EP2278210A1

EP2278210A1 - Method for the gasification of a liquid hydrocarbon stream and an apparatus therefore

Info

Publication number: EP2278210A1
Application number: EP10169479A
Authority: EP
Inventors: Philippe Robert Becart; Peter Marie Paulus
Original assignee: Shell Internationale Research Maatschappij BV
Current assignee: Shell Internationale Research Maatschappij BV
Priority date: 2009-07-16
Filing date: 2010-07-14
Publication date: 2011-01-26

Abstract

The present invention provides a method of and apparatus for the gasification of a liquid hydrocarbon stream (15), to provide a gaseous hydrocarbon stream (45) and power, said method comprising at least the steps of:
(i) heat exchanging, in one or more hydrocarbon stream heat exchangers (20, 30, 40), a liquid hydrocarbon stream (15) comprising one or more hydrocarbons, against two or more working fluids, in two or more working fluid circuits (100, 200, 300) to transfer cooling duty from said one or more hydrocarbons to said two or more working fluids, to provide a gaseous hydrocarbon stream (45) and two or more liquefied working fluid streams (115, 215, 315);
(ii) heat exchanging the two or more liquefied working fluid streams (115, 215, 315) in one or more heat exchangers (20, 30, 40, 130, 140, 240, 340) to provide two or more vaporised working fluid streams (145, 245, 345);
(iii) dynamically expanding the two or more vaporised working fluid streams (145, 245, 345) to provide power; and
(iv) cooling at least one of the working fluids with at least a portion of the cooling duty transferred to at least another one of the two or more working fluids from the one or more hydrocarbons.

Description

The present invention provides a method for the gasification of a liquid hydrocarbon stream, such as a liquefied natural gas (LNG) stream, to provide power and a gaseous hydrocarbon stream, such as a natural gas stream, and an apparatus therefor. The apparatus and method are particularly suitable for use at a LNG regasification facility, such as an import terminal.
LNG is usually primarily liquefied methane containing varying quantities of ethane, propane and butane with trace quantities of pentanes and heavier hydrocarbon components. Usually the LNG is low in aromatic hydrocarbons and non-hydrocarbons such as H₂O, N₂, CO₂, H₂S and other sulphur compounds, because these compounds have usually been removed at least partially before liquefying the natural gas stream, which is then stored and transported in liquid form.
It is desirable to liquefy natural gas 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. It may not need to be stored at high pressures. Such liquefied natural gas can be stored at atmospheric pressure if maintained at cryogenic temperatures, such as at -160 °C or below. Alternatively, liquefied natural gas may be stored at temperatures above -160 °C if it is pressurised above atmospheric pressure.
In order to regasify the LNG stream, it is usually pressurised and vaporised. If desired, a selected amount of one or more additional components, for example nitrogen, can be added to obtain natural gas having a desired gas quality, for example a selected heating value (i.e. energy content when the natural gas is burned), according to gas specifications or the requirements of a consumer. Furthermore, the heating value of the natural gas may also be adjusted by removing or adding a desired amount of heavier hydrocarbons from natural gas.
An LNG regasification terminal pressurises and vaporises LNG so that pressurised natural gas can be injected into a gas network at ambient temperature. Although different kinds of heat exchangers are available for vaporising LNG, using an ambient heat source such as seawater is a common option. However, vaporising LNG against an ambient heat source represents lost energy. For instance, for each 1 MTPA (million metric tons per annum) LNG vaporised, a heat flow of between 20 and 30 MW is lost. This could be used for a number of practical purposes, such as power generation. It would be desirable if this unused heat flow could be harnessed to generate electricity, rendering the regasification facility self-sufficient in terms of its power requirements, such that power input from an external electricity grid or the burning of hydrocarbon fuels would not be required. A gasification terminal which does not require the direct or indirect burning of hydrocarbon fuels for its operation will be carbon dioxide neutral.
US Patent Publication No. 2006/0236699 discloses a power and regasification system for LNG. In one embodiment, a Rankine cycle is provided in which LNG can be used to condense a working fluid vapour in a heat exchanger. The condensed working fluid is sent to a vaporiser in which the condensed working fluid is vaporised against a heat source such as sea water, an exhaust gas from a steam turbine or low pressure steam from a condensing steam turbine. The vaporised working fluid is expanded in a turbine to produce power and the working fluid vapour.
A problem of this known method of gasifying is that the power generated from the Rankine cycle is insufficient to meet the requirements of a regasification facility. External power is therefore required, either from an electricity grid, or through generation within the facility itself. Consequently, the combustion of hydrocarbons and the associated carbon dioxide emissions may be required in order to operate the regasification facility.
The present invention seeks to address this problem by providing a more efficient method of regasifying a liquid hydrocarbon stream to produce power from the cold potential of the liquid hydrocarbon as a cold sink for a condenser. The power produced is preferably generated as electrical power. The generation of electrical power is advantageous because any excess electricity not required to operate the gasification facility can be exported to other processing units or to an external electricity grid, such as an external electricity grid.
It is desirable to minimise the carbon dioxide emissions from a facility, and more preferably to eliminate such emissions entirely to provide a "zero carbon dioxide emissions" regasification method. Such a facility could be autonomous in terms of its power requirements and give rise to carbon credits as a result of its carbon dioxide neutral power generation in steady state operation.
In a first aspect, the present invention provides a method for the gasification of a liquid hydrocarbon stream, to provide a gaseous hydrocarbon stream and power, said method comprising at least the steps of:

(i) heat exchanging, in one or more hydrocarbon stream heat exchangers, a liquid hydrocarbon stream comprising one or more hydrocarbons, against two or more working fluids, in two or more working fluid circuits to transfer cooling duty from said one or more hydrocarbons to said two or more working fluids, to provide a gaseous hydrocarbon stream and two or more liquefied working fluid streams;
(ii) heat exchanging the two or more liquefied working fluid streams in one or more heat exchangers to provide two or more vaporised working fluid streams;
(iii) dynamically expanding the two or more vaporised working fluid streams to provide power; and
(iv) cooling at least one of the working fluids with at least a portion of the cooling duty transferred to at least another one of the two or more working fluids from the one or more hydrocarbons.

In particular, the method and apparatus of the present invention can generate sufficient power from the gasification process to operate a regasification facility. For example, a power generation in the range of 2 to 3.5 MW/ MTPA is required for the normal daily operation of most LNG import terminals. By generating sufficient power to operate the facility, an autonomously powered gasification method utilising the cold energy from the liquefied hydrocarbon is provided.
In steady state operation, there would be no requirement for external power generation, eliminating carbon dioxide emissions associated with the burning of hydrocarbon fuels to generate power. In this way, a carbon dioxide neutral gasification process can be provided.
Furthermore, the present invention can operate using only heat sources at or below ambient temperature, eliminating emissions of carbon dioxide associated with the burning of hydrocarbon fuels to generate heat for the vaporisers and heaters, such a submerged combustion vaporisers.
In a further aspect, the present invention provides an apparatus for the gasification of a liquid hydrocarbon stream to provide a gaseous hydrocarbon stream and power, said apparatus comprising at least:

a first working fluid circuit having a first working fluid, said circuit comprising:
- a first hydrocarbon stream heat exchanger to condense an expanded vaporised first working fluid stream against a liquid hydrocarbon stream comprising one or more hydrocarbons to transfer cooling duty from the liquid hydrocarbon stream to the expanded evaporated first working fluid stream to provide a liquefied first working fluid stream and a warmed hydrocarbon stream;
- a first working fluid heat exchanger to heat the liquefied first working fluid stream or a stream derived therefrom against a heat source to provide a vaporised first working fluid stream;
- a first working fluid turbine to dynamically expand the vaporised first working fluid stream to provide the expanded vaporised first working fluid stream and drive a first working fluid generator to produce power;
a second working fluid circuit having a second working fluid, said circuit comprising:
- a second hydrocarbon stream heat exchanger to condense an expanded vaporised second working fluid stream against the warmed hydrocarbon stream to transfer cooling duty from the warmed hydrocarbon stream to the expanded vaporised second working fluid stream to provide a liquefied second working fluid stream and the gaseous hydrocarbon stream;
- a second working fluid heat exchanger to heat the liquefied second working fluid stream or a stream derived therefrom against a heat source to provide a vaporised second working fluid stream;
- a second working fluid turbine to dynamically expand the vaporised second working fluid stream to provide the expanded vaporised second working fluid stream and drive a second working fluid generator to produce power; and
a heat exchanger to cool at least one of the first and second working fluids with at least a portion of the cooling duty transferred to at least one of the first and second working fluids from one or both of the liquid hydrocarbon stream and the warmed hydrocarbon stream.

One major advantage of the present invention is that the method and apparatus utilise two integrated working fluid circuits to extract the cold energy from the liquid hydrocarbon to generate power. The first and second working fluid circuits are integrated such that the cold energy provided to the first working fluid by the liquid hydrocarbon is passed to the second working fluid. The integration allows cold energy from the first working fluid to be used in the power generation, via the dynamic expansion of the second working fluid, rather than be lost when the first working fluid is vaporised against an ambient heat source.
Thus, the latent heat of vaporisation from the first working fluid can be used to condense the second working fluid. Similarly, the latent heat of condensation of the second working fluid can be used to vaporise the first working fluid. In this way, more cold energy is provided to the second working fluid, allowing the mass flow in the second working fluid circuit to be increased.
The method and apparatus of the invention can generate, for instance, between 3.6 and 9 MW/ MTPA, depending upon the number of working fluid circuits and operational conditions. The operational conditions comprise the temperature and pressure of the liquid hydrocarbon stream, the temperature and pressure of the gasified gaseous hydrocarbon stream, the composition of the working fluids and the temperature and pressures of the working fluid streams.
In a preferred embodiment, the first working fluid is self-cooled and partially self vaporised, which increases the efficiency of the first working fluid circuit and the total mass flow of the first working fluid which can be condensed against the liquid hydrocarbon stream. The term "self-cooled and partially vaporised" relates to the use of the liquefied first working fluid stream, after optional pressurisation, to cool itself by passing a portion of its cold energy to the expanded vaporised first working fluid stream. The second working fluid can vaporise both the warmed hydrocarbon stream and the first working fluid. Such a line-up advantageously further increases the power which can be generated from the liquid hydrocarbon.
The heat sources used herein to warm the working fluid and hydrocarbon streams preferably have a temperature greater than the stream to be warmed. Preferably, the heat source has a temperature of greater than -40 °C, more preferably greater than -30 °C, still more preferably in the range of 0 to 30 °C. In a preferred embodiment, the upper temperature limit of the heat source is substantially equal to ambient temperature. One or more of the heat sources may be, for example, one or more of an ambient heat source such as ambient air or ambient water and a hydrocarbon stream generated in the gasification method. However, it is also possible to integrate the heat source with that from a combined power plant to achieve temperatures in excess of 30 °C.
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:

Figure 1 is a diagrammatic scheme of a method of and apparatus for gasifying a liquid hydrocarbon stream according to a first embodiment;
Figure 2 is a is a diagrammatic scheme of a method of and apparatus for gasifying a liquid hydrocarbon stream according to a second embodiment; and
Figure 3 is a is a diagrammatic scheme of a method of and apparatus for gasifying a liquid hydrocarbon stream according to a third embodiment.

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. The same reference numbers used in different Figures represent identical lines and streams.
As discussed above, it is often desirable to liquefy hydrocarbon compositions, such as natural gas, because it can be stored more conveniently and transported over long distances more readily as a liquid than in gaseous form. The liquefied natural gas can then be regasified at the desired destination, usually by a regasification facility such as an LNG import terminal, and passed, under pressure, to a gas network.
Referring to the drawings, Figure 1 shows a method and apparatus for the gasification of a liquid hydrocarbon feed stream 5. The liquid hydrocarbon feed stream 5 may comprise one or more hydrocarbons. The liquid hydrocarbon feed stream 5 is preferably a liquefied natural gas stream, or a synthetic hydrocarbon composition, such as that provided by the reaction of synthesis gas in a Fischer-Tropsch process. Liquefied natural gas can be provided by the treatment and liquefaction of natural gas in a manner known in the art.
Liquid hydrocarbon feed stream 5 is passed to a hydrocarbon stream pump 10, where it is pressurised to provide a liquid hydrocarbon stream 15. The liquid hydrocarbon stream 15 is preferably provided at a pressure at or above the minimum pipeline pressure. The minimum pipeline pressure is the required pipeline pressure of the consumer of the vaporised hydrocarbon, such as a gas network. This pressurisation step ensures that the liquid hydrocarbon stream 15 intended to provide the network gas is at a pressure equal to or higher than that of the pipeline pressure of the gas network. It is significantly more energy efficient to pressurise a liquid stream to the pipeline pressure or above, than to pressurise the corresponding evaporated gaseous stream.
The liquid hydrocarbon stream 15 is passed to a first hydrocarbon stream heat exchanger 20. The first hydrocarbon stream heat exchanger 20 may be selected from the group comprising a coil wound heat exchanger, a plate and fin heat exchanger and a printed circuit heat exchanger. The liquid hydrocarbon stream is warmed against at least an expanded vaporised first working fluid stream 105 comprising first working fluid in the first hydrocarbon stream heat exchanger 20 to provide a warmed hydrocarbon stream 25 and a liquefied first working fluid stream 115. The first working fluid is present in a first working fluid circuit 100, and is discussed in greater detail below.
The warmed hydrocarbon stream 25 is preferably a multi-phase stream comprising liquid and vaporised hydrocarbon. This stream is then passed to a second hydrocarbon stream heat exchanger 30, in which it is warmed against an expanded vaporised second working fluid stream 205 comprising a second working fluid to provide a gaseous hydrocarbon stream 45 and an at least partially liquefied second working fluid stream 255. The second working fluid is present in a second working fluid circuit 200, which is discussed in greater detail below.
If required, the gaseous hydrocarbon stream 45 can be heat exchanged against a heat source integrated with a combined power plant or having a temperature less than or substantially equal to ambient temperature, such as an ambient heat source, more preferably an ambient air or ambient water stream 405d, in a gaseous hydrocarbon stream heat exchanger 50. The gaseous hydrocarbon stream heat exchanger 50 warms the gaseous hydrocarbon stream 45 against the heat source to provide a warmed gaseous hydrocarbon stream 55 and a cooled heat source, such as a cooled air or cooled water stream 410d. In a preferred embodiment the gaseous hydrocarbon stream heat exchanger 50 is an ambient heat exchanger, more preferably an ambient superheater. The gaseous hydrocarbon stream heat exchanger 50 can be an open rack vaporiser (ORV), shell and tube heat exchanger or an intermediate fluid vaporiser.
The warmed gaseous hydrocarbon stream 55 is preferably provided at a temperature appropriate for further use, for instance in a gas network. For example, if the gaseous hydrocarbon stream 45 is provided at or below 0 °C, the gaseous hydrocarbon stream heat exchanger 50 provides additional warming to near ambient temperature in order to prevent condensation of water vapour on associated pipework. The gaseous hydrocarbon stream 55 should also be provided at a pipeline pressure appropriate for the gas network. The pipeline pressure can be set by the pressurisation of the liquid hydrocarbon feed stream 5 in the hydrocarbon stream pump 10.
The first and second working fluid circuits 100, 200 are used to generate the power from the gasification of the liquid hydrocarbon stream 15 in order to operate the gasification process, such as the various pumps present. The two or more working fluid circuits used herein preferably operate as Rankine cycles.
The Rankine cycle is a thermodynamic operation which converts heat into work. An expanded gaseous working fluid is condensed to provide a liquid working fluid. The liquid working fluid is pressurised in a pump to provide a pressurised liquid working fluid. The pressurised liquid working fluid is then vaporised to provide a gaseous working fluid. The gaseous working fluid is dynamically expanded in a turbine to produce useful work, and provide the expanded gaseous working fluid. The Rankine cycle is thus similar to a Carnot cycle, except that a pump is used to pressurise a liquid rather than a compressor to pressurise a gas. The compression of a liquid in a pump rather than a gas in a compressor requires less energy.
It will be apparent that the liquid and warmed hydrocarbon streams 15, 25 provide the cold energy to condense the working fluid in the Rankine cycle. One or more heat sources integrated with a combined power plant or having a temperature of less than or substantially equal to ambient temperature can be used to provide the heat to vaporise the liquefied working fluid after pressurisation. The use of a heat source having a temperature of less than or substantially equal to ambient temperature avoids the necessity to burn hydrocarbons or use electricity produced using hydrocarbon combustion to generate the heat for the gasification process.
Turning to the first working fluid circuit 100, the liquefied first working fluid stream 115 is produced by the condensation of the expanded vaporised first working fluid stream 105 in the first heat exchanger 20, which is preferably a first condenser. The liquefied first working fluid stream 115 is passed to a first working fluid pump 120, where it is pressurised to provide a liquefied first working fluid stream 115a, which is a pressurised stream.
As used herein, reference to a "liquefied working fluid stream" is also intended to refer to such a stream after pressurisation in a pump. Where appropriate, the pumped stream will also be referred to as a (pressurised) liquefied working fluid stream, using the same reference numeral as the stream upstream of the pump, but with an additional letter.
The (pressurised) liquefied first working fluid stream 115a is then passed to a common working fluid heat exchanger 130. The (pressurised) liquefied first working fluid stream 115a is warmed against the at least partially liquefied second working fluid stream 255 provided by the second heat exchanger 30 to provide a warmed first working fluid stream 135 and a liquefied second working fluid stream 215. The common working fluid heat exchanger 130 is thus preferably a second working fluid condenser.
The warmed first working fluid stream 135 can then be passed to a first working fluid heat exchanger 140, where it is heated against a heat source integrated with a combined power plant or having a temperature of less than or substantially equal to ambient temperature, such as an ambient air or ambient water stream 405a. The first working fluid heat exchanger 140 provides a vaporised first working fluid stream 145 and a cooled heat source, such as a cooled air or cooled water stream 410a. In a preferred embodiment the first working fluid heat exchanger 140 is an ambient heat exchanger, more preferably it is an ambient superheater. The first working fluid heat exchanger 140 can be an open rack vaporiser (ORV).
The vaporised first working fluid stream 145 can be passed to a first working fluid turbine 150. The turbine 150 converts the useful work released by the dynamic expansion of the vaporised first working fluid stream 145 into kinetic energy to turn the shaft 155 of a first working fluid electric generator 160. The electric generator 160 can provide a portion of the electrical power required to meet the needs of the gasification facility. The first working fluid turbine 150 provides the expanded vaporised first working fluid stream 105, which can then be passed back to the first hydrocarbon heat exchanger 20.
With regard to the second working fluid circuit 200, the liquefied second working fluid stream 215 provided by common working fluid heat exchanger 130 can be passed to a second working fluid pump 220. The second working fluid pump 220 provides a liquefied second working fluid stream 215a as a pressurised stream.
The (pressurised) second working fluid stream 215a can then be passed to a second working fluid heat exchanger 240, where it is heated against a heat source integrated with a combined power plant or having a temperature of less than or substantially equal to ambient temperature, such as an ambient air or ambient water stream 405b. The second working fluid heat exchanger 240 provides a vaporised second working fluid stream 245 and a cooled heat source, such as a cooled air or cooled water stream 410b. In a preferred embodiment, the second working fluid heat exchanger 240 is an ambient heat exchanger, more preferably it is an ambient superheater. The second working fluid heat exchanger 240 can be an open rack vaporiser (ORV).
The vaporised second working fluid stream 245 can be passed to a second working fluid turbine 250. The turbine 250 converts the useful work released by the dynamic expansion of the vaporised second working fluid stream 245 into kinetic energy to turn the shaft 255 of a second working fluid electric generator 260. The electric generator 260 can provide a portion of the electrical power required to meet the needs of the gasification facility. The second working fluid turbine 250 provides the expanded vaporised second working fluid stream 205, which can then be passed back to the second hydrocarbon heat exchanger 30.
Figure 1 shows a single heat source 405, such as an ambient air or ambient water source stream from which streams 405a, 405b, 405d are drawn.
The first and second working fluids can be single components or mixtures of components. The first and second working fluids may comprise the same components, but in different proportions. In some cases, a refrigerant is useful as the working fluid. Preferred working fluids are selected from one or more of the group comprising: tetrafluoromethane (R14), trifluoromethane (R23), ethane, pentafluoroethane (R125), propane and nitrogen.
The working fluids may be single components or mixtures of components. In one embodiment, the first working fluid is ethane and the second working fluid is propane. In a further embodiment, the first working fluid is tetrafluoromethane (R14) and the second working fluid is pentafluoroethane (R125). In a still further embodiment , the first and second working fluids can be mixtures of components with the first working fluid comprising substantially ethane and the second working fluid comprising substantially propane. In another embodiment, the first working fluid can comprise 45 mol% methane, 40 mol% ethane, 11 mol% propane and 4 mol% nitrogen, and the second working fluid can consist essentially of propane.
The line-up of the embodiment of Figure 1 can generate 3.6 MW power per million metric ton per annum (MTPA) LNG vaporised. This can be achieved utilising single component working fluids, such as ethane as the first working fluid and methane as the second working fluid. Mixed component working fluids may also be used, and may be expected to provide superior performance due to more efficient thermal transfer resulting from better matched cooling curves. Such a configuration is advantageous because it provides an integration of the first and second working fluid circuits 100, 200, such that heat exchange can occur between different working fluids. The first and second working fluids can exchange their latent heat of vaporisation or condensation respectively in the common working fluid heat exchanger 130, providing an efficient use of the cold energy recovered from the liquid hydrocarbon stream 15 and the warmed hydrocarbon stream 25.
In particular, the latent heat of vaporisation of the first working fluid can be used to condense the second working fluid in the common working fluid heat exchanger 130 and to increase the mass flow which can be circulated in the second working fluid circuit 200 because additional cold energy can be provided to the second working fluid by the (pressurised) liquefied first working fluid stream 115a.
In comparison, an alternative line-up may be considered which does not fall within the method and apparatus disclosed herein. In such a comparative line-up two independent working fluid circuits are provided in which there is no integration i.e. no common working fluid heat exchanger 130 to act as a vaporiser for the first working fluid by liquefying the second working fluid. There is thus no cooling of at least one of the working fluids with at least a portion of the cooling duty transferred to at least one of the two or more working fluids from the one or more liquid hydrocarbons. Such a comparative line-up would generate maximally approximately 2.6 to 2.9 MW/ MTPA, depending upon the choice of working fluids. For instance, single component working fluids such as an ethane first working fluid and a propane second working fluid can generate approximately 2.6 MW/ MTPA in the gasification process. The higher power generation of 2.9 MW/ MTPA can be achieved with one or more mixed component working fluids, such as a first working fluid comprising 45 mol% methane, 40 mol% ethane, 11 mol% propane and 4 mol% nitrogen, and a second working fluid consisting essentially of propane.
Figure 2 provides an alternative embodiment of the method and apparatus disclosed herein. In a similar manner to the embodiment of Figure 1, first and second working fluid circuits 100, 200 are provided. However, this line-up includes a self regenerative process in which the first working fluid is cooled against itself ("self-cooled") and partially vaporised against itself ("self-vaporised") in order to increase the efficiency and total mass flow which can be condensed with cold energy from the liquid hydrocarbon stream 15. Such a process is referred to as "self-regenerative" herein.
The liquefied hydrocarbon stream 15, which is a pressurised stream, is passed to the first hydrocarbon stream heat exchanger 20 where it is warmed against an expanded vaporised first working fluid stream 105. The first hydrocarbon stream heat exchanger 20 provides a warmed hydrocarbon stream 25 and a liquefied first working fluid stream 115. In a preferred embodiment, the warmed hydrocarbon stream is provided at a temperature in the range of -45 to -80 °C, more preferably about -55 °C.
The warmed hydrocarbon stream 25 can then be passed to a second hydrocarbon stream heat exchanger 30, in which it is further warmed against an expanded vaporised second working fluid stream 205. The second hydrocarbon stream heat exchanger 30 provides a gaseous hydrocarbon stream 45 and a liquefied second working fluid stream 215. In the case where the hydrocarbon is natural gas, the gaseous hydrocarbon stream may be provided at a temperature of about -25 °C.
The gaseous hydrocarbon stream 45 can then be passed to one or more gaseous hydrocarbon stream heat exchangers 50, where it is warmed, preferably superheated against a heat source integrated with a combined power plant or having a temperature of less than or substantially equal to ambient temperature, such as an ambient air or ambient water stream 405d. The one or more gaseous hydrocarbon stream heat exchangers 50 provide a warmed gaseous hydrocarbon stream 55, which can be passed to one or more gaseous hydrocarbon stream consumers, and a cooled heat source, such as a cooled air or cooled water stream 410d.
The embodiment of Figure 2 differs from that of the embodiment of Figure 1 in terms of the integration of the first and second working fluid circuits 100, 200.
In a preferred embodiment, the first working fluid comprises, by molar composition:

25 to 90% of one or both of methane and tetrafluoromethane;
10 to 60% of one or both of ethane and trifluoromethane; and
0 to 50% of one or both of propane and pentafluoroethane.

In a similar manner to Figure 1, the liquefied first working fluid stream 115 can be passed to a first working fluid pump 120, where it is pressurised.
The (pressurised) liquefied first working fluid stream 115a produced by first working fluid pump 120 is preferably at a pressure of less than 50 bar. Rather than being passed to a working fluid heat exchanger as in the line-up of Figure 1, the (pressurised) liquefied first working fluid stream 115a is instead returned to the first hydrocarbon stream heat exchanger 20 where it can cool itself, passing a portion of its cold energy to the expanded vaporised first working fluid stream 105. In doing so, the (pressurised) liquefied first working fluid stream 115a is warmed in the heat exchanger 20, to provide a first warmed first working fluid stream 135a.
The first warmed working fluid stream 135a can then be passed to the second hydrocarbon stream heat exchanger 30, where a portion of its cold energy can be transferred to the second working fluid to help liquefy the expanded vaporised second working fluid stream 205. The first warmed first working fluid stream 135a is preferably a liquid stream to avoid two phase flow in the second hydrocarbon heat exchanger 30. The first working fluid leaves the second hydrocarbon stream heat exchanger 30 as a second warmed first working fluid stream 135b. The second warmed first working fluid stream 135b is preferably fully vaporised in the second hydrocarbon stream heat exchanger 30.
The second warmed first working fluid stream 135b can then be passed to a first working fluid heat exchanger 140, where it is warmed to provide vaporised first working fluid stream 145. In a preferred embodiment, the first working fluid heat exchanger 140 is an ambient heat exchanger, more preferably an ambient superheater. The heat source may be integrated with a combined power plant or an ambient air or ambient water stream 405a. The first working fluid heat exchanger 140 can be an open rack vaporiser (ORV).
Power is generated in the first working fluid circuit 100 by dynamically expanding the vaporised first working fluid stream 145 in a first working fluid turbine 150, which mechanically drives first working fluid electric generator 160 via first shaft 155. The first working fluid turbine 150 provides the expanded vaporised first working fluid stream 105. For instance, the first working fluid circuit can be the primary source of power, generating up to 4.0 MW/ MTPA.
Turning to the second working fluid in the second working fluid circuit 200, this may preferably comprise:

propane; or
pentafluoroethane; or
a mixture of ethane and propane having at least 50 mol% propane; or
a mixture of ethane and pentafluoroethane having at least 60 mol% pentafluoroethane.

The liquefied second working fluid stream 215 provided by the second hydrocarbon stream heat exchanger 30 can be passed to a second working fluid pump 220, which pressurises the stream to provide a (pressurised) liquefied second working fluid stream 215a. (Pressurised) liquefied second working fluid stream 215a is preferably provided at a pressure of less than 20 bar. The (pressurised) liquefied second working fluid stream 215a may be passed to a second working fluid heat exchanger 240, where it is warmed to provide a vaporised second working fluid stream 245. The second working fluid heat exchanger 240 is preferably an ambient heat exchanger, more preferably a second working fluid vaporiser.
Power is generated in the second working fluid circuit 200 by dynamically expanding the vaporised second working fluid stream 245 in a second working fluid turbine 250, which mechanically drives second working fluid electric generator 260 via second shaft 255. The second working fluid turbine 250 provides the expanded vaporised second working fluid stream 205. The second working fluid circuit can generate 0.5 to 1.5 MW/MTPA.
Thus, in the embodiment of Figure 2, a heat source integrated with a combined power plant or having a temperature of less than or substantially equal to ambient temperature is used to superheat the gaseous hydrocarbon stream 45 and (second) warmed first working fluid stream 135b, and to vaporise the (pressurised) second working fluid stream 215a.
Such a configuration with two working fluid circuits 100, 200 is, for instance, capable of generating about 5 MW/MTPA when the one or more hydrocarbons are natural gas and the warmed gaseous hydrocarbon stream 55 is provided at 18 °C and a pipeline pressure of 75 bar. This degree of power generation corresponds to a saving of 15300 ton/year carbon dioxide, the generation of which would be avoided by such a process. A higher power generation can be achieved compared to the embodiment of Figure 1 because of the self regeneration of the first working fluid. Also, higher power generation can be achieved if the minimum required pipeline pressure is lower than 75 bar.
Figure 3 provides an alternative embodiment of the method and apparatus disclosed herein. Three integrated working fluid circuits 100, 200, 300 are provided. The addition of a third working fluid circuit 300 allows further cold to be extracted from the liquid hydrocarbon stream 15 and streams 25, 35, derived therefrom increasing the efficiency of the gasification process and the power generation. The addition of a third working fluid circuit 300 allows the first working fluid to cool the second and third working fluids and the second working fluid to cool the third working fluid.
The gasification of the liquid hydrocarbon stream 15 is carried out in two or more hydrocarbon stream heat exchangers 20, 30, 40. Preferably, the heat exchangers 20, 30, 40 may be selected from the group comprising coil wound heat exchangers, printed circuit heat exchangers and plate and fin heat exchangers. However it is pointed out that plate and fin heat exchangers are preferably used at pressures below 100 bar. More preferably, the plate and fin heat exchangers are plate and fin brazed aluminium heat exchangers.
The liquid hydrocarbon stream 15 is warmed against an expanded vaporised first working fluid stream 105 in the first hydrocarbon stream heat exchanger 20 to provide a first warmed hydrocarbon stream 25. It is preferred that the first warmed hydrocarbon stream 25 is provided at a temperature in the range of -90 to -130 °C, more preferably at about -100 °C.
The first warmed hydrocarbon stream 25 can then be passed to a second hydrocarbon stream heat exchanger 30, in which it is further warmed against an expanded vaporised second working fluid stream 205 to provide an at least partially vaporised hydrocarbon stream 35.
The at least partially vaporised hydrocarbon stream 35 can then be passed through the third hydrocarbon stream heat exchanger 40, such as a plate and fin or a printed circuit heat exchanger. The third hydrocarbon stream heat exchanger 40 can warm the at least partially vaporised hydrocarbon stream 35 against an expanded vaporised third working fluid stream 305 to provide a gaseous hydrocarbon stream 45 and a liquefied third working fluid stream 315.
The gaseous hydrocarbon stream 45 can then be passed to one or more gaseous hydrocarbon stream heat exchangers 50, where it is warmed, preferably superheated against a heat source integrated with a combined power plant or having a temperature of less than or substantially equal to ambient temperature, such as an ambient air or ambient water stream 405d. The one or more gaseous hydrocarbon stream heat exchangers 50 provide a warmed gaseous hydrocarbon stream 55, which can be passed to one or more gaseous hydrocarbon stream consumers, and a cooled heat source, such as a cooled air or cooled water stream 410d.
Turning to the liquefied first working fluid stream 115 in the first working fluid circuit 100, this may comprise one or both of methane and tetrafluoromethane. More preferably, the first working fluid comprises:

10 to 90 mol% of one or both of methane and tetrafluoromethane;
10 to 50 mol% ethane; and
0 to 15 mol% of one or both of propane or pentafluoroethane.

It is preferred that the liquefied first working fluid stream 115 is provided at a temperature of less than -100 °C. The liquefied first working fluid stream 115 can be passed to a first working fluid pump 120 where it is pressurised to provide a (pressurised) liquefied first working fluid stream 115a, preferably at a pressure in the range of 4 to 50 bar. In a similar manner to the embodiment of Figure 2, the (pressurised) liquefied first working fluid stream 115a is passed to the first hydrocarbon stream heat exchanger 20 where it will be warmed against the incoming expanded vaporised first working fluid stream 105. The first hydrocarbon stream heat exchanger 20 provides a (first) warmed first working fluid stream 135a. In this way, a portion of the (pressurised) liquefied first working fluid stream 105 can be warmed, preferably vaporised, against itself in the form of the expanded vaporised first working fluid stream 105, a portion of which will be correspondingly cooled, preferably liquefied. This is known as the regenerative effect.
In a preferred embodiment, a portion of the (pressurised) liquefied first working fluid stream 115a can be vaporised in the first hydrocarbon stream heat exchanger 20, such that (first) warmed first working fluid stream 135a is a (first) at least partly vaporised first working fluid stream.
Thus, the first hydrocarbon stream heat exchanger 20 warms the liquefied hydrocarbon stream 15 and the (pressurised) liquefied first working fluid stream 115a, and liquefies the expanded vaporised first working fluid stream 105.
The (first) warmed first working fluid stream 135a can be passed to the second hydrocarbon stream heat exchanger 30. In the second hydrocarbon stream heat exchanger 30, the (first) warmed first working fluid stream 135a can be warmed against an expanded vaporised second working fluid stream 205, to provide a (second) warmed first working fluid stream 135b and a liquefied second working fluid stream 215.
The (second) warmed first working fluid stream 135b can be passed to third hydrocarbon stream heat exchanger 40, where it can be warmed against an expanded vaporised third working fluid stream 305. The third hydrocarbon stream heat exchanger 40 provides a gaseous hydrocarbon stream 45, a (third) warmed first working fluid stream 135c and a liquefied third working fluid stream 315. In a preferred embodiment, the (third) warmed first working fluid stream 135c can be a fully vaporised stream.
The (third) warmed first working fluid stream 135c can be passed to a first working fluid heat exchanger 140, which is preferably an ambient superheater. The (third) warmed first working fluid stream 135c is heated against a heat source integrated with a combined power plant or having a temperature of less than or substantially equal to ambient temperature, such as an ambient air or ambient water stream 405a, to provide vaporised first working fluid stream 145. In a preferred embodiment in which the (third) warmed first working fluid stream 135c is a fully vaporised stream, gaseous first working fluid stream 145 can be a superheated stream.
The gaseous first working fluid stream 145 can then be passed to first working fluid turbine for dynamic expansion and power generation in a similar manner to the embodiments of Figures 1 and 2.
Turning to the second working fluid circuit 200, the liquefied second working fluid stream 215 preferably comprises:

0 to 20 mol% of one or both of methane and tetrafluoromethane;
10 to 60 mol% of one or both of ethane and trifluoromethane; and
30 to 80 mol% of one or both of propane and pentafluoroethane.

The liquefied second working fluid stream 215 is preferably provided at a temperature in the range of -50 to less than -100 °C. The liquefied second working fluid stream 215 can be passed to a second working fluid pump 220. The second working fluid pump 220 provides a (pressurised) liquefied second working fluid stream 215a, preferably at a pressure in the range of 3 to 25 bar, which is passed to the second hydrocarbon stream heat exchanger 30. In the second hydrocarbon stream heat exchanger 30, the (pressurised) liquefied second working fluid stream 215a is warmed against the expanded vaporised second working fluid stream 205 to provide a (first) warmed second working fluid stream 235a and the liquefied second working fluid stream 215. The second working fluid circuit 200 can thus also be a regenerative circuit, in which a portion of the cold energy of the (pressurised) liquefied second working fluid stream 215a is passed to the expanded evaporated second working fluid stream 245.
Thus, second hydrocarbon stream heat exchanger 30 warms the (first) warmed hydrocarbon stream 25, the (first) warmed first working fluid stream 135a and the (pressurised) liquefied second working fluid stream 215a, and liquefies the expanded vaporised second working fluid stream 205. Cold energy from the first working fluid (as well as the first warmed hydrocarbon stream 25) can thus be transferred to the second working fluid.
The (first) warmed second working fluid stream 235a can be passed to third hydrocarbon stream heat exchanger 40. In third hydrocarbon stream heat exchanger 40, the (first) warmed second working fluid stream 235a is warmed against an expanded vaporised third working fluid stream 305 in a third working fluid circuit 300. Third hydrocarbon stream heat exchanger 40 provides a gaseous hydrocarbon stream 45, a liquefied third working fluid stream 315 and a (second) warmed second working fluid stream 235b. The (second) warmed second working fluid stream 235b is preferably a partially or fully evaporated stream.
The (second) warmed second working fluid stream 235b can be passed to a second working fluid heat exchanger 240. Second working fluid heat exchanger 240 warms and preferably superheats the (second) warmed second working fluid stream 235b against a heat source integrated with a combined power plant or having a temperature of less than or substantially equal to ambient temperature, such as an ambient air or ambient water stream 405b, to provide a vaporised second working fluid stream 245 and a cooled heat source stream 410b, such as a cooled air or cooled water stream.
The vaporised second working fluid stream 245 can then be passed to a second working fluid turbine 250, where it is dynamically expanded. The second working fluid turbine is mechanically connected to a second working fluid electric generator 260 by a shaft 255 to provide electricity. The second working fluid turbine 250 provides the expanded vaporised second working fluid stream 205 to the second hydrocarbon stream heat exchanger 30.
Turning to the third working fluid circuit 300, the third working fluid may preferably comprise one or both of propane and pentafluoroethane. The third hydrocarbon stream heat exchanger 40 condenses an expanded vaporised third working fluid stream 305 to provide the liquefied third working fluid stream 315.
The third heat exchanger 40 warms the (second) warmed first working fluid stream 135b, the (first) warmed second working fluid stream 235a, and the (second) warmed hydrocarbon stream 35, and liquefies the expanded vaporised third working fluid stream 305. Cold energy from the first and second working fluids can thus be transferred to the third working fluid in the third heat exchanger 40.
The liquefied third working fluid stream 315 can be passed to a third working fluid pump 320 where it is pressurised to provide (pressurised) liquefied third working fluid stream 315a. The (pressurised) third working fluid stream 315a can be passed to a third working fluid heat exchanger 340, where it is vaporised against a heat source integrated with a combined power plant or having a temperature of less than or substantially equal to ambient temperature, such as an ambient air or ambient water stream 405c, to provide a vaporised third working fluid stream 345 and a cooled heat source, such as a cooled air or cooled water stream 410c.
The vaporised third working fluid stream 345 can be passed to a third working fluid turbine 350, where it is dynamically expanded to provide power and the expanded vaporised third working fluid stream 305. The third working fluid turbine 350 is mechanically connected to a third working fluid electric generator 360 by shaft 355.
The self-regenerative effect of transferring cold energy between two streams of a working fluid is maximal, in terms of heat transfer, when the pressures of the expanded vaporised working fluid stream and (pressurised) liquefied working fluid stream are similar. This allows maximal latent heat recovery. However, the larger the pressure ration between the expanded vaporised working fluid stream and (pressurised) liquefied working fluid stream, the more power generated, such that these two effects must be balanced.
The advantages of the regenerative effect are shown in the following Example, in which the power generation of a regenerative three working fluid circuits gasification processes is compared to a non-regenerative three working fluid circuits gasification process. Both of these methods fall within the scope of the methods and apparatus disclosed herein.
The regenerative effect gasification process can be contrasted with a non-regenerative gasification process. In a method involving three working fluid circuits, a regenerative effect may be provided when the first and second working fluids self-cool and self-vaporise.
In the case without the regenerative effect, the (pressurised) first working fluid stream 115a would be passed directly to the second hydrocarbon stream heat exchanger 30 and not first through the first hydrocarbon stream heat exchanger. Similarly, the (pressurised) second working fluid stream 215a would be passed directly through the third hydrocarbon stream heat exchanger 40, and not first through the second hydrocarbon stream heat exchanger 30.
The first working fluid was chosen to be methane, provided at a mass flow of 8-9 kg/s. The methane first working fluid was liquefied to -126 °C against a LNG stream in a first hydrocarbon heat exchanger and then pressurised to 30 bars in a first working fluid pump.
The (pressurised) liquefied methane first working fluid stream was then passed directly to the second hydrocarbon stream heat exchanger where it was pre-warmed against an expanded vaporised ethane second working fluid stream at a pressure of 2 bar. Pre-warmed methane first working fluid stream was than passed to a third heat exchanger where it was vaporised against an expanded vaporised propane third working fluid stream. The vaporised methane first working fluid stream was then superheated to 10 °C against an ambient heat source. The superheated vaporised first working fluid stream was then dynamically expanded to 10-11 bar in a first working fluid turbine to generate 0.5 MW/MTPA LNG.
The ethane second working fluid was provided at a mass flow of 22 kg/s and liquefied in the second hydrocarbon stream heat exchanger against the warmed LNG stream and the (pressurised) liquefied methane first working fluid stream to provide a liquefied ethane second working fluid stream at -80 °C. The liquefied ethane second working fluid stream was then pressurised to 11 bar in a second working fluid pump.
The (pressurised) liquefied ethane second working fluid stream was then passed to the third heat exchanger where it was vaporised against the propane third working fluid to provide a vaporised ethane second working fluid stream. The vaporised ethane second working fluid stream was then superheated to 15 °C against an ambient heat source. Dynamic expansion of the superheated vaporised ethane second working fluid stream to a pressure of 2 bar in a second working fluid turbine generated 1.5 MW/MTPA LNG.
The propane third working fluid stream was provided at a mass flow of 60 kg/s and liquefied in third hydrocarbon stream heat exchanger against the further warmed LNG, the pre-warmed methane first working fluid stream and the (pressurised) liquefied ethane second working fluid stream, from the second hydrocarbon stream heat exchanger, to provide a liquefied propane third working fluid stream at -30 °C.
The liquefied propane third working fluid stream was then pressurised to a pressure of 6.5 bar in a third working fluid pump and vaporised against an ambient heat source. The vaporised propane third working fluid stream was then passed to a third working fluid turbine where it was dynamically expanded to a pressure of 2 bar to generate 2 MW/MTPA LNG.
Thus, the entire three working fluid circuit non-regenerative gasification process yields 4 MW/MTPA LNG. Indeed, there would appear to be an upper power generation limit of approximately 4.2 MW/MTPA for single component working fluids in independent working fluid circuits i.e. working fluid circuits in which there is no thermal transfer between different working fluids.
This non-regenerative method can be contrasted with a regenerative process according to the embodiment of Figure 3. As an example, the first working fluid was chosen to comprise substantially 30 mol% methane, 35 mol% ethane and 35 mol% tetrafluoromethane (R14). The second working fluid comprised 10 mol% tetrafluoromethane (R14), 60 mol% pentafluoroethane (R125) and 30 mol% ethane. The third working fluid comprised 50 mol% propane and 50 mol% pentafluoroethane (R125).
The line-up had the stream parameters shown in Table 1. The first working fluid circuit generated about 2 MW/MTPA. The second working fluid circuit generated 2 MW/MTPA. The third working fluid circuit generated 1.5 MW/MTPA. Thus, the embodiment of Figure 3 can generate a total power or 5.5 MW/MTPA for the gasification of LNG. Under optimised conditions, such as a pipeline pressure of the gasified LNG of 20 bar, increased power generation can be achieved, for instance about 7.6 MW/MTPA.

It is apparent from this comparison that a regenerative process involving two, three or more integrated working fluid circuits can provide a significant increase in power generation compared to a non-regenerative process.

Table 1: Stream parameters for an embodiment according to Figure 3

Stream No.	Pressure (bar)	Temperature (°C)
5	8	-160
15	77	-155
25	76.5	-63
45a	76	-25
55	75	10
105	1.5
115	1.3	-149
115a	37
135a		-110
135b		-53
135c		-22
145	37
205	1.3
215	1.0	-26
215a	17
235b		-40
245	16.5

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, two or more of the first, second and third working fluid ambient heat exchangers 140, 240, 340 can be integrated into a single heat exchanger unit, such as a common seawater superheater.

Claims

A method for the gasification of a liquid hydrocarbon stream (15), to provide a gaseous hydrocarbon stream (45) and power, said method comprising at least the steps of:
(i) heat exchanging, in one or more hydrocarbon stream heat exchangers (20, 30, 40), a liquid hydrocarbon stream (15) comprising one or more hydrocarbons, against two or more working fluids, in two or more working fluid circuits (100, 200, 300) to transfer cooling duty from said one or more hydrocarbons to said two or more working fluids, to provide a gaseous hydrocarbon stream (45) and two or more liquefied working fluid streams (115, 215, 315);

(ii) heat exchanging the two or more liquefied working fluid streams (115, 215, 315) in one or more heat exchangers (20, 30, 40, 130, 140, 240, 340) to provide two or more vaporised working fluid streams (145, 245, 345);

(iii) dynamically expanding the two or more vaporised working fluid streams (145, 245, 345) to provide power; and

(iv) cooling at least one of the working fluids with at least a portion of the cooling duty transferred to at least another one of the two or more working fluids from the one or more hydrocarbons.
The method of claim 1 further comprising, between steps (i) and (ii), the step of:
- pressurising the two or more liquefied working fluid streams (115, 215, 315).
The method according to claim 1 or claim 2, in which step (iv) comprises cooling at least one of the working fluids with at least a portion of the cooling duty from a liquefied working fluid stream (115, 215) of the same working fluid.
The method of claim 2 or claim 3 wherein:
- step (i) comprises:
heat exchanging, in one or more first hydrocarbon stream heat exchangers (20), the liquid hydrocarbon stream (15) against at least an expanded vaporised first working fluid stream (105) comprising a first working fluid in a first working fluid circuit (100), to provide a warmed hydrocarbon stream (25) and a liquefied first working fluid stream (115);

heat exchanging an expanded vaporised second working fluid stream (205) in a second working fluid circuit (200) against at least the warmed hydrocarbon stream (25) in one or more second hydrocarbon stream heat exchangers (30) to provide a liquefied second working fluid stream (215);

- pressurising the two or more liquefied working fluid streams (115, 215, 315) comprises pressurising the liquefied first working fluid stream (115) to provide a pressurised liquefied first working fluid stream (115a) and pressurising the liquefied second working fluid stream (215) to provide a pressurised liquefied second working fluid stream (215a);

- step (ii) comprises:
heat exchanging the pressurised liquefied first working fluid stream (115a) in one or more heat exchangers (20, 30, 130, 140) to provide a vaporised first working fluid stream (145);

heat exchanging the pressurised liquefied second working fluid stream (215a) to provide a vaporised second working fluid stream (245);

- step (iii) comprises:
dynamically expanding the vaporised first working fluid stream (145) to provide power and the expanded vaporised first working fluid stream (105);

dynamically expanding the vaporised second working fluid stream (245) to provide power and the expanded vaporised second working fluid stream (205); and

- at least a portion of the cooling duty transferred to the first working fluid from the one or more hydrocarbons is used to cool at least the second working fluid.
The method according to claim 4 wherein step (ii) comprises heat exchanging the pressurised liquefied second working fluid stream (225) against a heat source (305b) having a temperature of less than or substantially equal to ambient temperature in a second working fluid heat exchanger (240) to provide the vaporised second working fluid stream (245).
The method according to claim 4 or claim 5 wherein step (ii) comprises:
- heat exchanging the pressurised liquefied first working fluid stream (115a) against an at least partially liquefied second working fluid stream (255) in a common working fluid heat exchanger (130), to provide a warmed first working fluid stream (135) and the liquefied second working fluid stream (215);

- heat exchanging the warmed first working fluid stream (135) in a first working fluid heat exchanger (140), against a heat source (305a) having a temperature of less than or substantially equal to ambient temperature to provide the vaporised first working fluid stream (145);

- heat exchanging the expanded vaporised second working fluid stream (205) against the warmed hydrocarbon stream (25) in the second heat exchanger (30), to provide the at least partially liquefied second working fluid stream (255) and a gaseous hydrocarbon stream (45).
The method according to claim 4 or claim 5 wherein step (ii) comprises:
- heat exchanging the pressurised liquefied first working fluid stream (115a) against the expanded vaporised first working fluid stream (105) in the first hydrocarbon stream heat exchanger (20) to provide the liquid hydrocarbon stream (115) and a first warmed first working fluid stream (135a);

- heat exchanging the first warmed first working fluid stream (135a) against the expanded vaporised second working fluid stream (205) in the second hydrocarbon stream heat exchanger (30) to provide a second warmed first working fluid stream (135b) and the liquefied second working fluid stream (215);

- heat exchanging the second warmed first working fluid stream (135b) against a heat source having a temperature of less than or substantially equal to ambient temperature in a heat exchanger (140) which is a first fluid heat exchanger, to provide the vaporised first working fluid stream (145).
The method according to claim 4 wherein in step (i) heat exchanging the expanded vaporised second working fluid stream (205) in the second working fluid circuit (200) against at least the warmed hydrocarbon stream (25) in one or more second hydrocarbon stream heat exchangers (30) further provides an at least partially vaporised hydrocarbon stream (35), step (i) further comprising
- heat exchanging an expanded vaporised third working fluid stream (305) in a third working fluid circuit against at least the at least partially vaporised hydrocarbon stream (35) in one or more third hydrocarbon stream heat exchangers (40) to provide the gaseous hydrocarbon stream (45) and a liquefied third working fluid stream (315);
step (ii) comprises:
- heat exchanging the pressurised liquefied first working fluid stream (115a) against the expanded vaporised first working fluid stream (105) in the first hydrocarbon stream heat exchanger (20) to provide the liquid hydrocarbon stream (115) and a first warmed first working fluid stream (135a);

- heat exchanging the first warmed first working fluid stream (135a) against the expanded vaporised second working fluid stream (205) in the second hydrocarbon stream heat exchanger (30) to provide a second warmed first working fluid stream (135b) and a liquefied second working fluid stream (215);

- heat exchanging the second warmed first working fluid stream (135b) against an expanded vaporised third working fluid stream (305) in the third hydrocarbon stream heat exchanger (40) to provide a third warmed first working fluid stream (135c) and the liquefied third working fluid stream (315);

- heat exchanging the third warmed first working fluid stream (135c) against a heat source having a temperature of less than or substantially equal to ambient temperature, to provide the vaporised first working fluid stream (145);

- heat exchanging a pressurised liquefied third working fluid stream (315a) against a heat source having a temperature of less than or substantially equal to ambient temperature to provide a vaporised third working fluid stream (345); and
dynamic expansion step (iii) further comprises:
- dynamically expanding the vaporised third working fluid stream (345) to provide power and the expanded vaporised second working fluid stream (305)
The method according to claim 8 in which heat exchange step (ii) further comprises:
- heat exchanging the pressurised liquefied second working fluid stream (215a) against the expanded vaporised second working fluid stream (205) in the second hydrocarbon stream heat exchanger (30) to provide a first warmed second working fluid stream (235a);

- heat exchanging the first warmed second working fluid stream (235a) against the expanded vaporised third working fluid stream (305) in the third hydrocarbon stream heat exchanger (40) to provide a second warmed second working fluid stream (235b);

- heat exchanging the second warmed second working fluid stream (235b) against a heat source having a temperature of less than or substantially equal to ambient temperature to provide a vaporised second working fluid stream (245).
The method according to any of the preceding claims wherein the dynamic expansion of the two or more vaporised working fluid streams to provide power comprises:
- dynamically expanding each vaporised working fluid stream (145, 245) in one or more turbines (150, 250), each said turbine (150, 250) driving an electric generator, to generate electrical power.
The method according to any of the preceding claims wherein the liquid hydrocarbon stream (15) is a liquefied natural gas stream and the gaseous hydrocarbon stream (45) is a natural gas stream.
An apparatus (1) for the gasification of a liquid hydrocarbon stream (15) to provide a gaseous hydrocarbon stream (45) and power, said apparatus comprising at least:
- a first working fluid circuit (100) having a first working fluid, said circuit comprising:
a first hydrocarbon stream heat exchanger (20) to condense an expanded vaporised first working fluid stream (105) against a liquid hydrocarbon stream (15) comprising one or more hydrocarbons to transfer cooling duty from the liquid hydrocarbon stream (15) to the expanded evaporated first working fluid stream (145) to provide a liquefied first working fluid stream (115) and a warmed hydrocarbon stream (25);

a first working fluid heat exchanger (140) to heat the liquefied first working fluid stream (115) or a stream derived therefrom (135) against a heat source (405) to provide a vaporised first working fluid stream (145)

a first working fluid turbine (150) to dynamically expand the vaporised first working fluid stream (145) to provide the expanded vaporised first working fluid stream (105) and drive a first working fluid generator (160) to produce power

- a second working fluid circuit (200) having a second working fluid, said circuit comprising:
a second hydrocarbon stream heat exchanger (30) to condense an expanded vaporised second working fluid stream (205) against the warmed hydrocarbon stream (25) to transfer cooling duty from the warmed hydrocarbon stream (25) to the expanded vaporised second working fluid stream (205) to provide a liquefied second working fluid stream (215) and the gaseous hydrocarbon stream (45) or an at least partially vaporised hydrocarbon stream (35);

a second working fluid heat exchanger (240) to heat the liquefied second working fluid stream (215) or a stream derived therefrom (235) against a heat source (405) to provide a vaporised second working fluid stream (245);

a second working fluid turbine (250) to dynamically expand the vaporised second working fluid stream (245) to provide the expanded vaporised second working fluid stream (205) and drive a second working fluid generator (260) to produce power; and

- a heat exchanger (30, 130) to cool at least one of the first and second working fluids with at least a portion of the cooling duty transferred to at least one of the first and second working fluids from one or both of the liquid hydrocarbon stream (15) and the warmed hydrocarbon stream (25).
The apparatus according to claim 12 further comprising:
- a third working fluid circuit (300), having a third working fluid, said circuit comprising:
a third hydrocarbon stream heat exchanger (40) to condense an expanded vaporised third working fluid stream (305) against the at least partially vaporised hydrocarbon stream (35) and one or both of the first and second working fluids to provide a liquefied third working fluid (315) and the gaseous hydrocarbon stream (45);

a third working fluid heat exchanger (340) to heat the third working fluid against a heat source (405) to provide a vaporised second working fluid stream (345);

a third working fluid turbine (350) to dynamically expand the vaporised third working fluid stream (345) to provide the expanded vaporised third working fluid stream (305) and drive a third working fluid generator (360) to produce power; and

- a heat exchanger (40) to cool at least one of the first, second and third working fluids with at least a portion of the cooling duty transferred to at least one of the first and second working fluids from one or both of the liquid hydrocarbon stream (15) and the warmed hydrocarbon stream (25).
The apparatus according to claim 12 or claim 13 wherein at least one, and preferably all, of the heat sources (405) have a temperature of less than or substantially equal to ambient temperature.