I would be very grateful if any fellow sailor has any academic materials on the deep processing of crude benzene

Refer to this: Literature on crude benzene processing http://bbs.hcbbs.com/viewthread.php?tid=188395&highlight=%B4%D6%B1%BD%BC%D3%B9%A4

http://bbs.hcbbs.com/viewthread.php?tid=377996&highlight=%B4%D6%B1%BD

Refining of crude benzene – Distillation stage. The purpose of refining crude benzene is to process it into products such as benzene, toluene, and xylene; these are all valuable chemical raw materials. Catalytic benzene refining includes acid washing or hydrogenation, distillation separation, the processing of cyclopentadiene in the initial fractions, and the extraction and processing of cumarone and indene from the high-boiling fraction. Main products of crude benzene refining and processing methods I. Composition of crude benzene and properties of its main components Crude benzene is primarily composed of benzene, toluene, xylene, and trimethylbenzene among other benzenoid hydrocarbons, as well as unsaturated compounds and small amounts of compounds containing sulfur, nitrogen, and oxygen. The contents of these various components vary significantly depending on the quality and composition of the coal blend, as well as the conditions of the coking process. The contents and properties of the main components in crude benzene below 180°C are shown in Table 7-1 below.
Table 7-1: Contents and properties of the main components in crude benzene below 180°C

| Name | Molecular formula | Structural formula | Relative molecular mass | Relative density d204 | Boiling point at 101.3 kPa / °C | Crystallization point / °C | Refractive index n200 | Mass fraction / % |
|------|-------------------|--------------------|------------------------|-----------------------|-------------------------------|---------------------------|----------------------|------------------|
| Benzene series hydrocarbons | | | | | | | | |
| Benzene | C6H6 | | 78.112 | 0.8790 | 80.1 | 5.53 | 1.50112 | 50–80 |
| Toluene | C6H5CH3 | | 92.138 | 0.8669 | 110.6 | -95.0 | 1.49693 | 12–22 |
| o-Xylene | C6H4(CH3)2 | | 106.85 | 0.8802 | 144.4 | -25.3 | 1.50545 | 0.4–0.8 |
| m-Xylene | C6H4(CH3)2 | | 106.165 | 0.8642 | 139.1 | -47.9 | 1.49722 | 2.0–3.0 |
| p-Xylene | C6H4(CH3)2 | | 106.165 | 0.8611 | 138.35 | 13.3 | 1.49582 | 0.5–1.0 |
| Ethylbenzene | C6H5C2H5 | | 106.165 | 0.8670 | 136.2 | -94.9 | 1.49583 | 0.5–1.0 |
| Pseudocumene (1,2,5-trimethylbenzene) | C6H3(CH3)3 | | 120.09 | 0.8652 | 164.7 | -44.8 | 1.50112 | 0.2–0.4 |
| Hemimellitene (1,2,3-trimethylbenzene) | C6H3(CH3)3 | | 120.09 | 0.894 | 176.1 | -22.5 | 1.5134 | 0.05–0.15 |
| Mesitylene (1,2,4-trimethylbenzene) | C6H3(CH3)3 | | 120.09 | 0.8758 | 199.3 | -43.8 | 1.50484 | 0.15–0.3 |
| Cumene | C6H5C3H7 | | 120.09 | 0.8618 | 152.4 | -96.03 | 1.49245 | 0.03–0.05 |
| n-Propylbenzene | C6H5C3H7 (H2C-CH2-CH3) | | 120.09 | 0.8620 | 159.2 | -90.5 | 1.49202 | 0.08–0.1 |
| m-Ethyltoluene | C9H12 (CH3 C2H5) | | 120.09 | 0.8645 | 161.3 | -95.55 | 1.49660 | |
| p-Ethyltoluene | C9H12 (CH3 C2H5) | | 120.09 | 0.8612 | 162 | -62.35 | 1.49500 | |
| o-Ethyltoluene | C9H12 | | 120.09 | 0.8807 | 165.15 | -80.83 | 1.50456 | 0.03–0.05 |
| Unsaturated compounds | | | | | | | | |
| 1-Pentene | C5H10 | C3H7CH=CH2 | 70.1 | 0.642 | 30 | -165 | 1.3712 | 0.5–0.8 |
| 2-Pentene | C5H10 | C2H5CH=CHCH3 | 70.1 | 0.650 | 36.5 | -138 | 1.3798 | |
| 2-Methylbutene-2 | C5H10 | CH3C=CHCH3 | 70.1 | 0.662 | 38.5 | -133.8 | 1.3878 | |
| Cyclopentadiene | C5H6 | | 66.101 | 0.804 | 42.5 | -85 | 1.4432 | 0.5–1.0 |
| Straight-chain olefins (C6–C8) | — | — | 0.69–0.73 | 66–122 | 1.38–1.42 | 0.6 | |
| Styrene | C6H5CHCH2 | | 104.149 | 0.907 | 145.2 | -30.6 | 1.5462 | 0.5–1.0 |
| Coumarone | C8H6O S | | 118.132 | 1.051 | 172.0 | -17.8 | 1.5642 | 0.6–1.2 |
| Indene | C9H8 | | 116.160 | 0.998 | 181.6 | -1.7 | 1.5784 | 1.5–2.5 |
| Sulfides | | | | | | | | |
| Hydrogen sulfide | H2S | H-S-H | 34.081 | — | -60.4 | -85.5 | 0.2 | |
| Carbon disulfide | CS2 | S====C====S | 76.141 | 1.263 | 46.3 | -110.8 | 1.6278 | 0.3–1.5 |
| Thiophene | C4H6S S | | 84.139 | 1.064 | 84.1 | -37.1 | 1.5288 | 0.2–0.1 |
| 2-Methylthiophene (α-methylthiophene) | C5H6S S | | 98.166 | 1.025 | 112.5 | -63.5 | 1.5240 | 0.1–0.2 |
| 3-Methylthiophene (β-methylthiophene) | C5H6S S | | 98.166 | 1.026 | 114.5 | -68.6 | 1.5266 | |
| Other impurities | | | | | | | | |
| Pyridine | C5H5N N | | 79.100 | 0.986 | 115.4 | -41.7 | 1.5092 | 0.1–0.5 |
| 2-Methylpyridine | C6H7N N | | 93.126 | 0.950 | 130 | -66 | 1.5092 | |
| 3-Methylpyridine | C6H7N N | | 93.126 | 0.9564 | 144.1 | -6.1 | 1.4971 | |
| 4-Methylpyridine | C6H7N N | | 93.126 | 0.9546 | 145.3 | 3.8 | 1.504 | |
| Phenol | C6H5OH | | 94.111 | 1.072 | 181.9 | 40.84 | 1.5425 | 0.1–0.6 |
| o-Cresol | C7H8O | | 108.138 | 1.0465 | 191.5 | 30 | 1.5453 | |
| m-Cresol | C7H8O | | 108.138 | 1.034 | 201.8–202.6 | 12.3 | 1.5398 | |
| p-Cresol | C7H8O | | 108.138 | 1.0347 | 202.5 | 34.8 | 1.5395 | |
| Naphthalene | C10H8 | | 128.170 | 1.148 | 217.9 | 80.2 | 1.5822 | 0.5–2.0 |
| Saturated hydrocarbons (C6–C8) | | | 0.68–0.76 | 49.7–131.8 | 65–126.6 | 0.5–2.0 | |

Benzene, toluene, and xylene account for over 90% of the content in crude benzene; they are the main products obtained through the refining of crude benzene. Benzene hydrocarbons are fluid, flammable, water-immiscible, colorless and transparent liquids; their vapors, when mixed with air, can form explosive mixtures. Explosion range at normal temperature and pressure: 1.4%–7.1% benzene vapor ； Toluene vapor 1.4%~6.7% ； Xylene 1.0%~6.0%. The content of unsaturated compounds in crude benzene ranges from 5% to 10%, and this value depends primarily on the testing temperature of the coking coal. The higher the carbonization temperature, the lower the content of unsaturated compounds. The distribution of unsaturated compounds in the crude benzene fraction is very uneven, with them being concentrated mainly in the high-boiling fractions above 140°C and in the low-boiling fractions below 79°C. The initial distillate before 79°C mainly consists of cyclopentadiene alkanes, while the heavy benzene above 140°C primarily contains guaiacol, indene, and styrene. It also contains methylindene and dimethylindene, among others. These unsaturated compounds are mainly cyclic olefins with one or two double bonds; they readily polymerize and resinate, and easily form brown resinous substances in reaction with oxygen in the air. They dissolve in benzene-based products, causing them to turn brown. Therefore, when producing benzene, toluene, and xylene, it is necessary to remove unsaturated compounds. The sulfur compound content in crude benzene is approximately 0.6% to 2%, mainly consisting of carbon disulfide, thiophene, and their homologs. The freshly produced crude benzene still contains about 0.2% hydrogen sulfide, but during its storage, it is gradually oxidized to elemental sulfur. Thiols are also present among the sulfides in crude benzene, in very small amounts; generally, they account for no more than 0.1% of the total sulfides. Crude benzene also contains pyridine and its derivatives as well as phenols; however, due to their very low concentrations, they are not extracted as W products. Crude benzene also contains small amounts of saturated hydrocarbons, with a total content generally ranging from 0.6% to 1.5%, and these are mostly concentrated in the high-boiling fraction. Because the yield of high-boiling-point fractions is low. Therefore, the content of saturated hydrocarbons is quite significant. For example, it can reach 3%–5% in xylene fractions, thereby reducing the density of the product. Pure benzene contains 0.2%–0.8% of saturated hydrocarbons, mainly cyclohexane and heptane, which can all form azeotropic compounds with benzene. II. Methods for refining crude benzene and the yields of major products. The methods for refining crude benzene are determined based on its composition and properties, as well as the requirements regarding the types and quality of the products to be obtained. The main components of crude benzene, such as benzene, toluene, xylene, and trimethylbenzene, can be separated by distillation due to the large difference in boiling points between adjacent components. The boiling points of certain unsaturated compounds and sulfur compounds differ only slightly from those of benzene derivatives; therefore, they cannot be separated by distillation and must be separated using chemical methods. Depending on the method used to remove unsaturated compounds and sulfur compounds, the main methods for refining crude benzene are acid washing and hydrogen washing. The pickling and refining method features a simple process flow, flexible operation, low equipment investment, readily available materials, and operation at normal temperature and pressure, but it generates liquid waste. This method is widely used in coking plants in China. The hydrorefining process is complex and requires high-quality materials for the equipment. It produces products of excellent quality with a wide range of applications, resulting in high sales prices. No liquid waste is generated, which is beneficial for environmental protection; therefore, it is suitable for use in centralized benzene processing plants. This method is used in coking plants in China. The yield of crude benzene refining products is related to the properties of the raw material and the operating conditions. The yield of benzene products produced through the refining of crude benzene in large-scale coking plants is shown in Table 7-2. Table 7-2 Refined Products and Yields of Crude Benzene and Light Benzene
Product | Raw Material | Product | Raw Material
---|---|---|---
Crude Benzene | Light Benzene | Yield of Crude Benzene | Yield of Light Benzene
Yield (based on raw material) (mass fraction) % | Yield (based on raw material) (mass fraction) %
Initial distillate | Pure benzene | Toluene | Xylene | Light solvent oil | Residue from blowing off benzene
0.9 | 69.0 | 12.8 | 3.0 | 0.8 | 2.2 | 1.0 | 74.5 | 13.9 | 3.3 | 0.9 | 2.4
Refined residue | Heavy benzene | Naphtha solvent oil | Washing losses | Refining losses | Total
0.8 | 3.0 | 4.0 | 1.9 | 1.6 | 100 | 0.9 | 2.0 | 1.1 | 100

III. Uses and Quality of the Main Products Obtained from Crude Benzene Refinement
The main products obtained from crude benzene refinement are benzene, toluene, xylene, and trimethylbenzene (light solvent oil). Benzene is the most significant component of crude benzene, accounting for 55%~80% of its content. Benzene is a colorless, volatile, and flammable liquid with an oily aromatic odor; it is insoluble in water but soluble in ethanol. Benzene is a fundamental raw material in the organic synthesis industry, with extremely wide applications. In China, it is currently mainly used in synthetic fibers, plastics, synthetic rubber, the production of pesticides, and the defense industry. The main uses of benzene are shown in Figure 7-1. Toluene has a yield second only to benzene; it can be used through methods such as chlorination, nitration, sulfonation, oxidation, and reduction to produce intermediates for dyes, pharmaceuticals, fragrances, etc., as well as saccharin. Additionally, it can be used to manufacture caprolactam for the production of nylon. Toluene has a very low freezing point (-95°C) and can be used as a dye in aerospace applications as well as an additive in internal combustion engine fuels. Xylene: Industrial xylene obtained through the purification of crude benzene is a mixture of p-xylene (21%), o-xylene (16%), m-xylene (50%), and ethylbenzene (7%). Industrial xylene can be used as a solvent in the rubber and paint industries, as well as an additive in aerospace and fuel applications. o-, m-, and p-xylene obtained from industrial xylene can be used to produce o-, m-, and p-terephthalic acid, of which o- and p-terephthalic acid are important raw materials for manufacturing plasticizers and polyester fibers. Solvent oil is a mixture that emerges during the distillation of crude benzene at temperatures between 145–180°C, and its composition is roughly as follows: ； Xylene 25%~40% ； Aliphatic hydrocarbons and naphthenes 8%~15% ； Propylbenzene and isopropylbenzene 10%~15% ； Cumene 10%~15% ； Trimethylbenzene 12%~20%. It is primarily used as a solvent in the paint and dye industries, and can also serve as a raw material for producing the isomers of xylene and trimethylbenzene. The toluene isomers separated from solvent oil can be used in the production of aniline dyes, pharmaceuticals, and more. 2,4,5-T (herbicide); p-***phenol—pesticide E605; pesticides—DDT (insecticide); p-aminobenzyl ether—dye viridion (VB); engineering plastics—silicone resins (high-temperature resistant insulating materials); p-phenylenediamine—dyes such as Fur Black D; epoxy resin coagulant; p-ethoxyacetylaniline—phenacetin (antipyretic and analgesic); o-aminochlorobenzene—dyes such as yellow salt GC; rubber accelerator M; solvent ***chlorobenzene; vanillin—flavoring; synthetic fibers; engineering plastics; synthetic detergents; pesticides; pharmaceuticals—anthelmintics, aspirin, etc.; trichlorophenol—**; benzidine—main intermediate for direct dyes; dyes; additives—antioxidants D, accelerators M; pharmaceuticals—antipyretic ice; diphenylamine—**stabilizer; p-aminobenzenesulfonic acid—weedicide for wheat; p-aminoethylbenzene—color-developing agent; dyes; m-***sodium sulfonate; p-aminosalicylic acid—drug PAS used specifically for treating lung diseases; synthetic rubber; engineering plastics. During the refining of crude benzene, in addition to benzene-based products, some unsaturated compounds and sulfides are also obtained. Synthetic resin is produced by using the initial distillate of crude benzene as raw material, through distillation and thermal polymerization. It can be used to produce the monomer cyclopentadiene. Synthetic resins can be produced through thermal polymerization of dimerics and monomers together with vegetable oils. Cyclopentadiene can also be used to produce organic pesticides and insecticides such as \"chlordane\" through chlorination polymerization. Gumaron indene is a raw material used in the production of paints, plastics, rubber products, and insulating materials. Styrene can be polymerized to produce a colorless resin used in the manufacture of insulating materials. Carbon disulfide is commonly used as a solvent in the chemical industry, as an insecticide in agriculture, as a flotation agent in mineral processing, and it can also be used to produce sulfonates. Thiophene can be used in organic synthesis as well as in the production of dyes, pharmaceuticals, and chemicals for color films. A derivative of thiophene—thiophenol trifluoropropyl ether—is also an extractant for separating the radioactive elements neptunium, plutonium, and uranium. The quality specifications for coked benzene products are shown in Tables 7-3(1), 7-3(2), 7-3(3), and 7-3(4). Table 7-3(1): Quality specifications for coked benzene. Parameter Name: Premium Grade, Grade 1, Grade 2, Grade 3. Appearance: A transparent liquid at room temperature (18–25°C); its color should not be darker than that of a solution containing 0.003 g of potassium dichromate per 1000 ml of water. Density (20°C)/g/cm³: 0.867–0.880, 0.867–0.880, 0.875–0.880, 0.874–0.880. Boiling range (at 101325 Pa atmospheric pressure, including 80.1°C)/°C: ≤ 0.7, 0.8, 0.9, 1.0. Acid wash colorimetry (compared to standard color solutions): Color should not be darker than 0.15, 0.20, 0.30, 0.40 respectively. Bromine value/g/100ml: ≤ 0.06, 0.15, 0.30, 0.40. Freezing point/°C: ≤ 5.2, 5.0, 4.9, 0.40. Carbon disulfide/g/100ml: ≤ 0.005, 0.006, 4.9, 0.40. Thiophene/g/100ml: ≤ 0.04, 0.06. Neutralization test: Neutral. Moisture: No visible insoluble water when inspected at room temperature (18–25°C). Copper sheet corrosion test: Color change should not be worse than Grade 1 (mild discoloration). Note: 1. The copper sheet corrosion test is a reference parameter. 2. Shipping is not permitted if the water layer height of benzene in the tank truck is greater than 5 mm, or if it is greater than 1 mm in the iron drum. If re-inspection upon delivery to the recipient exceeds the above regulations, it shall be agreed upon by both the supplier and the buyer. Distillation test: Indicates the degree of concentration of substances in the product. Pure substances have a definite boiling point, whereas mixtures have a range of boiling points. The purer the product, the narrower this range; the higher the initial boiling point and the lower the final boiling point. Acid washing colorimetry: Certain benzene-based products are mixed with sulfuric acid having a quality fraction of (95+05)%, and after a reaction occurs, impurities cause the sulfuric acid layer to change color. The depth of this colored layer is compared with that of a standard color solution; this is what is referred to as acid washing colorimetry. The higher the impurity content in the product, the darker the sulfuric acid coloring. Standard colorimetric solutions are potassium dichromate solutions of various concentrations prepared using a 1:1 sulfuric acid solution. There are 18 types, ranging from 0.05 to 5 g of potassium dichromate per liter. 　Bromine value: Unsaturated compounds can react with bromine. Under specified conditions, bromine reacts with a 100 ml sample of benzene-based product; the grams of bromine consumed correspond to the bromine value. The level of bromine value reflects the amount of unsaturated compounds in the sample. The bromine value method is widely used to determine the content of unsaturated compounds in benzene products produced by the light benzene washing and refining process. The content of unsaturated compounds in the benzene products obtained by catalytic hydrogenation refining is extremely low, so this parameter is not required. Crystal point: The crystal point of 100% pure benzene is 5.53°C, and the crystal point can be used to indicate the purity of the product. Table 7-3(2) Quality specifications for coked toluene Specification Name Premium Grade First Grade Second Grade Appearance Transparent liquid at room temperature (18~25°C); color not darker than that of a solution containing 0.003 g of potassium dichromate per 1000 ml of water Density (20°C)/ (g/cm3) 0.863~0.868 0.861~0.868 0.860~0.870 Boiling range (at 101325 Pa atmospheric pressure, up to 110.6°C)/°C ≤ 0.7 0.9 2.0 Acid wash color comparison (against standard color solution): color not darker than 0.15 0.20 0.30 Bromine value/(g/100ml) ≤ 0.1 0.2 0.3 Neutralization test Neutral Moisture No visible insoluble water when inspected at room temperature (18~25°C) Copper sheet corrosion test Color not darker than Grade 1 (mild discoloration) Note: 1. The copper sheet corrosion test is a reference specification. 2. Shipping is not permitted if the water layer height of benzene in the tank truck is greater than 5 mm, or if it is greater than 1 mm in the iron drum. If re-inspection upon delivery to the recipient exceeds the above regulations, it shall be agreed upon by both the supplier and the buyer. Table 7-3(3) Quality specifications for coked xylene Specification name Xylene at 3°C Xylene at 5°C Xylene at 10°C Appearance Transparent liquid at room temperature (18–25°C); color not darker than that of a potassium dichromate solution containing 0.003 g and 0.03 g per 1000 ml of water, respectively Color of potassium dichromate solution Density (20°C)/ (g/cm³) 0.857–0.866 0.856–0.866 0.840–0.870 Boiling range (at 101325 Pa atmospheric pressure) Initial boiling point/°C ≥ Final boiling point/°C ≤ 137.5 140.5 136.5 141.5 135.0 145.5 Acid wash colorimetry (compared to standard color solutions): color not darker than 0.6, 2.0, 4.0, respectively Moisture No visible insoluble water when inspected at room temperature (18–25°C) Neutrality test Neutral Copper sheet corrosion test Color change not worse than grade 2 (i.e., moderate discoloration) Note: 1. The copper sheet corrosion test is a reference specification. 2. Shipping is not permitted if the water layer height of benzene in the tank truck is greater than 5 mm, or if it is greater than 1 mm in the iron drum. If re-inspection upon delivery to the recipient exceeds the above regulations, it shall be agreed upon by both the supplier and the buyer. Preliminary distillation and acid washing purification of light benzene: To obtain high-quality benzene products, it is first necessary to separate crude benzene into light benzene and heavy benzene. The vast majority (over 98%) of benzene, toluene, and xylene, as well as most of the sulfides, and nearly 50% of the unsaturated compounds are concentrated in light benzene, while high-boiling-point unsaturated compounds such as styrene, cumene, and indene are concentrated in heavy benzene. I. Preliminary distillation of light benzene: The purpose of the preliminary distillation of light benzene is to separate low-boiling-point compounds such as carbon disulfide, cyclopentadiene, and alkenes from the aromatic hydrocarbons, thereby obtaining a first distillate and an aromatic mixture composed of benzene, toluene, xylene, etc. (also known as the unprocessed mixed fraction). The yield of the initial fraction (relative to light benzene) is approximately 1.0%–1.2%. Its approximate composition (by mass fraction) is shown in Table 7-4. Table 7-4 Composition of the initial distillate. Components (mass percentage): Crude benzene initial distillate, Light benzene initial distillate. Components (mass percentage): Crude benzene initial distillate, Light benzene initial distillate – Carbon disulfide %, Cyclopentadiene, dicyclopentadiene %, Other unsaturated compounds %. Values range from 15–25 for crude benzene initial distillate and light benzene initial distillate, respectively; from 25–40 and 20–30 for the corresponding categories. Benzene/% and Saturated compounds/%: Values range from 30–50 for crude benzene initial distillate, and from 3–6, 5–15 for light benzene initial distillate respectively; from 4–8 for the other categories. The initial distillate is required to have a boiling point not exceeding 70°C, with a benzene content not exceeding 15%. The benzene mixture fraction is required to have a initial boiling point of over 82°C, a bromine value of less than 9 g/100 ml, a carbon disulfide content of less than 0.087%, and to be free of water. The process flow diagram for the distillation of light benzene is shown in Figure 7-2. Figure 7-2: Process flow for continuous distillation of light benzene. 1—Distillation tower; 2—Condenser; 3—Oil-water separator; 4—Feed pump; 5—Reflux pump; 6—Standby pump; 7—Filter; 8—Cooler; 9—Reboiler of the distillation tower; 10—View glass. After being dehydrated by standing in a storage tank, light benzene is pumped into the distillation tower using a feed pump for distillation. The initial distillate is evaporated at the top of the tower, with the temperature maintained between 45 and 50°C, and the reflux ratio controlled at 0.8 to 0.9 relative to the feed rate. The distilled light ends vapor passes through a cooler, where it is cooled to 25–30°C before entering an oil-water separator, where it is divided into two parts. The oil separated out goes into the light ends storage tank via a sight glass, while the other part is pumped back to the top of the light ends tower as reflux. The temperature at the bottom of the initial distillation tower is controlled at 90–95°C. The mixed distillate discharged from the bottom of the tower is cooled to 25–30°C in a cooler, after which it flows automatically into the intermediate tank for mixed distillates, to be used as raw material for acid washing. The initial distillation tower is heated by a reboiler; since the residue at the bottom of the tower contains polymers formed from unsaturated compounds, these polymers must be filtered out before they reach the reboiler in order to prevent blockages in the equipment. The initial distillation column typically uses a 35-stage floating valve column (or bubble column); the gas velocity in the empty column can range from 0.6 to 0.9 m/s, while the number of feed stages can be between 15 and 232. The optimal location for the feed depends on the composition of the raw material. To ensure the quality of the mixed fraction, reduce the carbon disulfide content in it, and lower the benzene content in the initial distillate, the method of increasing the feed layer can be employed. Two-phenyl towers: There are mainly two types of two-phenyl towers, namely bubble column towers and floating valve towers. The structure of the bubble column is shown in Figure 6-23. The upper section of the two-phene column is the distillation section, while the lower section is the stripping section. The distillation section is equipped with 8 fast trays, each tray having several circular bubble caps, with a spacing of 600 mm between the trays. The second tray and the bottommost tray in the distillation section are dead trays, which serve to direct the liquid containing condensed water on those trays to the oil-water separator; after the water is separated, it is returned to the lower trays within the tower, thereby preventing the accumulation of condensed water from disrupting the normal operation of the distillation tower. The tower section is equipped with three rapid trays, with a tray spacing of approximately 1000 mm. Each fast tray is equipped with several circular high bubble caps and coiled tube heaters, which maintain a high liquid level on the tray to ensure that the heaters are submerged. Heavy benzene is discharged from the bottom of the distillation section. The crude benzene vapor from the fractionator enters the bottom of the distillation section, while the tower top is refluxed with light benzene. Direct steam is introduced at the bottom of the stripping section. The mass of light benzene and heavy benzene is controlled by the return flow of the supplied light benzene and direct steam. The structure of the two-benzene column with vapor-phase fine material floating valve is shown in Figure 6-24. The distillation section has 13 trays, while the stripping section has 5 trays. Each tray is equipped with several cross-shaped floating valves; their structure and arrangement on the tray are shown in Figure 6-25. The tray spacing in the two-benzene column with floating valves is 300~400 mm. The steam flow velocity at the cross-section of the empty tower can be taken as 0.8 m/s. A distillation column equipped with 30 trays is used to fractionate crude benzene into three products: light benzene, refined heavy benzene, and solvent oil, to facilitate further processing and purification. The two-phase tower with liquid-phase feed is shown in Figure 6-26. Generally, there are 35 trays, and crude benzene is pumped into the middle of the biphenyl tower. There is a reboiler on the outside of the tower; within this reboiler, steam is used to indirectly heat the crude benzene introduced from the bottom of the tower, and the vaporized crude benzene then enters the tower. Light benzene gas is led out from the top of the tower, and there is an inlet for the return of light benzene at the top level. The tower side line yields refined heavy benzene, while the bottom discharge contains naphtha solvent oil. In the two towers for producing light benzene and heavy benzene, the yield of light benzene at temperatures below 150°C, which is distilled from crude benzene at temperatures above 180°C, is generally 93%–95%, while the yield of heavy benzene is 5%–8%. The extraction temperature of light benzene at the top of the two-bottle column is 73–78°C. The amount of water vapor condensed within the column is equal to the amount of water vapor carried in by the crude benzene vapor, plus the difference between the amount of direct vapor supplied from the bottom of the column and the amount of water vapor carried away by the light benzene. This portion of the condensate must be separated using a separator to ensure the proper operation of the two benzene towers. Washing section: The main chemical reactions involved in acid cleaning and purification. The benzene mixture obtained through the initial distillation of light benzene contains unsaturated compounds and sulfides with boiling points similar to those of aromatic hydrocarbons; these substances cannot be removed by distillation. To produce high-quality products, it is necessary to remove them in advance, and the method used for this purpose is treatment with sulfuric acid. When washed with sulfuric acid, multiple chemical reactions occur simultaneously, among which the main reactions are as follows. 1. Reactions for removing unsaturated compounds (1) Polymerization of unsaturated compounds Unsaturated compounds readily undergo polymerization in the presence of concentrated acids, forming various complex polymers. The first stage of the polymerization reaction is the formation of the acid ester. The second stage of the polymerization reaction is the reaction of the acid ester with unsaturated compounds to form dimers. For example, (CH3)3C===CH2 + HOSO3H → (CH3)3COSO3H; isobutylene acid ester. (CH3)2C====CH2 + (CH3)3COSO3H → (CH3)2C====CHC(CH3) + H2SO4; isobutylene acid ester. Isobutylene acid ester. This reaction can continue to proceed, resulting in higher-order polymers. Deep polymers have a higher density and can be separated from the washed mixed fraction, while products with a lower degree of polymerization remain dissolved in the washed mixed fraction and are separated during the next step that does not involve distillation. Sulfuric acid in its free state is separated from the mixed fraction; after dilution with water, it forms regenerated acid. (2) Addition reactions of unsaturated compounds: The addition reaction of sulfuric acid with unsaturated compounds yields acid esters and mixed esters. The acid ester dissolves in sulfuric acid and water, thereby being separated from the washed mixed fraction. Chinese esters are insoluble in sulfuric acid and water, but soluble in the washed mixed distillate. During final distillation, Chinese esters decompose into carbon disulfide, hydrogen sulfide, sulfur trioxide, carbon dioxide, certain unsaturated compounds, and carbon residue. Acidic gases such as sulfur dioxide cause corrosion of the equipment, while other decomposition products degrade the quality of the product. Therefore, during the initial distillation of light benzene, the initial distillate must be thoroughly removed; during the acid washing process, deep polymerization should be suppressed, and the washed mixed distillate should remain slightly alkaline after neutralization. For example, OC(CH3)3 2(CH3)2C==CH¬¬2 + H2SO4 → O2S + OC(CH3)3 2. The reaction for removing sulfides: In the mixed fraction, carbon disulfide does not react with sulfuric acid, and the amounts of other sulfides are low; therefore, acid washing primarily serves to remove thiophene and its derivatives. (1) Sulfonation of thiophene Thiophene and its homologs undergo sulfonation with sulfuric acid to produce thiophene sulfonic acid. As shown in the reaction below: C4H4S + H2SO4 → C4H3SSO3H + H2O. Thiophene, thiophenesulfonic acid. The relationship between the reaction rate constant for thiophene sulfonation and the concentration of the acid as well as temperature is shown in Table 7-5. Table 7-5: Relationship between the reaction rate constant for thiophene sulfonation and acid concentration and temperature. Mass percentage of acid %; Reaction rate constant K; Relative reaction rate K30/K15; Temperatures: 15°C, 30°C. Values: 88.7, 93.0, 95.4, 96.5, 98.5, 101.3; 0.0005, 0.002, 0.007, 0.013, 0.031, 0.065; 0.0012, 0.003, 0.014, 0.029, 0.056, 0.098. At concentrations of 0.25, 1.0, 3.5, 6.5, 15.5, 32.5%, the corresponding values are 1.0, 3.5, 6.5, 15.5, 32.5. At concentrations of 0.4, 1.0, 4.6, 9.6, 18.6, 32.6%, the values are 2.4, 1.5, 2.0, 2.2, 1.8, 1.5. As can be seen from Table 7-5, the formation of thiophenesulfonic acid occurs quite slowly, and the effect of acid concentration on the reaction rate is more significant than that of temperature. To accelerate the reaction, acid with a concentration of over 93% must be used. The resulting thiophene sulfonic acid is dissolved in sulfuric acid and water, and separated from the washed mixture fraction. (2) Formation of copolymers from thiophene and unsaturated compounds The polymerization of thiophene and its homologs with unsaturated compounds, especially those with high boiling points, proceeds very rapidly and completely under the catalysis of a small amount of sulfuric acid. For example, in S-thiophene-indene copolymers, in addition to the various reaction types mentioned above, during pickling and purification, pyridine bases also react with sulfuric acid and are removed. The resulting copolymers are generally soluble in the washed mixed fraction and end up in the bottom residue during final distillation. 3. Side reactions: While the above four reactions occur, the following two side reactions that result in losses of benzene-based products also take place. (1) Copolymerization of aromatic hydrocarbons with unsaturated compounds: Under the catalysis of concentrated sulfuric acid, aromatic hydrocarbons react in copolymerization with unsaturated compounds; for example, +CH2===CHC(CH3)3 and CH3CHC(CH3)3 – hexene and hexylene. The products of this copolymerization are high-boiling-point compounds that are often soluble in the mixed distillate fractions, and they end up in the residue at the bottom of the reactor during final distillation. The copolymerization reaction not only reduces the yield of benzene but also increases the yield of tar residues and acid tar. (2) Sulfonation of aromatic hydrocarbons: During acid washing, the sulfonation of aromatic hydrocarbons also occurs. For example, in the pickling reaction of +H2SO4 +H2O, the higher the reaction temperature and the longer the washing time, the more intense such sulfonation reaction becomes, and the greater the amount of sulfuric acid consumed. To reduce the progression of this reaction, it is required that the washing operation be carried out in the shortest possible time with minimal acid consumption. Process requirements and production flow for acid cleaning 1. Process requirements for acid cleaning The acid cleaning process not only requires the removal of unsaturated compounds and sulfides present in the mixed distillate as much as possible, but also demands low consumption of sulfuric acid, minimal loss of aromatic hydrocarbons, low formation of acid tar, and that the reaction proceed in such a way as to produce polymers that can dissolve in the cleaned mixed distillate. Therefore, it is necessary to select appropriate pickling operating conditions. (1) Reaction temperature: The appropriate reaction temperature is 35–45°C. At too high temperatures, the sulfonation of aromatic hydrocarbons and the copolymerization of unsaturated compounds intensify, leading to increased loss of aromatic hydrocarbons; at too low temperatures, the purification reaction cannot take place. The pickling and purification reaction is an exothermic reaction, and the amount of heat released depends on the content and composition of unsaturated compounds in the mixed fraction. When the mixed fraction contains 2%–3% saturated compounds, the temperature increase usually does not exceed 4–6℃ ； When the content of unsaturated compounds in the mixed fraction exceeds 4%–5%, the temperature can rise by 12–20°C. Considering the thermal effects of the pickling process, the preheating temperature of the mixed fraction before pickling is generally set at 25–30°C. (2) Pickling concentration: The appropriate mass percentage for pickling is 93%~95%, with a consumption of about 5% (for the mixed fraction). At too high a concentration, the sulfonation reaction intensifies, leading to increased loss of aromatic hydrocarbons ； If the concentration is too low, the purification effect of the reaction cannot be achieved. In actual production, the concentration of sulfuric acid should be determined based on the composition of light benzene. When the benzene content in light benzene is low and the bromine number of the unprocessed mixed fraction is also low, 93% sulfuric acid can achieve a good purification effect ； When the benzene content in light benzene is high and the bromine value of the unwashed mixed fraction is also high, sulfuric acid with a mass fraction of over 94% is advisable. (3) Reaction time: The purification effects at various stages of the sulfuric acid purification mixer are shown in Table 7-6. Table 7-6: Purification effects at various stages of the sulfuric acid purification mixer. Parameter: Before purification, After pump, Spherical mixer, Reactor; Time taken for the process/s: 0, 5, 60, 300. Unsaturation level: 3.27, 0.52, 0.13, 0.11; Thiophene level: 1.32, 0.28, 0.13, 0.11; Slag level: 6.4, 4.1. Acid tar yield/%: 2.0, 5.07; Loss of benzene hydrocarbons/%: 8.1, 3.73. Increasing the reaction time improves the washing effect, resulting in a significant reduction in the colorimetric and bromine value of the washed mixed fractions. However, an excessively long reaction time will also exacerbate the sulfonation reaction, leading to the loss of aromatic hydrocarbons. The reaction time is insufficient; relying solely on adding acid to improve the reaction efficiency not only results in high acid consumption but also increases the amount of acid tar, making neutralization difficult and the separation of acid from oil challenging. The typical reaction time is about 10 minutes. The amount of acid consumed during the pickling and purification process is not large, and most of it can still be recovered using a method involving washing with water. The recovery rate of the regenerated acid varies between 65% and 80% depending on the washing conditions and the composition of the mixed fraction, with the mass fraction of the regenerated acid being 40% to 50%. The less acid tar is produced, the higher the recovery rate of the acid. The amount and consistency of acid tar are related to the properties of the unprocessed mixed fraction as well as the operating conditions; when the carbon disulfide content in the mixed fraction is high, more viscous acid tar is produced ； Conversely, a dilute acid tar that is easy to separate is produced. The amount of acid tar produced from raw materials with different compositions is generally 0.5%–0.6% (as a percentage of the mass of the raw material fraction). The mass fraction of the components in acid tar is: sulfuric acid 15%~30% ； Benzene derivatives 15%~30% ； Polymer 40%~60%. 2. Acid cleaning and purification process flow: Except that the high-purity benzene production units in small coking plants use intermittent washing, large and medium-sized coking plants all employ continuous washing systems, as shown in the process diagram 7-3. The unwashed mixed fraction obtained from the initial distillation of light benzene is preheated to 25–30°C using heating jacket 1. After being initially mixed with concentrated sulfuric acid having a mass fraction of 93%–95%, which is added continuously before washing pump 2, it is sent to mixing sphere 3 where the main acid cleaning reaction takes place. The mixing sphere consists of two steel or cast iron hemispheres lined with an acid-resistant layer, connected together by flanges. The individual mixing balls are connected to each other by short, thin tubes at 90° angles to one another, allowing the fluid to achieve a highly turbulent state within the balls. Since the liquid remains inside the ball for only about 1 minute, the pickling process is not complete; therefore, a further pickling and purification reaction is required in pickling reactor 4. The liquid remains in the reactor for 10 minutes to ensure complete evolution of the reaction. The mixed distillate and sulfuric acid coming out of the pickling reactor enter the water addition mixer 5, where water accounting for 3% to 4% of the unprocessed mixed distillate is continuously added to stop the reaction from proceeding to produce crude sulfuric acid. Then, it enters the acid-oil separator where it stays for 1 hour to undergo clarification and separation; the regenerated acid and acid tar settle down, while the mixed fraction is discharged from the upper part of the acid-oil separator to the alkali-oil mixer 7. Alkali with a mass fraction of 12%–16% is added continuously before the alkali-oil mixer to ensure uniform mixing, thereby rendering the washed mixed fraction slightly alkaline and converting the phenols present into phenates. This mixture is then sent to another acid-oil separator for further separation, with a separation time of 1–1.5 hours. The oil separated from the alkali solution is sent to the intermediate tank containing the washed mixed fraction; after further separation of the residual liquid, this liquid is used as raw material for the benzene stripping tower. The alkali solution discharged from the lower part of the alkaline oil separator is used to neutralize acid tar. The regenerated acid discharged from the bottom of the acid oil separator enters the regenerated acid sump, and after further separation of acid tar, it is pumped to the regenerated acid storage tank. The acid tar from the acid oil separator 6 and the regenerated acid precipitation tank is fed into the acid tar distillation vessel 11, where it is neutralized using the alkaline solution discharged from the bottom of the alkali oil separator 8; additional alkali may be added as needed for further neutralization, and the benzene derivatives contained therein are distilled out using direct steam. The benzene vapor obtained through distillation is separated by cooler 12 and oil-water separator 13, and then sent to the intermediate tank containing the washed or unwashed mixed fraction; the residue remaining in the reactor is discharged into a sedimentation tank. Finally, it is sent together for coal blending. Sulfuric acid is sent from the acid pump 15 to the acid sump 16, passes through the filter 23 and the automatic flow control device for adjusting the ratio of acid to oil 24, and is then added to the unprocessed mixed distillate. The prepared alkaline solution is pumped by the alkali pump 18 into the alkali sump 19, and then added to the mixed fraction after pickling via a metering device. Based on 100% sulfuric acid, the consumption of sulfuric acid is 4% to 5% of the unprocessed mixed distillate ； Based on 100% sulfuric acid, the consumption of sodium hydroxide is 0.5% lower than that in the unwashed mixed fraction ； For every 100% of sulfuric acid, 1.7 kg of recycled acid can be recovered, which corresponds to 40% of it. The quality parameters of the washed mixed fraction are: colorimetric value < 0.5, bromine value < 0.2 g/100 mg; it is slightly alkaline. Figure 7-3 Process flow for continuous washing of unwashed mixed distillates 1—Heating jacket ； 2—Continuous washing pump ； 3—Mixed Ball ； 4—Pickling Reactor ； 5—Heating mixer ； 6—Acid oil separator ； 7—Alkali oil mixer 8—Alkali oil separator ； 9—Regenerated acid precipitation tank ； 10—In the acid production pump ； 11—Acid tar distillation and blowing vessel ； 12—Steam-driven benzene condensation cooler ； 13—Oil-water separator ； 14—Sulfuric acid tank ； 15—Acid pump ； 16—Sulfuric acid high-level tank ； 17—Alkali mixing tank ； 18—Alkali pump ； 19—Alkali high-level tank ； 20—Regenerated acid storage tank ； 21—Viewing mirror ； 22—Discharge trough ； 23—Acid filter ； 24—Automatic flow regulation device ； 25—Flow transmitter indicator device ； 26—Flow indication device ； 27—Temperature indicating device ； Distillation of the washed mixed fraction: The washed mixed fraction is slightly alkaline; first, benzene stripping is carried out (simple benzene stripping), and then the benzene obtained from this stripping (a mixture of benzenoid hydrocarbons) is subjected to final distillation. Depending on the production scale of the benzene refining plant, there are continuous distillation systems and semi-connected distillation systems. I. Blowing off benzene from the washed mixed fraction: The washed mixed fraction is fed into a benzene-blowing tower for continuous vaporization and blowing off; this is a process of flash separation. Its purposes are: ○1 To decompose the esters dissolved in the mixed fraction into sulfur dioxide, sulfur trioxide, carbon dioxide, and carbon residue under high temperatures, thereby separating them out; ○2 To remove various polymers dissolved in the mixed fraction as benzene-blowing residues, so as to avoid affecting the quality of the distilled products and preventing equipment blockages. The benzene-blowing residues can be used as raw materials for producing gumar resin. To prevent the acidic gases emitted during benzene blowing from corroding the distillation equipment, the benzene vapor blown out needs to be neutralized by alkali solution washing. The process flow for continuous benzene stripping is shown in Figure 7-4. Figure 7-4: Process flow diagram for continuous benzene stripping of washed mixed distillates. 1 – Benzene stripping tower ； 2—Neutralizer ； 3—Condensing cooler ； 4—Oil-water separator ； 5—Alkali oil separator ； 6—Raw material pump ； 7—Standby pump ； 8—Circulating alkali pump ； 9—Heater ； 10—Benzene blowing residue tank ； 11—Air pump ； 12—View mirror ； 13—Casing condenser: After the washed mixed distillate is allowed to settle in the intermediate tank to separate the alkaline solution, it is pumped by a feed pump into the benzene-blowing tower heater 9, where it is heated to 105–110°C. It then enters the flash section at the top of the benzene-blowing tower in a gas-liquid mixture state, with the light fractions being blown out. The temperature of the vapor escaping from the top of the tower is related to atmospheric pressure; when the atmospheric pressure is around 101.3 kPa, this temperature ranges from 100 to 105°C. The benzene vapor is then introduced into the bottom of the neutralizing gas stream, where it comes into contact with a sodium hydroxide solution having a mass fraction of 12% to 16%, thereby undergoing a neutralization reaction with the acidic gases present in the benzene vapor. The neutralized steam is cooled to 25–30°C by the condensation cooler 3, and after being separated by the oil-water separator 4, it flows into the benzene blowout intermediate tank. The quality criteria for the benzene blown out are: colorimetry < 0.5, and the reaction is neutral. The alkaline solution discharged from the bottom of neutralizer 2 is cooled by sleeve cooler 13 before entering the alkali-oil separator 5; the oil separated out is sent to the intermediate tank for blowing off benzene or for the washed mixed fraction, while the alkaline solution is continuously pumped by a circulating alkali pump back to the neutralizer for reuse. The concentration of the alkaline solution sprayed by the neutralizer must not be less than 4%, to prevent the alkalis from failing to separate properly from benzene-based substances. The temperature of the alkali solution should be above 70°C to reduce the condensation of benzene vapor and water vapor within the alkali solution, thereby preventing the concentration of the alkali solution from dropping too rapidly. Maintaining a high temperature of the alkali solution also facilitates the progress of the neutralization reaction. The spent alkali solution is discharged from the neutralizer intermittently, while new alkali solution is added simultaneously. The temperature at the bottom of the benzene stripping tower is 130–140°C, and the polymer is discharged from the bottom of the tower in the form of sludge into the benzene stripping residue tank 10. To ensure that the residue from the benzene blowing process meets the specifications regarding oil and water content, an indirect steam heater is installed at the bottom of the tower in addition to direct steam supply. The residue is used as a raw material for producing guaiac resin. The benzene yield from the blowing of the washed mixed fraction was 97.5%, while the residue yield was 2.5%. II. Semi-continuous distillation for extracting benzene: Semi-continuous distillation uses benzene as the raw material; it first involves feeding this benzene continuously into a pure benzene column to extract pure benzene, and then other products are obtained from the residue left after purifying the benzene using semi-continuous distillation. Depending on the feeding method of the pure benzene residue, semi-continuous distillation systems mainly comprise two process configurations: batch still distillation and discontinuous continuous distillation. 1. Batch distillation: The pure benzene residue is subjected to continuous distillation in a batch reactor, with the process flow shown in Figure 7-5. Figure 7-5 Process flow of batch distillation 1—Feed pump ； 2—Distillation pump ； 3—Distillation tower ； 4—Condensing cooler ； 5—Oil-water separator ； 6—Measurement tank ； 7—Distillation reflux pump ； 8—View mirror: The pure benzene residue is charged into the rectifying still 2 from the storage tank once using the feed pump 1, and full reflux is achieved by heating with steam. When the kettle temperature reaches 124–125°C, the initial fractions (benzene-toluene fraction) begin to be removed, and when the top of the tower reaches 110°C, pure benzene can be extracted. When the liquid level in the kettle dropped by 1/3, pure benzene residue began to be continuously pumped into distillation column 3 (the feed point being between layers 15 and 23), and toluene was continuously extracted. Feeding is stopped once the high-boiling components in the reactor have accumulated to a certain extent and the reactor temperature reaches 145°C; thereafter, the toluene-xylene fraction, xylene, and light solvent oil are separated sequentially. Each product flows automatically into its respective storage tank through the metering tank 6. The distillation residue discharged from the bottom of the tank is sent to the storage tank via a gas pump and a jacket cooler. When the atmospheric pressure is 101.3 kPa, the operating conditions for batch distillation are shown in Table 7-7. Process: Separation of toluene and xylene (steam distillation). Reflux ratio (relative to the distillate): Top of tower temperature/°C: 2–3; 110±0.5. Bottom of tower temperature/°C: 2–3; 90–100. Tower pressure/MPa: 145–154; <0.03. 145–160; <0.03. Note: The temperatures within the tower vary depending on local atmospheric pressure and pipeline resistance; the values listed are those used in coking plants located in coastal areas. When extracting xylene and solvent oil, appropriate amounts of direct steam are introduced into the distillation vessel to carry out steam distillation; a steam ejector can also be used to create a certain degree of vacuum, thereby reducing the distillation temperature and minimizing the consumption of direct steam. The batch feeding method offers greater operational flexibility, but requires frequent adjustments, which makes it unsuitable for automatic control. The intermediate fraction contains a high amount of steam, typically accounting for about 20% of the pure benzene residue. 2. Discontinuous continuous distillation: In a discontinuous continuous distillation system, the same continuous distillation column is used to treat pure benzene residue and toluene residue in stages. The benzene to be blown out is continuously fed into the pure benzene column by a pump from the storage tank. The pure benzene column is a floating valve column with 30–35 trays, and the feed tray is generally located between the 17th and 21st trays. The temperature of the steam at the top of the pure benzene tower is controlled at 80°C. The vapor that escapes at this temperature is collected as a product through the condensation section. The temperature of the pure benzene residue at the bottom of the tower is 124–128°C; it is continuously discharged from the bottom of the tower, cooled in a jacketed cooler, and then sent to the pure benzene residue tank. Once a certain amount of pure benzene residue has accumulated, it is continuously fed into the distillation tower using a feed pump; the feed plate is usually located between the 13th and 19th layers. The bottom of the distillation tower is heated by indirect steam, and toluene is continuously extracted from the top of the tower. After the toluene vapor is condensed and cooled, as well as after cooling in the oil-water separator, part of it is sent to the top of the tower as reflux, while the remaining part is collected as a product. The toluene residue discharged from the bottom of the tower is cooled in a residue oil cooler before being sent to the toluene residue storage tank. Toluene residue is required to be free of toluene, with a initial boiling point greater than 138°C. Once a certain amount of toluene residue has accumulated, the treatment of pure benzene residue should be stopped, and instead treatment of the toluene residue should proceed. Similarly, the toluene residue is continuously pumped into the distillation tower using a feed pump; the bottom of the tower is heated with indirect steam, while direct steam is also supplied. The xylene vapor escaping from the top of the tower is cooled by condensation and separated from oil and water; part of the xylene is used as reflux, while another part is extracted as a product. The operating procedures for batch continuous distillation and continuous distillation are shown in Table 7-8. Table 7-8 Operating Conditions for Discontinuous Continuous Distillation Item Separation of Toluene Separation of Xylenes (Steam Distillation) Item Separation of Toluene Separation of Xylenes (Steam Distillation) Reflux Ratio (relative to feedstock) Top of Tower Temperature/°C 1–1.5 110±0.5 1.5–2.5 89–96 Bottom of Tower Temperature/°C Tower Pressure/MPa 150–155 <0.035 140–150 <0.035 Note: The temperature inside the tower varies depending on atmospheric pressure and pipeline resistance; the temperatures listed in the table represent the operating parameters for coking plants located in coastal areas. To extract light solvent oil, heavy solvent oil, and other regular fractions, a set of side streams is installed above and below the feed zone of the distillation tower; the extracted fractions are sent to the oil storage facility after oil-water separation. The distillation residue discharged from the bottom of the tower enters the distillation residue tank after cooling. After processing the toluene residue, the process was changed to processing pure benzene residue in order to extract toluene. This method is simple to operate and easy to automate, with the intermediate fraction containing approximately 5% steam. The yield of the product increases and power consumption is reduced, but its operational flexibility is not as good as that of a batch reactor continuous distillation system. III. Fully continuous distillation for benzene extraction: A continuous distillation process can be employed in benzene refining plants that handle 20,000 tons or more of light benzene per year. That is, xylene is continuously extracted starting from the initial distillation of light benzene. For larger-scale installations, trimethylbenzene can also be extracted from xylene residue. The fully continuous distillation process has the advantages of stable production, high product yield and good quality, simple operation, low investment, and ease of automatic control. There are mainly two types of systems for fully continuous refining of light benzene. 1. Fully continuous distillation process with hot oil for feeding the material (1) Process flow: Fully continuous distillation using hot oil for feeding the material involves using untreated pure benzene residue and toluene residue as feed materials, which are pumped directly into the next distillation column by hot oil pumps; the process flow is shown in Figure 7-6. Pure benzene is continuously fed into pure benzene column 3 using raw material pump 2 during the start-up and shutdown phases of the column. The pure benzene vapor at the top of the column is cooled by condenser 5 and separated from water in oil separator 6; part of this vapor is sent back to the top of the column as reflux using reflux pump 7, while the remaining portion is used as a product. The pure benzene residue is fed directly into tower 10 without cooling. The toluene vapor at the top of the tower is cooled by condensation and then separated from water; part of this vapor is pumped back to the top of the tower as reflux, while the remaining portion is collected as a product. The toluene residue at the bottom of the tower is also sent directly to xylene column 17. The xylene vapor at the top of the tower is cooled by steam condensation and subjected to oil-water separation; part of it is pumped back to the top of the tower as reflux, while the rest is collected as a product. The xylene residue at the bottom of the tower is cooled by the casing cooler 19 before being sent to the xylene residue tank 22. Each tower is heated using a reboiler. The continuous hot oil process requires relatively stable raw material composition, product amount, reflux ratio, steam pressure, top temperature of the towers, and bottom liquid level. If one of the products in the three towers is found to be defective, a full cycle of re-vaporization is required; at the same time, the feed rate to the benzene stripping tower should be reduced appropriately or the tower should be shut down to avoid material imbalance. (2) Process operation procedures: The operation procedures for continuous distillation with hot oil as the feed are essentially the same as those for semi-continuous distillation in the benzene stripping system and earlier stages; the operation procedures for the pure benzene column and subsequent units are shown in Table 7-9. 7-9 Operating parameters for full continuous distillation with hot oil feed Item Benzene tower Toluene tower Xylene tower (steam distillation) Top temperature of tower/°C 80±0.5 110±0.5 89±96 Bottom temperature of tower/ 124~128 150~155 140~150 Tower pressure/MPa <0.035 <0.035 <0.035 Reflux ratio (relative to feed) 1~1.5 1.5~2.0 0.8~1 Oil outlet temperature of condenser/°C 20~30 20~30 20~30 Water outlet temperature of condenser/°C <45 <45 <45 Liquid level at tower bottom 1/3~1/2 1/3~1/2 1/3~1/2 Note: The temperature values for each tower refer to the operating conditions in coking plants located in coastal areas. The atmospheric pressure in western China is generally lower than that along the coast, and various temperature indicators will vary accordingly. (3) Quality of intermediate products: In continuous distillation production, in order to obtain benzene, toluene, xylene, and other products of qualified quality, it is necessary to ensure the quality of pure benzene residue, toluene residue, and xylene residue. The device quality indicators are shown in Table 7-10. Name Quality Specifications Residual pure benzene: Initial boiling point > 113°C; colorimetric value < 0.5; neutral in reaction; free of benzene. Residual toluene: Initial boiling point > 138°C; free of toluene. Residual xylene: Initial boiling point > 168°C; free of xylene. Table 7-10: Quality specifications for intermediate products in light benzene refining. Figure 7-6: Process flow diagram for continuous distillation using hot oil as the feed medium. 1 – Tank for starting up and shutting down the pure benzene tower ； 2—Raw material pump for benzene tower ； 3—Pure benzene tower ； 4—Reboiler of pure benzene tower ； 5—Condenser cooler for pure benzene tower ； 6—Pure benzene oil-water separator ； 7—Reflux pump for pure benzene ； 8—Toluene tower start-up and shutdown sequence ； 9—Hot oil feed pump for toluene tower ； 10—Toluene tower ； 11—Toluene column reboiler ； 12—Toluene tower condenser ； 13—Toluene reflux pump ； 15—Xylene start-up and shutdown tank ； 16—Xylenes hot oil feed pump ； 17—Xylene Tower ； 18—Xylenes tower reboiler ； 19—Xylenes condensation cooler ； 20—Xylene oil-water separator ； 21—Xylene reflux pump ； 22—Xylene residue tank ； 23—Xylene residue pump ； 24—Cooling sleeve ； 25—Viewing mirror. 2. Continuous distillation with gas connection between reactors. At present, some coking plants in China have implemented a production process that involves gas connection between the benzene stripping tower and the pure benzene tower; the benzene stripping vapor coming out of the stripping tower enters the pure benzene tower directly as a material for distillation, after which continuous distillation continues using hot oil as the medium for heat transfer. The advantages of gas-phase series continuous distillation are stable and simple operation, increased processing capacity, reduced equipment requirements, and savings in water, electricity, and steam consumption. However, benzene towers are prone to water accumulation; therefore, in some cases, tubular furnaces are used to heat the residue at the bottom of the tower, with a heat carrier being employed instead of steam to heat this residue, thereby reducing steam consumption.

I’ve learned it~; it seems that the pickling method is now an obsolete process.

You on the 4th floor, you’re really so professional! It has to be no less than 1,000 words!