Removal of acid gases from post-combustion process streams

Abstract

A method for removing acid gases from a post-combustion process stream, the method comprising the steps of: receiving the post-combustion process stream into hollow fibers of at least one MBC unit, each hollow fiber being gas permeable, liquid impermeable; flowing a lean solvent in contact with the outer surfaces of the hollow fibers; exchanging the acid gas through the hollow fibers into the solvent; discharging the treated lean gas stream; the acidic gas-rich solvent is allowed to exit.

Description

Technical Field

[0001] The present invention relates to systems and methods for removing acid gases such as CO2 and H2S from post-combustion process streams. Background Art

[0002] For clarity, references to CO2 will also apply to H2S and other acid gases unless the application would make such an extension infeasible.

[0003] Process streams such as those for natural gas may be high pressure, medium temperature, high driving force, allowing for various methods of managing acid gases within the process stream.

[0004] However, post-combustion (flue gas) (which also requires CO2 removal, but is done at atmospheric pressure and high temperature with low driving force) makes conventional methods inefficient or unable to achieve the desired results, especially in terms of processing relatively low pressures. Syngas applications requiring both CO2 and H2S may 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 the steps of: receiving the post-combustion process stream into hollow fibers of at least one MBC unit, each hollow fiber being gas permeable and liquid impermeable; flowing a lean solvent therethrough in contact with an outer surface of the hollow fibers; exchanging the acid gases into the solvent through the hollow fibers; discharging the treated lean gas stream; and allowing the acid gas-rich solvent to exit.

[0006] In a second aspect, the present invention provides a gas exchange system comprising: at least one membrane contactor unit; the at least one membrane contactor unit having a cavity in which a gas-permeable, liquid-impermeable hollow fiber is placed or the opposite configuration in which a liquid is placed in the cavity; each hollow fiber having a membrane inlet arranged to receive a gas from an inlet chamber and a membrane outlet for discharging the gas; the cavity being arranged to allow a lean solvent to flow through in contact with the outer surface of the hollow fiber to allow gas to be exchanged through the gas-permeable, liquid-impermeable membrane, and; a pressure differential control system arranged to monitor the pressure of a post-combustion process stream and a lean solvent; wherein the pressure differential control system is configured to control the solvent and process streams and maintain a pressure differential such that the lean solvent pressure is greater than the gas pressure.

[0007] The present invention relates to systems and methods for removing acid gases such as CO2 and H2S from post-combustion process streams.

[0008] By combining membranes and solvents, membrane contactor systems can provide an advantageous approach for gas-liquid absorption of post-combustion gases. The microporous membrane acts as a non-selective phase barrier, allowing the liquid and gas phases to contact each other without dispersing one phase into the other. This barrier prevents overflow or foaming problems, simplifying MBC operation. Packing into hollow-fiber membrane (HFM) modules provides a higher mass transfer area than conventional packed columns, giving MBCs high enhancement potential.

[0009] It will be appreciated that the gas may pass through the fibres with the solvent in the shell, or vice versa. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] It will be convenient to further describe the invention with respect to the accompanying drawings which illustrate possible arrangements of the invention. Other arrangements of the invention are possible, and therefore the particularity of the accompanying drawings should not be understood as superseding the generality of the foregoing description of the invention.

[0011] Figure 1 is a process flow chart according to one embodiment of the present invention;

[0012] Figure 2 is a process flow chart according to another embodiment of the present invention;

[0013] Figures 3A to 3C are multiple cross-sectional views of fibers in a membrane contactor unit according to another embodiment of the present invention;

[0014] Figures 4A to 4D shows various views of a regeneration module according to one embodiment of the present invention;

[0015] Figure 5A and Figure 5B are multiple views of an enrichment region of a module according to another embodiment of the present invention;

[0016] Figure 6A and Figure 6B are multiple views of a membrane contactor unit according to another embodiment of the present invention, and;

[0017] Figure 7A 、 Figure 7B and Figure 7C are multiple views of a membrane contactor unit according to another embodiment of the present invention. DETAILED DESCRIPTION

[0018] Generally speaking, the present invention relates to systems and methods for removing acid gases such as CO2 and H2S from post-combustion process streams. The present invention may include at least one membrane contactor unit that receives an inflow of flue gas characterized by low pressure and high temperature. The membrane contactor unit includes a hollow membrane that is permeable to gas but impermeable to liquid, or the opposite configuration in which the liquid is placed in the cavity. It will be noted that in one embodiment, the gas can pass through the fibers, and the solvent through the shell. By passing the lean solvent through the cavity of the unit to be close to the outer surface of the membrane, more efficient gas exchange will occur. The gas is then discharged from the membrane contactor unit, at which point the rich solvent flows out of the unit, potentially for regeneration. It will be understood that although the system according to the present invention is well suited for low pressure regeneration, high pressure regeneration is also possible at pressures approaching 10 barg.

[0019] Figure 1 One embodiment of a process flow 5 is shown 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 supplied to the array of membrane contactor cells (MBCs) 15 as is the lean liquid solvent.

[0020] The flue gas or post-combustion gas stream 14 may be fed to a product gas knockout (KO) drum 40 to remove any entrained liquid from the treated gas, and the flue gas or post-combustion gas stream 14 is discharged 45 from the product gas knockout drum 40. The now CO2-rich solvent is cross-exchanged through cold 50 and hot 55, which extract the CO2 (and H2S, if the original gas feed is syngas) 60 for subsequent discharge from the acid gas KO drum 70. The remaining solvent is heated 75 and regenerated 80 for reuse 85, 95. The acid gas KO drum is arranged to extract the last of the residual solvent 90, which is fed to the regeneration column 80.

[0021] Thus, the MBC process for treating post-combustion gases allows the formation of a low-pressure, medium-temperature influent to separate CO2 from the raw gas feed. The availability of a lean solvent supply and regeneration equipment further improves the efficiency of CO2 extraction.

[0022] Figure 2 Another embodiment of the present invention is shown. Figure 2 The process flow diagram divides the different processes of CO2 absorption 100 and solvent regeneration 105.

[0023] Referring to the absorption process, syngas or post-combustion gas 110, which has a combination of CO2 and H2S, is directed through valve 115 into an MBC array 120. Array 120 receives a lean solvent 125, which absorbs the CO2, leaving behind a syngas (SG)-rich gas stream, or purified flue gas, which is fed to another valve 130 for subsequent discharge 140. The now CO2-rich solvent stream exits array 120 and is fed to a rich solution flash tank 160. The remaining SG gas is extracted and discharged to a flash gas system 165.

[0024] To maintain a suitable solvent concentration within flash tank 160, a portion of the lean solvent stream is also fed to flash tank 160, where flash gas is removed and the solvent is passed to lean-rich exchanger 170. Lean-rich exchanger 170 also receives a lean solvent stream from a reflux tank, which receives lean solvent from MBC array 190, which is used to extract CO2 from the rich solvent stream received from flash tank 160. The gas removed from the rich solvent stream passes through condenser 200 and enters flash gas device 210 via KO tank 205. The gas removed from the rich solvent can then be vented or used for CO2 utilization or CO2 storage purposes. The remaining solvent is then returned 220 to the reflux tank for exchange through lean-rich exchanger 170. The lean solvent stream is then fed back to MBC array 120 via amine tank 150, which also includes solvent makeup 145.

[0025] Both MBC arrays 120 , 190 feature the addition of a pressure differential device 135 , 215 .

[0026] If you will refer to Figures 3A to 3C As illustrated, maintaining a pressure differential between the incoming (treated or outlet) flue gas 110 and the liquid solvent 125 yields significant advantages in process efficiency.

[0027] Figures 3A to 3C A cross section of membrane contactor unit 225 is shown.

[0028] Figure 3A The gas / liquid interface 235 is shown where the pressure differential between the liquid 245 and the gas 230 keeps the liquid outside the pores 240 and thus prevents the gas 230 from being "wetted" as it passes through the MBC cell.

[0029] If the pressure differential is allowed to increase above a certain limit, e.g. Figure 3B and Figure 3C As shown in , the gas / liquid interface 250, 255 will intrude into the pores, thereby wetting the gas and limiting or preventing efficient transfer of the gas into the solvent (which is affected by the pores). Therefore, if the pores 240 are flooded with solvent, the process may stop and MBC maintenance may be required.

[0030] In one embodiment, the pressure control can maintain the pressure difference between the liquid and gas of the MBC at 0.3 bar to 0.8 bar. In this example, 0.3 bar to 0.8 bar may be sufficient to prevent Figure 3B and Figure 3C The gas bubbling and overflow shown in , and thus provide as Figure 3A conditions shown in .

[0031] For this example of a post-combustion process stream, the set points for the gas and solvent (liquid) can be 0.4 barg and 0.7 barg, respectively. The differential pressure control can be set to maintain the pressure difference between the liquid and gas at 0.3 barg. By having this cascaded SP controller, the liquid pressure will always follow the actual gas pressure at the MBC, ensuring reliable operation and preventing membrane wetting in the event of high pressure fluctuations or disturbances.

[0032] Table #1 shows that a series of CO2 experiments were performed using MBC where the CO2 removal efficiency was greater than >90%.

[0033]

[0034]

[0035] Table #1

[0036] Due to differences in gas composition and removal efficiency, different solvents can be used for flue gas and synthesis gas applications (flue gas: CO2 removal; synthesis gas: CO2 + H2S removal). The process is mature, but it still has the disadvantages of large volume occupation, high equipment height, energy intensiveness, and some operational problems such as flooding, channeling, entrainment and foaming.

[0037] One of the reasons why removing CO2 from post-combustion is energy-intensive is that the driving force is low, so the flue gas pressure is at atmospheric pressure. For syngas, the pressure is between low and medium pressure (up to 50 bar) and has strict H2S requirements (<5 ppm) to protect the catalysts in downstream processes.

[0038] The present invention provides process intensification because the surface area per unit volume can be 30 times that of conventional packed towers, thereby reducing size, energy consumption, and cost. Compared with conventional absorption processes, the present invention can provide advantages including higher packing density, independent control of gas and absorbent flows, and a compact modular structure that provides flexibility for scale-up or scale-down.

[0039] In one embodiment, woven PTFE fibers with small pore size can be used in conjunction with alkanolamines for acid gas removal to increase the surface area per unit volume. Additionally, multi-barrel MBC designs can have a central tubesheet, or baffles, and pressure control.

[0040] In an alternative embodiment, the fibers may be hydrophobic. In this embodiment, the hydrophobic fibers resist complete immersion by solvents that could damage or limit the efficiency of the fibers. Because they are hydrophobic, the pressure of the liquid may be allowed to exceed the pressure of the gas within predetermined limits controlled by the pressure differential device. For example, P 液体 Can exceed P 气体 0.3 bar gauge to avoid the intrusion of liquids that could adversely affect the efficiency and operation of the MBC. In this case, it is permissible to Figure 3B In addition, maintaining close control can also allow Figure 3C The degree of immersion shown in .

[0041] Figures 4A to 4D as well as Figure 5A and Figure 5B A membrane contactor module 305 according to one embodiment of the present invention is shown.

[0042] Figure 4A A front view of a module 305 is shown having a housing 310 with end caps 315, 320. The end caps 315, 320 respectively include a gas inlet 325 and a gas outlet 330. In addition, the housing 310 includes an inlet 335 for receiving a liquid solvent and an outlet 40 for the liquid solvent to exit.

[0043] Figure 4B and Figure 4C A plurality of cartridges 355 are shown grouped in parallel and held in place by support plates 367, 375. The inlet end cap 315 and the inlet support plate 367 define an inlet chamber into which gas is injected through the inlet 325. Each cartridge 355 includes an open end 357 that allows flue gas from the inlet chamber to enter the cartridge 355, and specifically through the hollow longitudinal membrane 377. Each cartridge can have one or more membranes located therein, depending on the desired flow rate and the optimal size of the membrane.

[0044] The cartridge 355 also includes an interstitial space 359 within the cartridge cavity for receiving the solvent, as will be discussed below. The entire cartridge 355 is then sealed around the periphery by the housing 353. The gas is allowed to exit 351 into an outlet chamber defined by the outlet support plate 375 and the outlet end cap 320, which feeds the gas until it reaches the outlet 330.

[0045] Solvent enters housing 310 through inlet 335, which allows liquid solvent to flow around cartridge 355 within interstitial space 380. However, cartridge housing 353 prevents direct contact between the solvent within interstitial space 380 and membrane 377. In accordance with the present invention, solvent entering 345 housing 310 is directed to flow into inlet aperture 365 to flow through cartridge 355 within interstitial space 359 and thereby contact membrane 377 before exiting cartridge 355 through inlet aperture 370.

[0046] This arrangement provides a fluid path for the solvent that places the solvent in close proximity to the membrane and thus solves the problem of having sufficient flow for efficient gas transfer.

[0047] In another embodiment, the present invention provides a baffle 360 that defines an enrichment zone.

[0048] Reference Figure 5A and Figure 5B , the baffle 360 defines an enrichment zone 385 that separates the interstitial space 390 near the inlet, thereby ensuring that all solvent flows 395 into the inlet hole 365, and the solvent is not directly discharged through the outlet 340. Therefore, the use of the baffle 360 defines an enrichment zone 385 separated from the outlet 340, thereby ensuring a flow path of the solvent close to the membrane and thus fully utilizing each of the membranes in the cartridge group. After the solvent flows through the length of the cartridge 355, it exits from the inlet hole 370 and flows 400 into the interstitial space 395 near the outlet 340.

[0049] will understand, though Figure 4B The baffle 360 is shown positioned approximately two-thirds of the way along the length of the module 305, but in reality the position will be a function of the flow rate 345 entering the module 305, the size of the holes 365, 370, and the desired gas flow rate during in-cartridge gas transfer. Thus, the size of the enrichment zone can vary depending on the application, influenced by various design parameters based on permeability, flow rate, etc.

[0050] Figure 6A and Figure 6B A further improvement in the MBC unit is shown. Within the unit is a cartridge 410 having a plurality of individual hollow fibers 420, 415, which may be PTFE fibers. In this embodiment, the hollow fibers are not held in a linear arrangement, but are braided 425, thereby providing a larger surface area within which the gas / liquid transfer required by the MBC unit is permitted. The braiding can take several different configurations, all of which fall within the braided hollow fibers of the present invention.

[0051] Figure 7A and Figure 7BA further improvement is shown in the MBC unit 427. The unit comprises a housing 440, an inlet 430 for introducing flue gas, and an outlet 475 for exhausting the flue gas. Within the unit 427 is a chamber with a cartridge 445, separated by two baffles 450, into which a cooling medium 465 is introduced to exchange heat, thereby maintaining a temperature range that allows for better CO2 removal. The cooling medium is subsequently released 460 after heat transfer. A lean amine solvent 470 is introduced into the chamber to receive the acid gas, while an amine-rich solvent 455 is released, allowing the deacidified flue gas to be exhausted 475.

[0052] Figure 7C Another configuration of unit 485 is shown where the solvent enters 490 shell side and exits 495 at the opposite end, while the flue gas enters the tube side 505. The solvent will be cooled with an external intercooler and re-enter the second stage before exiting 495 as amine-rich solvent.

Claims

1. A method for removing acid gases from a post-combustion process stream, the method comprising the steps of: receiving the post-combustion process stream into hollow fibers of at least one MBC unit, each hollow fiber being gas permeable and liquid impermeable; allowing a lean solvent to flow through the hollow fiber in such a manner as to contact the outer surface of the hollow fiber; exchanging the acid gas into the solvent through the hollow fiber; discharging a treated lean gas stream; The solvent rich in acid gases is allowed to leave.

2. The method according to claim 1, further comprising the steps of: monitoring the pressure of the post-combustion process stream; monitoring the pressure of the lean solvent; controlling the solvent and / or natural gas flow to thereby; Maintaining a pressure differential, whereby the solvent pressure is greater than the gas pressure, and maintaining an optimum solvent temperature allows for higher CO2 removal performance.

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. The method according to any one of claims 1 to 3, further comprising the following steps: regenerating the acid gas-rich solvent, thereby; generating a lean solvent stream, and; A regenerated lean solvent stream is combined with the lean solvent stream.

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

6. A gas exchange system, comprising: at least one membrane contactor unit; The at least one membrane contactor unit has a lumen in which gas-permeable, liquid-impermeable hollow fibers are placed, or a reverse configuration in which a liquid is placed in the lumen; Each hollow fiber has a membrane inlet arranged to receive gas from the inlet chamber and a membrane outlet for discharging the gas; The chamber is arranged to flow a lean solvent therethrough in contact with the outer surfaces of the hollow fibers to allow gas exchange through the gas permeable and liquid impermeable membrane, and; a differential pressure control system arranged to monitor the pressures of the post-combustion process stream and the lean solvent; wherein the pressure differential control system is arranged to control the solvent and the process stream and to maintain a pressure differential whereby the pressure of the lean solvent is greater than the pressure of the gas.

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

9. The unit contains a chamber separated by two baffles, into which a cooling medium is introduced to exchange heat, thereby maintaining a certain temperature range.

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