Chemical Engineering August 2021 - 34

If
the
exponent
is
rounded
off to 3.00, the pre-constant
changes to 0.40. So, for pitchedblade
turbines with Po = 1.50,
Equation (30) applies:
(30)
It is not clear why the ratio of
blend to circulation times has a different
dependency on the impeller
to vessel diameter ratio for Rushton
and pitched-blade turbines,
although the primary flow that they
generate is significantly different.
The Rushton turbine generates a
radial jet that splits when it reaches
the vessel wall with flow loops created
above and below the impeller.
On the other hand, pitched-blade
turbines generate an axial jet with a
single flow loop.
Final thoughts
The macro-timescales of mixing in
a well-designed stirred tank are on
the order of tens of seconds and
the
References
Authors
1. Bourne, J. R., Mixing and the Selectivity of Chemical Reactions,
Org. Proc. Res. Dev., 7, 471-508, 2003.
2. Levenspiel, O., " Chemical Reaction Engineering, " 3rd ed.,
John Wiley & Sons Inc., Hoboken, N.J., pp. 257-282,
1999.
3. Ibid, pp. 943-100.
4. Moore, R. L., " Environmental Protection by the Neutralization
of Wastewater using pH Control, " Instrumentation
Society of America (ISA), Research Triangle Park, N.C.,
pp. 190-194, 1995.
5. Nienow,
A. W., Constant
Turnover
up Criterion for Agitated Tanks, " Chem. Eng.
pp. 1,043-1,044, 1974.
7. Jones, P. N.,
Time as a ScaleSci.,
29,
6. Samaras, K., Mavros, P. and Zamboulis, D., Effect of Continuous
Feed Stream and Agitator Type on CFSTR Mixing State,
Ind. Eng. Chem. Res., 45, pp. 4,805-4,815, 2006.
Özcan-Taskin, N. G. & Yianneskis,
M.,
The Use of Momentum Ratio to Evaluate the Performance
of CSTRs, Chem. Eng. Research & Design, 87,
pp. 485-491, 2009.
8. Marchdo, M. B., Nunhez, J. R., Nobes, D. & Kresta, S. M.,
Impeller Characterization and Selection: Balancing Efficieint
Hydrodynamics with Process Requirements, AIChE Journal,
58, pp. 2,573-2,588, 2012.
9. Hemrajani, R. R. & Tatterson, G. B., " Handbook of Industrial
Mixing: Science and Practice, " John Wiley & Sons Inc.,
Hoboken, N.J., p. 352, 2004.
10. Thomas, J. A., Giacomelli, J. J. and Grenville, R. K., LES
simulation of Continuous Stirred Tank Reactors: Comparison
to Theory and Experiment, AIChE Annual Meeting,
Salt Lake City, UT, 2015.
11. Grenville R. K. and Giacomelli, J. J., Modelling of RTD in
a Complex Geometry using Large-Eddy Simulation, iAIChE
Annual Meeting, San Francisco, CA, 2016.
12. Connolly, J. R. and Winter, R. L., Approaches to Mixing
Operation Scale-up, Chem. Eng. Prog., 65, pp. 70-78,
August 1969.
process can be considered
to be well-mixed under most circumstances,
since this timescale
will be short compared to other
steps, such as filling and emptying
the contents.
For CSTR design, a good firstpass
analysis can be conducted
based on the method described
here, especially if the impeller-tofeed-momentum
ratio is considered.
Large-eddy simulation CFD
models can also be reliably used to
assess CSTR performance [10].
If the timescale of a chemical
reaction is long compared to the
blend time, for instance on the
order of several minutes, the reactants
will exist in a well-mixed environment
and the yield or selectivity
of the reaction will be governed by
the reaction kinetics. Conversely, if
the rate of reaction is faster than the
rate of mixing, the local conditions
at the feed point will determine the
selectivity. Essentially, the reaction
is over before the vessel contents
are blended, and this must be taken
into account in the design of the
mixing equipment.
■
Edited by Mary Page Bailey
32
13. Hicks, R. W., Morton, J. R. and Fenic, J. G., How to Design
Agitators for Desired Process Response, Chem. Eng., pp.
102-110, April 1976.
14. Grenville, R. K., Giacomelli, J. J., Padron, G. and Brown, D.
A. R., Impeller Performance in Stirred Tanks, Chem. Eng.,
42-51, August 2017.
15. Leng, D.
E., Succeed at Scale-up, Chem. Eng. Prog.,
pp. 23-31, June 1991.
16. Patterson, G. K., Paul, E. L., Kresta, S. M. and Etchells III, A.,
" Handbook of Industrial Mixing: Science and Practice, " John
Wiley & Sons Inc., Hoboken, N.J., p. 764, 2004.
17. Fox, E. A. and Gex, V. E., Single Phase Blending of Liquids,
AIChE Journal, 2, pp. 539-544, 1956.
18. Norwood, K. W. and Metzner, A. B., Flow Patterns and
Mixing Rates in Agitated Vessels, AIChE Journal, 6,
pp. 432-437, 1960.
19. Landau, J. and Procházka, J., Studies on Mixing XI: Experimental
Methods for Following the Homogenization of Miscible
Liquids in Rotary Mixers, " Coll. Czech. Chem. Commun.,
26, pp. 1,976-1,990, 1961.
20. Havas, G., Sawinsky, J., Deák, A. and Fekete, A., Investigation
of the Homogenization Efficiency of Various Propeller Agitator
Types, Period. Polytech., 22, pp. 331-343, 1978.
21. Shiue, S. J. and Wong, C. W., Studies in Homogenization Efficiency
of Various Agitators in Liquid Blending, Can. J. Chem.
Eng., 62, pp. 602-609, 1984.
22. Procházka, J. and Landau, J., Studies on Mixing XII: Homogenization
of Liquids in the Turbulent Region, Coll. Czech.
Chem. Commun., 26, pp. 2,961-2,973, 1961.
23. Khang, S. J. and Levenspiel, O., New Scale-up and Design
Method for Stirrer Agitated Batch Mixing Vessels, Chem. Eng.
Sci., 31, pp. 569 - 577, 1976.
24. Grenville, R. K., Blending of Viscous Newtonian and PseudoPlastic
Fluids, PhD Thesis, Cranfield Inst. of Tech., 1992.
25. Strand, A. and Hensel, A., Investigation of Blend Time for Turbulent
Newtonian Fluids in Stirred Tanks: a Second Analysis " ,
Mixing XXVI Conference, San Juan, P.R., 2018.
26. Holmes, D. B., Vonken, R. M. and Dekker, J. A., Fluid Flow in
Turbine Stirred Baffled Tanks - 1: Circulation Times, Chem.
Eng. Sci., 19, pp. 201-208, 1964.
27. Roberts, R. M., Gray, M. R., Thompson, B. and Kresta, S.
M., The Effect of Impeller and Tank Geometry on Circulation
Time Distributions in Stirred Tanks, Trans. IChemE., 73A,
pp. 78-86, 1995.
Richard K. Grenville is director
of Mixing Technology at Philadelphia
Mixing Solutions, an SPX
FLOW Brand (1221 East Main
Street Palmyra,
PA 17078;
Phone: +1 717 202 7976; Email:
rkgrenville@philamixers.com),
and has worked in the field of
mixing for nearly 40 years. He is
an adjunct professor at Rowan
University and the University of Delaware, where he
co-teaches courses on mixing and regularly presents
seminars to customers and for AIChE local and student
chapters. He has a B.S.Ch.E. from the University of
Nottingham and Ph.D. from Cranfield Institute of Technology.
Grenville is a chartered engineer and fellow of
both IChemE and AIChE. He is a past president of the
North American Mixing Forum and winner of its award
for sustained contributions in the field of mixing. He
has co-authored several papers and conference presentations
on subjects including jet mixing, mixing of
non-Newtonian fluids and solids suspension.
Jason J. Giacomelli is a research
and development engineer
at Philadelphia Mixing Solutions,
an SPX FLOW Brand (same
address as above; Phone: 717832-8884;
Email: jgiacomelli@
philamixers.com), where he is responsible
for running pilot-scale
experimental programs for both
internal product development and
process development on behalf of customers. He also
runs computational fluid dynamic (CFD) models to support
these programs. He is a member of AIChE and
works with the local chapter to help with community
outreach programs, such as the Science Technology
Engineering and Math (STEM) Festival in Washington,
D.C. Giacomelli has a B.S.Ch.E. from Rowan University
and is currently studying for a Ph.D. on the subject of
solids suspension in stirred vessels at the University of
Limerick in Ireland. He has also co-authored a number
of papers and presentations.
Benjamin A. Boyer is a research
and
development
engineer at
Philadelphia Mixing Solutions, an
SPX FLOW brand (Same address
as above; Phone: +1-717-8328896;
Email: bboyer@philamixers.
com). With 10 years at the company,
he focuses on CFD modelling
of mixing systems, as well as
the design and implementation of
reduced-scale and full-scale test equipment supporting
the physical validation of CFD models. Boyer has a
B.S. in mechanical engineering from Lafayette College,
is co-author of a number of papers and presentations
through AIChE events, and has spoken at the North
American Mixing Forum (NAMF).
Sarah Jean Johnson is a research
and development engineer
at Philadelphia Mixing Solutions,
an SPX FLOW brand (Same
address as above; Phone: +1717-644-3011;
Email:
johnson@spxflow.com,
johnson@mines.sdsmt.edu).
sarah.
sarah.
In
this role, she is responsible for
internal research and development
experimental efforts. Johnson has a B.S.Ch.E.
from South Dakota School of Mines and Technology, an
M.S.Ch.E. from Penn State University and is currently
studying for a Ph.D. in chemical engineering from
South Dakota School of Mines and Technology.
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