KR101269914B1 - Method and apparatus for liquefying a natural gas stream - Google Patents

Abstract

The present invention relates to a process for liquefaction of a natural gas stream, wherein the natural gas stream (10) is provided at a pressure of 30 to 80 bar, expanded to less than 35 bar, and is fed to a gas / liquid separator (31), the gas / Liquid separator is separated into a gaseous stream 40 and a liquid stream 30. The pressure of the gaseous stream is increased to a pressure above 70 bar and the pressurized gaseous stream 90 is liquefied to obtain a liquefied natural gas stream.

Natural gas streams, liquefaction methods, liquefaction devices

Description

METHOD AND APPARATUS FOR LIQUEFYING A NATURAL GAS STREAM}

The present invention relates to a method of liquefying a natural gas stream.

Various methods of liquefying natural gas streams to obtain liquefied natural gas (LNG) are known. For many reasons it is desirable to liquefy a natural gas stream. For example, natural gas can be more easily stored and transported over longer distances in a liquefied state than in a gaseous state, because in the liquefied state the capacity is smaller and does not need to be stored at high pressure. .

Examples of known liquefaction methods are described in US Pat. No. 6,272,882 and DE 102 26 597 A1.

According to FIG. 1 of DE 102 26 597 A1 a natural gas stream having a pressure of 70 to 100 bar is expanded to a pressure range of 40 to 70 bar (expander (X)) and cooled (heat exchanger (E1)), Heavy hydrocarbon (HHC) column (T1) is introduced. The C ₂ concentrated fraction obtained from the top of the HHC column is further cooled (E2) and introduced into another column (D). The top stream of the other column (D) is pressurized in the range from 50 to 100 bar (V) and then liquefied.

The problem with the method according to DE 102 26 597 is that it is unnecessarily complex. Another problem with this process is that the recovery of heavier compounds (especially propane and butane) is less than methane.

It is an object of the present invention to minimize this problem.

Another object of the present invention is to increase the recovery of compounds heavier than methane, especially propane.

Another object of the present invention is to provide another method of liquefaction of a natural gas stream.

One or more of the above or other objects are achieved in accordance with the present invention by providing a process for liquefying a natural gas stream, the process for liquefying a natural gas stream,

(a) providing a feed stream comprising natural gas at a pressure of 30 to 80 bar,

(b) expanding the feed stream of step (a) to obtain an expanded feed stream having a pressure of less than 35 bar,

(c) feeding the expanded feed stream to a gas / liquid separator,

(d) separating the expanded feed stream into a gaseous stream and a liquid stream in a gas / liquid separator, where the gas stream is concentrated in methane compared to the feed stream and the liquid stream is reduced in methane compared to the feed stream. Steps,

(e) increasing the pressure of the gaseous stream obtained in step (d) to a pressure of at least 70 bar, preferably at least 84 bar, and

(f) liquefying the pressurized gaseous stream obtained in step (e) to obtain a liquefied natural gas stream,

The pressure in the feed stream provided in step (a) is not increased until the pressure in step (e) increases.

Surprisingly, it has been found that the process according to the invention can be used to significantly increase the recovery of compounds heavier than methane. An important advantage of the present invention is that this increase in recovery can be achieved in a surprisingly simple manner.

Another advantage of the present invention is that a given refrigeration power can be used to increase the production of liquefied natural gas. Thus, with a given refrigeration power (eg using a given line-up comprising one or more cryogenic heat exchangers, compressors, etc.), the process according to the invention provides more LNG than known processes. In accordance with the present invention, it has been found that it is possible to increase LNG production by 20% while keeping the refrigeration power constant.

The natural gas stream can be any suitable gas stream to be liquefied, but is typically obtained from natural gas or oil reservoirs. Alternatively, natural gas can also be obtained from other sources, including synthetic sources such as the Fischer-Tropsch process.

Typically, the natural gas stream consists essentially of methane. Preferably, the feed stream comprises at least 60 mol% methane, more preferably at least 80 mol% methane, most preferably at least 90 mol% methane.

Depending on the source, natural gas may contain various aromatic hydrocarbons as well as heavier amounts of hydrocarbons (ethane, propane, butane and pentane) than methane. The natural gas stream may also include non-hydrocarbons such as H ₂ O, N ₂ , CO ₂ , H ₂ S and other sulfur compounds.

If desired, the feed stream comprising natural gas may be pretreated before being expanded and fed to the gas / liquid separator. Such pretreatment may include the removal of undesirable components such as CO ₂ and H ₂ S or other steps such as precooling or prepressurization. Such steps are well known to those skilled in the art and are not described further herein.

The gas / liquid separator may be any suitable means for obtaining a gaseous stream and a liquid stream, for example a scrubber, a distillation column, and the like. If desired, there may be two or more gas / liquid separators.

Those skilled in the art will readily appreciate that the pressure increase of the gaseous stream can be carried out in various ways (assuming a pressure of at least 70 bar, preferably at least 84 bar can be obtained).

It will also be readily understood by those skilled in the art that liquefaction of the pressurized gaseous stream can be carried out in a variety of ways, for example using one or more cryogenic heat exchangers.

Furthermore, those skilled in the art will readily understand that after liquefaction, liquefied natural gas can be further processed if desired. For example, the obtained LNG can be depressurized by a Joule-Thomson valve or cryogenic turboexpander. In addition, another intermediate processing step can be carried out between the gas / liquid separation step and the liquefaction step.

Preferably, the pressure in step (e) is increased to at least 86 bar, preferably at least 90 bar. In this way, the amount of LNG produced can be increased. By using a relatively high pressure, the gaseous stream can be supercritical depending on the prevailing pressure and composition of each gaseous stream. The gaseous stream is preferably in a supercritical state because it can prevent phase changes in the liquefaction process.

It is also preferred that the gaseous stream obtained in step (d) has a C ₅ ⁺ content of 0.5 mol% or less, preferably 0.1 mol% or less. This minimizes operational problems in the downstream liquefaction unit. "C ₅ ⁺ content" means the content of a hydrocarbon component having at least 5 carbon atoms.

According to a preferred embodiment, the pressure in step (e) is increased by compressing the gaseous stream in a compressor to obtain a compressed stream. For this purpose, more than one compressor can be used.

In addition, the gaseous stream obtained in step (e) is preferably cooled, for example in an ambient heat exchanger. In addition, the compressed stream is preferably heat exchanged with respect to the gaseous stream obtained in step (d).

According to a particularly preferred embodiment of the method according to the invention, the expander for expanding the feed stream in step (b) is functionally connected to a compressor for compressing the gaseous stream. As a result, the power generated by the expander is at least partly used to drive the compressor functionally connected to the expander. Thus, the expander and the compressor form a so-called "compressor-expander configuration", as a result of which the energy consumption of the whole process is minimized. Although the "compressor-expander configuration" is not described herein, those skilled in the art will readily understand.

In another aspect, the invention relates to LNG products, in particular liquefied methane, obtained by the process according to the invention.

In another aspect, the invention relates to a device suitable for carrying out the method according to the invention, the device comprising:

Means for providing a feed stream comprising natural gas at a pressure between 30 and 80 bar,

An expander which expands the feed stream to obtain an expanded feed stream having a pressure of less than 35 bar,

A gas / liquid separator that separates the expanded feed stream into a gaseous stream and a liquid stream, the gaseous stream having a higher concentration of methane than the feed stream and the liquid stream having a reduced methane relative to the feed stream; Liquid separator,

A pressurizing unit for increasing the pressure of the gaseous stream obtained in the gas / liquid separator to a pressure of at least 70 bar, preferably at least 84 bar, and

A liquefaction unit that liquefies a gaseous stream having a pressure of at least 70 bar, preferably at least 84 bar, and also comprises at least one cryogenic heat exchanger.

Preferably, the pressurizing unit comprises a compressor.

In addition, the apparatus preferably further comprises a heat exchanger for heat-exchanging the emissions from the compressor for the gaseous stream obtained in the gas / liquid separator.

In addition, the apparatus preferably further comprises an expander for expanding the feed stream.

According to a particularly preferred embodiment, the compressor and the expander are functionally connected, forming a so-called "compressor-expander configuration".

The invention is further described below with reference to the following non-limiting drawings.

1 is a schematic diagram of a processing method according to one embodiment of the present invention, and

2 is a schematic view of a process method in accordance with another embodiment of the present invention.

For illustration purposes, a single reference numeral is given to the line as well as the stream carried on this line. Like reference numerals denote like elements.

1 schematically shows a base rod liquefied natural gas (LNG) sending process and apparatus 1 for carrying out the same. The feed stream 10 comprising natural gas is expanded in the expander 12 and then fed to the gas / liquid separator 31 at any inlet pressure and inlet temperature. Typically, the pressure of the stream 10 is 30 to 80 bar (preferably greater than 60 bar and less than 70 bar), and the temperature of the stream is around ambient temperature, typically 5 to 50 ° C.

If desired, the feed stream 10 may be pretreated before entering the expander 12. For example, feed stream 10 may be precooled to the refrigerant in a heat exchanger (not shown) or a series of heat exchangers (including two or more heat exchangers operating at different refrigerant pressure levels, for example). have.

Expansion in the expander 12 is selected to form a partially condensed expanded feed stream 25. In addition, expansion in inflator 12 is selected to optimize subsequent separation steps in separator 31.

The expanded stream 25 enters the gas / liquid separator 31. There, the feed stream of line 25 is separated into gaseous top stream 40 and liquid bottom stream 30. Top stream 40 is concentrated in methane (and typically ethane) compared to expanded feed stream 25.

Bottom stream 30 is typically liquid and also typically contains some components that are refrigerated when the temperature reaches the temperature at which methane is liquefied. Separator 31 may be a distillation column or separation vessel, such as a scrub column, depending on the separation required to remove the refrigerated components from the feed stream. Typically refrigerated components are hydrocarbon components having a molecular weight of at least CO ₂ , H ₂ S and pentane. These freezing components may also be at least partially removed from the feed stream before entering the separator 31.

Bottom stream 30 may also include hydrocarbons that can be treated separately to form liquefied petroleum gas (LPG) products.

Typically, the bottom stream 30 is subjected to one or more fractionation steps to collect various natural gas liquid products. Top stream 40 is compressed via compressor 52 to obtain a compressed stream.

The compressed stream exits line 65 at a pressure of at least 70 bar, preferably at least 84 bar. The pressure increase in this compression step is selected from 30 bar to 150 bar depending on the respective separation and liquefaction pressures.

Some of the heat added in the compression step is removed from the stream 65 with respect to the surroundings, for example using an air cooler 61 or a water cooler. Thereafter, the resultant ambient-cooled stream 75 is further cooled in at least one external cooling stage. This external cooling stage may comprise a preliminary cooling stage (shown as heat exchanger 81). Instead, a series of continuous heat exchangers can be used.

Thereafter, the precooled stream 90 is further cooled and liquefied in a liquefaction unit 5 comprising at least a main cryogenic heat exchanger 91. Any suitable type of heat exchanger can be used. A cryogenic heat exchanger 91 operated by a mixed refrigerant is shown, wherein the light and heavy fractions are first autocooled in a tube (not shown) formed parallel to the precooled stream and then the inlet means. (95, 96) is expanded to the shell side through each. The spent heavy fractions and the light fractions are withdrawn from the shell side of the main cryogenic heat exchanger 91 via the outlet 97. The spent refrigerant in line 97 can be recompressed and cooled to form a liquid or, in the case of a mixed refrigerant, to form a mixed gaseous hard fraction and a liquid heavy fraction.

Referring again to stream 65, the liquefaction pressure is selected to exceed a pressure of at least 84 bar, more preferably at least 86 bar. As a result, the vapor in stream 65 may be supercritical.

In the next step, the liquefied stream exiting the main cryogenic heat exchanger 91 via line 100 is further cooled in the flash stage, where the pressure is reduced through the valve or liquid expander 101. Suitably, the pressure after expansion is approximately atmospheric pressure. The heat of expansion is removed from the liquefied stream so that the temperature is lowered to the temperature at which the liquefied product remains liquid at atmospheric pressure. Flash gas 130, which typically contains nitrogen and some methane, is separated from stream 110 in flash tank 111. A portion of flash gas 130 may be used as fuel gas to provide energy to the liquefaction process. The liquid portion of stream 110 exits the bottom of flash tank 111 in line 120. This discharged liquid portion can be stored and transported as LNG.

Preferably, the compressor train 52 uses at least expansion energy from the expander 12. To this end, one or more compressors of the compressor train 52 are functionally connected to the expander 12 to form a so-called "compressor-expander configuration". However, additional compression power can be provided to obtain a pressure above 84 bar. Preferably the additional compressor motor power consumed by the compressor 52 is chosen to be the same as or close to the power required for the refrigerant compressor (not shown) so that the same driver can be used for both of these compressors. This can provide cost and maintenance benefits.

Unlike the embodiment shown in FIG. 2, in the embodiment of FIG. 1, since no integrated heat is applied to the coolant imparted to the upper stream 40 (as in the heat exchanger 41 of FIG. 2), the line After the compressed top stream in 65 is cooled to about ambient (in cooler 61), the stream is sent directly to the external cooling stage of the heat exchanger 81 via line 75.

Table 1 shows the pressure and temperature of the stream at various parts in the example process of FIG. In addition, methane is represented by mol%. The feed stream in line 10 of FIG. 1 roughly comprised the following composition.

80% methane,

8% ethane,

5% propane,

-4% butane,

-1% C ₅ ⁺ ,

-2% N ₂ .

Frozen components such as H ₂ S, CO ₂ and H ₂ O were removed in advance.

2 schematically depicts another embodiment of the process according to the invention.

In this embodiment, the top stream 40 passes through the discharge stream heat exchanger 41, where it is indirectly heated to the stream at approximately ambient temperature (stream 70). The stream 50 discharged from the discharge stream heat exchanger 41 is then compressed through a compressor 52 or a series of two or more compressors. The compressed stream is withdrawn to line 60 at a pressure of 84 bar or higher and, for example, cooled in air cooler 61 to obtain stream 70. The resultant ambient-cooled stream 70 is then introduced into the discharge stream heat exchanger 41, where the stream is cooled by indirect heat exchange with the cold upper stream 40 to cool the stream 80. And this stream is cooled further in the heat exchanger (81).

Table 2 shows the increase in propane and butane recovery using the process as shown in FIG. 1 in accordance with the present invention. The same lineup was used and compared as in FIG. 1, but expanded to about 45 bar in the inflator 12 (in contrast to the present invention). As shown in Table 2, the present invention increased the recovery of propane and butane in stream 30 (16% and 9%, 36% and 20%, respectively).

Table 3 shows the increase in LNG product using the process as shown in FIG. 1 according to the present invention. For comparison, the same refrigeration power and lineup as in FIG. 1 was used, but not in the compressor train 52 (in contrast to the present invention), so that the compression pressure in line 65 was reduced to line 40. The compressive pressure at was equal to about 30.4 bar. As shown in Table 3, the increase in LNG product was about 19%.

Claims (17)

As a method of liquefaction of a natural gas stream, (a) providing a feed stream comprising natural gas at a pressure between 30 and 70 bar, (b) expanding the feed stream of step (a) to obtain an expanded feed stream having a pressure of less than 35 bar, (c) feeding the expanded feed stream to a gas / liquid separator, (d) separating the expanded feed stream into a gaseous stream and a liquid stream in a gas / liquid separator, where the gas stream is concentrated in methane compared to the feed stream and the liquid stream is reduced in methane compared to the feed stream. Steps, (e) increasing the pressure of the gaseous stream obtained in step (d) to a pressure of at least 70 bar, (f) liquefying the pressurized gaseous stream obtained in step (e) in a main cryogenic heat exchanger to obtain a liquefied natural gas stream; And reducing the pressure of the liquefied natural gas stream obtained in step (f), A process for liquefying a natural gas stream that does not increase the pressure of the feed stream provided in step (a) until the pressure in step (e) increases. The method of claim 1, wherein the pressure in step (e) is increased to at least 84 bar. The process of claim 1 wherein the gaseous stream obtained in step (d) has a C ₅ ⁺ content of 0.5 mol% or less. The method of claim 1, wherein the pressure in step (e) is increased by compressing the gaseous stream to obtain a compressed stream. The method of claim 1, wherein the gaseous stream obtained in step (e) is cooled. 5. The method of claim 4, wherein the compressed stream is heat exchanged with the gaseous stream obtained in step (d) before being liquefied in step (f). The method of claim 1, wherein the expander for expanding the feed stream in step (b) is connected to a compressor for compressing the gaseous stream. 8. Process according to any of the preceding claims, wherein the pressure in step (a) is greater than 60 bar and less than 70 bar. 8. Process according to any one of the preceding claims, wherein the temperature of the feed stream in step (a) is 5 to 50 ° C. A liquefaction apparatus of a natural gas stream, Means for providing a feed stream 10 comprising natural gas at a pressure between 30 and 70 bar, An expander 12 which expands the feed stream 10 to obtain an expanded feed stream 25 having a pressure of less than 35 bar, A gas / liquid separator 31 which separates the expanded feed stream 25 into a gaseous stream 40 and a liquid phase stream 30, wherein the gaseous stream 40 concentrates methane as compared to the feed stream 10. And the liquid stream 30 is a gas / liquid separator 31 with reduced methane as compared to the feed stream 10, A pressurizing unit 52 for increasing the pressure of the gaseous stream obtained in the gas / liquid separator 31 to a pressure of at least 70 bar, and A liquefaction unit 5 which liquefies a gaseous stream having a pressure of at least 70 bar and also comprises at least one cryogenic heat exchanger 91, A valve or liquid expander 101 for reducing the pressure of the liquefied stream 100 exiting the cryogenic heat exchanger 91, Apparatus for liquefying a natural gas stream characterized in that there is no other pressurizing unit between the pressurizing unit and the means for providing a feed stream (10) at said pressure of 30 to 70 bar. 11. Apparatus according to claim 10, characterized in that the pressurizing unit (52) comprises a compressor. 12. Liquefaction of a natural gas stream as claimed in claim 11, further comprising a heat exchanger (41) for exchanging heat from the compressor (52) for the gaseous stream obtained in said gas / liquid separator (31). Device. 11. Apparatus according to claim 10, characterized in that the compressor (52) and expander (12) are connected. The apparatus of claim 10, wherein the pressurizing unit is arranged to increase the pressure of the gaseous stream from the gas / liquid separator to a pressure of at least 84 bar. The method according to any one of claims 10 to 14, characterized in that the means for providing said feed stream (10) comprising natural gas feeds said feed stream at a temperature of 5 to 50 ° C. Liquefaction apparatus of natural gas stream. delete delete

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