(554g) Gas-Liquid Flow in a Rectangular Partially Aerated Bubble Column: Combined Effect of Aspect Ratio and Superficial Gas Velocity

ABSTRACT:

Bubble columns are multiphase
reactors frequently used in the chemical, biological and petrochemical
industry. The main objective of the design and operation of this multiphase
equipment is the maximization of its performance, that is, the calculation of
the optimum conditions for mass and heat transfer. Transfer phenomena taking
place across the gas-liquid interface depend strongly on the mixing efficiency
and, therefore, on the existing flow regimes inside the bubble column.
Consequently, the correct design of bubble columns requires the accurate
identification of the existing flow patterns at different experimental
conditions, since this identification is the key in the selection of the
appropriate models for mass and heat transfer.

This work presents the
characterization of the flow regimes in a two-dimensional bubble column using a
centrally aerated plate. Two variables are studied: the superficial gas
velocity (U_G) and the aspect ratio (H/W). The selection of these
variables is based on their crucial role on the development of different flow
regimes and scale-up operations respectively. Visual observations and the
measurement of wall pressure fluctuations are the experimental methods used for
a qualitative and a quantitative description of the flow respectively.
Different values of U_G and (H/W) result in different pressure time
series that are analyzed from both average and instantaneous perspectives. The
steadiness or unsteadiness of the flow as well as the instantaneous liquid flow
patterns and time-averaged flow regimes are analyzed together. As a result, a
complete description of the flow regimes encountered at different values of U_G
and (H/W) is obtained, unifying previous descriptions that did not take into
account the combined effect of these two variables.

In this way, the existence of pseudo
steady state flow regimes for all values of U_G is only observed at
(H/W) = 1.25. At low values of U_G, a single liquid circulation cell
of the width of the column is observed (single circulation bubbly flow (SCBF))
while at high values of U_G, the flow is characterized by a couple of
symmetrical vortices with the liquid phase moving up in the column's centerline
and moving down along the sidewalls (double cell turbulent flow (DCTF)).  At 1.50 ≤ (H/W) ≤ 2.00, an
increasing value of U_G delimits the existence of unsteady flow
structures (vortical flow (VF)) and the DCTF while the flow is unsteady (VF)
for all values of U_G at (H/W) = 2.25. The VF is characterized by the
existence of unsteady flow structures consisting on two or three transient
circulation cells (depending on (H/W)) together with an oscillatory bubble
plume. Quantitative analysis of the transition velocities between different
flow regimes is also presented in this work.