Reasons for temperature fluctuations at the third fractionation outlet and in the lower section of the atmospheric tower in constant pressure and reduced pressure processes

Liquid flooding refers to the situation in a distillation column where, for various reasons, the liquid phase accumulates beyond the available space. Foaming can be divided into downcomer foaming, mist entrainment foaming, etc. ? Introduction: Liquid flooding in the downcomer refers to the accumulation of the liquid phase in the downcomer up to the upper tray. The main reasons for liquid flooding in the downcomer include a low clearance height at the bottom of the downcomer and an excessive flow rate of the liquid phase. Foam entrainment flooding occurs when the gas flow velocity in the open spaces of the tray reaches a certain level, causing the liquid phase on the tray to be carried upward with the rising gas to the upper tray. The main cause of foam entrainment and flooding is an excessive gas phase velocity. The operating condition at which flooding occurs is called the flooding point. When designing a distillation column, it is necessary to control and maintain the flooding rate within a certain range to ensure its stable operation. Liquid flooding of the tower? In a tower with a fixed diameter, the cross-sectional area available for free flow of both gas and liquid phases is limited. If the flow rate of either one of them increases to a certain level, the liquid in the downcomer cannot flow downward smoothly ； When the liquid in the tube rises to the top of the overflow weir on the upper plate, it will spill over onto that upper plate, resulting in abnormal accumulation of liquid; eventually, this can lead to the space between the two plates being filled with foam. This phenomenon is called flooding. If flooding is severe enough and the trays as well as the interior of the tower are filled with liquid, this is known as tower flooding. What is flooding in liquid-overflow tower equipment and what are its causes? When flooding begins, the pressure drop in the tower increases sharply, and its efficiency drops significantly. Subsequently, the operation of the tower was disrupted. The main factors that contribute to flooding are as follows: a. Backflow of liquid in the downcomer back to the upper tray. Due to the resistance exerted by the tray on the rising gas flow, the pressure above the lower tray is higher than that above the upper tray. Only when the static head corresponding to the height of the foam in the downcomer is sufficient to overcome this pressure difference can the liquid flow downward. When the liquid flow rate remains constant while the gas flow rate increases, the pressure difference between the lower and upper plates also increases, causing the liquid level in the downcomer to rise. If the gas flow rate increases to such an extent that the liquid in the downcomer rises to the top of the weir, the liquid in the tube can no longer flow downward; instead, it begins to flow back upward toward the upper tray, causing liquid to accumulate on that tray ； As it is operated, liquid is continuously fed in from outside the tower, eventually filling the entire tower with liquid. This results in flooding. If the gas flow rate remains constant while the liquid flow rate increases, the resistance to the flow of liquid through the downcomer rises, and the liquid layer on the plate thickens, which increases the pressure difference between the upper and lower sides of the plate. All these factors cause the liquid level in the downcomer to rise, leading to flooding. b. Excess foam is carried up to the upper plate. The foam carried by the airflow to the upper plate can cause the liquid layer on the plate to thicken; under normal conditions, however, this increase is not significant. At a certain liquid flow rate, if the gas flow rate is increased to a certain level, the thickening of the liquid layer becomes significant (more liquid accumulates on the plate, and more bubbles form and grow larger). The mist carried away by the airflow through the thickened liquid layer increases further. This excessive entrainment of liquid foam reduces the distance between the top of the foam layer and the bottom of the upper plate. The entrainment of liquid foam continues to increase, with large droplets easily being ejected onto the upper plate; foam can also rise to that plate, until eventually the entire tower is filled with liquid. Of the two reasons mentioned above that lead to flooding, excessive entrainment of liquid foam is the more common one.