280 THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING VOLUME 85, JUNE 2007 INTRODUCTION B ubble column reactors are gaining importance as a simple and inexpensive means of achieving gas-liquid contactor where gas or mixture of gases is introduced in a continu- ous liquid phase as a dispersed phase of bubbles. Before the 1970s, no great scientific interest was shown in bubble column reactors. Very little literature was available from either industries or universities. However, since the mid-1970s, the research and development interest in various types of bubble columns has dramatically increased. A lot of studies have been reported on the hydrodynamics of two-phase co-current flow but the majority of these studies deal with either horizontal two-phase flow or vertical two-phase upflow. Reports on two-phase vertical downflow are very scanty. These studies can be categorized either under a plunging jet or sparger type system. In the plunging jet system, a jet of liquid while plunging into a pool of the same liquid carries along with it some ambient gas, which disperses into bubbles due to momentum transfer of the jet. The liquid and gas bubbles move down through the liquid pool to some distance and the gas bubbles then move up. In the sparger type system, gas is allowed to pass through the sparger, and the Energy Efficiency of Two-Phase Mixing in a Modified Bubble Column Subrata Kumar Majumder 1* , Gautam Kundu 2 and Dibyendu Mukherjee 2 1. Department of Chemical Engineering, Indian Institute of Technology Guwahati, India, 781039 2. Department of Chemical Engineering, Indian Institute of Technology, Kharagpur, India, 721302 liquid flowing past the sparger shears the gas and carries it down the column. Recently, a number of researchers have shown a keen interest in two-phase gas-liquid downflow bubble columns due to certain recompenses such as bubbles are finer and more uniform in size, coalescence is negligible, homogenization of gas bubbles can be obtained and gas-liquid contact area is high (Majumder et al., 2006c). Some authors have published works (Burgess et al., 1973; Kulkarni et al., 1984; Bando et al., 1988; Bin, 1993; Evans et al., 2001; Majumder et al., 2006a, b) that are relevant to the liquid jet ejector systems. These studies are said to be attractive. However, energy efficiency of gas-liquid mixing has not been reported in detail. In spite of the wide application of bubble columns, the design procedures are still closer to empiricism. This is principally because of the complexity of fluid mechanics of the two-phase flows in bubble columns. The knowledge of energy efficiency of phase mixing of the system is of direct importance because of obvious reasons, indirectly also; it is very useful as it gives an Energy efficiency for gas liquid mixing in a modified downflow bubble column reactor has been analyzed in this paper. Efficiencies of the differ- ent parts of the bubble column have been assessed on the basis of energy dissipation. Prediction of the energy dissipation coefficient as well as energy utilization efficiency due to gas-liquid mixing as a function of different physical, geometric and dynamic variables of the system has been done by correlation method. The distribution of energy utilization in the different zones of the column has also been analyzed. Experiments were carried out with air-water and air-aqueous solutions of carboxy methyl cellulose with different concentrations. Dans cet article on a analysé l’efficacité énergétique pour le mélange gaz-liquide dans un réacteur à colonne à bulles à écoulement descendant modifié. Les efficacités des différentes régions de la colonne à bulles ont été évaluées sur la base de la dissipation d’énergie. On a utilisé une méthode de corrélation pour prédire le coefficient de dissipation d’énergie ainsi que l’efficacité d’utilisation de l’énergie due au mélange gaz- liquide en fonction des différentes variables physiques, géométriques et dynamiques du système. La distribution de l’utilisation de l’énergie dans les différentes régions de la colonne a également été analysée. Des expériences ont été menées avec des solutions air-eau et air-solutions aqueuses de carboxyméthylcellulose à différentes concentrations. Keywords: two-phase mixing, downflow bubble column, energy dissipation, ejector system * Author to whom correspondence may be addressed. E-mail address: rsmaju@yahoo.com