KR20030075336A - Purification method of 2,6-dimethylnaphthalene - Google Patents

Abstract

PURPOSE: A method for separating and purifying 2,6-dimethylnaphthalene is provided, to obtain 2,6-dimethylnaphthalene from naphtha cracking raffinate with a high purity and a high production yield. CONSTITUTION: The method comprises the steps of selectively separating a mixture of dimethylnaphthalene isomers containing 2,6-dimethylnaphthalene from a mixture of naphtha cracking raffinate and dimethylnaphthalene isomers by melting crystallization; and separating highly pure 2,6-dimethylnaphthalene from the mixture of dimethylnaphthalene isomers containing 2,6-dimethylnaphthalene by extraction crystallization. Preferably the first step is carried out at a cooling temperature of 60 to -40 deg.C and a cooling velocity of 0.1-10 K/min; and the second step is carried out at a cooling temperature of 100 to -6 deg.C, and the extraction solvent in the second step is selected from the group consisting of an alcohol of C1-C6, acetone, toluene, p-xylene, o-xylene, hexane, heptane, octane, decane, dodecane, cyclopentane, cyclohexane, methylcyclopentane, methylcyclohexane, cyclooctane, methylcyclooctane, ether, tetrahydrofuran and tetrahydrofuran methanol.

Description

Purification method of 2,6-dimethylnaphthalene {Purification method of 2,6-dimethylnaphthalene}

The present invention relates to a separation and purification method of 2,6-dimethylnaphthalene, and more specifically, to a high purity 2,6-dimethylnaphthalene (a combination of melt crystallization and extraction crystallization from naphtha cracking residue oil and dimethylnaphthalene isomeric mixture) The present invention relates to a separation and purification method in which 2,6-dimethylnaphthalene can be obtained with high purity and high yield by separating '2,6-DMN').

2,6-naphthalene dicarboxylic acid as a raw material of a high performance polyester used in the production of polyethylene naphthalate fibers, films and the like is required to be of high purity, and thus 2,6-naphylene dicarboxylic acid, which is a raw material of the 2,6-naphylene dicarboxylic acid, 6-DMN is also required to be of high purity.

Dimethylnaphlenene (hereinafter referred to as 'DMN') has 10 isomers depending on the position of the two methyl groups, and 2,6-DMN to the 2,6-naphthalene dicarboxylic acid raw material substantially contains other isomers. High purity is needed.

The known method for preparing 2,6-DMN was prepared by distillation or a process using a catalyst from a mixture of DMN isomers, but the boiling point of each DMN isomer is quite close as shown in Table 1 below. Separation and purification of 2,6-DMN by distillation is quite difficult.

As can be seen from Table 1, among the DMN isomers, the melting point of 2,6-DMN is the highest. Therefore, 2,6-DMN can be separated and purified through the melt crystallization process. The separation method of 2,6-DMN is a method of crystallization, a method of adsorption, a method of forming a complex with 2,6-DMN using some kind of organic compound, and then separating and decomposing the complex. Is being proposed.

Looking at the prior art related to the method of separating 2,6-DMN as follows.

Korean Patent Publication No. 2001-33746 discloses that 2,6-DMN, which is used for the production of polyethylene naphthalate, is not limited to the specific isomers present in the feedstock, but is separated from the DMN isomer mixture through a series of fractionation, crystallization and adsorption steps. After the crystallization step to prepare 2,6-DMN with high purity and high yield, a process of adsorbing and separating 2,6-DMN by dissolving in p-, o-xylene as a final purification step was proposed. In Japanese Unexamined Patent Publication No. 1997-301900, 2,6-DMN is precipitated by crystallization from a DMN isomerization reaction product, high purity 2,6-DMN is obtained in high yield, and preferable 2,6-DMN crystal having good filterability. To prepare 2,6-DMN through crystallization of 2,6-DMN from DMN isomerization reaction product in the presence of solvent as a method of precipitation and industrially advantageously separating and recovering high purity 2,6-DMN. Suggested. The solvent used for the isomerization reaction is pentene, hexane, hetan, octane, nonane, decane, undecane, dodecane, cyclopentane, cyclohexane, methylcyclopentane, methylcyclohexane, cyclooctane, methylcyclooctane, decaine, methyl Decaine, dimethyldecaine and the like were used. In Japanese Patent Laid-Open No. 1997-249586, it is industrially possible to precipitate 2,6-DMN by crystallization from a mixture of DMN isomers, to obtain high purity 2,6-DMN, to be stable over a long period of time, and to maintain a predetermined purity or more. Advantageous separation and recovery methods are disclosed.

However, it is pointed out that among these methods, the method by crystallization is the simplest and most suitable as an industrial separation method, but the process is complicated, the yield is relatively low, and the use of expensive solvents causes relatively high fixed investment and production costs. There is. In particular, even in the case of using the separation process through crystallization, there is no detailed description, and most of the forms are simply crystallized by cooling. The focus is on isomerization process or adsorption process using a catalyst rather than crystallization process. A solvent was used for the isomerization process. In addition, in the operation performed through crystallization, the purity of 2,6-DMN is 45 wt% or more, and a lower purity has a problem that is difficult to use.

Thus, the present inventors have completed the present invention by separating the high purity 2,6-dimethylnaphthalene by combining the melt crystallization method and the extraction crystallization method from naphtha cracked residue oil and dimethyl naphthalene isomeric mixture in order to solve the above problems. .

Accordingly, the present invention provides a method for separation and purification of 2,6-dimethylnaphthalene, which is simple in operation and simple in operation, which can reduce fixed investment and production costs and obtain 2,6-dimethylnaphthalene in high purity and high yield. There is a purpose.

1 is a process diagram schematically showing a separation and purification method of 2,6-dimethylnaphthalene according to the present invention.

2 is a graph showing the purity and yield according to the amount of the extraction solvent (hexane) in the secondary extraction crystallization method according to the present invention.

The present invention

1) separating a mixture of dimethylnaphthalene isomers, optionally including 2,6-dimethylnaphthalene, from the naphtha cracking residue oil and the dimethylnaphthalene isomer mixture by melt crystallization;

2) separating high purity 2,6-dimethylnaphthalene from the mixture of dimethylnaphthalene isomers containing 2,6-dimethylnaphthalene in the first step through extraction crystallization;

It is characterized by the separation and purification method of 2,6-dimethylnaphthalene consisting of.

Referring to the present invention in more detail as follows.

The present invention is a high-purity, high-purity combination of 2,6-dimethylnaphthalene contained in naphtha cracking residue oil, low energy consumption type, simple apparatus, simple operation, melt and crystallization and extraction crystallization method that can reduce fixed investment and production cost The present invention relates to a method for separating and purifying 2,6-DMN in yield.

The method for separating and purifying 2,6-DMN according to the present invention will be described in more detail with reference to FIG. 1 as follows.

First, a first step of separating the mixture of dimethyl naphthalene isomer including 2,6-dimethyl naphthalene selectively through melt crystallization from naphtha cracking residue oil and dimethyl naphthalene isomer mixture in the primary separation purification process of FIG. Looking at it as follows.

The naphtha cracking residue is a mixture containing 10 isomers of DMN, monomethylnaphlenene isomers and low boiling hydrocarbons which are by-products of naphtha cracking processes, 2,6-DMN, 2,7-DMN, 2,3-DMN and others. The mixtures of are each included in the amounts shown in Table 2 below.

As shown in Table 2, the present invention can separate and purify industrially useful high purity 2,6-DMN from naphtha cracked residue oil containing 2,6-DMN in a low content of 2 to 20% by weight. Conventional 2,6-DMN has the advantage that can be separated more efficiently than the method of separating 2,6-DMN from the naphthalene-based compound containing a higher content.

The primary separation and purification process uses a melt crystallization method to prepare a mixture of dimethylnaphthalene isomers containing 2,6-DMN containing 2,6-DMN with a purity of 45% by weight or more from the naphtha cracking residue oil and the dimethylnaphthalene isomer mixture. In the separation process, 2,6-DMN crystals are produced with the cooling temperature of the crystallizer in the range of 60 ° C to -40 ° C and the cooling rate in the range of 0.1 to 10 K / min. At this time, the reason why the cooling temperature is within the above range is because the crystal formation range of 2,6-DMN in the raw material is in the above range. If the temperature is outside the above temperature range, other components other than 2,6-DMN also grow into solid crystals. In addition, there is a problem that crystals do not form at high temperatures, and if the cooling rate is out of the above range, the operation of the crystallizer is not only difficult but also the crystal growth rate is fast. There is.

The melt crystallization method may be a film crystallization method or suspended crystallization method as a purification process for improving the purity and yield of the crystal.

The flotation crystallizer used in the flotation crystallization method is composed of a cooling crystallizer with an outer jacket, a thermostat and a stirrer. As the stirrer, a marine-type stirrer (MS280D, MS) equipped with a stirring speed controller is used to mix the solution in the crystallizer, and the stirring speed is 1000 rpm at a stirring. The temperature of the solution and the thermostat in the cooling crystallizer is recorded with a digital temperature measuring instrument (180L, Yokogawa) connected with a K-type thermocouple.The thermostat is recorded using a refrigerated thermostat (RBC20, JEIO TECH) equipped with a PID controller. Control to an error range of ± 0.05K. The heat medium (refrigerant) of the thermostat is a mixture of ethylene glycol and water in a 1: 3 ratio and industrial methanol. The solution temperature inside the crystallizer is controlled by the heating medium circulated through the outer jacket, the cooling rate of the crystallizer is controlled by PID programming, and the final cooling temperature control range is based on the concentration of naphtha cracking residue and dimethylnaphthalene isomeric mixture. The resulting crystals are then separated from the mother liquor using a vacuum filter at 60 ° C to -40 ° C. At this time, the cooling rate is in the range of 0.1 to 10 K / min. The composition of the crystal and the residual liquid was analyzed by gas chromatography equipped with Flame Ionization Detector (hereinafter referred to as 'GC': DS 6200, DONAM) and the purity of 2,6-DMN was 45 wt% or more. When it is transferred to the secondary separation and purification process through extraction crystallization.

The film crystallization apparatus used in the film crystallization method includes a film-like crystallizer for attaching crystals, an external crystallization vessel for placing a sample, a thermostat equipped with a PID controller for controlling the temperature of the refrigerant, a digital temperature recorder for recording a temperature profile, and Consists of GC for analyzing the sample taken, the crystallizer was made of Pyrex glass. The general procedure of the film crystallization method is as follows. The temperature of the refrigerant tank is maintained at 60 to 100 ° C. for about 30 minutes, and then cooled to 60 to -40 ° C. by the refrigerant circulation in a constant temperature bath equipped with a PID temperature controller. The surface of the crystallizer is cooled to 60 to -40 ° C by a commercial methanol and a refrigerant in which water circulating inside the crystallizer is mixed with ethylene glycol in a 3: 1 ratio. At this time, the cooling rate is in the range of 0.1 to 10 K / min. The amount of crystals formed on the crystallizer wall was determined by the amount of the residual melt, and the composition of the crystal and the residual liquid was referred to as gas chromatography (hereinafter referred to as 'GC') equipped with a Flame Ionization Detector (FID: DS 6200, DONAM). When the purity of 2,6-DMN is 45% by weight or more, it is transferred to the secondary separation and purification process through extraction crystallization.

The separation and purification process by the melt crystallization method as described above is a single or continuous multi-stage crystallization process, when the purity of 2,6-DMN reaches 45% by weight or more, it is transferred to the secondary separation and purification process through extraction crystallization. Multistage crystallization separates the crystals and the residual liquid from the crystallizer (I), the crystal product is sent to the secondary separation and purification process in two stages, and the residue is sent to the crystallizer (II) to carry out the crystallization. -Method for separating and purifying DMN (see FIG. 1).

Next, a high purity of 99% by weight or more through extraction crystallization from a mixture of dimethylnaphthalene isomers containing 2,6-DMN of 45% by weight or more obtained by the first separation and purification process of the secondary separation purification process of FIG. 1. Looking at in detail for the 2 step of separating the 2,6-dimethylnaphthalene as follows.

The secondary separation purification process is a process of separating 2,6-DMN having a purity of 99% by weight or more from the mixture of the dimethylnaphthalene isomer including 2,6-DMN by using an extraction crystallization method, and cooling temperature of the crystallizer to 100 2,6-DMN crystal | crystallization is produced in the range of -6 degreeC from -degreeC, and cooling rate in the range of 0.1-10K / min. At this time, the reason why the cooling temperature is in the above range is not only a crystal forming temperature of 2,6-DMN at the above range, but also most crystalline phase, and there is no formation of crystal at a high temperature outside the above range. This is because it is not separated and other impurities are present in the crystal phase at low temperatures, and when the cooling rate is out of the above range, many other impurities in the crystals crystallize 2,6-DMN into the same solid phase. In the extraction crystallization method, the mixing ratio of the extraction solvent and the mixture of the dimethylnaphthalene isomer including 2,6-DMN is preferably 1: 1 to 100 weight ratio, and if the mixing ratio is out of the range, high purity is at a low mixing ratio. It is difficult to obtain the crystals, and it is difficult to obtain crystals in the above temperature range because the amount of the extraction solvent is relatively too large and the supersaturation does not occur at a high mixing ratio. In particular, the extraction solvent used in the extraction crystallization method is easy to separate the solvent and crystals by using a solvent which is inert to 2,6-DMN, has a low viscosity, and has a large boiling point difference from 2,6-DMN. It is preferable in that a large number of crystallizations can be obtained, and the extraction solvent may be alcohol having 1 to 6 carbon atoms, acetone, toluene, p-xylene, o-xylene, hexane, heptane, octane, decane, dodecane, cyclopentane, Cyclohexane, methylcyclopentane, methylcyclohexane, cyclooctane, methylcyclooctane, ether or tetrahydrofuran, tetrahydrofuranmethanol can be used, more preferably methanol or ethanol. The extraction crystallization process forms a eutectic system in which 2,6-DMN and 2,7-DMN coexist. In the melt crystallization process, it is difficult to separate in the temperature range below the eutectic point. Therefore, by changing the behavior of the phase using an extraction solvent, a third material, we can obtain the crystal of 2,6-DMN that we want.

As described above, the separation and purification method of 2,6-dimethylnaphthalene according to the present invention is a combination of melt crystallization method and extraction crystallization method from a mixture of naphtha cracking residue oil and dimethylnaphthalene isomer containing low purity 2,6-DMN. It is possible to separate 2,6-dimethylnaphthalene having a purity of 99% by weight or more. In addition, crystallization using the extraction crystallization method in the first purification process can more effectively obtain 2,6-DMN of high purity.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited by Examples.

[First-Phase Separation Purification Using Melt Crystallization in a Floating Crystallizer]

Example 1

100 g of naphtha cracking residue (crude) containing about 10% by weight of 2,6-DMN is placed in a crystallizer with a jacket, and the heat medium is circulated through the jacket of the crystallizer to maintain the temperature at 25 ° C. for 30 minutes. Cool down to -12 ° C at a rate of 1 K / min and hold at -12 ° C for 1 hour. Stirring speed was maintained at 500 rpm. The suspended crystals were filtered using a vacuum filter to separate the residual liquid and the crystal product, and each component was analyzed using GC. The results are shown in Table 3 below. The purity of the obtained crystal product was 45.1 wt% and the yield was 75%.

The yield of the generated 2,6-DMN was calculated as in the following equation (1).

[Secondary Separation Purification Using Extraction Crystallization in Floating Crystallizers]

Example 2

To 10 g of the product of Example 1 (45.1 wt%) produced in the primary crystallization, 100 g of cyclohexane was added as an extraction solvent and placed in a jacketed crystallizer, and the heat medium was circulated through the jacket of the crystallizer. After maintaining for 30 minutes, the mixture was cooled to -20 ° C at a rate of 1 K / min and maintained at -20 ° C for 1 hour. Stirring speed was maintained at 500 rpm. The suspended crystals were filtered using a vacuum filter to obtain a crystal product.

The crystals thus obtained were filtered using a vacuum filter to separate the remaining liquid and the crystal product, and the respective components were analyzed using GC. The results are shown in Table 3 above. The purity of the obtained crystal product was 83.3 wt% and the yield was 32.4%.

Example 3

To 50 g of the product of Example 1 (45.1 wt%) produced in the first crystallization, 500 g of hexane was added as an extraction solvent and placed in a jacketed crystallizer, and the heat medium was circulated through the jacket of the crystallizer to bring the temperature to 60 ° C. After holding for 30 minutes, the mixture was cooled to -20 ° C at a rate of 1 K / min and maintained at -20 ° C for 1 hour. Stirring speed was maintained at 500 rpm. The suspended crystals were filtered using a vacuum filter to obtain a crystal product.

The crystals thus obtained were filtered using a vacuum filter to separate the residual liquid and the crystal product, and each component was analyzed using GC. The results are shown in Table 3 above. The purity of the obtained crystal product was 65.4 wt% and the yield was 98.7%.

Example 4

To 25 g of the product of Example 1 (45.1 wt.%) Produced in the first crystallization, 500 g of hexane was added as an extraction solvent and placed in a jacketed crystallizer, and the heat medium was circulated through the jacket of the crystallizer. After holding for 30 minutes, the mixture was cooled to -10 ° C at a rate of 1 K / min and maintained at -10 ° C for 1 hour. Stirring speed was maintained at 500 rpm. The suspended crystals were filtered using a vacuum filter to obtain a crystal product.

The crystals thus obtained were filtered using a vacuum filter to separate the residual liquid and the crystal product, and each component was analyzed using GC. The results are shown in Table 3 above. The purity of the obtained crystal product was 98.2% by weight and the yield was 92.4%.

Example 5

To 10 g of the product of Example 1 (45.1 wt%) produced in the primary crystallization, 500 g of hexane was added as an extraction solvent and placed in a jacketed crystallizer, and the heat medium was circulated through the jacket of the crystallizer to bring the temperature to 60 ° C. After holding for 30 minutes, the mixture was cooled to -10 ° C at a rate of 1 K / min and maintained at -10 ° C for 1 hour. Stirring speed was maintained at 500 rpm. The suspended crystals were filtered using a vacuum filter to obtain a crystal product.

The crystals thus obtained were filtered using a vacuum filter to separate the residual liquid and the crystal product, and each component was analyzed using GC. The results are shown in Table 3 above. The purity of the obtained crystal product was 99.7% by weight and the yield was 78.9%.

Example 6

To 5 g of the product of Example 1 (45.1 wt%) produced in the primary crystallization, 500 g of hexane was added as an extraction solvent and placed in a jacketed crystallizer, and the heat medium was circulated through the jacket of the crystallizer to bring the temperature to 60 ° C. After holding for 30 minutes, the mixture was cooled to -8 ° C at a rate of 1 K / min and maintained at -8 ° C for 1 hour. Stirring speed was maintained at 500 rpm. The suspended crystals were filtered using a vacuum filter to obtain a crystal product.

The crystals thus obtained were filtered using a vacuum filter to separate the residual liquid and the crystal product, and each component was analyzed using GC. The results are shown in Table 3 above. The purity of the obtained crystal product was 99.9% by weight and the yield was 75.8%.

As shown in Examples 3 to 6, the purity and yield of 2,6-DMN according to the addition amount of the extraction solvent are shown in a graph in FIG. 2, and the higher the amount of the extraction solvent, the higher the purity of 2,6-DMN can be obtained. Confirmed.

In addition, in Examples 2 to 4, since 2,6-DMN having a purity of 99% by weight or more could not be separated in a second separation and purification process using an extraction crystallization method, to obtain a purity of 99% by weight or more. As shown in Figure 1, a multistage crystallization process was performed to separate 2,6-DMN having a purity of 99% by weight or more.

[First-Phase Separation Purification Using Melt Crystallization in Dura-type Crystallizer]

Example 7

100 g of naphtha cracking residue (crude) containing about 10% by weight of 2,6-DMN is placed in an external crystallization vessel containing a jacketed sample, and the heat medium is circulated into the crystallizer to maintain the temperature at 25 ° C. The crystallization rod was cooled at a rate of 1 K / min from 25 ° C. to −12 ° C. and maintained at −12 ° C. for 1 hour. The crystals thus crystallized and separated into crystallization rods were separated from the residual liquid, and each component was analyzed using GC. The results are shown in Table 4 below. The purity of the obtained crystal product was 27.5% by weight and the yield was 50.4%.

As shown in Table 4, in the primary separation and purification process using a film-type crystallizer (melt crystallization method), since 2,6-DMN having a purity of 27.5 wt% is separated in one step, the secondary separation and purification process In order to transfer 2,6-DMN to 2,6-DMN purity of 45% by weight or more is required, as shown in Figure 1 was subjected to a multi-stage crystallization process to remove 45.3% by weight of 2,6-DMN.

[Secondary Separation Purification Using Extraction Crystallization Method in Dural Crystallizers]

Example 8

To 10 g of the product of Example 7 (purity 45.3% by weight) produced in the primary crystallization, 700 g of cyclohexane was added as an extraction solvent, placed in an external crystallization vessel containing a jacketed sample, and the heat medium was circulated into the crystallizer. The temperature was maintained at 45 ℃ and the film crystallization rod was cooled at a rate of 1 K / min from 45 ℃ to -20 ℃ was maintained at -20 ℃ for 2 hours. The crystals thus obtained were filtered using a vacuum filter to separate the residual solution and the crystal product, and each component was analyzed using GC. The results are shown in Table 4 above. The purity of the obtained crystal product was 95.5% by weight and the yield was 35%. Since the purity of 2,6-DMN according to Example 8 was low as 95.5% by weight, 2,6-DMN having a purity of 99.5% by weight was separated by performing a multistage crystallization process to obtain a purity of 99% by weight or more.

Example 9

To 10 g of the product of Example 7 (purity 45.3 wt%) produced in the primary crystallization, 300 g of hexane was added as an extraction solvent, placed in an external crystallization vessel containing a jacketed sample, and the heat medium was circulated into the crystallizer. The temperature was maintained at 45 ° C., and the film was cooled to a temperature of 1 K / min from 45 ° C. to −10 ° C. and maintained at −8 ° C. for 1 hour.

The crystals thus obtained were filtered using a vacuum filter to separate the residual solution and the crystal product, and each component was analyzed using GC. The results are shown in Table 4 above. The purity of the obtained crystal product was 95.8% by weight and the yield was 73%. Since the purity of 2,6-DMN according to Example 9 was low as 95.8% by weight, 2,6-DMN having a purity of 99.7% by weight was separated by performing a multistage crystallization process to obtain a purity of 99% by weight or more.

As described above, the separation and purification method of 2,6-DMN by the melt crystallization method (using a floating type crystallizer and a film-like crystallizer) and the extraction crystallization method according to the present invention is about 1 of the heat of vaporization used in the distillation operation. By using the heat of fusion of / 5, energy is saved and high purity 2,6-DMN can be separated with high yield by simple solid-liquid separation operation. The present invention has the advantage that the separation and purification apparatus is simple and the operation is simple, so that the fixed investment cost and production cost can be reduced. In addition, there is an advantage that the extraction crystallization process as an additional process can effectively separate high yield, high purity 2,6-DMN.

Claims (8)

1) separating a mixture of dimethylnaphthalene isomers, optionally including 2,6-dimethylnaphthalene, from the naphtha cracking residue oil and the dimethylnaphthalene isomer mixture by melt crystallization; 2) separating 2,6-dimethylnaphthalene of high purity from the mixture of dimethylnaphthalene isomers including 2,6-dimethylnaphthalene in the first step by extracting crystallization method Separation Purification Method. The method of claim 1, wherein the melt crystallization method of the first step is a separation and purification method of 2,6-dimethylnaphthalene, characterized in that the cooling temperature range 60 ~ -40 ℃. The method of claim 1, wherein the melt crystallization method of step 1 is a separation and purification method of 2,6-dimethylnaphthalene, characterized in that the cooling rate is in the range of 0.1 ~ 10 K / min. The method of claim 1, wherein the extraction crystallization method of the second step is a separation and purification method of 2,6-dimethylnaphthalene, characterized in that the cooling temperature is in the range of 100 ~ -6 ℃. 2. The 2,6-dimethylnaphthalene according to claim 1, wherein the mixing ratio of the extraction solvent and the mixture of the dimethyl naphthalene isomer including 2,6-dimethylnaphthalene in the two-step extraction crystallization method is 1: 1 to 100 by weight. Separation and purification method of. The method of claim 1, wherein the two extraction solvents are alcohols having 1 to 6 carbon atoms, acetone, toluene, p-xylene, o-xylene, hexane, heptane, octane, decane, dodecane, cyclopentane, cyclohexane, methyl Separation and purification method of 2,6-dimethylnaphthalene, characterized in that selected from cyclopentane, methylcyclohexane, cyclooctane, methylcyclooctane, ether and tetrahydrofuran, tetrahydrofuranmethanol. The method of claim 1, wherein the naphtha cracking residue oil and the dimethyl naphthalene isomeric mixture comprise 2 to 20 wt% of 2,6-dimethylnaphthalene. 2. The method of claim 1, wherein the mixture of dimethylnaphthalene isomers including 2,6-dimethylnaphthalene, which is subjected to the separation and purification method of step 1, comprises 2,6-dimethylnaphthalene in an amount of 45 to 60% by weight. -Separation and purification method of dimethyl naphthalene.