Crystallization

CRYSTALLIZATION

Dr. Leonardo C. Medina Jr.

Perry’s Chemical Engineers Handbook 8th Edition

7th Edition

Table 2-122 Solubilities of Table 2-120 Solubilities of Inorganic Compounds in Inorganic Compounds Water at Various in Water at Various Temperature Temperature Tables 2-182 & 2-183 Heats of Solution

Tables 2-224 & 2-225 Heats of Solution

Tables 2-33 to 2-122 Densities of Aqueous Inorganic & Organic Solutions at 1 atm

Tables 2-31 to 2-117 Densities of Aqueous Inorganic & Organic Solutions at 1 atm

Crystallization  – process whereby a

solution is super saturated so as to cause the formation of crystals. Methods of Supersaturating a Solution: 1. Super saturation by cooling – e.g. Swenson-Walker Crystallizer  2. Super saturation by evaporation of solvent – “Salting-out” evaporator, Oslo crystallizer  3. Adiabatic evaporation with cooling – Vacuum crystallizer  4. Adding a third component where the solubility of the solute is simply reduced by seeding. 5. Addition of a third component to form

Solubility Curves for (1) KNO3, (2) NaCl and (3)MnSO4 H2O in Aqueous Solution

Phase Diagram MgSO4 H2O System

Integral Heats of Solution for Sulfates in Water at 25°C

Enthalpy Concentration Diagram MgSO4 H20 System

Swenson Walker Continuous Cooling Crystallizer 

Circulating Batch Cooling Crystallizers

Swenson Draft Tube Baffled Crystallizer 

Continuous Crystallizer 

Draft Tube Baffle Crystallizer with internal system for fines separation and removal

Brodie Purifier Countercurrent Cooling Crystallizer 

Process for the Production of MgSO4 7H2O

Crystallizer for Production of MgSO4 7H2O Crystals

Principles of Crystallization

 x F 

 x L

MATERIAL BALANCE:

F   C  L  V 

Over-all Material Balance: Solute Balance:

Fx F   Cx c  Lx  L

Solvent Balance: F 1  xF    C 1  xc   L1  x L  + V where: XL is the solubility of the solute at TL XC is the MW of solute/MW of hydrated crystal For the solubility of some solids, see Fig. 18-56; Fig. 18-57 Perry

NOTE: Tie substance method may be applied: (a) If the crystals are anhydrous, water becomes the tie substance between the original solution and the mother liquor  (b) If the crystals are in the form of hydrates, free or excess water is the tie substance.

ENTHALPY BALANCE If Enthalpy-Concentration data are available:

Fh F   VH V    Lh L  Chc  q HEAT BALANCE

:

If the specific heats, heat of solution and heat of crystallization are available

q V   L  FcF T F   T c   CH c For the area of the crystallizer





 L

CRYSTALLIZATION BY SEEDING Mechanism of crystallization: (a) Nucleation and (b) Crystal Growth Assumptions in the use of seed crystals: 1. no new nucleation occurs 2. uniform linear growth, i.e., the ∆L or ∆D law applies ∆L or ∆D law = the rate of linear growth is independent of size.

Rate of Deposition of crystals:

dW 

 k   f   x   x i 

dt  Volume of crystal:

V   aD

3

or  2

dV   3aD dD also

dW    dV 

2

Substituting:    3aD dD dt  dD dt 

 k  f  x  xi 

 K   f   x   x i 

Relation between seed and product crystals:  D   D   D  p

s

For ideal growth, is the same for all 3 3 crystals: W   a    D  a   D   D  p

 p



3

s



therefore:

for a single sized crystal 3

   D    W S  W P   1     D S    For an entire range of particle sizes: 3

W P 

W S 

o

   D   dW S  1     DS   

which is solved by trial and error.

Effect of Supersaturation on Crystal Growth Quality and Type of Nucleation for MgSO4 7H2O