MIXING
Syllabus:
Fundamentals.
Factors influencing the
selection of mixers.
Mixing mechanism. Study of
solid-solid, solid-liquid and liquid-liquid mixers used in industry.
Questions:
1. Mixing of viscous masses - short note. (99) [4]
2. Define mixing index for pastes and solids. How do they
vary with time of mixing? (98) [5]
What are the different types of impellers used for
mixing liquids? How vortex formation is controlled in mixing tanks? (98) [5]
3. Power requirement in mixing - short note (98) [4]
4. What factors influenced the selection of mixers? (97) [4]
5. Write short note on solid-solid mixers. (97) [4]
6. Write short note on solid-solid mixing. (96) [4]
7. Write short note on the operation of a V-type mixer.
(95) [4]
8. Define mixing. What are the objectives of mixing?
Explain the mechanism for powder mixing. (94) [16]
Definition:
Mixing may
be defined as an operation in which two or more components, in a separate or
roughly mixed condition, are treated so that each particle lies as nearly as
possible in contact with a particle of each of the other ingredients.
Objectives of mixing:
1. Simple physical mixture
This may be
simply the production of a blend of two or more miscible liquids or two or more
uniformly divided solids. In pharmaceutical practice, the degree of mixing must
commonly be of a high order, as many such mixtures are dilutions of a potent
substance (e.g. small amount of steroid mixed in large amount of inert
diluents), and correct dosage must be ensured.
2. Physical change
Mixing may
aim at producing a change that is physical, for example the solution of a
soluble substance. In case, a lower efficiency of mixing will often be
acceptable because the mixing merely accelerates a process that could occur by
diffusion, without agitation.
3. Dispersion
This
includes the dispersion of two immiscible liquids to form an emulsion or the
dispersion of a solid in a liquid to give a suspension or paste. Usually good
mixing is required to ensure stability.
4. Promotion of reaction
Mixing will
usually encourage (and control at the same time) a chemical reaction, so
ensuring uniform products. Examples of this type include products or processes
where accurate adjustment to pH is required and the degree of mixing will
depend on the process.
TYPES OF MIXTURES
Mixtures
may be divided into three types that differ fundamentally in their behavior:
Positive mixtures
Positive
mixtures are formed from materials such as gases and miscible liquids, where
irreversible mixing would take place, by diffusion, without the expenditure of
energy provided time is unlimited. In general, such materials do not present
any problems in mixing.
Negative mixtures
Suspensions
of solids in liquids are examples of negative mixtures that require work for
their formation, and the components of which will separate unless work is
continually expended on them.
Negative mixtures are more difficult to form and a higher
degree of mixing efficiency is required.
Neutral mixtures
Neutral
mixtures are static in their behavior, the components having no tendency to mix
spontaneously, nor do they segregate when mixed.
e.g. Pastes, ointments and mixed powders.
MIXING LIQUID WITH LIQUID
The mixing operation has two requirements:
1.
Localized mixing, sufficient to apply shear to the
particles of the fluid.
2.
A general movement, sufficient to take all parts of the
bulk of the materials through the shearing zone and to ensure that a uniform
final product is obtained.
For readily miscible liquids - flow alone is sufficient.
For two immisible liquids to produce an emulsion -
shear force is essential.
Mixers:
Liquid mixing is usually performed with a
(i)
mixing element, commonly a rotational device, which
provides the necessary shear force and flow,
(ii)
a tank in which the mixing element will be fitted.
Mechanism of mixing:
The movement of the liquid at any point in the vessel will
have three velocity components and the complete flow pattern will depend upon
variations in these three components in different parts of the vessel.
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The three velocity components are:
(i)
Radial component, acting in a direction vertical to the impeller shaft.
(ii)
Longitudinal component, acting in a direction parallel to the impeller shaft.
(iii) A
tangential component, acting in a direction that is a tangent to the circle of rotation round the impeller shaft.
A satisfactory flow pattern will depend on the balance of
these three components.
Examples: Assuming that the impeller is placed
vertically in a mixing tank.
·
Excessive radial movement, especially if
solids are present, will take materials to the container wall, when they fall
to the bottom and may rotate as a mass beneath the impeller.
·
If the tangential component is dominant,
a vortex forms and may deepen until it reaches the impeller, when
aeration occurs.
·
If the longitudinal component is
inadequate, liquids and solids may rotate in layers without mixing, even when
rotation is rapid and in the presence of vortexing.
Factors affecting the flow pattern of liquids:
(i)
Form of impeller and its position; e.g. whether it is
high or low in the vessel, whether mounted centrally or to one side, or whether
the shaft is vertical or inclined.
(ii)
Container shape.
(iii) Presence
of baffles.
(iv) Liquid
properties -
It has been found that the optimum speed of rotation (v) of the mixing element
and the ratio of the diameter of the container (D), to the diameter of the
mixing element (d) are both inversely proportional to the apparent viscosity (h) of
the liquid.
i.e. v µ 1/h
D/d µ 1/h
·
Hence, a liquid of low viscosity will use
an impeller with a D/d ratio of the order of 20 and rotating at high speed.
·
A liquid of high viscosity, such as
paste, will need a D/d ratio of 1 and low speed of rotation. i.e., blades of
the impellers are used so that they more slowly and scrape the side of the
vessel.
MIXING EQUIPMENT FOR LIQUID
Shaker mixers:
They agitate the material either in a oscillatory or a
rotary movement.
·
Oscillatory movement is applicable to
small-scale working only.
·
Rotary movement can be applied to large vessels,
which are usually cylindrical in shape and rotated in a similar manner to the
ball mill.
Factors affecting the
mixing efficiency in rotary type are:
·
Viscosity of liquid,
·
Construction of vessel (mixer container) e.g.
baffles within the vessel.
·
Use: these have limited use.
Propeller mixers:
·
The propellers are small impellers that produce
an longitudinal movement of liquids.
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·
Generally they are small in relation to the
container i.e. container diameter to propeller diameter ratio (D/d) » 20.
·
They generally operates at high speeds: upto
8000 rpm.
·
Propeller mixer is not normally effective with
liquids of viscosity greater than about 5 Ns/m2 ; which is some what
greater than glycerin or castor oil.
Vortexing and its remedies:
Due to the high speed of the propellers vortexing and
finally aeration may occur; i.e. air may get entrapped which may be difficult
to remove from the product and the air may encourage oxidation in some cases.
To avoid vortexing the following strategies can be worked
out:
(i) The propeller should be deep into the liquid and [fig
(a)]
(ii) Symmetry should be avoided:
(a) propeller shaft may be off-set
from the center. [fig (b]
(b) propeller shaft may be mounted
at an angle to the vertical wall of the container. [fig (c)]
(c) the shaft may enter side of the
vessel [fig (d)]
(d) or, a vessel other than
cylindrical may be used, (N.B. although this is liable to give rise to ‘dead
spots’ in corners)
(iii) A push-pull type of propeller may be used in which two
propellers of opposite pitch are mounted on the same shaft so that the rotating
effects are in opposite directions and cancel each other. [fig (e)]
(iv) One or more baffles may be used which are usually
vertical strips attached to the wall of the vessel. [fig (f)]
Use:
(i) Propellers are suitable when strong vertical currents
are required e.g. in suspensions of solids in liquids.
(ii) They are not suitable when considerable shear is
required, as in emulsification.
Turbine mixers
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A turbine mixer uses a circular disc impeller, to which are
attached a number of vertical blades,
which may be straight or curved.
Characteristics:
(i)
They are usually rotated at a some what lower speed
than the propeller type.
(ii)
D/d ratio is lower than that of propeller type.
(iii) The
blades are usually flat, hence, very little axial or tangential flow, the
liquid moves rapidly in a radial direction.
(iv) They
give rise to greater shear forces than propeller type and these shear forces
can be increased further by fitting a diffusing ring. This is a stationary
perforated or slotted ring which surrounds the impeller, so that the discharged
liquid must pass through the apertures. The diffuser reduces rotational
swirling and vortexing , but is most useful in increasing shear forces.
(v)
They can deal with more viscous liquids than the
propeller mixer, having a range upto 100Ns/m2 approximately the
consistency of liquid glucose.
Use:
(i)
Suitable for viscous liquids.
(ii)
Not suitable for suspensions, because no vertical flow
is there.
(iii) The
higher shear forces and the greater viscosity range give it a special
application in the mixing of liquids that may stratify with a propeller and,
particularly, in the preparation of emulsions of immiscible liquids.
Paddle mixers
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Paddle mixers use an agitator consisting usually of flat
blades attached to a vertical shaft and rotating at low speed (100 rpm).
Characteristics:
(i)
For liquids of low viscosity simple flat paddles are
used and the emphasis is on radial and tangential movements.
(ii)
Paddles for more viscous liquids generally have a
number of blades, often shaped to fit closely to the surface of the vessel,
avoiding ‘dead spot’ and deposited solids.
(iii) An
alternative design for the more viscous range of liquids is the planetary motion mixer, which has a
smaller paddle that rotates on its own axis, but travels also, in circular path
round the mixing vessel. The agitator is fixed at the side of the vessel, to
eliminate ‘dead spots’.
(iv) The
width of the agitator is not more than 1/2 to 2/3r of the diameter of the
vessel, which requires less power than that needed for a full width central
agitator, improves the circulation in the vessel, and increases mixing
efficiency.
SOLID MIXING
MECHANISM
It has been generally accepted that solids mixing proceeds
by a combination of one or more of the following mechanisms:
1. Convective mixing:
A relatively large mass of material is moved from one part
of the powder bed to another - this is called convection.
Depending on the type of mixer employed, convective mixing can occur by an
inversion of the powder bed, by means of blades
or paddles, or by means of a revolving screw etc.
2. Shear mixing
As a result of forces within the particulate mass, slip
planes are set up. Depending on the flow characteristics these can occur singly
or in such a way that it give rise to laminar flow. When shear occurs between
regions of different composition and parallel to their interface, it reduces
the scale of segregation by thinning the dissimilar layers. Shear occur in a
direction normal to the interface of such layers is also effective since it too
reduces the scale of segregation.
3. Diffusive mixing
Mixing by “diffusion” is said to occur when random motion of
particles within a particle bed causes them to change position relative to one
another. Such as exchange of positions by single particles result in reduction
of the intensity of segregation. Diffusive mixing occurs at the interfaces of
dissimilar regions that are undergoing shear and therefore results from shear
mixing.
MIXING EQUIPMENT
·
The ideal mixer should produce a complete blend rapidly with as gentle as
possible a mixing action to avoid product damage.
·
It should be cleaned and discharged easily,
·
be dust-tight and
·
require low maintenance and
·
low power consumption.
Size reduction equipment can be used for mixing.
For example, attrition
type of mill giving good shearing action near the moving surfaces. In general,
these are not good mixers, however, because the batch is small, the material is
not dilated, and there is very little convective movement.
Impact mills (e.g.
hammer mills) are effective in dealing with aggregates, but the hold-up is too
small to permit bulk mixing.
BATCH MIXERS
In tumbling
mixers, rotation of the vessel imparts movement to the materials by tilting the
powder until the angle of the surface exceeds the angle of repose when the
surface layers of the particles go into a slide.
A common
type of mixer consists of a container of
one of several geometrical forms, which is mounted so that it can be rotated
about an axis. The resulting tumbling motion is accentuated by means of baffles or simply by virtue of the shape of the container.
Rotating -Shell
Mixers
The drum
type, cubical-shaped, double-cone and twin shell blenders ar eall examples of
this class of mixers.
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Drum-type blenders with their axis of rotation
horizontal to the centre of the drum are used quite commonly.
Disadvantages:
This suffers from poor cross flow along the axis.
Remedy:- The addition of baffles or inclining
the drum on its axis increases cross flow and improves the mixing action.
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Cubical and polyhedron shaped blenders with the
rotating axis set at various angles also are available.
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Disadvantages:- In
the polyhedron type blender, because of their flat surface, the powder is
subjected more to a sliding than a rolling action which is not conducive to the
most efficient mixing.
Double cone blender provides a good cross flow with a
rolling rather a sliding motion. Normally no baffles are required so that
cleaning is simplified.
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Twin shell blender combines the efficeincy of the
inclined drum-type with the intermixing that occurs when two such mixers
combinetheir flow. The twin-shell blender takes the form of a cylinder that has
been cut in half, at approximately a 450-angle with its long axis,
and then rejoined to form a “V”-shape. This is rotated so that the material is
alternatively collected in the bottom of the V and then split into two portions
when the V is inverted.
MOA
This is
quite effective because the bulk transport and shear, which occur in tumbling
mixers generally, are accentuated by this design.
A bar containing blades that rotate in a direction opposite
to that of the twin shell often is used to improve agitation of the powder bed,
and may be replaced by a hollow tube for the injection of liquids.
The efficiency of tumbling mixers is highly depended on the
speed of rotation.
·
If the rotation speed is too slow Þ does not produce desired tumbling or cascading
motion nor does
it generate rapid shear rates.
·
If the rotation speed is too high Þ produce centrifugal force sufficient to hold
the powder to the sides
of the mixer and thereby reduce efficiency.
·
If the rotation speed is optimumÞ depends on the size, shape, r.p.m. Commonly in
the range of 30
to 100 rpm.
Agitator mixers
Agitator mixer for powders can take a similar form to paddle
mixers for liquids, but their efficiency is low. Planetary motion mixers are
effective, but special design are to be preferred.
This type of mixers employs a stationary container to hold
the material and brings about mixing by means of moving screws, paddles or blades.
Use: Since the mixers do not depend entirely on
gravity as do the tumblers, it is useful in mixing wet solids, sticky pastes
etc.
The high shear force effectively break up lumps or
aggregates.
Examples:
(i) The ribbon blender
consists of a relatively long trough-like shell with a semicircular botom. The
shell is fitted with a shaft on which are mounted spiral ribbons, paddles or
helical screws, alone or in combination. These mixing baldes produce a
continuous cutting and shuffling of the charge by circulating the powder from
end to end of the trough as well as rotationally . The shearing action that
develops between the moving blade and the trough serves to break down powder
agglomerates.
Disadvantages:
They are not precesion blenders and they are difficult to clean.
RIBBON BLENDER
(ii) Sigma blade and
planetary mixers are also used at a step prior to addition of liquid (e.g
just before wet massing in tablet wet-granulation). Fig. See semisolid mixers
for sigma blenders and
(iii) In helical flight mixer powders are lifted
by a centrally located and vertical screw and allowed to cascade to the bottom
of the tank.
MIXER FOR SEMISOLIDS
Agitator mixers
(i) Planetary motion
mixers:
Agitator arms are designed to give
a pulling and kneading action. Shape is such that it clears the mass from all
sides and corners of the vessel.
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(ii) Sigma arm mixer:
It uses two mixer blades, the shape
of which resembles the Greek letter “sigma” (S).The two blades rotates
towards each other and operate in a mixing vessel which has a double trough
shape, each blade fitting into a trough.
The two blades rotate at different
speeds, one usually about twice the speed of the other, resulting in a lateral
pulling of the material and divisions into two troughs, while the blade shape and
difference in speed causes end-to-end movement.
Use:
·
This types of mixers are of sturdy construction
and high power, hence, they can handle even the heaviest plastic materials and
products like tablet granule moves, and ointments are mixed readily.
·
To reduce the entrainment of air in ointment
masses the sigma mixer can be enclosed and operated under reduced pressure,
which is an excellent method for avoiding entrainment of air and may assist in
minimizing decomposition of oxidizable materials, but it must be used with
caution if mix contains volatile ingredients.
·
As with many other mixers, the vessel is jacket
for heating or cooling and, in this case, the blades can be hollow for the same
purpose. This can be very useful in practice, since some semi-solids may be
reduced in viscosity by heating, while with other materials it may be necessary
to dissipate the heat resulting from the energy put into the mixing process.
Shear mixers
Colloid mill as a
stator and a rotor with conical working surfaces. The rotor works at speeds of
the order of 3000 to 15000 rev/min and the clearance can be adjusted between 50
to 500 mm.
A roughly mixed suspension or dispersion is introduced through a funnel and is
thrown out between the working surfaces by centrifugal force.
Ultrasonic Mixers
An effective method for dealing with certain forms of mixing
problems is to subject the material to ultrasonic vibration. This has a special
application in mixing in the preparation of emulsions.
POWER REQUIREMENTS OF MIXERS
Large amount of energy is required for mixing. Only part of
the energy supplied to the mixer is directly useful for mixing, and in many
machines the useful part is small.
·
Mixers that work intensively on small quantities of material, dividing
it into very small elements, make more effective use of energy than those that
work more slowly on large quantities.
·
Machines
that weigh little per pound of material processed waste less energy than
heavier machines.
·
The shorter
the mixing time required to bring the material to the desired degree of
uniformity, the larger the useful fraction of the energy supplied will be.
·
Regardless of the design of the machine,
however, the power needed to drive a mixer for pastes and deformable solids is
many times greater than that needed by a mixer for liquids.
·
The energy supplied appears as heat, which must
ordinarily be removed to avoid damaging the machine or the material.
MIXER SELECTION
Factors that must be taken into consideration before
selection of a mixer include:
1.
the physical properties of the materials to be mixed,
such as density, viscosity and miscibility,
2.
economic considerations regarding processing, e.g. time
required for mixing and the power expenditure necessary, and
3.
cost of equipment and its maintenance.
For monophasic
system:
·
The viscosity and the density of the fluid(s) to
be mixed determine the type of flow that can be produced.
·
Fluids
of relatively low viscosity are best
mixed by methods that generate a high degree of turbulence and at the same time
circulate the entire mass of material. e.g.various high speed impeller mixers
can be used.
·
For viscous creams, ointments, and pastes it is
impossible to produce turbulence within their bulk hence such preparations can
be mixed in turbine mixers with flat blade of radial flow type.
Polyphase systems
The mixing of systems composed of several liquid or solid
phases primarily involves the subdivision or deaggregation of one of the phases
present, with subsequent dispersal throughout the mass of material to be mixed.
·
The mixing of two immiscible liquids require the
subdivision of one of the phases into globules, which are then distributed
throughout the bulk of the liquid. So high shear rate is required in mixing tow
immiscible liquids.
·
For low-viscosity liquids high shear rates are
commonly produced by passing the liquid under high pressure through small
orifices or by bringing it into contact with rapidly moving surfaces. e.g.
homogenizer and colloid mill.
·
For high-viscosity liquids e.g. in ointments are
efficiently dispersed by the shear action of two surfaces in close proximity
and moving at different velocities with respect to each other. e.g. paddle
mixers, sigma blenders etc.
·
Mixing of finely divided solids with liquids of
low viscosity i the production of suspension depends n the separation of
aggregates into primary particles and the distribution of these particles
throughout the fluid. High speed turbines, frequently fitted with stators, to
produce increased shear, are often employed.
·
If the solid percentage is high or the liquid is
of high viscosity the it will take the consistency of a paste or dough. In this
case the machine should be of heavy design. Mixers that either knead or mull
the material are required. e.g. Sigma blender is required.
·
Mulling mixers are efficient in deaggregation of
solids. e.g. Roller mills consists of one or more rollers. Of these three roll
type is preferred for semisolid preparations. The rollers rotate at different
speed. The material is placed in the hopper which then passes through roller B
and C. Materials coming into the rollers are crushed, depending on the gap
between the rollers. The gap between c and D (lesser than the previous one)
reduces the particles further and smoothes the mixture. a scrapper E
continuously removes the materials from the roller D.
·
When small amount of liquid is mixed with large
amount of solid powder (e.g. wet granulation method of tablet preparation) then
mixers used for pastes are used e.g. planetary mixers, sigma blenders etc.
MIXING INDEX
The degree
of uniformity of a mixed product is expressed by mixing index.
Mixers act on two or more separate material in a random
fashion. Once a material is randomly
distributed through another, mixing may be considered to be complete.
Procedure:
Let two powders A and B are mixed in a mixer. After mixing a
number of small samples, at random from various locations in the mixture are
taken and the fraction of A (or B) is determined in the samples.
Let the fractions are = xi in each
Let the number of spot samples = N
Average value of the measured concentrations = 
When N is very large x will become m.
So the standard deviation of the samples :
As mixing proceeds the standard deviation, s, decreases
towards zero. So a very low value means good mixing.
Now, before mixing starts two powders remain in two separate
layers:
In the layer containing powder B concentration of powder A =
0 i.e. x1
= 0
In the layer containing powder A concentration of powder A =
1 i.e. x2
= 1
Under this conditions the standard deviation is given by 
where m is the overall concentration of A in the mixture. The
mixing index for the mixture is then expressed by:
Significance:
In any batch mixing process, I is unity (i.e. I = 1) at the
start and increases as mixing proceeds. In theory it should become infinite at
long mixing time but practically it does not, for two reasons:
1 mixing is
never quite complete
2 unless the
analytical methods are extraordinarily precise the measured value of xi never agree exactly
with each other or with x and I is finite even with perfectly mixed materials.
The maximum limiting value of I for “completely mixed”
materials varies with the consistency of the materials being processed, the
effectiveness of the mixer, and the precision of the analytical method.
Typically it falls between the range of 10 and 150.
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