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Kinetic Modelling for Deactivation by Coke Deposition of a HZSM-5 Zeolite Catalyst in the Transformation of Aqueous Ethanol into Hydrocarbons
Studies in Surface Science and Catalysis, Vol. 139 J.J. Spivey, G.W. Roberts and B.H. Davis (Editors) 9 2001 Elsevier Science B.V. All rights reserved.
455
Kinetic Modelling for Deactivation by Coke Deposition of a HZSM-5 Zeolite Catalyst in the Transformation of Aqueous Ethanol into Hydrocarbons A.G. Gayubo*, A.T. Aguayo, A.M. Tarrio, M. Olazar, J. Bilbao Departamento de Ingenieria Quimica. Universidad del Pals Vasco. Apartado 644, 48080 Bilbao. Spain. Fax: 34 4 4648500, E-mail: [email protected] A kinetic model for the deactivation by coke deposition of a catalyst (based on a HZSM-5 zeolite) used in the transformation of aqueous ethanol into hydrocarbons has been proposed. The experiments have been carded out in an isothermal fixed bed reactor by feeding the reactor with ethene, ethanol/water with different mass ratios and diethyl ether. The kinetic model quantifies the effect on coke deposition of the concentration of the organic components in the reaction medium, and takes into account the attenuating effect of water on coke deposition. 1. INTRODUCTION The transformation of ethanol into hydrocarbons on acid catalysts is an interesting approach to upgrading renewable resources, such as biomass, via fermentation [1 ]. There is currently a great interest in the catalytic transformation of aqueous ethanol, as it avoids the costly operations involved in the total elimination of water for obtaining pure ethanol [2]. The aim of the catalytic transformation is to obtain ethene, which is the objective of the BETE process (bioethanol to ethene) [3] or obtain C5+ hydrocarbons (BTG process) (bioethanol to gasoline) [1]. The latter process requires a reaction temperature above 350 ~ when deactivation by coke is significant. There is a great similarity in the catalytic transformation of ethanol and of methanol on a HZSM-5 zeolite, either concerning the reaction mechanism [4], or in the spectra of products [5,6]. Nevertheless, the high content of water in the feed in the BTG process markedly influences the distribution of products and catalyst deactivation [5-7]. It has been proven that water attenuates deactivation by coke at moderate temperatures [6], but causes irreversible deactivation by dealumination of the zeolite at high temperatures. For this process, there are gaps in the literature in aspects such as the establishment of a kinetic model for the main reaction and for deactivation, in order to quantify the product distribution and the effect of operating conditions on this distribution. A kinetic model for deactivation by coke deposition is proposed in this paper, which pays special attention to the role of water on the attenuation of coke deposition, and takes as a reference the recent results in the kinetic modelling of deactivation by coke in the MTG process (methanol to gasoline) on a HZSM-5 zeolite [8] and in the MTO process (methanol to olefins) on a SAPO-34 [9]. 2. EXPERIMENTAL The preparation of the HZSM-5 zeolite has been carried out following Mobil patents [ 10].
456 The catalyst has been prepared by agglomerating the HZSM-5 zeolite (15 wt%) with bentonite (30 wt%) and using alumina (45 wt%) as an inert charge. Prior to use, the catalyst is calcined at 843 K for 2 h (for the experimental results to be reproducible under reaction-regeneration cycles) [ 11 ]. The Si/A1 ratio of the HZSM-5 zeolite is Si/Al=24. The physical properties of the catalyst, determined by N2 adsorption-desorption in a Micromeritics ASAP 2000, are: surface area, 131 2 -1 -1 m g ; pore volume, 0.43 cm3 g ; 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 is: dp < 10-3 l.tm (micropores), 8.1%; 10-3 l.tm < dp < 10-2 l.tm (mesopores), 14.7%; 10 -2 l-tm < dp < 2 l-tm (macropores), 77.2%. The distribution of the catalyst acid strength, obtained by adsorption-desorption of NH3 carried out in a SDT 2960 thermobalance (TA Instruments), shows the uniformity of the catalyst acid structure, as the catalyst sites release a similar energy in the adsorption of NH3 (between 125 and 150 kJ (mmol de NH3)-I). The reaction equipment is operated by means of a data acquisition and control program. The reactor is of stainless steel 316, with 9 mm internal diameter. It is provided with a fixed bed of catalyst diluted with alumina as inert and operates in isothermal regime. The reaction products are analysed by gas chromatography (Hewlett Packard 6890) by means of detectors based on thermal conductivity (TCD) and flame ionization (FID). The separation of products is 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); b) remaining products (C5+). 2) SUPEL-Q Plot semicapillary column for individually separating out both volatile components and polar components, which will be subsequently analysed by TCD and FID. 3) PONA capillary column for separation of C5+ hydrocarbons, which will be analysed by FID. Three sets of experiments have been carded out at atmospheric pressure: one by feeding ethene into the reactor, another by feeding ethanol and water with different mass ratios (6, 50 and 75 wt% of water), and the third by feeding pure diethyl ether. The experimental conditions were: temperature range between 350-450 ~ space time up to 0.83 g catalyst h (g ethanol) -1 partial pressure 75 kPa; time on stream up to 60 h. This feed is diluted with a flow of 30 cm3 min-1 of He as an inert gas, with the aim of ensuring bed isothermallity. 3. KINETIC STUDY 3.1. Kinetic model for the main reaction
The kinetic transformation of aqueous ethanol into hydrocarbons on a HZSM-5 zeolite can be represented by means of a kinetic scheme similar to the one common in the literature for the transformation of methanol [12-14]. A difference is the convenience of taking ethene as an individual component, separated from the remaining components or lumps in the reaction medium. The adopted kinetic scheme is shown in Table 1, together with the corresponding kinetic constants [15,16]. The first step corresponds to the dehydration of ethanol or diethyl ether (A=oxygenate). The second step corresponds to the oligomerization-cracking of ethene (E) to light olefins (propene and butenes, O). Ethene, by means of oligomerization-crackingaromatization, can produce hydrocarbons corresponding to C5+ lump (gasoline, G) or it can directly produce gaseous paraffins (P). The lump of gasoline can be generated by condensation of olefins. From the cracking of gasoline, paraffins as well as ethene+olefins can be generated.
457 Table 1 Kinetic scheme for the transformation of ethanol into hydrocarbons and kinetic constants Kinetic Scheme
Kinetic constants
A
kl
E
k2 > O
k 2 - 2.74(+0.34) exp [- (5750(_+600)) (1/T - 1/623)]
E
k3 > G
k 3 - 1.89(+0.31) exp [- (4350(+650)) (1/T -1/623)]
E
k4 >G
k 4 - 3.93(_+1.22) exp [- (2750(+900)) (1/T -1/623)]
E
k5 > P
k 5 = 0.835(+0.16) exp [- (6950(_+1000)) (1/T -1/623)]
G
k6 >P
k 6 - 0.304(_+0.02) exp [- (4000(+1500)) (1/T - 1/623)]
G
k7 >E+O
k 7 - 3.52(_+0.20) exp [- (11050(_+1000)) (1/T -1/623)]
>E
k 1 - 283(+_83.35) exp [- (17000(+1850)) (1/T - 1/623)]
kw0 = 0.459(_+0.04) exp [- (600(+_250)) (1/T - 1/623)] A first order kinetics for each one of the individual steps of the kinetic scheme has been assumed and the kinetic equations at zero time on stream are expressed in terms of the mass fraction of the lumps by mass unit of the organic components in the reaction medium [15,16]. The kinetic parameters in Table 1 have been obtained from the results for zero time on stream in isothermal integral reactor, in the 350-450 ~ range. The attenuation of conversion due to water is taken into account by multiplying each kinetic constant, ki, by a function dependent on water content, 0(Xw), which is given by: 0(Xw) = exp (-kw0Xw)
(1)
The term Xw is the ratio between the mass flow of water, mw, and of organic components, mo, in the reaction medium. The mass flow of water in the reaction medium is the sum of the water formed as product, mwf, and of the mass flow of water in the feed, mwo. Taking into account the total mass balance, the mass flow of water is calculated as: m T ( m W f / m o ) + mwo (2) 1 + (mwf / m o ) In eq. (2), the amount of water formed by mass unit of organic components, mwf/mo, can be calculated by means of the stoichiometry from the amount of ethanol or diethyl ether converted in the reaction, by eqs. (3) and (4), respectively. mw =
(mwf / mo) - 18(1 - XA)/46
(3)
(mwf / mo) - 18(1 - X A) / 74
(4)
3.2. Kinetics of the deactivation by coke deposition The following equation has been proposed in literature [8] for the deactivation of the catalyst in the transformation of aqueous methanol into hydrocarbons: da_ _(~k~Xi)ad dt -
(5)
458 Due to the similarity of the catalyst and of the characteristics of the deactivation in the transformation of methanol and of ethanol, eq. (5) has been taken as a basis for the establishment of possible deactivation equations for the transformation of ethanol into hydrocarbons. In eq. (5), Xi is the mass fraction (based on the organic components in the reaction medium) of the lumps of the kinetic scheme that can be considered coke precursors. This is the way in which the composition of the reaction medium is commonly expressed in the literature for the kinetic study of the processes of transformation of methanol on a HZSM-5 zeolite [8,12-14,16] and on a SAPO-34 [9]. In eq. (5), activity, a, is defined as the ratio between reaction rate at t time and at zero time: a-
(ri)j (rio)j
for j= 2,....,7
(6)
where j refersto each one of the steps in the kinetic scheme of Table I. Eq. (6) corresponds to a non-selective deactivation kinetic model and, consequently, it considers the same deactivation rate for all the steps in the kinetic scheme, except for the dehydration of ethanol into ethene. For this step the deactivation is not significant (a=l) for any experimental conditions (which include experiments of up to 60 h under the conditions corresponding to the more severe deactivationfor the other steps in the kinetic scheme). k ~ is the apparent kineticconstant for deactivation,which is calculated as follows: !
kdi = kdi0 d (X w )
(7)
where 0d(Xw) is a function that quantifies the attenuation of deactivation due to water. Different mathematical expressions of one parameter, kw, have been tested, and the following has been adopted as the more suitable: 0 d(Xw) = exp(-kwXw)
(8)
The different expressions that have been tested for catalyst deactivation are summarized in Table 2. In these equations different lumps are considered as coke precursors. The lump of oxygenates (ethanol and diethyl ether) has not been considered as coke precursor, as its composition in the reaction medium is almost zero under experimental conditions. The formation of coke from paraffins has not been taken into account, given their low reactivity. Consequently, the possible coke precursors are ethene, the lump of olefins (propene and butenes) and the lump of gasoline. This hypothesis is supported by the oligomerization capacity of light olefins to give compounds that will remain trapped in the porous structure [ 17] and by the cyclization and condensation capacity of aromatics (whose proportion is high in the lump of gasoline) [ 18,19]. Model 1 in Table 2 considers that deactivation does not depend on the composition of the reaction medium (deactivation independent of the main reaction). In model 2, deactivation takes place in parallel with the main reaction and ethene (which is formed at the entrance of the reactor) is the coke precursor. In model 3 the lumps of products (olefins and gasoline) are considered as coke precursors (deactivation in series with the main reaction). Models 4 and 5 consider the three lumps (ethene, olefins and gasoline) as coke precursors (deactivation in series-parallel with the main reaction). In model 5, the same contribution to coke formation is considered for olefins and for gasoline.
459 Table 2 Possible kinetic equations for deactivation in the transformation of aqueous ethanol into hydrocarbons and error objective functions calculated for each one of them. Model
E.O.F. (eq. (14))
Kinetic equation for deactivation !
1
2.09.10 -3
da = _kdad dt
2
1.42.10 .3
3
4.33.10 -3
da =_kdEXEa d dt da , ) d-~ - doXo +kdGXG a d
4
1.23-10 -3
da = dt
5
1.25.10 3
ddta -- - (k'dEXE + k d' o G ( X o + XG) ) ad
i
, , ) X E + k d o X o + kdGX G a d
(9) (10) (11) (12) (13)
The selection of the more suitable deactivation kinetic model and the calculation of the kinetic parameters have been carried out by minimizing the following error objective function: n I nexp ~ (Xi,j - Xi(cal),j)2 (14) nlnexp where Xi,j are the experimental values of weight fractions based on the organic components, for component i in the experimental point j (under conditions in which irreversible deactivation by dealumination due to steam have been avoided) and Xi(calc),j are the values of composition calculated by solving the corresponding mass conservation equations for each lump or component of the kinetic scheme. The mass conservation equation for i lump in the reactor, assuming plug flow and expressing the concentration as mass fraction based on the organic product stream and the longitudinal position as dimensionless, ~, can be written as: E.O.F. = i = l j = l
c3Xi 0t
(l-e) e
- - = ~ p
RT m O uc3X i _ ri - PM FAo Z c3~
(15)
where e is the bed voidage; p is the apparent catalyst density; P is the partial pressure of the organic components; M is the average molecular weight of the organic components; FAo is the mass flowrate of oxygenates in the feed; u is the gas linear velocity; Z is the reactor length; ri is the reaction rate at t time, corresponding to the formation of the different lumps of the kinetic scheme: rA = - k l 0 ( X w ) X A
(16)
rE = kl0(Xw)X A - 0(Xw)((k 2 + k 3 + k5)X E - k7XG)a
(17)
rO - 0 ( X w ) ( k 2 X E + k7X G - k4Xo] a
(18)
460 rp = 0(X w)(k5X E + k6X G ) a
(19)
A set of (n- 1) equations such as eq. (15), corresponding to (n- 1) lumps of the kinetic scheme, must be solved, together with the deactivation kinetic equation. A program in FORTRAN has been developed for this calculation, which uses the LSODE subroutine from the DSSP library. The composition of the remaining lump is calculated by difference to unity. The correlation between the estimations of frequency factor and activation energy has been minimized by reparameterization [20]. Thus, the parameters to optimize are the kinetic constants at a reference temperature, 623 K, and the activation energies. The 90% confidence intervals of the estimated parameters have been obtained by non-linear regression using the Marquardt algorithm. The results of the objective function corresponding to each one of the proposed kinetic models are summarized in Table 2. As is observed, model 4 is the one with the best fit, although the difference with model 5 is very small. As this latter model is simpler, we adopted it for subsequent studies. The kinetic parameters corresponding to the kinetic model 5 (with 90% confidence intervals calculated by means of the Mardquardt algorithm for non-linear regression) are: !
kdE - 0.480 (+0.078) exp [- (5800(+800)) (1/T - 1/623)]
(20)
k'do G
(21)
0.175(_+0.077) exp [- (1000(+_2050)) (1/T
-
1/623)]
-
kdw - 0.481(+0.083) exp [- (3400(+1500))(1/T - 1/623)]
(22)
d= 1.78+0.08
(23)
The fitting of the kinetic model calculated to the experimental results is shown in Figures 1 and 2, where the evolution with time on stream of the mass fraction (by mass unit of organic components in reaction medium) of each lump of the kinetic scheme has been plotted. As is observed in Figure 1, deactivation attenuates as water content in the feed increases, at 350 ~ 1
1
Xi
W/FAo = 0.219 gcatalysth/gEtOH
Xi
Xwo = 0.045
0.8
9
99 ~
0.6 9
~eoe~ 9 ethene = olefins
9
Xwo = 3.0
0.6
, C5+ 9 paraffins
0.4
W/FAo = 0.709 gcatalysth/gEtOH
0.8
9 ethene 9 olefins
9 C5+
0.4
9 paraffins
4tOO"b . . . . . . . '~-____e_e.ti._~_.~._..#..,~.,.."
0.2 0 |
=
0
,
I
1
i
""-m-'t i
2
i
-----
0.2
m-urnr = ~ r - . r n i
3
i
i
4
time on stream, h
J
=-w tm m-mtm i
5
=
AAAAA JJ,rlBi-il
0
6
0
9
1 1 4 1 1 , n 1 - r i l ~ 1 . 1 1 I,
1
2
3
9 9
,U-i'l'-ll -Jl;lrm 11111-F nl-.-i
4
5
6
time on stream, h
Fig. 1. Fitting of calculated (lines) to experimental (points) evolution of mass fraction of each lump with time on stream, for 350 ~ and different water contents in the feed.
461
1
I
Xi 0.8
T= 350 ~
* ethene
W/FAo= 0.387 UcatalystnuUEtOH
9 olefins 9 C5+ 9 paraffins
Xi
0.6
I
* elhene
T= 400 ~
0.8
9 olefins
W/FAo = 0.387 gcatalysth/gEtOH
* C5+ 9 paraffins
0.6 000
0.4
9 ~176
0.4
i b l , I~
..... % ~ . t , .
0.2
0.2 n9 ill"rat i
0
,"
--" -w't
0
~,
.
""
9o
i
1
! n'm i
ii'-h-B-,i-" i
2
i
-i" i
3
-rI
-Ilrlnrmr i
4
time on stream, h
rlri-B i
lrt i
5
9
----n-..-._~.~,._t
.......
!1i
i
0 6
0
[
1
i
I
2
i
9I 3
i
9 "-~,-i-,--r I i I 4
5
i
[ II 6
t i m e on s t r e a m , h
Fig. 2. Fitting of calculated (lines) to experimental (points) evolution of mass fraction of each lump with time on stream, for different temperatures and 50 wt% of water in the feed. Figure 2 shows the increase in deactivation with temperature for a feed of ethanol-water with 50 wt% of water. By comparing eqs. (20) and (21) it is observed that at the reference temperature (350 ~ the deactivation constant corresponding to the evolution of ethene to coke, kdE, is three times higher than that corresponding to the evolution of olefins and gasoline to coke, kdo~. Consequently, ethene is the main coke precursor. The important effect of temperature in the kinetic modelling is noteworthy. Thus, although model 5 fits better in the whole range of temperatures studied, 350-450 ~ model 2 (parallel deactivation) is suitable at high temperatures and model 1 (independent deactivation) is acceptable at low temperatures. In order to understand the different effect of temperature in the evolution of coke from individual components in the reaction medium, cracking of components (oligomers) generated by condensation of the constituents of the lump of olefins and gasoline above 400 ~ must be taken into account. Consequently, at these temperatures, the effect of concentration of these coke precursors is less important, which justifies the validity of the parallel deactivation kinetic model under these conditions. Taking this result into account, an improvement in the deactivation kinetic model may be suggested by introducing a factor of deactivation attenuation due to this partial cracking of coke. Eq. (22) shows that the attenuating effect of water on deactivation increases with temperature, which may be explained by an increase in the ability for water to displace and desorb the coke precursors that are evolving on the active sites, or by the activation of coke precursor hydrocracking mechanisms as temperature is increased. 4. CONCLUSIONS It has been proven that the deactivation of the HZSM-5 zeolite used in the transformation of aqueous ethanol into hydrocarbons in the 350-450 ~ range is explained by a similar kinetic scheme to that already established for the transformation of aqueous methanol. The kinetic equation, eq. (13), is applicable to all the steps of the kinetic scheme except to ethanol dehydration, which is not affected by catalyst deactivation. Eq. (13) takes into account the effect of concentration of ethene and of the lump of olefins and gasoline in the reaction medium on the deactivation, although ethene is the main coke precursor on the basis of the values of the kinetic constants. The water present in the reaction medium plays an important
462 role in the attenuation of deposition and evolution of coke, which is considered in the kinetic model and quantified by means of a constant that increases with temperature. The kinetic model proposed, eq. (13), is suitable for the 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. ACKNOWLEDGEMENTS
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 (334-98). REFERENCES
1. D.R. Whitcrafl, X.E. Verykols, R. Mutharasan, Ind. Eng. Chem. Process Des. Dev., 22 (1983)452. 2. G.A. Aldridge, X:E. Veryklos, R. Muthasarn, R., Ind. Eng. Chem. Process Des. Dev., 23 (1984) 733. 3. R. Le Van Mao, T.M. Nguyen, G.P. McLaughlin, Appl. Catal., 48 (1989) 265. 4. J.P. Van den Berg, J.P. Wolthuizen, J.H.C. van Hoof, Proceedings of the 5th International Zeolite Conference; Rees, L.V.V., Ed., Heydon, London, p. 649, 1981. 5. E. Costa, A. Uguina, J. Aguado, P.J. Hernhndez, Ind. Chem. Eng. Process Des. Dev., 24 (1985) 239. 6. A.K. Talukdar, K.G. Bhattacharyya, S. Sivasanker, Appl. Catal., 148 (1997) 357 7. C.B. Phillips and R. Datta, Ind. Eng. Chem. Res., 36 (1997) 4466. 8. A.G. Gayubo, A.T. Aguayo, A.L. Mor~in, M. Olazar, J. Bilbao, AIChE J., submitted, 2001 9. A.G. Gayubo, A.T. Aguayo, A.E. Shnchez del Campo, P.L Benito, J. Bilbao, Stud. Surf. Sci. Catal., 26 (1999) 129. 10. R.J. Argauer, G.R. Landolt, U.S. Patent, 3,702,886 (1972). 11. P.L. Benito, A.G. Gayubo, A.T. Aguayo, M. Olazar, J. Bilbao, Ind. Eng. Chem. Res., 35 (1996) 3991. 12. C.D. Chang, Hydrocarbons from Methanol, Heinemann, H., Ed., Marcel Dekker, Inc., New York, 1983. 13. P.H. Schipper and F.J. Krambeck, Chem. Eng. Sci., 41 (1986) 1013. 14. A.T. Aguayo, A.G. Gayubo, J.M. Ortega, M. Olazar, J. Bilbao, Catal. Today, 37 (1997) 239. 15. A.M. Tarrio, Ph.D: Thesis, University of the Basque Country, Bilbao, 2000. 16. A.G. Gayubo, A.M. Tarrio, A.T. Aguayo, M. Olazar, J. Bilbao, Ind. Eng. Chem. Res., submitted, 2001. 17. A.K. Ghosh, R.A. Kydd, J. Catal., 100 (1986) 185. 18. J. Bilbao, J., A.T. Aguayo, J.M. Arandes, Ind. Eng. Chem. Product Res. Dev., 24 (1985) 531. 19. S. Bhatia, J. Beltramini, D. Do, Catal. Rev.-Sci.Eng., 31 (1990) 431. 20. J.R. Kittrell, R. Mezaki, C.C. Watson, Ind. Eng. Chem., 57 (1965) 19.
455
Kinetic Modelling for Deactivation by Coke Deposition of a HZSM-5 Zeolite Catalyst in the Transformation of Aqueous Ethanol into Hydrocarbons A.G. Gayubo*, A.T. Aguayo, A.M. Tarrio, M. Olazar, J. Bilbao Departamento de Ingenieria Quimica. Universidad del Pals Vasco. Apartado 644, 48080 Bilbao. Spain. Fax: 34 4 4648500, E-mail: [email protected] A kinetic model for the deactivation by coke deposition of a catalyst (based on a HZSM-5 zeolite) used in the transformation of aqueous ethanol into hydrocarbons has been proposed. The experiments have been carded out in an isothermal fixed bed reactor by feeding the reactor with ethene, ethanol/water with different mass ratios and diethyl ether. The kinetic model quantifies the effect on coke deposition of the concentration of the organic components in the reaction medium, and takes into account the attenuating effect of water on coke deposition. 1. INTRODUCTION The transformation of ethanol into hydrocarbons on acid catalysts is an interesting approach to upgrading renewable resources, such as biomass, via fermentation [1 ]. There is currently a great interest in the catalytic transformation of aqueous ethanol, as it avoids the costly operations involved in the total elimination of water for obtaining pure ethanol [2]. The aim of the catalytic transformation is to obtain ethene, which is the objective of the BETE process (bioethanol to ethene) [3] or obtain C5+ hydrocarbons (BTG process) (bioethanol to gasoline) [1]. The latter process requires a reaction temperature above 350 ~ when deactivation by coke is significant. There is a great similarity in the catalytic transformation of ethanol and of methanol on a HZSM-5 zeolite, either concerning the reaction mechanism [4], or in the spectra of products [5,6]. Nevertheless, the high content of water in the feed in the BTG process markedly influences the distribution of products and catalyst deactivation [5-7]. It has been proven that water attenuates deactivation by coke at moderate temperatures [6], but causes irreversible deactivation by dealumination of the zeolite at high temperatures. For this process, there are gaps in the literature in aspects such as the establishment of a kinetic model for the main reaction and for deactivation, in order to quantify the product distribution and the effect of operating conditions on this distribution. A kinetic model for deactivation by coke deposition is proposed in this paper, which pays special attention to the role of water on the attenuation of coke deposition, and takes as a reference the recent results in the kinetic modelling of deactivation by coke in the MTG process (methanol to gasoline) on a HZSM-5 zeolite [8] and in the MTO process (methanol to olefins) on a SAPO-34 [9]. 2. EXPERIMENTAL The preparation of the HZSM-5 zeolite has been carried out following Mobil patents [ 10].
456 The catalyst has been prepared by agglomerating the HZSM-5 zeolite (15 wt%) with bentonite (30 wt%) and using alumina (45 wt%) as an inert charge. Prior to use, the catalyst is calcined at 843 K for 2 h (for the experimental results to be reproducible under reaction-regeneration cycles) [ 11 ]. The Si/A1 ratio of the HZSM-5 zeolite is Si/Al=24. The physical properties of the catalyst, determined by N2 adsorption-desorption in a Micromeritics ASAP 2000, are: surface area, 131 2 -1 -1 m g ; pore volume, 0.43 cm3 g ; 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 is: dp < 10-3 l.tm (micropores), 8.1%; 10-3 l.tm < dp < 10-2 l.tm (mesopores), 14.7%; 10 -2 l-tm < dp < 2 l-tm (macropores), 77.2%. The distribution of the catalyst acid strength, obtained by adsorption-desorption of NH3 carried out in a SDT 2960 thermobalance (TA Instruments), shows the uniformity of the catalyst acid structure, as the catalyst sites release a similar energy in the adsorption of NH3 (between 125 and 150 kJ (mmol de NH3)-I). The reaction equipment is operated by means of a data acquisition and control program. The reactor is of stainless steel 316, with 9 mm internal diameter. It is provided with a fixed bed of catalyst diluted with alumina as inert and operates in isothermal regime. The reaction products are analysed by gas chromatography (Hewlett Packard 6890) by means of detectors based on thermal conductivity (TCD) and flame ionization (FID). The separation of products is 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); b) remaining products (C5+). 2) SUPEL-Q Plot semicapillary column for individually separating out both volatile components and polar components, which will be subsequently analysed by TCD and FID. 3) PONA capillary column for separation of C5+ hydrocarbons, which will be analysed by FID. Three sets of experiments have been carded out at atmospheric pressure: one by feeding ethene into the reactor, another by feeding ethanol and water with different mass ratios (6, 50 and 75 wt% of water), and the third by feeding pure diethyl ether. The experimental conditions were: temperature range between 350-450 ~ space time up to 0.83 g catalyst h (g ethanol) -1 partial pressure 75 kPa; time on stream up to 60 h. This feed is diluted with a flow of 30 cm3 min-1 of He as an inert gas, with the aim of ensuring bed isothermallity. 3. KINETIC STUDY 3.1. Kinetic model for the main reaction
The kinetic transformation of aqueous ethanol into hydrocarbons on a HZSM-5 zeolite can be represented by means of a kinetic scheme similar to the one common in the literature for the transformation of methanol [12-14]. A difference is the convenience of taking ethene as an individual component, separated from the remaining components or lumps in the reaction medium. The adopted kinetic scheme is shown in Table 1, together with the corresponding kinetic constants [15,16]. The first step corresponds to the dehydration of ethanol or diethyl ether (A=oxygenate). The second step corresponds to the oligomerization-cracking of ethene (E) to light olefins (propene and butenes, O). Ethene, by means of oligomerization-crackingaromatization, can produce hydrocarbons corresponding to C5+ lump (gasoline, G) or it can directly produce gaseous paraffins (P). The lump of gasoline can be generated by condensation of olefins. From the cracking of gasoline, paraffins as well as ethene+olefins can be generated.
457 Table 1 Kinetic scheme for the transformation of ethanol into hydrocarbons and kinetic constants Kinetic Scheme
Kinetic constants
A
kl
E
k2 > O
k 2 - 2.74(+0.34) exp [- (5750(_+600)) (1/T - 1/623)]
E
k3 > G
k 3 - 1.89(+0.31) exp [- (4350(+650)) (1/T -1/623)]
E
k4 >G
k 4 - 3.93(_+1.22) exp [- (2750(+900)) (1/T -1/623)]
E
k5 > P
k 5 = 0.835(+0.16) exp [- (6950(_+1000)) (1/T -1/623)]
G
k6 >P
k 6 - 0.304(_+0.02) exp [- (4000(+1500)) (1/T - 1/623)]
G
k7 >E+O
k 7 - 3.52(_+0.20) exp [- (11050(_+1000)) (1/T -1/623)]
>E
k 1 - 283(+_83.35) exp [- (17000(+1850)) (1/T - 1/623)]
kw0 = 0.459(_+0.04) exp [- (600(+_250)) (1/T - 1/623)] A first order kinetics for each one of the individual steps of the kinetic scheme has been assumed and the kinetic equations at zero time on stream are expressed in terms of the mass fraction of the lumps by mass unit of the organic components in the reaction medium [15,16]. The kinetic parameters in Table 1 have been obtained from the results for zero time on stream in isothermal integral reactor, in the 350-450 ~ range. The attenuation of conversion due to water is taken into account by multiplying each kinetic constant, ki, by a function dependent on water content, 0(Xw), which is given by: 0(Xw) = exp (-kw0Xw)
(1)
The term Xw is the ratio between the mass flow of water, mw, and of organic components, mo, in the reaction medium. The mass flow of water in the reaction medium is the sum of the water formed as product, mwf, and of the mass flow of water in the feed, mwo. Taking into account the total mass balance, the mass flow of water is calculated as: m T ( m W f / m o ) + mwo (2) 1 + (mwf / m o ) In eq. (2), the amount of water formed by mass unit of organic components, mwf/mo, can be calculated by means of the stoichiometry from the amount of ethanol or diethyl ether converted in the reaction, by eqs. (3) and (4), respectively. mw =
(mwf / mo) - 18(1 - XA)/46
(3)
(mwf / mo) - 18(1 - X A) / 74
(4)
3.2. Kinetics of the deactivation by coke deposition The following equation has been proposed in literature [8] for the deactivation of the catalyst in the transformation of aqueous methanol into hydrocarbons: da_ _(~k~Xi)ad dt -
(5)
458 Due to the similarity of the catalyst and of the characteristics of the deactivation in the transformation of methanol and of ethanol, eq. (5) has been taken as a basis for the establishment of possible deactivation equations for the transformation of ethanol into hydrocarbons. In eq. (5), Xi is the mass fraction (based on the organic components in the reaction medium) of the lumps of the kinetic scheme that can be considered coke precursors. This is the way in which the composition of the reaction medium is commonly expressed in the literature for the kinetic study of the processes of transformation of methanol on a HZSM-5 zeolite [8,12-14,16] and on a SAPO-34 [9]. In eq. (5), activity, a, is defined as the ratio between reaction rate at t time and at zero time: a-
(ri)j (rio)j
for j= 2,....,7
(6)
where j refersto each one of the steps in the kinetic scheme of Table I. Eq. (6) corresponds to a non-selective deactivation kinetic model and, consequently, it considers the same deactivation rate for all the steps in the kinetic scheme, except for the dehydration of ethanol into ethene. For this step the deactivation is not significant (a=l) for any experimental conditions (which include experiments of up to 60 h under the conditions corresponding to the more severe deactivationfor the other steps in the kinetic scheme). k ~ is the apparent kineticconstant for deactivation,which is calculated as follows: !
kdi = kdi0 d (X w )
(7)
where 0d(Xw) is a function that quantifies the attenuation of deactivation due to water. Different mathematical expressions of one parameter, kw, have been tested, and the following has been adopted as the more suitable: 0 d(Xw) = exp(-kwXw)
(8)
The different expressions that have been tested for catalyst deactivation are summarized in Table 2. In these equations different lumps are considered as coke precursors. The lump of oxygenates (ethanol and diethyl ether) has not been considered as coke precursor, as its composition in the reaction medium is almost zero under experimental conditions. The formation of coke from paraffins has not been taken into account, given their low reactivity. Consequently, the possible coke precursors are ethene, the lump of olefins (propene and butenes) and the lump of gasoline. This hypothesis is supported by the oligomerization capacity of light olefins to give compounds that will remain trapped in the porous structure [ 17] and by the cyclization and condensation capacity of aromatics (whose proportion is high in the lump of gasoline) [ 18,19]. Model 1 in Table 2 considers that deactivation does not depend on the composition of the reaction medium (deactivation independent of the main reaction). In model 2, deactivation takes place in parallel with the main reaction and ethene (which is formed at the entrance of the reactor) is the coke precursor. In model 3 the lumps of products (olefins and gasoline) are considered as coke precursors (deactivation in series with the main reaction). Models 4 and 5 consider the three lumps (ethene, olefins and gasoline) as coke precursors (deactivation in series-parallel with the main reaction). In model 5, the same contribution to coke formation is considered for olefins and for gasoline.
459 Table 2 Possible kinetic equations for deactivation in the transformation of aqueous ethanol into hydrocarbons and error objective functions calculated for each one of them. Model
E.O.F. (eq. (14))
Kinetic equation for deactivation !
1
2.09.10 -3
da = _kdad dt
2
1.42.10 .3
3
4.33.10 -3
da =_kdEXEa d dt da , ) d-~ - doXo +kdGXG a d
4
1.23-10 -3
da = dt
5
1.25.10 3
ddta -- - (k'dEXE + k d' o G ( X o + XG) ) ad
i
, , ) X E + k d o X o + kdGX G a d
(9) (10) (11) (12) (13)
The selection of the more suitable deactivation kinetic model and the calculation of the kinetic parameters have been carried out by minimizing the following error objective function: n I nexp ~ (Xi,j - Xi(cal),j)2 (14) nlnexp where Xi,j are the experimental values of weight fractions based on the organic components, for component i in the experimental point j (under conditions in which irreversible deactivation by dealumination due to steam have been avoided) and Xi(calc),j are the values of composition calculated by solving the corresponding mass conservation equations for each lump or component of the kinetic scheme. The mass conservation equation for i lump in the reactor, assuming plug flow and expressing the concentration as mass fraction based on the organic product stream and the longitudinal position as dimensionless, ~, can be written as: E.O.F. = i = l j = l
c3Xi 0t
(l-e) e
- - = ~ p
RT m O uc3X i _ ri - PM FAo Z c3~
(15)
where e is the bed voidage; p is the apparent catalyst density; P is the partial pressure of the organic components; M is the average molecular weight of the organic components; FAo is the mass flowrate of oxygenates in the feed; u is the gas linear velocity; Z is the reactor length; ri is the reaction rate at t time, corresponding to the formation of the different lumps of the kinetic scheme: rA = - k l 0 ( X w ) X A
(16)
rE = kl0(Xw)X A - 0(Xw)((k 2 + k 3 + k5)X E - k7XG)a
(17)
rO - 0 ( X w ) ( k 2 X E + k7X G - k4Xo] a
(18)
460 rp = 0(X w)(k5X E + k6X G ) a
(19)
A set of (n- 1) equations such as eq. (15), corresponding to (n- 1) lumps of the kinetic scheme, must be solved, together with the deactivation kinetic equation. A program in FORTRAN has been developed for this calculation, which uses the LSODE subroutine from the DSSP library. The composition of the remaining lump is calculated by difference to unity. The correlation between the estimations of frequency factor and activation energy has been minimized by reparameterization [20]. Thus, the parameters to optimize are the kinetic constants at a reference temperature, 623 K, and the activation energies. The 90% confidence intervals of the estimated parameters have been obtained by non-linear regression using the Marquardt algorithm. The results of the objective function corresponding to each one of the proposed kinetic models are summarized in Table 2. As is observed, model 4 is the one with the best fit, although the difference with model 5 is very small. As this latter model is simpler, we adopted it for subsequent studies. The kinetic parameters corresponding to the kinetic model 5 (with 90% confidence intervals calculated by means of the Mardquardt algorithm for non-linear regression) are: !
kdE - 0.480 (+0.078) exp [- (5800(+800)) (1/T - 1/623)]
(20)
k'do G
(21)
0.175(_+0.077) exp [- (1000(+_2050)) (1/T
-
1/623)]
-
kdw - 0.481(+0.083) exp [- (3400(+1500))(1/T - 1/623)]
(22)
d= 1.78+0.08
(23)
The fitting of the kinetic model calculated to the experimental results is shown in Figures 1 and 2, where the evolution with time on stream of the mass fraction (by mass unit of organic components in reaction medium) of each lump of the kinetic scheme has been plotted. As is observed in Figure 1, deactivation attenuates as water content in the feed increases, at 350 ~ 1
1
Xi
W/FAo = 0.219 gcatalysth/gEtOH
Xi
Xwo = 0.045
0.8
9
99 ~
0.6 9
~eoe~ 9 ethene = olefins
9
Xwo = 3.0
0.6
, C5+ 9 paraffins
0.4
W/FAo = 0.709 gcatalysth/gEtOH
0.8
9 ethene 9 olefins
9 C5+
0.4
9 paraffins
4tOO"b . . . . . . . '~-____e_e.ti._~_.~._..#..,~.,.."
0.2 0 |
=
0
,
I
1
i
""-m-'t i
2
i
-----
0.2
m-urnr = ~ r - . r n i
3
i
i
4
time on stream, h
J
=-w tm m-mtm i
5
=
AAAAA JJ,rlBi-il
0
6
0
9
1 1 4 1 1 , n 1 - r i l ~ 1 . 1 1 I,
1
2
3
9 9
,U-i'l'-ll -Jl;lrm 11111-F nl-.-i
4
5
6
time on stream, h
Fig. 1. Fitting of calculated (lines) to experimental (points) evolution of mass fraction of each lump with time on stream, for 350 ~ and different water contents in the feed.
461
1
I
Xi 0.8
T= 350 ~
* ethene
W/FAo= 0.387 UcatalystnuUEtOH
9 olefins 9 C5+ 9 paraffins
Xi
0.6
I
* elhene
T= 400 ~
0.8
9 olefins
W/FAo = 0.387 gcatalysth/gEtOH
* C5+ 9 paraffins
0.6 000
0.4
9 ~176
0.4
i b l , I~
..... % ~ . t , .
0.2
0.2 n9 ill"rat i
0
,"
--" -w't
0
~,
.
""
9o
i
1
! n'm i
ii'-h-B-,i-" i
2
i
-i" i
3
-rI
-Ilrlnrmr i
4
time on stream, h
rlri-B i
lrt i
5
9
----n-..-._~.~,._t
.......
!1i
i
0 6
0
[
1
i
I
2
i
9I 3
i
9 "-~,-i-,--r I i I 4
5
i
[ II 6
t i m e on s t r e a m , h
Fig. 2. Fitting of calculated (lines) to experimental (points) evolution of mass fraction of each lump with time on stream, for different temperatures and 50 wt% of water in the feed. Figure 2 shows the increase in deactivation with temperature for a feed of ethanol-water with 50 wt% of water. By comparing eqs. (20) and (21) it is observed that at the reference temperature (350 ~ the deactivation constant corresponding to the evolution of ethene to coke, kdE, is three times higher than that corresponding to the evolution of olefins and gasoline to coke, kdo~. Consequently, ethene is the main coke precursor. The important effect of temperature in the kinetic modelling is noteworthy. Thus, although model 5 fits better in the whole range of temperatures studied, 350-450 ~ model 2 (parallel deactivation) is suitable at high temperatures and model 1 (independent deactivation) is acceptable at low temperatures. In order to understand the different effect of temperature in the evolution of coke from individual components in the reaction medium, cracking of components (oligomers) generated by condensation of the constituents of the lump of olefins and gasoline above 400 ~ must be taken into account. Consequently, at these temperatures, the effect of concentration of these coke precursors is less important, which justifies the validity of the parallel deactivation kinetic model under these conditions. Taking this result into account, an improvement in the deactivation kinetic model may be suggested by introducing a factor of deactivation attenuation due to this partial cracking of coke. Eq. (22) shows that the attenuating effect of water on deactivation increases with temperature, which may be explained by an increase in the ability for water to displace and desorb the coke precursors that are evolving on the active sites, or by the activation of coke precursor hydrocracking mechanisms as temperature is increased. 4. CONCLUSIONS It has been proven that the deactivation of the HZSM-5 zeolite used in the transformation of aqueous ethanol into hydrocarbons in the 350-450 ~ range is explained by a similar kinetic scheme to that already established for the transformation of aqueous methanol. The kinetic equation, eq. (13), is applicable to all the steps of the kinetic scheme except to ethanol dehydration, which is not affected by catalyst deactivation. Eq. (13) takes into account the effect of concentration of ethene and of the lump of olefins and gasoline in the reaction medium on the deactivation, although ethene is the main coke precursor on the basis of the values of the kinetic constants. The water present in the reaction medium plays an important
462 role in the attenuation of deposition and evolution of coke, which is considered in the kinetic model and quantified by means of a constant that increases with temperature. The kinetic model proposed, eq. (13), is suitable for the 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. ACKNOWLEDGEMENTS
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 (334-98). REFERENCES
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