DE2440215A1 - Liquefaction of low-boiling gases - by partial liquefaction with mixed liquid coolant and further cooling with expanded gas coolant - Google Patents

Abstract

Low boiling gases are liquified and further cooled by coolant circulated in multiple cycles, in which (A) cooling and at least partial liquefaction takes place by heat exchange with a liquid multi-component mixt., followed by (B) complete liquefactin and further cooling in heat exchange with an expanded gaseous coolant. Used in the liquefaction of natural gas supplies, esp. in base load plants. Plant is simplified and energy requirements reduced in comparison with known processes using initial cooling by a propane cycle followed by several multi-component cycles. Vaporisation of the multi-component mixt. occurs not at a fixed but at a sliding temp., which by choice of vaporisation pressure and compsn. allows the temp. range of vaporisation to be matched to the cooling curve of the gas. Temp. difference in heat exchange and hence energy losses, are low.

Description

LINDE AKTIENGESELLSCHAFT

(H 823)

H 74/048

La / p 21.8.74

Process for liquefying and subcooling a low-boiling gas

The invention relates to a method for liquefying and ' Subcooling of a low-boiling gas by cooling with refrigerants in several refrigeration circuits.

A method for liquefying and subcooling natural gas is already known, in which the natural gas in the Heat exchange with a propane circuit cooled and liquefied in heat exchange with a mixture circuit and is hypothermic. Propane becomes propane within the propane circuit compressed, liquefied and relaxed in three stages. After each expansion stage, part of the propane is

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gas and the multi-component mixture of the mixture cycle evaporates whereby the natural gas is pre-cooled and the multi-component mixture, which essentially consists of nitrogen, methane, ethane and Propane exists, is partially condensed. The partially condensed multicomponent mixture then becomes one Phase separation subjected. The resulting liquid fraction is supercooled, relaxed and against liquefying Natural gas, against the gaseous fraction resulting from the phase separation, which also differs during this heat exchange liquefies, and evaporates against itself, whereby the hypothermia takes place. The liquefied gaseous Fraction is also supercooled, expanded and vaporized against natural gas and itself, thereby creating the natural gas and the liquefied gaseous fraction are supercooled. After evaporation, both fractions become common again the circuit compressor of the mixture circuit fed. (DT-OS 1 9b0 301.).

This known method is energetically unfavorable because in particular the heat conversion in the mixture circuit is very high. The entire for subcooling or for liquefaction and Subcooling of the fractions resulting from the phase separation required in addition to the cold Liquefaction and subcooling of the natural gas required cold to be applied by the circuit. Come in addition, that the evaporation of the two fractions must necessarily take place at relatively low pressure, with the result that the suction pressure of the cycle compressor is low and

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thus its effective compression work is high. In addition, it has been shown that due to the fact that that a liquid mixture is evaporated in the coldest heat exchangers, the pressure drop in these heat exchangers is disadvantageously high.

The outlay on equipment for carrying out the known method is also very high, since in the area of pre-cooling for Achieving a reasonably good approximation of the heating curve for propane to the cooling curve for natural gas at least three expansion stages with a corresponding number of expansion valves and heat exchangers are required are.

The invention is based on the object of a simple and yet energy-efficient method for liquefying and supercooling to develop a low-boiling gas.

This object is achieved in that the cooling and at least partial liquefaction of the gas in heat exchange with a liquid multi-component mixture and the complete liquefaction and subcooling of the gas in heat exchange takes place with a relaxed gaseous refrigerant.

The method according to the invention is very advantageous both from the point of view of equipment and energy. Through the

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use of a mixture cycle for precooling and at least partial liquefaction of the gas can be transferred to the gas to be liquefied by means of a single circuit at constant Pressure can be given off cold over a large temperature difference. This is not the case with the known method the case. Here are to bridge a similar temperature difference several different pressure levels and consequently several relief valves and Heat exchanger required. In addition, the subject of the invention in the area of cooling and liquefaction of the low-boiling gas, the evaporation of the liquid multicomponent mixture does not occur at constant but at sliding rates Temperatures takes place. Each evaporation temperature corresponds to the boiling diagram of the multi-component mixture assigned a certain mixture composition. By choosing the evaporation train accordingly and the composition of the multicomponent mixture, the temperature range of the evaporation can therefore be determined very well adapt to the cooling curve of the low-boiling gas. The temperature differences in the heat exchanges are small and the energy losses are therefore low.

The deep freezing of the gas, that is to say its complete liquefaction and subcooling, takes place according to the invention in Heat exchange with a gaseous refrigerant, which, according to a further feature, is advantageous initially in heat exchange cooled with the multicomponent mixture and then

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is relaxed while doing work. With this measure, a very good temperature range can also be achieved in the lowest temperature range Adjustment of the heating curve of the refrigerant to the cooling curve of the gas, as both curves in this The area is relatively flat and therefore there is little temperature difference are given even in the coldest court. A Another advantage is that the heat conversion of such an expander circuit is relatively low, since the entire Cold generated by the relaxation can be transferred directly to the gas to be treated. It doesn't need how it is the case with the known method, a part of the cold generated in the circuit for liquefaction and subcooling of the circulating medium itself. Moreover, it has been shown; * ,, that in contrast to the known Process the pressure loss in the heat exchanger, in which the warming up of the expanded gaseous refrigerant takes place, are very low. In addition, the expansion of the gaseous refrigerant to a relatively high final pressure take place, which in turn has a beneficial effect on the required compression work of the circuit compressor.

The method according to the invention is particularly suitable for liquefying natural gas in so-called "base-load systems".

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In this case, a multi-component mixture is expediently used, which is composed of hydrocarbons with a, composed of two, three and four and possibly more carbon atoms, while as a gaseous refrigerant Nitrogen, a gas that boils lower than natural gas, is an option.

Further explanations of the invention can be found in the exemplary embodiments shown schematically in the figures refer to, wherein the same parts of the device are shown with the same reference numerals.

Show it:

Figure 1 shows an embodiment of the invention for liquefaction of natural gas

FIG. 2 shows a further embodiment; FIG. J5 shows a third embodiment

According to Figure 1, in which an exemplary embodiment of the invention for the liquefaction and subcooling of natural gas is shown, the natural gas to be treated is fed to the plant via a line 1 under a pressure of about 50 ata. After his Cooling down and complete liquefaction in the heat exchangers 2 and 3 is the natural gas in a heat exchanger

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4 subcooled and via a valve 5 into a storage container 6 relaxed. The flash gas produced during the expansion is counteracted in the heat exchangers 4, 3 and 2 Natural gas is heated and drawn off as fuel gas from the system via a line 7.

The one required for cooling and liquefying the natural gas Cold is generated by a mixture cycle and the cold required for subcooling by a nitrogen expander cycle made available.

In the mixed cycle, a multicomponent mixture consisting of methane, ethane, propane and butane is compressed in the cycle compressor 8, partially condensed in the "water cooler 9 and subjected to a phase separation in the separator 10. The liquid fraction obtained in the separator 10 is expanded in the valve 11 and counteracted in the heat exchanger 2 cooled natural gas, against the nitrogen expander cycle and against the accumulating in the separator 10 gaseous fraction which liquefies in this heat exchanger, evaporated. the resulting separator 10 gaseous fraction is GESS to liquefy in the heat exchanger 2 in the heat exchanger 3 ^en itself subcooled expanded in an expansion valve 12 and in the heat exchanger J against liquefying natural gas, against the nitrogen of the expander

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cycle and evaporated against itself. After further heating in the heat exchanger 2, it is together with the liquid fraction from the separator 10 is fed back to the circulation compressor 8.

Within the nitrogen expander circuit, the nitrogen compressed in the brake fan 15 to the required final pressure and cooled in the water cooler 14 is first cooled in the heat exchangers 2 and 3 against the multicomponent mixture and then in a further heat exchanger 15 against itself. Thereupon, the nitrogen is expanded in a working manner in a turbine 16 and warmed in the heat exchanger 4 against natural gas, which is supercooled during this heat exchange. The energy released by the work-performing expansion in the turbine 16 is transmitted directly to the brake fan IJ coupled to the turbine Io. In the heat exchanger 15, the nitrogen which has been expanded to perform work is further heated in the heat exchange with compressed nitrogen and fed to the first stage 17 of a two-stage cycle compressor 18 and compressed to a medium pressure. The nitrogen is then first cooled again in the water cooler 19 and then in the heat exchanger 2 against the multicomponent mixture. The nitrogen is then further compressed in the second compression stage 20 of the cycle compressor 18. After renewed cooling in the water cooler 21, the nitrogen is now fed to the final compression in the brake fan 13. The intermediate cooling of the nitrogen after its compression in the first

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Compression stage 17 in the heat exchanger 2 proves to be very advantageous because it is reduced due to the cooling Volume flow, the energy requirement of the second compression stage 20 is significantly reduced and this is also smaller can be interpreted.

From an energetic point of view, it has proven to be particularly favorable that natural gas exchanges heat with the multicomponent mixture in the heat exchangers 2 and 3 already completely liquefied and the nitrogen expander circuit exclusively to be used for subcooling the natural gas.

Another exemplary embodiment of the invention is shown in FIG shown. As in the exemplary embodiment according to FIG. 1 the cooling and liquefaction of the natural gas takes place in the heat exchangers 2 and 3 in heat exchange with a mixture circuit and the subcooling in the heat exchanger 4 Im heat exchange with a nitrogen expander circuit, but now the natural gas in the heat exchanger 4 is so strong is undercooled so that no more flash gas is produced during the subsequent expansion in valve 5. In contrast to FIG. 1 also takes place in this exemplary embodiment the compression of the nitrogen in the cycle compressor 18 in one stage.

There is an essential difference between the two embodiments but in the conception of the Gomisch cycle.

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According to FIG. 2, the multi-component mixture is compressed in two stages in compression stages 22 and 23. After the first compression stage 23, the multi-component mixture is partially condensed under medium pressure in the water cooler 24 and subjected to a phase separation in the separator 23. The resulting liquid fraction is supercooled against itself in the heat exchanger 2, expanded in the valve 2b and then evaporated and heated in the heat exchanger 2 against the nitrogen of the expander circuit, against natural gas, against the gaseous fraction from the separator 25 and against itself. The gaseous fraction occurring in the separator 25 is compressed in the second compression stage 23 to the final pressure of the circuit, cooled in the water cooler 27 and liquefied in the heat exchanger 2. This fraction is then supercooled in the heat exchanger 3, expanded in the valve 28 and ^{evaporated in the heat exchanger 3 S e} ß ^en the nitrogen expander circuit, against liquefying natural gas and against itself. Both fractions are now fed together again to the first compression stage 22 of the cycle compressor. The concept described results in the following advantages: Due to the partial condensation and phase separation of the multicomponent mixture after the first compression stage, i.e. at intermediate pressure, the multicomponent mixture can be used to a greater extent with higher-boiling hydrocarbons,

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such as propane, butane and possibly even higher boiling, which is due to the relatively high heat of evaporation this higher-boiling hydrocarbons has a beneficial effect on the refrigeration capacity of the circuit. It has been shown that such an increase in the concentration of the multicomponent mixture of higher hydrocarbons is not readily possible in a mixture circuit according to FIG. Here were at an increase in concentration higher hydrocarbons are dragged along in the lowest temperature range of the circuit, whereby the evaporation temperature in this area is undesirable Way would be increased and, if necessary, blockages in the corresponding heat exchanger cross-sections due to solid separations could occur. Due to the partial condensation and separation of the higher-boiling hydrocarbons already after the intermediate compression, however, the partial pressure of these hydrocarbons in the in the The multicomponent mixture reaching lower temperature ranges is kept so low that there is no undesirable increase the evaporation temperature occurs.

Another exemplary embodiment of the invention is shown in FIG shown.

According to this figure, the cooling and liquefaction of the natural gas drawn in via line 1 takes place in the heat exhaust

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exchangers 2, 3 and 29 against evaporating multi-component mixture and the subcooling in heat exchanger 4 against work-performing relaxed nitrogen. The nitrogen expander cycle is very similar to that shown in FIG. However The deepest cooling of the nitrogen takes place here before it is expanded in the turbine 16 in the course of heat exchange Multi-component mixture which evaporates in the heat exchanger 29.

The mixture circuit according to FIG. 3 differs from that described in FIG. 2 essentially in that the gaseous fraction obtained during the intermediate condensation of the multicomponent mixture in the separator 25 is not directly fed in the second compression stage 25, but im Heat exchanger 2 is partially condensed again and subjected to a further phase separation in separator 30. The resulting liquid fraction is supercooled in the heat exchanger 3 and expanded in the valve 31, while the in the separator 30 resulting gaseous fraction now in the second compression stage 23 is compressed to the circuit pressure. This F -action is used in the heat exchangers 2, 3 and 29 liquefied and supercooled, relaxed in valve 32 and evaporated in the heat exchanger 29 against the nitrogen of the expander circuit against natural gas and against itself.

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Claims (11)

2441121b LINDE AKTIENGESELLSCHAFT - ir - claims 1._ 'Process for liquefying and subcooling a low-boiling gas by cooling with several refrigeration circuits guided refrigerants, characterized in that the cooling and at least partial liquefaction of the Gas in heat exchange with a liquid multicomponent mixture and complete liquefaction and subcooling of the gas takes place in heat exchange with a relaxed gaseous refrigerant. 2. The method according to claim 1-, characterized in that the gaseous refrigerants cooled after their compression in heat exchange with the multi-component mixture and then is relaxed while doing work. 3. The method according to claims 1 and 2, characterized in that that the compression of the gaseous refrigerant takes place in several stages and that at least after one compression stage the gaseous refrigerant exchanges heat with the Multi-component mixture is cooled. 609810/0132 Z4 4U2Ib LINDE AKTIENGESELLSCHAFT - 14 - 4. The method according to claims 1 to 3 »characterized in that that the gaseous refrigerant used is a gas whose boiling point is lower than the boiling point of the one to be liquefied Gas. 5- method according to claims 1 to 4, characterized in that that the low-boiling gas to be liquefied is natural gas and the gaseous refrigerant is nitrogen. 6. The method according to claims 1 to 5 *, characterized in that that the low-boiling gas to be liquefied is completely liquefied in heat exchange with the multicomponent mixture will. 7 · Process according to claims 1 to 6, characterized in that the multicomponent mixture according to its at least single-stage compression is partially condensed in at least one condensation stage and that in each condensation stage accumulating gaseous fraction separated and in heat exchange with the relaxed liquid fraction is liquefied. 8. The method according to claims 1 to J, characterized in that the fractions obtained in each condensation stage are supercooled before they are relaxed. 6 03810/0132 2UU21S LINDE AKTIENGESELLSCHAFT - 15 - 9. The method according to claims 1 to 8, characterized in that that the compression of the mixture of multiple components takes place in two stages, that after the first compression stage resulting multicomponent mixture is partially condensed and subjected to a phase separation and that the The liquid fraction arising from the phase separation is supercooled and relaxed, while the liquid fraction arising from the phase separation gaseous fraction is fed directly to the second compression stage. 10.Verfahren according to claim 9, characterized in that the resulting in the partial condensation gaseous fraction is partially condensed in heat exchange with the relaxed liquid fraction and that the resulting liquid fraction is supercooled and relaxed, while the resulting gaseous fraction immediately the second Compression stage is supplied. 11. The method according to claims 1 to 10, characterized in that that the multicomponent mixture of hydrocarbons with one, two, three, four and optionally also five and consists of more carbon atoms. 6uy8 1 υ / '■ 1 32 , 4fr Blank page