AU2011221424B2

AU2011221424B2 - Natural gas liquefaction

Info

Publication number: AU2011221424B2
Application number: AU2011221424A
Authority: AU
Inventors: Heinz Bauer
Original assignee: Linde GmbH
Current assignee: Linde GmbH
Priority date: 2010-09-09
Filing date: 2011-09-09
Publication date: 2016-03-31
Anticipated expiration: 2031-09-09
Also published as: CN102410702A; BRPI1104609A2; CN102410702B; CH703773A2; US20120060553A1; CH703773B1; AR082919A1; DE102010044869A1; AU2011221424A1; NO20111212A1

Abstract

Abstract Natural qas liquefaction A method is described for liquefying a hydrocarbon-rich feed fraction, preferably natural gas, against a nitrogen refrigeration cycle, wherein the feed fraction is cooled against 5 gaseous nitrogen that is to be warmed and the feed fraction is liquefied against liquid nitrogen that is to be vaporized. According to the invention - the feed fraction is cooled and liquefied in an at least three-stage heat-exchange 10 process (E1a - E1c), - wherein, in the first section of the heat-exchange process (E1a), the feed fraction (1) is cooled against superheated gaseous nitrogen (9) to the extent that an essentially complete separation (D2) of the relatively heavy components (2') is achievable, 15 - in the second section of the heat-exchange process (E1b), the feed fraction (2) freed from relatively heavy components is partially liquefied against gaseous nitrogen that is to be superheated (9), and - in the third section of the heat-exchange process (Elc), the feed fraction (2) is liquefied against nitrogen that is to be partially vaporized (8). co, w j. c o' 1~ (N w - (N) 0)0 (4D0

Description

- 1 AUSTRALIA PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT ORIGINAL Name of Applicant: Linde Aktiengesellschaft Actual Inventor: Heinz Bauer Address for Service is: SHELSTON IP 60 Margaret Street Telephone No: (02) 9777 1111 SYDNEY NSW 2000 Facsimile No. (02) 9241 4666 CCN: 3710000352 Attorney Code: SW Invention Title: Natural gas liquefaction The following statement is a full description of this invention, including the best method of performing it known to me/us: File: 71446AUP00 2 Description Natural qas liquefaction The invention relates to a method for liquefying a hydrocarbon-rich feed fraction, preferably natural gas, against a nitrogen refrigeration cycle, wherein the feed fraction 5 is cooled against gaseous nitrogen that is to be warmed and the feed fraction is liquefied against liquid nitrogen that is to be vaporized. Hydrocarbon-rich gases, in particular natural gases, are liquefied commercially in a capacity range from 10 to 30 000 tonnes of LNG per day (tato). In plants of medium 10 capacity - this is taken to mean liquefaction processes having a capacity between 300 and 3000 tato of LNG - and large capacity - this is taken to mean liquefaction processes having a capacity between 3000 and 30 000 tato of LNG - those skilled in the art attempt to optimize the operating costs by means of high efficiency. In contrast, in the case of smaller plants - this is taken to mean liquefaction processes having a 15 capacity between 10 and 300 tato of LNG - low capital costs are in the foreground. In such plants, the capital cost proportion of a dedicated refrigeration plant in which the working medium used is, for example, nitrogen or a nitrogen-hydrocarbon mixture, is considerable. Therefore, generation of cold in the liquefaction plant is if possible dispensed with and a suitable refrigerant imported. Customarily, in this case, liquid 20 nitrogen is used and after its use as refrigerant, is given off to the atmosphere in the gaseous state. If in nearby air separation plants unused product amounts of liquid nitrogen can be provided inexpensively, this concept for small liquefaction plants is absolutely commercially expedient. 25 For reasons of costs, in small liquid-nitrogen-cooled plants, brazed aluminium plate heat exchangers are generally used. These appliances, however, are sensitive to high thermal stresses such as can arise, for example, by an oversupply of refrigerant and/or large temperature differences between warm and cold process streams. The resultant mechanical stresses can lead to damage to these appliances. 30 In addition, care must be taken to ensure that, during operation of the liquefaction process, the feed fraction does not fall below the freezing temperature. The solid point of methane at -182*C is markedly above the atmospheric boiling temperature of 3 nitrogen, which is -196C Freezing of the plant always causes an unwanted operating fault and can in addition have lasting damage as a consequence. A method of the type in question for liquefying a hydrocarbon-rich feed fraction 5 is known from US patent 5,390,499. This method is suitable, in particular, for plants of small capacity, as explained at the outset. In the liquefaction method described in US patent 5,390,499, the gas that is to be liquefied is cooled and liquefied against nitrogen in two separate heat exchangers. In this case the liquid low-boiling nitrogen is completely vaporized in the second heat exchanger and warmed up to a temperature 10 at which relatively heavy crude gas components can be taken off in the liquid state by means of a separator from the gas that is to be liquefied. In a process procedure as described in US patent 5,390,499, however, the point at which the nitrogen vaporizes completely can vary considerably according to load. This can lead to unwanted process conditions which have the abovementioned disadvantages as a consequence, 15 Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field. 20 It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art or to provide a useful alternative. According to a first aspect the present invention provides method for liquefying a hydrocarbon-rich feed fraction against a nitrogen refrigeration cycle, comprising: 25 cooling said hydrogen-rich feed fraction against gaseous nitrogen that is to be warmed, and liquefying said hydrogen-rich feed fraction against liquid nitrogen that is to be vaporized wherein 30 - the hydrocarbon-rich feed fraction is cooled and liquefied in an at least three stage heat-exchange process, - in a first stage of said at least three-stage heat-exchange process, said hydrocarbon-rich feed fraction is cooled to achieve essentially complete separation of relatively heavy components against superheated gaseous 35 nitrogen, 3a - in a second stage of said at least three-stage heat-exchange process, the portion of said hydrocarbon-rich feed fraction that is freed from relatively heavy components is partially liquefied against gaseous nitrogen, whereby said gaseous nitrogen is superheated to form said superheated gaseous nitrogen, 5 and - in a third stage of said at least three-stage heat-exchange process, the portion of said hydrocarbon-rich feed fraction that is freed from relatively heavy components is liquefied against boiling liquid nitrogen, whereby said nitrogen is partially vaporized, 10 wherein the boiling pressure of the gaseous nitrogen that is superheated, against said portion of said hydrocarbon-rich feed fraction that is freed from relatively heavy components, is adjusted to a value of 5-30 bara, wherein liquid nitrogen is fed to a separator wherein said liquid nitrogen is converted to said boiling liquid nitrogen, 15 said boiling liquid nitrogen is removed from said separator and conducted through said third stage of said at least three-stage heat-exchange process wherein said boiling liquid nitrogen is partially vaporized to form partially vaporized nitrogen, and said partially vaporized nitrogen is fed back to the separator. 20 Unless the context clearly requires otherwise, throughout the description and the claims, the words "comprise", "comprising", and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of "including, but not limited to". 25 In a broad embodiment, the present invention provides, a method for liquefying a hydrocarbon-rich feed fraction, wherein - the feed fraction is cooled and liquefied in an at least three-stage heat exchange process, 30 - wherein in the first section of the heat-exchange process, the feed fraction ?s cooled against superheated gaseous nitrogen to the extent that an essentially complete separation of the relatively heavy components is achievable, 3b in the second section of the heat-exchange process, the feed fraction freed from relatively heavy components is partially liquefied against gaseous nitrogen that is to be superheated, and in the third section of the heat-exchange process, the feed fraction is liquefied 5 against nitrogen that is to be partially vaporized.

4 The expression "heavy components" may be taken to mean hereinafter hydrocarbons from ethane. Further advantageous embodiments of the method according to the invention for 5 liquefying a hydrocarbon-rich feed fraction include - the three-stage heat-exchange process is achieved in one or more heat exchangers, 10 - the condensation pressure of the feed fraction freed from relatively heavy components is adjusted to values between 1 and 15 bara, preferably between 1 and 8 bara, and - the boiling pressure of the gaseous nitrogen that is to be superheated is 15 adjusted to values between 5 and 30 bara, preferably between 10 and 20 bara. The method according to the invention for liquefying a hydrocarbon-rich feed fraction and other advantageous embodiments of the same may be described in more detail hereinafter with reference to the exemplary embodiment shown in the figure. 20 The hydrocarbon-rich feed fraction that is to be liquefied is fed via line 1 to a heat exchanger El. This is subdivided into three sections or stages a to c. The boundaries between these sections or stages are shown by the two dashed lines. In the warmest section a of the heat exchanger El, the hydrocarbon-rich feed fraction is cooled 25 against superheated gaseous nitrogen which is fed via line 9 to the heat exchanger El to the extent that a separation of the heavy components from the feed fraction is possible in a separator D2 downstream of the heat exchanger El. For this purpose, the cooled feed fraction is fed from the heat exchanger El via line 1' to the separator D2. From the bottom phase thereof, via line 2', in which a valve V1 is provided, the 30 unwanted heavy components are taken off in liquid form and released from the process. Instead of the separator D2 shown in the figure, a rectification column can be used to achieve a more precise separation of relatively heavy components or higher 35 hydrocarbons from the feed fraction.

5 At the top of the separator D2, via line 2, the feed fraction that is freed from heavy components is taken off and fed to the second section b of the heat exchanger El. Therein, the feed fraction that is freed from heavy components is partially liquefied 5 against gaseous nitrogen that is to be superheated 9. Then, in the third stage c of the heat exchanger El, the feed fraction is completely liquefied against nitrogen to be partially vaporized which is fed to the heat exchanger El via the line 8. The liquefied feed fraction, after passage through the heat exchanger El via line 3, in 10 which a control valve V3 is arranged, is fed to a storage vessel D4. The liquefied product (LNG) can be discharged therefrom via line 4. The control valve V3 serves for expanding the liquefied feed fraction to the product delivery pressure, which corresponds at least approximately to atmospheric pressure. 15 If the nitrogen is vaporized in the third section c of the heat exchanger El at a pressure of greater than 15 bara, the boiling temperature thereof is no longer low enough in order to subcool the liquefied feed fraction to the extent that outgassing after expansion thereof in the control valve V3 can be prevented. In this case, the boil-off gas formed in the storage vessel D4 is advantageously taken off via line 5, compressed in the 20 compressor C3 and fed back to the feed fraction 2 which is freed from heavy components before liquefaction thereof and re-liquefied in the heat exchanger El. This process procedure should be selected, in particular, in the case of significant temporary storage of the LNG product in an atmospheric flat-bottom tank D4, since the resultant boil-off gas is also processed thereby. 25 The nitrogen required for providing cold is fed to the liquefaction process via line 6. Advantageously, a buffer tank D3 is provided which serves for compensating for quantitative fluctuations of the feed fraction that is to be liquefied and/or of the refrigerant nitrogen. By means of a pump P1, liquid nitrogen is fed in the amount 30 required to a separator D1 via line 7. From the bottom phase of the separator D1, boiling nitrogen is taken off and conducted via line 8 through the coldest section c of the heat exchanger El. The nitrogen that is partially vaporized in this case is then fed via line 8' back to the separator D1.

6 If the reliquefaction process that is still to be described is operated, at least temporarily the generation of cold by the reliquefaction of the nitrogen can exceed the refrigeration requirement of the natural gas liquefaction. An oversupply resulting therefrom of liquid nitrogen can be delivered into the buffer tank D3 via line 8" and valve V6. 5 At the top of the separator D1, gaseous nitrogen is taken off via line 9 and fed to the middle section b of the heat exchanger El. The gaseous nitrogen is conducted through the second and first sections of the heat exchanger El in countercurrent flow to the feed fraction 2 that is to be cooled and partially liquefied, and is warmed and 10 superheated in this process. The superheated nitrogen is then taken off from the process via the line sections 10 and 11. By means of the control valve V4, the boiling pressure of the gaseous nitrogen that is to be superheated 9 can be controlled. Advantageously, this boiling pressure is 15 adjusted to values between 5 and 30 bara, preferably between 10 and 20 bara. Similarly, the condensation pressure of the feed fraction 2 that is freed from relatively heavy components can be controlled by means of the control valve V2. This condensation pressure is preferably adjusted to values between 1 and 15 bara, 20 preferably between 1 and 8 bara. By means of the control valves V2 and/or V4, the temperature profile in the third section c of the heat exchanger El can be controlled thereby. Whereas, by means of the control valve V2, the condensation pressure of the feed fraction is established in 25 the section between the control valves V2 and V3, by means of the control valve V4, the boiling pressure of the nitrogen in the separator D1 and the third section c of the heat exchanger El is controlled. Owing to the above-described subdivision of the heat exchange process into a second and third section and with the phase separation in separator D1 it can then be established exactly in what section of the heat exchanger 30 El a (partial) vaporization or superheating of the nitrogen is taking place. By means of the subdivision of the heat-exchange process El into three sections a to c, it is possible to reliably prevent the phase boundary between liquid and gaseous refrigerant from migrating within the heat exchanger El and thereby causing unwanted 35 thermal and mechanical stresses within the heat exchanger El.

7 If the nitrogen boiling pressure (pN 2 ) and the crude gas condensation pressure (pRG) are selected according to the inequality pRG (bara) 0.3 pN 2 (bara) -1, a thermal overload of the heat exchanger El due to impermissibly high temperature differences can be safely avoided. 5 By restricting the boiling pressure of the liquid nitrogen in the third section c of the heat exchanger El and of the separator D1 to at least 5 bara - the associated boiling temperature is -179*C - it is possible to prevent reliably a temperature below the freezing temperature of methane occurring in the heat exchanger El. Operating 10 problems and possible damage due to solids formation are thereby excluded. The superheated nitrogen taken off from the heat exchanger El via line 10 can, alternatively to a removal via line 11, be at least partially reliquefied. For this purpose the nitrogen is fed via the line sections 12 and 13 to a compression - shown in the 15 figure by a two-stage compressor unit C1/C2, wherein a heat exchanger, E3 or E4 respectively, is connected downstream of each compressor unit - and then is fed via line 14 to a heat exchanger E2. Therein, the nitrogen is reliquefied and then fed to separator D1 via line 15. Pressure regulation of the compressor C2 is performed by the control valve V5. For the purpose of providing cold in the heat exchanger E2, via line 20 16 a substream of the compressed nitrogen stream is taken off, preferably expanded in a multistage manner - shown by the gas expanders X1 and X2 - and then conducted via line 17 through the heat exchanger E2 in countercurrent flow to the nitrogen stream that is to be liquefied. The shafts of the compressors C1 and C2 are preferably coupled to the shafts of the gas expanders X2 and X1. 25 If the above-described reliquefaction process is operated, it is advantageous to feed to the heat exchanger El via line 9 only the amount of gaseous nitrogen that is required for a small positive temperature difference of approximately 3*C between streams 1 and 10 at the warm end of the heat exchanger El. The excess amount of cold gaseous 30 nitrogen is used via line 9' proportionately for reliquefaction in the heat exchanger E2. In principle, the liquefaction process can proceed by means of "imported" nitrogen - in this case, the superheated nitrogen is taken off from the heat exchanger El via the line sections 10 and 11 - by means of reliquefied nitrogen, or by any desired combination 35 of both modes of operation.

Claims

2. Method according to claim 1, wherein said hydrocarbon-rich feed fraction is natural gas,
3. Method according to clairn I or claim 2, wherein the at least three-stage heat 5 exchange process is achieved in one heat exc'ianger.
4. Method according to claim 1 or claim 2, wherein the at east three-stage heat exchange process is achieved in more than one heat exchanger. 10 5. Method according to any one of claims 1 to 4, wherein the condensation pressure of the portion of said hydrocarbon-rich feed fraction that is freed from relatively heavy components is adjusted to a value between 1 and 15 bara before being introduced into said second stage of said at least three-stage heat-exchange process 15
6. Method according to claim 5, wherein the condensation pressure of the portion of said hydrocarbon-rich feed fraction that is freed from relatively heavy components is adjusted to a value between 1 and 8 bara before being introduced into said second stage of said at least three-stage heat-exchange process. 20 7 Method according to any one of claims 1 to 6, wherein the boiling pressure of the gaseous nitrogen that is to be superheated, against said portion of said hydrocarbon-rich feed fraction that is freed from relatively heavy components, is adjusted to a value between 10 and 20 bara. 25
8. Method according to any one of claims I to 7, wherein gaseous nitrogen is removed from said separator and is used as the gaseous nitrogen in said second stage of said at least three-stage heat-exchange process to partially liquefy said portion of said hydrocarbon-rich feed fraction that is freed from relatively heavy 30 components.
9. MV ethod according to claim 8, wherein a portion of the gaseous nitrogen removed from said separator is heated in a separate heat exchanger and is combined with superheated gaseous nitrogen removed from said second stage of said at least 35 three-stage heat-exchange process, and the resultant combined gaseous nitrogen 10 stream is compressed and cooled and then used at a heating medium in said separate heat exchanger to heat said portion of the gaseous nitrogen removed from said separator. 5 10 Method according to claim 9, wherein at east a portion of the combined gaseous nitrogen stream is liquefied in said separate heat exchanger and then fed back to said separator.
11. Method according to claim 10, wherein another portion of the combined gaseous 10 nitrogen stream is removed from said separate heat exchanger at an intermediate point thereof, expanded and then fed back to said separate heat exchanger to provide cooling in said separate heat exchanger. 12 Method according to claim 11, wherein said another portion of the combined 15 gaseous nitrogen stream, after being expanded; is combined with said portion of the gaseous nitrogen removed from said separator before being fed back to said separate heat exchange to provide cooling.