Coal chemical phenolic wastewater extraction and solvent recovery process

Abstract

The application discloses a coal chemical phenol-containing wastewater extraction and solvent recovery process. After the phenol-containing wastewater is separated by an extraction tower, the extract is separated into light oil, heavy aromatic hydrocarbon and crude phenol by an extract separation tower. Part of the light oil is refluxed to the extract separation tower, and the other part is washed by a water washing tower to obtain light oil products. Part of the crude phenol is refluxed to the extract separation tower, and the other part is directly used as crude phenol products. After the raffinate wastewater is separated by a water tower and an oil-water separator, the extraction solvent is recovered, the treated wastewater is sent to biochemical treatment for further treatment or is directly reused. The process can improve the content of phenol and homologues in the crude phenol, reduce the content of neutral oil, and make the crude phenol products meet the standard requirements. Meanwhile, the process can also remove light oil and other impurities in the extraction solvent in time, thereby improving the quality of the recovered extraction solvent, and further improving the extraction treatment effect of the wastewater and reducing the treatment load of the biochemical device.

Description

Technical Field

[0001] This invention belongs to the field of wastewater treatment technology, specifically relating to an extraction and solvent recovery process for phenol-containing wastewater from coal chemical industry. This process further improves effluent quality and solves wastewater pollution problems, while simultaneously recovering and purifying phenolic substances in the wastewater into qualified crude phenol that meets standard requirements. Background Technology

[0002] my country is rich in coal resources, with over 50% being low-rank coal. The processing and utilization of low-rank coal is of great significance for ensuring my country's energy security and promoting economic development. During the gasification and dry distillation (pyrolysis) of low-rank coal, large amounts of high-concentration phenol-containing wastewater are generated, with phenol content reaching 5000–25000 mg / L. On the one hand, phenols have strong biological toxicity and pose significant environmental risks; on the other hand, phenols are important chemical products and raw materials with high economic value. Therefore, utilizing phenol recovery devices and employing liquid-liquid extraction technology to extract phenols from wastewater as crude phenol byproducts, while simultaneously reducing the biological toxicity of the wastewater and improving its biodegradability, is a common practice in the current coal chemical industry. The extracted crude phenol must meet the requirements of standard YB / T 5079-2012, namely, the content of phenol and its homologues in the crude phenol should not be less than 83%, and the neutral oil content should not exceed 0.8%. Currently used extraction solvents mainly include methyl isobutyl ketone, diisopropyl ether, and methylpentenone.

[0003] Because coal chemical wastewater has a very complex composition, during the extraction process, in addition to various phenols, aliphatic hydrocarbons, benzenes, naphthalenes, and other organic compounds are also extracted and enter the extract. Current phenol recovery equipment typically uses binary distillation to separate the extract into solvent and crude phenol. This configuration, for conversion processes with high-temperature dry distillation sections or when using highly coalified coal as raw material, generally reduces the COD of the treated wastewater to below 3000 mg / L, and the crude phenol quality meets the requirements of standard YB / T 5079-2012. However, when using less coalified lignite as raw material and employing medium- and low-temperature dry distillation to produce semi-coke, the above solvent recovery process often results in high oil content and COD in the effluent, and the homologue content in the crude phenol is far below 83%, with the neutral oil content in the crude phenol often far exceeding the standard value. Analysis of the extract composition revealed that the extract from the semi-coke production process is more complex than that from gasification wastewater, especially the components with distillation ranges of 130–160℃ and above 210℃, which have significantly higher contents. When binary distillation is used for solvent recovery, the content of neutral oils, primarily naphthalene, in the crude phenol often reaches over 1.5%, while the oil content in the solvent can also reach 1%–2%, leading to reduced extraction efficiency. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides a process for extracting and recovering solvents from phenol-containing wastewater in coal chemical industry. This process can increase the content of phenols and homologues in crude phenols and improve the purity of the recovered extraction solvent, thereby improving the wastewater treatment effect.

[0005] The coal chemical phenol-containing wastewater extraction and solvent recovery process provided by this invention includes the following steps:

[0006] Step 1: Extraction

[0007] The phenol-containing wastewater is divided into two parts. Most of the phenol-containing wastewater enters the extraction tower and comes into contact with the extraction solvent from the solvent circulation tank to complete the extraction process. The other part of the phenol-containing wastewater enters the water washing tower. The extract obtained after extraction overflows into the extract tank and is then pumped to the extract separation tower for separation of the extract. The raffinate wastewater after extraction is pumped from the bottom of the extraction tower to the water tower for stripping to recover the dissolved and entrained extraction solvent in the wastewater.

[0008] Step 2: Extract separation

[0009] The extract obtained in step 1 enters the feed inlet of the extract separation tower, and the extraction solvent is collected from the top of the extract separation tower. The light oil collected from the first side outlet of the extract separation tower is cooled and enters the light oil tank. Part of it is returned to the extract separation tower, and the other part is sent to the water washing tower for washing. The crude phenol collected from the second side outlet of the extract separation tower is cooled and enters the crude phenol tank 8. Part of it is returned to the extract separation tower, and the other part is sent to the crude phenol product tank. The heavy aromatics in the tower bottom are cooled and sent to the heavy aromatics product tank.

[0010] Step 3: Solvent stripping

[0011] The effluent obtained in step 1 enters from the top of the water tower. After the effluent from the top of the water tower is condensed, it enters the oil-water separator for oil-water separation. The upper layer of extraction solvent overflows into the solvent circulation tank for recycling, while the lower layer of aqueous phase is returned to the water tower for re-stripping. The treated wastewater discharged from the bottom of the water tower is sent to the biochemical plant for further treatment or directly reused.

[0012] Step 4: Light oil wash

[0013] In step 2, the light oil entering the water washing tower comes into countercurrent contact with a small portion of the phenol-containing wastewater that entered the water washing tower in step 1 to complete the water washing. The washed light oil is then sent to the light oil product tank. The extraction solvent entrained in the light oil is washed off into the phenol-containing wastewater, which is then pumped from the bottom of the water washing tower to the extraction tower for extraction treatment.

[0014] Furthermore, in step 1 above, it is preferable that the phenol-containing wastewater is divided into two parts at a split ratio of 1:10 to 24.

[0015] Furthermore, in step 1 above, the extraction solvent is preferably one of methyl isobutyl ketone, methyl pentenone, or a mixture of both.

[0016] Furthermore, in step 1 above, the extraction temperature of the extraction tower is preferably 30–80°C.

[0017] Furthermore, in step 2 above, the operating pressure of the extract separation tower is preferably -0.09 to 0.03 MPa, the number of theoretical plates is 20 to 30, and the feed inlet is located at the 8th to 17th theoretical plate.

[0018] Furthermore, in step 2 above, the first side-stream outlet is preferably located at the 3rd to 6th theoretical plate below the feed inlet. The temperature of the first side-stream outlet is 65 to 185°C. After the light oil is extracted, it is cooled to 30 to 60°C and then enters the light oil tank. A portion is refluxed to the extract separation tower with a reflux ratio of 0.2 to 1.0, and the other portion is sent to the water washing tower for washing.

[0019] Furthermore, in step 2 above, the second side stream outlet is preferably located at the 6th to 14th theoretical plate below the feed inlet, and the temperature of the second side stream outlet is 120 to 215°C. After the crude phenol is collected, it is cooled to 60 to 80°C and then enters the crude phenol tank. Part of it is refluxed to the extract separation tower with a reflux ratio of 0.2 to 0.3, and the other part is sent to the crude phenol product tank.

[0020] Furthermore, in step 2 above, the preferred temperature at the top of the column is 46–115°C, and the extraction solvent is collected from the top of the column; the temperature at the bottom of the column is 160–245°C, and the heavy aromatics in the bottom of the column are cooled and sent to the heavy aromatics product tank.

[0021] Furthermore, in step 4 above, the theoretical number of stages of the water washing tower is preferably 1 to 4.

[0022] The beneficial effects of this invention are as follows:

[0023] This invention involves extracting and separating phenol-containing wastewater using an extraction tower. The extract is then separated into light oil, heavy aromatics, and crude phenol using an extractant separation tower. A portion of the light oil is returned to the extractant separation tower, while the remainder is washed in a water washing tower and sent to a light oil product tank. A portion of the crude phenol is returned to the extractant separation tower, while the remainder is sent to a crude phenol product tank. The remaining wastewater is collected in a water tower and separated by an oil-water separator. The extraction solvent is recovered, and the treated wastewater is sent to a biochemical facility for further treatment or directly reused. This invention utilizes a water washing tower to increase the content of phenol and its homologues in the crude phenol and reduce the content of neutral oil, ensuring the crude phenol product meets standard requirements. Simultaneously, the water washing tower improves the quality of the recovered extraction solvent, promptly removing impurities such as light oil, thereby improving the wastewater extraction treatment effect and reducing the processing load on the biochemical unit. Attached Figure Description

[0024] Figure 1 This is a process flow diagram of the present invention. In the diagram, 1 is an extract tank, 2 is an extraction tower, 3 is a water tower, 4 is an oil-water separator, 5 is an extract separation tower, 6 is a light oil tank, 7 is a water washing tower, 8 is a crude phenol tank, and 9 is a solvent circulation tank. Detailed Implementation

[0025] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments, but the scope of protection of the present invention is not limited to these embodiments.

[0026] Example 1

[0027] A coal pyrolysis plant generates 120 tons / hour of phenol-containing wastewater with a phenol content of 15000 mg / L. According to... Figure 1 The process involves extraction and solvent recovery, and the specific steps are as follows:

[0028] Step 1: Extraction

[0029] The phenol-containing wastewater is divided into two parts with a split ratio of 1:10. Most of the phenol-containing wastewater enters the extraction tower 2, while a small portion enters the water washing tower 7. The phenol-containing wastewater entering the extraction tower 2 comes into contact with the extraction solvent methyl isobutyl ketone from the solvent circulation tank 9 to complete the extraction process at an extraction temperature of 30–40°C. The extracted extract overflows into the extract tank 1 and is then pumped to the extract separation tower 5 for extract separation. The raffinate wastewater after extraction is pumped from the bottom of the extraction tower 2 to the water tower 3 for stripping to recover the dissolved and entrained extraction solvent from the wastewater.

[0030] Step 2: Extract separation

[0031] The extract obtained in step 1 enters the upper part of the extractant separation column 5 for distillation and separation. The operating pressure of the extractant separation column 5 is -0.09 MPa, with 20 theoretical plates, and the feed inlet is located at the 8th theoretical plate. The extraction solvent is collected from the top of the extractant separation column 5 at a temperature of 46–48°C. The first side stream outlet of the extractant separation column 5 is located at the 3rd theoretical plate below the feed inlet, with a temperature of 65°C. After being collected, the light oil is cooled to 30°C and then enters the light oil tank 6. A portion of the light oil in the light oil tank 6 is refluxed back to the extractant separation column 5 at a reflux ratio of 0.3, while the other portion is sent to the water washing column 7 for washing. The second side stream outlet of the extractant separation column 5 is located at the 7th theoretical plate below the feed inlet, with a temperature of 120–124°C. After being collected, the crude phenol is cooled to 60–80°C and then enters the crude phenol tank 8. A portion of the crude phenol in crude phenol tank 8 is refluxed to extractant separation tower 5 at a reflux ratio of 0.2, while the other portion is sent to the crude phenol product tank. The bottom temperature of extractant separation tower 5 is 160–162°C, and the cooled heavy aromatics in the bottom are sent to the heavy aromatics product tank.

[0032] Step 3: Solvent stripping

[0033] The effluent obtained in step 1 enters from the top of water tower 3. After the effluent from the top of water tower 3 is condensed, it enters oil-water separator 4 for oil-water separation. The upper layer of extraction solvent overflows into solvent circulation tank 9 for recycling; the lower layer of aqueous phase returns to water tower 3 for re-stripping; the wastewater discharged from the bottom of water tower 3 is the treated wastewater, which is sent to biochemical treatment for further treatment or directly reused.

[0034] Step 4: Light oil wash

[0035] In step 2, the light oil entering the washing tower 7 is countercurrently contacted with a small portion of the phenol-containing wastewater that entered the washing tower 7 in step 1 to complete the washing process. The washing tower 7 is configured as a liquid-liquid extraction tower, with a theoretical number of stages of 1. The washed light oil is sent to the light oil product tank; the extraction solvent entrained in the light oil is washed away into the phenol-containing wastewater, which is then pumped from the bottom of the washing tower 7 to the extraction tower 2 for phenol removal extraction.

[0036] In this embodiment, compared with the prior art, the content of phenol and homologues in the crude phenol product tank increased from about 55% to 85%, and the neutral oil content decreased from over 2.0% to below 0.5%; the oil content in the recovered solvent decreased from about 1.5% to below 0.05%; the oil content in the treated wastewater discharged from the bottom of water tower 3 decreased from 180 mg / L to below 60 mg / L, and the COD decreased from over 3800 mg / L to below 2800 mg / L. The obtained light oil and heavy aromatics also meet the corresponding product quality standards.

[0037] Example 2

[0038] A coal pyrolysis plant generates 80 tons / hour of phenol-containing wastewater with a phenol content of 20,000 mg / L. According to... Figure 1 The process involves extraction and solvent recovery, and the specific steps are as follows:

[0039] Step 1: Extraction

[0040] The phenol-containing wastewater is divided into two parts with a split ratio of 1:20. Most of the phenol-containing wastewater enters the extraction tower 2, while a small portion enters the water washing tower 7. The phenol-containing wastewater entering the extraction tower 2 comes into contact with the extraction solvent methyl isobutyl ketone from the solvent circulation tank 9 to complete the extraction process at an extraction temperature of 30–40°C. The extracted extract overflows into the extract tank 1 and is then pumped to the extract separation tower 5 for extract separation. The raffinate after extraction is pumped from the bottom of the extraction tower 2 to the water tower 3 for stripping to recover the dissolved and entrained extraction solvent.

[0041] Step 2: Extract separation

[0042] The extract obtained in step 1 enters the upper part of the extractant separation column 5 for distillation and separation. The operating pressure of the extractant separation column 5 is 0.02 MPa, with 30 theoretical plates, and the feed inlet is located at the 13th theoretical plate. The extraction solvent is collected from the top of the extractant separation column 5 at a temperature of 110–112°C. The first side stream outlet of the extractant separation column 5 is located at the 4th theoretical plate below the feed inlet, with a temperature of 135°C. After being collected, the light oil is cooled to 40°C and then enters the light oil tank 6. A portion of the light oil in the light oil tank 6 is refluxed back to the extractant separation column 5 at a reflux ratio of 0.8, while the other portion is sent to the water washing column 7 for washing. The second side stream outlet of the extractant separation column 5 is located at the 9th theoretical plate below the feed inlet, with a temperature of 210–215°C. After being collected, the crude phenol is cooled to 60–80°C and then enters the crude phenol tank 8. A portion of the crude phenol in crude phenol tank 8 is refluxed to extractant separation tower 5 at a reflux ratio of 0.3, while the other portion is sent to the crude phenol product tank. The bottom temperature of extractant separation tower 5 is 238–245°C, and the heavy aromatics in the bottom of the tower are cooled and then sent to the heavy aromatics product tank.

[0043] Step 3: Solvent stripping

[0044] The effluent obtained in step 1 enters from the top of water tower 3. After the effluent from the top of water tower 3 is condensed, it enters oil-water separator 4 for oil-water separation. The upper layer of extraction solvent overflows into solvent circulation tank 9 for recycling; the lower layer of aqueous phase returns to water tower 3 for re-stripping; the wastewater discharged from the bottom of water tower 3 is the treated wastewater, which is sent to biochemical treatment for further treatment or directly reused.

[0045] Step 4: Light oil wash

[0046] In step 2, the light oil entering the washing tower 7 is countercurrently contacted with a small portion of the phenol-containing wastewater that entered the washing tower 7 in step 1 to complete the washing process. The washing tower 7 is configured as a liquid-liquid extraction tower, with a theoretical number of 4 stages. The washed light oil is sent to the light oil product tank; the extraction solvent entrained in the light oil is washed away into the phenol-containing wastewater, which is then pumped from the bottom of the washing tower 7 to the extraction tower 2 for phenol removal extraction.

[0047] In this embodiment, compared with the prior art, the content of phenol and homologues in the crude phenol product tank increased from about 45% to 85%, and the neutral oil content decreased from over 3.0% to below 0.6%; the oil content in the recovered solvent decreased from about 2% to below 0.03%; the oil content in the treated wastewater discharged from the bottom of water tower 3 decreased from 260 mg / L to below 80 mg / L, and the COD decreased from over 4500 mg / L to below 2800 mg / L. The obtained light oil and heavy aromatics also meet the corresponding product quality standards.

Claims

1. A coal chemical phenolic wastewater extraction and solvent recovery process, characterized in that, Includes the following steps: Step 1: Extraction The phenol-containing wastewater is divided into two parts at a split ratio of 1:10 to 24. Most of the phenol-containing wastewater enters the extraction tower and comes into contact with the extraction solvent from the solvent circulation tank to complete the extraction process. The other part of the phenol-containing wastewater enters the water washing tower. The extract obtained after extraction overflows into the extract tank and is then pumped to the extract separation tower for separation of the extract. The raffinate wastewater after extraction is pumped from the bottom of the extraction tower to the water tower for stripping to recover the dissolved and entrained extraction solvent in the wastewater. Step 2: Extract separation The extract obtained in step 1 enters the feed inlet of the extractant separation tower, and the extraction solvent is collected from the top of the extractant separation tower. The first side stream outlet of the extractant separation tower is located at the 3rd to 6th theoretical plates below the feed inlet, where light oil is collected at a temperature of 65–185°C. After being cooled to 30–60°C, it enters the light oil tank, with a portion flowing back to the extractant separation tower and the other portion sent to the water washing tower for washing. The second side stream outlet of the extractant separation tower is located at the 6th to 14th theoretical plates below the feed inlet, where crude phenol is collected at a temperature of 120–215°C. After being cooled to 60–80°C, it enters the crude phenol tank, with a portion flowing back to the extractant separation tower and the other portion sent to the crude phenol product tank. The heavy aromatics in the tower bottom are cooled and sent to the heavy aromatics product tank. Step 3: Solvent stripping The raffinate obtained in step 1 enters from the top of the water tower. After the extract from the top of the water tower is condensed, it enters the oil-water separator for oil-water separation. The upper layer of extraction solvent overflows into the solvent circulation tank for recycling, and the lower layer of aqueous phase is returned to the water tower for re-stripping. The treated wastewater discharged from the bottom of the water tower is sent to biochemical treatment or directly reused. Step 4: Light oil wash In step 2, the light oil entering the water washing tower comes into countercurrent contact with a small portion of the phenol-containing wastewater that entered the water washing tower in step 1 to complete the water washing. The washed light oil is then sent to the light oil product tank. The extraction solvent entrained in the light oil is washed off into the phenol-containing wastewater, which is then pumped from the bottom of the water washing tower to the extraction tower for extraction treatment.

2. The coal chemical phenolic wastewater extraction and solvent recovery process according to claim 1, characterized in that, In step 1, the extraction solvent is one or a mixture of methyl isobutyl ketone and methylpentenone.

3. The coal chemical phenolic wastewater extraction and solvent recovery process according to claim 1, characterized in that, In step 1, the extraction temperature of the extraction tower is 30–80°C.

4. The coal chemical phenolic wastewater extraction and solvent recovery process according to claim 1, characterized in that, In step 2, the operating pressure of the extract separation tower is -0.09 to 0.03 MPa, the number of theoretical plates is 20 to 30, and the feed inlet is located at the 8th to 17th theoretical plate.

5. The coal chemical phenolic wastewater extraction and solvent recovery process according to claim 1, characterized in that, In step 2, the reflux ratio of the light oil extracted from the first side stream outlet to the extractant separator is 0.2 to 1.

0.

6. The coal chemical phenolic wastewater extraction and solvent recovery process according to claim 1, characterized in that, In step 2, the reflux ratio of the crude phenol collected from the second side stream outlet to the extract separation tower is 0.2 to 0.

3.

7. The coal chemical phenolic wastewater extraction and solvent recovery process according to claim 1, characterized in that, In step 2, the temperature at the top of the column is 46–115°C, and the extraction solvent is collected from the top of the column; the temperature at the bottom of the column is 160–245°C, and the heavy aromatics in the bottom of the column are cooled and sent to the heavy aromatics product tank.

8. The coal chemical phenolic wastewater extraction and solvent recovery process according to claim 1, characterized in that, In step 4, the theoretical number of stages in the water washing tower is 1 to 4.

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