The method of liquefying a gas, which is at elevated pressure disclosed in this publication comprises the steps of: (a) cooling in six heat exchangers arranged in series, each having at least a first warm side, a second warm side and a cold side the compressed gaseous stream by passing the compressed gaseous stream at elevated pressure through the first warm sides of the heat exchangers, wherein heat is transferred to a refrigerant stream passing through the cold sides of the heat exchangers at refrigerant pressure, to obtain a cooled gaseous stream; (b) expanding the cooled gaseous stream to a low pressure to obtain an expanded fluid stream; (c) liquefying the expanded fluid stream by indirect heat exchange with evaporating liquefied refrigerant to obtain a liquefied gaseous stream and a gaseous refrigerant stream; (d) withdrawing the liquefied gaseous stream as product stream ; (e) returning the gaseous refrigerant stream via the cold sides of the six heat exchangers to obtain warm refrigerant; (f) compressing the warm refrigerant to obtain a compressed refrigerant stream at high pressure ;

(g) cooling the compressed refrigerant stream by passing the stream at high pressure through the second warm sides of the six heat exchangers to obtain a cooled refrigerant stream, and expanding the cooled refrigerant stream to a pressure at which the refrigerant is liquefied to obtain the liquefied refrigerant used in step (c); (h) withdrawing from the compressed refrigerant stream upstream of the second heat exchanger a first side stream, expanding the first side stream to an intermediate pressure, cooling the expanded first side stream in the third, fourth and fifth heat exchangers by passing it through further warm sides of these heat exchangers; (i) removing from the first side stream at intermediate pressure, upstream of the fourth heat exchanger, a second side stream, expanding the second side stream to the refrigerant pressure, and adding the expanded second side stream to the gaseous refrigerant stream directly downstream of the fourth heat exchanger; and (j) expanding remainder of the first side stream at intermediate pressure to the refrigerant pressure and adding the expanded remainder to the gaseous refrigerant stream directly downstream of the sixth heat exchanger.

In the above method the intermediate pressure is a pressure between the refrigerant pressure and the high pressure.

In this known method, the refrigerant is hydrogen, similar to the gas to be liquefied. The known process comprises six heat exchange stages and three expansion stages; thus the ratio of heat exchange stage to expansion stages is two.

An improvement of this method is described in Wasserstoff-Energietechnik II, VDI Berichte No. 725, 1989, pages 163-177. In the method disclosed in this publication, the hydrogen feed is precooled with

evaporating liquid nitrogen, which has a boiling temperature that is higher than the condensation temperature of hydrogen, the gas to be liquefied.

Precooling with liquid nitrogen has the advantage that two heat exchange stages and one expansion stage can be omitted. However, the ratio of heat exchange stages to expansion stages remains two.

In WO 90/08295 is disclosed in Figure 2 a method for liquefying hydrogen with a ratio of heat exchange stages to expansion stages of one. Helium gas is used as refrigerant. In the method of WO 90/08295, hydrogen at 35 bar and 300 K (20) is cooled in three heat exchangers (100A-C) arranged in series, each having at least a first warm side, a second warm side and a cold side, and one further heat exchanger (lOOD) having a warm side and a cold side. Hydrogen at 35 bar (20) is first passed through the first warm sides of the three heat exchangers (100A-C) and then through the warm side of the further heat exchanger (100D), wherein heat is transferred to helium passing through the cold sides of the heat exchangers at refrigerant pressure (1 bar), to obtain a cooled gaseous hydrogen stream (24), which is expanded to a low pressure to obtain an expanded fluid hydrogen stream (27), which expanded stream (27) is then liquefied in a final heat exchanger by indirect heat exchange with helium to obtain liquefied hydrogen (28). The helium (3), leaving the cold side of the first heat exchanger (100A) is compressed in compressor 108 and recycled via the three heat exchangers (100A-C) and an expander (110D) to the final heat exchanger. Just upstream of each of the three heat exchangers (100A-C), a side stream is withdrawn from the compressed helium and expanded to refrigerant pressure in an expander (110A-C) and the expanded helium is added to the refrigerant downstream of the heat exchanger (100A-C).

In a publication published by the American Institute of Aeronautics and Astronautics (AIAA), Inc. , titled Thermoeconomics of hydrogen liquefiers operating on the modified Collins cycle by M. T. Syed et al., Vol 2 (2000), p. 1383-1393, a similar helium-refrigerated hydrogen liquefier, i. e. with a ratio of heat exchange stages to expansion stages of one, is disclosed in Figure 1. On page 1389 of this publication, it is mentioned that hydrogen is not suitable for refrigeration purposes in the liquefier of Figure 1. Therefore, a modified, more complicated, liquefier as shown in Figure 2 on page 1391 is used in case of hydrogen as refrigerant.

It has now been found that the liquefier of Figure 2 of WO 90/08295 or of Figure 1 of the above-mentioned AIAA publication can be improved by using evaporating liquefied natural gas for 1) precooling the hydrogen to be liquefied to a temperature well below ambient temperature and 2) removing the heat of compression from the refrigerant stream. Thus, use is made of the available cold from evaporating liquefied natural gas that would otherwise be wasted. The resultant pre-cooling has the advantage that at least one heat exchanger/expansion stage can be omitted and that the liquefier can also be used with hydrogen as refrigerant with an acceptable number of expanders.

Accordingly, the present invention relates to a method of liquefying hydrogen at elevated pressure comprising: (a) precooling the hydrogen by indirect heat exchange with evaporating liquefied natural gas to obtain a precooled gaseous hydrogen stream at elevated pressure; (b) cooling in at least two heat exchangers arranged in series, each having a first warm side, a second warm side and a cold side, the gaseous hydrogen stream at elevated pressure by passing the gaseous hydrogen stream at

elevated pressure through the first warm sides of the heat exchangers, wherein the heat is transferred to a refrigerant stream passing through the cold sides of the heat exchangers at refrigerant pressure, to obtain a cooled gaseous hydrogen stream at elevated pressure and a warm refrigerant stream; (c) liquefying the cooled gaseous hydrogen stream at a low pressure by heat exchange with evaporating liquefied refrigerant to obtain a liquefied hydrogen stream and a gaseous refrigerant stream; (d) withdrawing the liquefied hydrogen stream as product stream; (e) returning the gaseous refrigerant stream via the cold sides of the at least two heat exchangers to obtain warm refrigerant; (f) compressing the warm refrigerant and removing the heat of compression from the compressed refrigerant stream by indirect heat exchange with evaporating liquefied natural gas to obtain an after-cooled compressed refrigerant stream at high pressure; (g) cooling the after-cooled compressed refrigerant stream by passing the stream at high pressure through the second warm sides of the at least two heat exchangers to obtain a cooled refrigerant stream, and expanding the cooled refrigerant stream to a pressure at which the refrigerant is liquefied to obtain the liquefied refrigerant used in step (c); and (h) withdrawing from the compressed refrigerant stream upstream of each heat exchanger a side stream, expanding the side stream to the refrigerant pressure, and adding the expanded side stream to the gaseous refrigerant stream directly downstream of the heat exchanger.

The invention will now be described by way of example in more detail with reference to the accompanying drawing, which shows schematically a process flow scheme

of the method of liquefying hydrogen according to the present invention.

Hydrogen to be liquefied is supplied through conduit 5 to a first heat exchanger 7. In the first heat exchanger 7 the hydrogen is precooled by indirect heat exchange with evaporating liquefied natural gas to obtain a precooled gaseous hydrogen stream. The liquefied natural gas is supplied through conduit 10.

The precooled gaseous hydrogen stream is compressed in compressor 12 to an elevated pressure. The gaseous hydrogen stream at elevated pressure is supplied through conduit 15 to a second heat exchanger 17, where the heat of compression is removed by indirect heat exchange with evaporating liquefied natural gas to obtain a precooled gaseous hydrogen stream at elevated pressure. The liquefied natural gas is supplied through conduit 18.

If gaseous hydrogen is supplied through conduit 5 at elevated pressure, then compressor 12 and the heat- exchanger 17 for removal of the heat of compression may be omitted in the method according to the invention.

The precooled gaseous hydrogen stream at elevated pressure thus-obtained preferably has a temperature below 200K, more preferably has a temperature in the range of from 115 to 150K. The evaporating liquefied natural gas used for the precooling is preferably at elevated pressure, i. e. above ambient pressure, more preferably has a pressure in the range of from 1.0 to 10 MPa, even more preferably of from 2.5 to 9.0 MPa. A pressure in the range of from 4.0 to 8.0 MPa is particularly preferred.

The precooled gaseous hydrogen stream at elevated pressure is supplied through connection conduit 19 to a set of at least two heat exchangers for cooling. In the embodiment as shown there are three heat exchangers 20, 21 and 23. Each of the heat exchangers 20,21 and 23 has a first warm side 20a, 21a and 23a, a second warm side

20b, 21b and 23b, and a cold side 20c, 21c and 23c. The heat exchangers 21,22 and 23 each represent a heat exchange stage.

In each heat exchanger 20,21 and 23 the gaseous hydrogen stream at elevated pressure passes through the first warm sides 20a, 21a and 23a and is cooled by indirect heat exchange with a refrigerant passing through the cold sides 20c, 21c and 23c. For the sake of simplicity the connection conduits connecting the warm sides of the heat exchangers are referred to with reference numeral 19. Thus the reference numeral 19 is used to refer to all conduits through which the hydrogen to be liquefied passes. A cooled gaseous hydrogen stream is obtained from the first warm side 23a of the last heat exchanger 23 of the set of at least two heat exchangers.

A warm refrigerant stream is removed from the cold side 20c of the first heat exchanger 20 of the set of at least two heat exchangers through conduit 25.

The next step is liquefying the cooled gaseous hydrogen stream at a low pressure by heat exchange with evaporating liquefied refrigerant to obtain a liquefied hydrogen stream and a gaseous refrigerant stream. To this end the cooled gaseous hydrogen stream is expanded to a low pressure in a suitably expansion device, such as a Joule-Thompson valve 27, to obtain an expanded fluid hydrogen stream. This expanded fluid hydrogen stream may be partially liquid.

The expanded fluid hydrogen stream is supplied through connection conduit 19 to the warm side 30a of a final heat exchanger 30. The expanded fluid hydrogen stream is further liquefied by indirect heat exchange with evaporating liquefied refrigerant in the cold side 30c of the final heat exchanger 30 to obtain a liquefied hydrogen stream and a gaseous refrigerant stream. The liquefied hydrogen stream is withdrawn from the warm side

30a of the final heat exchanger 30 through connection conduit 19 as product stream.

Reference herein to elevated pressure of the gaseous hydrogen stream is to a pressure above the critical pressure for hydrogen, i. e. above 1.3 MPa. Preferably, the elevated pressure at which the precooled gasous stream is passed through the first warm sides of the heat exchangers is in the range of from 1.3 to 5.0 MPa, more preferably of from 2.0 to 4.0 MPa.

The low pressure to which the cooled gaseous hydrogen stream is expanded in expansion device 27 is the product pressure. Preferably the product pressure is in the range of from 0.2 to 0.5 MPa.

We will now discuss how the liquefied refrigerant that is used in the cold side 30c is obtained. To this end we follow the gaseous refrigerant stream that is withdrawn from the cold side 30c of the final heat exchanger 30 through conduit 25. The reference numeral 25 is used to refer to all conduits through which the gaseous refrigerant stream passes.

The gaseous refrigerant stream is returned via the cold sides 23c, 21c and 20c of the at least two heat exchangers to obtain warm refrigerant. In the heat exchangers 23,21 and 20 the gaseous refrigerant is warmed by the heat removed from the gaseous hydrogen stream at elevated pressure that is being cooled.

The warm refrigerant steam exiting the cold side 20c of the first heat exchanger 20 is passed to a refrigerant compressor 33. In the refrigerant compressor 33 the warm refrigerant stream is compressed to obtain a compressed refrigerant stream at high pressure that is supplied to the heat exchangers 20,21 and 23 through connection conduit 35. In heat exchanger 37 the heat of compression is removed from the compressed refrigerant stream by indirect heat exchange with evaporating liquefied natural

gas. The liquefied natural gas is supplied through conduit 38. The heat of compression is preferably removed to such extent that the temperature of the after-cooled compressed refrigerant is substantially the same as that of the warm refrigerant stream exiting the cold side 20c of the first heat exchanger 20. Typically, the after- cooled compressed refrigerant has a temperature which is 1 to 5K above the temperature of the warm refrigerant stream. The liquefied natural gas supplied through conduit 38 preferably has the same pressure as the liquefied natural gas used for the precooling of the hydrogen in step (a).

The after-cooled compressed refrigerant stream is cooled by passing the stream at high pressure through the second warm sides 20b, 21b and 23b of the at least two heat exchangers 20,21 and 23 to obtain a cooled refrigerant stream. Cooling is effected by indirect heat exchange with the refrigerant passing through the cold sides 20c, 21c and 23c. The connection conduits through which the refrigerant stream passes at high pressure are referred to with reference numeral 35.

The cooled refrigerant stream exiting the last heat exchanger 23 is expanded in a suitably expansion device, such as a Joule-Thompson valve 39, to a pressure at which the refrigerant is liquefied to obtain liquefied refrigerant. The liquefied refrigerant is evaporated in the cold side 30c of the final heat exchanger 30 to liquefy the expanded fluid stream passing through the warm side 30a of the final heat exchanger 30. The pressure at which the liquefied refrigerant evaporates is the refrigerant pressure. The refrigerant pressure is so selected that the temperature at which the refrigerant evaporates is below the temperature at which the hydrogen becomes liquefied at the low pressure. Preferably, the refrigerant pressure is 50 to 90% of the low pressure.

From the compressed refrigerant stream upstream of each heat exchanger 20, 21 and 23 a side stream is withdrawn through conduits 40,41 and 42. Each side stream is expanded to the refrigerant pressure in a suitable expansion device, for example turbo expanders 45,46 and 47. Each expanded side stream at refrigerant pressure is added to the gaseous refrigerant stream directly downstream of the heat exchanger through conduits 48,49 and 50. The reference for the relative terms upstream and downstream is the direction of flow of the gaseous hydrogen stream to be liquefied.

In the method according to the invention, refrigerant is compressed from refrigerant pressure to high pressure in compressor 37 and expanded from high pressure to refrigerant pressure in each of expanders 45,46, 47. It will be appreciated that the ratio between high pressure and refrigerant pressure depends inter alia on the number of heat exchanger/expander stages in the process, the refrigerant used, and the temperature of the pre-cooled gaseous hydrogen stream, since the pressure difference over the expander is preferably such that the resultant temperature drop matches the temperature difference over the heat exchanger in the same stage. For a three-stage process as illustrated in the Figure and hydrogen as refrigerant, the ratio is preferably in the range of from 5 to 11, for a two-stage process and hydrogen as refrigerant the ratio is preferably in the range of from 10 to 20.

Preferably, the method according to the invention has two to four heat exchanger/expander stages in step (b), more preferably two or three such stages.

In order to liquefy 1 kg/s of hydrogen with the process discussed with reference to the Figure, the hydrogen is compressed in compressor 12 to a pressure of 2.5 MPa, and cooled in heat exchanger 17 by indirect heat

exchange with evaporating liquefied natural gas to a temperature of 115 K. The temperature of the gaseous hydrogen stream downstream the first warm side 20a of the first heat exchanger 20 is 76 K, downstream the first warm side 21a of the second heat exchanger 21 the temperature of the hydrogen stream is 48 K, and downstream the first warm side 23a of the third heat exchanger 23 the temperature of the hydrogen stream is 31 K. The temperature of the expanded hydrogen stream leaving expansion device 27 is 27 K and its pressure is 0.4 MPa. The temperature of the liquefied hydrogen stream leaving the final heat exchanger 30 is 26 K and its pressure is 0.4 MPa. The temperature of the liquefied hydrogen refrigerant entering into the cold side 30c of the final heat exchanger 30 is 25 K and its pressure is 0.3 MPa. The temperature of the hydrogen refrigerant before it enters into the cold side 23c is 28 K, before it enters into the cold side 21c its temperature is 45 K and before it enters into the cold side 20c its temperature is 73 K. The mass flow rate of the refrigerant supplied through conduit 25 to the compressor 33 is 7.43 kg/s and its pressure is 0.3 MPa. In the compressor 33, the pressure is increased to 2.0 MPa. The mass flow rate of the first side stream, withdrawn through conduit 40, is 1.56 kg/s, the mass flow rate of the second side stream, withdrawn through conduit 41, is 2.66 kg/s and the mass flow rate of the third side stream, withdrawn through conduit 42, is 3.07 kg/s.

Suitably, the method of the present invention further comprises further cooling the cooled gaseous hydrogen stream in additional heat exchanger 55 before expanding this stream in expansion device 27 by indirect heat exchange with the gaseous refrigerant stream exiting the last heat exchanger 30.

The refrigerant in conduit 38 is evaporating liquefied natural gas, i. e. the same refrigerant as the one supplied through conduits 10 and 18.

Not shown is that the liquefied hydrogen stream can pass through one or more o-p conversion reactors that are suitably included in the heat exchangers.

Please note that compression of the refrigerant stream in compressor 33 can be done in more than one stage with inter-stage cooling.

Preferably, the refrigerant passing through conduit 25 and connection conduit 35 is hydrogen, i. e. the same gas as the gas to be liquefied. An advantage of the use of hydrogen as refrigerant is that no contamination of the hydrogen to be liquefied with refrigerant will take place in case of leaks in the refrigerant system.

Hydrogen liquefaction is suitably carried out at a mass flow rate between 0.1 and 10 kg hydrogen per second (which corresponds to a production of between about 10 and 1 000 ton liquefied hydrogen per day).

The method of liquefying hydrogen according to the present invention provides an efficient liquefaction process.