Ind. Eng. Chem. Res. 1989, 28, 572-516
572
Knight, J. R.; Doherty, M. F. Ind. Eng. Chem. Fundam. 1986,25, 279. Ladisch, M. R.; Voloch, M.; Hong, J.; Blenkowski, P.; Tsao, G. T. Ind. Eng. Chem. Process Des. Dev. 1984, 23, 431. Lee, F. M.; Pahl, R. H. Ind. Eng. Chem. Process Des. Dev. 1985,24, 168. Leeper, S . A.; Wankat, P. C. Ind. Eng. Chem. Process Des. Deu. 1982, 21, 331.
Levy, S.G.; Doherty, M. F. Ind. Eng. Chem. Fundam. 1986a, 25,269. Levy, S. G.; Doherty, M. F. Chem. Eng. Sci. 1986b, 41, 3155. Levy, S.G.; Van Dongen, D. B.; Doherty, M. F. Ind. Eng. Chem. Fundam. 1985,24,463. Mehta, G. D. J . Membrane Sci. 1982, 12, 1.
Peters, M. S.; Timmerhaus, K. D. Plant Design and Economics f o r Chemical Engineers, 3rd. ed.; McGraw-Hill: New York, 1980. Pham, H. N.; Doherty, M. F. Chem. Eng. Sci. 1989, in press. Prokopakis, G. J.; Seider, W. D. AIChE J . 1983, 29, 49. Robertson, G. H.; Doyle, L. R.; Pavlath, A. E. Biotechnol. Bioeng. 1983, 25, 3133. Tedder, D. W.; Taefik, W. Y.; Poehlein, S. R. 4th Symposium of Energy Conservation and Technology, Knoxville, TN, Oct 1985. Whitcraft, D. R.; Verykios, X. E.; Mutharasan, R. Znd. Eng. Chem. Process Des. Deu. 1983, 22, 452. Received for review August 1, 1988 Accepted December 5, 1988
Separation of p -Xylene and Ethylbenzene from C8 Aromatics Using Medium-Pore Zeolites Tsoung Y. Yan Central Research Laboratory, Mobil Research and Development Corporation, P.O. Box 1025, Princeton, New Jersey 08540
Medium-pore ZSM-5 zeolites were found t o be excellent for the separation of p-xylene and ethylbenzene from C8 aromatics. Their adsorption capacity and selectivity for p-xylene were high. T h e selectivity for p-xylene over ethylbenzene, Pple,was 5.5 at the optimum Si02/A1203of 700. The competitive adsorption capacity for p-xylene from a simulated reformer product was 120 mg/g. The p-xylene selectivity, Pple, increased sharply and then leveled off when p-xylene loadings reached 50 and 90 mg/g, respectively. T h e high p-xylene selectivity and its dependence on p-xylene loading are believed to be related to the unique packing of p-xylene in the crystalline cavities. The C8 aromatics are important raw materials for petrochemicals. The most important isomer is p-xylene for terephthalic ester production. o-Xylene, m-xylene, and ethylbenzene are raw materials for phthalic anhydride, isophthalic acid, and styrene, respectively. Because of their close boiling points, their separation by distillation is impractical and uneconomical. Separation processes based on other principles have been developed and reviewed (Wada, 1974; Milewski, 1981). Selective adsorption by the use of a zeolitic adsorbent is generally considered to be the most economical among the industrial processes for separation of Ca aromatics. The Parex process by UOP (Boughton, 1983) and the Asahi process by Asahi Chemicals (Seko et al., 1979) are both based on zeolitic adsorbents. The Parex process has been successfully operated since 1971; 34 Parex units have been licensed throughout the world (Boughton, 1983). Seko et al. (1979) pointed out that, for the separation of xylenes that have large molecular diameters, it is essential to use large-pore-size crystals of natural faujasite, zeolite X, or zeolite Y. On the basis of the patent literature, the zeolite used in the Parex process appears to be metalion-exchanged Y (Neuzil, 1976). Santacesaria et al. (1982) also found that potassium-exchanged Y is a good material for the separation of p - and m-xylenes. Milewski and Berak (1982) have studied the effects of preparation procedures on the selectivity for xylene isomer separation of potassium-barium-exchanged natural faujasite. Due to a disproportionately large demand, p-xylene is removed from the Caaromatic mixtures and the rest of the stream is recycled to the isomerization unit for reequilibration. In the typical isomerization process, ethylbenzene conversion is more difficult and less selective to p-xylene than o- and m-xylenes. Some processes have been developed to overcome this problem. On the other hand, ethylbenzene has a ready market for styrene production. Unfortunately, it has been difficult to separate ethyl-
benzene from the mixtures along with the p-xylene. In fact, p-xylene separation is limited by ethylbenzene contamination in the conventional separation based on elution chromatography. Thus, in order to improve xylene separation and to separate ethylbenzene from the mixture, an adsorbent of high p-xylene selectivity relative to ethylbenzene is required. The characteristics of improved adsorbents for xylene separations are high selectivity for p-xylene over ethylbenzene, high adsorption capacity, and insensitivity to variation in feed composition and impurities. Much progress has been made in the industry to improve natural faujasite and zeolite Y through modification and preparation procedures. Venuto and Cattanach (1971) of Mobil Oil discovered that p-xylene can be separated from the mixtures of o-, p - , and m-xylenes using ZSM-5 zeolites as a selective adsorbent. Dessau (1980) also showed that the ZSM-5 zeolite selectively adsorbs p-xylene in the mixtures of o- and p-xylenes with a para-to-ortho selectivity of about 5. In this study, the potential of using medium-pore-size zeolites, instead of large-pore zeolites, for xylene separation was explored. On the basis of the equilibrium adsorption data, the proprietary zeolite ZSM-5 was found to be an excellent adsorbent for xylene separation.
Experimental Section Chemicals. The chemicals p-xylene, o-xylene, ethylbenzene, mesitylene, and 1,2,3,5-tetramethylbenzene were the finest commercial grades and were pretreated over fresh ZSM-5 catalyst before use. Adsorbents. ZSM-5 zeolites were prepared according to the general procedure of Argauer and Landolt (1972). The zeolites were crystallized from mixtures containing a tetrapropyl ammonium compound, sodium oxide, alumina, silica, and water. Through synthesis, the Si02/A1203 ratios varied between 70 and 1600 (Table I). The synthesized ZSM-5 was calcined in Nz at 1 OC/min to 500 "C
0888-5885/89/2628-0572$01.50/0 0 1989 American Chemical Society
Ind. Eng. Chem. Res., Vol. 28, No. 5, 1989 573 Table I. Xylene Adsorption Capacity and Selectivity of ZSM-5(Temperature = 25 "C) 1 2 3 run catalyst 2-2 2-3 z-1 code 198 350 70 SiOz/AlZO3 eq sol, w t % feed 19.5 18.5 19.2 19.7 ethylbenzene 21.3 20.8 19.0 18.2 p-xylene 41.1 41.3 39.7 40.5 o-xylene 20.8 19.5 20.2 20.7 mesitylene adsorption, mg/g 49.2 42.4 30.9 ethylbenzene 36.6 102.0 127.1 p-xylene 103.9 173.9 187.4 total selectivity 0.66 2.63 4.45 &/e 7.49 3.93 9.80 &lo 9.29 3.08 2.20 Bole
and then calcined in air at 535 OC for 3 h. The calcined ZSM-5 in the form of powder was used directly in the experiments. Procedures. The hydrocarbon feed mixtures were prepared from the above chemicals to simulate the typical aromatic product stream from reformers. Their compositions were determined by using gas chromatography (GO. In the experiments, 1.5 g of the feed solution was contacted with 0.5 g of the zeolite for 1-2 h at 25 "C. The supernatant solution was analyzed with GC for calculating the adsorption. In run 1, the mixture, after 1h of contact, was recontacted with the zeolite for 45 h and reanalyzed. The results were essentially identical, suggesting that the adsorption equilibrium was reached within 1-2 h. This behavior is consistent with the theoretical prediction. If the diffusivities of xylenes are cm2/s, the zeolite hydrocarbon adsorption equilibrium can be reached in
4
5
2-4 600
2-5 1600
19.8 18.4 41.2 20.6
19.7 18.6 41.1 20.6
22.7 116.5 160.2
25.6 110.8 160.2
5.52 12.42 2.25
4.59 10.37 2.26
.-120
-
100
-
.oi
E"
s 80 -
u
4
g -
:.
60
*
40
-
20
-
S.
In comparison with the xylenes and ethylbenzenes at the testing condition, the adsorptions of mesitylene and tetramethylbenzene are small because the critical diameters of these compounds are much larger than the pore diameter of ZSM-5 and the exterior surface of the zeolite is small. They can be considered as diluents without affecting adsorption of xylenes in ZSM-5 zeolites and used as the internal standard in GC analysis of the products. The concentrations of the diluents were varied to change the loadings of the xylene and ethylbenzene in the zeolites. The adsorption of the adsorbable compounds, Ai, and selectivity, pi,,, can be calculated as follows:
where Aiis the adsorption of i, gram/gram of zeolite; M , is the mesitylene charge in the feed, gram/gram of zeolite; Ci,, is the concentration of i in the feed, gram/gram; Ci is the concentration of i in equilibrium solution, gram/gram; C, is the concentration of mesitylene in equilibrium solution, gram/gram; Pili is the selectivity for i relative to j ; and i and j are components i and j , respectively.
Results and Discussion (1) Adsorption Capacities for C8 Aromatics. Except for the adsorbent Z-1 with a low SiOz/A1,O3 ratio of 70, the total adsorption capacities of ZSM-5 for C8 aromatics were between 160 and 190 mg/g (Table I). It is interesting to note that Santacesaria et al. (1982) have estimated the
40
60
80 100
200
400
600 800 1000
2000
Si02/A1203 R a t i o of Z e o l i t e
Figure 1. Competitive adsorption of C8 aromatics versus SiOz/A1203 ratio.
loading capacities of zeolite Y for all isomers of xylenes to be 1.75 X mol/g (185.5 mg/g). The low total adsorption of C8 aromatics for the low Si02/A1,03 zeolite z-1 was due to preadsorbed water in the zeolite before the experiments. Every aluminum ion and the corresponding proton in the unit cell strongly hold four molecules of water, leading to reduced adsorption capacity for C8 aromatics. Since high-silica ZSM-5 contains less aluminum, its C8 aromatics adsorption capacity is rather insensitive to moisture from the air and the feed. The effects of moisture on ZSM-5-type zeolites for adsorption of C8 aromatics are the subject of a forthcoming communication. (2) Effect of Si02/A1,0, Ratio on Adsorption of C8 Aromatic Isomers. In these experiments, the ZSM-5 zeolites were almost fully loaded because the feed is high in C8 aromatics and contains only 19.5 wt 70 mesitylene diluents. As the SiOz/A1203ratio increased, the adsorption of p-xylene increased rapidly, reaching a plateau at a SiO2/A1,O3 ratio of 400 (Figure 1). The maximum competitive adsorption capacity for p-xylene in the simulated xylene mixture feed is about 120 mg/g, which is much higher than those estimated from the literature. On the other hand, adsorption of ethylbenzene decreased and then leveled off as the Si02/A1203ratio was increased to 600. It is this opposite trend in competitive adsorption
Ind. Eng. Chem. Res., Vol. 28, No. 5, 1989
574
2:
1
160
140
120
i
m
2 f
100
-
i
2 e
80
e
0 (c
g 60 i
a
0
o.iJ 7C
I
40
60 80 100 200 400 600 800 1000 S1O2/kl2O3 Ratio o f Zeolite
m
2000 40
Figure 2. p-Xylene selectivity versus Si02/A1,03 ratio.
for p-xylene and ethylbenzene that makes the selectivity for p-xylene vary significantly with the Si02/A1203ratio. The adsorption of o-xylene is only slightly affected by the Si02/A1,03 ratio of ZSM-5. (3) Optimum Si02/A1,03 Ratio for Maximum p Xylene Selectivity. The Si02/A1,03 has a profound effect on adsorption selectivities for C, aromatic isomers (Figure 2). For the compositions typical of the reformer streams, the selectivity for p-xylene over ethylbenzene, Pple, increased with Si02/A1203ratio, peaked a t the optimum Si02/A1203of about 700, and decreased gradually as the Si02/A1203further increased. Seko et al. (1979) found that, for a faujasite-type zeolite, the selectivity for p-xylene increases linearly with Si02/A1203ratio between 2.5 and 5.5. The optimum Si02/A1203ratio could vary somewhat with the compositions of the feed. Fortunately, the optimum Si02/A1203is quite broad so that a near optimum operation can be achieved when the Si02/A1203is around 700. At the optimum Si02/A1203,the selectivities for p-xylene over ethylbenzene and o-xylene, PPle and PPI,,, were about 6 and 13, respectively. The corresponding selectivities for potassium-exchanged Y are estimated to be 1.7 and 4 . 5 based on the results of Santacesaria et ai. (1982). (4) Possible Mechanism for Selective Adsorption of p-Xylene. The selective adsorption observed in this study is due to higher affinity of the high Si02/A1203 ZSM-5 for p-xylene and not to shape selectivity or differences in diffusivity of the isomers because adsorption equilibria were reached in the experiments. Apparently, the affinity of ZSM-5 for p-xylene relative to ethylbenzene increases with the Si02/A1,03 ratio. The mechanism for selective adsorption of p-xylene by ZSM-5 is yet to be elucidated. However, the following mechanism can be postulated: (A) Preferential Packing of p-Xylene in the Cell. The high Si02/A1203ZSM-5 is unique among the known zeolites. Olson et al. (1981) found that at low temperatures the adsorption capacity for p-xylene is almost twice as much as that for o-xylene or benzene. They attribute this difference between benzene and p-xylene to entropy contributions resulting from more favorable packing of pxylene in the crystalline cavities. Thus, the entropy change
20
0 3
20 40 60 80 L i q u i d Phase Concentration of C8 Aromatics M i x t u r e , w t Z
Figure 3. Adsorption isotherm of CBaromatics mixture.
for p-xylene, J A S a d l p , should be greater than that for ethylbenzene, l A S a d l e , and A G a d = m a d - T A S a d for Pxylene should be more negative than that for ethylbenzene. This led to the conclusion that I m a d ( p > l m a d l e . Indeed, the heat of adsorption of p-xylene and ethylbenzene on ZSM-5 at loadings of 5 4 molecules per unit cell have been experimentally determined to be about 80 and 75 kJ/mol (Thamm, 1987). Santacesaria et al. (1982) observed that a difference in the thermodynamics of adsorption among xylene isomers is the change in entropy and concluded that the selectivity of the potassium-exchanged Y zeolite must be mainly attributed to the different arrangement of the molecules in the intracrystalline cavities of the zeolite. Apparently, the causes of p-xylene selectivity are different for ZSM-5 and potassium-exchanged Y zeolites. (B) Selectivity Increases with Si02/A1203Ratio. The factors important in affecting favorable packing of p-xylene are critical in determining p-xylene selectivity. As the Si02/A1203ratio decreases, the aluminum ions per unit cell increase. They seriously interfere with p-xylene packing, leading to lower p-xylene adsorption. On the other hand, the same factors affect ethylbenzene adsorption to a much lesser extent because it never packs well to begin with or it is more ”flexible” to accommodate the intrusion. The alternative mechanism is that the slight change in unit cell parameters due to an increase in SiOz/A1203ratio could be important in p-xylene packing. (5) Adsorption Isotherm of Xylenes. To study the adsorption isotherm, the stock solution containing xylene and ethylbenzene was diluted with 1,2,3,5-tetramethylbenzene to the desired concentration. The adsorption results are shown in Table 11. The total adsorption of xylenes is shown in Figure 3. The total adsorption increases rapidly with the concentration of the xylenes in the equilibrium solution and levels off at about 160 mg/g, leading to a typical adsorption isotherm.
Ind. Eng. Chem. Res., Vol. 28, No. 5, 1989 575 Table 11. Adsorption Capacity and Selectivity" run 10 composition, wt 70 feed 2.17 ethylbenzene p-xylene 2.36 4.41 o-xylene mesitylene 2.17 TMBb 88.89 equilibrium ethylbenzene 0.91 1.18 p-xylenes 4.71 o-xylene 2.35 mesitylene TMB 90.84 adsorption, mg/g 39.9 ethylbenzene 38.1 p-xylene o-xylene 1.8 total 79.8 selectivity 0.74 &/e 83.44 @Pi0 112.76 @e/,
7
9
6
8
4
3.90 4.26 7.94 3.90 80.00
6.50 7.10 13.23 6.50 66.67
9.75 10.65 19.75 9.75 50.00
13.00 14.20 26.47 13.00 33.33
19.50 21.30 39.70 19.50 0.00
2.63 2.58 8.14 4.21 82.44
5.62 4.55 13.65 6.89 69.29
9.60 7.97 21.19 10.42 50.82
12.82 11.09 27.55 13.76 37.76
19.80 18.40 41.20 20.60 0.00
43.9 56.1 12.0 112.0
35.9 84.2 10.6 130.7
23.1 96.3
27.1 118.1 11.6 156.8
22.7 116.1 21.0 160.2
1.30 14.77 11.36
2.89 23.88 8.26
5.08 25.46 4.39
5.52 12.42 2.25
O? 119.4? 5.01
Adsorbent, ZSM-5;code, 2-4;SiOz/AlZO3,600;solution/zeolite, g/g, 3; temperature, "C, 25. bTMB= 1,2,3,5-tetramethylbenzene.
1
120
p-Xylene
. m
40
i
5
30
x I
20
0
0
o-Xylene
0 0
20
, 4
8 Llquid
12 16 20 P h a s e C o n c e n t r a t i o n of Components, w t %
24
28
Figure 4. Competitive adsorption versus liquid-phase concentration.
(6) Effect of Loading on Competitive Adsorption of
C8Aromatics. In competitive adsorption, p-xylene adsorption increased monotonically and approached the asymptotic value of about 120 mg/g as the concentration of p-xylene in the equilibrium liquid phase increased (Figure 4). Similarly, o-xylene adsorption increased monotonically with increased o-xylene concentration even though the increase in adsorption was rather small. On the contrary, ethylbenzene adsorption increased rapidly, peaked at an ethylbenzene concentration of about 2 w t 70, and reached a lower asymptotic level as the ethylbenzene concentration was further increased. This is an abnormal adsorption behavior and only happened in competition with p-xylene. Apparently, ethylbenzene loses its competitiveness for adsorption to p-xylene at higher concentrations and loadings. Competitive adsorptions of ethylbenzene and o-xylene are plotted against the adsorption of p-xylene in Figure 5. It is noted that in this study the relative concentrations of ethylbenzene, p-xylene, and o-xylene in the feed were kept constant and their absolute concentrations were varied by use of a diluent, 1,2,3,5-tetramethylbenzene.The increased adsorption of p-xylene is the result of increased
20
PI 40
60
80
100
120
p-Xylene A d s o r p t i o n , mg/g
Figure 5. Ethylbenzene and o-xylene adsorption versus p-xylene adsorption.
p-xylene concentration in the feed and, in turn, the equilibrium liquid phase. As the p-xylene adsorption increased over 50 mg/g, the ethylbenzene adsorption decreased in spite of further increases in ethylbenzene concentration in the liquid phase. On the other hand, the o-xylene adsorption increased slightly with an increase in p-xylene adsorption due to the increase of o-xylene concentration in the liquid phase. (7) Effect of Loading on p-Xylene Selectivity. The selectivity for p-xylene over ethylbenzene, Pple,varied with p-xylene and total hydrocarbon loadings on the zeolite (Figure 6). At low loading, PPlewas less than 1. The increased rapidly as p-xylene and total loadings were increased to 50 and 100 mg/g, respectively, and finally leveled off to 5.5 at p-xylene and total loadings of 90 and 150 mg/g, respectively. Even though the data fluctuated, PPI, appears to be rather constant and independent of the loadings of p-xylene and total hydrocarbons. The variation of p-xylene selectivity with loading is the result of competitive adsorption phenomena unique to the ZSM-5 system discussed above and is entirely unexpected. In normal competitive adsorption systems, the selectivity remains constant as the total loading changes. (8) Mechanism of Loading Effect on p-Xylene Selectivity. The preferential adsorption of p-xylene by ZSM-5 is due to its favorable packing in the crystalline
PPI,
Ind. Eng. Chem. Res., Vol. 28, No. 5, 1989 adsorption capacity is between 160 and 190 mg/g, and the competitive adsorption capacity for p-xylene from typical xylene mixtures is 120 mg/g. The PPle increases sharply as the p-xylene loading exceeds 50 mg/g and reaches the asymptotic value of 5.5 as the p-xylene loading is increased to 90 mg/g. This variation of PPI, with p-xylene loading is unique to the ZSM-5 system. The high selectivity for p-xylene and its variation with p-xylene loading appear to be related to the unique packing of p-xylene in the crystalline cavities.
Acknowledgment
2
40
E5
eo r4,O.ptln"
103 P$
!2C
14:
1'E
,
Figure 6. p-Xylene selectivity versus adsorption.
cavities. The p-xylene and ethylbenzene loadings where ethylbenzene adsorption starts to decrease were about 50 and 40 mg/g, respectively (Figure 6). This is consistent with the idea of favorable packing, suggested by Olson et al. (1981). They found that 4 molecules/unit cell of pxylene, equivalent to 73 mg/g of adsorption, is uniquely packed. Theoretical calculations by Reischman et al. (1988) indicate that the most energetically favored sorption sites are the channel intersections for p-xylene loading 14 molecules per unit cell of zeolites with an adsorption energy of -82 kJ/mol. At p-xylene loadings >4 molecules/unit cell, molecules begin to fill the sinusoidal channels with an adsorption energy of -57 kJ/mol. Richards and Rees (1988) also experimentally found that the heat of adsorption of p-xylene for loadings less than 4 molecules/unit cell was -80 kJ/mol. At the initial stage of competitive adsorption, there is enough room to accommodate all species so that both p-xylene and ethylbenzene have equal access to the adsorption sites and are adsorbed independently without interfering with each other. It appears that at this stage p-xylene is adsorbed on the preferred packing positions while ethylbenzene goes to the same type of positions that remain available. Thus, in this stage, the loading for each species is determined by their concentration in the liquid phase according to their adsorption isotherms. As the loading of p-xylene is increased to 50 mg/g, the preferred position is nearly exhausted, and a further increase in p-xylene loading is achieved by replacing ethylbenzene, leading to a decrease in ethylbenzene adsorption and improved p-xylene selectivity. The data clearly show that the late-coming p-xylene displaces the ethylbenzene already adsorbed on the preferred sites (Figure 4).
Summary In lieu of conventional large-pore zeolites, a proprietary medium-pore ZSM-5 zeolite was tested for separation of p-xylene and ethylbenzene from C8 aromatics mixtures. ZSM-5 with a high Si02/A1203ratio is highly selective for p-xylene adsorption, and the selectivity for p-xylene over ethylbenzene, ppls, can be as high as 5.5. The total
Valuable discussions with W. 0. Haag, D. H. Olson, R. M. Dessau, W. K. Bell, and A. B. Schwartz are gratefully acknowledged. This study benefitted from the related work conducted by E. H. Unger, P. T. Allen, P. E. Keown, C. C. Meyers, and P. Grandio at Mobil Chemical Co. This work has been presented at the 1988 AIChE Spring National Meeting in New Orleans. Registry No. Ethylbenzene, 100-41-4; p-xylene, 106-42-3; o-xylene, 95-47-6; mesitylene, 108-67-8.
Literature Cited Argauer, R. J.; Landolt, G. R. (to the Mobil Oil Corporation) US Patent 3,702,886, 1972. Boughton, D. B. Development of the Parex Process for Separation of p-Xylene from C8 Hydrocarbon Mixtures. Prepr.-Am. Chem. SOC.,Diu.Pet. Chem. 1983, 28(4), 1072. Dessau, R. M. Selective Sorption Properties of Zeolites. In Adsorption and Ion Exchange with Synthetic Zeolites; Flank, W. H., Ed.; ACS Symposium Series No. 135; American Chemical Society: Washington, DC, 1980; pp 123-135. Milewski, M. Separation of Mixtures of Ethylbenzene and Xylene Isomers by Adsorption Methods. Przem. Chem. 1981,60(2),71-3. Milewski, M.; Berak, J. M. Effect of Adsorbent Preparation Parameters on the Selectivity of Xylene Isomers Separation. Sep. Sci. Technol. 1982, 17(2), 369-374. Neuzil, R. W. Process for Separating p-Xylene. (assigned to UOP), US Patent 3,997,620, Dec 14, 1976. Olson, D. H.; Kokotailo, G . T.; Lawton, S. L.; Meier, W. M. Crystal Structure and Structure-Related Properties of ZSM-5. J. Phys. Chem. 1981, 2238-2243. Reischman, P. T.; Schmitt, K. D.; Olson, D. H. A Theoretical and NMR Study of p-Xylene Sorption into ZSM-5. J . Phys. Chem. 1988,92, 5165-5169. Richards, R. E.; Rees, L. V. C. The Sorption of p-Xylene in ZSM-5. Zeolites 1988, 8, 35-39. Santacesaria, E.; Morbidelli, Massimo; Danise, P.; Mercenair, M.; Carra, S. Separation of Xylenes on Y Zeolites. 1. Determination of the Adsorption Equilibrium Parameters, Selectivities, and Mass-Transfer Coefficients through Finite Bath Experiments. Ind. Eng. Chem. Process Des. Deu. 1982,21, 440-44. Seko, M.; Miyake, T.; Inada, K. Economical p-Xylene and Ethylbenzene Separated from Mixed Xylene. Ind. Eng. Chem. Prod. Res. Deu. 1979, 18(4),263-268. Thamm, H. Calorimetric Study on the State of Aromatic Molecules Sorbed on Silicalite. J . Phys. Chem. 1987, 91, 8-11. Venuto, P. B.; Cattanach, J. Mobil R+D Corporation, private communication, 1971. Wada, M. Reviews of Xylene Isomers Separation Technology. Sek i p Gakkai Shi 1974, 17(5), 364-9. Received for review July 29, 1988 Accepted January 24, 1989
