Issues with heat recovery reflux and side-stream discharge in the middle section of atmospheric and vacuum distillation units

I found some explanations on the forum, and I’d like to share them with everyone: The main function of the middle return is to balance the gas load in the tower and reduce its diameter. If no mid-return is used and top return is employed exclusively, the tower diameter will be very large, and the cooling load at the tower top will be high; moreover, it is difficult to recover heat due to the low temperature at the top, resulting in significant waste. Therefore, installing one or more mid-returns at appropriate positions within the tower allows for effective heat recovery, though this comes at the cost of reducing the efficiency of several trays between the mid-return inlet and outlet. Extracting heat at intermediate stages can affect the separation of the products; in my opinion, intermediate stages should not be used as a means to adjust product quality. Finally, speaking of top circulation, the purpose of it is to reduce the cold reflux at the top of the tower. The general operating principle here is to appropriately decrease the amount of cold reflux, in order to reduce the cooling load on the tower top and increase the heat output from the reflux at the lower levels. During operation, do not adjust the mid-stage reflux frequently, and do not use it to control product quality; the goal is to maintain a stable balance in the distribution of vapor and liquid loads in the atmospheric pressure tower. The mid-stage reflux is adjusted only when there are significant changes in the refining volume, variations in the density of crude oil, or alterations to the product mix. This helps maintain a good heat balance throughout the entire tower ； Adjust the internal recirculation to suit the product design; the product must meet quality standards. The principle for heat extraction is to make full use of the heat in the middle section, by extracting more heat from the lower part of this middle section at higher temperatures. But at the same time, it is necessary to avoid excessive temperature differences during withdrawal, as this can affect the efficiency of the return tray; therefore, there should be lower and upper limits! The return temperature of the mid-section reflux should not be too low; the temperature difference with the temperature at the returning tray should not be excessive, otherwise vaporization will occur, disrupting the normal thermal balance. It is advisable for the reflux temperature to be slightly lower than the temperature of that tray. In principle, it is still advisable to consider a control scheme that features a small temperature difference and a high flow rate. Advantages: 1. It helps to make the vapor-liquid load more uniform throughout different sections of the tower; 2. It increases the processing capacity of the equipment; 3. The diameter of the tower can be reduced; 4. It facilitates heat recovery, thereby reducing the consumption of fuel and cooling water. Disadvantages: 1. It complicates the process and increases the number of pumps and heat exchangers; 2. It requires a large volume of liquid to be transported, resulting in high power consumption; 3. The reflux entering the tower is subcooled liquid, and to achieve vapor-liquid equilibrium on the tray surfaces, several trays that serve primarily as heat exchangers are needed, which reduces the separation efficiency.