WO2023064977A1

WO2023064977A1 - A process and plant of vacuum pressure swing adsorption for producing pure carbon dioxide from industrial off-gas containing co2

Info

Publication number: WO2023064977A1
Application number: PCT/AU2022/051245
Authority: WO
Inventors: Penny Y. Xiao; Daniel Lefu Tao; Ming S. Liu
Original assignee: Dimer Technologies Pty. Ltd.
Priority date: 2021-10-18
Filing date: 2022-10-16
Publication date: 2023-04-27

Abstract

A vacuum-pressure swing adsorption process and multiple column plant for separating carbon dioxide (CO2) from low-pressure CO2 -containing industrial off-gas and purifying it to 99 CO2 by vol%. The vacuum-pressure swing adsorption (VPSA) cycles includes a feed step, two or three sets of pressure equalization steps, a CO2 product gas purge step, a desorption step, and a feed gas re-pressurisation step. The plant to carry out the said VPSA process comprises of five or more parallel adsorption columns, six solenoid control valves for each column, and storage tank, a CO2 product gas vacuum pump and storage tank, a CO2 gas purging blower/compressor and storage tank, a CO2 lean gas storage tank, and corresponding pipes connecting the valves, tanks, and rotatory devices to the adsorption columns.

Description

Title of Invention: A process and plant of vacuum pressure swing adsorption for producing pure carbon dioxide from industrial off-gas containing CO2

Technical Field

[0001 ] The present invention relates to the field of carbon dioxide separation and purification from low-pressure industrial off-gas. The invention designs a vacuumpressure swing adsorption process that includes a CO2 product gas purging step to obtain high purity CO2 (> 99 vol%) from superatmospheric pressure CO2- containing industrial off-gas, and correspondingly a vacuum -pressure swing adsorption (VPSA) plant to implement the said process.

Background Art

[0002] Carbon dioxide (CO2) is the major greenhouse gas that contributes to climate changes and global warming. The industry and society are swiftly transiting to low carbon operation by adaption to renewable energies, reduction of carbon dioxide emissions and improvement of energy efficiency. Therefore, carbon capture, utilization and storage (CCLIS) with various industrial off-gases has huge industrial, environmental and economic benefits, which is under intensive development in line with the "resource-product-renewable resources" circulation economy. With the increasing CCLIS demands, many separation and purification technologies, including cryogenic condensation, liquid absorption, solid adsorption and membrane separation, have been researched and developed to recover and process carbon dioxide, but they all have certain advantages and disadvantages. Pressure swing adsorption technology (PSA) based on solid adsorption/desorption method have been widely used in gas industries for separating and purifying hydrogen (H2), oxygen (O2), nitrogen (N2), methane (CH4), and carbon monoxide (CO). To date, conventional PSA processes have been applied in carbon dioxide recovery and separation for the ammonia synthesis-shift gas of the fertilizer plants or the off-gas from petrochemical refineries, usually by pressurizing the off-gas, and carbon dioxide is adsorbed and separated under high pressure. In these CO2 separation processes, the PSA process typically uses a compressor to boost the feed off-gas up to a pressure greater than 0.8MPa gauge and utilizes the PSA principle to recover/remove CO2 from the off-gas. The process can achieve a recovery rate of >80%, and a carbon dioxide purity of 90%~95%. Because of the less high purity of the product CO2, it is not conducive to further utilization of CO2, such as, further to food-grade or liquefication. At the same time, due to the overall pressurization of the off-gas, the energy consumption is quite high.

[0003] Major challenge still remains to recover/capture CO2 with both high purity (i.e. > 99 vol%) and high recovery rate, yet other challenges are the inflated capital costs of the plant with increasingly complex processes. These challenges are more critical for low pressure industrial off-gases, wherein the pressure is normally at low in the superatmospheric range of 20~200kPa.g, and they typically contain many more impurities other than CO2. Based on conventional PSA technologies, innovative PSA technologies, such as vacuum -pressure swing adsorption and the similar need to be invented and developed to enhance the CO2 recovery and capture capabilities.

[0004] In the prior art of PSA and VPSA technologies, US patent 2010/010449 A1 describes a device with high thermal efficiency carbon dioxide recovery. This patent application shows that a solvent purge system or a PSA system can be utilized for CO2 separation, with hydrogen-containing gas recycled back to the system. But apparently the description is superficial, there are no details of the actual decarburization process or implementation. US Patent 2010/0287981 A1 describes various hydrogen and carbon dioxide recovery processes in steam reforming systems. The target gas in this invention is the water-gas shift product. After using conventional PSA for H2 recovery, the tail gas is compressed to a certain pressure and firstly sent to pressure-vacuum swing adsorption (PVSA) integrated with membrane system for carbon dioxide recovery. However, there is neither an example nor a specific flow (loop) or detailed performance in this invention. Also, in US patent 2008/0072752 A1 , a process flow based on ‘PVSA+PSA’ is used to separate carbon dioxide and hydrogen. The process steps disclosed mainly include a feed adsorption step, a series of pressure reduction step, the emptying step, the pressure equalization step after the emptying, a series of pressure equalization steps and the repressure step. The pressure requirement of the feed gas in this patent technology is significantly high (e.g. 0.8~3.5MPa.a), and the high-pressure gas is reduced to near atmospheric pressure through a series of depressurization steps, thereby releasing the non- CO2 gas adsorbed in the adsorbent and improving the CO2 purity by subsequent evacuation. Similarly, European patent application EP3733264A1 depicts the recovery of CO2 from a feed gas at 3.0MPa absolute using PVSA process, whereas a CO2-rich purge and product-CO2 purge were used to improve the purity of product CO2. Eventually a CO2 purity of 95% and recovery of >89% are achieved. These patent applications use high pressure of the feed off-gas and undergo complex process steps, and induce the expensive equipment investment and operation costs, although the treatment targets of these PSA-PVSA processes are the low-pressure off-gas of the water-gas shift reactions. Chinese patent CN106698429A developed two-stage VSA+PSA processes for recovery CO2 from flue-gas without pressurizing the feed-gas into high pressure in the first stage VSA. Australian patent All-2016201267A1 and Chinese patent CN- 201611219796.5 invented one-stage VSAA/PSA processes for recovery CO2 from refinery tail-gas without pressurizing feed gas into high pressure. However, in these VSA processes CO2 has been adsorbed and desorbed multiple times, this results in lower adsorbent efficiency and higher energy consumption.

Therefore, more advanced VSA and VPSA processes need to be designed and developed.

[0005] In conclusion, the prior-art technologies mainly aimed at purifying non-carbon dioxide gas, so the designed PSA- or PVSA-column height is generally tall (e.g. exceeding 4 meters), and such column height causes a large pressure drop and excessive energy due to the compression. Meanwhile for off-gas from heavy industries such as petrochemical refineries, coal-chemical, or ammonia fertilizers, carbon dioxide concentration is in the range of mid to high-level (for example, varying from 20% to 85%), likely they also contain saturated water vapor at low temperature and many types of impurity of CO, N2, H2 and hydrocarbons. The previously developed PSA, or VPSAA/SA processes are based on the ‘stripping cycle’ mechanism, which have not purpose-designed or optimised for efficient recovery of CO2 with the higher purity and lower carbon footprint.

Summary of Invention [0006] The purpose of the present invention is to provide a novel vacuum swing adsorption method for separating and purifying CO2 from low-pressure CO2- containing industrial off-gas, which is essentially a creative multiple-column VPSA process including a CO2 product gas purging step and feed gas at superatompheric pressure. The present invention can overcome the shortcomings of the prior arts. In the case of using at least 5 adsorption columns, a CO2 product gas with a purity of more than 99% and desirable recovery rate can be achieved from various industrial off-gas containing 30% to 70% CO2 at a minimum pressure of 120 kPa absolute.

[0007] The said process of the present invention includes the following steps.

[0008] Feed and adsorption step. Feed industrial off-gas enters an adsorption column where CO2 is adsorbed by the adsorbent. Subsequently, CO2 lean gas is produced and exits the adsorption column from the top.

[0009] Co-current pressure equalization depressurising steps. After the feed and adsorption step, the adsorption column is depressurised by connecting another adsorption column which has finished the desorption step and at a lower pressure. At the end of each pressure equalization steps, the pressure within the two connected adsorption columns is about the same. In the present invention, there are 2 or 3 sets of pressure equalization steps.

[0010] CO2 product gas purging step. On finishing the first pressure equalization depressurising step, the adsorption column is then connected to the CO2 purging storage tank and the purging blower (or compressor) to let the pure CO2 product gas enter the adsorption column from the bottom. The propagation of high concentration CO2 region from the bottom of the adsorption column drives more lean CO2 gas exiting the column from the top of the column.

[0011] Vacuum desorption step. After a series of column interactive depressurising steps, the adsorption column is finally connected to the desorption vacuum pump to extract the remaining gas as well as the components desorbed from the adsorbent at the vacuum pressure level. The gas extracted by the vacuum pump at this moment is mainly CO2 and is passed to the CO2 product storage tank.

[0012] Counter current pressure equalization pressurising steps. Corresponding to the co-current pressure equalization depressurising steps, the adsorption column after desorption is pressurised by connecting another adsorption column at a higher pressure, so that the pressure within the adsorption column can be restored before the next adsorption cycle.

[0013] Receiving purging step. The adsorption column at the receiving purging step is pressurised further using the produced lean CO2 gas in the CO2 product gas purging step before the final repressurising step.

[0014] Final repressurising step. Upon finishing other pressurising steps, the feed industrial off-gas enters the adsorption column from the bottom but without opening exit at the top. The pressure within the adsorption column is eventually pressurised to the pressure level of the feed gas

[0015] The above steps are the operating adsorption steps of one single adsorption column in a cyclic process. The said at least five adsorption columns repeat the above steps in sequence synchronically but at different time, so as to realize that the feed gas continuously enters one of the adsorption columns at any moment.

[0016] In the above steps, the number of the co-current pressure equalization depressurising steps and the number of counter current pressure equalization pressurising steps are coupled. Preferably, the number of coupling pressure equalization steps is 2 or 3, and it is adjustable to adapt to different feed streams with varying CO2 compositions in the range of 30% to 70%.

[0017] The CO2 product gas purging step and the receiving purging step occur normally after the first pressure equalization coupling steps. The method of the present invention does not include the step of purging the adsorption column with the CO2 lean product gas.

[0018] Preferably, the volume percentage of CO2 in the feed industrial off-gas is in the mid to high-level (e.g. varying from 30% to 70%). More preferably, for example, the CO2-containing feed gas comes from, but not limited to, the off-gas of a hydrogen production unit of methane steam reforming (SMR), methanol cracking and the off-gas of a coal-to-syngas process.

[0019] In the above method, the pressure of the CO2-containing industrial off-gas is in the at superatmospheric low pressure range of 120 to 250 kPa absolute. The lower pressure feed gas entering the adsorption column can be pressurized to, or more than, 120 kPa absolute. The pressure in the evacuation step is 10 to 20 kPa absolute achieved by vacuum pump(s). The pressure after the desorption vacuum pump in the CO2 product gas storage tank is around 110 kPa absolute. To purge the adsorption column, the CO2 product gas is re-pressurized to a pressure level as close as the feed pressure, and stores in another storage tank before entering the adsorption column.

[0020] Preferably, adsorbents are one two or more layers packed with a layer of water-selective adsorbent and the layer(s) of CO2-selective adsorbents from zeolite A, zeolite X, zeolite Y, activated carbon, activated alumina, metal organic framework, silica gel and/or the combination thereof. The selected adsorbents and its packed multi-layers are optimised for high selectivity, large capacity, fast kinetics and low adsorption heat.

[0021 ] Another purpose of the present invention is to provide a plant for implementing the said VPSA method of separating pure CO2 from a CO2- containing industrial off-gas of the present invention. The plant comprises a feed gas inlet pipe, a feed gas storage tank, at least five adsorption columns, a desorption pipe, a desorption vacuum pump, a CO2 product gas storage tank, a CO2 product gas purging pipe, a CO2 product gas purging blower/com pressor, a CO2 product gas purging storage tank, a first pressure equalizing pipe, a second pressure equalizing pipe, a lean CO2 gas pipe, and a lean CO2 gas storage tank. Each of the adsorption columns is connected with a feed gas supply valve, a desorption gas valve, a CO2 product gas purging valve, at least two pressure equalization control valves, and a CO2 lean gas valve.

[0022] Preferably, adsorbents packing in the adsorption column is in a dense layered packing manner.

[0023] Preferably, the plant of the present invention also includes pressure monitoring instrumentations within the adsorption column.

[0024] Compared with traditional CO2 capture PSA or PVSA processes, the said VPSA method of the present invention can achieve a CO2 product gas purity of more than 99% from a feed gas containing 30% to 70% CO2. The energy requirement to the blowers/compressors are competitive low for pressurising the feed gas from 100 kPa to 200 kPa absolute, while in traditional CO2 capture PSA or PVSA process, the compression duty is normally rising the pressure from 100 kPa to a range of 800 to 1000 kPa absolute. Meanwhile, the vacuum requirements in the said VPSA method of the present invention are lower than that of the traditional CO2 vacuum pressure swing adsorption process. A vacuum degree of 20 kPa absolute could yield satisfied production results with high CO2 purity and high recovery rate.

[0025] The object of the present invention is to construct a plant to implement the said VPSA process for producing high-purity CO2 from the low-pressure CO2- containing industrial off-gas. The present invention plant is a one-stage vacuum pressure swing adsorption plant ideally for off-gas treatment.

[0026] The invented VPSA process and plant can produce CO2 with a purity of greater than 99% from low pressure industrial off-gas containing CO2 with desirable recovery rate. In the present invention, the feed off-gas directly enters the vacuum pressure swing adsorption column(s) or with limited compression. Meanwhile, the present invention designs an optimised CO2 product gas purging step and unique process steps to replace the traditional vacuum pressure swing adsorption method. It can produce high-purity CO2 and high calorific value fuel gas with ideal recovery rate, significantly overcoming the problems of, particularly the low efficiency of adsorbent usage and high energy consumption of vacuum pump, as existing with the traditional VSAA/PSA process.

Brief Description of Drawings

[0027] In the accompanying drawing:

Fig.1

[0028] Figure 1 is a schematic diagram showing the invented five-bed VPSA plant for carrying out the invented VPSA process of the present invention.

Fig.2

[0029] Figure 2 is a flow chart showing the circulation steps of a single adsorption column of the present invention when 3 sets of coupled pressure equalization steps are applied.

Fig.3

[0030] Figure 3 is a table of the coupling cycle of an embodiment of the present invention based on the VPSA cycles as defined in Figure 2. In Figure 3, the horizontal axes of l-a, l-b, l-c, , V-c are sequential adsorption steps, and the vertical axes C01 , C02,..., C05 refer to the individual adsorption column.

Fig.4

[0031 ] Figure 4 is a flow chart showing the circulation steps of a single adsorption column of the present invention when 2 sets of coupled pressure equalization steps are applied.

Fig.5

[0032] Figure 5 is a table of the coupling cycle of an embodiment of the present invention based on the VPSA cycles as defined in Figure 4.

[0033] In Figure 2 to Figure 5, the step symbols respectively represent: AD, feed adsorption step; PPE1 , pressure equalization depressurising step 1 ; PPu, CO2 product gas co-current purging step; PPE2, pressure equalization depressurising step 2; PPE3, pressure equalization depressurising step 3; V, vacuum desorption step; RPE3, pressure equalization pressurising step 3; RPE2, pressure equalization pressurising step 2; RPE1 , pressure equalization pressurising step 1 ; RPu, receiving product gas counter current purging step; IDLE, waiting step; FRP, feed gas pressurising step.

Description of Embodiments

[0034] According to the first aspect of the present invention, there is a provided process for producing CO2 with a purity of higher than 99% from various low- pressure industrial off-gas by vacuum -pressure swing adsorption using five or more adsorption columns each filled with adsorbents. Each adsorption column is programmed to repeatedly perform the same sequential cycle of steps. However, at one certain moment, they are operated at different steps so that they can be synchronized to satisfy the continuous utilisation of the feed gas and provide continuous production of pure CO2.

[0035] In one variation, accordance to the Figure 2 and Figure 3, the process undergoes with 3 sets of coupled pressure equalization steps as described below, and it is conducted by focusing on one of the five adsorption columns. The repeatedly performed cycle of steps includes: [0036] Adsorption step (AD): passing the feed gas through the adsorption column to adsorb most of CO2 on the adsorbents. The lean CO2 gas is passing through the adsorption column and exiting the adsorption column from the top.

[0037] No.1 co-current depressurising equalization step (PPE1 ): after the adsorption step, the content in the adsorption column flows co-current to another adsorption column which is undertaking a counter-current pressurising equalization step until pressures within two connected adsorption columns reach the same level.

[0038] CO2 product gas purging step (PPu): On finishing the first co-current pressure equalization depressurising step, the adsorption column is then connected to the CO2 purging storage tank and the purging blower/compressor to let the pure CO2 product gas enter the adsorption column from the bottom. The propagation of high concentration CO2 region from the bottom of the adsorption column drives more lean CO2 gas exiting the co-current depressurising column.

[0039] No.2 co-current depressurising equalization step (PPE2): after the CO2 product gas purging step, the content in the adsorption column flows co-current to another adsorption column which is undertaking the No.2 counter-current pressurising equalization step, until pressures within two connected adsorption columns reach the same level.

[0040] No.3 co-current depressurising equalization step (PPE3): after the second depressurising equalization step, the content in the adsorption column flows cocurrent to another adsorption column which is undertaking the No.3 countercurrent pressurising equalization step until pressures within two connected adsorption columns reach the same level.

[0041 ] Vacuum desorption step (V): the content in the adsorption column forcedly flows counter-current out of the bottom of the adsorption column, while the evacuation force is provided by the vacuum pump connected to the adsorption column. The extracted gas is mostly CO2, and the exiting gas from the bottom is regarded as the CO2 product.

[0042] No.3 counter-current pressurising equalization step (RPE3): from the top of the adsorption column, a gas stream from another adsorption column, which is undertaking a No.3 co-current depressurising equalization step, flows counter- current to pressurise the column until pressures within two connected adsorption columns reach the same level.

[0043] Idle step (IDLE): after the No. 3 counter-current pressurising equalization step, the adsorption column is placed in the idle step. In idle step, the adsorption column has no interaction with all other parallel adsorption columns. The disconnection is achieved by switching off the rotatory valves connecting to the top and bottom of the adsorption column. The idle step is placed to achieve synchronization among all five adsorption columns.

[0044] No.2 counter-current pressurising equalization step (RPE2): from the top of the adsorption column, a gas stream from another adsorption column, which is undertaking the No.2 co-current depressurising equalization step, flows countercurrent to pressurise the column until pressures within two connected adsorption columns reach the same level.

[0045] No.1 counter-current pressurising equalization step (RPE1 ): from the top of the adsorption column, a gas stream from another adsorption column, which is undertaking the No.1 co-current depressurising equalization step, flows countercurrent to pressurise the column until pressures within two connected adsorption columns reach the same level.

[0046] Receiving purging step (RPu): the adsorption column at the receiving purging step is pressurised further using the lean CO2 gas produced in the CO2 product gas purging step.

[0047] Feed gas re-pressurising step (FRP): from the top of the adsorption column, a gas stream from the product buffer gas tank flows counter-current to pressurise the column. After this step, the adsorption column will again operate the adsorption step to conduct a new and repeated VPSA cycle.

[0048] In another variation, accordance to the Figure 4 and Figure 5, the process can undergo with 2 sets of coupled pressure equalization steps is described below:

[0049] Adsorption step (AD): passing the feed gas through the adsorption column to adsorb most of CO2 on the adsorbents. The lean CO2 gas is passing through the adsorption column and exiting the adsorption column from the top. [0050] No.1 co-current depressurising equalization step (PPE1 ): after the adsorption step, the content in the adsorption column flows co-current to another adsorption column which is undertaking a counter-current pressurising equalization step until pressures within two connected adsorption columns reach the same level.

[0051 ] CO2 product gas purging step (PPu): On finishing the first co-current pressure equalization depressurising step, the adsorption column is then connected to the CO2 purging storage tank and the purging blower/compressor to let the pure CO2 product gas enter the adsorption column from the bottom. The propagation of high concentration CO2 region from the bottom of the adsorption column drives more lean CO2 gas exiting the co-current depressurising column.

[0052] No.2 co-current depressurising equalization step (PPE2): after the CO2 product gas purging step, the content in the adsorption column flows co-current to another adsorption column which is undertaking the No.2 counter-current pressurising equalization step, until pressures within two connected adsorption columns reach the same level.

[0053] Vacuum desorption step (V): the content in the adsorption column forcedly flows counter-current out of the bottom of the adsorption column, while the evacuation force is provided by the vacuum pump connected to the adsorption column. The extracted gas is mostly CO2, and the exiting gas from the bottom is regarded as the CO2 product.

[0054] Idle step (IDLE): after the No. 3 counter-current pressurising equalization step, the adsorption column is placed in the idle step. In idle step, the adsorption column has no interaction with all other parallel adsorption columns. The disconnection is achieved by switching off the rotatory valves connecting to the top and bottom of the adsorption column. The idle step is placed to achieve synchronization among all five adsorption columns.

[0055] No.2 counter-current pressurising equalization step (RPE2): from the top of the adsorption column, a gas stream from another adsorption column, which is undertaking the No.2 co-current depressurising equalization step, flows countercurrent to pressurise the column until pressures within two connected adsorption columns reach the same level. [0056] No.1 counter-current pressurising equalization step (RPE1 ): from the top of the adsorption column, a gas stream from another adsorption column, which is undertaking the No.1 co-current depressurising equalization step, flows countercurrent to pressurise the column until pressures within two connected adsorption columns reach the same level.

[0057] Receiving purging step (RPu): the adsorption column at the receiving purging step is pressurized further using the lean CO2 gas produced in the CO2 product gas purging step.

[0058] Feed gas re-pressurising step (FRP): from the top of the adsorption column, a gas stream from the product buffer gas tank flows counter-current to pressurise the column. After this step, the adsorption column will again operate the adsorption step to conduct a new and repeated VPSA cycle.

[0059] Preferably, the vacuum pump vacuums the adsorption column to the absolute pressure level of 10-20 kPa at the vacuum desorption step for both variations.

[0060] According to the second aspect of present invention, a purpose-built plant, i.e. a VPSA plant using five or more adsorption columns filled with designed adsorbents, can produce CO2 product with a purity higher than 99% from various industrial off-gas containing CO2 within the range of 30% to 70%.

[0061 ] This VPSA plant can carry out the process according to the first aspect of the present invention, and hence has the same advantages as those described above with respect to the present invention. Below the Example section shows the application examples of the present invention.

[0062] The following describes a preferred embodiment of the apparatus of the present invention based on the accompanying Figure 1 :

[0063] Adsorption columns C01 , C02, C03, C04, and C05 are provided in parallel to each other. The adsorption column is packed with two or more layers of adsorption, with a layer of water-selective adsorbent and another layer of CO2- selective adsorbents. The CO2-selective adsorbents could be silicas, active carbon, zeolites A/X/Y-type, and metal organic frameworks, and the combination thereof or the equivalent. The selection of the adsorbents and its packed layers are optimised for high selectivity, large capacity, fast kinetics and low adsorption heat. The adsorption columns are configured to produce a CO2 product discharging in path 7 by adsorbing most of the CO2 and letting other components from the feed gas pass through the adsorption column.

[0064] In the said vacuum-pressure swing adsorption process, the flowrate ratio of the CO2 product gas purging stream in the purge step to the CO2 product gas obtaining in the desorption step is preferably between 15% to 50%.

[0065] The feed gas supply path 1 for supplying a feed gas is connecting the feed gas buffer tank 1 a to the lower portion of the five adsorption columns. Feed gas supply valves 2a, 3a, 4a, 5a, and 6a on the feed supply path of each adsorption column open and close the feed gas supply to the corresponding individual adsorption column.

[0066] The desorption path 7 for discharging the CO2 product is connected to the lower portion of the five adsorption columns. A vacuum pump 7a is installed on the desorption path to reduce the pressure in the desorption path to an absolute pressure value of at most 20 kPa absolute. Desorption gas valves 2b, 3b, 4b, 5b, and 6b on the desorption path of the corresponding adsorption column open and close the desorption gas discharging path to the vacuum pump 7a. The off-gas side of vacuum pump 7a is connected to the CO2 product tank 7b.

[0067] Another set of valves, 2c, 3c, 4c, 5c, and 6c, connect the CO2 purging tank 8b, the purging blower (or the compressor) 8a, and the adsorption columns via CO2 purging path 8. The purging blower (or the compressor) 8a pressurises the CO2 product gas from the CO2 product tank 7b to a level same as the feed gas stream in the CO2 purging tank 8b. The CO2 purging valves 2c, 3c, 4c, 5c, and 6c open and close the corresponding purging path allowing the CO2 product gas from the CO2 purging tank to backflow to and purge the adsorption columns.

[0068] The CO2 lean gas path 11 for letting CO2 lean gas pass the adsorption columns is connecting between the upper portions of the five adsorption columns and the CO2 lean gas buffer tank 11a. CO2 lean gas valves 2f, 3f, 4f, 5f, and 6f on the product gas path of each adsorption column open and close the product gas discharging path of the corresponding adsorption column to the CO2 lean gas tank.

[0069] Further, two sets of pressure-equalization adjustment valves are connected to the upper portion of the adsorption column. On the first pressure-equalization path 9, pressure-equalization control valves 2d, 3d, 4d, 5d, and 6e enable/disable the interactions between any of the two adsorption columns by opening/closing the corresponding control valves. Similarly, on the second and third pressureequalization (if applicable) path 10, pressure-equalization control valves 2e, 3e, 4e, 5e, and 6e enable/disable the interactions between any of the two adsorption columns by opening/closing the corresponding control valves.

[0070] Referring to the embodiment of the plant described above, the designed VPSA cycle with 3 sets of coupled pressure equalization steps (as shown in Figure 2 and Figure 3) can be therewith implemented. Specifically, all steps of the designed VPSA cycles are arranged to perform in respective adsorption columns in parallel. The following description is explained accordance to the Figure 1 , Figure 2, and Figure 3 to demonstrate how to operate the designed VPSA cycle with 3 sets of coupled pressure equalization steps in the described embodiment of the plant.

[0071] In Step (l-a), the adsorption column C01 is undertaking the first part of the adsorption step (AD). Specifically, the feed gas is introduced to the bottom of the adsorption column C01 by switching on feed gas supply valve 2a. Meanwhile, the CO2 lean gas valve 2f is switched on to discharge CO2 lean gas from the top of the adsorption column C01 to the CO2 lean gas tank 11a. The adsorption column C02 and adsorption column C05 are performing the No. 1 pressure equalization coupling steps (PPE1 and RPE1 ) using the pressure equalization path 9. Specifically, the adsorption column C05 is providing a co-current gas stream via the opening pressure-equalization valve 6d, and in a result the pressure within the adsorption column C05 decreases. The adsorption column C02 is receiving a gas stream counter-current via the opening pressure-equalization valve 3d, and in a result that the pressure within the adsorption column C02 increases. The overall consequence of the No. 1 pressure equalization coupling steps in step (I- a) is that the pressure within the adsorption column C05, which is relatively higher at the initial stage of Step (l-a), and the pressure within the adsorption column C02, which is relatively lower, reach to the same level at the end stage of Step (l-a). The adsorption column C03 and adsorption column C04 are performing the No. 3 pressure equalization coupling steps (PPE3 and RPE3) using the pressure equalization path 10. Specifically, the adsorption column C04 is providing a co-current gas stream via the opening pressure equalization valve 5d, and in a result the pressure within the adsorption column C04 decreases. The adsorption column C03 is receiving a gas stream counter-current via the opening pressure equalization valve 3d, and in a result the pressure within the adsorption column C03 increases. The overall consequence of the No. 3 pressure equalization coupling steps in Step (l-a) is that the pressure within the adsorption column C04, which is relatively higher at the initial stage of Step (l-a), and the pressure within the adsorption column C03, which is relatively lower, reach to the same level at the end stage of Step (l-a).

[0072] In Step (l-b), the adsorption column C01 is in the second part of the adsorption step (AD), which is similar to Step (l-a) by opening the feed gas supply valve 2a and CO2 lean gas valve 2f. The adsorption column C02 is in the counter-current receiving purge step (RPu), while the adsorption column C05 is performing CO2 product gas purging step (PPu). Specifically, the CO2 purging valve 6c is switched on, and the CO2 product gas is introduced from the bottom of the adsorption column C05 via the CO2 purging tank 8b and the purging blower/com pressor 8a. The pressure-equalization valve 6d is kept opening to let the purging gas discharge from the top of the adsorption column C05. Meanwhile, the pressure equalization valve 3d is opened for purging gas stream from the adsorption column C05 to purge the adsorption column C02. The adsorption column C03 in Step (l-b) is in idle step (IDLE), and all control valves attached to the adsorption column C03 are switched off. The adsorption column C04 is undertaking the first part of the vacuum desorption step (V), that the CO2 gas product is evacuated from the bottom of the adsorption column C04 by the vacuum pump 7a via the opened desorption valve 5b, and the CO2 product gas is temporarily stored in the CO2 product gas tank 7b. The pressure within the adsorption column C04 decreases to the desorption pressure which is decided by the power of the vacuum pump.

[0073] In Step (l-c), the adsorption column C01 is in the third part of the adsorption step (AD), which is like Step (l-a) and Step (l-b) by opening the feed gas supply valve 2a and CO2 lean gas valve 2f. The adsorption column C02 is carrying out feed gas re-pressurising step (FRP). The feed gas is introduced via feed gas supply path 1 by opening the feed gas supply valve 3a. The pressure within the adsorption column C02 increases to the same level as the feed gas at the end stage of Step (l-c). The adsorption column C03 and adsorption column C05 are performing the No. 2 pressure-equalization coupling steps (PPE2 and RPE2) using the pressure equalization path 10. Specifically, the adsorption column C05 is providing a co-current gas stream via the opening pressure equalization valve 6e, in a result that the pressure within the adsorption column C05 decreases. The adsorption column C03 is receiving a counter-current gas stream via the opening pressure equalization valve 4e, in a result that the pressure within the adsorption column C03 increases. The overall consequence of the No. 2 pressureequalization coupling steps is that the pressure within the adsorption column C05, which is relatively higher at the initial stage of Step (l-c), and the pressure within the adsorption column C03, which is relatively lower, reach to the same level at the end stage of Step (l-c). The adsorption column C04 is undertaking the second part of the vacuum desorption step (V), that the CO2 gas product is evacuated from the bottom of the adsorption column C04 by the vacuum pump 7a via the opened desorption valve 5b.

[0074] Step (l-a), Step (l-b) and Step (l-c) contains all VPSA steps including: the adsorption step (AD), No. 1 pressure-equalization coupling steps (PPE1 and RPE1 ), the CO2 product gas purging step and receiving purging step (PPu and RPu), No. 2 pressure-equalization coupling steps (PPE2 and RPE2), No. 3 pressure-equalization coupling steps (PPE3 and RPE3), vacuum desorption step (V), and idle step (IDLE). Thereafter, Step (l-a), Step (l-b) and Step (l-c) are the basic “building block units” and can be expanded to produce the whole VPSA cycle by swapping adsorption columns to perform every individual step, as illustrated as Step (I l-a) to Step (V-c) in Figure 3.

[0075] The same operation method, as described above to implement designed VPSA cycle with 3 sets of coupled pressure equalization steps, can be adopted to carry out Step (l-a), Step (l-b), and Step (l-c) of the designed VPSA cycle with 2 sets of coupled pressure equalization steps in Figure 5. Therefore, the whole VPSA cycle with 2 sets of coupled pressure equalization steps that is built on the “building block units” can be implemented with the same embodiment of the plant and method that described above. [0076] The sequential proceeding of adsorption cycle steps is governed by orderly switching on/off the program -control led valves. The program-controlled valves in the embodiment can be controlled remotely and/or wirelessly through an automatic control and communication system. As a preferred embodiment of the present invention, the plant of the present invention includes an automatic control system, such as a programmable logic controller (PLC) with a communication module or a distributed control system (DCS) with a communication module. Skilled and trained personnel can remotely/wirelessly control the plant of the present invention through an automatic PLC/DCS control system according to the requirements of gas separation.

[0077] To better describe the present invention and facilitate the understanding of the implementation of the invention, the present invention will be described in further detail below along with the drawings and examples. However, these embodiments are only simple examples of the present invention, and do not fully represent or limit the scope of the present invention.

[0078] Although an embodiment of the present invention has been described above, the present invention is not limited to the embodiment. The specific VPSA method and plant to produce high purity CO2 from industrial off-gas referring to this invention can be further modified in various ways without deviating from the scope of the present invention. For example, although five adsorption columns are described above, the number of the adsorption column is not limited to five. With any numbers of adsorption columns (no less than five) coupled with matching VPSA cycle including adsorption step, various sets of pressure equalization steps, purge step, desorption step, vacuum desorption step, and product re-pressurisation steps etc., can perform the same and provide the same advantage of the present invention.

Examples

[0079] Example 1 :

[0080] In this example, a feed gas of low-pressure off-gas introduced from a refinery SMR hydrogen-PSA system is purified to pure CO2 by the described VPSA cycle with 3 sets of coupled pressure equalization steps in the foregoing embodiment. Specifically, CO2 recovery and purification are performed under the following conditions.

[0081 ] Table 1.

Adsorption pressure (kPa.A) 230

Desorption pressure (kPa.A) 20

CO2 product purity 99.6 vol%

CO2 recovery rate 84.4% Feed gas Product gas

(after drying, vol%)

Carbon dioxide, CO2 51.66 99.62

Methane, CH₄ 14.44 0.00

Carbon monoxide, CO 9.77 0.25

Hydrogen, H2 22.78 0.00

Water, H2O 0.94 0.13

Nitrogen, N2 0.37 0.00

[0082] The five adsorption columns are in a cylinder shape with the same diameter of 2.8 meters. The height of the adsorbent packing section is 3.5 meters. The feed industry off-gas from a refinery SMR hydrogen-PSA system contains 51 .66 vol% of CO2 with other impurities. The feed gas is supplied at a volumetric flow rate of 15,300 Nm3/hr. The adsorption pressure (maximum pressure) in the adsorption step is 230 kPa (absolute pressure: this holds true in every case of the pressures described hereinafter), and the desorption pressure achieved by the vacuum pump is 20 KPa. The CO2 purity achieved after preliminary drying process is 99.62% and the CO2 recovery is 84.4%. Further drying process can be applied to enhance the CO2 product quality. Table 1 above discloses the operating conditions and the VPSA process performance.

[0083] Example 2:

[0084] In this example, a same feed gas as example 1 of low-pressure off-gas introduced from a refinery SMR hydrogen-PSA system is purified to pure CO2 by the described VPSA cycle with 2 sets of coupled pressure equalization steps in the foregoing embodiment. Specifically, CO2 recovery and purification are performed under the following conditions.

[0085] Table 2.

Adsorption pressure (kPa.A) 230

Desorption pressure (kPa.A) 20

CO2 product purity 99.02 vol% CO2 recovery rate 85.9% Feed gas Product gas

(after drying, vol%)

Carbon dioxide, CO2 51.66 99.02

Methane, CH₄ 14.44 0.02

Carbon monoxide, CO 9.77 0.83

Hydrogen, H2 22.78 0.00

Water, H2O 0.94 0.13

Nitrogen, N2 0.37 0.00

[0086] The operating condition of example 2 are same as those of example 1 , except the VPSA cycle with 2 sets of coupled pressure equalization steps as illustrated in Figure 5, instead of 3 sets as illustrated in Figure 3. The CO2 purity achieved after preliminary drying process is 99.02% and the CO2 recovery is 85.9%. Further drying process can be applied to enhance the CO2 product quality. Table 2 above discloses the operating conditions and the VPSA process performance.

[0087] Example 3:

[0088] In this example, an off-gas stream from a coal gasification process is fed to the designed plant to produce pure CO2 by the VPSA cycle with 3 sets of coupled pressure equalization steps in the foregoing embodiment. Specifically, CO2 recovery and purification are performed under the following conditions.

[0089] Table 3.

Adsorption pressure (kPa.A) 330

Desorption pressure (kPa.A) 20

CO2 product purity 99.36 vol%

CO2 recovery rate 70.55%

Composition Feed gas Product gas

Carbon dioxide, CO2 29.58 99.36

Methane, CH₄ 1.54 0.00

Carbon monoxide, CO 16.56 0.62

Hydrogen, H2 51.58 0.00

Water, H2O 0.01 0.02

Nitrogen, N2 0.73 0.00

[0090] The adsorption column used in this example is with a diameter of 2.8 meters and a height of 3.5 meters. The feed gas from a coal gasification process contains 29.58 vol% of CO2 with other impurities. The feed gas is supplied at a volumetric flow rate of 28,600 Nm3/hr. The adsorption pressure in the adsorption step is 330 kPa and the desorption pressure achieved by the vacuum pump is 20 KPa. The CO2 purity achieved in this example is 99.36% and the CO2 recovery is 70.55%. Table 3 above discloses the operating conditions and the VPSA process performance.

[0091 ] Example 4:

[0092] In this example, an off-gas stream from a process producing hydrogen from methanol cracking is fed to the designed plant to produce pure CO2 by the VPSA cycle with 2 sets of coupled pressure equalization steps in the foregoing embodiment. Specifically, CO2 recovery and purification are performed under the following conditions.

[0093] Table 4.

Adsorption pressure (kPa.A) 150

Desorption pressure (kPa.A) 20

CO2 product purity 99.79 vol%

CO2 recovery rate 90.7%

Composition Feed gas Product gas

(vol%) (after drying, vol%)

Carbon dioxide, CO2 67.21 99.79

Methane, CPU 0.14 0.00

Carbon monoxide, CO 2.80 0.21

Hydrogen, H2 29.01 0.00

Water, H2O 0.83 0.00

Nitrogen, N2 0.00 0.00

[0094] The adsorption column used in this example is with a diameter of 2.8 meters and a height of 3.2 meters. The feed gas from a process producing hydrogen from methanol contains 67.21 vol% of CO2 with other impurities. The feed gas is supplied at a volumetric flow rate of 18,400 Nm3/hr. The adsorption pressure in the adsorption step is 150 kPa and the desorption pressure achieved by the vacuum pump is 20 KPa. The CO2 purity after drying process achieved in this example is 99.79% and the CO2 recovery is 90.7%. Table 4 above discloses the operating conditions and the VPSA process performance.

[0095] Those skilled in the art of the invention will appreciate that many variations and modifications may be made to the specific embodiment and examples without departing from the spirit and scope of the present invention. It should also be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in any countries.

Claims

[Claim 1] A vacuum-pressure swing adsorption (VPSA) cycle for producing pure carbon dioxide (CO2 > 99 vol%) from low-pressure industrial off-gas containing CO2 by stepwise adsorption steps going through multiple adsorption columns packed with adsorbents selective for at least one more strongly CO2-absorbable component. Each of the multiple adsorption columns in each process step operates in a cyclic and coordinated manner. The said VPSA process undergoes 3 sets of coupled pressure equalization steps comprising the following steps: a. feed and adsorption step b. the first co-current pressure equalization depressurising step c. purge step by introducing CO2 product gas from the bottom of the adsorption column d. the second co-current pressure equalization depressurising step e. the third co-current pressure equalization depressurising step f. desorption step by depressurising the adsorption column using vacuum pump to a pressure level of at most 20 kPa absolute g. the third counter current pressure equalization pressurising step h. idle step i. the second counter current pressure equalization pressurising step j. the first counter current pressure equalization pressurising step k. receiving purge step with the adsorption column which is undertaking the purge step l. re-pressurising step by the feed gas

[Claim 2] Another vacuum-pressure swing adsorption (VPSA) cycle for producing pure carbon dioxide (CO2 > 99 vol%) from low-pressure industrial off-gas containing CO2. The said VPSA process undergoes 2 sets of coupled pressure equalization steps comprising the following steps: a. feed and adsorption step b. the first co-current pressure equalization depressurising step c. purge step by introducing CO2 product gas from the bottom of the adsorption column d. the second co-current pressure equalization depressurising step e. desorption step by depressurising the adsorption column using vacuum pump to a pressure level of at most 20 kPa absolute f. idle step g. the second counter current pressure equalization pressurising step h. the first counter current pressure equalization pressurising step i. receiving purge step with the adsorption column which is undertaking the purge step re-pressurising step by the feed gas

[Claim 3] The vacuum-pressure swing adsorption process of Claim 1 & Claim 2, wherein the purity of CO2 produced by both said VPSA cycles is greater than 99 vol% with a desirable production yield, preferably with a yield greater than 80% and more preferably greater than 90%.

[Claim 4] The vacuum-pressure swing adsorption process of Claim 1 & Claim 2, wherein the pressure of the feed gas and adsorption step is at superatmospheric pressure of at least 120 kPa absolute.

[Claim 5] The vacuum-pressure swing adsorption process of Claim 1 & Claim 2, wherein the feed gas contains CO2 component in the range of mid to high- level, i.e. varying from 30% to 70%. The feed gas also contains impurity of, including but limited to, carbon monoxide, nitrogen, hydrogen, hydrocarbons, water (dry to saturated at the feed conditions) and others.

[Claim 6] The vacuum-pressure swing adsorption process of Claim 1 & Claim 2, wherein the pressure of the desorption step is at most 20 kPa absolute.

[Claim 7] The vacuum-pressure swing adsorption process of Claim 1 Claim 2, wherein the adsorbents in the adsorption column are two or more layers with at least one layer of water-selective adsorbent and another layer of CO2-selective adsorbents of silicas, active carbon, zeolites A/X/Y-type, and metal organic frameworks, and the combination thereof or the equivalent. The selection of the adsorbents and its packed layers are optimised for high selectivity, large capacity, fast kinetics and low adsorption heat.

[Claim 8] The vacuum-pressure swing adsorption process of Claim 1 & Claim 2, wherein the flowrate ratio of the CO2 product gas purging stream in the purge step to the CO2 product gas obtaining in the desorption step is preferably between 15% to 50%.

[Claim 9] A plant can carry out the VPSA cycles of Claim 1 & Claim 2 for separating pure CO2 from industrial off-gas containing CO2. The said plant comprising: a. feed gas supply path b. feed gas buffer tank c. at least five adsorption columns d. desorption gas path e. desorption gas vacuum pump f. CO2 product gas tank g. CO2 purging path h. CO2 purging tank i. CO2 purging blower/com pressor j. the first pressure equalization path k. the second pressure equalization path l. CO2 lean gas path m. CO2 lean gas tank n. feed gas supply valves o. desorption gas valves p. CO2 purging valves q. the first pressure equalization valves r. the second pressure equalization valves s. CO2 lean gas valves

[Claim 10] The plant of Claim 9, wherein a) feed gas supply path connects with b) feed gas buffer tank, and n) f feed gas supply valves, to let the feed gas containing CO2 enter the adsorption column from bottom; wherein d) desorption gas path connects with e) desorption gas vacuum pump, f) CO2 product gas tank and all o) desorption gas valves, to let the CO2 product gas exit the adsorption column from the bottom; wherein g) CO2 purging path connects h) CO2 purging tank, i) CO2 purging blower/compressor, and all p) CO2 purging valves, to allow the CO2 product gas to flow back and purge the adsorption column; wherein j) the first pressure equalization path connects with all q) the first pressure equalization valves, to let the adsorption columns carry out interaction steps, such as the first co-current and counter-current pressure equalization steps and the CO2 gas purging step; wherein k) the second pressure equalization path connects with all r) second pressure equalization valves, to let the adsorption columns carry out interaction steps, such as the second and the third co-current and counter current pressure equalization steps; wherein I) CO2 lean gas path connects with m) CO2 lean gas tank and all s) CO2 lean gas valves, to let the CO2 lean gas exit the adsorption column from the top. The plant of Claim 9, wherein the adsorbents in the columns are packed by optimally designed layers. ;