AN APPARATUS AND METHOD FOR COMPRESSING FLUID

Abstract

The invention provides an apparatus for compressing a first fluid. The apparatus comprises a compressor piston comprising a piston cylinder and a piston assembly slidably mounted therein. The piston assembly comprises first and second spaced apart piston members defining a space therebetween. The space is configured to contain a second fluid used to cause compression of the first fluid. The piston assembly further comprises means for feeding second fluid to the space between the first and second piston members.

Claims

1. An apparatus for compressing a first fluid, the apparatus comprising a compressor piston comprising a piston cylinder and a piston assembly slidably mounted therein, wherein the piston assembly comprises first and second spaced apart piston members defining a space therebetween, which space is configured to contain a second fluid used to cause compression of the first fluid, and means for feeding second fluid to the space between the first and second piston members.

2. An apparatus according to claim 1, wherein the apparatus comprises a storage tank configured to store the second fluid therein, and the means for feeding the second fluid to the space between the first and second piston members comprises a pump and at least one second fluid feed conduit extending between the storage tank and the space between the piston members along which the fluid is fed, preferably wherein the pump is activated during the non-compression stage and the second fluid disposed in the space between the first and second piston members is fluidly connected to the storage tank via at least one second fluid leakage conduit.

3. An apparatus according to claim 2, wherein the second piston member comprises a valve configured to control the flow of the second fluid through the at least one second fluid feed conduit into the space between the piston members, preferably wherein the valve comprises biasing means configured to bias the valve into a closed configuration in the second fluid feed conduit, more preferably wherein the biasing means comprises a spring, optionally a helical spring or a cup spring.

4. An apparatus according to claim 3, wherein the second piston member comprises actuation means configured to activate the valve in response to a change in pressure on the first seal or in response to the position of the first piston member with respect to an actuation set-point, preferably wherein the pump is configured to pump the second fluid from the storage tank, through the open valve activated by the actuation means, and along one or more conduit into the space between the first and second piston members.

5. An apparatus according to claim 4, wherein the actuation means is configured to open the valve when the pressure on the first seal increases, and activate the pump to pump second fluid through the valve and/or the actuation means is configured to close the valve when the pressure on the first seal decreases, and deactivate the pump to prevent pumping of second fluid, and/or wherein when the pressure on the first fluid side of the first seal increases due to leakage of the second fluid through the second seal, the first piston member is configured to be urged towards the second piston member, thereby resulting in the actuation means opening the valve, and/or wherein the pre-stress tension of the biasing means exerted on the actuating means substantially corresponds to the weight of the first piston member and the friction created between the cylinder tube and the first seal.

6. An apparatus according to claim 4, wherein the actuation means is configured to open the valve when the position of the first piston member reaches the actuation set-point and/or the actuation means is configured to close the valve when the position of the first piston member moves beyond the actuation set-point.

7. An apparatus according to claim 1, wherein the first piston member is configured to oscillate within the cylinder tube, and is sealed with the cylinder tube by a first radial seal, optionally a stem seal or a piston seal, and the second piston member is configured to oscillate within the cylinder tube, and is sealed with the cylinder tube by a second radial seal, optionally a stem seal or a piston seal, and the first piston member is substantially centrally mounted on the second piston member, and is guided concentrically thereby.

8. An apparatus according to claim 1, wherein the first fluid that is to be compressed contacts one side of the first piston member, and the second fluid contacts the opposite side of the first piston member, preferably wherein the apparatus comprises a leakage cycle return line to the space between the first and second piston members, because, during use of the compressor piston, any leakage of the second fluid at the second seal is automatically balanced out by a replenishment flow of second fluid.

9. An apparatus according to claim 1, wherein the second piston member comprises one or more conduits which extend radially outwardly from the valve to the space between the piston members, preferably wherein the one or more conduits extend diagonally from the valve to the space between the piston members.

10. An apparatus according to claim 1, wherein the pressure difference between the side of the first piston member contacting the first fluid, and the side contacting the second fluid is less than 75 Bar, 50 Bar, 25 Bar, 15 Bar, 10 Bar, 5 Bar, or less than 3 Bar and the compressor piston is configured to increase the pressure of the first fluid to between 100 bara and 1500 bara.

11. An apparatus according to claim 1, wherein the first fluid comprises gas, such as natural gas, fuel gas, hydrogen, gaseous hydrocarbon, liquefied combustion gas, nitrogen, helium, oxygen, and a noble gas, such as argon, or a mixture thereof and the second fluid comprises liquid, which is substantially incompressible, preferably wherein the second fluid comprises an ionic liquid, an LOHC (liquid organic hydrogen carrier), semiheavy water (HDO), deuterium oxide (heavy water), water, or hydraulic oil, or a mixture thereof.

12. An apparatus according to claim 1, wherein the apparatus is configured to use an ionic liquid cushion disposed between the piston assembly and the first fluid to be compressed, preferably wherein the ionic liquid cushion comprises or consists of a substantially pure ionic liquid, or a mixture of an ionic liquid and LOHC.

13. An apparatus according to claim 1, wherein the apparatus comprises: an oscillating compressor and/or a hydraulically driven compressor; and/or a liquid piston compressor and/or an ionic compressor; and/or a single-stage compressor or a multi-stage compressor; and/or a plunger functionally connected to one or more displacement pistons configured to oscillate within a housing, and configured to displace the second fluid to and from the compressor piston, thereby compressing the first fluid therein, preferably wherein oscillation of the or each displacement piston driven by the plunger is facilitated by a lubricant which is fed into the housing via at least one inlet, wherein the lubricant is hydraulic oil, LOHC or an ionic liquid, or mixtures thereof.

14. A method of compressing a first fluid, the method comprising: feeding a first fluid into a compressor piston comprising a piston cylinder and a piston assembly slidably mounted therein, wherein the piston assembly comprises first and second spaced apart piston members defining a space therebetween, which space is configured to contain a second fluid used to cause compression of the first fluid; and feeding a second fluid to the space between the first and second piston members, and compressing the first fluid.

15. A method according to claim 14, wherein the method uses an apparatus comprising a compressor piston comprising a piston cylinder and a piston assembly slidably mounted therein, wherein the piston assembly comprises first and second spaced apart piston members defining a space therebetween, which space is configured to contain a second fluid used to cause compression of the first fluid, and means for feeding second fluid to the space between the first and second piston members.

Description

[0050] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0051] FIG. 1 is a schematic diagram of a first embodiment of a gas compressor according to the invention having two spaced apart piston compressors (left and right-hand sides) each one having a piston assembly, which is slidably mounted in a cylinder tube;

[0052] FIG. 2 is a schematic diagram of a second embodiment of the gas compressor according to the invention having two spaced apart piston compressors (left and right-hand sides) each having a slidably mounted piston assembly. The piston assembly in each compressor uses an ionic liquid cushion thereon, and the piston assembly of the left-hand piston compressor has moved to the top of its cylinder tube, thereby compressing gas therein via the ionic cushion, and the piston assembly of the right-hand piston compressor is positioned towards the middle of its cylinder tube, such that the gas remains substantially uncompressed;

[0053] FIG. 3 is a cross-sectional side view of the compressor shown in FIG. 1, in which the piston assembly of the left-hand piston compressor has moved to the top of its cylinder tube, thereby compressing gas therein, and the piston assembly of the right-hand piston compressor is positioned at the base of its cylinder tube, such that the gas remains uncompressed. Fresh gas is sucked in towards the bottom dead centre;

[0054] FIG. 4 is an enlarged cross-sectional side view of the top of the left-hand piston compressor shown in FIG. 3 with the piston assembly positioned at the top of its cylinder tube having compressed the gas; and

[0055] FIG. 5 is an enlarged cross-sectional side view of one piston assembly of a piston compressor present in the compressor of the invention.

EXAMPLE

[0056] Referring to FIGS. 1-3, there are shown embodiments of a compressor 2 for compressing gas 14, such as natural gas (CNG), fuel gas, hydrogen, gaseous hydrocarbons, liquefied combustion gas, nitrogen, helium, oxygen, and noble gases such as argon. For example, the compressor 2 can be used to compress hydrogen which is used as fuel in hydrogen-driven vehicles. Compression is hydraulically driven, for example by means of an ionic compressor or by a piston compressor, as shown in the Figures. It will be appreciated therefore that the compressor 2 is a liquid piston compressor.

[0057] FIGS. 1 and 2 show first and second embodiments of the compressor 2, respectively. In each embodiment, the compressor 2 includes two, spaced apart piston compressors 4 in parallel, into which uncompressed gas 14 is fed via an inlet 40, and from which compressed gas 14 exits via outlet 41. The pressure of the inlet gas 14 is about 6 Bar, and the pressure of the outlet, compressed gas 14 is about 350 Bar. The inlet 40 and outlet 41 are fitted with multichannel valves 44 with very low frequency expectation values (a compressor frequency 0.1 Hz-5 Hz, more preferably 0.5 Hz-1.5 Hz means low actuating frequencies for the valves as well) to allow the passage of gas 14 therethrough.

[0058] As can be seen in the Figures, the illustrated compressor 2 is a single-stage compressor (i.e. 1-stage). The piston compressors 4 in the illustrated 1-stage system are in parallel and are driven by a single plunger 30 which causes the reciprocal oscillation of pistons 32 connected thereto within a housing 58. Each piston 32 is connected to a corresponding pump 42, which is arranged to displace hydraulic driving fluid 16 disposed in a reservoir 60 to and from its corresponding compressor piston 4, thereby compressing the gas 14 therein.

[0059] However, multi-stage compressors are also envisaged in which at least two of the compressors 2 in FIG. 3 are connected in series, so that the discharge through outlet 41 of both same pressure stage compressors 4 are connected to the suction inlet port 40 of the higher pressure stage. For example, there may be four compressor 2 stages, in which the pressure of the inlet gas 14 into the first compressor 2 is 6 bara, and the pressure of the outlet, compressed gas 14 is 16.6 bara; the pressure of the inlet gas 14 into the second compressor 2 is 16.6 bara, and the pressure of the outlet gas 14 is 45.7 bara; the pressure of the inlet gas 14 into the third compressor 2 is 45.7 bara, and the pressure of the outlet gas 14 is 126 bara; and the pressure of the inlet gas 14 into the fourth compressor 2 is 126 bara, and the pressure of the outlet gas 14 is 350 bara.

[0060] The hydraulic driving fluid 16 is incompressible, and can be any ionic liquid, an LOHC (liquid organic hydrogen carrier), heavy water, deuterium oxide, water, or hydraulic oil, or mixtures thereof. The overall hydraulic system needs to be designed for the lower lubricity of the heavy water, for example, compared to a standard lubricant like oil. Oscillation of the pistons 32 driven by the plunger 30 is facilitated by a lubricant 34 which is fed into the housing 58 via inlets 54. In some embodiments, the lubricant 34 for the plunger 30 may be hydraulic oil 34, LOHC or an ionic liquid, or mixtures thereof. The lubricant 34 should be kept separate from the driving fluid 16 because it needs to have a different compression ratio.

[0061] In FIGS. 2 and 3, the compressor 2 is shown with its left-hand piston compressor 4 in a configuration such that it is compressing the gas 14, and with its right-hand piston compressor 4 in a configuration in which gas 14 remains substantially uncompressed, after fresh gas has been sucked in through suction valve 40. Position sensors 46 connected to each pump 42 detect the configuration of each piston compressor 4, and facilitate respective oscillations therein, such that gas 14 is automatically fed into the piston compressors 4 through inlets 40, and compressed, and then expelled at high pressure through outlets 41.

[0062] In prior art compressors, gas seals disposed the gas being compressed 14 and the pistons are subjected to the full gas pressure, which lead to wear during continued use. However, referring to FIGS. 3 and 4, the compressor 2 of the invention is fitted with a mechanism by which the life-time of gas seals 18 within the piston compressors 4 is significantly extended by reducing wear and tear thereon. As can be seen most clearly in FIG. 5, each piston compressor 4 includes a cylinder tube 6 in which a piston assembly 7 (also known as a dummy piston) is slidably mounted. Each piston assembly 7 consists of a floating piston 10 connected to a spaced apart main piston 8. The floating piston 10 is arranged to oscillate within the cylinder tube 6, and is sealed therein by a radial gas seal 18, such as a V-piston ring. One side of the floating piston 10 (i.e. the upper side shown in FIGS. 1, 2 and 5) is in contact with the gas 14 that is to be compressed (e.g. hydrogen, or compressed natural gas, CNG). On its opposite side (i.e. the lower side shown in FIGS. 1, 2 and 5), the floating piston 10 is in contact with a thin layer of the same incompressible hydraulic driving fluid 16, which is displaced by pistons 32 to cause the piston assembly 7 to oscillate within the cyclinder tube 6.

[0063] The floating piston 10 is centrally embedded within the main piston 8, and is guided concentrically thereby. The main piston 8 is also slidably mounted within the cylinder tube 6 and is sealed therewith by a radial hydraulic seal 20, such as a V-piston ring. The incompressible hydraulic driving fluid 16 disposed in the space between the floating piston 10 and the main piston 8 is fluidly connected, via a duct 26 along which any leaked hydraulic fluid through seal 20 is fed, to a storage tank 28 in which replenishment hydraulic driving fluid 16 is stored, which is shown in FIGS. 1 and 2.

[0064] Referring to FIG. 5, the storage tank 28 creates a leakage cycle return line to the space between the pistons 8, 10, because, during use of the compressor piston 4, any leak of hydraulic driving fluid 16 at the hydraulic seal 20 can be automatically balanced out by a replenishment flow of driving fluid 16, as follows. The main piston 8 has a hydraulic fluid replenishment feed valve 24, which is fluidly connected by conduits 38, 50 to the storage tank 28. The valve 24 is biased into a closed position by a helical spring 22 or a cup spring 22 acting thereon. However, if the pressure on the gas side of the first seal 18 increases due to leakage of driving fluid 16 through seal 20, the floating piston 10 is urged towards the main piston 8, resulting in the replenishment feed system being activated via an actuating unit 12 connected to the valve 24. The valve 24 is opened by the actuating unit 12, and hydraulic fluid 16 is pumped by pump 48 from the storage tank 28 along conduits 50, 38, through the open valve 24, and along diagonal conduits 36, which lead directly into the space between the main piston 8 and the floating piston 10. Accordingly, a constant depth of hydraulic driving fluid 16 is maintained between the floating piston 10 and main piston 8. The replacement driving fluid 16 can be pumped back into the space between the floating piston 10 and main piston 8 at any stage in the process. However, in the embodiment shown in the Figures, the pump 48 is activated when the piston assembly 7 is disposed at the bottom of the cylinder tube 6, i.e. the non-compression stage.

[0065] The pressure between the gas side of the floating piston 10 and the incompressible hydraulic fluid 16 is designed to be constantly balanced out. The spring's 22 pre-stress tension on the actuating unit 12 corresponds to the weight of the floating piston 10 and the friction created between the cylinder tube 6 and the gas seal 18. The pressure difference between the side of the floating piston 10 contacting the gas 14, and the side contacting the hydraulic fluid 16 is less than 2 Bar, and small radial forces between the floating piston 10 and the seal 18 results in less wear and tear.

[0066] The system described above therefore always attempts to subject the first gas seal 18 only to the pre-tension pressure that is defined by the seal 18, in order to minimise wear on the seal 18. In prior art compressors, the gas seal in contact with the compressed gas 14 is exposed to a pressure equivalent to the gas pressure, which causes wear, whereas by splitting the piston assembly 7 into two (i.e. the floating piston 10 and the main piston 8), the gas seal 18 in the compressor 2 of the invention is exposed to a reduced pressure of only 2 bar. Hence, the invention results in the significant reduction of load on the gas seal 18. Although the hydraulic seal 20 is exposed to the similar pressures to that experienced in the prior art compressor, it does not affect the system as a whole because any leakage of hydraulic fluid 16 is immediately re-injected back into the space between the pistons 10, 12 along conduits 36 from storage tank 28.

[0067] The embodiment of the compressor 2 shown in FIG. 3 is essentially the same as that shown in FIG. 2 except that, in FIG. 3, an ionic liquid cushion 56 is provided in between the piston assembly 7 and the gas 14 being compressed. This is useful in embodiments when the hydraulic driving fluid 16 is itself an ionic liquid, and may not be necessary when the driving fluid 16 is an LOHC. The ionic liquid cushion 56 is on top of the floating piston 10 and fills out all of the dead space whilst in the compression phase. The ionic liquid cushion 56 comprises a fluid with a low vapour pressure, and can be made up of any pure ionic liquid, or a mixture of ionic liquid and LOHC.

[0068] Advantages of the compressor 2 reside in extended wear time for the piston seals 18, 20 (>20.000 h), due to very good lubrication created between the pistons 8, 10 and the cylinder tube 6. This provides excellent corrosion protection, and mechanical protection against compressor knocking, and so results in low noise emission. Further advantages include longer service lives leading to lower maintenance costs. This results in increases plant availability, and lower requirements on the opposite contact face because of lower contact pressure forces, which again, minimises maintenance costs.