Reflux issue in the middle section of the paraffin oil hydrocracking distillation column

The amount of reflux simply affects the temperature distribution within the tower; temperature determines the efficiency of product distillation

Mid-section reflux, top reflux, and top circulation reflux work together to remove the remaining heat from the atmospheric pressure tower. Whether it is cold reflux or hot reflux, top reflux is an important means of controlling the temperature at the top of the distillation column; therefore, the amount of top reflux cannot be adjusted arbitrarily. The amounts of top circulation reflux and mid-section reflux, on the other hand, can be adjusted based on the even distribution of the vapor-liquid load in the distillation column, as well as taking into account the heat conditions within the column. With the increasing emphasis on energy-saving measures for equipment, efforts are made, whether in the design of new equipment or in the renovation of existing ones, to increase the proportion of heat removed through reflux from the lower section, thereby facilitating heat recovery through heat exchange and reducing the condensation cooling load at the top of the tower. However, for the tray at the mid-section reflux inlet, the mid-section reflux is subcooled liquid; on the heat exchange tray of the circulating reflux, it primarily serves for heat exchange as well as for one-way mass transfer that causes some of the vapor phase to condense into the liquid phase, resulting in a relatively poor distillation effect. The fractionation efficiency of heat exchange plates is only 30% to 50% of that of ordinary tray plates. As the reflux at the middle section increases, the reflux ratio of the upper tray decreases accordingly, resulting in a reduced separation efficiency in the upper part. Therefore, in production operations, it is appropriate to increase the reflux flow at the middle section of the high-temperature zone in a tower. The exact amount by which this flow should be increased depends on the fact that the quality of the product at the tower top and the products obtained from the side streams must remain satisfactory. When adjusting the temperature at the tower top, a ratio in which the heat removed by the reflux at the middle section is proportional to the heat removed by the reflux at the tower top proves to be both sensitive and stable. The heat absorbed during circulation reflux is low; the reflux flow rate at the tower top is high, resulting in good distillation performance at the tower top, while the heat utilization efficiency of the tower is low ； A large reflux flow in the top loop is beneficial for the distillation process in the upper part of the tower, but utilizing reflux from the middle section at higher temperatures in the lower part allows for better heat utilization. The reflux flow rate in the middle section is high, which has a significant impact on the light naphtha and heavy naphtha at the top of the atmospheric tower; as a result, the product composition tends to become lighter and the yield decreases ； The reflux flow rate in the second section is high, affecting the distillation efficiency of jet fuel and diesel (in units equipped with a diesel side stream)

High extraction volume means low internal recirculation, while low extraction volume means high internal recirculation; it’s easy to understand

Consider the material balance and heat balance of the tower!