Abstract:
Apparatus for liquefying natural gas, 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, expansion means for isentropically expanding at least two separate streams of the compressed refrigerant, said expanded streams of refrigerant communicating with a cool end of a respective one of the heat exchangers, and a precooling refrigeration system for precooling the natural gas to a temperature below 0° C. before it is fed to the series of heat exchangers, and for precooling the compressed refrigerant discharged from a warm end of the series of heat exchangers to a temperature below 0° C. before it is fed back into the series of heat exchangers or to the expansion means.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of application Ser. No. 09/581,341, filed Aug. 21, 2000, now abandoned, which was a U.S. national phase of PCT/GB98/03708, filed Dec. 11, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a liquefaction process and apparatus. 
     In the liquefaction of natural gas with a refrigerant it is known to try to match the natural gas cooling curve with the refrigerant warning curve by splitting the refrigerant into two streams which are cooled to different temperatures. This is described, for example, in ore WO-A-9527179. 
     In our WO-A-9713108 there is disclosed a compact LNG plant for use in the offshore liquefaction of natural gas. FIG. 1 of the attached drawings illustrates a natural gas liquefaction apparatus of the general type disclosed in WO-A-9713108, although there are differences between FIG.  1  and the disclosure of WO-A-9713108. 
     In FIG. 1 pretreated natural gas is fed via a conduit  101  to a heat exchanger  166  at a pressure of about 8.3 MPa. In one example, the natural gas in conduit  101  would have the following composition: 4.2 mol% nitrogen; 85.1 mol% methane; 8.2 mol% ethane; and 2.5 mol% propane. The natural gas in the conduit  101  is cooled to a temperature in the range about 5° C. to 10° C. by heat exchange with chilled water, and is discharged into a conduit  102 . 
     The natural gas exiting the heat exchanger  166  is fed to the warm end of a CWHE (coil wound heat exchanger)  150  via the conduit  102 . The CWHB  150  comprises a single shell, which houses two separate heat exchanger bundles  151  and  152 . The natural gas is cooled in the CWHE  150  by countercurrent heat exchange with a nitrogen refrigerant. The cooled natural gas leaves the CWHE  150  at a temperature around −90° C., and is fed to a farther heat exchanger  153  via a conduit  104 . the heat exchanger  153  may be an aluminum PFHE (platefin heat exchanger). The natural gas is cooled to a temperature of about −150° C. in the heat exchanger  153 , and 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 −160° C., and it exits the cool end of the exchanger  154  into a conduit  107 . The natural gas in conduit  107  is fed to the top of a nitrogen stripper column  157 . The column  157  is needed when the nitrogen content of the feed gas is high and the required composition of the LNG product cannot be achieved using one or two stages of flash separation drums. 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 a conduit  167 , tom 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 . 
     LNG is fed from the bottom of the column  157  to a conduit  111  and then to a transfer pump  158 . The pump  158  pumps the LNG into a conduit  112  and on to a LNG storage tank  186 . 
     The flash gas, which contains methane and a high proportion of nitrogen, exits from the top end of the column  157  to a conduit  109 . The flash gas in conduit  109 , which is at a temperature of about −167° C., is fed to the cool end of a heat exchanger  155 , in which the gas is warmed to a temperature of about −40° C. The warmed gas is fed from the warm end of the exchanger  155  to a conduit  110 , from which it is fed to a multistage fuel gas compressor  180 . The compressor  180  has at least four stages of compression with intercooling between each stage using cooling water. The flash gas is compressed in the compressor  180  from just above atmospheric pressure to a pressure which is typically in the range 2.7 to 5.5 MPa, and is then fed to a turbine  173  of a refrigerant compressor  159 , as described in more detail below. High fuel gas pressures are required when the turbine is an aeroderivative turbine, owing to the high compression ratios used in such turbines. The fuel gas compressor  180  thus has a significant power requirement, owing to the high discharge pressure and high nitroen content of the gas, such that a gas turbine drive is usually used from economic considerations, rather than an electric motor drive. As described below, the flash gas fed through the conduit  110  is used to provide the bulk of the fuel gas requirements of the liquefaction plant. 
     The nitrogen refrigeration cycle which cools the natural gas to a temperature at which it can liqueur will now be described. Nitrogen refrigerant is discharged from the warm end of the CWHE  150  into a conduit  132  at a temperature of about 5° C. The nitrogen is fed to a multistage compressor unit  159 , which comprises at least two compressor stages  169  and  170 , with at least one intercooler  171 , and an aftercooler  172 . The compressor stages  169  and  170  are driven by a gas turbine  173 . The operation of the compressor unit  159  consumes almost all of the power required by the nitrogen refrigeration cycle. The gas turbine  173  is driven by the fuel gas derived from conduit  110 . 
     The compressed nitrogen is discharged from the compressor unit  159  into a conduit  133  at a pressure of about 5.1 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 8.5 MPa, and is then fed from the compressor  162  to a conduit  136 . The nitrogen in the conduit  135  is fed to a compressor  163  in which it is compressed to a pressure of about 8.5. MPa, and is then fed from the compressor  163  to a conduit  137 . The nitrogen in both the conduits  136  and  137  is fed to a conduit  138  and then to a heat exchanger  164 , where it is cooled to ambient temperatures. The nitrogen is fed from the heat exchanger  164  through a conduit  139  to a heat exchanger  165  in which it is cooled to a temperature of 5° C. to 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% of the nitrogen in conduit  140  flows through the conduit  141 . art 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. by countercurrent heat exchange with the flash gas from the column  157 . The cooled nitroen is discharged from the cool end of the exchanger  155  to a conduit  142 . 
     The conduit  120  is connected to the warm end of the CWHE  150 , whereby the nitrogen is fed to the warm end of the heat exchanger bundle  151 . The nitrogen from conduit  120  is pre-cooled to about 13° C. in the heat exchanger bundle  151 . A majority of the nitrogen refrigerant is withdrawn from the CWHE  150 , after passing through the bundle  151 , via a conduit  122 . The remainder of the nitrogen refrigerant passes through the bundle  152 , is cooled to a temperature of about −90° C., and is discharged from the CWHE  150  into a conduit  124 . 
     The nitrogen in the conduit  122  is fed to a turbo expander  160 , in which it is work expanded to a pressure of about 1.9 MPa and a temperature of about −95° C. The expanded nitrogen is discharged from the expander  160  into a conduit  128 . The nitrogen in the conduit  124  is mixed with the nitrogen in the conduit  142 , and is then fed to a turbo expander  161  in which it is work expanded to a pressure of about 1.9 MPa and a coolest nitrogen temperature of about −151° C. The expanded nitrogen is discharged 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. 
     The nitrogen in the conduit  126  is fed to the cool end of the heat exchanger  153 , and cools the natural gas therein by countercurrent heat exchange. In the heat exchanger  153  the nitrogen is warmed to an intermediate nitrogen temperature of about −95° C. The nitrogen exits the warn end of the heat exchanger  153  and is mixed with the nitrogen in the conduit  128  before being fed to the cool end of the CWHE  150 . The nitrogen in the CWHE  150  cools the natural gas therein by countercurrent heat exchange. 
     The heat exchangers  153 ,  154  and  155 , and the column  157  are arranged within a cold box  181 . 
     Inlet combustion air for the gas turbine  173  is fed to a beat exchanger  182  where it is cooled to 5° C. to 10° C. by heat exchange with chilled water. The combustion air is then discharged into a conduit  183  and is fed to the turbine  173 . 
     Chilling of the inlet air to the gas turbine increases the power output where the ambient air temperature is high. 
     As in most large scale LNG plants, the most expensive items of equipment are the gas turbine drives and compressors as well as the main CWHE cooling exchangers, such as the bundles  151  and  152  which are normally made of aluminum. 
     It is an object of the present invention to improve the efficiency and lower the capital cost of prior art processes for liquefying natural gas. 
     SUMMARY OF THE INVENTION 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a method and apparatus for liquefying natural gas, comprising a series of heat exchangers for cooling the natural gas in countercurrent heat exchange relationship with a refrigerant, compression 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. 
     One important aspect of the invention involves the use of a precooling refrigeration system to precool the natural gas to a low temperature below 0° C., before it is fed to the warm end of the series of heat exchangers. The precooling refrigeration system is also used to precool the high pressure refrigerant to a temperature below 0° C. before it is fed to any of the heat exchangers in the series of heat exchangers or to the expansion means. This has been found to reduce significantly power requirements of liquefaction apparatus, and reduce the number of components of the equipment. Advantageously, substantially all the refrigerant in the refrigeration cycle is precooled by the precooling refrigeration system. 
     It is desirable that the refrigerant in a first of said separate refrigerant streams is cooled in at least one of the series of heat exchangers; this takes place after it has been precooled in the precooling refrigeration system. Furthermore, the use of the precooling refrigeration system makes it unnecessary to cool more than one of the refrigerant streams in the series of beat exchangers, so we prefer that each refrigerant stream other than the first is fed directly to its respective expansion means without farther cooling in the series of heat exchangers. 
     The refrigerant of the refrigerant streams may be precooled before or after being separated into said streams, although it is more convenient and economical to carry out the precooling before separation. Preferably the refrigerant is split into two refrigerant streams. 
     We have unexpectedly found that, by using a precooling refrigeration system, it is only necessary to use two heat exchangers in the series of heat exchangers, which is fewer than in WO-A-9527179 and WO-A-9713108, and which leads to significant savings in the cost of manufacturing, operating and maintaining the heat exchangers. 
     Accordingly, in the preferred embodiment there are two heat exchangers in the series of heat exchangers, the refrigerant is split into first and second refrigerant streams, and only the refrigerant in the first refrigerant stream is cooled in a first, warmest, of said two heat exchangers. Thus, when two refrigerant streams are used, the first refrigerant stream can be fed through the warmest of the heat exchangers in the series of heat exchangers, and the second refrigerant stream can be fed directly to the expansion means without passing through the series of heat exchangers. This allows the heat transfer area in the first heat exchanger to be reduced by about 35% compared with the arrangement shown in FIG. 1, and reduces the complexity of the equipment. This makes it easier to use less expensive types of heat exchanger, such as an aluminum PFHE or a printed circuit heat exchanger (PCHE), instead of a CWHE. 
     The intercooler  171 , shown in FIG. 1, is an expensive piece of equipment because of its high design pressure, large area requirement, and titanium construction used for the parts in contact with sea water cooling medium. With the apparatus of the present invention it is possible to dispense with the intercooler  171  of FIG. 1, because the compressed refrigerant discharged from the compression means is within normal bounds. 
     Furthermore, the present invention makes it possible to reduce the complexity of the compression means, because the lower refrigerant temperature and, therefore, lower head requirement, allows either a smaller number of compressor wheels or a reduction in wheel diameter; additionally, the number of nozzles required on the compressor case can be reduced from 4 to 2, leading to further cost savings. Another advantage is that the power requirement for the compression means is reduced by about 16% for the same natural gas capacity, which makes it possible to reduce the rating of a turbine used to power the compressor. This makes it possible to replace the two compressor/intercooler arrangement of FIG. 1 with a single compressor stage and no intercooler. 
     The expanded refrigerant of the first refrigerant stream is preferably fed to a cool end of the second heat exchanger, and the expanded refrigerant of the second refrigerant stream is preferably fed to the cool end of the first heat exchanger. Before being fed to the cool end of the first heat exchanger, the expanded refrigerant of the second refrigerant stream is preferably mixed with the expanded refrigerant of the first refrigerant stream discharged from the warm end of the second heat exchanger. 
     The two heat exchangers of said series of heat exchangers may be separate, or may be provided in a single heat exchanger shell having two heat exchange bundles therein; each bundle corresponds to one of the heat exchangers of said series of heat exchangers. The use of a single heat exchanger has the advantage that the cold box can be omitted without any significant disadvantageous effects on efficiency. 
     The natural gas and the refrigerant are preferably precooled in the precooling refrigeration system to a temperature in the range 0° C. to −40° C., preferably −10° C. to −30° C. It is preferred to cool the natural gas and the refrigerant to substantially the same temperature with the precooling refrigeration system. The refrigerant is typically discharged from the warm end of the warmest heat exchanger at a temperature below −20° C. 
     An industry standard refrigeration system can be used, to precool the nitrogen and natural gas streams in two or more stages. The number of refrigeration stages for the system is selected depending on the final precooling temperature and by optimising the power requirements of the refrigeration system against the increase in cost for the larger number of equipment items. 
     There is a range of types of heat exchanger that could be used as the precooling heat exchangers For example, the precooling heat exchanger may be of the aluminum core-in-kettle type or an aluminum plate-fin heat exchanger PFHE or a PCHE. However, it is preferred for economic reasons that the precooling heat exchangers are conventional kettle type shell and rube chillers constructed of carbon steel. 
     The precooling refrigerant system precools both the natural gas and the natural gas refrigerant using a separate precooling refrigerant. The precooling refrigerant may be, for example, propane, propylene, ammonia or a Freon refrigerant. It is preferred that the precooling refrigerant is R410a Freon, because it is relatively safe and environmentally benign with a high capacity. 
     In the present invention, the precooling refrigerant is advantageously compressed by a single compression unit comprising two or more compressors stages driven by a precooling gas turbine, instead of using a plurality of separate electric motor driven chiller units as in FIG. 1. A two-stage refrigeration system is usually suitable, but in some cases a three or four stage system may be advantageous The reduction in the overall electric power requirements of the plant brought about by the elimination of the electric motor driven chiller units allows the precooling gas turbine to economically power an electric generator in addition to the compression unit for the precooling refrigeration system. This electric generator can meet all the normal power requirements of the apparatus according to the invention and allows a substantial reduction in the investment required for separate gas turbine driven electric generators required in FIG.  1 . 
     In a preferred embodiment, the natural gas discharged from the series of heat exchangers is fed to a nitrogen stripper column. The natural gas discharged from the series of heat exchangers may be fed to a heat exchanger within the stripper column, at or near the bottom of the stripper column, in order to provide reboil heat for the column; apart from this, it is preferred that the natural gas discharged from the series of heat exchangers is not subjected to any other heat exchange before being fed into the stripper column. 
     The stripper column generates a gaseous top product containing nitrogen and methane, and it is preferred that this top product is used as a fuel gas to power a turbine for driving the compression means for the refrigerant. The top product is preferably compressed in a fuel gas compressor before being fed to the turbine, and desirably the top product is not subjected to any heat exchange before being fed to the fuel gas compressor. 
     The arrangement of the stripper column, in accordance with the invention, makes it possible to use a cold box of a smaller size, and containing less equipment, than the cold box  181  in FIG. 1 (or even eliminate the cold box when the series of heat exchangers is provided in a single heat exchanger shell). 
     Furthermore, by feeding the top product directly to the fuel gas compressor, without any intermediate heat exchange, the suction temperature to the fuel gas compressor is lower, which reduces the power requirements and complexity. With the present invention, the fuel gas compressor may comprises a single compressor or two compressor stages with a single intercooler, and the power requirement can be reduced by up to about 50% compared with the compressor  180  in FIG.  1 . Furthermore, an electric motor driven compressor may be used instead of the more expensive gas turbine. 
     The refrigerant is preferably nitrogen, and it is preferred that the refrigerant in the gaseous phase through the refrigerant cycle. 
     The relative flow rates of the first and second refrigerant streams can be controlled to match as closely as possible the natural gas cooling curve with the nitrogen warming curve. This is described in more detail in, for example, WO-A-9713108 and WO-A-9527179. 
     The apparatus according to the invention may be used in an offshore apparatus for liquefying natural gas, as described in WO-A-9713108. In this embodiment, the apparatus can be provided on a support structure (for example a ship) which is floatable or is otherwise adapted to support the apparatus at least partially above sea level. 
     With the arrangement of FIG. 1 a specific power of about 15.8 KW/tonne LNG produced/day is required. With the apparatus according to the invention a specific power of about 14.75 KW/tonne LNG produced/day is required. It will be appreciated that this is a significant power saving and is additional to the capital cost savings mentioned above. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Reference is now made tot he accompanying drawings, in which: 
     FIG. 1 is a schematic diagram showing a natural gas liquefaction apparatus of the general type disclosed in WO-A-9713108; 
     FIG. 2 is a schematic diagram showing one embodiment of an apparatus according to the invention; 
     FIG. 3 is a schematic diagram showing another embodiment of an apparatus according to the invention; and 
     FIG. 4 is a schematic diagram showing an embodiment of a precooling refrigeration system for use with the embodiments of FIGS.  2  and  3 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 2 pretreated natural gas is fed via a conduit  201  to a first precooling heat exchanger  266  at a pressure of about 8.3 MPa. In one example the natural gas in conduit  201  would have the following composition: 4.2 mol% nitrogen; 85.1 mol% methane; 8.2 mol% ethane; and 2.5 mol% propane. The heat exchanger  266  is a carbon steel kettle type chiller using R410a as a refrigerant. The natural gas in the conduit  201  is cooled to −19° C. in the heat exchanger  266 , and is discharged into a conduit  202 . 
     The natural gas exiting the heat exchanger  266  is fed to the warm end of a first heat exchanger  250  via the conduit  202 . The heat exchanger  250  is a CWHE and comprises a single shell, which houses a single heat exchanger bundle  251 . The natural gas is cooled in the heat exchanger  250  by countercurrent heat exchange with a nitrogen refrigerant. The cooled natural gas leaves the heat exchanger  250  at a temperature around −95° C. and is fed to a second heat exchanger  253  via a conduit  204 . A throttle valve  285  is provided in the conduit  204 , through which the natural gas can, optionally, be expanded. The natural gas is cooled to a temperature of about −152° C. in the heat exchanger  153 , and exits the cool end of the exchanger  253  into a conduit  206 . 
     The natural gas in conduit  206  is fed directly to a heat exchange arrangement  254  disposed within a nitrogen stripper column  257 . The natural gas fed to the heat exchange arrangement  254  provides reboil heat at the bottom of the column  257 , and is cooled by the natural gas at the bottom of the column  257 . The natural gas is discharged from the heat exchange arrangement  254  into a conduit  207  through which the natural gas is fed to the top of the nitrogen stripper column  257 . A throttle valve  256  is provided in the conduit  207 , through which the natural gas can, optionally, be expanded. 
     LNG is discharged from the bottom of the column  257  into a conduit  211  and then to a pump  258 . The pump  258  pumps the LNG into a conduit  212  and on to an LNG storage tank  286 . 
     The flash gas, which contains methane and a high proportion of nitrogen, exits from the top end of the column  257  to a conduit  209 . The flash gas in conduit  209 , which is at a temperature of about −165° C., is fed to a fuel gas compressor  280 . The compressor  280  is either a single stage compressor, or a two stage compressor with a single intercooler. The compressor  280  is driven by a  3 MW electric motor. The flash gas is compressed in the compressor  280  from just above atmospheric pressure to a pressure which is typically in the range 2.7 to 5.5 MPa. High pressure fuel gas is discharged from the compressor  280  into a conduit  210 . As described below, the methane-containing gas fed to the conduit  210  is used to provide the bulk of the fuel gas requirements of the liquefaction plant. 
     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 heat exchanger  250  into a conduit  232  at a temperature of about −26° C. The nitrogen is fed to a single compressor stage  259 ; unlike the apparatus shown in FIG. 1, there is only one compressor stage, and, therefore, no intercooler is required. The compressor  259  is driven by a gas turbine  273  which may be an m Trent @ 54MW. The operation of the compressor  259  consumes almost all of the power required by the nitrogen refrigeration cycle. 
     The compressed nitrogen is discharged from the compressor  259  into a conduit  287  at a pressure of about 5.2 MPa. The nitrogen in the conduit  287  is fed to a heat exchanger  288 , in which the compressed nitrogen is cooled to ambient temperatures by countercurrent heat exchange with sea water. The compressed nitrogen is discharged from the heat exchanger  288  into a conduit  233 . 
     The conduit  233  leads to two conduits  234  and  235  between which the nitrogen from the conduit  233  is split according to the power absorbed by the compressor. The nitrogen in the conduit  234  is fed to a compressor  262  in which it is compressed to a pressure of about 8.5 MPa, and is then fed from the compressor  262  to a conduit  236 . The nitrogen in the conduit  235  is fed to a compressor  263  in which it is compressed to a pressure of about 8.5 MPa. and is then fed from the compressor  263  to a conduit  237 . The nitrogen in both the conduits  236  and  237  is fed to a conduit  289  and then to a heat exchanger  290 , where it is cooled to ambient temperatures by countercurrent heat exchange with sea water. 
     The nitrogen is discharged from the heat exchanger  290  into a conduit  238  through which it is fed to a second precooling heat exchanger  264 . The nitrogen is fed from the heat exchanger  264  through a conduit  239  to a third precooling heat exchanger  265 . The heat exchangers  264  and  265  are similar to the heat exchanger  266 , i.e., they are carbon shell kettle type chillers using R410a is a refrigerant. The compressed nitrogen is cooled to about 7° C. in the heat exchanger  264 , and is cooled to about −19° C. in the heat exchanger  265 . 
     The cooled compressed nitrogen is discharged from the exchanger  265  to a conduit  240 , which leads to two conduits  220  and  222 . The conduits  220  and  222  split the nitrogen into first and second refrigerant streams respectively. The conduit  220  is connected to the warm end of the heat exchanger  250 . The nitrogen passing through the heat exchanger  250  is cooled to about −95° C. before being discharged into a conduit  221 . 
     The nitrogen in the conduit  222  is fed to a turbo expander  260 , in which it is work expanded to a pressure of about 1.9 MPa and a temperature of about −10° C. The expanded nitrogen is discharged from the expander  260  into a conduit  228 . The nitrogen in the conduit  221  is fed to a turbo expander  261  in which it is work expanded to a pressure of about 1.9 MPa and a coolest nitrogen temperature of about −154° C. The expanded nitrogen is discharged from the expander  261  into a conduit  226 . The turbo expander  260  is arranged to drive the compressor  262 , and the trirbo expander  261  is arranged to drive the compressor  263 . In this way the majority of the work produced by the expanders  260  and  261  can be recovered. 
     The nitrogen in the conduit  226  is fed to the cool end of the heat exchanger  253 , and cools the natural gas in therein by countercurrent heat exchange. In the heat exchanger  253  the nitrogen is warmed to an intermediate nitrogen temperature of about −100° C. The nitrogen exits the warm end of the heat exchanger  253  and is mixed with the nitrogen in the conduit  228  before being fed to the cool end of the heat exchanger  250 . The nitrogen in the heat exchanger  250  cools the natural gas therein by countercurrent heat exchange. 
     The heat exchanger  253 , the throttle valve  256  and the column  257  are arranged within a cold box  298 . 
     The gas turbine  273  is driven by the fuel gas derived from conduit  210 . The combustion air for the turbine is fed to a fourth precooling heat exchanger  282 , in which it is cooled to a temperature of about 10° C. Inlet air is discharged from the heat exchanger  282  into a conduit  283  which is connected to the air inlet of the turbine  273 . The heat exchanger  282  is a finned tube exchanger using R410a as a refrigerant. 
     FIG. 3 shows a modification of the apparatus shown in FIG.  2 . Many of the parts shown in FIG. 3 are similar to the parts shown in FIG.  2  and like parts have been designated with like reference numerals. 
     The difference between the embodiments of FIGS. 2 and 3 are: 
     (i) The first and second heat exchangers  250  and  253  have been replaced with a single CWHE  350  comprising a shell housing first and second heat exchanger bundles  351  and  353 . 
     (ii) The cold box  289  has been omitted. 
     FIG.  4 . shows the precooling refrigeration system for the heat exchangers  264 ,  265 ,  266  and  282  providing refrigeration at −23° C. and 3° C. temperature levels. The heat exchangers  264 ,  265 ,  266  and  282  can be considered as first, second, third and fourth precooling heat exchangers respectively. The system includes a two stage, single case, API type refrigeration compressor unit  410  driven by a gas turbine  412 . The compressor unit  410  has two compressor stages  414  and  416 . In this example a two-stage refrigeration system is shown but it may be advantageous to use 3 or 4 stages for other situations. The precooling refrigerant is R410a, but other refrigerants may be used instead, including other Freons such as R134a. The turbine  412  also drives an electric generator G. which serves most of the electrical power requirements of the apparatus shown in FIGS. 2 and 3. 
     It will be appreciated that modifications may be made to the invention described above.