Kinetic Modeling of Methanol Transformation into Olefins on a SAPO

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Kinetic Modeling of Methanol Transformation into Olefins on a SAPO-34 Catalyst Ana G. Gayubo,* Andre´ s T. Aguayo, Ana E. Sa´ nchez del Campo, Ana M. Tarrıó, and Javier Bilbao Departamento de Ingenierıá Quı´mica, Universidad del Paı´s Vasco, Apartado 644, 48080 Bilbao, Spain

A kinetic model for the methanol to olefins (MTO) process on a catalyst based on a silicoaluminophosphate SAPO-34 has been proposed. The model takes into account four individual steps for the production of ethene, propene, butenes, and remaining hydrocarbons (pentenes + paraffins). The kinetic parameters have been obtained by experimentation in an isothermal fixedbed reactor, in the 623-748 K range. In virtue of the results, it has been proven that the water introduced in the feed (required for attenuation of coke deactivation) inhibits olefin formation. This inhibition is taken into account in the kinetic model. Introduction The transformation of methanol into light olefins (the MTO process) is gaining interest in view of the strong demand for light olefins by the petrochemical industry. Originally, the MTO process was an alternative to the methanol to gasoline (MTG) process developed on HZSM-5 in which C2-C4 olefins are intermediate in the kinetic scheme.1-4 Several paths have been followed to increase selectivity to light olefins in the transformation of methanol into hydrocarbons: (1) optimization of the operating conditions using a HZSM-5 zeolite;5-7 (2) modifications in the HZSM-5 zeolite;8-12 (3) testing of other natural and synthetic zeolites.5,13,14 The review of Froment et al. approaches several aspects of these studies.15 The Union Carbide Corp. synthesized silicoaluminophosphates, SAPO-n,16,17 which are proposed as catalysts for the selective transformation of methanol into C2-C4 olefins.18,19 SAPO-n are a wide range of catalysts with shape selectivity and carefully controlled acid strength.16,20 SAPO-34 permits one to obtain 90% selectivity to light olefins (selectivity to ethene and propene is 60%) in the 623-748 K range, even though deactivation by coke is very high.21-24 SAPO-34 has an isomorphous structure to chabazite.25 It is a tridimensional structure consisting of channels (spaces between 8 atoms of oxygen), with a maximum diameter of 4.4 Å and a minimum diameter of 3.1 Å, and by intersections between channels, which give way to cavities of 10 Å maximum diameter and 6.7 Å minimum diameter.26 Its structure is also similar to that of AlPO-18, although there is a small difference in the geometry of the cavities because of the different atomic arrangement.27 The MTO process at industrial scale on SAPO-34 must be carried out in a fluidized-bed reactor with catalyst circulation, because of the high deactivation level of the catalyst. This is the strategy considered as the most suitable one by Mobil for the MTO process on ZSM-5 zeolites,28 but this choice is even more evident for a catalyst like SAPO-34, whose deactivation by coke * To whom correspondence should be addressed. Telephone: 34-94-6012620. Fax: 34-94-4648500. E-mail: [email protected].

Figure 1. Kinetic scheme proposed by Bos et al.31 for transformation of methanol on SAPO-34.

is very rapid. Since 1995, a large reactor-regeneration demonstration unit processing methanol feed of 500 kg day-1 has been operating in Norway; its interest lies in the availability of natural gas reserves.29 Although the design of the reactor for this strategy requires rigorous kinetic models for both the main reaction and the deactivation,30 unfortunately, the literature information concerning the kinetic model of the MTO process with SAPO-34 is scarce. Pop et al.24 proposed (and experimentally proved in an isothermal fixed bed at 673 K) a kinetic model on the basis of a mechanism made up of intermediate carbenes: k1

A 98 B + H2O k2

A + B 98 C + H2O k3

C + B 98 P where A ) oxygenates (methanol and dimethyl ether), B ) carbene intermediate, C ) olefins, and P ) paraffins. Bos et al.31 carried out a discrimination of the formation steps of each olefin (Figure 1). The model of Bos et al.31 suitably fits the experimental data obtained by the authors at 723 K in a fixed-bed microreactor where methanol diluted at 50 wt % with He was fed by pulses. This model agrees with the carbon pool mechanism proposed by Dahl and Kolboe.32-34

10.1021/ie990188z CCC: $19.00 © 2000 American Chemical Society Published on Web 01/19/2000

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The aim of this paper has been to carry out an experimental kinetic study in an isothermal integral fixed-bed reactor to propose a kinetic model valid in a wide range of operating conditions. This kinetic model will be required for establishing a future kinetic model for deactivation. Both models will be needed for designing industrial-scale reactors. Special attention has been paid to the role of water on activity and selectivity. Water is present because it is a reaction product and because it is fed together with methanol to reduce deactivation by coke. The dealuminating capacity of water has been extensively studied in the literature, and steaming treatments are very well-known for regulating the acid structure of HY35 and HZSM-5 zeolites.36,37 Nevertheless, information on the role of water in the reaction mechanisms and in the distribution of the reaction products is scarce. To quantify this, the effect of water must be isolated from the other phenomena that take place simultaneously when there is water in the reaction medium, such as dealumination,38,39 the role of water as an inert diluent, and the attenuation of deactivation.40 For this purpose, a rigorous study like the one carried out by Zhao and Wojciechowski is required,41,42 which has proven that an alteration in the mechanism of 2-methylpentane cracking over a USHY zeolite is produced because of the presence of water in the reaction medium. Experimental Section Catalyst. The preparation of SAPO-34 (whose composition is H0.09(Si0.08Al0.51P0.4)O2) is carried out following the patent of Union Carbide.16 Aluminum isopropoxide, Al(OC3H7)3, is added to an aqueous solution at 28 wt % of orthophosphoric acid. A colloidal solution of 30 wt % SiO2 is added to the resulting mixture, and it is stirred until homogenization. Subsequently, a solution of 40 wt % tetraethylammonium bromide is added. The crystallization is carried out in a 1 L stainless steel Teflon-lined reactor, where a stirring speed of 200 rpm is maintained (double turbine) at 463 K for 7 days. The solid obtained is cooled to room temperature, separated by centrifugation, and washed with distilled water. The organic template is eliminated by calcination in a muffle oven at 848 K for 10 h. The verification that the product obtained is a SAPO-34 (on the basis of its crystalline structure) is carried out by X-ray diffraction.16 The catalyst that is to be used in the reaction is prepared by agglomerating the SAPO-34 (25 wt %) with bentonite (Exaloid) (30 wt %), using fused alumina (Martinswerk) as the inert charge (45 wt %). Particle catalysts with high sphericity and with size in the 0.10.3 mm range are obtained by grinding cylindrical extrudates of 1.5 mm diameter. The catalyst is calcined at 848 K for 4 h in a N2 stream. From the isotherm of CO2 adsorption-desorption (ASAP 2000 from Micromeritics), a BET surface area of 875 m2 g-1 and a micropore volume of 0.33 cm3 g-1 were determined for SAPO-34 (pure). The micropore volume calculated is 0.06 cm3 (g of catalyst)-1, which shows that 27% of the microporous structure of the original SAPO-34 is lost in the preparation of the final catalyst, which must be attributed to partial occlusion of the SAPO-34 crystals by the bentonite (binder). The surface area corresponding to the micropores is 153 m2 (g of catalyst)-1.

The mesopores of the catalyst are a contribution of the bentonite (binder), which has a characteristic peak for a diameter of around 40 Å. The surface area of the catalyst when micro- and mesopores are taken into account is 228 m2 (g of catalyst)-1. The mesopore volume is 0.19 cm3 (g of catalyst)-1, and the micropore + mesopore volume is 0.26 cm3 (g of catalyst)-1. In a previous paper it was determined that the surface acidic structure of SAPO-34 consists mainly of skeletal hydroxyl groups with a wide characteristic FTIR peak at 3610 cm-1.43 The acidity strength of the sites, which are mainly Bronsted, is moderate, so that 80% of the sites have NH3 adsorption heat between 33.6 and 36.0 kcal (mol of NH3)-1. Reaction and Product Analysis. The automated reaction equipment used was previously described.44,45 A fixed-bed reactor of 0.007 m internal diameter, which has a coil to preheat the gases, is used. The reactor is placed in an oven of electric resistances, in whose design the inertia to the computer-controlled temperature response has been minimized. Temperature is measured by means of three thermocouples introduced in the bed, one at the middle point of the bed axis, another at the outlet of the bed axis, and finally one on the inside of the bed wall. The products pass through a 10-port valve that allows for a sample to be sent to the Hewlett-Packard 5890 series II chromatograph. The chromatograph has an HP 3390-A integrator, which is provided with a card enabling the results of the analysis to be sent to a computer by means of RS-232-C interface. The feedreaction-analysis system is controlled by a computer routine. The calculation of the weight fraction of oxygenates (methanol and dimethyl ether), of light olefins (ethene, propene, butenes, and pentenes), and of the remaining products is carried out by means of a program in FORTRAN, on the basis of the composition of each individual product. This composition is obtained by chromatographic analysis, where a system of three columns is used: (1) a HP-1 semicapillary column of 0.53 mm diameter and 5 m length, which separates the fraction of volatile (C1-C4) and polar compounds (methanol, water, and dimethyl ether) from the remaining products; (2) a SupelQ Plot semicapillary column of 0.53 mm diameter and 30 m length for total separation of the light fraction that is to be analyzed by thermal conductivity (TCD) and flame ionization (FID) detectors; (3) a PONA capillary column of 0.2 mm diameter and 50 m length to separate the second fraction, which is to be analyzed by FID. Results Kinetic Data. The kinetic data have been obtained in runs under the following conditions: temperature, 623, 648, 673, 698, and 748 K; space time, between 0.01 and 0.44 (g of catalyst) h (g of methanol)-1 (space time was changed by modifying the catalyst mass); time on stream, 1 h; water/methanol ratios in the feed, 0, 1, and 3, in weight; total liquid flow in the feed, 0.40 cm3 min-1; catalyst particle size, between 0.1 and 0.3 mm. The catalyst is diluted to 25 wt % with inert alumina to achieve bed isothermallity (the MTO process is highly exothermal, ∼4.6 kcal mol-1). Because catalyst deactivation is very rapid, the composition of the product stream corresponding to the fresh catalyst was determined by extrapolation to zero time on stream of the evolution of the composition of

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Figure 3. Evolution with time on stream of the mass fraction of the reaction components, for different values of water concentration in the feed. Temperature, 698 K. Upper plot: space time, 0.104 (g of catalyst) h (g of methanol)-1; Xwo ) 0. Lower plot: space time, 0.088 (g of catalyst) h (g of methanol)-1; Xwo ) 3.

Figure 2. Evolution with time on stream of the mass fraction of the reaction components, for different values of temperature. Space time, 0.0931 (g of catalyst) h (g of methanol)-1. Water/methanol ratio in the feed, Xwo ) 1.

each component. This solution is the one proven as the more suitable in kinetic studies of reactions with rapid catalyst deactivation.46 By replication of the experiments and taking 8-11 results of concentration vs time on stream in each experiment, the average relative error estimated in the determination of concentration at zero time on stream is (5%. As an example of the results, Figures 2 and 3 show the evolution with time on stream of the mass fraction (calculated on a water-free basis) of oxygenates (methanol and dimethyl ether), ethene, propene, and butenes. The concentration of pentenes is very low (lower than 10% of butenes), and that of paraffins is even lower (mainly methane). The results in Figure 2 correspond to three reaction temperatures, 648 (Figure 2a), 698 (Figure 2b), and 748 K (Figure 2c), to the same value of space time, 0.0931 (g of catalyst) h (g of methanol)-1,

and to a water/methanol ratio in the feed of Xwo ) 1. The results in Figure 3 correspond to different values of the water/methanol ratio in the feed, Xwo ) 0 and 3, and to the same value of temperature, 698 K, and they were obtained under very similar values of space time, close to 0.1 (g of catalyst) h (g of methanol)-1. The results in Figures 2 and 3, as well as those corresponding to the other operating conditions are within the ranges of the results in the literature for catalysts prepared based on SAPO-34.16,22-24,31 The selectivity to light olefins C2-C4 is above 90% in the range of conditions studied when enough space time is used. The main byproducts are dimethyl ether, pentenes, methane (lower quantities of other paraffins like ethane, propane, and traces of butane), and CO2. The butene stream contains 1-butene, cis- and trans-butene, and isobutene (the latter in smaller proportion). The amount of hydrocarbons C5+ detected is insignificant. When the composition of the reactor outlet stream is compared in Figures 2b and 3 (corresponding to the same values of temperature and space time), a noticeable decrease in the conversion to olefins at zero time on stream is observed when water is fed with methanol. This effect takes place simultaneously with a noticeable attenuation of deactivaction. Kinetic Scheme. The kinetic scheme proposed by Bos et al.31 (Figure 1) has been taken as the starting point. Our study is aimed at the following objectives: (1) simplification of the kinetic scheme by eliminating the slower steps, in order for the model to be easily used in the design of the reactor for optimum production of olefins C2-C4; (2) study of the effect of water in the


Figure 4. Kinetic scheme proposed.

reaction mechanism and, consequently, in the kinetic scheme. Bearing in mind these objectives and taking into account our experimental results, our study has been focused on the kinetic scheme shown in Figure 4. Coke formation steps, that is, steps 7, 10, and 13 in Figure 1, have not been taken into account in Figure 4, because consideration of these steps implies a high degree of uncertainty and does not improve the kinetic information on olefin formation. Bos et al.31 do not take into account these steps in their kinetic study. Coke deposition is extraordinarily rapid, because under the more severe conditions a deposition rate of 3-4% of the methanol in the feed was measured in the first minute of time on stream. Nevertheless, this deposition slightly attenuates with time on stream and as space time, reaction temperature, and methanol dilution with water in the feed are increased. Consequently, the nonconsideration of the coke formation steps in the kinetic scheme does not significantly affect the calculation of the olefin formation kinetics. The important effect of the operating conditions on coke deposition requires a more detailed study which is beyond the scope of this paper. Nevertheless, it is evident that the simplification of not considering the conversion of methanol to coke has a minimum significance when the kinetic model calculated in this way is applied in conditions under which deactivation is minimized. Thus, when water is fed together with methanol, its conversion to coke is lower than 1% even in the first minute of time on stream (maximum value). On the other hand, in the kinetic scheme shown in Figure 4 two minor modifications to the original kinetic scheme of Bos et al.31 have been carried out: (1) steps corresponding to the formation of each olefin (ethene, propene, butenes, and pentenes) and a step corresponding to the formation of the lump of paraffins have been established. This scheme allows for quantification of the production of butenes and pentenes instead of the sums C4 and C5; (2) the formation of ethene, also from pentenes, has been taken into account. This route is supported by the experimental verification of Abbot and Wojciechowski47 about the cracking capacity of pentenes on a HZSM-5 zeolite at 673 K via a dimerizationcracking route. Study of the Inhibiting Effect of Water in the Kinetic Model. When the results in Figures 2b and 3 (corresponding to the same operating conditions) are compared, an important effect of water concentration on the distribution of the components for zero time on stream is observed. The water may play, a priori, the role of an inert gas that decreases the real concentration of the components in the reaction medium. On the other hand, water may also take part in the reaction mechanism (given the slightly hydrophilic nature of SAPO34) by interaction with the intermediate adsorbed on the cationic centers. In any case, as is shown in Figures

2b and 3, the presence of water in the feed has an important effect of attenuating coke deactivation. When operating in reaction-regeneration cycles (by burning the coke with air at 823 K for 2 h), it has been proven that the catalyst completely recovers its activity, even for the more severe reaction conditions among those studied, which a priori favor dealumination (for a water/methanol mass ratio in the feed of 3/1 at 748 K). This hydrothermal stability of SAPO-34 gives this catalyst possibilities of application under severe reaction or regeneration conditions,48 for which the HZSM-5 zeolite has stability problems caused by dealumination.38,49 In the MTO process on HZSM-5 zeolite, the presence of water gives way to an increase in the selectivity to light olefins (by decreasing the formation of aromatics) and also changes their distribution by increasing the ethene selectivity. This latter effect has also been observed in the literature when methanol was diluted in the feed with an inert gas.5,50,51 When the reaction is carried out with SAPO-34, Liang et al.22 and Marchi and Froment23 have proven that the presence of water in the feed gives way to a considerable increase in the selectivity to olefins C2-C4, which, nevertheless, is not observed when nitrogen is used as the diluent. Marchi and Froment23 attribute this result to the competition of water against olefins for adsorption on the strong acid sites. To quantify the effect of water (used in the feed to attenuate deactivation but which is also a reaction product that is unavoidably present in the reaction medium) on the formation rate of each component of the kinetic scheme in Figure 4, ri, the following kinetic equations have been proposed, which are formulated as a function of the concentration (expressed on a waterfree basis, Xi) of the corresponding reactants and of water:

For steps 1-5: (r1-5)i )

k1-5XA 1 + KwXw

(1)

where Kw is a parameter that quantifies the resistance to formation of component i in the corresponding reaction step, because of the presence of water in the reaction medium. This resistance has been assumed to be the same for the different lumps in the kinetic scheme.

For step 6: (r6)C2) ) For step 7: (r7)C2) )

k6XC3)

(2)

1 + KwXw k7XC4)

(3)

1 + KwXw

For step 8: (r8)C2) ) (r8)C3) )

k8XC5) 1 + KwXw

(4)

The water content in the reaction medium, Xw, is the summation of the contributions of the water formed, Xwf, and of the water fed, Xwo. The water content formed has been calculated as a function of the oxygenate content, XA, by using the following equation:52

Xwf ) 0.566 - 0.280XA + 0.247XA2 - 0.311XA3

(5)

296


Calculation of the Kinetic Parameters. The method for calculation of the kinetic parameters has consisted of solving the mass conservation equations for each component in the reactor under the plug-flow assumption:

∂Xi W r ) ∂ξ FMo i

of each lump have been confirmed independently, with the aim of accurately analyzing the selectivity to each individual component in the kinetic scheme. The values calculated for the kinetic parameters are

Kw ) 1.00 ((0.15) (6)

where the formation rate of each product, ri, is

(14)

1 (T1 - 698 )] (15) [ 1 1 (16) k ) 14.2 ((0.6) exp[(-9700 ((750) T 698)] 1 1 (17) k ) 5.66 ((0.43) exp[-8250 ((1150)( T 698)] 1 1 k ) 0.812 ((0.343) exp[-6900 ((1500)( T 698)] k1 ) 9.92 ((0.35) exp -11300 ((600) 2

rC2) )

dXC2) d(W/FMo)

rC3) )

) (r1)C2) + (r6)C2) + (r7)C2) + (r8)C2) (7)

dXC3) d(W/FMo)

rC4) ) rC5) )

) (r2)C3) - (r6)C3) + (r8)C3)

dXC4)

) (r3)C4) - (r7)C4)

d(W/FMo) dXC5)

d(W/FMo)

rpar )

4

(8)

(9)

) (r4)C5) - 2(r8)C5)

dXpar d(W/FMo)

) (r5)par

(10)

[

k5 ) 0.341 ((0.094) exp -11500 ((4400)

1 (T1 - 698 )]

(Xi,j - Xi(calc),j)2 ∑ ∑ i)1 j)1 n1nexp

(12)

where Xi(calc),j are the calculated values (by solving the mass conservation equation, eq 6) of weight fractions on a water-free basis, at zero time on stream, for component i in the experimental point j (corresponding to a given value of space time). Xi,j are the experimental values. The calculation is detailed in previous papers dealing with the MTG process on HZSM-5 zeolites.53-55 To reduce the correlation between the estimations for the frequency factor and activation energy, reparametrization was carried out.56,57 The reparametrized kinetic constant-temperature relationship is

1 1 [-E R (T 698)]

[

k6 ) 1.14 ((0.36) exp -10200 ((5200)

1 (T1 - 698 )] (20)

(11)

n1 nexp

ki ) ki0 exp

(18)

(19)

For solving the set of n - 1 differential equations corresponding to the mass conservation equations of the n - 1 components of the kinetic scheme (Figure 4), a calculation program in FORTRAN, which uses the subroutine DGEAR of IMSL library, was developed. The kinetic parameters that best fit the experimental results are those that minimize the error objective function:

EOF )

3

(13)

Thus, the parameters to optimize are the kinetic constants at a reference temperature, T0 ) 698 K, and the activation energies. The 95% confidence intervals of the estimated parameters have been obtained by nonlinear regression using the Marquardt algorithm.58 By means of this procedure, the confidence intervals of the constants involved in the formation/disappearance

1 (T1 - 698 )] (21) 1 1 ) 0.916 ((3.953) exp[-8200 ((34000)( T 698)]

[

k7 ) 1.19 ((0.34) exp -9700 ((3900) k8

(22) The corresponding value of the error objective function, eq 12, is 0.645 × 10-3. As Kw ) 1 in eqs 1-4, the corresponding kinetic equations are of first-order referred to the mass fraction (on a water-present basis) of component i, xi, which is

xi )

Xi 1 + Xw

(23)

From the values of the kinetic constants, it is concluded that the formation of ethene takes place mainly from the lump of oxygenates and to a much lesser extent from propene and butenes. Thus, at 698 K the kinetic constants corresponding to the formation of ethene from methanol, from propene, and from butenes are k1 ) 9.92 h-1, k6 ) 1.14 h-1, and k7 ) 1.19 h-1, respectively. The low reaction rate of steps 4 and 5 is also noteworthy. Their kinetic constants at 698 K are k4 ) 0.812 h-1 and k5 ) 0.341 h-1. The method followed for step discrimination and simplification of the kinetic scheme has consisted of the application of the F test for the successive elimination of steps for which the confidence interval of the kinetic constant is high (including the zero value) and of the steps for which the kinetic constant was small (insignificant).59 Thus, subsequent to the first fitting for the kinetic scheme of Figure 4, k8 has been eliminated on the basis of its high confidence interval (including the zero value; eq 22). Successively, steps 7, 6, and 5 of the kinetic scheme in Figure 4 have been eliminated.


Figure 5. Simplified kinetic model.

Figure 6. Mass fraction on a water-free basis, for zero time on stream, of each olefin and of oxygenates (methanol + dimethyl ether) for different values of space time. Temperature, 698 K. Xwo ) 0. Points, experimental results. Lines, calculated with the kinetic model.

As a result of the simplifications, in the new kinetic scheme of Figure 5 pentenes and paraffins have been grouped into a lump (remaining hydrocarbons). Despite its simplicity, the kinetic scheme in Figure 5 is suitable for quantifying the production of ethene, propene, and butenes. This kinetic scheme, given its simplicity, is especially useful for establishing a kinetic model for coke deactivation. This objective is of great interest because the rapid deactivation of the catalyst conditions the design of the industrial reactor. From the fitting of the experimental results to the kinetic scheme in Figure 5 and taking Kw ) 1 (as a consequence of the previous results for the kinetic scheme in Figure 4), the following values are obtained for the kinetic parameters of the single steps:

1 1 k1 ) 10.5 ((0.4) exp -11800 ((600) T 698

Figure 7. Mass fraction on a water-free basis, for zero time on stream, of each olefin and of oxygenates (methanol + dimethyl ether) for different values of space time and of reaction temperature. Xwo ) 1. Points, experimental results. Lines, calculated with the kinetic model.

( )] (24) [ 1 1 (25) k ) 13.7 ((0.5) exp[-9500 ((600)( T 698)] 1 1 (26) k ) 5.49 ((0.37) exp(-7100 ((1000)( T 698)) 1 1 k ) 1.00 ((0.22) exp[-9000 ((3600)( (27) T 698)]

the kinetic fitting, and the curves have been determined by solving eq 6 using the kinetic parameters calculated.

The value of the error objective function is 0.677 × 10-3. The adequacy of the simplified kinetic model proposed is shown in Figures 6-8, which correspond to different values of water concentration in the feed. In each plot, corresponding to a given temperature, the mass fraction on a water-free basis for each olefin and for oxygenates (methanol + dimethyl ether) is plotted against space time. The points are the experimental results used in

As a consequence of the results, the fulfillment of the kinetic scheme in Figure 5 is deduced. It is noteworthy that the formation of ethene occurs mainly from methanol and that the steps of oligomerization-cracking of heavier olefins are negligible. The effect on the main reaction of water in the feed is that of dilution of methanol and of attenuation of all of the single reactions of the kinetic scheme. From the result calculated, Kw ) 1.0, the individual rates for olefin formation are referred to fraction of total reactant mass

2

3

4

Conclusions

298


trial process, where the serious problem caused by the rapid deactivation of this catalyst by coke will have to be solved by means of a suitable reactor design and the provision of a regeneration unit. In the kinetic modeling of deactivation, the concentration of water, which efficiently attenuates deactivation, will play an important role. Acknowledgment This work was carried out with the financial support of the Department of Education, University and Research of the Basque Country (Project No. PI96/10), and of the Ministry of Education and Culture of the Spanish Government (Project CICYT PB97-0644). Nomenclature

Figure 8. Mass fraction on a water-free basis, for zero time on stream, of each olefin and of oxygenates (methanol + dimethyl ether) for different values of space time and of reaction temperature. Xwo ) 3. Points, experimental results. Lines, calculated with the kinetic model.

(water in basis), xi:

For ethene formation: rC2) ) k1xA

(28)

For propene: rC3) ) k2xA

(29)

For butenes: rC4) ) k3xA

(30)

For remaining hydrocarbons: r(C5)+par) ) k4xA (31) where k1-k4 are calculated using eqs 24-27. This kinetic model proposed is useful for quantifying the individual formation of each olefin C2-C4 on SAPO34 in a wide range of operating conditions. Moreover, it is simple enough to combine it in future papers with kinetic models for deactivation, which will allow for evaluation of the performance of SAPO-34 in an indus-

A ) oxygenates (methanol + dimethyl ether) E ) activation energy, cal mol-1 EOF ) error objective function FMo ) mass flow of methanol in the feed, g h-1 Kw ) parameter that quantifies the resistance to the formation of component i in the corresponding reaction step, because of the presence of water in the reaction medium ki ) kinetic constant of step i in the kinetic scheme ki0 ) kinetic constant of step i in the kinetic scheme at the reference temperature R ) constant of gases, cal mol-1 K-1 ri ) reaction rate for formation of component i, g h-1 (g of catalyst)-1 T ) temperature, K T0 ) reference temperature for reparametrization of kinetic constants t ) time on stream, h W ) catalyst weight, g Xi, xi ) weight fraction of component i, on a water-free basis and on a total basis Xw ) weight fraction of water in the reaction medium, on a water-free basis Xwf ) weight fraction of water formed in the reaction medium, on a water-free basis Xwo ) weight fraction of water in the feed, on a water-free basis (or water/methanol ratio in mass) ξ ) dimensionless longitudinal coordinate of the reactor

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Received for review March 15, 1999 Revised manuscript received October 29, 1999 Accepted November 11, 1999 IE990188Z

Kinetic Modeling of Methanol Transformation into Olefins on a SAPO

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