RU2537480C2 - Method of liquidising flow with high content of hydrocarbons - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: described is method of liquidising fraction with high content of hydrocarbons with simultaneous removal of fraction with high C₂₊ content with cooling and fraction liquefying taking place with indirect heat-exchange by means of mixture of refrigerants of circulation circuit of refrigerant mixture, in which mixture of refrigerants is subjected to at least two-step compression, and separation of fraction with high C₂₊ content taking place at regulated temperature level, with mixture of refrigerants being separated into gaseous and liquid fractions, both fractions are overcooled, expand, in fact, to pressure of absorption of first stage of compressor and are at least partly evaporated. In accordance with invention at least one partial flow (19, 24) of liquefied, first of all gaseous fraction of refrigerant mixture (15) is expanded (j, h) and admixed to expanded liquid fraction of refrigerant mixture (22).

EFFECT: invention is aimed at creation of reliable method of liquefying fraction with high content of hydrocarbons and at provision of effective and controlled removal of ethane and higher hydrocarbons in the course of natural gas liquefying.

6 cl, 2 dwg

Description

The invention relates to a method for liquefying a high hydrocarbon fraction while removing a high C ₂₊ fraction, while cooling and liquefaction of the high hydrocarbon fraction occurs by indirect heat exchange using a mixture of refrigerants in the circulation circuit of the refrigerant mixture, in which the mixture of refrigerants is subjected to at least two-stage compression and removing the fraction having a high content of C ₂₊ occurs at a controlled temperature level, the refrigerant mixture section etsya into a gaseous and a liquid fraction, both fractions supercooled, extend substantially up to the suction pressure of the first compressor stage and at least partially evaporated.

Such a method of liquefying a high hydrocarbon fraction is known, for example, from DE-A 19722490. Such liquefaction methods are used, for example, in liquefying natural gas. With methods of this kind, in the standard case, it is necessary to remove certain components, since at the required low temperatures they would precipitate and / or degrade the quality of the product according to the specification. In the simplest case, it is enough to provide a separator, which serves to remove undesirable components from the fraction with a high hydrocarbon content to be liquefied. Selective removal of the lighter constituents of natural gas, such as, for example, ethane, on the contrary, poses higher requirements both to the implementation of the method and to the possibility of regulation under changing boundary conditions.

In technological processes of liquefying natural gas with a capacity of small to medium - this should be understood as the norms of productivity from 30,000 to 1 million tons of LNG (liquefied natural gas, LNG) per year - circulation circuits of the mixture, which include only one cycle compressor, are often used they are also called SMR (Single Mixed Refrigerant) processes. They have the disadvantage that the liquid phase of the refrigerant can only evaporate at one pressure level. Therefore, the purposeful establishment and regulation of the desired temperature profile is difficult, since the number of possibilities of intervention or, accordingly, the degrees of freedom in such technological processes is limited. Corresponding temperature profiles are necessary, for example, in order to facilitate, up to a certain temperature, the partial condensation of the high-hydrocarbon fraction to be liquefied, which is necessary to remove undesirable components.

It is an object of the present invention to indicate such a method of liquefying a high hydrocarbon fraction while removing a high C ₂₊ fraction that will prevent the disadvantages described above. In particular, such a method of liquefying a fraction with a high hydrocarbon content should be indicated, which, on the one hand, is reliable, and on the other hand, provides the possibility of efficient and controlled removal of ethane and higher hydrocarbons during the process of liquefying natural gas. Therefore, the mode of evaporation of the flow of the mixture of refrigerants should be such that it can be used directly to control the removal of ethane and higher hydrocarbons.

To solve this problem, this kind of method is proposed for liquefying a fraction with a high content of hydrocarbons while removing a fraction with a high content of C ₂₊ , characterized in that at least temporarily at least one partial stream of a liquefied, previously gaseous fraction of a mixture of refrigerants expands and mixes with expanded liquid fraction of a mixture of refrigerants.

By varying the ratios of the quantities of the liquid fraction and the liquefied, previously gaseous fraction, it is possible to influence the temperature profile during the evaporation of the refrigerant mixed from the two above-mentioned fractions, so that, in accordance with the task, the temperature of the mixed refrigerant in the upper region of one or those heat exchangers that serve for The cooling and partial condensation of the high hydrocarbon fraction to be liquefied was always lower than the temperature of the fraction to be liquefied. The implementation of the method proposed by the invention allows sufficient control of the temperature of the high hydrocarbon fraction to be liquefied at the inlet to the removal device or separation column provided for removing the high C ₂₊ fraction, so that the desired concentration of C ₂₊ hydrocarbons can be set in liquefied product or LNG (Liquided Natural Gas - Liquefied Natural Gas, LNG).

Other preferred embodiments of the inventive method for liquefying a high hydrocarbon fraction while removing a high C ₂₊ fraction that are subject to the dependent claims are characterized in that

- a partial stream of the liquefied, previously gaseous fraction of the mixture of refrigerants at the cold end of the heat exchanger between the high hydrocarbon fraction to be liquefied and the mixture of refrigerants and / or at an appropriate intermediate temperature is diverted, expanded and mixed with the expanded liquid fraction of the refrigerant mixture, while the appropriate intermediate temperature is established when the refrigerant mixture is at least 5 ° C, preferably 10 ° C, relative to the boiling state,

- heat exchange between the high hydrocarbon fraction to be liquefied and the mixture of refrigerants is carried out in a multi-threaded heat exchanger, which is preferably made of a plate heat exchanger or a twisted heat exchanger,

- since the removal of the high C ₂₊ fraction is carried out in at least one separation column, at least temporarily a partial stream of the high hydrocarbon fraction to be liquefied is supplied to the head region and / or lower region of the separation column, and

- since the removal of the fraction with a high content of C _{2+ is} carried out in at least one separation column, the temperature of the lower part of the separation column is set by means of the boiler with which the separation column is equipped.

The inventive method of liquefying a high hydrocarbon fraction while removing a high C ₂₊ fraction, as well as other preferred embodiments thereof, which are the subject of the dependent claims, will be explained in more detail below using the exemplary embodiments shown in FIG. .1 and 2.

Below, when explaining the embodiment shown in FIG. 2, only differences from the implementation of the method shown in FIG. 1 will be described.

The embodiments of the inventive method for liquefying a high hydrocarbon fraction shown in FIGS. 1 and 2 include a separation column T, which serves to remove the high C ₂₊ fraction from the high hydrocarbon fraction to be liquefied. The liquefied fraction with a high hydrocarbon content, hereinafter referred to as the natural gas stream, is fed via line 1 to the E3 multi-flow heat exchanger.

This heat exchanger is preferably made in the form of a brazed aluminum plate heat exchanger. Depending on the size of the installation, preferably from 1 to 6 parallel heat exchanger blocks are provided. Alternatively, the multi-threaded heat exchanger E3 can be made in the form of a twisted heat exchanger. In this case, aluminum plate heat exchangers are used, preferably with a liquefaction capacity of 30,000 to 500,000 tons of LNG per year, twisted heat exchangers are preferably with a liquefaction capacity of 100,000 to 1,000,000 tons of LNG per year.

The natural gas stream in the heat exchanger E3 is cooled, partially condensed and then expanded by valve a in the head region of the separation column T. At the head of the separation column T, a gaseous fraction with a high methane content is discharged through line 2, in the heat exchanger E3, it is liquefied and also cooled, and then through the pipeline 3, in which the control valve e is provided, is diverted and used for other purposes or, accordingly, is transferred to temporary storage. This fraction is a liquefied product (LNG). From the lower part of the separation column T through line 4, which is also equipped with a control valve d, a fraction with a high content of C ₂₊ is discharged and sent for further use.

By supplying a partial stream from the natural gas stream through conduit 5 and control valve b, it is possible to influence the temperature in the head of the separation column T and, at the same time, the composition of the gaseous fraction with high methane content discharged through conduit 2. It is also possible to influence the temperature in the lower part of the separation column T, as well as the composition of the liquid fraction discharged through line 4 using a boiler E4 and / or supplying a partial stream from the natural gas stream through line 6 and expansion valve c.

The circulation circuit of the refrigerant mixture consists of a two-stage compressor unit, consisting of one first and one second stage C1 or, respectively, C2 compressor. After each compressor stage there is a cooler E1 or E2 respectively. In addition, a low pressure separator D1, a medium pressure separator D2, and a high pressure separator D3 are provided.

From the head of the low-pressure separator D1, which serves to protect the first stage C1 of the compressor, the mixture of refrigerants circulating in the circulation circuit through line 11 is supplied to the first stage C1 of the compressor. At this stage, the mixture of refrigerants is compressed to the desired intermediate pressure - this pressure is usually from 7 to 35 bar, preferably from 10 to 25 bar, then it is cooled in cooler E1, partially condensed and fed through line 12 to medium-pressure separator D2. While the liquid fraction is discharged from this separator through line 20, which will be discussed in more detail below, the gaseous phase of the refrigerant mixture discharged via line 13 from the head of the separator D2 is fed to the second stage C2 of the compressor and is compressed at this stage to the desired final pressure - this pressure is usually from 30 to 80 bar, preferably from 40 to 60 bar. The refrigerant mixture is then cooled in cooler E2, partially condensed, and fed through line 14 to high pressure separator D3. The liquid fraction that precipitates in the lower part of the separator D3 is returned via line 16 to the expansion valve k to the medium pressure separator D2.

In the head part of the separator D3, through the pipeline 15, a gaseous fraction of the refrigerant is discharged, in the heat exchanger E3 is liquefied, and also cooled, and from this heat exchanger it is discharged through the pipe 17. In the expansion valve g, this fraction or, correspondingly, partial flow of this fraction expands to the lowest circulation pressure circuit before it is piped 18 through the heat exchanger E3 and is completely evaporated. Through line 10, the fully evaporated fraction is then fed to separator D1.

In the implementation of the method shown in FIG. 1, the liquid fraction of the refrigerant is discharged through a pipe 20 from the lower part of the separator D2, is supplied to the heat exchanger E3 and in this heat exchanger it is supercooled. Through the pipe 21, the supercooled liquid fraction is discharged from the heat exchanger E3, in the valve f expands to the lowest pressure of the circulation circuit, and then through the pipe 22 it is again fed to the heat exchanger E3. The evaporated fraction in it through the pipeline 23 is mixed with the already mentioned evaporated fraction in the pipeline 10.

Valves f and g typically expand to a pressure that, with the exception of unavoidable pressure drops, corresponds to the suction pressure of the first compressor stage C1. By proper selection of the composition, amount and / or pressure of evaporation of the mixture of refrigerants, both the final temperature and the mass flow of the fraction to be liquefied with a high nitrogen content or, accordingly, the liquefied natural gas stream can be established.

In contrast to the implementation of the method depicted in FIG. 1, in the embodiment depicted in FIG. 2, the liquid fraction of the mixture of refrigerants supplied to the heat exchanger E3 is no longer discharged from the separator D2, but from the separator D3 through a pipe 20 '. Therefore, the liquid fraction that precipitates in the lower part of the separator D2 is supplied to the separator D3 via a line 16 'in which the pump P is located.

The implementation of the method depicted in FIG. 2 is somewhat more efficient than the implementation of the method depicted in FIG. 1 — it allows to improve the efficiency from 1 to 5% — however, it requires a pump, increased investment costs and requires a lot of effort maintenance. Therefore, the implementation of the method shown in Fig. 1 will be applied preferably at lower plant capacities (from 30,000 to 500,000 tons of LNG per year), while the implementation of the method depicted in Fig. 2 will be implemented preferably at higher plant capacities ( from 100,000 to 1,000,000 tons of LNG per year).

Due to the expansion of the liquid supercooled, as well as the liquefied, previously gaseous fraction of the mixture of refrigerants in the valves f and g to a substantially identical evaporation pressure, the free choice of the temperature regime of the refrigerant flow in the heat exchanger E3 downstream of the valve f is not possible. The compositions of the gaseous and liquid fractions of the refrigerant, in turn, are interconnected with the equilibria in the separators D2 and D3. Therefore, the position of the valve f cannot sufficiently affect the temperature in the upper or hotter part of the heat exchanger E3.

Therefore, in accordance with the invention, at least temporarily a partial stream of the liquefied, previously gaseous fraction of the mixture of refrigerants 15 expands and mixes with the expanded liquid fraction of the mixture of refrigerants in the pipe 22. The figures show two possible partial flows 19 and 24 of a mixture of refrigerants, which after expansion in the valve h or j respectively can be mixed with the expanded mixture of refrigerants in the pipe 22. In practice, in most cases, either valve h or j will be provided. However, in principle, it is true that the partial flows 19 and 24 of the mixture of refrigerants separately or together can be used to control the temperature or, accordingly, the temperature profile.

In this case, one or both of the partial flows of the refrigerant mixture 19 and 24 at the cold end of the heat exchanger E3 and / or at an appropriate intermediate temperature are discharged through a pipe 19 or 24, expand in valve h or j and mix with the expanded liquid fraction of the mixture of refrigerants 22. A suitable intermediate temperature is established when the mixture of 15 refrigerants is at least 5 ° C, preferably 10 ° C, relative to the boiling state.

By implementing the method of the invention, it is possible to control the temperature of the high-hydrocarbon fraction to be liquefied or, accordingly, the natural gas stream 1 at the inlet to the separation column T, which is necessary to establish the desired concentration of C ₂₊ hydrocarbons in the liquefied product or LNG, respectively.

Claims (7)

1. A method of liquefying a high hydrocarbon fraction while removing a high C ₂₊ fraction, while cooling and liquefying a high hydrocarbon fraction by indirect heat exchange by means of a refrigerant mixture in a circulation circuit of a refrigerant mixture in which at least a refrigerant mixture is subjected two-stage compression, and the removal of the fraction with a high content of C ₂₊ occurs at an adjustable temperature level, while the mixture of refrigerants is divided into gaseous and a dry fraction, both fractions are supercooled, expand essentially to the suction pressure of the first stage of the compressor and at least partially evaporate, characterized in that at least temporarily at least one partial stream (19, 24) of the liquefied, previously gaseous fraction of the mixture (15) the refrigerant expands (j, h) and mixes with the expanded liquid fraction of the mixture (22) of refrigerants, and at least temporarily at least one partial stream (19, 24) of the liquefied, previously gaseous fraction of the mixture (15) of refrigerants is lighter nym by itself. 2. The method according to claim 1, characterized in that partial stream (19, 24) of the liquefied, previously gaseous fraction of the mixture (15) of refrigerants at the cold end of the heat exchanger (E3) between the high-hydrocarbon fraction (1, 2) to be liquefied and the mixture (15, 17, 18, 20, 20 ' , 22) the refrigerants and / or at an appropriate intermediate temperature are removed, expanded (j, h) and mixed with the expanded liquid fraction of the refrigerant mixture (22), while the appropriate intermediate temperature is established when the refrigerant mixture (15) is at least supercooled 5 ° C, preferably 10 ° C relative boiling state. 3. The method according to claim 1 or 2, characterized in that the heat exchange between the liquefied fraction (1, 2) with a high content of hydrocarbons and the mixture (15, 17, 18, 20, 20 ', 22) of refrigerants is carried out in a multi-flow heat exchanger ( E3), which is preferably in the form of a plate heat exchanger or a twisted heat exchanger. 4. The method according to claim 1 or 2, in which the removal of the fraction with a high content of C _{2+ is} carried out in at least one separation column, characterized in that at least temporarily a partial stream (5) of the fraction to be liquefied with a high content of hydrocarbons is supplied into the head region of the separation column (T). 5. The method according to claim 1 or 2, in which the removal of the fraction with a high content of C _{2+ is} carried out in at least one separation column, characterized in that at least temporarily a partial stream (5) of the fraction to be liquefied with a high content of hydrocarbons is supplied into the lower region of the separation column (T). 6. The method according to claim 1 or 2, in which the removal of fractions with a high content of C _{2+ is} carried out in at least one separation column, characterized in that by means of the boiler (E4), which is equipped with the separation column (T), the lower temperature is set parts of the separation column.

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