Method and an apparatus for liquefying hydrogen

The present invention relates to a method and an apparatus for liquefying hydrogen including nitrogen precooling and a hydrogen cooling cycle.

Background

Hydrogen liquefaction processes including nitrogen precooling and a hydrogen cooling cycle are known. For example, an article by U. Cardella et al., “Economically viable large-scale hydrogen liquefaction”, IOP Conf. Series: Materials Science and Engineering 171 (2017) 012013, illustrates in Figure 1 a hydrogen Claude cycle in which a hydrogen precooling heat exchanger arrangement is provided. Such an arrangement or any modification thereof may form the basis of the present invention.

In such an arrangement, a gaseous hydrogen feed is cooled in the hydrogen precooling heat exchanger arrangement and thereafter, typically including an ortho/para-conversion in catalytic beds, further cooled and ultimately liquefied in a series of further heat exchangers of a hydrogen liquefaction heat exchanger arrangement.

In the hydrogen cooling cycle, hydrogen is compressed at a non-cryogenic temperature level to a certain pressure level (“first pressure level” hereinbelow) in a hydrogen compressor arrangement (“high pressure hydrogen”). A first part of the hydrogen at the first pressure level is cooled in the hydrogen precooling heat exchanger arrangement and thereafter in the hydrogen liquefaction heat exchanger arrangement to a certain temperature level (“first temperature level”). This first part is expanded to a second pressure level below the first pressure level (“medium pressure hydrogen”). A second part of the hydrogen at the first pressure level is also cooled in the hydrogen precooling heat exchanger arrangement and thereafter in the hydrogen liquefaction heat exchanger arrangement, but to a lower temperature level than the first part (“second temperature level”). This second part is also expanded, but to a pressure level below the second pressure level (“low pressure hydrogen”). Both parts are then used as coolant in the hydrogen liquefaction heat exchanger arrangement and the hydrogen precooling heat exchanger arrangement and thereby heated to the non-cryogenic temperature level at which they are recompressed to the first pressure level in the hydrogen compressor arrangement, closing the hydrogen cooling cycle.

The hydrogen precooling heat exchanger arrangement is also operated with nitrogen which may particularly be passed through the hydrogen precooling heat exchanger arrangement in gaseous form. Such nitrogen may also initially be provided in liquid form and then be expanded to form a liquid and a gaseous phase of which the liquid phase may be evaporated in a separate heat exchanger. The gas phase and the evaporated liquid phase may then be used for cooling. The nitrogen heated in the hydrogen precooling heat exchanger arrangement may be reliquefied in a nitrogen cooling cycle or vented to the atmosphere.

It is an object of the present invention to improve operation of a hydrogen precooling heat exchanger arrangement of the type mentioned.

Disclosure of the invention

In view of the above, the present invention provides a method and an apparatus for liquefying hydrogen including nitrogen precooling and a hydrogen cooling cycle comprising the features of the independent claims, respectively. Preferred embodiments are subject of the dependent claims and of the description below.

Herein, terms such as “pressure level” and “temperature level” are used in order to express that no exact pressures but pressure ranges can be used in order to realise aspects of the present invention and advantageous embodiments thereof. Different pressure and temperature levels may lie in distinctive ranges or in ranges overlapping each other. They also may cover expected and unexpected, particularly unintentional, pressure or temperature changes, e.g. inevitable pressure or temperature losses. Values expressed for pressure levels in bar units are absolute pressure values.

The term “liquefaction” may, as used herein, refer to a transfer of a fluid from the gaseous to the liquid state but likewise to a transfer of a fluid from the supercritical to the liquid state. The latter is also referred to as “pseudo-liquefaction” at other places. According to the present invention, a method for liquefying hydrogen is provided, wherein a gaseous hydrogen stream is subjected to precooling in a hydrogen precooling heat exchanger arrangement and thereafter to further cooling and liquefaction in a hydrogen liquefaction heat exchanger arrangement operated with a hydrogen cooling cycle, wherein said precooling is performed using nitrogen streams heated in a hydrogen precooling heat exchanger of the hydrogen precooling heat exchanger arrangement. According to the present invention, said nitrogen streams include a first nitrogen stream supplied to the hydrogen precooling heat exchanger at the cold end with a proportion of liquid of at least 80%, particularly completely in the liquid state, and at a first nitrogen pressure level of 1 to 1 .5 bar absolute pressure, and a second nitrogen stream supplied to the hydrogen precooling heat exchanger at an intermediate position under a cryogenic temperature and at a second nitrogen pressure level of near, at or above its critical pressure, in particular at 30 to 55 bar absolute pressure.

A "cryogenic temperature" is a temperature of less 120 K. A pressure "near" the critical pressure of nitrogen is a absolute pressure not more than 5 bar different from the critical pressure of nitrogen.

In embodiments of the present invention, an additional stream of gaseous nitrogen is heated in the hydrogen precooling heat exchanger in a proportion of less than 30% by mass of the total nitrogen which is heated in the hydrogen precooling heat exchanger, and in other embodiments by less than 20% or 10%.

The specific form of nitrogen precooling provided herein allows for significantly improving the cooling performance (such as e.g. demonstrated by the cooling curves) in the hydrogen precooling heat exchanger. According to embodiments of the present invention, a mean logarithmic temperature difference LMTD of only about 3.5 K can be reached. The product of the heat exchange area A and the heat transfer coefficient U may increase significantly, e.g. by 50%, as compared to standard design.

According to an embodiment of the present invention, the nitrogen streams are provided using liquefied nitrogen provided at a supercritical third nitrogen pressure level above the second nitrogen pressure level, wherein said liquefied nitrogen at the third nitrogen pressure is provided by cooling gaseous nitrogen at the third nitrogen pressure level. Such an embodiment allows for a particularly advantageous coupling with a standalone nitrogen liquefaction unit (NLU), i.e. a nitrogen liquefaction unit which is not part of an air separation unit.

Such an embodiment may particularly include that a first part of the liquefied nitrogen at the third nitrogen pressure level is depressurized from the third pressure level to the second pressure level to form the second nitrogen stream. Nitrogen used accordingly may advantageously withdrawn directly downstream of a nitrogen liquefaction heat exchanger which is used for cooling the gaseous nitrogen at the third nitrogen pressure level in the nitrogen liquefaction unit. Such an embodiment allows for coupling a nitrogen liquefaction unit and a nitrogen precooling heat exchanger without substantially modifying the latter.

Providing the second nitrogen stream at the second nitrogen pressure level (such as at about 34 bar) from nitrogen at the third nitrogen pressure level (such as at about 58 bar) and at a well subcooled condition (such as at about 97 K), i.e. by throttling from the liquefied nitrogen at the third nitrogen pressure level to approx. 34 bara (wherein thermodynamic losses are quite low) enables feeding it back to a nitrogen liquefaction unit as indicated below. This embodiment can allow to have advantages in capital expenses, while operating expenses may be similar to that of a non-inventive option as mentioned before, including cooling with liquid nitrogen and substantial amounts of gaseous nitrogen. This is due to the fact that the liquefied nitrogen at the third pressure level is, as mentioned, well subcooled and, therefore, the mass (and especially the volumetric) flow of this stream is significantly lower as compared to gaseous nitrogen. This leads to reduction of pipes and valves diameters for the “cold” as well as for the “warm” piping. The nitrogen precooling heat exchanger can therefore be built more compact due to a significantly higher portion of high-pressure streams in it as well as due to a lower impact of pressure losses on power at high pressures. Corresponding embodiments are universally applicable due to the fact that the nitrogen liquefaction machinery used may be operated very close to standard operating conditions where an optimized Q-T-Diagram for liquefaction process may be obtained. In summary, embodiments of the present invention may result in a more than 20% reduction in for the nitrogen liquefaction unit, if no additional liquefied nitrogen is produced and exported or in a perceptible increase of liquid nitrogen for the merchant market with an unchanged power consumption. In such embodiments, a second part of the liquefied nitrogen at the third pressure level may be expanded, particularly using one or more throttle valves, from the third nitrogen pressure level to a fourth nitrogen pressure level between the third nitrogen pressure level and the first nitrogen pressure level, forming a so-called throttle stream. Further nitrogen, which may also be cooled at the third pressure level, particularly together with the nitrogen liquefied at the third pressure level, but which may not or at least not completely liquefied, may be expanded in an expansion turbine, forming a so-called throttle stream. By expanding the throttle stream, a relatively low proportion of gas, such as less than 20%, is formed, while by expanding the turbine stream, a considerable amount of liquid, such as about 10%, may be formed.

Expanding the second part of the liquefied nitrogen at the third pressure level, and optionally the further nitrogen, a gaseous nitrogen fraction and a liquid nitrogen fraction are formed, wherein such fractions may initially be parts of one or more biphasic streams, wherein at least a part of the liquid nitrogen fraction is used to form the first nitrogen stream. That is, according to embodiments of the present invention, either the gaseous and liquid phase may be passed separately or together through the nitrogen precooling heat exchanger.

The second part of the liquefied nitrogen at the third nitrogen pressure level may be subcooled or further subcooled before the expansion from the third nitrogen pressure level to the fourth nitrogen pressure level and/or or the liquid nitrogen may subcooled after said expansion. Subcooling particularly reduces flash losses when reducing the pressure to he first pressure level. This is particularly relevant for liquid nitrogen passed on to a tank as cold would be lost into flash gas. If no liquid nitrogen is stored at the first pressure level in larger amounts, subcooling may also be omitted.

In embodiments of the present invention, a hydrogen liquefaction section and a nitrogen provision section may be used, wherein the nitrogen provision section may comprise a nitrogen liquefaction heat exchanger used in providing the liquefied nitrogen at the third nitrogen pressure level, wherein the hydrogen liquefaction section includes the hydrogen precooling heat exchanger, and wherein the subcooling may be performed using a subcooler arranged in closer proximity to the nitrogen liquefaction heat exchanger than to the hydrogen precooling heat exchanger and/or using a subcooler arranged in closer proximity to the hydrogen precooling heat exchanger than to the nitrogen liquefaction heat exchanger. In the former embodiment, a subcooler already present in a known nitrogen liquefaction unit may be used. In the latter embodiment, a subcooler may be specifically adapted to the nitrogen precooling heat exchanger and to the requirements for precooling.

In embodiments of the present invention, the subcooling may be performed using a subcooler integrated into the hydrogen precooling heat exchanger. This allows for a particularly compact arrangement which may e.g. be particularly easily housed in a coldbox due to its advantageous dimensions.

In embodiments of the present invention, a third nitrogen stream is supplied in gaseous form to the hydrogen precooling heat exchanger. This stream may be formed from nitrogen expanded in an expander from cooled, but not liquefied, nitrogen. Such nitrogen may be at least a part of the gas phase formed when expanding the second part of the liquefied nitrogen at the third pressure level mentioned above, i.e. the throttle stream, and optionally the turbine stream. Further gaseous nitrogen, in a considerably smaller amount, may also be formed when nitrogen is expanded for forming the first nitrogen stream. Such nitrogen may be supplied to the nitrogen precooling heat exchanger as well (preferably into the same channel used by the first nitrogen stream).

The gaseous nitrogen liquefied at the third nitrogen pressure level may, in embodiments of the present invention, be compressed to the third nitrogen pressure level using a nitrogen compressor unit and thereafter a first compressor/expander unit and a second compressor/expander unit. As such an arrangement particularly corresponds to that of conventional nitrogen liquefaction units, corresponding embodiments of the present invention are universally applicable to such units.

In such embodiments, the first nitrogen stream may, after being heated in the hydrogen precooling heat exchanger, be recycled to a position upstream of the nitrogen compressor unit and the second nitrogen stream may, after being heated in the hydrogen precooling heat exchanger, be recycled to a position upstream of the first compressor/expander unit or between the first compressor/expander unit and the second compressor/expander unit. The hydrogen cooling cycle may, in embodiments of the present invention, be operated with hydrogen compressed to a first hydrogen pressure level which is cooled in the hydrogen precooling heat exchanger and thereafter in the hydrogen liquefaction heat exchanger arrangement, and which is expanded in a first part to a second hydrogen pressure level below the first hydrogen pressure level and in a second part to a third hydrogen pressure level below the first hydrogen pressure level, wherein the first part and the second part are heated in in the hydrogen liquefaction heat exchanger arrangement and thereafter in the hydrogen precooling heat exchanger.

An apparatus for liquefying hydrogen, comprising means adapted to subject a gaseous hydrogen stream to precooling in a hydrogen precooling heat exchanger arrangement and thereafter to further cooling and liquefaction in a hydrogen liquefaction heat exchanger arrangement operated with a hydrogen cooling cycle, and comprising means adapted to perform said precooling using nitrogen streams heated in a hydrogen precooling heat exchanger of the hydrogen precooling heat exchanger arrangement is also be part of the present invention. The apparatus is adapted to supply a first nitrogen stream of said nitrogen streams to the hydrogen precooling heat exchanger at the cold end with a proportion of liquid of at least 80% and at a first nitrogen pressure level of 1 to 1 .5 bar absolute pressure, and to supply a second nitrogen stream of said nitrogen streams to the hydrogen precooling heat exchanger at an intermediate position in a subcooled and liquid state at a second nitrogen pressure level of 30 to 55 bar absolute pressure.

As to specific further features and embodiments of such an apparatus, reference is made to the explanations above relating to the method according to the invention and its advantageous embodiments. This equally applies for a corresponding apparatus which is adapted to perform a corresponding method or one of its embodiments. Such an apparatus may particularly include a control unit programmed or adapted to control the apparatus accordingly.

The present invention will further be described with reference to the appended drawings which relate to a preferred embodiment of the present invention.

Short description of the Figures Figure 1 schematically illustrates a hydrogen liquefaction apparatus not forming part of the present invention.

Figures 2a and 2b schematically illustrate hydrogen precooling heat exchangers not forming part of the present invention.

Figures 3 to 7, including Figure 3a, schematically illustrate hydrogen liquefaction apparatuses and methods according to embodiments of the present invention.

Detailed description of the Figures

Elements of identical or corresponding function and/or technical realization are indicated with like reference numerals hereinbelow. Repeated explanations are omitted for reasons of generality only. If, hereinbelow, reference is made to method steps, the corresponding explanations likewise apply to technical components of device parts used or adapted to realize such method steps and vice versa.

Figure 1 illustrates a hydrogen liquefaction apparatus such as mentioned at the outset in connection with the prior art such as Cardella et al. The apparatus includes a compression arrangement 10 including a first compressor or compressor stage 11 , such as a dry piston type compressor (stage) and a second compressor or compressor stage 12 such as a dry piston type compressor (stage) which may be bypassed by bypass lines illustrated as dash-dotted lines and valves not specifically indicated.

The apparatus further includes a hydrogen precooling heat exchanger arrangement 20 including a first hydrogen precooling heat exchanger 21 , a second hydrogen precooling heat exchanger 22 and a third hydrogen precooling heat exchanger 23. The second hydrogen precooling heat exchanger 22 and the third hydrogen precooling heat exchanger 23 may also be combined to form one heat exchanger block. Also a common hydrogen precooling heat exchanger integrating the functions of the first, second, and third hydrogen precooling heat exchangers 21 to 23 may be provided. Such a heat exchanger is illustrated with a dotted line and indicated 50. Furthermore, a hydrogen liquefaction heat exchanger arrangement 30 is provided which includes, in the specific example, six hydrogen liquefaction heat exchangers 31 to 36. As illustrated with hatched regions, the third hydrogen precooling heat exchanger 23 and the hydrogen liquefaction heat exchangers 31 to 36 are provided with catalytic beds adapted for ortho/para conversion of a hydrogen feed stream A. All heat exchangers mentioned may be integrated in an evacuated cold box as shown with a dashed line.

In the hydrogen compressor arrangement 10, hydrogen is compressed at a non- cryogenic temperature level using the first and second compressors or compressor stages 11 , 12 to a pressure level which is also referred to as “first pressure level” herein, forming a hydrogen stream B at the first pressure level, which is also referred to as “high pressure hydrogen” herein. A water cooler 13 may be used for aftercooling. Hydrogen stream B is thereafter passed through the hydrogen precooling heat exchanger 21 and the second hydrogen precooling heat exchanger 22 of the hydrogen precooling heat exchanger arrangement 20, and thereafter through an adsorber 24 and the hydrogen liquefaction heat exchanger 31 of the hydrogen liquefaction heat exchanger arrangement 30.

Thereafter, hydrogen stream B is split to form a partial stream C and a partial stream D. Partial stream C is expanded in an expansion turbine 41 , cooled in the hydrogen liquefaction heat exchanger 33 and expanded in two further expansion turbines 42 and 43 to a pressure level below the first pressure level which is referred to as “second pressure level” herein. Partial stream B is also referred to as “medium pressure hydrogen” herein. The hydrogen expanded to the second pressure level is heated in the hydrogen liquefaction heat exchangers 34 to 31 and in the hydrogen precooling heat exchanger 21 and recompressed in the hydrogen compressor arrangement 10. Partial stream D is further cooled in the hydrogen liquefaction heat exchangers 32 to 35 and thereafter expanded in a valve not specifically indicated to a pressure level below the first pressure level which is referred to as “second pressure level” herein, forming a gaseous and a liquid phase which are separated in a separation vessel 37. Partial stream D is, in its entirety and including the gaseous and the liquid phase, also referred to as “low pressure hydrogen” herein. The liquid phase is evaporated in hydrogen liquefaction heat exchanger 36 before being combined with the gaseous phase and being heated in the hydrogen liquefaction heat exchangers 35 to 31 and in the hydrogen precooling heat exchanger 21. The low pressure hydrogen D It is thereafter recompressed in the hydrogen compressor arrangement 10.

A gaseous nitrogen stream N is passed through the hydrogen precooling heat exchanger 21. This gaseous nitrogen stream N is, in the example shown, formed using a gas phase and a liquid phase of a saturated liquid nitrogen stream L expanded via a valve not specifically indicated into a vessel 25 for phase separation. The amount of the liquid fraction is about 98.5% of the total amount of the liquid nitrogen expanded. The liquid phase H is withdrawn as saturated liquid nitrogen from the head of the vessel 25 in the second hydrogen precooling heat exchanger 22 before being combined with the gas phase H which is withdrawn as saturated gaseous nitrogen from the head of the vessel 25 to form the nitrogen stream N. As shown below, the gaseous nitrogen stream N may be reliquefied in an arrangement not shown here to form liquid nitrogen stream L.

Feed hydrogen A to be liquefied is passed through the hydrogen precooling heat exchanger 21 and the third hydrogen precooling heat exchanger 23 before being purified in an adsorber arrangement 26. A purified hydrogen stream A' thus formed is thereafter being cooled in the third hydrogen precooling heat exchanger 23 and ortho/para converted and liquefied in hydrogen liquefaction heat exchangers 31 to 36. Ortho/para-conversion after purification may be performed in the third hydrogen precooling heat exchanger but also in a dedicated vessel. Any heating and cooling steps required may be performed. An auxiliary hydrogen stream X may be supplied to an ejector 38 in which the hydrogen may be flash expanded between being cooled in hydrogen liquefaction heat exchangers 35 and 36.

The liquid nitrogen N is typically supplied by a nitrogen liquefaction unit at about 5 bar absolute pressure in subcooled condition (at about 80 to 81 K) and throttled into vessel 25. The nitrogen is then used at the pressure level of about 1 .2 bar absolute pressure to provide the lowest possible precooling temperature for the process.

Precooling with liquid nitrogen only would not be efficient due to the fact that too much of refrigeration is provided at the lowest temperature level by evaporation of liquid. This results in considerable temperature differences in the low temperature region of the heat exchanger and leads to additional thermodynamical losses. Figures 2a and 2b illustrate details of possible improvements not forming part of the present invention. In Figure 2a, a heat exchanger 50' is illustrated which could potentially be used as the heat exchanger 50 according to Figure 1 . As to streams A to C and A', reference is made to Figure 1 . An absorber 26 or means for ortho/para conversion may be present but is not illustrated for reasons of clarity. Instead, a heat exchanger 27 is illustrated which may stand for any of these means.

In the non-inventive embodiment illustrated in Figure 2a, liquid nitrogen N is evaporated at two pressure levels, i.e. about 1 .2 bar absolute pressure after expansion into vessel 25 in the form of a stream N' and at about 5 bar absolute pressure in the form of an additional stream N". In the non-inventive embodiment illustrated in Figure 2b, nitrogen is provided in the form of a liquid nitrogen stream L1 and a gaseous nitrogen stream L2.

In Figure 3, an apparatus 100 for liquefying hydrogen according to an embodiment of the present invention is illustrated. Apparatus 100 includes a hydrogen liquefaction section 110 and a nitrogen provision section 120. The former may be provided in a way generally known to the skilled person, such as shown in Figure 1 or any modification thereof, and includes a hydrogen precooling heat exchanger arrangement of which a hydrogen precooling heat exchanger 50 is illustrated. Again, as to streams A to C and A', reference is made to Figure 1 . An absorber 26 or means for ortho/para conversion may be present but is not illustrated for reasons of clarity. Instead, a heat exchanger 27 is illustrated which may stand for any of these means.

Nitrogen provision section 120 includes a nitrogen liquefaction arrangement 60 comprising a nitrogen liquefaction heat exchanger 61 , a nitrogen subcooler 62, a separation vessel 63, a first compressor/expander unit 64, a second compressor/expander unit 65. Nitrogen provision section 120 further includes a nitrogen compression arrangement 70 with a first compressor or compressor section 71 , also referred to as “feed compressor” and a second compressor or compressor section 72 also referred to as “recycle compressor”. In all compression steps or units, aftercoolers may be used which are not specifically illustrated. In nitrogen provision section 120, feed or makeup nitrogen F may be provided. A nitrogen stream N1 passed from the hydrogen liquefaction section 110 to the nitrogen provision section 120 (see below), a first recycle stream R1 recycled in the nitrogen provision section 120 (see below), and the feed or makeup nitrogen F, if applicable, are compressed in the first compressor or compressor section 71 . A compressed nitrogen stream thus formed is combined with a second recycle stream R2 recycled in the nitrogen provision section 120 (see below) and thereafter compressed in the first and second compressor sections 71 , 72, forming a high pressure nitrogen stream P.

High pressure nitrogen stream P is split in partial streams P1 and P2, of which partial stream P1 is further compressed in first compressor/expander unit 64, combined with a nitrogen stream N2 passed from the hydrogen liquefaction section 110 to the nitrogen provision section 120 (see below), thereafter yet further compressed in the second compressor/expander unit 65, and in part liquefied in the nitrogen liquefaction heat exchanger 61 , to form a subcooled high pressure liquid nitrogen stream P1 L at a pressure level of about 58 bar absolute pressure and a temperature level of about 97 K. A further part is withdrawn from the nitrogen liquefaction heat exchanger 61 at an intermediate withdrawal position as a stream P1G and, forming a biphasic stream, expanded in the first compressor/expander unit 64 into separation vessel 63.

A partial stream N2, which was already mentioned above, is formed from the subcooled high pressure liquid nitrogen stream P1 L and passed from the nitrogen provision section 120 to the hydrogen liquefaction section 110. Partial stream N2 is throttled in a valve 51 to an evaporation pressure level of about 34 bar absolute pressure, supplied to the hydrogen liquefaction heat exchanger 51 at an intermediate feed position, and evaporated therein to transform the partial stream N2 to a gaseous nitrogen stream at a pressure level slightly lower than the evaporation pressure level, e.g. at about 33.5 bar absolute pressure. Partial stream N2 may then be passed back to the nitrogen provision section 120 as explained above. A remainder of the high pressure liquid nitrogen stream P1 L is expanded, using a valve not specifically indicated, and forming a biphasic stream, into separation vessel 63.

Partial stream P2 is expanded in the second compressor/expander unit 65 and fed into the nitrogen liquefaction heat exchanger 61 at an intermediate feeding position where it is combined with top gas from the separation vessel 63 to form the second recycle stream R2 which is recycled and recompressed to form the high pressure nitrogen stream P as already explained above. Bottom liquid from the separation vessel 63 is subcooled in subcooler 62 against a part which is expanded to form the first recycle stream R1 . This part is recycled and recompressed as also explained above.

A subcooled nitrogen stream, which is indicated L as in the previous Figures, is passed from the nitrogen provision section 120 to the hydrogen liquefaction section 110 at a pressure level of about 5 bar absolute pressure and a temperature level of about 80.15 K and expanded in a valve not specifically indicated to a pressure level of about 1 .2 bar absolute pressure, forming a biphasic stream, into vessel 25. Liquid from the sump of vessel 25 is evaporated in hydrogen precooling heat exchanger 50 to form nitrogen stream N1 which is passed back to the nitrogen provision section 120 and used as explained. Top gas from the vessel 25 may be fed into hydrogen precooling heat exchanger 50 as well, as illustrated with a dashed line. As a proportion of liquid in the biphasic stream formed from stream L may be about 0.985, a vessel 25 may be omitted and the biphasic stream may be passed to the hydrogen precooling heat exchanger 50 in its entirety. Subcooled nitrogen not being passed from the nitrogen provision section 120 to the hydrogen liquefaction section 110 may be withdrawn from apparatus 100 in the form of a stream L2 and e.g. stored in a tank.

In Figure 3a, a modification of apparatus 100 as shown in Figure 3 is illustrated. This modification lacks subcooler 63, which may, as mentioned above, be the case when nitrogen is not passed into a tank as a product. In this case, all liquid from vessel 63 may be used as the stream L. All embodiments illustrated herein and further embodiments of the invention may be configured accordingly.

In Figure 4, an apparatus 200 for liquefying hydrogen according to an embodiment of the present invention is illustrated. For explanations of parts and functions of apparatus 200, reference is made to the explanations above.

In contrast to apparatus 100 as shown in Figure 3, liquid nitrogen stream L is not formed from liquid nitrogen subcooled in subcooler 62 of the nitrogen liquefaction unit 60 of the nitrogen provision section 120 but as a part of pressurized liquid nitrogen stream P1 L downstream of the nitrogen liquefaction heat exchanger 61 . In this connection, a subcooler 28 can be used in which this liquid nitrogen stream L is subcooled before being expanded into vessel 25 against top gas thereof. Said top gas and liquid from the sump of vessel 28 are heated in hydrogen precooling heat exchanger 50 to form nitrogen stream N1 as explained above.

In Figure 5, an apparatus 300 for liquefying hydrogen according to an embodiment of the present invention is illustrated. For explanations of parts and functions of apparatus 300, reference is made to the explanations above.

Like in the apparatus 200 as shown in Figure 4, liquid nitrogen stream L is not formed from liquid nitrogen subcooled in subcooler 62 of the nitrogen liquefaction unit 60 of the nitrogen provision section 120 but as a part of pressurized liquid nitrogen stream P1 L downstream of the nitrogen liquefaction heat exchanger 61 . In contrast to the apparatus 200 as shown in Figure 4, a subcooling section 28', which is used as described above for subcooler 28, is provided. Subcooling section 28' is integrated in the form of passages into hydrogen precooling heat exchanger 50.

In Figure 6, an apparatus 400 for liquefying hydrogen according to an embodiment of the present invention is illustrated. For explanations of parts and functions of apparatus 400, reference is made to the explanations above.

In apparatus 400, a part of the top gas from separation vessel 63 is passed as a further nitrogen stream N3 from the nitrogen provision section 120 to the hydrogen liquefaction section 110, fed at an intermediate position into hydrogen precooling heat exchanger 50, heated therein, and recycled to a position corresponding to that of stream R2.

In Figure 7, an apparatus 500 for liquefying hydrogen according to an embodiment of the present invention is illustrated. For explanations of parts and functions of apparatus 500, reference is made to the explanations above.

In apparatus 500, as compared to apparatus 100 shown in Figure 3, compressor/expander units 64 and 65 are interchanged.

Be it noted that all features shown before for one of the embodiments in the form of apparatus 100, 200, 300, 400 and 500 may be used in any other embodiments and that features may be compared as applicable and if technically useful.