WO2017129373A1

WO2017129373A1 - Compressor

Info

Publication number: WO2017129373A1
Application number: PCT/EP2017/025003
Authority: WO
Inventors: Robert Adler; Ekkehardt Klein; Christoph Nagl; Lukas TOBEINER
Original assignee: Linde Aktiengesellschaft
Priority date: 2016-01-28
Filing date: 2017-01-13
Publication date: 2017-08-03
Also published as: GB201601601D0

Abstract

The invention relates to use of a liquid organic hydrogen carrier (LOHC) in a piston compressor. Particular aspects of the invention relate to the use of an LOHC as a driving liquid and/ or a liquid cushion in a piston compressor. The invention also extends to piston compressors comprising an LOHC and methods of compressing gas using an LOHC.

Description

Compressor

The present invention relates to a compressor. More specifically, the present invention relates to a compressor comprising a liquid piston. Numerous methods and devices for conveying and compressing fluids are known. For instance, in a traditional reciprocating compressor, gas is drawn into a cylinder and is subsequently compressed by a piston. The piston is traditionally driven in a reciprocating motion by a crankshaft, and to allow the reciprocating compressor to function efficiently, appropriate sealing systems are necessary which can result in high production, maintenance and running costs.

To overcome the problems associated with reciprocating compressors, ionic compressors were developed in which a liquid salt column is used to compress a gas. However, a problem with ionic compressors is that the ionic liquid may become entrained in the gas during the compression step, and so a separator is provided downstream of the compressor in order to remove entrained ionic liquid from the gas. This is particularly important if the compressed gas is being used to power an energy converting system, such as a fuel cell or internal combustion engine. For instance, if a separator is not used, and the compressed gas containing the ionic liquid is introduced into an internal combustion engine, the combustion residues due to the ionic liquid can cause excessive thermal stresses to the engine.

The present invention arises from the inventors' work in trying to overcome the problems associated with the prior art.

In accordance with a first aspect of the invention, there is provided use of a liquid organic hydrogen carrier (LOHC) in a piston compressor. Preferably, the LOHC is used to compress a gas.

Preferably, the piston compressor is a liquid piston compressor.

In accordance with a second aspect, there is provided use of a liquid organic hydrogen carrier (LOHC) as a driving liquid in a liquid piston compressor. Advantageously, liquid organic hydrogen carriers (LOHCs) combust in a soot-free manor. Accordingly, LOHCs which have become entrained in a compressed gas may be viewed as unimportant as these impurities should not adversely affect an internal combustion engine which receives the compressed gas.

The inventors have produced a gas compressor which is driven by an LOHC.

In accordance with a third aspect, there is provided a piston compressor for

compressing gas, the compressor comprising at least one compressor chamber configured to receive the gas therein, the or each compression chamber comprising a liquid piston comprising a driving liquid and the piston compressor further comprising a driving liquid supply means configured to vary the volume of the driving liquid in the at least one compressor chamber, characterised in that the driving liquid comprises one or more liquid organic hydrogen carrier (LOHC).

The driving liquid may consist substantially of one or more LOHC. For example, the driving liquid may consist of ioo LOHC.

Alternatively, the driving liquid may also comprise an ionic liquid.

Advantageously, if the gas is wet then the ionic liquid would become saturated thereby removing water, semi heavy water and/or heavy water from the gas.

Accordingly, the driving liquid may also comprise water, semi heavy water and/or heavy water.

The volumetric ratio of the one or more LOHC to the ionic liquid may be between 99.9:0.01 and 0.01:99.9. Preferably, the volumetric ratio of the one or more LOHC to the ionic liquid may be between 75:25 and 5:95, more preferably between 50:50 and 10:90, and most preferably between 25:75 and 15:85.

If a preferred embodiment, an anhydrous driving liquid may comprise at least 10% (v/v), 20% (v/v), 30% (v/v) or 40% (v/v) ionic liquid, more preferably at least 50% (v/v) or 60% (v/v) ionic liquid, and most preferably at least 70% (v/v) ionic liquid. In a preferred embodiment, an anhydrous driving liquid may comprise up to 80% (v/v) ionic liquid.

Preferably, the apparatus is configured to regenerate the driving liquid by removing water, semi heavy water and/or heavy water therefrom. The apparatus may be configured to regenerate the driving liquid when the driving liquid comprises at least 10 % (v/v) water, semi heavy water and/or heavy water, preferably at least 20 % (v/v) or 30 % (v/v) water, semi heavy water and/ or heavy water, and most preferably at least 40 % (v/v) water, semi heavy water and/ or heavy water.

The apparatus may comprise regeneration means configured to remove the water, semi heavy water and/or heavy water from the driving liquid.

In a preferred embodiment, the apparatus is configured to regenerate the driving liquid when the driving liquid comprises up to 50 % (v/v) water, semi heavy water and/or heavy water.

Accordingly, the driving liquid preferably contains 50 % (v/v) or less water, semi heavy water and/or heavy water.

In accordance with a fourth aspect, there is provided use of a liquid organic hydrogen carrier (LOHC) as a liquid cushion in a piston compressor.

In accordance with a fifth aspect, there is provided a piston compressor for

compressing gas, the compressor comprising at least one compressor chamber configured to receive the gas therein, the or each compression chamber comprising a piston configured to move therein and a liquid cushion disposed between the piston and the gas to be compressed, characterised in that the liquid cushion comprises one or more liquid organic hydrogen carrier (LOHC).

The liquid cushion may consist substantially of one or more LOHC. For example, the liquid cushion may consist of 100% LOHC.

Alternatively, the liquid cushion may also comprise an ionic liquid. Advantageously, if the gas is wet then the ionic liquid would become saturated thereby removing water, semi heavy water and/ or heavy water from the gas.

Accordingly, the liquid cushion may also comprise water, semi heavy water and/or heavy water.

The volumetric ratio of the one or more LOHC to the ionic liquid maybe between 99.9:0.01 and 0.01:99.9. Preferably, the volumetric ratio of the one or more LOHC to the ionic liquid may be between 75:25 and 5:95, more preferably between 50:50 and 10:90, and most preferably between 25:75 and 15:85.

If a preferred embodiment, an anhydrous liquid cushion may comprise at least 10 % (v/v), 20 % (v/v), 30 % (v/v) or 40 % (v/v) ionic liquid, more preferably at least 50 % (v/v) or 60 % (v/v) ionic liquid, and most preferably at least 70 % (v/v) ionic liquid.

In a preferred embodiment, an anhydrous liquid cushion may comprise up to 80 % (v/v) ionic liquid.

Preferably, the apparatus is configured to regenerate the liquid cushion by removing water, semi heavy water and/or heavy water therefrom. The apparatus may be configured to regenerate the liquid cushion when the liquid cushion comprises at least 10 % (v/v) water, semi heavy water and/or heavy water, preferably at least 20 % (v/v) or 30 % (v/v) water, semi heavy water and/or heavy water, and most preferably at least 40 % (v/v) water, semi heavy water and/or heavy water.

The apparatus may comprise regeneration means configured to remove the water, semi heavy water and/or heavy water from the liquid cushion.

In a preferred embodiment, the apparatus is configured to regenerate the liquid cushion when the liquid cushion comprises up to 50 % (v/v) water, semi heavy water and/or heavy water.

Accordingly, the liquid cushion preferably contains 50 % (v/v) or less water, semi heavy water and/ or heavy water.

The or each piston may comprise a solid piston. Alternatively, the or each piston may comprise a liquid piston comprising a driving liquid and a dummy piston disposed between the driving liquid and the liquid cushion. Each piston may comprise a v-piston ring configured to provide a seal between a peripheral surface of the dummy piston and an inner surface of the chamber.

Preferably, the v-piston ring is configured to move with the dummy piston, and thereby to maintain the seal as the dummy piston moves.

The piston compressor may comprise driving liquid supply means configured to vary the volume of a driving liquid in the at least one compressor chamber.

The driving liquid may comprise an LOHC, an ionic liquid, water, semi heavy water, heavy water and/or hydraulic oil. The one or more LOHC may comprise an LOHC with a low vapour pressure.

It will be appreciated that the vapour pressure of the LOHC will depend on the temperature. Accordingly, in one embodiment the LOHC has a vapour pressure between l x lo^~12 bar and 1 bar at 20°C, preferably between 1 x lo^~10 bar and 0.75 bar at 2o°C, and more preferably between 1 x lo^~9 bar and 0.5 bar at 20°C.

Alternatively or additionally, the LOHC has a vapour pressure between 1 x io^-6 bar and 1 bar at 127°C, preferably between 1 x 10-5 bar and 0.5 bar at 127°C, and more preferably between 1 x lcr bar and 0.1 bar at 127°C.

Advantageously, this reduces the amount of the LOHC which becomes entrained in the gas.

Preferably, the one or more LOHC comprises a cycloalkane, cycloalkene, N-heterocycle and/or formic acid. Preferably, the one or more LOHC comprises N-ethylcarbazole, dodecahydro-N-ethylcarbazole, dibenzyltoluene, toluene, benzene, naphthalene, formic acid and/or saturated analogues thereof.

The one or more LOHC may comprise an unsaturated LOHC and/or a saturated LOHC. For instance, it will be appreciated that toluene is an unsaturated LOHC and the saturated analogue thereof is methyl-cyclohexane. The piston compressor maybe configured to compress any gas. In a preferred embodiment, the piston compressor is configured to compress a flammable gas. The flammable gas may comprise hydrogen, natural gas, methane, ethane, propane and/or butane. Alternatively, the piston compressor may be configured to compress an inert gas. The inert gas may comprise nitrogen or a noble gas, such as helium, neon or argon.

In an embodiment in which the piston compressor is configured to compress hydrogen, the one or more LOHC may comprise a saturated LOHC. Advantageously, hydrogen does not react with a saturated LOHC.

The piston compressor may comprise one compressor chamber. However, preferably the piston compressor comprises a multistage piston compressor comprising a plurality of compressor chambers connected in series.

Preferably, the piston compressor comprises between one and twenty compressor chambers. More preferably, the piston compressor comprises between two and ten compressor chambers. Most preferably, the piston compressor comprises between three and five compressor chambers.

The piston compressor may comprise a multistage piston compressor comprising a plurality of compressor stages connected in parallel. Advantageously, this would increase the throughput of the compressor.

Accordingly, in one embodiment, the multistage piston compressor may a plurality of series, wherein each series comprise a plurality of compressor stages connected in a series and the plurality of series are connected parallel. Preferably, each compressor chamber comprises a gas inlet configured to allow the gas to flow into the chamber, and a gas outlet configured to allow the gas to flow therefrom. Preferably, the gas inlet comprises a one-way valve. Preferably, the gas outlet comprises a one-way valve. The chamber may comprise a cylinder. The piston compressor may comprise an inlet conduit configured to transport a gas to be compressed to the at least one compressor chamber. In the embodiment where the piston compressor comprises one compressor chamber the inlet conduit may be configured to transport the gas to be compressed to the inlet valve of the one compressor chamber. Alternatively, in the embodiment where the plurality of compressor chambers connected in a series the inlet conduit may be configured to transport the gas to be compressed to the inlet valve of a first compressor chamber in the series.

Preferably, the multistage piston compressor comprises at least one intermediate conduit, wherein the or each intermediate conduit extends between two compressor stages. Preferably, the or each intermediate conduit extends between the gas outlet of one compressor stage and the gas inlet of a further compressor stage.

The piston compressor may comprise an outlet conduit configured to transport the compressed gas away from the at least one compressor chamber. In the embodiment where the piston compressor comprises one compressor chamber the outlet conduit may be configured to transport the compressed gas from the outlet valve of the one compressor chamber. Alternatively, in the embodiment where the plurality of compressor chambers connected in a series the outlet conduit may be configured to transport the compressed gas from the outlet valve of a final compressor chamber in the series.

A heat controlling device configured to control the heat of the gas may be disposed upstream of the at least one compressor chamber. Accordingly, the heat controlling device maybe disposed on the inlet conduit.

Alternatively, or additionally, at least one heat controlling device configured to control the heat of the gas may be disposed between each of the compressor stages connected in a series. Accordingly, the or each heat controlling device maybe disposed on the or each intermediate conduit.

The or each heat controlling device may comprise a heat exchanger. The or each heat controlling device may be configured to maintain the gas at a temperature below 150°C, more preferably below 125°C and most preferably below ioo°C. Advantageously, in embodiments where the driving liquid or liquid cushion comprises a saturated LOHC any degassing therefrom should be minimised. Preferably, the or each heat controlling device is configured to maintain the gas at a temperature between 5°C and 8o°C or between io°C and 6o°C, more preferably between 15°C and 40°C, and most preferably between 20°C and 30°C.

Alternatively, the or each heat controlling device maybe configured to maintain the gas at a temperature greater than 90°C.

Advantageously, N-ethylcarbazole in its unsaturated form is a liquid at temperatures greater than 90°C.

In this embodiment, preferably the or each heat controlling device is configured to maintain the gas at a temperature between go°C and i8o°C. Advantageously, by maintaining the temperature below i8o°C this reduces the stress on the materials.

Additionally, or alternatively, a heat controlling device configured to control the heat of the gas maybe disposed downstream of a final compressor stage in the series.

Accordingly, the heat controlling device may be disposed on the outlet conduit. The or each heat controlling device may comprise a heat exchanger.

Advantageously, the heat controlling device controls the temperature of the compressed gas which leaves the piston compressor. Preferably, the heat controlling device is configured to maintain the gas at a temperature between 5°C and 8o°C or between io°C and 6o°C, more preferably between 15°C and 40°C, and most preferably between 20°C and 30°C.

Preferably, each of the plurality of compressors is configured to increase the pressure of the gas received therein.

In embodiments where the piston compressor comprises a liquid piston compressor, each chamber may be fluidly connected to the driving liquid supply means. Each chamber may be connected to a separate driving liquid supply means. Alternatively, a plurality of chambers could be connected to one driving liquid supply means. In a preferred embodiment, each chamber is connected to one driving liquid supply means. Preferably, the piston compressor is configured to increase the pressure of the gas to between loo bara and 1500 bara. More preferably, the piston compressor is configured to increase the pressure of the gas to between 150 bara and 1250 bara. Most preferably, the piston compressor is configured to increase the pressure of the gas to between 300 bara and 1000 bara.

It may be appreciated that the desired pressure of the gas varies depending upon the gas which is being compressed. Accordingly, when the piston compressor is configured to compress hydrogen, the piston compressor may be configured to increase the pressure of the gas to between 500 bara and 1500 bara, more preferably to between 700 bara and 1400 bara, and most preferably to between 800 bara and 1300 bara.

Alternatively, when the piston compressor is configured to compress natural gas, methane, ethane, propane, butane and/ or nitrogen the piston compressor may be configured to increase the pressure of the gas to between 100 bara and 700 bara, more preferably to between 200 bara and 600 bara, and most preferably to between 300 bara and 500 bara. Alternatively, when the piston compressor is configured to compress noble gas the piston compressor may be configured to increase the pressure of the gas to between 100 bara and 800 bara, more preferably to between 300 bara and 700 bara, and most preferably to between 400 bara and 600 bara. Preferably, the noble gas is helium. The piston compressors of the third and fifth aspects may be included in an apparatus configured to feed compressed gas to a gas fed device.

Hence, in accordance with a sixth aspect, there is provided an apparatus comprising a gas source configured to provide a gas to be compressed, a piston compressor according to the third or fifth aspect disposed downstream of, and in fluid communication with, the gas source and configured to compress the gas received therefrom, and a gas-fed device disposed downstream of, and in fluid communication with, the piston compressor and configured to receive the compressed gas from the piston compressor. Advantageously, the apparatus is configured to feed the gas-fed device with compressed gas. The gas source may comprise a tank configured to store a gas therein. Alternatively, the gas source may comprise a tank configured to store a liquefied gas therein and a vaporiser configured to vaporise the liquefied gas and disposed downstream of, and in fluid communication with, the tank. A conduit may extend between the tank and the vaporiser.

The gas source may be configured to provide a flammable gas. The flammable gas may comprise hydrogen, natural gas, methane, ethane, propane and/or butane.

Alternatively, the gas source may be configured to provide an inert gas. The inert gas may comprise nitrogen or a noble gas, such as helium, neon or argon.

The gas source may be configured to provide a gas at a pressure between 2 bara and 300 bara.

The gas source may comprise a high pressure gas source configured to provide the gas at a pressure between 50 bara and 250 bara, more preferably between 100 bara and 200 bara. Alternatively, the gas source may comprise a low pressure gas source configured to provide the gas at a pressure between 3 bara and 15 bara, and more preferably between 4 bara and 10 bara.

A conduit may extend between the gas source and the piston compressor. The conduit may comprise the inlet conduit.

Alternatively, the gas source may be configured to provide a gas at about atmospheric pressure. In this embodiment, the apparatus may comprise a pre-compressor.

Preferably, the pre-compressor is disposed downstream of, and in fluid

communication, with the gas source. A conduit may extend between the gas source and the pre-compressor. Preferably, the pre-compressor is configured to receive the gas from the gas source.

Preferably, the pre-compressor is disposed upstream of, and in fluid communication with, the piston compressor. A conduit may extend between the pre-compressor and the piston compressor. The conduit may comprise the inlet conduit. The pre-compressor maybe configured to increase the pressure of the gas to a pressure between 2 bara and 20 bara, more preferably between 3 bara and 15 bara, and most preferably between 4 bara and 10 bara. Advantageously, the pre-compressor can raise the pressure of the gas from about atmospheric pressure to a higher pressure more efficiently than the piston compressor.

The pre-compressor may comprise a compressor turbine, a multistage centrifugal compressor, a screw compressor, a pressure wave super charger or a positive displacement compressor. Preferably, the pre-compressor comprises a compressor turbine.

A conduit may extend between the piston compressor and the gas-fed device. The conduit may comprise the outlet conduit.

Alternatively, the apparatus may comprise liquid separation means configured to separate gas and the driving liquid or liquid cushion. The liquid separation means may be disposed downstream of, and in fluid communication with, the piston compressor. Preferably, a conduit extends between the piston compressor and the liquid separation means. The conduit may comprise the outlet conduit. Preferably, the conduit is configured to transport the gas from the piston compressor to the liquid separation means.

The liquid separation means may be disposed upstream of, and in fluid communication with, the gas-fed device. Preferably, a conduit extends between the liquid separation means and the gas-fed device. Preferably, the conduit is configured to transport the gas from the liquid separation means to the gas-fed device.

The liquid separation means may comprise a coalescing filter, a molecular sieve, a centrifugal separator or a metal hydride separator. Preferably, the separator comprises a coalescing filter.

Advantageously, the liquid separation means removes any driving liquid or liquid cushion which is present in the compressed gas. This increases the purity of the gas received by the gas-fed device. The liquid separation means may be in fluid communication with the driving liquid supply means. Preferably, a conduit extends between the liquid separation means and the driving liquid supply means. Preferably, the conduit is configured to transport recovered driving liquid from the liquid separation means to the driving liquid supply means.

The gas-fed device may comprise an engine or a fuel cell. Preferably, the gas-fed device comprises an engine. The engine may comprise a gas-fuelled engine or a hybrid fuel engine. The engine may comprise a four-stroke engine or a two-stroke engine.

Preferably, the engine comprises a two-stroke engine.

The engine may be configured to create a propulsive force. Alternatively, the engine may comprise an electrical generator. In accordance with a seventh aspect, there is provided use of the compressor of the third or fifth aspect to compress a gas.

In accordance with an eighth aspect, there is provided use of the apparatus of the sixth aspect to feed a gas-fed device with a compressed gas.

In accordance with a ninth aspect, there is provided a method of compressing a gas, the method comprising:

feeding gas to a compressor comprising one or more pistons, wherein the or each piston comprises a liquid piston; and

- compressing the gas using the one or more pistons;

characterised in that the one or more pistons are driven by a driving liquid comprising one or more liquid organic hydrogen carrier (LOHC).

Preferably, the method of the ninth aspect uses the compressor of the third aspect.

Preferably, compressing the gas using the one or more liquid pistons comprises feeding the gas into a chamber and then feeding the driving liquid into the chamber to thereby compress the gas. In accordance with a tenth aspect, there is provided a method of compressing a gas, the method comprising: feeding gas to a compressor comprising one or more compressor chambers, wherein the or each compressor chamber comprises a piston and a liquid cushion disposed between the piston and the gas; and compressing the gas using the one or more pistons;

characterised in that the liquid cushion comprises one or more liquid organic hydrogen carrier (LOHC).

Preferably, the method of the tenth aspect uses the compressor of the fifth aspect. Preferably, compressing the gas using one or more pistons comprises compressing the gas with a series of pistons, wherein each piston is configured to further compress the gas.

The method may comprise controlling the temperature of the gas prior to compressing the gas using one or more pistons. Alternatively, or additionally, the method may comprise controlling the temperature of the gas between compressing the gas in subsequent pistons in the series of pistons.

Controlling the temperature of the gas may comprise passing the gas through a heat exchanger.

Controlling the temperature of the gas may comprise causing the gas to be at a temperature below 150°C, more preferably below 125°C and most preferably below loo°C.

Controlling the temperature of the gas may comprise causing the gas to be at a temperature between 5°C and 8o°C or between io°C and 6o°C, more preferably between 15°C and 40°C, and most preferably between 20°C and 30°C. Alternatively, controlling the temperature of the gas may comprise causing the gas to be at a temperature greater than 90°C. Controlling the temperature of the gas may comprise causing the gas to be at a temperature between 90°C and i8o°C.

The method may comprise splitting the flow of gas into a plurality of parallel streams upstream of the one or more pistons. Each of the plurality of streams may then be compressed, in parallel, by one or more pistons. Preferably, each of the plurality of parallel streams is compressed by a series of pistons, wherein each piston is configured to further compress the gas.

Preferably, compressing the gas with the one or more pistons comprises compressing the gas with between one and twenty pistons in series, more preferably between two and ten pistons in series, most preferably, between three and five pistons in series.

The method may comprise controlling the temperature of the gas subsequent to the step of compressing the gas with the one or more pistons. Controlling the temperature of the gas may comprise passing the gas through a heat exchanger.

Controlling the temperature of the gas may comprise causing the gas to be at a temperature between 5°C and 8o°C or between lo°C and 6o°C, more preferably between 15°C and 40°C, and most preferably between 20°C and 30°C.

Preferably, compressing the gas with the one or more pistons comprises compressing the gas to a pressure between loo bara and 1500 bara, more preferably to between 150 bara and 1250 bara, most preferably to between 300 bara and 1000 bara. In an embodiment where the gas is hydrogen, the method may comprise compressing the gas to a pressure between 500 bara and 1500 bara, more preferably to between 700 bara and 1400 bara, and most preferably to between 800 bara and 1300 bara.

In an embodiment where the gas is natural gas, methane, ethane, propane, butane and/or nitrogen the method may comprise compressing the gas to a pressure between 100 bara and 700 bara, more preferably to between 200 bara and 600 bara, and most preferably to between 300 bara and 500 bara.

In an embodiment where the gas is a noble gas the method may comprise compressing the gas to a pressure between 100 bara and 800 bara, more preferably to between 300 bara and 700 bara, and most preferably to between 400 bara and 600 bara.

In accordance with a eleventh aspect, there is provided a method of providing a gas-fed device with compressed gas, the method comprising:

- feeding a gas to be compressed from a gas source to a compressor; compressing the gas using the method according to the ninth or tenth aspect; and

feeding the compressed gas to the gas-fed device. Preferably, the method of the eleventh aspect uses the apparatus of the sixth aspect.

Prior to feeding the gas to be compressed from the gas source to the compressor the method may comprise vaporising a liquefied gas to obtain the gas to be compressed. The method may comprise feeding the gas to be compressed from the gas source to the compressor at a pressure between 2 bara and 300 bara.

The method may comprise feeding the gas to be compressed from a high pressure gas source to the compressor at a pressure between 50 bara and 250 bara, more preferably between 100 bara and 200 bara.

Alternatively, the method may comprise feeding the gas to be compressed from a low pressure gas source to the compressor at a pressure between 3 bara and 15 bara, and most preferably between 4 bara and 10 bara.

Alternatively, the method may comprise feeding the gas to be compressed from the gas source to a pre-compressor and pre-compressing the gas to a pressure of between 2 bara and 20 bara, more preferably to between 3 bara and 15 bara, and most preferably to between 4 bara and 10 bara. Pre-compressing the gas may comprise feeding the gas through a compressor turbine, a multistage centrifugal compressor, a screw

compressor, a pressure wave super charger or a positive displacement compressor.

Preferably, the method then comprises feeding the pre-compressed gas to the compressor.

Preferably, subsequent to compressing the gas with the one or more pistons and prior to feeding the compressed gas into the gas-fed device, the method comprises separating the gas and the driving liquid or liquid cushion. Separating the gas and the driving liquid or liquid cushion may comprise feeding the gas through a coalescing filter, a molecular sieve, a centrifugal separator or a metal hydride separator. Preferably, - l6 - separating the gas and the driving liquid or liquid cushion may comprise feeding the gas through a coalescing filter.

Subsequent to feeding the compressed gas into the gas-fed device, the method may comprise combusting the compressed gas. The method may comprise using the heat energy produced by combusting the gas to create a propulsive force or electricity.

All features described herein (including any accompanying claims, abstract and drawings), and/ or all of the steps of any method or process so disclosed, may be combined with any of the above aspects in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

For a better understanding of the invention, and to show how embodiments of the same maybe carried into effect, reference will now be made, by way of example, to the accompanying Figures, in which: -

Figure 1 is a schematic diagram of an embodiment of a compressor of the invention comprising liquid pistons; and

Figure 2 is a diagram of a compressor cylinder of an alternative embodiment of the invention.

Example 1: LOHC as a driving liquid

Compressors comprising a liquid piston are known. To aid this understanding, Figure 1 shows a schematic diagram of an apparatus 2 comprising a gas tank 4, a compressor 6 and an engine 8.

The gas tank 4 stores a gas 10 at a pressure of about 6 bara. A conduit 12 extends between the gas tank 4 and the compressor 6, and is configured to transport gas 10 to the compressor 6, as described below. The compressor 6 is a liquid piston compressor comprising five compressor cylinders 14, 16, 18, 20, 22 in series. Each compressor cylinder 14, 16, 18, 20, 22 comprises an inlet valve 24 configured to allow the gas 10 to flow into each compressor cylinder 14, 16, 18, 20, 22, and an outlet valve 26 configured to allow the compressed gas 10 to flow out of each cylinder 14, 16, 18, 20, 22. Both the inlet valves 24 and outlet valves 26 comprise non-return valves. The five compressor cylinders 14, 16, 18, 20, 22 are connected in series. Accordingly, the conduit 12 extends between the gas tank 4 and the inlet valve 24 on the first compressor cylinder 14, and an intermediate conduit 28 extends between the outlet valve 26 of the first compressor cylinder 14 and the inlet valve 24 of the second compressor cylinder 16, and so on. As shown in Figure 1, further intermediate conduits 3°_> 3² _> 34 similarly connect the third, fourth and fifth compressor cylinders 18, 20, 22 in series.

The compressor 6 comprises a radial piston machine 36, which comprises five cylinder chambers 38. In each cylinder chamber 38, a piston (not illustrated) is displaceably mounted. Each cylinder chamber 38 is connected by means of a connection duct 40 with a compressor cylinder 14, 16, 18, 20, 22. Driving liquid 42 is disposed in the compressor cylinders 14, 16, 18, 20, 22, and the volume disposed in each compressor cylinder 14, 16, 18, 20, 22 maybe varied by the piston machine 36.

Accordingly, as the volume of driving liquid 42 in the first compressor cylinder 14 is decreased, the gas 10 is drawn from the conduit 12, through the inlet 24 and into the compressor cylinder 14. Conversely, as the volume of driving liquid 42 in the first compressor cylinder 14 is increased, the gas 10 will be compressed and forced out of the outlet 26 into the intermediate conduit 28.

Accordingly, by decreasing and increasing the volume of the driving liquid 42 in each compressor cylinder 14, 16, 18, 20, 22, the gas 10 maybe transported into and out of each compressor cylinder 14, 16, 18, 20, 22, and the pressure of the gas 10 will increase at each stage.

It will be appreciated that the temperature of the gas 10 will increase as it is

compressed. Some of this heat will be transferred to the driving liquid 42, which is cooled in a heat exchanger (not shown). Additionally, further heat exchangers 44 are provided on the intermediate conduits 28, 30, 32, 34 and are configured to maintain the gas 10 within a desired temperature range.

Finally, a conduit 46 extends from the outlet valve 26 of the fifth and final compressor cylinder 22, and is arranged to transport the compressed gas from the compressor 6 to the engine 8. A final heat exchanger 48 is disposed on the conduit 46 and is provides to ensure that the gas 10 delivered to the engine 8 is within a desirable temperature range. In prior art systems, the driving liquid 42 is an ionic liquid. However, a portion of this liquid becomes entrained in the gas during compression. Accordingly, the apparatus requires a separator, such as a coalescing filter, downstream of the final compressor cylinder 22, for removing entrained gas from driving liquid 42.

However, in the apparatus shown in Figure 1, the driving liquid 42 comprises a liquid organic hydrogen carrier (LOHC) instead of an ionic liquid. It is generally desirable to use an LOHC with a low vapour pressure as this reduces the amount of the driving liquid 42 which becomes entrained in the gas. Suitable LOHCs and there vapour pressures are given in table 1 below.

Table 1: Suitable LOHCs and their respective vapour pressures

The hydrogenation of the substances allows properties of the working medium to be matched to the application requirements in a controlled manner. For instance, N- ethylcarbazole would be used in hydrogenated form or at temperatures greater than 90°C, since it is a solid form at room temperature when unladen. In embodiments where N-ethylcarbazole was used in its unladen form, each compressor cylinder 14, 16, 18, 20, 22 would also comprise a heater configured to heat the driving liquid therein to a temperature greater than 90°C when the apparatus was initiated. The heater could comprise a heat exchanger configured to deliver heat transfer fluid to the outside of the cylinder 14, 16, 18, 20, 22.

Similarly, where the gas being compressed is hydrogen, the driving liquid 42 is preferably hydrogenated to prevent the gas from reacting with the driving liquid 42. The heat exchangers 44 are configured to ensure that the compression temperatures are kept moderate, i.e. preferably less than ioo°C, to prevent constant

hydrogenation/ dehydrogenation. If the moderate temperature is not maintained, this would result in a lower conveying performance since less fresh gas 10 could be sucked into a compressor cylinder 14, 16, 18, 20, 22 as a result of the driving liquid 42 outgassing therein.

Advantageously, most LOHCs combust in a soot-free manner. Accordingly, impurities in the compressed gas 10 due to LOHCs disposed therein are unimportant as these impurities should not adversely affect the internal combustion engine 8. Therefore, unlike in the prior art, it is not necessary to provide the apparatus 2 with a separator, such as a coalescing filter, to separate the gas 10 from the driving liquid 42 prior to introducing the compressed gas 10 into the internal combustion engine 8. However, it will be appreciated that in some embodiments a separator disposed between the compressor 6 and the engine 8 could still be provided. Example 2: LOHC as a cushion in a liquid compressor

Figure 2 shows a compressor cylinder 50 which is part of a multistage liquid piston compressor. Similar to the compressor cylinders described in example 1, the compressor cylinder 50 shown in Figure 2 comprises an inlet valve 24 configured to allow a gas 10 to flow into the cylinder 50, and an outlet valve 26 configured to allow the compressed gas 10 to flow out of the cylinder 50. Both the inlet valves 24 and outlet valves 26 comprise non-return valves.

The compressor cylinder 50 shown in Figure 2 is the first of a series of compressor cylinders. Accordingly, a conduit 12 extends between the gas tank (not shown) and the inlet valve 24 on the compressor cylinder 50, and an intermediate conduit 28 extends between the outlet valve 26 of the compressor cylinder 50 and the inlet valve of the second compressor cylinder in the series (not shown). The multistage compressor comprises a radial piston machine (not shown). As described in example 1, the radial piston machine comprises a plurality of cylinder chambers where a piston is displaceably mounted. A cylinder chamber (not shown) is connected by means of a connection duct 40 with the compressor cylinder 50. Driving liquid 42 is disposed in the compressor cylinder 50, and the volume disposed in the compressor cylinder 50 may be varied by the piston machine. Unlike the embodiment disclosed in example 1, it is not essential that the driving liquid 42 comprises an LOHC. Alternatively, or additionally, the driving liquid 42 may comprise an ionic liquid, water, semi heavy water, heavy water, hydraulic oil, or a mixture thereof. As the volume of driving liquid 42 in the first compressor cylinder 14 is decreased, the gas 10 is drawn from the conduit 12, through the inlet 24 and into the compressor cylinder 14. Conversely, as the volume of driving liquid 42 in the first compressor cylinder 14 is increased, the gas 10 will be compressed and forced out of the outlet 26 into the intermediate conduit 28. Accordingly, by decreasing and increasing the volume of the driving liquid 42 in the compressor cylinder 50, the gas 10 maybe transported into and out of the compressor cylinder 50, increasing the pressure of the gas as it is transported.

The compressor cylinder 50 differs from the compressor cylinders described in example 1 because it comprises a dummy piston 52, which separates the gas 10 from the driving liquid 42. During an operation cycle, the dummy piston 52 remains on top of the driving liquid 42 and moves up and down due to the variation in the level thereof. V- piston rings 54 are used to provide sealing between the peripheral surface of the dummy piston 52 and the inner surface of the compressor stage 50. The v-piston rings 54 move with the dummy piston 52 to maintain the seal as the dummy piston 52 moves. A liquid cushion 56 comprising an LOHC is provided in between the dummy piston 54 and the gas 10 being compressed. The liquid cushion 56 is on top of the floating piston 10 and fills out all of the dead space whilst in the compression phase. While not illustrated, it will be appreciated that a similar liquid cushion could also be used in piston compressors where the piston comprises a solid piston.

Example 3: Alternative embodiments

It will be appreciated that in an alternative embodiment the compressors described in examples 1 and 2 may comprise a hydraulic drive instead of the radial piston machine 36. The hydraulic drive would also be configured to vary the volume of driving liquid 42 disposed in each compressor cylinder.

Additionally, the driving liquid 42 used in example 1, or the liquid cushion 56 used in example 2, does not need to comprise pure LOHCs. Instead the liquid 42, 56 could comprise a mixture of LOHCs and an ionic liquid. Accordingly, an anhydrous driving liquid 42 or liquid cushion 56 may comprise up to 80 % (v/v) ionic liquid. Advantageously, if the gas is wet then the ionic liquid becomes saturated, removing the water from the gas. Accordingly, the apparatus can comprise regeneration means configured to remove the water, semi heavy water and/ or heavy water from the driving liquid 42 or liquid cushion 56.

The regeneration means may be configured to remove water, semi heavy water and/ or heavy water before the driving liquid 42 or liquid cushion 56 comprises 50 % (v/v) water, semi heavy water and/ or heavy water.

It may also be advantageous for the apparatus to comprise a separator disposed downstream of the compressor 6 and upstream of the internal combustion engine 8. The separator may be less complex than separators required for prior art devices where the compressor is configured to compress a flammable gas to feed a combustion engine. This is because if a small portion of LOHCs remain within the gas then they will simply become a part of the fuel.

However, where the driving liquid 42 or liquid cushion 56 comprises an ionic liquid, or there is a demand for the compressed gas to be a high purity, then the separator may be or the sort used in the prior art, such as a coalescing filter.

Claims

Claims l. Use of a liquid organic hydrogen carrier (LOHC) in a piston compressor.

2. Use of a liquid organic hydrogen carrier (LOHC) as a driving liquid in a liquid piston compressor.

3. Use of a liquid organic hydrogen carrier (LOHC) as a liquid cushion in a piston compressor.

4. A piston compressor for compressing gas, the compressor comprising at least one compressor chamber configured to receive the gas therein, the or each compression chamber comprising a liquid piston comprising a driving liquid and the piston compressor further comprising a driving liquid supply means configured to vary the volume of the driving liquid in the at least one compressor chamber, characterised in that the driving liquid comprises one or more liquid organic hydrogen carrier (LOHC).

5. A piston compressor according to claim 4, wherein the driving liquid consists substantially of one or more LOHC, or wherein the driving liquid also comprises an ionic liquid and the volumetric ratio of the one or more LOHCs to the ionic liquid is between 75:25 and 5:95.

6. A piston compressor for compressing gas, the compressor comprising at least one compressor chamber configured to receive the gas therein, the or each compression chamber comprising a piston configured to move therein and a liquid cushion disposed between the piston and the gas to be compressed, characterised in that the liquid cushion comprises one or more liquid organic hydrogen carrier (LOHC).

7. A piston compressor according to claim 6, wherein the liquid cushion consists substantially of one or more LOHC, or wherein the liquid cushion also comprises an ionic liquid and the volumetric ratio of the one or more LOHCs to the ionic liquid is between 75:25 and 5:95.

8. A piston compressor according to either claim 6 or claim 7, wherein the compressor comprises a liquid piston compressor, and the or each piston comprises a liquid piston comprising a driving liquid and a dummy piston disposed between the driving liquid and the liquid cushion, and the driving liquid comprises an LOHC, an ionic liquid, water, semi heavy water, heavy water and/or hydraulic oil.

9. A piston compressor according to any one of claims 4 to 8, wherein the LOHC has a vapour pressure between 1 x io^~12 bar and 1 bar at 20°C and/or the LOHC has a vapour pressure between 1 x lo^-6 bar and 1 bar at 127°C, and/or wherein the one or more LOHC comprises a cycloalkane, cycloalkene, N-heterocycle and/or formic acid.

10. A piston compressor according to any one of claims 4 to 9, wherein the piston compressor is configured to compress hydrogen and the one or more LOHC comprises a saturated LOHC.

11. A piston compressor according to any one of claims 4 to 10, wherein the compressor comprises a heat controlling device disposed upstream of the at least one compressor chamber and configured to control the heat of the gas, and the or each heat controlling device is configured to maintain the gas at a temperature between 90°C and 150°C.

12. An apparatus comprising a gas source configured to provide a gas to be compressed, a piston compressor according to any one of claims 4 to 11 disposed downstream of, and in fluid communication with, the gas source and configured to compress the gas received therefrom, and a gas-fed device disposed downstream of, and in fluid communication with, the piston compressor and configured to receive the compressed gas from the piston compressor.

13. A method of compressing a gas, the method comprising:

compressing the gas using the one or more pistons;

14. A method of compressing a gas, the method comprising:

feeding gas to a compressor comprising one or more compressor chambers, wherein the or each compressor chamber comprises a piston and a liquid cushion disposed between the piston and the gas; and compressing the gas using the one or more pistons;

15. A method of providing a gas-fed device with compressed gas, the method comprising:

feeding a gas to be compressed from a gas source to a compressor; compressing the gas using the method according to either claim 13 or claim 14; and

feeding the compressed gas to the gas-fed device.