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Title:
EXTRACTION OF NATURAL GAS LIQUIDS AND COOLING OF TREATED RECOMPRESSED GAS
Document Type and Number:
WIPO Patent Application WO/2015/158395
Kind Code:
A1
Abstract:
An apparatus for processing of natural gas feed gas NG comprises: an upstream natural gas liquid extraction part (11) using a distillation column (28), where natural gas liquids NGL are extracted from the bottom of the distillation column (28) and a treated natural gas stream can be taken from the top of the distillation column (28); and a compression and cooling part (21) where treated natural gas from the top of the distillation column (28) is compressed and cooled; wherein after the natural gas liquid extraction part (11) and before the compression and cooling part (21) the treated natural gas process stream passes through a pre-cooling part (31), which is used to generate cooling duty for the treated natural gas process stream NG' after it has passed the compression and cooling part (21).

Inventors:
MARÅK KNUT ARILD (NO)
JØRSTAD ODDVAR (NO)
Application Number:
PCT/EP2014/057946
Publication Date:
October 22, 2015
Filing Date:
April 17, 2014
Export Citation:
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Assignee:
STATOIL PETROLEUM AS (NO)
International Classes:
C10L3/10; C10G5/06; F25J3/00; B63B35/44
Foreign References:
US5685170A1997-11-11
US6182469B12001-02-06
US20130180282A12013-07-18
US20050204774A12005-09-22
US20100206003A12010-08-19
US20030005722A12003-01-09
US20110005273A12011-01-13
Attorney, Agent or Firm:
TAYLOR, Adam (10 Salisbury Square, London EC4Y 8JD, GB)
Download PDF:
Claims:
CLAIMS:

1. An apparatus for processing of natural gas feed gas, the apparatus comprising:

an upstream natural gas liquid extraction part using a distillation column, where natural gas liquids are extracted from the bottom of the distillation column and a treated natural gas stream can be taken from the top of the distillation column; and a compression and cooling part where treated natural gas from the top of the distillation column is compressed and cooled;

wherein after the natural gas liquid extraction part and before the

compression and cooling part the treated natural gas process stream passes through a pre-cooling part, which is used to generate cooling duty for the treated natural gas process stream after it has passed the compression and cooling part.

2. An apparatus as claimed in claim 1 , wherein pre-cooling part comprises a pre-cooling heat exchanger for heat exchange between treated natural gas process stream within the pre-cooling part with the treated natural gas process stream after it passes out of the pre-cooling part and through the compression and cooling part of the apparatus.

3. An apparatus as claimed in claim 2, wherein the pre-cooling part includes an expander for reducing the temperature of the treated process stream before it passes through the pre-cooling heat exchanger.

4. An apparatus as claimed in claim 2 or 3, wherein the pre-cooling heat exchanger receives process fluid both after completion of treatment in the natural gas liquid extraction part and also directly from the top of the distillation column of the natural gas liquid extraction part, wherein a part of the output of the top of the distillation column is used in the pre-cooling heat exchanger whilst the remainder of the output of the top of the distillation column provides cooling duty in a feed stream heat exchanger within the natural gas liquid extraction part.

5. An apparatus as claimed in any preceding claim, wherein the natural gas liquid extraction part has a natural gas feed gas inlet coupled via a feed stream heat exchanger and an expander to the distillation column; and wherein the treated natural gas process stream after distillation is used for heat exchange to cool the incoming feed stream in the feed stream heat exchanger, and then subsequently is compressed before further treatment in the pre-cooling part and the compression and cooling part.

6. An apparatus as claimed in any preceding claim, wherein the pre-cooling part incorporates a recycling line for taking a part of the treated natural gas process stream after compression in the compression and cooling part and mixing this part into the process stream as it enters a/the pre-cooling expander.

7. An apparatus as claimed in any preceding claim, comprising a by-pass line for selectively by-passing the pre-cooling part by connecting the output of the natural gas extraction part directly to the compression and cooling part.

8. A floating liquid natural gas production system incorporating an apparatus as claimed in any preceding claim.

9. A process for separation of natural gas liquids from natural gas feed gas and for cooling the treated gas, the process comprising:

receiving a natural gas feed stream;

using an upstream natural gas liquid extraction stage with a distillation column to extract natural gas liquids from the feed gas stream, where natural gas liquids are extracted from the bottom of the distillation column and a treated natural gas stream can be taken from the top of the distillation column;

passing the treated natural gas process stream through a pre-cooling stage, which is used to generate cooling duty;

compressing and cooling the treated natural gas process stream after it has been used to generated cooling duty in the pre-cooling stage and further cooling the treated natural gas process stream after it has passed the compression and cooling stage by means of the cooling duty generated by the pre-cooling stage.

10. A process as claimed in claim 9, wherein the pre-cooling stage uses a pre- cooling heat exchanger for heat exchange between treated natural gas process stream within the pre-cooling stage with the treated natural gas process stream after it passes out of the pre-cooling stage and through the compression and cooling stage of the apparatus.

11. A process as claimed in claim 10, wherein the pre-cooling stage includes compression, cooling and expansion of the process stream, for reducing the temperature of the treated process stream before it passes through the pre-cooling heat exchanger.

12. A process as claimed in claim 10 or 1 1 , wherein the pre-cooling stage heat exchanger receives the treated natural gas process stream both after completion of treatment in the natural gas liquid extraction part and directly from the top of the distillation column, whereby a part of the output of the top of the distillation column is separated and passed through in the pre-cooling heat exchanger whilst the remainder of the output of the top of the distillation column is used to provide cooling duty to cool the feed stream within the natural gas liquid extraction stage.

13. A process as claimed in any of claims 9 to 12, wherein the natural gas liquid extraction stage includes cooling and then expansion of a natural gas feed gas before passing the process stream to the distillation column; and wherein the treated natural gas process stream after distillation is used for heat exchange to cool the incoming feed stream, and then subsequently is compressed before further treatment in the pre-cooling stage and the compression and cooling stage.

14. A process as claimed in any of claims 9 to 13, wherein the pre-cooling stage incorporates a step of recycling a part of the treated natural gas process stream after compression in the compression and cooling stage, where the part is mixed into the process stream as it enters a/the pre-cooling expander.

15. A process for producing liquid natural gas on a floating platform, the process comprising a process as claimed in any of claims 9 to 14.

16. A process or an apparatus for processing natural gas substantially as hereinbefore described with reference to Figure 5, 6 or 7 of the accompanying drawings.

Description:
EXTRACTION OF NATURAL GAS LIQUIDS AND

COOLING OF A TREATED RECOMPRESSED GAS

The invention relates to an apparatus for extraction of natural gas liquids from a natural gas feed gas, and to pre-cool the natural gas feed gas. In preferred implementations the invention relates to an apparatus and a method for use with Floating Liquefied Natural Gas (FLNG) systems, where there are space and weight restrictions.

When liquefying a natural gas feed gas, natural gas liquids (NGL - ethane, propane and heavier components) are extracted from the natural gas feed gas for three main reasons:

• The C6+ components (hexanes and heavier) will freeze during the liquefaction process and the concentration of these must hence be reduced to a low concentration before liquefaction.

• Ethane, propane and butanes are often needed as refrigerants in the refrigeration system. They are conveniently found in the feed gas and extracted in the NGL-extraction system.

• Propane and butanes (LPG) as a separate product can have a higher value than their value as a constituent in the liquefied natural gas (LNG) product.

Processes are known for extracting the NGLs, as described below in connection with Figures 1 to 3. The known processes can require large amounts of space and heavy equipment. They are therefore not well suited to FLNG systems where the available space is limited and the weight of the equipment is restricted. In prior art arrangements that are more compact, there is an undesirable loss in efficiency. There is therefore a need for a process and apparatus for NGL extraction and precooling that is compact and simple but also maintains energy efficiency when used, for example, with a LNG system.

Viewed from a first aspect the invention provides an apparatus for processing of natural gas feed gas, the apparatus comprising: an upstream natural gas liquid extraction part using a distillation column, where natural gas liquids are extracted from the bottom of the distillation column and a treated natural gas stream can be taken from the top of the distillation column; and a compression and cooling part where treated natural gas from the top of the distillation column is compressed and cooled; wherein after the natural gas liquid extraction part and before the compression and cooling part the treated natural gas process stream passes through a pre-cooling part, which is used to generate cooling duty for the treated natural gas process stream after it has passed the compression and cooling part.

Hence, the treated natural gas process stream, which is the process stream after treatment to remove the natural gas liquids (NGL), is used for auto refrigeration, providing pre-cooling for itself. The apparatus provides treated natural gas at a lower temperature than apparatuses without the pre-cooling part. This arrangement leads to a compact and relatively lightweight apparatus, which is ideal for use in FLNG systems. There is no requirement for a separate pre-cooling system, which avoids the need for additional parts for storage and handling of a separate refrigerant. For example, in some prior art systems C0 2 refrigerant is used. This might be available from an upstream C0 2 -removal system, but the C0 2 must be dried of water, purified and liquefied. This requires equipment which adds weight and takes up space. This equipment is avoided by the currently proposed process and apparatus.

The inventors have made the realisation that by careful selection and combination of elements to form a new process as described above, then it is possible to devise an optimal solution that forms a compromise between known compact but inefficient systems and known efficient but bulky and/or weighty systems. Integrating a pre-cooling process into NGL extraction process allows it to operate more efficiently. The process can run independently of the liquefaction part of a LNG production system that it may precede. The LNG liquefaction technology need not change when used with the proposed process. In preferred embodiments the apparatus is for NGL extraction for a LNG production apparatus, for example for use on-board a FLNG vessel. The invention extends to an LNG or FLNG production apparatus incorporating the proposed NGL extraction apparatus.

The pre-cooling part may comprise a pre-cooling heat exchanger for heat exchange between treated natural gas process stream within the pre-cooling part with the treated natural gas process stream after it passes out of the pre-cooling part and through the compression and cooling part of the apparatus. The pre-cooling part in preferred embodiments includes an expander for reducing the temperature of the treated process stream before it passes through the pre-cooling heat exchanger. In some examples the pre-cooling part also includes a compressor and cooler for applying further cooling to a gas stream, for example there may be a compressor, cooler and expander, in sequence, that cool the treated process stream before it passes through the pre-cooling heat exchanger. The expander and compressor may advantageously be interconnected, for example as a compander. Optionally, a bypass line may be provided to enable the process stream to by-pass the expander and to pass through an expansion valve instead. This is useful in the event of failure of the expander (or the compander) of the pre-cooling part.

In a preferred embodiment the natural gas liquid extraction part has a natural gas feed gas inlet coupled via a feed stream heat exchanger and an expander to the distillation column. There may be a flash tank between the heat exchanger and expander, with the expander receiving the gas from the flash tank and the liquid from the flash tank passing, optionally via an expansion valve, to the distillation column. The treated natural gas process stream after distillation may be used for heat exchange to cool the incoming feed stream in the feed stream heat exchanger, and then subsequently it is preferably compressed before it is passed to the pre-cooling part and/or the compression and cooling part. In some embodiments, after compression in the natural gas extraction part compressor the treated natural gas process stream is passed to the compressor of the pre-cooling part. An additional cooler may optionally be used between the natural gas extraction part compressor and the compressor of the pre-cooling part. Preferably the compressor and expander of the natural gas liquid extraction part of the apparatus are coupled together, for example as a compander.

As well as receiving the treated natural gas process stream after compression in the natural gas extraction part the pre-cooling heat exchanger may also be connected directly to the top of the distillation column of the natural gas liquid extraction part, whereby a part of the output of the top of the distillation column is used in the pre-cooling heat exchanger whilst the remainder of the output of the top of the distillation column provides cooling duty in the feed stream heat exchanger. In this case, it is preferred for the part of the output used in the pre-cooling heat exchanger to be returned to the remainder after the remainder has passed through the feed stream heat exchanger, such that the process stream is re-joined before compression in the compressor of the natural gas liquid extraction part. As an optional added feature, or as an alternative to splitting the output of the top of the distillation column, the pre-cooling heat exchanger and feed stream heat exchanger may be combined into a single heat exchanger arrangement.

The pre-cooling part may optionally incorporate a recycling line for taking a part of the treated natural gas process stream after compression in the compression and cooling part and mixing this part into the process stream as it enters the pre- cooling expander. The recycling line may for example be coupled to the output from the cooler of the compression and cooling part before the pre-cooling heat exchanger. Alternatively, recycling line may for example be coupled to the output of the pre-cooling heat exchanger. Recycling lines from both sides of the pre-cooling heat exchanger could be used.

The compression and cooling part preferably comprises a compressor with a driver, and a cooler. The cooler may cool the process stream to ambient temperature or close to the ambient temperature, for example using water as a coolant. The driver for the compressor may be an electric motor, a gas turbine driver or alternatively it may be a turbine such as an expander used in another processing system.

The apparatus may incorporate a by-pass line for connecting the output of the natural gas extraction part directly to the compression and cooling part. This by-pass line allows for the pre-cooling part to be by-passed, for example for maintenance purposes.

The various features of the apparatus may each operate with temperatures and pressures as given below in discussion of preferred embodiments with reference to the Figures. The disclosed ranges of temperatures and/or pressures apply generally to the component parts and will apply in the same way even if not all parts of the preferred embodiment are present.

Viewed from a second aspect the invention provides a process for separation of natural gas liquids from natural gas feed gas and for cooling the treated gas, the process comprising: receiving a natural gas feed stream; using an upstream natural gas liquid extraction stage with a distillation column to extract natural gas liquids from the feed gas stream, where natural gas liquids are extracted from the bottom of the distillation column and a treated natural gas stream can be taken from the top of the distillation column; passing the treated natural gas process stream through a pre- cooling stage, which is used to generate cooling duty; compressing and cooling the treated natural gas process stream after it has been used to generated cooling duty in the pre-cooling stage and further cooling the treated natural gas process stream after it has passed the compression and cooling stage by means of the cooling duty generated by the pre-cooling stage.

Hence, as with the apparatus of the first aspect, in this process the treated natural gas process stream, which is the process stream after treatment to remove the natural gas liquids (NGL), is used for auto refrigeration, providing pre-cooling for itself. The process may include equivalent features to those described above in connection with the apparatus. The process can run independently of the liquefaction part of a LNG production system for which it may provide treated natural gas as an input feed stream. The process may be a part of an LNG or FLNG process and the invention extends to a LNG or FLNG production process incorporating the process of the second aspect.

The pre-cooling stage of the process may use a pre-cooling heat exchanger for heat exchange between treated natural gas process stream within the pre-cooling stage with the treated natural gas process stream after it passes out of the pre- cooling stage and through the compression and cooling stage of the apparatus. The pre-cooling stage in preferred embodiments includes compression, cooling and expansion of the process stream, for reducing the temperature of the treated process stream before it passes through the pre-cooling heat exchanger. The compression and expansion may be provided by an interconnected expander and compressor, for example a compander.

In a preferred embodiment the natural gas liquid extraction stage includes cooling and then expansion of a natural gas feed gas before passing the process stream to the distillation column. The cooling may occur within a feed stream heat exchanger. There may be a flash tank between the heat exchanger and expander, with the expander receiving the gas from the flash tank and the liquid from the flash tank passing, optionally via an expansion valve, to the distillation column at a lower pressure. The treated natural gas process stream after distillation may be used for heat exchange to cool the incoming feed stream in the feed stream heat exchanger, and then subsequently it is preferably compressed before further treatment in the pre-cooling stage and/or the compression and cooling stage. In some embodiments, after compression in the natural gas extraction stage the treated natural gas process stream is passed to the pre-cooling stage for compression. An additional cooling step may optionally be present between the natural gas extraction stage

compression and compression in the pre-cooling stage. Preferably the compressor and expander of the natural gas liquid extraction stage are coupled together, for example as a compander.

The pre-cooling stage heat exchanger preferably receives the treated natural gas process stream after compression in the natural gas extraction stage and, for example, after compression, cooling and expansion in the pre-cooling stage. The pre-cooling stage heat exchanger may also receive a process stream directly from the top of the distillation column, whereby a part of the output of the top of the distillation column is separated and passed through in the pre-cooling heat exchanger whilst the remainder of the output of the top of the distillation column is used to provide cooling duty in the feed stream heat exchanger. In this case, it is preferred for the part of the output used in the pre-cooling heat exchanger to be returned to the remainder after the remainder has passed through the feed stream heat exchanger, such that the process stream is re-joined before compression in the compressor of the natural gas liquid extraction part. As an optional added feature, or as an alternative to splitting the output of the top of the distillation column, the pre- cooling heat exchanger and feed stream heat exchanger may be combined into a single heat exchanger arrangement.

The pre-cooling stage may optionally incorporate a step of recycling a part of the treated natural gas process stream after compression in the compression and cooling stage, where the part is mixed into the process stream as it enters the pre- cooling expander. The recycling line may for example be coupled to the output from the cooler of the compression and cooling part before the pre-cooling heat exchanger. Alternatively, recycling line may for example be coupled to the output of the pre-cooling heat exchanger. Recycling lines from both sides of the pre-cooling heat exchanger could be used.

Certain preferred embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings in which:

Figure 1 shows NGL extraction using pre-cooling provided by a separate refrigerant and a scrub column;

Figure 2 illustrates NGL extraction using a distillation column without pre- cooling;

Figure 3 shows distillation column NGL extraction similar to Figure 2 with the addition of compression of the purified natural gas stream;

Figure 4 shows a LNG production system using the Niche LNG process;

Figure 5 shows an NGL extraction process with auto refrigeration for pre- cooling of the treated natural gas;

Figure 6 is a diagram of a process that is similar to the process of Figure 5, with some alternative and/or optional features;

Figure 7 shows some more alternative and/or optional features in another process that is similar to the process of Figure 5; and

Figure 8 shows a further alternative arrangement for NGL extraction with auto refrigeration for pre-cooling of the treated natural gas

For understanding the proposed new process it is instructive to first understand the basic operation of LNG production plants and to also understand typical processes for NGL extraction.

In some LNG production plants, the NGL is extracted by an integrated scrub column which is located after the pre-cooling system. A typical scrub column configuration is illustrated in Figure 1. Important features of this arrangement are that the pressure is the same as in the feed gas and that large amounts of reflux are therefore needed to wash out the heavy components.

As can be seen, the first part of the system is a pre-cooler, which uses heat exchangers 2, 4, 6 in combination with high, medium and low pressure suction drums, 8, 10, 12, compressor 14 and condenser 16, as well as expansion valves and ancillary parts. The second part of the system, on the right in Figure 1 , is for NGL extraction and it uses a scrub column 18 with a reboiler 20 and a reflux condenser 22, which is coupled to the scrub column 18 via a reflux flash drum 24. Reflux condenser 22 is often integrated into the much larger liquefaction heat exchanger of an LNG production system (not shown). The pre-cooling system receives one or more refrigerant stream(s) R and a natural gas feed stream NG. Refrigerant stream(s) R are passed to a liquefaction heat exchanger after they are used in the pre-cooler stage for the natural gas stream NG and may also provide the refrigeration duty in the reflux condenser 22. The pre-cooled feed stream NG is passed to the scrub column 18 in the NGL-extraction stage, and the output of the NGL extraction stage consists of natural gas liquids (NGL) from the bottom of the column 18 and refined natural gas NG' from the reflux flash drum 24.

This arrangement has a simple construction, but it requires a large reflux stream and this has an efficiency penalty. In addition, as the pressure during separation is high, there are physical limits to how much of the C3+ components can be taken out from the gas stream. The NGL will also contain some methane that must be taken out in the subsequent NGL-fractionation, where the methane will have to be recompressed before it is mixed with "natural gas to liquefaction".

Another possible method is upstream NGL removal. Upstream NGL removal is used in a few LNG plants. An example of this type of system is shown in Figure 2. It is characterized in that the natural gas feed stream NG is first cooled to a certain degree and the cooled gas is then expanded to reduce the pressure and thereby also the temperature. This expansion can take place in a valve or more often in an expander turbine 26 of a compressor-expander 26, 27 as in Figure 2. The incoming feed stream NG is cooled by a heat exchanger 32, which uses process streams from elsewhere in the system. The feed stream passes through flash tank 30 before the gas part is passed to the expander. The liquid part from the flash tank 30 is used in heat exchanger 32.

The now low pressure and low temperature gas/liquid mixture enters a distillation column 28, which is coupled to a reboiler and reflux flash tank/reflux pump as is known in the art. The lower pressure makes the distillation process much easier than in the scrub column 18 of Figure 1. Therefore, the amount of reflux is much smaller. The distillation column 28 is used to provide a feed stream for the heat exchanger 32 and also receives the liquid from the flash tank 30, after it is heated by heat exchange in heat exchanger 32.

The gas exiting the top of the distillation column 28 is usually sent via a condenser 34 to a heat exchanger 32 to provide refrigeration for the incoming feed stream NG. Then it is compressed in a compressor 27 which can be connected to the expander turbine 26 in the compressor-expander. The gas then exits as refined natural gas NG' at close to ambient temperature and at a lower pressure than the incoming gas. It is this net pressure reduction through the process that drives the NGL-extraction. The NGL extracted by the system is output from the bottom of the distillation column 28, which has a lower content of methane than the bottom product from the scrub column in Figure 1.

Various configurations are used in upstream NGL removal systems and these are generally conventional ways of extracting liquid products from a natural gas as used in other gas processing plants (i.e. non-LNG plants). There are various known ways to take out heavier components, both to achieve a dew-point specification by also to take out valuable components such as propane and butanes from natural gas. In contrast to the scrub column in Figure 1 , upstream NGL-removal processes require no external cooling.

The term upstream in this context is used where the NGL-extraction can be done independently from the other parts of the plant (i.e. not integrated as in Figure 1 ), as well as indicating that it is done upstream of the pre-cooling part of an LNG plant. It is also characterized in that the treated gas temperature is at around ambient temperature as it exits the NGL-extraction system.

Liquefaction for LNG production gives rise to further requirements for an optimal NGL extraction system. For thermodynamic reasons, the energy

consumption for liquefying a natural gas stream into LNG is decreased if the pressure of the natural gas is higher. It is therefore advantageous if the refined stream NG' is at an elevated pressure. The reason for this is two-fold; firstly, the enthalpy that must be removed from the natural gas in the liquefaction process is reduced when the pressure is higher. Secondly, the heat that must be removed from the natural gas is removed at a higher temperature.

Compression beyond 50-60 bar is normally not desired upstream of a scrub column as the density increases and separation in the scrub column is made more difficult.

As all compression of a gas leads to temperature increase, compressing the gas out of the scrub column leads to a temperature after compression being close to ambient temperature. As a result, the natural gas must undergo pre-cooling twice; one before the scrub column and one after the compression. Feed gas compression is therefore usually not suited for NGL-extraction where scrub columns are used.

On the other side, gas compression is therefore well suited after upstream NGL-removal such as the process shown in Figure 2, which could make this a more attractive option for use with NGL extraction used for LNG production processes. After the upstream NGL-removal system, the gas temperature is close to the ambient temperature. The temperature increase due to the compression can then easily be removed to the surroundings by air-cooling or water cooling. The gas then goes to pre-cooling after this compression, and further to liquefaction and sub cooling without further treatment.

Figure 3 is an example showing upstream NGL-extraction combined with compression. The NGL extraction part of this, in the lower part 11 of the Figure, can be regarded as a simplified illustration of Figure 2 and hence has a feed stream NG passing through heat exchanger 32 and via flash tank 30 and an expander 26 of a compressor-expander 26, 27 to a distillation column 28. The distillation column 28 produces the natural gas liquid product NGL at its bottom, and gas from the top of the column is used in heat exchanger 32, compressed in compressor 27 and cooled before being sent for further optional compression in a compressor 34 of a compression stage of the system (shown in the upper part 21 of Figure 3). This compressor 34 is driven by a driver 35, for example an electric motor or a gas turbine driver. After the compressor 34 the gas is cooled in an air or water cooler 36. From there, the refined natural gas stream NG' enters the LNG liquefaction plant. The temperature of the gas is now typically around 20 - 40°C and the pressure somewhere between 50 bar and 100 bar.

For LNG production pre-cooling of the incoming process stream NG' is also important. The pre-cooling of a natural gas for LNG production is the first cooling step, cooling the process streams to approx. -30 to -50 °C. In most LNG plants, this pre-cooling also provides cooling and part- of full condensation of the refrigerants. It also provides cooling and light condensation of the natural gas stream.

In most known land-based LNG-plants, pre-cooling is done by a propane refrigeration system. This system often involves large units and a large content of liquid propane. For Floating Liquefied Natural Gas (FLNG) systems, where weight and deck space is scarce and health and safety risks must be minimized, propane pre-cooling is not regarded as suitable.

As alternatives to propane, the pre-cooling refrigerant can be -134a or C0 2 . These refrigerants are safer since they are not flammable and yield around the same thermodynamic efficiency as propane. Also mixed hydrocarbon refrigerants can be used for pre-cooling, such as in the Mixed Fluid Cascade (MFC®) process developed by Linde or Shell's Double Mixed Refrigerant (DMR) process. The system of Figure

1 shows an example of a system using a single component refrigerant cycle for pre- cooling. Whatever process is used, a pre-cooling system by using a refrigerant cycle will always involve a system with a lot of piping, heat exchangers, storage, purification/import and handling, which takes up space and weight. This is a significant disadvantage when space and weight are constraints, as in FLNG systems.

In place of cooling with a separate refrigerant cycle, cooling can be provided by expanding natural gas in turbo-expanders. This is used in the processes of Figure

2 and Figure 3 with expansion in the compressor-expanders 26. The cooled process stream can be used as a refrigerant to cool other streams by heat transfer. This arrangement can be put to use for pre-cooling in a LNG production process. One example of this is found in the Niche LNG process. US 6412302 describes this type of system. The same principle is used in the exemplary LNG production system illustrated in Figure 4.

Here, the incoming natural gas NG' (i.e. the feed stream with NGL removed) is pre-cooled and a part of the pre-cooled stream is taken out, expanded and used as refrigerant for its own cooling. The low pressure natural gas (Low Pressure NG) is then recompressed and reinjected into the feed stream. The natural gas stream in this way indirectly cools itself, which sometime is denoted "auto-refrigeration". A separate nitrogen cooling cycle 38 is used to further cool the natural gas before a final expansion step into a flash tank 40 creates the required LNG output stream, as well as flash gas. In Figure 4 the nitrogen cooling cycle 38 is on the right hand side, with the auto refrigeration cycle on the left hand side, both providing cooling to the main process stream for LNG production via a multi-stream heat exchanger 42.

Figure 5 illustrates an example embodiment for an integrated NGL extraction and pre-cooling process. This is a combined process where NGL-extraction, compression and pre-cooling of the natural gas is integrated into one process module without using external refrigerant such as propane, C0 2 or R-134a. Advantageously, the process can be run independently of the liquefaction part of the LNG plant. The NGL extraction part 1 1 and compression part 21 are broadly similar to the NGL extraction part 1 1 and compression part 21 described above in relation to the lower and upper parts 1 1 , 21 of Figure 3. Thus, after NGL extraction using heat exchanger 31 , flash tank 30, distillation column 28 and expander-compressor- 26, 27 a purified natural gas stream is compressed and cooled via compressor 34 and cooler 36. The process of Figure 5 differs considerably from the process of Figure 3 in that a new pre-cooling part 31 is added. This part 31 takes compressed gas from the compressor 27 of the expander-compressor- 26, 27 in the NGL extraction part 1 1 and uses it for generating refrigeration in a heat exchanger 44 to pre-cool the refined natural gas stream NG' by auto-refrigeration.

Thus, in Figure 5, the gas leaving the compressor 27 in the NGL extraction part 1 1 is passed to a compressor 46 for further compression, cooled to near ambient temperature in cooler 48 and expanded in expander 50 to provide refrigeration in the heat exchanger 44. Then the gas is compressed in compressor 34 before being cooled in aftercooler 36 by using air or water to ambient temperature (around 10-40 °C). The now treated gas NG', which is at a high pressure, is sent through 44 where it is pre-cooled to a temperature ranging from -10°C to -40°C.

The net exergy for the pre-cooling is provided by the change in conditions for the compressor 34, which has to compress the gas from a lower pressure compared to the equivalent compressor 34 in the process of Figure 3. Advantageously, the expander (or turbine) 50 of the pre-cooling part 31 is connected to the same shaft as the pre-cooling compressor 46 and so no driver is used here.

An additional, optional, feature is that if expander 50 and compressor 46 are out of service then the system may still operate. A valve 52 can be provided to bypass the expander 50, as shown by the dashed line, or alternatively to by-pass both the compressor 46 and the expander 50 as shown by the dashed and dotted lines. In the case of a failure then the gas is sent through valve 52 (along the dotted line), which will provide some cooling but in a less efficient way than the use of the expander 50. In this case the compressor 34 has to provide some additional compression.

To allow for the pre-cooling part 31 to be completely taken out of service a bypass line 54 can be provided, which essentially switches the system back to the configuration of Figure 3, assuming that the heat exchanger 44 has no effect or is bypassed (for example as with the by-pass 60 described in relation to Figure 7 below).

Some optional and alternative features are illustrated in Figure 6. Extra refrigeration could perhaps be provided in heat exchanger 44 if a part of the pre- cooled, high pressure treated gas NG' is recycled through a valve 56 back into the process stream prior to expander 50 in the pre-cooling stage 31. This will increase the mass flow through heat exchanger 44 and the compression stage 21 compressor 34 will have to compress more gas. The extra refrigeration can, for example, lower the temperature of the natural gas NG' or alternatively it may enable other refrigerant stream(s) ' to be pre-cooled in heat exchanger 44. The other refrigerant stream(s) R' may for example be for subsequent liquefaction and sub-cooling in production of LNG.

Alternatively, or in addition, a further cooler 58 can be mounted after the compressor 27 in the NGL-extraction part 1 1. This is beneficial if the pressure increase and the gas outlet temperature from this compressor are high.

In a further alternative the two heat exchangers 32 and 44 may be combined into one block. This may give a better heat integration. Two separate blocks may however be advantageous from an operational point of view.

Figure 7 shows a preferred embodiment with further additional features, which could be used separately or combined with other features from Figure 5 or Figure 6. It may occur that the cold overhead product from distillation column 28, which normally is passed to the NGL extraction part 1 1 heat exchanger 32, can provide a larger refrigeration duty than needed in the NGL extraction part 1 1 heat exchanger 32. In this case, some of the gas out of the top of the distillation column 28 can be routed to provide additional cold duty in the pre-cooling heat exchanger 44 as shown in Figure 7. This can enable a lower pre-cooling temperature of the high pressure natural gas out of the pre-cooling heat exchanger 44. In one example, around 70% of the gas exiting the distillation column 28 goes through NGL extraction part 1 1 heat exchanger 32 while the remaining 30% goes through the pre-cooling part 31 heat exchanger 44.

It is a large advantage if the liquefaction plant can operate at reduced capacity when the pre-cooling functionality is out of order. This could be the case during a malfunction or maintenance of the compressor-expander 46, 50. The bypass valve 52 shown indicated in Figure 5 is one possibility for this but it is anticipated that this will provide a considerably reduced performance than that of the expander 50. The preferred fall-back solution if the pre-cooling should fall out is therefore to send the gas directly from the compressor 27 of the NGL extraction part 1 1 to the compression part 21 and hence by-pass the pre-cooling part 31 via by-pass line 54 and via a by-pass 60 for the heat exchanger 44. This means that the high- pressure treated gas NG' leaves the pre-cooling at close to ambient temperature. In this case, the feed flow rate to any subsequent LNG plant should be reduced as the liquefaction plant will not be designed to handle the same flow rate entering at ambient temperature.

As an example, for a high pressure lean feed gas, a simulation showed that the flow rate must be reduced to around 85% if the pre-cooling is out of order and the pre-cooling part 31 is bypassed using the by-pass line 54. In this case, with the process of Figure 7, the splitting of the gas out of the distillation column 28 described above should be changed so that the temperature profile in the NGL extraction part heat exchanger 32 remains constant.

The splitting of the gas out of the distillation column 28 can be avoided when the two heat exchangers 32, 44 are combined into one heat exchanger. However, this can be problematic when the pre-cooling functionality is out of order. Then, the combined heat exchanger probably will be subject to temperature spans as the flow out of the cooler 36 of the compression part 21 by-passes the combined heat exchanger 32, 44, for example via by-pass line 60 shown in Figure 7.

A further possible alternative relates to the feature of recycling a part of the high pressure treated gas NG' as an additional refrigerant stream entering between cooler 48 and expander 50 in the pre-cooling part 31. In Figure 6 this recycling is achieved via by valve 56. In Figure 7 this valve 56 is also present, but there is also an additional valve 62 that recycles the refined natural gas stream NG' into the pre- cooling part 31 from a point prior to the pre-cooling heat exchanger 44.

A possible disadvantage of using recycle through the valve 56 as in Figure 6 is that this cold stream has undergone cooling through the heat exchanger 44, meaning that the additional cooling it can provide mostly will be used to cool itself. By applying a recycle from before the heat exchanger 44 through the valve 62 instead then the recycle stream is at ambient temperature (as is the gas out of the cooler 48 with which it is mixed) and can provide a more efficient way to provide extra refrigeration if, for example, the refrigeration demand in the NGL extraction part 1 1 heat exchanger 32 is so great that all the gas exiting the distillation column 28 must go through this heat exchanger 32, with no gas being sent from the top of the distillation column 28 to the pre-cooling part 31. Whichever valve is used for recycling, there will be a pressure drop requiring an increase in the work that must be provided by the compression part compressor 34.

Figure 8 illustrates another possible arrangement. In this example each of the NGL extraction part 1 1 , the compression part 21 and the pre-cooling part 31 has the same basic components, but they are coupled together in an alternative way. In particular, cooling in the heat exchanger 44 is generated by using the gas stream from the top of the distillation column 28 and by using the gas stream from the heat exchanger 32 as an input stream for the expander 50 of the pre-cooling part 31. After the gas from the top of the distillation column 28 has passed through the heat exchanger 44 it is joined with the gas from the heat exchanger 32 and passed to the expander 50. The expander 50, cooler 48 and compressor 46 are all still present in the pre-cooling part 31 , but used in a different order. In Figure 8 the compressor 46 is fed by gas that is first expanded in expander 50 and then passed through the heat exchanger 44 before compression. After compression in the compressor 46 this gas is cooled by the cooler 48 and then passed to the compressor 27 for further compression, before being passed to the compression part 21 for yet further compression.

In a study comparing the proposed process (for example, Figure 5, 6 or 7) with conventional pre-cooling (i.e. the traditional process in Figure 1 ) the

performance of the auto refrigeration process varies with the feed gas pressure and the composition of the natural gas feed. The largest advantage is achieved when the inlet gas has a high pressure and a lean composition.

A high inlet gas pressure favours the current proposed process using a compander combination 26, 27 in the NGL extraction part 1 1. This is because parts of the inlet pressure can be recovered by the compressor 27. In contrast, the pressure in the scrub column type system (Figure 1 ) has an upper limit of 55-60 bars and so a solution with recompression is not suited for scrub columns. This means that a high pressure inlet gas must be throttled to 55-60 bar before the scrub column and that this pressure (or lower) persists throughout the liquefaction process. For a NGL-extraction solution using a compander, the pressure reduction needed in the NGL-column can be partly recovered. When the inlet pressure is high, this is advantageous for solution not using a scrub column.

A lean composition means that the natural gas has a relatively low concentration of propane, butanes, pentanes and heavier components and correspondingly high concentration of methane. When the natural gas feed is a lean gas, less condensation occurs through heat exchanger the NGL extraction part 1 1 heat exchanger 32. This reduces the heat duty in this heat exchanger 32, which allows a higher share of the gas out of the distillation column 28 can be routed through the pre-cooling part heat exchanger 44, and hence enables a lower temperature for the high pressure, treated gas NG' that is output from the system. There are clear advantages to this when the treated gas NG' is the input to a LNG system.

As well as the option of by-passing the pre-cooling phase 31 using the bypass line 54 it is also possible to operate a downstream liquefaction plant without using the compressor 34 in the compression part. Then the pressure of the natural gas NG' will be lower during liquefaction and the plant yields a reduced capacity. In any case, the NGL distillation column 28 operates as normal and the heating values of the treated gas are unchanged.

The driver 35 providing power to the compressor 34 in the compression part 21 can be an electric motor. Alternatively, it can be a gas turbine. In this way, the driver 35 can increase the production capacity of the downstream liquefaction plant, both by increasing the pressure of the gas stream and also by providing a pre-cooled gas stream.

As the pressure and temperature in distillation column 28 determines the NGL-content in the treated gas NG', the system can be adapted to a different feed gases by changing the temperature into flash tank 30, the expansion in expander 26 and the conditions in a reflux condenser and reboiler (not shown) used with distillation column 28.

Thus, as will be understood, the proposed addition of a pre-cooling part 31 to an NGL extraction system with NGL extraction part 1 1 and compression part 21 will provide pre-cooling for an LNG plant without a dedicated pre-cooling refrigeration system. The pre-cooling is combined with a NGL-extraction system and pressure increase for the natural gas NG'. Pre-cooling is provided by only the addition of a further heat exchanger 44 (which may be an extension of NGL extraction heat exchanger 32), an expander 50 and a compressor 46. These parts are not weight or space consuming. The proposed process is therefore well suited for use where weight and space are at a premium, such as for FLNG.

This type of pre-cooling (auto-refrigeration using the natural gas) can be expected to be more energy consuming than traditional pre-cooling by using propane, C0 2 or -134a. Surprisingly, the inventors have found that any disadvantage is outweighed by the benefit of reduced space and weight, making it, contrary to expectations, advantageous to use the proposed auto refrigeration especially for FLNG systems. Moreover, the operation of the cooling is simple and this can hence add LNG production capacity in a straightforward fashion.