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.