Just because it's small doesn't mean it's mixed: ensuring good mixing in meso-scale reactors

The advent of high throughput experimentation (HTE) for molecular discovery and rapid screening of new catalyst formulations has led to interest in the mixing characteristics of small stirred vessels at scales below those previously studied. In this paper, particle image velocimetry (PIV) is used to obtain macromixing characteristics for single phase fluids of two different viscosities (mu = 0.001 and 0.433 Pa s) and for two-phase air-water mixtures in a 45 mm diameter vessel using a 6 blade up-pumping pitched-blade turbine. Eccentric agitation is examined as a means of improving the mixing performance since the vessels are used without baffles, and the global circulation of the fluid was comparable with that of a conventional baffled configuration for single-phase mixing at a constant power input per unit mass (0.168-5.5 W kg(-1)). For high viscosity fluids, the characteristic flow patterns resembled those for a radial device. For two-phase mixing at gassing rates of 0.25 and 0.5 vvm, two sparger configurations were used with the sparger being placed either beneath the impeller axis (alpha) or away from the impeller (beta). The beta configuration appeared to be a better choice because of the smaller size of bubbles generated compared with the a configuration and the satisfactory levels of global mixing observed in the vessel.