JPH0420411B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention produces cyclopentadiene (hereinafter referred to as CPD) from the cracked gasoline fraction, which is produced as a by-product during thermal or steam cracking of petroleum fractions such as naphtha, kerosene, and gas oil, for the purpose of producing ethylene. This relates to a method of separation and recovery. The advantage of the method of the present invention is that high-purity CPD can be efficiently separated by simply adding simple equipment without changing the conventional cracked gasoline treatment process and operating conditions; By suppressing dimerization as much as possible during distillation and distilling CPD in the form of monomers, the heat cost for separation and recovery can be reduced. CPD is a useful substance that is used, for example, as a curing agent for epoxy resins, as a raw material for CPD resins, and as a raw material for Diels-Alder reactions. (Prior Art) CPD is contained in about 3 to 7% of cracked gasoline, which is produced as a by-product during thermal or steam cracking of petroleum fractions such as naphtha, kerosene, and diesel oil for the purpose of producing ethylene. The method for separating and recovering CPD from this cracked gasoline is to distill the cracked gasoline fraction to obtain a C ₅ - _{C 7} fraction, heat treat it to remove the light fraction, and then crack the produced dimer. Distill it
A method of recovering CPD in the form of monomers (USP2733279) or a method of distilling the fraction containing this dimer under reduced pressure to concentrate and recover the CPD dimer, followed by decomposition distillation to recover CPD in the form of monomers (USP2733280) etc. have been proposed. However, as mentioned above, the content of CPD in cracked gasoline is as low as about 3 to 7%, and it is not easy to separate it by distillation. Furthermore, CPD is thermally unstable and easily dimerizes, co-dimerizes with other substances, and polymerizes to such a high molecular weight that it does not easily become a monomer even when thermally decomposed, making it difficult to handle. As a result, separation and recovery operations tend to be complicated and inefficient. Therefore, these methods are not economically advantageous because they require a large amount of cracked gasoline to be processed, require large equipment, and require a large amount of energy for distillation, dimerization, and cracking operations. The present inventors previously proposed in Japanese Patent Application Laid-Open No. 59-80618 that it can be easily added to a normal treatment device for cracked gasoline fractions, which are produced as by-products when petroleum fractions such as naphtha, kerosene, and light oil are cracked by heat or steam. As a method for easily recovering CPD with high purity without requiring any change in the treatment conditions of the gasoline fraction, it is possible to decompose CPD dimers or codimers present in the cracked gasoline supplied to the rerun tower. Benzene, toluene and xylene fractions (hereinafter referred to as B, T,
We proposed a method to separate the That is, the method is to extract B・T・X from the cracked gasoline fraction.
The reflux liquid is extracted from the recovery section of the distillation column (rerun column), which collects the gas as an overhead fraction, and sent to the cracking distillation column where it is thermally decomposed at 170 to 230°C for 10 to 200 minutes, and the bottom stream of the cracking distillation column is rerun. Returning to the column below the withdrawal point, sending the column overhead stream to a CPD column, operating the CPD column at a bottom temperature of 160 to 230°C and a residence time of 0.25 to 6 hours to distill CPD from the top of the column. This method recovers CPD from cracked gasoline fraction consisting of (Problems to be Solved by the Invention) However, in this method, since xylene and the like are mainly extracted from the bottom of the cracking distillation column, the feed liquid is diluted by the reflux liquid. Therefore, not only is it difficult for the temperature at the bottom of the tower to rise,
The mole fraction of CPD dimer decreases, it becomes difficult to obtain the necessary residence time, and the proportion of CPD dimer extracted from the bottom of the column without decomposition increases.
The recovery rate of CPD is still insufficient, and even if the bottom temperature is further increased by increasing the bottom pressure to increase the dimer decomposition rate, a large amount of high molecular weight substances that are difficult to re-decompose are generated. It was uneconomical. (Means for Solving the Problems) Therefore, as a result of various studies, the present inventors extracted xylene, etc. from the bottom of the cracking distillation column, and installed a distillation stage near the bottom of the column or below it. We discovered a method to extract a portion of the reflux liquid from the tower. According to this method, the rate at which the dimer at the bottom of the column is diluted by refluxed xylene is reduced, and therefore the temperature at the bottom of the column can be increased to efficiently decompose the dimer.
Costs are significantly reduced and CPD recovery rates are improved. At this time, the temperature of the reflux liquid in the distillation stage to be extracted is 140 to 210°C in terms of normal pressure, and the amount of liquid to be extracted is an amount corresponding to 0.1 to 0.8 times the amount of liquid supplied to the cracking distillation column. In addition, the amount of liquid extracted from the bottom of the column is set to be an amount corresponding to the amount of the remaining liquid relative to the total amount of the reflux liquid extracted and the top stream in the amount of liquid supplied to the cracking distillation column. The object of the present invention cannot be achieved if a certain amount of the reflux liquid at a certain temperature is withdrawn.
In this case, the tower top pressure should be 0 to 5°C/cm ² G, the tower top temperature should be 130 to 200°C, and the concentration of CPD and methylcyclopentadiene (hereinafter referred to as MCPD) in the tower vapor should not be too high. But for example CPD
It is better to suppress the total concentration of CPD monomer and MCPD to 90% or less at most to reduce
It is preferable because co-dimerization with MCPD can be prevented. On the other hand, when operating at a high evaporation rate with almost no internal reflux, the CPD concentration in the vapor distilled from the top of the column becomes extremely low, making it difficult to distill and recover high-purity CPD in the next CPD distillation column. In this case, it is necessary to reflux a large amount of CPD, which is the overhead fraction.
This not only increases the operating load on the CPD distillation column, but also increases the chance of CPD changing into high molecular weight products that do not easily thermally decompose by refluxing a large amount of highly concentrated CPD, resulting in a higher CPD recovery rate. This will lead to a decrease in Therefore, by attaching a dephlegmator to the top of the cracking distillation column, the CPD concentration of the overhead vapor of the cracking distillation column can be reduced to at least 20
% or more is desirable. The decomposition rate of CPD dimer in the decomposition distillation column is
Of course, it is a function of pressure and temperature, but while monomers polymerize in the liquid state, dimer decomposition into monomers is easy to decompose in the gas state, so it is difficult to decompose unless the dimer or codimer is evaporated. Moreover, since a certain amount of residence time is required as a gas, when the CPD dimer concentration in the feedstock is particularly low as in the present invention, the evaporation rate at the bottom of the column, in other words, the extraction rate of the bottom fraction depends on the In other words, increasing the evaporation rate (reducing the extraction rate from the bottom)
The present inventors have found that the recovery rate of CPD is improved significantly by doing the above, so it is thought that this is at least one reason why the recovery rate of CPD is improved by doing as described above. The present invention will be described in detail below with reference to the drawings.
For the sake of simplicity, the drawings omit pumps, heat exchangers, reflux drums, etc. that are not particularly necessary for the explanation, and only show the parts necessary for understanding the invention. Figure 1 shows an example of the method proposed in JP-A-59-80618, and Figure 2 shows the method shown in Figure 1 for more economical and efficient separation and recovery of CPD. 1 is a process flow of an example of the method of the present invention, which is a further improvement of the steps shown in FIG. Second
In the figure, parts common to those in FIG. 1 are given the same numbers. In Figures 1 and 2, cracked gasoline as a by-product from the ethylene production equipment is passed through conduit 1 to the first pipe.
It is supplied to a distillation column (depentanizer) 2, and the C ₅ ^- fraction is extracted from the top of the column through a conduit 3, and the remaining fraction is supplied through a conduit 4 to a second distillation column (rerun column) 5. Therefore, the C ₉ ⁺ fraction is extracted from the bottom of the column through conduit 7, while the fraction containing mainly B, T, and X is extracted from the top of the column through conduit 6 and sent to a hydrogenation reactor (not shown). After being sent, it is fed to an aromatic extractor. In the second distillation column (rerun column) 5, all or part of the reflux liquid in the column is extracted from a distillation stage as close as possible to the bottom of the column or a liquid reservoir installed below it, and is passed through a conduit 8 to the cracking distillation column 9. send to the bottom. The temperature of the reflux liquid at the outlet is approximately 170 to 200°C, and the residence time of the reflux liquid in the second distillation column (rerun column) 5 is generally 10 minutes or less. CPD and its homologues, MCPD dimers and codimers, which are supplied to CPD, are sent to the decomposition distillation column 9 without being decomposed in a significant amount. The composition of CPD, MCPD dimer, and codimer in this extracted liquid varies depending on the ethylene production raw material, decomposition temperature, distillation conditions, etc., but CPD dimer is 3 to 6 wt%, and MCPD dimer and codimer are approximately 0.4 to 1.5 wt%. be. In the decomposition distillation column 9, the bottom temperature is set at 160 to 230°C.
Preferably 200-230℃, residence time of liquid at the bottom of the column
By distilling from 10 minutes to 200 minutes, CPD,
Most of the MCPD dimer and codimer are decomposed into monomers, which are sent to the CPD distillation column 12 from the top of the column through a conduit 10 along with some xylene fraction. On the other hand, heavy fractions other than CPD and MCPD (xylene fraction and C ₉ ⁺ fraction) are circulated from the bottom of the column to the second distillation column (rerun column) 5 through a conduit 11. This returning position is preferably one step below the extraction stage where the conduit 8 is installed. By the way, in this cracking distillation stage, the dimer is decomposed and evaporated as monomer, and a large amount of xylene fraction etc. is also evaporated, but in the method shown in Fig. 1, this xylene fraction is evaporated from the top of the column together with the monomer. Except for a portion that is distilled off, most of the liquid is extracted from the bottom of the column as a reflux liquid, further reducing the concentration of CPD dimer supplied to the bottom of the column. Therefore, as shown in Fig. 2, a part of the reflux liquid in the column is pumped 0.1 to 0.8 times the amount of liquid supplied to the cracking distillation column using a conduit 18 from a distillation stage near the bottom of the column or a reservoir installed below it. By withdrawing the amount, the decrease in the concentration of CPD dimer supplied to the bottom of the tower can be reduced.
Therefore, the temperature at the bottom of the column and the evaporation rate are increased, making it possible to efficiently decompose the dimer. It is also possible to raise the bottom temperature by increasing the pressure of the cracking distillation column, but this is not preferable because it promotes the increase in the molecular weight of the dimer and causes a decrease in the recovery rate. Usually, it is desirable that the tower top pressure be 0 to 5 kg/cm ² ·G and the temperature be 130 to 200°C. Note that the reflux liquid in the column, which is extracted through the conduit 18 and is equivalent to 0.1 to 0.8 times the amount of liquid supplied to the cracking distillation column, may be extracted outside the system as shown in the figure.
If necessary, it may be circulated to the second distillation column (rerun column) 5 together with the heavy components extracted from the bottom of the column through the conduit 11. On the other hand, a dephlegmator is attached to the top of the cracking distillation column to selectively condense the xylene fraction.
The CPD concentration in the overhead fraction can be increased without going through a liquid state that causes CPD polymerization. At this time, efficient heat recovery can be achieved by appropriately selecting the tower top pressure (tower top temperature) and generating steam in a partial condenser. In Figures 1 and 2, CPD distillation column 1
2, tower bottom temperature 140~230℃, tower top pressure 0~2Kg/ ^cm2・
CPD is extracted from the top of the column through a conduit 13 by operating the column at a temperature of 35 to 50° C. in terms of normal pressure. MCPD is dimerized in the column and extracted in dimer form through a conduit 19 together with a xylene fraction and partially dimerized CPD dimer. In FIG. 1, which is a flow sheet of a known method, the residue from the CPD column 12 is transferred to a methylcyclopentadiene column (MCPD column) 15 via a conduit 14.
The MCPD obtained here is recovered through a conduit 16, and the residue from the MCPD column is returned to the decomposition distillation column 9 through a conduit 17. The process of the invention is not limited to the steps shown in FIG. 2, but can be carried out, if desired, by, for example, conduit 19 flowing after conduit 17 in FIG. In 12,
CPD distillation column 12 from a distillation stage with an appropriate reflux liquid temperature.
By extracting a certain amount of reflux liquid and/or vapor from the amount of MCPD supplied to the MCPD, or by further supplying it to the MCPD distillation column, MCPD can be separated and recovered with considerable or high purity along with CPD. It is also possible to do so. (Effects of the Invention) As described above, according to the method of the present invention, in a cracking distillation column, xylene, etc. are not only extracted from the bottom of the column, but also a liquid reservoir is installed in a distillation stage near the bottom of the column or under it. By withdrawing part or all of the reflux liquid in the column, the rate at which the dimer at the bottom of the column is diluted by the reflux xylene is reduced, thereby increasing the temperature at the bottom of the column and increasing the evaporation rate at the bottom of the column. can be efficiently decomposed, the manufacturing cost of CPD can be significantly reduced and the recovery rate of CPD can also be improved. (Example) Hereinafter, the present invention will be explained with reference to Examples. Example 1 A liquid having the composition shown in Table 1 was produced using the distillation stage near the bottom of the second distillation column (rerun column) 5 under substantially the same conditions as the example described in JP-A-59-80618. I pulled it out.

[Table] The table above shows the codimers of CPD and MCPD or their codimers with other components.
CPD and MCPD are shown in terms of CPD dimer and MCPD dimer. The fraction having the above composition is transferred to the cracking distillation column 9 in Fig. 2.
supplied. The operating conditions of the cracking distillation column 9 are shown in Table 2, and the composition of the resulting column top liquid is shown in Table 3. Also, the fraction contained in the extraction stage at this time
CPD was 0.23% and MCPD was 0.11%, and CPD and MCPD contained in the bottom liquid were 0.40% and 0.20%, respectively.

【table】

[Table] The reflux liquid shown in Table 2 was extracted at 200°C from a reservoir installed under the distillation stage closest to the bottom of the six distillation stages. The recovery rate of CPD from the top of the column was 91.3%. At this time, heat was recovered by supplying 3 kg/cm ² ·G of saturated water to the partial condenser and generating 3 kg/cm ² ·G of steam. The samples up to this point were obtained from actual equipment. That is, the reflux liquid of the rerun column 5 was collected from an actual device in commercial operation, and the composition and flow rate of the top stream of the cracking distillation column, the fraction of the withdrawal stage, and the bottom liquid are suitable for the purpose of the present invention. These are the results of a test run of an actual device in which the operating conditions were changed to achieve this. The overhead stream of cracking distillation column 9 was cooled with dry ice to prevent dimerization and co-dimerization of CPD and MCPD, and the sample was then transported to the laboratory and the following experiment was conducted. The composition liquid shown in Table 3 above was supplied at a rate of 176.5 g/hr to the sixth stage from the top of the CPD distillation column 12 (an older Schau distillation column with a perforated plate diameter of 32 mmφ, a stage interval of 30 mm, and a number of stages of 25 stages). The operating conditions are shown in Table 4.
92.3g/hr) is shown in Table 5.

【table】

[Table] Regarding CPD dimer in the feed liquid at this time
The CPD recovery rate was 97.6%.

[Brief explanation of drawings]

Figure 1 is a systematic diagram of an example of the method previously proposed in JP-A-59-80618, and Figure 2 is a system diagram of an example of the method proposed in JP-A-59-80618, and Figure 2 is a combination of the extraction process from the distillation stage, which is a further improvement on the method shown in Figure 1. FIG. 2 is a system diagram of an example of the method of the present invention. 2...First distillation column (depentanizer), 5...Second distillation column (rerun column), 9...Cracking distillation column, 12...CPD
Distillation column, 13... CPD collection tube, 18... Distillation stage reflux liquid extraction tube.

Claims (1)

[Scope of Claims] 1. From the recovery section of a distillation column (rerun column) that collects benzene, toluene, and xylene as an overhead fraction from a cracked gasoline fraction produced as a by-product during thermal or steam cracking of liquid hydrocarbon oil. In a method for separating and recovering cyclopentadiene (CPD) from the extracted reflux liquid, the reflux liquid is supplied to a decomposition distillation column and thermally decomposed at 160 to 230°C, so that the temperature of the reflux liquid in the column reaches 140 °C in terms of normal pressure. A reflux liquid equivalent to 0.1 to 0.8 times the volume of the feed liquid is extracted from the distillation stage or a liquid reservoir installed below it at ~210°C, and the feed liquid is extracted from the distillation stage of the cracking distillation column from the bottom of the column. The reflux liquid and the bottom liquid in an amount corresponding to the total amount of the overhead stream of the cracking distillation column described later are extracted and returned to the recovery section of the distillation column (rerun column), and the overhead stream is transferred to the cyclopentadiene distillation column. (CPD Tower)
A method for separating and recovering cyclopentadiene from a cracked gasoline fraction, comprising operating the CPD tower at a bottom temperature of 140 to 230°C and separating cyclopentadiene from the top of the tower. 2 In the cracking distillation column, the column top pressure is 0 to 5 kg/
The method according to claim ¹ , wherein the temperature at the top of the column is 130 to 200°C.