Process for Producing Liquefied Hydrogen

Description

FIELD OF THE INVENTION

The present invention relates to a method for liquefying hydrogen gas, in particular a method of liquefaction by means of a partially-liquefying expander machine or turbine.

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, shipping and other transportation purposes. A need for large-scale storage and transport of hydrogen in liquid form will develop as the use of hydrogen as a fuel increases.

As is well known, hydrogen at ambient temperature (“normal” hydrogen) exists as a mixture of two forms, 75% ortho-hydrogen and 25% para-hydrogen, while at liquid hydrogen temperatures circa −250deg C. the equilibrium composition is almost completely para-hydrogen. Accordingly hydrogen liquefaction technology generally includes one or more stages of catalytic conversion at low temperatures in the range of −200deg C. to −250deg C. approximately, in which the ortho-hydrogen content of the incoming feed hydrogen is exothermically converted into para-hydrogen upstream of the final liquefaction step. In the absence of this catalysed conversion step, the ortho-hydrogen content of the liquefied hydrogen would slowly convert exothermically to para-hydrogen in the storage tank, with the result that most or all of the product would evaporate and be lost.

Existing and proposed hydrogen liquefaction processes generally comprise

• • a step or steps of conversion of the ortho-hydrogen content of the feed gas into para-hydrogen, followed by
  • a step or steps of liquefaction of the resulting para-hydrogen gas or vapour by means of indirect heat exchange with a colder fluid.

Helium is already used, and is proposed for future use, as the said colder fluid, due to its low boiling temperature range relative to hydrogen (helium −269deg C., hydrogen −253 deg at atmospheric pressure).

Helium is accordingly an excellent refrigerant for hydrogen liquefaction, but it is expensive, and its price is expected to rise with growth in use in hydrogen liquefaction and for other purposes. Also there may be logistical difficulties in replenishing a closed-circuit helium system in a large industrial plant following a significant leak or an accident of some kind.

So as to overcome these potential difficulties associated with use of helium, hydrogen itself may be considered for use as the colder fluid in the final heat exchange stages. As proposed by U Cardella (doctoral dissertation, Technische Universität München, 2018 and perhaps others), “normal” hydrogen, i.e. with content 75% ortho-hydrogen+25% para-hydrogen, may be used in a closed refrigeration circuit, with only the quantity of hydrogen to be liquefied as product being passed over a catalyst for conversion of its ortho-hydrogen content into para-hydrogen.

The use of hydrogen as the cold-end refrigerant avoids the above-mentioned economic and practical objections to helium, but very small temperature differences are necessary in the coldest heat exchangers, and this may be difficult to arrange when the product hydrogen is required to be delivered at near-atmospheric pressure.

SUMMARY OF THE INVENTION

The invention relates to the final stage of a process for liquefaction of hydrogen, in particular to the use of hydrogen as the refrigerating fluid.

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

The term expander where used in this application describes a process duty only. More than one expander machine or rotor in series may be required for an individual process duty.

The invention, which has two aspects, has the aim of avoiding reliance on a final condensing heat exchanger with close temperature approaches, and of facilitating the production of liquid hydrogen at near-atmospheric pressures.

According to the first aspect of the invention, a final heat exchanger as described in prior art, typically requiring close temperature approaches of around 1deg C., in which hydrogen in the form of para-hydrogen is condensed by indirect heat exchanger with colder “normal” hydrogen, is replaced with an expander machine or turbine, having an outlet stream consisting of vapour and liquid. The expander outlet stream flows to a vapour-liquid separator, which may be integrated with a storage tank, in which the liquid fraction is separated to form the liquefied hydrogen product of the process, and the vapour fraction is re-compressed and recycled.

According to the second aspect of the invention, the extent of reconversion of para-hydrogen into ortho-hydrogen during the above-said re-compression and recycling steps is minimised by providing for the re-compressor to operate with one or more compression stages having cryogenic inlet temperatures.

Inevitably some back-conversion of para-hydrogen into ortho-hydrogen will take place during the said re-compressed and recycling steps, and provision is made for conversion of the resulting relatively small amount of ortho-hydrogen by means of a further body of catalyst located in a lower temperature region of the process.

The Applicant respectfully submits that the described combination of (1) production of liquid hydrogen in a partially liquefying para-hydrogen expander machine, and (2) recompression and recycle of the separated vapour fraction of the expander outlet stream at cryogenic temperature (so as to minimise reverse conversion of para-hydrogen into ortho-hydrogen) is both novel and inventive.

From the viewpoint of practical application in a hydrogen liquefaction plant in which hydrogen is used as the refrigerating fluid, use of the invention

• • removes in the expander, in the form of mechanical work, the heat of condensation of the product hydrogen, thereby significantly reducing or eliminating the need for low temperature heat exchangers with small temperature approaches, and
  • by compressing the recycled hydrogen at low temperature and hence increased density, reduces the power required for the hydrogen recycle compression and facilitates the use of a centrifugal hydrogen compressor.

Accordingly there is provided as follows a description of a process for liquefying hydrogen, illustrating the main aspects 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] at a pressure of between 10 bar and 150 bar;
  • cooling stream [1] in heat exchanger [A], having an outlet stream [2] with a temperature of between −150deg C. and −210deg C.;
  • admitting stream [2] to assembly [B] comprising a catalyst [C] for conversion of ortho-hydrogen to para-hydrogen having an outlet stream [3] and a heat exchanger [D] having an outlet stream [4], the said assembly [B] comprising multiples of catalyst [C] and heat exchanger [D], such that the composition of stream [4] is essentially para-hydrogen, with a temperature of between −210deg C. and −250deg C.;
  • providing a recycle stream of hydrogen [11] having a composition essentially of para-hydrogen and having the same pressure as stream [4];
  • combining stream [4] and stream [11] to form stream [5];
  • admitting stream [5] to an expander machine [E] having an outlet stream [6] with a pressure of between 10 bar and 1 bar and containing both liquid and vapour;
  • passing stream [6] to a vessel [F] wherein the liquid fraction stream [7], comprising the liquid hydrogen product from the process, is separated from the vapour fraction stream [8];
  • providing vessel [F] with a catalyst which converts residual ortho-hydrogen in stream [6] into para-hydrogen;
  • reheating stream [8] in a heat exchanger [G] to form outlet stream [9], having a temperature of between −100deg C. and −240deg C.;
  • admitting stream [9] to a compressor [H];
  • providing compressor [H] with an outlet stream [10] having a pressure between 10 bar and 150 bar;
  • cooling stream [10] in a heat exchanger [I], having as outlet stream the aforementioned stream [11].

Drawing 2/3 shows a version of Drawing 1/3 in which the catalyst in assembly [B] is incorporated in a hot passage of heat exchanger [D].

In a further aspect of the foregoing description, the expander machine [E] may be split into two or more stages in series, when in a particular use of the invention it is desirable for streams [4] and stream [11] to have different pressures or temperatures. In such a case one of the two said streams may be admitted to expander machine [E] at intermediate stage(s).

Moreover one or more additional heat exchangers may be introduced between stages of the said expander machine [E];

EMBODIMENT OF THE INVENTION

There is furthermore also provided a description of an embodiment of the invention (reference is made to Drawing 3/3 and the equipment tags and stream numbers shown thereon):

• • providing a stream of pure hydrogen feed gas [21] at a pressure of between 10 bar and 150 bar;
  • cooling stream [21] in a hot passage of heat exchanger [a], having an outlet stream [22] with a temperature of between 0deg C. to −150deg C.;
  • further cooling stream [22] in a first hot passage of heat exchanger [b], having an outlet stream [23] with a temperature of between −150deg C. and −210deg C.;
  • admitting stream [23] to assembly [c] comprising a catalyst [d] for conversion of ortho-hydrogen to para-hydrogen with outlet stream [24] and a heat exchanger [e] provided with a first hot passage having an outlet stream [25], such that the composition of stream [25] is essentially para-hydrogen, with a temperature of between −210deg C. and −250deg C.;
  • providing a recycle stream of hydrogen [35] having a composition essentially of para-hydrogen and having the same pressure as stream [25];
  • combining stream [25] and stream [35] to form stream [26];
  • admitting stream [26] to a first expander machine [f] having an outlet stream [27] with a pressure of between 10 bar and 1 bar and containing both liquid and vapour;
  • passing stream [27] to a vessel [g] wherein the liquid fraction stream [28], comprising the liquid hydrogen product from the process, is separated from the vapour fraction stream [29];
  • providing vessel [g] with a catalyst which converts residual ortho-hydrogen in stream [27] into para-hydrogen;
  • reheating stream [29] successively in a first cold passage of heat exchanger [e] to form outlet stream [30] and in a first cold passage of heat exchanger [b] to form outlet stream [31] having a temperature of between −100deg C. and −240deg C.;
  • admitting stream [31] to compressor [h] as a first, lower pressure inlet stream;
  • providing compressor [h] with a second, higher pressure inlet stream [39] having a composition essentially of para-hydrogen;
  • providing compressor [h] with an outlet stream [32] having a pressure of between 10 bar and 150 bar;
  • cooling stream [32] in a second hot passage of heat exchanger [b], having an outlet stream [33] and the same temperature as stream [23];
  • dividing stream [33] into two parts, stream [34] and stream [36];
  • cooling stream [34] in a second hot passage of heat exchanger [e] having as its outlet stream the above-mentioned stream [35];
  • passing stream [36] to a second expander machine [i], having an outlet stream [37] with a pressure of between 30 bar and 2 bar;
  • reheating stream [37] successively in a second cold passage of heat exchanger [e] to form outlet stream [38] and in a second cold passage of heat exchanger [b] to form the aforementioned outlet stream [39];
  • providing a stream of refrigerant fluid [40], and passing said stream [40] through a cold passage of heat exchanger [a], having an outlet stream [41];
  • providing a stream of refrigerant fluid [42], and passing said stream [42] through a third cold passage of heat exchanger [b], having an outlet stream [43].