554g

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.

See more of #554 - Multiphase Mixing I: Gas-Liquid Mixing (06000)

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See more of North American Mixing Forum

See more of The 2006 Annual Meeting