Adsorption Equilibrium of Xylene Isomers and p-Diethylbenzene on a

Several studies on adsorption isotherms or on coadsorption of xylene isomers in the liquid or in the gaseous phase on NaY, BaY, BaX,3-8KY,9,10 and Kba...
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Ind. Eng. Chem. Res. 2001, 40, 5983-5990

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Adsorption Equilibrium of Xylene Isomers and p-Diethylbenzene on a Prehydrated BaX Zeolite H. Tournier, A. Barreau,* B. Tavitian, and D. Le Roux Department of Applied Chemistry and Physical Chemistry, Institut Franc¸ ais du Pe´ trole, 92852 Rueil Malmaison Cedex, France

J.-C. Moı1se, J.-P. Bellat, and C. Paulin Laboratory of Researches on Reactivity of Solids, University of Bourgogne, Dijon, France 21078

The present work deals with the study of adsorption equilibria of xylenes and p-diethylbenzene on a prehydrated BaX zeolite at 175 and 130 °C. The adsorption isotherms of single components recorded by a thermogravimetric method show that the adsorption affinity of the zeolite and the adsorbate/adsorbent interaction energy at low loading are about the same for the three aromatics and that, at 175 °C, the adsorption capacities are the same for the three components. The adsorption of binary and ternary mixtures in the liquid phase has been performed by a chromatographic method. The results show that the selectivity depends on the composition of the liquid phase and on the temperature. Moreover, for the two mixtures p-xylene/m-xylene and p-diethylbenzene/m-xylene, the effect of temperature appears only on the preferentially adsorbed component, p-xylene and p-diethylbenzene, respectively. Tentative interpretations in terms of adsorption sites are proposed. Last, the Langmuir-Freundlich model describes the variation of the selectivity and the adsorbed amounts with the composition of the liquid phase in the binary mixtures. The extension of the model to ternary mixtures provides very qualitative results. 1. Introduction The petrochemistry of benzene, toluene, and xylenes (BTX) undergoes a strong economic development, the main reason being the increasing use of p-xylene. p-Xylene is the starting material for the synthesis of polyesters. Thus, separation of p-xylene from C8 isomers is one of the classical separation problems in the petrochemical industry. The most attractive industrial technique for separating p-xylene is selective adsorption on molecular sieves. The most frequently used adsorbent are faujasite-type zeolites, among which prehydrated BaX zeolite holds an important place. The separation is generally performed by adsorption in the liquid phase, using a simulated moving bed technology at a temperature close to the boiling point of xylenes (∼150 °C).1,2 The selective adsorption process is relatively complex, and the quality of the separation depends on many chemical and physical parameters. Thus, the improvement of the separation process requires often a very complete database, a better understanding of the adsorption process, and a suitable thermodynamic model. Measurements of adsorption equilibria provide databases from which correlations can be developed and models fitted. Several studies on adsorption isotherms or on coadsorption of xylene isomers in the liquid or in the gaseous phase on NaY, BaY, BaX,3-8 KY,9,10 and KbaY11 are available in the literature. These studies have shown that the faujasite zeolite may be selective for one isomer or the other, depending on the nature of the exchanged cations, the loading of the adsorbent, the composition of the mixture, and the presence and amount of preadsorbed water in the zeolite.6,12 The work presented here deals with the study of adsorption equilibria of xylene para and meta isomers and of p-diethylbenzene on a prehydrated BaX zeolite

at 175 and 130 °C. The adsorption isotherms of single components have been recorded in the gaseous phase by a thermogravimetric method, and the adsorption equilibria of binary and ternary mixtures have been recorded in the liquid phase by a chromatographic method. 2. Experimental Section 2.1. Materials. The BaX zeolite, used in the form of extruded pellets, is a lab scale zeolite prepared by cation exchange from NaX. The Si/Al ratio is 1.54, yielding a unit cell formula: [Ba32(Al02)76(SiO2)116]. Because of the large amount of binder, the Dubinin volume is 0.216 cm3/g as measured by nitrogen adsorption at 77 K.The adsorbent water content is 5.7% in weight. 2.2. Thermogravimetry. Thermogravimetry under controlled vapor pressure and temperature is used to record the adsorption isotherms of single gaseous aromatics on the prehydrated BaX zeolite. The experimental equipment is a McBain balance. The adsorption isotherm is measured step by step using a static method. Once a plateau of mass is recorded, the next equilibrium is reached by changing the pressure of the cold point. The mass of the BaX sample is 15 mg. Before each experiment, the zeolite is activated in situ at 400 °C under 10-2 Pa during 12 h. The heating rate is low enough (about 2 °C/min) to avoid a partial dealumination of the BaX zeolite by streaming. The range of investigated pressure, controlled with a MKS Baratron absolute pressure transducer (10-2 to 1000 Pa), lies from 10 to 1000 Pa for xylenes and to 400 Pa for pdiethylbenzene, and the adsorption isotherms are measured at 130 and 175 °C, with the temperature being controlled by a chromel-alumel thermocouple located

10.1021/ie0011371 CCC: $20.00 © 2001 American Chemical Society Published on Web 11/10/2001

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near the sample. After activation, the temperature of the sample is decreased to 90 °C under vacuum. Prehydration of the BaX zeolite is then performed by introducing water vapor in the balance in order to obtain a water content of about 10% in weight. The sample is submitted to vacuum, and its temperature is increased to the adsorption temperature. During this step, water is desorbed, but the rate of desorption is low because water is strongly adsorbed on BaX zeolite. As the water content reaches around 5.7% in weight in the zeolite, the vacuum is cut. Water being strongly adsorbed, the water pressure above the zeolite is then negligible. Then the adsorption of the aromatic component is carried out by successive increments of pressure in the balance. For each pressure, the adsorbed amount is measured by weighing and recorded. The experimental accuracy is (0.02 mg for the mass of adsorbate, (0.5 °C for the temperature, and (10 Pa for the pressure. 2.3. Chromatographic Method. 2.3.1. Experimental Setup. A chromatographic method is used to study the adsorption of aromatic mixtures in the liquid phase on the BaX zeolite. Adsorption equilibria are obtained by an entirely automated equipment, which consists of an experimental and an analytical part. This equipment and the experimental method have been thoroughly described.11 The experimental part is composed of a thermostated air bath allowing to work in a large range of temperatures (from 50 to 250 °C). Inside the air bath are two cells, called “zeolite cell” and “liquid-phase cell”. The zeolite cell, with a volume of about 30 cm3 and two metal seals, is made of stainless steel and contains the molecular sieve. The liquid-phase cell, equipped with a stirrer, has a volume of about 150 cm3. Temperature is measured by a PT100 probe, with an accuracy of (0.1 °C. The airtightness is ensured by a metal seal. On the top of the cell, an opening allows introduction of the components of the mixture. The allowed working conditions are 200 °C and 30 bar. A circulation pump, located outside the air bath, is installed to circulate the mixture through the cells and through a bypass. The heating device, the general power supply, and the computer, which allow us to carry out and to monitor the experiment, are placed outside the air bath. The analytical part consists of a gas chromatograph (GC) Hewlett-Packard 5890 equipped with a splitless injector, a flame ionization detector and a polar capillary column (HP FFAP, L ) 0.2 mm, length ) 50 m). An on-line sampling valve between the GC and the circuit is devoted to the analysis of the residual liquid phase with the gas chromatograph. 2.3.2. Experimental Conditions. The mass of BaX sample is about 20 g. Before each experiment, new zeolite with a water mass content of 5.7% is introduced in the zeolite cell. Adsorption equilibria of binary and ternary aromatic mixtures on the prehydrated BaX zeolite are measured at 130 and 175 °C. The experiments are carried out with an inert solvent component, isooctane, and with a quantity of aromatics sufficient to saturate the zeolite. The total mass of aromatics is about 10 g, and the mass ratio isooctane/aromatics is about 90/10. The chromatographic method is based on the use of an inert component to perform mass balance calculations in order to have access to the composition of the adsorbed phase. At saturation, isooctane is not

Figure 1. Adsorption isotherms of single aromatics on prehydrated BaX zeolite at 130 °C. [(b), p-xylene; (9), m-xylene; (2), p-diethylbenzene].

adsorbed, and it has no selectivity toward the aromatics.11 The experimental accuracies are (0.2 mg for each weighing, (0.02% for the mass fractions, and thus (0.2 for the experimental selectivity. 2.3.3. Experimental Procedure. After placing the zeolite cell filled with adsorbent in the thermostated air bath, the whole equipment is filled with nitrogen. Then, the components of the mixture are introduced into the liquid-phase cell. Their masses are known exactly by weighing. The liquid phase is then mixed and moved through a bypass by means of the circulation pump. A chromatographic analysis of the liquid phase before adsorption is performed in order to calculate the response factor of each component of the mixture. The step of adsorption begins with the heating and the stabilization of the air bath at the first temperature chosen for the adsorption equilibrium. The bypass is then closed, and the liquid phase is circulated through the zeolite cell for 2 h. Several circulation times were tested, and it was found that after 2 h of circulation the adsorption equilibrium is reached. After this delay, four chromatographic analyses of the liquid phase at equilibrium are realized. The time between each analysis is about 10 min. Knowing the initial mass composition of the liquid phase and the final mass composition of the residual liquid phase, the mass composition of the adsorbed phase can be calculated by mass balance calculations. Then, the step of adsorption starts again with the heating of the air bath at the second temperature chosen. 3. Results and Discussion 3.1. Adsorption of Single Aromatics. The adsorption isotherms of single aromatics (p-xylene, m-xylene, and p-diethylbenzene) on prehydrated BaX zeolite at 175 and 130 °C are shown in Figures 1 and 2. Experimental data are reported in the Supporting Information (Tables 1 and 2). As observed on anhydrous X and Y zeolites, the adsorption isotherms have the type I shape of the IUPAC classification,13 which is characteristic of the adsorption in a microporous solid. At very low pressure (p < 20 Pa), the adsorption follows Henry’s law. The values of the Henry constants are given in Table 1. The deviations on these values are quite large (around 10%) because an important and abnormal deviation on the

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Figure 2. Adsorption isotherms of single aromatics on prehydrated BaX zeolite at 175 °C. [(b), p-xylene; (9), m-xylene; (2), p-diethylbenzene]. Table 1. Henry Constants for the Adsorption of p-Xylene, m-Xylene and p-Diethylbenzene on Prehydrated BaX Zeolite at 130 and 175 °C KH (g/g of zeolite /Pa) 130 °C 175 °C

p-xylene

m-xylene

p-diethylbenzene

0.006 0.0035

0.006 0.0024

0.0057 0.0033

adsorbed amount is observed in the low-pressure region. Indeed, the relative deviation on the adsorbed amount reaches 20% for pressures lower than 5 Pa. This poor accuracy can be attributed to either an adsorption of gas on the stainless wells of the balance, molecular interactions between water and aromatics in the gaseous phase, or a partial desorption of water by aromatic hydrocarbons. For these reasons, it would be very hazardous to analyze these Henry constants in more detail. However, it can be noticed that the Henry constants are of the same order of magnitude for the three aromatics at constant temperature. This means that the adsorption affinity of the zeolite and the adsorbate/adsorbent interaction energy at low loading is quite the same for the xylene isomers and p-diethylbenzene. Obviously, as the temperature increases, the adsorption affinity is weakened, and consecutively, the Henry constants decrease. At 175 °C, no significant differences are observed in the adsorption capacities of p-xylene, m-xylene, and p-diethylbenzene. The adsorption isotherm of p-xylene is superimposed on that of m-xylene in the whole range of pressures investigated. This result has already been observed at 150 °C with anhydrous BaY and BaX zeolites and partially hydrated BaX zeolite which were in the form of pure crystalline powder.3,14 With regard to p-diethylbenzene under 400 Pa, the adsorption capacity is slightly higher than those of xylene isomers. At 130 °C, more significant differences are observed in the adsorption capacities. The adsorption isotherm of p-xylene lies above that of m-xylene in the whole range of pressures and above that of p-diethylbenzene at the plateau of the isotherms, as the filling of the micropores is almost complete. It is thus the less bulky component which is the more adsorbed. At low loading, the adsorption quantities of p-xylene and p-diethylbenzene are comparable, and the Henry constant of mxylene is sensibly lower than those of the two other components. These results suggest that some of the adsorption sites accessible to p-xylene and p-diethyl-

benzene could be inaccessible to m-xylene. The maximum adsorption capacity would thus be limited by steric hindrance. Indeed, with its methyl groups in the meta position, m-xylene is a more bulky molecule with a nonsymmetrical geometry and a dipolar moment. The effect of this distribution between adsorption sites of different interaction energies would decrease as the temperature rises to 175 °C. This tentative interpretation agrees with previous work performed by other techniques. A large number of microscopic measurements such as neutron diffraction, calorimetric, and molecular simulation studies15-21 have been carried out on p-xylene/m-xylene pure compound or mixture adsorption in faujasite zeolites. Neutron diffraction experiments on prehydrated BaX zeolite at high loading18,20 and at low loading15,17 have shown that whatever the loading there is only one type of adsorption site for metaxylene, which is located in the supercages close to the compensation cations in cristallographic position II, in front of the hexagonal windows of the sodalite cages. The interaction energy of a metaxylene molecule adsorbed in this position with the zeolite is important. For paraxylene, two adsorption sites have been found: the first is the same as that for metaxylene molecules, facing site II cations, and corresponds to a high interaction energy with the zeolite. It is the only site populated at low loading. At high loading, another site was found to be populated as well. It is located close to the 12-ring window separating two supercages of the zeolite. A paraxylene molecule adsorbed in this site does not develop strong interactions with a baryum cation; the interaction energy with the zeolite is thus sensibly lower. The results on the interaction energies have been fairly confirmed by molecular simulation studies19,21 and calorimetry.14 To our knowledge, no similar studies on p-diethylbenzene adsorption in faujasite-type zeolites have been carried out. However, we can make a remark similar to that formulated for p-xylene and assume the existence of one preferential adsorption site and one less favorable adsorption site for p-diethylbenzene, the latter being more sensitive to a variation of temperature. 3.2. Adsorption of Binary Mixtures. The adsorption equilibria of three aromatic mixtures on prehydrated BaX zeolite were studied at 130 and 175 °C: p-xylene/m-xylene, p-diethylbenzene/m-xylene, and pdiethylbenzene/p-xylene. Each adsorption equilibrium was characterized by the adsorption selectivity for component i relative to component j:

Rij )

xi yj xj yi

(1)

where xi and xj and yi and yj are the molar fractions of components i and j in the adsorbed phase and the liquid phase, respectively. For each mixture, the aromatics were diluted in isooctane. In this section, all concentrations are given on a solvent-free basis. Experimental results for adsorption equilibria of the three binary mixtures on the prehydrated BaX zeolite at 130 and 175 °C are reported in the Supporting Information (Tables 3-5). Experimental selectivities of the three binary mixtures versus composition of the liquid phase are plotted in Figures 3-5. Figures 6-8 represent the variation of the adsorbed amounts with the composition of the liquid phase in the different aromatic mixtures.

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Figure 3. p-Xylene/m-xylene binary selectivities on prehydrated BaX zeolite at 175 (b) and at 130 °C (O) [Solid line, LangmuirFreundlich model at 175 °C; dotted line, Langmuir-Freundlich model at 130 °C].

Figure 4. p-Diethylbenzene/m-xylene binary selectivities on prehydrated BaX zeolite at 175 (b) and at 130 °C (O). [Solid line, Langmuir-Freundlich model at 175 °C; dotted line, LangmuirFreundlich model at 130 °C].

Figure 6. Variation of the adsorbed amounts with the composition of the liquid phase for the p-xylene/m-xylene mixture on prehydrated BaX zeolite at 175 (full symbols) and 130 °C (empty symbols). [Circles, p-xylene; triangles, m-xylene; squares, total. Solid line, Langmuir-Freundlich model at 175 °C; dotted line, Langmuir-Freundlich model at 130 °C.]

Figure 7. Variation of the adsorbed amounts with the composition of the liquid phase for the p-diethylbenzene/m-xylene mixture on prehydrated BaX zeolite at 175 (full symbols) and 130 °C (empty symbols). [Circles, p-diethylbenzene; triangles, m-xylene; squares, total. Solid line, Langmuir-Freundlich model at 175 °C; dotted line, Langmuir-Freundlich model at 130 °C.]

Figure 5. p-Diethylbenzene/p-xylene binary selectivities on prehydrated BaX zeolite at 175 (b) and at 130 °C (O). [Solid line, Langmuir-Freundlich model at 175 °C; dotted line, LangmuirFreundlich model at 130 °C].

The results show that at constant temperature and whatever the mixture the binary selectivity depends on the composition of the mixture. This variation of the selectivity is very moderate in the p-xylene/m-xylene mixture whatever the temperature, whereas it is more important in the p-diethylbenzene/m-xylene mixture, mainly in the range of contrasting concentrations. At 130 and 175 °C, the p-xylene/m-xylene and the pdiethylbenzene/m-xylene binary selectivities are greater than unity in all of the range of concentrations, which means that the prehydrated BaX zeolite is selective for p-xylene and p-diethylbenzene, respectively. However, at constant temperature, the p-diethylbenzene/m-xylene selectivities are higher (between 3.5 and 8.5) than the p-xylene/m-xylene selectivities (around 3).

Figure 8. Variation of the adsorbed amounts with the composition of the liquid phase for the p-diethylbenzene/p-xylene mixture on prehydrated BaX zeolite at 175 (full symbols) and 130 °C (empty symbols). [Circles, p-diethylbenzene; triangles, p-xylene; squares, total. Solid line, Langmuir-Freundlich model at 175 °C; dotted line, Langmuir-Freundlich model at 130 °C.]

For the p-diethylbenzene/p-xylene mixture, the selectivity is around unity, between 0.9 and 1.9, whatever the temperature. The prehydrated BaX zeolite does not seem to be very selective for one component of the mixture or the other. As with the two other mixtures, the p-diethylbenzene/p-xylene selectivity depends on the composition of the mixture: it decreases very slightly when the molar fraction in p-xylene increases in the mixture, and this, mainly at 130 °C.

Ind. Eng. Chem. Res., Vol. 40, No. 25, 2001 5987 Table 2. Experimental Data for the p-Diethylbenzene/p-Xylene/m-Xylene Mixture on Prehydrated BaX Zeolite at 175 and 130 °C temp (°C)

175

130

yPDEB

yp-xyl

0.0637 0.0808 0.0987 0.1413 0.1898 0.2365 0.2490 0.3289 0.3381 0.4053 0.4222 0.6378 0.7478 0.8353 0.0615 0.0830 0.1035 0.1394 0.1988 0.2389 0.2408 0.2543 0.3314 0.3339 0.4111 0.4188 0.6304 0.7410 0.8316

0.0693 0.3800 0.7634 0.1596 0.5563 0.2628 0.3631 0.2829 0.1838 0.4493 0.2498 0.1648 0.1137 0.0740 0.0635 0.3642 0.7562 0.1499 0.5426 0.2518 0.3153 0.3502 0.2748 0.1801 0.4420 0.2470 0.1682 0.1172 0.0762

adsorbed amounts (Qi in g/g of zeolite) ym-xyl PDEB p-xyl m-xyl 0.8670 0.5393 0.1379 0.6991 0.2540 0.5006 0.3879 0.3882 0.4781 0.1453 0.3280 0.1974 0.1385 0.0907 0.8750 0.5528 0.1403 0.7107 0.2586 0.5093 0.4439 0.3955 0.3939 0.4860 0.1469 0.3342 0.2014 0.1418 0.0921

0.0213 0.0185 0.0150 0.0389 0.0312 0.0535 0.0487 0.0659 0.0735 0.0631 0.0782 0.1020 0.1074 0.1148 0.0223 0.0186 0.0144 0.0402 0.0301 0.0545 0.0515 0.0490 0.0672 0.0764 0.0639 0.0815 0.1071 0.1132 0.1195

0.0190 0.0705 0.1021 0.0346 0.0814 0.0443 0.0554 0.0395 0.0269 0.0501 0.0307 0.0162 0.0102 0.0062 0.0209 0.0767 0.1087 0.0379 0.0877 0.0488 0.0524 0.0608 0.0432 0.0288 0.0546 0.0328 0.0162 0.0099 0.0060

Temperature plays an important role on the adsorption properties of the zeolite. Of course, adsorption being an exothermic phenomenon, an increase of temperature entails a decrease of the total adsorption capacity. Moreover, at constant temperature and for the two binary mixtures p-xylene/m-xylene and p-diethylbenzene/m-xylene, the total adsorption capacity depends slightly on the composition of the mixture: it decreases when the mixture becomes impoverished in the preferentially adsorbed component, p-xylene and p-diethylbenzene, respectively (Figures 6 and 7). For the p-xylene/m-xylene and p-diethylbenzene/mxylene mixtures with not too contrasting compositions, a decrease of temperature entails an increase of the binary selectivity. Moreover, in these two mixtures, the effect of temperature appears only on the preferentially adsorbed component, p-xylene and p-diehtylbenzene, respectively. Only the adsorbed amounts of these components vary with temperature. In light of results obtained for p-xylene/m-xylene mixtures adsorption in various faujasites by neutron diffraction16,18 and molecular simulation,19,21 this observation can be interpreted in much the same way as has been explained in the preceding section for pure compounds adsorption. Whereas m-xylene molecules are always adsorbed in front of extraframework cations in crystallographic position II, whatever the loading or the composition, at high loading or for mixtures rich in p-xylene or pdiethylbenzene, another adsorption site is available to these two latter compounds, close to the 12-ring windows separating two supercages. Being less energetic than the site in crystallographic position II, it would thus be more sensitive to an increase of temperature. To our knowledge, no similar studies on p-diethylbenzene/m-xylene mixtures adsorption in faujasite zeolite have been carried out. However, as the same kind of behavior is observed, we can suppose an interpreta-

0.0739 0.0288 0.0055 0.0433 0.0106 0.0228 0.0164 0.0141 0.0179 0.0043 0.0108 0.0048 0.0030 0.0019 0.0710 0.0293 0.0058 0.0450 0.0109 0.0241 0.0197 0.0170 0.0151 0.0183 0.0047 0.0114 0.0048 0.0029 0.0020

total

Rp-xyl/m-xyl

selectivity RPDEB/m-xyl

RPDEB/p-xyl

0.1142 0.1178 0.1226 0.1168 0.1231 0.1206 0.1206 0.1195 0.1182 0.1175 0.1197 0.1230 0.1206 0.1229 0.1212 0.1245 0.1288 0.1231 0.1287 0.1274 0.1236 0.1268 0.1255 0.1235 0.1232 0.1257 0.1280 0.1260 0.1274

3.21 3.48 3.32 3.50 3.52 3.70 3.62 3.84 3.91 3.77 3.72 4.07 4.18 3.91 3.66 3.98 3.49 4.00 3.83 4.09 3.75 4.05 4.10 4.25 3.85 3.91 4.07 4.19 3.66

3.11 3.40 2.99 3.52 3.12 3.93 3.67 4.35 4.60 4.16 4.44 5.25 5.31 5.10 3.21 3.34 2.66 3.61 2.84 3.81 3.82 3.56 4.18 4.80 3.84 4.52 5.70 6.00 5.33

0.97 0.98 0.90 1.00 0.89 1.06 1.01 1.13 1.18 1.10 1.19 1.29 1.27 1.30 0.88 0.84 0.76 0.90 0.74 0.93 1.02 0.88 1.02 1.13 1.00 1.16 1.40 1.43 1.46

tion similar to that proposed above for p-xylene/mxylene mixtures. Last, one can observe that the influence of temperature on the p-diethylbenzene/p-xylene selectivity is less pronounced. We can underline that an increase of temperature causes a slight decrease of the binary selectivity when the mixture becomes impoverished in p-xylene, but the variations of the selectivity are within the range of experimental accuracies. Moreover, the adsorbed amounts of p-xylene and p-diethylbenzene depend on the temperature. This seems coherent with the adsorption mechanism proposed: when the temperature increases, the population of the less energetic adsorption sites, occupied with p-xylene and p-diethylbenzene, decreases. 3.3. Adsorption of Ternary Mixtures. To have a full database on adsorption equilibria of aromatics on prehydrated BaX zeolite, we have studied p-diethylbenzene/p-xylene/m-xylene ternary mixtures in which one of the initial mass ratios p-xylene/m-xylene, p-diethylbenzene/p-xylene, or p-diethylbenzene/m-xylene is close to unity. All of the compositions of ternary mixtures studied are plotted in a ternary diagram (Figure 9). The definition of the binary selectivity is the same as that in the binary mixtures. Experimental values for adsorption equilibria of ternary mixtures are reported in Table 2. The binary selectivities A/B versus the molar fraction in component C in the mixture for a constant molar ratio A/B are plotted in Figures 10 and 11. The results allow us to evaluate the influence of the third component on the binary selectivity, through a comparison of the measures in binary mixtures with those in ternary mixtures. It appears that the influence of the temperature on the selectivity is not very pronounced. Influence of p-Diethylbenzene on p-Xylene/m-Xylene Selectivity. At 175 °C, the p-diethylbenzene has entailed

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obtained in the ternary mixture depends very slightly on temperature. Whatever the temperature, it decreases from 4.8 to 3 when the concentration in p-xylene rises. The prehydrated BaX zeolite is thus always selective for p-diethylbenzene, but the values of selectivities are much lower than those measured in the p-diethylbenzene/m-xylene mixture. In the binary mixture for a same molar ratio p-diethylbenzene/m-xylene, the selectivity was around 6 at 175 °C and 8 at 130 °C. Finally, p-xylene has entailed a decrease of the p-diethylbenzene/m-xylene selectivity. Influence of m-Xylene on the p-Diethylbenzene/pXylene Selectivity. Whatever the temperature and whatever the concentration in m-xylene, the p-diethylbenzene/ p-xylene selectivity is always around unity, as in the binary mixture. Figure 9. Molar compositions of binary and ternary mixtures.

4. Modeling In this section, the adsorption equilibria of aromatic mixtures are described by the Langmuir-Freundlich model.22 The binary data are used to fit the model parameters. Then, from the parameters values of the model, the adsorption equilibria of ternary mixtures could be predicted and compared to the experimental data. The general Langmuir-Freundlich-type equation for calculating the competitive adsorption equilibria in the liquid phase is Figure 10. Binary selectivities in ternary mixtures on prehydrated BaX zeolite at 175 °C. (b): p-xylene (A)/m-xylene (B) selectivities (with a molar ratio A/B ∼ 0.80). (9): PDEB (A)/mxylene (B) selectivities (with a molar ratio A/B ∼ 0.70). (2): PDEB (A)/p-xylene (B) selectivities (with a molar ratio A/B ∼ 0.90).

Kixiγi Qi ) Qi∞ 1 + Kjxjγj

∑j

(2)

where the variables are Qi, the adsorbed amount of component i per adsorbent mass, Qi∞, the limiting adsorbed amount of component i per adsorbent mass, Ki, the parameter of the Langmuir-Freundlich model, xi, the molar fraction of component i in the liquid phase, and γi, the exponent parameter of the LangmuirFreundlich model. In a binary mixture, if the limiting adsorbed amounts are the same (Q1∞ ) Q2∞), the binary selectivity can be written as

Figure 11. Binary selectivities in ternary mixtures on prehydrated BaX zeolite at 130 °C. (b): p-xylene (A)/m-xylene (B) selectivities (with a molar ratio A/B ∼ 0.75). (9): PDEB (A)/mxylene (B) selectivities (with a molar ratio A/B ∼ 0.70). (2): PDEB (A)/p-xylene (B) selectivities (with a molar ratio A/B ∼ 0.95).

an increase of the p-xylene/m-xylene selectivity, which rises to 3.5-4 depending on the concentration of pdiethylbenzene, whereas in the binary mixture, the selectivity is around 3. At 130 °C, the p-xylene/m-xylene selectivity obtained in the ternary mixture is almost constant (around 4) with the concentration in p-diethylbenzene, and it is slightly higher than the selectivity measured in the binary mixture. Finally, p-diethylbenzene has a rather beneficial influence on the p-xylene/ m-xylene selectivity. Influence of p-Xylene on p-Diethylbenzene/m-Xylene Selectivity. The p-diethylbenzene/m-xylene selectivity

R12 )

M2 K1 x1γ1-1 M1 K2 x γ2-1

(3)

2

where there are three model parameters, K1/K2, γ1, and γ2, and Mi is the molar mass of component i. In this work, three binary mixtures have been studied. Thus, there are five model parameters: K1/K2, K3/ K2, γ1, γ2, and γ3. When the adjustment of the parameters is performed on each binary mixture individually, the experimental data are very well described.9 Because we wish to predict the adsorption equilibria of ternary mixtures from binary data, the adjustment of the five parameters at constant temperature is performed simultaneously on all of the experimental selectivities of the binary mixtures. The Qi∞ values are taken equal to the average of total experimental adsorbed quantities. From the parameter values, the adsorbed quantities can

Ind. Eng. Chem. Res., Vol. 40, No. 25, 2001 5989 Table 3. Langmuir-Freundlich Parameters at 175 and 130 °C Kp-xyl/Km-xyl KPDEB/Kp-xyl KPDEB/Km-xyl γp-xyl γm-xyl γPDEB

175 °C

130 °C

3.519 2.307 8.123 1.039 0.943 1.158

4.539 2.627 11.937 1.075 0.849 1.272

Table 4. Average Absolute Deviations [A.A.D. (%)] Obtained on the Selectivity for Each Binary Mixture 175 °C 130 °C

Rp-xyl/m-xyl

RPDEB/m-xyl

RPDEB/p-xyl

7.5 9.4

9.4 10.3

19.6 27.7

Table 5. Average Absolute Deviations [A.A.D. (%)] Obtained on the Whole Adsorbed Amounts for Each Binary Mixture

175 °C 130 °C

p-xylene/ m-xylene

PDEB/ m-xylene

PDEB/ p-xylene

3.8 4.8

4.3 5.0

7.0 9.7

be calculated and compared with the experimental adsorbed quantities. The results of the adjustment are given in Table 3. Figures 3-5 allow us to make the comparison between the calculated curves and the experimental curves. In Table 4 are reported the average absolute deviations obtained on the selectivities of binary mixtures. First, it appears that the model allows us to describe the variation of the p-xylene/m-xylene selectivity with the composition of the mixture. However, for mixtures poor in p-xylene, the model overestimates the selectivity. The gap between the experimental data and the calculated data is over 10% in this range of concentration. Then, as for the p-diethylbenzene/m-xylene selectivity, the model does not allow us to describe correctly the bellshaped experimental curves, mainly at 130 °C. The average absolute deviations are around 10% and thus higher than those obtained with the p-xylene/m-xylene mixture. Finally, from Figure 5, it appears that the calculated p-diethylbenzene/p-xylene selectivity is strongly overestimated in the whole range of concentrations, with the average absolute deviations being about 20% at 130 °C and 28% at 175 °C. The difficulty in achieving a good fit of the selectivities derives certainly from the mathematical form of the Langmuir-Freundlich model. In fact, the selectivity as expressed from the Langmuir-Freundlich model (eq 3) tends to zero or infinity, depending on the values of parameters γ1 and γ2 relative to unity, when the molar fraction in the liquid phase tends to 0 or 1. With the number of adjustable parameters in the model being small, it is difficult to achieve a really good fit of the selectivity data with these constraints. This problem does not arise with adsorbed quantities. Indeed, the experimental adsorbed amounts in binary mixtures are better described by the Langmuir-Freundlich model than are the selectivities, whatever the temperature. The average absolute deviations obtained on the adsorbed amounts for each binary mixture are lower (Table 5) than those obtained for selectivity. For the two mixtures, p-xylene/m-xylene and p-diethylbenzene/m-xylene, the decrease of the total amount adsorbed when the mixture is impoverished in the preferentially adsorbed component is correctly reproduced by the Langmuir-Freundlich model.

Figure 12. Binary selectivities in ternary mixtures (with a molar ratio p-xylene/m-xylene about 0.80) on prehydrated BaX zeolite at 175 °C. [(b): p-xylene/m-xylene selectivities. (9): PDEB/mxylene selectivities. (2): PDEB/p-xylene selectivities. Solid line: Langmuir-Freundlich model.]

Figure 13. Binary selectivities in ternary mixtures (with a molar ratio p-xylene/m-xylene about 0.80) on prehydrated BaX zeolite at 130 °C. [(b): p-xylene/m-xylene selectivities. (9): PDEB/mxylene selectivities. (2): PDEB/p-xylene selectivities. Solid line: Langmuir-Freundlich model.]

Figure 14. Adsorbed amounts in ternary mixtures (with a molar ratio p-xylene/m-xylene about 0.80) on prehydrated BaX zeolite at 175 (full symbols) and 130 °C (empty symbols). [Circles, p-xylene; diamonds, m-xylene; triangles, p-diethylbenzene; squares, total. Solid line, Langmuir-Freundlich model at 175 °C; dotted line, Langmuir-Freundlich model at 130 °C].

Ternary equilibria may be predicted from binary data. The equations used are the same as those presented previously. Figures 12 and 13 allow a comparison between experimental data and predicted ternary data. The results show that the description of the selectivity in ternary mixtures is very qualitative, with the gaps being very important and the variations of the selectivity being not well described. Figure 14 shows the experimental and calculated adsorbed amounts for ternary mixtures where the molar ratio of p-xylene/mxylene is about 0.80. It appears that the adsorbed amounts are better described than are the selectivities.

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5. Conclusion The objective of this work was to provide a very complete database from which models can be fitted. Thermogravimetry has allowed us to measure isotherms adsorption of single components (xylene isomers and p-diethylbenzene) in the gaseous phase on BaX with a water content of about 5.7% in weight. The adsorption equilibria of single components show that there are no important differences between p-xylene, m-xylene, and p-diethylbenzene. The adsorption affinity of the partially hydrated BaX zeolite is the same for the three isomers. Thus, at 175 °C, the adsorption isotherms are almost superimposed. It is only at 130 °C and at high filling of the zeolite that some significant differences in the adsorbed amounts are observed. The adsorption processes of p-xylene and p-diethylbenzene seem similar but different than that of m-xylene. We have studied the adsorption equilibria of binary and ternary mixtures in the liquid phase on BaX by a chromatographic method. The experiments show that all measured selectivities depend strongly on the composition of the liquid phase and on the temperature. The p-xylene/m-xylene selectivity is greater than unity whatever the temperature, with the BaX zeolite being selective for p-xylene. Moreover, the p-diethylbenzene/ m-xylene selectivity is in favor of p-diethylbenzene. The study of ternary mixtures on prehydrated BaX zeolite has shown that the presence of a third component causes a change in the binary selectivity. The temperature plays an important role on adsorption properties of the adsorbent. As expected, an increase of temperature entails a decrease of the total adsorption capacity. Moreover, examination of the effect of temperature on the adsorbed amounts supports an interpretation in terms of the existence of less energetic adsorption sites for paraxylene, as has also been demonstrated elsewhere by neutron diffraction experiments and molecular simulation studies. Our results suggest that this could also be the case for paradiethylbenzene. Thus, an increase of temperature entails a depopulation of these adsorption sites. These results and observations comfort the idea that adsorption selectivity in these systems is governed mainly by entropic effects (steric hindrance, preferential adsorption sites, orientation of the adsorbates in these sites, etc.). These effects appear mostly at high loading and in mixtures. Last, the Langmuir-Freundlich model is used to describe the variation of the selectivity and the adsorbed amounts with the composition of the liquid phase. When the adjustment of the parameters is performed simultaneously on the whole binary data, the description of adsorption equilibria is not very good. The extension of the model to ternary mixtures provides very qualitative results. The gap between experiments and calculations are important. However, the adsorbed amounts are better predicted than are the selectivities. The Langmuir-Freundlich model has a simple analytic expression, but it remains a strongly empirical thermodynamic model. Nomenclature KH ) Henry constant Ki ) parameter of the Langmuir-Freundlich model (dimensionless) p ) pressure (Pa)

Qi ) adsorbed amount of component i per adsorbent mass (g/g of zeolite) xi ) molar fraction of component i in the adsorbed phase yi ) molar fraction of component i in the liquid phase at adsorption equilibrium Greek Symbols Rij ) binary selectivity of components i and j (dimensionless) γi ) parameter of the Langmuir-Freundlich model (dimensionless) Superscripts ∞ ) value at saturation of the adsorbent Subscripts m-xyl ) m-xylene PDEB ) p-diethylbenzene p-xyl ) p-xylene

Supporting Information Available: Tables 1-5 containing experimental data. This material is available free of charge via the Internet at http://pubs.acs.org. Literature Cited (1) Neuzil, R. W.; U. S. Patent, No. 3,558,730,1971. (2) Ash, G.; Barth, K.; Mank, L; Renard, P. Rev. Inst. Fr. Pet. 1994, 49 (5), 541-549. (3) Cottier, V. Ph.D. Thesis, University of Bourgogne, Dijon, France, 1997. (4) Cottier, V.; Bellat, J.-P.; Simonot-Grange, M.-H.; Me´thivier, A. J. Phys. Chem. B 1997, 101, 4798-4802. (5) Bellat, J.-P.; Cottier, V.; Moı¨se, J-C.; Pilverdier, E.; Me´thivier, A.; Simonot-Grange, M-H. Fundam. Adsorpt., Conf. Fundam. Adsorp., 6th; 1998, 225-230. (6) Bellat, J.-P.; Moı¨se, J.-C.; Cottier, V.; Paulin, C.; Me´thivier, A. Sep. Sci. Technol. 1998, 33 (15), 2335-2348. (7) Bellat, J.-P.; Simonot-Grange, M.-H.; Jullian, S. Zeolites 1995, 15, 124. (8) Bellat, J.-P.; Simonot-Grange, M.-H. Zeolites 1995, 15, 219. (9) Ruthven, D. M.; Goddard, M. Zeolites 1986, 6, 275-282. (10) Santacesaria, E.; Morbidelli, M.; Danise, P.; Mercenari, M.; Carra, S. Ind. Chem. Process. Des. Dev. 1982, 21, 440-445. (11) Tournier, H.; Barreau, A.; Tavitian, B.; Le Roux, D.; Sulzer, C.; Beaumont, V. Microporous Mesoporous Mater. 2000, 39, 537547. (12) Furlan, L. T.; Chaves, B. C.; Santana, C. Ind. Eng. Chem. Res. 1992, 31, 1780. (13) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquero, J.; Siemieniewska, T. Pure Appl. Chem. 1985, 57, 603. (14) Moı¨se, J.-C. Ph.D. Thesis, University of Bourgogne, Dijon, France, 1999. (15) Mellot, C. Ph.D. Thesis, University of Bourgogne, Dijon, France, 1993. (16) Descours, A. Ph.D. Thesis, University of Bourgogne, Dijon, France, 1997. (17) Mellot, C.; Simonot-Grange, M.-H.; Pilverdier, E.; J-Bellat, P.; Espinat, D. Langmuir 1995, 11, 1726-1730. (18) Pichon, C.; Me´thivier, A.; Simonot-Grange, M.-H.; Baerlocher, C. J. Phys. Chem. B 1999, 103, 10197-10203. (19) Lachet, V.; Boutin, A.; Tavitian, B.; Fuchs, A. H. Langmuir 1999, 15, 8678-8685. (20) Pichon, C. Ph.D. Thesis, University of Bourgogne, Dijon, France, 1999. (21) Lachet, V. Ph.D. Thesis, University of Paris, Sud, Paris, France, 1998. (22) Koble, R. A.; Corrigan, T. E. Ind. Eng. Chem. 1952, 44 (2), 383.

Received for review December 30, 2000 Revised manuscript received July 19, 2001 Accepted August 12, 2001 IE0011371