1. A method for separating 2, 5-dichlorotoluene from a dichlorotoluene mixed solution is characterized by comprising the following steps:

(1) removing weight: removing the heavy component of the mixed dichlorotoluene in a rectifying tower to remove 2, 3-dichlorotoluene;

(2) and (3) extraction and rectification: and (2) taking N-N-butyl pyrrolidone as an extracting agent, carrying out rectification extraction on the mixed dichlorotoluene subjected to the weight removal in the step (1) in an extraction rectification tower, and collecting 2, 5-dichlorotoluene at the tower top.

2. The separation method as claimed in claim 1, wherein the number of theoretical plates of the rectifying tower in the step (1) is 120-140, the temperature of a tower kettle is 140-155 ℃, the temperature of a tower top is 135-143 ℃, the pressure of the tower is-0.90-0.98 bar, and the reflux ratio is 3-6.

3. The separation method according to claim 1, wherein the number of theoretical plates of the extractive distillation column in step (2) is 100, the feed positions of the extractant and the de-weighted mixed dichlorotoluene are respectively 3 rd and 36 th, the feed temperature of the extractant is 80 to 100 ℃, and the mass ratio of the de-weighted mixed dichlorotoluene to the extractant is 1.0: 4.0-10.0, the temperature of the tower kettle is 130-145 ℃, the temperature of the tower top is 125-138 ℃, the pressure of the tower is-0.90-0.98 bar, and the reflux ratio is 5-20.

4. The separation process according to claim 1, wherein the dichlorotoluene in the step (2) is mixed with dichlorotoluene

The compound was a mixture of 2, 3-dichlorotoluene, 2, 4-dichlorotoluene and 2, 5-dichlorotoluene.

5. The separation method according to claim 1, wherein the bottom component of the extractive distillation column in the step (2) is a mixture of mixed dichlorotoluene and an extractant.

Background

In the catalytic or thermal chlorination of toluene and chlorine to produce p-chlorotoluene, o-chlorotoluene and 2, 6-dichlorotoluene, a mixed dichlorotoluene byproduct containing 2, 4-dichlorotoluene and 2, 5-dichlorotoluene is usually generated, and the mixed dichlorotoluene generally accounts for about 17% of the main product yield. At present, the partial mixed dichlorotoluene is often used as solvent oil (the utilization value is low), the low-value utilization of the partial byproducts obviously improves the production cost of a toluene chlorination production device, and the great waste of resources is caused.

Since the boiling points of 2, 5-dichlorotoluene and 2, 4-dichlorotoluene are 198.5 ℃ and 200.0 ℃ respectively, and the difference between the boiling points is only 1.5 ℃, the conventional rectification method is adopted for separation, and the 2, 5-dichlorotoluene with the content of more than 99.5 percent is difficult to obtain. For this reason, a comprehensive utilization scheme of selective hydrogenation of mixed dichlorotoluene to chlorotoluene by adopting a hydrogenation technology is proposed in documents, for example, chinese patent CN201810682893.0 proposes a method for preparing o-chlorotoluene, which uses a strip-shaped Pd/SiC catalyst containing 0.5% of Pd in a tubular fixed bed reactor at a reaction pressure of 0.01-0.10 MPa, a temperature of 120-250 ℃ and a mass space velocity of mixed dichlorotoluene of 0.2-1.5 h^-1When the molar ratio of the fed hydrogen to the dichlorotoluene is 0.2-2: 1, the yield of the o-chlorotoluene is over 50 percent, and the service life of the catalyst is as long as 1000 hours. The American patent US4749817A uses a partially vulcanized palladium or platinum-containing catalyst and adopts a gas phase hydrogenation process, the temperature is 225-450 ℃, the reaction pressure is normal pressure, the molar ratio of the fed hydrogen to the polychlorobenzene is 1: 1, and the mass space velocity is 0.5-5.0 h^-1Under the condition of (2), the polychlorobenzene is selectively converted into dichlorobenzene and chlorobenzene, the amount of generated benzene is very small, and the selectivity of the hydrogenation process is good. The US patent No. 5545779A adopts a catalyst with palladium or platinum as an active component to hydrodechlorinate chlorobenzene at a temperature of 100-250 ℃. US patent 6482996A developed a method of using Pd/Al₂O₃-SiO₂Catalyst, a process for preparing 2, 6-dichlorotoluene by hydrogenation of polychlorinated toluene.

Although the hydrogenation technical scheme can solve the problem of comprehensive utilization of the mixed dichlorotoluene, the high-purity 2, 5-dichlorotoluene product cannot be obtained, and the treatment cost of the mixed dichlorotoluene is increased due to the treatment of hydrogen chloride generated in the hydrogenation process, the service life of a hydrogenation catalyst and the overhigh temperature required by hydrogenation, so that a novel process route is developed, and the method for separating and obtaining the high-purity 2, 5-dichlorotoluene product from the mixed dichlorotoluene is an effective and economic method for improving the comprehensive utilization value of the mixed dichlorotoluene.

Disclosure of Invention

Aiming at the problems of low comprehensive utilization value and high cost of a mixed dichlorotoluene byproduct and low purity of the separated 2, 5-dichlorotoluene in the prior art, the invention provides a method for separating 2, 5-dichlorotoluene from a dichlorotoluene mixed solution, which takes N-N-butyl pyrrolidone as an extractant and separates high-purity 2, 5-dichlorotoluene from the mixed dichlorotoluene by extractive distillation.

The invention is realized by the following technical scheme:

a method for separating 2, 5-dichlorotoluene from a dichlorotoluene mixed solution is characterized by comprising the following steps:

(1) removing weight: removing the heavy component of the mixed dichlorotoluene in a rectifying tower to remove 2, 3-dichlorotoluene;

(2) and (3) extraction and rectification: and (2) taking N-N-butyl pyrrolidone as an extracting agent, carrying out rectification extraction on the mixed dichlorotoluene subjected to the weight removal in the step (1) in an extraction rectification tower, and collecting 2, 5-dichlorotoluene at the tower top.

Further, the number of theoretical plates of the rectifying tower in the step (1) is 120-140, the temperature of a tower kettle is 140-155 ℃, the temperature of a tower top is 135-143 ℃, the pressure of the tower is-0.90 to-0.98 bar, and the reflux ratio is 3-6.

Further, the number of theoretical plates of the extractive distillation column in the step (2) is 100, the feeding positions of the extractant and the de-weighted mixed dichlorotoluene are respectively the 3 rd and the 36 th blocks, the feeding temperature of the extractant is 80-100 ℃, and the mass ratio of the de-weighted mixed dichlorotoluene to the extractant is 1.0: 4.0-10.0, the temperature of the tower kettle is 130-145 ℃, the temperature of the tower top is 125-138 ℃, the pressure of the tower is-0.90-0.98 bar, and the reflux ratio is 5-20.

Further, the dichlorotoluene mixture in the step (2) is a mixture of 2, 3-dichlorotoluene, 2, 4-dichlorotoluene and 2, 5-dichlorotoluene.

Further, the tower bottom component of the extractive distillation tower in the step (2) is a mixture of mixed dichlorotoluene (mainly 2, 4-dichlorotoluene) and an extractant.

Since the mixed dichlorotoluene mainly contains two components of 2, 5-dichlorotoluene and 2, 4-dichlorotoluene, the boiling points of the two components are 198.5 ℃ and 200.0 ℃ respectively, and only the difference is 1.5 ℃, if a conventional rectification method is adopted for separation, 2, 4-dichlorotoluene with the content of more than 99.5 percent is obtained, the number of theoretical plates of a required rectification tower is too large, and the theoretical plates are feasible, but difficult to realize in actual industrial production. The invention selects N-N-butyl pyrrolidone as an extractant for extracting and rectifying mixed dichlorotoluene, not only can effectively pull open the relative volatility (alpha) of 2, 5-dichlorotoluene and 2, 4-dichlorotoluene, but also can not achieve the separation technical effect by commonly used extractants such as acetonitrile, DMF, N-methyl pyrrolidone, ethylenediamine, N-butanol, sulfolane, dibutyl phthalate and diphenylamine, and the N-N-butyl pyrrolidone and the 2, 4-dichlorotoluene can be mutually dissolved, the boiling point is 243.8 ℃, the boiling point difference with the latter is 43.8 ℃, and the separation of the two can be realized by rectification. The dichlorotoluene is separated by rectification and extractive rectification, because the separation is a physical process, no chemical reaction occurs, the yield can reach more than 95 percent, and in addition, the N-N-butyl pyrrolidone has high thermal stability, good chemical stability, no toxicity, no corrosion to equipment and easy recovery.

Advantageous effects

According to the invention, N-N-butyl pyrrolidone is used as an extracting agent, the mixed dichlorotoluene raw material is treated by an extractive distillation process, and 2, 5-dichlorotoluene with the content of more than 99.5% is obtained by separation, so that the cheap production of the product is realized, the additional value of the mixed dichlorotoluene used as a solvent is obviously improved, the production process is simple, the production cost has obvious advantages, and the method is suitable for continuous production of industrial devices.

Detailed Description

In order to make those skilled in the art better understand the technical solution of the present invention, the following clear and complete description of the technical solution of the present invention shall be included in the scope of the present application, and other similar embodiments obtained by those skilled in the art without creative efforts shall be included based on the embodiments in the present application.

Example 1

(1) Removing weight: the mixed dichlorotoluene is sent into a rectifying tower with the theoretical plate number of 140 for weight removal, the temperature of the tower bottom is 140 ℃, the temperature of the tower top is 135 ℃, the pressure of the tower is-0.98 bar, the reflux ratio is 3, the contents of the raw materials, the rectified tower top and the tower bottom are analyzed by GC, and the results are shown in Table 1;

TABLE 1 analysis table of dichlorotoluene components before and after weight removal

(2) And (3) extraction and rectification: taking N-N-butyl pyrrolidone as an extractant, feeding the tower bottom liquid (the mixed dichlorotoluene after weight removal) in the step (1) into an extractive distillation tower for extractive distillation, extracting 2, 5-dichlorotoluene (the purity of the 2, 5-dichlorotoluene is 99.58%) from the tower top, and extracting a mixture of the dichlorotoluene and the extractant from the tower bottom; the theoretical plate number of the extractive distillation tower is 100, the feeding positions of the extractant and the de-weighted mixed dichlorotoluene are respectively 3 rd and 36 th, the feeding temperature of the extractant is 80 ℃, the mass ratio of the de-weighted mixed dichlorotoluene to the extractant is 1.0: 4.0, the temperature of a tower kettle is 130 ℃, the temperature of a tower top is 125 ℃, the pressure of the tower is-0.98 bar, and the reflux ratio is 5;

(3) desorbing: feeding the tower bottom liquid (mixture of dichlorotoluene and extractant) in the step (2) into a desorption tower for desorption, extracting mixed dichlorotoluene (the content of 2, 4-dichlorotoluene is 98.48 percent) from the top of the desorption tower, and recycling the extractant extracted from the tower bottom; the theoretical plate number of the desorption tower is 50, the temperature of a tower kettle is 150 ℃, the temperature of a tower top is 136 ℃, the pressure of the tower is-0.98 bar, and the reflux ratio is 3;

GC analysis is carried out on the components of the sample before the extractive distillation in the step (2), the overhead fraction after the extractive distillation and the overhead fraction after the desorption in the step (3), and the analysis results are shown in Table 2:

TABLE 2 analysis table of mixed dichlorotoluene components before and after extractive distillation and desorption

。

Example 2

(1) Removing weight: feeding the mixed dichlorotoluene into a rectifying tower with a theoretical plate number of 120 for removing weight, wherein the temperature of a tower kettle is 155 ℃, the temperature of a tower top is 143 ℃, the pressure of the tower is-0.90 bar, and the reflux ratio is 6; GC analysis is adopted for the contents of the mixed dichlorotoluene before rectification and the components at the top and the bottom of the tower after rectification, and the results are shown in Table 3;

TABLE 3 analysis table for dichlorotoluene components before and after weight removal

(2) And (3) extraction and rectification: taking N-N-butyl pyrrolidone as an extractant, feeding the tower bottom liquid (the mixed dichlorotoluene after weight removal) in the step (1) into an extractive distillation tower for extractive distillation, extracting 2, 5-dichlorotoluene (the purity of the 2, 5-dichlorotoluene is 99.78%) from the tower top, and extracting a mixture of the dichlorotoluene and the extractant from the tower bottom; the theoretical plate number of the extractive distillation tower is 100, the feeding positions of the extractant and the de-weighted mixed dichlorotoluene are respectively 3 rd and 36 th, the feeding temperature of the extractant is 100 ℃, the mass ratio of the de-weighted mixed dichlorotoluene to the extractant is 1.0: 10.0, the temperature of a tower bottom is 145 ℃, the temperature of a tower top is 138 ℃, the pressure of the tower is-0.90 bar, and the reflux ratio is 20;

(3) desorbing: feeding the tower bottom liquid (mixture of dichlorotoluene and extractant) in the step (2) into a desorption tower for desorption, extracting mixed dichlorotoluene (the content of 2, 4-dichlorotoluene is 98.68 percent) from the top of the desorption tower, and recycling the extractant extracted from the tower bottom; the theoretical plate number of the desorption tower is 50, the temperature of a tower kettle is 165 ℃, the temperature of a tower top is 140 ℃, the pressure of the tower is-0.90 bar, and the reflux ratio is 6;

GC analysis is carried out on the components of the sample before the extractive distillation in the step (2), the overhead fraction after the extractive distillation and the overhead fraction after the desorption in the step (3), and the analysis results are shown in the following table 4:

TABLE 4 analysis table for dichlorotoluene components before and after extractive distillation and desorption

。

Example 3

(1) Removing weight: feeding the mixed dichlorotoluene into a rectifying tower with a theoretical plate number of 130 for weight removal, wherein the temperature of a tower kettle is 145 ℃, the temperature of a tower top is 140 ℃, the pressure of the tower is-0.95 bar, and the reflux ratio is 5; GC analysis is adopted for the contents of the raw materials, the components at the top of the tower and the components at the bottom of the tower, and the results are shown in the following table 5;

TABLE 5 analysis table for dichlorotoluene components before and after weight removal

(2) And (3) extraction and rectification: taking N-N-butyl pyrrolidone as an extractant, feeding the tower bottom liquid (the mixed dichlorotoluene after weight removal) in the step (1) into an extractive distillation tower for extractive distillation, extracting 2, 5-dichlorotoluene (the purity of the 2, 5-dichlorotoluene is 99.88%) from the tower top, and extracting a mixture of the dichlorotoluene and the extractant from the tower bottom; the theoretical plate number of the extractive distillation tower is 100, the feeding positions of the extractant and the de-weighted mixed dichlorotoluene are respectively 3 rd and 36 th, the feeding temperature of the extractant is 90 ℃, the mass ratio of the de-weighted mixed dichlorotoluene to the extractant is 1.0: 7.0, the temperature of a tower bottom is 135 ℃, the temperature of a tower top is 132 ℃, the pressure of the tower is-0.95 bar, and the reflux ratio is 12;

(3) desorbing: feeding the tower bottom liquid (mixture of dichlorotoluene and extractant) in the step (2) into a desorption tower for desorption, extracting mixed dichlorotoluene (the content of 2, 4-dichlorotoluene is 98.78%) from the top of the desorption tower, and recycling the extractant extracted from the tower bottom; the theoretical plate number of the desorption tower is 50, the temperature of the tower kettle is 158 ℃, the temperature of the tower top is 138 ℃, the pressure of the tower is-0.95 bar, and the reflux ratio is 4;

GC analysis is carried out on the components of the sample before the extractive distillation in the step (2), the overhead fraction after the extractive distillation and the overhead fraction after the desorption in the step (3), and the analysis results are shown in the following table 6:

TABLE 6 analysis table of mixed dichlorotoluene components before and after extractive distillation and desorption

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