GB2091121A

GB2091121A - Separation of gas mixtures

Info

Publication number: GB2091121A
Application number: GB8201032A
Authority: GB
Original assignee: Cryoplants Ltd
Current assignee: Cryoplants Ltd
Priority date: 1981-01-16
Filing date: 1982-01-14
Publication date: 1982-07-28
Also published as: GB2091121B

Abstract

In a pressure swing adsorption process typically for the separation of oxygen from air relatively low power consumption can be achieved with smaller beds than those used in conventional plants by using zeolite molecular sieve of particle size less of 0.4 to 3 mm and operating a cycle of passing air through each bed; venting the bed; purging the bed with product oxygen, and raising the pressure of the bed with product oxygen. Furthermore, a short, as compared to conventional processes, cycle time is employed (up to 45 secs). There are typically two beds operating such cycles 180 DEG out-of-phase with one another, the purge gas and pressuring gas for one cycle being taken from the other cycle. In the venting, purging and pressurising steps the flow of gas is in the opposite direction to the flow of air through the bed. The difference in pressure between the feed pressure and the average pressure of the product gas (before any compression thereof) is less than R x 48 kPa m<-1> where R is the height of each bed.

Description

SPECIFICATION Separation of gas mixtures This invention relates to a process for the separation of gas mixtures.

It is known to separate a gas mixture such as air into one stream enriched in one component of the mixture and another stream depleted in said component by passing the gas mixture through a bed of molecular sieve that selectively or preferantially adsorbs another component of the mixture. In commercial practice, such processes employ two or more beds oftheadsorbenttoenableeach bed in turn to be regenerated while a continuous stream of the enriched gas mixture is produced. Regeneration of each bed is always performed at a lower pressure than that at which adsorption takes place. Hence, such processes are often referred to as 'pressure swing adsorption' or 'PSA' processes.

Commercial plants exist for enriching air in oxygen by a PSA process in which beds of zeolite molecular sieve having a particle size in the range 3 to Smm are employed. Typically there may be three beds and each bed may perform a cycle of operations as set out in Table A or Table B accompanying UK specification no. 1 449 864. Thus, the cycle performed on each bed may be as follows. Typically, air is compressed to a pressure in the range 1.1 to 10 bars and passed through a bed of adsorbent (the 'operating bed'). The operating bed is then returned to atmospheric pressure by venting the adsorbed gas in a direction counter to that in which the gas mixture passed through the operating bed. Residual gas is evacuated by means of a vacuum pump in the same direction as the venting.The adsorption pressure is restored by passing oxygen of product purity from another bed into the operating bed in the opposite direction to that followed by the compressed air. Finally, oxygen of less than product purity is passed from the other bed into the operating bed in the same direction as that followed by the compressed air; and product oxygen is simultaneously taken from the outlet of the bed. During the part of the cycle in which compressed air is passed through the operating bed, oxygen of product purity is used to repressurise another bed. Restoration of the pressure is typically achieved as the purity of the oxygen produced by the operating bed begins to fall. The impure oxygen is passed to the other bed and product oxygen withdrawn from its outlet end.

Typically, the complete cycle of operations last for three minutes for a plant employing three beds and is repeated throughout the period for which the plant is operated.

Although the above-described process has been operated successfully on a commerical basis we believe that it is capable of improvement so far as its operating efficiency is concerned. In particular, we believe it to be possible to improve the power consumption (per unit volume of gas produced) and reduce the size of the adsorbent beds (for a given rate of production of gas).

US patent specification No. 4 194891 relates to a process which seeks interalla to make it possible to achieve such improved power consumption with smaller beds than are used in conventional plant. The aforementioned US patent specification discloses a rapid adiabatic pressure swing process for air separation to produce at least 35 mole per cent oxygen product gas in which:: (i) feed air at 10 to 50 psig (170 to 446 kPa) is introduced to thefirst end of an adsorbent bed having an end-to-end length of 1 to 3.5 feet and comprising crystalline zeolite molecular sieve of at least 5 Angstrom apparent pore size and 40 to 120 mesh particle size; (ii) nitrogen is selectively adsorbed and oxygen product gas is continuously discharged from the second end of the bed during a feed air introduction period; (iii) a reverse outward flow period follows the feed air introduction period during which oxygen purging-nitrogen desorbate gas is released from the first end of the bed; (iv) the feed air introduction period and the reverse outward flow period follow one another in a repetitive two step cycle; (v) there are two or three beds arranged in an alternative flow sequence with a single product manifold joined to the second end of such beds;; (vi) each bed has a feed air introduction period of 0.1 to 6 seconds, and a reverse outward flow period with oxygen product gas flowing directly from one bed to the second end of the other as purge such that the reverse outward flow period/feed air introduction period time ratio is at least 0.5 but less than 2; (vii) the total cycle time is 0.2 to 18 seconds; and (viii) prior to the succeeding feed air introduction period, oxygen product gas discharged from a different bed flows directly to the second end without release of gas from the first end as a product repressurisation period not exceeding 1.5 times the feed air introduction period.

One disadvantageous feature of this process is that there is a relatively high pressure drop from end-to-end of the bed. This pressure drop is disclosed to be in the order of 12 psi per ft bed length (270 kPa m-1). Typical practical pressure drops are illustrated in Figure 11 of US patent specification No. 194891.

One of the consequences of such a pressure drop is that in certain uses of the gas supplied by the process it becomes necessary to recompress the product gas. Where the product gas is oxygen a special compressor is required for this purpoe including a number of safety features whereby any fire or explosion risk consequent upon the compression of the oxygen is avoided. In view of the running and capital costs of such a compressor it is desirable to avoid its use wherever possible and accordingly it is desirable to avoid the excessive pressure drops from end-to-end of the beds that are a feature of the process described in US patent specification No. 4194 891.

It is also known from US patent specification No. 4194 892 to use a single bed in a rapid cycling PSA process. Such a process suffers from the disadvantages that large end-to-end pressure drops occur as the feed air flows through the bed and that it is typically necessary to have periods in the operating cycle in which no airflows into the bed.

It is an aim of the present invention to provide a process which (at least in a preferred form) is capable of being operated more economically than those specifically described in UK patent specification No. 1 449864 without giving occasion to large individual pressure drops from end-to-end of an adsorption bed.

According to the present invention there is provided a process of separating a gas mixture comprising two or more components so as to form a product gas whose mole fraction of one component is greater than the mole fraction of said component in the gas mixture, in which process the following cycle of steps is repeated:: (a) passing said gas mixture, under pressure, through a bed comprising particles of zeolite molecular sieve which preferentially adsorb one or more other components of the gas mixture and thereby obtaining, as the non-adsorbed gas, a stream of product gas; (b) stopping the flow of said gas mixture into said bed; (c) venting gas from said bed in a direction counter to that followed by the said gas mixture in passing through the bed; (d) passing a purge gas through said bed in a direction counter to that followed by the said gas mixture as it passes through the bed; (e) increasing the pressure in the bed by passing a pressurising gas into the bed (typically in a direction counter to that followed by the said gas mixture as it passes through the said bed) while restricting or preventing the escape of gas from the bed; and in which process; (i) at least two such cycles are simultaneously performed out-of-phase with one another and at least two adsorbent beds are employed, one in each cycle; (ii) the purge gas and the pressurising gas for each cycle are taken from product gas obtained from the other or another cycle; (iii) the particle size of substantially all said particles of molecular sieve is in the range 0.4 to 3mm; (iv) the difference between the pressure at which the said gas mixture enters each bed and the average pressure of the product gas (before any compression thereof) is less than R x 48 kPa m-1 where R is the height of each bed in metres; and (v) each said cycle lasts up to 45 seconds.

The process according to the present invention is particularly suited to the separation of a gas stream rich in oxygen from air. For example, 90% pure oxygen may be produced. In a preferred process according to the invention suitable for producing approximately 90% pure oxygen, substantially all the particles of the molecular sieve have a particle size in the range 0.5 to 3mm and most preferably in the range 0.5 to 1.2 mm and preferably the cycle time is in the range 15 to 30 seconds and most preferably is in the range 18 to 24 seconds. By using such relatively small particles and short cycle times (in comparison to those conventionally employed) it is possible we believe to produce, from air, oxygen of ninety per cent purity without the necessity to use a vacuum pump to assist in venting gas from the beds.Moreover, it is possible to obtain such a product at a pressure only marginally below the air feed pressure without having to employ a compressorforthe product.

Preferably,thegas mixtureto be separated is supplied orfedto each bed ata pressure of at least 170 kPa (10 psig). If the gas mixture is not available at the desired feed pressure a compressor may be employed to raise it to the desired pressure.

Since step (a) of the cycle is performed immediately after step (e) the bed contains product gas at the time in the cycle at which the gas mixture starts to flow into the bed.

Preferably the product gas is passed into a reservoir communicating with each bed. Product may be withdrawn from the reservoir at a rate determined by the setting of a flow control valve associated with an outlet from the reservoir. Use of the reservoir facilitates supply of product gas at a substantially constant pressure and flow rate. Instead of employing a reservoir downstream of the beds it is possible to provide one upstream of the beds in communication with the source of the gas mixture to be separated.

In the method according to the present invention there is preferably no flow of product gas into a bed during the period in which the said gas mixture is flowing into or through said bed.

If the gas mixture to be separated is air, provision is preferably made for removing water vapour therefrom upstream of the zeolite molecular sieve. For this purpose, activated alumina, silica gel or other drying agents may be employed. If desired, particles of the drying agent may be included in each bed as a lower layer.

Typically, in performing the method according to the present invention the gas velocities through each bed will be sufficient to tend to fluidise or 'lift' the particles of zeolite. Accordingly, it is desirable to restrain the particles against being fluidised or lifted. Such restraint may be effected by means of a compressive force applied to the bed by means of, for example, a spring-loaded grid or perforated plate.

Typically, if there are n beds and hence n cycles being formed simultaneously (n being an integer greater than 1 ) step (a) of each cycle is performed for a period of time equal to 1/n x the total cycle time. Preferably, there are just two beds and hence two cycles operating simultaneously, and thus step (a) of each cycle occupies half the cycle time.

Typically, gas is vented from each bed by placing it in communication with the surrounding atmosphere and thus the pressure in the bed will fall from its upper working pressure (that is the pressure of the gas mixture to be separated) to atmospheric pressure. If, however, the method according to the invention is operated on board an aeroplane in flight the pressure at the level at which the aeroplane is flying will be less than 1 bar and it is within the scope of this invention in such circumstances to vent the bed by placing it in communication with the atmosphere. We prefer, however, not to use a vacuum pump to assist in the venting of each bed as this will add both to the operating and capital costs of plant for performing the process according to the present invention.

In a typical plant for performing the process according to the invention, each bed is located in a cylindrical or columnar having an inlet at its base for the gas mixture to be separated, an outlet at its top for product gas, an inlet in its top for purge gas, and an inlet in its top for pressurising gas, and a valve in each inlet. Changing over from one step of the cycle to the next may thus be carried out by appropriately opening and closing the inlet valves. Thus, the flow of gas mixture into a bed may be stopped by closing the air inlet valve. This is preferably done simultaneously with opening the purge valve, ie there is no interval of time between the completion of step (b) and the start of step (c). Similarly, we prefer there to be no substantial interval between the end of step (d) and the start of step (e) and the start of step (a).The end of step (c) and the start of step (d) is typically merely marked by the opening of the purge valve.

Each bed is preferably pressurised in step (e) of the cycleto a pressure intermediate the pressure under which the gas mixture enters said bed and atmospheric pressure (1 bar (a)). The pressurisation step (step (e)) is preferably performed by passing the pressurising gas into the bed in a direction counter to that in which the gas mixture flows through the bed. Typically, each pressurisation step (step (e)) has a duration of from 0.2 to 0.25 times the duration of step (a).

We have found that the specific power consumption in operating the process according to the invention varies with the pressure at which the gas mixture is fed to the beds. With 'bound' 5A molecuar sieve having a particle size in the range 0.5 to 1.2 mm the specific power consumption decreases with increasing feed pressure until a minimum is reached and then increases again, and we prefer the gas mixture to be fed to each bed at a pressure in the range 198 to 515 kPa (14 to 60 psig).

We have also found that product purity tends to increase with increasing purge time and accordingly prefer that the duration of step (d) is from 55 to 70% of the duration of step (a) and that the ratio of the duration of step (d) to that step (c) is in the range 1.5:1 to 4:1.

Typically, in step (e) of each cycle the absolute pressure in the bed is increased to a value in the range of 50 to 80% of the absolute pressure at which the feed gas is passed into the bed.

The process according to the invention is typically performed under generally adiabatic conditions.

The process according to the present invention will now be described by way of example with reference to the accompanying drawings, of which: Figure 1 is a schematic flow diagram illustrating a plant suitable for use in separating oxygen of 90% by volume purity from air by the process according to the present invention.

i:igure 2 is a table illustrating suitable timings for opening and closing valve shown in Figure 1 so as to perform the process according to the invention; Figure 3 is a graph illustrating the variation in estimated power consumption with feed pressure in the process according to the invention; Figure 4 is a graph illustrating the variation in specific product with feed pressure in the process according to the invention, and Figure 5 is a graph illustrating the variation in product purity with purge time and the volume of purge gas in the process according to the invention.

Referring to Figure 1 of the accompanying drawings, the illustrated plant has a pipeline 2 for incoming air leading to a compressor 4. The outlet of the compressor 4 communicates with a pipeline 6 which is adapted to feed an inlet pipe 8 of a first upright cyclindrical vessel 12 and an inlet pipe 10 of a second upright cyclindrical vessel 14. The inlet pipes 8 and 10 terminate in the bottom of their respective vessels 12 and 14.

The vessels 12 and 14 contain beds 16 and 18 respectively comprising zeolite molecular sieve. The choice of the particular zeolite to be used will depend on the composition of the gas mixture to be separated and the product. Typically, for obtaining oxygen (of 90% by volume purity) from air a bound 5A type of zeolite may be used, that is to say a zeolite with an apparent pore size of 5 Angstroms. The zeolite may be naturally occurring or may be synthetic. Typically, the zeolite having a particle size in the range 0.5 to 1.2 mm is used.

Each bed is supported on a lower bed support grid or plate 20 formed with perforations or orifices smaller than the smallest particle size of zeolite in the bed. Pressure is exerted on the top of each bed by means of an upper grid or plate 24 substantially identical to the lower plate or grid 20. The grids or plates 24 are typically loaded by compression springs 26.

The vessels 12 and 14 have outlet pipes 28 and 30 respectively. In the pipes 28 and 30 are disposed non-return valves 34 and 36 respectively. The outlet pipes 28 and 30 terminate in a pipeline 32 leading to a reservoir 40. A non-return valve 38 is disposed in the pipeline 32 just upstream of the inlet to the reservoir 40.

The reservoir 40 has a single outlet which communicates with a product outlet pipeline 42 having a flow control valve 44 disposed therein.

Associated with the bed 16 and 18 are eight valves each of which has two positions and is operable automatically on a predetermined time sequence by means of a suitable controller (not shown). In one position each such valve is shut and in its other position it offers substantially no impedance to the flow of gas. Such valves shall be referred to in the ensuing description as 'on-off' valves so as to distinguish them from flow control valves used to set various flow rates in the plant.

In the bed inlet pipes 8 and 10 are 'on-off' valves 50 and 52 respectively. These valves shall be referred to as 'feed' valves in the ensuing description.

A purge pipeline 66 communicates with the pipeline 32 and is adapted to conduct oxygen of product purity therefrom to either one of two purge pipes 78 and 80 which communicate with pipes 28 and 30 respectively and in which are disposed on-off valves 58 and 60 respectively. The valves 58 and 60 shall be referred to in the ensuing description as purge valves. Upstream of its union with the pipes 78 and 80 the purge pipeline 66 has disposed in it a flowmeter 68 and, downstream of the flowmeter 68, a flow control valve 70.

Also communicating with the pipeline 32 is a backfill or pressurising pipeline 82 which serves backfill or pressurising pipes 86 and 88 communicating with pipes 28 and 30 respectively. In the pipes 86 and 88 are on-off valves 62 and 64 respectively, referred to in the ensuing description as pressurising or backfill valves.

Extending between the inlet pipes 8 and 10 is a vent pipeline 72. A vent pipe 74 has an inlet terminating in said pipeline 72 and an outlet communicating with the ambient atmosphere. An on-off valve 54 is disposed in the pipeline 72 intermediate its unions with the inlet pipe 8 and vent pipe 74, and on-off valve 56 is disposed in the pipeline 72 intermediate its unions with the inlet pipe 10 and vent pipe 74. The valves 54 and 56 are described in the ensuing description as vent valves.

The plant shown in Figure 1 may be operated according to the cycle shown in Figure 2. Air is compressed in the compressor 4 to a pressure typically in the range 170 to 446 kPa (10 to 50 psig) and then freed from oil, liquid water and particulate matter. At the time T = 0 with feed valve 50 open and the other on-off valves associated with the bed 16 shut the compressed air is fed into the bottom of the bed 16 containing gas of product purity (90% oxygen) having been pressurised to a pressure intermediate the feed pressure and atmospheric in the pressurising step of the immediately preceding cycle. At the time T = 0 thereservoir 40 contains gas at a pressure created in the previous feed step and is thus at a higher pressure than the bed 16.

Thus, at time T = O no gas is flowing from the bed 16 into the reservoir 40. (The non-return valve 38 prevents gas flow in the opposite direction.) However, the pressure in the bed 16 rapidly rises to a value in excess of that in the reservoir 40 and thus gas will flow from the bed 16 into the reservoir. There is thus a flow of air through the bed 16. As the air encounters zeolite molecular sieve so nitrogen is adsorbed and thus only oxygen-enriched air leaves the bed and passes into the outlet pipe 28. The period of time for which air flows through the bed before the feed valve is closed is chosen so as to ensure that the purity of the product oxygen does not substantially decline. As shown in Figure 2 the feed step may last 10 seconds.

The aforesaid steps (a) to (e) are performed using bed 18 simultaneously with the cycle prformed using the bed 16 but 180 out-of-phase therewith. Thus at time T = 0 the vent valve 56 is opened and unadsorbed gasin the bed 18 at the feed pressure flows back through the bed 18 in the opposite direction to that followed by air in the preceding part cycle and vents therefrom through the inlet 10, vent pipeline 72, and vent pipe 74.

The pressure in the bed 18 thus falls to atmospheric pressure, and as the pressure falls so nitrogen is desorbed from the zeolite molecular sieve, and some of the desorbed gas may also be vented. In order to assist in exhausting the bed 18 of desorbed nitrogen the purge valve 60 is opened and thus a portion of the oxygen product flowing along the pipeline 32 is diverted into the pipeline 66 and flows into the bed 18 in the same direction as the gas being vented. This oxygen purges the desorbed nitrogen from the bed 18. The opening of the purge valve 60 marks the end of the vent step and the start of the purge step and typically takes place at time T = 1.8 seconds as shown in Figure 2.

Although it is possible to operate a cycle of steps in which the pressurisation of each bed is omitted, performing such a pressurisation (or backfilling) step enhances the purity of the product gas. Accordingly, at a chosen time after the start of the cycle being performed with the bed 18, the purge and vent valves 60 and 56 are closed and the pressurising valve 64 is simultaneously opened. As shown in Figure 2, this may take place at time T = 8.2 seconds. A portion of the oxygen product flowing from bed 16 through the pipeline 32 is then diverted into the pressurising (or backfilling) pipeline 82 from which it flows through the pressurising valve 64 into the bed 18 in the direction opposite to that followed by the incoming air in previous cycles. The pressure in the bed 18 rises as it fills with such oxygen.

The pressurising step may be arranged to last a chosen time. Thus at, say, time T = 10 seconds, as shown in Figure 2, the pressurising step may be ended and a feed step simultaneoulsy started by closing pressurising valve 64 and opening feed valve 52. This occurs simultaneously with the bed 16 being switched from a feed step to vent step by the closing of feed valve 50 and the opening of vent valve 54. The period of pressurisation and the setting of the valve 84 are chosen such that at the end of the pressurisation step the bed 18 is at a chosen pressure intermediate the feed pressure and atmospheric pressure. As an alternative, it is of course possible to monitor the pressure in the bed 18 by means of pressure sensor 96 communicating with inlet 10 through a pipe 92 and stop the pressurising step when the desired pressure has been reached provided that the pressurising step is never allowed to encroach into the period during which air is to be fed into the bed 18.

Between times T = 10 seconds and T = 20 seconds the bed 16 is vented, purged and pressurised as indicated in Figure 2 in a manner precisely analogous the venting, purging and pressurising of the bed 18 as described hereinabove. (A pressure sensor 94 may be provided in communication with the inlet 8 through a pipe 90 so as to sense the pressure in the bed 16 during pressurisation or backfilling). Also between times T = 10 seconds and T = 20 seconds the bed 18 receives compressed air from the compressor 4 and produces product oxygen typically of 90% purity in the manner described hereinabove with reference to bed 16.

The cycle shown in Figure 2 is continuously repeated to enable a continuous stream of oxygen to be supplied from the plant shown in Figure 1. Apart from at the start of each feed step there is continuous flow of oxygen into the reservoir 40. It is thus possible to withdraw oxygen from the reservoir 40 continuously at a substantially constant pressure and at a chosen flow rate depending on the setting of the valve 44. With molecular sieve of size in the range 0.5 to 1.2 mm there is no substantial pressure drop through the beds and thus the product oxygen can generally be supplied from the reservoir 40 at a pressure not more than 34 kPa (5 psi) below the feed pressure. In general, the pressure in the reservoir will fluctuate througout a cycle varying from 1 to 5 psi (7 to 34 kPa) less than the feed pressure.

The product flow rate will depend in part on the volume (ie quantity) and particle size of the zeolite molecular sieve and thus the overall volume of the beds will depend in part on the demand which the plant shown in Figure 1 is required to meet. Nonetheless, it can be stated that for a given oxygen purity the specific product (that is the flow rate per unit volume of adsorbent per unit time) is greater than it is in conventional (ie relatively slow cycling) PSA processes.

The relative timings of the different parts of the cycle may be different from that shown in Figure 2, and the duration of the cycle may be different from that shown in Figure 2.

The choice that is made will depend inter alia on the pressure at which the air is fed to the beds; the desired product purity, flow rate and pressure, and the particle size of the zeolite molecular sieve. For a given product purity it is desirable to keep the power consumption to a minimum. In Figure 3 is shown a graph of estimated power consumption against maximum bed pressure (which is substantially equal to the feed air pressure). It will be seen that as the pressure rises from 170 kPa so the power consumption (per unit volume (standard cubic metre) of product) decreases until it reaches a minimum at about 250 kPa and then increases again.From Figure 4, which is a graph of specific product (that is standard cubic metres of product per hour per cubic metre of sieve in each bed) against maximum pressure for the production of 90% pure oxygen, it can be seen that the higher the maximum bed pressure and hence the higher feed air pressure, the greater the specific product. If the demand for oxygen permits it is preferable to work with a feed pressure in the range 195 to 320 kPa.

In obtaining the results for Figures 3 and 4, a minimum bed pressure (ie the pressure to which the bed is vented) of 1 bar (a)(c98 kpa) was employed and the zeolite molecular sieve was Grace SP7 11430104(0.5 to 1.20 m particle size).

The specific product also varies with cycle time. In Table 1 below this variation is shown for a given ratio of feed time: vent time: purge time: pressurise time, a feed air pressure of 308 kPA, a minimum bed pressure of 98 kPa (ie 1 bar), and an oxygen product of 90% purity.

TABLE 1 Cycle Time Specific Product lin seconds) Un sm3/hr/m3 or sieve in each bed) 16 214 22 181 30 158 40 128 The purity of the product oxygen depends in part on the relative durations of the feed, vent, purge and pressurising (or backfilling) steps of the cycle. This is illustrated in Figure 5 which is a graph of product purity against purge volume. In obtaining the results shown in Figure 5 zeolite molecular sieve of particle sizes in the range 1 to 3mm was used. In order to be able to test the effect of variations of purge time, purge volume and backfill pressure, the product flowrate was held constant at a value which gave product purities in the range 60% to 75% oxygen and allowed the effect of the variables to be demonstrated. Accordingly, the oxygen purities achieved were relatively low.A cycle in which the combined vent and purge times equalled 12 seconds, the pressurising time 3 seconds, and the air feed time 15 seconds was employed. Within these parameters, the ratio of vent time to purge time was adjusted and measurements of product purity were taken for purge times of 5,7 and 9 seconds. In addition, for each purge time measurements of oxygen purity were taken for different volumes of purge gas.

One set of experiments was performed with a backfill pressure of 135 kPA (5 psig) and another set with a backfill pressure of 204 kPA (15 psig). For both sets of experiments the feed pressure was 377 kPA (40 psig) and the vent pressure (ie minimum bed pressure) 98 kPa (1 bar(a)).

The results obtained show that a higher product purity is achieved when the bed is pressurised (backfilled) to 204 kPa than when it is pressurised to 135 kPa. For a given backfill pressure the product purity increases with increasing purge time from 3 to 9 seconds. For a given backfill pressure and purge time, the product purity does not vary greatly with purge value at the higher backfill pressure. At the lower backfill pressure the purity increases with purge volume until (at least at 9 seconds purge time) a maximum is reached when it begins to fall again. Thus, the product purity is more sensitive to backfill pressure and purge time than purge volume.

It can be appreciated from the foregoing description that by appropriately selecting cycle times and relative durations of the feed, vent, purge and pressurising (backfilling) steps, and the pressure to which the bed is pressurised (backfilled) the feed pressure and hence the specific power may be minimised for a given product purity.

The process according to the present invention is illustrated by the following example which was performed on an experimental rig generally similar to the plant shown in Figure 1. The example is set out in Table 2: TABLE 2 Molecular Sieve Grace SP 7 11430104 Particle size of 0.5 to 1.2 mm molecular sieve No of beds 2 Volume of each bed 7.8 litres Feed Pressure 305 kPa (30 psig) Product Pressure Varying between 274 and 301 kPa (25 and 29 psig) Pressure to which bed is vented Atmospheric (0 psig) Pressure to which bed is backfilled 184 kPa (12 psig) Feed time 11.0 seconds Vent time 2.0 seconds Purge time 7.0 seconds Pressurising time 2.0 seconds Purge volume 9.7 litres/part cycle ' Product flow rate 25 standard litres per minute Purity or product oxygen 90% (by volume) Volume of reservoir 14.5 litres Height of each bed 760 mm ie in each purge step 9.7 litres of purge gas are employed

Claims

1. A process of separating a gas mixture comprising two or more components so as to form a product gas whose mole fraction of one component is greater than the mole fraction of said component in the gas mixture, in which process the following cycle of steps is repeated: (a) passing said gas mixture, under pressure, through a bed comprising particules of zeolite molecular sieve which preferentially adsorb one or more other components of the gas mixture and thereby obtained, as the non-adsorbed gas, a stream of product gas; (b) stopping the flow of said gas mixture into said bed; (c) venting gas from said bed in a direction counter to that followed by the said gas mixture in passing through the bed; (d) passing a purge gas through said bed in a direction counter to that followed by the said gas mixture as it passes through the bed;; (e) increasing the pressure in the bed by passing a pressurising gas into the bed (in a direction counter to that followed by the said gas mixture as it passes through the said bed) while preventing the escape of gas from the bed; and in which process: (i) at least two such cycles are simultaneously performed out-of-phase with one another and at least two adsorbent beds are employed, one in each cycle; (ii) the purge gas and the pressurising gas for each cycle are taken from product gas obtained from the other or another cycle; (iii) the particle size of substantially all said particles of molecular sieve is in the range 0.4mm to 3mm;; (iv) the difference between the pressure at which the said gas mixture enters each bed and the average product gas (before any compression thereof) is less than R x 48kPam- (R x 6.7 psig where R is the height of each bed; (v) each said cycle lasts up to 45 seconds.

2. A process as claimed in claim 1, in which in step (c) the bed is vented to atmospheric pressure without the aid of a vacuum pump.

3. A process as claimed in claim 1 or claim 2, in which the gas is air and the said component is oxygen.

4. A process as claimed in any one of the preceding claims, in which there are only two beds.

5. A process as claimed in any one of the preceding claims, in which substantially all the particles of the molecular sieve have a particle size in the range of 0.5 to 1.2mm.

6. A process as claimed in any one of the preceding claims, in which each cycle lasts for a period of time in the range of 15 to 30 seconds.

7. A process as claimed in claim 6, in which each cycle lasts for a period of time in the range of 18 to 24 seconds.

8. A process as claimed in any one of the preceding claims, in which the pressure at which the gas mixture is passed into the bed is at least 170 kPa (10 psig).

9. A process as claimed in claim 8, in which the said pressure is in the range 195 to 320 kPa (14 to 32 psig).

10. A process as claimed in any one of the preceding claims, in which step (a) of the cycle occupies a period of time equal to 1/n times the total period of time occupied by the cycle, where n is the number of beds.

11. A process as claimed in any one of the preceding claims, in which there is no substantial interval of time separating the end of step (a) from the start of step (c) in each cycle.

12. A process as claimed in any one of the preceding claims, in which step (e) of the cycle is performed by passing the pressurising gas into the bed in a direction counter to that in which the gas mixture flows through the bed.

13. A process as claimed in claim 12, in which in each cycle step (e) has a duration of from 0.2 to 0.25 times the duration of step (a).

14. A process as claimed in any one of the preceding claims, in which in each cycle the duration of step (d) is from 55 to 70% of the duration of step (a).

15. A process as claimed in any one of the preceding claims, in which in each cycle the ratio of the duration of step (d) to that of step (c) is in the range of 3:1 to 4:1.

16. A process as claimed in any one of the preceding claims, in which in each cycle the pressure of the bed is increased to a value in the range of 60 to 70% of the pressure at which the feed gas is passed into the bed.

17. A process of separating a gas mixture substantially as described herein with reference to Figures 1 and 2 of the accompanying drawings.

18. A process of separating a gas mixture substantially as described in the Example.