Gas holdup and mass transfer in bubble column reactors operated at elevated pressure

Abstract

Measurements of the total gas holdup, ε, have been made in a 0.15 m diameter bubble column operated at pressures ranging from 0.1 up to 1.3 MPa. The influence of the increasing system pressure is twofold: (1) a shift of the flow regime transition point to higher gas fractions, and (2) a decrease of the rise velocity of "large" bubbles in the heterogeneous regime. The large bubble rise velocity is seen to decrease with the square root of the gas density, √ ρ_G. This square root dependence can be rationalized by means of a Kelvin-Helmholtz stability analysis. The total gas holdup model of Krishna and Ellenberger (1996, A.I.Ch.E. J. 42, 2627-2634), when modified to incorporate the √ ρ_G correction for the large bubble rise velocity, is found to be in good agreement with the experimental results. The influence of system pressure on the volumetric mass transfer coefficient, k_Lα, is determined using the dynamic pressure-step method of Linek et al. (1993, Chem. Engng Sci. 48, 1593-1599). This pressure step method was adapted for application at higher system pressures. The ratio (k_Lαε) is found to be practically independent of superficial gas velocity and system pressure up to 1.0 MPa; the value of this ratio is approximately equal to one half. This result provides a simple method for predicting k_Lα using the model developed for estimation of ε.