Power Dissipation and Power Number Correlations for a Retreat-Blade Impeller under Different Baffling Conditions

Abstract

Glass-lined tanks/reactors provided with a torispherical (dish) bottom and equipped with a retreat-blade impeller (RBI) are commonly used in the industrial syntheses of Active Pharmaceutical Ingredients (APIs). The power, P, dissipated by the RBI in the liquid in the tank is critical to many mixing processes, especially since the power per unit liquid volume, P/V, controls many mixing phenomena. Despite their common industrial use, limited information on P for RBIs and the corresponding nondimensional Power number, Po, is available. Here, P values for an RBI in liquids of very different viscosities at different agitation speeds and for different baffling configurations, i.e., unbaffled, partially baffled (with a single “beavertail” baffle), and fully baffled (four rectangular baffles), were measured experimentally in a scaled-down version of a typical reactor for API synthesis. The corresponding Po values were obtained for a large range of the Reynolds number, Re (1 < Re < 400,000). Po was found to vary significantly with baffling type and Re. Equations correlating Po with Re and baffling type are presented.

Digital Design of Batch Cooling Crystallization Processes: Computational Fluid Dynamics Methodology for Modeling Free-Surface Hydrodynamics in Agitated Crystallizers

“…a single 21 and two beavertail baffles 19,20 , with the exception of reference 22 with a single beavertail baffle where the measured Np is lower than the predicted values, the reason for which is not clear.…”

“…A liquid volume fraction of 90% was used to define the air-liquid interface as suggested in previous studies in unbaffled agitated vessels. 10,22 The predicted instantaneous liquid free-surface profiles of water due to the transient nature of the surface topography on the 90270 o angular position (see Figure 3) for impeller speeds of 100, 150, 200 and 250 rpm are illustrated in Figure 14. As can be seen, a single baffle is unable to supress vortex formation, particularly at higher impeller speeds (> 100 rpm), and the vortex depth increases with increasing impeller speed.…”

“…An increase in power number should be expected with an increase in baffle number/size due to the shift of flow pattern from a rotational flow to a predominantly vertical recirculating flow. Experimental measurements in dish-bottom vessels with a RCI: two beavertail baffles, 19,20 and a single beavertail baffle, 21,22 . Experimental measurements in flat-and conical-bottom vessels with a RCI and a single beavertail baffle.…”

Overview on the hydrodynamic conditions found in industrial systems and its impact in (bio)fouling formation

“…The agitation in STR is usually concentric (agitator shaft centrally located), but the eccentrical agitation (off-centred agitator shaft) has demonstrated better mixing performances under laminar flow [135,154]. In fact, concentric agitation (mainly radial flow) under laminar flow divides the fluid bulk into two independent compartments and a central vortex is created under high stirrer speeds [167]. While the eccentrical agitation disrupts the fluid symmetry and compartmentalization phenomena, promoting the axial flow and increasing turbulence [135].…”

“…On the other hand, the use of baffles in tank walls inhibits vortex formation and improves the mixing conditions [161]. In addition, baffling can reduce power requirement of agitation under turbulent flow, without influence under laminar flow [167].…”