Isomerization of Xylene over Hydrogen Mordenite. A Comprehensive Model George H. Norman, Dennis S. Shigemura, and Jack R. Hopper' Chemical Engineering Department, Lamar University, Beaumont, Texas 77710
A comprehensive kinetic model has been developed for the liquid-phase isomerization of xylene over hydrogen mordenite. Product composition of the xylene isomerization reaction was successfully predicted as a function of time using the model. This model is more comprehensive than any previously reported because it includes the effects of feed composition, catalyst activity, and temperature. A Langmuir-Hinshelwood kinetic model was developed for the isomerization of xylene which assumes first-order interconversion among the three isomers. The effect of composition was included in the reaction rate equations by the Langmuir-Hinshelwood expression which contains separate adsorption parameters and reaction rate parameters. The catalyst was observed to deactivate exponentially with the weight of feed through the reactor per unit weight of catalyst. The relative activity decreased 50% from the initial level at a feed/catalyst ratio of 15 f 2. Temperature was included in the model with an Arrhenius expression. The observed activation energy was 27.5 f 2 kcal/mol from 425 to 525 O F . Data were obtained in a tubular-flow integral reactor at space velocities from 1.1 to 10.3 W/HR/W at 400 psig.
Introduction Hydrocarbon isomerization reactions have received considerable attention in the past two decades. Significant progress in paraffin isomerization capability was achieved from the employment of a crystalline zeolite base in the dual-function type catalyst. The use of the crystalline zeolite catalysts for the aromatic isomerization seemed to be a natural extension because of the success of these catalysts with paraffin isomerization. Xylene isomerization studies have been performed in both the liquid and the vapor phase. Liquid phase studies have generally been made with homogeneous acidic halide catalysts (Allen and Yats, 1959; Brown and Jungk, 1959; Kemp, 1950; McCaulay, 1964) which suffer from corrosion and recovery problems. Vapor phase reactions over heterogeneous silica-alumina and dual-function type catalysts avoid these problems but lead to coke formation and lower yield of desired products because of the higher temperature required (Hanson and Engel, 1967; Silvestri and Prater, 1964; Ciapetta and Hunter, 1953; Pitts et al., 1955). High activity of a large number of zeolites relative to silica-alumina catalyst was reported by Hansford and Ward (1969). Lanewala and Bolton (1969) used a Type Y zeolite to obtain a large conversion to transalkylated products in the xylene isomerization reaction. A rare earth-exchanged zeolite X (REX) showed high activity with a low rate of deactivation for the same reaction (Wise, 1968); however, substantial Cg+ aromatics were also observed. Matsumato et al. (1968) observed a first-order reaction using several exchanged zeolites in a vapor phase isomerization. In most studies the reaction rate parameters and the effect of catalyst deactivation were not reported. Also, significant Cg+ aromatics were observed. Almost total elimination of side reactions and high activity for the xylene isomerization was achieved using a special zeolite catalyst of undisclosed composition (Grandio et al., 1971; Bowes et al., 1971). Kinetic data were not reported. Two recent kinetic investigations of the isomerization of xylene in the liquid phase over a zeolite-type catalyst were made by Hopper and Shigemura (1973) and by Aly et al. (1973). Aly et al. (1973) reported a kinetic model for the low-
temperature isomerization of xylene which would predict successfully the degree of reaction and isomer distribution as a function of charge composition. Excellent isomerization selectivity was observed. The type of zeolite used in the study was not disclosed. Catalyst aging effects were observed but were considered to be small and were not included in the kinetic model, Relative reaction rate constants were reported for only one feed composition. Hopper and Shigemura (1973) studied the catalytic isomerization of orthoxylene over hydrogen mordenite a t 450 O F , 400 psig, and 23 mol % toluene diluent. Excellent selectivity to the xylene isomerization reaction was obtained, and a first-order reversible reaction with exponential decay of catalyst activity was used to obtain a satisfactory model for this system. Variations in rate constants with feed composition and temperature were not included.
Experimental System The equipment for this study consisted of a micro-catalytic reactor and auxiliary equipment similar to that previously described (Shigemura, 1972; Norman, 1974). A simplified flow diagram is shown in Figure 1. The basic system consisted of a Ruska pump for liquid metering, a fluidized sand bath to maintain constant reactor temperature, a back-pressure regulator in the effluent line, a liquid product collecting system, and a wet-test meter for gas measurement. Gas metering was provided for catalyst activation and regeneration and for purging the reactor system. The position of the reactor in the fluidized sand bath is shown in Figure 2. The top and bottom sections of the bath were flanged to support porous stainless steel plates which allowed passage of air and supported the sand particles. The reactor was a 14-in. section of 316 stainless steel Ii4-in. tubing and was charged with 2.5 g of catalyst. Specially fitted porous stainless steel disks at each end of the reactor held the catalyst in place. The catalyst was a zeolon 900 H-mordenite produced by Norton Company. The catalyst was crushed and screened to 50-100 mesh particle size, calcined in air a t 1000 O F for 1 h, and activated a t 700 O F with hydrogen, After use, the catalyst was regenerated a t 800 O F with oxygen for 24 h. Nitrogen was used to purge the catalyst and reactor system. The catalyst properties include a theoretical mole ratio, Ind. Eng. Chem., Prod. Res. Dev., Vol. 15, No. 1, 1976
41
LIQUID XYLENE
'MI
A= B= C= Bi
PARA-XYLENE ORTHO-XYLENE META-XYLENE APPARENT RATE CONSTANT
Figure 3. Isomerization model. age reaction system density, l/&,and the equilibrium constant, K , as follows.
d7
=
1 [B1($ c2- cl) + B5 (c3- K 3 4t
Figure 1. Process flow diagram.
AIR OUT
The kinetic model described by eq 1, 2, and 3 may become more generalized by replacing the rate constants, B,, with expressions that include the effects of adsorption, catalyst deactivation, and temperature. A Langmuir-Hinshelwood adsorption model is used to modify the rate constants in terms of adsorption constants, A Langmuir-Hinshelwood adsorption model is used to modify the rate constants in terms of adsorption constants, K,, reaction rate constants, k,, and the Langmuir-Hinshelwood expression, \k, as follows
SANDBATH
B, = \kk,K, W PRODUCT OUT POROUS FRIT AIR IN
=
Figure 2. Fluidized sand bath. SiOz/A1203, of l O / l , a pore volume of 0.208 cm3/g, particle porosity of 0.274, and a surface area of 386 m2/g.
Kinetic Model A simplified reaction scheme for the xylene isomerization is illustrated in Figure 3. This scheme represents a general three-component unimolecular reaction system with interconversion among all three isomers. This scheme, or a modification for which there is no interconversion between the ortho and para isomers, has been proposed for several other studies in which a zeolite catalyst was not used (Hanson and Engel, 1967; Brown and Jungk, 1959). A first-order reversible reaction among the three isomers was assumed as a basis for the kinetic model. Since there is no change in the number of moles during the reaction, the equations can be expressed in terms of the toluene-free concentration of each isomer, Ci, the space time, 7, an aver42
Ind. Eng. Chem., Prod. Res. Dev., Vol. 15, No. 1, 1976
\k = 1/(1+KaCa
+ KBCB+ KcCc + KTCT)
(4) (5)
The catalyst deactivation model is used to modify the rate constants in the adsorption model in terms of reactant-to-catalyst weight ratio, % / s ,as follows
h, = koIe-at(S/T)
(6)
where ko, is the reaction rate constant a t initial catalyst activity and a, is the decay constant. The temperature model uses an Arrhenius expression to include the effects of temperature on the reaction rate constants in terms of activation energy, E,, and temperature, T , as follows
koL= A,e-EJRT
(7)
The final form of the equation for the B,'s in the generalized model is as follows
B, = K , A , 3 exp{-[a,(%/7)+ E,/RT]I
(8)
It should be emphasized that \k changes with concentration during the reaction and cannot be considered a constant. The subscript j is related to the subscript i such that when j = A, B, or C then i = 1 , 3 , or 5 respectively.
Experimental Results The generalized kinetic model for the catalytic isomerization of xylene over hydrogen mordenite has been examined by varying the range of several experimental conditions. Integral reactor data were processed to obtain prod-
uct concentrations as a function of space time. These concentrations were used as a basis for determining kinetic parameters in the reaction rate equations and for evaluating the performance of the kinetic model in predicting concentration. The data for the simplified model and the adsorption model were obtained a t conditions of 450 O F , 400 psig, at various space velocities, and for four feed compositions. The data for the deactivation model were obtained by further processing the data for the absorption model. Data for the temperature model were obtained using one feed composition and one space velocity. The temperature was varied from 425 to 525 O F . Experiments were performed by Shigemura (1972) which showed the effects of mass-transfer and diffusional resistances were negligible for the range of reaction conditions used in this study. An illustration of the type of plots used to obtain concentration as a function of time is shown in Figure 4. The samples for each experimental run were collected a t incremental process times, 0, and analyzed to determine product compositions. The space velocity, 117, (g of feed/hr/g of cat.), was held constant during each run. Thus, each experimental run yielded values of concentration at one space time, T , and several values of oil-to-catalyst ratio, 017. Experimental runs were made a t several different space velocities for different feed compositions at 450 O F . Concentrations for each xylene isomer were plotted against space time a t constant values of 01s. A smooth curve was drawn through the experimental values of concentration so that concentration could be read a t any value of space time. These values were prepared on punched cards for computer calculations. Analysis of Results The constants for the generalized model were selected to obtain the least sum of squares in the differences between calculated and experimental values of concentration. The results were evaluated graphically to determine if a satisfactory representation of the experimental data was obtained from the model. Adsorption Model. Rate constants were determined for the basic reaction model, eq 1, 2, and 3, for each feed and each 017. These data showed that the rate constants varied as the starting feed composition was varied. Thus the basic model was observed to be inadequate to account for a change in feed composition. A sample of these data for each feed containing 77% total xylene and 23% toluene at a 017 of 20 and 450 OF,400 psig is shown in Table I. The adsorption model was developed to account for the variation of rate constants with reactant composition. Equations 1, 2, 3, and 4 were used to determine the calculated values of reaction rate, dCldr, to be compared to experimentally determined values, AC1A.r. The residual which was optimized to obtain reaction rate constants a t each 017 for all time intervals ( N ) , components (P), and feed compositions ( M ) ,is as follows
where Rijk = residual that is minimized a t each 017 and
-A c i j k - Ci+l,j,k - ci-l,j,k AT1 T i + l - Ti-1 The optimized residual function used to obtain the adsorption constants was the same as eq 9 except that it was summed over all 017 (Q) as follows
FEED COMPOSITION
39.6% P-XYLENE 37.4% 0-XYLENE 0.0% M-XYLENE 23.0% TOLUENE
I
=*.1
;; .4 Y
E. z
.2
.2
=
0 c o
i
z
w
y
8
10 15 20 25 30 (W/HR/W) (HRS ON STREAM)
5
dc
.3
1.
Y
2z 7U
Li
E
.6 .4
.2
ov
.05 '
.1
.15
.2
.25
.3
r,l/(W/HR/W)
Figure 4. Simplified illustration for obtaining concentration vs. space time data from raw experimental data. Table I. Rate Constants for Xylene Isomerization Obtained from the Basic Rate Equationsa Feed composition 100% Ortho
100% Para
100% Meta
50% Ortho/ 50% Para
0 5.38 0 6.60 3.82 3.56 B, 2.10 1.82 1.73 a All feeds contained 23% toluene diluent. Reactions were at 450 "F,400 psig. @ / T = 20.
B,
B3
0.45 3.58 0.90
Table 11. Constants for the Adsorption Modela k, k3 rijk ____k , 1.986 12.961 5.910 0.121 15 0.956 9.521 4.636 0.081 20 0.684 7.332 3.795 0.068 25 0.602 5.896 3.064 0.051 30 0.500 4.787 2.555 0.043 a K A = 0.940; K B = 1.029; Kc = 0.990; KT = 1.094; rijkl = 0.073. 6IT
10
A single set of rate constants was calculated a t each O/r using eq 9. The optimization was performed over the four feed compositions. A single set of adsorption constants was calculated for all 017 and all feed compositions using eq 10. An iterative procedure was used to optimize rate constants and adsorption constants alternately until the value of Rijkl decreased no more than some arbitrary small value (Le., t = 0.0001) for each iteration in the optimization steps. The results are listed in Table 11. The average values of the residuals are listed to give a basis for comparing the relative agreement between experimental and calculated values of rate. A relatively small amount of interconversion between oand p-xylene is indicated by the small values of kl compared to k 3 and k 5 in Table 11. It is apparent in Table I1 that the rate constants decrease with increasing 017, thus verifying the experimentally observed decrease in catalyst activity. Ind. Eng. Chem., Prod. Res. Dev., Vol. 15,No. 1, 1976
43
I \
A -
-
= EXPERIMENTAL VALUES CALCULATED VALUES
Table 111. Comparison of Activation Energies with Values from Other Sources Activation energy, kcal/mol
10
Source
K
I
I
This study Aly et al. (1973) Hansford and Ward (1969) Brown and Jungk (1959) Hanson and Engel (1967)
P 8
5
27i 2
39 20 i- 3 22i 1 25.5
Y
t
Table IV. Numerical Parameters for the Generalized Modela
Bi B, B3
r
Y
.5
In A i 30.9802 29.5419 30.3786
Figure 5. Comparison between calculated and experimental values of reaction rate constants as a function of oil throughput.
"i
0.089 0.052 0.042
B, Subscripts: i = 1,j = A ;i = = C . + = 1/ ( 1 + K A C A + K B C B + K C C C + K T C T ) . K A = 0.940; K B = 1.029; K c = 0,990; KT = 1.094; R = 1.98719; T i n K. Q
8/7, OIL THROUGHPUT, W/HRM
Ei 29 600 26 600 28 315 3, j = B ; i = 5, j
EXPERIMENTAL VALUES 0
EXPERIMENTAL VALUES = In k a
A
c
A
z
P
$
0 = In ko5
aI
0
1
= P-XYLENE = 0-XYLENE = M-XYLENE
SOLID LINE -CALCULATED CURVE
v 2
PI c -2
4
0
gw
N
c
a -
!i
3 1
x "
1
2
a= g
ta Y
$
1
I
, 1.85
,
1
.sa
, 1.95
, 2.00
lTT(°Klxl$
Figure 6. Arrhenius plot of In hol vs. 1/T using experimental values and least-squares optimization curves.
Deactivation Model. The model for catalyst deactivation may be represented by eq 6 which can be written in linear form as follows In ki = In KO;
- cyi(8/7)
(11)
A least-squares optimization technique was used to calculate a ; and In hoi from the adsorption model rate constants. The values of ai and In k0i were used to make the plot of In hi vs. 8/7 shown in Figure 5. The solid lines in Figure 5 are theoretical values calculated by eq 11using the parameters, In koi and ai. The experimental values are shown as symbols in the graph. The values of ai represent the rate of decay of the catalyst. The values of a3 and a5 agree within 20%. However, the value of 011 is much larger, or about 80% larger than cyg. The values of a3 and cy5 are considered to be more reliable than 011 because of the sensitivity of the model to values of k l on which cy1 is based. In terms of relative magnitude, the catalyst activity drops t o 50% of its initial level after passing 15 weight units of reactant over a unit weight of catalyst. 44
I
3
7, l / I W / H R M l
W
0 c
1
I
Figure 7. Comparison between concentrations obtained experimentally and from the generalized model at 450 O F ; o-xylene-pxylene feed, BIT = 10.
2
8 2
2
Ind. Eng. Chem., Prod. Res. Dev., Vol. 15, No. 1, 1976
Temperature Model. The effect of temperature on the rate parameters is given by eq 7. The experimental data were obtained using one feed composition of 39.6% p-xylene, 37.4% o-xylene and 23% toluene. Rate data were obtained a t 425, 450, 475, 500, and 525 O F a t a space velocity of 10.339 (g of feed/hr/g of catalyst). Rate constants for one value of composition a t one specific space time were selected so that the integrated rate equations would pass through the experimental values. This procedure assumes that only one set of rate constants exists such that the curves of concentration vs. time pass through the experimental values. The values of In k03 and In k 0 5 were plotted against the reciprocal absolute temperature, 1/T, as shown in Figure 6. According to eq 7, each plot should be a straight line with a slope of -EJR and should intercept the ordinate a t In A,. A least-squares optimization technique was used to determine the values of the activation energies, E,, and the logarithm of the frequency factors, In A,. The solid line in Figure 6 was obtained using eq 7 and the optimized values of E, and In A,. The experimental and theoretical values are shown to be in close agreement in the figure. The activation energies, E3 and Eg, are compared to values reported by other investigators in Table 111. The activation energies obtained in this study are about 44% lower than the value reported by Aly et al., 26% higher than the value reported by Hansford and Ward and 5% lower than the value of Hanson and Engel (1967). The o - and p xylene interconversion was very small and values of kol obtained from the Optimization calculation were observed to
be very erratic. In order to include values of El and A1 in the generalized model, these values were estimated from values of kol at 450 O F (Cremer, 1955; Norman, 1974). Values of 29 600 cal/g-mol and 30.9802 were estimated for E1 and In A l , respectively. Generalized Model. The effect of each of the separate models is incorporated into one comprehensive generalized model using eq 1, 2, 3, and 8. The numerical parameters to be used in the generalized model are given in Table IV. These parameters are values for K,, In Ai, Ei, and ai which are substituted into the equations for Bi. The Bi's are then substituted into the basic rate equations. The capability of the generalized model to calculate concentration as a function of space time, 7,is demonstrated in Figure 7 for an o-xylene-p-xylene feed mixture a t 8/r = 10, 450 OF. The rate equations were numerically integrated to obtain concentration versus time. Good representation of experimental data is given by the model in this case. Results a t least as good as this were obtained with all other cases checked.
Acknowledgment Appreciation is expressed to E. I. du Pont de Nemours, Lamar Research Center, and Diamond Shamrock for financial support. Notation A = p-xylene Ai = frequency factor for reaction i , cc/g-h B = o-xylene Bi = reaction rate constants, cclg-h C = m-xylene Ci,,,k = concentration of component j at time interval i with k feed composition number, mol of i per total volume Ei = activation energy for reaction i , callmol Kj = adsorption constant for component j , cclmol ki = reaction rate constants, adsorption model R = gas constant, cal/g-mol K RijkL = total sum of residuals over all time intervals, components, feed compositions, and feed-to-catalyst ratios T = temperature, K Subscripts A = p-xylene B = o-xylene C = m-xylene i = reaction number for conversion of: 1, p-xylene to oxylene; 2, o-xylene to p-xylene; 3, o-xylene to m-xylene;
4, m-xylene to o-xylene; 5, m-xylene to p-xylene; 6, p -
xylene to m-xylene
i = time interval for 1to P intervals j = component number for 1to M components k = feed composition number for 1 to N compositions as follows: 1, 51.4% p-xylene and 48.6% o-xylene; 2, 100% p-xylene; 3, 100%o-xylene; 4,100% m-xylene 1 = feed-to-catalyst ratio number for 1 to Q ratios as follows: 1,8ls = 10; 2,8/r = 15; 3,817 = 20; 4,81r = 25; 5, 8/r = 30 0 = parameter for fresh catalyst, or initial time. Greek Letters catalyst decay constant = thermodynamic equilibrium constant for reaction i, dimensionless 8 = process time, h r = space time, h dt = sum of Xi/pi, cc of feedltotal g of feed \k = 1/(1+ KACA KBCB KcCc KTCT) ai =
+
+
+
Literature Cited Allen, R., Yats, L., J. Am. Chem. SOC., 81, 5289 (1959). Aly, A. F., Rope, B. W.. Wise, J. J., "A Kinetic Model for Low Temperature Isomerization of Xylenes Over Zeolite Catalyst," Paper Presented at the 74th National A.1.Ch.E. Meeting, New Orleans, La., March 1973. Bowes, E., Wise, J. J., U.S. Patent 3 578 723 (1971). Brown, H., Jungk, H., J. Am. Chem. SOC., 81, 5289 (1959). Ciapetta, F. G., Hunter, J. B., Ind. Eng. Chem., 45, 147 (1963). Cremer. E., Adv. Catal., 7, 75 (1955). Grandio. P. F., Schneider, F. H., Wise, J. J., Paper Presented at the 162nd National Meeting, American Chemical Society, D Chemistry, Sept 1971. Hansford, R. C., Ward, J. W., J. Catal., 13, 316 (1969). Hanson, K. L., Engel, A. J.. A.I.Ch.E. J., 2, 260 (1967). Hopper, J. R.. Shigemura, D. S..A.I.Ch.E. J., 19, (5) (1973). Kemp, J. D..U.S. Patent 2 527 824 (1950). Lanewala. M. A., Bolton, A. P., J. Org. Chem., 34, 3107 (1969). Matsumoto, H.. Take, J., Yoneda, Y., J. Catal., 74, 6246 (1952). Myers, J.. Mabry, L., U.S. Patent 2 488 510 (1958). Norman, G. H., Jr. M.E.S. Thesis, Lamar University, Beaurnont, Texas, 1974. Pachovsky, R. A., Best, D. A.. Wojciechowski, B. W., Ind. Eng. Chem., Process Des. Dev., 12, 254 (1973). Pitts, P. M., Comer, J. E., Jr.. Leum, L. N., Ind. Eng. Chem., 47, 770 (1955). Shigemura, D. S., M.E.S. Thesis, Department of Chemical Engineering, Lamar University, Beamont, Texas, 1972. Silvestri, A. J., Prater, C. D.,J. Phys. Chem., 88, 3268 (1964). Taylor, W. J., Wagman, D. D., Williams, M. G., Pitzer, K. S., Rossini, F. D.,J. Res. Nat. Bur. Std., 57, 95 (1946). Voorhies. A,, Jr., Ind. Eng. Chem., 37 (4), 318 (1945). Weekman. V. W., Jr., Nace, D. M.. A.1.Ch.E. J., 16 (3), 397 (1970). Wise, J. J., U.S. Patent 3 377 400 (1968). Wojciechowski, B. W., Can. J. Chem. Eng., 46, 48 (1968).
Received for reuiew March 20, 1975 Accepted November 5 , 1975 Presented a t the Symposium on "Recent Advances in Petroleum and Petrochemical Reaction Engineering," 79th National Meeting of the AIChE, Houston, Texas, Mar 16-20,1975,
Ind. Eng. Chem., Prod. Res. Dev., Vol. 15, No. 1, 1976
45
