Packed Column Distillation

By Craig D. Mansfield

Some Background on Packed Column Distillation

• Commonly uses– High value products– Heat sensitive products– Usually run in small/medium batches

• Used since ~1907– Patent for Raschig rings by Dr. Raschig in 1907

Comparison to Tray ColumnsPacked Advantages• Lower • Smaller column diameter• Cheaper corrosive seps• Less foaming• Low liquid holdup• Efficient batch operation• Greater thermal control

Tray Advantages• Can handle solids• High liquid rates• Large column diameter• Allows complex ops• Easier alt. feed locations• Better performance

predictions• Higher residence time• Weigh less• Better wetting

Research Problem Statement

• Design/build a new packed distillation column for the UOL

• Separate isopropanol and water• Operate in batch or continuous mode

Basic Design Algorithm Used

• Mixture properties• Flooding point data• Size/capacity of reboiler heat exchanger– Determine power source

• Determine mass transfer performance• Size/capacity of reflux heat exchanger• Size/capacity of components and throughput– Volume of tanks/reboiler

Mixture Properties

• Used UniSim Design software– Viscosity, thermal cond., surface tension

• Thermo models– gen. NRTL w/ PR

• Diffusivity models– Wilke and Chang (diluted in water)– Sitaraman et al. (diluted in isopropanol)– Leffler and Cullinan (liquid mixture)– Gilliland (vapor mixture)

T-X Diagram

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 180

82

84

86

88

90

92

94

96

98

100

C3H7OH-H2O System T-X DiagramP = 1 atm

T_BubbleT_Dew

x_C3H7OH, y_C3H7OH

T_Bu

bble

, T_D

ew (d

egre

es C

)

X-Y Diagram

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

C3H7OH-H2O System X-Y DiagramP = 1 atm

yx

x_C3H7OH

y_C3

H7O

H

Viscosity vs X

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10.25

0.3

0.35

0.4

0.45

0.5

C3H7OH-H2O System Liquid Viscosity vs X DiagramP = 1 atm

Liq. Viscosity

x_C3H7OH

Liqui

d Vi

scos

ity x

10^3

(Pa*

s)

Flooding Point

• Column filled w/ liquid holdup from high vapor flow

• Common flooding models– Sherwood et al.– GPDC

• Used Sherwood et al. as model for design• Determined flooding vapor/liquid flow rates

Power Source

• Required power is 6.34 KW• Choices are electric or steam• Electric power (via resistance) requires a min.

52.8 amps of current• Steam is already available and efficient• Steam was chosen as the main power source

Size of Reboiler Heat Exchanger

• Used a vapor rate below flood point to find min. power requirement

• Modeled reboiler w/ nucleate pool boiling• Correlations used:– Modified Thöme and Shakir model– Mostinski model

• Calculated the area (“size”) required

Mass Transfer Correlations

• Onda et al.– Effective specific area– Interfacial Mass Transfer Coefficients

Determination of Mass Transfer Performance (Transfer Units)

• Used packed column design integral(s):

vs

-0.199999999999999 6.10622663543836E-16 0.200000000000001 0.400000000000001 0.6000000000000010

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

C3H7OH-H2O System Column Height vs x_B DiagramP = 1 atm, x_D = 0.6

Series1

x_B

Z_t (

m)

Size of Reflux Heat Exchanger

• Sized to match or exceed max reboiler power– At flood– At highest transfer capacity

• Model used: Nusselt horizontal pipe theory• Size was the transfer area required (again)

The Nominal Model

Column/Operation Specs• ID = 3 in.• = 2 m• = 1• = 6.34 KW• = 0.577 m• = 0.154 m• = 3.42• HETP = 0.21 m

Reboiler/Condenser Specs• = 94.96 W/K• = 3.18 W/K• Tube NPS = 0.5 in.• = 0.320 m• = 0.355 m• = 0.01598 • = 0.0177

The Nominal Model

Compositions• = 0.1• = 0.6• = 0.2

Flow Rates• B = 0.28 mol/s = 6.59 USGPH• F = 0.34 mol/s = 10.42 USGPH• D = 0.069 mol/s = 3.878 USGPH• L = 0.069 mol/s = 3.878 USGPH

Average Efficiencies• = 0.80, z = 0.577 m• = 0.23, z = 2 m

Core System Diagram

Acknowledgements

• Dr. Lewis E. Johns• Dr. Ranga Narayanan• Dr. Spyros Svoronos• The University of Florida

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