This document provides an overview of crystallization processes. It discusses how crystals form from solutions or melts via nucleation and growth. Primary and secondary nucleation are described. Mass transfer and population balance theories are used to model crystal growth rates and size distributions. The document outlines how continuous crystallizers like MSMPR systems operate and how residence time affects crystal size distribution. Methods for controlling crystal size like double draw-off, fines removal, and classified product removal are also summarized.

1. Crystallization
Mass Transfer 2
B.Tech. 3rd Year
Instructor
U. K. Arun Kumar

2. Introduction
• Finished solid products from process industries are
often in the form of crystals
• Examples: Sugar, NaCl, citric acid, ammonium
sulphate etc.
• Sugar production:
– Juice extraction from crushed cane
– Removal of suspended and colloidal materials (juice)
– Clear juice is then concentrated and cooled to get raw
sugar crystals

3. • Sugar production:
– Raw sugar is refined to remove residual colours and
impurities
– Crystals are redissolved in limited water.
– Impurities and colouring matters are removed by
adsorption
– Clear and purified solution is again evaporated and
cooled to get snow white crystals.
• Crystallization produce products in an acceptable
and granular form.
• An important quality of crystal products are size
distribution – should be narrow range

4. • A crystal is a solid body with plane faces in
which the atoms are arranged in an orderly
repetitive array.
• Crystalline materials generally have high
degree of purity -
• Even when obtained from impure solution.
• Size distribution (also shape) is an important
parameter of commercial importance.
• The process of formation or production of
crystals from a solution or a melt is called
crystallization.

5. • When crystals are formed by cooling a
saturated solution, it is called solution
crystallization.
• Melt crystallization: Crystals are generated by
cooling a molten solid in the absence of any
solvent.
• Example: Separation of xylene isomers – by
melt crystallization.

6. • Crystallization process:
• Saturation of solution
– When the solution concentration is increased
(either by evaporating solvent or by cooling),
solution becomes saturated.
• Supersaturated - When concentration of
solution is more than the solubility of the solid
at a particular temperature it is supersaturated.
• Supersaturation can be created and maintained
for almost all systems.

7. • The level of supersaturation depends upon
the system, also on how calm and free from
disturbances the system is.
• Sugar solution can have very high degree of
supersaturation.
• Up to 80 % concentration more than
saturation level.
• But in NaCl, it is too small.

8. • Spontaneous formation and growth of tiny
crystals called nuclei, take place in a
supersaturated solution.
• If a nucleus or a seed crystal is added to a
supersaturated solution, it gradually grows to
larger size.
• Supersaturation in a solution - is the driving
force for transport of solute from bulk
solution to crystal surface.

9. • On reaching the surface, solute molecules get
oriented and integrated into crystal lattice.
• Commercially, solution is heated in a heat
exchanger and led to crystallization chamber.
• In the chamber, some more solvent is
evaporated.
• Solution gets supresaturated and
crystallization takes place.

10. • The suspension or slurry with crystals and
solution is called magma.
• Solution remaining after removal of crystals is
mother liquor.
• Crystal production steps
– Crystallization
– Separation of crystals from mother liquor by
filtration or centrifugation.
– Washing crystals with fresh water to remove
adhering mother liquor.
– Drying of moist crystals.

13. Solid-liquid phase equilibrium
• Crystallization is opposite to dissolution of a
solid in a liquid or solvent.
• Solids dissolve as long as it reaches saturated
value.
• Conversely, in a supersaturated solution,
addition of few seed crystals -
• - creates a driving force for the transportation
of solute from bulk to crystal surface.

14. • The extent of supersaturation in a solution is
the driving force for crystallization.
• The solubility of very small particles may be
significantly larger than the normal solubility
of a substance
• This is given by Gibbs-Thomson equation

15. • Smaller particles have more than normal
solubility.
• They dissolve even in a saturated solution
• Thus rising the concentration creating
supersaturation.
• Due to this the larger particles present in the
suspension starts growing
• Overall process is: dissolution of smaller
particles making the larger particles to grow.
• This process is called Ostwald ripening.

16. • Solubility of a solid in a liquid at different
temperatures is conveniently shown in a solid-
liquid phase diagram.

18. • This is SLE diagram for benzene – naphthalene
system is shown above.
• This a typical SLE graph for a binary system.
• Curve AE – is freezing point curve of C6H6
• Curve EB – is freezing point curve of C10H8
• Line AEB represents the equilibrium solubility
curve or saturation concentration curve.

19. • Curve AEB separates single phase and two
phase region.
• Region above AEB represents unsaturated
homogeneous solution of (C6H6 + C10H8)..
(single layer (phase) region)
• Curve AEB is called liquidus.
• Point E is called Eutectic Point.
• Between curve AEB and line CED the
heterogeneous solution exists in a mixture of
solution and solid.

20. • In the region of ACE, the solution is a mixture
of solid benzene and solution of (C6H6 + C10H8).
• In the region BED, the solution is a mixture of
pure solid naphthalene + solution of (C6H6 +
C10H8).
• Considering the molten liquid of composition
P1.
• When the liquid is cooled to P2, the system
enters the two phase region - liquid + solid
C10H8

21. • Almost pure solid C10H8 begins to come out of
solution,
• The remaining liquid becomes rich in Benzene.
• When cooled to further below than P2, more
of C10H8 forms.
• The relative amounts are given by lever rule.
• The corresponding composition of benzene is
given by drawing a horizontal line to reach the
equilibrium line.

22. • The liquid is richer in benzene than before.
• P3 represents unsaturated solution,
• When it reaches P4, it reaches the two phase
region – solid benzene and solution.
• On further cooling below P4, the solution
reaches, CED line, corresponding to point E.
• Point E – represents Eutectic point. The
freezing point, is lower than the two pure
components.

23. • At Eutectic point – three phases co-exists
together – solid benzene + solid naphthalene
+ solution.
• Below line CED, only mixture of solids (C6H6
and C10H8) are presents at different
compositions.
• Line CED – is called SOLIDUS.

24. Nucleation and Crystal Growth.
• Nucleation – is formation of tiny new crystals
in a supersatured solution.
• A new crystal that is formed is called nucleus.
• Different types of nucleation

25. Primary Nucleation
• Primary nucleation – phenomenon of
formation of new crystals, independent of
presence of other crystals.
• Primary nucleation – two types
– Homogeneous nucleation
– Heterogeneous nucleation
• Homogeneous nucleation – it is formed by
the clustering of the solute molecules or ions
in a supersaturated solution.

26. • At normal levels of supersaturation, the
homogeneous nucleation is slow.
• It increases rapidly, if supersaturation is high.
• Solution Viscosity – if solution viscosity is
high, homogeneous nucleation rate is slowed
down.
• Temperature: Increase in T, decreases μ, so B0
increases.
• Heterogeneous nucleation: Formation of
crystals on tiny suspended foreign solid
particles. Also on crystallizer surface.

27. Secondary Nucleation(SN)
• Secondary nucleation: Formation of new
crystals from the existing crystals, is called
secondary nucleation.
•
• SN can happen by
– Fracture and attrition
• Fracture of existing crystals with impeller blades
• Attrition between two crystals.
– Contact nucleation – it is predominant

28. • Contact nucleation:
– Diffusion or convection of ion-pairs or molecules
from bulk solution to crystal surface.
– Continuous adsorption on crystals forming lattice
leading to crystal growth.
• Loosely adsorbed molecules on crystals gets
displaced by agitator or pump disturbances.
• Breeding: secondary nucleation is also called
as breeding.

29. Nucleation & Crystal Growth Rate

30. • Power law –Nucleation Correlation
• A simple equation for the overall rate of
nucleation by all mechanism
• B0 = nucleation rate, number/cc. s
• K (T)= a temperature dependent coefficient
• W= a measure of mechanical agitation (speed)
• MT= total mass of crystals per unit volume of
suspension or density of the suspension (Kg/m3)
• s = degree of supersaturation s= (C-Cs)/Cs
• m, n, p =exponents

31. Crystal Growth
• Crystal growth theories
– Adsorption theory
– Mass transfer theory
• Mass Transfer Theory of crystal growth
– Assumption – two steps are involved in crystal
growth.
• 1) Convective transport of solute – from
solution bulk to crystal surface
• 2) Surface integration or accommodation of
solute in growing layers of a crystal.

32. • Crystals growth is determined by the
resistances of above two steps.
• The second step: sometimes resembles first
order (reaction) process.
• Its rate depends on extent of supersaturation
(C-Cs)
• Mass transfer theory – combines diffusion
and reaction processes.
• Its some times called diffusion-reaction
theory of crystal growth.

33. • Rate of increase in mass of crystal growth –
can be written as
• dmc/dt = KLAc(C-Ci) = KrAc(Ci-Cs)

35. • KL – is overall mass transfer coefficient –
combination of two resistances
• The above equation give growth rate of a
crystal in terms of rate of change of mass
• The growth rate can also be expressed in
terms of rate of change of characteristic size.
• It is related to mass and area of crystal as

37. • dL/dt = G - is the measure of crystal growth rate
• If the process of solute integration on crystal
surface is nonlinear,
• A power law type correlation for growth rate is
used,
• Parameters k or k’ can be found by fitting
experimental data

38. McCabe ΔL law
• McCabe in 1929, based on several research
work, showed that
• The rate of crystal growth G, is independent
of crystal size, L.
• dG/dL =0
• This is well-known McCabe ΔL law
• Not all system obey this law.
• Sometime G, depends on L.

39. Crystal Size Distribution (CSD)
• Supersaturation of a solution is ‘lost’ by the
competition between nucleation and crystal
growth.
• Their relative rates depend upon various
factors including supersaturation.
• In a continuous crystallizer, a nuclei remain for
a short period of time and then leaves with
the mother liquor.

40. • At any time - crystals in the apparatus vary in
size
• It is determined by Crystal Size Distribution
(CSD).
• CSD is extremely important in design and
analysis of crystallizers.
• A narrow size distribution is always preferred.
• Theoretical analysis of CSD is conveniently
done by population balance technique.

41. • The following analysis of CSD is applicable to a
mixed suspension mixed product removal
(MSMPR) type crystallizer.
• Most continuous crystallizers used in
industries are of MSMPR type.

42. • Assumption of MSMPR
– Particles remain uniformly distributed in the
suspension
– Feed enters continuously and product is removed
continuously
– The suspension density and particle size
distribution inside vessel and in the outlet stream
are same.
– Negligible breakage of crystal occurs
– Steady-state operation takes place.

43. • Let n (L) - CSD based on length of crystals.
• n(L)dL – number of crystals with the
infinitesimal range L to L + dL in unit volume
of suspension.
• n(L) – also called population density function.
• Making a population balance of crystal
• Considering crystals only the arbitrary size
range L1 to L2

44. • Population density for the size L1 is n1
• Population density for the size L2 is n2
• The average population density in the size
range L1 to L2 be - ñ
• Over a small time, dt
• Some crystals little small than L1, would grow
and enter the size range [L1, L2]

45. • Similarly,
• Some crystals little smaller than L2 would
become oversize in time dt, and leave the
range [L1, L2]
• Number of crystals in suspension volume V is
given below
• Number of crystals that crosses size L1 and
enters the range [L1, L2] over a time dt,

46. • Number of crystals that crosses size L2 and
leaves the range [L1, L2] over a time dt,
• Feed source (inlet): crystal in feed in range
[L1,L2]
– Crystal in the range [L1,L2] may enter along with
the feed
• In exit stream:
– crystal in range [L1,L2] may leave the device with
suspension withdrawn as product.

47. • Let ni –population density function of crystal
in feed.
• Average value of the density function in the
size range [L1,L2] is given by ñi
• If Qi – flow rate of liquor into crystallizer,
• Number of crystals entering with the feed in
time dt,

48. • Let Qo - flow rate of product (exit) stream
with crystal size range [L1,L2] -
• Number of crystal in product stream is
• Writing a population balance equation for
crystal size range [L1,L2]

49. • {No. of particles that grow and enter the size
range [L1,L2]
+ No. of particles that enter the vessel with
feed} =
• {No. of particles that grow (>L2) and leave the
range [L1,L2]
• + No. of particles that leave the vessel (with
product)}

50. • If feed does not contain any crystal then
• Dividing both sides by ΔL.dt and taking limit
• ΔL -- 0
• As ΔL -- 0, the average population density
ñi --- n

51. • - is residence time or holding time
or draw-down time.
• The above equation is basic – population
balance equation for an MSMPR crystallizer.
• If growth is independent of L, above equation
reduces to

52. • n0 – is the population density of crystals having
a vanishingly small size ( as L-> 0)
• n0 and Gt are two parameters
• CSD or the population distribution in an
MSMPR is similar to residence time distribution
of fluid element in a CSTR.

53. Average Particle Size
• Ordinary average size – length, area and mass
average size.
• Length average particle size = ratio of total
length of all particles to the total number of all
particles

54. • Area average size particle
• Mass-average size particle

55. Weighted Average Particle Size
• Population weighted average particle size
• Length-weighted average particle size
• Area-weighted average particle size
• Mass-weighted average particle size

56. Crystal size control
• An ideal MSMPR crystallizer give a product of
wide size distribution.
• Few modifications in operation produces
crystals of narrow size range.
• It is done by controlling the residence time of
the crystals.
• If smaller particles are not allowed to remain
in the vessel for long,
• Product will contain more bigger crystals and
less of fines.

57. • This is because the fines, do not get sufficient
time to grow.
• Three common options for controlling are
– Double draw-off (DDO) or Clear liquid overflow
(CLO) or clear liquid advance (CLA)
– Fines removal with destruction
– Classified product removal

58. Double draw-off strategy
• Two streams are drawn from the crystallizer
• A clear liquid stream having few fine crystals
from near the top of the vessel (overflow)
and
• A slurry or suspension from a lower level
(underflow) is removed
• If some ‘fines’ are continuously removed from
the vessel,
• The concentration of the fines can be reduced.

60. Crystallizer with fines removal and
destruction
• A stream containing small crystals (L < LF) is
withdrawn from the top at a volumetric rate
Q0
• The crystals are dissolved by heating in a heat
exchanger
• The solution is recycled back into the
crystallizer, with volume V
• Product stream is drawn with a rate Qp

62. • The cumulative weight fraction W(x) is
• X – is dimensionless crystal size

63. • For cyrstallizer with fines removal and
destruction

64. Crystallizers
• Nucleation and growth both depend on
supersaturation (SS) of solution.
• At high SS level – nucleation is high
• At low SS level – rate of nucleation is slow.
• Secondary nucleation rate can be controlled
by maintaining low mechanical energy input.

65. Mixed Suspension Mixed Product Removal
Crystallizer or Circulating Magma Crystallizer
• In this growing crystals are kept in suspension
by agitation: 20-40 % solids in suspension
• Two types of such crystallizers are
– Forced-circulation (evaporative) crystallizer
– Draft-tube-baffle (DTB) crystallizer

66. Forced circulation crystallizer
• It has four major components:
• The slurry is pumped through an external
steam heated vertical heat exchanger to raise
T from 2 to 6 °C

68. • Heat exchanger tubes : 11/4 to 13/4 inch
• Liquid velocity: 2-3 m/s
• No boiling occurs in the heat exchanger.
• Hot liquid is introduced into crystallizer –
evaporation of liquid occurs.
• Thus generating supersaturation for crystal
growth.
• A condenser removes the vapor at the top.
• Product is removed from the circulating pipe.

69. • The feed also enters the vessel through this
pipe, at a lower level
• Pump speed should be low to reduce
mechanical energy input – to maintain small
secondary nucleation.
• Ammonium sulphate, NaCl, citric acid, sugar
are crystallized by using FC crystallizer.
• Crystals typically of size range 30-60 mesh are
produced.

70. Draft Tube Crystallizer (DTB)

71. • It contains an inner baffle tube and a skirt
baffle.
• A long-shaft slow moving impeller throws
liquid upwards through baffle tube towards
the boiling surface.
• This causes circulation of magma in the
cyrstallizer
• More circulation is achieved at the same
power input.
• Fouling is also less than in a FC.

72. • Magma flows out of body through annular
space between skirt baffle and the wall.
• Then enters a steam –heated exchanger.
• Then is recycled back to the crystallizer vessel.
• Some of the fines may dissolve in the heat
exchanger.
• Thus the crystallizer has a fine destruction
feature.
• Settling of large crystals occurs in the annular
region,

73. • So that only the fines leave the re-circulating
slurry.
• However this happens only when the mother
liquor and the crystals have sufficiently
different densities.
• Crystals ranging from 8 to 30 mesh may be
produced.
• Applications: adipic acid, KCl, K2SO4 etc.

74. Circulating Liquor Cyrstallizer

75. • In this crystallizer - crystals are retained
within the vessel
• Only the liquor is circulated through an
external heat exchanger
• In this liquor is either heated by steam or
cooled to generate supersaturation.
• Crystals are kept in suspension by the
upflowing liquid in the vessel.

76. • Liquid velocity is maintained so that hardly
any crystals are present in the top region of
the vessel
• Virtually clear liquid enters the circulating pipe
• An Oslo-krystal crystallizer is shown above.
• Significance: supersaturation is created in a
separate region before the liquor enters the
vessel.

77. • The level of supersaturation drops down as
the liquor from (SS) zone mixes with the
slurry.
• This helps to achieve uniform crystal growth
and low rate of secondary nucleation
• This type of crystallizer produces narrow range
CSD