Effect of a continuous feed on the fluid-dynamics of a mechanically stirred tank

This paper focuses on the fluid dynamic characteristics of a laboratory scale fully baffled vessel stirred by a Rushton turbine and provided with one inlet and one outlet for the continuous feed of the working liquid. The specific aim of the investigation is the assessment of the operating conditions, namely the agitation speed and the inlet flow rate, at which the influence of feed stream on the overall behavior of the stirred tank starts to be significant from an engineering point of view. Due to the widespread adoption of stirred tanks in continuous industrial processes, the evaluation of the feed effect is expected to provide a useful contribution to evaluate to what extend the current knowledge on batch stirred tanks can be confidently extended to continuous systems. The effect of the inlet jet on the vessel local fluid dynamics is quantified by means of coupled Particle Image Velocimetry (PIV) and Planar Laser Induced Fluorescence (PLIF) measurements, thus resulting in a full characterization of both the flow field and the homogenization dynamics under a number of different operating conditions. The results show that the inlet stream modifies the mean flow field features more importantly the smaller the agitation speed is, as compared with the feed flow rate, i.e. when the ratio between the power input associated to the feed inlet and that provided by the agitator overcomes a critical value. Similarly, a clear dependence of mixing time on inlet flow rate is found, whose entity is larger the smaller the agitation speed is.