Brief description of the process flows in each section of coking gas purification -- Final cooling and benzene washing section

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Why is final cooling performed on the gas in the final cooling and benzene washing section before it enters the benzene absorption tower? One of the main factors affecting crude benzene recovery is the recovery temperature. When the content of benzene and its hydrocarbons in the gas remains constant, a lower absorption temperature for the wash oil to absorb benzene increases the absorption driving force, thereby raising the benzene recovery rate. Practice has shown that the optimal temperature is 20–28°C. However, after passing through the saturator, the temperature of the gas is generally 50–65°C. To effectively use the washing oil to absorb crude benzene from the gas, final cooling of the gas is necessary. 1. The process flow: The gas at approximately 55°C coming from the ammonium sulfate plant first enters the final cooling tower, where the gas is cooled in two stages. The lower section uses circulating cooling water at about 37°C, while the upper section uses circulating cooling water at about 24°C to cool the gas to ~25°C before it enters the benzene washing tower. After being washed with oil-poor liquid to remove crude benzene and passing through a mist catcher, the gas is sent to the end users. The circulating cooling water in the lower section of the final cooling tower enters this section from the middle of the tower; it comes into contact with the gas in the opposite direction to cool it, and is then pumped out. After passing through the lower section’s circulating spray cooler, it is cooled to 37°C by the circulating water before being sent back to the middle section of the final cooling tower for reuse. The circulating cooling water in the upper section of the final cooling tower enters this section from the top of the tower to cool the gas; afterward, it is pumped out and cooled to 24°C using low-temperature water in the upper section’s circulating spray cooler, before being returned to the top of the final cooling tower for reuse. At the same time, a certain amount of alkaline solution is added to the upper section of the final cooling tower to further remove H2S from the gas, ensuring that its concentration remains at ≤20 mg/m3. The condensed liquid discharged from the lower section is sent for treatment in the phenol-cyanide wastewater treatment process, while the alkaline-containing condensed liquid discharged from the upper section is sent to the top of the ammonia distillation tower in the ammonia distillation unit to decompose the fixed ammonium in the remaining ammonia water. The lean oil coming from the crude benzene distillation unit is sprayed from the top of the benzole scrubber, where it comes into countercurrent contact with coal gas to absorb benzene from the gas. The rich oil at the bottom of the tower is pumped by a rich oil pump to the crude benzene distillation section for debenzolization, after which it is recycled. The washing oil consumed by the system is regularly replenished into the system from the washing oil tank via the oil-enriched pump inlet. 2. Process characteristics: 1) Final cooling is achieved through direct air cooling, resulting in lower capital investment. 2) The final gas cooling employs a closed-loop continuous blowdown process, which not only reduces the volume of wastewater discharged but also helps protect the atmospheric environment. 3. Key technical operating parameters: Gas temperature at the final cooling tower is 25–27°C; oil-poor gas temperature entering the benzene washing tower is 27–29°C. The resistance in the final cooling tower is <1.5 kPa, and the resistance in the benzene washing tower is also <1.5 kPa. The benzene content in the gas after passing through the benzene washing tower is approximately 4 g/m3. 4. Key environmental protection measures: 1) The waste water and waste oil generated within the system are collected together and sent to underground drainage tanks, from where they are returned to the system rather than being discharged outside. 5. Working principle of the main equipment: The final cooling tower is a baffled tower divided into two sections. In the lower section, circulating water from the cooling tower is sprayed to cool the gas to around 40°C. In the upper section, low-temperature circulating water at 20–23°C is used for spraying, further cooling the gas to approximately 25°C. At the bottom of the hot water cascade final cooling tower, the water flows to the hot water tank through a water seal tube; it is then pumped to the cold water tower, cooled by a refrigerator, and flows back into the cold water tank before being pumped to the lower section of the final cooling tower. Benzene washing tower, packed benzene washing tower: The packing distributed on the trays creates tortuous pathways for the coal gas, thereby increasing its residence time and simultaneously enhancing the absorption rate of benzene. To ensure the uniform distribution of wash oil across the cross-section of the tower, a liquid redistributor equipped with a gas swirl cap is installed at regular intervals within the tower. The gas swirl caps are arranged in concentric circles on the liquid redistributor plate; the direction of the bent pipe outlets is tangent to the circumference. The outlet directions on the same circumference are consistent, while they are opposite on adjacent circumferences. Due to the guiding effect of the elbow, as the gas flows out of the vortex hood, multiple upward swirling air currents are formed, thereby mixing the gas so that it enters the upper packing layer at a uniform concentration. The washing oil collected on the liquid redistribution plate flows, via the elbow in the lift pipe, to the surface of the circular rod located at the center of the lift pipe; from there it flows to the toothed circular plate at the lower end, where it is splashed into droplets by gravity and falls onto the packing in the lower section. This can eliminate the phenomenon of wash oil flowing down along the tower wall and its uneven distribution. 6. The main factor affecting benzene washing is the absorption temperature: When the content of benzene hydrocarbons in the gas remains constant, the lower the temperature, the higher the content of crude benzene in the washing oil that is in equilibrium with it. Absorption capacity of the washing oil and amount of circulating oil: Under constant other conditions, a decrease in the relative molecular mass of the washing oil leads to an increase in the content of crude benzene in it, thereby improving its absorption capacity. Increasing the oil washing circulation volume can reduce the content of crude benzene in the washing oil and enhance the absorption driving force, thereby improving the recovery rate of crude benzene. Benzene content in lean gas: The benzene content in lean gas is one of the main factors determining the amount of benzenoid hydrocarbons in the gas exiting the tower. In addition, there are also influencing factors such as the absorption surface area, gas pressure, and flow rate. 7. The quality requirements for the washing oil are such that it can meet the needs of recovering and producing crude benzene from gas; therefore, the washing oil should possess the following properties: 1) It should have a good ability to absorb aromatic hydrocarbons at room temperature, and it should also be able to effectively separate these aromatic hydrocarbons when heated ； 2) It has chemical stability, meaning its absorption capacity remains essentially constant over long-term use ； 3) No solid precipitates should form at the absorption operation temperature ； 4) It separates easily from water without forming an emulsion ； 5) It has good fluidity, can be pumped, and spreads evenly over the packing. The main tar wash oils and petroleum wash oils used in coking plants for benzene absorption. Tar wash oil is the fraction of high-temperature coal tar with a temperature range of 230–300°C; it is required that the naphthalene content be less than 13%. A certain amount of naphthalene in the tar helps to lower the temperature at which precipitates form in the wash oil. Petroleum washing oil refers to light diesel, which is the fraction obtained during petroleum distillation after gasoline and kerosene have been distilled off. (The fraction at 270–350°C, C9–C14) Production experience shows that using petroleum wash oil for benzene purification offers advantages such as low consumption of wash oil, easy separation of oil from water, and simple operation. Oil washing has a strong ability to remove naphthalene but a weak ability to remove benzene; therefore, the amount of circulation is large, and steam consumption is high as well. The ammonia vaporization unit 1 and the process flow are designed for a residual ammonia water treatment capacity of 30 m3/h. The remaining ammonia water exchanges heat with the ammonia vapor wastewater discharged from the bottom of the ammonia vaporization tower before entering the tower. A portion of the ammonia vapor wastewater at the bottom of the tower is heated by steam in a steam reboiler and then flows back into the tower where it flashes to produce steam. Alkaline-containing condensate discharged from the upper section of the final cooling tower is also added to the top of the tower to decompose the fixed ammonia in the remaining ammonia water. The ammonia vapor at the top of the ammonia evaporation tower is condensed and cooled by a fractionator and an ammonia vapor condenser, turning into concentrated ammonia water, which then enters the desulfurization unit’s desulfurization regeneration tower. The ammonia-evaporated wastewater after heat exchange is cooled in a wastewater cooler before being sent to the phenol-cyanide wastewater treatment plant. 2. Process characteristics: 1) The ammonia vaporization tower is a stainless-steel floating valve tower, which features stable operation, high distillation efficiency, and a long service life. 2) The fractionator of the ammonia vaporization tower is made of titanium, which is corrosion-resistant. 3. Key technical operating parameters: Temperature at the top of the ammonia evaporation tower – 96°C; Pressure at the bottom of the ammonia evaporation tower – 0.03–0.04 MPa; Temperature of the ammonia-evaporation wastewater after passing through the wastewater cooler – 40°C. 4. Working principle of the main equipment: Ammonia solution remaining in the system is introduced at the third tray at the upper part of the tower; the ammonia solution flowing down along the various trays comes into contact with the vapor that is introduced from the bottom of the tower. Water vapor boils through the ammonia layer on the tray via the gaps between the bubble cap teeth, resulting in bubbling. On the tray, ammonia water meets steam and is heated to its boiling point; ammonia, carbon dioxide, hydrogen sulfide, and other substances present in the water gradually transfer into the gas as it rises. This process continues all the way to the bottom of the tower. Eventually, the wastewater discharged from the bottom of the tower contains less than 0.1 g/L of ammonia. When the remaining ammonia solution is a mixed ammonia solution and the inlet temperature to the tower is 60–70°C, the direct steam consumption per cubic meter of ammonia solution is approximately 200 Kg. Ammonia fractionators: Hydrogen sulfide, cyanides, and other substances present in the gas mixture escaping from the ammonia distillation tower have a strong corrosive effect on ordinary steel pipes; therefore, cast iron pipes are currently used for buried fractionators. In the reductor, ammonia flows inside the tubes while cooling water flows outside them. This type of retractor has good acid resistance and a long service life, but it is relatively bulky. 5. The main environmental protection measure is to direct the vent liquid into an underground vent tank, from where it returns to the system without being discharged outside.