2. Definition
In industrial process engineering, mixing is
a unit operation that involves manipulating a
heterogeneous physical system, with the
intent to make it more homogeneous.
OR
Mixing is a phenomena in which two or
more separate components are intermingled
to form more or less uniform product.
3. Types of Mixing
Mixing of cohesive solids
Mixing of noncohesive solids
Mixing of liquids
4. Cont…
Single phase liquid mixing
Mixing of immiscible liquids
Gas liquid mixing
Liquid solid mixing
Solid liquid gas mixing
Solid solid mixing
6. Mechanism of Mixing
Shear Mixing;
In shear mixing, shear stresses give rise to slip zones
and mixing takes place by interchange of particles
between layers within the zone.
Diffusive Mixing;
Diffusive mixing occurs when particles roll down a
sloping surface.
Convective Mixing.
Convective mixing is by deliberate bulk movement of
packets of powder around the powder mass.
7. Types of Mixer
Tumbling mixers.
Closed vessel rotating about its axis
Shapes of vessels include v-mixer, double cone, rotating cube
Mechanism is diffusive mixing.
Quality of mixture is limited.
Baffles may be installed in an attempt to reduce segregation
8. Types of Mixer
Convective mixers
Circulation patterns are set up within a static shell by
rotating blades or paddles
Mechanism is convective mixing
Accompanied by some diffusive and shear mixing
Ribbon blender in which helical blades or ribbons rotate
on a horizontal axis in a static cylinder
Rotational speeds are typically < 60 rpm
16. Mixer Selection
Choice of impeller can also affect mixing time
Propellers typically require longer mixing times
compared to turbines
Propellers have lower power consumption
Gas bubbles, liquid drops, or solid particles also
increase blending time
No direct relation between power consumed and
amount or degree of mixing.
17. Degree of the Mixing
Difficult to quantify the degree of mixing
If dealing with the solid particles, the statistical
variation in composition among withdrawn at
any time from a mix is commonly used a
measure of the degree of mixing
Standard deviation S or the variance s2
is
generally used.
No amount of mixing will lead to the formation of
a uniform mosaic but only to a condition, where
there is an overall uniformity but not point
uniformity.
18. Agitation vs. Mixing
Agitation – induced motion of a material in a
specified way
Usually a circulatory pattern inside a container
Mixing – random distribution, into and
through one another, of two or more initially
separate phases
Various degrees of homogeneity
19. Purposes of Agitation
Suspending solid particles
Blending miscible liquids
Dispersing a gas through a liquid in the form
of small bubbles
Dispersing a second liquid, immiscible with
the first, to form an emulsion or suspension of
fine drops
Promoting heat transfer between the liquid
and a coil or jacket
20. Agitation Equipment
Tank or vessel
Cylindrical in form with a vertical axis
Rounded tank bottom
Depth ≅ diameter
Impellers
Axial-flow – generate currents parallel with the
axis of the impeller shaft
Radial-flow – generate currents in a tangential or
radial direction
Propellers, Turbines & High-efficiency Impellers
22. “Standard” Turbine Design
Type and location of impeller, proportions of
vessel, number and proportions of baffles
No. of baffles – 4
No. of impeller blades – 4-16 (usually 6-8)
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25. Flow Patterns
Depends on type of impeller, characteristics
of fluid, size and proportions of tank, baffles,
and agitator
Swirling – stratification at various levels with
no longitudinal flow between levels
