GB2487898A

GB2487898A - Gas separation

Info

Publication number: GB2487898A
Application number: GB201101584A
Authority: GB
Inventors: Rajan Sooraj; Subrata Sarkar; Sateesh Verma; Nilesh Tendulkar; Sahebrao Tekale; Alan Ernest Massey; Kevin Connolly
Original assignee: Eaton Aerospace Ltd
Current assignee: Eaton Aerospace Ltd
Priority date: 2011-01-31
Filing date: 2011-01-31
Publication date: 2012-08-15
Also published as: GB201101584D0; WO2012104272A1

Abstract

A gas separation apparatus comprises a hollow fibre separation membrane arrangement for separating a feed gas into a permeate gas fraction and a retentate gas fraction. The hollow fibre separation membrane arrangement has an inlet 42 for receiving the feed gas, a permeate passage 50 for passing the permeate gas fraction, and a retentate passage for passing a retentate gas fraction. Also provided is a pump arrangement for compressing the feed gas and a pump arrangement for decompressing the permeate gas. The pump arrangements are commonly driven. Preferably, a double acting pump comprising a double acting piston in a double acting cylinder defines two chambers wherein each of the two pump chambers provides one of the said pump arrangements. Preferably the feed gas is air, the permeate is an oxygen (O2) enriched air fraction and the retentate is a nitrogen (N2) enriched air fraction.

Description

Gas Separation Apparatus and Methods This invention relates to apparatus and methods for gas conditioning in which a feed gas is supplied to a gas separation module to separate the gas into permeate and retentate fractions. In particular, but not exclusively, the invention is concerned with applications where the feed gas is air possibly containing moisture, the permeate gas is oxygen-enriched air along with moisture if present, and the permeate is nitrogen-enriched air.

Membrane based gas separation has evolved as a rapidly developing technology during the past decade. The major commercial processes for gas separation are Pressure Swing Adsorption (PSA) using coalescence material, use of a Hollow Fibre Membrane Air Separation Module (HFM-ASM), use of an Ion Transparent Membrane using heated ceramic tiles, and cryogenic distillation based on Reverse-Brayton Cycle and Joule-Thomson Cycle. Out of these four processes, HFM-ASM has several advantages such as being simple in constmction, easy to maintain and operate and having fewer moving parts, contributing to higher reliability. In addition, the low weight and high rate of permeation of oxygen and water through a hollow fibre membrane make it attractive for producing Nitrogen Enriched Air (NEA) for aircraft fuel inciting and fire protection in cargo compartments by reducing the level of oxygen concentration below a level at which combustion can be sustained or initiated. The main disadvantages of HFM-ASM are that it requires a high pressure at the inlet and hence, in conventional arrangements, requires a high pressure ratio compressor (for bleed air from a gas turbine engine), and also that the rate of permeation depends on the pressure differential across the thickness of the hollow fibre. Given these requirements, most commercial HFM-ASM systems consist of a rotary compressor to provide compressed air and/or a vacuum pump to accelerate the rate of permeation. Some existing designs also use a portion of the moisture-free NEA as a purge gas to carry away the moisture and oxygen from the permeate side of the HFM, thereby reducing the efficiency of the HFM-ASM system.

U56755894, US7393390, U56540818 and US20070157803 describe examples of earlier devices.

It is an aim of this invention to provide a highly efficient and reliable system which operates for example to remove moisture and oxygen from air to provide a high purity of NEA that can be supplied to an aircraft fuel tank to assist inerting. We have designed a system in which compression on the feed side of the separator module is applied simultaneously with decompression at the permeate side by a commonly driven arrangement, so that high pressure differentials across the membrane can be achieved without requiring rotary compressors or high pressure bleed air.

Accordingly, in one aspect, this invention provides a gas separation apparatus comprising: a hollow fibre separation membrane arrangement for separating a feed gas into a permeate gas fraction and a retcntate gas fraction, said arrangement having an inlet flow passage for receiving said feed gas, a permeate flow passage for passing said permeate gas fraction, and a retentate gas passage for passing a retentatc gas fraction; a pump arrangement for compressing said feed gas, and a pump arrangement for decompressing said permeate gas, thereby to provide a pressure differential across said hollow fibre membrane arrangement, wherein the pump arrangements are commonly driven.

By this arrangement a suitable pressure differential can be maintained which provides beneficial operating conditions for the membrane and also provides suction at the permeate side of the membrane to help draw the permeate gas away from the membrane to assist operation and without necessitating active flushing. Using co-acting or commonly driven pump arrangements allows coordinated compression and decompression.

Preferably the apparatus includes a double acting pump comprising a double acting piston in a double acting cylinder thereby defining two pump chambers and wherein each of the two pump chambers provides a respective one of said pump arrangements.

In this manner the compression and decompression occur simultaneously upon movement of the piston in a given direction.

Conveniently said double acting pump is operable alternately to: i) draw in a charge of feed gas whilst exhausting a charge of permeate gas, and ii) to compress and deliver a charge of feed gas to said hollow fibre separation arrangement whilst decompressing a charge of permeate gas from said hollow fibre separation arrangement.

Although they may be different, preferably the working fluid volumes at each end of the double acting piston are the same. This means that, in general, because the pump compresses the feed air, but decompresses just the permeate fraction, the pressure differential at the permeate side of the membrane will tend to be greater than at the inlet, and this can fUrther assist dissipation and discharge of the permeate away from the permeate side of the filter where it might otherwise collect.

Preferably two double acting pumps are provided which operate substantially in antiphase whereby, as one pump draws in a charge of feed gas whilst exhausting a charge of permeate gas, the other pump is compressing a charge of feed gas whilst decompressing a charge of permeate gas, and vice versa.

Preferably the double acting pistons of the two double acting pumps are fixedly coupled and connected to a drive which causes the double acting pistons to reciprocate in their respective cylinders.

Preferably one chamber of each double acting pump has a feed gas delivery passage connected in common to the inlet flow passage of the hollow fibre separation membrane.

Preferably, one chamber of each double acting pump has a permeate gas inlet passage, the permeate flow inlet passages being connected in common to the permeate gas passage of the hollow fibre separation membrane arrangement.

Preferably, a heat exchanger arrangement is provided to extract heat from the gas passing into the hollow fibre separation membrane arrangement.

Although applicable to separation of other gas mixtures, in a preferred application said feed gas is air (possibly with moisture), and said permeate is an oxygen-enriched air fraction and said retentate is a nitrogen-enriched air fraction (and moisture if present).

In another aspect, this invention provides a gas separation apparatus which incorporates a hollow fibre membrane arrangement having an inlet feed for inlet feed gas, a flow passage for a permeate gas fraction that permeates the hollow fibre membrane and a flow passage for a retentate gas fraction that does not permeate the hollow fibre membrane arrangement, said method comprising compressing said feed gas whilst simultaneously decompressing said permeate gas, thereby to provide a pressure differential across said hollow fibre membrane arrangement, wherein said compressing and decompressing is done using commonly driven pumping arrangements.

S Whilst the invention has been described above, it extends to any inventive combination of the features set out above or in the following description or claims.

Whilst the invention may be performed in various ways, an embodiment thereof will now be described by way of example only, reference being made to the accompanying Figure which is a schematic view of an embodiment of gas conditioning system in accordance wfth this invention.

Referring to the drawing, the gas conditioning system consists of six major sub-units, namely an electrical drive motor 10, a piston assembly 12 driven by the motor, two cylinders 14, 16, a heat exchanger 18 and a hollow fibre membrane based air separation (HFM-ASM) module 20. In this system, the piston assembly 12 and cylinders 14, 16 operate as two back to back double acting synchronously running pumps which cooperate to compress feed gas and supply it to the HFM-ASM module 20 at the same time as decompressing the permeate from the module. In this manner, a high pressure differential is applied across the hollow fibre membranes of the module 20 made up of the pressure increase caused by compression of feed air on the inlet side and the pressure reduction caused by decompression of the permeate fraction on the permeate side. The retentate is discharged into ambient pressure in this example. In one particular application, the feed gas may be moisture-laden air, with the permeate being oxygen- enriched air and moisture, and the retentate being nitrogen-enriched air. The nitrogen-enriched air may be used for the inerting purposes etc as discussed above.

Referring now in more detail to the arrangement of the drawings, the piston assembly 12 comprises two double acting pistons 22, 24 connected by an interconnecting rod 26. Each piston is slideably mounted within a respective double acting cylinder 14, 16. The interconnecting rod 26 slides within a passage 28 connecting the two cylinders 14, 16 and is suitably sealed. The upper piston 22 in this arrangement is reciprocally driven by a connecting rod 30 that passes through a sealed aperture 32 in the wall to the motor 10. A suitable gearing or drive mechanism is provided, and motion is transmitted to the lower piston 24 by the interconnecting rod 28.

Referring initially to the upper double acting pistonleylinder combination 22, 14, the upper chamber 34 has an inlet 36 connected to a common feed gas supply 37, with a non-return valve 38 allowing inward flow only, and an outlet 40 connected via the heat exchanger 18 to the inlet 42 of the HFM-ASM module 20, with backflow being prevented by a non-return valve 44. The lower chamber 46 of the upper arrangement has an inlet 48 connected to the permeate outlet 50 of the HFM-ASM module 20 with a non-return valve 52 preventing backflow. The outlet 54 of the lower chamber is connected to a common permeate outlet 56, with a non-return valve 58 preventing baekflow.

In the lower double acting piston/cylinder combination 24, 16, the inlet 60 of the upper chamber 62 is connected to the permeate output 50 of the HFM-ASM module 20, with a non-return valve 64 preventing baekflow. The outlet 66 of the upper chamber is connected to the common permeate outlet 56, with a non-return valve 68 preventing baekflow. In the lower chamber 70, the inlet 72 is connected to the common gas feed supply 37, with a non-return valve 74 preventing backflow. The outlet 76 of the lower chamber 70 is connected to join the flow path to the heat exchanger 18, with a non-return valve 78 preventing baekflow. At the junction between the outlets 40, 76 of the upper and lower double acting piston/cylinder arrangements, a further non-return valve is provided.

In use, the piston alTangement is driven reciprocally with each pistonlcylinder combination 22, 14; 24, 16, alternately compressing feed gas into the HFM-ASM module 20 and decompressing permeate at the permeate outlet. The piston/cylinder combinations work in antiphase so that as, say, the upper piston cylinder combination nears the end of its compression cycle, the lower piston cylinder combination is just about to start, and so on. The operation is summarised in the table below.

___ PISTON STROKE

___ UP DOWN

C Compress and expel Draw in feed gas Y T feed gas to HFM + L 0 + Expel permeate from I P Decompress and draw cylinder N in permeate from

D ___ HFM ______________

E B

R 0 Expel permeate from Decompress and draw T cylinder in permeate from HEM T + + 0 Draw in feed gas Compress and expel M feed gas to HEM By this arrangement, each up and down stroke of the pistons compresses the inlet feed gas and decompresses the permeate gas.

In this arrangement, the pump is shown as a double acting piston/cylinder arrangement which is driven by a motor in a reciprocal fashion. It will of course be appreciated that other types of pump arrangements may be used where the compression and decompression actions are similarly co-ordinated. Suitable pump types may include a swash plate pump, liquid ring pump, vane pump, diaphragm pump, bellows pump or similar devices.

Although in the illustrated embodiment the double acting pistonlcylinder combinations are similar to a tandem cylinder, different arrangements with multiple piston-cylinder arrangements or combinations of both types may be used.

Although just a single HFM-ASM module is shown it will be appreciated that multiple HFM-ASM in series and/or parallel could be used as necessary.

Although the above embodiment is applicable to inerting in aerospace applications, it may be used in other gas separation applications such as e.g. for nitrogen generation for industrial use; for oxygen generation for medical use; or for moisture separation in fuel cell applications.

Claims

CLAIMS1. A gas separation apparatus comprising: a hollow fibre separation membrane arrangement for separating a feed gas into a permeate gas fraction and a retentate gas fraction, said arrangement having an inlet flow passage for receiving said feed gas, a permeate flow passage for passing said permeate gas fraction, and a retentate gas passage for passing a retentate gas fraction; a pump arrangement for compressing said feed gas, and a pump arrangement for decompressing said permeate gas, thereby to provide a pressure differential across said hollow fibre membrane arrangement, wherein the pump arrangements are commonly driven.
2. A gas separation apparatus according to Claim 1, wherein the apparatus includes a double acting pump comprises a double acting piston in a double acting cylinder thereby defining two pump chambers, wherein each of the two pump chambers provides a respective one of said pump arrangements.
3. A gas separation apparatus according to Claim 2, wherein said pump double acting pump is operable alternately to: i) draw in a charge of feed gas whilst exhausting a charge of permeate gas, and ii) to compress and deliver a charge of feed gas to said hollow fibre separation arrangement whilst decompressing a charge of permeate gas from said hollow fibre separation arrangement.
4. A gas separation apparatus according to Claim 2 or Claim 3, wherein the displacement volume is the same at each end of the double acting piston.
5. A gas separation apparatus according to Claims 2 to 4, wherein two double acting pumps are provided, to operate substantially in antiphase whereby, as one pump draws in a charge of feed gas whilst exhausting a charge of permeate gas, the other pump is compressing a charge of feed gas whilst decompressing a charge of permeate gas, and vice versa.
6. A gas separation apparatus according to Claim 5, wherein the two double acting pistons are fixedly coupled and connected to a drive which causes the double acting pistons to reciprocate in their respective cylinders.
7. A gas separation apparatus according to Claim 5 or Claim 6, wherein one chamber of each double acting pump has a feed gas delivery passage connected in common to the inlet flow passage of the hollow fibre separation membrane.
8. A gas separation apparatus according to any of the preceding Claims, wherein one chamber of each double acting pump has a permeate gas inlet passage, the permeate gas inlet passages being connected in common to the permeate gas passage of the hollow fibre separation membrane arrangement.
9. A gas separation apparatus according to any of the preceding Claims, wherein a heat exchanger arrangement is provided to extract heat from the gas passing in use into the hollow fibre separation membrane arrangement.
10. A gas separation apparatus according to any of the preceding Claims, wherein said feed gas is air, and said permeate is an oxygen-enriched air fraction and said retentate is a nitrogen-enriched air fraction.
11. A method of operating a gas separation apparatus which incorporates a hollow fibre membrane arrangement having an inlet feed for inlet feed gas, a flow passage for a permeate gas fraction that permeates the hollow fibre membrane and a flow passage for a retentate gas fraction that does not permeate the hollow fibre membrane arrangement, said method comprising compressing said feed gas whilst simultaneously decompressing said permeate gas, thereby to provide a pressure differential across said hollow fibre membrane arrangement, wherein said compressing and decompressing is done using commonly driven pumping arrangements.
12. A gas separation apparatus substantially as herein described with reference to, and as illustrated in, the accompanying Figure.
13. A method of operating as gas separation apparatus substantially as herein described.