Patent Publication Number: US-2013247610-A1

Title: Method of preparing a cooled hydrocarbon stream and an apparatus therefor

Description:
This application claims the benefit of Indian Application No. 1022/CHE/2012 filed Mar. 20, 2012, which is incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to a method and apparatus for preparing a cooled hydrocarbon stream from a hydrocarbon feed stream. The cooled hydrocarbon stream may be cooled to such an extent that the hydrocarbon stream is in a fully condensed condition. 
     BACKGROUND OF THE INVENTION 
     An example of a hydrocarbon feed stream that in the industry often requires to be cooled is natural gas. Natural gas is a useful fuel source, as well as a source of various hydrocarbon compounds. It is often desirable to liquefy natural gas in a liquefied natural gas (LNG) plant at or near the source of a natural gas stream for a number of reasons. As an example, natural gas can be stored and transported over long distances more readily as a liquid than in gaseous form because it occupies a smaller volume and does not need to be stored at high pressure. 
     Conventionally, the hydrocarbons heavier than methane are removed as far as needed to produce a liquefied hydrocarbon product stream in accordance within a desired specification. Hydrocarbons heavier than butanes are removed as far as efficiently possible from the natural gas prior to any significant cooling for several reasons, such as having different freezing or liquefaction temperatures that may cause them to block parts of a methane liquefaction plant. 
     A process and apparatus for cooling a natural gas stream to a fully condensed condition is described in U.S. Pat. No. 6,370,910. The natural gas stream is pre-cooled before it enters into a scrub column. In the scrub column heavier hydrocarbons are withdrawn from the natural gas stream, to obtain a gaseous overhead stream at the top of the scrub column. This gaseous overhead stream is partly condensed by indirect heat exchanging against an (auxiliary) multicomponent refrigerant evaporating at a low (auxiliary) refrigerant pressure in a(n auxiliary) heat exchanger. A condensate stream is separated from the so partly condensed gaseous overhead stream, and returned to an upper part of the scrub column as reflux. The pre-cooling of the natural gas stream is effected by indirect heat exchange with a bleed stream from the multicomponent refrigerant. To this end the bleed stream is passed to a pre-cooling heat exchanger via an expansion valve. The multicomponent refrigerant that has evaporated in the (auxiliary) heat exchanger is removed from the heat exchanger, re-united with the bleed stream that is removed from the pre-cooling heat exchanger, and subsequently recompressed. 
     The process and apparatus described above has an inherent less than optimal efficiency, because the reflux is produced with the same refrigerant composition and pressure as are used for pre-cooling the natural gas. 
     SUMMARY OF THE INVENTION 
     In one aspect, the present invention provides a method of preparing a cooled hydrocarbon stream from a hydrocarbon feed stream, comprising:
     circulating a cooling fluid consisting of a mixed refrigerant composition in a loop along a circulation direction wherein, in consecutive order,   

     passing the cooling fluid through an expander to provide an expanded cooling fluid, 
     allowing the expanded cooling fluid to progressively evaporate as the expanded cooling fluid flows through a cold side heat exchanging channel, by allowing the expanded cooling fluid to flow through a first section of the cold side heat exchanging channel in contact with a first cold surface of a first heat exchanging fluid barrier whereby liquid from the expanded cooling fluid is continuously transformed to vapour thereby forming a residual liquid portion of not evaporated expanded cooling fluid, and subsequently allowing the residual liquid portion to continue its flow through a second section of the cold side heat exchanging channel in contact with a second cold surface of a second heat exchanging fluid barrier whereby the residual liquid is continuously vaporized, 
     compressing the vapour and the vaporized residual liquid to provide a compressed vapour, 
     transferring heat from the compressed vapour to ambient, and 
     closing the loop by again passing the cooling fluid through the expander;
     progressively cooling a hydrocarbon feed stream as it flows through a second warm section of a warm side heat exchanging channel in contact with a second warm surface of said second heat exchanging fluid barrier, thereby forming a pre-cooled hydrocarbon feed stream, by allowing the hydrocarbon feed stream to lose heat to the evaporating residual liquid passing through the second section of the cold side heat exchanging channel;   passing the pre-cooled hydrocarbon feed stream into a column;   drawing an overhead vapour hydrocarbon stream from the column;   progressively condensing the overhead vapour hydrocarbon stream as it flows through a first warm section of the warm side heat exchanging channel in contact with a first warm surface of said first heat exchanging fluid barrier, until the overhead vapour hydrocarbon stream is partially condensed and forms a partially condensed hydrocarbon stream, by allowing the overhead vapour hydrocarbon stream to lose heat to the evaporating expanded cooling fluid passing through the first section of the cold side heat exchanging channel;   separating the partially condensed hydrocarbon stream into a liquid component and a vaporous component, wherein the vaporous component comprises the cooled hydrocarbon stream;   feeding the liquid component into the column as reflux stream.   

     In another aspect, the present invention provides an apparatus for preparing a cooled hydrocarbon stream from a hydrocarbon feed stream, comprising:
     a cooling fluid consisting of a mixed refrigerant composition;   a loop containing the cooling fluid for circulating the cooling fluid in a circulation direction, which loop, described in consecutive order in the circulation direction, comprises:   

     an expander to provide an expanded cooling fluid, 
     a cold side heat exchanging channel comprising a first section and a second section, wherein the first section is fluidly connected to the expander to receive the expanded cooling fluid, wherein the first section comprises a first heat exchanging fluid barrier with a first cold surface facing into the first section of the cold side heat exchanging channel and arranged to allow passage of the expanded cooling fluid in contact with the first cold surface of the first heat exchanging fluid barrier, and wherein the second section of the cold side heat exchanging channel is arranged to receive at least a residual liquid portion from the first section of the cold side heat exchanging channel, and wherein the second section comprises a second heat exchanging fluid barrier with a second cold surface facing into the second section of the cold side heat exchanging channel and arranged to allow passage of the residual liquid in contact with the second cold surface of the second heat exchanging fluid barrier, 
     a first compressor train in fluid communication with at least the second section of the cold side heat exchanging channel and comprising at least one first compressor for compressing vaporised expanded cooling fluid and vaporized residual liquid originating from the cold side heat exchanging channel to provide a compressed vapour, 
     an ambient heat exchanger arranged to receive the compressed vapour and to transfer heat from the compressed vapour to ambient, and 
     a cooling fluid connection fluidly extending between the ambient heat exchanger and the expander by which the loop is closed;
     a warm side heat exchanging channel comprising a first warm section and a second warm section, whereby a first warm surface of said first heat exchanging fluid barrier faces into the first warm section, which first warm surface is in heat exchanging contact with the first cold surface through the first heat exchanging fluid barrier; and whereby a second warm surface of said second heat exchanging fluid barrier faces into the second warm section of the warm side heat exchanging channel, which second warm surface is in heat exchanging contact with the second cold surface through the second heat exchanging fluid barrier;   a hydrocarbon feed stream;   a source of the hydrocarbon feed stream;   a column, comprising an overhead discharge outlet, and comprising a first column inlet and a second column inlet, which column is fluidly connected to the source of the hydrocarbon feed stream via the first column inlet and via the second warm section of the warm side heat exchanging channel to allow passage of the hydrocarbon feed stream from the source to the column in contact with the second warm surface;   a reflux separator comprising a separator inlet and a liquid discharge outlet and a vapour discharge outlet, whereby said reflux separator is in fluid communication with the column via the overhead discharge outlet, the separator inlet and via the first warm section of the warm side heat exchanging channel to allow passage of an overhead vapour hydrocarbon stream from the column to the reflux separator in contact with a first warm surface, whereby said first warm section of the warm side heat exchanging channel extends between the overhead discharge outlet and the separator inlet;   a reflux conduit fluidly connecting the reflux separator and the column via the liquid discharge outlet and the second column inlet;   a cooled hydrocarbon stream conduit connected to the vapour discharge outlet of the reflux separator arranged to remove a vaporous component from the reflux separator which vaporous component comprises the cooled hydrocarbon stream.   

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be further illustrated hereinafter, using non-limiting examples and with reference to the drawing in which; 
         FIG. 1  represents a schematic process flow scheme representing a method and apparatus according to an embodiment of the invention; 
         FIG. 2  schematically represents a longitudinal section of a part of a heat exchanger used in  FIG. 1 ; 
         FIG. 3  represents a schematic process flow scheme representing a method and apparatus for producing a liquefied hydrocarbon stream, such as a liquefied natural gas stream, wherein the embodiment of  FIG. 1  is embedded; 
         FIG. 4  schematically represents an alternative compressor train arrangement that can optionally be used in the embodiment of  FIG. 3 ; 
         FIG. 5  schematically represents a schematic process flow scheme representing an alternative method and apparatus for producing a liquefied hydrocarbon stream, such as a liquefied natural gas stream, wherein the embodiment of  FIG. 1  is embedded; 
         FIG. 6  schematically represents an alternative heat exchanger arrangement that can optionally be used in the embodiments of  FIGS. 1 ,  3 ,  4  and  5 ; 
         FIG. 7  schematically represents another alternative heat exchanger arrangement that can optionally be used in the embodiments of  FIGS. 1 ,  3 ,  4  and  5 . 
     
    
    
     In these figures, same reference numbers will be used to refer to same or similar parts. Furthermore, a single reference number will be used to identify a conduit or line as well as the stream conveyed by that line. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Described below will be a method and apparatus for preparing a cooled hydrocarbon stream from a hydrocarbon feed stream. The hydrocarbon feed stream is partially condensed. The partially condensed hydrocarbon feed stream is then sent to a column. An overhead vapour hydrocarbon stream from the column is then partially condensed by indirect heat exchanging against an expanded cooling fluid flowing through a first section of a cold side heat exchanging channel. The cooling fluid consists of a mixed refrigerant composition, and liquid from the expanded cooling fluid is continuously transformed to vapour thereby forming a residual liquid portion of not evaporated expanded cooling fluid. The residual liquid is used to progressively condense the hydrocarbon feed stream to produce the partially condensed hydrocarbon feed stream that is sent to the column, by allowing the hydrocarbon feed stream to lose heat to the residual liquid passing through a second section of the cold side heat exchanging channel. The liquid component that is condensed out of the overhead vapour hydrocarbon stream is used as reflux for the column. 
     Relatively volatile components from the mixed refrigerant composition evaporate in the first section of the cold side heat exchanging channel using heat from the overhead vapour hydrocarbon stream from the column, leaving relatively less volatile components in the residual liquid. This residual liquid is evaporated using heat from the hydrocarbon feed stream. Therefore, the reflux can be at lower temperature than the partially condensed hydrocarbon feed stream being fed to the column, while at the same time optimal use is made of the heat absorbing capacity that is available in the cooling fluid. 
     Moreover, the composition of the cooling fluid being evaporated in the first section is different from the residual liquid that is evaporated in the second section of the cold side heat exchanging channel, while advantageously no phase separator is necessary to achieve the different compositions. 
     The cooling fluid can be kept at essentially the same pressure in the first and second sections of the cold side heat exchanging channel, other than dynamic pressure loss inherently caused by passing through the cold side heat exchanger channel and passing from the first section to the second section of the cold side heat exchanger channel. This safes equipment (e.g. an expansion turbine and/or expansion valve) and simplifies re-compression of the evaporated cooling fluid over alternative approaches where the cooling fluid is evaporated at deliberately different pressure levels in the respective first and second sections of the cold side heat exchanging channel. For instance, the residual liquid portion may be passed from the first section of the cold side heat exchanging channel to the second section of the cold side heat exchanging channel without changing the pressure of the residual liquid portion by more than 1 bar anywhere between these first and second sections. 
     Only one single heat exchanger is required to prepare the cooled hydrocarbon stream. Both the first and second sections of the cold side heat exchanging channel can be located within one single heat exchanger. Being located in a single heat exchanger may mean being located within a single shell. 
     Referring now to  FIG. 1 , there is schematically shown an apparatus for preparing a cooled hydrocarbon stream  50  from a hydrocarbon feed stream  10 . It comprises a loop in the form of a cooling fluid loop  100  having a circulation direction  101 . The cooling fluid loop  100  contains a cooling fluid consisting of a mixed refrigerant composition. The present embodiment uses a so-called tube-in-shell heat exchanger  200 , which may be provided in the form of coil-wound heat exchanger. 
     Described in consecutive order in the circulation direction  101 , the cooling fluid loop  100  comprises an expander  110 ; a cold side heat exchanging channel  120 , here as example shown as a shell side of the tube-in-shell heat exchanger  200 ; a first compressor train  130  in fluid communication with the cold side heat exchanging channel  120 ; an ambient heat exchanger  140 ; and a cooling fluid connection  150  fluidly extending between the ambient heat exchanger  140  and the expander  110 . Generally, the expander  110  may be provided in any suitable form, for instance an expansion turbine, an expansion valve (such as a Joule Thomson (JT) valve) or a combination thereof. In the example as shown the expander  110  is represented in the form of a JT valve. 
     In the present embodiment, the cold side heat exchanging channel  120  occupies the entire shell side of the tube-in-shell heat exchanger  200 . A bundle break is provided in the tube-in-shell heat exchanger  200 , which separates the cold side heat exchanging channel  120  into a first section  124  and a second section  126 . The location of the bundle break is schematically indicated by a dashed line  220 . In the embodiment as shown, the first section  124  is located gravitationally higher than the second section  126  so that non-evaporated residual liquid of a cooling fluid can traverse the bundle break and flow downward from the first section  124  into the second section  126  by pull of gravity. 
     The first section  124  of the cold side heat exchanging channel  120  is on an upstream end fluidly connected to the expander  110  and on a downstream end fluidly connected to the second section  126  of the cold side heat exchanging channel  120 , via which second section  126  the first section  124  is connected to the first compressor train  130 . 
     Preferably, the cooling fluid loop  100  does not comprise any phase separator between the expander  110  and the second section  126  of the cold side heat exchanging channel  120 , when the cooling fluid loop  100  is considered in the circulation direction and in a single pass of the cooling fluid through the cooling fluid loop  100 . 
     A warm side heat exchanging channel  220  is arranged in the heat exchanger, provided with a heat exchanging fluid barrier that a warm side of the heat exchanger from a cold side. In the example, the warm side heat exchanging channel  220  is arranged within the shell  201  of the shell-and-tube heat exchanger  200  as a bundle of tubes traversing the shell side of the shell-and-tube heat exchanger  200 . The warm side heat exchanging channel  220  comprises a first warm section  230  and a second warm section  210 . 
     A specific example of a structure of the warm side heat exchanging channel within the shell is illustrated schematically in  FIG. 2 , wherein a longitudinal cross section is shown though one of the tubes of the tube-in-shell heat exchanger  200 . In this example, the first section  124  of the cold side heat exchanging channel  120  comprises a first heat exchanging fluid barrier  231 , with a first cold surface  232  facing into the first section  124  of the cold side heat exchanging channel  120 . The second section  126  comprises a second heat exchanging fluid barrier  211  with a second cold surface  212  facing into the second section  126  of the cold side heat exchanging channel  120 . Likewise the first heat exchanging fluid barrier  231  has a first warm surface  233  that faces into the first warm section  230 . The first warm surface  233  faces away from the first cold surface  232 , and is in heat exchanging contact with the first cold surface  232  through the first heat exchanging fluid barrier  231 . The second heat exchanging fluid barrier  211  has a second warm surface  213  that faces into the second warm section  210  of the warm side heat exchanging channel  220 . The second warm surface  213  faces away from the second cold surface  212 , and is in heat exchanging contact with the second cold surface  212  through the second heat exchanging fluid barrier  211 . 
     In this case, the first and second heat exchanging fluid barriers ( 231 , 211 ) are formed by the collective tube walls of the relevant tube bundle in the coil-wound heat exchanger. It should be noted that for reason of providing clarity within the drawing the tubes are drawn straight, while in a practical embodiment according to normal design principles known in the art the tube bundle is often arranged spiralling through the shell whereby the tubes within the bundle are spread through the majority of the available cross section within the shell, optionally intertwined with tubes belonging to other tube bundles. 
     Referring, again, to  FIG. 1 , the first compressor train  130  comprises at least one first compressor  131 , and may optionally comprise a plurality (not shown) of first compressors  131  arranged in a parallel configuration (in which the respective suction inlets of parallel configured first compressors are fluidly connected to each other) and/or in a serial configuration (in which the suction inlet of one of the serially configured first compressor is fluidly connected to the discharge outlet of another one of the serially configured first compressor). 
     In the embodiment of  FIG. 1 , the cooling fluid connection  150  that fluidly extends between the ambient heat exchanger  140  and the expander  110  comprises an auxiliary warm side heat exchanging channel  160  arranged in heat exchanging relationship with the cold side heat exchanging channel  120 . Similar to the warm side heat exchanging channel  220  described above, the auxiliary warm side heat exchanging channel  160  is arranged in the heat exchanger, and it comprises a third warm section  164  arranged within the second section  126  of the cold side heat exchanging channel  120  and a fourth warm section  166  arranged with in the first section  124  of the cold side heat exchanging channel  120  Like the warm side heat exchanging channel  220 , the auxiliary warm side heat exchanging channel  160  may be provided in the form of an auxiliary tube bundle of which tubes are helically arranged within the shell preferably intertwined with the other tubes. 
     As best viewed in the schematic illustration of  FIG. 2 , the auxiliary warm side heat exchanging channel  160  comprises a third heat exchanging fluid barrier  161 , and a fourth heat exchanging fluid barrier  166 . The third heat exchanging fluid barrier  161  has a third surface  162  facing into the second section  126  of the cold side heat exchanging channel  120 , and a third warm surface  163  facing into the third warm section  164  of the auxiliary warm side heat exchanging channel  160  and facing away from the third cold surface  162 . The third warm surface  163  is in heat exchanging contact with the third cold surface  162  through the third heat exchanging fluid barrier  161 . Likewise the fourth heat exchanging fluid barrier  169  has a fourth warm surface  167 , which faces into the fourth warm section  166  of the auxiliary warm side heat exchanging channel  160 , and which faces away from the fourth cold surface  168 . The fourth warm surface  167  is in heat exchanging contact with the fourth cold surface  168  via the fourth heat exchanging fluid barrier  166 . 
     Again with reference to  FIG. 1 , a source  5  of the hydrocarbon feed stream  10  is in fluid communication with a column  25  via the second warm section  210  of the warm side heat exchanging channel  220 . The column  25  comprises an overhead discharge outlet  26 , a bottom liquid outlet  22 , a first column inlet  21 , and a second column inlet  27 . The column  25  may optionally be provided with distillation internals, such as a contacting section  28  containing a plurality of gas/liquid contacting trays or structured packing. Preferably, the second column inlet  27  is arranged gravitationally higher than the contacting section  28 , as is the overhead discharge outlet  26 , whereas the first column inlet  27  is preferably arranged gravitationally lower than the contacting section  28  as is the bottom liquid outlet  22 . 
     The column  25  is fluidly connected to the source  5  of the hydrocarbon feed stream  10  via the first column inlet  21 , and via the second warm section  210  of the warm side heat exchanging channel  220  to allow passage of the hydrocarbon feed stream  10  from the source  5  to the column  25  in contact with the second warm surface  213 . 
     A reflux separator  45  is associated with the column  25 . The reflux separator  45  comprises a separator inlet  41 , a liquid discharge outlet  42  and a vapour discharge outlet  43 . The reflux separator  45  is in fluid communication with the column  25  via the overhead discharge outlet  26  of the column  25 , the separator inlet  41  and the first warm section  230  of the warm side heat exchanging channel  220  which is located between the overhead discharge outlet  26  of the column  25  and the separator inlet  41 . The reflux separator  45  is also in fluid communication with the column  25  via a reflux conduit  47  fluidly connecting the reflux separator  45  and the column  25  via the liquid discharge outlet  42  and the second column inlet  27 . 
     A cooled hydrocarbon stream conduit  50  is fluidly connected to the vapour discharge outlet  43  of the reflux separator  45 . 
     The purpose of the column  25  is to extract heavier hydrocarbons from the hydrocarbon feed stream  10  in the form of the bottom liquid  52  that is removed from the column  25  via the bottom liquid outlet  22 . The column  25  may be provided in the form of a distillation column suitable for the purpose, such as an NGL extraction column or a scrub column. The column  25  optimized according to its intended purpose. For instance, if the hydrocarbon feed stream  10  contains methane, and heavier hydrocarbons including C 2 -C 4  and C 5 + hydrocarbons, it can be adapted or optimized to extract as much of the C2-C4 components as possible. It may also be adapted or optimized to produce a cooled hydrocarbon stream in cooled hydrocarbon stream conduit  50  that has less than 0.1 mol. % of C 5 + hydrocarbons. In that case the cooled hydrocarbon stream in cooled hydrocarbon stream conduit  50  can ultimately be liquefied without creating solidified hydrocarbon components. 
     The apparatus of  FIG. 1  can be employed as follows in a method of preparing a cooled hydrocarbon stream  50 , from a hydrocarbon feed stream  10 . 
     The cooling fluid, consisting of a mixed refrigerant composition, is circulated in a cooling fluid loop  100  along the circulation direction  101 . During the circulation, in consecutive order the cooling fluid passes through: the expander  110 , the cold side heat exchanging channel  120 , the compressor train  130 , the ambient heat exchanger  140 , and the cooling fluid connection  150  that fluidly extends between the ambient heat exchanger  140  and the expander  110 . 
     The passing the cooling fluid through the expander  110  provides an expanded cooling fluid. The expanded cooling fluid is allowed to progressively evaporate as the expanded cooling fluid flows through the cold side heat exchanging channel  120 . 
     In more detail, expanded cooling fluid is allowed to progressively evaporate by first allowing the expanded cooling fluid to flow through the first section  124  of the cold side heat exchanging channel  120  in contact with the first cold surface  232  of the first heat exchanging fluid barrier  231 , whereby liquid from the expanded cooling fluid is continuously transformed to vapour. Hereby, a residual liquid portion of not evaporated expanded cooling fluid is formed. Subsequently, the residual liquid portion is allowed to continue its flow through the cold side heat exchanging channel  120  through the second section  126  thereof, and in contact with the second cold surface  212  of the second heat exchanging fluid barrier  211  whereby the residual liquid is continuously vaporized. 
     Preferably, during any single pass of the cooling fluid through the cooling fluid loop  100 , the expanded cooling fluid does not pass through any phase separator between the expander  110  and the second section  126  of the cold side heat exchanging channel  120 . 
     In the next part of the circulating of the cooling fluid, the vapour and the vaporized residual liquid (in discharge line  128 ) are compressed as a combined vapour, thereby providing a compressed vapour. In the embodiment of  FIG. 1 , the vapour is allowed to flow from the first section  124  to and through the second section  126  of the cold side heat exchanging channel  120 . Thus the vaporized residual liquid is mixed with the vapour from the first section  124  already in the heat exchanger. Alternatives are possible. For instance, the vapour could be removed from the heat exchanger separately from the vaporised residual liquid, and then combined to form a combined vapour that can be fed to the compressor train  130  as a single stream. Still alternatively, the separately removed vapour and vaporized residual liquid can be separately compressed and brought together somewhere else in the cooling fluid loop  100 . 
     Next, heat is transferred from the compressed vapour to ambient thereby producing an ambient cooled compressed cooling fluid. The heat comprises heat added during compression as well as heat gained while passing through the cold side heat exchanging channel  120  and being evaporated therein. The loop is closed by again passing the cooling fluid through the expander  110 . 
     Optionally, but preferably, during passing of the compressed vapour (in the form of the ambient cooled compressed cooling fluid) from the ambient heat exchanger  140  to the expander  110 , the compressed vapour flows through the optional auxiliary warm side heat exchanging channel  160 . This comprises flowing through the third warm section  164  in contact with the third warm surface  162  of the third heat exchanging fluid barrier  161  and subsequently through the fourth warm section  166  in contact with the fourth warm surface  167  of the fourth heat exchanging fluid barrier  169 . The cooling fluid in the cold side heat exchanging channel  120  is in contact with the respective fourth ( 168 ) and third ( 162 ) cold surfaces. Thereby, the compressed vapour can lose heat to the evaporating residual liquid passing through the second section  126  of the cold side heat exchanging channel  120  and subsequently to the evaporating expanded cooling fluid passing through the first section  124  of the cold side heat exchanging channel  120 . By these losses of heat, the compressed vapour can condense and in so far as it has already condensed it can be subcooled prior to being expanded in the expander  110 . 
     The hydrocarbon feed stream  10  is progressively cooled as it flows through the second warm section  210  of the warm side heat exchanging channel  220  in contact with the second warm surface  213  of the second heat exchanging fluid barrier  211 . Herewith a pre-cooled hydrocarbon feed stream  20  is formed, by allowing the hydrocarbon feed stream  10  to lose heat to the evaporating residual liquid passing through the second section  126  of the cold side heat exchanging channel  120  in contact with the second cold surface  212  of the second heat exchanging fluid barrier  211 . The pre-cooled hydrocarbon feed stream  20  preferably consists of a mixture of vapour and liquid phases. 
     The thus pre-cooled hydrocarbon feed stream  20  is then removed from the heat exchanger and passed into the column  25 , suitably via the first column inlet  21 . An overhead vapour hydrocarbon stream  30  is drawn from the column  25  and passed back to the heat exchanger. Here the overhead vapour hydrocarbon stream  30  is progressively condensed as it flows through the first warm section  230  of the warm side heat exchanging channel  220  in contact with the first warm surface  233  of the first heat exchanging fluid barrier  231 . The overhead vapour hydrocarbon stream  30  is partially condensed by allowing the overhead vapour hydrocarbon stream  30  to lose heat to the evaporating expanded cooling fluid passing through the first section  124  of the cold side heat exchanging channel  126  in contact with the first cold surface  232  of the first heat exchanging fluid barrier  231 . Hereby a partially condensed hydrocarbon stream  40  is formed out of the overhead vapour hydrocarbon stream  30 . 
     The partially condensed hydrocarbon stream  40  is passed into the reflux separator  45 , suitably via the separator inlet  41 , in which reflux separator  45  the partially condensed hydrocarbon stream  40  is phase separated into a liquid component and a vaporous component. The vaporous component, which comprises the cooled hydrocarbon stream, is discharged via the vapour discharge outlet  43  into the cooled hydrocarbon stream conduit  50 . The liquid component is discharged via the liquid discharge outlet  42  into the reflux conduit  47  and passed to and fed as reflux stream into the column  25 . This may be done for instance by force of gravity and/or with assistance of a reflux pump (not shown). 
     Interestingly, the temperature gradient in the column  25  is determined by the mixed refrigerant composition as the mixed refrigerant composition determines the temperature profile within the heat exchanger  200 . 
     The hydrocarbon feed stream  10  to be cooled, and ultimately preferably liquefied as will be described in embodiments below, may be derived from any suitable gas stream to be refrigerated and optionally liquefied. An often used example is a natural gas stream, for instance obtained from natural gas or petroleum reservoirs, shale, or coal beds. As an alternative the hydrocarbon feed stream  10  may also be obtained from another source, including as an example a synthetic source such as a Fischer-Tropsch process. 
     When the hydrocarbon feed stream  10  is a natural gas stream, it is usually comprised substantially of methane. Preferably the hydrocarbon feed stream  10  comprises at least 50 mol % methane, more preferably at least 80 mol % methane. 
     Depending on the source, natural gas may contain varying amounts of hydrocarbons heavier than methane such as in particular ethane, propane and the butanes (together indicated by the abbreviation C 2 -C 4 ), and possibly lesser amounts of pentanes and aromatic hydrocarbons (C 5+  hydrocarbons). The composition varies depending upon the type and location of the gas. 
     The column  25  in the present invention suitably serves to extract such C 5 + hydrocarbons so as to produce a cooled hydrocarbon stream in cooled hydrocarbon stream conduit  50  that has less than 0.1 mol. % of these C 5+  hydrocarbons. Moreover, natural gas liquids consisting mainly of C 2 -C 4 , hydrocarbons, particularly petroleum gas liquids in the form of C 3 -C 4  hydrocarbons (LPG) are typically recovered as well. 
     The natural gas may also contain non-hydrocarbons such as H 2 O, N 2 , CO 2 , Hg, H 2 S and other sulphur compounds, and the like. Thus, if desired, the source  5  of the hydrocarbon feed stream  10  may comprise equipment to perform pre-treatment steps comprising one or more of reduction and/or removal of undesired components such as CO 2  and H 2 S or other steps such as early cooling, pre-pressurizing or the like. As these steps are well known to the person skilled in the art, their mechanisms are not further discussed here. As part of such pre-treatment, the natural gas may be dried in accordance with WO 2012/000998, which disclosure is incorporated herein by reference. 
     Reference is now made to  FIG. 3 , which schematically illustrates a method and apparatus for producing a liquefied hydrocarbon stream  90 , such as a liquefied natural gas stream, wherein the invention as described above is embedded. In addition to the elements described hereinabove, such as the cooling fluid loop  100  and the heat exchanger  200 , the method and apparatus for producing the liquefied hydrocarbon stream  90  in accordance with  FIG. 3  further comprises a main refrigeration loop  300  comprising a main cooling fluid. The main cooling fluid consists of a main mixed refrigerant composition, that is different from the mixed refrigerant composition described above in that it contains relatively more volatile constituents. The main refrigeration loop  300  in this embodiment is separate from the cooling fluid loop  100  in which the cooling fluid is circulated as described above. This means that under normal circulation within the respective loops, the cooling fluid and the main cooling fluid are kept separated from each other. 
     Described in consecutive order in a circulation direction  301  of the main refrigeration loop  300 , the main refrigeration loop  300  comprises one or more expanders  310   a,   310   b  here as example shown in the form of expansion valves; a main cryogenic heat exchanger  400 ; a second compressor train  330  in fluid communication with the main cryogenic heat exchanger  400 ; a main ambient heat exchanger  340 ; and a main cooling fluid connection  350  fluidly extending between the main ambient heat exchanger  340  and the one or more expanders  310   a,   310   b.    
     In the embodiment as shown the main cooling fluid connection  350  passes through the shell-in-tube heat exchanger  200  of the cooling fluid loop  100  via a second auxiliary warm side heat exchanging channel  360  extending through the shell side of the shell-in-tube heat exchanger  200  similar to the auxiliary warm side heat exchanging channel  160 . From there the second auxiliary warm side heat exchanging channel  360  is connected to the main cryogenic heat exchanger  400  via a main cooling fluid separator  365  wherein the main cooling fluid can be separated in a light mixed refrigerant stream  370   a  to be discharged from the top of the main cooling fluid separator  365  and a heavy mixed refrigerant stream  370   b  to be discharged from the bottom of the main cooling fluid separator  365 . However, alternative main liquefaction processes and line ups exist which may be employed if desired. 
     The second compressor train  330  comprises at least one second compressor. In the embodiment as shown in  FIG. 3  the at least second compressor is provided in the form of an LP compressor  331  in a first casing and an MP/HP compressor  333  combined as two successive stages in a second casing being a separate casing from the first casing. The LP compressor  331  discharges into the suction inlet of the MP/HP compressor  333  via a first ambient intercooler  332 . A second ambient intercooler  334  may be provided between the MP and HP stages in the MP/HP compressor  333 . Alternative compressor trains can be selected instead of the specific embodiment described here. 
     The cooled hydrocarbon stream conduit  50  connects to a liquefaction passage  55  extending through the main cryogenic heat exchanger  400 . The liquefaction passage  55  extends through the main cryogenic heat exchanger  400  in heat exchanging contact with the main cooling fluid that has been expanded in the one or more expanders  310   a,   310   b  and fed to the main cryogenic heat exchanger  400 . By indirectly heat exchanging the cooled hydrocarbon stream  50  against the main cooling fluid that is evaporating, the cooled hydrocarbon stream is liquefied and preferably subcooled and thus converted into a raw liquefied hydrocarbon product  60 . 
     The raw liquefied hydrocarbon product  60  may be further treated by end treatment system  80  to yield for instance the liquefied hydrocarbon stream  90  and a by-product stream  70 . Such by-product stream may be in vapour phase and it could be end compressed to a desired pressure in an end compressor  85  and subsequently heat exchanged against the ambient in end heat exchanger  86 . Usually the by-product stream  70  has a much lower temperature than the cooled hydrocarbon stream in cooled hydrocarbon stream conduit  50 . In such as case, preferred embodiment embodiments provide for cold recovery. One suitable way is by indirectly heat exchanging the by-product stream  70  before any end compression against a slipstream of the light mixed refrigerant stream  370   a  which is split off between the main cooling fluid separator  365  and the main cryogenic heat exchanger  400  and indirectly heat exchanged with the by-product stream  70  instead of in the evaporating main cooling fluid main cryogenic heat exchanger  400 . Another suitable way is by indirectly heat exchanging the by-product stream  70  before any end compression against a slipstream of the cooled hydrocarbon stream which is split off from the cooled hydrocarbon stream conduit  50  between the reflux separator  45  and the liquefaction passage  55  in the main cryogenic heat exchanger  400  and is and indirectly heat exchanged with the by-product stream  70  instead of the evaporating main cooling fluid in the main cryogenic heat exchanger  400 . 
     In a typical liquefaction plant, the end treatment system  80  contains one or more expanders to depressurize the raw liquefied hydrocarbon product  60 . The by-product stream  70  may suitably contain flash vapours that are generated by such depressurization. The end treatment system may be selected with the aim to bring the liquefied hydrocarbon stream within a maximum specified content of light contaminants such as nitrogen and helium in the case the liquefied hydrocarbon stream consists of LNG. Numerous suitable end treatment systems are known in the art and the present invention is not limited to any one specific selection of end treatment system. 
     In the embodiment of  FIG. 3 , the compressing of the vapour and the vaporized residual liquid is performed with the first compressor train  130  comprising at least the one first compressor  131 , while the circulating of the main cooling fluid comprises compressing the main cooling fluid in the second compressor train comprising the at least one second compressor ( 331 , 333 ). Each of the compressors in the first compressor train  130  and of the second compressor train  330  may be provided with its own dedicated one or more compressor drivers. Suitable drivers include a steam turbine, a gas turbine (industrial frame type or, preferably, of aero derivative type), an electric motor. Sets comprising several suitable drivers in combination may be employed. Several of the compressors may be jointly driven by one or more combined drivers. For instance, the LP and MP/HP compressors  331 , 333  may be jointly driven by one set of one or more drivers, whereas the at least one first compressor  131  of the first compressor train  130  may be driven by another set of one or more drivers. Another option is to employ one set of drivers to drive every compressor of the first compressor train  130  and a subset of compressors of the second compressor train  330  or the other way round. This option offers advantages in terms of load balancing between the two separate cooling fluid loops. 
       FIG. 4  illustrates a special case wherein all compressors ( 131 ) of the first compressor train  130  are jointly driven together with all compressors ( 331 , 333 ) of the second compressor train  330  by one single set  335  of drivers. The single set  335  of drivers may consist of a single gas turbine or a single steam turbine or a single electric motor, or any combination thereof. In this special case, a common drive shaft  336  mechanically drives the at least one first compressor and any other compressor in the first compressor train  130  as well as the at least one second compressor and any other compressor in the second compressor train  330 . 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Reference numbers correspond to FIG. 3. 
               
            
           
           
               
               
            
               
                   
                 Ref. 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 10 
                 20 
                 30 
                 40 
                 47 
                 50 
                 52 
                 60 
                 70 
                 90 
               
               
                   
                   
               
            
           
           
               
            
               
                 Physical conditions 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Phase 
                 V 
                 V + L 
                 V 
                 V + L 
                 V + L 
                 V 
                 V 
                 L 
                 V 
                 L 
               
               
                 (V/L) 
               
               
                 Flow rate 
                 2.69 
                 2.69 
                 2.68 
                 2.68 
                 0.09 
                 2.59 
                 0.11 
                 2.59 
                 0.15 
                 2.43 
               
               
                 (kmol/s) 
               
               
                 Temp (° C.) 
                 37 
                 −6 
                 −10 
                 −23 
                 −23 
                 −23 
                 −6 
                 −153 
                 −161 
                 −161 
               
               
                 Pressure 
                 58 
                 56 
                 50 
                 53 
                 56 
                 53 
                 56 
                 47 
                 1.1 
                 5.0 
               
               
                 (bara) 
               
            
           
           
               
            
               
                 Composition (mol. %) 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 N2 
                 0.83 
                 0.83 
                 0.84 
                 0.84 
                 0.13 
                 0.86 
                 0.11 
                 0.86 
                 9.2 
                 0.34 
               
               
                 C1 
                 88.0 
                 88.0 
                 88.5 
                 88.5 
                 40.8 
                 90.1 
                 34.2 
                 90.1 
                 90.8 
                 90.1 
               
               
                 C2 
                 5.6 
                 5.6 
                 5.6 
                 5.6 
                 12.4 
                 5.4 
                 9.8 
                 5.4 
                 0.01 
                 5.7 
               
               
                 C3 
                 2.7 
                 2.7 
                 2.7 
                 2.7 
                 16.9 
                 2.3 
                 13.1 
                 2.3 
                 0.00 
                 2.4 
               
               
                 C4 (i + n) 
                 2.2 
                 2.2 
                 2.2 
                 2.2 
                 27.6 
                 1.3 
                 23.5 
                 1.3 
                 0.00 
                 1.4 
               
               
                 C5+ 
                 0.7 
                 0.7 
                 0.11 
                 0.11 
                 2.3 
                 0.04 
                 19.3 
                 0.04 
                 0.00 
                 0.04 
               
               
                   
               
            
           
         
       
     
     Table 1 shows physical conditions and compositions of the hydrocarbon stream in the process of  FIG. 3  as calculated for an example hydrocarbon feed gas using heat and material balance software. For this example, the mixed refrigerant composition of the cooling fluid is (in mol. %) 1.0 of methane, 48.2 of ethylene, 4.2 of propane, 16.6 of butanes; the main mixed refrigerant composition of the main cooling fluid is (in mol. %) 6.5 of nitrogen, 34.7 of methane, 40.2 of ethylene, 14.2 of propane, 4.4 of butanes. The amount of CO 2  in the hydrocarbon feed stream  10  was less than 50 ppm (by mol) which could be achieved by applying a CO 2  removal process in a pre-treatment. 
     The embodiment of  FIG. 4  shows how the invention can be embedded in a so-called double mixed refrigerant (DMR) process.  FIG. 5  is an example wherein the invention is embedded in a so-called single mixed refrigerant (SMR) process. In this case a combined ambient heat exchanger  540  fulfils the functions of both the ambient heat exchanger  140  and the main ambient heat exchanger  340  of  FIG. 1 . Between the combined ambient heat exchanger  540  and the various expanders  110 ,  310   a,    310   b  a cooling fluid separator  560  is provided that discharged into the main cooling fluid connection  350  and the cooling fluid connection  150 . A combined SMR compressor train  530  fulfils the function of the first ( 130 ) and second ( 330 ) compressor trains described above. As shown, an SMR MP/HP compressor  533  corresponds to the first compressor train  130  of  FIG. 3  while an SMR LP compressor  531  together with the SMR MP/HP compressor  533  corresponds to the second compressor train  330  of  FIG. 3 . Similar to the LP compressor  331  and the MP/HP compressor  333  of  FIG. 3 , the SMR LP compressor  531  discharges into the suction inlet of the SMR MP/HP compressor  533  via a first SMR ambient intercooler  532 . A second SMR ambient intercooler  534  may be provided between the MP and HP stages in the SMR MP/HP compressor  533 . Alternative compressor train configurations can be selected instead of the specific embodiment described here. 
     In the embodiments described above, the first heat exchanging fluid barrier  231  and the second heat exchanging fluid barrier  211  are both located within a single heat exchanger  200 . Advantageously, the residual liquid portion is passed from the first section  124  of the cold side heat exchanging channel  120  to the second section  126  of the cold side heat exchanging channel  120  without changing the pressure of the residual liquid portion by more than 1 bar anywhere between these first ( 124 ) and second sections ( 126 ). This is easily attainable by arranging the first ( 124 ) and second ( 126 ) sections of the cold side heat exchanging channel  120  within a single shell of a single heat exchanger. An important advantage is that the compressor train  130  can be kept simple because it does not have to handle multiple input vapour streams at mutually different pressure levels. 
     Furthermore, the residual liquid portion is advantageously passed from the first section  124  of the cold side heat exchanging channel  120  to the second section  126  of the cold side heat exchanging channel  120  without changing the composition of the residual liquid portion anywhere between these first ( 124 ) and second sections ( 126 ). This has as advantage that a minimum of equipment is needed if the composition does not need to be changed. Finally, the residual liquid portion is passed from the first section  124  of the cold side heat exchanging channel  120  to the second section  126  of the cold side heat exchanging channel  120  without changing the flow rate of the residual liquid portion anywhere between these first ( 124 ) and second sections ( 126 ). 
     While these conditions can easily be met using a single heat exchanger, for completeness,  FIGS. 6 and 7  show alternative embodiments wherein the first warm section  230  (comprising the first heat exchanging fluid barrier) is located within a first heat exchanger  200 A and the second warm section  210  (comprising the second heat exchanging fluid barrier) is located within a second heat exchanger  200 B. The first ( 200 A) and second ( 220 B) heat exchangers are interconnected to allow fluid communication between the first ( 124 ) and second ( 126 ) sections of the cold side heat exchanging channel  120 . In the case of  FIG. 6 , only the residual liquid portion  129 , which is separated off in phase separator  127  from the vapour that consists of the evaporated expanded cooling fluid, is conveyed from the first heat exchanger  200 A to the second heat exchanger  200 B. The vapour  128   a  from the first heat exchanger  200 A and the vaporized residual liquid  128   b  from the second heat exchanger  200 B are combined before compressing in the compressor train  130 . 
     An advantage of the embodiment of  FIGS. 6 and 7  is that it may be easier to connect the warm side heat exchanging channel  220  to the column  25 . However, a disadvantage of the embodiment of  FIG. 6  is that the interconnection that allows fluid communication between the first and second sections of the cold side heat exchanging channel may cause an additional pressure drop on the cooling fluid. 
     In an alternative embodiment, the cooling fluid loop  100  does not comprises any phase separator between the first section  124  and the second section  126  of the cold side heat exchanging channel  120 , when the cooling fluid loop  100  is considered in the circulation direction and in any single pass of the cooling fluid through the cooling fluid loop  100 . 
     In  FIG. 7  the first and second heat exchangers ( 200 A,  200 B) are provided in the form of plate-fin type heat exchangers. An advantage is that the phase separator  127  of the embodiment of  FIG. 6  can suitably be avoided, as it is less challenging to convey the vapour and residual liquid portion from the first heat exchanger  200 A to the second heat exchanger  200 B as a two-phase fluid in the case of plate-fin type heat exchangers than in the case of tube-in-shell type heat exchangers.