AU2006222326A1

AU2006222326A1 - Helium production in LNG plants

Info

Publication number: AU2006222326A1
Application number: AU2006222326A
Authority: AU
Inventors: Hans Schmidt
Original assignee: Linde GmbH
Current assignee: Linde GmbH
Priority date: 2005-03-04
Filing date: 2006-02-28
Publication date: 2006-09-14
Anticipated expiration: 2026-02-28
Also published as: AU2006222326B2; US20090211297A1; DE102005010053A1; WO2006094676A1; RU2007136599A

Description

lox 259. HVneton. Vic 3444 AUSTRALIA o www.ocodemyXL.com o info@ocademyXt.com - a business of Tenco Services Pty Ltd o ABN 72 892 315 097 Free'9 1800 637 640 Interx +61 3 54 232558 Fox A 03 54 232677 Inter A +61 3 54 232677 TRANSLATION VERIFICATION. CERTIFICATE This is to certify that the attached document is an English translation of the -- German-language Patent Application PCT/EP2006/001805 and Academy Translations declare that the translation thereof is to the best of their knowledge and ability true and correct. August 27, 2007 m .................. PO Box 259, Kynet z Vic A STRALIA Date Stamp/Signature: AT Ref.: dcc-1966 Multilingual Technical Documentation Translation from German of PCT Application PCT/EP2006/001805 Helium production in LNG plants 5 The invention relates to a method for separation of a helium-rich fraction from a liquefied natural gas stream. Helium is usually recovered in large quantities from natural gas or natural gas fractions - such as those 10 which occur in so-called LNG baseload plants - i.e. from a gas mixture consisting primarily of methane, a high proportion of nitrogen and hydrocarbons. Smaller quantities of helium can also be separated from 15 the air and thus recovered in cryogenic air separation plants by means of low temperature air fractionation. Helium occurs in known natural gas reserves in concentrations up to about 0.2 mol percent. For this reason, technical recovery is only practical as part of 20 the aforementioned LNG baseload plants, since in these the inert helium is concentrated in the flash gas of the LNG storage tank. With helium recovery at the so-called "cold end" of LNG baseload plants it is therefore desirable to produce a constant quantity of helium even 25 with different natural gas compositions, although the different nitrogen concentrations of the natural gas lead particularly to different flash conditions for the helium-rich flash. Usually the liquefied natural gas present, which is under high pressure and the temperature 30 of which is nearly constant due to the coolant(s) of the LNG baseload plant, is first throttled to a medium pressure between 3 and 10 bar; the helium-rich flash gas produced in this process - which typically has a helium 2 content between 5% and 20% - is heated and supplied to a helium recovery plant as a feed fraction. Before this feeding, the helium-rich flash gas is warmed, 5 for example, against a purified stream of natural gas present under high pressure - which is drawn off before the actual liquefaction step of the liquefaction process and thus originates from the so-called "warm area" of the LNG baseload plant - in order to be able to use the cold 10 of the helium-rich flash gas for cooling and liquefaction of this additional natural gas stream. The amount of this additional natural gas stream is to be chosen here such that no noticeable change of the liquefaction capacity of the LNG baseload plant occurs, which is the case in a 15 broad range of stream quantities. However, with this procedure differences in natural gas quality with respect to the content of helium and nitrogen, which in turn have large differences in the 20 helium and nitrogen content in the flash gas separated from the liquefied natural gas as a consequence, cannot be taken into account. It is the object of the present invention to specify a generic method which avoids the aforementioned disadvantages in the separation of a 25 helium-rich fraction from a liquefied natural gas stream. To achieve this object, a generic method is proposed comprising the following process steps: a) expansion of the liquefied natural gas stream and 30 separation of a helium-rich fraction, b) warming of the helium-rich fraction against a natural gas stream to be cooled and liquefied and 3 c) feeding the natural gas stream, which was liquefied against the helium-rich fraction to be warmed, to the depressurised liquefied natural gas stream before and/or in the separation of the helium-rich fraction, 5 d) in which the total enthalpy of the mixture of the two aforementioned natural gas streams fed to the separation of the helium-rich fraction can be varied. Here the quantity flow of the aforementioned natural gas 10 stream to be cooled and liquefied is preferably set so no significant change of the liquefaction capacity of the LNG baseload plant occurs. In principle, the total enthalpy of the mixture of the 15 two aforementioned natural gas streams fed to the separation of the helium-rich fraction - here it is a two-phase stream - can result from - varying the supercooling conditions of the liquefied natural gas at the so-called "cold end" of the 20 liquefaction process; however, this would require an intervention in the operation of the LNG baseload plant, which is usually not desired, - warming the supercooled natural gas stream from the "cold end" of the LNG baseload plant against one or 25 more coolant streams of the LNG baseload process; this variant would also result in a typically undesired intervention in the operation of the LNG baseload plant, or - warming the supercooled natural gas stream from the 30 "cold end" of the LNG baseload plant by addition of a warmer natural gas stream from the "warm end" of the LNG baseload plant; this alternative results in an 4 increase of the throughput for the LNG baseload plant, which is why this alternative is preferred. Henceforth, the method according to the invention makes 5 it possible to handle the greatest variety of helium and nitrogen contents in the natural gas stream to be liquefied and in the liquefied natural gas stream. The helium-rich fraction and the natural gas stream to be cooled and liquefied, which are brought together in heat 10 exchange, can now be warmed or cooled against each other in a selective temperature-controlled manner. Thus the conditions for the expansion of the liquefied natural gas stream and the separation of the helium-rich fraction can be selectively regulated so that for various compositions 15 of liquefied natural gas streams a maximum separation and/or yield of helium is possible by expansion and separation of the helium-rich fraction. Extending the method according to the invention, it is 20 proposed that - according to the composition of the natural gas stream - the quantity flow of the helium-rich fraction supplied to the heat exchange and/or the quantity flow of the natural gas stream to be cooled and liquefied which is supplied to the heat exchange be 25 varied in such a way that the helium yield of the helium rich fraction remains essentially constant and/or is maximised. Further advantageous embodiments of the inventive method 30 are characterised in that - the helium-depleted, liquefied natural gas stream is depressurised and subjected to a fuel gas separation, 5 - the fuel gas fraction recovered in the fuel gas separation is warmed against the natural gas stream to be cooled and liquefied, - at least one substream of the natural gas stream to 5 be cooled and liquefied, at least one substream of the helium-rich fraction to be warmed and/or at least one substream of the fuel gas fraction to be warmed bypass(es) the heat exchange between the helium-rich fraction to be warmed and the natural gas stream to 10 be cooled and liquefied, - the heat exchange between the helium-rich fraction to be warmed and the natural gas stream to be cooled and liquefied takes place in at least one spiral-wound heat exchanger and/or at least one TEMA heat 15 exchanger, - the separation of the helium-rich fraction is implemented in a separator or washing column. The method according to the invention and further 20 embodiments thereof which represent the objects of dependent claims are explained in more detail below based on the embodiments depicted in the figures 1 and 2. As shown in figure 1, a liquefied natural gas stream 25 obtained from any natural gas liquefaction process is introduced via line 1, depressurised to a pressure between 3 and 10 bar in valve a and subsequent fed via line 2 to a separator D. A helium-rich gas fraction is drawn off at the head of this separator D via line 3. 30 In the heat exchanger E, which is preferably a spiral wound heat exchanger or a TEMA heat exchanger, the helium-rich gas fraction is warmed against a natural gas 6 stream to be cooled and liquefied, which is described in more detail below, and afterward supplied via line 4 for its further use, such as in a process in which a pure helium fraction is recovered. 5 A helium depleted liquid fraction is drawn off from the sump of the separator D via line 5, depressurised in valve b to a pressure between 1 and 5 bar and supplied via line 6 to its further use - possibly after prior 10 conveyance with a pump and intermediate storage in a storage tank at atmospheric pressure. The aforementioned natural gas stream to be cooled and liquefied is supplied via line 9 to the heat exchanger E. 15 This gaseous natural gas stream is drawn off from the natural gas liquefaction process, for example, after the separation of heavy hydrocarbons which is generally required. The quantity of this natural gas stream is preferably set so that no noticeable change in the 20 liquefaction capacity of the LNG baseload plant results from the helium separation D. The cold of the helium-rich fraction supplied via line 3 to the heat exchanger E is utilised to cool and liquefy 25 the natural gas stream supplied via line 9 to the heat exchanger E. It is subsequently added via line 10, which is provided with an expansion valve c, to the liquefied natural gas stream in line 2 before it is fed to the separator D. 30 Depending on the mixing temperature and pressure, different helium concentrations and quantities now result 7 in the helium-rich gas fraction 3 drawn off at the head of the separator D. The figure also shows two bypass lines, in which 5 regulating valves d and e are arranged respectively. By means of these bypass lines 7 and 11 the fractions carried in the lines 3 and 9 can bypass the heat exchanger E entirely or at least partially. 10 In accordance with the invention, the quantity flow of the natural gas stream fed to the heat exchanger E via line 9 can now be varied by means of the expansion valve c and/or the bypass line 11. The same applies for the helium-rich fraction fed to the heat exchanger E via line 15 3, since its quantity flow through the heat exchanger E can be regulated by means of the bypass line 7. Using the aforementioned regulation options, maximised helium yields or quantities in the helium-rich fraction 3 20 drawn off at the head of the separator D can be set or achieved even for different compositions of the liquefied natural gas stream. The supercooling temperature of the natural gas stream 25 carried via line segments 9 and 10 is set after the heat exchange E according to the quantity, composition, degree of supercooling and pre-pressure and thus the total enthalpy of the liquefied natural gas stream carried via line segments 1 and 2 as well as the quantity, 30 composition, pressure and temperature and thus the total enthalpy of the natural gas stream carried via line segments 9 and 10 in order to maximise the helium yield in the helium-rich fraction 3.

8 The objective of this procedure is to adjust the conditions in the separator D, i.e. the total enthalpy of the mixture, so that even with different compositions of 5 the natural gas streams 1 and 9 a maximum helium yield is achieved in the helium-rich flash gas 3 and 4 and at the same time the production of the LNG baseload process is not influenced or is only marginally influenced. In this manner an optimal feed fraction can be provided for a 10 downstream process for recovery of pure helium. If comparatively small temperature differences between 5 and 30 K occur in the heat exchanger E, it is preferably implemented as a plate heat exchanger. In the case of 15 larger temperature differences, it is advantageous to implement the heat exchanger E as a spiral-wound heat exchanger and/or TEMA heat exchanger. In particular when the quantities of the streams 20 introduced via the line segments 1 and 2 and/or line segments 9 and 10 remain approximately constant but their helium and nitrogen proportions vary, bypass line 11 can be used to set and regulate at the outlet of the heat exchanger E the supercooling temperature required in the 25 natural gas stream carried via line segments 9 and 10 for a maximum yield of helium. However, a maximum of 97% of the helium contained in the natural gas stream to be liquefied can be recovered with 30 the embodiment of the inventive method depicted in figure 1.

9 If - as shown in figure 2 - the separator D is replaced by a washing column (K), a helium yield up to 99.9% can be achieved. 5 For this it is necessary to feed the natural gas stream, which is cooled against the helium-rich gas fraction to be warmed and which is liquefied in the heat exchanger E' via line 10' to the washing column (K) as a stripping stream, while the liquefied natural gas stream 10 depressurised in valve a' is supplied via line 2' to the washing column (K) as a reflux. This increase in the helium yield does in fact require additional equipment and processing expense, but this 15 appears acceptable in view of the value of helium. The inventive method for separation of a helium-rich fraction from a liquefied natural gas stream thus enables the helium yields of even the most diverse liquefied 20 natural gas streams to be maximised. The expense of the regulation technology required for this is limited, so the implementation of the inventive method leads to additional costs which are insignificant.

Claims

1. A method for separation of a helium-rich fraction from a liquefied natural gas stream, which comprises 5 the following process steps: a) depressurisation (a, a') of the liquefied natural gas stream (1, 1') and separation (D, K) of a helium-rich fraction (3, 3'), b) warming (E, E') of the helium-rich fraction (3, 10 3') against a natural gas stream (9, 9') to be cooled and liquefied and c) addition of the natural gas stream (10, 10'), which was liquefied in the heat exchange (E, E') with the helium-rich gas fraction to be warmed, 15 to the depressurised liquefied natural gas stream (1, 1') [sic] before and/or in the separation (D, K) of the helium-rich fraction (3, 3'), d) in which the total enthalpy of the mixture (2, 2' [sic]) of the two aforementioned natural gas 20 streams fed to the separation (D, K) of the helium-rich fraction can be varied.

2. A method according to claim 1, characterised in that the temperature of the natural gas stream (10, 10') 25 liquefied in the heat exchange (E, E') with the helium-rich fraction (3, 3') to be warmed can be varied.

3. A method according to claim 1 or 2, characterised in 30 that the quantity flow of the helium-rich fraction (3, 3') supplied to the heat exchange (E, E') and/or the quantity flow of the natural gas stream (9, 9') to be cooled and liquefied, which is supplied to the 1l heat exchange (E, E'), is varied in such a way that the helium yield of the helium-rich fraction (3, 3', 4, 4') remains essentially constant and/or is maximised. 5

4. A method according to one of the claims 1 to 3, characterised in that at least one substream (11, 11') of the natural gas stream (9, 9') to be cooled and liquefied and/or at least one substream (7, 7') 10 of the helium-rich fraction (3, 3') to be warmed bypass (es) the heat exchange between the helium-rich fraction (3, 3') to be warmed and the natural gas stream (9, 9') to be cooled and liquefied. 15

5. A method according to one of the claims 1 to 4, characterised in that the heat exchange (E, E') between the helium-rich fraction (3, 3') to be warmed and the natural gas stream (9, 9') to be cooled and liquefied takes place in at least one spiral-wound 20 heat exchanger and/or at least one TEMA heat exchanger.

6. A method according to one of the claims 1 to 5, characterised in that the separation of the helium 25 rich fraction (3, 3') is implemented in a separator (D) or washing column (K).