What is the impact of excessive changes in the vapor and liquid phase loads in a tower on its operation? How does the phenomenon of tower flooding occur?

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If the vapor load is too low and the gas velocity inside the tower is too low, liquid leakage will occur (that is, liquid drips down through the openings in the upper tray levels), also known as liquid spillage or leakage. Clearly, liquid leakage reduces the separation efficiency of the tower and hinders its stable operation. An excessive vapor load causes intense agitation of the liquid layer on the tray, resulting in a higher foam layer and an increased amount of liquid entrained into the upper trays. If the entrainment level exceeds 10%, it will disrupt normal operation. If the liquid phase load is too low, below 50% of the design load, it becomes difficult to operate due to hydraulic constraints within the tower. If the liquid load increases suddenly, the flow velocity of the liquid in the downcomer speeds up, preventing the gas components entrained in the liquid from separating in time; these gases then reach the lower tray levels, reducing the separation efficiency of those trays. It is generally required that the flow velocity of the liquid in the downcomer shall not exceed 0.12 m/s. A sudden excessive increase in the feed rate will increase both the liquid-phase load and the vapor-phase load. This leads to an increase in the liquid level height at the outlet weir, an increase in the gas velocity through the valve orifice, greater resistance as the liquid flows through the downcomer, and an increased pressure drop across the tray. As a result, the liquid level height (h_liquid) in the downcomer rises. There is the following relationship among the various factors: H_liquid = △p_plate + h1 + h2 + h_weir. Here, △p_plate represents the pressure drop across the tray, in mm of liquid column ； h1—Height of the liquid layer at the outlet weir, in mm of liquid column ； h2—Resistance to fluid flow through the downcomer, in mm of liquid column ； h weir—exit weir height, in mm of liquid column. If the liquid level in the downcomer reaches its peak and the downcomer is filled with liquid, the upper and lower trays become connected by the liquid, completely disrupting the distillation process. This is what is known as tower flooding or liquid flooding phenomenon. It is generally required that the liquid level height h in the downcomer be ≤ 0.5 plate H (where plate H is the plate spacing).