Adsorption of mixed vapors on solids. II. Cab-O-Sil

Adsorption of Mixed Vapors on Solids II. Cab-O-Sil 1

GRACIA A. P E R F E T T I ~ AND J. P. WIGHTMAN 3 Chemistry Department, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061 Received June 20, 1975; accepted January 28, 1976 The adsorption isotherms for ethanol-cyclohexane, ethanol-benzene, and benzene-cyclohexane vapor mixtures on Cab-O-Sil at 20, 30, and 40°C were measured at constant total pressure. The adsorption isotherms for the pure components were also obtained. The amounts of the pure vapors adsorbed on Cab-O-Sil followed the order ethanol > benzene > cyclohexane. Isosteric heats of adsorption and BET cross sectional areas were calculated for the pure adsorbates on Cab-O-Sil. The data indicated that the three adsorbates do not form close-packed monolayers on the Cab-O-Sil surface. The binary vapor adsorption isotherms were compared to the pure component isotherms. In several instances, the amounts of the components adsorbed from the mixtures were greater than from the pure states. For the Cab-O-Sil systems, selective adsorption of ethanol occurred from ethanol-cyclohexane and ethanol-benzene mixtures; benzene was selectively adsorbed from benzenecyclohexane mixtures. The temperature dependence of the selectivity for the systems studied followed no consistent trend. Comparison of the binary vapor adsorption isotherms with the analogous solution adsorption isotherms indicated that selectivity is generally higher in adsorption from solution. The experimental binary vapor adsorption isotherms were compared to those calculated from the pure vapor adsorption isotherms using the ideal adsorbed solution model. It was found that the adsorbed solutions were ideal or slightly nonideal for all three mixtures on Cab-O-Sil. It was concluded that the ideal adsorbed solution model is ~ useful one for predicting binary vapor adsorption equilibria. INTRODUCTION

Physical adsorption of pure gases and vapors onto solids has been studied extensively (1, 2). In contrast, data on adsorption from gas mixtures are scarce (3). Adsorption of gas mixtures is important in industrial separation (4, 5), in removal of trace impurities (6), and in reduction of air pollution (7). To determine if separations by adsorption are feasible, data on adsorption of mixtures Presented at the 49th National Colloid Symposium, Potsdam, New York, June 16-18, 1975. Based in part on the Ph.D. thesis of Gracia A. Perfetti. 3 Author to whom inquiries should be addressed. Present addless: School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS, England.

onto solids are required. While pure vapor adsorption data are easily obtained, measurement of adsorption of vapor mixtures is more complicated and time consuming. It would be advantageous to be able to calculate mixed vapor adsorption from adsorption data for the pure components. Several models have been developed for this purpose. But because of the scarcity of mixed vapor adsorption data, the validity of these models has not been adequately investigated. This work was undertaken to obtain experimental adsorption isotherms for binary vapor mixtures onto silica along with pure component adsorption data. The adsorbates chosen for study were ethanol, benzene, and cyclohexane.

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Journal of Colloid and Interface Science, VoL 55, No. 2, May 1976

Copyright ~ 1976 by Academic Press, Inc. All rights of reproduction in any form reserved.

MIXED VAPOR ADSORPTION AND THERMODYNAMICS Preliminary results for these systems have been reported by Perfetti and Wightman (8). Isosteric heats of adsorption were obtained from the pure component isotherms. The results of binary vapor adsorption were compared to the results of binary solution adsorption for the same systems. Finally, the isotherms obtained for the binary vapor mixtures were compared to those calculated from the pure component isotherms using several thermodynamic models. A general review of the literature on adsorption of binary vapor mixtures onto solids prior to 1957 is available (2). Cines and Ruehlen (9) have studied the effect of surface coverage and multilayer formation on selective adsorption by measuring adsorption of benzene-2,4dimethylpentane vapor mixtures on silica gel at 65.6°C. Isotherms of amounts adsorbed versus pressure for three different equilibrium vapor phase compositions were obtained. Benzene was selectively adsorbed from the mixtures. Selectivity increased with increasing pressure, indicating that benzene replaced dimethylpentane in the adsorbed phase as surface coverage increased. Lewis et al. (4) have obtained adsorption data for binary mixtures of the lower gaseous hydrocarbons on silica and on carbon. Isotherms were measured at 25°C and at a total pressure of one atm. For mixtures involving components with the same degree of unsaturation but different vapor pressures, both carbon and silica selectively adsorbed the less volatile component. For mixtures involving components with nearly equal vapor pressures, silica selectively adsorbed the more unsaturated component. Bering, Serpinskii, and Surinova (10) have shown that the temperature dependence of the selectivity a, is related to the isosteric heats of adsorption (qs,~) of the components in the mixture. If qst 1 > qst2, ot decreases with increasing temperature and vice versa. Shen and Smith (11) have studied adsorption of benzene-hexane mixtures on silica at low surface coverage (8 < 0.1) from 70 to 130°C. Amounts adsorbed were measured as a function

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of pressure for several constant vapor phase compositions. Preferential adsorption of benzene occurred. Isosteric heats of adsorption for benzene and hexane, from the pure states and from the mixtures, were calculated. For the pure components, qs, decreased rapidly with increasing surface coverage, indicating that the silica surface was heterogeneous. The heat of adsorption of benzene from the mixture, qst b, was independent of the amount of hexane adsorbed. But qsth decreased as the amount of adsorbed benzene increased. The authors, therefore, concluded that benzene occupied the most energetic sites on the surface, leaving the low energy sites for hexane. The heat of adsorption of hexane from the mixtures was independent of the amount of hexane present, which indicated that the sites available for hexane all had about the same energy. It was postulated that selective adsorption of benzene was due to interactions between its ,r electrons and the hydroxyl groups on the silica surface. Many models for predicting binary vapor adsorption from the isotherms of the pure components have been proposed. These models have been reviewed by Buelow, Grossmann, and Schirmer (12), and by Sircar and Myers (13). Myers and Prausnitz (14, 15) have developed the ideal adsorbed solution model. The authors defined an adsorbed solution analogous to a liquid solution. If the adsorbed solution is ideal, then P y i = Pi* x~,

[-1-]

where xi and y~ are the mole fractions in the adsorbed and vapor phases, respectively, P is the total vapor phase pressure, and P~* is the pressure of component i at the same temperature and spreading pressure r as that of the adsorbed solution. If the adsorbed solution is ideal, binary vapor isotherms can be calculated from the pure component isotherms. Spreading pressure curves are obtained for the pure components by application of the Gibbs adsorption equation to the pure vapor isotherms: P

,r/eT= fo (.~/P)de,

[23

dournaI of Colloid and InterJace Science, Vol. 55, No. 2, May 1976

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PERFETTI AND WIGHTMAN

where N is the number of moles adsorbed per unit area of adsorbent. Values of Pi* at a particular value of 7r may then be interpolated from the spreading pressure curves. If the binary vapor isotherm is desired at constant total pressure, xl is first calculated from: p -- p~* x~ =

E3-]

PI* -- P"*' and y, is calculated from Eq. [-1-]. The ideal adsorbed solution model has been applied to a variety of literature data with good results. Sircar and Myers (13) have pointed out, however, that the ideal adsorbed solution model sometimes cannot be used for cases in which one of the components of the mixture is close to saturation (16, 17). To use the ideal adsorbed solution model, the pressures of the pure components at the same spreading pressure as that of the adsorbed solution P~*, must be known. If component 1 of a binary mixture is close to saturation, some adsorbed phase compositions may have spreading pressures greater than the saturation spreading pressure of component 2. Therefore, P2* would not be defined. Sircar and Myers have developed an extension of the ideal adsorbed solution model which m a y be used when one of the components of a mixture is near saturation. The binary vapor isotherms are calculated from :

spreading pressure of the mixture at saturation 7r*, can be obtained from solution adsorption data for the same system: 71.s __ 7r2s

RT

fal

nl

Jo (1 - xl')al d~l'

[7-]

where nl is the number of moles of component 1 adsorbed from solution, xx' is the mole fraction in the bulk liquid solution, and ax is the activity. The use of this model is straightforward if binary vapor isotherms of amounts adsorbed versus pressure for constant vapor phase composition are desired. If isotherms at constant total pressure are desired, Eq. [4-] must be solved for 7rr by successive approximations. Sircar and Myers have applied this model to the data of Cines and Ruehlen (9) for adsorption of benzene-dimethylpentane mixtures onto silica with good results. EXPERIMENTAL

The adsorbates used were benzene (Fisher: 99 mole°-/o grade), cyclohexane (Fisher: 99 mole% grade), and ethanol. Benzene and cyclohexane were used as received; ethanol was stored over Cab-O-Sil to remove trace quantities of water. The adsorbent, Cab-O-Sil, was obtained from Cabot Corporation. Cab-O-Sil is a flamehydrolyzed silica having a surface that is 25% hydroxylated (18, 19). The B E T surface area of Cab-O-Sil determined by nitrogen adsorp1 y, F~-/Trd -- ~-~)1 --=E--exp , [4-] tion was 214 + 2 m2/g. P • , The pure vapor adsorption isotherms were measured by a volumetric technique. The conPyi=Pi*xiexp . - - - , [-5-] stant volume apparatus consisted of a gas [- Ni*RT buret, a mercury manometer, an adsorbate reservoir, and a sample container. A residual where Irr, the reduced spreading pressure, is pressure of 10-5 Torr was obtained with a defined by: mechanical pump and a liquid nitrogen trapped 7r 7ri* mercury diffusion pump. The mercury heights lrr = -- = - [-6-] in the manometer were read to the nearest "/I"8 7ri* 0.1 m m with a cathetometer. The entire adIn Eq. [6], ~-i* is the spreading pressure at Pi*. sorption system was surrounded by an air bath The spreading pressures of the pure compo- in which the temperature could be raised to nents at saturation ~rd, may be obtained from 45°C. A 0.0756 g sample of Cab-O-Sil was the pure vapor isotherms using Eq. [-2]. The used for all of the pure vapor isotherms.

Journal oJ Colloid and Interface Science, Vol. 55, No. 2, M a y 1976

MIXED VAPOR ADSORPTION AND THERMODYNAMICS Prior to measuring each adsorption isotherm, the adsorbent was heated under vacuum to ll0°C for 1 hr. The adsorbent container was then surrounded by a constant temperature bath for at least 30 min before adsorption measurements were begun. The water bath could be held to within 0.1°C of the desired adsorption temperature. Although equilibrium appeared to be reached after 5 rain, an equilibration time of 15 rain was allowed. Data were obtained up to a relative pressure of about 0.95 for each isotherm. Adsorption isotherms for the binary vapor mixtures were measured by a modified volumetric technique, in which the partial pressures of each component of the mixture before and after equilibration with the adsorbent were measured with a gas chromatograph. The experimental apparatus and the procedure has been described in detail previously (8). Prior to measuring each point on an isotherm, the adsorbent was treated the same as for pure vapor adsorption. A 1.6264 g sample of Cab-O-Sil was used to obtain the 30°C isotherms of the three mixtures and the 40°C ethanol-benzene isotherm. For the remainder of the isotherms, a 1.8646 g sample was used. The binary vapor adsorption isotherms were obtained at a constant total pressure of 30 Torr with the exception of the 40°C ethanol-benzene isotherm, which was measured at a total pressure of 40 Torr.

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14 12 10

I

0

40

80 120 P(tor r)

160

FIG. 2. Benzene/Cab-O-Sil isotherms at 20, 30, and 40°C. RESULTS AND DISCUSSION The adsorption isotherms at 20, 30, and 40°C for ethanol, benzene, and cyclohexane onto Cab-O-Sil are shown in Figs. 1, 2, and 3, respectively. B E T cross sectional areas (a) of 37.5, 59.3, and 103. A2/mol. were calculated from the isotherms for ethanol, benzene, and cyclohexane, respectively. The calculated values of ~ are all higher than the areas based on liquid packing (20). Similar results have been reported previously by Whalen (21) and Pierce

14 12

20[ o 20oc / ~30~C

~ I

~_ 10 I

~m

O

~6

I

20

40

60 80 P(torr)

100

120

140

FIO. 1. Ethanol/Cab-O-Sil isotherms at 20, 30, and 40°C.

0

/ / 40

80 120 P(torr)

/ 160

FIG. 3. Cyclohexane/Cab-O-Sil isotherms at 20, 30, and 40°C. Journal of Colloid and Interface Science, Vol. 55, No. 2, May 1976

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For systems exhibiting lateral interactions, isosteric heat of adsorption usually drops sharply near monolayer coverage (23). For ethanol on Cab-O-Sil, q, starts to decrease ~ 13 around 0 = 1.4. The discrepancy could be related to the fact that ethanol is not packed to liquid density at monolayer coverage, as was concluded from the BET cross sectional area ' ' i ~,~6 1.0 14 18 2I2 2.6 for ethanol on Cab-O-Sil. The isosteric heat e continues to decrease for O > 1.4, and gradually FIG. 4. Isosteric h e a t of adsorption for ethanol on approaches the heat of vaporization of ethanol. Cab-O-Sil. The isosteric heat curves for benzene and cyclohexane on Cab-O-Sil are shown in Fig. 5. and Ewing (22). The higher areas were atThe heats of vaporization of benzene and tributed to less than close packed monolayers. cyclohexane are almost identical, and are Figure 4 shows the isosteric heat of adsorprepresented by the dashed line in Fig. 5. Both tion versus surface coverage for ethanol on heat curves appear to contain maxima at low Cab-O-Sil. The isosteric heat curve shows a coverages. Hockey and Pethica (26) have atpronounced maximum. Maxima in heat curves tributed the maxima at low coverage in the are attributed to lateral interactions in the heat curves for benzene on dehydroxylated adsorbed phase (23). For ethanol, the lateral silicas to cooperative adsorption into surface interactions are probably caused by hydrogen micropores. bonding between adjacent adsorbate molecules. The heat curve for cyclohexane decreases Isosteric heat curves for alcohols on fully hygradually with increasing coverage, and does droxylated silica surfaces do not contain not approach the heat of vaporization until maxima (24). For adsorption onto a fully hywell into the multilayer region. This result droxylated surface, lateral interactions would supports the previous conclusion that cyclonot be important because ethanol would be hexane is not packed to liquid density in the hydrogen bonded to the surface hydroxyl rnonolayer region. The isosteric heat curve obgroups. But the hydroxyl groups on the 250-/0 tained for benzene on Cab-O-Sil is anomalous hydroxylated Cab-O-Sil surface are isolated in two respects. First, it lies below the heat (25); adsorbed ethanol molecules would, thus, curve for cyclohexane. It was expected to lie be free to interact laterally through hydrogen above the cyclohexane heat curve because of bonding. interactions of the ~r electrons of benzene with the hydroxyl groups on the Cab-O-Sil surface. o Benzene Second, the benzene heat curve drops below I '~::~ the heat of vaporization. These two anomalies may be related to the fact that the Cab-O-Sil surface is only 250-/0 hydroxylated. 17

i

Binary Vapor Adsorption Isotherms

02

0.6

10

1,4

1.8

2.2

2.6

e

FI6. 5. Isosteric beats of adsorption for benzene a n d eyclohexane on Cab-O-Sil. Journal of Colloid and Interface Science, Vol. $$, No. 2, May 1976

The binary vapor adsorption isotherms for ethanol-cyclohexane, ethanol-benzene, and benzene-cyclohexane vapor mixtures on CabO-Sil at 20°C are shown in Figs. 6-8. Binary vapor isotherms for the three mixtures were also obtained at 30 ° and 40°C. The isotherms

MIXED VAPOR ADSORPTION AND THERMODYNAMICS oEthanol

•o Ethanol O8enzene

/- / / / "// / / o o

E /f E

257

//

%4 2

0

FIG. 6.

0.2

ON

O6

08

0.2

10

Ethanol-eyclohexane/Cab-O-Sil isotherm

at 200C. have been plotted as N, the number of moles of each component adsorbed per square meter of adsorbent, versus vapor phase composition, yl. The corresponding pure component isotherms, represented by the dashed curve, have been included on these graphs for comparison purposes. In this case, N values were interpolated from pure component isotherms at values of PC corresponding to yl values. Although this method of representing binary vapor adsorption data has not been used previously, it illustrates some interesting features. The adsorption isotherms for ethanolcyclohexane mixtures on Cab-O-Sil, shown in Fig. 6, indicate that ethanol is the dominant component in the adsorbed phase. Adsorption of cyclohexane is appreciable only at low vapor mole fractions of ethanol. The amounts of cyclohexane adsorbed from the mixtures are much less than from the pure state. An anomalous enhancement of adsorption of ethanol over the pure state is observed at high ethanol mole fractions. This was also observed in the 40°C isotherm. The ethanol-benzene/Cab-O-Sil isotherms at 20°C, shown in Fig. 7, indicate that ethanol again dominates the adsorbed phase, but not to the extent that it does when mixed with cyclohexane. Adsorption of both ethanol and benzene from the mixtures is generally less than from the pure states. However, at 40°C enhancement of adsorption of ethanol and benzene was observed at high and low ethanol mole fractions, respectively.

0,4

0.6

(~B

~.0

YE

FI6. 7. Ethanol-benzene/Cab-O-Sil isotherm at 20°C. For adsorption of benzene-cyclohexane mixtures onto Cab-O-Sil at 20°C, shown in Fig. 8, there are appreciable amounts of both components adsorbed over the entire mole fraction range. Unlike the other two mixtures, neither component is capable of dominating the Cab-O-Sil surface. For benzene-cyclohexane mixtures at all three temperatures, adsorption of both components is greater from the mixtures than from the pure states, in contrast to the general behavior observed for the other two mixtures. The relative amounts of adsorption of a species from mixtures and from the pure state has not been widely discussed. Myers (15) has observed that for systems showing fairly strong adsorbate-adsorbent interactions, adsorption from mixtures is generally less than from the pure state. This behavior has been attributed to competition of the components for surface o Benzene 5 -aCyc~ohexane

/

4

o /

]

/ /

J

m i

O

O12

0;4

06

08

1.0

Y8 Fie. 8. Benzene-cyclohexane/Cab-O-Sil isotherm

at 20°C. Journal of Colloid and Interface Science, Vol. 55, No. 2, May 1976

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sites. An enhancement of adsorption over the pure state has been observed by Pavlyuchenko (27) for chloroform-acetone vapor mixtures onto carbon. Bering, Serpinskii, and Surinova (28) have postulated that for tile chloroformacetone/carbon system, in which adsorbateadsorbent interactions are fairly weak, the enhancement is casued by attractive forces between the two components in the adsorbed phase. The pronounced enhancements in adsorption from the mixtures observed for the benzene-cyclohexane/Cab-O-Sil systems, and the slight enhancements observed in some of the other isotherms, are difficult to rationalize. Although Figs. 6-8 give some indication of selective adsorption, selectivity is best illustrated by adsorbate-vapor composition diagrams. The 20°C isotherms for the three mixtures have been replotted in Fig. 9 as adsorbed phase composition x, versus vapor phase composition y. The asterisks in Fig. 9 denote the component of each mixture whose mole fractions have been plotted. If no selective adsorption were occurring in these systems, the compositions of the adsorbed and vapor phases would be equal, and the isotherms would lie along the dotted line in Fig. 9. If the component whose compositions are being plotted is selectively adsorbed from a mixture, the isotherm will lie above the dotted line. The further the isotherm lies from the dotted line, the

greater the selectivity. The trends in selectivity observed for the three mixtures on Cab-O-Sil can be explained qualitatively on the basis of adsorbate-adsorbent interactions. Since ethanol is capable of polar interactions with the Cab-O-Sil surface, it is selectively adsorbed from ethanol-cyclohexane and ethanol-benzene mixtures. The selective adsorption of benzene from benzene-cyclohexane mixtures may be attributed to interactions between the ~r electrons of benzene and the hydroxyl groups of the Cab-O-Sil surface, and is consistent with the observations of Lewis et al. (4) which show that for mixtures of components having nearly equal vapor pressures, the more unsaturated component is selectively adsorbed on silica. The selectivity observed for the benzene-cyclohexane mixtures is much less pronounced than for the other two mixtures. The 30 and 40°C composition diagrams for the three mixtures on Cab-O-Sil show the same trends in selectivity. For ethanol-cyclohexane mixtures, selectivity follows the trend 40 > 20 > 30°C. The composition diagrams for ethanol-benzene mixtures indicate that selectivity increases with increasing temperature. For benzenecyclohexane mixtures, the greatest selectivity is observed at 40°C; selectivities at 20 and 30°C are almost identical. As was pointed out by Bering, Serpinskii, and Surinova (10), selectivity can either increase or decrease with temperature, depending on the relative magnitudes of the heats of adsorption of the components in the mixture. The dashed lines in Fig. 9 represent the 30°C solution adsorption isotherms obtained by Matayo and Wightman (29). In contrast to adsorption from the vapor phase, no temperature dependence was observed in the solution adsorption isotherms over a temperature range from 25 to 35°C. Comparing the solution isotherms to the 30°C vapor phase isotherms, it 0 0.2 04 06 0.8 1.0 is observed that selectivity is greater from ¥i solution than from the vapor phase for ethanol-cyclohexane and benzene-cyclohexane FIG. 9. Composition Diagrams for ethanol-cyclomixtures on Cab-O-Sil. For ethanol-benzene hexane, ethanol-benzene, and benzene-cyclohexane mixtures on Cab-O-Sil at 20°C. mixtures, selectivity is greater from solution

°I

02~

I

Journal of Colloid and Interface Science, VoL 55, No. 2, May 1976

MIXED VAPOR ADSORPTION AND THERMODYNAMICS for ethanol mole fractions <0.5. Comparisons between adsorption from solution and from the vapor phase on the same systems are rare. This is the first reported systematic study of adsorption from the vapor phase and from solution. Kiselev and Pavlova (30) have found that selectivity is greater from solution for adsorption of benzene-hexane mixtures on silica gel.

Models Since data for adsorption of binary vapor mixtures onto solids are scarce, the various models proposed for calculating binary vapor adsorption isotherms have not been adequately tested. The ideal adsorbed solution model (14, 15) has been fairly successful in predicting binary vapor adsorption equilibria from pure component adsorption data. The success of this model may be attributed to the fact that it was rigorously derived from thermodynamics with a minimum number of assumptions. This section, therefore, will be devoted primarily to comparisons between the experimental binary vapor adsorption isotherms and the isotherms calculated from the experimental pure vapor isotherms using the ideal adsorbed solution model. Such comparisons are also useful in interpreting some of the binary vapor adsorption data that were presented in the previous section. The models proposed by Kidnay and Myers (31) and by Fernbacker and Wenzel (32) are simplifications of the ideal adsorbed solution model. They are much easier to use since spreading pressure curves for the pure components need not be calculated. None of the pure vapor isotherms obtained in this work fulfilled the conditions necessary for these two models. Use of the ideal adsorbed solution model first requires knowledge of spreading pressure as a function of equilibrium gas phase pressure for each of the components in the mixture. Spreading pressure curves were obtained from the pure vapor isotherms by use of the Gibbs adsorption equation [2"]. Except at very low pressures, the integration was performed numerically using the trapezoidal rule. Since the integrand in Eq. [2] is undefined when P = 0,

259

it was necessary to use Henry's law to integrate in the low pressure region. The calculated spreading pressure curves for ethanol, benzene, and cyclohexane on Cab-O-Sil at 20°C are shown in Fig. 10. The 30 and 40°C spreading pressure curves are similar in appearance. According to the ideal adsorbed solution model, the composition xx, of a mixture having a particular value of spreading pressure is calculated from the pressures of the pure components having this same spreading pressure using Eq. [3]. Examination of Fig. 10 shows that this procedure cannot be used for calculating adsorption of ethanol-cyclohexane or ethanol-benzene vapor mixtures onto Cab-OSil. For example, for ethanol-benzene mixtures on Cab-O-Sil at a total pressure of 30 Torr, the spreading pressures of interest lie between the spreading pressure of benzene at 30 Torr (,r/RT = 5 X 10- Gmole/m 2) and the spreading pressure of ethanol at 30 Torr (Tr/RT = 20 X 10.6 mole/m~). Figure 10 illustrates that for Ir/RT > 11 X 10. 6 mole/ms, Ps* is not defined. The modification of the ideal adsorbed solution model developed by Sircar and Myers (13) applies to such cases, and therefore, was used to calculate the adsorption isotherms for ethanol-benzene and ethanolcyclohexane vapor mixtures on Cab-O-Sil. In the Sircar and Myers model, the isotherms are calculated from Eqs. [4], [5], and [6].

20 "~

16

O

ol x/ aBenzene / ACyclohexane

20

40

60

80

P(torr)

FIG. 10. Spreading pressure curves for the pure adsorbates on Cab-O-Sil at 20°C. J o u r n a l of Colloid and Interface Science, Vol. 55, No. 2. May 1976

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PERFETTI AND WIGHTMAN

°8I

y

oi# 0'4F(/ /~//

o'~E-C

o27-;

/f/C"/

od#// 0

0.2

04

0.6

08

10

1 FIG. 11. Calculated ethanol-cyclohexane, e t h a n o l -

benzene, and benzene-cyclohexane/Cab-O-Silisotherms at 20°C. The spreading pressure of pure component i at saturation, rd, was obtained by integrating Eq. [2] to saturation vapor pressure. The reduced spreading pressure ~'r, was then calculated from Eq. [6]. Ni*, the amount of pure component i adsorbed at Pi*, was obtained from the pure vapor adsorption isotherms. The spreading pressure of the mixture at saturation lr ~, which is a function of vapor phase composition y~, was obtained from Eq. [-7]. The solution adsorption data obtained by Matayo and Wightman (29) for ethanol-cyclohexane and ethanol-benzene mixtures on Cab-O-Sil at 30°C were used in Eq. [7]. Since no temperature dependence was observed for the solution adsorption isotherms, the 30°C data were used to calculate the binary vapor isotherms at all three temperatures. Activities of ethanolcyclohexane solutions at 20 and 30°C and of ethanol-benzene solutions at 40°C are available (33). The activities of ethanol-cyclohexane solution at 40°C were calculated from the 20 and 30°C activities by assuming that the heat of mixing is constant over the temperature range involved. Similarly, activities for ethanol-benzene solutions at 20 and 30°C were calculated from experimental activities at 40 and 50°C. The integration in Eq. [7] was then performed numerically using the trapezoidal rule. Since the experimental binary vapor adsorption isotherms were measured at constant Journal of Colloid and Interface Science, Vol. 55, No. 2, May 1976

total pressure, for each vapor phase composition, yl, it was necessary to solve Eq. [,4,1 for 7rr by successive approximations. These values of 7rr, along with the corresponding values of -Pi* and Ni*, were then used to calculate the adsorbed phase compositions xi from Eq. [-5'1. The isotherms calculated from the Sircar and Myers model for ethanol-cyclohexane and ethanol-benzene vapor mixtures on Cab-O-Sil at 20°C are shown in Fig. 11. The agreement between calculated and experimental isotherms implies that these two mixtures form ideal adsorbed solutions on the Cab-O-Sil surface. In contrast, these same mixtures form extremely nonideal liquid solutions. It has been observed (28) that for a system involving fairly strong adsorbate-adsorbent interactions, the adsorbed solution is more ideal than the corresponding liquid solution. This behavior was attributed to the decreased importance of intermolecular interactions between molecules in the adsorbed state. While adding benzene or cyclohexane to liquid ethanol disrupts the intermolecular interactions, addition of these same substances to an adsorbed film of ethanol has little effect, since ethanol is interacting primarily with the surface. The benzene-cyclohexane/Cab-O-Sil isotherms were calculated from the ideal adsorbed solution model of Myers and Prausnitz (14, 15). From the spreading pressure curves and Eq. [3-1, the adsorbed phase compositions were determined. The corresponding vapor phase compositions were then calculated from Eq. [1]. The calculated benzene-cyclohexane/ Cab-O-Sil isotherm at 20°C is shown in Fig. 11. The agreement between experimental and calculated isotherms is good, indicating that benzene and cyclohexane form a slightly nonideal adsorbed solution on the Cab-O-Sil surface. SUMMARY The pure vapor adsorption isotherms for ethanol, benzene, and cyclohexane on Cab-OSil were measured at 20, 30, and 40°C. The amounts adsorbed followed the order ethanol > benzene > cyclohexane. The B E T cross

MIXED VAPOR ADSORPTION AND THERMODYNAMICS sectional areas of ethanol, benzene and cyclohexane on Cab-O-Sil were 37.5, 59.3, and 103 A 2, respectively. These values are much higher than the cross sectional areas based on liquid molar volumes. I t was concluded that the three adsorbates do not form close-packed monolayers on the Cab-O-Sil surface. The maximum in the isosteric heat curve for ethanol on Cab-O-Sil was attributed to hydrogen bonding interactions between ethanol molecules in the adsorbed phase. The heat curves for benzene and cylohexane both contained maxima at low coverages, but the data were not accurate enough to determine whether these maxima were significant. No sharp changes in the isosteric heats of adsorption of either benzene or cyclohexane were observed at monolayer coverage. The adsorption isotherms at 20, 30, and 40°C for ethanol-cyclohexane, ethanol-benzene, and benzene-cyclohexane vapor mixtures on CabO-Sil measured at a total pressure of 30 Torr, were compared with the pure component isotherms. For ethanol-cyclohexane and ethanolbenzene mixtures, tile amounts of the components adsorbed from the mixtures were generally less than from the pure states. For some of these systems, adsorption of a component (i) from a mixture at high y~ values was greater than from the pure state at corresponding equilibrium pressures. For the benzene-cyclohexane/Cab-O-Sil isotherms, the amounts adsorbed from the mixtures were greater than from the pure states. None of these enhancements in adsorption could be adequately explained. From the composition diagrams for the three vapor mixtures on Cab-O-Sil, it was concluded that selective adsorption of ethanol occurs from ethanol-cyclohexane and ethanol-benzene mixtures, and selective adsorption of benzene occurs from benzene-cyclohexane mixtures. Selectivities were highest for the ethanol-cyclohexane mixtures, and lowest for the benzene-cyclohexane mixtures. The variation of selectivity with temperature for the isotherms showed no particular trend. Comparison of the 30°C binary vapor ad-

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sorption isotherms with the corresponding solution adsorption isotherms showed that selectivity is generally higher from solution. The experimental binary vapor adsorption isotherms were compared to those calculated from the pure component isotherms using the ideal adsorbed solution model. I t was concluded that ethanol-cyclohexane and ethanol-benzene mixtures form ideal adsorbed solutions on the Cab-O-Sil surface. Benzene-cyclohexane mixtures form slightly nonideal adsorbed solutions on Cab-O-Sil. On the basis of this work, it may be concluded that the ideal adsorbed solution model is a useful one for predicting binary vapor adsorption equilibria. ACKNOWLEDGMENTS Financial support under NSF Grant GP-29225 including a graduate research assistantship for one of us (G.A.P.) is gratefully acknowledged. We recognize the capable assistance of Dave McCommon, Rick Miller, and Andy Mollick during the course of this work. REFERENCES 1. BRUNAUER, S., "The Adsorption of Gases and Vapors," Vol. I. Princeton Univ. Press, Princeton, New Jersey, 1943. 2. YouNG, D. M., AND C~OWELL,A. D., "Physical Adsorption of Gases," Butterworths, London, 1962. 3. ARNOLD,J. R., J. Amer. Chem. Soc. 71, 104 (1949). 4. LEwis, W. K., GILLILAND,E. R., CHERTOW,B., ANDCADOCAN,W. P., Ind. Eng. Chem. 42, 1319 (1950). 5. ELLIS, S. R. M., ANDTHOMPSON,D. W., Birmingham Univ. Chem. Eng. 16, 99 (1965). 6. CooK, W. H., AND BASMADJIAN,D., Canad. J. Chem. Eng. 43, 78 (1965). 7. "Calgon Water Report," Environ. Sci. Technol. 8, 871 (1974). 8. PERleETTI, G. A., AND WIGHT•AN, J. P., J. Colloid Interface Sci. 49, 313 (1974). 9. CINES,M. R., ANDRUEHLEN,F. N., J. Phys. Chem.

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(12), 2487 (1967). 11. SHEN, J., AND SMITH, J. M., Ind. Eng. Chem. Fundam. 7, 100 (1968). 12. BUELOW, M., GROSSMANN,A., AND SCHIRMER, W., Z. Chem. 12, 161 (1972). Journal of Colloid and Inlerface Science. Vol. 55. No. 2. May 1976

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13. SIRCAR,S., AND MYERS, A. L., Chem. Eng. Sci. 28, 25. BASSETT, D. R., BOUCHER, E. A., AND ZETTLE~OYER, A. C., J. Colloid Interface Sci. 34, 436 489 (1973). (1970). 14. MYERS, A. L., AND PRAUSNITZ, ~. M., AIChE J. 11, 121 (1965). 26. HOCKEY,J. A., ANDPETaICA, B. A., Trans. Faraday 15. MYERS, A. L., Ind. Eng. Chem. 60, 45 (1968). Soc. 58, Part 10, 2017 (1962). 16. REEDS, J. ikI., AND KAMMERMEYER,K., Ind. Eng. 27. PAVLYUCHENKO, N. M., Zh. Fiz. Khim. 44, 271 Chem. 51, 707 (1959). (1970); Russ. J. Phys. Chem. 44, 152 (1970). 17. HENSON, T. L., AND KABEL, R. L., AIChE J. 12, 28. BERING, B. P., SERPINSKII, V. V., AND SURINOVA, 606 (1966). S. I., Izv. Akad. Nauk SSSR, Set. Khim. (1), 18. BASSETT, D. R., BOUCHER, E. A., AND ZETTLE7 (1973); Bull. Acad. Sei. USSR Chem. Set. (1), MOYER, A. C., J. Colloid Interface Sci. 34, 436 5 (1973). (1970). 29. MATAYO, D. R., AND WIGHTMAN,J. P., J. Colloid 19. HAIR, M. L., AND HERTL, W., J. Phys. Chem. 73, Interface Sci. 44, 162 (1973). 4269 (1969). 20. WAS~SURN, E. W., (Ed.), "International Critical 30. KISELEV, A. V., AND PAVLOVA,L. F., Kolloid. Zh. Tables," Vol. III. McGraw-Hill, New York, 25, 537 (1963); Chem. Abstr. 60, 6242d (1964). 1928. 31. KIDNAY, A. J., AND MYERS, A. L., AIChE J. 12, 21. WHALEN,J. H., J. Phys. Chem. 71, 1557 (1967). 981 (1966). 22. PIERCE, C., AND EWING, B., J. Amer. Chem. Soc. 32. FERNBACHER, J. M., ANDWENZEL,L. A., Ind. Eng. 84, 4070 (1962). Chem. Fundam. 11, 457 (1972). 23. BEEJ3E, R. A., AND YOUNG, D. M., J. Amer. Chem. 33. TI~mSR~rANS,J., "Physico-Chemical Constants of Soc. 58, 93 (1954). 24. BORELLO, E., ZECCHINA, A., MORTERRA, C., AND Binary Systems," Vol. II. Interscience, New GalOTTI, G., J. Phys. Chem. 71, 2945 (1967). York, 1959.

Journal of Colloid and InterfaceScience,VoL 55, No. 2, May 1976