HEAT-INTEGRATED TRANSFORMATIVE CARBON DIOXIDE CAPTURE PROCESS

Abstract

An apparatus includes an absorber having a first packing section, a second packing section and a third packing section. The first packing segment includes a first structured packing, having a first specific surface area SA1, the second packing segment includes a second structured packing, having a second specific surface area SA2, and the third packing segment includes a third structured packing, having a third specific surface area SA3 where SA1<SA2<SA3. The structured packing in the various packing segment may be periodically interrupted with one or more layers of random packing.

Claims

1. A method of capturing carbon dioxide from an acid gas stream, comprising: providing an absorber tower with a first packing segment, having a first specific surface area SA1, a second packing segment, having a second specific surface area SA2, and a third packing segment, having a third specific surface area SA3, where SA1<SA2<SA3 to control local liquid-to-gas mass ratios, CO.sub.2 absorption rates and temperatures in order to reduce a packing height required to achieve a desirable carbon loading; subjecting the acid gas stream to a countercurrent flow of a carbon dioxide lean CO.sub.2 absorbent in the absorber tower to produce a carbon dioxide rich CO.sub.2 absorbent; separately discharging treated acid gas and the carbon dioxide rich CO.sub.2 absorbent from the absorber tower; and recovering the carbon dioxide from the carbon dioxide rich CO.sub.2 absorbent in a stripper and returning the carbon dioxide lean CO.sub.2 absorbent to the absorber tower.

2. The method of claim 1, including positioning the second packing segment between the first packing segment and the third packing segment.

3. The method of claim 2, further including separating the first and second structured packing segments with a first layer of a higher pressure drop random packing and the second and third structured packing segments with a second layer of higher pressure drop random packing whereby the first and second layers of the higher pressure drop random packing act to redistribute the carbon dioxide lean and the carbon dioxide rich CO.sub.2 absorbent in the absorber tower.

4. The method of claim 1, further including controlling an exothermic CO.sub.2 absorption reaction rate by controlling a local liquid-to-gas mass ratio inside the absorber tower.

5. A method of capturing carbon dioxide from an acid gas stream, comprising: providing an absorber tower with a plurality of packing segments having different specific surface areas at different heights in the absorber tower to control local liquid-to-gas mass ratios, CO.sub.2 absorption rates and temperatures in order to reduce a packing height required to achieve a desirable carbon loading; subjecting the acid gas stream to a countercurrent flow of a carbon dioxide lean CO.sub.2 absorbent in the absorber tower to produce a carbon dioxide rich CO.sub.2 absorbent; separately discharging treated acid gas and the carbon dioxide rich CO.sub.2 absorbent from the absorber tower; and recovering the carbon dioxide from the carbon dioxide rich CO.sub.2 absorbent in a stripper and returning the carbon dioxide lean CO.sub.2 absorbent to the absorber tower.

6. The method of claim 5, further including separating the plurality of structured packing segments with a higher pressure drop random packing which acts to redistribute the carbon dioxide lean and the carbon dioxide rich CO.sub.2 absorbent in the absorber tower.

Description

BRIEF DESCRIPTION OF THE DRAWING FIGURES

[0030] The accompanying drawing figures incorporated herein and forming a part of the specification, illustrate several aspects of the apparatus and method and together with the description serve to explain certain principles thereof.

[0031] FIG. 1 is a schematic illustration of one possible embodiment of the new and improved apparatus including an absorber, a stripper and a secondary stripper.

[0032] FIG. 2 is a detailed schematic view of the absorber illustrating the new and improved discretized packing arrangement that improves the efficiency of the carbon dioxide capture process.

[0033] FIG. 3 is a detailed schematic view of the stripper illustrating the new and improved discretized packing arrangement that improves the efficiency of the carbon dioxide stripping process.

[0034] FIG. 4 is a detailed schematic view of the secondary stripper illustrating the new and improved discretized packing arrangement that improves the efficiency of the carbon dioxide stripping process.

[0035] FIG. 5 is a graph of packing height vs. temperature that illustrates how the discretized packing arrangement described in this document modifies the temperature profile of the absorber column resulting in a 5-11% increase in rich loading, depending upon solvent lean loading.

[0036] FIG. 6 is a graph illustrating a small plot CO.sub.2 absorber temperature profile where O is the top and 14 is the bottom of the absorber tower. A 100% draw-off intercooler is at 9 m. The temperature bulge of more than 20° C. occurs in the top third of the tower rendering the heights from 6 to 9 m ineffective.

[0037] FIG. 7 is a plot illustrating that a higher temperature absorption rate is attainable with an ideal absorber temperature profile and a lower CO.sub.2 absorption rate is attainable with a typical temperature profile, leading to a taller tower requirement to achieve the same CO.sub.2 capture efficiency.

[0038] Reference will now be made in detail to the illustrated embodiments of the apparatus, examples of which are illustrated in the accompanying drawing figures.

DETAILED DESCRIPTION

[0039] Reference is now made to FIG. 1 generally illustrating at a high level the new and improved apparatus 10. Flue gas from a coal fired power plant (not shown) is delivered to a direct contact cooler 12 (note action arrow A) where the flue gas is contacted with water being circulated by a pump 14. That water is first cooled by heat exchange with a cooling water supply at heat exchanger 16. As a result, the temperature of the flue gas is reduced to 37.78-48.89° C. Further, water vapor is removed from the flue gas. This minimizes the chemicals used in the downstream pretreatment column 18.

[0040] Next, a blower 20 delivers the flue gas to the pretreatment column 18 where the flue gas is treated with an aqueous solution of soda ash (Na.sub.2CO.sub.3) or sodium hydroxide (NaOH) to remove sulfur dioxide (SO.sub.2). More particularly, the pump 22 circulates the caustic solution through a second cooling water supply heat exchanger 24 if needed and the pretreatment column 18. The removal of the sulfur dioxide serves to minimize thermal stable salt formation and degradation of the solvent in the CO.sub.2 capture block.

[0041] The flue gas is then passed through a membrane 26, of a type known in the art, to split the flue gas into two streams. Such a membrane is more fully described in U.S. Pat. No. 9,409,120 (owned by the assignee of the present invention), the full disclosure of which is incorporated herein by reference.

[0042] The first stream 28 is a CO.sub.2-enriched permeate stream comprising approximately 24% of the total flue gas and containing approximately 25% CO.sub.2 after removal of water vapor at 40° C. The second stream 30 is a CO.sub.2-lean reject stream (approximately 76% of the total flue gas flow rate) containing approximately 10% CO.sub.2 after water removal at 40° C.

[0043] More particularly, the two streams 28 and 30 are both delivered to the absorber tower or absorber 32. There the flue gas is subjected to a concurrent stream of an CO.sub.2 absorbent or amine solvent of a type known in the art to be suitable for CO.sub.2 capture. Such a CO.sub.2 absorbent includes primary, secondary and tertiary amines including, for example, 1-amino-2-propanol (A2P), 2-amino-2-methyl-1-propanol (AMP), piperidine (PZ), methyldiethanolamine (MDEA) and other compounds, including, for example, anti-oxidant.

[0044] Following CO.sub.2 removal, the treated flue gas is delivered from the top of the absorber 32 to the solvent recovery column 34 where CO.sub.2 absorbent entrained in the treated flue gas is recovered using a countercurrent flow of wash water and an amine nucleation agent circulated through a separator element 36, such as a screen or filter, by a pump 38. As shown, the wash water, amine nucleation agent and entrained amine solvent may be cooled by a cooling water supply in the heat exchanger 40. Wash water, including entrained amine solvent separated from the amine nucleation agent by the separator element 36 is returned to the absorber 32 by means of a return circuit not shown.

[0045] The solvent recovery column 34 and the method for recovering the amine solvent from the treated flue gas are more fully described in U.S. patent application Ser. No. 16/460,229 entitled APPARATUS AND METHOD FOR RECOVERING AN AMINE SOLVENT FROM AN ACID GAS STREAM and filed on Jul. 2, 2019, the full disclosure of which is incorporated herein by reference. As disclosed therein, the amine nucleation agent may comprise an activated carbon having a density of 997 kg/m.sup.3+/−600 kg/m.sup.3 and a diameter less than 1.0 millimeter. The treated flue gas may then be discharged.

[0046] The now carbon dioxide rich CO.sub.2-absorbent or amine solvent is discharged from the bottom of the absorber 32 and directed by the pump 42 through the heat exchanger 44 to the stripper 46 where carbon dioxide is stripped from the carbon rich CO.sub.2-absorbent or amine solvent. The carbon dioxide exits at the top of the stripper 46 and is routed through a primary heat recovery exchanger 48 before exiting as a CO.sub.2 product stream that may be stored or undergo other chemical processing (not shown). The reboiler 52 functions to recycle carbon dioxide rich CO.sub.2-absorbent or amine solvent through the stripper 46 to ensure more efficient processing.

[0047] Carbon dioxide semi-lean CO.sub.2-absorbent or amine solvent exits the bottom of the stripper 46 and is transferred by pump 54 through the heat exchanger 56 to the secondary stripper 50 where even more carbon dioxide is stripped from the carbon dioxide semi-lean CO.sub.2-absorbent or amine solvent. This additional carbon dioxide exits the top of the secondary stripper 50 and is routed through the secondary heat exchanger 44 before being recycled to the power plant boiler. The now carbon dioxide lean CO.sub.2-absorbent or amine solvent exits the bottom of the secondary stripper 50 and is returned by the pump 58 through the lean heat recovery exchanger and the solvent polishing exchanger 62 to the absorber 32 where it is used to capture carbon dioxide from the flue gas as previously described. The water evaporator 61 supplies makeup water to amine loop from energy recovered from the heat exchangers 60 and 48 by operation of the pump 63.

[0048] Reference is now made to FIG. 2 which illustrates the absorber 32 in detail. As illustrated, the absorber 32 includes a first or uppermost packing segment 64, a second or intermediate packing segment 66 and a third or lowermost packing segment 68. The first packing segment 64 includes a first structured packing 65 of high capacity and low efficiency having a first specific surface area SA1. The second packing segment 66 includes a second structured packing 67 of high efficiency and high capacity having a second specific surface area SA2. The third packing segment 68 includes a third structured packing 69 of low capacity and high efficiency having a third specific surface area SA3. SA1 is less than SA2 and SA2 is less than SA3.

[0049] In one possible embodiment of the apparatus 10, the first structured packing has a first specific surface area SA1 of less than 34 ft.sup.2/ft.sup.3, the second structured packing has a second specific surface area SA2 of between 34 ft.sup.2/ft.sup.3 and 129 ft.sup.2/ft.sup.3 and the third structured packing has a third specific surface area SA3 of above 152 ft.sup.2/ft.sup.3.

[0050] In the illustrated embodiment, a first in-situ “random packing” liquid/gas distributor 70 is provided between the first packing segment 64 and the second packing segment 66. A second in-situ “random packing” liquid/gas distributor 72 is provided at an intermediate point of the second packing segment 66. Two additional in-situ “random packing” liquid/gas distributors 74, 76 are provided at spaced points in the third packing segment 68. Each of the in-situ liquid/gas distributors 70, 72, 74, 76 may comprise a layer of random packing. That random packing is characterized by a first pressure drop per height PD1 that is greater than any pressure drop per height PD2 of the first structured packing 65 in the first packing segment 64, the second structured packing 67 in the second packing segment 66 and the third structured packing 69 in the third segment 68.

[0051] In one possible embodiment of the apparatus 10, the first or upper segment 64 comprises the top 20-30% of the total height of the top packing 64, 66 and 68. The second segment 66 comprises the middle 30-50% of the total height of the packing 64, 66 and 68. The third segment 68 comprises the bottom 30-40% of the total height of the packing 64, 66 and 68.

[0052] In one possible embodiment of the apparatus 10, every 60.96 to 243.84 cm of structured packing is interrupted by 7.62-15.24 cm of in-situ liquid/gas distributor in the form of random packing. This includes the first structured packing 65 used to form the first packing segment 64, the second structured packing 67 used to form the second packing segment 66 and/or the third structured packing 69 used to form the third packing segment 68.

[0053] It has been found that due to the low CO.sub.2 absorption driving force in utility flue gas and the highly viscous nature of second generation advanced amine solvents, the low pressure drop structured packing used in the packing sections 64, 66 and 68 suffers from a lack of macro-mixing/turbulence between the bulk solvent and the gas-liquid interface. This results in localized channel flow and significantly reduces column effectiveness. The application of short sections 7.62-15.24 cm of high pressure drop random packing in the form of in-situ liquid/gas distributors 70, 72, 74 and 76 re-adjusts the pressure and redistributes the liquid within the structured packing 65, 67, 69. As a result, the efficiency of the absorber 32 is significantly enhanced.

[0054] In one possible embodiment of the absorber 32, the first packing segment 64 includes multiple layers of the first structured packing 65 separated by in-situ gas/liquid distributor of random packing. The multiple layers of the first structured packing 65 may have a first thickness T1 of 121.92-182.88 cm and the layer of random packing may have a second thickness T2 of 7.62-15.24 cm. The second packing segment 66 may include multiple layers of the second structured packing 67 having a first thickness T1 of 121.92-182.88 cm separated by the layer of random packing 72 having a thickness T2 of 7.62-15.24 cm. The third packing segment 68 may have multiple layers of the third structured packing 69 having a thickness T1 of 121.92-182.88 cm separated by the layer of random packing 74 or 76 having a thickness T2 of 7.62-15.24 cm.

[0055] Reference is now made to FIG. 3 which schematically illustrates the stripper 46 in detail. In the illustrated embodiment, the stripper 46 includes an upper packing section 78 and a lower packing section 80. The upper packing section 78 includes the third structured packing 69 having the third specific surface area SA3 while the lower packing section 80 includes the first or the second structured packing 65 having the first specific surface area SA1 and second specific surface area SA2. Both the upper packing section 78 and the lower packing section 80 may include multiple layers of the structured packing 69, 65 separated by in-situ liquid/gas distributors 82, 84 in the form of a layer of random packing of the type previously described. Every 60.96 to 243.84 cm of structured packing 69, 65 may be interrupted by 7.62-15.24 cm of in-situ liquid/gas distributor 82, 84 in the form of random packing. Once again, the alternating layered arrangement of structured packing and random packing re-adjusts the pressure and redistributes the liquid within a section 78, 80 of the structured packing thereby increasing stripper efficiency.

[0056] Reference is now made to FIG. 4 which schematically illustrates the secondary stripper 50 in detail. In the illustrated embodiment, the secondary stripper 50 includes a top packing section 86 and a bottom packing section 88. The top packing section 86 includes the first structured packing 65 having the first specific surface area SA1 while the lower packing section 80 includes a fourth structured packing 89 having fourth specific surface area SA4 where SA4=SA2 or SA3. Both the top packing section 86 and the lower packing section 88 may include multiple layers of the structured packing 65, 89 separated by in-situ liquid/gas distributors 90, 92 in the form of a layer of random packing of the type previously described. Every 60.96 to 243.84 cm of structured packing 65, 89 may be interrupted by 7.62-15.24 cm of liquid/gas distributor 90, 92 in the form of random packing. Once again, the alternating layered arrangement of structured packing and random packing re-adjusts the pressure and redistributes the liquid within a section 86, 88 of the structured packing thereby increasing secondary stripper efficiency.

[0057] The apparatus 10 described herein is useful in a method of capturing carbon dioxide from an acid gas stream. That method includes a number of step including providing an absorber tower with a plurality of packing segments having different specific surface areas at different heights in the absorber tower to control local liquid-to-gas mass ratios, CO.sub.2 absorption rates and temperatures in order to reduce a packing height required to achieve a desirable carbon loading. The method also includes the step of subjecting the acid gas stream to a countercurrent flow of a carbon dioxide lean CO.sub.2 absorbent in the absorber tower to produce a carbon dioxide rich CO.sub.2 absorbent. Still further, the method includes the steps of separately discharging treated acid gas and the carbon dioxide rich CO.sub.2 absorbent from the absorber tower and recovering the carbon dioxide from the carbon dioxide rich CO.sub.2 absorbent in a stripper and returning the carbon dioxide lean CO.sub.2 absorbent to the absorber tower. The method also includes the step of separating the plurality of structured packing segments with a higher pressure drop random packing which acts to redistribute the carbon dioxide lean and the carbon dioxide rich CO.sub.2 absorbent in the absorber tower.

[0058] Controlling the exothermic CO.sub.2 absorption reaction rate is accomplished by controlling the local liquid-to-gas mass ratio inside the absorber tower. The reaction is strongest at the top, after the lean solvent enters the tower. As the reaction proceeds, the temperature increases. As the temperature increases the driving force for CO.sub.2 capture decreases and the reaction slows. Also, as the reaction proceeds the amount of gas (G) decreases, as CO.sub.2 is transferred from the gas phase to the liquid phase. Typically, a temperature bulge occurs in the top section of the absorber tower and the tower height below this bulge is relatively ineffective, as illustrated in FIG. 6. The effect of the temperature bulge on the CO.sub.2 absorption rate is illustrated in FIG. 7.

[0059] Changing the type of structured packing allows for different liquid-gas interface areas to be applied at different heights of the absorber tower. This allows the temperature and therefore the reaction rate to be controlled, which, in turn, allows advantage to be taken of the entire tower height, minimizing the ineffective zone below the temperature bulge. The magnitude of the bulge can be reduced and the temperature profile can be optimized in the CO.sub.2 absorber tower, as illustrated in FIG. 5, which is a graph of packing height versus temperature. This graph illustrates how the discretized packing arrangement, including the alternating layers of structured packing and random packing as described above, modify the temperature profile of the absorber column resulting in a 5-11% increase in CO.sub.2 rich loading, depending upon solvent lean loading This maximizes the effectiveness of the CO.sub.2 absorber tower and allows for a smaller tower to be applied, saving both capital and operating cost.

[0060] Traditionally, liquid and gas is redistributed inside an absorber tower with specialized internals every 5-10 ft of structure packing height. Significantly, these specialized internals require several feet of additional column height greatly increasing the overall height of the absorber tower. Instead, we use a few inches of random packing applied between sections of structured packing to cause short sections of higher gas pressure drop that work to redistribute the liquid and gas. Keeping the CO.sub.2 absorber tower or column short saves both capital and operating costs.

[0061] The foregoing has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the embodiments to the precise form disclosed. Obvious modifications and variations are possible in light of the above teachings. All such modifications and variations are within the scope of the appended claims when interpreted in accordance with the breadth to which they are fairly, legally and equitably entitled.