Crystallization Equipment (Crystallizers) : Swenson-Walker Crystallizer

PAMANTASAN NG LUNGSOD NG MAYNILA
Intramuros, Manila
College of Engineering and Technology

Chemical Engineering Department
Crystallization equipment (crystallizers)

Crystallization equipment is classified according to the method by which
supersaturation is brought about:
1. Supersaturation by cooling
2. Supersaturation by evaporation
3. Supersaturation by adiabatic evaporation (cooling and evaporation)
SUPERSATURATION BY COOLING
Swenson-Walker Crystallizer
The Swenson-Walker Crystallizer is
an equipment used for continuous
operation of crystallization in which the
supersaturation is achieved solely by
cooling. It is an example of the scraped
surface crystallizer and probably the
most widely used crystallizer. It
consists of an open rectangular trough
with a semi-cylindrical bottom of width
0.6m and length of 3 to 6m. The trough
is jacketed externally for circulation of
cooling water during operation. A spiral
agitator/stirrer is provided for the
following purposes:
1. To prevent the accumulation of crystals on the cooling surface for
better heat transfer
2. To lift the crystals that have already been formed and shower them
down through the solution
In this manner, the crystals grow while they are freely suspended in the
liquid and therefore they are:
1. Fairly perfect individuals
2. Uniform in size
3. Free from inclusions or aggregation

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The setup consists of a top open jacketed round bottom trough. A ribbon mixer is
provided in crystallizer trough, which rotates at low rpm for agitation of saturated
solution. The saturated solution is prepared in a tank fitted with heater and a stirrer.
This hot saturated solution is fed at one end of the crystallizer and flows slowly
towards the other end while being cooled and is allowed to flow back in hot feed
tank making the process continuous. The crystals sediments at the bottom of the
trough, and can be washed out at the end of process. Arrangement for Cooling
water circulation and flow measurement in jacket is made by using storage type
water tank, a pump and a rotameter. Necessary instrumentation is done for temp
measurement. In order to control crystal size, it is sometimes desirable to introduce
an extra amount of water into certain sections in the jacket.
Other Swenson Crystallizers

Swenson has an unusually broad background in the crystallization field and
can supply a wide range of crystallizers and evaporators to meet the requirements
for almost any application. Thousands of evaporators, crystallizers, dryers and other
process equipment have been installed around the world and bear the Swenson
name. Swenson equipment is designed and fabricated to provide optimum results
under customer specified operating conditions. Each project is unique and requires
its own unique process design solution.
The Swenson Draft Tube Baffle (DTB) crystallizer is the most successful of the
crystallizers designed to make the large, uniform crystals required for fertilizer and
similar applications where superior filtration, centrifugation, washing, drying, and
storage characteristics are required. This design is the result of years of research
and development as well as proven plant scale operation in hundreds of
installations.
Associated Products
Potassium Chloride
Associated Equipment
Draft Tube Baffle (DTB) Crystallizer
Zero Liquid Discharge
Surface Cooled Crystallizer
Associated Industries

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Pulp & Paper
Pan Cooler
Crystallization pan is a vessel equipped with heat exchange areas (for heating or
cooling) and providing agitation. The heat exchange areas may be provided by a
jacket or by a tubular heat exchange element. Agitation may be provided either by
a mechanical stirrer or by recycling. Crystallization pans can operate in batch or
continuous mode.
These pans are
widely used in food
industry, especially
in sugar production.
Recycling pans with
external
heat
exchangers
are
frequently used in
the production of
salt and citric acid
but not in the sugar
industry because of
the high viscosity of
material. Agitation
must be sufficiently
vigorous to facilitate
mass transfer and good mixing, but gentle enough to prevent crystal fracture,
abrasion, and secondary nucleation.
Even though the sugar industry increasingly changes to continuous pan
boiling, batch pans are still required, e.g. for the production of seed.
The first step of seeding proceeds in a cooling crystallizer which is equipped
with a stirrer specifically designed for a high shear rate. The low temperature and
the clearly defined supersaturation ensure uniform growth of the added slurry seed
crystals, avoiding the formation of aggregates. The syrup used (preferably thick
juice) is subjected in the cooling crystallizer to evaporation to the required dry
substance content, and the concentrated syrup is then cooled. When a
supersaturation of 1.1 has been reached, it is seeded with the required quantity of

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slurry. Thereafter, the suspension is further cooled (slowly) to approx. 30C. The
massecuite-to-cooling-water temperature difference serves to control the cooling
gradient and, consequently, to adapt the supersaturation to be maintained during
the cooling crystallization process. After the crystals have grown to a mean size of
approx. 0.08 to 0.11mm (subject to requirements) at a crystal content of
approximately 20% at the end of the cooling phase, the 1st-seed massecuite is
dropped into a receiver.
In a second seed production step, batch pans produce crystallizate with a
mean crystal size of approx. 0.3 to 0.5mm. Optimum plants use stirrers providing a
high shear rate. New batch pans are optimally designed for the production of 2nd
seed. The feed solution is concentrated until the seeding point is reached at a
supersaturation of approx. 1.1 and the 1st-seed massecuite is drawn in as quickly
as possible. After a short phase of homogenization and temperature equalization,
growing can start immediately. After growth, the resultant 2nd-seed magma
containing approx. 45% to 50% crystals is dropped into the receiver. By adjusting
the quantities of slurry and seed, the mean crystal size of white sugar can be
varied within a range of approx. 0.5 to 1mm. For raw sugar, the product crystal size
usually is 0.4 to 0.5mm, and for low-grade sugar approx. 0.3 to 0.35mm.
Agitated Batch Cooler
In
these
crystallizers,
supersaturation
takes
place
by
cooling with the help of cooling coils.
In this crystallizer, feed is introduced
into the crystallizer from the top.
Supersaturation by cooling is done
through the cooling coils. An agitator
is present which keeps the solution in
random motion throughout the
process. Blender does not only
accelerate the heat, it also brings the
solution of some more uniform
temperature that can promote the
emergence and nucleating of crystal
growth. The main disadvantage in
this is the deposition of crystals around the coils hindering heat exchange. In short,
water is circulated though the cooling coils and the solution is agitated by the
propellers mounted on the central shaft. Product is collected at the bottom of the
crystallizer.
This agitation performs two functions:
1. It increases the rate of heat transfer and keeps the temperature of the

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solution more uniform.

2. It keeps the fine crystals in suspension, thus it gives them an opportunity to
grow uniformly instead of forming large crystals or aggregates. (Production of
uniform crystals)
Advantages
1. The agitation increases the rate of heat transfer and keeps the temperature
of the solution uniform throughout the crystallizer.
2. Agitation keeps the smaller crystals in suspension and allows them to grow
uniformly thus finer crystals can be obtained.
Disadvantages
1. It is a batch or discontinuous apparatus.

2. Since the solubility is least at the cooling surface hence the crystals growth is
more rapid on the cooling coils. Thus the crystals deposited on the cooling
coils reduces the heat transfer rate.

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PAMANTASAN NG LUNGSOD NG MAYNILA

College of Engineering and Technology