Measurement of maximum stable drop size in aerated dilute liquid–liquid dispersions in stirred tanks

Abstract

Turbulence intensity, or hydromechanical stress, is a controlling parameter in many industrially relevant processes. Especially fermentation processes are often characterized by intense aeration and agitation, operating conditions for which the measurement of turbulence intensity is extremely difficult. Since the maximum stable drop diameter in a break-up controlled dispersion is directly correlated with turbulence intensity, the measurement of drop sizes can enable an indirect access to the intensity of turbulence under such operating conditions. This work presents the constraints and the development of a method for the measurement of maximum stable drop size in aerated liquid–liquid dispersions in stirred tanks for the purpose of characterizing turbulence intensity. Continuous and dispersed phase properties were selected to achieve a break-up controlled dispersion with negligible coalescence. This was accomplished mainly by applying a dilute dispersion, a low ionic strength and by incorporating a dispersed phase, paraffin oil, with a negative spreading coefficient. The negative spreading coefficient prevents coalescence due to drop–bubble interactions for aerated operating conditions. It was demonstrated that the off-line measured drop size distributions are representative for the conditions in the bioreactor and are not altered by sample handling. The sampling and measurement procedure was found to be highly reproducible with a standard deviation for the maximum stable drop size for independent experiments of approximately 10%. Relevant constraints for the application of the method in large-scale experiments were discussed and accounted for during method development to allow a later application in production scale equipment.