WO2019193740A1

WO2019193740A1 - Natural gas treatment method, and natural gas treatment device

Info

Publication number: WO2019193740A1
Application number: PCT/JP2018/014669
Authority: WO
Inventors: 謙角谷; 直樹一町
Original assignee: 日揮株式会社
Priority date: 2018-04-06
Filing date: 2018-04-06
Publication date: 2019-10-10

Abstract

[Problem] To provide technology for reducing the content of benzene mixed into methane during a treatment for separating natural gas into methane and a C2 or higher heavy component. [Solution] A natural gas treatment method using a natural gas distillation column 17 to separate natural gas into methane and a C2 or higher heavy component, wherein natural gas containing benzene is received from a supply line 106, cooled, and then separated into a gas and a liquid. By decompressing and expanding the gas obtained from the separation and thus lowering the temperature thereof, a gas-liquid mixture is obtained, and the gas-liquid mixture and the gas are introduced into a natural gas distillation column 174 and distilled to discharge methane and a heavy component. A rectification unit 2 separates, from the heavy component, absorption liquid main components which include butane and isopentane and are for absorbing the benzene in the natural gas. Natural gas from a natural gas supply line 101 and an absorption liquid containing the absorption liquid main components are brought into contact, and the benzene contained in the natural gas is absorbed by the absorption liquid.

Description

Natural gas processing method and natural gas processing apparatus

The present invention relates to a technology for processing natural gas containing benzene.

In natural gas processing equipment that processes natural gas (NG: NaturalNGas), after removing impurities such as water and acid gas contained in NG produced from the well source, a natural gas distillation tower is used. Then, a process of separating NG into a gas containing methane as a main component (methane gas) and a heavy component heavier than methane is performed.

For example, in Patent Document 1, a fluid extracted from a demethanizer that is a natural gas distillation tower (gas flowing out from the top of the demethanizer or a part of the liquid being distilled) is used as a self-refrigerant and cooled NG. Describes a technique for distilling the slag with a demethanizer. The demethanizer of Patent Document 1 is supplied with NG cooled to about -70 to -90 ° C.
On the other hand, the heavy component contained in NG may contain a component that solidifies at a low temperature and closes a device that performs NG processing.

Further, Patent Document 2 discloses heavy carbonization to the “volatile residual gas (methane, hydrogen, nitrogen, and other volatile gases)” extracted from the top of a fractionation tower, which is a natural gas distillation tower. In order to reduce the outflow of hydrogen (C ₂ + component), a C ₂ -C ₅ hydrocarbon distillate from a middle height region of a fractionation column (also described as “distillation column, demethanizer”) In which a lighter component than C ₃ is stripped in a depropanizer and C ₄ -C ₅ hydrocarbons are re-supplied to the top of the fractionation column. The C ₄ -C ₅ hydrocarbon acts as an absorbent that absorbs the C ₂ + component contained in the vapor rising in the fractionating column.
Here, Patent Document 2 mentions the problem that benzene contained in NG solidifies and solidifies at the processing temperature of NG, but does not disclose a technique focusing on the processing of benzene in NG.

US Pat. No. 4,157,904 Japanese Patent No. 5798127

The present invention has been made under the background as described above, and in the process of separating natural gas into methane and heavy components having 2 or more carbon atoms, the content of benzene mixed in methane is reduced, Provided is a natural gas processing method capable of preventing benzene from solidifying in a facility in which low-temperature methane flows.

The natural gas processing method of the present invention uses a natural gas distillation tower to separate natural gas into methane and a heavy component having 2 or more carbon atoms.
Receiving a natural gas supplied from a natural gas supply line at a pressure of 5 to 7 MPa and containing benzene at a concentration of 10 mol ppm or more;
Extracting a low-temperature fluid distilled in the natural gas distillation tower and using it as a self-cooling medium, and cooling the natural gas received from the natural gas supply line;
Separating the natural gas after cooling into gas and liquid in a gas-liquid separator;
Reducing the temperature by expanding the gas obtained in the gas-liquid separation part under reduced pressure in the pressure-lowering part, and obtaining a gas-liquid mixture;
Introducing the liquid from the gas-liquid mixture and the gas-liquid separation unit into the different height positions of the natural gas distillation column from the upper side in ascending order of temperature;
These gas-liquid mixture and liquid are distilled in the natural gas distillation tower, the methane is flowed out from the top of the tower, and the heavy component is flowed out from the bottom of the tower;
In order to absorb benzene in natural gas, including butane and isopentane, in a rectifying section that has a plurality of distillation columns and separates the heavy components flowing out from the natural gas distillation column Separating the absorption liquid main component, a fraction lighter than the absorption liquid main component, and a condensate heavier than the absorption liquid main component,
A step of bringing the natural gas from the natural gas supply line into contact with an absorption liquid containing the main component of the absorption liquid and causing the absorption liquid to absorb benzene contained in the natural gas.

The natural gas processing method may have the following characteristics.
(A) The absorption liquid is a mixed liquid of the absorption liquid main component and a fraction lighter than the absorption liquid main component.
(B) The gas obtained in the gas-liquid separation unit is separated into a part of the gas that is decompressed and expanded in the pressure-decreasing unit and the remaining gas, and after the remaining gas is further cooled, Obtaining a gas-liquid mixture by decompressing and expanding in the pressure-lowering part, and obtaining a gas-liquid mixture obtained by decompressing and expanding in the pressure-lowering part by first gas-liquid mixture and decompressing and expanding in the other pressure-lowering part When the obtained gas-liquid mixture is called a second gas-liquid mixture, in the step of introducing the gas-liquid mixture and the liquid, the first gas-liquid mixture, the second gas-liquid mixture, and The liquid is introduced into the natural gas distillation column at different height positions from the upper side in ascending order of temperature.
(C) In the step of absorbing the benzene into the absorption liquid, the absorption liquid is supplied to the gas-liquid separation unit, and the gas separated from the liquid is brought into countercurrent contact with the absorption liquid, whereby benzene in the gas Is absorbed in the absorption liquid, and the absorption liquid that has absorbed benzene is combined with the liquid. The gas-liquid separation unit is supplied with an absorption liquid supercooled by heat exchange with the self-refrigerant extracted from the natural gas distillation tower. Alternatively, the gas-liquid separator is supplied with an absorption liquid supercooled by heat exchange with an external refrigerant.
(D) In the step of absorbing the benzene into the absorption liquid, the absorption liquid is combined with the natural gas before the step of cooling the natural gas through the natural gas supply line and the natural gas is gasified. In the process of separating into liquid and liquid, the liquid containing the absorption liquid after absorbing benzene is separated from the gas.

(E) The rectifying section is a distillation tower for separating ethane, propane, butane, and isopentane from the heavy fraction in this order, an ethane separation tower, a propane separation tower, a butane separation tower, and an isopentane separation tower And in the step of separating the heavy component, the main component of the absorbing solution is obtained from the butane and isopentane obtained in the butane separation tower and the isopentane separation tower.
(F) The rectification section includes an ethane separation tower, a propane separation tower, and a butane-isopentane, which are distillation towers for separating ethane, propane, and a mixture of butane and isopentane in this order from the heavy component. A separation tower is provided, and in the step of separating the heavy component, the main component of the absorbing solution is obtained from the mixture obtained in the butane-isopentane separation tower.
(G) The rectification section includes an ethane separation tower, which is a distillation tower for separating a mixture of ethane and propane, butane, and isopentane in this order from the heavy fraction, and a stabilizer, In the step of separating the components, the main component of the absorbent is obtained from the mixture obtained by the stabilizer.
(H) The natural gas supplied from the natural gas supply line contains 90 mol% or more of methane, and liquefies methane flowing out from the top of the natural gas distillation column to obtain liquefied natural gas. thing.

The present invention uses a natural gas distillation column to separate the natural gas into methane and a heavy component having 2 or more carbon atoms, and the main component of the absorption liquid containing butane and isopentane separated from the natural gas is natural gas. Therefore, benzene contained in the natural gas can be taken into the heavy component side.

It is explanatory drawing of the NG processing apparatus which concerns on embodiment. It is explanatory drawing of the NG processing apparatus which concerns on 2nd Embodiment. It is explanatory drawing of the NG processing apparatus which concerns on 3rd Embodiment. It is explanatory drawing of the NG processing apparatus which concerns on 4th Embodiment. It is explanatory drawing of the NG processing apparatus which concerns on 5th Embodiment. It is explanatory drawing of the NG processing apparatus which concerns on 6th Embodiment.

FIG. 1 shows a configuration example of a natural gas processing apparatus (NG processing apparatus) to which the natural gas processing method of the present invention is applied. The NG processing apparatus includes an NG supply line 101 to which natural gas (NG) to be processed is supplied,

cold boxes

11 and 12 for cooling NG supplied from the NG supply line 101, and

cold boxes

11 and 12. A feed separator (gas-liquid separation unit) 13 for gas-liquid separation of NG that has been cooled and partially liquefied, and a demethanizer (naturally-occurring) that performs distillation of NG and separates it into methane and heavy components having 2 or more carbon atoms A gas distillation column) 17 and a rectifying section 2 for further distilling and separating components contained in the heavy component.
In the following description, the region obtained by dividing the demethanizer 17 in the height direction as shown in FIG. 1 is also referred to as “upper region S1, middle region S2, lower region S3” from the top. Further, these “upper region S1, middle region S2, and lower region S3” may be further divided into two parts in the vertical direction and referred to as “upper side, lower side” (for example, “lower side of upper region S1”). 2 to 6, S1, S2, and S3 are not attached. However, in FIGS. 2 to 6, as in FIG. 1, the portions corresponding to the demethanizer 17 in FIG. 1 are “upper region S1, middle region S2. , Lower region S3 ".

From the NG supply line 101, NG from which impurities such as mercury, moisture, and acid gas have been removed in a pretreatment unit (not shown) is supplied at a pressure of 5 to 7 MPa (step of receiving natural gas). The NG supplied from the NG supply line 101 handles NG called low-calorific value gas (lean gas) containing 90 mol% or more of methane and having a heavy content of 2 or more carbon atoms of less than 10 mol%. This is an example.

Furthermore, the NG treatment apparatus of this example is suitable for the treatment of NG containing benzene having a concentration of 10 mol ppm or more. While it is difficult to remove benzene by pretreatment, benzene is solidified at a low temperature, and the filling P filled in the upper region S1 in the demethanizer 17 and the methane gas outflow line 105 through which the methane flowing out from the demethanizer 17 flows. There is a risk of blocking the piping. Therefore, the benzene concentration in the methane gas at about −160 ° C. flowing through the methane gas outflow line 105 is 1 mol ppm or less, and the benzene concentration in the upper region S1 of the demethanizer 17 that is −80 ° C. or less is 10 mol ppm or less. Is operated.

However, in the NG processing apparatus, the higher the benzene concentration of NG supplied to the demethanizer 17, the more likely benzene is mixed into the methane side that is the product. The benzene mixed in the methane is solidified in the process of circulating in the demethanizer 17 or the methane gas outflow line 105, and there is a possibility that the equipment through which the methane flows is blocked by the solidified benzene. In particular, it was found that when the concentration of benzene in NG is 10 mol ppm or more, the problem of this blockage becomes remarkable and improvement is necessary.

For such a problem, the NG treatment apparatus of this example can treat NG containing 10 mol ppm or more of benzene. Since the concentration of benzene contained in NG produced from the natural world is diverse, it is difficult to specify the upper limit of the benzene content in general, especially when processing NG containing 100 mol ppm or more of benzene. The effect of taking benzene into the heavy side becomes higher. The NG treatment apparatus of this example is also capable of treating NG containing about 10 mol% of benzene as long as it can be treated with an acid removal equipment (ASRU) provided in the previous stage of the NG treatment equipment as a pretreatment equipment for NG. It can be designed to be possible.

NG supplied from the NG supply line 101 is cooled in the

cold boxes

11 and 12 to become a gas-liquid mixed fluid.
The cold box 11 cools NG by using methane gas, which is part of the fluid distilled by the demethanizer 17 and flowing out from the top of the demethanizer 17, as a self-cooling agent (step of cooling natural gas). The cold box 11 corresponds to the natural gas cooling unit of this example. In the cold box 12, cold heat is recovered by heat exchange between NG supplied from the NG supply line 101 and a liquid condensed from NG in a feed separator 13 described later, and this cold heat is supplied from the NG supply line 101. Used to cool part of NG.

In addition, here, for example, a fluid (for example, a liquid) extracted from the middle region S2 or the lower region S3 of the demethanizer 17 is used as a self-refrigerant, and cooling of NG supplied from the NG supply line 101 (step of cooling natural gas) is performed. You may employ | adopt the variation which provides the cold box to perform.

The gas-liquid mixed fluid of NG cooled in the

cold boxes

11 and 12 is supplied to a feed separator 13 which is a gas-liquid separator, and is separated into gas and liquid (step of separating into gas and liquid). . In the NG treatment apparatus shown in FIG. 1, the NG gas-liquid mixed fluid is gas-liquid separated by the feed separator 13, so that benzene flows out to the upper region S 1 of the demethanizer 17 and the methane gas outflow line 105. Can be suppressed. Details of the gas-liquid separation will be described later.

Part of the gas obtained by the feed separator 13 is diverted to the expander 14 constituting the pressure-lowering part, and the gas-liquid mixture (first gas-liquid mixture) is reduced by expanding the pressure under reduced pressure. Obtained (step of obtaining a gas-liquid mixture). The gas / liquid mixture obtained by the expander 14 is introduced to the lower side of the upper region S <b> 1 of the demethanizer 17 through the first gas / liquid mixture supply line 103. The temperature at which the gas-liquid mixture (first gas-liquid mixture) is supplied to the demethanizer 17 is, for example, −50 ° C. to −80 ° C.

Further, as shown in FIG. 1, the step-down unit may be configured such that, for example, a JT (Joule-Thomson) valve 15 is attached to the expander 14. In this case, when the flow rate for operating the expander 14 is small as a part of the gas obtained in the feed separator 13 and the expander 14 cannot be operated, a part of the gas is not diverted to the expander 14. The entire amount may be diverted to the JT valve 15. In addition, when a part of the gas obtained by the feed separator 13 is excessive and cannot be processed only by the expander 14, the excess may be diverted to the JT valve 15. Further, when the expander 14 is efficient and part of the gas obtained in the feed separator 13 cools more than expected and causes a problem in the process, the JT valve 15 is used together. It may be diverted to the expander 14 and the JT valve 15. Not only when the expander 14 is installed as described above, but by installing only the JT valve 15, a part of the gas obtained by the feed separator 13 may be diverted to the JT valve 15. .

The gas obtained by the feed separator 13 except for the gas supplied to the step-down unit is supplied to the cold box 16 which is a cooling unit. The remaining gas is further cooled by heat exchange with the low-temperature methane gas flowing out from the top of the demethanizer 17 in the cold box 16, and then decompressed (more specifically, adiabatically expanded) by the decompression valve 104a. The pressure reducing valve 104a corresponds to “another pressure reducing unit” in this example. Thus, a gas-liquid mixture (second gas-liquid mixture) is obtained from the remaining gas (step of obtaining a gas-liquid mixture). The gas / liquid mixture obtained by flowing through the cold box 16 and the pressure reducing valve 104a is supplied to the upper side of the upper region S1 of the demethanizer 17 via the second gas / liquid mixture supply line 104. The temperature at which the gas-liquid mixture (second gas-liquid mixture) is introduced into the demethanizer 17 is, for example, −70 ° C. to −100 ° C.

Note that it is not an essential requirement to provide a cold box 16 for further cooling a part of the gas obtained by the feed separator 13. For example, the entire amount of gas obtained by the feed separator 13 may be supplied to the pressure-lowering unit (expander 14 or JT valve 15), and a gas-liquid mixture may be obtained by decompression expansion.

On the other hand, the liquid obtained in the feed separator 13 is subjected to heat exchange with the NG supplied from the NG supply line 101 in the cold box 12, and after the cold energy is recovered, the liquid outflow line 102 and the distribution are obtained. It is introduced into the upper side of the lower region S3 of the demethanizer 17 via the 172. The temperature at which the liquid is introduced into the demethanizer 17 is, for example, −10 ° C. to 30 ° C.

As described above, with respect to the demethanizer 17, corresponding to the temperature distribution in the height direction of the demethanizer 17, the demethanizer 17 is arranged at different height positions in descending order from the upper side (second gas-liquid mixture, In the order of the first gas-liquid mixture and the liquid, these gas-liquid mixture and liquid are introduced (step of introducing the gas-liquid mixture and liquid).

Further, the demethanizer 17 is provided with a reboiler 171 as an external heat source. The reboiler 171 heats the liquid extracted from the lower side of the lower region S3 of the demethanizer 17 and returns it to the lower side than the extraction position of the liquid. The liquid heated in the reboiler 171 serves as a heat source, and light components in the demethanizer 17 are distilled.

As mentioned above as a variation of the cooling method for NG, the liquid extracted from the middle region S2 and the lower region S3 of the demethanizer 17 is used as a self-cooling agent, and cold is performed to cool the NG supplied from the NG supply line 101. A box may be provided. In this case, the self-refrigerant whose temperature has increased due to heat exchange with NG is returned again to the bottom side of the demethanizer 17 (below the extraction position), thereby providing a heat source for the distillation operation performed in the demethanizer 17. You can also.

The demethanizer 17 is provided with packings P or trays for promoting gas-liquid contact of fluid distilled in the tower in each of the upper region S1, the middle region S2, and the lower region S3. In FIG. 1, the case where the filler P is installed in the demethanizer 17 is illustrated. NG is distilled in the demethanizer 17 having the above-described configuration, and is distilled into methane and a heavy component having 2 or more carbon atoms.

For example, methane gas at −70 ° C. to −120 ° C. flows out from the top of the demethanizer 17 toward the methane gas outflow line 105, and heavy components at 40 ° C. to 140 ° C., for example, are heavy from the bottom of the demethanizer 17. It flows out toward the separation line 106 (a step of flowing out methane from the top of the tower and flowing out heavy components from the bottom of the tower).

The methane gas that flowed out to the methane gas outflow line 105 (collected from NG) was supplied from the NG supply line 101 in the

cold boxes

16 and 11 in order to partially cool the gas obtained in the feed separator 13. Cool NG. Thereafter, the pressure of the methane gas is increased by the compressor 311 and the booster compressor 312, cooled by the air fin cooler 32, and then shipped by pipeline through the gas discharge line 301.

On the other hand, the heavy component that has flowed out to the heavy component discharge line 106 (collected from NG) is sent to the rectification unit 2 equipped with a plurality of distillation columns 21 to 24 using the extraction pump 173. The rectifying unit 2 separates useful components (such as propane and butane described later) and heavier fractions of condensate from the heavy component (step of separating the heavy component).

Here, in the conventional NG processing apparatus, in order to obtain propane (C3) and butane (C4) as LPG having high added value, ethane is separated from a deethanizer 21 that separates ethane (C2) from the heavy components. In some cases, a rectifying unit 2 including a depropanizer 22 that separates C3 from the heavy component after separation and a debutizer 23 that separates C4 from the heavy component after separation of C3 may be provided. In this case, a fraction heavier than C4 (C5 +) after C4 is separated by the debutizer 23 is discharged from the bottom of the debutizer 23 as a condensate.

In contrast to such a conventional NG processing apparatus, the NG processing apparatus of this example is a liquid (absorbing liquid main component) containing C4 and isopentane (i-C5) contained in the heavy component flowing out from the bottom of the demethanizer 17. This is characterized in that benzene contained in NG is taken into the heavy component side.

From this point of view, the rectifying unit 2 of the NG treatment apparatus shown in FIG. 1 is a distillation column for separating C2, C3, C4, and i-C5 in this order from the heavy component flowing into the heavy component discharge line 106. A deethanizer 21, a depropanizer 22, a debutanizer 23, and a deisopentanizer 24.
C2 and C3 correspond to lighter fractions than the main component of the absorbing liquid (in this example, C4 and i-C5 separated by the debutanizer 23 and the deisopentanizer 24). These C2 and C3 components are combined with the main component of the absorbing solution containing C4 and i-C5 to form a mixed solution, which is supplied to the feed separator 13 as an absorbing solution for absorbing benzene. The flow of the absorbing liquid will be described later.

The deisopentaizer 24 separates i-C5 from the heavy component after C4 is separated by the debutanizer 23. Then, a component heavier than i-C5, more specifically a component containing normal pentane (n-C5) after i-C5 is separated by deisopentaizer 24, is discharged as condensate. It is paid out from the line 204.

The next heaviest component after i-C5 is normal pentane (n-C5). Since n-C5 is difficult to be separated from benzene by distillation, if n-C5 is used as the main component of the absorbing solution, benzene may be re-supplied to NG supplied from the NG supply line 101. is there. Therefore, i-C5 is separated by the deisopentanizer 24, and a fraction heavier than n-C5 and n-C5 is used as a condensate, thereby suppressing resupply of benzene to NG.

C2 obtained by the deethanizer 21, C3 obtained by the depropanizer 22, C4 obtained by the debutanizer 23, and i-C5 obtained by the deisopentaizer 24 are supplied to the absorbent raw material line 201. The gas and liquid are separated by the separator 27. Note that surplus C2, C3, and C4 that are not used as the main component of the absorbing solution are paid out as products. Surplus i-C5 is mixed with condensate.

The gas that is separated from the liquid by the separator 27 and mainly contains C2 is discharged out of the system via the fuel gas line 202 and used as fuel gas. Further, the gas containing C2 flowing out of the separator 27 is pressurized using a compressor (not shown), and the pressurized gas is joined to the upstream side of the booster compressor 312 so that the gas containing C2 is used as a pipeline. You may ship.

The liquid separated from the gas by the separator 27 is supplied as an absorption liquid to the upstream side of the demethanizer 17 via the liquid feed pump 28 and the absorption liquid supply line 203. The more heavy components (2 or more carbon atoms) contained in the NG supplied from the NG supply line 101, the easier benzene falls to the bottom of the demethanizer 17, and the benzene is supplied to the feed separator 13 to absorb benzene. It is possible to reduce the absorption liquid main component and the absorption liquid.

The absorption liquid supplied through the absorption liquid supply line 203 is supercooled to about −20 ° C. to −45 ° C. in the cold box 11 by heat exchange with methane gas, which is a self refrigerant (each component of the absorption liquid is liquefied). After cooling to a temperature lower than the temperature at which the feed separator 13 is supplied. The supercooling of the absorbing solution is performed in order to improve the ability of benzene to absorb the absorbing solution. In the feed separator 13, the NG from the NG supply line 101 and the absorption liquid from the absorption liquid supply line 203 are brought into contact with each other, and the gas benzene contained in NG is absorbed into the absorption liquid. The absorption liquid supply line 203 corresponds to the absorption liquid supply unit of this example.

The feed separator 13 is filled with a filler Q for promoting contact between the NG gas and the absorbing liquid. Absorbing liquid from the absorbing liquid supply line 203 is introduced into the feed separator 13 from a position above the packing Q layer, and NG from the NG supplying line 101 is fed from a position below the packing Q layer. Introduced into the separator 13. As a result, the NG gas rising in the feed separator 13 and the descending absorbing liquid come into countercurrent contact, and benzene contained in a trace amount in the gas is efficiently absorbed by the absorbing liquid based on the gas-liquid equilibrium ( Process of absorbing benzene in the absorbing solution).

The absorbing liquid that has absorbed benzene in the feed separator 13 joins with the NG liquid separated in the feed separator 13 and is supplied to the demethanizer 17 and flows out from the bottom of the demethanizer 17 as a heavy component.
On the other hand, since the gas brought into contact with the absorbing solution has a small amount of entrained benzene, the amount of benzene in the upper region S1 of the demethanizer 17 and the methane gas outflow line 105 decreases. As a result, compared with the case where benzene is not absorbed by the absorbing liquid, low-temperature methane gas flows, such as the packing P provided in the upper region S1 of the demethanizer 17 and the equipment installed downstream of the demethanizer 17. It is possible to prevent benzene from solidifying at the locations.

According to the NG liquefaction apparatus according to the present embodiment, when performing the process of separating NG into methane and heavy components of C2 or higher using the demethanizer 17, the absorbing liquid containing C4 and i-C5 separated from NG Since the main component is resupplied to the supply side of NG, benzene contained in the NG can be taken into the heavy component side.

Here, the technology for absorbing benzene in NG using the absorbing liquid of this example is an NG process in which methane gas obtained by distilling NG is shipped as in the example described with reference to FIG. The present invention is not limited to the case where it is applied to an apparatus. For example, as in the example shown in FIG. 2, the present invention can also be applied to an NG processing apparatus that is a natural gas liquefying apparatus (NG liquefying apparatus) that liquefies NG to produce LNG.
Hereinafter, in the embodiments described with reference to FIGS. 2 to 6, the same reference numerals as those used in FIG. 1 are given to the same components as those described with reference to FIG.

In the case of the NG liquefaction apparatus, the methane gas flows out from the top of the demethanizer 17 to the methane gas outflow line 105, and then heat exchange in the

cold boxes

16 and 11, compression by the compressor 311 and the booster compressor 312, the air fin cooler 32. Then, it is further cooled by the cooler 33. Thereafter, the methane gas is cooled using MCHE (Main Cryogenic Heat Exchanger) 34, liquefied and supercooled to become LNG. The LNG is depressurized by the liquid expander 35 and delivered through the LNG delivery line 302.

Further, the gas (mainly C2) separated from the heavy component in the rectifying unit 2 and liquefied and supercooled in the MCHE 34 is also liquefied and supercooled and joined with the LNG obtained from the methane gas. You may pay out via the payout line 302.

Similar to the example described with reference to FIG. 1, in the NG liquefying apparatus having the above-described configuration, for example, in the feed separator 13, an absorption liquid containing C4 and i-C5, an NG gas containing benzene, By bringing them into countercurrent contact, benzene in the gas can be taken into the heavy component side.

Next, an embodiment according to a variation of the rectifying unit 2 will be described with reference to FIGS.
The rectifying unit 2 shown in FIG. 3 is provided with a debut / deisopentaizer 25 instead of the debutanizer 23 and the deisopentanizer 24 described in FIG. That is, the rectifying unit 2 is a distillation tower for separating C2, C3, and “mixture of C4 and i-C5” from the heavy components in this order, the deethanizer 21, the depropanizer 22, and the debuter A deisopentaizer 25 is provided. Then, the main component of the absorbent is obtained from the mixture obtained by the debut / deisopentanizer 25 (step of separating heavy components).

Then, a mixture of C2 obtained by the deethanizer 21, C3 obtained by the depropanizer 22, and C4 obtained by the debut / deisopentanizer 25 and i-C5 is absorbed in the absorbent raw material line 201. The gas and liquid are separated by the separator 27. As a result, an absorption liquid containing a mixture of C4 and i-C5, which are the main components of the absorption liquid, and light components (C2, C3) is obtained, and this absorption liquid is supplied to the feed separator via the absorption liquid supply line 203. 13 is supplied.

Further, in the rectifying section 2 shown in FIG. 4, a stabilizer 26 is provided in place of the depropanizer 22, the debutizer 23, and the deisopentanizer 24 described in FIG. That is, the rectification unit 2 includes a deethanizer 21 and a stabilizer 26 which are distillation towers for separating C2 and “mixture of C3, C4, and i-C5” in this order from the heavy component. And an absorption liquid main component is obtained from the mixture obtained in the stabilizer 26 (process which isolate | separates a heavy part).

Then, C2 obtained by the deethanizer 21 and a mixture of C3, C4 and i-C5 obtained by the stabilizer 26 are merged in the absorbent raw material line 201, and gas-liquid separation is performed in the separator 27. As a result, an absorption liquid containing a mixture of C3, C4, and i-C5, which are the main components of the absorption liquid, and a light component (C2) is obtained, and this absorption liquid is fed through the absorption liquid supply line 203 to the feed separator. 13 is supplied.

Next, FIG. 5 shows an absorption liquid cooler 18 that uses an external refrigerant instead of the cooling method that uses the methane gas that is the self refrigerant in the cold box 11 described in FIG. The example which provided is shown. Examples of the external refrigerant include liquefied C3. Further, an absorbent cooler 18 using an external refrigerant may be provided as an auxiliary to the cold box 11 that supercools the absorbent using the self-refrigerant shown in FIG.

Further, FIG. 6 shows a case where NG (natural) before being cooled in the

cold boxes

11 and 12 through the NG supply line 101 instead of the example in which the supply destination of the absorbing liquid described in FIG. An example is shown in which the absorbent is joined to NG) before the gas cooling step is performed. In this example, benzene is absorbed from the NG side to the absorption liquid side until the mixture of NG and the absorption liquid flows into the feed separator 13a which is a gas-liquid separator. According to such a configuration, it is not necessary to provide a flow path for allowing the absorbent to flow through the cold box 11, and the cold box 11 can be configured simply.

Thereafter, in the feed separator 13a, the liquid containing the absorbing liquid that has absorbed benzene is separated from the mixture of NG and the absorbing liquid. Here, unlike the example of the feed separator 13 provided with the layer of the packing Q described with reference to FIG. 1, the feed separator 13 a that performs gas-liquid separation of NG is cooled by the

cold boxes

11 and 12. Any drum that can separate NG, which is a gas-liquid mixed fluid, into gas and liquid may be used.

The technology according to each embodiment described with reference to FIGS. 3 to 6 can be applied to the NG liquefying apparatus shown in FIG.

In the above embodiment, NG supplied from the NG supply line 101 has been described with respect to the case where 90 mol% or more of methane is contained and the heavy component having 2 or more carbon atoms is less than 10 mol%, but is not limited thereto. It is not something. This NG liquefaction device treats NG with a benzene concentration that is extremely high, for example, even if the amount of methane contained in NG is less than 90 mol%, for example, containing 10 mol% or more of heavy components having 2 or more carbon atoms It can also be applied to.

Although not particularly mentioned in the above embodiment, for example, when an LNG facility is provided, the C2, C3, C4, and i-C5 components obtained in the rectifying unit 2 are the MCHE 34 shown in FIG. It may be used as a refrigerant. In this case, as in the above-described embodiment described in FIGS. 1 to 3, 5, and 6, the rectifying unit 2 includes a plurality of distillation columns, so that C2, C3, The C4 and i-C5 components can be extracted as a single substance or as a mixed refrigerant, and the degree of freedom by combining the refrigerant compositions used in the MCHE 34 can be increased. If no LNG facility is provided, it is not necessary to extract the refrigerant used in the MCHE 34. Therefore, the rectifying unit 2 may be configured to include a deethanizer 21 and a stabilizer 26 as shown in FIG.

Although not particularly mentioned in the above embodiment, the present NG liquefaction apparatus can be employed as a ground facility or an offshore facility. In the limited space on the ocean, the demethanizer 17 and the rectifying unit 2 are installed closer to each other than in the case of ground facilities. Therefore, when adopting the present NG liquefaction apparatus as an offshore facility, the absorbent supply line 203 can be designed to be short, which is advantageous.

Moreover, in the embodiment shown in FIGS. 3 and 4, the number of distillation columns installed in the rectifying unit 2 can be reduced. Therefore, when employing this NG liquefaction apparatus as an offshore facility, a distillation tower can be installed in a space-saving manner in a limited space on the ocean, and the space can be used effectively.

101

NG supply line

13, 13a
Feed separator 17 Demethanizer 2 Rectifier 21 Deethanizer 22 Depropanizer 23 Debutizer 24 Deisopentaizer 25 Debuta deisopentaizer 26 Stabilizer

Claims

In a natural gas processing method for separating natural gas into methane and heavy components having 2 or more carbon atoms using a natural gas distillation tower,
Receiving a natural gas supplied from a natural gas supply line at a pressure of 5 to 7 MPa and containing benzene at a concentration of 10 mol ppm or more;
Extracting a low-temperature fluid distilled in the natural gas distillation tower and using it as a self-cooling medium, and cooling the natural gas received from the natural gas supply line;
Separating the natural gas after cooling into gas and liquid in a gas-liquid separator;
Reducing the temperature by expanding the gas obtained in the gas-liquid separation part under reduced pressure in the pressure-lowering part, and obtaining a gas-liquid mixture;
Introducing the liquid from the gas-liquid mixture and the gas-liquid separation unit into the different height positions of the natural gas distillation column from the upper side in ascending order of temperature;
These gas-liquid mixture and liquid are distilled in the natural gas distillation tower, the methane is flowed out from the top of the tower, and the heavy component is flowed out from the bottom of the tower;
In order to absorb benzene in natural gas, including butane and isopentane, in a rectifying section that has a plurality of distillation columns and separates the heavy components flowing out from the natural gas distillation column Separating the absorption liquid main component, a fraction lighter than the absorption liquid main component, and a condensate heavier than the absorption liquid main component,
Natural gas from the natural gas supply line, and the absorption liquid containing the main component of the absorption liquid is brought into contact with each other, and the absorption liquid absorbs benzene contained in the natural gas. Gas processing method.
The natural gas treatment method according to claim 1, wherein the absorption liquid is a mixed liquid of the absorption liquid main component and a fraction lighter than the absorption liquid main component.
The gas obtained in the gas-liquid separation unit is separated into a part of the gas that is decompressed and expanded in the pressure-decreasing unit and the remaining gas, and after the remaining gas is further cooled, Obtaining a gas-liquid mixture by expanding under reduced pressure,
When the gas-liquid mixture obtained by decompression expansion in the pressure-lowering part is called a first gas-liquid mixture, and the gas-liquid mixture obtained by decompression expansion in the other pressure-lowering part is called a second gas-liquid mixture. In the step of introducing the gas-liquid mixture and the liquid, the first gas-liquid mixture, the second gas-liquid mixture, and the liquid are placed on the upper side at different height positions of the natural gas distillation column. The natural gas treatment method according to claim 1, wherein the natural gas treatment method is introduced in ascending order of temperature.
In the step of absorbing the benzene in the absorption liquid, the absorption liquid is supplied to the gas-liquid separation unit, and the gas separated from the liquid is brought into countercurrent contact with the absorption liquid, whereby the benzene in the gas is absorbed. The natural gas treatment method according to claim 1, wherein an absorption liquid that has been absorbed in water and benzene is combined with the liquid.
The natural gas processing method according to claim 4, wherein the gas-liquid separation unit is supplied with a supercooled absorption liquid by heat exchange with the self-refrigerant extracted from the natural gas distillation tower. .
The natural gas processing method according to claim 4, wherein the gas-liquid separation unit is supplied with an absorption liquid supercooled by heat exchange with an external refrigerant.
In the step of absorbing the benzene into the absorption liquid, the absorption liquid is combined with the natural gas before the step of cooling the natural gas is performed through the natural gas supply line, and the natural gas is gas and liquid. 2. The natural gas processing method according to claim 1, wherein in the step of separating the liquid, the liquid containing the absorption liquid after absorbing benzene is separated from the gas.
The rectifying section includes an ethane separation tower, a propane separation tower, a butane separation tower, and an isopentane separation tower, which are distillation towers for separating ethane, propane, butane, and isopentane from the heavy fraction in this order. 2. The natural gas processing method according to claim 1, wherein in the step of separating the heavy component, the main component of the absorption liquid is obtained from butane and isopentane obtained in the butane separation tower and the isopentane separation tower.
The rectification section includes an ethane separation tower, a propane separation tower, and a butane-isopentane separation tower, which are distillation towers for separating ethane, propane, and a mixture of butane and isopentane in this order from the heavy fraction. The natural gas treatment method according to claim 1, wherein the main component of the absorbent is obtained from the mixture obtained in the butane-isopentane separation column in the step of separating the heavy component.
The rectifying section is provided with an ethane separation tower and a stabilizer for separating ethane and a mixture of propane, butane, and isopentane in this order from the heavy fraction, and separates the heavy fraction. The natural gas processing method according to claim 1, wherein the absorbing liquid main component is obtained from the mixture obtained by the stabilizer in the step of performing.
The natural gas supplied from the natural gas supply line contains 90 mol% or more of methane, and is for liquefying methane flowing out from the top of the natural gas distillation tower to obtain liquefied natural gas. The natural gas processing method according to claim 1.
In a natural gas processing apparatus equipped with a natural gas distillation tower and separating natural gas into methane and heavy components having 2 or more carbon atoms,
A natural gas supply line in which natural gas containing benzene at a concentration of 10 mol ppm or more is supplied at a pressure of 5 to 7 MPa;
A natural gas cooling section for extracting a low-temperature fluid distilled in the natural gas distillation tower and using it as a self-refrigerant and cooling natural gas received from the natural gas supply line;
A gas-liquid separator that separates the cooled natural gas into gas and liquid;
A pressure reducing unit that lowers the temperature by expanding the gas obtained in the gas-liquid separation unit under reduced pressure to obtain a gas-liquid mixture;
The liquids from the gas-liquid mixture and the gas-liquid separation unit are introduced into different height positions from the upper side in ascending order of temperature, the gas-liquid mixture and the liquid are distilled, and the methane is discharged from the top of the tower. A natural gas distillation tower for letting out the heavy component from the bottom of the tower,
A plurality of distillation towers, including butane and isopentane, an absorbent main component for absorbing benzene in natural gas, a fraction lighter than the main component of the absorbent, and a fraction heavier than the main component of the absorber A condensate, and a rectifying section for separating the heavy component flowing out from the natural gas distillation tower,
In order to make the absorption liquid absorb benzene contained in the natural gas, an absorption liquid supply unit that makes the absorption liquid containing the main component of the absorption liquid contact the natural gas supplied from the natural gas supply line, A natural gas processing apparatus characterized by that.