BENZENE-HEXANE M I X T U R E S JOHN SHEN AND J . M. S M I T H University of California, Davis, Calif;
Equilibrium isotherms were obtained for n-hexane and benzene on silica gel, both separately and from mixtures of the two gases. The measurements were made at low surface coverages and indicated a preferential adsorption of benzene. In the mixture studies at a fixed hexane partial pressure, increasing the benzene in the gas significantly reduced the adsorption of n-hexane. The effect of hexane on benzene was much less. Data were taken from 70" to 130' C. and heats of adsorption calculated from these measurements decreased sharply with increasing surface coverage. Results for the mixture measurements showed that AH for hexane decreased with increasing amounts of benzene. The heat of adsorption of benzene was not affected by hexane in the gas.
HE separation of hydrocarbon mixtures by adsorption is T i n f l u e n c e d by the interactions between the adsorbed species. Single-component adsorption rates are reasonably well understood; for linear equilibrium (linear adsorption isotherm) solutions are available (Brunauer, 1945) for predicting breakthrough curves in terms of resistances to inter- and intraparticle diffusion and adsorption. When more than one component is adsorbed, linear isotherms are rare, because adsorption of one component on part of the surface influences the availability and energy characteristics of the adjacent vacant surface. Practical separation problems frequently involve small concentrations of adsorbable species in an otherwise inert gas. I t is of interest to know if these interaction effects play an important role at low concentrations. The first step in studying the separation problem is to determine adsorption isotherms. This paper presents such equilibrium data for benzene-n-hexane mixtures on silica gel at surface coverages (fractions of an estimated monomolecular layer) from 0.2 to 10% and from 70' to 130' C. These components where chosen because they exhibited strong differences in quantity and rates of adsorption, yet had about the same molecular weight. Only a t the highest temperature and up to partial pressures of less than 0.01 atm. were the isotherms linear. Experimental data apparently have not been published for this system and mixture adsorption data are rare.
injected through valve 19. The calibration was made immediately following each run. The results were corrected for the hydrocarbon in the gas space in the adsorber by prior calibration. The completeness of the desorption procedure was checked by repeating the heating and desorbing procedure. No detectable hydrocarbons were observed in the helium stream flowing through the bed. Pretreatment of fresh gel prior to a series of runs was required to remove adsorbed impurities such as water, air, and carbon dioxide. The weakly adsorbed materials were removed by purging the bed, held a t 250" C., with pure helium at a flow rate of 50 cc. per minute for 30 minutes. The outgassing step consisted of passing helium, at 10 cc. per minute, through the bed a t 250' C. for 30 hours. Complete removal of chemisorbed impurities is probably not possible without permanently injuring the surface of the gel (Brunauer, 1945), so that the purpose of the pretreatment was to obtain a reproducible, if not clean, surface. The saturator, 9, consisted of a Dewar flask filled with water maintained a t a constant temperature, usually 20" C., and a glass bottle containing the liquid hydrocarbon. The helium was introduced in small bubbles into the bottom of the bottle with a glass filter. The performance of the system was checked by liquefying and weighing the hydrocarbon removed from
2 -
. .
h
21
Apparatus
A flow-type apparatus (Perkin-Elmer Shell sorptometer) employing helium as the carrier gas was used. As shown in Figure 1, for adsorption of a single component, part of the helium was diverted through the hydrocarbon saturator, 9, and rejoined the main carrier gas stream a t 16. The combined stream then flowed through the reference side of the detector (four-filament thermal conductivity cell, 17) and into the adsorption bed of silica gel particles, 18. From the adsorber the gas passed through the sample side of the detector and was discharged through soap-film meter 21. The bed was first saturated with adsorbent by running the equipment until the composition indicated by the sample side of the detector was the same as that indicated by the reference side. Then the amount of hydrocarbon desorbed was measured. The desorption was carried out by first heating the bed to 200' C. and flushing the hydrocarbon gas in the connecting lines with helium. Then helium was passed through the bed and the area of the recorded e.m.f. curve from the detector was computed automatically with a Perkin-Elmer integrator. The moles of gas desorbed were obtained by comparing the area with that resulting when a known amount of hydrocarbon was 100
IhEC FUNDAMENTALS
L Figure 1.
Flow diagram for pure-component adsorption 0 , 7, 22. Silica gel-activated charcoal beds 1 , 2, 1 1. Three-way valves 3. Pressure gage 4 3 . Ball flowmeters 6, 21. Soap fllm meters 9. Hydrocarbon saturator 8, 10. Needle valves 12, 13. Pressure-reguloting valves 14, 15. Restricting tube 17. Detector 18. Fixed-bed adsorber 19. Injection valve 20. Recorder 23. Bypass valve 24. Three-way stopcock
the helium stream in a dry ice bath. These results agreed well with those obtained by assuming complete saturation of the helium stream. The fixed-bed adsorber was made of glass tubing ( i d . = 0.75 cm.) and surrounded by an electric heater for a length of 11 cm. The bed of silica was approximately 2.0 cm. in length and both ends included short sections of glass beads and glass wool to hold the gel in place. The temperature was measured with a Chromel-Alumel thermocouple. Axial variations were less than 1' C. Temperature variations in the bed were not a problem because of the low concentrations of adsorbent gases (maximum = 2%). Properties of Materials. The properties of the silica gel (Davison Chemical Co.) are as follows: Surface area, S Void volume, V Void fraction Particle density Av. pore radius
EXPERIMENTAL DATA
REPRDDUCIBI L I T Y TESTS ' I AGED S I L I C A 7 FRESH S I L I C A
A
V
7OoC 0 90°C
llD°C 0 13PC
I -
CALC.
FROM EO. ( I )
12.5
-
€35
IO w
832 sq. m./g. 0.43 cc./g. 0.486 1.13 g./cc. = 2 V / S = 11 A.
= = = =
W
a W
w
0 U W
u
U LL
3
v,
Both hydrocarbons were spectrograde, of more than 99 weight % purity, from the Eastman Organic Chemicals Co. The helium was stated to have a purity of 99.99 weight %. All gas streams were further purified by silica-activated charcoal beds, as indicated in Figure 1. Absorption equilibrium data for pure benzene and pure nhexane were obtainrd under the following conditions: Temperature. 70°, 9 O 0 , l l O o , 130' C. Hydrocarbon partial pressure. 0.0 to 0.02 atm. Average particle diameter, silica gel. 0.0137 cm. Total pressure. 1 atm. Max. pressure drop across bed. 0.01 atm. Mixture Adsorption, Separate equilibrium isotherms were obtained for each component when mixtures of benzene and n-hexane were adsorbed by analyzing the total stream desorbed from a previously saturated bed. The procedure was that described for single-component adsorption, except that separate hydrocarbon 5,aturators were used for each hydrocarbon. This permittrbd a known mixture of benzene, nhexane, and helium to be passed through the bed of silica gel. The desorbed gases were analyzed with a gas chromatograph (Wilkens Model 90-P3). A seven-way valve was employed to inject the sample gas into the chromatograph and to introduce synthetic samples for calibration. A dual column consisting of a 2-foot, 3-inch length of SE 30;followed by a 2-foot, 10-inch bed of Ucon gave complete separation between benzene and n-hexane peaks and a short resolution time (80 seconds). Details of the supports, liquid phase absorbent, and operating conditions for the dual column apparatus are available (Shen, 1967). The relative composiiion of the mixture was described by w , the ratio, po/ph, of the individual partial pressures in the gas mixture in equilibrium with the bed of gel. Isotherms for each component were obtained at the same total pressure and temperature as for the single-component data and for w = 0.087, 0.24, 0.94, and 5.3. Partial pressures of the individual hydrocarbons in the bed ranged from 0 to 0.012 atm.
5
0
0.008
0.0211
0.016
PARTIAL PRESSURE OF n-HEXANE ( am. 1
Figure 2. hexane
Equilibrium adsorption isotherms of pure nHigh pressure range for 90' to 130' C. l o w pressure range for 70' C.
REPRODUCI B i 1I TY TESTS
EXPERIMENTAL DATA
A
90.C 0 13O0C
ACED S I L I C A FRESH S I L I C A
110.C
-
CALC. FRM EO. ( I )
Results
Single Component. The adsorption isotherms for pure n-hexane are shown in Figures 2 and 3 and for pure benzene in Figures 4 and 5. Ruins a t different times with the same bed of gel were reproducible within 5%. Some tests were made with fresh and aged gel to test the stability of the gel to the regeneration procedure. These runs, shown in the figures, also indicated a reproducibility of 570 or better. T h e nonlinear data could not be represented accurately by either the Freundlich or Langmuir isotherm over the whole pressure range. However, an empirical form of the Langmuir equation
0.002
0.0011
0.0065
PARTIAL PRESSUREOF n-HEXANE I: a b . 1
Figure 3. hexane
Equilibrium adsorption isotherms of pure nlow pressure range
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101
EXPERIMENTAL DATA
V 7OoC 0 90%
A
7 AGED S I L I C A
llO°C
7
0 130°C
-CALC.
represented the data a t any one temperature within about 8%. Values of the constants are given in Table I, and the curves computed using them are the solid lines shown in the figures. According to theory, K2 should be the amount adsorbed for a full monomolecular layer and hence be nearly independent of temperature. The variation of K z from 70' to 130' C., shown in Table I, emphasizes the lack of agreement with the Langmuir theory. The values of the constants were obtained by a least mean square fit of the linear form of Equation 1:
REPROOUCI E l l 1 TY TESTS FRESH S I L I C A
FROM EQ. ( 1 )
The per cent surface coverage shown as left-side ordinates in Figures 2 to 5 are estimates based upon the surface area, 832 sq. meters per gram, measured by low temperature nitrogen adsorption. Thus the moles adsorbed for a monomolecular layer are given by
o.o2r(
0.016
0.008
0
PARTIAL PRESSURE OF BENZENE ( a t # . 1
Figure 4. benzene
Equilibrium adsorption isotherms of pure
Values of the surface area, A , per molecule adsorbed, were taken from Livingston (1949) and Young (1962) and are shown in Table I along with corresponding Nmo calculated from Equation 2. The surface coverage is then NOIN,'. The absolute values of the coverage are subject to error, since nitrogen adsorption was employed to establish the monolayer area. U p to small partial pressures such as covered in Figures 3 and 5 the isotherms are approximately linear. In this region a constant adsorption equilibrium constant may be defined as
High pressure range
(3)
EXPERIMENTAL DATA
REPROOUCI E I L I T Y TESTS
V
V
70°C
0 90'C
AllOOC 0 130'C
-CALC.
A
0
7 4T
Combination of Equation 1, simplified to apply for low pressures, and Equation 3 gives
AGEO S I L I C A FRESH S I L I C A
Kt = KztKitR, T
(4)
Adsorption equilibrium constants computed from Equation 4 are as follows:
FROM EO. ( 1 )
1.5
30
Adsorption Equilibrium Constant, K , Cc./G. n-Hexane Benzene
Temp.,
c.
be
w W p? 4
5g
20
1470 946 375 208
70 90 110 130
5380 3040 1340 657
V W V 4 LL
a ( 3 0
2.5
IO
Other data for K for these components on silica gel do not seem to be available. Schneider and Smith (1968) have reported adsorption equilibrium constants evaluated from low concentration measurements for light paraffins a t 50' C. These results, for the same type of silica gel, are: KCIHa=
Table 1.
0
0
0.002
0.m
0.0065
PARTIAL PRESSM OF BENZENE ( a b . ) Figure 5.
Equilibrium adsorption isotherms of pure benzene low pressure range
102
IhEC FUNDAMENTALS
70 90 110 130
68.6 52.4 33.1 32.0
7.63 4.79 3.60 1.96
58.5 59.8 61.2 63.0
Langrnuir Constants
23.6 23.1 22.6 21.0
115 99.6 49.0 30.8
Nmo: X lo4 g. moles/(g. gel) Ah, Ab: X 10'6 Sq. cm. K1: (atm.)-'
K1,
16.6 10.3 8.69 6.45
33.6 34 1 34.8 35.5
41.1 40.5 39.7 39.0
14.5, K c ~ H = ~63, and K c , H ~=~ 308 cc. per gram. T h e two sets of data indicate the rapid increase in K with molecular weight for aliphatic hydrocarbons and also show the effect of structure, at constant (approximately) molecular weight. Mixture Data. The adsorption of n-hexane was strongly influenced when benzene was also absorbed, while the isotherms for benzene were not greatly influenced by n-hexane. The results at 90' C. shown in Figures 6 and 7 are typical. Results for all temperatures are available (Shen, 1967).
The modified BET theory, first proposed by Hill (1946), has been found effective (Day, 1965; Mason and Cooke, 1965) for predicting the isotherm for a component in a mixture from isotherms for the pure component. Both investigations were for mixtures of the lower aliphatic hydrocarbons. Attempts to predict mixture results such as shown in Figures 6 and 7 from the pure component correlations, using the constants K1 and K1 of Table I, led to deviations as large as 100%. The theory of Myers and Prausnitz (1965), which treats the adsorbed layer as a two-dimensional ideal solution, showed similar deviations. The accuracy of the computed adsorptions by the theory was limited by the range of partial pressures for which the spreading pressure could be calculated. The partial pressure range studied by Day (1965) and Mason and Cooke (1965) was at least an order of magnitude higher than the level reported here. Difficulties in correlating low pressure data by methods which gave good results a t higher pressures have been reported (Hanson and Stafford, 1965; Lewis et al., 1950). Adsorption in the low pressure region involves only the most active sites, and such sites prnbably vary greatly in activity. Evidence for this surface heterogeneity is found in the heat of adsorption results. While no completely successful prediction method was found, the mixture data could be correlated reasonably well using the following equation of Lewis et al. (1950).
Here (h'b)pb means the moles of benzene adsorbed from the is mixture a t a benzene partial pressure, p b , and (ivbo)pb+ph the moles of benzene adsorbed as a pure component at a partial pressure equal toPo p h in the mixture. The data for all temperatures are plotted according to Equation 5 in Figure 8. The data points group according to values of w , as noted in the figure. The purely empirical Equation 5, which is in agreement with the Langmuir adsorption equation, satisfactorily accounts for the competition between hexane and benzene for the active sites on the gel surface. Equation 5 is not sufficient to predict adsorption isotherms for a component in a mixture from the pure component isotherm. I t permits only the calculation of the isotherm of one component in the mixture from that of the other. Heats of Adsorption. The isotherm data at different temperatures can be used to evaluate heats of adsorption, defined as the partial enthalpy, Etr", of adsorbent i in the gas phase minus its partial enthalpy, Etr', on the solid surface.
+
0
0.OOY
0.008
0.012
O.OI+
PARTIAL PRESSURE OF n-HEXANE ( a h 1
Figure 6.
Equilibrium data for n-hexane in mixtures at
90" C.
1 .o
0.8
- EWATION OF LEWIS 0.6
0.q 0.2 0.a 0
0.00Y
0.008
0.012
0.013
0.0
(
PARTIAL PRESSURE OF BENZEE ( ab1
Figure 7.
90" C.
Equilibrium data for benzene in mixtures at
0.4
0.2
Figure 8. isotherms
0.6
0.8
1.0
n.HEXlNE
Correlation of mixture equilibrium adsorption
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103
Rf’ and Rf”- Ri’ will in general
be a function of the amount of adsorption, since the adsorbent and solid surface do not form an ideal solution. If the gas phase is ideal, conventional application of the first and second laws of thermodynamics (Young and Crowell, 1962) gives, for single-component adsorption,
To evaluate Ani,curves of In pi: us. 1/T at constant amount adsorbed, Ni0, were prepared from data obtained by crossplotting the isotherms shown in Figures 2 to 5 . The resulting plots were straight lines, indicating that the variation of ARt with temperature was not significant in the range 70’ to 130’ C. A typical plot for benzene is Figure 9. The slope of the
straight lines, according to Equation 6, is (l/Ro)(AR~).The heats of adsorption for n-hexane and benzene obtained in this way are shown by the solid points in Figures 10 and 11. The ordinate in these figures is AR‘ - AH,,,, where AHvspwas evaluated a t 100’ C. and is 6.0 kcal. per gram mole for nhexane and 7.0 kcal. per gram mole for benzene. For both components decreases rapidly with amount adsorbed, or surface coverage. This is additional evidence of the heterogeneity in activity of the most active sites on the gel surface. Under these circumstances it is not strange that the Langmuir theory is not applicable. The heat of adsorption for each component in mixture adsorption can be determined in a similar way. The equation for (ARb)mix: for example, is (Shen, 1967)
Ani
with an analogous expression available for hexane. To obtain (ARJrnix adsorption isotherms were first plotted for hexane at constant NO and for benzene at constant ivh. Then cross plots from these curves gave partial pressure us. temperature data at constant -hTb and h’h, as required by Equation 7. Plotted as In pf us. l / T these data gave straight lines from which AB1 was evaluated. The results are shown by the open points in Figures 10 and 11. There are two significant features of the hexane data in Figure 10. First, ARh is a strong function of the amount of
8.0 =1.5 x
2.45
2.05
/
4.0
0
20
10
30
40
Nbenzene for
Figure 1 1.
Heat of adsorption of benzene
6.2 5 .V
Y
P
( 9 . moles/g. o f d r y
V.6
0.0
0
1.0 15.0 27.0
0
3.0 I
0
I
Y
Figure 10. l&EC FUNDAMENTALS
0
I Nh x
104
v A
3.8 b
50
Nb x IO5 g. m o l e s 8. o f d r y silica
7.0
-
slllca
2.95
103 T , ( I / K ) 2’65 Figure 9. Plot of In pb vs. I / r a t constant pure benzene adsorption equilibrium data
g. m o l e s g 0 1 d r y
0 IO5,
12 g. moles/g. o f d r y f i I I c a
Heat of adsorption of n-hexane
16
silica
)
1 20
60
Table II. Heats of Adsorption for Single-Component Adsorption (on Silica Gel) Heat o f Adsorptibn, AR, Kca1.l Surface Ref. Adsorbate G . Mole Coverage, % >loo MasonandCooke (1965) n-Hexane 9.05 8.6 100 Schulman (1957)
Benzene
11 .o 11.4 12.6 9.5 12.0 12.0 13.6
5 10 0.5 100 10 10
0.5
Schulman (1957) This work This work Schulman (1957) Schulman (1957) This work This work
the gas mixture. However, AB for hexane did decrease in the presence of benzene and became independent of surface coverage. These results suggest that after the most active sites were occupied by benzene, the next group of sites, available for hexane, were of more or less equal activity. Acknowledgment
The financial assistance of the Petroleum Research Fund, American Chemical Society, through Grant 1633 is gratefully acknowledged. Nomenclature
A benzene adsorbed. When benzene is present, it apparently occupies preferentially the most active sites, leaving for the hexane the less active ones for which the heat of adsorption is relatively low. Second, A R h is independent of the amount of hexane adsorbed when benzene is present. This suggests that the sites available for hexane have about the same activity. The lack of dependence of A n * in Figure 11 on n-hexane agrees with the concept of preferential adsorption of benzene on the most active sites. Previous data on heats of adsorption from mixtures of nhexane and benzene were not found. Several investigations report AB values for single-component adsorption for both substances on silica gel, but not at surface coverages as low as studied here. Comparisons with these results are given in Table 11. The values obtained in this study at 0.5% coverage are larger than published values a t higher surface coverages. At the 10% level the published values agree reasonably well with the present data. Insufficient information is available for comparison of the surface properties of the several gels referred to in the table. According to Schulman (1957), surface properties, such as the degree of dehydration, can influence the heat of adsorption. The similarities in the results in Table I1 at equivalent coverages indicate that large differences in surface structure either did not exit or did not affect AR significantly. Conclusions
Measurement of simultaneous adsorption of benzene and n-hexane at lo^. surface coverages indicates competition for the most active sites, with benzene being preferentially adsorbed. This may be due to the interactions between the ?r electrons in benzene and the hydroxyl ions on the surface of silica gel. Kiselev (1958) and Shapiro and Kolthoff (1950) report that residual hydroxyl ions remain on silica gel up to very high temperatures and also that such ions increase the affinity of the gel for benzene. Kiselev (1958) found that the hydroxyl ions had little effect on the adsorption of n-hexane. The heats of adsorption for both n-hexane and benzene, a t the very low surface coverages (0.5y0)studied, were higher than results previously reported at higher coverages. This indicates surface heterogeneity among the most active sites and explains the difficulty mcountered in representing the adsorption equilibrium data by the usual theories, and the lack of success in predicting simultaneous adsorption of benzene and n-hexane from single-component data. Heats of adsorption of benzene decreased with increasing coverage and were independent of the amount of n-hexane in
=
surface area occupied by one molecule, sq. cm.
= gas concentration, g. moles/cc. =
Ri
=
ARi
= = =
K2
=
Ki
= = =
= = Pb
Ph Pi Pt
=
=
T
= = = = =
V
=
W
= =
4 S
i
partial enthalpy of component i in gas, cal./g. mole; 8,” = Hi“for ideal solution partial enthalpy of i adsorbed on silica gel, cal./g. mole heat of adsorption, cal./g. mole heat of vaporization, cal./g. mole constant analogous to adsorption equilibrium constant in Langmuir isotherm, atm.-‘ constant analogous to amount of adsorption for a monomolecular layer in Langmuir isotherm, g. moles/g. silica gel adsorption equilibrium constant for linear isotherm, component i, cc./g. Avogadros’ number adsorption of i in single component adsorption, g. moles/g. adsorption of i from a mixture, g. moles/g. adsorption corresponding to a monomolecular layer, g. moles/g. partial pressure of benzene, atm. partial pressure of n-hexane, atm. partial pressure of component i, atm. total pressure, atm. gas constant surface area, sq. cm./g. temperature, K. void volume ratio of partial pressures subscript, denoting component i
literature Cited
Brunauer, S., “The Adsorption of Gases and Vapors,” Vol. 1, “Physical Adsorption,” Princeton University Press, Princeton, N. J., 1945. Day, J. J., Ph.D. thesis, University of Oklahoma, Norman, Okla., 1965. Hanson, J. C., Stafford, F. E., J . Chem. Educ. 4 2 , 8 8 (1965). Hill, T. L., J . Chem. Phys. 14, 268 (1946). Kiselev, as reported in “The Structure and Properties of Porous Materials,” D. H. Everett and F. S. Stone, Eds., p. 195, Butterworths, London, 1958. Lewis, \V. K., Gilliland, E. R., Chertow, B., Cadogan. W. P., Ind. Eng. Chem. 42, 1319, 1326 (1950). Livingston, H. K., J . Colloid Chem. 4, 447 (1949). Mason, J. P., Cooke, C. E., “Adsorption of Hydrocarbon Gas Mixtures at High Pressures,” 57th Annual Meeting, A.I.Ch.E., Boston, December 1965. Myers, A. L., Prausnitz, J. M., A.I.Ch.E. J . 11, 121 (1965). Schneider, P., Smith, J. M., to bepublished, A.I.Ch.E. J . (1968). Schulman, J. H., Ed., “Second International Congress of Surface Activity,” Vol. 11, p. 179, Butterworths, London, 1957. Shapiro, I., Kolthoff, I. M., J . A m . Chem. SOC.72, 776 (1950). Shen, John, Ph.D. thesis, University of California, Davis, January 1967. Young, D. M., Crowell, A. D., “Physical Adsorption of Gases,” Butterworths, London, 1962. RECEIVED for review March 3, 1967 ACCEPTEDOctober 18, 1967
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