Sorption Equilibrium and Thermal Regeneration of Acetone and

Adsorption and thermal regeneration dynamics of acetone on activated carbon were compared to those of toluene. The adsorption isotherms of acetone on ...
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Ind. Eng. Chem. Res. 2007, 46, 4584-4594

Sorption Equilibrium and Thermal Regeneration of Acetone and Toluene Vapors on an Activated Carbon Jong-Hwa Kim, Sang-Jin Lee, Min-Bae Kim, Jang-Jae Lee, and Chang-Ha Lee* Department of Chemical Engineering, Yonsei UniVersity, Sodaemun-ku, Shinchon-dong 134, Seoul, Korea

Adsorption and thermal regeneration dynamics of acetone on activated carbon were compared to those of toluene. The adsorption isotherms of acetone on the activated carbon were type-II, but they approached typeIII with an increase in temperature. On the other hand, those of toluene were type-I in the experimental range. Although the temperature excursion of toluene was higher than that of acetone in the activated carbon bed, the breakthrough shape of toluene was steeper due to the strong adsorption affinity. Compared to toluene, more concentrated acetone within shorter period of time could be obtained from the activated carbon bed by hot nitrogen purge regeneration because its isotherm approached type-III at high temperature. Therefore, the energy requirement and purge gas consumption for acetone desorption were significantly changed with purge gas velocity, regeneration temperature, and initial bed loading, which was different from toluene. A nonequilibrium and nonadiabatic/nonisothermal model was used to fit temperature and concentration profiles of adsorption and thermal regeneration. Even though the adsorption of acetone was performed in a low concentration range, the multilayer adsorption isotherm model should be applied for the regeneration step to design the activated carbon bed system more accurately. 1. Introduction Volatile organic compounds (VOCs), which are commonly found as solvents in industrial processes and domestic use, are considered as major air contaminants for inducing directly health troubles and for being precursors of tropospheric ozone.1,2 There are several chemical engineering processes that are commonly used in the industry to deal with VOC emissions. Incineration and biofiltration are classified as destructive techniques whereas condensation, scrubbing, and adsorption are considered as recuperative ones.3-6 Adsorption has been widely recognized as an effective means of controlling emissions to the atmosphere and, in some applications, of recovering recyclable materials from process exhaust streams. The solvent recovery process is a particularly common application of adsorption for VOCs emission control. Adsorption by porous carbon such as a granular activated carbon and an activated carbon fiber is used extensively compared with other methods because these materials have a large specific area and a high adsorption capacity. In the adsorption process, micropores in the porous carbon, due to the overlap of attractive forces of opposite pore walls, are primarily responsible for the adsorption of gases at low concentration.7 Until now, activated carbon is generally acknowledged to be a very efficient material for VOCs emission control in the industrial field. When the adsorbent becomes exhausted or the effluent from the bed reaches an allowable discharged level, the spent adsorbent must be regenerated for reuse. Therefore, chemical industries have widely used periodic adsorption processes such as thermal swing adsorption (TSA) and pressure swing adsorption (PSA). The TSA has not been conventional for bulk separation processes, but it is more preferable for solvent recovery, purification and, drying than the PSA.8,9 The regeneration process for activated carbon can be carried out by hot steam, dc electrical heating, and supercritical CO2, etc.10,11 In addition, many research works related to adsorption * To whom correspondence should be addressed. Fax: +82 2 312 6401. E-mail address: [email protected].

and hot purge regeneration on activated carbon applications were reported because this method has been extensively operated in the industrial field.12-18 However, very few focused on the multilayer adsorption of VOCs on an activated carbon. Moreover, the proper model of hot purge regeneration in this case has been little reported. Since the regeneration step in the TSA cycle involves time needed to heat and cool the bed, it is often the most timeconsuming step in the TSA cycle. Consequently, in the design and economic assessment of the TSA process, one is principally interested in reducing the energy consumption and time involved in regeneration time, purge gas requirements, and heat load in the regeneration step.15 In addition, the prediction for interaction of adsorbent and adsorbate is as important as the development of adsorbents in the view of process design and process energy consumption. The characteristics of adsorption and thermal desorption on an activated carbon were studied. As representative nonpolar and polar adsorbates, toluene and acetone were selected. Toluene (nonpolar; dipole moment ) 0.4 debye) is one of the most typical VOCs emitted in many industries, while acetone (polar; dipole moment ) 2.9 debye) can be recovered from the flue gas in an acetate process. The adsorption isotherms of two adsorbates on the activated carbon used were presented. The dynamic characteristics of adsorption and hot purge regeneration of acetone on the activated carbon bed were compared to those of toluene. A mathematical model incorporating a multilayer isotherm was developed in order to describe the adsorption and hot purge regeneration in the activated carbon bed of nonisothermal and nonadiabatic conditions. From this work, predictions of adsorption and thermal regeneration can contribute to the design and optimization for the activated carbon process such as initial bed loading, the gas quantity of regeneration required, the total energy required, and total regeneration time. 2. Experimental 2.1. Materials. The adsorbent used in this study was a bituminous-based activated carbon (BPL 4 × 10) in granular type manufactured by Calgon Carbon Co. (USA). Before the

10.1021/ie0609362 CCC: $37.00 © 2007 American Chemical Society Published on Web 05/27/2007

Ind. Eng. Chem. Res., Vol. 46, No. 13, 2007 4585 Table 1. Properties and Characteristics of Adsorbent type

granular type (g/cm3)

bulk density (FB) BET surface area (m2/g) micropore area (m2/g) average pore diameter (nm) average particle size (mm)

0.48 993 956 2.0 3.35

experiment, the activated carbon was regenerated for more 24 h at 423 K with a vacuum pump. Acetone and toluene were obtained from Yakuri Pure Chemicals Co. (Osaka, Japan), and their purity was over 99.5%. The physical properties of the adsorbent are presented in Table 1. 2.2. Measurements of Adsorption Equilibrium. The adsorption equilibria of acetone vapor on the activated carbon were measured in the range of 298∼378 K and 0∼21 kPa. In this study, a static volumetric system was used to measure the adsorption equilibria. It was composed of a manifold, an adsorption cell, a vacuum pump, a pressure transducer, and two resistance temperature detectors (RTDs, Pt 100 Ω). The total quantity of gas admitted into the system and the amount of gas in the vapor phase remaining after adsorption equilibrium were determined by appropriate P-V-T measurements. The volumes of the manifold and adsorption cell in the adsorption system were determined by the expansion of helium gas and the experimental temperature. The system pressure was measured by an absolute pressure transducer (MKS, type 690A13TRA) with a high accuracy signal conditioner (MKS, type 270D), and two RTDs (Pt 100 Ω) were installed inside the adsorption cell and the manifold in the system. During experiments, the adsorption cell was placed in a water bath and the temperature was maintained constant by a refrigerating circulator (Haake type E3). Prior to the experiments, the adsorbent in the cell was activated by evacuation at 423 K over 12 h. The desired amount of vapor was supplied to the manifold controlled by a valve. When the adsorption cell reached the desired temperature, the vapor was admitted into the adsorption cell. During the experiment, the temperatures and pressures were recorded automatically on a computer. By using the pressure, temperature, and gaseous volume before and after each adsorption step, the number of moles adsorbed could be calculated. Details of the equipment and operating procedures used are described in previous works.19,20 2.3. Measurements of Adsorption and Thermal Regeneration Breakthrough. The schematic of the experimental apparatus for adsorption and hot purge regeneration at nonisothermal and nonadiabatic conditions is shown in Figure 1. In the adsorption experiment, the nitrogen gas line was divided into two branches: one was for pure nitrogen gas as a carrier and the other was connected to a vessel filled with the toluene or acetone liquid. In the method, the desired concentration was produced by adjusting the amounts of nitrogen entering the lines and was identified several times by gas chromatography before the each experimental runs. An electric furnace was installed at an inlet section of the adsorption column to heat the nitrogen purge gas. While the feed flowed upward in the adsorption bed in adsorption breakthrough, hot purge gas flowed downward along the bed in regeneration breakthrough. The adsorption beds were made of stainless steel pipe with a length of 30 cm, i.d. of 2.2 cm, and wall thickness of 0.147 cm. The bed was insulated with about 5.0 cm of thick glass wool. The characteristics of the adsorption beds are listed in Table 2. Six calibrated RTDs (Pt 100 Ω) were installed at the positions of 3, 9, 15, 21, 27, and 30 cm from the bottom of the

Figure 1. Schematic of the apparatus for the breakthrough experiment. Table 2. Characteristics of Adsorption Bed and Parameter Values bed length, L (cm) bed inside radius, RBi (cm) bed outside radius, RBo (cm) heat capacity of column, Cpw (J/g‚K) density of column, Fw (g/cm3) internal heat transfer coefficient, hi (J/cm2‚K‚s) external heat transfer coefficient, ho (J/cm2‚K‚s) axial dispersion coefficient (cm2/s) LDF parameter for adsorption (1/s) LDF parameter for desorption (1/s)

30.0 1.1 1.247 0.502 9.83 6.28 × 10-3 4.60 × 10-4 1 × 10-4 0.0003 0.00026

bed. The two pressure transducers were installed above and below the column to measure the bed pressure variation. The gas flow to the column for adsorption and regeneration steps was controlled by a mass flow controller (Hastings, 202D-799). The system was fully automated by a personal computer with a developed control program, and all measurements including pressure and temperature were saved on the personal computer through the AD (analog to digital) converter. During the adsorption and regeneration steps, adsorbate concentrations were measured by an on-line gas chromatograph with a flame ionization detector (Hewlett-Packard 6890 series II) and helium was used as carrier gas. The feed and hot purge lines were wrapped by the heating tapes with temperature controllers to prevent vapors from condensing. An automatic 6-port sampling value (Valco Instruments Co. Corp.) controlled by the gas chromatograph was used to take samples at programmed time intervals. The effect of heat transfer at the wall is important in thermal regeneration because the temperature variation of hot purge gas gives a significant effect on the adsorption isotherm. In this study, prior to the breakthrough experiments, the separate experiments were performed in order to estimate the heat transfer coefficients in the bed packed with the clean activated carbon. The inner and outer heat transfer coefficients at the wall were determined by matching the both energy balances in the bed and the wall to the experimental data.12,13,15 2.4. Dynamic Model for Adsorption and Regeneration Breakthrough. To understand the dynamic behaviors of acetone and toluene at adsorption and hot purge regeneration, a nonisothermal and nonadiabatic model including mass and energy balances was developed on the basis of the following assumptions: (i) the gas phase behaves as an ideal gas; (ii) radial concentration and temperature gradients are negligibl;, (iii) the

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pressure gradient across the bed is neglected; (iv) the solid and gas phases reach thermal equilibrium instantaneously; (v) the flow pattern is described by the axially dispersed plug flow model; (vi) the mass transfer rate is represented by a linear driving force (LDF) model. These assumptions were widely accepted by several studies on adsorption process.9,13,15,16 Since the ratio of bed to adsorbent size was relatively high in this study, this equation contributes to minimizing the deviation stemmed from the assumption of no radial temperature gradient. In addition, the heat transfer coefficients were determined by matching both energy balances in the bed and wall to the experimental data at various temperature and flow conditions. Therefore, the coefficients were reflected on all the thermal effects in the bed. Applying the above assumptions for a single component mass balance through a packed bed, the following equation was obtained:

-DL

∂2Ci 2

∂z

+

∂(uCi) ∂Ci 1 -  ∂qi + + Fp )0 ∂z ∂t  ∂t

(

)

(1)

where q is the adsorbed phase concentration (mol/g), C is concentration (mol/cm3), DL is the axial dispersion coefficient (cm2/s), Fp is the particle density (g/cm3),  is bed void fraction, u is the interstitial velocity (cm/s), z is axial distance along the adsorption bed (cm), and t is time (s). The depth of the temperature variation has a great effect on the adsorption and hot purge regeneration of activated carbon because the adsorption isotherm and gas velocity are changed by the temperature. The energy balance in gas and solid phases with the heat transfer to the column wall was constructed as follows.

-KL

∂T ∂2T + (tFgCpg + FBCps + FBqjCpaMa) + 2 ∂t ∂z 2h i ∂ ∂qj  (FgCpguT) - FB(∆H) + (T - Tw) ) 0 (2) ∂z ∂t RBi

Where, t is the total void fraction ()  + (1 - )R), FB is the bed density, () (1 - )Fp), KL is the effective axial thermal conductivity (J/cm2‚s‚K), ∆H is the isosteric heat of adsorption (J/mol), Cp is the heat capacity (J/g‚K), hi is the heat transfer coefficient at the outer wall (J/cm2‚s‚K), and R is radius of the bed (cm). Even though the bed was wrapped by the glass wool, the bed could not be perfectly adiabatic and the temperature change was monitored in the experiments. Therefore, in this study, heat loss through the column wall and heat accumulation in the wall was considered. Another energy balance in the column wall was used as follows:

FwCpwAw

∂Tw ) 2πRBihi(T - Tw) - 2πRBoho(Tw - Tatm) ∂t Aw ) π(RBo2 - RBi2)

(3)

where subscripts g, p, a, B, w, i, and o represent gas phase (g), particle (p), adsorbed phase (a), bed (B), wall (w), inner (i), and outer (o), respectively. The heat capacity of the adsorbed phase was considered in the second term in eq 2 due to the large heat capacity of toluene and acetone. The heat capacity of adsorbate was expressed by the following temperature-dependent equation.22

Cpg )

∑i yiCpgi

Cpgi ) ai + biT + ciT2 + diT3

(4-1) (4-2)

In eq 4-2, the heat capacity parameters for toluene/acetone and nitrogen came from the work of Reid et al.23 The sorption rate of the gas and solid phase is explained by the LDF model.

∂qj ∂th

) k(q* - qj)

(5)

where k is the LDF coefficient (1/s) and superscript * is equilibrium state. Even though several correlations can be used to estimate the value of the coefficient k,16,26 it is also possible to consider the k Value as an adjustable parameter. The adsorption equilibrium is described by the temperaturedependent form of the Langmuir equation and Aranovich and Donohue (A-D) equation,24 respectively. The boundary conditions for the adsorption breakthrough at z ) 0 and L, and for t > 0, could be written by

-DL -kz

∂C ∂C | ) uf(Cf - Cz)0) |z)L ) 0 ∂z z)0 ∂z

∂T ∂T | ) ufFgCpg(Tf - Tz)0) | )0 ∂z z)0 ∂z z)L

(6)

As for the countercurrent regeneration case, the boundary conditions are

-DL -kz

∂C ∂C | ) uf(Cf - Cz)L) | )0 ∂z z)L ∂z z)0

∂T ∂T | ) ufFgCpg(Tf - Tz)L) |z)0 ) 0 ∂z z)L ∂z

(7)

where subscript f represents feed. The purity of N2 for adsorption and regeneration experiments was 99.9%. The N2 gas was supplied to the system after passing a silica gel bed. Therefore, Cf in the regeneration step became zero. A “clean bed” condition was used as the associated initial condition for the adsorption breakthrough.

C(0,z) ) q(0,z) ) 0 T(0,z) ) Tw(0,z) ) Tatm

(8)

A “saturated bed” condition was used as the initial condition for the regeneration breakthrough together with the condition of the uniformly adsorbed phase.

C(0,z) ) C0 T(0,z) ) Tw(0,z) ) Tatm

(9)

Model simulation was carried out in the gPROMS modeling tool.25 The method of line adopted consists of two steps: (1) the discretization of the continuous spatial domains into finite grid of points, thus reducing partial differential-algebraic equations (PDAEs) to differential algebraic equations (DAEs). and (2) integration of the DAEs over time by employing an integrator, called a DASOLV code, based on backward differentiation formulas (BDF). The main advantage of this method is to minimize an integral error efficiently by this DAE integration technique. The centered finite difference method

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(CFDM) in the context of the MOL was used for the discretization with 60 nodes. 3. Adsorption Equilibria The adsorption isotherms for acetone on the activated carbon were obtained at 293, 313, 333, 353, and 373 K and pressure up to 21 kPa. The recursively calculated isotherms according to the theories of Langmuir, Aranovich-Donohue (A-D), BET, Aranovich, and FHH are compared with the experimental data. The Langmuir theory is based on a kinetic principle, that is, that the rate of adsorption is equal to the rate of desorption from the surface.26 The A-D equation starts in that the characteristic feature of a multilayer adsorption isotherm is its sigmoidal shape with apparent divergence of q to infinity as P approaches Ps. Hence, the mathematical function used to describe the curve must have a singularity at P, which is equal to the saturation pressure, Ps. The A-D proposed an isotherm model as the following form24

q)

f(P) (1 - P/Ps)d

(10)

In this study, the Langmuir equation, eq 11 was substituted into f(P). Monolayer adsorption is formed by the same concept as the Langmuir type adsorption while adsorption above monolayers is equivalent to condensation of the adsorbate molecules, giving rise to the BET. The form of the Aranovich equation is very similar to that of the BET equation, but the difference is in the exponent of the term (1 - P/P0) in the denominator of the two equations. Frenkel, Halsey, and Hill (FHH) suggested the multilayer adsorption model. In the model, the parameter b is regarded as a measure of the rate of decline in the adsorption potential with distance from the surface.26 The following Langmuir, A-D, Aranovich, and FHH isotherms were applied to the isotherm data.

bP Langmuir isotherm: q ) qm 1 + bP

(11)

bP A-D isotherm: q ) qm (1 + bP)(1 - P/Ps)d

(12)

CP BET isotherm: q ) qm (Ps - P)[1 + (C - 1)(P/Ps)] (13) Aranovich: q ) qm

C(P/Ps)

x1 - (P/Ps)(1 + C(P/Ps))

(14)

FHH: ln(P/Ps) ) -A(q)-b

(15)

mP Toth: q ) qm (1 + bPt)1/t

(16)

where q is the adsorbed amounts (mol/kg), qm is the saturation capacity (mol/kg), P is the gas pressure (kPa), and Ps is the saturated pressure (kPa) As shown in Figure 2, the isotherm of acetone on the activated carbon exhibits a type-II (Brunauer classification) multilayer adsorption isotherm in the experimental range. On the other hand, the isotherm of toluene was well presented by the Langmuir model as described in the previous study.27 In

Figure 2. Multilayer adsorption isotherms of acetone at 293 K and of toluene at 298 K on activated carbon. Table 3. Isotherm Parameter Values Estimated for Acetone-Activated Carbon System temperature [K] 293

313

333

353

373

qm [mol/kg] K [1/kPa] d [-] ∆q [%]

3.82 4.92 0.61 2.36

A-D 2.91 2.01 2.50 5.33

2.45 0.69 6.31 3.11

2.38 0.21 12.7 4.34

1.10 0.38 29.4 3.12

qm [mol/kg] B [-] ∆q [%]

26.4 2.04 19.7

FHH 7.03 0.92 18.8

4.81 0.50 5.66

4.30 0.31 8.24

4.20 0.22 12.9

addition, the amount of adsorption was smaller than that of acetone on the activated carbon. At low pressure (lower than 6 kPa), the equilibrium relation of acetone to the activated carbon was highly favorable and matched type-I isotherm models such as the Langmuir model. As the pressure approaches the saturation pressure of acetone, however, multilayer adsorption or capillary condensation occurs in the activated carbon. Therefore, the isotherm model that considers these phenomena should be applied to predict the equilibrium adsorption amount, as the adsorption and desorption mechanism changed with pressure. In addition, the BET curve showed large deviation above P ) 9 kPa even though it presented the type-II isotherm. On the other hand, the A-D, FHH, and Aranovich models presented the type-II isotherm reasonably over the entire pressure range, but the Aronovich model showed relatively larger deviation in the high-pressure region than the other two models. In this study, the adsorption isotherms measured at various temperatures were fitted by the A-D, FHH, and Langmuir models. The parameter values of these isotherm models were listed in Table 3 with the average percent deviations (∆q) calculated from the following equation:

- qcal qexp j j ∆q(%) ) | | k j-1 qexp j 100

k

∑

(17)

In addition, Figure 2 shows a comparison of the acetone isotherm with the toluene isotherm on the same activated carbon.

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Ind. Eng. Chem. Res., Vol. 46, No. 13, 2007 Table 4. Experimental Conditions of Adsorption and Regeneration Breakthrough Runs of Acetone in Activated Carbon Bed Adsorption Breakthrough temperature [K]

flow rate [LSTP/min]

run no.

Tatm

Tf

Q

feed conc [ppm]

ATCV ATC1V ATCV1 AT1CV AT2CV

293. 293. 294. 294. 294.

293. 292. 294. 295. 294.

10. 10. 10. 6. 10.

4210. 64860. 4370. 3790. 4550.

Regeneration Breakthrough

run no. Figure 3. Equilibrium isotherms of acetone on activated carbon at various temperatures.11

In the low pressure region, both adsorbates showed strong favorable adsorption on the activated carbon. However, the acetone isotherm was type-II while the toluene isotherm was type-I. Furthermore, a greater amount of acetone with a smaller molecular weight was adsorbed on the activated carbon than on toluene. As shown in Figure 3 and Table 3, the experimental isotherms of acetone on the activated carbon could be accurately fitted by the multilayer isotherm models and the A-D model was slightly better than the FHH model. In the low temperature region, the adsorption affinity of acetone on the activated carbon is strong and the strong adsorbate-adsorbate interactions lead to the multilayer adsorption on the activated carbon. However, due to the lower boiling point of acetone (Tbp ) 329.3 K) than toluene (Tbp ) 383.9 K), the adsorption isotherm was highly affected by temperature. Moreover, the isotherm seems to be an unfavorable shape such as type-III at 373 K. The zero-loading isosteric heats of adsorption (-∆Hs) calculated from the temperature dependence of the equilibrium capacity using the Clausius-Clapeyron equation26 were about 14.0 kcal/mol for acetone and 15.0 kcal/mol for toluene. This implies that the stronger adsorbate-adsorbate interaction of acetone, stemmed from its polarity, contributes to a greater amount of adsorption, but toluene shows slightly stronger adsorption affinity on the activated carbon than acetone. In addition, it was expected that the desorption characteristics of acetone on the activated carbon would be different from that of toluene. 4. Characteristics of Adsorption at the Fixed Bed To study the effects of operation variables on the adsorption breakthrough curves, five experimental adsorption runs for acetone and toluene in an activated carbon bed were performed, respectively. The operating conditions are summarized in Tables 4 and 5. Figure 4a-d shows the adsorption breakthrough curves and temperature excursion for acetone and toluene in the activated carbon bed. In Figure 4a and c, the temperature inside the bed was steeply increased due to the high heat of adsorption. Since the isosteric heat of adsorption of toluene is higher than that of acetone, the maximum temperature excursion of toluene was higher than that of acetone. Moreover, since the adsorption affinity of toluene in a low concentration range was stronger than that of acetone, the small second excursions of temperature

RATCV RATC1V RATCV1 RAT1CV RAT2CV

temperature [K] Tatm Tf 295. 296. 295. 295. 296.

423. 423. 423. 473. 393.

flow rate [LSTP/min] Q

related adsorption run

10. 10. 6. 10. 10.

ATCV ATC1V ATCV1 AT1CV AT2CV

Table 5. Experimental Conditions of Adsorption and Regeneration Breakthrough Runs of Toluene in Activated Carbon Bed Adsorption Breakthrough temperature [K]

flow rate [LSTP/min]

run no.

Tatm

Tf

Q

feed conc [ppm]

TTCV TTC1V TTCV1 TT1CV TT2CV

295. 293. 294. 296. 294.

293. 293. 293. 293. 293.

10. 10. 10. 10. 10.

4650. 3030. 4690. 4610. 4600.

Regeneration Breakthrough

run no. RTTCV RTTC1V RTTCV1 RTT1CV RTT2CV

temperature [K] Tatm Tf 295. 296. 295. 295. 296.

423. 423. 423. 473. 393.

flow rate [LSTP/min] Q

related adsorption run

10. 10. 6. 10. 10.

TTCV TTC1V TTCV1 TT1CV TT2CV

were also observed in the temperature profiles of toluene after passing a certain period of time. In this study, to confirm the reproducibility of experiments, the experimental runs were repeated at similar condition as shown in Figure 4b and d. The breakthrough time was naturally extended with a decrease in feed concentration and flow rate. In addition, as expected in the results of temperature, the breakthrough time of acetone was almost similar to that of toluene under the similar experimental condition. However, the breakthrough curves of toluene were slightly steeper than those of acetone in the activated carbon bed because the adsorption affinity and less dispersive temperature profile. The toluene breakthrough curves in Figure 4d could be well fitted by the mathematical model using the Langmuir equation. In the case of acetone in Figure 4b, the simulated results using the A-D equation were more accurate, but those using the Langmuir equation were reasonable because the adsorption breakthrough experiments were conducted at low concentration before the multilayer adsorption range. It is noted that this result does not imply the validity of the Langmuir equation at thermal regeneration. Even though the acetone is adsorbed on the activated carbon at low concentration, it can be highly concentrated at the thermal desorption step. It will be discussed at the later section in detail.

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Figure 4. Temperature profiles of run AT2CV (a) and TTCV (c) and adsorption breakthrough curves of acetone (b) and toluene (d) at various conditions in the adsorption breakthrough curve.

Figure 5. Profiles of the concentration in the gas, equilibrium, and solid phases and the temperature at t ) 10, 50, and 80 min in the adsorption breakthrough curve of run AT2CV (a-c) and TTCV(d-f).

Figure 5 shows the internal profiles of concentration in the gas and solid phase and the temperature at different times. Figure

5a-c presents the internal profiles at run AT2CV, and Figure 5d-f present those at TTCV. In these figures, the dimensionless

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Figure 6. Temperature profiles of run RAT2CV (a) and RTTCV (c) and effluent concentration for acetone (b) and toluene (d) in the thermal regeneration step.

parameters were defined as follows.

Y ) y/yin, Q ) q/q/in, Q* ) q*/q/in, Θ ) T/Tf (18) Y is gas phase concentration, Q and Q* are solid phase and equilibrium solid phase concentrations, and Θ is a gas phase temperature. In Figure 5a and d, the bed temperature at the product end (X ) 1) increased over ambient temperature even though the concentration wave front only reached close to 6 (X ) 0.2) and 12 cm (X ) 0.4) from the feed end. This means that the propagation of the temperature wave front to the product end was faster than that of the concentration wave front. The mass transfer zone (MTZ) approached the bed end as time passed, and the corresponding concentration in the solid phase and temperature moved with the MTZ. Furthermore, since the thermal wave front preceded the concentration wave front, the bed temperature was increased at the bed end and the concentration wave front did not approach the bed end. As shown in Figure 5a and d, the concentration and thermal wave front in TTCV (activated carbon-toluene system) is steeper than those in AT2CV (activated carbon-acetone system). In Figure 5, Q*, the dimensionless equilibrium amount of adsorbate, could be calculated from the isotherm equation for a corresponding concentration and temperature. The bed temperature at the bed end increased over ambient temperature even though the concentration wave front only reached the feed end. This means that the propagation of the temperature wave front to the product end was faster than that of the concentration wave front. Because the temperature excursion in the activated carbon-toluene system was greater than that in the activated carbon-acetone system, Figure 5d-f for toluene showed clearly this phenomenon. In addition, the self-sharpening effect in toluene (run TTCV) was more dominant in the concentration wave front than in acetone (run AT2CV). As a result, the

difference between the concentration wave front in the gas and adsorbed phases in toluene was small, but did slightly increase with time. 5. Characteristics of Hot Purge Regeneration at the Fixed Bed The thermal regeneration curves of acetone and toluene in the activated carbon bed are shown in Figure 6a-d as a representative case. The experimental regeneration conditions for both acetone and toluene were almost the same conditions except for regeneration temperature. As shown in Figure 6a and c, at each position in the bed, the increasing rate of temperature became slower due to the heat of desorption and heat transfer through the column wall. In addition, the experimental temperature profiles approached to limiting values after a steep temperature increase at the initial time. This implies that the adsorbate desorbed at each position of the bed was re-adsorbed at the lower part of the bed and the bed temperature was sustained for a while because heat energy was used to desorb the saturated adsorbent.15 Such phenomena were more clearly shown in Figure 6a than in Figure 6c because a lower regeneration temperature was applied to acetone (acetone 393 K; toluene 423 K). In the initial period of the regeneration step, the effluent concentration of the adsorbate increased rapidly and reached a maximum value. Then, the effluent concentration decreased to the feed concentration at the previous adsorption step, and thereafter, it leveled off slowly to zero concentration. However, the effluent concentration of acetone approached zero much faster than that of toluene despite the lower regeneration temperature applied as shown in Figure 6b and d. In addition, a rollup phenomenon of acetone was greater than toluene in the activated carbon bed. Because the isotherm of acetone on the activated carbon at high temperature was similar to type-III, the adsorption capacity and affinity of acetone became small with an increase in temperature. On the

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Figure 7. Profiles of the concentration in the gas, equilibrium, and solid phases and the temperature at t ) 6, 16.6, and 33.3 min in the regeneration curves of run RAT2CV (a-c) and RTTCV(d-f).

other hand, the isotherm of toluene at even high temperature still showed a favorable shape. Furthermore, as shown in Figure 6b, the simulated result using the A-D isotherm showed clearly a difference from that using the Langmuir isotherm. Because the desorption concentration at the initial period was much higher than the adsorption concentration, the isotherm at this condition approached the multilayer adsorption region more closely. Figure 7 shows the internal profiles of the concentrations in the gas and adsorbed phases and the bed temperature at different times in the hot nitrogen regeneration. As shown in Figure 7a and b, the desorbed acetone at the upper part of the bed was re-adsorbed at the lower part of the bed. Therefore, the re-adsorption led to a decrease of concentration in the gas phase along the bed. However, after a certain period time in Figure 7c, the re-adsorption phenomena disappeared, but the concentration profiles in the gas and solid phases remained a favorable shape. In the case of toluene, such phenomena were observed only at the initial period in Figure 7d. After passing a certain period of time, the concentration profile of the gas phase (y) became favorable with an increase in temperature profile. In addition, when the bed temperature became higher, unfavorable and linear concentration profiles in the gas and solid phases (y and Q), respectively, were formed in Figure 7f due to its strong adsorption affinity. Therefore, this result led to a long tailing in the regeneration curve as shown in Figure 6d. 6. Parametric Study for Regeneration In parametric study of thermal regeneration, an effort was made to reach the same desired conditions insofar as was possible. However, since the experimental regeneration run was performed right after the previous adsorption run, the initial adsorbed condition in the bed, except for the parameter to be studied, might not be the same to each other. Three parameters are of importance in the thermal regeneration of a fixed bed adsorber: the quantity of regeneration

required, the total energy required, and the total regeneration time. The first two parameters are generally discussed in terms of the quantity of purge per kilogram of adsorbent and energy per kilogram of adsorbent to represent regeneration efficiency. The third parameter is important in the design of a cyclic adsorption process. The ratio of regeneration time to adsorption time determines the number of beds required to maintain continuous operation. Figure 8a and d shows the effect of adsorption concentration in the thermal regeneration. As shown Figure 8a, the rollup of desorption in the run RATC1V was observed to be very small due to its relatively large adsorption capacity at the high feed concentration condition. However, the regeneration time was independent of the initial solid phase concentration in both acetone and toluene in the activated carbon bed.28 However, the regeneration time was independent of initial the solid phase concentration in both acetone and toluene in the activated carbon bed.12,28 Figure 8b and e show the effect of purge gas velocity on the thermal regeneration of acetone and toluene on the activated carbon bed. As shown in these figures, the regeneration curves were strongly affected by the purge gas velocity for a constant bed length. The slower purge gas velocity in the regeneration step leads to slower increase of bed inside temperature with time even though the same inlet temperature is applied to the bed. As a result, the rollup phenomena and tailing were increased as the purge gas velocity was decreased. On the other hand, as shown in Figure 8c and f, as the regeneration temperature was increased, the rollup of desorption was increased but the tailing was decreased. Compared to toluene, the effect of such purge gas velocity and regeneration temperature on the activated carbon bed was more clearly shown in the acetone-activated carbon bed because of the weak adsorption affinity of acetone. Figure 8 shows a comparison of the effluent concentration profiles of acetone fitted by using the Langmuir isotherm model

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Figure 8. Effect of initial bed loading(a and d), purge gas velocity(b and e), and purge gas temperature(c and f) for acetone and toluene on the activated carbon bed, respectively.

and A-D isotherm model. The A-D model and Langmuir model are applied to acetone and toluene, respectively. In Figure 4a, the differences of predicted results using two models were little shown in the adsorption step because the applied concentration range was much lower than multilayer adsorption. However, in the thermal desorption, the difference between the two models was more clearly shown because multilayer adsorption of acetone on the activated carbon in the regeneration step occurred by a rollup phenomenon. As a result, although the adsorption was performed in a low concentration range, the dynamic model using multilayer adsorption such as the A-D isotherm model was strongly recommended to design the activated carbon bed process more accurately. With the purge gas requirement, the energy requirement is one of the most important parameters in the design of a TSA process. As the regeneration temperature increases, the quantity of purge gas required decreases. Regeneration efficiency at different purge temperatures was defined as the specific energy requirement.

EP ) NpCpg(TR - T0)

(19)

Np ) td × U

(20)

In this equation, T0 is a reference temperature, and the temperature of 293 K is employed in this study. Np (mol/g) is the purge gas required per gram adsorbent, and TR is temperature of purge gas. The term td,1% is the time when the exit concentration is equal to 1% of the feed concentration, and U is the purge flow rate.12 Figure 9a and c show the comparison of adsorption breakthrough curve and thermal regeneration curve. The inlet

temperature history with time is supposed to be constant. At 5000 ppm of acetone and toluene in feed, the breakthrough shape of toluene in the activated carbon bed was sharper. However, the regeneration time and tailing of toluene was longer than acetone. In addition, if a too low regeneration temperature and purge gas velocity (333 K and 6 L/min) were applied to the acetone loaded bed, the regeneration advantage of acetone in the activated carbon bed was lost, compared to toluene. With an increase in temperature, the Np value of acetone at td,1% was changed from 0.62 to 0.24 mol/g while that at td,5% was changed from 0.55 to 0.21 mol/g. In the case of the Ep of acetone, the value at td,1% was changed from 1160 to 1985 cal/g while that at td,5% was changed from 1038 to 1793 mol/g. On the other hand, the Np value of toluene at td,1% was changed from 2.10 to 0.46 mol/g while that at td,5% was changed from 1.37 to 0.36 mol/g. And, the Ep value of toluene at td,1% was changed from 881 to 862 cal/g while that at td,5% was changed from 572 to 674 mol/g. The purge gas amount was plotted against the regeneration temperature in Figure 9b and d. As the regeneration temperature was increased in the acetone-activated carbon bed, the amount of hot nitrogen consumed during the regeneration breakthrough was decreased but the energy requirement was increased drastically. In addition, the effect of td on the specific energy requirement was larger than the required nitrogen amount. Moreover, with an increase in regeneration temperature, the difference of the required nitrogen amount between td,1% and td,5% was decreased, while the increase of the specific energy requirement in both cases was almost parallel. Such phenomena stemmed from the adsorption isotherm change with an increase in temperature.

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Figure 9. Comparison of purge gas quantity and energy requirement for acetone (a and b) and toluene (c and d) in the activated carbon bed regeneration.

In the case of toluene, the variation shape of the required nitrogen amount with an increase in regeneration temperature was similar to acetone, but a greater amount of nitrogen was need at the same td. However, the energy requirement of toluene was totally different from that of acetone. The energy requirement in the toluene-activated carbon system was not changed severely with a change of regeneration temperature although the difference of the energy requirement between td,1% and td,5% was almost same as the case of acetone. There results came from the strong adsorbed toluene on the activated carbon surface as expected in the tailing phenomena in Figure 9c. 7. Conclusions The sorption equilibria of acetone vapor on the activated carbon were studied at 293, 313, 333, 353, and 373 K and pressures up 21 kPa by using a volumetric method. The adsorption isotherms of acetone were type-II, but the isotherms approached type-III with an increase in temperature. As the pressure approached the saturation pressure of acetone (above 6 kPaP), multilayer adsorption or capillary condensation occurred in the activated carbon. In this study, the A-D isotherm model fitted successfully the adsorption isotherms of acetone on the activated carbon. On the contrary, the adsorption isotherm of toluene on the activated carbon with type-I behavior could be well presented by the Langmuir model. Adsorption and thermal regeneration dynamics of acetone were compared to those of toluene with a type-I isotherm in a fixed bed of activated carbon. Due to the strong adsorption affinity of toluene on the activated carbon, the temperature excursion of toluene was higher. In addition, the breakthrough shape and time of toluene was steeper and slightly shorter than those of acetone at similar conditions, respectively.

Compared to toluene, more concentrated acetone within a shorter period of time could be obtained from the activated carbon bed by thermal regeneration because its isotherm approached type-III behavior at high temperature. Therefore, the specific energy requirement and purge gas consumption for acetone desorption were significantly changed with purge gas velocity, regeneration temperature, and initial bed loading. On the contrary, the change of rollup and tailing in the toluene was relatively small with temperature. Therefore, the variation of energy consumption with the purge gas amount and regeneration temperature was smaller in the toluene-activated carbon system than in the acetone-activated carbon system. A nonequilibrium and nonadiabatic/nonisothermal model was used to fit temperature and concentration profiles of adsorption and hot nitrogen purge regeneration. Since the heat transfer coefficients play a key role in the thermal regeneration step, these values were obtained from the separate experimental data. The simulated result using the A-D isotherm showed a little difference from that using the Langmuir isotherm in the adsorption of acetone because the experiments were performed in a low concentration range. However, in the thermal regeneration step, the difference in the fitted results between the A-D isotherm model and the Langmuir model was clearly observed. Even though the adsorption of acetone was performed in a low concentration range, a multilayer adsorption isotherm model should be applied for the regeneration step due to the rollup and re-adsorption phenomena. Acknowledgment This study was supported by the “National RD&D Organization For Hydrogen & Fuel Cell” in the “Ministry of Commerce, Industry and Energy” (2004-N-HY12-P-01-0-000) and the “Korea Research Foundation Grant” (KRF-2005-005-J01401).

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Nomenclature AW ) cross sectional area of the wall (cm2) Ci ) i component concentration in bulk phase (mol/cm3) Cpg, Cps, Cpw ) gas, pellet, and wall heat capacity, respectively (cal/g‚K) d ) parameter in A-D isotherm (-) De ) effective diffusivity defined by solid diffusion model (cm2/ s) Dc ) intracrystalline diffusivity (cm2/s) DL ) axial dispersion coefficient (cm2/s) hi ) internal heat transfer coefficient (cal/cm2‚K‚s) ho ) external heat transfer coefficient (cal/cm2‚K‚s) -∆H h ) average heat of adsorption (cal/mol) k ) LDF coefficient (1/s) K ) proportionality parameter for LDF model (-) KL ) axial thermal conductivity (cal/cm‚s‚K) L ) bed length (cm) Ma ) mole weight (g/mol) Np ) purge gas per adsorbent (mol/g) P ) total pressure (atm) q, q*, qj ) amount adsorbed, equilibrium amount adsorbed and average amount adsorbed, respectively (mol/g) qm ) equilibrium parameter for Langmuir-Freundlich model (mol/g) Q ) solid phase concentration (-) Q* ) equilibrium solid phase concentration (-) R ) gas constant (cal/mol‚K) Rp ) radius of pellet (cm) RBi, RBo ) inside and outside radius of the bed, respectively (cm) t ) time (s) td ) desorption time (s) Tatm ) temperature of atmosphere (K) T, Tw ) pellet or bed temperature and wall temperature, respectively (K) u ) interstitial velocity (cm/s) U ) purge flow rate (mol/g‚s) yi ) mole fraction of species i in gas phase Y ) gas phase concentration (-) z ) axial distance in bed from the inlet (cm) Greek Letters R ) particle porosity (-) , t ) voidage of adsorbent bed and total void fraction, respectively (-) Fg, Fp, FB, Fw ) gas density, pellet density, bulk density, and bed wall density, respectively (g/cm3) θ ) dimensionless adsorbed amount () qi/qmi) (-) Subscripts B ) bed f ) feed i ) component i in ) inlet condition p ) pellet g ) gas phase s ) solid w ) wall Literature Cited (1) Monneyron, P.; Manero, M.-H.; Foussard, J.-N. Measurement and modeling of single- and multi-component adsorption equilibria of VOC on high-silica zeolite. EnViron. Sci. Technol. 2003, 37, 2410.

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ReceiVed for reView July 18, 2006 ReVised manuscript receiVed April 3, 2007 Accepted April 26, 2007 IE0609362