Deactivation Kinetics for Direct Dimethyl Ether Synthesis on aCuO-ZnO-Al2O3/γ-Al2O3 Catalyst

Irene Sierra, Javier Erena,* Andres T. Aguayo, Martin Olazar, and Javier Bilbao

Departamento de Ingenierıa Quımica, UniVersidad del Paıs 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 thedirect synthesis of dimethyl ether. The model allows calculating the effect of the operating conditions and theevolution 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 ofconditions: 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 cokedeposition on the metallic function and that this coke is formed by degradation of oxygenates (dimethyl etherand methanol). The effect of water in the reaction medium has been quantitatively considered in the kineticmodel, whose drawback is the attenuation of the methanol synthesis rate and whose benefit is the attenuationof deactivation by coke deposition. The kinetic model is useful for predicting the influence of water in thefeed with syngas as a strategy to attenuate deactivation, which is insignificant for a H2O/(H2 + CO) molarratio 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 setto supplement oil in the next decades to meet the increasingdemand for automotive fuels and petrochemical raw materials.Dimethyl ether (DME) has aroused great interest because it isan eco-friendly fuel (sulfur-free and with a low emission ofparticles and NOx) which may substitute oil LPG as a domesticfuel and may be used as an additive for gasoline and automotivediesel.1–4 Furthermore, DME is an intermediate raw material(alternative to methanol) for the production of hydrocarbons(light olefins, BTXE, conventional fuels), chemicals (dimethylsulfate, methyl acetate), and H2, by means of catalytic steamreforming.5–9 Furthermore, DME has low toxicity and itsaverage lifetime in the atmosphere is approximately 5 days;consequently, it is also used as an aerosol propellant to replacechlorofluorocarbons (CFCs).10

The production of DME from syngas in a single-step process(STD process) on a bifunctional catalyst shifts the thermody-namic equilibrium of methanol synthesis (due to the lowerconcentration of methanol in the reaction medium), andconsequently, the STD process may be carried out with highyields of DME at lower pressure and higher temperature thanmethanol synthesis.11,12 The advantage is that CO2 cofeedingrequires a lower pressure than in methanol synthesis.13–15

Great attention has been paid in the literature to the formula-tion of the catalyst with CuO-ZnO-Al2O3 being the metallicfunction, which has been commonly used for methanol synthesisand whose preparation and properties are key factors for catalystactivity.16–18 The catalyst acid function exceeds that requiredfor methanol dehydration to enhance the aforementionedthermodynamic advantage. A high selectivity to DME requiresmoderate strength of the acid function sites to avoid secondaryreactions of DME transformation into hydrocarbons. Thismoderate acid strength (mainly Lewis acid sites and a fewBrønsted sites) is attained with γ-Al2O3,

16–20 HZSM-5 zeoliteswith high Si/Al ratio or partially sodium ones,21,22 microporouszeolites with a high Si/Al ratio,23–26 and with SAPO ones.27

The possible causes of catalyst deactivation are Cu sinteringand mainly coke deposition.19,28–30 Furthermore, attention hasalso been paid to the attenuation of activity by water adsorptionon the acid functions of a hydrophilic nature.19 The cause ofdeactivation is related to the composition of the catalyst, and ifthis is prepared with an excess of acid function, the depositionof coke on this function has no bearing on catalyst activity.Likewise, operating below 325 °C (limit temperature to avoidsintering of Cu-Zn function), this cause of deactivation isavoided. These hypotheses have been proven in a previous paperfor a CuO-ZnO-Al2O3/γ-Al2O3 catalyst, based on runs inwhich the metallic and acid functions have been used separatelyin the single-step process and in the individual reactions ofmethanol synthesis and methanol dehydration. The results ofevolution with time on stream of product yields and propertiesof catalyst functions (particularly metallic dispersion and acidity)allowed establishing that the deactivation cause is the depositionof coke on the metallic function. The coke initially deposits onCu-ZnO sites and on the interphase between these sites andthe support (Al2O3), and as time on stream elapses, it is alsodeposited on the support. A significant deposition on the acidfunction (γ-Al2O3) is observed only for very high coke con-tents.31

The interpretation of the kinetic results in the literature ishindered by the complexity of the kinetic scheme, in which theproducts of each step are the reactants in the following one.Consequently, a kinetic model is required for reactor designand the optimization of operating conditions. Kinetic modelinghas been approached in the literature for the single-step processat zero time on strean,32,33 by combining the kinetic equationspreviously proposed for both individual reactions, i.e., methanolsynthesis 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 individualreactions. Pressure is lower and temperature is higher than inmethanol synthesis, and methanol dehydration is carried out atroom temperature. Furthermore, the composition of the reactionmedium in the single-step process is different due to theaforementioned synergisms between individual reactions. To fill

* To whom correspondence should be addressed. Tel.: 34-94-6015363. Fax: 34-94-6013500. E-mail: javier.erena@ehu.es.

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

10.1021/ie900978a 2010 American Chemical SocietyPublished on Web 11/19/2009

this gap, a specific kinetic model has been established in aprevious paper, which considers the following steps for DMEsynthesis:38

hydrogenation of CO:

CO + 2H2 798k1

CH3OH (1)

rMeOH ) k1[fCO fH2

2 -fCH3OH

K1]θ (2)

dehydration of methanol to DME:

2CH3OH 798k2

CH3OCH3 + H2O (3)

rDME ) k2[fCH3OH2 -

fCH3OCH3fH2O

K2] (4)

formation of CO2 (water gas shift):

CO + H2O 798k3

CO2 + H2 (5)

rCO2) k3[fCO fH2O -

fCO2fH2

K3] (6)

synthesis of hydrocarbons (C1-C10 paraffins):

nCO + (2n + 1)H2 798k4

CnH2n+2 + nH2O (7)

rHC ) k4[fCO fH2

3 -fHC fH2O

K4]θ (8)

Equations 2, 4, 6, and 8 have been established by consideringelementary 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 paraffinformation from H2 + CO (eq 7) is due to the metallic functioncapacity for synthesis under the high pressure used in theprocess, which has been experimentally proven.

A term (θ) has been included in the rates for the steps ofmethanol synthesis (eq 2) and hydrocarbon formation (eq 8) toquantify the attenuation in the rate of these steps by theadsorption of water on the catalyst:

θ ) 11 + KH2O fH2O

(9)

This attenuation is explained by the adsorption competence ofwater and reactants on the metallic sites or by the adsorptionof water on Al2O3 (support of this function), given that thissupport has a major influence on the activity of Cu-Zn sites.17

This great influence of water content on the kinetics of theprocess explains why the kinetic equations do not needadsorption terms for reactants and products, given that theirinfluence on the attenuation of reaction kinetics is lower thanthat of water.

The KH2O constant in eq 9 depends on temperature as follows:

KH2O ) KH2O* exp[∆HH2O

R (1T- 1

548)] (10)

Given that eqs 9 and 10 are empirical, the relationship betweenthe parameter ∆HH2O and water adsorption heat is not straight-forward, although this parameter is related to this heat and tothe hydrophilic nature of the Al2O3 that supports the metallicfunction.

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 extendedin this paper by incorporating a new kinetic equation fordeactivation, which will allow a kinetic model applicable to anyvalue of time on stream. This model considers two opposingeffects created by the water in the reaction medium, which are,on the one hand, the unfavorable attenuation of methanolsynthesis (apart from the shift of methanol dehydration equi-librium) and, on the other, the attenuation of deactivation bycoke deposition on this function.19,31,38

2. Experimental Section

2.1. Equipment and Reaction Conditions. The reactionequipment used (PID Eng. & Tech. Microactivity) has beendescribed elsewhere and is provided with a fixed bed, whichallows operating at high temperatures and pressures.38 Onlineproduct analysis has been carried out by means of a Varian CP-4900 gas microchromatograph. The reaction mixture is passedthrough 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 thefollowing 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 temper-ature); 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. Thecalculation of contact time for different H2/CO molar ratios inthe feed is based on the fact that the reactant molar flow rate inthe feed is maintained constant, 1 mmol of (H2 + CO) min-1.Consequently, the contact time (h-1) is the result of multiplyingthe 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, correspondingto the values of H2/CO molar ratio of 2/1, 3/1, and 4/1,respectively. Under the operating conditions used, diffusionalrestrictions are not significant, which are more important whenthe 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, andCH4; (ii) Porapak Q, PPQ (10 m × 20 µm), which separatesCO, CH4, C2H6, C3H8, C4H10, methanol, and DME; (iii) CPSil(8 m × 2 µm) to separate C4-C10 fraction. The response factorof each component has been determined by means of mixturesprepared with known compositions.

2.2. Catalyst. The bifunctional catalyst, CuO-ZnO-Al2O3/γ-Al2O3, is composed of a metallic function for hydrogenatingCO to produce methanol (CuO-ZnO-Al2O3), and an acidfunction for the dehydration of methanol to DME (γ-Al2O3).The metallic function has been prepared by coprecipitating thecorresponding metallic nitrates with Na2CO3 at pH 7.0.39

The γ-Al2O3 acid function has been prepared by coprecipi-tation of a NaAlO2 suspension with HCl at 70 °C until the pHreaches a value of 9.0. The following steps are the aging of thecatalyst at 70 °C for 1 h, filtering, washing, drying (at 20 °Cand at 120 °C for 12 h each), and calcination (550 °C, 2 h).This function has low acid strength to minimize the transforma-tion of methanol and DME into light olefins and heavyhydrocarbons, which may undergo degradation to produce coke.

This catalyst has been prepared by mixing the dry metallicfunction and the acid function in an aqueous solution, using amass ratio of 2:1, which is the ratio that provides the best catalystperformance.15 Furthermore, this mass ratio between the metallicfunction and the acid one is appropriate for ensuring that

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

methanol synthesis is the limiting step, with a rapid transforma-tion of methanol into DME. Subsequent to the mixing of metallicand acid functions, the suspension is centrifuged and the solidis washed, dried (in two steps, at 20 and 120 °C for 12 h eachstep), 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 is11.7 m2 (g of catalyst)-1 and has been determined by N2Ochemisorption (Micromeritics AUTOCHEM 2920 online witha Balzers Instruments Omnistar mass spectrometer). The physi-cal properties, determined by N2 adsorption-desorption (Mi-cromeritics, 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 aBalzers 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 thetemperature programmed desorption curve.

Prior to use, the bifunctional catalyst has been subjected toan equilibration treatment by oxidation-reduction in the reactoritself, which consists in exposing it to a H2 stream diluted inHe (at 10%) at 200 °C for 14 h and, subsequently, to a secondstream (at 20%) for 1 h at 300 °C. This process has alreadybeen optimized and its aim is to reduce the CuO contained inthe metallic function to Cu0, which is the component of higheractivity in the methanol synthesis step.41

3. Results

3.1. Deactivation Cause. The analysis of coke compositionand evolution of properties of metallic and acid functions withtime on stream has established that deactivation is produced bymetallic site blockage by coke, which is presumably formedby 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 ofthis support. As coke is being deposited, there is a progressiveblockage of first the metallic sites, then the Al2O3 that supportsthese sites, and finally the acid function (γ-Al2O3), althoughhigh coke content is required for γ-Al2O3 blockage and,consequently, for its deactivation. It should be noted that thedeactivation due to CuO-ZnO metallic function deteriorationis a consequence of the well-known relationship between itsactivity and a suitable dispersion on Al2O3, which is highlyaffected by coke.17

The effect of operating conditions on deactivation confirmsthe origin of the coke.15,22 Figure 1 shows as an example theeffect of space time on the evolution of DME, methanol, andparaffin yields with time on stream. The remaining operatingconditions are constant. The yields have been calculated as theratio between the organic carbon molar flow rate in the productstream and the CO molar flow rate in the feed:

YDME )2nDME

(nCO)0100 (11)

YMeOH )nMeOH

(nCO)0100 (12)

YHC )∑

i

nCini

(nCO)0100 (13)

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

reactor outlet stream, nCiis 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 ratiobetween DME and paraffin yields increases for zero time onstream. 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 synthesisstep hinders DME formation, whereas the capacity for paraffinformation according to the FT reaction is unaffected. Conse-quently, DME yield steadily decreases with time on stream,whereas that of paraffins increases. Coke formation presumablyoccurs by the degradation of methoxy groups due to 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.

Ind. Eng. Chem. Res., Vol. 49, No. 2, 2010 483

dehydrogenating capacity of the metallic function for activatingcoke formation steps such as condensation and dehydrocycliza-tion.42

3.2. Kinetic Models Proposed for Deactivation. Previouspapers have shown that deactivation is a consequence of cokedeposition 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 fH2

2 -fCH3OH

K ]θa (14)

Activity, a, is defined as the ratio between the reaction rates att time and zero time on stream:

a )rMeOH

(rMeOH)0(15)

The remaining steps of the kinetic scheme are assumed not tobe directly affected by coke deposition.

Several expressions of separable variables and first order fordeactivation have been considered to describe the relationshipbetween activity and operating conditions (Table 1). Theseequations correspond to different alternatives for coke origin:(i) Model 1 considers a deactivation kinetics that is independentof concentration. No precursor is identified and temperature isthe main variable. (ii) Model 2 considers that coke is formedin parallel with methanol synthesis, and consequently from COreactant, according to a kinetics depending on its concentration.(iii) Model 3 considers that deactivation occurs in series withthe main reaction kinetic scheme, with a rate that is dependenton oxygenate compound concentration. This model considersthat methanol/DME is degraded to evolve toward coke con-stituents, presumably through methoxy ion intermediates formedat the interphase between the metallic function and Al2O3

support.31 (iv) Model 4 assumes deactivation kinetics dependenton the concentration of paraffins formed as a byproduct.Consequently, deactivation also occurs in series with the mainreaction step.

The attenuating effect of water in the reaction medium oncoke deposition has also been proven, which is a fact that hasalso been confirmed in previous papers dealing with thetransformation of methanol/DME into gasoline (MTG process)on HZSM-5 zeolite catalysts,43 and with the transformation ofmethanol into olefins (MTO process), on SAPO-3444 and SAPO-18 catalysts.45 The attenuating effect of water is explained byits adsorption on the acid sites, in which it competes with theintermediate compounds in the steps of coke evolution on acid

catalysts, namely, oligomerization, cyclization, aromatization,and polycondensation. As in these processes, this effect has beentaken into account in the kinetic modeling by considering thedeactivation kinetic constant, kd′, in the models shown in Table1, as an apparent kinetic constant defined as the product of thereal kinetic constant, kd, by a function, θd, that quantifies theattenuating effect of water in the reaction medium:

kd′ ) kdθd (20)

Once the fitting has been tested by using different expressionsfor the θd function, the following hyperbolic expression has beenadopted as suitable:

θd ) 11 + (KH2O)d fH2O

(21)

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

(KH2O)d ) (KH2O)d* exp[(∆HH2O)d

R (1T- 1

548)] (22)

where (KH2O)d* is the value of the constant at a referencetemperature (275 °C).

3.3. Methodology for Kinetic Parameter Calculation. Themethodology used for calculating the evolution of concentrationswith time on stream has consisted in simultaneously solvingthe mass conservation equations for each component with thekinetic equations at zero time and the corresponding kineticequation for deactivation. The models and kinetic parametersof best fit have been obtained by minimizing an error objectivefunction (Φ) that compares the experimental and calculatedresults:

Φ ) ∑i)1

nv

wiφi ) ∑i)1

nv

wi(yi* - yi)2 (23)

where wi and φi are, respectively, the distribution weightingfactor and the sum of square residuals for each i dependentvariable, nv is the number of dependent variables of the modelto be fitted, yi* is the vector of experimental molar fractionvalues for i, and yi is the vector of molar fraction valuescalculated by solving the mass balance in the reactor. The qualityof the chromatographic analyses is noteworthy, which haveprovided a mass balance closure above 98% in all the runs usedfor the kinetic study.

This rigorous calculation method is required for consideringthe catalyst’s “past history” and avoiding the errors of ap-proximate calculation methods. This requirement is especiallyappropriate when the kinetic model is dependent on componentconcentration and reaction schemes are complex.46–48

The calculation of kinetic parameters for each model has beencarried out by means of a program written in MATLAB, inwhich experimental data and operating conditions (compositionof each component for different temperatures, pressures, spacetimes, and reaction times) are introduced and the kineticparameters of best fit are calculated by multivariable nonlinearregression. The calculation has been restricted to the parametersof the deactivation equation, given that the values determinedin a previous paper and shown in Table 2 have been adoptedfor reaction rates at zero time on stream.38 The high value ofthe activation energy for methanol dehydration (352.21 kJmol-1) is explained because it is an apparent activation energy,given that it is a reaction following a mechanism with severalsteps.49 Bercic and Levec also obtained a high value (143.70kJ mol-1) for γ-Al2O3 pellets by feeding pure methanol under

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

model kinetic equation Φ

1 -dadt

) kd′a (16) 7.51

2 -dadt

) -kd′(fCO)a (17) 6.24

3 -dadt

) kd′(fMeOH + fDME)a (18) 5.13

4 -dadt

) kd′(fHC)a (19) 6.74

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

internal diffusion control.36,37 It has also been proven that theactivation energy increases as water is fed with methanol,50 andthis paper specifically approaches the kinetics of DME synthesiswith high water contents in the reaction medium.

3.4. Kinetic Parameters. The results of the error objectivefunction (eq 23) are set out in Table 1 for the fittingscorresponding to the deactivation kinetic models proposed.

A comparison of the results for models 1-4 shows that theone of poorer fitting to the experimental results is model 1,corresponding to independent deactivation. Model 2 (whichassumes deactivation in parallel to the main reaction) and model

4 (series deactivation with paraffins as coke precursors) fitsimilarly, although model 2 fits slightly better. Model 3, whichassumes series deactivation and oxygenate compounds (metha-nol and DME) as coke precursors, is the one of best fit. Thisresult confirms the conclusions obtained by Erena et al.concerning coke origin by degradation of methoxy ions formedat the interface between the metallic function and Al2O3

support.31 It is essential to consider a function to account forthe attenuation of deactivation due to the presence of water inthe reaction medium. When this function, defined in eq 21, isnot 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 inTable 3 for the model chosen as more suitable (model 3), whichconsiders catalyst deactivation as a consequence of cokegeneration from oxygenate compounds in series with the mainreaction. The kinetic constants are calculated at a referencetemperature of 275 °C.

3.5. Effect of Operating Conditions on Deactivation. Thekinetic model established allows calculating the evolution ofcomponent concentration in the reaction medium with time onstream in a wide range of conditions. Figures 2-5 compare theexperimental results (points) with those calculated using themodel (lines) for the evolution of the molar fraction of DMEand paraffins at the reactor outlet with time on stream. It shouldbe noted that the experimental conditions correspond to asignificant deactivation, as required for a kinetic study.

The comparison of experimental and calculated results showsthat the fit is satisfactory in a wide range of conditions and fora reaction that is very sensitive to these conditions. Figure 2

Table 2. Parameters for the Zero Time on Stream Kinetic Model38

parametera value

k1*, molMeOH (molH2)-1 gcat

-1 h-1 bar-3 (3.37 ( 0.04) × 10-6

k2*, molDME (molH2)-1 gcat

-1 h-1 bar-2 1.57 ( 0.09k4*, molHC (molH2

)-1 gcat-1 h-1 bar-4 (2.81 ( 0.09) × 10-7

E1, kJ mol-1 112.79 ( 2.68E2, kJ mol-1 352.21 ( 7.87E4, kJ mol-1 227.27 ( 7.20KH2O*, bar-1 1.04 ( 0.05∆HH2O, kJ mol-1 276.10 ( 11.92

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

Table 3. Parameters for the Deactivation Kinetic Model

parametera value

kd*, h-1 bar-1 (3.91 ( 0.24) × 10-2

Ed, kJ mol-1 71.80 ( 3.90(KH2O)d*, bar-1 2.43 ( 0.22(∆HH2O)d, kJ mol-1 42.97 ( 2.38

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

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

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

Ind. Eng. Chem. Res., Vol. 49, No. 2, 2010 485

corresponds to different temperatures and shows a significantincrease in DME yield as temperature is increased from 250 to275 °C (Figure 2a), as well as an increase in paraffin yield astemperature is increased from 275 to 325 °C (Figure 2b). Asmentioned above, the decrease in DME yield with time onstream occurs simultaneously to a significant increase in paraffinyield.

When the effect of pressure is studied (Figure 3), a steadyincrease in DME yield is observed as pressure is increased,whereas paraffin yield (very low at 20 bar) significantly increaseswhen the operation is carried out at 30 bar. It is noteworthythat deactivation is faster as pressure is increased, due to theenhancement of coke condensation reactions.

There is an optimum for a H2/CO molar ratio in the feed ofaround 3/1 to maximize DME yield (Figure 4a), which alsocorresponds to the maximum DME selectivity, due to the smallerinfluence of this ratio on the formation of paraffins (Figure 4b).Deactivation attenuates as H2/CO molar ratio is increased from3/1 to 4/1, due to the inhibition of coke formation by a higherH2 concentration.

An increase in space time has a relevant effect of increasingDME yield (Figure 5a) and decreasing paraffin yield (Figure5b). Figure 5a shows that, as space time is increased, thedecrease in DME yield with time on stream is more pronounced,which is consistent with a deactivation that occurs in series withthe main reaction in which the cause of coke formation is theproduct DME.

Special attention should be paid to the effect of water in thefeed 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

evolution of the molar fraction of DME (Figure 6a) and paraffins(Figure 6b) at the reactor outlet with time on stream. Deactiva-tion is significant without water in the feed. Deactivation isinsignificant when water is in the feed at a molar ratio of H2O/(H2 + CO) ) 0.20, which means that the θd factor (eq 21) mustbe considered in the model for deactivation. Nevertheless, waterin the feed also attenuates methanol synthesis (an effectquantified by the factor θ, eq 9) and, in fact, DME formation isinsignificant for a molar ratio of H2O/(H2 + CO) ) 0.60.

A fact to be noted is the satisfactory fit between the resultsof the model and those for the evolution of concentration withtime on stream corresponding to all the components consideredin the reaction scheme (eqs 1, 3, 5, and 7). The aforementionedresults refer to the concentrations of DME, paraffins, andmethanol. Figure 7 (an example under given conditions) showsthe fit of experimental results (points) and those calculated(lines) for the evolution of CO, CO2, and H2O concentrationswith time on stream at the reactor outlet.

The fitting of the kinetic model to the experimental resultshas been carried out by expressing these in terms of concentra-tion. The model allows calculating derived magnitudes (conver-sion, product yields, selectivities), which will be useful for futurestudies dealing with the optimization of process conditions andreactor operation strategy. Figure 8a shows the evolution of COconversion with time on stream under the same experimentalconditions as in Figure 7. The points are experimental results,and the line has been calculated using the model. CO conversionhas been defined as

XCO )(nCO)0 - nCO

(nCO)0100 (24)

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

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

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

where (nCO)0 and nCO are the molar flow rates of CO at thereactor inlet and outlet.

The calculation of activity (eq 15) is the key for solving themodel, and its evolution with time on stream is shown in Figure8b for the same run as in Figure 7.

4. Conclusions

The deactivation kinetic model established for a single-stepDME synthesis from H2 + CO quantifies the evolution ofcomponent concentrations in the reaction medium (DME,methanol, paraffins, H2, CO, CO2, and H2O) with time on streamin 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; H2/COmolar ratio in the feed, 2-4; time on stream, 30 h.

The model is based on previous results for the evolution withtime on stream of the metallic function and acid catalystproperties and is consistent with the hypothesis that thedeactivation cause is the deposition of coke on the metallicfunction. This coke is presumably formed by degradation ofmethoxy ions generated from oxygenates (DME and, to a lesserextent, methanol) in the reaction medium. This hypothesisconfirms that deactivation only has a direct effect on themethanol synthesis reaction and that deactivation kinetics isdependent on the concentration of oxygenates in the reactionmedium. The complex reaction scheme explains the evolutionof composition for the remaining compounds with time onstream at the reactor outlet.

The presence of water in the reaction medium plays asignificant role in the attenuation of deactivation, and conse-quently, 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 timeon stream due to the attenuation in catalyst activity.

The deactivation kinetic model proposed is useful for futurestudies concerning the optimization of process conditions, andthe methodology for its development may also be useful forother catalysts involving similar problems, in which thedetermining factor is deactivation by coke deposition. It isnoteworthy that the model has been established under conditionsin which there is a high water content in the reaction medium.Consequently, it faithfully considers the effect of water, on theone hand, by attenuating the rate of the kinetic scheme steps(unfavorable effect) and, on the other, by attenuating thedeactivation by coke (favorable effect). Consequently, the

Figure 6. Effect of water in the feed on the evolution of the molar fractionof 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 calculatedwith the kinetic model (lines) for the evolution of the molar fraction ofinorganic products at the reactor outlet with time on stream. Reactionconditions: temperature, 275 °C; pressure, 30 bar; H2/CO molar ratio inthe feed, 3/1; space time, 12.8 (g of catalyst) h (mol of reactants)-1.

Figure 8. Evolution with time on stream of CO conversion (a, in whichthe line has been calculated with the deactivation kinetic model and thepoints 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.

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content of water in the feed is a key factor in processoptimization.

Acknowledgment

This paper has been financed by the University of the BasqueCountry and the Department of Education of the BasqueGovernment (Project GIC07/24-IT-220-07) and by the SpanishMinistry of Science and Innovation (Project CTQ2007-66571/PPQ).

Notation

a ) activity, eq 15DME, HC, MeOH ) dimethyl ether, hydrocarbons, and methanol,

respectivelyEd ) parameter of the deactivation kinetic modelEj ) 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 adsorptionin main reaction, and its value at reference temperature (275 °C)

(KH2O)d, (KH2O)d* ) term that quantifies capacity for water adsorptionduring deactivation, and its value at reference temperature (275°C)

Kj ) equilibrium constant for j reactionkj, 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 20LPG ) liquefied petroleum gasMTG ) methanol to gasoline processMTO ) methanol to olefins processni ) 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 fittedrMeOH ) reaction rate for the formation of methanol(rMeOH)0 ) reaction rate for formation of methanol at zero time on

streamSTD ) syngas to DME processwi ) distribution weighting factors for i dependent variablesXCO ) CO conversion, eq 24Yi ) yield of i productyi ) vector of molar fraction values for i components, calculated

by solving mass conservation equations in reactoryi* ) vector of experimental molar fraction values for i components

Greek Symbols

Φ ) sum of square residuals, defined in eq 23φi ) sum of square residuals for i dependent variablesθ ) term that quantifies attenuation in reaction rate by adsorption

of water on catalyst, eq 9θd ) term that quantifies attenuation in catalyst deactivation by

water adsorption, eq 21

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ReceiVed for reView June 17, 2009ReVised manuscript receiVed October 19, 2009

Accepted November 4, 2009

IE900978A

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