Deactivation Kinetics for Direct Dimethyl Ether Synthesis on a CuO

Ind. Eng. Chem. Res. 2010, 49, 481–489

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Deactivation Kinetics for Direct Dimethyl Ether Synthesis on a CuO-ZnO-Al2O3/γ-Al2O3 Catalyst Irene Sierra, Javier Erenã,* Andre´s T. Aguayo, Martin Olazar, and Javier Bilbao Departamento de Ingenierıá Quı´mica, UniVersidad del Paı´s Vasco, Apartado 644, E-48080 Bilbao, Spain

A kinetic model has been established for the deactivation of a CuO-ZnO-Al2O3/γ-Al2O3 catalyst in the direct synthesis of dimethyl ether. The model allows calculating the effect of the operating conditions and the evolution of component concentration in the reaction medium (dimethyl ether, methanol, paraffins, H2, CO, CO2, and H2O) with time on stream. The results of the model fit the experimental results in a wide range of conditions: 225-350 °C; 10-40 bar; space time, 0.1-68.0 (g of catalyst) h (mol of reactants)-1; molar ratio (H2/CO) in the feed, 2-4; time on stream, 30 h. The model considers the cause of deactivation to be coke deposition on the metallic function and that this coke is formed by degradation of oxygenates (dimethyl ether and methanol). The effect of water in the reaction medium has been quantitatively considered in the kinetic model, whose drawback is the attenuation of the methanol synthesis rate and whose benefit is the attenuation of deactivation by coke deposition. The kinetic model is useful for predicting the influence of water in the feed with syngas as a strategy to attenuate deactivation, which is insignificant for a H2O/(H2 + CO) molar ratio of around 0.20, although it causes a slight decrease in initial catalyst activity. 1. Introduction Syngas (obtained from coal, natural gas, or biomass) is set to supplement oil in the next decades to meet the increasing demand for automotive fuels and petrochemical raw materials. Dimethyl ether (DME) has aroused great interest because it is an eco-friendly fuel (sulfur-free and with a low emission of particles and NOx) which may substitute oil LPG as a domestic fuel and may be used as an additive for gasoline and automotive diesel.1–4 Furthermore, DME is an intermediate raw material (alternative to methanol) for the production of hydrocarbons (light olefins, BTXE, conventional fuels), chemicals (dimethyl sulfate, methyl acetate), and H2, by means of catalytic steam reforming.5–9 Furthermore, DME has low toxicity and its average lifetime in the atmosphere is approximately 5 days; consequently, it is also used as an aerosol propellant to replace chlorofluorocarbons (CFCs).10 The production of DME from syngas in a single-step process (STD process) on a bifunctional catalyst shifts the thermodynamic equilibrium of methanol synthesis (due to the lower concentration of methanol in the reaction medium), and consequently, the STD process may be carried out with high yields of DME at lower pressure and higher temperature than methanol synthesis.11,12 The advantage is that CO2 cofeeding requires a lower pressure than in methanol synthesis.13–15 Great attention has been paid in the literature to the formulation of the catalyst with CuO-ZnO-Al2O3 being the metallic function, which has been commonly used for methanol synthesis and whose preparation and properties are key factors for catalyst activity.16–18 The catalyst acid function exceeds that required for methanol dehydration to enhance the aforementioned thermodynamic advantage. A high selectivity to DME requires moderate strength of the acid function sites to avoid secondary reactions of DME transformation into hydrocarbons. This moderate acid strength (mainly Lewis acid sites and a few Brønsted sites) is attained with γ-Al2O3,16–20 HZSM-5 zeolites with high Si/Al ratio or partially sodium ones,21,22 microporous zeolites with a high Si/Al ratio,23–26 and with SAPO ones.27 * To whom correspondence should be addressed. Tel.: 34-946015363. Fax: 34-94-6013500. E-mail: [email protected].

The possible causes of catalyst deactivation are Cu sintering and mainly coke deposition.19,28–30 Furthermore, attention has also been paid to the attenuation of activity by water adsorption on the acid functions of a hydrophilic nature.19 The cause of deactivation is related to the composition of the catalyst, and if this is prepared with an excess of acid function, the deposition of coke on this function has no bearing on catalyst activity. Likewise, operating below 325 °C (limit temperature to avoid sintering of Cu-Zn function), this cause of deactivation is avoided. These hypotheses have been proven in a previous paper for a CuO-ZnO-Al2O3/γ-Al2O3 catalyst, based on runs in which the metallic and acid functions have been used separately in the single-step process and in the individual reactions of methanol synthesis and methanol dehydration. The results of evolution with time on stream of product yields and properties of catalyst functions (particularly metallic dispersion and acidity) allowed establishing that the deactivation cause is the deposition of coke on the metallic function. The coke initially deposits on Cu-ZnO sites and on the interphase between these sites and the support (Al2O3), and as time on stream elapses, it is also deposited on the support. A significant deposition on the acid function (γ-Al2O3) is observed only for very high coke contents.31 The interpretation of the kinetic results in the literature is hindered by the complexity of the kinetic scheme, in which the products of each step are the reactants in the following one. Consequently, a kinetic model is required for reactor design and the optimization of operating conditions. Kinetic modeling has been approached in the literature for the single-step process at zero time on strean,32,33 by combining the kinetic equations previously proposed for both individual reactions, i.e., methanol synthesis and methanol dehydration to DME.34–37 Nevertheless, the single-step process is carried out under different conditions (pressure and temperature) from those used in the individual reactions. Pressure is lower and temperature is higher than in methanol synthesis, and methanol dehydration is carried out at room temperature. Furthermore, the composition of the reaction medium in the single-step process is different due to the aforementioned synergisms between individual reactions. To fill

10.1021/ie900978a  2010 American Chemical Society Published on Web 11/19/2009

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Ind. Eng. Chem. Res., Vol. 49, No. 2, 2010

this gap, a specific kinetic model has been established in a previous paper, which considers the following steps for DME synthesis:38 hydrogenation of CO: k1

CO + 2H2 798 CH3OH

[

rMeOH ) k1 fCO fH22 -

(1)

fCH3OH

]

θ

(2)

2CH3OH 798 CH3OCH3 + H2O

(3)

K1

dehydration of methanol to DME: k2

[

rDME ) k2 fCH3OH2 -

fCH3OCH3 fH2O K2

]

(4)

formation of CO2 (water gas shift): k3

CO + H2O 798 CO2 + H2

[

rCO2 ) k3 fCO fH2O -

fCO2 fH2 K3

(5)

]

(6)

synthesis of hydrocarbons (C1-C10 paraffins): k4

nCO + (2n + 1)H2 798 CnH2n+2 + nH2O

[

rHC ) k4 fCO fH23 -

fHC fH2O K4

]

(7)

(8)

θ

Equations 2, 4, 6, and 8 have been established by considering elementary reactions. The stoichiometry of hydrocarbon (HC) formation in eq 8 corresponds to that of methane formation, given that methane is the main paraffin. The cause of paraffin formation from H2 + CO (eq 7) is due to the metallic function capacity for synthesis under the high pressure used in the process, which has been experimentally proven. A term (θ) has been included in the rates for the steps of methanol synthesis (eq 2) and hydrocarbon formation (eq 8) to quantify the attenuation in the rate of these steps by the adsorption of water on the catalyst: θ)

1 1 + KH2O fH2O

(9)

This attenuation is explained by the adsorption competence of water and reactants on the metallic sites or by the adsorption of water on Al2O3 (support of this function), given that this support has a major influence on the activity of Cu-Zn sites.17 This great influence of water content on the kinetics of the process explains why the kinetic equations do not need adsorption terms for reactants and products, given that their influence on the attenuation of reaction kinetics is lower than that of water. The KH2O constant in eq 9 depends on temperature as follows:

[

)]

∆HH2O 1 1 (10) R T 548 Given that eqs 9 and 10 are empirical, the relationship between the parameter ∆HH2O and water adsorption heat is not straightforward, although this parameter is related to this heat and to the hydrophilic nature of the Al2O3 that supports the metallic function. KH2O ) KH2O* exp

(

Furthermore, the direct synthesis of methanol by CO2 hydrogenation has been proven to have an insignificant rate.38 The validity of the kinetic model described has been extended in this paper by incorporating a new kinetic equation for deactivation, which will allow a kinetic model applicable to any value of time on stream. This model considers two opposing effects created by the water in the reaction medium, which are, on the one hand, the unfavorable attenuation of methanol synthesis (apart from the shift of methanol dehydration equilibrium) and, on the other, the attenuation of deactivation by coke deposition on this function.19,31,38 2. Experimental Section 2.1. Equipment and Reaction Conditions. The reaction equipment used (PID Eng. & Tech. Microactivity) has been described elsewhere and is provided with a fixed bed, which allows operating at high temperatures and pressures.38 Online product analysis has been carried out by means of a Varian CP4900 gas microchromatograph. The reaction mixture is passed through several purifiers to eliminate possible traces of oxygen, water, and iron carbonyl that may deactivate the catalyst. Runs have been carried out by feeding H2 + CO under the following conditions: 225-375 °C (only the results up to 350 °C have been used in the calculation of the kinetic parameters, given that irreversible deactivation occurs above this temperature); pressure, 10-40 bar; space time, 0.1-68.0 (g of catalyst) h (mol of reactants)-1; H2/CO molar ratio in the feed, 2-4; time on stream, 30 h; catalyst particle size, 0.4 mm. The calculation of contact time for different H2/CO molar ratios in the feed is based on the fact that the reactant molar flow rate in the feed is maintained constant, 1 mmol of (H2 + CO) min-1. Consequently, the contact time (h-1) is the result of multiplying the inverse of space time ((g of catalyst) h (mol of reactants)-1) by each one of the factors 10.67, 8.50, and 7.20, corresponding to the values of H2/CO molar ratio of 2/1, 3/1, and 4/1, respectively. Under the operating conditions used, diffusional restrictions are not significant, which are more important when the process is carried out in the liquid phase.38 The microchromatograph is provided with three columns: (i) molecular sieve MS5 (10 m × 12 µm) to separate H2, CO, and CH4; (ii) Porapak Q, PPQ (10 m × 20 µm), which separates CO, CH4, C2H6, C3H8, C4H10, methanol, and DME; (iii) CPSil (8 m × 2 µm) to separate C4-C10 fraction. The response factor of each component has been determined by means of mixtures prepared with known compositions. 2.2. Catalyst. The bifunctional catalyst, CuO-ZnO-Al2O3/ γ-Al2O3, is composed of a metallic function for hydrogenating CO to produce methanol (CuO-ZnO-Al2O3), and an acid function for the dehydration of methanol to DME (γ-Al2O3). The metallic function has been prepared by coprecipitating the corresponding metallic nitrates with Na2CO3 at pH 7.0.39 The γ-Al2O3 acid function has been prepared by coprecipitation of a NaAlO2 suspension with HCl at 70 °C until the pH reaches a value of 9.0. The following steps are the aging of the catalyst at 70 °C for 1 h, filtering, washing, drying (at 20 °C and at 120 °C for 12 h each), and calcination (550 °C, 2 h). This function has low acid strength to minimize the transformation of methanol and DME into light olefins and heavy hydrocarbons, which may undergo degradation to produce coke. This catalyst has been prepared by mixing the dry metallic function and the acid function in an aqueous solution, using a mass ratio of 2:1, which is the ratio that provides the best catalyst performance.15 Furthermore, this mass ratio between the metallic function and the acid one is appropriate for ensuring that


483

methanol synthesis is the limiting step, with a rapid transformation of methanol into DME. Subsequent to the mixing of metallic and acid functions, the suspension is centrifuged and the solid is washed, dried (in two steps, at 20 and 120 °C for 12 h each step), and calcined (300 °C, 6 h). The Cu:Zn:Al atomic ratio determined by X-ray fluorescence (Philips, Minipal PW 4025) is 2.1:1.0:2.8. The metal surface is 11.7 m2 (g of catalyst)-1 and has been determined by N2O chemisorption (Micromeritics AUTOCHEM 2920 online with a Balzers Instruments Omnistar mass spectrometer). The physical properties, determined by N2 adsorption-desorption (Micromeritics, ASAP 2000) are BET surface area, 125.3 m2 g-1; pore volume, 0.23 cm3 g-1; and mean pore diameter, 74.5 Å. By combining thermogravimetry and calorimetry of NH3 adsorption at 150 °C (SETARAM TG-DSC 111 online with a Balzers Thermostar mass spectrometer),40 a total acidity of 0.03 (mmol of NH3) (g of catalyst)-1 and acid strength of 100 kJ (mol of NH3)-1 are determined, with a peak at 250 °C in the temperature programmed desorption curve. Prior to use, the bifunctional catalyst has been subjected to an equilibration treatment by oxidation-reduction in the reactor itself, which consists in exposing it to a H2 stream diluted in He (at 10%) at 200 °C for 14 h and, subsequently, to a second stream (at 20%) for 1 h at 300 °C. This process has already been optimized and its aim is to reduce the CuO contained in the metallic function to Cu0, which is the component of higher activity in the methanol synthesis step.41 3. Results 3.1. Deactivation Cause. The analysis of coke composition and evolution of properties of metallic and acid functions with time on stream has established that deactivation is produced by metallic site blockage by coke, which is presumably formed by the degradation of methoxy groups.31 These methoxy groups (very reactive) are formed from methanol and DME on Al2O3 (metallic function support) and are favored by the acidity of this support. As coke is being deposited, there is a progressive blockage of first the metallic sites, then the Al2O3 that supports these sites, and finally the acid function (γ-Al2O3), although high coke content is required for γ-Al2O3 blockage and, consequently, for its deactivation. It should be noted that the deactivation due to CuO-ZnO metallic function deterioration is a consequence of the well-known relationship between its activity and a suitable dispersion on Al2O3, which is highly affected by coke.17 The effect of operating conditions on deactivation confirms the origin of the coke.15,22 Figure 1 shows as an example the effect of space time on the evolution of DME, methanol, and paraffin yields with time on stream. The remaining operating conditions are constant. The yields have been calculated as the ratio between the organic carbon molar flow rate in the product stream and the CO molar flow rate in the feed: YDME )

2nDME 100 (nCO)0

(11)

YMeOH )

nMeOH 100 (nCO)0

(12)

YHC )

∑n i

Cini

(nCO)0

100

(13)

where (nCO)0 is the molar flow rate of CO in the feed, nDME and nMeOH are the molar flow rates of DME and methanol in the

Figure 1. Evolution with time on stream of the yields of DME, methanol, and paraffins for different values of space time: (a) 2.6, (b) 6.4, and (c) 12.8 (g of catalyst) h (mol of reactants)-1. Reaction conditions: temperature, 275 °C; pressure, 30 bar; H2/CO molar ratio in the feed, 3/1.

reactor outlet stream, nCi is the carbon atom number of each paraffin, and ni is the molar flow rate of these paraffins. Figure 1 shows that paraffin formation by Fischer-Tropsch (FT) is the prevailing reaction for very low values of space time (Figure 1a). As space time is increased (Figure 1b,c), the ratio between DME and paraffin yields increases for zero time on stream. Of these C1-C10 paraffins, over 90 mol % are C1-C6 paraffins. As time on stream is increased under these conditions of high DME selectivity, deactivation by coke in the methanol synthesis step hinders DME formation, whereas the capacity for paraffin formation according to the FT reaction is unaffected. Consequently, DME yield steadily decreases with time on stream, whereas that of paraffins increases. Coke formation presumably occurs by the degradation of methoxy groups due to the

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dehydrogenating capacity of the metallic function for activating coke formation steps such as condensation and dehydrocyclization.42 3.2. Kinetic Models Proposed for Deactivation. Previous papers have shown that deactivation is a consequence of coke deposition on the catalyst metallic function.19,31 Consequently, the methanol synthesis rate (eq 2) decreases with time on stream, which is considered by introducing an activity term:

[

rMeOH ) k fCO fH22 -

fCH3OH K

]

(14)

θa

Activity, a, is defined as the ratio between the reaction rates at t time and zero time on stream: rMeOH a) (rMeOH)0

kd′ ) kdθd

Table 1. Kinetic Equations Proposed for the Deactivation and the Corresponding Values of the Error Objective Function (eq 23) kinetic equation

Φ

1

-

da ) kd′a dt

(16)

7.51

2

-

da ) -kd′(fCO)a dt

(17)

6.24

3

-

da ) kd′(fMeOH + fDME)a dt

(18)

5.13

4

-

da ) kd′(fHC)a dt

(19)

6.74

(20)

Once the fitting has been tested by using different expressions for the θd function, the following hyperbolic expression has been adopted as suitable: θd )

(15)

The remaining steps of the kinetic scheme are assumed not to be directly affected by coke deposition. Several expressions of separable variables and first order for deactivation have been considered to describe the relationship between activity and operating conditions (Table 1). These equations correspond to different alternatives for coke origin: (i) Model 1 considers a deactivation kinetics that is independent of concentration. No precursor is identified and temperature is the main variable. (ii) Model 2 considers that coke is formed in parallel with methanol synthesis, and consequently from CO reactant, according to a kinetics depending on its concentration. (iii) Model 3 considers that deactivation occurs in series with the main reaction kinetic scheme, with a rate that is dependent on oxygenate compound concentration. This model considers that methanol/DME is degraded to evolve toward coke constituents, presumably through methoxy ion intermediates formed at the interphase between the metallic function and Al2O3 support.31 (iv) Model 4 assumes deactivation kinetics dependent on the concentration of paraffins formed as a byproduct. Consequently, deactivation also occurs in series with the main reaction step. The attenuating effect of water in the reaction medium on coke deposition has also been proven, which is a fact that has also been confirmed in previous papers dealing with the transformation of methanol/DME into gasoline (MTG process) on HZSM-5 zeolite catalysts,43 and with the transformation of methanol into olefins (MTO process), on SAPO-3444 and SAPO18 catalysts.45 The attenuating effect of water is explained by its adsorption on the acid sites, in which it competes with the intermediate compounds in the steps of coke evolution on acid

model

catalysts, namely, oligomerization, cyclization, aromatization, and polycondensation. As in these processes, this effect has been taken into account in the kinetic modeling by considering the deactivation kinetic constant, kd′, in the models shown in Table 1, as an apparent kinetic constant defined as the product of the real kinetic constant, kd, by a function, θd, that quantifies the attenuating effect of water in the reaction medium:

1 1 + (KH2O)d fH2O

(21)

The constant (KH2O)d depends on temperature according to the following equation:

[

(KH2O)d ) (KH2O)d* exp

(∆HH2O)d 1 1 R T 548

(

)]

(22)

where (KH2O)d* is the value of the constant at a reference temperature (275 °C). 3.3. Methodology for Kinetic Parameter Calculation. The methodology used for calculating the evolution of concentrations with time on stream has consisted in simultaneously solving the mass conservation equations for each component with the kinetic equations at zero time and the corresponding kinetic equation for deactivation. The models and kinetic parameters of best fit have been obtained by minimizing an error objective function (Φ) that compares the experimental and calculated results: nv

Φ)

∑ i)1

nv

wiφi )

∑ w (y * - y ) i

i

i

2

(23)

i)1

where wi and φi are, respectively, the distribution weighting factor and the sum of square residuals for each i dependent variable, nv is the number of dependent variables of the model to be fitted, yi* is the vector of experimental molar fraction values for i, and yi is the vector of molar fraction values calculated by solving the mass balance in the reactor. The quality of the chromatographic analyses is noteworthy, which have provided a mass balance closure above 98% in all the runs used for the kinetic study. This rigorous calculation method is required for considering the catalyst’s “past history” and avoiding the errors of approximate calculation methods. This requirement is especially appropriate when the kinetic model is dependent on component concentration and reaction schemes are complex.46–48 The calculation of kinetic parameters for each model has been carried out by means of a program written in MATLAB, in which experimental data and operating conditions (composition of each component for different temperatures, pressures, space times, and reaction times) are introduced and the kinetic parameters of best fit are calculated by multivariable nonlinear regression. The calculation has been restricted to the parameters of the deactivation equation, given that the values determined in a previous paper and shown in Table 2 have been adopted for reaction rates at zero time on stream.38 The high value of the activation energy for methanol dehydration (352.21 kJ mol-1) is explained because it is an apparent activation energy, given that it is a reaction following a mechanism with several steps.49 Bercic and Levec also obtained a high value (143.70 kJ mol-1) for γ-Al2O3 pellets by feeding pure methanol under


485

38

Table 2. Parameters for the Zero Time on Stream Kinetic Model parametera -1

value -1

-1

-3

k1*, molMeOH (molH2) gcat h bar k2*, molDME (molH2)-1 gcat-1 h-1 bar-2 k4*, molHC (molH2)-1 gcat-1 h-1 bar-4 E1, kJ mol-1 E2, kJ mol-1 E4, kJ mol-1 KH2O*, bar-1 ∆HH2O, kJ mol-1 a

(3.37 ( 0.04) × 10-6 1.57 ( 0.09 (2.81 ( 0.09) × 10-7 112.79 ( 2.68 352.21 ( 7.87 227.27 ( 7.20 1.04 ( 0.05 276.10 ( 11.92

For a parameter, an asterisk (*) indicates the value at 275 °C.

Table 3. Parameters for the Deactivation Kinetic Model parametera -1

-1

kd*, h bar Ed, kJ mol-1 (KH2O)d*, bar-1 (∆HH2O)d, kJ mol-1 a

value (3.91 ( 0.24) × 10-2 71.80 ( 3.90 2.43 ( 0.22 42.97 ( 2.38

For a parameter, an asterisk (*) indicates the value at 275 °C.

Figure 3. Comparison of experimental results (points) and those calculated with the kinetic model (lines) for the evolution of the molar fraction of DME (a) and paraffins (b) at the reactor outlet with time on stream for different pressures. Reaction conditions: temperature, 275 °C; H2/CO molar ratio in the feed, 4/1; space time, 8.4 (g of catalyst) h (mol of reactants)-1.

Figure 2. Comparison of experimental results (points) and those calculated with the kinetic model (lines) for the evolution of the molar fraction of DME (a) and paraffins (b) at the reactor outlet with time on stream for different temperatures. Reaction conditions: pressure, 40 bar; H2/CO molar ratio in the feed, 4/1; space time, 8.4 (g of catalyst) h (mol of reactants)-1.

internal diffusion control.36,37 It has also been proven that the activation energy increases as water is fed with methanol,50 and this paper specifically approaches the kinetics of DME synthesis with high water contents in the reaction medium. 3.4. Kinetic Parameters. The results of the error objective function (eq 23) are set out in Table 1 for the fittings corresponding to the deactivation kinetic models proposed. A comparison of the results for models 1-4 shows that the one of poorer fitting to the experimental results is model 1, corresponding to independent deactivation. Model 2 (which assumes deactivation in parallel to the main reaction) and model

4 (series deactivation with paraffins as coke precursors) fit similarly, although model 2 fits slightly better. Model 3, which assumes series deactivation and oxygenate compounds (methanol and DME) as coke precursors, is the one of best fit. This result confirms the conclusions obtained by Erenã et al. concerning coke origin by degradation of methoxy ions formed at the interface between the metallic function and Al2O3 support.31 It is essential to consider a function to account for the attenuation of deactivation due to the presence of water in the reaction medium. When this function, defined in eq 21, is not included, the value of the error objective function (eq 23) is 6.24 instead of 5.13. The values of the kinetic parameters of best fit are set out in Table 3 for the model chosen as more suitable (model 3), which considers catalyst deactivation as a consequence of coke generation from oxygenate compounds in series with the main reaction. The kinetic constants are calculated at a reference temperature of 275 °C. 3.5. Effect of Operating Conditions on Deactivation. The kinetic model established allows calculating the evolution of component concentration in the reaction medium with time on stream in a wide range of conditions. Figures 2-5 compare the experimental results (points) with those calculated using the model (lines) for the evolution of the molar fraction of DME and paraffins at the reactor outlet with time on stream. It should be noted that the experimental conditions correspond to a significant deactivation, as required for a kinetic study. The comparison of experimental and calculated results shows that the fit is satisfactory in a wide range of conditions and for a reaction that is very sensitive to these conditions. Figure 2

486


Figure 4. Comparison of experimental results (points) and those calculated with the kinetic model (lines) for the evolution of the molar fraction of DME (a) and paraffins (b) at the reactor outlet with time on stream for different feed compositions. Reaction conditions: temperature, 275 °C; pressure, 40 bar; space time, 8.4 (g of catalyst) h (mol of reactants)-1.

corresponds to different temperatures and shows a significant increase in DME yield as temperature is increased from 250 to 275 °C (Figure 2a), as well as an increase in paraffin yield as temperature is increased from 275 to 325 °C (Figure 2b). As mentioned above, the decrease in DME yield with time on stream occurs simultaneously to a significant increase in paraffin yield. When the effect of pressure is studied (Figure 3), a steady increase in DME yield is observed as pressure is increased, whereas paraffin yield (very low at 20 bar) significantly increases when the operation is carried out at 30 bar. It is noteworthy that deactivation is faster as pressure is increased, due to the enhancement of coke condensation reactions. There is an optimum for a H2/CO molar ratio in the feed of around 3/1 to maximize DME yield (Figure 4a), which also corresponds to the maximum DME selectivity, due to the smaller influence of this ratio on the formation of paraffins (Figure 4b). Deactivation attenuates as H2/CO molar ratio is increased from 3/1 to 4/1, due to the inhibition of coke formation by a higher H2 concentration. An increase in space time has a relevant effect of increasing DME yield (Figure 5a) and decreasing paraffin yield (Figure 5b). Figure 5a shows that, as space time is increased, the decrease in DME yield with time on stream is more pronounced, which is consistent with a deactivation that occurs in series with the main reaction in which the cause of coke formation is the product DME. Special attention should be paid to the effect of water in the feed as a strategy for attenuating deactivation (Figure 6). First, a suitable fit is achieved between the experimental results (points) and those calculated with the model (lines) for the

Figure 5. Comparison of experimental results (points) and those calculated with the kinetic model (lines) for the evolution of the molar fraction of DME (a) and paraffins (b) at the reactor outlet with time on stream for different values of space time. Reaction conditions: temperature, 275 °C; pressure, 40 bar; H2/CO molar ratio in the feed, 3/1.

evolution of the molar fraction of DME (Figure 6a) and paraffins (Figure 6b) at the reactor outlet with time on stream. Deactivation is significant without water in the feed. Deactivation is insignificant when water is in the feed at a molar ratio of H2O/ (H2 + CO) ) 0.20, which means that the θd factor (eq 21) must be considered in the model for deactivation. Nevertheless, water in the feed also attenuates methanol synthesis (an effect quantified by the factor θ, eq 9) and, in fact, DME formation is insignificant for a molar ratio of H2O/(H2 + CO) ) 0.60. A fact to be noted is the satisfactory fit between the results of the model and those for the evolution of concentration with time on stream corresponding to all the components considered in the reaction scheme (eqs 1, 3, 5, and 7). The aforementioned results refer to the concentrations of DME, paraffins, and methanol. Figure 7 (an example under given conditions) shows the fit of experimental results (points) and those calculated (lines) for the evolution of CO, CO2, and H2O concentrations with time on stream at the reactor outlet. The fitting of the kinetic model to the experimental results has been carried out by expressing these in terms of concentration. The model allows calculating derived magnitudes (conversion, product yields, selectivities), which will be useful for future studies dealing with the optimization of process conditions and reactor operation strategy. Figure 8a shows the evolution of CO conversion with time on stream under the same experimental conditions as in Figure 7. The points are experimental results, and the line has been calculated using the model. CO conversion has been defined as XCO )

(nCO)0 - nCO 100 (nCO)0

(24)


Figure 6. Effect of water in the feed on the evolution of the molar fraction of DME (a) and paraffins (b) at the reactor outlet with time on stream. Points, experimental results. Lines, calculated results. Reaction conditions: temperature, 275 °C; pressure, 30 bar; H2/CO molar ratio in the feed, 3/1; space time, 12.8 (g of catalyst) h (mol of reactants)-1.

Figure 7. Comparison of experimental results (points) and those calculated with the kinetic model (lines) for the evolution of the molar fraction of inorganic products at the reactor outlet with time on stream. Reaction conditions: temperature, 275 °C; pressure, 30 bar; H2/CO molar ratio in the feed, 3/1; space time, 12.8 (g of catalyst) h (mol of reactants)-1.

where (nCO)0 and nCO are the molar flow rates of CO at the reactor inlet and outlet. The calculation of activity (eq 15) is the key for solving the model, and its evolution with time on stream is shown in Figure 8b for the same run as in Figure 7. 4. Conclusions The deactivation kinetic model established for a single-step DME synthesis from H2 + CO quantifies the evolution of component concentrations in the reaction medium (DME, methanol, paraffins, H2, CO, CO2, and H2O) with time on stream in a wide range of conditions: 225-350 °C; 10-40 bar; space

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Figure 8. Evolution with time on stream of CO conversion (a, in which the line has been calculated with the deactivation kinetic model and the points are experimental) and calculated activity (b). Reaction conditions: temperature, 275 °C; pressure, 30 bar; H2/CO molar ratio in the feed, 3/1; space time, 12.8 (g of catalyst) h (mol of reactants)-1.

time, 0.1-68.0 (g of catalyst) h (mol of reactants)-1; H2/CO molar ratio in the feed, 2-4; time on stream, 30 h. The model is based on previous results for the evolution with time on stream of the metallic function and acid catalyst properties and is consistent with the hypothesis that the deactivation cause is the deposition of coke on the metallic function. This coke is presumably formed by degradation of methoxy ions generated from oxygenates (DME and, to a lesser extent, methanol) in the reaction medium. This hypothesis confirms that deactivation only has a direct effect on the methanol synthesis reaction and that deactivation kinetics is dependent on the concentration of oxygenates in the reaction medium. The complex reaction scheme explains the evolution of composition for the remaining compounds with time on stream at the reactor outlet. The presence of water in the reaction medium plays a significant role in the attenuation of deactivation, and consequently, a flow rate in the feed at a molar ratio of around H2O/ (H2 + CO) ) 0.20 makes deactivation insignificant for 30 h, although there is a slight decrease in DME yield from zero time on stream due to the attenuation in catalyst activity. The deactivation kinetic model proposed is useful for future studies concerning the optimization of process conditions, and the methodology for its development may also be useful for other catalysts involving similar problems, in which the determining factor is deactivation by coke deposition. It is noteworthy that the model has been established under conditions in which there is a high water content in the reaction medium. Consequently, it faithfully considers the effect of water, on the one hand, by attenuating the rate of the kinetic scheme steps (unfavorable effect) and, on the other, by attenuating the deactivation by coke (favorable effect). Consequently, the

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content of water in the feed is a key factor in process optimization. Acknowledgment This paper has been financed by the University of the Basque Country and the Department of Education of the Basque Government (Project GIC07/24-IT-220-07) and by the Spanish Ministry of Science and Innovation (Project CTQ2007-66571/ PPQ). Notation a ) activity, eq 15 DME, HC, MeOH ) dimethyl ether, hydrocarbons, and methanol, respectively Ed ) parameter of the deactivation kinetic model Ej ) activation energy of j reaction, kJ mol-1 fi ) fugacity of i component, bar ∆HH2O ) energetic coefficient in eq 10, kJ mol-1 (∆HH2O)d ) energetic coefficient in eq 22, kJ mol-1 KH2O, KH2O* ) term that quantifies capacity for water adsorption in main reaction, and its value at reference temperature (275 °C) (KH2O)d, (KH2O)d* ) term that quantifies capacity for water adsorption during deactivation, and its value at reference temperature (275 °C) Kj ) equilibrium constant for j reaction kj, kj* ) kinetic constant for j reaction and its value at reference temperature (275 °C) kd, kd* ) kinetic constant for deactivation and its value at reference temperature (275 °C) kd′ ) apparent kinetic constant for deactivation, eq 20 LPG ) liquefied petroleum gas MTG ) methanol to gasoline process MTO ) methanol to olefins process ni ) molar flow rate of each i product, mol s-1 nCi ) number of carbon atoms in each paraffin nCO, (nCO)0 ) molar flow rate of CO at reactor outlet and in feed, mol s-1 nv ) number of dependent variables for the model to be fitted rMeOH ) reaction rate for the formation of methanol (rMeOH)0 ) reaction rate for formation of methanol at zero time on stream STD ) syngas to DME process wi ) distribution weighting factors for i dependent variables XCO ) CO conversion, eq 24 Yi ) yield of i product yi ) vector of molar fraction values for i components, calculated by solving mass conservation equations in reactor yi* ) vector of experimental molar fraction values for i components Greek Symbols Φ ) sum of square residuals, defined in eq 23 φi ) sum of square residuals for i dependent variables θ ) term that quantifies attenuation in reaction rate by adsorption of water on catalyst, eq 9 θd ) term that quantifies attenuation in catalyst deactivation by water adsorption, eq 21

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ReceiVed for reView June 17, 2009 ReVised manuscript receiVed October 19, 2009 Accepted November 4, 2009 IE900978A

Deactivation Kinetics for Direct Dimethyl Ether Synthesis on a CuO

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