mixture thereof. Suitable amines include, but are not limited  to,  1,4-bis[hydroxyethyl]piperazine, 4-[hydroxyethyl]pip› erazine,  1,4-diazabicyclo[2.2.2]octane  (DABCO),  2-[2- aminoethyl]pyridine,  2-aminomethylpyridine,  3-amino  5-methylpyrazole,  3-aminopyrazole,  3-methylpyrazole,  N,N,N’,N’-tetraethyldiethylenetriamine, N,N,N’,N’-tetram› ethyldiethylenetriamine, 2-piperazinone 1,4-bis[2-hydroxy› ethyl], or a combination thereof.  The amine-based absorbent may be a diamine represented  by the structural formula:  wherein R  1  is an alkylene of two or three carbon atoms as a  straight chain or as a branched chain, R 2 , R 3 , R 4 , and R5 may  be the same or different and can be hydrogen, alkyl (e.g.,  lower alkyl of 1 to 8 carbon atoms including cycloalkyls),  hydroxyalkyl (e.g., lower hydroxy alkyl of 2 to 8 carbon  atoms), aralkyl ( e.g., 7 to 20 carbon atoms), aryl (may be, for  example, monocyclic or bicyclic), or alkaryl (e.g., 7 to 20  carbon atoms), and any of R 2 , R 3 , R 4 , and Rs may form  cyclic structures.  The diamines may also be tertiary diamines. For instance,  the tertiary diamine may be of the formula:  wherein R$^{1 }$is as defined above, and R 2 ,  R 3 ,  R 4 ,  and Rs are  as defined above with the exception that none are hydrogen.  In an exemplary embodiment, each ofR 2 , R 3 , R 4 , and Rs is  the same or different and is an alkyl group (e.g., methyl or  ethyl) or a hydroxy-alkyl group (e.g., 2-hydroxyethyl).  Other diamines in which one or both of the nitrogen atoms  is primary or secondary and which otherwise meet the  parameters discussed herein may also be suitable, provided  mild oxidative or thermal conditions exist to minimize side  reactions of the solvent, including oxidation.  Suitable diamines, according to some invention embodi› ments, have one amine with a lower  pKa  and the other amine  with a higher pKa wherein the higher  pKa  is above 6.5 and,  in some instances, above 7.5 and the lower pKa is less than  5.0 and, in some instances, less than 4.0. The stronger amine  (the one with the higher pKJ may react to form heat stable  salts (HSS). For instance, the stronger amine may react with  a strong acid (e.g., sulfuric acid) to obtain a HSS. In some  embodiments, the lean amine-based absorbent, which is  exposed to the gas stream, is therefore in its half-salt form.  Accordingly, only the weaker, more moderate amine is  available for reacting with the feed gas stream and releas› ably absorbing SO$_{2}$ .  In some embodiments, the diamine in half salt form has  a  pKa  value for the free nitrogen atom of 3.0 to 5.5 and, in  some instances, 3.5 to 4.7 at a temperature of 20°  C. in an  aqueous medium. The free amine form of the amine salt  absorbent may have a molecular weight less than 300 g/mol  and, in some instances, less than 250 g/mol.  In some embodiments, the amine salt absorbents have a  hydroxyalkyl group as a substituent on an amine group.  The organic acid may comprise one or more of formic  acid, acetic acid, glycolic acid, malonic acid, propanoic acid,  succinic acid, phthalic acid, citric acid, adipic acid, tartaric  acid, malic acid, and oxalic acid. In some embodiments, the  The organic acid may have a  pKa  of 1.2-6 and, in some  instances, 3.5-5.5.  In some other embodiments, the chemical solvent may  comprise an organic acid. The organic acid may have a pKa  such that, at the pH of the lean aqueous medium, the organic  acid is substantially in its basic form and, at the pH of the  spent absorbent medium, the organic acid is substantially in  its acidic form. For example, if the organic acid is formic  acid, then at the pH of lean absorbent stream, the formic acid  is present as formate and, at the pH of the spent absorbing  medium (SO2 rich absorbent stream), the organic acid is  substantially in the form of formic acid. By substantially, it  is meant that at least 30% or, in some instances, at least 50%,  of the organic acid is in the particular form at the specified  pH.  The amine-based absorbent may be in an amount suffi› cient to provide a spent absorbing medium containing at  least 180 grams of SO$_{2 }$ per kilogram of absorbing medium.  The amount of amine-based absorbent, however, may not be  so great as to either (a) unduly increase the viscosity of the  absorbing medium such that undesirable pressure drops are  incurred in the feed gas stream passing through an absorber  vessel or (b) render the absorbing medium difficult to  atomize in, for example, a Waterloo scrubber.  It will be appreciated that, in some embodiments, one or  more amines may be used as the absorbent and one or more  amines may be used with other heat regenerable sulfur  dioxide absorbents.  Without being limited by theory, it is believed that use of  amine-based absorbents that generate HSS at a controllable  low level may permit an increase in the concentration of the  physical solvent in absorbing medium of the present inven› tion, and maintain a one-phase solution of the absorbing  medium. One example of such an amine is 2-piperazinone  1,4-bis(2-hydroxyethyl) (Amide-DiHep).  According to some embodiments of the invention, the  diamine may be selected from the group comprising  hydroxyethyl piperazine, bis-hydroxyethyl piperazine, pip› erazine, hydroxyethylethylenediamine, bis-hydroxyethyl- ethylenediamine and mixtures thereof. For example, the  diamine may comprise 1,4-bis[hydroxyethyl]piperazine,  4-[hydroxyethyl]piperazine, or a combination thereof.  Suitable diamine compounds may include, but are not  limited to, N,N’N’-(trimethyl)-N-(2-hydroxyethyl)-ethylene› diamine  (pKa  =5.7); N,N,N’,N’-tetrakis(2-hydroxyethyl)eth- ylenediamine  (pKa  =4.9);  N,N’-dimethylpiperazine  (pKa  =4.8);  N,N,N’,N’-tetrakis(2-hydroxyethyl)-1,3-di- aminopropane;  and  N’,N’-dimethyl-N,N-bis(2-hydroxy- ethyl)ethylenediamine. Useful diamines may also comprise,  in some embodiments, heterocyclic compounds, such as  piperazine  (pKa  =5.8), N-(2-hydroxyethyl)piperazine, N,N’› di(2-hydroxyethyl)piperazine and 1,4-diazabicyclo[2.2.2]  octane  (pKa  =4.9). The  pKa  values identified in the brackets  are for the weaker, sorbing nitrogen.  Without being limited by theory, it is believed that a hydroxy  substituent may increase the solubility of the amine salt  absorbents in water. Without being limited by theory, it is  further believed that a hydroxy substituent may retard the  oxidation of sulphite or bisulphite to sulfate, which can  result in the formation of HSS. As discussed below, it may  be desirable to minimize the formation of HSS.  US 9,919,262 B2  6  5 