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

M. Elena Diaz, Francisco J. Montes, and Miguel A. Galan. Chemical Engineering Department, University of Salamanca, Plaza de Los Caidos, 1-5, Facultad de Ciencias Quimicas. Departamento de Ingenieria Quimica, Salamanca, 37008, Spain

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 (UG) 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 UG 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 UG 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 UG is only observed at (H/W) = 1.25. At low values of UG, a single liquid circulation cell of the width of the column is observed (single circulation bubbly flow (SCBF)) while at high values of UG, 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 UG delimits the existence of unsteady flow structures (vortical flow (VF)) and the DCTF while the flow is unsteady (VF) for all values of UG 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.