Seeking help regarding “overflow””

This post was last edited by LQ198619 on 2017-2-21 at 14:10. The original poster should use a more detailed title for the next post! OP, why do I think your post is asking about liquid flooding in distillation columns?

This post was last edited by ardance1977 on 2017-2-21 at 14:19. What he wanted to ask about was flooding; he had never heard of the term ‘overflow’. I must have heard it from someone else; I didn’t understand it. For a tower of fixed diameter, the cross-section through which both gas and liquid phases can flow freely 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 pipe rises to the top of the overflow weir on the upper plate, it will spill over onto that upper plate, causing abnormal accumulation of liquid; ultimately, this can result in the space between the two plates being filled with foam. This phenomenon is known as flooding, also referred to as tower flooding. At the onset of flooding, 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 lead to flooding are as follows: 1. 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 liquid bubble 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, where liquid starts to accumulate ； As operations proceed, 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. 2. 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 increases to a certain level, the thickening of the liquid layer becomes significant (more liquid accumulates on the plate, and there are more bubbles as well as larger bubbles). The droplets carried away by the airflow through the thickened liquid layer increase 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. Among the causes of flooding, excessive liquid foam entrainment is a relatively common one.

The answer on the 2nd floor was very professional; I’ll learn from it too.

Foaming is a precursor to tower flooding; if you really understand this, it’s best to go straight to Baidu – it’s a faster way to solve the problem!