Ind. Eng. Chem. Res. 2003, 42, 897-903
897
Deactivation Model for Desorption of Tricholoroethylene Vapor from Granular Activated Carbon Y. Suyadal* Chemical Engineering Department, Ankara University, Tandogˇ an, 06100 Ankara, Turkey
Desorption breakthrough curves of tricholoroethylene (TCE) vapor were investigated in a laboratory-scale packed-bed adsorber (PBA) using granular activated carbon (GAC) at atmospheric pressure. The PBA was operated batchwise with a single charge of 2.5-10 g of GAC to obtain TCE adsorption-desorption breakthrough curves. TCE adsorption breakthrough curves were obtained at constant operating temperature (25 °C) and TCE feedstock concentration (10 000 ppm) within the range of gas space velocity 10 000 e ϑ/h-1 e 40 000. In addition, TCE desorption breakthrough curves were obtained at various temperatures in the range 25 e T/°C e 55. By using the analogy between the desorption of TCE and the deactivation of catalyst particles, a deactivation model (DM) for the gas-phase concentration of TCE obtained from these curves was tested. Observed desorption rate constants (k′′) and first-order deactivation rate constants (kd) were obtained from the model. It was found that the deactivation model provides a good description of the experimental desorption breakthrough curves of TCE obtained from the GAC. The deactivation model was compared to other adsorption-desorption isotherms reported in the literature. Introduction
Table 1. Ranges of Experimental Conditions
The removal of volatile organic compounds (VOCs) from industrial gas streams by adsorption is one of the most proven methods for controlling their emissions. The most widely used macroscopic transport model for laboratory and controlled field-scale investigation is onedimensional sorption. The sorption term in this equation complicates the analysis of transport in porous solid media. Equilibrium partitioning might or might not adequately describe species partitioning between the pores and grain. To understand the transport of organic vapors through porous solid media, equilibrium partitioning and sorption kinetics have frequently been studied.1-6 Although it might seem reasonable to model adsorption-desorption processes using isotherms, such as Langmuir, Freundlich, BET, and others encountered in the literature,1-6 the good description of adsorption breakthrough curves provided by the deactivation model (DM) presented in a previous paper7 makes that model more appropriate for these curves. Sorption capacities and rates of adsorption and desorption can be measured using the differential adsorption bed technique. Sorption equilibrium is well described by nonlinear isotherms. The isotherms gradually change from nonlinear to linear, and granular sorption capacities decrease. For dry solids, adsorption occurs more rapidly than desorption; for moist solids, adsorption and desorption kinetics are symmetric and significantly faster. The relative rates of adsorption and desorption are related to the shape of the equilibrium isotherm.8-10 The phenomena or mechanisms of deactivation in adsorption are related to the coating of the pore surface with vapor molecules. The deactivation of adsorbent is first-order with respect to the solid surface and can be described in terms of an exponential decrease with time in available solid surface area. * E-mail:
[email protected].
parameter
range
operating pressure operating temperature mass of adsorbent feedstock concentration space velocity
101.3 kPa 25 e T/°C e 55 2.5 e mads/g e 10.0 10 000 ppm TCE 10 000 e ϑ/h-1 e 40 000
As a result, a number of simpler, semiempirical models have been developed.8-10 These researchers made use of the analogy between the adsorption of VOCs and the deactivation of catalyst particles, both of which can be described in terms of an exponential decrease in available surface area with time. The current work concerns, on one hand, an exploration of the desorption breakthrough curves of tricholoroethylene (TCE) with the changing operating temperature and mass of sorbent and, on the other hand, testing of the DM for the first time for a description of the desorption breakthrough curves in adsorption-desorption isotherms obtained in a packed-bed adsorber (PBA). Experimental Section Apparatus. The experimental setup consisted mainly of a granular activated carbon (GAC) column as the PBA and auxiliary equipment for the preparation of TCE vapor-air mixtures, gas analysis, and on-line data logging. The schematic diagram of the experimental setup, which was presented for TCE adsorption breakthrough curves in a previous paper,7 was modified and is shown in Figure 1. Materials. GAC provided by the Turkish Machinery and Chemistry Corporation (MKE Co., Ankara, Turkey) and TCE from Merck (Darmstadt, Germany) were used in the experiments for obtaining TCE adsorptiondesorption breakthrough curves. The properties and characterization tests of GAC were reported in a previous paper.7
10.1021/ie020229i CCC: $25.00 © 2003 American Chemical Society Published on Web 01/24/2003
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Figure 1. Schematic diagram of the experimental setup.
Figure 2. Test of the DM (eq 9) for kd and k′′ as a function of the PBA operating temperature.
Procedure. The concentrations of TCE in the mixed streams were adjusted by varying the temperature of liquid TCE in a circulating water bath and the relative flow rates of TCE-rich air and the main stream with a rotameter. During the adsorption-desorption experiments, the inlet and outlet TCE concentrations were continuously measured by a nondispersive infrared (NDIR) gas analyzer (MIR 9000, Environnement S.A., Poissy Cedex, France). The PBA was operated batchwise with the operating conditions given in Table 1. Model Development. The following adsorption reaction (eq 1) between TCE and a vacant site (s) of the
activated carbon surface results in the formation of activated site (TCE‚ ‚ ‚s). In addition, desorption (eq 2) results in the formation of a vacant site and TCE.
adsorption
C2HCl3 + s f C2HCl3‚ ‚ ‚s
(1)
desorption
C2HCl3‚ ‚ ‚s f C2HCl3 + s
(2)
To formulate the desorption of TCE from the GAC, the following assumptions were made: (1) The system is isothermal. (2) External mass-transfer limitations are neglected. (3) The PBA can be approximated as an ideal
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Figure 3. Effect of operating temperature on the solid-phase concentration of TCE vs time curves.
Figure 4. Arrhenius plots of desorption rate constant (k′′) and deactivation rate constant (kd).
batch-solids reactor with a plug flow of fluid according to Levenspiel.11 (4) The pseudo-steady-state assumption is valid in the PBA. (5) Deactivation of the sorbent is first-order with respect to the solid surface area and can be described in terms of an exponential decrease with time in the available surface area as follows
S ) S0 exp(-kdt) w
dS ) -kdS dt
(3)
Because the amount of adsorbed TCE is equal to the amount of TCE captured at the initial time of desorption in the solid phase, the TCE desorption ratio (XS) can be calculated from the amount of TCE measured in the PBA outlet stream. The TCE desorption ratio is given by
∫0t(C/C0)des dt]/[∫0∞(1 - C/C0)ads dt]
XS ) [
(4)
where C is the concentration of TCE in the outlet stream and C0 is the feedstock concentration of TCE. According to the desorption ratio, the solid surface concentration of TCE at any time can be given by
CS ) 1 - XS CS0
(5)
where CS and CS0 are the surface concentration of TCE at any time and at the initial time, respectively. The mass balance for adsorbed TCE on activated sites of the solid gives the desorption rate of TCE. The following equation (eq 6) represents the mass balance based on a unit activated solid surface area at unsteady-state conditions
-
dCS 1 d(VCS) ) kSCS w ) -k′′CS exp(-kdt) (6) S dt dt
where k′′ ) kSS0/V and kd are the observed desorption rate constant and deactivation rate constant, respectively. Equation 6 can be integrated to give
[
CS k′′ ) exp - [1 - exp(-kdt)] CS0 kd As t f ∞, eq 7 can be rearranged to read
]
(7)
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Figure 5. Effect of operating temperature and mass of GAC on TCE adsorption-desorption breakthrough curves.
[ ]
CS∞ k′′ ) exp CS0 kd
(8)
Combining eqs 7 and 8, the result can be rearranged
to give
[(
ln ln
CS/CS0 CS∞/CS0
)]
) ln(k′′/kd) - kdt
(9)
Ind. Eng. Chem. Res., Vol. 42, No. 4, 2003 901
Figure 6. Comparison of the models in describing the experimental data according to Table 2.
where CS∞ is the TCE concentration of activated sites at t f ∞. Thus, if ln[ln(CS/CS∞)] is plotted versus reaction time (t), a straight line should be obtained with a slope equal to -kd and an intercept giving ln(k′′/kd), from which k′′ can be obtained. In addition, The mass balance for TCE in the fluid phase gives the following equation at pseudo-steady-state conditions
Q0C - Q0(C + dC) + kg(CS - C) dS ) 0
(10)
Similarly, the mass balance for TCE in the solid phase at pseudo-steady-state conditions is given by
kSCSdS - kg(CS - C) dS ) 0
(11)
Combining eqs 10 and 11 gives the plug-flow design equation, which can be rearranged to
-Q0 dC + kSCS dS ) 0
(12)
Substituting eqs 3 and 7 into eq 12 gives
Q0
∫CC dC ) (-kd)(k′′)(VCS0)∫0texp(-kdt) ×
[
0
exp -
]
k′′ [1 - exp(-kdt)] dt (13) kd
Eq 13 can be integrated to give
(C/C0) ) 1 -
[∫ {
∞
0
]
(1 - C/C0)ads dt ×
[
kd 1 - exp -
k′′ [1 - exp(-kdt)] kd
]}
(14)
where the integral (first term in brackets) gives the area. The dimensionless concentrations of TCE in the solid phase (y) and fluid phase (x) were determined according
to the following equations
solid phase
∫0t(CS/CS0) dt y) ∞ ∫0 (CS/CS0) dt
(15)
∫0t(C/C0) dt x) ∞ ∫0 (C/C0) dt
(16)
fluid phase
Results and Discussion Test of DM (Eq 9) for Experimental Desorption Breakthrough Curves with Varying Temperature. To test the proposed deactivation model, ln[ln(CS/CS∞)] was plotted as a function of time for each operating temperature. The linearity of the data points for each temperature can be seen in Figure 2. It is therefore concluded that eq 9 provides an adequate representation of the experimental points at various temperatures. The good fit of the DM predictions to the solid surface concentration data can be seen by inserting the corresponding values of k′′ and kd tabulated in the Appendix into the eq 7, as shown in Figure 3. As can be seen in Figure 3, the results indicated a shift in the solid surface concentration curves of TCE toward the bottom with increased temperature, which can be attributed to an increase in the amount of TCE desorbed from the GAC. Arrhenius plots of the observed desorption rate constant (k′′) and deactivation rate constant (kd) are shown in Figure 4. As can be seen in Figure 4, the observed desorption rate constant increased with increasing operating temperature, but the deactivation rate constant did not. These plots indicatethat the deactivation rate constant in the desorption of TCE exhibits nonArrhenius behavior. According to the Arrhenius plot of the rate constants, the values of the activation energies for desorption (Ea) and deactivation (Ed) were found to 13.95 and 28.54 kJ/mol, respectively. The values of kd that appear only in the DM were found to vary between 1.3 × 10-3 and 7.2 × 10-3 s-1 in this study. In addition, the values of k′′ in the DM were found to vary between 1.0 × 10-3 and 5.8 × 10-3 s-1.
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Ind. Eng. Chem. Res., Vol. 42, No. 4, 2003
Table 2. Selected Isotherms13 to Fit the Data for Comparison with DM
adsorption-desorption isotherm
mathematical representation
linearized form
x
Harkins-Jura (HJ)
y)
Brunauer-Emmett-Teller (BET)
x y) (1 - x)(a + bx)
x ) a + bx y(1 - x)
Freundlich (F)
y ) axb
ln(y) ) ln(a) + b ln(x)
Langmuir (L)
y)
Dubinin-Raduskevich-Kagener (DRK)
y ) a exp[-b ln2(x)]
ln(y) ) ln(a) - b ln2(x)
deactivation model (this study)
y according to eq 15 x accroding to eq 16
ln[ln(CS/CS∞)] ) I I ) ln({k′′}/{kd}) - kdt
b ln(a/x)
ln(x) ) ln(a) - (b/y2)
ax (1 + bx)
According to the experimental data for the adsorption TCE onto GAC at 25 °C, the adsorption capacity of GAC was found to be 0.65 g of TCE/g of GAC. A comparison of the model predictions with the experimental data in the desorption breakthrough curves of TCE can be seen in Figure 5. The good fit of the DM predictions to the experimental data can be seen by inserting the corresponding values of k′′ and kd tabulated in the Appendix into eq 14, as shown in Figure 5. It is therefore concluded that eq 14 provides an adequate representation of the experimental points for various temperatures and masses of GAC. As can be seen in Figure 5, the results indicate a shift in the desorption breakthrough curves of TCE toward the right with increased temperature, which can be attributed to an increase in the amount of TCE desorbed from the GAC. The similar behavior for the adsorption-desorption breakthrough curves of other VOCs has been reported in the literature.6,8-10,12 Conclusions The DM was applied successfully for the first time to VOC desorption from GAC in this study, as can be seen from comparison of the models given in Figure 6 based on Table 2.13 The models considered are the so-called adsorption-desorption isotherms encountered frequently in the literature.1,6,8-10,12,14 It can be concluded that the two rate constants can be incorporated into a PBA model that assumes the bed to behave essentially as a plugflow adsorber. The results of the semibatch experiments in a PBA can establish the basis for the design of fixedbed processes. In addition, an estimation of the desorption isotherms from the breakthrough curves of TCE was carried out successfully under the present experimental conditions in a batchwise manner for different masses of GAC. According to eqs 15 and 16, the dimensionless concentrations of TCE in the solid phase (y) and in the fluid phase (x) were determined and fitted to the models listed in Table 2. This relationship determined is shown graphically in Figure 6. As can be seen from Table 2 and Figure 6, the proposed DM fitted the data with the highest correlation coefficient (r2) of 0.995. The equilibria for single-solute sorption given in the literature are frequently presented as dimensionless
1 b 1 ) + y ax a
parameter values and correlation coefficient a ) 0.0467 b ) 1.0383 r2 ) 0.989 a ) -5.3067 b ) 18.6647 r2 ) 0.969 a ) 0.7036 b ) 1.8811 r2 ) 0.989 a ) 0.2801 b ) -0.6629 r2 ) 0.989 a ) 0.6157 b ) 6.0031 r2 ) 0.981 k′′ ) 3.3 × 10-3s-1 kd ) 7.2 × 10-3s-1 Ea ) 13.95 kJ/mol Ed ) 28.54 kJ/mol r2 ) 0.995
Table 3. Deactivation Model Parameters temperature observed desorption rate deactivation rate (T, K) constant (k′′ × 103, s-1) constant (kd × 103, s-1) 298.15 308.15 318.15 328.15
mAcC ) 0.0025 kg, P ) 101.3 kPa 3.3333 3.9167 4.6500 5.7667
7.1667 5.0167 3.6833 2.8833
298.15 308.15 318.15 328.15
mAcC ) 0.0050 kg, P ) 101.3 kPa 1.7333 1.8333 2.0000 2.2500
3.6500 2.8167 2.2333 1.8000
298.15 308.15 318.15 328.15
mAcC ) 0.0100 kg, P ) 101.3 kPa 1.0000 1.1000 1.1833 1.3333
3.3667 2.2000 1.7000 1.2667
concentration isotherms as functions of the constant separation factor (r), which are analogous to relative volatility or its reciprocal in distillations. It is known that r values greater than unity give favorable desorption isotherms, whereas the linear isotherm, or Raoult’s law case, corresponds to r ) 1, as shown in Figure 6.15 Acknowledgment The author gratefully acknowledges financial support from the Ankara University Research Fund (Project 9125-00-53), as well as the Scientific and Technical Research Council of Turkey (TU ¨ BITAK) under Grant MI˙ SAG/KTC¸ AG-116. Appendix: Model Parameters In the Table 3 are listed all k′′ and kd values obtained from the linearized form of the deactivation model (DM) as a function of operating temperature and mass of GAC. Nomenclature C ) exit concentration of TCE at any time, kmol m-3 C0 ) feedstock concentration of TCE, kmol m-3 CS ) surface concentration of TCE at any time, kmol m-3
Ind. Eng. Chem. Res., Vol. 42, No. 4, 2003 903 CS0 ) surface concentration of TCE at the initial time, kmol m-3 CS∞ ) surface concentration of TCE at t f ∞, kmol m-3 Ea ) activation energy for desorption, kJ mol-1 Ed ) activation energy for deactivation, kJ mol-1 kd ) first-order deactivation rate constant, s-1 kg ) gas-solid mass transfer coefficient based on solid surface, m s-1 k′′ ) observed desorption rate constant, s-1 kS ) desorption rate constant based on a unit of activated solid surface area, m s-1 m ) mass of GAC, kg P ) operating pressure, kPa Q0 ) volumetric flow rate of air, m3 s-1 s ) vacant site of solid surface, m2 S ) activated site of solid surface at any time, m2 S0 ) activated site of solid surface at initial time, m2 t ) time, s T ) operating temperature, K V ) volume of adsorbent, m3 XS ) desorption ratio of TCE x ) dimensionless fluid-phase concentration of TCE y ) dimensionless solid-phase concentration of TCE
(4) Cal, M. P.; Rood, M. J.; Larson, S. M. Removal of VOCs from Humidified Gas Streams Using Activated Carbon Cloth. Gas. Sep. Purif. 1996, 10 (2), 117-121. (5) Cal, M. P.; Rood, M. J.; Larson, S. M. Gas-Phase Adsorption of Volatile Organic Compounds and Water Vapor on Activated Carbon Cloth. Energy Fuels 1997, 11, 311-335. (6) Chou, M.-S.; Chiou, J.-H. Modelling Effects of Moisture on Adsorption Capacity of Activated Carbon for VOCs. J. Environ. Eng. 1997, 123 (5), 437-443. (7) Suyadal, Y.; Ogˇuz, H. Deactivation Model for the Adsorption of Tricholoroethylene Vapor on an Activated Carbon Bed. Ind. Eng. Chem. Res. 2000, 39, 724-730. (8) Lin, T.-F.; Little, J. C.; Nazaroff, W. W. Transport and Sorption of Organic Gases in Activated Carbon. J. Environ. Eng. 1996, 122 (3), 169-175. (9) Lin, T.-F.; Nazaroff, W. W. Transport and Sorption of Water Vapor in Activated Carbon. J. Environ. Eng. 1996, 122 (3), 175180. (10) Lin, T.-F.; Van Loy, M. D.; Nazaroff, W. W. Gas-Phase Transport and Sorption of Benzene in Soil. Environ. Sci. Technol. 1996, 30, 2178-2186. (11) Levenspiel, O. Chemical Reaction Engineering, 3rd ed.; John Wiley & Sons Inc.: New York, 1999.
ϑ ) space velocity based on bed volume, h-1
(12) Yeo, S.-D.; Tuncer, E.; Akgerman, A. Adsorption of Volatile Organic Compounds on Soil and Prediction of Desorption Breakthroughs. Sep. Sci. Technol. 1997, 32, 2497-2512.
Literature Cited
(13) Gregg, S. J.; Sing, K. S. Adsorption, Surface Area and Porosity, 2nd ed.; Academic Press: London, 1982.
Greek Letter
(1) Werner, M. D. The Effects of Relative Humidity on the Vapor Phase Adsorption of Trichloroethylene by Activated Carbon. Ind. Hyg. Assoc. 1985, 46 (10), 585-590. (2) Lorgooei, M.; Carmichael, K. R.; Kelly, T. W.; Rood, M. J.; Larson, S. M. Activated Carbon Cloth Adsorption-Cryogenic System to Recover Toxic Volatile Organic Compounds. Gas. Sep. Purif. 1996, 10 (2), 123-130. (3) Goss, K.-L.; Eisenreich, S. J. Adsorption of VOCs from the Gas Phase to Different Minerals and a Mineral Mixture. Environ. Sci. Technol. 1996, 30, 2135-2142.
(14) Jonas, L.; Rehrmann, R. The Rate of Gas Adsorption by Activated Carbon. Carbon 1974, 12 (2-A), 95-101. (15) Perry, R. H.; Green, D. Chemical Engineer’s Handbook, 7th ed.; McGraw-Hill Book Co.: New York, 1997.
Received for review March 28, 2002 Revised manuscript received November 4, 2002 Accepted November 4, 2002 IE020229I
