OA21979A - Single Mixed Refrigerant LNG Production Process. - Google Patents

Abstract

A simple and efficient single mixed refrigerant process for cooling and liquefying a hydrocarbon feed stream, such as natural gas. The process employs a closed-loop single mixed refrigerant process for refrigeration duty. The refrigerant compressed to a high pressure using at least three stages of compression and two intercoolers (both producing liquid). A hydraulic turbine is used to expand the high pressure refrigerant before it flows into the main heat exchanger.

Description

SINGLE MIXED REFRIGERANT LNG PRODUCTION PROCESS

BACKGROUND

Production of liquefied natural gas (LNG) through indirect heat exchange against a single mixed réfrigérant (SMR) is well-known in the art. A simple, well-known prior art SMR process is described herein and shown in Figure 3.

There hâve been many attempts to improve the efficiency of SMR processes. For example, U.S. Patent No. 10,139,157 describes a single mixed réfrigérant LNG production cycle in which a single mixed réfrigérant stream is cooled and liquefied in a cryogénie exchanger, before passing through a Joule-Thompson valve. Similarly, U.S. Patent No. 6,334,334 teaches a single mixed réfrigérant LNG production cycle in which two mixed réfrigérant (vapor and liquid) streams are cooled and liquefied separately in cryogénie exchangers before the resulting liquid is passed through a work producing turbine. The resulting liquefied vapor stream passes through a JouleThompson valve. In this process, three compression stages are provided, with liquid produced in the first intercooler being mixed with the discharge of the second stage then cooled in a second intercooler to produce a second liquid and vapor stream, which is subsequently separated. The liquid stream is sent directly to a cryogénie exchanger. In addition, only a portion of the mixed réfrigérant is passed through the work producing turbine.

Many of the attempts to improve the efficiency of the SMR process results in processes that are complex to build and/or operate. Accordingly, there is a need for an improved SMR process that better balances increased efficiency and complexity.

SUMMARY

Disclosed herein is a simple and efficient SMR process that cools and liquéfiés a single high pressure ambient two phase mixed réfrigérant stream in a cryogénie heat exchanger, expands the liquid réfrigérant at the cold end, then vaporizes it in the exchanger to provide réfrigération duty to a natural gas stream being liquefied and a high-pressure mixed réfrigérant stream.

An important feature of the exemplary embodiments disclosed herein a synergistic combination of three stages of compression, two intercoolers (both producing liquid), and the use of a hydraulic turbine to expand the réfrigérant before it flows into the main heat exchanger. Providing three stages of compression (with liquid formation as described) enables a high mixed réfrigérant discharge pressure to be achieved efficiently. The high mixed réfrigérant discharge pressure improved the réfrigération performance of hydraulic turbine and unexpectedly improved the performance of liquéfaction System.

Several aspects of the Systems and methods are outlined below.

Aspect l. A method for liquefying a hydrocarbon stream using a mixed réfrigérant, the method comprising:

(a) cooling and condensing the hydrocarbon stream and a coolcd two-phase high pressure réfrigérant stream in a main heat exchangcr against an expanded réfrigérant stream to form a liquefied hydrocarbon stream, a condensed réfrigérant stream, and a vaporized réfrigérant stream;

(b) compressing the vaporized réfrigérant stream in a first compression stage to a first pressure to form a low pressure compressed réfrigérant stream;

(c) cooling the low pressure compressed réfrigérant stream in a first ambient cooler to form a cooled two-phase réfrigérant stream;

(d) separating the cooled two-phase réfrigérant stream into a first cooled vapor stream and a first cooled liquid stream;

(e) compressing the first cooled vapor stream in a second compression stage to a second pressure to form a medium-pressure compressed stream;

(f) cooling the medium-pressure compressed stream in a second ambient cooler to form a cooled medium-pressure compressed stream;

(g) separating the cooled medium pressure compressed stream into a second cooled vapor stream and a second cooled liquid stream;

(h) compressing the second cooled vapor stream in a third compression stage to a third pressure to form a two-phase high-pressure compressed stream;

(i) cooling a first two-phase high-pressure stream comprising the two-phase highpressure compressed stream in a third ambient cooler to form a cooled two-phase high-pressure compressed stream;

(j) expanding the condensed réfrigérant stream to form the expanded réfrigérant stream, wherein at least a portion of the expansion is performed using a hydraulic turbine.

(k) combining the first cooled liquid stream with a fluid stream that is downstream from the cooled two-phase réfrigérant stream and upstream from the cooled two-phase highpressure compressed stream; and (l) combining the second cooled liquid stream with a fluid stream that is downstream from the cooled two-phase réfrigérant stream and upstream from the cooled two-phase highpressure compressed stream.

Aspect 2. The method of Aspect l, further comprising:

(m) changîng the pressure of the first cooled liquid stream prior to performing step (k).

Aspect 3. The method ofAspect 2, wherein step (m) comprises increasing the pressure of the first cooled liquid stream to the second pressure or third pressure prior to perfonning step (k).

Aspect 4. The method of Aspect l, further comprising:

(n) changing the pressure of the second cooled liquid stream prior to perfonning step (l).

Aspect 5. The method of Aspect 4, wherein step (n) comprises reducing the pressure of the second cooled liquid stream to the first pressure prior to perfonning step (l).

Aspect 6. The method ofAspect 4, wherein step (n) comprises increasing the pressure of the second cooled liquid stream to the third pressure prior to perfonning step (l).

Aspect 7. The method ofAspect l, wherein the expanded réfrigérant stream provides the sole réfrigération duty for step (a).

Aspect 8. The method of Aspect l, wherein flow of the réfrigérant in steps (a) through (!) defines a closed-loop réfrigération cycle and ail of the réfrigérant flows through the hydraulic turbine in step (n).

Aspect 9. The method of Aspect 8, wherein the main heat exchanger comprises a wann end and a cold end and the expanded réfrigérant stream is introduced into the main heat exchanger at the cold end.

Aspect 10. The method ofAspect l, wherein the vaporized réfrigérant stream has a first flow rate in step (b) and the expanded réfrigérant stream has a second flow rate in step (n), the first flow rate being equal to the second flow rate.

Aspect 11. The method ofAspect l, wherein the cooled two-phase high pressure réfrigérant stream has a pressure of at least 1000 PS IA (68.95 bara).

Aspect 12. The method of Aspect 1, wherein the composition of the réfrigérant is the same in the vaporized réfrigérant stream, the two phase high pressure réfrigérant stream, the condensed réfrigérant stream, and the expanded réfrigérant stream.

Aspect 13. A method for liquefying a hydrocarbon stream using a mixed réfrigérant, the method comprising:

(a) cooling and condensing the hydrocarbon stream and a cooled two-phase high pressure réfrigérant stream in a main heat exchanger against an expanded réfrigérant stream to form a liquefied hydrocarbon stream, a condensed réfrigérant stream, and a vaporized réfrigérant stream;

(b) compressing the vaporized réfrigérant stream in a first compression stage to a first pressure to form a low pressure compressed réfrigérant stream;

(c) cooling the low pressure compressed réfrigérant in a first ambient cooler to form a cooled two-phase réfrigérant stream;

(d) separating the coolcd two-phase réfrigérant stream into a first cooled vapor stream and a first cooled liquid stream;

(e) compressing the first cooled vapor stream in a second compression stage to a second pressure to form a medium-pressure compressed stream;

(f) pumping the first cooled liquid stream to the second pressure to form a pumped first cooled liquid stream;

(g) combining the pumped first cooled liquid stream with the medium pressure réfrigérant stream to form a combined medium-pressure réfrigérant stream;

(h) cooling the combined medium-pressure réfrigérant stream in a second ambient cooler to form a cooled combined medium pressure réfrigérant stream;

(i) separating the cooled combined medium pressure réfrigérant stream into a second cooled vapor stream and a second coolcd liquid stream;

(j} compressing the second cooled vapor stream in a third compression stage to a third pressure to form a high-pressure compressed stream;

(k) pumping the second cooled liquid stream to the third pressure to form a pumped second cooled liquid stream;

(l) combining the pumped second cooled liquid stream with the high-pressure compressed stream to form a two-phase high-pressure réfrigérant stream;

(m) cooling the two-phase high-pressure réfrigérant stream in a third ambient cooler to form the cooled two-phase high-pressure réfrigérant stream; and (n) expanding the condensed réfrigérant stream through a hydraulic turbine to form the expanded réfrigérant stream.

Aspect 14. The method of Aspect 13, wherein the expanded réfrigérant stream provides the sole réfrigération duty for step (a).

Aspect 15. The method of Aspect 13, wherein fiow of the réfrigérant in steps (a) through (n) defines a closed-loop réfrigération cycle and ail of the réfrigérant flows through the hydraulic turbine in step (n).

Aspect 16. The method of Aspect 15, wherein the main heat exchanger comprises a warm end and a cold end and the expanded réfrigérant stream is introduced into the main heat exchanger at the cold end.

Aspect 17. The method of Aspect 13, wherein the vaporized réfrigérant stream has a first flow rate in step (b) and the expanded réfrigérant stream has a second fiow rate in step (n), the first fiow rate being equal to the second fiow rate.

Aspect 18. The method of Aspect 13, wherein the cooled two-phase high pressure réfrigérant stream has a pressure of at least 1000 PSIA (68.95 bara).

Aspect 19. The method of Aspect 13, wherein the composition of the réfrigérant is the same in the vaporized réfrigérant stream, the two phase high pressure réfrigérant stream, the condensed réfrigérant stream, and the expanded réfrigérant stream.

Aspect 20. The method of Aspect 13, wherein the main heat exchanger comprises a wann bundle and a cold bundle and the method further comprises:

(o) providing a first réfrigération duty in the wann bundle when perfonriing step (a);

(p) providing a second réfrigération duty in the cold bundle when perfonning step (a), the second réfrigération duty being less than the first réfrigération duty.

Aspect 21. The method of Aspect 20, wherein the wann bundle and the cold bundle are each contained within separate shells.

Aspect 22. The method of Aspect 20, wherein the main heat exchanger further comprises a middle bundle and the method further comprises:

(q) providing a third réfrigération duty in the middle bundle when perfonning step (a), the third réfrigération duty being less than the first réfrigération duty.

Aspect 23. The method of Aspect 22, wherein the wann bundle, the cold bundle, and the middle bundle are each contained within separate shells.

Aspect 24. The method of Aspect 13, wherein the hydrocarbon stream comprises natural gas.

Aspect 25. The method of Aspect 13, wherein step (i) further comprises selectively expanding the condensed réfrigérant stream through an expansion valve located on a bypass circuit instead of through the hydraulic turbine.

Aspect 26. A method for liquefying a hydrocarbon stream using a mixed réfrigérant, the method comprising:

(a) cooling and condensing the hydrocarbon stream and a cooled two-phase high pressure réfrigérant stream in a main heat exchanger against an expanded réfrigérant stream to fonn a liquefied hydrocarbon stream, a condensed réfrigérant stream, and a vaporized réfrigérant stream;

(b) expanding the condensed réfrigérant stream to fonn the expanded réfrigérant stream, wherein at least a portion of the expansion is performed using a hydraulic turbine;

(c) compressing the vaporized réfrigérant stream in a first compression stage to a first pressure to fonn a low pressure compressed réfrigérant stream;

(d) cooling the low pressure compressed réfrigérant stream in a first ambient cooler to fonn a cooled two-phase réfrigérant stream;

(e) separating a combined cooled two-phase réfrigérant stream into a first cooled vapor stream and a first cooled liquid stream;

(f) compressing the first cooled vapor stream in a second compression stage to a second pressure to form a medium-pressure compressed stream;

(g) pumping the first cooled liquid stream to a third pressure to fomi a pumped first cooled liquid stream;

(h) cooling the medium-pressure compressed stream in a second ambient cooler to fonn a cooled medium-pressure compressed stream;

(i) separating the cooled medium pressure compressed stream into a second cooled vapor stream and a second cooled liquid stream;

(j) compressing the second cooled vapor stream in a third compression stage to the third pressure to fonn a two-phase high-pressure compressed stream;

(k) combining the pumped first cooled liquid stream with the two-phase high-pressure compressed stream to fonn a combined two-phase high-pressure compressed stream;

(l) cooling the combined two-phase high-pressure compressed stream in a third ambient cooler to form the cooled two-phase high-pressure compressed stream;

(m) expanding the second cooled liquid stream through an expansion valve to fonn an expanded cooled stream; and, (n) combining the expanded cooled stream with the cooled two-phase réfrigérant stream to fonn the combined cooled two-phase réfrigérant stream.

Aspect 27. The method of Aspect 26, wherein the expansion of step (b) is provided by a hydraulic turbine followed by an expansion valve.

Aspect 28. The method of Aspect 26, wherein the second compression stage opérâtes at a température of approximately 96.8° F.

Aspect 29. The method of Aspect 26, wherein the expanded réfrigérant stream provides the sole réfrigération duty for step (a).

Aspect 30. The method of Aspect 26, wherein the flow of the réfrigérant in steps (a) through (n) defines a closed loop réfrigération cycle and ail of the réfrigérant flows through the hydraulic turbine in step (l).

Aspect 31. The method of Aspect 26, wherein the main heat exchanger comprises a warm bundle and a cold bundle contained within separate shells.

Aspect 32. The method of Aspect 26, wherein the main heat exchanger additionally comprises a middle bundle located between the warm bundle and the cold bundle.

Aspect 33. The method of Aspect 26, wherein the hydrocarbon stream comprises natural gas.

Aspect 34. The method of Aspect 26, wherein the main heat exchanger comprises a warm end and a cold end and the expanded réfrigérant sircam is introduced into the main heat exchanger at the cold end.

Aspect 35. A method ofdesigning and fabricating a System for liqucfying natural gas using a closed loop single mixed réfrigérant process that supplies réfrigération duty to a cryogénie heat exchanger having a plurality of coil wound bundles, each of the plurality of coil wound bundles having an overall tube length, the method comprising:

(a) selecting a réfrigération duty for each of a plurality of coil wound bundles that minimizes différences in the overall tube length of each of the plurality of coil wound bundles; and (b) fabricating the System to provide the réfrigération duties selected in step (a) wherein the sole réfrigération duty for the cryogénie heat exchanger is a stream of the single mixed réfrigérant that has been compressed to a pressure of at least I000 PSIA (6S.95 bara) and expanded by a hydraulic turbine.

Aspect 36. The method of Aspect 35, wherein the plurality of coil wound bundles comprises a warm bundlc and a cold bundle, the selected réfrigération duty of the warm bundle being less than the selected réfrigération duty of the cold bundle.

BR1EF DESCRIPTION OF THE DRAWING(S)

The présent invention will hereinafter be described in conjunction with the appended drawing figures wherein like numerals dénoté like éléments.

Figure l is a schematic flow diagram dcpicting an improved SMR process.

Figure 2 is a schematic flow diagram depicting the improved SMR process of Figure l, modified to include a coil-wound heat exchanger with multiple shells.

Figure 3 is a schematic flow diagram depicting an alternative improved SMR process modified to include an expansion valve to reduce the température of fluids during the process and reduce power requirements.

Figure 4 is a schematic flow diagram depicting the prior art PRICO® LNG process.

Figure 5 is a table comparing key parameters from the prior art process shown in Figure 4 with several variants of exemplary embodiments of the présent invention shown in Figures l through 3.

DETAILED DESCRIPTION OFTHE PREFERRED EMBODIMENTS

The ensuing délailed description provides preferred exemplary embodiments only, and is not intended to limit the scope, applicability, or configuration of the invention. Rather, the ensuing 7 dctailed description of the preferred exemplary embodiments will provide those ski lied in the art with an enabling description for implementing the preferred exemplary embodiments of the invention. It being understood that varions changes may be made in the function and arrangement of éléments without departing front the spirit and scope of the invention.

In the claims, letters are used to identify claimcd steps (e.g. (a), (b), and (c)). These letters are used to aid in referring to the method steps and are not intended to indicate the order in which claimed steps are performed, unless and only to the extent that such order is specifically recited in the claims.

In order to aid in describing the invention, directîonal tenns may be used in the spécification and claims to describe portions of the présent invention (e.g., upper, lower, left, right, etc.). These directional tenus are merely intended to assist in describing and claiming the invention and are not intended to limit the invention in any way. In addition, référencé numérale that are introduced in the spécification in association with a drawing figure may be repeated in one or more subséquent figures without additional description in the spécification in order to provide context for other features.

Unless otherwise indicated, the articles “a” and “an” as used herein mean one or more when applied to any feature in embodiments of the présent invention described in the spécification and claims. The use of “a” and “an” does not limit the meaning to a single feature unless such a limit is specifically stated. The article “the” preceding singular or plural nouns or noun phrases dénotés a particular specified feature or particular specified features and may hâve a singular or plural connotation depending upon the context in which it is used.

Unless otherwise stated herein, introducing a stream at a location is intended to mean introducing substantially ail of the said stream at the location. Ail streams discussed in the spécification and shown in the drawings (typically represented by a line with an arrow showing the overall direction of fluid flow during normal operation) should be understood to be contained within a corresponding conduit. Each conduit should be understood to hâve at least one inlet and at least one outlet. Further, each piece of equipment should be understood to hâve at least one inlet and at least one outlet.

The tenu “conduit,” as used in the spécification and claims, refers to one or more structures through which fluids can be transported between two or more components of a System. For example, conduits can include pipes, ducts, passageways, and combinations thereof that transport liquids, vapors, and/or gases.

As used in the spécification and claims, the term “flow communication” is intended to mean that two or more éléments are connected (either directly or indirectly) in a manner that enables fl nids to flow between the éléments, including connections that may contain valves, gates, tees, or other devices that may selectively restrict, merge, or separate fluid flow.

The tenn “natural gas”, as used in the spécification and claims, means a hydrocarbon gas mixture consîsting primarily of méthane.

The terms “hydrocarbon’’, “hydrocarbon gas”, or “hydrocarbon fluid, as used in the spécification and claims, means a gas/fluid comprising at least one hydrocarbon and for which hydrocarbons comprise at least 80%, and more preferably at least 90% of the overall composition of the gas/fluid.

The tenu “mixed réfrigérant”, as used in the spécification and claims, means a mixture of hydrocarbons, typically comprising hydrocarbon components containing one to five carbon atoms and may contain saturated and/or non-saturated components and/or straight chain and/or branched components, and nitrogen.

The tenn “ambient cooler”, as used in the spécification and claims, means a heat exchange device the cools a fluid against an ambient fluid (typically ambient air).

As used in the spécification and claims, the tenns “high-high”, “high”, “medium”, “low”, and “low-Iow” are intended to express relative values for a property of the éléments with which these tenns are used. For example, a high-high pressure stream is intended to indicate a stream having a higher pressure than the corresponding high-pressure stream or medium pressure stream or low pressure stream described or claimed in this application. Similarly, a high-pressure stream is intended to indicate a stream having a higher pressure than the corresponding medium pressure stream or low-pressure stream described in the spécification or claims, but lower than the corresponding high-high pressure stream described or claimed in this application. Similarly, a medium pressure stream is intended to indicate a stream having a higher pressure than the corresponding low-pressure stream described in the spécification or claims, but lower than the corresponding high-pressure stream described or claimed in this application.

Unless otherwise stated herein, any and ail percentages identified in the spécification, drawings and claims should be understood to be on a mass percentage basis. Unless otherwise stated herein, any and ail pressures identified in the spécification, drawings and claims should be understood to mean gauge pressure.

As used in the spécification and claims, the tenn “compression System” is defined as one or more compression stages. For example, a compression System may comprise multiple compression stages within a single compresser. In an alternative example, a compression System may comprise multiple compressors.

As used herein, the term “hydraulic turbine” is intended to refer to a work producing liquid expander. In the context of this invention, the primary purpose of the hydraulic turbine is to 9 provide réfrigération to the process by rcmoving enthalpy from the réfrigérant stream, and the work produced may be recovered using an electrical gcnerator, used to compress another fluid, or simply dissipated as heat released to the surroundings.

Figure I shows a single mixed réfrigérant (SMR) natural gas liquéfaction process. in this exemplary process, a feed gas stream 100 and a two-phase high-pressurc réfrigérant stream 128 is cooled against an expanded réfrigérant stream 136. For the processes of Figure l and 2, the term “réfrigérant” should be understood to mean a mixed réfrigérant.

In this example, the feed gas stream 100 is natural gas, which is preferably pre-treated to remove water, acid gases (carbon dioxide and sulfur dioxide) and freezable hcavy hydrocarbons. The feed gas stream 100 is preferably near ambient température or may hâve been pre-cooled by another process using known réfrigération techniques (fluid boiling, gas expansion etc.). Typically, the feed gas stream 100 enters a warm end 160 of the cryogénie heat exchanger 130 at a pressure of40 bara to 80 bara, then exits a cold end 161 of the cryogénie heat exchanger 130 as a product stream 102 in liquid phase, at a température of typically between -140 degrees C and 150 degrees C. The product stream 102 preferably passes through pressure réduction device 138 which may be a Joule-Thompson valve or a work producing hydraulic turbine before being sent to storage (not shown).

In this exemplary process, the cryogénie heat exchanger 130 consists of a single shell 131. Examples of suitable types of heat exchanger types for the cryogénie heat exchanger 130 include a plate and fin heat exchanger or a coil wound heat exchanger. In the case of a coil wound heat exchanger, the expanded réfrigérant stream 136 flows through a shell side of the cryogénie heat exchanger 130. If the cryogénie heat exchanger 130 is a plate fin-style exchanger, it may be désirable to employ devices to ensure even distribution of two-phase high-pressure réfrigérant stream 128 among parallel passages and/or exchangers using techniques well known in the art, such as a phase separator liquid pumps and the like.

The feed gas stream 100 could optionally be removed from the cryogénie heat exchanger 130 at an intermediate location (stream 103) and sent to a séparation device 150 for heavy hydrocarbon removal, then a predominately méthane stream 105 is reintroduced into cryogénie heat exchanger130.

After providing réfrigération duty, a vapor réfrigérant stream 104 is withdrawn from the warm end 160 of the cryogénie heat exchanger 130. The vapor réfrigérant stream 104 is préférable at near-ambient température and at a pressure of 3-5 bara. The vapor réfrigérant stream 104 is then compressed in compresser stage 106 to a pressure typically from 10 to 20 bara, forming a low-pressure réfrigérant stream 107. The low-pressure réfrigérant stream 107 is then cooled by ambient cooler 108, using cooling water or air. The resulting cooled two-phase réfrigérant stream 10

109 is separated into a liquid stream I 13 and a vapor stream 111, using a separator 110. The vapor stream 111 is compressed (via compression stage l14)to a pressure typically between 25 and 30 bara to form a medium pressure réfrigérant stream 115. The liquid stream 113 is pumped (using pump 112) to suhstantially the same pressure as the medium pressure réfrigérant stream 115 then combined with the medium pressure réfrigérant stream 115. The combined stream H7 is then cooled by ambient cooler 116 using eooling water or air.

The resulting two-phase réfrigérant stream 118 is separated into a liquid stream 123 and a vapor stream I2l using a separator 120. The vapor stream I2l is further compressed (via a compression stage 124) to a pressure typically between 40 and 70 bara for fonn a high-pressure réfrigérant stream 125. The liquid stream 123 is pumped (via pump 122) to substantially the same pressure as the high-pressure réfrigérant stream 125, then recombined with the high pressure réfrigérant stream 125. The combined stream 127 is then cooled by ambient cooler I26 using eooling water or air to fonn the two-phase high-pressure réfrigérant stream 128.

The two-phase high-pressure réfrigérant stream 128 is then cooled and condensed in the cryogénie heat exchanger 130, exiting as a condensed réfrigérant stream 132 in liquid phase and at a température of typically between -140 degrees C and -150 degrees C. The condensed réfrigérant stream 132 is sent to a hydraulic turbine 134 and expanded to fonn an expanded réfrigérant stream 136. Optionally, the hydraulic turbine 134 may hâve a single-phase liquid outlet followed by a valve or may include a work producing liquid expander with a two-phase outlet.

An optîonal bypass circuit 139 with an expansion valve 137 (such as a Joule-Thompson valve) could be provided to enable the System to continue to function when the hydraulic turbine 134 is being serviced or in the case of a failure. In addition, an optional expansion valve 162 (such as a Joule-Thompson valve) could be provided downstream from the hydraulic turbine 134 to provide for further expansion of the expanded réfrigérant stream 136. The optional bypass circuit 139 and optional expansion valve 162 could also be included in the embodiment shown in Figure 2.

The expanded réfrigérant stream 136 is then introduced into the cold end I6l of the cryogénie heat exchanger 130, vaporîzed (typically at a pressure of 3-5 bara) (providing réfrigération duty for the process), and exits the wann end 160 as the vapor réfrigérant stream 104.

In Figure 2, ail items are labeled with référencé numerals of the format 2XX. Unless otherwise described herein, éléments in Figure 2 with a reference numéral having the same last two digits as an elcment in Figure l should be understood to be substantially the same as the corresponding element of Figure l. For example, the separator 210 of Figure 2 is substantially the same as the separator 110 of Figure l.

H

The process of Figure 2 is very similar to the process of Figure l, with the exception ofthe cryogénie heatexchanger I30of Figure l being replaced by a cryogénie heat exchanger 230 having three shells 230a, 230b and 230c, each arranged in sériés and each containing a coil wound tube bundle (not shown). The duty may be split between two or three coil wound exchangers in separate shells with interconnecting piping (253-257) as shown. Altematively, the cryogénie heat exchanger 230 could be built with two or three coil wound bundles in a common shell. Splitting the service may help to keep the individual bundles within manufacturing limitations, for example limitations on the maximum length to stay within the capability of current manufacturing facilities. The structure ofthe cryogénie heat exchanger 230 enables a réfrigération duty to be split unevenly between the bundles and selected in a manner that reduces manufacturing time.

It ail of the shells 230a, 230b and 230c and bundles of the cryogénie heat exchanger 230 are fabricated at the same time, overall fabrication time is dictated by the bundle that requires the most time to fabricate. If réfrigération duty is split equally among the bundles contained within each ofthe shells 230a, 230b and 230c, then the bundle contained within the shell 230a would take longer to fabricate than the bundles contained within shells 230b and 230c. The feed gas stream 200 and the two phase high pressure réfrigérant stream 228 hâve a relatively high vapor fraction and relatively low density when flowing through shell 230a, as compared to those same streams when flowing through shell 230b or 230c. This is due to a density increase as the streams 200, 228 are cooled and condensed. This results in the bundle of shell 230a requiring more tubes than the bundles of shell 230b or 230c. If réfrigération duty is shifted from shell 230a to shells 230b and 230c by making the bundle contained within shell 230a shorter and increasing the length of the bundles contained within shells 230b and 230c, the same pressure drop can be achieved with fewer tubes because the length of the tubes is reduced. This reduces the fabrication time of the bundle for shell 230a, and therefore, the overall manufacturing time.

For example, for the exemplary embodiment shown in Figure 2, the duty may be split such that shell 230a has 27% of the total duty, shell 230b lias 35% of the total duty and shell 230c has 38% of the total duty. In general, for a SMR System having three heat exchanger shells, the duty split preferably results in the warmest shell (shell 203a in Figure 2) having less than 30% ofthe total réfrigération duty of the heat exchanger. Similarly, for an SMR System having only two heat exchanger shells, the duty split preferably results in the warmest shell having less than 45% ofthe total réfrigération duty of the heat exchanger.

In Figure 3, ail items are labeled with reference numerals of the format 3XX. Unless otherwise described herein, éléments in Figure 3 with a référencé numéral having the same last two digits as an element in Figure l or Figure 2 should be understood to be substantially the same as the corresponding element of Figure l or Figure 2. For example, the phase separator 310 of 12

Figure 3 is substantially the same as the phase separator 110 of Figure ! and the phase separator 210 of Figure 2.

The process of Figure 3 is very similar to the process of Figure 2. After providing réfrigération duty, a vapor réfrigérant stream 304 is withdrawn front the warm end of the cryogénie heat exchanger 330 and is then compressed in compressor stage 306 to form a low-pressure compressed réfrigérant stream 319. It should be noted that the wami end of the heat exchanger 330 is at the top end of the heat exchanger 330 in this example. In some other applications, such as a braised aluminum heat exchanger (“BAHX”), the warm end would be at the bottom end. The lowpressure compressed réfrigérant stream 319 is then cooled by ambient cooler 308. The resulting cooled two-phase réfrigérant stream 309, which is combined with an expanded cooled stream 354 (discussed herein), then separated into a first cooled liquid stream 313 and a first cooled vapor stream 311 using a phase separator 310. Both phases within the phase separator 310 are below ambient température. The vapor stream 311 is compressed via compression stage 314 to form a medium pressure compressed stream 315. The first cooled liquid stream 313 is pumped using pump 3 12 to substantially the same pressure as a high pressure compressed stream 331 (discussed herein) to form a high-pressure cooled liquid stream high-pressure compressed stream. The medium pressure compressed stream 315 is then cooled with an ambient cooler 316 to form a cooled medium pressure compressed stream 318, then separated into a liquid stream 323 and a vapor stream 325 using a phase separator 320. The vapor stream 325 is compressed via a compression stage 324 to form a two-phase high-pressure compressed stream 331.

The liquid stream 323 is cooled and expanded through a suitable expansion valve 352, e.g., a Joule-Thompson valve, to form the expanded cooled stream 354, at least some of which is vapor. The expanded two-phase stream 354 is mixed with the two-phase réfrigérant stream 309 upstream oi the phase separator 310. This mixîng reduces the température of the two-phase réfrigérant stream 309 entering the phase separator 310 which, in tum, reduces the power requirement of second compressor stage 3 14 and total power required to operate the System. This arrangement also requires one less pump than the illustrative examples of Figures l and 2.

In this illustrative example, the high-pressure cooled liquid stream 322 is combined with the high-pressure réfrigérant stream 331 downstream from the compressor stage 324, and the combined stream is then cooled by ambient cooler 326 to form the two-phase high pressure réfrigérant stream 328, which is then cooled and condensed in the cryogénie heat exchanger 330.

In Figure 4, ail items arc labeled with référencé numerals of the format 4 XX. Unless otherwise described herein, éléments in Figure 4 with a référencé numéral having the same last two digits as an element in Figure l or Figure 2 should be understood to be substantially the same as the corresponding élément of Figure l or Figure 2. For example, the phase separator 410 of Figure 4 is substantially the saine as the phase separator 110 of Figure l or 210 of Figure 2.

Figure 4 shows the PRICCT LNG process, which is a prior art single mixed réfrigérant process that is often used in small-scale and mid-scale LNG plants. In this process, two compression stages 406, 414 and a single intercooler 408 and drum 410 are used. Liquid 413 from the drum 410 is pumped (using pump 412), then mixed with the vapor stream 411 from the final compression stage 414 before being coolcd in an aftercooler 416. A Joule-Thompson valve 434 is used to expand liquefied mixed réfrigérant stream 432 before it is sent to exchanger 430 where it is vaporized to provide réfrigération. This process is provided to form a basis for comparing performance with the processes disclosed herein in the examples provided below.

EXAMPLE

Figure 5 is a table that compares, for a fixed production rate, the prior art process shown in Figure 4 with several variants of exemplary embodiments shown in Figures l and 3. The data in Figure 5 was generated in a process simulator with the operating parameters of pressures, températures and mixed réfrigérant composition selected using a numerical optimization program. Design assumptions such as LNG production rate, ambient température, pressure drops, exchanger minimum approach températures, and compressor efficiencies were the same for ail five examples. The relative power value for each configuration is a ratio of the total power required to operate the System in each configuration to the power required to operate the System using the embodiment shown in Figure 4.

As shown in columns l and 2, there is a 6.0% benefit (i.e., réduction in total power requirement) to adding a hydraulic turbine to the prior art process of Figure 4. A comparison of columns I and 3 shows a 5.2% benefit to adding a third compression stage, second intercooler and second pump to the prior art process of Figure 4. Based on these results, the total expected benefit of the process of Figure l versus that of Figure 4 would be expected to be 11.2%. The actual improvement was 13.2% — significantly greater than expected. This unexpected resuit is believed to be due to the synergistic effect of providing a higher mixed réfrigérant discharge pressure (due to the additional compression stage I24) to the hydraulic turbine 134. A comparison of the prior art process of Figure 4 (column l) and the process of Figure 3 (column 5) shows a 15.6% benefit by adding the hydraulic turbine 334, the third compression stage 324, the second intercooler 316, the second pump 312, and the expansion valve 352 to the prior art process of Figure 4. The suction température of the second compression stage 3 14 is reduced due to the Joule-Thompson effect of liquid from phase separator 320 therein reducing the total power required to operate the System. These modeled calculations did not account for any recovery of work performed by the hydraulic turbine 134. Accordingly, it is possible that additional benefit could be realized from the 14 embodiments that include a hydraulic turbine.

As such, an invention has been disclosed in terrns of preferred embodiments and altemate embodiments thereof. Of course, various changes, modifications, and alterations from the teachings of the présent invention may be contemplated by those skilled in the art without 5 departing from the intended spirit and scope thereof It is intended that the présent invention only be limited by the ternis of the appended claims.

Claims (25)

1. l. A method for liquefying a hydrocarbon stream using a mixed réfrigérant, the method comprising:

   (a) cooling and condensing the hydrocarbon stream and a cooled two-phase high pressure réfrigérant stream in a main heat exchanger against an expanded réfrigérant stream to form a liquefied hydrocarbon stream, a condensed réfrigérant stream, and a vaporized réfrigérant stream;

   (b) compressing the vaporized réfrigérant stream in a first compression stage to a first pressure to form a low pressure compressed réfrigérant stream;

   (c) cooling the low pressure compressed réfrigérant stream in a first ambient cooler to form a cooled two-phase réfrigérant stream;

   (d) separating the cooled two-phase réfrigérant stream into a first cooled vapor stream and a first cooled liquid stream;

   (e) compressing the first cooled vapor stream in a second compression stage to a second pressure to form a medîum-pressure compressed stream;

   (f) cooling the medium-pressure compressed stream in a second ambient cooler to form a cooled medium-pressure compressed stream;

   (g) separating the cooled medium pressure compressed stream into a second cooled vapor stream and a second cooled liquid stream;

   (h) compressing the second cooled vapor stream in a third compression stage to a third pressure to form a two-phase high-pressure compressed stream;

   (i) cooling a first two-phase high-pressure stream comprising the two-phase highpressure compressed stream in a third ambient cooler to form a cooled two-phase high-pressure compressed stream;

   (j) expanding the condensed réfrigérant stream to form the expanded réfrigérant stream, wherein at least a portion of the expansion is performed using a hydraulic turbine.

   (k) combining the first cooled liquid stream with a fluid stream that is downstream from the cooled two-phase réfrigérant stream and upstream from the cooled tw^ro-phase highpressure compressed stream; and (1) combining the second cooled liquid stream with a fluid stream that is downstream from the cooled two-phase réfrigérant stream and upstream from the cooled two-phase highpressure compressed stream.
2. 2. The method of claim 1, further comprising:

   (m) changing the pressure of the first cooled liquid stream prier to perfbrming step (k), wherein step (m) comprises increasing the pressure of the first cooled liquid stream to the second pressure or third pressure prior to performing step (k).
3. 3. The method of claim l, further comprising:

   (n) changing the pressure of the second cooled liquid stream prior to performing step (l), wherein step (n) comprises reducing the pressure of the second cooled liquid stream to the first pressure prior to performing step (l), or wherein step (n) comprises increasing the pressure of the second cooled liquid stream to the third pressure prior to performing step (l).
4. 4. The method of claim l, wherein the expanded réfrigérant stream provides the sole réfrigération duty for step (a).
5. 5. The method of claim l, wherein flow of the réfrigérant in steps (a) through (l) defines a closed-loop réfrigération cycle and ail of the réfrigérant flows through the hydraulic turbine in step (n, wherein the main heat exchanger comprises a warm end and a cold end and the expanded réfrigérant stream is introduced into the main heat exchanger at the cold end.
6. 6. The method of claim l, wherein the vaporized réfrigérant stream has a first flow rate in step (b) and the expanded réfrigérant stream has a second flow rate in step (n), the first flow rate being equal to the second flow rate.
7. 7. The method of claim l, wherein the cooled two-phase high pressure réfrigérant stream has a pressure of at least 1000 PSIA (68.95 bara).
8. 8. The method of claim I, wherein the composition of the réfrigérant is the same in the vaporized réfrigérant stream, the two phase high pressure réfrigérant stream, the condensed réfrigérant stream, and the expanded réfrigérant stream.
9. 9. A method for liquefying a hydrocarbon stream using a mixed réfrigérant, the method comprising:

   (a) cooling and condensing the hydrocarbon stream and a cooled two-phase high pressure réfrigérant stream in a main heat exchanger against an expanded réfrigérant stream to form a liquefied hydrocarbon stream, a condensed réfrigérant stream, and a vaporized réfrigérant stream;

   (b) compressîng the vaporized réfrigérant stream in a first compression stage to a first pressure to form a low pressure compressed réfrigérant stream;

   (c) eooling the low pressure compressed réfrigérant in a first ambient cooler to form a cooled two-phase réfrigérant stream;

   (d) separating the cooled two-phase réfrigérant stream into a first cooled vapor stream and a first cooled liquid stream;

   (e) compressîng the first cooled vapor stream in a second compression stage to a second pressure to form a medium-pressure compressed stream;

   (f) pumping the first cooled liquid stream to the second pressure to form a pumped first cooled liquid stream;

   (g) combining the pumped first cooled liquid stream with the medium pressure réfrigérant stream to form a combined medium-pressure réfrigérant stream;

   (h) eooling the combined medium-pressure réfrigérant stream in a second ambient cooler to form a cooled combined medium pressure réfrigérant stream;

   (i) separating the cooled combined medium pressure réfrigérant stream into a second cooled vapor stream and a second cooled liquid stream;

   (j) compressîng the second cooled vapor stream in a third compression stage to a third pressure to form a hîgh-pressure compressed stream;

   (k) pumping the second cooled liquid stream to the third pressure to form a pumped second cooled liquid stream;

   (l) combining the pumped second cooled liquid stream with the high-pressure compressed stream to form a two-phase high-pressure réfrigérant stream;

   (m) eooling the two-phase high-pressure réfrigérant stream in a third ambient cooler to form the cooled two-phase high-pressure réfrigérant stream; and (n) expanding the condensed réfrigérant stream through a hydraulic turbine to form the expanded réfrigérant stream.
10. 10. The method of claim 9, wherein the expanded réfrigérant stream provides the sole réfrigération duty for step (a).
11. 11. The method of claim 9, wherein flow of the réfrigérant in steps (a) through (n) defines a closed-loop réfrigération cycle and ail of the réfrigérant flows through the hydraulic turbine in step (n), wherein the main heat exchanger comprises a warm end and a cold end and the expanded réfrigérant stream is introduced into the main heat exchanger at the cold end.

    I2. The method of claim 9, wherein the vaporized réfrigérant stream has a first flow rate in step (b) and the expanded réfrigérant stream has a second flow rate in step (n), the first flow rate being equal to the second flow rate.

    I 3. The method of claim 9, wherein the cooled two-phase high pressure réfrigérant stream has a pressure of at least 1000 PSIA (68.95 bara).
12. 14. The method of claim 9, wherein the composition of the réfrigérant is the same in the vaporized réfrigérant stream, the two phase high pressure réfrigérant stream, the condensed réfrigérant stream, and the expanded réfrigérant stream.
13. 15. The method of claim 9, wherein the main heat exchanger comprises a warm bundle and a cold bundle and the method further comprises:

    (o) providing a first réfrigération duty in the warm bundle when performing step (a);

    (p) providing a second réfrigération duty in the cold bundle when performing step (a), the second réfrigération duty being less than the first réfrigération duty.

    , wherein the warm bundle and the cold bundle are each contained within separate shells and , wherein the main heat exchanger further comprises a middle bundle and the method further comprises:

    (q) providing a third réfrigération duty in the middle bundle when performing step (a), the third réfrigération duty being less than the first réfrigération duty, wherein the warm bundle, the cold bundle, and the middle bundle are each contained within separate shells,
14. 16. The method of claim 9, wherein the hydrocarbon stream comprises natural gas.
15. 17. The method of claim 9, wherein step (i) further comprises selectively expanding the condensed réfrigérant stream through an expansion valve located on a bypass circuit instead of through the hydraulic turbine.
16. 18. A method for liquefying a hydrocarbon stream using a mixed réfrigérant, the method comprising:

    (a) cooling and condensing the hydrocarbon stream and a cooled two-phase high pressure réfrigérant stream in a main heat exchanger against an expanded réfrigérant stream to fonn a liquefied hydrocarbon stream, a condensed réfrigérant stream, and a vaporized réfrigérant stream;

    (b) expanding the condensed réfrigérant stream to form the expanded réfrigérant stream, wherein at least a portion of the expansion is performed using a hydraulic turbine;

    (c) compressing the vaporized réfrigérant stream in a first compression stage to a first pressure to form a low pressure compressed réfrigérant stream;

    (d) cooling the low pressure compressed réfrigérant stream in a first ambient cooler to form a cooled two-phase réfrigérant stream;

    (e) séparaiing a combined cooled two-phase réfrigérant stream into a first cooled vapor stream and a first cooled liquid stream;

    (f) compressing the first cooled vapor stream in a second compression stage to a second pressure to form a medium-pressure compressed stream;

    (g) pumping the first cooled liquid stream to a third pressure to form a pumped first cooled liquid stream;

    (h) cooling the medium-pressure compressed stream in a second ambient cooler to form a cooled medium-pressure compressed stream;

    (i) separating the cooled medium pressure compressed stream into a second cooled vapor stream and a second cooled liquid stream;

    (j ) compressing the second cooled vapor stream in a third compression stage to the third pressure to form a two-phase high-pressure compressed stream;

    (k) combining the pumped first cooled liquid stream with the two-phase highpressure compressed stream to fonn a combined two-phase high-pressure compressed stream;

    (I) cooling the combined two-phase high-pressure compressed stream in a third ambient cooler to fonn the cooled two-phase high-pressure compressed stream;

    (m) expanding the second cooled liquid stream through an expansion valve to form an expanded cooled stream; and, (n) combining the expanded cooled stream with the cooled two-phase réfrigérant stream to fonn the combined cooled two-phase réfrigérant stream.
17. 19. The method of claim 18, wherein the expansion of step (b) is provided by a hydraulic turbine followed by an expansion valve.
18. 20. The method of claim 18, wherein the second compression stage opérâtes at a température of approximately 96.8° F.

    2I. The method of claim I 8, wherein the expanded réfrigérant stream provides the sole réfrigération duty for step (a).
19. 22. The method of claim 18, wherein the tlow of the réfrigérant in steps (a) through (n) defines a closed loop réfrigération cycle and ail of the réfrigérant flows through the hydraulic turbine in step (l).
20. 23. The method of claim 18, wherein the main heat exchanger comprises a wann bundle and a cold bundle contained within separate shells.
21. 24. The method of claim 18, wherein the main heat exchanger additionally comprises a middle bundle located between the wann bundle and the cold bundle.
22. 25. The method of claim 18, wherein the hydrocarbon stream comprises natural gas.
23. 26. The method of claim 18, wherein the main heat exchanger comprises a wann end and a cold end and the expanded réfrigérant stream is introduced into the main heat exchanger at the cold end.
24. 27. A method of designing and fabricating a System for liquefying natural gas using a closed loop single mixed réfrigérant process that supplies réfrigération duty to a cryogénie heat exchanger having a plurality of coil wound bundles, each of the plurality of coil wound bundles having an overall tube length, the method comprising:

    (a) selectîng a réfrigération duty for each of a plurality of coil wound bundles that minimizes différences in the overall tube length of each of the plurality of coil wound bundles; and (b) fabricating the System to provide the réfrigération duties selected in step (a) wherein the sole réfrigération duty for the cryogénie heat exchanger is a stream of the single mixed réfrigérant that has been compressed to a pressure of at least 1000 PSIA (68.95 bara) and expanded by a hydraulic turbine.
25. 28. The method of claim 27, wherein the plurality of coil wound bundles comprises a warm bundle and a cold bundle, the selected réfrigération duty of the wann bundle being less than the selected réfrigération duty of the cold bundle.