The influence of powder properties on the imbibation rate.

Mixing of powders into liquids is a common unit operation. Mixing can be divided into several steps, imbibation of the powder into the liquid being the first. Under some circumstances, such as when the powder has poor wetting properties, imbibation can be the rate-determining step. The effects of different powder properties on the imbibation rate were evaluated using an experimental imbibation model based on the imbibing process employed in an industrial powder-in-liquid mixer. A multivariate analysis of the results suggests that bulk density and capillary penetration rate, and to some extent cohesivity, play an important role in determining the powder imbibation rate. The results also suggest that the capillary penetration rate is increasing on a large particle radius, slow solubilisation of the particles and a low ability of the particles to generate viscosity.

Physiological responses to mixing in large scale bioreactors.

Escherichia coli fed-batch cultivations at 22 m3 scale were compared to corresponding laboratory scale processes and cultivations using a scale-down reactor furnished with a high-glucose concentration zone to mimic the conditions in a feed zone of the large bioreactor. Formate accumulated in the large reactor, indicating the existence of oxygen limitation zones. It is suggested that the reduced biomass yield at large scale partly is due to repeated production/re-assimilation of acetate from overflow metabolism and mixed acid fermentation products due to local moving zones with oxygen limitation. The conditions that generated mixed-acid fermentation in the scale-down reactor also induced a number of stress responses, monitored by analysis of mRNA of selected stress induced genes. The stress responses were relaxed when the cells returned to the substrate limited and oxygen sufficient compartment of the reactor. Corresponding analysis in the large reactor showed that the concentration of mRNA of four stress induced genes was lowest at the sampling port most distant from the feed zone. It is assumed that repeated induction/relaxation of stress responses in a large bioreactor may contribute to altered physiological properties of the cells grown in large-scale bioreactor. Flow cytometric analysis revealed reduced damage with respect to cytoplasmic membrane potential and integrity in cells grown in the dynamic environments of the large scale reactor and the scale-down reactor.

An Experimental Study of the Kinetics of Particle Deposition in a Wall-Jet Cell Using Total Internal Reflection Microscopy.

The deposition of polystyrene latex particles of 0.46 µm diameter was studied in situ using a wall-jet cell in combination with total internal reflection microscopy. The particles were deposited onto an indium tin oxide surface in a laminar flow field for up to 13 h. The initial particle flux was found to be mass-transfer-controlled. It was shown that one period of the time-dependent deposition rate period was of first-order nature. The effective particle transfer coefficient during this period appeared to be correlated inversely to the wall shear rate. With the help of three characteristic empirical constants, one of them being a mass transfer coefficient, the overall deposition process was described by a model equation. The concentration dependency was elucidated using a Langmuir-type pseudoisotherm. Copyright 2000 Academic Press.