Process for capturing acid gases

Abstract

An absorption column 1 for separating CO.sub.2 and a second acid gas from a gas stream, the column comprising a first and second section (4, 5) for the absorption of CO.sub.2 and the second acid gas; a solvent inlet in the second section for the addition of liquid stream 3 including an absorbent liquid for CO.sub.2 and the second acid gas; a gas inlet (21) in the first section for the addition of a gas stream (2) containing CO.sub.2 and the second acid gas; a gas outlet (15) in the second section of the column; a first solvent outlet (22) for the removal of at least a portion of the solvent (6) from the second section of the column and a second solvent outlet (23) for solvent stream (11) from the first section of the column; and a liquid flow distributor arrangement (8) to allow a portion of the solvent to flow from the second section of the column to the first section. A method of operating the apparatus and method of solvent extraction is also disclosed.

Claims

1. A post combustion capture process for removing CO.sub.2 and at least one second acid gas from a flue gas stream including the steps of: providing a flue gas stream containing CO.sub.2 in the range of 1 to 30 vol %, and at least one second acid gas to an absorption column, the absorption column having at least separate first and second sections within said absorption column, the gas stream being provided to the first section of the column, providing a solvent comprising an absorbent liquid for CO.sub.2 and the second acid gas to the second section to flow counter current to the gas stream, said solvent successively passing through both the second section and the first section of the column, with at least a portion of said solvent being removed from the absorption column once said solvent has passed through the second section and prior to said solvent passing through the first section, passing the gas stream through the absorption column preferentially absorbing the second acid gas into said solvent in the first section of the absorption column before passing to the second section of the absorption column where CO.sub.2 is absorbed into said solvent, recovering gas depleted in CO.sub.2 and the second acid gas from the second section of the column.

2. The process of claim 1 wherein a liquid flow distributor prevents liquid from flowing directly between the second and first sections of the column while allowing gas to pass through, the solvent from the first section is removed and regenerated, and the regenerated solvent streams are returned to the second section of the absorption column.

3. The process of claim 1 wherein a liquid flow distributor between the first and second section allows liquid to flow directly from the second section to the first section and a portion of the solvent is removed from the column prior to entering the first section of the column.

4. The process of claim 1 wherein the solvent removed prior to the first section is subsequently processed to remove the CO.sub.2 before being returned to the absorption column.

5. The process of claim 1 wherein the solvent removed from the absorption column prior to the first section is subsequently processed to remove the second acid gas and then returned to the absorption column.

6. The process of claim 1 wherein at least a portion of the solvent removed from the absorption column prior to the first section is recycled to the top of the first section.

7. The process of claim 1 wherein the second acid gas is selected from the group of SO.sub.2, H.sub.2S, HF, HCl and NO.sub.2.

8. The process of claim 1 wherein the gas stream contains CO.sub.2 in the range of 3 to 30 vol %.

9. The process of claim 1 wherein the solvent stream entering the first section of the column has a CO.sub.2 content of between 0 and 200% of the saturated CO.sub.2 content.

Description

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

(1) FIG. 1 is a graph showing the impact of gas phase SO.sub.2 concentration on the CO.sub.2 absorption flux into a thin film of 30% w/w aqueous MEA at 40 C. as determined by modelling; and

(2) FIG. 2 is a graph showing the impact of gas phase SO.sub.2 concentration on the CO.sub.2 absorption flux into a thin film of 30% w/w aqueous MEA at 40 C. as determined by experiment; and

(3) FIG. 3 is an illustration of the wetted-wall contactor used to experimentally determine the CO.sub.2 and SO.sub.2 absorption flux into a falling thin liquid film of 30% w/w MEA; and

(4) FIG. 4 is a schematic diagram of an embodiment of the invention which would allow CO.sub.2 and SO.sub.2 removal from a single absorber column and single solvent stream.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(5) While the invention will be described with reference to CO.sub.2 and SO.sub.2 gases, it is intended that the invention is equally applicable to CO.sub.2 in the presence of a second acid gas where the acid formed is a stronger acid than that formed from CO.sub.2. These include SO.sub.2, H.sub.2S, NO.sub.2, HF and HCl.

(6) The present invention is a process that allows both CO.sub.2 and SO.sub.2 removal from a gas stream using a single absorber tower and single aqueous solvent. The invention utilises the differences in physical solubility, absorption rate and acidity of CO.sub.2 and SO.sub.2 to achieve this.

(7) A schematic diagram of the process is shown in FIG. 4. Flue gas (2) enters at the bottom of a packed absorber column (1). The design of the column (1) itself may be similar in design to those in existing use for gas treating processes.

(8) The absorption column (1) for separating CO.sub.2 and a second acid gas from a gas stream, the column includes a first and second section (4, 5) for the absorption of CO.sub.2 and the second acid gas and a liquid flow distributor (8). The second section (5) includes a solvent inlet (20) for the addition of liquid stream (3) including an absorbent liquid for CO.sub.2 and the second acid gas, a gas outlet (15) and a first solvent outlet for the removal of at least a portion of the solvent as stream (6) from the second section (5) of the column.

(9) First section (4) includes a gas inlet (21) for the addition of a gas stream (2) containing CO.sub.2 and the second acid gas and a second solvent outlet (23) for solvent stream (11) from the first section of the column. The liquid flow distributor arrangement (8) is provided with a liquid distributor to allow a portion of the solvent to flow from the second section of the column to the first section.

(10) An aqueous solvent, suitable for CO.sub.2 capture such as but not limited to an aqueous amine, lean in CO.sub.2 and SO.sub.2 enters at the top of the absorber column (liquid stream 3). As the gas stream moves up the column SO.sub.2 absorption occurs in the bottom first section (4). This first section (4) of column (1) may also act as a quench to cool the flue gas to a temperature suitable for CO.sub.2 capture (40 C.) from its original temperature (normally above 80 C.). This first section (4) of column (1) is exposed to a stream (7) of absorbent liquid. Stream (7) is a side stream comprising a small portion of the solvent stream (6). Solvent stream (6) is the result of contact with the gas stream in top second section (5) of the absorption column, now CO.sub.2 rich, which originally entered at the top of the absorber as solvent stream (3). Even though the solvent is CO.sub.2 rich effective SO.sub.2 removal still occurs in first section (4) due to the selectivity for SO.sub.2. Some CO.sub.2 desorption may also occur at this point increasing the CO.sub.2 content of the gas stream. The SO.sub.2 lean gas stream then moves into the mid and upper second sections (5) of the column.

(11) In mid and upper second sections (5) of the column CO.sub.2 absorption occurs as in a traditional CO.sub.2 capture process. At the interface between the second section (5) of the column and the bottom section, a flow distributor arrangement (8) is shown which allows gas to continue to rise up the column while restricting the flow of solvent down the column. The solvent progressing down the column (1) after passing through the second section (5) is the now CO.sub.2 rich and the SO.sub.2 lean solvent. This solvent at the base (9) of the second section is removed and a return stream (7) which is a small portion of stream (6) returned to the first section of the column (1).

(12) The flow distributor arrangement (8) may also take the form of a restrictor such as a sieve plate or orifice (not shown) which allows gas to pass upwardly in the column from the first section (4) to the second section (5) while allowing a small amount of solvent to continue down the column (1). The flow restrictor causes a hold up in the solvent at the base of the second section equivalent to the difference to the solvent flow through the second and first sections (5, 4) which is removed from the column 1 in a stream similar to stream 6.

(13) Solvent stream 6 then passes to a CO.sub.2 regeneration unit (10) for CO.sub.2 stripping and solvent regeneration.

(14) As mentioned above, a portion of the solvent which has passed through the second section (5) of the column passes into the bottom first section (4) of the column where SO.sub.2 removal occurs. The fraction of the remainder of the total process liquid stream 3 needed to provide bulk capture of SO.sub.2 will depend on the ratio of SO.sub.2/CO.sub.2 content in the flue gases and typically will range between 0.1% and 3% of the total process stream needed for CO.sub.2 capture. Given this much reduced flow, it may be desirable to recirculate the solvent in the first section (4) multiple times in the first section (4) of the column to provide adequate contact. Alternatively a dedicated gas/liquid contactor able to operate at high gas/liquid ratios, such as a membrane contactor, might be used.

(15) The fraction of the process stream (6) used for selective SO.sub.2 absorption might also be obtained as a product stream from a separation step, which will result in partial rejection of amines, thus preventing them from entering the bottom part of the absorber. This will avoid excessive oxidation in the bottom (SO.sub.2-removal) stage of the absorber. The SO.sub.2 rich solvent (11) from the first section (4) of the column is then removed and passed to an SO.sub.2 regeneration unit (12) for sulfur recovery and solvent regeneration. The regenerated solvent streams (13, 14) are then mixed and returned to the top of the absorber as process stream (3).

(16) In an alternative embodiment (not shown), at least a portion of the SO.sub.2 rich solvent stream (11) may be recycled and added to the top of first section (4) with solvent stream (7). The recycle of the SO.sub.2 rich solvent stream (11) potentially will provide some cost and performance benefits, by increasing the concentration of SO.sub.2 in solvent stream (11).

(17) CO.sub.2 stripping and solvent regeneration for the CO.sub.2 rich and SO.sub.2 lean solvent stream carried out in CO.sub.2 regeneration unit (10) uses a standard CO.sub.2 stripping process. A general description of a CO.sub.2 stripping process follows, however this does not preclude the use of any other CO.sub.2 stripping process. The solvent stream exiting the absorber is preheated (generally via a heat exchange with the lean CO.sub.2 solvent stream from the stripper bottom) and enters the top of a packed column. At the base of the column liquid is heated to 120-160 C. via a reboiler to generate stripping steam and heat the solvent. The solvent entering at the top of the column is contacted with and heated by the stripping steam. At this elevated temperature, the CO.sub.2 absorption process is reversed and CO.sub.2 comes out of solution. The gaseous CO.sub.2 stream passes upwards through the column and exits through the top for further purification and compression for transport. The CO.sub.2 lean solvent stream is removed from the bottom of the column, cooled via heat exchange, and returned to the absorber.

(18) Sulfur recovery and solvent regeneration of the SO.sub.2 rich stream (11) may be carried out using a sulfur recovery process suitable for use with aqueous amines. Due to oxidation of sulfite species as a result of the presence of oxygen in the flue gas both sulfites and sulfates may be present in this stream. Options include but are not limited to: metathesis via the addition of NaOH/NaCO.sub.3 or other hydroxides/carbonates to form Na.sub.2SO.sub.3(s) and Na.sub.2SO.sub.4(s) precipitates or others; ion exchange resins to separate sulfites and sulfates from other species; and membrane electrodialysis to separate sulfites and sulfates from other species. The sulfur recovery step might also beneficially be integrated with the amine reclaimer. The reclaimer might be a distillation column in which the amine is recovered as the overhead product as a result of its higher volatility. Degradation products, heat stable salts, including sulfur products will then be left in the bottom fraction of the distillation column. Alternatively, as is the case for non-volatile amines, the amine reclaimer might be based on the removal of degradation products and heat stable salts via a combination of electrodialysis, filtration, adsorption and ion-exchange. These steps can also be used to remove the sulfur products.

(19) The present invention provides an improved CO.sub.2 capture process for SO.sub.2 containing flue gas streams that does not require SO.sub.2 removal (FGD) prior to its application. It also utilises existing process technologies already in use for gas cleaning applications. The ability to carry out CO.sub.2 capture in the presence of SO.sub.2 is extremely desirable from an industrial perspective as it eliminates the need to install FGD equipment (where it is not already installed) to allow a CO.sub.2 capture process to be used. In situations where FGD is installed it avoids the installation of additional capacity in existing columns or the use of an additional clean-up step using an additional column. This has significant benefits in terms of reducing cost and the overall technical complexity of applying CCS to flue gas streams that contain SO.sub.2.

(20) It will be understood that the invention disclosed and defined in this specification extends to all alternative combinations of two or more of the individual features mentioned or evident from the text or drawings. All of these different combinations constitute various alternative aspects of the invention.