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Ind. Eng. Chem. Res. 1998, 37, 604-608
Influence of Oxygenated Contaminants on the Separation of C8 Aromatics by Adsorption on Faujasite Zeolites Alain Methivier Institut Francais du Petrole, 1 et 4, av. de Bois Pre´ au, 92852 Rueil Malmaison Cedex, France
The influence of the formation of oxygenated compounds on the adsorption properties of a Y zeolite were investigated. A procedure was developed which allows the zeolite to age under a flow of toluene or p-xylene containing dissolved oxygen. The separation properties of the sieve were tested by a chromatographic method involving the separation of p-xylene and m-xylene. It was found that aging in the presence of oxygen leads to a dramatic decrease in both the capacity and selectivity of the sieve, while aging in a clean feed leads to stable separation properties. Together with the evolution of the sieve, impurities which are mainly benzaldehyde and water are produced in the feed. Heavy aromatic compounds also occur during aging. The oxidation of toluene to benzaldehyde and the subsequent formation of water may be responsible for the decreased performance of the sieve in the separation of p-xylene from C8 aromatics. Introduction The process of separating xylene isomers by adsorption on X- or Y-exchanged zeolites was developed in the 1960s, and numerous units are running all over the world (Balannec, 1993; Broughton, 1961, 1965). Today, the p-xylene demand is still high and this process remains of interest. It is well-known that one of the keys to the success of the process concerns the stability of the sieve which is very sensitive to impurities in the feeds. Particularly, unsaturated compounds such as olefins are detrimental to the process because they react and may lead to the formation of coke. As mentioned by Yih et al. (1988) benzaldehyde or acetophenone compounds are also known as major impurities of the xylene feeds. These compounds could be formed by an oxidation process of the feed which contains small amounts of oxygen (Sun, 1994; Wei, 1969). The occurrence of these oxygenated compounds may lead to a degradation of the sieve (Yih, 1988). In this paper, we focus on the oxidation of toluene and p-xylene in the presence of oxygen and on the subsequent degradation of the sieve. This study was conducted with an appropriate device which allows the sieve to age in the presence of a feed containing oxygen or an inert gas. The separation properties of the sieve were then studied with a chromatographic apparatus which allowed testing of the zeolite after various aging times. The results, consisting of an analysis of the performances of the sieve and of the evolution of the composition of the aging feed, are discussed in order to explain this degradation. Experimental Section Sieve. Faujasite exchanged zeolite pellets were used as a starting material for this study. The samples consisted of faujasite zeolite crystals formed into spherical pellets of diameter 0.3-0.8 mm. The pellets were loaded in a stainless steel column which was used both for aging of the sieve and for the chromatographic experiments. Prior to aging, the column was dehydrated at 400 °C for 10 h in a nitrogen flow.
Figure 1. Aging device.
Sieve Aging. The influence of various poisoning impurities may be studied with the apparatus which is described in Figure 1. The experiment involves circulating a toluene or p-xylene feed on the column which contains the sieve. This column is maintained at the required temperature by a furnace. Gas injection allows the feed to be saturated with a gas which is first dehydrated with 3 Å molecular sieve. This gas may be either helium for removing the oxygen molecules from
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Ind. Eng. Chem. Res., Vol. 37, No. 2, 1998 605
Figure 3. Breakthrough curve obtained with a sieve before aging. Table 2. Feed Composition Used for Adsorbent Characterization p-xylene m-xylene isooctane
Figure 2. Testing device. Table 1. Description of the Various Kinds of Aging Experiments experiment
description
A
toluene feed without occluded oxygen circulating on the sieve column toluene or p-xylene feed saturated with oxygen circulating on the sieve column toluene feed saturated with oxygen circulating on an empty column
B C
the feed or air for saturating the toluene feed with oxygen. The system was operated at 15 bar and 200 °C. Various kinds of aging tests were performed as described in Table 1. Chromatographic tests were then performed after various aging times. Samples of the circulating feed were also collected for analysis. Sieve Testing. A dynamic adsorption unit was used for sieve characterization. This apparatus is described in Figure 2 and is very similar in principle to those described in the literature (De Rosset, 1976; Furlan, 1992). It allowed the sieve to be tested by separating a xylene feed. During the experiment the whole column was maintained at 20 bar and 150 °C in a furnace. The feed sample whose composition appears in Table 2 was injected in the adsorbent column which initially contained a solvent. The feed is injected until it moves
weight (%)
purity
45 45 10
99.8 99.8 99.9
manufacturer Fluka Fluka Haltermann
through the column with a constant outlet concentration. A Gilson (201) sample collector allowed collection of the liquid fractions which were analyzed by a Hewlett Packard (5890) gas chromatograph with a FID detector. The analytical column was a FFAP, and the analyses were carried out at 90 °C. The concentrations at the outlet of the column could then be plotted as fractional concentrations as a function of elution time. Since the paraffinic tracer is weakly adsorbed in the presence of xylenes, it emerges first and establishes the void volume of the bed. The amount of solvent recovered establishes the total volume of the bed, and the microporous volume may be obtained by subtracting the void volume from the total volume. A mass balance allows the establishment of the selectivity for p-xylene which may be expressed as
RPX/MX ) (YPXXMX)/(YMXXPX) where RPX/MX is the sieve selectivity for p-xylene, XMX and XPX are the mass fractions of m-xylene and p-xylene in the liquid phase, and YMX and YPX are the mass fractions of m-xylene and p-xylene in the adsorbed phase. Liquid Phase Analysis. The analysis of the liquid phase was conducted in order to determine the presence of contaminants in the liquid feed and to measure their concentrations as a function of aging time. The analyses were performed by gas chromatography (PONA column) associated with a mass spectrometer to identify the various contaminants. The water content in the feeds was qualitatively obtained from Panametrics probes. Results Sieve Aging. Sieve-aging experiments were conducted with the apparatus described in the previous section with a toluene feed and with a xylene feed which was either free of oxygen or saturated with oxygen according to procedures A and B described in Table 1. The sieve characterization was carried out before and after aging. Figures 3 and 4 give two breakthrough curve examples. Figure 3 refers to an adsorbent column
606 Ind. Eng. Chem. Res., Vol. 37, No. 2, 1998
Figure 4. Breakthrough curve obtained with a sieve aged for 200 h with B experiment.
Figure 6. Relative selectivity evolution as a function of time for A and B experiments. Table 3. Various Compounds Identified in the Feed Stream molecular weight experiment
compound benzene benzaldehyde benzylic alcohol (phenylmethyl)toluene heavy aromatics water
MW ) 106 MW ) 108 MW ) 182 MW > 400
comment
B B, C C B, C
very few meta and ortho occur as well as para
B, C B, C
Table 4. Formula of Various Aromatic Impurities compound
Figure 5. Relative capacity evolution as a function of aging time for A and B experiments.
before aging, and Figure 4 refers to the same column aged for 200 h with the B procedure. The comparison of the two figures immediately shows a degradation of the sieve, as shown by a smaller area between the m-xylene and the p-xylene curves in Figure 4 than in Figure 3. Sieve-aging experiments were conducted with the apparatus described in the previous section with a toluene feed and with a xylene feed which was either free of oxygen or saturated with oxygen according to procedure A and B described in Table 1. The sieve characterization was performed before and after aging. These experiments have been carried out with procedures A and B, and the degradation was quantified for increasing aging time. Figures 5 and 6 represent the changes of capacity and selectivity (in percentage of the initial performances) as a function of time for experiments A and B. The results obtained with a toluene feed and with a p-xylene feed are reported in these figures. Figure 5 exhibits a decrease of the adsorption capacity of the sieve for experiment B for toluene as well as for p-xylene while there is no significant change for A. The same trend may be observed in Figure 6 which shows that the selectivity is dramatically affected by the presence of oxygen in the feed. Taking into account both selectivity and capacity, the separation performances are decreased by 25% for a toluene feed and by 30% for a p-xylene feed after 200 h aging in the presence of oxygen.
formula
toluene
CH3
benzadehyde
CHO
benzylic alcohol
CH2OH
(phenylmethyl)toluene
CH3
CH3 CH3
Aging Feed Concentration Evolution. The aging toluene feed composition was studied as a function of aging time for the three experimental paths (A, B, and C). The analysis were carried out by gas chromatography, and components were identified by mass spectroscopy as described in the Experimental Section. Table 3 gives the compounds obtained for the three kinds of experiments, and Table 4 gives the formulas of the various compounds. It may be noticed that the presence of oxygen during aging (experiments B and C) leads to oxygenated compounds as well as to heavy compounds (MW ) 182, MW > 400). Moreover, it appears that the benzylic
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Discussion
Figure 7. Impurity concentration evolution as a function of time for B experiment.
Figure 8. Impurity concentration evolution as a function of time for C experiment.
alcohol occurs only when there is no sieve in the system (C), while benzaldehyde occurs in both cases (B and C). The heavier nonoxygenated compounds have been divided into two families. The first family, whose molecular weight is 182, is composed of the three isomers (para, meta, and ortho) of (phenylmethyl)toluene. The second family is composed of heavier compounds whose molecular weight is over 400. At least, we may notice the occurence of water. We have studied the changes of the concentrations of the major aromatic impurities as a function of time for both experiments B and C. The results appear in Figures 7 and 8. In the experiment B, the major impurities are the heavy nonoxygenated compounds (MW ) 182 and MW > 400) while benzaldehyde concentration remains very low. Water concentration was also qualitatively studied and shows the same trend, i.e., to an increasing concentration as a function of aging time. Values of approximately 100 ppm were obtained after aging for 200 h. For experiment C, the major impurities are the oxygenated compounds (benzaldehyde and benzylic alcohol) while the heaviest aromatic compound (MW > 400) remain at a very low level. As aging in a p-xylene feed also leads to a decreasing in capacity and selectivity, the aging feed concentration evolution was also studied. The same trend was observed with the formation of methylbenzaldehyde instead of benzaldehyde.
These results show that the aging of the sieve with toluene (or other aromatic compounds such as p-xylene) which contains oxygen leads to the formation of impurities whose presence may be related to the degradation of the separation properties. The key impurities are probably the oxygenated compounds (benzaldehyde and benzylic alcohol) which are usually observed (Sun, 1994; Wei, 1969). Benzaldehyde, particularly, whose concentration is constant for experiment B and increasing for experiment C, could act as a reaction intermediate for the formation of heavier compounds in the presence of the sieve (B). This fact could be an explanation for the constant concentration of this compound in B aging. Another explanation could be the strong adsorption of benzaldehyde on the sieve (Yih and co-workers, 1988). In this study, they reported the adsorption of acetophenone (aldehyde corresponding to ethylbenzene oxidation) in faujasites. They found that the amount adsorbed for 0.1% acetophenone is 0.01 cm3/g which is not negligible compared to the total adsorption capacity of the sieve. This observation could be still valid for the explanation of the absence of benzylic alcohol in experiment B. This component, strongly adsorbed in experiment B, could be revealed only in the absence of sieve, i.e., in experiment C. This observation is confirmed by the work of Sun et al. (1994) in their paper concerning the oxidation of toluene. They found that the oxidation leads selectively to benzaldehyde formation in the presence of calcium- or barium-exchanged Y zeolite. The presence of the sieve together with oxygen seems to be responsible for the production of heavier compounds (MW ) 182 and MW > 400). However, compounds whose molecular weight is 182 are also produced in experiment C but surprisingly only one of the three isomers is present. So, one isomer is selectively produced when there is no sieve. The formation of heavy compounds is more difficult for experiment C, i.e., without the sieve. So, the sieve leads to the occurrence of heavy compounds (MW > 400) and is probably related to catalytic effects. These phenomena may, in part, lower the separation performances of the sieve by decreasing the capacity and modifying the selectivity. Moreover, the presence of water in the feed may also be responsible for decreasing both the capacity and selectivity as it is well-known that the preadsorption of polar molecules may greatly affect the selectivities of faujasites for xylenes (Furlan, 1992). These phenomenon may also occur by aging in the presence of xylenes instead of toluene and indicate a very general behavior. Conclusion This study showed that the presence of oxygen in the feeds may dramatically affect the separation performances of an exchanged faujasite used in the separation of xylenes. Particularly, a decrease of both capacity and selectivity is observed during aging of the sieve in a toluene or p-xylene feed containing oxygen while helium has no effect. This phenomenon is caused by the oxidation of toluene and the subsequent formation of heavy compounds which strongly adsorb onto the surface. These strongly adsorbed species lead to a decrease in capacity and selectivity which greatly affect the separation performances.
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Literature Cited Balannec, B.; Hotier, G. From batch elution to simulated counter current chromatography. In Preparative Scale Chromatography, Chromatographic Science Series 301; Ganestos, G., Barker, P. E., Eds.; Marcel Dekker: New York, 1993; Vol. 6. Broughton, D. B. (U. O. P. Corp.). U.S. Patent 2,985,589, 1961. Broughton, D. B. (U. O. P. Corp.). U.S. Patent 3,204,247, 1965. De Rosset, A. J.; Neuzil, R. W.; Korous, D. J. Liquid Column Chromatography as a Predictive Tool for Continuous Countercurrent Adsorption Separations. Ind. Eng. Chem. Process Des. Dev. 1976, 15 (2), 261. Furlan, L. T.; Chaves, B. C.; Santana, C. C. Separation of Liquid Mixtures of p-Xylenes and o-Xylenes in X Zeolites: The Role of Water Content on the Adsorbent Selectivity. Ind. Eng. Chem. Res. 1992, 31, 1780. Sun, H.; Blatter, F.; Frei, H. Selective Oxydation of Toluene to Benzaldehyde by O2 with Visible Light in Barium and Calcium Exchanged Zeolite Y. J. Am. Chem. Soc. 1994, 116, 7951.
Wei, K. S.; Adelman, A. H. The photooxydation of Toluene: The Role of an Excited Charge Complex Transfer. Tetrahedron Lett. 1969, 38, 3297. Yih, S. M.; Hsiao, H. C. The Effect of Contaminants on Xylene Separation by Zeolite Adsorption. Adsorpt. Sci. Technol. 1988, 5 (2), 116.
Received for review October 7, 1996 Revised manuscript received September 24, 1997 Accepted September 24, 1997X IE9606294
X Abstract published in Advance ACS Abstracts, November 15, 1997.
