A method for extracting indole from coal-tar wash oil

Abstract

The present application relates to a kind of method for extracting indole from coal tar wash oil, it is with coal tar wash oil as raw material, first low concentration acid wash removes quinoline salt base, then through phase transfer catalyst makes indole and 30%~35% of sulfuric acid concentration successfully reaction generates indole sulfate, after neutralization, industrial grade indole product is obtained by rectification.Indole extraction rate is improved by the addition of phase transfer catalyst, avoids the generation of indole oligomer, while taking into account the production of beta-methylnaphthalene, the method has many advantages such as simple operation, good product quality, high yield and the like.

Description

Technical Field

[0001] The indole in this invention is a deep-processed product of coal tar, belonging to the field of coal tar deep processing technology, and in particular, a method for extracting indole from coal tar wash oil. Background Technology

[0002] Indole is an important component of coal tar and has a wide range of uses. Indole can be used as a flavor preservative, in the synthesis of tryptophan, which promotes animal growth, and in the synthesis of plant growth hormones such as β-indoleacetic acid and β-indolepropionic acid.

[0003] Indole is mainly found in coal tar wash oil, with a mass fraction of 1.73% (Coal Tar Chemical Engineering, 2nd Edition, Xiao Ruihua, p. 164). Besides indole, wash oil also contains important components such as β-methylnaphthalene, α-methylnaphthalene, quinoline, biphenyl, acenaphthene, and fluorene. Indole (boiling point 253℃) and biphenyl (boiling point 254.9℃) have similar boiling points, making them difficult to separate by distillation. Although indole and β-methylnaphthalene (boiling point 241℃) have significantly different boiling points, indole and β-methylnaphthalene can form an azeotropic mixture, which is also difficult to separate by distillation.

[0004] In his article "An Overview of Research on Indole Recovery from Coal Tar Wash Oil Fractions," Xiao Ruihua systematically describes indole extraction methods, including alkali fusion, azeotropic distillation, solvent extraction, complexation, and acid polymerization. In his conclusion, he points out that "dual-solvent extraction" and "acid polymerization" are relatively feasible methods for industrialization. The advantage of "dual-solvent extraction" is that, using selective extractants and suitable extraction equipment and operating conditions, it can yield high-purity indole with a high recovery rate. Its main disadvantage is that the selectivity of currently used extractants is not ideal, and the miscibility of polar and non-polar solvents leads to loss of the target product and difficulty in solvent recovery, resulting in high production costs, especially when the indole content in the raw material wash oil is low. "Acid polymerization" includes three steps: acid polymerization, washing and neutralization, and thermal decomposition. Its results are considered a trade secret. Its advantage is that it can completely recover indole regardless of the indole content in the raw material, typically achieving a one-step recovery rate of over 80% (for raw material wash oil), under mild conditions and with a simple process.

[0005] Patent CN1974552A describes an azeotropic distillation method. Azeotropic distillation involves distilling the feedstock and the azeotropic agent simultaneously, resulting in high energy consumption. To reduce energy consumption, narrow fractions of feedstock are typically used. This patent uses residual oil from β-methylnaphthalene production as feedstock, meaning that indole and β-methylnaphthalene are not separated during β-methylnaphthalene production. Because they exhibit azeotropic properties during distillation, the quality of the β-methylnaphthalene product is low, and some indole is lost within the β-methylnaphthalene, affecting the indole yield. Patent CN1424311A is also an azeotropic distillation method using industrial methylnaphthalene as feedstock, which consumes even more energy. Patent CN1746159A is a dual-solvent extraction method, a relatively mature industrial method. It is suitable for feedstocks with high indole content; otherwise, solvent loss is high and production costs are high. Therefore, this patent also uses residual oil from β-methylnaphthalene production as feedstock. Patent CN103951604A essentially represents the highest level of the dual-solvent extraction method. Its examples clearly demonstrate that the product quality using residual oil from the production of β-methylnaphthalene as raw material is significantly superior to that using mixed methylnaphthalene as raw material. Patent CN104876847A is an improvement on the acid polymerization method. Using methylnaphthalene fraction as raw material, the acid concentration is controlled to obtain indole oligomers. The indole oligomers are separated from the quinoline base solution through freeze crystallization. After depolymerization of the indole oligomers, crude indole with 90% purity and an indole yield of over 90% are obtained.

[0006] In his master's thesis, "Study on the Extraction of Indole from Coal Tar Wash Oil," Chen Xiaoping conducted a detailed discussion on the steps and conditions for extracting indole by acid polymerization. Using wash oil as raw material, indole oil with a content of 73% was obtained through four steps: extraction of quinoline, indole acid polymerization, washing and neutralization of acid polymer salt, and thermal decomposition of acid polymer. The yield was 77%, and the indole extraction rate exceeded 96%.

[0007] The above literature indicates that the "dual-solvent extraction method" is more suitable for using narrow fractions with high indole content, such as β-methylnaphthalene residue oil, as raw materials, while the "acid polymerization method" is more suitable for using fractions with low indole content, such as wash oil. Although the "acid polymerization method" has a high indole extraction rate, which facilitates the subsequent production of β-methylnaphthalene products, the thermal decomposition of indole oligomers into indole monomers is difficult and results in significant indole loss.

[0008] The *Encyclopedia of Chinese Metallurgy—Coking Chemical Industry* (Metallurgical Industry Press, 1992, p. 404) describes the extraction of indole using sulfuric acid washing. This involves adding 30-35% sulfuric acid to the wash oil, reacting to obtain sulfates of indole and quinoline. Neutralization with alkali precipitates indole and quinoline. First, distillation is used to separate indole from quinoline, concentrating the indole in a fraction at 250-265℃. Then, 10-20% sulfuric acid is added to this fraction to wash away the quinoline (indole does not react at this acidity). Simultaneously, a suitable amount of benzene is added to obtain a benzene solution of indole. This solution is distilled to remove the benzene, yielding crude indole. Further distillation, crystallization, and pressing yield industrial-grade indole.

[0009] The sulfuric acid washing method yields indole monomers after acid washing and alkali neutralization, without the formation of indole oligomers. Therefore, thermal decomposition is unnecessary, resulting in minimal indole loss. However, the inventors' experiments revealed a strong correlation between the indole extraction rate from the wash oil and the sulfuric acid concentration. Higher concentrations lead to higher extraction rates. Sulfuric acid concentrations below 20% show minimal reaction with indole. At 30-35% concentrations, indole undergoes neutralization to form indole sulfate, but the extraction rate is low. Concentrations exceeding 50% result in polymerization, forming oligomers. Higher concentrations lead to higher indole extraction rates, but also to deeper polymerization and greater losses during thermal decomposition. Therefore, while the sulfuric acid washing method avoids thermal decomposition and minimizes indole loss, its extraction rate is limited. The inventors' experimental results indicate an extraction rate of approximately 60%, meaning that about 40% of indole remains in the wash oil, resulting in reduced indole yield and impacting the quality of the β-methylnaphthalene product.

[0010] Based on a summary of the advantages and disadvantages of the above technologies, this invention develops a new indole extraction method using wash oil as raw material. This method not only improves the indole extraction rate from wash oil and increases the indole yield while ensuring the quality of β-methylnaphthalene production, but also avoids the formation of oligomers, thus preventing the loss of indole due to thermal decomposition. It has many advantages, such as simple operation, high indole purity, and high yield. Summary of the Invention

[0011] The purpose of this invention is to provide a method for extracting indole, which uses coal tar wash oil as raw material. First, the quinoline salt is removed by low-concentration acid washing. Then, indole and sulfuric acid with a concentration of 30% to 35% are reacted smoothly through a phase transfer catalyst to generate indole sulfate. After neutralization, the product is obtained by distillation to obtain industrial-grade indole.

[0012] The technical solution of this invention consists of the following steps, and its technical roadmap is shown in the figure. Figure 1 :

[0013] Step 1. Add the wash oil and a sulfuric acid solution with a mass fraction of 15% to 20% to a washing machine equipped with a stirrer at a mass ratio of 100:20 to 25. Stir for 20 to 30 minutes, let stand and separate into layers to obtain the upper layer of wash oil after washing and removal of quinoline salt groups and the lower layer of quinoline sulfate solution. The quinoline sulfate solution can be further used to produce quinoline products.

[0014] Step 2. Add the quinoline-free wash oil obtained in Step 1, a 30%–35% sulfuric acid solution, and the phase transfer catalyst polyethylene glycol-400 to a stirred scrubber in a mass ratio of 100:5:0.1–0.2. Heat the mixture to 70°C–80°C, stir for 2–3 hours, and allow it to stand to separate into layers. The upper layer is de-indole wash oil, and the lower layer is a sulfuric acid-indole solution. The de-indole wash oil can be used for benzene washing or further processing.

[0015] Step 3. Add the indole sulfate solution obtained in Step 2 and the solvent toluene in a mass ratio of 5:1 into the neutralizer. Under stirring, add 15% sodium carbonate solution dropwise to carry out the neutralization reaction. Stop adding sodium carbonate solution when the pH value of the reaction solution reaches 7-8. Continue stirring for 30-60 minutes and let it stand to separate into layers. The lower layer is sodium sulfate solution and the upper layer is toluene solution of indole. After recovering the solvent toluene, crude indole is obtained.

[0016] Step 4. The crude indole obtained in Step 3 is subjected to intermittent vacuum distillation to obtain an indole product with a mass fraction greater than 98%.

[0017] The intermittent vacuum distillation column is a packed column with 80 to 100 theoretical plates, a top vacuum of -0.095 to -0.085 MPa, a reflux ratio of 20 to 30, and a top outlet temperature of 180℃ to 200℃.

[0018] Wash oil contains various components such as indole, β-methylnaphthalene, α-methylnaphthalene, quinoline, isoquinoline, and biphenyl. These components are widely used products, but they are impurities to each other, interfering with each other during separation and purification. For example, indole and biphenyl have similar boiling points, and indole and β-methylnaphthalene are azeotropic. Therefore, a comprehensive approach is needed to achieve the best results. The indole extraction method developed in this invention retains the advantages of sulfuric acid washing (no polymerization reaction, no thermal decomposition, and minimal indole loss) and the high indole extraction rate of acid polymerization, while also being relatively simple to operate.

[0019] The main technical improvement of this invention is to first separate the quinoline base and indole, as well as other components of the wash oil, using sulfuric acid with a concentration of 15%–20%. Indole does not react much, thus achieving the separation of the quinoline base and indole without losing indole. Then, through a phase transfer catalyst, the indole reacts smoothly with sulfuric acid with a concentration of 30%–35%, thereby increasing the indole extraction rate. This ensures that indole does not interfere with the subsequent purification of β-methylnaphthalene and biphenyl, and that indole does not polymerize, avoiding thermal decomposition that would increase operational complexity and indole loss. Attached Figure Description

[0020] Figure 1 This is a technical roadmap for the present invention. Implementation

[0021] The present invention is further illustrated by the following examples: Example 1

[0022] Gas chromatography analysis revealed that the mass fraction of quinoline salts in coal tar wash oil was 4.98%, and the mass fraction of indole was 1.68%. (At 5m...) 3 2000 kg of coal tar wash oil and 500 kg of 15% sulfuric acid solution were added to the scrubber in a mass ratio of 100:25. After stirring for 20 minutes, the mixture was allowed to stand for 60 minutes. First, 600 kg of the lower layer of quinoline sulfate solution was released, followed by 1900 kg of wash oil with quinoline sulfate removed. The quinoline sulfate solution can be further used to produce quinoline products.

[0023] At 5m 3 1900 kg of quinoline-free wash oil, 95 kg of 30% sulfuric acid solution, and 1.9 kg of polyethylene glycol-400 phase transfer catalyst were added to the scrubber in a mass ratio of 100:5:0.1. The mixture was heated to 70°C, stirred for 2 hours, and allowed to stand for 60 minutes. First, 129.9 kg of the lower layer of sulfuric acid-indole solution was released, followed by 1867 kg of the upper layer of quinoline-free and indole-free wash oil. Gas chromatography analysis showed that the indole content in the wash oil was trace and virtually undetectable.

[0024] In a 500L neutralizer, 129.9 kg of indole sulfate solution and 26 kg of toluene obtained earlier were added in a mass ratio of 5:1. Under stirring, a 15% sodium carbonate solution was added dropwise. At this time, a large number of bubbles were overflowed. The pH value of the reaction solution was measured. When the pH value reached 7, the addition of sodium carbonate solution was stopped. Stirring was continued for 30 min, and then the mixture was allowed to stand for 60 min to separate into layers. The lower layer of sodium sulfate solution was released first, followed by 60 kg of the upper layer of indole-toluene solution. After recovering the solvent toluene, 34 kg of crude indole was obtained. Gas chromatography analysis showed that the purity (mass fraction) of the crude indole was 94.81%, and the indole yield was 95.9% based on the indole in the wash oil.

[0025] The crude indole was distilled using a batch vacuum distillation apparatus. The distillation vessel had a volume of 50 L, and the distillation column was packed with triangular spiral packing with 80 theoretical plates. 34 kg of the previously obtained crude indole was added to the distillation vessel, heating was started, and the vacuum pump was activated to maintain a vacuum of -0.09 MPa. The reflux ratio was set to 20. The fraction collected from the top of the column at temperatures between 185℃ and 195℃ yielded 26.4 kg of indole product with a purity of 98.06%. Example 2

[0026] Add 2000 kg of the same wash oil as in Example 1 and 440 kg of 17% sulfuric acid solution to the scrubber in a mass ratio of 100:22. Stir for 25 min and let stand for 60 min. First, release 540 kg of the lower layer of quinoline sulfate solution, then release 1900 kg of wash oil with quinoline sulfate removed. Add the previously obtained 1900 kg of quinoline sulfate-removed wash oil, 95 kg of 32% sulfuric acid solution, and 2.8 kg of phase transfer catalyst polyethylene glycol-400 to the scrubber in a mass ratio of 100:5:0.15. Heat to 75°C, stir for 2.5 h, and let stand for 60 min. First, release 130.8 kg of the lower layer of indole sulfate solution, then release 1867 kg of the upper layer of wash oil with quinoline sulfate and indole removed. The indole content in the wash oil was undetectable. 130.8 kg of indole sulfate solution and 26.2 kg of toluene obtained earlier were added to the neutralizer. Under stirring, a 15% sodium carbonate solution was added dropwise. When the pH value reached 7, the dropwise addition was stopped, and stirring was continued for 45 min. After standing for 60 min, the lower layer of sodium sulfate solution was released first, followed by the upper layer of indole-toluene solution, 60.4 kg. After recovering the toluene, 34.2 kg of crude indole was obtained with a purity of 94.85% and an indole yield of 96.5%.

[0027] The same distillation apparatus as in Example 1 was used. The 34.2 kg of crude indole obtained earlier was added to the distillation vessel, heating was started, and the vacuum pump was started to maintain the vacuum degree of the system at -0.09 MPa. The reflux ratio was set to 25, and the fraction with a temperature of 185℃ to 195℃ from the top of the column was taken to obtain 27.1 kg of industrial-grade indole with a purity of 98.15%. Example 3

[0028] Add 2000 kg of the same wash oil and 400 kg of 20% sulfuric acid solution to the scrubber in a mass ratio of 100:20. Stir for 30 min and let stand for 60 min. First, release 500 kg of the lower layer of quinoline sulfate solution, then release 1900 kg of wash oil with quinoline sulfate removed. Add the previously obtained 1900 kg of quinoline sulfate-removed wash oil, 95 kg of 35% sulfuric acid solution, and 3.8 kg of phase transfer catalyst polyethylene glycol-400 to the scrubber in a mass ratio of 100:5:0.2. Heat to 80℃, stir for 3 h, and let stand for 60 min. First, release 131.8 kg of the lower layer of indole sulfate solution, then release 1867 kg of the upper layer of wash oil with quinoline sulfate and indole removed. The indole content in the wash oil was undetectable. 131.8 kg of indole sulfate solution and 26.4 kg of toluene obtained earlier were added to the neutralizer. A 15% sodium carbonate solution was added dropwise under stirring. Once the pH reached 7, the addition was stopped, and stirring continued for 60 minutes, followed by standing for 60 minutes. The lower layer of sodium sulfate solution was first discharged, followed by the upper layer of indole-toluene solution (60.9 kg). Toluene was recovered to obtain 34.5 kg of crude indole with a purity of 94.89% and an indole yield of 97.4%. Using the same distillation apparatus, 34.5 kg of crude indole was added to the distillation vessel. Heating was started, and the vacuum pump was activated to maintain a vacuum of -0.09 MPa. The reflux ratio was set to 30. The fraction collected from the top of the column at a temperature of 185℃–195℃ yielded 27.9 kg of industrial-grade indole with a purity of 98.34%.

Claims

1. A method for extracting indole from coal tar wash oil, characterized in that... Includes the following steps: Step 1. Add the wash oil and a sulfuric acid solution with a mass fraction of 15% to 20% to a washing machine equipped with a stirrer at a mass ratio of 100:20 to 25. Stir for 20 to 30 minutes, let stand and separate into layers to obtain the upper layer of wash oil after washing and removal of quinoline salt groups and the lower layer of quinoline sulfate solution. The quinoline sulfate solution can be further used to produce quinoline products. Step 2. Add the quinoline-free wash oil obtained in Step 1, a 30%–35% sulfuric acid solution, and the phase transfer catalyst polyethylene glycol-400 to a stirred scrubber in a mass ratio of 100:5:0.1–0.

2. Heat the mixture to 70°C–80°C, stir for 2–3 hours, and allow it to stand to separate into layers. The upper layer is de-indole wash oil, and the lower layer is a sulfuric acid-indole solution. The de-indole wash oil can be used for benzene washing or further processing. Step 3. Add the indole sulfate solution obtained in Step 2 and the solvent toluene in a mass ratio of 5:1 into the neutralizer. Under stirring, add 15% sodium carbonate solution dropwise to carry out the neutralization reaction. Stop adding sodium carbonate solution when the pH value of the reaction solution reaches 7-8. Continue stirring for 30-60 minutes and let it stand to separate into layers. The lower layer is sodium sulfate solution and the upper layer is toluene solution of indole. After recovering the solvent toluene, crude indole is obtained. Step 4. The crude indole obtained in Step 3 is subjected to intermittent vacuum distillation to obtain an indole product with a mass fraction greater than 98%.

2. A method for extracting indole from coal tar wash oil as described in claim 1, characterized in that... The intermittent vacuum distillation column in step 4 is a packed column with 80 to 100 theoretical plates, a top vacuum of -0.095 to -0.085 MPa, a reflux ratio of 20 to 30, and a top outlet temperature of 180℃ to 200℃.

Images