Bubble size distribution and gas holdup in bubble columns employing non‐Newtonian liquids: A CFD study

Abstract

The hydrodynamics involved with the rise of air bubbles in shear‐thinning non‐Newtonian liquids in bubble columns was investigated using computational fluid dynamics (CFD). In bubble columns, the bubble size distribution (BSD) and gas holdup significantly affect the mass transfer rates and the reactor design. Consequently, the influence of superficial gas velocity, flow index, consistency index, sparger bubble size, and yield stress were analyzed on a local and global scale. The bubble sizes and the coalescence and breakage phenomena were incorporated using the homogeneous discrete method of the population balance model (PBM). The bubbles underwent coalescence during ascent and exhibited bimodal distribution. The radial homogeneity of the gas phase increased axially. Moreover, the zones of low liquid dynamic viscosity produced zones of a high gas holdup. Though the dual effect of viscosity on gas holdup was non‐existent, the gas holdup surged after 35.1 mPa s at the mid‐zone and remained constant thereafter. The noteworthy differences in simulation results put further emphasis upon the importance of sparger bubble size in CFD modelling. A decrease in the overall gas holdup and the axial air velocity with yield stress was observed. CFD simulation proved capable of providing results in reasonable agreement with experiments.

A CFD Investigation on the Aerosol Drug Delivery in the Mouth–Throat Airway Using a Pressurized Metered-Dose Inhaler Device

“…Based on the flow Reynolds number (300 < Re < 10 4 ), for an inhalation flow rate and geometrical irregularities, a laminar-to-transitional and even fully turbulent flow was expected [9]. The k-ω turbulence model was employed in this work instead of the k-ε model based on the ability to accurately predict a pressure drop, velocity profiles, and shear stresses for transitional and turbulent flows, especially near the wall, which is significantly important for particle deposition [41]. The two parameters used to account for the turbulent nature of the flow field were turbulence kinetic energy, k, and turbulent dissipation rate, ω.…”

“…The convergence of the simulation was evaluated by monitoring the drop of continuity residuals below 10 −4 while the other residuals were required to fall below 10 −5 . The velocity and pressure were linked using the phase-coupled SIMPLE method [41]. The second-order upwind was adopted for the turbulent dissipation rate to limit the numerical diffusion errors [49].…”

Novel Identifier of Transitions in Bubble Columns Operated with Water and Aqueous Alcohol Solutions

“…Both gas holdup and BSD depend on the superficial gas velocity U g , which is a key operational parameter. The BSD changes gradually from uniform, narrow, and monomodal in the homogeneous FR to bimodal having large bubbles in the heterogeneous FR 4. The boundaries of the main FRs depend strongly on U g and thus they define the regime‐specific degrees of liquid mixing, turbulence, and mass and heat transfer.…”

“…Both parameters are very important for BC reactor design and they affect significantly the mass transfer efficiency. Paul and Pakzad argued that bubble wakes, bubble coalescence and breakage, and liquid vortices result in a heterogeneous spatial-temporal distribution of bubble size and gas holdup [4]. Numerous empirical correlations for gas holdup prediction have been proposed [5,6].…”

“…heterogeneous FR [4]. The boundaries of the main FRs depend strongly on U g and thus they define the regime-specific degrees of liquid mixing, turbulence, and mass and heat transfer.…”