Catalyst Deactivation by Coke in the Transformation of Aqueous

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Ind. Eng. Chem. Res. 2002, 41, 4216-4224

Catalyst Deactivation by Coke in the Transformation of Aqueous Ethanol into Hydrocarbons. Kinetic Modeling and Acidity Deterioration of the Catalyst Andres T. Aguayo,* Ana G. Gayubo, Alaitz Atutxa, Martin Olazar, and Javier Bilbao Departamento de Ingenierıá Quı´mica, Universidad del Paı´s Vasco, Apartado 644, 48080 Bilbao, Spain

Basic aspects (coke deposition, acidity deterioration) of the deactivation by coke of a catalyst (prepared from a HZSM-5 zeolite) used in the transformation of aqueous ethanol into hydrocarbons in the 350-450 °C range have been studied. The experiments were carried out in an isothermal fixed-bed reactor with feeds of ethene, mixtures of ethanol and water at different mass ratios, and diethyl ether. The importance of temperature and water content in the reaction medium on the mechanism of coke evolution is demonstrated. The effect of these variables on deactivation is explained by a kinetic model in which the role of coke precursors (ethene, light olefins, and gasoline) is considered. This role depends on the operating conditions, particularly the temperature. 1. Introduction An interesting method for upgrading natural carbohydrates (vegetable biomass and residues from the agroforestry and sugar industries) is the catalytic transformation of the fermentation liquid product once it has been subjected to treatment for the partial elimination of water. The aims are to avoid the costly operations for total water elimination and to obtain pure ethanol.1,2 The purpose of the catalytic transformation might be either to obtain ethene, which is the objective of the BETE (bioethanol-to-ethene) process,3-6 or to obtain C5+ hydrocarbons [BTG (bioethanol-to-gasoline) process].2 The BTG process requires a reaction temperature above 350 °C to achieve high yields, and at these temperatures, deactivation by coke is important. In studies of the catalytic transformation of ethanol on HZSM-5 zeolites, it has been shown that the mechanism occurs, similarly to the transformation of methanol [MTG (methanol-to-gasoline) process], via oxonium ions as intermediate compounds.7-13 Likewise, the ranges of hydrocarbons obtained in the product streams of the catalytic transformations of the two alcohols on HZSM-5 zeolites are similar,14-16 and consequently, the conclusions referring to the effects of the operating conditions on the yields and composition of the product stream of the MTG process are generally applicable to the BTG process.17,18 Nevertheless, in the catalytic transformation of aqueous ethanol into hydrocarbons on a HZSM-5 zeolite, the considerable proportion of water in the feed has a major influence on important aspects, such as product distribution3,4,14,15,19-21 and catalyst deactivation. In the literature, two facts have been confirmed: on one hand, the role of water as an agent that attenuates deactivation by coke19 at moderate temperatures and, on the other, the inconvenience of the presence of water at high temperatures because it causes zeolite dealumination. Taking into account the importance of deactivation in the economic viability of a process, in this paper, the * Corresponding author. Tel.: 34-94-6012580. Fax: 34-944648500. E-mail: [email protected].

main aspects of deactivation have been studied, as well as coke deposition and the deterioration in the acidity of the catalyst active sites. Furthermore, the soundness of a kinetic model that accords with these facts is verified. The kinetic model is similar to those recently proposed for deactivation by coke in the MTG (methanolto-gasoline) process on a HZSM-5 zeolite22 and in the MTO (methanol-to-olefins) process on a SAPO-34 silicoaluminophosphate.23 2. Experimental Section The catalyst was prepared by agglomerating the HZSM-5 zeolite (25 wt %) with bentonite (30 wt %) and using alumina (45 wt %) as an inert charge. The preparation of the HZSM-5 zeolite was carried out following Mobil patents and using a Si/Al ratio of 24.24,25 The calcination temperature of 570 °C is suitable for obtaining an acid structure, and as has been shown, this structure is hydrothermally stable in the MTG (methanol-to-gasoline) process carried out following reactionregeneration cycles.26 The physical properties of the catalyst, determined by N2 adsorption-desorption in a Micromeritics ASAP 2000 instrument, are as follows: surface area, 131 m2 g-1; pore volume, 0.43 cm3 g-1; apparent density, 1.21 g cm-3; real density, 2.53 g cm-3. The contribution of pores of different size to the total pore volume was dp < 10-3 µm (micropores), 8.1%; 10-3 µm < dp < 10-2 µm (mesopores), 14.7%; 10-2 µm < dp < 2 µm (macropores), 77.2%. The Bro¨nsted/Lewis site ratio, determined by FTIR analysis (Nicolet 740 spectrometer provided with a Spectra Tech chamber) of adsorbed pyridine, was 2.9. The reaction equipment was operated by means of a data adquisition and control program. The reactor was constructed of stainless steel 316, with a 9-mm internal diameter and a 100-mm length. It was provided with a fixed bed of catalyst diluted with alumina as an inert component and operated in the isothermal regime. The reaction products were analyzed by gas chromatography (Hewlett-Packard 6890) with detection by thermal conductivity (TCD) and flame ionization (FID).

10.1021/ie020068i CCC: $22.00 © 2002 American Chemical Society Published on Web 07/27/2002

Ind. Eng. Chem. Res., Vol. 41, No. 17, 2002 4217

Figure 1. Kinetic scheme for the transformation of ethanol into hydrocarbons (steps 2-7 for ethene transformation).

The separation of products was carried out by means of a system made up of three columns: (1) HP-1 semicapillary column for splitting the sample into two fractions, (a) volatile hydrocarbon components (C4-) and polar components (ethanol, water and diethyl ether) and (b) remaining products (C5+); (2) SUPEL-Q Plot semicapillary column for individually separating both volatile components and polar components, which were subsequently analyzed by TCD and FID; and (3) PONA capillary column for separation of C5+ hydrocarbons, which were analyzed by FID. The results in terms of reaction component concentration, expressed as weight percentages by organic mass unit, were determined from the chromatographic results by means of a program written in Fortran. This program performs both the carbon atom and mass balances by calculating the water formed from the stoichiometry of its formation as a function of time on stream. Three sets of experiments were carried out at atmospheric pressure: one by feeding ethene into the reactor, another by feeding ethanol and water with different mass ratios, and the third by feeding pure diethyl ether. This feed was diluted with a flow of 30 cm3 min-1 of He as an inert gas, with the aim of ensuring bed isothermality. The coke content deposited in the catalyst was determined by its combustion with air once the deactivated catalyst sample had been subjected to a stripping treatment with He at 550 °C for 0.5 h. The equipment used was a SDT 2960 (TA Instruments) thermobalance provided with a Thermostar (Balzer Instruments) mass spectrometer. The coke H/C ratio was determined by measuring the combustion gases, that is, H2O, CO, and CO2.27 3. Reaction Scheme and Kinetic Model of the BTG Process The catalytic transformation of aqueous ethanol into hydrocarbons on a HZSM-5 zeolite can be described by a kinetic scheme similar to the one commonly expressed in the literature for methanol transformation.17,28-32 One difference is the convenience of taking ethene as a differentiated component from the remaining components and lumps in the reaction medium. The first step of the kinetic scheme, shown in Figure 1, corresponds to ethanol dehydration. The second step corresponds to oligomerization-cracking, for which the

reactant is ethene and the products are light olefins (propene and butenes). Oligomerization-cracking-aromatization of ethene can give rise to hydrocarbons of lump C5+ (gasoline) (third step in Figure 1). The gasoline lump can be obtained by olefin condensation (fourth step). Cracking of the gasoline lump can also give rise to ethene and olefins (seventh step). Paraffins are obtained (mainly propane and butane, along with a very low concentration of ethane and an insignificant concentration of methane) as a result of the oligomerization-cracking of ethene (fifth step) and cracking of the gasoline lump (sixth step). The presence of paraffins in the reaction medium obtained by oligomerizationcracking of ethene has already been reported in the literature.14 Coke formation has not been taken into account in the kinetic scheme (which would require the addition of three individual coke formation steps from each possible precursor, E, O, and G, as will be explained later) because its yield is insignificant. Thus, from the coke content deposited at the end of reaction, it was determined that, under the more severe conditions of deactivation (24 h of reaction at 450 °C with a feed of pure ethene) the yield of coke is lower than 0.2 wt % of the amount of ethene fed. The kinetic model describes the formation rate of the components of the kinetic scheme of Figure 1 by means of the following expressions, where the steps of Figure 1 are considered as elementary

rAo ) -θ(XW)k1XA

(1)

rEo ) θ(XW)[k1XA - (k2 + k3 + k5)XE + k7XG] (2) rOo ) θ(XW)[k2XE - k4XO + k7XG]

(3)

rPo ) θ(XW)[k5XE + k6XG]

(4)

where θ(XW) is a function that quantifies the attenuating effect of water given by

θ(XW) ) exp(-kWoXW)

(5)

In eqs 1-5, the concentration of each component and lump of the kinetic scheme, Xi, is expressed as the weight fraction by mass unit of organic components. XW is the water/organic components mass ratio in the reaction medium. From experiments carried out in an isothermal integral reactor in the 350-450 °C range with different values of the space time, the kinetic constants listed in Table 1 were calculated by experiments in which ethene, ethanol, and diethyl ether (either in pure form or with varying water content) were fed into the reactor.33 4. Results 4.1. Coke Deposition. The coke content in the catalyst depends on the reaction conditions, time on stream, temperature, and space time, as well as on the feed composition (particularly water content). Figure 2 shows the effect of temperature on the coke content in the catalyst bed for different feeds. The results correspond to 6 h time on stream, and they are average values for the bed because they were determined by the combustion of catalyst samples obtained after homogenizing the deactivated bed. As can be seen in Figure 2, ethene is the feed that produces the highest

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Ind. Eng. Chem. Res., Vol. 41, No. 17, 2002

Figure 2. Effect of reaction temperature on coke deposition for different feeds. Reaction conditions: W ) 2 g; t ) 6 h. Table 1. Kinetic Parameters for the Steps of the Reaction Scheme in Figure 1 for the 350-450 °C Range

k1 ) 283 ((83) exp

[

-33 800 ((3000) 1 1 R T 623

k2 ) 2.74 ((0.34) exp

k5 k6 k7

[ [ [ [ [ [

)]

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

(

(6)

)]

( )] -5500 ((1800) 1 1 ) 3.93 ((1.22) exp (T - 623 )] R -13 800 ((2000) 1 1 ) 0.84 ((0.16) exp (T - 623 )] R -7950 ((3000) 1 1 ) 0.30 ((0.02) exp (T - 623 )] R -22 000 ((2000) 1 1 ) 3.50 ((0.20) exp (T - 623 )] R -1200 ((500) 1 1 ) 0.46 ((0.04) exp[ (T - 623 )] R

k3 ) 1.89 ((0.31) exp k4

(

kWo

-8650 ((1300) 1 1 R T 623

(7) (8) (9) (10) (11) (12) (13)

coke content. The coke content produced by the ethene feed is similar to that determined for propene in the same temperature range.34 In Figure 2, it can be observed that coke content decreases as the water content in the reaction medium increases, either because of the formation of water as a product (as in the case of the transformation of diethyl ether into ethene) or, apart from this formation, because water was fed into the reactor (aqueous ethanol in the feed). It is noteworthy that, when the water content in the feed is doubled, the coke content is reduced by onehalf. A similar effect of the water content was observed in the transformation of aqueous methanol into olefins on a SAPO-34 catalyst.23,35 When the effect of temperature is analyzed, a maximum in coke deposition is observed at 400 °C, which can be explained by the cracking of coke precursor oligomers above this temperature.36 The small effect of temperature on coke deposition for a feed of 25 wt % ethanol diluted with water is explained by the fact that water also attenuates these cracking reactions of coke precursors.

Figure 3. Effect of space time on coke deposition for different feeds. Time on stream ) 6 h. (a) 350 °C. (b) 450 °C.

At 350 °C (Figure 3a), an increase in the coke content in the catalyst bed is observed as the space time is increased. This rise is explained by the increase of product concentration (olefins and gasoline) in the reaction medium and is less pronounced as the water content in the reaction medium increases. At 450 °C (Figure 3b), the effect of the water concentration in the reaction medium follows the same trend in attenuating coke deposition. Nevertheless, the effect of space time is more complex, and the coke content in the catalyst bed passes through a maximum value, which is explained by the cracking of coke precursors beyond a certain value of space time. Values of the coke H/C ratio were measured and found to lie within the range between 1.2 and 1.5. These results are evidence that coke is slightly developed in this process, but because this ratio lies within a narrow range, an assessment of the effect of the operating conditions on this parameter was not possible. It was found, however that, when the coke is subjected to severe aging treatment (under a N2 flow for 0.5 h at 550 °C), the H/C ratio decreases, and the coke samples corresponding to very different operating conditions are homogenized. As a result, the H/C ratios of these samples lie within the range between 0.45 and 0.60. The fact that the coke deposited in this process is slightly developed and has a high H/C ratio is in agreement with the results reported in the literature for methanol transformation into hydrocarbons on


Figure 5. Relationship between total catalyst acidity and deposited coke content.

Figure 4. Distribution of acid strength measured by differential adsorption of NH3 for (a) fresh and (b) deactivated catalyst. Reaction conditions: 350 °C, 0.709 (g of catalyst) h (g of ethanol)-1, XWo ) 3.

HZSM-5 zeolites.37-42 It is a consequence of the shape selectivity of the zeolite (which limits the condensation of aromatic rings in the internal channels of the crystals);43 the moderate strength of the acid sites; the presence of water, which attenuates the evolution of coke precursors; and the capacity of the zeolite for cracking the oligomers that make up the coke forming above 400 °C.36 4.2. Deterioration of Catalyst Acidity. In Figure 4, the distributions of acidity strength determined for the fresh catalyst (upper graph) and for a deactivated catalyst (lower graph) are compared. These results were obtained by calorimetric measurements of NH3 adsorption at 150 °C in a SDT 2960 (TA Instruments) thermobalance provided with a Thermostar (Balzer Instruments) mass spectrometer. The results are evidence of the uniformity of the catalyst acid structure, as the catalyst sites release similar amounts of energy in the adsorption of NH3 [between 125 and 150 kJ (mmol of NH3)-1]. It can be observed that the distribution of the acidity strength of the sites is not affected by coke deposition. This conclusion was also reached when samples of catalyst deactivated under other operating conditions were analyzed. Nevertheless, the decrease in total acidity was significant, as shown in Figure 5, where a pronounced decrease in acidity is observed for incipient coke deposition, followed by a linear decrease

in acidity with increasing coke content. The two results that deviate from the general trend correspond to experiments at high temperature (450 °C) and high water content in the feed (XWo ) 3), which are conditions for which a decrease in acidity can be attributed to irreversible deactivation by dealumination.44 Unfortunately, differential adsorption of NH3 does not provide any information on the nature (Lewis or Bro¨nsted) of the acid sites, whose monitoring might provide interesting information on their role in deactivation. The limiting conditions (reaction temperature and water content in the feed) that provoke irreversible catalyst deactivation were studied by carrying out cyclic experiments of reaction-regeneration, It was determined that the catalyst fully recovers its activity at reaction temperatures up to 450 °C, as long as the water content in the feed does not exceed 50 wt %. Consequently, these are the limiting conditions used for the kinetic study of reversible deactivation (by coke). 4.3. Kinetic Model for Deactivation. Gayubo et al.45 have proposed the following kinetic equation for catalyst deactivation

-

da )( dt

∑i k′diXi)ad ) [∑i kdiθd(XW)Xi ]ad

(14)

where Xi is the weight fraction by mass of organic components in each lump or component of the kinetic scheme that can act as coke precursor. θd(XW) is a function that quantifies the attenuating effect of water, so that the apparent deactivation constant, k′di, is the product of the constant for coke formation from each coke precursor lump, kdi, and this function. After analyzing different mathematical expressions with only one kinetic parameter, kW, for quantifying the attenuation of deactivation by water, the following expression was adopted for the function θd(XW)

θd(XW) ) exp(-kWXW)

(15)

In eq 14, the activity, a, is defined as the ratio of the rate of formation of lump i at time t on stream to the rate of formation of lump i corresponding to zero time on stream

a)

(ri)j (rio)j

for j ) 2, ..., 7

(16)

4220


where j refers to each step of the kinetic scheme in Figure 1. For the dehydration of ethanol to ethene (step 1 in Figure 1), deactivation was insignificant (a ) 1) under all of the experimental conditions studied (including experiments of 60-h duration under the conditions corresponding to the more severe deactivation for the remaining steps).16 The selection of the kinetic model and the calculation of the corresponding parameters were carried out by fitting the experimental concentration data to the mass conservation equation in the reactor for each component and lump of the kinetic scheme. The procedure is described in detail in previous papers for the MTG process22,46 and for this process.45 The best-fit deactivation kinetic model (among the different cases of the general expression in eq 14) is

-

da ) θd(XW)[kdE XE + kdOG(XO + XG)]ad dt

(17)

This kinetic model considers ethene, the olefin lump (propene and butenes), and the gasoline lump as coke formation precursors. This hypothesis is well supported by the oligomerization capacity of light olefins to produce compounds that will remain trapped in the porous structure47,48 and by the cyclization and condensation capacity of aromatic compounds (at high ratios in the gasoline lump).43,49,50 Equation 17 does not take into account the possible effect of ethanol or diethyl ether on deactivation, because the conversions of these species to ethene are very rapid and complete at the very beginning of the reactor. Consequently, the concentrations of these reactants are zero throughout the reactor under all of the experimental conditions studied. This circumstance does not change with time because the dehydration step shown in Figure 1 is not affected by deactivation, as was indicated above. The corresponding kinetic parameters (with their 90% confidence intervals obtained by the Mardquardt algorithm for nonlinear regression) are listed in Table 2. From a comparison of eqs 18 and 19 from Table 2, it can be seen that, at the reference temperature (350 °C), the deactivation constant corresponding to the evolution of ethene to coke, kdE, is 3 times that corresponding to the evolution of light olefins and gasoline, kdOG. Consequently, although ethene is the main coke formation precursor, at 350 °C, the contribution of olefins and gasoline to deactivation is also important. Nevertheless, as the temperature is increased, the contribution of the lumps of products (olefins and gasoline) is less important than that of ethene, as can be deduced from the activation energy values for the constants kdE and kdOG. Equation 20 shows that, as the reaction temperature is raised, the attenuating effect of water on deactivation Table 2. Parameters of the Kinetic Equation of Deactivation, Eq 17, for the 350-450 °C Range

( )] -2000 ((4100) 1 1 ) 0.175 ((0.077) exp[ (T - 623 )] R -6700 ((3000) 1 1 ) 0.481 ((0.083) exp[ (T - 623 )] R

kdE ) 0.480 ((0.078) exp

[

-11 500 ((1600) 1 1 R T 623

(18)

kdOG

(19)

kW

(20)

d ) 1.78 ((0.08)

(21)

Figure 6. Evolution with time on stream of the mass fractions, by unit mass of organic components in the reaction medium, of ethene and of the lumps of products in the reactor outlet stream at 350 °C at XWo ) 1 for different values of space time. (a) 0.194 (g of catalyst) h (g of ethanol)-1. (b) 0.387 (g of catalyst) h (g of ethanol)-1. Lines, calculated with the kinetic model. Points, experimental results.

increases, which is due either to the increase in the capacity for water to displace and desorb coke precursors that are evolving on the active sites or to the activation of cracking mechanisms for coke precursors as the temperature increases. Concerning the validity of eq 17, which corresponds to a deactivation model in series-parallel with the main reaction, it must be pointed out that its fitting to the experimental results in the 350-450 °C temperature range is adequate. The validity of the model is shown in Figures 6 and 7, where the fittings of the results corresponding to 350 and 400 °C are shown as examples. The evolution with time on stream of the product stream composition at the reactor outlet is plotted, and each graph corresponds to different values of water content in the feed (aqueous ethanol) and space time. The lines are the results calculated with the kinetic model, and the points are the experimental results. In previous papers, the validity of the kinetic model has clearly been shown for the entire range of operating conditions studied.44,45 Nevertheless, at extreme conditions of high concentrations of either a coke precursor (ethene) or the other coke precursors as a whole (olefins and gasoline), the following points should be taken into account concerning the use of alternative models to eq 17:


Figure 7. Evolution with time on stream of the mass fractions, by unit mass of organic components in the reaction medium, of ethene and of the lumps of products in the reactor outlet stream at 400 °C for different values of operating conditions. (a) XWo ) 1.0, space time ) 0.387 (g of catalyst) h (g of ethanol)-1. (b) XWo ) 3.0, space time ) 0.355 (g of catalyst) h (g of ethanol)-1. Lines, calculated with the kinetic model. Points, experimental results.

First, for temperatures below 400 °C (when cracking of gasoline is not significant) and high space time and, in general, under conditions in which the concentrations of the olefin and gasoline lumps are high, a better fitting to the experimental data is obtained when similar values of the constants kdOG and kdE are used in eq 17, which shows that ethene and olefin/gasoline lump contribute to coke formation to the same degree. This result corresponds to a deactivation that takes place in series-parallel with the main reaction. On the other hand, under conditions in which the ethene concentration in the reaction medium is favored, an exclusively parallel deactivation model [without the term depending on (XO + XG) in eq 17] gives a better fitting to the experimental data. This situation arises above 400 °C, when the cracking of coke components also occurs, as was mentioned previously. The kinetic results that reveal the lesser importance of the olefin and gasoline lumps compared to ethene as coke precursors above 400 °C are explained by the higher crackability of the oligomers formed from these lumps. Taking this result into account, an improvement in the deactivation kinetic model can be made by introducing a factor of deactivation attenuation due to this partial cracking of coke.

Figure 8. Evolution of the activity at the reactor outlet with time on stream for different values of temperature and space time. XWo ) 0.045. (a) 350 °C. (b) 400 °C. (c) 450 °C.

These explanations justify the wide range (for the given confidence interval) of the activation energy of the constant for deactivation by coke formed from olefins and gasoline (eq 19). Nevertheless, the generally suitable fitting obtained with eq 17 enabled this equation to be used in a reactor simulation with the aim of optimizing the reaction step and the combined global process (reactor-regenerator) for directly upgrading aqueous ethanol by catalytic transformation. 4.4. Parametric Study of Deactivation. In Figure 8, the evolution of the activity at the reactor outlet is plotted as a function of time on stream for a feed of almost pure ethanol (the water/organic components mass ratio in the feed is XWo ) 0.045). Each graph corresponds to a given reaction temperature. The results were calculated by solving the kinetic equation for deactivation, eq 17, with the parameters calculated according to eqs 18-21. It can be seen that activity decreases more rapidly as the temperature is increased. When the effect of space time (which is equivalent to longitudinal position along the reactor) is analyzed, deactivation is more rapid for all three temperatures when the space time is lower, which is a consequence of the higher ethene content in the reaction medium.

4222


Figure 11. Relationship between the activity at the reactor outlet and the total acidity for different operating conditions.

still indicates a trend in the a-Cc relationship that follows the equation

a ) exp[-(1.6Cc - 0.1Cc2)]

Figure 9. Evolution of the activity at the reactor outlet with time on stream at 400 °C for different values of water content in the feed and space time.

(22)

The activity calculated at the reactor outlet for different operation conditions is plotted in Figure 11 against the total acidity of the catalyst. Each symbol corresponds to a different feed. The results reveal a linear relationship between activity and acidity. It can also be observed that the completely deactivated catalyst maintains a residual acidity level. This residual acidity corresponds to very weak acid sites whose activity is sufficient for the ethanol dehydration step. Concerning the results of Figure 11, it must be pointed out that NH3 is smaller in size than the reactant molecules, and consequently, its possible diffusional limitation in the porous structure due to coke deposition will be lower. In other words, the activity results might be affected by this limitation, even though it has little effect on the acidity results. Therefore, a reduction in catalyst activity might be caused not only by the blockage of acid sites by coke but also by the decrease in the effective pore size resulting from coke deposition, which will limit the diffusion of reactants and products. 5. Conclusions

Figure 10. Relationship between the activity at the reactor outlet and the average coke content in the catalyst bed for different operating conditions.

The attenuating effect of water is clear when the results of graph b in Figure 8 (corresponding to 400 °C and XWo) 0.045) are compared with the results in Figure 9, which correspond to the same temperature as in Figure 8b but to different water contents in the feed (XWo) 1.0 in graph a, and XWo) 3.0 in graph b). Figure 10 shows the relationship between the activity calculated at the reactor outlet and the average coke content in the catalyst bed for experiments carried out with different feeds (aqueous ethanol and pure diethyl ether) and under different operating conditions (temperature, space time, and time on stream). Although this average coke content is only an approximate indication of the catalyst state, the curve in Figure 10

It has been shown that the deactivation of HZSM-5 zeolite used in the transformation of aqueous ethanol into hydrocarbons in the 350-450 °C range can be described by a kinetic model similar to that previously established for aqueous methanol transformation. The resulting kinetic equation, eq 17, is applicable to all of the steps of the kinetic scheme except for ethanol dehydration, which is not affected by catalyst deactivation. Equation 17 considers the effects on deactivation of the concentrations of ethene and of the lumps of olefins and gasoline in the reaction medium, although, judging from the values of the kinetic parameters calculated, ethene is the main coke precursor, especially at high temperatures. The water present in the reaction medium, partly as a result of introduction in the feed and partly as a product, has an important role in the attenuation of coke deposition and evolution. This effect is taken into account in the kinetic model and quantified by means of a constant that increases with temperature.


As a consequence of the attenuation of coke evolution by water, the coke is slightly developed and has a high H/C ratio, which explains its significant cracking above 400 °C and the resulting contribution to attenuating coke evolution. The kinetic model for deactivation is suitably rigorous in considering the effects of the operating conditions while still maintaining simplicity. This simplicity is important for its use in simulations of reactors using different possible strategies of fixed and fluidized beds, as wellas the combined global process (reactor-regenerator) for directly upgrading, by catalytic transformation, the liquid product obtained by the fermentation of natural carbohydrates. Acknowledgment This work was carried out with the financial support of the Department of Education, University and Research of the Basque Country (Project PG98/9), and of the University of the Basque Country (Project G34-98). Nomenclature A, E, G, O, P ) ethanol and/or diethyl ether, ethene, lump of gasoline (C5+), lump of olefins, and lump of paraffins, respectively a ) activity (eq 16) Cc ) coke content in the catalyst, wt % d ) deactivation order FAo ) mass flow rate of ethanol or diethyl ether in the feed, g h-1 Fo ) mass flow rate of ethene, ethanol, or diethyl ether in the feed, g h-1 ki ) kinetic constant of step i in the kinetic scheme, h-1 k′i ) apparent kinetic constant of step i in the kinetic scheme (including the attenuating effect of water present in the reaction medium), h-1 kdi ) kinetic constant for deactivation by coke formed from component i, h-1 k′di ) apparent kinetic constant for deactivation by coke formed from component i (including the attenuating effect of water present in the reaction medium), h-1 kWo, kW ) parameters that quantify the resistance to formation of component i in the corresponding reaction step and the formation of coke due to the presence of water in the reaction medium rio, ri ) rates of formation of component i at zero time on stream and at any time on stream, respectively, g h-1 (g of catalyst)-1 T ) temperature, K t ) time on stream, h W ) catalyst weight, g Xi ) weight fraction of component i in the reaction medium by unit mass of organic components XW, XWo ) water/organic components mass ratios in the reaction medium and in the feed, respectively Greek Letters θ(XW), θd(XW) ) functions for quantifying the effect of water present in the reaction medium on the rates of the steps of the kinetic scheme and on the kinetics of deactivation by coke, respectively

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Received for review January 23, 2002 Revised manuscript received May 29, 2002 Accepted May 29, 2002 IE020068I

Catalyst Deactivation by Coke in the Transformation of Aqueous

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