Method and apparatus in a cryogenic liquefaction process

Abstract

Methods and apparatus for the efficient cooling within air liquefaction processes with integrated use of cold recovery from an adjacent LNG gasification process are disclosed.

Claims

1. A cryogenic liquefaction device comprising: a primary heat exchanger having a length; a phase separator; an expansion device; a first arrangement of conduits, arranged such that a pressurised main stream of gas from a first source is directed through the length of the primary heat exchanger, the expansion device and the phase separator, whereby a portion of the main stream of gas is converted into cryogen; a cold recovery circuit including a first heat transfer fluid and a second arrangement of conduits arranged such that the first heat transfer fluid is directed along the second arrangement of conduits adjacent to the first arrangement of conduits through a first portion of the length of the primary heat exchanger in a counter-flow direction to a flow direction of the pressurised main stream of gas through the primary heat exchanger for cooling the pressurised main stream of gas; and a refrigerant circuit including a second heat transfer fluid and a third arrangement of conduits arranged such that the second heat transfer fluid is directed along the third arrangement of conduits adjacent to the first arrangement of conduits through a second portion of the length of the primary heat exchanger in a counter-flow direction to the flow direction of the pressurised main stream of gas through the primary heat exchanger for further cooling the pressurised main stream of gas; each of the second and third arrangements of conduits forming a pressurised closed-loop circuit; the first and second portions of the length of the primary heat exchanger being arranged in succession so that the pressurised main stream of gas through the primary heat exchanger is cooled by the cold recovery circuit before the further cooling by the refrigerant circuit, at least one secondary heat exchanger and at least one secondary arrangement of conduits arranged such that a cold stream from a liquefied natural gas regasification terminal is directed through the at least one secondary heat exchanger; and the second and third arrangement of conduits being arranged such that the first and second heat transfer fluids are directed through the at least one secondary heat exchanger in a counter-flow direction to a flow direction of the cold stream through the at least one secondary heat exchanger for cooling the first and second heat transfer fluids, wherein the second arrangement of conduits of the closed-loop cold recovery circuit conveys the first heat transfer fluid to the primary heat exchanger after the first heat transfer fluid has been reduced in temperature by passing through the at least one secondary heat exchanger, wherein the closed-loop refrigerant circuit further comprises an expander, and wherein the third arrangement of conduits of the closed-loop refrigerant circuit conveys the second heat transfer fluid to the primary heat exchanger after the second heat transfer fluid has been reduced in temperature by passing through the at least one secondary heat exchanger and then further reduced in temperature by passing through the expander so that the temperature of the second heat transfer fluid that is directed to the primary heat exchanger is less than the temperature of the first heat transfer fluid that is directed to the primary heat exchanger.

2. The cryogenic liquefaction device of claim 1, configured such that an output stream from the expansion device has a liquid fraction of at least 95% of the pressurised main stream of gas from the first source.

3. The cryogenic liquefaction device of claim 2, configured such that the pressurised main stream of gas exits the primary heat exchanger at a pressure of between 55 and 56 bar and a temperature of 97 k.

4. The cryogenic liquefaction device of claim 1, wherein the cold stream is a liquefied natural gas.

5. The cryogenic liquefaction device of claim 2, wherein the cold stream is a liquefied natural gas.

6. The cryogenic liquefaction device of claim 1, wherein the cold recovery circuit further comprises means for circulating the first heat transfer fluid through the second arrangement of conduits.

7. The cryogenic liquefaction device of claim 6, wherein the second arrangement of conduits is arranged such that the first heat transfer fluid is directed from the at least one secondary heat exchanger and through the means for circulating the heat transfer fluid before being directed through the primary heat exchanger.

8. The cryogenic liquefaction device of claim 7, wherein the means for circulating the first heat transfer fluid is a mechanical blower.

9. The cryogenic liquefaction device of claim 1, wherein the refrigerant circuit further comprises a compression device, and wherein the third arrangement of conduits is arranged such that the second heat transfer fluid is directed from the primary heat exchanger and through the compression device before being directed through the at least one secondary heat exchanger.

10. The cryogenic liquefaction device of claim 1, wherein the expansion device is a joule-Thomson valve.

11. The cryogenic liquefaction device of claim 10, wherein the second portion of the length of the primary heat exchanger is closer to the expansion device, in the flow direction of the pressurised main stream of gas, than the first portion of the length of the primary heat exchanger.

12. A method for balancing a cryogenic liquefaction process comprising: directing a pressurised main stream of gas from a first source through successive first and second portions of a length of a primary heat exchanger, an expansion device and a phase separator along a first arrangement of conduits, thereby converting a portion of the main stream of gas into cryogen; directing a first heat transfer fluid in a pressurised closed-loop cold recovery circuit along a second arrangement of conduits adjacent to the first arrangement of conduits through the first portion of the length of the primary heat exchanger in a counter-flow direction to a flow direction of the pressurised main stream of gas for cooling the pressurised main stream of gas; and directing a second heat transfer fluid in a pressurised closed-loop refrigerant circuit along a third arrangement of conduits adjacent to the first arrangement of conduits through the second portion of the length of the primary heat exchanger in a counter-flow direction to the flow direction of the pressurised main stream of gas such that the pressurised main stream of gas through the primary heat exchanger is cooled by the cold recovery circuit before being cooled by the refrigerant circuit for further cooling the pressurised main stream of gas, wherein the step of directing the first and second heat transfer fluids includes directing the first and second heat transfer fluids through at least one secondary heat exchanger, and further comprising a step of directing a cold stream from a liquefied natural gas regasification terminal through the at least one secondary heat exchanger in a counter-flow direction to flow directions of the first and second heat transfer fluids through the at least one secondary heat exchanger for cooling the first and second heat transfer fluids, wherein the second arrangement of conduits of the closed-loop cold recovery circuit conveys the first heat transfer fluid to the primary heat exchanger after the first heat transfer fluid has been reduced in temperature by passing through the at least one secondary heat exchanger, wherein the closed-loop refrigerant circuit further comprises an expander, and wherein the third arrangement of conduits of the closed-loop refrigerant circuit conveys the second heat transfer fluid to the primary heat exchanger after the second heat transfer fluid has been reduced in temperature by passing through the at least one secondary heat exchanger and then further reduced in temperature by passing through the expander so that the temperature of the second heat transfer fluid that is directed to the primary heat exchanger is less than the temperature of the first heat transfer fluid that is directed to the primary heat exchanger.

13. The method of claim 12, wherein the cold stream is a liquefied natural gas.

14. The method of claim 12, further comprising: directing the first heat transfer fluid from the at least one secondary heat exchanger and through a means for circulating the heat transfer fluid before directing the first heat transfer fluid through the primary heat exchanger.

15. The method of claim 12, further comprising: directing the second heat transfer fluid from the primary heat exchanger and through a compression device before directing the second heat transfer fluid through the at least one secondary heat exchanger.

16. The method of claim 12, wherein an output stream from the expansion device has a liquid fraction of at least 95% of the pressurised main stream of gas from the first source.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments of the present invention will now be described with reference to the figures in which:

(2) FIG. 1 shows a profile of the relative change in total enthalpy in which the process gas undergoes during the cooling process (Relative Change of Total Enthalpy vs Process Gas Temperature)

(3) FIG. 2 shows profiles of the relative change in total enthalpy in which the cooling streams must undergo during the cooling process for systems with and without the use of large quantities of cold recycle (Relative Change of Total Enthalpy vs Process Gas Temperature)

(4) FIG. 3 shows profiles of the relative change in total enthalpy in which the cooling streams must undergo during the cooling process for ‘ideal’ and ‘state of art’ systems with the use of large quantities of cold recycle (Relative Change of Total Enthalpy vs Process Gas Temperature)

(5) FIG. 4 shows a typical state of the art air liquefaction plant arrangement

(6) FIG. 5 shows a schematic of a cryogenic energy system liquefaction process with ‘cold recovery circuit’ using typical state of the art air liquefaction plant arrangement; and

(7) FIG. 6 shows a schematic of a cryogenic energy system liquefaction process according to a first embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

(8) The first simplified embodiment of the present invention is shown in FIG. 6. The system in FIG. 6 is similar to that of the conventional layout shown in FIG. 5 in that a pressurised stream of gas (the main process gas stream (31, 35)) is cooled to a temperature using the cold energy recovered from a stream of LNG (60), after which additional cooling is provided before the stream (31, 35) is expanded through a Joule-Thomson Valve (1) to produce liquid air.

(9) However, whereas the additional cooling in the layout shown in FIG. 5 is provided by the a portion of the main process gas stream (31, 35) itself, the additional cooling in the embodiment of FIG. 6 according to the present invention is provided by cold energy recovered from a stream of LNG (80) in a refrigerant circuit (140). The stream of LNG (80) used in the refrigerant circuit (140) may be the same stream as the stream of LNG (60) used in the cold recovery circuit (120) or it may be a different stream. Likewise, the heat exchanger (102) used in the refrigerant circuit (140) may be the same heat exchanger (101) used in the cold recovery circuit (120) or it may be a different heat exchanger.

(10) In the first embodiment, the main process gas stream (31, 35) is compressed to high pressure, preferably of at least the critical pressure (which for air is 38 bar), but more preferably 56 bar, at ambient temperature (≈298 k). The main process gas stream (31, 35) enters inlet 31, from which point it is directed through a first heat exchanger (100) and is cooled progressively by the cold recovery circuit (120) HTF passing through passage (52). The HTF in the cold recovery circuit (120) may comprise gas or a liquid, at high or low pressure. In the preferred case, a gas such as Nitrogen at a pressure of 5 bar is used.

(11) The cold recovery circuit (120) consists of a means of circulation (5) such as a mechanical blower. A second heat exchanger 101 is provided in addition to the first heat exchanger 100 described above. The HTF is circulated around the cold recovery circuit by the mechanical blower and enters the second heat exchanger 101 at 185 k. The HTF is progressively cooled by virtue of its proximity to the waste stream of LNG (60) passing through the first heat exchanger, and exits the second heat exchanger at around 123 k. The HTF is then directed to the first heat exchanger 100, through which it passes via passage 52 providing cooling to the high pressure main process gas stream (31, 35) by virtue of its proximity thereto.

(12) At point 35 the main process gas stream (31, 35) has been cooled to a temperature of between 110-135 k, but in the preferred case 124 k, and continues to pass through the first heat exchanger (100) in which it continues to be cooled progressively by a refrigerant circuit (140) HTF passing through passage (71) as described in more detail below.

(13) The use of a refrigerant circuit (140) in the present invention enables the greater utilisation of lower quality cold energy to provide high quality cold energy which has hitherto been carried out by expanding a proportion of the high pressure main process gas stream, such as in the conventional system shown in FIG. 5.

(14) In addition to the first heat exchanger (100), the refrigerant circuit (140) consists of a compressor (7), a third heat exchanger (102), and an expander (6). The refrigerant circuit (14) contains a HTF which may comprise of a gas or a liquid, at high or low pressure. However, in the preferred case, a gas such as Nitrogen at a pressure of between 1.4 and 7 bar is utilised. At point 72, the HTF is at a temperature of 122 k and a pressure of 1.4 bar. The HTF is compressed to higher pressure (for example between 5 bar and 10 bar, but preferably 7 bar) by compressor (7). The HTF exits the compressor (7) at temperature 206 k, before entering the third heat exchanger 102 where it is progressively chilled by virtue of its proximity to waste stream of LNG (80) passing through the third heat exchanger. The HTF then enters expander (6) at pressure 6.9 bar and temperature 123 k, where it is expanded to 1.5 bar and 84 k. The HTF then enters the first heat exchanger (100), where it is directed through passage 71 providing cooling to the high pressure main process gas stream (31, 35) by virtue of its proximity thereto.

(15) Using Nitrogen as the HTF in both the cold recovery and refrigerant circuits of the present invention provides a level of isolation between the potentially hazard cold source and process gas which in the preferred case is LNG and gaseous air containing oxygen.

(16) Finally the main process gas stream (31, 35) exits the first heat exchanger (100) at approximately 55-56 bar and 97 k, where it is expanded through a Joule-Thompson valve 1 (or other means of expansion device) creating a typical composition of an output stream with liquid fraction >95% (optimally >98%), which is directed in to the phase separator 2. The liquid fraction is collected through stream 33 and vapour fraction expelled through 34.

(17) It will of course be understood that the present invention has been described by way of example, and that modifications of detail can be made within the scope of the invention as defined by the following claims.