JPH0780794B2 - Separation and purification method of hydrocarbon pyrolysis gas - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to a method for separating and refining hydrocarbon pyrolysis gas obtained from an ethylene production apparatus for pyrolyzing naphtha, natural gas and the like, and particularly to hydrocarbon pyrolysis. The present invention relates to a gas cooling method.

[Background of the Invention]

Hydrocarbon pyrolysis gas obtained from ethylene production equipment by pyrolysis contains hydrogen, methane (C ₁ ), ethylene, ethane (C ₂ ), propylene, propane, butylene, butadiene (C ₃ +), etc., and ethylene, propylene Etc. are raw materials for petrochemicals, and particularly for polymerization raw materials, they do not adversely affect the catalyst and must be purified to a very high degree of purity.

FIG. 3 shows a flow sheet of a typical example of a conventional separation and purification method for hydrocarbon pyrolysis gas. A hydrocarbon pyrolysis gas F composed of hydrogen, C ₁ , C ₂ and C ₃ + hydrocarbon is introduced from line 1 into the cooling process. In the cooling process, the pyrolysis gas is the first cooler.
It is cooled by the propylene refrigerating refrigerant in E1 and the second cooler E2, and then introduced into the first-stage gas-liquid separation tank D1 via the line 2. C ₂ condensed in the first-stage gas-liquid separation tank D1
Most of the hydrocarbons and C ₃ or more hydrocarbons are returned to the previous step via line 4, and the rest are introduced into demethanizer T1 via line 5. The uncondensed gas composed of hydrogen, methane, ethylene, and ethane cooled in the first-stage gas-liquid separation tank D1 is introduced into the heat exchanger E3 via the line 3 and cooled by the ethylene refrigerating refrigerant, and then via the line 6 to the first It is introduced into the two-stage gas-liquid separation tank D2. Line 8 is a condensate composed of methane, ethylene, ethane, and a small amount of hydrogen condensed in the second-stage gas-liquid separation tank D2.
Through the first stage gas-liquid separation tank D1 and the condensate in line 5 to be introduced into the demethanizer T1. The uncondensed gas is sent to the ethylene recovery system via line 7.

Next, in the demethanization step, the condensate from the lines 8 and 5 is supplied to the demethanizer T1 at an operating pressure of about 30.
It is distilled at kg / cm ² · G, the methane fraction is withdrawn from the top of the column, and the ethylene / ethane fraction is withdrawn from the bottom of the column. The methane fraction is sent to the condenser E4 from the line 9 and cooled by the ethylene refrigerating refrigerant, separated into uncondensed gas and condensed liquid in the third gas-liquid separation tank D3, and the uncondensed gas is sent to the ethylene recovery system via line 11. The condensate is returned to the demethanizer T1 via the line 10. In the demethanizer T1, a part of the bottom liquid is reboiled by the reboiler E5.

Next, in the ethylene separation step, the bottom liquid from the demethanizer T1 is decompressed and expanded by the expander V1 via the line 12 to an operating pressure of about 7 kg / cm ² · G and a temperature of about -56 ° C. ,
It is fed to the ethylene column T2 via the line 13 in a state where the ratio of the substrate is about 44 mol%. The ethylene fraction of the uncondensed gas distilled in the ethylene tower T2 is condensed in the condenser E6 via the line 14, and part of it is refluxed as a reflux liquid in the ethylene tower.
It is returned to T2 and the rest is taken off as product ethylene P via line 16. Further, the ethane fraction separated from the bottom of the ethylene column T2 is recycled to the pyrolysis raw material after collecting cold heat through the line 17. A part of the bottom liquid is reboiled by the reboiler E7.

Hydrogen contained in the hydrocarbon pyrolysis gas as described above,
Hydrocarbons above C ₁ , C ₂ , C ₃ have extremely low boiling points and must be cooled to a low temperature for separation. The cooling step is the step in which the amount of frozen refrigerant used is the largest in the separation and purification apparatus, and many processes have been implemented to date in order to reduce the amount of frozen refrigerant used in this step. Refrigerant such as ethylene or propylene having various temperature levels is used as the refrigerating refrigerant, but a large amount of refrigerating refrigerant must be used in order to condense the ethylene / ethane fraction. Is an expensive refrigerant that requires a large amount of energy, and there has been a demand for a method of reducing the amount used.

[Object of the Invention]

The object of the present invention is to solve the above-mentioned problems of the prior art,
Provided is a method for separating and refining hydrocarbon cracked gas, which significantly reduces the power required for a refrigeration compressor related to an expensive refrigerating refrigerant used in a low temperature cracking and refining process of hydrocarbon pyrolysis gas and production of a refrigerating refrigerant. .

[Outline of Invention]

The method for separating and purifying hydrocarbon pyrolysis gas of the present invention is
Hydrocarbon pyrolysis gas consisting of C ₁ , C ₂ , C ₃ or more hydrocarbons is cooled in the cooling process, then separated into hydrogen / methane fraction and ethylene / ethane fraction in the demethanization process, and then ethylene separation In the purification method of separating ethylene fraction and ethane fraction in the step, the high-pressure ethylene-ethane fraction separated in the demethanization step is decompressed to a low temperature, and the low-temperature ethylene-ethane fraction is used as a refrigerant. It is a method of introducing it into the ethylene separation step after it has been used as.

Further, the method of the present invention will be described in detail. In the demethanization process, the operating pressure is 25 to 35 kg for the fraction mainly consisting of ethylene and ethane extracted from the demethanizer as bottom liquid.
/ cm ² · G, usually 30kg / cm ² · G before and after temperature -15～10 ° C., is a state of before and after the normal -5 ° C., operated by being decompressed and expanded pressure 5~20kg / cm ² · G, Usually around 7kg / cm ² · G,
Temperature can be drastically lowered to -70 to -30 ℃, usually -50 ℃ or less. This extremely low temperature ethylene-ethane fraction is used as a refrigerant for a heat exchanger in the cooling step or the like, and the amount of frozen refrigerant used is greatly reduced. When ethylene / ethane fraction is used as a refrigerant, enthalpy increases,
When introduced into the ethylene column of the final ethylene separation step, the reflux liquid must be increased. Therefore, it has never been considered to use a low-temperature ethylene-ethane fraction as a refrigerant. However, the present inventor has analyzed the operating conditions of the ethylene column, examined the increasing tendency of the reflux amount due to the introduction conditions of this distillate, and the energy balance of the entire pyrolysis gas separation and purification device.・
It has been clarified that the ethane fraction does not increase significantly with respect to the increase in the enthalpy of the ethylene ethane fraction as long as the ethane fraction does not become excessively superheated steam.

Fig. 2 shows the relationship between the increase in the enthalpy of the ethylene ethane fraction introduced into the ethylene column and the increase in the ethylene column reflux rate. Point A is the case where the ethylene / ethane fraction is not used as a refrigerant in the cooling step, points B and C are used as the refrigerant, point B is the case of the examples described later, point C is the total amount of the fraction. This is the case when saturated steam is reached. The paper proportions at points A, B and C are 44, 89 and 100 mol%, respectively. If the cold heat of the ethylene / ethane fraction is used as the refrigerant in the cooling step and the increase in the enthalpy of the fraction is C point or less, preferably B point or less, where the entire amount becomes saturated vapor, the ethylene column recirculation amount will be greatly increased. Instead, energy saving is achieved as a whole of the apparatus for separating and purifying hydrocarbon pyrolysis gas.

The effective use of low-temperature cold heat obtained by decompressing and expanding the high-pressure ethylene / ethane fraction separated in the demethanization step is, in addition to the cooling step, other low-temperature separation / purification step or frozen refrigerant production. Although it can be used in a process or the like, it is preferably used as a refrigerant in the cooling process.

Example of Invention

A flow sheet of the separation and purification apparatus of the present invention is shown in FIG.
The method will be described. Compared with FIG. 3 of the conventional device, this device is provided between the second cooler E2 and the first-stage gas-liquid separation tank D1 in the cooling process, and between the first-stage gas-liquid separation tank D1 and the third cooler E3. A column bottom liquid first heat exchanger E8 and a column bottom liquid second heat exchanger E9 are provided between them, respectively, and the column bottom liquid ethylene / ethane fraction withdrawn from the demethanizer T1 is decompressed by the expander V1. A device used as a refrigerant after being expanded.

Hydrogen and a hydrocarbon pyrolysis gas F composed of C ₁ , C ₂ , C ₃ or higher hydrocarbons is introduced from line 1 and cooled with propylene refrigeration refrigerant in the first cooler E1 and the second cooler E2, It is introduced into the column bottom liquid first heat exchanger E8, cooled by the ethylene-ethane distillate refrigerant, and introduced into the first stage gas-liquid separation tank D1. Next, the uncondensed gas from the first-stage gas-liquid separation tank D1
It is introduced into the second bottom heat exchanger E9 via 20 and cooled by the ethylene-ethane fraction refrigerant. The ethylene-ethane fraction as a refrigerant is withdrawn from the demethanizer T1 as a bottom liquid, expanded under reduced pressure by an expander V1 via a line 12 to a low temperature, and passed through a line 18 to generate a second heat of the bottom liquid. After passing through the exchanger E9, it proceeds to the bottom liquid first heat exchanger E8 where the steam ratio is about 90.
It becomes a mol% state and is introduced into the ethylene tower T2 through lines 19 and 13. On the other hand, the uncondensed gas of pyrolysis gas is
Via the column bottom liquid second heat exchanger E9, and further cooled with ethylene refrigerating refrigerant in the third cooler E3 via line 21;
It is introduced into the second stage gas-liquid separation tank D2. Second stage gas-liquid separation tank D2
Of the condensate passes through the line 8 and the first-stage gas-liquid separation tank D1
Part of the condensate from the is combined via line 5 and introduced into demethanizer T1. The heat exchange capacity of the third cooler E3 is reduced because the column bottom liquid first heat exchanger E8 and the column bottom liquid second heat exchanger E9 are provided, and the amount of ethylene refrigeration refrigerant used can be reduced. The ethylene / ethane fraction from the demethanizer T1 passes through the line 12 and is operated by the expander V1 at an operating pressure of 31 kg / cm ² · G.
Decompressed and expanded to 7.5kg / cm ² · G, and temperature from −5.5 ℃ to −
After being greatly reduced to 53 ° C., introduced into the cooling step via line 18 as described above, and used as a refrigerant, line 1
It is introduced into the ethylene tower T2 via 9,13. In the ethylene tower T2, it is separated into the ethylene fraction P of the product and the ethane fraction 17 recycled as the raw material.

Table 1 shows a comparison between the case of the present invention and the case of the conventional example, in which the same hydrocarbon pyrolysis gas was supplied to the facility for ethylene production of 450,000 tons / year. From Table 1, the method of the present invention shows that the amount of ethylene and propylene finally used as the refrigerating refrigerant is reduced and the required power of the refrigerating compressor is reduced, that is, ethylene refrigerating compressor and propylene refrigerating compressor. Regarding the total required power of the machine, it was clear that the present invention reduced the power by 1600 KW as compared with the conventional example.

The above method is one embodiment of the present invention, and other than this method, the following method can be performed. Although the tower bottom liquid heat exchanger E8 is provided between the second cooler E2 and the first-stage gas-liquid separation tank D1 in the cooling step in the embodiment shown in FIG. 1, this is omitted and the tower bottom liquid heat exchanger is omitted. Only E9 is enough. Further, in FIG. 1, the ethylene-ethane fraction refrigerant is passed through the series in each of the bottom liquid heat exchangers E8 and E9, but the refrigerant may be divided into two and passed in parallel.

Further, in addition to using the ethylene ethane fraction that has been expanded under reduced pressure to a low temperature in the above-mentioned refrigerant step, it is introduced into the refrigerating refrigerant manufacturing step (not shown) from the line 18 and used as a refrigerant for refrigerating refrigerant manufacturing. After that, it can also be introduced into the ethylene tower T2. At this time, the power required for the refrigerating refrigerant manufacturing process can be greatly reduced.

The essential point of the present invention is to effectively utilize the ethylene-refrigerant refrigerant and the ethylene-ethane fraction refrigerant which has been decompressed and expanded in place of the propylene refrigerant refrigerant in the cooling step and other general cooling steps within a range that does not result in an excessively superheated state. Is the way.

〔The invention's effect〕

The method for separating and purifying a hydrocarbon pyrolysis gas according to the present invention comprises expanding the ethylene / ethane fraction withdrawn from the demethanizer under reduced pressure to an extremely low temperature, and cooling the low temperature ethylene / ethane fraction with a pyrolysis gas. By using it as a refrigerant for a heat exchanger in a refrigerating refrigerant manufacturing process, etc., the amount of expensive refrigerating refrigerant used can be drastically reduced, and the power required for manufacturing the refrigerating refrigerant can be reduced. The effect is great, and the economic effect is also great.

[Brief description of drawings]

FIG. 1 shows a flow sheet of a separation and purification apparatus for hydrocarbon pyrolysis gas according to the present invention, FIG. 2 shows a relationship between an increase in enthalpy of ethylene / ethane fraction and an increase in ethylene column reflux amount, and FIG. 3 shows a flow sheet of the apparatus of FIG. F ... Hydrocarbon pyrolysis gas P ... Ethylene product E1 ... First cooler E2 ... Second cooler E3 ... Third cooler E8 ... Bottom liquid first heat exchanger E9 ... Bottom Liquid second heat exchanger D1 …… First-stage gas-liquid separation tank D2 …… Second-stage gas-liquid separation tank T1 …… Demethanizer T2 …… Ethylene tower V1 …… Expander

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shinichiro Kodai 3-2 Ariakidaihito Ichihara, Chiba Prefecture (72) Inventor Hiroshi Takagi 3-2 Ariakidaihito Ichihara, Chiba Prefecture (56) References Japanese Patent Publication Sho 59 -35895 (JP, B2)

Claims (2)

[Claims] 1. A hydrocarbon pyrolysis gas comprising hydrogen and C ₁ , C ₂ , C ₃ or higher hydrocarbons is cooled in a cooling step, and then in a demethanization step, hydrogen / methane fractions and ethylene / ethane fractions are obtained. In the purification method of separating and then separating into ethylene fraction and ethane fraction in the ethylene separation step, the ethylene-ethane fraction separated in the demethanization step is decompressed to a low temperature, and the ethylene A method for separating and purifying a hydrocarbon pyrolysis gas, which comprises using an ethane fraction as a refrigerant and then introducing it into the ethylene separation step. 2. A patent characterized in that the ethylene / ethane fraction separated in the demethanization step is expanded under reduced pressure to a low temperature and the low temperature ethylene / ethane fraction is used as a refrigerant in the cooling step. The method for separating and purifying a hydrocarbon pyrolysis gas according to claim 1.