Crystallization

1. Crystallization
Crystallization is the formation of solid particles within a homogeneous
phase.It may occure as the formation of solid particles in a vapour, as in
snow; as solidification from a liquid melt, as in the manufacture of large
single crystals; or as crystallization from liquid solution.
Crystallization from solution is important industrially because of the
variety of materials that are marketed in the crystalline form. Its wide use
has a twofold basis; a crystal formed from an impure solution is itself
pure(unless mixed crystals occur),and crystallization affords a practical
method of obtaining pure chemical substances in a satisfactory condition
for packaging and storing.
MAGMA: In industrial crystallization from solution, the two-phase
mixture of mother liquor and crystals of all sizes, which occupies the
crystallizer and is withdrawn as product, is called a magma.
Crystal habit: The term crystal habit is used to denote the relative
development of the different types of faces. For example, sodium
chloride crystallizes from aqueous solutions with cubic faces only. On
the other hand, if sodium chloride is crystallized from an aqueous
solution containing a small amount of urea,

2. the crystals obtained will have octahedral faces. Both types of crystals belong to the
cubic system but differ in habit. The word habit is sometimes incorrectly used to
designate these features of external form, but when properly used it refers to the
type of faces developed and not to the shape of the resulting crystal.
• Classification of crystallizers: Crystallization equipment is most easily classified by
the methods by which supersaturation is brought about. These are as follows:
• 1. Supersaturation by cooling
• 2. Supersaturation by the evaporation of the solvent
• 3. Supersaturation by adiabatic evaporation (cooling plus evaporation)
• 4. Salting out by adding a substance that reduces the solubility of the substances.
The classification at the beginning of this section may be somewhat elaborated as follows:
1.Supersaturation by cooling alone
A. Batch processes
(i) Tank crystallization
(ii) Agitated batch crystallizers

3. B. Continuous processes
(i) Swenson-Walker
(ii) Other
• 2. Supersaturation by adiabatic cooling
• A. Vacuum crystallizers
• (i) Without external classifyeing seed bed
• (ii) With external classifying seed bed
• 3. Supersaturation by evaporation
• A. salting evaporators
• B. crystal evaporators

4. Commercial crystallizers may operate either continuously or batchwise.Except for
special applications, continuous operation is prefered. The first requirement of any
crystallizer is to create a supersaturated solution,because crystallization cannot
occure without supersaturation.
•Vacuum crystallizer: Most modern crystallizers fall in the category of
vacuum units in which adiabatic evaporative cooling is used to create
supersaturation. In its original and simplest form, such a crystallizer is a closed
vessel in which a vacuum is maintained by a condenser, usually with the help of a
steam-jet vacuum pump, or booster, placed between the crystallizer and the
condenser. A warm saturated solution at a temperature well above the boiling
point at the pressure in the crystallizer is fed to the vessel. A magma volume is
maintained by controlling the level of the liquid and crystallizing solid in the
vessel and the space above the magma used for release of vapor and elimination of
entrainment. The feed solution cools spontaneously to the equilibrium
temperature; since both the enthalpy of cooling and the enthalpy of crystallization
appear as latent enthalpy of vaporization, a portion of the solvent evaporates. The
supersaturation generated by both cooling and evaporation causes nucleation and
growth. Product magma is drawn from the bottom of the crystallizer. The
theoretical yield of crystals is proportional to the difference between

5. the concentration of the feed and the solubility of the solute at equilibrium
temperature. Figure shows a continuous vacuum crystallizer with the conventional
auxiliary units for feeding the unit and processing the product magma. The essential
action of a single body is much like that of a single effect evaporator, and in fact
• these units can be operated in multiple effect. The magma circulates from the cone
bottom of the crystallizerbody through a downpipe to a low-speed low-head circulating
pump, passes upward through a vertical tubular heater with condensing steam in the
shell, and thence into the body. The heated stream enters through a tangential inlet just
below the level of the magma surface. The supersaturation thus generated provides the
driving potential for nucleation and growth.
• Feed solution enters the downpipe before the suction of the circulating pump. Mother
liquor and crystals are drawn off through a discharge pipe positioned above the feed
inlet in the downpipe. Mother liquor is separated from the crystals in a continuous
centrifuge; the crystals are taken off as a product or for further processing,and the
mother liquor is recycled to the downpipe. Some of the mother
• liquor is bled from the system by a pump to prevent accumulation of impurities.

7. Material balances: If the material precipitates as a hydrated salt, this simple
method of calculation will not be correct, since the solid salt contains a definite
amount of water that does not remain in the mother liquor and therefore the total
water does not pass through the process unchanged. The key to calculations of such
a process is to express all compositions in terms of hydrated salt and excess
water,since it is this latter quantity that remains constant during the crystallization
process, and composition expressed on the basis of this excess water can be
subtracted to give a correct result.
•Energy balances: In addition to the use of material balances to calculate
the yield from a crystallization operation, energy balances are used to calculate the
cooling requirements or are necessary to determine final conditions. Consider the
case of a steady-state operation in which only cooling is used and no evaporation
occurs. This corresponds to the operation of the Swenson-walker crystallizer.
Figure illustrates this schematically. For purposes of the energy balance it is
convenient to show two streams leaving,i,e., crystals of the solid phase and
saturated solution, although the actual product from the crystallizer is aslurry or
magma of these two phases.If the feed condition(temperature and composition)
and the final temperature are set, the composition of the saturated solution leaving
the crystallizer and the yield are both fixed.

8. Consequently, the quantities and compositions of all streams are known or may be
calculated. The energy balance is FhF= LhL+ ChC + q
where, F= feed , Ib/hr L= mother liquor, Ib/hr C=crystals, Ib/hr
h=enthalpy,Btu/Ib , q=Btu/hr

9. The Miers supersaturation theory: Consider the curves of fig. The curve AB is
the ordinary solubility(equilibrium) curve and represents the maximum
concentration of solutions that can be obtained by bringing solid solute into
equilibrium with solvent.

10. It also represents the ultimate limit toward which crystallization from supersaturated
solutions tends.If a sample of material having the composition and temperature of
point C is cooled in the direction shown by the arrow, it first crosses the solubility
• Curve, and one would suppose that here it should begin to crystallize. If one starts
with pure solutions,carefully freed from all solid particles, not only of the
substance itself but of any foreign solid matter, the solution will not begin to
crystallize until it has supercooled considerably past the curve AB. Somewhere in
the neighborhood of the point D, according to the Miers theory,crystallization
begins, and the concentration of the substance then follows roughly according to
the curve DE.
• In the absence of any solid particles the curve FG(called the supersolubility curve)
represents the limit at which nucleus formation begins spontaneously and
consequently, the point where crystallization can start. According to the Miers
theory, short of this point(i,e, at any position along the line CD), nuclei cannot
form and crystallization cannot then occur.

11. Limitation of the Miers theory: Such an explanation for the formation of nuclei
scarcely justifies the assumption of an exact supersolubility curve FG. The general
tendency at the present time is to consider this critical supersolubility range not as a
• definite line but as an area. For instance, it is known that with sufficiently great
lengths of time nuclei can form even well below the supersolubility curve. If the
formation of such nuclei depends on such accidental collisions of molecules of
solutes into aggregates large enough to persist, it would seem that the larger the
volume of the solution the more chance there would be for such a collision to form
somewhere, and this is actually found to be true, namely, that nuclei appear in
large volumes of solution quicker than they do in very small samples. But as long
as the formation of the basic nucleus is dependent on the accidental combination
of the molecules of solute to form permanent aggregates, it makes it doubtful that
any exact line such ;as FG can be drawn. Actually in practice this is still more of a
problem.
• The discussion so far has been based on the postulation that the solution consists
of pure solvent and pure solute without the presence of any solid particles,
whether of solute itself or of any foreign material. It has been found repeatedly
that a solid particle, not necessarily of the solute, can act as the nucleus.

12. . It is also unavoidable that in commercial practice, where solutions are exposed to
the air and where the plant air is full of dust of the product being made, many
millions of dust particles of the solute could fall into the solution . Even in closed
vessels protected from atmospheric contamination it is always possible for
fragments of crystals to persist in the apparatus.
• The colloidal and amorphous particles of insoluble dust can also act as nuclei.
• In order to justify the Miers supersolubility curve it is necessary to deal with pure
solutions completely free from every particle of solid matter. Since it has been
shown that if (a)the time be long enough, (b) the volume of the solution be large
enough, (c) there be particles of the solute introduced as dust, or(d) any foreign
solid particles be introduced(even colloidal and amorphous material),
crystallization can occure, it follows that as far as actual practice is concerned the
existence of a fixed curve FG according to the Miers theory is no longer possible.

13. Rate of crystal growth: Since in commercial crystallization it is highly desirable to have
the product not only of uniform size but of a particular uniform size, it follows that if too
many million nuclei are started in a crystallization process there may not
• be material enough to grow them up to the desired size before the solution is
brought down to saturation. In this case the partly grown crystals must be kept in
suspension through several such cycles. Consequently it is also desirable in the
commercial process to control the rate of nucleus formation and keep the number
of nuclei that start to grow down to the number that can be grown to the desired
crystal size within the limits of the amount of material handled.
• Once the nucleus has formed and has started to grow, the laws for the rate of
crystal growth are still not all understood. It might be supposed that the
controlling factor would be the rate of diffusion of the solute from the mass of the
solution to the interface. If this were true, then the rate of growth of all the faces of
a crystal would have to be the same. In practice it is found that different faces of a
crystal grow at different rates. Further, if diffusion of the solute from the solution
to the interface were the controlling factor, then as viscosity increased, the rate of
crystal growth should decrease, because this would decrease the rate of diffusion
of solute to the crystal surface. In some cases, at least, this is not true, and
crystallization has been found to be independent of solution viscosity. Probably
the impression created by the above paragraphs is that not much is known at the
present time

14. about how nuclei start or how crystals grow. An enormous amount of work has been
done on pure solutions, and an enormous amount of work has been done on the
crystallization of pure substances from melts. However , none of this has reached
the point where it is of any quantitative value in the actual operation of a practical
crystallization process.
• Caking of crystals: A serious problem that is often met in handling crystalline
products is their tendency to cake or bind together. This is often troublesome in bulk
storage or in barreled products but is most serious in those cases where crystals are sold
in small packages. The difficulty may exist in degrees, varying from loose aggregates
that fall apart between the fingers to solid lumps that can be crushed only by
considerable force. The demand of the average consumer that the material shall flow
freely from the package makes the prevention of caking a serious problem for the
manufacturer.
• Prevention of caking: Suppose a sample of sodium chloride be exposed for a short
time to an atmosphere more moist than its critical humidity then that it be removed to an
atmosphere less moist than its critical humidity. During the first period it will absorb
moisture, and during the second period it will loss this moisture. If the crystals are large,
so that there are relatively few points of contact there is a large free volume between the
crystals, there will probably be no appreciable bonding of

15. the crystals due to this solution and reevaporation, if the time of exposure were not
too great. If, on the otherhand, the crystals are fine or have a small percentage of
voids, 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
has been reevaporated the crystals will lock into solid mass. Consequently, to
prevent the caking of such salts the following conditions are desirable: first, the
highest possible critical humidity; second, a product containing uniform grains
with the maximum percentage of voids and the fewest possible points of contact;
third, a coating of powdery inert material that can absorb moisture.