Influence of feed pipe diameter on mesomixing in stirred tank reactors

Abstract

The product distribution of competitive reactions depends upon how the reagents are mixed when reaction is fast relative to mixing. Different mixing scales are considered here. The rate of micromixing is determined by the engulfment of small fluid elements. Mesomixing refers to the turbulent dispersion of a feed stream shortly after it enters a reactor. Turbulence creates the concentration field within which micromixing proceeds. The source of this field is the feed pipe, which had previously been modelled as a point, resulting in a Gaussian distribution of concentration downstream from the feed pipe outlet. This pipe can however be sufficiently large so that it represents a finite source: the relevant solution of the turbulent dispersion equation is given and contains a new dimensionless group, namely the ratio of the velocities inside and outside the feed pipe. With incresing size of the feed pipe opening it is predicted that (a) the critical feed time of a semi-batch reactor should decrease, thereby raising the productivity, and (b) when mesomixing is important and the feed time is constant, reagents should mix faster. These predictions were verified in experiments whereby sodium hydroxide was added through feed pipes of 1, 2, 3, 4 and 5 mm bore to a mixture of hydrochloric acid and ethyl monochloroacetate in a 0.021 m 3 stirred tank reactor. It was also confirmed that product distribution is independent of the feed pipe, when only micromixing controls. Backmixing of reagents from the tank into the feed pipe occurred with the larger openings when the feed time was long. It was directly visualized and could have a large effect on product distribution. The modelling of micromixing and mesomixing does not include backmixing, which should be studied in more detail.