Chemical Engineering October 2010 - 31

MSMPR Crystallization
equipment
Department Editor: Scott Jenkins
C
rystallization is a key purification technique
for various sectors of the chemical process
industries (CPI). Several approaches for
industrial crystallization have evolved over time
and highly specialized crystallizer designs have
been developed, especially in long-established
industries. Solution crystallization is an important
unit operation because the
process can generate high-purity
products from solutions containing
significant levels of impurities
with relatively low energy input.
One scheme for classifying
this equipment is according to
the method used to suspend the
growing crystals. In this scheme,
a class of equipment known as
mixed-suspension, mixed-product-removal
(MSMPR) crystallizers
is most important for the CPI.
Recirculation
pipe
Steam
inlet
Heat
exchanger
Condensate
outlet
MSMPR
MSMPR crystallizers, also
known as magma-circulation
crystallizers, have found
widespread application in the
CPI for continuous crystallization.
In this configuration, a feed
solution is continuously introduced
into the crystallizer, which is equipped with
a mechanism to create supersaturation. An
agitation device allows mixing of the feed
with the contents of the crystallizer and also
maintains a uniform suspension in the mother
liquor inside the crystallizer. A stream of slurry
is continuously removed from the crystallizer in
such a way that the fractions of solid particles
and the particle size distribution in the slurry
inside the equipment equals that of the slurry
removed from the crystallizer.
Circulation
pipe
ExaMPlES
Among the group of MSMPR-type crystallizers,
some of the most common are the forced-circulation
(FC), draft-tube (DT) and draft-tube-baffle
(DTB) crystallizers.
Forced-circulation crystallizers
This type of MSMPR crystallizer consists of a
body through which a slurry of growing crystals
flows, and a tube-and-shell heat exchanger,
which increases temperatures without vaporization
(diagram, left). Heated slurry returns to the
body via a recirculation line, where it mixes
with the body slurry and raises the temperature
locally, near the point of entry. During the consequent
cooling and vaporization to achieve
equilibrium between liquid and vapor, the
supersaturation created causes deposits on the
swirling body of suspended crystals until they
leave again through the circulating pipe. The
lower limit for economic continuous operation
of forced-circulation crystallizers is 1-4 ton/d
of crystals, and the upper limit for a single
vessel is 100-300 ton/d of crystals. Units in
parallel can reach higher capacities.
DTB and DT evaporator-crystallizers
A number of designs have been developed
with circulators located within the body of
the crystallizer in an effort to reduce the head
Expansion
joint
Feed
inlet
Product
discharge
Feed
inlet
Condensate
Forced-circulation (evaporative) crystallizer
Draft-tube-baffle (DTB) crystallizer
against which the circulator must pump.
Internal circulators reduce the power input and
circulator tip speed, and thereby reduce the
rate of nucleation, which is influenced significantly
by mechanical circulation. The DTB is an
example (diagram, right). In this type, a large,
slow-moving propeller is surrounded by a draft
tube within the body that directs the slurry to
the liquid surface to prevent solids from shortcircuiting
the zone of most intense supersaturation.
Slurry that has been cooled is returned
to the bottom of the vessel and recirculated
through the propeller, where heated solution is
mixed with the recirculating slurry. A finesdestruction
feature is common, where a heating
element is used to raise the temperature of the
solution removed from a settler to destroy the
small crystalline particles that are withdrawn.
The baffle can be omitted in cases where fines
destruction is not needed or wanted.
In DT and DTB crystallizers, the circulation
rate is generally much greater than that
achieved in a forced circulation device. Therefore,
DT and DTB crystallizers are applied
when it is necessary to circulate large quantities
of slurry and minimize supersaturation
levels within the equipment. These types are
commonly used in the production of granular
materials, such as ammonium sulfate, potassium
chloride and other inorganic and organic
crystals for which product in the range of 8 to
30 mesh is required.
OPERating PRinciPlES
Basic good-operating principles for solution
crystallization apply regardless of what type
of crystallization equipment is used. The following
represent some of these concepts:
* Control the level of supersaturation to ensure
low nucleation rates
* Maintain an adequate slurry density to
provide sufficient surface area to relieve
supersaturation by the deposition of solute.
Crystallizers should operate with a minimum
10 wt.% suspended crystals (slurry density)
* Contact the supersaturated liquor quickly with
crystals to avoid losses due to time decay
* Destroy excess nuclei via fines destruction
configurations. Seeding the crystallizer with
fines will lower crystal size
* Minimize secondary nucleation by keeping
mechanical energy input and crystal attrition
as low as possible
* Maintain high slurry densities. In general,
high densities can produce larger average
crystal size as long as crystal attrition is not
a negative influence
* Minimize solids buildup by eliminating localized
heat- and mass-transfer gradients
* Ensure adequate velocities and operation
at low temperature gradients across heatexchange
equipment
* Avoid fluctuations in operating conditions,
such as vacuum, residence time and concentrations.
Employ wash nozzles at liquid
interfaces
* Provide a chemical environment (impurities
and additives) that favors the desired crystal
shape, purity and size distribution
* Maintain longer crystal retention times,
which can result in less liquor occlusions in
the crystals
* Keep the feed to the crystallizer slightly
unsaturated
References
1. Sutradhar, B.C. Coping with Crystallization Problems.
Chem. Eng., March 2004, pp. 46-52.
2. Schweitzer, P.A. " Handbook of Separation Techniques
for Chemical Engineers, " 3rd ed. McGraw-Hill, New
York, 1997.
3. Perry's Chemical Engineer's Handbook, 7th ed.
McGraw-Hill, New York, 1997.
4. Couper, James. " Chemical Process Equipment:
Selection and Design, " Gulf Professional Publishing,
Houston, 2010.
Swirl breaker
Circulating pipe
Cooling
water
Cooling
water inlet
Propeller
drive
Non-condensable
gas outlet
Barometric
condenser
Boiling
surface
Body
Draft
tube
Settling
zone
Propeller
Body
Skirt baffle
Slurry
Clarified
mother
liquor
Settler
Circulating
pipe
Source for
diagrams:
Elutriation leg
Heating element
Steam
Product
discharge
Perry's Chemical
Engineers'
Handbook,
Swenson Process
Equipment
Air
ejector
Barometric
condenser
Chemical Engineering October 2010

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