Ind. Eng. Chem. Res. 1988,27, 1665-1668
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SEPARATIONS Effects of Moisture in Separation of C8 Aromatics Using Medium-Pore Zeolites Tsoung Y.Y a n Mobil Research and Development Corporation, P.O. Box 1025, Princeton, New Jersey 08540 High-silica medium-pore zeolites, such as ZSM-5, are excellent adsorbents for the separation of p-xylene and ethylbenzene from C8aromatics. In addition to high adsorption selectivity and capacity, the separation performance of high-silica ZSM-5 is not affected by the presence of moisture due to its hydrophobicity. The ZSM-5 becomes essentially hydrophobic as its SiOz/Al2O3ratio exceeds -500 (or for an A1203content of 2 w t % (326 and 331), is significantly impaired by moisture. For 326 (A1203 = 4.47 wt %), the hydrocarbon adsorption capacity decreased from 120 to 88 mg/g, as the moisture content was increased by varying PIPo for water from 22 to 97 mg/g. Apparently, the surface of high-A1203ZSM-5 becomes sufficiently hydrophilic, and adsorption of water is preferred to hydrocarbon. The preadsorbed moisture could not be displaced readily and completely by the incoming hydrocarbon, leading to a lower hydrocarbon adsorption capacity. At a certain moisture level, the hydrocarbon adsorption appears to increase slightly with an increase in Si02/A1203 ratio of zeolites (Figure 4) even though the data available are too few to quantify their relationship. The combined adsorption of hydrocarbon and water increases with water adsorption, particularly for ZSM-5 with a high A1203content (Table I). These results suggest that, in addition to competing with hydrocarbons for the adsorption sites, water can be adsorbed on sites which do not adsorb hydrocarbons well. These hydrophilic sites are associated with A1203. 3. Effect of Moisture on Adsorption Pattern of Hydrocarbons. The moisture adsorbed on the zeolites
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1668 Ind. Eng. Chem. Res., Vol. 27, No. 9, 1988
interesting to point out that the moisture makes the lowsilica ZSM-5 nonselective to p-xylene adsorption. These results are consistent with the fact that the separation efficiency of faujasites of low SiO2/AlZO3ratio is sensitive to moisture content (Seko et al., 1979).
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Figure 5. p-Xylene selectivity versus water adsorption of zeolite.
has little effect on adsorption patterns of and capacity for hydrocarbons for high-silica ZSM-5. However, it has a profound impact on low-silica ZSM-5 (Table I). For high-silica ZSM-5 (321 and 343), the presence of moisture seems to increase the relative adsorption of o-xylene to p-xylene slightly (cf. XSE 92 versus 106 and 89 versus 103). However, for the low-silica ZSM-5 (326 and 331), the moisture on the zeolites lowers p-xylene and total hydrocarbon adsorption without affecting adsorption of ethylbenzene and o-xylene (Table I). These results indicate that hydrocarbons prefer different types of sites in the zeolite cavities in agreement with Olson et al. (1981). Other studies suggest that p-xylene is preferentially adsorbed on the intersection of the channels over ethylbenzene and o-xylene (Yan, 1988). Most importantly, the water competes for sites preferred by p-xylene, leading to preferential loss in p-xylene for the low-silica zeolites. 4. Effect of Moisture on p-Xylene Selectivity. Moisture has no detrimental effect on p-xylene selectivity of ZSM-5 with a low A1203content or high Si02/A1203ratio (Figure 5). The p-xylene selectivities for 321 and 343 remained essentially constant and increased only slightly from 4 to 4.5 as the moisture was increased from 14 to 57 mg/g. This is due to the hydrophobicity of the high-silica zeolites and due to the fact that water does not interfere with the adsorption of hydrocarbon and p-xylene, in particular. Because of its insensitivity to water, high-silica ZSM-5 is desirable for p-xylene separation processes with variable water content in the feed. However, moisture is significantlydetrimental to p-xylene selectivityof low-silica ZSM-5. The p-xylene selectivity of 326 (SiOz/Alz03= 38) dropped from 1.71 to 0.61 when the moisture content increased from 22 to 97 mg/g (Table I). It is particularly
Summary Based on equilibrium measurements, the p-xylene selectivity and adsorption capacity of high silica ZSM-5 are not affected by the presence of moisture, in contrast to the conventional faujasites. This unique characteristic makes high-silica ZSM-5 an ideal adsorbent for xylene separation from CB aromatics, particularly in applications where moisture in the feed is variable. Water is adsorbed on the zeolites both chemically and physically. The amount of chemisorbed water decreases sharply with decreasing A1203content, and the zeolite becomes hydrophobic at Si02/A1203of >500. The physisorbed water also decreases linearly with decreasing AZO3 content. Moisture has little effect on total hydrocarbon adsorption of high-silica ZSM-5, but it impairs the hydrocarbon adsorption capacity of low-silica ZSM-5. Moisture has no effect on the relative adsorption of hydrocarbons for high-silica ZSM-5. However, for lowsilica ZSM-5, the moisture competes with and preferentially decreases the adsorption of p-xylene. Due to its hydrophobicity, the p-xylene selectivity and adsorption capacity of high-silica ZSM-5 are not affected by the presence of water. Registry No. PX, 106-42-3; OX, 95-47-6; EB, 100-41-4; MT, 108-67-8; H20,7732-18-5.
Literature Cited Argauer, R. J.; Landolt, G. R. “Crystalline Zeolite ZSM-5 and Method of Preparing the Same”. U S . Patent 3 702 886, November 14, 1972 (to the Mobil Oil Corporation). Chen, N. Y. “Hvdrophobic Properties of Zeolites”. J . Phys. Chem. 1976, 80(1), 60-64. Dwver. “ , F. G.: Jenkins. E. E. “Crvstalline Silicates and Method of Preparing ’Same”. U.S.Paten; 3 941 871, March 2, 1976 (to the Mobil Oil Corporation). Milewski, M. “Separation of Mixtures of Ethylbenzene and Xylene Isomers by Adsorption Methods”. Prezem. Chem. 1981, 60(2), 71-73. Olson, D. H.; Kokotailo, G. T.; Lawton, S. L.; Meier, W. M. “Crystal Structure and Structure-Related Properties of ZSM-5”. J. Phys. Chem. 1981,85, 2238-2243. 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. Wada, M. “Reviews of Xylene Isomer Separation Technology”. Sekiyu Gakkai Shi 1974, 17(5), 364-369. Yan, T. Y. “Separation of p-Xylene and Ethylbenzene From C8 Aromatics Using Medium-Pore Zeolites”. Presented at the AIChE 1988 Spring National Meeting, New Orleans, LA, March 6-10, 1988; paper 44F.
Received for review December 3, 1987 Revised manuscript received April 11, 1988 Accepted April 29, 1988
