Langmuir 1997, 13, 2083-2089
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Kinetics of Water Vapor Adsorption on Activated Carbon N. J. Foley, K. M. Thomas,* P. L. Forshaw, D. Stanton,† and P. R. Norman† Northern Carbon Research Laboratories, Department of Chemistry, Bedson Building, University of Newcastle upon Tyne, Newcastle upon Tyne, NE1 7RU, U.K. Received April 9, 1996. In Final Form: December 9, 1996X The adsorption of water vapor on a highly microporous carbon derived from the carbonization of coconut shell has been studied. This material was characterized by the adsorption of nitrogen at 77 K and carbon dioxide at 273 K. The micropore size distribution was determined using probe molecule vapors at p/p0 ) 0.5 and 301 K. The adsorption and desorption characteristics of water vapor on the activated carbon were investigated over the pressure range 0-2.41 kPa (p/p0 0-0.9) in a static water vapor system. The adsorption and desorption kinetics were studied with different amounts of preadsorbed water for changes in vapor pressure of 0.303 kPa. The adsorption rate constants were also studied for three relative humidities for a dynamic flow system at a constant temperature. In these experiments the changes in vapor pressure were much higher than in the static vapor pressure experiments. The kinetic results for both the static atmosphere and dynamic flow systems are discussed in relation to their relative position on the equilibrium isotherm and the adsorption/desorption mechanism.
1. Introduction Active carbons are used widely for the removal of gases and vapors from air.1,2 When respirator filters are used in practical applications, toxic material is removed from air containing water vapor. Hence there will be competition between the material to be adsorbed on the carbon surface and the air containing water vapor. Water vapor is known to have a large effect on the adsorption of pollutants/contaminants from air/gas streams which involves a decrease in the breakthrough time for the carbon bed and is therefore detrimental to the performance of the carbon filter bed. Little work has been carried out on the kinetics of adsorption/desorption of water on activated carbons and how this affects the kinetics of adsorption of the toxic material to be adsorbed. Thus there is a growing interest in the area of competitive adsorption as the effect of water which varies in concentration in air must be taken into account for optimum prediction of filter adsorption performance. The behavior of water in porous carbons is very different from that of nonpolar gases, due to the hydrophobic nature of the carbon surface3,4 and the hydrogen-bonded nature of bulk water.5 The behavior of water in porous networks is to a large extent determined by the influence of the pore structure on this hydrogen bonding and on the adsorption-desorption process. Activated carbons prepared by heating carbonaceous materials in the presence of water, oxygen, or carbon dioxide have a variety of surface chemical sites, such as hydroxyl, carboxyl, quinone, peroxide, and aldehyde.6 These surface sites act as primary adsorption centers for water. The adsorption of water molecules on a carbon surface is strongly influenced by the presence of hydrophilic functional groups which play the role of primary adsorption centers due to the formation of hydrogen bonds between the adsorbed * Author to whom correspondence should be addressed. † PLSD, Porton Down, Salisbury, Wiltshire, SP4 0JQ, U.K. X Abstract published in Advance ACS Abstracts, February 1, 1997. (1) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity, 2nd ed.; Academic Press: London, 1982. (2) Hall, C. R.; Sing, K. S. W. Chem. Br. 1988, 24, 670. (3) Stoeckli, F.; Jakubov, T.; Lavanchy, A. J. Chem. Soc., Faraday Trans. 1994, 90 (5), 783-786. (4) Chepurnoi, S. G. Bull. Acad. Sci. USSR Div. Chem. Sci. 1985, 35 (Pt. 1), 1-5. (5) Ulberg, D. E.; Gubbins, K. E. Mol. Phys. 1995, 84 (6), 1139-1153. (6) Boehm, H. P. Carbon 1994, 32, 759.
S0743-7463(96)00339-3 CCC: $14.00
molecules and the functional groups.7 The ability of the water molecule to form hydrogen bonds with the primary adsorption centers helps compensate for the loss of waterwater hydrogen bonds in the bulk water.8 An increase in the water vapor pressure leads to the formation of clusters of associated water molecules around the primary adsorption centers. The adsorption isotherm is a function of the concentration and distribution of primary adsorption centers, the pore structure, and the vapor pressure. The adsorption of water molecules starts with adsorption on the hydrophilic sites followed by the formation of either individual isolated clusters in equilibrium with saturated vapor or a continuous adsorption film on the surface while volume filling of the porosity occurs at high vapor pressures. Clearly the distribution and distance between the primary adsorption centers is of critical importance. The uptake of water vapor is enhanced by functional group or primary site density. If the distribution of the primary sites is appropriate, water molecules may bridge between adsorbed water molecules and this represents a fluidfluid cooperative effect. There have been many investigations of the isotherms for adsorption of water vapor on activated carbons.5-8 However, investigations of the adsorption kinetics of water vapor on active carbons are very scarce while the kinetics are of critical importance in assessing the performance of active carbon beds for the adsorption of environmentally unfriendly species. This investigation is intended as an initial stage in a systematic study of the effect of water vapor on the adsorption and desorption of organic species on active carbons and the role of competitive adsorption in determining the adsorption characteristics of the carbon. The initial study has involved a comparison of kinetic rates and equilibrium uptakes for water adsorption on a highly microporous carbon in static atmosphere conditions and dynamic flow conditions. The objective of this study was to investigate the adsorption and desorption kinetics of water vapor on carbons as a function of relative partial pressure and as a function of relative humidity, in order to understand the effect of changes in atmospheric conditions on the performance of activated carbons in real situations. (7) Vartapetyan, R. S. Colloids Surf. AsPhysicochem. Eng. Aspects 1995, 101 (2-3), 227-232. (8) Muller, E. A.; Rull, L. F.; Vega, L. F.; Gubbins, K. E. J. Phys. Chem. 1996, 100, 1189-1196.
© 1997 American Chemical Society
2084 Langmuir, Vol. 13, No. 7, 1997
Foley et al.
2. Diffusion Models for Adsorption
Mt/Me ) ktn
The transport of gases into an activated carbon is a very complex process. There is only a limited amount of information available in the literature relating to the adsorption kinetics of gases and vapors on activated carbons. The studies have been associated mainly with the carbon molecular sieves where the gas separation is achieved using differences in the rates of adsorption, for example, in the separation of air into nitrogen and oxygen. The adsorption process involves diffusion in slit-shaped micropores with pore widths considerably smaller than the mean free path of the gas molecules at atmospheric pressure. It is likely that processes such as molecular diffusion, Knudsen diffusion, surface diffusion, diffusion in micropores and the chemical interaction with the functional groups, etc. could all make significant contributions to water adsorption on activated carbons. Therefore, bearing in mind the pore size distribution in carbons, the modeling of the kinetic process is very difficult.4 The two simplest approaches are to use either Fick’s diffusion laws for homogeneous materials or describe the process by phenomenological models. These approaches are described below. 2.1. Fickian Diffusion. The solutions of short and long time diffusion behavior can be derived9 from Fick’s law for isothermal diffusion for a concentration C into a homogeneous sphere of radius a with D being the diffusivity which is a constant.9 The differential equation for unsteady state diffusion in the radial direction r can be represented as follows:
( ) ( )( )
δC δ2C 2 δC + )D 2 δr r δr δr
(1)
The solution of this equation is
Mt Me
)1-
6
∞
∑
() ( 1
π2 n)1 n2
exp
)
-Dn2π2t a2
(2)
where Mt ) gas uptake at time t, Me ) gas uptake at equilibrium, and a ) particle radius. Equation 2 contains a rapidly converging series with the diffusion coefficient D and time t being complementary parameters combining to give a dimensionless time parameter for a given particle size. A graph of ln(1 Mt/Me) against t for Fickian diffusion into a spherical particle can thus be plotted. A graph of ln(1 - Mt/Me) against time is linear in the region for Mt/Me > 0.5, thereby allowing the diffusion coefficient to be calculated from the gradient with 99% accuracy when compared with the theoretical value. The solution to the equation assumes isothermal behavior which is not always valid for rapid uptake. However, because of the slow kinetics observed and the nature of the adsorbate used in this study, this complication is not considered relevant. The assumption that the diffusion coefficient is constant is unlikely to hold for adsorption on activated carbons. In general, diffusivity is a strong function of concentration. Hence the above description is a simplification and the true description is likely to be more complicated than that described above. 2.2. Empirical Descriptions. An alternative simple empirical description of the adsorption of gases on carbon surfaces to that of Fickian diffusion is the empirical diffusion equation5 (9) Crank, J. The Mathematics of Diffusion, 2nd ed.; Clarendon Press: Oxford, 1975.
where Mt ) gas uptake at time t, Me ) gas uptake at equilibrium, t ) time, k ) constant, and n ) diffusional exponent. The diffusional exponent (n) can be evaluated from a plot of ln(Mt/Me) against ln(t) using the data for short times. When n ) 0.5 then the diffusion mechanism is Fickian, and if n ) 1 then it is described as case II. If n lies between 0.5 and 1 then the mechanism is said to be anomalous, and the mechanism is super case II if n is greater than 1. The gas uptake may also be described10-13 by the following phenomenological model which is equivalent to a linear driving force mass transfer model:
Mt/Me ) 1 - e-kt In this case, a graph of ln(1 - Mt/Me) against time gives a straight line and k is the rate constant for the process. This behavior is observed for diffusion into carbon molecular sieves with kinetic selectivity produced by carbon deposition.13 At gas uptakes where Mt/Me > 0.5, the graphs of the Fickian and linear driving force mass transfer models described are similar with a graph of ln(1 - Mt/Me) against time being linear. It is only in the initial uptake where different behavior is observed. The carbon used in this study is activated to a small extent. Therefore, the analysis of the adsorption kinetics is expected to be more complex than for a carbon molecular sieve material due to the wider pore size distribution. In this study, the graph of ln(1 - Mt/Me) against time was used as this allowed examination of the complex adsorption mechanisms since this graph is observed for both Fickian description (Mt/Me > 0.5) and for the linear driving force mass transfer model. The graph of ln(1 - Mt/Me) allowed calculation of a kinetic rate constant by calculation of the gradient from the linear portion of the graph. 3. Experimental Section 3.1. Materials Used. The carbon was derived from coconut shell carbon by activation in carbon dioxide. The char was heated to 1173 K in a vertical tube furnace in an argon atmosphere, and on reaching the desired temperature, the gas was switched to CO2 and held at 1173 K for the required reaction time. The active carbon product was then cooled under an argon atmosphere. The weight loss during the activation procedure which represented loss of adsorbed moisture, volatile matter, and carbon dioxide gasification was 20 wt %. Thermogravimetric analysis showed that the moisture and volatile matter present in the unactivated carbon represented 10 wt %. Hence the carbon dioxide gasification corresponded to 10 wt %. The particle size fraction used throughout the study was 0.7-1.2 mm. 3.2. Surface Area Measurements. The CO2 (273 K) and N2 (77 K) surface areas were obtained using a McBain spring apparatus. The surface areas were calculated using areas of 1.62 × 10-19 and 1.7 × 10-19 m2 for the nitrogen and carbon dioxide molecules, respectively. 3.3. Continuous Flow Adsorption Kinetics. A CI vacuum microbalance was connected to a Robal balance controller and via a RS232 serial interface to an IBM PC. This was used to (10) Sykes, M. L.; Chagger, H. K.; Thomas, K. M. Proceedings of the Conference on Carbon 92, 22-26 June, Essen, Germany; Deutsche Keramische Geselleschaft, 1992; pp 263-265. (11) Sykes, M. L.; Chagger, H. K.; Thomas, K. M. Twenty-First Biennial Conference on Carbon, 13-18 June, 1993, Buffalo, USA; American Carbon Society: 1993; pp 436-437. (12) Chagger, H. K.; Ndaji, F. E.; Sykes, M. L.; Thomas, K. M. Carbon, Proceedings Carbon 94, Granada, Spain, 3-8 July 1994; Grupo Espanol del Carbon: 1994; p 320. (13) Ndaji, F. E.; Chagger, H. K.; Sykes, M. L.; Thomas, K. M. Carbon 1995, 33, 1405.
Water Vapor Adsorption on Activated Carbon measure the rate of uptake of gases in a flowing system as well as the equilibrium capacity of the carbon sample. The reaction tube size was minimized in order to cut down the dead volume. The gas was passed through a sinter in order to ensure that the gas flow was distributed evenly and to minimize fluctuations in flow rate on changing the gas over from helium to adsorptive/ helium mixture or vice versa. The reaction tube was also enclosed within a thermostatically controlled water jacket. The system was positioned within a heating mantle to facilitate outgassing at 383 K to remove any moisture and adsorbed gases on the sample. The procedure for measurement of equilibrium gas capacity and kinetics of gas uptake consisted of the following stages: 1. The water jacket was drained and the sample tube and heating mantle were placed in position with the vacuum joints being regreased. 2. The sample was outgassed at 383 K under vacuum until the weight stabilized. 3. The pressure was gradually increased to atmospheric pressure by purging 100 cm3 min-1 helium gas through the balance head along with a preadjusted flow of 100 cm3 min-1 of helium which was introduced through the bottom of the sample tube. Water was circulated through the water jacket at the desired temperature in order to maintain isothermal conditions. 4. After the balance stabilized in the helium flow, the helium flow through the bottom of the sample tube was replaced with adsorptive of the same flow rate. In this case the adsorptive was water vapor with helium acting as the carrier gas. The water partial pressure was maintained constant using a DG3 humidity generator in conjunction with a S3000 dewpoint meter and sensor supplied by Mitchell Instruments Ltd. The weight uptake with time was measured and the data stored on an IBM PC. 3.4. Static Atmosphere Adsorption Kinetics. The apparatus used was an intelligent gravimetric analyzer (IGA) supplied by Hiden Analytical Ltd. This apparatus allows water isotherms and the corresponding kinetics of adsorption and desorption during the pressure steps to be determined. This apparatus is fully computer-controlled and measures gravimetric adsorption and desorption on a microbalance. A nine-point water isotherm was obtained at 295 K by setting equal pressure intervals of 0.303 kPa between vacuum and 2.43 kPa (saturation vapor pressure of 2.68 kPa at 295 K). Initially the carbon was outgassed until constant weight at 10-4 Pa. The pressure was then increased gradually to the desired pressure to prevent disruption of the balance. This was carried out over a time period of 60 s. The weight change was then measured as a function of time. The sample temperature was also monitored throughout the adsorption process, and the variation was found to be minimal. The weight uptake for each pressure step was used to calculate the adsorption kinetic parameters. After equilibrium was achieved, the vapor pressure was increased to the next desired vapor pressure and the next weight uptake was measured until equilibrium was re-established. The reverse procedure was used to study the desorption process. The above procedure allowed the kinetics associated with both increase and decrease in vapor pressure to be studied over the complete isotherm. Thus the water vapor adsorption/desorption kinetics for a carbon with preadsorbed water were studied for a series of changes in vapor pressure. 3.5. Micropore Size Distribution Determination. The effective micropore size distribution was determined using the molecular probe method14 involving the adsorption of organic vapors of the active carbon in a greaseless McBain spring apparatus. The carbon samples (200 mg) were outgassed under vacuum (0.1 Pa) for 16 h at 110 °C prior to the adsorption studies. The following vapors were used because they have sufficient vapor pressure at a reasonable temperature and a suitable molecular size, CH2Cl2 (400 pm), CHCl3 (460 pm), i-C5H12 (490 pm), and CCl4 (600 pm). The adsorption studies were carried out at 301 K and with the carbon exposed to the vapors at a relative pressure (p/p0) of 0.5. The uptake of each vapor was measured 24 h after initial exposure. (14) Braymer, T. A.; Coe, C. G.; Farris, T. S.; Gaffney, T. R.; Schork, J. M.; Armor, J. N. Carbon 1994, 32, 445.
Langmuir, Vol. 13, No. 7, 1997 2085 Table 1. Adsorption Capacity Measurements for the Carbon
adsorbate
temperature (K)
N2 CO2 H2O
77 273 295
apparent surface area (m2/g)
apparent pore volume (cm3/g)
622 533 (476a)
0.222 0.212 0.166b
a From Langmuir isotherm. b Calculated from the adsorption data obtained obtained in the continuous gas flow rig.
a
b
Figure 1. (a) Micropore size distribution of the carbon. (b) Water adsorption isotherm of the carbon at 295 K.
4. Results 4.1. Adsorption Capacity Measurements. The nitrogen and carbon dioxide adsorption isotherms were both type 1 while the H2O isotherm was type V. The surface area and pore volumes were estimated by analysis of the adsorption isotherms, and these are shown in Table 1. The nitrogen adsorption capacity was estimated using the Langmuir isotherm. The adsorption of carbon dioxide at 273 K and pressures up to 100 kPa only covers p/p0 values 90% of the capacity change in the specific pressure range increment. Table 2 shows the adsorption/ desorption rate constants and capacities over a range of pressures for water vapor at 295 K which were performed in a static atmosphere using the IGA apparatus. The rate constants were calculated from the gradient of the ln(1 - Mt/Me) versus time graph. It is apparent that the rate constants for adsorption and desorption vary for the pressure ranges corresponding to the position on the isotherm and reach a minimum in the pressure range 1.22-1.52 kPa. The final stage of desorption has a very
Water Vapor Adsorption on Activated Carbon
Langmuir, Vol. 13, No. 7, 1997 2087
Table 2. Adsorption/Desorption Rate Constants and Capacities Obtained from the IGA Adsorption Instrument (p0 ) 2.68 kPa) pressure range (kPa)
adsorption rate constanta (×10-4 s-1)
0-0.3 0.007-0.3
115.0
0.3-0.61 0.61-0.91 0.91-1.22 1.22-1.52 1.52-1.82 1.82-2.11 2.11-2.41
72.1 44.2 17.7 3.7 7.4 19.8 27.0
desorption rate constanta (×10-4 s-1) (1) 33.0 (2) 1.2 87.7 39.2 16.1 5.5 9.3 26.2 31.4
pressure (kPa)
adsorption capacityb (mmol g-1)
desorption capacityb (mmol g-1)
0.007 0.3
0.61
0.24 0.77
0.61 0.91 1.22 1.52 1.82 2.13 2.43
0.72 1.12 1.93 5.26 7.66 8.49 9.01
1.24 2.00 3.61 7.20 8.17 8.60
a Standard deviations obtained from the kinetic graphs were typically less than 0.5% of the determined values. b Adsorption capacity refers to the pressure column.
a
Figure 3. Adsorption/desorption rate constants over the isotherm pressure range (arrows indicate direction of pressure change).
b
Table 3. Adsorption Rate Constants and Capacities from the Continuous Flow Kinetics Apparatus at 295 K relative humidity (% RH)
partial pressure (kPa)
equilibrium capacity, n (mmol g-1)
60 80 100
1.608 2.144 2.68
8.02 9.07 9.20
adsorption rate constantsa (×10-4 s-1) (1)
(2)
(3)
3.9 3.7 4.9
9.0 6.6 6.9
1.9 1.6 1.3
a (1), (2), and (3) refer to the rate constants for the initial, intermediate, and final stages of adsorption process, see Figure 4a-c. Standard deviations obtained from the kinetic graphs were typically less than 0.5% of the determined values.
slow rate constant which can be ascribed to the difficulty in removing water from the ultramicroporosity. It is also apparent that the adsorption and desorption rate constants are very similar for a given pressure range with the exception of the lowest pressure range ( 0.95). The value of the initial rate constant for the dynamic flow system compares well to the rate constant for the static atmosphere system for the pressure range 1.22-1.52 kPa; i.e., the slowest process is occurring in this pressure range. The intermediate rate constant is
2088 Langmuir, Vol. 13, No. 7, 1997
similar to that observed for the pressure range 1.52-1.82 kPa in the static atmosphere system. The rate adsorption experiments at 80% RH and 100% RH show similar graphs. In all three cases there is a slow final stage which corresponds to the uptake of the last 10% of water vapor. 5. Discussion The adsorption of water molecules on an activated carbon surface involves hydrophilic functional groups that play the role of primary adsorption centers which form hydrogen bonds with the water molecules.3 These functional groups are located at the edges of the graphene layers. As the water vapor pressure is increased, the adsorption increases by the formation of hydrogen bonds with the adsorbed molecules, resulting in the formation of clusters of water molecules located around the primary adsorption centers. Molecular simulation studies have shown8 that the water adsorption isotherm is dependent on the distribution and concentration of the primary adsorption centers. The adsorption of water molecules starts with adsorption on the primary sites and develops with the formation of either individual isolated clusters in equilibrium with saturated vapor or a continuous adsorption film on the surface eventually leading to pore filling at high vapor pressures. In the former case, the distance between the primary adsorption centers is relatively large compared with the water clusters. The pore volume calculated from the water adsorption capacity was slightly lower than that obtained for nitrogen adsorption at 77 K. This observation has been reported previously15-17 with values for the ratio of the pore volume calculated from water adsorption at p/p0 ∼ 0.9 to total pore volume from nitrogen adsorption at 77 K in the range 0.22-0.9. Arnell et al. reported15 values of 0.7-0.9 for the ratio, which is in good agreement with the ratio 0.78 reported in this paper. However, Bradley et al. reported16 surprisingly low values of 0.22 and 0.28 for the ratios of two commercial active carbons. Freeman et al. have shown17 that the ratio may exceed 1 for Kevlar carbons with low extents of activation where activated diffusion effects are apparent. The ratio decreased to 0.74 at 80% carbon burn-off in agreement with this and previous work. A similar study18 of the adsorption of water vapor on carbon molecular sieves used for air separation has shown that water vapor is able to access the porosity leading to pore filling while the nitrogen adsorption at 77 K is very low due to activated diffusion effects. This suggests that the water molecules have access to the carbon pore structure even in a carbon molecular sieve where the pore size distribution is very narrow. There are a number of possible explanations, and these are possibly related to different mechanisms of the adsorption processes for the two adsorptives. The adsorbed water molecule clusters associated with the primary adsorption centers and adjacent to hydrophobic surfaces have different structures than that of bulk water. The latter is thought to have a structure with more hydrogen bonds between the water molecules, making it more like the structure of ice, which has a lower density than water. Carrot et al.19 have proposed that the low ratio of water uptake compared to nitrogen for silicalite was due to the inability of water to (15) Arnell, J. C.; McDermott, H. L. Can. J. Chem. 1952, 30, 177. (16) Bradley, R. H.; Rand, B. Carbon 1991, 29, 1165. (17) Freeman, J. J.; Tomlinson, J. B.; Sing, K. S. W.; Theocharis, C. R. Carbon 1993, 31, 865. (18) Thomas, K. M.; O’Koye, I. P.; Benham, M. Submitted for publication in Langmuir. (19) Carrot, P. J. M.; Kenny, M. B.; Roberts, R. A.; Sing, K. S. W.; Theocharis, C. R. Characterization of Porous Solids II; RodriguezReinoso, F., Rouquerol, J., Sing, K. S. W., Unger, K. K., Eds.; Elsevier: Amsterdam, 1991; pp 685-692.
Foley et al.
form a three-dimensional hydrogen-bonded structure in the cylindrical micropores which have a diameter of 500 pm. The pore size distribution for the carbon determined from the probe molecule studies indicates that it contains a substantial amount (∼80%) of the pore volume in porosity with 400 pm size and below. It is evident that the proposal involving the inability of adsorbed water to form a threedimensional structure in ultra microporosity is a possible explanation of the lower pore volume determined from water vapor adsorption compared with nitrogen adsorption. The similarities between the rate constants obtained for a static water vapor and water vapor/helium flow situations show unequivocally that the adsorption kinetics relate the adsorption mechanisms and diffusion into the porous structure. It is perhaps somewhat surprising that the adsorption and desorption rate constants are very similar in the region where hysteresis is relatively large. The values in Table 2 correspond to the progressive stepwise adsorption/desorption for the carbon over a range of pressures. The results show that the slowest stage of the adsorption process is in the pressure range 1.22-1.52 kPa, i.e., the steepest part of the adsorption isotherm (see Figure 1b). Also, the results show that the pressure range 1.22-1.52 kPa, i.e., the steepest part of the desorption isotherm (see Figure 1b), is the rate-determining step for desorption. Therefore, for water adsorption on activated carbon, the mid vapor pressure range has the slowest rates of adsorption and desorption. This corresponds to the development of bridging between pore walls and water molecule clusters leading to pore filling and the reverse for emptying of the porosity. Table 2 also shows that the adsorption rate constants for all the pressure steps follow a general trend of very fast uptake on the primary adsorption sites in the microporosity followed by the slower uptake involving the development of clusters of water molecules associated with the primary adsorption sites, some bridging between clusters of water molecules, filling of microporosity, and, in the final stage, an increased rate for adsorption. Cooperative effects occur and involve both fluid-fluid interactions and fluid-solid interactions with suitably located sites. Molecular simulations show that adsorption is enhanced when surface sites are located such that bridges can be made between clusters of water molecules.8 The reverse of this applies for desorption down to the pressure range 0.3-0.007 kPa, where the rate slows down considerably. Table 2 shows that the kinetics of desorption at very low pressures deviates for the pressure range 0.3-0.007 kPa from the trend as illustrated in Figure 2. This may have been due to the water in the smallest micropores being strongly adsorbed due to the overlap of potential fields from the pore walls, and there is the possibility of diffusional limitations. Therefore, it will be more difficult to break the hydrogen bonds between primary sites on the carbon surface and the adsorbed water molecules that are formed during the initial adsorption stage. The results in Table 2 obtained using the IGA were produced in a static environment, i.e., only water vapor. Table 3 contains the kinetic rate constants of adsorption and equilibrium uptakes for a saturated vapor flow system for a range of humidities. The three humidity values were chosen as they covered the main region of pore filling on the isotherm. The equilibrium uptake results in Table 3 are in good agreement with those of the same vapor pressure in Table 2. The kinetic rates of adsorption are different from those in Table 2. The reason for this is that the carbon was degassed thoroughly at 110 °C in high vacuum prior to the adsorptive being introduced in the case of the adsorption kinetics continuous flow rig.
Water Vapor Adsorption on Activated Carbon
However, the initial adsorption rates in Table 3 correspond closely to those in Table 2 for the pressures 1.22-1.52 kPa. This shows that although the sample had reached equilibrium with preadsorbed water at 1.22 kPa when this pressure range increment was performed on the IGA, the kinetic rate constant (3.7 × 10-4 s-1) for adsorption for this pressure range where the pore filling process starts to occur is similar. This is the rate-determining or slowest step for the initial uptake for Mt/Me ) 0-0.63 in the adsorption of the larger vapor pressure steps in the continuous flow rig. The intermediate faster water vapor adsorption for Mt/Me ) 0.63-0.88 has a rate constant of 6.6 × 10-4 s-1, which is similar to the rate constant for adsorption in the pressure range 1.52-1.82 kPa obtained using static vapor in the IGA. The enhanced rate constant for adsorption in this region is thought to be due to cooperative effects in the pore filling process. The final very slow rate constant observed in the gas flow rig only occurs for a very small part of the adsorption process, i.e., Mt/Me > ∼0.9, and this is possibly associated with the final approach to equilibrium involving reorganization of the adsorbed water structure in the porosity allowing increased accessibility to the pore structure. The observation that the slowest rates of adsorption and desorption occur in the pressure range 1.22-1.52 kPa, corresponding to 40-70% relative humidity, is critical in determining the performance of the carbon adsorbents since this is the main relative humidity range experienced by carbons in actual use. The continual slow water vapor adsorption/desorption cycles which occur in use will affect the adsorption dynamics of the carbon for the particular gas or vapor. Previous studies of the CH3I/CO2 system show that competitive adsorption has little or no effect on the capacity but has a marked effect on the rate of adsorption.20 The adsorption of CH3I in the active carbon gave a type 1 isotherm. It is apparent that adsorption kinetics are more susceptible to changes due to competitive adsorption. In addition, in this case the rate constants (20) Malik, A. A.; Thomas, K. M.; Meddings, P. R.; Patel, A. Carbon 1996, 34, 439.
Langmuir, Vol. 13, No. 7, 1997 2089
were directly proportional to the partial pressure of the CH3I vapor. The results outlined above show a different trend and suggest that the development of bridges between the adsorbed water molecule clusters around the primary adsorption centers when significant amounts of free surface are available is the rate-determining or slowest process in the adsorption of water vapor on active carbons. At high humidities this process leads to pore filling. This bridging process takes place throughout the pore structure and may involve adjacent clusters and those on opposite pore walls. The process is critically dependent on the concentration and distribution of primary adsorption sites, i.e., the functional groups located on the edges of the graphene layers in the carbon structure. 6. Conclusions The presence of water vapor in the air affects the performance of carbon adsorbents. The adsorption kinetics of water vapor on active carbons for rapid changes in vapor pressure follow a linear driving force mass transfer rate law for >90% of the adsorption process. The results clearly show that the rates of adsorption and desorption vary with the position on the isotherm and hence the mechanism of the adsorption process. The fastest rates were observed for adsorption on the primary adsorption centers at low relative pressures. The slowest rates of both adsorption and desorption were observed in the region of relative humidities in the range 40-70% where the adsorption process involves the growth of clusters of water molecules on the primary adsorption centers. This leads to water molecules bridging between pore walls and adjacent water molecule clusters when significant amounts of free surface are available and eventually leads to condensation of water vapor in the pores. The relative humidity range of 40-70% is the most commonly encountered range in real situations. Hence the dynamics of water vapor adsorption/desorption are an important consideration for the characterization of carbon adsorbents. LA960339S
