Coadsorption of Organic Compounds and Water Vapor on BPL

240

Ind. Eng. Chem. Res. 1999, 38, 240-250

Coadsorption of Organic Compounds and Water Vapor on BPL Activated Carbon. 4. Methanol, Ethanol, Propanol, Butanol, and Modeling Syed M. Taqvi† Department of Chemical Engineering, University of Virginia, Charlottesville, Virginia 22903

W. Scot Appel‡ and M. Douglas LeVan* Department of Chemical Engineering, Vanderbilt University, Nashville, Tennessee 37235

A volumetric apparatus was used to measure adsorption equilibria for alcohols and their mixtures with water on BPL activated carbon. Pure alcohol isotherms are reported for methanol, ethanol, propanol, and butanol at 25, 50, 75, and 100 °C over a large pressure range. Binary equilibria are reported for each alcohol coadsorbed with water at 25 and 100 °C including hysteresis for all systems. Enhanced water adsorption was observed in the presence of all alcohols, and enhanced alcohol adsorption was observed for methanol and ethanol. Pure alcohol isotherms are described using a multitemperature Toth equation, and binary equilibria are modeled with the virial mixture coefficient method. Introduction Activated carbon adsorption columns are used for many separation and purification processes. Water, present as moisture in humid air, is frequently a component in the feed streams to these columns. Therefore, the performance of these processes depends on the adsorption of organic species in the presence of water. Unfortunately, while much is understood about the adsorption of organic compounds and water separately, including the hysteresis behavior of the latter, fairly little is known about the coadsorption problem, even at the qualitative level. The molecular interactions that account for the various nonidealities exhibited during the coadsorption of mixtures are classified as either adsorbate-adsorbent or adsorbate-adsorbate. These interactions are similar, however, in that they are due to the same types but different extents of physical adsorption forces: dispersion, hydrogen bonding, electrostatic (including interactions involving permanent or induced dipoles or quadrupoles), and repulsion. There is a significant difference between the adsorption of organic compounds and water on activated carbon. Adsorption of organic compounds, which exhibit type I isotherms, is characterized by strong dispersion interactions and superposition of the adsorption potential energy functions in the micropores, resulting in a strong increase in the adsorption energy. As a result, much of the pore volume is filled at low relative pressures. Water does not interact strongly with carbonaceous solids and exhibits a type V (S-shaped) * To whom correspondence should be addressed: Vanderbilt University, Box 1604, Station B, Nashville, TN 37235. Tel: (615) 322-2441. Fax (615) 343-7951. E-mail: mdl@ vuse.vanderbilt.edu. † Present address: UOP, P.O. Box 5015, Des Plaines, IL 60017. ‡ Present address: Westvaco, P.O. Box 118005, Charleston, SC 29423.

isotherm on activated carbon. Initially, adsorption occurs by formation of hydrogen bonds between water molecules and oxide sites present on the surface.1 The adsorbed molecules then act as secondary sites for further adsorption due to the ability of the water molecules to form hydrogen bonds with each other. Clusters of water molecules are thus formed and grow until bulk condensation occurs to fill the available pore volume. Another characteristic of water adsorption on activated carbon is the presence of a hysteresis loop in the microporous region. Reviews of much of the available literature dealing with water and water-organic adsorption are given in the previous papers of this series,2-4 which include discussions of both experiments and modeling. While most experimental measurements for pure water were obtained near room temperature,5-10 Rudisill et al.2 obtained adsorption and desorption water isotherms on BPL-activated carbon at five different temperatures. The coadsorption of organic compounds and water has mostly been studied by measuring organic isotherms at constant relative humidity. In contrast, LeVan and coworkers2,3 have reported the coadsorption of water with a variety of organic species at two different temperatures. In their experiments, the organic loading was held constant and the partial pressure of water was varied from low pressures to near-saturation. Both the adsorption and desorption branches of the isotherm were measured, and the partial pressure of the organic compound was found to exhibit significant hysteresis. The coadsorption of alcohols and water has been considered in previous studies. Okazaki et al.11 obtained isotherms for the adsorption of two water-soluble compounds (methanol and acetone) and two water-insoluble compounds (benzene and toluene) in the presence of water. They assumed the absence of water hysteresis during coadsorption of water with water-soluble compounds. Ripperger and Germerdonk12 studied the adsorption of toluene and butanol from dry and humid air. These authors found that at high relative humidities

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Ind. Eng. Chem. Res., Vol. 38, No. 1, 1999 241

adsorption of both toluene and butanol was suppressed. Matsumura et al.13 studied the coadsorption of water with benzene and methanol vapors on carbons with different concentrations of hydrophilic sites. These authors observed that a decrease in the concentration of oxide sites decreased the adsorption affinity for water and methanol but not for benzene. They also observed a cooperative enhancement in methanol adsorption in a humid atmosphere. Similar water-vapor-promoted enhancements on carbonaceous adsorbents have been reported recently for methanol by Taqvi and LeVan14 and for trichloroethylene based on fixed-bed breakthrough experiments by Kane et al.15 Recent models to describe water-organic adsorption include the empirical power-law mixing rules of Russell and LeVan4 and the virial mixture coefficient (VMC) method of Appel et al.16 The former is able to describe competition between adsorbates for surface sites. The VMC method applies virial-type coefficients as an adjustment to pure-component isotherms. The focus of the investigation in this work is to study the coadsorption of several primary alcohols and water on BPL activated carbon. Alcohols can contribute to our understanding of the effects of variation in molecular size, polarity, water solubility, and degree of hydrogen bonding on the coadsorption of water and organic compounds. These macroscopic properties are related to the fundamental molecular interactions mentioned above. The alcohols chosen for this study were methanol, ethanol, n-propanol, and n-butanol. As the chain length in this series increases, the hydrocarbon character increases and the ability to hydrogen bond extensively decreases. Pure-component isotherms of the four alcohols are reported at four different temperatures and are described using the Toth equation. Isotherms for the binary adsorption of water and the four alcohols are given at 25 and 100 °C and are described using the VMC method.16 We provide a qualitative interpretation of the data in terms of thermodynamic, physical, and chemical properties. Description of Experiments Apparatus. The apparatus used in this study is of the volumetric type and is based on recirculating a gas through a closed loop of known volume containing a small bed of activated carbon until equilibrium is reached. The apparatus and operating procedures have been discussed in detail by Rudisill et al.2 and Eissmann and LeVan.3 Materials. The adsorbates used in these experiments were methanol (99.9+% pure ACS HPLC grade, SigmaAldrich), ethanol (dehydrated 200 proof, Pharmco), n-propanol (99.5+% pure HPLC grade, Sigma-Aldrich), n-butanol (99.8+% HPLC grade, Sigma-Aldrich), and distilled, deionized water. The carbon used was 6 × 16 mesh type BPL activated carbon (Calgon Corp., Lot No. 4814-J). This is the carbon used in our previous studies.2-4 Operating Procedure. The volume of the system was kept small (99.57 cm3) during the measurement of pure alcohol isotherms in order to ensure that essentially all of the organic compound in the system resides in the adsorbed phase, as described previously.2 (For the pure-component isotherm data reported here, the percentages in the adsorbed phase range from approximately 99.98% for butanol at the lowest temperature and loading to 90% for methanol at the highest

temperature and loading.) The volume of the sample loop of the gas chromatograph was 50 µL. The inert gas in the system was nitrogen. Isotherms obtained with helium as the inert gas did not show any variation in the measured loading as compared to the use of nitrogen. The amount of carbon used in the bed was roughly 3.3 g. Before the start of an experiment, activated carbon was regenerated at 200 °C in a stream of nitrogen as described previously.2,3 The bed was then placed in the apparatus at 25 °C, and a small amount of alcohol was injected. The system was heated to 110 °C and then slowly cooled to 100 °C. This was done to ensure that the measured data point was an adsorption point and not a desorption point. The temperature was kept constant at 100 °C until equilibrium was established. The vapor phase was then sampled; typically 3 or 4 samples were taken to ensure that equilibrium was established. The temperature was then lowered to 75 °C and the sampling procedure repeated. Then, the temperature was decreased to 50 °C and finally to 25 °C. Another injection was then made, and the system was heated to 110 °C. As in our previous work,2,3 mixture experiments were designed to maintain a relatively constant organic loading as water was added to the system. Also as before, the volume of the loop and the amount of carbon used were adjusted in order to examine hysteresis fully, and we set conditions such that approximately half of the water would be in the vapor phase near saturation. For the mixture experiments at 100 °C, the volume of the loop was 631 cm3, the volume of the gas sampling valve sample loop was 100 µL, and the amount of carbon used was roughly 0.55 g. At 25 °C, the volume of the system was increased to 2.437 L and the volume of the gas sampling loop was increased to 250 µL. The amount of carbon used was roughly 0.15 g. The amount of alcohol in the loop was a constant for a given experiment and was set by an initial injection. Thus, if the amount of alcohol in the vapor phase changed, then that in the adsorbed phase changed in the opposite direction. We report below the variations in the adsorbed-phase concentrations from the averages. As should be expected, these are more pronounced for the more volatile organics and at high temperatures, where vapor-phase concentrations are greater. Our operating procedure for the mixture experiments was as follows. Alcohol was first injected into the loop to reach the desired loading on carbon. The vapor phase was sampled until equilibrium was reached. Water was then injected, and the system was heated to 10-15 °C above the temperature of interest. The system was then cooled slowly (2-4 °C/h) to the desired isotherm temperature. The vapor phase was then sampled for the concentrations of both the alcohol and water. Material balances were made to determine the loadings of both compounds on carbon. The system was then cooled to between 15 and 20 °C below the temperature of interest and then heated slowly (2-4 °C/h) to the desired temperature. The vapor phase was sampled again to determine the desorption branch of the isotherm. Another injection of water was made, and this procedure was repeated until the saturation pressure of the water was reached. Experimental Results Pure-Component Alcohol Isotherms. The isotherms obtained for methanol, ethanol, propanol, and

242 Ind. Eng. Chem. Res., Vol. 38, No. 1, 1999

Figure 1. Isotherms for methanol on BPL activated carbon. Vertical lines indicate saturation pressures.

Figure 2. Isotherms for ethanol on BPL activated carbon. Curves given by the multitemperature Toth equation with ns ) 10.5 mol/ kg, b0 ) 7830, t ) 0.441, and ∆Ha ) 57.9 kJ/mol. Vertical lines indicate saturation pressures.

Figure 4. Isotherms for butanol on BPL activated carbon. Curves given by the multitemperature Toth equation with ns ) 6.4 mol/ kg, b0 ) 98, t ) 0.281, and ∆Ha ) 65.9 kJ/mol. Vertical lines indicate saturation pressures.

Figure 5. Isosteric heat of adsorption as a function of loading. Curves have been scaled by liquid molar volumes equal to 118, 167, 219, and 275 cm3/mol for methanol, ethanol, propanol, and butanol, respectively.

in the loop, as evidenced by points lying on nearly horizontal lines. However, at higher vapor pressures, dropping the temperature results in noticeably greater loadings. The measured pure-component isotherms were used to determine the variation in the isosteric heat of adsorption with loading using the thermodynamic relation

∆Ha ) RT2

Figure 3. Isotherms for propanol on BPL activated carbon. Curves given by the multitemperature Toth equation with ns ) 7.3 mol/kg, b0 ) 1855, t ) 0.375, and ∆Ha ) 66.7 kJ/mol. Vertical lines indicate saturation pressures.

butanol at 25, 50, 75, and 100 °C are shown in Figures 1-4, respectively. In these figures the amount adsorbed is plotted as a function of the alcohol partial pressure on a logarithmic scale so that the low-pressure data are not obscured. The isotherms shown are over 3-4 decades of pressure. Tabulated data for all experiments are given by Taqvi.17 The results show that the loading remains relatively constant on decreasing the temperature at low pressures with a fixed amount of alcohol

|

∂ ln p ∂T n

(1)

where ∆Ha is the isosteric heat of adsorption, T is the temperature, p is the adsorbate partial pressure, and n is the loading. Using the assumption that ∆Ha is temperature-independent, the slope obtained from plotting for p against 1/T gives values for ∆Ha as a function of loading. The variation in ∆Ha with loading for the alcohols is shown in Figure 5. The loading has been scaled using the critical molar volume of each alcohol. The magnitude of the isosteric heat of adsorption decreases as loading increases for all four alcohols. This trend is characterized by a sharp decrease at low and high loadings separated by a region of near-constant ∆Ha at moderate loadings. Figure 5 gives some indication of the heterogeneous nature of BPL activated carbon. However, adsorbate-adsorbent interactions are very complex, and isosteric heat data alone are insuf-


ficient to develop any general conclusions about the nature of the surface. A Polanyi potential theory analysis was used to check the consistency of the pure-component adsorption data.17 The cumulative micropore volume was calculated using the molar volume of each alcohol at the isotherm temperature. The Polanyi potential was scaled using the liquid molar volume of the alcohol at the normal boiling point. Most of the data are described by a common correlation curve. The microporous adsorption capacity of BPL activated carbon estimated from the curve was 487 cm3/kg. Coadsorption of Methanol and Water. Experiments for the coadsorption of water and methanol were conducted at 25 °C with nominal methanol loadings of 0.94 ( 0.09 and 2.05 ( 0.18 mol/kg of carbon. These loadings correspond to 8% and 17%, respectively, of the micropore volume filled. A constant pure-component adsorbate molar volume is assumed in calculating the fractional micropore volume filled. The micropore volume calculated from the pure alcohol experiments was used in calculating the fractional micropore volume occupied by the alcohols. Rudisill et al.2 have obtained the micropore volume as 430 cm3/kg from the pure water isotherms. The fractional micropore volume occupied by water is calculated assuming that the micropore volume is 430 cm3/kg. This is equivalent to using a reduced density for water and a single micropore volume and is consistent with previous work.18 Figure 6a shows the pure water isotherm obtained by Rudisill et al.2 and the water isotherms obtained on coadsorption with methanol at 25 °C. Because of the presence of methanol, the mixture water isotherms are shifted to the right of the pure water isotherm as less space is available for water adsorption. When the methanol loading is increased from 8% to 17%, the water isotherm shifts to the left. Thus, more water is adsorbed at the same partial pressure even though more methanol is present on the carbon and less space is available for adsorption. Near saturation of the vapor phase, based on the micropore volumes (or densities) for the pure components, 80% of the micropore volume is filled by water and 8% is filled by methanol for the experiment at the lower methanol loading. For the second experiment, 67% of the micropore volume is filled by water and 18% is filled by methanol near saturation. Parts b and c of Figure 6 show the variation in the methanol partial pressure versus water loading for the two experiments. There is a decrease in the partial pressure of methanol as water is injected into the system at low water loadings. At higher water loadings, an increase in the partial pressure of methanol is observed in both experiments. Another observation to be made from this figure is that the methanol partial pressure is a function of the water loading as well as the adsorption path. The adsorption branch is at a greater partial pressure than the desorption branch for the same water loading. Thus, methanol shows hysteresis in the same direction as water during coadsorption with water. This trend was observed during the coadsorption of all four alcohols with water and is the same trend observed in our previous studies.2-4 At 100 °C, two experiments for the coadsorption of methanol and water were carried out with nominal loadings of 1.23 ( 0.27 and 2.99 ( 0.43 mol/kg of carbon corresponding to 11% and 28% of the micropore volume filled, respectively. Water isotherms are shown in Figure

Figure 6. Coadsorption of methanol and water at 25 °C. (a) Water isotherms. Pure water shows data of Rudisill et al.2 Effect of water loading on methanol partial pressure for (b) nm ) 0.94 mol/kg and (c) nm ) 2.05 mol/kg. Filled symbols denote adsorption, and open symbols denote desorption. pr ) p/ps with psw ) 3.17 kPa and psm ) 16.85 kPa.

7a. The water isotherms for the mixture lie to the left of the pure water isotherm. Thus, more water is adsorbed in the presence of methanol compared to the amount of water adsorbed as a pure component. Near saturation of the vapor phase, based on the micropore volumes (or densities) for the pure components, 99% of the micropore volume is filled by water for the experiment at the lower methanol loading and 82% of the micropore volume is filled by water at the higher loading. Methanol fills 10% and 24% of the micropore volume in the two experiments as saturation of the vapor phase is approached. The trends observed in the variation of the partial pressure of methanol and the


Figure 8. Coadsorption of ethanol and water at 25 °C. (a) Water isotherms. Pure water shows data of Rudisill et al.2 (b) Effect of water loading on ethanol partial pressure for ne ) 1.71 mol/kg. Filled symbols denote adsorption, and open symbols denote desorption. pr ) p/ps with psw ) 3.17 kPa and pse ) 7.82 kPa.

Figure 7. Coadsorption of methanol and water at 100 °C. (a) Water isotherms. Pure water shows data of Rudisill et al.2 Effect of water loading on methanol partial pressure for (b) nm ) 1.23 mol/kg and (c) nm ) 2.99 mol/kg. Filled symbols denote adsorption, and open symbols denote desorption. pr ) p/ps with psw ) 101.33 kPa and psm ) 346.39 kPa.

loading on carbon as a function of the water loading are very similar to those observed at 25 °C. There is a decrease in the partial pressure of methanol at low water loadings followed by an increase at high loadings as shown in Figure 7b,c. Coadsorption of Ethanol and Water. An experiment involving the coadsorption of ethanol and water at 25 °C was conducted at a nominal ethanol loading of 1.71 ( 0.15 mol/kg of carbon, which corresponds to approximately 20% of the micropore volume filled. The mixture and pure water adsorption isotherms are shown in Figure 8a. In the presence of ethanol, the water isotherm is shifted to the right because there is less

space available for adsorption. However, the overall shape of the isotherm, including the hysteresis loop, is very similar to that of the pure water isotherm. Near saturation of the vapor phase, approximately 72% of the available micropore volume is occupied by water and 19% by ethanol. The variations in the partial pressure of ethanol as a function of the water loading are shown in Figure 8b. A slight decrease in the ethanol partial pressure and a corresponding increase in the ethanol loading are observed at low water loadings. This trend is more apparent for the desorption branch. At high loadings, however, a rapid increase in the ethanol partial pressure and a corresponding decrease in the loading on carbon are observed just as for the watermethanol system. A water-ethanol coadsorption experiment was carried out at 100 °C with an ethanol loading of 0.76 ( 0.11 mol/kg of carbon with 10% of the micropore volume filled with ethanol. The results are shown in Figure 9. An unusual feature of this system is the shape of the water hysteresis loop. The hysteresis loop starts to close at prw ) 0.6 but opens up again and finally closes at the saturation pressure of water. (The apparatus was exposed to a temperature swing twice without making a water injection to reproduce the odd point where the hysteresis loop begins to close.) Ethanol occupies 9% of the available micropore volume near saturation of the vapor phase, and water fills 102% of the micropore volume (based on the micropore volumes or densities deduced from the pure-component isotherms). Although


Figure 9. Coadsorption of ethanol and water at 100 °C. (a) Water isotherms. Pure water shows data of Rudisill et al.2 (b) Effect of water loading on ethanol partial pressure for ne ) 0.76 mol/kg. Filled symbols denote adsorption, and open symbols denote desorption. pr ) p/ps with psw ) 101.33 kPa and pse ) 224.89 kPa.

Figure 10. Coadsorption of propanol and water at 25 °C. (a) Water isotherms. Pure water shows data of Rudisill et al.2 (b) Effect of water loading on propanol partial pressure for np ) 2.26 mol/kg. Filled symbols denote adsorption, and open symbols denote desorption. pr ) p/ps withpsw ) 3.17 kPa and psp ) 2.70 kPa.

less pronounced than methanol, the partial pressure of ethanol decreases as water is added to the system and then shows a rapid increase at higher water loadings. Coadsorption of Propanol and Water. At 25 °C, the water-propanol coadsorption experiment was conducted with a propanol loading of 2.26 ( 0.14, which corresponds to 35% of the micropore volume occupied by propanol. The water isotherm obtained during this experiment is shown in Figure 10a. The isotherm is shifted to the right of the pure water isotherm. Near saturation of the vapor phase, water occupies approximately 66% of the available micropore volume and propanol occupies 33% of the micropore volume. As shown in Figure 10b, the partial pressure of propanol increased continuously during the course of the experiment. Thus, the strong cooperative interactions observed between water and the lower alcohols are not apparent for propanol. The loading of propanol decreased by 11% over the course of the experiment, corresponding to a 7-fold increase in the partial pressure in the vapor phase. For the water-propanol experiment conducted at 100 °C, the propanol loading was 0.40 ( 0.03 mol/kg of carbon with approximately 7% of the micropore volume filled. Results for the experiment are shown in Figure 11. In this case also, the mixture water isotherm lies to the left of the pure water isotherm. Near saturation of the vapor phase, 103% of the available micropore volume is filled by water and 6% by propanol. Figure 11b shows the dependence of the propanol partial

pressure on the water loading. The trends are similar to those at 25 °C. The increase is gradual at first and more rapid later. Near saturation of the vapor phase, the propanol partial pressure is 4 times the initial partial pressure. The propanol loading decreases as more water is injected into the system, and the final loading is 15% less than the initial loading. Coadsorption of Butanol and Water. For the coadsorption of butanol and water at 25 °C, the butanol loading was 3.34 ( 0.08 mol/kg of carbon with 63% of the micropore volume filled with butanol. Figure 12a shows the water isotherm obtained in the presence of butanol and the pure water isotherm. The mixture water isotherm is shifted to the right of the pure water isotherm. In this experiment, near the saturation pressure of the vapor phase, 55% of the available micropore volume was filled by water and 62% by butanol. As seen in Figure 12b, there is a gradual increase in the butanol partial pressure as water is injected into the system at low water loadings. At higher loadings, a more rapid increase in the partial pressure is observed. The butanol loading remained relatively constant at low water loading but decreased as the concentration of water approached saturation. The water-butanol mixture experiment at 100 °C is shown in Figure 13 and was carried out with a butanol loading of 1.19 ( 0.07 mol/kg of carbon, which corresponds to 25% of the micropore volume filled by butanol. The mixture water isotherm lies slightly to the left of the pure water isotherm. As in the previous experiment,


Figure 11. Coadsorption of propanol and water at 100 °C. (a) Water isotherms. Pure water shows data of Rudisill et al.2 (b) Effect of water loading on propanol partial pressure for np ) 0.40 mol/kg. Filled symbols denote adsorption, and open symbols denote desorption. pr ) p/ps with psw ) 101.33 kPa and psp ) 111.79 kPa.

Figure 12. Coadsorption of butanol and water at 25 °C. (a) Water isotherms. Pure water shows data of Rudisill et al.2 (b) Effect of water loading on butanol partial pressure for nb ) 3.34 mol/kg. Filled symbols denote adsorption, and open symbols denote desorption. pr ) p/ps with psw ) 3.17 kPa and psb ) 0.81 kPa.

the increase in butanol partial pressure is gradual at first and more rapid at higher water loadings. In this experiment, water fills 96% of the micropore volume near saturation of the vapor phase and butanol fills 23%.

satisfied; substituting eqs 2 and 3 into eq 1 recovers the isosteric heat of adsorption. Toth isotherm fits for ethanol, propanol, and butanol are shown in Figures 2-4. The multitemperature Toth isotherm was not able to describe the methanol data as well as the other alcohols and is not shown. Values for ns, t, b0, and ∆Ha were all treated as adjustable parameters for describing the pure-component data and are given in the figure captions. Although ∆Ha has been shown to be a function of loading, for use with the Toth isotherm, it was considered loading- and temperatureindependent. Alcohol-Water Mixtures. The alcohol-water mixtures were analyzed using the VMC method,16 which is able to describe trends exhibited in the experiments. The VMC method rather than the simple empirical model of Russell and LeVan4 was chosen because the simple empirical model is not able to describe the cooperative interactions present in the water-methanol and water-ethanol experiments. A detailed description of the VMC method has been given by Appel et al.16 and Appel.20 The method uses pure-component equations of state and virial-type mixture terms to describe multicomponent equilibrium. The method was developed for isothermal systems; however, because alcohol-water data were measured at two temperatures (25 and 100 °C), we extended the VMC method to describe multitemperature equilibrium data. The multitemperature VMC method requires purecomponent isotherms for water and the alcohols as

Analysis of Experimental Data Pure Alcohol Isotherms. Isotherms for the pure alcohols were described using a multitemperature Toth isotherm. The Toth isotherm has been previously written with a temperature dependence by Malek and Farooq.19 The form that we use is

p)

[( ) ] b

n ns

-t

1/t

(2)

-1

with

(

b ) b0 exp -

)

t∆Ha RT

(3)

where ns is the monolayer capacity, t and b are Toth parameters, and ∆Ha is the isosteric heat of adsorption. We have expressed the isotherm in a slightly different form than Malek and Farooq,19 but the only significant difference is the inclusion of a t in the exponential term of eq 3. The t was included in order to maintain thermodynamic consistency such that eq 1 would be


The VMC method is extended to multitemperature systems by including a temperature dependence for the virial coefficients which were written as

C ) C0 + C1/T

(8)

where C represents the virial coefficients Bij, Cijk, Dijkl, etc. Combining the Toth isotherm, the association theory isotherm, and the multitemperature virial coefficients results in isotherms for the alcohols and water given by

ln pa ) ln pa,pure + 2Bawnw + 3Caawnanw + 3 C n 2 + ... (9) 2 aww w 3 ln pw ) ln pw,pure + 2Bawna + Caawna2 + 2 3Cawwnanw + ... (10) with

ln pa,pure )

(

(11)

Ψ + (1 HΨ + kΨ) n

(12)

ln pw,pure ) ln

Figure 13. Coadsorption of butanol and water at 100 °C. (a) Water isotherms. Pure water shows data of Rudisill et al.2 (b) Effect of water loading on butanol partial pressure for nb ) 1.19 mol/kg. Filled symbols denote adsorption, and open symbols denote desorption. pr ) p/ps with psw ) 101.33 kPa and psb ) 51.41 kPa.

functions of temperature. The Toth isotherm given by eq 2 was used to describe the adsorption of the pure alcohols. Talu and Meunier21 used a model based on the association of water molecules to describe the data of Rudisill et al.2 for pure water on BPL activated carbon as a function of temperature. The association theory defines the partial pressure of water as21

pw )

()

HΨ Ψ exp s 1 + kΨ n

(4)

where H is Henry’s law constant, k is a reaction constant for association, ns is the saturation capacity of water, and Ψ is given by

Ψ)

-1 + x1 + 4kζ 2k

(5)

with ζ ) nsn/(ns - n). The temperature dependence is expressed through H and k by

H ) exp(H0 + H1/T)

(6)

k ) exp(k0 + k1/T)

(7)

and

The parameters obtained by Talu and Meunier21 for ns, H0, H1, k0, and k1 were utilized for this work.

)

1 b ln s -t t (na/na) - 1

s w

where the subscripts a and w represent respectively alcohol and water, respectively, and the temperature dependencies of b, H, k, and the virial coefficients are given by eqs 3, 6, 7, and 8, respectively. Equations 9 and 10, which give deviations from purecomponent behavior, were used to describe the adsorption branches of the alcohol-water isotherms. Although, binary equilibrium data were measured at nearconstant alcohol loadings, the actual loadings were used for the purpose of modeling. For the alcohols, small differences between experiments resulted in purecomponent points in the mixture experiments that did not fall exactly on the pure-component isotherms because of sample-to-sample variations in the activated carbon and experimental error. Therefore, for alcohols, we chose to compare experimental data and predictions on the basis of the ratio of the partial pressure for a component in the mixture to the pressure of that component as a pure component at the initial experiexp pred pred /pexp mental loading. Thus, pa,mix a,pure and pa,mix/pa,pure are compared, where numerators are measured and predicted pressures at experimental loadings and denominators are measured (for that particular experiment) and predicted pressures for pure adsorbed alcohol at its initial loading, prior to the addition of any water. For two of the mixture experiments (propanol and butanol at 25 °C), no pure alcohol point was recorded (see Figures 10b and 12b). In these cases, the purecomponent point was estimated by extrapolation. Data descriptions based on the VMC method are shown in Figure 14. Virial terms through E were used for all of the binary fits. Figure 14a compares the predicted and experimental pressures for the alcohols from all of the alcohol-water experiments with the predicted values given by eq 9. The axes in this figure are the ratios just described multiplied by ppred a,pure and divided by the pure-component vapor pressure of the alcohol. Figure 14b compares the predicted and experimental pressures for water from all of the alcohol-water


Figure 14. Comparison of measured and predicted partial pressures for all experiments. Predicted values given by eqs 9 and pred exp s ) pexp 10. (a) Alcohol pressures. pexp,*,r a a,mix(pa,pure/pa,pure)/pa and s exp,r s pred,r ppred,r ) ppred ) pexp a a,mix/pa. (b) water pressures. pw w,mix/pw and pw pred s ) pw,mix/pw.

experiments with the predicted values given by eq 10. As seen in Figure 14, the description of alcohol partial pressures is accurate for all experiments while the description of water partial pressures is less accurate. For all experiments, the measured alcohol partial pressures varied significantly from the pure-component values, and use of the VMC correction was necessary in order to obtain a reasonable description of the data. Values of all VMC coefficients are given by Appel.20 Discussion Pure-Component Alcohol Isotherms. All four alcohols considered in this study exhibit type I isotherms in the Brunauer classification.18 Methanol is the most polar of the alcohols considered, and butanol is the least polar. Considering this and volatility, butanol should be the most strongly adsorbed and methanol should be the least strongly adsorbed on activated carbon, which has a higher affinity for nonpolar compounds. This is validated by the experimental data. In making a comparison, we have looked at the number of decades of pressure it takes at 25 °C for the fractional loading to increase from 25% to 75% of saturation. The smaller the number of decades, the weaker the adsorption of the compound. For butanol, the number of decades is about 3.2, for propanol, it is 2.7, and for weakly adsorbing ethanol and methanol, the number of decades is 1.4 and 1.0, respectively. The isosteric heat of adsorption decreased as loading increased for all of the alcohols. These heats can be

compared to heats of vaporization of liquid, which are 39.2, 40.5, 43.6, and 45.9 kJ/mol for methanol, ethanol, propanol, and butanol, respectively. As expected, the calculated isosteric heats of adsorption are larger in magnitude than the heats of vaporization for all alcohols at all loadings, indicating a thermodynamic preference for adsorption over condensation. Coadsorption of Alcohols and Water. Several interesting trends are observed during the coadsorption of alcohols and water on activated carbon because of the ability of alcohols to form hydrogen bonds with water molecules. There is an enhancement in the amount of water adsorbed at 100 °C. At this temperature the mixture water isotherms lie to the left of the pure water isotherm. Therefore, at the experimental conditions studied, the amount of water adsorbed at any partial pressure was greater in the presence of alcohols. The micropore volume is also filled very efficiently at 100 °C. Additionally, the fraction of micropore volume filled by alcohol and water is greater than 100% for all of the experiments. As explained earlier, this is based on using the micropore volumes (or densities) deduced from purecomponent isotherms. Thus, at this temperature, water molecules adsorb more efficiently, occupying less volume in the presence of alcohols than they would as a pure component. At 25 °C, the mixture water isotherms lie to the right of the pure water isotherm. The fraction of total micropore volume filled by alcohol and water is slightly less than unity for all experiments except for the coadsorption of water and butanol. Rudisill et al.2 observed poor pore filling by water at 25 °C and suggested that the coalescence of water molecules at 25 °C might be more difficult as the vapor-phase concentration of water at 25 °C is about 2 orders of magnitude less than its concentration at 100 °C. Our results seem to support their hypothesis. Micropore volume is not filled as efficiently at 25 °C as it is at 100 °C. However, the fraction of micropore volume filled near saturation of the vapor phase is close to unity even at 25 °C. This is similar to the results of Rudisill et al.2 for acetone but in contrast to those of Rudisill et al.2 and Eissmann and LeVan3 for hydrophobic compounds, which show very little pore filling near saturation at 25 °C. Therefore, at 25 °C, the presence of alcohols does not enhance adsorption of water as much as it does at 100 °C. However, some enhancement is indicated by some of the experimental results. For the water-methanol system, when the loading of methanol is increased for the second mixture experiment, the mixture water isotherm shifts to the left. Thus, more water is adsorbed at the same partial pressure even though more methanol is present on the carbon. The fraction of the micropore volume filled by alcohol and water near saturation of the vapor phase increases as the alcohol loading increases. For the water-butanol experiment, the butanol loading on carbon is high and the fraction of micropore volume filled by water and butanol is significantly greater than unity. This indicates that a large amount of alcohol in the adsorbed phase can promote a high packing density in the mixed adsorbed phase, resulting in considerably better micropore filling. The variation of the partial pressure of alcohols as a function of water loading shows some very interesting features. For higher alcohols (propanol and butanol), the partial pressure increases gradually at first and then shows a sharp increase at higher water loadings.


Similar behavior was observed by Rudisill et al.2 and Eissmann and LeVan3 in their experiments. For ethanol and methanol, the partial pressure starts to decrease initially, reaches a minimum, and then starts to increase at higher water loadings. These trends can be qualitatively explained in terms of the cooperative mechanism and the competition for adsorption sites. The true micropore volume available for the adsorbates is fixed, but all may not be available to all adsorbates because of steric factors. Moreover, at high loadings in our experiments, water has a higher concentration in the vapor phase compared to the alcohol concentration. Therefore, as more water is injected into the system and the micropore volume begins to fill up, water drives the alcohol from the pores of the carbon in a manner consistent with common competition among adsorbate molecules. Near saturation of the vapor phase, most of the micropore volume is filled by water. This competitive advantage for water results in an increase in the alcohol partial pressure at high water loadings. As mentioned earlier, alcohols are able to form hydrogen bonds with water. Therefore, alcohol molecules on the surface of carbon do not hinder the growth of water clusters in comparison to water-immiscible organic compounds such as hexane. In fact, alcohols provide hydroxyl groups for linkage of water molecules and aid the growth of clusters. Thus, there is a cooperation between alcohol and water molecules. This accounts for the enhanced water adsorption during coadsorption with alcohols. Moreover, if the alcohols can form an extensive hydrogen bound network with water molecules with a large heat of mixing (adsorption), further adsorption of alcohols becomes thermodynamically possible. Therefore, for more polar alcohols, a decrease in the partial pressure is observed at low water loadings when there is little competition for adsorption sites. As the pores begin to fill up, competition for adsorption sites starts to dominate the coadsorption process and overshadows the cooperative mechanism. Thus, an increase in the alcohol partial pressure is observed at higher water loadings. Conclusions Alcohols exhibit a type I isotherm on activated carbon. As expected, butanol was the most strongly adsorbed and methanol was the least strongly adsorbed of the alcohols. Pure component equilibrium data were described using a multitemperature Toth isotherm which gave good agreement with experimental data for ethanol, propanol, and butanol. Calculated isosteric heats of adsorption decreased with alcohol loading. Coadsorption of water and alcohols was studied at 25 and 100 °C. Water was found to promote the adsorption of the lower alcohols and to inhibit the adsorption of the higher alcohols. The observed trends are explained in terms of molecular interactions. The ability of the alcohols to form hydrogen bonds with water molecules results in enhanced water adsorption and also enhanced alcohol adsorption for lower alcohols. Hydrogen bond formation also results in better filling of the micropore volume available for adsorption. At 100 °C, for the loadings of the alcohols considered, the amount of water adsorbed in the presence of alcohols is more than the amount of pure water adsorbed at the same partial pressure. Competition for adsorption sites is present at

high loadings irrespective of the nature of interactions between water molecules and alcohol molecules. Binary equilibrium was described using the VMC method of Appel et al.16 The model was extended to a multitemperature form utilizing the multitemperature Toth isotherm and the association theory of Talu and Meunier.21 Predicted partial pressures for the alcohols and the water give good agreement with measured values. Acknowledgment We gratefully acknowledge financial support from the U.S. Army Edgewood Research, Development and Engineering Center and the National Aeronautics and Space Administration. Nomenclature B, C, ... ) virial coefficients b ) constant in the Toth isotherm equation H ) Henry’s law constant, mol/(m2 kPa) ∆Ha ) heat of adsorption, kJ/mol k ) equilibrium constant for association, kg/mol n ) loading, mol/kg ns ) saturation loading, mol/kg p ) pressure, kPa pr ) p/ps ) reduced pressure ps ) saturation pressure R ) gas constant, J/(mol K) T ) temperature, K t ) exponent in the Toth isotherm equation Greek Letters Ψ, ζ ) association theory groups, mol/kg Subscripts 0, 1 ) indices for temperature-dependent parameters a ) alcohol b ) butanol e ) ethanol i, j, k ) component indices m ) methanol p ) propanol w ) water

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(16) Appel, W. S.; LeVan, M. D.; Finn, J. E. Nonideal Adsorption Equilibria Described by Pure Component Isotherms and Virial Mixture Coefficients. Ind. Eng. Chem. Res. 1998, 37, 4774-4782. (17) Taqvi, S. M. Studies on Adsorption Equilibrium and Pressure Swing Adsorption. Ph.D. Dissertation, University of Virginia, Charlottesville, VA, 1996. (18) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity. Academic Press: London, 1982. (19) Malek, A.; Farooq, S. Comparison of Isotherm Models for Hydrocarbon Adsorption on Activated Carbon. AIChE J. 1996, 42, 3191-3201. (20) Appel, W. S. Adsorption Equilibrium and Fixed-Bed Dynamics for Organic Compounds and Water Vapor on Activated Carbon. Ph.D. Dissertation, University of Virginia, Charlottesville, VA, 1998. (21) Talu, O.; Meunier, F. Adsorption of Associating Molecules in Micropores and Application to Water on Carbon. AIChE J. 1996, 42, 809-819.

Received for review May 22, 1998 Revised manuscript received October 15, 1998 Accepted October 20, 1998 IE980324K

Coadsorption of Organic Compounds and Water Vapor on BPL

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