Study on vapor-rise-up through hanging downcomers

Abstract

A common problem in high capacity trays is the difficulty of establishing a liquid seal of hanging or truncated downcomers since the hanging or truncated downcomers came out. When the downcomer seal is lost, the vapor rises up through the downcomer, which may cause tray instability and poor separation. For the downcomer unseal or vapor-rise-up, it was found that there exist two regimes of free vapor-rise-up regime and dynamic vapor-rise-up regime. The mechanism of free vapor-rise-up is lower liquid height than the tray pressure drop, whereas the mechanism of dynamic vapor-rise-up is the driving force of the tray pressure drop. Using the hydraulic model of dualflow tray, the vapor-rise-up model in dynamic regime was developed. The experiments were carried out in a 500 mm-diameter transparent plexiglass column with air–water system. From the experimental data, the vapor-rise-up correlation was obtained. The free vapor-rise-up regime converts into dynamic regime with the increase of the vapor rate, and the transition point is a certain real velocity of rise-up vapor. For preventing the vapor-rise-up, a seal line is required. With the definition of the rise-up vapor being limited to a certain ratio of rise-up vapor rate to liquid rate, the seal line is qualitatively obtained from the vapor-rise-up model. The seal line has the minimum liquid rate, which is in accord with the seal line by Li et al. [Li, Y.M., Yu, X.M. and Yao, K.J., 2004, Dynamic seals on hanging downcomers. Chem Eng Res Des, 82: 1].