Chemical Engineering July 2011 - 31

2
3
4
5
Increasing fines
withdrawal flow
1
Appreciable production loses
1 x 10-4
Cut size, Lf (m)
2 x 10-4
FIGURE 4. Increasing Lf yields moderate crystal
size improvement, but also results in production
loss due to dissolution of the fines. The plot is for
an inorganic salt
ing surface where the supersaturation
created by evaporation and cooling is
relieved with nucleation and growth.
The high flowrates from the agitator
are mixed with the feed stream to limit
surface cooling to approximately 1-2°F
during flashing. This limitation keeps
the supersaturation at a low value.
Cooled slurry is returned to the bottom
of the vessel through the space between
the outside of the draft tube and an
annular baffle. It then mixes with the
heated slurry returning from the heat
exchanger or with an adiabatic feed.
The active volume includes volumes
inside and outside the draft tube, but
not areas behind the baffle. DTBs usually
contain slurry equal to 25-50% of
apparent settled volume. This active
volume is circulated one to four times
per minute, which limits nucleation
and scaling on the vessel walls while
promoting growth on the existing crystal
surfaces. Crystal residence times
are usually 4-6 h, with the production
rates often ranging from 60-85 kg/m3
h. The period between washouts often
increases with longer residence time
and slurry density as a result of decreased
supersaturation and scaling.
The annular baffle area functions
as a settling zone through which a
stream of mother liquor and fines is
separated from the slurry in the active
volume. The segmentation is due to
differential gravitational settling. The
result is a residence time for the fines
that is less than the residence time for
the product. The CSD in the body is
controlled by adjusting the vertical velocity
of the mother liquor in the baffle
areas and controlling the maximum
crystal size that will be removed and
dissolved. This can be achieved either
by changing the flowrate in the baffle
area or the amount of area used for
the baffles. The maximum
size of the crystals (Lf) that is
theoretically removed by the
baffle can be used to modify
the CSD of the product. The
term Lf has a hindered settling
velocity equal to the
free vertical velocity. If Lf is
too large, the increased supersaturation
generated by
recrystallizing the dissolved
fines can cause nucleation
in the unit, resulting in periodic
upsets in the system and cycling
of the CSD.
Increasing the percent solids in the
slurry to a concentration greater than
the " natural make " (the percent solids
without other means of building
slurry density) is at times achieved
by withdrawing a stream of mother
liquor from the upper portion of the
baffle zone. This procedure thickens
the slurry in the body. This practice
is referred to as clear liquor advance
(CLA) or double-discharge operation
(DDO). It is employed for systems that
have low natural make. A larger crystal
size may result due to lower levels
of supersaturation (more crystal
surface area in the slurry), increased
residence time for the solids and removal
of fines in the CLA. Potential
downsides could
include increases
in secondary nucleation and crystal
breakage. High slurry densities tend
to reduce the baffle efficiency. To improve
performance, a lamella plate is
installed in the crystallizer body to
direct flow vertically at the baffle entrance,
or by the installation of altering
doughnut baffles in the settling
zone behind the regular baffles. These
doughnuts dissipate large liquid eddies
that can trap and carry out undesirable
larger crystals.
A fines-destruction system produces
larger crystals with a narrower size distribution.
Ideally, the heat exchanger
would supply enough heat to both satisfy
the evaporation requirements and
to raise the temperature of the lowsolids
slurry removed from the baffle
to destroy unwanted fines. For applications
where the only heat removed
is that which is required for adiabatic
cooling of the feed, fines destruction
is achieved by selectively removing a
weak slurry stream containing fines
and redissolving them via dilution
prior to their return to the crystallizer,
often in the region below the impeller.
This technique increases supersaturation
in the crystallizer body, which
increases both crystal growth and
nucleation rates. The reason is that
excess fines are destroyed by the heat
of dilution and then the dissolved solute
is recrystallized in the body. The
result is a unit that operates at higher
internal production rates, due to production
of both product crystals and
recrystallization of dissolved fines.
The fines-destruction stream, while
low in percentage of suspended solids,
can often represent a flow that is
a multiple of the product underflow
stream. Thus, the recrystallization
of the dissolved fines can represent a
significant percentage of the internal
production of the unit. In comparison
to the product stream, this configuration
deliberately reduces the residence
time of the crystals of size Lf or less.
Some fines also migrate to the product
stream. The ratio of the product crystal
residence time to the fines residence
time is referred to as R, or τP/τF.
Fines destruction, in the form of accelerated
removal and destruction of
fine crystals ≤ Lf, can be used to increase
the product size. With fines dissolution
and recycling to the crystallizer, the
supersaturation and production rate
increase, thereby increasing the nucleation
rate (fines are destroyed) and
the growth rate, yielding a larger CSD.
Under some conditions, a bimodal distribution
with a small peak in the 0 to
Lf size range is obtained. A first-order
estimate of size improvement is:
(6)
This indicates that the size increase
is dependent on R, Lf and i. Lm is the
median crystal size.
One can then determine the impact
of various combinations of Lf and R
in order to yield the desired product
size. Frequently, Lf is in the 20-80
µm range, and there is a large impact
of increasing R, which is often in the
4-10 range. Note that, as expected, the
ability to improve the size distribution
CHEMICAL ENGINEERING WWW.CHE.COM JULY 2011 31
R = 10
R = 5
R = 4
R = 3
R = 2
Size enlargement factor,
(Lm)2
(Lm)1
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Chemical Engineering July 2011

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