Kinetic Modelling of the Transformation of Aqueous Ethanol into

A kinetic model for the transformation of aqueous ethanol into hydrocarbons on a HZSM-5 zeolite has been proposed for the 623-723 K range on the basis...
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Ind. Eng. Chem. Res. 2001, 40, 3467-3474

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Kinetic Modelling of the Transformation of Aqueous Ethanol into Hydrocarbons on a HZSM-5 Zeolite Ana G. Gayubo,* Ana M. Tarrı´o, Andres T. Aguayo, Martin Olazar, and Javier Bilbao Departamento de Ingenierı´a Quı´mica, Universidad del Paı´s Vasco, Apartado 644. 48080 Bilbao, Spain

A kinetic model for the transformation of aqueous ethanol into hydrocarbons on a HZSM-5 zeolite has been proposed for the 623-723 K range on the basis of experiments carried out in an isothermal fixed-bed reactor with feeds of ethene, ethanol-water mixtures of different mass ratios, and diethyl ether. The kinetic model is of interest for directly upgrading the liquid product obtained by the fermentation of carbohydrate wastes. It is based on the latest progress made in the catalytic transformation of methanol and quantifies the attenuating effect of water in the reaction medium in all the steps of the kinetic scheme. Introduction The liquid product obtained from the fermentation of natural carbohydrates (vegetable biomass, agroforest residues, and sugar-industry wastes) has a high water content (up to 95 wt %), which is a problem for the use of ethanol as a raw material or fuel. The operations for separating water (conventional distillation in one or two stages, azeotropic distillation, and extraction with liquid solvents or CO2) are expensive and require high capital costs for their installation on a large scale.1,2 The goal of research aimed at upgrading natural carbohydrates consists of improving the liquid product obtained by fermentation, once it has been subjected to treatment for the partial elimination of water. The upgrading method involves a catalytic transformation into hydrocarbons that are appropriate as petrochemical raw materials or as motor fuel.1,2 This latter process can be called BTG (bioethanol-to-gasoline). It is accepted that, similarly to the transformation of methanol (MTG process), the mechanism for the transformation of ethanol into ethene (as the primary product) occurs via oxonium ions as intermediate products.3-9 Likewise, similar hydrocarbons are obtained in the product streams of the catalytic transformation of the two alcohols on a HZSM-5 zeolite.10,11 Consequently, the conclusions concerning the mechanism of hydrocarbon evolution and the effect of operating conditions on the yields and composition of the products obtained in the transformation of methanol are largely applicable to the BTG process.12-16 The kinetic modeling of the MTG process has undergone great development, which has allowed for progress to be made in the design of the reactor, with the alternatives of adiabatic fixed beds17,18 and fluidized beds with catalyst circulation.19,20 In this paper, kinetic modeling of the transformation of aqueous ethanol has been studied with the aim of quantifying the formation rate of lumps that are composed of products similar to those considered in the kinetic models proposed for methanol transformation.17,22-28 In this study, it has been taken into account that, in the kinetic scheme of the catalytic transforma* Corresponding author. Tel.: 34-94-6015449. Fax: 34-944648500. E-mail: [email protected].

tion of ethanol, ethene can be considered as the true reactant, because of the high reactivity of the HZSM-5 zeolite for ethanol dehydration at the temperatures required for the formation of C5+ hydrocarbons (above 573 K). Special interest was paid to quantification of the attenuating effect of water in the reaction medium on the rates of the steps of the kinetic scheme. Studies on the catalytic transformation of aqueous ethanol have shown that, when the water content in the feed is increased, the yield of aromatics decreases and the yield of ethene increases.10,11,29-33 This result lies at the base of the BETE (bioethanol-to-ethene) process as an alternative to gasoline production.30,31,34,35 Experimental Section The catalyst was prepared by agglomerating the HZSM-5 zeolite (15 wt %) with bentonite (30 wt %) and using alumina (45 wt %) as an inert charge. The preparation of the HZSM-5 zeolite was carried out following Mobil patents and using a Si/Al ) 24.36,37 The calcination temperature of 843 K is suitable for obtaining an acid structure, and as has previously been shown,38 it is hydrothermally stable in the MTG (methanol-to-gasoline) process carried out following reactionregeneration cycles. The physical properties of the catalyst, determined by N2 adsorption-desorption in an ASAP 2000 instrument from Micromeritics, are as follows: surface area, 131 m2 g-1; pore volume, 0.43 cm3 g-1; apparent density, 1.21 g cm-3; true density, 2.53 g cm-3. The contributions of pores of different size to the total pore volume are dp < 10-3 µm (micropores), 8.1%; 10-3 µm < dp < 10-2 µm (mesopores), 14.7%; and 10-2 µm < dp < 2 µm (macropores), 77.2%. Figure 1 shows the distribution of the catalyst acid strength (upper graph) and the TPD (temperatureprogrammed desorption) curve (lower graph). These results were obtained by the adsorption-desorption of NH3 carried out in an SDT 2960 thermobalance (TA Instruments) connected on-line to a Thermostar (Balzer Instruments) mass spectrometer. These results are evidence of the uniformity of the catalyst acid structure, as the catalyst sites release similar energies in the adsorption of NH3 [between 125 and 150 kJ (mmol of NH3 at 423 K)-1]. The Bronsted/Lewis site ratio, deter-

10.1021/ie001115e CCC: $20.00 © 2001 American Chemical Society Published on Web 07/07/2001

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flow of 30 cm3 min-1 of He as an inert gas, with the aim of ensuring bed isothermicity. Kinetic Study Methodology for the Kinetic Study. The choice of the kinetic model and the calculation of the corresponding parameters were carried out by fitting the experimental data to the mass conservation equation in the reactor for each component and lump of the kinetic scheme. The expression of the mass conservation equation at zero time on stream, assuming plug flow, is

-u

∂Ci ∂u (1 - ) - Ci ) F[rio(Xi,T)] ∂z ∂z 

(1)

Defining the dimensionless longitudinal position along the reactor, ξ, eq 1 is transformed into the expression

∂Xi Z(1 - )F RT mO Z(1 - )FS ) rio(Xi,T) ) rio(Xi,T) ∂ξ FAo PM u FAo (2)

Figure 1. Catalyst acidity. Upper plot: acidic strength distribution determined by calorimetric measurement of NH3 adsorption. Lower plot: Temperature-programmed desorption (TPD) of NH3 (heating rate, 5 K min-1).

mined by FTIR analysis (with a Nicolet 740 spectrometer provided with a Spectra Tech chamber) of adsorbed pyridine, is 2.9. The reaction equipment is operated by means of a data adquisition and control program. The reactor is of stainles steel 316, with a 9-mm internal diameter and a 100-mm length. It is provided with a fixed bed of catalyst diluted with alumina as an inert species and operates in the isothermal regime. The reaction products are analyzed by gas chromatography (Hewlett-Packard 6890 instrument) by means of detectors based on thermal conductivity (TCD) and flame ionization (FID). The separation of products is carried out by means of a system made up of three columns: (1) HP-1 semicapillary column for splitting the sample into two fractions, namely, (a) volatile hydrocarbon components (C4-) and polar components (ethanol, water and diethyl ether) and (b) remaining products (C5+); (2) SUPEL-Q Plot semicapillary column for separating out individually both volatile components and polar components, which will be subsequently analyzed by TCD and FID; and (3) PONA capillary column for separation of C5+ hydrocarbons, which will be analyzed by FID. Three sets of experiments were carried out at atmospheric pressure, one by feeding ethene into the reactor, another by feeding ethanol and water with different mass ratios (corresponding to different amounts of catalyst between 0.125 and 4.0 g), and the third by feeding pure diethyl ether. This feed is diluted with a

where Xi is the mass fraction of lump i based on the organic components. To calculate the parameters of the kinetic equation, rio(Xi,T), a set of (n - 1) differential equations similar to eq 2, corresponding to the (n - 1) lumps of the kinetic scheme must be solved. For this calculation, a program was written in Fortran by using the DGEAR subroutine of the IMSL library. The composition of the remaining lump is calculated by subtraction, given that

Σ Xi ) 1

(3)

For the experiments carried out by feeding ethanol or diethyl ether and water, the water/organic components ratio, in terms of mass, is calculated as

XW )

mW mO

(4)

where the water mass flow rate in the reaction medium, mW, is the sum of the flows of the water formed as a product, mWf, and of the water in the feed, mWo

mW ) mWf + mWo ) mO

mWf + mWo mO

(5)

On the other hand, the global mass balance must be fulfilled

mT ) mO + mW

(6)

Combining eqs 5 and 6, we obtain

mWf + mWo mT mO mW ) mWf 1+ mO

(7)

In eq 7, the amount of water formed per unit mass of organic components can be calculated by means of the stoichiometry; that is, from the amount of ethanol consumed in the reaction

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1 - XA mWf ) 18 mO 46

(8)

When diethyl ether is fed

1 - XA mWf ) 18 mO 74

(9)

The calculation of the kinetic parameters was carried out by minimizing the error objective function (EOF) by means of the complex algorithm39 nl nexp

EOF )

(Xi,j - Xi(calc),j)2 ∑ ∑ i)1 j)1 nlnexp

(10)

where Xi(calc),j represents the calculated values (from the solution of the mass conservation equation, eq 2) of the weight fractions based on the organic components at zero time on stream for component i at the jth experimental point (corresponding to given values of the space time and temperature). Xi,j represents the experimental values. With the aim of reducing the correlation between the estimations of the frequency factor and of the activation energy, the variables to be optimized were reparametrized.40,41 Thus, these parameters are the kinetic constants at the reference temperature of 623 K and the activation energies. The 95% confidence intervals of the estimated parameters were obtained by nonlinear regression using the Marquardt algorithm.42 By means of this procedure, these intervals of the constants involved in the formation/disappearance of each lump were confirmed independently, with the aim of accurately analyzing the selectivity to each individual component in the kinetic scheme. Kinetic Model for the Transformation of Ethene into Hydrocarbons. With the aim of isolating the attenuating effect of water, the kinetic model was studied in two steps. In this section, the results obtained in the first set of experiments are collected. In these experiments pure ethene was fed, and consequently, there was no water in the reaction medium. The results obtained in the other set of experiments, with different water contents in the reaction medium, are analyzed in the following section. Following an evaluation of the fits of the experimental results obtained by feeding pure ethene to the different kinetic schemes, the kinetic model shown in Figure 2 is proposed as the most suitable. The method followed for step discrimination and simplification of the kinetic scheme consisted of the application of the F test for successive elimination of steps for which the confidence interval included the zero value and of the steps for which the kinetic constant was small (insignificant).43 The first step of the kinetic scheme of Figure 2 corresponds to ethanol dehydration. The second corresponds to oligomerization-cracking, whose reactant is ethene and whose products are light olefins (propene and butenes). The oligomerization-cracking-aromatization of ethene can give way to hydrocarbons of lump C5+ (gasoline) (third step of Figure 2). The gasoline lump can be obtained by olefin condensation (fourth step). Cracking of the gasoline lump can also give way to

Figure 2. Kinetic scheme for the transformation of ethanol into hydrocarbons (steps 2-7 for ethene transformation).

ethene and olefins (seventh step). Paraffins are obtained (mainly propane and butane, with a very low concentration of ethane and an insignificant concentration of methane) as a result of the oligomerization-cracking of ethene (fifth step) and cracking of the gasoline lump (sixth step). The presence of paraffins in the reaction medium as intermediate compounds (obtained by the oligomerization-cracking of ethene) has already been observed in the literature.10 The composition of lump C5+ is important in evaluating its potential as a fuel. This lump undergoes reconversion steps as a consequence of the different cracking capacities of its components. Consequently, its aromatic content increases because these are the components with lowest cracking capacity. The study of the evolution of the lump-C5+ composition with the operating conditions is beyond the scope of this paper. The scheme of Figure 2 takes into account the equimolecular formation of ethene and olefin lumps (composed of propene and butenes) by cracking of the gasoline lump. It must be pointed out that this consideration has been studied in detail, and it has been shown that the fitting of the experimental results to the scheme of Figure 2 is not improved by taking two separate steps, one for the formation of ethene and another for the formation of the olefin lump, instead of the seventh step. This regularity in the formation of alkenes of different molecular weight has also been observed in the literature.44,45 The steps of the kinetic scheme are taken as elemental, so the expressions for the rate formation of each lump, rio, needed for solving eq 2 are

rAo ) -k′1XA

(11)

rEo ) k′1XA - (k′2 + k′3 + k′5)XE + k′7XG

(12)

rOo ) k′2XE - k′4XO + k′7XG

(13)

rPo ) k′5XE + k′6XG

(14)

From the experiments carried out by feeding pure ethene into the reactor (without considering step 1 of the kinetic scheme of Figure 2) the kinetic parameters k′2 - k′7 were calculated. The suitability of the kinetic model is shown in Figure 3. The results correspond to the values calculated using the kinetic model (lines) and to the experimental values of the mass fraction of ethene and of the lumps of products (points) for two tempera-

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Figure 4. Comparison of the experimental (points) and calculated results for the mass fraction of ethene in the product stream for different feeds (ethene and ethanol/water) at 723 K. Dashed lines, those calculated by taking the effect of water into account in the kinetic model by means of eq 16. Solid lines, those calculated by taking the effect of water into account in the kinetic model by means of eq 17.

When there is water in the reaction medium, the kinetic constants k′1 - k′7 defined in the set of kinetic eqs 11-14 are, in fact, apparent constants because they depend on the water content in the feed. To quantify this dependency, an attenuation function, θ(XW), is introduced Figure 3. Mass fraction of ethene and of the lumps of products when ethene is fed at 623 K (upper graph) and 723 K (lower graph) for different values of space time. Lines, calculated with the kinetic model. Points, experimental results.

tures, 623 K (upper graph) and 723 K (lower graph), and for different values of space time. Although a study of the catalyst deactivation is beyond the scope of this paper, it is well-known that the BTG process at temperatures above 623 K undergoes an important deactivation of the catalyst by coke deposition.29,46 The coke content levels are similar to those described in the literature when ethene and propene are fed into the reactor and a HZSM-5 zeolite is the catalyst.47 By monitoring the combustion of the coke deposited with air by thermogravimetry, it has been shown that the amount of coke deposited depends on the operating conditions and that this amount is considerably attenuated by the presence of water in the reaction medium. With the aim of avoiding the effect of deactivation on the kinetic study, the experimental results used in the kinetic modeling were obtained by extrapolating to zero time on stream the results of the evolution with time on stream of the concentrations of the components in the reaction scheme. By carrying out experiments in cycles of reaction and regeneration, it has been shown that the catalyst fully recovers its activity after coke combustion. Kinetic Model for the Transformation of Ethanol into Hydrocarbons. This section deals with the results from experiments carried out with water in the reaction medium. Water is a product of the dehydration of the ethanol or diethyl ether fed into the reactor, and water is also fed into the reactor in varying proportions with ethanol.

k′i ) ki θ(XW)

(15)

Two mathematical expressions have been proposed for the θ(XW) function:

θ(XW) )

1 1 + kWXW

θ(XW) ) exp(-kWXW)

(16) (17)

In these equations, the constant kW quantifies the attenuating effect of water. This effect is explained either by a reduction in the acid strength of the active sites due to water (poisoning of Bronsted sites)32 or by a competition of water with oxonium ions in their adsorption on the acid sites, which is the explanation given by Marchi and Froment48 and Gayubo et al.21 in the MTO (methanol-to-olefins) process on SAPO-34. Equation 17 is the integrated expression of the equation

dθ(XW) ) -kWθ(XW) dXW

(18)

Consequently, the physical meaning of eq 17 is that the attenuating effect of water follows first-order kinetics with respect to the attenuation level. The results corresponding to the experiments carried out by feeding ethanol and water at different mass ratios were fitted to the proposed kinetic model. Deactivation was taken into account, and consequently, the data for zero time on stream were obtained by extrapolating the evolution of the concentrations of the components in the reaction medium with time on stream.

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The values of the error objective function, eq 10, are EOF ) 2.33 × 10-3 for eq 16 and EOF ) 1.00 × 10-3 for eq 17. Consequently, eq 17 is more suitable. To illustrate the difference in the fits of eqs 16 and 17 to the experimental results, Figure 4 shows the results of the mass fraction of ethene in the product stream at 723 K for different space times and different water contents in the feed. The results when ethene was fed are also included. The points are the experimental results, and the lines were calculated using eq 16 (dashed lines) and eq 17 (solid lines), with the value of kW corresponding to the best fit. The unsuitable fitting of eq 16 is evident for all of the results corresponding to high contents of water in the reaction medium. The values of the kinetic constants corresponding to the best fit are

[

-33 800 ((3000) 1 1 k1 ) 283 ((83) exp R T 623

(

(19)

k2 ) 2.74 ((0.34) exp

[

k3 ) 1.89 ((0.31) exp

[

k4 ) 3.93 ((1.22) exp

k5 ) 0.84 ((0.16) exp

)]

)]

-11 400 ((1200) 1 1 R T 623 (20)

(

-8650 ((1300) 1 1 R T 623

(

)]

(21)

[

)]

-5500 ((1800) 1 1 R T 623

(

(22)

[

)]

-13 800 ((2000) 1 1 R T 623 (23)

(

[

-7950 ((3000) 1 1 k6 ) 0.30 ((0.02) exp R T 623

(

(

[

kW ) 0.46 ((0.04) exp

)]

(24)

k7 ) 3.50 ((0.20) × 10-2 -22 000 ((2000) 1 1 exp R T 623

[

Figure 5. Mass fractions of ethene and of the lumps of products when ethanol is fed at 623 K and XWo) 0.045 for different values of space time. Lines, calculated with the kinetic model. Points, experimental results.

)] (25)

-1200 ((500) 1 1 R T 623

(

Figure 6. Mass fractions of ethene and of the lumps of products when ethanol is fed at 673 K and XWo ) 3.0 for different values of space time. Lines, calculated with the kinetic model. Points, experimental results.

)]

(26)

As can be seen, the effect of temperature on the constant corresponding to water attenuation, kW, is small. This result is explained by the similar influences of the temperature on the competing mechanisms of adsorption of water and the other reaction components on the active sites of the catalyst. It must be pointed out that, under the reaction conditions studied, the value of k1 is so large that it does not affect the calculation of the remaining kinetic constants. The value of k1 was calculated from a set of experiments at low reaction temperatures in which the dehydration of ethanol was studied separately because, under these conditions, ethanol and diethyl ether are present in the reaction medium in different proportions.46 Figures 5-7 show the adequate fitting of the experimental results to the kinetic model proposed for the different conditions. Each figure corresponds to a dif-

Figure 7. Mass fractions of ethene and of the lumps of products when ethanol is fed at 723 K and XWo ) 1.0 for different values of space time. Lines, calculated with the kinetic model. Points, experimental results.

ferent reaction temperature and water content in the feed. The points are the experimental results, and the lines were determined by introducing the kinetic constants calculated using eqs 19-26 into the kinetic model. As can be seen, ethanol dehydration to diethyl ether was not taken into account in the kinetic scheme, Figure

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Conclusions It has been shown that the catalytic transformation of aqueous ethanol into hydrocarbons on a HZSM-5 zeolite can be described by means of a kinetic scheme similar to the one well-known in the literature for methanol transformation, but with the incorporation of an empirical term that takes into account the attenuating effect of water in the steps of the kinetic scheme. A difference from the transformation of methanol is the fact that it is advisable to consider ethene as differentiated from the remaining components and lumps in the reaction medium. The formation of C5+ hydrocarbons, corresponding to the lump of gasoline obtained by the catalytic transformation of aqueous ethanol, requires a reaction temperature above 573 K. The yield is similar to that of the MTG process, and consequently, the results encourage subsequent studies on the economic viability of a possible industrial process for upgrading the liquid product obtained by fermentation, once it is subjected to water elimination treatment. The economy of the global process (stage 1, partial elimination of water; stage 2, catalytic transformation) implies a minimization of the requirements of the first stage and a consideration of the attenuation of the reaction steps in the second stage by the water present in the reaction medium. The kinetic model proposed in this paper, combined with subsequent studies on deactivation kinetics and catalyst regenerability, are of interest for optimization of the reaction steps and of the combined global process for upgrading the liquid product of fermentation. Figure 8. Mass fractions of ethene and of the lumps of products when pure diethyl ether is fed (upper graph) and when ethanol with XWo ) 0.045 is fed (lower graph), at 723 K and for different values of space time. Lines, calculated with the kinetic model. Points, experimental results.

2. Because this reaction is extremely fast, diethyl ether is not present in the reaction medium throughout the entire range of experimental conditions, and consequently, the reactivities of ethanol and diethyl ether cannot be quantified separately. With the aim of verifying that this step has no influence on the kinetic modeling, a series of experiments was carried out by feeding pure diethyl ether into the reactor. The production of water in diethyl ether dehydration (to form two ethene molecules) is equimolecular to the consumption of diethyl ether, as is the case in ethanol dehydration to ethene. Figure 8 shows the adequate fitting of the experimental results (points) to the values calculated using the kinetic model (lines) when pure diethyl ether is fed (upper graph) and when ethanol with XWo) 0.045 is fed (lower graph). These results correspond to 723 K and to different values of space time. The adequate fitting of the kinetic model for both feeds, each one corresponding to an individual component of the lump of oxygenates, indicates that the kinetic scheme proposed in Figure 2, in which the lump A corresponds to either diethyl ether or ethanol, is correct. In the experiments in Figure 8, the water contents in the reaction media are very similar. However, unfortunately it is not possible to make an accurate comparison to show that the kinetic behaviors are similar when diethyl ether/ water and ethanol are fed into the reactor, in terms of reaching the same water content in the reaction medium, because diethyl ether is immiscible with water.

Acknowledgment This work was carried out with the financial support of the Department of Education, University and Research of the Basque Country (Project PG98/9), and of the University of the Basque Country (Project G34-98). Nomenclature A, E, G, O, P ) oxygenates (ethanol and/or diethyl ether), ethene, lump of gasoline (C5+), lump of olefins, and lump of paraffins, respectively Ci ) concentration of lump i, mol L-1 Fo ) organic mass flow in the feed, g h-1 FAo ) mass flow rate of ethanol or diethyl ether in the feed, g h-1 FEo ) mass flow rate of ethene in the feed, g h-1 ki ) kinetic constant of step i in kinetic scheme, h-1 k′i ) apparent kinetic constant of step i in the kinetic scheme (including the attenuating effect of water present in the reaction medium), h-1 kW ) parameter that quantifies the resistance to formation of component i in the corresponding reaction step that is due to the presence of water in the reaction medium mO, mW, mT ) mass flow rates of organic components and water and total mass flow rate, respectively, g h-1 mWo, mWf ) mass flow rates of water present in the feed and produced in the reaction, respectively, g h-1 M ) average molecular weight of organic components, g mol-1 P ) partial pressure of organic components, Pa R ) gas constant, cal mol-1 K-1 rio ) rate formation of component i, g h-1 (g of catalyst)-1 S ) cross-sectional area of the reactor, m2

Ind. Eng. Chem. Res., Vol. 40, No. 16, 2001 3473 T ) temperature, K u ) gas linear velocity, m h-1 W ) catalyst weight, g Xio ) weight fraction of component i based on the organic components at zero time on stream XW, XWo ) water/organic components mass ratios in the reaction medium and in the feed, respectively Z ) total length of the reactor, m z ) longitudinal coordinate along the reactor, m Greek Letters  ) bulk porosity F ) catalyst density, g L-1 ξ ) dimensionless longitudinal coordinate along the reactor (z/Z) θ(XW) ) function for quantifying the effect of water present in the reaction medium on the steps of the kinetic scheme

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Received for review December 18, 2000 Revised manuscript received May 14, 2001 Accepted May 15, 2001 IE001115E