Ind. Eng. Chem. Res. 2006, 45, 6563-6569
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Studies on Adsorption of Ethyl Acetate Vapor on Activated Carbon Sampatrao D. Manjare*,† and Aloke K. Ghoshal‡ Chemical Engineering Group, Birla Institute of Technology and Science (BITS), Pilani 333 031, Rajasthan, India, and Department of Chemical Engineering, IIT Guwahati, North Guwahati, Guwahati 781 039, Assam, India
Adsorption of ethyl acetate, a toxic volatile organic compound, on activated carbon is studied. Adsorption isotherms at temperatures of 30, 45, and 55 °C are obtained. Langmuir isotherm parameters, saturation capacity, and heat of adsorption are found from the experimental data. The overall mass-transfer coefficient of ethyl acetate is evaluated using the uptake curve method. A mathematical model, with consideration of nonisothermal and nonadiabatic processes without axial dispersion of mass and energy and on the basis of the linear driving force approximation, well-predicted the experimental breakthrough data. Results show that the isotherms are of Brunauer type I and well-fitted with the Langmuir isotherm model. The saturation capacity of ethyl acetate on activated carbon is 0.73-0.487 kg/kg in the temperature range of 303-328 K. The heat of adsorption both by van’t Hoff and by Clausius-Clapeyron equations are on the order of 105 J/kg. The overall masstransfer coefficient is on the order of 10-4 s-1. Introduction
Table 1. Physical Properties of Adsorbents Supplied by E-merck India
Forthcoming regulatory developments and public concern over the perceived health risks associated with volatile organic compounds (VOCs) exposure make it imperative to reduce VOC emissions. A substantial number of the VOCs are solvents, emitted from processes such as drying, gluing, and coating. Ethyl acetate (EA), an important solvent used in petrochemical and polymer industries, is a clear, volatile, inflammable liquid with a characteristic fruity odor detectable at 7-50 ppm. It is commonly emitted from industrial plants and is often a constituent of industrial wastes.1 Ethyl acetate vapor present in air is one of the VOCs.2 To recover VOCs and/or to reduce its concentration in effluent streams from environmental concern, adsorption with activated carbon is customarily employed. Several researchers have been concerned for a few decades with the removal/recovery of VOCs/hydrocarbons, which include hexane, benzene, n-heptane, isopentane, propane, propylene, heptane, methylbenzene, methyl chloride, trichloroethylene, ethylene, ethane, dimethyl chloride, toluene, acetone, and methane. Ghoshal and Manjare, and Moretti focused on the selection of appropriate abatement technology for removal of VOCs.3,4 Most of the reported studies on adsorptive separation were related to the equilibrium and kinetic studies5-7 and their adsorption behavior as a single component8-10 as well as multicomponent systems.11,12 Adsorbents used are activated carbon, molecular sieves, and, in some cases, specifically developed adsorbents. A few desorption studies are also reported to conclude upon the ease of recovery.1,13 A few available reported literature provide information on the adsorptive capacity of fibrous carbon cloth and crushed activated carbon for ethyl acetate from inert gas5 and from aqueous solution.1 Gales et al.14 studied hysteresis in the cyclic adsorption of ethyl acetate on BASF activated carbon. Their study was mainly on the adsorption behavior when adsorption/desorption cycles are applied, which is important for design and operation of the pressure swing adsorption unit design. They used a standard * To whom correspondence should be addressed. Fax: 01596244183. E-mail:
[email protected]. † BITS. ‡ IIT Guwahati.
parameters
adsorbent: activated carbon (granular)
average particle diameter, mm pore diameter, Å particle bulk density, kg/m3 BET surface area, m2/g macroporosity
1.50 10-50 500 1200 0.38
volumetric system to measure the isotherms. Gales et al., in their other paper,15 experimentally investigated the possibility of recovering ethyl acetate along with ethanol and acetone from a nitrogen stream by thermal pressure swing adsorption on activated carbon. They developed a mathematical model considering a nonisothermal, nonadiabatic, and nonequilibrium process that combines principles of thermal swing and pressure swing adsorption. As a kinetic parameter, they estimated intraparticle effective diffusivity by zero length column technique. Heat of adsorption was calculated from Langmuir constants at different temperatures. In the present investigation we report equilibrium capacities generated from the adsorption isotherm experimentation of ethyl acetate vapor present in air using activated carbon supplied from E-Merck, India. A suitable adsorption isotherm representing the equilibrium data is reported. We also estimated experimentally the overall mass-transfer coefficient using the uptake curve method16 and heat of adsorption using both the van’t Hoff equation and the Clausius-Clapeyron equation. The simplified mathematical model without axial dispersion of heat and mass and based on a linear driving force approximation is simulated with the estimated input parameters to predict the experimental breakthrough data. It is worth mentioning that the model used for thermal swing adsorption in the present work on activated carbon is widely different from that of Gales et al.15 used for thermal pressure swing adsorption in their work. The validity of the model and the accuracy of the input parameters obtained by the present methods are analyzed. Experimental Section Adsorption Isotherm Study. The adsorption isotherm setup10 consisting of a U-tube containing a known weight (ap-
10.1021/ie0603060 CCC: $33.50 © 2006 American Chemical Society Published on Web 08/12/2006
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Table 2. Model Parameters and Heat of Adsorption for Ethyl Acetate Adsorption on Activated Carbon -∆H,a J/kg × 10-5 Langmuir
iosteric heat of adsorption
sr no
temp, °C
qs, kg/ kg
b, m3/kg
Langmuir model
1 2 3
30 45 55
0.73 0.584 0.487
50.92 41.99 39.58
0.97
a
q*(kg/kg) ) 0.1
q*(kg/kg) ) 0.2
q*(kg/kg) ) 0.3
q*(kg/kg) ) 0.4
2.82
3.27
4.04
5.88
-∆H is the heat of adsorption.
proximately 1 g) of preheated (at 150 °C overnight and thereafter cooled) granular activated carbon kept in a constant-temperature water bath. Physical properties of the activated carbon are given in Table 1. Compressed air was passed through the air regulatory valve to supply air without fluctuations to the bottom of the pretreatment column packed with silica gel, glass wool, and activated alumina. This column reduces any moisture content, carbon dioxide content, and particulates when it is passed through the column. The pretreated air was then passed through saturator(s) filled with ethyl acetate (99% extra pure) and then through knockout pot(s) to avoid any possible entrainment. The inlet concentration of ethyl acetate was varied by maintaining different flow rates as well as different levels of ethyl acetate in the saturator(s). Weight change of the U-tube containing activated carbon was measured time-to-time using an electronic Metler balance until the saturation limit. Further details can be seen from our recent publication.10 Adsorption Dynamics Study. The apparatus for the dynamic studies9 consisted of a dehydration column, saturators, knockout pot(s), flow meter, packed-bed adsorption column, and a constant-temperature water bath. The fixed bed adsorber column was a S. S. pipe of i.d. of 10 cm, thickness of 0.2 cm, and length of 40 cm. Four thermocouple probes at equal spacing (6.0 cm) were provided on the wall of the column at different radial locations to measure the temperature of the packed bed column along its length. Two thermocouple probes were also provided at the bottom and at the top conical section to monitor the temperatures of inlet and outlet streams, respectively. Air was pretreated after passing through the dehydration column packed with glass wool, activated alumina, and silica gel to remove dust particles, moisture, and carbon dioxide, respectively, from it. A metered amount of this air was bubbled through ethyl acetate kept in a series of saturators. Before performing each run of the adsorption experiment, the adsorbent was heated to 150 °C overnight and then cooled to room temperature in glass desiccators. Further details can be seen from our recent publication.9 Data Analysis. The concentration of ethyl acetate vapor in the inlet stream was measured with the help of a gas chromatography (GC) with AT 1000 Poropack Q column and a flame ionization detector (FID). The column used was made up of stainless steel with a length of 1.83 m, an outer diameter of 3.175 mm, and an inner diameter of 2 mm. The oven temperature was maintained at 80 °C. The detector and injector temperature was maintained at 120 °C. Nitrogen was used as an inert gas, and hydrogen was used as a fuel gas with air. A calibration chart was made by taking a known volume of a (vapor + air) mixture in the microsyringe (500 µL capacity, Hamilton Co.) collected from just above the surface of liquid ethyl acetate from the ethyl acetate bottle and diluting it for different concentrations. The concentration of ethyl acetate is calculated using the following two steps: (a) Calculation of Concentration of Ethyl Acetate in the Vapor from Ethyl Acetate Bottle. The Antoine equation was used to determine the vapor pressure of ethyl acetate at the
operating temperature. For a temperature range of 260-385 K, the constants A-C for ethyl acetate are A ) 16.15, B ) 2790.50, and C ) -57.15. At 307 K the vapor pressure calculated from the above equation is 145.66 mmHg. According to Dalton’s law, partial pressure ) (total pressure) × (gas-phase mole fraction). For pure liquid, according to Raoult’s law, partial pressure ) vapor pressure. So, the mole fraction of pure ethyl acetate in air, y ) partial pressure/(total pressure). Therefore y ) 145.66/760 ) 0.192 at ambient condition (307 K) assuming the air in the bottle is fully saturated. (b) Calculation of Ethyl Acetate Concentration in the Syringe after Dilution, i.e., the Ethyl Acetate Final Concentration. For A vol. % of air-ethyl acetate mixture from the bottle present in the 100 unit volume of the syringe containing air-ethyl acetate from the bottle and ethyl acetate free air from ambient, ethyl acetate present is Ay mol/(100 mol of final ethyl acetate and air mixture). Then ethyl acetate concentration is (Ay × 88 × 1.165)/(100 × 29.1) kg/m3, where the molecular weights of ethyl acetate and air are 88 and 29.1, respectively, and the density of the air is 1.165 kg/m3 at 307 K. Mathematical Model for Dynamic Study. The following important assumptions are made while developing the mathematical model: (1) radial mixing is complete; (2) there is negligible axial dispersion of heat and mass; (3) air free from moisture, carbon dioxide, and particulates acts as an inert carrier gas; (4) the equilibrium relationship is represented by the Langmuir isotherm with Langmuir constant b, showing the normal exponential temperature dependence; (5) the masstransfer rate is represented by the linear driving force rate expression, where the mass-transfer coefficient is the overall mass-transfer coefficient (lumped parameter); (6) an overall heattransfer coefficient is used to account for heat loss from the system. The temperature of the column wall is assumed to be the same as the feed temperature, implying uniform temperature across the column with all heat-transfer resistances concentrated at the wall. (a) Component Mass Balance.
fluid-phase component mass balance: Vo ∂c Fb ∂q ∂c )∂t ∂z ∂t
(1)
solid-phase component mass balance: ∂q ) k(q* - q) ∂t
(2)
(b) Energy Balance.
fluid-phase energy balance: Vo ∂Tg 2hw(Tg - Tw) hap (1 - ) ∂Tg ) (Ts - Tg) ∂t FgCpg ∂z RFgCpg (3)
Ind. Eng. Chem. Res., Vol. 45, No. 19, 2006 6565
solid-phase energy balance: ∂Ts (-∆H)∂q hap (T - Tg) ) ∂t Cps ∂t FsCps s
(4)
(c) Initial and Boundary Conditions.
fluid-phase component mass balance: initial condition: at t ) 0 for all z, c ) 0, i.e., c|t)0 ) 0 boundary condition: at z ) 0 for t > 0, co ) cin initial condition for solid-phase component mass balance: at t ) 0 for all z, q ) 0, i.e., q|t)0 ) 0 fluid-phase energy balance: initial condition:
at t ) 0 for all z, Tg ) Tgo, i.e., Tg|t)0 ) Tgo
initial condition for solid-phase energy balance: at t ) 0 for all z, Ts ) Tso, i.e., Ts|t)0 ) Tso (d) Isotherm Model. The Langmuir isotherm equation, q*/ qs ) bc/1 + bc, is used to fit in the experimental equilibrium data of ethyl acetate in activated carbon and model parameters such as Langmuir constant b and saturation capacity qs are obtained. The temperature dependency of Langmuir constant b is calculated by the van’t Hoff equation:16
ln c ) constant -∆Hs/RT
(6)
where ∆Hs is the isosteric heat of adsorption. The heat of adsorption is calculated by two ways. First one is by the van’t Hoff equation (eq 5). The other one, the isosteric heat of adsorption, is by eq 6 using the plot of ln c versus 1/T for constant q*. The results obtained by these two methods are presented in Table 2. (f) Determination of Overall Mass-Transfer Coefficient. For most particle shapes, representation as an equivalent sphere is an acceptable approximation and transport may therefore be described by a diffusion equation, written in spherical coordinate with the assumption of constant diffusivity as16
(
)
∂2q 2 ∂q ∂q ) De 2 + ∂t r ∂r ∂r
(7)
The solution of this equation with suitable initial and boundary conditions16 for the uptake curve is given by the familiar expression
q*
)1-
6
∞
∑
1
π2 n)1n2
( )
(9)
In the long time region a plot of ln(1 - q/q*) versus t should be linear with slope -π2De/Rp2 and intercept ln(6/π2). Such a plot provides, in principle, a simple method of both checking the conformity of an experimental uptake curve with the diffusion equation and determining the diffusional time constant. The overall mass-transfer coefficient in the linear driving force rate equation is expressed as k ) 15De/Rp2. The validity of this approximation has been confirmed for many different initial and boundary conditions.18
(5)
(e) Heat of Adsorption. The Clausius-Clapeyron equation16 after integrating for constant adsorbed phase concentration, q (while ∆Hs is independent of temperature), and converting pressure in terms of the fluid-phase concentration of adsorbate takes the following form:
q
De/Rp2, where q/q* is the fractional approach to equilibrium.17 This expression (eq 8) converges rapidly in the long time region since the higher terms of the summation become vanishingly small. For fractional uptakes greater than 70%, only the first term may be retained to obtain16,18
π2Det q 6 1 - ≈ 2 exp - 2 q* π Rp
boundary condition: at z ) 0 for t > 0, Tgo ) Tgin
b ) boe-∆H/RT
Figure 1. Ethyl acetate adsorption isotherms at different temperatures in activated carbon.
( )
exp -
n2π2Det Rp2
(8)
Equation 8 can be used to find the diffusional time constant,
Results and Discussion Comparison of Adsorption Isotherms. Isotherm experiments were performed for several concentration levels of ethyl acetate at three different temperatures, 30, 45, and 55 °C. The corresponding three isotherms are presented in Figure 1. It is observed from the figure that the isotherms are typically of Brunauer type I. Again, temperature dependencies of the adsorption capacities as evident from the figure showed that at higher temperature the equilibrium capacity was less. This establishes the fact that higher temperatures do not favor adsorption. The Langmuir constant, saturation capacities, and heat of adsorption values for activated carbon are evaluated from the equilibrium data, and the results are tabulated in Table 2. Results show that the model parameters such as Langmuir constant b and saturation capacity qs are functions of temperature. As expected, qs is decreased with an increase in temperature due to a decrease in adsorption capacities at higher temperatures. It can also been noticed that the difference in capacities at two different temperatures goes on increasing with an increase in concentration and reaches a constant value at a higher concentration level of ethyl acetate. This may be attributed to the fact of a nonlinear temperature dependency of the equilibrium adsorption capacity. The decrease in saturation capacity with an increase in temperature can be explained on the basis of the fact that in the original Langmuir formulation the saturation limit was assumed to coincide with the saturation of a fixed number of identical surface sites and as such it should be independent of temperature. In fact a modest decrease in saturation capacity with temperature is generally observed and is indeed to be expected, if the saturation limit corresponds with filling of the micropore volume, rather than with the saturation of a set of surface sites.10
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Figure 2. Variation of isosteric heat of adsorption with equilibrium adsorption capacity.
The saturation capacity of ethyl acetate adsorption on activated carbon in the present study is 0.73-0.487 kg/kg for the temperature range of 303-328 K. Gales et al.14 in their adsorption study on BASF activated carbon reported the saturation capacity in terms of adsorbed phase concentration at saturation limit. According to them the adsorbed phase concentration at the saturation limit decreases with temperature and has values of about 0.53 and 0.326 kg/kg at 25 and 80 °C, respectively. The above comparison shows that the present results are somewhat closer to the results presented by Gales et al.14 The existing differences may be attributed to the type and properties of the activated carbon used from the different sources. Thus, the used E-Merck activated carbon in the present study shows higher adsorption capacity for ethyl acetate than that in the BASF activated carbon reported by Gales et al.14 Determination of Heat of Adsorption. The Langmuir constant b values obtained and eq 5 are used to calculate the values of heat of adsorption. Isosteric heat of adsorption is calculated by interpolating adsorption isotherms at different values of equilibrium adsorption capacity. These data were plotted (not shown here) to determine isosteric heat of adsorption based on eq 6, and the results are reported in Table 2. It is found that isosteric heat of adsorption increases nonlinearly with an increase in the saturation adsorption capacity (Figure 2), but all the values lie in the order of 105 J/kg, as is observed by heat of adsorption from model parameters. According to the ideal Langmuir model the heat of adsorption should be independent of coverage. But, in practice, heat of adsorption varies with coverage, depending upon the surface heterogeneity and sorbate-sorbate interaction. With nonpolar sorbates an increase in the heat of adsorption with coverage is commonly observed, and this is commonly attributed to the effect of intermolecular attraction forces.16 Thus, with ethyl acetate being a weakly polar compound, the observed variation of isosteric heat of adsorption with equilibrium capacity is in line with the above discussion. The observed lower value of the heat of adsorption from eq 5 than the values of isosteric heat of adsorption may be explained as follows. At higher temperature the apparent value of qs is lower than the true saturation limit.16 In the present paper, if the true saturation limits are higher at temperatures 45 and 55 °C than the apparent qs values obtained experimentally, then corresponding the Langmuir constant values will be lower. The slope of the plot between ln b vs 1/T will increase. This in turn may increase the value of the heat of adsorption and may make it at par with isosteric heat of adsorption values (Table 2). Determination of Overall Mass-Transfer Coefficient. The shape of the particle is very important where diffusive transport is concerned. However, as discussed earlier, the overall masstransfer coefficient can be obtained from the following equation: k ) 15De/Rp2, where the diffusional time constant De/Rp2 can be found from the slope of the plot of ln(1 - q/q*) versus t. Figure 3 is the sample plot for the determination of mass-
Figure 3. Sample plot for the determination of the mass-transfer coefficient using uptake rates for ethyl acetate in activated carbon.
transfer coefficient using uptake rates for ethyl acetate in activated carbon. The intercept of the lines (Figure 3) from the fitting of the experimental data is not equal to ln(6/π2) or -0.4977. The model used is the simplest model. So, there is every possibility that the model would not predict the experimental data perfectly. However, the model gives us the scope of first hand calculation of the mass-transfer coefficient. The values thus obtained are of the same order of magnitude with the values used for validation of the adsorption breakthrough data reported later. Apart from the model simplicity, it may also be noted that at higher times (Figure 3), when the adsorbent becomes almost saturated with ethyl acetate, very precise measurement of the change in weight should be recommended since during that time the weight change is much less. In view of that, Figure 3 was redrawn (not reported here) after removing the points of uptake at much higher times when equilibrium is almost reached. It was observed that the intercepts of the lines become closer to -0.4977. But the mass-transfer coefficients did not differ much from the previously obtained values, and the order of magnitude remained the same. Therefore, application of a time-dependent very precise weight measuring device could have given more accurate values of the mass-transfer coefficient. It is also important to note that the adsorbent used for the present study is not a single particle but a few granules. Therefore, there is a possibility of an additional diffusional resistance associated with the system studied here.10 This could be another reason the simplest model used does not accurately predict the experimental uptake data. Furthermore, there can be a vapor-phase concentration distribution in the U-tube in the direction of vapor flow. However, in the present study, a very small amount of (approximately 1 g) adsorbent is taken only. Therefore, same vapor-phase concentration in the U-tube in the direction of vapor flow is assumed.10 The values of the overall mass-transfer coefficient found from the uptake curve method (k) are reported in Table 3 for different temperatures of study. It is observed from the table that the values of k are of the order of 10-4 s-1 and lie mostly in the range of (1.25-3.0) × 10-4 s-1. There are a few cases when k is slightly less than 1.25 × 10-4 s-1or more than 3.0 × 10-4 s-1. This may be due to experimental error associated with collecting the data required for evaluation of k. The k values determined for all the different operating conditions (Table 3) do not show any distinct effect of flow rate on the overall masstransfer coefficient. This indicates the less importance of the external film resistance for mass transfer within the range of flow rates studied. It is also clear from the table that there is no distinct effect of temperature on the overall mass-transfer coefficient. This signifies that k is independent of temperature in the range of temperature studied in the present investigation.
Ind. Eng. Chem. Res., Vol. 45, No. 19, 2006 6567 Table 3. Equilibrium Capacities and Mass-Transfer Coefficients for Ethyl Acetate in Activated Carbon at Different Temperatures 30 °C sr no. 1 2 3 4 5 6 7 a
Q×
106,a
m3/s
1.49 1.49 1.49 4.0 4.0 6.5 4.0
c, g/L
q*, kg/kg
0.027 0.047 0.12 0.27 0.43 0.56 0.6
0.41 0.55 0.62 0.66 0.6914 0.7 0.705
k×
45 °C
104 s-1
2.20 3.37 2.83 1.32 1.90 1.28 1.45
R2
q*, kg/kg
0.9447 0.9167 0.9634 0.9516 0.9319 0.9505 0.9675
0.3 0.42 0.4803 0.525 0.5507 0.5533 0.5553
k×
55 °C
104 s-1
2.98 1.44 2.64 2.00 1.65 2.08 1.90
R2
q*, kg/kg
k × 104 s-1
R2
0.9938 0.9698 0.9657 0.9761 0.9736 0.9853 0.966
0.24 0.3561 0.405 0.43 0.4543 0.4561 0.4569
4.73 5.90 2.51 2.03 1.57 3.42 2.31
0.9934 0.9677 0.9767 0.978 0.9593 0.9874 0.9446
Q is the flow rate.
Table 4. Variables Studied in Packed-Bed Adsorption Experiments system
adsorbent
variable
EA-air
activated carbon
co,a kg/m3 Vo × 103,b m/s L,c m
range of study 0.45-0.70 3.0-7.0 0.06-0.18
a c is the inlet EA concentration. b V is the inlet velocity. c L is the bed o o height.
Table 5. Operating Conditions for Adsorption of Ethyl Acetate on Activated Carbon run no. DS1 DS 2 DS 3 DS 4 DS 5 DS 6 DS 7
co,a kg/m3
Vo × 103,b m/s
L,c m
Tgo ) Tso, °C
0.564 0.450 0.700 0.564 0.564 0.564 0.564
3.0 3.0 3.0 5.0 7.0 3.0 3.0
0.06 0.06 0.06 0.06 0.06 0.12 0.18
35 35 34 35 35 34 34
Table 6. Values of Model Input Parameters for Simulation of Experimental Runs parameters adsorption bed diameter, m × 102 average particle diameter, m adsorbate molecular diffusivity, m2/s × 106 bed voidage inlet fluid temperature, K initial solid temperature, K particle external surface area, m2/m3 of bed solid density, kg/m3 of solid fluid thermal conductivity, J/(m/s/K) overall mass-transfer coefficient, 1/s × 104 wall to bed heat-transfer coefficient, J/(m2/s/K) heat of adsorption, J/kg × 10-5 Langmuir constants, qs, bo
value or range of values 10 0.00158 8.9 0.4 307-308 307-308 1524.0 500 0.0267 7.0 6.0 0.97105 0.73, 1.696
a c is the inlet EA concentration. b V is the inlet velocity. c L is the bed o o height.
So from the values of k obtained by the uptake curve method, it may be concluded that k is independent of temperature, concentration, and flow rate in the range of experimental conditions in the present study. Furthermore, independency of k on the flow rate apparently led to the conclusion that pore diffusion is predominant for the present system of study. This can be confirmed through further experimentation on single particles and/or further analysis through a more realistic model for evaluation of the mass-transfer coefficient from the experimental uptake data reported in the present work Dynamic Adsorption Behavior. Experimental and theoretical dynamic studies are performed for ethyl acetate adsorption on activated carbon, and the results are presented in the form of concentration and temperature breakthrough curves. These concentrations and temperatures at the bed exit are nondimensionalized with respect to inlet concentration and inlet gas temperature, respectively. The range of variables studied during the experimentations is presented in Table 4. The operating conditions for each experimental run are shown in Table 5. Mathematical models in the form of partial differential equations for transient mass and energy balances of adsorbate (ethyl acetate) in the gas and solid phases over an elemental section of an adsorption column are presented under the model development section. A Langmuir type adsorption isotherm is considered for the model. Verification of the validity of the model is attempted through the comparison of the concentration and temperature histories measured at the end of the bed with those obtained from the proposed model. The model is simulated after taking values of the input parameters such as Langmuir constant, equilibrium adsorption capacity, heat of adsorption, and overall mass-transfer coefficients as calculated from equilibrium studies. The other input parameter values are taken from standard literature (Table 6).
Figure 4. Comparison of the experimental concentration breakthrough curve of ethyl acetate in activated carbon with simulated results for run DS2.
Figure 5. Comparison of the experimental temperature breakthrough curve of ethyl acetate in activated carbon with simulated results for run DS2.
Figures 4 and 5 present the sample experimental concentration and temperature breakthrough curves of ethyl acetate adsorption on activated carbon, respectively, for run no. 2 with the corresponding model predictions. It is found both from concentration and temperature breakthrough curves that the model shows good agreement with the experimental results. In all cases the overall mass-transfer coefficients are increased to 7.0 × 10-4 s-1, which is a little higher than the calculated values (1.25-
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Figure 6. Comparison of the experimental concentration breakthrough curve of ethyl acetate in activated carbon with simulated results for calculated k and increased k for run DS 1.
Figure 7. Comparison of the experimental temperature breakthrough curve of ethyl acetate in activated carbon with simulated results for calculated k and increased k for run DS 1.
3.0) × 10-4 s-1. Sample plots of predictions of concentration and temperature breakthrough curves by the model with the calculated overall mass-transfer coefficient (2.0 × 10-4 s-1) showed poor fitting (Figures 6 and 7, respectively). The simulated results with the increased value of overall mass transfer very well predict the experimental concentration and temperature breakthrough data. This indicates that the kinetics studies for activated carbon may have underpredicted the value of the overall mass-transfer coefficient. The possible reasons are discussed in detail in the section Determination of Overall Mass-Transfer Coefficient. However, the suitability of no axial dispersion terms in the model may lead to a conclusion that adsorption of ethyl acetate in activated carbon is an example of a strong adsorption phenomenon. Conclusion Ethyl acetate adsorption in activated carbon follows a Brunauer type I adsorption isotherm and is well-described by a Langmuir isotherm model. The overall mass-transfer coefficient calculated by the uptake curve method produces satisfactory values for the systems considered here. The overall mass-transfer coefficient is not influenced by the fluid flow rate, and its value is on the order of 10-4 s-1. The heat of adsorption value is on the order of 105 J/kg. The mathematical model without axial mass and energy dispersion well-represents the air-ethyl acetate activated carbon system. Nomenclature ap ) external surface area of adsorbent per volume of adsorbent (m2/m3) b ) Langmuir constant (m3/kg) bo ) preexponential factor in van’t Hoff equation (m3/kg)
c ) concentration of EA in air (kg/m3) co ) concentration of adsorbate in the fluid phase at the inlet (kg/m3) Cpg ) specific heat of gas (kJ/(kg‚K)) Cps ) specific heat of solid (kJ/(kg‚K)) De ) effective diffusivity (m2/s) Dm ) molecular diffusivity (m2/s) -∆H ) heat of adsorption (J/kg) h ) heat-transfer coefficient between solid and gas (kJ/(s‚m2‚ K)) hw ) heat-transfer coefficient between fluid and wall (kJ/(s‚ m2‚K)) K ) Freundlich constant k ) mass-transfer coefficient (s-1) kc ) thermal conductivity of the gas (kJ/(s‚m‚K)) L )bed length (m) n ) Freundlich exponent Q ) flow rate of air (m3/s) q* ) amount adsorbed at equilibrium (kg/kg) q ) amount adsorbed at any time (kg/kg) qs ) saturation capacity of adsorbent (kg/kg) r ) radial coordinate for particle (m) R ) universal gas constant R2 ) coefficient of determination Rp ) particle radius (m) t ) time (s) T ) temperature (K) V ) interstitial velocity (m/s) Vo ) superficial velocity (m/s) z ) column length (m) Greek Symbols Nomenclature ) bed porosity F ) density (kg/m3) Superscripts and Subscripts Nomenclature o ) inlet condition b ) bed g ) gas p ) pellet s ) solid ∆ ) gradient Literature Cited (1) Tan, C. S.; Liou, D. C. Desorption of Ethyl Acetate from Activated Carbon by Supercritical Carbon Dioxide. Ind. Eng. Chem. Res. 1988, 27, 988. (2) Khan, F. I.; Ghoshal, A. K. Removal of Volatile Organic Compounds from Polluted Air. J. Loss PreV. Process Ind. 2000, 13, 527. (3) Ghoshal, A. K.; Manjare, S. D. Selection of Appropriate Adsorption Technique for Recovery of VOCs: An Analysis. J. Loss PreV. Process Ind. 2002, 15, 413. (4) Moretti, E. C. Reduce VOC and HAP Emissions. Chem. Eng. Prog. 2002, 98 (6), 30. (5) Dolidovich, A. F.; Akhremkova, G. S.; Efremtsev, V. S. Novel Technologies of VOC Decontamination in Fixed, Moving and Fluidized Catalyst-Adsorbent Beds. Can. J. Chem. Eng. 1999, 77, 342. (6) Huang, M. C.; Chou, C. H.; Teng, H. Pore-Size Effects on ActivatedCarbon Capacities for Volatile Organic Compound Adsorption. AIChE J. 2002, 48 (8), 1804.
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(14) Gales, L.; Mendes, A.; Costa, C. Hysteresis in the Cyclic Adsorption of Acetone, Ethanol and Ethyl Acetate on Activated Carbon. Carbon 2000, 38, 1083. (15) Gales, L.; Mendes, A.; Costa, C. Recovery of Acetone, Ethyl Acetate and Ethanol by Thermal Pressure Swing Adsorption. Chem. Eng. Sci. 2003, 58, 5279. (16) Ruthven, D. M. Principles of Adsorption and Adsorption Processes; Wiley: New York, 1984. (17) Barrer, R. M.; Gregg, S.; Sing, K. S. W.; Stoechli, H. F. In Characterization of Porous Solids; Society of Chemical Industry: London, 1979. (18) Yang, R. T. Gas Separation by Adsorption Processes; Butterworth: Boston, 1987.
ReceiVed for reView March 14, 2006 ReVised manuscript receiVed May 11, 2006 Accepted July 7, 2006 IE0603060
