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Kinetic modelling for selective deactivation in the skeletal isomerization of n-butenes
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Chemical Engineering Science, Vol. 52, No. 16, pp. 2829-2835, 1997 c!? 1997 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0009 2509/97 $17.00 + 0.00
Pergamon PII:
S0009-2509(97)00103-6
Kinetic modelling for selective deactivation in the skeletal isomerization of n-butenes (Received 28 N o v e m b e r 1996; accepted in revised form 3 M a r c h 1997)
1. INTRODUCTION Skeletal isomerization of n-butenes is an interesting process for increasing the production of isobutene, by means of the catalytic transformation of the n-butene stream produced in catalytic cracking units (FCC). This increase in the concentration of isobutene allows for exploiting online processes with the aim of obtaining methyl tert-butyl ether (MtBE), by the reaction of isobutene with methanol over an acidic catalyst (Harandi and Owen, 1989a, b). In the literature (Sun and Gastinger, 1987; Gajda, 1992; Cheng and Ponec, 1994; Xu et al., 1994; Simon et al., 1994; Gielgens et al., 1995; Guisnet et al., 1996; Asensi et al., 1996) different catalysts have been proposed to improve the low selectivity towards isobutene, which is due to thermodynamic restrictions and to the formation of byproducts (by oligomerization, preferably of isobutene). Nevertheless, the use of microporous catalysts, which seek shape selectivity, unfortunately increases the inconvenience of catalyst deactivation. This deactivation is due to active site blockage by coke, which is made up of a fraction of heavy reaction byproducts. The kinetic modelling of deactivation for this reaction has been studied in the literature in a simplified way (Smirnova et al.. 1981; Szabo et al., 1993), without taking into account the complex kinetic scheme of the reaction, Fig. 1 (Szabo et al., 1993). The knowledge of the kinetic model of deactivation in this and in other industrial processes, such as catalytic cracking and reforming, is rendered difficult by the complexity of the reaction network, which is made up of different single reactions that can be affected in a different way by catalyst deterioration. The cause of the selective deactivation lies simply in the different role that each active site plays in each single reaction. The problem of the kinetic modelling becomes complicated when the sites that take part in each single reaction are of different strength or efficiency (Butt and Petersen, 1988; Froment and Bischoff, 1990; Froment, 1991). In virtue of the results in the literature that relate coke deposition in acidic catalysts to the nature and strength of the sites, it is clearly proven that the strongly acidic sites will be affected by coke deposition to a greater degree than the moderately or weakly acidic sites (Blackmond et al., 1982; Bilbao et aL, 1985; Aguayo et al., 1987, Gayubo et al., 1993a; Aguayo et al., 1994). The consideration of selective deactivation in the kinetic model was originally introduced by Froment and Bischoff (1962), for parallel reaction systems. Later, it was taken into account for a triangular network of reactions by Rickert and Wei (1968), for catalytic cracking by Campbell and Wojciechowski (1969, 1970) and for naphtha reforming by Schipper et al. (1984). Corella and Asfia (1982), Corella et al. (1985, 1989), Corella and Men~ndez (1986) and Corella and Monzbn
(1988) have studied the kinetic model of selective deactivation by taking into account the surface heterogeneity of the acidic active sites, which are deactivated in a nonuniform way. Thus, for each singlej reaction, the reaction rate on sites of q strength is rj.q = (rj, q)oaj. ~
(1)
where a¢.q is the activity forj reaction, for the active sites of q strength. In this way, a deactivation equation for each group of q strength sites for each j reaction is established. This equation has the general expression: .t4j,aj ~
d~tq
",
g
~ q(Pi T ) a j
",
qdj..
(2)
Gayubo et al. (1994) analyzed the different simplifications used in the literature for solving eq. (2), in order to obtain the kinetic parameters for different reactions. In the present paper, the selective deactivation by coke deposition in the skeletal isomerization of n-butenes over a chlorinated alumina catalyst has been studied. The methodology used is based on that described in previous work (Gayubo et al., 1993b), where the selective deactivation of a silica-alumina catalyst used in the double bond isomerization of n-butenes was studied. For this reaction, the triangular kinetic scheme establishes the interconversion between 1-, cis- and trans-butene. 2. EXPERIMENTAL 2.1. C a t a l y s t The catalyst used, named AICI2, is a chlorinated alumina prepared by impregnation of a ),-alumina support (Rhone Poulenc SCS 250, with particle size in 0.15-0.5 mm, which was previously activated at 550°C for 4 h). The impregnation is carried out by immersion for 3 h in an ammonium chloride solution. The catalyst is dried at ll0°C for 16 h and calcined at 500°C for 1 h. The resulting chlorine content in the catalyst is 2 wt%. By comparing the kinetic behavior of this catalyst with that of other catalysts (silica-aluminas, ZSM5 zeolites, SAPO34, aluminas which have been fluorinated, bromated or chlorinated, with different halogen content), this catalyst has been proven (LLorens et al., 1996) to be highly active and selective towards skeletal isomerization. In Fig. 2 the acidic strength distribution for this catalyst is shown, which has been obtained by differential adsorption of NH3 at 250°C in a differential scanning calorimeter. The presence of certain strongly acidic sites (with adsorption heat higher than 150 kJ/mol) together with moderately or weakly acidic sites is observed. It has been proven (LLorens et al., 1996) that at
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Shorter Communications cis-butene
S
kc
kcs ks%
<
l-butene
~
ks1
ktl~k
kd-
isobutene
ksp
~-- byproducts
S
kct
trans-butene
Fig. 1. Kinetic scheme for n-butene isomerization. The formation of byproducts, p, from cis-butene, c, transbutene, t, and isobutene, s, such as is indicated in the scheme of Fig. 1, has been quantified by means of a second-order kinetics (Szabo et al., 1993):
200
AICI2 catalyst 180
(rv)o = (r,p)o + (rc~,)o+ (%)0 = 2kspP~(P~ + P,).
gz
m O
160
(4)
The restrictions imposed by the equilibria between butenes, which relate the direct and reverse kinetic constants, have been taken into account:
140
kj_2 = ( x Q
kji
= ,,,.
(5)
\ X; ]
120
100
' 0
'
'
'
'
0.03
'
'
'
0.06 rnmol
NH3
' 0.09
'
' 0.12
Taking into account eqs, (3)-(5), the rate of formation for each one of the species, at any time on stream, expressed in terms of molar fraction, will be given by the following expressions:
Ig
X,
Fig. 2. Acidic strength distribution for AIC12 catalyst. temperatures below 500°C the catalyst is stable, without CI loss and, consequently, without irreversible deactivation. 2.2. Reaction equipment and operating conditions The reaction equipment (LLorens et aL, 1996) consists of an isothermal fixed-bed reactor of 7 mm internal diameter and 8.8 cm length, and is provided with a coil to preheat the feed. The system has two feed lines: a line for reactant (1-butene) and another one for the inert gas (Helium) used for diluting the feed, The reactor is placed in an oven with electric resistances of 1250 W, and the reaction temperature is measured with thermocouples placed at the middle point of the bed axis and on the inside of the bed wall. The reaction products are analyzed by gas chromatography (Perkin Elmer 8700) with a Chrompack Plot column, 5 0 m × 0.32 mm, of silica covered with A1203/KCI. For obtaining the kinetics at zero time on stream the following operating conditions have been adopted: temperature, between 350 and 500°C; contact time, up to 1.0 (g of catalyst h (g of 1-butene)-1; partial pressure of 1-butene in the feed, between 0.15 and 1.0 atm. The kinetic experiments for the deactivation study have been carried out in the range between 400 and 475°C, since deactivation by coke deposition is hardly noticeable below 400°C. 3. KINETIC MODELLING 3.1. Initial kinetics The study of the initial kinetics has been carried out by applying the model proposed by Szabo et al. (1993), Fig. 1, which takes into account each one of the single reactions of interconversion between the different isomers, together with the formation of byproducts. The rate of formation of each j isomer from i isomer has been quantified by first-order kinetics (Szabo et al., 1993):
(rij)o = kiiPi.
(3)
K,c/
\
Kc,][ ar
-[k~Po(Xc-~---~[,)+kwp2x~X~]as
(7)
r, =~f =Ikl,Po(Xl-~It) +k"P°(X~--K'aa)]aT x,
', X.'~l
2
+k,sPo X, -_-v-Has -kspPoX,(Xc + X,)as (9) 1%/j re = dXp d---( = 2kspP2oXAXc + X,)as.
(10)
The kinetics accounting for deactivation (which is quantified by the decrease in as and ar activities) will be dealt with in the next section. For the initial kinetics, as = 1 and aT = 1. The equilibrium constants, K u, have been obtained from the thermodynamic data of Choudhary (1974). The kinetic constants have been obtained by minimizing the objective function: OF =
. . . . .Z~=~(I Xj,. - X ? . l ) : .=l nexpncomp
(I1)
where n represents each experimental point; j is the number of components (4 isomers + byproducts); X*. is the
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Shorter Communications calculated value of molar fraction of each component, which is obtained by numerical solution (by using a DGEAR subroutine of IMSL library) of the differential equation corresponding to plug flow with the rate equations, eqs (6)-(10); and Xj., is the experimental value of molar fraction obtained by extrapolating the conversion vs time on stream data at zero time on stream. The complex optimization algorithm (Box, 1965) has been used for a first estimation of the kinetic parameters, and then, the Marquardt algorithm (1963) for nonlinear regression has been used for a more accurate estimation of the parameters and for obtaining their confidence intervals, By fitting the experimental data of initial molar fraction to the error objective function, eq. (11), the following expressions for the kinetic constants are obtained (with a 95% confidence interval): kl, =(0.41 + 0.02) x 103exp[(-5200+_500)/RT]
(12)
klc=(O.16+_O.O1)×lO3exp[(-3300+_200)/RT]
(13)
k,=(O.17+_O.O1)xlO3exp[(-4300+_700)/RT]
(14)
kl~=(O.62+_O.O1)xlO2exp[(-5900+_200)/RT]
(15)
k , = (0.53 _+0.05) × 104 e x p [ ( - 12900 _+ IO00)/RT] (16) k,~ = (0.49 + 0.03) × 107 exp[( -19100 _+ 1200)/RT] (17) ksp = (0.43 _+ 0.01) × 107 exp[(-20500 +_ 2100)/RT]. (18) 3.2. Kinetic model accounting for selective deactivation In order to quantify the catalyst deactivation a selective deactivation kinetic model has been proposed and this takes into account the participation in the kinetic scheme of acidic sites of different strength and of different deactivation rate. As has been shown in previous work (LLorens et al., 1996), the steps of skeletal isomerization (1-iso, cis-iso and transiso) and of byproduct formation require the participation of stronger acidic sites than those needed for the steps of double bond migration (1-cis, 1-trans) and for cis-trans isomerization, which can take place over all acidic sites (Aguayo et al., 1990). Two different activities have been considered for the single reactions in the kinetic scheme of Fig. 1: an activity, as, for quantifying the effect of deactivation upon the single reactions of skeletal isomerization and formation of byproducts, which take place over strongly acidic sites; and another different activity, at, for the reactions of double bond migration and cis-trans isomerization, in which all the acidic sites participate. These activities are defined by the following equations: r~
as
r.
rts
(rl~)O (rc~)o=(r~)o rlc
glt
rp
(rp)o
(19)
rct
a~. = (rlc)-'-~= (rx,)o = (r,)o"
(20)
The terms of the numerator and denominator of each one of the fractions of eqs (19) and (20) must be calculated for the same values of composition and of temperature. This condition is needed for considering the past history of the catalyst and must be taken into account in the methodology for calculation of activity from the experimental results of concentration vs time on stream (Gayubo et al., 1993c, d, 1994). Taking into account eqs (19) and (20), the formation rate of each one of the reaction components, at any time on stream, will be given by eqs (6)-(10). For the deactivation functions (corresponding to each one of the activities, as and ar) of the deactivation kinetic equation, eq. (2), the following expression has been considered:
~ttj(Pi, T) = £ kaoPi i--I
(21)
8 ,~ v~ 6 4
O o ~ 2 o o 0
I
0
0.04
I
0.08
I
0.12
I
0.16
0.2
contact time (geataiysth/gl.bute.o) Fig. 3. Effect of contact time upon coke content. Reaction temperature, 450°C; time on stream, 30 h.
where Pi corresponds to each one of the possible coke precursors, and kd,jis the deactivation constant for the precursor i in the reaction j, which is activated by the acidic sites of strength q (S or T). Thus, in eq. (21) the contribution to deactivation of acidic sites of different strength (S or T ) by coke stemming from the degradation of different i components of the kinetic scheme is considered. For the identification of possible i coke precursors in the deactivation kinetic model, an experimental study of coke deposition along the catalytic bed has been carried out. Coke content deposited on the catalyst under different operating conditions has been determined from thermogravimetric curves (TG) obtained by coke combustion. In Fig. 3 the effect of contact time upon coke deposition is shown, at 450°C and for 30 h time on stream. It can be observed that the coke content increases almost linearly with contact time (or with longitudinal position in the reactor). This behavior corresponds to a deactivation mechanism in series with the main reaction, in which the reaction products are responsible for deactivation (Froment and Bischoff, 196 I; Bilbao et al., 1985). In fact, the increase in coke content takes place in parallel to the increase in the conversion to isobutene (and also to the formation of byproducts). Hence, isobutene can be taken as one of the precursors of deactivation by coke deposition. Although the concentration of byproducts also increases with contact time, this concentration is always very small within the operating conditions used for the kinetic study of deactivation, and so, the influence of byproducts on coke formation has been taken to be negligible compared with the contribution of isobutene, which is always present in higher amounts. On the other hand, in previous works (Gayubo et al., 1993b, c) in which the isomerization between linear butenes over a silica-alumina catalyst has been studied, it has been proven that cis-butene and trans-butene are coke precursors, but not 1-butene, whose influence upon catalyst deactivation is negligible. These results have been taken into account in this work. Cis-butene, trans-butene and isobutene have been considered as coke precursors. In virtue of the previous results, it is foreseeable that the effect of isobutene upon coke formation is more important than that of linear isomers, and that these have a similar effect. Consequently, the contribution of isobutene to deactivation has been quantified by two constants, ka.s and ka,~, each one corresponding to one of the activities defined (as and at), while the contribution of the linear isomers has also been quantified by another two constants, kd.~ and kn.7, which are equal for both linear isomers.
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By introducing these considerations in eq. (20, the possible deactivation equations for the two activities proposed are:
das d t = [kd~sP~+ ka.s(P~+ P,)]as
(22)
dar - [kd~P~ + kd,,(P~ + P,)]aT. dt
(23)
Consequently, the product of the kinetic constant (which represents the potential deactivating capacity) and the composition of the corresponding component will represent the real contribution to deactivation of each component under given operating conditions. 3.3. Methodology for analysis of kinetic data on deactivation The method for kinetic data analysis consists of fitting, by nonlinear regression, the experimental results of concentration of each reaction component to the mass conservation equation for each component, which is solved together with the proposed deactivation equation (Gayubo et al., 1993c, d, 1994). The mass conservation equation for each i component in an isothermal plug flow reactor without radial gradients, and without variation in mol number (which is the case for isomerization, where the small variation in tool number due to the formation of byproducts is considered negligible), has the following expression:
OX, (u~OXi + ( 1 - e ) RT O'--t=-\ZJ ~ e p ~ o [roi(Xi, T)]a
(24)
where [ro/(X/, T )] is the rate of formation of i component for zero time on stream. The boundary conditions at the reactor inlet are:
Xi(¢ =0, t) = Xio
(25)
as(~ =0, t) = as(Xio, t)
(26)
aT(~ =0, t) = aT(Xio, t)
(27)
and the initial conditions (initial profile of molar fractions along the reactor and initial activities):
x,(~, t=0)=x,0(¢)
(28)
as(¢,t=O)=l
(29)
ar(¢, t = 0 ) =1.
(30)
The initial profile for molar fractions along the bed is calculated by solving the mass conservation equation, eq. (24), when the first term is zero and the activity is equal to unity:
-~- =
--P-~0
[~°(x'' r)]"
(31)
In order to solve the set of partial differential equations made up of eq. (24) for each component together with the deactivation equations, eqs (22) and (23), each partial differential equation has been transformed into a set of ordinary differential equations by means of orthogonal collocation (Villadsen and Michelsen, 1978), using Lagrange polynomials as test functions, as has been detailed in previous papers (Gayubo et al., 1993c, d, 1994). The optimization algorithm is the same as that used for the study of initial kinetics. The molar fraction profile has been obtained by interpolating as has been previously described. The objective function to be minimized is given by eq. (11), where the values of composition correspond to different times on stream. 3.4. Kinetic parameters for the deactivation By following the previously described methodology, the parameters of the proposed kinetic model, with a 95%
confidence interval, are given by the following equations:
kd,~ = (0.32 + 0.03) 105 exp[(--14950 + 1200)/RT]
(32)
kd,s = (0.17 ___0.01) 104 exp[(--22200 ___1800)/RT]
(33)
kd,T = (0.44 + 0.05) 103 exp [( -- 12200 + 1100)/R T ]
(34)
kd,~ = (0.92 __+0.01) 102 exp[(--12500 + 1200)/RT]. (35) The goodness of fit is shown in the plots of Fig. 4, in which the experimental results (points) and calculated values (curves) of molar fraction evolution with time on stream corresponding to each isomer are compared at 450°C for two values of contact time. From the eqs (32)-(35), the following conclusions can be drawn: The contribution to deactivation of isomers cis- and trans-butene (rate constants kd,s and kd.T) is much lower than that of isobutene (rate constants kd~ and kd~T).This difference is more noticeable for as activity (kd,s >>kd,s) in the range of temperature studied. Taking into account that kd,s values are negligible compared with those of the kd,s, deactivation equation for as, eq. (22) (which corresponds to strongly acidic sites over which skeletal isomerization and byproduct forming reactions take place) can be simplified to the following expression:
dos
----=kd,sPsas. dt
(36)
- The values of the kinetic constant for deactivation for the skeletal isomerization and the formation of byproducts from the degradation of isobutene are much higher than the values of the deactivation constant corresponding to the double bond isomerization reactions, (kd,s >> kd~). Consequently, the decrease in as activity has to be much larger than that in aT. These conclusions concerning the kinetic model, together with the values of concentration of the reaction components, explain the experimental results of deactivation shown in Fig. 4. The concentration of isobutene always decreases with time on stream, while the concentration of the linear isomers increases slightly or is maintained almost constant. Only for high values of time on stream, for which the double bond isomerization rate will be noticeably diminished, a decrease in the conversion to cis-butene and to trans-butene would be observed. As a consequence of the kinetic model for deactivation, the evolution of the composition with time on stream is highly dependent on operating conditions: Comparing plots a and b in Fig. 4 it is observed that the decrease in the concentration of isobutene and the increase in the concentration of linear isomers is more noticeable as contact time increases. Thus, as an example, for time on stream 7 h, the values of activity are: as =0.22 and aT =0.68 for contact time 0.053 (g of catalyst) h (g of 1-butene) - l [Fig. 4(a)], and as =0.02 and aT =0.38 for 0.148 (g of catalyst) h (g of 1-butene)- 1 [Fig. 4(b)]. - The effect of temperature is similar, although it is less pronounced than that of contact time. As an example, for contact time 0.148 (g of catalyst) h (g of 1-butene)- 1, partial pressure of 1-butene in the feed 1.0 atm and time on stream 9 h, the activities are: as =0.08 and a r =0.66 at 450°C, and as =0.34 and aT =0.81 at 400°C. - The increase in partial pressure of 1-butene in the feed results in an increase in deactivation. As an example, for contact time 0.148 (g of catalyst) h (g of 1-butene) - l ,
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methodological aspects presented here may be useful for those reactions.
0.5
A. G. G A Y U B O *
0.4
0.3
X
AA==
•
•
•
Departamento de Ingenieria Quimica, Universidad del Pais Vasco. Facultad de Ciencia~ Apartado, 644. 48080 Bilbao, Spain
A•••
• A==e--e-=ee
l!..-'-~
i
6
i
T= 450 °C Plo = 1.0 atm
0.2
Xt • Xl• Xs* XC •
W/Flo= 0.053 gcatalysth/glo
0.1
6
12
18
24
F. J. L L O R E N S E. A. C E P E D A
Facultad de Farmacia. Apartado 450, 01006 Vitoria, Spain M. O L A Z A R J. BILBAO
30
time on stream (h) Departamento de lngenierla Quimica, Universidad del Pais Vasco, Facultad de Ciencias, Apartado, 644 48080 Bilbao, Spain
0.6 T= 450 °C Po = 1.0 atm
0.5
Xt • Xl • Xs,~ Xc A
W/Flo = 0.148 gcatalysth/gl_butene 0.4 o•
Xl
oo
a
0.3
a j, q
0.2
as, aT
0.1
Ci dj.q Flo
0 0
6
12
18
24
30
time on stream (h) Fig. 4. Comparison of the experimental results of molar fraction vs time on stream with the calculated ones, for different values of contact time.
ka,s, ka, r
kd.s, ka.r
kij
temperature 450°C and for time on stream 27 h, the activities are: as =0.35 and ar =0.80 for partial pressure of 1-butene in the feed 0.3 atm, and as =0.02 and aT =0.38 for 1.0 atm.
Kij
OF ~,~,~
4. CONCLUSIONS The kinetic model for the selective deactivation by coke deposition proposed in this paper is based upon the kinetic scheme proposed by Szabo et al. (1993) for skeletal isomerization of n-butenes. The combination of both models can be used in reactor simulation down to very low catalyst activity levels. The model proposed takes into account two experimentally proven circumstances: the participation of active sites with different acidic strength in different steps of the kinetic scheme; and the different deactivation rate of these sites of different acidic strength. The identification of the deactivation of sites of only two acidic strength levels is a simplification that can be adopted in the reaction studied here, but in other reactions such as reforming or catalytic cracking, in which single reactions of very different nature take place, the characterization of a higher number of acidity levels m a y be necessary. Nevertheless,
Plo ri
ri~, (rij)o rj. a
R T u
W Xi
xl
NOTATION activity of the catalyst at t time on stream, defined as ratio of reaction rates activity for j reaction, for active sites of q strength catalyst activity attributable to strong acidic sites and to the total number of acidic sites concentration of each i component, tool cm 3 deactivation order for active sites of q strength in j reaction mass flow of l-butene in the feed, (g of lbutene) h deactivation kinetic constants corresponding to the degradation of isobutene over the strongly acidic sites and over all the acidic sites, h - 1 a t m - 1 deactivation kinetic constants corresponding to the degradation of cis-butene and transbutene over the strongly acidic sites and over all the acidic sites, h - 1 a t m kinetic constant for the single reaction of formation o f j component from i component, tool (g of catalyst)- 1 h - 1 a t m equilibrium constant for the single reaction of formation o f j component from i component objective function partial pressure of cis-butene, trans-butene and isobutene, respectively, atm partial pressure of each i component, atm partial pressure of 1-butene in the feed, atm global formation rate of component i, at time t, mol (g of catalyst)- ~ h - t rate of the single reaction of formation of j component from i component, at time t and at zero time, mol (g of catalyst) ~ h reaction rate for j reaction, for active sites of q strength constant of the gases, J m o l - t K reaction temperature, K superficial gas velocity, m h mass of catalyst, g molar fraction of i component molar fraction of i component at the equilibrium
*Corresponding author
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2
Z
Shorter Communications molar fraction o f j component at the experimental point n, calculated by numerical solution of the plug flow equation longitudinal coordinate of the reactor, m total length of the reactor, m
Greek letters voidage of the catalyst bed dimensionless longitudinal coordinate in the reactor particle density of the catalyst, kg m 3 contact time (W/Flo), (g of catalyst) h (g of 1-butene)- 1 Subscripts c, t, l, s, p 0 i0
cis-butene, trans-butene, 1-butene, isobutene and byproducts, respectively zero time conditions reactor inlet conditions
REFERENCES Aguayo, A. T., Arandes, J. M., Romero, A. and Bilbao, J. (1987) Optimization of the preparation of a catalyst under deactivation. 1. Control of its kinetic behavior by electing the preparation conditions. Ind. Engng Chem. Res. 26, 2403-2408. Aguayo, A. T., Arandes, J. M., Olazar, M. and Bilbao, J. (1990) Isomerization of butenes as test reaction for measurement of solid catalysts acidity. Ind. Engng Chem. Res. 29, 1172-1178. Aguayo, A. T., Benito, P. L., Gayubo, A. G., Olazar, M. and Bilbao, J. (1994) Acidity deterioration and coke deposition in a H-ZSM-5 zeolite in the MTG process. Stud. Surf Sci. Catal. 88, 567-572. Asensi, M. A., Corma, A. and Martinez, A. (1996) Skeletal isomerization of 1-butene on MCM-22 zeolite catalyst. J. Catal. 158, 561-569. Bilbao, J., Aguayo, A. T. and Arandes, J. M. (1985) Coke deposition on silica-alumina catalysts in dehydration reactions. Ind. Engng Chem. Prod. Res. Dev. 24, 531-539. Blackmond, D. G., Goodwin, J. G. and Lester, J. E. (1982) In situ fourier transform infrared spectroscopy study of HY cracking catalysts: coke formation and the nature of the active sites. J. Catal. 78, 34-43. Box, M. J., (1965) A new method of constrained optimization and a comparison with other methods. Comput. J. 1, 42-52. Butt, J. B. and Petersen, E. E. (1988) Activation, Deactivation and Poisoning of Catalysts, Academic Press, San Diego. Campbell, D. R. and Wojciechowski, B. W. (1969) Theoretical patterns of selectivity in aging catalysts with special reference to the catalytic cracking of petroleum. Can. J. Chem. Engn 9 47, 413-417. Campbell, D. R. and Wojciechowski, B. W. (1970) Selectivity of aging catalysts in static, moving and fluidized bed reactors. Can. J. Chem. Engng 48, 224 228. Corella, J. and AsOa, J. M. (1982) Kinetic of deactivation of a solid catalyst in a nonsimple reaction Isobutene oxidation in gaseous phases by a parallel reaction network. Ind. Engng Chem. Process Des. Dev. 21, 551-558. Corella, J., Bilbao, R., Molina, J. A. and Artigas, A. (1985) Variation with time of the mechanism, observable order, and activation energy of the catalyst deactivation by coke in the FCC process. Ind. Engn O Chem. Process Des. Dev. 24, 625-636. Corella, J. and Men6ndez, M. (1986) The modelling of the kinetics of deactivation of nonfunctional catalysts with an acid strength distribution in their non-homogeneous surface. Application to the deactivation of commercial catalysts in the FCC process. Chem. Enon 9 Sci. 41, 1817-1826. Corella, J. and Monz6n, A. (1988) Modeling of the deactivation kinetics of solid deactivation by two or more simulataneous and different causes. Ind. Engng Chem. Res. 27, 367-374.
Corella, J., Morales, F. G., Provost, M., Espinosa, A. and Serrano, J. (1989) The selective deactivation kinetic model applied to the kinetics of the catalytic cracking (FCC process). In Recent Advances in Chemical Engineering, ed. D. N. Saraf and D. Kunzru, pp. 192-210. McGraw Hill, New Delhi. Cheng, Z. X. and Ponec, V. (1994) Fluorinated alumina as a catalyst for skeletal isomerization of n-butene. Appl. Catal. 118, 127-138. Choudhary, V. R. (1974) Catalytic isomerization of n-butene to isobutene. Chem. Ind. Dev. 8, 32-41. Froment, G. F. (1991) The modeling of catalyst deactivation by coke formation. In Catalyst deactivation, ed. C. H. Bartholomew and J. B. Butt, pp. 53-85. Elsevier, Amsterdam. Froment, G. F. and Bischoff, K. B. (1961) Non-steady state behaviour of fixed bed catalytic reactors due to catalyst fouling. Chem. Engn 9 Sci. 16, 189-201. Froment, G. F. and Bischoff, K. B. (1962) Kinetic data and product distributions from fixed bed catalytic reactors subject to catalyst fouling. Chem. Engng Sci. 17, 105 114. Froment, G. F. and Bischoff, K. B. (1990) Chemical Reactor Analysis and Design, 2nd Edn, Chap. 5. Wiley, New York. Gadja, G. J. (1992) Butene isomerization process. U.S. Patent, 5,132,484, 21 July 1992. Gayubo, A. G., Arandes, J. M., Aguayo, A. T., Olazar, M. and Bilbao, J. (1993a). Deactivation and acidity deterioration of a SIO2/A1203 catalyst in the isomerization of cis-butene. Ind. Engng Chem. Res. 32, 588 593. Gayubo, A. G., Arandes, J. M., Aguayo, A. T., Olazar, M. and Bilbao, J. (1993b) Selective kinetic deactivation model for a triangular reaction scheme. Chem. Engn9 Sci. 48, 2273 2282. Gayubo, A. G., Arandes, J. M., Aguayo, A. T., Olazar, M. and Bilbao, J. (1993c) Calculation of the kinetics of deactivation by coke in integral reactor for a triangular scheme reaction. Chem. Engn9 Sci. 48, 1077-1087. Gayubo, A. G , Arandes, J. M., Olazar, M., Aguayo, A. T. and Bilbao, J. (1993d) Calculation of the kinetics of deactivation by coke for a silica-alumina catalyst in the dehydration of 2-ethylhexanol. Ind. Engn O Chem. Res. 32, 458-465. Gayubo, A. G., Arandes, J. M., Aguayo, A. T., Olazar, M. and Bilbao, J. (1994) Contributions to the calculation of coke deactivation kinetics. Comparison of methods. Chem. Engng. J. 55, 125-134. Gielgens, L. H., van Kampen, M. G. H., Brock, M. M., van Hardeveld, R. and Ponec, V. (1995) Skeletal isomerization of 1-butene on tungstene oxide catalysts. J. Catal. 154, 201-207. Guisnet, M., Andy, P., Gnep, N. S., Benazzi, E. and Travers, C. (1996) Skeletal isomerization of n-butenes. I. Mechanism of n-butene transformation on a nondeactivated Hferrierite catalyst. J. Catal. 158, 551-560. Harandi, M. N. and Owen, H. (1989a) Integrated etherification and oxygenates to gasoline process. U.S. Patent No, 4,826,507, 2 May 1989. Harandi, M. N. and Owen, H. (1989b) Production of ethers from methanol. U.S. Patent No. 4,831,195, 16 May 1989. LLorens, F. J., Gayubo, A. G., Cepeda, E. A., Aguayo, A. T. and Bilbao, J. (1996) The role of the shape selectivity and intrinsic selectivity of acidic sites of the catalyst in the skeletal isomerization of n-butenes. Ind. Engng Chem. Res. submitted. Marquardt, F. W. (1963) An algorithm for least-squares estimation of nonlinear parameters. J. Soc. Ind. Appl. Math. 11, 431-441. Rickert, L. and Wei, J. (1968) Kinetics of coupled first-order reactions with time-dependent rate coefficients in ternary systems. Ind. Engng Chem. Fundam. 7, 125-131. Schipper, P. H., Graziani, K. R., Choi, B. C. and Ramage, M. P. (1984) The extension of Mobil's kinetic reforming model to include catalyst deactivation. I. Chem. E. Symp. Series (ISCRE 8) 87, 33-44.
Shorter Communications Simon, M. W., Suib, S. L. and O'Young, C.-L. (1994) Synthesis and characterization of ZSM-22 zeolites and their catalytic behavior in 1-butene isomeration reactions. J. Catal. 147, 484-493. Smirnova, I. M., Fel'dblyum, V. S., Tsailingol'd, T. A. (1981) Reaction kinetics of isomerization of n-butenes to isobutylene. Neftekhimiya 21, 46-51. Sun, H. N. and Gastinger, R. G. (1987) Skeletal Isomerization of olefins over bromided aluminas. US Patent No 6,654,463, 31 March 1987.
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Szabo, J., Szabo, G., van Gestel, J., Cornet, D. (1993) Isomerization of n-butenes to isobutene catalyzed by fluorinated alumina. Reaction kinetics. Appl. Catal. 96, 319-330. Villadsen, J. and Michelsen, M. L. (1978) Solution of Differen-
tial Equation Models of Polynomial Approximation: Prentice-Hall, New Jersey. Xu, W.-Q., Yin, Y.-G., Suib, S. L. and O'Young, C-L. (1994) Selective conversion of n-butene to isobutylene at extremely high space velocities on ZSM-23 zeolites. J. Catal. 150, 34-45.
~
~
~
~
~
~
~
~
~
~
Chemical Engineering Science, Vol. 52, No. 16, pp. 2829-2835, 1997 c!? 1997 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0009 2509/97 $17.00 + 0.00
Pergamon PII:
S0009-2509(97)00103-6
Kinetic modelling for selective deactivation in the skeletal isomerization of n-butenes (Received 28 N o v e m b e r 1996; accepted in revised form 3 M a r c h 1997)
1. INTRODUCTION Skeletal isomerization of n-butenes is an interesting process for increasing the production of isobutene, by means of the catalytic transformation of the n-butene stream produced in catalytic cracking units (FCC). This increase in the concentration of isobutene allows for exploiting online processes with the aim of obtaining methyl tert-butyl ether (MtBE), by the reaction of isobutene with methanol over an acidic catalyst (Harandi and Owen, 1989a, b). In the literature (Sun and Gastinger, 1987; Gajda, 1992; Cheng and Ponec, 1994; Xu et al., 1994; Simon et al., 1994; Gielgens et al., 1995; Guisnet et al., 1996; Asensi et al., 1996) different catalysts have been proposed to improve the low selectivity towards isobutene, which is due to thermodynamic restrictions and to the formation of byproducts (by oligomerization, preferably of isobutene). Nevertheless, the use of microporous catalysts, which seek shape selectivity, unfortunately increases the inconvenience of catalyst deactivation. This deactivation is due to active site blockage by coke, which is made up of a fraction of heavy reaction byproducts. The kinetic modelling of deactivation for this reaction has been studied in the literature in a simplified way (Smirnova et al.. 1981; Szabo et al., 1993), without taking into account the complex kinetic scheme of the reaction, Fig. 1 (Szabo et al., 1993). The knowledge of the kinetic model of deactivation in this and in other industrial processes, such as catalytic cracking and reforming, is rendered difficult by the complexity of the reaction network, which is made up of different single reactions that can be affected in a different way by catalyst deterioration. The cause of the selective deactivation lies simply in the different role that each active site plays in each single reaction. The problem of the kinetic modelling becomes complicated when the sites that take part in each single reaction are of different strength or efficiency (Butt and Petersen, 1988; Froment and Bischoff, 1990; Froment, 1991). In virtue of the results in the literature that relate coke deposition in acidic catalysts to the nature and strength of the sites, it is clearly proven that the strongly acidic sites will be affected by coke deposition to a greater degree than the moderately or weakly acidic sites (Blackmond et al., 1982; Bilbao et aL, 1985; Aguayo et al., 1987, Gayubo et al., 1993a; Aguayo et al., 1994). The consideration of selective deactivation in the kinetic model was originally introduced by Froment and Bischoff (1962), for parallel reaction systems. Later, it was taken into account for a triangular network of reactions by Rickert and Wei (1968), for catalytic cracking by Campbell and Wojciechowski (1969, 1970) and for naphtha reforming by Schipper et al. (1984). Corella and Asfia (1982), Corella et al. (1985, 1989), Corella and Men~ndez (1986) and Corella and Monzbn
(1988) have studied the kinetic model of selective deactivation by taking into account the surface heterogeneity of the acidic active sites, which are deactivated in a nonuniform way. Thus, for each singlej reaction, the reaction rate on sites of q strength is rj.q = (rj, q)oaj. ~
(1)
where a¢.q is the activity forj reaction, for the active sites of q strength. In this way, a deactivation equation for each group of q strength sites for each j reaction is established. This equation has the general expression: .t4j,aj ~
d~tq
",
g
~ q(Pi T ) a j
",
qdj..
(2)
Gayubo et al. (1994) analyzed the different simplifications used in the literature for solving eq. (2), in order to obtain the kinetic parameters for different reactions. In the present paper, the selective deactivation by coke deposition in the skeletal isomerization of n-butenes over a chlorinated alumina catalyst has been studied. The methodology used is based on that described in previous work (Gayubo et al., 1993b), where the selective deactivation of a silica-alumina catalyst used in the double bond isomerization of n-butenes was studied. For this reaction, the triangular kinetic scheme establishes the interconversion between 1-, cis- and trans-butene. 2. EXPERIMENTAL 2.1. C a t a l y s t The catalyst used, named AICI2, is a chlorinated alumina prepared by impregnation of a ),-alumina support (Rhone Poulenc SCS 250, with particle size in 0.15-0.5 mm, which was previously activated at 550°C for 4 h). The impregnation is carried out by immersion for 3 h in an ammonium chloride solution. The catalyst is dried at ll0°C for 16 h and calcined at 500°C for 1 h. The resulting chlorine content in the catalyst is 2 wt%. By comparing the kinetic behavior of this catalyst with that of other catalysts (silica-aluminas, ZSM5 zeolites, SAPO34, aluminas which have been fluorinated, bromated or chlorinated, with different halogen content), this catalyst has been proven (LLorens et al., 1996) to be highly active and selective towards skeletal isomerization. In Fig. 2 the acidic strength distribution for this catalyst is shown, which has been obtained by differential adsorption of NH3 at 250°C in a differential scanning calorimeter. The presence of certain strongly acidic sites (with adsorption heat higher than 150 kJ/mol) together with moderately or weakly acidic sites is observed. It has been proven (LLorens et al., 1996) that at
2829
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Shorter Communications cis-butene
S
kc
kcs ks%
<
l-butene
~
ks1
ktl~k
kd-
isobutene
ksp
~-- byproducts
S
kct
trans-butene
Fig. 1. Kinetic scheme for n-butene isomerization. The formation of byproducts, p, from cis-butene, c, transbutene, t, and isobutene, s, such as is indicated in the scheme of Fig. 1, has been quantified by means of a second-order kinetics (Szabo et al., 1993):
200
AICI2 catalyst 180
(rv)o = (r,p)o + (rc~,)o+ (%)0 = 2kspP~(P~ + P,).
gz
m O
160
(4)
The restrictions imposed by the equilibria between butenes, which relate the direct and reverse kinetic constants, have been taken into account:
140
kj_2 = ( x Q
kji
= ,,,.
(5)
\ X; ]
120
100
' 0
'
'
'
'
0.03
'
'
'
0.06 rnmol
NH3
' 0.09
'
' 0.12
Taking into account eqs, (3)-(5), the rate of formation for each one of the species, at any time on stream, expressed in terms of molar fraction, will be given by the following expressions:
Ig
X,
Fig. 2. Acidic strength distribution for AIC12 catalyst. temperatures below 500°C the catalyst is stable, without CI loss and, consequently, without irreversible deactivation. 2.2. Reaction equipment and operating conditions The reaction equipment (LLorens et aL, 1996) consists of an isothermal fixed-bed reactor of 7 mm internal diameter and 8.8 cm length, and is provided with a coil to preheat the feed. The system has two feed lines: a line for reactant (1-butene) and another one for the inert gas (Helium) used for diluting the feed, The reactor is placed in an oven with electric resistances of 1250 W, and the reaction temperature is measured with thermocouples placed at the middle point of the bed axis and on the inside of the bed wall. The reaction products are analyzed by gas chromatography (Perkin Elmer 8700) with a Chrompack Plot column, 5 0 m × 0.32 mm, of silica covered with A1203/KCI. For obtaining the kinetics at zero time on stream the following operating conditions have been adopted: temperature, between 350 and 500°C; contact time, up to 1.0 (g of catalyst h (g of 1-butene)-1; partial pressure of 1-butene in the feed, between 0.15 and 1.0 atm. The kinetic experiments for the deactivation study have been carried out in the range between 400 and 475°C, since deactivation by coke deposition is hardly noticeable below 400°C. 3. KINETIC MODELLING 3.1. Initial kinetics The study of the initial kinetics has been carried out by applying the model proposed by Szabo et al. (1993), Fig. 1, which takes into account each one of the single reactions of interconversion between the different isomers, together with the formation of byproducts. The rate of formation of each j isomer from i isomer has been quantified by first-order kinetics (Szabo et al., 1993):
(rij)o = kiiPi.
(3)
K,c/
\
Kc,][ ar
-[k~Po(Xc-~---~[,)+kwp2x~X~]as
(7)
r, =~f =Ikl,Po(Xl-~It) +k"P°(X~--K'aa)]aT x,
', X.'~l
2
+k,sPo X, -_-v-Has -kspPoX,(Xc + X,)as (9) 1%/j re = dXp d---( = 2kspP2oXAXc + X,)as.
(10)
The kinetics accounting for deactivation (which is quantified by the decrease in as and ar activities) will be dealt with in the next section. For the initial kinetics, as = 1 and aT = 1. The equilibrium constants, K u, have been obtained from the thermodynamic data of Choudhary (1974). The kinetic constants have been obtained by minimizing the objective function: OF =
. . . . .Z~=~(I Xj,. - X ? . l ) : .=l nexpncomp
(I1)
where n represents each experimental point; j is the number of components (4 isomers + byproducts); X*. is the
2831
Shorter Communications calculated value of molar fraction of each component, which is obtained by numerical solution (by using a DGEAR subroutine of IMSL library) of the differential equation corresponding to plug flow with the rate equations, eqs (6)-(10); and Xj., is the experimental value of molar fraction obtained by extrapolating the conversion vs time on stream data at zero time on stream. The complex optimization algorithm (Box, 1965) has been used for a first estimation of the kinetic parameters, and then, the Marquardt algorithm (1963) for nonlinear regression has been used for a more accurate estimation of the parameters and for obtaining their confidence intervals, By fitting the experimental data of initial molar fraction to the error objective function, eq. (11), the following expressions for the kinetic constants are obtained (with a 95% confidence interval): kl, =(0.41 + 0.02) x 103exp[(-5200+_500)/RT]
(12)
klc=(O.16+_O.O1)×lO3exp[(-3300+_200)/RT]
(13)
k,=(O.17+_O.O1)xlO3exp[(-4300+_700)/RT]
(14)
kl~=(O.62+_O.O1)xlO2exp[(-5900+_200)/RT]
(15)
k , = (0.53 _+0.05) × 104 e x p [ ( - 12900 _+ IO00)/RT] (16) k,~ = (0.49 + 0.03) × 107 exp[( -19100 _+ 1200)/RT] (17) ksp = (0.43 _+ 0.01) × 107 exp[(-20500 +_ 2100)/RT]. (18) 3.2. Kinetic model accounting for selective deactivation In order to quantify the catalyst deactivation a selective deactivation kinetic model has been proposed and this takes into account the participation in the kinetic scheme of acidic sites of different strength and of different deactivation rate. As has been shown in previous work (LLorens et al., 1996), the steps of skeletal isomerization (1-iso, cis-iso and transiso) and of byproduct formation require the participation of stronger acidic sites than those needed for the steps of double bond migration (1-cis, 1-trans) and for cis-trans isomerization, which can take place over all acidic sites (Aguayo et al., 1990). Two different activities have been considered for the single reactions in the kinetic scheme of Fig. 1: an activity, as, for quantifying the effect of deactivation upon the single reactions of skeletal isomerization and formation of byproducts, which take place over strongly acidic sites; and another different activity, at, for the reactions of double bond migration and cis-trans isomerization, in which all the acidic sites participate. These activities are defined by the following equations: r~
as
r.
rts
(rl~)O (rc~)o=(r~)o rlc
glt
rp
(rp)o
(19)
rct
a~. = (rlc)-'-~= (rx,)o = (r,)o"
(20)
The terms of the numerator and denominator of each one of the fractions of eqs (19) and (20) must be calculated for the same values of composition and of temperature. This condition is needed for considering the past history of the catalyst and must be taken into account in the methodology for calculation of activity from the experimental results of concentration vs time on stream (Gayubo et al., 1993c, d, 1994). Taking into account eqs (19) and (20), the formation rate of each one of the reaction components, at any time on stream, will be given by eqs (6)-(10). For the deactivation functions (corresponding to each one of the activities, as and ar) of the deactivation kinetic equation, eq. (2), the following expression has been considered:
~ttj(Pi, T) = £ kaoPi i--I
(21)
8 ,~ v~ 6 4
O o ~ 2 o o 0
I
0
0.04
I
0.08
I
0.12
I
0.16
0.2
contact time (geataiysth/gl.bute.o) Fig. 3. Effect of contact time upon coke content. Reaction temperature, 450°C; time on stream, 30 h.
where Pi corresponds to each one of the possible coke precursors, and kd,jis the deactivation constant for the precursor i in the reaction j, which is activated by the acidic sites of strength q (S or T). Thus, in eq. (21) the contribution to deactivation of acidic sites of different strength (S or T ) by coke stemming from the degradation of different i components of the kinetic scheme is considered. For the identification of possible i coke precursors in the deactivation kinetic model, an experimental study of coke deposition along the catalytic bed has been carried out. Coke content deposited on the catalyst under different operating conditions has been determined from thermogravimetric curves (TG) obtained by coke combustion. In Fig. 3 the effect of contact time upon coke deposition is shown, at 450°C and for 30 h time on stream. It can be observed that the coke content increases almost linearly with contact time (or with longitudinal position in the reactor). This behavior corresponds to a deactivation mechanism in series with the main reaction, in which the reaction products are responsible for deactivation (Froment and Bischoff, 196 I; Bilbao et al., 1985). In fact, the increase in coke content takes place in parallel to the increase in the conversion to isobutene (and also to the formation of byproducts). Hence, isobutene can be taken as one of the precursors of deactivation by coke deposition. Although the concentration of byproducts also increases with contact time, this concentration is always very small within the operating conditions used for the kinetic study of deactivation, and so, the influence of byproducts on coke formation has been taken to be negligible compared with the contribution of isobutene, which is always present in higher amounts. On the other hand, in previous works (Gayubo et al., 1993b, c) in which the isomerization between linear butenes over a silica-alumina catalyst has been studied, it has been proven that cis-butene and trans-butene are coke precursors, but not 1-butene, whose influence upon catalyst deactivation is negligible. These results have been taken into account in this work. Cis-butene, trans-butene and isobutene have been considered as coke precursors. In virtue of the previous results, it is foreseeable that the effect of isobutene upon coke formation is more important than that of linear isomers, and that these have a similar effect. Consequently, the contribution of isobutene to deactivation has been quantified by two constants, ka.s and ka,~, each one corresponding to one of the activities defined (as and at), while the contribution of the linear isomers has also been quantified by another two constants, kd.~ and kn.7, which are equal for both linear isomers.
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Shorter Communications
By introducing these considerations in eq. (20, the possible deactivation equations for the two activities proposed are:
das d t = [kd~sP~+ ka.s(P~+ P,)]as
(22)
dar - [kd~P~ + kd,,(P~ + P,)]aT. dt
(23)
Consequently, the product of the kinetic constant (which represents the potential deactivating capacity) and the composition of the corresponding component will represent the real contribution to deactivation of each component under given operating conditions. 3.3. Methodology for analysis of kinetic data on deactivation The method for kinetic data analysis consists of fitting, by nonlinear regression, the experimental results of concentration of each reaction component to the mass conservation equation for each component, which is solved together with the proposed deactivation equation (Gayubo et al., 1993c, d, 1994). The mass conservation equation for each i component in an isothermal plug flow reactor without radial gradients, and without variation in mol number (which is the case for isomerization, where the small variation in tool number due to the formation of byproducts is considered negligible), has the following expression:
OX, (u~OXi + ( 1 - e ) RT O'--t=-\ZJ ~ e p ~ o [roi(Xi, T)]a
(24)
where [ro/(X/, T )] is the rate of formation of i component for zero time on stream. The boundary conditions at the reactor inlet are:
Xi(¢ =0, t) = Xio
(25)
as(~ =0, t) = as(Xio, t)
(26)
aT(~ =0, t) = aT(Xio, t)
(27)
and the initial conditions (initial profile of molar fractions along the reactor and initial activities):
x,(~, t=0)=x,0(¢)
(28)
as(¢,t=O)=l
(29)
ar(¢, t = 0 ) =1.
(30)
The initial profile for molar fractions along the bed is calculated by solving the mass conservation equation, eq. (24), when the first term is zero and the activity is equal to unity:
-~- =
--P-~0
[~°(x'' r)]"
(31)
In order to solve the set of partial differential equations made up of eq. (24) for each component together with the deactivation equations, eqs (22) and (23), each partial differential equation has been transformed into a set of ordinary differential equations by means of orthogonal collocation (Villadsen and Michelsen, 1978), using Lagrange polynomials as test functions, as has been detailed in previous papers (Gayubo et al., 1993c, d, 1994). The optimization algorithm is the same as that used for the study of initial kinetics. The molar fraction profile has been obtained by interpolating as has been previously described. The objective function to be minimized is given by eq. (11), where the values of composition correspond to different times on stream. 3.4. Kinetic parameters for the deactivation By following the previously described methodology, the parameters of the proposed kinetic model, with a 95%
confidence interval, are given by the following equations:
kd,~ = (0.32 + 0.03) 105 exp[(--14950 + 1200)/RT]
(32)
kd,s = (0.17 ___0.01) 104 exp[(--22200 ___1800)/RT]
(33)
kd,T = (0.44 + 0.05) 103 exp [( -- 12200 + 1100)/R T ]
(34)
kd,~ = (0.92 __+0.01) 102 exp[(--12500 + 1200)/RT]. (35) The goodness of fit is shown in the plots of Fig. 4, in which the experimental results (points) and calculated values (curves) of molar fraction evolution with time on stream corresponding to each isomer are compared at 450°C for two values of contact time. From the eqs (32)-(35), the following conclusions can be drawn: The contribution to deactivation of isomers cis- and trans-butene (rate constants kd,s and kd.T) is much lower than that of isobutene (rate constants kd~ and kd~T).This difference is more noticeable for as activity (kd,s >>kd,s) in the range of temperature studied. Taking into account that kd,s values are negligible compared with those of the kd,s, deactivation equation for as, eq. (22) (which corresponds to strongly acidic sites over which skeletal isomerization and byproduct forming reactions take place) can be simplified to the following expression:
dos
----=kd,sPsas. dt
(36)
- The values of the kinetic constant for deactivation for the skeletal isomerization and the formation of byproducts from the degradation of isobutene are much higher than the values of the deactivation constant corresponding to the double bond isomerization reactions, (kd,s >> kd~). Consequently, the decrease in as activity has to be much larger than that in aT. These conclusions concerning the kinetic model, together with the values of concentration of the reaction components, explain the experimental results of deactivation shown in Fig. 4. The concentration of isobutene always decreases with time on stream, while the concentration of the linear isomers increases slightly or is maintained almost constant. Only for high values of time on stream, for which the double bond isomerization rate will be noticeably diminished, a decrease in the conversion to cis-butene and to trans-butene would be observed. As a consequence of the kinetic model for deactivation, the evolution of the composition with time on stream is highly dependent on operating conditions: Comparing plots a and b in Fig. 4 it is observed that the decrease in the concentration of isobutene and the increase in the concentration of linear isomers is more noticeable as contact time increases. Thus, as an example, for time on stream 7 h, the values of activity are: as =0.22 and aT =0.68 for contact time 0.053 (g of catalyst) h (g of 1-butene) - l [Fig. 4(a)], and as =0.02 and aT =0.38 for 0.148 (g of catalyst) h (g of 1-butene)- 1 [Fig. 4(b)]. - The effect of temperature is similar, although it is less pronounced than that of contact time. As an example, for contact time 0.148 (g of catalyst) h (g of 1-butene)- 1, partial pressure of 1-butene in the feed 1.0 atm and time on stream 9 h, the activities are: as =0.08 and a r =0.66 at 450°C, and as =0.34 and aT =0.81 at 400°C. - The increase in partial pressure of 1-butene in the feed results in an increase in deactivation. As an example, for contact time 0.148 (g of catalyst) h (g of 1-butene) - l ,
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methodological aspects presented here may be useful for those reactions.
0.5
A. G. G A Y U B O *
0.4
0.3
X
AA==
•
•
•
Departamento de Ingenieria Quimica, Universidad del Pais Vasco. Facultad de Ciencia~ Apartado, 644. 48080 Bilbao, Spain
A•••
• A==e--e-=ee
l!..-'-~
i
6
i
T= 450 °C Plo = 1.0 atm
0.2
Xt • Xl• Xs* XC •
W/Flo= 0.053 gcatalysth/glo
0.1
6
12
18
24
F. J. L L O R E N S E. A. C E P E D A
Facultad de Farmacia. Apartado 450, 01006 Vitoria, Spain M. O L A Z A R J. BILBAO
30
time on stream (h) Departamento de lngenierla Quimica, Universidad del Pais Vasco, Facultad de Ciencias, Apartado, 644 48080 Bilbao, Spain
0.6 T= 450 °C Po = 1.0 atm
0.5
Xt • Xl • Xs,~ Xc A
W/Flo = 0.148 gcatalysth/gl_butene 0.4 o•
Xl
oo
a
0.3
a j, q
0.2
as, aT
0.1
Ci dj.q Flo
0 0
6
12
18
24
30
time on stream (h) Fig. 4. Comparison of the experimental results of molar fraction vs time on stream with the calculated ones, for different values of contact time.
ka,s, ka, r
kd.s, ka.r
kij
temperature 450°C and for time on stream 27 h, the activities are: as =0.35 and ar =0.80 for partial pressure of 1-butene in the feed 0.3 atm, and as =0.02 and aT =0.38 for 1.0 atm.
Kij
OF ~,~,~
4. CONCLUSIONS The kinetic model for the selective deactivation by coke deposition proposed in this paper is based upon the kinetic scheme proposed by Szabo et al. (1993) for skeletal isomerization of n-butenes. The combination of both models can be used in reactor simulation down to very low catalyst activity levels. The model proposed takes into account two experimentally proven circumstances: the participation of active sites with different acidic strength in different steps of the kinetic scheme; and the different deactivation rate of these sites of different acidic strength. The identification of the deactivation of sites of only two acidic strength levels is a simplification that can be adopted in the reaction studied here, but in other reactions such as reforming or catalytic cracking, in which single reactions of very different nature take place, the characterization of a higher number of acidity levels m a y be necessary. Nevertheless,
Plo ri
ri~, (rij)o rj. a
R T u
W Xi
xl
NOTATION activity of the catalyst at t time on stream, defined as ratio of reaction rates activity for j reaction, for active sites of q strength catalyst activity attributable to strong acidic sites and to the total number of acidic sites concentration of each i component, tool cm 3 deactivation order for active sites of q strength in j reaction mass flow of l-butene in the feed, (g of lbutene) h deactivation kinetic constants corresponding to the degradation of isobutene over the strongly acidic sites and over all the acidic sites, h - 1 a t m - 1 deactivation kinetic constants corresponding to the degradation of cis-butene and transbutene over the strongly acidic sites and over all the acidic sites, h - 1 a t m kinetic constant for the single reaction of formation o f j component from i component, tool (g of catalyst)- 1 h - 1 a t m equilibrium constant for the single reaction of formation o f j component from i component objective function partial pressure of cis-butene, trans-butene and isobutene, respectively, atm partial pressure of each i component, atm partial pressure of 1-butene in the feed, atm global formation rate of component i, at time t, mol (g of catalyst)- ~ h - t rate of the single reaction of formation of j component from i component, at time t and at zero time, mol (g of catalyst) ~ h reaction rate for j reaction, for active sites of q strength constant of the gases, J m o l - t K reaction temperature, K superficial gas velocity, m h mass of catalyst, g molar fraction of i component molar fraction of i component at the equilibrium
*Corresponding author
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Greek letters voidage of the catalyst bed dimensionless longitudinal coordinate in the reactor particle density of the catalyst, kg m 3 contact time (W/Flo), (g of catalyst) h (g of 1-butene)- 1 Subscripts c, t, l, s, p 0 i0
cis-butene, trans-butene, 1-butene, isobutene and byproducts, respectively zero time conditions reactor inlet conditions
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