HYDRODYNAMICS AND GAS-LIQUID MASS TRANSFER IN A RECTANGULAR BUBBLE COLUMN
Advancing the chemical engineering fundamentals
Multifase Flows (T2-5P)
Keywords: hydrodynamic, gas liquid mass transfer, volumetric mass transfer coefficient, bubble column
HYDRODYNAMICS AND GAS-LIQUID MASS TRANSFER IN A RECTANGULAR BUBBLE COLUMN
ELENA M. CACHAZA, ELENA DIAZ MARTÍN, FRANCISCO J. MONTES AND MIGUEL A. GALAN
ABSTRACT:
Bubble columns (BC) are multiphase reactors widely employed in chemical, biochemical and petrochemical industrial processes due to their advantages such as simple construction and excellent heat and mass transfer. 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 of the bubbles, depend strongly on the mixing capacity of the system and, therefore, on the existing flow regimes inside the bubble column. This work presents the estimation of the efficiency of oxygen mass transfer in a two dimensional bubble column using an air- water system related to the hydrodynamics developed inside the reactor at different aspect ratios and superficial gas velocities.
For volumetric mass transfer coefficient (kLa) measurements in BCs, the dynamic gas absorption method has been popularly used in BCs for its simplicity and reliability. In the dynamic method, the measured transient concentration profiles of the introduced gas represent not only the extent of gas–liquid mass transfer rate but also the phase mixing. Hence, a reactor model is needed to estimate kLa values by fitting the experimental data. Based on its advantages, the dynamic method has been chosen in this work and kLa values have been estimated using two reactor models: (1) the continuous stirred tank reactor (CSTR) model, which assumes perfect mixing for both phases and constant gas concentration in the gas phase; and (2) the axial dispersion model (ADM). Both of them are completed with another model provided by the gas sensor used for the measurement of the dissolved gas concentration.
Despite the extended use of the dynamic method, there are still some unsolved problems when calculating accurate values of kLa. Among them, the position of the oxygen sensor in the bubble column has been pointed out as a variable determining the resulting kLa value. This variable is also studied in this research in order to determine its influence on the calculated mass transfer coefficients.
In this way, the work presented has three main parts:
(1) The study of the influence of the hydrodynamics in the determination of the volumetric mass transfer coefficients.
(2) The fitting of the experimental results to the CSTR and AD mass transfer models considering the existence of a sensor delay in the measurements and the comparison of the kLa estimated by both of them in the 2D bubble column reactor at the tested operating conditions.
(3) The relationship between mass transfer and the position of the gas sensor in the BC.
Presented Tuesday 18, 13:30 to 15:00, in session Multifase Flows (T2-5P).
