Mechanical Agitator Power Requirements for Liquid Batches - a PDH Online Course for Engineers and Surveyors

Mechanical Agitator Power Requirements for Liquid Batches

Course Outline

This two hour course will focus on deriving the general Application Equation used for calculating the mechanical agitator power requirements for liquid batches. The power requirement is based on a relationship of the agitator type, agitator speed, and liquid physical properties.

Following the Application Equation derivation, correlations are presented from the open literature which define the methodology for calculating the power requirements for various agitator types.

An industrial example is given in order to demonstrate the calculation techniques for determining specific power requirements. The example calculates the power required to mix water in a vessel with a flat blade turbine at an agitator speed of 60 rpm.

This course includes a True-False quiz at the end.

Learning Objective

It is the intention of this course to enable process engineers or other plant operations personnel to be able to calculate the power requirements for a mechanical agitator used to mix a liquid batch. The course will cover the typical areas of turbulent liquid batch mixing power requirements. The systems included in this course as related to fluids and properties, include Newtonian fluids. Most liquids, such as water, oils, and acids have constant stress versus velocity gradients and are classified as Newtonian fluids. Mechanical configurations included in the scope of this course include:

top-center mounted agitators,
baffled vessels,
axial impellers, such as marine props and pitched blade turbines,
radial impellers, such as flat-blade and disk-blade turbines and paddles.

Several correlations are presented from the open literature which can be utilized to determine the specific power required for a selected agitator type. Other correlations are available in the literature for process situations not covered by this course. The student is encouraged to investigate these. A good starting point are references #1, #2, and #9 listed in the Related Reference section.

An example of the power calculation is given: mixing water in a vessel using a flat blade turbine impeller. The calculation techniques required by the example will utilize all of the background covered in the Application Equation and correlation development. At the conclusion of the course the student will:

Have an understanding of the mechanisms in an agitated vessel system which affect power requirements.
Be knowledgeable in the assumptions used to develop power correlations for mechanical agitators.
Calculate design motor power requirements for various agitator types and fluid properties.

Course Introduction

This course provides a step-by-step development for the calculation of the power requirements for a mixed liquid batch at known fluid properties and vessel geometry. This course does not cover several special areas of liquid batch mixing. The areas not covered by the course include:

gassed systems,
non-Newtonian fluids, such as Dilatant liquids, and Pseudoplastic Bingham plastic fluids,
non-baffled vessels,
off-center or bottom agitator placement configurations,
non-turbulent agitation with N_Re < 10,000,
multiple impeller configuration.

Several of the open literature power correlations are presented so mechanical agitator requirements could be determined based on specified process conditions.

An industrial example follows the Application Equation development and the correlation review. The example demonstrates how the design motor power is calculated for a vessel containing water using a flat blade turbine agitator impeller.

Course Content

The course material is contained in the following content file:

K103content.pdf (PDF, 306 KB)

You need to open or download this document for your study.

Course Summary

Mechanical agitator power requirements for liquid batches are calculated by determining the power number, N_P , for a given system and correcting for motor, gearing and bearing losses. Design specification of the motor is then determined by selecting the closest higher standard size.

The system turbulence is checked by calculating the Reynolds number for the agitation system. Shape factors are calculated using the mixing vessel geometry. A power number is then obtained by comparing the shape factor results with published power correlations.

Specific agitator power correlations were presented so that the power requirements could be determined based on process conditions. An example using an agitated vessel was given and worked through so that power requirements for a process system could be calculated.

Related References

1. McCabe, Warren L. and Julian C. Smith, Unit Operations of Chemical Engineering, McGraw-Hill Book Company, New York, 3^rded., 1976, pp. 222-244.
2. Don W. Green, Editor, Perry's Chemical Engineers' Handbook, McGraw-Hill Book Company, New York, 7^th ed., 1997, p. 18-12.
4. Perry, Robert H. and Cecil H. Chilton, Chemical Engineer's Handbook, McGraw-Hill Book Company, New York, 5^th ed., 1973, pp. 21-6 to 21-9.
5. Yaws, Carl L., Chemical Properties Handbook, McGraw-Hill Book Company, New York, 1999, pp. 56, 185, 472, and 531.
6. Foust, Alan S. et. al., Principles of Unit Operations, John Wiley & Sons, New York, 1960, p. 412-417.
7. Himmelblau, David M., Basic Principles and Calculations in Chemical Engineering, Prentice-Hall, Inc., Englewood Cliffs, NJ, 1967, Appendix C: Steam Tables.
8. Treybal, R.E, Mass Transfer Operations, McGraw-Hill, New York, 1980, pp. 146-153.
9. Harnby, N., et. al., Mixing in the Process Industries, Buttersworth, Boston, 1987, pp. 131-140.
10. Rushton, J.H., et. al., Chemical Engineering Progress, Vol.46, No. 9, 1950, pp. 467-476.
11. Glover, T. J., Pocket Ref., 1^st ed. Sequoia Publishing, Inc., Littleton, CO, 1993, p. 125.
12. Aiba, S., et. al., Biochemical Engineering, 2^nd ed., Academic Press, Inc., New York, 1973, pp. 174-75, 304.
13. Fukuda, H., et. al., "Scale-up of Fermentators. Part ii. Modified Equation for Power Requirement", J. Ferm. Tech. (Japan), 1968, Vol. 46, p. 838.

Quiz

Once you finish studying the above course content, you need to take a quiz to obtain the PDH credits.

DISCLAIMER: The materials contained in the online course are not intended as a representation or warranty on the part of PDHonline.com or any other person/organization named herein. The materials are for general information only. They are not a substitute for competent professional advice. Application of this information to a specific project should be reviewed by a registered professional engineer. Anyone making use of the information set forth herein does so at their own risk and assumes any and all resulting liability arising therefrom.