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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: 

<img src='content_image/929807.jpg'>

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: 

<img src='content_image/929808.jpg'>

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. 

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. 

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. 

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. 

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). 

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. 

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. 

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 organic acid may have a pKa of 1.2-6 and, in some instances, 3.5-5.5. 

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 

