Volume of catalytic cracking slurry returned to the tower

I understand your question — the operating procedures state that going upward in the tower increases the liquid level, while going downward reduces it; you find this logic a bit confusing, right? In fact, once you try it out, you’ll find that these two opposite changes aren’t contradictory; the explanation provided is quite thorough. I would like to add two points based on my own experience operating this device: the \"return to the tower\" process actually serves as a means of heat exchange regulation; it is used primarily to control the temperature distribution at the top and middle sections of the tower. Increasing the amount of fluid that returns to the tower enhances the internal recirculation in the upper part of the tower, and this liquid flowing down from the upper section naturally causes the liquid level at the bottom of the tower to rise. The increase in liquid level refers to the amount of liquid, not heat; therefore, \"heat being removed\" and \"increasing liquid level\" are not contradictory. The key function of the lower section of the tower is heat absorption and vaporization: The cooled oil slurry enters directly at the bottom of the tower, where it absorbs heat from that area and causes a large amount of light components to vaporize. Although the oil slurry itself increases the volume of material entering the tower, the vaporization effect is stronger, resulting in a reduction in the total liquid volume at the bottom of the tower and a drop in the liquid level. In practice, if the amount of fluid drawn back down the tower is too large, it may lead to vacuum conditions at the bottom of the tower or in the pump; it is important to maintain a balance. It is recommended that you review the situation by referring to the material balance diagram of the fractionation tower, or make slight adjustments to a certain amount of reflux on the DCS and observe the changes in liquid level for 10 minutes – this will give you a much clearer understanding than just reading the text. Of course, there are slight differences in the operating conditions of different devices; the specific parameters should be based on your workshop’s operating procedures and process sheets.

The counter-current contact between the rising gas from the tower and the reaction gas oil causes the heavier components in the reaction gas oil to condense; as a result, an increase in the amount of gas rising from the tower leads to an elevation in the liquid level at the bottom of the tower