(163c) Mixing Assessments for Industrial Stirred Tank Reactors with Gas Entrainment from the Headspace

Cloeter, M. D., The Dow Chemical Company
Deshpande, S., The Dow Chemical Company
This mixing study was prompted by a difference in reaction rates in two semi-batch reactors that make the same product. The reactors are located at different production sites and have differing geometries. A gaseous reactant is introduced into the headspace of the stirred reactors containing a liquid phase co-reactant. The mixing is intended to provide a degree of contacting between the two reactant phases.

The pressure in the reactor headspace is sensitive to the consumption rate of the gaseous reactant, and thus provides a convenient metric for mass transfer behavior. The pressure data for the vessel with the slower reaction rate clearly shows long periods of poor contacting separated by short period of good mixing when liquid level occurs at impeller location. This is confirmed by motor amp readings. Laboratory experiments were performed to confirm the existing aeration behaviors and identify a suitable (i) impeller type, (ii) agitation speed, and (iii) number of impellers to improve surface aeration.

Significant capacity gains can be brought about in the sold-out reactor by improving the mixing performance. To give an estimate of potential, a reduction of < 30 minutes per batch equates to several million additional pounds of product per year (several percent gain in capacity).

Based on the plant data and experiments, a new agitation system was proposed. This design is currently moving toward implementation.