MAIN HEAT EXCHANGER AND A PROCESS FOR COOLING A TUBE SIDE STREAM

Abstract

A process for cooling a tube side stream in a main heat exchanger is described. The process comprises: a) supplying a first mass flow of a tube side stream to a first zone of individual tubes in the tube bundle; b) supplying a second mass flow of the tube side stream to a second zone of individual tubes in the tube bundle, the second zone being offset from the first zone; c) supplying a refrigerant stream on the shell side for cooling the first and second mass flows; d) removing the evaporated refrigerant stream from the warm end of the main heat exchanger; and, e) adjusting the first mass flow of the tube side stream relative to the second mass flow of the tube side stream to maximise the temperature of the removed evaporated refrigerant stream.

Claims

1. A process for cooling a tube side stream in a main heat exchanger having a warm end and a cold end, the main heat exchanger comprising a wall defining a shell side within which a coil-wound tube bundle is arranged around a central mandrel, the process comprising the steps of: a) supplying a first mass flow of a tube side stream to the warm end of a first zone of individual tubes in the tube bundle via a first nozzle; b) supplying a second mass flow of the tube side stream to the warm end of a second zone of individual tubes in the tube bundle via a second nozzle, the second zone being offset from the first zone along a radius extending from the central mandrel to the wall of the main heat exchanger; c) supplying a refrigerant stream on the shell side for cooling the first and second mass flows to form an evaporated refrigerant stream; d) removing the evaporated refrigerant stream from the warm end of the main heat exchanger; and, e) adjusting the first mass flow of the tube side stream relative to the second mass flow of the tube side stream to maximise the temperature of the evaporated refrigerant stream removed in step d) wherein step e) comprises equalizing the temperature of the first mass flow of the tube side stream at a first axial location relative to the length of the mandrel with the temperature of the second mass flow of the tube side stream at said first axial location by adjusting the mass flow supplied to one or both of the first and second nozzles.

2. (canceled)

3. The process of claim 1 wherein a first temperature sensor generates a first signal indicative of the temperature of the first mass flow and a second temperature sensor generates a second signal indicative of the temperature of the second mass flow and step e) comprises using a controller to adjust the first mass flow of the tube side stream relative to the second mass flow of the tube side stream to equalise the first signal with the second signal.

4. The process of claim 1 wherein the first axial location is at or adjacent to the cold end of the main heat exchanger.

5. The process of claim 1 wherein the first zone is an inner zone of the tube bundle and the second zone is an outer zone of the tube bundle.

6. The process of claim 1 wherein the mass flow through the first nozzle is controllably adjusted using a first valve and the mass flow through the second nozzle is controllably adjusted using a second valve.

7. The process of claim 6 wherein one or both of the first and second valves is external to the main heat exchanger.

8. The process of claim 6 wherein one or both of the first and second valves is a fail-safe open low pressure drop valve.

9. The process of claim 6 wherein one or both of the first and second valves is located at one or both of the warm end and the cold end of the tube side stream.

10. The process of claim 1 wherein the first nozzle supplies the tube fluid to the first zone via a first tube sheet and the second nozzle supplies the tube side fluid to the second zone via a second tube sheet.

11. The process of claim 1 wherein the tube bundle comprises a warm tube bundle arranged towards the warm end of the main heat exchanger, and a cold tube bundle arranged towards the cold end of the main heat exchanger, each of the warm tube bundle and the cold tube bundle having a warm end and a cold end and the first location is at or adjacent to the cold end of the warm tube bundle.

12. The process of claim 11 wherein the tube side stream is a first tube side stream which enters the warm end of the warm tube bundle as a liquid and exits the cold end of the cold tube bundle as a sub-cooled liquid.

13. The process of claim 11 wherein the first tube side stream enters the warm end of the warm tube bundle as a gaseous, methane-rich feed which has been liquefied by the time it passes from the warm end of the warm tube bundle into the warm end of the cold tube bundle.

14. The process of claim 13 wherein the first tube side stream enters the warm end of the cold tube bundle as a liquid and exits the cold end of the cold tube bundle as a sub-cooled liquid.

15. The process of claim 14 wherein the sub-cooled liquid is removed from the cold end of the cold tube bundle of the main heat exchanger before being directed to storage.

16. The process of claim 15 wherein the first tube side stream exchanges heat with a predominately liquid light refrigerant stream which is progressively boiled off on the shell side of the cold tube bundle.

17. The process of claim 16 wherein evaporated refrigerant removed from the warm end of the shell side of the main heat exchanger is fed to first and second refrigerant compressors in which the evaporated refrigerant is compressed to form a high pressure refrigerant stream.

18. The process of claim 17 wherein the high pressure refrigerant stream is directed to a heat exchanger in which it is cooled so as to produce a partly-condensed refrigerant stream which is then directed in a separator to separate out a heavy refrigerant fraction in liquid form and a light refrigerant fraction in gaseous form.

19. The process of claim 18 wherein the heavy refrigerant fraction becomes a second tube side stream which is supplied at the warm end of the warm tube bundle as a liquid and exits at the cold end of the warm tube bundle as a sub-cooled heavy refrigerant stream in liquid form.

20. The process of claim 19 wherein the sub-cooled heavy refrigerant stream removed at the cold end of the warm tube bundle is expanded across a first expansion device to form a reduced pressure heavy refrigerant stream that is then introduced into the shell side of the main heat exchanger at a location intermediate between the cold end of the warm tube bundle and the warm end of the cold tube bundle, and wherein said reduced pressure heavy refrigerant stream is allowed to evaporate in the shell side, thereby cooling the fluids in the first, second and third tube side streams as they pass through the warm tube bundle.

21. The process of claim 20 wherein part of the light refrigerant fraction from the separator becomes a third tube side stream which is introduced into the warm end of the warm tube bundle as a gas and exits at the cold end of the cold tube bundle as a sub-cooled liquid.

22. The process of claim 21 wherein the third tube side stream is cooled from a gas to a liquid as it passes through the warm tube bundle and is cooled from a liquid to a sub-cooled liquid as it passes through the cool bundle.

23. The process of claim 22 wherein the sub-cooled light refrigerant stream removed from the cold end of the cold tube bundle is expanded through a second expansion device to cause a reduction in pressure and produce a reduced pressure light refrigerant stream.

24. The process of claim 23 wherein the reduced pressure light refrigerant stream is introduced into the shell side of the main heat exchanger at its cold end, and wherein said reduced pressure light refrigerant stream is allowed to evaporate in the shell side, thereby cooling the fluids in the first and third tube side streams as they travel through the cold tube bundle as well as providing cooling to the fluids in the first, second and third tube side streams as they travel through the warm tube bundle.

25. (canceled)

26. (canceled)

27. (canceled)

28. (canceled)

29. (canceled)

30. (canceled)

31. (canceled)

32. (canceled)

33. (canceled)

34. (canceled)

35. (canceled)

36. (canceled)

37. (canceled)

Description

DESCRIPTION OF THE DRAWINGS

[0032] In order to facilitate a more detailed understanding of the nature of the invention embodiments of the present invention will now be described in detail, by way of example only, with reference to the accompanying drawings, in which:

[0033] FIG. 1 shows schematically the distribution of flows to each layer of a prior art spiral wound main heat exchanger;

[0034] FIG. 2 shows schematically a flow scheme of a plant for liquefying natural gas; and,

[0035] FIG. 3 shows schematically the distribution of flows to each layer of the main heat exchanger of one embodiment of the present invention.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0036] Particular embodiments of the process and apparatus of the present invention are now described, with particular reference to a plant for liquefying a gaseous, methane-rich feed gas in the form of natural gas in a main heat exchanger to produced liquefied natural gas, by way of example only.

[0037] The present invention is equally applicable to a main heat exchanger used for other applications such as the production of ethylene or other process requiring on two tube side streams instead of the three tube side streams described in detail below. The terminology used herein is for the purpose of describing particular embodiments only, and is not intended to limit the scope of the present invention. Unless defined otherwise, all technical and scientific terms used herein have the same meanings as commonly understood by one of ordinary skill in the art to which this invention belongs. In the drawings, it should be understood that like reference numbers refer to like parts.

[0038] Using a typical prior art spiral wound main heat exchanger such as the one illustrated schematically in FIG. 1, the tube bundle is spiral wound whereby each tube side stream is introduced to the tube bundle via one or more flow control nozzles arranged to evenly distribute the mass flow of any given type of tube side stream into a plurality of individual tubes arranged randomly yet evenly across the full radius of the tube bundle when viewed in cross-section. More specifically, each nozzle causes the mass flow of each tube side stream to be distributed evenly between each layer of individual tubes within the tube bundle. When the tube bundle is wound so as to have a plurality of layers of individual tubes, the mass flow of any given tube side stream from any given nozzle is split evenly across each of the plurality of layers. The net result is that every nozzle distributes an even amount of its mass flow across any given cross-section taken through the tube bundleaxially and radially. In an analogous manner, the mass flow of light refrigerant entering the shell side at the cold end of a cold tube bundle in the main heat exchanger is distributed across the shell side using a first distributor (not shown), and the mass flow of heavy refrigerant entering the shell side at the cold end of a warm tube bundle is distributed across the shell side using a second distributor (not shown). This prior art arrangement is advocated for use in maintaining as even a heat balance across the main heat exchanger as possible at all times.

[0039] The present invention is based in part on the realisation that it is difficult to fix any imbalance in the temperature, composition or mass flow rate distribution of the reduced pressure light and heavy refrigerant streams on the shell side of the main heat exchanger. Whilst the vapour phase present is capable of mixing in the radial direction to some degree, the liquid phase present on the shell side does not to any significant extent with the result that any imbalance in temperature across the tube bundle cannot be corrected by making adjustments on the shell side. Instead, the Applicants have realised that an improvement in efficiency can be achieved by adjusting the mass flow of at least one of the tube side streams to compensate for any imbalance on the shell side. The present invention is further based in part on a realisation that this traditional method of construction of a spiral would heat exchanger provides no mechanism to address problems which arise in the event of an imbalance of cooling on the shell side of the main heat exchanger.

[0040] Using the process of the present invention, the tube bundle is wound in such a way that any given nozzle supplies a tube side stream into only one zone of the tube bundle, each zone comprising a plurality of layers of individual tubes, so that the mass flow of that tube side stream to each zone within the tube bundle can be separately controlled. By providing this level of control, the mass flow of each tube side stream to each zone of the bundle can be adjusted to compensate for an uneven distribution of cooling on the shell side, wherever and whenever this occurs. Advantageously the adjustable mass flows through each separate nozzle (and thus each separate zone) can also be used to redress heat transfer imbalance issues that might otherwise arise due to changes in feed gas composition over time, or from a change in vertical alignment of the main heat exchanger, such as may occur on a vessel. In other words, the temperature of the evaporated refrigerant stream removed from the shell side at the warm end of the main heat exchanger is maximised by separately adjusting the mass flow of the tube side stream in each zone of the tube bundle as described in greater detail below. Another way to achieve maximum efficiency is to ensure that the exit temperature of the tube side streams for each zone is as uniform as possible. The overarching aim is to match the tube side duty to the shell side dutyeven when the shell side duty is imbalanced.

[0041] FIGS. 2 and 3 illustrate one embodiment of a process or plant (10) for cooling a tube side stream in a main heat exchanger (12) according to the present invention. The main heat exchanger (12) has a wall (14) defining a shell side (16) within which a coil-wound tube bundle (18) is arranged around a central mandrel (19), the main heat exchanger (12) having a warm end (20) and a cold end (22). A first mass flow (28) of a tube side stream is supplied to the warm end (20) of a first zone (24) via a first nozzle (25). A second mass flow (30) of a tube side stream is supplied to the warm end (20) of a second zone (26) via a second nozzle (27). The second zone (26) is offset from the first zone (24) a radius extending from the central mandrel (19) to the wall (14) of the main heat exchanger (12). In the embodiment illustrated in FIG. 3, the tube bundle (18) further includes an optional third intermediate zone (35) arranged between the first zone (24) and the second zone (26), said third zone (35) being supplied with a third mass flow (37) of the tube side stream by the third nozzle (39). It is to be understood that any number of zones may be used provided only that supply to each zone is controlled by separate nozzles. It is to be further understood that within each zone, the individual tubes remain evenly distributed and may be arranged in a plurality of layers.

[0042] With reference to FIGS. 2 and 3, a single or mixed refrigerant stream (31) is introduced at the cold end (22) of the main heat exchanger and evaporated on the shell side (16) to provide cooling to the first and second mass flows (28 and 30, respectively) of the tube side stream. An evaporated refrigerant stream (74) is removed from the warm end (20) of the main heat exchanger (12). The first mass flow (28) which flows only through the first zone (24) is separately adjusted relative to the second mass flow (30) which flows only through the second zone (26) to maximise the temperature of the evaporated refrigerant stream (74) removed from the warm end (20) of the main heat exchanger (12).

[0043] In one embodiment of the present invention, the temperature of the evaporated refrigerant stream (74) removed from the warm end (20) of the main heat exchanger (12) is maximised by equalising the temperature of the first mass flow (28) as measured at a first axial location (33) relative to the length of the mandrel (19) with the temperature of the second mass flow (30) as measured at said first axial location (33). The mass flow supplied by one or both of the first and second nozzles (25 and 27, respectively) is adjusted in this way to ensure that the temperature of said tube side stream in the first zone (24) is matched to the temperature of said tube side stream in the second zone (26) at any give axial location along the length of the tube bundle (18). While, by way of example, it would be ideal for the exit temperature of the first mass flow (28) to be equal to the exit temperature of the second mass flow (30) at the cold end (22) for maximum efficiency, the term equalise is used throughout this specification and the appended claims to refer to incremental adjustment of at least one of the first and second mass flows (28 and 30, respectively) to achieve the result that the exit temperature of the first mass flow (28) more closely approaches the exit temperature of the second mass flow (30) at the cold end (22).

[0044] In the embodiment illustrated in FIG. 3, the temperature of the first mass flow (28) is measured using a first temperature sensor (32) with the temperature of the second mass flow (30) being measured using a second temperature sensor (34). With reference to FIG. 2, the temperature of the evaporated refrigerant stream (74) removed from the warm end (20) of the main heat exchanger (12) is measured using a third temperature sensor (75).

[0045] For the purposes of automation of one embodiment of the process, a first signal (35) indicative of the temperature measured by the first temperature sensor (32) is compared with a second signal (41) indicative of the temperature measured by the second temperature sensor (34) using a controller (40). The controller (40) is then used to adjust the mass flow supplied to the first zone (24) by the first nozzle (25) separately relative to the mass flow supplied to the second zone (26) via the second nozzle (27) so as to equalise the first and second signals (35 and 41). Alternatively or additionally, a third signal (77) indicative of the temperature measured by the third temperature sensor (75) is provided to the controller (40). The controller (40) is then used to adjust the mass flow supplied to the first zone (24) by the first nozzle (25) relative to the mass flow supplied to the second zone (26) via the second nozzle (27) so as to maximise the temperature of the evaporated refrigerant stream (74). When the tube bundle (18) further includes an optional third intermediate zone (35), the controller (40) may receive a fourth signal indicative of the temperature in the third intermediate zone in an analogous manner to allow adjustment of the third mass flow (37) supplied via the third nozzle (39).

[0046] It is to be understood that the total mass flow into the main heat exchanger (12) is controlled either upstream or downstream of the main heat exchanger (12). Consequently, an adjustment made by controller (40) to any of the nozzles (25, 27 or 39) will change the relative mass flow through the other nozzles (25, 27 or 39) whilst the overall mass flow through the main heat exchanger remains constant.

[0047] In the embodiment illustrated in FIG. 3, each nozzle is provided with a flow valve, for example a low pressure butterfly valve, located either at the inlet or outlet of the tube side stream (either upstream or downstream of the cold end of the tube bundle) to facilitate adjustment of the mass flow through that nozzle. Thus, the mass flow through the first nozzle (25) is controllably adjusted using a first valve (45) while the mass flow through the second nozzle (27) is controllably adjusted using a second valve (47). Advantageously, when one or both of the first and second valves (45 and 47, respectively) are external to the main heat exchanger, adjustment of the mass flow rate through the first and second nozzles (25 and 27, respectively) can occur without having to take the main heat exchanger offline, thus avoiding the disruptive loss of production associated with shutdowns.

[0048] Reference is now made to FIG. 2 which illustrates schematically a plant (10) for liquefying a gaseous, methane-rich feed gas in the form of natural gas in a main heat exchanger (12). In this embodiment, the wall (14) of the main heat exchanger (12) defines a shell side (16) within which is arranged two tube bundles, being a warm tube bundle (50) having a warm end (52) and a cold end (54) and a cold tube bundle (56) having a warm end (58) and a cold end (60). The warm tube bundle (50) is arranged towards the warm end (20) of the main heat exchanger (12) and the cold tube bundle (56) is arranged towards the cold end (22) of the main heat exchanger (12). In the embodiment illustrated in FIG. 2, the tube bundle is arranged to received a first tube side stream (62), a second tube side stream (64), and a third tube side stream (66) as described in greater detail below. However, the present invention applies equally to main heat exchanger operating with only one or two tube side streams provided only that a first mass flow of any given tube side stream is directed to flow through a first subset of individual tubes and a second mass flow of said tube side stream is directed to flow through a second subset of individual tubes, with each of the first and second subsets of individual tubes being radially offset across the coil-wound tube bundle.

[0049] In the embodiment illustrated in FIG. 2, the first tube side stream (62) enters the warm tube bundle (50) at elevated pressure as a gaseous, methane-rich feed which has been liquefied and partially sub-cooled by the time it passes from the cold end (54) of the warm tube bundle (50) into the warm end (58) of the cold tube bundle (56). The first tube side stream (62) enters the warm end (58) of the cold tube bundle (56) as a partially sub-cooled liquid and exits the cold end (60) of the cold tube bundle (56) as a further sub-cooled liquid. As it passes through the cold tube bundle (56), the first tube side stream (62) exchanges heat with a predominately liquid light refrigerant stream (68) which is progressively boiled off on the shell side (16) of the cold tube bundle (56). The resulting sub-cooled liquefied first tube side stream (70) is removed from the cold end (22) of the main heat exchanger (12) before being directed to storage (72).

[0050] An evaporated mixed refrigerant stream (74) removed from the shell side (16) at the warm end (20) of the main heat exchanger (12) is fed to first and second refrigerant compressors (76 and 78) in which the evaporated refrigerant stream (74) is compressed to form a high pressure refrigerant stream (80). The high pressure refrigerant stream (80) is then directed to one or more heat exchangers (82) in which it is cooled so as to produce a partly-condensed mixed refrigerant stream (84) which is then directed in a separator (86) to separate out a heavy refrigerant fraction in liquid form (88) and a light refrigerant fraction in gaseous form (90). The heavy refrigerant fraction (88) becomes the second tube side stream (64) which enters at the warm end (52) of the warm tube bundle (50) as a liquid and exits at the cold end (54) of the warm tube bundle (50) as a sub-cooled heavy refrigerant stream (92). In this way, the heavy refrigerant second tube side stream remains a liquid at all times as it passes through the warm tube bundle of the main heat exchanger.

[0051] The sub-cooled heavy refrigerant stream (92) removed at the cold end (54) of the warm tube bundle (50) is expanded across a first expansion device (94), for example a Joule-Thompson valve (J-T valve), to form a reduced pressure heavy refrigerant stream (96) that is then introduced into the shell side (16) of the main heat exchanger (12) at a location intermediate between the cold end (54) of the warm tube bundle (50) and the warm end (58) of the cold tube bundle (56). The reduced pressure heavy refrigerant stream (96) is thus one of the refrigerant streams (31) that is allowed to evaporate in the shell side (16), thereby cooling the fluids in the first, second and third tube side streams (62, 64 and 66, respectively) as they pass through the warm tube bundle (50).

[0052] Part of the light refrigerant fraction (90) from the separator (86) becomes the third tube side stream (66) which is introduced into the warm end (52) of the warm tube bundle (50) as a gas and exits at the cold end (60) of the cold tube bundle (56) as a sub-cooled liquid light refrigerant stream (100). More specifically, the third tube side stream (66) is cooled from a gas to a liquid and partially sub-cooled as it passes through the warm tube bundle (50) and is further cooled to a sub-cooled liquid as it passes through the cool bundle (56). The sub-cooled light refrigerant stream (100) removed from the cold end (22) of the main heat exchanger (12) is expanded through a second expansion device (102), for example a J-T valve to cause a reduction in pressure and produce a reduced pressure light refrigerant stream (104). The reduced pressure light refrigerant stream (104) is thus another of the refrigerant streams (31) introduced into the shell side (16) of the main heat exchanger (12). In this case, the reduced pressure light refrigerant stream (104) starts to evaporate in the shell side (16) to provide cooling to the cold tube bundle (56), thereby cooling the fluids in the first and third tube side streams (62 and 66, respectively) as they travel through the cold tube bundle (56) as well as providing cooling to the fluids in the first, second and third tube side streams (62, 64 and 66, respectively) as they travel through the warm tube bundle (50).

[0053] When the process and apparatus of the present invention is used for liquefaction of a gaseous methane-rich feed to obtain a liquefied natural gas, the tube side stream can be one or more of: the first tube side stream; the second tube side stream; or, the third tube side stream. The selection of which tube side stream(s) require rebalancing will depend on the size of the temperature differentials measured for different zones across the cold end of the tube bundle at the tube side stream exits.

[0054] By way of example, the temperature of a first tube side stream exiting a first zone at the cold end of the tube bundle may be compared with the temperature of the first tube side stream exiting a second zone of the cold end of the tube bundle. In this example, the mass flow of the first tube side stream into the warm end of the tube bundle is rebalanced until the temperature of the first tube side stream exiting the first zone at the cold end of the tube bundle moves closer to the temperature of the first tube side stream exiting the second zone at the cold end of the tube bundle. If the temperature of the first tube side stream exiting the first zone at the cold end of the tube bundle is higher than the temperature of the first tube side stream exiting the second zone at the cold end of the tube bundle, the step of rebalancing of the mass flow is achieved by restricting the flow of the first tube side stream to the first zone at the warm end of the tube bundle. In this way, the mass flow of the first tube side stream to the second zone at the warm end of the tube bundle is essentially increased as the overall mass flow rate of the first tube side stream into the warm end of the tube bundle does not change.

[0055] Analogously, by way of further example, the temperature of the second tube side stream exiting a first zone at the cold end of the warm tube bundle may be compared with the temperature of the second tube side stream exiting a second zone at the cold end of the warm tube bundle. In this example, the mass flow of the second tube side stream into the warm end of the warm tube bundle is rebalanced until the temperature of the second tube side stream exiting the first zone at the cold end of the warm tube bundle moves closer to being equal to the temperature of the second tube side stream exiting the second zone at the cold end of the warm tube bundle. If the temperature of the second tube side stream exiting the first zone at the cold end of the warm tube bundle is lower than the temperature of the second tube side stream exiting the second zone at the cold end of the warm tube bundle, the step of rebalancing of the mass flow is achieved by restricting the flow of the second tube side stream to the second zone at the warm end of the warm tube bundle. In this way, the mass flow of the second tube side stream to the first zone at the warm end of the warm tube bundle is essentially increased as the overall mass flow rate of the second tube side stream into the warm end of the warm tube bundle does not change.

[0056] Restriction of the mass flow of a tube side stream to any given zone within the bundle can be achieved by adjusting the mass flows through the nozzle or valve responsible for directing the mass flow of that side stream to said zone. It is considered a matter of routine for a person skilled in the art to determine the degree to which flow through a nozzle needs to be adjusted for any given zone of the tube bundle to compensate for the difference in temperature of said tube side stream exiting the cold end of the tube bundle for said zone. This can be achieved using modelling techniques well known in the art.

[0057] Now that embodiments of the invention have been described in detail, it will be apparent to persons skilled in the relevant art that numerous variations and modifications can be made without departing from the basic inventive concepts. For example, a plurality of shell side temperature sensors (71) may be used to provide a corresponding plurality of signals indicative of the temperature of each zone within the tube bundle. This plurality of signals may be fed to the controller (40) to facilitate controlled adjustment of the mass flow of a tube side stream to said zones. All such modifications and variations are considered to be within the scope of the present invention, the nature of which is to be determined from the foregoing description and the appended claims.

[0058] Any patents cited in this specification, are herein incorporated by reference. It will be clearly understood that, although a number of prior art publications are referred to herein, this reference does not constitute an admission that any of these documents forms part of the common general knowledge in the art, in Australia or in any other country. In the summary of the invention, the description and claims which follow, except where the context requires otherwise due to express language or necessary implication, the word comprise or variations such as comprises or comprising is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.