GB2090161A

GB2090161A - Process an apparatus for separating a mixed gas such as air

Info

Publication number: GB2090161A
Application number: GB8137079A
Authority: GB
Original assignee: Mitsubishi Heavy Industries Ltd
Current assignee: Mitsubishi Heavy Industries Ltd
Priority date: 1980-12-26
Filing date: 1981-12-09
Publication date: 1982-07-07
Also published as: KR830008536A; GB2090161B; FR2497120A1; DE3150688A1; FR2497120B1; KR860000190B1

Abstract

The invention relates to a process and apparatus for separating mixed gas by adsorption in which adsorption and desorption steps are repeatedly carried out in a plurality of adsorption towers in staggered timings, wherein to commence a desorption step, mixed gas delivered from an adsorption tower which has completed adsorption during a primary pressure-reducing period in said adsorption tower, is fed to another adsorption tower which has been raised in pressure up to an intermediate pressure between the adsorption pressure and the desorption pressure, after termination of the desorption step, by an unadsorbate gas delivered as a result of low-pressure adsorption and/or mixed gas delivered during the primary pressure-reducing period, to carry out a low-pressure adsorption step in which the adsorbate constituent gas in said mixed gas is made to be adsorbed, whereby a pressurized mixed gas having a low adsorbate constituent concentration which is delivered during the primary pressure-reducing period can be effectively utilized.

Description

SPECIFICATION Process and apparatus for separating a mixed gas such as air The present invention relates to improvements in a process and apparatus for separating a mixed gas such as air by adsorption.

The composition ratio of an N2 gas to an O, gas in air is, as is well known, about 4:1, and the partial pressure ratio of the N2 gas to the 02 gas is also about 4:1. The amount of gas adsorbed by an adsorbent will vary in proportion to these partial pressures of the gases. For instance, when making an adsorbent adsorb an 02 gas in air under an air pressure of 5 ata, the partial pressure of the 02 gas is about 1 ata. If this air pressure of 5 ata is reduced to 1 ata, then the partial pressure of 02 gas is lowered to about 0.2 ata and most of the o2 gas adsorbed by the adsorbent would be desorbed.

However, in adsorption-separation apparatus of a practical size, by merely-lowering the air pressure to 1 ata then O2 gas adsorbed by the adsorbent will not be desorbed. The reason is because when the air pressure in the adsorption tower is lowered to 1 ata, the 02 gas is desorbed from the adsorbent, hence the partial pressure of the O2 gas around the adsorbent becomes high, but the 02 gas partial pressure around the adsorbent is not lowered since the flow rate of the air flowing through the adsorption tower is too small, therefore the 02 gas stays around the adsorbent, thus desorption of the 02 gas does not proceed, and the 02 gas is kept adsorbed by the adsorbent.

One known form of adsorption-separation apparatus for air which can resolve the abovementioned problem will now be described with reference to Figure 1. Thus, raw air is fed via a main feed pipe a to an air compressor b, which may be provided with an after-cooler, drain separator, etc. for cooling and dehumidifying the compressed air, which would normally be at a high-temperature, and if riecessary, also provided with a refrigerator, or the like. Downstream of the compressor, the main feed pipe ais split into two branch pipes C1, C2.Cavity gas discharge pipes d1, e, and d2, e2 are provided in juxtaposition to respective feed pipes c, and c2, and valve fr to f6 are provided in the above-mentioned respective pipes C1, d1, ea c2, d2 and e2. A main discharge pipe section g communicates with the discharge pipes d1und d2, and connects to a vacuum pump h, from which a main discharge pipe section i extends. Also a main discharge pipe section j communicates with the discharge pipes ea and e2. A common header pipe k is provided for respective pipes cX, d, and e1, whilst a common header pipe / is provided for the respective pipes c2, d2 and e2.The header pipes connect into respective adsorption towers m and n filled with adsorbents, designated mr and n1, respectively. N2 gas delivery pipes o and p having respective valves q and r provided therein lead from respective adsorption towers m and n to a common delivery pipe section s which leads to a tank t to which a main delivery pipe n is connected.

The adsorption towers m and n are arranged to alternately carry out an adsorption step and a desorption step. Assuming now that the adsorption tower m has entered an adsorption step, then the valves fa q are opened and the valves 2, f3 are closed. Consequently, compressed raw air from the air compressor b is fed through the main feed pipe a, feed pipe cr, valve f1 and header pipe k to the adsorption tower m, and when the raw air passes through the adsorption tower m, the 02 gas (an adsorbate constituent gas) in the raw air is adsorbed by the adsorbent m1, while an N2 gas (an unadsorbate constituent gas) is condensed and delivered through the delivery pipe o, valve q and pipe delivery section s to the tank t and is further extracted externally of the system through the main delivery pipe u as a product. In addition, when the adsorption tower m enters an adsorption step as described above, the other adsorption tower n enters a desorption step, and so, the valve f, is opened, while the valves 4, f, and r are closed. Therefore, residual air around the adsorbent nr is discharged externally of the system through the header pipe I, valve 6, discharge pipe e2 and discharge pipe sectionj. Subsequently, the valve f, is closed and the valve f5 is opened.Hence, the residual air around the adsorbent n, is sucked away and discharged externally of the system through the header piDe /, valve 5, discharge pipe 2, discharge pipe section a, vacuum pump h and discharge pipe section i, so that the pressure in the adsorption tower n is lowered to the partial pressure of the 02 gas upon adsorption, hence the 02 gas being adsorbed by the adsorbent n, is desorbed, and thus the adsorbent n, can be re-activated.

in the above-described adsorption-separation apparatus for air, although the above-referred problem can be resolved, there was a shortcoming in that pressurized gas having a high O2 concentration is discharged externally of the system through the main discharge pipe sectionsj, i without being utilized.

Various methods for effectively utilizing this pressurized gas have been proposed, and one example of the proposed methods is illustrated in Figure 2. In the adsorption-separation apparatus shown in Figure 2, a part of the N2 gas which has been separated and condensed during an adsorption step, is fed through a flow rate regulating valve w into an adsorption tower which has finished a pressure-reducing step, thus by lowering the partial pressure of the residual air air around an adsorbent, O2 gas adsorbed by the adsorbent is desorbed and the desorbed 02 gas is scavenged and discharged to the discharge pipe g and, while a vacuum pump can be omitted, a part of the N2 gas which has been separated and condensed with much labour and expense would be consumed in the adsorption tower during a desorption step. Moreover, since the gas discharged through the discharge pipe g is not utilized, in the case of extracting N gas as a product there was a problem that the cost of the product is increased.

The present invention deals with the above-mentioned problem, and relates to process and apparatus for separating a mixed gas by adsorption, of the kind in which an adsorption step of feeding raw gas into an adsorption tower filled with an adsorbent to make said adsorbent adsorb an adsorbate constituent gas and extracting an unadsorbate constituent gas from said adsorption tower, and a desorption step of reducing the pressure in said adsorption tower to make said adsorbent desorb the adsorbate constituent gas adsorbed thereby and recovering the adsorbate constituent gas, are repeatedly carried out in that sequence in a plurality of adsorption towers in staggered timings.

In accordance with the invention, such process and apparatus are characterised by having features as set out in the appended patent Claims.

In order that the present invention well be readily understood and the various features thereof made apparent a number of embodiments will now be described, with reference to the accompanying drawings in which Figures 1 and 2 are block diagrams of two forms of conventional adsorption-separation apparatus for carrying out the process for separating air by adsorption, and Figures 3 to 7 are block diagrams of various embodiments of adsorption-separation apparatus in accordance with the invention for separating air by the adsorption process.

Referring to Figure 3 in a first embodiment of adsorption-separation apparatus for practising the process, 1 designates a main feed pipe for raw air, 2 designates an air compressor for compressing and feeding raw air, and this air compressor 2 is provided with an after-cooler, a drain separator, etc. for cooling and dehumidifying the compressed high-temperature air, and if necessary, it can also be provided with a refrigerator or the like. In addition, 3a, 3b, 3c designate raw air feed pipes branched from the above-referred main feed pipe 1. Cavity gas pipes 4a, 4b and 4c are provided in juxtaposition to the feed pipes 3a, 3b and 3c.Also cavity gas discharge pipes Sa, Sb and 5c are provided in juxtaposition to the cavity gas discharge pipes 4a, 4b and 4c, and cavity discharge pipes 6a, 6b and 6c are provided in juxtaposition to the cavity gas discharge pipes Sa, Sb and 5c. A common cavity gas discharge pipe 7 communicates with the above-referred discharge pipes 4a, 4b and 4c, whilst a cavity gas main discharge pipe 8 communicates with the above-referred to discharge pipes Sa, Sb and 5c.The discharge pipe 8 is provided in series with a flow rate regulating valve 9, a surge tank 10 and a valve 11, and is connected into a cavity gas main discharge pipe 12 which communicates with the above-referred to discharge pipes 6a, 6b and 6c. In addition, 1 3a and 13b designate valves in a line by-passing valve 11 and in which there is provided a compressor 14. Valves 15a151 are provided in the above-referred respective pipes 3a, 4a, 5a, 6a 3b, 4b, 5b. 6b, 3c, 4c, 5c, and 6c.A common header pipe 16 is provided for the above-mentioned respective pipes 3a, 4a and 5a and 6a, whilst a common header pipe 1 7 is provided for the above-mentioned respective pipes 3b, 4b, 5b and 6b, and a common header pipe .8 is provided for the above-mentioned respective pipes 3c, 4c, 5e and 6c. Three adsorption towers 19, 20 and 21 are provided and each is filled with an adsorbent designated 1 9a, 20a, 21 a respectively. N2 gas delivery pipes 25a, 25b and 25c are provided and juxtaposed to respective N2 gas delivery pipes 26a, 26b and 26c.Also respective cavity gas discharge pipes 28a, 28b and 28c are provided in juxtaposition to the delivery pipes 26a, 2Sb and 26c, which discharge pipes communicate with the abovedescribed cavity gas main discharge pipe 12. In addition a header pipe 22 is provided for the above pipes 25a, 26a and 28a, whilst a header pipe 23 is provided for the pipes 25b, 26b and 28b and a header pipe 24 is provided for the pipes 25c, 26e and 28c. Valves 29a-29l are provided in the abovereferred to respective pipes 25a, 26a, 28a, 25b, 26b, 28b, 25c, 26e and 28c.A common N2 gas delivery pipe 30 communicates with the above-mentioned delivery pipes 25a, 25b and 25c and is connected to a surge tank 31, to which an N2 gas main delivery pipe 32 is connected. A common N2 gas delivery pipe 33 communicates with the above-mentioned delivery pipes 26a, 26b and 26c, which is also connected to a surge tank 34, to which an N2 gas main delivery pipe 35 is connected.

The common cavity gas discharge pipe 7 is connected to a vacuum pump 36 to which a cavity gas main discharge pipe 37 is connected. It is to be noted that, while an automatic control device for operating the respective valves would be provided for the apparatus, it is omitted from the block diagram. In addition, four or more adsorption towers could be provided.

In operation of the above-described adsorption-separation apparatus, the three adsorption towers 19. 20 and 21 carry out: I a high-pressure adsorption step, II primary and secondary pressure-reducing steps and a desorption step, and Ill a low-pressure pressure-raising step, a low-pressure adsorption step and a high-pressure pressure-raising step, repeatedly in the sequence of I-li-Ill in staggered timings. Assuming now that the adsorption tower 19 has entered the step I above, then only the valves 1 Sa, 29a are opened. Consequently, raw air compressed by the air compressor 2 is fed through the main feed pipe 1 , feed pipe 3a, valve 15a and header pipe 16 to the adsorption tower 19, and when it passes through the adsorption tower 19, the 02 gas (an adsorbate constituent gas) in the raw air is adsorbed by the adsorbent 19a, while the N2 gas (an unadsorbate constituent gas) is condensed and delivered through the header pipe 22, delivery pipe 25a, valve 29a and common delivery pipe 30 to the surge tank 31, and is further extracted externally of the system through the main delivery pipe 32 as a product.When the adsorption tower 19 enters the step 1, the adsorption tower 21 enters the step II, so that the valves 1 so, 291 and the valve 11 are opened and the valves 1 Si, 1 5j, 1 5k, 29i, 29j and the valves 1 Sa, 1 Sb are closed, and thus the primary pressurereducing step is commenced in the adsorption tower 21 which has finished the step I. In this step, a cavity gas at a relatively high pressure around the adsorbent 21 a is passed through the header pipe 18, valve 1 51 and discharge pipe 6c to the main discharge pipe 12, and also passed through the header pipe 24, discharge pipe 28c and valve 291 to the main discharge pipe 12, and further it is passed from the main discharge pipe 12 via the valve 11 to the tank 10.When the internal pressures of the adsorption tower 21 and the tank 10 have become nearly equal to each other, the valve 11 is closed and the valves 1 Sa, 1 3b are opened to enter the secondary pressure reducing step. In this step, the cavity gas passed from the adsorption tower 21 to the main discharge pipe 12 is raised in pressure by the compressor 14 up to the internal pressure of the tank 10, and thereafter passed into the tank 10. When the internal pressure of the adsorption tower 21 has become nearly equal to the partial pressure of the O2 gas in the raw air being fed to the adsorption tower 19, the valves 1 51, 291, 11, 1 3a and 13b are closed, and then a series of pressure-reducing steps has been terminated.Subsequently, the adsorption tower 21 enters the desorption step by opening the valve 15j. As a matter of course, the above-mentioned switching pressure could be varied depending upon the conventration of the O2 gas delivered upon the desorption step. When the valve 1 5j is opened at the adsorption tower 21, cavity gas in the adsorption tower 21 is sucked away and discharged through the header pipe 18, valve 15j, discharge pipe 4c, common discharge pipe 7 and vacuum pump 36, so that the 02 gas partial pressure of the cavity gas around the adsorbent 21 a is lowered, hence the O2 gas being adsorbed by the adsorbent 21 a is desorbed, and the desorbed gas is scavenged to the main discharge pipe 37 and recovered.When the internal pressure of the adosorption tower 21 has been lowered to a predetermined pressure, the valve 15j is closed and the desorption step is terminated. When the adsorption tower 19 enters the step I, the adsorption tower 20 enters the step Ill and only the valve 29f is opened. Therefore, low-pressure N2 gas in the tank 34 is passed through the main delivery pipe 33, valve 29f, delivery pipe 26b and header pipe 23 into the adsorption tower 20 which has finished the desorption step, and then a low-pressure pressure-raising step is commenced. When the internal pressures of the tank 34 and the adsorption tower 20 have become nearly equal to each other, the valve 1 5g is opened to enter the low-pressure adsorption step.

In this step, the cavity gas stored in the tank 10 during the pressure-reducing steps is passed through the flow rate regulating valve 9, main discharge pipe 8, discharge pipe 5b, valve 15g and header pipe 17 into the adsorption tower 20, hence 02 gas in that gas is adsorbed by the adsorbent 20a, the condensed and separated N2 gas is delivered through the header pipe 23, delivery pipe 26b, valve 29f and main delivery pipe 33 to the tank 34, and while watching and determining the moment when the concentration of the O2 gas in that gas begins to rise, the valves 1 Sg and 29f are closed to terminate the low-pressure adsorption step.It is to be noted that the low-pressure pressure-raising step and the lowpressure adsorption step could be combined into one step by simultaneously opening the valves 159 and 29f. Also, depending upon the O2 gas concentration in the low-pressure N2 gas delivered from the tank 34, a modification could be made such that the valve 1 5g is opened instead of opening the valve 29f to carry out the low-pressure pressure-raising step by feeding the cavity gas into the adsorption tower 20.When the low-pressure adsorption step has been finished in the adsorption tower 20, the valve 29e is opened, thereby the high-pressure N2 gas in the tank 31 is passed through the common delivery pipe 30, valve 29e, delivery pipe 25b and header pipe 23 into the adsorption tower 20 to carry out the high-pressure pressure-raising step, and when the internal pressures of the tank 31 and the adsorption tower 20 have become nearly equal to each other, the valve 29e is closed to finish the highpressure pressure-raising step. It is to be noted that the valve 29e could be kept open in preparation for the subsequent high-pressure adsorption step.Since the cavity gas stored in the tank 10 is consumed during the low-pressure adsorption step and the consumed cavity gas is supplemented during the secondary pressure-reducing step, it is nacessary for the tank 10 to have adequate volume for conducting such operations. In addition, depending upon the O2 gas concentration in the high-pressure N2 gas fed from the tank 31, a modification could be made such that the valve 1 sue is opened instead of opening the valve 29e during the high-pressure pressure-raising step to feed the raw air into the adsorption tower 20 and thereby the high-pressure pressure-raising step is carried out.Moreover, further modification could be made such that after termination of the low-pressure pressure-raising step, the adsorption tower 20 enters the high-pressure adsorption step and the valves 15e 29e are simultaneously opened to carry out the high-pressure pressur-raising step and the high-pressure adsorption step at the same time (the above-mentioned being a first preferred embodiment).

In the above-described first preferred embodiment, the internal pressure of the adsorption tower 21 was lowered by the vacuum pump 36 to make the 02 gas being adsorbed by the adsorbent 21 a to be desorbed. However, in a second preferred embodiment, as shown in Figure 4, during the desorption step the valves 1 5j and 29k are opened to feed the low-pressure N2 gas in the tank 34 through a flow rate regulating valve 39, main delivery pipe 38, valve 29k, delivery pipe 27c and header pipe 24 into the adsorption tower 21 to lower the partial pressure of the 02 gas in the adsorption tower 21 and thereby cause the O2 gas being adsorbed by the adsorbent 21 a to be desorbed, and on the other hand, the desorbed gas is discharged externally through the header pipe 18, valve 15j, discharge pipe 4c and common discharge pipe 7 and then recovered. In addition, while watching and determining the moment when this O2 gas concentration begins to fall, the valves 1 Sj, 29k are closed to finish the desorption step, and with respect to the other portions the second preferred embodiment illustrated in Figure 4 is identical to the first preferred embodiment. It is to be noted that in both these preferred embodiments, the tank 10 can be omitted without any inconvenience. In addition, depending upon the circumstance of the N2 gas consuming facility, the tanks 31,34 can also be omitted without any inconvenience.

When the process according to the present invention described above, and the processes in the prior art were compared by filling each one of the adsorption towers in Figures 1 and 2 and the adsorption towers in Figures 3 and 4 with 10 kg of adsorbent (Fe-K-Na-A), the results indicated in Table1 and Table2 were obtained.It is to be noted that in this case, since Fe-K-Na-A is used as an absorbent, the absorbate constituent gas is oxygen 02 and the unadsorbate gas is nitrogen N2. TABLE 1

Process According to the Present Invention Process in the Prior Art Fig.3 Fig.4 Fig.1 Fig.2 Test Apparatus & Test P@ocoss First Embodiment Second Embodiment Evacuating Desorption Scavenging Desorption Feed Pressure ata 6 6 6 6 Air Flow Rate Nm/H 10 10 10 10 Temperature "C 25 25 25 25 Delivered Flow Rate High Pressure ata 6 6 6 6 N2 gas Low Pressure ata 2.7 3 - Pressure High Pressure Nm3/H 5.3 5.3 4.9 4.5 Low Pressure Nm3/H 2.1 4.9 - Totat Nm3/H 7.4 7.2 4.9 4.5 O2 Concentration vol. % 1 or less 1 or less 1 or less 1 or less Delivered Pressure ata 1.2#0.2 1.2 1.2#0.2 1.2 O2 gas Flow Rate Nm3/H 2.6 2.8 1.4 1.7 O2 Concentration vol. % 79 73 70 55 Pure O2 Flow Rate Nm3/H 2.05 2.04 0.98 0.94 Secondary Cavity Gas Flow Rate Nm3/H 2.0 2.5 - TABLE 2

Fig. 3 Fig. 4 Fig. 1 Fig.2 Enthalph Difference between Compressor (2) Kcal/Kg 50.1 50.1 50.1 50.1 an Outlet and an Inlet in the Compressor (14) Kcal/Kg 18.0 21.5 - Case of Adiabatic Change Vacuum Pump (36) Kcal/Kg 48.3 - 48,3 Necessary Eiection Power Compressor (2) Kcal/H 649/# 649/# 649/# 649/# Compressor (14) Kcal/H 23/# 35/# - Vacuum Pump (36) Kcal/H 90/# - 48/# Total Kcal/H 762/# 684/# 697/# 649/# Necessary Energy per Unit N2 gas Kcal/Nm3 103/# 90/# 142/# 114/# Volume of Product pure O2 Kcal/Nm3 372/# 335/# 711/# 690/# N2 gas + pure O2 Kcal/Nm3 87/# 74/# 119/# 119/# As will be apparent from Table1, the delivery flow rates of N2 and O2 gases and the concentration of the delivery O2 gas have been greatly improved as compared to the process in the prior art.

On the other hand, it will be apparent from Table2 that the consumed power has been reduced to about 2/3 of that of the prior art process. More particularly, while the compressor 14 has a smaller pressure difference between outlet and inlet than the compressor 2 and the vacuum pump 36 and its efficiency w1 must be higher, assuming that these efficiencies are identical and roughly calculating the relevant power by making use of an i-s diagram for air, the results indicated in Table2 have been obtained. Here, it is assumed that the temperature of the raw air introduced through the pipe 1 is 250C, the pressure rise caused by the compressor 2 is 6.5 ata, the average necessary power for the compressor 14 and the vacuum pump 36 is one-half the maximum value, and the delivery pressure of the O2 gas is 1.2 ata.

Referring now to Figure 5, in a third preferred embodiment of the invention it is to be noted that component parts identical to those inthe second preferred embodiment (Figure 4) are identified by the same reference designations and in the following description only those portions of the apparatus which are different from the second preferred embodiment will be explained.

In this embodiment the compressor 14 in Figure 4 is omitted from themain discharge pipe 12 and therefore the valve 1 3b becomes redundant. The pipe 12 now discharges externally to the system at the outlet 40. In all other respects the components of Figure 5 are identical to those of Figure 4.

In use the process of the third embodiment is identical to that of the second embodiment except that, when the adsorption tower 19 has entered step I and the adsorption tower 21 step II, only valves 151 and 1 1 are opened whilst valve 291 is closed along with valves 1 5i, 1 5j, 1 5k, 29i, 29j, 29k and 1 3a, valve 1 3b having been deleted. In this step the cavity gas in contact with the adsorbent 21 a passes directly from the main discharge pipe 12 via the valve 1 1 to the tank 10 and since valve 1 3b has been deleted it is only necessary to open valve 1 3a and close valve 1 1 when the pressures in tower 21 and tank 10 have become equalized to embark upon the secondary pressure reducing step. In this step the cavity gas in the main discharge pipe 12 is discharged externally of the system through the outlet 40.

When the internal pressure in the adsorption tower 21 almost equals the partial prcssure of the O2 gas in the said air being fed to the adsorption tower 19, the valve 1 3a is closed and the series of pressure reducing steps is terminated. Subsequently the process enters the desorption step in tower 21 by opening valves 1 5j and 29k. As a matter of course at the start of or during the secondary pressure reducing step, as soon as the partial pressure of the O2 gas has been attained the subsequent pressure reducing step is instantly terminated. With respect to further workings of the process they are identical to those of the first embodiment. However, by introducing a modification to this process, a fourth embodiment becomes possible.Thus, instead of delivering the cavity gas from only one side of the adsorption tower 21 by opening the valve 1 51, the cavity gas can be delivered from both sides of the tower simultanteously by opening valve 291 in addition to 151, leaving the process the same in all other respects. In a further modification of this process (a fifth embodiment), by opening valves 151 and 291 throughout the primary and secondary pressure reducing steps, cavity gas can be delivered from both sides of the tower 21.

When the process according to the present invention descirbed with reference to Figure 5 and the processes in the prior art were compared to each other by filling each one of the adsolption towers in Figures 1 and 2 and the adsorption towers in Figure 5 with 10 ks. of absorbent (Fe-K-Na-A), the results indicated in Table 3 were obtained. Again, since (02) and the unadsorbate gas is nitrogen (N2). TABLE 3

Process in the Prior Art Processes According to the Present Invention Fig. 1 Fig. 2 Fig. 5 Test Apparauts & Test Process Evacuating Scavenging 3rd 4th 5th Desorption Desorption Embodiment Embodiment Embodiment Feed Air Pressure ate 6 6 6 6 6 Flow Rate Nm3/H 10 10 10 10 10 Temperature C 25 25 25 25 25 Delivered Pressure N2 gas High-Pressure ate 6 6 6 6 6 Low-Pressure ate - - 2.2 2.7 2.9 Flow Rate High-Pressure Nm3/H 4.9 4.5 4.7 5.9 6.0 Low-Pressure Nm3/H - - 1.2 0.3 0.1 Total Nm3/H 4.9 4.5 5.9 6.2 6.1 O2 Concentration vol. % 1 or less 1 or less 1 or less 1 or less 1 or less Delivered Pressure ate 1.2#0.2 1.2 1.2 1.2 1.2 O2 gas Flow Rate Nm3/H 1.4 1.7 2.6 2.0 2.1 O2 Concentration vol. % 70 55 55 62 74 Pure O2 Flow Rate Nm3/H 0.98 0.94 1.43 1.24 1.55 From Table 3 it will be seen that, despite the fact that onlythe compressor 2 was used and not the vacuum pump and hence the power consumption is reduced, the delivery flow rates of the N2 gas and the O2 gas were increased in comparison with the prior art processes, and so, the manufacturing cost was also reduced.

With regard to the delivery of the cavity gas from both sides of the adsorption tower, if it is carried on throughout the series of pressure-reducing steps, a greater delivery flow rate of O2 gas can be obtained and therefore a greater reduction of manufacturing cost can be obtained than when such delivery is only carried on during the primary pressure-reducing step. This means that in the case of delivering the cavity gas from only one side (the side for feeding raw air) of the adsorption tower during the third pressure-reducing step it takes a value intermediate between the above-mentioned results.

In the process according to the present invention, the delivery flow rate of an N2 gas is large as described above, and this implies that the O2 gas concentration in the gas discharged through the discharge pipes 7, 40 is high. Especially, in the case of the fifth preferred embodiment, the O2 gas concentration amounts to as high as 74%, and it was effective for reducing the manufacturing cost of an O2 gas.

Referring now to Figure 6 in a sixth preferred embodiment of the invention, it is to be noted that component parts identical to those in the first preferred embodiment (Figure 3) are identified by the same reference designations and in the following description only those portions of the apparatus which are different from the first embodiment will be explained.

In this embodiment the compressor 14 in Figure 3 is omitted from the main discharge pipe 12 and therefore the valve 1 3b becomes redundant. The surge tank 10 is also omitted from the cavity gas main discharge pipe 8 and consequently the valve 1 1 is also redundant. The pipe 12 now discharges externally of the apparatus at the outlet 40.

In all other respects the apparatus shown in Figure 6 is identical to that shown in Figure 3.

In use the process of the sixth embodiment is identical to that of the first except that in this case limitations are placed upon the elapsed times for each of the steps in the process in consequence of the removal of the surge tank 10. Thus, if the elapsed time for the high pressure adsorption step I is T minutes the primary pressure reducing part of step II is T1 minutes, the secondary pressure reducing part of Step II is T2 minutes and the adsorbed gas desorption part of step 11 is T3 minutes then the sum of T,.

T2 and T3 must be equal to or less than T minutes. And if the elapsed time of the parts of step Ill are the low pressure adsorption part T1 minutes and the pressure raising part T4 minutes, then the sum of T and T4 must also be equal to or less than T minutes. The process continues with Step I as in the first embodiment but when the tower 21 embarks on Step II only valves 151 and 291 are opened. This causes any residual gas remaining in the cavities of the adsorption material 21 a to be discharged through the main discharge pipe 12 either via the header pipe 18, valve 151 and pipe 6c or via the header pipe 24, pipe 28c and valve 291 upon the termination of the high pressure adsorption step.

At this juncture the low pressure adsorption step is being carried on in the tower 20 so the valves 15g and 29f are opened which causes low pressure N2 gas to flow through the valve 29f, pipe 26b, and header pipe 23 to the tower 20 whilst simultaneously the cavity gas in the main pipe 12 is directed to the tower 20 via the regulating valve 9, pipes 8 and 5b, valve 15g and header pipe 17. When the internal pressure in the tower 20 has reached the low pressure adsorption pressure the 02 gas in the cavity gas is adsorbed by the adsorbent filling 20a whilst the ocndensed N2 gas is being discharged to the surge tank 34 via the header pipe 23, pipe 26b, valve 29f and main pipe 33.After an inteival of T minutes has elapsed and the preliminary pressure reducing step in tower 20 and low pressure adsorption step in tower 21 are concluded, valve 15g is closed and valve 1 3a is opened which causes the remaining cavity gas in the tower 21 to be discharged externally of the system. However, if it is desired to extract 02 gas as a product depending upon the 02 gas concentration, it is also possible to close the valves 1 3a, 151 and 291 and open the valve 1 5j either at the beginning of or during the secondary cavity gas discharge step for delivering a part or the whole of the cavity gas jointly with the product gas.At the adsorption tower 21, 1 subscquently the valves 1 so, 291 and 13a are closed and the valve 15j is opened. Consequently, an adsorbed gas principally consisting of O2 gas which has been adsorbed by the adsorbent 21 a is desorbed due to the pressure reduction caused by the vacuum pump 36 via the header pipe 18, valve 15j, discharge pipe 4c and main discharge pipe 7, and it is discharged externally of the system through the common discharge pipe 37. This desorbed gas has a high 02 gas concentration. At the above-described instant, the adsorbed gas desorption step is terminated.On the other hand at the adsorption tower 20, when the lowpressure adsorption step has been finished, the valve 1 sag, and 29f are closed and the valve 29e is opened. Consequently, N2 gas at high pressure flowing through the common delivery pipe 30 flows into the adsorption tower 20 through the valve 29e, delivery pipe 25b and header pipe 23 to raise the pressure up to the pressure required fo r the high-pressure adsorption step, and then the pressureraising step is finished. Thereafter the adsorption towers 1 9, 20 and 21 repeatedly carry out the abovementioned steps in such manner that if the adsorption tower 19 enters the step II above, then the adsorption tower 20 enters the step I and the adsorption tower 21 enters the step ill, and thereby the raw air is separated into N2 gas and a gas having a high O2 gas concentration.While the cavity gas is delivered from the both sides of the adsorption tower 21 by opening the valves 151 and 291 of the adsorption tower 21 and the valve 13a during the secondary pressure-reducing step in the abovedescribed sixth preferred embodiment, the cavity gas could be delivered from only the side of the valve 1 51 of the adsorption tower 21 by opening only the valves 151 and 1 3a (the above-mentioned being a seventh embodiment).In addition, while the cavity gas delivered from the adsorption tower 21 is fed into the adsorption tower 20 by opening the valves 1 so, 291, 1 59 and 29f of the adsorption towers 20 and 21 during the primary pressure-reducing step and the cavity gas delivered from the adsorption tower 21 is discharged externally of the system through the common discharge pipe 12, valve 1 3a and outlet pipe 40 during the secondary pressure-reducing step in the above-mentioned sixth preferred embodiment, modification could be made such that the cavity gas is discharged externally of the system by opening the valves 1 so, 291 and 13a during the primary pressure-reducing step, then the cavity gas is fed into the adsorption tower 20 by opening the valves 151,291,159 and 29f and closing the valve 13a during the secondary pressure-reducing step, and further as a third pressure-reducing step, surplus cavity gas is discharged externally of the system by closing the valves 1 5g and 29f and opening the valves 151,291 and 13a (the above-mentioned being an eighth embodiment).

When the process in the prior art and the process according to the present invention described with reference to Figure 6 were compared, by filling each one of the adsorption towers in Figures 1 and 2 and the adsorption towers in Figure 6, the results indicated in Table 4 were obtained. It is to be noted that the indication "Modification of 8th Embodiment" implies the case where, in the eighth embodiment, the apparatus was operated with the internal pressure of the adsorption tower lower upon termination of the primary pressure-reducing step so that the third pressure-reducing step can be omitted.

TABLE 4

Fig. 1 Fig. 2 Fig.6 Process According to the Present invention Test Apparatus & Test Process Process Process Modification in the in the 6th 7th 8th to 8th Prior Art Prior Art Embodiment Embodiment Embodiment Embodiment Adsorption Pressure ata 6 6 6 6 6 6 Tower Feed {Flow Rate Nm3/H 10 10 10 10 10 10 Gas Temperature C 25 25 25 25 25 25 High- ata 6 6 6 6 6 6 Pressure Pressure { Low- ata - - 2.5 2.5 1.8 1.2 Pressure High Flow Pressure Nm3/H 4.9 6.4 5.4 5.9 5.2 4.8 Delivered {Rate { Low Pressure Nm3/H - - 1.1 0.8 0.4 0.2 Total Nm3/H 4.9 6.4 6.5 6.7 5.6 5.0 O2 Concentration vol. % 1 or less 1 or less 1 or less 1 or less 1 or less 1 or less Pressure ata 1.2#0.2 1.2#0.2 1.2#0.2 1.2#0.2 1.2#0.2 1.2#0.2 Delivered Flow Rate Nm3/H 1.4 1.7 2.0 1.8 2.1 2.0 O2 gas { O2 Concentration vol. % 70 74 80 75 80 80 Pure O2 Flow Rate Nm3/H 1.0 1.25 1.6 1,38 1.88 1.6 As will be apparent from Table 4, despite the fact that the conditions for feeding a gas to the adsorption towers which uses most of the consumed power are identical, in every preferred embodiment of the present invention both the 02 concentration and the flow rate of the delivered O2 gas are improved as compared to the process in the prior art, and, therefore, when extracting O2 gas a product, the cost can be reduced.

In the sixth preferred embodiment, although the delivery flow rate of the N2 gas is the same as that in the prior art process, both the O2 concentration and the flow rate of the delivered 02 gas are improved, and therefore, this embodiment is suitable for the case where both N2 and O2 gases are extracted as products.

In the seventh preferred embodiment, although the flow rate of the delivered O2 gas is low as compared to the sixth preferred embodiment, the delivery flow rate of N2 gas is increased, and therefore, this embodiment is suitable for the case where N2 gas is extracted as a product.

In the eighth preferred embodiment, although the flow rate of the delivered N2 gas is lower as compared to the sixth and seventh preferred embodiments, the flow rate of the delivered 02 gas and the like are improved, and hence this embodiment is suitable for the case where O2 gas is extracted as a product.

Referring now to Figure 7 in a ninth preferred embodiment of the invention it is to be noted that component parts identical of those in the sixth preferred embodiment (Figure 6) are identified by the same reference designations and in the following description only those portions of the apparatus which are different from the sixth embodiment will be explained.

In this embodiment a surge tank 10 and a check valve 11 have been inserted in the main delivery pipe 8. In all other respects the apparatus shown in Figure 7 is identical to that of Figure 6. In use the process of the ninth preferred embodiment only differs from the sixth embodiment in that it is now no longer necessary to synchronise the timings of the low pressure adsorption step on the tower 20 with the primary and secondary pressure reducing steps in the tower 21 since the cavity gas delivered from the tower 21 is fed indirectly to the tower 20 via the surge tank 10 and check valve 11.

The inclusion of the surge tank 10 and check valve 11 in the pipe 8 also enable the workings of the processes of the seventh and eighth embodiments to be modified in the same manner, by removing the necessity of synchronising the timings. Modifications can be made to this embodiment, similar to those described above for the seventh and eighth embodiments to produce tenth and eleventh embodiments.

In the ninth to eleventh preferred embodiments, similar effects to those of the sixth to eighth embodiments can be expected. Moreover, in the ninth to eleventh preferred embodiments, since there is no need to synchronize the low-pressure adsorntion step with the primarY and secondary pressurereducing steps there is mora freedom in the arrangement of steps. Furthermore, since the cavity gas can be delivered in a short period of time, the total time taken by the steps can be shortened and thus the processible quantity of raw air per unit time can be increased.

While the present invention has been described above in connection with various preferred embodiments thereof, it is a matter of course that the present invention should not be limited to only these embodiments but various changes and modifications in design could be made without departing from the spirit of the present invention. For instance, the invention is equally applicable to the case where a carbon dioxide gas is separated from a burn off gas by adsorption. In addition, for the adsorption, besides Fe-K-Na-A and Fe-Na-A, carbon molecular sieve (adsorbent for oxygen), or an adsorbent for nitrogen can be used.

Claims

1. A process for separating a mixed gas by adsorption in which an adsorption step of feeding a raw gas into an adsorption tower to make said adsorbent adsorb an adsorbate constituent gas and extracting an unadsorbate gas from said adsorption tower, and desorption step of reducing the pressure in said adsorption tower to make said adsorbent desorb the adsorbate constituent gas adsorbed thereby and recovering the adsorbate constituent gas, are repeatedly carried out in a plurality of adsorption towers in staggered timings; characterised in that to commence a desorption step, mixed gas delivered from an adsorption tower which has completed adsorption during a primary pressurereducing period in said adsorption tower, is fed to another adsorption tower which has been raised in pressure up to an intermediate pressure between the adsorption pressure and the desorption pressure, after termination of the desorption step, by an unadsorbate gas delivered as a result of low-pressure adsorption and/or mixed gas delivered during the primary pressure-reducing period, to carry out a lowpressure adsorption step in which the adsorbate constituent gas in said mixed gas is made to be adsorbed, whereby a pressurized mixed gas having a low adsorbate constituent gas concentration which is delivered during the primary pressure-reducing period can be effectively utilized.

2. A process for separating a gas by adsorption as claimed in Claim 1, characterised in that an adsorption tower which has completed primary pressure reduction is subjected to secondary pressure reduction by releasing the interior of said adsorption tower to the atmosphere, and further by reducing the pressure in an adsorption tower which has completed secondary pressure reduction by means of a vacuum pump an adsorbate constituent gas which has been adsorbed by an adsorbent is made to be desorbed.

3. A process for separating a gas by adsorption as claimed in Claim 1, characterised in that the interior of an adsorption tower which has completed primary pressure reduction is secondarily reduced in pressure nearly to the partial pressure of the adsorbate constituent gas in the raw gas, and further the adsorbate constituent gas adsorbed by an adsorbent is made to be desorbed by scavenging the interior of an adsorption tower which has completed secondary pressure reduction, with an unadsorbate gas delivered during a low-pressure adsorption step.

4. A process for separating a gas by adsorption as claimed in Claim 1, 2 or 3, characterised in that a mixed gas delivered during a secondary pressure-reducing step is raised in pressure up to the lowpressure adsorption pressure, and then fed into an adsorption tower which has been raised in pressure up to the low-pressur adsorption pressure either jointly with a mixed gas delivered during a primary pressure-reducing period or subsequently to said mixed gases to be absorbed, whereby yields of the both adsorbate and unadsorbate constituent gases in the raw gas can be raised and also power consumption can be reduced.

5. A process for separating a gas by adsorption as claimed in Claim 1,2 or 3, characterised in that within a tolerance of an adsorbate constituent gas concentration in the gas delivered externally of the system which is rich in an adsorbate constituent gas, a part or whole of a mixed gas delivered during the secondary pressure-reducing step is made to be delivered externally of the system jointly with the gas delivered from an adsorption tower during the desorption step, whereby a yield of the adsorbate constituent gas can be raised.

6. A process for separating a gas by adsorption as claimed in Claim 1,2,3,4 or 5, characterised in that an adsorption tower which has completed the last step of low-pressure adsorption is subjected to high-pressure pressure-raising by means of either one or both of a feed high-pressure raw gas and a part of a high-pressure unadsorbate gas delivered during the high-pressure adsorption step.

7. A process for separating a gas by adsorption as claimed in Claim 1,2,3,4 or 5, characterised in that a mixed gas delivered during the primary pressure-reducing step and/or the secondary pressurereducing step is fed to an adsorption tower which is carrying out the low-pressure adsorption step by the intermediary of a tank, whereby the necessity for synchronizing the low-pressure adsorption step with the primary and secondary pressure-reducing steps can be eliminated.

8. A process for separating a gas by adsorption as claimed in Claim 1,2,3,4,5,6 or 7, characterised in that during the secondary pressure-reducing step, a mixed gas is delivered from the raw gas feeding side of the adsorption tower or from both the raw gas feeding side and the unadsorbate gas delivery side of said adsorption tower.

9. A process for separating a gas by adsorption as claimed in Claim 1, 2, 3, 4, 5, 6, 7 or 8, characterised in that the adsorbent to be filled in the adsorplion towers is an adsorbent adapted to selectively adsorb oxygen from air, which is produced by adding at least "iron having a valency of two or more" to industrially pure A4 type zeolite.

10. Apparatus for separating a mixed gas by adsorption, comprising at least two adsorption towers, a main mixed gas feed pipe having a compressor therein, and valved connections at each each end of each tower so that mixed gas can be fed through either tower as required for adsorption therein, charactrerised in that the towers have mixed gas feed pipes, and cavity gas discharge pipes connected via a common header pipe, to their inlet ends, and constituent gas delivery pipes and cavity gas discharge pipes connected via a common header pipe at their outlet ends, in that valves are provided in all said pipes, and other equipment such as surge tanks, vacuum pumps and additional compressors are provided as appropriate for the particular feed conditions of the mixed gas and delivery requirement of the apparatus is such that it can carry out a process according to any one of the precoding claims.