EP2483615B1

EP2483615B1 - Method of fractionating a hydrocarbon stream and an apparatus therefor

Info

Publication number: EP2483615B1
Application number: EP10759659.5A
Authority: EP
Inventors: Thor Frette; Peter Hicks; David Bertil Runbalk; Peter Marie Paulus
Original assignee: Shell Internationale Research Maatschappij BV
Current assignee: Shell Internationale Research Maatschappij BV
Priority date: 2009-09-30
Filing date: 2010-09-29
Publication date: 2019-01-23
Anticipated expiration: 2030-09-29
Also published as: BR112012007167B1; AP3423A; EP2483615A2; BR112012007167A2; KR20120081602A; CN102893108A; CN102893108B; AP2012006144A0; WO2011039279A3; AU2010302667B2; AU2010302667A1; CY1121417T1; WO2011039279A2

Description

The present invention provides a method and apparatus for fractionating a hydrocarbon stream, said stream comprising at least a first hydrocarbon component, to provide at least a first hydrocarbon component reservoir stream.
The hydrocarbon stream may be obtained by extraction from a hydrocarbon feed stream. Natural gas is a common hydrocarbon feed stream. The first hydrocarbon component may constitute ethane.
Natural gas is a useful fuel source, as well as being a source of various hydrocarbon compounds. It is often desirable to liquefy natural gas in a liquefied natural gas (LNG) plant at or near the source of a natural gas stream for a number of reasons. As an example, natural gas can be stored and transported over long distances more readily as a liquid than in gaseous form because it occupies a smaller volume and does not need to be stored at high pressure.
It has been proposed for a number of years to liquefy natural gas off-shore in an off-shore plant. This could be a plant on a floating structure such as a floating vessel. Such concepts are advantageous because they provide off-shore alternatives to on-shore liquefaction plants. These structures can be moored off the coast, or close to or at a gas field, in waters deep enough to allow off-loading of the LNG product onto a carrier vessel. They also represent movable assets, which can be relocated to a new site when the gas field is nearing the end of its productive life, or when required by economic, environmental or political conditions.
U.S. Patent No. 4,504,296 discloses a process for pre-cooling, liquefying and subcooling a methane-rich feed stream, such as natural gas, with two closed circuit multicomponent refrigerant cycles. Refrigerant component make-up can be produced during the process. Ethane, propane and higher alkyl condensates can be removed from the feed stream by distillation separation. Ethane, propane and butanes can be stored for use in the first pre-cooling and second main multicomponent refrigerant compositions. Methane make-up for the second main cooling refrigerant composition can be drawn from the warm bundle of a coil wound main liquefying heat exchanger.
Publication US 2003/0177786 shows a de-ethanizer tower in a method of producing liquefied natural gas. The de-ethanizer tower is operated at about 24.8 barg. The de-ethanizer overhead vapour comprising about 99% ehtane and about 1% propane is condensed substantially completely by cooling with propane. A portion of this liquid overhead stream is returned as reflux to the top of the de-ethanizer tower. Moreover, a pure ethane stream is produced with can be reinjected into the LNG product or sold as a separate product. It can also be used as a component in a mixed refrigerant process.
Refrigerant component make-up constituents are normally individually stored in tanks on site at or close to ambient temperature under high pressure, such as pressures in the range of 10 to 20 bara. As and when required, the refrigerant component make-up can be passed from the pressurised storage tanks to the closed multicomponent refrigerant cycles to replace any refrigerant deficit.
These refrigerant component make-up constituents may be flammable hydrocarbons, such that any leakage of the hydrocarbon vapours can present a fire and/or explosion hazard. Such hazards are particularly pertinent in an off-shore facility typically as a result of space-constraints.
In a first aspect, the present invention provides a method of fractionating a hydrocarbon stream comprising at least a first hydrocarbon component to provide at least a first hydrocarbon component reservoir stream, said method comprising at least the steps of:

providing a hydrocarbon stream comprising at least a first hydrocarbon component;
separating the hydrocarbon stream in a first fractionation device to provide an overhead first hydrocarbon component stream and a first hydrocarbon component depleted bottoms stream;
cooling the overhead first hydrocarbon component stream to provide a liquefied first hydrocarbon component stream;
drawing a part of the liquefied first hydrocarbon component stream to provide a liquefied first hydrocarbon component reservoir feed stream;
cooling the liquefied first hydrocarbon component reservoir stream against a refrigerant stream to provide a cooled liquefied first hydrocarbon component reservoir stream and a warmed refrigerant stream;
reducing the pressure of the cooled liquefied first hydrocarbon component reservoir stream to provide a reduced pressure liquid first hydrocarbon component reservoir stream;
storing the reduced pressure liquid first hydrocarbon component reservoir stream in a liquid first hydrocarbon component reservoir at a first hydrocarbon component storage pressure of at most just above atmospheric pressure for use as a first refrigerant component make-up constituent for in at least one refrigerant circuit
passing a first hydrocarbon component supply stream from the liquid first hydrocarbon component reservoir to the at least one refrigerant circuit, wherein the step of passing the first hydrocarbon component supply stream from the first liquid hydrocarbon component reservoir to the at least one refrigerant circuit comprises: - heating the first hydrocarbon component supply stream to provide a warmed first hydrocarbon component stream; - passing the warmed first hydrocarbon component stream to the at least one refrigerant circuit.

In a further aspect, the present invention provides an apparatus for fractionating a hydrocarbon stream comprising at least a first hydrocarbon component to provide at least a first hydrocarbon component reservoir stream, said apparatus comprising at least:

a first fractionation device to separate a first hydrocarbon component from a hydrocarbon stream to provide an overhead first hydrocarbon component stream and a first hydrocarbon component depleted bottoms stream;
a first hydrocarbon component heat exchanger to cool the overhead first hydrocarbon component stream to provide an at least partially liquefied first hydrocarbon component stream;
a first splitting arrangement to split-off a liquefied first hydrocarbon component reservoir feed stream from the at least partially liquefied first hydrocarbon component stream;
a first hydrocarbon component reservoir heat exchanger to cool the liquefied first hydrocarbon component reservoir stream against a refrigerant stream to provide a cooled liquefied first hydrocarbon component reservoir stream and a warmed refrigerant stream;
a first hydrocarbon component reservoir pressure reduction device to reduce the pressure of the cooled liquefied first hydrocarbon component reservoir stream to provide a reduced pressure liquid first hydrocarbon component reservoir stream at a first hydrocarbon component storage pressure of at most just above atmospheric pressure;
a liquid first hydrocarbon component reservoir in fluid communication with the liquid first hydrocarbon component reservoir stream;
at least one refrigerant circuit;
a make-up system arranged to allow fluid communication between the at least one refrigerant circuit and the liquid first hydrocarbon component reservoir
wherein the make-up system comprises a first hydrocarbon component supply stream heat exchanger arranged between the at least one refrigerant circuit and the liquid first hydrocarbon component reservoir arranged to supply heat to the first hydrocarbon component to provide a warmed first hydrocarbon component stream upstream of the at least one refrigerant circuit.

Embodiments of the present invention will now be described by way of example only and with reference to the accompanying non-limited drawings in which:
Figures 1 and 2 schematically illustrate an apparatus and method for fractionating a hydrocarbon stream; Figure 3 schematically illustrates an alternative scheme for passing a cooled second fraction main refrigerant side stream through a first hydrocarbon component reservoir heat exchanger; and Figure 4 schematically illustrates a single hydrocarbon component supply stream heat exchanger that may be used in the embodiment of Figure 2.
Figures 1 and 2 may be viewed as complementary to each other: Figure 1 highlights details of the hydrocarbon product streams, while Figure 2 highlights details of refrigerant circuits.
For the purpose of this description, a single reference number will be assigned to a line as well as a stream carried in that line.
Embodiments of the present invention described herein include a method and apparatus for fractionating a hydrocarbon stream, which provides at least a liquid first hydrocarbon component reservoir stream at a first hydrocarbon component storage pressure of at most just above atmospheric pressure for use as a first refrigerant component make-up constituent.
The meaning of at most just above atmospheric pressure in the context of the present specification depends on the circumstances of the application, but in any case pressures from atmospheric pressure to about 1 bar above atmospheric pressure (i.e. from zero to about 1 barg) are considered to fall within the meaning of just above atmospheric pressure. Expressed in terms of absolute pressure values, it could mean a pressure of less than about 2 bara, or of less than 2 bara.
The potential hazard associated with hydrocarbon vapour leakage is reduced by storing the first and optionally any further hydrocarbon components in a reservoir as a liquid phase at a pressure of at most just above atmospheric pressure, such as at a pressure of less than 2 bara, preferably in the range of from 1 bara upto but not including 2 bara, more preferably in the range of from 1.0 bara to 1.2 bara, and still more preferably in the range of from 1.0 bara to 1.1 bara.
The method and apparatus are especially suitable for use on or in an off-shore hydrocarbon processing facility, such as for instance an offshore natural gas liquefaction plant on a floating structure, such as on a floating vessel. In such an off-shore setting space is generally limited and extra safety precautions with regard to fire and/or explosion resulting from the ignition of flammable hydrocarbon vapour may be required.
In a preferred embodiment, wherein the hydrocarbon stream further comprises a second hydrocarbon component, said method may further comprise the steps of:

separating the first hydrocarbon component depleted bottoms stream in a second fractionation device to provide an overhead second hydrocarbon component stream and a second hydrocarbon component depleted bottoms stream;
cooling the overhead second hydrocarbon component stream to provide a liquefied second hydrocarbon component stream;
drawing a part of the liquefied second hydrocarbon component stream to provide a liquefied second hydrocarbon component reservoir stream;
cooling the liquefied second hydrocarbon component reservoir stream against a refrigerant stream to provide a cooled liquefied second hydrocarbon component reservoir stream and a warmed refrigerant stream;
reducing the pressure of the cooled liquefied second hydrocarbon component reservoir stream to provide a reduced pressure liquid second hydrocarbon component reservoir stream;
storing the reduced pressure liquid second hydrocarbon component reservoir stream in a liquid second hydrocarbon component reservoir at a second hydrocarbon component storage pressure of less than 2 bar for use as a second refrigerant component make-up constituent.

In a similar manner, where the hydrocarbon stream further comprises a third hydrocarbon component, said method may optionally further comprise the steps of:

separating the second hydrocarbon component depleted bottoms stream in a third fractionation device to provide an overhead third hydrocarbon component stream and a third hydrocarbon component depleted bottoms stream;
cooling the overhead third hydrocarbon component stream to provide a liquefied third hydrocarbon component stream;
drawing a part of the liquefied third hydrocarbon component stream to provide a liquefied third hydrocarbon component reservoir stream;
cooling the liquefied third hydrocarbon component reservoir stream against a refrigerant stream to provide a cooled liquefied third hydrocarbon component reservoir stream and a warmed refrigerant stream;
reducing the pressure of the cooled liquefied third hydrocarbon component reservoir stream to provide a reduced pressure liquid third hydrocarbon component reservoir stream;
storing the reduced pressure liquid third hydrocarbon component reservoir stream in a liquid third hydrocarbon component reservoir at a third hydrocarbon component storage pressure of less than 2 bara for use as a third refrigerant component make-up consitutent.

In a preferred aspect, the refrigerant stream or streams for the cooling of the first and any further liquefied hydrocarbon component reservoir streams is drawn from the at least one refrigerant circuit. Such an integration with the at least one refrigerant circuit dispenses with the need to provide dedicated refrigerant circuits to cool the hydrocarbon component reservoir stream or streams, saving CAPEX. For instance, cooling duty can be provided to the first and any further refrigerant component reservoir streams from a pre-cooling or main refrigerant circuit use for liquefying a hydrocarbon stream, e.g. a natural gas stream to produce LNG.
In one group of embodiments, the at least one refrigerant circuit comprises a pre-cooling refrigerant circuit and/or a main cooling refrigerant circuit.
The main cooling refrigerant circuit may comprise a main cooling refrigerant in the form of a main mixed refrigerant. In an embodiment, the refrigerant stream that provides cooling duty to the first hydrocarbon component reservoir heat exchanger to cool the liquefied first hydrocarbon component reservoir stream is derived from the main mixed refrigerant or a fraction thereof. The cooling duty may be provided at a temperature below ambient to cool the liquefied first hydrocarbon component reservoir stream.
In a further embodiment, the pre-cooling refrigerant circuit may comprise a pre-cooling refrigerant. It is particularly advantageous when the pre-cooling refrigerant is a mixed refrigerant composition, because the pre-cooling mixed refrigerant can be provided at different temperatures and pressures to match the heating curve of the pre-cooling mixed refrigerant with the cooling curve of the hydrocarbon component to be cooled, increasing the efficiency of the process.
For instance, if the method and apparatus disclosed herein further provides second and/or third hydrocarbon component reservoir streams that have a higher specific density than the first hydrocarbon component reservoir stream, these can be cooled against the pre-cooling mixed refrigerant which is typically better optimized for extracting heat at a higher temperature than the main cooling refrigerant.
In another aspect, the cooling of streams other than the hydrocarbon component reservoir streams can be provided by the at least one refrigerant circuit.
In one embodiment, the at least one refrigerant circuit may comprise a pre-cooling refrigerant circuit comprising a pre-cooling mixed refrigerant. The overhead first hydrocarbon component stream may be cooled by exchanging heat against the pre-cooling mixed refrigerant or a fraction thereof at a temperature below ambient, to provide the liquefied first hydrocarbon component stream. The pre-cooling mixed refrigerant in the pre-cooling mixed refrigerant circuit may be available at a plurality of pressures, in which case a suitable pressure level may be selected for this heat exchanging depending on the cooling curve.
The apparatus comprises a make-up system arranged to allow fluid communication between the at least one refrigerant circuit and the liquid first hydrocarbon component reservoir. When the stored reduced pressure liquid first hydrocarbon component reservoir stream needs to be used as the first refrigerant component make-up constituent, it may be passed via the make-up system from the liquid first hydrocarbon component reservoir to the at least one refrigerant circuit in the form of a first hydrocarbon component supply stream.
The make-up system may comprise one or more hydrocarbon component supply stream heat exchangers, preferably at least one per hydrocarbon component supply stream. The passing of the first hydrocarbon component supply stream from the first liquid hydrocarbon component reservoir to at least one refrigerant circuit may comprises the steps of:

heating the first hydrocarbon component stream, in such a hydrocarbon component supply stream heat exchanger, to provide a warmed first hydrocarbon component stream;
passing the warmed first hydrocarbon component stream, optionally via a warmed first hydrocarbon component stream control valve, to the at least one refrigerant circuit.

A similar method can be used to pass any second and/or third refrigerant component make-up streams to the at least one refrigerant circuit.
In a preferred embodiment, the warmed hydrocarbon component streams are liquid streams. These can be used as liquid refrigerant component make-up for a mixed refrigerant composition. Large quantities of liquid hydrocarbon components for a refrigerant composition may be required as a result of the decompression of a part of the refrigerant circuit, such after the trip of the refrigerant compressor. The fine tuning of the refrigerant composition can be achieved using vapour overhead from the first and any second and/or third hydrocarbon component gas/liquid separators.
Figure 1 is a diagrammatic scheme of an apparatus 1 for fractionating a hydrocarbon stream 200. The hydrocarbon stream 200 comprises at least a first hydrocarbon component. Preferably the hydrocarbon stream 200 comprises first, second, third, fourth and further hydrocarbon components. The fourth hydrocarbon component may have a higher formula weight and therefore boiling point than the third hydrocarbon component, which may have a higher formula weight and therefore boiling point than the second hydrocarbon component, which may have a higher formula weight and therefore boiling point than the first hydrocarbon component.
In a preferred embodiment the hydrocarbon stream 200 may comprise one or more of the hydrocarbon components defined as follows:

first component: ethane;
second component: propane;
third component: butane; and
fourth component: condensate.

The hydrocarbon stream 200 is preferably extracted from natural gas obtained from natural gas or petroleum reservoirs, but may alternatively be obtained from another source, including a synthetic source such as a Fischer-Tropsch process. The hydrocarbon stream 200 may be the result of pre-treatment, e.g. in a pre-cooling and extraction unit, as discussed in greater detail below.
The hydrocarbon stream 200 may be a pressured stream, having a pressure greater than 2 bara. The hydrocarbon stream 200 can be passed to a first fractionation device 205, in which it is separated to provide an overhead first hydrocarbon component stream 210 and a first hydrocarbon component depleted bottoms stream 300. In one embodiment the first hydrocarbon component may be ethane and the first fractionation device 205 may be a deethanizer such that the overhead first hydrocarbon component stream 210 is an ethane enriched stream and the first hydrocarbon component depleted bottoms stream 300 is an ethane depleted stream.
The overhead first hydrocarbon component stream 210 can be passed to a first hydrocarbon component heat exchanger 215, in which it is cooled to provide an at least partially liquefied first hydrocarbon component stream 220. In a preferred embodiment, the overhead first hydrocarbon component stream 210 is cooled against a pre-cooling mixed refrigerant stream, or a fraction thereof, circulating in a pre-cooling mixed refrigerant circuit. For simplicity, the pre-cooling mixed refrigerant circuit is not shown in Figure 1, but this is discussed in greater detail in relation to the embodiment of Figure 2.
The at least partially liquefied first hydrocarbon component stream 220 can be passed to a first hydrocarbon component gas/liquid separator 225 to provide a liquefied first hydrocarbon component stream 230a, as a bottoms stream, and an overhead first hydrocarbon component vapour stream 227. The overhead first hydrocarbon component vapour stream 227 may be used as fuel gas, or as vapourous first refrigerant component make-up in a mixed refrigerant circuit.
It will be apparent that adjusting the cooling duty of the first hydrocarbon component heat exchanger 215 will vary the amount of overhead first hydrocarbon component vapour stream 227 produced. For instance, if the cooling duty is lowered, more overhead first hydrocarbon component vapour stream 227 and less liquefied first hydrocarbon component stream 230a will be produced. The latter can be carried out when vapourous first refrigerant component make-up is required.
The liquefied first hydrocarbon component stream 230a can be passed to an optional first hydrocarbon component pump 235, to provide the (pumped) liquefied first hydrocarbon component stream 230b to a first hydrocarbon component splitting device 245. The first hydrocarbon component splitting device 245 can separate the (pumped) liquefied first hydrocarbon component stream 230b into two parts, a liquefied first hydrocarbon component reflux stream 240a and a liquefied first hydrocarbon component reservoir stream 250.
The liquefied first hydrocarbon component reflux stream 240a can be passed through a first hydrocarbon component reflux pressure reduction device 255, such as a Joule-Thomson valve, to provide an expanded first hydrocarbon component reflux stream 240b. The expanded first hydrocarbon component reflux stream 240b can be passed to the first fractionation device 205 to improve separation therein. Preferably, the expanded first hydrocarbon component reflux stream 240b is added to the first fractionation device 205 at a point gravitationally higher than the hydrocarbon stream 200.
The liquefied first hydrocarbon component reservoir stream 250 is then passed to a first hydrocarbon component reservoir heat exchanger 265, in which it is heat exchanged against a refrigerant stream. The first hydrocarbon component reservoir heat exchanger 265 provides a cooled liquefied first hydrocarbon component reservoir stream 260 and a warmed refrigerant stream. In a preferred embodiment, the refrigerant stream 2160 is a main mixed refrigerant or a fraction thereof from a main cooling refrigerant circuit. For simplicity, the main cooling refrigerant circuit is not shown in Figure 1, but is discussed in greater detail in relation to the embodiment of Figure 2.
The cooled liquefied first hydrocarbon component reservoir stream 260 may be a pressurised stream having a pressure greater than 2 bara. Prior to storage, the cooled liquefied first hydrocarbon component reservoir stream 260 is passed to a first hydrocarbon component reservoir pressure reduction device 275, such as a Joule-Thomson valve, to provide a reduced pressure liquid first hydrocarbon component reservoir stream 270. The first hydrocarbon component reservoir pressure reduction device 275 preferably reduces the pressure of the cooled liquefied first hydrocarbon component reservoir stream 260 to approximately the storage pressure of less than 2 bara.
It will be apparent that the cooling of the liquefied first hydrocarbon component reservoir stream 250 in the first hydrocarbon component reservoir heat exchanger 265 is important to reduce the temperature of the stream 250 to such an extent that the first hydrocarbon component will remain substantially in the liquid phase upon expansion in first hydrocarbon component reservoir pressure reduction device 275. In practice this means that the cooled liquefied first hydrocarbon component reservoir stream 260 is in sub-cooled condition.
The reduced pressure liquid first hydrocarbon component reservoir stream 270 is then passed to a liquid first hydrocarbon reservoir 285 for storage at a pressure of less than 2 bara. The liquid first hydrocarbon reservoir 285 may be a cryogenic storage tank. Preferably, the storage pressure is in the range of 30-70 mbar above atmospheric pressure (i.e. in the range of 30-70 mbarg). In one example, the storage pressure is about 50 mbarg. When the first hydrocarbon component is ethane, the storage temperature will be below -89 °C to minimise vaporisation of the liquid.
Any boil off gas from the liquid first hydrocarbon reservoir 285 may be removed as first hydrocarbon component boil off gas stream 290. The first hydrocarbon component boil off gas stream 290 can passed to at least one refrigerant circuit as vapourous first refrigerant component make-up for a mixed refrigerant composition, after optional heating, if required.
A first hydrocarbon component supply stream 280, which is a liquid stream, may be drawn from the liquid first hydrocarbon reservoir 285 (optionally assisted by a first submerged pump provided in the first hydrocarbon reservoir 285) and passed to at least one refrigerant circuit, preferably in liquid form, as first refrigerant component make-up for a mixed refrigerant composition. An optional transfer pump 283 may be provided in the first hydrocarbon component supply stream 280. This is discussed in greater detail in relation to the embodiment of Figure 2.
Returning to the first fractionator 205, the first hydrocarbon depleted bottoms stream 300 may be a pressured stream, having a pressure greater than 2 bara. The first hydrocarbon depleted bottoms stream 300 is depleted in the first hydrocarbon component and rich in at least second and preferably third, fourth and further hydrocarbon components.
The first hydrocarbon depleted bottoms stream 300 can be passed to a second fractionation device 305, in which it is separated to provide an overhead second hydrocarbon component stream 310 and a second hydrocarbon component depleted bottoms stream 400. In one embodiment the second hydrocarbon component may be propane and the second fractionation device 305 may be a depropanizer such that the overhead second hydrocarbon component stream 310 is a propane enriched stream and the second hydrocarbon component depleted bottoms stream 400 is a propane (and ethane) depleted stream.
The overhead second hydrocarbon component stream 310 can be passed to a second hydrocarbon component heat exchanger 315, in which it is cooled to provide an at least partially liquefied second hydrocarbon component stream 320. Cooling duty can be provided to the second hydrocarbon component heat exchanger 315 by cooling water, such as seawater.
The at least partially liquefied second hydrocarbon component stream 320 can be passed to a second hydrocarbon component gas/liquid separator 325 to provide a liquefied second hydrocarbon component stream 330a, as a bottoms stream, and an overhead second hydrocarbon component vapour stream 327. The overhead second hydrocarbon component vapour stream 327 may be used as fuel gas, or as vapour make-up of the second hydrocarbon component in a mixed refrigerant circuit.
The liquefied second hydrocarbon component stream 330a can be passed to an optional second hydrocarbon component pump 335, to provide the (pumped) liquefied second hydrocarbon component stream 330b to a second hydrocarbon component splitting device 345. The second hydrocarbon component splitting device 345 can separate the (pumped) liquefied second hydrocarbon component stream 330b into two parts, a liquefied second hydrocarbon component reflux stream 340a and a liquefied second hydrocarbon component reservoir stream 350.
The liquefied second hydrocarbon component reflux stream 340a can be passed through a second hydrocarbon component reflux pressure reduction device 355, such as a Joule-Thomson valve, to provide an expanded second hydrocarbon component reflux stream 340b. The expanded second hydrocarbon component reflux stream 340b can be passed to the second fractionation device 305 to improve separation therein. Preferably, the expanded second hydrocarbon component reflux stream 340b is added to the second fractionation device 305 at a point gravitationally higher than the first hydrocarbon component depleted bottoms stream 300.
The liquefied second hydrocarbon component reservoir stream 350 is then passed to a second hydrocarbon component reservoir heat exchanger 365, in which it is heat exchanged against a refrigerant stream. The second hydrocarbon component reservoir heat exchanger 365 provides a cooled liquefied second hydrocarbon component reservoir stream 360 and a warmed refrigerant stream. In a preferred embodiment, the refrigerant stream is a pre-cooling mixed refrigerant or a fraction thereof from a pre-cooling refrigerant circuit. Preferably, the second hydrocarbon component reservoir stream 350 is heat exchanged against the pre-cooling mixed refrigerant or a fraction thereof at a temperature below ambient temperature. For simplicity, the pre-cooling refrigerant circuit is not shown in Figure 1, but is discussed in greater detail in relation to the embodiment of Figure 2.
The cooled liquefied second hydrocarbon component reservoir stream 360 may be a pressurised stream having a pressure greater than 2 bara. Prior to storage, the cooled liquefied second hydrocarbon component reservoir stream 360 can be passed to a second hydrocarbon component reservoir pressure reduction device 375, such as a Joule-Thomson valve, to provide a reduced pressure liquid second hydrocarbon component reservoir stream 370. The second hydrocarbon component reservoir pressure reduction device 375 preferably reduces the pressure of the cooled liquefied second hydrocarbon component reservoir stream 360 to approximately the storage pressure of less than 2 bara.
It will be apparent that the cooling of the liquefied second hydrocarbon component reservoir stream 350 in the second hydrocarbon component reservoir heat exchanger 365 is important to reduce the temperature of the stream 350 to such an extent that the second hydrocarbon component will remain substantially in the liquid phase upon expansion in second hydrocarbon component reservoir pressure reduction device 375. In practice this means that the cooled liquefied second hydrocarbon component reservoir stream 360 is in sub-cooled condition.
The reduced pressure liquid second hydrocarbon component reservoir stream 370 can then be passed to a liquid second hydrocarbon reservoir 385 for storage at a pressure of less than 2 bara. The liquid second hydrocarbon reservoir 385 may be a cryogenic storage tank. Preferably, the storage pressure is in the range of 30-70 mbar above atmospheric pressure. In one example, the storage pressure is about 50 mbarg. When the second hydrocarbon component is propane, the storage temperature will be below -43 °C to minimise vaporisation of the liquid.
Any boil off gas from the liquid second hydrocarbon reservoir 385 may be removed as second hydrocarbon component boil off gas stream 390. The second hydrocarbon component boil off gas stream 390 can passed to at least one refrigerant circuit as vapourous second refrigerant component make-up for a mixed refrigerant composition, after optional heating, if required.
A second hydrocarbon component supply stream 380, which is a liquid stream, may be drawn from the liquid second hydrocarbon reservoir 385 (optionally assisted by a second submerged pump provided in the second hydrocarbon reservoir 385) and passed to at least one refrigerant circuit, preferably in liquid form, as second refrigerant component make-up for a mixed refrigerant composition. An optional transfer pump 383 may be provided in the second hydrocarbon component supply stream 380. This is discussed in greater detail in relation to the embodiment of Figure 2.
Returning to the second fractionator 305, the second hydrocarbon depleted bottoms stream 400 may be a pressured stream, having a pressure greater than 2 bara. The second hydrocarbon depleted bottoms stream 400 is depleted in the first and second hydrocarbon components and rich in at least third and preferably fourth and further hydrocarbon components.
The second hydrocarbon depleted bottoms stream 400 can be passed to a third fractionation device 405, in which it is separated to provide an overhead third hydrocarbon component stream 410 and a third hydrocarbon component depleted bottoms stream 500. In one embodiment the third hydrocarbon component may be butane and the third fractionation device 405 may be a debutanizer such that the overhead third hydrocarbon component stream 410 is a butane enriched stream and the third hydrocarbon component depleted bottoms stream 500 is a butane (propane and ethane) depleted stream.
The third hydrocarbon component depleted bottoms stream 500, which can be a liquid stream, is passed to a third hydrocarbon component bottoms heat exchanger 510, in which it is cooled to provide a cooled liquid third hydrocarbon component depleted stream 520. The third hydrocarbon component depleted bottoms stream 500 can be cooled against a chilled cooling water stream in the third hydrocarbon component bottoms heat exchanger 510.
The cooled liquid third hydrocarbon component depleted stream 520 can then be passed to a third hydrocarbon component depleted reservoir pressure reduction device 525, such as a Joule-Thomson valve, to provide a reduced pressure liquid third hydrocarbon component depleted reservoir stream 530. The third hydrocarbon component depleted reservoir pressure reduction device 525 preferably reduces the pressure of the cooled liquid third hydrocarbon component depleted stream 520 to approximately the storage pressure of less than 2 bara.
The reduced pressure liquid third hydrocarbon component depleted stream 530 can then be passed to a liquid third hydrocarbon component depleted reservoir 535 for storage at a pressure of less than 2 bara. The liquid third hydrocarbon component depleted reservoir 535 may be a storage tank. Preferably, the storage pressure is in the range of 30-70 mbar above atmospheric pressure. The liquid third hydrocarbon component depleted stream may be hydrocarbon condensate, such that the liquid third hydrocarbon component depleted reservoir 535 may be a condensate reservoir.
The overhead third hydrocarbon component stream 410 can be passed to a third hydrocarbon component heat exchanger 415, in which it is cooled to provide an at least partially liquefied third hydrocarbon component stream 420. Cooling duty can be provided to the third hydrocarbon component heat exchanger 415 by cooling water, such as seawater.
The at least partially liquefied third hydrocarbon component stream 420 can be passed to a third hydrocarbon component gas/liquid separator 425 to provide a liquefied third hydrocarbon component stream 430a, as a bottoms stream, and an overhead third hydrocarbon component vapour stream 427. The overhead third hydrocarbon component vapour stream 427 may be used as fuel gas.
The liquefied third hydrocarbon component stream 430a can be passed to an optional third hydrocarbon component pump 435, to provide a (pumped) liquefied third hydrocarbon component stream 430b to a third hydrocarbon component splitting device 445. The third hydrocarbon component splitting device 445 can separate the (pumped) liquefied third hydrocarbon component stream 430b into two parts, a liquefied third hydrocarbon component reflux stream 440a and a liquefied third hydrocarbon component reservoir stream 450.
The liquefied third hydrocarbon component reflux stream 440a can be passed through a third hydrocarbon component reflux pressure reduction device 455, such as a Joule-Thomson valve, to provide an expanded third hydrocarbon component reflux stream 440b. The expanded third hydrocarbon component reflux stream 440b can be passed to the third fractionation device 405 to improve separation therein. Preferably, the expanded third hydrocarbon component reflux stream 440b is added to the third fractionation device 405 at a point gravitationally higher than the second hydrocarbon component depleted bottoms stream 400.
The liquefied third hydrocarbon component reservoir stream 450 is then passed to a third hydrocarbon component reservoir heat exchanger 465, in which it is heat exchanged against a refrigerant stream. The third hydrocarbon component reservoir heat exchanger 465 provides a cooled liquefied third hydrocarbon component reservoir stream 460 and a warmed refrigerant stream. In a preferred embodiment, the refrigerant stream is a pre-cooling mixed refrigerant or a fraction thereof from a pre-cooling refrigerant circuit. Preferably, the third hydrocarbon component reservoir stream 450 is heat exchanged against the pre-cooling mixed refrigerant or the fraction thereof at a temperature below ambient temperature. For simplicity, the pre-cooling refrigerant circuit is not shown in Figure 1, but is discussed in greater detail in relation to the embodiment of Figure 2.
The cooled liquefied third hydrocarbon component reservoir stream 460 may be a pressurised stream having a pressure greater than 2 bara. Prior to storage, the cooled liquefied third hydrocarbon component reservoir stream 460 can be passed to a third hydrocarbon component reservoir pressure reduction device 475, such as a Joule-Thomson valve, to provide a reduced pressure liquid third hydrocarbon component reservoir stream 470. The third hydrocarbon component reservoir pressure reduction device 475 preferably reduces the pressure of the cooled liquefied third hydrocarbon component reservoir stream 460 to approximately the storage pressure of less than 2 bara.
It will be apparent that the cooling of the liquefied second hydrocarbon component reservoir stream 450 in the third hydrocarbon component reservoir heat exchanger 465 is important to reduce the temperature of the stream 450 to such an extent that the third hydrocarbon component will remain substantially in the liquid phase upon expansion in third hydrocarbon component reservoir pressure reduction device 475. In practice this means that the cooled liquefied third hydrocarbon component reservoir stream 460 is in sub-cooled condition.
The reduced pressure liquid third hydrocarbon component reservoir stream 470 can then be passed to a liquid third hydrocarbon reservoir 485 for storage at a pressure of less than 2 bara. The liquid third hydrocarbon reservoir 485 may be a cryogenic storage tank. Preferably, the storage pressure is in the range of 30-70 mbar above atmospheric pressure. In one example, the storage pressure is about 50 mbarg. When the third hydrocarbon component is butane, the storage temperature will be below 0 °C to minimise vaporisation of the liquid.
A third hydrocarbon component supply stream 480 may be drawn from the liquid third hydrocarbon component reservoir 485 (optionally assisted by a third submerged pump provided in the third hydrocarbon reservoir 485) and passed to at least one refrigerant circuit (1000, 2000) as third refrigerant component make-up supply stream, preferably in liquid form. An optional transfer pump 483 may be provided in the third hydrocarbon component supply stream 480.
In a preferred embodiment, the method and apparatus disclosed herein can be utilised as part of a cooling, preferably liquefaction, process for a hydrocarbon feed stream 40. Figure 1 further provides a diagrammatic scheme of an apparatus 1 for treating and cooling, preferably liquefying, a hydrocarbon feed stream 40. The hydrocarbon stream 200 is prepared from the hydrocarbon feed stream 40 in a pre-cooling and extraction unit 10. Moreover, the pre-cooling and extraction unit 10 produces a pre-cooled methane-enriched stream 170 from the hydrocarbon feed stream 40. The pre-cooled methane-enriched stream 170 is subsequently liquefied in at least one main heat exchanger 175 to provide an at least partially, preferably fully, liquefied hydrocarbon stream 180.
There are many configurations known in the art for such a pre-cooling and extraction unit 10 that can prepare the hydrocarbon stream 200, for instance in the form of a natural gas liquids stream, and the pre-cooled methane enriched stream, and generally such a unit involves at least a separating step and a cooling step. One such configuration is shown in more detailed in Figure 1 as an example. It will be further described hereinbelow.
The hydrocarbon feed stream 40 may be any suitable gas stream to be cooled and liquefied, but is usually a natural gas stream. Usually a natural gas stream is a hydrocarbon composition comprised substantially of methane. Preferably the hydrocarbon feed stream 40 comprises at least 50 mol% methane, more preferably at least 80 mol% methane.
Hydrocarbon compositions such as natural gas may also contain non-hydrocarbons such as H₂O, N₂, CO₂, Hg, H₂S and other sulphur compounds, and the like. If desired, the natural gas may be pre-treated before cooling and any liquefying. This pre-treatment may comprise reduction and/or removal of undesired components such as CO₂ and H₂S or other steps such as early cooling, pre-pressurizing or the like. As these steps are well known to the person skilled in the art, their mechanisms are not further discussed here.
Thus, the term "hydrocarbon feed stream" 40 may also include a composition prior to any treatment, such treatment including scrubbing, as well as any composition having been partly, substantially or wholly treated for the reduction and/or removal of at least one compound or substance, including but not limited to sulphur, sulphur compounds, carbon dioxide, water and Hg.
Depending on the source, natural gas may contain varying amounts of hydrocarbons heavier than methane such as in particular ethane, propane and butanes, and possibly lesser amounts of pentanes and aromatic hydrocarbons. The composition varies depending upon the type and location of the gas.
Conventionally, the hydrocarbons heavier than methane are removed to various extents from the hydrocarbon feed stream prior to any significant cooling for several reasons, such as having different freezing or liquefaction temperatures that may cause them to block parts of a methane liquefaction plant or to provide a desired specification for the liquefied product. C₂+ hydrocarbons can be separated from, or their content reduced in a hydrocarbon feed stream by a demethanizer, which will provide an overhead hydrocarbon stream which is a methane enriched stream and a bottoms methane-lean stream comprising the C₂+ hydrocarbons.
The bottoms methane-lean stream comprising the C₂+ hydrocarbons is a preferred hydrocarbon stream 200 as used herein. The bottoms methane-lean stream is passed to further separators as discussed above to provide the individual hydrocarbon components and condensate.
After separation, the methane enriched stream is cooled. The methane enriched stream is passed against at least one refrigerant stream in at least one refrigerant circuit. Such a refrigerant circuit can comprise at least one refrigerant compressor to compress an at least partly evaporated refrigerant stream to provide a compressed refrigerant stream. The compressed refrigerant stream can then be cooled in a cooler, such as an air or water cooler, to provide the refrigerant stream. The refrigerant compressors can be driven by at least one turbine or electric motor.
The cooling of the methane enriched stream can be carried out in at least one stage. Initial cooling, also called pre-cooling or auxiliary cooling, can be carried out using a pre-cooling refrigerant, such as a single or mixed refrigerant, of a pre-cooling refrigerant circuit, in at least one pre-cooling heat exchanger, to provide a pre-cooled methane enriched stream. The pre-cooled methane enriched stream is preferably partially liquefied, such as at a temperature below 0°C.
Preferably, such pre-cooling heat exchangers could comprise a pre-cooling stage, with any subsequent cooling being carried out in at least one main heat exchanger to liquefy a fraction of the pre-cooled methane enriched stream in at least one main and/or sub-cooling cooling stages.
In this way, two or more cooling stages may be involved, each stage having at least one step, part etc.. For example, each cooling stage may comprise one to five heat exchangers. The or a fraction of the methane enriched hydrocarbon and/or the refrigerant may not pass through all, and/or all the same, heat exchangers of a cooling stage.
In one embodiment, the hydrocarbon may be cooled and liquefied in a method comprising two or three cooling stages. A pre-cooling stage is preferably intended to reduce the temperature of a methane enriched stream to below 0°C, usually in the range -20°C to -70°C.
Heat exchangers for use as the two or more pre-cooling heat exchangers are well known in the art. The pre-cooling heat exchangers are preferably shell and tube heat exchangers.
A main cooling stage is preferably separate from the pre-cooling stage. That is, the main cooling stage comprises at least one separate main heat exchanger. The main cooling stage is preferably intended to reduce the temperature of a hydrocarbon, usually at least a fraction of a methane enriched stream cooled by a pre-cooling stage, to below -100°C.
At least one of any of the main heat exchangers is preferably a spool wound heat exchanger or a shell and tube heat exchanger. Optionally, the main heat exchanger could comprise more than two cooling sections within its shell, and each cooling section could be considered as a cooling stage or as a separate 'heat exchanger' to the other cooling locations.
In another embodiment, one or both of the pre-cooling refrigerant stream and any main refrigerant stream can be passed through at least one heat exchanger, preferably two or more of the pre-cooling and main heat, to provide cooled refrigerant streams.
If the refrigerant is a mixed refrigerant in a mixed refrigerant circuit, such as one or both of any pre-cooling refrigerant circuit and any main refrigerant circuit, the mixed refrigerant may be formed from a mixture of two or more components selected from the group comprising: nitrogen, methane, ethane, ethylene, propane, propylene, butanes, pentanes, etc. At least one other refrigerant may be used, in separate or overlapping refrigerant circuits or other cooling circuits.
Any pre-cooling refrigerant circuit may comprise a mixed pre-cooling refrigerant. Any main cooling refrigerant circuit may comprise a mixed main refrigerant. A mixed refrigerant or a mixed refrigerant stream as referred to herein comprises at least 5 mol% of two different components. More preferably, the mixed refrigerant comprises two or more of the group comprising: nitrogen, methane, ethane, ethylene, propane, propylene, butanes and pentanes.
A common composition for a pre-cooling mixed refrigerant can be:

Methane (C1) 0-20 mol%

Ethane (C2) 5-80 mol%

Propane (C3) 5-80 mol%

Butanes (C4) 0-15 mol%

The total composition comprises 100 mol%.
A common composition for a main cooling mixed refrigerant can be:

Nitrogen 0-25 mol%

Methane (C1) 20-70 mol%

Ethane (C2) 30-70 mol%

Propane (C3) 0-30 mol%

Butanes (C4) 0-15 mol%

The total composition comprises 100 mol%.
Preferably, the cooled, preferably liquefied, methane enriched stream described herein can be stored in at least one storage tanks.
In a further preferred embodiment, if the hydrocarbon feed stream 40 is derived from natural gas, the cooled, preferably liquefied, methane enriched stream may be a LNG stream.
A number of methods of treating and liquefying hydrocarbon streams are known in the art. Figure 1 provides one such exemplary method.
A hydrocarbon feed stream 40 is provided, such as a stream derived from natural gas. The hydrocarbon feed stream 40 preferably comprises methane and at least the first and optionally second and third hydrocarbon components, such as ethane, propane and butane, discussed above. The hydrocarbon feed stream 40 is preferably in a form suitable for cooling, such that it may have been pre-treated to reduce and/or remove undesired components such as CO₂ and H₂S.
The hydrocarbon feed stream 40 is preferably a pressurised stream which can be passed to the pre-cooling and extraction unit 10. In the example as shown in Figure 1, the hydrocarbon feed stream 40 is first passed to a hydrocarbon feed separator 75 that is included in the pre-cooling and extraction unit. The hydrocarbon feed separator may be any type of gas/liquid separator. The hydrocarbon feed separator 75 provides an overhead hydrocarbon feed vapour stream 80 and a hydrocarbon feed liquid bottoms stream 90.
The hydrocarbon feed vapour stream 80 can be expanded in an overhead hydrocarbon vapour stream expansion device 95, such as a turbo expander, to provide an expanded overhead hydrocarbon feed stream 100. The expanded overhead hydrocarbon feed stream 100 can be passed to a feed fractionation device 115, such as a scrub column or demethanizer, to provide a methane enriched overhead stream 120 and a hydrocarbon stream 200. The hydrocarbon stream 200 is further fractionated according to the method disclosed herein and discussed above.
The hydrocarbon feed liquid bottoms stream 90 from the hydrocarbon feed separator 75 can be expanded in a bottoms feed stream expansion device 105, such as a Joule-Thomson valve, to provide an expanded bottoms hydrocarbon feed stream 110. The expanded bottoms hydrocarbon feed stream 110 can be passed to the feed fractionation device 115 to improve the separation of the hydrocarbon components therein. It is preferred that the expanded bottoms hydrocarbon feed stream 90 is passed to the feed fractionation device 115 at a point gravitationally lower than the expanded overhead hydrocarbon feed stream 100.
The methane enriched overhead stream 120 from the feed fractionation device 115 can be passed to at least one methane enriched stream compressors 125, 135. In the embodiment shown in Figure 1, a first methane enriched stream compressor 125 is provided which is mechanically driven by the overhead hydrocarbon vapour stream expansion device 95 via shaft 97. The first methane enriched stream compressor 125 provides a first compressed methane enriched overhead stream 130. The first compressed methane enriched overhead stream 130 can then be compressed by a second methane enriched stream compressor 135 mechanically driven by a methane enriched stream compressor driver 137. The methane enriched stream compressor driver 137 may be selected from a gas turbine, a steam turbine and an electric motor.
The second methane enriched stream compressor 135 provides a methane enriched stream 140. The methane enriched stream 140 can be passed to at least one pre-cooling heat exchanger 145, 155, 165, in which it is cooled against a pre-cooling refrigerant. The pre-cooling refrigerant may be a mixed pre-cooling refrigerant. Figure 1 shows first, second and third pre-cooling heat exchangers 145, 155, 165 respectively, providing a first cooled methane enriched stream 150, second cooled methane enriched stream 160 and pre-cooled methane enriched stream respectively. It is preferred that the plurality of pre-cooling heat exchangers 145, 155, 165 are used with a mixed pre-cooling refrigerant which can be provided at a different pressure in each pre-cooling heat exchanger 145, 155, 165 as discussed in relation to the embodiment of Figure 2.
The at least one pre-cooling heat exchanger 145, 155, 165 ultimately provide a pre-cooled methane enriched hydrocarbon stream 170. The pre-cooled methane enriched hydrocarbon stream 170 can be passed to a main heat exchanger 175 for cooling and preferably liquefaction. The main heat exchanger 175 may be a shell and tube or a spool wound heat exchanger.
The pre-cooled methane enriched hydrocarbon stream 170 can be cooled and preferably liquefied in the main heat exchanger 175 against a main refrigerant in a main refrigerant circuit, to provide an at least partially, preferably fully, liquefied hydrocarbon stream 180, such as LNG.
In an alternative embodiment, in an alternative pre-cooling and extraction unit that is not detailed in Figure 1, the hydrocarbon feed stream can be pre-cooled against a pre-cooling refrigerant prior to passing it for separation in a scrub column. The scrub column provides a methane enriched overhead stream and the hydrocarbon stream which can be fractionated according to the method and apparatus disclosed herein. The methane enriched overhead stream can be passed to an overhead stream heat exchanger in which it is cooled to provide a cooled methane enriched over head stream. The cooled methane enriched overhead stream can then be passed to an overhead stream accumulator which provides a methane enriched accumulator overhead stream and an overhead stream accumulator bottoms stream which can be returned to the scrub column as reflux. The methane enriched accumulator overhead stream can be passed to the main heat exchanger for cooling and preferably liquefaction against a main cooling refrigerant to provide an at least partially, preferably fully, liquefied hydrocarbon stream 180.
Figure 1 does not show exemplary pre-cooling and main cooling refrigerant circuits which can be used in the apparatus and method described herein. Figure 2 provides a diagrammatic scheme of an apparatus for cooling and preferably liquefying a methane enriched stream 140a showing an exemplary pre-cooling refrigerant circuit 1000 comprising a mixed pre-cooling refrigerant and an exemplary main cooling refrigerant circuit 2000 comprising a mixed main cooling refrigerant.
The methane enriched stream 140a can be provided by compressing a methane enriched overhead stream from a feed fractionation device as discussed above. The methane enriched stream 140a can be passed to a first pre-cooling heat exchanger 145a. The first pre-cooling heat exchanger 145a may be a high pressure pre-cooling heat exchanger 145a. The methane enriched stream 140a is cooled and indirectly heat exchanged with a mixed pre-cooling refrigerant evaporating at a high pressure in the shell side of the high pressure pre-cooling heat exchanger 145a. It is preferred that the methane enriched stream 140a is partly condensed in the high pressure pre-cooling heat exchanger 145a.
The cooled, preferably partly condensed, hydrocarbon exits the high pressure pre-cooling heat exchanger 145a as a first cooled methane enriched stream 150a. The operation of the pre-cooling refrigerant, which is in a pre-cooling refrigerant circuit 1000, is discussed in greater detail below.
The first cooled methane enriched stream 150a can be passed to a second pre-cooling heat exchanger 155a. The second pre-cooling heat exchanger 155a may be an intermediate pressure pre-cooling heat exchanger 155a. The first cooled methane enriched stream 150a is cooled and indirectly heat exchanged with a mixed pre-cooling refrigerant operating at an intermediate pressure in the shell side of the intermediate pressure pre-cooling heat exchanger 155a. It is preferred, if the methane enriched stream 140a is not partly condensed in the high pressure pre-cooling heat exchanger 145a, that the first cooled methane enriched stream 150a is partly condensed in the intermediate pressure pre-cooling heat exchanger 155a.
The cooled, preferably partly condensed, hydrocarbon exits the intermediate pressure heat exchanger 155a as a second cooled methane enriched stream 160a. The second cooled methane enriched stream, 160a can be passed to a third pre-cooling heat exchanger 165a. The third pre-cooling heat exchanger 165a may be a low pressure pre-cooling heat exchanger 165a. The second cooled methane enriched stream 160a is cooled and indirectly heat exchanged with a mixed pre-cooling refrigerant operating at a low pressure in the shell side of the low pressure pre-cooling heat exchanger 165a. It is preferred, if the first cooled methane enriched stream 150a is not partly condensed in the intermediate pressure pre-cooling heat exchanger 155a, that the second cooled methane enriched stream 160a is partly condensed in the low pressure pre-cooling heat exchanger 165a.
The cooled, preferably partly condensed, hydrocarbon exits the low pressure pre-cooling heat exchanger 165a as a pre-cooled methane enriched stream 170a. The terms "high pressure", "intermediate pressure" and "low pressure" describing the pre-cooling heat exchangers are used in a relative sense. That is, the shell side pressure of the low pressure pre-cooling heat exchanger 165a is less than the shell side pressure of the intermediate pressure pre-cooling heat exchanger 155a. The shell side pressure of the intermediate pressure pre-cooling heat exchanger 155a is less than the shell side pressure of the high pressure pre-cooling heat exchanger 145a. These pressures may vary depending upon the mixed pre-cooling refrigerant composition and the compositions of the streams to be pre-cooled. Suitable operating pressures are known to the skilled person.
In an alternative embodiment not shown in the Figures, two rather than three pre-cooling heat exchangers may be provided for use with a mixed pre-cooling refrigerant. For instance, high pressure and low pressure pre-cooling heat exchangers, with the high pressure pre-cooling heat exchanger operating at a higher shell side pressure than the low pressure pre-cooling heat exchanger can be used.
The pre-cooled methane enriched stream 170a can be passed to an optional main heat exchanger knock out drum 185 prior to passage to the main heat exchanger 175a. The main heat exchanger knock out drum 185 provides a pre-cooled methane enriched vapour stream 190 overhead.
The pre-cooled methane enriched vapour stream 190 can be passed to the main heat exchanger 175a, where it is at least partially, preferably fully, liquefied against a mixed main refrigerant to provide an at least partially, preferably fully liquefied hydrocarbon stream 180a.
The line-up of Figure 2 further discloses the cooling of streams in the first, second and third pre-cooling heat exchangers 145a, 155a, 165a. The method disclosed herein is particularly advantageous for the cooling of the mixed main refrigerant, which is used in the further cooling and at least partial liquefaction of the pre-cooled methane enriched vapour stream 190 in the main heat exchanger 175a.
The mixed main refrigerant is preferably cooled, and more preferably partially condensed in four stages. The mixed main refrigerant can be passed through one or more main refrigerant coolers 2015 and the high, intermediate and low pressure pre-cooling heat exchangers 145a, 155a, 165a in the pre-cooling stage.
A main refrigerant stream 2010, which can be a compressed stream provided by at least one main cooling refrigerant compressor 2225, can be passed to one or more coolers 2015, such as air or water coolers, to provide a first cooled mixed main refrigerant stream 2020.
The first cooled main refrigerant stream 2020 can be passed to the high pressure pre-cooling refrigerant heat exchanger 145a. The mixed refrigerant is cooled by indirect heat exchange with the mixed pre-cooling refrigerant evaporating at high pressure in the shell side of the high pressure pre-cooling heat exchanger 145a. Cooled mixed main refrigerant exits the high pressure pre-cooling heat exchanger 145a as the second cooled main refrigerant stream 2030.
The second cooled main refrigerant stream 2030 can be passed to the intermediate pressure pre-cooling refrigerant heat exchanger 155a. The mixed refrigerant is cooled by indirect heat exchange with the mixed pre-cooling refrigerant evaporating at intermediate pressure in the shell side of the intermediate pressure pre-cooling heat exchanger 155a. Cooled mixed main refrigerant exits the intermediate pressure pre-cooling heat exchanger 155a as the third cooled main refrigerant stream 2040.
The third cooled main refrigerant stream 2040 can be passed to low pressure pre-cooling heat exchanger 165a. The mixed refrigerant is cooled and preferably partly condensed by indirect heat exchange with pre-cooling refrigerant evaporating a low pressure in the shell side of the low pressure pre-cooling heat exchanger 165a. The cooled mixed main refrigerant exits the low pressure pre-cooling heat exchanger 165a as the pre-cooled mixed main refrigerant stream 2050.
The pre-cooled mixed main refrigerant stream 2050 can be passed to a main refrigerant separation device 2055, such as a gas/liquid separator. The main refrigerant separation device 2055 provides the first and second fraction main refrigerant streams 2060, 2110 respectively which are passed to the main heat exchanger 175a. The first fraction main refrigerant stream 2060 is preferably a vapour stream drawn overhead from the main refrigerant separation device 2055. The second fraction main refrigerant stream 2110 is preferably a liquid stream drawn from the bottom of the main refrigerant separation device 2055.
The first and second fraction main refrigerant streams 2060, 2110 are auto-cooled in the main heat exchanger 175a, expanded and passed to the shell side of the exchanger to provide cooling.
In particular, the first fraction main refrigerant stream 2060 is cooled, and preferably at least partially liquefied, in the main heat exchanger 175a against mixed main refrigerant and withdrawn from the exchanger to provide a cooled first fraction main refrigerant stream 2070. The cooled first fraction main refrigerant stream 2070 can then be passed to at least one first fraction main refrigerant expansion device, such as a Joule-Thomson valve 2075, to provide an expanded first fraction main refrigerant stream 2080. The expanded first fraction main refrigerant stream 2080 can then be passed to the shell side of the main heat exchanger 175a to provide cooling.
The second fraction main refrigerant stream 2110 is cooled in the main heat exchanger 175a against mixed main refrigerant and withdrawn from the exchanger to provide a cooled second fraction main refrigerant stream 2120. The cooled second fraction main refrigerant stream 2120 can be split in a cooled second fraction splitting device 2125 to provide a cooled second fraction main refrigerant continuing stream 2130 and a cooled second fraction main refrigerant side stream 2160, as a refrigerant stream.
The cooled second fraction main refrigerant continuing stream 2130 can be expanded in at least one second fraction main refrigerant expansion device 2135, such as a Joule-Thomson valve, to provide an expanded second fraction main refrigerant stream 2140. The expanded second fraction main refrigerant stream 2140 can be merged with an expanded second fraction main refrigerant side stream 2180, discussed below, in a second fraction stream combining device 2145, to provide a combined expanded second fraction main refrigerant stream 2150. The combined expanded second fraction main refrigerant stream 2080 can then be passed to the shell side of the main heat exchanger 175a to provide cooling.
The cooled second fraction main refrigerant side stream 2160 can be used to provide cooling duty to the first hydrocarbon component reservoir heat exchanger 265a. In such an embodiment, the cooled second fraction main refrigerant side stream 2160 is heat exchanged against a liquefied first hydrocarbon component reservoir stream (Figure 1, 250) to provide a warmed second fraction main refrigerant side stream 2170, as a warmed refrigerant stream and a cooled liquefied first hydrocarbon component reservoir stream (Figure 1, 260).
The main refrigerant is indirectly heat exchanged with the pre-cooled methane enriched vapour stream 190 and the first and second fraction main refrigerant streams 2060, 2110 to cool the streams and warm the main refrigerant. The warm main refrigerant is withdrawn from at or near the bottom of the main heat exchanger 175a, as warmed main refrigerant stream 2210.
The warmed main refrigerant stream 2210 is passed to a main refrigerant compressor knock-out drum 2215. The main refrigerant compressor knock-out drum 2215 provides a main refrigerant compressor feed stream 2220. The main refrigerant compressor feed stream 2220 is substantially gaseous.
The main refrigerant compressor feed stream 2220 can be passed to a main cooling refrigerant compressor 2225 in which it is compressed to provide the main refrigerant stream 2010 as a compressed stream. The main cooling refrigerant compressor 2225 is mechanically driven by a main refrigerant compressor driver 2235 such as a gas or stream turbine or an electric motor, via a main refrigerant compressor drive shaft 2245.
Figure 2 shows one convenient way in which the warmed second fraction main refrigerant side stream 2170 can be returned to the main cooling refrigerant circuit 2000. The warmed second fraction main refrigerant side stream 2170 can be passed to at least one warmed second fraction main refrigerant expansion device 2175 to provide the expanded second fraction main refrigerant side stream 2180. The expanded second fraction main refrigerant side stream 2180 can be merged with the expanded second fraction main refrigerant stream 2140 and passed to the main heat exchanger 175a as combined expanded second fraction main refrigerant stream 2150 as discussed above.
Alternatively, as is illustrated in Figure 3, the cooled second fraction main refrigerant side stream 2160 is passed through at least one cooled second fraction main refrigerant expansion device 2176, preferably in the form of a Joule-Thomson valve, before being fed to the first hydrocarbon component reservoir heat exchanger 265a. In this case, the warmed second fraction main refrigerant side stream 2170 can be returned to the main cooling refrigerant circuit 2000 by feeding it directly into the main refrigerant compressor knock-out drum 2215. This way, the temperature of the first hydrocarbon component reservoir stream 260 at the outlet of the first hydrocarbon component reservoir heat exchanger 265a can be controlled by manipulating the cooled second fraction main refrigerant expansion device 2176. Optionally, the warmed second fraction main refrigerant side stream 2170 can also be passed to the warmed second fraction main refrigerant expansion device 2175 to allow matching of the pressure to the pressure of the warmed main refrigerant stream 2210.
Still alternatively (not shown), the cooled second fraction main refrigerant side stream 2160 can be drawn from the expanded second fraction main refrigerant stream 2140.
Turning to the pre-cooling refrigerant circuit 1000 illustrated in Figure 2, a pre-cooling refrigerant stream 1010 of mixed pre-cooling refrigerant is provided as a compressed stream by a pre-cooling refrigerant compressor 1505. The pre-cooling refrigerant compressor 1505 is mechanically driven by a pre-cooling refrigerant compressor driver 1515, such as a gas or stream turbine or an electric motor, via a pre-cooling refrigerant compressor drive shaft 1525. The pre-cooling refrigerant stream 1010 is preferably provided under very high pressure.
The pre-cooling refrigerant stream 1010 can be cooled in one or more pre-cooling refrigerant cooling devices 1015, such as air or water coolers, to provide a first cooled pre-cooling refrigerant stream 1020. The first cooled pre-cooling refrigerant stream 1020 can be passed to a first cooled pre-cooling combining device 1025, which is connected to a make-up system 600 arranged to establish a fluid communication between the pre-cooling refrigerant circuit 1000 and one or more of the first or higher hydrocarbon component reservoirs 285, 385. In the embodiment of Figure 2, the make-up system 600 provides a pre-cooling refrigerant make-up stream 630, which can be merged with first cooled pre-cooling refrigerant stream 1020 in the combining device 1025 to provide a pre-cooling refrigerant accumulator feed stream 1030.
The pre-cooling refrigerant make-up stream 630 may comprise one or both of the first and second hydrocarbon components. For instance, first hydrocarbon component supply stream 280, which is a liquid stream from a liquid first hydrocarbon component supply reservoir, can be passed to a first hydrocarbon component supply stream heat exchanger 605. The first hydrocarbon component supply stream heat exchanger 605 warms the first hydrocarbon component stream 280 to provide a warmed first hydrocarbon component stream 610, which can be a liquid stream. The first hydrocarbon component stream 280 can be warmed by heat exchanging against any suitable heating medium 606, such as a water/glycol stream, a seawater stream or a propane stream, depending upon design preference.
Alternatively, the heating medium 606 is provided in the form of a vapour stream at a pressure selected such that the vapour condenses under the influence of warming the first hydrocarbon component stream 280. An advantage of employing a condensing vapour stream as the heating medium, is that the risk of solidifying of the heating medium as a result of freezing against the cold first hydrocarbon component supply stream 280 is lower than if a liquid or a subliming vapour would be used. Moreover, a relatively high amount of heat can be added per mass unit of the heating medium, in the form of latent heat, if the vapour is allowed to condense during its heat exchanging against the cold first hydrocarbon component supply stream 280.
A good example of a suitable vapour stream is a steam stream. Steam may be generated in any known way such as in a fired boiler, or in a waste heat recovery boiler. Such waste heat recovery boiler may be heated by a hot gas turbine exhaust stream from a gas turbine. Suitably, this may be a gas turbine that is used as compressor driver in the methods and apparatuses described herein, for instance the pre-cooling refrigerant compressor driver 1515, the main refrigerant compressor driver 2235 or the methane enriched stream compressor driver 137, and/or as driver of a generator for producing electric power for use in one or more of these compressor drivers.
Preferably, the steam stream is at a pressure of between 2 and 10 bara. An advantage of employing steam at fairly low pressure is that it may be derived from an exhaust steam stream of a unit that uses a high pressure steam stream, such as a steam turbine. Suitably, this may be a steam turbine that is used as compressor driver in the methods and apparatuses described herein, for instance the pre-cooling refrigerant compressor driver 1515, the main refrigerant compressor driver 2235 or the methane enriched stream compressor driver 137, and/or as driver of a generator for producing electric power for use in one or more of these compressor drivers.
Moreover, with a low pressure steam stream (e.g. with a pressure of 10 bara) the pressure-vessel requirements for the first hydrocarbon component supply stream heat exchanger 605 are more easily met than with a high pressure steam stream.
The warmed first hydrocarbon component stream 610 can be passed through a warmed first hydrocarbon stream control valve 615, to provide a controlled first hydrocarbon component stream 620. The controlled first hydrocarbon component stream 620 can be passed to the pre-cooling refrigerant make-up stream 630 by a hydrocarbon component stream combining device 625.
The pressure of the warmed first hydrocarbon component stream 610 may be above 30 bara, for instance between 30 and 55 bara, preferably to slightly exceed the pressure of the refrigerant in the at least one refrigerant circuit where the warmed first hydrocarbon component stream 610 is to be injected into. The temperature may be in the range of from 5 to 35 °C. In one example, the pressure in the warmed first hydrocarbon component stream 610 was 41 bara, and its temperature 25 °C. In another example, the pressure in the warmed first hydrocarbon component stream 610 was 41 bara, and its temperature 10 °C.
The second hydrocarbon component supply stream 380, which is a liquid stream from a liquid second hydrocarbon component supply reservoir, can be passed to a second hydrocarbon component supply stream heat exchanger 635. The second hydrocarbon component supply stream heat exchanger 635 warms the second hydrocarbon component stream 380 to provide a warmed second hydrocarbon component stream 640, which can be a liquid stream. The second hydrocarbon component stream 380 can be warmed against any suitable heating medium 636 such as described above with regard to the first hydrocarbon component supply stream heat exchanger 605, such as a water/glycol stream, a seawater stream, a propane stream, or a steam stream depending upon design preference.
The warmed second hydrocarbon component stream 640 can be passed through a warmed second hydrocarbon stream control valve 645, to provide a controlled second hydrocarbon component stream 650. The controlled second hydrocarbon component stream 650 can be passed to the pre-cooling refrigerant make-up stream 630 by the hydrocarbon component stream combining device 625.
The pressure of the warmed second hydrocarbon component stream 640 may be above 30 bara, for instance between 30 and 55 bara. The temperature may be in the range of from 5 to 35 °C. In one example, the pressure in the warmed second hydrocarbon component stream 640 was 41 bara, and its temperature 25 °C. In another example, the pressure in the warmed second hydrocarbon component stream 640 was 41 bara, and its temperature 10 °C.
The pre-cooling refrigerant accumulator feed stream 1030 can be passed to a pre-cooling refrigerant accumulator 1035. The pre-cooling refrigerant can be withdrawn from the pre-cooling refrigerant accumulator 1035 as a pre-cooling refrigerant supply stream 1040. The pre-cooling refrigerant supply stream 1040 can be passed to the high pressure pre-cooling heat exchanger 145a. The very high pressure mixed pre-cooling refrigerant is auto-cooled by indirect heat exchange with the mixed pre-cooling refrigerant evaporating at high pressure in the shell side of the high pressure pre-cooling heat exchanger 145a. Cooled mixed pre-cooling refrigerant exits the high pressure pre-cooling heat exchanger 145a as the first cooled pre-cooling refrigerant stream 1050.
The first cooled pre-cooling refrigerant stream 1050 can be passed to a first cooled pre-cooling refrigerant separation device 1055 to provide the continuing first cooled pre-cooling refrigerant stream 1110 and first cooled pre-cooling refrigerant split stream 1060.
The first cooled pre-cooling refrigerant split stream 1060 is passed to a first cooled pre-cooling refrigerant expansion device 1065, such as a Joule-Thomson valve, to provide a high pressure pre-cooling refrigerant stream 1070. The first cooled pre-cooling refrigerant split stream 1060 is expanded to the shell side operating pressure of the high pressure pre-cooling heat exchanger 145a.
The high pressure pre-cooling refrigerant stream 1070 is then passed to the shell side of the high pressure pre-cooling heat exchanger 145a to provide cooling to the pre-cooling refrigerant supply stream 1040, the methane enriched stream 140a and the first cooled mixed main refrigerant stream 2020. The high pressure pre-cooling refrigerant is warmed and at least partially vaporised in the high pressure pre-cooling heat exchanger 145a. The warmed and at least partially vaporised high pressure pre-cooling refrigerant is withdrawn from the high pressure pre-cooling refrigerant heat exchanger 145a as a high pressure pre-cooling refrigerant return stream 1080.
The high pressure pre-cooling refrigerant return stream 1080 can be passed to a high pressure pre-cooling refrigerant knock out drum 1085 to remove any liquid phase prior to passing the high pressure pre-cooling refrigerant to the pre-cooling refrigerant compressor 1505 as high pressure pre-cooling refrigerant vapour return stream 1090.
The continuing first cooled pre-cooling refrigerant stream 1110 provided by the first cooled pre-cooling refrigerant separation device 1055 can be passed to intermediate pressure pre-cooling refrigerant heat exchanger 155a. The high pressure mixed pre-cooling refrigerant is auto-cooled by indirect heat exchange with the mixed pre-cooling refrigerant evaporating at intermediate pressure in the shell side of the intermediate pressure pre-cooling heat exchanger 155a. Cooled mixed pre-cooling refrigerant exits the intermediate pressure pre-cooling refrigerant heat exchanger 155a as the second cooled pre-cooling refrigerant stream 1120.
The second cooled pre-cooling refrigerant stream 1120 can be passed to a second cooled pre-cooling refrigerant separation device 1125 to provide a continuing second cooled pre-cooling refrigerant stream 1210 and second cooled pre-cooling refrigerant split stream 1130.
The second cooled pre-cooling refrigerant split stream 1130 can be passed to a second cooled pre-cooling refrigerant expansion device 1135, such as a Joule-Thomson valve, to provide an intermediate pressure pre-cooling refrigerant stream 1140. The second cooled pre-cooling refrigerant split stream 1130 is expanded to the shell side operating pressure of the intermediate pressure pre-cooling heat exchanger 155a.
The intermediate pressure pre-cooling refrigerant stream 1140 can then be passed to the shell side of the intermediate pressure pre-cooling heat exchanger 155a to provide cooling to the continuing first cooled pre-cooling refrigerant stream 1110, the first cooled methane enriched stream 150a and the second cooled mixed main refrigerant stream 2030. The intermediate pressure pre-cooling refrigerant is warmed and at least partially vaporised in the intermediate pressure pre-cooling heat exchanger 155a. The warmed and at least partially vaporised intermediate pressure pre-cooling refrigerant is withdrawn from the intermediate pressure pre-cooling refrigerant heat exchanger 155a as an intermediate pressure pre-cooling refrigerant return stream 1150.
The intermediate pressure pre-cooling refrigerant return stream 1150 can be merged with combined warmed intermediate pressure pre-cooling refrigerant side stream 1260 discussed below using an intermediate pressure pre-cooling refrigerant combining device 1155 to provide a combined intermediate pressure pre-cooling refrigerant return stream 1160.
The combined intermediate pressure pre-cooling refrigerant return stream 1160 can be passed to an intermediate pressure pre-cooling refrigerant knock out drum 1165 to remove any liquid phase prior to passing the intermediate pressure pre-cooling refrigerant to the pre-cooling refrigerant compressor 1505 as intermediate pressure pre-cooling refrigerant vapour return stream 1170.
The continuing second cooled pre-cooling refrigerant stream 1210 provided by the second cooled pre-cooling refrigerant separation device 1125 can be passed to continuing second cooled pre-cooling refrigerant separation device 1215. The continuing second cooled pre-cooling refrigerant separation device 1215 provides a further continuing second cooled pre-cooling refrigerant stream 1310 and a second cooled pre-cooling refrigerant side stream 1220.
The second cooled pre-cooling refrigerant side stream 1220 is passed to a second cooled pre-cooling refrigerant side stream separation device 1225 to provide first and second part second cooled pre-cooling refrigerant side streams 1230a, 1230b respectively. The first and second part second cooled pre-cooling refrigerant side streams 1230a, 1230b are passed to first and second part second pre-cooling refrigerant expansion devices 1235a, 1235b to provide first and second part intermediate pressure pre-cooling refrigerant side streams 1240a, 1240b respectively. The first and second part second cooled pre-cooling refrigerant side streams 1230a, 1230b can be expanded to the intermediate pressure of the shell side of the intermediate pressure pre-cooling heat exchanger 155a.
The first part intermediate pressure pre-cooling refrigerant side stream 1240a can provide cooling duty to a first hydrocarbon heat exchanger 215a. The first part intermediate pressure pre-cooling refrigerant side stream 1240a can be heat exchanged against an overhead first hydrocarbon component stream (Figure 1, 210) to provide a warmed first part intermediate pressure pre-cooling refrigerant side stream 1250a and an at least partially liquefied first hydrocarbon component stream (Figure 1, 220) .
The second part intermediate pressure pre-cooling refrigerant side stream 1240b can provide cooling duty to a third hydrocarbon component reservoir heat exchanger 465a. The second part intermediate pressure pre-cooling refrigerant side stream 1240b can be heat exchanged against a liquefied third hydrocarbon component reservoir stream (Figure 1, 450) to provide a warmed second part intermediate pressure pre-cooling refrigerant side stream 1250b and a cooled liquefied third hydrocarbon component reservoir stream (Figure 1, 460).
The warmed first and second part intermediate pressure pre-cooling refrigerant side streams 1250a, 1250b can be merged in a warmed intermediate pressure pre-cooling refrigerant side stream combining device 1255 to provide the combined warmed intermediate pressure pre-cooling refrigerant side stream 1260.
The further continuing second cooled pre-cooling refrigerant stream 1310 provided by the continuing second cooled pre-cooling refrigerant separation device 1215 can be passed to low pressure pre-cooling refrigerant heat exchanger 165a. The intermediate pressure mixed pre-cooling refrigerant is auto-cooled by indirect heat exchange with the mixed pre-cooling refrigerant evaporating at low pressure in the shell side of the low pressure pre-cooling heat exchanger 165a. Cooled mixed pre-cooling refrigerant exits the low pressure pre-cooling refrigerant heat exchanger 165a as a third cooled pre-cooling refrigerant stream 1320.
The third cooled pre-cooling refrigerant stream 1320 can be passed to a third cooled pre-cooling refrigerant separation device 1325 to provide a continuing third cooled pre-cooling refrigerant stream 1410 and third cooled pre-cooling refrigerant split stream 1330.
The third cooled pre-cooling refrigerant split stream 1330 can be passed to a third cooled pre-cooling refrigerant expansion device 1335, such as a Joule-Thomson valve, to provide a low pressure pre-cooling refrigerant stream 1340. The third cooled pre-cooling refrigerant split stream 1330 is expanded to the shell side operating pressure of the low pressure pre-cooling heat exchanger 165a.
The low pressure pre-cooling refrigerant stream 1340 can then be passed to the shell side of the low pressure pre-cooling heat exchanger 165a to provide cooling to the further continuing second cooled pre-cooling refrigerant stream 1310, the second cooled methane enriched stream 160a and the third cooled mixed main refrigerant stream 2040. The low pressure pre-cooling refrigerant is warmed and at least partially vaporised in the low pressure pre-cooling heat exchanger 165a. The warmed and at least partially vaporised low pressure pre-cooling refrigerant is withdrawn from the low pressure pre-cooling refrigerant heat exchanger 165a as a low pressure pre-cooling refrigerant return stream 1350.
The low pressure pre-cooling refrigerant return stream 1350 can be merged with combined warmed low pressure pre-cooling refrigerant continuing stream 1450 discussed below using a low pressure pre-cooling refrigerant combining device 1355 to provide a combined low pressure pre-cooling refrigerant return stream 1360.
The combined low pressure pre-cooling refrigerant return stream 1360 can be passed to an low pressure pre-cooling refrigerant knock out drum 1365 to remove any liquid phase prior to passing the low pressure pre-cooling refrigerant to the pre-cooling refrigerant compressor 1505 as low pressure pre-cooling refrigerant vapour return stream 1370.
The continuing third cooled pre-cooling refrigerant stream 1410 provided by the third cooled pre-cooling refrigerant separation device 1325 can be passed to a third cooled pre-cooling refrigerant continuing stream separation device 1415 to provide first and second part third cooled pre-cooling refrigerant continuing streams 1420a, 1420b respectively. The first and second part third cooled pre-cooling refrigerant continuing streams 1420a, 1420b can be passed to first and second part third pre-cooling refrigerant expansion devices 1425a, 1425b to provide first and second part low pressure pre-cooling refrigerant continuing streams 1430a, 1430b respectively. The first and second part third cooled pre-cooling refrigerant continuing streams 1420a, 1420b can be expanded to the low pressure of the shell side of the low pressure pre-cooling heat exchanger 165a.
The first part low pressure pre-cooling refrigerant continuing stream 1430a can provide cooling duty to a hydrocarbon feed stream heat exchanger 65a. The first part low pressure pre-cooling refrigerant continuing stream 1430a can be heat exchanged against a hydrocarbon feed stream (Figure 1, 40) to provide a warmed first part low pressure pre-cooling refrigerant continuing stream 1440a and a cooled hydrocarbon feed stream. The cooled hydrocarbon feed stream can be passed to a hydrocarbon feed separator (Figure 1, 75).
The second part low pressure pre-cooling refrigerant side stream 1430b can provide cooling duty to a second hydrocarbon component reservoir heat exchanger 365a. The second part low pressure pre-cooling refrigerant continuing stream 1430b can be heat exchanged against a liquefied second hydrocarbon component reservoir stream (Figure 1, 350) to provide a warmed second part low pressure pre-cooling refrigerant continuing stream 1440b and a cooled liquefied second hydrocarbon component reservoir stream (Figure 1, 360).
The warmed first and second part low pressure pre-cooling refrigerant continuing streams 1440a, 1440b can be merged in a warmed low pressure pre-cooling refrigerant side stream combining device 1445 to provide the combined warmed low pressure pre-cooling refrigerant continuing stream 1450.
In the embodiment of Figure 2, the main refrigerant compressor 1505 is shown as a multi-stage compressor. The high pressure pre-cooling refrigerant vapour return stream 1090 can be passed to a high pressure stage of the pre-cooling refrigerant compressor 1505. The intermediate pressure pre-cooling refrigerant vapour return stream 1170 can be passed to an intermediate pressure stage of the pre-cooling refrigerant compressor 1505. The low pressure pre-cooling refrigerant vapour return stream 1370 can be passed to a low pressure stage of the pre-cooling refrigerant compressor 1505. These streams can be compressed to provide the pre-cooling refrigerant stream 1010 at very high pressure.
Alternatively, the main refrigerant compressor may be one or more main refrigerant compressors in series having high, intermediate and low pressure suction levels.
In a preferred embodiment, illustrated in Figure 4, the second hydrocarbon component supply stream heat exchanger 635 and the second hydrocarbon component supply stream heat exchanger 635 of Figure 2 are provided in the form of a single hydrocarbon component supply stream heat exchanger 1600. The single hydrocarbon supply stream heat exchanger 1600 is arranged in a manifold of lines provided with selection valves 1601a,b,c,d, in order to selectively pass exclusively one of the hydrocarbon component supply streams through the single hydrocarbon supply stream heat exchanger 1600. It will be understood by the person skilled in the art that the manifold may optionally be extended to be able to selectively pass one or more other streams other than the the first and second hydrocarbon component supply streams through the single hydrocarbon supply stream heat exchanger 1600. The heating medium 1606 may be of any of the types described above.
Figure 4 also illustrates the optional first and second submerged pumps 284 and 384 that can facilitate the drawing of the respective hydrocarbon component supply streams from the respective liquid second hydrocarbon reservoirs. These had not been shown in Figure 2 although they could be present in that embodiment as well.
The single hydrocarbon supply stream heat exchanger 1600, or the separate first and second hydrocarbon supply stream heat exchangers 605, 635, may be of any suitable type including, plate-fin, printed circuit, and tube-and-shell type heat exchangers. Tube-and-shell type heat exchangers are preferred, whereby preferably the hydrocarbon supply streams pass the hydrocarbon supply stream heat exchangers through the tube side and the heating medium through the shell side.
Typically, the pressure of the hydrocarbon supply streams 280, 380 at the inlet of the employed hydrocarbon supply stream heat exchanger may be between 35 and 60 bara. In one example the pressure was about 43 bara. When using low pressure steam as the heating medium, in one example the pressure at the inlet of the employed hydrocarbon supply stream heat exchanger was 4 bara, in another example it as 5 bara. The temperature at the inlet of the employed hydrocarbon supply stream heat exchanger is preferably above, for instance between 10 and 100 °C above, the condensation temperature at the prevailing pressure. At a pressure of 4.3 bara, the condensation temperature of steam is approximately 146 °C. In that case, the temperature at the inlet may be about 198 °C.
The person skilled in the art will understand that the present invention can be carried out in many various ways without departing from the scope of the appended claims. For instance, the present invention is applicable to many methods other than the specific line-up discussed above. The method may be applied to, for example, AP-X liquefaction processes such as those described in US Patent No.4,404,008 , C₃MR processes such as those described in US Patent No. 4,404,008 and Double Mixed Refrigerant (DMR) processes, such as those described in US Patent No. 6,370,910 .
In a similar manner to the line-up of Figure 2, the first and any second hydrocarbon component can be added to the main refrigerant stream after compression and cooling of these liquefaction processed as make-up to the main mixed refrigerant. Preferably, the refrigerant make-up hydrocarbon components are added prior to the main refrigerant accumulator or to the main refrigerant accumulator itself.

Claims

A method of fractionating a hydrocarbon stream (200) comprising at least a first hydrocarbon component to provide at least a first hydrocarbon component reservoir stream (270), said method comprising at least the steps of:
- providing a hydrocarbon stream (200) comprising at least a first hydrocarbon component;

- separating the hydrocarbon stream (200) in a first fractionation device (205) to provide an overhead first hydrocarbon component stream (210) and a first hydrocarbon component depleted bottoms stream (300);

- cooling the overhead first hydrocarbon component stream (210) to provide a liquefied first hydrocarbon component stream (230);

- drawing a part of the liquefied first hydrocarbon component stream (230) to provide a liquefied first hydrocarbon component reservoir stream (250);

- cooling the liquefied first hydrocarbon component reservoir stream (250) against a refrigerant stream (2160) to provide a cooled liquefied first hydrocarbon component reservoir stream (260) and a warmed refrigerant stream (2170);
characterized in that the method further comprises:
- reducing the pressure of the cooled liquefied first hydrocarbon component reservoir stream (260) to provide a reduced pressure liquid first hydrocarbon component reservoir stream (270);

- storing the reduced pressure liquid first hydrocarbon component reservoir stream (270) in a liquid first hydrocarbon component reservoir (285) at a first hydrocarbon component storage pressure of at most just above atmospheric pressure for use as a first refrigerant component make-up constituent for in at least one refrigerant circuit (1000,2000);

- passing a first hydrocarbon component supply stream (280) from the liquid first hydrocarbon component reservoir (285) to the at least one refrigerant circuit (1000,2000),
wherein the step of passing the first hydrocarbon component supply stream (280) from the first liquid hydrocarbon component reservoir (285) to the at least one refrigerant circuit (1000,2000) comprises:
- heating the first hydrocarbon component supply stream (280) to provide a warmed first hydrocarbon component stream (610);

- passing the warmed first hydrocarbon component stream (610) to the at least one refrigerant circuit (1000, 2000) .
The method according to claim 1, wherein the refrigerant stream (2160) used to cool the liquefied first hydrocarbon component reservoir stream (270) is drawn from the at least one refrigerant circuit (1000,2000) .
The method according to claim 1 or 2, wherein the at least one refrigerant circuit (1000,2000) comprises a pre-cooling refrigerant circuit (1000) comprising a pre-cooling mixed refrigerant at a plurality of pressures, in which the step of cooling the overhead first hydrocarbon component stream (210) to provide a liquefied first hydrocarbon component stream (230) comprises heat exchanging the pre-cooling mixed refrigerant or a fraction thereof at a temperature below ambient temperature against the overhead first hydrocarbon component stream (210).
The method according to any of the preceding claims wherein the at least one refrigerant circuit (1000,2000) comprises a main cooling refrigerant circuit comprising a main cooling refrigerant (2000), in which the refrigerant stream (2160) in the step of cooling the liquefied first hydrocarbon component reservoir stream (250) against a refrigerant stream (2160) to provide a cooled first hydrocarbon component reservoir stream (260) and a warmed refrigerant stream (2170) is derived from the main mixed refrigerant or a fraction thereof.
The method according to any of the preceding claims wherein the step of passing the warmed first hydrocarbon component stream (610)comprises passing the warmed first hydrocarbon component stream (610) to the at least one refrigerant circuit (1000,2000) via a warmed first hydrocarbon component stream control valve (615).
The method according to any of the preceding claims, wherein the hydrocarbon stream (200) is derived from natural gas.
The method according to any of the preceding claims, wherein the first hydrocarbon component is ethane.
The method according to any of the preceding claims, wherein the step of cooling the overhead hydrocarbon component stream (210,310,410) to provide a liquefied first hydrocarbon component stream (230,330,430) comprises the steps of:
- cooling the overhead hydrocarbon component stream (210,310,410) to provide an at least partially liquefied hydrocarbon component stream (230,330,430);

- separating the at least partially liquefied hydrocarbon component stream (230,330,430) in a hydrocarbon component gas/liquid separator (235,335,435) to provide a liquefied hydrocarbon component stream (230,330,430).
The method according to any of the preceding claims, further comprising the steps of:
- providing a hydrocarbon feed stream (40) comprising methane and at least the first hydrocarbon component;

- preparing the hydrocarbon stream (200) and a pre-cooled methane-enriched stream (170) from the hydrocarbon feed stream (40) comprising a separating step and a cooling step;

- at least partially liquefying the pre-cooled methane-enriched stream (170) in at least one main heat exchanger (175) to provide an at least partially liquefied hydrocarbon stream (180).
The method according to claim 9, wherein said at least partially liquefying of the pre-cooled methane-enriched stream (170) comprises fully liquefying the pre-cooled methane-enriched stream (170), to provide a fully liquefied hydrocarbon stream (180), and a subsequent step of depressurizing the fully liquefied hydrocarbon (180) stream to obtain the liquefied hydrocarbon stream at at most just above atmospheric pressure.
The method according to claim 9 or 10, wherein the hydrocarbon feed stream (40) is derived from natural gas and the at least partially liquefied hydrocarbon stream (180) is a liquefied natural gas stream.
The method according to any one or claims 9 to 11, wherein said preparing the hydrocarbon stream (200) and said pre-cooled methane-enriched stream (170) comprises:
- separating the hydrocarbon feed stream (40) to provide a methane enriched stream (140) and the hydrocarbon stream (200);

- cooling the methane enriched stream (140) against pre-cooling refrigerant in at least one pre-cooling heat exchanger (145,155,165) to provide the pre-cooled methane-enriched stream (170).
An apparatus (1) for fractionating a hydrocarbon stream (200) comprising at least a first hydrocarbon component, to provide at least a first hydrocarbon component reservoir stream (270), said apparatus (1) comprising at least:
- a first fractionation device (205) to separate a first hydrocarbon component from a hydrocarbon stream (200) to provide an overhead first hydrocarbon component stream (210) and a first hydrocarbon component depleted bottoms stream (300);

- a first hydrocarbon component heat exchanger (215) to cool the overhead first hydrocarbon component stream (210) to provide an at least partially liquefied first hydrocarbon component stream (220);

- a first splitting arrangement to split-off a liquefied first hydrocarbon component reservoir feed stream (250) from the at least partially liquefied first hydrocarbon component stream (230);

- a first hydrocarbon component reservoir heat exchanger (265) to cool the liquefied first hydrocarbon component reservoir stream (250) against a refrigerant stream (2160) to provide a cooled liquefied first hydrocarbon component reservoir stream (260) and a warmed refrigerant stream (2170);
characterized in that the apparatus further comprises:
- a first hydrocarbon component reservoir pressure reduction device (275) to reduce the pressure of the cooled liquefied first hydrocarbon component reservoir stream (260) to provide a reduced pressure liquid first hydrocarbon component reservoir stream (270) at a first hydrocarbon component storage pressure of at most just above atmospheric pressure;

- a liquid first hydrocarbon component reservoir (285) in fluid communication with the liquid first hydrocarbon component reservoir stream (270);

- at least one refrigerant circuit (1000,2000);

- a make-up system (600) arranged to allow fluid communication between the at least one refrigerant circuit (1000,2000) and the liquid first hydrocarbon component reservoir (285),
wherein the make-up system (600) comprises a first hydrocarbon component supply stream heat exchanger (605) arranged between the at least one refrigerant circuit (1000,2000) and the liquid first hydrocarbon component reservoir (285) arranged to supply heat to the first hydrocarbon component to provide a warmed first hydrocarbon component stream (610) upstream of the at least one refrigerant circuit (1000,2000).
Apparatus according to claim 13, wherein the first splitting arrangement comprises a hydrocarbon component gas/liquid separator (225) to separate the at least partially liquefied first hydrocarbon component stream (220) to provide a liquefied first hydrocarbon component stream (230), and a first hydrocarbon component splitting device (245) in the liquefied first hydrocarbon component stream (230) to draw the liquefied first hydrocarbon component reservoir feed stream (250) from the liquefied first hydrocarbon component stream (230).
Apparatus according to claim 13 or 14, further comprising a warmed first hydrocarbon component stream control valve (615) for passing the warmed first hydrocarbon component stream (610) to the at least one refrigerant circuit (1000,2000) via said warmed first hydrocarbon component stream control valve (615).