Abstract--Rapid coagulation by turbulence in stirred tanks was studied for particles and aggregates smaller than the Kolmogorov microscale. The coagulation kinetics are determined by the floc structure and by the hydrodynamic and colloidal interactions between the colliding particles. The collision efficiency for doublet formation in the heterogeneous shear field of a stirred tank follows from particle trajectory analysis of solid particles in simple shear flow, provided the simple shear rate is made to correspond to the residence time weighted turbulent shear rate. Experimentally, the resulting aggregates proved to be fractal-like with their porosity increasing with aggregate size. Porosity within the aggregates results in penetration of the floc surface by the fluid flow, giving rise to enhanced collision efficiencies compared to solid particles. The collision efficiencies between porous flocs may be estimated by a model that pictures a porous floc as consisting of an impermeable core and a completely permeable shell. With the collision efficiencies from this shell-core model the aggregate growth could be described adequately.

Previous studies have shown that hybrid distillation processes using either pervaporation or vapor permeation can be very attractive for the separation of mixtures. In this paper, a comparison between these two hybrid processes has been made. A tool has been presented that can assist designers and engineers to decide which process is more convenient for a specific application. Water removal from acetonitrile has been used as an example. A hybrid process with vapor permeation is preferred when the membrane is used either for water removal at high water concentration or just for overcoming the azeotropic composition. When the membrane removes water at water concentrations lower than the azeotropic point, pervaporation is more effective. Recycling part of the product as permeate (product sweep) and applying different pressures in the distillation columns and the membrane unit strongly reduce the required membrane area and the total cost of the process. Relatively low membrane selectivities are required for an economically optimal hybrid membrane-distillation process.

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