EP3361196A1 - Method for the liquefaction of a fraction rich in hydrocarbon - Google Patents

Abstract

It is a method for liquefying a hydrocarbon-rich stream, in particular a natural gas stream, wherein
the liquefaction of the hydrocarbon-rich stream takes place against a mixed refrigerant cycle cascade consisting of three refrigerant mixture cycles,
wherein the first of the three refrigerant mixture cycles of the pre-cooling, the second refrigerant mixture cycle of the liquefaction and the third refrigerant mixture cycle of the supercooling of the liquefied hydrocarbon-rich stream is used, and
- The compressed refrigerant mixture of the first refrigerant mixture cycle is condensed against ambient air and fed to a container.

According to the invention
- At least when a complete condensation (E4) of the compressed refrigerant mixture (2) of the first refrigerant mixture cycle can not be realized, in the container (D1) resulting gas phase (6) of the partially condensed refrigerant mixture compressed (C1 '), at least partially condensed against ambient air (E8), relaxed (V5) and returned to the container (D1) (7),
- wherein the gas phase (6) to a pressure which is at least 1.5 times, preferably 2 to 2.5 times the pressure in the container (D1) corresponds, is compressed (C1 '), and
- The refrigerant mixture of the first refrigerant mixture cycle of at least two of the components N ₂ , CH ₄ , C ₂ H ₄ , C ₂ H ₆ , C ₃ H ₆ , C ₃ H ₈ , iC ₄ H ₁₀ and nC ₄ H ₁₀ , wherein the Proportion of components N ₂ and CH _{4 is} at most 1 mol%.

Description

• die Verflüssigung des Kohlenwasserstoff-reichen Stromes gegen eine aus drei Kältemittelgemischkreisläufen bestehende Kältemittelgemischkreislaufkaskade erfolgt,
• wobei der erste der drei Kältemittelgemischkreisläufe der Vorkühlung, der zweite Kältemittelgemischkreislauf der Verflüssigung und der dritte Kältemittelgemischkreislauf der Unterkühlung des verflüssigten Kohlenwasserstoff-reichen Stromes dient, und
• das verdichtete Kältemittelgemisch des ersten Kältemittelgemischkreislaufes gegen Umgebungsluft kondensiert und einem Behälter zugeführt wird.

The invention relates to a method for liquefying a hydrocarbon-rich stream, in particular a natural gas stream, wherein

• the liquefaction of the hydrocarbon-rich stream takes place against a mixed refrigerant cycle cascade consisting of three refrigerant mixture cycles,
• wherein the first of the three refrigerant mixture precooling cycles, the second mixed refrigerant cycle of the liquefaction, and the third mixed refrigerant cycle is for undercooling the liquefied hydrocarbon-rich stream, and
• the compressed refrigerant mixture of the first refrigerant mixture cycle is condensed against ambient air and fed to a container.

Natural gas liquefaction often uses a combination of two or three refrigeration cycles in the capacity range between two and ten million tonnes per year LNG. Here are different working principles (phase change or work-performing relaxation) and different refrigerants (pure substance or mixture) are used.

In the DE-A 102004054674 a generic method for liquefying a hydrocarbon-rich stream, in particular a natural gas stream is described, which has three arranged in cascade mixture cycles with four equal power compressor strands. Here, the refrigerant of the precooling circuit is completely condensed against the environment by means of air or water.

In cold locations with large day and season temperature differences (eg Russia, Canada, Alaska, etc.) most of the time (> 70% of annual hours) are ambient conditions that allow efficient liquefaction of the precooling refrigerant against air at moderate pressure (<30 bar). preferably <25 bar).

In addition, the optimum mixture composition of the pre-cooling circuit differs depending on the condensation temperature, which is achievable with air cooling. While a mixture adjustment depending on the season is technically feasible and economically justifiable, a daily mixture optimization in practice is not feasible. Therefore, a non-optimal refrigerant mixture must be selected, which can be safely condensed at any time of day to prevent failure of the liquefaction plant. This leads to increased production costs or, given the drive power of the refrigeration cycle compressors, to significant LNG production losses. During the "summer period" only a reduced LNG production is possible with a given drive power, since the specific power requirement increases with increasing air temperature (and thus increasing condensation temperature of the precooling circuit).

The object of the present invention is to specify a generic method for liquefying a hydrocarbon-rich stream, in particular a natural gas stream, which enables energetically optimal plant operation based on air cooling throughout the year. Furthermore, the expected LNG production loss during the "summer period" should be minimized.

• zumindest dann, wenn eine vollständige Kondensation des verdichteten Kältemittelgemisches des ersten Kältemittelgemischkreislaufes nicht realisiert werden kann, die in dem Behälter anfallende Gasphase des teilkondensierten Kältemittelgemisches verdichtet, gegen Umgebungsluft zumindest teilkondensiert, entspannt und in den Behälter zurückgeführt wird,
• wobei die Gasphase auf einen Druck, der wenigstens dem 1,5-fachen, vorzugsweise dem 2- bis 2,5-fachen des Drucks in dem Behälter entspricht, verdichtet wird, und
• das Kältemittelgemisch des ersten Kältemittelgemischkreislaufes aus wenigstens zwei der Komponenten CH₄, N₂, C₂H₄, C₂H₆, C₃H₆, C₃H₈, iC₄H₁₀ und nC₄H₁₀ besteht, wobei der Anteil der Komponenten CH₄ und N₂ maximal 1 mol-% beträgt.

To solve this problem, it is proposed that

• at least when a complete condensation of the compressed refrigerant mixture of the first mixed refrigerant cycle can not be realized, the resulting in the container gas phase of the partially condensed refrigerant mixture, at least partially condensed against ambient air, relaxed and returned to the container,
• wherein the gas phase is compressed to a pressure which is at least 1.5 times, preferably 2 to 2.5 times the pressure in the container, and
• the refrigerant mixture of the first refrigerant mixture cycle of at least two of the components CH ₄ , N ₂ , C ₂ H ₄ , C ₂ H ₆ , C ₃ H ₆ , C ₃ H ₈ , iC ₄ H ₁₀ and nC ₄ H ₁₀ , wherein the proportion the components CH ₄ and N _{2 is} at most 1 mol%.

According to the invention, the compressor of the precooling circuit can now be operated independently of the air temperature with a constant discharge pressure. For a given Refrigerant mixture composition is no longer possible from a certain air temperature, a total condensation of the compressed refrigerant. Consequently, in the container, in addition to a first liquid refrigerant phase, a gas phase, which is enriched with highly volatile components of the refrigerant mixture.

The withdrawn from the container gas phase of the refrigerant mixture is subjected according to the invention an additional compression and compressed to a pressure corresponding to at least 1.5 times, preferably from 2 to 2.5 times the pressure in the container. The compressed refrigerant is then cooled against ambient air and thereby preferably at least partially condensed. As long as the dissipated in the condensation heat output is greater than the mechanical power supplied by the compression, falls in the subsequent expansion of the compressed gas phase in addition to a remaining gas phase, a second liquid refrigerant fraction. Overall, a cooling of the refrigerant in the container below the outlet temperature of the heat exchanger used for the partial condensation of the compressed refrigerant mixture, which eventually leads to the total condensation of Vorkühlkältemittels. Together with the first liquid refrigerant phase, the fully liquefied refrigerant can now be removed from the sump of the container.

The inventive method requires that in addition to the usual Kältemittekomponenten C ₂ H ₄ , C ₂ H ₆ , C ₃ H ₆ , C ₃ H ₈ , iC ₄ H ₁₀ and nC ₄ H ₁₀ in the refrigerant mixture of the precooling refrigeration cycle, the proportion of components CH ₄ and N _{2 is} at most 1 mol%.

All compressor trains can be powered by any combination of electric motor, gas turbine and steam turbine. In this case, the power equality of the drives for the compressors of the refrigerant mixtures of the three refrigerant mixture cycles is preferably maintained. The power requirement of the additional compressor required for the compression of the gas phase in the tank depends on the location and the mode of operation. In general, the power of this additional compressor is less than the individual power of each of the four other machine trains. Among the terms and terminology used "power equality", "substantially equal performance compressor" or "substantially identical and / or equal power drives" are compressors or Drives whose performances do not differ by more than +/- 2%.

Due to the additionally installed compressor or drive power for the precooling circuit, the power reduction of the gas turbines caused by the higher air temperature can be at least partially compensated and the annual system capacity increased.

The inventive method for liquefying a hydrocarbon-rich stream makes it possible to optimize the mixture composition and final pressure of the cycle compressor of the pre-cooling refrigeration cycle for operation at low air temperature. At a low air temperature, the additional compressor to be provided according to the invention can be taken out of operation. As the air temperature rises, the pressure in the tank rises until the power limit of the precooled refrigeration cycle compressor has been reached and the additional compressor has to be started. The procedure according to the invention requires no mixture adaptation and can therefore be completely automated in a simple manner. Thus, day / night fluctuations of the air temperature can be optimally considered energetically. The limit value of the air temperature, from which the additional compressor must be operated, is selected so that the cycle compressor of the pre-cooling refrigeration cycle - and the compressors of the liquefaction and the subcooling refrigeration cycle - are always in the energetically favorable characteristic range. Thus, the annual liquefaction power can be maximized, since it is always driven in the optimum efficiency range.

The inventive method for liquefying a hydrocarbon-rich stream and other embodiments thereof, which form the subject of the dependent claims, are described below with reference to in the FIG. 1 illustrated embodiment explained in more detail.

When using the FIG. 1 cooling, liquefaction and supercooling of the hydrocarbon-rich stream, which is supplied via line A to the heat exchanger E1, against a refrigerant mixed cycle cascade, consisting of three mixed refrigerant circuits.

The hydrocarbon-rich stream A to be liquefied is cooled in the heat exchanger E1, which is preferably a so-called wound heat exchanger, against the evaporating refrigerant mixture stream 5 of the first mixture cycle, and then fed via line B to the heat exchanger E2. In this, the hydrocarbon-rich stream is liquefied against the evaporating refrigerant mixture stream 15 of the second refrigeration cycle. After liquefaction, the hydrocarbon-rich stream C is supplied to a third heat exchanger E3 and subcooled therein against the evaporating refrigerant mixture stream 28 of the third refrigeration cycle. The supercooled liquid product D is then fed to its further use and / or (intermediate) storage. Also, the heat exchangers E2 and E3 are preferably formed as a wound heat exchanger.

While the subcooling refrigeration cycle comprises two compressors C3 and C3 'connected in series, the precooling and liquefaction refrigerating circuits each have a compressor C1 and C2, respectively. The compressors C1, C2, C3 and C3 'are identical or substantially identical in their performance. As a result, the power requirement of each compressor can be provided by an identical or substantially identical drive. As drives for the compressors preferably gas turbines, steam turbines and / or electric motors are used.

The compressed in the compressor C1 refrigerant mixture of the first mixture cycle is cooled in the heat exchanger E4 against ambient air and at least partially condensed. It is then fed to the container D1, from whose sump the liquid refrigerant mixture 4 is withdrawn and fed to the heat exchanger E1. In this, the refrigerant mixture is cooled and expanded after deduction at the cold end of the heat exchanger E1 in the valve V1. The expanded refrigerant mixture 5 is supplied to the jacket space of the heat exchanger E1, vaporized in this against the hydrocarbon-rich stream A and the still to be described, cooling high-pressure streams of the mixture cycles and then fed via line 1 again to the cycle compressor C1.

The compressed in the compressor C2 refrigerant mixture 11 of the second refrigeration cycle is preferably completely liquefied in the heat exchanger E5; If only a partial condensation in the heat exchanger E5 can be realized, the total condensation takes place in the downstream heat exchanger E1, to which the at least partially condensed refrigerant mixture 12 is supplied. In the heat exchangers E1 and E2, the refrigerant mixture is cooled 12/13 and relaxed after deduction at the cold end of the heat exchanger E2 in the valve V2, so that a gas phase and a liquid phase formed. The expanded refrigerant mixture 14 is fed to a refrigerant collector D2. For this, the liquid phase 15 and the gas phase 15 'via the control valves a and b are supplied to the jacket space of the heat exchanger E2. The two-phase mixture is evaporated in the jacket space against the hydrocarbon-rich stream B to be liquefied and the high-pressure streams of the second and third mixture circuits 13/25 to be cooled, and then fed again via line 10 to the cycle compressor C2.

The compressed in the low-pressure compressor C3 refrigerant mixture 21 of the third refrigeration cycle is cooled in the heat exchanger E6 and then compressed in the high-pressure compressor C3 'to the circuit pressure. In the heat exchanger E7, the compressed refrigerant mixture 23 is cooled to ambient air and in the downstream heat exchangers E1, E2 and E3, the refrigerant mixture 24/25/26 is condensed and supercooled. After deduction at the cold end of the heat exchanger E3, the refrigerant mixture is expanded in the valve V3, so that a gas phase and a liquid phase formed. The expanded refrigerant mixture 27 is supplied to a refrigerant collector D3. For this, the liquid phase 28 and the gas phase 28 'via the control valves c and d are supplied to the jacket space of the heat exchanger E3. The two-phase mixture is evaporated in the jacket space against the hydrocarbon-rich stream C to be subcooled and the high-pressure stream of the third mixture circuit 26 to be cooled, and then fed again via line 20 to the low-pressure compressor C3.

According to the invention, an additional high-pressure compressor C1 'is provided, to which the gas phase 6 arising in the container D1 is supplied. As described, a total condensation of the compressed refrigerant 2 in the heat exchanger E4 is no longer possible for a given refrigerant mixture composition above a certain air temperature. Consequently, in the container D1 in addition to a first liquid refrigerant phase, a gas phase, which is enriched with volatile components of the refrigerant mixture. The withdrawn from the container D1 gas phase 6 is compressed by the compressor C1 'to a pressure which corresponds to at least 1.5 times, preferably from 2 to 2.5 times the pressure in the container D1. The compressed refrigerant 7 is cooled in the heat exchanger E8 against ambient air and thereby preferably at least partially condensed. As long as the dissipated in the condensation heat output is greater than the mechanical power supplied by the compression, falls in the subsequent expansion in the valve V5 in addition to a remaining gas phase, a second liquid refrigerant fraction. Overall, a cooling of the refrigerant in the container D1 below the outlet temperature of the heat exchanger used for the partial condensation of the compressed refrigerant mixture E4, which eventually leads to the total condensation of Vorkühlkältemittels. Together with the first liquid refrigerant phase, the completely liquefied refrigerant 4 can now be taken from the sump of the container D1.

The inventive method, however, requires that the refrigerant mixture of the first refrigerant mixture cycle of at least two of the components N ₂ , CH ₄ , C ₂ H ₄ , C ₂ H ₆ , C ₃ H ₆ , C ₃ H ₈ , iC ₄ H ₁₀ and nC ₄ H ₁₀ , wherein the proportion of the components N ₂ and CH _{4 is} at most 1 mol%.

The additionally to be provided compressor C1 'is preferably carried out only single-stranded and with electric motor drive. Zweisträngigkeit and other drives such as gas turbine or steam turbine are in principle also possible.

Claims (3)

A process for liquefying a hydrocarbon-rich stream, in particular a natural gas stream, wherein the liquefaction of the hydrocarbon-rich stream takes place against a mixed refrigerant cycle cascade consisting of three refrigerant mixture cycles, wherein the first of the three refrigerant mixture cycles of the pre-cooling, the second refrigerant mixture cycle of the liquefaction and the third refrigerant mixture cycle of the supercooling of the liquefied hydrocarbon-rich stream is used, and the condensed refrigerant mixture of the first mixed refrigerant cycle is condensed against ambient air and fed to a container, characterized in that - At least when a complete condensation (E4) of the compressed refrigerant mixture (2) of the first refrigerant mixture cycle can not be realized, in the container (D1) resulting gas phase (6) of the partially condensed refrigerant mixture compressed (C1 '), at least partially condensed against ambient air (E8), relaxed (V5) and returned to the container (D1) (7), - wherein the gas phase (6) to a pressure which is at least 1.5 times, preferably 2 to 2.5 times the pressure in the container (D1) corresponds, is compressed (C1 '), and - The refrigerant mixture of the first refrigerant mixture cycle of at least two of the components N ₂ , CH ₄ , C ₂ H ₄ , C ₂ H ₆ , C ₃ H ₆ , C ₃ H ₈ , iC ₄ H ₁₀ and nC ₄ H ₁₀ , wherein the Proportion of components N ₂ and CH _{4 is} at most 1 mol%. A method according to claim 1, characterized in that the compressors (C1, C2) of the first and second mixed refrigerant cycle and the compressors (C3, C3 ') of the third mixed refrigerant cycle are driven by two substantially identical and / or equal power drives, being used as drives for the compressors preferably gas, steam turbines and / or electric motors are used. A method according to claim 1 or 2, characterized in that in the container (D1) resulting gas phase (6) of the partially condensed refrigerant mixture in an additional compressor (C1 '), which is preferably driven by an electric motor, is compressed.

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