Investigation of Local and Overall Volumetric Mass Transfer Coefficient and Gas Holdup in Aerated Coaxial Mixing Systems Containing Non-Newtonian Fluids through Tomography and Computational Fluid Dynamics
Gas dispersion in Newtonian and non-Newtonian fluids is one of the most important processes in biochemical industries. Aerated mixing tanks are preferred due to their superior performance in increasing the contact area and mass transfer between two phases. To enhance the operation of aerated mixing tanks, especially in the presence of non-Newtonian fluids, a configuration consisting of two central impellers and a wall scraping anchor is recommended. In this study, the performance of the coaxial system for gas dispersion in a non-Newtonian fluid was investigated using electrical resistance tomography (ERT), and computational fluid dynamics (CFD). For the first time, the effects of the central and anchor impeller speed, air flow rate, rheology, impeller type, pumping direction, and anchor rotation mode were studied on the performance of coaxial mixers comprised of two central impellers at the presence of a pseudoplastic fluid in terms of local and overall gas hold-up, bubble size, and volumetric mass transfer coefficient (kLa). Modified correlations were proposed for the power number and Reynolds number of the aerated coaxial systems. Additionally, empirical correlations for power consumption, overall gas holdup, and volumetric mass transfer coefficient of the coaxial mixer were also developed. The results showed the downward pumping co-rotation mode of the pitched blade impellers resulted in the highest gas holdup, while the numerical simulations revealed a non- uniform distribution of the gas phase in the system with the pitched blade impeller in the downward pumping mode. It was further discovered that the Sauter mean bubble size increased with the fluid concentration resulting in a lower amount of interfacial area, and consequently decreasing the kLa of the system. A comparison between the results of local and overall kLa revealed the superiority of the coaxial mixing system furnished with Rushton turbines. The results of this study lead to the development of: (i) new approaches, based on the local distribution of the gas holdup and kLa, to characterize aerated coaxial mixing tanks and evaluate coaxial mixers performance; (ii) a modified correlation for equivalent speed and diameter of the coaxial mixers; and (iii) empirical correlations that can be used for scaleup purposes.
- Doctor of Philosophy
- Chemical Engineering
Granting InstitutionRyerson University
LAC Thesis Type