Computers and Chemical Engineering 26 (2002) 1265 – 1279
The need for using rigorous rate-based models for simulations of
ternary azeotropic distillation
P.A.M. Springer, S. van der Molen, R. Krishna *
Department of Chemical Engineering, Uniersity of Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands
Received 24 September 2001; received in revised form 18 March 2002; accepted 18 March 2002
Abstract
Experiments were carried out in a bubble cap distillation column operated at total reflux with the system: water – ethanol –
methylacetate. This system has two binary azeotropes (water – ethanol and water – methylacetate), which gives a simple distillation
boundary connecting the two azeotropes. All experiments were restricted to the homogenous region without liquid phase splitting.
For certain starting compositions the measured distillation composition trajectories clearly demonstrate that crossing of the
distillation boundary is possible. In order to rationalize our experimental results, we develop a rigorous nonequilibrium (NEQ)
stage model, incorporating the Maxwell – Stefan diffusion equations to describe transfer in either fluid phase and a fundamental
description of tray hydrodynamics. The developed NEQ model anticipates the boundary crossing effects, and is in excellent
agreement with a series of experiments carried out in different composition regions. In sharp contrast, an equilibrium (EQ) stage
model fails even at the qualitatie level to model the experiments. The differences in the NEQ and EQ trajectories emanates from
differences in the component Murphree efficiencies, which in turn can be traced to differences in the binary pair vapor phase
diffusivities Ð
y,ij
. It is concluded that for reliable design of azeotropic distillation columns we must take interphase mass transfer
effects into account in a rigorous manner. © 2002 Elsevier Science Ltd. All rights reserved.
Keywords: Azeotropic distillation; Residue curve maps; Rate-based models; Maxwell – Stefan equations; Distillation boundary; Nonequilibrium
stage; Equilibrium stage
Nomenclature
a interfacial area per unit volume of vapor bubbles (m
2
m
-3
)
NRTL parameters; see Table 1 (K) B
ij
molar concentration of species i (mol m
-3
) c
i
mixture molar density (mol m
-3
) c
t
d
b
bubble diameter (m)
Fick diffusivity in binary mixture (m
2
s
-1
) D
12
Maxwell – Stefan diffusivity for pair i – j (m
2
s
-1
) Ð
ij
E
i
MV
component Murphree point efficiency, dimensionless
Fo Fourier number, Fo 4Ð
y
V
/d
b
2
, dimensionless
NRTL parameters; see Table 1, dimensionless G
ij
g acceleration due to gravity (m s
-2
)
distance along froth height (m) h
height of dispersion (m) h
f
molar diffusion flux of species i relative to the molar average reference velocity u (mol m
-2
s
-1
) J
i
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* Corresponding author. Fax: +31-20-52-55604.
E-mail address: krishna@science.uva.nl (R. Krishna).
0098-1354/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII:S0098-1354(02)00039-X