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Licensed Unlicensed Requires Authentication Published by De Gruyter July 12, 2019

CFD Modeling to Predict Mass Transfer in Multicomponent Mixtures

Mitra Sadat Lavasani, Rahbar Rahimi, Mortaza Zivdar and Mohammad Kalbassi

Abstract

A novel three-dimensional computational fluid dynamics mass transfer (CMT) model in Eulerian–Eulerian frame work is deploys for investigating the concentration profiles, and trays efficiencies in multicomponent distillation columns. The proposed model is based on Maxwell Stefan equations, and CFD was employed as a powerful tool to model the hydrodynamics and mass transfer. The two phases are modelled as two interpenetrating phases with interphase momentum, heat and mass transfer. The Closure model is developed for mass interphase transfer rate in ternary mixtures. The predictability of the mass transfer behaviours of multicomponent can result in a more efficient and predictable design of distillation trays. Two non-ideal ternary mixtures were studied. The tray geometry and operating conditions are based on the experimental works of Kalbassi and the composition profiles, tray efficiencies, and point efficiencies of mixtures were presented. The obtained results were confirmed by the experimental data. The results indicate that the values of individual component tray efficiencies and point efficiencies for these multicomponent systems were considerably different which confirm the interactive nature of the mass transfer in multicomponent mixtures. These mixtures also illustrated different point efficiencies across the tray because of the composition dependency of these mixtures. The average relative error for the prediction of efficiencies is about 8 %, which indicates the accuracy of the model.

Acknowledgments

The authors appreciatively acknowledge financial support by the Iran National Science Foundation(Grant number 96001257) and the Research Council of the University of Sistan and Baluchestan.

Nomenclature

a

effective interfacial area per unit volume, m−1

a/

interfacial area per unit bubbling area

AB

tray bubbling area, m2

AH

area of holes, m2

b

weir length per unit bubbling area, m−1

CD

drag coefficient, dimensionless

dB

bubble diameter, m

dG

the diameter of gas bubble, m

dh

hole diameter, m

DG

Gas molecular diffusivity, m2/s

DL

liquid molecular diffusivity, m2/s

EOV

vapor phase Murphree point efficiency

F

Fraction hole area per unit bubbling area

Fbba

vapor F-factor on bubbling area, (m/s) (kg/m3)0.5

FP

flow parameter

g

acceleration due to gravity, 9.81 m/s2

GM

molar gas rate per unit of bubbling area, kmoles/hr. m2

hf

froth height, m

hL

clear liquid height, m

hW

weir height, m

kG

binary over all gas phase mass transfer coefficient, m/s

KG

ternary over all gas phase mass transfer coefficient, m/s

kL

liquid phase mass transfer coefficient, m/s

Lw

weir length, m

m

slope of equilibrium line

MGL

interphase momentum transfer, N/m3

p

pitch of holes in sieve plate, m

P

pressure (N/m2)

QL

liquid volumetric flow rate, m3/s

SLG

rate of inter phase mass transfer, kg/m3s

t

Time, s

u

velocity vector, m/s

UL

time average liquid velocity on bubbling area, m.s

Us

time average superficial vapor velocity, m.s−1

Vslip

slip velocity between gas and liquid, m/s

VH

hole velocity, m/s

x

mass fraction in liquid phase, dimensionless

y

mass fraction in gas phase, dimensionless

Greek letters

μ

the viscosity of phase, Pa.s

ρ

the density of phases, kg/m3

θG

contact time in the gas phase, s

k

kinematic viscosity, m2/s

Subscripts

Eff

Effective

G

referring to gas phase

L

referring to liquid phase

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Received: 2019-05-12
Revised: 2019-06-25
Accepted: 2019-06-25
Published Online: 2019-07-12

© 2019 Walter de Gruyter GmbH, Berlin/Boston

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