Vertical
Crystallizer Concepts in Sugar Plant | Mono Vertical Crystallizer
Vertical Crystallizer Application in low grade massecuite cooling
The advantage of continuous crystallizer is
that all cooling surface remain covered with massecuite at all times. In
Continuous cooling system, used series
of batch type “U” shaped cystallizers and vertical crystallizer.
Vertical crystallizers have a number of
advantages over horizontal
crystallizers.
a) Large volume for small floor space occupied.
b) They can be placed on the ground without the need for any supporting
steel work. and suitable for installation out of doors.
c) High efficiency, due to better contact between massecuite and
heat-exchange surface.
d) They are cheaper to install than
the equivalent capacity of horizontal crystallizers.
e) Simplicity in
operation and Saving in
power
The major drawback of this type of
crystallizers is liquidating pumps are necessary to
empty the crystallizers.
Vertical crystallizers have been used in both the beet and cane sugar industries for
over 4 decades.
The Vertical
crystallizers Design aspects that have to be considers as
follow as:
Vessel diameter and height
Flow path of massecuite and water
Heat Transfer coefficient & Type of
Cooling Element
Cooling surface requirement (S/V ratio)
Massecuite agitation system.
Vessel diameter and height
The height cum diameter ratio ranging maximum
5 to minimum 2.5. A long narrow crystallizer should gives a better flow
pattern. Individual vertical crystallizer vessel volumes vary from 64 to 300 M3 with
diameters from 3 m to over 6 m, with heights raging from 8 to 20 mtrs. Some
larger sizes are used with beet sugar massecuites.
Flow path of massecuite and water
A vertical
crystallizer, it is better to have the massecuite flowing
vertically downward, because the density change with temperature tends to
promote plug flow. Achieving efficient counter counter flow, maintaining
acceptable pressure drops and avoiding air locking problems are all important design considerations for
vertical crystallizer cooling water circulation. According to massecuite flow
path different designs have been proposed and installed.
a) Twin Vertical crystallizer.
b) Mono Vertical crystallizer. (MVC )
c) Riser type Vertical Crystallizers.
Twin Vertical
crystallizer:
Twin vertical
crystallizer consists of two
horizontal vessels adjacent to each other. Both the vessels are connected with
pipe or duct of about 750 mm height or more. The hot massecuite enters in
vessel one and flows in downward
direction. Through interconnecting pipe the massecuite flows to
the second vessel
at bottom. In second vessel, the massecuite travels in upward direction.
For upward flow, the hydro-static
head of first vessel is used. During this massecuite
travel, massecuite get cooled with the help of water.
The cold water inlet is provided in cooled
massecuite i.e. at
the bottom of first vessel. The water cooling coils are
interconnected and the cooling water leaves from the top water coil. This
is satisfying the counter current cooling arrangement and the massecuite may
be cooled by using minimum quantity of water. In second vessel, the cold water
inlet is provided at the top second water coil and water leaves from the bottom
coil. This also ensures counter
current flow.
Mono Vertical
crystallizer: (MVC)
The mono vertical crystallizer (MVC
) consists of two cylindrical shells, one
of which is smaller than the other. Smaller shell is built inside the bigger
one. The holding capacity of the space is in between two shells is about
30% of the inner shell. Stirrer is provided in inner shell. The
massecuite is pumped from the receiver crystalliser to the inner shell
where cooling /
reheating elements are provided. The massecuite flows from
the top of the outer
shell to the pug mill of centrifugals.
In case of mono vertical crystallizers the
massecuite transfer from inner shell to outer shell is more smooth as compared
to twin vertical
crystallizer. The rising massecuite in outer shell is also
get heated due to the downward flow of hot massecuite in the inner shell. This
prevents progressive cooling to some extent.
Riser type Vertical
Crystallizer
The Riser type vertical crystallizer is design contains main shell at the center. From
the bottom of the shell, four
pipes of adequate size (650 mm dia or more) are provided
for rising
massecuite. The risers are also equipped with peripheral cell
to circulate controlled hot water. With the help of this hot water, the rising
massecuite temperature is increased to 4 –6 0C to increase the fluidity of cold
massecuite. The common gutter provides to collect the massecuite for all four
risers and sent to pugmill.
Heat Transfer coefficient (HTC) & Type of Cooling Element:
HTC:
Heat transfer
coefficients in low grade massecuite crystallizers are low,
particularly when high brix masssecuite are cooled to low temperatures. The HTC
depends upon number of factors like consistency
of massecuite, amount of shear applied to the cooling surfaces,
purity & viscosity of
the massecuite.
The HTC value is considered different range
by different designers. The average HTC
value for vertical crystallizes in the rang of 25 to 35 Kcal/m2/hr/oC
Type of Cooling Element:
Vertical crystallizer cooling elements can be
classified as two types of configurations like static and moving elements.
Moving cooling elements : It can be contracted as either oscillating or rotating units,
so it functioning as both agitation
and cooling devices. The moving cooling element design has
some advantages like to avoids the requirement for large motor and gearbox
drive unit with very high torque rating for very large crystallizers. However,
It is more complex then
fixed cooling element system and another disadvantage of this design is high mechanical stresses on
cooling element, which can be result over time, in fatigue crack leaks
forming at the weld joints.
Static Cooling element : Generally in
present scenario, stationary
cooling elements are
chosen for large vertical
crystallizers. The cooling surface may take many forms,
being made up of fine
tubes, coils, plates or discs. The design of cooling coils can have a significant effect on massecuite flow in
the crystallizer. The plain
tubes types cooling elements are used most commonly
in vertical
crystallizers.
Size of the plain tubes : The small tube diameter should be advantageous as
it should be easier to move cooled massecuite away from the tube surface. But
the disadvantage in small size tubes are increased in tube length, which is
required to be added to each bank gives a greater restriction to the massecuite
flow and higher water pressure drops in large crystallizers operating with full
counter-current flow. The several manufactures have been used pipes
of 50 to 65 mm
diameter tubes, and 180o and 90o standard
bends for contraction of cooling
element in vertical crystallizers.
Cooling surface requirement (S/V ratio):
The cooling surface / massecuite volume ( S/V) ratio will be give better choice to estimate of cooling surface area in vertical crystallizer. The S/V ratio ranging from 1.0 to 2.0 m2/m3 and it will be depend upon the massecuite viscosity and purity, tube pitching and diameter. Generally its ratio for
” B” massecuite vertical crystallizers having in the range of 1 to 1.2 and
for ” C” massecuite having 1.6
to 2.0.
Retention Time of
Massecuite: The massecuite
cooling rate depend upon the efficient
cooling element design, its surface area and time. The cooling
rate varies from 1.2 oC/hr to 0.6 oC/hr. According
to E.Hugot ,
the massecuite cooling time required 72 hours for C massecuite
while using ordinary
crystallizers and 36 to 50 hours requires in case of
vertical crystallizes. Different authors reported that residence time in range
of 30 hours to 50 hours according to design of cooling element. In present
scenario manufactures follows the S/V ration using 2.0 m2/m3 and
retention time provided around 24 to 30 hours for ” C” massecuite vertical
crystallizers design.
Massecuite agitation system:
In Vertical crystallizers massecuite stirring is
impotent for breaking up thermal wakes and creating a uniform temperature field
between cooling tube banks and massecuite. The agitator creation of a uniform
temperature field helps to avoid heat transfer from wasted on already cooled
massecuite and to eliminate excessively hot and cold regions.
Different drive arrangements have been used
for massecuite
stirring in Crystallizer vessels.
·
Electric motor and conventional worm and wheel gear boxes type
arrangement.
·
A single electric motor with flange mounted planetary
gearbox. The drive may be a fixed speed or a variable speed AC motor.
Now a days used planetary gear box for this
application due to its advantages like simple construction and it operates with
very less power.
As per Peter Rein –
Good vertical crystallizer design aspects consists of following points :
a) A long narrow flow path, in which all
massecuite has the same residence time.
b) Sufficient stirring in a direction perpendicular
to the flow direction to ensure a uniform temperature distribution.
c) Stirring elements which cover the full
diameter of the vessel, and preferably come close to scraping the internal
cylindrical walls.
d) Static cooling elements which provide
a uniform flow resistance across the full cross section of the vessel, to
ensure that no short-circuiting occurs.
e) No unstirred regions or sections where
massecuite can stagnate.
f) Sufficient surface area to achieve the
required cooling.
d) Cooling elements that are sheared by the
moving massecuite and do not trap cold massecuite.
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