JPH05228326A - Method for recovering dilute carbon dioxide - Google Patents

Abstract

PURPOSE:To recover concd. CO2 at a high recovery rate by returning a gas leaving the parallel-flow purging stage of the second-stage PSA-II process to the second-stage adsorption stage or countercurrent pressure restoring stage. CONSTITUTION:An adsorption tower 16b with the CO2 adsorption zone moved backward after the adsorption stage is switched to a parallel-flow purging stage by opening valves 20b and 21b, and a highly concentrated CO2 from a product holder 27 is passed through a passage 19 and the valve 20b and then concurrently through the adsorption tower 16b. Consequently, gaseous nitrogen staying in the tower is purged and discharged outside the tower through the valve 21b and a passage 22, the discharged gas having a considerably high content of CO2 is returned directly before a blower 14 and introduced into the adsorption tower 16b in the second adsorption stage to recover CO2. An adsorption tower 16c after the second adsorption stage is switched to a reduced- pressure recovery stage, and CO2 is desorbed and stored in the product holder 27. A part of the CO. is taken out, and a part is returned to the adsorption tower 16c to purge the tower.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for recovering CO ₂ which is a basic substance in the chemical industry by a pressure swing adsorption method (hereinafter referred to as PSA method).

[0002]

2. Description of the Related Art FIG. 12 is a flow sheet of an apparatus for concentrating CO ₂ by a conventional two-column pressure swing adsorption method (hereinafter referred to as PSA-I method). The adsorption tower 36 has Na
Adsorption tower 3 in the adsorption step, which is filled with a CO ₂ adsorbent 35 such as -X zeolite (SiO ₂ / Al ₂ O ₃ = 2.7)
The valves 34a and 37a of 6a are opened. The CO ₂ -containing gas 31 is pressurized from 1 atm to 3 atm by the blower 32, is introduced into the adsorption tower 36a via the flow path 33 and the valve 34a, and adsorbs CO ₂ to remove the hardly adsorbed component gas from the valve 3.
7a, flows out of the system through the flow path 38. (Adsorption step) In the adsorption tower 36b in which the CO ₂ adsorption zone has moved to the vicinity of the outlet of the tower and the adsorption step has been completed, the valve 39b is opened to reduce the pressure inside the tower to a predetermined level by the vacuum pump 40, and then the valve 42b is turned on. By opening the flow path 38 in the adsorption process.
A part of the hardly adsorbed component gas flowing out from the pressure reducing valve 41,
It is introduced into the adsorption tower 36b through the valve 42b to carry out countercurrent purging, and CO ₂ is desorbed and recovered from the adsorbent 35.
(Decompression countercurrent purge process)

C of the gas recovered in the reduced pressure countercurrent purging process
The O ₂ concentration C ₂ is expressed by the following equation, where G _{CO2 is} the amount of CO ₂ adsorbed, G _{COADS is} the amount of the hardly adsorbed component gas remaining in the adsorption tower, and Gp is the countercurrent purge gas amount. C ₂ = G _CO2 / (G _CO2 + G _COADS + Gp) ... According to the test by the present inventors, the co-adsorbed nitrogen is CO ₂
Since the partial pressure is reduced in the desorption process, the required amount of Gp is S
Since 0.4, which is considerably smaller than α = 1.2 proposed by the Karsstrom rule, is good, it is expressed by the following equation. Gp = α (Pd / Pa) Go (however, α ≧ 0.4) ... Also, when the ratio of the CO ₂ amount G _CO2 and the co-adsorbed component amount G _COADS is the selectivity β, it is expressed by the following equation. It β = G _CO2 / G _COADS ... And CO ₂ concentration in the raw material gas is C _0, and G _CO2 =
Assuming Go · C ₀ , the CO ₂ concentration C ₂ in the recovered gas is
From the above equations, the following equation can be obtained. C ₂ = 1 / [1+ (1 / β) + (αPd / C ₀ Pa)] As is clear from this equation, CO ₂ is regenerated as the selectivity β and the adsorption pressure Pa increase. The smaller the pressure Pd, the higher the concentration rate.

In the method of FIG. 12 (PSA-I method),
As a CO ₂ adsorbent, Na-X zeolite (SiO ₂ /
When Al ₂ O ₃ = 2.7) is used, C ₀ is 10 vol.
% = Β = 1.5, Pa = 1.2 atm, Pd =
If it is 0.2 atm, C ₂ will be about 42 vol%. In this method, when the CO ₂ recovery rate is set to near 100%, the CO ₂ concentration of the raw material gas suitable for the treatment is 40 vol.
% Is a relatively low concentration gas.

As a method suitable for treating a high-concentration gas having a CO ₂ concentration of more than 40 vol% as a raw material gas, there is a four-column pressure swing adsorption method (hereinafter referred to as PSA-II method) shown in FIG. The four adsorption towers 56 are filled with Na—X zeolite (SiO ₂ / Al ₂ O ₃ = 2.7) 55, and the adsorption tower 56a in the adsorption step has a valve 54a.
The valve 57a is opened, and the raw material gas 51 containing a high concentration of CO ₂ is compressed by the blower 52 from 1 atm to 3 atm, and the adsorption tower 56 is passed through the flow passage 53 and the valve 54a.
is supplied to a and adsorbs CO ₂ and releases the hardly adsorbed component gas to the outside of the system via the valve 57a and the flow path 58. CO ₂
The adsorption process is terminated when the adsorption zone has moved to the rear of the tower.

In the adsorption tower 56b which has completed the adsorption process, the valves 60b and 62b are opened, and the high-concentration CO ₂ -containing gas recovered in the next reduced pressure recovery process is passed from the product holder 66 through the flow path 59 and the valve 60b. Is introduced into the adsorption tower 56b, the hardly adsorbed component remaining in the tower is cocurrently purged, and is discharged from the system through the valve 62b and the flow passage 63. In the adsorption tower 56c after completion of the co-current purging step, the valve 64c is opened, the pressure inside the tower is reduced to 0.05 to 0.3 atm by the vacuum pump 65, CO ₂ is desorbed from the adsorbent 55, and high concentration of CO
_The gas containing ₂ is stored in the product holder 66. Then, a part thereof is sampled from the flow channel 67 as a product gas. The adsorption tower 56d which has completed the regeneration of the adsorbent 55 in the reduced pressure recovery step has reached the maximum vacuum degree, and the valve 69d is opened to allow the raw material gas 51 to flow through the flow passage 68 and the valve 69d. Introduced into and returned to atmospheric pressure.

Here, when the amount of gas collected by the vacuum pump is Go and the amount of gas used in the co-current purging step is Gp, the purge rate R (%) is defined as follows. R = (Gp / Go) × 100 If the CO ₂ concentration of the source gas is 65 vol% and the purge rates are 55%, 65%, and 80%, the CO ₂ concentration of the product gas is 95 vol%. , 99vol
%, 99.9 vol%. Thus, PSA-
Method II is a method with an extremely high concentration rate in which the product concentration reaches a maximum of 99.9 vol%. However, the recovery rate is 40-7
Blind to 0%, and the CO ₂ concentration of the inlet gas falls below 40 vol% it is impossible to maintain the PSA performance for not performed countercurrent purge Skarstrom type. Table 1 shows a comparative evaluation of the above PSA-I method and PSA-II method.
become that way.

[0008]

[Table 1]

[0009]

The above-mentioned PSA-I method can obtain a very high recovery rate in a low concentration range, but the product has a low CO ₂ concentration, and is desorbed when it is carried out on the high concentration side. The amount of gas increases, it takes time to reach the maximum vacuum degree, and desorption is sufficiently performed by this operation, so even if countercurrent purging is performed, it is not very effective. On the other hand, the PSA-II method can obtain a product with a very high CO ₂ concentration in the high concentration range, but the recovery rate is low, and when it is carried out on the low concentration side, the countercurrent purge is not adopted. The reproduction rate is low,
Requires a large amount of adsorbent. Then, the amount of product gas required for the cocurrent purge cannot be prepared. Thus, P
40 vol% even if SA-I method or PSA-II method is adopted
It was difficult to obtain a high-concentration gas of 90 vol% or more with a high recovery rate from the following low-concentration CO ₂ -containing gas. Therefore, the present invention solves the above-mentioned drawbacks and makes it possible to obtain a high concentration gas of 90 vol% or more with a high recovery rate from a low concentration CO ₂ containing gas of 40 vol% or less as a raw material.
It is intended to provide a method for recovering CO ₂ using the method.

[0010]

The present invention provides a method for recovering CO ₂ from a low-concentration CO ₂ -containing gas of 40 vol% or less by using an adsorption tower packed with a CO ₂ adsorbent in two stages. In the first stage adsorption tower, (1) an adsorption step of supplying the above-mentioned gas at a relatively low temperature and high pressure to adsorb CO ₂ and collecting the accompanying hardly adsorbed gas from the rear part of the tower, and (2) adsorption After the end of the process, the pressure was reduced from the front part of the adsorption tower, and then a part of the hardly adsorbed gas was introduced in a counterflow to adjust the CO ₂ concentration to 40
CO ₂ is continuously collected by alternately switching the step of reducing the volume to more than ol% and collecting the CO _2. Then, in the second-stage adsorption column, (3) the volume-reduced concentrated CO ₂ -containing gas is collected. An adsorption step of supplying CO ₂ at a relatively low temperature and high pressure to adsorb CO ₂ and collecting the accompanying hardly adsorbed gas from the rear part of the tower;
(4) A high-concentration CO ₂ -containing gas is passed in a cocurrent flow from the front part of the second-stage adsorption column after the adsorption step to release the hardly adsorbed gas remaining in the column to the outside of the column. Flow purging step, (5) decompression recovery step of decompressing the highly concentrated CO ₂ -containing gas from the front part of the second-stage adsorption tower after completion of the co-current purging step, and (6) decompression recovery step The CO ₂ gas having a high concentration is continuously recovered by alternately switching the step of flowing the gas countercurrently to the adsorption tower after the completion to restore the pressure, and the parallel flow of the above (4) of the second stage adsorption tower. The gas flowing from the purging step is adsorbed in the above (3) or the above (6)
The CO ₂ recovery method by the pressure swing adsorption method is characterized by returning to the countercurrent re-pressurization step. In the above method, the gas flowing from the adsorption step (3) of the second stage adsorption tower is returned to the adsorption step (1) of the first stage adsorption tower to the extent that it does not cause air pollution. It is preferable to facilitate the direct release to the atmosphere by reducing the CO ₂ concentration.

[0011]

The present invention organically combines the above-mentioned PSA-I method and PSA-II method in two steps to obtain 40 vo
The high concentration CO ₂ gas can be recovered from the low concentration CO ₂ containing gas of 1% or less at a high recovery rate.
That is, it is a drawback of the conventional PSA-II method by returning the gas flowing from the co-current purging step of the second PSA-II method to the adsorption step or the countercurrent recompression step of the second step. By improving the low recovery rate and returning the gas flowing from the adsorption step of the second stage PSA-II method to the first stage adsorption step, the recovery rate is further improved and the gas is released to the outside of the system. The CO ₂ concentration in the gas is further reduced to enable disposal into the atmosphere.

[0012]

EXAMPLE Using the PSA apparatus shown in FIG. 1, CO ₂ 1
C from raw material gas consisting of 0 vol% and 90 vol% nitrogen
O ₂ was concentrated to 99 vol%. The first two adsorption towers 6 are each filled with 500 kg of CO ₂ adsorbent 5, the valves 4a and 7a of the adsorption tower 6a in the adsorption step are opened, and the raw material gas 1 is blown by compressed into 5 atm, the flow path 3, adsorbed CO ₂ is introduced into the adsorption tower 6a past valve 4a, the valve 7a, to recover the flame adsorbable nitrogen gas through the flow path 8. The CO ₂ adsorption zone is the adsorption tower 6
The adsorption process was completed when the film was moved to the rear part of a.

In the adsorption tower 6b which has completed the adsorption step, the valves 9b and 12b are opened to communicate with the vacuum pump 10 and a part of the nitrogen gas recovered in the adsorption step is flowed while the pressure is reduced to 38 to 380 Torr. It was introduced into the adsorption tower 6b through the passage 8, the pressure reducing valve 11 and the valve 12b, and was subjected to countercurrent purging to desorb CO ₂ from the adsorbent 5 and to collect it from the valve 9b, the vacuum pump 10 and the passage 13. The CO ₂ concentration in the recovered gas was set to be 40 vol% or higher. In the adsorption tower 6b having completed the countercurrent purging step, only the valve 4b was opened to introduce the raw material gas, and the atmospheric pressure was restored.

The four adsorption towers 16 of the second stage are each filled with 250 kg of CO ₂ adsorbent 5, and the gas recovered in the depressurizing countercurrent purging step of the first stage is blown from the flow passage 13 to the blower 1.
4 and compress to 1-5 atm. The valves 15a and 17a of the adsorption tower 16a in the adsorption step are opened, and the recovered gas is introduced into the adsorption tower 16a via the valve 15a to introduce C
O ₂ was adsorbed and the hardly adsorbed nitrogen gas was recovered through the valve 17 a and the flow path 18. Since this recovered gas has a higher CO ₂ concentration than the gas flowing from the first-stage adsorption step,
It cannot be released directly into the atmosphere. Therefore, this recovered gas is returned immediately before the blower 2 via the flow path 18 and introduced into the adsorption tower 6a in the adsorption step of the first stage, whereby CO ₂ is adsorbed and separated to make the CO ₂ concentration extremely low. Nitrogen gas was recovered from the flow path 8 and discharged into the atmosphere except for the amount used for the countercurrent purging in the first stage.

The CO ₂ adsorption zone moves to the rear part of the adsorption tower,
The adsorption tower 16b that has completed the adsorption step shifts to the cocurrent purge step by opening the valve 20b and the valve 21b, and CO ₂ highly concentrated from the product holder 27 is passed through the flow path 19 and the valve 20b. By flowing in parallel to 16b, the nitrogen gas staying in the tower is purged and discharged outside the tower through the valve 21b and the flow path 22. Since this released gas has a considerably high CO ₂ concentration, it was returned immediately before the blower 14 and introduced into the adsorption tower 16a in the second stage adsorption step to recover CO ₂ .

Adsorption tower 16c which has completed the co-current purging process
Shifts to the reduced pressure recovery step by opening the valve 25c, and the CO ₂ adsorbed on the adsorbent 5 is desorbed and recovered by suctioning to a high vacuum of the regeneration pressure by the vacuum pump 26,
Stored in product holder 27. Part of the stored CO ₂ is taken out of the system from the channel 28 as a product, and
A part of them was returned to the adsorption tower 16c in the above parallel flow purging step and used for purging.

The adsorption tower 16d which has completed the reduced pressure recovery step is
By opening the valve 24d, the countercurrent decompression process is started, and a part of the gas released from the adsorption tower 16b in the cocurrent purge step is introduced into the adsorption tower 16d through the flow path 22 and the flow control valve 23. Then, the pressure was restored to prepare for the next adsorption step. The sequence of the first-stage adsorption tower during this period was as shown in FIG. 2, and the sequence of the second-stage adsorption tower was as shown in FIG. The unit of time required for each step is seconds.

In order to select the optimum adsorbent, the CO ₂ adsorbents shown in Table 2 were used, a gas with a CO ₂ concentration of 10 vol% and nitrogen of 90% was used as the raw material, and the adsorption pressure of the first stage adsorption tower was adjusted. 1.2 atm, adsorption temperature 50 ° C, regeneration pressure 0.2
Atm, the purge rate α is 40%, the cycle time is 5 minutes, the first stage adsorption operation is performed, and the CO ₂ concentration (v
ol%) and the treatment capacity (Nm ³ / Ton) of the raw material gas converted into an adsorbent of 1 Ton are shown in Table 2. As is clear from Table 2, in the Na—X zeolite CO ₂ adsorbent, the conventional C having an SiO ₂ / Al ₂ O ₃ ratio of 2.7 or more was used.
It can be seen that, as compared with the O ₂ adsorbent, the X-type one having the lowest practical value of 2.5 is superior. Also, 30mo of Na
Ca-X zeolite (SiO) with 1% or more exchanged for Ca
It can be seen that ₂ / Al ₂ O ₃ = 2.7) is superior to that having a low Ca exchange rate.

[0019]

[Table 2]

Therefore, as a CO ₂ adsorbent for the first stage adsorption tower, Na-X zeolite (SiO ₂ / Al ₂ O ₃ = 2.
5) is used as a CO ₂ adsorbent in the second stage adsorption tower, and Ca-X zeolite (SiO ₂ / Al ₂ O) having a Ca exchange rate of 60% is used.
₃ = 2.5) was used to evaluate the entire system as follows.

With respect to the first-stage adsorption tower, the adsorption pressure was 1.
2 atm, regeneration pressure 0.2 atm, CO ₂ concentration of raw material gas in the first stage adsorption tower 10 vol%, purge rate 40%
And the adsorption temperature is changed to change the CO in the recovered gas of the first stage.
_Two concentrations (vol%) were measured, and the relationship between the adsorption temperature and the CO ₂ concentration is shown in FIG. The Na-X adsorbent is 25 to 12
While it is applicable to a relatively low temperature range of 0 ° C, the above Ca
It can be seen that the -X adsorbent is applicable to a relatively high temperature range of 40 to 200 ° C.

Regarding the first-stage adsorption tower, the adsorption temperature is set to 50 ° C. among the above-mentioned conditions, and CO _{2 in} the inlet gas of the first stage is set.
CO _{2 in} the recovery gas of the first stage when changing the concentration
FIG. 5 shows the measured concentrations for comparison. When the inlet gas having a CO ₂ concentration of 8 vol% is used, the CO ₂ concentration in the recovered gas of the first stage reaches 40 vol% and is 40 vol%.
When using a% inlet gas, the CO ₂ concentration in the first stage recovered gas reached 80 vol%.

Regarding the first stage adsorption tower, the adsorption temperature is 50 ° C., the purge rate is 40%, the CO ₂ concentration in the inlet gas of the first stage is 10 vol%, and the regeneration pressure of the first stage is CO _{2 in} the recovery gas of the first stage when changing the
FIG. 6 shows the measured concentrations for comparison. The higher the ultimate vacuum pressure is, the smaller the amount of purge gas can be reduced, and theoretically, a purge at 1 Torr or less can be considered. However, considering the efficiency of the vacuum pump and the leak of the valve,
The lower limit is about 30 Torr.

Regarding the first stage adsorption tower, of the above conditions, the regeneration pressure is 0.2 atm, the adsorption temperature is 50 ° C., the purge rate is 40%, and the CO ₂ concentration in the first stage inlet gas is 10 v.
FIG. 7 shows that the CO ₂ concentration in the recovered gas in the first stage was measured and compared with the adsorption pressure in the first stage changed with the ol%. It is also possible to improve the recovery concentration by reducing the purge gas amount as the adsorption pressure rises. However, when the partial pressure of CO ₂ exceeds 1 atm, the amount of adsorption tends to saturate, so 4
atm is the upper limit. In order to save energy, it is preferable to operate at an adsorption pressure of 1.05 to 1.3 atm, which corresponds to the pressure loss of the adsorption tower.

Regarding the second stage adsorption tower, the adsorption pressure is 1.2 atm, the regeneration pressure is 0.2 atm,
Adsorption temperature is 50 ° C, CO ₂ concentration at the second stage inlet is 55 vo
If the second stage adsorption / separation is carried out in the same manner as above with 1%, the relationship between the purge rate and the CO ₂ concentration of the second stage recovered gas is shown in FIG.
And the purge rates of 60%, 70%, and 85% for both Na-X and Ca-X, and C of the recovered gas of the second stage.
O ₂ concentration is 95 vol%, 99 vol%, 99.9vo
reached 1%.

Regarding the second stage adsorption tower, the adsorption pressure is 1.2 atm, the regeneration pressure is 0.2 atm,
The adsorption temperature was fixed at 50 ° C, the purge rate was fixed at 65%, the CO ₂ concentration at the inlet of the second stage was changed, and the CO ₂ concentration of the recovered gas at the second stage was changed.
_The measurement of the _two concentrations is as shown in FIG. 9, and it can be seen that when the CO ₂ concentration at the inlet of the second stage exceeds 40 vol%, the CO ₂ concentration of the recovered gas of the second stage also exceeds 90 vol%.

Regarding the second-stage adsorption tower, the adsorption pressure was 1.2 atm, the adsorption temperature was 50 ° C., the purge rate was 70%, and the CO ₂ concentration at the second-stage inlet was 55 v.
It was set to ol%, the regeneration pressure was changed, and the CO ₂ concentration of the recovered gas in the second stage was measured.
The higher the regeneration pressure, the higher the CO
_{2 The} concentration increases, but if it is less than 0.05 atm, the concentration increase slows down, and the capacity of the vacuum pump increases, which is not economical.

Regarding the second stage adsorption tower, the regeneration pressure was 0.2 atm, the adsorption temperature was 55 ° C., the purge rate was 70%, the CO ₂ concentration at the second stage inlet was 55 vol%, and the adsorption pressure was When the CO ₂ concentration of the second-stage recovered gas was measured, it was as shown in FIG. 11. The higher the adsorption pressure, the higher the CO ₂ concentration of the second-stage recovered gas, but above 3 atm. And it is not economical from the power consumption of the blower.

(Example 1) After grasping the above tendency, CO ₂ was recovered under the following operating conditions, and the CO ₂ concentration of the recovered gas in the second stage adsorption tower and the CO ₂ recovery rate were compared. In case I, the outflow gas from the adsorption step of the second-stage adsorption tower is returned to the previous stage of the blower of the first-stage adsorption tower and introduced into the adsorption step together with the raw material gas, and the second-stage co-current purge is performed. The effluent gas of the process was countercurrently supplied to the adsorption tower after the decompression recovery process of the second-stage adsorption tower was completed to restore the pressure. In case II,
The effluent gas from the adsorption step of the second stage adsorption tower and the effluent gas of the second stage co-current purging step were directly discharged to the outside of the system.

First-stage adsorption tower Adsorbent Na-X (silica / alumina = 2.5) Adsorption pressure 1.2 atm Regeneration pressure 0.2 atm Countercurrent purge ratio 40% Adsorption temperature 50 ° C. CO ₂ concentration of inlet gas 10 vol% outlet of the CO ₂ concentration of 2 vol% recovered gas in the gas CO ₂ concentration 43 vol% second stage adsorption tower adsorbent Na-X (silica / alumina = 2.5) adsorption pressure 1.2atm regeneration pressure 0.2atm cocurrent purge rate 75 % adsorption temperature 50 ° C. inlet CO ₂ concentration 99 vol% of CO ₂ concentration 43 vol% recovered gas in the gas (case I) the CO ₂ concentration 95 vol% of the recovered gas (case II) recovery of overall CO ₂ are cases I It is 95%, whereas Case II is 40%, which shows that Case I is extremely effective.

(Example 2) CO ₂ was recovered under the conditions of Example 1, and in case I, the outflow gas from the adsorption step of the second stage adsorption tower was returned to the stage before the blower of the first stage adsorption tower. Was introduced into the adsorption step together with the raw material gas, and the outflow gas from the co-current purging step of the second-stage adsorption tower was returned to the front stage of the blower of the second-stage adsorption tower and introduced into the second-stage adsorption step. In Case II, the outflow gas from the adsorption process of the second stage adsorption tower and the outflow gas of the second stage co-current purging process were directly discharged to the outside of the system. The CO ₂ concentration in the recovered gas of the second stage adsorption tower is case I
In case II, the total CO ₂ recovery rate is 90%, whereas in case II, it is 40%, and case I is extremely effective. I know there is.

[0032]

The present invention uses the adsorption tower in two stages,
The effluent gas of the adsorption step of the two-stage adsorption tower is returned to the first-stage adsorption step, and the effluent gas of the co-current purging step of the second-stage adsorption tower is returned to the countercurrent recompression step or adsorption step of the second-stage adsorption tower. By flowing, it has become possible to recover a high concentration of CO ₂ at a high recovery rate, which has the advantages of the conventional countercurrent purge method and the parallel flow purge method.

[Brief description of drawings]

FIG. 1 is a flow sheet of an apparatus for carrying out the PSA method of the present invention.

FIG. 2 is a diagram showing a sequence of a first-stage adsorption tower in an example.

FIG. 3 is a diagram showing the sequence of the second-stage adsorption tower in the examples.

FIG. 4 is a graph showing the relationship between the adsorption temperature and the CO ₂ concentration of the recovered gas in the first stage adsorption tower in the examples.

[FIG. 5] In Example, C of the inlet gas of the first-stage adsorption tower
It is a graph showing the relationship between the O ₂ concentration and the CO ₂ concentration of the recovered gas of the first stage adsorption tower.

FIG. 6 shows the regeneration pressure of the first-stage adsorption column in the example,
CO in the recovered gas of the first stage adsorption tower ₂Showed the relationship with concentration
It is a graph.

FIG. 7 shows the adsorption pressure of the first-stage adsorption tower in Examples,
CO in the recovered gas of the first stage adsorption tower ₂Showed the relationship with concentration
It is a graph.

FIG. 8 shows the purge rate of the second-stage adsorption tower in Examples,
CO of recovered gas in the second stage adsorption tower ₂Showed the relationship with concentration
It is a graph.

FIG. 9 is a graph showing the relationship between the hydrogen sulfide concentration of the inlet gas of the second-stage adsorption tower and the CO ₂ concentration of the recovered gas of the second-stage adsorption tower in the examples.

FIG. 10 shows the regeneration pressure of the second-stage adsorption tower in the examples.
And CO of the recovered gas of the second stage adsorption tower ₂Show relationship with concentration
It is a graph.

FIG. 11 is an adsorption pressure of the second-stage adsorption tower in Examples.
And CO of the recovered gas of the second stage adsorption tower ₂Show relationship with concentration
It is a graph.

FIG. 12 is a flow sheet of an apparatus for performing a conventional countercurrent purging method.

FIG. 13 is a flow sheet of an apparatus for performing a conventional co-current purging method.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kazuaki Oshima 1-1 No. 1 Atsunoura, Nagasaki City, Nagasaki Prefecture Mitsubishi Heavy Industries, Ltd. Nagasaki Shipyard (72) Inventor Hiroshi Nohara 1-1 No. 1 Atsunoura, Nagasaki City, Nagasaki Prefecture Mitsubishi Heavy Industries, Ltd. Nagasaki Shipyard (72) Inventor Kiichiro Ogawa 2-5-1, Marunouchi, Chiyoda-ku, Tokyo Sanryo Heavy Industries Co., Ltd. (72) Hideo Nawata 2-5-1 Marunouchi, Chiyoda-ku, Tokyo Sanryo Heavy Industries Within

Claims (2)

[Claims] 1. A method for recovering CO ₂ from a low-concentration CO ₂ -containing gas of 40 vol% or less by using an adsorption tower packed with a CO ₂ adsorbent in two stages, wherein (1) is used in the first-stage adsorption tower. The above gas is supplied at a relatively low temperature and high pressure to produce CO ₂
Adsorption step of adsorbing the adsorbed gas and collecting the accompanying hardly adsorbed gas from the rear part of the tower, and (2) decompressing from the front part of the adsorption tower after the adsorption step, and then counterflowing a part of the hardly adsorbed gas. CO ₂ concentration is reduced to 40 vol% or more for concentration and recovery, and CO ₂ is continuously recovered, and then in the second-stage adsorption tower, (3) above-mentioned volume reduction is carried out. An adsorption step of supplying a concentrated CO ₂ -containing gas at a relatively low temperature and a high pressure to adsorb CO ₂ and collecting the accompanying hardly adsorbed gas from the rear part of the tower, and (4) after the adsorption step Second
Co-current purging step of flowing a highly concentrated CO ₂ -containing gas in a co-current direction from the front part of the adsorption tower and discharging the hardly adsorbed gas remaining in the tower to the outside of the tower, (5) co-current flow After the purging process is completed, a pressure reduction recovery process is performed in which pressure is reduced from the front part of the second-stage adsorption tower to recover highly concentrated CO ₂ -containing gas; And the step of re-pressurizing are alternately switched to continuously produce high-concentration CO ₂
And the gas flowing through from the parallel flow purging step (4) of the second stage adsorption tower is collected,
Alternatively, the method of recovering CO ₂ by the pressure swing adsorption method is characterized by returning to the countercurrent recompression step of (6) above. 2. The gas flowing from the adsorption step (3) of the second stage adsorption tower is returned to the adsorption step (1) of the first stage adsorption tower. CO ₂ recovery method.