New advances in agitation technology for exothermic reactions in very large reactors

Abstract

Organic reactions, such as polymerisations, often require precisely controlled homogeneous reaction conditions in order to achieve high product quality, minimise waste or rework, and therefore reduce environmental impact. Many such processes were originally manufactured in moderately sized reactors with a height–diameter ratio of around 1. Operators have subsequently driven up their production rates by intensifying the reaction and by increasing the reactor size (and often increased the reactor's height–diameter geometry) in response to the economic requirement for world-scale manufacturing plants. However, as reaction intensities and reactor sizes increase, product quality can be affected due to poorer homogeneity, since reaction rates are faster but bulk mixing is slower. This can lead to a “Limit to Scale” beyond which product quality is unacceptable. Better bulk mixing within very large reactors would increase the Limits to Scale. Laboratory trials were undertaken to compare the homogeneity achieved by different impeller configurations in a model of a typical large reactor. This leads to a new agitator design concept, which achieves rapid mixing in large vessels with high height–diameter ratios using a series of impellers that produce a narrowly confined axial flow, effectively a virtual draft tube, in the centre of the reactor. Fluid returns to the top of the reactor near the walls creating a loop flow pattern within the reactor, effectively an “internal loop reactor”. CFD simulations of the reactor were undertaken to better understand the hydrodynamics, and were validated against experimental results.