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Ind. Eng. Chem. Res. 1991,30, 2456-2463
Whisman, M. L.; Goetzinger, J. W.; Cotton, F. 0. Some Innovative Approaches to Reclaiming Used Crankcase Oil. Report RI-7925; Bartlesville Energy Research Center: Bartlesville, OK, 1974. Whisman, M. L.; Reynolds, J. W.; Goetzinger, J. W.; Cotton, F. 0. Process for Preparing Lubricating Oil from Used Waste Lubricating Oil. U.S.Patent 4 073 719, Feb 1978a. Whisman, M. L.; Goetzinger, J. W.; Cotton, F. 0. Method for Reclaiming Waste Lubricating Oil. U.S. Patent 4 073 720, Feb
1978b. Whiaman, M. L.; Reynolds, J. W.; Goetzinger, J. W.; Cotton, F. 0.; Brinkman, D. W. Re-refining Makes Quality Oils. Hydrocarbon Process. 1978c, 57 (Oct), 141-145.
Receiued for reoiew April 16, 1991 Reuised manuscript received July 8, 1991 Accepted July 17, 1991
Adsorptive Separations Based on the Differences in Solute-Sorbent Hydrogen-Bonding Strengths Nirmalya Maity, Gregory F. Payne,* and Jennifer L. Chipchosky Department of Chemical and Biochemical Engineering and Center f o r Agricultural Biotechnology, Uniuersity of Maryland, Baltimore County, Baltimore, Maryland 21228
Selectivity in adsorptive separations can be enhanced by limiting solute-sorbent interactions to a single or a few specific mechanisms. This work examines the potential of exploiting solute-sorbent hydrogen bonding as a selective adsorption mechanism, for solute adsorption from a nonpolar solvent onto a polycarboxylic ester sorbent. The hydrogen bond is believed to be formed between a proton-donating group on the solute and the carbonyl group on the sorbent. Studies were conducted for three classes of solutes, all of which can hydrogen bond, to determine whether differences in the strengths of adsorption can be exploited for separations. The enthalpies for adsorption from the nonpolar solvent onto the polycarboxylic ester sorbent were determined from calorimetry to be -5.1, -6.4, and -8.2 kcal/mol for the adsorption of N-methylaniline, alcohols, and phenols, respectively. In single-solute-adsorption studies with these solutes, we also observed a strong correlation between the adsorption affinity and the adsorption enthalpy. In studies on the adsorption from mixtures of two solutes, we observed that the solute with the higher adsorption enthalpy was preferentially adsorbed and that the temperature dependency of the separation factor could be related to the difference in the adsorption enthalpies of the two solutes. A simple thermodynamic framework, using data from single-solute studies, was capable of successfully predicting separation factors and the temperature dependence of separation factors.
Introduction Adsorption is gaining wider acceptance for large-scale separation from liquids (Sircar and Myers, 1986). The low-energy nature of adsorptive separation process can be advantageous compared to distillation for the separation of liquid mixtures of low volatilities or for mixtures with relative volatilities close to 1 (Ruthven, 1984). In comparison to extraction, the high concentrating abilities of adsorption have been exploited especially for the more efficient removal of solutes from dilute aqueous solutions (Faust and Aly, 1987). Currently, however, because of the difficulties in establishing multistaged contacting, adsorption operations are generally conducted in semibatch fixed bed mode, although complex equipment can be utilized to stimulate moving bed operations (Broughton et al., 1970). Fixed bed operation is ideally suited for two types of problems. In the first type, fixed bed operation is utilized with a nonselective sorbent to adsorb a wide range of solutes from a stream. An example of this problem is the removal of of organics from wastewater streams using activated carbon (Faust and Aly, 1987). In the second type of problem the goal would be to selectively adsorb one solute from a solute mixture. To achieve this goal, highly selective sorbents are needed. An example of such sorbents are molecular sieves which limit nonspecific adsorption through steric constraints. Thus, by using molecular sieves, it is possible to achieve high separation factors and therefore greatly reduce the need for multistage contacting to achieve separations of one solute from a mixture of solutes (Collins, 1968; Breck, 1974). The use of molecular sieves has thus demonstrated the potential
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of adsorptive separation when highly selective sorbents are available. These results have also stimulated the development of alternative sorbents which confer selectivity not through steric limitations, but rather by limiting adsorption to specific chemical interactions (Alexandratos and Kaiser, 1990). The development of such sorbents has been greatly facilitated by advances in polymer synthesis methods, which has made it possible to produce polymeric sorbents of well-characterized and uniform chemical surfaces (Albright, 1986). In our research, we have focused on hydrogen bonding as an appropriate adsorption mechanism. Hydrogen bonding is a useful mechanism for separations because the low energy of this bond ensures reversibility (Le., for recovery through desorption), while the directionality and the short range of this "bond" confer selectivity. In initial studies, we observed that solutes capable of hydrogen bonding were adsorbed onto a polycarboxylic ester sorbent (Amberlite XAD-7). Solutes unable to donate a proton for the formation of a hydrogen bond (e.g., benzene, anisole, and N-methylindole) were not adsorbed onto this sorbent (Payne et al., 1989). Subsequently we examined the potential of this polycarboxylic ester sorbent for use in separations and obtained high separation factors when one solute which could hydrogen bond was adsorbed from a mixture containing a second solute which was unable to hydrogen bond. Further, we observed that although the primary and the secondary amines aniline and Nmethylaniline, respectively, were adsorbed with similar enthalpies, the primary amine was selectively adsorbed from the mixture with a separation factor of 2 (Payne and 0 1991 American Chemical Society
Ind. Eng. Chem. Res., Vol. 30,No. 11, 1991 2457 Ninomiya, 1990). Because of the similarities in the enthalpies, we believe the adsorption of both aniline and N-methylaniline involves the formation of a single hydrogen bond and the selective adsorption of aniline results because the primary amine has two hydrogen atoms capable of hydrogen bonding while the secondary amine has a single hydrogen atom capable of hydrogen bonding. The goal of the work described here is to examine the potential for exploiting the hydrogen-bonding mechanism for the selective adsorption of solutes which are all capable of hydrogen bonding with the sorbent (through a single hydrogen atom) but with differing binding strengths. We have also used a simple thermodynamic framework to explain our experimental observations on the adsorption of individual solutes and the selective adsorption of solutes from solute mixtures.
Material and Methods The solutes used in this work included the phenols phenol and p-cresol; the alcohols benzyl alcohol, phenylpropanol, and phenylbutanol; and the aromatic amine N-methylaniline, Phenol (crystalline, 99% + pure) was purchased from J. T. Baker & Co. while p-cresol, benzyl alcohol, phenylpropanol, phenylbutanol, and N-methylaniline were purchased from Aldrich Chemicals (99% + pure). Two nonpolar solvents, n-hexane and isooctane (optima grade), were obtained from Fischer Scientific. All solutes and solvents were used without further purification. Since n-hexane was used as the model nonpolar solvent in most of our previous studies (Payne et al., 1989; Payne and Ninomiya, 1990; Maity and Payne, 1991),we continued to use this nonpolar solvent. However, a solvent of lower volatility, isooctane, was more desirable for use in calorimetric studies. Preliminary studies (not shown) demonstrated that the adsorption affinity was the same independent of whether hexane or isooctane was used as the solvent. The sorbent used was the polycarboxylic ester sorbent Amberlite XAD-7, which is commercially available in the form of macroporous beads from Rohm and Haas. Solvent-phase solute concentrations were measured using a Gilford Response UV spectrophotometer. Generation of Equilibrium Data for Single-Solute Adsorption. Adsorption studies were conducted in which the solutes were adsorbed from a nonpolar solvent (hexane or isooctane) onto the macroporous beads of the polycarboxylic ester sorbent. Prior to use, the sorbent was rinsed with methanol and the nonpolar solvent and the sorbent was heated to evaporate the residual solvent. Equilibrium data were generated for each solute at three different temperatures in the range 2 0 OC. In each case, weighed amounts of the sorbent were added to a dilute solution of the solute in a nonpolar solvent. After equilibration a t a fixed temperature the liquid-phase solute concentration was measured by UV spectrophotometry at the wavelength of peak absorbance for that solute. The adsorbed (solid-phase) solute concentrations were calculated using the following equation: q = (C" - C ) V / A (1) where q is the equilibrium solid-phasesolute concentration (mmol/g sorbent); Co and C are the initial and final (equilibrated) liquid-phase solute concentrations (mmol/L), respectively; V is the liquid volume (L); and A is the amount of sorbent (g). The adsorption affinity ( q / C ) is then defined as the equilibrium ratio of the solid-phase solute concentration (mmol/g sorbent) to the liquid-phase solute concentration (mmol/L). As will be discussed, the adsorption affinity has been used in our studies as a measure of the adsorption equilibrium.
Measurement of Adsorption Enthalpies. Adsorption enthalpies were measured using both van't Hoff relationships and direct calorimetry. 1. Measurement of Enthalpies Using van't Hoff Plots. In the van't Hoff method the temperature dependence of the adsorption equilibrium constant can be related to the adsorption enthalpy. The van't Hoff methods utilizes two thermodynamic relationships, the first being AGO = -RT In K = -Rt In (\k(q/C)) (2) where AGO is the standard free energy of adsorption, R is the universal gas constant, T is the absolute temperature in degrees kelvin, and K is the equilibrium constant for the adsorption process. By confining our studies to low solute concentration, adsorption was limited to the linear region of the isotherm, and therefore we believe that the equilibrium constant can be directly related to the adsorption affinity (q/C). The proportionality factor \k includes terms for the activity coefficients of the solute in the two phases and the activity of the unbound adsorption sites. Since we confined our studies to a narrow range of solute concentrations (in both the solid and liquid phases), it is possible that \k will be constant over the conditions studied. The second thermodynamic relationship used in the van't Hoff method is AGO = AHo- TASO (3) where AHo and ASo are the standard enthalpy and entropy changes of adsorption. Combining eqs 2 and 3, we then have
If AHo, ASo, and \k remain constant over the temperature range studied, eq 4 suggests that a plot of In ( q / C ) versus 1/T should yield a straight line with a slope of
-AHO/R. 2. Measurement of Enthalpies Using Direct Calorimetry. The second method used to measure adsorption enthalpy was calorimetry. This latter method is direct in that it does not require measurements of adsorption equilibria at various temperatures. An adiabatic solution calorimeter was used for this purpose. The solution (with isooctane as the solvent) was placed in a well-lagged double-walled Dewar flask, and the sorbent was placed in a glass cell above the Dewar. The sorbent was added by opening the glass cell and emptying its contents into the solution in the Dewar. The solution was constantly agitated to ensure proper contact with the sorbent, and the temperature was measured at all times using a solid-state thermistor. The temperature-time plot thus generated (i.e., the thermogram) was used to determine the enthalpy of adsorption. The macroporous sorbent beads were crushed to remove any diffusional resistances and speed up adsorption. Also the sorbent was soaked with a small amount of isooctane prior to use so that temperature changes, due to the heat produced by the dry sorbent contacting the pure solvent (similar to the heat of wetting), could be avoided. A typical thermogram has been shown in Figure 1. The point A on the time axis of the thermogram corresponds to the time when the sorbent is added. It can be seen that prior to sorbent addition the temperature-time plot is linear. As can be seen on the thermogram, there is a sharp increase in temperature, at time A, immediately following the sorbent addition. This temperature rise stops at time B. Independent concentration measurements showed that, after time B, there was no change in the liquid-phasesolute
2458 Ind. Eng. Chem. Res., Vol. 30, No. 11, 1991 0.4
23.2
i
0.3 0 z
0
-E
0.2 I
U
0
5
10 t
15
20
25
0.1
(minutes)
Figure 1. Typical temperature versus time plot (thermogram) generated by adiabatic calorimetry, for the adsorption of phenol from isooctane onto the polycarboxylic ester sorbent (Amberlite XAD-7). Initial phenol concentration in the liquid phase was 0.02 mol/L, the amount of sorbent used was 20 g, and the mass of solution used was 70 g.
concentration, demonstrating that adsorption was complete at time B. To obtain a consistent measurement of the temperature rise, AT,, from the thermogram, a graphical method suggested in the literature (Ramette, 1984; Hemminger and Hohne, 1984) has been followed. The preadsorption baseline (before time A) and the postadsorption baseline (after time B) are extended and the temperature difference, AT,, between them is measured (as shown in Figure 1)at the time the adsorption process is approximately 63% complete. The standard enthalpy change due to adsorption AHo, in kcal/mol can then be estimated from the relation AHo = S ( A T , ) / M (5) where S is the heat capacity of the system and is given by S = (Cpimi+ Cpsms+ S,) (6) Here Cpiis the heat capacity of the solution (in isooctane) in kcal/(g°C), miis the mass of the solution in grams, C is the specific heat capacity of the sorbent in kcall m, is the mass of sorbent in grams, and S, is the heat capacity of the calorimeter in kcal/OC. The values used for Cpi, Cpa,mi, and S, are listed in the Nomenclature section. M is the number of moles of the solute adsorbed, which is obtained from the difference in the initial and the final solute concentration in the liquid phase and the liquid volume. Determination of the Separation Factors for Adsorption from a Mixture of Solutes. In each of the adsorptive separation studies, the sorbent was contacted with a solution containing a mixture of two solutes in a nonpolar solvent (either hexane or isooctane). After equilibration the solvent-phase concentrations for each of the solutes were determined using a method based on UV spectrophotometry. Assuming that the solutes in the mixture do not interact in a way that affects each other's UV absorption spectrum, the UV spectrum for the mixture is simply the algebraic sum of the UV spectra for each of the individual solutes. This assumption was verified experimentally for the solutes used in these studies. Also, it should be noted that the use of UV spectrophotometry to measure the concentrations of two solutes was facilitated by the fact that the absorption maxima for the individual solutes in our mixtures are different. Thus by measuring the W absorbance at two wavelengths (Le., the wavelength of maximum absorbance for the individual solutes), it was possible to calculate the liquid-phase concentrations of both the solutes in the mixture.
(pod
n ...-n
, 0
.
,
amine
.
1
; .
i '
C
(mmollL)
; .
; '
phenol 0
pcresol
I
phenyl methanol
0
phenyl propanol
I
phenyl butanol
0
N-methylaniline
Figure 2. Adsorption isotherms a t 25 "C for the adsorption of the three classes of solutes (viz., the phenols, the alcohols, and the aromatic amine) from hexane onto the polycarboxylic ester sorbent. Adsorption affinity values a t 25 " C ,for each of the solutes, are also listed in Table I. Adsorption equilibrium data shown here have been obtained from previously reported work (Payne and Ninomiya, 1990; Maity and Payne, 1991).
The adsorbed, solid-phase, concentrations for the individual solutes of the mixture, q1 and q2, are calculated from relationships similar to eq 1. The separation factor a12(which is a measure of the selectivity of adsorption) is then defined as "12
=
(Ql/Cl)/((?2/C2)
(7)
where q l / C 1and q2/C2are the affinities for adsorption of the individual solutes 1 and 2 from the mixture. The separation factor has also been determined at three temperatures in the range 20-50 "C.
Results and Discussion Adsorption Equilibria. To understand how the chemical nature of the solute affects adsorption, we examined the adsorption equilibria for three classes of solutes-phenols, alcohols, and an aromatic amine. The isotherms for the adsorption of these solutes from a nonpolar solvent onto the polycarboxylic ester sorbent are shown in Figure 2. As can be seen from Figure 2, the phenols (phenol and p-cresol) have the highest adsorption affinities while the alcohols (benzyl alcohol, phenylpropanol, and phenylbutanol) have intermediate adsorption affinities. It can also be seen that all the alcohols have similar adsorption affinities irrespective of the number of their methylene groups. The aromatic amine, N methylaniline, showed the lowest adsorption affinity. It should be noted that throughout our work we employed two nonpolar solvents, hexane and isooctane, and that adsorption was not affected by our choice of solvent. For all three classes of solutes, adsorption from nonpolar solvents onto the polycarboxylicester sorbent is believed
Ind. Eng. Chem. Res., Vol. 30,No.11, 1991 2459
A slopes of adsorption isotherms
4
