Download 30(Unit Operations)

Solids Handling Unit Operations
Section 2.7
Figure 2.7.6-1:
Crystallizer
All crystallizers have some degree of mixing supplied by an agitator and/or pumparound. The limiting case is ideal mixing, where conditions in the crystallizer are
uniform throughout, and the effluent conditions are the same as those of the crystallizer contents. Such a unit is commonly known by the name of Mixed Suspension
Mixed Product Removal (MSMPR) crystallizer or Continuous Stirred Tank Crystallizer (CSTC). A further assumption made in the development of the crystallizer
model is that breakage or agglomeration of solid particles is negligible.
Crystallization Kinetics and Population
Balance Equations
Growth Rate:
(1)
GEXP
G = k GS
where:
G=
growth rate of crystals, m/sec
kG =
growth rate constant, m/sec
S=
supersaturation ratio =
Xsolute − Xeq
solute
Xeq
solute
Xsolute = actual mole fraction of solute in liquid
Xeq
solute = equilibrium mole fraction of solute in liquid at the crystallizer temperature
Nucleation Rate:
Bo = kB MT

BEXP1  BEXP2  BEXP3

S
 G

 RPM
BEXP4

(2)
where:
Bo =
crystal nucleation rate, number/sec.m3
kB =
nucleation rate constant
MT =
magma density, i.e., concentration of crystals in slurry, kg
crystals/m3 slurry
RPM = impeller speed, revolutions/min
BEXP1, BEXP2, BEXP3, BEXP4 = exponents
II-172
Crystallizer
May 1994