TWO-STAGED REFRIGERATION CYCLE USING A MULTICONSTITUANT REFRIGERANT

Abstract: A refrigeration cycle utilizing a multi-component refrigerant wherein the compression of low pressure refrigerant vapour is effected in at least two stages of compression (42, 44) with cooling (48, 58), partial condensation and separation (52, 62) from the uncondensed vapour of the condensate formed after each of at least two of the said stages thereby providing two or more condensate streams (64, 70) of different compositions and at different pressures and wherein at least two of said condensate streams are expanded and injected into the returning low pressure refrigerant (40) at different temperatures of said low pressure refrigerant (40).

Description: TWO-STAGED REFRIGERAΗON CYCLE USING A MULΗCONSTΪTUANT REFRIGERANT This invention relates to refrigeration cycles using a mixed refrigerant. In a refrigeration cycle, low pressure vapour is compressed and the compressed vapour is thereafter cooled and condensed and the high pressure condensed stream expanded back to the low pressure to form a returning low pressure refrigerant stream which is vaporised to re-form the low pressure vapour stream for return to the compressor. The final cooling and condensation of the compressed vapour is effected in indirect counter-current heat exchange with the vaporising low pressure stream. Cooling of the material to be refrigerated is by heat exchange with the vaporising low pressure stream. In a refrigeration cycle utilizing a multi-component refrigerant, sometimes known as a mixed refrigerant refrigeration cycle, the refrigerant stream is made up of a plurality of components having differing boiling points. The compressed vapour thus condenses over a range of temperatures and likewise the condensed refrigerant boils over a range of temperatures. These mixed refrigerant refrigeration cycles are used extensively, especially but not exclusively for natural gas. Because of the wide use of these systems, improvements in efficiency are always being sought, both in the sense of economy of operation and in simplification of the plant. Because of the scale of the plants, especially for the liquefaction of natural gas, even small improvements can substantially affect the viability of a plant. This invention provides an improvement to the mixed refrigerant refrigeration cycles currently in use. According to the present invention there is provided a refrigeration cycle utilizing a multi-component refrigerant wherein the compression of low pressure refrigerant vapour is effected in at least two stages of compression with cooling, partial condensation and separation from the uncondensed vapour of the condensate formed after each of at least two of the said stages thereby providing two or more condensate streams of different compositions and at different pressures and wherein at least two of said condensate streams are expanded and injected into the returning low pressure refrigerant at different temperatures of said low pressure refrigerant. In one preferred embodiment, at least one of the said condensate streams is sub- cooled prior to the said expansion. In one aspect of this embodiment, at least two of said streams are sub-cooled to the same temperature; alternatively, however, they may be sub-cooled to different temperatures. It is to be understood, however, that even if sub-cooled to the same temperature at least two of the said condensate streams must be injected into the returning low pressure refrigerant at different temperatures. In one embodiment of the invention, vapour recovered from the last stage of compression is subjected to two or more steps of cooling and partial condensation with separation of condensate from uncondensed vapour after each step and each separated condensate is thereafter expanded and injected into returning low pressure refrigerant. Preferably, at least one of the separated condensates is sub-cooled. One or more of these cooling steps may be effected by indirect countercurrent heat exchange with returning low pressure refrigerant. Further improvements may be achieved if a refluxing exchanger is employed in the generation and separation of condensate from uncondensed vapour in one or more of the vapour/liquid separation steps in the cycle. The use of a refluxing exchanger in the generation and separation of condensate from uncondensed vapour in a refrigeration cycle utilizing a multi-component refrigerant and including at least one step of cooling and partially condensing compressed vapour and separating the condensate so formed from uncondensed vapour is the subject of GOB patent application 9712301.2 filed on 12 June 1997. The invention will now be described in greater detail with reference to preferred embodiments thereof and with the aid of the accompanying drawings in which Figure 1 is a flow sheet of a known mixed refrigerant refrigeration cycle for use in the liquefaction of natural gas; Figure 2 is a flow sheet of another known mixed refrigerant refrigeration cycle; Figure 3 is a flow sheet of a refrigeration cycle in accordance with the present invention; Figure 4 is a flow sheet of a modification of the refrigeration cycle of Figure 3 wherein two stages of aftercooling are provided; Figure 5 is a flow sheet of another modification of the refrigerant cycle of Figure 3 wherein a refluxing exchanger is employed to generate and separate a condensate stream; and Figure 6 is a flow sheet of a modification of the refrigeration cycle of Figure 5 in which condensate formed in the compressed vapour during cooling by indirect countercurrent heat exchange with returning low pressure refrigerant is separated from the uncondensed vapour at an intermediate point of the heat exchanger. In the following description, the invention will be described with reference to the liquefaction of natural gas; however, it is to be understood that the use of the refrigeration cycles of this invention is not so limited and that they are also suitable for use in other applications, eg. for other gas liquefaction processes or for purification by partial condensation techniques. Referring now to Figure 1 of the drawings, which provides a flow sheet of a known mixed refrigerant refrigeration cycle for the liquefaction of natural gas, the natural gas which is to be liquefied, is supplied at an elevated pressure to a heat exchanger 4 through line 2 and the liquefied product is recovered through line 6. The details of the arrangement for recovering the liquefied product are not relevant to the invention and many variants are possible but in the embodiment illustrated the gas is first cooled and partially condensed to recover a heavy hydrocarbon fraction. The condensate is separated from uncondensed gas in liquid/vapour separator 8. Condensate is recovered in line 10 and the uncondensed gas is returned to a cooler section of the heat exchanger in line 12 for a further step of cooling and partial condensation with the further condensate being separated from the uncondensed gas in liquid/vapour separator 14. The uncondensed gas is again returned to the heat exchanger, this time to the cold end, in line 16 for final cooling and condensation after which it is recovered, expanded to an intermediate pressure through valve 18 and supplied to liquid/vapour separator 20 for separation of any uncondensed gas. Condensate recovered from the separator 20 in line 22 is further expanded to its final pressure in expansion valve 24 and supplied to liquid/vapour separator 26 from which the liquefied gas is recovered in line 6 as mentioned above. Uncondensed gas from separator 20 is returned via line 30 to be reheated in heat exchanger 4 and is then combined with condensed liquid in line 32 from separator 14 which has been expanded through valve 34. The combined stream is further warmed in heat exchanger 4 and then recovered therefrom in line 36. It is thereafter joined by the cold uncondensed gas from separator 26 in line 38. The cooling and liquefaction of the natural gas is effected in heat exchanger 4 by indirect countercurrent heat exchange with a vaporising mixed refrigerant stream in line 40. For the liquefaction of natural gas, the mixed refrigerant preferably comprises a mixture of nitrogen and C, to C 5 hydrocarbons. The low pressure vaporised stream recovered from the heat exchanger in line 40 is recycled for recompression in a two stage compressor having first and second stages 42, 44. After compression in the first stage 42, the vapour is transferred via line 46 for cooling in inter-cooler 48 and then passed via line 50 to vapour/liquid separator 52 for the separation of condensate formed by the cooling in the inter-cooler. The uncondensed vapour is recovered in line 54 and transferred to the second stage 44 of the compressor, the compressed vapour therefrom being collected in line 56 for transfer to after cooler 58 where it is cooled and partially condensed. The partially condensed high pressure stream is recovered in line 60. Condensate formed as a result of cooling in the inter-cooler 48 is recovered from vapour/liquid separator 52 in line 64, pumped up to the same pressure as the stream in line 60 by pump 66, and combined with that stream for supply to the vapour/liquid separator 62. Uncondensed vapour from vapour/liquid separator 62 is recovered overhead in line 68. Condensate recovered in line 70 is pumped by pump 72 to rejoin the overhead vapour via line 74. The combined stream is then passed through heat exchanger 4 in line 76 where the vapour is cooled and condensed in indirect countercurrent heat exchange with the vaporising refrigerant stream in line 40 and thereafter expanded through valve 78 into the low pressure line 40 to form the returning low pressure refrigerant stream. A known modification of the refrigeration cycle shown in Figure 1 will now be described and illustrated with reference to Figure 2, wherein pipelines and apparatus components common to the embodiment illustrated in Figure 1 are accorded the same reference numerals. In this modification, the condensate recovered from vapour/liquid separator 62 is not injected back into the overhead vapour in line 68 but is recovered in line 90 and directed into the warm end of heat exchanger 4 where it is sub-cooled in indirect countercurrent heat exchange with the vaporising returning low pressure refrigerant in line 40. It is then recovered from heat exchanger 4 at an intermediate point thereof, expanded to about the pressure of that low pressure stream and injected into that stream through line 94. An arrangement according to the present invention will now be described with reference to Figure 3 which illustrates the application of the invention to the prior art process illustrated in Figure 2 and in which all lines and apparatus components common with those of Figure 2 are accorded the same reference numerals. In the refrigeration cycle illustrated in Figure 3, the modification of the arrangement of Figure 2 lies in the treatment of the condensate formed in inter-cooler 48. In accordance with the invention, condensate formed in inter-cooler 48 and separated from uncondensed vapour in vapour/liquid separator 52 is recovered in line 102, in which it is directed into heat exchanger 4 at the warm end thereof and wherein it is sub-cooled by indirect countercurrent heat exchange with vaporising returning low pressure refrigerant in line 40, expanded to substantially the same pressure as said returning low pressure refrigerant in valve 104 and injected through line 106 into said low pressure refrigerant in line 40 at a higher temperature than that at which the condensate in line 94 is injected. It will be understood that the condensate in line 102, which was formed in the inter-cooler will be at a lower pressure than that in line 90 which was formed in the aftercooler. The condensate in line 102 is injected into the returning vaporising low pressure refrigerant stream at a higher temperature than that at which the condensate in line 90 is injected because the condensate in line 102 will have a higher boiling range than that in line 90. As will be seen with reference to Figure 3, in the refrigeration cycle according to the invention, the heavier liquid condensed in the compressor interstage cooler 48 is used as a separate refrigerant stream from the liquid condensed in the aftercooler 58. The interstage liquid is subcooled separately and is injected into the returning stream 40 at a higher temperature level than the liquid from the aftercooler. This in effect creates a complete additional refrigerant stage. This results in lower power consumption and/or a reduction in heat exchanger size due to the following: a) the interstage liquid is no longer pumped to discharge pressure; b) the heavier components condensed in the interstage are injected into the vaporising return low pressure refrigerant at a temperature level where they can be most beneficial, thus improving the match between the low pressure refrigerant H/T curve and the combined cooling curve of the high pressure streams. This results in an improvement in thermodynamic efficiency; c) less heavy components are contained in the liquid leaving the separator after the aftercooler. Thus less total refrigerant fluid is processed by the heat exchanger below the point of injection of the interstage liquid. Also, the return refrigerant below this point is lighter and therefore evaporates more easily, thus improving the heat transfer efficiency and reducing the heat exchanger duty; d) the degree of sub-cooling of the separate liquid streams can be more easily optimised to minimise the amount of flash on expansion to the common low pressure, and therefore reduce the complexity and cost of equipment required to achieve good two-phase distribution. A further benefit of the refrigeration cycle according to the invention is that it permits greater operational flexibility to cope with variation in gas composition, temperature and/or pressure and in changes in ambient conditions. While in the embodiment of Figure 3 there are two stages of compression and two condensate steams are obtained, the invention is also applicable to three or more stages of compression in which case any two or more of the condensate streams so obtained may be expanded and injected into the returning low pressure refrigerant. Preferably, at least one is sub-cooled to an appropriate temperature before expansion. If desired, two or more steps of cooling and separation of condensate may be effected after each stage of compression. It will be understood that the resultant condensate streams will be at substantially the same pressure although of 'different composition. An application of the invention to such an arrangement is illustrated in Figure 4 which is a modification of the arrangement of Figure 3 wherein two stages of after cooling are provided. In this Figure, wherein the same pipelines and apparatus components as those of Figure 3 are accorded the same reference numerals, the compressed refrigerant stream recovered from final compressor stage 44 in line 56 is cooled and partially condensed in first after-cooler 58A and the partially condensed stream is conveyed via line 60A to a first vapour/liquid separator 62A condensate from which is recovered in line 90A, subcooled in heat exchanger 4 in indirect countercurrent heat exchange with returning vaporising low pressure refrigerant in line 40, expanded in expansion valve 92A to substantially the pressure of said low pressure refrigerant and injected into it through line 94A. The uncondensed vapour from liquid/vapour separator 62A is further cooled and partially condensed in second after cooler 58B and the condensate formed therein is separated from uncondensed vapour in liquid/vapour separator 62B. recovered in line 90B and likewise sub-cooled, expanded (through expansion valve 92B) and injected (via line 94B) into the returning low pressure refrigerant stream. It will be understood that the condensate streams in lines 90A and 90B will be of different composition but at substantially the same pressure which will be a higher pressure than that of the condensate in line 102 which has been formed in inter-cooler 48. Still further improvement is achievable if a refluxing exchanger is employed in the generation and separation of a condensate. An application of this embodiment of the invention is illustrated in Figure 5 which is a modification of the arrangement of Figure 3 and where all pipelines and apparatus components common with Figure 3 are accorded the same reference numerals. In the arrangement of Figure 5, the after cooler 58 and liquid/vapour separator 62 of the arrangement of Figure 2 are replaced by a reflux exchanger 120. The compressed refrigerant recovered from the final stage 44 of compression is directed via line 56 to reflux exchanger 120 where it is cooled and partially condensed while being directed upwardly through the exchanger. Uncondensed vapour is recovered from the top of the exchanger through line 68 while condensate formed in the exchanger travels back down through the exchanger in direct countercurrent contact with the rising vapour and is collected from the bottom of the exchanger in line 90. As a result of replacing the after-cooler 58 and liquid/vapour separator 62 by the refluxing exchanger 120, the concentration of light components in the condensate in line 90 can be minimised thus enabling the condensate to be subcooled to a temperature where little or no flash occurs on expansion into the returning low pressure refrigerant. This greatly reduces the complexity and cost of equipment necessary for achieving good two-phase distribution. There is also a concomitant reduction in the heavy hydrocarbon content of the vapour leaving the reflux condenser, thus reducing the circulating flow and improving the thermodynamic efficiency in the lower temperature sections of the refrigerant circuit. A refrigeration cycle utilizing a multi-component refrigerant and including at least one step of cooling and partially condensing compressed vapour and separating the condensate so formed from uncondensed vapour to form a condensate stream which is thereafter expanded and injected into returning low pressure refrigerant, wherein a refluxing exchanger is employed in at least one of said steps of cooling and partially condensing to effect at least a part of the cooling and to separate condensate from uncondensed vapour, is the subject of GB Patent application 9712301.2 filed on 12 June 1997. While the invention has so far been described with reference to the treatment of condensates formed in the compressor inter cooler and after cooler stages, it will be understood that as the refrigerant comprises a mixture of components having different boiling points, condensate may also be recovered at one or more points in the course of the cooling of the compressed refrigerant by indirect countercurrent heat exchange with the low pressure refrigerant in heat exchanger 4. This embodiment is illustrated in Figure 6 which is a modification of the arrangement of Figure 5 and wherein pipelines and apparatus components common with Figure 5 are accorded the same reference numerals. In the arrangement of Figure 6 the compressed refrigerant vapour recovered overhead from the refluxing exchanger 120 in line 68 and passed through heat exchanger 4 in line 76 is withdrawn from heat exchanger 4 at an intermediate point where it is not fully condensed. The condensate is separated from uncondensed vapour in liquid/vapour separator 202, subcooled in line 204 in indirect countercurrent heat exchange with vaporising returning low pressure refrigerant, expanded through expansion valve 206 to about the same pressure as said low pressure refrigerant and thereafter injected into it. By this means, condensate formed in the compressed refrigerant may be recovered from it close to its dewpoint and then re-injected into returning low pressure refrigerant stream close to its boiling point, thereby further improving heat transfer efficiency and reducing the heat exchanger duty. Whilst this modification has been described with reference to the arrangement of Figure 5 wherein a refluxing exchanger is employed to generate and recover condensate from the compressed stream recovered from the last stage of the compressor, it will be understood that it is also applicable to the refrigeration cycles illustrated in Figures 3 and 4. It is further to be understood that while the condensate in line 204 of Figure 6 is shown as being sub-cooled prior to expansion, it need not be sub-cooled although the benefit obtained is then reduced. While the separation of only one such condensate and its treatment is shown, more than one may be thus separated and treated if desired, by repeatedly separating condensate from the condensing compressed refrigerant stream as it travels from the warm end to the cold end of heat exchanger 4 in line 76 (Figure 2), expanding each condensate so obtained and injecting it into the returning low pressure refrigerant. Each such condensate may or may not be sub-cooled prior to expansion, as desired. While in Figures 5 and 6, a refluxing condenser is shown as replacing the compressor after cooler and associated vapour/liquid separator, it will be understood that it may also be employed, additionally or alternatively, to replace a compressor inter-cooler such as inter-cooler 48 and associated vapour/liquid separator, such as separator 50, and possibly also even in the generation and separation of other condensate streams in the refrigeration cycle by partial condensation of compressed refrigerant. Each refluxing exchanger may also be used to provide less than all the cooling and thus used in series with a conventional inter-cooler or after cooler as well as a total replacement therefor. One or more of the expansion valves employed for the expansion of condensate in any part of the refrigeration cycle may, if desired, be replaced by devices in which expansion is effected with performance of external work, e.g. a turbine expander. It will be understood that while heat exchanger 4 is shown as being a single heat exchanger, its overall function may be supplied by a plurality of exchangers. It will generally be preferred for at least any heat exchanger employed in the indirect counter-current heat exchange of compressed refrigerant with returning low pressure refrigerant to be a multi-stream plate fin type heat exchanger because such heat exchangers provide greater flexibility to efficiently process a multiplicity of different streams. While the invention has been described with particular reference to the liquefaction of natural gas, it may also. be used in other cryogenic applications e.g. for the liquefaction of streams or for purification where one or more contaminants is or are removed by cooling and partial condensation. Examples include air separation, the treatment of refinery off gas, and the liquefaction of ethylene and ethane. Any suitable combination of two or more refrigerants may be used in the mixed refrigerant cycle and the choice will depend upon the composition of the material to be refrigerated and the temperature to which it is to be cooled. Examples of suitable refrigerants include nitrogen, low boiling halogenated hydrocarbons, eg. chlorofluorocarbons, and low boiling hydrocarbons. In general however, the mixed refrigerant will usually comprise two or more of nitrogen and C,-C 5 hydrocarbons. If desired, one or more of the condensate streams formed in the refrigeration cycle of the invention may be divided into at least two sub-streams having the same composition and the said sub-streams may each be expanded and injected into the returning low pressure refrigerant stream at different temperatures of the returning low pressure refrigerant. This enables the evaporation characteristics of the low pressure refrigerant to be changed progressively to better match the combined cooling curve of the high pressure streams, thereby still further improving the efficiency of the refrigeration cycle. A refrigeration cycle utilizing a multi-component refrigerant and including at least one step of partially condensing compressed vapour, forming a condensate stream by separating condensate so formed from uncondensed vapour and thereafter expanding said condensate stream and injecting said expanded condensate stream into returning low pressure refrigerant, characterised in that said expanded condensate stream is injected into said returning low pressure refrigerant in the form of at least two sub-streams formed by division of said condensate, at least two of said sub-streams being injected into the returning low pressure refrigerant at different temperatures of the returning low pressure refrigerant is the subject of GB patent application 9712302.0 filed on 12 June 1997. EXAMPLE The invention is now illustrated by the following Example. First, in a comparative experiment, a multi-component refrigerant stream of the composition shown (as mol. %) in Table 1 Table 1 is incorporated into the prior art mixed refrigerant refrigeration cycle shown in Figure 2 at 40. The vapour fractions, temperatures, pressures, flow rates and compositions of the various refrigerant streams are recorded in Table 2 below. This is used to cool a natural gas feed and streams produced therefrom. The vapour fractions, temperatures, pressures, flow rates and compositions of the various streams on the natural gas side are recorded in Table 3 below. In a second experiment, the same multi-component refrigerant stream is incorporated into the mixed refrigerant refrigeration cycle of the present invention shown in Figure 3 at 40 and this is used to cool the same natural gas feed. The vapour fractions, temperatures, pressures, flow rates and compositions of the various streams on the natural gas side are as shown in Table 3 while those for the various refrigerant streams are shown in Table 4. The use of the mixed refrigerant refrigerator cycle of the present invention is found to give improved efficiency. Thus, the total power consumed in the first experiment is 53784 KW while that second according to the invention is only 52860 KW, a saving of nearly 1MW (1.7%). This results in a lower capital and operating cost for the refrigeraton system. The total UA 1 was also measured in both cases. In the comparative experiment, the value was 34.99 MW/°C while in the experiment in accordance with the invention the value was 34.92 MW/°C. This value is a measure of heat exchanger surface area and shows that the experiment in accordance with the invention gives a similar surface area for a reduced power consumption. This results in a similar capital cost for this item of plant. The reduced capital cost for the refrigerant compression thus gives a net cost benefit. 'This term is derived form the equation Q=UAΔT where Q is the energy transferred; U is a heat transfer coefficient; A is the heat exchange area and ΔT is the temperature differential. Table 2 Table 3 I Table 4 (0 c CD (A H H C H rπ w I x m m H c m t σ>

Claims: CLAIMS 1. A refrigeration cycle utilizing a multi-component refrigerant wherein the compression of low pressure refrigerant vapour is effected in at least two stages of compression with cooling, partial condensation and separation from the uncondensed vapour of the condensate formed after each of at least two of the said stages thereby providing two or more condensate streams of different compositions and at different pressures and wherein at least two of said condensate streams are expanded and injected into the returning low pressure refrigerant at different temperatures of said low pressure refrigerant. 2. A refrigeration cycle as claimed in Claim 1 wherein at least one of said condensate streams is sub-cooled prior to said expansion. 3. A refrigeration cycle as claimed in Claim 1 wherein at least two of said condensate streams are sub-cooled to the same temperature prior to said expansion. 4. A refrigeration cycle as claimed in Claim 1 wherein at least two of said condensate streams are sub-cooled to different temperatures prior to said expansion. 5. A refrigeration cycle as claimed in any one of Claims 1 to 4 in which vapour recovered from the last stage of compression is subjected to two or more steps of cooling and partial condensation with separation of condensate from uncondensed vapour after each step and each separated condensate is thereafter expanded and injected into returning low pressure refrigerant. 6. A refrigeration cycle as claimed in Claim 5 in which compressor after cooling is effected in stages with separation of condensate formed in each stage to form two or more condensates having different compositions at substantially the same pressure and said condensates are separately sub-cooled, expanded and injected into returning low pressure refrigerant. 7. A refrigeration cycle as claimed in Claim 5 or Claim 6 wherein at least one of said separated condensates is subcooled prior to said expansion. 8. A refrigeration cycle as claimed in any one of the preceding claims in which a refluxing exchanger is employed in the generation and separation of condensate from uncondensed vapour in at least one of the steps of cooling, partial condensation and separation of condensate from uncondensed vapour. 9. A refrigeration cycle as claimed in Claim 8 in which a refluxing exchanger is employed in the generation and separation of condensate from compressed vapour recovered from the last stage of compression. 10. A refrigeration cycle as claimed in any one of the preceding claims utilized for the liquefaction of natural gas. 11. A refrigeration cycle as claimed in any one of the preceding claims wherein the refrigerant comprises a mixture comprising any combination of two or more of nitrogen and C, to C 5 hydrocarbons. 12. A refrigeration cycle as claimed in any one of the preceding claims wherein one or more multi-stream plate fin type heat exchangers is or are employed in the cooling and partial condensation of compressed refrigerant.

Inventors: BOUSTEAD PHILIP, FINN ADRIAN JOSEPH, ISALSKI WIESLAW HENRYK, LAVIN JOHN TERENCE, LINNETT DAVID THOMAS, TOMLINSON TERENCE RONALD, BOUSTEAD, PHILIP, FINN, ADRIAN, JOSEPH, ISALSKI, WIESLAW, HENRYK, LAVIN, JOHN, TERENCE, LINNETT, DAVID, THOMAS, TOMLINSON, TERENCE, RONALD
Applicants: BOC GROUP PLC, BOUSTEAD PHILIP, COSTAIN OIL GAS & PROCESS LIMI, FINN ADRIAN JOSEPH, ISALSKI WIESLAW HENRYK, LAVIN JOHN TERENCE, LINNETT DAVID THOMAS, TOMLINSON TERENCE RONALD, BOUSTEAD, PHILIP, COSTAIN OIL, GAS & PROCESS LIMITED, FINN, ADRIAN, JOSEPH, ISALSKI, WIESLAW, HENRYK, LAVIN, JOHN, TERENCE, LINNETT, DAVID, THOMAS, THE BOC GROUP PLC, TOMLINSON, TERENCE, RONALD
Application year: 1998
Main classification: F25J3
IPC codes: F25J1