EP2564139B1

EP2564139B1 - Process and apparatus for the liquefaction of natural gas

Info

Publication number: EP2564139B1
Application number: EP11717717.0A
Authority: EP
Inventors: Grant Johnson; Timothy Eastwood
Original assignee: Costain Oil Gas and Process Ltd
Current assignee: Costain Oil Gas and Process Ltd
Priority date: 2010-04-30
Filing date: 2011-04-19
Publication date: 2020-06-17
Anticipated expiration: 2031-04-19
Also published as: GB201007297D0; GB2479940B; WO2011135335A3; AU2011247081B2; EP2564139A2; WO2011135335A2; GB2479940A; AU2011247081A1

Description

This invention relates to processes and apparatus for the liquefaction of natural gas. In particular, the invention relates to processes and apparatus in which expansion of a refrigerant through a turbo-expander as part of a refrigeration cycle is used to drive a compressor which increases the pressure of the natural gas feed to the liquefaction process.
Natural gas fields are often located at large distances from consumer markets and effective and economical methods for the transportation of natural gas are an important aspect of the natural gas industry. The majority of natural gas is transported by pipelines, and pipeline networks are well-established in Europe, North America and the former Soviet Union. However, pipelines are costly to construct and are not economically viable for transporting gas from remote gas fields. Liquefaction of natural gas is therefore an important technology in enabling the exploitation of remote gas fields and the supply of natural gas to markets that are not adequately served by pipeline gas supplies.
Liquefied natural gas (LNG) has approximately 1/600 the volume of natural gas in the gaseous state. The reduction in volume makes the transportation of natural gas over large distances much more economical, for example using specially designed cryogenic sea vessels (LNG carriers) or cryogenic road tankers.
Processes for the liquefaction of natural gas involve condensing the natural gas to produce a liquid at or around atmospheric pressure. The natural gas fed to liquefaction processes typically comprises greater than 80% methane together with small amounts of ethane, propane and butane. Heavier hydrocarbons, such as pentane, hexane and benzene, which freeze above the temperature of liquefied natural gas (approximately -162 °C at atmospheric pressure), are usually removed from the natural gas feed by upstream processing, together with other impurities such as water and carbon dioxide which also freeze.
The use of turbo-expander refrigeration cycles is well established in the production of liquefied natural gas. As used herein, the term "turbo-expander" is used to refer to a radial or axial flow turbine through which a pressurised gas is expanded to produce work. The work extracted from the expanding pressurised gas may be used to drive a compressor or generator. Expansion using turbo-expanders is a near isentropic process resulting in a low temperature expanded gas which is used to provide refrigeration in many industrial processes.
In onshore applications, turbo-expander based liquefaction cycles have found use mainly in small-scale plants producing up to approximately 0.1 million tonnes LNG per annum. For larger onshore plants, including the very largest plants producing in excess of 5.0 million tonnes LNG per annum, vapour-compression refrigeration cycles employing mixed hydrocarbon refrigerants or cascaded refrigerants predominate, primarily due to their high thermodynamic efficiency / low power requirements. Turbo-expander cycles have been used at high production capacities for sub-cooling of LNG due to their high thermodynamic efficiency over this low temperature range.
In recent years, advances in offshore technologies and reduced project costs have led to increased interest in the use of offshore facilities, and particularly floating facilities, for the production of liquefied natural gas. Offshore liquefaction of natural gas has the advantage of avoiding the major infrastructure costs associated with onshore facilities. In addition, floating LNG facilities can easily be moved to new, remote gas fields as existing fields decline.
Turbo-expander cycles using gaseous refrigerants are highly appropriate for floating LNG facilities, even at plant capacities of between 0.5 and 3.0 million tonnes per annum typically considered, as they are largely insensitive to vessel movement. They also avoid the liquid hydrocarbon refrigerant inventories associated with other liquefaction processes, giving improved safety, and importantly giving reduced space and weight requirements.
Furthermore, the use of multiple turbo-expanders operating over different temperature levels is typically appropriate in liquefaction of natural gas.
An example of a typical double turbo-expander cycle using nitrogen refrigerant is shown in Figure 1. Liquefaction feed gas 400 is fed to a multi-stream liquefaction heat exchanger 505 where it is condensed and sub-cooled against multiple nitrogen refrigerant streams, exiting the heat exchanger 505 as a sub-cooled liquid 425. Sub-cooled liquid 425 is let down to storage pressure across valve 430 to give a two-phase stream 435, which passes to a vapour-liquid separator 440 to separate a low pressure LNG product stream 450 for storage and a flash gas stream 445.
Refrigeration to produce the LNG product stream is provided by a dual nitrogen turbo-expander refrigeration cycle. Nitrogen is compressed in a two-stage cycle compressor 580/590, incorporating inter- and after-coolers 575/585/595 (typically against air or water) to produce a high-pressure nitrogen stream 500.
High-pressure nitrogen stream 500 is split between warm and cold turbo-expander cycles. Warm nitrogen cycle feed gas is cooled in the liquefaction heat exchanger 505 to produce a cooled nitrogen cycle gas stream 510 at an intermediate temperature by heat exchange with returning cold low-pressure nitrogen cycle gas 520/535. The resulting cooled nitrogen cycle gas 510 is work-expanded in a warm cycle turbo-expander 515 to give a cold low-pressure nitrogen cycle gas 520 and to drive the warm cycle brake compressor 545. Cold low pressure nitrogen cycle gas 520 is reheated in liquefaction heat exchanger 505, cooling the warm and cold nitrogen cycle feed gas streams and cooling, condensing and sub-cooling the natural gas stream 400. The reheated warm cycle low-pressure nitrogen gas stream 540 is fed to the warm cycle turbo-expander brake compressor 545, driven by the warm cycle turbo-expander 515.
Cold nitrogen cycle feed gas is cooled in the liquefaction heat exchanger 505 to produce a cooled nitrogen cycle gas stream 525 at a temperature approaching that of the warm cycle turbo-expander discharge stream 520 by heat exchange with returning cold low-pressure nitrogen cycle gas 520/535. Cooled nitrogen cycle gas 525 is work-expanded in a cold cycle turbo-expander 530 to form a cold low-pressure nitrogen stream 535 and to drive cold cycle brake compressor 560. Cold low-pressure nitrogen cycle gas 535 has a pressure similar to that of the warm expander discharge stream 520 and is reheated in the liquefaction heat exchanger 505, cooling the warm and cold nitrogen cycle feed gas streams and cooling, condensing and sub-cooling the natural gas stream 400. The reheated warm cycle low-pressure nitrogen gas stream 555 is fed to the cold cycle turbo-expander brake compressor 560, driven by the cold cycle turbo-expander 530.
The compressed reheated warm and cold cycle nitrogen gas streams 550/565 are fed to the cycle compressors 585/595 for compression. Document US5768912A discloses a ical prior art nitrogen expander process for natural gas liquefaction.
Liquefaction process thermodynamic efficiency is normally expressed as specific power consumption - for example kWh/kg LNG produced. Reduced specific power consumption can translate either to: (i) lower power consumption for a given capacity; or (ii) higher capacity for a given power consumption. The latter can be of significant benefit to project economics if power is constrained by the output of a particular compressor driver or a particular power generation configuration.
There are a number of methods of reducing specific power consumption, one or more of which may be appropriate, for example:

(i) cooling natural gas feed or refrigerant using mechanical refrigeration, e.g.:
- vapour compression refrigeration;
- absorption refrigeration; or
- low temperature cooling water (e.g. sea water drawn from deep water or chilled water from a refrigeration unit);
(ii) increasing natural gas feed pressure to the liquefaction plant;
(iii) using multiple turbo-expanders operating over different temperature levels; and/or
(iv) using liquid expanders for pressure let down of sub-cooled LNG to reduce flash vapour and LNG production (or to reduce the degree of LNG sub-cooling required for a given LNG production).

All of these methods conventionally require additional equipment and typically increase space requirements and weight.
Increasing the feed gas pressure to the liquefaction plant is an attractive approach to reduce the specific power consumption of natural gas liquefaction processes. Specific power consumption required for compression of refrigerant in the liquefaction refrigeration cycle is reduced at higher feed pressures. This is largely due to the effect of 'straightening' the enthalpy-temperature 'curve' of the natural gas as it is cooled, condensed and subcooled, enabling a better match to be made with the cold refrigerant streams. Furthermore, the power required for compression to boost the feed pressure can be more than offset by the reduction in power required for refrigerant compression in the liquefaction refrigeration cycle. However, inclusion of a feed gas compressor adds to equipment count, and importantly in an offshore application increases space requirements and weight.
Low feed pressure to the liquefaction plant may be a consequence of a low field reservoir pressure or may result from upstream processing for extraction of heavy hydrocarbons and other impurities. As noted above, the removal of heavy hydrocarbons and other impurities is usually required to prevent solidification of these components during the liquefaction process. In some cases deeper hydrocarbon removal is also used to recover a saleable liquid petroleum gas (LPG) product comprising C3 and C4 hydrocarbons in addition to a condensate product to increase revenue, or to adjust LNG composition to meet gas quality specifications.
Several methods can be used to extract the heavy hydrocarbons from natural gas. These usually include the use of one or more stages of heat exchange, pressure reduction and separation, with refrigeration generated through adiabatic expansion across a Joule Thomson valve or by near isentropic expansion across a turbo-expander.
The use of feed pressure to generate refrigeration gives a simple, flexible process for heavy hydrocarbon removal upstream of the liquefaction plant. However, the consequential reduction in liquefaction feed gas pressure leads to increased specific power consumption by the liquefaction plant.
Other heavy hydrocarbon removal processes may also require pressure reduction to achieve the required separation, particularly to avoid operation close to the critical pressure of the vapour.
In the process of the present invention, power is recovered from work-expansion of the refrigerant in the refrigeration cycle to boost feed gas pressure instead of boosting refrigerant pressure. More specifically, a feed gas compressor is driven directly by a turbo-expander in the refrigeration cycle. All power to drive the feed gas compressor is therefore indirectly provided by the refrigeration cycle compressor and its driver. In this way, a reduction in specific power consumption is achieved without the requirement for additional feed gas compression equipment. The process of this invention can therefore facilitate upstream processing for the removal of heavy hydrocarbons and other impurities by enabling feed pressure to the liquefaction plant to be specified largely independently of the upstream processing without the need for additional feed gas compression equipment.
In a first aspect, this invention provides a process for liquefaction of natural gas according to claim 1.
Power input to the process is provided by compression of the fluid refrigerant in step (i), and heat is removed from the refrigeration cycle by heat exchange between the refrigerant and a cooling fluid in step (ii). Any suitable cooling fluid may be used, for instance, water or air. In particular, sea water may be a suitable cooling fluid for use with offshore processes or for onshore processing plants which are conventionally located close to the sea.
Preferably, the compressed natural gas stream is fully condensed and sub-cooled in step (d).
The process of the present invention is not limited to refrigeration cycles using one turbo-expander. For instance, double turbo expander refrigeration cycles may be used. In one preferred embodiment, the first refrigeration cycle further comprises the steps of:

(vi) expanding a second portion of the cooled compressed refrigerant from step (ii);
(vii) passing the expanded cooled refrigerant from step (vi) to the liquefaction heat exchange system to provide a reheated refrigerant; and
(viii) returning the reheated refrigerants from steps (iv) and (vii) to step (i).

Optionally, the reheated refrigerants from steps (iv) and (vii) may be combined prior to being returned to step (i).
In a further preferred embodiment, the second portion of the cooled compressed refrigerant is work-expanded in step (vi) in a second turbo-expander. The second turbo-expander may be used to drive a compressor to provide a portion of the compression in step (i). Alternatively, the second turbo-expander may be used to drive a compressor to provide additional compression of the natural gas feed stream in step (c).
In further embodiments of the invention, one or more further portions of the cooled refrigerant may be work-expanded in one or more further turbo-expanders. The one or more further turbo expanders may be used to drive one or more compressors to provide a portion of the compression in step (i), and/or to provide additional compression of the natural gas stream in step (c).
In the process of the present invention, the first portion and/or the second portion of the cooled compressed refrigerant from step (ii) may be further cooled by heat exchange in the liquefaction heat exchange system prior to expansion.
Where the first refrigeration cycle comprises both a first turbo-expander and a second turbo-expander, there is preferably a temperature differential between the first portion of the cooled compressed refrigerant fed to the first turbo-expander and the second portion of the cooled compressed refrigerant fed to the second turbo-expander. In one embodiment, the first portion of the cooled compressed refrigerant is fed to the first turbo-expander at a higher temperature than the second portion of the cooled compressed refrigerant fed to the second turbo-expander. In a further embodiment, the first portion of the cooled compressed refrigerant is fed to the first turbo-expander at a lower temperature than the second portion of the cooled compressed refrigerant fed to the second turbo-expander. The temperature of the respective portions of the cooled compressed refrigerant may be controlled by heat exchange in the liquefaction heat exchange system, as described above.
In the current invention, the first refrigeration cycle is a gas refrigeration cycle. In a gas refrigeration cycle, the fluid refrigerant is a gas which is successively compressed, cooled, expanded and reheated without changing phase. In the current invention, the gaseous fluid refrigerant consists essentially of nitrogen. Other gases which do not undergo a phase transition in the refrigeration cycle may also be used, for example air, helium or neon. Gas refrigeration cycles are advantageous when used in offshore applications, and particularly when used in floating applications, as the avoidance of liquid refrigerants through use of an inert gaseous refrigerant translates into reduced space and weight, insensitivity to floating vessel motion, and improved safety.
In most cases, the liquefied natural gas product obtained from the liquefaction heat exchange system is at a higher pressure than the storage systems commonly used in the art, which are usually at or slightly above atmospheric pressure, and usually no more than 125 kPa (absolute). Thus, the process of the present invention desirably comprises the step of:

(f) expanding the cooled and at least partly condensed natural gas product withdrawn from the liquefaction heat exchange system to storage pressure.

In a further embodiment, the cooled and at least partly condensed natural gas product withdrawn from the liquefaction heat exchange system is expanded to storage pressure using a liquid expander. The liquid expander will typically be coupled to an electrical generator but may be used to drive a compressor to further compress the fluid refrigerant in step (i) or to further compress the natural gas feed stream in step (c). Alternatively, the cooled and at least partly condensed natural gas product withdrawn from the liquefaction heat exchange system may be expanded to storage pressure under adiabatic conditions using a Joule-Thomson valve. After expansion, the natural gas product will usually comprise a mixture of liquid and vapour, which may be separated in a vapour-liquid separator to provide a LNG product for storage and a vapour stream known as "flash gas". The flash gas may optionally be recompressed and combined with the natural gas feed stream and returned to the liquefaction process.
The liquefaction heat exchange system used in the process of the present invention may function by passing the expanded cooled refrigerant from step (iii) in direct heat exchange with the compressed natural gas stream in step (d).
In other embodiments of the invention, the liquefaction heat exchange system used in the process of the present invention may function by passing the expanded cooled refrigerant from step (iii) in indirect heat exchange with the compressed natural gas stream in step (d), via an intermediate heat exchange fluid or via an intermediate refrigeration cycle.
In accordance with the present invention, one or more additional refrigeration cycles may be used to supplement the refrigeration provided by the first refrigeration cycle. The one or more additional refrigeration cycles may run in parallel and/or in series with the first refrigeration cycle and may be of any type, including gas and vapour-compression refrigeration cycles. In a vapour-compression refrigeration cycle, the refrigerant changes phase between vapour and liquid at the various stages of the refrigeration cycle. Suitable refrigerants for vapour-compression refrigeration cycles include light hydrocarbons, such as methane, ethane, propane and butanes or mixtures thereof. For example, a portion of the natural gas feed may be diverted to act as the refrigerant in a vapour-compression refrigeration cycle. Other suitable refrigerants include the class of hydrofluorocarbon refrigerants. In a preferred embodiment, one or more additional refrigeration cycles may be of the type described above, in which a turbo-expander is used to drive a compressor to compress the natural gas feed stream.
Where the one or more additional refrigeration cycles are run in series with the first refrigeration cycle, the cooling fluid in step (ii) may be a fluid refrigerant from an additional refrigeration cycle. Alternatively, or in addition, the expanded cooled refrigerant from step (iii) may be used as a cooling fluid for an intermediate refrigeration cycle. Cascades of refrigeration cycles are known in the art and, in accordance with the invention, the first refrigeration cycle may occupy any stage of a cascade comprising two or more refrigeration cycles.
Where one or more additional refrigeration cycles are run in parallel with the first refrigeration cycle, the expanded cooled refrigerant from step (iii) and the one or more additional refrigerants may each be passed in direct or indirect heat exchange with the compressed natural gas feed stream (step (d)) in the liquefaction heat exchange system.
For example, one or more additional refrigerants may be used to pre-cool the compressed natural gas stream in step (d) prior to heat exchange contact with the expanded cooled refrigerant from step (iii). Alternatively, the one or more additional refrigerants may be used to sub-cool the cooled and at least partially condensed natural gas from step (d) following heat exchange contact with the expanded cooled refrigerant from step (iii). As a further alternative, the one or more additional refrigerants and the expanded cooled refrigerant from step (iii) may simultaneously be used to cooled and at least partially condense the compressed natural gas stream in step (d), for example where a multi-stream heat exchanger is used in the liquefaction heat exchange system.
The expanded cooled refrigerant from step (iii) and the one or more additional refrigerants may also each be used as cooling fluids for an intermediate refrigeration cycle.
In a preferred embodiment, the compressed natural gas stream is cooled and at least partially condensed in step (d) entirely by heat exchange with the expanded cooled refrigerant from step (iii). More preferably, in accordance with this embodiment, the process of the invention does not involve the use of an additional refrigeration cycle. Where only the first refrigeration cycle is used, space and weight requirements are reduced. As noted above, this is advantageous in offshore processes.
The heat exchangers used in the liquefaction heat exchange system may be of any type known in the art of natural gas liquefaction. For example, shell and tube heat exchangers, plate heat exchangers, plate-fin heat exchangers, spiral wound heat exchangers and diffusion bonded heat exchangers may all be used in the process of the invention. In addition, the liquefaction heat exchange system may comprise more than one heat exchanger, for example when an additional refrigeration cycle is run in parallel with the first refrigeration cycle. Multi-stream heat exchangers may also be used.
The natural gas feed stream in step (b) preferably has a pressure in the range of from 2000 to 5000 kPa (absolute).
The compressed natural gas feed stream from step (c) preferably has a pressure in the range of from 4000 to 10000 kPa (absolute).
In a further embodiment, the natural gas feed stream in step (b) is obtained from an upstream process for the removal of heavy hydrocarbons and/or LPG components and/or water and/or mercury and/or acid gas components. Alternatively, the compressed natural gas stream from step (c) is passed to a process for the removal of heavy hydrocarbons and/or LPG components and/or water and/or mercury and/or acid gas components prior to being passed to the liquefaction heat exchange system in step (d).
In a second aspect, this invention provides an apparatus for liquefaction of natural gas according to claim 12.
Preferably, the apparatus of the invention is adapted to fully condense and sub-cool the compressed natural gas stream in step (d).
In a preferred embodiment, the first refrigeration system further comprises:

(vi) means for expanding a second portion of the cooled compressed refrigerant from step (ii);
(vii) means for conveying the expanded cooled refrigerant from step (vi) to the liquefaction heat exchange system to provide a reheated refrigerant; and
(viii) means for conveying the reheated refrigerant from steps (iv) and (vii) to step (i).

Optionally, the first refrigeration system may further comprise means for combining the reheated refrigerant from step (iv) with the reheated refrigerant from step (vii) prior to step (viii).
The means for expanding a second portion of the cooled compressed refrigerant from step (ii) may comprise a second turbo-expander. The second turbo-expander may advantageously be adapted to drive a second refrigerant compressor in step (i). Alternatively, the second turbo-expander may be adapted to drive a compressor to provide supplementary compression of the natural gas feed stream in step (c).
The apparatus of the present invention may comprise one or more further turbo expanders adapted to work-expand one or more further portions of the cooled refrigerant. The one or more further turbo-expanders may be adapted to drive one or more compressors to provide a portion of the compression in step (i), or to provide additional compression of the natural gas stream in step (c).
The apparatus of the invention may also comprise means for conveying the first portion and/or the second portion of the cooled compressed refrigerant from step (ii) to the liquefaction heat exchange system for further cooling prior to expansion.
The apparatus of the invention preferably comprises means for expanding the cooled and at least partly condensed natural gas product from the liquefaction heat exchange system to storage pressure. For example, the means for expanding the cooled and at least partly condensed natural gas product from the liquefaction heat exchange system to storage pressure comprises a Joule-Thomson valve or a liquid expander. Where the apparatus comprises a liquid expander, the liquid expander may be adapted to drive a compressor to provide further compression of the natural gas feed stream or to supplement the compression of the fluid refrigerant in step (i).
The apparatus of the present invention may further comprise one or more additional refrigeration systems, running in parallel or in series with the first refrigeration system, and which are adapted to supplement the refrigeration provided by the first refrigeration system.
In a preferred embodiment, the apparatus comprises means for removing heavy hydrocarbons and/or LPG components and/or water and/or mercury and/or acid gas components from the natural gas feed stream prior to liquefaction, which may be disposed upstream or downstream from the natural gas feed compressor.
In another aspect, the present invention provides a ship comprising an apparatus as defined above.
In a further aspect, the present invention provides an offshore platform comprising an apparatus as defined above. In accordance with the invention, the offshore platform may be, for example, a fixed platform, a compliant tower platform, a semi-submersible platform, a jack-up platform, a tension-leg platform, a spar platform or a conductor-support platform.
The invention will now be described in greater detail by way of example, with reference to preferred embodiments, and with the aid of the accompanying figures, in which:

Figure 1 shows a conventional double turbo-expander refrigeration cycle;
Figure 2 shows a single turbo-expander apparatus for the liquefaction of natural gas in accordance with the present invention;
Figure 3 shows a double turbo-expander apparatus for the liquefaction of natural gas in accordance with the present invention;
Figure 4 shows the double turbo-expander apparatus of Figure 3 in conjuction with an upstream pre-treatment process for removal of heavy hydrocarbons, carbon dioxide and water; and
Figure 5 shows the double turbo-expander apparatus of Figure 3 in conjunction with another embodiment of an upstream pre-treatment process for removal of heavy hydrocarbons, carbon dioxide and water.

In the embodiment of the invention shown in Figure 2, liquefaction feed gas 400 is compressed in cold expander brake compressor 405, driven by the nitrogen refrigeration cycle cold turbo-expander 530. Compressed feed 410 is cooled to ambient conditions in a heat exchanger 415 (typically against air or water) and the cooled compressed gas stream 420 is fed to a multi-stream liquefaction heat exchanger 505 where it is de-superheated, condensed and sub-cooled against nitrogen refrigerant, exiting the heat exchanger 505 as a sub-cooled liquid 425. Liquid product 425 is let down to storage pressure across valve 430 to give two-phase stream 435, which passes to vapour-liquid separator 440 separating an flash gas stream 445 and low pressure LNG product stream 450 for storage.
Refrigeration to produce the LNG product stream is provided by a nitrogen refrigeration cycle. Nitrogen is compressed in a two-stage cycle compressor 580/590, incorporating inter and after-coolers 585/595 (typically against air or water) to produce a high-pressure nitrogen stream 500.
High-pressure nitrogen stream 500 is cooled in the liquefaction heat exchanger 505 by heat exchange with returning cold low-pressure nitrogen cycle gas 535. Cooled nitrogen cycle gas 525 is work-expanded in a turbo-expander 530 to give a cold low-pressure nitrogen cycle gas 535, which is reheated in the liquefaction heat exchanger 505, cooling the high pressure nitrogen stream 500 and cooling, condensing and sub-cooling the natural gas stream 420.
The re-warmed low-pressure nitrogen cycle gas stream 550 is fed to the cycle compressors 580/590 for compression.
In the embodiment of the invention shown in Figure 3, liquefaction feed gas 400 is compressed in cold cycle brake compressor 405, driven by the cold cycle turbo-expander 530. Compressed feed 410 is cooled to ambient conditions in a heat exchanger 415 (typically against air or water) and the cooled compressed gas stream 420 is fed to a multi-stream liquefaction heat exchanger 505 where it is de-superheated, condensed and sub-cooled against multiple nitrogen refrigerant streams, exiting the exchanger as a sub-cooled liquid 425. Liquid product 425 is let down to storage pressure across valve 430 to give two-phase stream 435, which passes to vapour-liquid separator 440 separating an LNG flash gas stream 445 and low pressure LNG product stream 450 for storage.
Refrigeration to produce the LNG product stream is provided by a dual nitrogen refrigeration cycle. Nitrogen is compressed in a two-stage cycle compressor 580/590, incorporating inter- and after-coolers 585/595 (typically against air or water) to produce a high-pressure nitrogen stream 500.
High-pressure nitrogen stream 500 is split between warm and cold turbo-expander cycles. Warm nitrogen cycle gas is cooled in the liquefaction heat exchanger 505 to produce a cooled nitrogen gas stream 510 at an intermediate temperature by heat exchange with returning cold low-pressure nitrogen cycle gas 520/535. The resulting cooled nitrogen cycle gas 510 is work-expanded in a warm cycle turbo-expander 515 to give a cold low-pressure nitrogen cycle gas 520 and to drive the warm cycle brake compressor 545. Cold low pressure nitrogen cycle gas 520 is reheated in liquefaction heat exchanger 505, cooling the warm and cold nitrogen cycle feed gas streams and cooling, condensing and sub-cooling the natural gas stream 420.
Cold nitrogen cycle gas is cooled in the liquefaction heat exchanger 505 to produce a cooled nitrogen cycle gas stream 525 at a temperature approaching that of the warm cycle turbo-expander discharge stream 520 by heat exchange with returning cold low-pressure nitrogen cycle gas 520/535. Cooled nitrogen cycle gas 525 is work-expanded in a cold cycle turbo-expander 530 to form a cold low-pressure nitrogen stream 535 and to drive cold cycle brake compressor 405. Cold low-pressure nitrogen cycle gas 535 has a pressure similar to that of the warm expander discharge stream 520 and is reheated in the liquefaction heat exchanger 505, cooling the warm and cold nitrogen cycle feed gas streams and cooling, condensing and sub-cooling the natural gas stream 420.
The reheated warm and cold cycle low-pressure nitrogen gas streams 540 are fed to the warm cycle turbo-expander brake compressor 545, driven by the warm cycle turbo-expander 515. The nitrogen cycle gas, boosted in pressure 550, is cooled in a heat exchanger 575 (typically against air or water) and fed to the cycle compressors 580/595 for compression.
In the embodiment shown in Figure 4, the liquefaction apparatus shown in Figure 3 is integrated with a heavy hydrocarbon removal process based on Joule Thomson expansion of feed gas.
Wet natural gas 100 enters the process at elevated pressure. The natural gas feed stream 100, together with liquid stream 395 removed in downstream heavy hydrocarbon removal facilities is fed to an Inlet Separation and Condensate Stabilisation system 105. A liquid stream exits the system as stabilised condensate 110. A vapour stream 200 with reduced heavy hydrocarbon content is fed to the pre-treatment system 210 for removal of acid gas (carbon dioxide) 205 and water 215.
Treated gas 300 is cooled to an intermediate temperature and partially condensed by heat exchange with cold residue gas 360 and liquid hydrocarbon 390 in multi-stream heat exchanger 305. The two-phase stream 310 is let down in pressure across valve 315 and fed to a vapour-liquid separator 325. Liquid stream 330 is let down in pressure across valve 370 and combined with liquid stream 380 from the second cold separator 355.
Vapour stream 335, at an intermediate pressure is further cooled and condensed in heat exchanger 305. The two-phase stream 340 is let down in pressure across valve 345 and fed to a second vapour-liquid separator 355. Liquid stream 365 is let down in pressure across valve 375 and combined with the liquid stream 385 from the first separator. Combined liquid stream 390 is partially vaporised in heat exchanger 305, providing refrigeration to cool the high pressure gas stream 300 and passed to the upstream Inlet Separator and Condensate Stabilisation system 105.
Vapour product 360, with low heavy hydrocarbon content is reheated by heat exchange with high-pressure feed gas 300. The warmed vapour stream 400 is fed to the liquefaction plant.
The liquefaction system and refrigeration cycle configuration are as described for Figure 3.
In the embodiment shown in Figure 5, the liquefaction apparatus shown in Figure 3 is integrated with a heavy hydrocarbon removal process based on work-expansion of feed gas.
Wet natural gas 100 enters the process at elevated pressure. The natural gas feed stream 100, together with liquid stream 380 removed in downstream heavy hydrocarbon removal facilities is fed to an Inlet Separation and Condensate Stabilisation system 105. A liquid stream exits the system as stabilised condensate 110. A vapour stream 200 with reduced heavy hydrocarbon content is fed to the pre-treatment system 210 for removal of acid gas (carbon dioxide) 205 and water 215.
The treated gas stream 300 is cooled and partially condensed by heat exchange with cold residue gas 345 and liquid hydrocarbon 375 in a multi-stream heat exchanger 305. The two-phase stream 310 is fed to a vapour-liquid separator 315. Liquid stream 325 is let down in pressure across valve 365 and combined with the liquid stream 360 from a second cold separator 340.
Vapour stream 320 is work-expanded in turbo-expander 330. The resulting two-phase stream 335 is fed to a second vapour-liquid separator 340. Liquid stream 350 is let down in pressure across valve 355 and combined with the liquid stream 370 from the first separator 315. Combined liquid stream 375 is partially vaporised in heat exchanger 305, providing refrigeration to cool the high pressure gas stream 300 and passed to the upstream Inlet Separator and Condensate Stabilisation system 105.
Vapour product 345, with low heavy hydrocarbon content is reheated by heat exchange with high-pressure feed gas 300. Warmed vapour stream 385 is compressed in the turbo-expander brake compressor 390, driven by the turbo-expander 330. Treated natural gas, boosted in pressure 395, is cooled in heat exchanger 399 (typically against air or water) and fed to the liquefaction plant as stream 400.
The liquefaction system and refrigeration cycle configuration are as described for Figure 3.

Examples

These Examples consider a typical natural gas feed at 6000 kPa (absolute). The carbon dioxide and water concentration is reduced to typical levels of 50 ppm and 0.1 ppm respectively in an upstream pre-treatment system. These components are not shown in the heat and mass balance provided. An upstream heavy hydrocarbon removal system reduces benzene levels to 1.0ppm to avoid solidification during liquefaction.
Predicted performance data is provided for two Examples of this invention. Both Examples demonstrate the recovery of power from work expansion of the refrigeration cycle gas to boost the pressure of the feed gas to liquefaction.
In both Examples the LNG product is sub-cooled such that 5 mol% is flashed on let down to storage pressure across a valve to provide at least part of the plant fuel gas demand. However, an alternative would be to use a liquid expander, which would generate power, reduce flash vapour generation and reduce refrigeration requirements for sub-cooling.

Example 1

This Example corresponds to the embodiment of the invention shown in Figure 4, in which a Joule Thomson process is included for upstream removal of heavy hydrocarbons. This involves cooling and letting down the pre-treated natural gas across a valve to a pressure sufficient to condense the heavy hydrocarbons for removal in a vapour-liquid separator. The number of separation stages required to achieve the required heavy hydrocarbon removal is dependent on the natural gas feed conditions and composition, in this example two separation stages are required. The Joule Thomson process of for heavy hydrocarbon removal is relatively simple, and is able to handle a wide range of flows with high reliability, low space requirements and low weight.
The composition and conditions of the various streams (as shown in Figure 4) is given in Table 1.

Liquefaction System Power / Polytropic Efficiency:

Power Efficiency

Refrigeration Cycle Compressor Shaft Power (580/590): 74.8 MW 85%

Warm Expander-Brake Shaft Power (515) 29.6 MW 82%

Cold Expander-Brake Shaft Power (530) 7.1 MW 82%
Compressor interstage/after cooler process streams cooled to 40°C Warm/Cold Liquefaction cycle expander isentropic efficiencies taken as 88%

Example 2

This Example corresponds to the embodiment of the invention shown in Figure 5, in which a turbo-expander process is included for upstream removal of heavy hydrocarbons. This involves cooling and letting down the pre-treated natural gas in a turbo-expander to a pressure sufficient to condense the heavy hydrocarbons for removal in a vapour-liquid separator. The expansion work generated is used to boost the feed to liquefaction. The number of separation stages required to achieve the required heavy hydrocarbon removal is dependent on the natural gas feed conditions and composition, in this example two separation stages are required.
The turbo-expander process for heavy hydrocarbon removal of Example 2 has the benefit of recovering power from work expansion of the inlet gas. This is used to drive a feed gas compressor, enabling even higher feed pressure to liquefaction and reduced specific power.
The composition and conditions of the various streams (as shown in Figure 5) is given in Table 2.

Upstream Hydrocarbon Removal Power / Polytropic Efficiency:

Power Efficiency

Expander-Brake Shaft Power (390): 1.4 MW 82%

Turbo-expander isentropic efficiency 86%

Liquefaction System Power / Polytropic Efficiency:

Power Efficiency

Refrigeration Cycle Compressor Shaft Power (580/590): 4.8 MW 85%

Warm Expander-Brake Shaft Power (515) 29.6 MW 82%

Cold Expander-Brake Shaft Power (530) 7.1 MW 82%
Compressor inter-stage/after cooler process streams cooled to 40°C. Warm/Cold Liquefaction cycle expander isentropic efficiencies taken as 88%.

Claims

A process for liquefaction of natural gas comprising the steps of:
(a) providing a refrigeration cycle comprising the steps of:
(i) compressing a gaseous refrigerant (550, 540) that consists essentially of nitrogen gas;

(ii) cooling the compressed refrigerant from step (i) in heat exchange with a cooling fluid to provide a cooled compressed gaseous refrigerant (510, 525);

(iii) work-expanding at least a first portion (525) of the cooled compressed gaseous refrigerant (510, 525) from step (ii) in a first turbo-expander (530) to provide an expanded cooled refrigerant (535);

(iv) reheating the expanded cooled refrigerant (535) from step (iii) in a liquefaction heat exchange system (505) to provide a reheated refrigerant (550, 540); and

(v) returning the reheated refrigerant (550, 540) from step (iv) to step (i);

(b) providing a natural gas feed stream (400);

(c) compressing the natural gas feed stream (400);

(d) passing the compressed natural gas feed stream (410) directly or indirectly to heat exchange with the expanded cooled refrigerant (535) from step (iii) in the liquefaction heat exchange system (505); and

(e) withdrawing a cooled and at least partly condensed natural gas product (425) from the liquefaction heat exchange system (505);
wherein the term "turbo-expander" refers to a radial or axial flow turbine through
which a pressurised gas is expanded; characterised in that the first turbo-expander (530) is used to drive a compressor (405) to compress the natural gas feed stream (400) in step (c).
A process according to Claim 1, wherein the compressed natural gas feed stream (410) is fully condensed and sub-cooled in step (d).
A process according to Claim 1 or Claim 2, wherein the refrigeration cycle further comprises the steps of:
(vi) expanding a second portion (510) of the cooled compressed gaseous refrigerant (510, 525) from step (ii);

(vii) passing the expanded cooled refrigerant (520) from step (vi) to the liquefaction heat exchange system (505) to provide a reheated refrigerant (540); and

(viii) returning the reheated refrigerant (540) from steps (iv) and (vii) to step (i).
A process according to Claim 3, wherein the reheated refrigerant (550, 540) from steps (iv) and (vii) are combined prior to being returned to step (i), and optionally wherein the second portion (510) of the cooled refrigerant (510, 525) from step (ii) is work-expanded in step (vi) in a second turbo-expander (515), preferably wherein the second turbo-expander (515) is used to drive a compressor (545) to provide a portion of the compression in step (i) or to drive a compressor to provide additional compression of the natural gas feed stream (400) in step (c).
A process according to Claim 3 or Claim 4, wherein:
(1) the first portion (525) of the cooled compressed gaseous refrigerant (510, 525) from step (ii) is further cooled by heat exchange in the liquefaction heat exchange system (505) prior to expansion; and/or

(2) the second portion (510) of the cooled compressed gaseous refrigerant (510, 525) from step (ii) is further cooled by heat exchange in the liquefaction heat exchange system (505) prior to expansion; and/or

(3) wherein the first portion (525) of the cooled compressed gaseous refrigerant (510, 525) is fed to the first turbo-expander (530) at a lower temperature than the second portion (510) of the cooled compressed gaseous refrigerant (510, 525) fed to the second turbo-expander (515), or the first portion (525) of the cooled compressed gaseous refrigerant (510, 525) is fed to the first turbo-expander (530) at a higher temperature than the second portion (510) of the cooled compressed gaseous refrigerant (510, 525) fed to the second turbo-expander (515).
A process according to any of the preceding claims wherein the refrigerant consists of nitrogen gas.
A process according to any of the preceding claims, wherein one or more additional refrigeration systems are used in parallel or in series with the first refrigeration cycle to supplement the refrigeration provided by the first refrigeration cycle, preferably wherein the one or more additional refrigeration systems comprise a vapour-compression refrigeration cycle.
A process according to any of the preceding claims, wherein the cooling fluid in step (ii) is sea water and/or air, or a refrigerant circulating in a further refrigeration cycle.
A process according to any of the preceding claims, further comprising the step of:
(f) expanding the cooled and at least partly condensed natural gas product (425) withdrawn from the liquefaction heat exchange system (505) to storage pressure, preferably using a Joule-Thomson valve (430) or a liquid expander, wherein the liquid expander is preferably used to drive a compressor to further compress the gaseous refrigerant (550, 540) in step (i) or to further compress the natural gas feed stream (400) in step (c).
A process according to any of the preceding claims, wherein the liquefaction heat exchange system (505) comprises a multi-stream heat exchanger, and/or wherein the liquefaction heat exchange system (505) comprises more than one heat exchanger.
A process according to any of the preceding claims, wherein the natural gas feed stream (400) in step (b): 1) has a pressure in the range of from 2000 to 5000 kPa (absolute); and/or 2) the natural gas feed stream (400) is obtained from an upstream process for the removal of heavy hydrocarbons and/or LPG components and/or water and/or mercury and/or acid gas components; and/or 3) the compressed natural gas feed stream (410) from step (c) has a pressure in the range of from 4000 to 10000 kPa (absolute); and/or 4) the compressed natural gas stream (410) is passed to a process for the removal of heavy hydrocarbons and/or LPG components and/or water and/or mercury and/or acidic components prior to being passed to the liquefaction heat exchange system.
An apparatus for liquefaction of natural gas comprising:
(a) a liquefaction heat exchange system (505);

(b) a refrigeration system comprising:
(i) at least a first refrigerant compressor (580, 590, 545) for compressing a gaseous refrigerant (550, 540) that consists essentially of nitrogen gas;

(ii) means for cooling (575, 585, 595) the compressed refrigerant from step (i) in heat exchange with a cooling fluid to provide a cooled compressed gaseous refrigerant (525);

(iii) a first turbo-expander (530) adapted to work-expand at least a first portion (525) of the cooled compressed gaseous refrigerant (510, 525) from step (ii) to provide an expanded cooled refrigerant (535); and

(iv) means for reheating the expanded cooled refrigerant (535) from step (iii) in the liquefaction heat exchange system (505) to provide a reheated refrigerant (550, 540); and

(v) means for conveying the reheated refrigerant (550, 540) from step (iv) to step (i);

(c) a natural gas feed compressor (405) for compressing a natural gas feed stream (400);

(d) means for conveying a compressed natural gas feed stream (410) from the natural gas compressor (405) directly or indirectly to heat exchange with the expanded cooled refrigerant (535) from step (iii) in the liquefaction heat exchange system (505); and

(e) means for withdrawing a cooled and at least partly condensed natural gas product (425) from the liquefaction heat exchange system (505);
wherein the term "turbo-expander" refers to a radial or axial flow turbine through
which a pressurised gas is expanded; characterised in that the first turbo-expander (530) is adapted to drive the natural gas feed compressor (405) to compress the natural gas feed stream (400) in step (c).
An apparatus according to Claim 12, which is adapted to fully condense and sub-cool the compressed natural gas stream (410) in step (d).
An apparatus according to Claim 12 or Claim 13, wherein the refrigeration system further comprises:
(vi) means for expanding a second portion (510) of the cooled compressed gaseous refrigerant (510, 525) from step (ii), preferably wherein the means for expanding a second portion (510) of the cooled compressed gaseous refrigerant (510, 525) from step (ii) comprises a second turbo-expander (515), more preferably wherein the second turbo-expander (515) is adapted to drive a second refrigerant compressor (545) in step (i), or to drive a compressor adapted to provide supplementary compression of the natural gas feed stream (400) in step (c);

(vii) means for conveying the expanded cooled refrigerant (520) from step (vi) to the liquefaction heat exchange system (505) to provide a reheated refrigerant (540); and

(viii) means for conveying the reheated refrigerant (540) from steps (iv) and (vii) to step (i); and
optionally further comprising means for combining the reheated refrigerant (550) from step (iv) with the reheated refrigerant (540) from step (vii) prior to step (viii), and/or means for conveying the first portion (525) and/or the second portion (510) of the cooled compressed gaseous refrigerant from step (ii) to the liquefaction heat exchange system (505) for further cooling prior to expansion.
An apparatus according to any of Claims 12 to 14, comprising means for expanding the cooled and at least partly condensed natural gas product (425) from the liquefaction heat exchange system (505) to storage pressure, preferably wherein the means for expanding the cooled and at least partly condensed natural gas product from the liquefaction heat exchange system to storage pressure comprises a Joule-Thomson valve (430) or a liquid expander.
An apparatus according to any of Claims 12 to 15, wherein the apparatus further comprises one or more additional refrigeration systems running in parallel or in series with the first refrigeration system, and which are adapted to supplement the refrigeration provided by the first refrigeration system; and/or wherein the apparatus comprises means for removing heavy hydrocarbons and/or LPG components and/or water and/or mercury and/or acidic components from the natural gas feed stream (400) prior to liquefaction.
A ship or an offshore platform comprising an apparatus as defined in any of Claims 12 to 16.
Use of one or more turbo-expanders (530) to compress the natural gas feed (400) to the apparatus for the liquefaction of natural gas as defined in claim 12 wherein the one or more turbo-expanders (530) are driven by expansion of the refrigerant that consists essentially of nitrogen gas as part of the refrigeration cycle used to provide cooling to the liquefaction plant, wherein the term "turbo-expander" refers to a radial or axial flow turbine through which a pressurised gas is expanded.