Patent Document

CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application is a continuation application of parent application Ser. No. 09/051,210, filed Jul. 13, 1998 which was derived from PCT International application no. PCT/GB96/02434, filed Oct. 4, 1996. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a liquefaction apparatus, and more particularly relates to an offshore apparatus for liquefying natural gas. 
     2. Description of the Related Art 
     Natural gas is obtained from gas, gas/condensate and oil fields occurring in nature, and generally comprises a mixture of compounds, the most predominant of which is methane. Usually, natural gas contains at least 95% methane and other low boiling hydrocarbon (although it may contain less); the remainder of the composition comprises mainly nitrogen and carbon dioxide. The precise composition varies widely, and may include a variety of other impurities including hydrogen sulphide and mercury. 
     Natural gas may be “lean” gas or “rich” gas. These terms do not have a precise meaning, but it is generally understood in the art that a lean gas will tend to have less higher hydrocarbons than a rich gas. Thus, a lean gas may contain little or no propane, butane or pentane, whereas a rich gas would contain at least one of these materials. 
     Since natural gas is a mixture of gases, it liquifies over a range of temperatures; when liquefied, natural gas is called “LNG” (liquefied natural gas). Typically, natural gas compositions will liquefy, at atmospheric pressure, in the temperature range −165° C. to −155° C. The critical temperature of natural gas is about −90° C. to −80° C., which means that in practice it cannot be liquefied purely by the application of pressure it must be also be cooled below the critical temperature. 
     Natural gas is often liquefied before being transported to its point of end use. Liquefaction enables the volume of natural gas to be reduced by a factor of about 600. The capital costs, and running costs, of the apparatus required to liquefy the natural gas is very high, but not as high as the cost of transporting unliquefied natural gas. 
     The liquefaction of natural gas can be carried out by cooling the gas in countercurrent heat exchange relationship with a gaseous refrigerant, rather than with the liquid refrigerants used in conventional liquefaction methods, such as the cascade or propane-precooled mixed refrigerant processes. At least part of the refrigerant is passed through a refrigeration cycle which involves at least one compression step and at least one expansion step. Before the compression step, the refrigerant is usually at ambient temperature (ie the temperature of the surrounding atmosphere). During the compression step, the refrigerant is compressed to a high pressure, and is warmed by the compression process. The compressed refrigerant is then cooled with the ambient air, or with water if there is a water supply available, to return the refrigerant back to ambient temperature. The refrigerant is then expanded in order to cool it further. There are basically two methods of achieving the expansion. One method involves a throttling process, which may take place through a J-T valve (Joule-Thomson valve), wherein the refrigerant is expanded substantially isenthalpically. The other method involves a substantially isentropic expansion, which may take place through a nozzle, or, more usually, through an expander or turbine. The substantially isentropic expansion of the refrigerant is known in the art as “work expansion”. When the refrigerant is expanded through a turbine, work may be recovered from the turbine: this work can be used to contribute to the energy required to compress the refrigerant. 
     It is generally recognised that work expansion is more efficient than throttling (a greater temperature drop can be achieved for the same pressure reduction), but the equipment is more expensive. As a result most processes usually use only work expansion, or a mixture of work expansion and throttling. 
     When natural gas of a particular composition is cooled at a constant pressure, then for any given temperature of the gas there will be a particular value for the rate of change of enthalpy (Q) of the gas. The temperature (T) can be plotted against Q to produce a “cooling curve” for natural gas. The cooling curve is highly dependent upon pressure: if the pressure is below the critical pressure, then the T/Q cooling curve is highly irregular, ie, it contains several portions of different gradient, including a portion of zero, or close to zero, gradient. With increases in pressure, particularly above the critical pressure, the T/Q cooling curve tends towards a straight line. 
     Reference is now made to FIG. 1, which is a graph of temperature vs. rate of change of enthalpy for the cooling of natural gas below and above critical pressure. The curve A, which is for the cooling of natural gas below critical pressure, will be considered in more detail. The curve A has a characteristic shape, which can be divided into a number of regions. Region  1  has a constant gradient and represents the sensible cooling of the gas. Region  2  has a decreasing gradient and is below the dew point temperature of the gas as heavier components begin to condense. Region  3  corresponds to the bulk liquefaction of the gas and has the lowest gradient in the curve: the curve in this portion is almost horizontal. Region  4  has an increasing gradient and is above the bubble point temperature of the liquid as the lightest components are condensed. Region  5  is below the bubble point temperature and is of a constant gradient, which is greater than the gradient of regions  3  and  4 . Region  5  corresponds to the sensible cooling of the liquid; this is known as the “sub-cooling” region. 
     Reference is now made to FIG. 2 of the drawings, which is a graph of T/Q showing the combined cooling curve for natural gas and nitrogen, for a natural gas pressure of about 5.5 MPa. The graph also shows the warming curve for nitrogen over the same temperature range. This graph is representative of a liquefaction system in which natural gas is cooled in a series of heat exchangers by a simple nitrogen expander cycle. The nitrogen refrigerant exiting the series of heat exchangers is compressed, cooled with ambient air, cooled to about −152° C. by work expansion, then fed to the cold end of the series of heat exchangers. The nitrogen refrigerant is precooled, before work expansion, by being passed through at least one heat exchanger at the warm end of the series of heat exchangers; thus, the cooling curve is a combined natural gas/nitrogen cooling curve. 
     The gradient of the cooling and warming curves at any particular point in FIG. 2 is dT/dQ. It is well known in the liquefaction field that the most efficient process is one which, for any given value of Q, the corresponding temperature on the cooling curve of the natural gas is as close as possible to the corresponding temperature on the warming curve of the refrigerant. This has the implication that dT/dQ for the cooling curve of the natural gas is as close as possible to dT/dQ for the warming curve of the refrigerant. However, for any given Q, the closer the temperature of the natural gas and the refrigerant, the higher the surface area needed for the heat exchanger. Thus, there has to be a certain trade off between minimising the temperature difference, and minimising the heat exchanger surface area. For this reason, it is generally preferred that for any given Q, the temperature of the natural gas is at least 2° C. higher than that of the refrigerant. 
     In FIG. 2, the nitrogen warming curve is approximately a single straight line (ie, it has constant gradient). This is representative of a single stage refrigeration cycle, wherein the all the refrigerant nitrogen is cooled by work expansion to a low temperature of about −160° C. to −140° C., and is then passed in countercurrent heat exchange relationship with the natural gas. It is clear that at most parts of the T/Q curve there is a large temperature difference between the natural gas and the nitrogen refrigerant, and this indicates that the heat exchange is highly inefficient. 
     It is also known that the gradient of the warming curve of the refrigerant can be altered by changing the flow rate of the refrigerant through the heat exchangers: specifically, the gradient can be increased by decreasing the refrigerant flow rate. In the system shown in FIG. 2 it is not possible to decrease the nitrogen flow rate, because the increase in gradient will cause the nitrogen warming curve to intersect with the natural gas cooling curve. An intersection of the two curves is indicative of a temperature “pinch” or “cross-over” in the heat exchanger between the nitrogen and the natural gas, and under this condition it is impossible for the process to work. 
     However, if the nitrogen flow is split into two streams it is possible to make the nitrogen warming curve change from a single straight line into two intersecting straight line portions of different gradient. An example of such a process is disclosed in U.S. Pat. No. 3,677,019. This specification discloses a process in which the compressed refrigerant is split into at least two portions, and each portion is cooled by work expansion. Each work expanded portion is fed to a separate heat exchanger for cooling the gas to be liquefied. This causes the refrigerant warming curve to comprise at least two straight line portions of different gradient. This aids in the matching of the warming and cooling curves and improves the efficiency of the process. This specification was published over twenty years ago, and the process disclosed therein is inefficient by modern standards. 
     In U.S. Pat. No. 4,638,639 there is disclosed a process for liquefying a permanent gas stream, which also involves splitting the refrigerant stream into at least two portions in order to match the cooling curve of the gas to be liquefied with the warming curve of the refrigerant. The outlet of all the expanders in this process is at a pressure above about 1 MPa. The specification suggests that such high pressures increase the specific heat of the refrigerant, thereby improving the efficiency of the refrigerant cycle. In order to realise an efficiency improvement it is necessary for the refrigerant to be at, or near, its saturation point at the outlet of one of the expanders, because the specific heat is higher near to saturation. If the refrigerant is at the saturation point, then under these conditions there will be some liquid in the refrigerant that is fed to the heat exchangers. This leads to additional expense, because either the heat exchanger needs to be modified in order to handle a two-phase refrigerant, or the refrigerant needs to be separated into liquid and gaseous phases before being fed to the heat exchanger. 
     U.S. Pat. No. 4,638,639 is primarily concerned with processes in which the refrigerant comprises a portion of the gas to be liquefied, ie the refrigerant is the same as the gas to be liquefied. The specification is particularly concerned with a system in which nitrogen is liquefied using a nitrogen refrigerant. The specification does not specifically disclose a process in which natural gas is cooled by nitrogen, nor would it be expected to be useful in such a process, because all modern large-scale processes for liquefying natural gas use a mixed refrigerant cooling cycle. Furthermore, in U.S. Pat. No. 4,638,639 the gas being liquefied is cooled to a temperature just below its critical temperature. A series of three J-T valves are provided to sub-cool the gas being liquefied. 
     The earliest refrigerant cycle used for the liquefaction of natural gas was the cascade process. Natural gas can be cooled in the cascade process by successive cooling with, for example, propane, ethylene and methane refrigerants. The mixed refrigerant cycle, which was developed later, involves the circulation of a multi component refrigerant stream, usually after precooling to −30° C. with propane. The nature of the mixed refrigerant cycle is such that the heat exchangers in the process must routinely handle the flow of a two phase refrigerant. This requires the use of large, specialised heat exchangers. The mixed refrigerant cycle is the most thermodynamically efficient of the previously known natural gas liquefaction processes: it enables the warming curve of the refrigerant to be closely matched to the cooling curve of the natural gas over a wide temperature range. Examples of mixed refrigerant processes are disclosed in U.S. Pat. Nos. 3,763,658 and 4,586,942, and in European Paten No 87,086. 
     One of the reasons for the widespread use of the mixed refrigerant cycle in the cooling of natural gas is the efficiency of that process. The installation of a typical mixed refrigerant liquefaction plant for natural gas would cost upward of $US 1,000,000,000, but the high cost can be justified by the efficiency gains. In order to be cost effective through economy of scale the mixed refrigerant plants typically need to be able to produce at least 3 million tonnes of LNG per annum. 
     The size and complexity of mixed refrigerants liquefaction plants is such that, to date, they have all been constructed, and located, on land. Due to the size of natural gas liquefaction plants, and the requirement for deep water harbours, they cannot always be located near to the natural gas fields. Gas from the natural gas fields is usually transported to the liquefaction plant by pipeline. In the case of offshore natural gas fields, there are severe practical limitations on the maximum length of the pipeline. This means that offshore natural gas fields that are more than about 200 miles from land are seldom developed. 
     BRIEF SUMMARY OF THE INVENTION 
     According to one aspect of the present invention there is provided an offshore apparatus for liquefying natural gas, comprising a support structure which is either floatable or is otherwise adapted to be disposed in an offshore location at least partially above sea level, and natural gas liquefaction means disposed on or in the support structure, the natural gas liquefaction means comprising a series of heat exchangers for cooling the natural gas in countercurrent heat exchange relationship with a refrigerant, compression means for compressing the refrigerant, and expansion means for isentropically expanding at least two separate streams of the compressed refrigerant, wherein said expanded streams of refrigerant communicate with a cool end of a respective one of the heat exchangers. 
     The support structure may be a fixed structure, ie a structure that is fixed to the seabed, and is supported by the seabed. Preferred forms of fixed structure include a steel jacket support structure and a gravity base support structure. 
     Alternatively, the support structure may be a floating structure, ie a structure that floats above the seabed. In this embodiment, the support structure is preferable a flatable vessel having a steel or concrete hull, such as a ship or a barge. 
     In one preferred embodiment, the support structure is a floating production storage and off-loading unit (FPSO). 
     Pretreatment means is usually provided for pretreating the natural gas before it is delivered to the liquefaction means. The pretreatment means may include separation stages for removing impurities, such as condensate, carbon dioxide and produced water. 
     The natural gas liquefaction apparatus may be provided in combination with storage means for receiving and storing the natural gas after it has been liquefied. The storage means may be provided on or in the support structure. Alternatively, the storage means may be provided on a separate support structure, which is either floatable or otherwise adapted to be disposed in an offshore location at least partially above sea level; the separate support structure may be of the same type as, or of a different type to, the support structure for the liquefaction means. It is particularly preferred that the support structure is a ship, and that the liquefaction means and the storage means are provided on said ship. 
     In a preferred embodiment, the support structure comprises two spaced gravity bases, and a platform bridging said gravity bases, wherein said storage means comprises a storage tank provided on or in at least one of said gravity bases, and wherein the liquefaction means is provided on or in said bridging platform. 
     Means can be provided for connecting said apparatus to a subsea well, whereby the natural gas can be delivered to the liquefaction means at a pressure above 5.5 MPa, said pressure being derived directly or indirectly from the pressure in the subsea well. To facilitate this, the apparatus according to the invention can be located sufficiently close to the natural gas producing formation that the pressure of the natural gas in the series of heat exchangers can be provided substantially entirely by the pressure inherent in the natural gas producing formation. In certain gas fields, some of the gas may be recompressed for re-injection, and therefore may be available at a high pressure if passed through one or more compression stages of the re-injection apparatus before being passed to the liquefaction means. 
     According to another aspect of the invention there is provided natural gas liquefaction apparatus, for offshore installation, comprising: natural gas liquefaction means having (i) a series of heat exchangers for cooling the natural gas in countercurrent heat exchange relationship with a refrigerant, (ii) compression means for compressing the refrigerant, and (iii) expansion means for isentropically expanding at least two separate streams of the compressed refrigerant, wherein said expanded streams of refrigerant communicate with a cool end of a respective one of the heat exchangers; and a support frame carrying the components of the liquefaction means as a single unit for transportation to, and installation at, the offshore location. 
     Preferably, the liquefaction means further comprises cooling means for cooling the refrigerant after it has been compressed and before it is isentropically expanded, said cooling means comprising a heat exchanger, a liquid coolant and a refrigeration unit for cooling the coolant to a temperature between −10° C. and 20° C., wherein the compressed refrigerant is cooled in said heat exchanger in countercurrent relationship with said coolant. 
     The expansion means may comprise a work expander disposed in each of said compressed refrigerant streams, and the compression means may comprise at least one compressor. 
     The compression means preferably comprises a first compressor adapted to compress the refrigerant to an intermediate pressure, and a second compressor adapted to compress the refrigerant to a higher pressure. The second compressor is desirably operatively connected to the refrigerant expander means, whereby substantially all the work required to compress the refrigerant from the intermediate pressure to the higher pressure is provided by the expansion means. In one construction the expansion means comprises two turbo expanders, and the second compressor comprises two compressors each operatively connected to a respective one of the turbo expanders. In another construction the refrigerant expander means comprises two turbo expanders, and the second compressor comprises a single compressor operatively connected to both the turbo expanders by means of a common shaft. An aftercooler is generally provided for cooling the compressed refrigerant from the second compression means. 
     The first compressor may comprise a single compressor with an aftercooler for cooling the compressed refrigerant, but it is preferred that the first compressor comprises a series of at least two compressors with an intercooler between each compressor of the series, and an aftercooler after the last compressor of the series. 
     The series of heat exchangers preferably comprises an initial heat exchanger, an intermediate heat exchanger and a final heat exchanger, and the natural gas is passed sequentially through the initial, the intermediate and the final heat exchangers in order to cool it to successively cooler temperatures; refrigerant in a first of said refrigerant streams is delivered to the final heat exchanger, and refrigerant in a second of said refrigerant streams is delivered to the intermediate heat exchanger. 
     The refrigerant may be cooled in the initial heat exchanger after being compressed, but before being isentropically expanded, and the refrigerant in said first refrigerant stream may be cooled in the intermediate heat exchanger after being cooled in the initial heat exchanger, but before being isentropically expanded. 
     The apparatus is preferably operated such that the final heat exchanger receives refrigerant from the first refrigerant stream, and the relative flowrates of the first and second refrigerant streams are such that the warming curve for the refrigerant comprises a plurality of segments of different gradient, the refrigerant is warmed in said final heat exchanger to a temperature below −80° C., and the coolest refrigerant temperature and the flowrate of refrigerant in said first refrigerant stream are such that a part of the refrigerant warming curve relating to the final heat exchanger is at all times within 1 to 10° C., preferably 1 to 5° C., of the corresponding part of the cooling curve for the natural gas. 
     It will usually be most efficient to operate the heat exchangers such that the temperature difference between the natural gas cooling curve and the corresponding part of the refrigerant warming curve is between 1° C. and 5° C. Typically this temperature difference will be above 2° C., because smaller temperature differences require larger, more expensive, heat exchangers, and there is a greater risk that a temperature pinch will be inadvertently created in the heat exchanger. However, in circumstances where there is a surplus of energy available, it can be sensible to operate with temperature differences above 5° C., and perhaps as high as 10° C.: this enables the size of the heat exchangers to be reduced, thereby saving capital costs. 
     The apparatus is preferably operated such that the coolest refrigerant temperature is no greater than −130° C., whereby the natural gas is sub-cooled substantially in said series of heat exchangers. Most preferably, the coolest refrigerant temperature is in the range −140° C. to −160° C. 
     The liquefaction means may further comprise a gas turbine for generating power for the compression means. The gas turbine preferably comprises an aero-derivative gas turbine; this is advantageous because it has a smaller size and weight than the alternative industrial type gas turbines commonly used in onshore LNG plants. In addition, the aero-derivative turbine has high thermal efficiency, and it is easy to maintain due to its light weight components. The number and rating of the turbines depends upon the amount of LNG that it is desired to produce; for example, to produce about 2 million tonnes LNG/annum would require two aero-derivative turbines each rated at about 40 MW. 
     It is preferred that the liquefaction means further comprises a second series (or “train”) of heat exchangers, said second series of heat exchangers being arranged in parallel with said first series of heat exchangers, and a separate refrigerant compression means and refrigerant expansion means for each series of heat exchangers. At least some of the or each series of heat exchangers and pipework connected thereto are preferably disposed within a single, common heat insulating housing—this is known as a “cold box”, and it usually contains pearlite or rock wool. When there is more than one heat exchanger train, it is preferred that each heat exchanger train is disposed in its own cold box. 
     The liquefaction means may further comprise natural gas expansion means adapted to receive and expand sub-cooled natural gas from the series of the heat exchangers; the expansion means serves to expand the sub-cooled natural gas to a sub-critical pressure, thereby simultaneously cooling and liquefying the natural gas. The expansion means may be substantially isenthalpic expansion means, such as a J-T valve, or substantially isentropic expansion means, such as a liquid or hydraulic turbine expander. When the expansion means comprises a liquid or hydraulic turbine expander, or other work-producing expansion means, it is preferred that an electrical generator is provided. The generator is arranged to convert the work produced by the expansion means into electrical energy. 
     The liquefaction means may further comprise a flash vessel adapted to receive expanded natural gas from the natural gas expansion means. In practice the expanded natural gas comprises a two phase mixture of liquid and gas. The flash vessel is provided with a fuel gas exit, through which natural gas comprising mainly methane and a lesser amount of nitrogen is taken, and a LNG exit through which LNG is taken. It is preferable that the flash vessel is provided in the form of a fractionating column having a reboiler which comprises a heat exchanger arranged to warm a liquid stream, taken from the column, in countercurrent heat exchange relationship with natural gas exiting said series of heat exchangers. A fuel gas compressor means can be provided to compress the fuel gas to a suitable pressure for use in a gas turbine, after the gas is warmed in a heat exchanger. The flash vessel is preferably disposed within the cold box. It is desirable that the gas turbine is powered by fuel gas derived from the fuel gas exit of the flash vessel: by means of this arrangement, all the work required to compress the refrigerant is provided to the first compressor means, and this work is entirely provided by fuel gas created by the liquefaction process. 
     There are a number of suitable embodiments for the heat exchangers in the series. Aluminum plate-fin heat exchangers can only be manufactured up to a certain size and a number of individual cores must be manifolded together in parallel to handle the flowrates involved in the process and apparatus of the present invention. The single phase nature of the refrigerant makes it possible for these cores to be manifolded together relatively easily, without the difficulties encountered with two phase systems. However, aluminium plate-fin heat exchangers are constrained by the fact that the allowable design pressure decreases with increasing core size: in order to maintain the number of cores to a practical limit, the natural gas pressure should be below about 5.5 MPa. If higher pressures are desired, then it is preferred to use a spiral wound heat exchanger, a PCHE (printed circuit heat exchanger) or spool wound heat exchanger. Each heat exchanger in the series may comprise a plurality of heat exchanger cores in parallel. Each heat exchanger in the series may comprise more than one heat exchanger. In the preferred arrangement, the heat exchangers in the series are integrated into a single unit with appropriate inlet and outlet conduits. 
     It is possible for the natural gas to be cooled by the refrigerant in further intermediate heat exchangers arranged upstream of the final heat exchanger. However, it is preferred to use only one intermediate heat exchanger, because this reduces the complexity of the equipment, and makes it possible to achieve lower pressure drops across the heat exchanger train. 
     Whilst it is preferred that the refrigerant is divided into two streams, because this is the arrangement uses the least space, it is possible to divide the refrigerant into three, four or more streams. Each stream may be isentropically expanded in parallel with the other streams. It is also possible for one or more of the isentropic expansion steps to be carried out in stages using a series of isentropic expanders. 
     It is preferred that the refrigerant comprises at least 50 mol % nitrogen, more preferably at least 80 mol % nitrogen, and most preferably substantially 100 mol % nitrogen. Nitrogen has a substantially linear warming curve over the temperature range −160° C. to 20° C. In one preferred embodiment the refrigerant comprises nitrogen and up to 10 vol %, preferably 5-10 vol %, methane. 
     The refrigerant is ideally provided in a closed loop refrigerant cycle. The refrigerant could be, but need not be, taken from the stream of natural gas to be liquefied. Make-up refrigerant can be provided from a refrigerant source external to the refrigerant cycle. 
     The apparatus according to the invention is preferably operated in accordance with a process described in our copending PCT application of even date entitled “Liquefaction Process”. According to this process there is provided a natural gas liquefaction process comprising passing natural gas through a series of heat exchangers in countercurrent relationship with a gaseous refrigerant circulated through a work expansion cycle, said work expansion cycle comprising compressing the refrigerant, dividing and cooling the refrigerant to produce at least first and second cooled refrigerant streams, substantially isentropically expanding the first refrigerant stream to a coolest refrigerant temperature, substantially isentropically expanding the second refrigerant stream to an intermediate refrigerant temperature warmer than said coolest refrigerant temperature, and delivering the refrigerant in the first and second refrigerant streams to a respective heat exchanger for cooling the natural gas through corresponding temperature ranges, wherein the refrigerant in the first stream is isentropically expanded to a pressure at least 10 times greater than, and usually more than 10 times greater than, the total pressure drop of the first refrigerant stream across said series of heat exchangers, said pressure being in the range 1.2 to 2.5 MPa. 
     Preferably, the refrigerant is compressed to a pressure in the range 5.5 to 10 MPa. It is preferred that the first stream is isentropically expanded to a pressure in the range 1.5 to 2.5 MPa. The refrigerant in the first stream is preferably isentropically expanded to a pressure at least 20 times greater than the total pressure drop of the first refrigerant stream across said series of heat exchangers. It is possible to operate the process such that the first stream is isentropically expanded to a pressure at least 100 times greater than the total pressure drop of the first refrigerant stream across said series of heat exchangers. However, for most practical installations the refrigerant in the first stream will be isentropically expanded to a pressure not more than 50 times greater than the total pressure drop of the first refrigerant stream across said series of heat exchangers. 
     In one particularly desirable embodiment the refrigerant is compressed to a pressure in the range 7.5 to 9.0 MPa, the refrigerant in the first refrigerant stream is expanded to a pressure in the range 1.7 to 2.0 MPa, and the refrigerant in the first stream is isentropically expanded to a pressure in the range 15 to 20 times the total pressure drop of the first refrigerant stream across said series of heat exchangers. 
     The process is usually operated such that the temperature of each refrigerant stream after each isentropic expansion is greater than 1-2° C. above the saturation temperature of the refrigerant. Under these conditions, the refrigerant is well into the single phase, and is not close to saturation, there will be substantially no liquid in the isoentropically expanded refrigerant portions. However, there may be circumstances when it is desirable to operate the process such that a small amount of liquid is formed during expansion. For example, if the refrigerant comprises nitrogen with up to 10 vol % methane, preferably 5-10 vol % methane, then the process will be most efficient if some liquid is allowed to form during expansion. 
     The ratio of the pressure of the refrigerant, immediately prior to the isentropic expansion, to the pressure of the refrigerant, immediately after the isentropic expansion, is preferably in the range 3:1 to 6:1, more preferably 3:1 to 5:1. 
     In practice the best value for the intermediate refrigerant temperature depends upon the composition of the natural gas, and its pressure. However, in general the optimum value for the intermediate refrigerant temperature will be in the range −85° C. to −110° C. 
     The apparatus according to the invention can be used to produce LNG on a commercial scale, typically 0.5 to 2.5 million tonnes of LNG per annum. In an offshore natural gas liquefaction apparatus comprising two heat exchanger trains each in a cold box, it is possible to produce around 3 million tonnes/annum of LNG. The heat exchanger trains, including power generators and other associated equipment can be fitted on a single platform of about 35 m by 70 m, having a weight around 9000 tonnes. This size is small enough for the liquefaction means to be installed on an offshore production platform or a floating production and storage vessel. 
     The use of the present invention to liquefy gas at an offshore location has a number of advantages. The equipment is simple, particularly compared with the mixed refrigerant cycle; the refrigerant can be non-flammable; a relatively small amount of space is required; and the invention can be operated entirely with known, readily available equipment. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Reference is now made to the accompanying drawings in which: 
     FIG. 1 is a graph of temperature vs. rate of change of enthalpy showing the cooling curve of natural gas above and below critical pressure; 
     FIG. 2 is a graph of temperature vs. rate of change of enthalpy showing the combined cooling curve for natural gas and nitrogen, and the warming curve for nitrogen, in a simple expander process; 
     FIG. 3 is a schematic diagram showing one embodiment of apparatus for the process according to the present invention; 
     FIG. 4 is a graph of temperature vs. rate of change of enthalpy showing the combined cooling curve for natural gas and nitrogen, and the warming curve for nitrogen for the process illustrated in FIG. 3, when the natural gas has a lean gas composition and the natural gas pressure is about 5.5 MPa; 
     FIG. 5 is a graph of temperature vs. rate of change of enthalpy showing the combined cooling curve for natural gas and nitrogen, and the warming curve for nitrogen for the process illustrated in FIG. 3, when the natural gas has a rich gas composition and the natural gas pressure is about 5.5 MPa; 
     FIG. 6 is a schematic diagram of another embodiment of apparatus for the process according to the present invention; 
     FIG. 7 is a graph of temperature vs. rate of change of enthalpy showing the combined cooling curve for natural gas and nitrogen, and the warming curve for nitrogen for the process illustrated in FIG. 6, in which the natural gas has a lean gas composition and the natural gas pressure is about 5.5 MPa; 
     FIG. 8 is a graph of temperature vs. rate of change of enthalpy showing the combined cooling curve for natural gas and nitrogen, and the warming curve for nitrogen for the process illustrated in FIG. 6, in which the natural gas has a rich gas composition and the natural gas pressure is about 7.7 MPa; 
     FIG. 9 is a graph of temperature vs. rate of change of enthalpy showing the combined cooling curve for natural gas and nitrogen, and the warming curve for nitrogen for the process illustrated in FIG. 6, in which the natural gas has a rich gas composition and the natural gas pressure is about 8.3 MPa; 
     FIG. 10 is a schematic diagram of one embodiment of a natural gas liquefaction apparatus according to the present invention; 
     FIG. 11 is a schematic diagram of another embodiment of a natural gas liquefaction apparatus according to the present invention; 
     FIG. 12 is a schematic diagram of another embodiment of a natural gas liquefaction apparatus according to the present invention; 
     FIG. 13 is a schematic diagram of one embodiment of a part of the apparatus shown in FIGS. 10 to  12 ; and 
     FIG. 14 is a schematic diagram of another embodiment of a part of the apparatus shown in FIGS. 10 to  12 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1 and 2 have already been discussed above. Referring to FIG. 3, an apparatus for liquefying natural gas is shown. Lean natural gas, at a pressure of about 5.5 MPa, is fed from a pre-treatment plant (not shown) to conduit  1 . The natural gas is conduit  1  comprises 5.7 mol % nitrogen, 94.1 mol % methane and 0.2 mol % ethane. Various pre-treatment arrangements are known in the art and the exact configuration depends on the composition of the natural gas recovered from the ground, including the level of undesirable contaminants. Typically the pre-treatment plant would remove carbon dioxide, water, sulphur compounds, mercury contaminants and heavy hydrocarbons. 
     The natural gas in conduit  1  is fed to heat exchanger  66 , where it is cooled to 10° C. with chilled water. The exchanger  66  could be provided as part of the pre-treatment plant. In particular, the exchanger could be provided upstream of a water removal unit of the pre-treatment plant, in order to allow condensation and separation of the water contained in the natural gas, and to minimise the size of equipment. 
     The natural gas exiting the heat exchanger  66  is fed to conduit  2  from where it is passed to the warm end of a series of heat exchangers comprising an initial heat exchanger  50 , two intermediate heat exchangers  51  and  52 , and a final heat exchanger  53 . The series of heat exchangers  50  to  53  serves to cool the natural gas to a temperature sufficiently low that it can be liquefied when flashed to a pressure (usually about atmospheric pressure) below the critical pressure of the natural gas. 
     The natural gas in conduit  2 , at a temperature of about 10° C., is first fed to the warm end of the heat exchanger  50 . The natural gas is cooled in heat exchanger  50  to −23.9° C., and is passed from the cool end of the exchanger  50  to a conduit  3 . The natural gas in conduit  3  is fed to the warm end of the exchanger  51 , in which it is cooled to a temperature of −79.5° C. The natural gas exits the cool end of the exchanger  51  into a conduit  4 , from which it is fed to the warm end of the exchanger  52 . The exchanger  52  cools the natural gas to a temperature of −102° C., and natural gas exits the cool end of exchanger  52  into a conduit  5 . The natural gas in conduit  5  is fed to the warm end of exchanger  53 , in which it is cooled to a temperature of −146° C. The natural gas exits the cool end of the exchanger  53  into a conduit  6 . 
     The natural gas in conduit  6  is fed to the warm end of a heat exchanger  54 , in which it is cooled to a temperature of about −158° C., and it exits the cool end of the exchanger  54  into a conduit  7 . The natural gas in conduit  7 , which is still at supercritical pressure, is fed to a liquid expansion turbine  56  in which the natural gas is substantially isentropically expanded to a pressure of about 150 kPa. In the turbine  56  the natural gas is liquefied, and is reduced in temperature to about −166° C. The turbine  56  drives an electrical generator G to recover the work as electrical power. 
     The fluid exiting the turbine  56  is fed to a conduit  8 . This fluid is predominantly liquid natural gas, with some natural gas in the gaseous state. The fluid in conduit  8  is fed to the top of a fractionating column  57 . The natural gas feed in column  1  contains about 6 mol % of nitrogen: the fractionating column  57  serves to strip this nitrogen from the LNG. The stripping process is assisted by using the exchanger  54  to provide reboil heat transferred from the natural gas in conduit  6 . LNG is fed from the column  57  to conduit  67 , through which the LNG is fed to the cool end of the exchanger  54 . The exchanger  54  warms the LNG to a temperature of about −160° C.; the LNG exits the warm end of the exchanger  54  into conduit  68 , through which it is fed back to the column  57 . 
     Stripped nitrogen gas is fed from the top end of the column  57  to the conduit  9 . The conduit  9  also contains a large percentage of methane gas, which is also stripped in the column  57 . The gas in conduit  9 , which is at a temperature of −166.8° C. and a pressure of 120 kPa, is fed to the cool end of a heat exchanger  55 , in which the gas is warmed to a temperature of about 7° C. The warmed gas is fed from the warm end of the exchanger  55  to a conduit  10 , from which it is fed to a fuel gas compressor (not shown). The methane fed through the conduit  10  is used to provide the bulk of the fuel gas requirements of the liquefaction plant. 
     LNG is fed from the bottom of the column  57  to a conduit  11  and then to a pump  58 . The pump  58  pumps the LNG into a conduit  12  and on to a LNG storage tank (see FIGS.  10  and  11 ). The LNG in conduit  12  is at a temperature of −160.2° C. and a pressure of 170 KPa. 
     The nitrogen refrigeration cycle which cools the natural gas to a temperature at which it can liquefy will now be described. Nitrogen refrigerant is discharged from the warm end of the exchanger  50  into a conduit  32 . The nitrogen in conduit  32  is at a temperature of 7.9° C. and a pressure of 1.14 MPa. The nitrogen is fed to a multistage compressor unit  59 , which comprises at least two compressors  69  and  70 , with at least one intercooler  71 , and an aftercooler  72 . The compressors  69  and  70  are driven by a gas turbine  73 . The cooling in the intercooler  71  and the aftercooler  72  is provided to return the nitrogen to ambient temperatures. The operation of the compressor unit  59  consumes almost all of the power required by the nitrogen refrigeration cycle. The gas turbine  73  can be driven by the fuel gas derived from conduit  10 . 
     The compressed nitrogen is fed from the compressor unit  59  to a conduit  33  at a pressure of 3.34 MPa and a temperature of 30° C. The conduit  33  leads to two conduits  34  and  35  between which the nitrogen from the conduit  33  is split according to the power absorbed by the compressor. The nitrogen in the conduit  34  is fed to a compressor  62  in which it is compressed to a pressure of about 5.6 MPa, and is then fed from the compressor  62  to a conduit  36 . The nitrogen in the conduit  35  is fed to a compressor  63  in which it is compressed to a pressure of about 5.6 MPa, and is then fed from the compressor  63  to a conduit  37 . The nitrogen in both the conduits  36  and  37  is fed to a conduit  38  and then to an aftercooler  64 , where it is cooled to 30° C. The nitrogen is fed from the aftercooler  64  through a conduit  39  to a heat exchanger  65  in which it is cooled to a temperature of about 10° C. by chilled water. The cooled nitrogen is fed from the exchanger  65  to a conduit  40 , which leads to two conduits  20  and  41 ; the pressure in conduit  40  is 5.5 MPa. The nitrogen flowing through the conduit  40  is split between the conduits  20  and  41 : about 2 mol % of the nitrogen in conduit  40  flows through the conduit  41 . 
     The nitrogen flowing through the conduit  41  is fed to the warm end of the heat exchanger  55 , where it is cooled to a temperature of about −122.7° C. The cooled nitrogen is fed from the cool end of the exchanger  55  to a conduit  42 . The conduit  20  is connected to the warm end of the heat exchanger  50 , whereby the nitrogen is fed to the warm end of the heat exchanger  50 . The nitrogen from conduit  20  is pre-cooled to −23.9° C. in the heat exchanger  50 , and is fed from the cool end of the heat exchanger  50  to a conduit  21 . 
     The conduit  21  leads to two conduits  22  and  23 . The nitrogen flowing through the conduit  21  is split between the conduits  22  and  23 : about 37 mol % of the total nitrogen flowing through the conduit  21  is fed to the conduit  23 . The nitrogen in the conduit  22  is fed to a turbo expander  60 , in which it is work expanded to a pressure of 1.18 MPa and a temperature of −105.5° C. The expanded nitrogen exits from the expander  60  into a conduit  28 . 
     The nitrogen in the conduit  23  is fed to the warm end of the heat exchanger  51 , in which it is cooled to a temperature of −79.6° C. The nitrogen exits the cool end of the exchanger  51  into a conduit  24 , which is connected to a conduit  25 . The conduit  42  is also connected to the conduit  25 , so that the cooled nitrogen from the heat exchangers  51  and  55  is all fed to the conduit  25 . The nitrogen in conduit  25 , which is at a temperature of −83.1° C., is fed to a turbo expander  61  in which it is work expanded to a pressure of 1.2 MPa and a coolest nitrogen temperature of −148° C. The expanded nitrogen exits from the expander  61  into a conduit  26 . 
     The turbo expander  60  is arranged to drive the compressor  62 , and the turbo expander  61  is arranged to drive the compressor  63 . In this way the majority of the work produced by the expanders  60  and  61  can be recovered. In a modification the compressors  62  and  63  can be replaced with a single compressor that is connected to the conduits  33  and  38 . This single compressor can be arranged to be driven by the turbo expanders  60  and  61 , for example by being connected to a common shaft. 
     The nitrogen in the conduit  26  is fed to the cool end of the exchanger  53  to cool the natural gas fed to the exchanger  53  from the conduit  5  by countercurrent heat exchange. In the heat exchanger  53  the nitrogen is warmed to an intermediate nitrogen temperature of −105.5° C. The warmed nitrogen exits the warm end of the exchanger  53  into a conduit  27 , which is connected to a conduit  29 . The conduit  28  is also connected to the conduit  29 , whereby the nitrogen from the warm end of the heat exchanger  53  is recombined with the nitrogen from the turbo expander  60 . 
     The nitrogen in the conduit  29 , which comprises 100% of the total refrigerant flow, is fed to the cool end of the heat exchanger  52 . The nitrogen from the conduit  29  serves to cool the natural gas fed to the exchanger  52  from the conduit  4  by countercurrent heat exchange. The nitrogen flowing through the exchanger  52  is warmed by the natural gas to a temperature of −83.2° C., and exits from the exchanger  52  into a conduit  30 . 
     The nitrogen is fed from the conduit  30  to the cool end of the heat exchanger  51 , in which it serves to cool the natural gas fed to the exchanger  51  from the conduit  3 , and serves to cool the nitrogen refrigerant fed to the exchanger  51  from the conduit  23 , by countercurrent heat exchange. The nitrogen fed to the heat exchanger  51  from the conduit  30  is warmed to about −40° C., and exits the exchanger  51  into a conduit  31 . 
     The nitrogen is fed from the conduit  31  to the cool end of the heat exchanger  50 , in which it serves to cool the natural gas fed to the exchanger  50  from the conduit  2 , and serves to cool the nitrogen refrigerant fed to the exchanger  50  from the conduit  20 , by countercurrent heat exchange. The nitrogen fed to the heat exchanger  50  from the conduit  31  is warmed to 7.9° C., and exits the exchanger  50  into the conduit  32 . 
     Reference is now made to FIG. 4, which is a temperature-enthalpy graph representing the process of FIG. 3, in which the natural gas has the lean composition described above. The graph shows a combined cooling curve for the natural gas and the nitrogen refrigerant and a warming curve for the nitrogen refrigerant. 
     The cooling curve has a plurality of regions identified as  4 - 1 ,  4 - 2 ,  4 - 3  and  4 - 4 . The region  4 - 1  corresponds to cooling in the heat exchanger  50 : the gradient in this region is less than what would be the gradient of the cooling curve of natural gas alone over this region; in other words, the presence of the nitrogen refrigerant in the exchanger  50  lowers the gradient in this region. The region  4 - 2  corresponds to cooling in the heat exchanger  51 . The gradient is steeper here, due to the removal of part of the nitrogen refrigerant in conduit  22 ; the slope of the curve in region  4 - 2  is closer to the natural gas cooling curve than in region  4 - 1 . The region  4 - 3  corresponds to cooling in the heat exchanger  52 . The gradient here represents the natural gas cooling curve only, because there is no refrigerant being cooled in the heat exchanger  52 . This part of the curve represents the region over which liquefaction would take place if the pressure of the natural gas were below the critical pressure. The critical temperature is within the temperature range of region  4 - 3 . The region  4 - 4  corresponds to cooling in the heat exchanger  53 . The gradient is steepest in region  4 - 4  and represents the sub-cooling of the natural gas. If the natural gas were just below the critical pressure in this region, then it would be a liquid. 
     The warming curve has two regions identified as  4 - 5  and  4 - 6 : the region  4 - 5  corresponds to refrigerant warming in the heat exchanger  53 ; and the region  4 - 6  corresponds to refrigerant warming in the heat exchangers  50 ,  51  and  52 . The gradient of the warming curve in region  4 - 5  is greater than the gradient in the region  4 - 6 : this is due to the smaller mass flow rate of nitrogen in the heat exchanger  53  compared with the mass flow rate in the heat exchangers  50 ,  51  and  52 . A point  4 - 7  represents the nitrogen temperature in the conduit  26  as it enters the cool end of the heat exchanger  53 . A point  4 - 8  represents the nitrogen temperature in the conduit  32  as it exits the warm end of the heat exchanger  50 . The points  4 - 7  and  4 - 8  set the end points of the nitrogen warming curve. 
     The regions  4 - 5  and  4 - 6  intersect at a point  4 - 9 , which represents the nitrogen at the nitrogen intermediate temperature as it exits the heat exchanger  53 . It is highly advantageous that the point  4 - 9  is set as warm as possible within the constraints of the system. The nitrogen represented by the point  4 - 7  should be 1° C. to 5° C. cooler than the temperature of the natural gas exiting the heat exchanger  53  into the conduit  6 , and the nitrogen represented by the point  4 - 9  should be 1° C. to 10° C. cooler than the temperature of the natural gas entering the heat exchanger  53  from the conduit  5 ; these conditions are necessary to obtain a close match between the natural gas cooling curve and the nitrogen warming curve over the regions  4 - 4  and  4 - 5 . The temperature of the nitrogen represented by the point  4 - 9  should be below the critical temperature of the natural gas; this condition is also necessary to obtain a very close match between the natural gas cooling curve and the nitrogen warming curve over the regions  4 - 4  and  4 - 5 . Finally, the temperature of then nitrogen represented by the point  4 - 9  needs to be low enough that the straight line region between the point  4 - 9  and  4 - 8  does not intersect the natural gas/nitrogen cooling curve in the regions  4 - 1 ,  4 - 2  or  4 - 3 . A point  4 - 10  on the nitrogen warming curve and  4 - 11  on the natural gas/nitrogen cooling curve represents the point of closest approach between the natural gas/nitrogen cooling curve and the nitrogen warming curve. An intersection of the two curves at the point  4 - 10  and  4 - 11  (or anywhere else) represents a temperature pinch in the heat exchangers. In practice, the point  4 - 9  should be chosen so that there is a 1° C. to 10° C. temperature difference between the natural gas/nitrogen being cooled at the point  4 - 11  and the nitrogen being warmed at the point  4 - 10 . 
     The specific process parameters are heavily dependent upon the natural gas composition. The description in relation to FIGS. 3 and 4 was for a lean gas composition. The process could be used with a rich gas composition, comprising, for example, 4.1 mol % nitrogen, 83.9 mol % methane, 8.7 mol % ethane, 2.8 mol % propane and 0.5 mol % butane. Using such a composition, assuming a feed pressure in conduit  1  of about 5.5 MPa and a natural gas temperature in conduit  2  of 10° C., the pressures in the process are substantially the same as those described above with reference to the lean gas example. However, some of the temperatures are different. 
     The natural gas emerging from heat exchanger  50  to conduit  3  is at −14° C., the natural gas emerging from heat exchanger  51  to conduit  4  is at −81.1° C., the natural gas emerging from heat exchanger  52  to conduit  5  is at −95.0° C., and the natural gas emerging from heat exchanger  53  to conduit  6  is at −146° C. 
     As in the FIG. 3 embodiment, about 2.5 mol % of the total nitrogen flowing through the conduit  40  flows through the conduit  41 , while the rest flows through the conduit  20 . The nitrogen flowing through the conduit  41  emerges from the heat exchanger  155  into the conduit  42  at a temperature of about −105° C. The nitrogen in the conduit  22  is divided between the conduits  22  and  23 : about 33 mol % flows through the conduit  23  and about 67 mol % flows through the conduit  22 . The nitrogen refrigerant exiting the heat exchanger  50  to the conduit  21  is at −14° C. and the nitrogen refrigerant exiting the heat exchanger  51  to the conduit  24  is at −81.1° C. After mixing the nitrogen from the conduit  24  with the nitrogen from the conduit  42 , the nitrogen in the conduit  25  is at a temperature of −83.0° C. The nitrogen refrigerant from the conduit  22  is expanded in the turbo expander  60  to a temperature of 31 98.5° C., while the nitrogen refrigerant from the conduit  25  is expanded in the turbo expander  61  to a temperature of −148° C. 
     The nitrogen refrigerant exits from the heat exchanger  53  to the conduit  27  at −98.5° C., is combined with the refrigerant from the conduit  28 , is passed through the heat exchanger  52 , and exits from the heat exchanger  52  to the conduit  30  at a temperature of −92.1° C. Subsequently, the nitrogen refrigerant exits from the heat exchanger  51  to the conduit  31  at a temperature of about −24.4° C. 
     The temperature of the nitrogen exiting from the top of the column  57  to the conduit  9  is −164.1° C., and the temperature of the LNG product in conduit  12  is −158.4° C. 
     FIG. 5 is similar to FIG. 4, and shows a temperature-enthalpy graph representing the process of FIG. 3, where the natural gas has the rich composition described above. The graph shows a combined cooling curve for the natural gas and the nitrogen refrigerant and a warming curve for the nitrogen refrigerant. The cooling and warming curves have a plurality of regions identified as  5 - 1  to  5 - 6 , which correspond to regions  4 - 1  to  4 - 6  respectively of FIG. 4, and have a plurality of temperature points  5 - 7  to  5 - 11 , which correspond to regions  4 - 7  to  4 - 11  respectively of FIG.  4 . The description above, relating the FIG. 4, also applies to FIG. 5, with the exception that in FIG. 5, the natural gas critical temperature is in the region  5 - 2 , rather than  5 - 3 . 
     Referring now to FIG. 6, another embodiment of an apparatus for the present invention is shown. The FIG. 6 embodiment bears many similarities to the FIG. 3 embodiment, and the reference numerals given to the parts in FIG. 6 are exactly 100 higher than the equivalent parts in the FIG. 3 embodiment. The embodiment shown in FIG. 6 is preferred to the embodiment shown in FIG. 3, because fewer heat exchangers are required. 
     Lean natural gas is fed from a pre-treatment plant (not shown) to conduit  101 . The natural gas in conduit  101  comprises 5.7 mol % nitrogen, 94.1 mol % methane and 0.2 mol % ethane, and is at a pressure of about 5.5 MPa. As discussed above, various pre-treatment arrangements are known in the art and the exact configuration depends on the composition of the natural gas recovered from the ground, including the level of undesirable contaminants. Typically the pre-treatment plant would remove carbon dioxide, water, sulphur compounds, mercury contaminants and heavy hydrocarbons. 
     The natural gas in conduit  101  is fed to heat exchanger  166 , where it is cooled to 10° C. with chilled water. The exchanger  166  could be provided as part of the pre-treatment plant. In particular, the exchanger could be provided upstream of a water removal unit of the pre-treatment plant, in order to allow condensation and separation of the water contained in the natural gas, and to minimise the size of equipment. 
     The natural gas exiting the heat exchanger  166  is fed to conduit  102  from where it is passed to the warm end of a series of heat exchangers  150 ,  151  and  153 . The series of heat exchangers  150  to  153  cool the natural gas to a temperature sufficiently low that it can be liquefied when flashed to a pressure (usually about atmospheric pressure) below the critical pressure of the natural gas. It will be noted that in the embodiment of FIG. 6 there is no heat exchanger equivalent to the heat exchanger  52  of FIG.  3 . 
     The natural gas in conduit  102 , at a temperature of about 10° C., is first fed to the warm end of the heat exchanger  150 . The natural gas is cooled in heat exchanger  150  to −41.7° C., and is passed from the cool end of the exchanger  150  to a conduit  103 . The natural gas in conduit  103  is fed to the warm end of the exchanger  151 , in which it is cooled to a temperature of about −98.2° C. The natural gas exits the cool end of the exchanger  151  into a conduit  104 , from which it is fed to the warm end of the exchanger  153 , in which it is cooled to a temperature of −146° C. The natural gas exits the cool end of the exchanger  153  into a conduit  106 . 
     The natural gas in conduit  106  is fed to the warm end of a heat exchanger  154 , in which it is cooled to a temperature of about −158° C., and it exits the cool end of the exchanger  154  into a conduit  107 . The natural gas in conduit  107 , which is still at supercritical pressure, is fed to a liquid expansion turbine  156  in which the natural gas is substantially isentropically expanded to a pressure of about 150 kPa. In the turbine  56  the natural gas is liquefied, and is reduced in temperature to about −167° C. The turbine  156  drives an electrical generator G′ to recover the work as electrical power. 
     The fluid exiting the turbine  156  is fed to a conduit  108 . This fluid is predominantly liquid natural gas, with some natural gas in the gaseous state. The fluid in conduit  108  is fed to the top of a fractionating column  157 . The natural gas feed in conduit  1  contains about 6 mol % of nitrogen: the fractionating column  57  serves to strip the nitrogen from the LNG. The stripping process is assisted by using the exchanger  154  to provide reboil heat transferred from the natural gas in conduit  106 . LNG is fed from the column  157  to conduit  167 , from where the LNG is fed to the cool end of the exchanger  154 . The exchanger  154  warms the LNG to a temperature of about −160° C.; the LNG exits the warm end of the exchanger  154  into a conduit  168 , through which it is fed back to the column  157 . 
     Stripped nitrogen gas is fed from the top end of the column  157  to the conduit  109 . The conduit  109  also contains a large percentage of methane gas, which is also stripped in the column  57 . The gas in conduit  109 , which is at a temperature of −166.8° C. and pressure of 120 kPa, is fed to the cool end of a heat exchanger  155 , in which the gas is warmed to a temperature of about 7° C. The warmed gas is fed from the warm end of the exchanger  105  to a conduit  110 , from which it is fed to a fuel gas compressor (not shown). The methane fed through the conduit  110  is used to provide the bulk of the fuel gas requirements of the liquefaction plant. 
     LNG is fed from the bottom of the column  157  to a conduit  111  and then to a pump  158 . The pump  158  pumps the LNG into a conduit  112  and on to a LNG storage tank (see FIGS.  10  and  11 ). 
     The nitrogen refrigeration cycle which cools the natural gas to a temperature at which it can liquefy will now be described. Nitrogen refrigerant is discharged from the warm end of the exchanger  150  into a conduit  132 . The nitrogen in conduit  132  is at a temperature of about 7.9° C. and a pressure of 1.66 MPa. The nitrogen is fed to a multistage compressor unit  159 , which comprises at least two compressors  169  and  170 , with at least one intercooler  171 , and an aftercooler  172 . The compressors  169  and  170  are driven by a gas turbine  173 . The cooling in the intercooler  171  and the aftercooler  172  is provided to return the nitrogen to ambient temperatures. The operation of the compressor unit  159  consumes almost all of the power required by the nitrogen refrigeration cycle. The gas turbine  173  can be driven by the fuel gas derived from conduit  110 . 
     The compressed nitrogen is fed from the compressor unit  159  to a conduit  133  at a pressure of 3.79 MPa. The conduit  133  leads to two conduits  134  and  135  between which the nitrogen from the conduit  133  is split according to the power absorbed by the compressor. The nitrogen in the conduit  134  is fed to a compressor  162  in which it is compressed to a pressure of about 5.5 MPa, and is then fed from the compressor  162  to conduit a  136 . The nitrogen in the conduit  135  is fed to a compressor  163  in which it is compressed to a pressure of about 5.5 MPa, and is then fed from the compressor  163  to conduit a  137 . The nitrogen in both the conduits  136  and  137  is fed to a conduit  138  and then to an aftercooler  164 , where it is cooled back to ambient temperatures. The nitrogen is fed from the aftercooler  164  through a conduit  139  to a heat exchanger  165  in which it is cooled to a temperature of 10° C. by chilled water. The cooled nitrogen is fed from the exchanger  165  to a conduit  140 , which leads to two conduits  120  and  141 . The nitrogen flowing through the conduit  140  is split between the conduits  120  and  141 : about 2 mol % of the nitrogen in conduit  140  flows through the conduit  121 . 
     The nitrogen flowing through the conduit  141  is fed to the warm end of the heat exchanger  155 , where it is cooled to a temperature of about −123° C. The cooled nitrogen is fed from the cool end of the exchanger  155  to a conduit  142 . The conduit  120  is connected to the warm end of the heat exchanger  150 , whereby the nitrogen is fed to the warm end of the heat exchanger  150 . The nitrogen from conduit  120  is pre-cooled to −41.7° C. in the heat exchanger  150 , and is fed from the cool end of the heat exchanger  150  to a conduit  121 . 
     The conduit  121  leads to two conduits  122  and  123 . The nitrogen flowing through the conduit  121  is split between the conduits  122  and  123 : about 26 mol % of the total nitrogen flowing through the conduit  121  is fed to the conduit  123 . The nitrogen in the conduit  122  is fed to a turbo expander  160 , in which it is work expanded to a pressure of 1.73 MPa and a temperature of −102.5° C. The expanded nitrogen exits from the expander  160  into a conduit  128 . 
     The nitrogen in the conduit  123  is fed to the warm end of the heat exchanger  151 , in which it is cooled to a temperature of about −98.2° C. The nitrogen exits the cool end of the exchanger  151  into a conduit  124 , which is connected to a conduit  125 . The conduit  42  is also connected to the conduit  125 , so that the cooled nitrogen from the heat exchangers  151  and  155  is all fed to the conduit  125 . The nitrogen in conduit  125 , which is at a temperature of −100.3° C., is fed to a turbo expander  161  in which it is work expanded to a pressure of 1.76 MPa and a coolest nitrogen temperature of about −148° C. The expanded nitrogen exits from the expander  161  into a conduit  126 . 
     The turbo expander  160  is arranged to drive the compressor  162 , and the turbo expander  161  is arranged to drive the compressor  163 . In this way the majority of the work produced by the expanders  160  and  161  can be recovered. In a modification the compressors  162  and  163  can be replaced with a single compressor that is connected to the conduits  133  and  138 . This single compressor can be arranged to be driven by the turbo expanders  160  and  161 , for example by being connected to a common shaft. 
     The nitrogen in the conduit  126  is fed to the cool end of the exchanger  153  to cool the natural gas fed to the exchanger  153  from the conduit  104  by countercurrent heat exchange. In the heat exchanger  153  the nitrogen is warmed to an intermediate nitrogen temperature of −102.5° C. The warmed nitrogen exits the warm end of the exchanger  153  into a conduit  127 , which is connected to a conduit  129 . The conduit  128  is also connected to the conduit  129 , whereby the nitrogen from the warm end of the heat exchanger  153  is recombined with the nitrogen from the turbo expander  160 . 
     The nitrogen is fed from the conduit  129  to the cool end of the heat exchanger  151 , in which it serves to cool the natural gas fed to the exchanger  151  from the conduit  103 , and serves to cool and nitrogen refrigerant fed to the exchanger  151  from the conduit  123 , by countercurrent heat exchange. The nitrogen fed to the heat exchanger  151  from the conduit  129  is warmed to about −57.9° C., and exits the exchanger  151  into a conduit  131 . 
     The nitrogen is fed from the conduit  131  to the cool end of the heat exchanger  150 , in which it serves to cool the natural gas fed to the exchanger  150  from the conduit  102 , and serves to cool the nitrogen refrigerant fed to the exchanger  150  from the conduit  120 , by countercurrent heat exchange. The nitrogen fed to the heat exchanger  150  from the conduit  131  is warmed to 7.9° C., and exits the exchanger  150  into the conduit  132 . 
     FIG. 7 is similar to FIG. 4, and shows a temperature-enthalpy graph representing the process of FIG. 6, where the natural gas has the lean composition described above. The graph shows a combined cooling curve for the natural gas and the nitrogen refrigerant and a warming curve for the nitrogen refrigerant. 
     The cooling curve has a plurality of regions identified as  7 - 1 ,  7 - 2  and  7 - 4 . The region  7 - 1  corresponds to cooling in the heat exchanger  150 : the gradient in this region is less than what would be the gradient of the cooling curve of natural gas alone over the region; in other words, the presence of the nitrogen refrigerant in the exchanger  150  lowers the gradient in this region. The region  7 - 2  corresponds to cooling in the heat exchanger  151 . The gradient is steeper here, due to the removal of part of the nitrogen refrigerant in conduit  122 ; the slope of the curve in region  7 - 2  is closer to the natural gas cooling curve than in region  7 - 1 . This part of the curve also represents the region over which liquefaction would take place if the pressure of the natural gas were below the critical pressure: the critical temperature is within the temperature range of region  7 - 2 . The region  7 - 4  corresponds to cooling in the heat exchanger  153 . The gradient is steepest in region  7 - 4  and represents the sub-cooling of the natural gas. Note that there is no region  7 - 3  in FIG. 7, because there is no heat exchanger  152 . 
     The nitrogen warming curve has two regions identified as  7 - 5  and  7 - 6 : the region  7 - 5  corresponds to refrigerant warming in the heat exchanger  153 ; and the region  7 - 6  corresponds to refrigerant warming in the heat exchangers  150  and  151 . The gradient of the warming curve in region  7 - 5  is greater than the gradient in the region  7 - 6 : this is due to the smaller mass flow rate of nitrogen in the heat exchanger  153  compared with the mass flow rate in the heat exchangers  150  and  151 . A point  7 - 7  represents the nitrogen temperature in the conduit  126  as it enters the cool end of the heat exchanger  153 . A point  7 - 8  represents the nitrogen temperature in the conduit  132  as it exits the warm end of the heat exchanger  150 . The points  7 - 7  and  7 - 8  set the end points of the nitrogen warming curve. 
     The regions  7 - 5  and  7 - 6  intersect at a point  7 - 9 , which represents the nitrogen at the nitrogen intermediate temperature as it exits the heat exchanger  153 . It is highly advantageous that the point  7 - 9  is set as warm as possible within the constraints of the system. The nitrogen represented by the point  7 - 7  should be 1° C. to 5° C. cooler than the temperature of the natural gas exiting the heat exchanger  153  into the conduit  106 , and the nitrogen represented by the point  7 - 9  should be 1° C. to 10° C. cooler than the temperature of the natural gas entering the heat exchanger  153  from the conduit  105 ; these conditions are necessary to obtain a very close match between the natural gas cooling curve and the nitrogen warming curve over the regions  7 - 4  and  7 - 5 . The temperature of the nitrogen represented by the point  8 . 9  should be below the critical temperature of the natural gas: this condition is also necessary to obtain a very close match between the natural gas cooling curve and the nitrogen warming curve over the regions  7 - 4  and  7 - 5 . Finally, the temperature of the nitrogen represented by the point  7 - 9  needs to be low enough that the straight line region between the point  7 - 9  and  7 - 8  does not intersect the natural gas/nitrogen cooling curve in the regions  7 - 1  or  7 - 2 . A point  7 - 10  on the nitrogen warming curve and  7 - 11  on the natural gas/nitrogen cooling curve represents the point of closest approach between the natural gas/nitrogen cooling curve and the nitrogen warming curve. An intersection of the two curves at the point  7 - 10  and  7 - 11  (or anywhere else) represents a temperature pinch in the heat exchangers. In practice, the point  7 - 9  should be chosen so that there is a 1° C. temperature difference between the natural gas/nitrogen being cooled at the point  7 - 11  and the nitrogen being warmed at the point  7 - 10 . 
     The process of FIG. 6 will now be considered for a rich gas composition, comprising 4.1 mol % nitrogen, 83.9 mol % methane, 8.7 mol % ethane, 2.8 mol % propane and 0.5 mol % butane, using a natural gas feed pressure in conduit  1  of about 7.6 MPa and a natural gas temperature in conduit  102  of 10° C. 
     Under these new conditions, the natural gas would exit from the heat exchanger  150  into the conduit  103  at a temperature of −8.0° C., the natural gas would exit from the heat exchanger  151  into the conduit  104  at a temperature of −87° C., and the natural gas would exit from the heat exchanger  153  into the conduit  106  at a temperature of −146° C. 
     The nitrogen refrigerant exiting from the heat exchanger into the conduit  132  is at a temperature of 7.9° C. and a pressure of 2.31 MPa. The nitrogen refrigerant is compressed in the compressor unit  159  to a pressure of 6.08 MPa, and is then further compressed in the compressors  162  and  163  to a pressure of about 10 MPa. 
     The nitrogen refrigerant in the conduit  140  is at a temperature of 10.0° C., as a result of the cooling in the aftercooler  164  and the heat exchanger  165 . About 2.2 mol % of the nitrogen flowing through the conduit  140  flows through the conduit  141 , while the remainder flows through the conduit  120 . The nitrogen flowing through the conduit  141  is reduced in temperature to about −108° C. in the heat exchanger  155 . 
     The nitrogen refrigerant exiting the heat exchanger  150  into the conduit  121  it at a temperature of −8° C. About 25 mol % of the nitrogen in the conduit  121  flows through the conduit  123 , while the remaining 75 mol % flows through the conduit  122 . The nitrogen flowing through the conduit  123  emerges from the heat exchanger  151  at a temperature of −87° C., from where it flows into the conduit  125  along with the nitrogen from the conduit  142 : the temperature of the nitrogen in the conduit  125  is −88.7° C. The nitrogen flowing through the conduit  122  is expanded in the turbo expander  160  to a pressure of 2.39 MPa and a temperature of −90.5° C., and the nitrogen flowing through the conduit  125  is expanded in the turbo expander  161  to a pressure of 2.42 MPa and a temperature of −148° C. 
     The nitrogen refrigerant emerging from the heat exchanger  153  into the conduit  127  is at a temperature of −90.5° C., and the nitrogen refrigerant emerging from the heat exchanger  151  into the conduit  131  is at a temperature of about −18° C. 
     FIG. 8 is similar to FIG. 7, and shows a temperature-enthalpy graph representing the process of FIG. 6, where the natural gas has the rich composition described above, and is supplied at a pressure of about 7.6 MPa. The graph shows a combined cooling curve for the natural gas and the nitrogen refrigerant and a warming curve for the nitrogen refrigerant. The cooling and warming curves have a plurality of regions  8 - 1  to  8 - 6 , which correspond to regions  7 - 1  to  7 - 6  respectively of FIG. 7, and have a plurality of temperature points  8 - 7  to  8 - 11 , which correspond to temperature points  7 - 7  to  7 - 11  respectively of FIG.  7 . The description above, relating to FIG. 7, also applies to FIG.  8 . 
     The process of FIG. 6 will now be considered for a rich gas composition, comprising 4.1 mol % nitrogen, 84.1 mol % methane, 8.5 mol % ethane, 2.6 mol % propane and 0.7 mol % butane, using a natural gas feed pressure in conduit  1  of about 8.25 MPa and a natural gas temperature in conduit  102  of 10° C. There is one slight modification to the process described above with respect to FIG.  6 : boil-off gas from LNG storage tanks is combined with the top product from column  157  in conduit  109 , and the combined contents of the conduit  109  are fed to the heat exchanger  155 . 
     Under these new conditions, the natural gas would exit from the heat exchanger  151  into the conduit  104  at a temperature of −86.2° C., and would exit from the heat exchanger  153  into the conduit  106  at a temperature of −148.3° C. 
     The nitrogen refrigerant exiting from the heat exchanger into the conduit  132  is at a temperature of 3.0° C. and a pressure of 1.77 MPa. The nitrogen refrigerant is compressed in the compressor unit  159  to a pressure of 4.97 MPa, and is then further compressed in the compressors  162  and  163  to a pressure of about 8.3 MPa. 
     The nitrogen refrigerant in the conduit  140  is at a temperature of 10.0° C., as a result of the cooling in the aftercooler  164  and the heat exchanger  165 . About 1.7 mol % of the nitrogen flowing through the conduit  140  flows through the conduit  141 , while the remainder flows through the conduit  120 . The nitrogen flowing through the conduit  141  is reduced in temperature to about −143° C. in the heat exchanger  155 . 
     The nitrogen refrigerant exiting the heat exchanger  150  into the conduit  121  is at a temperature of −7° C. About 31 mol % of the nitrogen in the conduit  121  flows through the conduit  123 , while the remaining 69 mol % flows through the conduit  122 . The nitrogen flowing through the conduit  123  emerges from the heat exchanger  151  at a temperature of −86.2° C., from where it flows into the conduit  125  along with the nitrogen from the conduit  142 ; the temperature of the nitrogen in the conduit  125  is −89.3° C. The nitrogen flowing through the conduit  122  is expanded in the turbo expander  160  to a pressure of 1.84 MPa and a temperature of −93.2° C., and the nitrogen flowing through the conduit  125  is expanded in the turbo expander  161  to a pressure of 1.87 MPa and a temperature of −152.2° C. 
     The nitrogen refrigerant emerging from the heat exchanger  153  into the conduit  127  is at a temperature of −93.2° C. 
     FIG. 9 is similar to FIG. 7, and shows a temperature-enthalpy graph representing the process of FIG. 6, where the natural gas has the rich composition described above, and is supplied at a pressure of about 8.25 MPa. The graph shows a combined cooling curve for the natural gas and the nitrogen refrigerant and a warming curve for the nitrogen refrigerant. The cooling and warming curves have a plurality of regions  9 - 1  to  9 - 6 , which correspond to regions  7 - 1  to  7 - 6  respectively of FIG. 7, and have a plurality of temperature points  9 - 7  to  9 - 11 , which correspond to temperature points  7 - 7  to  7 - 11  respectively of FIG.  7 . The description above, relating to FIG. 7, also applies to FIG.  9 . 
     In FIG. 9 the minimum temperature difference between the two curves is 3.9° C., while in FIGS. 4,  5 ,  7  and  8 , the minimum temperature difference is 2° C. 
     Referring to FIG. 10 an embodiment of an apparatus for producing LNG is generally designated  500 . The apparatus comprises a floating platform in the form of a ship  501 , which carries a natural gas liquefaction plant  502  and LNG storage tanks  503 . The LNG is fed from the plant  502  to the storage tanks  503  via a conduit  504 . The natural gas is supplied to the plant  502  via a pipeline  505 , which extends to a natural gas rig  506 , and via a riser and manifold arrangement  510 , which extends from the ship  501  to the pipeline  505 . It is possible for the natural gas to be supplied from a plurality of said gas rigs  506 . A pre-treatment plant (not shown) may be provided for the natural gas, before it is fed to the plant  502 . The pre-treatment plant may be provided on the rig  506 , on a separate unit (not shown) or on the ship  501 . 
     The ship  501  also includes accommodation  507 , mooring lines  508 , and means  509  for supplying LNG from the storage tanks  503  to an LNG carrier (not shown). 
     Referring to FIG. 11 another embodiment of an apparatus for producing LNG is generally designated  600 . The apparatus comprises platform  601 , which is supported above the water level  607  by legs  609 , a natural gas liquefaction plant  602  and an LNG storage tank  603 . The LNG is fed from the plant  602  to the storage tank  603  via a conduit  604 . The storage tank  603  is supported by a concrete gravity base  610 , which rests on seabed  608 . The natural gas is supplied to the plant  602  via a pipeline  605 , which communicates with a natural gas rig  606 . It is possible for the natural gas to be supplied from a plurality of said gas rigs  606 . A pre-treatment plant (not shown) may be provided for the natural gas, before it is fed to the plant  602 . The pre-treatment plant may be provided on the rig  606 , on a separate unit (not shown), on the platform  601  or on the gravity base  610 . Means  611  is provided for supplying LNG from the storage tanks  603  to a LNG carrier (not shown). In a modification the apparatus  600  could be provided on the rig  606 . 
     FIG. 12 shows a modification of the LNG apparatus  600  shown in FIG.  11 . In FIG. 12 the modified LNG apparatus is generally designated  600 ′ and comprises two spaced concrete gravity bases  610 ′, which rest on the seabed  608 ′, so that they project above the water level  607 ′. A liquefaction plant  602 ′ is provided on a platform  601 ′, which rests on the gravity bases  610 ′ and bridges the gap between the gravity bases  610 ′. An LNG storage tank  603 ′ is provided on each of the gravity bases  610 ′. 
     The platform  601 ′ can be installed by supporting it on a barge (not shown): floating the barge into the gap between the gravity bases  610 ′ so that the platform  601 ′ projects over the upper surface of each gravity base  610 ′; lowering the barge so that the platform  601 ′ rests on the gravity bases  610 ′; and finally floating the barge out of the gap between the gravity bases  610 ′. 
     Referring to FIG. 13, the natural gas liquefaction plants  502 ,  602  and  602 ′ of FIGS. 10 to  12  are shown in more detail. In general, the components of the plant shown in FIG. 13 are similar to the components shown in FIGS. 3 and 6. Natural gas is supplied to conduit  450  of the plant at high pressure, which may be supercritical; the natural gas may have been pre-treated to remove contaminants using conventional processes. The natural gas in conduit  450  is fed to a heat exchanger  401  in which it is cooled with chilled water supplied from a chilled water refrigeration unit  415 . The heat exchanger  401  may, instead, be incorporated in the pretreatment process. The heat exchanger  401  may be a conventional shell and tube heat exchanger, or any other type of heat exchanger suitable for cooling natural gas with chilled water, including a PCHE. 
     The cooled natural gas exits from the heat exchanger  401  to a conduit  451 , through which it is fed to a cold box  402 , where the gas is progressively cooled to a low temperature in a series of heat exchangers (not shown) within the box  402 . The heat exchanger arrangement in the cold box  402  may be the same as the arrangement of heat exchangers  50 ,  51 ,  52  and  53  shown in FIG. 3, or may be the same as the arrangement of heat exchangers  150 ,  151  and  153  shown in FIG.  6 . The type of heat exchangers used depends on the pressure at which the natural gas is supplied. If the pressure is below about 5.5 MPa, then each heat exchanger comprises a number of aluminium plate heat exchangers manifolded in series. If the pressure is above about 5.5 MPa, then each heat exchanger comprises, for example, a spiral wound heat exchanger, a PCHE or a spool wound heat exchanger. However, when a spiral wound heat exchanger is used, the embodiment shown in FIG. 14 is more appropriate. The cold box  402  is filled with pearlite or rock wool to provide insulation. 
     The are many advantages to using a the cold box  402 . First, it enables the majority of the cold equipment and piping to be contained within a single space that requires a much smaller plot area than if the equipment and piping were installed separately. The quantity of external insulation required is much less than if the equipment and piping were installed separately, and this reduces the cost and time of installation and future maintenance. In addition, the number of flanges required for connections of piping and equipment is reduced, because all the connections within the box are fully welded—this reduces the possibility of leaks from cold flange during normal operation and during cool-down and warm-up operations. The entire cold box installation can be constructed in a controlled industrial location and can be delivered to the construction site fully leak tested, dry and ready for commissioning—this would otherwise have to be done on the individual bits of equipment and piping in the field in remote locations and under less than ideal conditions. The cold box steel shell and insulation provides protection from the salt air environment in an offshore location, and affords a measure of fire protection for the equipment containing the hydrocarbon inventory. It should be noted that, when spiral wound heat exchangers are used, the first and intermediate exchanger bundles may both be included in a single vertical exchanger shell and may be installed separately to the cold box. In this case, the spiral wound heat exchanger is externally insulated and the cold box containing the remaining cold exchangers and vessel is significantly smaller. 
     The sub-cooled natural gas is withdrawn from the cold box  402 , at its lowest temperature of about −158° C., into a conduit  452 , through which it is fed to a liquid or hydraulic turbine expander disposed within a suction vessel  413  in which the sub-cooled natural gas is work expanded to a low pressure (which is sub-critical), with a concomitant reduction in temperature and the formation of LNG. The work generated in the liquid or hydraulic turbine expander in the suction vessel  413  is used to turn an electrical generator; the electrical generator is also housed within the suction vessel  413 . It is possible for the liquid or hydraulic turbine expander and the suction vessel  413  to be replaced with a throttle valve: this will simplify the equipment, saving on capital costs and space, but there will be a small loss in process efficiency. 
     The LNG exits the liquid or hydraulic turbine expander in the suction vessel  413  into a conduit  453 , which is fed back into the cold box  402  to a nitrogen stripper located within the cold box  402 . The nitrogen stripper within the cold box  402  may be the same as the nitrogen stripper  57  in FIG. 3, or the nitrogen stripper  157  in FIG.  6 . The cold flash gas from the top of the nitrogen stripper is then reheated in another heat exchanger in the cold box  402 , which may be the same as the heat exchanger  55  shown in FIG. 3, or the heat exchanger  155  shown in FIG.  6 . The reheated flash gash exits the cold box  402  into a conduit  454 , which is equivalent to the conduit  10  of FIG. 3, or the conduit  110  of FIG.  6 . The reheated flash gas in the conduit  454  is fed to a compressor unit  414  in which it is compressed to the required fuel gas system pressure. Cooling is provided in the compressor unit  414  by cooling water, which enters the unit  414  via conduit  455  and leaves the unit  414  via conduit  456 . The compressed fuel gas exits the compressor unit  414  into a conduit  457 . The compressor unit  414  may be an integrally geared multistage centrifugal compressor driven by an electric motor and complete with integral intercoolers and aftercoolers. Alternatively, the unit  414  may be an API specification centrifugal compressor with several compressor cases driven by an electric motor or a small gas turbine. The power requirements for the unit  414  may be provided by part of the fuel gas produced therein. 
     The LNG product exits the nitrogen stripper into a conduit  458 , through which it is fed to a submerged pump  412 . The submerged pump  412  pumps the LNG into a conduit  459 , through which it is fed to storage tanks (see FIGS. 10 or  11 ). 
     The refrigeration of the natural gas in the cold box  402  is provided by a nitrogen refrigeration cycle, the components of which will now be described. Nitrogen refrigerant exits the cold box  402  into conduit  460 , having been warmed to ambient temperatures by countercurrent heat exchange with the natural gas. The nitrogen in the conduit  460  is fed to a first stage compressor  405  where it is compressed to high pressure. The compressed nitrogen exits the compressor  405  into a conduit  461 , through which it is fed to an intercooler  462 , where the nitrogen is cooled with cooling water. The compressed nitrogen exits the intercooler  462  into a conduit  463  through which it is fed to a second stage compressor  406 , where it is compressed to an even higher pressure. The compressed nitrogen exits the compressor  406  into a conduit  464 , through which it is fed to an aftercooler  465 , where the nitrogen is cooled with cooling water. The compressors  405  and  406  may be multi wheel API type compressors; alternatively, axial flow compressors may be used if the suction pressure is low enough and/or the circulation rate is high enough. The compressors  405  and  406  may be provided in the form of a single compressor. 
     The compressors  405  and  406  are driven by a gas turbine  403 . The gas turbine  403  is an aero-derivative type of gas turbine because of its smaller size and weight compared to the alternative industrial type gas turbines commonly used in onshore LNG plants. The temperature of the ambient air locations where the plant is located is often high, and this can substantially reduce the site rating of gas turbine  403 . This problem can be solved by cooling the gas turbine inlet air with chilled water in a heat exchanger  404 . The turbine air is taken in through an inlet manifold  467  of the turbine  403 , in which the heat exchanger  404  is disposed. The chilled water can be provided from the unit  15 . 
     The high pressure nitrogen refrigerant exits the aftercooler  465  into a conduit  466 , from which the flow is subsequently divided between conduits  470  and  471 . The nitrogen flowing through the conduit  470  is fed to the compressor side of the expander/compressor unit  408 , while the nitrogen flowing through the conduit  471  is fed to the compressor side of the expander/compressor unit  409 . The compressed nitrogen exits the units  408  and  409  into conduits  472  and  473  respectively at an even higher, supercritical, pressure. The nitrogen flowing through the conduits  472  and  473  is recombined in a conduit  474 , through which it is fed to an aftercooler  410 , where it is cooled with cooling water. The nitrogen refrigerant exits the aftercooler  410  into a conduit  475 , through which it is fed to a heat exchanger  411 , where it is further cooled by countercurrent heat exchange with chilled water provided by the unit  15 . The heat exchangers  462 ,  465 ,  410  and  411  are all stainless steel PCHE exchangers; a closed circuit of fresh water is used for cooling in exchangers  462 ,  465  and  410 . Alternatively, direct seawater cooling may be used for these exchangers, if suitable materials of construction are employed. 
     The nitrogen refrigerant exits the heat exchanger  411  into a conduit  476 , through which it is fed to the cold box  402 , where it is pre-cooled in the series of heat exchangers in a similar manner to that shown in FIG. 3 or FIG. 6. A portion of the pre-cooled nitrogen (50-80 mol % of the total nitrogen flow) is withdrawn from the cold box  402  into a conduit  477 , through which it is fed to the turbo expander end of the expander/compressor unit  409 . The nitrogen in the expander compressor unit  409  is expanded to a lower pressure, with concomitant temperature drop. The work produced during this expansion stage is used to drive the compressor end of the expander/compressor unit  409 . The expander nitrogen exits the turbo expander of the expander/compressor unit into a conduit  478 . 
     Another portion of the pre-cooled nitrogen (20-50 mol % of the total nitrogen flow) is withdrawn from the cold box  402  into a conduit  479 , through which it is fed to the turbo expander end of the expander/compressor unit  408 ; the nitrogen withdrawn into the conduit  479  has been cooled to a lower temperature than that withdrawn through the conduit  478 . The nitrogen in the expander compressor unit  408  is expanded to a lower pressure, with concomitant temperature drop. The work produced during this expansion stage is used to drive the compressor end of the expander/compressor unit  408 . The expanded nitrogen exits the turbo expander of the expander/compressor unit into a conduit  480 . 
     The nitrogen in the conduits  478  and  480  is fed back to the series of heat exchanger within the cold box  402 , and serves to cool the natural gas entering the cold box  402  via the conduit  451  and to pre-cool the nitrogen entering the cold box  402  via the conduit  476 . The nitrogen flowing in the conduits  478  and  480  may follow the same path as the nitrogen in conduits  28  and  26  respectively in FIG. 3, or as the nitrogen in conduits  128  and  126  respectively in FIG.  6 . As explained above, the warmed nitrogen is subsequently withdrawn from the cold box  402  via the conduit  460 . 
     The expander/compressor units  408  and  409  may be conventional radial flow expander units. If desired the expander of expander/compressor unit  409  may be replaced by two expander units in parallel or in series. All the expander/compressor units  408 / 409  may be installed on a single skid to save on plot area and interconnecting pipework; they may also have a common lube oil skid, thereby saving further in plot area and cost. Another possibility is to connect the expanders to a single compressor or a multi-stage compressor, this would avoid the need to split the nitrogen flow into conduits  470  and  471 . 
     The chilled water refrigeration unit  415  comprises one or more standard, commercially available units, which can use refrigerants such as Freon, propane, ammonia, etc. The chilled water is circulated to the heat exchangers  401 ,  404  and  411  in a closed circuit by centrifugal pumps (not shown). This unit has the advantage that it requires only a small inventory of refrigerant, and takes up very little space. 
     The cooling water system is also a closed circuit system—it uses fresh water to allow the use of PCHE exchangers. The PCHE heat exchangers have the advantage that they are considerably smaller and cheaper than the conventional shell and tube heat exchangers normally used for this type of system. 
     The nitrogen refrigeration system is a closed circuit system containing an initial inventory of dry nitrogen gas. This nitrogen must be replenished during normal operation, due to small losses of refrigerant from the circuit. These losses are caused by, for example, leaks to atmosphere from compressor seals and pipework flanges etc. A small amount of nitrogen is continuously added to the refrigeration system by nitrogen make-up unit (not shown), in order to compensate for the leakages. The nitrogen is extracted from the instrument air system on the plant. The make-up unit may be a commercially available unit, which can be of the membrane type or the pressure swing absorption type. 
     FIG. 14 shows another embodiment of the apparatus shown in FIG.  13 . Many of the parts illustrated in FIG. 14 are identical to the parts illustrated in FIG.  13 —like parts have been designated with like reference numerals. The differences are as follows: 
     The embodiment shown in FIG. 14 uses a series of heat exchangers in the form of a spiral wound heat exchanger (also known as a coil wound heat exchanger)  480  in place of the series of heat exchangers located within the cold box  402  in the apparatus shown in FIG.  13 . The heat exchanger  480  is provided with its own thermal insulation, so there is no need to locate it within a cold box. Cooled natural gas at supercritical pressure is withdrawn from the heat exchanger  480  via a conduit  482 , and is fed to a nitrogen stripper located within a cold box  484 . The nitrogen stripper within the cold box  484  may be the same as the nitrogen stripper  57  or  157 . 
     The five refrigeration cycles described above, and shown in FIGS. 4,  5 ,  7 ,  8  and  9 , were simulated in order to make comparisons between the relative performance. 
     The first cycle, as illustrated in FIG. 4, used lean gas at a pressure of 5.5 MPa cooled with refrigerant at 1.2 MPa. The total power requirement was found to be 17.1 kW/tonne natural gas produced/day. 
     The second cycle, as illustrated in FIG. 5, used rich gas at a pressure of 5.5 MPa cooled with refrigerant at 1.2 MPa. The total power requirement was found to be 15.0 kW/tonne natural gas produced/day. 
     The third cycle, as illustrated in FIG. 7, used lean gas at a pressure of 5.5 MPa cooled with refrigerant at 1.7 MPa. The total power requirement was found to be 17.4 kW/tonne natural gas produced/day. However, although the power requirement was higher than the first and second cycle, the increased pressure allows the heat exchanger sizes to be reduced. 
     The fourth cycle, as illustrated in FIG. 8, used rich gas at a pressure of 7.6 MPa cooled with refrigerant at 2.4 MPa. The total power requirement was found to be 13.0 kW/tonne natural gas produced/day. 
     The fifth cycle, as illustrated in FIG. 9, used rich gas at a pressure of 8.25 MPa cooled with refrigerant at 1.8 MPa. The total power requirement was found to be 14.6 kW/tonne natural gas produced/day. 
     For comparison, the power requirement of a conventional propane pre-cooled mixed refrigerant cycle would be in the range 13 to 14 kW/tonne natural gas produced/day, and the power requirement of the simple nitrogen refrigeration cycle shown in FIG. 2 is about 27 kW/tonne natural gas produced/day. This shows that the process of the present invention is much more efficient than the simple refrigeration cycle. 
     Whilst certain embodiments of the invention have been described herein, it will be appreciated that the invention may be modified. 
     For the avoidance of doubt, the term “comprising” as used in this specification means “includes”.

Technology Category: 4