Mixed Refrigerant Liquefaction System and Method with Pre-Cooling

Abstract

A system for cooling a gas includes a pre-cool heat exchanger and a liquefaction heat exchanger. The pre-cool heat exchanger uses a pre-cool refrigerant to pre-cool a feed gas stream prior to the stream being directed to a liquefaction heat exchanger. The liquefaction heat exchanger uses a mixed refrigerant to further cool the pre-cooled gas. The pre-cool heat exchanger also pre-cools the liquefaction mixed refrigerant used by the liquefaction heat exchanger.

Description

CLAIM OF PRIORITY

This application claims the benefit of U.S. Provisional Application No. 62/660,518, filed Apr. 20, 2018, the contents of which are hereby incorporated by reference.

FIELD OF THE DISCLOSURE

The present invention relates generally to systems and methods for cooling or liquefying gases and, more particularly, to a mixed refrigerant liquefaction system and method that uses cold vapor separation to fractionate high pressure mixed refrigerant vapor into liquid and vapor streams and that includes a sub-system for pre-cooling the feed gas stream and one or more mixed refrigerant streams using a second refrigerant.

BACKGROUND

Natural gas, which is primarily methane, and other gases, are liquefied under pressure for storage and transport. The reduction in volume that results from liquefaction permits containers of more practical and economical design to be used. Liquefaction is typically accomplished by chilling the gas through indirect heat exchange by one or more refrigeration cycles. Such refrigeration cycles are costly both in terms equipment cost and operation due to the complexity of the required equipment and the required efficiency of performance of the refrigerant. There is a need, therefore, for gas cooling and liquefaction systems having improved refrigeration efficiency and reduced operating costs with reduced complexity.

Use of a mixed refrigerant in the refrigeration cycle(s) for a liquefaction system increases efficiency in that the warming curve of the refrigerant more closely matches the cooling curve of the gas. The refrigeration cycle for the liquefaction system will typically include a compression system for conditioning or processing the mixed refrigerant. The mixed refrigerant compression system typically includes one or more stages, with each stage including a compressor, a cooler and a separation and liquid accumulator device. Vapor exiting the compressor is cooled in the cooler, and the resulting two-phase or mixed phase stream is directed to the separation and liquid accumulator device, from which vapor and liquid exit for further processing and/or direction to the liquefaction heat exchanger.

Separated liquid and vapor phases of the mixed refrigerant from the compression system may be directed to portions of the heat exchanger to provide more efficient cooling. Examples of such systems are provided in commonly owned U.S. Pat. No. 9,441,877 to Gushanas et al., U.S. Patent Application Publication No. US 2014/0260415 to Ducote et al. and U.S. Patent Application Publication No. US 2016/0298898 to Ducote et al., the contents of each of which are hereby incorporated by reference.

Further increases in cooling efficiency and decreases in operating costs in gas cooling and liquefaction systems are desirable.

SUMMARY OF THE DISCLOSURE

There are several aspects of the present subject matter which may be embodied separately or together in the methods, devices and systems described and claimed below. These aspects may be employed alone or in combination with other aspects of the subject matter described herein, and the description of these aspects together is not intended to preclude the use of these aspects separately or the claiming of such aspects separately or in different combinations as set forth in the claims appended hereto.

In one aspect, a system for cooling a gas with a pre-cool refrigerant and a mixed refrigerant includes a pre-cool heat exchanger having a feed gas inlet adapted to receive a feed gas stream and a feed gas outlet, a pre-cool refrigerant inlet and a pre-cool refrigerant outlet and a liquefaction mixed refrigerant inlet and a liquefaction mixed refrigerant outlet. The pre-cool heat exchanger is configured to use the pre-cool refrigerant to cool feed gas passing through the pre-cool heat exchanger between the feed gas inlet and outlet and to cool liquefaction mixed refrigerant passing through the pre-cool heat exchanger between the liquefaction mixed refrigerant inlet and outlet. A pre-cool compressor system includes a pre-cool compressor having an inlet in fluid communication with the pre-cool refrigerant outlet of the pre-cool heat exchanger. The pre-cool compressor system also has a pre-cool condenser having an inlet in fluid communication with an outlet of the pre-cool compressor. The pre-cool condenser also has outlet in fluid communication with the pre-cool refrigerant inlet of the pre-cool heat exchanger. A liquefaction heat exchanger includes a liquefying passage in fluid communication with the feed gas outlet of the pre-cool heat exchanger, a primary refrigeration passage, a high pressure vapor cooling passage and a cold separator vapor cooling passage, where the cold separator vapor cooling passage has an outlet in fluid communication with the primary refrigeration passage. A mixed refrigerant compression system includes a mixed refrigerant compressor having an inlet in fluid communication with an outlet of the primary refrigeration passage and a mixed refrigerant cooler having an inlet in fluid communication with an outlet of the mixed refrigerant compressor. The mixed refrigerant cooler also has an outlet in fluid communication with the liquefaction mixed refrigerant inlet of the pre-cool heat exchanger. The mixed refrigerant compression system also has a high pressure accumulator having an inlet in fluid communication with the liquefaction mixed refrigerant outlet of the pre-cool heat exchanger and a vapor outlet in fluid communication with an inlet of the high pressure vapor cooling passage of the liquefaction heat exchanger. A cold vapor separator has an inlet in fluid communication with an outlet of the high pressure vapor cooling passage of the liquefaction heat exchanger, a vapor outlet in fluid communication with an inlet of the cold separator vapor cooling passage of the liquefaction heat exchanger and a liquid outlet in communication with the primary refrigeration passage of the liquefaction heat exchanger.

In another aspect, a method for cooling a feed gas stream includes the steps of: pre-cooling the feed gas stream in a pre-cool heat exchanger using a first refrigerant to form a pre-cooled feed gas stream and further cooling the pre-cooled feed gas stream by i) cooling a high pressure second refrigerant stream in the pre-cool heat exchanger to form a cooled high pressure second refrigerant stream, ii) separating the cooled high pressure second refrigerant stream to form a high pressure vapor stream and a high pressure liquid stream, iii) cooling the high pressure vapor stream in a liquefaction heat exchanger, to form a mixed phase stream, iv) separating the mixed phase stream with a cold vapor separator to form a cold separator vapor stream and a cold separator liquid stream, v) condensing the cold separator vapor stream in the liquefaction heat exchanger using the second refrigerant and flashing, to form a cold temperature refrigerant stream, vi) directing the cold temperature refrigerant stream to the liquefaction heat exchanger, vii)subcooling the high pressure liquid stream to form a subcooled high pressure liquid stream and combining with the cold temperature refrigerant stream in the liquefaction heat exchanger, viii) subcooling the cold separator liquid stream to form a subcooled cold separator liquid stream and combining with the cold temperature refrigerant stream in the liquefaction heat exchanger and ix) thermally contacting the pre-cooled gas stream in the liquefaction heat exchanger with the cold temperature refrigerant stream

In another aspect, a system for cooling a feed gas with a mixed refrigerant includes a pre-cool heat exchanger having a pre-cool refrigerant inlet configured to receive a stream of pre-cool refrigerant and a pre-cool refrigerant outlet and a liquefaction mixed refrigerant inlet and a liquefaction mixed refrigerant outlet. The pre-cool heat exchanger is configured to use the pre-cool refrigerant to cool liquefaction mixed refrigerant passing through the pre-cool heat exchanger between the liquefaction mixed refrigerant inlet and outlet. A liquefaction heat exchanger includes a liquefying passage configured to receive a stream of the feed gas, a primary refrigeration passage, a high pressure vapor cooling passage and a cold separator vapor cooling passage, where the cold separator vapor cooling passage has an outlet in fluid communication with the primary refrigeration passage. A mixed refrigerant compression system includes a mixed refrigerant compressor having an inlet in fluid communication with an outlet of the primary refrigeration passage. The mixed refrigerant compression system also includes a mixed refrigerant cooler having an inlet in fluid communication with an outlet of the mixed refrigerant compressor. The mixed refrigerant cooler has an outlet in fluid communication with the liquefaction mixed refrigerant inlet of the pre-cool heat exchanger. The mixed refrigerant compression system also includes a high pressure accumulator having an inlet in fluid communication with the liquefaction mixed refrigerant outlet of the pre-cool heat exchanger and a vapor outlet in fluid communication with an inlet of the high pressure vapor cooling passage of the liquefaction heat exchanger. A cold vapor separator has an inlet in fluid communication with an outlet of the high pressure vapor cooling passage of the liquefaction heat exchanger, a vapor outlet in fluid communication with an inlet of the cold separator vapor cooling passage of the liquefaction heat exchanger and a liquid outlet in communication with the primary refrigeration passage of the liquefaction heat exchanger.

In another aspect, a method for cooling a feed gas stream includes the steps of: directing the feed gas stream into a liquefaction heat exchanger; cooling a high pressure mixed refrigerant stream in a pre-cool heat exchanger to form a cooled high pressure mixed refrigerant stream and cooling the feed gas stream in the liquefaction heat exchanger by: i) separating the cooled high pressure mixed refrigerant stream to form a high pressure vapor stream and a high pressure liquid stream, ii) cooling the high pressure vapor stream in the liquefaction heat exchanger to form a mixed phase stream, iii) separating the mixed phase stream with a cold vapor separator to form a cold separator vapor stream and a cold separator liquid stream, iv) condensing the cold separator vapor stream in the liquefaction heat exchanger and flashing, to form a cold temperature refrigerant stream, v) directing the cold temperature refrigerant stream to the liquefaction heat exchanger, vi) subcooling the high pressure liquid stream in the liquefaction heat exchanger to form a subcooled high pressure liquid stream and combining with the cold temperature refrigerant stream in the liquefaction heat exchanger, vii) subcooling the cold separator liquid stream to form a subcooled cold separator liquid stream and combining with the cold temperature refrigerant stream in the liquefaction heat exchanger; and viii) thermally contacting the gas stream in the liquefaction heat exchanger with the cold temperature refrigerant stream.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a process flow and schematic illustrating a first embodiment of the system and method of the disclosure;

FIG. 2 is a process flow and schematic illustrating a second embodiment of the system and method of the disclosure;

FIG. 3 is a is a process flow and schematic illustrating a third embodiment of the system and method of the disclosure;

FIG. 4 is a process flow and schematic illustrating a fourth embodiment of the system and method of the disclosure; and

FIG. 5 is a process flow and schematic illustrating a fifth embodiment of the system and method of the disclosure.

DETAILED DESCRIPTION OF EMBODIMENTS

Embodiments of the mixed refrigerant liquefaction system and method of the disclosure are illustrated in FIGS. 1-5. It should be noted that while the embodiments are illustrated and described below in terms of liquefying natural gas to produce liquid natural gas, the invention may be used to liquefy or cool other types of gases.

Embodiments of the disclosure may use the mixed refrigerant liquefaction system and process described in commonly owned U.S. Pat. No. 9,441,877 to Gushanas et al.; U.S. Patent Application Publication No. 2014/0260415, U.S. patent application Ser. No. 14/218,949, to Ducote et al., and U.S. Patent Appl. No. 62/561,417 to Ducote et al., the contents of each of which are hereby incorporated by reference.

It should be noted herein that the passages and streams are sometimes both referred to by the same element number set out in the figures. Also, as used herein, and as known in the art, a heat exchanger is that device or an area in the device wherein indirect heat exchange occurs between two or more streams at different temperatures, or between a stream and the environment. As used herein, the terms “communication”, “communicating”, and the like generally refer to fluid communication unless otherwise specified. Furthermore, although two fluids in communication may exchange heat upon mixing, such an exchange would not be considered to be the same as heat exchange in a heat exchanger, although such an exchange can take place in a heat exchanger. As used herein, the term “reducing the pressure of” (or variations thereof) does not involve a phase change, while the term “flashing” (or variations thereof) involves a phase change, including even a partial phase change. As used herein, the terms, “high”, “middle”, “mid”, “warm” and the like are relative to comparable streams, as is customary in the art.

Generally, with reference to FIG. 1, a first embodiment the system of the disclosure includes a mixed refrigerant liquefaction system, indicated in general at 8, including a multi-stream liquefaction heat exchanger, indicated in general at 10, having a warm end 12 and a cold end 14. The heat exchanger receives a pre-cooled natural gas feed stream 16 that is liquefied in cooling or liquefying passage 18 via removal of heat via heat exchange with refrigeration streams in the heat exchanger. As a result, a stream 20 of liquid natural gas (LNG) product is produced. The multi-stream design of the heat exchanger allows for convenient and energy-efficient integration of several streams into a single exchanger. Suitable heat exchangers include brazed aluminum heat exchangers, which may be purchased from Chart Energy & Chemicals, Inc. of The Woodlands, Texas. Such a plate and fin, multi-stream heat exchanger offers the further advantage of being physically compact.

The system of FIG. 1, including heat exchanger 10, may be configured to perform other gas processing options known in the prior art. These processing options may require the gas stream to exit and reenter the heat exchanger one or more times and may include, for example, natural gas liquids recovery or nitrogen rejection.

The removal of heat is accomplished in the heat exchanger using a mixed refrigerant that is processed and reconditioned using a liquefaction system mixed refrigerant compressor system indicated in general at 22. The mixed refrigerant compressor system includes a first stage suction drum 24, which receives a mixed refrigerant vapor stream 26 from the primary refrigeration passage 28 of the heat exchanger 10. The vapor stream is compressed in a first stage compressor 32 (which may be an individual compressor or a stage of a single, multi-stage compressor) and then cooled by first stage heat exchanger or cooler 34. The resulting mixed refrigerant vapor stream travels to a second stage suction drum 35 and then to a second stage compressor 36 (which may be an individual compressor or a stage of the single, multi-stage compressor) and, after compression, is cooled in second stage heat exchanger or cooler 38.

As is known in the art, the first and second stage suction drums 24 and 35, and the remaining suction drums noted below, guard against liquid delivery to their following compressors, and are optional.

In addition to the liquefaction heat exchanger 10, and associated components described below and in U.S. patent application Ser. No. 14/218,949, to Ducote et al., incorporated by reference above, and mixed refrigerant compressor system 22, the system of FIG. 1 includes a pre-cooling system, indicated in general at 40. The pre-cooling system includes a pre-cool warm heat exchanger, indicated in general at 42a, and a pre-cool cold heat exchanger, indicated in general at 42b. Warm and cold heat exchangers 42a and 42b may be, as an example only, CORE-IN-KETTLE heat exchangers, available from Chart Energy & Chemicals, Inc. of The Woodlands, Texas. Alternative types of heat exchangers including, but not limited to, shell and tube or thermosiphon type heat exchangers may be used for warm and cold heat exchangers 42a and 42b. The pre-cooling system may alternatively feature a single pre-cool heat exchanger or more than two pre-cool heat exchangers.

The pre-cooling system also includes a compressor system, indicated in general at 44, for processing and reconditioning a pre-cooling system refrigerant, such as propane, butane, ammonia or a chlorofluorocarbon. While the pre-cooling systems in the embodiments described herein use propane, alternative refrigerants including, but not limited to, butane, ammonia or liquid fluorinated hydrocarbons may be used.

The pre-cooling compressor system 44 includes a first stage suction drum 46 that receives a propane refrigerant vapor stream 48 from cold heat exchanger 42b, as described in greater detail below. Vapor stream 52 from the first stage suction drum travels to a pre-cooling compressor 54, and the resulting compressed stream travels to pre-cooling condenser 56. A resulting propane refrigerant liquid stream travels to pre-cooling refrigerant accumulator 62. A propane refrigerant liquid stream 64 travels from the accumulator to an expansion device 66 so that a two-phase stream 72 enters a shell 74 of the warm heat exchanger 42a. A liquid level sensor 76 controls the setting of the expansion device 66 so that a proper liquid level is maintained within the shell 74.

As in the case of all expansion devices referenced herein, expansion device 66 may be an expansion valve, such as a Joule-Thomson valve, or another type of expansion device including, but not limited to, a turbine or an orifice.

The shell 74 of the pre-cool warm heat exchanger 42a houses a core 78 that receives a natural gas feed stream 82. The core 78 of the warm feed gas heat exchanger, and all such cores discussed below, as an example only, may be a brazed aluminum heat exchanger (BAHX) or other heat exchanger type such as micro-channel or welded plate, tubes or coils, printed circuit heat exchanger, etc. The natural gas stream is cooled by the propane liquid refrigerant in the core 78, and the cooled natural gas stream exits the warm heat exchanger 42a as stream 84. In an alternative embodiment, where the natural gas stream 82 is cooler than the warm heat exchanger 42a, the gas stream may be routed directly to cold heat exchanger 42b as indicated by dashed line 84′ in FIG. 1. In such an embodiment, core 78 may be omitted.

A warm propane refrigerant vapor stream 86 exits the shell 74 of the pre-cool warm heat exchanger 42a and travels to a second stage suction drum 88 and to an inlet of pre-cooling compressor 54.

A propane refrigerant liquid stream exits the shell of the warm heat exchanger as stream 92 and travels to expansion device 94 so that a two-phase stream 96 enters a shell 98 of the pre-cool cold heat exchanger 42b. A liquid level sensor 102 controls the setting of the expansion device 94 so that a proper liquid level is maintained within the shell 98.

The shell 98 of the cold heat exchanger 42b houses a core 104 that receives the natural gas feed stream 84 (or natural gas feed stream 84′). The natural gas stream 84 is further cooled (or cooled) by the propane liquid refrigerant in the core 104, and the cooled natural gas stream exits the cold heat exchanger 42b as pre-cooled stream 16 and travels to liquefying passage 18 of the liquefaction heat exchanger 10. In an alternative embodiment, where the natural gas stream 82 is cooler than both the warm heat exchanger 42a and 42b, the gas stream 84′ of FIG. 1 may be routed directly to liquefying passage of the liquefaction heat exchanger. In such an embodiment, core 104 may also be omitted.

The propane refrigerant vapor stream 48 exits the shell 98 of the pre-cool cold heat exchanger 42b and travels to the first stage suction drum 46.

The high pressure mixed refrigerant stream 112 from the second stage compressor 36 and heat exchanger 38 of the mixed refrigerant compression system travels to a core 114 positioned within the shell 74 of the pre-cool warm heat exchanger 42a. The mixed refrigerant flowing through core 114 is cooled by the liquid propane refrigerant within shell 74, and the resulting cooled mixed refrigerant stream 116 is directed to the cold mixed refrigerant core 118 positioned within the shell 98 of the pre-cool cold heat exchanger 42b. The mixed refrigerant flowing through core 118 is cooled by the liquid propane refrigerant within shell 98, and a resulting mixed refrigerant (MR) mixed phase stream 122 is directed to a high pressure accumulator 124. While an accumulator drum is illustrated as high pressure accumulator 124, alternative separation devices may be used, including, but not limited to, another type of vessel, a cyclonic separator, a distillation unit, a coalescing separator or mesh or vane type mist eliminator. The same applies for the remaining separation devices or drums discussed herein.

High pressure vapor refrigerant stream 126 exits the vapor outlet of the accumulator 124 and travels to the warm end of the heat exchanger 10.

High pressure liquid refrigerant stream 128 exits the liquid outlet of accumulator 124 and also travels to the warm end of the heat exchanger. After cooling in the heat exchanger 10, via high pressure liquid cooling passage 125, it is flashed at 129 and travels to warm temperature separator 131. Vapor stream 127 and liquid stream 133 travel from the warm temperature separator 131 to the primary refrigeration passage 28 of the heat exchanger 10.

The heat exchanger 10 also receives and cools, via high pressure vapor cooling passage 135, the high pressure vapor stream 126 from the high pressure accumulator 124 and cools it so that it is partially condensed. The resulting mixed phase cold separator feed stream 132 is provided to a cold vapor separator 134 so that cold separator vapor stream 136 and cold separator liquid stream 138 are produced.

The cold separator vapor stream 136 is cooled and condensed in the heat exchanger 10, via cold separator vapor cooling passage 141, into liquid stream 142, flashed through expansion device 144 and directed to cold temperature separator 146 to form a cold temperature liquid stream 152 and a cold temperature vapor stream 154, which are directed to the primary refrigeration passage 28 of the heat exchanger 10 as a cold temperature refrigerant stream.

The cold separator liquid stream 138 is cooled in the heat exchanger 10, via cold separator liquid cooling passage 143, to form subcooled cold separator liquid 160, which is flashed at 162 and directed to mid temperature separator 164. A resulting liquid stream 166 and a resulting vapor stream 168 are directed to the primary refrigeration passage 28 of the heat exchanger 10.

The combined refrigerant streams from the warm temperature separator 131, the mid temperature separator 164 and the cold temperature separator 146 provide the refrigeration for liquefying pre-cooled feed gas stream 16 within the liquefying or cooling passage 18 of the heat exchanger 10, and exit the primary refrigeration passage 28 of the liquefaction heat exchanger as a combined return refrigerant stream 26, which preferably is in the vapor phase. The return refrigerant stream 26 flows to the suction drum 24, which results in vapor mixed refrigerant stream 27, as referenced previously.

The liquefied natural gas stream 172 exits the cold side of the heat exchanger and may be optionally expanded, using expansion device 174, and delivered to storage or a process.

The embodiment of FIG. 1 therefore shows a propane (C3) pre-cooled mixed refrigerant (MR) process in combination with a cold vapor separator (CVS) located in the main liquefaction section of the process. The combination of C3 pre-cooling and MR with a CVS results in a more efficient process than pre-cooling without the CVS and with lower equipment cost and also facilitates higher plant capacities. The combination of pre-cooling and CVS allows the C3 system to operate at a significantly warmer temperature such as, as an example only, approximately −5° C. vs. −35 to −40° C., with high efficiency, which reduces the propane system cost and power consumption.

The process of FIG. 1 can be used with any MR liquefaction process that utilizes a CVS.

It should be noted that while FIG. 1 shows two stages of pre-cooling in the pre-cooling system 40, one or more stages of pre-cooling may alternatively be used.

Furthermore, while FIG. 1 shows an MR liquefaction system 8 featuring separate warm, mid and cold temperature separators, any of these may be combined or, in certain cases, the separators may be eliminated. Furthermore, while these separators are illustrated as stand pipes, alternative types of separators known in the art may be used.

With the exceptions discussed below, the embodiments of FIGS. 2-4 feature the same mixed refrigerant compressor system, mixed refrigerant liquefaction system and pre-cooling compressor system components and operation as described above with reference to FIG. 1, and thus common reference numbers are used to indicate these portions, and common components, of the systems.

A second embodiment of the system of the disclosure is presented in FIG. 2. In this embodiment, two high pressure MR accumulators are used, instead of the single high pressure MR accumulator 124 of FIG. 1. More specifically, stream 182 exiting the second stage compression and cooling cycle of the MR compressor system 22 is directed to the core 114 of the warm pre-cool heat exchanger 42a. The core 114 cools the stream 182 using the liquid propane refrigerant within shell 74. The resulting cooled MR stream 186 travels to a first high pressure MR accumulator 188. The resulting vapor MR stream 192 travels to a core 194 positioned within the pre-cool cold heat exchanger 42b where it is cooled by the liquid propane refrigerant within shell 98. The resulting cooled stream 198 travels to a second high pressure MR accumulator 202.

The vapor stream 204 leaving the second high pressure MR accumulator 202 is cooled within the liquefaction heat exchanger 10, via passage 206, and is directed to cold vapor separator 208. The vapor stream exiting the cold vapor separator is processed as described above with regard to FIG. 1.

The liquid stream 212 leaving the second high pressure MR accumulator 202 is cooled within the liquefaction heat exchanger 10, via passage 214, is flashed via expansion device 216 and is directed to mid temperature separator 164, where it is combined with the cooled and flashed liquid stream from the cold vapor separator 208. The vapor and liquid streams exiting the mid temperature separator are directed to the primary refrigeration passage 28.

The liquid MR stream exiting the first high pressure MR accumulator 188 travels to a core 196 positioned within the pre-cool cold heat exchanger 42b where it is cooled by the liquid propane refrigerant within shell 98. The resulting cooled stream 218 is cooled in the liquefaction heat exchanger 10 via passage 220, and the resulting cooled liquid stream is flashed via expansion device 222 and delivered to warm temperature separator 131. The vapor and liquid streams exiting the warm temperature separator are directed to the primary refrigeration passage 28.

In addition, in the embodiment of FIG. 2, the pre-cooling system is used to cool the discharge stream 224 exiting the first stage compression and cooling cycle of the MR compressor system 22. More specifically, the pre-cool warm heat exchanger 42a contains a core 226 which receives the stream 224 through an interstage mixed refrigerant inlet and cools it using the propane liquid refrigerant within the shell 74. The resulting cooled stream exits the core through an interstage mixed refrigerant outlet and travels to an Interstage or MR low pressure accumulator 228. The resulting vapor stream 232 is directed to an input of the second stage compressor 36 of the MR compressor system 22. The liquid stream 234 exiting the MR low pressure accumulator 228 is received by a core 236 positioned within the shell 98 of the cold heat exchanger 42b. The resulting cooled stream 238 is cooled in passage 242 of the liquefaction heat exchanger 10, flashed via expansion device 244 and directed to the primary refrigeration passage 28 of the heat exchanger 10.

It is to be understood, with regard to the embodiment of FIG. 2, that pre-cooling the discharge stream (224) of the first compression and cooling stage of the MR compressor system 22 before compressing in the second stage and incorporating first and second MR high pressure accumulators (188 and 202) in the process are distinct and independent and may be utilized in combination or separately.

Furthermore, the pre-cooled liquid stream 224 from the first compression and cooling stage may be introduced into the MR liquefaction system 8 separately, as shown in FIG. 2, or combined with any of the other refrigeration streams in the separators of the MR liquefaction system 8 or in some cases without any separators.

A third embodiment of the system of the disclosure is presented in FIG. 3. In this embodiment, a warm mixed refrigerant (MR) pre-cooling system, indicated in general at 252 is used in place of the propane pre-cooling system of FIGS. 1 and 2.

The MR pre-cooling system includes a warm MR pre-cooling heat exchanger, indicated in general at 254, that includes a pre-cooling passage 256 that receives natural gas feed stream 82.

The MR pre-cooling system also includes a pre-cooling compressor system 262 that includes a first stage suction drum 264 that receives a pre-cooling MR vapor stream 266 from a pre-cooling primary refrigeration passage 268 of the heat exchanger 254. Vapor stream 272 from the first stage suction drum travels to an inlet of pre-cooling compressor 272, and the resulting compressed stream travels to pre-cooling condenser 274. A resulting MR liquid stream travels to pre-cooling MR accumulator 276. The vapor stream from the accumulator 276 may either be vented via valve 278 or directed via a second valve to a second stage suction drum 284. The vapor stream 286 from the second stage suction drum 284 travels to an inlet of pre-cooling compressor 272.

A liquid pre-cooling MR stream 292 travels from accumulator 276 through cooling passage 294 of the heat exchanger 254, and the resulting cooled liquid stream travels to an expansion device 296 and is flashed, with the resulting mixed phase stream entering pre-cooling cold separator 302. A portion of (or all of) the cooled liquid stream leaving passage 294 of the heat exchanger may be directed to a secondary pre-cooling refrigeration passage 304 of the heat exchanger using valve 298 depending on the system temperature and duty needs. The vapor stream 306 exiting the secondary pre-cooling refrigeration passage 304 is directed to second stage suction drum 284. Both the vapor and liquid pre-cooling MR streams (308 and 312, respectively) from the pre-cooling cold separator 302 are directed to the pre-cooling primary refrigeration passage 268 of the heat exchanger 254.

The natural gas feed stream flowing through pre-cooling passage 256 of the pre-cooling heat exchanger 254 is pre-cooled via refrigeration passages 268 and 304 of the heat exchanger, and the resulting cooled natural gas stream 314 is directed to the liquefaction heat exchanger 10 to be liquefied.

The liquefaction compressor system 316, similar to the embodiments of FIGS. 1 and 2, features a first stage compression and cooling cycle, that produces first stage liquefaction MR stream 318, and a second stage compression and cooling cycle, that produces second stage liquefaction MR stream 322. Liquefaction MR streams 318 and 322 are further cooled in the pre-cooling heat exchanger 254 via passages 324 and 326, and the resulting mixed phase stream 328 exiting passage 324 travels to a liquefaction MR low pressure accumulator 332, while the resulting mixed phase stream 334 travels to liquefaction MR high pressure accumulator 336.

Liquefaction MR vapor stream 338 travels from the liquefaction MR low pressure accumulator 332 to second stage suction drum 342 of the liquefaction compressor system 316, with the resulting vapor stream being directed to the second stage compression and cooling cycle. Liquefaction MR liquid stream 344 from the liquefaction MR low pressure accumulator 332 is cooled in passage 346 of the liquefaction heat exchanger 350, flashed via expansion device 348 and directed to the primary refrigeration passage 352 of the heat exchanger 350.

The liquefaction MR vapor stream 354 leaving the liquefaction MR high pressure accumulator 336 is cooled within the liquefaction heat exchanger 350, via passage 356, and is directed to cold vapor separator 358. The vapor stream exiting the cold vapor separator may be processed as described above with regard to FIG. 1.

The liquid stream 362 leaving the liquefaction MR high pressure accumulator 336 is cooled within the liquefaction heat exchanger 350, via passage 364, is flashed via expansion device 366 and is directed to mid temperature separator 368, after it is combined with the cooled and flashed liquid stream from the cold vapor separator 358 (which is functionally equivalent to combining the streams in the mid temperature separator, as indicated in FIG. 2). The vapor and liquid streams exiting the mid temperature separator are directed to the primary refrigeration passage 352 of the heat exchanger 350.

It should be noted that, with regard to the embodiment of FIG. 3, pre-cooling the liquefaction MR compression system 316 first stage discharge (318) before compressing in the second stage is an optional feature and may be utilized in combination with the other features or not used at all. In addition, the mixed refrigerants used in the pre-cooling system and the liquefaction system may be of the same or different compositions.

In addition, it should be noted that the MR pre-cooling system illustrated at 262 in FIG. 3 is merely an example of a suitable MR system—other MR systems, and non-mixed refrigerant systems, known in the art may be used instead as the pre-cooling system.

The embodiment of the system illustrated in FIG. 4 is essentially the same as the embodiment of FIG. 1, including the propane pre-cooling system, indicated in general at 370, with the exception of the configuration of the pre-cooling heat exchangers. More specifically, in the embodiment of the system illustrated in FIG. 4, the pre-cooling system 370 includes a pre-cool warm heat exchanger, indicated in general at 372a, and a pre-cool cold heat exchanger, indicated in general at 372b. Warm and cold heat exchangers 372a and 372b may be, as an example only, CORE-IN-KETTLE heat exchangers, available from Chart Energy & Chemicals, Inc. of The Woodlands, Texas. Alternative types of heat exchangers including, but not limited to, shell and tube or thermosiphon type heat exchangers may be used.

In the embodiment of FIG. 4, a core 374 (which, as an example only, may be a brazed aluminum heat exchanger (BAHX) or other heat exchanger type such as micro-channel or welded plate, etc.) extends thru the internal head 376 between the shells 378 and 382 of the warm and cold heat exchangers 372a and 372b such that the process stream, which is the discharge stream 384 from the second compression and cooling stage of the liquefaction MR compressor system 386, is continuous thru the core 374. The benefit of this arrangement is that the cooled and partially condensed process stream is not subject to two-phase flow mal-distribution, which adversely effects system performance, as could be encountered if the heat exchanger design was multiple cores piped in series, as shown in FIG. 1. The arrangement of FIG. 4. reduces the power consumption of the process, either the propane system or the liquefaction system or both, attributed to mal-distribution or simplifies the equipment count and reduces cost to eliminate mal-distribution effects.

It should be noted that the warm and cold heat exchangers 372a and 372b can utilize an internal head 376 of any shape, including flat plate. Furthermore, while FIG. 4 shows a propane (C3) pre-cooled MR process, the embodiment of FIG. 4 can be used with any process that utilizes at least two boiling refrigerant cooling steps. In addition, while propane (C3) is described as the coolant for the pre-cooling system of FIG. 4, any refrigerant may be used, such as, but not limited to, butane, ammonia or liquid fluorinated hydrocarbons, etc. Furthermore, while the system of FIG. 4 shows two stages of pre-cooling, two or more stages of cooling may be used. In addition, while FIG. 4 shows a separate feed exchanger, the feed exchanger may be combined with MR exchanger.

In the embodiment illustrated in FIG. 5, a chilled water cooling system, indicated in general at 402, is used to pre-cool the discharge stream 404 from the second compression and cooling stage of the liquefaction MR compressor system 406. More specifically, water is pumped via pump 412 to a coolant heat exchanger 414. The heat exchanger also receives the MR discharge stream 404 and cools it. The chilled water is water or water/glycol mixture cooled in a pre-cool refrigerant system that may be, but not limited to, a mechanical chiller or adsorption chiller or thermoelectric chiller or thermoacoustic refrigerator and is always colder than the temperature that can be achieved by either air cooling or water evaporative cooling.

The cooled MR stream 416 then flows to high pressure accumulator 124, with the resulting liquid and vapor streams directed to the liquefaction heat exchanger 420 of the MR liquefaction system 408, as in previous embodiments.

While a single chiller heat exchanger 414 is illustrated in FIG. 5, multiple chiller heat exchangers, in parallel or in series, may be used instead.

As in previous embodiments, the liquefaction MR compressor system provides refrigerant to an MR liquefaction system 408 that includes a cold vapor separator (CVS) 410. The combination of pre-cooling with a chilled water cooling system and MR with CVS results in a more efficient process than pre-cooling without the CVS and with lower equipment cost and also facilitates higher plant capacities. The combination of pre-cooling and CVS allows the chilled water cooling system to operate at a significantly warmer temperature, approximately −5° C. vs. −35 to −40° C. It also allows the chiller equipment to be located away from the hydrocarbon containing equipment, which reduces the system cost and provides plot plan flexibility. The process can be used with any MR liquefaction process that utilizes a CVS.

While FIG. 5 shows a chilled water pre-cooled MR process, any chilled cooling fluid may be used, such as, but not limited to, ammonia, water, water glycol mix, lithium bromide solution, liquid fluorinated hydrocarbons, liquid hydrocarbons, etc. In addition, while FIG. 5 shows a shell and tube heat exchanger for the pre-cooling system heat exchanger 414, any heat exchanger type may be used. Furthermore, while FIG. 5 shows separate warm, mid and cold temperature stand pipes 422, 424 and 426, any of these may be combined or in certain cases, the stand pipe may be eliminated. Although not explicitly shown, the chilled water cooling system may also be used to cool the feed gas and/or cool the 1st stage discharge as shown in FIG. 2 or provide cooling for turbine inlet air for the gas turbine driver or cool multiple liquefaction systems.

There are several aspects of the present subject matter which may be embodied separately or together in the methods, devices and systems described and claimed below. These aspects may be employed alone or in combination with other aspects of the subject matter described herein, and the description of these aspects together is not intended to preclude the use of these aspects separately or the claiming of such aspects separately or in different combinations as set forth in the claims appended hereto.

While the preferred embodiments of the invention have been shown and described, it will be apparent to those skilled in the art that changes and modifications may be made therein without departing from the spirit of the invention, the scope of which is defined by the appended claims.

Claims

1. A system for cooling a gas with a pre-cool refrigerant and a mixed refrigerant comprising:

a. a pre-cool heat exchanger having a feed gas inlet adapted to receive a feed gas stream and a feed gas outlet, a pre-cool refrigerant inlet and a pre-cool refrigerant outlet and a liquefaction mixed refrigerant inlet and a liquefaction mixed refrigerant outlet, said pre-cool heat exchanger configured to use the pre-cool refrigerant to cool feed gas passing through the pre-cool heat exchanger between the feed gas inlet and outlet and to cool liquefaction mixed refrigerant passing through the pre-cool heat exchanger between the liquefaction mixed refrigerant inlet and outlet;

b. a pre-cool compressor system including: i) a pre-cool compressor having an inlet in fluid communication with the pre-cool refrigerant outlet of the pre-cool heat exchanger; ii) a pre-cool condenser having an inlet in fluid communication with an outlet of the pre-cool compressor, said pre-cool condenser also having outlet in fluid communication with the pre-cool refrigerant inlet of the pre-cool heat exchanger:

c. a liquefaction heat exchanger including a liquefying passage in fluid communication with the feed gas outlet of the pre-cool heat exchanger, a primary refrigeration passage, a high pressure vapor cooling passage and a cold separator vapor cooling passage, where the cold separator vapor cooling passage has an outlet in fluid communication with the primary refrigeration passage;

d. a mixed refrigerant compression system including: i) a mixed refrigerant compressor having an inlet in fluid communication with an outlet of the primary refrigeration passage; ii) a mixed refrigerant cooler having an inlet in fluid communication with an outlet of the mixed refrigerant compressor, said mixed refrigerant cooler having an outlet in fluid communication with the liquefaction mixed refrigerant inlet of the pre-cool heat exchanger, iii) a high pressure accumulator having an inlet in fluid communication with the liquefaction mixed refrigerant outlet of the pre-cool heat exchanger and a vapor outlet in fluid communication with an inlet of the high pressure vapor cooling passage of the liquefaction heat exchanger;

e. a cold vapor separator having an inlet in fluid communication with an outlet of the high pressure vapor cooling passage of the liquefaction heat exchanger, a vapor outlet in fluid communication with an inlet of the cold separator vapor cooling passage of the liquefaction heat exchanger and a liquid outlet in communication with the primary refrigeration passage of the liquefaction heat exchanger.

2. The system of claim 1 wherein the pre-cool heat exchanger includes a warm pre-cool heat exchanger and a cold pre-cool heat exchanger.

3. The system of claim 2 wherein each of the warm pre-cool heat exchanger and the cold pre-cool heat exchanger includes a shell that receives the pre-cool refrigerant and at least one of the warm pre-cool heat exchanger and the cold pre-cool heat exchanger includes a feed gas core that receives the feed gas.

4. The system of claim 2 wherein each of the warm pre-cool heat exchanger and the cold pre-cool heat exchanger includes a liquefaction mixed refrigerant core configured to cool liquefaction mixed refrigerant passing through the pre-cool heat exchanger between the liquefaction mixed refrigerant inlet and outlet.

5. The system of claim 4 wherein a single liquefaction mixed refrigerant core extends within both of the shells of the warm and cold pre-cool heat exchangers and is configured to cool liquefaction mixed refrigerant passing through the pre-cool heat exchanger between the liquefaction mixed refrigerant inlet and outlet.

6. The system of claim 5 wherein an internal head extends between interior spaces of the shells of the warm and cold pre-cool heat exchangers and the single liquefaction mixed refrigerant core extends through the internal head.

7. The system of claim 1 wherein the mixed refrigerant compression system further includes a mixed refrigerant second compressor or compression stage having an inlet in fluid communication with the outlet of the mixed refrigerant cooler, a second mixed refrigerant cooler having an inlet if fluid communication with an outlet of the mixed refrigerant second compressor or compression stage, said second cooler having an outlet in fluid communication with the liquefaction mixed refrigerant inlet of the pre-cool heat exchanger.

8. The system of claim 7 wherein the pre-cool heat exchanger includes an interstage mixed refrigerant inlet and an interstage mixed refrigerant outlet, and wherein the mixed refrigerant compressor has an outlet in fluid communication with the interstage mixed refrigerant inlet of the pre-cool heat exchanger and the interstage mixed refrigerant outlet of the pre-cool heat exchanger is in fluid communication with an interstage accumulator having a vapor outlet in fluid communication with the inlet of the second compressor or second compression stage and a liquid outlet in fluid communication with the primary refrigeration passage of the liquefaction heat exchanger.

9. The system of claim 1 wherein the high pressure accumulator includes a liquid outlet and the liquefaction heat exchanger further comprises a high pressure liquid cooling passage having an inlet in fluid communication with the liquid outlet of the high pressure accumulator and an outlet in fluid communication with the primary refrigeration passage of the liquefaction heat exchanger.

10. The system of claim 1 wherein the pre-cool refrigerant is propane, butane, ammonia or a chlorofluorocarbon.

11. The system of claim 1 wherein the pre-cool refrigerant is a mixed refrigerant.

12. The system of claim 11 wherein the pre-cool refrigerant heat exchanger is a plate and fin heat exchanger.

13. A method for cooling a feed gas stream comprising the steps of:

a. pre-cooling the feed gas stream in a pre-cool heat exchanger using a first refrigerant to form a pre-cooled feed gas stream;

b. further cooling the pre-cooled feed gas stream by: i) cooling a high pressure second refrigerant stream in the pre-cool heat exchanger to form a cooled high pressure second refrigerant stream ii) separating the cooled high pressure second refrigerant stream to form a high pressure vapor stream and a high pressure liquid stream; iii) cooling the high pressure vapor stream in a liquefaction heat exchanger, to form a mixed phase stream; iv) separating the mixed phase stream with a cold vapor separator to form a cold separator vapor stream and a cold separator liquid stream; v) condensing the cold separator vapor stream in the liquefaction heat exchanger using the second refrigerant and flashing, to form a cold temperature refrigerant stream; vi) directing the cold temperature refrigerant stream to the liquefaction heat exchanger; vii) subcooling the high pressure liquid stream to form a subcooled high pressure liquid stream and combining with the cold temperature refrigerant stream in the liquefaction heat exchanger; viii) subcooling the cold separator liquid stream to form a subcooled cold separator liquid stream and combining with the cold temperature refrigerant stream in the liquefaction heat exchanger; and ix) thermally contacting the pre-cooled gas stream in the liquefaction heat exchanger with the cold temperature refrigerant stream.

14. The method of claim 13 wherein the high pressure liquid stream and the cold separator liquid stream are subcooled in the liquefaction heat exchanger.

15. The method of claim 13 wherein step b. further comprises the steps of cooling a low pressure mixed refrigerant stream in the pre-cool heat exchanger, separating the cooled low pressure mixed refrigerant stream to form a low pressure mixed refrigerant vapor stream and a low pressure mixed refrigerant liquid stream, compressing the low pressure mixed refrigerant vapor stream to form a high pressure mixed refrigerant steam and then cooling the high pressure mixed refrigerant stream to form the cooled high pressure mixed refrigerant stream and directing the low pressure mixed refrigerant liquid stream to the liquefaction heat exchanger.

16. The method of claim 15 wherein the high pressure mixed refrigerant stream is cooled in the pre-cool heat exchanger to form the cooled high pressure mixed refrigerant stream.

17. The method of claim 15 wherein the high pressure mixed refrigerant stream is cooled in both the pre-cool heat exchanger and the liquefaction heat exchanger to form the cooled high pressure mixed refrigerant stream.

18. The method of claim 13 wherein the pre-cool refrigerant is propane, butane, ammonia or a chlorofluorocarbon.

19. The method of claim 13 wherein the pre-cool refrigerant is a mixed refrigerant.

20. The method of claim 13 wherein step a. includes a first pre-cooling stage using a warm pre-cool heat exchanger and a second pre-cooling stage using a cold pre-cool heat exchanger.

21. A system for cooling a feed gas with a mixed refrigerant comprising:

a. a pre-cool heat exchanger having a pre-cool refrigerant inlet configured to receive a stream of pre-cool refrigerant and a pre-cool refrigerant outlet and a liquefaction mixed refrigerant inlet and a liquefaction mixed refrigerant outlet, said pre-cool heat exchanger configured to use the pre-cool refrigerant to cool liquefaction mixed refrigerant passing through the pre-cool heat exchanger between the liquefaction mixed refrigerant inlet and outlet;

b. a liquefaction heat exchanger including a liquefying passage configured to receive a stream of the feed gas, a primary refrigeration passage, a high pressure vapor cooling passage and a cold separator vapor cooling passage, where the cold separator vapor cooling passage has an outlet in fluid communication with the primary refrigeration passage;

c. a mixed refrigerant compression system including:

i) a mixed refrigerant compressor having an inlet in fluid communication with an outlet of the primary refrigeration passage;

ii) a mixed refrigerant cooler having an inlet in fluid communication with an outlet of the mixed refrigerant compressor, said mixed refrigerant cooler having an outlet in fluid communication with the liquefaction mixed refrigerant inlet of the pre-cool heat exchanger,

iii) a high pressure accumulator having an inlet in fluid communication with the liquefaction mixed refrigerant outlet of the pre-cool heat exchanger and a vapor outlet in fluid communication with an inlet of the high pressure vapor cooling passage of the liquefaction heat exchanger;

d. a cold vapor separator having an inlet in fluid communication with an outlet of the high pressure vapor cooling passage of the liquefaction heat exchanger, a vapor outlet in fluid communication with an inlet of the cold separator vapor cooling passage of the liquefaction heat exchanger and a liquid outlet in communication with the primary refrigeration passage of the liquefaction heat exchanger.

22. The system of claim 21 wherein the pre-cool heat exchanger also includes a feed gas inlet adapted to receive a feed gas stream and a feed gas outlet and said pre-cool heat exchanger is configured to use the pre-cool refrigerant to cool feed gas passing through the pre- cool heat exchanger between the feed gas inlet and the feed gas outlet.

23. The system of claim 21 wherein the pre-cool heat exchanger includes a plurality of heat exchangers connected in series or parallel.

24. The system of claim 21 wherein the pre-cool refrigerant is selected from the group consisting of propane, butane, ammonia, water, water glycol mix, lithium bromide solution, liquid fluorinated hydrocarbons and liquid hydrocarbons.

25. The system of claim 21 further comprising a pump configured to pump a stream of pre-cool refrigerant to the pre-cool refrigerant inlet of the pre-cool heat exchanger.

26. The system of claim 21 further comprising a pre-cool refrigerant system selected from the group consisting of a mechanical chiller, an adsorption chiller, a thermoelectric chiller and a thermoacoustic refrigerator wherein the pre-cool refrigerant system is configured to cool the pre-cool refrigerant.

27. A method for cooling a feed gas stream comprising the steps of:

a. directing the feed gas stream into a liquefaction heat exchanger;

b. cooling a high pressure mixed refrigerant stream in a pre-cool heat exchanger to form a cooled high pressure mixed refrigerant stream;

c. cooling the feed gas stream in the liquefaction heat exchanger by: i) separating the cooled high pressure mixed refrigerant stream to form a high pressure vapor stream and a high pressure liquid stream; ii) cooling the high pressure vapor stream in the liquefaction heat exchanger to form a mixed phase stream; iii) separating the mixed phase stream with a cold vapor separator to form a cold separator vapor stream and a cold separator liquid stream; iv) condensing the cold separator vapor stream in the liquefaction heat exchanger and flashing, to form a cold temperature refrigerant stream; v) directing the cold temperature refrigerant stream to the liquefaction heat exchanger; vi) subcooling the high pressure liquid stream in the liquefaction heat exchanger to form a subcooled high pressure liquid stream and combining with the cold temperature refrigerant stream in the liquefaction heat exchanger; vii) subcooling the cold separator liquid stream to form a subcooled cold separator liquid stream and combining with the cold temperature refrigerant stream in the liquefaction heat exchanger; and viii) thermally contacting the gas stream in the liquefaction heat exchanger with the cold temperature refrigerant stream.

28. The method of claim 27 further comprising the step of pre-cooling the feed gas stream in the pre-cool heat exchanger prior to step a.

29. The method of claim 27 wherein step b. is performed using a pre-cool refrigerant that is cooled using a pre-cool refrigerant system selected from the group consisting of a mechanical chiller, an adsorption chiller, a thermoelectric chiller and a thermoacoustic refrigerator.

30. The method of claim 29 wherein the pre-cool refrigerant is cooled to a temperature colder than a temperature that can be achieved by either air cooling or evaporative cooling.