JP2007253104A - Gaseous carbon dioxide separator and gaseous carbon dioxide separation method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a gaseous carbon dioxide separator capable of separating carbon dioxide from off-gases containing hydrogen and the carbon dioxide with high purity. <P>SOLUTION: The gaseous carbon dioxide separator is equipped with a means for bringing a gaseous mixture containing the carbon dioxide etc., into contact with an absorption liquid containing alkanol amine to make the carbon dioxide be absorbed in the absorption liquid, a means of transmitting the absorption liquid absorbing the carbon dioxide through a thin film of a hollow membrane to diffuse the carbon dioxide from the absorption liquid, a means for fractionating the carbon dioxide from the mixture composed of the absorption liquid diffusing the carbon dioxide and a means of bringing the gas failing to be absorbed in the first absorption liquid into contact with the second absorption liquid containing the alkanol amine to make the carbon dioxide be absorbed into the second absorption liquid, and a means for heating the second absorption liquid absorbing the carbon dioxide to diffuse the carbon dioxide from the second absorption liquid. The intra-system pressure from the downstream side of the hollow membrane in the diffusing means to the fractionating means is below the atmospheric pressure. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a carbon dioxide gas separation device and a carbon dioxide gas separation method.

  Conventionally, steam reforming processes, partial oxidation processes, autothermal reformers, and the like are known as hydrogen production processes in oil refining plants and petrochemical plants. In these processes, an acid gas such as carbon dioxide gas is usually produced as a by-product. The by-produced acid gas is removed by a chemical absorption process, a physical absorption process, a direct oxidation process, or the like. According to these acid gas removal processes, the carbon dioxide removed is of high purity. High purity carbon dioxide is industrially useful as a raw material for various chemical substances.

  On the other hand, in recent years, a PSA (Pressure Swinging Adsorption) process has been adopted as a process for removing by-product acid gas (see, for example, Non-Patent Document 1). The PSA process does not discharge alkaline waste liquid like the conventional chemical absorption process, the construction cost is relatively low, and the recovery of hydrogen gas in various off-gases discharged from oil refinery plants is possible. For some reason, the demand is increasing, and many plants using the PSA process for hydrogen production have been constructed. However, the off-gas discharged from such a PSA process contains a lot of other light gases such as hydrogen in addition to carbon dioxide, and the purity of carbon dioxide is higher than that of the conventional acid gas removal process described above. It is low. Therefore, in order to use the offgas from the PSA process as a raw material for the chemical substance, a means for separating carbon dioxide from the offgas with high purity and recovering it with a high yield is required.

By the way, as means for separating acidic gas such as carbon dioxide from general off-gas discharged from petroleum refining plants and petrochemical plants, means using an alkanolamine-containing absorbing liquid is known (for example, patent document). 1). According to this means, after carbon dioxide is highly selectively absorbed by the absorbing liquid, the carbon dioxide is diffused from the absorbing liquid by heating.
JP-A-8-10565 (Japan) Petroleum Institute, “Oil Refinery Process”, Kodansha, May 20, 1998, p324-326

  When the PSA process is applied to a hydrogen production system, the off gas contains a large amount of hydrogen, so carbon dioxide is separated from the off gas containing at least hydrogen and carbon dioxide with a sufficiently high purity and recovered in a high yield. There is a need to. However, in the prior art, none of the means for separating carbon dioxide contained in such off-gas has been sufficiently studied.

  Further, when the separation means as described in Patent Document 1 is applied to off-gas treatment from the PSA process to separate carbon dioxide with sufficiently high purity and to recover it with a high yield, a dioxide dioxide is obtained. It has been found that extremely high energy is required for heating to dissipate carbon from the absorbing solution.

  Therefore, the present invention has been made in view of the above circumstances, and can separate carbon dioxide from off-gas containing hydrogen and carbon dioxide with sufficiently high purity and can be recovered with sufficiently high yield. An object of the present invention is to provide a carbon dioxide gas separation device and a carbon dioxide gas separation method capable of sufficiently reducing energy required for separating carbon dioxide.

  The present invention is a carbon dioxide gas separation device for separating carbon dioxide from a mixed gas containing hydrogen and carbon dioxide, wherein the mixed gas is brought into contact with a first absorbent containing alkanolamine, and the first A first absorbing means for absorbing carbon dioxide in the first absorbing liquid, and a first absorbing liquid that has absorbed carbon dioxide permeated through the membrane wall of the hollow fiber membrane to dissipate carbon dioxide from the first absorbing liquid. A gas that has not been absorbed by the first absorbing liquid among the above-mentioned mixed gas, and a separating means that separates the carbon dioxide from the mixture of the first absorbing liquid that diffuses carbon dioxide and the carbon dioxide. Contacting the second absorption liquid containing alkanolamine, heating the second absorption liquid that absorbs carbon dioxide in the second absorption liquid, and heating the second absorption liquid that has absorbed carbon dioxide, 2 absorption liquids And a second dissipating means for dissipating carbon dioxide, system pressure from the downstream side of the hollow fiber membranes in the first dissipating means until separation means provides a carbon dioxide gas separation unit is less than atmospheric pressure.

  Further, the present invention is a carbon dioxide gas separation method for separating carbon dioxide from a mixed gas containing hydrogen and carbon dioxide, wherein the mixed gas is brought into contact with the first absorbent containing alkanolamine, The membrane wall of the hollow fiber membrane in which the first absorption step in which carbon dioxide is absorbed by the first absorbent and the first absorbent that has absorbed carbon dioxide are adjusted so that the downstream system pressure is less than atmospheric pressure. And the carbon dioxide from the mixture of the first absorbent and the carbon dioxide that diffused carbon dioxide under a pressure lower than atmospheric pressure. Separation step for fractionation, and of the mixed gas described above, the gas that has not been absorbed by the first absorbent is brought into contact with the second absorbent containing alkanolamine, and carbon dioxide is added to the second absorbent. Second suction to absorb A step, by heating the second absorbent that has absorbed carbon dioxide, to provide a carbon dioxide gas separation process and a second stripping step of dissipating carbon dioxide from the second absorption liquid.

  In the present invention, first, a mixed gas containing hydrogen and carbon dioxide is brought into contact with the first absorbing liquid containing alkanolamine, and carbon dioxide is absorbed into the absorbing liquid. The first absorbent that has absorbed carbon dioxide then permeates the membrane wall of the hollow fiber membrane, thereby releasing carbon dioxide. When the first absorbing liquid permeates the membrane wall of the hollow fiber membrane, pressure loss occurs in the hollow fiber membrane, so that the system pressure on the downstream side is lower than the upstream side of the hollow fiber membrane. Further, by reducing the system pressure downstream of the hollow fiber membrane to less than atmospheric pressure, carbon dioxide can be sufficiently dissipated from the first absorbent. Subsequently, the released carbon dioxide and the first absorption liquid are separated into gas and liquid in a system whose pressure is reduced to less than atmospheric pressure.

  In the conventional chemical absorption process of by-product acidic gas as described above, an amine-based absorption liquid that has absorbed an acidic gas such as carbon dioxide is heated in a regeneration tower equipped with a reboiler, so that the absorption liquid Acid gas is released and separated. In this case, since the absorbing solution is denatured and deteriorated by heating and the acid gas absorption ability is lowered, it is necessary to discharge the denatured and deteriorated absorbing solution out of the system and introduce a new absorbing solution into the system. Further, when the deteriorated absorbing liquid remains in the system, not only the absorption efficiency of the acid gas is lowered, but also a corrosion factor of piping equipment in the system such as generation of sludge. Therefore, in order to suppress corrosion of the piping equipment, it is necessary to improve the grade of the material of the piping equipment. One of the major reasons for introducing the PSA process into the hydrogen production equipment is the reduction of alkaline waste liquid discharge. Therefore, treating the off-gas of the PSA process directly with the conventional chemical absorption process has the advantage of the PSA process. It leads to destruction. In addition, the PSA off-gas processing apparatus generally has a significant increase in throughput (throughput) as compared with the above-mentioned chemical absorption process of by-product acid gas. As a result, the environmental impact associated with the discharge of the absorbent to the outside of the system, the increase in the amount of absorbent to be newly introduced, the increase in capital investment associated with upgrading the material quality of the piping equipment, and maintenance when the piping equipment is corroded In consideration of an increase in cost and the like, it is not practical to apply a conventional chemical absorption process as it is as an off-gas treatment apparatus of a PSA process.

  In the present invention, the first absorbent that has absorbed carbon dioxide is allowed to pass through the membrane wall of the hollow fiber membrane, and the downstream side of the hollow fiber membrane is depressurized to a pressure lower than atmospheric pressure, thereby reducing the first absorbent. The carbon dioxide emission treatment and the first absorption liquid and carbon dioxide fractionation treatment are separated. Thereby, even if it does not heat an absorption liquid, a carbon dioxide can be diffused and fractionated. The factor that can sufficiently dissipate carbon dioxide from the absorbent by reducing the pressure downstream of the hollow fiber membrane to a pressure lower than atmospheric pressure has not been clarified in detail at present. However, the present inventors consider that there are such factors as the fine structure of the hollow fiber membrane (for example, the pore diameter) and the change in the absorption equilibrium between carbon dioxide and the absorbent due to the decompression to less than atmospheric pressure. thinking. However, the factor is not limited to this.

  Further, some carbon dioxide still remains in the gas that has not been absorbed by the first absorbent from the mixed gas (hereinafter referred to as “unabsorbed gas”). This is because carbon dioxide that has not been dissipated remains in the first absorption liquid in contact with the mixed gas, and the carbon dioxide absorption capacity of the first absorption liquid is lower than when no carbon dioxide is contained. It is thought to be caused by If this unabsorbed gas is treated as an exhaust gas as it is, the yield of carbon dioxide becomes insufficient. Then, the unabsorbed gas is brought into contact with the second absorbing liquid containing alkanolamine, the carbon dioxide existing in the unabsorbed gas is absorbed into the second absorbing liquid, and the carbon dioxide is further absorbed. Carbon dioxide is diffused from the absorbing solution 2 under heating conditions. Thereby, the carbon dioxide which the 1st absorption liquid could not absorb can be further collected. As a result, in the present invention, the yield of carbon dioxide from the mixed gas can be sufficiently increased.

  Comparing the emission of carbon dioxide from the absorption liquid that has absorbed carbon dioxide through the hollow fiber membrane described above with the emission of carbon dioxide due to heating of the absorption liquid, in general, it is more More carbon can be released and carbon dioxide can be recovered in high yield. On the other hand, the amount of energy used can generally be reduced by passing through the hollow fiber membrane than by heating the absorbent. Therefore, the amount of energy used is low while maintaining the carbon dioxide recovery rate by adopting absorption using absorption liquid and dissipation by heating in the treatment of unabsorbed gas that does not contain much carbon dioxide in the mixed gas. Can be suppressed.

  In addition, according to the present invention, most of the carbon dioxide is absorbed in the first absorption liquid in the first absorption step, and it is not necessary to heat the first absorption liquid. It is extremely difficult to occur, and there is almost no need to discharge the first absorbing liquid out of the system or introduce a new absorbing liquid into the system. Furthermore, since deterioration of the first absorbent is sufficiently suppressed, corrosion of the piping equipment is difficult to occur. As a result, the opportunity for maintenance associated with corrosion of the piping equipment is reduced, and it is not necessary to make the material of the piping equipment high-grade, so that the capital investment can be reduced. Therefore, it is well suited for use in separating off carbon dioxide in off-gas in a PSA process having a high throughput.

  According to the present invention, carbon dioxide can be separated from off gas containing hydrogen and carbon dioxide with sufficiently high purity and can be recovered with sufficiently high yield, and at the same time, it is necessary for separating carbon dioxide. A carbon dioxide gas separation device and a carbon dioxide gas separation method capable of sufficiently reducing energy can be provided.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings as necessary. In the drawings, the same elements are denoted by the same reference numerals, and redundant description is omitted. Further, the dimensional ratios in the drawings are not limited to the illustrated ratios.

FIG. 1 is a flowchart showing a carbon dioxide gas separation device according to a preferred embodiment of the present invention. The carbon dioxide gas separation device 100 according to the present embodiment makes a mixed gas containing hydrogen and carbon dioxide contact a first absorbing liquid containing alkanolamine, and absorbs carbon dioxide in the first absorbing liquid. A CO ₂ absorption tower 10 which is a first absorption means to be allowed to pass through and a first absorption liquid which absorbs carbon dioxide through the membrane wall of the hollow fiber membrane to dissipate carbon dioxide from the first absorption liquid. Among the mixed gas, the membrane unit 20 which is a means, the decompression tank 30 which is a separation means for separating carbon dioxide from the mixture of the diffused carbon dioxide and the first absorption liquid, and the mixed gas were not absorbed by the first absorption liquid. CO ₂ absorption, which is a second absorption means for bringing a gas (hereinafter referred to as “unabsorbed gas”) into contact with a second absorption liquid containing alkanolamine and causing the second absorption liquid to absorb carbon dioxide. Tower 50 Comprises heating the second absorbent that has absorbed carbon dioxide, and a stripping column 60 which is the second dissipating means for dissipating carbon dioxide from the second absorption liquid.

The CO ₂ absorption tower 10 is a plate tower including one or two or more plate 12 similar to the conventional gas absorption tower, and a mixed gas supply line L1 is connected to a lower side surface thereof. The mixed gas containing hydrogen and carbon dioxide is supplied to the CO ₂ absorption tower 10 from the supply line L1. Gas mixture other than those gas components may be contained in the mixed gas containing hydrogen and carbon dioxide. When the mixed gas is off-gas discharged from the PSA process of the hydrogen production apparatus, methane and carbon monoxide are usually included as main gas components other than carbon dioxide and hydrogen.

The upper side surface of the CO ₂ absorption tower 10 is supplied with a lean absorbent (hereinafter referred to as “first lean absorbent”) that is a first absorbent that has flowed out from the bottom of the decompression tank 30 described in detail later. The lean absorbent supply line L10 is connected. The first lean absorbent is an absorbent containing an alkanolamine typified by monoethanolamine (MEA), diethanolamine (DEA), diisopropanolamine (DIPA) and / or methyldiethanolamine (MDEA). In the first lean absorbent, alkanolamine is diluted with water. Although the density | concentration of the alkanolamine in a 1st lean absorption liquid is not specifically limited, It is preferable in it being 12-60 mass%. When the concentration of alkanolamine is less than 12% by mass, the amount of carbon dioxide absorbed per system circulation amount of the absorbing liquid is small, and a large amount of circulation tends to be required. When the concentration of alkanolamine exceeds 60% by mass, the viscosity of the absorbing solution increases, and the flow resistance when penetrating through a hollow fiber membrane described later increases. Therefore, it is necessary to increase the pressure difference between the inside and outside of the hollow fiber membrane or increase the membrane area of the hollow fiber membrane.

  In this specification, in order to facilitate understanding, if necessary, an absorption liquid that does not contain carbon dioxide or contains almost no carbon dioxide is referred to as a “lean absorption liquid”, and a large amount of carbon dioxide is contained. The absorbed liquid that has been absorbed is referred to as a “rich absorbent liquid”.

The mixed gas supplied from the lower part of the CO ₂ absorption tower 10 comes into contact with the first lean absorbing liquid falling from the upper side in the tower, and carbon dioxide therein is selectively absorbed by the first lean absorbing liquid. Is done. At this time, since the CO ₂ absorption tower 10 includes the shelf 12, the first lean absorbent can efficiently contact the mixed gas, and can sufficiently capture and absorb carbon dioxide in the mixed gas. The number of shelves 12 is not particularly limited, and may be determined as appropriate based on the absorption efficiency of the desired carbon dioxide into the first lean absorbent, the supply amount of the mixed gas to the CO ₂ absorber 10, and the like.

The unabsorbed gas that has not been absorbed by the first lean absorbent from the mixed gas is supplied to the CO ₂ absorption tower 50 via the tower top gas line L12 connected to the CO ₂ absorption tower 10 top. This unabsorbed gas contains most of hydrogen, methane and carbon monoxide contained in the mixed gas, a small amount of carbon dioxide not absorbed by the first lean absorbent, and a small amount of volatilized water. On the other hand, the first rich absorbent that is the first absorbent that has selectively absorbed carbon dioxide in the mixed gas flows out from the rich absorbent outflow line L < _b > ₂ connected to the bottom of the CO ₂ absorber 10. Most of the gas components absorbed in the first rich absorption liquid are carbon dioxide, but there are cases where methane, hydrogen, and carbon monoxide are included slightly.

In the CO ₂ absorber 10, the supply amount of the first lean absorbing fluid at the rate of alkanolamine supplied amount to carbon dioxide to be recovered (Kgmol- amine / kgmol-CO _2), When it is 25~100kgmol / kgmol preferable. When this ratio is below the lower limit, the carbon dioxide recovery rate tends to decrease. When the ratio is higher than the upper limit, the amount of membrane permeate in the hollow fiber membrane increases and is necessary for dissipating carbon dioxide. The membrane area tends to increase.

The bottom temperature of the CO ₂ absorption tower 10 is preferably 25 to 35 ° C., and the bottom pressure is preferably 1.8 to 10 atm. If the bottom temperature and pressure are higher than the above upper limit values, the carbon dioxide absorption efficiency decreases, and the evaporation of moisture increases, which tends to be uneconomical. When the bottom temperature and pressure are less than the above lower limits, the carbon dioxide absorption efficiency is improved, but the viewpoint of gas-liquid equilibrium in the subsequent stage of carbon dioxide emission and the decrease in membrane permeation due to the increase in liquid viscosity. It is not preferable from the viewpoint. The bottom temperature is adjusted by the heat exchanger T1, and the bottom pressure is adjusted by a gas compressor (not shown) installed in the front stage of the mixed gas supply line L1.

The first rich absorption liquid flowing out from the CO ₂ absorption tower 10 is preferably circulated in detail later through a valve V1 whose valve opening is controlled in accordance with the liquid level at the bottom of the CO ₂ absorption tower 10. The first rich absorption liquid flowing out from the bottom of the tank 40 joins and flows through the line L3. Next, the membrane L is supplied to the membrane unit 20 through the membrane unit supply line L4 connected to the bottom of the membrane unit 20 from the line L3.

  The membrane unit 20 includes one or two or more hollow fiber membrane modules 22 in a pressure vessel. The hollow fiber membrane module 22 may have the same mode as a known hollow fiber membrane module. For example, the hollow fiber membrane module 22 is formed by accommodating a plurality of hollow fiber membranes bundled in a cylindrical shape in a cylindrical pressure-resistant container having both ends opened and having an inner diameter slightly larger than the cylindrical outer diameter. Has been. In order to allow the first absorbent to permeate reliably from the inner diameter side to the outer diameter side of the membrane wall of the hollow fiber membrane, the bundle of hollow fiber membranes is blocked by portions other than the inner diameter side of the hollow fiber at both ends thereof. . Further, the hollow fiber membrane module 22 is installed in the pressure resistant tank in a state where the axial direction of the hollow fiber membrane is substantially matched with the vertical direction. When a plurality of the hollow fiber membrane modules 22 are installed, the hollow fiber membrane modules 22 may be connected in parallel to the flow of the absorbing liquid, or may be connected in series. In consideration of pressure loss in the hollow fiber membrane, the hollow fiber membrane module 22 is preferably installed in parallel with the flow of the absorbent from the viewpoint of increasing the throughput.

  The material of the hollow fiber membrane must have physical and chemical characteristics such as high heat resistance and chemical resistance under process conditions, and can be porous or fibrous so that the first absorbent can permeate the membrane wall. Thus, there is no particular limitation as long as the film wall has pores and carbon dioxide can be effectively dissipated from the permeated first absorption liquid. Examples of known hollow fiber membrane materials include alumina, silicon nitride, polyethersulfone, polysulfone, and polyethylene. Of these, alumina, polyethersulfone, or polyethylene is preferred because carbon dioxide is more efficiently dissipated from the first absorbent.

  The inner diameter of the hollow fiber membrane is preferably 0.5 mm to 10 mm. When the inner diameter falls below the lower limit, the pressure loss of the liquid feeding tends to increase. On the other hand, when the inner diameter is less than the upper limit, the volume of the hollow fiber membrane module tends to be too large.

  The film thickness of the hollow fiber membrane is preferably 0.2 mm to 2 mm. When the film thickness is below the lower limit, the pressure resistance tends to be inferior. Moreover, when the film thickness exceeds the above upper limit, the liquid permeability tends to be inferior.

  The pore diameter in the hollow fiber membrane is preferably 0.01 μm to 1 μm. If the pore diameter is below the lower limit, the liquid permeability tends to be poor. On the other hand, when the pore diameter is less than the above upper limit value, the amount of carbon dioxide diffused tends to decrease as the liquid permeation amount increases.

  The length of the hollow fiber membrane, the number of hollow fiber membranes in one hollow fiber membrane module 22, and the number of hollow fiber membrane modules 22 in the membrane unit 20 are the amount of the first rich absorption liquid supplied to the membrane unit 20 and What is necessary is just to determine suitably based on process conditions.

  The 1st rich absorption liquid supplied to the membrane unit 20 from the bottom part flows into the inner diameter side space from the lower opening part of the hollow fiber membrane, and flows upward. While flowing in the inner diameter side space of the hollow fiber membrane, the first rich absorbent gradually permeates from the inner diameter side to the outer diameter side of the membrane wall of the hollow fiber membrane, and simultaneously dissipates carbon dioxide. When the first rich absorption liquid permeates the membrane wall of the hollow fiber membrane, pressure loss occurs, and therefore the inner diameter side space on the upstream side in the hollow fiber membrane is higher than the outer diameter side space on the downstream side. Has pressure.

  The outer diameter side space of the hollow fiber membrane is adjusted to a pressure lower than atmospheric pressure by a vacuum compressor (not shown) described later. More specifically, the pressure in the outer diameter side space of the hollow fiber membrane is preferably 50 to 150 mmHg. When this pressure is less than 50 mmHg, the proportion of water accompanying the separated carbon dioxide increases remarkably, and the construction cost and power consumption of the vacuum compressor tend to increase. As the volume increases, the construction cost and operation cost of the entire apparatus tend to increase too much. When this pressure exceeds 150 mmHg, the carbon dioxide from the first absorbing liquid in the membrane unit is insufficiently dissipated, so that the amount of circulating absorbent liquid tends to increase remarkably. Although it is preferable that the pressure difference between the inner diameter side space and the outer diameter side space of the hollow fiber membrane is larger, it also depends on the limit value of the durable pressure of the hollow fiber membrane.

  The first absorbent after passing through the membrane wall of the hollow fiber membrane and releasing carbon dioxide, that is, the mixture of the regenerated first lean absorbent and the diffused carbon dioxide is the outer diameter side of the hollow fiber membrane. Flow through space. The space on the outer diameter side of the hollow fiber membrane leads to a mixture line L5 connected to the lower side of the decompression tank 30, and the mixture of the first lean absorbent and carbon dioxide enters the decompression tank 30 via the mixture line L5. Supplied. Further, part of the diffused carbon dioxide and volatilized water are supplied to the decompression tank 30 via the gas line L6 connected to the upper side surface of the decompression tank 30 from the outer diameter side space of the hollow fiber membrane. . Both the mixture line L5 and the gas line L6 are adjusted to a pressure lower than the atmospheric pressure. The carbon dioxide once released from the first absorbing liquid is difficult to be reabsorbed by the first absorbing liquid while flowing in the system adjusted to a pressure lower than atmospheric pressure.

  A part of the first rich absorption liquid that has flowed into the inner diameter side space of the hollow fiber membrane overflows from the upper opening of the hollow fiber membrane without passing through the membrane wall of the hollow fiber membrane, and passes through the overflow line L7. And is supplied to the circulation tank 40 connected to the overflow line L7. The first rich absorbent supplied to the circulation tank 40 is sent out via the circulation line valve V5 by the circulation line bottom pump P2 provided in the circulation line L11 connected to the bottom of the circulation tank 40, and rich. The first rich absorption liquid from the absorption liquid outflow line L2 joins and flows through the line L3.

The decompression tank 30 may be a known gas-liquid separator. A gas line L6 connected to the membrane unit 20 is connected to the upper part of the side surface, and a mixture line L5 connected to the membrane unit 20 is connected to the lower part of the side surface. A CO ₂ recovery line L9 is connected to the top of the decompression tank 30, and a lean absorbent supply line L10 is connected to the bottom. A vacuum compressor is provided in the middle of the CO ₂ recovery line L9. The lean absorbent supply line L10 is provided with a lean absorbent feed pump P1, a heat exchanger T1, and a valve V3 in this order from the decompression tank 30 side toward the CO ₂ absorption tower 10.

The mixture of the first lean absorbing liquid and the carbon dioxide supplied to the decompression tank 30 flows out from the lean absorbing liquid supply line L10 connected to the bottom of the decompression tank 30 and is connected to the top of the decompression tank 30. The carbon dioxide is separated by flowing out from the CO ₂ recovery line L9. The first lean absorbent at the bottom of the decompression tank 30 flows out to the lean absorbent supply line L10 by the operation of the lean absorbent feed pump P1. The first lean absorbing liquid that has passed through the lean absorbing liquid delivery pump P1 passes through the heat exchanger T1, is appropriately temperature-adjusted, and is then supplied to the CO ₂ absorption tower 10 via the valve V3. As the heat source of the heat exchanger T1, steam or a heating fluid of another process is used.

What is necessary is just to select suitably the kind of alkanolamine in a 1st lean absorption liquid according to the kind of gas component contained in mixed gas other than hydrogen and a carbon dioxide. A off-gas mixed gas is discharged from the PSA process of the hydrogen production device, if also containing methane and carbon monoxide, from the viewpoint of high selectively absorbing carbon dioxide in the CO ₂ absorber 10, MEA, DEA and Preferred is / or DIPA, more preferred is MEA and / or DEA. Further, from the viewpoint of carbon dioxide emission efficiency in the membrane unit 20, MEA and / or DEA are preferable, and DEA is more preferable. Considering the above comprehensively, the alkanolamine in the first lean absorbent is preferably MEA and / or DEA, more preferably DEA.

A small amount of water vapor flows into the CO ₂ recovery line L9 together with carbon dioxide. The discharged water vapor is condensed into water by a heat exchanger (not shown) as a condenser, and returned to the decompression tank 30 again in a liquid state from a reflux line (not shown). Further, a line (not shown) for supplying makeup water may be connected to the reflux line in order to supplement the water discharged from the system from the CO ₂ absorption tower 10 or the like.

  The pressure in the decompression tank 30 is adjusted to less than atmospheric pressure by a vacuum compressor. Since the decompression tank 30 does not include a reboiler, the pressure secures a suitable pressure in the outer diameter side space of the hollow fiber membrane, as well as the viewpoint of more reliably separating the first lean absorbent and carbon dioxide. From the viewpoint, it is preferably (50−α) mmHg to (150−α) mmHg. Here, α represents a pressure loss (unit: mmHg) from the outer diameter side space of the hollow fiber membrane to the decompression tank 30. α is set in consideration of the balance between construction cost and operation cost when designing the device.

The CO ₂ absorption tower 50 is a distillation tower similar to a conventional gas absorption tower, and may be a known plate tower provided with one or two or more stages, and an irregular packing or a regular packing in the tower. It may be a known packed distillation column packed with products and the like. The top side gas line L12 from the CO ₂ absorption tower 10 which is a supply line for unabsorbed gas is connected to the lower side surface. Unabsorbed gas is supplied to the CO ₂ absorption tower 50 from the supply line L12. The unabsorbed gas includes most of hydrogen, methane, and carbon monoxide contained in the mixed gas, a small amount of carbon dioxide that has not been absorbed by the first lean absorbent, and a small amount of volatilized water.

The upper side surface of the CO ₂ absorption tower 50 is supplied with a lean absorption liquid (hereinafter referred to as “second lean absorption liquid”) that is a second absorption liquid flowing out from the bottom of the stripping tower 60 described in detail later. The lean absorbent supply line L19 is connected. The second lean absorbent is an absorbent containing an alkanolamine typified by monoethanolamine (MEA), diethanolamine (DEA), diisopropanolamine (DIPA) and / or methyldiethanolamine (MDEA). In the second lean absorbent, alkanolamine is diluted with water. The concentration of the alkanolamine in the second lean absorbent is not particularly limited, but is preferably 25 to 100 kgmol / kgmol with respect to the total amount of alkanolamine and water from the viewpoint of economy and convenience.

The unabsorbed gas supplied to the bottom of the CO ₂ absorption tower 50 comes into contact with the second lean absorbent that falls from the upper side in the tower, and carbon dioxide therein selectively becomes the second lean absorbent. Absorbed. At this time, since the CO ₂ absorption tower 50 is provided with a shelf and / or a packing, the second lean absorption liquid can efficiently contact the unabsorbed gas, and sufficiently capture and absorb carbon dioxide in the unabsorbed gas. it can.

The gas that has not been absorbed by the second lean absorbent from the unabsorbed gas is discharged from the tower top gas line L20 connected to the top of the CO ₂ absorber 50. This gas contains most of the hydrogen, methane and carbon monoxide contained in the unabsorbed gas, very little carbon dioxide not absorbed by the second lean absorbent, and a small amount of volatilized water. . On the other hand, the second rich absorption liquid, which is the second absorption liquid that selectively absorbs carbon dioxide in the unabsorbed gas, flows out from the rich absorption liquid outflow line L14 connected to the bottom of the CO ₂ absorption tower 50. Most of the gas components absorbed in the second rich absorption liquid are carbon dioxide, but there are cases where methane, hydrogen, and carbon monoxide are included slightly.

The supply amount of the second lean absorption liquid in the CO ₂ absorption tower 50 is preferably 0.5 to 2.5% of the supply amount of the first lean absorption liquid in the CO ₂ absorption tower 10. If this supply amount is below the lower limit value, the construction cost of the device tends to be too high, and if it exceeds the upper limit value, the energy saving effect tends to be reduced.

The bottom temperature of the CO ₂ absorption tower 50 is preferably 40 to 50 ° C. Since the CO ₂ absorption tower 50 is directly connected to the CO ₂ absorption tower 10, the top pressure is preferably (1.6−β) atm to (10−β) atm. Β represents a pressure loss (unit: atm) from the CO ₂ absorption tower 10 to the CO ₂ absorption tower 50. By keeping the bottom temperature and the top pressure within these numerical ranges, it becomes possible to absorb carbon dioxide in the unabsorbed gas even more selectively. The bottom temperature is adjusted by the heat exchanger T3.

The second rich absorbing solution flowing out from the CO ₂ absorber 50, via a heat exchanger T2, it is supplied to stripping tower 60. The stripping tower 60 is a distillation tower similar to a conventional so-called gas regeneration tower, and may be a well-known plate tower having one or two or more plates, and an irregular packing or a regular packing in the column. It may be a known packed distillation column packed with the like. A rich absorbent outflow line L14 from the CO ₂ absorption tower 50, which is a second rich absorbent supply line, is connected to the upper side surface. The second rich absorption liquid is supplied to the diffusion tower 60 from the supply line L14.

  The second rich absorbing liquid supplied to the stripping tower 60 is heated in the tower to dissipate carbon dioxide. The heat treatment of the second rich absorbent is performed by passing the heat from the bottom of the stripping tower 60 to the heat exchanger T5 functioning as a reboiler and returning it to the tower through the reboiler line L18. The heat exchanger T5 is for heating the second absorption liquid, and uses steam or another process heating fluid as a heat source. The temperature in the stripping tower 60 is controlled by adjusting the flow rate and temperature of the heat source and the flow rate of the second absorption liquid in the heat exchanger T5.

Carbon dioxide and water vapor released from the second rich absorbent by heating are extracted from the tower top line L15. Among these, the water vapor is condensed into water by the condenser T4 and returned to the stripping tower 60 via the line L17. As a cooling method in the condenser T4, water cooling or air cooling is used. Carbon dioxide is recovered through the CO ₂ recovery line L16.

  In the stripping tower 60, by controlling the heating temperature in the tower, the carbon dioxide in the second rich absorbent can be sufficiently selectively diffused.

The second lean absorbing liquid at the bottom of the stripping tower 60 flows out to the lean absorbing liquid supply line L19 by the operation of the lean absorbing liquid delivery pump P3. The second lean absorbent that has passed through the lean absorbent feed pump P3 passes through the heat exchanger T2, is cooled to some extent, further passes through the heat exchanger T3, is temperature-adjusted, and then is supplied to the CO ₂ absorber 50. Is done. In the heat exchanger T2, the second rich absorption liquid is heated to the second lean absorption liquid, so that the heating amount in the heat exchanger T5 that functions as a reboiler can be reduced. As the refrigerant in the heat exchanger T3, water or a cooling fluid of another process is used.

  The bottom temperature of the stripping tower 60 is preferably 101 to 105 ° C. By keeping the bottom temperature within this numerical range, it is possible to sufficiently dissipate carbon dioxide while reducing the heating amount in the heat exchanger T5 as much as possible.

Next, a carbon dioxide gas separation method according to a preferred embodiment of the present invention will be described with reference to FIG. The carbon dioxide gas separation method of the present embodiment is such that, in the CO ₂ absorption tower 10, a mixed gas containing hydrogen and carbon dioxide is brought into contact with a first lean absorbing liquid containing alkanolamine, and the first lean absorbing liquid. In the first absorption step in which carbon dioxide is absorbed, and in the membrane unit 20, the first rich absorption liquid that has absorbed carbon dioxide is permeated through the membrane wall of the hollow fiber membrane, and the carbon dioxide is diffused from the first rich absorption liquid. In the decompression tank 30, a fractionation step in which carbon dioxide is separated from the mixture of the diffused carbon dioxide and the first lean absorbing liquid in the decompression tank 30, and a non-absorbed state in the CO ₂ absorption tower 50. A gas is brought into contact with a second lean absorbing solution containing alkanolamine, and a second absorbing step in which carbon dioxide is absorbed by the second lean absorbing solution; Te, and heating the second rich absorbing liquid having absorbed carbon dioxide, and a second stripping step of dissipating carbon dioxide from the second rich absorbing liquid.

In the first absorption step, the mixed gas supplied to the bottom of the CO ₂ absorber 10, is contacted with a first lean absorbent solution falling down from the upper side in the CO ₂ absorber 10, the carbon dioxide in the mixed gas Is absorbed into the absorbent. At this time, since the CO ₂ absorption tower 10 includes the shelf 12, the first lean absorbent can efficiently contact the mixed gas, and can sufficiently capture and absorb carbon dioxide in the mixed gas.

  Although the density | concentration of the alkanolamine in a 1st lean absorption liquid is not specifically limited, It is preferable in it being 12-60 mass%. When the concentration of alkanolamine is less than 12% by mass, the amount of carbon dioxide absorbed per system circulation amount of the absorbing liquid is small, and a large amount of circulation tends to be required. When the concentration of alkanolamine exceeds 60% by mass, the viscosity of the absorbing solution increases and the flow resistance when permeating through the hollow fiber membrane increases, so that the pressure difference tends to increase. In this case, in order to suppress an increase in pressure difference, it is necessary to increase the membrane area of the hollow fiber membrane.

In the first absorption step, the supply amount of the first lean absorbing fluid at the rate of alkanolamine supplied amount to carbon dioxide to be recovered (Kgmol- amine / kgmol-CO _2), preferably a 25~100kgmol / kgmol . When this ratio is below the lower limit, the carbon dioxide recovery rate tends to decrease. When the ratio is higher than the upper limit, the amount of membrane permeate in the hollow fiber membrane increases and is necessary for dissipating carbon dioxide. The membrane area tends to increase.

In the first absorption step, the bottom temperature of the CO ₂ absorption tower 10 is preferably 25 to 35 ° C., and the bottom pressure is preferably 1.8 to 10 atm. If the bottom temperature and pressure are higher than the above upper limit values, the carbon dioxide absorption efficiency decreases, and the evaporation of moisture increases, which tends to be uneconomical. When the bottom temperature and pressure are less than the above lower limits, the carbon dioxide absorption efficiency is improved, but the viewpoint of gas-liquid equilibrium in the first emission process at the latter stage and the viewpoint of the decrease in membrane permeation amount due to the increase in liquid viscosity. Is not preferable. The bottom temperature is adjusted by the heat exchanger T1, and the bottom pressure is adjusted by a gas compressor (not shown) installed in the front stage of the mixed gas supply line L1.

  In the first diffusion step, first, the first rich absorption liquid obtained through the first absorption step is supplied to the membrane unit 20 from the bottom, and from the lower opening of the hollow fiber membrane included in the membrane unit 20, It flows into the inner diameter side space and flows upward. The first rich absorbent gradually permeates from the inner diameter side to the outer diameter side of the membrane wall of the hollow fiber membrane while flowing in the inner diameter side space of the hollow fiber membrane, and at the same time dissipates carbon dioxide, It becomes an absorbing solution. When the first rich absorption liquid permeates the membrane wall of the hollow fiber membrane, pressure loss occurs, and therefore the inner diameter side space on the upstream side in the hollow fiber membrane is higher than the outer diameter side space on the downstream side. Has pressure. Further, the pressure in the outer diameter side space of the hollow fiber membrane is adjusted to a pressure lower than atmospheric pressure by a vacuum compressor.

  The pressure in the outer diameter side space of the hollow fiber membrane is preferably 50 to 150 mmHg. When this pressure is less than 50 mmHg, the proportion of water accompanying the separated carbon dioxide increases remarkably, and the construction cost and power consumption of the vacuum compressor tend to increase. As the volume increases, the construction cost and operation cost of the entire apparatus tend to increase too much. When this pressure exceeds 150 mmHg, the carbon dioxide from the first absorbing liquid in the membrane unit is insufficiently dissipated, so that the amount of circulating absorbent liquid tends to increase remarkably. Although it is preferable that the pressure difference between the inner diameter side space and the outer diameter side space of the hollow fiber membrane is larger, it also depends on the durability pressure of the hollow fiber membrane.

In the separation step, a mixture of the first lean absorbing liquid and carbon dioxide obtained through the first releasing step is supplied to the decompression tank 30, and the first from the lean absorbing liquid supply line L <b> 10 connected to the bottom of the decompression tank 30. together to efflux lean absorption liquid, by flowing out the carbon dioxide from the CO ₂ recovery line L9 that is connected to the top of the vacuum vessel 30, fractionating the carbon dioxide first lean absorption liquid. The 1st lean absorption liquid obtained through the classification process is used again in the above-mentioned 1st absorption process.

  The pressure in the decompression tank 30 is adjusted to less than atmospheric pressure by a vacuum compressor. Since the decompression tank 30 does not include a reboiler, the pressure secures a suitable pressure in the outer diameter side space of the hollow fiber membrane, as well as the viewpoint of more reliably separating the first lean absorbent and carbon dioxide. From the viewpoint, it is preferably (50−α) mmHg to (150−α) mmHg. Here, α represents a pressure loss (unit: mmHg) from the outer diameter side space of the hollow fiber membrane to the decompression tank 30. α is set in consideration of the balance between construction cost and operation cost when designing the device.

In the second absorption step, the unabsorbed gas supplied to the bottom of the CO ₂ absorber 50, is contacted with a second lean absorbent solution falling down from the upper side in the CO ₂ absorption tower 50, in the unabsorbed gas Carbon dioxide is selectively absorbed into the absorbent.

  The concentration of the alkanolamine in the second lean absorbent is not particularly limited, but is preferably 25 to 100 kgmol / kgmol with respect to the total amount of alkanolamine and water from the viewpoint of economy and convenience.

The supply amount of the second lean absorption liquid in the CO ₂ absorption tower 50 is preferably 0.5 to 2.5% of the supply amount of the first lean absorption liquid in the CO ₂ absorption tower 10. If this supply amount is below the lower limit value, the construction cost of the device tends to be too high, and if it exceeds the upper limit value, the energy saving effect tends to be reduced.

In the second absorption step, the bottom temperature of the CO ₂ absorption tower 50 is preferably 40 to 50 ° C. Since the CO ₂ absorption tower 50 is directly connected to the CO ₂ absorption tower 10, the top pressure is preferably (1.6−β) atm to (10−β) atm. Β represents a pressure loss (unit: atm) from the CO ₂ absorption tower 10 to the CO ₂ absorption tower 50. By keeping the bottom temperature and the top pressure within these numerical ranges, it becomes possible to absorb carbon dioxide in the unabsorbed gas even more selectively. The bottom temperature is adjusted by the heat exchanger T3.

  The second rich absorption liquid obtained through the second absorption step is appropriately preheated as necessary, and then supplied to the diffusion tower 60 and heated to a predetermined temperature in the second diffusion step, whereby carbon dioxide is heated. Dissipate. The second lean absorbing liquid obtained through the second diffusion step is cooled to a predetermined temperature as necessary, and then used again in the second absorption step described above.

  The bottom temperature of the stripping tower 60 is preferably 101 to 105 ° C. By keeping the bottom temperature within this numerical range, it is possible to sufficiently dissipate carbon dioxide while reducing the heating amount in the heat exchanger T5 as much as possible.

  Next, Table 1 shows representative examples (design values) of the main material balance and each process condition in this embodiment when the off-gas of the PSA process in the hydrogen production apparatus is used as a mixed gas and DEA is used as the alkanolamine. It is shown in 2. In this representative example, the recovery rate of carbon dioxide from the mixed gas (L1) is 80.0% by volume at L9, 19.6% by volume at L16, and 99.6% by volume when both are combined. Further, the recovery rate of carbon dioxide from the unabsorbed gas (L12) is 98.0% by volume in L9.

The amount of carbon dioxide in the mixed gas (L1) is 3080 Nm ³ / h, whereas the amount of carbon dioxide in the unabsorbed gas (L12) is 616 Nm ³ / h, which is about 1/5. In view of the fact that there is almost no need to heat the fluid in the system when the carbon dioxide is diffused and separated from the first rich absorbent, in this representative example, the conventional process in which carbon dioxide is diffused from the absorbent by heating. It can be inferred that the amount of energy used can be significantly reduced as compared with the case where the entire amount of the mixed gas is supplied. Moreover, as described above, the carbon dioxide recovery rate can be extremely high at 99.6% by volume.

According to the preferred embodiment of the present invention described above, the first rich absorption liquid that has absorbed carbon dioxide in the CO ₂ absorption tower 10 is first permeated through the membrane wall of the hollow fiber membrane in the membrane unit 20. Dissipate carbon dioxide from the rich absorbent. Further, the mixture of the second lean absorbent and carbon dioxide after the carbon dioxide is diffused is fractionated in the decompression tank 30. And by maintaining the process pressure from the outer diameter side space of the hollow fiber membrane to the pressure reducing tank 30 at a pressure lower than atmospheric pressure, it is sufficient that the carbon dioxide once released is reabsorbed by the first absorbent. To prevent. Thus, carbon dioxide can be sufficiently diffused and separated without heating the first rich absorbent. The factor that can sufficiently dissipate carbon dioxide from the first rich absorbent by reducing the pressure downstream of the hollow fiber membrane to less than atmospheric pressure has not been clarified in detail at present. However, the present inventors are concerned with the fine structure of the hollow fiber membrane (for example, the pore diameter) and the change in the absorption equilibrium between the carbon dioxide and the first absorbent due to the decompression to less than atmospheric pressure. I believe there is. However, the factor is not limited to this.

  In addition, unlike the conventional acidic gas absorption process in which carbon dioxide is diffused and separated from the rich absorbent at a time by heating the rich absorbent in a regeneration tower equipped with a reboiler, the first rich absorbent is There is no need to heat. Furthermore, even if the first rich absorbent is not heated, the carbon dioxide absorbed in the first lean absorbent is small. Therefore, the modification and deterioration accompanying heating of the first rich absorption liquid are extremely difficult to occur, and it is almost unnecessary to discharge the modified and deteriorated absorption liquid out of the system or introduce a new lean absorption liquid into the system. . In addition, since the deterioration of the absorbing solution is sufficiently suppressed, material corrosion in the piping equipment in which the absorbing solution circulates or stays is less likely to occur. As a result, maintenance costs and frequency associated with material corrosion of piping equipment can be reduced. Further, it is not necessary to select a high-grade material having high corrosion resistance as the material of the piping equipment, so that the capital investment can be kept low. Therefore, the carbon dioxide gas separation device 100 of the present embodiment is preferably used when carbon dioxide is separated from off-gas discharged from the PSA process of the hydrogen production device having a large throughput (processing amount).

Further, the carbon dioxide that could not be absorbed by the first lean absorbent in the CO ₂ absorber 10 is absorbed by the second lean absorber in the CO ₂ absorber 50 and further dissipated from the second rich absorber in the diffusion tower 60. By collecting the carbon dioxide, it is possible to recover carbon dioxide with a very high yield. In addition, most of the carbon dioxide in the mixed gas is absorbed and released on the first absorbent side and recovered. Therefore, it is possible to reduce the load of the absorbing liquid per off-gas processing amount in the entire apparatus with respect to the absorbing liquid used in the conventional off-gas recovery process. This means that the energy consumption accompanying the heating of the absorption liquid is reduced, and that it contributes to material corrosion of piping equipment in the system and reduction of the absorption liquid usage.

  The preferred embodiment of the present invention has been described above, but the present invention is not limited to the above embodiment. For example, in the above-described embodiment, the flow method of the rich absorbent in the membrane unit 20 is an upper flow method that flows into the inner diameter side space from the lower opening of the hollow fiber membrane and flows upward. On the contrary, it may be a downflow system that flows into the inner diameter side space from the upper opening and flows downward, or may be a counterflow system that combines the upper flow system and the downflow system. Good. However, in the downflow method and the counterflow method, the absorption liquid that flows downward may interfere with the flow of gas components such as the diffused carbon dioxide and volatile components, so that the upper where such a phenomenon is unlikely to occur. A flow system is preferred.

Further, the CO ₂ absorption tower 10 is a plate tower, but instead, a known packed distillation tower in which an irregular packing or a regular packing is packed in the tower may be used. This packed distillation column is useful when it is desired to reduce the pressure loss in the CO ₂ absorption column for the convenience of the process. However, if it is necessary to increase the processing amount, a plate tower is preferable.

  Furthermore, although the membrane unit 20 is provided independently in the above-described embodiment, a plurality of membrane units may be provided instead. As a result, when a defect occurs in a part of the hollow fiber membrane module, only the supply of the rich absorbent to the membrane unit provided with the hollow fiber membrane module is stopped, and the membrane unit is isolated from the carbon dioxide gas separation device. In addition, the rich absorbent can be supplied to other membrane units. Alternatively, when the carbon dioxide gas separation device is normally operated, a switching method of supplying the rich absorbent to only a part of the plurality of membrane units and switching the membrane unit to which the rich absorbent is supplied at regular time intervals. It may be adopted. Thereby, the hollow fiber membrane in the membrane unit to which the rich absorbent is not supplied can be dried. Since the hollow fiber membrane has the highest carbon dioxide emission efficiency immediately after the rich absorption liquid starts to flow from the dry state, the above-mentioned switching method is adopted, so that the carbon dioxide emission efficiency in the entire carbon dioxide gas separation device is achieved. Can be further increased.

  Also, a plurality of gas separation devices similar to the carbon dioxide gas separation device described above are connected in series, and the material of the hollow fiber membrane in each gas separation device is appropriately selected, or in place of the hollow fiber membrane. It is also possible to sequentially isolate gas components other than carbon dioxide in the mixed gas by using the separation membrane (planar membrane, spiral membrane, etc.). In addition to or in place of this, carbon dioxide in the mixed gas is similarly selected by appropriately selecting an alkanolamine in the absorbing solution, or by using an organic solvent capable of absorbing other gas components instead of alkanolamine. It is also possible to sequentially isolate other gas components.

  In the above-described embodiment, the space extending from the outer diameter side space of the hollow fiber membrane to the decompression tank 30 is adjusted to a pressure lower than the atmospheric pressure by a vacuum compressor. The pressure may be adjusted to less than atmospheric pressure.

  As another embodiment, the same means as the first absorbing means and the separating means may be used instead of the above-mentioned second absorbing means as means for diffusing and collecting carbon dioxide in the unabsorbed gas. Even if these means are used, the yield of carbon dioxide can be sufficiently increased as compared with the prior art.

It is a schematic flowchart of the carbon dioxide gas separation device concerning a suitable embodiment.

Explanation of symbols

10, 50 ... CO ₂ absorption tower, 12 ... tray, 20 ... membrane unit, 22 ... hollow fiber membrane module, 30 ... vacuum vessel, 40 ... circulation tank, 60 ... stripping column, 100 ... carbon dioxide gas separation unit, L1 -L20 ... line, V1-V3, V5 ... valve, P1-P3 ... pump, T1-T5 ... heat exchanger.

Claims (2)

A carbon dioxide gas separation device for separating carbon dioxide from a mixed gas containing hydrogen and carbon dioxide,
First absorbing means for bringing the mixed gas into contact with a first absorbing liquid containing alkanolamine, and absorbing the carbon dioxide in the first absorbing liquid;
First diffusion means for allowing carbon dioxide to be diffused from the first absorbent by allowing the first absorbent to absorb carbon dioxide to pass through the membrane wall of the hollow fiber membrane;
A separation means for separating the carbon dioxide from the mixture of the first absorbing liquid and the carbon dioxide from which carbon dioxide has been released;
The second absorption in which the gas that has not been absorbed by the first absorption liquid in the mixed gas is brought into contact with the second absorption liquid containing alkanolamine, and carbon dioxide is absorbed by the second absorption liquid. Means,
A second diffusion means for heating the second absorption liquid that has absorbed carbon dioxide to diffuse carbon dioxide from the second absorption liquid;
With
A carbon dioxide gas separation apparatus, wherein the system pressure from the downstream side of the hollow fiber membrane to the separation means in the first diffusion means is less than atmospheric pressure. A carbon dioxide gas separation method for separating carbon dioxide from a mixed gas containing hydrogen and carbon dioxide,
A first absorption step in which the mixed gas is brought into contact with a first absorption liquid containing alkanolamine, and carbon dioxide is absorbed by the first absorption liquid;
The first absorbing liquid that has absorbed carbon dioxide is permeated through the membrane wall of the hollow fiber membrane whose downstream system pressure is adjusted to be lower than atmospheric pressure, and carbon dioxide is diffused from the first absorbing liquid. A first diffusion step to be performed;
A separation step of separating the carbon dioxide from the mixture of the first absorbent and the carbon dioxide that released carbon dioxide under a pressure lower than atmospheric pressure;
The second absorption in which the gas that has not been absorbed by the first absorption liquid in the mixed gas is brought into contact with the second absorption liquid containing alkanolamine, and carbon dioxide is absorbed by the second absorption liquid. Process,
A second emission step of heating the second absorption liquid that has absorbed carbon dioxide to dissipate carbon dioxide from the second absorption liquid;
A carbon dioxide gas separation method comprising:

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