Patent Publication Number: US-2013243676-A1

Title: Amine treating process for acid gas separation using blends of amines and alkyloxyamines

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is related to and claims priority benefit under 35 USC 120 from U.S. Patent Application Ser. No. 61/610,599, filed 14 Mar. 2012. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the absorption of acidic gases from mixed gas streams containing acidic and non-acidic components. 
     BACKGROUND OF THE INVENTION 
     The treatment of gases and liquids containing acidic gases such as CO 2 , H 2 S, CS 2 , HCN, COS and sulfur derivatives of C 1  to C 4  hydrocarbons with amine solutions to remove these acidic gases is well established. The amine usually contacts the acidic gases and the liquids as an aqueous solution containing the amine in an absorber tower with the aqueous amine solution passing in countercurrent to the acidic fluid. In typical cases using common amine sorbents such as monoethanolamine (MEA), diethanolamine (DEA), methyldiethanolamine (MDEA), diisopropylamine (DIPA), or hydroxyethoxyethylamine (DGA). The liquid amine stream contained the sorbed acid gas is typically regenerated by desorption of the sorbed gases in a separate tower with the regenerated amine and the desorbed gases leaving the tower as separate streams. The various gas purification processes which are available are described, for example, in  Gas Purification , Fifth Ed., Kohl and Neilsen, Gulf Publishing Company, 1997, ISBN-13: 978-0-88415-220-0. 
     The treatment of acid gas mixtures containing CO 2  and H 2 S with amine solutions typically results in the simultaneous removal of substantial amounts of both the CO 2  and H 2 S. It is often desirable, however, to treat acid gas mixtures containing both CO 2  and H 2 S so as to remove the H 2 S selectively from the mixture, thereby minimizing removal of the CO 2 . Selective removal of H 2 S results in a relatively high H 2 S/CO 2  ratio in the separated acid gas which simplifies the conversion of H 2 S to elemental sulfur using the Claus process. Selective H 2 S removal is applicable to a number of gas treating operations including treatment of hydrocarbon gases from oil sands, coal and shale pyrolysis, refinery gas and natural gas having a low H 2 S/CO 2  ratio and is particularly desirable in the treatment of gases wherein the partial pressure of H 2 S is relatively low compared to that of CO 2  because the capacity of an amine to absorb H 2 S from the latter type gases is very low. Examples of gases with relatively low partial pressures of H 2 S include synthetic gases made by coal gasification, sulfur plant tail gas and low-Joule fuel gases encountered in refineries where heavy residual oil is being thermally converted to lower molecular weight liquids and gases. 
     Although primary and secondary amines such as MEA, DEA, DPA, and DGA absorb both H 2 S and CO 2  gas, they have not proven especially satisfactory for preferential absorption of H 2 S to the exclusion of CO 2  because in aqueous solution, the amines undergo more selective reaction with CO 2  to form carbamates. The tertiary amine, MDEA, has been reported to have a high degree of selectivity toward H 2 S absorption over CO 2  (Frazier and Kohl,  Ind. and Eng. Chem.,  42, 2288 (1950)), but its commercial utility is limited because of its restricted capacity for H 2 S loading and its limited ability to reduce the CO 2  content of the gas. Similarly, diisopropylamine (DIPA) is relatively unique among secondary amino alcohols in that it has been used industrially, alone or with a physical solvent such as sulfolane, for selective removal of H 2 S from gases containing H 2 S and CO 2 , but contact times must be kept relatively short to take advantage of the faster reaction of H 2 S with the amine compared to the rate of CO 2  reaction. This greater selectivity was attributed to the relatively slow chemical reaction of CO 2  with tertiary amines as compared to the more rapid chemical reaction of H 2 S. 
     A number of severely sterically hindered etheramine compounds have been developed for the selective removal of H 2 S in the presence of CO 2 . U.S. Pat. Nos. 4,405,581; 4,405,583; 4,405,585; 4,471,138 and 4,894,178 disclose these highly effective hindered selective absorbents. The following typical types of absorbent are disclosed in these patents to which reference is made for a full description of these materials and their use in acidic gas sorption processes: 
     U.S. Pat. No. 4,405,581: The hindered aminoalcohol compounds disclosed in this patent are defined by the formula: 
     
       
         
         
             
             
         
       
     
     where R 1  is usually a C 1 -C 8  alkyl group such as tertiary butyl, secondary-butyl, isopropyl, tertiary-amyl or cyclohexyl, R 2  and R 3  are usually hydrogen, or C 1 -C 4  alkyl groups, with the certain provisos to define the adequately hindered molecule, x is an integer from 2 to 4, i.e., the aminoalcohols can be regarded as hindered aminated derivatives of ethylene glycol, propylene glycol or butylene glycol. Specific non-limiting examples of the severely sterically hindered secondary amino alcohols of this type include tertiarybutylaminoethanol, 2-(tertiarybutylamino)-1-propanol, 2-(isopropylamino)-propanol, 3-(tertiarybutylamino)-n-butanol, 3-(tertiarybutylamino)-1-propanol and 3-aza-2,2-dimethyl-1,6-hexanediol. 
     U.S. Pat. No. 4,405,583: The hindered diamino etheramines disclosed in this patent are defined by the formula: 
     
       
         
         
             
             
         
       
     
     where R 1  and R 8  are C 3 -C 8  secondary alkyl or secondary hydroxyalkyl, or C 4 -C 8  tertiary alkyl or tertiary hydroxyalkyl radicals, R 2  and R 6  are each hydrogen or C 1 -C 4  alkyl, with the proviso that when R 1  and R 8  are secondary alkyl, R 2  and R 6  are C 1 -C 4  alkyl radicals, and 0 is either zero or a positive integer ranging from 1 to 4. Representative di-secondary etheramines include, for example, bis-(tertiarybutylaminoethyl)ether; 1,2-bis(tertiarybutylaminoethoxy) ethane; 1,2-bis-(tertiarybutylaminoethoxyethoxy) ethane; bis[2-(iso-propylamino)propyl)ether and 1,2-[2-(isopropylamino)-propoxy]ethane. 
     U.S. Pat. No. 4,405,585: This patent discloses the selective removal of H 2 S from acidic gas mixtures using severely sterically hindered secondary etheramine alcohols for including those defined by the general formula: 
     
       
         
         
             
             
         
       
     
     where R 1  is primary C 1 -C 8  alkyl or primary C 2 -C 8  hydroxyalkyl branched chain alkyl or other selected groups; R 2 , R 3 , R 4  and R 5  are each independently hydrogen, C 1 -C 4  alkyl or C 1 -C 4  hydroxyalkyl, with the proviso that when R 1  is primary alkyl or hydroxyalkyl, both R 2  and R 3  bonded to the carbon atom directly bonded to the nitrogen atom are alkyl or hydroxyalkyl and that when the carbon atom of R 1  directly bonded to the nitrogen atom is secondary at least one of R 2  or R 3  bonded to the carbon atom directly bonded to the nitrogen atom is an alkyl or hydroxyalkyl, x and y are each positive integers independently ranging from 2 to 4 and z is a positive integer ranging from 1 to 4. Specific etheramine alcohols whose use is comprehended by this patent include: 
     
       
         
         
             
             
         
       
     
     Tertiarybutylaminoethoxyethanol 
     
       
         
         
             
             
         
       
     
     2-(2-tertiarybutylamino)propoxyethanol 
     
       
         
         
             
             
         
       
     
     (1-methyl-1-ethyl propylamino)ethoxyethanol 
     
       
         
         
             
             
         
       
     
     2-(2-isopropylamino)propoxyethanol 
     
       
         
         
             
             
         
       
     
     Tertiaryamylaminoethoxyethanol 
     
       
         
         
             
             
         
       
     
     (1-methyl-1-ethylpropylamino)ethoxyethanol 
     U.S. Pat. No. 4,471,138 is directed to a class of selective H 2 S absorbents which are secondary tertiary and etheramine alcohols of the formula: 
     
       
         
         
             
             
         
       
     
     where: 
     R 1 =R 2 =R 3 =CH 3 ; R 4 =R 5 =R 6 =H; R 1 =R 2 =R 3 =CH 3 ; R 4 =H or CH 3 ; R 5 =R 6 =H; 
     R 1 =R 2 =R 3 =R 6 =CH 3 ; R 4 =R 5 =H; R 1 =R 2 =R 3 =CH 3 CH 2 ; R 4 =R 5 =R 6 =H; or 
     R 1 ≠R 2 ≠R 3 =H, CH 3 , CH 3 CH 2 , R 4 ≠R 5 ≠R 6 =H or CH 3 , and x=2-3. 
     U.S. Pat. No. 4,894,178: This patent discloses the selective H 2 S absorbents which are a mixture of a severely hindered tertiary dietheramine with a severely hindered tertiary etheramine alcohol with the formulae: 
     
       
         
         
             
             
         
       
     
     with x being an integer from 2 to 6 and the weight ratio of the first amine to the aminoalcohol ranging from 0.43:1 to 2.3:1. The preferred absorbent is a combination of bis-(tert.-butylaminoethoxy) ethane (BTEE) and ethoxyethoxyethanol-tert.-butylamine (EEETB). These mixtures can be prepared in a one-step synthesis, by the catalytic tertiary butylamination of the polyalkenyl ether glycol, HO—(CH 2 CH 2 O)-x-CH 2 CH 2 —OH. For example, the mixture of BTEE and EEETB can be obtained by the catalytic tertiarybutylamination of triethylene glycol. The severely hindered amine mixture, e.g., BTEE/EEETB, in aqueous solution can be used for the selective removal of H 2 S in the presence of CO 2 . 
     U.S. 2010/0037775 discloses alkylamine alkyloxy alkyl ethers which are selective for the sorption of H 2 S from acidic gas mixtures containg CO 2 . The sorbents are produced by the reaction of an alkyloxy alcohol with a hindered primary alkylamine such as tert-butylamine. 
     US 2009/0308248 describes a different class of absorbents which are selective for H 2 S removal in the presence of CO 2 , the hindered amino alkyl sulfonate, sulfate and phosphonate salts, with the sulfonate and phosphonates being the preferred species. The formula of these compounds is: 
       R 1 R 2 N—(—CR 3 R 4 —) n —X
 
     where R 1 , R 2 , R 3  and R 4  are typically hydrogen, C 1 -C 9  substituted or unsubstituted alkyl, C 6 -C 9  aryl provided both R 1  and R 2  are not hydrogen; and wherein when n is 2 or more, R 3  and R 4  on adjacent carbon or on carbons separated by one or more carbons can be a cycloalkyl or aryl ring and wherein, when substituted, the substituents are heteroatom containing substituents, and n is an integer of 1 or more, and X is a metal salt group, such as —SO 3   − , —SOS 3   − , —NHSO 3   − , —PO 3   2− , —PO 3 H − , —OPO 3   2− , —NHPO 3   2−  or —CO 2   −  where the valence(s) of the salt group are satisfied by a metal cation such as sodium or potassium. Preferred absorbents of this type include sodium tert-butylaminomethylsulfonate; sodium 2-(tert-butylamino) ethylsulfonate; sodium 3-(tert-butylamino)propylsulfonate; diethyl tert-butylaminomethylphosphonate and disodium tert-butylaminomethylphosphonate. 
     Proposals have been made for using selective amine absorbents in combination with other materials affecting the sorption properties. U.S. Pat. No. 4,892,674, for example, discloses a process for the selective removal of H 2 S from gaseous streams using an absorbent composition comprising a non-hindered amine and an additive of a severely-hindered amine salt and/or a severely-hindered aminoacid. The amine salt is the reaction product of an alkaline severely hindered amino compound and a strong acid or a thermally decomposable salt of a strong acid, i.e., ammonium salt. 
     The potential of using amine blends was disclosed by Lunsford et al in  Optimization of Amine Sweetening Units , Proc. 1996 AlChE Spring National Meeting, New York, N.Y., which showed that a blend of MDEA in a 30% DEA solution, increased CO 2  take up. The use of physical solvents such as sulfolane with MDEAS or DIPOA is also reported to increase removal of species such as COS and mercaptans. 
     SUMMARY OF THE INVENTION 
     While the severely hindered etheramine alcohols and their derivatives such as the alkoxy derivatives of US 2010/003775 have excellent selectivity for H 2 S in acidic gas mixtures which also contain CO 2 , there are occasions when it is desired to absorb both H 2 S and CO 2 , for example, to remove CO 2  from natural gas which comes from wells with a high CO 2  content where it is desired to re-inject the CO 2  for pressure maintenance and for carbon sequestration but where it is also necessary to meet maximum H 2 S specifications for pipelining, e.g. with gas from fields such as LaBarge, Wyo. In these cases, the overall selectivity of CO 2  pickup may need to be optimized when maximum selectivity is not required. 
     We have now found that the overall selectivity of CO 2  pickup can be secured while maintaining good H 2 S sorption selectivity by carrying out the absorption with a severely hindered tertiary alkyletheramine alcohol derived from triethylene glycol in combination with a secondary absorbent amine component such as methyldiethylamine (MDEA), monoethanolamine (MEA), 2-amino-2-methyl-1-propanol (AMP), piperazine (PZ), diethanolamine (DEA), triethanolamine (TEA), diglycolamine (aminoethoxyethanol, DGA) and diisopropylamine (DIPA) or one or more of the alkyletheramines. 
     According to the present invention, the process for absorbing H 2 S and CO 2  from a gas mixture containing both these gases comprises contacting the gas mixture with an absorbent combination of (i) a primary absorbent component which comprises a severely sterically hindered tertiary alkyletheramine, and (ii) a secondary absorbent component which comprises an amine absorbent for acidic gases. The absorbent combination of the primary and secondary components will normally be used in the form of a liquid absorbent solution, typically an aqueous solution. While the ability to absorb both H 2 S and CO 2  is useful in certain circumstances as noted above, improved H 2 S selectivity is also useful asset as is the capability of loading (moles of absorbed gas per mole of amine) and the capacity (moles of gas absorbed by solution relative to the moles desorbed from the solution, that is the relative amount absorbed and released in each absorption/desorption cycle). For this purpose, combinations of etheramine compounds have been found to be advantageous as described in more detail below. 
    
    
     
       DRAWINGS 
       In the accompanying drawings: 
         FIG. 1  is a graph showing the H 2 S selectivity at different total gas loadings (H 2 S plus CO 2 ) with different etheramine mixtures. 
         FIG. 2  is a graph showing the H 2 S selectivity at different times with different ethoxyamine mixtures. 
         FIG. 3  is a graph showing the H 2 S selectivity of a preferred etheramine mixture in comparison with individual etheramines. 
     
    
    
     DETAILED DESCRIPTION 
     Glossary of Abbreviations 
     In order to facilitate understanding of various abbreviations of the compounds that may be named in the specification, the following glossary is provided:
     DEG Diethylene glycol   TEG Triethylene glycol   TBA Tertiary-butyl amine   MAE Methylaminoethanol   EEA Ethoxyethanolamine   EETB Ethoxyethanol-t-butylamine (tertiary-butyl-ethoxyethanol)   EEETB EthoxyEETB (Ethoxyethoxyethanol-t-butylamine)   DEGM Diethylene glycol monomethyl ether   TEGM Triethylene glycol monomethyl ether   MDEGTB Diethylene glycol t-butylamine monoethyl ether   MEETB MethoxyEETB (methoxy ethoxyethoxyethanol-t-butylamine)   BEETB ButoxyEETB   TEGTB Triethylene glycol-t-butylamine (ethoxyethoxyethanol-t-butylamine or t-butylamino-ethoxyethoxyethanol)   MEEETB MethoxyTEGTB (methoxyethoxyethoxyethanol-tert-butylamine or t-butylamino-ethoxyethoxyethyl methyl ether)   Bis-SE Bis-(t-butylamino)-DEG   Bis-TEGTB Bis-(t-butylamino)-TEG (TEG(TB) 2 )   DEGTB Diethylene glycol-t-butylamine (ethoxyethanol-t-butylamine or t-butylamino-ethoxyethanol)   Bis-DEGTB Bis-(t-butylamino)-DEG (DEG(TB) 2 )   

     Primary Absorbent Component—Severely Hindered Etheramine Absorbent 
     The preferred severely sterically hindered etheramine derivatives described below are preferably derived from triethylene glycol (TEG) although derivatives of diethylene glycol (DEG) as well as other etheramines particularly the polyglycolamines may also be found suitable. Thus, while any of the severely hindered amino derivatives described above may be used in combination with one or more of the more conventional amine absorbents, the TEG derivatives form a preferred class in view of their high selectivity for H 2 S absorption and absorption capacity which can then be balanced against the CO 2  absorption of the conventional amine. 
     In general, the preferred etheramine derivatives are made by the reaction of triethylene glycol (TEG) with a severely hindered amine which may be a primary or secondary amine. The preferred amines for reaction with the TEG are primary amines with a tertiary alkyl group, especially C3-C8 alkyl, to form secondary or tertiary amino derivatives of the glycol. Tertiary butyl is the preferred tertiary alkyl group. As derivatives of triethylene glycol (TEG), the severely hindered etheramineetheramines of the present process will have the characteristic group derived from this glycol: 
       —(CH 2 CH 2 —O—) 3 —
 
     Diethylene glycol derivatives will contain the characteristic grouping: 
       —(CH 2 CH 2 —O—) 2 —
 
     Various groups will be attached at the two ends of the polyglycol chain. For example, according to a first variant, secondary or tertiary amino groups may be attached at each end of the TEG moiety to form a dietheramine according to the preferred formula given in U.S. Pat. No. 4,405,583: 
     
       
         
         
             
             
         
       
     
     where R 1  and R 8  are each C 3  to C 8  secondary alkyl or hydroxyalkyl or C 4  to C 8  tertiary alkyl or hydroxyalkyl groups, R 2  and R 6  are each hydrogen, and where, in this case, o is 1. Representative di-alkyletheramines derivatives of TEG of this type include, for example, 1,2-bis-(tertiarybutylaminoethoxy) ethane. 
     Alternatively, following the formula of U.S. Pat. No. 4,405,585, the TEG derivatives may be etheramine alcohols of the formula: 
     
       
         
         
             
             
         
       
     
     where R 2 , R 3 , R 4  and R 5  are H, R 1  is C 3 -C 8  branched chain alkyl, preferably tertiary alkyl, e.g., tert.-butyl, x and y are each 2 and z is 2 (z is 1 for the corresponding DEG derivatives). An example of such an absorbent is ethoxyethoxyethanol-tert.-butylamine (EEETB) which, as described in U.S. Pat. No. 4,894,178, is preferably used in combination with the DEG derived diamino ethers of U.S. Pat. No. 4,405,583, for example, 1,2-bis(tert.-butylaminoethoxy)ethane (BTEE), with a preferred ratio of the two components being in the weight ratio of 0.43:1 to 2.3:1. 
     TEG derivatives following the general formula of U.S. Pat. No. 4,471,138 may also be blended with conventional amine absorbents; in this case, the TEG derivatives will adhere to the formula: 
     
       
         
         
             
             
         
       
     
     where R 1 =R 2 =R 3 =C 1 -C 4  alkyl, preferably CH 3 ; R 4 =R 5 =R 6 =H; x=y=2 and z=2. The corresponding DEG derivatives are formed when z=1. 
     If an alkoxy-capped TEG is reacted with the severely hindered amine to result in a hindered alkylamine alkoxy (alcohol) monoalkyl ether according to the reaction scheme set out in US 201/0037775, the starting alkoxy alcohol will be an alkoxy-triethylene glycol and the alkylamine will typically be a sterically hindered amine of the formula R 2 R 5 NH where R 2  is C 3 -C 6  alkyl, preferably C 3 -C 6  branched chain alkyl, R 5  is H or C 1 -C 6  alkyl; the preferred amine is tert-butylamine. 
     When the TEG derivative is an alcohol, e.g., an etheramine alcohol such as EEETB, the hydroxyl group may be esterified with a lower carboxylic acid (C 2 -C 6 ) to yield a etheramine ester such as 2-(ethoxyethoxy-tert.-butylamino) ethyl acetate, propionate or butyrate which may then be used as a component in the blend with the other amine. The hydroxyl group may, alternatively, be converted to an ether group by reaction with an lower (C 1 -C 4 ) alkyl halide 
     When the TEG etheramine has more than one amino group, improved solubility in water may be conferred by conversion of one of the amino groups to their corresponding aminosulfonate or aminophosphonate salts by reaction with the appropriate sulfonic acid or phosphonic acid although at the expense of decreased loading capacity for the acidic gases as the reacted amino group becomes inactive for acid gas removal. 
     Secondary Absorbent Components 
     The amine absorbents which are used as the secondary absorbent component in combination with the primary (hindered etheramine) absorbents comprise the amines which are effective for chemisorbing CO 2 . In this way, the relative sorption properties of the absorbent solution may be balanced between the H 2 S and CO 2  contents of the incoming gas stream so that the desired removal of each gas is obtained. As described below, the secondary absorbent component may be one or more etheramines. In general, the weight ratio of the two components of the blend may typically vary between 5:95 to 95:5, or over a more limited range from 10:90 to 90:10, more usually from 20:80 to 80:20 and in some cases an approximately equal weight of each in the absorbent solution, e.g. from 40:60 to 60:40. 
     Amines such as the ethanolamines, e.g., monoethanolamine (MEA), diethanolamine (DEA), triethanolamine, (TEA), methylaminoethanol (MAE) and ethoxyethylamine (EEA), methyldiethanolamine (MDEA), or hydroxyethoxyethylamine (diglycolamine, DGA), as well as other amines such as piperazine (PZ), diisopropylamine (DIPA), are all likely to be found useful as the secondary component in blends with the hindered etheramine absorbents. The preferred blends are, however, blends of etheramine compounds including EETB/MEETB, EEETB/MEETB, EETB/MEEETB, EEETB/MEEETB, EEETB/EEE(TB) 2 . The blends may include blends of dietheramines such as TEG(TB) 2  with DEG(TB) 2 , blends of aminoalcohols with other aminoalcohols such as EETB with EEETB, EETB with MEETB, EETB with MEEETB and blends of aminoether alcohols with diamino etheramines such as TEGTB with TEG(TB) 2 , DEGTB with DEG(TB) 2  etc. 
     The blended absorbent combination will typically be used in the form of an aqueous solution in the absorption process, normally at a concentration from 5 to 40 wt. percent total amine with most processing carried out at 5-30 wt. percent. Physical solvents (as opposed to the amino compounds which are chemical absorbents) may also be used. Solvents which are physical absorbents are described, for example, in U.S. Pat. No. 4,112,051, to which reference is made for a description of them; they include, for example, aliphatic acid amides, ethers, esters such as propylene carbonate, N-alkylated pyrrolidones such as N-methyl-pyrrolidone, sulfones such as sulfolane, sulfoxides such as DMSO, glycols and their mono- and diethers such as glyme. The preferred physical absorbents are the sulfones, most particularly, sulfolane. These physical solvents may also be used in combination with water. If the solvent system is a mixture of water and a physical absorbent, the typical effective amount of the physical absorbent employed may vary from 0.1 to 6 moles per litre of total solution, and preferably from 0.5 to 3 moles per litre, depending mainly on the type of amino compound being utilized. 
     The primary and secondary absorbent components may be used together over a wide range of ratios. As shown below, the addition of only a minor amount of a second absorbent is capable of effecting a significant change in the H 2 S selectivity. For example, the addition of just 5% MEEETB to EETB boosts the selectivity by approximately 5 percentage points over a broad range of total loadings (H 2 S plus CO 2 ) up to about 5% (total moles per mole of amine). The use of a 50/50 mixture of EETB and MEEETB may boost H 2 S selectivity by about 8 to 10 percentage points over the same range, as shown in  FIG. 1  below. The two components of the blend may therefore be used over a wide range of molar ratios typically extending from 95:5 to 5:95, e.g., from 90:10 to 10:90, from 80:20 to 20:80, from 25:75 to 75:25, 606:40 to 40:60 and in approximately equal molar proportions. 
     Processing of the acidic gas stream will follow the normal lines of an amine absorption process using an aqueous absorbent solution, usually in a cyclic absorption-regeneration unit of the type described in U.S. Pat. No. 4,471,138; 4,894,178 or 4,405,585, as referenced above. 
     The absorbent solution may include a variety of additives typically employed in selective gas removal processes, e.g., antifoaming agents, anti-oxidants, corrosion inhibitors, and the like. The amount of these additives will typically be in the range that they are effective, i.e., an effective amount. 
     One advantage of the triethylene glycol selective absorbents is that they may be readily mixed with the secondary absorbent component including the conventional amine absorbents such as MDEA, DEA, etc. as well as other etheramines in all proportions. A gas processing unit filled with a conventional amine absorbent can therefore be converted to operation with one of the triethylene glycol absorbents by simply topping up the unit with the triethylene glycol absorbent to replace losses of the conventional amine as they occur. Alternatively, a portion of the conventional amine may be withdrawn and replaced by the triethylene glycol derivative if a greater degree of selectivity for H 2 S is desired, for example, by a change in the composition of the feed or a requirement to increase the selectivity. 
     The absorbent solution ordinarily has a concentration of amino compound of about 0.1 to 6 moles per liter of the total solution, and preferably 1 to 4 moles per liter, depending primarily on the specific amino compound employed and the solvent system utilized. 
     Example 1 
     Mixtures of two etheramines, t-butylaminoethoxyethanol (EETB) and methoxy-triethylene glycol-t-butylamine (MEEETB, t-butylamino-ethoxyethoxyethyl methyl ether) in varying ratios were tested for their absorption characteristics by bubbling a gas mixture containing 10% v/v CO 2 , 1% H 2 S, balance N 2 , through a stirred 2.17 molar aqueous amine mixture at 40° C. (absorbent and gas), 138 kPag (20 psig) at a gas flow rate of 600 mL/min. The five gas ratios tested were (EETB/MEEETB): 100/0; 95:5; 90/10; 80/20 and 50:50. 
     The gas was introduced into the solvent solution down a dip tube with the outlet submerged just below (8 mm) the surface of the solvent. These parameters were found to provide stable and repeatable data for both MDEA and other solutions. The test gas was water saturated before entering the test cell. A variable speed paddle mixer circulated solvent past the dip tube at a controlled rate. The cell was run at atmospheric pressure. Gas venting from the cell was passed through a collection pot where it was sampled and analyzed for H 2 S and CO 2  concentration. using a GASTEC™ stain tube (colorimetric quantification). 
     The selectivities of the mixtures were calculated as the ratio of H 2 S and CO 2  absorbed in the solution to the H 2 S and CO 2  in the feed gas (moles/moles).  FIG. 1  shows that the addition of the MEEETB at quite low fractions of the overall composition makes a significant difference in the H 2 S selectivity with the greatest increase in selectivity at loadings up to about 0.35 moles per mole of amine being achieved with 50/50 mix.  FIG. 2  shows that the MEEETB appears to enhance selectivity through accelerated H 2 S absorption compared with the EETB base case rather than through inhibiting CO 2  pickup, implying that optimal gas/liquid contact times for H 2 S selectivity will be lower than those needed for maximal absorption (loading). 
     Example 2 
     Further studies with etheramines and blends of etheramines carried out in the same manner showed that the blends possessed potential advantages in H2S selectivity and loading in comparison with single etheramines, as shown by Table 1 below: 
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                 Loading 
                 Capacity 
                 Selectivity- 
               
               
                 Compound 
                 Mol. Wt. 
                 Selectivity 
                 (%) 
                 (%) 
                 Reabsorption 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 EETB 
                 161.24 
                 14.5 
                 17.4 
                 61.0 
                 15.3 
               
               
                 Bis-SE 
                 216.36 
                 16.76 
                 28.2 
                 80.0 
                 25.2 
               
               
                 MEEETB 
                 219.32 
                 64.4 
                 24.2 
                 98.4 
                 69.7 
               
               
                 TEG(TB) 2   
                 260.42 
                 23.3 
                 19.4 
                 65.1 
                 39.2 
               
               
                 TEGTB 
                 205.26/ 
                 128.2 
                 45.4 
                 82.6 
                 131.2 
               
               
                 (32.2%)/ 
                 260.42 
               
               
                 TEG(TB)2 
               
               
                 (67.4%) 
               
               
                   
               
               
                 Bis-SE = Bis-(t-butylamino)-diethylene glycol 
               
               
                 TEGTB = Triethylene glycol-t-butylamine 
               
               
                 TEG(TB) 2  = Bis-(t-butylamino)-triethylene glycol 
               
               
                 Loading = Moles of H 2 S/Moles of absorbent 
               
               
                 Capacity = Moles of H 2 S absorbed by solution/Moles of H 2 S after desorption from solution. 
               
            
           
         
       
     
     Thus, even though the mixture of TEGTB and TEG(TB) 2  has a molecular weight disadvantage (weighted average mol. wt of 241.61) compared to MEEETB (219.32) resulting in fewer moles of absorbent per unit weight purchased, the increased H 2 S selectivity and loading resulting from the two reaction sites on the two amine groups, approximately double that of the MEEETB, makes the use of the blend attractive since the capital and operating costs of the unit will be substantially reduced. Further, the selectivity, loading and other performance parameters for the blend are also greatly better than those of the bis-(amino) compound on its own. 
     Example 3 
     The evaluation was continued by the same method using MDEA, EETB, MEEETB and a mixture of TEGTB and TEG(TB) 2  (57.8%/35% with unreacted TEG as balance) to show the relationship of H 2 S selectivity with over a range of loadings. The results are shown in  FIG. 3 . MDEA is approximately as selective as EETB but only at very low loadings after which the selectivity becomes sharply worse at higher rates. EETB has the virtue of having a linear selectivity at all loadings. MEEETB and the TEG blend are significantly more selective than EETB at low to moderate loadings with MEETB having a marginal advantage but given the doubling in loading afforded by the bis-(amino) derivative in the mixture (see Example 2), the blend has a clear advantage in selectivity over the other material.