WO1983001908A1

WO1983001908A1 - Apparatus for separating gases

Info

Publication number: WO1983001908A1
Application number: PCT/GB1982/000340
Authority: WO
Inventors: Limited Oxymaster
Original assignee: Gardner, Ian, Richard; Gaskell, Noel
Priority date: 1981-12-02
Filing date: 1982-12-01
Publication date: 1983-06-09
Also published as: GB2122508B; EP0096685A1; GB8320667D0; GB2122508A

Abstract

A continuous flow of oxygen is provided from units (17, 18, 19) containing molecular sieves, into which air is passed. The number of units in operation at any time is varied according to the overall rate of utilisation of oxygen, so that the purity of the oxygen produced is not significantly affected by changes in this rate.

Description

Title: "Apparatus for separating gases"

Description of Invention

This invention relates to apparatus for separating gases from a gaseous mixture and which apparatus includes a unit comprising a vessel containing a separation medium which reversibly retains a first constituent of the mixture preferentially with respect to a second constituent of the mixture. Such a unit is hereinafter referred to as being of the kind specified..

When the gaseous mixture is passed into contact with the separation medium of a unit of the kind specified, the proportion of the first constituent in the gaseous phase in reduced . and the proportion of the second constituent in the gaseous phase is correspondingly increased. The resulting gaseous phase is called herein the product. Zeolites are examples of known separation media used in this way.

When the effectiveness of the separation medium of a unit of the kind specified falls substantially, owing to a cumulation of the first constituent, the effectiveness can be restored by allowing or causing the retention of the first constituent to be reversed, for example by purging the separation medium or by reducing the gas pressure to which the separation medium is subjected. It will be understood that the term "separation" as used herein includes partial separation which results in a change in the proportions of the constituents of ,a gaseous mixture.

The invention has been developed in connection with apparatus, for separating nitrogen from air so as to provide a supply of oxygen, and the following description

OMPI will therefore be confined to this application of the invention. However, it is to be understood that there is no limitation in this regard as the invention is also applicable to apparatus for separating other appropriate

5 gases.

It is known to use for separating nitrogen from air a gas-separating unit comprising a pressure vessel which contains a Zeolite, the vessel having an air inlet and an oxygen outlet. One known form of gas-separating unit of

10 the kind specified has two pressure vessels and means operable automatically to bring the vessels into opera¬ tion alternately, such a unit being hereinafter referred to as a unit of the form specified. The arrangement is such that, when the material in one pressure vessel to

1*5 which air is being fed has retained a substantial amount of nitrogen, the other vessel is brought into operation by feeding air into that other vessel and the pressure in the first vessel is reduced so that nitrogen is released from the separation medium therein and is permitted to

²⁰ vent from the vessel to the atmosphere. The vessel may be purged by venting air through the vessel to atmos¬ phere. The vessel is then ready to be brought into operation again when the separation medium in the second .vessel has retained a substantial amount of nitrogen. In

²⁵ this way, a continuous supply of almost pure oxygen is maintained.

The proportion of nitrogen remaining in the product obtained by treating air in a unit of the form specified depends upon the rate of flow of product from the unit.

30 Increasing the flow rate increases the proportion of nitrogen in the product.

Units of the form specified are supplied in relatively small sizes for use in the home to provide a supply of oxygen for medical purposes. Such units are

-55 satisfactory in this situation, since a constant rate of flow is normally required and the proportion of oxygen in the product can therefore be maintained within a narrow

P range. Since the product is delivered to the patient through an open mask, the requirements of the patient will not normally affect the rate of flow. Product not required by the patient simply escapes into the atmos- phere.

It is an object of the present invention to provide an apparatus for separating gases which is suitable for use in applications where there is required a supply of gas at flow rates which can vary considerably from time to time and where it is required that large changes in the flow rate should not be accompanied by large changes in the composition of the product.

According to a first aspect of the invention, there is provided apparatus comprising control means_$ a product discharge passage and a plurality of units of the kind specified, wherein each unit of the kind specified has an inlet for the mixture and a product outlet for the product, all of the units which constitute said plurality of units are adapted to deliver the product from their respective product outlets to the product discharge passage and wherein the control means is operable to vary the number of said units in operation at any one time to deliver the product to the product discharge passage in accordance with the demand for the product made on the apparatus.

According to a further aspect of the invention, there is provided a method of separating a gaseous mixture wherein the mixture is fed through a selected number of units of the kind specified to a common dis- charge passage and the number of units of the kind specified through which the mixture passes concurrently is varied according to conditions in the discharge passage.

If required, provision may be made for purging each of the vessels of each separator unit with product from the other vessel of that unit after the vessel has been vented to atmosphere and before the vessel is reconnected with the product discharge passage.

OMPI An example of apparatus embodying the invention will now be described, with reference to the accompanying drawings, wherein:

FIGURE 1 shows the apparatus diagra maticallyj and t- FIGURE 2 is a diagrammatic illustration of a gas- separating unit of the apparatus of Figure 1.

The apparatus shown in the accompanying drawings comprises an air reservoir 10, with which there are connected the outlets of a number of electrically _Q energisable compressors, two of which are shown at 11 and 12 respectively. A larger number of compressors may be provided.

For controlling operation of the compressors 11 and 12, there is provided control means which includes a

₁₅ transducer 13 responsive to changes in .the pressure within the air reservoir 10 to provide an electrical output signal which represents the air pressure. One suitable form of transducer provides an output signal having a voltage which represents the air pressure. This

2_Q output signal is processed by a processor 14 of the control means to provide a signal having a voltage which varies stepwise. Each step corresponds to a respective . threshold pressure in the air reservoir 10.

The processor 14 is arranged to control operation of

25 switching means 15 which controls energisation of the compressors 11 and 12. When the pressure in the reservoir 10 reaches a predetermined maximum value, all of the compressors are de-energised. If the pressure in the reservoir 10 subsequently falls to a first threshold value, the processor 14 causes one of the compressors to be energised. If the pressure in the reservoir 10 continues to fall and reaches a second threshold value, the processor 14 causes a second of the compressors to be energised." Depending upon the number of compressors, this procedure will be repeated until the pressure rises once more through the threshold values, whereupon the processor de-energises corresponding compressors in

OM succession as the pressure in the reservoir approaches the predetermined maximum value. In the event of the pressure in the reservoir 10 falling below a pre¬ determined minimum value, the processor 14 provides an alarm signal which operates an audible and/or visible alarm.

The processor 14 is also provided along lines 16 with signals representing respective output pressures of the compressors 11 and 12. If, when a particular compressor is energised, its output pressure does not promptly rise to a predetermined threshold, the processor again provides the alarm signal. Malfunction of one compressor may result in the pressure in the reservoir 10 being somewhat below the pressure which would be main- tained if that compressor was functioning perfectly but, provided the total number of compressors is adequate, malfunction of a single compressor will not result in the pressure in the reservoir 10 falling to the predetermined minimum value. The pressure in the reservoir can be maintained above this minimum value when a compressor is taken out of service for maintenance or replacement.

The apparatus further comprises a number of separator units, of which three are shown in the drawing at 17, 18 and 19 respectively. Each of these units has a respective air inlet connected with the reservoir 10. This connection may include a common pipe 20 and the separator units may be remote from the reservoir. Each of the separator units also has a respective product outlet, these being connected with a common product discharge passage which includes an oxygen reservoir 21 having an outlet 22 and an oxygen line 23 leading from the separator units to the reservoir 21.

The separator units 17_» 18 and 19 are identical one with the other and accordingly only one of these, the unit 17, will now be described and is illustrated in • Figure 2. This unit comprises two pressure vessels 24, 25 containing a Zeolite or a mixture of Zeolites which constitutes a molecular sieve for retaining nitrogen. The vessels 24 and 25 have respective inlets connected to

5 a valve 26 and respective outlets, spaced from the inlets by the Zeolite, connected with a valve 27.

The valve 26 has a port connected with the air line 20 and further ports open to the atmosphere. The valve can be set in a selected one of three alternative condi-

10 tions. In a first of these, an inlet of the vessel 24 is connected with the air line 20 and the vessel 25 is vented to atmosphere. In a second of the conditions, neither vessel is connected with the air line. In the third condition, the inlet of the vessel 25 is^* connected

25 with the air line and the vessel 24 is vented to atmos¬ phere.

The valve 27 also can be set in a selected one of three alternative conditions. In the first of these, the outlet of the vessel 24 is connected with the oxygen line

20 23_> in the second condition, neither vessel is connected with the oxygen line and in the third condition the outlet of the vessel 25 is connected with the oxygen line. Electrical signals for setting the valves 26 and 27 in selected conditions are provided by a controller 28

25 of the unit 17.

The valve 27 may be so arranged that, when in its first condition, oxygen can flow from the outlet of the vessel 24 to both the oxygen line 23 and the vessel 25, in order to purge the vessel 25 with oxygen. Similarly, ^"30 when in its third condition, this modified valve would permit flow of oxygen from the outlet of the vessel 25 through the vessel 24 to the atmosphere, in addition to flow from the outlet of the vessel 2S to the oxygen line 23. In place of the valves 26 and 27, there may be

35 provided separate valves for controlling the inlets and outlets of the vessels 24 and 25, in order that purging of a vessel with oxygen flowing from the outlet of the

OM other vessel can be confined to a period shorter than the period during which oxygen flows from- that other vessel to the oxygen line.

The controller 28 forms a part of control means which further includes a transducer 29 associated with the oxygen reservoir 21 and adapted to provide an electrical demand signal representing the pressure of oxygen in the reservoir. This signal is processed in a processor 30 which provides signals to the controllers of 0 the separator units 17, 18 and 19. The arrangement is such that, when the pressure in the oxygen reservoir 21 rises to a predetermined maximum value, the pressure vessels of all of the separator units are isloated from

4 the air line 20 and from the oxygen line 23 so that there 5 is no further flow of oxygen into the reservoir. When the pressure within the reservoir falls to a first threshold value, the processor 30 provides a signal which turns on one of the separator units, that is to say communication is established between one pressure vessel o of this unit and both the air line 20 and the oxygen line 23. Air then flows into the pressure vessel and oxygen flows from the pressure vessel to the reservoir 21. Whilst that separator unit remains in its On¹ condition, the controller 28 of the unit causes the condition of the valves of that unit to change cyclically between the first and third conditions so that the pressure vessels are used alternately for separating nitrogen from the air. Typically, the cycle time is less than one minute. If the pressure in the oxygen reservoir 21 continues to fall and reaches a second threshold value, the processor 30 provides a signal to set a further one of the separator units in an 'on' condition. If the pressure in the oxygen reservoir continues to fall, this procedure is repeated a number of times depending on the number of separator units provided until a predetermined minimum pressure is reached, when the processor 30 provides an alarm signal. It will be understood that the

OMPI ^_ reservoir can continue to supply oxygen through its outlet 22 to points of use even when the minimum pressure has been reached.

In place of each of the transducers 13 and 29, there may be provided a number of pressure-actuated switches, each of these switches providing a demand signal to the associated processor when the pressure in the reservoir concerned falls below a particular value associated with that switch. There is provided to the processor 30 along a line 31 from each separator unit 17, 18, 19, when that separator unit is in an 'on* condition, a signal indicat¬ ing that cycling of the valves is taking pl.ace. The processor 30 monitors these signals and, if cycling of a separator unit ceases whilst that unit is required to be 'on'i the processor provides an alarm signal and also provides a signal which sets the valves of that unit in their second conditions, isolating the pressure vessels of the unit from the air and oxygen lines. it will be understood that the supply of oxygen to the reservoir 21 can be maintained even when one of the separator units fails to cycle correctly or is taken out of service for maintenance or replacement.

There is connected with the oxygen line 23 an analyser 32 through which a sample of gas is bled from the oxygen line and tested for oxygen content. The analyser may be a known device, for example one which responds to the paramagnetic character of oxygen. In the event of the concentration of oxygen in the gas flowing along the oxygen line 23 falling to a threshold value, typically 94?, the analyser 32 provides a signal to the processor 30 and the latter responds by providing an alarm sign.al and by turning, on all of the separator units. This has the effect of decreasing the rate of product output . from each separator unit and thereby increasing the concentration of oxygen in the gas leaving each separator unit. There may be associated with the oxygen reservoir 21 further devices responsive to predetermined pressures in the reservoir to provide additional signals to the processor 30. If the information provided by these devices is inconsistent with that provided by the trans¬ ducer 29, the processor provides an alarm signal.

There may be interposed between the valve 27 and the oxygen line 23 a reservoir or a flow regulator or both. Non-return valves may be provided to prevent reverse flow at various positions in the separator unit. .

The oxygen reservoir 21 has a volume many times^" greater than that of the oxygen line 23 and normally contains sufficient oxygen for several hours supply. By varying the number of separator units operating in accordance with the pressure in the reservoir 21, in the manner hereinbefore described, the concentration of oxygen in the gas which flows along the oxygen line 23 can be maintained within a narrow range, for example within the range 94? to 98?.

Claims

CLAIMS :

1. Apparatus comprising control means, a product dis¬ charge passage and a plurality of units of the kind specified, wherein each of said units has an inlet for the mixture and a product outlet for the product, all of said units are adapted to deliver the product from their respective product outlets to the product discharge passage and wherein the ^"control means is operable to vary the number of said units in operation at any one time to deliver the product to the product discharge passage in accordance with the demand for the product made on the apparatus.

2. Apparatus according to Claim 1 wherein the control means further comprises signalling means for providing a demand signal whenever the pressure at a selected position in said discharge passage falls below a pre¬ determined threshold value.

3. Apparatus according to Claim 1 or Claim 2 wherein the discharge passage includes a vessel adapted to store a volume of the product which is a plurality of times greater than the internal volume of a pipe leading from the units to the vessel.

4. Apparatus according to any preceding claim wherein each unit is of the form specified.

5. A method of separating a gaseous mixture wherein the mixture is fed through a selected number of units of the kind specified to a common discharge passage and the number of .units of the kind specified through which the mixture passes concurrently is varied according to condi¬ tions in the discharge passage.

6. A method according to Claim 5 wherein, when the pressure in the discharge passage falls below a pre¬ determined threshold, there is provided a demand signal which is used to initiate flow of the mixture through one of said units.

7. Apparatus substantially as herein described with reference to and as shown in the accompanying drawings.

8. A method of separating a gaseous mixture substan¬ tially as herein described with reference to the accompanying drawings.

_*

9. Any novel feature or novel combination of features disclosed herein and/or in the accompanying drawings.