Ind. Eng. Chem. Res. 1990, 29, 2005-2012
2005
Disproportionation of Toluene and of Trimethylbenzene and Their Transalkylation over Zeolite Beta Ikai Wang,* Tseng-Chang Tsai,?and Sheng-Tai Huang Department of Chemical Engineering, Tsing-Hua University, Hsinchu, Taiwan, Republic of China
Zeolite Beta has been demonstrated as having the potential for catalysis in xylene production reactions, including toluene disproportionation, trimethylbenzene disproportionation, and toluene-trimethylbenzene transalkylation. I t exhibits excellent catalytic stability and transalkylation selectivity, In trimethylbenzene disproportionation, blending toluene into feedstocks can shift the production selectivity of tetramethylbenzene to xylene, whereas in toluene disproportionation, mixing trimethylbenzene into feedstocks can not only reduce the reaction temperatures but also raise the xylene yield. The shift of reaction selectivity is discussed from the viewpoints of both thermodynamics and kinetics. A methyl group transfer model has been proposed to rationalize the enhancement of xylene production efficiency.
Introduction Among benzene, toluene, and xylenes (BTX),the price of toluene is always the lowest. The process of toluene disproportionation over ZSM-5, developed by Mobil Co., was reported to be able to effectively upgrade toluene into benzene and xylenes (Grandio et al., 1972). By transalkylation with Cg+aromatics, toluene can be also converted to xylenes. Transalkylation processes are especially attractive to those refineries having BTX extraction units, where excessive Cg+aromatics are produced. It is found in Cg+ aromatics, produced by BTX units, that three trimethylbenzene isomers are the major components (Yang et al., 1984). The isomers are always in thermodynamic equilibrium, and the fraction of 1,2,4trimethylbenzene in total trimethylbenzenes is in the vicinity of 65%. Thus, 1,2,44rimethylbenzene could be a representative model compound of Cg+aromatics to be studied in transalkylation reactions. Mordenite and faujasite Y, which both belong to the zeolite family with a 12-membered-ringpore structure, are the only two zeolites that have been reported in the studies of transalkylation of toluene and trimethylbenzene (Csicsery, 1971; Yashima et al., 1972; Inoue and Sato, 1981; Wu and Leu, 1983; Barakat et al., 1987; Mikhail et al., 1987; Chao and Leu, 1989). Yashima et al. (1972) in the study of transalkylation over zeolites Y and mordenite showed that among various types of cations in the zeolites the hydrogen form exhibits the greatest activity. By controlling the hydrogen ion exchanged level of Nay, Chao and Leu (1989) demonstrated similar results. From the above literature survey, the pore size and acidity of zeolites may be the two factors of a good catalyst for transalkylation. First, only zeolites with 12-membered-ring openings possess a pore size large enough for transalkylation of Cg+ aromatics. Second, the higher acidity of the zeolites, the better is the activity. Zeolite Beta, recently unveiled as a new member of the zeolitic family of 12-membered-ringpore structure (Higgin et al., 1988), has stronger acidity than zeolite Y does (Hedge et al., 1989). These properties suggest the great potential of applying the zeolite Beta to catalyze transalkylation reactions. The purposes of this study are to explore this possibility and also to rationalize the mech-
* To whom all correspondence
should be addressed. 'On leave from the Refinery & Manufacturing Research Center, Chinese Petroleum Corp. 0888-5885/90/2629-2005$02.50/0
anism of transalkylation in terms of xylene production efficiency. Disproportionation of toluene and of trimethylbenzene and their transalkylation are studied. Experimental results are compared with those previously reported in the literature.
Experimental Section Zeolite Beta was prepared by the hydrothermal method from Si02-A1203-NaOH-tetraethylammonium hydroxide-H20 mixtures, following the procedures reported by Waldinger et al. (1967). Solid products from each preparation were filtrated, water washed, and dried overnight at 120 "C. The products were then characterized by a Simens D-500 X-ray diffractometer. The diffraction spectrum is shown in Figure 1. Through the use of inductive coupling plasma, the Si02/A1203 molar ratio of the products was found to be 31.3. The dried products were calcined in air at 540 "C for 4 h. Ion-exchange procedures, including (1) contacting solids with 1 N ammonium nitrate solution at 80 "C in a solid-to-solution ratio of 1:20 wt/wt for 8 h, (2) filtrating the solids, and (3) washing the solids with hot deionized water, were repeated 6 times. The NH4+ion exchanged solid was then dried overnight at 120 "C and calcined in air at 540 "C for 4 h. Zeolite H-USY (HSZ-360HUA) with a Si02/A1203ratio of 14.0 was supplied by the Tosoh Corp. Zeolite ZSM-5 was prepared according to the procedures reported by Chao (1981). The parent ZSM-5 has a Si02/A1203ratio of 35.8. Zeolite HY and HZSM-5 were prepared from NaY (Strem Chemical Co.) and the parent ZSM-5 by the ionexchange procedure described above. Reactions were conducted in a continuous fixed-bed reactor system. Mixtures of 2.0 g of 12-20-mesh zeolite and 4 g of Pyrex glass chips were packed into the reactor and activated in air at 540 "C for 2 h. The reactor was then cooled down to the desired reaction temperatures under nitrogen gas. Mixtures of toluene (Jassen Chemica) and trimethylbenzene (Jassen Chemica) with a different blending ratio were charged into the reactor by a metering pump. Reaction products were collected and analyzed with a Varian 3700 gas chromatograph equipped with a Bentone 34 column. The tetramethylbenzene isomers were analyzed with a dimethylsiloxanecapillary, HP Ultra l. After the reaction, the reactor was purged with nitrogen at the reaction temperature for 3 h, and then the amounts of coke deposition on catalysts were measured with a Dupont 951 thermogravimeter. 0 1990 American Chemical Society
Ind. Eng. Chem. Res., Vol. 29, No. 10, 1990
7a 7a 77 77
I
I
6752 5627 . 4502
Table I. Results of Toluene Disproportionation over Zeolite H-Beta and HZSM-5 HZ-
SMH-Beta
.
3377 . 2252
0 02
LL 50 90
130 170 210 250 290 330 370 L10 L50 20 Degrees
Figure 1. X-ray powder diffraction spectrum of zeolite Beta.
The conversions, either by disproportionation or transalkylation, of toluene, XT,and of trimethylbenzene (TMB), XTMB,are defined as XT =
(toluene wt % ) F - (toluene wt O / O ) ~ (toluene wt YO)F
XTMB=
(TMB wt Yo)F - (TMB wt (TMB wt % ) F
(1)
%)p
(2)
where the subscripts F and P represent the composition of components in the feed stream and in the product stream, respectively. Results and Discussion A. Disproportionation of Toluene. Zeolite Beta can catalyze toluene disproportionation reaction selectively and effectively, as shown in Table I. In the temperature range 300-450 " C , a nearly equal number moles of benzene and xylenes were produced. However, at reaction temperature above 350 "C, small amounts of trimethylbenzene produced by transalkylation of toluene and xylenes are observed. The conversion a t 450 O C reaches 20.24 wt % with a benzene-to-xylene (B/X) molar ratio of 1.04. Results of toluene disproportionation over zeolite Beta are also compared in Table I with that over HZSM-5. A t a comparable conversion level, 20.24 wt % for zeolite Beta and 24.61 w t % for ZSM-5, ZSM-5 gives a higher B / X ratio, 1.13, and a higher light gases yield, 0.13 wt %, than zeolite Beta does. The results of product distribution of ZSM-5 are very similar to the results obtained by Kaeding et al. (1981). It reveals that ZSM-5 is more liable to catalyze cracking reactions than zeolite Beta. Instead, zeolite Beta possesses a better transalkylation activity, shown by a higher trimethylbenzene yield, than ZSM-5 does. As shown in Table I, p-xylene exceeded the equilibrium value in the xylene isomers at low conversion levels. However, as conversion increases, the xylene isomers distribution approaches their equilibrium value. Thus, xylene isomerization is believed to be a secondary reaction. B. Disproportionation a n d Isomerization of 1,2,4Trimethylbenzene. Reaction results of 1,2,4-trimethylbenzene over zeolite Beta a t different reaction temperatures are presented in Table 11. Disproportionation of 1,2,4-trimethylbenzene produces xylenes and tetramethylbenzenes (TEMB) whereas isomerization gives 1,3,5- and 1,2,3-isomers. Unity of the xylene-to-tetramethylbenzene molar ratio (X/TEMB) should be obtained if there is no secondary transalkylation or dealkylation. This ratio, as shown in Table 11, is nearly unity. However,
reaction temp, O C WHSV, g h-' gcat,-l conversion, wt % product yields, wt 70 light gases benzene toluene xylene trimethylbenzene benzene/xylene, mol/mol xylene isomers, 70 p-xylene m-xylene o-xylene p-lo-xylene trimethylbenzene isomers, % 1,3,5-trimethylbenzene 1,2,4-trimethylbenzene 1,2,3-trimethylbenzene
5
-
300 2.82 3.29
350 2.82 7.53
400 450 405 2.82 2.82 2.82 14.45 20.24 24.61
0.04 1.38 96.71 1.87 0 1.00
0.03 3.13 92.47 4.25 0.12 1.00
0.03 6.05 85.55 8.00 0.33 1.03
0.03 8.51 79.76 11.03 0.60 1.05
34.30 49.42 16.28 2.10
26.83 52.68 20.49 1.31
25.19 52.67 22.14 1.14
25.92 24.32 51.00 52.25 23.08 23.42 1.12 1.04
0.13 10.93 75.39 13.18 0.37 1.13
33.33 12.50 25.37 70.04 66.67 75.00 64.18 25.50 0 12.50 10.45 4.46
toluene yield and benzene yield are increased at higher conversion levels. The benzene and toluene might come from the transalkylation of xylenes with trimethylbenzenes and dealkylation of alkylbenzenes. Since 1,2,4-trimethylbenzene can simultaneously undergo reactions of isomerization and disproportionation, we define the overall apparent selectivity of these reactions as
SI (mol/mol) = (1,2,3-TMB wt % ) p + (1,3,5-TMB wt 100 - (1,2,4-TMB wt % ) p
%)p
2(TEMB wt %)p/134 SD(mol/mol) = [lo0 - (1,2,4-TMB w t %)p]/120
(3) (4)
SD - (mol/mol) = SI
2(TEMB wt %)p/134 [(1,2,3-TMBwt %)p + (1,3,5-TMB wt %)p]/120 (5) where SIis selectivity of isomerization and SDis selectivity of disproportionation; the notation of subscripts is the same as those used in eq 1. At low conversion levels, the &/SI ratio is greater than unity, as shown in Table 11, which indicates that 1,2,4trimethylbenzene preferentially undergoes the disproportionation to isomerization. In isomerization, it gives more 1,3,5-trimethylbenzenethan 1,2,3-trimethylbenzene. According to the Fisher-Hirschfelder-Taylor molecular models, among the three isomers, the molecular size of 1,3,5-trimethylbenzene is the largest, followed by the 1,2,3-isomer and the 1,2,4-isomer (Csicsery, 1971). Therefore, 1,2,3-trimethylbenzene has a diffusion advantage over the 1,3,5-isomer. If shape selectivity of the trimethylbenzene isomers comes into effect, selectivity of the 1,2,3-isomerformation should be greater than the selectivity of the 1,3,5-isomer. Nevertheless, the 1,3,5-isomer is thermodynamically favorable over the 1,2,3-isomer (Earhart, 1982). The ratio of 1,3,5-to-1,2,3-isomer approaches thermodynamic equilibrium at 344 "C. Along with the results of nearly equilibrium conversions, it may be concluded that the disproportionation of 1,2,4-trimethylbenzene over zeolite Beta is a thermodynamically controlled reaction. The isomer distribution of tetramethylbenzene is an exception to the thermodynamic
Ind. Eng. Chem. Res., Vol. 29, No. 10, 1990 2007 Table 11. Results of Reaction of 1,2,4-Trimethylbenzeneover Zeolite H-Beta, HY, and H-USYa H-Beta reaction temp, "C on-stream time, min conversion, wt % coke, wt %/gat.* selectivity, mol/mol SI SD sD/sl
product yields, wt % benzene toluene xylene 1,3,5-trimethylbenzene 1,2,4-trimethylbenzene 1,2,3-trimethylbenzene tetramethylbenzene X/TEMB, mol/mol 1,3,5-/1,2,3-TMB xylene isomers, % p-xylene m-xylene o-xylene p-lo-xylene tetramethylbenzene isomers, % 1,2,4,5-TEMB 1,2,3,5-TEMB 1,2,3,4-TEMB 1,2,4,5/ 1,2,3,5-TEMB
HY -
USY -
200 90 13.57
250 510 29.20
300 630 61.32
344 720 70.48" 3.88
348 30 68.1OC 6.67
348 30 70.1e 6.62
0.38 0.62 1.62
0.40 0.60 1.50
0.34 0.65 1.92
0.26 0.72 2.78
0.25 0.74 2.97
0.22 0.72 3.27
0 0.04 3.74 2.90 86.43 2.22 4.67 1.01 1.31
0 0.11 7.70 7.17 70.80 4.45 9.77 1.00 1.61
0 0.94 16.99 16.15 38.68 4.85 22.39 0.96 3.33
0.10 3.06 20.88 13.66 29.52 4.34 28.44 0.93 3.15c
0.04 2.10 20.86 12.60 31.90 4.31 28.19 0.94 2.92e
0.04 2.63 22.60 11.59 29.91 4.29 28.99 0.99 2.70e
10.96 41.45 47.59 0.23
15.84 42.47 41.69 0.38
18.79 50.88 30.33 0.62
22.62 54.66 22.72 0.99
20.45 54.25 25.30 0.81
20.52 53.55 25.93 0.79
92.95 7.05
91.41 8.59
13.18
10.64
85.87 10.12 4.01 8.49
82.17 11.73 6.10 7.Olc
76.93 15.03 8.04 5.1lC
82.82 10.18 7.01 8.14c
OWHSV: 2.8 g h-l gat.-1. *Measured on the catalyst samples after 3 h of on-stream-time. eThermodynamic equilibrium value at 344 "C (Earhart, 1982): conversion, 74.4 w t %; 1,3,5-/1,2,3-TMB, 3.5; 1,2,4,5-/1,2,3,5-TEMB,0.74.
equilibrium value. It is possible that the formation of such big molecules is a shape-selective reaction. Unlike toluene disproportionation, 1,2,4-trimethylbenzene produces more o-xylene than p-xylene a t low conversion levels. The mechanism based on biphenylmethane carbonium ion intermediates (Csicsery, 1984) is proposed to explain this phenomenon. Nine intermediates of three types, classified by the positions of the carbonium ions, can be drawn from trimethylbenzene disproportionation and are given in Figure 2. Type I1 intermediates would lead to the formation of p-xylene, while type I produces o-xylene and type I11 gives m-xylene as their primary products. Since type I1 has more steric hindrance than types I and I11 do, less preference of p-xylene is observed. The formation of o-xylene is favorable at low conversion levels, and it could be the primary product. In addition, the selectivity of 1,2,4,5-tetramethylbenzene is far beyond thermodynamic equilibrium values. Therefore, intermediates c and i (Figure 2) are the most likely ones. Intermediate c leads to the formation of o-xylene and should be the major transition complex. Results of 1,2,4-trimethylbenzene reactions over zeolite H-Beta are compared in Table I1 with those over zeolite HY and H-USY. These three zeolites have comparable initial activity in isomerization and disproportionation, but USY shows a slightly lower isomerization selectivity. The ratio of 1,3,5-to l72,3-trimethylbenzeneover zeolite Beta is nearly thermodynamically equilibrated, while that over Y is lower. The amounts of coke deposition on catalysts after 3 h of on-stream time are shown in Table 11. At comparable initial conversion and reaction temperatures, zeolite Beta shows a much lower coke deposition than zeolite Y and USY do. Although zeolite USY shows a little bit higher initial activity and thus a slightly higher deactivation rate, the coke deposition rates of both Y zeolites are essential the same. Therefore, zeolite Beta has far better stability than both zeolite Y and USY, as shown in Figure 3a. As the activ-
Figure 2. Reaction intermediates of disproportionation of 1,2,4trimethylbenzene.
ities of zeolite Y decay, isomerization of o-xylene to pxylene also slows down, and the p-xylenelo-xylene ratio decreases accordingly, as shown in Figure 3b. Moreover, the ratio of 1,3,5-/ 1,2,3-trimethylbenzene decreases with the activity decay of zeolite Y, as shown in Figure 3c. Zeolite USY has only a slightly higher deactivation, but its 1,3,5-/ 1,2,3-trimethylbenzene ratio decreases sharper than that of zeolite Y. Therefore, the decrease of 1,3,5-
2008 Ind. Eng. Chem. Res., Vol. 29, No. 10, 1990 Table 111. Results of Transalkylation of 1,2,4-Trimethylbenzene a n d Toluene over Zeolite H-Beta" reaction temp, "C 350 350 352 350 feed compositions, w t YO toluene 100.00 86.63 67.30 18.17 0 1,2,4-trimethylbenzene 13.36 32.70 81.73 0.00 trimethylbenzene/ toluene 0.15 0.49 4.50 average methyl group numberb 1.21 1.54 1.0 2.56 conversions, wt YO toluene 7.51 (59.0) 14.56 (57.7) 29.36 (55.3) 38.14 (36.6) 62.80 (51.9) 1,2,4-trimethylbenzene 58.81 (67.2) 62.76 (74.5) product yields, wt % light gas 0.04 0.03 0 0 benzene 3.11 (27.1) 1.70 (22.3) 0.44 (14.8) 0.28 (2.1) toluene 47.54 (30.2) 92.49 (41.0) 74.02 (36.8) 11.24 (11.5) xylene 4.24 (26.2) 27.50 (34.4) 15.85 (29.4) 28.46 (30.6) 1,3,5-trimethylbenzene 0.04 (0.5) 2.12 (0.9) 5.79 (1.5) 12.36 (2.9) 1,2,4-trimethylbenzene 0.08 (3.5) 4.97 (6.3) 13.47 (10.6) 30.44 (20.9) 0 (1.1) 1,2,3-trimethylbenzene 0.67 (2.1) 1.80 (3.5) 3.94 (7.0) 0 tetramethylbenzene 0.37 (2.2) 2.00 (4.8) 12.96 (21.9) xylene isomers, 90 p-xylene 26.83 (23.7) 27.13 26.18 23.89 m-xylene 52.78 52.68 (52.6) 50.98 49.35 24.47 o-xylene 23.33 20.49 (23.7) 21.89 p-lo-xylene 1.02 1.31 (1.0) 1.24 1.07 reaction selectivity, wtlwt 9.32 (1.3) xylene/ benzene 1.36 (1.0) 62.5 (2.3) 101.6 (14.6) 44.03 (13.3) xylene/ tetramethylbenzene (43.7) 13.8 (7.2) 2.2 (1.4) 2.8 g h-'
gcat,-I.
*Average methyl group number per benzene ring.
344 0 100.00
3.0 70.48 (75.8) 0 0.10 (0.3) 3.06 (4.1) 20.88 (22.2) 13.66 (3.4) 29.52 (24.2) 4.34 (8.1) 28.44 (32.6)
22.62 (23.5) 54.66 (52.6) 22.72 (23.9) 0.99 (0.98) 208.2 (74.0) 0.73 (0.7)
Data in parentheses are the equilibrium compositions.
70 65 60
55 50
O8
t
3
Beta
0
HY A USY
t A
I
9
1
yL
96 -
A-
04 30
-
g-o----c
25-
\o
.;a h
20. 15.
13
, 30
50
0
'o-o-3-o\o
70
2
3
1,
5
Methyl Group Number per Benzene Ring
90
110 130 150 170
Time on Stream, min Figure 3. Comparison of stability (a, top), activity (b, middle), and isomer distribution (c, bottom) among. zeolite H-Beta, H-Y, and H-USY. WHSV: 2.8 g h-' gCat.-l. Reaction temperature: H-Beta, 344 "C; H-Y, 348 " C ; H-USY, 348 "C.
trimethylbenzene selectivity results from the loss of its isomerization activity accompanying the decrease of overall activity. C. Transalkylation of Toluene and Trimethylbenzene. The results discussed above show that trimethylbenzene is much more reactive than toluene in the disproportionation reaction. By mixing trimethylbenzene into toluene, it is hypothesized that trimethylbenzene would initiate the biphenylbenzene mechanism: one methyl group of trimethylbenzene could have been transferred to toluene. Therefore, toluene conversion could be increased. The methyl group transfer model, based on
Figure 4. Thermodynamic equilibrium compositions of aromatics as a function of number of methyl groups per benzene ring in feedstocks. Temperature 700 K. Data from Hastings and Nicholson (1961).
the kinetic viewpoints, will be discussed in great detail in the last section. On the other hand, from the thermodynamic viewpoint, mixing feed of toluene and trimethylbenzene would be also beneficial to transalkylation conversions and selectivities. A plot of aromatic compositions as a function of the number of methyl group per benzene ring in feedstocks is shown in Figure 4. The thermodynamic effect of mixing feedstocks on the equilibrated conversion and yield could be studied with the average methyl group number. By blending trimethylbenzene into toluene, the average methyl group number per benzene ring is increased. At a methyl group number of two, there is a maximum value of the xylene composition. Reaction results of toluene and 1,2,4-trimethylbenzene at various trimethylbenzene-to-toluene ratios (TMB/T) are shown in Table 111. Their reactions include toluene disproportionation, isomerization and disproportionation of trimethylbenzene, and transalkylation between toluene and trimethylbenzene. Benzene yield is a good indicator of toluene disproportionation. It is substantially reduced in accordance with the increases in TMB/T ratio, as shown in Figure 5. The selectivity improvement may be gained either thermodynamically or kinetically. However, as
Ind. Eng. Chem. Res., Vol. 29, No. 10, 1990 2009
t
TMBlT
0 (pure totuene)
o
TMBlT o
>
A
A
10
30
20 Toluene
LO
1.2.4-Trimethylbenzene Conversbn,wt %
Figure 5. Plot of benzene yield against toluene conversion at varratios and reaction temperaious 1,2,4-trimethylbenzene-to-toluene tures between 250 and 500 O C . WHSV: 2.8 g h-' gCat,-l.
100 aJ
c
!%
Figure 7. Plot of tetramethylbenzene yield vs conversion of 1,2,4trimethylbenzene at various reaction temperatures between 200 and 450 "C and a wide range of 1,2,4-trimethylbenzene-to-tolueneratios. WHSV: 2.8 g h-l
I
-
80 -
N
.
Experimental
/'
/
50
Conversion,wt0/.
0
0.29 0.49 L.50 pure TMB/
60
-
40 -
\ P
Toluene Conversion,wt 'lo
Figure 6. Plot of thermodynamic compositions and experimental compositions of xylene-to-benzene ratio against toluene conversions of transalkylation over H-Beta. Trimethylbenzene-to-toluene: 0.49 wt/wt. Reaction temperature: 2OC-450 O C . WHSV 0.8-11.9 g h-'
shown in Table 11, the reactivity of toluene increases tremendously with the blending of trimethylbenzenes. Thus, in the presence of trimethylbenzene, toluene preferentially reacts with 1,2,4-trimethylbenzene as opposed to its own disproportionation. In addition, conversion of toluene is increased. Toluene conversion levels approach the thermodynamic conversion ceiling, shown in the parentheses, a t a TMB/T ratio of 4.5 whose average methyl group number is 2.56. It should be noted that the nearly equilibrated xylene-to-benzene ratio (X/B) and X/TEMB ratio are observed only for the pure feed of toluene and trimethylbenzene, respectively. For the mixed feed, these two ratios are higher than the equilibrium ratios even though the toluene and trimethylbenzene conversions approach their equilibrium value ceilings. For a fixed TMB/T ratio, 0.49, whose average methyl group number is 1.54, the X/B ratio against toluene conversion is shown in Figure 6. At low conversion levels, the X/B ratio is far beyond the equilibrium ratio. As the toluene conversion increases by raising reaction severities,the ratio approaches the equilibrium value. Therefore, it is believed that the benefits of increasing xylene yields mainly come from kinetics. Comparing the last two columns in Table 111, the yield of tetramethylbenzene and the disproportionation selectivity of trimethylbenzene, X/TEMB is lowered by adding less than 20% of toluene into the trimethylbenzene feed. The above effect over a wide range of operating conditions is given in Figure 7 . Besides, there are some decreases of
- 0
10
o
0.49
A
4.50
20 30 LO Toluene Conversion, wto/lo
50
Figure 8. Plot of xylene yield against toluene conversion at various 1,2,4-trimethylbenzene-to-toluene ratio and operating conditions. Reaction temperature: 250-450 O C . WHSV: 0.8-11.9g h-l gaL-I.
trimethylbenzene conversion with the blending of toluene into trimethylbenzene, as shown in Figure 7 and Table 111. As far as economics is concerned, the utilization of toluene and trimethylbenzene can be promoted by each other. In toluene-dominated feed, the presence of trimethylbenzene can increase toluene conversion, lower reaction temperature, and raise xylene production efficiency. In trimethylbenzene-dominated feed, the addition of only a small amount of toluene can shift the production of tetramethylbenzene to xylene. Despite the difference of the TMB/T ratio in feed, the xylene yields against the toluene conversions can be correlated in one curve which lies far above the curve of pure toluene feed (Figure 8). This figure clearly indicates that by blending trimethylbenzene into toluene, xylene yield is boosted. The maximum xylene yield, 31.1 wt %, is obtained at a TMB/T ratio of 0.49 and at 399 "C whereas conversion of toluene is 28.5 wt % and that of 1,2,4-trimethylbenzene is 66.7 wt %. Further increases in TMB/T ratios would increase tetramethylbenzene yield and retard the further enhancement of xylene yield. At low TMB/T ratios, where toluene conversions are low, p-xylene dominates over o-xylene. As TMB/T ratios are increased, trimethylbenzene becomes the major component in feed, and toluene conversion is increased. Both these factors favor the formation of o-xylene to that of p-xylene, as discussed in Tables I and 11. Therefore, high TMB/T ratios result in an approximately thermodynamically equilibrated xylene distribution, as shown in Table 111. As toluene conversion reaches beyond 30 wt %, the
2010 Ind. Eng. Chem. Res., Vol. 29, No. 10, 1990 Table IV. Results of Transalkylation of 1,3,5-Trimethylbenzene a n d Toluene over Zeolite H-Beta" reaction temp, "C 203 252 302 351 400 conversions, wt 70 toluene 2.99 9.95 21.36 29.33 33.27 1,3,5-trimethylbenzene 49.62 74.40 81.91 86.44 86.95 isomerization selectivity (TMB), mol/mol SI 0.33 0.26 0.20 0.16 0.16 product yields. wt olc benzene 0.01 0.16 0.49 1.39 2.11 toluene 59.53 55.26 48.26 43.37 40.95 xylene 4.29 13.14 25.67 34.33 35.78 1,3,5-trimethylbenzene 19.46 9.89 6.99 5.24 5.04 1.2,4-trimethylbenzene 14.98 17.41 14.79 12.15 12.20 1,2,3-trimethylbenzene 1.47 1.90 1.75 1.58 1.73 tetramethylbenzene 0.26 2.24 2.05 1.94 2.19 xylene isomers, % p-xylene 33.57 31.63 29.29 24.80 24.28 m-xylene 35.66 44.28 48.58 52.90 52.61 o-xylene 30.77 24.09 22.13 22.30 23.11 p - / o-xylene 1.09 1.31 1.32 1.11 1.05
WHSV. 2.8 g h-l
gcat-I.
Trimethylbenzene-to-toluene: 0.63
wt/wt.
p - lo-xylene ratio approaches unity. Maximum p-xylene yield, 7.94 wt %, is obtained again at a TMB/T ratio of 0.49 with a toluene conversion of 28.5 wt %. Similar to the results of Figure 8, an excellent correlation between toluene conversion and xylene yield in transalkylation between toluene and 1,3,5-trimethylbenzeneis revealed in Figure 9. The maximum xylene yield of 35.9 wt 7'0 at 33.3 wt % of toluene conversion and 86.9 wt '70 of 1,3,5-trimethylbenzene conversion, obtained a t a TMB/T ratio of 0.63 and 400 OC,is higher than 31.1 w t 70 of xylene yield obtained in transalkylation of 1,2,4trimethylbenzene. On the other hand, the p-lo-xylene ratio from the transalkylation of toluene and 1,3,5-trimethylbenzene, as shown in Table IV, follows almost the same trend as that obtained from the transalkylation of toluene and 1,2,4trimethylbenzene. Transalkylation of toluene and both trimethylbenzene isomers favors the production of p xylene to o-xyleneat low toluene conversion. A comparable ortho to para isomer yield is obtained at high TMB/T ratio and high toluene conversion. Primary xylene products obtained from 1,3,5-trimethylbenzenecan only generate m-xylene. Due to good isomerization activity of zeolite Beta, there is no significant difference in xylene distribution between the transalkylation of toluene with these two trimethylbenzene isomers. Maximum xylene yield and conversions along with the operating conditions of transalkylation over zeolite Beta obtained from this study are compared in Table V with what was previously reported in the literature. The greatest xylene yield was reported by Inoue and Sat0 (1981). The reaction was catalyzed by mordenite and carried out at 30 kg/cm2. However, mordenite frequently
40 I
t
1:
TMBlT
1 0 0 0 6 A
"0
L
",f: // pure tduene
8
12
16
20
24
28
32
Toluene Conversion,wt% Figure 9. Plot of xylene yield against toluene conversion a t various 1,3,5-trimethylbenzene-to-tolueneratios and reaction temperatures between 200 and 400 "C. WHSV: 2.8 g h-l gCat-l.
has a serious deactivation problem in C9+ transalkylation (Wu and Leu, 1983). The xylene yield over HY (Inoue and Sato, 1981) is comparable to that over H-Beta, but its stability has been shown in Figure 3 to be poorer than that of zeolite Beta. Thus, it can be concluded that H-Beta gives very satisfactory results in terms of xylene yield and catalytic stability. D. Methyl Group Transfer Model. It has been disclosed above in Tables I11 and IV that transalkylation of toluene with trimethylbenzene results in the enhancement of xylene yield and toluene conversion but leads to the reduction of trimethylbenzene conversion. Thermodynamics favors xylene yields and trimethylbenzene conversion. However, the elevation of toluene reactivity and the enhancement of xylene selectivity, as discussed earlier, cannot be explained only by thermodynamics. Therefore, a methyl group transfer model, based on reactivity and diffusivity, is proposed to rationalize these results. Taking the operation with the best xylene yield as an example, the methyl group transfer model can be verified by theoretical calculation. The results of transalkylation of 1,3,5-trimethylbenzene and toluene at a TMB/T value of 0.63 and different reaction temperatures are presented in Table IV. Assuming that xylene is produced only by transalkylation, 1 mol of toluene and trimethylbenzene would produce 2 mol of xylenes. According to this assumption, xylene production efficiency (on a molar basis) of toluene, ET,and of trimethylbenzene, Em, is calculated as 0.5(xylene wt %)p/106 ET = [(toluene wt %)F - (toluene wt %)p]/92 (6)
ETMB =
0.5(xylene wt %)p/106 [(1,3,5-TMBwt O/O)F - (1,3,5-TMBwt %)p]/120 (7)
Table V. Comparison of Transalkylation at Maximum Xylene Yields over Zeolite H-Beta w i t h the Results over a n d Mordenite Reported in the Literature feed" conversiono xylene" catalvst TMB/T To1 TMB vield D/O 1.3.5-/1.2.3-TMB oDeratine conditions H-Beta 0.49 28.5 66.7 31.1 1.12 2.59 399 O C , 2.8 WHSV, 1 atm H-Betab 0.63 33.3 86.9 35.9 1.05 2.91 400 "C, 2.8 WHSV, 1 atm H-M 1.30 43.6 45.0 37.6 1.09 2.72-3.41 360 "C, 0.9 WHSV, 30 atm 8.9 30.1 22.6 480 O C , 1.7 WHSV, 21 atm Cu-Pd-M 0.23 H-Y 1.30 30.9 43.9 30.8 1.15 3.20 250 O C , 0.9 WHSV, 30 atm H-Y 3.74 6.5 64.9 17.5 0.73 2.50 460 "C, 0.1 WHSV, 1 atm Pt-La-Y 1.04 31.3 28.3 25.4 0.75 1.81 400 "C, 1 atm
Zeolites Y
ref Inoue (1981) Wu (1983) Inoue (1981) Chao (1989) Mikhail (1987)
All are expressed in wt %. Tol: toluene. TMB: trimethylbenzene. Results are obtained from transalkylation between toluene and 1,3,5-trimethylbenzene. All other data in this table are measured from transalkylation between toluene and 1,2,4-trimethylbenzene.
Ind. Eng. Chem. Res., Vol. 29, No. 10, 1990 2011 in
0
TMB
Toluene
I
203
240
280 320 Reaction Temp, 'C
I
1
360
LOO
Figure 10. Plot of theoretical xylene production efficiency of toluene and 1,3,5-trimethylbenzene against reaction temperatures of transalkylation over H-Beta. WHSV: 2.8 g h-' gult,-l. Trimethylbenzene-to-toluene: 0.63 wt/wt.
and selectivity of isomerization is defined as eq 8 (which is similar to eq 3 above) (1,2,3-TMB wt % ) p (1,2,4-TMB wt X ) p SI = (1,3,5-TMB wt %)F - (1,3,5-TMB wt % ) p (8) Their levels of efficiency are plotted in Figure 10 against reaction temperatures. It is shown that a t a reaction temperature of 203 "C the efficiency of toluene is almost 100% while that of 1,3,5-trimethylbenzene is much lower.' As shown in Tables I and 11, a t a reaction temperature of 200 "C toluene exhibits almost no conversion and trimethylbenzene shows a substantial conversion. Therefore, it is reasonable to believe that transalkylation is initiated by trimethylbenzene. Trimethylbenzene molecules are first adsorbed on active sites inside zeolite pores. The adsorbed trimethylbenzene molecules then form carbonium ions, which are ready for monomolecular isomerization, bimolecular disproportionation with another trimethylbenzene molecules, or transalkylation with toluene molecules. Diffusivity of benzene in ZSM-5 has been reported to be 4 orders of magnitude higher than that of trimethylbenzene (Meisel et al., 1976). Although the diffusivity in zeolite Beta has not been reported, the diffusivity of trimethylbenzene is very possibly much lower than that of toluene. Therefore, the toluene concentration inside the zeolite pores should be much higher than the trimethylbenzene concentration. The adsorbed trimethylbenzene carbonium ions then have a higher possibility of reacting with toluene molecules rather than with trimethylbenzene molecules. Thus, a methyl group is transferred from a trimethylbenzene carbonium ion to a toluene molecule and forms 2 mol of xylenes. Consequently, if toluene is blended into trimethylbenzene, trimethylbenzene disproportionation is greatly retarded, resulting in a lower trimethylbenzene conversion (Tables I11 and IV). On the other hand, observable toluene conversion is detected as low as 203 "C (Table IV), which is almost 100 "C lower than the minimum reaction temperature required for toluene disproportionation (Table I). Although the toluene concentration inside the zeolite pores is high, toluene fails to compete at the active sites with trimethylbenzene to form carbonium ions at low reaction temperatures. Toluene then plays a role as a scavenger, removing methyl group from trimethylbenzene, and becomes the controlling species of transalkylation. Accordingly, nearly 100 mol % xylene production efficiency of toluene is observed. Consequently, both toluene conversion (Tables I11 and IV) and xylene yield are increased (Figures 8 and 9).
+
As reaction temperature increases, substantial toluene disproportionation occurs and benzene is formed, as shown in Table IV. Toluene efficiency then decreases with increasing reaction temperatures. In contrast, a t high reaction temperatures, trimethylbenzene preferentially undergoes disproportionation to isomerization (Table IV)and the xylene production efficiency of trimethylbenzene is raised accordingly. However, the xylene production efficiency of trimethylbenzene is always lower than that of toluene. On the basis of the above discussion, it is clearly revealed that the methyl group is exclusively transferred from trimethylbenzene to toluene. Conclusion It has been demonstrated that zeolite Beta has a satisfactory catalytic capability in transalkylation of toluene and trimethylbenzene. Owing to its excellent stability, zeolite Beta exhibits very promising results in application of transalkylation reaction. The enhancement of xylene yield is discussed from the viewpoints of both thermodynamics and kinetics. It is found that thermodynamics alone cannot give a full picture of the observed phenomena. Therefore, a methyl group transfer model has been proposed. By transalkylation with trimethylbenzene, toluene reactivity and transalkylation selectivity are both upgraded. Toluene plays the role of the controlling species in methyl group transfer in transalkylation. It is found that at low reaction temperatures toluene has nearly 100% theoretical xylene production efficiency, but the efficiency decreases with increasing reaction temperatures. On the other hand, the efficiency of trimethylbenzene increases with increases of the reaction temperatures. Acknowledgment We deeply appreciate the efforts made by Mr. T. F. Chen for the analyses of thermogravimetry and the supply of USY samples from the Tosoh Corp. and Mitsui & Co., Ltd. This work was financially supported by the National Science Council, Republic of China. Registry NO.1,2,4,5TEMB, 95-93-2; 1,2,3,5-TEMB, 527-53-7; 1,2,3,4-TEMB, 488-23-3; xylene, 1330-20-7; toluene, 108-88-3; benzene, 71-43-2; 1,2,4-trimethylbenzene, 95-63-6; p-xylene, 106-42-3; m-xylene, 108-38-3; o-xylene, 95-47-6; 1,3,5-trimethylbenzene, 108-67-8; tetramethylbenzene, 25619-60-7; 1,2,3-trimethylbenzene, 526-73-8.
Literature Cited Barakat, Y.; Mikhail, S.; Ayoub, S. M. Effect of Feed Composition on Product Distribution in the Catalytic Conversion of Trimethylbenzene over (Pt/La)-Y Zeolite Catalyst. Zeolites 1987, 7, 235-239. Chao, K. J.; Leu, L. J. Conversion of Toluene and Trimethylbenzene over NaHY Zeolites. Zeolites 1989, 9, 193-196. Chao, K. J.; Tsai, T. C.; Chen, M. S.; Wang, I. Kinetic Studies on the Formation of Zeolite ZSM-5. J . Chem. Soc., Faraday Trans. 1 1981, I , 547. Csicsery, S. M. The Cause of Shape Selectivity of Transalkylation in Mordenite. J. Catal. 1971,23, 124-130. Csicsery, S. M. Shape-selective Catalysis in Zeolites. Zeolites 1984, 4,202-213. Earhart, H. W. Polymethylbenzenes. Kirk-Othmer Encyclopedia of Chemical Technology; Wiley: New York, 1982; Vol. 18, p 882. Grandio, P.; Schneider, F. H.; Schwartz, A. B.; Wise, J. J. Toluene for Benzene and Xylenes. Hydrocarbon Process., Int Ed. 1972, 8,as-a. Hastings, S. H.; Nicholson, D. E. Thermodynamic Equilibria Among Benzene and the Methylbenzenes from Spectroscopic Data. J . Chem. Eng. Data 1961,6, 1-5. Hedge, S. M.; Kumar, R.; Bhat, R. N.; Ratnasamy, P. Characterization of the Acidity of Zeolite Beta by FTIR Spectroscopy and T.P.D. of NH3. Zeolites 1989, 9, 231-237.
2012
Ind. Eng. Chem. Res. 1990,29, 2012-2020
Higgin, J. B.; Lappierre, R. B.; Schlenker, J. L.; Rohrman, A. C.; Wood, J. D.; Kerr, G. T.; Rohrbaugh, W. J. The Framework Topology of Zeolite Beta. Zeolites 1988, 8, 446-452. Inoue, T.; Sato, M. Conversion of Aromatic Hydrocarbons with Zeolite Catalysts (Part I) Transalkylation of Toluene and Trimethylbenzene on Synthetic Zeolite Catalysts. J . J p n . Petrol. Inst. 1981, 24 (2), 136-141. Kaeding, W. W.; Chu, C.; Young, L. B.; Butter, S. A. Shape-Selective Reactions with Zeolite Catalysts 11. Selective Disproportionation of Toluene to Produce Benzene and pXylene. J . Catal. 1981,69, 392-398.
Meisel, S. L.; McCullough, J. P.; Lechthaler, C. H.; Weisz, P. B. Gasoline from Methanol in One Step. CHEMTECH 1976,86-89. Mikhail, S.; Ayoub, S. M.; Barakat, Y. Conversion of Trimethyl-
benzene over Y-Zeolite Catalyst. Zeolites 1987, 7, 231-234. Waldinger, R. L.; Kerr, G. T.; Rosinki, E. J. Catalytic Composition of A Crystalline Zeolite. US Patent 3,308,069, 1967. Wu, J. C.; Leu, L. J. Toluene Disproportionation and Transalkylation Reaction over Mordenite Zeolite Catalysts. Appl. Cat a l . 1983, 7, 283-294.
Yang, H. M.; KO, J. W.; Wu, J. Unpublished data, 1984. Yashima, T.; Matsuoka, Y.; Maeshima, T.; Hara, N. Transalkylation of Toluene with Trimethylbenzene on synthetic Zeolites. J . Jpn. Petrol. Inst. 1972, 15, 487-492. Received for review December 4, 1989 Revised manuscript received May 15, 1990 Accepted May 23, 1990
Wiped Film Reactor Model for Nylon 6,6 Polymerization David D. Steppan,+Michael F. Doherty,’ and Michael F. Malone*’$ Department of Polymer Science and Engineering a n d Department of Chemical Engineering, University of Massachusetts, Amherst, Massachusetts 01003
A wiped film model for the design and performance analysis of continuously mixed thin-film nylon 6,6 polymerizers has been developed. The description is based on composition-dependent rate and equilibrium constants, a realistic degradation scheme, and finite gas-phase condensate concentrations. Even small amounts of mixing yield very large improvements in both the mass transfer and molecular weight generation in a wiped film reactor. Nylon 6,6 degradation reactions reduce the molecular weight generation by about 15% under typical operating conditions. These wiped film reactors should be operated between 280 and 285 “C and a t low pressure (5300mmHg) in order to achieve maximum molecular weight and good product quality with the smallest reactor. Higher temperatures severely affect the product quality through degradation, and increasing the pressure lowers the maximum attainable molecular weight. A catalyst for the main amidation reaction could yield improved reactor performance a t all but the lowest mixing rates. Introduction The “finishing” stage of continuous polycondensation largely determines the final product molecular weight and “quality”. Other investigators have attempted to formulate realistic process models for this important operation. For example, Ault and Mellichamp (1972a-c) developed a “periodically mixed” film model in which they assumed that a stationary polymer film was laid down and subsequent polymerization and mass transfer occured for a certain exposure time, after which the film was instantaneously well-mixed and the process repeated. They used a simplified kinetic model that included no side reactions or degradation reactions and ignored any diffusional resistance at the gas-film interface. Another model for wiped film polymerization reactors was developed by Amon and Denson (1980, 1983). They assumed that only a fraction of the material was laid down in the well-mixed film while the majority remained in a bulk pool adjacent to the moving wiper blade. The reaction was assumed to occur only in the bulk and condensate removal only in the film. They approximated the film thickness as infinite as far as mass transfer was concerned and, consequently, did not investigate the effect of film thickness. In addition, they also used a simplified kinetic model that did not include any side or degradation reactions. Gupta et al. (1983) modified the model of Amon and Denson to include the effect of film thickness. They compared the model of Ault and Mellichamp to that of Department of Polymer Science and Engineering. *Department of Chemical Engineering.
0888-5885/90/2629-2012$02.50/0
Table I. Nylon 6,6 Reactions C-SE+W degradation L-SE+A SE COz + SB SB + 2A X t 2NH, polyamidation A+C L+W
- -
1.1 1.2 1.3 1.4
1.5
Amon and Denson and found the two models gave nearly identical predictions. More recently, Kumar et al. (1984) studied the finishing stage of poly(ethy1ene terephthalate) (PET) polymerization with this modified Amon and Denson model and a kinetic scheme that included reactions with monofunctional compounds as well as redistribution and cyclization reactions. The finishing stages of PET polymerization have also been studied by Ravindranath and Mashelkar (1984) with the mixing film model similar to that of Ault and Mellichamp. However, they included degradation reactions in their kinetic scheme as well as an “effective flashing” technique to account for a changing interfacial concentration. A wiped film model for the design and performance analysis of continuously mixed thin-film nylon 6,6 polymerizers will be developed. Realistic kinetic and equilibrium correlations as well as a degradation scheme (Steppan et al., 1987, 1990b) will be used. The model predictions will be compared to the results of Kumar et al. (1984) and Ravindranath and Mashelkar (1984) for PET. The comparison is especially interesting since PET and nylon 6,6 have very different equilibrium constants (about unity versus several hundred) and degradation reactions. We will use a mixing film model similar to that of Ault and Mellichamp. However, we will assume a plug flow velocity profile in the film, which enables us to relate a 0 1990 American Chemical Society
