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Methanol conversion to light olefins over SAPO-34: kinetic modeling of coke formation
Microporous and Mesoporous Materials 35–36 (2000) 121–135 www.elsevier.nl/locate/micromeso
Methanol conversion to light olefins over SAPO-34: kinetic modeling of coke formation D. Chen a, H.P. Rebo a, A. Grønvold b,1, K. Moljord b,2, A. Holmen a, * a Department of Chemical Engineering, Norwegian University of Science and Technology (NTNU), N-7491 Trondheim, Norway b SINTEF Applied Chemistry, N-7465 Trondheim, Norway Received 8 April 1999; received in revised form 6 July 1999; accepted for publication 9 July 1999 Dedicated to the late Werner O. Haag in appreciation of his outstanding contributions to heterogeneous catalysis and zeolite science
Abstract Coke deposition during methanol conversion to light olefins over SAPO-34 has been studied in an oscillating microbalance (TEOM ) reactor as a function of space velocity (57–384 g/g h), temperature (673–823 K ) and cat methanol partial pressure (7.2–83 kPa). Two kinetic models were tested for their ability to describe the coking rate at different operating conditions. A modified Voorhies model related coke deposition to the amount of hydrocarbons formed per gram of catalyst. The model could be used to calculate the average coke selectivity and catalyst capacity for olefin formation. The average coke selectivity increased and the catalyst capacity decreased with increasing temperature, while no effect of methanol partial pressure or space velocity was observed. A kinetic model based on a mechanism with strongly adsorbed reaction intermediates as the coke precursors, which either desorb as olefins or react further into coke, was also fitted to the experimental data, and the deactivation functions were found to be linear in coke content for both olefin and coke formation. The rate of deactivation of the coke-forming reaction decreased, while the rate of deactivation of the olefin formation increased with increasing temperature, and no effect of methanol partial pressure or space velocity was observed. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Coke deposition; Deactivation; Kinetics; Oscillating microbalance; SAPO-34
1. Introduction Zeolite-type catalysts are used widely in petrochemical and petroleum refining processes [1]. The * Corresponding author. Tel: +47-73-59-41-51; fax: +47-73-59-50-47. E-mail address: [email protected] (A. Holmen) 1 Present address: Norsk Hydro Research Center, Box 2560, N-3907 Porsgrunn, Norway. 2 Present address: Statoil Research Center, Postuttak, N-7005 Trondheim, Norway.
catalytic processes using zeolites often include side reactions, leading to the formation of carbonaceous material with catalyst deactivation as a result. More exact knowledge about the mechanism and kinetics of the coke formation is a basis for improving these catalytic processes. The catalytic conversion of methanol to lower olefins (MTO) is an interesting and promising way of converting natural gas and coal to chemicals via methanol [2,3]. Coke deposition is known to be the major cause of deactivation in the MTO reaction over SAPO-34, and both activity and selectiv-
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Nomenclature a empirical constant in the modified Voorhies model b empirical constant in the modified Voorhies model CAHF cumulative amount of hydrocarbons formed per gram of catalyst (g/g ) cat C coke content (g /100 g , dry) coke cat C coke content in the ith CSTR i Cexp experimental coke content at the jth experiment j Ccal calculated average coke content at the jth experiment av,j C1 concentration of the coke precursor C initial concentration of oxygenates A0 C concentration of oxygenates in the ith CSTR A,i C inlet concentration of oxygenates to the ith CSTR Ain,i CAMF cumulative amount of methanol fed to the catalyst (g /g ) MeOH cat F molar flow rate of MeOH (mol/h) A0 k0 initial reaction rate constant of oxygenates (s−1) oxy k0 initial rate constant for coke formation C m number of experiments N number of CSTRs used in the model n reaction order OF objective function r0 initial rate of oxygenates conversion, on a CH basis (kmol/kg s) oxy 2 cat r0 initial rate of coke formation (kg /kg h) C coke cat RTC reactant to catalyst ratio S average coke selectivity (wt%) av,C S instantaneous coke selectivity (wt%) C T reaction temperature ( K ) t time on stream (h) W catalyst loading (g ) cat WHSV weight hourly space velocity (on a methanol basis) (kg /kg h) MeOH cat X conversion of oxygenates until the ith CSTR, CH basis i 2 Xexp experimental total conversion of oxygenates, CH basis j 2 Xcal calculated total conversion of oxygenates, CH basis j,N 2 Dx concentration of oxygenates in the ith CSTR i a empirical deactivation constant defined by Eq. (1) c2 R-squared value r catalyst density (kg /m3) S cat t space time in the ith CSTR (kg h/kg ) i cat MeOH w deactivation function for the coke formation C w deactivation function for the conversion of oxygenates oxy
ity are influenced by coke deposition [4–8]. Most authors [4–7] so far have studied the deactivation process as a function of time on stream and at a methanol conversion close to 100%. However, it is difficult to abstract a detailed mechanism of
coke formation and deactivation when coke deposition is not measured or conversions are so high that the effect of secondary reactions of olefins dominates. A kinetic model for the MTO reaction over
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SAPO-34 was recently reported [9] where coke formation and deactivation were studied by an integrated pulse method in a fixed-bed reactor as a function of the coke content of precoked catalyst samples. The rate of coke formation was not investigated in detail due to the difficulty of coke deposition measurements. Grønvold et al. [7] have studied coke formation and deactivation during MTO over SAPO-34 in a conventional microbalance reactor. As discussed previously [10,11], it is very difficult to obtain kinetic data for the coking reaction and the resulting deactivation during MTO over SAPO-34 in a conventional microbalance, owing to the high reaction rates including coke formation and poor exchange of mass and heat between the catalyst and the flowing gas. However, the oscillating microbalance (tapered element oscillating microbalance, TEOM ) reactor with its fixed-bed characteristics is a much more suitable technique for studying fast coke formation and deactivation processes [10,12,13]. The purpose of the present work was to gain a better quantitative understanding of coke formation during the MTO reaction over SAPO-34, using a TEOM reactor. The modeling of coke deposition and deactivation was reviewed by Froment in 1976 [14] and 1991 [15]. Earlier work was aimed at relating the coke deposition, as well as the deactivation, empirically to the time on stream based on the observation that coke formation was not dependent on the space velocity [16 ]. This might not be valid for different feeds and catalysts. However, Voorhies’ rate law has been widely accepted [17,18] and generalized beyond the scope of its original contribution. As pointed out by Froment [14], this type of equation has limited predictive power because it completely ignores the origin of the coke. This led Froment [14] to propose that one should relate catalyst deactivation to its coke content, and use the gas composition, temperature and catalyst activity as parameters to describe coke formation. The present work deals with the detailed study of coke formation as a function of operating conditions such as space velocity, temperature and partial pressure of methanol. Models for coke formation are proposed starting from the simple modified Voorhies model and proceeding to a more elaborate model.
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2. Experimental The TEOM reactor, the experimental procedures and the catalyst properties were as described previously [10,19]. Calcined SAPO-34, with a unit-cell composition of (Si Al 2.88 18 P )O , was obtained from SINTEF Applied 15.12 72 Chemistry. The SAPO-34 particles (52–140 mesh) were dried in situ at 500°C in flowing helium for more than 3 h. The MTO reaction was performed at a WHSV ranging from 57 to 385 g/g h, a cat methanol partial pressure ranging from 7.2 to 83 kPa, and temperatures between 673 and 823 K. The runs with different space velocities were carried out at 698 K and a methanol partial pressure of 7.2 kPa, to obtain relatively low coking rate. The runs with different methanol partial pressures were also performed at 698 K. The space velocity was adjusted to ensure an identical conversion when the partial pressure of methanol was increased. For high methanol partial pressures, very high space velocities were required, as a result of very rapid coke formation. In order to follow the rapid activity change, methanol was introduced by pulses which allowed the completion of gas chromatography (GC ) analyses between each pulse. The small void volume in the tapered element makes TEOM suitable for pulse experiments and TEOM is therefore also referred to as a pulse mass analyzer. Pulses of 9 s duration were selected in the present work for high methanol pressures, while 3 min pulses were used for methanol partial pressures less than 30 kPa. Pulses 1 min long were used for high-temperature experiments (773– 823 K ). The pulse method was compared to continuous experiments with respect to coke formation and the deactivating effect of coke. The effect of pulse sizes on coke formation and deactivation was investigated at 30 kPa and WHSV= 768 g/g h. cat The reactor effluent including unconverted MeOH and dimethylether (DME ) was analyzed on either on an HP 5890 or an HP 6890 gas chromatograph. The HP 5890 was equipped with FID and a GS-Q column (30 m, 0.53 mm). The HP 6890 was equipped with FID and TCD, GS-Q (30 m, 0.53 mm) and HP plot molsieve 5A columns. In the latter case, N was mixed with the 2
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reactor effluent and used as an internal standard. The oxygenates were converted to methane over a nickel catalyst in the GC and the calculated carbon mass balance was close to 100%. Some definitions used in the discussion are summarized in the following. $ Cumulative amount of methanol fed to the catalysts (CAMF )=t×WHSV (g/g ). cat $ Cumulative amount of methanol fed to the catalysts (CAMF, CH basis)=CAMF×14/32. 2 $ Cumulative amount of hydrocarbon formed (CAHF ) (g/g )=∆CAMF X d(CAMF ). cat 0 $ For 3 min pulses CAHF is approximately CAHF=SX (3/60)WHSV, where j is the j number of pulses. $ Catalyst capacity (g /g )=amount of hydrocarbon cat hydrocarbon formed per gram of catalyst until a certain loss in catalyst activity. $ Average coke selectivity (wt%)=the ratio of the coke content to the amount of hydrocarbon formed. $ The amount of coke on the catalyst is given as g of coke/100 g of dry, coke-free catalyst.
3.1. Effect of helium stripping The mass response curves for pulses during the MTO reaction over SAPO-34 were shown in our previous work [19]. The permanent mass increase after stripping for about 40 min in helium was defined as the coke formed for each pulse. However, the effect of stripping in helium on the coke formed must also be considered. Stripping in helium could influence the nature of the coke or the location of the coke; for example, coke molecules might grow into large entities when given time at high temperature. Such effects were checked by comparing pulse and continuous experiments at 698 K, 7.2 kPa and WHSV= 57 g/g h. Fig. 1 shows the changes in the convercat sion of methanol to DME and hydrocarbons and the coke formation with time on stream for both 3 min pulses and continuous experiments, where the data in the continuous experiments were obtained from two separate runs. It was found that stripping in helium did not significantly influence the coke deposition and the deactivating effect of coke. The residual 40% conversion at 20 wt% coke was due to DME formation. The stripping effect was also investigated by
3. Results As discussed previously [10], one of the main advantages of the oscillating microbalance reactor is that it provides a packed bed of catalyst whose mass is measured continuously and through which all the reactants are forced to flow. The classical methods for checking for mass transfer limitations in a fixed-bed reactor can therefore be applied to the TEOM reactor. Both the catalyst loading and the flow rate were adjusted to keep the space velocity identical (385 g/g h) at 698 K and a cat methanol partial pressure of 7.2 kPa. Catalyst loadings of 3 and 5.5 mg gave at this space velocity almost identical initial reaction rates, coke deposition and deactivation rates, indicating that external mass transfer limitation can be neglected. However, intracrystalline diffusion limitations could not be avoided at the reaction conditions employed in the present work [19].
Fig. 1. Coke deposition and methanol conversion for 3 min pulse experiments and continuous experiments during MTO over SAPO-34 at 698 K, WHSV=57 g/g h, P =7.2 kPa. cat MeOH Coke formation: line, continuous experiment; $, 3 min pulses. Conversion: &, continuous experiments; #, 3 min pulses.
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stripping in helium. This implies that the carbonaceous material defined as ‘coke’ (i.e., the material left on the catalyst after initial desorption of reactants and products) is adsorbed or trapped in the catalyst pores and does not change its deactivating effect during the stripping periods. The integrated pulse method is therefore used for the kinetic measurements in MTO. 3.2. Effect of space velocity In this series of experiments, space velocity was adjusted by varying the catalyst loading while maintaining a constant total flow rate. Fig. 3 shows the coke deposition as a function of the cumulative amount of methanol fed to the reactor. The results in Fig. 3 demonstrate that the coke deposition depends on the space velocity, and thereby on the conversion of methanol or oxygenates (methanol and DME ) to olefins. Lower space velocities gave higher conversion of oxygenates [Fig. 4(A)], resulting in a higher coking rate. However, when the space velocity was lowered from 82 to 57 g/g h, the coking rate changed only slightly cat ( Fig. 3). The reason is that the conversion depends to only a small extent on space velocity at higher
Fig. 2. Coke formation (A) and methanol conversion (B) during MTO over SAPO-34 as a function of the cumulative amount of methanol fed (CAMF ). T=698 K, WHSV=768 g/g h and P =30 kPa for different pulse cat MeOH sizes: 6, 3 min; 1, 1 min; ), 0.5 min; #, 0.15 min.
using different pulse lengths. Fig. 2 shows the coke deposition [Fig. 2(A)] and the conversion of methanol to DME and hydrocarbons [Fig. 2(B)] as a function of the cumulative amount of methanol fed to the catalysts at a methanol partial pressure of 30 kPa and WHSV=768 g/g h for pulse cat lengths between 9 s and 3 min. Fig. 2(A) and Fig. 2(B) clearly show that the coke deposition and the deactivation were not influenced by the
Fig. 3. Coke formation during MTO over SAPO-34 versus the cumulative amount of methanol fed to the catalysts, CAMF (g/g ), at 698 K and P =7.2 kPa for different space velocicat MeOH ties, WHSV (g/g h): %, 57; 1, 82; 6, 113; #, 270; ), 385. cat Lines: predicted by Model II.
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be described by: X=X exp(−aCAMF ). (1) 0 Both the conversion X and CAMF in Fig. 4(B) and Eq. (1) are on a CH basis. The initial 2 conversion X and the constant a were determined 0 by curve fitting. The CAHF at any time on stream or CAMF can then be calculated by integration of Eq. (1), as defined in the Experimental section. Fig. 5 illustrates the coke deposition versus the amount of hydrocarbon formed at different reaction conditions. The coke deposition is independent of WHSV at a given temperature, but increases with increasing temperature. A higher coke formation was found for the case of WHSV=57 g/g h (Fig. 5). This is probably a cat result of the high conversion of oxygenates (close to 100%), and possible temperature gradients in the catalyst bed at high conversions caused by the highly exothermic reaction [20]. Some important catalyst properties can be deduced from the results given in Fig. 5, e.g., catalyst capacity and coke selectivity. If, for example, a coke content of 18 wt% is defined as a final
Fig. 4. Conversion of oxygenates at 698 K and P =7.2 kPa for different space velocities, WHSV (g/g h) MeOH cat (1, 82; +, 113; n, 385), versus the coke content (A) and the cumulative amount of methanol fed (CAMF ) (B).
conversions, which is generally the case for firstorder reactions. The oxygenate conversion at different space velocities is plotted against the coke content in Fig. 4(A) and against the cumulative amount of methanol fed to the catalysts in Fig. 4(B). From Fig. 4(B), the cumulative amount of hydrocarbon formed per catalyst mass (CAHF, g/g ) at a cat certain time on stream or CAMF can be determined. The change in conversion with CAMF can
Fig. 5. Coke formation versus the amount of hydrocarbon formed (CAHF ) at different temperatures: %, 823 K (P =13 kPa, WHSV=270 g/g h); #, 773 K (P = MeOH cat MeOH 13 kPa, WHSV=270 g/g h); 698 K [P =7.2 kPa and cat MeOH WHSV=57 (&), 82 (1), 113 (+), 270 ($) and 385 (n) g/g h]; 6, 673 K (P =7.2 kPa, WHSV=385 g/g h). cat MeOH cat Lines: predicted by Eq. (3).
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state, the catalyst capacity at different temperatures can be directly obtained from Fig. 5, as presented in Table 1. The catalyst capacity decreases and the average coke selectivity increases with increasing temperature. 3.3. Effect of partial pressure of methanol Preliminary experiments showed that the conversion and the coke deposition increased with increasing partial pressure of methanol. Relatively low conversions are required to obtain useful kinetic data, and the effect of higher methanol partial pressure was compensated for by a corresponding increase in space velocity. In this way the conversion was kept nearly constant, illustrating that the MTO reaction over SAPO-34 has close to first-order characteristics in oxygenate conversion. Fig. 6 shows the coke deposition as a function of the cumulative amount of methanol fed to the catalysts for different methanol partial pressures and space velocities but at identical oxygenate conversion at 698 K. The fact that the coke deposition was almost identical for the different experiments implies that coke deposition can be directly related to the total amount of hydrocarbons formed during the reaction, regardless of the methanol partial pressure. This conclusion is valid at least at the conditions used in the present work.
Fig. 6. Coke formation versus the cumulative amount of methanol fed to the catalysts (CAMF ) (g /g ) at 698 K, for different cat P and WHSV values: ), 15 kPa and 384 g/g h; $, MeOH cat 30 kPa and 768 g/g h; +, 60 kPa and 1538 g/g h; %, 83 kPa cat cat and 2558 g/g h. cat
3.4. Effect of temperature Fig. 7 shows the coke deposition versus CAMF at different temperatures. Higher temperatures Table 1 Catalyst capacities and average coke selectivities in MTO over SAPO-34 at different temperatures. 18 wt% coke is defined as the final state Temperature (K)
Catalyst capacity (g /g ) hydrocarbons cat
Average coke selectivity (g /g , %) coke hydrocarbon
673 698 773 823
29.3 15.2 8.8 4.0
0.6 1.2 2.0 4.5
Fig. 7. Coke formation versus the cumulative amount of methanol fed to catalysts (g/g ) at different conditions: cat WHSV=270 g/g h and P =13 kPa — &, 823 K; $, cat MeOH 773 K; n, 698 K; and at WHSV=385 g/g h and cat P =7.2 kPa — %, 698 K; #, 673 K. Lines: predicted by MeOH Model II.
resulted in higher coke deposition. This is consistent with the results reported for methanol conversion to gasoline (MTG) over HZSM-5 [21]. Both Fig. 5 and Table 1 show that the average coke
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selectivity increases with temperature, meaning that higher temperatures favor the coke-forming reaction. Since we also observe that the selectivity to light paraffins increases with increasing temperature, the effect might be due to an increasing rate of hydrogen transfer between two coke molecules or between an olefin and a coke molecule. The coke formed during MTO over SAPO-34 has been found to consist of mono-aromatics at low coke content with an increasing fraction of di- and triaromatics at higher coke contents [11]. The formation of coke involves oligomerization, cyclization, hydrogen transfer and alkylation [22]. The final coke content was about 20 wt%, rather independent of the operating conditions. As a result of the higher coke selectivity, the catalyst capacity for olefin production decreases with increasing temperature ( Table 1).
Fig. 8. Coke formation (g/100 g , dry) during MTO over cat SAPO-34 versus time on stream (min) at 698 K and a methanol partial pressure of 7.2 kPa at different space velocities, WHSV (g/g h): %, 57; 1, 82; 6, 113; ), 385. cat
4. Discussion and modeling 4.1. Mechanism of coke formation Coke deposition is often studied as a function of time on stream [21]. Marchi and Froment [5] studied deactivation during MTO over SAPO-34 as a function of the cumulative amount of methanol fed to the catalyst. Results obtained from both approaches are given and discussed in the present work. Figs. 3 and 6 display the coke content as a function of CAMF, while Figs. 8 and 9 present the coke content as a function of time on stream at different space velocities and partial pressures, respectively. The interpretation based on these two types of figure leads to contradictory results. Fig. 8 reveals that a higher space velocity (or lower contact time and lower conversion) gives a higher coke deposition, as observed for coke deposition during MTG over HZSM-5 [21]. This result indicates that oxygenates might be the coke precursors. However, the fact that the coke content is proportional to the cumulative amount of hydrocarbon formed per catalyst mass (CAHF ) regardless of space velocity (Fig. 5) and methanol partial pressure ( Fig. 6) suggests that the olefins are the coke precursors.
Fig. 9. Coke formation versus time on stream at constant conversion for different methanol partial pressures and space velocities: ), 15 kPa and 384 g/g h; $, 30 kPa and 768 g/g h; cat cat +, 60 kPa and 1538 g/g h; %, 83 kPa and 2558 g/g h. cat cat
It has been shown that the coking rate for propene conversion is much lower than for methanol conversion over SAPO-34 [8], as has also been observed in ethene and propene conversion over SAPO-34 using ethanol and propanol as the feed
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[23]. The lower coke deposition from olefins during MTO might be explained by the difference in adsorption strength between olefins and polar molecules, such as methanol, DME and water. The adsorption strength of olefins is weaker than of methanol at the reaction conditions studied [19]. The competitive adsorption between polar molecules (methanol, DME and water) and olefins during MTO over SAPO-34 could give less olefins adsorbed and thus a lower coke deposition directly from olefins [5]. Dahl and Kolboe [24,25] demonstrated by isotopic tracer studies that the reactivity of ethene and propene was quite low during the MTO reaction over SAPO-34. Salehirad and Anderson also found by nuclear magnetic resonance spectroscopy (NMR) that olefin methylation is not the dominating route for olefin formation over SAPO-34 [26 ]. In addition, an experiment at low space velocity (20 g/g h) in cat the TEOM shows that the deposition of coke is higher at the entrance of the reactor, which is consistent with the observation of Bos et al. [9] in an isothermal fixed-bed reactor. If olefins are the major coke precursors, the coke deposition should be higher at the reactor exit. These results indicate that the olefins are not the major coke precursors during the MTO reaction. From a mechanistic point of view, surface oxonium ions can be formed from methanol and dimethylether through a chain growth process, and carbenium ions can be formed from these surface intermediates via b-elimination [26–28]. Carbenium ions will either form olefins and desorb or react to form larger molecules such as coke. A higher conversion of oxygenates results in a higher concentration of carbenium ions, and thus a higher rate of olefin formation as well as a higher coking rate, indicating that the carbenium ions inside the pores might be the coke precursors. The lower coking rate during olefin conversion over SAPO-34 might also be explained by the lower rate of carbenium ion formation. Although NMR [26 ] and isotopic labeling studies [24,25] have been used to study the detailed reaction mechanism, the exact nature of the intermediates inside the pores during methanol conversion as well as coke formation over SAPO-34 remain to be determined. If we ignore the complex-
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ity of the intermediates inside the pores, a simple mechanism of coke deposition can be proposed:
(2) where the oxygenates include methanol and dimethylether, and all hydrocarbons are lumped together based on the observation that the olefins are formed from the oxygenates in parallel [29]. The intermediates are considered to be a mixture of carbenium ions with different carbon numbers inside the pores. This simple mechanism for coke formation is similar to the hydrocarbon pool mechanism [24,25].
4.2. Criteria in catalyst evaluation The catalyst lifetime is normally defined as the time on stream before catalyst deactivation has reached a certain level, and is widely used in catalyst screening. At identical reaction conditions, the conversion level during screening experiments can vary for different samples due to their different activities, and the rate of catalyst deactivation may vary with the conversion. Conversion can be kept constant by adjusting space velocity in such experiments. A change in space velocity includes a change in the amount of methanol fed to the catalyst, hence the amount of hydrocarbon formed and the coke content on the catalyst. Therefore, using catalyst lifetime for catalyst screening could lead to a wrong conclusion, as it ignores the effect of the amount of methanol fed to the catalyst. The amount of hydrocarbons formed (CAHF ) as a measure of the catalyst capacity is suggested as a more precise parameter for catalyst screening, similar to the concept of the catalyst utilization value (CUV ) used by van Niekerk et al. [30]. They defined the catalyst utilization value as the mass of methanol/DME converted to hydrocarbons per gram of catalyst, before the conversion level drop below half of the maximum conversion.
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Table 2 Parameters in Model I (C (wt%)=a[CAHF ]b) for MTO over SAPO-34 at different temperatures Temperature ( K )
a
b
673 698 773 823
1.09 2.09 1.71 3.24
0.83 0.80 1.08 1.23
4.3. Modeling of coke deposition 4.3.1. Model I A simple Voorhies model [16 ] relating coke formation to time on stream is too simple for the MTO reaction over SAPO-34. Experimental results show that the coke deposition is influenced by the reaction conditions including space velocity, methanol partial pressure and temperature. The parameters in the Voorhies model depend on the operating conditions, as has been shown also for coke deposition during MTG over HZSM-5 [21], limiting its application. However, the coke deposition during the MTO reaction over SAPO-34 at constant temperature essentially depends on the cumulative amount of hydrocarbon formed (CAHF ), regardless of space velocity (Fig. 5) and methanol partial pressure (Fig. 6), indicating that CAHF is a more precise parameter than time on stream. The Voorhies coking model can therefore be modified and the coke formed can be described by: C (wt%)=a[CAHF ]b,
S
av,C
(wt%)=
C [CAHF ]
=a1/bC1−1/b.
(5)
By defining a coke content of 18 wt% as the final state, the average coke selectivity can easily be calculated by Eq. (5), and the catalyst capacity by Eq. (3). These parameters are presented in Table 1. Eq. (3) can be used to estimate the coke content on SAPO-34 based on the rate of hydrocarbon formation. Since it is difficult to measure the coke content in practice, the deactivation can be directly related to the cumulative amount of hydrocarbons formed in this case, as has been demonstrated by Sedran et al. [31] in modeling of the deactivation behavior of methanol conversion to hydrocarbons over HZSM-5. However, Voorhies’ relationship provides little information about the mechanism of coke deposition, since it ignores the coking reaction itself. In addition, this type of equation is relatively difficult to apply in reactor design, since the amount of hydrocarbons formed is a function of the coke content.
(3)
where a and b are parameters to be estimated. Their values are presented in Table 2 for different temperatures. a and b did only depend on the temperature. The instantaneous coke selectivity is a useful value for a detailed understanding of the effects of coke deposition on the coking and olefin formation inside the pores. The instantaneous coke selectivity (S , wt%) can be calculated by derivation of C Eq. (3): dC =ab[CAHF ]b−1. S = C d CAHF
By substituting CAHF in Eq. (4) with Eq. (3), the coke selectivity can be correlated with the coke content. The calculated instantaneous coke selectivity at a given temperature decreased with the coke content, indicating that the coke deposition deactivated more rapidly than the olefin formation, and this has been explained in terms of a shapeselectivity effect induced by coke [10]. The average coke selectivity (S , wt%) is a useful parameter av,C for catalyst evaluation. From Eq. (3) the coke selectivity can be expressed as:
(4)
4.3.2. Model II Due to the restrictions of Model I [Eq. (3)], a model proposed by Froment and Bischoff [32] can be a better choice for application in reactor design. A proper kinetic model should describe all the experimental observations, such as the changes in the coke content versus time on stream (Figs. 8 and 9), versus the cumulative amount of methanol fed to the catalyst (Figs. 3 and 4) and versus the amount of hydrocarbon formed ( Fig. 5). The conventional model of Froment and Bischoff relates the coking rate (dC/dt) to the concentration of coke precursor and the deactivation function of
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the coking reaction: dC dt
=k0 (C1)nw , C C
(6)
where k0 is the initial coking rate constant, n is C the reaction order and w is the deactivation funcC tion for the coking reactions. C1 is the concentration of the coke precursor, which is assumed to be proportional to the conversion of oxygenates (C1=X C , C being the initial concentration oxy A0 A0 of methanol and X being the oxygenate converoxy sion). Bos et al. [9] used this type of model with ethene and propene as the coke precursors to describe coke deposition during MTO over SAPO-34 in a fixed-bed reactor. Preliminary modeling showed a proper description of the coke content versus time on stream, but the model seems to be unappropriate to describe the effect of space velocity illustrated in Fig. 3. A modified model described in the following section was therefore developed. Although WHSV is rather high, the initial conversion is still relatively high due to the high catalyst activity, and the reactor should be treated as an integral plug-flow reactor instead of a differential reactor. The concentration of coke precursors will then vary in the reactor, and a distribution of coke is expected. In order to simplify the calculation, a series CSTR model is used for calculation of the coke deposition and the conversion of the oxygenate. The coke is assumed to be uniformly distributed in each CSTR reactor element, and the kinetic model is based on the simple reaction scheme described in Eq. (2). The deactivation of the transformation of the oxygenate is a transient process. However, the superficial gas velocity in the TEOM is about 1 m/s, and the residence time in the reactor is in the range of milliseconds. Since no appreciable deactivation occurs over a time period of milliseconds, the process can be treated as pseudo steady-state. The mass balance for the ith CSTR in the series is: Dx =D(W/F )r /r i A0 A,i S and
(7)
=k0 w C , oxy oxy A,i
(8)
r
A,i
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where W is the catalyst mass (g ), F is the cat A0 molar flow rate of methanol (mol/s), r is the A reaction rate of the oxygenate (mol/cm3 s), r is S the catalyst density (1.48 g/cm3, [19]), D(W/F )=(W/F )/N is the space time in the ith A0 A0 CSTR and w is the deactivation function for the oxy conversion of oxygenates. The oxygenates conversion is treated as a first-order reaction in Eq. (8), where C is the concentration of oxygenates A,i (mol/cm3) in the ith CSTR and k0 is the initial oxy rate constant (s−1). Dx is the conversion of i oxygenates in the ith CSTR, which can be written as: 1 , Dx =1− i 1+D(W/F )/r C k0 w A0 S Ain,i oxy oxy where
(9)
=C (1−Dx ). (10) Ain,i Ain,i−1 i−1 C is the inlet concentration of ith CSTR. The Ain,i conversion of oxygenates until ith CSTR is then calculated by: C
C −C Ain,i+1 . X = A0 i C A0 The coking rate in ith CSTR is: dC dt
i =r0 w . C,i C
(11)
(12)
The initial coking rate in the ith CSTR (r0 ) is C,i considered to be proportional to the amount of intermediates, which is a function of the conversion of oxygenates and the reactant to catalyst ratio ( RTC, g /g ). The initial coking rate is then feed cat described by: r0 =k0∞RTCDx . (13) C,i C i The RTC is similar to the oil-to-catalyst ratio in FCC [33]. The ratio of reactant to catalyst is an easily available parameter in a fluidized-bed reactor, but this parameter is not well defined for a fixed-bed reactor. Eq. (13) must therefore be rewritten in the form of: r0 =k0 Dx /t , (14) C,i C i i where the space time t in ith CSTR is defined as i
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t =(DWHSV )−1 (g h/g ). It has to be pointed i cat feed out that k0 in Eq. (14) is not identical to k0∞ in C C Eq. (13). Eq. (14) relates the initial coking rate directly to the rate of oxygenate conversion. Several empirical expressions given by Froment and Bischoff [32], relating the rate of oxygenates conversion and the coking rate to the coke content, were statistically tested. A linear relationship between the degree of deactivation and the coke content [Eq. (15)] was found to give the best fit for both the MTO reaction and the coke deposition: w
oxy
=1−a
oxy
C
and w =1−a C. C C
(15)
Integration of the continuity equation [Eq. (12)] combined with Eqs. (14) and (15) yields: 1 C= i a C
C
A
t 1−exp −a k0 Dx C Ct i i
BD
,
(16)
where t/t =D(CAMF ) (g /g ). This equation i MeOH cat therefore relates coke formation to the cumulative amount of methanol converted, which is in good agreement with the experimental observations. 4.4. Determination of the kinetics of the MTO reaction including coke deposition The procedure for developing a kinetic model of a process subject to deactivation due to coking usually consists of three steps. In the first step, the kinetics for the main reaction is studied in the absence of coking. In the second step, the deactivation functions are related to the coke content of the catalyst and, finally, the third step is development of a kinetic model for coke formation related to the process time. This procedure requires conversion data on the coke-free catalyst, which are normally obtained by extrapolating to zero time. However, the procedure might be hazardous for processes with very fast coking. For the MTO reaction, the coke content was already 2–3 wt% after only 2 min on stream using a methanol partial pressure as low as 7.2 kPa. Consequently, extrapolation to zero time to obtain the initial conversion or the initial reaction rate on a coke-free catalyst has large uncertainties. However, data for the
conversion versus coke content, which are obtained by the pulse technique in the TEOM reactor, made it possible to regress kinetic models for the main and coking reaction simultaneously, as well as for the catalyst deactivation. It has to be noted that the initial rate for conversion of oxygenates used in the model is a pseudo value, since an increasing conversion with coke formation has been observed at the start of the reaction by 9 s pulse experiments. For example, the conversion to hydrocarbons (CH basis) was 2.7% and 55.2% for the first and 2 the second pulse, respectively, at a methanol partial pressure of 83 kPa, temperature of 698 K and weight hourly space velocity of 2558 h−1. It has been shown that the increase in conversion of oxygenates with coke deposition depends on the effects of intracrystalline diffusion, and that strong diffusion limitations lead to a lower coke content corresponding to a maximum in the conversion of oxygenates [29]. This coke content corresponding to maximum conversion was about 1 wt% at 698 K on the SAPO-34 used in the present work. It is rather low compared with the 4 wt% on 2.5 mm crystals and 7 wt % on 0.25 mm crystals at identical reaction conditions, indicating strong diffusion limitations at the reaction conditions on the SAPO-34 employed in the present work. It is in good agreement with the results obtained by diffusion measurement [19]. The estimation of the parameters in the kinetic model has been carried out by the non-linear leastsquares routine in using the Levenberg– Marquardt method [34]. The objective function OF was calculated:
A
B
A
B
−Xcal 2 m Cexp −Ccal 2 m Xexp j j,N + ∑ j av,j , OF= ∑ Xexp Cexp j=1 j=1 j j (17) where Xcal are the calculated values of the total j,N conversion of the oxygenates at a given coke content and at the experimental point j. Xexp are j the corresponding experimental values. Ccal are av,j the calculated average coke content at the experimental point j, which is calculated by Eq. (18) and Cexp is the corresponding experimental value. j
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Due to the large variation in conversion and coke content in the experiments, 1/Xexp and 1/Cexp are j j used as the weighting factors:
A
B
N (18) C = ∑ C /N. i av i=1 The effect of N in the model was tested and N=20 was selected. The estimated parameters are presented in Table 3. The comparisons between the calculated values and the experimental values for coke deposition are shown in Fig. 3 for different space velocities at 698 K and a methanol partial pressure of 7.2 kPa, and in Fig. 7 at different temperatures. The model fitted the experimental data well, except for the low space velocities in Fig. 3. This is possibly a result of a temperature raise in the catalyst bed due to the high conversion, as discussed previously. Fig. 7 clearly shows the higher coking rate at the higher temperature. The model shows that the olefin formation and the coke formation during MTO have identical deactivation functions, but the deactivation rate constants (a for coke deposition and a for C oxy oxygenates conversion) are different. Both a oxy and a are independent of WHSV and methanol C partial pressure, but they depend on the temperature. a decreases and a increases with increasC oxy ing temperature ( Table 3). Higher a indicates a oxy higher deactivation rate. It also indicates that the toxicity of the coke on the olefin formation is higher at higher temperatures. The faster deactivation for oxygenates conversion and the slower deactivation for coke deposition result in higher coke selectivity at a higher temperature, which is in good agreement with the experimental observations ( Fig. 5). The temperature dependence of k0 C and k0 for coke deposition and conversion of oxy
133
oxygenates respectively are presented in Fig. 10. The data fitted the Arrhenius correlations well: k0 =3.96×1012 exp(−117 735/8.314T ), oxy c2=0.9897
(19)
and k0 =1.27 exp(−6707/8.314T ), c2=0.9968. C (20) The apparent activation energy (6.7 kJ/mol ) for the coke deposition was calculated from the coking rates at 698, 773 and 823 K. Between 673 and 698 K, the apparent coking activation energy is much larger (30 kJ/mol ). The reasons for the extremely low apparent activation energy for coke formation are not yet clear, but could be explained by diffusion limitations. The apparent activation energy for the olefin formation is 118 kJ/mol, which is very close to the value of 129.3 kJ/mol reported by Sedran et al. [31] for olefin formation from methanol over HZSM-5. The true activation energy should be larger if the effect of intracrystalline diffusion limitation is taken into account.
Table 3 Parameters in Model II for MTO over SAPO-34 at different temperatures Temperature ( K )
k0 C
a C
k0 (s−1) oxy
a oxy
673 698 773 823
0.33 0.40 0.45 0.48
0.061 0.058 0.035 0.033
2684 7235 35 132 150 941
0.043 0.052 0.057 0.070
Fig. 10. Arrhenius plot for the MTO reaction and the coking rate constant: %, MTO; #, coking reaction.
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5. Conclusions
References
Methanol conversion to olefins and coke over SAPO-34 has been studied in an oscillating microbalance ( TEOM ) as a function of temperature, methanol partial pressure and WHSV. It has been found that the coke deposition in MTO must be compared based on the amount of methanol fed to the catalyst instead of time on stream. Kinetic modeling was performed on the basis of a mechanism with carbenium ions as the coke precursors, which either desorb as olefins or react further into coke. Two kinetic models were tested for their ability to describe the rate of coke formation. A modified Voorhies model related coke deposition to the amount of hydrocarbons formed per gram of catalyst. This model can be used to calculate the average coke selectivity and catalyst capacity for olefin formation. The average coke selectivity increased and the catalyst capacity decreased with increasing temperature, while no effect of methanol partial pressure or WHSV was observed. The second model considered the effect of deactivation on olefin formation and on coke formation. Olefin formation was considered to be of first order in oxygenates. The initial coking rate was considered to be a function of oxygenate conversion and the ratio of reactant to catalyst. The deactivation functions were found to be linear in coke content for both olefin and coke formation. The rate of deactivation of the coking reaction decreased, while the rate of deactivation of olefin formation increased with increasing temperature. The model shows that the higher average coke selectivity at higher temperatures is caused mainly by the faster deactivation of olefin formation than of coke deposition. The apparent activation energy for olefin formation was estimated to be 118 kJ/mol.
[1] W.O. Haag, in: D. Olson, A. Bisio ( Eds.), Proceedings of 6th International Zeolite Conference, Butterworths, UK, 1984, p. 466. [2] B.V. Vora, T.L. Marker, P.T. Barge, H.E. Fullerton, H.R. Nilsen, S. Kvisle, T. Fuglerud, in: M. de Pontes, R.L. Espinosa, C.P. Nicolaides, J.H. Scholtz, M.S. Scurrel (Eds.), Nature Gas Conversion IV, Studies in Surface Science and Catalysis vol. 107, Elsevier, Amsterdam, 1997, p. 87. [3] C.D. Chang, Cat. Rev. — Sci. Eng. 26 (1984) 323. [4] J. Liang, H. Li, S. Zhao, W. Guo, R. Wang, M. Ying, Appl. Catal. A: General 64 (1990) 31. [5] A.J. Marchi, G.F. Froment, Appl. Catal. A: General 71 (1991) 139. [6 ] S. Nawaz, S. Kolboe, S. Kvisle, K.P. Lillerud, M. Sto¨cker, H.M. Øren, in: A. Holmen, K.J. Jens, S. Kolboe ( Eds.), Natural Gas Conversion I, Studies in Surface Science and Catalysis vol. 61, Elsevier, Amsterdam, 1991, p. 421. [7] A. Grønvold, K. Moljord, T. Dypvik, A. Holmen, in: H.E. Curry-Dyde, R.F. Howe ( Eds.), Natural Gas Conversion II, Studies in Surface Science and Catalysis vol. 81, Elsevier, Amsterdam, 1994, p. 399. [8] D. Chen, H.P. Rebo, K. Moljord, A. Holmen, in: B. Delmon, J.T. Yates ( Eds.), Catalyst Deactivation 1997, Studies in Surface Science and Catalysis vol. 111, Elsevier, Amsterdam, 1997, p. 159. [9] R.A.N. Bos, P.J.J. Tromp, H.N. Akse, Ind. Eng. Chem. Res. 34 (1995) 3808. [10] D. Chen, A. Grønvold, H.P. Rebo, K. Moljord, A. Holmen, Appl. Catal. A: General 137 (1996) L1. [11] A. Grønvold, Conversion of methanol to lower alkenes over molecular sieve type catalysts. Ph.D. thesis, The Norwegian Institute of Technology, Trondheim, Norway, 1994. [12] F. Hershkowitz, P.D. Madiara, Ind. Eng. Chem. Res. 32 (1993) 2969. [13] D. Chen, H.P. Rebo, K. Moljord, A. Holmen, Ind. Eng. Chem. Res. 36 (1997) 3473. [14] G.B. Froment, in: Proceedings of the Sixth International Congress of Catalysis vol. 1 (1976) 10. [15] G.B. Froment, in: C.H. Barthholomew, J.B. Butt ( Eds.), Catalyst Deactivation 1991, Studies in Surface Science and Catalysis vol. 68, Elsevier, Amsterdam, 1991, p. 53. [16 ] A. Voorhies, Ind. Eng. Chem. 37 (1945) 318. [17] S. Sezpe, O. Levenspiel, Eur. Symp. Chem. React. Eng. 41 (1968) 265. [18] B.W. Wojciechowski, Can. Chem. Eng. 46 (1968) 48. [19] D. Chen, H.P. Rebo, K. Moljord, A. Holmen, Ind. Eng. Chem. Res. 38 (1999) 4242. [20] M.O. Lewis, in: J.W. Ward (Ed.), Catalysis 1997, Elsevier, Amsterdam, 1988, p. 199. [21] P.L. Benito, A.G. Gayubo, A.T. Aguayo, M. Olazar, J. Bilbao, Ind. Eng. Chem. Res. 35 (1996) 3991. [22] K. Moljord, P. Magnoux, M. Guisnet, Appl. Catal. A: General 122 (1995) 21.
Acknowledgements The support by the Norwegian Research Council and Norsk Hydro ASA is gratefully acknowledged.
D. Chen et al. / Microporous and Mesoporous Materials 35–36 (2000) 121–135 [23] I.M. Dahl, R. Wendelbo, A. Andersen, D. Akporiaye, H.B. Mostad, T. Fuglerud, Microporous Mesoporous Mater. 29 (1999) 159. [24] I.M. Dahl, S. Kolboe, J. Catal. 149 (1994) 458. [25] I.M. Dahl, S. Kolboe, J. Catal. 161 (1994) 304. [26 ] F. Salehirad, M.W. Anderson, J. Catal. 164 (1996) 301. [27] G.J. Hutching, R. Hunter, Catal. Today 6 (1990) 279. [28] G.J. Hutchings, D.F. Lee, M. Lynch, Appl. Catal. A: General 106 (1993) 115. [29] D. Chen, K. Moljord, T. Fuglerud, A. Holmen, Microporous Mesoporous Mater. 29 (1999) 191.
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[30] M.J. van Niekerk, J.C.Q. Fletcher, C.T. O’Connor, Appl. Catal. A: General 138 (1996) 135. [31] U. Sedran, A. Mahay, H.I. de Lasa, Chem. Eng. J. 45 (1990) 33. [32] G.B. Froment, K.B. Bischoff, Chemical Reactor Analysis and Design, John Wiley & Sons, New York, 1979. [33] M.A. den Hollander, M. Makkee, J.A. Moulijn, in: B. Delmon, J.T. Yates ( Eds.), Catalyst Deactivation 1997, Studies in Surface Science and Catalysis vol. 111, Elsevier, Amsterdam, 1997, p. 295. [34] D. Marquardt, SIAM J. Appl. Math. 11 (1963) 431.
Methanol conversion to light olefins over SAPO-34: kinetic modeling of coke formation D. Chen a, H.P. Rebo a, A. Grønvold b,1, K. Moljord b,2, A. Holmen a, * a Department of Chemical Engineering, Norwegian University of Science and Technology (NTNU), N-7491 Trondheim, Norway b SINTEF Applied Chemistry, N-7465 Trondheim, Norway Received 8 April 1999; received in revised form 6 July 1999; accepted for publication 9 July 1999 Dedicated to the late Werner O. Haag in appreciation of his outstanding contributions to heterogeneous catalysis and zeolite science
Abstract Coke deposition during methanol conversion to light olefins over SAPO-34 has been studied in an oscillating microbalance (TEOM ) reactor as a function of space velocity (57–384 g/g h), temperature (673–823 K ) and cat methanol partial pressure (7.2–83 kPa). Two kinetic models were tested for their ability to describe the coking rate at different operating conditions. A modified Voorhies model related coke deposition to the amount of hydrocarbons formed per gram of catalyst. The model could be used to calculate the average coke selectivity and catalyst capacity for olefin formation. The average coke selectivity increased and the catalyst capacity decreased with increasing temperature, while no effect of methanol partial pressure or space velocity was observed. A kinetic model based on a mechanism with strongly adsorbed reaction intermediates as the coke precursors, which either desorb as olefins or react further into coke, was also fitted to the experimental data, and the deactivation functions were found to be linear in coke content for both olefin and coke formation. The rate of deactivation of the coke-forming reaction decreased, while the rate of deactivation of the olefin formation increased with increasing temperature, and no effect of methanol partial pressure or space velocity was observed. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Coke deposition; Deactivation; Kinetics; Oscillating microbalance; SAPO-34
1. Introduction Zeolite-type catalysts are used widely in petrochemical and petroleum refining processes [1]. The * Corresponding author. Tel: +47-73-59-41-51; fax: +47-73-59-50-47. E-mail address: [email protected] (A. Holmen) 1 Present address: Norsk Hydro Research Center, Box 2560, N-3907 Porsgrunn, Norway. 2 Present address: Statoil Research Center, Postuttak, N-7005 Trondheim, Norway.
catalytic processes using zeolites often include side reactions, leading to the formation of carbonaceous material with catalyst deactivation as a result. More exact knowledge about the mechanism and kinetics of the coke formation is a basis for improving these catalytic processes. The catalytic conversion of methanol to lower olefins (MTO) is an interesting and promising way of converting natural gas and coal to chemicals via methanol [2,3]. Coke deposition is known to be the major cause of deactivation in the MTO reaction over SAPO-34, and both activity and selectiv-
1387-1811/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S1 3 8 7 -1 8 1 1 ( 9 9 ) 0 0 21 3 - 9
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Nomenclature a empirical constant in the modified Voorhies model b empirical constant in the modified Voorhies model CAHF cumulative amount of hydrocarbons formed per gram of catalyst (g/g ) cat C coke content (g /100 g , dry) coke cat C coke content in the ith CSTR i Cexp experimental coke content at the jth experiment j Ccal calculated average coke content at the jth experiment av,j C1 concentration of the coke precursor C initial concentration of oxygenates A0 C concentration of oxygenates in the ith CSTR A,i C inlet concentration of oxygenates to the ith CSTR Ain,i CAMF cumulative amount of methanol fed to the catalyst (g /g ) MeOH cat F molar flow rate of MeOH (mol/h) A0 k0 initial reaction rate constant of oxygenates (s−1) oxy k0 initial rate constant for coke formation C m number of experiments N number of CSTRs used in the model n reaction order OF objective function r0 initial rate of oxygenates conversion, on a CH basis (kmol/kg s) oxy 2 cat r0 initial rate of coke formation (kg /kg h) C coke cat RTC reactant to catalyst ratio S average coke selectivity (wt%) av,C S instantaneous coke selectivity (wt%) C T reaction temperature ( K ) t time on stream (h) W catalyst loading (g ) cat WHSV weight hourly space velocity (on a methanol basis) (kg /kg h) MeOH cat X conversion of oxygenates until the ith CSTR, CH basis i 2 Xexp experimental total conversion of oxygenates, CH basis j 2 Xcal calculated total conversion of oxygenates, CH basis j,N 2 Dx concentration of oxygenates in the ith CSTR i a empirical deactivation constant defined by Eq. (1) c2 R-squared value r catalyst density (kg /m3) S cat t space time in the ith CSTR (kg h/kg ) i cat MeOH w deactivation function for the coke formation C w deactivation function for the conversion of oxygenates oxy
ity are influenced by coke deposition [4–8]. Most authors [4–7] so far have studied the deactivation process as a function of time on stream and at a methanol conversion close to 100%. However, it is difficult to abstract a detailed mechanism of
coke formation and deactivation when coke deposition is not measured or conversions are so high that the effect of secondary reactions of olefins dominates. A kinetic model for the MTO reaction over
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SAPO-34 was recently reported [9] where coke formation and deactivation were studied by an integrated pulse method in a fixed-bed reactor as a function of the coke content of precoked catalyst samples. The rate of coke formation was not investigated in detail due to the difficulty of coke deposition measurements. Grønvold et al. [7] have studied coke formation and deactivation during MTO over SAPO-34 in a conventional microbalance reactor. As discussed previously [10,11], it is very difficult to obtain kinetic data for the coking reaction and the resulting deactivation during MTO over SAPO-34 in a conventional microbalance, owing to the high reaction rates including coke formation and poor exchange of mass and heat between the catalyst and the flowing gas. However, the oscillating microbalance (tapered element oscillating microbalance, TEOM ) reactor with its fixed-bed characteristics is a much more suitable technique for studying fast coke formation and deactivation processes [10,12,13]. The purpose of the present work was to gain a better quantitative understanding of coke formation during the MTO reaction over SAPO-34, using a TEOM reactor. The modeling of coke deposition and deactivation was reviewed by Froment in 1976 [14] and 1991 [15]. Earlier work was aimed at relating the coke deposition, as well as the deactivation, empirically to the time on stream based on the observation that coke formation was not dependent on the space velocity [16 ]. This might not be valid for different feeds and catalysts. However, Voorhies’ rate law has been widely accepted [17,18] and generalized beyond the scope of its original contribution. As pointed out by Froment [14], this type of equation has limited predictive power because it completely ignores the origin of the coke. This led Froment [14] to propose that one should relate catalyst deactivation to its coke content, and use the gas composition, temperature and catalyst activity as parameters to describe coke formation. The present work deals with the detailed study of coke formation as a function of operating conditions such as space velocity, temperature and partial pressure of methanol. Models for coke formation are proposed starting from the simple modified Voorhies model and proceeding to a more elaborate model.
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2. Experimental The TEOM reactor, the experimental procedures and the catalyst properties were as described previously [10,19]. Calcined SAPO-34, with a unit-cell composition of (Si Al 2.88 18 P )O , was obtained from SINTEF Applied 15.12 72 Chemistry. The SAPO-34 particles (52–140 mesh) were dried in situ at 500°C in flowing helium for more than 3 h. The MTO reaction was performed at a WHSV ranging from 57 to 385 g/g h, a cat methanol partial pressure ranging from 7.2 to 83 kPa, and temperatures between 673 and 823 K. The runs with different space velocities were carried out at 698 K and a methanol partial pressure of 7.2 kPa, to obtain relatively low coking rate. The runs with different methanol partial pressures were also performed at 698 K. The space velocity was adjusted to ensure an identical conversion when the partial pressure of methanol was increased. For high methanol partial pressures, very high space velocities were required, as a result of very rapid coke formation. In order to follow the rapid activity change, methanol was introduced by pulses which allowed the completion of gas chromatography (GC ) analyses between each pulse. The small void volume in the tapered element makes TEOM suitable for pulse experiments and TEOM is therefore also referred to as a pulse mass analyzer. Pulses of 9 s duration were selected in the present work for high methanol pressures, while 3 min pulses were used for methanol partial pressures less than 30 kPa. Pulses 1 min long were used for high-temperature experiments (773– 823 K ). The pulse method was compared to continuous experiments with respect to coke formation and the deactivating effect of coke. The effect of pulse sizes on coke formation and deactivation was investigated at 30 kPa and WHSV= 768 g/g h. cat The reactor effluent including unconverted MeOH and dimethylether (DME ) was analyzed on either on an HP 5890 or an HP 6890 gas chromatograph. The HP 5890 was equipped with FID and a GS-Q column (30 m, 0.53 mm). The HP 6890 was equipped with FID and TCD, GS-Q (30 m, 0.53 mm) and HP plot molsieve 5A columns. In the latter case, N was mixed with the 2
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reactor effluent and used as an internal standard. The oxygenates were converted to methane over a nickel catalyst in the GC and the calculated carbon mass balance was close to 100%. Some definitions used in the discussion are summarized in the following. $ Cumulative amount of methanol fed to the catalysts (CAMF )=t×WHSV (g/g ). cat $ Cumulative amount of methanol fed to the catalysts (CAMF, CH basis)=CAMF×14/32. 2 $ Cumulative amount of hydrocarbon formed (CAHF ) (g/g )=∆CAMF X d(CAMF ). cat 0 $ For 3 min pulses CAHF is approximately CAHF=SX (3/60)WHSV, where j is the j number of pulses. $ Catalyst capacity (g /g )=amount of hydrocarbon cat hydrocarbon formed per gram of catalyst until a certain loss in catalyst activity. $ Average coke selectivity (wt%)=the ratio of the coke content to the amount of hydrocarbon formed. $ The amount of coke on the catalyst is given as g of coke/100 g of dry, coke-free catalyst.
3.1. Effect of helium stripping The mass response curves for pulses during the MTO reaction over SAPO-34 were shown in our previous work [19]. The permanent mass increase after stripping for about 40 min in helium was defined as the coke formed for each pulse. However, the effect of stripping in helium on the coke formed must also be considered. Stripping in helium could influence the nature of the coke or the location of the coke; for example, coke molecules might grow into large entities when given time at high temperature. Such effects were checked by comparing pulse and continuous experiments at 698 K, 7.2 kPa and WHSV= 57 g/g h. Fig. 1 shows the changes in the convercat sion of methanol to DME and hydrocarbons and the coke formation with time on stream for both 3 min pulses and continuous experiments, where the data in the continuous experiments were obtained from two separate runs. It was found that stripping in helium did not significantly influence the coke deposition and the deactivating effect of coke. The residual 40% conversion at 20 wt% coke was due to DME formation. The stripping effect was also investigated by
3. Results As discussed previously [10], one of the main advantages of the oscillating microbalance reactor is that it provides a packed bed of catalyst whose mass is measured continuously and through which all the reactants are forced to flow. The classical methods for checking for mass transfer limitations in a fixed-bed reactor can therefore be applied to the TEOM reactor. Both the catalyst loading and the flow rate were adjusted to keep the space velocity identical (385 g/g h) at 698 K and a cat methanol partial pressure of 7.2 kPa. Catalyst loadings of 3 and 5.5 mg gave at this space velocity almost identical initial reaction rates, coke deposition and deactivation rates, indicating that external mass transfer limitation can be neglected. However, intracrystalline diffusion limitations could not be avoided at the reaction conditions employed in the present work [19].
Fig. 1. Coke deposition and methanol conversion for 3 min pulse experiments and continuous experiments during MTO over SAPO-34 at 698 K, WHSV=57 g/g h, P =7.2 kPa. cat MeOH Coke formation: line, continuous experiment; $, 3 min pulses. Conversion: &, continuous experiments; #, 3 min pulses.
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125
stripping in helium. This implies that the carbonaceous material defined as ‘coke’ (i.e., the material left on the catalyst after initial desorption of reactants and products) is adsorbed or trapped in the catalyst pores and does not change its deactivating effect during the stripping periods. The integrated pulse method is therefore used for the kinetic measurements in MTO. 3.2. Effect of space velocity In this series of experiments, space velocity was adjusted by varying the catalyst loading while maintaining a constant total flow rate. Fig. 3 shows the coke deposition as a function of the cumulative amount of methanol fed to the reactor. The results in Fig. 3 demonstrate that the coke deposition depends on the space velocity, and thereby on the conversion of methanol or oxygenates (methanol and DME ) to olefins. Lower space velocities gave higher conversion of oxygenates [Fig. 4(A)], resulting in a higher coking rate. However, when the space velocity was lowered from 82 to 57 g/g h, the coking rate changed only slightly cat ( Fig. 3). The reason is that the conversion depends to only a small extent on space velocity at higher
Fig. 2. Coke formation (A) and methanol conversion (B) during MTO over SAPO-34 as a function of the cumulative amount of methanol fed (CAMF ). T=698 K, WHSV=768 g/g h and P =30 kPa for different pulse cat MeOH sizes: 6, 3 min; 1, 1 min; ), 0.5 min; #, 0.15 min.
using different pulse lengths. Fig. 2 shows the coke deposition [Fig. 2(A)] and the conversion of methanol to DME and hydrocarbons [Fig. 2(B)] as a function of the cumulative amount of methanol fed to the catalysts at a methanol partial pressure of 30 kPa and WHSV=768 g/g h for pulse cat lengths between 9 s and 3 min. Fig. 2(A) and Fig. 2(B) clearly show that the coke deposition and the deactivation were not influenced by the
Fig. 3. Coke formation during MTO over SAPO-34 versus the cumulative amount of methanol fed to the catalysts, CAMF (g/g ), at 698 K and P =7.2 kPa for different space velocicat MeOH ties, WHSV (g/g h): %, 57; 1, 82; 6, 113; #, 270; ), 385. cat Lines: predicted by Model II.
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be described by: X=X exp(−aCAMF ). (1) 0 Both the conversion X and CAMF in Fig. 4(B) and Eq. (1) are on a CH basis. The initial 2 conversion X and the constant a were determined 0 by curve fitting. The CAHF at any time on stream or CAMF can then be calculated by integration of Eq. (1), as defined in the Experimental section. Fig. 5 illustrates the coke deposition versus the amount of hydrocarbon formed at different reaction conditions. The coke deposition is independent of WHSV at a given temperature, but increases with increasing temperature. A higher coke formation was found for the case of WHSV=57 g/g h (Fig. 5). This is probably a cat result of the high conversion of oxygenates (close to 100%), and possible temperature gradients in the catalyst bed at high conversions caused by the highly exothermic reaction [20]. Some important catalyst properties can be deduced from the results given in Fig. 5, e.g., catalyst capacity and coke selectivity. If, for example, a coke content of 18 wt% is defined as a final
Fig. 4. Conversion of oxygenates at 698 K and P =7.2 kPa for different space velocities, WHSV (g/g h) MeOH cat (1, 82; +, 113; n, 385), versus the coke content (A) and the cumulative amount of methanol fed (CAMF ) (B).
conversions, which is generally the case for firstorder reactions. The oxygenate conversion at different space velocities is plotted against the coke content in Fig. 4(A) and against the cumulative amount of methanol fed to the catalysts in Fig. 4(B). From Fig. 4(B), the cumulative amount of hydrocarbon formed per catalyst mass (CAHF, g/g ) at a cat certain time on stream or CAMF can be determined. The change in conversion with CAMF can
Fig. 5. Coke formation versus the amount of hydrocarbon formed (CAHF ) at different temperatures: %, 823 K (P =13 kPa, WHSV=270 g/g h); #, 773 K (P = MeOH cat MeOH 13 kPa, WHSV=270 g/g h); 698 K [P =7.2 kPa and cat MeOH WHSV=57 (&), 82 (1), 113 (+), 270 ($) and 385 (n) g/g h]; 6, 673 K (P =7.2 kPa, WHSV=385 g/g h). cat MeOH cat Lines: predicted by Eq. (3).
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127
state, the catalyst capacity at different temperatures can be directly obtained from Fig. 5, as presented in Table 1. The catalyst capacity decreases and the average coke selectivity increases with increasing temperature. 3.3. Effect of partial pressure of methanol Preliminary experiments showed that the conversion and the coke deposition increased with increasing partial pressure of methanol. Relatively low conversions are required to obtain useful kinetic data, and the effect of higher methanol partial pressure was compensated for by a corresponding increase in space velocity. In this way the conversion was kept nearly constant, illustrating that the MTO reaction over SAPO-34 has close to first-order characteristics in oxygenate conversion. Fig. 6 shows the coke deposition as a function of the cumulative amount of methanol fed to the catalysts for different methanol partial pressures and space velocities but at identical oxygenate conversion at 698 K. The fact that the coke deposition was almost identical for the different experiments implies that coke deposition can be directly related to the total amount of hydrocarbons formed during the reaction, regardless of the methanol partial pressure. This conclusion is valid at least at the conditions used in the present work.
Fig. 6. Coke formation versus the cumulative amount of methanol fed to the catalysts (CAMF ) (g /g ) at 698 K, for different cat P and WHSV values: ), 15 kPa and 384 g/g h; $, MeOH cat 30 kPa and 768 g/g h; +, 60 kPa and 1538 g/g h; %, 83 kPa cat cat and 2558 g/g h. cat
3.4. Effect of temperature Fig. 7 shows the coke deposition versus CAMF at different temperatures. Higher temperatures Table 1 Catalyst capacities and average coke selectivities in MTO over SAPO-34 at different temperatures. 18 wt% coke is defined as the final state Temperature (K)
Catalyst capacity (g /g ) hydrocarbons cat
Average coke selectivity (g /g , %) coke hydrocarbon
673 698 773 823
29.3 15.2 8.8 4.0
0.6 1.2 2.0 4.5
Fig. 7. Coke formation versus the cumulative amount of methanol fed to catalysts (g/g ) at different conditions: cat WHSV=270 g/g h and P =13 kPa — &, 823 K; $, cat MeOH 773 K; n, 698 K; and at WHSV=385 g/g h and cat P =7.2 kPa — %, 698 K; #, 673 K. Lines: predicted by MeOH Model II.
resulted in higher coke deposition. This is consistent with the results reported for methanol conversion to gasoline (MTG) over HZSM-5 [21]. Both Fig. 5 and Table 1 show that the average coke
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selectivity increases with temperature, meaning that higher temperatures favor the coke-forming reaction. Since we also observe that the selectivity to light paraffins increases with increasing temperature, the effect might be due to an increasing rate of hydrogen transfer between two coke molecules or between an olefin and a coke molecule. The coke formed during MTO over SAPO-34 has been found to consist of mono-aromatics at low coke content with an increasing fraction of di- and triaromatics at higher coke contents [11]. The formation of coke involves oligomerization, cyclization, hydrogen transfer and alkylation [22]. The final coke content was about 20 wt%, rather independent of the operating conditions. As a result of the higher coke selectivity, the catalyst capacity for olefin production decreases with increasing temperature ( Table 1).
Fig. 8. Coke formation (g/100 g , dry) during MTO over cat SAPO-34 versus time on stream (min) at 698 K and a methanol partial pressure of 7.2 kPa at different space velocities, WHSV (g/g h): %, 57; 1, 82; 6, 113; ), 385. cat
4. Discussion and modeling 4.1. Mechanism of coke formation Coke deposition is often studied as a function of time on stream [21]. Marchi and Froment [5] studied deactivation during MTO over SAPO-34 as a function of the cumulative amount of methanol fed to the catalyst. Results obtained from both approaches are given and discussed in the present work. Figs. 3 and 6 display the coke content as a function of CAMF, while Figs. 8 and 9 present the coke content as a function of time on stream at different space velocities and partial pressures, respectively. The interpretation based on these two types of figure leads to contradictory results. Fig. 8 reveals that a higher space velocity (or lower contact time and lower conversion) gives a higher coke deposition, as observed for coke deposition during MTG over HZSM-5 [21]. This result indicates that oxygenates might be the coke precursors. However, the fact that the coke content is proportional to the cumulative amount of hydrocarbon formed per catalyst mass (CAHF ) regardless of space velocity (Fig. 5) and methanol partial pressure ( Fig. 6) suggests that the olefins are the coke precursors.
Fig. 9. Coke formation versus time on stream at constant conversion for different methanol partial pressures and space velocities: ), 15 kPa and 384 g/g h; $, 30 kPa and 768 g/g h; cat cat +, 60 kPa and 1538 g/g h; %, 83 kPa and 2558 g/g h. cat cat
It has been shown that the coking rate for propene conversion is much lower than for methanol conversion over SAPO-34 [8], as has also been observed in ethene and propene conversion over SAPO-34 using ethanol and propanol as the feed
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[23]. The lower coke deposition from olefins during MTO might be explained by the difference in adsorption strength between olefins and polar molecules, such as methanol, DME and water. The adsorption strength of olefins is weaker than of methanol at the reaction conditions studied [19]. The competitive adsorption between polar molecules (methanol, DME and water) and olefins during MTO over SAPO-34 could give less olefins adsorbed and thus a lower coke deposition directly from olefins [5]. Dahl and Kolboe [24,25] demonstrated by isotopic tracer studies that the reactivity of ethene and propene was quite low during the MTO reaction over SAPO-34. Salehirad and Anderson also found by nuclear magnetic resonance spectroscopy (NMR) that olefin methylation is not the dominating route for olefin formation over SAPO-34 [26 ]. In addition, an experiment at low space velocity (20 g/g h) in cat the TEOM shows that the deposition of coke is higher at the entrance of the reactor, which is consistent with the observation of Bos et al. [9] in an isothermal fixed-bed reactor. If olefins are the major coke precursors, the coke deposition should be higher at the reactor exit. These results indicate that the olefins are not the major coke precursors during the MTO reaction. From a mechanistic point of view, surface oxonium ions can be formed from methanol and dimethylether through a chain growth process, and carbenium ions can be formed from these surface intermediates via b-elimination [26–28]. Carbenium ions will either form olefins and desorb or react to form larger molecules such as coke. A higher conversion of oxygenates results in a higher concentration of carbenium ions, and thus a higher rate of olefin formation as well as a higher coking rate, indicating that the carbenium ions inside the pores might be the coke precursors. The lower coking rate during olefin conversion over SAPO-34 might also be explained by the lower rate of carbenium ion formation. Although NMR [26 ] and isotopic labeling studies [24,25] have been used to study the detailed reaction mechanism, the exact nature of the intermediates inside the pores during methanol conversion as well as coke formation over SAPO-34 remain to be determined. If we ignore the complex-
129
ity of the intermediates inside the pores, a simple mechanism of coke deposition can be proposed:
(2) where the oxygenates include methanol and dimethylether, and all hydrocarbons are lumped together based on the observation that the olefins are formed from the oxygenates in parallel [29]. The intermediates are considered to be a mixture of carbenium ions with different carbon numbers inside the pores. This simple mechanism for coke formation is similar to the hydrocarbon pool mechanism [24,25].
4.2. Criteria in catalyst evaluation The catalyst lifetime is normally defined as the time on stream before catalyst deactivation has reached a certain level, and is widely used in catalyst screening. At identical reaction conditions, the conversion level during screening experiments can vary for different samples due to their different activities, and the rate of catalyst deactivation may vary with the conversion. Conversion can be kept constant by adjusting space velocity in such experiments. A change in space velocity includes a change in the amount of methanol fed to the catalyst, hence the amount of hydrocarbon formed and the coke content on the catalyst. Therefore, using catalyst lifetime for catalyst screening could lead to a wrong conclusion, as it ignores the effect of the amount of methanol fed to the catalyst. The amount of hydrocarbons formed (CAHF ) as a measure of the catalyst capacity is suggested as a more precise parameter for catalyst screening, similar to the concept of the catalyst utilization value (CUV ) used by van Niekerk et al. [30]. They defined the catalyst utilization value as the mass of methanol/DME converted to hydrocarbons per gram of catalyst, before the conversion level drop below half of the maximum conversion.
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Table 2 Parameters in Model I (C (wt%)=a[CAHF ]b) for MTO over SAPO-34 at different temperatures Temperature ( K )
a
b
673 698 773 823
1.09 2.09 1.71 3.24
0.83 0.80 1.08 1.23
4.3. Modeling of coke deposition 4.3.1. Model I A simple Voorhies model [16 ] relating coke formation to time on stream is too simple for the MTO reaction over SAPO-34. Experimental results show that the coke deposition is influenced by the reaction conditions including space velocity, methanol partial pressure and temperature. The parameters in the Voorhies model depend on the operating conditions, as has been shown also for coke deposition during MTG over HZSM-5 [21], limiting its application. However, the coke deposition during the MTO reaction over SAPO-34 at constant temperature essentially depends on the cumulative amount of hydrocarbon formed (CAHF ), regardless of space velocity (Fig. 5) and methanol partial pressure (Fig. 6), indicating that CAHF is a more precise parameter than time on stream. The Voorhies coking model can therefore be modified and the coke formed can be described by: C (wt%)=a[CAHF ]b,
S
av,C
(wt%)=
C [CAHF ]
=a1/bC1−1/b.
(5)
By defining a coke content of 18 wt% as the final state, the average coke selectivity can easily be calculated by Eq. (5), and the catalyst capacity by Eq. (3). These parameters are presented in Table 1. Eq. (3) can be used to estimate the coke content on SAPO-34 based on the rate of hydrocarbon formation. Since it is difficult to measure the coke content in practice, the deactivation can be directly related to the cumulative amount of hydrocarbons formed in this case, as has been demonstrated by Sedran et al. [31] in modeling of the deactivation behavior of methanol conversion to hydrocarbons over HZSM-5. However, Voorhies’ relationship provides little information about the mechanism of coke deposition, since it ignores the coking reaction itself. In addition, this type of equation is relatively difficult to apply in reactor design, since the amount of hydrocarbons formed is a function of the coke content.
(3)
where a and b are parameters to be estimated. Their values are presented in Table 2 for different temperatures. a and b did only depend on the temperature. The instantaneous coke selectivity is a useful value for a detailed understanding of the effects of coke deposition on the coking and olefin formation inside the pores. The instantaneous coke selectivity (S , wt%) can be calculated by derivation of C Eq. (3): dC =ab[CAHF ]b−1. S = C d CAHF
By substituting CAHF in Eq. (4) with Eq. (3), the coke selectivity can be correlated with the coke content. The calculated instantaneous coke selectivity at a given temperature decreased with the coke content, indicating that the coke deposition deactivated more rapidly than the olefin formation, and this has been explained in terms of a shapeselectivity effect induced by coke [10]. The average coke selectivity (S , wt%) is a useful parameter av,C for catalyst evaluation. From Eq. (3) the coke selectivity can be expressed as:
(4)
4.3.2. Model II Due to the restrictions of Model I [Eq. (3)], a model proposed by Froment and Bischoff [32] can be a better choice for application in reactor design. A proper kinetic model should describe all the experimental observations, such as the changes in the coke content versus time on stream (Figs. 8 and 9), versus the cumulative amount of methanol fed to the catalyst (Figs. 3 and 4) and versus the amount of hydrocarbon formed ( Fig. 5). The conventional model of Froment and Bischoff relates the coking rate (dC/dt) to the concentration of coke precursor and the deactivation function of
D. Chen et al. / Microporous and Mesoporous Materials 35–36 (2000) 121–135
the coking reaction: dC dt
=k0 (C1)nw , C C
(6)
where k0 is the initial coking rate constant, n is C the reaction order and w is the deactivation funcC tion for the coking reactions. C1 is the concentration of the coke precursor, which is assumed to be proportional to the conversion of oxygenates (C1=X C , C being the initial concentration oxy A0 A0 of methanol and X being the oxygenate converoxy sion). Bos et al. [9] used this type of model with ethene and propene as the coke precursors to describe coke deposition during MTO over SAPO-34 in a fixed-bed reactor. Preliminary modeling showed a proper description of the coke content versus time on stream, but the model seems to be unappropriate to describe the effect of space velocity illustrated in Fig. 3. A modified model described in the following section was therefore developed. Although WHSV is rather high, the initial conversion is still relatively high due to the high catalyst activity, and the reactor should be treated as an integral plug-flow reactor instead of a differential reactor. The concentration of coke precursors will then vary in the reactor, and a distribution of coke is expected. In order to simplify the calculation, a series CSTR model is used for calculation of the coke deposition and the conversion of the oxygenate. The coke is assumed to be uniformly distributed in each CSTR reactor element, and the kinetic model is based on the simple reaction scheme described in Eq. (2). The deactivation of the transformation of the oxygenate is a transient process. However, the superficial gas velocity in the TEOM is about 1 m/s, and the residence time in the reactor is in the range of milliseconds. Since no appreciable deactivation occurs over a time period of milliseconds, the process can be treated as pseudo steady-state. The mass balance for the ith CSTR in the series is: Dx =D(W/F )r /r i A0 A,i S and
(7)
=k0 w C , oxy oxy A,i
(8)
r
A,i
131
where W is the catalyst mass (g ), F is the cat A0 molar flow rate of methanol (mol/s), r is the A reaction rate of the oxygenate (mol/cm3 s), r is S the catalyst density (1.48 g/cm3, [19]), D(W/F )=(W/F )/N is the space time in the ith A0 A0 CSTR and w is the deactivation function for the oxy conversion of oxygenates. The oxygenates conversion is treated as a first-order reaction in Eq. (8), where C is the concentration of oxygenates A,i (mol/cm3) in the ith CSTR and k0 is the initial oxy rate constant (s−1). Dx is the conversion of i oxygenates in the ith CSTR, which can be written as: 1 , Dx =1− i 1+D(W/F )/r C k0 w A0 S Ain,i oxy oxy where
(9)
=C (1−Dx ). (10) Ain,i Ain,i−1 i−1 C is the inlet concentration of ith CSTR. The Ain,i conversion of oxygenates until ith CSTR is then calculated by: C
C −C Ain,i+1 . X = A0 i C A0 The coking rate in ith CSTR is: dC dt
i =r0 w . C,i C
(11)
(12)
The initial coking rate in the ith CSTR (r0 ) is C,i considered to be proportional to the amount of intermediates, which is a function of the conversion of oxygenates and the reactant to catalyst ratio ( RTC, g /g ). The initial coking rate is then feed cat described by: r0 =k0∞RTCDx . (13) C,i C i The RTC is similar to the oil-to-catalyst ratio in FCC [33]. The ratio of reactant to catalyst is an easily available parameter in a fluidized-bed reactor, but this parameter is not well defined for a fixed-bed reactor. Eq. (13) must therefore be rewritten in the form of: r0 =k0 Dx /t , (14) C,i C i i where the space time t in ith CSTR is defined as i
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t =(DWHSV )−1 (g h/g ). It has to be pointed i cat feed out that k0 in Eq. (14) is not identical to k0∞ in C C Eq. (13). Eq. (14) relates the initial coking rate directly to the rate of oxygenate conversion. Several empirical expressions given by Froment and Bischoff [32], relating the rate of oxygenates conversion and the coking rate to the coke content, were statistically tested. A linear relationship between the degree of deactivation and the coke content [Eq. (15)] was found to give the best fit for both the MTO reaction and the coke deposition: w
oxy
=1−a
oxy
C
and w =1−a C. C C
(15)
Integration of the continuity equation [Eq. (12)] combined with Eqs. (14) and (15) yields: 1 C= i a C
C
A
t 1−exp −a k0 Dx C Ct i i
BD
,
(16)
where t/t =D(CAMF ) (g /g ). This equation i MeOH cat therefore relates coke formation to the cumulative amount of methanol converted, which is in good agreement with the experimental observations. 4.4. Determination of the kinetics of the MTO reaction including coke deposition The procedure for developing a kinetic model of a process subject to deactivation due to coking usually consists of three steps. In the first step, the kinetics for the main reaction is studied in the absence of coking. In the second step, the deactivation functions are related to the coke content of the catalyst and, finally, the third step is development of a kinetic model for coke formation related to the process time. This procedure requires conversion data on the coke-free catalyst, which are normally obtained by extrapolating to zero time. However, the procedure might be hazardous for processes with very fast coking. For the MTO reaction, the coke content was already 2–3 wt% after only 2 min on stream using a methanol partial pressure as low as 7.2 kPa. Consequently, extrapolation to zero time to obtain the initial conversion or the initial reaction rate on a coke-free catalyst has large uncertainties. However, data for the
conversion versus coke content, which are obtained by the pulse technique in the TEOM reactor, made it possible to regress kinetic models for the main and coking reaction simultaneously, as well as for the catalyst deactivation. It has to be noted that the initial rate for conversion of oxygenates used in the model is a pseudo value, since an increasing conversion with coke formation has been observed at the start of the reaction by 9 s pulse experiments. For example, the conversion to hydrocarbons (CH basis) was 2.7% and 55.2% for the first and 2 the second pulse, respectively, at a methanol partial pressure of 83 kPa, temperature of 698 K and weight hourly space velocity of 2558 h−1. It has been shown that the increase in conversion of oxygenates with coke deposition depends on the effects of intracrystalline diffusion, and that strong diffusion limitations lead to a lower coke content corresponding to a maximum in the conversion of oxygenates [29]. This coke content corresponding to maximum conversion was about 1 wt% at 698 K on the SAPO-34 used in the present work. It is rather low compared with the 4 wt% on 2.5 mm crystals and 7 wt % on 0.25 mm crystals at identical reaction conditions, indicating strong diffusion limitations at the reaction conditions on the SAPO-34 employed in the present work. It is in good agreement with the results obtained by diffusion measurement [19]. The estimation of the parameters in the kinetic model has been carried out by the non-linear leastsquares routine in using the Levenberg– Marquardt method [34]. The objective function OF was calculated:
A
B
A
B
−Xcal 2 m Cexp −Ccal 2 m Xexp j j,N + ∑ j av,j , OF= ∑ Xexp Cexp j=1 j=1 j j (17) where Xcal are the calculated values of the total j,N conversion of the oxygenates at a given coke content and at the experimental point j. Xexp are j the corresponding experimental values. Ccal are av,j the calculated average coke content at the experimental point j, which is calculated by Eq. (18) and Cexp is the corresponding experimental value. j
D. Chen et al. / Microporous and Mesoporous Materials 35–36 (2000) 121–135
Due to the large variation in conversion and coke content in the experiments, 1/Xexp and 1/Cexp are j j used as the weighting factors:
A
B
N (18) C = ∑ C /N. i av i=1 The effect of N in the model was tested and N=20 was selected. The estimated parameters are presented in Table 3. The comparisons between the calculated values and the experimental values for coke deposition are shown in Fig. 3 for different space velocities at 698 K and a methanol partial pressure of 7.2 kPa, and in Fig. 7 at different temperatures. The model fitted the experimental data well, except for the low space velocities in Fig. 3. This is possibly a result of a temperature raise in the catalyst bed due to the high conversion, as discussed previously. Fig. 7 clearly shows the higher coking rate at the higher temperature. The model shows that the olefin formation and the coke formation during MTO have identical deactivation functions, but the deactivation rate constants (a for coke deposition and a for C oxy oxygenates conversion) are different. Both a oxy and a are independent of WHSV and methanol C partial pressure, but they depend on the temperature. a decreases and a increases with increasC oxy ing temperature ( Table 3). Higher a indicates a oxy higher deactivation rate. It also indicates that the toxicity of the coke on the olefin formation is higher at higher temperatures. The faster deactivation for oxygenates conversion and the slower deactivation for coke deposition result in higher coke selectivity at a higher temperature, which is in good agreement with the experimental observations ( Fig. 5). The temperature dependence of k0 C and k0 for coke deposition and conversion of oxy
133
oxygenates respectively are presented in Fig. 10. The data fitted the Arrhenius correlations well: k0 =3.96×1012 exp(−117 735/8.314T ), oxy c2=0.9897
(19)
and k0 =1.27 exp(−6707/8.314T ), c2=0.9968. C (20) The apparent activation energy (6.7 kJ/mol ) for the coke deposition was calculated from the coking rates at 698, 773 and 823 K. Between 673 and 698 K, the apparent coking activation energy is much larger (30 kJ/mol ). The reasons for the extremely low apparent activation energy for coke formation are not yet clear, but could be explained by diffusion limitations. The apparent activation energy for the olefin formation is 118 kJ/mol, which is very close to the value of 129.3 kJ/mol reported by Sedran et al. [31] for olefin formation from methanol over HZSM-5. The true activation energy should be larger if the effect of intracrystalline diffusion limitation is taken into account.
Table 3 Parameters in Model II for MTO over SAPO-34 at different temperatures Temperature ( K )
k0 C
a C
k0 (s−1) oxy
a oxy
673 698 773 823
0.33 0.40 0.45 0.48
0.061 0.058 0.035 0.033
2684 7235 35 132 150 941
0.043 0.052 0.057 0.070
Fig. 10. Arrhenius plot for the MTO reaction and the coking rate constant: %, MTO; #, coking reaction.
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5. Conclusions
References
Methanol conversion to olefins and coke over SAPO-34 has been studied in an oscillating microbalance ( TEOM ) as a function of temperature, methanol partial pressure and WHSV. It has been found that the coke deposition in MTO must be compared based on the amount of methanol fed to the catalyst instead of time on stream. Kinetic modeling was performed on the basis of a mechanism with carbenium ions as the coke precursors, which either desorb as olefins or react further into coke. Two kinetic models were tested for their ability to describe the rate of coke formation. A modified Voorhies model related coke deposition to the amount of hydrocarbons formed per gram of catalyst. This model can be used to calculate the average coke selectivity and catalyst capacity for olefin formation. The average coke selectivity increased and the catalyst capacity decreased with increasing temperature, while no effect of methanol partial pressure or WHSV was observed. The second model considered the effect of deactivation on olefin formation and on coke formation. Olefin formation was considered to be of first order in oxygenates. The initial coking rate was considered to be a function of oxygenate conversion and the ratio of reactant to catalyst. The deactivation functions were found to be linear in coke content for both olefin and coke formation. The rate of deactivation of the coking reaction decreased, while the rate of deactivation of olefin formation increased with increasing temperature. The model shows that the higher average coke selectivity at higher temperatures is caused mainly by the faster deactivation of olefin formation than of coke deposition. The apparent activation energy for olefin formation was estimated to be 118 kJ/mol.
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Acknowledgements The support by the Norwegian Research Council and Norsk Hydro ASA is gratefully acknowledged.
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