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)
3. – Clear juice is then concentrated and cooled to get
raw sugar crystals
• 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.
4. • Crystallization produce products in an acceptable
and granular form.
• An important quality of crystal products are size
distribution – should be narrow range
• 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.
5. • 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.
• 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.
6. • Example: Separation of xylene isomers – by melt
crystallization.
• Crystallization process:
• Saturation of solution
– When the solution concentration is increased (either
by evaporating solvent or by cooling), solution
becomes saturated.
7. • 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.
• The level of supersaturation depends upon the
system, also on how calm and free from
disturbances the system is.
8. • Sugar solution can have very high degree of
supersaturation.
• Up to 80% concentration morethan
saturation level.
• But in NaCl, it is too small.
• Spontaneous formation and growth of tiny
crystals called nuclei, take place in a
supersaturated solution.
9. • 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.
• On reaching the surface, solute molecules get
oriented and integrated into crystal lattice.
10. • Commercially, solution is heated in a heat
exchanger and led to crystallization chamber.
• In the chamber, somemoresolvent is
evaporated.
• Solution gets supresaturated and crystallization
takes place.
• The suspension or slurry with crystals and
solution is called magma.
11. • 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.
12.
13.
14. 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 -
15. • - creates a driving force for the transportation
of solute from bulk to crystal surface.
• 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
16. • Smaller particles have morethan normal
solubility.
• They dissolve even in a saturated solution
• Thus risingthe concentration creating
supersaturation.
17. • 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.
• Solubility of a solid in a liquid at different
temperatures is conveniently shown in a
solidliquid phase diagram.
18.
19.
20. • 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.
21. • 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.
22. • Between curve AEB and line CED the
heterogeneous solution exists in a mixture of
solution and solid.
• 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.
23. • When the liquid is cooled to P2, the system
enters the two phase region - liquid + solid
C10H8
• 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.
24. • The corresponding composition of benzene is
given by drawing a horizontal line to reach the
equilibrium line.
• 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.
25. • Point E – represents Eutectic point. The freezing
point, is lower than the two pure components.
• 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.
26. 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
27. Primary Nucleation
• Primary nucleation – phenomenon of
formation of new crystals, independent of
presence of other crystals.
• Primary nucleation – two types
28. – Homogeneous nucleation
– Heterogeneous nucleation
• Homogeneous nucleation – it is formed by the
clustering of the solute molecules or ions in a
supersaturated solution.
• At normal levels of supersaturation, the
homogeneous nucleation is slow.
• It increases rapidly, if supersaturation is high.
29. • 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.
30. 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.
31. – Contact nucleation – it is predominant
• 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.
35. • 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)
36. • s = degree of supersaturation s= (C-Cs)/Cs
• m, n, p =exponents
Crystal Growth
• Crystal growth theories
– Adsorption theory
– Mass transfer theory
• Mass Transfer Theory of crystal growth
– Assumption – two steps are involved in crystal
growth.
37. • 1) Convective transport of solute – from
solution bulk to crystal surface
• 2) Surface integration or accommodation of
solute in growing layers of a crystal.
• 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
38. (C-Cs)
• Mass transfer theory – combines diffusion and
reaction processes.
• Its some times called diffusion-reaction theory
of crystal growth.
• Rate of increase in mass of crystal growth – can
be written as
• dmc/dt = KLAc(C-Ci) = KrAc(Ci-Cs)
39.
40.
41. • 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.
42. • It is related to mass and area of crystal as
43.
44.
45. • 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
46. 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.
47. • Sometime G, depends on L.
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.
48. • In a continuous crystallizer, a nuclei remain for
a short period of time and then leaves with the
mother liquor.
• 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.
49. • A narrow size distribution is always preferred.
• Theoretical analysis of CSD is conveniently done
by population balance technique.
• The following analysis of CSD is applicable to a
mixed suspension mixed product removal
(MSMPR) type crystallizer.
50. • Most continuous crystallizers used in industries
are of MSMPR type .
51. • Assumption of MSMPR
– Particlesremain 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.
52. • 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
53. • 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
54. • Some crystals little small than L1, would grow
and enter the size range [L1, L2]
55. •
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,
56. •
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]
57. •
– 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.
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
58. •
• If Qi – flow rate of liquor into crystallizer,
• Number of crystals entering with the feed in
time dt,
59. • Let Qo - flow rate of product (exit) stream with
crystal size range [L1,L2] -
• Number of crystal in product stream is
60. • Writing a population balance equation for
crystal size range [L1,L2]
• {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]
61. • + No. of particles that leave the vessel (with
product)}
• If feed does not contain any crystal then
• Dividing both sides by ΔL.dt and taking limit
• ΔL --0
62. • As ΔL --0, the average population density
ñi ---n
• - 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
63. • n0
– is the population density of crystals having
a vanishingly small size ( as L-> 0)
• n0
and Gt are two parameters
64. • CSD or the population distribution in an
MSMPR is similar to residence time
distribution of fluid element in a CSTR.
Average Particle Size
• Ordinary average size – length, area and mass
average size.
65. • Length average particle size = ratio of total
length of all particles to the total number of all
particles
• Area average size particle
66. • Mass-average size particle
Weighted Average Particle Size
• Population weighted average particle size
• Length-weighted average particle size
67. • Area-weighted average particle size
• Mass-weighted average particle size
Crystal size control
• An ideal MSMPR crystallizer give a product of
wide size distribution.
• Few modifications in operation produces
crystals of narrow size range.
68. • 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.
• This is because the fines, do not get sufficient
time to grow.
69. • 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
Double draw-off strategy
• Two streams are drawn from the crystallizer
70. • 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.
71.
72. 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
73. • The solution is recycled back into the
crystallizer, with volume V
• Product stream is drawn with a rate Qp
74.
75. • The cumulative weight fraction W(x) is
• X – is dimensionless crystal size
77. 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.
78. 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
79. 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
80.
81. • 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.
82. • Product is removed from the circulating pipe.
• 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.
86. • 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.
87. • Fouling is also less than in a FC.
• 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.
88. • Settling of large crystals occurs in the annular
region,
• 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.
92. • 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.
93. • 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.
94. • 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
