Dual mixed refrigerant cycle for gas liquefaction

Abstract: A pressurised gas (102), especially natural gas is liquefied using high and low level heat exchangers (104, 122) in each of which is provided by a respective mixed refrigerant (110, 124) which is vaporised at a respective essentially constant pressure. The mixed refrigerant vapour (140) from the high level heat exchanger (404) is compressed in at least two stages of compression which generates an interstage compressed refrigerant. The interstage compressed refrigerant is cooled, partially condensed, and separated into an interstage refrigerant vapour and an interstage refrigerant liquid (144). The interstage refrigerant vapour is compressed to yield a further compressed refrigerant (146) which is cooled, condensed, optionally subcooled, and flashed (450) to provide the first vaporising mixed refrigerant (410) and the interstage liquid refrigerant (144) is subcooled and flashed (468) at the first essentially constant pressure to yield additional refrigeration in the first heat exchange zone (404). The process and apparatus of the invention are especially well-suited for installation on ships (including barges) and offshore platforms.

Description: The liquefaction of natural gas at remote sites, transportation of the liquefied natural gas (LNG) to population centres, and storage and vaporisation of LNG for local consumption have been successfully practised for many years around the world. LNG production sites are usually located on land at remote sites having docking facilities for large LNG tankers which transport the LNG to end users. Numerous process cycles have been developed for LNG production to provide the large refrigeration requirements for liquefaction. Such cycles typically utilise combinations of single-component refrigeration systems using propane or single chlorofluorocarbon refrigerants operated in combination with one or more mixed refrigerant (MR) systems. Well-known mixed refrigerants typically comprise light hydrocarbons and optionally nitrogen and utilise compositions tailored to the temperature and pressure levels of specific process steps. The objectives in the design and operation of current LNG process cycles and equipment have been to minimise energy consumption and maximise LNG production while operating at changing product demand rates and varying ambient temperature conditions. Since LNG production facilities are typically land-based in remote locations, the land area required for plant battery limits has not been a critical factor in plant design and layout. Numerous mixed refrigerant (MR) LNG cycles have been disclosed in the art. These cycles generally use a first refrigerant which vaporises at a higher temperature (i.e., the warm or high level MR) in a first heat exchanger (i.e., the warm or high level exchanger) and a second refrigerant which vaporises at a lower temperature (i.e., the cold or low level MR) in a second heat exchanger (i.e., the cold or low level exchanger). US-A-4,274,849 describes a dual mixed refrigerant process in which feed gas is first cooled in a separate exchanger using the refrigerant fluid exiting the cold or low level MR heat exchanger. The precooled feed is then further cooled and liquefied in the cold MR exchanger. The vaporised low level refrigerant after compression is cooled against the warm or high level refrigerant in the warm or high level MR exchanger. A disadvantage of this process is that an extra heat exchanger is required for feed precooling. US-A-4,112,700 discloses a dual MR process in which the high level MR is boiled at three different pressure levels with interstage compression. This requires the use of multiple heat exchangers or multiple heat exchange zones, which requires multiple return streams to the compressor. Such multiple heat exchange/compression stages have a disadvantage from a thermodynamic perspective, since non-equilibrium streams of differing compositions are mixed interstage in the warm mixed refrigerant compression train. The mixing of streams causes a thermodynamic irreversibility which will result in reduced cycle efficiency. A dual mixed refrigerant process is described in US-A-4,525,185 wherein the high level MR is boiled at three different pressure levels. This requires the use of multiple heat exchangers or heat exchange zones, and leads to multiple vessels, valves, and piping associated with the interstage feeds to the high level MR compressor, and increases the area required for the plant. In this process, the feed Is first cooled using low level MR exiting the low level MR heat exchanger. The disadvantage of this approach is that an extra heat exchanger is required as in US-A-4,274,849 cited above. In this process cycle, non-equilibrium streams are mixed interstage in the high level mixed refrigerant compression train, which causes thermodynamic irreversibility and reduces cycle efficiency. US-A-4,545,795 discloses a dual MR process wherein the high level MR is boiled at three different pressure levels. This requires the use of multiple heat exchangers or heat exchange zones in the high level MR heat exchanger. In this process, the feed is first cooled using the fluid exiting the low level MR exchanger, and this requires an additional heat exchanger as in US-A-4,274,849 cited above. This flowsheet also has a disadvantage from a thermodynamic perspective, since non-equilibrium streams are mixed interstage in the high level MR compression train which causes thermodynamic irreversibility as earlier discussed. A dual mixed refrigerant process is described in US-A-4,539,028 in which the high level MR is boiled at three different pressure levels, which requires the use of multiple heat exchangers or heat exchange zones. The low level mixed MR is boiled at two different pressure levels, which also requires the use of multiple heat exchangers or heat exchange zones. In this process, the feed is first cooled using the low level MR, which requires an extra heat exchanger, a disadvantage shared by several of the processes cited above. This cycle also has a disadvantage from a thermodynamic perspective, since non-equilibrium streams are mixed interstage in the mixed refrigerant compression train. This mixing causes a thermodynamic irreversibility which will result in reduced cycle efficiency. A paper entitled "Liquefaction of Associated Gases" by H. Paradowski et al presented at the 7 th International Conference on LNG, May 15-19, 1983 describes a dual MR process in which the high level mixed refrigerant is boiled at three different pressure levels. This requires the use of multiple heat exchangers or heat exchange zones. In addition, the feed is first cooled using the low level MR exiting the low level MR exchanger, and this requires an extra heat exchanger. This process also has a disadvantage from a thermodynamic perspective, since high level MR streams are generally not in thermal equilibrium with the interstage stream before the high level and interstage MR streams are mixed in the refrigerant compression train. This mixing of streams into the main flow of the compressor causes a thermodynamic irreversibility which will result in reduced cycle efficiency. US-A-4,911,741 discloses a dual MR process in which the high level MR is boiled at three different pressure levels. This requires the use of multiple heat exchangers or heat exchange zones and also has a disadvantage from a thermodynamic perspective as earlier discussed, since streams which are potentially at different temperatures are mixed interstage in the high level mixed refrigerant compression train. This mixing of streams causes thermodynamic irreversibility which will result in reduced cycle efficiency. A dual MR process is described in US-A-4,339,253 in which the high level MR is boiled at two different pressure levels. In addition, an interstage liquid stream from the high level MR is boiled at a third pressure. This requires the use of multiple heat exchangers or heat exchange zones. In this process, the feed is initially cooled before heavier hydrocarbon removal by heat exchange with the low level MR vapour exiting warm end of the low level MR exchanger. The disadvantage of this approach is that an extra heat exchanger is required. This heat exchange also increases the pressure drop of the low level MR stream before compression. As in several of the processes described above, this process has a thermodynamic disadvantage since non-equilibrium streams are mixed interstage in the high level MR compression train. The mixing of streams into the main flow causes thermodynamic irreversibility which will result in reduced cycle efficiency. US-A-4,094,655 describes a dual MR process where the low level MR is boiled at two different pressure levels, which requires the use of multiple heat exchangers or heat exchange zones. In this process, the high level MR is first cooled using the fluid from the low level MR exchangers, rather than being cooled by the high level mixed refrigerant loop itself. The disadvantage of this approach is that an extra heat exchanger is required. As in several of the processes described above, this process has a thermodynamic disadvantage since non-equilibrium streams are mixed interstage in the high level MR compression train. The mixing of streams into the main flow causes thermodynamic irreversibility which will result in reduced cycle efficiency. Additional dual MR processes in which the high level MR is boiled at several different pressure levels are described in US-A-4,504,296; US-A-4,525,185; US-A-4,755,200; and US-A-4,809,154. The LNG processes described above typically are utilised at land-based locations, and the land area required for the plant battery limits generally is not a critical factor in plant design and layout. Recently, commercial interest has been increasing in the potential recovery of gas reserves not amenable to land-based liquefaction processes as described above. Such reserves are found in offshore locations, and the recovery of these reserves has generated a growing need for gas liquefaction systems amenable to installation on ships (including barges) and offshore platforms. Most large LNG production plants employ a propane refrigerant cycle to precool the feed gas prior to further cooling and liquefaction by means of multicomponent or mixed refrigerant (MR) cycles. The propane pre-cooled cycle, while very efficient and cost effective in land-based plants, has certain disadvantages for shipboard (including barge mounted) applications. The necessity of maintaining fairly large quantities of propane presents potential safety concerns, and the numerous propane evaporators consume scarce plot plan area. Several examples of dual mixed refrigerant cycles as described above reduce propane inventory in propane precooling systems, but require numerous heat exchangers and vessels which increase the required plot plan area, and therefore are not suitable for offshore applications. US-A-3964891; US-A-4504296 and WO-A-9960316 (published 25th November 1999) all disclose processes for liquefying a pressurized gas in which the pressurised feed gas is first cooled by indirect heat exchange in a first heat exchange zone with a high level vaporizing mixed refrigerant. This high level refrigerant is vaporized to yield a cooled feed gas and a first mixed refrigerant vapour. The cooled feed gas is further cooled and condensed by indirect heat exchange in a second heat exchange zone with a low level vaporizing mixed refrigerant to yield a liquid product and a second mixed refrigerant vapour. The first mixed refrigerant vapour is compressed, cooled and condensed and the resulting condensed refrigerant is flashed to provide the high level vaporizing mixed refrigerant. The low level mixed refrigerant vapour also is compressed, cooled and condensed and the resulting condensed second mixed refrigerant flashed to provide the low level vaporizing mixed refrigerant. At least a portion of the refrigeration for the cooling and condensing of the low level mixed refrigerant vapour is provided by indirect heat exchange in the first heat exchange zone with the vaporizing liquid mixed refrigerant. The present invention addresses the need for a natural gas liquefaction process having a minimum plot plan area which is suitable for offshore applications and which can operate at high efficiency without propane precooling in a cycle which is both compact and cost effective. A natural gas liquefaction process and system to meet these objectives is described below and defined in the claims which follow. The present invention is an efficient process and apparatus for gas liquefaction which is particularly useful for the liquefaction of natural gas on a ship or offshore platform where space is at a premium. The invention minimises both the size and number of required equipment items. According to one aspect of the present invention, there is provided a process for liquefying a pressurised feed gas which comprises: cooling the pressurised feed gas by indirect heat exchange in a first heat exchange zone with a first vaporising mixed refrigerant which is vaporised at a first essentially constant pressure to yield a cooled feed gas and a first (high level) mixed refrigerant vapour; further cooling and condensing the cooled feed gas by indirect heat exchange in a second heat exchange zone with a second vaporising (low level) mixed refrigerant which is vaporised at a second essentially constant pressure to yield a liquid product and a second mixed refrigerant vapour; compressing the first mixed refrigerant vapour; and cooling, condensing, and flashing the resulting compressed first mixed refrigerant vapour to provide the first vaporising mixed refrigerant; and compressing the second mixed refrigerant vapour; and cooling, condensing, and flashing the resulting compressed second mixed refrigerant vapour to provide the second vaporising mixed refrigerant, at least a portion of the refrigeration for the cooling and condensing of the second mixed refrigerant vapour being provided by indirect heat exchange in the first heat exchange zone with the first vaporising liquid mixed refrigerant characterized in that: the compression of the first mixed refrigerant vapour is carried out in at least two stages of compression which generates an interstage compressed refrigerant; the interstage compressed refrigerant is cooled, partially condensed, and separated into an interstage refrigerant vapour and an interstage refrigerant liquid; the interstage refrigerant vapour is compressed to yield a further compressed refrigerant which is cooled, condensed, optionally subcooled, and flashed to provide the first vaporising mixed refrigerant; and the interstage liquid refrigerant is subcooled and flashed at the first essentially constant pressure separately from the interstage refrigerant vapour to yield additional refrigeration in the first heat exchange zone. According to the invention, dual mixed component refrigerants are used to provide the refrigeration to the process, and no propane or other single hydrocarbon precooling systems are required. High level or warmer refrigeration can be provided in an optimum temperature range for removal of heavier hydrocarbons from the feed by distillation, and the refrigeration is provided at a single vaporising pressure for simultaneously precooling the natural gas feed and cooling the low level or cold mixed refrigerant. The low level mixed refrigerant provides refrigeration at a single vaporising pressure to achieve final cooling and liquefaction of the feed. The low level mixed refrigerant vapour preferably is compressed cold at approximately the minimum temperature provided by the high level mixed refrigerant. The high level refrigeration is provided using a mixed component circuit in which a high level mixed component stream is compressed and then cooled using an external cooling fluid such as air or cooling water. A portion of the mixed refrigerant may be liquefied by external cooling between compression stages. In an efficient embodiment of the invention this liquid is pumped, mixed with the gas exiting the final stage of compression, and cooled using external cooling. A portion of the compressed high level mixed refrigerant stream is liquefied after the external cooling. At least a portion of the compressed and cooled mixed refrigerant stream is further cooled and then reduced in pressure and vaporised by heat exchange while cooling the feed gas and low level mixed refrigerant. The evaporated and warmed mixed refrigerant steam is compressed and recirculated. The high level mixed refrigerant circuit suitably provides refrigeration at temperature levels from -20°C to -70°C as a fraction of the total refrigeration needed for natural gas liquefaction. The low level refrigeration is provided using a mixed component circuit in which a mixed component stream is compressed and cooled using an external cooling fluid such as air or cooling water. At least a portion of the compressed and cooled mixed refrigerant stream is further cooled in a heat exchanger using the high level mixed refrigerant, and after further cooling is reduced in pressure (flashed) and vaporised by heat exchange against the cooling and condensing feed gas stream. The evaporated and warmed mixed refrigerant steam is compressed cold without further heat exchange and recirculated. As indicated above, the compression of the first mixed refrigerant vapour is carried out in at least two stages of compression which generates an interstage compressed refrigerant; the interstage compressed refrigerant is cooled, partially condensed, and separated into an interstage refrigerant vapour and an interstage refrigerant liquid; the interstage refrigerant vapour is compressed to yield a further compressed refrigerant which is cooled, condensed, optionally subcooled, and flashed to provide the first vaporising mixed refrigerant above; and the interstage liquid refrigerant is subcooled and flashed at the first essentially constant pressure to yield additional refrigeration in the first heat exchange zone. At least a portion of the refrigeration for the cooling and condensing of the first mixed refrigerant vapour after compression can be provided by indirect heat exchange in the first heat exchange zone with the first vaporising liquid mixed refrigerant. Typically, the first vaporising mixed refrigerant is vaporised in a pressure range of one to 30 bar absolute (1 bar = 10 2 kPa) and the second vaporising mixed refrigerant is vaporised in a pressure range of one to 15 bar absolute. The first mixed refrigerant vapour normally comprises two or more components selected from nitrogen, methane, ethane, ethylene, propane, propylene, i-butane, butane, i-pentane, chlorinated hydrocarbons, and fluorinated hydrocarbons. The second mixed refrigerant vapour normally comprises two or more components selected from nitrogen, methane, ethane, ethylene, propane, propylene, i-butane, butane, i-pentane, chlorinated hydrocarbons, and fluorinated hydrocarbons. Preferably the pressurised feed gas is provided by treating a pressurised stream of natural gas to remove contaminants selected from water, carbon dioxide, sulphur-containing compounds, mercury, and mercury-containing compounds. If required, hydrocarbons heavier than methane can be removed from the pressurised feed gas by introducing the cooled feed gas into a distillation column at a first location, and withdrawing therefrom a methane-rich overhead stream and a bottoms stream of components heavier than methane; and separating the bottoms stream to obtain a first hydrocarbon stream comprising components with up to three or up to four carbon atoms and a second hydrocarbon stream comprising components with more than three or four carbon atoms respectively. Usually, this process further comprises cooling at least a portion of the first hydrocarbon stream by indirect heat exchange in the first heat exchange zone; and combining the resulting cooled hydrocarbon stream with the methane-rich overhead stream prior to the further cooling and condensing by indirect heat exchange in a second heat exchange zone. Hydrocarbons heavier than methane can be removed from the pressurised feed gas prior to further cooling and condensing by indirect heat exchange in the second heat exchange zone by cooling the pressurised feed gas and introducing the resulting cooled feed gas into a distillation column at a first location and withdrawing therefrom a methane-rich overhead stream and a bottoms stream comprising components heavier than methane; separating the bottoms stream to obtain a first hydrocarbon stream comprising components with up to three or up to four carbon atoms and a second hydrocarbon stream comprising components with greater than three or four carbon atoms respectively; cooling at least a portion of the first hydrocarbon stream by indirect heat exchange in the first heat exchange zone; and utilising at least a portion of the resulting cooled hydrocarbon stream as reflux for the distillation column. Optionally, a portion of the pressurised feed gas can be introduced into the distillation column at a second location which is below the first location. The liquid product can be a methane-rich liquid, and the methane-rich liquid product can be flashed and separated to yield a further enriched liquid methane product and an offgas stream comprising components lighter than methane. A portion of the refrigeration for the cooling and condensing of the second mixed refrigerant vapour after compression can be provided at least in part by indirect heat exchange in a third heat exchange zone with the offgas stream comprising components lighter than methane. A portion of the refrigeration for the cooling and condensing of the second mixed refrigerant vapour after compression can be provided at least in part by indirect heat exchange in the second heat exchange zone with the second vaporising mixed refrigerant. The second mixed refrigerant vapour after compression can be cooled by indirect heat exchange in the first heat exchange zone and withdrawn therefrom at a first temperature; the resulting cooled second mixed refrigerant stream introduced into the second heat exchange zone and further cooled therein by indirect heat exchange; the second mixed refrigerant vapour withdrawn from the second heat exchange zone at a second temperature which is lower than the first temperature; and the resulting further cooled second mixed refrigerant vapour is compressed directly without preheating. Additionally or alternatively, the cooling and condensing of the second mixed refrigerant vapour following compression can be effected by indirect heat exchange in the first heat exchange zone to yield a partially condensed second mixed refrigerant stream containing intermediate second mixed refrigerant vapour and intermediate second mixed refrigerant liquid; separating the partially condensed second mixed refrigerant stream to yield an intermediate second mixed refrigerant vapour and an intermediate second mixed refrigerant liquid; cooling, condensing, optionally subcooling, and flashing the intermediate second mixed refrigerant vapour to provide the second vaporising mixed refrigerant; and subcooling and flashing the intermediate second mixed refrigerant liquid at the second essentially constant pressure to yield additional refrigeration in the second heat exchange zone. The invention includes an apparatus for liquefying a pressurised feed gas by the process of the invention, which apparatus comprises: first heat exchange means for cooling the pressurised feed gas and compressed second mixed refrigerant, wherein cooling is effected at least in part by indirect heat exchange with the first mixed refrigerant which is vaporised at the first essentially constant pressure; means for compressing, cooling and condensing the first mixed refrigerant vapour to provide condensed first mixed refrigerant; pressure reduction means for flashing the condensed first mixed refrigerant to provide the first vaporising mixed refrigerant; second heat exchange means for further cooling and condensing the cooled feed gas, wherein cooling is effected at least in part by indirect heat exchange with the second mixed refrigerant which is vaporised at the second essentially constant pressure; means for compressing, cooling and condensing the second mixed refrigerant vapour to provide condensed second mixed refrigerant; and pressure reduction means for flashing the condensed second mixed refrigerant to provide the second vaporising mixed refrigerant characterized in that the means for compressing, cooling and condensing the first mixed refrigerant comprises at least two compressor stages, one stage of which generates an interstage compressed refrigerant, an interstage cooler in which the interstage compressed refrigerant is cooled and partially condensed thereby yielding a two-phase interstage refrigerant, a separator in which the two-phase interstage refrigerant is separated into an interstage refrigerant vapour and an interstage refrigerant liquid, and a further compressor stage in which the interstage refrigerant vapour is compressed to yield a further compressed refrigerant, and wherein the apparatus further comprises piping means to provide the further compressed refrigerant as the compressed first mixed refrigerant for cooling, condensing, and flashing to provide the first vaporising mixed refrigerant, piping means to transfer the interstage refrigerant liquid to the first heat exchange means, heat transfer means within the first heat exchange means for cooling the interstage refrigerant liquid, pressure reduction means to flash the resulting cooled interstage refrigerant liquid to provide additional vaporising mixed refrigerant at the first essentially constant pressure, and piping means to introduce the additional vaporising mixed refrigerant into the first heat exchange means to provide refrigeration therein. In a preferred embodiment, the first heat exchange means also cools the compressed first mixed refrigerant and the second heat exchange means further cools and condenses the cooled second compressed mixed refrigerant, wherein cooling is effected at least in part by indirect heat exchange with the second mixed refrigerant which is vaporised at the second essentially constant pressure. The apparatus can also include a separator for separating the cooled second compressed mixed refrigerant, when the cooled second compressed mixed refrigerant is partially condensed, to yield an intermediate second mixed refrigerant vapour and an intermediate second mixed refrigerant liquid; heat transfer means within the second heat exchange means to cool the intermediate second mixed refrigerant liquid; pressure reduction means to flash the resulting subcooled intermediate second mixed refrigerant liquid to provide additional vaporising mixed refrigerant at the second essentially constant pressure, and piping means to introduce the additional vaporising mixed refrigerant into the second heat exchange means to provide refrigeration therein. The apparatus may further comprise distillation means for separating cooled pressurised methane-rich feed gas obtained from natural gas, into an overhead stream further enriched in methane and a bottoms stream comprising components heavier than methane; separation means for separating the bottoms stream into a light hydrocarbon stream comprising components having up to three or up to four carbon atoms and a heavy hydrocarbon stream comprising components having more than three or four carbon atoms respectively; cooling means within the first heat exchange means for cooling the light hydrocarbon stream; and piping means for combining the resulting cooled light hydrocarbon stream with the overhead stream prior to the further cooling and condensing by indirect heat exchange in the second heat exchange means. Additionally or alternatively, the apparatus may comprise distillation means for separating cooled pressurised methane-rich feed gas obtained from natural gas into an overhead stream of further enriched in methane and a bottoms stream of components heavier than methane; separation means for separating the bottoms stream into a light hydrocarbon stream containing components having three or less or four or less carbon atoms and a heavy hydrocarbon stream comprising components having a more than three or more than four carbon atoms respectively; cooling means within the first heat exchange means for cooling the light hydrocarbon stream; and piping means for introducing a portion of the resulting cooled hydrocarbon stream as reflux for the distillation means. Optionally, the apparatus further comprises piping means for introducing a portion of the pressurised feed gas into the distillation means at a location which is below the location at which the cooled pressurised feed gas is introduced. The following is a description by way of example only and with reference to the accompanying drawings of presently preferred embodiments of the invention. In the drawings:- Fig. 1 is a schematic flowsheet of one preferred embodiment of the present invention; and Fig. 2 is a schematic flowsheet illustrating a variation of the second heat exchanger used in the present invention. The main embodiment of the invention is illustrated in Fig. 1. Feed gas 90, typically a methane-containing gas and preferably natural gas, at a pressure up to 300 bar absolute (all pressures reported herein are absolute pressures) is dried and cleaned by known methods in pretreatment section 100 for the removal of acid gases such as CO 2 and H 2 S along with other contaminants such as mercury or mercury-containing compounds. Pretreated gas 102 enters the first heat exchanger or heat exchange zone 404 and is cooled therein to an intermediate temperature of -20°C to -70°C. The actual temperature level of this cooling step depends on the feed composition and desired LNG product specification (e.g. heating value), and sometimes is dictated by a desire to achieve a certain power split between compression services. Cooling in heat exchanger 404 is effected by the warming and vaporisation of high level mixed refrigerant, which typically contains one or more hydrocarbons selected from methane, ethane, propane, i-butane, butane, and possibly i-pentane, and may contain other components such as nitrogen. Cooled feed stream 108 is introduced into reboiled stripper or scrub column 106 for the removal of hydrocarbons heavier than methane. Bottoms product stream 112 enters fractionation section 114 in which pentane and heavier components are separated and recovered in stream 116. A portion of the bottoms stream from scrub column 106 is vaporised in heater 172 to provide boilup or stripping gas to the column. Butane and lighter components are recovered as stream 118, which is cooled in heat exchanger 404 and combined with the overhead product of scrub column 106 to yield precooled feed stream 120. Alternatively, depending on LNG product specifications, fractionation may be carried out such that stream 118 contains propane and lighter components. Precooled feed stream 120 is further cooled and liquefied in heat exchanger 122 by indirect heat exchange by warming and vaporising low level mixed refrigerant stream. The resulting liquefied product stream 121, typically liquefied natural gas (LNG), is flashed by adiabatic pressure reduction to a low pressure across throttling valve 126. Alternatively, the pressure of liquefied product stream 121 may be reduced by work expansion across a turboexpander. The reduced pressure LNG product stream is introduced into storage tank 128, from which final liquefied product stream 130 is withdrawn. In some cases, depending on the natural gas feed composition and the temperature of stream 121 from heat exchanger 122, a significant quantity of light gas 132 can be evolved after the flashing across valve 126. Flash gas stream 132 typically is warmed, for example in heat exchanger 162, and compressed in offgas compressor 134 for use as fuel gas. Refrigeration to cool the natural gas feed from ambient temperature to a temperature of -20°C to -70°C is provided by a high level multi-component refrigeration as mentioned above. Stream 436 is a high level mixed refrigerant after compression and cooling, and typically contains some condensed liquid. The stream enters heat exchanger 404 at ambient temperature and an elevated pressure typically above 3 bar absolute, and is condensed, cooled, and optionally subcooled to a temperature of -20°C to -70°C exiting as stream 438. Stream 438 is flashed adiabatically to a low pressure in the range of one to 30 bar absolute across throttling valve 450 and reduced pressure stream 410 is introduced to the cold end of heat exchanger 404. Alternatively, the pressure of cooled refrigerant stream 438 can be reduced by work expansion across a turboexpander. The flashing step, which is defined as either isenthalpic or essentially isentropic pressure reduction, can include cooling or vaporisation and can be achieved either by throttling across a pressure reducing valve or by work expansion in a turboexpander or expansion engine. Flashed high level refrigerant stream 410 is warmed and vaporised in heat exchanger 404, and leaves the exchanger as vapour refrigerant stream 140, preferably at a temperature below the temperature of compressed refrigerant stream 436 returning to heat exchanger 404. Vapour refrigerant stream 140 is compressed in multi-staged intercooled compressor 142 to a pressure above 3 bar absolute. The compressed vapour refrigerant 146 is cooled and fed as stream 436 to exchanger 404, in which it is cooled and optionally subcooled to a temperature of -30°C to -70°C to provide stream 438, flashed across throttling valve 450, and fed to exchanger 404 as stream 410. Interstage refrigerant liquid stream 144 is fed to exchanger 404, cooled and optionally subcooled therein to a temperature warmer than stream 438, flashed across throttling valve 468, and introduced to exchanger 404 at an intermediate position in exchanger 404. Pressure drops across each of the throttling valves 450 and 468 are chosen such that the flashed fluids vaporise at essentially the same pressure. Final cooling of gas feed stream 120 from -20°C to -70°C to the final liquefaction temperature is accomplished using a low level mixed refrigerant loop containing refrigerant components as earlier described. Compressed low level mixed refrigerant stream 152 at about ambient temperature and a pressure greater than 3 bar absolute enters exchanger 404 and is cooled therein by indirect heat exchange to a temperature of -20°C to -70°C, exiting as cooled low level mixed refrigerant stream 154. Refrigerant stream 154 is further cooled and optionally subcooled in heat exchanger 122 to a final temperature below -125°C, and the cooled stream 158 is flashed isenthalpically across throttling valve 156 to a pressure of about 3.3 bar absolute. Alternatively, the pressure of cooled stream 158 can be reduced by work expansion across a turboexpander or reciprocating expansion engine. A small portion of refrigerant stream 154, as stream 160, can be cooled in heat exchanger 162 by flash gas stream 132. Flashed low level mixed refrigerant stream 124 is introduced into the cold end of heat exchanger 122, where it vaporises to provide refrigeration therein. Vaporised low level mixed refrigerant stream 164 leaves heat exchanger 122 at a temperature below the temperature of cooled refrigerant stream 154 returning to heat exchanger 122. Vaporised refrigerant stream 164 then is compressed directly in multi-stage intercooled compressor 166 to greater than 5 bar absolute to provide low level mixed refrigerant stream 152. Vaporised low level mixed refrigerant stream 164 is not used to precool feed gas or other process streams, and therefore passes directly to compression without preheating. Heat exchangers 404 and 122 can utilise any suitable heat exchange devices such as wound coil, shell and tube, or plate-fin exchangers known in the art. Wound coil exchangers are preferred for their compact dimensions and efficient heat transfer performance. In an alternative embodiment of the present invention stream 118 containing butane and lighter components is recycled to exchanger 404, cooled therein, and divided into two portions. One portion is used as reflux in scrub column 106 for the removal of heavy components such as benzene to very low levels. The remaining portion is combined with the overhead product of scrub column 106 to yield precooled feed stream 120. The relative flows of the two portions will depend upon the feed composition and the required degree of contaminant removal from the feed stream. Alternatively, the system can be operated such that stream 118 contains predominantly propane and lighter components. In a second alternative embodiment of the present invention a small portion of warm natural gas feed 102 is fed directly to scrub column 106 below the top location of feed stream 108 rather than being cooled in exchanger 404. This alternative reduces the amount of external heat required in reboiler exchanger 172 of scrub column 106 to generate stripping vapour. This alternative also decreases the refrigeration load in exchanger 404, and is beneficial when feed 102 contains high levels of heavier hydrocarbons and when the liquid fraction of stream 108 is high. The term "essentially the same pressure" as used here means that the pressure of the vaporising refrigerant varies within an exchanger only by the small hydraulic or pneumatic pressure drops or variations caused by flowing liquid or vapour. The refrigerant is not vaporised in separate heat exchange conduits or zones at different pressures as described in many of the prior art processes earlier described. A variation of the second heat exchanger is illustrated in Figure 2 wherein cooled low level mixed refrigerant stream 154 is a two-phase stream which is separated into vapour stream 568 and liquid stream 570 in drum 576. These refrigerant streams are introduced separately into heat exchanger 522. Vapour refrigerant stream 568 is liquefied and optionally subcooled to a low temperature to provide stream 558 and is flashed isenthalpically across throttling valve 556 to yield refrigerant stream 524, which is introduced into the cold end of heat exchanger 522 where it is vaporised to provide a portion of the refrigeration for product liquefaction. Liquid refrigerant stream 570 is subcooled in heat exchanger 522 to a temperature warmer than stream 558 and is flashed adiabatically across throttling valve 572 to low pressure refrigerant stream 574, which is introduced at an intermediate point into heat exchanger 522, where it is vaporised to provide the remaining portion of the refrigeration for product liquefaction. The vaporisation pressure of the two refrigerant streams in the heat exchanger is typically in the range of 1 to 30 bar absolute. The present invention provides for the liquefaction of feed gas using a minimum number of heat exchangers and utilises mixed refrigerants, wherein each refrigerant is vaporised at an essentially constant pressure in each of the exchangers. These characteristics reduce the process complexity and required plant plot plan area compared with known liquefaction processes. The process and apparatus of the invention can be used for installation on ships and offshore platforms by virtue of simplified process features and minimum plot plan area requirements. The present invention includes the feature wherein feed precooling, low level refrigerant precooling, and high level refrigerant cooling are effected in a single heat exchanger against high level refrigerant vaporising at a single essentially constant pressure. In addition, the process does not require a separate feed precooling heat exchanger, since feed precooling is achieved in combination with low level refrigerant precooling and high level liquid refrigerant cooling. Low level mixed refrigerant is not used to precool the feed gas and therefore passes directly to compression without preheating. This results in a compressor which requires fewer intercoolers for a given overall pressure ratio, since the pressure ratio in the first stage can be quite high.

Claims: A process for liquefying a pressurised feed gas which comprises: cooling the pressurised feed gas (102) by indirect heat exchange in a first heat exchange zone (404) with a first vaporising mixed refrigerant (410) which is vaporised at a first essentially constant pressure to yield a cooled feed gas (108) and a first mixed refrigerant vapour (140); further cooling and condensing the cooled feed gas by indirect heat exchange in a second heat exchange zone (122) with a second vaporising mixed refrigerant (124) which is vaporised at a second essentially constant pressure to yield a liquid product (121) and a second mixed refrigerant vapour (164); compressing, cooling and condensing the first mixed refrigerant vapour (140); and flashing (450) the resulting condensed first mixed refrigerant (438) to provide the first vaporising mixed refrigerant (410); compressing, cooling and condensing the second mixed refrigerant vapour (164); and flashing (156) the resulting condensed second mixed refrigerant (158) to provide the second vaporising mixed refrigerant (124), wherein at least a portion of the refrigeration for the cooling and condensing of the second mixed refrigerant vapour is provided by indirect heat exchange in the first heat exchange zone (404) with the first vaporising liquid mixed refrigerant (410) ; compression of the first mixed refrigerant vapour (140) is carried out in at least two stages of compression which generates an interstage compressed refrigerant; the interstage compressed refrigerant is cooled, partially condensed, and separated into an interstage refrigerant vapour and an interstage refrigerant liquid (144); the interstage refrigerant vapour is compressed to yield a further compressed refrigerant (146) which is cooled, condensed, optionally subcooled, and flashed (450) to provide the first vaporising mixed refrigerant (410); and the interstage liquid refrigerant (144) is subcooled and flashed (468) at the first essentially constant pressure to yield additional refrigeration in the first heat exchange zone (404), charaterized in that the subcooling and flashing at the first essentially constant pressure of the interstage liquid refrigerant is done separately from the interstage refrigerant vapour. A process as claimed in Claim 1 wherein the interstage liquid refrigerant is subcooled in the first heat exchange zone (404). A process as claimed in Claim 1 or Claim 2 wherein the resulting cooled interstage liquid refrigerant is flashed to produce additional vaporising mixed refrigerant which is introduced to the first heat exchange zone (404) at an intermediate location to provide the additional refrigeration therein. A process as claimed in any of Claims 1 to 3, wherein at least a portion of the refrigeration for the cooling and condensing of the first mixed refrigerant vapour after compression is provided by indirect heat exchange in the first heat exchange zone (104) with the first vaporising liquid mixed refrigerant (410). A process as claimed in any of Claims 1 to 4, wherein the pressurised feed gas is provided by treating a pressurised stream of natural gas to remove contaminants selected from water, carbon dioxide, sulphur-containing compounds, mercury, and mercury-containing compounds. A process as claimed in any one of the preceding claims, wherein a portion of the refrigeration for the cooling and condensing of the second mixed refrigerant vapour (164) after compression is provided at least in part by indirect heat exchange in the second heat exchange zone (122) with the second vaporising mixed refrigerant (124). A process as claimed in any one of the preceding claims, wherein the second mixed refrigerant vapour (164) after compression is cooled by indirect heat exchange in the first heat exchange zone (404) and is withdrawn therefrom at a first temperature, the resulting cooled second mixed refrigerant stream (154) is introduced into the second heat exchange zone (122) and is further cooled therein by indirect heat exchange, the second mixed refrigerant vapour is withdrawn from the second heat exchange zone at a second temperature which is lower than the first temperature, and the resulting further cooled second mixed refrigerant vapour is compressed directly without preheating. A process as claimed in any one of the preceding claims, wherein the cooling and condensing of the second mixed refrigerant vapour (152) following compression is effected by indirect heat exchange in the first heat exchange zone (104) to yield a partially condensed second mixed refrigerant stream (154) containing intermediate second mixed refrigerant vapour and intermediate second mixed refrigerant liquid, and further wherein the partially condensed second mixed refrigerant stream (154) is separated to yield an intermediate second mixed refrigerant vapour (568) and an intermediate second mixed refrigerant liquid (570); the intermediate second mixed refrigerant vapour (568) is cooled, condensed, optionally subcooled, and flashed (556) to provide the second vaporising mixed refrigerant (524); and the intermediate second mixed refrigerant liquid (570) is subcooled and flashed (572) at the second essentially constant pressure to yield additional refrigeration in the second heat exchange zone (522). An apparatus for liquefying a pressurised feed gas by a process as defined in Claim 1, which apparatus comprises: first heat exchange means (404) for the cooling pressurised feed gas (102) and compressed second mixed refrigerant (152), wherein cooling is effected at least in part by indirect heat exchange with the first mixed refrigerant (410) which is vaporised at the first essentially constant pressure; means (142,404) for compressing, cooling and condensing the first mixed refrigerant vapour (140) to provide condensed first mixed refrigerant (438); pressure reduction means (450) for flashing the condensed first mixed refrigerant (438) to provide the first vaporising mixed refrigerant (410); second heat exchange means (122) for further cooling and condensing the cooled feed gas (120), wherein cooling is effected at least in part by indirect heat exchange with the second mixed refrigerant (124) which is vaporised at the second essentially constant pressure; means (166, 104) for compressing, cooling and condensing the second mixed refrigerant vapour (164) to provide condensed second mixed refrigerant (558); and pressure reduction means (156) for flashing the condensed second mixed refrigerant (158) to provide the second vaporising mixed refrigerant (124) and wherein the means for compressing, cooling and condensing the first mixed refrigerant vapour comprises at least two compressor stages, one stage of which generates an interstage compressed refrigerant, an interstage cooler in which the interstage compressed refrigerant is cooled and partially condensed thereby yielding a two-phase interstage refrigerant, a separator in which the two-phase interstage refrigerant is separated into an interstage refrigerant vapour and an interstage refrigerant liquid, and a further compressor stage in which the interstage refrigerant vapour is compressed to yield a further compressed refrigerant, and wherein the apparatus further comprises piping means to provide the further compressed refrigerant as the compressed first mixed refrigerant for cooling, condensing, and flashing to provide the first vaporising mixed refrigerant characterized in that the apparatus further comprises piping means to transfer the interstage refrigerant liquid to the first heat exchange means (104), heat transfer means within the first heat exchange means (104) for cooling the interstage refrigerant liquid, pressure reduction means to flash the resulting cooled interstage refrigerant liquid to provide additional vaporising mixed refrigerant at the first essentially constant pressure, and piping means to introduce the additional vaporising mixed refrigerant into the first heat exchange means (104) to provide refrigeration therein. An apparatus as claimed in Claim 9 comprising said piping means to introduce the additional vaporising mixed refrigerant into the first heat exchange means (104) at an intermediate location to provide refrigeration therein. An apparatus as claimed in Claim 9 or Claim 10 wherein said means for cooling and condensing the first mixed refrigerant vapour (140) comprises said first heat exchange means (404). An apparatus as claimed in any of Claims 9 to 11, which comprises: first heat exchange means (104) for cooling pressurised feed gas (102), compressed first mixed refrigerant (148), and compressed second mixed refrigerant (152), wherein cooling is effected at least in part by indirect heat exchange with a first mixed refrigerant (110) which is vaporised at a first essentially constant pressure; first compression means (142) for compressing the first mixed refrigerant vapour (140) to provide the compressed first mixed refrigerant (148); pressure reduction means (150) for flashing the condensed first mixed refrigerant to provide the first vaporising mixed refrigerant (110); second heat exchange means (122) for further cooling and condensing the cooled feed gas (120) and for further cooling and condensing the cooled second compressed mixed refrigerant (154), wherein cooling is effected at least in part by indirect heat exchange with a second mixed refrigerant (124) which is vaporised at a second essentially constant pressure; second compression means (166) for compressing the second mixed refrigerant vapour (164) to provide the compressed second mixed refrigerant (152); and pressure reduction means (156) for flashing the condensed second mixed refrigerant (158) to provide the second vaporising mixed refrigerant (124). An apparatus as claimed in any of Claims 9 to 12, which further comprises a separator (576) for separating the cooled second compressed mixed refrigerant (154), when the cooled second compressed mixed refrigerant (154) is partially condensed, to yield an intermediate second mixed refrigerant vapour (568) and an intermediate second mixed refrigerant liquid (570); heat transfer means within the second heat exchange means (522) to cool the intermediate second mixed refrigerant liquid (570); pressure reduction means (572) to flash the resulting subcooled intermediate second mixed refrigerant liquid to provide additional vaporising mixed refrigerant at the second essentially constant pressure, and piping means (574) to introduce the additional vaporising mixed refrigerant into the second heat exchange means (522) to provide refrigeration therein.

Inventors: AGRAWAL RAKESH, ROBERTS MARK JULIAN, AGRAWAL, RAKESH, ROBERTS, MARK JULIAN
Applicants: AIR PROD & CHEM, AIR PRODUCTS AND CHEMICALS, INC.
Application year: 1999
Main classification: F25J1
IPC codes: F25J1