Kinetics of ethanol dehydrogenation into ethyl acetate

Article abstract of DOI:10.1134/S0023158414010078

The kinetics of gas-phase dehydrogenation of ethanol into ethyl acetate over a copper-zinc-chromium catalyst has been investigated in a flow reactor at pressures of 10-20 atm and temperatures of 230-290 C. For the process occurring under kinetic control, the rate constants of two reactions and the adsorption constants of five components have been determined using the Langmuir-Hinshelwood model. A kinetic model has been developed for the process. This model provides means to design a reactor for dehydrogenation of ethanol into ethyl acetate in different regimes.

Full text of DOI:10.1134/S0023158414010078

ISSN 0023ꢀ1584, Kinetics and Catalysis, 2014, Vol. 55, No. 1, pp. 12–17. © Pleiades Publishing, Ltd., 2014.
Original Russian Text © V.A. Men’shchikov, L.Kh. Gol’dshtein, I.P. Semenov, 2014, published in Kinetika i Kataliz, 2014, Vol. 55, No. 1, pp. 14–19.
Kinetics of Ethanol Dehydrogenation into Ethyl Acetate
V. A. Men’shchikov, L. Kh. Gol’dshtein*, and I. P. Semenov
OOO Tekhnologii VNIIOS, Moscow, 105005 Russia
*eꢀmail: l_goldstein@hotmail.com
Received November 27, 2012
Abstract—The kinetics of gasꢀphase dehydrogenation of ethanol into ethyl acetate over a copper–zinc–
chromium catalyst has been investigated in a flow reactor at pressures of 10–20 atm and temperatures of 230–
290°C. For the process occurring under kinetic control, the rate constants of two reactions and the adsorpꢀ
tion constants of five components have been determined using the Langmuir–Hinshelwood model. A kinetic
model has been developed for the process. This model provides means to design a reactor for dehydrogenation
of ethanol into ethyl acetate in different regimes.
DOI: 10.1134/S0023158414010078
Ethyl acetate is a very popular solvent. It is used in 30–55% at an ethyl acetate selectivity of 92–96%.
the production of food colorings, printing inks, paintꢀ The process is conducted in the gas phase.
work materials, medicines, etc. The manufacturers of
food colorings consume up to 30% of the total amount
of ethyl acetate produced. This is due to the very low
toxicity of this product: its TLV for work zone air is
200 mg/m3. The world output of ethyl acetate in 2010
was as high as ~2.6 million tons, including Russia’s
30000 t [1].
The entire ethyl acetate in Russia is produced by
acetic acid esterification with dry ethanol in the presꢀ
ence of sulfuric acid [1–3]. Most of the equipment
employed in this process is made from stainless steel
because of the high corrosiveness of acetic and sulfuric
acids. In addition, the esterification reaction yields
water, which is discharged into a sewage collector.
In order to rightly choose the type of industrial
reactor and to perform the corresponding design calꢀ
culation, it is necessary to study the kinetics of the
reaction. Experiments can be performed both in a difꢀ
ferential reactor and in an integral one. In the latter
case, for data processing it is necessary to have a speꢀ
cialꢀpurpose program taking into account the variaꢀ
tion of component concentrations along the catalyst
bed. In addition, the process should be conducted
under kinetic control, when there are no internal and
external diffusion limitations.
EXPERIMENTAL
Ethanol dehydrogenation was carried out over the
commercial catalyst NTKꢀ4 (CuO–ZnO–Cr2O3–
Al2O3)) in a reactor 18 mm in diameter having an axial
well intended for a movable thermocouple. This made
it possible to record the temperature profile along the
catalyst bed, whose height was 60 mm. The diameter
of the catalyst particles was 3–4 mm, and the correꢀ
sponding equivalent diameter was dequiv = 3.6 mm.
The reactor was placed in an electric furnace equipped
with a temperature controller. An analysis of the temꢀ
perature profile demonstrated that the catalyst operꢀ
ated at a constant temperature with a deviation of
Ethyl acetate is alternatively produced by ethanol
dehydrogenation [4], in which ethanol is the only
reactant. This process is based on the following reversꢀ
ible reactions [5]:
k1
←⎯⎯⎯
⎯⎯⎯→
(I)
C2Н5ОН
CН3CНО + Н2,
k−1
C2Н5ОН + CН3CНО
(II)
k2
←⎯⎯
⎯⎯→
CН CООC Н + Н .
3
2
5
2
k−2
The overall process is endothermic. The advantage of
this technology is that ethanol is the only reactant.
This process can also be carried out using bioethanol,
a renewable raw material, thus falling under the cateꢀ
gory of green chemistry.
Ethanol dehydrogenation proceeds efficiently over
copper–zinc catalysts [6, 7], and adding chromium to
these catalysts enhances their selectivity [7, 8]. The
optimum conditions for this reaction are the followꢀ
3°C.
The feed and product compositions were deterꢀ
mined on a Carlo Erba Strumentozione 4200 (Italy)
chromatograph (50ꢀmmꢀlong quartz capillary column
with supported OVꢀ101 phase, flameꢀionization
detector). Chromatograms were recorded and proꢀ
cessed using a Spectrophysics 4270 (Carlo Erba Struꢀ
mentazione, Italy) integrator.
ing: pressure, 10–15 atm; temperature, 230–290
°C;
volumetric flow rate of liquid ethanol, 0.6–1.5 L/h
The water concentration in the product stream was
[9]. Under these conditions, the ethanol conversion is measured on an LKhMꢀ80 (Russia) chromatograph
12

KINETICS OF ETHANOL DEHYDROGENATION INTO ETHYL ACETATE
13
fitted with a Polisorbꢀpacked column. The carrier gas
was helium.
The specific surface area and pore size distribution
of the catalyst were estimated from nitrogen adsorpꢀ
tion data.
If C G
g = 5 kg/m3, then max = 1.1 kg m–3 s. For a catꢀ
alyst bed volume of 15 mL, the maximum flow rate in
the reactor can reach 60 g/h, but it did not exceed
6.5 g/h in our experiments. Thus, there were no exterꢀ
nal diffusion limitations.
Now we will estimate the internal diffusion limitaꢀ
tions in the catalyst. The pore tortuosity coefficient for
the catalyst with aboveꢀspecified properties can be
RESULTS AND DISCUSSION
accepted to be σ ≈ 3 [10]. Ethanol transport in the catꢀ
The catalyst had the following parameters: apparꢀ
ent density, ρapp = 1.0 g/cm3; specific surface area,
×
alyst pores can occur via conventional molecular
(“bulk”) diffusion and Knudsen diffusion. The slowest
one should be involved in further calculations. The
conventional diffusion coefficient (Def,con) is deterꢀ
mined via the following formula [12]:
SBET = 77.4 m2/g; mean pore diameter, dpore = 1.3
10–8 m. The porosity of the catalyst calculated as
ϕ
=
SBETdpore/4,
(1)
was 0.25 cm3/g. The maximum ethanol dehydrogenaꢀ
Dg ϕ
−3 −1
tion rate (r) was 0.43
g cmCath .
(4)
Def,con
and its value is 0.15
=
,
σ
We preliminarily estimated the effect of external
and internal diffusion on the ethanol dehydrogenation
rate. The intensity of mass transfer between the flow
and the catalyst can be determined from the following
relationship [11]:
×
10–6 m2/s.
The Knudsen diffusion coefficient determined via
the formula [12]
2
ϕ
T
(5)
Def,Kn = 19400
,
Nuequiv =B ×Reexquiv ×Prdy,
(2)
σSBETρapp
M
wefdequivγ
where
М
is molar mass and Т is temperature (K), is
μ
where
Reequiv
=
,
Prd =
,
0.181 /s.
10–6 m2
×
μ
γDg
The formula for the effective diffusion coefficient
has the following form [12]:
βdequiv
β
is the mass transfer coefficient
Nuequiv
=
,
Dg
−1
⎛
⎞
1
1
(m/s), dequiv is the equivalent diameter of a catalyst
particle (m), Dg is the molecular diffusion coefficient
Def
0.082
=
+
,
⎜
⎟
⎠
⎝Def,con Def,Kn
10–6 m2/s.
of ethanol (m2/s),
w
ef is the flow velocity in the empty
is the flow density
is the flow viscosity (kg s m–2), and
is a
and Def
=
×
space of the catalyst (m/s),
γ
From the maximum rate of the reaction over the
catalyst and the average ethanol concentration in the
flow between the bed inlet and outlet, it is possible to
derive the parameter , which is defined as follows for
spherical particles [10]:
(kg/m3),
constant.
μ
B
The parameters of Eq. (2) depend on the Reynolds
number. Since the composition of the flow varies along
the catalyst bed, thermal properties were calculated
from mean values.
Φ
2
d
⎛
⎜
⎝
⎞
⎟
⎠
r
equiv
(6)
Φ =
.
2
×
DefCg
Under the experimental conditions, the
This parameter is 0.79
10–3, and the corresponding
reaction system had the following parameters: dequiv
3.6 g = 1.8 ef = 0.0031 m/s,
10–3 m, 10–6 m2/s,
= 11.3 kg/m3 10–5 kg s m–2, Reequiv = 0.59,
= 1.7
and Prd = 0.845.
=
×
D
,
×
w
operating efficiency of the catalyst granule bulk is ~1.
Therefore, the entire catalyst volume operates and
there are no external diffusion limitations.
γ
μ
×
For Reequiv = 0.1–2.0 and Prd = 0.6–10, the coeffiꢀ
cient of mass transfer from the flow to the catalyst surꢀ
face can be determined via the following formula [11]:
These estimates were verified experimentally.
Table 1 lists the data obtained for one catalyst sample
at
T = 270°C and P = 10 atm. The first three experiꢀ
ments were made on particles with different diameters.
Reducing the particle diameter by a factor of 10 did
increase the ethanol conversion or the reaction rate;
therefore, there were no internal diffusion limitations.
Experiments 1 and 4 were preformed at the same reacꢀ
tion time, but the liners flow velocity in experiment 4
was set to be 3 times higher than in experiment 1. This
exerted no effect on the rate of the process; there,
there were no external diffusion limitations.
Dg
0.85
equiv
βg =
0.515Re
Prd0.33
.
(3)
dequiv
Under our experimental conditions, βg = 1.55
10–4 m/s. The specific outer surface area of the cataꢀ
lysts (
) can be taken to be 1800 m2/m3. The maxiꢀ
mum amount of ethanol that can reach the catalyst
surface is
×
а
Gmax = βga(Cg − Ceq.),
where Cg is the average ethanol concentration in the
Developing a kinetic model of the process, we proꢀ
flow (kg/m3) and Ceq is the equilibrium ethanol conꢀ ceeded from Langmuir and Hinshelwood’s hypothesis
centration, which is ~0.2 Cg
.
that the components of the reaction mixture adsorb on
KINETICS AND CATALYSIS Vol. 55
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14
MEN’SHCHIKOV et al.
Table 1. Data for assessing the possibility of external and internal diffusion limitation taking place
Experiment no. Ethanol flow rate, mL/h
Catalyst particle size, mm
Bed height, cm Ethanol conversion, %
1
2
3
4
8
8
3–4
0.63–1.0
0.25–0.4
3–4
5
5
37.3
34.0
35.7
36.1
8
5
24
15
the catalyst. It was assumed that all components
adsorb on the same active sites. For reactions (I) and
(II), the time variation of the amounts of the reaction
components is described by the following system of
ordinary differential equations:
k1
3577.5
log
= −
+ 1.75logT + 1.0,
(13)
(14)
k_1
T
k2
2182.4
log
=
− 2.5.
kꢀ2
T
With the adsorption coefficients bi [13] taken into
account, the equilibrium constants will appears as
dN
1
1
(7)
(8)
= k1θ − k−1θ θ − k2θ θ + k−2θ θ ,
2
1
3
1
2
4 3
V dτ
dN
⎛ k1 ⎞ b2
1
2
K1 =
,
= −k1θ + k−1θ θ − k2θ θ + k−2θ θ ,
2
1
3
1
2
4
3
⎜
⎟
k
b b
1 3
V dτ
dN
⎝
⎠
−1
1
3
⎛ k2 ⎞ b1b2
(9)
= k1θ − k−1θ θ + k2θ θ − k−2θ θ ,
2
1
3
1
2
4
3
K2 =
.
⎜
⎟
V dτ
k
b b
3 4
⎝
⎠
−2
dN
1
The surface fraction that is active toward the
ponent is determined via the formula
i
th comꢀ
4
(10)
(11)
= k2θ θ − k−2θ θ ,
1
2
4 3
V dτ
5
dN
1
⎛
⎞
5
Ni
V
Ni
V
= 0.
(15)
θi = bi
1 + bi
,
⎜
⎜
⎝
⎟
⎟
⎠
∑
V dτ
i=1
Here, k1
ward and reverse reactions occurring according to
chemical equations (I) and (II), is time, MN is the
number of components (five), Ni is the number of
, k–1, k2, and k–2 are the rate constants of forꢀ
where bi is the adsorption coefficient of the ith compoꢀ
nent.
The rate constants of the reactions and adsorption
obey the Arrhenius relationship:
τ
moles of the
= 2, ethanol;
= 5, water),
components of the reaction mixture in the gas phase,
i
th components (
i
i
= 1, acetaldehyde;
i = 4, ethyl acetate;
i
i
= 3, hydrogen;
Ek
⎡
⎤
i
V is the flow volume occupied by all
(16)
(17)
ki = Ak exp −
,
i
⎢
⎣
⎥
⎦
R(T + 273)
and θi is the active catalyst surface occupied by the ith
Eb
⎡
⎤
i
bi = Ab exp
,
component.
i
⎢
⎣
⎥
⎦
R(T + 273)
The initial conditions are the following: at
components = 1, …, 5 Ni Ni0
τ
= 0, for
where the activation energies have dimensions of
cal/mol,
is the gas constant (1.987 cal K–1 mol–1),
and is temperature (° ).
i
=
.
R
The rate constants of the reverse reaction, k–1 and
k–2, can be eliminated from Eqs. (7)–(10) by involving
the equilibrium constants of reactions (I) and (II). The
k1/k–1 and k2/k–2 ratios can be determined via
Nernst’s approximate formula [5] using the heats of
reactions (I) and (II), which are 16360 and
T
C
The differential equations are integrated with
respect to the residence time of the flow in the catalyst
bed pf length l:
dl
dτ Sr
V
(18)
=
,
⎯
9980 cal/mol, respectively:
where
S
r (m2) is the crossꢀsectional area of the reactor
ki
−Qr
log
=
+
γ ×1.75logT + i*,
∑ ∑
(12)
with the dimensions of the thermocouple well taken
k−i 4.573T
where Qr is the heat of reaction (cal/mol), ∑γ is the
sum of stoichiometric coefficients, and
tional chemical constant.
into account (m2).
Since the overall reactions change the volume of
the system, the system of equations (7)–(11) is suppleꢀ
mented by the equation
i* is a convenꢀ
The ∑γ values for reactions (I) and (II) are 1 and 0,
respectively. The following values were accepted for
the chemical constants i*: hydrogen, 1.6; ethanol, 4.1;
acetaldehyde and ethyl acetate, 3.5. From formula
(12), we then obtain
(T + 273)MN
dNi
dV
dτ
(19)
= 22.4
,
∑
1
(
)
273P
dτ
where
Р is the absolute pressure in the reactor (atm).
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KINETICS OF ETHANOL DEHYDROGENATION INTO ETHYL ACETATE
15
Calculated concentration, wt %
Byꢀproduct concentration, wt %
9
90
80
70
60
50
40
30
20
10
0
+ 20%
– 230
– 250
– 270
– 290
°
°
°
°
C
C
C
C
8
6
4
2
0
– 20%
ethanol
ethyl acetate
acetaldehyde
0
10
20
30
40
50
Ethyl acetate concentration, wt %
0
10 20 30 40 50 60 70 80
90
Experimental concentration, wt %
Fig. 2. Total byꢀproduct concentration versus the concenꢀ
tration of the resulting ethyl acetate in the reaction mixture
at different temperatures.
Fig. 1. Calculated (solid line) versus experimental (points)
component concentrations in the reaction mixture at the
outlet of the catalyst bed.
ing differential equations were not considered. Below,
it will be demonstrated that their yield is unambiguꢀ
ously related with the ethyl acetate yield.
Because of the low flow resistance of the catalyst
bed, the pressure variation along its length can be
neglected.
The kinetic model has 14 unknown parameters,
namely, the preexponential factors of the rate conꢀ
stants and the activation energies of the two forward
reactions and the preexponential factors and activaꢀ
tion energies of adsorption of the five reaction compoꢀ
nents—ethanol, ethyl acetate, acetaldehyde, hydroꢀ
gen, and water.
In each experiment, the concentrations of ethyl
acetate, unreacted ethanol, and hydrogen and the
total concentration of byꢀproducts (butanol, hemiacꢀ
etal, butyl acetate, and acetic acid traces) were deterꢀ
mined at the reactor outlet.
In each of the 18 experiments, three product conꢀ
centrations were measured. Therefore, 14 unknowns
had to be determined from 54 data points. For this
purpose, it was necessary to carry out statistical proꢀ
cessing of the results by minimizing a functional of an
objective function (inverse problem). A possible objecꢀ
tive function is the sum of the squares of the absolute
or relative differences between the calculated and
experimental concentrations (meanꢀsquare deviaꢀ
tions):
Table 2 presents the experimental data used in the
determination of the kinetic parameters. The results of
18 experiments were processed. The amount of the
resulting hydrogen was measured in each experiment,
but this variable was not used in the calculations,
because it is directly correlated with the amount of the
resulting ethyl acetate and, therefore, is mot an indeꢀ
pendent variable. The discrepancy between the
amounts of water at the reactor inlet and outlet, which
was due to ethyl acetate hydrolysis, was within the
error of the analysis; for this treason, the inlet value
was involved in the calculations. The concentrations
of ethanol, and acetaldehyde, ethyl acetate and the
total concentration of byꢀproducts in each experiment
were used in the calculations.
L
MN
j
2
F =
F =
− Nijexp
)
[MN(L −1)],
[MN(L −1)]
(20)
∑∑(N
j=1 i=1
icalc
2
L
MN
j
j
⎛
⎞
N
icalc − Ni exp
(21)
,
∑∑⎜
⎝
j=1 i=1
Nijexp
⎟
⎠
where
= 1, …
of experiments (
i
= 1, …, 5 is the compound number
is the experiment number,
= 18), and the subscripts calc and
(
М
= 5),
L is the number
j
,
L
L
exp indicate calculated and experimental values.
Thus, in the calculations via formula (15), the conꢀ
tribution from acetaldehyde is insignificant; in the calꢀ
culations using formula (16) its contribution is domiꢀ
Since the byꢀproducts were different types of comꢀ
pounds and their yield was as low as 0.5–4.0%, their
chemical reactions and, accordingly, the correspondꢀ nant. In order to equalize, as far as possible, the conꢀ
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16
MEN’SHCHIKOV et al.
KINETICS AND CATALYSIS Vol. 55
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KINETICS OF ETHANOL DEHYDROGENATION INTO ETHYL ACETATE
17
tributions from all components to the functional
formula (16) was transformed to
F,
where Cby and CEA are the byꢀproduct and ethyl acetate
concentrations, respectively.
Thus, the kinetic relationships obtained here make
is possible to design a reactor for ethanol dehydrogeꢀ
nation into ethyl acetate in different operating
regimes.
2
L
MN
j
j
⎛
⎞
N
icalc − Ni exp
Nijexp
F =
[MN(L −1)].
∑∑δ
j=1 i=1
(22)
i
⎜
⎝
⎟
⎠
Here, δi is the weighting factor of component
following conditions were accepted:
i
. The
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A
, s–1 or cm3 mol–1 s–1;
E, cal/mol):
= 0.96
×
1010,
,
= –29500,
Ek
1
Ak
1
Ak = 0.82 ×109 Ek = − 21000,
2
2
,
,
,
AB = 6.6 EB = 2400
1
1
p. 359.
,
AB = 17.9 EB = 320
2
2
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3
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,
,
.
AB = 0.50 EB = 2360
4
4
,
AB = 102.5 EB = 830
5
5
Figure 1 presents a comparison between the calcuꢀ
lated and experimental component concentrations at
the outlet of the catalyst bed. Clearly, the deviation
does not exceed 20%.
An analysis of the experimental data concerning
byꢀproduct formation demonstrates that the byꢀprodꢀ
uct yield depends on the ethyl acetate yield and temꢀ
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obtained:
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in Heterogeneous Catalysis), Moscow: Khimiya, 1979.
Cby
6860
RT
(23)
= 1.4еxp −
,
(
)
Translated by D. Zvukov
CEA
KINETICS AND CATALYSIS Vol. 55
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2014