copper-chromite based catalyst

Catalysis Today 203 (2013) 202–210

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Kinetic study of ethanol dehydrogenation to ethyl acetate promoted by a copper/copper-chromite based catalyst G. Carotenuto, R. Tesser, M. Di Serio, E. Santacesaria ∗ University of Naples “Federico II”, Department of Chemistry, Complesso Universitario Monte S. Angelo, Via Cintia 4, IT 80126 Naples, Italy

a r t i c l e

i n f o

Article history: Received 8 November 2011 Received in revised form 2 February 2012 Accepted 24 February 2012 Available online 5 April 2012 Keywords: Bio-ethanol Ethyl acetate Dehydrogenation kinetics Copper chromite catalyst

a b s t r a c t A kinetic study of the ethanol dehydrogenation to ethyl acetate on a copper/copper-chromite catalyst has been performed. The used catalyst, in cylindrical pellets, contained also alumina as a support and barium chromate as a promoter. Support and promoter have the effect of increasing the activity, the selectivity and the stability of the catalyst, as shown in a previous work. The kinetic runs were carried out in a packed bed tubular reactor, alternatively filled with 2 or 50 g of catalyst, approximately isothermal, by feeding pure ethanol together with a mixture of nitrogen and hydrogen as carrier gas. Kinetic runs have been made by changing the temperature, in the range of 200–260 ◦ C, the pressure between 10 and 30 bar and the space time from 1 to 100 g h mol−1 . We have verified, at first, that inter-phase and intra-phase mass transfer limitations were negligible in the adopted conditions. Then, a Langmuir–Hinshelwood–Hougen–Watson kinetic model has been used for interpreting all the experimental data collected. This model corresponds to a mechanism in which the first step is the dissociative adsorption of ethanol on the surface, giving an adsorbed ethoxy group. Then, two other consecutive steps give place to respectively acetaldehyde as intermediate and ethyl acetate. This kinetic model allows a satisfactory fitting of all the performed experimental runs with a standard error below 15% for the runs performed with 2 g of catalyst and less than 12% for the runs made with 50 g of catalyst. © 2012 Elsevier B.V. All rights reserved.

1. Introduction The bio-ethanol production has grown very much in the world, in the last decade, in particular in some countries as Brazil, for example, because, it can be used as a renewable clean fuel with a low environmental impact, for a partial or total substitution of gasoline. Bio-ethanol is produced by fermentation of sugars and the main sources are today the bagasse from sugar cane and starch from maize. A challenge of the next future will be to produce bioethanol starting from second-generation raw materials such as ligno-cellulosic based materials that are not in competition with food and are much more abundant and cheap than the actual sources of fermentable sugars. Therefore, it is possible to foresee, a large availability of ethanol, as renewable energy vector, at much lower cost. In these conditions, bio-ethanol will become also an interesting building block for producing chemical commodities like ethylene, ethane, acetaldehyde, ethyl acetate, acetic acid, diethyl ether etc. and related derivatives. In this paper, we focused our attention on the possibility to obtain ethyl acetate directly in onestep from ethanol. The reaction was previously studied by some

∗ Corresponding author. Tel.: +39 081 674027; fax: +39 081 674026. E-mail address: [email protected] (E. Santacesaria). 0920-5861/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.cattod.2012.02.054

different authors [1–10] and recently by us [11]. Until now, two industrial processes were proposed [12,13]. Ethanol gives place in one-step to ethyl acetate by dehydrogenation, occurring according to the following overall stoichiometry: 2CH3 CH2 OH → CH3 COOCH2 CH3 + 2H2

(1)

As it can be seen, from this reaction both ethyl acetate and hydrogen are obtained as products. This route seems very promising, because, ethyl acetate is a largely employed solvent but it could become the raw material for obtaining acetic acid by hydrolysis and related derivatives. Copper is a good catalyst for promoting this reaction occurring in relatively mild conditions. In particular, it was observed that by operating at atmospheric pressure acetaldehyde is the main product, but moderately increasing the pressure up to 20–30 bar the selectivity is shifted toward ethyl acetate as main product [11]. Copper supported on alumina, for example, allows to obtain ethyl acetate, with interesting selectivity, in the range of 80–85% [11]. Unfortunately, these types of catalysts are subjected to fast deactivation probably due to both sintering and fouling [11]. Much higher is the selectivity obtained by using copper-chromite based catalysts that, for unsupported catalysts, can reach values of 94–95% and, very important, the catalysts are stable along the time. At last, very recently [11] we observed that a copper/copper chromite catalyst,

G. Carotenuto et al. / Catalysis Today 203 (2013) 202–210

203

Nomenclature

ethanol dehydrogenation to ethyl acetate may be described by the following reaction scheme:

r

CH3 CH2 OH → CH3 CHO + H2

(2)

CH3 CH2 OH + CH3 CHO → CH3 COOCH2 CH3 + H2

(3)

rj C0 Rp Deff Fi FE0 XE ,XEtOH Si Di εB B   kc n dp

v0 Ac U  nCi Aci z W,wcat ˛ij Nr L LBED ai N Sh i k1 k2 , k3 bi Kej Pi bi

observed reaction rate per unit particle volume [mol/(min cm3 )] reaction rate [mol/(min gcat )] reactant concentration at the external surface of the particle [mol/cm3 ] radius of the particle [cm] effective diffusivity coefficient [cm2 /min] molar flow rate of component i [mol/min] inlet ethanol flow rate [mol/min] ethanol conversion [–] selectivity to component i [–] molecular diffusivity of ethanol in nitrogen [cm2 /min] porosity of the bed [–] bulk density of the catalyst bed [g/cm3 ] tortuosity factor [–] constriction factor [–] mass transfer coefficient [cm/min] reaction order [–] pellets diameter [cm] volumetric flow rate [cm3 /min] cross sectional area of the reactor tube [cm2 ] superficial velocity = v0 /Ac [cm/min] kinematic viscosity [cm2 /min] number of carbon atoms in component i [–] chromatographic area of component i [–] dimensionless bed length [–] catalyst weight [g] stoichiometric coefficient of component i in reaction j [–] number of reactions [–] axial coordinate [cm] length of the catalytic bed [cm] conversion or selectivities [–] number of experimental measurements [–] Sherwood number [–] catalytic site occupied by component i [–] kinetic constant of reaction j [mol/(g h atm)] [mol/(g h atm2 )] adsorption parameters [atm−1 ] equilibrium constant of reaction j [–] partial pressure of component i [atm] adsorption constant of component i [atm−1 ]

Subscripts EtOH ethanol acetaldehyde AcH EA ethylacetate hydrogen H2

containing alumina as support and barium chromate as promoter, increases very much the selectivity level at 98.5–99%, maintaining a satisfactory activities (conversion about 65%) and showing high stability to sintering and fouling. We have explained, in a previous work, the reasons of this behavior also thanks to a deep investigation on the surface characteristics of the mentioned catalysts [11]. In the present paper, we have focused our attention on the study of the kinetics of the reaction by using the mentioned copper/copper chromite mentioned catalyst in the perspective of the process development. As reported in a previous work [11], the

That is, the formation of ethyl acetate is the consequence of two consecutive reactions, with the production of acetaldehyde as intermediate step. As explained in the already mentioned previous work a high selectivity to ethyl acetate is favored by a fast rate of reaction (3) draining the acetaldehyde concentration. This point is very important because acetaldehyde can give place, through the aldolic condensation, to several possible by-products lowering the selectivity and favoring catalyst deactivation by fouling. A reasonable complete reaction network has been proposed by Inui et al. [8] and is reported in the simplified Scheme 1. Our kinetic investigation starts from this scheme and from the reaction mechanisms proposed in the literature [4] with the aim to find the most reliable kinetic laws. According to a previous ethanol dehydrogenation to acetaldehyde kinetic study, made by Tu et al. [3], on unsupported Cu and Cr promoted Cu, this reaction should be of first order with respect to ethanol and should have an apparent activation energy of 12.2 kcal/mol. However, to our knowledge, only a paper has been published concerning the kinetics of ethyl acetate formation on Cu based catalysts, in particular on Cu/copper chromite catalyst [4]. Colley et al. [4] have demonstrated that, in a first step, ethanol adsorbs on the Cu component of Cu/Cr2 O3 catalysts, as an ethoxy specie, with an activation energy of 7.41 kcal/mol and, successively, the adsorbed ethoxy specie, is dehydrogenated to an acetyl specie with an activation energy of 22.50 kcal/mol. Finally, the ethoxy and acetyl species react each other to form adsorbed ethyl ethanoate that finally desorbs with an activation energy of 43.06 kcal/mol. In the present paper, we have studied the kinetics of this reaction performed on a commercial copper/copper chromite catalyst supported on alumina and promoted with barium chromate. In the kinetic approach, we have tested many different kinetic models but the best fittings have been obtained by applying a Langmuir–Hinshelwood–Hougen–Watson (LHHW) model based on a dual site adsorption. 2. Experimental 2.1. Employed catalysts and pre-treatments The commercial catalyst (BASF Cu-1234-1/16-3F), employed in the kinetic study of ethanol dehydrogenation and supplied by BASF, is a pre-reduced copper/copper chromite catalyst supported on alumina and containing BaCrO4 as promoter. The catalyst composition, provided by the supplier, is CuCrO4 /CuO/Cu/BaCrO4 /Al2 O3 (45:1:13:11:30% b.w.). As this is a pre-reduced catalyst, BaCrO4 , very probably, is present in a reduced form that is, as barium chromite or BaO/Cr2 O3 . Moreover, the presence of a low amount of CuO is due to the surface oxidation of Cu metal made by the supplier for stabilizing the catalyst before the use. The catalyst shape is characterized by cylindrical extrudates with 1.8 mm of diameter and irregular heights of 3–5 mm. Before the use in the kinetic runs the catalyst was submitted to a pretreatment consisting in a reduction with hydrogen, mixed with nitrogen (H2 /N2 = 6/94 mol/mol) and fed with a flow rate of 25 cm3 /min, keeping constant the temperature at 200 ◦ C, for 16–18 h. This treatment with hydrogen reduces CuO to Cu and about 50% of Cu2+ of the copper chromite to Cu0 according to the following reaction suggested by different authors [4,11,14]. Cu2+ + H2 ↔ 2H+ + Cu0

(4)

204

G. Carotenuto et al. / Catalysis Today 203 (2013) 202–210

Scheme 1.

Despite the Cu2+ reduction, the spinel structure of copper chromite is not destroyed, thanks to the formation of H+ inside crystalline lattice [4,14]. In Table 1 the main characteristics of the used catalyst are summarized. 2.2. Catalytic tests The catalytic tests were performed in a stainless steel packed bed reactor. A weighed amount of catalyst, in pellets (2 g) was charged in the reactor and, before the catalytic tests, was submitted to the already mentioned pretreatment with a flow stream of H2 –N2 mixture (H2 /N2 = 6/94 mol/mol) of 25 cm3 /min for about 16–18 h. The reaction was carried out at different temperatures in the range 200–260 ◦ C and different pressures in the range 10–30 bar. The kinetic laws, derived from the interpretation of the runs performed with 2 g of catalyst, have been verified by using more catalyst (50 g) in the same reactor. The liquid ethanol reactant (Fluka 99.8% b.w.) was vaporized at a temperature of 200 ◦ C into a pre-heater filled with inert material (glass balls) and mixed with the carrier flow constituted by the same mixture H2 /N2 used for the pre-treatment. The reaction was always conducted in the presence of a hydrogen stream. The ethanol space time W/F, for the runs made on 2 g of catalyst, has been taken in the range 1–20 g h mol−1 , where W and F are respectively the catalyst weight and the ethanol molar flow rate. After the reachment of steady state conditions (about 2–3 h were necessary), the unreacted ethanol and the condensable obtained products were collected, for about 30 min, in a trap cooled with liquid nitrogen. The obtained reaction products were injected and analyzed by a gas-chromatograph HP 6890, equipped with a Restek Rt-Q Plot 30 m × 0.32 mm column, pure hydrogen has been used as carrier gas. The conditions used for the analyses were as follows: the initial temperature of 80 ◦ C was increased at a rate of 10 ◦ C/min up to 220 ◦ C and then maintained at this value for 10 min. The flame ionization detector (FID) was kept at 280 ◦ C. The split–splitless injector was kept at 250 ◦ C. Three different sets of runs were performed in order to evaluate the effect of the

temperature, the pressure and the ethanol space time on the catalysts performances. Results are reported in terms of ethanol conversions and products selectivity. The ethanol conversion is defined as: XEtOH =

mols EtOHreacted mols EtOHfed



= 1−



1



(nCi /nCETOH )(Aci /AcEtOH )

(5)

While, the selectivities, determined on the basis of a carbon balance for each component, are determined as: Si = =

nCi mols producti formed × nCEtOH nEtOH reacted nCi Aci nCEtOH AcEtOH

1 − X

EtOH

XEtOH



(6)

where nCi and nCEtOH represent respectively the numbers of carbon atoms in the component i and in the ethanol fed, while Aci and AcEtOH are the normalized chromatographic peaks areas. The measured response factors for the main reaction products are: fcEtOH = 1, fcAcH = 1.5, fcAcOEt = 2.1, fcOthers = 2. 3. Results and discussion 3.1. Catalytic results Many different kinetic runs have been performed by changing the temperature, the pressure and the space time in the range previously mentioned. In correspondence with each experimental run different samples of the gaseous outlet mixture were withdrawn, condensed, and analyzed by the GC method previously described, to evaluate, when the reactor reached steady state conditions, the ethanol conversion and products yields. For each experiment, performed in steady-state conditions, three samples have been analyzed and the results averaged. All the collected data

3.90 83.84 2.05 BASF Cu-1234

CuCr2 O4 CuO Cu BaCrO4 (45–1–13–11–3% b.w.)

Al2 O3 127

0.41

1.22

12.27

20Å < r < 1000 A˚ r < 20 A˚

Cu surface area (m2 /g of catalyst) Composition given by the supplier Sample

Table 1 Main textural characteristics of the used catalyst.

Surface area (m2 /g)

Pores volume (cm3 /g)

Copper dispersion %

Pore distribution, vol.%

r > 1000 A˚

G. Carotenuto et al. / Catalysis Today 203 (2013) 202–210

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Table 2 Operative conditions and experimental results for runs performed by changing the pressure and the temperature of reaction but keeping constant the ethanol space time W/FEtOH = 4 g h mol−1 , nitrogen (5.7 × 10−2 mol/h) and hydrogen (4 × 10−3 mol/h) flow rates. 2 g of catalyst have been used in all the performed runs. T (◦ C)

P (atm)

XEtOH (%)

SACOEt (%)

SAcH (%)

SOthers (%)

200 200 200 220 220 220 240 240 240 260 260 260 200 200 200 220 220 220 240 240 240 260 260 260 200 200 200 220 220 220 240 240 240 260 260 260

10 10 10 10 10 10 10 10 10 10 10 10 20 20 20 20 20 20 20 20 20 20 20 20 30 30 30 30 30 30 30 30 30 30 30 30

16.16 15.47 16.79 25.94 24.55 23.07 39.80 38.59 40.62 46.46 42.18 47.26 10.60 10.89 10.24 25.33 25.27 24.29 32.08 31.99 31.74 42.79 39.76 42.18 17.30 16.94 17.03 20.89 20.30 22.64 25.47 23.60 23.21 35.35 28.76 29.40

62.79 59.38 63.16 67.60 66.99 66.94 70.64 68.21 71.78 70.21 70.84 70.33 81.07 85.73 82.96 79.18 80.19 77.59 78.37 78.70 77.00 80.81 80.29 80.54 76.02 77.68 79.85 80.23 80.08 84.41 70.36 70.39 70.58 73.74 72.14 70.55

37.21 40.62 36.84 26.35 27.02 27.09 21.10 24.55 22.22 21.84 21.80 21.61 18.93 14.27 17.04 16.85 16.46 18.93 17.27 16.94 18.25 12.14 13.59 13.32 20.9 18.58 16.85 15.63 15.20 12.12 25.02 23.15 24.89 19.45 22.17 23.49

0.01 0.01 0.01 6.05 5.99 5.97 8.26 7.24 6.00 7.95 7.36 8.06 0.01 0.01 0.01 3.97 3.35 3.48 4.36 4.36 4.75 7.05 6.12 6.14 3.10 3.70 3.30 4.12 4.72 3.46 1.38 6.46 4.53 6.81 5.69 5.96

were submitted to mathematical regression analysis. In Table 2, the results obtained in the runs performed at different temperatures (200–260 ◦ C) and pressures (10–30 bar) but keeping constant both the ethanol space time W/F = 4 g h mol−1 (ethanol flow rate = 0.5 mol/h), the nitrogen (5.7 × 10−2 mol/h) and hydrogen (4 × 10−3 mol/h) flow rates, are reported. In Table 3, the results obtained, by changing the ethanol space time W/FEtOH from 1 to 20 g h mol−1 , keeping constant the temperature at 220 ◦ C, the pressure at 20 bar and the nitrogen (5.7 × 10−2 mol/h) and hydrogen (4 × 10−3 mol/h) flow rates, are summarized. At last, in Table 4, the results of the kinetic runs performed, at a constant temperature of 220 ◦ C and pressure of 20 bar, by changing the ethanol space time from 1 to 20 W/FEtOH

Table 3 Operative conditions and experimental results for runs performed by changing the ethanol space time from 1 to 20 W/FEtOH (g h mol−1 ), keeping constant the temperature at 220 ◦ C, the pressure at 20 bar and nitrogen (5.7 × 10−2 mol/h) and hydrogen (4 × 10−3 mol/h) flow rates. W/FEtOH (g h mol−1 )

FEtOH (mol/h)

XEtOH (%)

SACOEt (%)

SAcH (%)

SOthers (%)

1 1 4 4 4 20 20

1.5 1.5 0.5 0.5 0.5 0.1 0.1

13.74 13.63 25.33 25.27 24.29 46.02 47.35

53.61 53.13 79.18 80.19 77.59 90.62 90.25

40.37 40 16.85 16.46 18.93 5.21 5.59

6.02 6.87 3.97 3.35 3.48 4.17 4.16

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G. Carotenuto et al. / Catalysis Today 203 (2013) 202–210

Table 4 Operative conditions and experimental results for runs performed by changing the ethanol space time from 1 to 20 W/FEtOH (g h mol−1 ), and the inlet hydrogen flow in the range 7.31 × 10−4 –3.66 × 10−3 mol/h, keeping constant the temperature at 220 ◦ C and the pressure at 20 bar. FEtOH (mol/h)

W/FEtOH (g h mol−1 )

FN2 (mol/h)

FH2 (mol/h)

X (%)

SACOEt (%)

SAcH (%)

SOthers (%)

0.1 0.1 0.1 0.1 0.1 0.1 0.5 0.5 0.5 0.5 0.5 0.5 0.5 1.5 1.5 1.5 1.5 1.5 1.5

20 20 20 20 20 20 4 4 4 4 4 4 4 1 1 1 1 1 1

0.057 0.057 0.034 0.034 0.011 0.011 0.057 0.057 0.057 0.034 0.034 0.011 0.011 0.057 0.057 0.034 0.034 0.011 0.011

3.66E−03 3.66E−03 2.19E−03 2.19E−03 7.31E−04 7.31E−04 3.66E−03 3.66E−03 3.66E−03 2.19E−03 2.19E−03 7.31E−04 7.31E−04 3.66E−03 3.66E−03 2.19E−03 2.19E−03 7.31E−04 7.31E−04

46.02 47.35 46.77 46.77 33.26 34.33 25.33 25.27 24.29 19.09 19.27 16.90 15.79 13.74 13.63 8.70 8.85 8.41 8.74

90.62 90.25 84.17 83.07 75.90 72.50 79.18 80.19 77.59 63.96 63.99 61.97 63.58 53.61 53.13 46.66 44.41 37.81 36.46

5.21 5.59 11.87 13.35 17.24 21.69 16.85 16.46 18.93 29.38 30.29 32.60 31.06 40.37 40.00 53.34 55.59 62.19 63.54

4.17 4.16 3.96 3.58 6.86 5.81 3.97 3.35 3.48 6.66 5.72 5.42 5.36 6.02 6.87 – – – –

(g h mol−1 ) and the inlet hydrogen flow rate (from 7.31 × 10−4 to 3.66 × 10−3 mol/h) have been reported.

3.3. Evaluation of the eventual external and internal diffusion limitation

3.2. Reactor model

Intra-particles transport effect has been analyzed by applying the Weisz and Prater criterium [15]. To ensure  > 0.95 in an isothermal spherical particle for a first-order reaction, the criteria requires:

The collected kinetic data, at different pressures, temperatures and space times, were interpreted by a mono-dimensional plug flow reactor model, for which an isothermal condition was assumed. This assumption is justified by the relatively low ethanol conversion obtained with 2 g of catalyst and by the high thermal capacity of the reactor body. Considering a mass balance for each component, this can be advantageously written in dimensionless form by introducing the dimensionless length of the bed, z, defined as z = L/LBED , as it follows:

 dFi = wcat ˛ij (rj ) dz Nr

(7)

J=1

The system of ordinary differential equations (7) has been numerically integrated using ODE45 function of MATLAB and a nonlinear least squares fitting has been simultaneously performed for the determination of the adjustable models parameters by using the following objective function that was minimized by mathematical regression analysis: 2 1  exp (ai − acalc ) RMS = i N n

(8)

i=1

In this relation aexp and acalc are respectively related to both experimental and calculated conversions and selectivities. The overall average error has been calculated by using the following expression:



exp  N  1  aji − acalc ji err = 100 exp N aji 4

j=1



(9)

i=1

where j indicates an index for respectively ethanol conversion, acetaldehyde selectivity, ethyl acetate selectivity and other byproducts selectivity.

rRp2 C0 Deff

<1

(10)

For power-law kinetics of different orders the criteria respectively of Weisz [16] (11) and Hudgins [17] (12) the following relation can be applied. rRp2

C0 Deff rRp2 C0 Deff

< 6 zero order < 0.6 first order < 0.3 second order

1 < n

(11)

(12)

For a second order kinetics a limit of 0.3 can be adopted according to Weisz criterium (11) and 0.5 according to Hudgins (12). Therefore, In our experiments we can exclude the internal diffusive limitations, because, we have found a maximum value of 0.15 for both the mentioned criteria (11) and (12). The average reaction rate r has been evaluated for all the performed runs with the following relationship: r=

FE0 XE (1 − εB )(wcat /B )

(13)

While the calculation of the effective diffusivity from molecular diffusion coefficient, porosity, tortuosity and constriction factor has been reported in Eq. (14). Deff = Di

ε ×   

(14)

By considering the properties reported in Table 5 and calculating the ratio of the Weisz and Prater criterium for the highest observed reaction rate we obtained a value of 0.15, that is, largely below the limit of 1. In conclusion internal diffusion limitation is not operative. Then, eventual external mass transfer limitation has been evaluated by adopting the Mears criterium [18]. rεb Rp n < 0.15 kc C0

(15)

G. Carotenuto et al. / Catalysis Today 203 (2013) 202–210

A

1.0

B

1.0

0.8

Calculated selectivities

Calculated conversion

0.8

207

0.6

0.4

0.2

0.6

0.4

Ethylacetate Acetaldehyde

0.2

Others

0.0

0.0

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.0

0.1

0.2

Experimental conversion

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Experimental selectivities

Fig. 1. (A and B) Parity plots related to respectively the ethanol conversion and selectivities by considering a catalyst bed of 2 g.

The mass transfer coefficient kc appearing in Eq. (15) has been evaluated by using the Thoenes–Kramers correlation [19]: 1 − εB kc = εB



Di dp



Sh

(16)

As in the previous case, in correspondence of the highest average reaction rates, we calculated a value for the expression (16) of less than 0.1, that is, also in this case, we can conclude that the external mass transfer has not influence. By concluding, our runs have been performed in chemical regime. 3.4. Equilibrium constants estimation The main occurring equilibrium reactions and the correspondent enthalpies are: CH3 CH2 OH ↔ CH3 CHO + H2

16.45 kcal/mol

(17)

for determining the equilibrium compositions and then the corresponding gas phase equilibrium constants have been estimated. Both the effects of pressure and of the non-ideality of the gaseous mixtures on equilibrium have been evaluated by adopting the PSRK equation of state. Our kinetic runs have been performed in a narrow range of temperature, therefore, we have applied the Van’t Hoff equation for evaluating the dependence of the equilibrium constants by the temperature (20). Kei = Keref × exp

−10.37 kcal/mol

5.98 kcal/mol

Tref

1 T



(20)

B T

(21)

(18)

The values of the constants A and B are the following: reaction (17) A = 16.5, B = −9136.4, reaction (18) A = −4.79, B = 4386.0. These expressions have directly been used in the developed kinetic model.

(19)

3.5. Kinetic approach and reaction mechanism

By adding the two reactions we obtain the overall reaction: 2CH3 CH2 OH ↔ CH3 COOCH2 CH3 + 2H2

R

−

The equilibrium constants have been estimated determining the outlet composition of an equilibrium reactor in ASPEN by considering the two main reactions, one related to acetaldehyde (Ke1 ) and the other one to ethylacetate (Ke2 ) formation. The dependence of the equilibrium constants on the temperature has been expressed by the following Eq. (21): ln Kei = A +

CH3 CH2 OH + CH3 CHO ↔ CH3 COOCH2 CH3 + H2

  1 Hi

As it can be seen, the overall reaction (19) is moderately endothermic and this justifies the assumption of isothermicity for a short catalytic bed. To evaluate the equilibrium constants of reaction (17) and (18) the method of Gibbs energy minimization has been used Table 5 Properties used for evaluating internal and external diffusion effects. Properties

Symbol

Value

Bed porosity Bulk density Tortuosity Constriction factor Molecular diffusivity EtOH in N2 Pellets diameter Cross sectional area of the tube Kinematic viscosity Assumed reaction order

εB B (g/cm3 )   Di (cm2 /min) dp (cm) Ac (cm2 )  (cm2 /min) n

0.4 0.95 0.4 0.8 6.12 0.26 2.54 0.0092 2

The kinetic approach has been developed on the following reaction scheme, considering three different main reactions: CH3 CH2 OH → CH3 CHO + H2

(22)

CH3 CH2 OH + CH3 CHO → CH3 COOCH2 CH3 + H2

(23)

2CH3 CHO → otherproducts

(24)

A first kinetic evaluation was done by using the following empirical power laws:



ˇ



1 PAcH PH2 1− ˛ ke1 PEtOH

r1 =

˛ k1 PEtOH

r2 =

ı ε k2 PEtOH PAcH







ω 1 PAcH PH2 1− ke2 P ı P ε EtOH A

(25)

 (26)

208

G. Carotenuto et al. / Catalysis Today 203 (2013) 202–210

A

1.0

B

Calculated selectivities

Calculated conversion

0.8

0.6

0.4

0.2

0.0 1.0

0.2

0.4

0.6

0.8

1.0 1.0

0.8

0.8

0.6

0.6

0.4

0.4

Ethylacetate 0.2

0.2

Acetaldehyde Others

0.0 0.0

0.2

0.4

0.6

0.8

1.0

0.0 0.0

0.2

Experimental conversion

0.4

0.6

0.0 1.0

0.8

Experimental selectivities

Fig. 2. (A and B) Parity plots related respectively to the ethanol conversion and selectivities by considering a catalyst bed of 50 g. 2 r3 = k3 PAcH

(27)

The best values obtained for the reaction orders ˛, ˇ, ε, , , ı,

suggested us a reaction mechanism in which the adsorption of both reactants and products plays an important role, that is, the powerlaw model suggested kinetic expressions with the partial pressures of different species at the denominator. According to the literature the adsorption of hydrogen on copper surface can occur by both an associative and/or a dissociative mechanism [20,21]. In the case associative adsorption, molecular hydrogen adsorbed on a single atom gives place to a weakened hydrogen–hydrogen bond and adsorbed hydrogen is more reactive. According to some authors [22] also the dissociative mechanism on two adjacent sites is possible, as normally occurs for other metals (palladium, platinum etc.). On the basis of these considerations, two models have been developed, one based on the dissociative adsorption (dual site mechanism DSM) and the other based on the molecular associative one (Langmuir–Hinshelwood–Hougen–Watson; LHHW). For example, the dual sites model, involving two catalytic sites, is based on a mechanism for the acetaldehyde formation, as it follows: CH3 CH2 OH(g) + 2 0 ⇔ CH3 CH2 O(a) + H(a)

(28.a)

CH3 CH2 O(a) +  0 ⇔ CH3 CHO(a) + H(a)

(28.b)

CH3 CHO(a) ⇔ CH3 CHO(g) +  0

(28.c)

2H(a) ⇔ H2 + 2 0

(28.d)

A similar approach can be developed also for the mechanism of ethyl acetate formation. CH3 CHO(a) +  0 ⇔ CH3 CO(a) + H(a)

(29.a)

CH3 CO(a) + CH3 CH2 O(a) ⇔ CH3 COOC2 H5 (a)

(29.b)

CH3 COOC2 H5 (a) ⇔ CH3 COOC2 H5 (g) + 2 0

(29.c)

At last, the by-products formation can be described through the following elementary reactions: CH3 CHO(a) + CH3 CHO(a) ⇔ CH3 CH(OH)CH2 CHO(a)

(30.a)

CH3 CH(OH)CH2 CHO(a) ⇔ CH3 CH(OH)CH2 CHO(g) + 2 0

(30.b)

By considering the two first mechanisms, i.e., the formation of respectively acetaldehyde and ethyl acetate, different rate determining steps (RDS) have been considered giving place to different

rate laws. The best performances have been obtained by considering as RDS for respectively the formation of acetaldehyde, ethyl acetate and others by-products, the three reactions (28.c), (29.c) and (30.b). This dual-site model has shown satisfactory performances in terms of average error (17%) but some of the parameters found were not meaningful. In particular, some activation energies resulted close to zero. Moreover, this model and all the others, based on the previously reported mechanisms, were not able to describe with a good accuracy the experimental data collected by using 50 g of catalyst (close to equilibrium). Then, we tested the performances of a LHHW model, based on the molecular associative hydrogen adsorption mechanism. The acetaldehyde formation is described through the following elementary steps: C2 H5 OH +  0 ⇔  EtOH EtOH + 0 → AcH + H2

(31.a) rate determining step

(31.b)

 AcH ⇔  0 + CH3 CHO

(31.c)

H2 → 0 + H2

(31.d)

Then, for the reaction to ethyl acetate we suggest the following mechanism C2 H5 OH +  0 ⇔  EtOH

(32.a)

CH3 CHO +  0 ⇔  AcH

(32.b)

EtOH + AcH ⇔ EA + H2

rate determining step

(32.c)

 EA ⇔ 0 + CH3 COOC2 H5

(32.d)

H2 ⇔ 0 + H2

(32.e)

In this case, the rate determining step should be the dual sites reaction between two adsorbed molecules of respectively ethanol and acetaldehyde to give adsorbed ethyl acetate and hydrogen. The third reaction to other by-products (24), occurring in a very small amount, between two molecules of adsorbed acetaldehyde to give other by-products, has been described in an approximated way by a second order not reversible reaction. Therefore, on the basis of the described mechanisms, the following kinetic rate laws can be derived (33)–(35): r1 =

k1 bEtOH PEtOH (1 − (1/Ke1 )(PAcH PH2 /PEtOH )) (1 + bEtOH PEtOH + bAcH PAcH + bH PH + bEA PEA )

2

(33)

G. Carotenuto et al. / Catalysis Today 203 (2013) 202–210 Table 6 Kinetic parameters of the LHHW dual site model determined by regression analysis on all the experimental runs performed by using the reactor loaded with 2 g of catalyst. Kinetic constants

Activation energy (kcal/mol)

97.1 ± 6.8 (mol/(gcat h atm)) 0.089 ± 9.8E−3 (mol/(gcat h atm2 )) 0.0011 ± 7.8E−4 (mol/(gcat h atm2 ))

k1-220 ◦ C k2-220 ◦ C k3-220 ◦ C

Adsorption parameters

r2 =

Adsorption enthalpy (kcal/mol)

10.4 ± 0.83 (atm−1 ) 98.4 ± 12.79 (atm−1 ) 41.2 ± 4.94 (atm−1 ) 2.5 × 10−4 ± 3.50E−5 (atm−1 )

bEtOH-220 ◦ C bAcH-220 ◦ C bEA-220 ◦ C bH-220 ◦ C

36.25 ± 4.35 12.95 ± 0.65 1.6E−4 ± 1.8E−05

−25.53 −7.02 −13.91 −13.34

± ± ± ±

2.55 0.35 0.14 1.47

k2 bEtOH bAcH PEtOH PAcH (1 − (1/Ke2 )(PEA PH2 /PEtOH PAcH )) (1 + bEtOH PEtOH + bAcH PAcH + bH PH + bEA PEA )

2

2 r3 = k3 PAcH

(34) (35)

In Table 6, all the kinetic parameters determined by regression analysis are reported. The equilibrium constants Ke1 and Ke2 have been calculated, as described in Section 3.4. From the kinetic parameters reported in Table 6 it is possible to observe that k3 has a very low value and a negligible value of the activation energy. This is the consequence of: (i) the approximation introduced by considering a pseudo-second order rate law; (ii) the fact that reaction (3) corresponds to an ensemble of different reactions considered as acetaldehyde that gives “others”; (iii) the very low amount of byproducts found corresponding to a low precision in the analytical determination. The parameters values are acceptable with the exclusion of the parameter related to the adsorption of hydrogen. However, the obtained value can be justified taking into account the fact that the catalyst is pre-reduced with hydrogen and very probably almost

209

all the catalytic sites are initially occupied by adsorbed hydrogen. Moreover, hydrogen is fed with the reagents. For these reasons a correct evaluation of this parameter is difficult in the described conditions. On the other hand, the co-feeding of hydrogen during the reaction is fundamental for limiting the acetaldehyde formation that is a prerequisite to increase the selectivity to ethyl acetate and preventing the acetaldehyde condensation to by products. Moreover, the presence of hydrogen prevents the catalyst fouling and consequent deactivation during the reaction [11]. Then, it is known, from the literature, that the presence of hydrogen is beneficial preventing the copper sintering [23]. Another unsuitable parameter is the activation energy of the reaction giving by-products. It is clear that in this case the assumption of second order kinetics and the difficulty of analytical response for very low concentrations of these substances are responsible of the poor result. However, the average standard error resulted less than 15% and the validity of the developed model can be appreciated in Fig. 1A and B in which are reported the parity plots for respectively the ethanol conversion and the selectivity to acetaldehyde, ethyl acetate and others.

3.6. Use of the kinetic model for the simulation of other kinetic runs As mentioned in a previous section, some kinetic runs have been performed by charging the same tubular reactor with 50 g of catalyst instead of 2 g. In this case, two different consecutive ovens have been used for heating the reactor with the aim to obtain as much as possible an isothermal profile along the catalytic bed. This has been experimentally verified and a variation of not more than ±5 ◦ C has been observed along the reactor. In Table 7 some of the performed runs are reported together with the adopted experimental conditions. In the same table are also reported for comparison experimental and calculated values of ethanol conversions and

Table 7 Comparison of experimental and calculated data by using LHHW model. The runs have been performed by using 50 g of catalyst, temperature range: 200–260 ◦ C, pressure range: 10–30 bar, at two different space time of 34.4–97.32 g h mol−1 , keeping constant the nitrogen (5.7 × 10−2 mol/h) and hydrogen (4 × 10−3 mol/h) flow rates. Operative conditions ◦

Experimental data 3

T ( C)

P (bar)

FEtOH (cm /min)

W/F (g h mol

200 220 240 260 200 200 220 220 240 240 260 200 220 220 240 260 200 220 240 260 200 220 240 260 200 220 240 240

10 10 10 10 20 20 20 20 20 20 20 20 30 30 30 30 10 10 10 10 20 20 20 20 30 30 30 30

0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5

97.32 97.32 97.32 97.32 97.32 97.32 97.32 97.32 97.32 97.32 97.32 97.32 97.32 97.32 97.32 97.32 34.40 34.40 34.40 34.40 34.40 34.40 34.40 34.40 34.40 34.40 34.40 34.40

−1

)

Calculated data with the LHHW model

XEtOH

SAcOEt

SAcH

SOthers

XEtOH

SAcOEt

SAcH

SOthers

45.26 51.14 61.8 69.7 49.58 48.63 57.33 60.54 64.83 64.33 70.63 54.29 61.3 63.21 63.21 67.17 34.81 40.5 59.94 60.38 35.32 47.21 61.17 67.54 35.93 42.37 60.87 59.19

96.19 96.1 96.53 93.41 98.79 98.3 99.33 98.76 99.58 93.43 94.25 98.95 98.94 96.84 96.84 96.04 78.18 86.91 84.05 84.18 93.53 96.91 96.1 95.23 94.79 96.08 95.8 96.66

3.18 2.9 1.11 1.52 1.21 1.28 0.66 1.23 0.42 0.47 0.52 11 0.39 0.59 0.56 0.8 10.58 6.31 6.3 4.88 5.21 1.92 1.8 1.75 3.68 1.99 2.11 1.47

0.63 1 2.36 5.07 0 0.42 0.01 0.01 0 6.1 5.23 1.05 0.67 2.57 2.6 3.16 11.24 6.78 9.65 10.94 1.26 1.17 2.1 3.02 1.53 1.93 2.09 1.87

50.60 63.28 70.13 74.00 43.73 43.73 55.68 55.68 63.29 63.29 67.93 43.73 51.19 51.19 59.16 64.34 35.90 50.43 63.84 72.42 30.86 43.26 55.78 65.33 27.85 39.10 51.01 51.01

97.27 96.74 95.33 92.78 96.97 96.97 95.99 95.99 94.03 94.03 90.58 96.97 94.99 94.99 92.36 87.80 94.78 94.81 94.44 93.13 95.01 94.31 93.22 91.28 94.55 93.29 91.51 91.51

1.74 1.91 2.68 4.16 1.29 1.29 1.44 1.44 1.97 1.97 2.97 1.29 1.23 1.23 1.64 2.42 3.98 3.66 3.64 4.36 2.82 2.75 2.81 3.27 2.36 2.36 2.43 2.43

0.99 1.35 1.99 3.06 1.74 1.74 2.57 2.57 4.00 4.00 6.45 1.74 3.78 3.78 6.00 9.79 1.24 1.53 1.92 2.51 2.17 2.94 3.97 5.45 3.08 4.36 6.06 6.06

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G. Carotenuto et al. / Catalysis Today 203 (2013) 202–210

1,0

Conversion and selectivities

0,9 0,8

50g

2g 0,7 0,6 0,5 0,4 0,3

LHHW Model SAcH

0,2

SAE XEtOH

0,1 0,0 0

20

40

60

80

100

-1

W/F (ghmol ) Fig. 3. Conversion and selectivities obtained for different space times. This plot has been obtained by considering all data collected in both the reactors containing respectively 2 and 50 g of catalyst working at 220 ◦ C, 20 bar with a constant flow of a mixture of 6% H2 in N2 of 25 cm3 /min that correspond to an hydrogen flow of 3.77 × 10−3 mol/h and nitrogen flow of 0.057 mol/h.

selectivities of respectively ethyl acetate, acetaldehyde and other by-products. Calculations have been made with the LHHW described model using the kinetic parameters reported in Table 6 and the agreement obtained is very satisfactory as it can be appreciated in Table 7 where experimental and calculated data are reported for comparison. In Fig. 2A and B are reported the parity plots of respectively conversions and selectivities for the runs performed with 50 g of catalyst. However, the results obtained in these runs correspond, very probably, to equilibrium conditions. This can be appreciated in Fig. 3, in which the profiles of respectively conversions and selectivities are reported as a function of the space time. As it can be seen, the runs made with 50 g of catalyst show the approaching to a plateau for both conversion and selectivities. For this reason these runs have not been considered together with the ones performed with 2 g of catalyst in the regression analysis but are used here to verify the model. The average standard error in simulating these runs is about 12%. 4. Conclusions In a previous work [11] we have shown that the ethanol dehydrogenation to ethyl acetate occurs on a copper/copper chromite catalyst, supported on alumina and promoted with barium chromate, with a conversion of 65–70% and a selectivity of 98–99% in the optimal conditions that are: 220 ◦ C, 20 bar and W/F = 100 g h mol−1 . A so high selectivity suggests the use of the mentioned catalyst in a new competitive process not requiring any post-treatment as for example the hydrogenation of co-produced acetaldehyde to

ethanol to be added to the ethanol recycle stream. A purge would be sufficient to maintain steady state conditions in the reactor. In this paper, the kinetics of the occurring reactions have been studied for the scope of reactor modeling and optimization. A Langmuir–Hinshelwood–Hougen–Watson kinetic model has been used for interpreting all the kinetic runs performed, that is, 62 runs performed in different operative conditions by using a tubular reactor filled with 2 g of catalyst and 28 runs made by using 50 g of catalyst. It has been shown that the runs with the lowest amount of catalyst have been performed in chemical regime and have been used to identify the best kinetic model, while, the runs performed with 50 g of catalyst give data that are near the equilibrium conditions and allow to verify both the model goodness and the validity of the equilibrium constants. The obtained agreements are satisfactory, considering the approximations introduced as the assumption of isothermal condition and the use of the equilibrium constants derived from theoretical calculations. At last, the model is based on a reliable mechanism and the kinetic parameters show physical mean. References [1] B.N. Dolgov, M.M. Koton, N.V. Siderov, Journal of General Chemistry of the USSR 6 (1936) 1456. [2] Y.J. Tu, Y.W. Chen, C. Li, Journal of Molecular Catalysis 89 (1994) 179–189. [3] Y. Tu, J. Li, C.Y.W. Chen, Journal of Chemical Technology and Biotechnology 59 (1994) 141–147. [4] S.W. Colley, J. Tabatabaei, K.C. Waugh, M.A. Wood, Journal of Catalysis 236 (2005) 21–33. [5] N. Iwasa, N. Takezawa, Bulletin of the Chemical Society of Japan 64 (1991) 2619–2623. [6] D.J. Elliot, E. Pennella, Journal of Catalysis 119 (2) (1989) 359–376. [7] K. Inui, T. Kurabayashi, S. Sato, Applied Catalysis A: General 237 (2002) 53–61. [8] K. Inui, T. Kurabayashi, S. Sato, N. Ichikawa, Journal of Molecular Catalysis A: General 216 (2004) 147–156. [9] K. Inui, T. Kurabayashi, S. Sato, Journal of Catalysis 212 (2002) 207–215. [10] A.B. Gaspar, F.G. Barbosa, S. Letichevsky, L.G. Appel, Applied Catalysis A: General 380 (2010) 113–117. [11] E. Santacesaria, G. Carotenuto, R. Tesser, M. Di Serio, Chemical Engineering Journal 179 (2012) 209–220. [12] S.W. Colley, C.R. Fawcett, M. Sharich, M. Tuck, D.J. Watson, M.A. Wood, US Patent 7,553,397, B1 June 30, 2009. [13] G. Carotenuto, M. Di Serio, E. Santacesaria, R. Tesser, New process for the production of ethylacetate and pure hydrogen from ethanol, Italian Patent NA2010A000009, 2010; WO2011/104738A2 assigned to EUROCHEM Engineering. [14] A.A. Khasin, T.M. Yur’eva, L.M. Plyasova, G.N. Kustova, H. Jobic, A. Ivanov, Yu.A. Chesalov, V.I. Zaikovskii, A.V. Khasin, L.P. Davydova, V.N. Parmon, Russian Journal of General Chemistry 78 (11) (2008) 2203–2213. [15] P.B. Weisz, C.B. Prater, Advances in Catalysis 6 (1954) 143. [16] P.H. Weisz, Zeitschrift fur Physikalische Chemie Neue Folge 11 (1957) 1. [17] R.R. Hudgins, Chemical Engineering Science 23 (1968) 93. [18] D.E. Mears, Industrial & Engineering Chemistry Process Design and Development 10 (4) (1971). [19] J.B. Butt, Reaction Kinetics and Reactors Design, 2001. [20] A.I. Seryhk, V.B. Kazansky, Physical Chemistry Chemical Physics 6 (2004) 5250–5255. [21] X.-J. Kuang, X.-Q. Wang, G.B. Liu, Journal of Chemical Sciences 123 (September (5)) (2011) 743–754. [22] L. Triguero, U. Wahlgren, P. Boussard, P. Siegbahn, Chemical Physics Letters 237 (1995) 550. [23] J.R. Lewis, H.S. Taylor, Journal of the American Chemical Society 60 (4) (1938) 877–879.