Practical production: What is a reasonable value for the reflux flow rate in the middle section of an atmospheric pressure tower?

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Mid-section reflux, top circulation reflux, and tower top reflux all play a role in removing heat from the atmospheric pressure tower; the way in which these processes share the heat removal task has a direct impact on product quality, separation accuracy, and the energy consumption of the plant. Based on actual on-site operations, there is only one core principle for determining the reflux flow rate at the middle section: maximize the utilization of heat from higher temperature areas, provided that all products meet the quality standards and the gas-liquid load within the tower is evenly distributed. I. First, determine whether the three types of reflux can be adjusted arbitrarily. 1. Top-of-tower reflux: Its primary function is to control the temperature at the top of the tower and ensure the dry point of the gasoline at the top; it serves as a key factor in determining the separation accuracy of the entire tower. It cannot be adjusted significantly, only slightly to maintain a stable temperature. 2. Top-loop recirculation primarily removes the residual heat from the upper part of the tower, reducing the load on the air/water cooling systems at the tower top; it is beneficial for separation in the upper section, but the temperature is low, resulting in low value of the recovered heat. 3. Mid-stage reflux (first and second stages) has a high temperature, which facilitates heat exchange and heat recovery; it is key to energy savings in the system ； However, it is a supercooled liquid; its return to the tower is primarily for heat exchange and condensation purposes. As a result, the fractionation efficiency of the corresponding tray is only 30% to 50% of that of a normal tray, and it also reduces the reflux ratio of the trays above it, thereby affecting the separation performance. Simply put: Tower-top reflux: Ensures product quality; must not be altered. Top-cycle reflux: Reduces the load and lowers the temperature. Mid-section reflux: Can save energy, but affects separation precision. II. In practice, what is the appropriate amount of mid-section reflux to use? 1. General principle: Priority should be given to increasing the reflux flow rate in the middle section of the lower high-temperature area, but one strict constraint must be met: the quality of all side-stream products, such as regular gasoline, Line 1 product, Line 2 product, and Line 3 product, must be satisfactory. The criterion for judgment is straightforward: if the top temperature of the tower responds sensitively to adjustments and remains stable without significant fluctuations, it indicates that the heat extraction ratio between the mid-section reflux and the top reflux is generally appropriate. 2. How to control the mid-stage reflux? The position of the mid-stage reflux is relatively high, and it has the most direct impact on the products in the upper section: Excessive mid-stage reflux often leads to gasoline being carried over; the first distillation fraction tends to be lighter, the dry point becomes lower, and the yield of the products decreases ； The diameter of the tower is too small: the load on the upper part increases, the cooling load at the top of the tower rises, energy consumption goes up, and the temperature at the top of the tower tends to be high. In practice, the amount is generally increased to an appropriate level while ensuring that the flash point and freezing point meet the required standards; there is no attempt to increase it excessively. 3. How to control the reflux in the second middle section? The temperature in this section is higher, which offers greater energy-saving benefits; it mainly affects the products in the middle and lower sections. If the amount of reflux in the second middle section is too high, it will significantly impact the separation efficiency of the first and third output streams, leading to increased overlap between these streams and resulting in substandard product quality ； The diameter of the second stage is too small: heat from the lower part cannot be removed, resulting in a high load at the bottom of the tower; less heat can be recovered through heat exchange, which increases the energy consumption of the system. Therefore, No. 2 Plant can absorb an appropriate amount more heat, but it must adhere to the strict requirement that the fractions in the second and third ranges be qualified and non-overlapping. III. Typical manifestations of an inappropriate heat extraction ratio: 1. Low mid-section reflux and high top reflux. The separation efficiency at the tower top is good, but the heat utilization rate is low, the cooling load is high, and energy consumption is high; thus, air cooling/water cooling operates at full capacity for extended periods. 2. Excessive mid-section reflux results in good energy-saving effects, but the products from the tower top and side streams tend to be too light, leading to low yields; the separation efficiency of the tray system decreases, and the products are likely to fail quality standards. 3. The top circulation is too large; separation in the upper part is acceptable, but the heat generated at high temperatures is not utilized, resulting in poor overall efficiency – it would be better to increase the height of the middle section. IV. One-sentence summary of on-site operations: Maximize reflux at the middle stage of the high-temperature section, as long as the product quality remains satisfactory ； The reflux at the top of the tower keeps the temperature stable, while the top circulation helps to reduce the load ； A sensitive and stable control of the tower top temperature represents the optimal heat extraction ratio.