Download Heat and Mass Transfer B4- OGILVIE

OGILVIE, Jeremy James E.
5ChEB
Introduction
Crystallization is a heat and mass transfer process that includes the solidification of materials in
slurry where the particles arrange themselves to form regular geometric patterns called crystals. In most
food and pharmaceutical companies, crystallizers have been widely used to produce their products to be
sold in the market that can be used in daily households and in hospitals. This process has been helpful in
the chemical industry because of its method of purification and in providing crystallized materials in a
desired size range. Compared to distillation, crystallization can save more power when trying to retrieve
solid materials.
The crystallization process comes in three steps. Some solids have this property to dissolve in a
solvent at different temperatures called solubility. Once a solution is saturated, the solute can no longer
be dissolved in the solvent therefore the solute remains visible on the solution making the solution
supersaturated. This is one step in crystallization, to induce supersaturation. This process can be carried
out by either heating the solution (to vaporize the solvent), cooling the solution (solubility decreases) or
by salting out (adding another substance that may induce crystallization). After achieving
supersaturation, nucleation will take place. During the random motion of the solute’s atoms or
molecules they form aggregates called clusters. These clusters would further form lattices called embryo
and when the embryos combine the nuclei crystals are formed. And one the nuclei crystals are formed
crystal growth begins.
Different types of crystallizers have developed through the years with different shapes and sizes
and different ways of inducing supersaturation like an Agitated Batch Crystallizer, Krystal Crystallizer,
and Vacuum Crystallizer. But in this paper a scraped surface crystallizer will be discussed specifically a
Swenson-Walker Crystallizer and on how this certain equipment will be designed.
In 1920, Swenson Co. conceived a continuous, semi cylindrical, horizontal, hollow trough
crystallizer that cools the slurry to induce supersaturation. The refrigerating fluid passes counter
currently with the feed through a jacket or a double wall. The heat transfer wall is scraped or agitated
such that the deposits cannot build up. At the end of the crystallizer, the products leave a drain where
the mother liquor is then centrifuged in order to obtain the crystals. Typically this equipment is used in
processing inorganic salts like sodium phosphate which has a high solubility with water.
Problem Statement
A solution containing 23 mass % of sodium phosphate is cooled from 313 to 298 K in a SwensonWalker crystallizer to form crystals of Na3PO4.12H2O. The company needs to produce 300 kg of crystals
in an hour. The mean heat capacity of the solution is 3.2 kJ/kg deg K and the heat of crystallization is
146.5 kJ/kg. If cooling water enters and leaves at 288 and 293 K, respectively. Given the following data
design a Swenson Walker Crystallizer.
In this process an overall material balance, solute balance and energy balance, were used in
order to acquire the other data needed in order to get the dimensions of the equipment.
Overall Material Balance
F= L + C
Solute Balance
Fxf = Lxl + Cxc
Heat Balance
q= wCp(t2-t1) = FCpf(tf-tl) +CLc = UAΔTlm
Given and solving the following data we obtained:
Feed Properties
F= 870.872 kg/hr
xf=0.237
Cpf=3200 J/kgK
tf= 313K
Crystals and Liquor Properties
C= 300 kg/hr
xc=0.432
L=570.872
xl=0.134
Lc=14600 J/kg tc=tl=298K
Cooling Water Properties
t1=288K
t2=293K
Cp=4184 J/kgK
U=350 W/m2K
w=4100 kg/hr
Dimensions of Swenson Walker Crystallizer
A= 4.717 m2
L= 3 m r=0.25 m
Data like xf, xc, and xl were obtained using Table 2-122 from Perry’s Chemical Engineering
Handbook. The overall heat transfer coefficient for a jacketed carbon and stainless steel vessel was
obtained from an internet source.
There were no rules of thumb available neither for the type of agitator needed nor its diameter.
So the a helical ribbon agitator was used since this was the common agitator used in a Swenson Walker
Crystallizer and it was stated that the agitator’s diameter should not touch the walls of the equipment
so a diameter of 0.2 meters was used.
In this equipment swaged and welded carbon steel would be used. Carbon steel can withstand
temperature until 900 °F. The design temperature for this process is only 154°F. The thickness of the
vessel would be 1.4mm according to the American Society of Mechanical Engineers Boilers and Pressure
Vessels Code Sec. VIII D.1 and a corrosion allowance of 4mm since the fluid being processed is an alkali
solution which can cause corrosion and a 2mm allowance for the double wall side since only water was
being used.
Rendered 3D Diagram
Equipment Specification Sheet
Swenson Walker Crystallizer Specification Sheet
Service of Unit: Trisodium Phosphate Crystals
Size
Type
Horizontal
Performance of Unit
Fluid Allocation
Fluid Name
Fluid Quantity(kg/hr)
Temperature (K)
in
out
Specific Heat (J/kgK)
Latent Heat (J/kg)
Double Wall
Shell Side
Side
trisodium phoshpate slurry water
~870
~4100
313
298
3200
146500
Heat Exchanged (J/hr)
Construction of Shell
Design Temperature (Max/Min F)
Wall thickness (mm)
Corrosion Allowance (mm)
Length (m)
Inside Diameter
Supports
Saddles
288
293
4184
85750 J/hr
154/79
1.5
4
3
0.5
1.5
2
3
0.54
Conclusion and Recommendation
A Swenson walker crystallizer producing trisodium phosphate crystals was designed using all the
knowledge gained from heat and mass transfer and equipment design.
I recommend anyone to try designing Swenson Walker Crystallizers in series or overlapping
equipment and compare which of the two arrangements would give a better yield. Also to show the
effects of using an insulating material like 85% magnesia or asbestos and diatomaceous earth comparing
the results or yield with an equipment without insulation. And lastly to have a further research on
designing a helical ribbon agitator like maximum diameter, number of ribbons, and position of the
agitator in order to achieve a better yield.
References:
McCabe & Smith (2000). Unit Operations of Chemical Engineering' McGraw-Hill, New York
Perry, R. H., & Green, D. W. (2008). Perry's Chemical Engineers' Handbook. New York: McGraw-Hill
Anantharam, N., & Begum, K.M. (2011). Mass Transfer: Theory and Practice. New Delhi: PHI Learning Private
Limited
http://www.pharmainfo.net/sudha/blog/crystallization
http://faculty.ksu.edu.sa/13042/Documents/crystalization%208-9.pdf
http://www.uobabylon.edu.iq/uobColeges/ad_downloads/4_13137_558.pdf
http://www.thermopedia.com/content/547/
Appendix
Feed Properties
tf  313
K
xf 
31
 0.237
131
J
Cpf  3200
kg  K
Crystals and Liquor Properties
xl 
15.5
115.5
 0.134
164
xc 
tl  298 K
164  12( 18)
J
Lc  146500 kg
 0.432
kg
C  300
hr
Cooling Water Properties
t1  288
t2  293
K
Cp  4184
K
J
kg  K
W
U  350
2
mK
Given
FL
 1 1 

 xf xl 
C
M  
F xf  L xl
C xc
F  870.872
 870.872 

 570.872 
lsolve( M N )  
L  570.872
w 
F Cpf  ( tf  tl)  C Lc
Cp ( t2  t1)
Kg
3
 4.099  10
hr
J
7
q  F Cpf  ( tf  tl)  C Lc  8.575  10
Tlm 
A 
( tf  t2)  ( tl  t1)
tf  t2 
ln 

 tl  t1 
q
U Tlm  3600
hr
 14.427
 4.717
2
m
For common Swenson Walker Crystallizers:
L  3 m
r 
A
2   L
 0.25
m
 C 

 C xc 
N  
Tables Used
Table 2-122