This invention relates to a method of and apparatus for the production of liquid oxygen and liquid hydrogen.

The use of liquid hydrogen as a fuel particularly for jet aircraft has a number of environmental and technological advantages over conventional fuels. It is recognised that a key problem in the development of hydrogen propulsion technology is the energy cost of liquefaction of oxygen and of hydrogen as discussed e.g. in the paper by H.P. Alder 'Hydrogen in air transportation. Feasibility study for Zurich Airport, Switzerland' (Int.J.Hydrogen Energy, Vol.12 No.8, pp.571-585. 1987).

Recent developments in aerospace engine technology envisage hydrogen burning propulsion systems which use oxygen taken from the atmosphere at lower altitudes and which make use of on-board supplies of liquid oxygen at higher altitudes. The mass ratio of liquid oxygen to liquid hydrogen to be carried by the aerospace vehicle ranges from 2.2. to 2.4 kg of liquid oxygen for each 1.0 kg of liquid hydrogen as reported in the article by J. Moxon 'HOTOL: where next?' (Flight International, pp 38-40, 1 March 1986) and the paper by B.R.A. Burns 'HOTOL space transport for the 21st century' (Aerotech '89, I.Mech.E. Seminar Papers C398/13, NEC Birmingham, 31 October - 3 November 1989).

In a paper by E.M. Smith entitled 'Slush hydrogen for aerospace applications' (int.J. for Hydrogen Energy, Vol.14, No.3, pp. 201- 213, 1989) thermodynamic analysis of the hydrogen liquefaction process shows one hydrogen liquefaction plant with a helium refrigeration system having a theoretical efficiency around 0.48. Alternative routes for the production of gaseous hydrogen which were discussed included high pressure electrolysis of water.

In a paper by E.M. Smith entitled 'Liquid oxygen for aerospace applications' (Int.J. for Hydrogen Energy, Vol. 14, No. 11, pp. 831-837, 1989) thermodynamic analysis of the oxygen liquefaction process describes one combined liquefaction plant capable of producing 2.25 kg of liquid oxygen and 1.0 kg of liquid hydrogen with a theoretical efficiency around 0.69. This compares well with the reported efficiency for water electrolysis alone which is around 0.80.

Future spaceflight missions may rely on the extraction of oxygen from lunar rocks, see the paper by N. Jarrett, S.K. Das and .E. Hampkin 'Extractions of oxygen and metals from lunar ores' (Space Solar Power Review, Vol. 1, pp. 281-287, 1980). The energy penalty of transporting oxygen from the moon to space stations is significantly less than transporting oxygen from the earth's surface. Liquefaction of oxygen gas, using only oxygen as refrigerant, may be desirable on the lunar surface.

Partial compression of oxygen produced by electrolysis in a

bipolar cell on the surface of the moon may be effected by first collecting the gas in an 'elasticated' balloon on the face of the moon exposed to the sun's heat which produces energy for the electrolysis, then optionally allowing the gas to be cooled and reduced in volume by contraction of the balloon membrane as it passes into shadow on the face of the moon away from the sun, finally connecting an evacuated gas bottle to the balloon and allowing some of the gas to flow into the bottle for storage, closing the bottle and disconnecting it from the balloon.

One object of the present invention is to gain improvement in the efficiency of liquefaction of a gas, more particularly oxygen.

What I propose is to split a stream of gas under pressure into a product stream and a refrigeration stream and to expand the refrigeration stream in order to provide work and/or c<~ ^• ling needed to liquefy the product stream. Conveniently and advantageously when the gas to be liquefied is oxygen, electrolysis of water under pressure as is know per se is performed to produce a stream of oxygen gas under pressure and also a hydrogen stream which can be liquefied in parallel with the oxygen stream, yielding liquid levels in the proportions required for aerospace applications described above. Alternatively, the reduction of lunar rocks or ores (on the lunar surface) in an electrolytic cell represents a convenient source of oxygen.

According to the present invention, I propose a method for the

production of a liquified gas in which a stream of the gas under pressure is divided into a product stream and a refrigerant stream, the method comprising:- initial cooling whether before or after division of the gas stream, of both the product and refrigeration streams against an inert medium and a return cooling flow provided by the refrigerant stream; expansion of at least the refrigerant stream to provide the said return cooling flow at low pressure; and further cooling the product stream (either directly or indirectly) by heat exchange with the return cooling flow.

The said expansion is preferably effected in two or more stages, producing first and second return cooling flow portions respectively after the first and second expansion stages, and the first return cooling flow portion is used for heat exchange in a first further cooling stage; the second return cooling flow portion is used for heat exchange in a second further cooling stage; after flowing through the first further cooling stage the first return cooling flow portion is further expanded and combined with the second return cooling flow portion after the second further cooling stage; and the recombined first and second return cooling flow portions are also used for heat exchange in the first further cooling stage.

Also according to this invention, I propose apparatus for the production of a liquefied gas in which a stream of the gas under pressure is divided into a product stream and a refrigerant, the

apparatus being intended and adapted for connection to a gas stream source and comprising first heat exc ange means for cooling both the product and refrigeration streams against an inert medium and a return flow provided by the refrigerant stream; means for expanding at least the refrigerant stream to provide the said return cooling flow at low pressure; and further heat exchange means for cooling the oxygen product stream (either directly or indirectly) by heat exchange with the return cooling flow.

Preferably, the expansion means comprises first and second expansion devices providing respectively first and second return cooling flow portions, and wherein the further heat exchange means comprises a first further heat exchanger to which the first return cooling flow portion is connected and a second further heat exchanger to which the second return cooling flow portion is connected, the apparatus further comprising a third expansion device connected to a first return cooling flow portion outlet from the first further heat exchanger, the second return cooling flow portion outlet of the second further heat exchanger and the outlet of the third expansion device being connected to a combined return cooling flow inlet of the first further heat exchanger, and the combined return cooling flow outlet of the first further heat exchanger being connected to a return cooling flow inlet of the first heat exchange means.

In one embodiment liquid oxygen and liquid hydrogen are produced in parallel by electrolysing water under pressure to generate

separate streams of oxygen and hydrogen gas under pressure hereinafter referred to as intermediate pressure, and removing heat from the hydrogen stream during liquefaction thereof by heat exchange with one or more suitable mediums as is known per se. The method of liquefying the oxygen comprises dividing the oxygen stream into an oxygen product stream and an oxygen refrigeration stream, raising the oxygen product stream to supercritical pressure before liquefaction thereof by heat exchange desirably with an inert medium and by heat exchange with return flow refrigerant oxygen streams at low pressure and by return vapour from the oxygen product stream achieving liquefaction of the cold supercritical pressure product oxygen stream by expansion to produce liquid oxygen and vapour. The intermediate pressure oxygen refrigeration stream is cooled by heat exchange desirably with the same inert medium and is further expanded to produce cooling for the oxygen product stream and work.

In this embodiment, apparatus for the production of liquid oxygen in which an oxygen stream is divided into an oxygen product stream and an oxygen refrigeration stream both under pressure hereinafter referred to as intermediate pressure, comprises compression means for raising the oxygen product stream to supercritical pressure, heat exchange means for cooling the oxygen product stream by an inert medium flowing in a cooling circuit and by low pressure return flow of the expanded oxygen refrigeration stream and by return vapour from the oxygen product stream expansion means for producing liquid oxygen and vapour, heat exchange means for cooling both the oxygen product stream

and the oxygen refrigeration stream by an inert medium flowing in a cooling circuit and by return flow refrigerant oxygen, expansion means for the cooled oxygen refrigeration stream to supply low pressure refrigeration oxygen, to heat exchangers to cool the oxygen product stream and the oxygen refrigeration stream.

In another embodiment, the method f liquefying the oxygen comprises cooling the oxygen stream by heat exchange desirably with an inert medium and by heat exchange with return flow refrigerant oxygen streams at low pressure further cooling the oxygen stream by expanding the oxygen stream to produce cooling and/or work, dividing the oxygen stream into a product stream and a refrigeration stream at low pressure, removing latent heat from the product oxygen stream by heat exchange against an evaporating inert medium.

In this embodiment, apparatus for the production of liquid oxygen from an oxygen stream at intermediate pressure comprises compression means to raise an inert medium to supercritical pressure, heat exchange means for cooling both the intermediate pressure oxygen stream and the supercritical pressure inert medium stream by an inert medium flowing in a cooling circuit and by heat exchange with the low pressure return flow of the expanded oxygen refrigeration stream and by return flow of cooled and expanded inert medium stream, expansion means connected to the cooled oxygen stream to supply low pressure oxygen to heat exchangers to further cool the inert medium at supercritical

pressure, expansion means for the inert medium at supercritical pressure to produce liquid and vapour colder than the temperature of liquid oxygen, division of the low temperature cold oxygen stream into an oxygen refrigeration stream and an oxygen product stream, heat exchange means to liquefy the oxygen product stream by evaporation of the cold liquid inert medium.

Apparatus for heat exchange between the oxygen product stream and the oxygen refrigeration streams and/or the oxygen and inert medium expansion means whether comprising machines and/or throttles is preferably contained in a evacuated cold box.

Various embodiments of the invention will now be described by way of example with reference to the accompanying drawings.

Fig. 1 is a block diagram of one embodiment of the plant for the production of liquid oxygen and liquid hydrogen;

Fig. 2 is a diagram concerning the liquefaction of hydrogen which shows details of module (f) and part of module (e) of Fig. 1;

Fig. 3 is a diagram of one embodiment of apparatus for the liquefaction of oxygen which shows details of module (d) and another part of the module (e) of Fig. 1;

Fig. 4 is a temperature/entropy diagram showing the thermodynamic liquefaction cycle relating to the arrangement shown in Fig. 3;

Fig. 5 is a diagram of another embodiment of apparatus for the liquefaction of oxygen which shows details of module (d) and another part of the module (e) of Fig. 1;

Fig. 6 is a temperature/entropy diagram showing the thermodynamic liquefaction cycle relating to the arrangement shown in Fig. 5.

In a preferred embodiment, a hydrogen liquefaction module receives a stream of hydrogen under intermediate pressure from the electrolysis means, an oxygen module receives a stream of oxygen under intermediate pressure from the electrolysis means. The oxygen module includes supercritical pressurisation means for part of the intermediate pressure oxygen stream before liquefaction and expansion means for the other part of the oxygen stream discharged at low pressure near ambient conditions from the module after providing cooling. One or more inert medium cooling modules supplying cooling for both the hydrogen and oxygen modules.

In another preferred embodiment, a hydrogen liquefaction module receives a stream of hydrogen under intermediate pressure from the electrolysis means, an oxygen module receives a stream of oxygen under intermediate pressure from the electrolysis means. The oxygen module includes expansion means for expansion to low pressure to cool to near liquefaction temperatures where the oxygen stream is divided into an oxygen refrigeration stream and an oxygen product stream. The oxygen product stream is liquefied by heat exchange against a cold evaporating inert

medium. The oxygen refrigeration stream is discharged at low pressure near ambient conditions from the module after providing cooling. One or more inert medium cooling modules supplying cooling for both the hydrogen and oxygen modules.

In this specification the term supercritical pressure refers to pressures above the critical pressure for the fluid or gas in question (typically, 2.5 times the critical pressure). The words subcritical pressure refer to pressures below the critical pressure for the fluid or gas in question. The words low pressure refer to pressure levels about which liquefaction conditions normally pertain (typically, atmospheric pressure). The words intermediate pressure refer to pressure levels between subcritical and low.

Typical supercritical pressure may be between 250 bar and the critical pressure for oxygen (say 51 bar), typical subcritical pressure between 51 bar and the critical pressure for hydrogen (say 13 bar), typical low pressure between 13 bar and zero bar, and typical intermediate pressure between 51 bar and zero.

In the final stages of liquefaction, two methods for liquefying a gas are available, viz;

1. assuming the product gas, initially at a pressure above the critical pressure, has been cooled to a temperature close to the liquefying temperature, the fluid is then expanded through an expansion device to the final pressure, producing a large amount of liquid and a correspondingly small amount

of vapour .

2. assuming the product gas has been cooled until it is available as vapour at the final pressure level and at a temperature just above the liquefying temperature, the latent heat can be removed and the gas condensed by heat exchange against another refrigerant fluid available as liquid and whose evaporating temperature lies below that of the gas to be liquefied.

The problem in case (1) is how to bring the product gas to its near final condition before expansion, by cooling it at supercritical pressure.

The problem in case (2) is how to bring the refrigerant gas to its near final condition by cooling at supercritical pressure and expanding to the desired condition, and additionally how to bring the product gas to its near final vapour condition by cooling at subcritical pressure conditions.

Since cooling of a product fluid at supercritical pressure is required in case (1) and cooling of a refrigerant fluid at supercritical pressure is required in case (2), henceforth the words supercritical fluid will be used to designate supercritical product stream or supercritical refrigerant stream as appropriate in discussing the common technical aspect.

Also since either a product fluid or a refrigerant fluid may act

as coolant, henceforth the word coolant will be used to designate subcritical product stream or subcritical refrigerant stream as appropriate in discussing the common technical aspect.

Note that a product stream may also act as both supercritical fluid and coolant, and that differing refrigerants may be employed in the several parts of a plant.

In reaching the final stages of liquefaction outlined above, it is essential to minimise temperature differences between the supercritical fluid stream and the several coolant streams. Large energy penalties are incurred if this is not done. As a rough rule-of-thumb, it is desirable that temperature differences should be kept to about l/30th of the absolute temperature level at any point.

As enthalpy/temperature plot of the supercritical fluid isobar reveals that the line is not straight, but that it can be broken into approximately straight-line segments. Each segment represents a heat exchanger in which the temperature gradients will be approximately linear (see Figures 4 and 6).

The coolant stream or streams for each heat exchanger are usually at low subcritical pressure levels in the superheated vapour field, where enthalpy/temperature lines are fairly straight. ^" Matched temperature gradients along each complete heat exchanger are achieved by adjusting the mass flow rate of each coolant stream appropriately.

Examination of the enthalpy/temperature line for the supercritical fluid stream usually permits division into three segments, the central segment having the steepest slope.

It is convenient to choose a supercritical level which will allow the central and lowest segments to be matched for temperature gradients together. Temperature gradients for the uppermost segment are matched independently.

Another constraint which has to be taken into consideration in configuring the plant is associated with how coolant streams are produced. An efficient method for producing cold coolant is to expand high pressure coolant gas in a single-stage inward radial flow nozzle-turbine set (IRF turbine). It is undesirable for the expanding gas to experience shock fronts (i.e. the flow should be subsonic everywhere), because shock fronts produce losses in the form of a temperature rise across each front. This limits the permissible design pressure ratio across every IRF turbine, and hence the achievable temperature reduction between inlet and outlet conditions.

The magnitude of the temperature ratio obtained depends on the pressure ratio, on the isentropic efficiency of the IRF turbine, and on the physical properties of the gas. Gases with a higher value of the isentropic index of expansion (gamma=Cp/Cv) permit greater pressure ratios across IRF turbines, and hence greater temperature reductions between inlet and outlet. Alternatively,

coolant gases with lower values of gamma require a higher pressure ratio to achieve the same temperature reduction.

At lower temperature levels, the same expansion pressure ratio in an IRF turbine produces the same temperature ratio between inlet and outlet conditions, but obviously smaller temperature reductions. It remains to match the achievable coolant stream temperature drops across the IRF turbines with the linear segments of the enthalpy/temperature line of the supercritical fluid stream which represents a heat exchanger. This is done by adjusting the supercritical pressure level of the supercritical fluid stream, and by selecting appropriate pressure levels for the various coolant streams.

Where the enthalpy/temperature line segment for the supercritical fluid stream is steep, more coolant mass flow is required. This suggests use of more than one expansion of the same coolant stream - as can be seen in the association of two IRF turbines with the middle multi-stream heat exchanger of Fig.3 and Fig.4.

When the central steepest segment has been matched, it is then appropriate to examine the lowest segment to assess whether an IRF turbine is capable of producing sufficient cold coolant flow to cool the supercritical fluid stream to the expansion point.

It remains to produce coolant gas for the first segment by independent choice of pressure ratio for the corresponding IRF turbine unit. The working fluid in this case can be the product

gas, or the refrigerant gas, or another suitable gas. Independent design of this coolant loop is possible, which makes design of the upper block heat exchanger straightforward.

Referring now to Fig. 1, feed water 1 at near ambient pressure and temperature is passed through a water filtration, deionisation and degassing module (a) which is connected at 2 to the inlet of an intermediate pressure feed pump (b) delivering at its outlet 3 water under intermediate pressure (typically 35 bar) to a water electrolysis module (c). Oxygen and hydrogen gas are generated by electrolysis at intermediate pressure, are cooled to near ambient temperature, and dried and have trace impurities removed in module (c) and then pass respectively along lines 4 and 10 to an oxygen module (d) and a hydrogen liquefaction module (f) . Oxygen gas which has been used as refrigerant leaves module (d) at 12 at near ambient conditions, liquid oxygen product leaves module (d) at 7, and liquid hydrogen product leaves module (f) at 11. Helium and/or argon and/or nitrogen refrigerant streams 5,6,8 and 9 are schematic, and possible internal working arrangements of modules (d), (e) and (f) are explained with reference to Fig. 2, Figs. 3 and 4, and Fig. 5 and 6.

Within the hydrogen liquefaction module (f) shown in fig. 2 the intermediate pressure hydrogen stream 20 passes through four or more successive heat exchanger arrangements 100A, B, C and D cooled by helium gas and optionally fitted with catalyst converters 102 for ortho-to-para hydrogen conversion before

reaching the expansion throttle at 104. The low pressure hydrogen discharge 27 (typically 1 bar) enters the storage tank 106 which is cooled by helium at a lower temperature than the vapour temperature of the low pressure hydrogen. An optional low pressure hydrogen return line from the storage tank, also provided with para-to-ortho hydrogen conversion catalysts, may pass from the storage tank 106 through each heat exchanger successively in counter-flow to the intermediate pressure hydrogen stream and is not shown in the diagram. The helium refrigeration cycle incorporating a compressor 108 and expansion machines 110A, B, C and D as shown in Fig. 2 forms part of module (e) of Fig. 1.

Within the oxygen module (d) shown in Fig. 3 the intermediate pressure oxygen gas is split into two streams, the oxygen product stream being compressed by compressor 300 to high pressure (typically 125 bar) and cooled to near ambient conditions before entering the first heat exchanger 302 at 15, the intermediate pressure oxygen refrigeration stream entering the first heat exchanger directly at 2. An argon refrigeration cycle shown cooling the first heat exchanger of Fig. 3 and incorporating a compressor 304 and an expansion machine 306, forms part of module (e) of Fig. 1. The cooled intermediate pressure refrigeration oxygen stream 3 is expanded in two stages by machines 308 and 310 to low pressure (typically 1 bar). After the first stage 310 of expansion to 4 the refrigeration flow is split into two streams 5 and 8. Stream 8 is expanded in the second stage 310 to low pressure at 9 to provide cooling for a third heat exchanger 312

while ream 5 is used first to cool the second heat exchanger 314 b- ore being expanded by machine 316 to low pressure at 7 to form a return cooling flow helping to cool the second heat exchanger 314 again. The high press., a oxygen product stream at 15 is cooled successively in the three heat exchangers before entering an expansion throttle 318 at 18 where it is expanded to low pressure and discharged to storage tank 320 at 19 with a low vapour fraction. The oxygen vapour return stream 21 joins return cooling flow 9 and enters the third heat exchanger 312 at 10, passing in counter-flow to the high pressure oxygen product stream. The oxygen return cooling flow 11 leaving the third heat exchanger 312 is augmented by the oxygen return cooling flow 7 before entering the second heat exchanger 314 at 12 in counter- flow to the high pressure oxygen product stream. The low pressure counter-flow oxygen return cooling flow 12 passes successively through the second 314 and first 302 heat exchangers before being discharged at near ambient conditions from the module at 14.

Within the oxygen module (d) shown in Fig. 5 the intermediate pressure oxygen gas stream (typically at 35 bar) enters the first heat exchanger 502 directly at 2. A closed cycle argon refrigeration plant incorporating compressor 504 and expansion machine 506 is shown cooling the first heat exchanger 502 and forms part of module (e) of Fig. 1. The cooled intermediate pressure oxygen stream 3 i.e. the product and refrigeration streams combined is expanded by machines 508 and 510 in two stages to low pressure (typically 1 bar). After the first stage

508 of expansion to 4 (typically 6 bar) a return cooling (refrigeration) flow 5 is split from stream 8 which is expanded in the second stage 510 to low pressure at 9 where it divides to provide a return cooling flow 10 for the third heat exchanger 512 and to form the liquefaction product stream 21. Return cooling flow 5 is used first to cool the second heat exchanger 514 before being expanded by machine 516 to low pressure at 7 (typically 1 bar) to help cool the second heat exchanger 514 again. The return cooling flow 11 leaving the third heat exchanger 512 is augmented by the expanded return cooling flow 7 before entering the second heat exchanger 514 at 12 in counterflow to an argon refrigerant stream raised by compressor 505 to supercritical pressure. The low pressure (combined) return cooling flow 12 passes successively through the second 514 and first 502 heat exchangers before being discharged at near ambient conditions from the module at 14. The supercritical pressure argon refrigeration stream at 25 is cooled successively in the three heat exchangers 502, 514 and 512 before entering an expansion throttle 518 at 28 which it leaves at low pressure (typically 1 bar) discharging at 29 with a low vapour fraction which enters a fourth heat exchanger 522 for heat exchange in counterflow with the oxygen product stream. The liquid argon evaporates to 30 liquefying the oxygen product stream 20. Cold argon vapour at 31 passes successively through the third 512 and second 514 heat exchangers to 33 where it combines with the auxiliary cooling argon flow 23 to enter the first exchanger 502 at 34. Argon leaves the first heat exchanger 502 at 24 and is compressed and cooled to 4 bar before dividing at 35 to enter the expansion

turbine 506 at 22 and the compressor 505 at 35 where it is compressed and cooled to 125 bar.

The helium refrigeration cycle shown in Fig. 2 forms part of module (e) of Fig. 1. The argon refrigeration cycle shown in Fig. 3 forms part of module (e) of Fig. 1. The argon refrigeration cycle shown in Fig. 5 forms part of module (e) of Fig. 1. It is possible for one or more parts of module (e) to be combined in a single refrigeration system using one refrigerant. The argon cycle of Fig. 5 (stations 25 to 36) may be replaced by a nitrogen cycle operating with a supercritical pressure level at around 80 bar.

Work from some or all of the gas expansion machines (typically inward radial flow turbines, possibly with one exhaust axial stage) may be used to help drive some or all of the gas compressors (typically multi-stage intercooled compressors) in the plants described.

To assist a more detailed understanding of the operation of the embodiment described above, with reference to Fig. 2 the FLOW conditions at various points (1 to 28) are set out in Table 1, with reference to Fig. 3 the flow conditions at various points (1 to 24) are set out in Table 2. With reference to Fig. 5 the flow conditions at various points are set out in Table 3, but as the intention is to allow direct comparison with Fig. 3 the station numbering system has gaps (explicitly at 1 and 15 - 19).

TABLE 1 Thermodynamic condition points for Fig.2

He => helium, eH2 => equilibrium hydrogen

TABLE 2 Thermodynamic condition points for Fig.3

2 => oxygen, Ar => argon

TABLE Thermodynamic condition points for Fig. 5

2 => oxygen, Ar => argon