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Unit Operations Lecture 10 23 June 2010 1
Unit OperationsLecture 10
23 June 2010
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Overview• Last timeLast time…• Column Internals; sizing• Batch (Rayleigh) Distillation
E ti• Evaporation• Crystallization
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EvaporationEvaporate a solvent from a solution to concentrate the solution; recover the solvent, or as a
preparation for further downstream processing (like crystallization or distillation)• Typical is removal of water or other solvents from:
o Aqueous sugar (or fructose) solutionso Salt solutionso Salt solutionso Acid/basic solutions (i.e., sulfuric acid or NaOH)o Glycerol (glycerine) solutionso Glueo Milko Fruit juices o Vegetable extractso Pharmaceuticalso Pharmaceuticalso Fine chemicals
• Things to consider:o Solute concentration (initial and final)
l bili f h lo Solubility of the soluteo Temperature sensitivity (time/temperature)o Foaming/frothingo Required T and P
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o equ ed a do Scale formation / Materials of Construction (MOC)
Equipment Types
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5
6
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Equipment Information
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Perry’s 8th ed.
Equipment Information
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Perry’s 8th ed.
Equipment Information
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Perry’s 8th ed.
Equipment InformationInformation
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Perry’s 8th ed.
Equipment Types
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Boiling Point Elevation• Solute insoluble, colloidal suspension: No boiling point change of the solventSo u e so ub e, co o da suspe s o : No bo g po c a ge o e so ve
• Solute soluble:oSolute non volatile – boiling point
l i ( lli i )elevation (colligative)oSolute volatile – boiling point
elevation/or decrease
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Boiling Point Elevation• Solute soluble, non volatile:So u e so ub e, o vo a e:
oColligative property, i.e. only depends on the number of dissolved particles but not their composition; (from thermodynamics or charts)
oFor ideal dilute solution the boiling point rise is given by:
dissolvedvaps
dissolvedb xhTRxKT
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oFor ideal dilute solution the boiling point rise is given by:
dissolvedvaps
dissolvedb h
where: is the so called ebullioscopic constantbKis the gas constantis the boiling point of the solventis the enthalpy of (heat of) vaporization of the solvent
R
vaphsT
is the enthalpy of (heat of) vaporization of the solventis the mole fraction of dissolve particles (moles of dissolved particles per mole of solution, i.e. solvent for dilute solutions)
sh
dissolvedx
[i N Cl ld h 2 l f di l d i l l f l i i f ll di i ]
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[i.e. NaCl would have 2 moles of dissolved particles per mole of salt, since it fully dissociates]
Boiling Point Elevation• Solute soluble, non So u e so ub e, o
volatile:• Solute soluble, and
slightly volatile:slightly volatile:oDetermine boiling
point elevation from Txy diagram orTxy diagram or
satOHOHOHOH Pxp
2222
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Evaporator ModelAssumptions:• Feed (thin-liquor) only has one
volatile solvent• Only vaporization energy comes
f l h f i ifrom latent heat of vaporization of the steam
• Contents are well mixed• Heat transfer driving force is• Heat transfer driving force is
difference between the steam temperature and the boiling solution temperature TTT solution temperature
• The P is found by knowing the boiling point elevation of the solution and that
ps TTT
epv TTT • Evaporator is well insulated
epv
16Seader & Henley, 2006
Evaporator ModelMass balance (mass units used)
pvf mmm Mass balance on solute
ppvvff mwmwmw
Energy balance on liquid solutionHmHmQHm ppvvff HmHmQHm
Energy from steam heat
vaps HmQ vapsQ
Heat transfer across the heat exchanger ps TTAUQ ps TTAkQ p pTypical HT Coefficients
U (overall)
Exchanger W/m2K btu/hft2oFShort tube vertical 1100‐2800 200‐500Horizontal 1100 2800 200 500
17Seader & Henley, 2006
Horizontal 1100‐2800 200‐500Long tube vertical 1100‐4000 200‐700Forced circulation 2300‐11000 400‐2000Agitated film 680‐2300 120‐400
Geankoplis, 2003
Multiple-Effect Evaporator Systems
18Seader & Henley, 2006
Mechanical Recompression
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McCabe, Smith, Harriott, 4th ed.
Overview• Last timeLast time…• Column Internals; sizing• Batch (Rayleigh) Distillation
E ti• Evaporation• Crystallization
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CrystallizationCool or evaporate a solution to cause crystal formation of the product in the desired (and
uniform): size, shape, and purity.
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Seader & Henley, 2006
Thermodynamics (review)
Solid – Liquid Equilibrium
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Seader & Henley, 2006
Thermodynamics
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Seader & Henley, 2006
MgSO47 H2OProduction
24Seader & Henley, 2006
MgSO47 H2OProduction
25Seader & Henley, 2006
Crystals• Crystal habitsy• Growth• Crystal Size Distribution (CSD)
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Crystals (Bravais, 1848)
27Seader & Henley, 2006
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Crystal Systems
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Seader & Henley, 2006
NaCl sucrose
30MgS04-7H2O
Crystals HabitsIncludes (for example): • Plates• Needles• PrismsC d d thi lik thCan depend on things like growth
rates, nucleation, and impuritiesBatch of crystals described by:
A ti l i• Average particle sizeCharacteristic dimensionSphericity
• Crystal Size Distribution (CSD)• Crystal Size Distribution (CSD)
Sphericity ()
)( volumesamewithsphereareasurface
V6
)()(
volumesameparticleareasurfacevolumesamewithsphereareasurface
P
P
sph SAV
D6
1h 1ti lh i l
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Seader & Henley, 2006
1sphere 1 particlesphericalnon
Sphericity (exercise)
Fi d th h i it f bi ti l ith id f 5
PV6Find the sphericity of a cubic particle with a side of 5 mm.
P
P
sph SAV
D6
3LV 3D2
3
6LSA
LV
cube
cube
spherecube VV
6
33 sphD
L
LDsph 2407.1sph
2
3
62407.16
LL
L 806.0
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Particle Size Measurement
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Seader & Henley, 2006
Particle Size MeasurementMeasurement
Screens (US/British Standard Screens)
PiPictures
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Particle Size MeasurementL li ht tt iC lt C t Laser light scatteringCoulter Counter Malvern
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Crystal Size Distribution (CSD)Screen analysis (for example)
Aperatureopening [mm]
Cum. wt. %
1 400 1001.400 1001.180 98.141.000 91.600.850 83.490 600 35 540.600 35.540.425 17.390.300 6.310.212 1.830 150 0 360.150 0.360.106 0.11
80
90
100
S d & H l 2006
Differential screen analysis50
60
70
80
ativ
e w
t. %
Seader & Henley, 2006
10
20
30
40C
umul
a
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Cumulative screen
analysis
0
10
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
Screen opening [mm]
Questions?
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