RU2362954C2 - Treating of liquefied natural gas - Google Patents

Abstract

FIELD: oil and gas production. ^ SUBSTANCE: disclosed method of treating liquefied natural gas (1) for generating flow (21) of liquid with reduced contents of components with low boiling point consists in expanding liquefied gas to produce two-phase flow; in introducing two-phase fluid medium into column (10) below single section (14) of gas and liquid contacting, in withdrawing flow (17') of liquid, characterised with reduced contents of components with low boiling point, from lower part of column (16), in withdrawing gaseous flow (25), rich with components with low boiling point, from upper part (23) of column (10), in heating gaseous flow in heat exchanger (27), in compressing (30) of gaseous flow to pressure generating fuel gas and producing fuel gas (33); in separation of re-circulating flow (34a) from fuel gas; in at least partial condensation (27) of re-circulating flow, producing flow (34b) of wet reflux; in introducing flow (34b) of wet reflux into column (10) over single section (14) of contacting. ^ EFFECT: implementation of the invention facilitates reducing contents of components with low boiling point and also facilitates simplifying of process. ^ 26 cl, 5 dwg

Description

The present invention relates to the purification of liquefied natural gas, and, in particular, to the purification of liquefied natural gas, which contains components having a boiling point lower than that of methane. An example of such a component is nitrogen. In this description and in the claims, the expressions “low boiling components” and “components having low boiling points” will be used to refer to components having boiling points lower than the boiling points of methane. The purification is aimed at extracting low-boiling components from liquefied natural gas in order to obtain liquefied natural gas, characterized by a reduced content of components with low boiling points. The improved method can be applied in two cases: (1) for purification of the same amount of liquefied natural gas as in the known method, or (2) for purification of a larger amount of liquefied natural gas in comparison with the known method. When applying the method in the first case, the content of low-boiling components in the liquefied natural gas purified in accordance with the method according to the present invention is lower than their content in the liquefied gas purified by the known method. When using this method in the second case, the content of low-boiling components is preserved, and the amount of liquefied gas increases.

US Pat. No. 6,199,403 A discloses a method for removing a highly volatile component, such as nitrogen, from a methane-rich feed stream. In accordance with US Pat. No. 6,199,403 A, the expanded liquefied natural gas stream enters the separation column at an intermediate level, i.e. not lower than the section of the gas-liquid contacting section.

US Pat. No. 5,421,165 A relates to a process for removing nitrogen from an initial liquefied hydrocarbon mixture. To solve this problem, US 5421165 A proposes a relatively complex process in which a nitrogen extraction column is used that contains a large number of theoretical stages of fractionation.

Another relatively complex process is described in international publication WO 02/50483. This international application describes several methods for recovering components having low boiling points from liquefied natural gas. In accordance with WO 02/50483, a liquid product stream with a reduced content of components with low boiling points is obtained.

A disadvantage of the above process described in WO 02/50483 is that the liquid product stream contains an undesirably high content of components having low boiling points.

The objective of the present invention is to minimize the above drawback.

In addition, an object of the present invention is to provide an alternative process.

Another objective of the invention is to provide a simplified process to reduce the number of components having low boiling points in a stream of liquefied natural gas.

One or more of these tasks or other tasks are solved in accordance with the present invention by a method for purifying liquefied natural gas supplied under a liquefaction pressure and containing low boiling components to obtain a liquid product stream with a low content of low boiling components, the proposed method includes:

(a) expanding the liquefied gas to a phase separation pressure to obtain an expanded biphasic fluid;

(b) introducing an expanded biphasic fluid into a column containing a single gas and liquid contacting section, wherein the expanded biphasic fluid is introduced below a single gas and liquid contacting section disposed in the column;

(c) accumulating in the lower part of the liquid column from the two-phase fluid and withdrawing from the lower part of the liquid stream column having a reduced content of components with low boiling points to obtain a liquid product stream;

(d) allowing vapor flow of the two-phase fluid through a single contacting section;

(e) withdrawing from the top of the column a gaseous phase stream that is enriched with low boiling point components;

(f) heating the gas stream obtained in step (e) in a heat exchanger to obtain a heated gaseous stream;

(g) compressing the heated gaseous stream obtained in step (f) to the pressure of the fuel gas to produce fuel gas;

(h) separating the recycle stream from the fuel gas obtained in step (g);

(i) at least partially condensing the recycle stream obtained in step (h) to obtain a reflux stream; and

(j) introducing the reflux stream obtained in step (i) above a single contacting section at a pressure inside the column corresponding to the phase separation pressure.

Applicants have found that the liquid product stream of the present invention has a lower component content with low boiling points than would be expected.

Such a desired result was unexpectedly obtained in accordance with the present invention using a simplified process using, in step (b), a column having only one single gas-liquid contacting section.

Using the simplified process in accordance with the present invention, a reduction in the amount of low boiling components in the liquid product stream can be achieved in a more economical manner.

In addition, it was found that the process according to this invention is especially suitable for flows of liquefied natural gas (which should be supplied at a liquefaction pressure) containing less than 7 mol.% Components with low boiling points.

The present invention will now be described in more detail by way of example and with reference to the accompanying drawings, not limiting the invention.

FIG. 1 is a flowchart illustrating an embodiment of a method according to the present invention.

Figure 2 is a diagram of an alternative to the process illustrated in figure 1.

3 is a flowchart for another embodiment of a method according to the present invention.

Figure 4 is a diagram of an alternative to the process illustrated in figure 3.

5 is an alternative embodiment of the allocated part V of the process flowchart illustrated in FIG. 4 is shown schematically and not to scale.

In accordance with the scheme shown in Fig. 1, liquefied natural gas containing components with low boiling points is supplied at a liquefaction pressure through line 1 to an expansion device made in the form of expander 3 and passed through a throttle valve 5, in which the effect is realized Joule-Thompson and which is installed in the outlet pipe 6 of the expander 3. In the expansion device, the liquefied gas expands to a phase separation pressure, whereby an expanded biphasic fluid is obtained. The liquefaction pressure is preferably in the range from 3 to 8.5 MPa, and the phase separation pressure is preferably from 0.1 to 0.5 MPa.

The expanded biphasic fluid is sent through a conduit 9 to the column 10. The expanded biphasic fluid is then introduced into the column 10 at a phase separation pressure through a suitable inlet device, for example, an inlet device 12 with vanes. The paddle inlet device 12, also known as the schoepentoeter inlet switchgear, provides an efficient separation of gas and liquid.

Column 10 is equipped with only one gas and liquid contacting section 14. This single contacting section 14 may contain any suitable means for contacting the gas and the liquid, for example contact plates and nozzles. Preferably, the single contacting section 14 comprises from two to eight horizontal contact plates 15 or a nozzle, the length of the nozzle section corresponding to the presence of two to eight contact plates in the column. The expanded biphasic fluid is introduced into the column 10 below the gas and liquid contacting section 14.

In the lower part 16 of the column 10, liquid is collected from the two-phase fluid, and a liquid stream having a low content of components with low boiling points is discharged from the lower part 16 of the column through a pipe 17 and is pumped into the storage tank 20 from the storage tank 20. From the storage tank 20, a liquid stream the product is discharged through line 21 and the gaseous stream through line 22. Such a gaseous stream is also known as evaporation gas.

The vapor of the two-phase fluid passes through the contacting section 14. From the upper part 23 of column 10, a gaseous stream enriched with low boiling components is discharged through a pipe 25. This gas stream is heated in a heat exchanger 27 to produce a heated gaseous stream, which is sent through a pipe 28 to a compressor 30. In the compressor 30, the heated gaseous stream is compressed to a pressure corresponding to fuel gas to produce fuel gas. The resulting fuel gas is discharged by means of a pipe 31 and cooled in a heat exchanger 32, which serves to remove the heat of compression. The fuel gas is removed from the installation through the pipeline 33. The pressure of the fuel gas is in the range from 1 to 3.5 MPa.

The recirculated fuel gas stream is sent to the heat exchanger 27 through a pipe 34a. In the heat exchanger 27, the recycle stream is at least partially condensed to produce a reflux stream, which is sent to the column 10 through a pipe 34b provided with a throttle valve 37, in which the Joule-Thompson effect is realized. The reflux stream is introduced at a phase separation pressure into the column 10 through an inlet device, for example a blade inlet device 39, located above the contacting section 14.

Table 1 summarizes the data of a hypothetical example in which the method illustrated in FIG. 1 is mapped to a base example. In a basic example, the recycle stream and feed are introduced into the column at the same level, so that the liquid phases from these two streams are mixed before they enter the column, and the column does not have a contact section. It has been found that the liquid stream discharged through conduit 17 in the base case contains more nitrogen than the same stream according to the present invention.

Table 1.
Comparison of data for a hypothetical example and an example implementation illustrated in figure 1 The example implementation according to figure 1 Basic example Number of plates in the contact section 3 - The consumption of source material coming through the pipeline 9 190.86 kg / s 190.86 kg / s The temperature of the source material introduced through the input device 12 -145 ° C -145 ° C The nitrogen content in the feed stream 3.05 mol% 3.05 mol% Recycle flow rate 26 kg / s 26 kg / s The temperature of the recycle stream introduced through the input device 39 -165.6 ° C -165.2 ° C The nitrogen content in the recycle stream the vapor phase contains 33 mol.%, the liquid phase contains 1.7 mol.% The total recycle stream contains 22 mol.% Product flow in the pipeline 21 169.25 kg / s 169.19 kg / s The nitrogen content of the product transported in the pipeline 21 0.65 mol% 0.82 mol% Fuel gas consumption in the pipeline 33 20.51 kg / s 20.59 kg / s Nitrogen content in fuel gas 24 mol% 22 mol% Compressor Required Electrical Power 30 20.8 MW 31.2 MW

From table 1 it is seen that in the product stream obtained using the method corresponding to the present invention, the nitrogen content is reduced.

In an alternative embodiment, the recycle stream separated from the fuel gas, before it is at least partially condensed in the heat exchanger 27, is further compressed in the axial compressor to an elevated pressure level. The high pressure recycle stream can be used in various ways, which will be discussed with reference to FIG. The circuit elements that have already been discussed with reference to FIG. 1 are indicated in FIG. 2 by the same reference numerals as in FIG. 1.

An axial compressor installed in conduit 34a is indicated at 35. The axial compressor 35 may be provided with a cooler (not shown) to remove the heat of compression from the compressed recycle stream. The compressed recycle stream is at least partially condensed in the heat exchanger 27. Part of the cold that is needed is provided by the gaseous stream, which is enriched by components having low boiling points passing through conduit 25. The rest of it is provided by the recycle stream. The cold from the recycle stream can be obtained by expanding part of the recycle stream to an intermediate pressure in the Joule-Thompson effect throttle valve 38, using the expanded fluid to cool the recycle stream in conduit 34a and directing the expanded fluid through conduit 38a to compressor 30. Intermediate the pressure to which a part of the recycle stream expands is in the pressure range from the suction pressure to the discharge pressure of the compressor 30 (including the boundary values cheniya this interval). The stage of the compressor 30 into which the expanded recycle stream enters is selected so that the pressure of the expanded recycle stream matches the pressure of the fluid in this stage of the compressor 30.

The rest of the recycle stream expands with the passage of the throttle valve 37, which implements the Joule-Thompson effect, and is introduced as reflux into the column 10, as noted above with reference to figure 1.

An advantage of the embodiment illustrated in FIG. 2 is that the recycle stream expands from a higher pressure and thus cools to a lower temperature. This allows you to get a warmer flow of the source material, for example, at a temperature of -142 ° C, compared with the temperature of the starting material equal to -145 ° C in the above embodiment. As a result, the temperature of the liquefied gas from the main cryogenic heat exchanger can be higher and, therefore, when the same amount of energy is consumed, more gas can be liquefied.

The increased pressure of the fluid exiting the axial compressor 35 is chosen such that the cost of energy required to drive the axial compressor is less than the cost of an increased amount of liquefied gas.

An example of an embodiment was considered above in which expansion is carried out in throttle valves 37 and 38. However, it should be understood that the expansion of the recycle stream can be carried out in two stages, first in an expansion device, for example expander 36, and then in throttle valves 37 and 38 in which the Joule-Thompson effect is realized.

Instead of supplying fluid after it has expanded through line 38a to compressor 30, the expanded fluid can be directed to an inlet (not shown) of compressor 35.

In the embodiments discussed with reference to FIGS. 1 and 2, liquid from a two-phase fluid stream accumulates in the lower part 16 of the column 10, and from this lower part 16 a liquid stream 17 having a reduced content of components with low boiling points is withdrawn, to obtain a liquid product stream. In an alternative embodiment, this process step involves accumulating liquid from a two-phase fluid at the bottom of the column and withdrawing from the bottom of the column a liquid stream having a reduced content of components with low boiling currents; introducing a fluid stream into the evaporation chamber at low pressure; the removal of the second gaseous stream from the upper part of the evaporation chamber; and withdrawing a liquid stream from the bottom of the evaporation chamber to obtain a liquid product stream.

This example of a two-chamber embodiment of the invention will now be described with reference to FIG. The circuit elements that have already been discussed with reference to FIG. 1 are indicated in FIG. 3 by the same reference numbers.

Column 10 'comprises an upper portion 10u and a lower portion 10l, wherein said upper portion functions as the column 10 shown in FIG. 1, and the lower portion 10l is an evaporation chamber operating at a pressure lower than the pressure in the upper portion 10u. It is acceptable that the pressure in the upper part 10u be in the range of 0.2 to 0.5 MPa, and the pressure in the evaporation chamber 10l is from 0.1 to 0.2 MPa.

During normal operation of the installation, the liquid of the two-phase fluid flowing through the pipeline 9 accumulates in the lower zone 16 'of the upper part 10u of the column 10'. From this lower zone 16 ', a liquid stream characterized by a reduced content of components with low boiling points is discharged through conduit 17'. This stream is then introduced into the evaporation chamber 10l at low pressure. Pressure reduction is achieved using a throttle valve 40 with a Joule-Thompson effect installed in the pipe 17 '. After the pressure decreases, a two-phase mixture is formed, which is introduced through the inlet 41 into the evaporation chamber 10l.

A liquid stream with a low content of low boiling components is discharged through a 17 "pipe and directed to a storage tank 20.

A second gaseous stream is discharged from the upper portion 23 "of the evaporation chamber 10l.

Accordingly, the second gaseous stream passes through a pipe 42 to a heat exchanger 27, where the second gaseous stream is heated by heat exchange with a recycle stream supplied through a pipe 34a. The heated stream is compressed in the compressor 45, the heat of compression is removed in the heat exchanger 48, after which the stream is transported through line 49 so as to connect the compressed second gaseous stream to another stream recirculated through line 34a.

It should be understood that compressors 45 and 30 can be combined into one compressor (not shown). In this case, the pipe 42 is connected to the compressor on the suction side, the pipe 28 to the intermediate inlet of the compressor, and the pipe 32 is connected to the discharge side of the compressor.

The advantage of this method is that it can be used for large gas liquefaction plants.

As in the embodiment described above with reference to FIG. 1, in the example discussed with reference to FIG. 3, an axial compressor can be used to compress the recycle stream separated from the fuel gas to an elevated pressure, before it is at least partially condensed in the heat exchanger 27. The high-pressure recycle stream can be used in various ways, which can be disclosed with reference to FIG. 4. The circuit elements that have already been discussed with reference to FIG. 3 are indicated in FIG. 4 by the same reference numbers.

An axial compressor included in conduit 34a is indicated at 35 in the drawing. This axial compressor may be provided with a cooler (not shown) for removing heat of compression from the compressed recycle stream. The compressed recycle stream is partially condensed by cooling it in the heat exchanger 27. Part of the required cold is provided by a gaseous stream rich in components with low boiling points, which is transported through line 25. The rest of the cold is provided by the recycle stream. The cold from the recycle stream can be obtained by expanding part of the recycle stream to an intermediate pressure in the throttle valve 38, which implements the Joule-Thompson effect, then using the expanded fluid to cool the recycle stream passing through conduit 34a, and supplying the expanded fluid through line 38a to compressor 30. The intermediate pressure to which a portion of the recycle stream expands is in the range from the suction pressure to the discharge pressure of the comp essora 30 (including the boundary values of the interval). The entry point of the expanded recycle stream to the compressor 30 is selected so that the pressure of the expanded recycle stream is consistent with the fluid pressure in the compressor 30, at the point of entry of the stream into it.

The remainder of the recycle stream expands in a throttle valve 37 with a Joule-Thompson effect and is introduced as reflux into column 10, as described above with reference to FIG. 1.

An advantage of this embodiment is that the recycle stream expands from a higher pressure and thus cools to a lower temperature. This provides a warmer feed stream, for example a feed stream at a temperature of −142 ° C., compared with a feed stream at a temperature of −145 ° C. in the above example. As a result, the temperature of the liquefied gas leaving the main cryogenic heat exchanger may be higher and, therefore, with the same amount of expended energy, a larger amount of gas can be liquefied.

The increased pressure of the fluid exiting the axial compressor 35 is chosen such that the cost of energy required to drive the axial compressor 35 is less than the cost of obtaining an increased amount of liquefied gas.

An embodiment was described above in which expansion is carried out in butterfly valves 37 and 38. However, it should be understood that expansion of the recirculation flow can be carried out in two stages: first in an expansion device, for example in expander 36, and then in butterfly valves 37 and 38, in which the Joule-Thompson effect is realized.

Figure 4 also shows that the gas obtained by evaporation of the liquefied natural gas is discharged from the storage tank 20 through the pipe 22 and is led to the suction side of the compressor 45.

It should also be understood that compressors 45 and 30 can be combined into one compressor (not shown). In this case, conduit 42 (directly connected to conduit 22) is connected to the suction side of this compressor, conduit 28 to the intermediate inlet of the compressor, and conduit 32 is connected to the discharge side of this compressor.

Instead of supplying the expanded fluid through line 38a to the compressor 30, the expanded fluid may be supplied to an inlet (not shown) of the compressor 35.

Fig. 5 shows an alternative embodiment to that illustrated in Fig. 4, in which a portion of the recycle stream transported through conduit 34a is separated from the common stream and sent through conduit 50 to heat exchanger 27. After this, the cooled recycle stream is expanded to an intermediate pressure in the expander 51 and is used to cool the recycle stream passing through the pipe 34a. The expanded stream is then introduced into the intermediate stage of the compressor 30.

It is acceptable that the recycle stream passing through conduit 34a contains from 10 to 90% by weight of fuel gas, which is transported through conduit 31.

In the embodiments described with reference to the drawings, a single contacting section 14 comprises contact plates, however, any other contact elements, for example a nozzle, can also be used. The length of the nozzle section in this case is preferably equivalent to the length of the section containing from two to eight contact plates.

The method according to the present invention provides a simple way to reduce the number of components having low boiling points in a stream of liquefied natural gas.

Claims (26)

1. The method of purification of liquefied natural gas supplied under
pressure liquefaction containing components with low boiling points, to obtain a stream of liquid product with a low content of components having low boiling points, which includes
(a) expanding the liquefied gas to a separation pressure to obtain an expanded biphasic fluid;
(b) introducing an expanded biphasic fluid into a column containing a single gas and liquid contacting section, wherein the expanded biphasic fluid is introduced below a single gas and liquid contacting section disposed in the column;
(c) accumulating in the lower part of the liquid column from the two-phase fluid and withdrawing from the lower part of the column a liquid stream having a reduced content of components with low boiling points to obtain a liquid product stream;
(d) allowing the flow of steam of the two-phase fluid through a single contacting section;
(e) withdrawing from the top of the column a gaseous phase stream that is enriched in components having low boiling points;
(f) heating the gas stream obtained in step (e) in a heat exchanger to obtain a heated gaseous stream;
(g) compressing the heated gaseous stream obtained in step (f) to the pressure of the fuel gas to produce fuel gas;
(h) separating the recycle stream from the fuel gas obtained in step (g);
(i) at least partially condensing the recycle stream obtained in step (h) to obtain a reflux stream; and
(j) introducing the reflux stream obtained in step (i) at a phase separation pressure into the column above a single contacting section. 2. The method according to claim 1, in which stage (c) includes the accumulation in the lower part of the column of liquid from a two-phase fluid and removing from the bottom of the column a liquid stream having a reduced content of components with low boiling points; introducing a fluid stream characterized by a reduced content of components with low boiling points into the evaporation chamber at low pressure; the removal of the second gaseous stream from the upper part of the evaporation chamber; withdrawal of a liquid stream from the lower part of the evaporation chamber to obtain a liquid product stream. 3. The method according to claim 2, further comprising heating a second gaseous stream in a heat exchanger; compressing the second gaseous stream to a fuel gas generation pressure; adding a second gaseous stream to the recycle stream. 4. The method according to claim 1, in which at least partial condensation of the recycle stream includes the implementation of indirect heat exchange of the recycle stream with a gaseous stream (flows) in the specified heat exchanger. 5. The method according to claim 2, in which at least partial condensation of the recycle stream includes the implementation of indirect heat exchange of the recycle stream with a gaseous stream (flows) in the specified heat exchanger. 6. The method according to claim 3, in which at least partial condensation of the recycle stream includes the implementation of indirect heat exchange of the recycle stream with a gaseous stream (flows) in the specified heat exchanger. 7. The method according to claim 1, wherein compressing the heated gaseous stream to a fuel gas generation pressure to produce fuel gas in step (g) further comprises removing heat of compression. 8. The method according to claim 2, in which the compression of the heated gaseous stream to the pressure of the formation of fuel gas, produced in order to obtain fuel gas in stage (g), further includes the removal of heat of compression. 9. The method according to claim 3, in which the compression of the heated gaseous stream to the pressure of formation of fuel gas, produced in order to obtain fuel gas in stage (g), further includes the removal of heat of compression. 10. The method according to claim 4, in which the compression of the heated gaseous stream to the pressure of formation of fuel gas, produced in order to obtain fuel gas in stage (g), further includes the removal of heat of compression. 11. The method according to claim 5, in which the compression of the heated gaseous stream to the pressure of the formation of fuel gas, produced in order to obtain fuel gas in stage (g), further includes the removal of heat of compression. 12. The method according to claim 6, in which the compression of the heated gaseous stream to the pressure of formation of fuel gas, produced in order to obtain fuel gas in stage (g), further includes the removal of heat of compression. 13. The method according to claim 1, wherein the recycle stream separated from the fuel gas in step (h) is compressed to an elevated pressure before at least partial condensation thereof. 14. The method of claim 2, wherein the recycle stream separated from the fuel gas in step (h) is compressed to an elevated pressure before at least partial condensation thereof. 15. The method according to claim 3, in which the recycle stream, separated from the fuel gas in stage (h), is compressed to an elevated pressure before at least partial condensation. 16. The method according to claim 4, in which the recycle stream separated from the fuel gas in stage (h), is compressed to an increased pressure before at least partial condensation. 17. The method according to claim 5, in which the recycle stream, separated from the fuel gas in stage (h), is compressed to an elevated pressure before at least partial condensation. 18. The method according to claim 6, in which the recycle stream separated from the fuel gas in stage (h), is compressed to an increased pressure before at least partial condensation. 19. The method according to claim 7, in which the recycle stream separated from the fuel gas in stage (h), is compressed to an increased pressure before at least partial condensation. 20. The method of claim 8, wherein the recycle stream separated from the fuel gas in step (h) is compressed to an elevated pressure before at least partial condensation thereof. 21. The method according to claim 9, in which the recycle stream, separated from the fuel gas in stage (h), is compressed to an elevated pressure before at least partial condensation. 22. The method of claim 10, wherein the recycle stream separated from the fuel gas in step (h) is compressed to an elevated pressure before at least partial condensation thereof. 23. The method according to claim 11, in which the recycle stream, separated from the fuel gas in stage (h), is compressed to an elevated pressure before at least partial condensation. 24. The method of claim 12, wherein the recycle stream separated from the fuel gas in step (h) is compressed to an elevated pressure before at least partial condensation thereof. 25. The method of claim 13, wherein the recycle stream separated from the fuel gas in step (h) is compressed to an increased pressure before at least partial condensation thereof. 26. The method according to one or more of claims 1 to 25, in which the incoming liquefied natural gas contains less than 7 mol.% Components with low boiling points.

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