Field of the Invention The present invention relates to a method for compressing and controlling a gas mixture for use in a cryogenic refrigeration cycle. In particular, the invention relates to a method for compressing and controlling a refrigerant gas mixture comprising neon, for use in a cryogenic refrigeration cycle

Background of the Invention

Cryogenic cooling and liquefaction of hydrogen and helium is performed at very low temperatures below 30 K. Therefore, cryogenic refrigeration cycles for this purpose are restricted to the use of hydrogen, helium, neon and/or mixtures of these fluids. A significant energy input is required in order to provide the cooling, and is mainly provided by the required gas compressors. Conventional hydrogen and helium liquefaction plants or refrigerators use reciprocating piston compressors and/or rotary screw compressors to compress the coolants in a closed refrigeration cycle e.g. mainly hydrogen and/or helium. These machines are, however, restricted in volumetric flow and/or efficiency.

New process concepts adopting turbo compressors for the compression of hydrogen, helium, neon and/or a mixture of these gases have been proposed. However, these concepts include only limited technical detail on the required solutions for the minimization of expensive refrigerant inventory losses due to leakage.

Hydrogen as a cryogenic coolant for a hydrogen liquefier is abundantly available and comparatively inexpensive. Due to the extremely low molecular mass of hydrogen, 2.02 g per mol, only a very small increase in pressure per turbo compressor stage can be achieved with conventional ambient temperature turbo compressors at conventional circumferential speeds (below 400 m/s). A correspondingly very high number of compressor stages of at least 15 stages would be required to provide the cooling for a hydrogen liquefier, which is technically and commercially highly unfavourable. Helium has a higher molecular mass than hydrogen, about 4 g per mol, but is comparatively still very light. In the range of conventional maximum operating speeds, only low turbo compressor stage pressure ratios can thus be generated.

Further, normal compressor leakage rates have to be avoided due to the high cost of helium as a refrigerant. Hermetically sealed machines can be used but are expensive and currently limited in both frame-size and efficiency. Neon as a cryogenic refrigerant is interesting due to the comparatively high molecular mass of nearly 20 g per mol. Neon can thus be efficiently compressed in turbo compressors near ambient temperature with an economically feasible number of turbo compressor stages. Neon as a potential refrigerant, however, is even more expensive than helium. Therefore, regular continuous leakage rates in the range of up to about 10 norm cubic meter per hour (Nm ³ /h) per turbo compressor stage, especially through the seals of conventional turbo compressors, must be strictly avoided in order to limit expensive inventory losses and to allow an economically practicable plant operation.

In order to use the advantages of the available fluids at cryogenic temperatures with conventional turbo compressors, refrigerant mixtures (particularly helium-neon and hydrogen-neon refrigerant mixtures) have been proposed, particularly with a molar mass above 7 g per mol.

Helium-neon mixtures used as refrigerants for hydrogen liquefiers achieve a lower energy efficiency as coolants and are made of two extremely expensive gases (helium and neon) compared to pure hydrogen or hydrogen-neon mixtures. Helium-neon gas mixtures thus require hermetically sealed turbo compressors in order to maintain commercially reasonable operating costs. These hermetically sealed machines cannot be operated at high rotational speeds due to their special construction and are thus limited also in energy efficiency and feasible stage pressure ratio, requiring a relatively high number of compression stages and a high capital investment even for gases or gas mixtures with a relatively high molar mass.

Mixtures of hydrogen and neon for cryogenic cooling, below the critical temperature of hydrogen, are of advantage due to the beneficial compromise between the inexpensive and energy efficient refrigerant, hydrogen, and neon with its high molecular weight. For this mixture, also highly efficient and significantly less expensive dry gas sealed turbo compressors can be used (instead of only hermetically sealed machines). To compress a valuable or hazardous gas medium, machines with dry gas seals have been employed in various applications e.g. for mixed-refrigerant compressors in natural gas liquefaction (LNG) plants. A dry gas seal is a device for the sealing of a turbo compressor whose shaft is sealed against the housing with the use of a sealing gas. Conventional dry gas seal turbo machines usually make use of a gas taken directly from the process refrigerant feed and/or nitrogen as external seal gas, since nitrogen is inert and inexpensive. A leakage of nitrogen into the process refrigerant feed can be tolerated in most applications due to the freezing point of pure nitrogen being at 63 K and thus significantly below the typical operational temperature for general

applications, in particular for LNG plants.

The use of nitrogen as primary external seal gas is, however, of concern in

conventional industrial cryogenic applications for hydrogen or helium

refrigerators/liquefaction, because the operating temperatures for these plants are well below 63 K which would lead to a freeze out of the nitrogen seal gas that is leaked into the process refrigerant feed. This would present serious risks for equipment and/or personnel at site. As an alternative to nitrogen as primary seal gas in turbo

compressors (having for example single or double gas seals), hydrogen, helium, neon or a gas mixture of these can be used as sealing gases and can be supplied either directly from the process gas compressor or from an external source, particularly for cryogenic refrigeration cycles involving hydrogen, helium, neon or mixtures of these coolants e.g. in hydrogen or helium liquefiers/refrigerators.

For the compression, for instance, of a hydrogen-neon process gas mixture, expensive continuous neon gas inventory leakage losses have to be avoided or kept to a minimum. A conventional double gas seal supplied with hydrogen-neon process gas from the process gas compressor can be implemented but will still lead to relatively high neon loss rates per stage. An external seal gas supply with pure hydrogen is a potential improvement as it leads to losses of the comparatively inexpensive external seal gas hydrogen and theoretically no continuous compressor losses of process gas. In a closed refrigeration cycle, the simple supply of a seal gas, such as pure hydrogen provided as external gas, would lead to a leakage of the seal gas into the process gas feed. This would result in a progressive hydrogen enrichment in the process gas mixture with operating time. This would lead to a continuous shift of the mixture composition, e.g. hydrogen-neon, from the optimal design point towards lighter mixtures with consequences to the feasible compressor stage pressure ratios and plant performance, and a potential loss of plant operability and/or (expensive) venting or refilling of the refrigerant inventory. Based on this background, it is the objective of the present invention to provide means and methods for compressing and controlling a gas mixture comprising neon, particularly comprising neon and hydrogen or neon and helium having an increased efficiency and/or a significantly decreased neon loss. Embodiments of the invention seek to provide an apparatus which overcome some or all of these problems.

Summary of Invention According to a first aspect of the present invention there is provided a method for compressing a gas mixture comprising neon, in a closed-loop cycle, the method comprising:

providing a flow of the gas mixture;

providing a compression device having at least one gas seal and providing a gas seal flow to the at least one gas seal;

compressing the gas mixture in the compression device, thereby forming a compressed gas mixture;

measuring a composition of the gas mixture in at least one location;

comparing the measured composition of the gas mixture with respect to a predetermined value or range;

determining if there is a deviation from the predetermined value or range; and when a deviation from the predetermined value is determined:

- adjusting at least one operating parameter of the process cycle to optimise the parameter for the change in composition; and/or - adjusting the composition of the gas mixture in the cycle, so that the measured composition of the gas mixture no longer deviates from the

predetermined value or range. In the following passages, the gas mixture is also referred to as the first process fluid (or refrigerant), and the one gas seals are also referred to as the second/third/fourth process fluid.

The compression device may be a compressor. The compression device may be a turbo compressor, preferably a radial (centrifugal) turbo compressor

The step of determining if there is a deviation may comprise evaluating whether the measured composition is above a predefined maximum value. The step of determining if there is a deviation may comprise evaluating whether the measured composition is below a predefined minimum value. The step of determining if there is a deviation may comprise evaluating whether the measured composition is outside a predefined range.

The gas mixture may comprise hydrogen. The or at least one gas seal may comprise hydrogen. The gas mixture may be a refrigerant mixture comprising or essentially consisting of hydrogen and neon. The gas mixture may be a refrigerant mixture comprising or essentially consisting of helium and neon.

The gas mixture may comprise or essentially consist of hydrogen and neon. The gas mixture may have a molecular mass in the range of 3.88 g ^* mol ^"1 to 13 g ^* mol ^"1

The gas mixture may have a molecular mass in the range of 6.5 g ^* mol ^"1 to 1 1.2 g ^* mol ^"1 .

The step of adjusting at least one process parameter may comprise adjusting the temperature and/or the pressure of said gas mixture. The step of adjusting at least one process parameter may comprise adjusting the temperature and/or pressure of said compressed gas mixture. The step of adjusting at least one process parameter may comprise adjusting the compression performance of the compression device. The step of adjusting at least one process parameter may comprise adjusting the compression ratio of the compression device. The step of adjusting at least one process parameter may comprise adjusting the pressure ratios at the inlet and/or outlet of the compression device (1 1 ) in order to adapt the operating point to the changing gas mixture composition.

The method may also include passing partial stream(s) of the compressed gas mixture through at least one turbo expander. The step of adjusting the at least one process parameter may comprise adjusting the pressure ratios at the inlet and/or outlet of the or each turbo expander in order to adapt the operating point to the changing gas mixture composition. The step of adjusting the at least one process parameter may comprise adjusting the cycle process temperature at an outlet of the or the coldest turbo expander. The cycle process temperature may be adjusted by manipulating the high/low gas pressure levels of the refrigeration cycle.

The step of adjusting at least one process parameter may comprise adapting the pressure ratio of the refrigeration cycle (HP to LP) as a function of the measured gas mixture composition.

The method may also include passing a partial stream of the compressed gas mixture through at least one turbo expander. The step of adjusting the at least one process parameter may comprise varying a cycle process temperature at an outlet of the or the coldest turbo expander.

The method may also include passing a first partial stream of the compressed gas mixture through a first turbo expander. The method may also include passing a second partial stream of the compressed gas mixture through a second turbo expander. The step of adjusting the at least one process parameter may comprise adjusting the cycle process temperature at the outlet of the coldest turbo expander.

The step of adjusting the at least one process parameter may comprise controlling at least one of the compressor operating speeds of the compression device.

The step of measuring a composition of the gas mixture may include measuring the volumetric fraction of at least one constituent of the gas mixture. The measurement of the composition may be performed before compression of the gas mixture in the compression device. The measurement of the composition may be taken after compression of the gas mixture in the compression device

The composition of the gas mixture may be measured using any suitable gas analysing device for example, a gas chromatograph.

The step of adjusting of said process parameter is performed automatically.

The step of determining if there is a deviation may include generating a data signal relating to the measured gas composition and conveying this signal to a control device. The step of determining if there is a deviation may be carried out by a control device. The control device determine the required adjustment at least one process parameter and convey a control signal to adjust the at least one process parameter. The step of adjusting the composition of the gas mixture in the cycle may comprise extracting a partial, high pressure stream from said compressed gas. The step of adjusting the composition of the gas mixture in the cycle may comprise separating the partial stream into a mainly liquid phase and a mainly vapour phase. The step of separating the partial stream may include passing the partial stream through a phase separator to produce the mainly liquid phase and the mainly vapour phase. The step of separating the partial stream may include passing the vapour phase through a second phase separator or second phase separator phase.

The gas mixture may comprise neon and hydrogen, such that the step of separating the partial stream involves separation into a mainly liquid neon phase and a mainly vapour hydrogen phase.

The partial stream may also be cooled (for example in a Joule-Thomson valve or other suitable device) prior to the separation step.

The partial stream may be extracted (or drawn off) continuously or intermittently. A control device, such as a valve, may be provided to control the partial flow (draw off). The method according to claim 9, wherein the method also includes passing the partial stream(s) of the compressed gas mixture through at least one turbo expander; and wherein the high pressure partial stream is extracted from a high pressure end of one turbo expander. In other words the high pressure partial stream may be extracted immediately upstream of the turbo expander. The gas mixture may comprise hydrogen and neon, such that the mainly liquid phase of the partial stream comprises mainly neon and the mainly vapour phase of the partial stream comprises mainly comprising hydrogen.

The method may include providing two turbo expanders. The method may include passing a first partial stream through a first turbo expander and a second partial stream through a second turbo expander. The high pressure partial stream is extracted from a high pressure end of the coldest turbo expander (in other words, upstream of the coldest turbo expander). Alternatively, the step of adjusting the composition of the gas mixture in the cycle may comprise extracting a partial stream from said gas mixture prior to the compression step. The partial stream may be separated into a mainly liquid phase comprising mainly neon and a mainly vapour phase comprising mainly hydrogen. The partial stream may also be cooled prior to the separation step.

The method may also include feeding at least part of said mainly liquid phase into the gas mixture flow or into the compressed gas mixture flow. At least part of said mainly liquid phase may be fed into the gas mixture flow or into the compressed gas mixture flow at any suitable point in the closed loop cycle.

The gas mixture may comprise neon and hydrogen, such that the step of separating the partial stream involves separation into a mainly liquid neon phase and a mainly vapour hydrogen phase. The compression device may include two gas seals. The method may include providing a first gas flow to the first gas seal and providing a second gas flow to the second gas seal.

The compression device may include three gas seals. The method may include providing a gas flow to each gas seal. The method may further comprise conveying the compressed gas mixture for use as a refrigerant in a cryogenic refrigerant cycle. The compressed gas mixture may be used as a refrigerant in a cryogenic cooling cycle. The compressed gas mixture may be used as a refrigerant in a cryogenic cooling cycle for liquefying hydrogen or helium. The compressed gas mixture may be used as a refrigerant, particularly in a cryogenic cooling cycle

According to a further embodiment, there is provided a method for liquefying hydrogen or helium, said method comprising the steps of:

- cooling a feed gas stream comprising hydrogen or helium with a compressed first refrigerant stream comprising neon, wherein said first compressed refrigerant stream is expanded to an expanded first refrigerant stream , thereby producing cold, and

wherein said compressed first refrigerant stream a compressed gas mixture as produced by the process according to any of the earlier embodiments.

According to a further embodiment, there is provided a turbo compressor, particularly a radial turbo compressor, having

- a first chamber comprising at least one compressor rotor,

- a second chamber separated from said first chamber by a first separating element and in fluidic connection with said first chamber by a first opening in said first separating element,

- a third chamber separated from said second chamberby a second

separating element and in fluidic connection with said second chamber by a second opening in said second separating element , and

- a fourth chamber separated from said third chamber by a third separating element and in fluidic connection with said third chamber by a third opening in said third separating element, and

- a rotating element being connected to the at least one compressor

rotor, wherein said rotating element extents through the first, second and third opening. The fourth chamber may be is separated from the ambient environment by a fourth separating element and in fluid connection with said ambient environment by a fourth opening in said fourth separating element . Said rotating element may extend through said fourth opening.

The process may comprise the following steps

providing a first process fluid comprising neon in a first chamber,

compressing the first process fluid to a first pressure in a compressing step, providing a second process fluid comprising hydrogen, neon, or a mixtures thereof in a second chamber with a second pressure being larger than the first pressure, wherein the second chamber is non-hermetically separated from the first chamber and in fluidic connection with the first chamber,

optionally expanding the compressed first process fluid to the first process fluid, monitoring the composition of the compressed first process fluid or the first process fluid, and

- adjusting the composition of the compressed first process fluid or a process parameter if the monitored composition deviates from a predefined value.

A partial stream may be separated from the compressed first process fluid, expanded and optionally cooled such that an expanded partial stream comprising a mainly liquid phase comprising mainly neon and a mainly vapour phase mainly comprising hydrogen is yielded.

Alternatively, a partial stream is separated from the first process fluid, optionally cooled and separated into said mainly liquid phase comprising mainly neon and a mainly vapour phase comprising mainly hydrogen.

The mainly liquid phase may be at least partly recycled (fed into) the gas mixture (refrigerant mixture). The mainly liquid phase may be at least partly recycled (fed into) into the first process fluid or the compressed first process fluid.

According to a further embodiment, a process for compressing and controlling a gas mixture comprising neon is provided. The process may comprise the steps of:

providing a first process fluid comprising neon in a first chamber,

- compressing the first process fluid to a first pressure in a compressing step, providing a second process fluid comprising hydrogen, neon, nitrogen or a mixtures thereof in a second chamber with a second pressure being larger than the first pressure, wherein the second chamber is non-hermetically separated from the first chamber and in fluidic connection with the first chamber,

- optionally expanding the compressed first process fluid yielding the first process fluid,

monitoring the composition of the compressed first process fluid or the first process fluid, and

- adjusting a process parameter of said compressing step if said monitored

composition deviates from a predefined value

Particularly, the second process fluid is designed to serve as a seal fluid.

The term "non-hermetically" in the context of the present specification particularly means that a fluid may flow from one chamber in the other chamber.

The term "mainly liquid stream" in the context of the present specification refers to a stream wherein the majority of the molecules within the stream are present in the liquid phase.

The term "mainly vapour stream" in the context of the present specification refers to a stream wherein the majority of the molecules within the stream are present in the vapour phase. The term "comprising mainly neon" in the context of the present specification refers to a feature of a stream or phase, which is characterized by a neon molecular fraction above 50 %, particularly at least 60 %, 70 %, or 80%.

The term "comprising mainly neon" in the context of the present specification refers to a feature of a stream or phase, having a hydrogen molecular fraction above 50 %, particularly at least 60 %, 70 %, or 80%.

In certain embodiments, the first chamber and the second chamber are comprised within a compressor, particularly a turbo compressor, wherein the first chamber and the second chamber are separated by a first separating element, and the first chamber comprises at least one compressor rotor. The turbo compressor may be a radial (centrifugal) turbo compressor.

In certain embodiments, the first process fluid and the second process fluid comprise hydrogen. Advantageously, leaking of the second process fluid into the first process fluid results only in a shift of the neon to hydrogen ratio without polluting the first process fluid, wherein particularly hydrogen may be separated from the first process fluid, thereby adjusting the composition to the predefined value. In certain embodiments, the first process fluid (refrigerant) comprises or essentially consists of hydrogen and neon and is has a molecular mass in the range of 3.88 g ^* mol ^{" 1} to 13 g ^* mol ^"1 . In certain embodiments, the first process fluid comprises or essentially consists hydrogen and neon and has a molecular mass in the range of 6.5 g ^* mol ^"1 to 1 1.2 g ^* mo|- ¹ .

In certain embodiments, the process further comprises removing from the first process fluid (refrigerant) at least one component of the second process fluid (first seal gas), (preferably hydrogen) that leaked from the second process fluid into the first process fluid in the compression step.

In certain embodiments, the second process fluid essentially consists of hydrogen. The hydrogen that has leaked from the second process fluid into the first process fluid during compression of the first process fluid, may be removed from the first process fluid. In other words, the second process fluid (first seal gas), that has leaked into the first process fluid (refrigerant) during compression of the refrigerant, may be removed from the first process fluid.

The method may further comprise the step of providing a third process fluid, preferable comprising or essentially consisting of hydrogen or nitrogen, in a third chamber with a third pressure . The third pressure may be between the ambient pressure and the second pressure, or above the second pressure.

The third pressure may be between ambient pressure and the second pressure. In this case, the second process fluid may leak into the first process fluid or into the third process fluid. In certain embodiments, the third pressure is above the second pressure. In this case the third process fluid acts or is designed to serve as an additional seal fluid.

Accordingly, the first process fluid is sealed by two pressure gradients.

In certain embodiments, the third chamber is comprised within the above mentioned turbo compressor, wherein the third chamber is separated from the second chamber by a second separating element. The method may further comprise the step of providing a fourth process fluid, preferably comprising or essentially consisting of nitrogen, in a fourth chamber with a fourth pressure, wherein particularly the fourth pressure lies between the third pressure and the ambient pressure. In certain embodiments, the fourth chamber is comprised within the above mentioned turbo compressor, wherein the fourth chamber is separated from the third chamber by a third separating element.

The method may further comprise the steps of

determining a volumetric fraction of at least one component of the first process fluid or the compressed first process fluid, preferably of neon or hydrogen, comparing the determined volumetric fraction with a predefined value,

- adjusting at least one process parameter of the compressing step in case of a deviation of the determined volumetric fraction from the predefined value.

The process parameter(s) which is/are adjusted may be one or more of the following:

- the temperature and/or the pressure of the first process fluid,

- the temperature and/or the pressure of the compressed first process fluid,

- the compression performance, or

- the compression ratio.

Particularly, in case of a turbo compressor, particularly a radial (centrifugal) turbo compressor may be used in the compressing step, the compression performance may be adjusted via the rotational speed of the compressor. The step of adjusting the process parameter may be performed automatically.

The compressed first process fluid may be used as a refrigerant, particularly in a cryogenic cooling cycle.

In certain embodiments, the cryogenic cooling cycles is comprised within a process for liquefying hydrogen or helium.

According to a further aspect, a process for liquefying hydrogen or helium is provided. The process comprises the steps of:

- cooling a feed gas stream comprising hydrogen or helium, particularly in a first cooling step by a first cooling cycle, with a first refrigerant stream consisting of or comprising neon, particularly comprising or essentially consisting of neon and hydrogen, wherein the first refrigerant stream is expanded to an expanded first refrigerant stream, thereby producing cold, and,

- optionally compressing the expanded first refrigerant stream to the first

refrigerant stream, wherein the first refrigerant stream is provided and/or the expanded first refrigerant stream is compressed by the process of the invention for compressing and controlling a gas mixtures comprising neon.

The feed gas stream may be provided with a pressure of at least 15 bar (a). The feed gas stream may be provided with an initial temperature and precooled to intermediate temperature, particularly in the range of 70 K to 150°K, before being cooled by the first refrigerant stream.

The feed gas stream may be cooled by the first refrigerant stream to a first

temperature, particularly from the intermediate temperature. The feed gas stream may be further cooled by a second refrigerant stream from the first temperature to a temperature below the critical temperature of hydrogen or helium, particularly below 24 K, and optionally expanded the cooled feed gas stream, yielding a liquid product stream comprising hydrogen or helium. A second refrigerant stream may be provided which comprises or consists of hydrogen and is expanded, thereby producing cold The feed gas stream may have hydrogen concentration of at least 99.99 Vol.%.

Preferably, ortho hydrogen comprised within the feed gas stream (about 75%) is converted to higher para hydrogen fractions preferably before liquefaction of the feed gas stream to avoid that the exothermic ortho to para reaction takes place in the liquid product possibly resulting in an undesired partial vaporization of the liquid hydrogen product during storage and transport.

The first temperature may lie in the range of 24.6 K to 44.5 K, particularly in the range of 26 K to 33 K.

The feed gas stream may be precooled to an intermediate temperature in the range of 80 K to 120 K, particularly 100 K, yielding the precooled feed gas stream, and the precooled feed gas stream is brought into contact with a catalyst being able to catalyse the ortho to para conversion of hydrogen, particularly before the first cooling step. The catalyst may be or may comprise hydrous ferric oxide. In certain embodiments, the catalyst is arranged within a heat exchanger, in which the feed gas stream is precooled.

The first cooling cycle may comprise the steps of:

- providing the first refrigerant stream with a first pressure by the process of the invention for compressing and controlling a gas mixture comprising neon, separating the first refrigerant stream at least into a first partial stream and a second partial stream,

expanding the first partial stream in a first expansion device to a second pressure yielding an expanded first partial stream,

expanding the second partial stream to a third pressure in a second expansion device yielding an expanded second partial stream,

guiding the expanded second partial stream and the feed gas stream or the precooled feed gas stream such that heat can indirectly be transferred between the expanded second partial stream and feed gas stream or the precooled feed gas stream, thereby particularly cooling the feed gas stream or the precooled feed gas stream to the first temperature,

merging the expanded first partial stream with the expanded second partial stream yielding an expanded first refrigerant stream, and compressing the expanded first refrigerant stream to the first pressure yielding the first refrigerant stream by the process of the invention for compressing and controlling a gas mixtures comprising neon. Particularly, the first refrigerant stream equates to the first process fluid of the method compressing and controlling a gas mixtures comprising neon.

The term "indirectl heat transfer" in the context of the present invention refers to the heat transfer between at least two fluid streams that are spatially separated such that the at least two fluid streams do not merge or mix but are in thermal contact, e.g. two fluid streams are guided through two cavities, for example of a plate heat exchanger, wherein the cavities are separated from each other by a wall or plate, and both streams do not mix, but heat can be transferred via the wall or the plate. The first expansion device and/or the second expansion device may comprise at least one turbo-expander.

A partial stream may be separated from the first refrigerant stream, particularly additionally to the aforementioned first and second partial stream. A partial stream may be expanded, optionally cooled, and separated into a neon rich partial stream and a hydrogen rich partial stream. Optionally the neon rich partial stream may be recycled into the first refrigerant stream or the expanded refrigerant stream. Alternatively or additionally, the neon rich partial stream may be stored. According to a further aspect of the invention, a refrigerant turbo compressor is provided. The refrigerant turbo compressor comprises

a first chamber comprising at least one compressor rotor,

a second chamber being separated from the first chamber by a first separating element and being in fluidic connection with the first chamber by a first opening in the first separating element,

a third chamber being separated from the second chamber by a second separating element and being in fluidic connection with the second chamber by a second opening in the second separating element, and a fourth chamber being separated from the third chamber by a third separating element and being in fluidic connection with the third chamber by a third opening in the third separating element, and

a rotating element being connected to the at least one compressor rotor, wherein the rotating element extents through the first, second and third opening.

A turbo compressor provided with third and fourth chambers, which in use are supplied with gas seal flow, provides significant operational advantages over known turbo compressors. The provisional of additional seal gas filled chamber provides an additional seal for the inner chambers, which achieves the overall result that the gas mixture to be compressed is more effectively sealed and leakage of this gas mixture, in particular the expensive neon component is reduced. This means that the inventory loss can be minimised. This is particularly advantageous, as discussed above, in the compression of a gas mixture comprising neon for use in a refrigerant cycle.

In certain embodiments, the rotating element is a shaft, particularly the drive shaft of the turbo compressor. In certain embodiments, the turbo compressor is a radial (centrifugal) turbo

compressor.

Particularly, the above described turbo compressor is designed or configured to perform the process of the invention for compressing and controlling a gas mixtures comprising neon, or the process of the invention for liquefying hydrogen or helium according to the above aspects or embodiments of the invention.

In certain embodiments, the fourth chamber is separated from the ambient environment by a fourth separating element and in fluidic connection by a fourth opening in the fourth separating element with the ambient environment, wherein the rotating element extends through the fourth opening.

In the following further features and advantages of the present invention as well as preferred embodiments are described with reference to the figures, wherein Fig. 1 shows a schematic representation of one embodiment of the process of the invention; and

Fig. 2 shows a schematic representation of a turbo compressor of the

invention.

Description of embodiments

The embodiments described below relate to a process to compress and control a refrigerant gas mixture composition comprising neon in a cryogenic refrigerant cycle. In addition, the embodiments relate a process to produce liquid hydrogen or helium.

An embodiment of the process of the invention can be understood from figure 1 . Figure 1 shows an embodiment of process for compressing and controlling a refrigerant gas mixture, where the compressed gas mixture is used in a cryogenic refrigeration cycle.

Figure 1 shows a refrigeration circuit for compressing a refrigerant 21 , 22. The refrigerant circuit comprises a compression device 1 1 , a water cooling device 12, turbo expanders 13, 14, a precooling heat exchanger 17, and a cooling (main) heat exchanger 18. The precooling heat exchanger 17 is supplied with a precooling refrigerant 51 , 52. The main heat exchanger 18 is also supplied with a second refrigerant stream 31 , 32. The second refrigerant stream may comprise hydrogen, or any other suitable refrigerant.

Preferably, the compression device 1 1 is a centrifugal refrigerant compressor. A particularly advantageous embodiment of a compressor is described below with reference to Figure 2.

The feed stream to be liquefied 41 is passed firstly through the precooling heat exchanger 17, through adsorber vessels 19, 20 to form a precooled feed stream 42. The precooled feed stream 42 is then conveyed through the main cooling heat exchanger 18 to produce cooled, liquefied feed stream. In the embodiment shown, the feed stream is taken to be hydrogen. It will be understood the arrangement of Figure 1 can be used to liquefy/cool other other feed streams, for example helium.

The composition of the first process fluid 21 or the compressed process fluid 22 (i.e. the composition of the refrigerant), is monitored after compression or at any suitable point of the above mentioned refrigerant cycle. If a deviation of the refrigerant composition from a predefined value or range is present, a process parameter is adjusted. The process parameter may be the pressure or the temperature of the first process fluid 21 or the compressed first process fluid 22, or a parameter of the compressor used for the compressing. By that, the compressing efficiency of the process can be maintained, and particularly undesired effects, such as two-phase discharge at the outlet of the expanders (29, 30) may be avoided.

Likewise, the composition of the refrigerant 21 , 22 may be adjusted in case of a deviation from a predefined value. For example, this can be advantageously achieved by separating hydrogen that has leaked into the refrigerant (first process fluid 21 or the compressed first process fluid 22) from the first seal gas (second process fluid) 33 during compression of the refrigerant21 . Preferably, gas a mixture comprising mainly hydrogen and neon is used as the refrigerant (first process fluid) 21 , with a molar mass between 3.83 g per mol and 13 g per mol, particularly for a hydrogen neon mixture with a molar mass between 6.5 g per mol and 1 1.2 g per mol. Particularly, the above described mixture compositions represent an optimised solution between:

1 . increased cooling efficiency (high hydrogen fraction beneficial)

2. reduced rotational speed and number of stages for refrigerant turbo compressor and turbo expander relative to state-of-the-art machinery (high neon fraction beneficial)

3. reduced cost of refrigerant inventory loss during operation and standstills (low neon fraction beneficial). An arrangement or device for monitoring and adjusting the refrigerant gas mixture is provided. With this arrangement, the gas composition or volumetric fraction of at least one constituent of the refrigerant gas mixture (first process fluid 21 or compressed first process fluid 22) is determined in at least one location within the refrigeration process cycle. In the event of deviation of the measured volumetric fraction compared to a preset target value or preset target range, and key process parameters are modified correspondingly.

For instance, the refrigeration cycle process temperature at the cold end (outlet of coldest turbo expander 14) can be varied depending on the measured neon mixture fraction of the first process fluid 21 (can be measured, for example, at the suction of the centrifugal refrigerant compressor 1 1 or at the inlet or outlet of turbo expander 14) to avoid two-phase turbine discharge by e.g. manipulating the high/low gas pressure levels of the refrigeration cycle. Also, for instance, rotational speed(s) of the centrifugal refrigerant compressor 1 1 can be monitored and adjusted, if required, in function of the determined volumetric fraction e.g. if the hydrogen fraction is increased.

The pressure ratios at the inlet and outlet of the centrifugal refrigeration compressor 1 1 and turbo expanders 13, 14 can thus be shifted in order to adapt the operating point to the changing process gas mixture (first process fluid 21 or compressed first process fluid 22) composition. The pressure ratio of the refrigeration cycle (HP to LP) can be adapted in function of the process gas mixture (first process fluid 21 or compressed first process fluid 22), if the composition of the mixture varies significantly due to the leakage of compressor seal gas 33 (hydrogen) into the process gas mixture (first process fluid 21 or compressed first process fluid 22). Preferably, these adjustments are performed automatically. To summarize, the device for monitoring and adjusting the process of the invention or the centrifugal refrigerant compressor 1 1 serves to compensate fluctuations in the hydrogen content within the process gas mixture (first process fluid 21 or compressed first process fluid 22).

Further, the object of controlling and regulating the process gas mixture composition 21 within a defined range can be achieved by the following described process. A required high-pressure refrigerant stream 25, preferably comprising neon and hydrogen, is drawn continuously or intermittently from the refrigeration cycle at cryogenic temperature (e.g. in the range of 50 K to 26 K), for example, by opening / closing of a regulating valve or other suitable flow control device. The flow of the high-pressure refrigerant stream 25 can be controlled by measuring the neon or hydrogen refrigerant fraction (gas mixture composition) and then adjusting the flow rate through stream 25. As depicted in the figure 1 , the high-pressure refrigerant stream 25 (neon recovery stream) is taken from a high-pressure line 22 upstream of the coldest turbo expander 14.

The high-pressure refrigerant stream (neon recovery stream 25) is further cooled down in the heat exchanger 18 to a temperature below the critical point of pure neon (44.5 K), particularly below 35 K. In alternative embodiments (not shown), the high pressure refrigerant stream 25 is not cooled in the heat exchanger 18.

T the high-pressure refrigerant stream 25is expanded and cooled in a Joule-Thomson throttle valve 15 to produce an expanded stream 26. The expanded stream's 26 temperature is constrained by the gas mixture freezing point, which when neon is present, is close to the triple point of neon (24.6 K) or slightly below, depending on the hydrogen fraction.

The high-pressure refrigerant stream (recovery stream) 25 flow rate as well as the low pressure downstream of the throttle valve 15 can be adjusted depending on monitored process conditions; depending on the designed refrigerant mixture composition (hydrogen-neon), and the volume of gas (hydrogen) that has to be removed/recovered (in function of seal gas leakage into process). For instance, if a hydrogen first seal gas stream 33 of 3 Nm ³ /h (normal cubic meter per hour) is continuously leaked into the process gas (first process fluid 21 or compressed first process fluid 22) in the compressor 1 1 , a hydrogen flow 28 of 3 Nm ³ /h can be extracted from the cycle at cryogenic temperature to keep the refrigerant mixture composition 21 , 22 within a defined range. In the embodiment of Figure 1 , this is achieve by passing the high-pressure refrigerant stream (recovery stream) 25 through at least one cryogenic vapor-liquid separation stage in a hydrogen-neon mixture phase separator 16. The expanded stream 26 downstream of the throttle valve 15 enters the phase separator 16 in a two-phase flow condition. A neon-rich liquid 27 can be drawn out at the bottom of the phase separator; a hydrogen-rich vapor 28 is extracted at the top of the phase separator. As an example, 3 Nm ³ /h hydrogen are extracted from the top of the phase separator 16. For this purpose, for instance, the required mass flow of approximately of a 50 mol.%-50 mol.% hydrogen-neon refrigerant mixture 25 is separated from the process cycle in the high pressure line at e.g. 25 bar(a). The separated stream 25 is cooled to a temperature (e.g. 32 K) and is then expanded in an isenthalpic throttle valve 15 to a suitable pressure, for example below 5 bar(a) or a temperature close to or below 26 K. The hydrogen-rich vapor 28 at the top of the separator vessel 16 is composed of approximately or above 80 mol. % hydrogen and approximately or below 20 mol. % neon. The neon-rich liquid 27 at the bottom of the separator vessel 16 is composed of above 80 mol.% neon and below 20 mol.% hydrogen. The hydrogen-rich vapor stream 28, corresponding to a pure hydrogen flow of approximately 3 Nm ³ /h, can be removed from the process to balance out the refrigerant mixture composition 21 , 22 in the cycle. The hydrogen-rich vapor stream 28 can be vented or stored in a buffer tank for hydrogen recovery or used, for instance, as a source for the seal gas 33, 34.

In a further embodiment (not shown in figure 1 ), the hydrogen-rich vapor 28 can be further cooled, expanded and guided to a second liquid-vapor phase separator stage to further reduce the quantity of neon loss.

In a further embodiment (not shown in Figure 1 ), the neon-rich liquid 27 can be used to recover the expensive neon and can be, for instance, routed back directly into the low pressure line 21 of the hydrogen-neon mixture cycle or can be stored in a buffer tank for neon recovery/make up. In this manner, a substantial reduction in continuous neon inventory losses can be achieved, below a total of 1 Nm ³ /h, particularly below 0.5 Nm ³ /h, compared to the significantly higher leakage losses with conventional double seal gas systems (approximately up to 10 Nm ³ /h per compressor stage).

When nitrogen is used as external compressor seal gas 33, 34there is the potential for nitrogen leakage into the refrigerant gas mixture. In a further embodiment (not shown in Figure 1 ) a nitrogen separation can be performed using a vapour-liquid phase separator with subsequent nitrogen absorber(s) in the refrigeration cycle above the melting point of nitrogen at 63 K particularly in order to avoid freeze out of nitrogen. As described above, the compressor 1 1 that can be used for this purpose is preferably a centrifugal (radial) turbo compressor, more particularly an integrally-geared centrifugal turbo compressor. Preferably, the compressor 1 1 is designed with up to 10 compressor stages, more particularly with up to 8 compressor stages with interstage cooling after at least every second compressor stage.

The compressor 1 1 is preferably a high-speed compressor that can run at high compressor blade tip speeds, particularly up to 650 m/s. The feasible compressor stage pressure ratio is dependent on the mentioned fluid or fluid mixture composition and rotational speed: preferably stage pressure ratios of at least 1 .15, and particularly between 1 .2 and 1 .4. The compressor is used for comparatively large gas volumes and a total compressor power above 1 000 kW at coupling, particularly for larger compressors above 5Ό00 kW coupling power. An embodiment of refrigerant compressor 1 1 is depicted in figure 2.

The compressor 1 1 comprises a first chamber 68 and, in this first chamber 68 there is a compressor rotor 61 . The refrigerant compressor 1 1 includes a second chamber 69, which is separated from the first chamber 68 by a first separating element 64a having a first opening 64b. The first chamber 68 is supplied with and substantially filled with the refrigerant gas or refrigerant gas mixture 21. The refrigerant 21 may comprise:

hydrogen, neon, helium or a mixture of these, preferably a gas mixture of hydrogen and neon. Thus, with the aid of the refrigerant compressor 1 1 the refrigerant gas 21 (for example a mixture of hydrogen and neon) is compressed. The second chamber 69 is supplied and filled with a first seal gas 33. The first seal gas 33 is preferably hydrogen 33 for a hydrogen-neon compressor 1 1 . Alternatively the first seal gas may comprise hydrogen, helium, neon, nitrogen or a gas mixture of these fluids depending on the refrigerant gas 21 to be compressed. The pressure p2 in the second chamber 69 is higher than the gas pressure p1 in the first chamber 68, as a result of which a seal gas sealing is realized. In this way, it is ensured that the refrigerant gas (or refrigerant gas mixture) 21 in the first chamber 68 cannot flow through the opening 64b of the first separating element 64a. Given that only the first seal gas 33, e.g. hydrogen, from the second chamber 69 can flow through the opening 64b of the first separating element 64a into the first chamber 68, there is no leakage of the valuable refrigerant gas 21 (e.g. hydrogen-neon gas mixture 21 ) in the first chamber 68, . Therefore, only a leakage of the first seal gas 33 present in the second chamber 69 (e.g. hydrogen) into the refrigerant gas 21 occurs. This leads to a shift in the designed refrigerant gas mixture composition 21 e.g. hydrogen-neon.

This arrangement means that the refrigerant compressor 1 1 can be operated with high rotational speeds and at comparatively high efficiencies.

The refrigerant compressor 1 1 includes a third chamber 70, which is separated from the second chamber 69 by a second separating element 65a with a second opening 65b. In the third chamber 70, there is a second seal gas (a third process fluid) 34. The second seal gas 34 preferably substantially consists of nitrogen. The second seal gas 34 pressure p3 is higher than atmospheric pressure and lower than the first seal gas 33 pressure.

The compressor 1 1 also includes a fourth chamber 71 , separated from the third chamber 70 by a third separating element 66a having a third opening 66b. The fourth chamber 70 is supplied with a third seal gas 35.

If the third chamber 70 is filled with hydrogen as the second seal gas 34 (instead of nitrogen), the fourth chamber 71 is substantially filled with nitrogen as a third seal gas. It is thereby achieved that between the second chamber 69 and the environment there is still at least one further chamber 70, 71 , in which the gas can be used to realize an additional seal of the second chamber 69 and thus also of the first space or chamber 68 from the environment. Furthermore, the fourth chamber 71 is separated from the environment by a fourth separating element 67a with a fourth opening 67b. The compressor 1 1 of Figure 2 further comprises a rotating element 62, particularly a drive shaft of the compressor, wherein the rotating element 62 is connected to the compressor rotor 61. The rotating element 62 extends through the first opening 64b of the first separating element 65a and further to a second opening 65b of the second separating element 65a; through the third opening 66b of the third separating element 66a and the fourth opening 67b of the fourth separating opening 67a.

The operation of the turbo compressor 1 1 of Figure 2 will now be described with reference to exemplary process fluids. The first process fluid (refrigerant) 21 comprises or essentially consists of neon and hydrogen is compressed in the first chamber 68 to a compressed first process fluid (compressed refrigerant) 22 with a first pressure. The first chamber 68 is non- hermitically separated from the second chamber 69 which filled with a second process fluid (first seal gas) 33 comprising or essentially consisting of hydrogen. The second process fluid (first seal gas) 33 is provided with a second pressure being larger than the first pressure. By that, the second process fluid 33 acts as a seal gas, which means that the first process fluid 21 cannot leak out of the first chamber 68 during

compression, and whereby only hydrogen of the second process fluid 33 may leak into the first chamber 68 and the first process fluid 21 or the compressed first process fluid 22, respectively.

The compressed first process fluid 22 can be used as a refrigerant in a refrigerant cycle (as shown in Figure 1 ).

The second process fluid 33 is preferably sealed from the environment by the third process fluid (second seal gas) 34 comprised within a third chamber 70 that is non- hermetically separated from the second chamber 69 and in fluidic connection with the second chamber 69. The third process fluid 34 preferably comprises or consists of nitrogen with a third pressure lying between the second pressure and the ambient pressure. Accordingly, the second process fluid 33 may leak into the first process fluid 21 or the third process fluid 34 during compression of the first process fluid 21.

Alternatively, the third process fluid 34 comprises or essentially consists of hydrogen and is provided with a third pressure being larger than the second pressure. In this case, first process fluid 21 is effectively sealed by the second process fluid 33 and third process fluid 34 during compression, whereby the possibility of leakage of the first process fluid 21 can be further decreased. In this alternative, the third process fluid 34 is sealed from the environment by the fourth process fluid (third seal gas) 35 comprising or essentially consisting of nitrogen, wherein the fourth process fluid 35 is comprised within a fourth chamber 71 being non-hermetically separated from the third chamber 70 and being in fluidic connection with the third chamber 70. Particularly in this case, the second process fluid 33 may comprise a mixture of neon and hydrogen instead of hydrogen only in order to reduce the potential dilution of the refrigerant 21 by hydrogen. With the described compressor 1 1 , a process for compressing a refrigerant gas and controlling the designed refrigerant gas composition during operation can be advantageously performed, in particular for a cryogenic refrigerant cycle for hydrogen and helium liquefiers/refrigerators, particularly for a refrigerant gas involving a gas mixture comprising neon, and particularly for a hydrogen-neon mixture.

To summarize, a refrigerant gas (preferably a mixture of hydrogen and neon) is compressed in a compression device having at least one gas seal (for example the turbo compressor 1 1 ) at high-speed, and the compressed refrigerant is maintained at a consistent, desired composition. The high quality compressed refrigerant can be used as a cryogenic refrigerant, while keeping the continuous losses of expensive gas (i.e. neon) inventory to a minimum and thus allowing the economically viable use of this refrigeration process for cryogenic applications, for example, in the production of liquid hydrogen or refrigeration of helium.

It will be appreciated that the method of the invention can also be applied to the compression of refrigerant gas in other compression devices in which seal gas(es) are provided and where therefore leakage of the seal gas(es) into the refrigerant gas flow may occur. For example, the compression device may be a turbocompressor having two chambers, with a single seal gas supplied to the second chamber. Alternatively the compression device may be a turbocompressor having three chambers, with two seal gases (which may be the same or different) being supplied to the second and third chambers.

It will also be understood that this process can be applied also to other refrigerant gas mixtures comprising neon, for example, neon and helium.

Advantages: significant decrease in specific energy demand and specific costs for the production of liquid hydrogen on a large-scale compared to prior known technologies

economically viable total costs of ownership for industrial hydrogen liquefaction plant due to the use of available and cheaper seal gases e.g. hydrogen; reduced losses of expensive neon inventory; use of cost efficient large-frame, high-speed and energy efficient dry gas sealed turbo compressors instead of expensive hermetically sealed compressors;

continuous expensive neon inventory losses can be kept below a total of 0.5 Nm ³ /h, compared to higher leakage losses with conventional double seal gas Systems using process gas as seal gas (approximately up to 10 Nm ³ /h per compressor stage);

cryogenic refrigerant mixture composition can be controlled and regulated within a defined range; turbo compressor machine parameters/performance and refrigeration cycle conditions can be regulated in function of actual mixture composition within the cycle;

screw compressors have comparatively low isentropic efficiency and relatively small available frame-sizes; reciprocating compressors are energy efficient but limited in frame size by the volumetric suction flow rate, thus requiring two or more very large reciprocating compressors to run in parallel for large-scale hydrogen liquefiers above 100 tpd (tons per day);

for pressure ratios required by refrigeration cycles in hydrogen and helium liquefiers or refrigerators, turbo-compressors operated at close to ambient temperature with pure helium or hydrogen as fluid would require complex designs with multiple machines and/or a high number of stages per machine a tubo compressor provided with a fourth chamber supplied with a third seal gas provides an .

Reference Numerals centrifugal (radial) turbo compressor

water cooling

, 14 expansion turbine

Joule/Thompson valve

phase separator

precooling heat exchanger

cooling heat exchanger

, 20 adsorber vessel low pressure refrigerant stream neon/hydrogen (first process fluid) high pressure refrigerant stream (neon/hydrogen) stream (compressed first process fluid)

high pressure first partial neon/hydrogen stream

high pressure second partial neon/hydrogen stream high pressure third partial neon/hydrogen stream

low pressure third partial neon/hydrogen stream

low pressure neon rich liquid

low pressure hydrogen rich vapour

low pressure first partial neon/hydrogen stream

low pressure second partial neon/hydrogen stream high pressure second refrigerant stream (hydrogen) low pressure second refrigerant stream (hydrogen)

First seal gas or second process fluid (neon/hydrogen)

Second seal gas or third process fluid (hydrogen)

Third seal gas or fourth process fluid (nitrogen)

First seal gas (second process fluid) stream

Second seal gas (third process fluid) stream

Third seal gas (fourth process fluid) stream

hydrogen feed stream

precooled hydrogen feed stream

cooled hydrogen feed stream cold precooling refrigerant

warmed precooling refrigerant compressor rotor

rotating element

rotating axis

a first separating element

b opening of the first separating elementa second separating element

b opening of the second separating elementa third separating element

b opening of the third separating elementa fourth separating element

b opening of the fourth separating element first chamber

second chamber

third chamber

fourth chamber