JPH0353289B2 - - Google Patents

Description

[Detailed description of the invention]

"Purpose of the Invention" [Industrial Application Field] The present invention is directed to cooling a high-pressure gas stream containing various kinds of medium- and low-class hydrocarbons including olefins as main components, and a mixture of the above-mentioned hydrocarbons obtained by this cooling. This invention relates to a method for cryogenic separation of methane from condensate. 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 two or more carbon atoms or mixtures thereof are thermally decomposed to produce, for example, ethylene,
For example, a raw material gas containing various kinds of medium and low hydrocarbons including olefins such as propylene and butylene as main components is produced, and desired components are separated from this raw material gas by a cryogenic separation method. This method is being used on a large scale to produce 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 described above by a cryogenic separation method, first, the raw material gas is
It is compressed to a pressure of 50 kg/cm ² G to form a high-pressure gas stream, and then this high-pressure gas stream is cooled to -120°C or below this temperature, and the hydrocarbons that are successively condensed during the cooling process are Generally used is a method in which each liquid mixture is separated from uncondensed gas and subjected to rectification depending on its composition. Among the general methods mentioned above, a method for cooling a high-pressure gas stream to obtain a hydrocarbon condensate is to use propylene or propane as a refrigerant,
A method of refrigeration in which this refrigerant is circulated through a repeated process consisting of compression → cooling (water cooling) liquefaction → depressurization → heat exchange evaporation → heat exchange heating → recompression, i.e., the refrigerant stream is used as raw material. A combination of both, a closed refrigeration cycle independent of the product gas stream, and a closed refrigeration cycle, also conducted with ethylene or ethane as the refrigerant, are most commonly used. 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 mentioned above consumes a lot of power and is a method that requires improvement.
The contents will be briefly explained below. FIG. 2 schematically illustrates the main steps of the most common method described above. Raw material gas is pipe 1
is sucked into compressor C-1, and after compression 20 to 50 kg/
It was discharged from tube 2 as a high-pressure gas stream of cm ² G, and was once indirectly cooled by water cooler W-1, and pre-treatment such as removal of moisture and other substances that would impede cryogenic separation was carried out. Afterwards (this pretreatment device is omitted in the drawing), it flows via pipe 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 low-temperature 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 Figure 2, heat exchanger H-1
4 low temperature side fluid passages R-1, R-2, R-
Although an example in which 3, R-4 passages are provided is described, the number of passages is not limited to four. In addition, other heat exchangers H- marked with the symbol H in FIG. 2 and FIG. 3 and FIG.
2, H-3, H-4, H-5, H-11, H-1
2, H-13, H-14, H-15, H-16, etc. 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. It is an exchanger. Heat exchanger H-1
In the closed refrigeration cycle, the high pressure gas stream is supplied to passage R-1, liquid propylene is supplied to passage R-1, and passages R-2, R-3, R-4.
In the lower temperature step, the gas is cooled by exchanging heat with each component in the separated raw material gas. 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 passed through the water cooler W.
-2, it is indirectly cooled and liquefied in pipe L-11.
It is produced by reducing the pressure of a part of the pressurized liquefied propylene taken out from the liquefied propylene. 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 closed 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 heat exchanger H-1 by the heat exchange described above, 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 condensed liquid is extracted from the pipe L-1, and after being depressurized, it is supplied to a predetermined position in the medium pressure rectification column D-1 depending on the composition of this liquid. On the other hand, uncondensed gas passes through pipe 5 to heat exchanger H
The high-pressure gas stream is introduced into the heat exchanger H-2 and is further cooled by exchanging heat with the gas stream at a lower temperature after being separated into each component, as in the heat exchanger H-1. -2, the high pressure gas flow is H
Since it is cooled to a lower temperature than in the case of H-1, circulating refrigerant ethylene is supplied to passage R-1 (this number is not attached in the figure) of H-2. 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. A portion of it was then depressurized. This circulating refrigerant ethylene is also used in heat exchangers H-2 and above, but all of the used circulating refrigerant ethylene is recycled to the suction port of 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, is supplied to a predetermined location in medium-pressure rectification column D-1 depending on the composition of the liquid. The uncondensed gas separated in the separator S-2 is taken out from the pipe 7 and introduced into the heat exchanger H-3,
It is further cooled in the same manner as above and passes through the pipe 8 to the separator S-3.
It is separated into condensed liquid and uncondensed 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 via pipe 9, where it is further cooled and -
The temperature reaches 120°C to -140°C and passes through pipe 10 to separator S-
4, the condensate produced during cooling is separated in separator S-4 and taken out from pipe L-4, and after pressure reduction is supplied to a predetermined location in the medium-pressure rectification column depending on the composition of this liquid. . The uncondensed gas in the separator S-4 is taken out from the pipe 11 and can be further cooled in the heat exchanger H-5 if necessary, but the temperature in the heat exchanger H-4 is about -120 to -140°C. The composition of the uncondensed gas removed from tube 11 in the cooled case usually consists of methane and the hydrogen entrained in the feed gas, and is substantially free of ethylene and hydrocarbons with a boiling point higher than ethylene. Therefore, if the purpose of cryogenic separation is to separate and obtain 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 not absolutely necessary. Each separator S-1, S-2,
Each condensate obtained in S-3 and S-4 is a mixture of methane, ethylene, ethane, propylene, propane, butanes, butenes, etc.
In addition, some hydrogen is dissolved, and a high-pressure gas flow -
These condensates produced during cooling to 120 to -140°C usually have a higher methane content as they are 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 transferred to heat exchanger H
-13, it is indirectly heated by another fluid.
As the heating fluid at this time, a part of the high-pressure gas flow 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, pipe 2 is connected from the top of the column.
The gas flowing out through the heat exchanger H-12 is cooled by a part of the circulating frozen ethylene, most of the methane in this gas is liquefied, and the methane liquid and uncondensed gas are separated in the separator S-12. After separation, the methane liquid is taken out from the pipe L-12, most of which is used as the reflux liquid of the medium pressure rectification column via the pipe L-14, and the remainder of the methane liquid is taken out as a product from the pipe L-14.
15 and used together with the uncondensed gas containing hydrogen and methane taken out from pipe 21, respectively, as a cooling cryogenic fluid in a desired heat exchanger. Due to the rectification 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. is collected at the top of the column. The methane liquid obtained from pipe L-15, the mixed gas of hydrogen and methane obtained from pipe 21, and the ethylene containing substantially no methane taken out via pipe L-13 at the bottom of the tower, and the boiling point higher than ethylene. It is separated into a mixture of various hydrocarbons. The carbonized mixture, which is obtained from the bottom of the medium-pressure rectification column and is substantially free of methane, is further separated into desired components by successive rectification methods, but this part is not shown in the figure. . In addition, the uncondensed gas and methane liquid obtained from the top of the medium pressure rectification column are transferred to a heat exchanger H.
-1, H-2, H-3, H-4, etc. The low temperature side fluid passages R-1, R-2, R-3, R-4, etc. are used appropriately to cool the high-pressure gas flow. Alternatively, it is used as a cold heat source 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 kg/cm ² G. The following is an outline of the most common conventional method, but this method has the drawback of high power consumption, as described below. 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 stream is transferred to heat exchangers H-1, H-2, H-3, H-4.
The hydrocarbon mixture that is cooled in the respective heat exchangers and condensed in the temperature range of each heat exchanger is separated from the uncondensed gas in the separators S-1, S-2, S-3, S-4, etc., respectively. The steps until the separated liquid is taken out through the pipes L-1, L-2, L-3, L-4, etc. are almost the same as in the case of FIG. has also been 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 D-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 bottom of the medium-pressure rectification column D-1 is heated by the heat exchanger H-13 as in the case of FIG. The method differs from the method shown in FIG. 2 in that the bottom liquid of the low-pressure rectification column is used as the cold heat source in heat exchanger H-12 for condensing into water. That is, the gas flowing out from the top of this medium pressure rectification column D-1 through the pipe 20, the heat exchanger H-
12, and most of this condensate is in the pipe L.
The latent heat of condensation released during condensation is used to heat the bottom liquid of the low-pressure rectification column. During this condensation, a part of the overhead gas is kept in gaseous form along with some methane in order to prevent the hydrogen dissolved in the condensate obtained in separators S-1 and S-2 from accumulating. The gas is taken out from the pipe 23 and fed to a low-pressure rectification column after the source pressure for the purpose of recovering a portion of the methane in this gas. Further, a part of the methane liquid condensed in the heat exchanger H-12 is taken out as a product from the pipe L-15. A gas consisting essentially of methane and hydrogen flows from the top of the low-pressure rectification column through pipe 22, and a liquid containing methane and other hydrocarbons flows from the bottom of the low-pressure rectification tower through pipe L-16. It is taken out after passing through.
This low-pressure rectification column bottom liquid is insufficiently separated from the residual methane, so it is pressurized by pump P-1 and further heated appropriately by heat exchanger H-14. -1 is supplied. 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 S- 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 methane liquid obtained from the top of the column via pipe L-15 and 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. The usage of the methane-removed hydrocarbon mixture taken out from pipe L-13 and the methane liquid taken out from pipe L-15 is the same as in the case of FIG. 2. The medium pressure rectification column according to this diagram 3 method is 27~
Under the pressure of 28Kg/cm ² G, the low pressure rectification column is 5 to 6
Each is operated under a pressure of Kg/cm ² G. The improved method shown in Figure 3 does not use the circulating refrigerant ethylene as the cold heat source for the 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, two drawbacks remain with the method of FIG. The first 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. The direct cause of
Since 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, the reflux ratios of both the low-pressure rectification column and the medium-pressure rectification column must be made sufficiently large. As a result, the pipe 22 at the top of the low pressure rectification column
In a gas consisting of methane and hydrogen obtained from
Alternatively, a non-negligible amount of ethylene will inevitably be mixed into any of the product methane liquid from the medium-pressure rectification column extracted from pipe L-15. The second drawback is that after methane and hydrogen have been removed,
Since ethylene and all of the hydrocarbons with higher boiling points than ethylene are obtained from L-13 as one liquid stream, a separate rectification column (not shown) is required to separate and obtain ethylene and ethane from this liquid stream. However, a large amount of heat source for heating the bottom liquid and a large amount of cold source for generating the reflux liquid in this separate rectification column are required. [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. Compared to the method shown in Figure 3, the yield of ethylene is improved and the size of the rectification column is reduced when separating ethylene and ethane from the hydrocarbon mixture after methane and hydrogen have been substantially removed. The purpose is to provide a means to do so. "Structure of the Invention" [Means for Solving the Problems] This inventive method consists of the following means as a gist. In other words, during the cooling process of the high-pressure gas flow, -120
℃ to -160℃ is divided into at least three temperature zones, a central column-shaped gas-liquid contact device equipped with an indirect heater at the bottom, an indirect heater at the bottom, and an overhead gas flowing into liquid ethylene. A medium-pressure rectification column is used, which is equipped with means for producing a reflux liquid that is condensed by heat exchange, and the condensate obtained from the high temperature zone whose lower limit temperature is -50°C or higher among at least three temperature zones is The gas is supplied to a medium-pressure columnar gas-liquid contact device, heated by a bottom indirect heater, and separated into a top effluent gas whose largest component is methane and a bottom effluent with a methane content of 0.5 mol% or less, Each condensate obtained from a temperature zone other than the high temperature zone and the gas effluent from the top of the medium-pressure gas-liquid contactor are supplied to a predetermined location in the medium-pressure rectification column according to their respective compositions. A rectification operation is performed while the bottom of the column is heated, and the methane product is separated into a methane product with a methane content of 90 mol% or more and a bottom effluent with a methane content of 0.5 mol% or less, and at least a part of the methane product is heated under high pressure. This method raises the temperature by exchanging heat with a 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. Not in. 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, substances that interfere with the cryogenic separation process, such as moisture contained in the high-pressure gas stream, are usually removed. In FIG. 1, the description of such a step of removing harmful substances is omitted.
From heat exchanger H-1 to pipes 4, 5, 6, 7, 8, 9,
10, 11, 12, 13, etc., each heat exchanger H-2, H-3, H
-4, H-5, etc. and each separator S-1, S-2, S-
3, S-4, S-5, etc., the configuration of the gas passages in each heat exchanger, and the configuration of the two closed refrigeration cycles that use propylene and ethylene as refrigerants, etc., are explained in Fig. 2. Since the smell is almost the same as the one described, the explanation will be omitted, but
In this example of FIG. 1, the temperature of the high pressure gas stream is -50°C at the outlet of heat exchanger H-1,
The heat transfer area of each heat exchanger is maintained so that the temperature at the outlet of -4 is -140°C. Heat exchanger H-
In No. 1, the high-pressure gas stream cooled to -50° C. by heat exchange with the low-temperature fluid contains some hydrocarbon condensate having a relatively high boiling point. 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 feed gas is naphtha pyrolysis gas, the composition of this condensate is 10-15 methane, 40-50 ethene, 5-15 ethane, 20-30 propylene, 0.2~1.5,
3 to 10 mol% of _C4 hydrocarbon and 0.1 to 1.0 mol% of hydrogen, but since the outlet temperature of heat exchanger H-1 -50°C is higher than the critical temperature of methane and hydrogen,
Methane and hydrogen in this condensate are mainly
It is thought that C ₂ , C ₃ and C ₄ hydrocarbon components are condensed and dissolved in a liquid, and most of it can be gasified by appropriately reducing pressure and heating. The gas-liquid contact device D-2 is a device that uses this development 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 tower flows down inside the tower, and the liquid that has reached the bottom of the tower passes through the heating heat exchanger H at the bottom of the tower.
-13, the hydrocarbon vapor is brought into countercurrent contact with the ascending flow of hydrocarbon vapor that is heated and boiled by another heating fluid, and the low boiling point components are preferentially evaporated, and the high boiling point components are preferentially condensed. This is realized as a result of material exchange such as
A bottom effluent consisting of C ₂ , C ₃ and C ₄ hydrocarbons is taken off through line L-13, and an overhead gas containing methane as the largest component is taken off through 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 described above, 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, only by rectification. In the method of the invention, methane can be removed by using a small amount of refluxing methane liquid in the methane separation process as a whole. According to the above principle, when pyrolysis gas of naphtha is used as raw material gas, methane 0.001 to 0.5, ethylene 35 to 55, ethane 5 to 15, propylene 25 to 45, propane 0.1~5.0,
_C4 hydrocarbon 2-15 and hydrogen 0-0.01 mol% liquid, from the top of the column, methane 25-60, ethylene 20-50,
Ethane 5-10, propylene 1-20, propane 0.1
~5.0, _C4 hydrocarbons 0.01~5.0 and hydrogen 0.5~3.0 mol% each can be obtained. The uncondensed gas in heat exchanger H-1 in the example in Figure 1 is further cooled to -140°C, and this period is divided into three temperature zones by heat exchangers H-2, H-3 and H-4. has been done. The condensate produced in heat exchanger H-2 and separated in separator S-2 passes through pipe L-2, and the condensate produced in heat exchanger H-3 and separated in separator S-3 passes through pipe L-2. L
-3, the condensate produced in the heat exchanger H-4 and separated in the separator S-4 passes through the pipe L-4 and is taken out from the top of the gas-liquid separator D-2 through the pipe L-4. The gas is cooled in the heat exchanger H-16 by another cooling fluid, a part of which is liquefied and flows out from the pipe 24, and depending on the composition of these liquids or gases, medium-pressure rectification is performed. Tower D
−1 is supplied to the predetermined position. Medium pressure rectification column D-1
The column is equipped with equipment for gas-liquid contact, such as trays or packing used in a rectification column for ordinary 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. Separator S- for separating this reflux liquid from uncondensed gas
12 are installed. In the medium-pressure rectification column D-1, the condensate and gas supplied to this column are subjected to the action of the reflux liquid, which is returned to this rectification column via pipes L-12 and L-14, and the heating of the bottom of the column. A mixed liquid of hydrocarbons with an extremely low methane content, which is rectified by the heating action of the column bottom liquid by heat exchanger H-15 and taken out from tube L-17 at the bottom of the column, tube L.
- Methane liquid taken out from 15 and heat exchanger H
-12, the methane liquid is not condensed and is separated into uncondensed gas containing hydrogen and methane, which are taken out from the pipe 21 separated from the methane liquid in the separator S-12. The methane liquid taken out from pipe L-15 and the uncondensed gas taken out from pipe 21 are used as the low temperature side fluid or gas when the high pressure gas stream is cooled in each heat exchanger from H-1 to H-5. Pipes L are connected to the bottom of the liquid contactor D-2 and the bottom of the medium pressure rectification column D-10, respectively.
-13 and pipe L-17, the hydrocarbon mixture is used as a cold source for generating the reflux liquid necessary when it is separated into each component by rectification, and is finally brought to room temperature. The temperature is raised to a temperature close to that of
It is discharged from the cryogenic separation process as a product gas. Due to this rectification action, methane 92~99.5, ethylene
Liquid methane with 0.001-0.5 and 0.5-8.0 mol% of hydrogen is also extracted from the bottom of the column, methane 0.001-0.5, ethylene 60-90, ethane 5-15, propylene 30-20, propane 0.01-3.0, _C4 hydrocarbons A hydrocarbon mixture containing 0.1 to 3.0 and 0 to 0.01 mole % 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. The reason why the lower limit temperature of the low temperature zone is -120 to -160℃ is that in this type of method, which normally aims to obtain fractionated ethylene and hydrocarbons with a higher boiling point than ethylene, this lower limit temperature is -120℃ or higher. In the case where ethylene is condensed and separated from the high-pressure gas stream insufficiently, ethylene loss occurs, and in the case where this lower limit temperature is set to be below -160°C, cooling to -160°C may cause ethylene loss. This is because even though the condensation separation of ethylene and hydrocarbons with higher boiling points than ethylene 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 this invention, it is desirable that the high pressure gas stream be cooled to below -160 DEG C. if it is desired to obtain fractionated methane and hydrogen. The reason why the low temperature zone is divided into at least two temperature zones is that the medium pressure rectification column D-1
This is to keep the reflux ratio small. That is, the first
In the example shown in the figure, during the cooling process of the high-pressure gas stream, condensation is generated in the heat exchanger H-4, where the temperature of the high-pressure gas stream is closest to the lower limit temperature of the low-temperature zone, and is separated in the separator S-4. Since the liquid has a very high methane content, by feeding this condensate to a position slightly below the top of the medium pressure rectification column, the reflux ratio required for the medium pressure rectification column is greatly increased. heat exchanger H-1 for the production of reflux liquid.
The amount of circulating refrigerant ethylene liquid required in 2 is significantly reduced, and finally the ethylene compressor C-
This is because the power required for 3 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 Separator S- with methane content in Figure 1
The optimum position for supplying this single liquid mixture to the medium pressure rectification column is the middle part of the medium pressure rectification column, which inevitably increases the necessary reflux ratio. I will do it. For these reasons, the preferred number of low temperature zones is three or four. 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
This increased advantage is outweighed by the increased number of heat exchangers and separators required. 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 separating 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 therein is lower than that obtained from the bottom of the 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 C ₃ and C ₄ hydrocarbons is produced. 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 fed between the top and the middle section of this another rectification column, and the bottom liquid of the gas-liquid contactor is fed into this another rectification column. compared to the case where a single stream of the hydrocarbon mixture obtained in the conventional method of FIGS. 2 and 3 is fed to the middle section of this separate rectification column. Rectification using a small reflux ratio becomes possible. In the example of FIG. 1, the gas flowing from the top of the gas-liquid contact device is
After being cooled by heat exchanger H-16 and partially liquefied, it is 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 can be used as a means to enhance the effects of the present invention by a similar action to the above. This is the preferred method. In addition, in this case, the gas flowing from the top of the gas-liquid contactor D-2 is
In the heat exchanger H-16, the gas is cooled 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. is also a preferred method. If the lower limit temperature of the high-temperature zone in the cooling process of the high-pressure gas stream is lower than -50°C, the concentration of ethylene and ethane in the condensate obtained in the high-temperature zone increases rapidly, resulting in a substantial increase in methane concentration. The amount of evaporation of the condensate required in the gas-liquid contact device to obtain a column bottom effluent that does not contain chlorine becomes excessive, and the amount of reflux liquid required for the next step, the medium-pressure rectification column, increases, making it impossible to achieve the purpose of the invention. I won't be able to do it. As the lower limit temperature rises above -50°C, the amount of condensate produced decreases rapidly.
Furthermore, the amount of methane dissolved in the condensate decreases rapidly, and the amount of methane that can be separated in the gas-liquid contact device also decreases, so the significance of installing the gas-liquid contact device decreases. Therefore, it is desirable that the upper limit of the lower limit temperature in this high temperature range is -30°C. 1st
In the example shown in the figure, this high temperature zone of the high pressure gas flow and the cooling process is controlled by heat exchanger H-1.
However, by using two or more heat exchangers followed by a separator, the high temperature zone is divided into multiple small temperature zones. The effects of the present invention are further enhanced by supplying the condensate obtained from the separators of each small temperature zone to positions at different heights of the gas-liquid contact device depending on 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 this method of using a gas-liquid contact device, the gas-liquid contact device functions similarly to a rectification 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 the liquid contact device is that such a cold source is not 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 selection, the amount of circulating refrigerant ethylene used for producing reflux liquid in heat exchanger H-12 is
The amount of methane obtained from No. 5 is reduced by the amount necessary to liquefy it, and the power saving effect of the present invention increases. Furthermore, this option allows the methane-rich gas obtained from the pipe 21 to be depressurized adiabatically while generating power in an expansion turbine (not shown);
This adiabatic depressurization lowers the temperature of this gas, thereby reducing the need for circulating refrigerant liquid by using this lower temperature gas as a source of cold in the desired heat exchanger, which is generated from the expansion turbine. Coupled with the reduction in power by utilizing the power thus obtained, the power required 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 of the medium-pressure rectification column is The methane content must be 90 mol% or more. The reason for this is that if the methane content in the bottom effluent of both columns is 0.5 mol% or more, it will be difficult to separate ethylene and ethane from both effluents using separate rectification columns as described above. If the methane content in the overhead product of the medium-pressure rectification column is less than 90 mol%, the ethylene content in this effluent gas will increase to a non-negligible extent, and this ethylene will be lost. It is for the sake of becoming. In this invention method, a gas-liquid contact device and a heating heat exchanger H-13 at the bottom of the medium pressure rectification column are used.
And as a heating fluid used in H-15, high pressure at a desired temperature is a branch of the gas stream, H-1, H-
A divided flow of the fluid that is used as a low-temperature side fluid in a heat exchanger such as No. 2 and heated to a desired temperature, a divided flow or the entire amount of the outlet flow G-1 of the propylene compressor C-2, and a divided flow of the fluid used as the low-temperature side fluid in the second class heat exchanger, and a divided flow of the outlet flow G-1 of the propylene compressor C-2. A divided flow or the entire amount of the outlet stream No. 3 at the desired temperature, and other fluids at the desired temperature can be used alone or in combination. A preferred pressure of the high pressure gas flow in the method of this invention is 20 to 50 kg/cm ² G. When the pressure of the high-pressure gas flow is lower than the above range, almost the entire amount of ethylene is liquefied and separated from the high-pressure gas flow, so the high-pressure gas flow reaches -160℃.
If the pressure of the high-pressure gas stream is higher than the above range, the power required to compress this gas will be unnecessarily large, both of which are uneconomical. However, as mentioned above, when using the method of the present invention and also carrying out the fractional acquisition of methane and hydrogen, pressures higher than the above range can be used. In the method of this invention, the preferred operating pressure of the gas-liquid contact device is 15 to
40 Kg/cm ² G, and a preferred operating pressure of the medium pressure rectification column is a pressure that is 0.1 to 10 Kg/cm ² lower than the operating pressure of the gas-liquid contact device. 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 partially evaporated 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. is smaller than if all the refrigerant liquid were evaporated to the lowest pressure required to cool the cooled fluid.
The power required to recompress the refrigerant gas can be significantly reduced. [Example] Hydrogen 16 and methane obtained by thermal decomposition of naphtha
Meta-separation was carried out by the method of the present invention using 75,000 Nm ³ /hour of a gas consisting of 29 mol %, 33 ethylene, 6 ethane, 12 mol % propylene, 1 propane, and 3 mol % of C ₄ hydrocarbon as a raw material gas. The process used is the same as that shown in Figure 1, with heat exchangers H-1 and H-
The cooling process of the high-pressure gas stream from room temperature to -133°C was divided into the following four temperature zones by H-2, H-3 and H-4. Temperature band due to H-1...+40℃~-39℃ Temperature band due to H-2...-39℃~-77℃ Temperature band due to H-3...-77℃~-100℃ Temperature band due to H-4 ...-100℃～-133℃ The above raw material gas is compressed by compressor C-1 at 33Kg/
cm ² G after being compressed to 40°C in water cooler W-1.
After cooling and separating the condensate, it is dehydrated by a molecular sieve adsorbent and flows into heat exchanger H-1. In addition, 35 plate plates were used for the gas-liquid contact device, and a 65 plate plate column was used for the medium pressure rectification column. Also, the heat source for heating the bottom of the gas-liquid contactor is hot water, and the heat source for heating the bottom of the medium-pressure rectification column is the propylene compressor C-
Appropriate amounts of the two discharge side streams were used. In addition, the condensed liquid and uncondensed gas separated in the separator S-5 are both depressurized and transferred to the heat exchangers H-1,
The low-temperature side passages of H-2, H-3, H-4, and H-5 are used to cool the high-pressure gas flow, and then taken out of the system as a product gas.
The bottom liquids obtained from No. 3 and L-17 were rectified into ethylene, propylene, propane, and _C4 hydrocarbon components by well-known methods not shown in FIG. The temperature of the high-pressure gas flow, the amount of condensate produced from the high-pressure gas flow, and its composition are shown in the table below when the apparatus shown in FIG. 1 was operated under the above conditions and a steady state was reached.

【table】

[Table] The operating conditions of the gas-liquid contactor and medium pressure rectification column in steady state were as shown in the table below.

[Table] "Effects of the Invention" The first advantage of this invention is that for the reasons mentioned above,
Compared to the conventional method shown in Figure 2, the power required to compress the circulating refrigerant ethylene and circulating refrigerant propylene is approximately 4~
This means that you can save 7%. Specifically, for example, in the case of an ethylene production 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
It is 18,000KWH/hour, while that of the method of this invention is about 17,000KWH/hour. Further, this reduction in power requirements is accompanied by a reduction in the size of both compressors, a reduction in the size of the power generation equipment, and a reduction in the size of the foundations for such equipment, resulting in significant economic benefits. The second advantage is that the yields of ethylene and ethane are lower than the conventional method explained in Figure 3.
The aim is to improve the content from 99.0 to 99.7 mol% in the conventional method to 99.75 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
Otherwise, it was inevitable that it would be mixed into the methane obtained from the top of the low-pressure rectification column D-3, resulting in a loss, but in the method of the present invention, this loss can be reduced to about 0.1%. At first glance, this reduction in loss does not seem to be a huge benefit, but in the production of ethylene, where large-scale equipment with an annual production capacity of 300,000 tons is the norm, this level of yield improvement can yield significant economic benefits. 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 in the rectification column for this separation is The reason is that the ratio 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,
Economic benefits can be obtained. Since the method of this invention has the above-mentioned advantages,
Aliphatic saturated hydrocarbon compounds having two or more carbon atoms and mixtures thereof, such as ethane gas, liquefied petroleum gas, naphtha, gas oil, etc., are thermally decomposed to produce raw material gases containing various olefins, methane, etc. This method is useful as a methane separation method when ethylene, propylene, C ₄ , olefin, etc. are produced from this raw material gas by cryogenic separation. In addition, it is particularly advantageous in that the operating power of a large number of existing ethylene production facilities constructed by the conventional method shown in Fig. 1 can be reduced by simply and small-scale addition of only the gas-liquid contact device and its ancillary equipment. Useful.

[Brief explanation of drawings]

Fig. 1 is an example of the process of this invention method, Fig. 2 is an example of the process of the conventional method, and Fig. 3 is an example of the process 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~
H-16... Heat exchanger, S-1 to S-12... Gas-liquid separator, L-1 to L-17... Liquefied gas pipe,
R-1 to R-4... Low-temperature side fluid passage in the heat exchanger, D-1... Medium pressure rectification column, D-2... Gas-liquid contact device, D-3... Low pressure rectification column, 1 ~24...Other tubes.

Claims (1)

[Scope of 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 from the condensate of the hydrocarbons obtained by the cooling. In the method in which methane is separated, during the cooling process of the high pressure gas stream from room temperature to -
A medium-pressure columnar gas-liquid contact device that is divided into at least three temperature zones from 120°C to -160°C and equipped with an indirect heater at the bottom, an indirect heater at the bottom, and a liquid effluent gas at the top of the tower. A medium-pressure rectification column is used which is equipped with means for producing a reflux liquid that is condensed by heat exchange with ethylene, and the reflux liquid obtained from a high temperature zone having a lower limit temperature of -50°C or higher among at least three temperature zones is used. The condensate is fed to the medium pressure columnar gas-liquid contactor and heated by the bottom indirect heater to separate the methane content from the top effluent gas whose maximum content is methane.
The condensate obtained from each temperature zone other than the high temperature zone and the tower top effluent gas of the medium pressure gas-liquid contactor are separated into 0.5 mol % or less of the tower bottom effluent, and the tower top effluent gas of the medium pressure gas-liquid contactor is divided into It is supplied to a predetermined part of the medium pressure rectification column, and the bottom of the medium pressure bottom rectification column is heated while the rectification operation is carried out to produce methane products with a methane content of 90 mol% or more and methane content.
A method for separating methane from a high-pressure gas stream, characterized in that at least a portion of the methane product is heated by heat exchange with the high-pressure gas stream. . 2. The method of 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 delivered to the top of the medium pressure columnar gas-liquid contact device. and each of the condensates produced in other small temperature zones is supplied to an intermediate point between the top and the bottom of the medium-pressure columnar gas-liquid contacting device according to their respective compositions. The method described in Section 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. Or the method described in Section 3. 5. The method according to claim 1, 2, 3, or 4, wherein the pressure in the medium pressure columnar gas-liquid contact device is 15 to 40 kg/cm ² G at the upper part of the column. 6 The pressure inside the medium-pressure rectification column is 0.1 below the pressure at the top of the medium-pressure columnar gas-liquid contact device at the top of the column.
~10Kg/cm ² Claim 1 which is defined as a low pressure
2. The method according to item 2, item 3, item 4, or item 5. 7. The method of claim 1, wherein the methane product is gaseous and at least a portion of it is subjected to heat exchange with the high pressure gas stream after the gaseous methane product is expanded while generating power. . 8. The bottom effluent of the intermediate pressure columnar gas-liquid contactor and the bottom effluent of the medium pressure rectification column are separated from each other in another rectification column for separating ethylene and ethane from both liquids. 2. The method according to claim 1, wherein the liquid is supplied to predetermined locations depending on the composition of each liquid. 9. The method according to claim 1, wherein the pressure of the high-pressure gas stream is between 20 and 50 kg/cm ² G.