GB1593538A

GB1593538A - Pressure swing adsorption process for gas separation

Info

Publication number: GB1593538A
Application number: GB4902977A
Authority: GB
Original assignee: Greene and Kellogg Inc
Current assignee: Greene and Kellogg Inc
Priority date: 1976-11-26
Filing date: 1977-11-24
Publication date: 1981-07-15
Also published as: JPS5377878A; DE2752801A1; DE2752801C2; FR2454834B1; FR2392706B1; GB1593539A; FR2454835A1; FR2454835B1; FR2392706A1; JPS6022964B2; FR2454834A1; CA1132918A; GB1593540A

Description

(54) PRESSURE SWING ADSORPTION PROCESS FOR GAS SEPARATION (71) We, GREENE & KELLOGG, INC., a corporation organised and existing under the laws of the State of Delaware, United States of America, of 790 Creekside Drive, Tokawanda, New York 14150, United States of America, formerly of 1716 Main Street, Buffalo, New York 14209, United States of America, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to a process for separating gas mixtures by pressure swing adsoprtion.

One area of use of the present invention is in fractionating air to provide a product stream of high purity oxygen, although the principles of the present invention can be applied in other fields.

In basic pressure swing adsorption processes and systems for separating air, adsorption is carried out at a high pressure and desorption is carried out at a low pressure. Compressed air is introduced into a fixed bed of adsorbent material so that the nitrogen thereof is then preferentially adsorbed to produce oxygen-rich product gas. When the adsorption bed is about saturated, the bed pressure is reduced to desorb nitrogen from the adsorbent material and to regenerate the adsorption capacity. A purge by some of the product or an intermediate process stream often is used in order to increase the efficiency of regeneration. To facilitate continuous operation, two or more adsorption beds are employed so that while one bed performs adsorption the other bed undergoes regeneration.

In the design and operation of pressure swing adsorption processes and systems, it would be highly desirable to provide maximum utilisation of adsorbent material in the adsorption beds, to reduce the energy requirements for operation of the system, to obtain a substantially constant degree of product purity, and to reduce adsorbent material requirements while maintaining a high degree of product purity along with improved efficiency and reliability.

Accordingly, the present invention provides a pressure swing process for fractionating at least one component from a gaseous mixture by selective adsorption in each of at least two adsorption beds by sequentially passing the gaseous mixture from a feed stream through a first adsorption bed until the first bed is about saturated while simultaneously purging and then pressurising a second adsorption bed, and then passing the gaseous mixture from the feed stream through the second adsorption bed until the second bed is about saturated while simultaneously purging and then pressurising the first adsorption bed part of the pressurising gas for each of the adsorption beds at the end of its purging phase being derived by pressure equalisation between the feed ends of the first and second beds, whereby the withdrawn gas from one of said adsorption beds at the end of the adsorption operation therein is in a direction countercurrent to feed flow and is introduced along with the gaseous mixture from the feed stream into the other of said adsorption beds in a direction cocurrent with feed flow and after partial pressurisation of the said other of the adsorption beds.

The invention also provides a pressure swing process for fractionating at least one component from a gaseous mixture by selective adsorption in each of at least two adsorption beds by sequentially passing the gaseous mixture from a feed stream through a first adsorption bed until the first bed is about saturated and the purity of the product gas reduces while simultaneously purging and then pressurising a second adsorption bed and then passing the gaseous mixture from the feed stream through the second adsorption bed until the second bed is about saturated while simultaneously purging and then repressurising the first adsorption bed; withdrawing product gas from the feed end of one of the said at least two adsorption beds countercurrent to feed flow when said one bed is at the end of the adsorption operation therein and introducing it along with the gaseous mixture from the feed stream to the feed end of another of said at least two adsorption beds after partial pressurisation thereof to repressurise said another bed before adsorption therein; withdrawing reduced purity product gas from said one of the said at least two adsorption beds in a direction cocurrent with feed flow as the pressure in said one bed decreases at the end of the adsorption operation therein and prior to purging of the said one bed; introducing the withdrawn reduced purity product gas to only one end of a storage adsorption bed which is segregated from the feed stream but is able to be communicated with an outlet end of each of said at least two adsorption beds, said withdrawn reduced purity product gas being the sole gas introduced into said segregated storage adsorption bed; then withdrawing gas from said one end of the segregated storage adsorption bed and passing said withdrawn gas into said one adsorption bed in a direction countercurrent to feed flow after purging of the said one bed and when the said one bed is at a relatively low pressure, the feed end of said one adsorption bed being closed during such passage, whereby said gas starts repressurising said one adsorption bed; and withdrawing product gas from the other end of said segregated storage adsorption bed and recovering it directly without further treatment.

In order that the present invention may more readily be understood, the following description is given, merely by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of a system for carrying out a pressure swing adsorption process according to the present invention; Figure 2 is a cycle sequence chart illustrating the pressure swing adsorption process of the present invention; Figure 3 is a schematic diagram of another embodiment of a system for carrying out a pressure swing adsorption process according to the present invention with some parts removed; and Figure 4 is a schematic block diagram of a control arrangement for a pressure swing adsorption system for carrying out a process according to the present invention Referring now to Figure 1, there is shown a system for carrying out the process according to the present invention, for fractionating at least one component from a gaseous mixture by a pressure swing adsorption. The gaseous feed mixture is supplied to the system by a feed gas stream which flows along an input conduit 10 and is moved therealong by means of a pump or compressor 12.

Although the present process is specifically described and illustrated in relation to the fractionation of air, by pressure swing adsorption, to produce an oxygen-rich stream, the present invention is broadly applicable to the separation of any organic and/or inorganic gas mixtures.

The system includes a bed A, in a first adsorption bed vessel 16 having a gas inlet 18 and a gas outlet 20. The system further includes at least one additional adsorption bed B in a bed vessel 24 having a gas inlet 26 and a gas outlet 28.

A preferred vessel construction includes an outer pressure cell with an inner annulus.

Many adsorbent materials are well-known in the art, and examples of typical adsorbent materials for use in adsorption beds include natural or synthetic zeolites, silica gel, alumina and the like. Generally, the adsorbent beds in a system contain the same adsorbent material, however, each bed may contain a different type of adsorbent material or different mixtures of adsorbent material as desired.

The system of the present invention further comprises a segregated storage adsorption bed C in a vessel 32, which does not communicate with the feed gas stream from conduit 10; in the system shown in Figure 1, gas is introduced to and withdrawn from bed C at the same end via a conduit 34. In the system shown, adsorption bed C is approximately the same size as the adsorption beds A and B and may contain the same type of adsorbent material, but the segregated storage adsorption bed C can instead be smaller in size, may include different adsorbent material, and may be operated at a different capacity as compared to the adsorption beds A and B.

The gas inlet 18 of adsorption bed A is connected to the feed gas stream conduit 10 by suitable conduits including an automatic valve 40A, and similarly the gas inlet 26 of adsorption bed B is connected to the feed gas stream conduit 10 by suitable conduits including an automatic valve 40B.

The system further includes a waste gas outlet 44 which can be open to the atmosphere or which can communicate with a waste gas stream. The gas inlets 18 and 26 of adsorption beds A and B, respectively, also are connected to the waste gas outlet 44 by suitable corresponding conduits including automatic valves 46A and 46B, respectively. The automatic valves 40A and 40B and 46A and 46B, and additional automatic valves to be described later, can be of the solenoid-operated type, but in any event are of the type which can be operated to be either fully open -or fully closed.

Suitable conduits define a gas flow path connected at one end to gas outlet 20 of adsorption bed A and at the opposite end to gas outlet 28 of adsorption bed B.

A first flow control valve 50A is positioned in the gas flow path between gas outlet 20 of adsorption bed A and adsorption bed B. Valve 50A allows unrestricted gas flow therethrough in the direction from bed A toward adsorption bed B, and provides controlled flow therethrough in the reverse direction. The controlled flow preferably is provided by manual adjustment. A second flow control valve 50B in the gas flow path between gas outlet 28 of adsorption bed B and the adsorption bed A allows unrestricted gas flow therethrough in the direction from bed B toward bed A, and provides controlled flow, preferably manually controlled, therethrough in the reverse direction. Valves 50A, 50B are preferably identical and can be of the type known commercially as Parker-Hannifin flow control valves. An isolation valve, in the form of an automatic valve 54, is provided in the gas flow path between gas outlets 20 and 28 of the adsorption beds A and B, preferably between gas outlet 20 of adsorption bed A and the flow control valve 50A.

A second gas flow path between beds A and B is provided by suitable conduits or piping which joins the gas outlets 20 and 28 of the beds. In this second path a first automatic valve 60A is connected adjacent outlet 20 of bed A, and a second automatic valve 60B is connected adjacent outlet 28 of adsorption bed B.

The segregated storage adsorption bed C is connected through an automatic valve 62 to a point in this second gas flow path between the automatic valves 60A and 60B.

The system further comprises a product outlet 66 and an output conduit for coupling the gas outlets of the adsorption beds to the product outlet 66. In the system shown the output conduit is connected to the said first flow path at a point between the flow control valves 50A and 50B and includes a first section 70 including an automatic valve 72 and a second section 74 including the series combination of a pressure regulator 76, a throttle valve 78 and a flow meter 80.

The flow rate of product to the product outlet 66 is controlled by the throttle valve 78 which preferably is a manually adjustable needle-type valve, and the flow rate is indicated visually by the meter 80.

The system further comprises a reservoir 84 which functions primarily to store product gas received through a conduit 86 and to serve as a reserve supply of product for maintaining output in the event of a system malfunction.

A first reservoir conduit is connected at one end to the second section 74 of the above-mentioned system output conduit and at the other end to an outlet conduit 86 of the reservoir 84 and includes flow control means in the form of check valve 90 which allows gas flow only in the direction from the system output conduit 74 to the reservoir 84. Another valve 92, in the form of a throttle valve which preferably is manually adjustable, is connected in the conduit, preferably between the check valve 90 and reservoir 84. The valve 92 can be used to control the rate of flow of gas product into reservoir 84.

A second reservoir conduit is connected at one end to reservoir 84 through conduit 86 and at the other end to the system output conduit and includes a valve 96 for controlling the flow of product gas from reservoir 84 to the output conduit section 74. A control unit 100 is connected by lines 102 and 103 to valves 72 and 96, respectively, and functions to open the normally-closed valve 96 in response to closing of valve 72. A pressure indicator meter 104 can be connected to the output of reservoir 84 for indicating the pressure of gas product remaining therein.

In general, the present invention is illustrated in terms of a process and system utilising a first adsorption bed A, a second adsorption bed B and a segregated storage adsorption bed C, but can employ more than one first adsorption bed, more than one second adsorption bed and more than one segregated storage adsorption bed provided the first and second adsorption beds communicate with the feed gas stream which supplies the gaseous mixture, and the segregated storage adsorption bed is never directly communicated with, or directly exposed to, the feed gas stream.

Although the process and system of the present invention are described with particular reference to fractionation of air to provide high purity oxygen by removal of nitrogen, essentially any gas mixture may be separated by the process and system of the present invention by the proper selection of the time for each cycle and step, and by the selection of a proper adsorbent material, adsorbent materials or mixtures of adsorbent materials.

As used herein, depressurising or depressurisation refers to the reduction of pressure in the vessel and associated piping of an adsorption bed, and the level to which pressure is reduced can be selected by those skilled in the art depending upon operating conditions and the nature of the gas mixture being fractionated.

Desorption and purging pressures are selected in a similar manner. Pressurising or pressurisation refers to the increase of pressure in the vessel and associated piping of an adsorption bed. The system illustrated has the capability of product gas delivery in a range of pressures down to about 2p.s.i.g. and up to about 40 p.s.i.g.

For example, in the fractionation of air to deliver high purity oxygen gas product, a delivery pressure of around 3 p.s.i.g. is employed for medical uses and breathing devices, whereas a high delivery pressure of up to about 40 p.s.i.g. is ideally suited for commercial uses such as in metal cutting or welding equipment.

Figure 2 illustrates a process timing sequence to enable the system of Figure 1 to be used for carrying out the present invention. Figure 2 the preferred times in seconds are indicated for each step, and the preferred pressures in each adsorption bed for each step are shown parenthetically and given in pounds per square inch gauge. The particular operation carried out in each adsorption bed during each step is shown in Figure 2 and most are abbreviated for concurrence in illustration. Thus "FEE" refers to feed end equalisation (to be explained in further detail presently), "ISOL" refers to isolation of a particular adsorption, "EQ" refers to pressure equalisation of two adsorption beds (to be explained in further detail presently), "REP" refers to repressurisation or repressurising to increase the pressure in an adsorption bed, and "PURGE" refers to introduction of purge gas or purging.

Prior to step I the gaseous mixture (i.e. atmospheric air) from the feed gas conduit 10 has been flowing through valve 40A into and through adsorption bed A wherein nitrogen is adsorbed. High purity oxygen gas leaves bed A through outlet 20 and flows through the open valve 54 and flow control valve 50A and then along the first conduit section 70 through the open valve 72, along second conduit section 74 and through the series combination of pressure regulator 76, needle valve 78 and flow meter 80 to the product outlet 66 for use.

Just prior to the beginning ofstep- of step, adsorption bed A will be about saturated and nearing the end of the adsorption operation therein. Also just prior to the beginning of step 1, the adsorption bed A is at a higher pressure than the adsorption beds B and C.

At the beginning of step 1, valve 40B is opened, while valve 40A and valve 72 are kept open. As indicated in Figure 2, at the beginning of step 1, typical pressures in beds A, B and C are 30, 7 and 7 p.s.i.g. respectively.

During step- 1, gas flows from the bottom or feed end of bed A out through inlet 18 and in reverse direction through valve 40A whereupon it mixes with the incoming feed air stream from conduit 10 and flows through valve 40B into the bottom or feed end of bed B. Bed A is very near the end of the adsorption step therein so the composition of this gas withdrawn from inlet 18 thereof is not appreciably different from the composition of air. As a result, during step 1, adsorption bed A is depressurised countercurrently to feed flow, and becomes pressure equalised with adsorption bed B causing the pressure in bed B to rise.

Also in step 1, adsorption bed A continues to supply oxygen gas product, as the pressure reduces, but this is terminated by the end of the step which preferably has a duration of about 7 seconds. Throughout step 1, and all other steps, there is continuous air flow into the system in conduit 10 and continuous product flow out through outlet 66.

Step 1 may be described as continuing to discharge product gas from the outlet of the first bed A while simultaneously equalising the pressures of the first and second adsorption beds A and B from the feed ends thereof.

At the transition between the end of step I and beginning of step 2, as shown in Figure 2, the pressures in beds A and B are equalised at 20 p.s.i.g. and the pressure in the segregated storage adsorption bed C has remained at 7 p.s.i.g.

At the beginning of step 2, valve 40B remains open, but valve 40A closes, and valve 60A opens. No product gas is now obtained from adsorption bed A. During this step, feed air continues to flow into the feed inlet 26 of bed B, and oxygen-rich gas is taken as product from the outlet 28 bed B and flows through flow control valve 50B into second conduit section 70 and to the product outlet 66. At the same time, low purity gas flows from the outlet 20 of bed A through the valve 60A and valve 62 and into the segregated storage adsorption bed C. As a result, during step 2 adsorption bed A is pressure equalised with the segregated storage adsorption bed C.

The automatic valve 62 can either remain open during all steps or be opened and closed when necessary.

Step 2 preferably has a duration of about 7 seconds.

The process of step 2 may be described as simultaneously terminating the pressure equalisation of step 1, adsorbing the- gaseous mixture from the feed gas stream in the second adsorption bed B, releasing product gas from the outlet of the second adsorption bed, and equalising the pressures of the first adsorption bed A and the segregated storage adsorption bed C.

At the transition between the end of step 2 and the beginning ofstep3, as shown in Figure 2, the pressures in adsorption beds A and C are equalised at 14 p.s.i.g. and the pressure in adsorption bed B has risen to 28 p.s.i.g.

At the beginning of step 3, valve 40B remains open, valve 60A closes and valve 46A opens. During step 3 feed air continues to enter bed B, and product quality oxygen-rich gas continues to be taken as product from the outlet of bed B and is available at product outlet 66. Also during step 3, adsorption bed A is depressurised to the atmosphere through valve 46A and waste outlet 44 in a direction countercurrent to feed flow. As a result, nitrogen rich waste gas is rejected to the atmosphere, and the pressure in bed A drops from 14 p.s.i.g. to 0 p.s.i.g. Simultaneously with the foregoing depressurisation, a portion of the oxygen gas product flowing from bed B through flow control valve 50B will flow through valve 50A and valve 54 into bed A. The product quality oxygen gas. flows through bed A in a direction opposite to that prevailing during air separation to give an oxygen purge flowing countercurrent to feed flow displacing nitrogen from the adsorbent material in bed A to become a nitrogen-rich stream leaving the system to the atmosphere through valve 46A and outlet 44. Step 3 preferably has a duration of about 39 seconds.

The process of step 3 may be described as simultaneously terminating the pressure equalisation of step 2, continuing adsorption, in the second adsorption bed B, of the gaseous mixture from the feed gas stream releasing product gas from the outlet of the second adsorption bed B, and depressurising the first adsorption bed A (in a direction countercurrent to feed flow) and purging the first adsorption bed by diverting some product gas from the outlet of the second adsorption bed B into the first adsorption bed A in a direction countercurrent to feed flow.

At the transition between the end of step 3 and the beginning ofstep 4, as shown in Figure 2, the pressure in bed A is at 0 p.s.i.g., the pressure in segregated storage adsorption bed C has remained at 14 p.s.i.g., and the pressure in bed B has risen to 30 p.s.i.g. At the beginning of step 4, valve 40B remains open, valve 46A closes, and valve 60A opens. Valve 62, if not already open, is opened at the beginning of step 4. During step 4 feed air continues to enter bed B, and product quality oxygen gas continues to be taken as product from the outlet of bed B and is available at product outlet 66. At the same time, gas flows from the segregated storage tank C, through valves 62 and 60A, into bed A through the outlet 20 thereof. This gas withdrawn from adsorption bed C during step 4 is a version of the gas supplied to bed C during step 2 which gas has been influenced by its travel into and dut of bed C.

As a result, during step 4 the segregated storage adsorption bed C is pressure equalised with the adsorption bed A. At least during the initial.portion of step 4, there may be some additional flow of purge gas from bed B through valves 50B, 50A and 54.

Step 4 preferably has a duration of about 7 seconds.

The process of step 4 may be described as simultaneously terminating the depressurising and purging of the first adsorption bed A, continuing adsorption of the gaseous mixture from the feed gas stream in the second adsorption bed B, releasing product gas from the outlet of the second adsorption bed B and equalising the pressures of the segregated storage adsorption bed C and the first adsorption bed A.

The foregiong process steps 1, 2, 3 and 4 are repeated consecutively, reversing the functions of the adsorption beds A and B beginning with pressure equalisation of the adsorption beds A and B from the feed ends thereof. At the transition between the end of step 4 and the beginning of step 5, as shown in Figure 2, the pressures in beds A and C are equalised at 7 p.s.i.g. and the pressure in bed B has remained at 30 p.s.i.g.

At the beginning of step 5, valve 40B remains open, valve 60A closes and valve 40A opens. During this step, gas flows from the bottom or feed end of bed B, which is near the end of its adsorption operation, in a reverse direction through valve 40B whereupon it mixes with the incoming feed air stream from conduit 10 and the resulting mixture flows through valve 40A into the bottom or feed end of bed A. As a result, adsorption bed B becomes pressure equalised with adsorption bed A, and bed A begins to adsorb the feed gas mixture. This feed end equalisation is similar to that which occurred during step 1 but in this step the roles of the beds A and B are interchanged. Also during step 5, while the pressure in bed B is falling during feed end equalisation, product quality oxygen-rich gas continues to be taken as product from bed B and is available at product outlet 66.

This step begins the second half of the process cycle wherein steps 5 to 8 are similar to 1 to 4 with the roles of beds A and B interchanged and with the valve sequence being the same but with the A and B designations interchanged.

For example, the process of step 6 (the same as step 2 but with the adsorption beds reversed) may be described as simultaneously terminating the pressure equalisation of step 5, repressurising the first adsorption bed A while withdrawing product gas therefrom, and equalising the pressures of the second adsorption bed B and the segregated storage adsorption zone C.

Carrying out the pressure equalising of the adsorption beds A and B at the feed ends thereof, as illustrated in step 1, advantageously reduces energy requirements and increases oxygen recovery. When an adsorption bed, at the end of the adsorption step therein, is depressurised countercurrently to feed flow, i.e.

as bed A from 30 p.s.i.g. to 20 p.s.i.g. in step 1, the composition of the gas obtained from the bed inlet is not greatly different from air and this gas can therefore be introduced into the feed end of an adsorbent bed in a repressurising phase without any appreciable loss in system performance as compared with the more usual step of repressurising using only the air from the system compressor 12. Feed end equalisation allows the repressurisation of one bed to be assisted cocurrently by the depressurisation air from another bed and thus greatly reduces the feed air requirement and increases oxygen recovery, i.e. decreases the size of compressor 12 required to produce a given amount of oxygen and to achieve the required rapid repressurisation for maintaining near constant output pressure. Feed end equalisation recovers energy, increases system efficiency and can be used for both low and high product delivery pressures. The foregoing advantages of course apply to both of the feed end equalisatiqns which occur during a single cycle as illustrated in steps 1 and 5.

Feed end equalisation requires less adsorbent material in a given bed as compared to product end equalisation, for the following reasons. In product or outlet end equalisation, the bed at the higher pressure depressurises in a direction cocurrent to feed flow during the pressure equalisation step. This causes the mass transfer zone to advance toward the product end of the bed as the pressure decreases. In order to contain the mass transfer zone during this step to maintain product purity, a larger bed, i.e. a greater quantity of adsorbent material, is required. In feed end equalisation, on the other hand, the bed at the higher pressure depressurises in a direction countercurrent to feed flow during the equalisation step. In this step, due to the direction of the gas flow, the mass transfer zone does not advance. The countercurrent depressurisation is also beneficial for the subsequent purge step because nitrogen starts to flow toward the feed end of the bed during this step. The combination of no advancing of the mass transfer zone and countercurrent depressurisation reduces the amount of adsorbent material required.

Bed size factor is a quantity used to compare the amount of adsorbent material required from one system or cycle to another. At a given bed size factor, it has been determined that using feed equalisation produces oxygen at a higher purity as compared to using product end equalisation.

The combination of equalising pressures between an adsorption bed and the segregated storage adsorption bed, when the adsorption bed is at the end of the adsorption operation therein and prior to purging of the bed as illustrated in step 2, and thereafter equalising pressures between these same two components after purging of the adsorption bed, when it is at a relatively low pressure as illustrated in step 4, maximises the utilisation of the adsorption bed while at the same time maximising purity of the product. In particular during step 2, as the depressurising bed A equalises cocurrently to feed flow into the segregated storage adsorption bed C, part of the nitrogen contained in the mass transfer zone of bed A will be transferred into bed C. This allows for maximum and continuous utilisation of bed A, i.e. the mass transfer zone can be moved along bed A from inlet to outlet as far as possible. At the beginning of the flow from bed A to bed C the gas is rich in oxygen but as flow continues the gas gradually reverts to the composition of fresh air. In addition, the segregated storage adsorption bed recovers some potential energy from the depressurising adsorber and this, in turn, reduces system blowdown pressure and increases recovery and efficiency. Providing the segregated storage adsorption bed C in effect provides a mixing volume to s in Table I is for oxygen product at a purity of 90% and the oxygen recovery percentage is presented for both low pressure and high pressure delivery conditions. The abbreviations used are: S.S.T. for segregated storage tank and F.E.E. for feed end equalisation.

TABLE I Low High Pressure Pressure Delivery Delivery S.S.T. Absent 21% 21% S.S.T. Present But Empty 25% 23% S.S.T. Half Full Of Adsorbent Material 35% 31% S.S.T. Full And With F.E.E. 49% 48% Figure 3 shows a detail of another embodiment of a system according to the present invention wherein gas product can be withdrawn from the other end of the segregated storage adsorption bed C. In the system shown in Figure 3, any components identical to those of Figure 1 have the same reference numerals but primed. The system, only a detail of which is shown in Figure 3 would of course also include adsorption beds A' and B' (not shown) identical to those designated A and B in the system of Figure 1, along with similar connections to the gas inlets and outlets, the gas feed stream, waste outlet, and product output.

The end of the segregated storage adsorption bed C' opposite its inlet 34' is connected by a conduit 108 which contains an automatic valve 110 to the second output conduit section 74 upstream from regulator 76'. Upon opening of valve 110, product quality gas can be withdrawn from the segregated storage adsorption bed C' and introduced to the output conduit and this can be advantageous in situations where low pressure rather than high pressure product delivery is needed. In addition, when product is delivered from the segregated storage adsorption bed C', the bed serves also as a product surge tank enabling product to be withdrawn from the system at a high flow rate for a short period of time before the mass transfer zone breaks through that end of the bed. On the other hand, recovery from a breakthrough condition can be relatively slow. Another advantage of withdrawing product gas from the segregated storage bed C' is that it provides a relatively higher rate of recovery of product. This is because withdrawal of product from storage bed C' reduces the pressure therein so that when the pressure next equalises with either of the adsorption beds A' and B' that adsorption bed will be at a lower pressure. The lower pressure, in turn, imposes a lower blow-down requirement for purging that adsorption bed with a result that less gas is released to the atmosphere. This reduction in the waste losses results in a higher percentage of product recovery. Another advantage associated with the double-connected segregated storage bed C' involves feed end equalisation which lowers the front of the mass transfer zone in each of the other two beds A' and B' so that when the beds are equalised from the tops with the segregated storage bed C' there is less nitrogen to be taken up by the segregated storage adsorption bed.

As shown in Figure 3, the system can also include a third reservoir conduit 114, connected at one end to the reservoir 84' and coupled at the other end to the adsorption beds. In the present illustration, the "bed" end of conduit 114 is connected to the flow path containing the automatic valves 60A' and 60B' and is connected between these valves. Conduit 114 contains an automatic valve 116 which, upQn opening, allows product gas from reservoir 84' to flow to the adsorption beds to be used for operations such as purging and repressurisation.

The primary role of the reservoir in the systems of Figures 1 and 3 is to hold a reserve supply of product gas in the event of equipment malfunction or power failure. This is of particular importance when the system supplies oxygen for medical use. Under normal operating conditions the reservoir is at a pressure of 28-29 p.s.i.g., and product oxygen flows through valve 72 and regulator 76 to product outlet 66. If electrical power fails or is otherwise interrupted the valve 72 closes and this is sensed by control 100 which will open valve 96. Oxygen flow continues from the reservoir through valve 96 to the output conduit to outlet 66 until the supply in the reservoir is depleted. An alarm can be sounded to indicate the power interruption.

The reservoir also can be used to supply part or all of the purge oxygen required, for an adsorption bed during its purge step, simply by opening valve 116 at the appropriate time. The reservoir also can be used as another surge tank.

Pressure equalisations to and from the adsorption beds can be accomplished through the correct sequencing of valves 116 and 62.

When the reservoir is supplying reserve oxygen, in the event of a malfunction, the length of time the reserve oxygen lasts depends on the pressure in the reservoir at the time of the malfunction. If the reservoir is only ever used as a back-up oxygen supply, the reservoir pressure will be at its maximum at all times, whereas if the reservoir is used to supply supplemental purge and/or repressurisation gas, the pressure in the reservoir will vary as will the reserve supply of oxygen. The reservoir can comprise an adsorption bed but it also can comprise an ordinary tank of larger size.

Figure 4 shows an arrangement for controlling the system of Figure 1 or 3.

The output conduit 74' can be connected to a tank or similar storage vessel 120, and gas product can be withdrawn therefrom through a conduit 122 for use. The sequencing and timing of the system including the control of the automatic valves is achieved by a system control unit designated 124, and control signals generated by the control unit 124 are transmitted by lines collectively designated 126 to the valves and other appropriate components of the system. Generally, the control 124 is responsive to the pressure of product gas within the storage vessel 120, and to this end a pressure sensor 130 is operatively connected to the storage vessel 120 by the connection 132. The output from the sensor 130 is connected by a line 134 to an additional control unit 136 which, in turn, is connected in controlling relation to the system control unit 124 by the connection 138. It has been determined that once operation of the system has begun there is an optimum time at which to terminate operation, both in terms of a minimum number of cycles to be completed and a point within a cycle to terminate operation. The additional control unit 136 functions to cause the system control unit 124 to ignore any "shut-down" signal and to maintain operation of the system; once begun, for a predetermined number of cycles. It has been determined that, in a system for producing oxygen from feed air, a total of two complete cycles provides desirable results. One complete cycle includes steps 1 to 8 tabulated in Figure 2.

Furthermore, it has been determined that there is an optimum point within a cycle at which operation of the system should be terminated, and this will be when the pressures are equal in the two adsorption beds A and B, i.e. at the beginning of step 2 or step 6. Thus, the additional control unit 136 also functions to ignore any "shut-down" signal and to stop the system only after two complete cycles have been completed and only at an optimum point within the next following cycle when the pressures are equal in the two adsorption beds A and B. The additional control unit 136 can, for example, be of the cam type or step switch type. Thus, the system control unit 124 is responsive to gas pressure in storage vessel 120, signalled by sensor 130, for stopping operation of the process and system normally when gas pressure in storage vessel 120 reaches a predetermined magnitude. The additional control unit 136 thus overrides the system control unit 124 to terminate operation of the process and system only at a predetermined instant in the first cycle after the completion of the minimum number of cycles as determined by system control unit 124.

Various aspects of the process and apparatus described above are also described and claimed in our co-pending Patent Applications Nos. 8002359 and 8002358 (Serial Nos. 1593540 and 1593539, respectively).

WHAT WE CLAIM IS: 1. A pressure swing process for fractionating at least one component from a gaseous mixture by selective adsorption in each of at least two adsorption beds by sequentially passing the gaseous mixture from a feed stream through a first adsorption bed until the first bed is about saturated while simultaneously purging and then pressurising a second adsorption bed, and then passing the gaseous mixture from the feed stream through the second adsorption bed until the second bed is about saturated while simultaneously purging and then pressurising the first adsorption bed, part of the pressurising.gas for each of the adsorption beds at the end of its purging phase being derived by pressure equalisation between the feed ends of the first and second beds, whereby the withdrawn gas from one of said adsrgtion beds at the end of the adsorption operation therein is in a direction countercurrent to feed flow and is introduced along with the gaseous mixture from the feed stream into the other of said adsorption beds in a direction cocurrent with feed flow and after partial pressurisation of the said other of the adsorption beds.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. Pressure equalisations to and from the adsorption beds can be accomplished through the correct sequencing of valves 116 and 62. When the reservoir is supplying reserve oxygen, in the event of a malfunction, the length of time the reserve oxygen lasts depends on the pressure in the reservoir at the time of the malfunction. If the reservoir is only ever used as a back-up oxygen supply, the reservoir pressure will be at its maximum at all times, whereas if the reservoir is used to supply supplemental purge and/or repressurisation gas, the pressure in the reservoir will vary as will the reserve supply of oxygen. The reservoir can comprise an adsorption bed but it also can comprise an ordinary tank of larger size. Figure 4 shows an arrangement for controlling the system of Figure 1 or 3. The output conduit 74' can be connected to a tank or similar storage vessel 120, and gas product can be withdrawn therefrom through a conduit 122 for use. The sequencing and timing of the system including the control of the automatic valves is achieved by a system control unit designated 124, and control signals generated by the control unit 124 are transmitted by lines collectively designated 126 to the valves and other appropriate components of the system. Generally, the control 124 is responsive to the pressure of product gas within the storage vessel 120, and to this end a pressure sensor 130 is operatively connected to the storage vessel 120 by the connection 132. The output from the sensor 130 is connected by a line 134 to an additional control unit 136 which, in turn, is connected in controlling relation to the system control unit 124 by the connection 138. It has been determined that once operation of the system has begun there is an optimum time at which to terminate operation, both in terms of a minimum number of cycles to be completed and a point within a cycle to terminate operation. The additional control unit 136 functions to cause the system control unit 124 to ignore any "shut-down" signal and to maintain operation of the system; once begun, for a predetermined number of cycles. It has been determined that, in a system for producing oxygen from feed air, a total of two complete cycles provides desirable results. One complete cycle includes steps 1 to 8 tabulated in Figure 2. Furthermore, it has been determined that there is an optimum point within a cycle at which operation of the system should be terminated, and this will be when the pressures are equal in the two adsorption beds A and B, i.e. at the beginning of step 2 or step 6. Thus, the additional control unit 136 also functions to ignore any "shut-down" signal and to stop the system only after two complete cycles have been completed and only at an optimum point within the next following cycle when the pressures are equal in the two adsorption beds A and B. The additional control unit 136 can, for example, be of the cam type or step switch type. Thus, the system control unit 124 is responsive to gas pressure in storage vessel 120, signalled by sensor 130, for stopping operation of the process and system normally when gas pressure in storage vessel 120 reaches a predetermined magnitude. The additional control unit 136 thus overrides the system control unit 124 to terminate operation of the process and system only at a predetermined instant in the first cycle after the completion of the minimum number of cycles as determined by system control unit 124. Various aspects of the process and apparatus described above are also described and claimed in our co-pending Patent Applications Nos. 8002359 and 8002358 (Serial Nos. 1593540 and 1593539, respectively). WHAT WE CLAIM IS:

1. A pressure swing process for fractionating at least one component from a gaseous mixture by selective adsorption in each of at least two adsorption beds by sequentially passing the gaseous mixture from a feed stream through a first adsorption bed until the first bed is about saturated while simultaneously purging and then pressurising a second adsorption bed, and then passing the gaseous mixture from the feed stream through the second adsorption bed until the second bed is about saturated while simultaneously purging and then pressurising the first adsorption bed, part of the pressurising.gas for each of the adsorption beds at the end of its purging phase being derived by pressure equalisation between the feed ends of the first and second beds, whereby the withdrawn gas from one of said adsrgtion beds at the end of the adsorption operation therein is in a direction countercurrent to feed flow and is introduced along with the gaseous mixture from the feed stream into the other of said adsorption beds in a direction cocurrent with feed flow and after partial pressurisation of the said other of the adsorption beds.

2. A process according to claim 1, wherein a segregated storage adsorption

bed is provided, isolated from direct communication with the feed gas stream; wherein after pressure equalisation of said adsorption beds from the feed ends thereof the gaseous mixture from the feed gas stream is adsorbed in one of said adsorption beds to release product gas from the outlet of said one bed, and the pressures of the other of said adsorption beds and said segregated storage adsorption bed are equalised by withdrawing low purity gas from said other of the adsorption beds in a direction cocurrent with feed flow and introducing said low purity gas into one end of said segregated storage adsorption bed; wherein thereafter said other of the adsorption beds is purged by diverting product gas from the outlet of the said one of the adsorption beds into said other adsorption bed in a direction countercurrent to feed flow; and wherein the pressure of said segregated storage adsorption bed is then again equalised with that of said other adsorption bed by withdrawing gas from said one end of said segregated storage adsorption bed and introducing the withdrawn gas into said other adsorption bed in a direction countercurrent to feed flow.

3. A process according to claim 2, wherein after the last pressure equalisation step set out in claim 2, all the steps of claim 2 are consecutively repeated beginning with pressure equalisation of said beds from the feed ends thereof, reversing the functions of said one of the adsorption beds and said other of the adsorption beds.

4. A process according to claim 2 or 3, further including withdrawing product gas from the other end of said segregated storage adsorption bed.

5. A process according to claim 3 or 4, wherein said process steps are controlled in a manner such that said process is continued for a predetermined number of identical cycles.

6. A process according to claim 3, 4 or 5, wherein said process steps are controlled in a manner such that the process is terminated only at- an optimum point in the process cycle.

7. A process according to any one of claims 2 to 6, wherein at the end of the adsorption operation and prior to a purging step in one of said first and second adsorption beds after said one bed is saturated, low purity gas is withdrawn therefrom in a direction cocurrent with feed flow; the withdrawn low purity gas is introduced to only one end of said segregated storage adsorption bed as the sole gas introduced into said segregated storage adsorption bed such that, during the withdrawal of low purity gas from said one adsorption bed and its introduction into said segregated storage adsorption bed, the mass transfer zone is moved from said one adsorption bed into said segregaged storage adsorption bed; and gas is withdrawn from said one end of said segregated storage adsorption bed and passed into said at least one adsorption bed in a direction countercurrent to feed flow after said purging step, the feed end of said one adsorption bed being closed during such passage whereby said gas aids in repressurising said one adsorption bed.

8. A process according to claim 7, wherein after withdrawal of gas from the outlet of said one adsorption bed into said segregated storage adsorption bed, the flow path between said one adsorption bed and said segregated storage adsorption bed is closed and said one adsorption bed is purged by flow therethrough of a purging gas in a direction which is countercurrent to the feed direction, before gas from said segregated storage adsorption bed is flowed back into said one adsorption bed to repressurise said one adsorption bed after purging.

9. A process according to claim 7 or 8, wherein the return of gas from said segregated storage adsorption bed into said one adsortion bed after purging takes place when said one adsorption bed is at a relatively low pressure and the feed end of said one adsorption bed is closed, whereby said returning gas repressurises said one adsorption bed.

10. A process according to any one of claims 2 to 6, wherein low purity gas is withdrawn from one of the first and second adsorption beds in a direction cocurrent with feed flow when said one bed is at the end of the adsorption operation therein and prior to purging of said one bed; the withdrawn low purity gas is introduced to only one end of said segregated storage adsorption bed as the sole gas introduced into said segregated storage adsorption bed; gas is withdrawn from said one end of the segregated storage adsorption bed and passed into said one adsorption bed in a direction countercurrent to feed flow after purging of said one adsorption bed and when the said one adsorption bed is at a relatively low pressure, the feed end of said one adsorption bed being closed during such passage, whereby said gas starts repressurising said one adsorption bed; and product gas is withdrawn from the other end of said segregated storage adsorption bed and is recovered directly without further treatment.

11. A process according to any one of the preceding claims, wherein the gaseous mixture is air and the product gas is mainly oxygen.

12. A process according to any one of the preceding claims, wherein during adsorption on each of said first and second adsorption beds the feed gas mixture is flowed into the inlet end of one of the respective beds and product gas is flowed out of the outlet of said respective bed along an outlet conduit system in an unrestricted manner in a flow direction away from the bed and towards a main product outlet conduit whereas during purging of said respective bed using product gas from the other of said second adsorption beds the flow or purging gas into the said respective bed passing along said outlet conduit system in a direction from the main product outlet conduit towards the respective bed is restricted.

13. A process according to claim 12, wherein said purging gas flow is restricted using means for simultaneously and automatically restricting flow towards said respective bed but allowing unrestricted flow in a direction away from said bed and towards said main product outlet conduit.

14. A process according to claim 13, wherein said means for simultaneously and automatically restricting flow comprises (a) a first polarised flow control valve, between the product gas outlet of said first adsorption bed and said second adsorption bed, designed to allow unrestricted gas flow in a direction from said outlet of said first bed toward said second adsorption bed but controlled flow in a direction towards said gas outlet of said first adsorption bed; and (b) a second polarised flow control valve, between the product gas outlet of said second adsorption bed and said first adsorption bed, allowing unrestricted gas flow in a direction from said outlet of said second bed toward said gas outlet of said first adsorption bed but controlled flow in a direction toward said gas outlet of said second bed, the main product outlet conduit being connected to a location in the flow path of gas between said first and second polarised flow control valves.

15. A process according to claim 14, and including the use of an isolation valve in the gas flow path which connects the product gas outlets of each of said first and second adsorption beds by way of said first and second polarised flow control valves.

16. A process according to any one of claims 12 to 15 and including the use of a product gas reservoir connected to the main product outlet conduit, and comprising the steps of normally preventing flow of product gas out of said reservoir, and flowing product gas from said reservoir to said main product outlet conduit when normal gas flow from said bed to said product outlet conduit fails, whereby product gas continues to be delivered to said product outlet conduit even after failure of normal product gas flow, until such time as said reservoir is exhausted.

17. A process according to claim 16, wherein normal flow of product gas in said main product outlet conduit is controlled by a valve which is designed to shut down in response to failure of mains power supply, and wherein flow of product gas from said reservoir to said main product outlet conduit is controlled to begin when the said mains power supply responsive valve closes.

18. A process according to claim 6, wherein said optimum point is at an instant when the pressures in the said first and second adsorption beds are equalised.

19. A process according to any one of the preceding claims, wherein during said selective adsorption the flow into the feed end of the respective adsorption bed and the flow of gas withdrawn from the other end of that bed are controlled such that said bed undergoes a decrease in pressure during said adsorption.

20. A pressure swing process for fractionating at least one component from a gaseous mixture by selective adsorption in each of at least two adsorption beds by sequentially passing the gaseous mixture from a feed stream through a first adsorption bed until the first bed is about saturated and the purity of the product gas reduces while simultaneously purging and then pressurising a second adsorption bed and then passing the- gaseous mixture from the feed stream through the second adsorption bed until the second bed is about saturated while simultaneously purging and then repressurising the first adsorption bed; withdrawing product gas from the feed end of one of the said at least two adsorption beds countercurrent to feed flow when said one bed is at the end of the adsorption operation therein and introducing it along with the gaseous mixture from the feed stream to the feed end of another of said at least two adsorption beds after partial pressurisation thereof to repressurise. said another bed before adsorption therein; withdrawing reduced purity product gas from said one of the said at least two adsorption beds in a direction cocurrent with feed flow as the pressure in said one bed decreases at the end of the adsorption operation therein .and prior to purging of the said one bed; introducing the withdrawn reduced purity product gas to only one end of a storage adsorption bed which is segregated from the feed stream but is able to be communicated with an outlet end of each of said at least two adsorption beds, said withdrawn reduced purity product gas being the sole gas introduced into said segregated storage adsorption bed; then withdrawing gas from said one end of the segregated storage adsorption bed and passing said withdrawn gas into said one adsorption bed in a direction countercurrent to feed flow after purging of the said one bed and when the said one bed is at a relatively low pressure, the feed end of said one adsorption bed being closed during such passage, whereby said gas starts repressurising said one adsorption bed; and withdrawing product gas from the other end of said segregated storage adsorption bed and recovering it directly without further treatment.

21. A process according to claim 20, wherein the repetition of pressurising, adsorbing, depressurising and purging is controlled to continue until an instant in the repetition; optimum for shut-down.

22. A process according to claim 21, wherein said optimum point is decided on the basis of both an optimum point during the cycle and the completion of a minimum number of complete cycles from start-up.

23. A process according to claim 22, wherein said minimum number of cycles is 2.

24. A process according to claim 21, 22 or 23, wherein said continuation of the repetition terminates at an optimum point in the process, and said optimum point is at an instant when the pressures in the said at least two adsorption beds are equalised.

25. A process according to any one of the preceding claims, wherein during adsorption on each of said first and second adsorption beds the feed gas mixture is flowed into the inlet end of one of the respective beds and product gas is flowed out of the outlet of said respective bed along an outlet conduit system in an unrestricted manner in a flow direction away from the bed and towards a main product outlet conduit whereas during purging of said respective bed using product gas from the. other of said first and second adsorption beds the flow of purging gas into the said respective bed passing along said outlet conduit system in a direction from the main product outlet conduit towards the respective bed is restricted.

26. A process according to claim 25, wherein said purging gas flow is restricted using means for simultaneously and automatically restricting flow towards said respective bed but allowing unrestricted flow in a direction away from said bed and towards said main product outlet conduit.

27. A process according to claim 26, wherein said means for simultaneously and automatically restricting flow comprises (a) a first polarised flow control valve, between the product gas outlet of said first adsorption bed and said second adsorption bed, designed to allow unrestricted gas flow in a direction from said outlet of said first bed toward said second adsorption bed but controlled flow in a direction towards said gas outlet of said first adsorption bed; and (b) a second polarised flow control valve, between the product gas outlet of said second adsorptiori bed and said first adsorption bed, allowing unrestricted gas flow in a direction from said outlet of said second bed toward said gas outlet of said first adsorption bed but controlled flow in a direction toward said gas outlet of said second bed, the main product outlet conduit being connected to a location in the flow path of gas between said first and second polarised flow control valves.

28. A pressure swing process for fractionating at least one component from a gaseous mixture by selective adsorption, such process being substantially as hereinbefore described with reference to the accompanying drawings.