Removal and recovery of sulphur dioxide from gas streams

Claim 2

A cyclic process for the removal of sulphur dioxide from a sulphur dioxide-containing gas stream using an absorbent medium and for the regeneration of the absorbent medium, which comprises: (a) contacting said gas stream at substantially atmospheric pressure with an aqueous absorbing medium containing a water-soluble half salt of a diamine at a temperature of from about 10° to about 60° C to absorb sulphur dioxide from said gas stream in amounts of at least about 100 g of sulphur dioxide per kg of absorbing medium, said diamine in free base form having a molecular weight of less than about 300 and said diamine in half salt form having a pKa value for the free nitrogen atom of about 5 to about 7.3, said aqueous absorbing medium containing at least 1 mole of water for each mole of sulphur dioxide to be absorbed and comprising less than about 80 wt. % of water; (b) stripping gaseous sulphur dioxide from said spent absorbing medium at a temperature of about 50° to about 110° C and at least about 30° C greater than said contact temperature to form a regenerated aqueous absorbing medium; (c) recovering the gaseous sulphur dioxide; and (d) recycling said regenerated aqueous absorbing medium to said contacting step.

Claim 3

The process of claim 1 wherein said aqueous absorbing medium has a pH in the range of about 6.5 to about 7.5 at the start of said contacting step and has a pH at least about 4.0 at the end of said contacting step.

Claim 4

The process of claim 2 wherein said half salt of a diamine has a molecular weight of the free base of less than about 250.

Claim 5

The process of claim 3 wherein said half salt of a diamine has a pKa value for the free nitrogen atom of about 5.7 to about 6.7.

Claim 6

The process of claim 1 wherein said diamine is a tertiary diamine.

Claim 7

The process of claim 5 wherein said tertiary diamine has the general formula: in which R₁ is an alkylene group and each R₂ is the same or different alkyl group or hydroalkyl group.

Claim 8

The process of claim 6 wherein said alkylene group contains from 2 to 3 carbon atoms.

Claim 9

The process of claim 7 wherein said alkylene group is a straight chain alkylene group.

Claim 10

The process of claim 8 wherein said alkyl group is methyl.

Claim 11

The process of claim 8 wherein said hydroxyalkyl group is a 2-hydroxyethyl group.

Claim 12

The process of claim 5 wherein said tertiary diamine is selected from the group consisting of N,N′ ,N′-(trimethyl)-N-­(2-hydroxyethyl)-ethylene diamine and N,N,N′,N′-­tetramethylethylene diamine.

Claim 13

The process of claim 1 wherein said diamine is piperazine.

Claim 14

The process of claim 5 wherein said half salt is the sulphite salt of the tertiary diamine.

Claim 15

The process of claim 5 wherein said half salt is the sulfate salt of the tertiary amine.

Claim 16

The process of claim 1 wherein said proportion of water present in the aqueous absorbing medium is from about 25 to about 50 wt. %.

Claim 17

The process of claim 1 wherein said contact is effected at a temperature of about 10° to about 50° C and said stripping is effected at a temperature of about 70° to about 90° C.

Claim 18

The process of claim 1 wherein the ratio of SO₂ absorbed in said contacting step to that desorbed in said stripping step is approximately 1:1.

Claim 19

The process of claim 17 wherein said stripping is effected by steam stripping.

Claim 20

The process of claim 17 wherein said stripping is effected by passing an inert gas through said spent absorbing medium.

Claim 21

The process of claim 1 wherein heat stable salts are removed from said regenerated aqueous absorbing medium.

Claim 22

A cyclic process for the removal of sulphur dioxide from a sulphur dioxide-containing gas stream using an absorbing medium and for the regeneration of the absorbing medium, which comprises; (a) contacting said gas stream with an absorbing medium containing (i) water in an amount of at least about 1 mole for each mole of sulphur dioxide to be absorbed up to about 80 weight percent of the medium and (ii) amine salt absorbent having at least two amine groups, at least one of which is a salt and at least one of which is, when in its free form, a group having a pKa in an aqueous medium at 25° C of about 4.5 to 7.3 ("sorbing nitrogen"), at a temperature up to about 60° C sufficient to enable under reference conditions of a partial pressure of 2 mm Hg of sulphur dioxide, at least about 0.5 mole of sulphur dioxide to be reversibly absorbed per mole equivalent of sorbing nitrogen, said absorbent being in an amount sufficient to provide a spent absorbing medium containing at least about 100 grams of sulphur dioxide per kilogram of absorbing medium; (b) removing gaseous sulphur dioxide from the spent absorbing medium at a temperature and pressure sufficient to remove at least about 70 mole percent of the sulphur dioxide absorbed in the contacting step, in the spent absorbing medium to provide a regenerated medium containing absorbent having at least one nitrogen as an amine salt and at least one free sorbing nitrogen; and (c) recovering the gaseous sulphur dioxide; and (d) recycling the regenerated absorbing medium to the contacting step.

Claim 23

A cyclic process for the removal of sulphur dioxide from a sulphur dioxide-containing gas stream using an absorbing medium and for the regeneration of the absorbing medium, which comprises; (a) contacting said gas stream with an absorbing medium containing (i) water in an amount of at least about 1 mole for each mole of sulphur dioxide to be absorbed up to about 80 weight percent of the medium and (ii) amine salt absorbent having at least two amine groups, at least one of which is a salt and at least one of which is, when in its free form, a group having a pKa in an aqueous medium at 25° C of about 4.5 to 7.3 ("sorbing nitrogen"), at a temperature up to about 60° C sufficient to enable under reference conditions of a partial pressure of 2 mm Hg of sulphur dioxide, at least about 0.5 mole of sulphur dioxide to be reversibly absorbed per mole equivalent of sorbing nitrogen, said absorbent being in an amount sufficient to provide a spent absorbing medium containing at least about 100 grams of sulphur dioxide per kilogram of absorbing medium; (b) removing gaseous sulphur dioxide from the spent absorbing medium at a temperature and pressure sufficient to remove at least about 70 mole percent of the sulphur dioxide absorbed in the contacting step, in the spent absorbing medium to provide a regenerated medium containing absorbent having at least one nitrogen as an amine salt and at least one free sorbing nitrogen; and (c) recovering the gaseous sulphur dioxide; and (d) recycling the regenerated absorbing medium to the contacting step.

Claim 24

The process of claim 21 in which the pH of the regenerated absorbing medium is between about 6.5 to 7.5.

Claim 25

The process of claim 21 in which the pKa of the sorbing nitrogen is between about 5 and 7.3.

Claim 26

The process of claim 21 in which the contacting temperature is about 10° to 50° C.

Claim 27

The process of claim 21 in which the molecular weight of the amine salt absorbent when in the free amine form is less than about 300.

Claim 28

The process of claim 21 in which the contacting is effected with the absorbing medium in finely divided droplets.

Claim 29

The process of claim 21 in which the amine groups of the amine salt absorbent are secondary or tertiary amines.

Claim 30

The process of claim 27 in which at least one amine group of the amine salt absorbent contains at least one hydroxyalkyl substituent.

Claim 31

The process of claim 21 in which the pH of the spent absorbing medium is about 4 to 6.

Claim 32

The process of claim 21 in which anion of the amine salt absorbent comprises at least one of sulphate, chloride, thiosulfate, dithionate, trithionate, and pyrosulphite.