Carbon 42 (2004) 2949–2962
www.elsevier.com/locate/carbon
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 208016, 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 other 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
(35–100 C), gas concentration (2000–10,000 ppm), gas flow rate (0.2–1.0 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.
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 photochemical 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
NOx, SO2, and CO in that the former usually requires
*
Corresponding author. Tel.: +91 512597704; fax: +91 512590104.
E-mail address: [email protected] (N. Verma).
0008-6223/$ - see front matter 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.carbon.2004.07.008
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) other 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.
2950
D. Das et al. / Carbon 42 (2004) 2949–2962
Nomenclature
a
a
C
CG
Cp
Cs
De
Df
Dk
Dm
DR
km
ka
kd
Lbed
M
Nr
P
Q
R
R1
R2
Rp
external surface area per unit volume of the
fiber, m2 /m 3
total adsorption surface area per unit volume
of the fiber, m2 /m3
concentration, mol/m3
inter-fiber concentration of the species mol/
m3
molar concentration of species inside the
pores, mol/m 3
molar surface concentration of adsorbed species inside the pores, mol/m2
effective diffusivity inside pore, m2/s,
diameter of the fiber, m
Knudsen diffusivity, m2/s
molecular diffusivity, m2/s
radial dispersion coefficient, m2/s
average mass film transfer coefficient around
the fiber, m/s
rate constant for adsorption, m/s
rate constant for desorption, mole/s-m2
length of the bed, m
molecular weight of gas, g/mol
molar diffusional flux of the component in
the radial direction, mol/m2 s
pressure, N/m2
volumetric flow rate of the gas, slpm
universal gas constant, J/kg K
inner radius of the bed, m
outer radius of the bed, m
pore radius, m
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 both equilibria and kinetics. A solid exhibiting favorable adsorption isotherm 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
Re
Sc
f
Reynolds number, qvD
l
l
Schmidt number, qDf
Sh
T
t
VR
Sherwood number, KDmmDf
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/m3
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
*
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 other
adsorbents. The regeneration of saturated (equilibrated)
adsorbents is another 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 (350–400 C) and high gas
flow rate are typically required for the regeneration of
GAC and other 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 isotherm [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
D. Das et al. / Carbon 42 (2004) 2949–2962
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 isothermal conditions. One
important point that may be noted 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. isothermal condition prevails in the
bed. In the recent study, it has been shown that the effect
of exothermic 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 correspond-
2951
ing to the inlet gas concentrations of 50,000, 10,000, and
1000 ppm, respectively. The assumption of isothermal
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 not 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.
2.1. 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 other 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 isothermal condition,
in which pressure drop is negligible and there is no variation in fluid velocity, Vr. By making a species balance
in the gaseous phase in the ACF bed of porosity, e, the
following equation is obtained:
oC G
oC G DR o
oC G
R
þVR
ot
oR
R oR
oR
1e
þ
ð1Þ
k m aðC G C sF Þ ¼ 0
e
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), CG is the concentration of the adsorbing species at any arbitrary location
R in the bed, while C sF is the concentration at the outer
2952
D. Das et al. / Carbon 42 (2004) 2949–2962
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.
2.2. 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, Cp,
in the pore may be obtained in terms of the radial molar
diffusion flux, (Nr of the component, and the rate of
change in the surface concentration of the adsorbed species, Cs, as follows:
oC p 1 o 2 oC s
r N r þ a
þ 2
¼0
ð2Þ
a
r or
ot
ot
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 total
adsorption surface area per unit volume of the fiber,
respectively.
2.3. Adsorption/desorption on the pore-walls
The rate of change of the surface concentration of the
s
adsorbed species oC
is determined from the individual
ot
kinetic rate expressions for adsorption and desorption:
N
N
oC s
Cs
Cs
¼ K aCp 1 kd
ð3Þ
ot
C s max
C s max
Here, ka and kd are the adsorption and desorption rate
constants. Under steady-state condition, when the
s
change in the surface concentration is zero, i.e. oC
¼ 0,
ot
the corresponding isotherm (Cs vs. Cp) for the adsorbing
species may be obtained from Eq. (3) by equating the
individual adsorption and desorption rates:
"
#
ðKC p Þ1=N
C s ¼ C s max
ð4Þ
1=N
1 þ ðKC p Þ
Here K is the adsorption equilibrium constant and
equals kkda C s max , and N are the other isotherm parameters, which are usually obtained from the experimental
adsorption data under equilibrium conditions. Eq. (4)
is better known as the Sips isotherm 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 isotherm has been found to
explain the equilibrium isotherm data reasonably well
for a number of VOCs, including benzene, toluene and
xylene, etc. [13]. For the model simulation, ka was determined from the kinetic theory of gases [14], whereas kd
together with Cs max 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 isotherm may closely
resemble the simple HenryÕs law (Cs = KCp) of solubility
applied mostly at low concentration levels, if the
exponent of the Sips isotherm N is set at 1 and the
term KCp 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 not 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 (both
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 SO2 with porous Ca-sorbents [15],
and adsorption of SO2 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 not presented
here. As a consequence of the aforementioned approximation, Eq. (2) may be simplified to the following
form:
oC p
3
a oC s
k m ðC G C sf Þ þ
¼0
Rf a
ot
a ot
ð5Þ
where, C p and C s are the volume average surface concentrations in the pores and on the surface, respectively
and Rf is the radius of the fiber. The concentration of the
adsorbing species at the pore-mouth within the pore, C sF
is determined by equating mass flux across the external
D. Das et al. / Carbon 42 (2004) 2949–2962
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 CG as follows:
C sF
k m C G þ 10ðDe =Df ÞC p
¼
ðk m þ 10De =Df Þ
ð6Þ
Here, De is the effective diffusivity inside the pores,
which takes into account both Knudsen (Dk) and molecular (Dm) 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 isothermal condition.
2953
Table 1
Different experimental conditions for VOC adsorption
Experimental conditions
Inlet VOC concentration (ppm)
Gas flow rate (slpm)
Bed temperature (C)
Sorbents amount (g)
BET areas (m2/g)
2000
0.2
35
2.2
4000
40
1000
6000
0.5
50
4.5
10,000
1.0
75
100
8.5
1700
2.4. 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 cloth form. These samples
had different BET surface area (1000 and 1700 m2/g,
respectively), although both 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.
3.1. 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 bottle (0.8 m long · 0.025 m
dia). The bubbler was essentially a SS made 1/4 in. tube
whose bottom 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 bottom end. The entire perforated section of the bubbler was submerged in the
liquid. The level of the liquid in the bottle 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 bottle 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 0.2–1.0 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
2954
D. Das et al. / Carbon 42 (2004) 2949–2962
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.
3.2. 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 bottle 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.
gen flow rate (0.1 slpm). A temperature of 150 C
and regeneration time of 60–75 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 (both 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 total 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
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 nitro-
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, QN2 ¼ 0:5 slpm, Cinlet = 10,000 ppm).
D. Das et al. / Carbon 42 (2004) 2949–2962
at the bed temperature of 50 C. However, further increase in the bed temperature to 75 C resulted in significant decrease in both breakthrough time (6 min) and
total 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 SO2 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
30–50 C. The VOC adsorption by ACF in a fixed bed
under isothermal condition is considered to be controlled by three rate mechanisms as discussed in the theoretical section:
2955
sion 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 35–50 C a marginal
increase in the pore diffusivity from 1.118 · 107 to
1.145 · 107 m2/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, kd and Cs max 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 both Type-1 and Type-2 ACF samples. The
numerical values of the adsorption equilibrium constants K, defined as the ratio of adsorption rate constant
(ka) to desorption rate constant (kd), 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
exothermic. Under the existing experimental conditions
the exothermic heats of adsorption determined from
the slope of ln K 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).
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
4.142 · 105, whereas Sc was found to marginally vary
(5%) in the range of 1.38–1.45. Hence, in each case the
effect of bulk diffusion was nearly the same. The diffu-
Fig. 4. Comparison between model predictions and data: (w = 5.0 g,
L = 10.0 cm, QN2 ¼ 0:5 slpm, Cinlet = 10,000 ppm).
2956
D. Das et al. / Carbon 42 (2004) 2949–2962
Table 2
Model parameters for various temperatures
Temperature (C)
Concentration
(ppm)
K (m3/mol)
Cls (mmol/g)
35
40
50
10,000
10,000
10,000
25.60
23.94
22.81
0.75
0.85
0.95
Table 3
Adsorption equilibrium constants at various bed temperatures and
toluene concentrations
Temperature
(C)
Concentration
(ppm)
35
40
50
50
50
50
50
10,000
10,000
10,000
2000
4000
6000
10,000
K (m3/mol)
ACF
(Type-1
sample)
ACF
(Type-2
sample)
25.60
23.94
22.81
61.79
34.91
32.22
25.60
21.76
20.95
20.14
120.84
67.14
37.32
20.14
compare to 10 kcal/mol for the adsorption of SO2 over
5A zeolites [20], 2.05 kcal/mol for the adsorption of benzene over ACF under supercritical CO2 [21], and 8 kcal/
mol for that of dichloromethane over hydrophobic Ytype 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.
4.2. 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 total 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 total 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 total adsorption time with the increase in concentration is
relatively larger at higher concentration levels (4000–
10,000) than lower concentration levels (2000–4000) in
Fig. 5. Concentration effects on breakthrough of toluene over ACF
(w = 5.0 g, L = 10.0 cm, Tbed = 50 C, QN2 ¼ 0:5 slpm).
the case of both 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 total adsorption times with the decrease in inlet concentration levels as observed from
Fig. 5 can be explained in terms of the total amount
of VOC. With decrease in the inlet concentration under
identical flow rates, the total 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 kd were required
to be adjusted over the concentration range of 2000–
10,000 ppm. The corresponding values of K obtained
are reported in Table 3 for both Type-1 and Type-2
ACF samples. The slopes of K vs. concentration plots
as depicted in the inset of Fig. 5 indicate a nonlinear
type of adsorption isotherm over the selected concentration range. This is consistent with the postulate of the
Sips isotherm 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
D. Das et al. / Carbon 42 (2004) 2949–2962
sample over the Type-1 sample in capturing VOC may
be attributed due to difference in the BET surface area.
4.3. 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 kd and Cs max
should not be adjusted with the variation in the flow
rates. Expectedly, the values of K (corresponding to
kd) and Cs max were kept constant at 25.6 m3/mol and
0.95 mmol/g, respectively in the model predictions for
each case. As also observed in Fig. 6, both the breakthrough time and the total 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 trans-
2957
fer 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 km, which in turn is a function of
k D
Sh, where, Sh ¼ mDm p :Dm and Dp 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 (4.6 · 107m) and low radial gas velocities (0.5–
2.5 · 103 m/s), although the particle Reynolds numbers
were very small (1–8 · 105), 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 km, the particle-gas mass transfer
coefficient and De, 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 1.0 · 107m2/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 km and 10De/Df as they appear in the numerator of Eq. (6), were found to be of the same order of
magnitude. Further model parametric study (not 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.
4.4. Effect of BET surface area
Fig. 6. Gas flowrate effects on breakthrough of toluene over ACF
(w = 5.0 g, L = 10.0 cm, Tbed = 50 C, Cinlet = 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 total adsorption times
for Type-2 sample having a BET surface area 1700 m2/
g were greater than that for Type-1 sample having a
BET area of 1000 m2/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-plotted
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
2958
D. Das et al. / Carbon 42 (2004) 2949–2962
Table 4
Calculated values of Re, Sh, Km and Deff under various operating conditions
S. No.
T (C)
Q (slpm)
Vr · 103 (m/s)
Re · 105
Sc
Sh · 102
Km (m/s)
Dk · 107 (m2/s)
Deff · 107 (m2/s)
1
2
3
4
5
6
7
8
9
30
30
30
40
40
40
50
50
50
0.2
0.5
1.0
0.2
0.5
1.0
0.2
0.5
1.0
0.463
1.159
2.318
0.479
1.197
2.395
0.494
1.230
2.471
1.656
4.141
8.283
1.656
4.141
8.283
1.656
4.141
8.283
1.359
1.359
1.359
1.404
1.404
1.404
1.449
1.449
1.449
5.060
6.867
8.652
5.115
6.942
8.746
5.169
7.010
8.838
1.042
1.414
1.782
1.053
1.429
1.801
1.044
1.445
1.820
5.281
5.281
5.281
5.367
5.367
5.367
5.452
5.452
5.452
1.109
1.109
1.109
1.127
1.127
1.127
1.145
1.145
1.145
Fig. 7. Effects of BET area on toluene breakthrough over ACF under
different bed temperatures (w = 5.0 g, L = 10.0 cm, QN2 ¼ 0:5 slpm,
Cinlet = 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, QN2 ¼ 0:5 slpm,
Tbed = 50 C.
ACF samples (Type-1 and Type-2). As observed from
Fig. 7, the breakthrough times and the total 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 total adsorption times. These results
are not reported here for brevity.
4.5. 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
D. Das et al. / Carbon 42 (2004) 2949–2962
Fig. 9. Comparative adsoprtion of toluene and xylene over ACF
(w = 3.7 g, L = 10.0 cm, Tbed = 50 C, QN2 ¼ 0:5 slpm).
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 both the
adsorbing species. However, as the inlet concentration
is decreased from 10,000 to 2000 ppm, the total 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 total
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 kd
(or K) were required, as expected, to be adjusted in
the model. Moreover, the values of kd 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. 9.
2959
Fig. 10. Relative adsorption performance of commercial adsorbents
(Tbed = 50 C, Cinlet = 4000 ppm, QN2 ¼ 1:0 slpm, w = 10 gm).
4.6. 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 other 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
2960
D. Das et al. / Carbon 42 (2004) 2949–2962
adsorption times were larger in the case of ACF packed
as a cloth 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.
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 cloth with the aid of an
electrical DC power supply. Since the ACF was wrapped
over the Teflon-tube, it was not necessary to remove the
cloth 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 N2 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, Cin =
10,000 ppm, and QN2 = 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 total 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
plots, 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, QN2 ¼ 0:5 slpm,
Cinlet = 10,000. ppm Regeneration: V = 15 V, I = 2.5 A, QN2 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,
QN2 ¼ 0:5 slpm, Cinlet = 10,000 ppm. Regeneration: t = 60 min,
V = 10–20 V, I = 2–3 A, QN2 ¼ 0:5 slpm.
4.7. Regeneration of ACF
D. Das et al. / Carbon 42 (2004) 2949–2962
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 25–30 times without
observing any appreciable loss in its adsorption capacity,
as evident in the corresponding breakthrough response.
2961
Appendix A
A.1. Dispersion efficient
The dispersion coefficient was determined by the following correlation for axial dispersion coefficient in a
fixed bed [12]
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 potential 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 35–50 C, the exothermic 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
other 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.
DR ¼
Dm
ð20 þ 0:5ScReÞ
e
A.2. Mass transfer coefficient
The gas-particle mass transfer coefficient was determined using the following correlation proposed by
Geankoplis [22]
Sh ¼ ð1:09=eÞSc1=3 Re1=3
for 0:0001 < Re < 72
A.3. Pore diffusivity
The effective diffusivity inside the pores was determined by a combination of both Knudsen and molecular diffusivity [14].
The Knudsen diffusivity is given by
1
12
2 rp 8RT 2
T
Dk ¼
¼ 97 Rpore
pM
M
3
where, Rpore = pore radius in m, M = molecular weight in
g/mol, T = temperature in K, Dk = Knudsen diffusivity in
m2/s. The combined diffusivity contributed by both molecular and Knudsen diffusion is given by D1 ¼ D1k þ D1m .
Finally, the effective diffusivity inside the pores is
determined by using the formula De ¼ aD
where,
s
a = intrafiber void fraction, s = tortuosity factor.
A.4. Adsorption and desorption rate constants
From the kinetic theory of gases, adsorption rate
constant, ka is defined as
0:5
RTx1000
; where s is the sticking coefficient:
ka ¼ s
2pM
Acknowledgements
References
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.
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