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

Measurements of the total gas holdup, epsilon, 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 how 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, root rho(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 root rho(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(L)a, 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(L)a/epsilon) 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(L)a using the model developed for estimation of epsilon. (C) 1999 Elsevier Science Ltd. All rights reserved.