KR101238172B1 - Treating liquefied natural gas - Google Patents

Abstract

According to the present invention, a process for treating liquefied natural gas (1) for obtaining a liquid stream (21) having a reduced content of low boiling point component, the step of expanding (3) the liquefied gas to obtain an expanded two-phase fluid ; Introducing a biphasic fluid into the column 10 under a single gas-liquid contacting zone 14; Withdrawing the liquid stream (17) with reduced content of low boiling point components from the bottom (16); Withdrawing the high boiling component gaseous stream 25 from the top 23 of column 10; Heating the gaseous stream in a heat exchanger (27); Compressing the stream to fuel gas pressure 30 to obtain fuel gas 33; Separating the recycle stream 34a from the fuel gas; At least partially condensing (27) the recycle stream to obtain a reflux stream (34b); And introducing a reflux stream 34b into the column 10 above the single contacting zone 14.

Liquefied natural gas, boiling point.

Description

Processing of liquefied natural gas {TREATING LIQUEFIED NATURAL GAS}

The present invention relates to the treatment of liquefied natural gas, in particular to the treatment of liquefied natural gas comprising components having a lower boiling point than methane. An example of such a component is nitrogen. In the description and claims, the expressions "low boiling point component" and "low boiling point component" are used to refer to components having a lower boiling point than methane. The treatment is for removing the low boiling point component from the liquefied natural gas in order to obtain a liquefied natural gas having a reduced content of the low boiling point component. The improved method has two ways: (1) to process the same amount of liquefied natural gas as in the conventional method, and (2) to process a greater amount of liquefied natural gas than in the conventional method. Can be applied as When applied in the first manner, the content of low boiling point components in the liquefied gas treated with the method of the present invention is lower than the content of low boiling point components in the liquefied gas treated with the conventional method. When applied in the second way, the content of low boiling point components is maintained and the amount of liquefied gas is increased.

A method for removing high volatility components such as nitrogen from a feed stream rich in methane is disclosed in US-A-6 199 403. According to US-A-6 199 403, the expanded liquefied natural gas stream enters the separation column at intermediate rather than below a single gas-liquid contact zone.

US-A-5 421 165 relates to a method for denitrogenating a feed of a liquefied mixture of hydrocarbons. To this end, US-A-5 421 165 proposes a relatively complex method of using a denitrification column comprising a plurality of theoretical fractionation steps.

Another relatively complex method is described in WO 02/50483. WO 02/50483 discloses several methods for removing low boiling components from liquefied natural gas. According to WO 02/50483 a liquid product stream with reduced content of components with low boiling points is obtained.

The problem with the process described in WO 02/50483 is that the liquid product stream contains undesirably high content of components with low boiling points.

It is an object of the present invention to minimize the above problems.

Another object of the present invention is to provide a simple method of reducing the amount of low boiling point components in a liquefied natural gas stream.

According to the invention, at least one of said or other objects is

A process for treating liquefied natural gas that is fed under a liquefaction pressre and comprising a component having a low boiling point to obtain a liquid product stream having a reduced content of components having a lower boiling point,

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

(b) introducing an expanded biphasic fluid into a column having a single gas-liquid contact zone, but below said single gas-liquid contact zone;

(c) collecting a liquid of the biphasic fluid at the bottom of the column and withdrawing the liquid stream having a reduced content of low boiling point component from the bottom of the column to obtain a liquid product stream;

(d) allowing vapor of the two-phase fluid to flow through the single contact zone;

(e) withdrawing a high boiling component gaseous stream from the top of the column;

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

(g) compressing the heated gaseous stream obtained in step (f) to fuel gas pressure to obtain 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) is achieved by providing a process for treating liquefied natural gas comprising introducing the reflux stream obtained in step (i) into a column under a separation pressure above a single contacting zone.

Applicants have found that the liquid product stream obtained according to the present invention has less content of components with lower boiling points than generally expected.

Surprisingly, according to the present invention, this preferred result is obtained by a simple method using a column having only a single gas-liquid contacting zone in step (b).

Using the simple method according to the invention, the amount of low boiling point component in the liquid product stream can be reduced more cost effectively.

It has also been found that the process according to the invention is particularly suitable for liquefied natural gas streams (supplied at liquefaction pressure) which contain less than 7 mol% of components with low boiling points.

Hereinafter, the present invention will be described in more detail with reference to the accompanying drawings.

1 is a schematic process flow diagram of an embodiment of the method of the present invention.

2 is a schematic view of a modification of the process of FIG. 1.

3 is a schematic process flow diagram of another embodiment of the method of the present invention.

4 is a schematic view of a modification of the process of FIG. 3.

FIG. 5 is a schematic diagram of a modification to part V of the process flow diagram of FIG. 4 (irrespective of scale). FIG.

See FIG. 1. Liquefied natural gas comprising a component having a low boiling point is subjected to Joule-Thomson in the expansion device in the form of an expansion engine 3 and an outlet pipe 6 of the expansion engine 3 via a conduit 1 under liquefaction pressure. Thompson) is supplied to the valve (5). In the expansion device, the liquefied gas can expand to the separation pressure and an expanded two-phase fluid is obtained. The liquefaction pressure is suitably in the range of 3 to 8.5 MPa, and the separation pressure is suitably in the range of 0.1 to 0.5 MPa.

The expanded two-phase fluid is delivered to the column 10 through the conduit 9. The expanded two-phase fluid is introduced into the column 10 under separation pressure through a suitable inlet device such as a vane inlet device 12. An inlet device, also known as a schopenpentoeter, allows for effective separation of gas and liquid.

The column 10 is provided with only a single gas-liquid contacting zone 14. The single contacting zone 14 may comprise any suitable means for contacting the gas and the liquid, such as a tray and packing. The single contact zone 14 preferably consists of one to eight horizontal contact trays 15, or one packing having a length corresponding to two to eight contact trays and having a packed section. The expanded two-phase fluid is introduced into column 10 under a single gas-liquid contact zone 14.

At the bottom 16 of the column 10, liquid is collected from the two-phase fluid, and the liquid stream with reduced content of low boiling point components is removed from the bottom 16 via the conduit 17 and the pump 18. Is sent to the storage tank 20. From the storage tank 20, the liquid product stream is removed through the conduit 21 and the gaseous stream is removed through the conduit 22. The gaseous stream is also known as a boil-off gas.

Vapor from the two-phase fluid flows through the single contact zone 14. From the upper part 23 of the column 10, a high boiling component gaseous stream is removed through the conduit 25. The gaseous stream is heated in a heat exchanger 27 to obtain a heated gaseous stream that is delivered through a conduit 28 to the compressor 30. In the compressor 30, the heated gaseous stream is compressed to fuel gas pressure to obtain fuel gas. The fuel gas is removed through the conduit 31 and cooled in the heat exchanger 32 to remove the heat of compression. Fuel gas exits through conduit 33. The fuel gas pressure is in the range of 1 to 3.5 MPa.

The recycle stream from the fuel gas is fed to the heat exchanger 27 via conduit 34a. In the heat exchanger 27, the recycle stream is at least partially condensed to obtain a reflux stream, which is passed through the conduit 34b with the Joule-Thomson valve 37. Is passed to. The reflux stream is introduced into the column 10 under separation pressure over a single contacting zone 14 through an inlet device such as a vane inlet device 39.

Table 1 summarizes the results of a hypothetical example comparing the method of FIG. 1 to the base case. In the basic case, the recycle stream and the feed are introduced into the column at the same level, so that the liquid phase of the two streams is mixed before being introduced into the column, and the column has no contact zone. In the basic case it has been found that the liquid stream exiting the conduit 17 contains more nitrogen than the same stream in the present invention.

TABLE 1 Summary of hypothetical examples in the embodiment of FIG. 1

Embodiment of FIG. 1 The basic case Within the contact area
Number of trays 3 - Via conduit (9)
Feed flow 190.86 kg / s 190.86 kg / s Via inlet device (12)
Feed temperature introduced -145 ℃ -145 ℃ Nitrogen content in the feed 3.05 mol% 3.05 mol% Recycle flow rate 26 kg / s 26 kg / s Via inlet device (39)
Temperature of recycling introduced -165.6 ℃ -165.2 ℃ Recycling stream
Nitrogen content The gas phase contains 33 mol%,
Liquid contains 1.7 mol%. The entire recycling stream
22 mol% inclusive. Flow rate of product in conduit 21 169.25 kg / s 169.19 kg / s In conduit 21
Nitrogen content of the product 0.65 mol% 0.82 mol% In conduit (33)
Flow rate of fuel gas 20.51 kg / s 20.59 kg / s Nitrogen content of fuel gas 24 mol% 22 mol% Compressor (30)
Power required 30.8 MW 31.2 MW

Table 1 shows that the nitrogen content in the product stream is lower for the process of the present invention.

In another embodiment of the invention, the recycle stream separated from the fuel gas is additionally compressed to elevated pressure in the subcompressor before it at least partially condenses in the heat exchanger 27. The high pressure recycle stream can be used in several ways, which will be described with reference to FIG. 2. The same reference numerals are used for the parts already described with reference to FIG. 1.

The auxiliary compressor included in conduit 34a is denoted by reference numeral 35. The subcompressor 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 by cooling in the heat exchanger 27. Some of the required cooling is provided by the low boiling component rich gaseous stream delivered through conduit 25. The remainder is provided by the recycle stream. Cooling by the recycle stream expands a portion of the recycle stream to medium pressure in the Joule-Thomson valve 38, uses the expanded fluid to cool the recycle stream in the conduit 34a, and expands the expanded fluid into the conduit ( By feeding it to the compressor 30 via 38a). The intermediate pressure at which a portion of the recycle stream is expanded is in the range above the suction pressure and below the discharge pressure of the compressor 30. Entering the expanded recycle stream into the compressor 30 is selected such that the pressure of the expanded recycle stream matches the pressure of the fluid in the compressor 30 at that stage.

The remainder of the recycle stream is expanded by the joule-thomson valve 37 and introduced as reflux to the column 10 as described with reference to FIG. 1.

An advantage of the embodiment described with reference to FIG. 2 is that the recycle stream is cooled to a lower temperature since it expands from a higher pressure. This can result in a feed stream that is slightly higher in temperature (in the example above) than the feed stream at -145 ° C, such as a feed stream at -142 ° C. Thus, the temperature of the liquefied gas coming from the main cryogenic heat exchanger can be higher, and thus more gas can be liquefied with the same amount of energy.

The elevated pressure of the fluid discharged from the subcompressor 35 is selected such that the cost of the power required to drive the subcompressor 35 is less than the value of the increase amount of the liquefied gas.

In the above, embodiment in which expansion is performed by expansion valves 37 and 38 was examined. However, expansion of the recycle stream may consist of two stages, a first stage in an expansion device such as inflator 36 and a second stage in joule-thomson valves 37, 38.

Instead of supplying the expanded fluid to the compressor 30 via the conduit 38a, the expanded fluid can be supplied to an inlet (not shown) of the compressor 35.

In the embodiment described with reference to FIGS. 1 and 2, the liquid of the biphasic fluid is collected at the bottom 16 of the column 10 and the bottom of the liquid stream 17 with reduced content of components having a low boiling point is present. Exit from (16) to obtain a liquid product stream. In another embodiment of the present invention, this step comprises the steps of: collecting a liquid of the two-phase fluid at the bottom of the column and withdrawing a liquid stream having a reduced content of components having a boiling point from the bottom of the column; Introducing the liquid stream into a low pressure flash vessel; Removing the second gaseous stream from the top of the flash vessel; And removing the liquid stream from the bottom of the flash vessel to obtain a liquid product stream.

In the following, this two-vessel embodiment is described with reference to FIG. 3. The same reference numerals are used for the parts already described with reference to FIG. 1.

The column 10 'includes an upper portion 10u and a lower portion 101, the upper portion serving as the column 10 in FIG. 1, and the lower portion 101 of the upper portion 10u. A flash vessel that operates at a pressure lower than the pressure. The pressure in the upper portion 10u is suitably in the range of 0.2 to 0.5 MPa, and the pressure in the flash container 101 is suitably in the range of 0.1 to 0.2 MPa.

During normal operation, liquid from the two-phase fluid supplied through the conduit 9 is collected at the bottom 16 'of the upper portion 10u of the column 10'. From its bottom 16 ′, a liquid stream with reduced content of low boiling point components exits the conduit 17 ′. This stream is then introduced into the low pressure flash vessel 101. Pressure reduction is achieved by the joule-thompson valve 40 in the conduit 17 '. As a result, a two-phase mixture is formed and introduced into the flash container 101 through the inlet device 41.

Through conduit 17 '', the liquid stream with reduced content of low boiling point components is removed and delivered to storage tank 20.

The second gaseous stream is removed from the upper portion 23 ″ of the flash vessel 101.

The second gaseous stream is suitably delivered to the heat exchanger 27 via conduit 42, in which the second gaseous stream is heated by heat exchange with a recycle stream supplied via conduit 34a. The heated stream is compressed in compressor 45, the heat of compression is removed in heat exchanger 48 and passed through conduit 49, and the compressed second gaseous stream is added to the recycle stream in conduit 34a.

Compressors 45 and 30 may be merged into one compressor (not shown). In such a case, the conduit 42 is connected to the suction end of the compressor, the conduit 28 is connected to the intermediate inlet, and the conduit 32 is connected to the outlet end of the compressor.

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

As with the embodiment described with reference to FIG. 1, in the embodiment described with reference to FIG. 3, in order to compress the recycle stream separated from the fuel gas to an elevated pressure before at least partially condensing in the heat exchanger 27, An auxiliary compressor may also be provided. The high pressure recycle stream can be used in a number of ways, which will be described with reference to FIG. 4. The same reference numerals are used for the parts already described with reference to FIG. 3.

The auxiliary compressor included in conduit 34a is indicated by reference numeral 35. The auxiliary 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 partially condensed by cooling in the heat exchanger 27. Some of the required cooling is provided by the low boiling component rich gaseous stream delivered through conduit 25. The remainder is provided by the recycle stream. Cooling by the recycle stream expands a portion of the recycle stream to medium pressure in the Joule-Thomson valve 38, uses the expanded fluid to cool the recycle stream in the conduit 34a, and expands the expanded fluid into the conduit ( By feeding it to the compressor 30 via 38a). The intermediate pressure at which a portion of the recycle stream is expanded is in the range above the suction pressure and below the discharge pressure of the compressor 30. The point at which the expanded recycle stream enters the compressor 30 is selected such that the pressure of the expanded recycle stream matches the pressure of the fluid in the compressor 30 at the inlet point.

The remainder of the recycle stream is expanded by the joule-thomson valve 37 and introduced as reflux to the column 10 as described with reference to FIG. 1.

An advantage of this embodiment is that the recycle stream is cooled to a lower temperature since it expands from a higher pressure. This can result in a feed stream that is slightly higher in temperature (in the example above) than the feed stream at -145 ° C, such as a feed stream at -142 ° C. Thus, the temperature of the liquefied gas coming from the main cryogenic heat exchanger can be higher, and thus more gas can be liquefied with the same amount of energy.

The elevated pressure of the fluid discharged from the subcompressor 35 is selected such that the cost of the power required to drive the subcompressor 35 is less than the value of the increase amount of the liquefied gas.

In the above, embodiment in which expansion is performed by expansion valves 37 and 38 was examined. However, expansion of the recycle stream may consist of two stages, a first stage in an expansion device such as inflator 36 and a second stage in joule-thomson valves 37, 38.

4 also shows that boil-off gas from the storage tank 20 is provided through the conduit 22 to the suction end of the compressor 45.

Compressors 45 and 30 may be merged into one compressor (not shown). In that case, the conduit 42 (with the conduit 22 open to this conduit) is connected to the suction end of the compressor, the conduit 28 is connected to the intermediate inlet, and the conduit 32 is connected to the compressor. Is connected to the discharge end of the.

Instead of supplying the expanded fluid to the compressor 30 via the conduit 38a, the expanded fluid can be supplied to an inlet (not shown) of the compressor 35.

Another form of the embodiment of FIG. 4 is shown in FIG. 5, wherein a portion of the recycle stream delivered through conduit 34a is separated from the conduit and delivered to heat exchanger 27 through conduit 50. The cooled recycle stream is then expanded to medium pressure in expander 51 and used to cool the recycle stream in conduit 34a. The expanded stream is then introduced to an intermediate compressor 30.

The recycle stream delivered through conduit 34a is suitably between 10 and 90 mass% of the fuel gas delivered through conduit 31.

In the embodiment described with reference to the drawings, the single contacting zone 14 comprises a tray, but any other contacting means such as packing may also be employed. And, the length of the packed zone preferably corresponds to two to eight contact trays.

The process of the present invention provides a simple way of reducing the amount of low boiling point components in a liquefied natural gas stream.

Claims (7)

A method of treating a liquefied natural gas comprising a component having a low boiling point and supplied under liquefaction pressure to obtain a liquid product stream having a reduced content of a component having a low boiling point, (a) expanding the liquefied gas to a separation pressure to obtain an expanded two-phase fluid; (b) introducing an expanded biphasic fluid into a column having a single gas-liquid contact zone comprising at least one of a tray and packing, but below said single gas-liquid contact zone; (c) collecting a liquid of the biphasic fluid at the bottom of the column and withdrawing the liquid stream having a reduced content of low boiling point component from the bottom of the column to obtain a liquid product stream; (d) allowing vapor of the two-phase fluid to flow through the single contact zone; (e) withdrawing a high boiling component gaseous stream from the top of the column; (f) heating the gaseous 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 fuel gas pressure to obtain 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) into the column under a separation pressure above a single contacting zone. The method of claim 1, wherein step (c) comprises: collecting a liquid of the two-phase fluid at the bottom of the column and withdrawing a liquid stream having a reduced content of low boiling components from the bottom of the column; Introducing a liquid stream with reduced content of low boiling point components into a low pressure flash vessel; Removing the second gaseous stream from the top of the flash vessel; And removing the liquid stream from the bottom of the flash vessel to obtain a liquid product stream. 3. The process of claim 2, further comprising: heating the second gaseous stream in a heat exchanger; Compressing the second gaseous stream to fuel gas pressure; And adding the second gaseous stream to a recycle stream. 4. The method according to any one of claims 1 to 3, wherein step (i) of at least partially condensing the recycle stream comprises indirectly exchanging the recycle stream with the gaseous stream (s) in a heat exchanger. Process for processing liquefied natural gas 4. Liquefaction according to any one of the preceding claims, wherein step (g) of compressing the heated gaseous stream to fuel gas pressure to obtain fuel gas further comprises removing the heat of compression. Method of processing natural gas. 4. A process according to any one of the preceding claims, wherein the recycle stream separated from the fuel gas in step (h) is compressed to elevated pressure before it is at least partially condensed. . 4. The process according to any one of claims 1 to 3, wherein the supplied liquefied natural gas contains less than 7 mol% of components having a low boiling point.

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