Coadsorption of Organic Compounds and Water Vapor on BPL

2380

Ind. Eng. Chem. Res. 1997, 36, 2380-2389

Coadsorption of Organic Compounds and Water Vapor on BPL Activated Carbon. 3. Ethane, Propane, and Mixing Rules Bradley P. Russell† and M. Douglas LeVan*,‡ Department of Chemical Engineering, University of Virginia, Charlottesville, Virginia 22903-2442

Experimental data are reported for adsorption of ethane and propane and their mixtures with water on BPL activated carbon. Pure alkane isotherms were measured at 273, 298, and 323 K from the Henry’s law region to approximately atmospheric pressure. Mixture data, including hysteresis measurements, were obtained at 298 K by fixing the water partial pressure (or relative humidity). Alkane adsorption is significantly affected by water, with loading reductions of up to 60% at a relative humidity of 50%. Our results show that alkanes do not adsorb independently of water, and greater loading reductions can be expected at higher relative humidities. Also, from our data we deduce that approximately 6% of the adsorbent surface strongly adsorbs water, probably on oxide sites. Pure alkane isotherms are analyzed in terms of a lattice model, and a simple empirical method is proposed for calculating mixture equilibria from pure-component isotherms. The method is applied to our data as well as to data for other organic-water mixtures adsorbed on BPL carbon. Results are compared with predictions of previously published methods. Introduction Activated carbons are often used as adsorbents in air purification and solvent recovery processes. Usually, the feed gas to such processes is humid, and steam can be used to regenerate the adsorbent. Commercial activated carbons are partially hydrophilic, and water vapor can affect the process behavior. Thus, there are important practical incentives for studying the equilibrium behavior of water-organic mixture adsorption on activated carbons. Previous experimental studies have shown that these systems exhibit interesting and exceptionally complex equilibrium behavior. Polar surface groups (primarily oxides) are generally present on activated carbons and are responsible for their partial hydrophilicity. There have been numerous studies of the surface chemistry of carbonaceous materials (for a recent review see Boehm, 1994), and there can be many different types of oxygen functional groups on the surface; these groups are generally believed to be located at the edges of carbon layers. Many experiments have confirmed that adsorption of water vapor is strongly affected by the extent of surface oxidation (e.g., Walker and Janov, 1968; Stoeckli et al., 1983; Barton et al., 1984; Miura and Morimoto, 1986; Dubinin et al., 1993; Bradley and Rand, 1993; Kaneko et al., 1995). Recent molecular simulations of adsorption in model activated carbons (Segarra and Glandt, 1994) also reveal a strong dependence of the water isotherm on the degree of surface polarity. Adsorption of other polar molecules on activated carbons is also affected by the extent of surface oxidation (Rodriguez-Reinoso et al., 1992), with the effect increasing with the dipole moment of the adsorbable component. Even adsorption of nonpolar molecules, such as n-hexane, can be influenced by surface oxygen groups on activated carbons (Bandosz et al., 1993). Thus, we expect that the surface chemistry * Author to whom correspondence is addressed. Phone: (615) 322-2441. FAX: (615) 343-7951. e-mail: mdl@ vuse.vanderbilt.edu. † Present address: UOP, P.O. Box 5017, Des Plaines, IL 60017. ‡ Present address: Department of Chemical Engineering, Vanderbilt University, P.O. Box 1604, Station B, Nashville, TN 37235. S0888-5885(96)00533-7 CCC: $14.00

of activated carbons is an important variable in the equilibrium behavior of organic-water-activated carbon systems. An additional variable is the porosity of the adsorbent. The signature of pure water adsorption on microporous carbons is an S-shaped (type 5) isotherm with a hysteresis loop. With nonporous carbons, however, hysteresis is generally absent (Dubinin, 1980). Therefore, the hysteresis behavior of pure water and organicwater mixtures is certainly related to the porous structure of the carbon. A generally accepted model for pure water is the initial adsorption on surface oxides (active sites or primary centers) followed by clustering of water molecules and filling of the porous structure (see Dubinin et al., 1955; Dubinin, 1980; Talu and Meunier, 1996). Pure organics, on the other hand, interact strongly with the carbon surface and exhibit concavedownward (type 1) isotherms. Hysteresis can also occur with pure organics if there is significant mesoporosity in the carbon structure (see Dubinin, 1980; Eissmann and LeVan, 1993). Very few experimental data have been reported for adsorption of organic-water gas mixtures on porous carbons, compared to data for the pure components. Most of the data for these mixtures are for binary mixtures of water and heavy, solvent-type molecules, such as aromatics, alcohols, or halocarbons (e.g., Bering and Serpinsky, 1953, 1959; Okazaki et al., 1978; Grant et al., 1983; Ripperger and Germerdonk, 1983; Matsumura et al., 1985; Cal et al., 1996). Extensive measurements have been performed by LeVan and coworkers (Rudisill et al., 1992; Eissmann and LeVan, 1993; Taqvi and LeVan, 1996) for binary mixtures of n-hexane, acetone, 1,1,2-trichloro-1,2,2-trifluoroethane (CFC-113), dichloromethane, and alcohols with water on BPL activated carbon. These measurements cover a relatively broad range of conditions, including the hysteresis properties of these systems. A general conclusion of the foregoing experimental studies is that water vapor has a significant effect on the organic adsorption under conditions of practical interest, even for strongly adsorbed organics such as toluene or n-hexane. Usually, the organic loading is reduced at a given organic partial pressure when water is present. An exception is methanol, the adsorption of © 1997 American Chemical Society

Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997 2381

which can be enhanced by water (Matsumura et al., 1985; Taqvi and LeVan, 1996). Methods for estimating equilibrium properties for the coadsorption of solvent and water vapors on activated carbons have been proposed by Okazaki et al. (1978), Manes (1983), and Doong and Yang (1987); these methods are reviewed by Tien (1994). There is a need for experimental equilibrium measurements for coadsorption of light organics and water on activated carbons. Light organics can be present in air purification processes, and the studies noted above with heavy organics suggest that water vapor could markedly affect the behavior of processes involving lighter organics. We are not aware of any experimental measurements heretofore for light organic-wateractivated carbon systems. This paper is a continuation of our experimental studies of organic-water mixture adsorption on BPL activated carbon. Previous papers in this series were concerned with heavy, solvent-type organics (Rudisill et al., 1992; Eissmann and LeVan, 1993). The primary purpose of this paper is to present and discuss experimental data for ethane-water and propane-water mixtures. Ethane and propane were chosen as simple, model light organics. Isotherms are reported for the pure alkanes at temperatures of 273, 298, and 323 K. Mixture data, including hysteresis measurements, were obtained at 298 K with a fixed relative humidity of 50%. A second objective of this paper is to interpret our measurements in terms of a model and compare results with predictions of other methods. A lattice model is used for the pure alkane data, and a simple empirical method is proposed for the mixtures. The mixture method can be used to obtain qualitative estimates of binary mixture equilibria for water and alkanes as well as for water and other organics from the pure-component isotherms. Description of Experiments Apparatus. We constructed a volumetric apparatus to measure adsorption isotherms for pure gases and binary organic-water mixtures. Figure 1 is a schematic diagram of our apparatus. The heart of the apparatus is housed in an environmental chamber (Thermotron S-8S-SL) with a programmable temperature controller. The temperature within the chamber can be maintained at a constant value of T ( 0.3 K, with a range of T ) 233-453 K. A variable-speed metal bellows pump (Senior Flexonics MB-41 with a dc motor) is used to circulate material through various loops of the apparatus (with flow rates on the order of 1 std L/min). A data acquisition/instrument control system (Apple Macintosh with National Instruments hardware and LabVIEW software) is used to periodically sample the gas phase (with a VICI gas sampling valve) and record pressure measurements (with MKS pressure sensors). The concentrations of water and alkane in the gas phase are determined with a gas chromatograph (HP 5890A with integrator); a flame ionization detector is used for the alkanes, and a thermal conductivity detector is used for water. A liquid-nitrogen cold trap is used to condense material outside of the environmental chamber in the mixture experiments, and there are adjustable volumes (stainless-steel cylinders) in the gasinjection section and in various loops of the apparatus. Materials. We used BPL granular carbon in 6 × 16 mesh form (Calgon Carbon Corp., Lot No. 4814-J) as the adsorbent in these experiments. This activated

Figure 1. Schematic diagram of the experimental apparatus. The symbols in the diagram denote the following: P, pressure sensor; V, adjustable volume; gsv, gas sampling valve; X, on/off valve.

carbon is predominantly microporous, with a BET specific surface area of approximately 1200 m2/g and a mean pore width (assuming independent, slit-shaped pores) of about 1 nm (Russell and LeVan, 1994). Rudisill et al. (1992), Eissmann and LeVan (1993), and Taqvi and LeVan (1996) used this activated carbon for their measurements of organic-water coadsorption. Purities of the alkanes were greater than or equal to 99.5 mol % in single-component experiments and 99.0 mol % in mixture experiments, and the water was distilled and deionized. Helium was used as the inert gas in all of the experiments. Operating Procedures. Pure alkane measurements were performed by making gas injections. A small quantity of pure ethane or pure propane was injected into a closed loop of known volume which contained the carbon bed. The amount of material injected was determined from the decrease in pressure in the gas-injection volume. The environmental chamber was then slowly cooled to the temperature of interest while the gas was recirculated around the closed loop. The gas phase was periodically sampled, and when the alkane concentration reached a constant value, we recorded the equilibrium concentration. The alkane partial pressure was calculated from the equilibrium concentration, and the amount adsorbed was the difference between the total amount of material in the loop and the amount in the gas phase. The loop volume and the carbon mass were chosen so that a large fraction of the material in the loop was in the adsorbed phase. The apparatus was then cooled to another equilibrium temperature or heated to a higher temperature where another gas injection was performed. The foregoing procedure was repeated, and in this fashion multiple pure alkane isotherms were measured in a single experiment. Prior to these pure alkane measurements,

2382 Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997

the adsorbent was regenerated in situ by passing pure helium through the bed at 373 K for several hours (typically 12-24 h). We confirmed that regeneration was sufficient by measuring the residual alkane concentration in the system. Mixture experiments were performed in an initially open system, where the carbon bed was saturated with a humidified stream of helium and alkane. Pure helium and alkane streams were mixed outside of the environmental chamber, and the mixed stream was saturated with water by sparging. The alkane partial pressure in the feed stream was varied by changing the ratio of pure alkane to pure helium flow rates. The total flow rate through the bed was maintained at approximately 1 std L/min. The water partial pressure was fixed by the temperature of the spargers. Two or three spargers connected in series were immersed in a water bath with a temperature controller (Lauda RM20-B). The bath temperature was maintained within (0.1 K of the set point, and the bath temperature was always less than ambient temperature and the temperature within the environmental chamber. We expect hysteresis when water is present. Therefore, results depend on the path to the equilibrium state. Measurements of the adsorption branch were performed either by slowly cooling the chamber to the equilibrium temperature (298 K) while the temperature of the spargers was held constant (thus fixing the water partial pressure) or by fixing the chamber temperature at 298 K while the temperature of the spargers was slowly increased (thereby increasing the water partial pressure). We confirmed that the adsorption data were independent of these two methods. Desorption measurements were performed by fixing the chamber temperature at 298 K while slowly decreasing the temperature of the spargers (thereby decreasing the water partial pressure). Again, the gas phase was periodically sampled, and the equilibrium concentrations were recorded. The alkane partial pressure was calculated from the measured equilibrium concentration. We allowed the mixture feed to continue for about 1 h after the alkane concentration reached a constant value before analyzing the adsorbed phase. After equilibrium was reached in a mixture experiment, we reconfigured the apparatus in a closed loop that contained the adsorbent bed and a liquid-nitrogen cold trap. The environmental chamber was then heated to 373 K, and the gas was recirculated around the closed loop for a few hours (typically 2-4 h). In this way, the adsorbent was regenerated, and the water and alkane were condensed in the cold trap. The gas-injection section of the apparatus was used as a helium reserve during regeneration, so that the loop pressure was maintained near atmospheric pressure. We confirmed that regeneration was sufficient by monitoring the residual alkane concentration in the loop. After regeneration, the adsorbent was isolated by closing the valves just upstream and downstream of the bed, and the apparatus was reconfigured in another closed loop that contained the condensed water and alkane but did not contain the carbon. The liquid nitrogen was removed, and the cold trap was heated while material was recirculated around the loop. Water and alkane concentrations in this loop of known volume were determined as before, and the individual amounts adsorbed were obtained by subtracting the amounts that were in the equilibrium gas phase from the loop amounts. The carbon mass and the loop volumes were

Figure 2. Isotherms for adsorption of pure ethane and pure propane on BPL carbon. The symbols are experimental data: (.) T ) 273 K, (4) T ) 298 K, and (0) T ) 323 K. The lines are plots of eq 1.

adjusted to avoid the problem of subtracting two numbers of approximately equal magnitudes. The foregoing procedure was repeated with different feed concentrations. We fixed the water partial pressure (relative humidity) and varied the alkane partial pressure. Each mixture data point represents a completely separate experiment. Results Pure Alkanes. Adsorption isotherms for pure ethane and pure propane at three different temperatures are plotted in Figure 2. These isotherms are concavedownward when plotted on rectangular coordinates (type 1). The pressure values cover a range of 2-3 decades for ethane and 4-5 decades for propane, from the Henry’s law region to approximately atmospheric pressure. The loadings were reproducible to within about 3%. Pure Water. Isotherms for adsorption of pure water vapor on BPL carbon at 298 K are plotted in Figure 3. As noted earlier, the signature of this system is an


Figure 3. Isotherms for pure water on BPL carbon at T ) 298 K. Symbols are experimental data: (4) obtained by Rudisill et al.; (]) obtained by sparging with pure helium. Filled and open symbols are for adsorption and desorption branches, respectively.

S-shaped isotherm with a pronounced hysteresis loop. Most of the data plotted in Figure 3 were obtained by Rudisill et al. by making liquid injections with a different experimental apparatus. We obtained four adsorption points and one desorption point by sparging with pure helium, as previously described for the mixture measurements; these points are plotted as diamond-shaped symbols in Figure 3. Our sparging measurements correspond closely with the independent results of Rudisill et al., giving additional confidence in our general procedure for mixture measurements. Alkane-Water Mixtures. Mixture data were obtained at 298 K and at a fixed relative humidity of 50%. From Figure 3 we see that the pure water loadings at this relative humidity (P/Psat ) 0.5) are 6 mmol/g on the adsorption branch and 14 mmol/g on the desorption branch. These loadings are approximately 30% and 60%, respectively, of the maximum water loading. The water loading is reduced when an alkane is present, as shown in Figure 4. The water loading decreases sharply as the alkane partial pressure increases, especially for propane. The sharper decrease in nw for propane compared to ethane is due, in part, to the greater concavity of the pure propane isotherm. That is, the pure-component loading increases more rapidly with pressure for propane than for ethane. An interesting observation from Figure 4 is that nw appears to level off to a nonzero asymptote. That is, a portion of adsorbed water molecules are not displaced by the alkane. This type of behavior was also found by Bering and Serpinsky (1953) in experiments with ethyl chloride and water coadsorbed on active charcoal. These results suggest that a portion of adsorbed water molecules are strongly bound to surface oxides. If this is true, then we can obtain an estimate of the degree of surface oxidation of our BPL carbon sample. From Figure 4, the “asymptotic” water loading is 0.9 mmol/g. If we assume that these strongly bound water molecules are in the form of a unimolecular layer on the surface oxides with a molecular area of 0.13 nm2 (calculated from a van der Waals constantssee McClellan and Harnsberger, 1967), then the specific area of the oxide surface is 70 m2/g. Comparing this with the BET area

Figure 4. Reduction in water loading with increasing alkane partial pressure. The relative humidity is fixed at 50% (i.e., Pw/ Psat w ) 0.50). Filled and open symbols are for adsorption and desorption branches, respectively.

(1200 m2/g), we conclude that approximately 6% of the surface is covered with polar groups that strongly adsorb water. The influence of water on adsorption of the alkanes is shown in Figure 5. The alkane loading is diminished when water is present. Also, hysteresis is induced in the alkane isotherms by water, since there are two water loadings (and therefore two alkane loadings) for a given alkane partial pressure. The reduction in alkane loading is greatest at low partial pressures. Comparing the pure-component isotherms with the mixture data, we find that the alkane loadings are reduced by approximately 40% and 60% for the adsorption and desorption branches, respectively, at the lowest partial pressures. As the alkane pressure increases, the loadings slowly converge to the pure-component values. Our results clearly show that the weakly bound water in the carbon structure influences the alkane adsorption, and the alkanes do not adsorb independently of water.


Figure 6. Isotherms for adsorption of n-hexane on BPL carbon. Symbols are experimental data of Rudisill et al.: (.) T ) 298 K, (4) T ) 323 K, (0) T ) 348 (]) T ) 373 K, and (×) T ) 398 K. The lines are plots of eq 1.

the density function is given by

h(η) )

ηa-1(1 - η)b-1 B(a,b)

(2)

where a and b are parameters of the distribution and B(a,b) is the β function. The local isotherm θ(T,P,η) is given implicitly by

P(T,θ,η) )

exp

Figure 5. Adsorption isotherms for alkanes in the absence of water (4) and in the presence of water with a relative humidity of 50% (]). With the mixture data, filled and open symbols are for adsorption and desorption branches, respectively.

Analysis of Experimental Data Pure Alkane Isotherms. We have interpreted our pure-component measurements, as well as data for adsorption of n-hexane on the same adsorbent (Rudisill et al., 1992), in terms of a lattice model (Russell and LeVan, 1996). The model is of localized adsorption of chainlike molecules on a patchwise heterogeneous surface. We use a group-contribution idea and assume that interior -CH2- groups and -CH3 end groups in the alkanes can be treated as a single type of group, CHx. A dimensionless adsorptive energy for the CHx group is defined as η ) /max, where max is the maximum adsorptive energy on the surface (with g 0). The adsorption isotherm is obtained by integrating over a distribution of energies:

∫01θ(T,P,η) h(η) dη

n(T,P) ) m

(1)

A β distribution is chosen for the energy distribution;

[

]

z - 2θ(r - 1) 1 θ K(T,η) 1 - rθ z(1 - rθ)

{

r-1

×

}

wθ(zr - 2r + 2)2[z - θ(r - 1)] RT[z - 2θ(r - 1)]2

(3)

where z is the coordination number of the lattice and r is the chain length of the alkane. The exponential term in eq 3 accounts for interactions among adsorbate molecules, with w as the potential energy of interaction between two neighboring CHx groups (w < 0 for attraction). The Henry constant K(T,η) for a homogeneous patch of energy η may be written as

(

K(T,η) ) K0(T) exp

)

rηmax RT

(4)

A comparison of eq 1 with experimental data is shown in Figure 2 for ethane and propane and in Figure 6 for n-hexane. The parameters in eq 1 were determined by minimizing the sum of the squares of the errors, as defined by

SSE )

(

∑

)

ncalc - nexper nexper

2

(5)

where the sum is over all pure-component data. Values of the fitted parameters are given in Table 1. The lattice coordination number z was chosen a priori as z ) 6 (see Russell and LeVan, 1996). Also included in Table 1 is a mean deviation between calculated and experimental loadings for each isotherm, defined by

D)

|

∑

100 K


|

ncalc - nexper nexper

(6)

where K is the number of data points in a particular isotherm and the sum is over all points in the isotherm. Agreement between calculated and experimental loadings in Figures 2 and 6 is excellent, and eq 1 fits the data within experimental error. The β distribution for CHx-BPL carbon adsorption energies is plotted in Figure 7. The density function is highly asymmetric and indicates that BPL carbon is very heterogeneous for alkanes. The third central moment (skewness) of the distribution is greater than zero (right-hand widened), as is commonly found with activated carbons (Rudzinski and Everett, 1992). An approximate temperature dependence may be determined for the preexponential factor K0. In the limit P f 0, eq 1 reduces to

n(T,P) ) mP

∫01K(T,η) h(η) dη ) rηmax 1 h(η) dη mPK0(T)∫0 exp RT

(

)

Table 1. Parameters and Deviations for Alkanes on BPL Carbona K0 (kPa-1)

r D

T (K)

ethane

propane

273 298 323 348 373 398

6.29 × 10-4 3.06 × 10-4 1.66 × 10-4

5.62 × 10-3 2.14 × 10-3 9.43 × 10-4

3.38 2.29 2.44 4.56

4.35 1.64 1.52 1.68

273 298 323 348 373 398

n-hexane 0.887 0.186 0.0593 0.0188 7.68 × 10-3 6.80 2.12 1.54 2.10 2.46 2.61

a m ) 29.66 mmol/g. For CH : x max ) 16.97 kJ/mol, w ) -0.03822 kJ/mol, a ) 2.421, and b ) 17.12.

(7)

Applying the relation

ln P (∂ ∂T )

qst ) RT2

n,m

(8)

for the isosteric enthalpy of adsorption to eq 7 and integrating gives

K0(T) )

A exp(q°st/RT)

∫0 exp(rηmax/RT)h(η) dη 1

(9)

if q°st * q°st(T) where A is a temperature-independent integration constant. Values of A and q°st can be determined from eq 9 and the values of K0 given in Table 1. Alkane-Water Mixtures. A complete model for adsorption of pure water or water-organic mixtures on activated carbons must predict hysteresis. Because it does not do so, a lattice model of monolayer adsorption is inappropriate for systems involving water. We would like to interpret coadsorption of organics and water using an equation based on the pore-filling concept. We seek to relate quantity adsorbed of a component to the pore volume left by the other adsorbing component. Of course, many such equations can be written. We chose one that would lend itself easily to graphical analysis. We assume that mixture loadings may be related to pure-component loadings by power-law mixing rules of the form:

( )

ni ) n* i 1 and

nw

l

( )

nw ) n* w 1 -

(10)

n∞w

ni

n∞i

k

(11)

where the subscript i denotes the organic component, the asterisk denotes a pure-component value, and the superscript ∞ denotes the maximum pure-component loading. The term 1 - nw/n∞w in eq 10 is the fraction of

Figure 7. β density function for CHx-BPL carbon adsorption energies, eq 2.

pore space that is not filled with water. Likewise, the term 1 - ni/n∞i in eq 11 is the fraction of pore space that is not occupied by the organic component. When l ) k ) 1, eqs 10 and 11 have a precise physical interpretation: viz., they describe a displacement phenomenon, where water excludes the pore space that it occupies from the organic component and the organic excludes the space that it occupies from water. Previous measurements of organic-water coadsorption (Rudisill et al., 1992; Eissmann and LeVan, 1993) have shown that the fraction of available pore volume that is filled by the mixture depends on the water solubility of the organic component. With waterinsoluble organics, such as n-hexane and CFC-113, only a fraction of the available pore volume is filled with water near saturation, while pore filling is more complete with water-soluble organics, such as acetone. In terms of the mixing rules presented above, incomplete pore filling is manifested in values of l and k that are greater than unity. In this case, the two components exclude more of the pore volume from the other component than they actually occupy. Conversely, pore filling is enhanced when l and k are less than unity. Therefore, we expect a dependence of the exponents l and k on water solubility of the organic component. Maximum pure-component loadings for vapors can be estimated from experimental pure-component isotherms if data are available near Psat. Saturation pressures are


Figure 8. Alkane-water coadsorption: ethane-water (O, l ) 1.8, k ) 5.3), propane-water (4, l ) 1.8, k ) 5.3 ), and n-hexanewater (0, l ) 0.67, k ) 3.4). Open symbols indicate T ) 298 K, and . symbols denote T ) 373 K. n∞i ) m/ri, with m and ri given in Table 1. n∞w ) 23 mmol/g.

easily accessible with water, whereas it may be necessary to estimate n∞i . Methods of estimating n∞i from the carbon micropore volume are discussed by Dubinin (1975). A disadvantage of this approach is that the adsorbate density must be introduced, which is uncertain (especially with supercritical adsorption). An alternative approach is to interpret the organic adsorption in terms of a lattice (flat-surface) model, as we did in the previous section. In this case, the limiting loading is simply n∞i ) m/ri (see Table 1). We have plotted our experimental data for ethanewater and propane-water mixtures, as well as data for n-hexane-water coadsorption (Rudisill et al., 1992), in the form of eqs 10 and 11 in Figure 8 using logarithmic coordinates. The value of n∞w was determined from pure water isotherms, and values of n∞i were calculated from m and ri as described above. If a mixture data point corresponded to an adsorption branch, then we used the pure-component value of n* w for the adsorption branch at the water partial pressure, and similarly for

desorption points the value of n*w on the desorption branch was chosen. Significant reductions in loadings, caused by the coadsorbing component, are apparent from the figure. The data for alkane adsorption, shown in Figure 8a, can be represented reasonably well with straight lines of slopes l ) 1.8 for the light alkanes and l ) 0.67 for n-hexane. The data for water adsorption shown in Figure 8b shows more scatter, with fits giving roughly k ) 5.3 for the light alkanes and k ) 3.4 for n-hexane. Propane-water data at high propane partial pressures deviate significantly because the strongly bound water is not displaced by propane; the model does not account for this type of surface chemical heterogeneity for water adsorption. Also, data for n-hexane-water coadsorption tend to line up in columns in Figure 8b, since these data were taken in an isosteric fashion (with ni fixed). If the model were completely accurate, then each of these columns would converge to a single point. The most noticeable outlier in Figure 8b, for n-hexanewater at 373 K at the top center of the figure, is for a high loading of n-hexane and a low relative humidity (34%, before the water hysteresis loop opens); water was still able to adsorb appreciably at this point relative to its low pure-component loading. The value of l ) 1.8 for ethane and propane indicates that water influences light-alkane adsorption more strongly than n-hexane adsorption; a value of l ) 0 would give ni ) n* i, or waterindependent organic adsorption. From the n-hexanewater data, we see that l and k do not appear to be temperature-dependent. Also, we found no strong, general trend in Figure 8 related to the adsorption and desorption branches of the hysteresis loop. We have compared eqs 10 and 11 with our mixture data in Figure 9. In general, the model is in qualitative agreement with the data. Agreement between calculated and experimental loadings appears to be better for the adsorption branches, although alkane loadings are somewhat too high. On desorption branches, calculated water loadings tend to be too large and alkane loadings somewhat too small. Also, as mentioned previously, the model does not reflect the “leveling off” of nw to a nonzero asymptote as Pi increases. Other Types of Organics. We have applied the power-law mixing rules to experimental data of Rudisill et al. (1993) and Eissmann and LeVan (1992) for mixtures of water with other types of organic molecules. Values of n∞i were obtained from the pure-component isotherms. An alternative procedure for estimating n∞i from critical properties of the organic components is discussed by Russell (1996). Data of Rudisill et al. (1992) for acetone-water coadsorption and of Eissmann and LeVan (1993) for halocarbon-water mixtures are plotted in the form of eqs 10 and 11 in Figure 10. All of these data were obtained in an isosteric fashion (with ni fixed), and we see again that the data tend to line up in columns in Figure 10b rather than converging to a single point. In other words, when ni is fixed, eq 11 indicates that nw is a constant multiple of n* w; that is, the basic shape of the pure water isotherm is preserved. However, the experimental data show that this is only approximately correct. Nevertheless, the data may be represented fairly well with straight lines of slopes l ) 0.27 and k ) 1.2 for acetone, l ) 0.38 and k ) 2.2 for dichloromethane, and l ) 1.0 and k ) 3.0 for CFC-113. Figure 10b shows considerable scatter; the values of k were obtained by least squares analysis. Trends in values of l and k are consistent with water solubility, as anticipated earlier.


Figure 9. Comparison of experimental data for ethane-water coadsorption (O) and propane-water coadsorption (]) with powerlaw mixing rules (dotted lines). l ) 1.8 and k ) 5.3.

They decrease from CFC-113 (essentially insoluble in water) to dichloromethane (slightly water-soluble) to acetone (infinitely soluble in water). Comparison of Methods. The power-law mixing rules are correlative as used above. Results can be compared with those of previously published predictive methods. Table 2 compares deviations, as defined by eq 6, for the power-law relations with predictions based on the methods of Doong and Yang (1987) and Manes (1983). The method of Okazaki et al. (1978) was not used because it requires liquid-phase adsorption data. Overall, the power-law mixing rules are superior, but again it should be stressed that they have been implemented in a correlative manner, not a predictive one. Conclusions Experimental equilibrium measurements were performed for adsorption of light alkanes and their mixtures with water vapor on BPL activated carbon. Purecomponent isotherms were measured for ethane and

Figure 10. Organic-water coadsorption: acetone-water (O, l ) 0.27, k ) 1.2), CFC-113-water (4, l ) 1.0, k ) 3.0), and dichloromethane-water (0, l ) 0.38, k ) 2.2). Open symbols indicate T ) 298 K, and . symbols denote T ) 373 K. n∞a ) 7 ∞ ∞ mmol/g, nCFC ) 4 mmol/g, ndcm ) 8 mmol/g, and n∞w ) 23 mmol/g. Table 2. Comparison of Methods for Calculating Organic-Water Adsorption Equilibria power-law mixing rule

Doong and Manes Yang method method

adsorbates

k

l

Di

Dw

Di

Dw

Di Dw

ethane-water propane-water n-hexane-water CFC-113-water dichloromethane-water acetone-water

5.3 5.3 3.4 3.0 2.2 1.2

1.8 1.8 0.67 1.0 0.38 0.27

16 11 16 37 9.8 6.1

30 41 46 36 20 20

26 17 50 17 75 51

73 140 250 99 68 41

38 71 22 68 30 180 21 86 54 63 N/A

propane. Mixture data were obtained for ethane and propane at 298 K and at a fixed relative humidity of 50%. Hysteresis is introduced in the alkane isotherms, and the alkane loadings are diminished when water is present. The diminution is greatest at low alkane partial pressures, with loading reductions of up to 40% and 60%, respectively, for adsorption and desorption branches. Our data clearly show that alkane adsorption


does not occur independently of water. Therefore, one can expect greater alkane loading reductions at higher relative humidities. Water is rapidly displaced from the adsorbent as the alkane partial pressure increases, but a small amount of water is strongly bound to surface oxides and is not displaced by the alkane. From our data, we deduced that approximately 6% of the adsorbent surface is covered with oxides that strongly adsorb water. Pure alkane isotherms were analyzed in terms of a lattice model that accounts for surface heterogeneity. The model fits the data within experimental error. The analysis indicates that BPL carbon is a heterogeneous adsorbent for alkanes, with a highly asymmetric energy distribution. Mixture data for coadsorption of light alkanes and water were analyzed using simple empirical mixing rules. Application of the relations to other organicwater mixtures gave similar trends. The method appears to be more successful in describing the effect of water adsorption on organic adsorption than vice versa. Results are in qualitative agreement with experimental data, and the mixing rules can be used to obtain rough estimates of binary mixture equilibria from purecomponent isotherms. The mixing rules compare favorably with available predictive methods. Acknowledgment Financial support from the U.S. Army ERDEC is gratefully acknowledged. Nomenclature a ) parameter of β distribution A ) temperature-independent parameter in eq 9, Pa-1 b ) parameter of β distribution D ) mean absolute percent deviation (eq 6) h ) density function for CHx adsorptive energies k ) exponent in mixing rules K ) Henry constant, Pa-1 K0 ) preexponential factor in Henry constant, Pa-1 l ) exponent in mixing rules m ) number of adsorption sites per unit mass of adsorbent, mol/g n ) amount adsorbed per unit mass of adsorbent, mol/g P ) pressure, Pa qst ) isosteric enthalpy of adsorption, J/mol q°st ) isosteric enthalpy of adsorption as P f 0, J/mol r ) parameter related to the size of the adsorbate molecule R ) gas constant, J/(mol K) T ) absolute temperature, K w ) interaction energy between two neighboring CHx groups, J/mol z ) coordination number of lattice Greek Letters ) adsorptive potential energy, J/mol η ) dimensionless adsorptive energy θ ) nlocal/m Subscripts e ) ethane i ) organic component p ) propane w ) water

Superscripts * ) pure-component value ∞ ) maximum pure-component loading

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Received for review August 26, 1996 Revised manuscript received February 11, 1997 Accepted February 27, 1997X IE960533+

X Abstract published in Advance ACS Abstracts, April 15, 1997.

Coadsorption of Organic Compounds and Water Vapor on BPL

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