Removal of volatile organic compound by activated carbon fiber

Transcription

1 Carbon 42 (2004) Removal of volatile organic compound by activated carbon fiber Debasish Das, Vivekanand Gaur, Nishith Verma * Department of Chemical Engineering, Indian Institute of Technology Kanpur, Kanpur , India Received 25 June 2004; accepted 5 July 2004 Abstract Experiments were carried out to study adsorption/desorption of volatile organic compound (VOC) on the activated carbon fiber (ACF) under dynamic conditions. The primary objective was to experimentally demonstrate the suitability of ACF in effectively adsorbing VOCs from inert gaseous stream under varying operating conditions, and compare its performance vis-à-vis that of the her commercially available adsorbents, such as granular activated carbon (GAC), silica gel, and zeolites. The adsorption experiments were carried out in a fixed tubular packed bed reactor under various operating conditions including temperature ( C), gas concentration ( ,000 ppm), gas flow rate ( slpm) and weight of the adsorbent (2 10 g). A mathematical model was developed to predict the VOC breakthrough characteristics on ACF. The model incorporated the effects of the gas-particle film mass transfer resistance, adsorbent pore diffusion and the adsorption/desorption rates within the pore. The experimental data and the corresponding model simulated results were compared and found to be in good agreement. The ACF repeatedly showed a good regeneration capability following desorption by DC electrical heating. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: A. Activated carbon, Carbon fibers; B. Adsorption; C. Modeling; D. Diffusion 1. Introduction The common examples of VOCs are BTX (benzene, toluene xylene), dichloromethane, and trichloroethylene. The emissions of VOCs have primary as well as secondary harmful impacts. Eye and throat irritation, damage to liver, central nervous system, all may occur due to the prolonged exposure to VOCs. VOCs may also have carcinogenic effects. The role of VOCs as a precursor in the formation of phochemical smog and a number of toxic byproducts is well established [1]. Due to the inherent presence of VOCs in the products (chemicals), the control strategy for VOCs emission distinctly differs from that for the common urban pollutants such as NO x,so 2, and CO in that the former usually requires * Corresponding author. Tel.: ; fax: address: nishith@iitk.ac.in (N. Verma). an end-of-pipe control strategy. The substitution of raw materials and equipment and process modification are applicable mostly in control of the latter pollutants. There are various VOCs control methods presently available. These include condensation, adsorption, catalytic oxidation and thermal oxidation [2]. The adsorption of gaseous VOCs onto porous adsorbents has been suggested as an innovative treatment process in the environmental applications, although adsorption has been successfully utilized for the past several years in bulk separation or purification processes. In general, the methods (especially condensation) her than adsorption are effective when VOCs concentrations are relatively at higher levels (>1%). In contrary, adsorption has been found to be effective at low concentration levels i.e. parts per million (ppm) [3]. The latter scenario (emissions at low concentration levels) is quite typical during handling, storage, and distribution of the chemicals containing VOCs /$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi: /j.carbon

2 2950 D. Das et al. / Carbon 42 (2004) Nomenclature a external surface area per unit volume of the fiber, m 2 /m 3 a tal adsorption surface area per unit volume of the fiber, m 2 /m 3 C concentration, mol/m 3 inter-fiber concentration of the species mol/ C G m 3 C p molar concentration of species inside the pores, mol/m 3 C s molar surface concentration of adsorbed species inside the pores, mol/m 2 D e effective diffusivity inside pore, m 2 /s, D f diameter of the fiber, m D k Knudsen diffusivity, m 2 /s D m molecular diffusivity, m 2 /s D R radial dispersion coefficient, m 2 /s k m average mass film transfer coefficient around the fiber, m/s k a rate constant for adsorption, m/s k d rate constant for desorption, mole/s-m 2 L bed length of the bed, m M molecular weight of gas, g/mol N r molar diffusional flux of the component in the radial direction, mol/m 2 s P pressure, N/m 2 Q volumetric flow rate of the gas, slpm R universal gas constant, J/kg K R 1 inner radius of the bed, m R 2 outer radius of the bed, m pore radius, m R p Re Sc Sh T t V R Reynolds number, qvd f l Schmidt number, l qd f Sherwood number, K md f D m temperature, K time, s gas velocity in the radial direction, m/s Greek letters e porosity of bed a porosity of fiber q gas density, kg/m 3 l gas viscosity, Pa s Subscripts 1 inner 2 outer max maximum P pore of the fiber a adsorption d desorption R radial direction of the bed r radial direction of the pore of the fiber Superscripts s at the fiber outer surface volume average quantities inside the pores of particle * non-dimensionalized variables The keyword of an adsorption process is a porous solid medium having high adsorptive capacity. A large surface area or large micro-pore volume can be achieved due to the porous structure of the solid. The success of the adsorption process depends on the performance of adsorbents in bh equilibria and kinetics. A solid exhibiting favorable adsorption isherm as well as faster kinetics is supposed to be a good adsorbent. Therefore, in order to be a good adsorbent, a solid must have (a) a reasonably larger surface area, and (b) a relatively larger pore network for the transport of molecules to the interior. The breakthrough curve (transient response of the adsorbent bed to a step-change in the influent concentration) is reflective of the adsorbents performance under dynamic conditions. A relatively larger breakthrough time and gradual increase in the concentration following breakthrough are desirable. There are a number of commercially available adsorbents, including activated carbon fiber (ACF), alumina, silica gel, granular activated carbon (GAC) and zeolites, etc. In recent times, ACF has been considered to be one of the promising adsorbents for controlling VOCs. Due to its high BET surface area and macro-pore size distribution, the adsorption capacity of ACF is higher than any her adsorbents. The regeneration of saturated (equilibrated) adsorbents is anher critical factor that must be considered while selecting an adsorbent. In the case of ACF, regeneration can be carried out at a temperature of 150 C and under a small flow rate of the purge gas, whereas a high temperature ( C) and high gas flow rate are typically required for the regeneration of GAC and her adsorbents [4]. Reviewing literature, it is fair to say that most of the studies so far carried out on the adsorption of VOCs by ACF pertain to the equilibrium conditions, with primary objective of these studies being the development of an appropriate isherm [5 8]. Only limited studies on the adsorption of VOC by ACF have been carried out under dynamic conditions [9 11]. It is in this context that the present study was undertaken with a view to

3 D. Das et al. / Carbon 42 (2004) ascertaining the suitability of ACF in controlling the VOCs emissions under dynamic adsorption/desorption conditions. The major objectives of this study were as follows: (1) set-up of an experimental test bench to study adsorption/desorption characteristics for the removal of VOCs by commercially available ACF, (2) obtain breakthrough curves under varying operating conditions such as bed temperature, concentrations, gas flow rate and weight of the adsorbent, (3) development of a mathematical model to understand the adsorption mechanism and predict the breakthrough profiles, (4) screening of various commercial adsorbents (zeolites, GAC, and silica gel) for determining their comparative adsorption performance, and (5) desorption or regeneration of ACF by electrical heating. 2. Theoretical analysis In this section, a mathematical model is presented to predict the time-dependent (unsteady-state) concentration profiles of the adsorbing species (VOCs) on a solid adsorbent (ACF) under ishermal conditions. One important point that may be ned is that, in reality the steady-state condition never exists in the bed of adsorbing materials during adsorption/desorption. Hence, a finite adsorption/desorption rate always prevails in the bed. The steady-state is achieved only when the bed reaches saturation levels. The principal aspect of the present model is that it is based on non-equilibrium approach and incorporates the individual kinetic rate expressions for adsorption and desorption rather than equilibrium or pseudo-stationary assumptions often made in most of the models developed for adsorption in a fixed bed. The model developed in this work also incorporates the effects of pore diffusion, in addition to that of the gas-fiber mass transfer resistance on the performance of adsorbents. The following assumptions were made in the theoretical analysis for developing a mathematical model for the adsorption of VOCs on ACF: (1) Temperature is assumed to be uniform throughout the bed, i.e. ishermal condition prevails in the bed. In the recent study, it has been shown that the effect of exhermic heat on the rate of VOC adsorption is mostly negligible, considering the fact that the concentration of VOCs are usually in ppm levels and an unsteady-state condition exists throughout a typical adsorption/desorption cycle [3]. In the aforementioned study, the simulations were carried out to estimate the temperature rise in the bed during an unsteady-state adsorption in a tubular reactor, incorporating the heat production rate during the VOC uptake by the surface. The maximum increase in the gas temperature was estimated to be approximately 7, 1 and 0.1 C corresponding to the inlet gas concentrations of 50,000, 10,000, and 1000 ppm, respectively. The assumption of ishermal condition was also validated in the present study with the experimental measurements. Under the varying experimental conditions of gas flow rates, VOC concentrations, and the reactor dimensions used, the gas temperature was found to vary by n more than 1 C in either axial or radial direction. (2) Negligible pressure drop exists in the bed. This (isobaric condition) was found to be consistent with the experimental measurements made in this study. The assumption of a constant pressure in the reactor bed also derives from the fact that the concentrations were at low levels [12]. (3) There is a constant fluid velocity throughout the bed. This assumption follows from those made in (2) above, i.e. low concentration levels and negligible pressure-drop. The present model is based on three governing equations: (a) species balance of the adsorbing component in the bed, (b) species balance of the component inside the pores of the fiber, and (c) adsorption/desorption rates at the pore-walls of the fiber Species balance in the adsorbent bed There are generally two types of arrangements for carrying out the adsorption experiment on fibrous materials. In one of the arrangements the materials (ACF in the present case) are packed in a tube and the gas is allowed to flow in the longitudinal direction of the tube. In the second arrangement, ACF is wrapped over a perforated tube (closed at one end) and the gas may be allowed to flow from the her end, in which case the bulk gas flow will be in the radial direction. In this work the latter type of arrangement was selected for carrying out the adsorption/desorption experiments of VOC on ACF (refer the subsequent section on experimentation). Consider a gas flow through an ACF bed wrapped over such a perforated tube under ishermal condition, in which pressure drop is negligible and there is no variation in fluid velocity, V r. By making a species balance in the gaseous phase in the ACF bed of porosity, e, the following equation is obtained: oc G þ V R or D R o R or R oc G or þ 1 e k m aðc G C s F e Þ¼0 oc G ð1þ The terms on the left hand side of this equation are transient, convection (radial direction), dispersion and mass transfer rate from the bulk gas phase to the fiber surface (pore-mouth), respectively. In Eq. (1), C G is the concentration of the adsorbing species at any arbitrary location R in the bed, while C s F is the concentration at the outer

4 2952 D. Das et al. / Carbon 42 (2004) surface of the fiber. In this equation the concentration is assumed to be uniform in the longitudinal direction (or tubeõs axial direction) and longitudinal dispersion is neglected. This is due to the fact that in the perforated tubular reactor used in the study, the velocity of the gas through the wrapped ACF over the tube is in the radial direction, unlike in the case of the conventional tubular reactor packed with the spherical pellets wherein the flow is in longitudinal direction in which case the concentration is usually assumed to be uniform in the radial direction and radial dispersion is neglected Mass balance of the component inside the pores of the fiber Assuming the shape of the particles to be spherical, an expression for the concentration of the species, C p, in the pore may be obtained in terms of the radial molar diffusion flux, (N r of the component, and the rate of change in the surface concentration of the adsorbed species, C s, as follows: a oc p þ 1 r 2 o or r2 N r þ a oc s ¼ 0 ð2þ In the above equation the terms in the left hand side are transient, diffusion and adsorption/desoprtion, respectively. a and a are intrafiber void fraction and the tal adsorption surface area per unit volume of the fiber, respectively Adsorption/desorption on the pore-walls The rate of change of the surface concentration of the adsorbed species ocs is determined from the individual kinetic rate expressions for adsorption and desorption: N N oc s C s C s ¼ K a C p 1 k d ð3þ C s max C s max Here, k a and k d are the adsorption and desorption rate constants. Under steady-state condition, when the change in the surface concentration is zero, i.e. ocs ¼ 0, the corresponding isherm (C s vs. C p ) for the adsorbing species may be obtained from Eq. (3) by equating the individual adsorption and desorption rates: " # ðkc p Þ 1=N C s ¼ C smax ð4þ 1 þðkc p Þ 1=N Here K is the adsorption equilibrium constant and equals k a k d C s max, and N are the her isherm parameters, which are usually obtained from the experimental adsorption data under equilibrium conditions. Eq. (4) is better known as the Sips isherm for a single component system. In the present study, the above kinetics rates for adsorption and desorption (Eq. 3) were selected due to the fact that the Sips isherm has been found to explain the equilibrium isherm data reasonably well for a number of VOCs, including benzene, toluene and xylene, etc. [13]. For the model simulation, k a was determined from the kinetic theory of gases [14], whereas k d together with C smax were used as adjusted model parameters. N was set constant at 1.5 throughout the simulation. Knowing the individual adsorption and desorption rate constants, the corresponding values of equilibrium constant K were obtained. Here, it is appropriate to point out that the Sips isherm may closely resemble the simple HenryÕs law (C s = KC p ) of solubility applied mostly at low concentration levels, if the exponent of the Sips isherm N is set at 1 and the term KC p in the denominator of Eq. (4) is assumed smaller than 1. In this study, however, the model assuming the first order adsorption and desorption rates, which is tantamount to HenryÕs law under equilibrium conditions could n explain the experimentally obtained breakthrough curves for the adsorption of VOC on ACF. The governing equations (1) (3) are the basis of the model developed in this study for predicting the adsorbents performance in the bed under dynamic conditions. These equations are coupled partial differential equations (PDEs), with time (t) and radial directions (bh R and r) as independent variables. Due to extensive computation involved in numerically solving such type of a set of PDEs, an approach was adopted in the present study to simplify the numerical computations and significantly reduce the CPU time without losing much of accuracy. Essentially, in this approach radial (r) concentration profiles within the solid pores are averaged assuming parabolic concentration profiles and the average surface and gas phase concentrations within the pores of the fiber are determined. This approach originally proposed by Yang [12] has been successfully implemented elsewhere in similar situations, viz. adsorption of VOC in GAC containing macro-pores [3], irreversible chemical reaction of SO 2 with porous Ca-sorbents [15], and adsorption of SO 2 in zeolites containing macro as well as micro-pores [16,17]. The salient advantage of this mathematical approximation is the reduction of the second order PDEs to first order PDE with variation only in R direction (bed radial length). For the sake of brevity, the detailed computational steps are n presented here. As a consequence of the aforementioned approximation, Eq. (2) may be simplified to the following form: oc p 3 R f a k mðc G C s f Þþa a oc s ¼ 0 ð5þ where, C p and C s are the volume average surface concentrations in the pores and on the surface, respectively and R f is the radius of the fiber. The concentration of the adsorbing species at the pore-mouth within the pore, C s F is determined by equating mass flux across the external

5 D. Das et al. / Carbon 42 (2004) gas film to that at the mouth of the pores within the fibers and is shown to be obtained in terms of independent variables C p and C G as follows: C s F ¼ k mc G þ 10ðD e =D f ÞC p ð6þ ðk m þ 10D e =D f Þ Here, D e is the effective diffusivity inside the pores, which takes into account bh Knudsen (D k ) and molecular (D m ) diffusion. In the present model, the simplified governing equations (1), (3) and (5) completely describe the adsorption/desorption behavior of a single component VOC in a perforated packed bed of ACF in a gas flow under ishermal condition. Table 1 Different experimental conditions for VOC adsorption Experimental conditions Inlet VOC concentration (ppm) ,000 Gas flow rate (slpm) Bed temperature ( C) Sorbents amount (g) BET areas (m 2 /g) Numerical solution technique For a system of single adsorbing component in an inert gas, we have three dependent variables C G ; C p ; C s, which are function of time and radial location, and as many 1st order partial differential equations (1), (3), and (5). These equations were solved simultaneously by finite difference method using a Fortran subroutine D03pcf of NAG Fortran library. On a Pentium III machine the CPU time of computation was found to be less than a minute. 3. Experimental studies The experiments were carried on the two types of ACF samples obtained in clh form. These samples had different BET surface area (1000 and 1700 m 2 /g, respectively), although bh of the samples were prepared from viscous rayon precursor. The several operating conditions under which the adsorption experiments were carried out were bed temperature, inlet concentration of VOCs, gas flow rate, and weight of ACF. Table 1 describes these conditions Experimental set-up Fig. 1 is the schematic diagram of the experimental set-up used in this study. The experimental set-up may be assumed to consist of three sections as shown in Fig. 1: (a) gas preparation section, (b) test, and (c) analysis sections. In the gas preparation section, carrier gas (nitrogen in this case) was bubbled in the liquid VOC contained in a vertical btle (0.8 m long m dia). The bubbler was essentially a SS made 1/4 in. tube whose btom end was closed and the outer surface was perforated with holes of diameter 0.08 mm up to a distance of 10 cm from the btom end. The entire perforated section of the bubbler was submerged in the liquid. The level of the liquid in the btle was maintained high enough to provide sufficient residence time Fig. 1. Schematic diagram of the experimental set-up to study VOC adsorption over ACF. to the bubbling gas for complete saturation with VOC. The btle was kept inside a Freon (R-11) refrigeration unit (AICIL, India). Therefore, by controlling the temperature the vapor pressure of VOCs and hence, the partial pressure of the vapor (VOC) in the carrier gas were controlled. This way the required VOC concentration in the carrier gas was obtained. The gas flow rates over the range of slpm were controlled and monitored by a mass flow controller (Bronkhorst, Netherlands). Prior to bubbling the carrier gas through the liquid VOC, the gas was passed through two gas purification sections. The first purification section containing silica gel was used to remove moisture and the second one containing molecular sieves was used to remove trace amount of hydrocarbons which may be present in the carrier gas. The test section consisted of a vertical Teflon made tubular reactor (ID = 2.5 cm, OD = 2.8 cm, L =10 cm) encapsulated in a SS shell (ID = 4.0 cm, L =20 cm) with provisions for the gas inlet and outlet. Inside the SS shell the Teflon reactor with one end closed was vertically and co-axially mounted. The outer surface of the reactor was perforated with holes of diameter 0.1 mm at center-to-center distances of 0.5 cm. The thermocouple used to monitor the bed temperature was mounted vertically at the center of the reactor. There was a provision for varying the bed temperature from 35 to 100 C with the aid of an electrical heater and a PID controller (FUJI Electric, Japan). The effluent gas stream from the reactor was passed to the analytical

6 2954 D. Das et al. / Carbon 42 (2004) section consisting of a gas chromatography (GC) with flame ionization detector (FID) and data station. A computer was connected to the data station to store the chromatograph and the peak areas Experimental procedure The required amount of ACF sample was weighed by an electronic balance (Anamed, USA) and then pretreated before carrying out the test runs. The pretreatment was carried out in a vacuum oven at a temperature of 150 C for 4 h. The ACF sample was then wrapped over the Teflon reactor. Prior to start of the experiment, the btle was filled with 140 ml of VOC and kept inside the refrigeration unit. For a particular VOC concentration in the carrier gas, the corresponding saturation temperature of the liquid VOC was set in the refrigeration unit. The refrigeration unit was allowed sufficient time (0.5 h) to attain the required temperature. The reactor was heated to the desired bed temperature (50 C) and further kept at that temperature for 1 h so that the system was stabilized and a uniform temperature existed in the bed. The required flow rate of the gas was controlled using MFC. The concentration of the inlet gaseous mixture in the line bypassing the reactor was measured by GC prior to the start of the adsorption process. As the adsorption was started, the transient concentrations of exit gas from the reactor (breakthrough data) were monitored and measured by GC. The schematic diagram of the experimental set-up for the regeneration (or desorption) of ACF is described in Fig. 2. The regeneration of ACF was carried out by electrical (DC 20 V and 3 A) heating under a small nitrogen flow rate (0.1 slpm). A temperature of 150 C and regeneration time of min were typically required for the complete regeneration of ACF. The temperature was monitored by a thermocouple fixed at the surface of the ACF sample. The voltage and current were measured by a voltmeter and an ammeter, respectively. The concentration of the exit gas from the reactor was measured with the help of a GC using FID. 4. Results and discussion 4.1. Adsorption temperature for ACF To determine the effects of temperature on the adsorption of VOC, the experiments were carried out at varying bed temperatures (35, 40, 50, 75 and 100 C). The inlet concentration of VOC (toluene) was kept constant at 10,000 ppm. In all the test runs, the weight of the ACF samples was taken to be 5 g and the gas flow rate was maintained constant at 0.5 slpm. Fig. 3 describes the experimentally obtained breakthrough curve during adsorption by ACF (bh Type-1 and Type-2 samples) carried out at five different temperatures. For Type-1 sample it may be observed in Fig. 3 that the breakthrough times are nearly the same (approximately 7 min) at these temperatures; although the tal adsorption time is slightly greater at 40 C (40 min) than at 35 C (30 min). The breakthrough and adsorption times were found to increase to 10 and 50 min, respectively 3.3. Regeneration of ACF Fig. 2. Schematic diagram of the experimental set-up for the regeneration of ACF. Fig. 3. Temperature effects on breakthrough of toluene over ACF (w = 5.0 g, L = 10.0 cm, Q N2 ¼ 0:5 slpm, C inlet = 10,000 ppm).

7 D. Das et al. / Carbon 42 (2004) at the bed temperature of 50 C. However, further increase in the bed temperature to 75 C resulted in significant decrease in bh breakthrough time (6 min) and tal adsorption time (30 min). As also observed from the figure, the breakthrough curve further shifted to the left as the bed temperature was increased to 100 C, indicating an early saturation of the bed. In fact, at temperatures exceeding 100 C, almost no adsorption occurred, resulting in a sharp rising breakthrough curve. This type of breakthrough characteristics (optimum adsorption at an intermediate temperature) was observed for all concentration levels less than 1%. The similar trend was observed for Type-2 sample over the identical temperature range (ref. Fig. 3), i.e. the breakthrough characteristic began to shift to the left as the temperature was gradually increased to 50 C and higher. Thus, from these results it was concluded that a temperature of 50 C is most favorable for ACF in capturing toluene vapor from the effluent. It is important to point out that the reverse trend in the breakthrough characteristic observed at higher temperatures is typical to most of the adsorbents/adsorbate system and is indicative of reduced adsorption due to possible loss of the active sites for adsorption at relatively higher temperatures. As reported in literature [18,19], GAC is found to be most effective in adsorbing a number of VOCs at around 75 C, whereas 13X zeolites exhibit maximum adsorption for SO 2 at 60 C. Beyond these temperatures, the adsorption was observed to be insignificant in each case. The model predictions were done under identical operating conditions to explain the breakthrough of VOC in the ACF bed over the temperature range of C. The VOC adsorption by ACF in a fixed bed under ishermal condition is considered to be controlled by three rate mechanisms as discussed in the theoretical section: The diffusion of the gas from the bulk to the fiber is dependent on the mass transfer coefficient. The mass transfer coefficient is shown to be dependent on Sherwood number, Sh, which in turn is a function of the particles Reynolds number, Re and Schmidt number, Sc (refer Appendix A). Under the existing experimental conditions Re was calculated to be constant at , whereas Sc was found to marginally vary (5%) in the range of Hence, in each case the effect of bulk diffusion was nearly the same. The diffusion of the gas in the pores is primarily dependent on the pore size of the adsorbents. With increase in the temperature the pore diffusion coefficient is expected to be higher and favorable for pore diffusion. However, over the temperature range of C a marginal increase in the pore diffusivity from to m 2 /s occurs. The main reason for longer breakthrough time at higher temperature was thus attributed solely to the higher rate constant because of its Arrhenius type of temperature dependence. In the present model, k d and C smax have been used, wherever necessary, as adjustable parameters to explain the experimental data. Fig. 4 compares the model predicted breakthrough curves with the experimental data for the Type-1 and Type-2 ACF samples. A reasonable good agreement is observed between the two results within the experimental and computational errors. The adjusted parameters for toluene are reported in Table 2 for bh Type-1 and Type-2 ACF samples. The numerical values of the adsorption equilibrium constants K, defined as the ratio of adsorption rate constant (k a ) to desorption rate constant (k d ), are reported in Table 3 for the two types of samples for various temperatures. The decrease in the value of K with increase in the temperature is indicative of the adsorption being exhermic. Under the existing experimental conditions the exhermic heats of adsorption determined from the slope of lnk vs. 1/T curve were determined to be 1.45 and 1.01 kcal/mol for Type-1 and Type-2 ACF, respectively, as shown in the inset of Fig. 4. These values (a) Diffusion of the species from bulk gas to the fiber surface through a gas film surrounding the fiber (bulk diffusion). (b) Diffusion of the gas within the pores (pore diffusion). (c) Adsorption /desorption of VOCs inside the pores (surface reaction). Fig. 4. Comparison between model predictions and data: (w = 5.0 g, L = 10.0 cm, Q N2 ¼ 0:5 slpm, C inlet = 10,000 ppm).

8 2956 D. Das et al. / Carbon 42 (2004) Table 2 Model parameters for various temperatures Temperature ( C) Concentration (ppm) K (m 3 /mol) 35 10, , , C ls (mmol/g) Table 3 Adsorption equilibrium constants at various bed temperatures and toluene concentrations Temperature ( C) Concentration (ppm) K (m 3 /mol) ACF (Type-1 sample) ACF (Type-2 sample) 35 10, , , , compare to 10 kcal/mol for the adsorption of SO 2 over 5A zeolites [20], 2.05 kcal/mol for the adsorption of benzene over ACF under supercritical CO 2 [21], and 8 kcal/ mol for that of dichloromethane over hydrophobic Y- type zeolite [13]. It may be pointed out that the values of the heat of adsorption reported in literature [13,20,21] were experimentally obtained under equilibrium conditions Effect of VOC concentration To determine the effects of VOC concentration on the breakthrough characteristics, the experiments were carried out for varying VOC inlet concentrations: 2000, 4000, 6000, and 10,000 ppm. For each run 5 g of the ACF sample was taken. The bed temperature was set at the predetermined optimum temperature of 50 C and the flow rate at 0.5 slpm. Fig. 5 describes the experimentally obtained breakthrough curves for toluene under the various gas inlet concentrations for Type-1 and Type-2 ACF samples. As observed from Fig. 5, the breakthrough times are less than 10 min in each case for Type-1 sample, whereas the tal adsorption time decreases from 70 to 40 min as the inlet concentration is increased from 2000 to 10,000 ppm. For Type-2 sample, the breakthrough times are observed to be approximately 10 min in each case. However, the tal adsorption time decreases from 80 to 50 min as the concentration level is increased from 2000 to 10,000 ppm. As also observed from these results, decrease in tal adsorption time with the increase in concentration is relatively larger at higher concentration levels ( ,000) than lower concentration levels ( ) in Fig. 5. Concentration effects on breakthrough of toluene over ACF (w = 5.0 g, L = 10.0 cm, T bed =50 C, Q N2 ¼ 0:5 slpm). the case of bh types of the samples, indicating suitability of ACF in capturing toluene vapor at the concentration levels less than 1% (10,000 ppm). The increase in the breakthrough and tal adsorption times with the decrease in inlet concentration levels as observed from Fig. 5 can be explained in terms of the tal amount of VOC. With decrease in the inlet concentration under identical flow rates, the tal amount of VOC (moles) entering the macro-pores of the fiber is less. Therefore, the saturation of the ACF bed is delayed and occurs in relatively longer time. Fig. 5 also shows that the model predictions agree well with the experimental data. For the model predictions of the breakthrough curves under identical experimental conditions, the values of only k d were required to be adjusted over the concentration range of ,000 ppm. The corresponding values of K obtained are reported in Table 3 for bh Type-1 and Type-2 ACF samples. The slopes of K vs. concentration pls as depicted in the inset of Fig. 5 indicate a nonlinear type of adsorption isherm over the selected concentration range. This is consistent with the postulate of the Sips isherm assumed in this study for the adsorption of toluene on ACF surface. As observed from Table 3 the adsorption equilibrium constant K in the case of Type-2 sample is consistently greater than that in the case of Type-1 sample over the entire concentration range. Hence, from these results it may be concluded that the adsorption capacity of Type-2 sample is greater than that for Type-1 sample under identical concentration levels. The superior performance of the Type-2

9 D. Das et al. / Carbon 42 (2004) sample over the Type-1 sample in capturing VOC may be attributed due to difference in the BET surface area Effect of gas flow rate The experiments were carried out at different gas flow rates: 0.25, 0.5 and 1.0 slpm. The primary objective was to determine the influence of the particle (film) mass transfer resistance on the adsorption by ACF. In each test run the bed temperature was held constant at the reaction temperature of 50 C. The weight of the sample and the inlet gas concentration were kept constant at 5 g and 10,000 ppm, respectively. Fig. 6 describes the experimentally obtained VOC breakthrough curves under the above mentioned flow rates. As observed in Fig. 6, the breakthrough time decreases from 20 to 3 min as the flow rate is increased from 0.25 to 1 slpm. The corresponding times to reach 10,000 ppm are 55 and 14 min, respectively. Fig. 6 also compares the experimental data with the model predictions. Reasonably good agreements are observed between the two results. In principle, values of the model parameters k d and C smax should n be adjusted with the variation in the flow rates. Expectedly, the values of K (corresponding to k d )andc smax were kept constant at 25.6 m 3 /mol and 0.95 mmol/g, respectively in the model predictions for each case. As also observed in Fig. 6, bh the breakthrough time and the tal adsorption time decreased with increase in flow rate. While the early saturation of the bed at higher gas flowrate is trivial to explain, it is worth ascertaining if the gas-particle film mass transfer controlled the removal process, especially at lower flowrate chosen in the experiment. The diffusion rate of the gas from the bulk phase to the fiber surface is determined by the particle mass transfer coefficient k m, which in turn is a function of Sh, where, Sh ¼ kmdp D m :D m and D p are the molecular diffusivity and particle diameter, respectively. As mentioned earlier in the text, Sh was calculated from a reported correlation (refer Appendix A) between Sh, Re and Sc [22]. Here, Re is dependent on the diameter of the fiber and gas superficial velocity. Hence, due to small fiber size ( m) and low radial gas velocities ( m/s), although the particle Reynolds numbers were very small ( ), the mass transfer coefficient was calculated to be quite significant and varying between 1 and 2 m/s under the experimental conditions (refer Appendix A). Referring Eq. (6) of the model equations, it is evident that k m, the particle-gas mass transfer coefficient and D e, the pore diffusional resistance may be considered to be in series. Under the existing conditions the pore diffusion coefficient was calculated to be approximately constant at m 2 /s. With the variation in the bed temperature (from 30 to 50 C), there was only a marginal improvement (<5%) in the diffusion coefficient. As a consequence, the numerical values of k m and 10D e /D f as they appear in the numerator of Eq. (6), were found to be of the same order of magnitude. Further model parametric study (n reported here) suggested that the second term of Eq. (5) representing the rates of mass transfer across the particle surface and within the pore were negligible in comparison with the last term representing the kinetic rates of adsorption/desorption. Table 4 lists the numerical values of various parameters, including a few dimensionless numbers required in the model simulation for breakthrough. From these results it was concluded that neither the fluid-to-fiber mass transfer resistance nor intraparticle (pore) resistance controlled the adsorption of VOC on ACF, and the entire dynamics was solely determined by the adsoption and desorption rates Effect of BET surface area Fig. 6. Gas flowrate effects on breakthrough of toluene over ACF (w = 5.0 g, L = 10.0 cm, T bed =50 C, C inlet = 10,000 ppm). The breakthrough data obtained for the two types of ACF samples (Type-1 and Type-2) under different temperatures and concentrations consistently revealed that the breakthrough times and the tal adsorption times for Type-2 sample having a BET surface area 1700 m 2 / g were greater than that for Type-1 sample having a BET area of 1000 m 2 /g. The superiority of Type-2 sample over Type-1 is clearly attributed due to the higher specific surface area responsible for gas adsorption. To elucidate the above observation the data were re-plted in Figs. 7 and 8 on a comparable format. Figs. 7 and 8 compare the experimentally obtained breakthrough curves of toluene during adsorption by two types of

10 2958 D. Das et al. / Carbon 42 (2004) Table 4 Calculated values of Re, Sh, K m and D eff under various operating conditions S. No. T ( C) Q (slpm) V r 10 3 (m/s) Re 10 5 Sc Sh 10 2 K m (m/s) D k 10 7 (m 2 /s) D eff 10 7 (m 2 /s) Fig. 7. Effects of BET area on toluene breakthrough over ACF under different bed temperatures (w = 5.0 g, L = 10.0 cm, Q N2 ¼ 0:5 slpm, C inlet = 10,000 ppm). Fig. 8. Effects of BET area on toluene breakthrough over ACF under different concentration levels (w = 5.0 g, L = 10.0 cm, Q N2 ¼ 0:5 slpm, T bed =50 C. ACF samples (Type-1 and Type-2). As observed from Fig. 7, the breakthrough times and the tal adsorption times are greater in the case of Type-2 sample than those in the case of Type-1 under two different bed temperatures (30 and 50 C). It is clear from Fig. 7 that at the temperature of 50 C the breakthrough time in the case of Type-1 ACF was less than 5 min in comparison with 20 min for the Type-2 ACF. Similarly, the times to reach the VOC inlet concentration were approximately 40 and 72 min, respectively. As observed in Fig. 8, the breakthrough curves follow the same trend under two different VOC inlet concentrations (2000 and 10,000 ppm), indicating the superior performance of the ACF having higher BET surface area than the ACF having relatively lower BET surface area. Figs. 7 and 8 also compare the experimental data with the model predictions. Good agreements are observed between two results within the experimental and computational errors. The effects of the amount of ACF on the breakthrough characteristics were observed to be qualitatively similar, i.e. increase in the amount resulted in increase in the breakthrough and tal adsorption times. These results are n reported here for brevity Relative adsorption of toluene and xylene over ACF The relative adsorption of toluene and m-xylene were determined by determining their breakthrough characteristics vis-à-vis ACF under identical experimental conditions. The test runs were carried out with toluene and xylene for varying gas inlet concentrations: 2000, 6000 and 10,000 ppm; the remaining operating

11 D. Das et al. / Carbon 42 (2004) Fig. 9. Comparative adsoprtion of toluene and xylene over ACF (w = 3.7 g, L = 10.0 cm, T bed =50 C, Q N2 ¼ 0:5 slpm). Fig. 10. Relative adsorption performance of commercial adsorbents (T bed =50 C, C inlet = 4000 ppm, Q N2 ¼ 1:0 slpm, w = 10 gm). variables (temperature, adsorbent weight, gas flowrate) were kept unchanged. Fig. 9 describes the breakthrough characteristics of the VOCs at these concentrations. As observed in the figure, the breakthrough times are approximately less than 15 min for bh the adsorbing species. However, as the inlet concentration is decreased from 10,000 to 2000 ppm, the tal adsorption time for toluene increases from approximately 40 to 80 min, while that for m-xylene increases from 60 to 100 min. As also observed in the figure, the tal adsorption time for xylene is larger at low concentration (2000 ppm) than high concentration (10,000 ppm), indicating a relatively larger adsorption of m-xylene by ACF. The model predictions are observed to be in good agreement with the data under identical conditions. Two clarifications are in order. First, similar to the study carried out on the adsorption of toluene, a series of test runs were also carried out a priori for determining the optimum temperature for the adsorption of xylene on ACF. The optimum temperature for the maximum adsorption of xylene reflected in terms of breakthrough and adsorption times was also observed to be around 50 C. Second, for explaining the breakthrough data of xylene, the values of only k d (or K) were required, as expected, to be adjusted in the model. Moreover, the values of k d for xylene were found to be smaller (approximately three times) than those required for predicting the toluene breakthrough under identical conditions, consistent with the observed breakthrough curves for these two VOCs in Fig Comparative performance of commercial adsorbents The experiments were carried out under varying operating conditions for screening the various commercially available adsorbents such as GAC, 5A zeolites and silica gel for adsorbing toluene and xylene vapors. Fig. 10 describes one such representative result of the breakthrough curves for these adsorbents obtained under the identical operating conditions (weight of the adsorbents, gas flowrate, and gas inlet concentration). As observed in the figure, the adsorption for either toluene or xylene over zeolite was insignificant resulting in almost an instantaneous breakthrough and the saturation of the bed following the breakthrough. The similar observations were made during the adsorption of xylene over silica gel, although in the case of toluene, the adsorption time following the breakthrough was greater (45 min). As seen in the figure, the performance of GAC was superior to that of silica gel as the breakthrough in the latter occurred instantaneously, in contrary to GAC for which the breakthrough was suppressed for approximately 25 min. However, for either adsorbent, the breakthrough and adsorption times were appreciably shorter than those for ACF (45 and 70 min, respectively) suggesting the superiority of ACF over silica gel and GAC in capturing VOCs. Here, it is worth pointing out that while ACF was wrapped over a perforated tubular reactor, the her adsorbents available in particles or pellets forms were packed in a SS tubular reactor. In one of our recent studies, we have experimentally demonstrated that the breakthrough and

12 2960 D. Das et al. / Carbon 42 (2004) adsorption times were larger in the case of ACF packed as a clh in a tubular reactor that those in the case of ACF (same weight) wrapped over a perforated tube [23]. It was due to the method employed for regeneration (DC electrical heating) that the latter arrangement was preferred in the present study, as described in the following section Regeneration of ACF The ability to retain its adsorption capacity after successive adsorption and desorption is one of the major characteristics of a good adsorbent. As described in Fig. 2, the regeneration (desorption) of saturated ACF was carried out by heating the clh with the aid of an electrical DC power supply. Since the ACF was wrapped over the Teflon-tube, it was n necessary to remove the clh after completion of the adsorption experiments and the heating could be carried out in situ by passing the DC current along the longitudinal direction of the fiber. Since ACF is reasonably a good conductor of electricity, the required temperature for regeneration could be attained in a very short time (1 min) by adjusting the voltage between 10 and 20 V. The flow rate of purge N 2 was kept nominal (0.1 slpm) to continuously remove the desorbed species. To determine the optimum temperature for regeneration, desorption experiments were performed at three temperatures: 75, 100, 150 C. In each case the fresh ACF samples were pre-saturated with toluene in a typical adsorption test run. The common adsorption conditions chosen were: T = 50 C, C in = 10,000 ppm, and Q N2 = 0.5 slpm. Fig. 11 describes the toluene desorption concentrations profiles during a typical regeneration experiment. As observed in the figure, at 100 and 50 C there is initial rise in the concentration levels above the initial concentration value. The concentration gradually decreases till the entire desorption is completed. The initial rise is significant (approximately 1.5 times the initial concentration level) at 150 C. No initial rise was observed at 50 C. As also evident in the figure, the area under the curve (proportional to the tal amount of VOC desorbed) is higher at 150 C than 100 and 50 C. The regeneration carried out at higher temperatures (maximum 200 C) indicated no further improvement in the desorption characteristic. In fact, the desorption profiles were found to be almost unchanged at temperatures exceeding 150 C, suggesting a temperature of 150 C to be effective in completely removing the adsorbed VOC. To further corroborate the above finding (an optimum desorption temperature of 150 C), each of the regenerated ACF samples was re-subjected to the adsorption test under the identical adsorption conditions. Fig. 12 describes the breakthrough profiles of these ACF samples. As seen in the pls, the adsorption performance of the sample regenerated at 150 C was found to be superior to those of the Fig. 11. Effect of regeneration temperature on desoprtion of toluene. Adsorption: w = 5.0 g, L = 10.0 cm, T =50 C, Q N2 ¼ 0:5 slpm, C inlet = 10,000. ppm Regeneration: V =15 V, I = 2.5 A, Q N2 ¼ 0:5 slpm. Fig. 12. Breakthrough profiles of regenerated ACF at various temperatures. Adsorption: w = 5.0 g, L = 10.0 cm, T =50 C, Q N2 ¼ 0:5 slpm, C inlet = 10,000 ppm. Regeneration: t = 60 min, V = V, I = 2 3 A, Q N2 ¼ 0:5 slpm.

13 D. Das et al. / Carbon 42 (2004) remaining samples and nearly similar to that of a fresh ACF. In this study, an ACF sample was subjected to adsorption/desorption cycles at least times without observing any appreciable loss in its adsorption capacity, as evident in the corresponding breakthrough response. 5. Conclusions The various conclusions that may be drawn from this study are summarized as follows: 1. The experimental results revealed that activated carbon fiber is a pential adsorbent for capturing VOCs at ppm levels under dynamic adsorption/desorption conditions, as the breakthrough times were observed to considerably decrease with increase in the concentration levels (typically greater than 1%). 2. The simulation results from the mathematical model developed to predict the breakthrough profiles were found to be in good agreement with the data. The model assumes a finite kinetic rate for adsorption and desorption and incorporates two resistances to mass transfer, between the gas and the adsorbents surface, and within the pores of the adsorbents. 3. A bed temperature of 50 C was found to be favorable in terms of longer breakthrough and adsorption times. 4. From the breakthrough curves obtained over the temperature range of C, the exhermic heats of adsorption were determined to be 1.45 and 1.01 kcal/mol for Type-1 and Type-2 ACF samples, respectively. 5. The commercially obtained ACF was shown to exhibit greater adsorption for VOC than most of the her adsorbents available in pellets or powders form, including granular activated carbon, zeolites, and silica gel, under identical operating conditions. 6. Under the experimental conditions, an ACF sample was adsorbed and desorbed repeatedly without exhibiting any appreciable degradation in its adsorption performance. 7. The regeneration of ACF could be effectively carried out by DC electrical heating. Appendix A A.1. Dispersion efficient The dispersion coefficient was determined by the following correlation for axial dispersion coefficient in a fixed bed [12] D R ¼ D m e ð20 þ 0:5ScReÞ A.2. Mass transfer coefficient The gas-particle mass transfer coefficient was determined using the following correlation proposed by Geankoplis [22] Sh ¼ð1:09=eÞSc 1=3 Re 1=3 for 0:0001 < Re < 72 A.3. Pore diffusivity The effective diffusivity inside the pores was determined by a combination of bh Knudsen and molecular diffusivity [14]. The Knudsen diffusivity is given by D k ¼ 2 r p 3 8RT pm 1 2 ¼ 97 Rpore 1 2 where, R pore = pore radius in m, M = molecular weight in g/mol, T = temperature in K, D k = Knudsen diffusivity in m 2 /s. The combined diffusivity contributed by bh molecular and Knudsen diffusion is given by 1 ¼ 1 D D k þ 1 D m. Finally, the effective diffusivity inside the pores is determined by using the formula D e ¼ ad where, s a = intrafiber void fraction, s = tortuosity factor. T M A.4. Adsorption and desorption rate constants From the kinetic theory of gases, adsorption rate constant, k a is defined as k a ¼ s RTx1000 0:5 ; where s is the sticking coefficient: 2pM Acknowledgements The authors thankfully acknowledge HEG Ltd., Bhopal (India) and Beijing Evergrow Resources Co., Ltd., Beijing (China) for supplying the ACF samples to conduct the present study. The authors also acknowledge the partial financial support (Grant No: 01(1599)/99/ EMR-II) from Central Scientific Industrial Research (CSIR), New Delhi to conduct this study. References [1] Nevers N. Air pollution control engineering. Singapore: Mc- Graw-Hill; [2] Ruddy EN, Carroll LA. Select the best VOC control strategy. Chem Eng Prog 1993;89: [3] Gupta VK, Verma N. Removal of volatile organic compounds by cryogenic condensation followed by adsorption. Chem Eng Sci 2002;57: