FRICTION ON A SOLID SPHERE EXPOSED TO GAS-LIQUID AND GAS-LIQUID-SOLIDFLOW IN BUBBLE-COLUMN AND FLUIDIZED-BED REACTORS

Local velocity gradients on a solid spherical surface have been studie d in a bubble column and in two- and three-phase fluidized beds, in or der to clarify the influence of gas flow. The electrochemical method, measuring apparent local mass transfer coefficients, was verified and used to obtain the local velocity gradients, shear stresses and total frictional forces. The observed mass transfer rate was independent of liquid velocity, owing to a non-changing how structure around the part icles and not to averaging opposing effects. The identity in flow stru cture also held for three-phase fluidized beds up to a superficial gas velocity of 5 cm s(-1). The dramatic increase in velocity gradient on gas introduction was not a result of decreased homogenous density, bu t was caused by a change in the turbulent structure around a particle, leaving a larger portion of the total drag as frictional drag, thus i mproving the mass transfer characteristics of the bed. Use of velocity gradient measurements, including span of fluctuations and exposure ti me, to predict biomass growth and mechanical degradation in a reactor is also discussed.