On the interrelation of Flooding- and Complete Dispersion Characteristics in Agitated Gas-Liquid Contactors
Advancing the chemical engineering fundamentals
Multifase Flows - I (T2-5a)
Keywords: gas-liquid systems, agitated vessels, flooding, complete dispersion
On the interrelation of Flooding- and Complete Dispersion Characteristics in Agitated Gas-Liquid Contactors
by Eva Babalona and Joannis Markopoulos
Gas-Liquid contactors agitated by Rushton turbines are widely used in chemical and biochemical processes, offering optimal dispersion conditions and greater efficiency of gas utilization in the reactor. Concerning the mechanism of gas dispersion, a particularly important criterion in designing the agitator is flooding. Several investigators have worked on the flooding-loading transition, giving each of them different results. These differences between the results arise from the difference in the definition of flooding. After a review of the literature it is found, that the definition for flooding-loading transition which seems to be used by most researchers, is the transition that is characterized by the ceasing of the radial motion of bubbles in the impeller stream. However, at this stage of agitation, the dispersion is ineffective, as the gas bubbles are sufficiently dispersed only in the zone above the impeller.
On the other hand, and despite the importance of flooding- and complete dispersion characteristics in mechanically agitated gas-liquid contactors, limited data, mainly in the form of regime maps, are available in literature on the interrelation of the flooding-loading and loading-complete dispersion transitions. Particularly, working at the flooding-loading transition, i.e. at the minimum stirrer speed at which flooding does not occur for constant gas flow rate (nF,qF)*, only 1/3 or in some cases even 1/2 of the tank volume is efficiently used; the same approximately occurs, when for constant gas flow rate the speed is smaller than the speed nCD* for complete dispersion. Consequently, it is not nF but nCD which determines the optimal working conditions. Thus, having nF and nCD experimentally predicted, the knowledge of the interrelation of nF and nCD (or, in dimensionless form, of FrF and FrCD), at a given gas flow range, would be of fundamental importance and of great practical use.
In this work, there has been an attempt to study the dependence of nCD on nF for gas flow numbers between 0.03 and 0.09, in aerated gas-liquid contactors agitated by Rushton turbines of two different diameters d=0.33D and d=0.49D. The accuracy of the method used is estimated to be better than 10%. The experimental results were determined either visually, or from the step change of power consumption when the gas flow changed at constant speed.
*(nF, nCD, etc. means: stirrer speed at flooding-loading transition and stirrer speed at loading-complete dispersion transition respectively, with "F" and "CD" symbol indexes. Analogously, for the other symbols)
Presented Monday 17, 11:15 to 11:35, in session Multifase Flows - I (T2-5a).
