Adsorption and Desorption of Carbon Dioxide onto and from Activated

2808

Ind. Eng. Chem. Res. 1997, 36, 2808-2815

Adsorption and Desorption of Carbon Dioxide onto and from Activated Carbon at High Pressures Jeng Hsin Chen, David Shan Hill Wong,* and Chung Sung Tan Department of Chemical Engineering, National Tsing Hua University, Hsinchu, Taiwan 30043, ROC

Ramkumar Subramanian, Carl T. Lira, and Matthias Orth Department of Chemical Engineering, Michigan State University, East Lansing, Michigan 48824

Adsorption of carbon dioxide near its critical point on DeGussa IV activated carbon is investigated in this study. A volumetric method was used to measure the adsorption/desorption isotherms at 284, 300, 305, 310, and 314 K over a large pressure range. At subcritical temperatures, adsorption isotherms display a discontinuity at the vapor pressure of carbon dioxide, and desorption hysteresis is observed. However, there is no desorption hysteresis if adsorption is terminated before vapor-liquid transition occurs. At supercritical temperatures, adsorption isotherms display a plateau, and the excess decreases beyond the critical pressure. No hysteresis occurs during the desorption process. The adsorption isotherms can be represented very well by the simplified local density model. Introduction The application of supercritical fluid (SCF) as a regeneration solvent for adsorbents has been extensively studied (Modell et al., 1979; DeFilippi et al., 1980; Dooley et al., 1987; Tan and Liou, 1988, 1989a,b, 1990; etc.). SCF chromatography has become an important analytical and separation technique (Brennecke and Eckert, 1989). However, the importance of the interaction between SCF and the adsorbent in such processes has been noted (King, 1987) but not extensively investigated. Tan and Liou (1990) found that adsorption of toluene from supercritical carbon dioxide on activated carbon would change with SCF carbon dioxide density. A phenomenological thermodynamic model was developed by Wu et al. (1991) to explain the data using the Peng-Robinson equation of states and ideal adsorption solution theory. They suggested that the adsorption of carbon dioxide must not be ignored in such processes. Experimental data on adsorption of high-pressure carbon dioxide are relatively scarce (Jones et al., 1959; Ozawa et al., 1975; Strubinger and Parcher, 1989). However, adsorption of high-pressure fluids is relevant in a variety of applications including storage of natural gas (Findenegg, 1983; Barton et al., 1983) and catalysis (Vaska and Selwood, 1953). In an extensive review, Menon (1968) summarized the characteristics of adsorption for various systems at high pressure. At supercritical temperatures the adsorption reaches a plateau and then decreases with increasing pressure. At subcritical temperatures, the adsorption increases with pressure until a discontinuity is reached at the vapor pressure of the adsorbate and is small above the vapor pressure. There are many types of models that can be used for describing the adsorption of high-pressure fluids, including multilayer adsorption BET theory (Lowell and Shields, 1991), continuum mechanic model (Kim et al., 1993), and statistical thermodynamic theory and computer simulation (Petersen and Gubbins, 1987; Petersen et al., 1988). Lira and co-workers (Rangarajan et al., 1995; Subramanian et al., 1995) developed the simplified local density (SLD) model to correlate adsorption * Corresponding author. E-mail: [email protected]. S0888-5885(96)00227-8 CCC: $14.00

isotherms of a pure component on a flat surface over a large pressure range. It has been successfully applied to adsorption of ethylene and krypton on graphitized carbon black and adapted to describe clustering phenomenon in SCF. Subramanian and Lira (1996) extended the model to account for the effect of different geometries (pore and slits) but only for large pore diameter or slit width relative to molecular diameter. A volumetric method was used to measure adsorption and desorption isotherms of carbon dioxide adsorbed on activated carbon from 284 to 314 K and up to 15 MPa. The purpose is to find out if adsorption-desorption hysteresis can be found at subcritical conditions. The slit SLD model of Subramanian and Lira (1996) is extended to small slits to correlate experimental data. Experiment and Material The activated carbon DeGussa IV was used as adsorbent in this study. It was screened to 5-6 mesh section with an average particle diameter of 0.386 cm. The screened section was boiled in deionized water to remove impurities and then was dried in an oven at 373 K for over 24 h. After drying, about 15-20 g of the prepared activated carbon was loaded in the adsorption cell. The sample has a BET surface area of 1699 m2/g measured by a porosity analyzer Coulter Omnisorp 100CX. The amounts of micropore and mesopore volumes were found to be 0.8609 and 0.1703 cm3/g, respectively. The pore size distributions are shown in Figure 1a,b. The solid density was found to be 2.581 g/cm3. A schematic diagram of the apparatus is shown in Figure 2. The volumes of loading and adsorption cell and connection tubing were determined to be 523.9 ( 0.3 and 97.0 ( 0.5 cm3, respectively, by the water displacement method. The carbon dioxide used was of 99.6% purity. It was first sent through a precolumn containing silica gel to remove any moisture contents and then compressed into the loading cell. The experimental system was placed in a water bath in which the temperature was controlled to within 0.1 K. The pressure was measured using a pressure transducer, ASIHI-Mes-T249, to within 0.034 MPa. In the adsorption experiments, the whole system was first evacuated to a vacuum state. Carbon dioxide was © 1997 American Chemical Society

Ind. Eng. Chem. Res., Vol. 36, No. 7, 1997 2809

a

long time for the system to become stable. This procedure was continued until the pressures of the two cells were balanced. To perform the desorption experiment, the loading cell was then evacuated through valves 8 and 11-1. Then both valves were closed. The expansion procedure was then carried out in a reverse direction (from the adsorption cell to the loading cell). Changes in pressure and temperature in the loading cell and the adsorption cell were recorded in each expansion step. The amount of carbon dioxide delivered from the loading cell to the adsorption cell (or vice versa) was calculated using an accurate equation of state (Angus et al., 1976) and the volume of the loading cell: l l ∆nli ) Vl[F(Tln,Pln) - F(Tn-1 ,Pn-1 )]

b

(1)

The free volume of the adsorption cell is defined as the difference in the volume of the adsorption cell and the volume occupied by the solid adsorbent:

Vf ) Va - ws/Fs

(2)

The amount of Gibbs adsorption is equal to the total amount delivered to (or remaining) in the adsorption cell minus the amount of bulk fluid that occupied the free volume: n

na ) Figure 1. (a) Incremental micropore volume. (b) Cumulative mesopore and macropore volume.

then delivered into the loading cell from the inlet valve 11-1 until pressure reached a desired value. After this, the valve 11-1 was closed. Valve 11-2 was opened for a short time to let an adequate amount of carbon dioxide from the loading cell expand to the adsorption cell. Temperature and pressure readings of both the loading cell and adsorption cell were taken after a sufficiently

Figure 2. Schematics of the experiment.

∑ ∆nl - VfF(Tan,Pan) k)1

(3)

Simplified Local Density Model The framework of the SLD model was outlined in the following. Let the distance z be the normal distance from the point of interest to the surface of the solid. The chemical potential of fluid inside a porous medium at a distance z is the result of fluid-fluid and fluid-solid interaction:

2810 Ind. Eng. Chem. Res., Vol. 36, No. 7, 1997

µ(z) ) µff(z) + µfs(z) ) µb

(4) )

At equilibrium, there will be no chemical potential gradient from the surface of the solid to the bulk fluid outside. For a nonideal bulk fluid, the chemical potential can be written in terms of fugacity:

µb ) µ+(T) + RT ln(fb/f+)

(5)

Let us define the fugacity due to fluid-fluid interaction inside a porous medium. At a distance z normal to the surface of solid using the same standard state as a bulk fluid:

µff(z) ) µ (T) + RT ln[fff(z)/f ] +

(6)

+

[

fff(z) ) fb exp -

]

Ψ(z) + Ψ(L-z) kT

(8)

The Peng-Robinson equation of state was used to calculate the fugacity for a bulk fluid:

ln fb )

bFb abFb 1 - bFb RT(1 + 2bF - b2F 2) b b

[

]

[

]

A similar expression was adopted for the fugacity due to fluid-fluid interaction inside the porous medium:

{

ln

a(z)F(z) bF(z) 1 - bF(z) RT{1 + 2bF(z) - b2[F(z)]2}

}

[

]

[1 - bF(z)] 1 + (1 + x2)F(z) a(z) ln (10) RTF(z) 2x2RT 1 + (1 - x2)F(z)

The equations for a(z) depend on the ratio of the slit width L to the molecular diameter σff. Since L is the distance between slit wall surfaces, L + σ is the normal distance between the centers of the atoms of the two solid surfaces. For L/σff g 3, a(z) is given by

[

a(z) 3 z 5 ) + ab 8 σff 6

1 L-z 3 - 0.5 σff

(

)

3

] 0.5 e

)

[

38 83

1 z 3 - 0.5 σff

(

-

z e 1.5 (11) σff

]

1 3 L-z 3 - 0.5 σff z L 1.5 e e - 1.5 σff σff

) ( 3

)

3

]

[

a(z) 3 z 5 ) + ab 8 σff 6

1 L-z 3 - 0.5 σff

(

)

3

]

z L e - 1.5 (12) σff σff

0.5 e

(

)

z L - 1.5 e e 1.5 σff σff

)

3 L -1 8 σff

)

3 L-z 5 + 8 σff 6

[

1 z 3 - 0.5 σff

(

)

3

]

1.5 e

z L e - 0.5 σff σff

For 1.5 e L/σff e 2

(1 - bFb) 1 + (1 + x2)Fb ab ln ln (9) RTFb 2x2RT 1 + (1 - x2)Fb

ln fff(z) )

(

Equations for L/σff g 3 were presented by Subramanian and Lira (1996). However, there was an error in a numerical constant of eq 7 of that publication. The equations presented here are correct. For 2 < L/σff < 3

(7)

L being the width of the slit. Equations 1-4 combine to give

1 z 3 - 0.5 σff

z L L - 1.5 e e - 0.5 σff σff σff

For a parallel slit, the chemical potential due to fluidsolid interaction can be written as

µfs(z) ) NA[Ψ(z) + Ψ(L-z)]

[

3 L-z 5 + 8 σff 6

)

(

)

a(z) 3 L ) -1 ab 8 σff

(13)

For a small slit where 1 e L/σff e 1.5 the mean field model breaks down. Equation 13 incorrectly approaches zero as L/σff approaches 1. Therefore, we use the values of eq 13 at L/σff ) 1.5, i.e. a(z)/ab ) 3/16 as an approximation, as discussed in the Appendix. For L/σff e 1 particles are excluded from the slit and the local density is zero. For the solid-fluid interaction, a 10-4 potential model (Lee, 1988) was used. 2

Ψ(z) ) 4πFatomsfsσfs

(

σfs10 -

5(z′)10

1

σfs4

4

∑

2i)1(z′ + (i - 1) × 3.35 Å)4

)

(14)

where fs is the fluid-solid interaction energy parameter. Fatoms ) 0.382 (Å)2. The distance z′ is the perpendicular distance between the center of a fluid molecule to the center of the first plane of solid atoms, i.e., z′ ) z + σss/2. We have truncated the interactions at the fourth plane of solid atoms. The interplanar spacing is 3.35 Å. In the SLD approach, the fluid-solid interaction energy parameter fs is fit to the experimental data. σff is taken from the literature (Reid, 1987). σfs is the distance parameter of the fluid-solid interaction:

σfs )

σff + σss 2


Figure 3. Adsorption and desorption isotherms at 284 K.

Figure 5. Adsorption and desorption isotherms at 295 K, adsorption experiment terminated before the vapor pressure of carbon dioxide was reached.


Given the bulk fluid pressure and temperature, eqs 5-7 are used to solve for a “local density” F(z). The Gibbs adsorption per unit weight area of adsorbent is calculated as follows: /2 [F(z) - Fb(z)] dz ∫σL-σ /2

Γ)A

ff

ff

(15)

A is the surface area per unit weight of the adsorbent. The limits for the integration consider adsorption to be negligible in the region closer than z ) σff/2. This arbitrary cutoff distance neglects any soft nature of the repulsive forces and will result in the incorrect prediction of Γ ) 0 in slits approaching the molecular size, so refinement of the limits will be required for applications in extremely narrow slits. This will be considered in more detail in a future publication. Results and Discussion The adsorption/desorption isotherms at 284 and 300 K are shown in Figures 3 and 4. The amount of Gibbs adsorption increases with pressure, shows a point of inflection, and then increases rapidly when the operating pressure approaches the vapor pressure. At low pressures, the adsorption isotherm is a typical type I isotherm, which is common for a microporous medium. Close to the vapor pressure, the adsorption isotherm behaves more like a type II isotherm, common for pores with large diameters. The observed behavior of adsorption isotherms is therefore consistent with the pore size distribution data obtained. If the pressure increases

beyond the vapor pressure of carbon dioxide, the amount adsorbed decreases abruptly. At pressures beyond the vapor pressure of carbon dioxide, the desorption isotherms coincide almost exactly with the adsorption isotherms. Ozawa et al. (1979) also presented data of adsorption carbon dioxide on five different activated carbons. The isotherms were at 298, 323, and 343 K. No desorption data were presented. The behavior we observed is similar to that of the sample ACL, which has a broad pore size distribution and a mean slit width of 1.46 nm. At 298 K the isotherm had a rapid increase close to the critical pressure. The supercritical isotherms demonstrated no discontinuity but only maxima. Error bars shown in the figures are estimated using uncertainties in pressure (0.034 MPa) and temperature (0.1 K) measurements. During initial stages of the adsorption experiment, the fluid in the loading cell is a compressed liquid. The compressibility and expansion coefficient of the fluid are relatively small. Therefore, uncertainties in pressure and temperature measurements lead to relatively small error in calculating density. As the experiment progresses, the load cell is depleted. The fluid expands, and the compressibility and expansion coefficient increase. Errors in pressure and temperature measurement are magnified through density calculations. This leads to an increase in error in estimating ∆nl. In the desorption stage, the fluid in the load cell is in a gaseous state. The compressibility and expansion coefficient are relatively large. Hence the error in ∆nl is much larger than that in adsorption. However, the differences between adsorption and desorption measurements are still significant compared to possible experimental errors. Moreover, this adsorption-desorption hysteresis loop is not found if the adsorption procedure is not carried beyond the vapor pressure of carbon dioxide (Figure 5). At 305 K, above the critical temperature, the discontinuity disappears. The isotherm possesses a sharp maximum near the critical temperature of carbon dioxide and decreases abruptly beyond this point (Figure 6). Adsorption-desorption hysteresis could not be observed. At higher supercritical temperatures (310 and 314 K, as shown in Figures 7 and 8), isotherms display a plateau instead of discontinuity or sharp maxima. If



Figure 9. Fitting of adsorption data of carbon dioxide on octadecyl-bonded silica provided by Strubinger and Parcher (1989). Table 1. Molecular Properties Used in the SLD Model for Carbon Dioxide system

eq

14 CO2-DeGussa IV CO2-octadecyl-bonded 16 silica


0.394

0.367 1.047

80 2500

1.06

not scale linearly with slit/pore sizes, this slit width seems to be in fair agreement with the pore size distribution data in Figure 1. For the subcritical isotherms, the SLD model does not predict the rapid rise seen experimentally immediately below the vapor pressure. It also fails to predict the peak of the experimental isotherm just above the critical temperature at 305 K. The calculations we have performed do not include representation of large pores or the external surface area, thus precluding the formation of thick adsorption layers which lead to this behavior. This is actually consistent with the observation by Ozawa et al. (1975). For four activated carbons with relatively narrow slit width distributions and average slit widths from 0.87 to 1.24 nm, the discontinuity was not observed. The inclusion of large pores in the model would produce the phenomena. Note in eq 11, in the limit of L approaching infinity, that the equations reduce to the flat wall results presented by Subramanian et al. (1995) where sharp maxima are present at the vapor pressure. Figure 9 illustrates that the SLD flat wall model provides a good correlation of adsorption of carbon dioxide on 5 µm octadecyl-bonded silica reported by Strubinger and Parcher (1989) which also has this kind of discontinuity. Because the true form of the potential is complex, we have used a singlelayer 10-4 potential:


Ψ(z) ) fs pressure increases beyond a certain value, the amount of adsorption starts to decrease. No hysteresis was found. The parameters of the SLD model obtained by fitting experimental data are listed in Table 1. The activated carbon used in this study has a wide pore size distribution; however, the behavior can be largely represented by an average slit width of L ) 1.06 nm. It has been shown that the SLD model predicts that a slit of 1.6 nm has about the same capacity as a pore of 2.45 nm for ethylene adsorbing on activated carbon (Subramanian and Lira, 1996). Although excess adsorption does

σff (nm) σfs (nm) fs/k (K) L (nm)

[ ( ) ( )] 1 σfs 5 z

10

-

1 σfs 2 z

4

(16)

The energy and distance parameters in Table 1 were obtained by fitting isotherm data at 303 K. The model extrapolates reasonably into higher temperatures. Conclusions A volumetric method was used to measure the adsorption/desorption isotherms at 284, 300, 305, 310, and 314 K over a large pressure range. Experimental data revealed that at subcritical temperatures, adsorption isotherms have a discontinuity at operating pressure


corresponding to the vapor pressure of liquid carbon dioxide. Adsorption-desorption hysteresis was also found. If adsorption is terminated before the vaporliquid transition can occur, there is no hysteresis. At supercritical temperatures, the adsorption isotherm would display a saturation plateau and decrease beyond a certain pressure. No hysteresis phenomenon occurred during the desorption process. The adsorption behavior is well represented by the SLD model. Acknowledgment The group from National Tsing Hua University would like to acknowledge the support of this research by the National Science Council, Taiwan (Grant NSC852214E007-010). D.S.H.W. would like to express his gratitude to the Department of Chemical Engineering, Hong Kong University of Science and Technology, for their hospitality during the period in which the manuscript was revised. Special thanks must be given to Dr. X. J. Hu of HKUST for providing the micropore analysis data. Nomenclature A ) surface area per unit weight of the adsorbent a, b ) constants of equation of state f ) fugacity k ) Boltzmann’s constant L ) slit width NA ) Avogadro’s number n ) molar number P ) pressure R ) gas constant T ) temperature w ) total weight of the adsorbent z ) distance from the surface of the wall Greek Symbols Γ ) amount adsorbed (mol/g) Ψ ) fluid-solid potential ) energy parameter of molecular interaction µ ) chemical potential F ) molar density σ ) molecular diameter

placed at the center of the particle of interest. The form of the integrals and definition of Φ are given by Rangarajan et al. (1995), who also provide more detail on the evaluation of integrals; however, here we define φ ) -4ff(σ6/r6), thus Φbulk ) -(16πffσ3NA/3)F. Case I. For a Particle near the Wall, 0.5 e z e 1.5σ. The slit integration is divided into the following regions: Region 1. x ranges from -(z - σ/2) to 0 0 ∫-(z-σ/2) ∫(σ∞ -x )

Φ1 ) -4ffσ6NA

2

Superscripts l ) property of the loading cell a ) property of the adsorption cell s ) property of the adsorbent f ) free volume + ) standard state

Appendix. Calculation of the Configurational Integral for Slits Definitions. L is the distance between the two wall surfaces. σ denotes the fluid diameter where the subscript is dropped to simplify notation. z is the distance of the center of the particle of interest from the surface of one of the walls. Integrations are performed in cylindrical coordinates. r is the radial distance. x is a dummy variable that denotes axial distance perpendicular to the walls, where the origin is

2πr F(x) 2 dr dx (x + r2)3

[σz - 21]

Φ1 ) -2ffσ3NAπF(z)

where F(x) and F(z) differ only in the coordinate system used to specify the location. Region 2. x ranges from 0 to σ

Φ2 ) -4ffσ6NA

∫0σ∫(σ∞ -x ) 2

2 1/2

2πr F(x) 2 dr dx (x + r2)3

Φ2 ) -2ffσ3NAπF(z) Region 3. x ranges from σ to L - σ/2 - z

Φ3 ) -4ffσ6NA

dr dx ∫σL-σ/2-z∫0∞F(x)(x22πr + r2)3

Φ3 ) -4ffσ6NAπ

∫σL-σ/2-z x4

F(x)

[

dx

2 1 Φ3 ) - ffσ3NAπF(z) 1 3 L 1 z3 - σ 2 σ

(

)

]

Φ ) Φ1 + Φ2 + Φ3

[

Φ ) -2ffσ3NAπF(z)

Subscripts b ) bulk property ff ) property from contribution of fluid-fluid interaction fs ) property from contribution of fluid-solid interaction i ) experimental index ss ) property of the solid molecule

2 1/2

z 5 + σ 6

1 L 1 z3 3 - σ 2 σ

(

)

]

Note that for a particle near the other wall, the result is a mirror image of the above result; i.e., replace z with L - z. Case II. For Slits Where L/σff g 3. The particle may also be located at 1.5σ e z < L - 1.5σ. In this case, the slit integration is divided into four regions: Region 1. x ranges from -(z - σ/2) to σ

Φ1 ) -4ffσ6NA

-σ dr dx ∫-(z-σ/2) ∫0∞F(x)(x22πr + r2)3

[

]

2 1 Φ1 ) - ffσ3NAπF(z) 1 3 z 13 σ 2

(

)

Region 2. x ranges from -σ to 0. The result is the same as case I, region 2. Region 3. x ranges from 0 to σ. The result is the same as case I, region 2. Region 4. x ranges from σ to L - σ/2 - z. The result is the same as case I, region 3.


[

Φ ) Φ1 + Φ2 + Φ3 + Φ4

a(z) 3 8 ) ab 83

1 z 3 - 0.5 σff

(

1 L-z 3 - 0.5 σff

) ( 3

-

)

]

3

Case III. In Slits 2 < L/σff < 3. The integration depends on the location of the particle: A. For 0.5 e z/σ < L/σ - 1.5, case I applies. B. For 1.5 e z/σ < L/σ - 0.5, the mirror image of case I applies. C. For L/σ - 1.5 e z/σ < 1.5, the integration will be divided into two regions. Φ1 will be the same as case I, region 1. For region 2, x going from 0 to L - σ/2 - z

∴Φ2 ) -4ffσ6NA

∫0L-σ/2-z∫(σ∞ -x ) 2

Φ2 ) -2ffσ3NAπF(z)

2 1/2

2πr F(x) 2 dr dx (x + r2)3

[Lσ - 21 - σz]

Φ ) Φ1 + Φ2 Φ ) -2ffσ3NAπF(z)

[Lσ - 1]

Case IV. For Slits L/σ e 2. The particle is always located by the constraints defined by case IIIc, and that result is applied with the qualifications discussed here. Note that Φ1 incorrectly approaches the value of zero as L/σ approaches one. The problem arises due to the shortcomings of mean field theory (that neglects particle size) at an interface where particle size is most important. Mean field theory usually assumes that fluid particles are present in all spaces between the walls of a slit. At an interface this is incorrect; the reason can be visualized by considering a flat wall. In flat walls, we begin integrating from z ) 0.5σ because molecular centers are excluded from being closer due to their finite size. Extending this approach to a confined slit, the SLD approach is applied for each wall, excluding a volume of integration within 0.5σ of each wall. When the width is just one molecular diameter, the excluded volumes from the two walls meet, resulting in exclusion of the entire volume. One approach to eliminate the incorrect limit would be to calculate the integral for a slit of one molecular width as a base reference and add this integral onto the other cases where the slits are larger. Presently, however, no clear and simple solution to this problem exists, because the problem of finite particle size is simply transferred from the first layer to each subsequent layer. In order to investigate the behavior of a/ab in confined spaces approaching σ, we can easily consider the packing of disks in a two-dimensional channel. Solving for the configurational energy in the closest packing in a two-dimensional channel for disks of a finite size by summing a 1/r6 potential, the configurational energy for a channel of one molecular diameter width is approximately 2% lower than the configurational energy of a channel 1.5 molecular diameters wide. Based on this two-dimensional result, it seems reasonable to assume that a/ab is constant for slit widths below 1.5σ. We choose 1.5σ because 1σ represents total constraint in packing, while 2σ represents total freedom in packing. This assumption could be evaluated by conducting molecular simulations in small slits and determining

the dependence of configurational energy on slit size to further refine a/ab. Therefore, we apply case IIIc for slits 1.5 e L/σ e 2, and for slits 1 e L/σ e 1.5 we use the value for L/σ ) 1.5. For L/σ e 1, particles are excluded from the slit and the local density is zero. Literature Cited Angus, B.; Armstrong, B.; deReuck, K. M. Carbon Dioxide: International Thermodynamics Tables of the Fluid State; Pergaman Press: New York, 1976. Barton, S. S.; Dacey, J. R.; Quinn, D. F. High Pressure Adsorption of Methane on Porous Carbons. In Fundamentals of Adsorption; Myers, A. L., Belfort, G., Eds.; Engineering Foundation: New York, 1983; p 65. Brennecke, J. F.; Eckert, C. A. Phase Equilibria for Supercritical Fluid Process Design. AIChE J. 1989, 35, 1409. DeFilippi, R. P.; Krukonis, V. J.; Robey, R. J.; Modell, M. Supercritical Fluid Regeneration of Activated Carbon for Adsorption of Pesticides. EPA Report; EPA: Washington, DC, 1980. Dooley, K. M.; Kao, C.-P.; Grambrell, R. P.; Knopf, F. C. The Use of Entrainers in the Supercritical Extraction of Soils Contaminated with Hazardous Organics. Ind. Eng. Chem. Res. 1987, 26, 2058. Findenegg, G. H. High-Pressure Physical Adsorption of Gases on Homogeneous Surfaces. In Fundamentals of Adsorption; Myers, A. L., Belfort, G., Eds.; Engineering Foundation: New York, 1983; p 207. Jones, W. M.; Isaac, P. J.; Phillips, D. The Adsorption of Carbon Dioxide and Nitrogen at High Pressure by Porous Plugs of Lampblack. Trans. Faraday Soc. 1959, 55, 1953. Kim, M. J.; Holste, J. C.; Hall, K. R.; Slattery, J. C. Supercritical Adsorption in Small Pores. J. Colloid Surface Sci. 1993, 158, 488. King, J. W. Supercritical Fluid Adsorption at the Gas-Solid Interface; ACS Symposium Series 329; American Chemical Society: Washington, DC, 1987; p 150. Lee, L. L. Molecular Thermodynamics of non-Ideal Fluids; Butterworth: Stoneham, MA, 1988. Lowell, S.; Shields, J. E. Powder Surface Area and Porosity, 3rd ed.; Chapman & Hall: New York, 1991. Menon, P. G. Adsorption at High Pressure. Chem. Rev. 1968, 68, 277. Modell, M.; Robey, R. J.; Krukonis, V. J.; DeFilippi, R. P.; Oestreich, D. Supercritical Fluid Regeneration of Activated Carbon. Presented the AIChE Meeting, Boston, 1979. Ozawa, S.; Kusumi, S.; Ogino, Y. High Pressure Adsorption of Carbon Dioxide on several activated carbons with different pore size distribution. Proceedings of the Fourth International Conference on High Pressure; Physical-Chemical Society of Japan: Kyoto, Japan, 1975; p 580. Peterson, B. K.; Gubbins, K. E. Phase transitions in a cylindrical pore: Grand canonical Monte Carlo, mean field theory and the Kelvin equation. Mol. Phys. 1987, 62, 215. Peterson, B. K.; Gubbins K. E.; Heffelfinger, G. S.; Marconi, U. M. B.; Swol, F. V. Lennard-Jones fluids in cylindrical pores: Nonlocal theory and computer simulation. J. Chem. Phys. 1988, 88, 6487. Rangarajan, B.; Lira, C. T.; Subramanian, R. Simplified Local Density Model for Adsorption over Large Pressure Ranges. AIChE J. 1995, 41, 838. Reid, R. C.; Prausnitz, J. M.; Poling, B. E. The Properties of Gases and Liquids, 4th ed.; McGraw-Hill: New York, 1987; p 734. Strubinger, J. R.; Parcher, J. F. Surface Excess (Gibbs) Adsorption Isotherms of Supercritical Carbon Dioxide on Octadecyl-bonded Silica Stationary Phases. Anal. Chem. 1989, 61, 951. Subramanian, R.; Lira, C. T. An Engineering Model for Adsorption of Gases onto Various Media. Fundamentals of Adsorption; LeVan, M. E., Ed.; Kluwer: Boston, MA, 1996; p 873. Subramanian, R.; Pyada, H.; Lira, C. T. An Engineering Model for Adsorption of Gases onto Flat Surfaces and Clustering in Supercritical Fluids. Ind. Eng. Chem. Res. 1995, 34, 3840. Tan, C. S.; Liou, D. C. Desorption of Ethyl Acetate from Activated Carbon by Supercritical Carbon Dioxide. Ind. Eng. Chem. Res. 1988, 27, 988.

Ind. Eng. Chem. Res., Vol. 36, No. 7, 1997 2815 Tan, C. S.; Liou, D. C. Regeneration of Activated Carbon Loaded with Toluene by Supercritical Carbon Dioxide. Sep. Sci. Technol. 1989a, 24, 111. Tan, C. S.; Liou, D. C. Supercritical Regeneration of Activated Carbon Loaded with Benzene and Toluene. Ind. Eng. Chem. Res. 1989b, 28, 1222. Tan, C. S.; Liou, D. C. Adsorption Equilibrium of Toluene from Supercritical Carbon Dioxide on Activated Carbon. Ind. Eng. Chem. Res. 1990, 29, 1412. Vaska, L.; Selwood, P. W. The Mechanism of Chemisorption: Hydrogen on Nickel at Elevated Pressures. J. Am. Chem. Soc. 1958, 80, 1331.

Wu, Y. Y.; Wong, D. S. H.; Tan, C. S. Thermodynamic Model for the Adsorption of Toluene From Supercritical Carbon Dioxide on Activated Carbon. Ind. Eng. Chem. Res. 1991, 30, 2493.

Received for review April 16, 1996 Revised manuscript received January 21, 1997 Accepted January 26, 1997X IE960227W X Abstract published in Advance ACS Abstracts, March 15, 1997.

Adsorption and Desorption of Carbon Dioxide onto and from Activated

Recommend Documents