of CO 2  (volume CO 2  per volume MEA). Thus, a sorbent comprising MEA is suitable to remove CO 2  from gases having low concentrations of CO 2 , typically 3 to 10% (v/v) of CO 2 . Preferably, the N,N′-di(hydroxyalkyl)piperazine is N,N′-d-(2-hydroxyethyl)piperazine and/or N,N′-di-(3-hydroxypropyl)piperazine. Preferably, the tetrakis-(hydroxyalkyl)-1,6-hexanediamine is N,N,N′,N′-tetrakis(2-hydroxyethyl)-1,6-hexanediamine and/or N,N,N′,N′-tetrakis(2-hydroxypropyl)-1,6-hexanediamine. Preferably, the tertiary alkylamine sulfonic compounds are selected from the group of 4-(2-hydroxyethyl)-1-piperazine-ethanesulfonic acid, 4-(2-hydroxyethyl)-1-piperazine-propanesulfonic acid, 4-(2-hydroxyethyl)piperazine-1-(2-hydroxypropanesulfonic acid) and 1,4-piperazinedi-(sulfonic acid). 
     An especially preferred sorbent comprises mixtures of a primary or secondary amine with a tertiary or a sterically hindered amine. Suitable tertiary or sterically hindered amines have been described hereinabove. The primary or secondary amine compound suitably has a pKb (at 25° C. in water) below 5.5, preferably below 5, more preferably below 4.5. A lower pKb results in improved process results in the form of increased CO 2  absorption. An especially preferred secondary amine is piperazine. 
     In the event that the process stream  212  comprises an appreciable quantity of oxygen, suitably in the range of from 1 to 20% (v/v) of oxygen, preferably a corrosion inhibitor is added to the absorbing liquid. Suitable corrosion inhibitors are described for example in U.S. Pat. No. 6,036,888. It will be understood that the conditions used for the removal of the sorbate depend inter alia on the type of sorbent used. In the event that the sorbent comprises an amine, the sorbate removal in solvent extraction reactor  220  is suitably carried out at a temperature between 15 and 90° C., preferably at a temperature of at least 20° C., more preferably between 25 and 80° C., still more preferably between 40 and 65° C., and even still more preferably at about 55° C. In the event that the absorbing liquid comprises ammonia, suitably the sorbate removal is performed at temperatures below ambient temperature, preferably in the range of from 0 to 10° C., more preferably from 2 to 8° C. 
     Returning to  FIG. 2 , the loaded sorbent  200  is removed from at or near the bottom of the solvent extraction reactor  220  and passed to sorbent heat exchanger  50   a  as loaded sorbent stream  206  where it is heated to release the sorbate component and regenerate the sorbent. The sorbent regeneration temperature is preferably between 100 and 200° C., more preferably between 120 and 180° C. The regenerated sorbent is returned to the solvent extraction reactor  220  as sorbent stream  238 . In this way a continuous sorbate component extraction and sorbent regeneration proves is provided. 
     Treated process stream  214 , which contains a diminished sorbate component content compared to process stream  212  is withdrawn from solvent extraction reactor  220 . If process stream  212  is a natural gas stream, then treated process stream  214  can be passed to one or more further treatment units, for instance for the optional removal of natural gas liquids, and subsequent liquefaction to provide LNG. 
       FIG. 3  is a diagrammatic scheme for an apparatus and method of regenerating a loaded sorbent according to a third embodiment. A CSP system  10  comprising concentrators  20 , receivers  30  and first thermal transfer circuit  40  is disclosed. First thermal transfer circuit  40  comprises a first thermal transfer fluid stream  12 , which is split into three parallel first thermal transfer fluid part streams  12   a,    12   b,    12   c.  Each first thermal transfer fluid part stream  12   a,    12   b,    12   c  is passed through three pairs of concentrators  20  and receivers  30  which capture solar thermal energy to provide three heated first thermal transfer fluid part streams  42   a ,  42   b,    42   c  respectively. The three heated first thermal transfer fluid part streams  42   a,    42   b,    42   c  are then combined to provide heated first thermal transfer fluid stream  42 . The first thermal transfer fluid is preferably selected from the group comprising: H 2 O, liquid sodium, molten salt, natural or synthetic oil and air 
     The CSP system  10  shown in  FIG. 3  is a parabolic trough system comprising nine parabolic trough concentrators  20  each having an associated tubular receiver  30 . Each parabolic trough concentrator  20  reflects and concentrates light from the sun onto a corresponding receiver  30 . A first thermal transfer fluid part stream  12   a,    12   b,    12   c  is carried within each receiver and is heated by the solar thermal energy captured in each receiver  30 . 
     The present invention is not limited to such a CSP system comprising an array of parabolic trough concentrators  20  and tubular receivers  30 . Alternative arrays comprising a plurality e.g. two, four, five or more parallel first thermal transfer fluid part streams are encompassed together with associated concentrators and receptors, which are not limited to trough and tube systems and may be linear Fresnel reflector arrays, solar tower, or parabolic reflector systems as discussed above. Such systems may comprise a plurality of concentrators and/or receivers in each first thermal transfer fluid part stream. 
     A thermal storage system  60  is provided in first thermal transfer fluid circuit  40 . This operates in an identical manner to the thermal storage system  60  shown in  FIG. 1 . 
     The heated first thermal transfer fluid stream  42  is heat exchanged in first heat exchanger  150  against a second thermal transfer fluid stream  244 , which in this embodiment is a sorbent stream, to provide a heated second thermal transfer fluid stream  152 , which is a heated sorbent stream, and the first thermal transfer fluid stream  112 . First heat exchanger  150  replaces the boiler-fired reboiler in conventional acid gas treatment systems. 
     The heated sorbent stream  152  is passed to a sorbent heat exchanger  50   b,  which can be a stripper column, where it is used to heat and thereby regenerate loaded sorbent stream  228   b.  This heating releases the captured one or more sorbate components as sorbate component stream  242 . Sorbate component stream  242  is cooled by first cooler  282 , which may be an air or water cooler, to provide cooled sorbate component stream  284 . Cooled sorbate stream  284  may be a multi-phase stream comprising the one or more sorbate components, such as carbon dioxide, and any residual sorbent, such as an aqueous amine solution, which will have been condensed by first cooler  282 . Cooled sorbate stream  284  is passed to a separation vessel  290 , such as a gas/liquid separator known in the art. Separation vessel  290  provides a sorbent bottoms stream  292  which is passed back to the sorbent heat exchanger  50   b,  and a sorbate top stream  294  which is passed to a first compressor  310 , powered by driver D 1 . First compressor  310  compresses the sorbate top stream  294  to provide a compressed sorbate stream  312 , such as a compressed carbon dioxide stream, which is passed to storage tank  320  for storage. The storage tank  320  may be in the form of a conventional constructed vessel for the storage of a gaseous product but also includes storage in an oil reservoir, for instance an undersea reservoir. In this way, the method and apparatus described herein can be used in a carbon capture process, for instance by transporting the compressed carbon dioxide stream to an undersea oil reservoir. In addition, the compressed carbon dioxide may also be used in an enhanced oil recovery process, where it is injected into an oil reservoir to increase the amount of oil removed. 
     In a further embodiment (not shown), the compressed sorbate stream  312 , which can be a compressed carbon dioxide stream, can be passed to a mineral carbonation zone. In this zone, an aqueous stream comprising dispersed silicate particles is passed to a mineral carbonation reactor where it is reacted with the compressed carbon dioxide stream to produce carbonate compounds. The carbonate compounds can then be used elsewhere or stored. This process is discussed in greater detail in WO2004/037391. 
     Returning to the sorbent heat exchanger  50   b,  at least a part of (regenerated) sorbent stream  244 , is passed to second heat exchanger  250  as sorbent part stream  244   b , where it its heat exchanged against loaded sorbent bottoms stream  228   a  to pre-heat the loaded sorbent bottoms stream, providing loaded sorbent stream  228   b  and heat exchanged sorbent stream  246 . Heat exchanged sorbent stream  246  is then cooled by second cooler  260 , which can be an air or water cooler, to provide cooled sorbent stream  248 . Cooled sorbent stream  248  is passed to solvent extraction reactor  220  where it is intimately contacted with the process stream  212  comprising one or more sorbate components. Loaded sorbent bottoms stream  228   a  can be removed from at or near the bottom of the solvent extraction reactor  220  and passed to the second heat exchanger  259 . 
     A person skilled in the art will readily understand that the present invention may be modified in many ways without departing from the scope of the appended claims. 
     For instance, natural gas is mentioned above as one example of the hydrocarbon stream comprising one or more sorbate components. Other hydrocarbon streams comprising one or more sorbate components may be employed, such as gasses being formed during decomposition of waste, particularly organic waste. 
     For instance, liquefaction of the treated process stream is mentioned as one example of a further treatment of the treated process stream. However, other further treatments of the treated process stream are possible, such as compressing and/or sending to a gas network, and/or use as feed to a chemical conversion process of the hydrocarbons in the treated process stream, such as oxidation, partial oxidation, etc.. Venting into the atmosphere is, however, not considered to be a “further treatment” in a further treatment unit.