Simulation Studies of Reactive Distillation Processes for Synthesis of Ethyl Acetate
Zhixian Huang , Hui Tian, Ting Qiu, Yanxiang Wu
School of Chemistry and Chemical Engineering
Fuzhou University
Fuzhou, China 350002
E-mail: [email protected], [email protected]
lines diagram and validate theoretical prediction through
performing a set of reactive distillation experiments in a
glass tray column with 80 bubble cap trays [4]. Calvar
investigated the reaction kinetics of the esterification of
acetic acid with ethanol catalyzed both homogeneously by
the acetic acid and heterogeneously by Amberlyst 15. A
packed bed reactive distillation column filled with Amberlyst
15 has been employed to obtain ethyl acetate. The influence
of feed composition and reflux ratio had been analyzed[5].
Lai used a complex two-column, consisting one RD column
(10 reactive trays plus structured packing for separation) and
a downstream stripper with a decanter in between, to product
high purity ethyl acetate[2]. Smejkal proposed a coupling of
a fixed bed reactor and a reactive distillation column for
acetic acid and ethanol esterification with strong acidic ionexchange resin as catalyst[6].
The aim of this study is to investigate in more detail the
performance of reactive distillation for the production of
ethyl acetate from HAc and EtOH. Experiment studies are
perform in our lab and simulation studies are carried out
using the ASPEN PLUS simulator. Emphasis is placed on
the analysis of the effects of key operating parameters, such
as reflux ratio and feed stage location, on the conversion of
EtOH and EtAc purity.
AbstractEthyl acetate (EtAc) is mainly used as solvent in
paints and coatings industry, inks production and as industrial
solvent in many other branches of industry. Esterification of
acetic acid with ethanol may be performed to synthesize ethyl
acetate. In this paper, a reactive distillation (RD) column with
packing cation exchange resins as a catalyst is applied.
Mathematical models of reactive distillation are based on the
conventional distillation process with supplementary equations
added to model the reactions present. Column simulations
performed here using Aspen Plus show excellent agreement
with experimental data for the EtAc system. A sensitive
analysis was performed to determine the effects of key design
and operating variables on column performance and,
subsequently, an optimal column configuration was obtained.
Keywords: reactive distillation, ethyl acetate, esterification,
simulation
I. INTRODUCTION
Ethyl acetate (EtAc) is mainly used as solvent in paints
and coatings industry, inks production and as industrial
solvent in many other branches of industry [1, 2]. In China,
the domestic demand of ethyl acetate shows a trend of
sustained growth, and over the next few years this demand is
expected to be kept increasing in a rate of 10% per year [3].
The most common strategy of producing ethyl acetate
consists in a simple esterification of ethanol (EtOH) with
acetic acid (HAc) in the presence of acidic catalyst, however
one of the key issues in the production of ethyl acetate is the
equilibrium limitation from the reversible reaction of acetic
acid and ethanol. In order to broke the equilibrium limitation
and achieve high conversion, reactive distillation (RD),
which contains simultaneously reaction and separation in a
column, has become an interesting alternative to produce
ethyl acetate. The reactive distillation is featured with its
merits not only in promoting the reaction conversion, but
also in reducing both the capital and operational costs as its
multifunctional nature. Since Eastman Chemical Company
owned a commercial reactive distillation process for the
production of methyl acetate. Later on, extensive researches
on the RD process appeared in the literature.
In last years, investigations of thermodynamic, reaction
kinetics and simulation for different processes to synthesis
ethyl acetate have been made. Kenig examined the feasibility
of ethyl acetate synthesis by using the reactive distillation
II.
COMPUTER SIMULATION
A. Reaction Chemistry
EtAc is produced from the reversible reaction of HAc
and EtOH over an acid catalyst, such as the acidic ionexchange resin, Amberlyst-36wet:
(1)
HAc + EtOH ↔ H 2O + EtAc
The reaction is equilibrium limited in the industrially
significant range of temperatures. The reaction has been
studied, and a detailed expressions of pseudo-homogeneous
model (eq.1-3) has been proposed [7], as shown below.
rA = wcat × ( k + C EtOH C HAc − k −C EtAc CWater )
(2)
Ea+
)
RT
E
k− = A− exp( a − )
RT
k + = A+ exp(
560
(3)
(4)
TABLE II.
where r is the reaction rate (mol·L-1·g-1·min-1), C is the
concentration (mol·L-1), wcat is catalyst weight (g), k is the
kinetic constant (L·mol-1·min-1·g-1), A is preexponential
factor ( L·mol-1·min-1·g-1 ), Ea is apparent activation energy
(kJ·mol-1), R is gas constant (J·mol−1·K−1).
Forward reaction
A l·mol-1·min-1·g-1
Ea kJ·mol-1
B.
Reactive Distillation Model
The reactive distillation is an extremely complex process,
which contains not only the mass transfer process but also
the separation process and the reaction process. In this paper,
simulation of the synthesis of ethyl acetate in a RD column
was carried out by using Aspen Plus. The RADFRAC
module, based on a rigorous equilibrium stage model for
solving the mass balance, phase equilibrium, summation and
energy balance (MESH) equations, was used in this study to
describe a multistage vapour-liquid separation in the
distillation column. The equilibrium stage model has been
often applied with great success for the simulation RD
column[8].
material balance (for seperation stage)
(5)
Lin + V in – Lout – Vout=0
material balance (for reaction stage)
(6)
Lin + V in – Lout – Vout +∑ri = 0
component balance
(7)
Linxi,in + Vinxi,in –Loutxi,out –Voutxi,out =0
energy balance
LinHLi,in + VinHVi,in – LoutHLi,out – VoutH Vi,out = 0 (8)
equilibrium
Pyi =γixiPVapi
(9)
(10)
∑yi =1
The phase equilibrium of this system is complex because
of the existence of several azeotropes. To account for the
non-ideal vapor–liquid equilibrium (VLE) and possible
vapor–liquid–liquid equilibrium (VLLE) for these quaternary
systems, the NRTL activity coefficient model was adopted.
The second viral coefficients of Hayden-O’Connell were
used to account for vapor phase association of acetic acid
due to dimerization and trimerization[9]. The Aspen Plus
built-in binary interaction parameters were used to compute
the fugacity coefficients. The compositions and temperatures
of the azeotropes for the system were shown in Table.
TABLE I.
TABLE III.
EtOH-H2O
EtAc-EtOH
EtAc-H2O
EtAc-EtOH-H 2O
EtAc
(wt.%)
H2O
(wt.%)
95.9
30.73
-10.70
-69.27
91.47
81.80
4.1
-8.53
8.53
Reverse reaction
81389
60.55
SPECIFICATION OF THE RD COLUMN AND FEED
total no. of stages
24
rectifying section
reactive section
stripping section
HAc feed composition
(mass fraction)
HAc
H2O
HAC feed flow rate L/h
1-8
9-14
15-24
HAC feed stage
EtOH feed composition
(mass fraction)
EtOH
H2O
9
0.999
0.001
0.513
0.999
0.001
EtOH feed flow rate L/h
0.2
EtOH feed stage
catalyst loading per stage
the mole feed ratio of acetic acid to ethanol
space velocity
m3(EtOH feeding)/h/m3(cat)
reflux ratio
15
29 g
3:1
0.213
1
III. PILOT PLANT EXPERIMENTS
A reactive distillation column (RD column) containing
solid catalyst was used for the synthesis of ethyl acetate from
ethanol and acetate acid. The strong acid iron resin
(Amberlyst-36Wet) was used as heterogeneous catalyst. The
experimental setup was given in Fig.1. The setup included a
RD column with internal diameter of 25 mm and a decanter,
which all were made of stainless steel. The section and the
stripping section of the RD column were packed with bulk θ
packing (φ, 3mm) while catalyst-bundles of ion exchange
resin catalyst were packed in the middle of the column as
reaction zones. The height of the rectifying section, the
reaction section, and the stripping section were 500mm,
1050mm, and 600mm, respectively. In order to quantitatively
describe the catalytic distillation column separation
efficiency, the number of theoretical stages of RD column
was estimated using Fenske method through separating a
mixture (ethanol and isopropyl alcohol) with close boil
point[11]. The results indicated that the separation efficiency
of rectifying section, reactive section, and stripping section
were equivalent to 8 TS, 6 TS, and 9 TS, respectively. The
temperature was measured at eight points in the column
while the liquid samples were taken from top, bottom and
along the column (as shown in Fig.1).
Acetate acid were fed above the reactive section at room
temperature while ethanol was pre-heated to 373.15K and
then introduced below the reactive section, with feed space
velocity of 0.213 m3 / (m3·h), acid-alcohol ratio of 3, the
reflux ratio 1. The overhead vapor was cooled to
Calculated Data
Azeotrope
131137
57.96
CONDITIONS
THE COMPOSITIONS AND TEMPERATURES OF THE
AZEOTROPES AT ATMOSPHERIC PRESSURE
EtOH
(wt.%)
PREEXPONENTIAL FACTORS AND ACTIVATION ENERGY [10]
T
( )
78.18
71.43
70.37
69.95
The chemical reaction was assumed to occur only in the
liquid phase in the reactive section, pre-exponential factors
and activation energy for pseudo-homogeneous model (eq.1)
was taken from Table. According to the experimental setup, the process parameters of the experiment and the
specification of the RD column for process simulation were
listed in Table .
561
Decanter
org. P.
aq. P.
Liquid Sample
Vapor mass fraction
environmental temperature and introduced to a decanter for
phase separation. The water formed by the reaction was
taken off from the phase separator. The smaller part of the
organic phase was refluxed back to the column as entrain to
carry out water. Another major part of the organic phase was
withdrawn as distillate.
Water
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
1
F1 Acetate acid
Product
4
7
10
13
16
19
22
Stage number
Figure 3. Comparison of experimental and simulation: vapor composition
profile of the column (H2O, ○ HAc, EtAc, ■ EtOH )
Reactive Zone
F2 Ethanol
V. SENSITIVE ANALYSIS
Reactive distillation columns behave substantially
differently from conventional distillation columns due to the
interactions between the chemical reactions present and
vapor-liquid equilibrium. Because of the constraints and
limitations in the experimental setup of the reactive
distillation column, some important process parameters can’t
be studied experimentally. So, the effects of key design and
operating variables are discussed in detail though computer
simulation. The effects described below should be
considered during design and operation of the column to
ensure optimal performance.
Figure 1. Flow sheet of the reactive distillation column for production of
ethyl acetate.
IV. MODEL VALIDATION
In order to check the suitable of the models for
simulating the reactive distillation, the models was used to
predict the temperature distribution and vapor phase
composition along the RD column for comparing with the
experimental data obtained in the present study. The
comparison of concentration and temperature profiles
obtained from the experiment and simulations results are
shown in Fig.2 and Fig.3.
Fig.2 demonstrates the calculated temperature trend is
similar with that obtained from experiment and the
maximum relative error is 3.4%, which is an acceptable
value considering the predictive nature of the process model.
From Fig.3, it can be seen that the composition in 6 sampling
points, calculated by Aspen Plus, are in good agreement with
the experimental data. The small difference may be due to
neglecting actual mass transfer, heat transfer and process
hydrodynamics in the equilibrium stage model. So the
equilibrium stage model can describe the column profiles
quantitatively and qualitatively.
A. Effect of reflux ratio
In a reactive distillation column, reflux ratio is a very
important parameter. Fig.4 presents the effect of the reflux
ratio on ethanol conversion and ethyl acetate purity. It can be
seen from this figure that with the increase of reflux ratio
(from 0.25 to 1.25) both the conversion of EtOH and the
purity of EtAc are enhanced. Because at low reflux ratios,
insufficient product separation limits the conversion and
traces of heavy component contaminate the product. The
increase of the reflux not only enhances separation efficiency
but recycles unreacted reactants and EtAc to the reaction
zone. In the quaternary system EtAc can be considered as
entrainer to carry water out off the RD column, which break
the chemical equilibrium constraints and subsequently
increase the forward reaction rate. However, when the reflux
ratio is increased from 1.25 to 2.5, it has a negative effect on
the EtOH conversion and EtAc purity. This is because the
increase of the reflux ratio increases the concentration of
EtAc in the reaction section. For equilibrium limited system,
the increase of EtAc concentration at the reaction section
shifts the equilibrium of the esterification of EtAc in the
reverse direction, further decreasing EtOH conversion and
EtAc purity. So the optimal reflux ratio is 1.25.
7HPSHUDWXUH 6LPXODWLRQ
([SHULPHQW
6WDJHQXPEHU
Figure 2. Comparison of experimental and simulation: temperature profile
of the column
562
90%
80%
EtOH conversion
EtAc purity
70%
60%
0
0.5
1
1.5
2
2.5
Reflux Ratio
Figure 4. Effect of reflux ratio on ethanol conversion and ethyl acetate
purity
B. Effect of the feed location of EtOH
Reactive distillation characters the simultaneous
implementation of reaction and separation. In the RD column,
the composition and temperature profiles can affect the
performance of the reactive section, and the feed location
appears to be one of the most important variables for these
profiles redistribution. Therefore, the selection of appropriate
feed location is a key to ensure high concentrations of
reactants in the reactive zone.
In this section, the effect of the feed location of ethanol
on the reactive distillation performance is investigated
through sensitivity analysis. The effect of the feed location of
ethanol on the conversion and EtAc purity is shown in Fig.5.
From the figure we can intuitively see that the optimal feed
stage of ethanol is 14 TS. When ethanol is fed above 9 TS,
most ethanol may be distillated directly from rectifying
section while only a small part of ethanol flow into the
reactive section due to the relative volatility difference
between ethanol and acetic acid, resulting in a low
conversion. As the ethanol feeding stage varies from 14 to 24
TS, the EtAc conversion and EtAc purity both appear a small
decrease. This is probably due to the fact that the residence
time of the reactants is decreased when the location of the
EtOH feed is below the 14 TS.
80%
60%
(kg/kg)
EtOH Conversion\ EtAc Purity
100%
40%
EtAc Purity
20%
EtOH Conversion
0%
1
3
5
7
9 11 13 15 17 19 21 23
Feed Stage of EtOH
Figure 5. Effect of the feed stage of ethanol on EtOH conversion and
EtAc purity
563
EtOH Conversion \ EtAc Purity (kg/kg)
EtOH conversion\ EtAc Purity (kg/kg)
C. Effect of the feed location of HAc
The effect of feed stage of HAc on the conversion of
ethonal and purity of ethyl acetate in the distillate was shown
in Fig.6. When the feed location of HAc is varied, as shown
in Fig.6, a characteristic maximum in conversion and purity
around a value of about 9 TS is found. An explanation for
this phenomenon can be derived from the concentration of
HAc in the reaction section. When HAc is feed above 9 TS,
inefficient separation of HAc from the rectifying section will
make part of HAc distillate, which causes decreasing
conversion and purity. Moreover, the concentration of HAc
in the reactive section decreases with the down-shift feed
location of HAc from 10 to 17 TS.
100%
100%
90%
80%
EtAc Purity
70%
EtOH Conversion
60%
1
3
5
7
9
11
13
15
17
Feed Stage of HAc
Figure 6. Effect of the feed stage of acetic acid on EtOH conversion and
EtAc purity
D. Effect of the molar feed ratio of HAc to EtOH
The effect of the molar feed ratio of HAc to EtOH on the
performance of the RD column is shown in Fig.7. As we all
know, the stoichiometric ratio of reactants significantly
affects the reaction conversion. If the reactant excess is too
low, product conversion is adversely limited. It is found that
as the molar feed ratio (molHAc : molEtOH ) increases from 0.5
to 2.97, the EtOH conversion increase linearly from 30% to
97%. This can be explained by the fact that with the
increased molar feed ratio, the concentration of HAc in the
reaction section increase, promoting the equilibrium reaction
to shift in the forward direction. It also can be seen that the
EtAc purity deceases rapidly when the molar feed ratio of
HAc to EtOH raise from 2.9 to 5.0. Esterification reaction of
HAc with EtOH is equimolar reaction, and excess unreacted
HAc may be distillated with EtAc from the top of the column,
resulting in reducing the EtAc purity. So the optimal mole
feed ratio of HAc to EtOH is 2.9.
REFERENCES
EtOH Conversion \ EtAc Purity(kg/kg)
100%
90%
[1]
Zhang, L.X., S.Z. Liu, H. Feng. Research progress on catalysts for
ethyl acetate[J]. Chem. Ind. and Eng. Process, 23, 1058-1061 (2004)
[2] Lai,I-K., Y.C. Liu, C.C. Yu, M. J. Lee, H.P. Huang. Production of
high-purity ethyl acetate using reactive distillation: Experimental and
start-up procedure[J]. Chem. Eng. Processing, 47, 1831–1843 (2008).
[3] Huang, H.S., K.L. Huang, B.Yang, et al. Status quo and research
progress in synthesis production technology of ethyl acetate[J]. Tech.
& Dev. Chem. Ind., 36, 11-16 (2007)
[4] Kenig, E.Y., H. Bäder, A. GÓrak, B. Beßling, T. Adrian, H.
Schoenmakers. Investigation of ethyl acetate reactive distillation
progress[J]. Chem. Eng. Sci., 56, 6185–6193 (2001,)
[5] Calvar, N., B. González, A. Dominguez. Esterification of acetic acid
with ethanol: Reaction kinetics and operation in a packed bed reactive
distillation column[J]. Chem. Eng. Processing, 46, 1317–1323 (2007)
[6] Smejkal, Q., J. Kolena, J. Hanika, Ethyl acetate synthesis by coupling
of fixed bed reactor and reactive distillation column - process
integration aspects, Chem. Eng. J., doi:10.1016/j.cej.2009.04.022
(2009)
[7] Pöpken, T., L. Götze, J. Gmehling, Reaction kinetics and chemical
equilibrium of homogenously and heterogeneously catalyzed acetic
acid esterification with methanol and methyl acetate hydrolysis[J],
Ind. Eng. Chem. Res., 39, 2601–2611 (2000)
[8] Lee, H.Y., H.P. Huang, I.L. Chen. Control of reactive distillation
process for production of ethyl acetate[J]. J. Process Control, 17, 363377 (2007)
[9] Zeng, G.B., S.J. Li, F. Qian. Dynamic simulation and control of acetic
acid dehydration system[J]. Computers and Applied Chemistry, 25(5),
533-537 (2008)
[10] Chen Y.B. Synthesis of ethyl acetate via catalytic reaction
distillation[D]. FuZhou university Master's thesis, (2007)
[11] Du Y.C., Guo J.B., Hu R.T Synthesis of n-butyl acetate by catalytic
rectification[J]. Petrochemical Technology, 2007, 36(4), 349-354
80%
70%
60%
50%
EtAc Purity
40%
EtOH Conversion
30%
20%
0
1
2
3
4
Feed ratio of HAc to EtOH (mol/mol)
5
Figure 7. Effect of the molar feed ratio on EtOH conversion and EtAc
purity
VI. CONCLUSIONS
Process simulation of the EtAc reactive distillation
columns can be performed using either the MESH distillation
equations, with appropriate additional equations to model the
chemical reaction(s). The MESH method was shown to be
accurate for the EtAc column using Aspen Plus. The
simulation results were in good agreement with experimental
results, the equilibrium stage model could describe the
distillation column profiles quantitatively and qualitatively.
Several useful results for EtAc synthesis via reactive
distillation were obtained from the sensitive analysis of
simulation columns: the feed location of ethanol in the 14
stage, the feed location of acetic acid in the 9 stage, the
molar feed ratio of acetic acid to ethanol 2.9:1, the reflux
ratio 1.25. Under the above conditions, the conversion of
ethanol was 97.16%, and ethyl acetate purity, 95.44%, was
achieved.
NOMENCLATURE
HAc
EtOH
EtAc
H2O
RD
TS
T
r
C
K
A
L
V
acetate acid
ethanol
ethyl acetate
water
reactive distillation
theoretical stage
temperature
reaction rate
concentration
the rate constant
preexponential factor
P
molar liquid flow
molar vapor flow
pressure
Ea
apparent activation energy
HL
HV
xi
yi
γi
ri
molar liquid enthalpy
molar vapor enthalpy
molar liquid concentration of component i
molar vapour concentration of component i
activity coefficient of component i
reaction rate of component i
564