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The following information comes from Haiyou, an experienced shift worker; it is for reference only. Of the heat brought into the atmospheric tower in a vacuum distillation process, after part of it is carried away by the products, the remaining heat is mainly removed by the top condenser and the intermediate reflux heat exchanger. Midsection reflux removes a portion of the heat from the middle part of the tower, thereby reducing the amount of reflux heat on the trays above the reflux extraction plate. As a result, both the liquid-phase reflux and the gas-phase load are decreased. Hence, the use of midsection reflux not only improves the processing capacity of the unit but also allows for the recovery of significant amounts of heat through the heat exchanger. When the processing capacity of the unit is at its limit, introducing reflux at the middle stage can reduce the gas-phase load on the column, decrease mist entrainment, and improve the distillation efficiency. However, when the processing capacity of the unit is relatively low, the reduction in the liquid-phase reflux above the mid-section reflux leads to a poor distillation effect in the column and a decrease in plate efficiency. Additionally, the few plates used for the mid-section reflux generally serve only for heat exchange, with limited distillation functionality. Therefore, for a tower, it is generally appropriate to increase the mid-section reflux flow at the high-temperature area in order to recover the heat there. The exact amount by which this flow should be increased depends on ensuring that the quality of the product at the tower top and the side products remains satisfactory. When adjusting the temperature at the tower top, this approach proves to be sensitive and stable; thus, it can be considered reasonable to use a proportionate balance between heat recovery through mid-section reflux and heat recovery through reflux at the tower top. The mid-section reflux absorbs less heat, resulting in a relatively large amount of reflux at the tower top; this yields good distillation effects at the top, but the tower’s heat utilization efficiency is low. The backflow rate in the middle section is high, which has a significant impact on the overhead product and the side-stream products; as a result, the products tend to become lighter, the yield of these products decreases, and their quality declines. Therefore, the adjustment of the mid-stage reflux volume should be based on the processing capacity of the device; if the processing volume is low, the mid-stage volume can be appropriately reduced, and vice versa. Under normal circumstances, the mid-stream reflux flow rate should not be used to adjust the quality of the product. Of course, the level of adjustment of the mid-stream reflux flow rate is also related to the design aspects of the tower; in some designs, the mid-stream reflux heat exchanger is made relatively large while the top condenser is smaller. In such cases, it is only possible to increase the mid-stream reflux as much as possible.

Cold reflux and overhead circulation reflux generally account for 30–40% of the total reflux in the tower, while mid-section circulation reflux accounts for 60–70%； The internal reflux under the recirculation extraction plate should be at least 30% of the internal reflux volume of the lower tray ； A high proportion of heat recovery from mid-stage reflux affects the quality and yield of the product; it is necessary to find an appropriate level of mid-stage reflux in order to increase product output ； Adjust the reflux ratio according to changes in the properties of the raw materials to achieve maximum product yield.

Generally, the heat removed by the overhead reflux in a catalytic fractionation tower (including recycle reflux and cold reflux) accounts for about 20% of the total reflux heat; the heat removed by the intermediate-stage reflux accounts for 30–50% of the total reflux heat, while the heat removed by the recycle slurry accounts for 50–30% of the total reflux heat. Regarding the distribution of heat absorbed from each circulation loop, it is generally determined based on the principle of increasing the proportion of high-temperature heat sources as much as possible, while ensuring product quality. The internal recirculation beneath the circulating return extraction plate should be at least 25% of the recirculation heat in that section. The heat extraction distribution ratio for each cycle varies significantly depending on the catalyst used and the production scheme. During the operation, heat extraction in each cycle should be adjusted according to different production plans.

I. Generally, for catalytic fractionation towers: 1. The heat removed by the tower top reflux (recycling reflux + cold reflux) accounts for 15–30% of the total reflux heat in the tower； 2. The heat removed by the mid-section reflux accounts for 25–50% of the total reflux heat. 3. The heat removed by the circulating slurry accounts for 30–45% of the total reflux heat. II. Regarding the distribution of heat absorbed from each circulating loop, it is generally determined based on the principle of increasing the proportion of high-temperature heat sources as much as possible, while ensuring product quality. III. The internal recirculation beneath the circulation return extraction plate should be at least 30% of the recirculation heat in that section. IV. Adjust the reflux ratio according to changes in the properties of the raw materials and different production plans, in order to achieve the highest possible product yield.

1. The proportions of cold reflux, overhead circulation reflux, and intermediate-stage reflux are approximately 20%:30%:30%:20%; the key is to consider the proportions of various fractions in the crude oil’s distillation curve and make adjustments accordingly. 2. The internal recirculation under the circulation return extraction plate should be at least 50%. 3. The higher the product yield, the greater the corresponding cycle reflux as well. However, it is necessary to ensure the quality of the product, including meeting requirements such as the initial boiling point and dry point, by making adjustments within a certain range. 4. While meeting the requirements of the product design, strive to increase the proportion of reflux at high temperatures, thereby raising the energy recovery rate, reducing the amount of circulating water used, and minimizing energy losses.

Cold reflux and overhead circulation reflux generally account for 30–40% of the total reflux in the tower, while mid-section circulation reflux accounts for 60–70%； The internal reflux under the recirculation extraction plate should be at least 30% of the internal reflux volume of the lower tray ； A high proportion of heat recovery from mid-stage reflux affects the quality and yield of the product; it is necessary to find an appropriate mid-stage reflux ratio in order to increase product output. While meeting the product requirements, efforts should be made to increase the proportion of reflux at high temperatures, thereby raising the energy recovery rate, reducing the amount of circulating water used, and minimizing energy losses.

For a distillation column, it is appropriate to increase the mid-section reflux flow at the high-temperature section in order to recover the high-temperature heat. When adjusting the tower top temperature, it is relatively sensitive and stable; thus, it can be considered that the ratio of heat extraction from the middle section reflux to that from the tower top reflux is reasonable. Generally, the amount of mid-stage reflux should be adjusted according to the processing capacity of the device; if the processing volume is low, the mid-stage amount can be appropriately reduced, and vice versa. Therefore, it is generally not advisable to use the mid-stage reflux flow to adjust the quality of the product. Of course, the level of mid-stage reflux flow also depends on the design factors of the tower; in some designs, the mid-stage reflux heat exchanger is made relatively large while the top condenser is smaller, and in such cases it is only possible to increase the mid-stage reflux as much as possible.

For catalytic fractionation towers, it is appropriate to increase the heat extraction from the high-temperature sections, especially at the bottom of the tower, while still meeting the separation requirements, in order to recover the thermal energy present in those high-temperature areas. 30–40% of the heat extracted is used to adjust the reflux at the top of the tower; depending on the specific circumstances, this proportion can vary and is usually around 20–30%. The amount of reflux in the middle section should be adjusted based on the processing capacity of the unit and the quantity of product produced there. If the processing volume is low, the amount of reflux in the middle section can be reduced accordingly; otherwise, the adjustment is done in the opposite direction, with this proportion accounting for 30–40%.

Generally, the heat carried away by the top reflux accounts for 20–30% of the total reflux heat. The amount of mid-stage reflux should be adjusted according to the processing capacity of the unit; the heat carried away by the mid-stage reflux makes up 30–50% of the total reflux heat, while the heat carried away by the circulating reflux accounts for 35–45% of the total reflux heat. If the processing volume is low, the amount of mid-stage reflux can be reduced appropriately; otherwise, the adjustment should be made in the opposite direction. Therefore, it is generally not advisable to use the mid-stage reflux flow to adjust the quality of the product. Of course, the level of mid-stage reflux flow also depends on the design factors of the tower; in some designs, the mid-stage reflux heat exchanger is made relatively large while the top condenser is smaller, and in such cases it is only possible to increase the mid-stage reflux as much as possible.