EXTRACTIVE DISTILLATION OF ISOBUTYL ALCOHOL (IBA) + ISOBUTYL
ACETATE (IBAc) USING N,N-DIMETHYLFORMAMIDE (DMF) AS ENTRAINER.
PILOT PLANT ANALYSIS
N. Martínez, M. C. Burguet, J. B. Montón
Departamento de Ingeniería Química, Escuela Técnica Superior de Ingeniería,
Universitat de València, 46100 Burjassot, Valencia, Spain.
Abstract
The present work shows the study of the extractive distillation of isobutyl alcohol (IBA) +
isobutyl acetate (IBAc) using N,N-dimethylformamide (DMF) as entrainer in a pilot plant
distillation column with 30 real bubble cap trays. The experimental distillate curves of the
ternary system obtained with the column working at total reflux have been analyzed and the
distillation curves with the column working with continuous addition of the azeotropic feed
and the entrainer has been obtained.
Keywords: extractive distillation, pilot plant, residue curve map, isobutyl alcohol, isobutyl
acetate, N,N-dimethylformamide.
1. Introduction
The present work analysis of the distillate curves the working at total reflux, and now the
study continues with the analysis of the experimental and simulated curves of continuous
distillations with different operation conditions. The binary system is a mixture of IBA(1) +
IBAc(2), which presents a minimum boiling azeotrope at 101.3 kPa. The azeotropic
composition of this mixture at normal pressure is 0.884 molar % in IBA. In this study DMF
has been used as entrainer. The vapor-liquid equilibrium (VLE) data of the binary and ternary
system have been reported in previous works1-3. The IBA + DMF and IBAc + DMF binary
systems and the IBA + IBAc + DMF ternary system do not present azeotropes; therefore there
are not different distillation regions in all the range of the ternary system. Moreover, the
system, at the experimental conditions, is also miscible in all the compositions range. The
problem is focused then on breaking the IBA + IBAc azeotrope.
1
Literature Review
Distillation is still the most used separation process in the chemical industry,
notwithstanding its elevated energetic consumption. The recent studies have the purpose to
reduce as much as possible this consumption improving the operation knowledge and
optimizing its structure and operation parameters.
In these studies, the simulation programs are turned into a powerful tool due to the facility
and quickness for changing the operational conditions and for analyzing the simulated results.
But the use of commercial simulators without an original data and results (both
thermodynamic and operational) critical analysis can justify decisions very far of the desired
goal.
The problem of the critical analysis of the simulation results is increased by the little
information available about the experimental data obtained in industrial columns or in pilot
plants. Against the plentiful bibliography of the thermodynamic data (equilibria, heat of
mixture, activity coefficients, etc) it is remarkable the small amount of studies in column
published.
Heterogeneus Azeotropic Distillation
Most of the published dynamic models for nonideal multicomponent distillation separations
are related to heterogeneous azeotropic distillation. This process is the most widely used to
separate azeotropic mixtures with low relative volatilities.
Heterogeneous azeotropic distillation uses a third component (entrainer) to form a
heterogeneous azeotrope in the reflux drum.
One of the phases is recovered as product, and the other is sent back as reflux to the column.
Although the reflux drum is used as a decanter, this process usually requires more than one
column to recover the entrainer.
The extractive (homoazeotropic) distillation is a very efficient separation method where the
two components (A and B) forming an azeotrope are separated by the aid of a separating
agent (solvent/entrainer). Extractive distillation in a continuous system is widely used in the
industry and it has an extensive literature Previous studies have concentrated on the effect of
the extractive agent on the separation process. For example, its effect on the relative volatility
of the mixture, the effect of feed concentration on specific consumption of extractive agent
and the effect of adding the extractive agent as one of the components of the mixtures [18,19].
The possibility of reducing the costs of energy consumption in auto-extractive distillation has
also been studied [20]. In this paper, we describe an experimental
En ultimos trabajos publicados, a escala de laboratorio, Present an experimental investigation
of the separation of the azeotropic ternary mixture via batch rectifier column4-14 , y en otros
menos, se realizan en sistemas continuos15-20 , both cases with different types from column
y configurations, using different models mathematical with the objective to find one effect
economic potencial of the system.
There are few publications on investigation pilot scale for extractive distillation. Rueda et al21
presents the results from the experimental validation of dynamic models of an azeotropic
distillation system of methanol, normal pentane, and cyclohexane. The model was validated
with experimental data from a pilot-scale size packed distillation unit operated at finite reflux.
The approach presented in this work links the process fundamental dynamic model (HYSYS)
with the control software used in the process. K. Terelak et al22, present results from pilotscale distillation of mixtures of formaldehyde, water and methanol, in column with 2 m.
packing, two variants of that packing were studied. Barbel Kolbe et al23, present an study
interesting of simulation for processing petrochemical cuts, based on the total annual cost ,
2
verified on a laboratory and pilot plant scale . Cusack24, designed and executed pilot-plant
program of the extraction system for the separation process.
However, also some few the publication on the computer simulation of extractive
distillation25-28, the best work R. Muñoz et al28., They simulated and evaluated economically
two separation alternatives of a mixture made up of 52 mole% of isobutyl alcohol and 48
mole% of isobutyl acetate by means of a practical case of a plant to treat 12,000 Tm/year of
the original mixture. The simulation has been carried out satisfactorily by means of a package
of commercial software (Aspen HYSYS®) using the thermodynamic model UNIQUAC with
binary parameters obtained experimentally by us.
In these work, based on the guidelines for the solvent screening, They have chosen three
solvents: N,N-dimethylformamide (DMF), 1-hexanol and butyl propionate (BUP). DMF was
recommended as a potential entrainer for alcohol–acetate azeotropic mixtures because of its
high polarity29 and 1-hexanol and BUP have been chosen because they are, respectively, in
the same homologous series with one of the key-components30. Therefore, in order to be able
to select the best solvent among them, They have carried out simulations with Aspen
HYSYS® v3.2 of Aspen Technology Inc., using the binary interaction parameters correlated
from experimental data obtained for all binaries involved1-3 . According to the results
obtained, the best solvent seems to be butyl propionate. Once the solvent has been selected,
we have designed the separation sequence and optimized the operating parameters.
The two processes evaluated (extractive distillation using n-butyl propionate as a solvent and
pressure-swing distillation) was optimized independently from each other and the best
configurations and evaluated economically. The simulation and economic evaluation of the
two separation alternatives that we have considered allow us to conclude that, for a 12,000
Tm/year plant, the pressure-swing distillation is more attractive than the extractive distillation
using n-butyl propionate as an entrainer.
Isobutyl acetate (IBAc) is a solvent widely used in Chemical Industry. It is used alone or in
solvent blends in applications including coatings, inks, adhesives, industrial cleaners and
degreasers. The IBAc is produced by estherification of aceticacid with isobutyl alcohol (IBA).
Final purification of acetate by traditional technologies is a relatively complex procedure due
to the existence of a minimum boiling point azeotrope in the IBA+ IBAc mixture at
atmospheric pressure.
Experimental System
The chemical system selected for the experiments performed in this research was a ternary
mixture of isobutyl alcohol (IBA) + isobutyl acetate (IBAc) using n,n-dimethylformamide
(DMF).
The distillation curves has been obtained, using the DISTIL program of Aspentech the
residue curves of the ternary system with UNIQUAC model and the binary parameters3, The
Figure 1, shows the residue curve map. An adequate topology is observed with only one
distillation region into the composition diagram.
The topology of this system can easily be deduced from the number and nature of the
existing one azeotrope. In table 1 these data are showed, along with the nodes represented by
pure compounds.
3
Node
IBA
components
TºC
108.09
Node Type Saddle
Tabla 1
IBAc
DMF
116.00
saddle
152.63
stable
Azeotropo
IBA+IBAc
107.93
unstable
Given the importance that the exact knowledge of the distillation curves shape has, he is very
suitable to have information on the coincidence between the distillation curves estimated by
simulator and real ones obtained in the column.
IBA + IBAc + DMF
IBAc
(116,90ºC)
0.0
1.0
0.2
0.8
0.4
0.6
0.6
0.4
0.8
0.2
Azeotrope
( 107.93ºC )
DMF 1.0
(145.69 ºC)
0.0
0.0 IBA
0.2
0.4
0.6
0.8
1.0
(108.09 ºC)
Figure 1. Residual curve map for IBA + IBAc + DMF at 101.3 kPa using UNIQUAC model; ( ) azeotrope; the
symbol of arrow indicates the residual curve direction.
Apparatus and procedure
Figure 2 shows an image of the experimental distillation column that has been manufactured
by Fischer. The pilot distillation column has 30 real bubble cup trays and an overall height
4100 cm and 9 cm of diameter. Each one of them has a sample intake that can be used to
4
remove liquid samples of the tray or to introduce a thermocouple or another temperature
measurement element.
In our column thermocouples have been settled in the plate number 3, 8, 13, 18 and 26. In
addition there are thermocouples in the reboiler, in the outlet of the distillate and in the feed
inlet.
The column consists of three sections of 10 real trays each one, isolated of the outside by a
vacuum-jacketed and with three possible feed inlets in each section. The reboiler takes several
control systems of the energy provided to the fluid in order to be able to control the vapour
flow generated and to avoid flooding of the trays.
The system has two computers to measure and control the most important operation
parameters, as well as an alarm system and automatic shutdown in case of a dangerous
operation (interruption of the refrigeration water, for instance). The distillate flow is regulated
by an automatic control of the opening time of the exit valve located in the high part of the
column and the residue takes up through a constant level system. Distillate and residue can be
mixed again and be given back to the feeding system by membrane pumps, although in this
case we have preferred to retire the distillate and the residue in order to keep the stationary
conditions.
5
Figure 2 Experimental distillation column
Total reflux
If the experimental was planed to determine a distillation curve, In order to reach the
stationary state the column starts working at total reflux until the temperatures of all the trays
stay constant and compositions ( reboiler and condensation), during half an hour (This
process can last around 2 hours) , small samples were taken from rebolier, steps and
condenser.
Continuous operation
After reaching the stationary state at total reflux the feed is introduced and it is expected
again until the temperatures of the reboiler and the trays remain stationary (around 4 hours).
The feed is introduced in the selected tray (in our case in the 10th tray in all the
experiments) by means of gravity drop from a constant level tank and after going through a
heating process that heat the feed until the boiling temperature. The solvent is introduced in
the high part of the column (24th and 29th trays) following an equivalent procedure to the one
of the feed. The flows are regulated with valves and are measured with individual calibrated
rotameters.
6
El caudal de vapor utilizado para todos los experimentos fue el mismos, correspondiendo al
80% del máximo permitido para que la columna trabaje con inundación,
Once the stationary state is reached small samples from the reboiler, trays and condenser
are taken (0.2 cc). Due to the total content of the column (about 2 litres), the extractions do
not destabilize the column.
Analysis
The analysis of the samples has been made by gas chromatography with the same
equipment and the same operation parameters that were described in the previous work where
is realized the study of the ternary vapour-liquid equilibrium2 .
Chemicals
IBA (99.5 mass%, HPLC grade), IBAc (>99 mass%, analytical grade) and DMF (>99.9
mass%, HPLC grade) were purchased from Aldrich Ltd. The reagents were used without
further purification after chromatography failed to show any significant impurities (purity +
99 mass %).
Results and discussion
Analysis of the distillation curves of total reflux
In the figure 3, the residual curves generated by the DISTIL Simulator have imagined using
UNIQUAC model have been presented [7]. The discontinuous lines show the estimation
obtained using the parameters of the Simulator data base, while the continuous lines are
residual curves with the parameters calculated from the experimental equilibrium data. The
experimental points corresponding to the composition of different plates from the column
working at reflux and in stationary regime have been also drawn. In order to make this
comparison a central point from each experiments has been obtained by means of program
DISTIL
It can be observed that the experimental points agree very well with the residual curves
predicted by simulator.
7
IBAc
0
0.2
1
0.8
0.4
0.6
0.4
0.6
0.2
0.8
DMF
0
1
0
0.2
0.4
Figure 3 Residue curve map.(
0.6
0.8
1
IBA
) Simulated by DISTIL. (▲) Experimental data
Analysis of the distillation curves in continuous operation.
Several experiments have been carried out with continuous addition of near azeotropic
feed, entrainer extraction of distillate and bottoms flow, changing the operation conditions of
the column. In Table 2 the more important operation parameters are indicated. In the same
table the values the food (XFi), distillate (XDi) and bottoms (XBi) compositions are indicated.
These values are expressed in molar fractions. In all the experiments the feed has been
introduced in the 10th tray (bottom up), a reflux ration of 19 has been used and the
temperature of the feed has been introduced in the column is near to its boiling point.
Table 2 Experiments and operation conditions
Experiment
Feed inlet stage
Solvent/Feed ratio
Feed (kmol/h)
XF1
XF2
Distillate (kmol/h)
XD1
XD2
Bottom (kmol/h)
XB1
XB2
6
24
4.8
4.112 10-3
0.7486
0.2513
8.28 10-4
0.4548
0.5252
2.175 10-2
0.1396
0.0333
7
29
3.8
3.701 10-3
0.7486
0.2513
3.401 10-4
0.0692
0.4856
1.635 10-2
0.1759
0.0445
8
29
4.3
3.524 10-3
0.7486
0.2513
7.433 10-4
0.1953
0.5507
1.742 10-2
0.1304
0.027
9
29
4.5
4.288 10-3
0.7486
0.2513
5.364 10-4
0.1088
0.5816
2.126 10-2
0.1603
0.0362
10
24
4.0
4.699 10-3
0.7486
0.2513
8.071 10-4
0.4047
0.5634
2.134 10-2
0.1404
0.0346
The composition and flow values of the Table 1 have been obtained as average of different
measurements made throughout each experiment (around 2 hours); therefore do not fulfil the
8
material balance exactly. In any case the errors between input and output values of the three
components are less than 5%.
IBAc
0.0
0.2
0.8
0.4
0.6
0.6
0.4
Feed
0.8
0.2
Azeotrope
0.0
DMF
0.0
0.2
0.4
0.6
0.8
IBA
Figure 4 Continuous operation. Experimental data: (  ) Exp. 7; ( ∆ ) Exp. 8;
( □ ) Exp. 9
In Figure 4 the experimental curves corresponding to the experiments 7, 8 and 9 have been
represented, in which entrainer is introduced in the 29th tray.
Figure 5 shows the experimental curves for the 6 and 10 experiments and in these cases the
entrainer inlet is in the 24th tray.
If it is compared the two figures (Fig. 4 and 5), it can be observed that when the entrainer
comes in the 29th tray the composition of the DMF in the distillate is high because there are
not enough stages to separate and incorporate to the liquid that goes down. Nevertheless,
when the DMF enters to the 24th tray the distillate is free of DMF, although immediately the
IBA + IBAc mixture tends to move towards the azeotropic composition.. This fact is due to
two opposing effects, by one side, the solvent reverses the volatility of the original mixture in
the extractive section (enriching the vapor phase in IBAc) [2] and, on the other hand, in the
upper rectifying section (without solvent) the original mixture behaves in the normal way
(enriching the vapor phase in IBA).
This study can be realized easily by simulation, provided that the simulation reproduces,
even if in approximately way, the experimental curves.
9
IBAc
0.0
0.2
0.8
0.4
0.6
0.6
0.4
Feed
0.8
0.2
Azeotrope
0.0
DMF
0.0
0.2
0.4
0.6
0.8
IBA
Figure 5 Continuous operation. Experimental data: ( ● ) Exp. 5; ( ∆ ) Exp. 6
Figure 6 shows the experimental curves to the 6th experiment and the simulation data made
by HYSYS fixing the conditions of the inlet streams, the reflux ratio and the distillate flow. In
order to make the simulation 50% effectiveness had been assumed to each stage, average
value that was obtained in experiments with binary systems.
IBAc
0.0
1.0
0.2
0.8
0.4
0.6
0.6
0.4
0.8
0.2
Azeotrope
1.0
DMF
0.0
0.0
0.2
0.4
0.6
0.8
1.0
IBA
Figure 6 ( --- ) simulated by HYSYS. ( ▲ ) Experimental data
10
Acknowledgements
Financial support from the Ministerio de Ciencia y Tecnología of Spain, through project
No. CTQ2004-04477/PPQ, the FEDER European Program and the Conselleria de Cultura,
Educació i Esport (Generalitat Valenciana) of Valencia (Spain) are gratefully acknowledged.
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