-
8/13/2019 Blending and Mixing
1/23
Blending and Mixing
-
8/13/2019 Blending and Mixing
2/23
Mixing vs Agitation
In process industries many operations are dependent oneffective
agitation and mixing of fluids.
Agitation refers to forcing a fluid by mechanical means toflow
in a circulatory or other pattern inside a vessel.
Mixing usually implies the blending of two or moreseparate
phases, such as a fluid and a powdered solid, ortwo fluids, and
causing them to be randomly distributedthrough one another.
-
8/13/2019 Blending and Mixing
3/23
Mixing vs Agitation
Mixing is much more difficult operation than agitation.
In agitation, velocity and flow patterns are complex
butreasonably definite and reproducible. Also, the powerconsumption
is readily measured.
In mixing studies, flow patterns are seldom highlyreproducible
and depend in large measure on howmixing is defined.
-
8/13/2019 Blending and Mixing
4/23
What is the difference between mixing and blending?
-
8/13/2019 Blending and Mixing
5/23
Blending in process vessels
For a standard six-blade turbine,
or
-
8/13/2019 Blending and Mixing
6/23
Figure . Mixing times in agitated vessels. !ashed lines are
for
unbaffled tan"s# solid lines are for baffled tan"s.
-
8/13/2019 Blending and Mixing
7/23
In the figure, the mixing times are appreciably greater when the
$eynoldsnumbers are in the range of %- %%%.
Figure &. 'orrelation of blending times for miscible li(uids
in aturbine-agitated baffled vessel.
-
8/13/2019 Blending and Mixing
8/23
A general correlation given by )orwood and Met*ner for turbines
+Figure &
For -/ in Figure
A general correlation by Fox and 0ex for propellers +Figure
-
8/13/2019 Blending and Mixing
9/23
Example 1
An agitated vessel 1 ft + .2/m in diameter contains asix-blade
straight%-blade turbine & ft +%.1 m in diameter,set one
impeller diameter above the vessel floor, and
rotating at 2% rpm. It is proposed to use this vessel
forneutrali*ing a dilute a(ueous solution of )a3 at 4% Fwith a
stoichiometrically e(uivalent (uantity ofconcentrated nitric acid.
5he final depth of the li(uid inthe vessel is to be 1 ft + .2/m .
Assuming that all theacid is added to the vessel at one time, how
long will itta"e for the neutrali*ation to be complete6
-
8/13/2019 Blending and Mixing
10/23
Jet mixers
Figure /. Flow a submerged circular 7et.
-
8/13/2019 Blending and Mixing
11/23
Jet mixers
5he velocity in the 7et issuing from the no**le is uniformand
constant.
5he core is surrounded by an expanding turbulent 7et, inwhich
the radial velocity decreases with distance fromthe centerline of
the 7et.
-
8/13/2019 Blending and Mixing
12/23
Suspension of solid particles
8articles of solids are suspended in li(uids for many purposes,
perhaps toproduce a homogeneous mixture for to a processing unit,
to dissolve thesolids, to cataly*e a chemical reaction, or to
promote growth of a crystallineproduct from a supersaturated
solution.
Degrees of Suspension
)early complete suspension with filleting.
'omplete particle motion.
'omplete suspension or complete off-bottom suspension.
9niform suspension.
-
8/13/2019 Blending and Mixing
13/23
orrelations for suspension
:wietering;s correlation to measure critical stirrer speed
+based ondata for < types of impellers in six tan"s from 1 in =
& ft in diameter
-
8/13/2019 Blending and Mixing
14/23
5able . >hape factor, > for critical stirrer speed
-
8/13/2019 Blending and Mixing
15/23
!ower consumption
Figure ?. 8ower re(uiredfor complete suspensionof solids in
agitatedtan"s using pitched-blade turbines.
-
8/13/2019 Blending and Mixing
16/23
Example "
An agitated vessel 1 ft + .2m in diameter with a wor"ing depth
of 2ft +&.??m is used to prepare a slurry of
-
8/13/2019 Blending and Mixing
17/23
#ispersion operations
Mean !iameter
E volume = surface mean diameter or >auter meandiameter E
holdup +vol. fraction of dispersed phase in the system
a E interfacial area per unit volume
-
8/13/2019 Blending and Mixing
18/23
$i%uid&li%uid dispersion
Beber number +ratio of fluid "inetic energy at the impeller tip
speedto surface-tension stress
where E density of continuous phase
E interfacial tension
'orrelation for the dispersion of li(uids with standard
six-bladeturbine
-
8/13/2019 Blending and Mixing
19/23
$i%uid&li%uid dispersion
>tatic mixers
where E pipe diameter
E average velocity
E friction factor
!rops from orifices
where E diameter of orifice
E density of dispersed phase
-
8/13/2019 Blending and Mixing
20/23
Example '
'yclohexane is dispersed in water at &< ' in a
baffledvessel /% cm in diameter with a normal depth of /< cm.5he
agitator is a standard six-blade turbine % cm indiameter. (a) If
the stirrer speed is 1 rGs and thesuspension is 2 cyclohexane by
volume, calculate thepower consumption and power per unit volume,
andestimate the mean droplet si*e. (b) If the li(uid mixture
ispumped at .& mGs through a Henics helical-element mixer
that is & cm in diameter and has &% elements, each /
cmlong, estimate the mean droplet si*e and the powerconsumption.
5he friction factor may be ta"en as %.?&.
-
8/13/2019 Blending and Mixing
21/23
(as&li%uid dispersion
Interfacial area
'ritical gas velocity
-
8/13/2019 Blending and Mixing
22/23
Figure
-
8/13/2019 Blending and Mixing
23/23
Agitator selection and Scaleup
xample /.
A pilot plant vessel ft +/%< mm in diameter is agitated bya
six-blade turbine impeller ? in + %& mm in diameter.Bhen the
impeller $e is %?, the blending time of twomiscible li(uids is
found to be < s. 5he power re(uired is& hp per %%% gal
+%.?"BGm/ of li(uid. (a) Bhat powerinput would be re(uired to give
the same blending time in
a vessel 1 ft + ,2/% mm in diameter6 (b) Bhat would bethe
blending time in the 1 ft vessel if the power input perunit volume
were the same as in the pilot-plant vessel6
