Power Consumption Correlation in an Aerated Stirred Vessel with a Void Fraction around the Stirring Impeller Using Electrical Resistance Tomography Measurements

M. Kaminoyama1, K. Nishi1, R. Misumi1and A. Tagawa2

1. Department of Materials Sciences and Chemical Engineering, Faculty of Engineering, Yokohama National University, 79-5 Tokiwadai Hodogaya-ku Yokohama-shi 240-8501 JAPAN

2. Department of Materials Sciences and Chemical Engineering, Graduate School of Engineering, Yokohama National University, 79-5 Tokiwadai Hodogaya-ku Yokohama-shi 240-8501 JAPAN

Tel and Fax: 81-45-339-3999, E-mail: kamin@ynu.ac.jp

ABSTRACT

The power consumption is important in evaluating mass transfer coefficients, kLa, in an aerated stirred vessel, and many correlation equations of the ratio of power with aeration to that without aeration, Pg/P0, have been proposed. However, they cannot cover a wide range, from a loading to a flooding state, because they are based on macroscopic physical variables throughout the vessel, and because Pg/P0 doesn’t decrease monotonically with an increase in the rotational speed of the stirring impeller, n. This last issue may be attributable to the gas hold-up behind the impellers. On the other hand, the

electrical resistance tomography technique makes it possible to grasp localized information in a stirred vessel. The tomography technique was, therefore, applied to an aerated stirred vessel, having a diameter of 0.2m and fitted with a Rushton turbine impeller. The distributions of gas hold-up were obtained for 5 horizontal planes in the vessel under various conditions of rotational speed of the stirring impeller, n, and gas flow rate, Qg. It was found that the gas hold-ups around the impeller were much larger than those out of the region, even in a loading state.

In observing the void fractions around the impeller in the region swept by the impeller, φimp, based on the tomography measurements, it was found that the total void fractions in the stirred vessel increased monotonically with an increase in n, regardless of being in a flooding or a loading state, while φimp varied complicatedly, in particular, in the loading state. Based on observations using pure water and aqueous solutions of 40, 60 and 80 wt.% of glycerin, the following correlation equation was proposed for Pg/P0, from a flooding to a loading state, using φimp and including viscosity variation: Pg/P0 = 0.21φiimp- 0.34. Further, equations were obtained with which it was possible to estimate kLa accurately under various conditions, based on the tomography measurements of φimp.

Keywords Aerated Stirred Vessel, Power Consumption Correlation, Void Fraction, Electrical Resistance Tomography