AN EXPERIMENTAL-STUDY TO CHARACTERIZE IMPERFECT MACROMIXING IN A STIRRED SEMIBATCH REACTOR

A first-order decay of a diazonium salt, rate constant similar to 10(- 3) s(-1), and a parallel, very fast, second-order coupling with the ad dition of pyrazolone, i.e., A --> S and A + B --> R, have been used to study macromixing in a stirred semibatch reactor. The experiments und ertaken include two types of impeller, two Liquid heights, two feed po sitions, three feed rates, and three semitechnical scales of vessel fr om 0.3-0.6 m diameter. At high impeller speeds, all conditions give si milar yields of around 90%. At lower speeds, typical of those found on the industrial scale, the yield fell as low as 65%. However, feeding into the impeller zone compared to the surface and using a larger 60 d egrees pitch blade impeller rather than a standard Rushton turbine inc reased yields at similar values of power/volume to about 85%. Yields u nder geometrically similar conditions at each scale were the same when operating at-constant agitator speed. Such results fit in well with t he concept that the same scale-up rule applies to macromixing as that for equal mixing times. It also matches the scale-up rule implied by t he ''network-of-zones'' model developed by Mann and co-workers, though the absolute values of yield predicted differ widely from those obtai ned experimentally at low speeds and with surface addition. The result s also show that improving bulk blending is a good strategy for increa sing yields. The performance of a 60-ton industrial-scale reactor used to produce reactive dyes was significantly enhanced following this wo rk by changing a feed point from the surface to the impeller region.