ADSORPTION SEPARATION FACTORS OF LIQUIDHYDROCARBON h/IIXTURES
787
Adsorption Separation Factors and Selective Adsorbent Capacities of Some Binary Liquid Hydrocarbon Mixtures1
by Carleton N. Rowe2and Robert W. Schiessler Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania (Received September 20,1966)
Separation of binary liquid mixtures by adsorption on solids can be adequately expressed in terms of the separation factor (a)and the selective adsorbent capacity ( 2 ) . Langmuir’s adsorption theory is extended to binary liquid mixtures. An expression has been derived for a single-stage system which permits the direct and simultaneous determination of the equilibrium constant and the selective adsorbent capacity, both for adsorption for mixtures. Under certain conditions the equilibrium constant is identical with the separation factor. This identification makes it possible to evaluate certain thermodynamic properties of the physical displacement reaction a t the surface. An equation for expressing the separation of multistage systems has been modified so that both a and z can be determined directly. Separation factors and selective adsorbent capacities have been determined for two low- molecular weight and ten high molecular weight hydrocarbon systems with alumina and silica gel. Values of a vary from 1.8 to 17.4 with alumina and 9.0 to 28.8 with silica gel. The selective adsorbent capacities vary from 0.011 to 0.130 cc of liquid per gram of adsorbent on alumina and 0.06 to 0.31 cc of liquid per gram of adsorbent on silica gel. These capacities bear no simple relationship to pore volume. Values of a and x are discussed in terms of the chemical constitution of the hydrocarbon systems studied.
Introduction The process of adsorption is an important separation tool for liquids because of its wide applicability, its simplicity and speed of operation, and the inexpensive equipment required. The separations obtained are the result of a concentration of one of the liquid components in a boundary layer on the surface of the solid. This concentrating action is a direct result of the heterogeneous affinities which normally exist between the surface of the solid and the molecular species in a contiguous phase. Thus, for a given solid adsorbent, the magnitude of the attractive forces will depend on the molecular structure and constitution of the species at the solid-liquid interface. The adsorptive process is a dynamic and an equilibrium one between the adsorbed and liquid phases. Quantitative separations of binary mixtures may be expressed adequately by two parameters : a dimensionless number expressing the relative adsorptive afinity of the components for the adsorbent (called the separa-
tion factor), and a number expressing the selective capacity of the adsorbent (i.e., that part of the adsorbent’s capacity which discriminates between the mixture’s components). It must be emphasized that the “selective adsorbent capacity” is here defined as the total volume of material (expressed as liquid adsorbed per unit of adsorbent) that is enriched in the preferred component and is in equilibrium with the contiguous liquid phase. Any segment of adsorbed phase having the same composition as the liquid phase is unselective and is not included in the “selective adsorbent capacity.” I n 1949, Mair, Westhaver, and Rossini3 introduced in adsorption the separation f a ~ t o ra, , ~which ~ ~ is defined as
(1) Taken from a Ph.D. Thesis by C. N. Rowe, 1955. (2) Research Department, Socony Mobil Oil Co., Inc., Princeton,
N. J. (3) B. J. Mair, J. W. Westhaver, and F. D. Rossini, Ind. Eng. Chem., 42, 1279 (1950).
Volume 70, Number 3 March 1966
788
where N indicates mole fraction, A and B represent the components with A being preferred by the adsorbent, and the superscripts a and 1 refer to the selectively adsorbed and liquid phases, respectively. This expression is analogous to the expression for relative volatility in fractional distillation. Because an unambiguous method has not been discovered for physically separating the “selectively” adsorbed phase from the equilibrium liquid phase, direct experimental analysis of the selectively adsorbed phase is still impossible, as is direct determination of its quantity. As a consequence of this rather troublesome difficulty, Mair and co-workers3 determined the capacity of a specified adsorbent for several low molecular weight hydrocarbons by vapor phase equilibration, and assumed the average of these capacity values equalled the selectively adsorbed phase in liquidadsorbate equilibria. Employing an equation for separations by column (hereafter called the dynamic method) which is rather difficult to conceive theoretically, they were able to approximate separation factors. Subsequently, Schiessler and ROW@ combined the vapor phase equilibration method in estimating the selective adsorbent capacity with a material balance equation a t equilibrium conditions to calculate adsorption separation factoi’s for a one-stage system (static method). Hirschler and ;\lertes6 have defined the selective adsorbent capacity in terms of pore volume, which was obtained by vapor phase equilibration studies, to calculate separation factors and adsorption isotherms for static systems. Separation factors determined by all three groups of ~ o r k e r sdecreased ~ ~ ~ ~ ~considerably with increasing concentration of the preferred component in the equilibrium liquid mixtures. This absence of constancy of separation factor over a range of concentration suggested that either the assumptions made in the measured “selective adsorptive capacities” were erroiteous, or else adsorption separation factors per se are not very meaningful if they are thus dependent 011 the composition of the equilibrium liquid phase. The initial objective of this study was to develop a method for determining accurate selective adsorbent capacities and accurate separation factors. Such a method mas discovered and was reported briefly.’ It involves the elimination of the preferred component’s composition between the separation factor expression and a material balance equation for equilibrium, followed by linearizing the resulting expression to The Journal of Physical Chemistry
CARLETON N. ROWEAND ROBERT W. SCHIESSLER
where V A ~is‘ the volume fraction of component A in liquid phase at equilibrium, V A l is the volume fraction of component A in initial mixture, VBel = 1 - VAe’, M is the mass of adsorbent, X is the volume of initial liquid, 2 is the volume of selectively adsorbed phase per unit mass adsorbent, and a is the separation factor. Linear plots of v A e ’ os. v A ~ ’ v B ~ ~ M / ( v A ~ - vAel)x permitted the simultaneous evaluation of the selective adsorbent capacity and the separation factor from the slope and the intercept, respectively. Using this method, our findings for several binary hydrocarbon mixtures on alumina and silica gel are now reported. In addition, it seemed desirable to extend Langmuir’s theory of adsorption to binary liquid mixtures by including the “selective adsorbent capacity” concept, and then to consider the thermodynamics derivable from such an extension.
Theoretical Extension of Langmuir’s Adsorption Theory. There are two important aspects of the extension of Langmuir’s concept for gaseous adsorption to the adsorption of binary liquid mixtures. First, the constant in gaseous adsorption signifying the volume of gas in a monolayer will be taken to signify the volume of liquid in the enriched adsorbed phase (any adsorbate having the composition of the equilibrium liquid phase will not be included in the constant), and second, the adsorption separation will be considered to proceed by a displacement reaction. A t the moment a solid surface comes in contact with a binary liquid, the composition of the adsorbed phase will be identical with that of the liquid phase. Separation will then occur by the reaction
A
+ BS,AS
ki
kz
+B
where A and B are the components with A being preferred by the adsorbent, and subscript S signifying molecules which are adsorbed. This reaction as written assumes that one molecule of A displaces one mole(4) The separation factor may be expressed equally well in terms of mole, weight, or volume fraction without influence on the numerical value.6 Analogously defined separation factors are useful in quantitative study of all physical separation processes, including distillation and extraction. (5) R. W.Schiessler and C. N. Rowe, J . Am. Chem. SOC.,75, 4611
(1953). (6) A. E.Hirschler and T. S. Mertes, Ind.Eng. Chem., 47, 193 (1955). (7) C. N. Rowe and R. W. Schiessler, J . Am. Chem. Soc., 76, 1202 (1954).
ADSORPTIONSEPARATION
FACTORS OF
LIQUIDHYDROCARBON RIIXTURES
cule of B. The equilibrium constant ( K ) for the reaction is
(3)
+
Let 0 = fraction of enriched adsorbed phase (A B) occupied by component A, VA' = volume fraction of A in liquid phase any time preceding equilibrium, V A ~= ' volume fraction of A in liquid phase a t equilibrium, z = volume of enriched adsorbed phase (A B) per gram of adsorbent, and Z A = volume of A in enriched adsorbed phase per gram of adsorbent. Derivation of the equation for the adsorption of component A is
+
rate of adsorption of A
=
kl(l
(Note that composition of B is considered in the desorption of A because the net A desorbed equals the net B adsorbed.) At equilibrium kl(1 - @he' = kzeVBe'
(4)
Solving for 8 and substituting K = kl/kz from eq 3
K vAe' KV~"'
+
vB"'
(5)
but because ZA = Bz ZA
= vB"'
K ZvAe' KV~"'
+
Because Z A cannot be directly experimentally determined for binary mixtures, a second relation involving A material balance equation ZA must be employed. for component A between the two phases is VA'X = VA"'(X
- Mz)+ MZA
(7)
where VA' is the volume fraction of A in initial liquid, X is the volume of initial liquid, M is the mass of adsorbent, and VAe', z, and ZA are as defined previously. Elimination of Z A between eq 6 and 7 gives (vAi
- VAel)X
+
v,elz
M
=
Straight-line plots of V A ~ us. ' VA~'VB"'M/(VA' V A " ' ) permit ~ the evaluation of x from the slope and K from the intercept, and demonstrate that z is in fact constant over a major portion of the concentration range. The evaluation of the equilibrium constant (K) of eq 10 assumes that one molecule of A desorbs one molecule of B and vice versa. This assumption will be correct when the cross-sectional areas occupied by both species on the surfaces of the selective sites are equal or nearly equal. Under these condit'ions the separation factor is identical with the equilibrium constant
- 8)Vi
rate of desorption of A = krOVB'
e=
789
KZV~"'
vBe'+ K V ~ ~ '
At very low concentrations, both VAe' and - VA~')Xin (10) approach zero, which VA~'VB"'M/(VA~ is to say, a or K becomes infinite. In visualizing the separation process a t low concentrations, there is an insufficient quantity of the preferred component to satisfy all those sites having a strong preference for A. Consequently, the value of z (the slope of a plot of eq 10) would be expected to decrease. It would also be expected that the preferred component would go to the most selective sites which would have the effect of an increased a in the separation process. Thus, VA"'will approach zero less rapidly than VAe'Vge'M/ ( V A ~- VAe')X. Experimental observations have supported this deduction by exhibiting curvature toward the ordinate a t extreme low concentrations of the preferred component. However, linearity should be observed over a wide concentration range. The extrapolation of the linear relationship to negative values of VA"'is only for convenience in determining the separation factor a efective in the linear range. The results of these studies are concerned with the linear region, and not with the low concentration region where a and z are changing constantly with concentration because they are responding to the spectrum of adsorptive site selectivities. The extrapolation to negative values of VAel for determining a can be avoided by rearranging eq 10 to
Rearrangement gives
K-1 vA"')X VAe1v,"'MZ VBe' KVAe'
(vAi
*-
Inverting and rearranging gives
+
No particular advantage is to be had by either method of plotting. Recently, Klinkenberg* has taken an expression similar to eq 9 and by differentiation obtains an equation relating the maximum amount of preferential (8) A. Klinkenberg, Rec. Trav. Chim., 78, 593 (1959).
Volume 70, Number 8 March 1966
CARLETON N. ROWEAND ROBERT W. SCHIESSLER
790
adsorption to the selective adsorbent capacity (2). Similarly, CY is related to the equilibrium concentration a t maximum conditions. By a plotting technique for which a hyperbola is obtained and using the law of conjugate diameters of conical sections, CY and z can be determined. This technique for determining CY and L is much more indirect than eq 10, but certainly appears to be valid. Thermodynamics of Adsorption Separations. The identification of the separation factor with the equilibrium constant for the displacement reaction a t the adsorbent-liquid interface, and the determination of separation factors at different temperatures, allows the evaluation of several thermodynamic properties for the separation process. The thermodynamic properties and equations are
AF
AS
=
-
-RT In
AH
(13)
CY
- AF
(15)
T
where AF, AH, and A S are the free energy, the heat, and the entropy of the displacement reaction, respectively. The calculation of AH assumes it is constant over the specified temperature range. It should be stressed that these thermodynamic values are for the displacement reaction underlying the separation process. For example, AH is not the heat of adsorption when an adsorbent is brought in contact with a binary liquid mixture, but is the heat evolved when the more strongly adsorbed component displaces the more weakly adsorbed component. Dynamic Adsorption Expression. Because of its simplicity and speed, our “static” method (one-theoretical stage) for determining the separation factor and the selective adsorbent capacity is preferred. However, when the degree of separation is small, increased precision can be obtained by the “dynamic” method since a number of theoretical stages are involved. Mair, Westhaver, and Rossini3 have derived an expression from rate relationships for a given cross section in a column of adsorbent which relates the separation factor with the net amount of either component transported across the cross section. Combining eq 40,41, and 51 of their work gives CY-l=
ZvA
V>(VF$lMpm -
XVA)
(16)
where E V A is the net volume of component A transported, and also equals net volume of component B transported in opposite direction, and Vgl are volume The Journal -of Physical Chemistry
fractions of components A and B in existing macroscopic amount of liquid phase a t equilibrium conditions, which in this case, would be the initial mixturelg M is the mass of adsorbent, and pm is the selective adsorbent capacity per unit mass adsorbent ( x in static method). In the studies reported by hlair and coworkers, p, was determined by vapor phase equilibration with pure hydrocarbon liquids in order to calculate CY. However, linearization of eq 16 to
permits the determination of the selective adsorbent capacity from the slope of the plots of VA1 us. VA’VB~M/ZVA and the separation factor from the intercep t
.
Experimental Section Adsorbents. The adsorbents were alumina (Alcoa, F-20 grade; 80-200 mesh) and silica gel (Davison Chemical Corp., No. 12-08-08-237 ; 28-200 mesh). A large quantity of each adsorbent was purchased in 5-lb lots, which were thoroughly mixed, and each adsorbent was stored in a tightly closed glass carboy to ensure uniformity throughout the studies. X o pretreatment of the adsorbents was made before use. Ntrogen adsorption studies showed that the alumina had a surface area of 190 m2/g and a pore size distribution of 12-20-A radius with a high concentration of pores having a radius of 19 A. The surface area of silica gel was not determined. Hydrocarbons. Toluene was purified by fractionation in a 200-plate helix-packed column a t a reflux ratio of 1 5 : l and collected over CaH2. The fractions used had T L ~ O D 1.49683. n-Heptane (Phillips; 99 mole % min) was passed twice over silica gel; product nZoD1.38765. Methylcyclohexane (Phillips, Technical Grade; 95 mole % pure) was purified by fractionation in a 40-plate column a t a reflux ratio of 20: 1. The material used had nZoD 1.42038. The high molecular weight hydrocarbons had been prepared and purified by various workers at the Pennsylvania State Universitylo and had been stored under pure dry nitrogen in sealed glass ampoules. The hydrocarbons ranged in molecular weight from 138 to 353 and differed widely in structure. Table I lists (9) A measurable quantity of liquid having the original composition must remain, since a quantity of adsorbent in excess of that required to give some separation of the entire charge would be incapable of changing the degree of separation, and therefore would be incapable of exhibiting its separation power. (10) R . W. Schiessler and F. C. Whitmore, Ind. Eng. Chem., 47, 1660 (1955).
ADSORPTION SEPARATION FACTORS OF LIQUIDHYDROCARBON MIXTURES
79 1
Table I : High Molecular Weight Hydrocarbons" Reported"
PSU no.
25
Name
9-n-Octylheptadecane
Structure
&
C,-C-C*
196
6-n-Octylperhydrobenz(de)anthracene
575
Perhydrochrysene
500
7-n-Hexyltridecane
csc-c,
2,2,3,3,5,6,6-Heptamethyl-
c c c c c-~--L-c-c-c-cI I
( 2 p
Mol wt
n%
352.7
1.4468
343.6
1.5063
246.4
1.5194
268.5
1.4389
198.4
1.4440
I
CS
556
heptane
AI!
I C
531
n-Tetradecane
198.4
1.4269
111
l-Cyclopentyl-4( 3-cyclopentylpropy1)dodecane
348.6
1.4688
19
l-Cyclohexyl-3( 2-cyclohexylethy1)hendecane
348.6
1.4738
18
l-Phenyl-3( 2-phenylethyl) hendecane
336.5
1.5173
87
9(2-Phenylethyl) heptadecane
344.6
1.4787
559
1-a-Naphthylhendecane
282.5
1.5379
567
l-Methylnaphthalene
142.2
1.6150
569
cis-Decahydronaphthalene
138.2
1.4789
'See ref 10.
the hydrocarbons, their molecular weights, reported refractive indices, and shows the carbon skeleton structures. All of the hydrocarbons, with the exception of 6-n-octylperhydrobenz(de)anthracene (PSU 196), have melting points below 25", which was the temperature employed for the separation studies. PSU 196 is a mixture of isomers and is normally a supercooled liquid a t ambient temperature. The data from several PSU 196-PSU 25 mixtures with silica gel indicated partial crystallization of PSU 196.
Similar evidence of crystallization was not found with alumina. Volume Fractim-Refractive Index Relationship. A refractive index-composition diagram was evaluated for each pair of hydrocarbons since the refractive index was the property used to analyze the equilibrium binary liquid mixtures. Eight to ten mixtures were prepared by weighing the individual pure components to the nearest 0.1 mg and mixing thoroughly with the aid of heat. The refractive index of the binary mixture Volume 70,Number 3 March 1966
792
CARLETON N. ROWEAND ROBERT W. SCHIESSLER
was determined using a five-place Valentine refractometer having a precision of =k0.00002unit. The composition of the mixtures in volume fraction units was calculated from the weights of the components and their densities. The experimental volume fraction and refractive index data were fitted to a three-constant equation by the method of least squares
V
=
A(n)2
+ B(n) + C
(18)
where V is the volume fraction of component preferred by the adsorbent, n is the experimental refractive index, and A , B, and C are constants. For those binary mixtures where volume change upon mixing was appreciable, a density-volume fraction diagram was approximated from the equation
Ad =
da
- dB
nA
-
nB
An
(19)
where Ad is the deviation of density from linearity with composition, d A and dB are the density of components A and €3, n A and nB are refractive indices of componenis A and B, and An is the deviation of refractive index from linearity with composition. The true density of a given composition approximately equals the algebraic sum of Ad and the density computed assuming volume additivity. Static Method. Six to eight 1-cc binary liquid mixtures of varying compositions were prepared in small screw-cap vials lined with aluminuni foil to give a tight seal. The refractive indices were determined to a precision of =k0.00002 unit. The liquid mixtures were weighed to the nearest 0.1 mg. Approximately 0.5 g of adsorbent weighed to the nearest 0.1 mg was added to the mixtures. The vials and contents were placed in a desiccator which in turn was placed in a constant-temperature box at 25.0 f 0.5'. An equilibration time of 4 hr with intermittent manual shaking was found to be sufficient for equilibration. After equilibration, the refractive index of the equilibrium phase was determined, and the composition in volume fraction units was determined using eq 18. On the average, depending upon the difference in the refractive indices of the pure components, a precision of 0.00001 refractive index unit was equivalent to approximately 0.0001 volume fraction unit. The experimental data were linearized according to eq 2, and the selective adsorbent capacity was determined from the slope and the separation factor from the intercept. Dynamic Method. Approximately six binary liquid mixtures of varying compositions 6 to 7 cc each were passed through columns containing 50 g of adsorbent. The columns were 8-mm i d . and 100 cm long. The T h e Journal
of
Physical Chemistry
direction of flow was upward, and the rate of flow was controlled by a pressure head of desorbent in a parallel column. Rates of flow were 0.1 cm/niin. Fractionally distilled methylene chloride (Fischer) was used as the desorbent. The refractive indices of the fractions were determined, and the compositions were evaluated according to eq 18. The volume of component B transported (VB) was determined graphically by plotting volume fraction of A us. volume per cent of charge. (Theoretically V A = U B , but experimentally V B is the more precise since V A may be in slight error due to the presence of desorbent.) The experimental data from all the column runs in the study were linearized according to eq 17, and both the separation factor and the selective adsorbent capacity were evaluated from the intercept and slope, respectively.
Results and Discussions Low Molecular Weight Hydrocarbons. Table I1 lists the separation factors and selective adsorbent capacities for tolueneln-heptane systems on alumina and silica gel a t -80 and 25". The reproducibility is considered good. Graphs of representative studies according to eq 2 are shown in Figures 1 and 2, denionstrating the constancy of the separation factor (a)and the selecthe adsorbent capacity ( z ) over a wide concentration range. For either adsorbent the separation factors are almost three times higher at the lower temperature, while the selective adsorbent capacity is evidently
Table I1 : Toluene-n-Heptane Systems 7 -
Expt no.
1 2 3 4 5 6
Av
----
Alumina--2,
a
c -
( . I
7.7 8.4 9.0 6.9 7.9
CC/P
Expt no.
Temp 25" 0.062 1 0.065 0 060 0.059 0.066 0.062
Silica gel---2.
a
CC/P
14.3
0.261
-
7.9 0.062 ( z t 0 . 5 ) (*0.002)
1 2
20.8
Temp -80" 0.066 1
39.5
0,261
16.2
0.069
41.7
0.267
3 4
21.1 22.8
0.068 0.058
Av
-
20.2 (3r2.0)
-
0.065 (iz0.004)
2
_ .
Av
40.6 (iS.1)
-
0.264 (i0.003)
ADSORPTION SEPARATION FACTORS OF LIQUIDHYDROCARBON MIXTURES
1.00
793
libria data by Lombardo'l were fitted to eq 2 in this study. The calculated values from Lombardo's data are 14.5 for a and 0.264 cc/g for x , which check our results shown in Table 11. KlinkenbergB has fitted the toluene-n-heptane data of Hirschler and llertes6 to eq 12 and found 20.5 and 0.28 cc/g for CY and z, respectively. Klinkenberg also used an indirect method of plotting, which involves the use of the law of conjugate diameters of conic sections, to determine CY and z. He found 18.5 and 0.28 cc/g for CY and x , respectively, by this method. These comparisons with the results in Table I1 illustrate the utility and reproducibility of the method as well as the apparent uniformity in silica gels. Table 111 gives the results for methylcyclohexanen-heptane systems on both alumina and silica gel. The methylcyclohexane was preferred by the alumina, whereas an S-type diagram was observed on silica gel. An S-shaped plot of eq 2 is observed when one component is preferred by the adsorbent over a portion of the concentration range while the other com-
- 0.50 2
-9
*i
E"
0.00
i
Figure 1. Toluentm-heptane on alumina: 0 , T = -80"; 0,T = 25'.
Table I11 : Methylcyclohexane-n-Heptane I
1.00
Alumina (dynamic method) Preferred component
Methylcyclohexane n-Heptane
-
Silica gel" (static method)
E,
2,
a
cc/g
a
cc/ g
2.0
0 017
2 2
0 066
2 5
0 057
"Composition of inseparable mixture = 0.59 to 0.60 volume fraction methylcyclohexane.
0.50
s' d
-9 E"
0.00
0
Figure 2.
4.0
Toluene-n-heptane on silica gel: 0,T = 25'.
0 , T = -80';
independent of temperature. The thermodynamic properties will be discussed later. The results with silica gel at 25" may be compared with data by other workers. "Static" method equi-
ponent is preferred over the remaining portion of the concentration range. The composition of the inseparable mixture on silica was about 0.59 to 0.60 volume fraction of methylcyclohexane and this composition was considered as a single component for calculating a and z. It is probably coincidental that the separation factors on both adsorbents have the same numerical value. A comparison of the selective adsorbent capacities of the above system with those of the toluene-nheptane system indicates the dependence of the selective adsorbent capacity on toluene's electron configuration. The pore volume or "adsorbent" capacity determined by vapor phase equilibration with alumina was 0.192 cc/g for both pure n-heptane and pure methylcyclohexane. Thus it is obvious that the selec(11)
R. J. Lombardo, Ph.D. Thesis, The Pennsylvania State Uni-
versity, 1951.
Volume 70,Number 3 March 1966
794
CARLETON N. ROWEAND ROBERT W. SCHIESSLER
Table IV : Separation Factors (a)and Selective Adsorbent Capacities High Molecular Weight Hydrocarbons on Alumina and Silica Gel
(2)
for Some Systems of
-Silica
-AluminaSystem
gelI,
2,
Method5
a
cc/d
a
S
1.8
0.076
S-type diagram8
S
2.8
0.051
S-type diagram
D
1.5
0.046
D
2.9
0.011
from vol fctn 0 . 1 to 0.9
D
3.6
0.013
S-type diagram
S
15.3
0.100
28.8
0,277
S
7.4
0.089
13.0
0.297
S
3.1
0.130
9.0
0.233
55945
S
15.0
0.097
27.6
0.310
5’67-569
S
17.4
0.058
28.5
0.247
PSU no.
196.25
Structures
C C / d
Ca
PSU 25 preferred from vol fctn 0 . 1 to 0.9
PSU 25 preferred
“SS = static; D = dynamic. * Adsorbent capacity in cubic centimeters of liquid selectively adsorbed per gram of adsorbent. type diagram” is one in which one component is preferentially adsorbed a t one end of composition range, and the other component is preferred a t the other end of the mixture range. The cross-over composition cannot be separated by the subject adsorbent in such systems. ~~
tive adsorbent capacity bears no simple relationship to pore volume. High Molecular Weight Hydrocarbons. Separation factors and selective adsorbent capacities on alumina were determined for ten binary mixtures comprising 13 high molecular weight hydrocarbons, and on silica gel for five binary hydrocarbon mixtures. Figures 3 and 4 show the results for l-phenyl-3(2-phenylethyl)hendecane-9-n-octylheptadecane on alumina and silica The Journal of Physical Chemistry
gel, respectively, which are typical of the results obtained for unsaturate-saturate systems. Figure 5 shows the results for perhydrochrysene-7-n-hexyltridecane on alumina, which is typical of the precision obtained for saturate systems. All the plots are linear in the concentration range studied. Table IV shows the binary systems studied, the corresponding separation factors, selective adsorbent capacities, and the method used for their determina-
ADSORPTION SEPARATION FACTORS OF LIQUIDHYDROCARBON MIXTURES
1.00
0
4.0
VAelVBeIM/( VA'
8.0
- VAe')X, g/CC.
Figure 3. l-Phenyl-3(2-phenylethyl)hendecane9-n-octylheptadecane on alumina.
0
1.0 2.0 V A ~ ~ V B ~ ' M / ( V-A 'V A ' ) ) ~ g/co. ,
Figure 4. 1-Phenyl-3( 2-phenylethy1)hendecane9-n-octylheptadecane on silica gel.
tion on alumina and silica gel. The hydrocarbon listed first in each binary system is the component preferentially adsorbed over the composition range studied for those systems for which values are listed.
795
I
0
10 V A ~ ' V B ' ~ ~ ' MVA' /(
I
-
20
,,
30
V A ~ ~ g/cc. )X,
Figure 5. Perhydrochrysene-7-n-hexyltridecane on alumina.
The static method values are the average from two or more experiments. Averaged over all mixtures, the reproducibility of the separation factor (a)was 5.6% for both alumina and silica gel. The average reproducibility of the selective adsorbent capacity (2) was 6.2% on alumina and 2.3% on silica gel. The dynamic method values are from a single study, but are considered to be reasonably precise because the values calculated from the net volume transported of either component are in agreement. Three of the saturated hydrocarbon systems exhibited S-type diagrams with silica gel, while for the remaining two the component preferred by silica gel was not the one preferred by alumina. If an S-type diagram existed in any of the systems for which values are listed, the inseparable mixture composition must be very close to the pure nonpreferred component, since an S-type diagram would have led to departure from linearity in the plots obtained . The separation factors in Table IV range from 1.5 to 17.4 on alumina, and from 9.0 to 28.8 on silica gel. The selective adsorbent capacities vary from 0.011 to 0.130 cc of liquid/g of adsorbent for alumina (a 12-fold difference), and from 0.233 to 0.310 cc of liquid/ g of adsorbent for silica gel. It is very significant that the selective adsorbent capacity (2) is not a constant for different systems, as was indicated by the vapor phase equilibrium data for liquids. A second important observation is that a and z do not appear to be related, Volume 70.Number 3 March 1966
796
even for systems of similar size molecules. The following qualitative correlations may be made between hydrocarbon structure and values of a and z. A comparison of binaries 196-25 and 575-500 is interesting because of the structural relationships (Table IV). The separation factors (a)on alumina are 1.8 and 2.8 for binary 196-25 and binary 575-500, respectively. The lower value for the 196-25 system may be attributed to the dilution effect on the more strongly adsorbed fused ring nucleus of the n-octyl group in PSU 196 (6-n-octylperhydrobenz(de)anthracene). The selective adsorbent capacities ( 2 ) for 19625 and 575-500 are 0.076 and 0.051 cc of liquid/g of adsorbent, respectively. The higher z for 196-25 may again be due to the n-octyl group, which may protrude away from the surface and thus not occupy sites selective for the fused ring. For this reason a larger liquid volume of PSU 196 would be involved in occupying the same selective sites as for PSU 575. I n fact, only 17 of the 25 carbons in PSU 196 are cycloparaffinic, and 17/25 of 0.076 is 0.051,the same as z for the 575-500 binary. The separation factor of 3.6 for the 556-531 binary is surprising because it is the highest value for the five systems composed of saturated hydrocarbons. The high CY is counteracted by the low value of 0.013 cc of liquid/g of adsorbent for 2 , resulting in less separation than that in the systems 196-25 and 575-500. Relatively few sites are selective for 556-531, but those that are selective have substantial power for giving separation. In contrast, the selective adsorbent capacity for the 575-500 binary is about 300% larger, but its separation factor is 22% less. A reasonable explanation is that the separation factor for a given site is greater for the 575-500 binary than for the 556-531 system, and because of this a greater number of sites are selective for the 57&500 system. However, the additional sites exhibit low separation factors and thus the over-all a! is relatively lower. (The separation factor associated with a heterogeneous surface is believed to be a statistical average of the values of a greater than unity.) A comparison of the 111-25 binary and the 19-25 binary is in effect a comparison of the cyclopentyl and cyclohexyl rings. The system containing the cyclopentyl groups (1 11-25) shows a value of z which is approximately four times that of the system containing the cyclohexyl groups (19-25), but has a value of CY which is only half as large. Once again the system exhibiting the higher a exhibits the lower x . Since the cyclopentyl ring is essentially planar, possibly it can achieve a larger “ordered” electron concentration than the cyclohexyl, giving it a greater adsorptive The J O U Tof~Physical Chemistry
CARLETON N.ROWEAND ROBERTW. SCHIESSLER
potential, and thus increasing the number of sites which have some selectivity for it. Again, such additional sites probably would be of low relative selectivity, and the average a! found would be less than the average a for binary 19-25. This large difference between cyclopentyl and cyclohexyl groups demonstrates that apparently small structural changes may cause large variations in adsorptive behavior. The binaries 19-25 and 18-25 differ only in that the rings are benzenoid in the latter system. In going from cycloparaffinic to aromatic, the separation factor increases fivefold and the selective adsorbent capacity ninefold. The effect of electron density is to increase greatly the number of selective sites, and possibly to orient the molecule on the surface so that the saturated parts of the molecule tend to protrude from the surface, thus permitting a more compact film. A comparison of binaries 18-23 and 87-25 on both alumina and silica gel shows that the separation factor for the diphenyl system is about twice that for the monophenyl system. Assuming no interaction of the molecules in each binary, this might be expected because the adsorbabilities of aromatic hydrocarbons 18 and 87 should be largely determined by the phenyl rings. A continuation of this reasoning would predict separation factors of 2 for binary 18-87 on both adsorbents, whereas the experimental values are 3.1 and 9.0 for alumina and silica gel, respectively. The selective adsorbent capacities for the systems 18-25, 87-25, and 18-87 do not show the same trend on both adsorbents. The significance of this variance and of the trends shown on the respective adsorbents is unknown. The separation factors for binaries 559-25 and 567569 are nearly equal on each adsorbent. The values of z for 559-25 are greater than for 567-569. This difference may again be attributed to the n-undecyl group protruding from the surface. Relative Separation Power of Silica Gel and Alumina. Table V shows the ratio of the separation factors on silica gel to those on alumina to be a constant of 1.78 (*4a/,) for all of the mixtures in which one component was partly aromatic and the other a saturated hydrocarbon. The single system in which both components were partly aromatic gave a ratio which differed from the constant mentioned. This correlation indicates that, having determined the a! for a specific binary system on one of these adsorbents, the CY for the other adsorbent may be estimated with an expected precision of about 4y0. It also suggests that this simple relationship could be used to relate the discriminating power of adsorbents to some “standard” adsorbent for a given
ADSORPTIONSEPARATION FACTORS OF LIQUIDHYDROCARBON MIXTURES
797
Table V: Comparison of the Separation Factors between Silica Gel and Alumina ---Ratio-
u
System
c
% Deviation from average
Silica gel
Alumina
U8iOdQAlnO#
14.3
7.9
1.81
1.7
27.6
15.0
1.84
3.4
28.5
17.4
1.64
7.8
28.8
15.3
1.88
5.6
13.0
7.4
1.75
1.7
Av 1.78
4.0
/
hydrocarbon-type system. An analogous relationship was not found for selective adsorbent capacity. Thermodynamics of the Toluene-n-Heptane System. Table I1 lists the results of a brief study of the effect of temperature on the separation factor for the toluene n-heptane binary. Assuming AH constant over the temperature range -80 to 25”, AF, AH, and A S were calculated according to eq 13, 14, 15 and are listed in Table VI. As previously stated, these equations are valid only when both molecular species have about the same cross-sectional area on the adsorbent’s surface. The differences in the free energy change a t the two Table VI : Thermodynamic Properties of Toluene*-Heptane Systems Property
Alumina
AF at -80”, cal/mole A F at 25”. cal/mole AH, cal/riole ’ A S at -80”, eu/mole A S at 25”, eu/mole
- 1150
Silica gel
- 1220 - 1020
- 1420 - 1580 - 1140
0.67 0.67
1.45 1.47
temperatures on either adsorbent are small. The heats of the displacement reaction on the two adsorbents are similar, with the reaction on silica gel being slightly more exothermic. The entropy changes are positive, indicating an increase in “randomness,” and apparently are constant with temperature for each adsorbent. The entropy increase in the system containing silica gel is approximately twice that of the system containing alumina. This can be interpreted as follows: the displacement reaction, as formulated above, considers the initial liquid phase to be pure component A (which is the component to be preferentially adsorbed), and the initial adsorbed phase to be pure component B. Thus the displacement of the nonpreferred component by the preferred component on the adsorbent’s surface causes mixing in both adsorbed and liquid phases, increasing the “disorder” of the system. Silica gel, exhibiting higher separation power than alumina, produces a higher degree of mixing, reflected then in a higher entropy increase than for alumina. Acknowbdgment. The authors are grateful to the American Petroleum Institute for the funds which made this study possible.
Volume 70,Number 3 March 1966
