DE2502959A1 - Natural gas liquefaction process - with circulating medium leakage compensated by higher boiling liquid fraction from feed stock - Google Patents

Abstract

In a closed refrigeration circuit for the liquefaction of natural gas, the circulating gas mixture is compressed, cooled and liquefied in several stages by partial condensation. The natural gas is partly split into fractions during its liquefaction. The liquid fraction consisting of hydrocarbons with a higher boiling point are recycled into the refrigeration circuit, after it has it has been evaporated and heated in a heat exchange with natural gas and the circulating medium. The corresponding part of the circulating medium is extracted from the closed circuit. This compensates for the leakage of components of the circulating medium. The capital investment and the operating costs for storage tanks of the individual components are saved.

Description

LINDE AKTIENGESELLSCHAFT

(H 712) ' H 75/02

La / Vo 21.1.1975

Process for liquefying natural gas

The invention relates to a method for liquefying natural gas with the aid of a closed refrigeration circuit in the one which serves as a circulating medium and contains natural gas components Oase mixture is compressed, cooled and gradually liquefied by partial condensation, with the in the individual temperature levels formed condensates relaxed, Vaporized, warmed up in the heat exchange with circulating medium and natural gas and returned to the cycle compressor.

A method for liquefying natural gas is already known, in which the required cold in a closed mixture circuit is generated. Within

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of the mixture cycle is the cycle medium that is made up of Methane, ethane, propane and higher boiling hydrocarbons, as well as nitrogen; compacted and a multi-level subjected to partial condensation at decreasing temperature levels. The accumulating in the condensation stages liquid fractions are against natural gas and circulating medium evaporated and warmed and then fed back to the cycle compressor (US Pat. No. 3,364,685).

A disadvantage of the known method is that, due to unavoidable leaks in the circuit, a constant Part of the circulating medium escapes from the circuit. To the To compensate for such leakage losses, the individual circuit components have so far been reintroduced into the circuit in an almost pure form introduced. However, this requires the individual components to be stored in a separate tank farm or the processing of already liquefied natural gas in a so-called "make-up unit". Both variants, however, have considerable implications additional investment and operating costs.

The invention is based on the object of developing a method of the type mentioned at the outset, which this Does not have any disadvantages.

According to the invention, this object is achieved in that the natural gas undergoes preliminary decomposition in the course of its liquefaction is subjected and that the

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falling fraction of higher boiling hydrocarbons at least is partially fed into the refrigeration cycle.

The invention succeeds in a simple one Way, at least 'part of the unavoidable leakage losses in a refrigeration cycle for liquefying natural gas, which is connected to a operated from several components existing circulating medium is to cover by those of the cycle gas components, which are contained in the natural gas to be liquefied anyway, immediately as a mixture of the natural gas in the course of its liquefaction separated and fed directly into the refrigeration cycle. It is therefore neither a stockpiling of the needed Circuit components in separate storage tanks also require processing of already liquefied natural gas in a so-called "makeup unit".

The amount not required to cover the leakage losses of the refrigeration circuit that is fed into the circuit Components are withdrawn from the circuit at a suitable point. The deductions are chosen in the same way as the stationary ones Condition of the circuit is maintained, i.e. that the enrichment or enrichment of one or more circuit components is prevented will. The excess circulating gas is withdrawn such that the sum> of the withdrawn cycle gas in ' its components is equal to the fraction fed into the circuit minus the leakage losses.

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Advantageously, the liquid fraction obtained during the pre-decomposition of the natural gas, which, in addition to nitrogen, already contains all the components of the cycle gas, i.e. in particular the higher-boiling hydrocarbons, is first evaporated and warmed up in the heat exchange with natural gas to be liquefied and then immediately after the last compression stage of the circuit compressor fed into the circuit. Feeding in at this point ensures that the individual components of the gas fed in can condense out again in the individual condensation stages of the circuit that correspond to their respective condensation levels.

Especially when the natural gas to be liquefied contains no or only very little nitrogen, so if the losses of the nitrogen cycle component are not immediate can be covered by nitrogen separated from the exhaust gas, it proves to be advantageous for maintenance the steady operating state of the circuit parts of the in the condensation stages of the circuit at higher temperature levels, resulting from the liquid fractions from the circuit deduct. In this case, the additional losses of the nitrogen cycle component are very low, as in these Fractions no or only a little nitrogen is dissolved. All fed into the circuit to cover leakage losses and

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now in excess in this containing hydrocarbons however, are contained in these liquid fractions.

If the natural gas to be liquefied is nitrogen is contained and this must be removed, it proves to be advantageous, in addition to the higher-boiling hydrocarbons the nitrogen fraction obtained in a further separation column to cover the nitrogen losses also in the Feed cycle.

In this case it proves to be useful to Maintaining the steady operating state of the circuit now parts of the in the higher condensation stages of the Circulation accumulating gaseous fraction to be withdrawn again from the circuit, as this is both nitrogen and the contain other hydrocarbons and are therefore particularly suitable for making the required adjustments.

Further explanations of the invention can be found in the exemplary embodiments shown schematically in the figures remove. In the figures, the same reference numbers are provided for the same parts of the device.

Show it:

Figure 1 shows an embodiment in which the liquefied

Natural gas contains little or no nitrogen Figure 2 shows an embodiment in which the liquefied Natural gas is enriched with nitrogen.

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According to Figure 1 is to be liquefied and already
Purified Erdgae in the composition
Methane - 94.8

Ethane. - 3.6

Propane - 0.8

(^ -Hydrocarbons - 0.8
via line 1 of the system under a pressure of about 40 ata
fed, in heat exchangers 2, 3 and 4 to about 200 K
cooled, partially condensed and subjected to a phase separation in a separator 5. The in the separator 5
Any liquid fraction already enriched in higher hydrocarbons is fed into a pre-separation column 6. The gaseous top product of the column 6, which only contains small amounts of higher-boiling hydrocarbons, is withdrawn via a line 7 and with the gaseous fraction from the
Separator 5 combined in heat exchanger 4 after it has been further cooled. Both fractions are now at least partially liquefied in a heat exchanger 8, subcooled in a heat exchanger 9 and then expanded via a throttle valve 10 into a storage tank 11 in which the natural gas is stored as a liquid end product.

The sump heating of the pre-decomposition column 6 takes place by a part of the natural gas to be liquefied, which is drawn off via a line 12, in a sump of the column 6 Heat exchanger 13 cooled and then via a Line 14 again with the rest of the natural gas to be liquefied

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is mixed. The head of the column 6 is cooled by a Part of the circuit running medium, which via a line 50 from the Circuit withdrawn, relaxed in a valve 51, in one Heat exchanger 52 evaporates and again via a line 53 is returned to the cycle.

The required cold is in a closed cooling circuit with a gas mixture in the composition Nitrogen ■ - 9j3 #

- 32

- 44 # C ^ H ₈ - 4.2 mol- # C ^ hydrocarbons -10.5 mol- #

generated as a circulatory medium. Here, the circulatory medium is in a first compressor stage 15 initially compressed to about 14 ata, partially condensed in a water cooler 16 and in a separator 17 subjected to a phase separation. The liquid fraction obtained in the separator 17 is in the heat exchanger 2 supercooled, expanded in valve 18, evaporated and warmed up in heat exchanger 2 against natural gas and circulating medium, and then again supplied to the suction side of the compressor stage 15. Through the partial condensation with subsequent phase separation of the cycle medium results already under an intermediate cycle pressure the advantage that the circulating medium is relatively strong higher boiling hydrocarbons can be enriched, the contribute significantly to the cooling capacity due to their relatively large heat of vaporization,

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without the risk of such high hydrocarbons being dragged along into the low-temperature part of the circuit and there leading to solid deposits and thus blockages of the corresponding heat exchanger cross-sections. The gaseous fraction obtained in the separator 17 is further compressed in a second compression stage 19 to the final circuit pressure of about 35 ata, partially condensed in the water cooler 20 and subjected to a phase separation in the separator 21. The liquid phase is supercooled in the heat exchanger 2, expanded in the valve 22 and then likewise evaporated and warmed up against natural gas and circulating medium. The gaseous fraction obtained in the separator 21 is partially condensed in the heat exchanger 2 and subjected to a phase separation in the separator 23. The liquid fraction occurring in this separator is also initially subcooled in the heat exchanger 3, then expanded in the valve 24 and finally evaporated in the heat exchanger 3 against natural gas and circulating medium, while the gas phase of the separator 2? partially condensed in the heat exchanger 3 and is now broken down in the separator 25 into a liquid and a gaseous phase. The liquid phase from the separator 25 is supercooled in the heat exchanger 4, expanded in the valve 26 and then evaporated in the heat exchanger 4 against natural gas and circulating medium. The gaseous fraction obtained in the separator 25 is partially condensed in a further condensation stage in the heat exchanger 4 and thereupon a phase separation in the separator 27,

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subjected. While the liquid fraction occurring in this separator is supercooled in the heat exchanger 8, expanded in the valve 28 and evaporated in the heat exchanger 8 against natural gas and circulating medium, the gaseous fraction from the separator 27 is at least partially liquefied in the heat exchanger 8, then supercooled in the heat exchanger 9 and finally relaxed in valve 29. This fraction, which at a temperature of about 108 K provides the peak cold required to liquefy the natural gas, is now evaporated in the heat exchanger 9, then heated together with the fractions from earlier condensation stages in the heat exchangers 8, 4, J> and 2 and then heated again supplied to the suction side of the first compressor stage 15.

To cover the inevitable leakage losses of circulating medium in such a circuit, according to the invention Bottom product of the pre-decomposition column 6, which contains all the components required in the refrigeration cycle except for the nitrogen are drawn off via a line J50, evaporated in the heat exchanger 2 and following the second compression stage 19 fed into the refrigeration cycle.

At the same time will _; To maintain the steady state, ie to prevent the enrichment or depletion of a circuit component, circuit gas is removed from the circuit in such a way that the sum of the withdrawn circuit gas is in its

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Components equal to the sum of the components of the sump product fed in minus those related to the individual components Leakage is. This requirement is met in the present exemplary embodiment, in the one to be liquefied Natural gas does not contain nitrogen, with which therefore the leakage losses of the cycle of "nitrogen do not get through from the natural gas separated nitrogen can be covered, thereby fulfilled that essentially parts of the accumulating in the separators 21 and 23 liquid fractions via lines 31 and 32 from the Circuit can be withdrawn. In these fractions, the cycle component nitrogen is not present at all or at most in a very small amount Contain quantities so that no nitrogen losses beyond the leakage losses of nitrogen due to the fume cupboards may occur.

It has been shown that due to their composition just the liquid fraction from the separator 21 is particularly suitable as a deduction. An extension the withdrawal possibility can also be achieved in that the liquid withdrawn via line 31 in a valve 33 relaxed and in a separator 3 ^ a repeated phase separation is subjected. As a result, the concentration shift occurring in the separator within the phases can also be avoided can be used to achieve the required comparisons. If necessary, a part of the can also be used for further correction The fraction fed to the suction side of the compressor stage 15 is withdrawn from the circuit via a line 35 and an expansion valve 36 will. 60983 1/0 132

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Overall, it thus results for the present example. That the residual gas withdrawn from the circuit via line 37 contains precisely those components which the circuit from were fed to the bottom of the column 6, but each component is reduced purely in terms of quantity by exactly the amount that the leakage losses of the circuit with regard to this component is equivalent to.

The invention thus makes it easy possible to compensate the leakage losses of a closed mixture circuit for the liquefaction of natural gas without doing this the individual circulatory components are available in pure form need to be asked.

The exemplary embodiment according to FIG. 2 shows the application of the invention to the liquefaction of nitrogen-containing natural gas. Here, the refrigeration circuit corresponds to that according to FIG. 1, so that it does not need to be explained again.

Since in this embodiment the natural gas to be liquefied also contains nitrogen, which is to be separated, there is now the possibility of compensating for the nitrogen leakage losses of the refrigeration circuit according to the invention. For this purpose, the natural gas is not only subjected to a pre-decomposition to separate hydrocarbons, but also to a further separation of nitrogen.

'• A

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According to Figure 2, the natural gas to be liquefied is in the composition

Nitrogen - 14 moles -; *

CH ₄ - 82

C ₂ H _6-3

C ₃ H ₈ - 0.5 mol #

C4 + KW - 0.5 MoI-Ji

fed to the system via line 1, in heat exchangers 2, 3 and 4 are at least partially liquefied and fed into the pre-decomposition column 6. The one from a valve 38 and one Line 39 existing bypass, through which part of the natural gas can bypass the heat exchanger 4, is used for setting Favorable temperature conditions in the pre-decomposition column 6. The bottom heating of the column 6 is carried out in this example part of the circulation medium of the refrigeration cycle, which withdrawn from the circuit via a line 40, in the heat exchanger 13 is cooled and fed back into the refrigeration circuit via a line 41.

The gaseous top product of the column 6, which consists essentially of lower-boiling hydrocarbons and Nitrogen, as well as a part of the return flow from column 6 will be cooled further in a heat exchanger 42 and in a second under a pressure of about 20 ata standing rectification column 43 relaxed, in which the natural gas is subjected to a nitrogen separation. The one in the heat exchanger

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The refrigeration required is available through the refrigeration cycle made by a portion of the supercooled in the heat exchanger 4 liquid fraction from the separator 25 via a line 44 withdrawn, relaxed in valve 45, evaporated in heat exchanger 42 and is then returned to the circuit via a line 46.

The bottom product of the column 43, which is essentially low-boiling hydrocarbons and contains only a little nitrogen, is further cooled in the heat exchangers 4, 8 and 9, expanded in the valve 10 to approximately atmospheric pressure and * then fed to the storage container 11 as a liquid end product.

The one that is relatively heavily enriched with nitrogen Top product of the column 43, however, is in the heat exchangers 4, 3 and 2 warmed and partly with part of the sump product which is expanded in valve 47 and evaporated in heat exchanger 2 from the column 6 mixed. This mixture is made via a Line 48 introduced immediately after the first compressor stage 15 in the refrigeration circuit, while not in the Remnants of the two fractions fed into the refrigeration cycle a line 49 is withdrawn from the system as residual gas. The gas mixture fed into the refrigeration circuit contains, as it is from the bottom product of column 6, which is rich in hydrocarbons and the nitrogen-rich overhead product of column 43.

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all components of the circulating medium carried in the refrigeration circuit and is therefore suitable for compensating for leakage losses all components carried in the cycle. To maintain the stationary operating state of the refrigeration circuit In this example, too, the part of the gas mixture introduced into the circuit does not have to cover the leakage losses more is needed, can be withdrawn from the cycle again without this leading to a depletion or enrichment of one or more Circulatory components leads. This is advantageously achieved by that now parts of the accumulating in the separators 23 and 25 gaseous fractions, which in addition to the hydrocarbons also contain nitrogen, via lines 50 and 51 from the

Circuit can be withdrawn.

2 sheets of drawings
10 claims

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Claims (1)

LINDE AKTIENGESELLSCHAFT (H 712) ^ * Claims 1.) Process for liquefying natural gas -with the help of a closed Refrigeration cycle, in which one is used as a circulating medium serving gas mixture containing natural gas components, compressed, cooled and gradually liquefied by partial condensation, with the in the individual temperature levels formed condensates relaxed, in heat exchange with Circulating medium and natural gas are vaporized, heated and returned to the circulating compressor, characterized in that that the natural gas is subjected to a preliminary decomposition in the course of its liquefaction, and that this is in the liquid Condition resulting fraction from higher boiling hydrocarbons at least partially fed into the refrigeration cycle will. 2. The method according to claim I _1, characterized in that the liquid fraction is evaporated and heated prior to being fed into the refrigeration circuit in heat exchange with natural gas and circulating medium. 609831/0132 LINDE AKTIENGESELLSCHAFT J5. Method according to claim 1 or 2, characterized in that that the higher-boiling hydrocarbons are fed into the refrigeration cycle immediately after the last Compression stage of the cycle compressor takes place. 4. The method according to one or more of claims 1 to 3, characterized in that part of the circulating medium is constantly its composition, the composition of the fraction fed in, minus the leakage losses of the circuit is withdrawn from the circuit. 5. The method according to claim 4, characterized in that parts those occurring in the warmer condensation stages of the circuit liquid fractions are withdrawn from the circuit. 6. The method according to claims 4 and 5, characterized in that part of the resulting in the second condensation stage liquid fraction is withdrawn from the circuit. 7. Process according to Claims 5 and 6, characterized in that that at least part of the withdrawn liquid fractions is relaxed and subjected to a phase separation, the the resulting liquid phase is withdrawn from the circuit and the gas phase is fed back to the circuit compressor will. 609831/0132 LINDE AKTIENGESELLSCHAFT 8. The method according to one or more of claims 1 to 4, characterized in that the natural gas to be liquefied is subjected to a nitrogen separation and that the nitrogen-rich fraction obtained in this case is at least partially together with at least some of the higher-boiling hydrocarbons obtained during pre-decomposition Fraction is fed into the cycle '. 9 · The method according to claim 8, characterized in that the Feeding takes place following an intermediate compression stage of the cycle compressor. 10. The method according to claims 8 and 9 »characterized in that that some of the gas phases occurring in the individual condensation stages of the circulating medium are withdrawn from the circuit will. 609831/01 32