(145a) Mixing Experiences

MIXING EXPERIENCES

Abstract

Chemical engineers design and develop processes to
convert raw materials into desired products.  In order to accomplish this, the raw
materials may be mixed, heated, reacted and separated as diagrammed in Figure
1.  Each process is designed with a
specific order to the steps, and each step requires the design of specific
equipment.  Chemical engineering
students are taught to design both the process and the equipment.  Full courses, or significant portions of
courses are devoted to the heating, reacting and separating steps.  However little course time is devoted to
the mixing steps and often hands-on opportunities are not available in
laboratories.

Mixing is a central operation in most
industries:  the food,
pharmaceutical, paper, plastics, ceramics and rubber industries all include
mixing steps.  Despite its widespread
use, the process is difficult to analyze. 
Advances in obtaining experimental measurements and analyzing the data
are helping to meet the challenges, but unexpected results due to mixing
continue to arise during the scale up of a new process.  Inefficiencies due to this lack of
understanding have been estimated to cost $1-10 billion per year¹.

Mixing experiments have been published, with the
purpose of providing students with practical experience^{2, 3}.  The experiments investigate mixing of
both Newtonian and non-Newtonian fluids using classic impellers.  The experiments provide a general
introduction and experience with the dimensionless variables.  However, the equipment is not available
at all institutions, and only two impellers are used.  

To
address these issues, students completed a mixing experiment at XX University that
was developed with a colleague at Philadelphia Mixing Solutions, Ltd¹.  The necessary equipment is likely available
in a standard chemical engineering laboratory.  Philadelphia Mixing Solutions generously
donated six impellers for the study, and will provide similar impellers to
institutions that wish to include mixing in the curriculum.  Details of the experiment procedure and
analysis will be provided in the full paper.

The
experiment was implemented for the first time in the Spring 2017 semester.  After a brief introduction to mixing, twelve
groups of four students measured the mixing time for an impeller at a variety
of stirring rates.  They calculated
the dimensionless mixing time, Reynolds number and power supplied.  A guest speaker from the mixing industry
presented mixing theory and its limitations and challenged the students to
compare their results to theory. 
One class period was also devoted to scale up methods.  In total, three class periods and two hours
in the laboratory were spent on the topic of mixing.

An
assessment of student learning showed that after completing the experiment:

á     
The students
had a greater understanding of what is ÒmixedÓ.  They were able to apply what they
learned in process controls (response to a step change) and determine when the
solution was 90% homogeneous.

á     
The students
are conversant in the language of mixing. 
In conversations with the instructor they correctly used phrases such as
radial and axial mixing, impeller Reynolds number and power number.  The written reports echoed this.

á     
The students
showed an understanding of the trade off between shear and efficiency among
different impellers.  In fact, when
shown six different impellers, the students can now
provide their name and their mixing characteristics.

While
these students are certainly not mixing experts, they now have knowledge that
will be valuable to future employers. 
Future offerings will include readings from the literature and small
hands on activities during class.

References:

1.   
Grenville,
R.K., ÒMixing in the chemical engineering curriculumÓ, Paper ID #11457,
American Society of Engineering Education Annual Conference, Seattle, WA
(2105).

2.   
Ascanio, G., Legros, R., Tanguy, P.A., ÒA Fluid-Mixing Laboratory for ChE UndergraduatesÓ, Chemical Engineering Education 37(3) :296-299 (2003).

3.   
Pour, S.B., Norca, G.B., Fradette, L., Legros, R., and Tanguy, P.A., ÒSolid-Liquid and
Liquid-Liquid Mixing Laboratory for ChE
UndergraduatesÓ, Chemical Engineering Education 41(2):  101-106 (2007).