Calcium alginate beads motion in a foaming three-phase bubble column

Abstract

Calcium alginate beads are frequently used to immobilize enzymes or microorganisms for fermentations carried out in agitated or pneumatic reactors. In this work, the well-known Radioactive Particle Tracking (RPT) technique is used to non-invasively extract relevant information of the motion of calcium alginate beads within a three phase bubble column under foaming conditions, which frequently appear in bioreactors operation. Special care is taken to produce a radioactive tracer that perfectly matches the features of the other particles in density and size. In addition, the tracer has the same texture and wettability since the adherence of gas to particles in foaming systems is crucial in determining the solid motion. Particles distribution, solid residence time, velocity fields, dispersion coefficients, shear stress and turbulence kinetic energy are determined from the radioactive tracer trajectories. Compared to previous works in non-foaming systems with denser particles, a relatively strong inward flow and less definite descending motion of the solid near the column wall is found. Turbulence intensities and shear stress are high in the disengagement zone, particularly for the churn-turbulent flow regime. However, since the biocatalyst damage would also depend on the actual exposure to harsh regions, the frequency of visit at different location was calculated to estimate maps of exposure risks as the product of turbulence stresses and these frequencies. Considering the particles motion, the region of largest risk for hydrodynamic damage is close to the gas entrance zone.