A model for gas holdup in bubble columns incorporating the influence of gas density on flow regime transitions

Abstract

The aim of this study was to develop a practically usable model to describe the influence of increased gas density on the gas holdup in bubble column reactors. In order to develop an insight into this effect, we performed extensive sets of experiments at pressures ranging from 0.1 MPa to 2 MPa and with several gases (nitrogen, carbon dioxide, argon, helium and sulphur hexafluoride) in de-ionized water in a 0.16 m diameter bubble column. A careful analysis of the experimental results shows that the major effect of increased gas density is to stabilize the regime of homogeneous bubble flow and, consequently, to delay the transition to the churn-turbulent flow regime. The superficial gas velocity at this regime transition point, U_trans, was found to be a unique function of the gas density, encompassing both effects of pressure and molar mass. To elucidate the hydrodynamics in the two regimes, dynamic gas disengagement experiments were carried out in a 0.19 m diameter bubble column with four liquids (water, turpentine, n-butanol and mono-ethylene glycol) using nitrogen at 0.1 MPa. These results showed that the churn-turbulent regime is characterized by a bi-modal bubble size distribution, consisting of fast rising large bubbles (typically 5 cm diameter or larger) and small bubbles (typically ≤ 5 mm diameter). In the churn-turbulent regime the holdup of the small bubbles was found to be virtually constant. The regime transition velocity U_trans was found to depend on the liquid properties. A simple model for describing the gas holdup is also proposed.