# Patent

## Title
Selective caustic scrubbing using a driver gas

## Abstract

Systems and methods are described for selectively removing an acid gas from a feed stream to reduce co-absorption of CO2. The system can include an absorber configured to contact at least a portion of the feed stream with a caustic solution or other basic solvents to produce a clean stream substantially depleted of the acid gas. To reduce co-absorption of CO2 by the caustic solution to less than 10% of the CO2 present in the feed stream, the absorber can be further configured to receive a driver gas that is substantially non-reactive with the caustic solution.

## Background
FIELD OF THE INVENTION
The field of the invention is caustic scrubbers.
BACKGROUND
Caustic solutions provide an effective means to remove H2S from acid-gas streams, but co-absorption of CO2 is often a problem because CO2 absorbs more strongly than H2S. Absorption of CO2 wastes the caustic solution and also may cause fouling due to the precipitation of carbonate solids (e.g., sodium carbonate decahydrate).
One solution has been to use a co-current contactor with very low residence times to minimize co-absorption of CO2. Since H2S can be absorbed much more quickly than CO2 at high pH levels, selective H2S removal is accomplished by limiting the contact time between the gas and the liquid in a static mixer. See, e.g., U.S. Pat. No. 4,824,645 to Jones, et al.
Maximizing the absorption of H2S and minimizing the absorption of CO2 is typically controlled by varying the rate addition of fresh caustic, solution circulating rate and column characteristics such as packing height or number of trays. See, e.g., GasTIPS, “Simulator Provides Guidance for Increasing CO2 Slip in Gas Treating Applications”, Weiland, Ralph H., et al., volume 8, no. 1, Winter 2002: 21-28. However, the low residence times used can limit the efficiency of the scrubber.
These and all other extrinsic materials discussed herein are incorporated by reference in their entirety. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.
Thus, there is still a need for improved caustic scrubbers that reduce co-absorption of CO2 by the caustic solution without requiring low residence times.
SUMMARY OF THE INVENTION
The inventive subject matter provides apparatus, systems and methods in which one can reduce co-absorption of CO2 from a feed stream through the addition of a driver gas. Preferred systems are configured to selectively remove an acid gas from the feed stream. Using an absorber configured to contact at least a portion of the feed stream with a strong, basic absorbent to produce a clean stream substantially depleted of the acid gas.
The absorber can be further configured to receive a driver gas that is substantially non-reactive with the absorbent, such that co-absorption of CO2 by the absorbent is reduced to less than 10% of the CO2 present in the feed stream. As used herein, the term “substantially non-reactive” means that the driver gas will not react with the caustic solution to any appreciable extent. One preferred absorbent is a caustic solution, although any commercially suitable solvent could be used that is basic (e.g., an alkaline solution) and absorbs CO2 more strongly than the acid gas.
In another aspect, methods of reducing co-absorption of CO2 from the feed gas can include the step of contacting at least a portion of the feed gas with a strong, basic absorbent in an absorber to produce a clean stream substantially depleted of an acid gas. In preferred embodiments, the feed stream can include a driver gas that is (a) substantially non-reactive with the absorbent, and (b) sufficient to reduce co-absorption of CO2 by the absorbent to less than 10% of the CO2 present in the feed gas.
Unless the context dictates the contrary, all ranges set forth herein should be interpreted as being inclusive of their endpoints, and open-ended ranges should be interpreted to include commercially practical values. Similarly, all lists of values should be considered as inclusive of intermediate values unless the context indicates the contrary.
Various objects, features, aspects and advantages of the inventive subject matter will become more apparent from the following detailed description of preferred embodiments, along with the accompanying drawing figures in which like numerals represent like components.



BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a schematic of one embodiment of a system configured to selectively remove an acid gas from a feed stream.
FIG. 2 is a chart of illustrating the effect of adding a driver gas to the feed gas on the amount of CO2 captured.
FIG. 3 is a flowchart of one embodiment of a method of reducing co-absorption of CO2 from a feed gas.



DETAILED DESCRIPTION
One should appreciate that the disclosed techniques provide many advantageous technical effects including the reduction of co-absorption of CO2 from a feed gas by utilizing a driver gas. Specifically, a simple wash process using a caustic solution or other basic absorbent can be used to minimize the co-absorption of CO2 in H2S removal from acid gas.
The following discussion provides many example embodiments of the inventive subject matter. Although each embodiment represents a single combination of inventive elements, the inventive subject matter is considered to include all possible combinations of the disclosed elements. Thus if one embodiment comprises elements A, B, and C, and a second embodiment comprises elements B and D, then the inventive subject matter is also considered to include other remaining combinations of A, B, C, or D, even if not explicitly disclosed.
In FIG. 1, system 100 is shown configured to selectively remove an acid gas from a feed stream 102 to reduce co-absorption of CO2. In a preferred embodiment, the acid gas is H2S, although the feed stream could comprise other acid gases including, for example, acetic acid and butyric acid, depending upon the upstream combustion.
System 100 can include an absorber 120 configured to contact at least a portion of the feed stream 102 with a caustic solution 106 such as sodium hydroxide to produce a clean stream 122 that is substantially depleted of the acid gas. Sodium hydroxide, potassium hydroxide and other solvents that are basic and absorb CO2 more strongly than the acid gas could be used for the caustic solution. The absorber 120 can be further configured to receive a driver gas 104 that is substantially non-reactive with the caustic solution 106, such that co-absorption of CO2 by the caustic solution 106 is reduced to less than 10% of the CO2 present in the feed stream 102. It is especially preferred that the addition of the driver gas 104 is sufficient to reduce co-absorption of CO2 by the caustic solution 106 to less than 5%, and more preferably less than 3%, of the CO2 present in the feed stream 102.
Driver gases preferably include inert gases such as He and N2, for example, although CH4, air, and any commercially suitable gas could be used that is substantially non-reactive with the caustic solution. The addition of the driver gas advantageously dilutes the feed gas and reduces the driving force for absorption of CO2 relative to the acid gas. In this manner, the driver gas can increase the absorption kinetics for the acid gas relative to CO2. Thus, the co-absorption of CO2 can be reduced without the need for low residence times.
In some contemplated embodiments, the system 100 can include a mixer 110 that is fluidly coupled upstream of the absorber 120 and configured to allow for mixing of the feed stream 102 and the driver gas 104 to produce a first feed stream 108, at least a portion of which can be fed to absorber 120. Use of mixer 110 upstream of the absorber 120 adds an additional degree of control, which enables the driver gas 104 to be mixed with the feed stream 102 containing the acid gas prior to feeding the feed stream 102 to the absorber 120.
In other contemplated embodiments, the driver gas can be fed as a separate stream to the absorber 120.
At least a portion of the caustic solution 124 can be collected at a bottom portion of the absorber 120 and sent to a separator 140, which separates the caustic solution from the spent caustic fluid 142 and other fluids. The separated caustic solution 106A can then be fed to the absorber to form a recirculating loop. It is contemplated that the recycled caustic solution 106A can be condensed through heat exchange in exchanger 130 before the caustic solution 106 is fed to the absorber 120.
Additional caustic solution 106B can be combined with the recycled solution 106A as needed as a make-up fluid to replace spent caustic fluid. The amount of make-up fluid needed will likely depend upon the overall reduction of co-absorption of CO2.
In FIG. 2, a chart is shown that illustrates an example of the effect of adding a driver gas to the feed gas has on the amount of CO2 captured. In the example depicted in FIG. 2, the driver gas is N2, and the acid gas is H2S.
Without the addition of the driver gas, the H2S slip is 0.02%, while about 10% of the CO2 is captured. When N2 is added to the feed gas such that its mole fraction is raised to 50%, the fraction of CO2 captured is reduced by a factor of about two (to about 5%), while the H2S slip is still low (about 0.04%). Thus, as the mole fraction of N2 is increased in the feed gas, the amount of CO2 captured is significantly reduced while maintaining the H2S slip at low levels.
FIG. 3 illustrates one embodiment of a method 300 of reducing co-absorption of CO2 from a feed stream.
In step 310, at least a portion of the feed stream can be contacted with a caustic solution or other strong, basic absorbent that absorbs CO2 more strongly than the acid gas in an absorber to produce a clean stream substantially depleted of an acid gas. In step 314, the acid gas comprises H2S, although it is contemplated that the method 300 could be used with other acid gases depending upon the upstream combustion.
The feed stream can preferably include a driver gas in step 320, which is (a) substantially non-reactive with the caustic solution, and (b) sufficient to reduce co-absorption of CO2 by the caustic solution to less than 10% of the CO2 present in the feed stream.
The driver gas preferably is an inert gas in step 322, and more preferably at least one of He and N2 in step 323. However, it is contemplated in step 324 that non-inert gases could be used including, for example, CH4 and air, so long as the chosen driver gas is substantially non-reactive with the caustic solution and sufficient to reduce co-absorption of CO2. The specific gas chosen will likely depend upon the caustic solution and the desired reduction of CO2 co-absorption.
It is especially preferred in step 326 that the driver gas is selected such that co-absorption of CO2 by the caustic solution is reduced to less than 5%, and more preferably, less than 3% in step 328.
In step 312, the feed stream can be mixed with the driver gas in a mixer disposed upstream of the absorber to thereby produce the feed stream. Alternatively, the driver gas can be fed directly to the absorber without pre-mixing the driver gas with the feed stream.
As used herein, and unless the context dictates otherwise, the term “coupled to” is intended to include both direct coupling (in which two elements that are coupled to each other contact each other) and indirect coupling (in which at least one additional element is located between the two elements). Therefore, the terms “coupled to” and “coupled with” are used synonymously.
It should be apparent to those skilled in the art that many more modifications besides those already described are possible without departing from the inventive concepts herein. The inventive subject matter, therefore, is not to be restricted except in the scope of the appended claims. Moreover, in interpreting both the specification and the claims, all terms should be interpreted in the broadest possible manner consistent with the context. In particular, the terms “comprises” and “comprising” should be interpreted as referring to elements, components, or steps in a non-exclusive manner, indicating that the referenced elements, components, or steps may be present, or utilized, or combined with other elements, components, or steps that are not expressly referenced. Where the specification claims refers to at least one of something selected from the group consisting of A, B, C . . . and N, the text should be interpreted as requiring only one element from the group, not A plus N, or B plus N, etc.

## Claims

1. A system configured to selectively remove an acid gas from a feed stream to reduce co-absorption of CO2, comprising:
an absorber configured to contact at least a portion of the feed stream with a basic absorbent to produce a clean stream substantially depleted of the acid gas; and
wherein the absorber is further configured to receive a driver gas that is substantially non-reactive with the basic absorbent, such that co-absorption of CO2 by the basic absorbent is reduced to less than 10% of the CO2 present in the feed stream.

2. The system of claim 1, further comprising a mixer fluidly coupled upstream of the absorber and configured to allow for mixing of the feed stream and the driver gas.

3. The system of claim 1, wherein the driver gas comprises an inert gas.

4. The system of claim 3, wherein the inert gas comprises at least one of He and N2.

5. The system of claim 1, wherein the driver gas comprises at least one of CH4 and air.

6. The system of claim 1, wherein addition of the driver gas is sufficient to reduce co-absorption of CO2 by the absorbent to less than 5% of the CO2 present in the feed stream.

7. The system of claim 1, wherein addition of the driver gas is sufficient to reduce co-absorption of CO2 by the absorbent to less than 3% of the CO2 present in the feed stream.

8. The system of claim 1, wherein the acid gas comprises H2S.

9. The system of claim 1, wherein the absorbent comprises a caustic solution.

10. A method of reducing co-absorption of CO2 from a feed stream, comprising:
contacting at least a portion of the feed stream with a basic absorbent in an absorber to produce a clean stream substantially depleted of an acid gas; and
wherein the feed stream comprises a driver gas that is (a) substantially non-reactive with the basic absorbent, and (b) sufficient to reduce co-absorption of CO2 by the basic absorbent to less than 10% of the CO2 present in the feed stream.

11. The method of claim 10, further comprising the step of mixing a flue gas with the driver gas in a mixer upstream of the absorber to produce the feed stream.

12. The method of claim 10, wherein the driver gas comprises an inert gas.

13. The method of claim 12, wherein the inert gas comprises at least one of He and N2.

14. The method of claim 10, wherein the driver gas comprises at least one of CH4 and air.

15. The method of claim 10, wherein the driver gas is sufficient to reduce co-absorption of CO2 by the absorbent to less than 5%.

16. The method of claim 10, wherein the driver gas is sufficient to reduce co-absorption of CO2 by the absorbent to less than 3%.

17. The method of claim 10, wherein the acid gas comprises H2S.

18. The method of claim 10, wherein the absorbent comprises a caustic solution.