JPS61100531A - How to separate methane - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] "Objective of the Invention" [Industrial Application Field] This invention is directed to the cooling of a high-pressure gas stream containing various kinds of medium and low hydrocarbons including olefins as a main component. The present invention relates to a method for cryogenically separating methane from the resulting mixed condensate of hydrocarbons.

More particularly, the invention provides a method by which the separation of methane contained therein from the hydrocarbon-based condensate obtained when the high-pressure gas stream is cooled can be carried out with low power consumption. Regarding.

[Prior Art] Aliphatic hydrocarbons having 2 or more carbon atoms or mixtures thereof are thermally decomposed and contain as a main component many kinds of middle and lower hydrocarbons including olefins such as ethylene, propylene, and butylene. BACKGROUND ART Raw material gas is produced and desired components are separated from this raw material gas by cryogenic separation, which is carried out on a large scale, for example, in the production of ethylene using naphtha as a raw material. As a method for separating desired components from a raw material gas mainly containing various kinds of medium- and low-grade hydrocarbons including olefins as mentioned above, by cryogenic separation, the raw material gas is first separated at 20 to 50'KfJ/
The high-pressure gas stream is compressed to a pressure of atG to form a high-pressure gas stream, and then this high-pressure gas stream is cooled to 1120°C or below, and each liquid mixture of hydrocarbons that is successively condensed and produced during the cooling process is A method is generally used in which it is separated from the uncondensed gas and, depending on its composition, subjected to rectification. Among the general methods mentioned above, in which a high pressure gas stream is cooled to obtain a hydrocarbon condensate,
Compression → cooling (water cooling) in which propylene or propane is used as a refrigerant and the refrigerant is not mixed with the high-pressure gas stream.
a closed refrigeration cycle in which the refrigerant flow is independent of the feedstock and product gas flows;
The most commonly used combination method is a closed refrigeration cycle in which ethylene or ethane is used as the refrigerant. Furthermore, when each condensate is subjected to rectification, the most common method is to first separate methane, which is a hydrocarbon with the lowest boiling point. The most common conventional method as described above consumes a large amount of power and is desired to be improved, and its contents will be briefly explained below.

FIG. 2 schematically illustrates the main steps of the most common method described above. Raw material gas is passed from pipe 1 to compressor C-
1, and after compression, it is discharged from pipe 2 as a high-pressure gas flow of 20-50 kGl/m G, and once it is sent to the water cooler W-
After being indirectly cooled by 1 and subjected to pretreatment such as removal of moisture and other substances that impede cryogenic separation (this pretreatment equipment is omitted in the drawing)
, tube 3 into the heat exchanger H-1 of the cryogenic separator.

The heat exchanger H-1 exchanges this high-pressure gas stream with components in the separated high-pressure gas stream whose temperature is lower than that of the high-pressure gas stream, a refrigerant circulated in the closed refrigeration cycle, etc. as desired. A heat exchanger for exchanging heat and cooling a high-pressure gas stream, the heat exchanger having at least one (usually two or more) passages for a cold side fluid to be heat exchanged with the high-pressure gas stream, the number of passages and The heat transfer area of each passage can be freely designed as desired in the heat exchanger. In FIG. 2, the heat exchanger H-1 has four low-temperature fluid passages R-1, R-2, R-3, R-
Although an example in which four passages are provided is described, the number of passages is not limited to four. In addition, other heat exchangers H-2, H-3, H-4, H-5, H-1 marked with the symbol H in FIG. 2 and FIG. 3 and FIG.
1, H-12, H-13, H-14, H-15, H-1
6 and the like are all heat exchangers having one or more high-temperature side fluid passages and one or more low-temperature side fluid passages, similar to the heat exchanger H-1. In the heat exchanger H-1, the high pressure gas stream is mixed with liquid propylene by a closed refrigeration cycle of the propylene refrigerant supplied to the passage R-1 and the passage R-
2, R-3, R-4, etc., and is cooled by exchanging heat with each component in the separated raw material gas in a lower temperature process. The liquid propylene supplied to the passage R-1 is a closed refrigeration cycle using propylene as a circulating refrigerant, that is, the circulating propylene is compressed in the compressor C-2, and this compressed propylene is indirectly compressed in the water cooler W-2. It is produced by reducing the pressure of a part of pressurized liquefied propylene that is cooled and liquefied and taken out from pipe L-11. The propylene that is evaporated through heat exchange with the high-pressure gas stream in this passage R-1 is further heated in another heat exchanger as necessary, and then the propylene that has been evaporated and heated in the same way in another process is heated. At the same time, it is recirculated to the suction port of compressor C-2 and reused.

That is, the circulating propylene forms an independent w1 chain refrigeration cycle that is isolated from the high pressure gas stream and the gas stream after the high pressure gas stream has been separated into its components. In the following description, liquid propylene and liquid ethylene, which have been compressed, cooled, liquefied, and then depressurized in this closed refrigeration cycle, will be referred to as circulating refrigerant propylene and circulating refrigerant ethylene, respectively. The high-pressure gas stream is cooled to about -40 DEG C. in the heat exchanger 1-1-1 by the above-mentioned heat exchange, with some relatively high-boiling hydrocarbons in the high-pressure gas stream being condensed as a mixture. This high-pressure gas stream containing the condensate is introduced via line 4 into separator S-1, where the condensate is separated from the uncondensed gas. The condensate is drawn off from pipe L-1;
After the pressure is reduced, the liquid is supplied to a predetermined position in the medium pressure rectification column D-1 depending on the composition of the liquid. On the other hand, the uncondensed gas is introduced into heat exchanger H-2 via pipe 5, where it is heated with the lower temperature gas stream etc. after the high-pressure gas stream has been separated into its components, as in heat exchanger H-1. The heat exchanger H-
In 2, the high-pressure gas stream is cooled to a lower temperature than in H-1, so passage R-1 (not numbered in the figure) of H-2 is supplied with circulating refrigerant ethylene. . The circulating refrigerant ethylene is used as the circulating refrigerant for a closed refrigeration cycle in which the refrigerant ethylene stream is made independent of the high-pressure gas stream and the separation gas stream into its respective components, as in the case of the circulating refrigerant propylene described above. The refrigerant ethylene compressed in step 3 is cooled by a water cooler W-3, and further cooled and liquefied by heat exchange with a part of the circulating refrigerant phlopylene in a heat exchanger H-11, and taken out through a pipe L-12. After that, part of it was depressurized. This circulating refrigerant ethylene is used in places other than the heat exchanger H-2, but all the used circulating refrigerant ethylene is recycled to the suction port of the compressor C-3 and reused. Due to such cooling in heat exchanger H-2, a portion of the remaining hydrocarbons is condensed again in the high-pressure gas stream, and the condensed liquid and uncondensed gas are introduced into separator S-2 via pipe 6. , the condensate is separated from the uncondensed gas. The separated condensate is taken out from pipe L-2, and after being depressurized, it is supplied to a predetermined location of medium-pressure rectification column D-1 depending on the composition of the liquid.

The uncondensed gas separated at minute 1118-2 is taken out through tube 7 and introduced into heat exchanger H-3, where it is further cooled in the same manner as above and enters through tube 8 at minute 1118-3, where it enters the condensate and uncondensed gas. Separated into gas. The condensed liquid is taken out from the pipe L-3, and after being depressurized, it is supplied to a predetermined location of the medium-pressure rectification column D-1 depending on the composition of the liquid. The uncondensed gas in separator S-3 is introduced into heat exchanger H-4 through pipe 9, where it is further cooled to reach -120°C to -140°C, and then introduced into separator s-4 through pipe 10. The condensate produced during this cooling is separated in the separator S-4 and taken out from the pipe L-4, and after the pressure is reduced, it is supplied to a predetermined portion of the medium-pressure rectification column depending on the composition of the liquid. The uncondensed gas in separator S-4 is taken off through tube 11 and can be further cooled in heat exchanger H-5 if necessary;
Pipe 11 when cooled to about 0 to -140°C
The composition of the uncondensed gas extracted from the reactor usually consists of methane and hydrogen entrained in the feed gas, and does not substantially contain ethylene or hydrocarbons with a boiling point higher than ethylene.

Therefore, when the purpose of cryogenic separation is to separate ethylene and hydrocarbon components with boiling points higher than ethylene contained in the high-pressure gas stream, further cooling of the high-pressure gas stream by heat exchanger H-5 is necessary. Not necessarily necessary. Each separator S-
1. Each condensate obtained in S-2, S-3, and S-4 is a mixture of methane, ethylene, ethane, propylene, propane, butanes, butenes, etc., and some hydrogen is also dissolved. are doing. Also, the high pressure gas flow is -120~-
These condensates produced during cooling to 140° C. typically have higher methane contents than those produced during the cooler cooling stage. In order to separate desired components, particularly ethylene and propylene, from these mixed liquids, it is advantageous to first remove methane and hydrogen, which have the lowest boiling points, from these mixed liquids. The medium pressure rectification column D-1 is a rectification column for removing methane and hydrogen from these mixed liquids. The bottom liquid of this rectification column is indirectly heated by another fluid in heat exchanger H-13. As the heating fluid at this time, a portion of the high-pressure gas stream or other fluids at an appropriate temperature can be used singly or in combination of two or more as desired. Also, at the top of this rectification column, the gas flowing out from the top of the column through the pipe 20 is passed through the heat exchanger H-12.
is cooled by a part of the circulating refrigerant ethylene, most of the methane in this gas is liquefied, and the separator 5-1
After separation into methane liquid and uncondensed gas in step 2, the methane liquid is taken out from pipe 12, most of which is passed through pipe 14 and used as the reflux liquid of the medium pressure rectification column, and the remainder of the methane liquid is taken out from pipe 12. is taken out as a product from pipe L-15 and used together with the uncondensed gas containing hydrogen and methane taken out from pipe 21, respectively, as a cooling cryogenic fluid in the desired heat exchanger. Due to the II distillation action in this medium pressure rectification column D-1, the condensate supplied to this column from separators S-1, S-2, S-3, S-4, etc. Methane liquid obtained from pipe L-15 at the top of the tower, a mixed gas of hydrogen and methane obtained from pipe 21, and ethylene and ethylene containing substantially no methane taken out through pipe L-13 at the bottom of the tower. It is separated into a mixture of various hydrocarbons with high boiling points.

The hydrocarbon mixture substantially free of methane obtained from the bottom of the medium-pressure rectification column is further separated into desired components by a sequential rectification process, but this part is not shown in the diagram. do not have. In addition, the uncondensed gas and methane liquid obtained from the top of the medium pressure rectification column are transferred to heat exchangers H-1, H-2, H-
3. Low temperature side fluid passages R-1, R-2, R-3 such as H-4
, R-4, etc., as appropriate, to cool a high-pressure gas stream, or as a source of cold heat when the methane-free hydrocarbon mixture is subjected to rectification separation. This medium pressure rectification column is normally operated under a pressure of 15 to 40 ka/crtG.

The above is an overview of the most common conventional method, but this method has the drawback of high power consumption, as will be described later.

There is a method disclosed in US Pat. No. 3,443,388 as a method for improving the drawbacks of such conventional methods. The general steps of the method disclosed in this US patent are shown in FIG. In the method shown in Figure 3, the high pressure gas flow is transferred to the heat exchanger H-1.
, H-2, l-1-3, H-4, etc., and condenses in the temperature range of each of these heat exchangers, the hydrocarbon mixture is cooled in separators S-1, S-2, and S-, respectively.
3. The process of separating the uncondensed gas in S-4, etc., and taking out the respective separated liquids through the pipes L-1, L-2, L-3, L-4, etc. is shown in Fig. 2 above. Since this is substantially the same as in the case of , the explanation is omitted and the diagram is simplified. The method shown in FIG. 3 is an improved method of the methane rectification separation process shown in FIG. Two rectification columns are used: a low pressure rectification column (3). The condensate obtained in the separators S-1 and S-2 is supplied to the medium pressure rectification column D=1, and the condensate obtained in the separator S-3 is supplied to the low pressure rectification column D-3. The methane-rich condensate obtained in the separator S-4 is supplied to the top of the low-pressure rectification column D-3 as a reflux liquid. The heating of the bottom of the medium-pressure rectification column D-1 is carried out by the heat exchanger H-13 as in the case of FIG. This method differs from the method shown in FIG. 2 in that the bottom liquid of the low-pressure rectification column is used as a source of cold heat in heat exchanger H-12 for condensation. That is, the gas flowing out from the top of this medium-pressure rectification column D-1 via pipe 20 is condensed in heat exchanger H-12, and most of this condensate is returned to this column via pipe -14. However, the latent heat of condensation released during this condensation is used to heat the bottom liquid of the low-pressure rectification column. During this condensation, a part of the top gas is kept in gaseous form along with some methane in order to prevent the hydrogen dissolved in the condensate obtained in separators 8-1 and S-2 from accumulating. tube 23
After reducing the pressure, the gas is extracted from the gas and fed to a low-pressure rectification column for the purpose of recovering a portion of the methane in this gas. Also heat exchanger H-1
A portion of the methane liquid condensed in step 2 is taken out from pipe L-15 as pure water. From the top of the low pressure rectification column, pipe 2
A gas consisting essentially of methane and hydrogen is taken off via pipe L-16, and a liquid containing methane and other hydrocarbons is removed from the bottom of this low-pressure rectification column via pipe L-16. The bottom liquid of this low-pressure rectification column has not yet been sufficiently separated from methane.
It is pressurized by pump P-1, further heated appropriately by heat exchanger H-14, and then supplied to medium-pressure rectification column D-1. In the medium pressure rectification column D-1, the bottom liquid of the low pressure rectification column is supplied from this pipe L-16, and the separator $- is supplied to this column from the above-mentioned pipes L-1 and L-2. The condensate from 1 and S-2 does not substantially contain the methane liquid obtained from the top of the column via pipe L-15 and the methane obtained from the bottom of the column via pipe L-13, and has a boiling point lower than that of ethylene. It is separated into a mixed liquid consisting of high hydrocarbons. Hydrocarbon mixture from which methane has been removed taken out from pipe L-13 and pipe L-
The method of using the methane liquid taken out from No. 15 is the same as that shown in FIG. The medium pressure rectification column according to this Figure 3 method is under a pressure of 27 to 28 kg/dG, and the low pressure rectification column is under a pressure of 5 kg/dG.
Each is operated under a pressure of ~6 ka/cnG.

The improved method shown in Figure 3 does not use the circulating refrigerant ethylene as the cold source for heat exchanger H-12 to generate the reflux liquid of the medium-pressure rectification column, as in the conventional method shown in Figure 2. The power required for the ethylene compressor C-3 for a closed refrigeration cycle using ethylene as the refrigerant is reduced by the amount equivalent to the amount of circulating ethylene refrigerant required in the heat exchanger H-12 in Figure 2. It has its advantages and is a great improvement. However, the method of FIG. 3 still has two drawbacks. The first
This is due to the composition of the high-pressure gas stream when conventional aliphatic saturated hydrocarbons are used as feedstock for pyrolysis, and also because the process uses two rectification columns, both of which require reflux. As a direct cause of this, there is a limit to the amount of reflux liquid supplied from separator S-4 to the low-pressure rectification column via pipe L-4, so the reflux ratio of the low-pressure rectification column and medium-pressure rectification column is cannot be made sufficiently large, and as a result, either in the gas consisting of methane and hydrogen obtained from the pipe 22 at the top of the low-pressure rectification column, or in the pipe L.
It is inevitable that a non-negligible amount of ethylene will be mixed into any of the product methane liquid from the medium-pressure rectification column extracted from -15. The second drawback is that after methane and hydrogen have been removed, all of the ethylene and higher-boiling hydrocarbons are removed as one liquid stream.
13, another rectification column (not shown) for separating and obtaining ethylene and ethane from this liquid stream is large, and a heat source for heating the bottom liquid in this other rectification column is required. Also, a cold heat source is often required to generate the reflux liquid.

[Problems to be Solved by the Invention] The present invention aims to improve the drawbacks remaining in the conventional method as described above, that is, to reduce power consumption compared to the method shown in FIG. Improve the yield of ethylene and reduce the size of the rectification column when separating ethylene and ethane from the hydrocarbon mixture after methane and hydrogen have been substantially removed compared to the method shown in the figure. The purpose is to provide a means by which this can be done.

"Structure of the Invention" Means for Solving Problem E] This inventive method consists of the following means as a gist. That is, ■In the cooling process of the high-pressure gas flow, from room temperature to -120℃~-
The temperature range up to 160°C is divided into at least three temperature zones, and a medium-pressure columnar gas-liquid contact device is equipped with an indirect heater at the bottom and an indirect heater at the bottom and the top effluent gas is mixed with liquid ethylene. A medium-pressure rectification column equipped with means for producing a reflux liquid that is condensed by heat exchange is used, and the lower limit temperature of at least three temperature zones is 150°C.
The condensate obtained from the above-mentioned high temperature zone is supplied to a medium-pressure columnar gas-liquid contacting device, and heated by a bottom indirect heater to reduce the methane content to 0.0. Each condensate obtained from the temperature zone other than the high-temperature zone and the tower top effluent gas from the medium-pressure gas-liquid contactor are separated into 5 mol% or less of the bottom effluent, and are subjected to medium-pressure purification according to their respective compositions. It is supplied to a predetermined point in the distillation column, and the rectification operation is carried out while the bottom of the medium-pressure rectification column is heated to produce a methane product with a methane content of 90 mol% or more and a methane product with a methane content of 0.5 mol% or less. (1) At least a portion of the methane product is heated by heat exchange with a high-pressure gas stream.

[Operation] The gist of this invention will be explained below using the process example shown in FIG. 1, and then the content of this invention will be explained in detail.However, this invention is limited by the process example shown in FIG. isn't it. In FIG. 1, low pressure feed gas is drawn into compressor C-1 through pipe 1 and compressed into a high pressure gas stream. This high-pressure gas stream is indirectly cooled by water in water cooler W-1 and fed into heat exchanger H-1. This heat exchanger H-1 and subsequent steps are a cryogenic separation step. Between the water cooler W-1 and the heat exchanger H-1,
Usually, substances that interfere with the cryogenic separation process, such as moisture contained in the high-pressure gas stream, are removed.
In the figure, the description of such a step of removing harmful substances is omitted. Heat exchanger H-1 to tube 4.5.6.7.8
．． 9.10.11.12.13 etc. In the cooling process of the high pressure gas stream, each heat exchanger H-2, l-1-3, H-
4, H-5 etc. and each separator S-1, S-2, S-3, S-
4. The arrangement of S-5, etc., the configuration of the gas passages in each heat exchanger, the configuration of the two closed refrigeration cycles that use propylene and ethylene as refrigerants, etc. are as described in the explanation of Fig. 2. 1, the temperature of the high-pressure gas flow is -50°C at the outlet of heat exchanger H-1, and -50°C at the outlet of heat exchanger H-4. The heat transfer area of each heat exchanger is maintained so that the temperature is 140°C. In the heat exchanger H-1, the high-pressure gas stream cooled to -50°C by heat exchange with the fluid on the low-temperature side as described above contains a condensate of some hydrocarbons with a relatively high boiling point. has been done. This condensed liquid is separated from uncondensed gas in separator S-1, and after being depressurized, passes through pipe L-1 to the top of medium-pressure columnar gas-liquid contactor (hereinafter simply referred to as gas-liquid contactor) D-2. Supplied nearby. For example, in the case where the high-pressure gas stream has a well-known composition in which the raw material gas is pyrolysis gas of naphtha, the composition of the condensate is 10-15% methane, 40-50% ethylene, 5-15% ethane,
Propylene 20-30, Propane 0.2-1.5, C4
The condensate contains 3 to 10 mol% of hydrocarbons and 0.1 to 1.0 mol% of hydrogen, but since the outlet temperature of heat exchanger H-1 is -50°C higher than the critical temperature of methane and hydrogen, this condensate The methane and hydrogen in it are mainly C2, C3 and C
It is thought that the hydrocarbon components of No. 4 were condensed and dissolved in the resulting liquid, and by appropriately reducing pressure and heating, most of it can be gasified.

The gas-liquid contact device 0-2 is a device that utilizes this phenomenon to separate the condensate into a tower top effluent gas rich in methane and a tower bottom effluent gas with a low methane content. The column is equipped with plates or packing for gas-liquid contact, similar to a conventional rectification column used in cryogenic separation. This separation occurs when the condensate supplied to this column flows down inside the column, and the liquid that reaches the bottom of the column is heated by another heating fluid in the heating heat exchanger H-13 at the bottom of the column. brought into countercurrent contact with an upward flow of hydrocarbon vapor generated by the process, resulting in a mass exchange such that low-boiling components are preferentially evaporated and high-boiling components are preferentially condensed; A bottom effluent consisting of C2, C3 and C4 hydrocarbons substantially free of methane is taken off from line L-13, and an overhead gas containing methane as the largest component is taken off from line 23. . As a result, this gas-liquid contactor converts a substantially methane-free bottom effluent from the top effluent gas without the use of cold sources that must be supplied from other processes to produce reflux. Compared to the conventional method in which methane is separated from the total amount of condensate separated in each heat exchanger in the process of cooling a high-pressure gas stream by rectification alone, the method of the present invention In the entire methane separation process, methane can be removed by using a small amount of refluxing methane liquid.

According to the above-mentioned principle, methane 0.001 to O25, ethylene 3
5-55, ethane 5-15, propylene 25-45, propane 0.1-5.0, C4 hydrocarbon 2-15 and hydrogen 0-0.01 mol% liquid, from the top of the column, methane 25-45
60, ethyl, lene 20-50, ethane 5-10, propylene 1-20, propane 0.1-5.01 G4 hydrocarbon 0.01-5.0 and hydrogen 0.5-3.0 mol% gas each can be obtained.

The uncondensed gas in the heat exchanger H-1 of the example in FIG.
It is divided into three temperature zones H-3 and H-4. The condensate produced in heat exchanger H-2 and separated in separator S-2 passes through pipe L-2 and is transferred to heat exchanger H-2.
The condensate produced in heat exchanger H-4 and separated in separator S-3 passes through pipe L-3, and the condensate produced in heat exchanger H-4 and separated in separator S-4 passes through pipe L-4. , the top outflow gas of the gas-liquid separator D-2 taken out from the pipe 23 is cooled by another cooling fluid in the heat exchanger H-16, a part of which is liquefied, and then flows out from the pipe 24. At the same time, these liquids or gases are supplied to predetermined positions of the medium pressure rectification column D-1 depending on their compositions. - The inside of the medium pressure rectification column D-1 is equipped with equipment for gas-liquid contact, such as trays or packing used in a rectification column during normal cryogenic separation. Further, at the top of this rectification column, there is a heat exchanger H-12 and a heat exchanger H-12 for cooling the top outflow gas of this column with ethylene liquid as a circulating refrigerant and condensing most of it to make it a reflux liquid of this column. A separator 5-12 is provided to separate this reflux liquid from uncondensed gas. In the medium-pressure rectification column [)-1, the condensate and gas supplied to this column undergo the action of the reflux liquid, which is returned to this rectification column via pipes L-12 and L-14, and the bottom of the column. A mixed liquid of hydrocarbons with an extremely low methane content is rectified by the heating action of the column bottom liquid by the heating heat exchanger H-15 and taken out from the tube L-17 at the bottom of the column, taken out from the tube L-15. The methane liquid is not condensed in the heat exchanger H-12, but is separated from the methane liquid in the separator 5-12 and taken out from the pipe 21 (separated into uncondensed gas containing hydrogen and methane. Pipe L) The methane liquid taken out from H-15 and the uncondensed gas taken out from pipe 21 are
The pipes L-13 are connected to the bottom of the low-temperature side fluid or gas-liquid contactor D-2 and the bottom of the medium-pressure rectification column D-1, respectively, when the high-pressure gas stream is cooled in each heat exchanger up to -5. The hydrocarbon mixture taken out through pipe 17 is used as a cold heat source to generate the reflux liquid necessary when it is separated into each component by the M distillation method, and is finally raised to a temperature close to room temperature. It is warmed and exits the cryogenic separation process as a product gas. Due to this rectification action, methane of 92 to 99.5 and ethylene of 0.0
Liquid methane containing 0.01 to 0.5 mol % and 0.5 to 8.0 mol % of hydrogen, and methane 0.001 to 0.5, ethylene 60 to 90, ethane 5 to 15, propylene 3 to 2
0, Propane 0.01~3゜0, C4 hydrocarbon 0.1~
A hydrocarbon mixture containing 3.0 and 0.01 mol % of hydrogen is obtained.

In the method of this invention, during the cooling process of the high-pressure gas flow, a temperature zone (hereinafter collectively referred to as the low temperature zone) other than the temperature zone (hereinafter simply referred to as the high temperature zone) in the heat exchanger H-1 of the example in FIG. ) is divided into at least two temperature zones, and the lower limit temperature of the lowest temperature zone (hereinafter simply referred to as the lower limit temperature of the low temperature zone) must be set to -120 to -160°C. Low temperature band lower limit temperature is -120 to -
The reason why it is set at 160°C is that in this type of method, where the usual purpose is to separate and obtain ethylene and hydrocarbons with a boiling point higher than ethylene, if this lower limit temperature is set to 1120°C or higher, it is necessary to If condensation and separation of ethylene is insufficient, resulting in loss of ethylene, and if this lower limit temperature is set at -160°C or lower, cooling to -160°C will remove ethylene and hydrocarbons with a boiling point higher than ethylene. This is because even though condensation separation from the high-pressure gas stream has been completed, the power required for further cooling of the high-pressure gas stream is increased more than necessary. The lower limit temperature of the low temperature zone is selected at a desired temperature within the above range depending on the pressure of the high pressure gas stream. However, even when using the method of the present invention, if it is desired to obtain methane and hydrogen separately, it is desirable that the high-pressure gas stream be cooled to below 1160°C. The reason why the low temperature zone is divided into at least two temperature zones is to reduce the reflux ratio in the medium pressure rectification column D-1. That is, in the example of FIG. 1, during the cooling process of the high-pressure gas stream, the high-pressure gas stream is generated in the heat exchanger H-4, where the temperature is closest to the lower limit temperature of the low-temperature zone, and the high-pressure gas stream is generated in the separator S-4. Since the condensate separated in the process has a very high methane content, this condensate is supplied to a position slightly below the top of the medium-pressure rectification column to meet the requirements of the medium-pressure rectification column. The reflux ratio is significantly reduced, and as a result, the amount of circulating refrigerant ethylene liquid required in heat exchanger H-12 for reflux liquid generation is significantly reduced, and ultimately the amount of circulating refrigerant ethylene liquid required in ethylene compressor C-3 is reduced. This is because the power required can be significantly reduced. On the other hand, if the low temperature zone is a single temperature zone and all of the condensate generated in this zone is supplied to the medium pressure rectification column as a single mixed liquid stream, the The methane content becomes smaller than that obtained in separator S-4 in Figure 1, and the optimum position for this single mixed liquid to be supplied to the medium pressure rectification column is the middle part of the medium pressure rectification column. Therefore, the necessary reflux ratio inevitably increases. For these reasons, the preferred number of low temperature zones is 3 or 4.
It is said that By increasing the number of temperature zones, the above advantages can be increased, but increasing the number of temperature zones to 5 or more in this part is more important than the increase in the number of heat exchangers and separators. The number of disadvantages increases as well. Similar advantages to those described above exist when separating the low-boiling components ethylene and ethane contained in a hydrocarbon liquid from which methane and hydrogen have been removed. That is, in the conventional method shown in FIGS. 2 and 3, the hydrocarbon mixture after separation of methane and hydrogen is obtained as a single stream from the bottom of the medium pressure rectification column D-1. The total content of ethylene and ethane in this single stream is lower than that obtained from the bottom of medium pressure rectification column D-1 of the process according to the invention (FIG. 1). The reason is that in the case of the method of the present invention, the total content of ethylene and ethane from the bottom of the gas-liquid contactor D-2 is lower than that obtained from the bottom of the medium pressure rectification column, Another hydrocarbon mixture stream rich in C3 and C4 hydrocarbons is generated.

Therefore, when ethylene and ethane are separated using another rectification column from the hydrocarbon mixture after methane and hydrogen have been separated, ethylene and ethane are separated for the same reason as above. The medium-pressure rectification column bottom liquid of the method of the present invention is supplied between the top and the middle part of this another rectification column, and the bottom liquid of the gas-liquid contactor is fed to the middle part of this another rectification column. and the bottom of the column, compared to the case where a single stream of the hydrocarbon mixture obtained in the conventional method shown in FIGS. 2 and 3 is fed to the middle section of this separate rectification column. Rectification by reflux ratio becomes possible. In the example shown in FIG. 1, the gas flowing from the top of the gas-liquid contactor is cooled by heat exchanger H-16, a portion of which is liquefied, and then supplied to the medium-pressure rectification column. Although the cooling of the gas flowing out from the top of the gas-liquid contactor by the heat exchanger H-16 is not an absolutely necessary step for the method of the present invention, it is a preferable method as a means for enhancing the effects of the present invention by a similar action to that described above. It is. In addition, in this case, the gas flowing from the top of the gas-liquid contactor D-2 is transferred to the heat exchanger H-16,
It is also a preferred method to cool the gas without being liquefied until the temperature at the outlet of the heat exchanger H-16 is approximately equal to the internal temperature at the supply position of the gas in the medium pressure rectification column.

If the lower limit temperature of the high-temperature zone in the cooling process of the high-pressure gas stream is lower than 150°C, the S degrees of ethylene and ethane in the condensate obtained in the high-temperature zone will increase rapidly and become substantially In order to obtain a bottom effluent that does not contain methane, the amount of evaporation of the condensate required in the gas-liquid contact device becomes excessive, and the amount of reflux liquid required for the medium pressure rectification column in the next step increases.
The purpose of the invention cannot be achieved. This lower limit temperature is 150
As the temperature rises above ℃, the amount of condensate produced rapidly decreases, and the amount of methane dissolved in this condensate also decreases rapidly, and the amount of methane that can be separated in the gas-liquid contact device also decreases. The significance of installing a liquid contact device is reduced.

Therefore, it is desirable that the upper limit of the lower limit temperature in this high temperature range is -30°C. In the example of FIG. 1, this high temperature zone during the cooling process of the high pressure gas stream is
However, by using two or more heat exchangers followed by separators, the high temperature zone can be divided into multiple small temperature zones. The effects of the present invention are further enhanced by dividing the condensate into zones and supplying the condensate obtained from the separator of each small temperature zone to different height positions of the gas-liquid contact device according to the respective compositions. . When the high-temperature zone of the high-pressure gas stream is divided into multiple small temperature zones in this way, the condensate obtained from the lowest temperature small zone is supplied to the top of the gas-liquid contact device, and the other In order to enhance the effects of the present invention, it is important that the condensate obtained from the low temperature zone is supplied between the top and the bottom of the gas-liquid contactor according to the respective compositions. In the usage of such a gas-liquid contact device, this gas-liquid contact device functions similarly to an N-reduction column, and it appears to be similar to the conventional method shown in Fig. 3 above. While the method shown in Figure 3 uses the bottom liquid of the low-pressure rectification column as a cold heat source to generate the reflux liquid necessary for the medium-pressure rectification column D-1, the method of this invention uses the bottom liquid of the low-pressure rectification column as a cold source. The major difference in liquid contact devices is that no such cold source is used at all.

In the example shown in FIG. 1, the products obtained from the top of the medium-pressure rectification column were methane liquid obtained from pipe L-15 and hydrogen gas containing methane obtained from pipe 21. In this case, it is also possible to select only the methane gas obtained from the pipe 21 containing hydrogen and a large amount of methane as the product obtained from the top of the medium pressure rectification column. In the case of this option, the amount of circulating refrigerant ethylene used for the production of reflux liquid in heat exchanger H-12 is reduced by the amount required to liquefy the methane obtained from pipe L-15; The power saving effect of this invention will be increased.

Furthermore, this option allows the methane-rich gas obtained from the pipe 21 to be adiabatically reduced in pressure while generating power in an expansion turbine (not shown), and this adiabatic reduction in pressure causes Since the temperature of this gas is reduced, the reduced temperature gas is used as a cold source in the desired heat exchanger, reducing the need for circulating refrigeration fluid and utilizing the power generated from the expansion turbine. Coupled with the power savings due to this, the power for compressing the circulating refrigerant is further reduced.

In the method of this invention, the methane content in the bottom effluent of the gas-liquid contactor and in the bottom effluent of the medium-pressure rectification column is both 0.5 mol% or less, and The methane content in the top product must be 90 mol% or more. The reason for this is that when the methane content m in the bottom effluent of both columns is 0.5 mol% or more, when ethylene and ethane are separated from both effluents by a separate rectification column as described above, If this occurs and the methane content in the overhead product of the medium-pressure rectification column is less than 90 mol%, the ethylene content in the effluent gas will increase to a non-negligible extent, and this ethylene This is because it will result in a loss.

In this invention method, a gas-liquid contact device and heating heat exchangers H-13 and H-15 at the bottom of the medium pressure rectification column are used.
Diversion of high-pressure gas flow at a desired temperature as heating fluid used for heating, diversion of fluid used as low-temperature side fluid in heat exchangers such as H-1 and H-2 and heated to desired temperature. , a divided stream or the entire amount of the outlet stream G-1 of the propylene compressor C-2, a divided stream or the entire amount of the outlet stream of the ethylene compressor 11C-3 at a desired temperature, and other fluids at a desired temperature alone or Can be used in combination. A preferred pressure of the high pressure gas flow in the method of this invention is 20 to 50 kO/ewrG. When the pressure of the high-pressure gas flow is lower than the above range, almost all of the ethylene is liquefied and separated from the high-pressure gas flow.
If a high-pressure gas stream needs to be cooled to a temperature below 1160°C and the pressure of the high-pressure gas stream is higher than the above range, the power required to compress this gas will be greater than necessary; It is also uneconomical. However, as mentioned above, when using the method of the present invention and also carrying out the fractional acquisition of methane and hydrogen, a pressure higher than the above range can be used. In the method of this invention, the preferred operating pressure of the gas-liquid contactor is 15 to 40 kg/cdG, and the preferred operating pressure of the medium-pressure rectification column is the operating pressure of the gas-liquid contactor, in order to fully exhibit the advantages described above. A pressure lower by 0.1 to 10 ka/cm can be mentioned. The above operating pressure range of the gas-liquid contact device and the medium-pressure rectification column covers the generation of reflux liquid at the top of the medium-pressure rectification column, the heating of the bottom of the gas-liquid contact device and the medium-pressure rectification column, and the pressure range of both columns. This pressure range is selected based on the necessity of further rectifying and separating the tower bottom effluent into each component.

In the method of this invention, when the circulating refrigerant propylene and the circulating refrigerant ethylene liquid are boiled and evaporated for cooling the fluid to be cooled, the pressure at the time of boiling and evaporation is gradually lowered. The power required for recompression can be saved. That is, before these circulating refrigerants are depressurized from one boiling pressure stage to the next, evaporated refrigerant gas and unevaporated refrigerant liquid are separated, and only the refrigerant liquid is depressurized to a lower pressure. If the evaporated refrigerant gas is not depressurized, but is heated through heat exchange with the fluid to be cooled and then recompressed, the compression ratio when recompressing the refrigerant gas can be reduced. This results in a significant reduction in the power required to recompress the refrigerant gas than if all the refrigerant liquid were evaporated to the lowest pressure required to cool the cooled fluid.

[Example] Hydrogen 16, methane 29, obtained by thermal decomposition of naphtha,
33 ethylene, 6 ethane, 12 propylene, 1 propane
7500 mol % of each of θC4 hydrocarbons and 3
Methane separation according to the method of the present invention was carried out using 011+j/hour as the raw material gas. The process used is the same as in Figure 1, with heat exchangers H-1, H-2, H-3 and H-
4, the cooling process of the high-pressure gas stream from room temperature to -133° C. was divided into the following four temperature zones.

Temperature band by H-1...+40℃~-39℃H-2
Temperature band by -39°C to -77°C Temperature band by H-3... -77°C to -100°C Temperature band by H-4... -100°C to -133°C The above raw material gas is Compressed to 33 ka/cdG by compression 1c-i, the condensate is cooled to 40°C in the condensate cooler W-1 and the condensate is separated, then dehydrated with a molecular sheep adsorbent and flows into the heat exchanger H-1. . A 35-plate plate column was used as the gas-liquid contactor, and a 65-plate plate column was used as the medium pressure rectification column. Further, appropriate amounts of hot water were used as the heat source for heating the bottom of the gas-liquid contactor, and appropriate amounts of the flow from the discharge side of the propylene compressor C-2 were used as the heat source for heating the bottom of the medium-pressure rectification column. Also separator S
The condensed liquid and uncondensed gas separated in -5 are transferred to heat exchangers H-1, H-2, H-3, H-
4 and H-5 to cool the high-pressure gas stream, and then taken out of the system as product gas, and the bottom liquid obtained from pipes L-13 and L-17. teeth,
It was rectified and separated into ethylene, propylene, propane, and C4 hydrocarbon components by a well-known method not shown in FIG.

The temperature of the high-pressure gas stream, the final temperature of the condensate from the high-pressure gas stream, and its composition when the apparatus shown in FIG. 1 was operated under the above conditions and a steady state was reached are shown in the table below.

Separation number S-I S-28-38-4 Companion of each separator fi -39-77-100-133 Amount of separated condensate 15°0◇ 600 200
350 to 9 mol/h Composition of condensate Mol% Hydrogen 0.5 0.6 0.7 1.
1 methane 14.0 30.0 51.5
89.7 Ethylene 43.2 53.5
42.7 8.8 Ethane 10.0 9,
0 4.5 0,4 Propylene 25,0
6.4 0.6 -Propane 0.8
0.2 - -C4 hydrocarbon 6.5 0
．． 3--Also, the operating conditions of the gas-liquid contact device and the medium pressure rectification column in steady state were as shown in the table below.

Gas-liquid contact medium pressure equipment 9/cdG 29.0 2 per column top
8. Composition of S tower overhead effluent mol% Hydrogen 1,6 2.8 Methane
49.0 97.2 ethylene
39.3 - Ethane
5.6- Propylene 4.1- Propane 0.1- 04 Hydrocarbon 0.3- Gas-liquid contact medium pressure device Composition of rectification column bottom liquid mol% Hydrogen -- Methane 0.01 0.01 Ethylene 44 ,79 80.49 Ethane 11.7 11.4 Propylene 33.5 7.5 Propane
1.0 0.2C4 hydrocarbon 9.
0 0,4 "Effects of the Invention" The first advantage of the present invention is that, for the reasons mentioned above, compared to the conventional method shown in FIG. This means that you can save 7%. Specifically, for example, in the case of an ethylene manufacturing facility with an annual output of 300,000 tons, the power required to compress the circulating refrigerants ethylene and propylene using the conventional method shown in Figure 2 is approximately 18,000 yen.
KWH/hour, whereas that of the method according to the present invention is about 17,000 KWH/hour. Further, this reduction in power requirements is accompanied by smaller compressors, smaller power generating equipment, smaller foundations for such equipment, etc., resulting in significant economic benefits. The second advantage is that compared to the conventional method explained in FIG.
The purpose is to improve the content to 5 to 99.95 mol%.

That is, in the method shown in FIG. 3, about 0.5% of the ethylene and ethane contained in the raw material gas is transferred to the medium pressure rectification column D-1 and/or the low pressure rectification column D-3. It was inevitable that the methane would be mixed into the methane obtained from the top of the tower, resulting in a loss, but in the method of this invention, this loss is reduced to approximately 0.
．． It can be reduced to 1%. At first glance, this reduction in loss does not seem to be a large gain, but in the production of ethylene, where large-scale equipment with an annual production capacity of 300,000 tons is the norm, this degree of yield improvement can provide a large economic gain. The third advantage of this invention is that when ethylene and ethane are separated by rectification from the hydrocarbon mixture from which methane has been removed by the method of this invention, the reflux ratio in the rectification column for this separation is The reason is that it can be reduced. The reason why this reduction in the reflux ratio is possible has already been explained, but this advantage reduces the amount of cold heat source required to generate the reflux liquid in the rectification column, the column diameter of the rectification column, and the heating of the bottom liquid. The amount of heat source required is reduced, and economic benefits are obtained.

Since the method of the present invention has the above-mentioned advantages, aliphatic saturated hydrocarbon compounds having 2 or more carbon atoms and mixtures thereof, such as ethane gas, liquefied petroleum gas, naphtha, gas oil, etc., can be thermally decomposed. This method is useful as a methane separation method when raw material gas containing various olefins, methane, etc. is produced, and ethylene, propylene, C4 olefins, etc. are produced from this raw material gas by cryogenic separation.

In addition, the conventional method shown in Fig. 1 is particularly advantageous in that the operating power of a large number of existing ethylene production facilities can be reduced through simple and small-scale addition of only the gas-liquid contactor and its ancillary equipment. Useful.

[Brief explanation of drawings]

Fig. 1 shows a process example of this invention method Fig. 2 shows a process example of a conventional method Fig. 3 shows a process example of another conventional method Symbol C-1... Raw material gas compressor C- 2...
・・・・・・Propylene compressor C-3・・・・・・
... Ethylene compressor W-1, W-2, W-3 ... Water cooler H-1 to H-16 ... Heat exchanger S-1 to $-12 ... Gas-liquid separator L-1~L-17...Liquid gas pipe R-1~R-
4...low temperature side fluid passage in heat exchanger

Claims (9)

[Claims] (1) A high-pressure gas stream containing various types of aliphatic medium-low hydrocarbons as main components is cooled to -120°C or below, and methane is separated from the condensate of the hydrocarbons obtained by the cooling. [1] In the cooling process of the high-pressure gas flow, from room temperature to -12
The temperature range from 0°C to -160°C is divided into at least three temperature zones, [2] A medium-pressure columnar gas-liquid contact device equipped with an indirect heater at the bottom, an indirect heater at the bottom, and an overhead outflow. A medium-pressure rectification column is used, which is equipped with a reflux liquid generating means in which the gas is condensed by heat exchange with liquid ethylene, and [3] the lower limit humidity of at least three temperature zones is -
The condensate obtained from the high temperature zone of 50°C or higher is supplied to the medium-pressure columnar gas-liquid contact device, and heated by the bottom indirect heater to separate the top effluent gas whose largest component is methane and methane. [4] The condensate obtained from each temperature zone other than the high temperature zone and the tower top effluent of the medium pressure gas-liquid contact device are are supplied to predetermined locations of the medium-pressure rectification column according to their respective compositions, and a rectification operation is carried out while the bottom of the medium-pressure rectification column is heated to produce methane containing 90 mol% or more of methane. product and a bottom effluent having a methane content of 0.5 mol % or less, [5] characterized in that at least a portion of the methane product is heated by heat exchange with the high-pressure gas stream. A method for separating methane from the high pressure gas stream. (2) The method according to claim 1, wherein the high temperature zone is a single temperature zone. (3) The high temperature zone is divided into two or more small temperature zones, and the condensate produced in the lowest temperature zone of these small temperature zones is the medium pressure columnar gas-liquid contactor. According to a patent claim, each condensate is supplied to the top of the column and generated in another small temperature zone is supplied to an intermediate point between the top and the bottom of the medium pressure columnar gas-liquid contacting device according to its respective composition. The method described in Scope 1. (4) Claims 1 and 2, in which the top effluent gas of the medium pressure columnar gas-liquid contact device is indirectly cooled and a part of it is liquefied before being supplied to the medium pressure rectification column. The method described in Section 2 or Section 3. (5) Claims 1, 2, 3, or 44, wherein the pressure within the medium-pressure columnar gas-liquid contact device is 15 to 40 kg/cm^2G at the upper part of the column. Method. (6) The pressure inside the medium-pressure rectification column is 0.1 to 10 k higher than the pressure at the top of the medium-pressure columnar gas-liquid contact device at the upper part of the column.
Claim 1, where the pressure is lower by g/cm^2;
The method according to item 2, 3, 4 or 5. (7) The methane product is gaseous and at least a portion thereof is subjected to heat exchange with the high pressure gas stream after the gaseous methane product is expanded while generating power. the method of. (8) The bottom effluent of the medium-pressure columnar gas-liquid contactor and the bottom effluent of the medium-pressure rectification column are transferred to another rectification column for separating ethylene and ethane from both liquids. At a separate predetermined location,
2. The method according to claim 1, wherein each liquid is supplied according to its composition. (9) The pressure of the high-pressure gas flow is 20 to 50 kg/cm^2
The method according to claim 1.