ERT MEASUREMENT OF GAS HOLD-UP DISTRIBUTION IN BUBBLE COLUMN WITH COALESCENCE INHIBITING SOLUTIONS Chirag B. Dave, Bawadi Abdullah, Dai-Viet N. Vo, Tuan Huy Nguyen, Cyrus G. Cooper, Adesoji A. Adesina* Reactor Engineering & Technology Group, School of Chemical Sciences and Engineering, University of New South Wales, Sydney, New South Wales, Australia 2052 *Corresponding author. Tel.: +61 2 9385 5268; Fax: +61 2 9385 5966 Email: a.adesina@unsw.edu.au Abstract: A study has been undertaken to investigate the gas hold-up variations in a 0.1 m ID bubble column using Electrical Resistance Tomography (ERT). Based on different inorganic solute concentrations in water, the continuous phases are prepared into coalescence inhibiting phases to study the effect of ionic strength on the changing electrical conductivity and the resultant gas hold-up. Nitrogen gas was selected as the non-conductive phase and the superficial gas velocity of nitrogen was varied between 0 to 0.064 m/s. Sintered metal plate with pore size of 10 μm was used as a sparger. Gas hold-up increased proportionally with changes in conductivities of the liquid mixture and superficial gas velocity. Increasing ionic strength, based on increasing aqueous solution concentration, leads to an increase in gas hold-up due to dissociation of ions in the solution and also inhibits bubble coalescence. Increased ionic charge also led to increased inhibition of the bubble coalescence. Keywords: bubble column; liquid phase properties; hold-up profiles; tomography, 1. INTRODUCTION Bubble column reactors have been extensively utilised in chemical, biochemical, petrochemical and metallurgy industries. They have many advantages such as high heat and mass transfer coefficients, compactness, low maintenance and operating cost. The performance of bubble column reactors is influenced by key variables including; gas hold-up, bubble size distribution, bubble slip velocities and flow regime characterisation (Kantarci et al., 2005). Bubble size distribution is a complex phenomenon which depends on various parameters such as bubbles breakage, coalescence, growth, nucleation and shrinkage and the relative slip velocities between the dispersed and the continuous phase. Mechanical and chemical properties such as shear forces, dissipation of turbulent energy, reactions, gas-liquid equilibrium also have a strong influence on bubble size distribution. Bubble coalescence is defined by considering bubble collisions due to turbulence, buoyancy and laminar shear and can be explained using a small process of two small bubbles colliding and during the collision a small amount of liquid is trapped between the bubbles. This liquid drains until a critical thickness of liquid is attained and the thin film rupture causes coalescence. Bubble break-up can be analysed in terms of bubble interactions with turbulent eddies. The turbulent eddies increase the surface energy of the bubbles through deformation and when it reaches a critical value; bubble break-up occurs (Prince and Blanch, 1990). Parasu Veera and Joshi (2000) carried out experiments on the effect of liquid phase properties on gas hold-up using gamma ray tomography. They showed that various factors such as liquid phase properties, sparger design, column diameter, height of dispersion and superficial gas velocity strongly influence gas hold-up profiles. For a coalescing liquid such as butanol, the gas hold-up profile is steep and for a non-coalescing liquid phase such as carboxyl methyl cellulose, the hold-up profile is relatively flat. Parasu Veera et al., (2004) applied gamma ray tomography in bubble column to investigate the effect of foaming agent concentration on the bubble size distribution in foaming liquids and observed that the rise velocity of a single bubble depends on the size of the bubble and average bubble size decreases as temperature and pressure are increased and liquid surface tension is decreased. Average bubble size increases as liquid viscosity increases. George et al., (2000) validated ERT measurements with Gamma Densitometry Tomography in a slurry bubble column reactor and found good agreement with the results for average gas volume fractions and radial gas volume fractions in gas-liquid flows and solid volume fractions in solid-liquid