Seminar - Richard Grenville

Characterizing Impeller Performance in Stirred Tanks with Examples of Process Results

Richard K. Grenville

Director of Mixing Technology
Philadelphia Mixing Solutions Ltd., Palmyra, PA

Characterizing Impeller Performance in Stirred Tanks with Examples of Process Results

 

Abstract

Impellers in stirred vessels are often described in terms such as high flow, high efficiency, high shear etc. These terms are qualitative and are not helpful when carrying out an agitator design / sizing calculation. The impellers are essentially pumps, they are machines that move fluid inside the vessel, and their performance characteristics can be defined in the same way as a pump; in terms of their power input, the flow and head generated and their efficiency. Impellers are also required to generate shear, either to disperse a second liquid, gas or solids phase in order to generate surface area for mass transfer or to promote coagulation / flocculation of fine particles suspended in the liquid phase.

The region of highest shear in a stirred tank is found in the trailing vortex at the tip of the impeller blades. The properties of the vortex must be taken into account in order to correctly calculate a local shear rate. In this presentation a method for quantifying the flow and shear characteristics of impellers will be presented demonstrating how this method may be applied to shear driven processes. The calculation method will be verified by comparing with results from droplet break-up and flocculation experiments.

The main conclusion of this work is that the Rushton turbine, traditionally considered “high shear”, actually generates larger droplets and flocs than a Hydrofoil, traditionally considered to be “low shear” at the same power input. Understanding this is important when selecting the appropriate impellers for processes where high or low-shear mixing are requirements.

Examples of processes that rely on flow, such as blending of tank contents, and shear, such as dispersion of immiscible liquids and flocculation of fine particles, will be used to demonstrate the application of this characterization approach.

Finally, the use of the word “shear” in the context of turbulent mixing will be discussed. While it is commonly used to convey the general idea of an impeller’s ability to reduce the size of a second phase, increasing interfacial area, it is not the best mechanistic description of the process which is driven by the local energy dissipation rate within the impeller’s trailing vortices.

 

Bio

Richard Grenville is Director of Mixing Technology at Philadelphia Mixing Solutions and has over 30 years of experience in the field of mixing.

He studied Chemical Engineering at the University of Nottingham in the UK, graduating in 1983, and started work as an Applications Engineer for Chemineer Ltd. sizing agitators and static mixers in response to customers’ inquiries.

After a brief period working as a Process Engineer at Unilever Research he went to work at the Fluid Mixing Processes consortium, which is managed by the British Hydromechanics Research Group, as a Project Engineer.  His main area of research was mixing of non-Newtonian fluids.  He also registered as a graduate student at Cranfield Institute of Technology and received his PhD in 1992.

In 1991 Richard joined DuPont as a mixing consultant in the Engineering department and worked on a wide variety of projects to manufacture, among others, fluorochemicals and polymers, various agricultural chemicals, water borne paint and fermentations.  The last major project he worked on before leaving DuPont was the design of agitators for the Cellulosic Ethanol plant currently being commissioned in Nevada, IA.

In 2013 Richard joined Philadelphia Mixing Solutions as Director of Mixing Technology where he is responsible for developing solutions for customers’ mixing problems which may include lab-scale testing and scale-up.  He regularly gives seminars for customers and AIChE student and local chapters.

Richard co-teaches courses on mixing at Rowan University in New Jersey and at the University of Delaware, is a Senior Member of the American Institute of Chemical Engineers, a Chartered Engineer and a Fellow of the Institution of Chemical Engineers.  He is currently the president of the North American Mixing Forum.