Phase hold­up, mixing and mass transfer in a bubble column reactor using ERT analysis

Chirag B. Dave, Bawadi Abdullah, Tuan­Huy Nguyen, Cyrus G. Cooper and Adesoji A. Adesina* Reactor Engineering & Technology Group, School of Chemical Engineering

The University of New South Wales, Sydney, New South Wales, Australia 2052

a.adesina@unsw.edu.au

ABSTRACT

A study has been undertaken to investigate the gas holdup variations, degree of gas dispersion and mass transfer in a 0.1 m ID bubble column using Electrical Resistance Tomography (ERT). ERT is a non­intrusive technique for the internal visualisation of electrical conducting processes in a multiphase system. A sintered metal plate (pore size of 10 µm) is used as a gas distributor. Air was used as the dispersed phases in water to determine the mass transfer behaviour in the column. In this study, gas­ phase Reynolds number (Reg) varied from 0 to 435 resulting in a sigmoidal increase in gas volume fraction and dimensionless gas dispersion time. The radial gas holdup profile is parabolic behaviour with a maximum at the centre. Correlation of gas holdup as a function of both the Reg and the radial coordinate was obtained. The degree of gas dispersion is importantly dependent on bubble coalescence and breakup phenomena where bubble coalescence occurs when absorption dominates and bubble breakup results during desorption. Gas dispersion is further influenced by the physicochemical properties of the liquid phase and the geometric dimensions of column vessel. Heterogeneity Index for pixel­based Tomograms (HIT) was used to quantify the degree of gas dispersion through the analysis and ranking of measured conductivity fluctuations to quantify heterogeneity in the image. Over the range of gas flow rate employed, three flow regimes existed in the two­phase mixture? homogeneous bubbly flow, transitional flow and churn­turbulent flow. The residence time distribution of the tomography data using a dispersion model for open vessel enabled to obtain Peclet Number, Per and thus the coefficient of gas dispersion. The increasing ratio of Reg to Per led to a power-­law increase in mass transfer coefficients which were found to be between 9 x 10-4s-­1 and 3 x 10-2 s-1.

Keywords gas holdup, mass transfer, degree of gas dispersion, ERT