Recovery of carbon dioxide plant vent gas using membranes

Abstract: A process for recovering CO₂ from a CO₂ liquifaction plant vent gas, which comprises feeding vent gas from a CO₂ liquifaction plant to a first semi-­ permeable gas separation membrane through which and at a pressure at which CO₂ is capable of diffusing therethrough, to form a CO₂-enriched permeate, and then returning the CO₂-enriched permeate to a feed side of said CO₂ liquifaction plant at a pressure capable of effecting the same.

Description: The present invention pertains to a passive membrane system which may be used to recover carbon dioxide from a carbon dioxide plant vent gas stream. Depending upon whether a gas mixture contains carbon dioxide (CO₂) in a high or low amount, a variety of techniques have been used to separate CO₂ from the mixture. For example, when the amount of CO₂ present in the gas mixture is low, and purification cannot be achieved directly by cooling and partial condensation, it is possible to scrub the gas mixture with a suitable solvent to dissolve the CO₂, and then to strip the CO₂ from the solution so obtained. The carbon dioxide obtained can then be compressed, dried, cooled and further purified by partial condensation or distillation. When the gas mixture contains a high amount of CO₂, however, the gas mixture may be compressed and then dried by absorption or other means. Finally, after removing undesirable impurities such as sulfur containing compounds, the mixture is cooled and after distillation CO₂ is obtained as a bottoms product. Unfortunately, the overhead product of the distillation column will always contain a significant amount of CO₂ which is inevitably wasted. This problem is particularly acute in conventional CO₂ liquifaction plants. In a typical CO₂ liquifaction plant, a significant percentage of the CO₂ feed is lost as stripper vent gas. For example, in a 200 ton/day CO₂ liquifaction plant, about 10 to 15% of the CO₂ feed is lost. The composition of this dry waste gas steam is usually 75% or more of CO₂ with the remainder being N₂, O₂, H₂, and CH₄ with trace amounts of NH₃, CO and sulfur containing compounds. In order to address this problem, at present, carbon dioxide is recovered from gas mixtures by subjecting the gas mixture to membrane separation, recovering from the membrane separation a permeate having a carbon dioxide concentration between the equilibrium concentration and about 98% by volume and then distilling the permeate at subambient temperature above the freezing temperature of the permeate and recovering carbon dioxide as a liquid bottoms product of the distillation. The above conventional process is described in U.S.-A-4,639,257. In this process, a distillation step is required and the only energy savings available is attained by recycling the overhead stream from the distillation step to the membrane separation step. However, such a separation technique requires an external energy source for operation. This renders the process economics very unfavorable. Thus, it would be extremely desirable if a method for separating CO₂ from a gas mixture could be effected without the use of an external energy source and in an economically advantageous manner. It would also be extremely desirable if vent gas which is lost from conventional CO₂ liquifaction plants could be treated to recover CO₂ in a completely passive manner using the pressure of the waste gas stream. Accordingly, it is an object of the present invention to provide a method for recovering CO₂ from the waste gas stream of a CO₂ liquifaction plant without the use of an external energy source. It is also an object of the present invention to provide a method for recovering CO₂ from the waste gas stream of a CO₂ liquifaction plant which can allow the recovered CO₂ to be recycled back to the feed side of the liquifaction plant at a purity compatible with the feed gas stream to the plant. Moreover, it is also an object of the present invention to provide a CO₂ recovery process which is completely passive using the pressure of the waste gas stream from a CO₂ liquifaction plant. These objects and others which will become apparent are provided by a process for recovering CO₂ from a CO₂ liquifaction plant vent gas according to claim 1. FIGURE 1 illustrates the effect of temperature on gaseous separation factors for mixtures of CO₂/N₂, CO₂/O₂ and CO₂/H₂. FIGURE 2 illustrates a single-stage membrane CO₂ recovery unit in accordance with the present invention. FIGURE 3 illustrates a dual-stage membrane CO₂ recovery unit in accordance with the present invention. FIGURE 4 illustrates the performance of the present single-stage and dual-stage membrane CO₂ recovery units of the present invention by displaying the recovery as a function of product purity. The process of the present invention utilizes a semi-permeable gas membrane to recover carbon dioxide from a waste gas stream of a CO₂ liquifaction plant. Notably, the recovery systems of the present invention are completely passive in that they contain no moving parts and use only the pressure energy of the waste gas stream to effect separation of CO₂ from the gas mixture. Further, the recovery systems of the present invention are advantageously operated at about the CO₂ plant temperature in order to enhance the CO₂/impurity separation and to improve process economics. Additionally, the degree of separation achieved by the present invention may be further increased by operating the membrane at the cold temperature of the off-gas stream. The enriched CO₂ strain that is produced by the membrane is then recycled back to the feed side of the liquifaction plant at a purity compatible with the feed gas stream to the plant. In accordance with the present invention, it has been found to be quite advantageous to use a membrane to achieve separation of the CO₂ from the gas mixture since the waste gas stream is already pressurized. Hence, no external energy source is required in order to operate the CO₂ recovery systems of the present invention. Further, as noted above, it is observed that at low temperature the separation factors between CO₂ and other gases present are quite high, resulting in a higher product purity than if the gas mixture were at room temperature. FIGURE 1 illustrates the effect of temperature on various separation factors for gaseous mixtures of CO₂/N₂, CO₂/O₂ and CO₂/H₂. With decreasing temperature, an improved separation factor is obtained. Thus, no external cooling of the gas is required in the present invention to achieve these high separation factors inasmuch as the vent gas temperature is already at low temperature, i.e., about -18.9°C (-2°F). Hence, the present CO₂ recovery units enjoy very favorable process economics. A single-stage membrane recovery unit of the present invention is illustrated in Figure 2. This recovery system and the operation thereof will now be described. The carbon dioxide-containing feed gas mixture is provided through a feed header pipeline to a compressor 1 and then the compressed mixture is fed via pipeline 2 to condenser 3 which contains refrigeration unit 6, an exit 4 at the bottom of the condenser for removal of liquid CO₂, and pipeline 5 through which the overhead stream or vent gas stream passes at elevated pressure through pressure regulator 7 to separation membrane 8. Generally, the term "elevated pressure" means that the pressure of the vent gas is sufficient to facilitate permeation of CO₂ through the separation membrane. Typically, however, pressures in the range of about 17.25 x 10⁵ Pa - 24.15 x 10⁵ Pa (250-350 psia) are used. This will be explained in more detail below. The non-permeating waste gas is exhausted through a vent or pressure regulator 9 to the atmosphere. The CO₂-enriched permeating gas passes through a back pressure regulator 10 on the permeate side of the membrane and is then recycled via conduit 11 to the feed side of the liquifaction plant for reprocessing. Additionally, the purity of the CO₂ obtained may be further increased by the addition of a catalytic burn-out system 10 in the permeate return line. Quite advantageously, it has been found that the passive catalytic unit reacts the remaining combustibles with oxygen in the product gas to raise the purity to above 99%. For this purpose, any commercially available catalytic unit will suffice. In the above single-stage recovery process, the vent gas is typically fed from the condenser at an elevated pressure in the range of about 17.25 x 10⁵ Pa - 24.15 x 10⁵ Pa (250-350 psia) and at a low temperature in the range of about -23.3°C - -15°C (-10°F to about 5°F). However, it is most desirable if a pressure of about 19.32 x 10⁵ Pa - 22.08 x 10⁵ Pa (280-320 psia) is used, and a temperature of about -20.6°C to -17.8°C (-5°F) to 0°F is used. It is particularly preferred that a pressure of about 20.7 x 10⁵ Pa - 21.39 x 10⁵ Pa (300-310 psia) is used. Most often, however, is used a pressure of about 21.05 x 10⁵ Pa (305 psia), and a temperature of about -18.9°C (-2°F). Additionally, prior to membrane separation, if desired, the gas mixture can be filtered to remove impurities. Further, a pressure regulator is advantageously used to provide a constant feed pressure on the membrane unit. Also, back pressure regulators in the permeate (CO₂ product) line and membrane vent line maintain adequate pressure on the module feed side and permeate side. Typically, a pressure in the range of about 0.69 x 10⁵ Pa - 2.76 x 10⁵ Pa (10 to 40 psia) is maintained on the permeate side and in the feed line returning to the CO₂ liquifaction plant. It is essential only that the pressure of the gas at this stage have sufficient pressure to be returned to the feed side of the CO₂-liquifaction plant. Non-permeating gas exits the membrane unit and is exhausted to the atmosphere. In Figure 3, a dual-stage membrane CO₂ recovery unit is illustrated. From Figure 3, it is seen that membranes 8 and 9a are positioned in series such that the permeate from the first stage becomes the feed to the second stage. In this recovery system, a pressure regulator and three back pressure regulators maintain gas pressures at the desired levels within the system. Pressures in this system are in accordance with the pressures used in the single-stage system. For example, the permeate pressures in the single-stage and dual-stage recovery systems are maintained in a range of about 0.69 x 10⁵ Pa - 2.76 x 10⁵ Pa (10 to 40 psia). It is more preferred, however, to maintain these pressures within the range of about 1.04 x 10⁵ Pa - 2.07 x 10⁵ Pa (15 to 30 psia). Most often, however, the permeate pressure in the single-stage and dual-stage recovery systems is maintained at about 1.59 x 10⁵ Pa (23 psia). In any event, in accordance with both single- and dual-stage recovery systems, it is sufficient if the permeate pressure is merely adequate to push the CO₂ product back to the feed side of the liquifaction plant. In the dual-stage recovery system, the non-permeate streams from stage 1 and stage 2 are vented to the atmosphere. In more detail, in Figure 3, it is seen that the carbon dioxide-containing feed gas mixture is provided through a feed header pipeline to a compressor 1 and then the compressed mixture is fed via pipeline 2 to condenser 3 which contains refrigeration unit 6, an exit 4 at the bottom of the condenser for removal of liquid CO₂, and pipeline 5 through which the overhead stream or vent gas stream passes at elevated pressure through pressure regulator 7 to separation membrane 8. The non-permeating gas is vented to the atmosphere through pressure regulator 8a. The term "elevated pressure" is defined hereinabove. The permeating gas from the first membrane 8, which is enriched in CO₂, is then passed through pipeline 9 to the second separation membrane 9a. The non-permeating gas from this membrane is vented to the atmosphere via pressure regulator 9b. The permeating gas from the second membrane, which is further enriched in CO₂, is then passed through pipeline 10 at a pressure sufficient to push the highly CO₂ enriched gas back to the feed side of the CO₂ liquifaction plant. Typically, pressures on the permeate side of the second membrane are in the range of about 0.69 x 10⁵ Pa - 2.76 x 10⁵ Pa (10 to 40 psia), preferably 1.04 x 10⁵ Pa - 3.11 x 10⁵ Pa (15 to 30 psia). Most often, however, a pressure of about 1.59 x 10⁵ Pa (23 psia) is used. In accordance with the present invention, any semi-permeable gas membrane may be used as the separation membrane or membranes provided that it exhibits sufficient permeability to CO₂ and has a sufficient separation factor with respect to impurity gases. Although any such membrane may be used, one such membrane is one which is in the form of a bundle of hollow fibers. In order to illustrate the present invention, reference will now be made to the following Examples which are provided solely for purposes of illustration and are not intended to be limitative. Example 1 A test membrane consisting of a bundle of hollow fibers, with a bore side diameter of approximately 0.35 mm, was inserted into a pressure shell. A gas mixture containing CO₂ at high pressure was fed to the feed side of the membrane module which allowed fast permeating CO₂ to diffuse through the membrane into the lower pressure permeate side where it was recovered. The remainder of the slow permeating gas exited the module at high pressure from the end opposite the feed side and was vented. The operating limits of the membrane were a 6.9 x 10⁵ Pa (100 psi) maximum pressure differential with a -28.9°C to -48.9°C (-20 to 120°F) temperature range. Example 2 This example and the results obtained thereby is illustrated in Figure 4. The results obtained were generated via computer model for a feed gas rate of 675 m³/h (25,000 SCFH) and a concentration of 75% CO₂. The permeation coefficients for the individual gases were experimentally determined using a sample of membrane material. It is seen that using a single-stage recovery unit, the maximum CO₂ purity using this membrane is 96.5% at 25% recovery. The recovery increases rapidly as the product purity decreases, however. For example, by operating the unit at a 70% CO₂ recovery in a single-stage producing a recycle gas of 95% CO₂, almost 22 tons of CO₂ gas per day is recovered. Example 3 Further to Example 2, an even higher purity CO₂ is attained when a dual-stage membrane is used. Product purity of about 98% CO₂ is attained at a 33% recovery. The best performance attained is a CO₂ product purity of 97% at a 60% recovery which corresponds to about 18 tons of CO₂ per day. Example 4 Further to Example 2 and Example 3, it is observed that when using a passive catalytic burn-out system with either the single- or dual-stage recovery system of the present invention to react the remaining combustibles and oxygen in the product gas, the purity of the resulting final product gas is raised to about 99%. The above Examples illustrate several processes in accordance with the present invention. However, it is understood that many variations and modifications would be apparent to one skilled in the art which would be within the ambit of the present invention. For example, more than two separation units might be used instead of a catalytic burn-out unit, or a filtration system might be used to remove gaseous impurities prior to membrane separation.

Claims: A process to improve the operation of a carbon dioxide liquefaction plant wherein a feed gas stream of carbon dioxide rich gas mixture is compressed, fed to a carbon dioxide liquefaction plant wherein liquid carbon dioxide is recovered from the bottom of the plant and wherein a gaseous mixture comprising carbon dioxide gas is vented from said plant, whereby said vent gas (5) is fed to at least one first semi-permeable gas separation membrane (8) to form a CO₂ enriched permeate gas (9) and a CO₂ depleted non permeating gas, said non permeating gas being vented to the atmosphere, characterised in that said CO₂ enriched permeate gas (9, 9c, 11) is recycled to a feed input of said CO₂ liquefaction plant, at a purity and a pressure compatible with the feed gas stream. A process according to claim 1, wherein said CO₂ permeate (9) is passed through a passive catalytic burn out unit (10). A process according to one of claims 1 or 2, wherein said vent gas is fed to said first semi-permeable gas membrane (8) at a pressure in the range of about 19.32 x 10⁵ Pa (280 Psia) - 22.08 x 10⁵ P (320 psia) and at a temperature of about -20.6°C (-5°F) to -17.8°C (0°F). A process according to claim 3, wherein said vent gas is fed to said first semi-permeable gas membrane (8) at a pressure in the range of about 20,7 x 10⁵ Pa (300 psia) to 21,4 x 10⁵ Pa (310 psia) and at a temperature in the range of about -20,5°C (-5°F) to -17,8°C (0°F). A process according to claim 4, wherein said vent gas is fed to said first semi-permeable gas membrane (8) at a pressure of about 21 x 10⁵ Pa (305 psia) and at a temperature of about -18,9°C (-2°F). A process according to one of claims 1 to 5, wherein the pressure at the first membrane (8) feed side and permeate side is maintained by pressure regulators (7, 8a, 9b, 9d). A process according to one of claims 1 to 6, wherein said pressure on the permeate side of said first membrane (8) is in the range of about 0.69 x 10⁵ Pa to 2,76 x 10⁵ Pa (10 to 40 psia). A process according to claim 7, wherein said pressure on the permeate side of said first membrane (8) is in the range of about 1 x 10⁵ Pa to 2 x 10⁵ Pa (15 to 30 psia). A process according to claim 8, wherein said pressure on the permeate side of said first membrane (8) is about 1,59 x 10⁵ Pa (23 psia). A process according to one of claims 1 or 3 to 9, wherein said CO₂-enriched permeate from said first semi-permeable gas separation membrane (8) is fed to a second semi-permeable gas separation membrane (9a). A process according to claim 10, wherein said CO₂-enriched permeate from said second semi-permeable gas separation membrane (9a) is fed to a passive catalytic burn-out unit (10). A process according to one of claims 1 to 11, wherein said semi-permeable gas membranes (8, 9a) comprise a bundle of hollow fibers. A process according to claim 12, wherein said semi-permeable gas membranes (8, 9a) have an increased selectivity for carbon dioxide with decreasing temperature. A process according to one of claims 1 to 13, wherein the gas membranes (8, 9a) are made from a polymer selected from the group comprising polyimide, polyaramide polyester and polyamide.

Inventors: PAGANESSI JOSEPH E, SAUER RICHARD A, PAGANESSI, JOSEPH E., SAUER, RICHARD A.
Applicants: AIR LIQUIDE, L'AIR LIQUIDE, SOCIETE ANONYME POUR L'ETUDE ET L'EXPLOITATION DES PROCEDES GEORGES CLAUDE
Application year: 1990
Main classification: B01D53
IPC codes: B01D53, C01B31, F25J3