Hydrodynamic and gas-liquid mass transfer considerations for the design of airlift bioreactors

This thesis deals with gas holdup, liquid circulation and gas-liquid mass transfer phenomena in airlift bioreactors. The study is conducted in model media (water, salt solution, cellulose fibre slurries) some of which are carefully formulated to simulate fermentations of mycelial fungi, solid substrates and filamentous bacteria. Relatively large (50-1000 L liquid volume) airlift reactors are investigated over a broad range of configurations including rectangular channel airlift, external-loop, concentric draught-tube internal-loop and split-cylinder devices. Significant work on bubble columns is also reported.; Useful recommendations on the positioning of gas spargers, on riser and downcomer diameters, on the clearances of the draught-tube (or the baffle) from the reactor base and on the location of gas-liquid (or slurry) dispersion surface from the top of the draught-tube are made. These should enable improved gas distribution, mixing and circulation performance in airlift reactors.; A new hydrodynamic model is propounded to describe the gas holdup phenomenon in Newtonian and non-Newtonian fluids. A new equation is developed for the prediction of liquid circulation rate in airlift reactors and it is shown to apply to water-like fluids and to shear thinning media. The circulation model holds for many distinct types of airlifts over extremely broad ranges of scale, including reactor heights of up to {dollar}sim{dollar}10 m (height-to-diameter ratio up to {dollar}sim{dollar}24).; The study pays much attention to reactor height associated effects and the author shows that the overall gas holdup and the overall volumetric mass transfer coefficient are affected by the height of airlift device. The height effects, which can be quite complex, are shown to not occur in bubble columns which are otherwise similar to the airlift reactors employed in this study.; In mass transfer work a new concept of scale-up based on a constant mass transfer coefficient-to-bubble diameter ratio is advanced and it is shown to apply to a number of pneumatically agitated bioreactors.