Low-temperature mixed-refrigerant for hydrogen precooling in large scale

Abstract

The present invention relates to a refrigerant composition. According to the invention it is envisioned that the composition comprises comprising an inert gas selected from nitrogen, argon, neon and a mixture thereof, and a mixture of at least two C.sub.1-C.sub.5 hydrocarbons. The present invention further relates to the use of the refrigerant composition in a method for liquefying a gaseous substance, particularly hydrogen or helium.

Claims

1. A method for liquefying a feed gas stream, the method comprising: providing said feed gas stream comprising a gas wherein said gas is hydrogen or helium, and wherein said feed gas stream has an initial temperature, precooling said feed gas stream from said initial temperature to an intermediate temperature in a precooling step by a closed loop cooling cycle with a precooling refrigerant stream yielding a precooled feed gas stream, cooling said precooled feed gas stream in a cooling step from said intermediate temperature to a temperature below the boiling temperature or the critical temperature of said gas, wherein said precooling refrigerant stream comprises a refrigerant composition selected from the following compositions (a), (b), and (c): composition (a) consisting of: a first component, a second component, a third component and a fourth component, wherein said first component is nitrogen in the range of 5 mol. % to 35 mol. %, said second component is methane in the range of 20 mol. % to 40 mol. %, said third component, is ethane or ethylene in the range of 10 mol. % to 45 mol. %, and said fourth component is one of n-butane, isobutane, 1-butene, n-pentane or isopentane in the range of 10 mol. % to 35 mol. %, or propane or propylene in the range of 10 mol.% to 45 mol. %, provided that the sum of the concentrations of the components does not exceed 100 mol. %; composition (b) consisting of: a first component, a second component, a third component, a fourth component, and a fifth component wherein said first component is nitrogen in the range of 5 mol. % to 35 mol. %, said second component is methane in the range of 20 mol. % to 40 mol. %, said third component is ethane or ethylene in the range of 10 mol.% to 45 mol. %, and said fourth component is one of n-butane, isobutane, 1-butene, n-pentane or isopentane in the range of 10 mol. % to 35 mol. %, or propane or propylene in the range of 10 mol. % to 45 mol. %, and said fifth component is one of n-butane, isobutane, propane, propylene, n-pentane and provided that the sum of the concentrations of the components does not exceed 100 mol. %; and composition (c) consisting of: a first component, a second component, a third component, a fourth component, and a fifth component, and a sixth component wherein said first component is nitrogen in the range of 5 mol. % to 35 mol. %, said second component is methane in the range of 20 mol. % to 40 mol. %, said third component is ethane or ethylene in the range of 10 mol.% to 45 mol. %, and said fourth component is one of n-butane, isobutane, 1-butene, n-pentane or isopentane in the range of 10 mol. % to 35 mol. %, or propane or propylene in the range of 10 mol. % to 45 mol. %, said fifth component is one of n-butane, isobutane, propane, propylene, n-pentane and isopentane, wherein said fifth component is different from said fourth component, and said sixth component is one of n-butane, isobutane, propane, propylene, n-pentane and isopentane, and wherein said sixth component is different from said fourth component and said fifth component, provided that the sum of the concentrations of the components does not exceed 100 mol. %.

2. The method according to claim 1, wherein said composition is composition (b).

3. The method according to claim 1, wherein said composition is composition (c).

4. The method according to claim 1, wherein said composition is composition (a).

5. The method according to claim 1, wherein said fourth component is isobutane, isopentane or 1-butene in the range of 10 mol. % to 35 mol. %, or propane or propylene in the range of 10 mol. % to 45 mol. %.

6. The method according to claim 1, wherein said composition is composition (a) and consists of: 11 mol. % nitrogen, 33 mol. % methane, 31 mol. % ethane and 25 mol. % n-butane, or 12 mol. % nitrogen, 32 mol. % methane, 31 mol. % ethane and 25 mol. % n-butane, or 14 mol. % nitrogen, 32 mol. % methane, 29 mol. % ethane and 25 mol. % isobutane, or 16 mol. % nitrogen, 31 mol. % methane, 27 mol. % ethane and 26 mol. % isobutane, or 11 mol. % nitrogen, 32 mol. % methane, 38 mol. % ethane and 19 mol. % isopentane, or 22 mol. % nitrogen, 30 mol. % methane, 24 mol. % ethane and 24 mol. % isobutane, or 17 mol. % nitrogen, 33 mol. % methane, 24 mol. % ethane and 26 mol. % isobutane, or 18 mol. % nitrogen, 29 mol. % methane, 36 mol. % ethane and 17 mol. % isopentane, or 18 mol. % nitrogen, 27 mol. % methane, 37 mol. % ethane and 18 mol. % isopentane, or 20 mol. % nitrogen, 30 mol. % methane, 26 mol. % ethane and 24 mol. % 1-butene, or 20 mol. % nitrogen, 30 mol. % methane, 24 mol. % ethane and 26 mol. % 1-butene, or 18 mol. % nitrogen, 27 mol. % methane, 37 mol. % ethane and 18 mol. % isopentane, or 23 mol. % nitrogen, 29 mol. % methane, 24 mol. % ethane and 24 mol. % isobutane.

7. The method according to claim 1, wherein said composition is composition (b) and consists of: 22 mol. % nitrogen, 29 mol. % methane, 28 mol. % ethane, 12 mol. % isobutane and 9 mol. % isopentane.

8. The method according to claim 1, wherein said precooling step comprises: providing said precooling refrigerant stream with a first pressure, expanding said precooling refrigerant stream in a first expansion device to a second pressure yielding an expanded precooling refrigerant stream, guiding said expanded precooling refrigerant stream and said feed gas stream such that heat can indirectly be transferred between the expanded precooling refrigerant stream and said feed gas stream, thereby cooling said feed gas stream to said intermediate temperature, and compressing said expanded precooling refrigerant to said first pressure in a first precooling compressor.

9. The method according to claim 1, wherein said feed gas stream comprises hydrogen and is precooled in said precooling step to a temperature equal or above 80 K, yielding said precooled feed gas stream and said precooled feed gas stream is brought into contact with a catalyst that is being able to catalyze conversion of ortho hydrogen to para hydrogen.

10. The method according to claim 9, wherein said feed gas stream is precooled in said precooling step to a temperature in the range of 85 K to 120 K to yield said precooled feed gas stream.

11. The method according to claim 1, wherein feed gas stream consists of hydrogen or helium.

12. The method according to claim 1, wherein the intermediate temperature is in the range of 80 K to 150 K.

13. The method according to claim 1, wherein said cooling of said precooled feed gas stream is performed using a second refrigerant which comprises neon and/or hydrogen.

14. A method of liquefying a feed gas stream, the method comprising: providing a feed gas stream comprising a gas wherein said gas is hydrogen or helium, and wherein said feed gas stream has an initial temperature, precooling said feed gas stream from said initial temperature to an intermediate temperature in a precooling step by a closed loop cooling cycle with a precooling refrigerant stream yielding a precooled feed gas stream, cooling said precooled feed gas stream in a cooling step from said intermediate temperature to a temperature below the boiling temperature or the critical temperature of said gas, wherein said preceding refrigerant stream comprises a refrigerant composition selected from the following compositions (a), (b), and (c): composition (a) consisting of: a first component, a second component, a third component, and a fourth component, wherein said first component is nitrogen in the range of 5 mol. % to 35 mol. %, said second component is argon in the range of 20 mol. % to 40 mol. %, said third component is ethane or ethylene in the range of 10 mol. % to 40 mol. %, and said fourth component is one of isobutane, isopentane or 1-butene in the range of 10 mol. % to 35 mol. %, or propane or propylene in the range of 10 mol. % to 45 mol. %, provided that the sum of the concentrations of the components does not exceed 100 mol. %; composition (b) consisting of: a first component, a second component, a third component, a fourth component, and a fifth component wherein said first component is nitrogen in the range of 5 mol. % to 35 mol. %, said second component is argon in the range of 20 mol. % to 40 mol. %, said third component is ethane or ethylene in the range of 10 mol. % to 45 mol. %, and said fourth component is one of isobutane, isopentane or 1-butene in the range of 10 mol. % to 35 mol. %, or propane or propylene in the range of 10 mol. % to 45 mol. %, and said fifth component is one of n-butane, isobutane, propane, propylene, n-pentane and isopentane, wherein said fifth component is different from said fourth component, provided that the sum of the concentrations of the components does not exceed 100 mol. %; and composition (c) consisting of: a first component, a second component, a third component, a fourth component, a fifth component, and a sixth component wherein said first component is nitrogen in the range of 5 mol. % to 35 mol. %, said second component is argon in the range of 20 mol. % to 40 mol. %, said third component is ethane or ethylene in the range of 10 mol. % to 40 mol. %, and said fourth component is one of isobutane, isopentane or 1-butene in the range of 10 mol. % to 35 mol. %, or propane or propylene in the range of 10 mol. % to 45 mol. %, and said fifth component is one of n-butane, isobutane, propane, propylene, n-pentane and isopentane, wherein said fifth component is different from said fourth component, and said sixth component is one of n-butane, isobutane, propane, propylene, n-pentane and isopentane, and wherein said sixth component is different from said fourth component and said fifth component, provided that the sum of the concentrations of the components does not exceed 100 mol. %.

15. The method according to claim 14, wherein said composition is composition (a) and consists of 12.5 mol. % nitrogen, 38 mol. % argon, 25.5 mol. % ethane and 24 mol. % 1-butene.

16. A method for liquefying a feed gas stream, the method comprising: providing a feed gas stream comprising a. gas wherein said gas is hydrogen or helium, and wherein said feed gas stream has an initial temperature, precooling said feed gas stream from said initial temperature to an intermediate temperature in a precooling step by a closed loop cooling cycle with a precooling refrigerant stream yielding a precooled feed gas stream. cooling said precooled feed gas stream in a cooling step from said intermediate temperature to a temperature below the boiling temperature or the critical temperature of said gas, wherein said precooling refrigerant stream comprises a refrigerant composition consisting of a first component, a second component, a third component, and a fourth component, wherein: (a) said first component is nitrogen in an amount of 8 mol. % to 18 mol. %, said second component is methane in an amount of 30 mol. % to 36 mol. %, said third component is ethane or ethylene in an amount of 28 mol. % to 35 mol. %, and said fourth component is n-butane or isobutane in an amount of 22 mol. % to 28 mol. %, or n-pentane or isopentane in an amount of 15 mol. % to 25 mol. %; or (b) said first component is nitrogen in an amount of 15 mol. % to 25 mol. %, said second component is methane in an amount of 28 mol. % to 35 mol. %, said third component is ethane or ethylene in an amount of 23 mol. % to 36 mol. %, and said fourth component is isobutane in an amount of 21 mol. % to 28 mol. %, isopentane in an amount of 15 mol. % to 22 mol. %, or propane in an amount of 30 mol. % to 40 mol. %; or (c) said first component is nitrogen in an amount of 10 mol. % to 35 mol. %, said second component is methane in an amount of 20 mol. % to 40 mol. %, said third component is ethane in an amount of 5 mol. % to 35 mol. %, and said fourth component is 1-butene in an amount of 10 mol. % to 35 mol. %; or (d) said first component is nitrogen in an amount of 18 mol. % to 25 mol. %, said second component is methane in an amount of 28 mol. % to 34 mol. %, said third component is ethane in an amount of 20 mol. % to 27 mol. %, and said fourth component is 1-butene in an amount of 20 mol. % to 28 mol. %, or propane in an amount of 30 mol. % to 40 mol. %.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the following, further features and advantages of the present invention as well as preferred embodiments are described with reference to the Figures, wherein

(2) FIG. 1 shows a schematic illustration of a method according to an embodiment of the invention;

(3) FIG. 2 shows another schematic illustration of a method according to another embodiment of the invention; and

(4) FIG. 3 shows a schematic illustration of a method according to further embodiment of the invention.

DESCRIPTION OF EMBODIMENTS

(5) The present invention relates to novel refrigerant mixtures and compositions which have been particularly optimized for energy and cost efficient hydrogen precooling in hydrogen liquefiers. The refrigerant mixtures are particularly designed for closed-loop Joule-Thomson refrigeration cycles for large-scale industrial hydrogen liquefaction plants. The refrigerant mixtures are optimized for a low number of fluid components, preferably up to 4, and particularly for low-temperature cooling in the range between 80 K and 120 K, particularly between 90 K and 110 K. The mixtures have been designed for clog-free plant operation with margins to potential mixture or component freeze out (solidification) e.g. also through selected and effective melting-point depression.

(6) The new proposed refrigerant mixtures and compositions allow a precooling cycle design and operation with comparatively low capital costs, industrially sound equipment e.g. heat exchanger size, and with reduced operational complexity. Compared to known technology and conceptual design for large-scale hydrogen liquefiers, the novel precooling refrigerant mixtures can reduce specific energy consumption of the liquefier by as much as 30%, thus enabling an economical production of liquid hydrogen on a large-scale. The present invention is used in the step of precooling the hydrogen feed gas stream as well as the precooling of other refrigerant streams to an intermediate temperature yielding a precooled feed gas stream. The invention is particular advantageous when, the intermediate temperature is in the range of 80 K to 150 K.

(7) The novel refrigerant mixtures and compositions are used to provide precooling duty in a closed-loop refrigeration cycle e.g. in highly efficient single mixed-refrigerant (MR) cycles. The MR compositions in this invention have been optimized for hydrogen precooling temperatures particularly between 80 K and 120 K, thus differentiating itself from other large-scale industrial applications with warmer cooling temperatures, as natural gas liquefaction.

(8) In the following the use of the refrigerant composition as a precooling refrigerant is exemplary illustrated in a process for hydrogen liquefaction. In other words the feed gas in the illustrative embodiments below comprises hydrogen. It will be appreciated that the invention includes embodiments in which the feed gas comprises helium, in accordance with the claims and statements above.

(9) Hydrogen Cooling and Liquefaction:

(10) A normal hydrogen (25% para) feed gas stream 11 from a hydrogen production plant is fed to the liquefaction plant 100 with a feed pressure above 15 bar(a), e.g. 25 bar(a), and a feed temperature near ambient temperature, e.g. 303 K. The feed stream 11 with a mass flow rate above 15 tpd, e.g. 100 tpd, is optionally cooled down between 283 K and 308 K, e.g. 298 K, with a cooling water system 75 or air coolers before entering the precooling cold-box 78 through plate-fin heat exchanger 81.

(11) The hydrogen feed 11 is cooled in the aforementioned heat exchanger 81 to the lower precooling temperature T-PC, e.g. 100 K, by the warming-up the low pressure streams of the single mixed-refrigerant cycle 41 and the cold hydrogen refrigeration cycle (26 and 33). At the outlet of the heat exchanger 81, residual impurities are removed from the precooled hydrogen feed gas 12 to achieve a purity of typically 99.99% in the adsorber vessels 76, 77 by physisorption. The feed gas 12 enters the adsorption unit 76, 77 at the temperature T-PC, e.g. 100 K, which can thus be designed at about 20 K higher than in prior known hydrogen liquefier applications. This allows to shift the start of the catalytic ortho-para conversion to higher temperatures, e.g. 100 K, which is thermodynamically convenient.

(12) After the feed gas purification 76, 77, the precooled feed gas stream 12 is routed back to the heat exchanger through 81 the catalyst filled passages (hatched areas in FIG. 1 or 2) of the plate-fin heat exchanger 81, where the normal hydrogen (25% para) is catalytically converted to about 39% para while being cooled to T-PC, while the exothermic heat of conversion is being removed by the warming up refrigerants 42 in the heat exchanger 81.

(13) The precooled feed gas stream 12 enters the vacuum-insulated liquefier cold-box 79 with T-PC (between 90 K and 120 K, e.g. 100 K). The feed stream 12 is subsequently cooled and liquefied as well as being catalytically converted to higher hydrogen para-fractions (hatched areas in FIG. 1 or 2) in plate-fin heat exchanger (82 to 90).

(14) The hydrogen gas feed stream 11 from battery-limits may be further compressed e.g. from 25 bar(a) to higher pressures, e.g. 75 bar(a), to increase process efficiency and to reduce volumetric flows and equipment sizes by means of a one or two stage reciprocating piston compressor at ambient temperature, or a one stage reciprocating piston compressor with cold-suction temperatures after precooling in the heat exchanger 81 or an ionic liquid piston compressor.

(15) Alternatively, an adiabatic ortho-para catalytic converter vessel may be used in the precooling cold box 78 to pre-convert normal-hydrogen (25%) para to a para-fraction near equilibrium in the feed gas stream 12 at the outlet of adsorber 76, 77, before routing the feed gas stream 12 back to the heat exchanger 81.

(16) Detailed Description of the Single Mixed-Refrigerant Precooling Cold Cycle

(17) A low pressure mixed refrigerant stream 42 is routed through suction drum 71 to avoid that entrained liquid droplets from the warmed-up refrigerant stream arrive to the suction side of compressor of stage one 63a of the compressor 63. The MR composition and the discharge pressure of the resulting refrigerant stream 43 (particularly in the range of 10 bar(a) to 25 bar(a)), after at least one compression stage, are optimized to produce the aforementioned stream 43 with a liquid fraction after intercooling. This reduces the mass-flow of refrigerant 43 that has to be compressed in stage two 63b of the compressor 63. The intercooled refrigerant stream 43 is separated into a liquid mixed refrigerant stream 45 that is pumped to the high pressure (particularly in the range of 30 bar(a) and 70 bar(a)) and into a vapour refrigerant stream 44, which is compressed to high pressure (particularly in the range of 25 bar(a) and 60 bar(a)) by the second stage 63b of compressor 63. Both the vapour 44 and the liquid stream 45 are mixed to a two-phase high pressure mixed-refrigerant stream 41 after compression 63. The first vapour stream 44 may be additionally separated into a second liquid phase and a second vapour phase, wherein preferably the first liquid phase 45 and the second liquid are unified, pumped together to high pressure and afterwards unified with the second vapour phase before entering the precooling cold box 78. Alternatively, the low pressure mixed refrigerant stream may be compressed by more than two stages. If compression and after-cooling results in the formation of a liquid phase, additionally phase separators may be arranged between the compressor stages.

(18) The two-phase high pressure mixed-refrigerant stream 41 enters the precooling cold-box 78 passing through the heat exchanger 81, where it is precooled to the lower precooling temperature of 100 K. A Joule-Thomson valve 64 expands the precooled mixed-refrigerant stream 41 to an expanded mixed refrigerant stream 42 that is characterized by an optimized low pressure level, particularly between 1.5 bar(a) and 8 bar(a). The refrigerant mixture of the high pressure mixed refrigerant stream 41 is designed to cool down from the temperature T-PC by at least 2.5 K, e.g. 96 K, through the Joule-Thomson expansion. The mixture temperature decrease is designed to maintain a feasible temperature difference between warming up and cooling down streams in the heat exchanger 81 as well as to assure that no component freeze-out occurs in the refrigerant mixture.

(19) Additionally, the two-phase high pressure mixed-refrigerant stream 41 may be further separated into a vapour 41a and a liquid phase 41b, wherein the liquid phase 41b may be additionally pumped to high pressure and unified with the vapour phase 41b before entering the precooling cold box 78. Alternatively, the vapour stream 41a of the above mentioned additional separation is guided through the heat exchanger 81 and an additional heat exchanger 81a or through two separate blocks 81, 81a of heat exchanger 81 in the precooling cold-box 78, expanded in a throttle valve 64b and guided again through both exchangers or blocks 81, 81a, whereby the liquid stream 41b of the additional separation is guided through the additional heat exchanger 81a, expanded in a throttle valve 64a and guided again through the additional exchanger 81a.

(20) Alternatively as depicted in FIG. 3, the two-phase high pressure mixed-refrigerant stream 41 may be guided through the additional heat exchanger 81a or block, and thereby cooled, and separated into a vapour 41a and a liquid phase 41b in a phase separator 73. The vapour stream 41a of the above mentioned additional separation is then guided through the heat exchanger 81 and the additional heat exchanger 81a or through two separate blocks 81, 81a of heat exchanger 81 in the precooling cold-box 78, expanded in a throttle valve 64b and guided again through both exchangers or blocks 81, 81a, wherein the liquid stream 41b of the additional separation is guided through the additional heat exchanger 81a or block, expanded in a throttle valve 64a and guided again through the additional exchanger 81a.

(21) Particularly, the vapour stream 41a may be merged after passing the heat exchanger 81 and expansion in the throttle valve 64b with the liquid stream 41b after passing the additional heat exchanger 81a and expansion in the throttle valve 64a, wherein the so merged expanded mixed-refrigerant stream 42 is then guided through the additional heat exchanger 81a.

(22) The MR composition can be regulated and controlled by the make-up system to adapt the mixture composition to ambient conditions and changed process conditions. The mixed-refrigerant is compressed in a two-stage MR turbo-compressor with interstage water cooling to decrease power requirement.

(23) Alternatively, in a very simplified configuration, the low pressure refrigerant stream 42 can be compressed within an at least two-stage compression 63 with inter-stage cooling and the refrigerant composition can be designed to avoid the appearance of a liquid fraction after the first compression stage 63a. Advantageously, no liquid pumps and no phase separator are required. However, a lower efficiency is expected.

(24) Low temperature precooling is efficiently achieved with a refrigerant mixture optimized specifically for hydrogen liquefaction, wherein the refrigerant preferably contains only four refrigerant components to maintain a manageable plant makeup system. A preferred mixture composition for a precooling temperature in the range of 90 K to 100 K consists of 18 mol. % nitrogen, 27 mol. % methane, 37 mol. % ethane and 18 mol. % isopentane. Ethylene may replace the ethane component for reasons of refrigerant availability and cost. For precooling temperatures between 90 K and 100 K, iso-butane may be replaced by 1-butene, iso-pentane, propane or propylene The mixture of the mixed-refrigerant may be adapted depending on the precooling temperatures. Accordingly, the mixture may contain nitrogen, methane, ethylene, and n-butane, isobutane, propane, propylene isopentane, iso-butane and/or n-pentane for precooling temperatures between 100 K and 120 K (or higher).

(25) For precooling temperatures above 85 K, the mixture may contain nitrogen, argon, neon, methane, ethane, propane, propylene, 1-butene.

(26) Also alternatively, the hydrogen feed stream 11 may be precooled to temperatures above 120 K, wherein in this case the mixed-refrigerant preferably contains nitrogen, methane, ethylene, n-butane.

(27) For slightly higher process efficiencies, a fifth or more refrigerant mixture component(s) can be added to the refrigerant mixture: iso-butane, iso-pentane, 1-butane, argon, neon, propane or propylene for precooling temperatures between 90 K and 100 K, or n-butane, iso-butane, iso-pentane, propane, propylene or pentane for the precooling temperature T-PC, particularly above 100 K, and additionally n-pentane, for precooling temperatures above 110 K.

(28) Additionally, conventional refrigeration units (chiller), e.g. vapour compression refrigerators, operated with e.g. propane, propylene or CO2, can be placed to cool down the high pressure lines 11, 21, 41 from ambient temperature, downstream the respective water coolers 75, to increase the overall energy-efficiency of the plant. Chiller(s) can be placed in the Single Mixed-Refrigerant stream 41 and/or the Hydrogen Cold Refrigeration cycle stream 21 and/or the Feed Hydrogen stream 11.

(29) Detailed Description of the Main Cooling High Pressure-Hydrogen Cycle

(30) The high pressure hydrogen (first refrigerant) stream 21 with a pressure of at least 25 bar(a), particularly 30 bar(a) to 70 bar(a) enters the precooling cold-box 78 and is precooled by the warming up streams 42, 33, 26 in the heat exchanger 81 to the precooling temperature T-PC. At the inlet of the liquefier cold-box 79, the first refrigerant stream 21 is further precooled by the warming up streams of the cold hydrogen refrigeration cycle (33 and 26). The high pressure stream 21 is then separated in at least three turbine-strings, at different temperature levels, to generate cooling by nearly isentropic expansions (polytropic) in min. five turbine-expanders. In the illustrated example, seven turbine-expanders are employed (51 to 57) in to four partial streams 22, 23, 24, 25 22, 23, 24, 25 which are routed through four turbine strings. The turbines 51 to 57 within the high-pressure process are designed with rotational speeds and frame-sizes that are industrially feasible and allow the partial recovery of process energy e.g. by the means of turbine brakes coupled with a turbo-generator to produce electricity and thus increase the total plant energy-efficiency. Alternatively, each of the above mentioned turbine strings may comprise only one turbo-expander, respectively, wherein the respective stream is directly expanded to low or medium pressure.

(31) In the preferred invention example, the high pressure hydrogen flow 21 is first separated after being cooled in a heat exchanger 82. One fraction, or partial stream (also referred to as a fourth partial stream) 25 is routed to a first turbine string (57 and 56), in which it is expanded in two-stages from high pressure to a medium pressure to form a medium pressure (fourth partial) stream 32, particularly in the range 6 bar(a) to 12.9 bar (a), more particularly in the range of 7 bar (a) to 11 bar(a), e.g. 9 bar(a), to achieve high isentropic efficiencies with moderate turbine rotational speeds. This medium pressure stream 32 provides cooling duty to the cooling down streams 12, 21 The remaining high pressure flow fraction is subsequently cooled in heat exchanger 83 to the temperature of a second turbine string. A further partial stream (also referred to as a third partial stream) 24 is then separated and expanded in two-stages (55 and 54) to the above-mentioned medium pressure level to form a partially expanded stream 31. This partially expanded (third partial) stream 31 is warmed up and mixed with the above-mentioned medium pressure stream 32 in order to provide additional cooling to duty to the cooling down streams 12, 21. The turbine strings for the streams 25 and 24 can, alternatively, be designed with intermediate cooling between the two expansion stages.

(32) A further remaining high pressure flow fraction, or partial stream (also referred to as the second partial stream) 23 routed to a third turbine string after being further cooled down by the warming up streams in heat exchanger(s) 85, 86. The following process feature is special to this hydrogen liquefaction process: the second partial stream 23 is expanded in turbo-expander 53 to an intermediate pressure between medium pressure and high pressure, to produce an intermediate pressure stream 29. The resulting intermediate pressure stream 29 preferably has a temperature above the critical temperature of the refrigerant, e.g. 34 K to 42 K. The intermediate pressure stream 29 is then re-warmed slightly in a further heat exchanger 88 before being expanded again in turbo-expander 52 to the medium pressure level yielding a medium pressure (first partial) stream 30. In this way, cooling with the third turbine string is generated at two different pressures (medium and intermediate pressure) and two different temperature levels. The heat exchanger enthalpy-temperature curve between the cooling down and warming up streams in a critical temperature range, e.g. 30 K to 50 K, can, hence, be matched more closely. This can reduce exergetical losses in the heat exchanger. This new process configuration is particularly beneficial for hydrogen feed cooling since: depending on the pressure, the specific isobaric heat capacity of the hydrogen feed stream possesses steep gradients in the region close to its critical temperature (particularly between 30 K and 50 K). Alternatively in an embodiment not shown, the third turbine string for the second partial stream 23 can be designed analogous to first and second turbine strings 25 and 24, with no intermediate warming-up after the first turbine, or with a slight cooling down between the expanders.

(33) The medium pressure stream 30 provides cooling duty to the cooling down streams in the heat exchanger 86 to 89 up to the temperature of turbine outlet 54, where it is mixed with the medium pressure stream 31 to form a mixed stream. The mixed stream is warmed approximately to the temperature of the turbine 56 outlet (between the precooling temperature and the temperature of cooled feed stream 13 at the cold end of the heat exchanger 89, where it is further mixed with the medium pressure stream 32. The total medium pressure hydrogen flow 33 is warmed up in the heat exchangers 84 to 81 to a temperature close to ambient temperature, thereby providing additional cooling duty to the cooling down streams 11, 21, 41.

(34) The outlet temperature and pressure of turbo-expander 52 are optimized in combination with the cold-end hydrogen cycle. The temperature of the medium pressure stream 30 at the turbine outlet is the cold-end temperature T-CE. For the newly proposed high pressure cycle, optimal cold-end temperatures T-CE for the high pressure cycle are set between 28 K and 33 K, particularly between 29 K and 32 K, for a dry-gas turbine discharge and an optimal MP1 pressure level, particularly in the range of 6 bar(a) and 12.9 bar(a), more particularly between 7 bar(a) and 11 bar(a), at the outlet of the turbo-expander 52 (medium pressure level between 7 bar(a) and 11 bar(a)). The warmed-up stream 33 is mixed with the compressed low pressure stream 26 from the compressor 61 to produce a mixed stream 34. The mixed stream 34 is compressed from medium pressure level in e.g. one or preferably two parallel running 100% reciprocating piston compressors 62, or alternatively three parallel running 50% reciprocating piston compressors to the high pressure level between 30 bar(a) and 75 bar(a). Temperature T-CE, medium and high pressure levels are optimized in function of precooling temperature TPC and liquid hydrogen production rate (feed mass flow rate). Piston compressors 61 and 62 are designed with at least two intercooled stages each (three stages preferred).

(35) Compared to prior known technology, this high pressure configuration with significantly higher turbine outlet pressure levels (medium and high) yields moderate effective volumetric flows at the suction of compressor 62, thus enabling the design of mechanically viable frame-sizes for the hydrogen piston compressor, even for very large liquefaction capacities e.g. up to 150 tpd (with two parallel compressors).

(36) At the cold end, the remaining high pressure hydrogen flow fraction, the first partial stream 22 in the cold-cycle provides the cooling for the final liquefaction and ortho-para conversion of the feed stream. The high pressure hydrogen refrigerant in the first partial stream 22 is expanded from high pressure to low pressure in at least one turbine string though at least one turbo-expander e.g. 51.

(37) If the turbo-expander 51 is to be designed with a dry-gas discharge, high pressure stream 22 is expanded from high pressure to an intermediate pressure, above the critical pressure, e.g. 13 bara, or to a pressure below, e.g. in the range of 5 bar(a) to 13 bar(a), if no two-phase is to be generated within the turbine 51 or at the outlet of the turbine 51. Subsequently, the cooled stream is expanded through a Joule-Thomson throttle valve 59 into a gas-liquid separator 74. For a turbo-expander with allowed two-phase discharge, e.g. a wet expander, the high pressure stream 22 can be expanded directly to low pressure level. The low pressure stream 26 stream is warmed-up to near ambient temperature providing cooling duty to the cooling down streams 11, 12, 21, 41 in the precooling and liquefier cold-box. The low pressure stream 26 is then compressed in one multistage reciprocating piston compressor 61 with interstage cooling.

(38) The hydrogen feed stream 12 is cooled by the warming up cold low pressure stream 26 down to a temperature equal to the high pressure stream 22, e.g. 29.5 K, and is catalytically converted to a para-fraction slightly below the equilibrium para-fraction. The cooled feed stream 13 is then expanded by the means of at least one turbo-expander 58 from feed pressure to an intermediate pressure e.g. 13 bar(a) or lower. Subsequently, the expanded and cooled feed stream is further expanded through the Joule-Thomson throttle valve 60 to the low pressure level that is required for final product storage pressure e.g. 2 bar(a) and particularly further cooled by the low pressure stream 26.

(39) For turbo-expanders allowing a two-phase discharge, the high pressure feed stream 13 can be directly expanded into the two-phase region to the product storage pressure e.g. 2 bar(a). For shaft power around 50 kW or above, as in large-scale liquefiers with e.g. 100 tpd capacity, a turbo-expander with energy-recovery via a turbo-generator can be employed to raise the plant energy-efficiency. Alternatively, a cold liquid piston expander can be employed to directly expand the feed stream from the intermediate pressure level, e.g. 13 bar(a), to the low pressure level near the final product storage pressure. In either case, the two-phase hydrogen feed stream 14 is finally cooled and can be further catalytically converted in the last part of plate-fin heat exchanger 91 with the aid of the warming-up low pressure cold-cycle refrigerant stream 26.

(40) With this configuration, a liquid hydrogen product stream 15 at the outlet can be generated as saturated liquid or even subcooled liquid. A final para-fraction of the liquid product stream 15 above 99.5% can be reached if desired.

(41) The method of the invention offers the following advantages:

(42) In summary: Significant decrease in total specific energy demand and specific costs for the production of liquid hydrogen on a large-scale compared to prior known technologies. Highly efficient and simple mixed refrigerant for low-temperature closed-loop Joule-Thomson precooling cycle in large-scale hydrogen liquefiers, compared to state-of-the art precooling via e.g. liquid nitrogen stream evaporation at about 78 K. Refrigerant mixture is specifically optimized for hydrogen liquefaction and enabling particularly low precooling temperatures between 80 K and 120 K, which are significantly lower than in other known large-scale industrial mixed-refrigerant technology application. The mixture can be adapted depending on the precooling temperature design. Low precooling temperature mixed-refrigerant combines the benefits of energy-efficient single mixed-refrigerant cycles with comparatively low precooling temperatures. Low precooling temperatures are beneficial to the liquefier plant design because of the decreased cooling duty to be provided by the colder refrigeration cycle(s) e.g. hydrogen closed-loop, thus reducing the size of cold refrigeration cycle equipment, which is the plant capacity limiting factor e.g. heat exchanger/liquefier cold box, compressors and turbines. Refrigerant mixture compositions optimized for cooling across Joule-Thomson expansion valve and for clog-free plant operation, by avoiding hazardous component freeze out within the process due to potential mixture melting. Low temperature refrigerant mixture for hydrogen liquefiers is particularly designed for energy efficient precooling with only 4 refrigerant components to design a manageable plant makeup and a simple plant gas management. Specified mixed refrigerant allows the design of the continuous catalytic ortho-para conversion directly after the precooling at a higher temperature level, e.g. 100 K, compared to prior known hydrogen liquefaction plants (around 80 K). Due to the removal of exothermic heat of conversion at a higher temperature level, the thermodynamic efficiency of the plant is improved. Advantageously, impurity adsorption in hydrogen feed stream, e.g. nitrogen removal, can be performed directly after low temperature precooling and prior to catalytic ortho-para conversion, e.g. at 100 K, thus reducing adsorber vessel dimensions. Physisorption process improves with decreasing temperature. Adsorption vessel removes residual impurities from the hydrogen feed which can poison hydrogen ortho-para catalysts.

(43) TABLE-US-00001 Reference numerals: 100 liquefaction plant  11 feed stream  12 precooled feed stream  13 cooled feed stream  14 expanded cooled feed stream  15 Liquid product stream  21 high pressure refrigerant stream  22 high pressure first partial stream  23 high pressure second partial stream  24 high pressure third partial stream  25 high pressure fourth partial stream  26 low pressure first partial stream  27 vapour phase of low pressure first partial stream  28 liquid phase of low pressure first partial stream  29 intermediate pressure second partial stream  30 medium pressure second partial stream  31 medium pressure third partial stream  32 medium pressure fourth partial stream  33 combined medium pressure stream  34 further combined medium pressure stream  35 medium pressure first partial stream  41 high pressure mixed (second) refrigerant stream  41a vapour phase of high pressure mixed refrigerant stream  41b Liquid phase of high pressure mixed refrigerant stream  42 low pressure mixed refrigerant stream  43 medium pressure mixed refrigerant stream  44 vapour phase of medium pressure mixed refrigerant stream  45 liquid phase of medium pressure mixed refrigerant stream  51, 52, 53, 54, turbo-expander  55, 56, 57, 58  59, 60, 64, throttle valve  64a, 64b  61 piston compressor  62 piston compressor  63a first compressor stage  63b second compressor stage  71 suction drum  72, 73, 74 phase separator  75 water cooling  76, 77 adsorber vessel  78 pre cooling cold box  79 liquefier cold box  81, 82, 83, 84, heat exchanger block or heat exchanger filled with  85, 86, 87, 88, ortho-para catalyst (hatched area)  89, 90, 91