GB2155805A

GB2155805A - Gas separation process and apparatus

Info

Publication number: GB2155805A
Application number: GB08505860A
Authority: GB
Inventors: Shigeo Matshui; Yasuto Suzu; Masahiko Kumagaya; Shuichi Kawagoe
Original assignee: Osaka Oxygen Industries Ltd
Current assignee: Osaka Oxygen Industries Ltd
Priority date: 1984-03-07
Filing date: 1985-03-07
Publication date: 1985-10-02
Also published as: JPS60191002A; AU583497B2; GB8505860D0; AU3956385A; GB2155805B

Abstract

A pressure swing adsorption process for separating a product gas comprising less and more readily adsorbable constituents employs four adsorption columns. The following sequence of steps is performed repeatedly. 1. A chosen column is pressurised with feed gas. 2. Less readily adsorbable gas is taken from the column and used to pressurise another column. 3. Product gas is collected from the column. 4. The pressure in the column is reduced to atmospheric pressure. 5. The column is evacuated. 6. The column is purged with gas from another column. 7. The column is pressurised with product gas produced by another column. This sequence of steps is followed by each column in appropriate time relationship with the other columns. A pressure equalisation step may for example be performed between steps 3 and 4. The process can operate with a low feed gas pressure. Flue gases can be treated to separate hydrogen and another gas, eg methane, oxygen, nitrogen, carbon monoxide and carbon dioxide.

Description

SPECIFICATION Gas separation process and apparatus The present invention relates to a gas separation process and apparatus, for example a method is provided for the simultaneous recovery of hydrogen and one component other than hydrogen from flue gases by the pressure swing adsorption (PSA) method. The flue gases that can be treated by the method of the present invention are mixed gases which contain at least hydrogen gas, such as those discharged from petro-chemical plants, reduction furnaces and annealing furnaces. The present invention also relates to a method that removes impurities from the gas evolved furing the fermentation of organic wastes and which increases the concentration of the first less-adsorbable component, thereby making it possible to use the combination of H2 and the recovered component as a valuable mixed gas.

The PSA method has long been used to separate only the first less-adsorbable gas, or H2, from a gaseous mixture by adsorption and various techniques have so far been proposed, such as those described in Japanese Patent Publication Nos. 25969/1963, 208/1964, 284/1968 and 122951198. These prior art methods use high adsorption pressures and require a great amount of energy to compress the mixed gas feed. In order to recover the used energy, a complicated process is necessary but this leads to an intricate process operation and the use of expensive equipment. The practice that has recently gained commercial acceptance comprises first removing impurities from a multi-component gas and, then, concentrating the remainder to a useful two-component or multi-component gas, instead of mixing the recovered single-- component gas with another useful gas.For example, reduction furnaces use hydrogen and nitrogen gases separated from ammonia decomposition gases (H2, N2, NH, and H2O). Gases that evolve either from the fermentation or organic wastes or at sewage treatment plants consist of H2, CH4, CO2, etc., and after removing the impurities only the useful gases (H2 and CH4) are concentrated for another use.

As already mentioned, many inventions have been made to separate a single component from a feed gas containing trace impurities. However, few attempts have been made to separate two components from multi-component gases such as flue gases by adsorption. A principal object of the present invention is to provide a method of recovering two or more useful gases at low cost with simple equipment-and using a smaller amount of energy to compress the mixed feed gas.

Before recovering the simple-component gases, the feed gas is cleaned of any impurities such as NH2, H2S, COS and water vapour.

The recovery of single-component gases is generally performed with an adsorbent at high pressures.

The recovery of the product gas in a high concentration can be ensured by ceasing the supply of the feed gas when the gases other than the product gas have reached a breakthrough point. In beds filled with commercial adsorbents, a weakly adsorbable gas, or a less adsorbable gas moves faster toward the exit end of the bed than a strongly adsorbable, or easily adsorbable gas. Adsorption equipment operating on the PSA method is commercially used in the purification of H2, production of 02 and N2 by air separation, and concentration/recovery of hydrocarbons. In these operations, steps of adsorption, evacuation, desorption and purging are successively repeated, and two or more units of adsorption bed are employed to yield of a product gas having a concentration of 95 vol % or more.One great disadvantage of these operations is that they incur huge initial and running costs because of the need to use complicated equipment and procedures.

In accordance with the method of the present invention, a gas that contains H2 and at least one useful component selected from among 02, CO, C02 and CH4 and which optionally contains trace amounts of impurities such as NH, and H2S is used as the feed, from which a concentrated gas containing at least 50 vol %, preferably at least 70 vol %, of the sum of H2 and one other useful gas is produced.

Illustrative feed gases are flue gases from reduction furnace (50 H2, 42% N2, 5% CO and 2% COJ and gases evolving from the fermentation of organic wastes (70% CO2, 27.7% CH4, 2% N2 abd 0.3% 02). Either gas contains trace levels of impurities such as NH2, COS, H2S and H2O. The adsorbability of the respective components varies with the adsorbent used, but generally, it increases in the following order: HE < H2 < 02 < N2 < CH4 < CO < CO2 < C2H2 Reduction gas composed of hydrogen and nitrogen is used for reducing iron oxide in a reduction furnace in an iron-making works.When the reduction gas is used for reducing iron oxide, the waste gas containing hydrogen, nitrogen, carbon monoxide and carbon dioxide is discharged from reduction furnace. In the prior art, the waste gas has been used for combustion. Thus, the present invention may be used to provide a process for removing CO and CO2 from the waste. The gas from which CO and CO2 are removed can be used for reducing iron oxide.

Another use of the present invention is to provide a process for recovering hydrogen and/or methane from fermentation gas.

According to the present invention, there is provided a process for separating a product gas comprising less readily and more readily adsorbable constituents from a feed gas including said constituents by pressure swing adsorption employing at least four adsorption columns containing suitable adsorbent for effecting said adsorption, comprising performing in sequence: (i) a feed gas pressurisation step in which a chosen adsorption column is pressurised with the feed gas.

(ii) A "first adsorption" step in which feed gas is passed into the said adsorption column and a gas comprising the less readily adsorbable gas flows out, at least part of which less readily adsorbable gas is used to pressurise another column.

(iii) A "second adsorption" step in which feed gas is passed into the said adsorption column and a gas mixture comprising the less readily adsorbable gas and the more readily adsorbable gas flows out, is collected and is used to form the product gas.

(iv) a pressure reduction step, in which the pressure in the chosen column is reduced to atmospheric pressure (or a pressure close to atmospheric pressure).

(v) an evacuation step in which the chosen column is evacuated to desorb the gas remaining in the column.

(vi) a purge step in which the chosen column is placed in communication with at least one other column so as to purge the chosen column with gas from said other column.

(vii) a "product gas" pressurisation step in which the chosen column is placed in communication with another column in which a "first adsorption" step is being performed so as to pressurise the chosen column with less adsorbable gas, and then repeating the sequence of steps, wherein the sequence of steps is followed by each column in appropriate time relationship with the other columns.

In one example of a process according to the invention, between the steps (iii) and (iv), a pressure equalisation step is performed in which the chosen column is in communication with another column which is simultaneously undergoing a "product gas" pressurisation step so as to depressurise the chosen column and pressurise the other column and thus bring the pressures in the two columns nearer to equality with one another, and during the "product gas pressurisation" step the chosen column is also in communication with a column undergoing such a pressure equalisation step whereby gas flows into the chosen column from the other column and the pressures in the two columns are thereby brought nearer to equality.If desired, during an early stage of the pressure equalisation step, gas is vented from the chosen column in a direction countercurrent to the flow of gas out of the chosen column into said other column. In this example of the process, during the pressure reduction step, gas withdrawn from the column is used to purge another column. (This example of the process is subsequently described herein in more detail and is referred to as the "second embodiment" of the process).

In a modification to the above example of a process according to the invention, between the second adsorption step and the pressure equalisation step, a "third adsorption" step is performed in which feed gas is passed into the chosen column and a gas mixture flows out of the column, at least part of which gas is used to purge another column. (This modified example of the process is subsequently described herein in more detail and is referred to as the "first embodiment" of the process).

In a yet further example of the process according to the invention the third adsorption step is performed between the second adsorption step and the pressure reduction step of the operating cycle. In this last example, no pressure equalisation steps are performed and the gas from the pressure reduction step is typically not used to purge another column but is instead vented to atmosphere.In this last example flow of gas out of the chosen column takes place in the opposite direction to passage of gas into the column during the "adsorption" steps, whereas in the other examples the direction of flow is the same as in the adsorption steps. (The last example of the process is subsequently described herein in more detail and is referred to as the "third embodiment" of the process.) Typically, in these examples of processes according to the invention, in the first and third adsorption steps another part of the gas mixture leaving the chosen column is collected and used to form product gas.

Examples of processes according to the invention will now be described in further detail with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram illustrating one apparatus suitable for performing the process according to the invention.

Figures 2, 4 and 6 are schematic representations of respectively first, second and third embodiments of the process according to the invention; and Figures 3, 5 and 7 are diagrams illustrating the schedule of operations that needs to be followed to perform the cycles shown in Figures 2, 4 and 6 respectively.

Referring to Figure 1 of the accompanying drawings, valves 1A, 2A, 3A, 5A, 6A, 7A, 8A, 9A, 10A, 1 IA, 1 B, 2B, 3B, 5B, 6B, 7B, 8B, 9B, 10B, 11B, 1 C, 2C, 3C, 5C, 6C, 7C, 8C, 9C, 1 OC, 11C, 1 D, 2D, 3D, 5D, 6D, 7D, 8D, 9D, 10D and 11D are on-off valves.

Valves 4, 12, 13 and 14 are valves being capable of adjusting the amount of gas flowed through the valve. A tank for product gas is shown at 15 and a vacuum pump is shown at 16. Figure 1 is designed for carrying out the following first embodiment. The second and third embodiments can be carried out in the apparatus shown in Figure 1. Not all the valves given in Figure 1 are necessary for carrying out the second and third embodiments. Other apparatuses may be used for carrying out the second and third embodiments. Apparatuses with 5 or more columns can be used in the present invention.

By the term "purge" in the specification and the claims is meant that purging step is effected while evacuating the column by a vacuum pump or other means. The term "adsorption column" signifies a vessel containing a bed of adsorbent.

The adsorbents employed in the practice of this invention may be selected from natural or synthetic zeolites, molecular sieves, activated alumina, activated carbon, silica gel and the like. In general, activated alumina or silica gel may be placed at the bottom of each adsorption column, and activated carbon or zeolite may be placed on the alumina or silica gel. Three layers of adsorbent comprising a first layer of activated alumina or silica gel, a second layer of activated carbon, and a third layer of zeolite may also be used.

The three embodiments of the process described below include the following process steps: STEP ( PRESSURISATION BY FEED GAS) In this step the feed gas is introduced into an adsorption column to increase pressure in the column.

The adsorption pressure may be in the range of 0.05 - 5 bar, and preferably 0.5 - 2 bar. (Although Step (i) is not termed an adsorption step herein, it is to be appreciated that adsorption takes place during this step).

STEP (il) (ADSORPTION (I)) In this step, the feed gas is introduced into the column to recover hydrogen, this step is continued until the second less adsorbable gas is withdrawn from the column.

Part of the hydrogen gas (or other less readily adsorbable constituents) withdrawn in this step is used for pressurisation by product of the other column. The ratio of amount of gas withdrawn as product gas to amount of gas introduced into the other column can be adjusted by valve 13.

STEP (iii) (ADSORPTION rip)) In this step, the feed gas is introduced into the column to recover hydrogen gas (or other less readily adsorbable gas) and second, more readily adsorbable gas. Product gas withdrawn in this step is introduced into tank for product gas.

STEP (IV) (ADSORPTION {111)) In this step, the feed gas is introduced into the column. The product gas (hydrogen gas and second less adsorbable gas) withdrawn in this step is used for purging step of the other column. This step is optional and is omitted in the second embodiment.

STEP (V) FDEPRESSURISA TION BY EOUALISA TIONI In this step, the column, in which adsorption was previously completed, is connected to the column, in which purge (II) was previously completed. The pressure in both the columns become equal or approximately equal. In this step, the gas remaining in the column may countercurrently be discharged from the other exit of the column. This step is optional and is omitted in the third embodiment.

STEP (Vl) (DEPRESSURIZA TION) In this step, the pressure in the column is reduced to one atmospheric pressure or a pressure close to it. The gas withdrawn in this step may be used for a purging step of another column in the first two embodiments and it flows out of the top of the chosen column for this purpose, while in the third embodiment, gas is vented to atmosphere from the bottom of the column.

STEP (VIZ) {EVACUATION) In this step, the column is evacuated to a pressure below one atmospheric pressure by means of vacuum pump, blower, ejector or the like. It is preferable that the column is evacuated to a pressure less than 300 Torr, more preferably to 50 - 150 Torr.

STEP (VIII) (PURGE) In this step, in the first two embodiments, the column is purged with the gas withdrawn in depressurisation step of another column. In the first embodiment, the column may further be purged with the product gas (comprising hydrogen gas (or other less readily adsorbable gas) and more readily adsorbable gas) withdrawn in the third adsorption step of another column. In the third embodiment, this purging with product gas is performed instead of the step of purging with gas withdrawn in a depressurisation step of another column, In this step, the gas remaining in the column is replaced with the purge gas.

STEP IIXI (PRESSURISA TION BY PRODUCT GAS) In this step, the chosen column is pressurised by introducing product gas (e.g. hydrogen gas) withdrawn from another column. The gas withdrawn from depressurisation by equalisation of a further column may be introduced into the chosen column in the first and third embodiments.

Referring now to Figures 2 to 7 of the accompanying drawings, in the cycles illustrated in Figures 2 and 6, that is the first and third embodiments of the process according to the invention, the above mentioned process steps are performed in a sequence of twelve operating steps, and in the cycle illustrated in Figure 4, that is the second embodiment of the process according to the invention, the process steps are performed in eight operating steps. The sequence of valve operations for the cycles illustrated in Figures 2, 4 and 6 are shown in Figures 3, 5 and 7 respectively.

In Figures 2, 4 and 6 the arrows indicate the direction of flow gas.

The invention is further illustrated by the following examples. All percentages are by volume in these examples.

EXAMPLE I A reduction furnace flue gas having the composition shown below was purified by a process which consisted of the following cycles; pressurisation of the feed gas, adsorption (I) adsorption (II), adsorption (III), depressurisation by equalisation, depressurisation, evacuation, purging, pressurisation by product gas/pressurisation by equalisation. Four steel adsorption beds (16B x 3.0 m) were used and each was packed with 30 kg of activated alumina for water adsorption (the alumina forming the bottom most layer), 30 kg of activated carbon and 150 kg of zeolite (Zeoherb ZE-502, product of Osaka Sanso Kogyo, Ltd).

PROCESS CONDITIONS: Flue gas composition: H2 = 65.5% N2 = 32% CO = 0.5% CO2 = 2.0% N H4 = 0.05% NH, = 600 ppm COS = 200 ppm Adsorption pressure: 3.0 bar Ultimate pressure in evacuation: 100 Torr Purge pressure: 120 Torr A product gas consisting of 80% H2 and 20% N2 was recovered in a volume of 52 m3 from 1 m3 of the feed gas. The recovery of H2 gas was 63.5%. The product gas contained 0.2% CO + CO2, 1 ppm of NH3 and zi ppm of COS. Five cubic metres of a purge gas were used.

EXAMPLE 2 An organic waste gas having the composition shown below was purified by a process which consisted of the following cycle; pressurisation of the feed gas, adsorption (I), adsorption (II), pressure equalisation, depressurisation, evacuation, purge, and pressurisation by product gas/pressurisation by equalisation.

Four adsorption beds were used and each was packed with a lower activated alumina layer for water adsorption and an upper activated carbon layer. The concentration of CH4 in the feed gas was increased and CO2 was removed by adsorption for the purpose of increasing the heat value of the gas.

CH4 = 27.7% CO2 = 70.0% 02 = 0.3% N2 = 2.0% Because of the adsorption selectivity of 02 ;; N2 < CH4 < CO2, in the feed gas passing through the adsorp tion beds was retained in the beds whereas 02, N2 and CH4 passed through the beds. By ceasing the supply of the feed gas before the breakthrough of CO2, a product gas consisting of CH4, and CO2 was obtained.

The product gas in the reservoir downstream of the adsorption beds had the following composition.

COMPONENTS AMOUNTS (%) CH4 60 CO2 35 02 0.7 N2 4.3 After completion of the adsorption steps, CO2 in the beds was purged by evacuation in combination with the blowing of part of the product gas a a purge gas.

As will be understood from the foregoing description, the method of the present invention will contribute greatly to the saving of natural resources and energy by recycling various flue and waste gases as a useful fuel containing H2 and/or another useful gas.

It can be appreciated from the foregoing examples that the process according to the present invention may be employed to separate a product gas comprising more and less readily adsorbable consistuents from a feed gas comprising these constituents, wherein the mole ratio of the less adsorbable gas to the more readily adsorbable gas in the product gas is greater than it is in the feed gas.

Claims

1. A process for separating a product gas comprising less readily and more readily adsorbable constituents from a feed gas including said constituents by pressure swing adsorption employing at least four adsorption columns containing suitable adsorbent for effecting said adsorption, comprising performing in sequence: (i) a feed gas pressurisation step in which a chosen adsorption column is pressurised with the feed gas.

(ii) A "first adsorption" step in which feed gas is passed into the said adsorption column and a gas comprising the less readily adsorbable gas flows out at least part of which less readily adsorbable gas is used to pressurise another column.

(iii) A "second adsorption" step inwhich feed gas is passed into the said adsorption column and a gas mixture comprising the less readily adsorbable gas and the more readily adsorbable gas flows out, is collected and is used to form the product gas.

2. A process as claimed in claim 1, in which between the steps (iii) and (iv) a pressure equalisation step is performed in which the chosen column is in communication with another column which is simultaneously undergoing a "product gas" pressurisation step so as to depressurise the chosen column and pressurise the other column and thus bring the pressures in the two columns nearer to equality with one another, and during the "product gas" pressurisation step the chosen column is also in communication with a column undergoing such a pressure equalisation step whereby gas flows into the chosen column from the other column and the pressures in the two columns are thereby brought nearer to equality with one another.

3. A process as claimed in claim 2, in which during an early stage of the pressure equalisation step, gas is vented from the chosen column in a direction countercurrent to the flow of gas out of the chosen column into said other column.

4. A process as claimed in claim 2 or claim 3, wherein during the pressure reduction step, gas withdrawn from the column is used to purge another column.

5. A process as claimed in any one of claims 2 to 4, in which, between the second adsorption step and the pressure equalisation step, a "third adsorption" step is performed in which feed gas is passed into the chosen column and a gas mixture flows out of the column at least part of which gas is used to purge another column.

6. A process as claimed in claim 5, in which in the third adsorption step another part of the gas mixture that flows out of the column is collected and is used to form the product gas.

7. A process as claimed in claim 1, in which between the second adsorption step and the pressure reduction step a "third adsorption" step is performed in which feed gas is passed into the chosen column and a gas mixture flows out of the column, at least part of which gas mixture is used to purge another column.

8. A process as claimed in claim 7, in which in the third adsorption step another part of the gas mixture that flows out of the column is collected and is used to form the product gas.

9. A process as claimed in any one of the preceding claims, in which an adsorption pressure of 0.05 to 5 bar (gauge) is created by the feed gas pressurisation step.

10. A process as claimed in claim 9 in which the adsorption pressure is in the range 0.05 to 2 bar (gauge).

11. A process as claimed in any one of the preceding claims, in which in the evacuation step the pressure in the chosen column is reduced to less than 300 Torr.

12. A process as claimed in claim 11, in which the pressure is reduced to a pressure in the range 50 to 100 Torr.

13. A process as claimed in any one of the preceding claims, in which the less readily adsorbable gas is hydrogen.

14. A process as claimed in claim 13, in which the more readily adsorbable gas is one or more of methane, oxygen, nitrogen, carbon monoxide and carbon dioxide.

15. A process as claimed in any one of claims 1 to 12, in which the less readily adsorbable gas is methane.

16. A process as claimed in claim 15, in which the more readily adsorbable gas is one or both of carbon monoxide and carbon dioxide.

17. A process as claimed in any one of the preceding steps, in which another part of the less readily adsorbable constituent that flows out of the adsorption column during the first adsorption step is collected and is used to form product gas.

18. A process as claimed in claim 13 or claim 14, in which the feed gas is of the kind set out in Example 1.

19. A process as claimed in claim 15 or claim 16, in which the feed gas is of the kind set out in Example 2.

20. A process for separating a product gas from a feed gas substantially as set out in Figure 2, Figure 4 or Figure 6 of the accompanying drawings.

21. Apparatus for separating a product gas from a feed gas, comprising at least four adsorption columns, means for evacuating the columns, and a plurality of valves and pipes arranged so as to enable a process as claimed in any one of the preceding claims to be performed.

22. Apparatus for separating a product gas from a feed gas, substantially as described herein with reference to Figure 1 of the accompanying drawings, and having its valves arranged to be operated in accordance with the schedule set out in Figure 3, Figure 5 or Figure 7 of the accompanying drawings.

23. A gas separation process having any novel feature or novel combination of features described herein.

24. A process for separating hydrogen gas and a useful gas or a second less adsorbable gas other than hydrogen gas (hereinunder one or two of the hydrogen gas and the second less adsorbable gas to be separated is referred to as product gas) from a feed gas containing hydrogen gas and the second less adsorbable gas through pressure swing adsorption by using at least four adsorption columns containing an adsorbent capable of selective adsorption to gases other than product gas (hereinunder the gas adsorbed in the adsorbent is referred to as easily adsorbable gas) which comprises; (i) a step of pressurising an adsorption column by the feed gas, in which the step (x) was previously completed;; (ii) a step of introducing the feed gas into the adsorption column, in which step (i) was previously completed, to recover hydrogen gas, part of the hydrogen gas obtained in this step being used in step (x) of the other adsorption column (hereinunder this step is referred to as adsorption (I); (iii) a step of introducing the feed gas into the adsorption column, in which step (ii) was previously completed, to recover hydrogen gas and the second less adsorbable gas (hereinunder this step is referred to as adsorption (ill);; (iv) optionally a step of connecting the adsorption column, in which step (iii) was previously completed, to the other adsorption column, in which step (viii) was previously completed, and introducing the feed gas into the former adsorption column, and part of the gas withdrawn from the former adsorption column being used in purging step of the latter column.

(v) optionally a step of connecting the adsorption column, in which step (iv) was previously completed, to the other adsorption column, in which step (ix) was previously completed, to concurrently depressurise the former column and pressurising the latter column until near pressure equalisation of the two column is reached, (in the earlier time of this step, the gas remaining in the former may countercurrently be removed simultaneously out of the adsorption columns); (vi) a step of reducing the pressure in the adsorption column, in which step (v) was previously completed, to one atmospheric pressure or a pressure close to it, the gas withdrawn from the column optionally being used in step (viii) of the other column; (vii) a step of evacuating the adsorption column, in which step (vi) was previously completed to desorb the gas remaining in the column;; (viii) optionally a step of connecting the adsorption column, in which step (vii) was previously completed, to the other column, in which step (v) was previously completed, to purge the former column; (ix) in the event that step (iv) is performed a step of connecting the adsorption column, in which step (viii) was previously completed, to the other adsorption column, in which step (iii) was previously completed, to purge the former column; and (x) a step of connecting the adsorption column, in which step (ix) was previously completed to the other adsorption column, in which step (ii) is being carried out, to introduce part of the gas withdrawn from the latter column into the former column; and at the same time connecting the adsorption column, in which step (ix) was previously completed, to the other column, in which step (iv) was previously completed, to introduce the gas withdrawn from the latter column into the former column, Periodically switching the flow between or among said adsorption column so as to repeat the above steps in the columns.