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65
Unit Operations
inlet and a fluid outlet; and at the base it is fitted with solids
outlet.
Working: The suspension is introduced tangentially at a high
speed so that a rotary movement takes place inside the vessel
and the fluid is removed by the fluid outlet fitted at the top.
The rotary movements within the cyclone causes the particles
to move by centrifugal force thereby the solids are thrown out
to the walls of the cyclone from where they fall to the conical
base and ultimately removed from there through the solids
outlet (Fig. 4.5).
Cyclone separators can be used for separating suspensions
of solids in liquids but preferably they are used for suspension
of a solid in a gas, usually air. Cyclone separator is a form of
sedimentation but instead of gravitational force the entrifugal
force is used. Hence depending on the velocity of the fluid all
particles can be separated with this cyclone or only the coarse
particles can be removed and the fine particles are allowed to
be carried with the fluid.
Fig. 4.5: Cyclone separator: (A) Elevation, (B) plan.
Introduction to Pharmaceutics
66
SIEVES AND STANDARDS OF SIEVES
Sieves
Sieves are the simplest and sieving is the most frequently used
method for size separation.
Construction: Sieves for pharmaceutical purposes are con struc-
ted from wire cloth with square meshes, woven from wires of
brass, bronze, stainless steel or any other suitable material. The
wires should be of uniform circular cross-section and should
not be coated or plated. There should not be any reaction
between the material of the sieve and the substance which is
being sifted from it. Sieves should not be coated or plated.
There must be no reaction between the material of the sieve
and the substance to be sieved.
Standards for sieves used to testing must specify the following:
Number of sieves: Sieve number indicates the number of meshes
in a length of 25.4 mm in each transverse direction parallel to
the wires (Fig. 4.6).
Fig. 4.6: Seives number.
Nominal size of aperture: Nominal size of
aperture indicates the distance between
the wires. It represents the length of the
side of the square aperture. The IP has
given the nominal mesh aperture size
for majority of sieves in mm or in cm
(Fig. 4.7).
Fig. 4.7: Nominal
aperture.
67
Unit Operations
Nominal diameter of the wire: Wire mesh sieves are made from the
wire having the specified diameter in order to give a suitable
aperture size and sufficient strength to avoid distortion of the
sieve.
Approximate percentage sieving area: This standard expresses the
area of the meshes a percentage of the total area of the sieve.
It depends on the size of the wire used for any particular
sieve number. Generally, the sieving area is kept within the
range of 35 to 40 percent in order to give suitable strength to the
sieve.
Approximate screen area =
Total sieve area – Area occupied
by the wire
Total area of the screen
× 100%
Tolerance average aperture size: Some variation in the aperture
size is unavoidable and when this variation is expressed as a
percentage, it is known as the ‘aperture tolerance average’.
MIXING
DEFINITION
The process of mixing is one of the most commonly used
operations in daily life. It is difficult to find a pharmaceutical
product in which mixing is not done at one stage or the other
during its manufacturing. A wide variety of materials such as
liquids, viscous liquids, semisolids and solids require mixing.
Mixing is done not only in pharmaceutical industries but it has
been widely used in food, plastic, glass, paint, paper, pesticides
and fertilizer industries.
Mixing may be defined as the process in which two or
more than two components in a separate or roughly mixed
condition are treated in such a way so that each particle of
any one ingredient lies as nearly as possible to the adjacent
particles of other ingredients or components.
It is not possible to ascertain the degree of mixing for each
and every product. It depends on the purpose of the product
and the objective of the mixing operation.
Introduction to Pharmaceutics
68
Objects of Mixing
Mixing may be done for following reasons:
To ensure uniformity of composition between the mixed
ingredients which may be determined by taking samples
from the bulk material and analysing them which should
represent overall composition of the mixture.
To initiate or to enhance the physical or chemical reactions,
e.g. diffusion, dissolution, etc. Sometimes physical change
also take place.
Generally mixing is carried out to obtain following types of
products:
When two or more than two miscible liquids are mixed
together, a solution is produced known as true solution.
When two immiscible liquids are mixed in the presence of
an emulsifying agent, an emulsion is produced.
When a solid is dissolved in a vehicle, a solution is obtained.
When an insoluble solid is mixed with a vehicle, a
suspension is obtained.
When a solid or liquid is mixed with a semisolid base an
ointment or a suppository is produced.
When two or more than two solid substances are mixed
together, a powder is obtained which when filled into
capsule shells is known as capsules and when compressed
under heavy pressure is called tablets.
Types of Mixtures
Mixtures may be classified as follows:
Positive mixtures.
Negative mixtures.
Neutral mixtures.
Positive Mixtures
These types of mixtures are formed when two or more than
two gases or miscible liquids are mixed together by means of
diffusion process. In this case no energy is required provided
the time allowed for solution formation is sufficient. Generally,
these types of materials do not create any problem in mixing.
69
Unit Operations
Negative Mixtures
These types of mixtures are formed when insoluble solids
are mixed with a vehicle to form a suspension or when two
immiscible liquids are mixed to form an emulsion. These
mixtures are more difficult to prepare and require a higher
degree of mixing with external force. The components of
these mixtures separate out unless they are continuously
stirred.
Neutral Mixtures
Many pharmaceutical products such as pastes, ointments and
mixed powders are the examples of neutral mixtures. They are
static in their behaviour. The components of such products do
not have any tendency to mix spontaneously but once mixed
they do not separate out easily.
Mechanism of Mixing
In all types of mixers, mixing is achieved by applying one or
more of the following mechanisms.
Convective mixing: During convective mixing transfer of
groups of particles in bulk take place from one part of
powder bed to another.
Shear mixing: During shear mixing, shear forces are created
within the mass of the material by using agitator arm or a
blast of air.
Diffusive mixing: During this mixing, the materials are tilted
so that the gravitational forces cause the upper layers to slip
and diffusion of individual particles take place over newly
developed surfaces.
Rate of Mixing
Generally speaking mixing is the process of achieving uniform
randomness which on subdivision to individual doses contains
the correct proportions of each component which depends
on the amount of mixing done. In the early stages of mixing,
the rate of mixing is very fast because the mixing particles
change their path of circulation quickly and find themselves in
Introduction to Pharmaceutics
70
different environment whereas at the end of the process rate of
mixing reaches to almost zero because the particles do not find
different envirornment.
Liquid Mixing
Liquid mixing may be divided into following two subgroups:
Mixing of liquids and liquids
y Mixing of two miscible liquids
y Mixing of two immiscible liquids
Mixing of liquids and solids
y Mixing of liquids and soluble solids
y Mixing of liquids and insoluble solids.
Mixing of two miscible liquids (homogeneous mixtures, e.g.
solutions): Mixing of two miscible liquids is quite easy and
occur by diffusion. Such type of mixing does not create any
problem. Simple shaking or stirring is enough but if the
liquids are not readily miscible or if they have very different
viscosities then electric stirrer may be used.
Mixing of two immiscible liquids (heterogeneous mixtures, e.g.
emulsions):
When two immiscible liquids are mixed together in the
presence of an emulsifying agent an emulsion is produced.
For the production of a stable emulsion, the mixing must be
very efficient, i.e. continuous without ceasing because the
components tend to separate out if continuous work is not
applied on them.
Mixing of liquids and soluble solids (homogeneous mixtures, e.g.
solutions):
In this case soluble solids are dissolved in a suitable liquid
by means of stirring. It is a physical change, i.e. a soluble
solid is converted to a solution.
Mixing of liquids and insoluble solids (heterogeneous mixtures,
e.g. suspensions):
When insoluble solids are mixed with a vehicle a suspension
is produced which is an unstable system. The ingredients of a
suspension separate out when allowed to stand for sometime.
Therefore to get a good suspension a suitable suspending agent
should be used. On small scale, suspensions may be prepared
in a pestle and mortar.
71
Unit Operations
MIXING EQUIPMENT
Double Cone Blender
Double cone blender is a competent and versatile machine
used for uniform blending of granules and dry powders. It
is made up of stainless steel. It is available in a wide range of
working capacities ranging from 5–500 litres.
Principle
Double cone blender produces a homogeneous solid-solid
mixture. This equipment involves axial mixing as the powder
moves in different sections. This blender provides thorough
mixing depending on its speed of rotation.
Construction
The body of double cone blender consists of two cone-shaped
sections (Fig. 4.8). These are joined to a central cylindrical
section at their bases. The rotational axis is perpendicular to
the cone axis and passes through the cylindrical section.
The blender body is held by two lateral supports. One of
these supports is fitted with the driving motor.
Working
The double cone blender’s angle is adjusted with the help of a
moving wheel so that the material can be conveniently loaded.
After loading, the lid of the blender is closed, and the lid is
secured by fitting the safety pin within its groove. Then the
safety guard is put and the equipment is switched on. After the
blending process is complete, the safety guard is removed (only
Fig. 4.8: Double cone blender without mixing blade.
Introduction to Pharmaceutics
72
after switching the equipment off), the blender is adjusted to an
angle (so that the material can be conveniently unloaded), the
safety pin is unlocked, and the lid is opened.
Uses
It is employed in different industries for the preparation of
different products such as pharmaceutical, food, chemical,
cosmetic products, etc.
It is used for uniform mixing of granules or dry powders.
It is used for the preparation of pharmaceuticals, food, and
chemical products.
It is suitable for homogeneous mixing of small quantity of
powders.
It is used for providing heating and cooling effect with the
help of jacketed construction.
Merits
Double cone blender has the following merits:
Large quantity of material can be easily handled.
It requires minimum maintenance cost.
It can be easily cleaned and maintained.
It can be easily operated with easy charging and discharging
of the material.
Demerits
Double cone blender has the following demerits:
It requires large headspace for installation.
Due to insufficient shearing force, it is not suitable for
materials having large differences in particle size distri-
bution or for systems containing fine particles.
TURBINE MIXER
Turbine mixer is another type of process agitator. They are used
as an alternative to propellers for mixing low viscosity liquids
and typically for the effective mixing of medium viscosity
liquids. The velocity of mixing using turbine systems is low
compared to propellers.
73
Unit Operations
Principle
Turbine mixer agitators can create a turbulent movement of
the fluids due to the combination of centrifugal and rotational
motion. These combined motions causes effective mixing of
low to medium viscosity fluids.
Construction
A turbine consists of a circular disc to which a number of short
blades are attached. Compared to propellers the diameter of
turbines is approximately 0.13–0.67 to that of the diameter
of the vessel, with 0.33:1 being most typical. There is a wide
range of turbine designs (Fig. 4.9). The blades may be straight,
pitched, curved or disk type. Turbine rotates at a lower speeds
usually 50–200 rpm than the propellers. Flat blade turbines
produce radial and tangential flow but as the speed increases
radial flow dominates. Pitched blade turbine produces axial
flow. Near the impeller zone of rapid currents, high turbulence
and intense shear is observed. Shear produced by turbines can
Fig. 4.9: Turbine impeller.
Introduction to Pharmaceutics
74
be further enhanced using a diffuser ring. The diffuser ring is a
stationary perforated ring which surrounds the turbine.
Working
A mixer is filled through with an opening at its top. Usually it
is a pan or drum within which mixing blades revolve about the
vertical axis. The variable speed drill with turbine mixer whip
air into the material mixture. The air in mixture yield bubbles
contributing mixing.
The top entry turbine mixer is fitted with various impellers
and turbines to suit heat and mass transfer in solids, suspen-
sions and liquids. This type of mixer does not damage pro duct.
Top entry high shear causes uniform emulsification and homo-
genization. The mixing blades revolve about the vertical axis.
Merits
Turbines are used in emulsification as they generate higher
shearing forces than propellers even at low pumping rates.
They are effective in mixing high viscous solutions with a
wide range of viscosities up to 7.0 Pa.S.
They are highly suitable for making dispersion containing
60% solids.
Turbines are suitable for liquids of large volume and high
viscosity with the additional provision of baffles in the tank.
As they generate high radial flow efficiency of mixing is high.
In low viscous materials of large volumes turbine create
a strong currents which spread throughout the tank
destroying stagnant pockets.
Demerits
They are not preferred for solvents with high viscosity such
as more than 20 cps.
There is possibility of air entrapment that may cause
oxidation of material being mixed.
Applications
Highly used in chemical reactions and extraction operations.
For example, liquid and gas reactions.
Used in preparing emulsions, suspensions and syrups.