Effect of shaft eccentricity on the laminar mixing performance of a radial impeller

Canadian Journal of Chemical Engineering, Dec, 2008 by F. Cabaret, Fradette L., P.A. Tanguy

INTRODUCTION

Fluid mixing in mechanically agitated tanks is a common unit operation in the chemical process industries. According to the fluid viscosity and process constraints, generally two types of impellers are encountered in mechanically agitated vessels: close clearance impellers and open impellers. Close clearance impellers are used in the laminar regime because they generate sufficient bulk movement to homogenize the fluid at an acceptable level of mixing. They are, however, disadvantaged by a high-energy consumption. On the other hand, open impellers draw less power, but they are inefficient for mixing in the laminar regime. Indeed, open impellers generate segregated regions in the laminar regime irrespective of the discharge flow type, whether radial or axial (see Figure 1). Recent studies on open impellers (Alvarez, 2000; Ascanio et al., 2002a,b; Alvarez et al., 2002; Hall et al., 2005x; Sanchez Cervantes et al., 2006) conclude that, in the laminar regime, the dynamic perturbations generated by using an eccentric shaft could help to break up the segregated regions, and increase the axial circulation in a tank even with radial agitator. This change in flow patterns prevents the formation of flow compartmentalization and helps to improve the mixing efficiency of the mixing device.

The idea of using off-centred impellers in the laminar regime is fairly recent but already pretty well established in the turbulent regime. Indeed, many authors have shown that, in turbulent flow, shaft eccentricity is equivalent to baffling (Joosten et al., 1977; Nishikawa et al., 1979; Novak et al., 1982; King and Muskett, 1985; Hall et al., 2004, 2005b; Karcz and Szoplik, 2004; Karcz et al., 2005; Montante et al., 2006). Nishikawa et al. (1979), Karcz and Szoplik (2004) and Karcz et al. (2005) have shown that for axial and radial type impellers the mixing time decreases with the increase of the shaft eccentricity. Many studies also report increased power with the shaft eccentricity with both axial and radial type impellers (Nishikawa et al., 1979; Novak et al., 1982; Karcz et al., 2005).

As only limited data is available to describe the effects of shaft eccentricity on mixing time and power consumption in the laminar regime, the objective of this article is to examine and quantify these effects when a Rushton turbine (RT) is used. Let us note that the results of this study might also be extended to axial impellers as they generate radial flow characteristics in laminar regime (Fangary et al., 2000). The purpose of this study is to determine the optimum shaft eccentricity in laminar flow.

[FIGURE 1 OMITTED]

MATERIALS AND METHODS

Equipment

The experiments were conducted using a non-baffled cylindrical polycarbonate tank with an open-top and a flat bottom, and having an internal diameter T = 0.215 m (see Figure 2). A liquid column height H = T was used yielding a liquid volume of 7.8 L. The open impeller examined in this study was a radial flow six-blade RT with a diameter of D = 6.53 x [10.sup.-2] m. The following notations will be used in the forthcoming: R denotes the radius of the tank; e, the distance from the middle of the shaft to the centreline of the tank (see Figure 2), and the eccentricity, E, is defined as E = e/R. In the different experiments E was varied between 0 and 56.2% (56.2% being the highest possible value of E for the RT without touching the tank wall), and the bottom clearance of the impeller was kept constant at C = T/3. The mixing shaft was driven by an AC motor (0.69 kW). To modify the value of the shaft eccentricity, the vessel was moved horizontally on the table, perpendicularly with the camera focal axis (see Figure 3). A torquemeter (Vibrac[TM], Model TQ-512) coupled to the agitation shaft was used for determining the power consumption in a range between 0 and 3.61 N m.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

In the experimental setup presented in Figure 3, the cylindrical mixing tank is installed in a second rectangular water-filled chamber in order to minimize optical distortions due to the curvature of the tank. To obtain homogeneous illumination, a white sheet of paper was used as light diffuser on the rectangular vessel. The mixing process was filmed with a digital mono CCD camera (Digital Handycam DCR-PC 101, Sony[TM]) linked to a computer via a 1394 IEEE (FireWire) connector.

Methods

Aqueous solutions of corn syrup (Glucose Enzose 62DE, Univar[TM]) were used as the viscous Newtonian fluids. In all the experiments the glucose solutions were allowed to settle for 24 h before starting the experiments in order to eliminate air bubbles. Knowing that the viscosity, [micro], of the viscous corn syrup solutions varies significantly with temperature, preheating of the solutions was achieved by leaving the impeller in rotation (using viscous dissipation as the source of heat). A constant temperature of 27.2[degrees]C was then reached and the temperature was monitored during the experiments to ensure it remained at this constant value. Newtonian viscosities of the solutions were determined with a viscometer (Visco 88, Bohlin[TM] Instruments) at 27.2[degrees]C with the Couette configuration. During the experiments, the viscosity of the solutions was found in the range 6.45-6.85 Pas and the liquid density, [rho], was 1360 kg/[m.sup.3].