Patent Application
20250073632
The invention relates to a system and method of removing acidic gas, such as CO2 and H2S, from a post combustion process stream.
For clarity, reference to CO2 will further apply to H2S and other acidic gases unless the application would make such an extension unworkable.
Process streams, such as for natural gas, may be high pressure, moderate temperature, high driving force, allowing varied methods of managing acidic gases within the process stream.
However, post-combustion (flue gas), which also requires the removal of CO2, but at atmospheric pressure and high temperature with low driving force, renders conventional means inefficient or incapable of achieving the desired result, particularly with regard to dealing with the relatively low pressure. Syngas applications, which involve both CO2 and H2S, may be as low as post-combustion, but generally in between post-combustion and natural gas application's typical operating pressure.
In a first aspect, the invention provides a method for removing acidic gases from a post combustion process stream, the method comprising the steps of: receiving the post combustion process stream into hollow fibres of at least one MBC cell, each hollow fibre gas permeable, liquid impermeable; passing a lean solvent in contact with an external surface of said hollow fibres; exchanging said acidic gas into the solvent through the hollow fibre; venting an acidic gas lean stream; exiting an acidic gas rich solvent.
In a second aspect, the invention provides a gas exchange system, said system comprising: at least one membrane contactor cell; said at least one membrane contactor cell having a bore in which is placed a gas permeable, liquid impermeable, hollow fibre; each hollow fibre having a membrane inlet arranged to receive a gas from an inlet chamber and a membrane outlet for venting said gas; said bore is arranged to flow a lean solvent in contact with an external surface of the hollow fibre so as to permit the exchange of gas through said gas permeable, liquid impermeable membrane, and; a pressure differential control system, said pressure differential control system arranged to monitor a pressure of the post combustion stream and the lean solvent; wherein said pressure differential control system is arranged to control the solvent and process stream, and maintain a pressure differential whereby the gas pressure is greater than the lean solvent pressure.
The invention involves a system and method for removing acidic gases, such as CO2 and H2S, from a post combustion process stream.
By combining membrane and solvent, a membrane contactor system may offer a advantageous way to perform gas-liquid absorption for post combustion gas. The microporous membrane acts as a non-selective phase barrier, allowing the liquid and gas phases to contact with each other, yet without the dispersion of one phase into the other. This barrier prevents flooding or foaming issues from happening, thereby making MBC simple to operate. Packaging into hollow-fiber membrane (HFM) modules offers a higher mass transfer area compared with conventional packed columns, giving MBC a high intensification potential.
It will be convenient to further describe the present invention with respect to the accompanying drawings that illustrate possible arrangements of the invention. Other arrangements of the invention are possible and consequently, the particularity of the accompanying drawings is not to be understood as superseding the generality of the preceding description of the invention.
In general terms the invention involves a system and method for removing acidic gases, such as CO2 and H2S, from a post combustion process stream. The invention may include at least one membrane contactor cell receiving an inflow of flue gas characterised by being low pressure and high temperature. The membrane contactor cell includes a hollow membrane being gas permeable but liquid impermeable. By passing a lean solvent through the bore of the cell so as to be proximate with the external surface of the membrane, a more efficient exchange of gas will occur. The gas is then vented from the membrane contactor cell, with the now rich solvent flowing out of the cell, potentially for regeneration.
A flue gas or post-combustion gas stream 14 is fed to a product gas knockout (KO) drum 40 to remove any entrained liquid from the treated gas, and from which the flue gas or post-combustion is vented 45. The now CO2 rich solvent is passed through cold 50 and hot 55 cross exchanges, which extract CO2 (and H2S if the original gas feed is syngas) 60 for subsequent venting 70 from an acid gas KO drum. 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 into the regeneration tower 80.
Thus, the MBC process for treating post combustion gas allows for a low pressure, high temperature inflow to have CO2 separated from the original gas feed. Accompanied with a lean solvent supply and regeneration plant, further enhances the efficiency of the CO2 extraction.
With reference to the absorption process, a syngas having a combination of CO2 and H2S or a post combustion gas 110 is directed through a valve 115 into the MBC array 120. The array 120 receives a lean solvent 125, which absorbs CO2, leaving a syngas (SG)rich gas flow being fed to a further valve 130 for subsequent venting 140. The now CO2 rich solvent stream exits the array 120 and is fed into a rich solution flash drum 160. Residual SG gas is extracted and vented to a flash gas system 165.
To maintain appropriate concentrations of solvent within the flash drum 160, parts of a lean solvent stream are also fed into the flash drum 160 with the flash gas removed and the solvent passed to a lean rich exchanger 170. The lean rich exchanger 170 further receives a lean solvent stream from a reflex drum which receives the lean solvent from an MBC array 190 used to extract CO2 from the rich solvent stream received from the flash drum 160. The gas removed from the rich solvent stream is passed through a condenser 200 and into a flash gas device 210 via a KO drum 205. The residual solvent is then returned 220 to the reflex drum for an exchange through the lean rich exchanger 170. The lean solvent stream is subsequently fed back into the MBC array 120 via an amine tank 150 which further includes a solvent top-up 145.
A characteristic of both of the MBC arrays 120, 190 is the addition of a pressure differential device 135, 215.
As will be explained with reference to
If the pressure differential is permitted to reduce, as shown in
In one embodiment, the pressure control may maintain the MBC's pressure difference between gas and liquid at 0.3 bar. In this example, 0.3 bar may be sufficient to prevent the flooding shown in
For this example, the set point 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 differential pressure between the gas and liquid at 0.3 barg. By having this cascade SP controller, the liquid pressure will always follow the actual gas pressure at MBC to ensure reliable operation and to prevent membrane wetting in the event of high-pressure fluctuations or upset.
Table #1 shows a series of CO2 experiments were conducted using MBC with CO2 removal efficiency above >90%.
Different solvents may be used for flue gas and syngas application due to the difference in gas composition and the removal efficacy (flue gas: CO2 removal; syngas: CO2+H2S removal). The process is mature, but it still has the disadvantages of large volume occupancy, high equipment height, energy intensive and some operation problems such as flooding, channeling, entrainment, and foaming.
One of the reasons why CO2 removal from post-combustion is energy intensive is because the driving force is low whereby the pressure of the flue gas is at atmospheric pressure. While for syngas, the pressure is between low to medium (up to 30 bar) and with stringent H2S requirement (<5 ppm) to safeguard the downstream process' catalyst.
The present invention provides a process intensification as the surface area per unit volume may be 30 times more than conventional packed column enabling a reduction in size, energy consumption and cost. The present invention may provide advantages compared to the conventional absorption process, including higher packing density, independent control of gas and absorbent flows, and compact modular structure which provides the flexibility for scale up or down.
In one embodiment, braided PTFE fibers may be used in combination with alkanolamines for acid gas removal to improve surface area per unit volume. In addition, multi-cartridge MBC design ay have a central tubesheet, or baffle plate, and pressure control.
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 sealed around the periphery by a casing 353. The gas is permitted to vent 351 into an outlet chamber defined by outlet support plate 375 and outlet end cap 320 which feed the gas through to the outlet 330.
The solvent enters the housing 310 through the inlet 335 which allows the liquid solvent to flow about the cartridges 355, within an interstitial space 380. The cartridge housing 353, however, prevents direct contact between the solvent within the interstitial space 380 from contacting the membranes 377. According to the present invention, the solvent entering 345 the housing 310 is directed to flow into ingress apertures 365 so as to flow through the cartridge 355 within the interstitial space 359 and thus contact with the membrane 377 before exiting the cartridge 355 through ingress apertures 370.
This arrangement provides a fluid path for the solvent that places the solvent in close proximity to the membranes, and so solves the issue of having a sufficient flow for an efficient gas transfer.
In a further embodiment, the invention provides baffle plates 360 which define concentration zones.
With reference to
It will be appreciated that whilst
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/CN2022/135236 | 11/30/2022 | WO |
