Chemical Engineering July 2011 - 29

Cooling
water
To vacuum
equipment
Barometric
condenser
To hotwell
Fines loop
Slurry
discharge
Steam
Heating element
Condensate
Circulating
pump
Figure 2. A draft tube is an efficient design for suspending
solids with low energy input. In a draft-tube
baffle crystallizer, slow-moving impellers move slurry
upward to the boiling surface, where surface cooling
and evaporation creates supersaturation, which is then
relieved by crystal nucleation and growth
mixture exists in the mixed slurry of
the active volume, where the percentage
of suspended solids and crystal
size distribution (CSD) are perfectly
uniform. This uniformity is also present
at the point of product discharge.
In addition, it is assumed that the
mother liquor has the same residence
time as the solids. Thus, there is no
deliberate attempt to control the residence
time of the crystals of any size.
Forced-circulation crystallizer
The FC crystallizer (Figure 1) is a type
of MSMPR unit. This design usually
has the lowest capital cost per pound
of product generated. The feed usually
enters the downpipe at a point following
the discharge of the product slurry
to the solid-liquid separation device.
The combined stream, consisting of
fresh feed and recirculated slurry,
is pumped via an axial flow pump
through the circulating pipe to a vertical
or horizontal heat exchanger,
Feed
Purge
where it is heated by condensing
steam.
This slurry-and-feed
stream is introduced into
the crystallizer below
the liquid surface in the
vapor body, the part of
the FC where flashing
occurs. It mixes with the
slurry at the point of feed
entry and raises the local
temperature to cause
flashing (evaporation
under vacuum) at the
liquid surface. The flashing
causes cooling at the
surface, resulting in supersaturation
that is relieved
as crystal growth
occurs or as new nuclei
are formed in the active
volume. FC units operate
with a high circulation
rate, which limits scaling.
The lowered scaling
enables the unit to evaporate
solutions that have
scaling solutes, such as
calcium sulfate.
At times, FC crystallizers
can be designed
for classified product discharge,
usually by suspending
an elutriation
step beneath the crystallizer body.
Introduction of mother liquor to the
lower portion of the leg fluidizes the
particles prior to discharge, and selectively
returns the smaller crystals
to the body for future growth. Many
NaCl plants have such a process step
for both classified product discharge
and for keeping a suspension of calcium
sulfate in the unit to reduce
scaling. The concept is that maintaining
small CaSO4 crystals in the vessel
encourages growth on the crystal,
rather than forming scale throughout
the system.
Another modification of an FC unit
involves a conical inlet for the slurry
to flow into the crystallizer. The conical
entrance improves mixing in the
body, allowing better dispersion of the
supersaturation and improving the
uniformity of the slurry at the boiling
surface. This installation may also
be combined with baffling, in order to
allow removal of fines and clear-liquor
advance (removing mother liquor without
removing crystals). This builds the
slurry density beyond its natural level.
An important point is that both
classified product removal and fines
destruction are not entirely consistent
with the MSMPR properties regarding
residence, because these practices
are deliberate attempts to modify the
residence time of the crystals based on
their size.
The heat exchanger in an FC unit
usually has a one- or two-pass configuration,
often with 1.5-in., 12-gauge
tubes. It is essential to limit the temperature
differential between the steam
and the slurry in the heat exchanger
to avoid boiling in the tubes, which can
lead to scaling, plugging and excessive
nucleation. In addition, there must be
adequate liquid head at the point of
entry to the body (submergence of the
slurry leaving the heat exchanger) to
avoid flashing in the recirculation line.
Doing this avoids premature flashing
in the heat exchanger tubes and the
inlet piping prior to entry into the crystallizer
tank. Another aspect of the FC
crystallizer is the use of an axial-flow
pump to achieve high slurry flow with
reduced crystal breakage and secondary
nucleation.
In an adiabatic evaporative-cooling
FC unit (where the heat exchanger is
omitted), the feed is introduced to the
body at a position adequately below
the liquid-vapor surface to avoid flashing
during the mixing process.
In both configurations, supersaturation
is minimized by high circulation
rates (typically about 7 ft/s) and by
limiting the temperature drop at the
liquid-vapor surface to 3-8°F, with
some inorganic salt slurries being limited
to a 1-5°F decrease.
For some applications, supersaturation
is generated by indirect cooling,
as opposed to evaporation. In this
case, the process flowrate is designed
to operate at a low temperature drop
(0.5-5°F) and low log-mean-temperature-difference
(LMTD) across the
heat exchanger. The goal is to stay
within the metastable zone to reduce
fouling on the tubes. The end result
is usually a large surface area, aimed
at achieving the required heat transfer
and improve on-stream time. The
difference between an effective design
ChemiCal engineering www.Che.Com July 2011 29
http://www.Che.Com
Chemical Engineering July 2011

Table of Contents for the Digital Edition of Chemical Engineering July 2011
