GB2226255A - Gas separation method and apparatus - Google Patents

Abstract

Apparatus for producing a supply of gas at substantially constant pressure P1 includes a pressure swing adsorption plant 1 and an alternative source 3 of gas sharing a buffer vessel 4. A valve 11 is operable to select whether gas flow from plant 1 to buffer vessel 4 is via an unrestricted flow path 14 or via a restricted flow path 12. Gas is also supplied to the vessel 4 from the source 3 when the pressure in the vessel 4 is at or less than P1. During normal operation of the plant gas passes through the unrestricted path 14, but in the event that i) the pressure in the buffer vessel is below P3 where P3 is a predetermined value below P1, ii) there is a recorded failure in the alternative source 3, iii) there is a recorded failure in the supply line 5 or iv) the pressure of the inlet mixture in line 2 is P4 where P4 is a predetermined value above P1, the gas from the PSA plant 1 is switched to the restricted path 12. <IMAGE>

Description

Gas Separation Apparatus and Method This invention relates to a pressure swing adsorption, gas separation apparatus and method for producing a supply of product gas at a substantially satisfactory purity level and at a relatively high pressure, despite possible variations in the take-off of the product gas or in the supply of gas mixture to the pressure swing adsorption plant.

It is conventional practice to provide a supply of product gas along a supply line by separating that gas from a gas mixture by use of a pressure swing adsorption (PSA) plant. Optimum operation of such a plant requires substantially uniform inlet and outlet pressures and pressure drop through the plant. Unfortunately the inlet and outlet pressures often vary, for instance due to take-off of inlet or outlet gases for other purposes.

In order to help maintain conditions within the pressure swing adsorption plant at near optimum, it is therefore conventional to provide one or more flow regulators in the lines leading to or from the plant.

For instance there is usually a flow regulator in the outlet line and there can also be one in the inlet line.

The provision of these one or more flow regulators means that there is inevitabn xt a signlf cant pressure drop (e.g., 2 bar) across the combination of the plant and the flow regulators, with the result that the pressure of product gas along the resultant supply line is inevitably lower than it would be if the flow regulators could be omitted.

Another difficulty with the use of a pressure swing adsorption plant is that, unless special adaptations are provided, the plant necessarily takes a finite time from start-up before it produces product gas of consistent pressure and quality. Yet another difficulty is that the plant is, of course, dependent upon the provision o a supply of inlet gas and of power for operating the compressors associated with the plant.

It is known for the outlet from the PSA plant to lead to a buffer vessel that holds a store of the product gas. If the pressure in the buffer vessel drops, the flow rate from the PSA plant increases and so the purity of the product gas in the buffer vessel drops.

Because it is often necessary to provide a guaranteed supply of product gas, for instance irrespective of power failures, it is known to provide a reservoir of pressurised or liquified product gas that feeds direct into the supply line in emergencies.

The object of the invention is to provide an apparatus and method by which a pressure swing adsorption plant can be operated efficiently, despite variations in the inlet pressure, to provide an outlet supply at a purity that is less susceptible to variations due to different rates of take off and at a pressure that is higher than would normally be obtainable, for a given inlet pressure, from that pressure swing adsorption plant.

Apparatus according to the invention for producing a supply of product gas along a supply line at a desired substantially constant pressure Pl comprises a pressure swing adsorption plant having a plant inlet for a pressurised inlet mixture of the product gas (that passes through the plant without being adsorbed) and a second gas, and a plant outlet for the product gas, a buffer vessel for holding product gas and from which the supply line leads, and an alternative source of product gas at a pressure greater than Pl, and the apparatus includes buffer valve means interconnecting the buffer vessel and the alternative source and which operate to permit flow from the alternative source when the pressure in the buffer vessel is below P2 where P2 equals Pl or is a predetermined value less than Pl, connecting means for flow of product gas from the plant outlet to the buffer vessel and that include a substantially unrestricted flow path wherein the flow is regulated substantially only by the pressure in the buffer vessel, and a restricted flow path including a flow regulator wherein the flow is regulated by the flow regulator, and selector means for selecting (a) the restricted flow path when (i)the pressure in the buffer vessel is below P3 where P3 is a predetermined value below P1 or (ii) there is a recorded failure in the alternative source, or (iii) there is a recorded failure in the supply line, or (iv) the pressure of the inlet mixture is P4 where P4 is a predetermined value above P1 and (b) the unrestricted flow path when the pressure in the buffer vessel is above P3 and when the pressure in the inlet mixture is below P4.

The invention also includes the method of producing product gas, as described herein.

During normal start-up, the buffer valve means are opened so that the buffer vessel is filled with the product gas and so supply of product gas is available substantially immediately upon start-up, irrespective of any delay in the start-up of the PSA plant. Thereafter, the alternative source is utilised to compensate for minor variations in the pressure in the buffer vessel caused by, for instance, variations in the rate of usage of product gas from the buffer vessel, thereby holding the pressure in the buffer vessel substantially constant at the desired pressure Pl.

During normal conditions the connecting means and the selector means result in the outlet from the pressure swing adsorption plant being regulated directly by a pressure that is substantially the pressure that is within the buffer vessel, instead of being regulated by a flow regulator. Thus the plant can be designed to operate at maximum efficiency and purity at this pressure and the pressure loss that is usually unavoidable (due to an outlet flow regulator) is avoided. However in abnormal conditions (when the pressure in the buffer drops too much or when the pressure in the inlet mixture to the plant rises too much) the efficiency of the plant and the purity of the product gas is maintained by regulating the output through a flow regulator.

Thus by the invention it is possible to use a PSA plant to provide a satisfactory supply of product gas despite changes in process conditions, such as relatively small variations in the rate of usage of the product gas or in the pressure of the inlet mixture to the PSA plant.

The supply of product gas has a pressure higher than would conventionally be available having regard to the inlet pressure of the gas mixture. It has a purity that can be maintained at a substantially uniform level (or at least at above a chosen level). Also the supply is substantially instantaneously available after start-up and is held at the desired pressure despite the changes in usage or inlet supply pressure.

The PSA plant comprises one or more pressure swing adsorption units. Each unit may comprise a single column or it may comprise two or more columns that provide parallel flow paths. The plant inlet is the inlet to the unit or the upstream unit and the plant outlet is the outlet from the unit or the downstream unit. Generally there are at least two units. The plant may include means for conventional processes such as pressure equalisation, backfilling or purging.

If the pressure in the buffer vessel becomes too high, for instance when the rate of utilisation of the product gas drops below its rate of production by the PSA plant, it is necessary to stop or reduce flow of product gas to the buffer vessel. For instance flow may be diverted to another store vessel or to another line, or may be vented. Preferably however the plant is placed on standby, when the pressure in the buffer vessel is at a value P5, where P5 is a predetermined value above P1.

For instance an inlet valve may shut, and the plant may go on to standby, when P5 is at a value between 105 and 130% of Pl, generally a value of at least 110 or 115% up to 120% of P1.

The alternative source of product gas can be a separate apparatus for separating the desired product gas from a gas mixture, provided that the mixture and the power for the separation process are both supplied totally independently of the gas mixture and power supply to the PSA plant. If the gas mixture and power supplies are not independent then the alternative source is likely to fail in the event of a failure in the gas mixture or power supply to the PSA plant. Generally however the alternative source comprises at least one vessel containing the product gas either liquified or in a gaseous form under pressure that is very much arger than the pressure Pl, for instance being at least 200%, and often at least 400% of P1. It is particularly preferred that the alternative source should comprise the product gas in liquified form.This is especially convenient when the product gas is nitrogen.

The buffer valve means can allow a small feed of the product gas from the alternative source into the buffer vessel substantially continuously throughout the process and to open to permit a greater feed when required.

However it is generally preferred for the buffer valve means to be held closed during normal operation and to open, to permit the flow of product gas from the alternative source to the buffer vessel, only when the pressure in the buffer vessel drops below a predetermined value P2. This value can be P1 but is generally slightly below P1. Thus the pressure P2 at which the valve is set to open can be, typically, a pressure within the range 100% to 80% of P1 but generally it is set at a value of 98 to 85%, most preferably around 96 to 90%, of P1.

The connecting means must be capable of providing, in response to the pressures P3 and P4, either an unregulated flow path wherein the outlet from the final bed of the PSA plant is regulated by the pressure in the buffer, or a regulated flow path including a flow regulator that regulates the flow from the final vessel of the PSA plant. These two paths may follow the same line through a flow regulator that is capable of being adjusted by the selector means to a fully open position, in which it provides the unregulated flow, but it is often preferred for the connecting means to comprise separate regulated and unregulated flow paths and the selecting means may then comprise valve means for directing the flow either through the regulated flow path or through the unregulated flow path which, typically, is in a bypass around the regulated flow path.The valve means should be of a type that produces little or no pressure drop.

The selector will divert the flow through the regulated flow path when abnormal conditions prevail such that it is necessary to regulate the flow out of the final column of the PSA plant in order, for instance, to maintain purity and optimum performance of the plant.

When normal conditions prevail, the flow from the final column of the plant is led direct to the buffer vessel and it is the pressure in the buffer vessel which effectively controls the rate of flow out of the final column of the PSA plant.

One abnormal situation that necessitates flow through the flow regulator is when the pressure in the buffer becomes seriously low, for instance due to failure of the alternative source or due to excess utilisation of the product gas supply or due to some emergency. The pressure P3 in the buffer vessel at which the flow regulator comes into operation is thus set below the value P2 since the alternative source is normally relied upon to compensate for pressures down to P2. The pressure P3 is generally set substantially below Pl, for instance at a value in the range 90% to 50%, often in the range 85 to 70%, of P1.

There can additionally or alternatively be means for selecting the regulated flow path in response to a recorded failure in the alternative source or anywhere else in the apparatus or supply line.

Another abnormal situation that necessitates the flow being passed through the regulator is when the inlet pressure to the PSA plant (i.e., to the first column of the plant) is higher than is acceptable for satisfactory operation of the plant. For instance it is an advantage of the invention that the pressure P4 of the inlet mixture is generally, during normal operation, at a value of 102 to 120%, usually 105 to 110%, of Pl, although higher values can be tolerated. However when the value P4 is above its predetermined maximum, the flow is then diverted through the flow regulator. Normally this occurs when P4 is at a value above 110%, more usually above 120 or 130%, of P1.

When the regulated flow path is being used, the degree of regulation can be selected in response to the measured pressure values in the inlet mixture or buffer vessel.

It is also possible to regulate the pressure of the inlet mixture by a flow regulator in the path leading to the plant inlet, but in order to avoid unwanted pressure drop this regulator should normally be fully open, so as to provide no flow regulation, during normal operation.

Similarly, a flow regulator can be provided in the line leading from the buffer vessel but again it is preferred that this should normally be fully open during normal operation.

The invention can be applied to any desired combination of product gas and second gas, with the absorbent for the PSA plant being selected appropriately.

Preferably the product gas is nitrogen and the second gas is oxygen, the inlet mixture preferably being air.

The invention is illustrated in the accompanying drawing, which is a diagrammatic representation of an apparatus according to the invention. The apparatus comprises a PSA plant 1 supplied with gas mixture from a line 2, an alternative source 3, a buffer vessel 4 and a supply line 5 for product gas from the buffer vessel.

The PSA plant 1 comprises columns 6 and 7 having a common plant inlet 8 for inlet mixture from the inlet line 2 and a common plant outlet 9 leading to an outlet line 10. An inlet valve 20 is provided in conventional manner in the inlet 8 to each column. Outlet valves in the outlet 9 from each colum, valved vent lines from each column and valved pressure equalisation lines for placing the columns in communication with each other at the top and bottom are not shown but may be provided in conventional manner. Non-return or other conventional valves may be provided in the lines 10 and 5.

The line 10 leads into a three-way valve 11 by which the outlet flow can be passed through a restricted flow path 12 that includes a flow regulator 13 or through a substantially unrestricted flow path 14, the two paths then merging to lead into the buffer vessel at 15. When flow is through the substantially unrestricted path 14, the pressure at the outlet 9 is substantially the same as, or close to, the pressure in the vessel 4 and so flow from the PSA plant is controlled primarily by this pressure. If desired tor if the apparatus that is available makes it unavoidable) some restriction may be present in the flow path 14 but it must not be such as to create a major restriction in the flow along that path.

However when flow is diverted through the path 12, the flow is controlled primarily by the regulator 13. This can be, for instance, a calibrated orifice.

The alternative source 3 comprises a pressure vessel 16 in which liquified or other pressurised gas may be stored and a line 17 leads, optionally through a heat exchanger 18, and through a valve 19 to the buffer vessel 4.

A sensor 22 is provided to record the pressure of the inlet mixture at the inlet 8 and one or more sensors 23 are provided in the buffer vessel for recording the pressure within the vessel. Appropriate electronic or other control means (not shown) are provided for actuating valves 11, 19 and 20 in response to the pressure recorded by the sensors 23 and for actuating valve 11 in response to the pressure recorded by sensor 22.

When the plant is designed to produce nitrogen from air, columns 6 and 7 contain molecular sieve which allows the desired gas to pass through unadsorbed. The undesired components are adsorbed by the sieve. While one column is used to provide product the other is regenerated by opening a vent line (not shown) to the atmosphere. The undesired components are desorbed and then vented to the atmosphere. The column Is then ready for production and the incoming air is switched to this column while the other column is regenerated. When one vessel has finished venting undesired components to the atmosphere, it is placed in communication with the other vessel (with the valves 20 and the vent line valves both being closed) so that the pressure in the two vessels is approximately equalised. In this way the yield of product gas may be increased.When one wants to shut down the plant, all one does is to leave the plant in a condition in which the two columns communicate with one another via the pressure equalisation lines, the two valves 20 being closed.

In typical operation, the air is supplied through the line 2 and there is a liquified nitrogen store 16.

Typically the pressure in the buffer vessel 4 and the supply line 5 is intended to be held constant at around 6.65 bar (P1) even though the rate of take-off from the supply line 5 may vary and the pressure in the line 2 may vary from, for instance, about 7.2 to 10.4 bar.

Generally the PSA plant 1 is designed for optimum efficiency and purity when the pressure drop between the inlet S and the outlet 9 is not more than about 0.5 bar.

For instance, at initial start-up, buffer vessel 4 is filled with air at 1 bar and this is flushed out of the vessel, through anent 25, by repeated flushing with nitrogen from vessel 16. Valve 19 is then opened to fill the buffer vessel 4 with nitrogen until the sensor 23 records a pressure P2 that is generally just below the desired optimum pressure P1 of around 6.65. For instance the sensor 23 may close the valve 19 when the pressure in vessel 4 reaches 6.35 bar (P2).

The PSA plant will previously have been purged with nitrogen and the valves 20 opened in turn to admit air to each column in turn at an inlet pressure, recorded by the sensor 22, typically of around 7.25 bar. Provided the take-off of nitrogen from the line 5 is at substantially the same rate as nitrogen is produced by plant 1 the sensor 23 will record a substantially constant pressure of around 6.65 bar and so will keep valve 19 shut and will cause the valve 11 to direct flow through path 14, so that the pressure at the outlet 9 is substantially the same as the pressure of around 6.65 bar in the vessel 4.

If the rate of take-off of nitrogen from the line 5 exceeds the capacity of the plant 1 then the pressure in the vessel 4 will start to drop and as soon as it falls below a preselected minimum value P2 of, say, 6.35 bar the valve 19 will be opened in response to the signal from the sensor 23 so as to permit the flow of nitrogen into the vessel 4 to restore the pressure in that vessel to the desired value. The valve 19 will then close again.

The pressure at the outlet 9 from the PSA vessel is thus kept substantially constant during these conditions but if the sensor 23 records that the pressure in the vessel 4 drops too far to P3 (for instance to below 6 bar) due to a failure in the alternative source 3 or some other emergency then the sensor 23 activates the 3-way valve 11 to divert the flow from the unregulated flow path 14 through the regulated flow path 12 including the flow regulator 13. This will have the effect of maintaining the PSA plant output at a constant level, and thus of maintaining efficiency and purity, even though the pressure drop between the inlet 8 and the buffer vessel is greatly in excess of the designed pressure drop for the PSA plant.

Although the inlet pressure may be intended to be, for instance, 7.25 bar, it may increase up to P4, for instance, 7.5 bar or even as much as 10.3 bar due to surges in the air supply and when the sensor 22 records such a surge, for instance above 7.5 bar, it activates the valve 11 to direct the flow through the restricted flow path. This will increase the pressure drop through the plant while maintaining the steady flow but will not cause pressure in the vessel 4 to drop below the desired value. When the sensor 22 records that the pressure has fallen again to a desired value of, for instance, 7.25 bar then it activates the valve 11 to direct the flow through the unrestricted flow path.

If the rate of usage of the nitrogen from the line 5 drops such that the pressure in the buffer vessel 4 increases above a desired maximum P5 of, for instance, 6.9 bar then the sensor 23 switches the PSA plant to standby by closing both valves 20, until the pressure in the vessel 4 has fallen to its desired value.

Claims (10)

1. Apparatus for producing a supply of product gas along a supply line (5) at a desired substantially constant pressure P1 comprising a pressure swing adsorption plant (1) having a plant inlet (8) for an inlet mixture of the product gas and a second gas, and a plant outlet (9) for the product gas, a buffer vessel (4) for holding product gas and from which the supply line (5) leads, and an alternative source (3) of product gas at a pressure greater than Pl, characterised in that the apparatus includes buffer valve means (19) interconnecting the buffer vessel (4) and the alternative source (3) and which operate to permit flow from the alternative source when the pressure in the buffer vessel is below P2 where P2 equals P1 or is a predetermined value less than P1, connecting means (10, 11 and 14 or 12 and 13) for flow of product gas from the plant outlet to the buffer vessel and that include a substantially unrestricted flow path (14) wherein the flow is restricted substantially only by the pressure in the buffer vessel, and a restricted flow path (12) including a flow regulator (13) wherein the flow is restricted by the flow regulator, and selector means (11, 22 and 23) for selecting (a) the restricted flow path (12) when (i) the pressure in the buffer vessel (4) is below P3 where P3 is a predetermined value below P1 or (ii) there is a recorded failure in the alternative source, or (iii) there is a recorded failure in the supply line, or (iv) the pressure of the inlet mixture is P4 where P4 is a predetermined value above P1 and (b) the unrestricted flow path (14) when the pressure in the buffer vessel (4) is above P3 and when the pressure in the inlet mixture is below P4.

2. Apparatus according to claim 1 including an inlet valve (20) in the plant inlet (8) and means (23) for closing this valve and putting the plant on standby when the pressure in the buffer vessel (4) is at a value P5 where P5 is a predetermined value above P1.

3. Apparatus according to claim 1 or claim 2 in which the alternative source (3) is a supply of product gas that is liquified or is under a pressure substantially above P1.

4. Apparatus according to any preceding claim including means (23) for maintaining the buffer valve means (19) shut during normal operation of the plant and for opening it only when the pressure in the buffer vessel (4) is below P2.

5. Apparatus according to any preceding claim in which the connecting means comprise a regulated flow path (12) and a separated unregulated flow path (14) that is in a by-pass, and a valve (11) operable to select the restricted or unrestricted flow paths.

6. Apparatus according to any preceding claim in which the selector means (11, 22 and 23) select the restricted flow path 12 when the pressure in the buffer vessel (4) is below P3 where P3 is a predetermined value below P1 or the pressure of the inlet mixture is P4 where P4 is a predetermined value above Pr.

7. A method for producing a supply of product gas along a supply line at a desired substantially constant pressure P1 comprising separating the product gas from an inlet mixture of the product gas and a second gas by means of a pressure swing adsorption plant and passing the resultant product gas along connecting means to a buffer vessel for holding the product gas and from which the supply line leads, and permitting flow of product gas from an alternative source to the buffer vessel when the pressure in the buffer vessel is below P2 where P2 equals P1 or is a predetermined value less than Pl, and in which the flow through the connecting means to the buffer vessel is restricted by a flow regulator when (i) the pressure in the buffer vessel is below P3 where P3 is a predetermined value below Pl, or (ii) there is a recorded failure in the alternative source or (iii) there is a recorded failure in the supply line for the product gas or (iv) the pressure of the inlet mixture is P4 where P4 is a predetermined value above Pi, and the flow through the connecting means is an unrestricted flow path wherein the flow is restricted substantially only by the pressure in the buffer vessel when the pressure in the buffer vessel is above P3 and when the pressure in the inlet mixture is below P4.

8. A method according to claim 7 in which the plant is put on standby when the pressure in the buffer vessel is at a value PS where P5 is a predetermined value above P1.

9. A method according to claim 7 or claim 8 in which the alternative source of product gas is product gas that is liquified or under a pressure substantially above P1.

10. A method according to any of claims 7 to 9 in which product gas is supplied from the alternative source to the buffer vessel only when the pressure in the buffer vessel is below P2.