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Packed beds are widely used in industrial mass transfer operations, including absorption, stripping, adsorption and distillation. The packing material offers a large surface area available for heat and mass transfer between gas-liquid or fluid-solid phases for a given volume. Distillation, absorption, adsorption and extraction are typical applications of packed columns. In the design of a packed column, the averaged mass transfer coefficient is usually used and assumed to be constant at all locations in the column. This is due to the fact that studies of mass transfer in a packed bed are generally based on a macroscopic approach. In this approach, the averaged mass transfer coefficient is determined based on the conditions of the inlet and outlet streams without consideration of local fluid dynamic and local mass transfer at different locations within the bed. Local mass transfer in a packed bed is in fact dependent on local liquid velocity. The local mass transfer coefficient thus varies with locations in the packed bed due to the variation of velocity and the random nature of liquid spreading in the bed. Therefore, the use of the averaged mass transfer coefficient often renders uncertainty in design and scaling-up of a packed column. Among different methods, dissolution (Kumar et al., 1977; Sedahmed et al., 1996; Guo and Thompson, 2001) and gas absorption (Aroonwilas et al., 2003; Linek et al., 2001) are the two popular methods that have been used to obtained the average mass transfer coefficient. More recently, direct measurements of the local mass transfer coefficient in a packed bed was developed using an electrochemical technique (Gostick et al., 2002), and a mathematical model for local mass transfer coefficient in a packed bed was proposed (Dang-Vu et al., 2006a). The mass transfer coefficient is strongly dependent on liquid distribution in a packed bed. Liquid distribution is in turn dependent on the packing size and type, and the design of the liquid distributor. Several studies on liquid velocity and distribution in a packed column have been carried out using different techniques, such as: liquid collecting method (Hoek et al., 1986; Kouri and Sohlo, 1996; Dang-Vu et al., 2006b), tracing method (Macias-Salinas and Fair, 1999; Inglezakis et al., 2001), conductance probe (Tsochatzidis et al., 2002), and tomographic measurements (Loser et al., 1999; Reinecke and Mewes, 1997; Yin et al., 2002; Bolton et al., 2004; Ruzinsky and Bennington, 2007). The liquid collecting method has been used widely to investigate liquid distribution in a packed column due to its simplicity in measurements

Measurement of Liquid Velocity and Liquid Distribution in a Packed Bed Using Electrical Resistance Tomography