Apparatus and method for transferring a fluid from a subcritical gaseous state into a supercritical state

The present invention relates to an apparatus and a method for transferring a fluid from a subcritical gaseous state into a supercritical state.

Background

Separation units that separate a specific component from a mixed fluid stream of various components oftentimes return a fluid stream of the corresponding separated component in a subcritical gaseous state, i.e. in a gaseous state with a temperature below a critical temperature T _c and with a pressure below a critical pressure p _c , wherein these critical values define the critical point. With a temperature and a pressure above the corresponding critical point the fluid would be in a supercritical state, in which distinct liquid and gaseous phases do no longer exist. Separation units of that kind can e.g. be CO ₂ separation units for separating a CO ₂ stream from a mixed gas stream, oftentimes yielding the separated CO ₂ in a fluid stream in a subcritical, gaseous state.

However, the subcritical gaseous state can oftentimes not be expedient for further use of the separated fluid, e.g. for storage or for long distance transportation e.g. in pipelines. For further use the corresponding fluid often is required to be in the supercritical state, i.e. with a temperature above the critical temperature T _c and a pressure above the critical pressure p _c . It is therefore desirable to provide a possibility for transferring a fluid from its subcritical gaseous state into its supercritical state.

Disclosure of the invention

The present invention relates to an apparatus and a method for transferring a fluid from a subcritical gaseous state into a supercritical state with the features of the independent claims. Further advantages and embodiments of the invention will become apparent from the description and the appended figures. Advantages and advantageous embodiments of the apparatus according to the present invention and the method according to the present invention shall arise from the present description in an analogous manner.

The apparatus comprises a compressor unit or compressor stage, a pump unit or pump stage, a drive unit and a liquefaction unit or liquefaction stage. The compressor unit can comprise one or several compressors. The pump unit can comprise one or several pumps. The compressor unit and the pump unit are commonly driven by the drive unit. The drive unit is thus provided as a common drive or engine coupled with both the compressor unit and the pump unit. The liquefaction unit is provided downstream of the compressor unit and upstream of the pump unit. The fluid, particularly CO ₂ , in its subcritical gaseous state is provided to the compressor unit.

The compressor unit is configured to compress the fluid from a first subcritical gaseous state, expediently from its initial state with an initial pressure level, to a first predetermined pressure level of a second subcritical gaseous state. This first predetermined pressure level is thus expediently below the corresponding critical pressure p _c of the fluid. After this compression, the fluid can e.g. have a temperature level above the corresponding critical temperature T _c . The correspondingly compressed fluid in its second subcritical gaseous state is then transported to the liquefaction unit.

The liquefaction unit is configured to reduce the temperature of the correspondingly compressed fluid in the second subcritical gaseous state downstream of the compressor unit to a predetermined temperature level such that the fluid is transferred from the second subcritical gaseous state to a liquid state. The temperature of the fluid is thus reduced below a liquefaction temperature or condensation temperature at the first pressure level. In the course of this liquefaction process, the fluid is particularly kept at the first predetermined pressure level or at least essentially at the first predetermined pressure level. Before this liquefaction process, the fluid can for example have a temperature level above the critical temperature T _c of the corresponding fluid. The predetermined temperature level after the liquefaction can expediently be below the corresponding critical temperature T _c . The correspondingly liquefied fluid is then transported to the pump unit. The pump unit is configured to compress the correspondingly liquefied fluid in the liquid state downstream of the liquefaction unit to a second predetermined pressure level such that the fluid is transferred from the liquid state to the supercritical state. The pressure level is thus particularly increased above the critical pressure p _c . The temperature level is expediently increased above the critical temperature T _c . The fluid in its supercritical state can then be provided downstream of the pump unit for further use.

To characterise pressures and temperatures, the present application uses the terms "pressure level" and "temperature level", which are intended to signify that corresponding pressures and temperatures do not necessarily have to be used as exact pressure and temperature values. However, such pressures and temperatures are typically within particular ranges which lie, for example ±1%, 5%, 10%, 20% or even 50% around an average. In this respect, corresponding pressure levels and temperature levels can lie within disjoint ranges or within overlapping ranges. In particular, for example pressure levels include pressure losses which are unavoidable or which are to be expected. The same applies accordingly to temperature levels. Pressure levels which are given here in bar are absolute pressures.

The present invention provides a single, highly integrated apparatus for compressing the subcritical fluid into its supercritical state in an effective, space saving, flexible manner with low footprint and low costs. In particular, commonly driving the compression unit and the pump unit with the same drive unit yields the possibility to consolidate corresponding compression steps and pumping steps into one single, integrated apparatus. It is therefore particularly not necessary to provide separate machines, especially separate pumps and compressors, independently driven by different engines. The present apparatus combines the compressor unit and pump unit into a common unit, thus reducing complexity and number of components and conserving energy and required space. The apparatus therefore requires little plot space and can easily be installed. The apparatus further allows to flexibly integrate the liquefaction process or liquefaction stage, thus increasing effectiveness and flexibility of the process of transferring the fluid into its supercritical state. The flexibility and the reduced plot space allows for the apparatus to be applied to a wide range of inlet and outlet fluid pressures and temperatures. Combining or integrating the compression stage and pumping stage allows for an optimization of machinery technology. Fluid compression by means of the compression unit allows to pre-compress the fluid given its first subcritical gaseous starting state. Pumping by means of the pump unit is an efficient and simple way to further compress the pre-compressed fluid to the required process pressure for the supercritical state. Pre-compressing the subcritical fluid, liquefying the pre-compressed fluid by means of the liquefaction unit, and then compressing the liquefied fluid to a pressure level above the critical pressure p _c provides an energetically efficient way to transfer the fluid into its supercritical state.

Advantageously, the liquefaction unit comprises or is provided as a cooling unit configured to perform liquefaction of the fluid by means of air and/or cooling water and/or refrigerated or chilled water and/or an external refrigeration medium. Depending on the site conditions of the apparatus location, e.g. in cold regions, ambient cooling can be performed, i.e. ambient air or water can be used to cool the fluid below its liquefaction temperature. In hot regions, refrigerated water or refrigeration medium can be used to cool down the fluid.

According to a preferred embodiment, the liquefaction unit is further configured to perform a process cooling of an external process unit. Thus, the liquefaction unit is particularly not only used for the liquefaction of the fluid in the present apparatus, but also for cooling process streams in another, external process unit, e.g. an LNG facility or plant. This external process unit can e.g. be a nearby apparatus of the same provider or operator as the present apparatus. This multiple use of the liquefaction unit is particularly beneficial in plants where a large refrigeration capacity is already available and the equipment is already installed.

According to a preferred embodiment, the liquefaction unit is provided as an open loop refrigerant system comprising a heat exchanging unit and a precooling unit. This open loop refrigerant system is preferably configured to extract a portion of the fluid from the compressor unit, for example upstream of the compressor unit or downstream of the compressor unit or from in-between the compressor unit itself. This extracted portion of the fluid is provided to the precooling unit in order to precool the extracted portion of the fluid. This precooling unit can preferably comprise an ambient (air or water) cooling unit and/or a valve unit, particularly for a cascade Joule-Thomson expansion. The precooled extracted portion of the fluid is provided as a refrigerant to the heat exchanging unit. -Further, the compressed fluid in the second subcritical gaseous state downstream of the compressor unit is provided to the heat exchanging unit. The temperature of the compressed fluid in the second subcritical gaseous state is reduced to the predetermined temperature level by means of the heat exchanging unit. In the course of the corresponding heat exchange, the temperature of the extracted portion of the fluid is increased again. Downstream of the heat exchanging unit, the extracted portion of the fluid is returned to the compressor unit. Therefore, the liquefaction unit can be provided as an open loop refrigerant system using the process fluid (e.g. CO ₂ ) as a refrigerant and using the compressor portion of the apparatus in combination with ambient (air or water) cooling and a cascade Joule-Thomson valve expansion of a side stream of the process fluid to provide the necessary refrigeration for the liquefaction. This will come at an energetic cost but will provide significant benefits in terms of installed equipment count and plot space if alternative refrigeration sources are not readily available.

Preferably, the liquefaction unit comprises or is provided as a hydrocarbon refrigerant unit configured to perform liquefaction of the fluid by means of a hydrocarbon refrigerant, especially using a cold hydrocarbon stream as a refrigerant. Preferably, a C3 refrigerant can be used, particularly propane C ₃ H ₈ or a fluid mixture comprising propane. Alternatively or additionally, a C4 refrigerant can preferably be used, particularly butane C4H10 or a fluid mixture comprising butane. A hydrocarbon refrigerant unit of that kind can expediently be used for process cooling of an external process unit. For example, corresponding C3 or C4 refrigeration compressors can also be used for an LNG facility, utilising a cold hydrocarbon side stream as a refrigerant.

Advantageously, the liquefaction unit comprises or is provided as a coldbox. A coldbox or packaged unit is an assembly of various cryogenic components in a steel containment. For example, interconnecting piping, vessels, valves and instrumentation can be included in a packaged unit of that kind, expediently filled with insulation material, e.g. perlite. The coldbox can e.g. further be utilised for a wide range of applications for the treatment of cryogenic fluids and gases, e.g. for external process units like separation plants, liquefaction plants, chemical and petrochemical plants, etc. According to a preferred embodiment, the drive unit is coupled to or connected with the compressor unit and the pump unit via a gearbox. The gearbox particularly distributes mechanical energy produced by the drive unit to the compressor unit and the pump unit. A driven shaft or output shaft of the drive unit is particularly coupled to the gearbox and the gearbox is particular coupled with both the compressor unit and the pump unit. Pinions of the pump unit and the compressor unit can expediently be integrated in this gearbox, driven by the drive unit.

According to a preferred embodiment, the drive unit comprises two output shafts or driven shafts. The drive unit is therefore expediently provided as a double-end drive unit. A first output shaft or driven shaft of the drive unit is preferably coupled with the compressor unit and a second output shaft or driven shaft of the drive unit is preferably coupled with the pump unit. Each output shaft can be coupled directly with the corresponding unit or via an individual gearbox. The output shafts can also be directly coupled with a common gearbox and this common gearbox can directly be coupled with each of the compressor unit and the pump unit.

Preferably, the drive unit is provided as an electric motor, a steam turbine, an expansion turbine, a hydraulic power recovery turbine (HPRT), a gas turbine or a combination thereof. The drive unit thus allows to realise high-pressure stages with pump casings and impellers.

Advantageously, the compressor unit is configured to perform centrifugal compression of the fluid in multiple stages with interstage cooling. The fluid is therefore compressed in a multitude of compressing steps, wherein after each compressing step a cooling step is performed in which the temperature of the compressed fluid is reduced, since each compression step particularly increases the temperature of the fluid. Centrifugal compressing with interstage cooling of that kind provides an expedient way to compress the fluid, depending on the starting pressure and temperature levels of its subcritical state, to the second pressure level and to prepare the fluid for subsequent liquefaction.

Advantageously, the pump unit is provided as a centrifugal pump. The pump unit is particularly configured to achieve an end pressure of the fluid required for subsequent use of the supercritical fluid. The centrifugal pump can expediently be provided without interstage cooling, especially since a temperature increase way above the critical temperature T _c can be desired. Thus, an overall efficiency of the apparatus can be increased. Costs and space requirements can be reduced.

According to an advantageous embodiment, the apparatus is provided as or used as a part of a heat recovery unit or waste heat recovery unit. For this purpose, the apparatus is preferably configured to use heat or waste heat to heat up the fluid in the supercritical state and expand it to a liquefaction pressure level. The apparatus can expediently be used to drive a refrigeration cycle in a corresponding refrigeration cycle unit, e.g. a Rankine cycle or an organic Rankine cycle. In a Rankine cycle, heat is supplied to a working fluid thereby bringing the working fluid in its gaseous state, which then drives a turbine. Afterwards, the working fluid is condensed back into its liquid state in the course of which waste heat is produced. This waste heat can partially be recovered by the present apparatus.

According to a preferred embodiment, the first predetermined pressure level is in a range between 50 bar and 70 bar, preferably between 55 bar and 65 bar. The subcritical fluid is thus expediently compressed from its initial pressure level to a medium pressure. The first predetermined pressure level can especially be in range between 60% and 95% of the critical pressure p _c , particularly between 75% and 90% of the critical pressure p _c .

According to a preferred embodiment, the predetermined temperature level is in a range between 10°C and 30°C, preferably between 15°C and 25°C. Expediently, the predetermined temperature level is below the liquefaction temperature at the first pressure level. Particularly, the liquefaction unit is configured to achieve a liquid state of the fluid with the highest possible temperature.

According to a preferred embodiment, the second predetermined pressure level lies above 100 bar, preferably above 125 bar, preferably above 150 bar. The second pressure level can especially be predetermined in dependence of an end pressure required for subsequent further use of the supercritical fluid.

According to a particularly advantageous embodiment, the fluid is CO2 or a fluid mixture comprising CO ₂ . In particular, the fluid is dry CO ₂ of high purity, especially >99% C0 ₂ . The fluid can expediently be an azeotropic mixture of CO ₂ and ethane C ₂ H ₆ . This fluid mixture can particularly address the topic of CO ₂ freeze in the case of rapid depressurization, further adding to the stable and reliable operation of the process.

Preferably, the fluid in the first subcritical gaseous state is provided or produced by a separation unit separating a specific component from a fluid mixture of various components. The apparatus can flexibly be adjusted to a variety of different input conditions of the subcritical, gaseous fluid. Expediently, the apparatus can therefore flexibly be provided downstream of different kinds of separation units. For example, the corresponding separation unit can use chemical or mechanical absorption, chemical or mechanical adsorption, pressure swing adsorption, temperature swing adsorption, membranes, cryogenic distillation, or a combination thereof to return a fluid stream of the separated component in the subcritical gaseous state. Since the subcritical gaseous state may not be expedient for further use of the separated fluid, the apparatus particularly provides the supercritical fluid with output conditions needed for further use.

Advantageously, the fluid in the supercritical state is used or provided for long distance transportation, pipeline transportation, storage, Enhanced Oil Recovery (EOR), sequestration or underground sequestration, or a combination thereof. For example, in the course of Enhanced Oil Recovery, crude oil can be extracted from oil fields by injecting the supercritical CO ₂ . For example, in the course of sequestration, the supercritical CO ₂ can be stored in carbon pools e.g. using subsurface saline aquifers, reservoirs, ocean water, aging oil fields, etc.

It should be noted that the previously mentioned features and the features to be further described in the following are usable not only in the respectively indicated combination, but also in further combinations or taken alone, without departing from the scope of the present invention.

The present invention will now be described further, by way of example, with reference to the accompanying drawings, in which Fig. 1 schematically shows preferred embodiments of an apparatus according to the present invention.

Fig. 2 schematically shows a pressure-temperature-diagram representing a transfer of CO ₂ from a subcritical, gaseous state into a supercritical state according to a preferred embodiment of the present invention.

Fig. 3 schematically shows a pressure-temperature-diagram representing a transfer of CO ₂ from a subcritical, gaseous state into a supercritical state according to a preferred embodiment of the present invention.

Fig. 4 schematically shows another preferred embodiment of an apparatus according to the present invention.

Detailed description

Fig. 1a schematically shows a preferred embodiment of an apparatus 100 according to the present invention for transferring, transforming or converting a fluid in a subcritical gaseous state into a supercritical state.

The subcritical gaseous fluid, particularly dry CO ₂ of high purity, can be provided by a separation unit 101 for separating a specific component from a fluid mixture of various components.

Supercritical CO ₂ can be further processed in a CO ₂ utilising unit 102, e.g. for long distance transportation, pipeline transportation, storage, Enhanced Oil Recovery, sequestration, or a combination thereof.

For example, the CO ₂ can be provided by the separation unit 101 in an initial or first subcritical gaseous state with a pressure p ₀ of ca. 5 bar and a temperature To of ca. 35°C. The critical pressure p _c of CO ₂ is 73.3 bar and the critical temperature T _c is 304.1 K or 31 °C.

In order to compress the CO ₂ from this first subcritical gaseous state to its supercritical state, the apparatus 100 comprises a compressor unit or compressor stage 110, e.g. a centrifugal compressor. This compressor unit 110 is configured to compress the fluid from its first subcritical gaseous state and from its initial pressure level po to a first predetermined pressure level pi of a second subcritical gaseous state, i.e. such that fluid remains in a subcritical gaseous state. For example, this first predetermined pressure level pi can be in the range between 55 bar and 65 bar. After this compression, the fluid can have a temperature level Ti above its critical temperature.

A liquefaction unit or liquefaction stage 120 is provided downstream of the compression unit 110 and is configured to reduce the temperature of the compressed fluid in the second subcritical gaseous state to a predetermined temperature level T ₂ such that the fluid is transferred from the second subcritical gaseous state to a liquid state. For example, this predetermined temperature level T ₂ can be in a range between 15°C and 25°C. The pressure of the fluid during and after liquefaction can remain at least essentially at the first predetermined pressure level pi.

The liquefaction unit 120 can for comprise a cooling unit configured for cooling and liquefying the fluid by means of air and/or cooling water and/or refrigerated water and/or an external refrigeration medium.

The liquefaction unit 120 can also be used for cooling purposes in an external process unit, e.g. for cooling process streams in a nearby LNG facility.

The liquefaction unit 120 can further comprise a hydrocarbon refrigerant unit configured to perform liquefaction of the fluid by means of a hydrocarbon refrigerant, e.g. a C3 refrigerant like propane C ₃ H ₈ and/or a C4 refrigerant like butane C4H10. For example, a C3 or C4 refrigeration of that kind can also be used for nearby LNG facilities.

It is also possible, that the liquefaction unit 120 comprises a coldbox, e.g. an assembly of various cryogenic components in a steel containment, which can further be used for the treatment of cryogenic fluids and gases in external process units.

A pump unit 130, e.g. a centrifugal pump, is provided downstream of the liquefaction unit 120 and is configured to compress the fluid in the liquid state to a second predetermined pressure level p ₂ such that the fluid is transferred from the liquid state to the supercritical state. This second predetermined pressure level p ₂ can e.g. be above 150 bar.

A common drive unit 140 is provided for commonly driving both the compressor unit 110 and the pump unit 130, e.g. indirectly via a common gearbox 150. For this purpose, an output shaft or driven shaft 141 of the drive unit 140 is directly connected or coupled with the gearbox 150 and the gearbox 150 is directly connected or coupled with both the compressor unit 110 and the pump unit 130. The drive unit 140 can e.g. be provided as an electric motor, a steam turbine, an expansion turbine, a hydraulic power recovery turbine (HPRT), a gas turbine or a combination thereof.

It is also possible that the drive unit is directly coupled with both the compressor unit 110 and the pump unit 130 as shall now be explained with reference to Fig. 1 b.

In Fig. 1 b, a preferred embodiment of an apparatus according to the present invention is schematically shown and referred to as 100'. In Fig. 1a and 1 b, identical reference signs refer to identical elements or to elements of at least the same function.

As shown in Fig. 1b, a corresponding drive unit 140' is provided as a double-end drive unit with two driven shafts or output shafts 141', 142'. A first output shaft or driven shaft 141' of this drive unit 140' is coupled with the compressor unit 110 and a second output shaft or driven shaft 142' of the drive unit 140' is coupled with the pump unit 130.

Fig. 2 schematically shows a pressure-temperature-diagram 200 representing the transfer of CO ₂ from the subcritical, gaseous state into the supercritical state by means of the apparatus 100 of Fig. 1a or the apparatus 100' of Fig. 1 b

Depending on its pressure and temperature, CO ₂ can exist in its solid state in a solid phase range 201 , in the liquid state in a liquid phase range 202, and in the gaseous phase in a gaseous state range 203. A pressure p _t of 5.2 bar and a temperature T _t of 216.6K or -56.6°C represents the triple point of CO ₂ , at which the gaseous state, the liquid state, and the solid state exist in thermodynamic equilibrium. In a supercritical range 204, with pressures above the critical pressure p _c of CO ₂ of 73.3 bar and with temperatures above the critical temperature T _c of 304.1 K or 31 °C, distinct liquid and gaseous phases do not exist anymore. Point 210 represents the CO2 in its first subcritical gaseous state as provided by the separation unit 101 with the initial pressure p ₀ of ca. 5 bar and the initial temperature To of ca. 35°C.

Arrow 215 represents the step of compressing the CO ₂ in its first subcritical gaseous state 210 by means of the compressor unit 110 from its initial pressure p ₀ to the first predetermined pressure level pi of e.g. 65 bar.

Point 220 represents the CO ₂ in the second subcritical gaseous state after compression step 215, still in its subcritical gaseous state, with the first predetermined pressure level pi of e.g. 65 bar. For example, after the compression, the CO ₂ can have a temperature level T1 of e.g. 40°C.

Arrow 225 represents the step of reducing the temperature of the CO ₂ in the second subcritical gaseous state 220 from the temperature level T1 to the predetermined temperature level T ₂ of e.g. 25°C, such that the fluid is transferred from the second subcritical gaseous state to a liquid state.

Point 230 represents the CO ₂ in its liquid state after this liquefaction step 225. In the course of this liquefaction, the temperature of the CO ₂ is reduced from the temperature level T1 to the predetermined temperature level T ₂ of e.g. 25°C.

Arrow 235 represents the step of compressing the CO ₂ in the liquid state 230 by means of the pump unit 130 to the second predetermined pressure level p ₂ such that the fluid is transferred to the supercritical state.

Point 240 represents the CO ₂ after this compression step 235 in its supercritical state with the second predetermined pressure level p ₂ of e.g. 150 bar.

The compression of the initially provided CO ₂ in its first subcritical gaseous state by means of the compressor unit 110 can also be performed by means of a centrifugal compression in multiple stages with interstage cooling, as shall now be explained with reference to Fig. 3. Fig. 3 also schematically shows a pressure-temperature-diagram 300 representing the transfer of CO2 from the subcritical, gaseous state into the supercritical state by means of the apparatus 100 of Fig. 1a or the apparatus 100' of Fig. 1 b.

Fig. 3 shows the liquid phase range 302, the gaseous phase range 303 and the supercritical range 304 of CO ₂ . Point 310 represents the CO ₂ in the first subcritical gaseous state provided by the separation unit 101 , in this example with an initial pressure of 5 bar and an initial temperature of 40°C.

The arrows 315 represent the step of compressing the CO ₂ in the first subcritical gaseous state 310 by means of a centrifugal compression in multiple stages with interstage cooling from the initial pressure to the first predetermined pressure level pi of e.g. 65 bar. As can be seen, the CO ₂ is compressed in a multitude of compressing steps, wherein after each compressing step a cooling step is performed in which the temperature of the compressed fluid is reduced, since each compression step particularly increases the temperature of the fluid.

Point 320 represents the CO ₂ in its second subcritical gaseous state after this compression step 315 with the first predetermined pressure level pi and with a temperature level than can e.g. correspond to the critical temperature T _c .

Arrow 325 represents the step of reducing the temperature of the CO ₂ in the second subcritical gaseous state 320 to the predetermined temperature level T ₂ of e.g. 15°C, such that the fluid is transferred from the second subcritical gaseous state to a liquid state.

Point 330 represents the CO ₂ in its liquid state after this liquefaction step 325 with the predetermined temperature level T ₂ of 15°C and with the first predetermined pressure level pi of 65 bar.

Arrow 335 represents the step of compressing the CO ₂ in the liquid state 330 to the second predetermined pressure level p ₂ such that the fluid is transferred to the supercritical state. In this example, the second predetermined pressure level p ₂ corresponds to 200 bar. Point 340 represents the CO ₂ after this compression step 335 in the supercritical state.

Fig. 4 schematically shows another preferred embodiment of an apparatus 400 according to the present invention.

In accordance with the apparatus 100 and 100' as shown in Fig. 1a and Fig. 1 b, the apparatus 400 of Fig. 4 is configured to transfer a fluid, e.g. CO ₂ , from a subcritical gaseous state provided by a separation unit 401 into a supercritical state for a CO ₂ utilising unit 402. For this purpose, the apparatus 400 comprises a compressor unit 410, a liquefaction unit 420, a pump unit 430 and a common drive unit 440.

The compressor unit 410 comprises several individual compressors 411 , 413, 416 and heat exchangers 414, 417 for performing the centrifugal compression in multiple stages with interstage cooling and for compressing the fluid from its first subcritical gaseous state to its second subcritical gaseous state. The compressors 411 , 413, 416 and the pump unit 430 are commonly driven by the drive unit 440. Further, a control element 418 can be provided.

The liquefaction unit 420 is provided as an open loop refrigerant system with a heat exchanging unit 422 and a precooling unit 421 , wherein this precooling unit 421 is provided as a valve unit, particularly for a cascade Joule-Thomson expansion.

The open loop refrigerant system 420 is configured to extract a portion of the fluid from the compressor unit 410, particularly downstream of the compressor unit 410 by means of a manifold 419. This extracted portion of the fluid is provided to the precooling unit 421 in order to precool the extracted portion of the fluid by means of a cascade Joule- Thomson expansion. This precooled extracted portion of the fluid is provided as a refrigerant to the heat exchanging unit 422. Further, the compressed fluid in the second subcritical gaseous state downstream of the compressor unit 410 is provided to the heat exchanging unit 422. The temperature of the compressed fluid in the second subcritical gaseous state is reduced to the predetermined temperature level by means of the heat exchanging unit 422, thereby transferring the fluid from the second subcritical gaseous state to its liquid state. In the course of the corresponding heat exchange, the temperature of the extracted portion of the fluid is increased again. Downstream of the heat exchanging unit 422, the extracted portion of the fluid is returned to the compressor unit 410, for example to a manifold 412 between the compressors 411 , 413. A control unit 423 can be provided, e.g. for measuring a temperature and flowrate of the returning portion of the fluid and for controlling the amount of fluid extracted downstream of the compressor unit 410 by means of the manifold 419.

The fluid in its liquid state downstream of the heat exchanger unit 422 is provided to the pump unit 430 via a scrubber or absorber 424 for absorbing gas components in the stream of liquid fluid. The pump unit 430 compresses the fluid in the liquid state to the second predetermined pressure level such that the fluid is transferred to the supercritical state.

Reference list

100 apparatus for transferring a subcritical gaseous fluid into a supercritical state 100' apparatus for transferring a subcritical gaseous fluid into a supercritical state

101 separation unit

102 CO ₂ utilising unit

110 compressor unit

120 liquefaction unit

130 pump unit

140 drive unit

141 output shaft of the drive unit 140

140' drive unit, double-end drive unit

141' first output shaft of the double-end drive unit 140'

142' second output shaft of the double-end drive unit 140'

150 gearbox

200 pressure-temperature-diagram of CO ₂

201 solid phase range

202 liquid phase range

203 gaseous phase range

204 supercritical range

210 CO ₂ in a first subcritical gaseous state with an initial pressure p ₀ and an initial temperature To

215 compressing the CO ₂ to a first predetermined pressure level pi

220 CO ₂ in a second subcritical gaseous state with the first predetermined pressure level pi

225 reducing the temperature of the CO ₂ to a predetermined temperature level T ₂

230 CO ₂ in a liquid state with the predetermined temperature level T ₂

235 compressing the CO ₂ to a second predetermined pressure level p ₂

240 CO ₂ in a supercritical state with a second predetermined pressure level p ₂

300 pressure-temperature-diagram of CO ₂

302 liquid phase range

303 gaseous phase range

304 supercritical range 310 CO ₂ in a first subcritical gaseous state with an initial pressure and an initial temperature

315 compressing the CO ₂ in multiple stages with interstage cooling

320 CO ₂ in a second subcritical gaseous state with a first predetermined pressure level pi

325 reducing the temperature of the CO ₂ to a predetermined temperature level T ₂

330 CO ₂ in a liquid state with the predetermined temperature level T ₂

335 compressing the C0 ₂ to a second predetermined pressure level p ₂

340 CO ₂ in a supercritical state with a second predetermined pressure level p ₂

400 apparatus for transferring a subcritical gaseous fluid into a supercritical state

401 separation unit

402 CO ₂ utilising unit

410 compressor unit

411 compressor

412 manifold

413 compressor

414 heat exchanger

415 manifold

416 compressor

417 heat exchanger

418 control element

419 manifold

420 liquefaction unit

421 precooling unit, valve unit

422 heat exchanger unit

423 control element

424 scrubber

430 pump unit

440 drive unit