KINETICS OF THERMAL HYDRODEALKYLATION OF MESITYLENE, m=XYLENE,AND TOLUENE SAMUEL E. S H U L L ' A N D A . N . H I X S O N University of Pennsylvania School of Chemical Engineering, Philadelphia, P a ,
1 3 h - o r d e r kinetics for thermal hydrodealkylation of mesitylene (1,3,5-trimethyIbenzene), m-xylene, and toluene have been determined. Dealkylation takes place by stepwise removal of methyl groups. The reverse reaction, methylation, does not occur to any appreciable extent. No isomerization of m-xylene and mesitylene to other CS and Cg isomers was found to occur with reaction. Results for toluene hydrodealkylation are in general agreement with those found in the literature. Differences in activation energies for the consecutive reactions could not be found. A constant activation energy of 98,100 B.t.u./lb. mole was used for correlating the experimental results.
HERMAL and catalytic hydrodealkylation of Ce+ alkylated Taromatics is a significant commercial process for producing benzene, naphthalene, and other such derivatives from higher order alkylated compounds. This study has been made to determine the reaction kinetics for the hydrodealkylation of mesitylene (1,3,5-trimethylbenzene)to rn-xylene, for which no data have been published. Kinetics for the hydrodealkylation of vi-xylene and toluene have been developed in support of the mesitylene work and for direct comparison with data available in the literature.
Equipment and Procedure
Experimental lvork \vas done in a small-scale flow reactor illustrated in Figure I . The reactor was of the fixed-bed cylindrical type having a removable stainless steel tube of l1lI6-inch i.d., 22l/2 inches long, connected to a flange a t one end. The tube fitted concentrically into an external casing and !vas bolted to the head \\hen the tube was in place. The temperature in the reactor was measured by six thermocouples located in a h/16-inch 0.d. thermoivell sealed into the reactor and extending along its central axis. The entire reactor was heated by an electrically Ivound bronze block surrounding the external casing. The entire stainless steel reactor tube was filled with 6- to 10- (Tyler) mesh quartz chips previously heat-treated for 2 hours a t 1400' F. in static air. Void volume of the packed reactor was measured by filling it with water and and found to be 135 cc. Piping to and from the reactor was '/*-inch i.d. stainless steel. The reactants entered from the top and passed down through the reactor. Products Lvere cooled by a water-jacketed condenser and then run to a high pressure gas-liquid separator where the liquid product was removed. The flash gas from the liquid drained from the separator was very small and was neglected in the material balance. The product gas from the separator \vas run through a pressure-control valve to a wettest meter. The liquid product \\'as analyzed by gas chromatograph under the folloiving conditions: Column packing Column size Carrier gas Flow rate Column temperature
50 HB-2000 (Polar) Ucon on GO- to 80-
mesh Chromosorb 25-foot X '/r-inch copper tubing Helium 50 to 100 ml. gas/min. 120' C.
fractions, respectively. Small quantities of unidentifiable heavy material representing less than 1 weight % total feed were ignored. Materials
Toluene was of nitration grade obtained from the Sun Oil Co. m-Xylene was obtained as pure-grade material from the Phillips Petroleum Co. Mesitylene was of approximately 95% purity from the Enjay Chemical Co. The major impurities in this material were pseudodocumene and o-ethyltoluene. All of these materials were used on an as-received basis without further purification. Commercial grade Air Products and Chemical Co. hydrogen was purified over a platinum catalyst under pressure and a t elevated temperature and then dried. Thermodynamics
Thermodynamic equilibria characterizing the consecutive reactions of mesitylene to rn-xylene, m-xylene to toluene, and toluene to benzene have been developed using published THERMOWELL
ROTAMETER
T I I b %:tFRF
146
I & E C PROCESS D E S I G N A N D D E V E L O P M E N T
/, fi
H20 IN
DEOXO CATALYST
L! 1
WET TEST METER
HIGH PRESSUR GAS-LIOUID SEPARATOR
LIQUID LEVEL S I G H T GLASS
Molar percentages in the liquid charge and product were calculated on a 100% benzene, toluene, m-xylene, and mesitylene basis. Small quantities of Cg and Cg isomers present were lumped together with the rn-xylene and mesitylene Present address, Houdry Process and Chemical Co., Marcus Hook, Pa.
4
MILTON ROY MINI PUMP
SUPPLY H2
B Figure 1. aromatics
Flow diagram
for thermal hydrodealkylation of
thermodynamic data ( 7). Equilibrium product distributions have been calculated for four cases (980’ and 1340’ F., 3 and 6 ,noles of Hz per mole of mesitylene) covering the approximate rang- of experimental work. Results from these calculations, summarized in Table I, show that the equilibrium aromatics distribution consists essentially of all benzene. Matsui et al. ( 6 ) have shown that for toluene hydrodealkylation under similar conditions (3 and 6 moles of Hz per mole of toluene) the equilibrium conversion of toluene to benzene is nearly quantitative. Weiss and Doelp ( 7 7) cite the following calculated thermodynamic equilibrium distribution for Cs isomers. Doelp e t al. ( 3 ) cite data of Scott sho\ving a typical distribution of Cg isomers occurring naturally in petroleum and similar to that found in reformate. Equilibrium Distribution o j Ca Isomers,
Mole %
F.
at 1160’
ni-Xylene o-Xylene $-Xylene Ethylbenzene
41.5 27.5 18.5 12.5
Mole Distribution of Cg Isomers Found in Petroleum Isopropylbenzene n-Propylbenzene I-Methyl-3-ethylbenzene 1-Methyl-4-ethylbenzene 1,3,5-Trimethylbenzene 1-Methyl-2-ethylbenzene 1,2,4-Trimethylbenzene ( pseudodocumene) 1,2,3-Trimethylbenzene
5.5 6.9 13.0 4.6 9.2 7.0 39.3
14.5
Determination of Effective Reactor Temperature and Residence Time
Variations existing in temperature with volume through the experimental reactor have been treated as proposed by Hougen and \\’atson (5) and Reiser and Watson (7) for evaluating data obtained from a nonisothermal unit. Only that portion of the remperature-void volume profile making a significant contribution to the conversion of reactants was considered. T h e low temperature or initial point on the profile was chosen so that the specific reaction rate a t this point was approximately l/20th of its value a t the maximum (outlet) temperature measured in the reactor. A literature activation energy of 98,100 B.t.u./lb. mole (70) was used for determining the significant portion of the temperature profile and for applying the Hougen and Watson method in the calculation of effective reactor volume and residence time corresponding to base temperature considered representative for the run.
had been reached. In general, a minimum time of 2 to 3 hours was required to achieve “lined-out” temperature operation. R u n conditions for each series of runs were set up to cover a wide variation in run temperature, so that the temperature effect on the rate constant could be determined. Other conditions were chosen a t random. Results from these runs are summarized in Table 11. Experiments were made in the order of increasing run number. Discussion of Results
Chromatographic analyses showed no evidence of skeletal isomerization of the m-xylene or mesitylene feeds during reaction. Thermodynamic considerations indicated that considerable quantities of other CSand C gisomers could be present if equilibrium was approached to any significant extent. Selectivity. An accurate estimate of reaction selectivity was not obtained. Spot “semiquantitative” analyses indicated that the selectivities were in the 95 to ’98% range as found by Fowle and Pitts ( 4 )for thermal toluene hydrodealkylation. The assumption of 100 mole % selectivity made in this work should be only in slight error. Kinetics. The rate of thermal hydrodealkylation can be described by 1.jth-order kinetic expressions, first-order with respect to aromatic and half-order with respect to hydrogen ( 9 , 70). Thermodynamic considerations have shown that a t equilibrium the production of benzene from toluene, mxylene, or mesitylene is overwhelmingly favored. Tsuchiya et al. (70) cite data which show that the benzene plus methane reaction is negligible even in the presence of a methane partial pressure three times that of hydrogen. Kinetics have been developed on the basis of the following “irreversible” reactions:
FH3
Experimental Results
The experimental runs were designed to cover temperatures from 1030’ to 1300’ F., pressures from 400 to 800 p.s.i.g. (28 to 56 atm.), and hydrogen-aromatic mole ratios of 3 to 9. A total of 21 runs was made with “pure” grade materials to define the kinetics of toluene, rn-xylene, and mesitylene hydrodealkylation. Seven runs were made for each feed. Each run consisted of a I-hour balance after stationary conditions
Table 1.
(3) Calculation of Absolute Rate Constants
The reaction rate for the hydrodealkylation of ”100%” mesitylene or rn-xylene or toluene can be expressed by:
Thermodynamic Equilibria for Mesitylene Hydrodealkylation
Temp., O F. Initial hydrogen-mesitylene mole ratio
Moles at equilibrium Mesitylene m-Xylene Toluene Benzene Hydrogen Methane
(4)
980
980
1340
1340
3
6
3
6
0.0000
. 0000 ~ . 0,0000 . 0.0000
0.0012
. 0000 0.0051 0.0000
0.0701
,0019
0.9288 0.0724 2,9276
0.1387 0.0084 ,9981 0.8561 0.9916 ,0019 0.1490 3.0084 ,9981 2.8510 2.9916
(5) Since hydrogen concentration was in large excess with respect to aromatics concentration, the 1.5th-order specific rate constants for all runs can be calculated by using a n average reactor hydrogen concentration without significantly affecting the accuracy of the calculated specific rate constants (pseudo first-order reactions). For m-xylene and mesitylene VOL. 5
NO. 2
APRIL 1966
147
Interpretation of Kinetic Results
dealkylation this was done to simplify the integrated form of the rate equation. For toluene hydrodealkylation hydrogen concentration could be related directly to toluene conversion and the 1.5th-order rate equation set up and integrated without difficulty. For toluene hydrodealkylation the following equations apply:
Activation Energy. The calculated rate constants summarized in Table I1 for each of the experimental runs have been plotted in Figure 2 on a logarithmic scale as a function of the reciprocal absolute temperature on a linear scale (Arrhenius plot). For each set of runs (toluene, m-xylene, and mesitylene) the activation energy was determined by calculating the slope and corresponding activation energy by the method of least squares. Energies of activation for toluene, m-xylene, and mesitylene thermal hydrodealkylation by this method were found to be 119,000, (106,000), and 105,000 B.t.u./lb. mole, respectively. Comparisons of data variance among the three sets of data a t the 90% confidence level showed that the measured differences among activation energies could not be considered real (statistically significant). Experimentally it was difficult in some instances to achieve a close approach to an isothermal bed temperature profile (equivalent adiabatic temperature rise for 100% conversion of toluene to benzene with 4 moles of HP per mole of toluene is 245' F.) because of the extremely exothermic nature of the reaction. Since temperatures could be measured only a t the center of the reactor, it is reasonably certain that the authors did not measure the true temperature maximum for all of the runs a t high conversion levels. For this reason an activation energy of 98,100 B.t.u./lb. mole (70) was considered more reasonable than those calculated above and was used in correlating the data. The experimental data were re-regressed by a least squares technique using this activation energy. The results of these calculations are shown in Figure 2.
For m-xylene and mesitylene hydrodealkylation the applicable equations are : (9)
Calculation of Relative Rate Constants Best Describing Mole Per Cent Aromatics Distribution in liquid Product
1 In -
Absolute rate constants were calculated from Equations 8 and 11. These results were interpreted in terms of a constant activation energy of 98,100 B.t.u./lb. mole for each of the three reactions considered. The rate constants expressed relative to toluene are independent of temperature. Relative rate constants also have been determined, using a 98,100 B.t.u./ lb. mole activation energy from product yield data alone for
Specific rate constants of 1.5th order were calculated for the experimental runs from Equations 8 and 11 and tabulated in Table 11.
Table II.
Summary of Experimental Results Obtained from Thermal Hydrodealkylation of Toluene, m-Xylene, and Mesitylene Charge Stocks Rate Const. k1.5, Contact CU.Ft.O.6/ Moles Press., Base Temp., Time, e, Mat. Bal., Conv., Lb. MoleQ.6 Run No. Hn/Oil P.S.I.G. O F. Seconds W t . yo Rec. W - Sec. Stock 0.032 0.00577 28.5 99.6 1090 6 3.0 800 Toluene
7
8
m-Xylene
Mesitylene
9 10 19 20 21 22 23 24 25 26 27 11
12 13 14 15 17 18
148
6.8 4.0 3.4 9.0 4.9 3.9 4.2 4.2 4.4 7.6 8.9 4.9 9.1 3.9 3.9 4.2 8.8
6.0 5.9 5.8
600 800 800 400 600 400 600 800 400 800 600 400 600 800 800 400 800 800 400 600
I & E C PROCESS D E S I G N A N D D E V E L O P M E N T
1130 1190 1260 1300 1270 1090 1050 1070 1220 1150 ....
1220 1280 1150 1030 1110 1060 1160 1240 1160 1250
10.6 14.4 30.1 5.8 9.1 13.1 19.7 27.1 11.7 13.6 9.6 10.0 8.3 40.7 33.7 16.3 18.4 12.5 6.8 7.7 ~~
101.4 98.4 96.0 97.5 97.1 97.1 100.1 98.6 99.4 98.9 97.1 99,4 98.5 94.5 96.2 98.5 105.0 97.0 100.3 97.4
0.036 0.184 0.801 0.591 0.306 0.015 0.028 0.047 0.247 0.155 0.394 0.653 0,137 0.097 0.404 0.038 0.730 0.903 0.139 0,637
0.0198 0.0751 0.316 1.11
0.249 0.00874 0.00830 0.00878 0.180 0.0610 0.302 0.810 0.100
0.0126 0.0803 0.0167 0.360 1.02 0.154 0.812
10.0 '-
-
LEGEND 0 TOLUENE, A.7.18 M-XYLENE,A;IS.O T
MESITYLENE, A.33.3
x
IO",E.
\\
98,100
X I O ' I , E n 96,100 X IO",€.
99,100
t
RELATIVE RATES TOLUENE. 1.0 M-XYLENE. 1.9 MESITYLENE a 4.6
( I1 SILSBY
SAWYER
(41
( 2 1 TSUCHIYA, HASHIMOT0,TOM INAGA AN0 MASAMUNE
0.0I
y6
\
i
( 2 ) A; 9.38 X I O i 1 , E = 9 8 , 1 0 0 ( 4 0 ATM.) (11 A = 7 . 5 2 X 1 0 9 , E : 8 4 , 6 0 0 (100 ATM.1 (41 A; 7.18 X IO'', E * 9 8 , 1 0 0 ( 2 8 TO 56 ATM.1
0.00 I 5.0
5.4
5.6
6.2
\
6.6
iI
1 0 4 / ~[ * R )
Figure 3. Experimental and published data on thermal hydrodealkylation of toluene
supplement data obtained from the analog. Results from these calculations are shown in Figures 4 and 5 .
runs in which rn-xylene and mesitylene were charged as feed. Relative rate constants best fitting the experimental yield data have been obtained by computer solutions of the applicable differential and integral equations. At constant temperature the following differential equations can be written :
Comparison of Results with Published Data
Results for toluene hydrodealkylation are compared with literature values in Figure 3. They are in general agreement in the comparable 28- to 56-atm. pressure range. Betts et al. (2) have suggested that the differences between their data a t 100 atm. and those of Silsby and Sawyer (9) a t atmospheric pressure may have been due to a change in reaction path occurring with temperature. Silsby and Sawyer's atmospheric pressure runs were made a t much higher temperatures than those of Betts et al. made a t 100 atm. Relative rate constants calculated from Equations 8 and 11 (shown in Figure 2) and the computer-calculated relative rate constants best fitting the experimental yield data (Figures 4 and 5) for m-xylene and mesitylene hydrodealkylation are shown in Table 111. The differences observed reflect inconsistericies and inaccuracies in the measurement of residence
Equations 12 to 14 have been expressed in terms of relative mole fractions and programmed for solution on a n analog computer. An analytical solution was also obtained for the relative mole fraction form of Equations 12 to 14. This solution was programmed on a digital computer to check and
Table 111.
Ref erencc Toluene rn-Xylene p-Xylene o-Xylene Mixed xylenes Mesitylene Mixed C o isomers
Comparison with Published Data of Relative (to Toluene) Rate Constants for C,+ Hydrodealkylation Doelp, Betts, Weiss, Brenner, Author's Silsby, popper, Doelp Weiss Best Sawyer Silsby (Catalytic) (Catalytic) Fig. 2 Fig. 4 Fig. 5 Projection (9) (2) (77) (3) 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 1. o 1.9 2.2 2.8 2.3 ... 3.5 2.6 ...
*..
... ...
4.6
...
... ... ... ... *..
...
... ...
3.9
...
... ... ...
4.2
...
1.5 2.5
... ... ...
2.9
2.4
6.4
4.6
...
...
.,.
VOL. 5
...
... ...
NO. 2
... ...
2.7
...
4.4
APRIL 1 9 6 6
149
-98,100 ktolueae
= 7.18
X 10“ e
cu. ft.O*5/lb.mole0.5-sec.
R* -98,100
LeOENP M-XYLENE 0 TOLUENE A BENZENE
km-xuiene =
16.5 X 10“ e
kmesitylene =
30.2 X 10l1 e
7 cu. ft.0e5/lb. mole0%ec. -98,100
/
RT
CU.
ft.O*s/lb.mole0.5-sec.
Acknowledgment
The authors thank W.Cohen and L. C. Eagleton, University of Pennsylvania, who assisted in setting up a program on the analog computer for determining the relative reaction rates and reviewing the reaction kinetics, respectively. The authors thank the Houdry Process and Chemical Co. for permission to use their laboratory facilities. L. C. Doelp, J. H. Terrell, and G. H. Van Wert, of Houdry and A. H. LVeiss, University of Pennsylvania, advised on the analytical and computational methods used in this work. Nomenclature
20
0
40 MOLE
%
60
BO
IO’
UNCONVERTED M-XYLENE
Figure 4. Determination of relative rate constants best fitting experimental data in m-xylene thermal hydrodealkyla-
tion
V
0 z
I
0
f
LEGEND MESITYLENE M-XYLENE
RELATIVE RATE CONSTANTS TOLUENE, 1.0 M-XYLENE,L.B MESITYLENE,LS
f
USED I N KISETICDEVELOPMENT k = I.5th-order specific rate constant, cu. ft.O,5/lb. mole0,jsec. r.hr = reaction rate lb. moles aromatic converted/cu. ft.-sec. w = fraction of starting aromatic converted c = Concentration, lb. mole/cu. ft. cAr = aromatic concentration, Ib. moIe//cu. ft. c A r o = inlet aromatics concentration, Ib. mole/cu. ft. CE = hydrogen concentration, lb. mole/cu. ft. C H ~= inlet hydrogen concentration, lb. moIe/cu. ft. F,, = aromatic feed rate, Ib. mole/sec. VE = equivalent reactor volume, cu. ft. 0 = residence time, sec. OE = equivalent residence time, sec. T = total pressure, p.s.i.a. R = gas constant = 10.71 p.s.i.-cu. ft./lb. mole-OR. = 1.987 B.t.u./lb. mole-OR. Ho = reactor inlet hydrogen to aromatic mole ratio H A v = arithmetic average hydrogen to aromatics mole ratio between inlet and outlet E = activation energy, B.t.u./lb. mole A = frequency factor, cu. ft.’J.s/lb. moleO%ec. T = temperature, O F . (or OR.) T B = base temperature, O F . (or OR.) literature Cited
M O L E % UNCONVERTED MESITYLENE
Figure 5. Determination of relative rate constants best fitting experimental data in mesitylene thermal hydrodealkylation
time, bed temperatures, and product analyses. T h e best projection of the true relative rate constants was taken as the average relative rate constants determined by the two methods. These constants are compared in Table I11 with literature values. The relative rate constant for rn-xylene hydrodealkylation was somewhat lower than that calculated by Betts et al. Better agreement was found with the relative rate constants for grouped xylenes and C Q isomers determined by Doelp et al. for catalytic hydrodealkylation of a C9 to C11 aromatic stock. For design considerations use of the following rate equations is recommended : 150
l&EC PROCESS DESIGN A N D DEVELOPMENT
(1) American Petroleum Institute Research Project 44, Tables ly, 15y, and Oy. (2) Betts, \V. D., Popper, F., Silsby, R. I., J . Appl. Chem. (London) 7, 497 (1957). ( 3 ) Doelp, L. C., Brenner, W.,LYeiss, A. H., “Production of Xylenes by Hydrodealkylation,” Division of Industrial and Engineerin? Chemistry, 147th Meeting, ACS, Philadelphia, Pa., April 9, 1964. (4) Fowle, M. J., Pitts, D. M., “Thermal Hydrodealkylation,” 46th Annual Meeting, American Institute of Chemical Engineers, Los Angeles, Calif., February 1962. (5) Hougen, 0. .4.,Lt’atson, K. M., “Chemical Process Principles,” p. 884, Wiley, New York, 1947. (6) Matsui, H., Amano, A,, Tokuhisa, H., Bull. Japan. Petrol. Inst. 1, 67 (March 1959). (7) Reiser, C. 0.. Watson, K. M., Nutl. Petrol. News 88, No. 14, ’R-260 (i946). . (8). Shull, S. E., “Determination of 1.5th Order Reaction Kinetics for Thermal Hvdrodealkvlation of Mesitvlene, m-Xylene. and Toluene,” University of Pennsylvania School of Chthical Engineering master’s thesis, April 20, 1964. (9) Silsby, R. I., Sawyer, E. IV., J . Appl. Chem. (London) 6 , 347 (1956). - -,(10) Tsuchiya, A., Hashimoto, A , , Tominaga, H., iMasamune, S., Bull. Japan. Petrol. Inst. 1, 73 (March 1959). (11) \Yeiss, A. H., Doelp, L. C., IND.ENG.CHEM. PROCESS DESIGN DEVELOP. 3, 73 (1964). RECEIVED for review February 1, 1965 ACCEPTED September 10, 1965 Taken from a master’s thesis presented to the University of Pennsylvania School of Chemical Engineering (8). ~
\ - -
