Ind. Eng. Chem. Res. 1993, 32, 2752-2757
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Coadsorption of Organic Compounds and Water Vapor on BPL Activated Carbon. 2. 1,1,2-Trichloro-1,2,2-trifluoroethane and Dichloromethane Roy N. Eissmannt and M. Douglas LeVan' Department of Chemical Engineering, University of Virginia, Charlottesoille, Virginia 22903-2442
A novel volumetric apparatus is used t o measure equilibria for mixed vapors of halocarbons and water coadsorbed on BPL activated carbon. Pure-component l,l,Z-trichloro-l,Z,Z-trifluoroethane (CFC-113) and dichloromethane (methylene chloride) isotherms are obtained at 0, 25,50,75, and 100 "C over wide ranges of pressure. A separate isotherm measured for CFC-113 a t 50 "C indicates the presence of pure component hysteresis in the mesopore structure of the activated carbon a t high halocarbon loadings. Mixture equilibria are measured for CFC-l13/water systems at 25 and 100 "C and for dichloromefhane/water systems a t 25 "C. All components exhibit hysteresis, and the halocarbon partial pressure increases as the water loading is increased at constant halocarbon loading. Results for halocarbon/water mixtures together with previous results for hydrocarbon/water mixtures show that the apparent total pore volume filled near saturation is dependent on the adsorption temperature and the solubility of the organic compound in water. Introduction Chlorofluorocarbons and other halocarbons are used as refrigerants, solvents, foaming agents, blowing agents, and fire-extinguishing chemicals. Concern over the environmental and health consequences of using these materials have risen very significantly in recent years. To design adsorption processes to recover these and other compounds effectively, adsorption equilibria are required. Water is known to have a significant effect on equilibrium adsorbed-phase organic loadings in many applications such as the use of carbon filters to remove organic contaminants from humid gas streams, fixed beds to reduce levels of undesirable compounds in closed environments, and adsorption columns that have a steam regeneration cycle. The knowledge of how water vapor affects the adsorption properties of the activated carbon will allow for more efficient design of adsorption systems for such applications. This paper is concerned with the determination of adsorption equilibria for two popular halocarbon solvents: 1,1,2-trichloro-l,2,2-trifluoroethane (CFC-113)and dichloromethane (methylene chloride). Pure-component isotherms are measured over a wide range of temperatures and pressures, and the coadsorption of water vapor over wide ranges of composition is considered. This paper extends the work of Rudisill et al. (1992), which was on the vapor phase adsorption of hexane, acetone, water, and hydrocarbon/water mixtures on BPL activated carbon. That paper gives a thorough review of prior work on the adsorption of water vapor, with and without a competing organic species, on activated carbon. In an additional recently published study using BPL activated carbon, Rube1 (1992) measured adsorption isotherms for water vapor adsorbed on nanogram microparticles of BPL carbon and other adsorbents by levitating them in electric fields; results agree reasonably well with previous measurements. Little research has been performed on adsorption of halocarbons, even to obtain pure-component isotherms. Among the more recent papers, Kodama et al. (1992) measured isotherms of CFC-113 on 13X zeolite, montmorillonite, and silica gel at 25 "C and on activated carbon
* Author
to whom correspondence should be addressed. Pont Chemicals, Deepwater, NJ.
+ Present address: Du
from 15 to 30 "C. Also, Kuo et al. (1991) and Kuo and Hines (1992) measured isotherms for chlorocarbons adsorbed on silica gel from 15 to 25 "C. Concerning reactivities, Barrer and Brook (1953) reported that fluoromethanes react with chabazite. Similarly, Kumar (1982) found that dichlorodifluoromethane (CFC-12) reacts with 4A zeolite, reducing the intraparticle diffusion coefficient for adsorbed molecules. The interpretation of adsorption equilibria for vapors of organic compounds and water coadsorbed on activated carbon poses interesting challenges. Even water adsorbed separately raises theoretical questions concerning mechanism and hysteresis as discussed by Rudisill et al. (1992). When water and a hydrophobic organic compound are coadsorbed in chemically and structurally heterogeneous microporous carbons, the problem becomes much more complex. This paper reports results of an experimental investigation. Discussion of results is limited to trends reflected in the data and comparisons with previously reported results. Methods developed to predict adsorption equilibria should be consistent with these trends.
Experimental Section Apparatus. The apparatus used in this work has been described in detail by Rudisill et al. (1992). Briefly, the apparatus is housed in an environmental chamber with programmable temperature control. A magnetic pump is used to recirculate gas through a closed loop of known volume which contains a bed of carbon. A known amount of material is injected into the loop, and the vapor phase is sampled using a gas chromatograph to determine its concentration and when equilibium is reached. A material balance is then made to determine the amount of material adsorbed on the activated carbon. There are several distinct advantages to using this type of apparatus. First, the system temperature can be varied widely so that isotherms over a wide range of temperatures (0-125 "C) can be measured. Second, the apparatusworks equally well for both pure components and mixtures, including systems that show hysteresis. Third, equilibrium can be achieved rapidly. Finally, equilibrium from very low pressures to saturation pressures can be measured accurately. The system volume and the amount of carbon in the system can be adjusted to ensure that the amount
Qs88-5885/93/2632-2752$04.QQlQ 0 1993 American Chemical Society
of material adsorbed on the carbon is a significant fraction of the total material in the loop. Thus, when the vaporphase concentration is measured directly, the error in the amount adsorbed determined by material balance is small. Materials. The adsorbates used in these experiments were 1,1,2-trichloro-1,2,2-trifluoroethane (99.9 mol 5% pure HPLC grade), dichloromethane (99.9 mol 5% pure HPLC grade), and distilled, deionized water. The carbon used was 6 X 16 mesh Type BPL activated carbon (Calgon Carbon Corp., lot No. 4814-5). OperatingProcedures. For the measurement of pure component halocarbon isotherms, the volume of the loop was kept small, 100.3 cm3. The experiment began by regenerating the activated carbon to remove any contaminants adsorbed on it. This was accomplished by heating the fixed bed of carbon in an oven at 150-175 "C with a nitrogen flow through the bed of 2-4 L/min until the bed showed no change in weight. The bed was then placed in the apparatus, and a small amount of halocarbon was injected. The system was heated to 100 "C and held until equilibrium was established as determined by sampling the gas phase. The temperature was then lowered to 75 "C and the sampling repeated. Then, the temperature was dropped to 50 "C, then 25 "C, and finally0 "C. Another injection was made, and the system was heated back up to 100 "C. This procedure was repeated until the region in which hysteresis occurs in mesopores was reached. An experiment resembling those for water coadsorption was also performed to determine the size and extent of the hysteresis loop that exists for pure-component halocarbons. For mixtures of halocarbon and water, it is very important that the experiment be set up so that changes in system temperature will significantly affect the loading of the adsorbate on the carbon. This is done to ensure that adsorption and desorption points are correctly measured. We seek to approach equilibrium via a path along which both components are either adsorbing or desorbing. The variables which can be varied so that this can occur are the system volume and the amount of carbon used. Since we are studying a mixture in which one compound is much more strongly adsorbed than the other, the ratio of carbon amount to system volume was such that most of the strongly adsorbing halocarbon was in the adsorbed phase, and between 20% and 80 5% of the weakly adsorbing water was in the adsorbed phase. In these experiments, the loading of the halocarbon was held constant and the loading of water was varied over the complete range of reduced water partial pressure. The water isotherm was measured to determine the extent and size of the hysteresis loop. The measurement of this isosteric mixture isotherm is possible since even for significant changes in the halocarbon vapor concentration the halocarbon which desorbs from the carbon is small compared to the total amount of halocarbon adsorbed. Additional injections of halocarbon can be made, if necessary, to maintain a constant loading. These experiments were performed by first injecting the halocarbon into the loop. After an initial pure component measurement was completed, water was injected and the system heated between 10 and 20 "C above the temperature of interest. The system was then cooled slowly (2-4 "C/h) to the temperature of interest and held there until equilibrium was established. The vapor phase was sampled for the concentrations of both halocarbon and water. Material balances could then be made to determine the loadings of both compounds. The system was then cooled between 10 and 25 "C below the tem-
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Pc~c-115(kPa) Figure 1. Isotherms for CFC-113adsorbed on BPL activated carbon. Vertical lines indiate saturation pressures.
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Figuret. Isotherm for dichloromethane adsorbed on BPL activated carbon. Vertical lines indicate saturation pressures.
perature of interest, heated slowly (2-4 "C/h) back up to the temperature of interest, and held until equilibrium was established. 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 vapor phase was saturated.
Results and Discussion Pure-ComponentHalocarbon Isotherms. Isotherms for CFC-113 and dichloromethane at 0,25,50,75, and 100 "C are shown in Figures 1and 2, respectively. The data are plotted as the amount adsorbed versus the pressure, which is shown on a logarithmic scale over many decades. Vertical lines indicate saturation pressures. The results or ~ D C M ,is show that the amount adsorbed, nc~c-113 relatively constant for the data at the different temperatures as isosteres are measured at low loadings. But once the vapor-phase concentration becomes large enough, dropping the temperature results in noticeably greater loadings for a given amount of halocarbon in the loop. To determine whether BPL carbon produces type 1or type 4 isotherms for a halocarbon, another experiment was performed. This experiment tested for hysteresis in the CFC-113 isotherm at 50 "C. If hysteresis is present, then the isotherm is of the type 4 classification, meaning
2754 Ind. Eng. Chem. Res., Vol. 32, No. 11, 1993 6
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Figure 4. CFC-113 loading as a function of water loading at 26 OC for experiments with t1-113 = 0.746 and 1.477 mol&. Filled squares denote adsorption, and open squares denote desorption. The solid lines represent the average CFC-113 loadings.
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Figure 3. Isotherm for CFC-113 at 50 OC on BPL activated carbon. PccFOil9 = PcFc-11s/Pc~113 with PcFCll3 = 110.06 P a . Filled squares denote adsorption and open squares desorption.
that the adsorbent has significant mesoporosity. Results are shown in Figure 3. As can be seen from the figure, the isotherm does indeed produce a region of hysteresis beginning at a relative pressure of 0.35. This is not unexpected on the basis of the pore size distribution of BPL carbon and results for other systems (Dubinin, 1966, 1980). The existence of hysteresis in these systems for adsorption of organic compounds and water can be explained in terms of the strength of the potential fields in the adsorption space. In micropores, the fields from neighboring walls overlap, causing the interaction energy between the solid and gas to be large for organics. As the loading increases beyond the point where the micropore volume is filled, the mesopores begin to fillwith the organic adsorbate by capillary condensation. This gives rise to a hysteresis loop through network and possibly other effects. For the organics, hysteresis occurs in mesopores but not in micropores because there is a decrease in the effect of the potential field from the walls as the pore size increases (Gregg and Sing, 19821,with a change in mechanism from strong gas-solid interaction to condensation. In contrast, the potential field has a weak impact on the adsorption of water (Dubinin, 1980). Thus, water shows hysteresis in the micropores where it is weakly adsorbed, although the mechanism is not certain, as we have discussed (Rudisill et al., 1992). Coadsorption of Halocarbons and Water. Coadsorption measurements with CFC-113 and water were made at 25 and 100"C. Experiments for dichloromethane and water were conducted only at 25 O C . Similar experiments at 100 "C could not be performed as attempts showed dichloromethane to be very reactive in the apparatus at this temperature when water was present. As discussed earlier, mixture experiments were conducted with halocarbon loadings held essentially constant. At 25 "C, two different CFC-113 loadings were used, 0.746 f 0.007 and 1.477 f 0.009 mol/kg of carbon. From Figure 1,these correspond to roughly 175% and 34 % filling of the micropore volume of BPL carbon. Figure 4 shows the slight variation that occurred in these loadings as the result of varying the water loading. The CFC-113 loadings decrease very slightly as the water loading is increased. The water isotherms at 25 "C for the two CFC-113 loadings are plotted in Figure 5. The top isotherm in the
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figure is the pure water isotherm of Rudisill et al. (1992). The figure shows that the size of the hysteresis loop decreases dramatically as the CFC-113loading is increased. Calculations indicate that, using apparent densities determined from pure-component adsorption, the micropore volume filled by the mixture near saturation of the vapor phase is significantly less than 100%. For ~ C F G 113 = 0.746 mol/kg, 83% of the pore volume is available for water adsorption but only 71% of that volume is filled by water. For nCFC.113 = 1.477 mol/kg, 66% of the pore volume is available and only 35% of that is filled. Figure 6 shows the dependence of the equilibrium CFC113 partial pressure at 25 "C on the amount of water adsorbed for the two loadings of CFC-113. While the partial pressure increases with the water loading and is path dependent, the trend is not dramatic. A pattern can be noted in Figure 6a that is present in almost all of our data. Specifically,for coadsorption of water and an organic compound of moderate volatility, the hysteresis observed for the organic compound is in the same direction as that for water. In other words, higher loadings (or lower partial pressures) for both CFC-113 and water occur on the desorption branch of the isotherm. Water isotherms a t 100 "C are shown in Figure 7 for CFC-113 loadings of 0.68 f 0.12 and 1.41 f 0.18 mol/kg
Ind. Eng. Chem. Res., Vol. 32, No. 11, 1993 2755 2 0 *
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Figure 6. Effect of water loading on CFC-113 partial pressure at 25 "C. PIcFClls= Pc,~l$Pccl13 with PIcFC113 = 44.59 Wa. Filled squares denote adsorption,and open squares denote desorption: (a) s 1.477 mol/kg. nCFc113 = 0.746 mol/kg, (b) n ~ p c l l =
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Figure 8. Effect of water loading on CFC-113 partial pressure at 100 OC. PIcFCll3 = PcFC1l$~cFClls with PIcmlls = 442.4Wa. Filled squares denote adsorption, and open squares denote desorption: (a) rL.CFC.113 = 0.68 mol/kg, (b) n ~ p c - 1 1=~ 1.41 mol&.
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Figure 7. Water isothermsat 100 "Cfor CFC-113loadingof ncfi113 = 0 (top), 0.68, and 1.41 (bottom) mol/kg. Filled squares denote adsorption, and open squares denote desorption.
of carbon. The data are similar to those at 25 OC but shifted to the right somewhat as also occurs for pure water isotherms (Rudisill et al., 1992). Furthermore, the hysteresis loop appears to be wider than that for comparable loadings at 25 "C. Of greater importance, however, is that the data at 100 "C show a signifiant increase in total micropore volume filled near saturation. For these data, within experimental accuracy, calculations using pure adsorbate densities indicate that the mixed adsorbate fills the micropore structure completely. The equilibrium partial pressures for CFC-113 at 100 "C as a function of the water loading are shown for the two CFC-113loadings in Figure 8. The partial pressures show large increases for both loadings as the water loading
increases. For nCFC.113 = 0.68 mol/kg, the partial pressure of CFC-113 increases by a factor of 20, while for nCFC-119 = 1.41 mol/kg the partial pressure increases by a factor of 6. Since these increases in partial pressures significantly decreased the CFC-113 loadings, small injections of CFC113were made during the experiment to keep the loading relatively constant. Dichloromethane and water were adsorbed at 25 OC at dichloromethane loadings of 1.38 f 0.14 and 2.74 f 0.19 mol/kg of carbon. Water isotherms are shown in Figure 9 for pure water and the two dichloromethane loadings. Only a slight decrease in the size of the water hysteresis loop is found as the dichloromethane loading is increased. Another observation, as with CFC-113at 25 "C, is that the total apparent pore volume filled by the dichloromethane and water near saturation of the vapor phase is significantly less than the total micropore volume available for adsorption. For ~ W =M1.38mol/kg, 83% of the pore volume is available for water adsorption and 89 % of this is filled. Similarly, for ~ D C M= 2.74 mol/kg, 66% of the pore volume is available and 80% of it is filled.
2756 Ind. Eng. Chem. Res., Vol. 32, No. 11, 1993 I
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Figure 9. Water isotherms at 25 O C for dichloromethaneloading of n m =~0 (top), 1.38,and 2.74 (bottom)mol&. Filled squares denote adsorption, and open squares denote desorption.
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Figure 10. Effect of water loading on dichloromethane partial = PmM/PmM withPmM = 57.33 P a . Filled pressure at 25 O C . PmV squares denote adsorption,and open squaresdenote desorption: (a) nm~ = 1.38 mol/kg, (b) n m =~ 2.74 mol/kg.
Figure 10shows the dependence of the dichloromethane partial pressure on the water loading. A modest increase in the partial pressure is found as the water loading is increased. Hysteresis is apparent with the dichloromethane exhibiting a greater partial pressure on the adsorption branch of the isotherm. The most striking feature of the CFC-llS/water and dichloromethane/water data of this work as well as the hexane/water and acetonelwater data of Rudisill et al. (1992) is shown in Figure 11,which gives the total apparent fractional pore volume filled as a function of the apparent fractional pore volume filled by the organic compound as the vapor phase approaches saturation at 25 "C. The most efficient pore filling occurs for the acetone system, followed
by dichloromethane, hexane, and CFC-113. If the water solubilities of these four compounds are examined, it is evident that the more soluble a compound is in water, the better it and water are able to fill the pore volume available during coadsorption. The acetone is infinitely soluble in the water, the dichloromethane is soluble up to 1.30 wt %, and the hexane and the CFC-113 are soluble up to 0.014 and 0.017 wt % , respectively. Similar conclusions about the solubility effects have been pointed out by other investigators (Okazaki et al., 1978; Rudisill et al., 1992). Okazaki et al. (1978) explain that once the water-soluble molecules absorb on the surface they then act as hydrophilic sites which attract water and thus allow for more complete pore filling. On the other hand, if the less watersoluble molecules adsorb on the surface, then water adsorption is further inhibited. The reason why hexane appears to give slightly higher pore filling than CFC-113 may be because it is somewhat more strongly adsorbed as a pure component. At 100 "C, although similar trends exist for solubility, the mixtures fill the available pore volume to a much greater extent. Explanations for this behavior include the higher vapor-phase concentrations of water at 100 OC, which may enable the water to compete more effectively for adsorption sites (Rudisill et al., 1992). Several additional observations can be made from the mixture data. First, except for one case (nCFC-113 = 1.41 mol/kg at 100 "C), all of the data including that of Rudisill et al. show that the desorption step produces both a greater organic loading and a greater water loading. Second, the partial pressure of the organic compound increases as the water loading increases for all conditions examined here and by Rudisill et al.; this indicates a competition for adsorption sites between the organic compound and water with the competition tending to favor the water more and more at high water loadings, even though it is much less strongly adsorbed on the carbon than the organic compound. Third, the temperature dependence of the hysteresis behavior is difficult to explain. The results in this study are exactly opposite to those found by Rudisill et al. Temperature increased the relative size of the CFC113 loop, and the more water soluble dichloromethane
Ind. Eng. Chem. Res., Vol. 32, No. 11,1993 2767 showed more hysteresis than CFC-113 at 25 "C. Rudisill et al. report the opposite temperature and solubility effects, and Okazaki et ai. (1978) report the opposite solubility effect.
Conclusions A novel volumetric apparatus was used to measure equilibria for vapors of CFC-113 and dichloromethane adsorbed separately and with water on BPL activated carbon over wide ranges of temperature and pressure. All components exhibited hysteresis, and the halocarbon partial pressure increased as the water loading was increased. Solubility and temperature have significant effects on the total loading that can be achieved for halocarbon/ water mixture adsorption. An increase in temperature is shown to significantly increase the extent to which the pore volume can be filled at apparent pure-component densities. Likewise, an increase in solubility also tends to allow for more complete pore filling, thus increasing the overall loading. Halocarbon/water coadsorption tends to allowfor higher loadings of both components on the desorption branch of the isotherm. The hysteresis loop found for CFC-113 indicates the presence of significant mesoporosity in BPL carbon. This finding adds further complication to attempts to model mixtures of organic compounds and water adsorbed on activated carbon. Acknowledgment We are grateful to the US.Army ERDEC for the support of this research.
Literature Cited Barrer, R. M.; Brook, D. W. Sorption and Reactivity of Simple Organic Molecules in Chabazite. Trans. Faraday SOC. 1953,49,940-948. Dubinin, M. M. Porous Structure and Adsorption Properties of Active Carbons. In Chemistry and Physics of Carbon; Walker, P. L., Ed.; Marcel Dekker: New York, 1966; pp 51-120. Dubinin, M. M. Water Vapor Adsorption and the Microporous Structures of Carbonaceous Adsorbenta. Carbon 1980,18,355364. Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity; Academic Press, 1982. Kodama, K.; Kaguei, S.; Wakao, N. Batch Adsorption of Trichlorotrifluoroethane (Freon-113) onto Activated Carbon-Surface Diffusivity and Pore Diffusivity. Can. J. Chem. Eng. 1992, 70, 244-249. Kumar, R. Effect of Freon-12 Exposure on the Sieving Property of 4A Zeolite. Can. J. Chem. Eng. 1982,60,577-578. Kuo, S.-L.;Hines, A. L. Adsorption of l,l,l-Trichloroethane and Tetrachloroethylene on Silica Gel. J. Chem. Eng. Data 1992,37, 1-3. Kuo, S.-L.; Hines, A. L.;Dural, N. H. Correlation of Methyl Chloride, Methylene Chloride, Chloroform, and Carbon Tetrachloride Data on Silica Gel. Sep. Sci. Technol. 1991,26, 1077-1091. Okazaki, M.; Tamon, H.; Toei, R.Prediction of Binary Adsorption Equilibria of Solvent and Water Vapor on Activated Carbon. J. Chem. Eng. Jpn. 1978,11, 209-215. Rubel, G. 0. Water Isotherm Measurements for Microparticles of Carbon. Carbon 1992,30, 1007-1011. Rudisill, E. N.; Hacskaylo, J. J., LeVan, M. D. Coadsorption of Hydrocarbons and Water on BPL Activated Carbon. Ind. Eng. Chem. Res. 1992,31,1122-1130. Received for review January 21, 1993 Revised manuscript received July 6, 1993 Accepted July 13, 1993. Abstract published in Advance ACS Abstracts, October 1, 1993.
