Introduction

Bubble columns are commonly used as contact reactors in chemical processing, bio-chemical, and metallurgical applications due to their simplicity (e.g., no moving parts), low operation cost, and high efficiency at heat and mass transfer. Understanding and modeling the transport phenomena as well as hydrodynamics of bubble columns requires a fundamental understanding of characteristics of the dispersed (gas) phase (i.e. bubbles). Bubble size (db ), population, and rise velocity (Ub ) significantly influence the physical behavior of the bubbly flow.1Bubble size distribution (BSD) is a primary aspect in the understanding of the physical behavior of the multiphase flow and was studied in this work. Note that the bubble rise velocity is a function of bubble size; therefore, any factor that effects the bubble size effects the rise velocity, which in turn effects the void fraction (ε ). Both bubble size and void fraction are impacted by gas superficial velocity, liquid properties, bubble column operation condition, column geometry, and gas injection method. Current work studies the effect of gas superficial velocity and liquid viscosity on bubble size and void fraction.
Shah et al.2 showed that the void fraction is predominately a function of the gas superficial velocity. The study of bubble columns with different system characteristics showed that there is a direct correlation between gas superficial velocity and void fraction.3-11 Lockett and Kirkpatrick12 and Kara et al.13showed that in the homogenous regime, void fraction exhibits a linear increase with increasing gas superficial velocity. However, in the heterogeneous regime the functional form between gas superficial velocity and void fraction is less apparent.13,14Liquid properties effect the void fraction by influencing the bubble formation as well as coalescence and breakup processes.1 The bubble column literature reports both increasing and decreasing void fraction with increasing liquid viscosity.15-21 Besgni et al.22argues that viscosity has a dual effect on void fraction. At low liquid viscosity, the coalescence is limited and increasing the viscosity increases the drag force acting on bubbles and, in turn, increases the bubble residence time and void fraction. However, in more viscous liquids, viscosity increases the coalescence rate and, consequently, produces larger bubbles with higher terminal velocity that decrease the void fraction. Bubble column literature provides numerous correlations for the prediction of the void fraction. Interested readers are referred to Besagni et al.23 for a summary of available correlations. Akita and Yoshida24 proposed a well-known correlation for void fraction scaling based on dimensional analysis. Their work suggests that the Froude number (Fr ), Archimedes number (Ar ), and Eötvös number (Eo ) scale the void fraction with a power law functional form, ε/(1- ε)4 = CFrΧArΨEoΩ . HereC is a proportionality constant and Χ,Ψ,Ω are the powers of each non-dimensional term. Similar functional forms are reported in the bubble column literature.16,25-28 Akita and Yoshida24 used the column diameter as a characteristic length scale to calculate the aforementioned dimensionless terms; however, in the present study using the bubble size as the characteristic length scale seems more appropriate since the bubble size is much smaller than the column diameter.
There is a general scarcity in bubble size data reported in the bubble column literature, partly because of the difficulties associated with bubble size measurements. While Leonard et al.29outline the inconsistencies in the bubble size distribution literature, there is a general consensus that in the homogenous regime the bubble sizes increase with increasing the gas superficial velocity while in the heterogeneous regime bubble size decreases with increasing the gas superficial velocity. Li and Prakash30 studied the spatial distribution of bubbles and found that smaller bubbles dominate the near wall region, and larger bubbles are more common in the central region of the column. In a highly viscous liquid, the bubble surface is more stable, larger bubbles form at the injector,31,32and the coalescence rate is larger than the breakage rate.2,33-35 The study of bubble size distribution shows that in viscous liquids the probability density function (PDF) of the BSD exhibits a bimodal shape.15,21,36,37 In the bubble column literature, scaling of the characteristic bubble length has been broadly approached assuming the sizing is dominated by either a breakage mechanism38 or bubble formation.39,40 The former attempts to find a stable bubble size under a given external (breakage) force in the heterogeneous regime, and the latter aims to find a characteristic bubble length scale in the homogenous regime using gravity, surface tension, and shear forces acting on a bubble.
The goal of the current work is to study the bubble size and void fraction in a batch bubble column with respect to operation regime and contribute to the current understanding of these multiphase parameters. This paper is organized as follows. Section 2 describes the experimental setup including instrumentation used. In Section 3, the results are presented for characterization and scaling of the bubble size and void fraction. Finally, conclusions and remarks on the current work are given in Section 4.