Process for Producing Liquefied Hydrogen

Field of the Invention

The present invention relates to a method for liquefying hydrogen gas, in particular a method of cooling the hydrogen to be liquefied to an intermediate temperature prior to liquefaction.

Background

Liquefied hydrogen is a potential substitute for carbon-containing fuels. In addition to its current use in space applications, larger quantities of liquid hydrogen will be required in the future for use as fuel for aviation and shipping. A need for large-scale storage and transport of hydrogen in liquid form will develop as the use of hydrogen as a fuel increases.

Existing and proposed hydrogen liquefaction processes mostly comprise:

- a first step of cooling (hereinafter “precooling”) of the incoming hydrogen to an intermediate temperature (hereinafter “intermediate temperature”) by means of heat exchange with an evaporating fluid (the “first refrigerant”); the most widely proposed first refrigerant fluid being liquid nitrogen, with liquid methane (LNG), and mixed refrigerants also proposed, and - a second step of further cooling and liquefaction of the precooled hydrogen, either by means of work-expansion of part of the precooled hydrogen or of a second refrigerant such as helium.

Although a hydrogen liquefaction process without any precooling, and comprising only the aforesaid second step (refrigeration by means of expansion of hydrogen or a second refrigerant) is feasible and may have been practised, the incorporation of a first step of precooling is preferred due to two factors (a) reduction in total compression power of the complete liquefaction process, and (b) the perceived lower investment and production cost resulting from a reduced circulation rate and compression power of the second refrigerant system.

In relation to factor (b), use of the lowest practical temperature of the hydrogen at the outlet of the first, precooling step (typically around -190 degC using liquid nitrogen as the first refrigerant) will minimise the required circulation rate and hence the compression power of the refrigerant in the said second step. However the lowest practical precooling temperature will not necessarily result in the lowest total compression power of the complete liquefaction process when the compression power requirement of the precooling system is taken into account. Summary of the Invention

The main aspect of the invention relates to the liquefaction of hydrogen, and discloses an improved method of precooling of the hydrogen stream to be liquefied to an intermediate temperature, typically of between -150 degC and -200 degC.

Where pressures are stated anywhere in this application as “bar”, these are bar absolute.

The disclosed means of precooling is a closed cycle containing a fluid, such as but not limited to methane or nitrogen or a mixture thereof, comprising:

- a high temperature gas expander machine having a gaseous outlet stream

- a low temperature gas expander machine having a partly liquefied outlet stream

- separation of liquid from the outlet stream from the low temperature gas expander machine

- depressurising the said separated liquid to near atmospheric pressure

- successive cooling of the feed hydrogen (and a second refrigerant if used) from near ambient temperature, first by heat exchange with the outlet stream from the said high temperature gas expander; secondly by heat exchange with the outlet stream from the said low temperature gas expander after the said separation of liquid; and thirdly to a said typical intermediate temperature of -150 degC and -200 degC by heat exchange with evaporation of the said depressurised liquid refrigerant

- recompression of the resulting low pressure refrigerant streams.

The arrangement of the precooling cycle described above resembles the process for methane liquefaction (LNG production) described in GB2486036, particularly in respect of the formation of liquid in the low temperature gas expander, followed by separation of the said liquid from the low temperature gas expander outlet stream. While in that referenced case the liquid formed in the low temperature gas expander contributes part of the total liquid (LNG) output of the process, in this application the said liquid is depressurised and then evaporated by heat exchanger with the hydrogen to be liquefied, so as to cool the said hydrogen to the said intermediate temperature, typically of -150 degC and -200 degC, in a hydrogen liquefaction process.

The invention includes the use in the methane as the refrigerant in the said high temperature gas expander machine while using nitrogen as the refrigerant in the said low temperature gas expander machine.

The Applicant submits that this method of cooling of the hydrogen to be liquefied, namely the formation of liquid refrigerant in a gas expander, separation and depressuring and evaporation of said liquid as a precoolant in a hydrogen liquefaction process, has not been disclosed in prior art and is novel. The production of the said liquid is thermally efficient, as it results from direct production of mechanical work in the low temperature gas expander. There is also the practical benefit of production of liquid refrigerants such as liquid methane or liquid nitrogen within the hydrogen liquefaction process, removing the need for costly and elaborate external supply of liquid first refrigerants such as mixed refrigerants.

Accordingly there is provided as follows a description of a process for liquefying hydrogen according to the main aspect of the invention (reference is made to Drawing 1/3 and the equipment tags and stream numbers shown thereon):

- providing a stream of pure hydrogen feed gas [1];

- providing a stream of recycled hydrogen gas [2] at a pressure of from 1 bar to 50 bar;

- admitting streams [1] and [2] to a hydrogen compressor [A], the said compressor having a combined discharge stream after cooling [3] with a pressure of between 10 bar and 200 bar, and more typically a pressure of between 20 bar and 100 bar;

- cooling said combined discharge stream [3] in a first hot passage of heat exchanger [B], said hot passage having an outlet stream [4];

- cooling said stream [4] in a first hot passage of heat exchanger [C], said hot passage having an outlet stream [5];

- cooling said stream [5] in a first hot passage of heat exchanger [D], said hot passage having an outlet stream [6];

- passing stream [6] to a hydrogen liquefaction unit [E]; hydrogen liquefaction unit [E] typically comprises dividing stream [6] into two parts; cooling a first part [e-1] in a first gas expander to form outlet stream [e-2]; cooling the second part [e-3] in a first heat exchanger to form stream [e-4]; dividing stream [e-4] into two parts; cooling a first part [e-5] in a second gas expander to form outlet stream [e-6]; cooling and liquefying the second part [e-7] in the second heat exchanger to form liquefied hydrogen product stream [7]; recycling stream [e-6) through a second heat exchanger to form stream [e-8]; combining streams [e-2] and [e-8] to form stream [e-9]; reheating stream [e-9] in the first heat exchanger to form recycle hydrogen stream [8]; provision of a catalyst in the second heat exchanger to facilitate conversion of ortho-hydrogen to para-hydrogen;

- the liquefied hydrogen product stream [7] has a temperature of between -240 degC and -255 degC;

- the recycle hydrogen stream [8] has a pressure between 1 bar and 30 bar; reheating stream [8] in a first cold passage of heat exchanger [D] forming outlet stream [9]; reheating stream [9] in a first cold passage of heat exchanger [C] forming outlet stream [10]; reheating stream [10] in a first cold passage of heat exchanger [B], the said reheated stream from heat exchanger [B] forming the above-said hydrogen recycle gas stream [2];

- heat exchangers [B], [C] and [D] may be physically combined in a single unit;

- providing a stream of refrigerant gas [21] at a pressure of from 10 bar to 150 bar;

- dividing the stream of refrigerant gas [21] into first [22] and second [25] parts;

- passing said first part [22] to a first refrigerant gas expander [L], the outlet stream [23] from said first refrigerant gas expander having a pressure between 5 bar and 50 bar;

- reheating the first refrigerant gas expander outlet stream [23] in a second cold passage of heat exchanger [B] to form reheated stream [24];

- compressing the reheated stream [24] in compressor [M] to a pressure of from 10 to 150 bar to form after cooling a first constituent of the above-said refrigerant gas [21];

- passing the second part [25] of the refrigerant gas to a second hot passage of heat exchanger [B], having an outlet stream [26];

- passing said cooled second part of the refrigerant gas [26] to a second refrigerant gas expander [N], the outlet stream [27] from said second refrigerant gas expander having a pressure of typically between 3 bar and 50 bar and comprising a mixture of vapour and liquid;

- separating the outlet stream [27] of the second gas expander [N] in vapour/liquid separator [O] to form a vapour stream [28] and a liquid stream [29];

- depressurizing said liquid stream [29], typically in a valve [P], to form stream [30] having a pressure of between 0.5 bar and 10 bar, and typically at near-atmospheric pressure; the temperatures of stream [30] are typically -160 degC with methane as the refrigerant and -195 degC with nitrogen as the refrigerant, both at essentially atmospheric pressure;

- evaporating and reheating stream [30] in a second cold passage of heat exchanger (D), so as to form outlet vapour stream [31];

- compressing stream [31] to the same pressure as the pressure of stream [28] by means of refrigerant compressor [Q] having outlet stream [32];

- combining stream [28] and stream [32] to form stream [34];

- reheating stream [34] in a second cold passage of heat exchanger [C] to form stream [35] and then in a third cold passage of heat exchanger [B] to form stream [36];

- compressing the reheated stream [36] in compressor [M] to a pressure of from 10 to 150 bar to form after cooling a second constituent of the above-said refrigerant gas [21].

A second aspect of the invention is takes advantage of the high efficiency of the two-stage expander precooling circuit described above to operate the hydrogen recycle compressor with a significantly sub-ambient suction temperature. The proposed flow scheme is shown on schematically on Drawing 2/3. Stream [9) enters the first part of compressor A typically at a temperature of -120 degC.

Alternatively the inlet stream to compressor [A] may be taken directly from the outlet stream [8] of the hydrogen liquefier unit [E], or from the outlet of the first cold passage [10] of heat exchanger [C] on Drawing 1/3;

Depending on the inlet temperature of compressor [A], the power of said compressor [A] may be reduced by approximately 50%, relative to the configuration with ambient inlet temperature shown on Drawing 1/3. There is an approximately equivalent increase in the power demand for the first refrigerant compressors [M] and [Q].

The Applicant submits that this arrangement of operation of the hydrogen compressor with a significantly sub-ambient inlet temperature is both novel and particularly advantageous in relation to prior art for hydrogen liquefaction:

- hydrogen compression generally requires use of reciprocating compressors, as the density of hydrogen may be too low for use in centrifugal compressors; taking into consideration the relatively high investment and operational costs of reciprocating compressors, particularly in large installations requiring multiple compressors in parallel, the reduction in power requirement of reciprocating compressors due to use of a sub-ambient inlet temperature will be significant;

- operation of the hydrogen compressor with a significantly sub-ambient inlet temperature increases the inlet density; for instance at -120 degC the inlet density is approximately 2 x the density at ambient temperature, facilitating the use of centrifugal compression in hydrogen liquefaction.

In a third aspect of the invention, illustrated on Drawing 3/3, part or all of the refrigeration required to cool further and liquefy the hydrogen stream in the hydrogen liquefaction unit [E] is provided by expansion of a second refrigerant in one or more stages in a closed circuit. With this arrangement, the amount of refrigeration produced in the hydrogen liquefaction unit [E] by expansion of a part of Stream [6] can be much reduced or even eliminated, and consequently the flow rate of Stream [8] and the power required for compressor [A] may be significantly lower than in the flow scheme illustrated on Drawing 1/3.

According to this third aspect of the invention:

- a stream of a second refrigerant [11] is cooled successively in heat exchangers [B], [C] and [D] to form stream [14] which typically has the same temperature as the hydrogen inlet stream [6] to the hydrogen liquefier unit [E];

- in addition to the typical internal arrangement of the hydrogen liquefaction unit [E] described in respect of the said main aspect of the invention and shown on Drawing 1/3, the hydrogen liquefaction unit [E] typically further comprises division of stream [14] into two parts; cooling a first part [e-11] in a first expander to form outlet stream [e-12]; cooling the second part [e-13] in a first heat exchanger to form stream [e-14]; reheating stream [e-14] in the first heat exchanger to form stream [e-15]; further cooling stream [e-12] in a second expander to form outlet stream [e-16]; reheating stream [e-16] in a second heat exchanger to form stream [e-17]; and combining streams [e-15] and [e-17] to form stream [15];

- stream [15] leaves the hydrogen liquefier unit [E] at a lower pressure than stream [14];

- stream 15 is then reheated successively in heat exchangers [D], [C] and [B] to form reheated stream [18] at near-ambient temperature;

- stream [18] is then recompressed in compressor [F] to form after cooling the above-mentioned second refrigerant [11].

The second refrigerant may comprise hydrogen, helium, or neon or mixtures thereof.

In the case of the use of hydrogen as the second refrigerant, no significant conversion of ortho-hydrogen to para-hydrogen is expected in the absence of a conversion catalyst in the second refrigerant circuit. Due to the above- mentioned resulting lower flow of stream [6] in this third aspect of the invention, the flow of hydrogen passing over the said conversion catalyst in the hydrogen liquefier unit [E] may be lower in than in the main aspect of the invention shown on Drawing 1/3, and as a result the quantity of ortho- to para-hydrogen conversion catalyst may also be reduced.

The invention has been extensively simulated by means of widely used process simulation software.

Description of Preferred Embodiments

The invention will be described with reference to the accompanying drawings in which represent flow diagrams illustrating embodiments of the process in accordance with the invention.

The exact flow sheets are subject to variation, but will generally contain these basic elements.

In a first embodiment of the invention, illustrated on Drawing 1/3, the feed stream of hydrogen to be liquefied [1] with pressure 25 bar is admitted to a compressor [A]. The compressor also receives a stream of recycle hydrogen [2], described below. The combined stream of feed hydrogen and recycle hydrogen after cooling [3] is discharged from the compressor at 75 bar.

The combined stream [3] is cooled to -50 degC by passing through the first hot passage of heat exchanger [B] to form stream [4]; then further cooled to -120 degC by passing through the first hot passage of heat exchanger [C], to form stream [5]; the necessary refrigeration being provided as described below by a closed circuit of methane refrigerant.

The outlet stream [5] from heat exchanger [C] is further cooled to -158 degC by evaporation of a low pressure methane refrigerant stream to form stream [6].

Stream [6] then flows to a hydrogen liquefaction unit [E] comprising one or more hydrogen expanders, one or more heat exchangers and one or more ortho-to-para hydrogen catalytic conversion stages.

The hydrogen liquefaction unit [E] has an outlet stream of liquid hydrogen [7] with a temperature of -244 degC and a pressure of 7.5 bar, and an outlet stream of gaseous hydrogen stream [8] having at temperature of -161 degC and a pressure of 6.8 bar.

Stream [8] is reheated first in a cold passage of heat exchanger [D] to form Stream [9] with temperature -123 degC, and then is further reheated in a first cold passage of heat exchanger [C] to form Stream [10] with a temperature of -53 degC, and then is further reheated in a first cold passage of heat exchanger [B], the reheated stream at near-ambient temperature forming the above- mentioned hydrogen recycle Stream [2], The above-mentioned closed refrigeration circuit containing methane refrigerant has stream [21] with a pressure of 90 bar at the discharge of refrigerant compressor [M].

The outlet stream [21] from compressor [M] is divided into a first part [22] and a second part [25],

The first part [22] passes to a first refrigerant gas expander [L] having outlet stream [23] with pressure 26 bar and temperature -54 degC. The second part [25] is passed through a second hot passage of heat exchanger [B], which has an outlet stream [26] having the same outlet temperature of -50 degC as the above-mentioned hydrogen stream [4].

Stream [23] is reheated to near-ambient temperature in a second cold passage of heat exchanger [B]. The reheated stream [24] flows to refrigerant compressor [M] at near-ambient temperature as a first constituent after cooling of the above-said refrigerant gas stream [21],

The outlet stream [26] from heat exchanger [B] flows to a second refrigerant gas expander [N], having outlet stream [27] with pressure 10 bar and temperature -124 degC and containing both vapour and liquid. Stream [27] is separated in vapour/liquid separator [O] to form vapour stream [28] and liquid stream [29].

Liquid stream [29] is depressurizing in valve [P] to near-atmospheric pressure, so as to form a mixture of liquid and vapour in the outlet Stream [30] with a temperature of -158 degC.

Stream [30] is fully evaporated and reheated in a second cold passage of heat exchanger (D), so as to form outlet vapour stream [31] having the same temperature of -123 degC as above-mentioned hydrogen stream [9]. Stream [31] is compressed by refrigerant compressor [Q] which has outlet stream [32] having the same pressure of 9.7 bar as stream [28]. Streams [28] and [33] are then combined to form stream [34].

Stream [34] is reheated first in a second cold passage of heat exchanger [C] to form stream [35] having a temperature of -53 degC and then in a third cold passage of heat exchanger [B], The reheated stream [36] flows to compressor [M] at near-ambient temperature as a second constituent after cooling of the above-said refrigerant gas stream [21].

The invention will be further described by reference to the accompanying Drawing 2/3 representing a second embodiment of the invention. This second embodiment, which is described in concept above, comprises a variant of the first embodiment, whereby the hydrogen recycle compressor [A] receives an inlet stream with a significantly sub-ambient suction temperature.

In an example of this second embodiment, the hydrogen recycle stream [9] flows directly to compressor [A) at a temperature of -123 degC and at a pressure of 6.6 bar. The temperature of the outlet stream [3] from compressor [A] is then reduced to near-ambient temperature.