Systems and methods for removing acid gases from post-combustion process streams

Abstract

1. A method for removing acid gases from a post-combustion process stream, comprising: receiving the post-combustion process stream into hollow fibers of at least one MBC cell, each hollow fiber being gas permeable and liquid impermeable; passing a lean solvent in contact with the outer surfaces of the hollow fibers; exchanging the acid gases through the hollow fibers into the solvent; discharging an acid gas-lean stream; and discharging an acid gas-rich solvent.

Description

[Technical Field]

[0001] The present invention relates to a system and method for removing acid gases, such as CO2 and H2S, from a post combustion process stream. [Background technology]

[0002] For clarity, references to CO2 would also apply to H2S and other acid gases, unless application makes such an extension impracticable.

[0003] Process streams, such as those relating to natural gas, can be at high pressures, moderate temperatures, and high driving forces, allowing for a variety of methods for managing acid gases within the process stream.

[0004] However, post-combustion (flue gas) applications, which still require CO2 removal but at atmospheric pressure, high temperatures, and low driving forces, make traditional means insufficient or unable to achieve the desired results, especially at relatively low pressures. Syngas applications involving both CO2 and H2S can be as low as post-combustion, but are generally between the typical operating pressures of post-combustion and natural gas applications. Summary of the Invention

[0005] In a first aspect, the present invention provides a method for removing acid gases from a post-combustion process stream, the method comprising: receiving the post-combustion process stream into hollow fibers of at least one MBC cell, each hollow fiber being gas permeable and liquid impermeable; passing a lean solvent in contact with the outer surfaces of the hollow fibers; exchanging the acid gases into the solvent through the hollow fibers; venting an acid gas-lean stream; and exiting an acid gas-rich solvent.

[0006] In a second aspect, the present invention provides a gas exchange system comprising: at least one membrane contactor cell having holes with gas-permeable, liquid-impermeable hollow fibers disposed therein; each hollow fiber having a membrane inlet disposed to receive gas from an inlet chamber and a membrane outlet for releasing the gas; and a membrane contactor cell configured to allow lean solvent to flow through the holes while in contact with the outer surfaces of the hollow fibers to allow gas exchange through the gas-permeable, liquid-impermeable membrane; and a pressure differential control system configured to monitor the pressure of the post-combustion stream and the lean solvent; and the pressure differential control system configured to control the solvent and process streams to maintain a pressure differential, thereby increasing the gas pressure above the lean solvent pressure.

[0007] The present invention includes a system and method for removing acid gases, such as CO2 and H2S, from post-combustion process streams.

[0008] By combining a membrane and a solvent, a membrane contactor system can provide a suitable method for carrying out gas-liquid absorption of post-combustion gases. The microporous membrane acts as a nonselective phase barrier, allowing the liquid and gas phases to contact each other but not dispersing one phase into the other. This barrier prevents flooding or foaming problems from occurring, thereby simplifying MBC operation. Packaging within hollow fiber membrane (HFM) modules provides a higher mass transfer area compared to conventional packed columns, giving MBCs a high enhancement potential.

[0009] It will be convenient to further describe the invention in connection with the accompanying drawings which illustrate possible configurations of the invention. Other configurations of the invention are possible, and consequently the details of the accompanying drawings should not be understood to supersede the generality of the preceding description of the invention. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a process flow diagram according to one embodiment of the present invention. [Figure 2] FIG. 2 is a process flow diagram according to a further embodiment of the present invention. [Figure 3] 3A-3C show various views of a membrane contactor cell according to a further embodiment of the present invention. [Figure 4] 4A-4D show various views of a regeneration module according to one embodiment of the present invention. [Figure 5] 5A and 5B are various views of enrichment zones for modules according to further embodiments of the present invention. [Figure 6] 6A and 6B are various views of a membrane contactor cell according to a further embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0011] Generally, the present invention includes a system and method for removing acid gases, such as CO2 and H2S, from a post-combustion process stream. The system may include at least one membrane contactor cell that receives an inflow of flue gas characterized by low pressure and high temperature. The membrane contactor cell includes a gas-permeable, liquid-impermeable hollow membrane. By passing lean solvent through the holes in the cell so that it is in close proximity to the membrane's outer surface, more efficient exchange of gases will occur. Gases are then released from the membrane contactor cell, with the resulting enriched solvent flowing out of the cell, which may be used for regeneration.

[0012] 1 shows one embodiment of a process stream 5 having an array 15 of membrane contactor cells 10. Flue gas 25 passes through a quench tower 35 and a filter coalescer 30 and is then subsequently fed to the membrane contactor cell (MBC) array 15, as well as a lean liquid solvent.

[0013] The flue gas or post-combustion gas stream 14 is fed to a product gas knockout (KO) drum 40 to remove any entrained liquids from the treated gas, from which the flue gas or post-combustion gas is released 45. The now CO2-rich solvent is passed through low temperature 50 and high temperature 55 cross exchanges, which extract CO2 (and H2S, if the original gas feed was syngas) 60, and subsequently released from the acid gas KO drum 70. The remaining solvent is heated 75, regenerated 80, and recycled 85, 95. The acid gas KO drum is configured to extract a final residual solvent 90, which is fed into a regeneration tower 80.

[0014] Thus, the MBC process for treating post-combustion gas allows for a low-pressure, high-temperature inlet to separate the CO2 from the original gas feed. When accompanied by a lean solvent feed and regeneration plant, the efficiency of CO2 extraction is further enhanced.

[0015] A further embodiment of the present invention is shown in Figure 2. The process flow diagram of Figure 2 separates the separate processes of CO2 absorption 100 and solvent regeneration 105.

[0016] In connection with the absorption process, syngas or post-combustion gas 110 having a combination of CO and HS is directed through valve 115 into MBC array 120. Array 120 receives lean solvent 125, which absorbs the CO, leaving a syngas (SG) rich gas stream that is fed to a further valve 130 and subsequently discharged 140. The now CO rich solvent stream exits array 120 and is fed into rich solution flash drum 160. Residual SG gas is extracted and discharged to flash gas system 165.

[0017] To maintain the appropriate concentration of solvent in the flash drum 160, a portion of the lean solvent stream is also fed into the flash drum 160 to remove flash gas, and the solvent passes to the lean-to-rich exchanger 170. The lean-to-rich exchanger 170 also receives a lean solvent stream from a reflex drum, which receives the lean solvent from an MBC array 190 used to extract CO from the rich solvent stream received from the flash drum 160. Gas removed from the rich solvent stream passes through a condenser 200 and enters a flash gas device 210 via a KO drum 205. The residual solvent then returns 220 to the reflex drum for further exchange through the lean-to-rich exchanger 170. The lean solvent stream then returns to the MBC array 120 via an amine tank 150, which further contains a solvent top-up 145.

[0018] Unique to both MBC arrays 120, 190 is the addition of a pressure differential device 135, 215.

[0019] As will be explained in connection with Figures 3A-3C, maintaining a pressure differential between the incoming flue gas 110 and the liquid solvent 125 provides significant benefits with respect to the efficiency of the process.

[0020] 3A-3C show cross-sectional views of the membrane contactor cell 225. FIG.

[0021] FIG. 3A shows the gas / liquid interface 235 in a situation where the pressure difference between the gas 230 and the liquid 245 keeps the liquid outside the pores 240, thus preventing the gas 230 from "wetting" as it passes through the MBC cell.

[0022] 3B and 3C, if the pressure differential is allowed to decrease, the gas / liquid interfaces 250, 255 will invade the pores, wetting the gas and limiting or preventing the efficient transfer of gas into the solvent achieved by the pores. Thus, if the pores 240 become flooded with solvent, the process will stall and the cell may be irreparably damaged.

[0023] In one embodiment, the pressure control can maintain the pressure difference between the gas and liquid MBC at 0.3 bar. In this example, 0.3 bar can be sufficient to prevent the flooding shown in Figures 3B and 3C, and thus provide the condition shown in Figure 3A.

[0024] For this example, the set points for the gas and solvent (liquid) may be 53.8 barg and 54.1 barg, respectively. The pressure differential control may be set to maintain a pressure differential between the gas and liquid of 0.3 barg. By having this cascade SP controller, the liquid pressure will always track the actual gas pressure at the MBC, ensuring reliable operation and preventing membrane wetting in the event of a high pressure excursion or upset.

[0025] Table 1 shows that a series of CO2 experiments were performed using MBC and the CO2 removal efficiency was >90%.

[0026] [Table 1]

[0027] Due to the difference in gas composition and removal efficiency (flue gas: CO2 removal; syngas: CO2 + H2S removal), different solvents may be used for flue gas and syngas applications. Although the process is complete, it still has the disadvantages of occupying a large volume, being high in equipment height, being energy intensive, and some operational problems, such as flooding, channeling, entrainment, and foaming.

[0028] One of the reasons why post-combustion CO2 removal is energy intensive is the low driving force and therefore atmospheric pressure of the flue gas. For syngas, the pressure is low to moderate (up to 30 bar) and the strict H2S requirement (<5 ppm) protects catalysts in downstream processes.

[0029] The present invention provides process intensification because it has 30 times the surface area per unit volume of conventional packaging columns, which may allow for reductions in size, energy consumption, and cost. The present invention may offer advantages over conventional absorption processes, including higher packing density, independent control of gas and absorbent flow, and a compact, modular design that provides flexibility for scaling up or down.

[0030] In one embodiment, braided PTFE yarns may be used in combination with alkanolamines to improve surface area per unit volume for acid gas removal. Additionally, multi-cartridge MBC designs may have a central tube plate, or baffle plate, and pressure control.

[0031] 4A-4D and 5A and 5B show a membrane contactor module 305 according to one embodiment of the present invention.

[0032] 4A shows a front view of a module 305 having a housing 310 with end caps 315, 320. End caps 315, 320 each include a gas inlet 325 and a gas outlet 330. Additionally, housing 310 includes an inlet 335 for receiving a liquid solvent and an outlet 330 for exiting the liquid solvent.

[0033] 4B and 4C show multiple cartridges 355 grouped in parallel and held in place by support plates 367, 375. Inlet end cap 315 and inlet support plate 367 define an inlet chamber into which gas is injected through inlet 325. Each cartridge 355 includes an open end 357 that allows flue gas to enter the cartridge 355 from the inlet chamber, specifically through hollow longitudinal membrane 377. Each cartridge may have one or more membranes located therein, depending on the required flow rate and the optimal size of the membrane.

[0034] The cartridge 355 further includes interstitial spaces 359 within the bore of the cartridge for receiving solvent, as will be discussed below. The entire cartridge 355 is then peripherally sealed by a casing 353. Gas is allowed to be released 351 into an outlet chamber defined by an outlet support plate 375 and an outlet end cap 320, which supplies the gas to an outlet 330.

[0035] Solvent enters housing 310 through inlet 335, which allows the liquid solvent to flow around cartridge 355 within interstitial space 380. However, cartridge housing 353 prevents the solvent within interstitial space 380 from directly contacting membrane 377. In accordance with the present invention, solvent 345 entering housing 310 flows through cartridge 355 within interstitial space 359 and is therefore directed to flow into entry opening 365 to contact membrane 377 before exiting cartridge 355 through entry opening 370.

[0036] This arrangement provides a solvent flow path that places the solvent in close proximity to the membrane, thus solving the problem of having sufficient flow for efficient gas transfer.

[0037] In a further embodiment, the present invention provides a baffle plate 360 ​​that defines an enrichment zone.

[0038] 5A and 5B, the baffle plate 360 ​​defines a concentration zone 385 that separates the interstitial spaces 390 near the inlets, ensuring that all solvent flows 395 into the entry openings 365 without leaking directly out the outlets 340. The use of the baffle plate 360 ​​thus defines a concentration zone 385 separate from the outlets 340, ensuring a solvent flow path close to the membranes, and therefore full utilization of each membrane in the cartridge group. Once the solvent has flowed the entire length of the cartridge 355, it exits the entry openings 370 and flows 400 into the interstitial spaces 395 close to the outlets 340.

[0039] 4B shows the location of the baffle plate 360 ​​as being approximately two-thirds the length of the module 305, it will be appreciated that in practice this location will be a function of the flow rate 345 into the module 305, the size of the openings 365, 370, and the desired gas flow rate during gas transfer within the cartridge. Thus, the size of the enrichment zone may vary from application to application and will depend on a variety of design parameters based on permeability, flow rate, etc.

[0040] 6A and 6B show a further improvement in the MBC cell. Within the cell is a cartridge 410 having a plurality of individual hollow fibers 420, 415, which may be PTFE fibers. In this embodiment, rather than maintaining the hollow fibers in a linear arrangement, the hollow fibers are braided 425, thus providing a larger surface area for gas / liquid transfer required for the MBC cell. The braid type may take several different configurations, all of which are within the scope of the present invention for braided hollow fibers.

Claims

1. 1. A method for removing acid gases from a post-combustion process stream, comprising: receiving the post-combustion process stream into hollow fibers of at least one MBC cell, each hollow fiber being gas permeable and liquid impermeable; passing a lean solvent through the hollow fibers while contacting the outer surfaces of the hollow fibers; exchanging the acid gas through the hollow fibers into the solvent; discharging the acid gas lean stream; emitting an acid gas-rich solvent; A method comprising:

2. monitoring the pressure of the post-combustion process stream; monitoring the pressure of the lean solvent; controlling said solvent and process streams, thereby; maintaining a pressure differential whereby said gas pressure is greater than said solvent pressure; The method of claim 1 further comprising:

3. 3. The method of claim 2, wherein the pressure differential is configured to prevent the solvent from entering the pores of the hollow fibers.

4. regenerating the acid gas rich solvent, thereby; generating a lean solvent stream; combining the regenerated lean solvent stream with the lean solvent stream; The method of any one of claims 1 to 3, further comprising:

5. The method according to any one of claims 1 to 4, wherein the hollow fibers are braided.

6. 1. A gas exchange system, comprising: at least one membrane contactor cell; the at least one membrane contactor cell having bores with gas permeable, liquid impermeable hollow fibers disposed therein; each hollow fiber having a membrane inlet positioned to receive gas from the inlet chamber and a membrane outlet for exiting said gas; a membrane contactor cell, the membrane contactor cell being positioned to allow a lean solvent to flow through the membrane in contact with the outer surfaces of the hollow fibers, the holes allowing gas exchange through the gas-permeable, liquid-impermeable membrane; a pressure differential control system configured to monitor the pressure of the post-combustion process stream and the lean solvent; a pressure differential control system configured to control the solvent and process flows and maintain a pressure differential, thereby causing the gas pressure to be greater than the lean solvent pressure; Including, the system.

7. 7. The system of claim 6, wherein the pressure differential is configured to prevent the solvent from entering the pores of the hollow fibers.

8. 8. The system of claim 6 or 7, wherein the hollow fibers are braided.

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