1. Crystallization
o Pharmaceutical significance
o Crystal forms and crystal habit
o Solubility curves
o The mier’s supersaturation theory
o Factors affecting rate of crystal growth
o Caking of crystals
o Crystallization equipment
2. Batch crystallizers
Outline of the lecture:
A. Agitated batch crystallizer
B. Swenson walker crystallizer
C. Wulf bock crystallizer
3. Continuous crystallizer
A. Oslo crystallizers
B. Oslo cooler crystallizer
C. Oslo evaporative crystallizer
D. Oslo vacuum crystallizer
E. Howard crystallizerCrystallization is normally carried out either from a solution or from a melt.
Rarely, crystals are formed directly by condensation from the vapor phases.
Crystallization from solutions is important industrially.
Crystal forms:
The only logical and accepted method for the classification of crystals is
according to the angles between the faces.
In this system, the types of crystal form have no relationship to the
relative sizes of the faces since the relative development of the faces is
not a constant characteristic of a specific material.
Any substance always crystallizes in such a way that the angle between a
given Pair of faces is the same in all specimens and is characteristic of
that particular substance.Crystal habit is affected by the following factors
1. Presence of impurities
2. Temperature
3. Degree of supersaturation.
In some respects crystallizeation can be regarded as the inverse of
dissolution but there are important differences.
The number of particles present during dissolution will remain constant or
decrease whereas in crystallization the number of nuclei on which material
is deposited may continuously increase.Effect of Temperature on solubility
Generally, increase in the temperature of the solution usually increases the
solubility of the solute. In some cases, the temp coefficient of solubility is
negative and sometimes it is zero.
KNO3 has a large positive temperature coefficient and is therefore readily
crystallized by cooling a saturated solution. NaCL has a small coefficient and
very little crystallization occurs on cooling and evaporation of solvent is
therefore essential. (The solubility of KNO3 is more affected by temperature
than NaCL).
When the stable crystal form changes as temp is altered (e.g. with hydrated
salts), the curve is discontinuous, the coefficient may be positive over part of
the range of temp. (Na2CO3. 10H2O) and negative over the remainder
(Na2CO3.H2O).2. Rate of agitation: The rate of crystallization is improved by increasing the rate
of agitation. The crystallization rate initially rises very rapidly by increasing
agitation but a point is reached where further increase in the agitation produces
no effect on the rate of crystal growth.
3. Degree of supersaturation: This increases the crystallization rate.
4. Presence of impurities: Any foreign solid bodies act as a nucleus and enhance
crystallization.
5. Viscosity: As the viscosity increases, the rate of crystal growth decreases,
because the viscosity decreases the rate of diffusion of solute to the crystal
surface.This depends on
1. Vapour pressure of the solution.
2. Relative humidity (partial pressure of water in atmosphere)
If a saturated solution is brought into contact with air in which the partial
pressure of water is less than the vapour pressure of the solution, the solution
will evaporate. On the other hand, if the air contains more moisture than this
limiting amount, the solution will absorb water until it is so dilute that its vapor
pressure is equal to the partial pressure of the moisture of the air with which it
is in contact.Relative humidity:
• \ Relative humidity of saturated NaCL solution = (14.63)/ 18.76 x 100 = 77.8
%
Relative humidity =
V.p. of saturated solution
V.p. of the solvent
x 100
• \ If salt at 70 °F is brought into contact with air its relative humidity > 78%,
the partial pressure of water vapour in the air is more than that of saturated
salt solution so moisture will be absorbed and condensed on the salt.
• On the other hand, if NaCL is exposed to air its relative humidity < 78%, it
will stay dry, due to evaporation of H2O from the solution and caking Doesn’t
occurs.
• \ 78% in referred to the critical humidity of NaCI. If the crystals are large so that there are few points of contact and there is
a large free volume between the crystals so there is no appreciable
bonding of the crystals, this will lead to caking minimization. On the
other hand, if the crystals are fine or have small percentage of voids or are
in contact with a moist atmosphere for a long time, sufficient moisture
may be absorbed to fill the voids entirely with saturated solution and
when this have been reevaporated, the crystals will lock into a solid mass.
o Pharmaceutical significance
o Crystal forms and crystal habit
o Solubility curves
o The mier’s supersaturation theory
o Factors affecting rate of crystal growth
o Caking of crystals
o Crystallization equipment
2. Batch crystallizers
Outline of the lecture:
A. Agitated batch crystallizer
B. Swenson walker crystallizer
C. Wulf bock crystallizer
3. Continuous crystallizer
A. Oslo crystallizers
B. Oslo cooler crystallizer
C. Oslo evaporative crystallizer
D. Oslo vacuum crystallizer
E. Howard crystallizerCrystallization is normally carried out either from a solution or from a melt.
Rarely, crystals are formed directly by condensation from the vapor phases.
Crystallization from solutions is important industrially.
Crystal forms:
The only logical and accepted method for the classification of crystals is
according to the angles between the faces.
In this system, the types of crystal form have no relationship to the
relative sizes of the faces since the relative development of the faces is
not a constant characteristic of a specific material.
Any substance always crystallizes in such a way that the angle between a
given Pair of faces is the same in all specimens and is characteristic of
that particular substance.Crystal habit is affected by the following factors
1. Presence of impurities
2. Temperature
3. Degree of supersaturation.
In some respects crystallizeation can be regarded as the inverse of
dissolution but there are important differences.
The number of particles present during dissolution will remain constant or
decrease whereas in crystallization the number of nuclei on which material
is deposited may continuously increase.Effect of Temperature on solubility
Generally, increase in the temperature of the solution usually increases the
solubility of the solute. In some cases, the temp coefficient of solubility is
negative and sometimes it is zero.
KNO3 has a large positive temperature coefficient and is therefore readily
crystallized by cooling a saturated solution. NaCL has a small coefficient and
very little crystallization occurs on cooling and evaporation of solvent is
therefore essential. (The solubility of KNO3 is more affected by temperature
than NaCL).
When the stable crystal form changes as temp is altered (e.g. with hydrated
salts), the curve is discontinuous, the coefficient may be positive over part of
the range of temp. (Na2CO3. 10H2O) and negative over the remainder
(Na2CO3.H2O).2. Rate of agitation: The rate of crystallization is improved by increasing the rate
of agitation. The crystallization rate initially rises very rapidly by increasing
agitation but a point is reached where further increase in the agitation produces
no effect on the rate of crystal growth.
3. Degree of supersaturation: This increases the crystallization rate.
4. Presence of impurities: Any foreign solid bodies act as a nucleus and enhance
crystallization.
5. Viscosity: As the viscosity increases, the rate of crystal growth decreases,
because the viscosity decreases the rate of diffusion of solute to the crystal
surface.This depends on
1. Vapour pressure of the solution.
2. Relative humidity (partial pressure of water in atmosphere)
If a saturated solution is brought into contact with air in which the partial
pressure of water is less than the vapour pressure of the solution, the solution
will evaporate. On the other hand, if the air contains more moisture than this
limiting amount, the solution will absorb water until it is so dilute that its vapor
pressure is equal to the partial pressure of the moisture of the air with which it
is in contact.Relative humidity:
• \ Relative humidity of saturated NaCL solution = (14.63)/ 18.76 x 100 = 77.8
%
Relative humidity =
V.p. of saturated solution
V.p. of the solvent
x 100
• \ If salt at 70 °F is brought into contact with air its relative humidity > 78%,
the partial pressure of water vapour in the air is more than that of saturated
salt solution so moisture will be absorbed and condensed on the salt.
• On the other hand, if NaCL is exposed to air its relative humidity < 78%, it
will stay dry, due to evaporation of H2O from the solution and caking Doesn’t
occurs.
• \ 78% in referred to the critical humidity of NaCI. If the crystals are large so that there are few points of contact and there is
a large free volume between the crystals so there is no appreciable
bonding of the crystals, this will lead to caking minimization. On the
other hand, if the crystals are fine or have small percentage of voids or are
in contact with a moist atmosphere for a long time, sufficient moisture
may be absorbed to fill the voids entirely with saturated solution and
when this have been reevaporated, the crystals will lock into a solid mass.
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