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Journal of Hazardous Materials 185 (2011) 1609–1613
Contents lists available at ScienceDirect
Journal of Hazardous Materials
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / j h a z m a t
Short communication
Sorption of SO2 and NO from simulated flue gas over rice husk ash
(RHA)/CaO/CeO2 sorbent: Evaluation of deactivation kinetic parameters
Irvan Dahlan a, Keat Teong Lee b, Azlina Harun Kamaruddin b, Abdul Rahman Mohamed b,∗
a School of Civil Engineering, Universiti Sains Malaysia, Engineering Campus, Seri Ampangan, 14300 Nibong Tebal, Pulau Pinang, Malaysiab School of Chemical Engineering, Universiti Sains Malaysia, Engineering Campus, Seri Ampangan, 14300 Nibong Tebal, Pulau Pinang, Malaysia
a r t i c l e i n f o
Article history:
Received 22 June 2010Received in revised form 7 October 2010
Accepted 13 October 2010
Available online 20 October 2010
Keywords:
Rice husk ash (RHA)
Sorbent
SO2/NO sorption
Breakthrough curves
Deactivation kinetic model
a b s t r a c t
In this study, the kinetic parameters of rice husk ash (RHA)/CaO/CeO 2 sorbent for SO2 and NO sorptions
were investigated in a laboratory-scale stainless steelfixed-bed reactor. Dataexperiments were obtained
from our previous results and additional independent experiments were carried out at different condi-
tions. The initial sorption rate constant (k0) and deactivation rate constant (kd) for SO2/NO sorptions
were obtained from the nonlinearregression analysisof the experimental breakthrough datausing deac-
tivation kinetic model. Both the initial sorption rate constants and deactivation rate constants increased
with increasing temperature, except at operating temperature of 170 ◦C. The activation energy and fre-
quencyfactorfor theSO2 sorption were found to be 18.0 kJ/moland 7.37×105 cm3/(g min), respectively.
Whereasthe activation energyand frequencyfactor forthe NOsorption,wereestimated to be 5.64 kJ/mol
and 2.19×104 cm3/(g min), respectively. The deactivation kinetic model was found to give a very good
agreement with the experimental data of the SO2/NO sorptions.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
Cleaning flue gases from sulfur oxides (SO x) and nitric oxides
(NO x) has become an issue of great importance to governmental
regulatory agencies and general public due to their negative effect
towards the environment and human health. Normally SO x and
NO x, which consists of more than 98% of sulfur dioxide (SO 2) [1]
andover 90–95% of nitricoxide(NO) [2], are generated mainly from
the combustion of fossil fuels in power stations as well as chemical
plants and metallurgy processes.Attempts havebeen madeto finda
suitable methodfor theremovalof SO2 and NO simultaneously.Dry
sorption method is now considered to be the most attractive way
to treat waste gases containing SO2 and NO due to the drawbacks
of wet sorption methods [3,4]. There are several dry-type sorbents
that have been considered in the previous study for simultaneous
removal of SO2 and NO.RHA, which is produced from the burning of rice husk, has been
chosenin this study as a rawmaterial in thepreparationof dry-type
sorbent since it is available abundantly in rice-producing countries
like Malaysia. RHA also contains high amount of silica. However,
RHA has low sorption capacity when used alone to remove acidic
gases. Therefore, this agricultural waste-siliceous starting material
needs to be activated with other materials and the silica in RHA
∗ Corresponding author. Tel.: +60 4 5996410; fax: +60 4 5941013.
E-mail address: chrahman@eng.usm.my (A.R. Mohamed).
plays an important role in the formation of reactive species which
is responsible for high sorption capacity [5,6].Previously, we had reported the sorption characteristics of SO2
and NO over rice husk ash (RHA)-based sorbent at low temper-
ature [5–11]. Nevertheless, our previous reports only dealt with
activity measurement related to sorbent preparation conditions
and effects of reactor operating conditions. Our previous results
alsoshowed thatthe highest sorption capacity for the simultaneous
removal of SO2 andNO wasobtainedusing RHA/CaO/CeO2 sorbent.
Currently,the optimum preparative parameters for this kind of sor-
bent had also been reported [12]. On the other hand, the reaction
between the siliceous/calcium dry-type sorbents and SO2/NO is
very scarcely reported. The reaction between this siliceous/calcium
dry-type sorbents and SO2/NO are very complicated due to the
complex composition of the sorbent. The sorption of these pol-
lutant gases (SO2/NO) on the sorbents is not a simple physicalsorption processes, but also may be described as chemisorption or
as gas–solid non-catalytic reactions.
There are various kinetic models that have been employed to
estimate kinetic parameters in gas–solid reaction, mainly involves
single component sorbent (such as CaO, Ca(OH)2 and CaCO3)andit
wascarried outmainly at high operating temperature. These kinet-
ics models included shrinking unreacted core model [13], changing
grain size model [14] and random pore model [15]. Most of these
models contain large number of adjustable parameters related to
the pore structure, to the product layer and pore diffusion resis-
tances as well as the surface sorption rate parameters. In addition,
0304-3894/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jhazmat.2010.10.053
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it is complicated to incorporate them without having to perform
lengthy computer programs. Therefore in this study, the simplified
deactivation kinetic model was used to estimate kinetic parame-
ters against other models. The breakthrough curves data obtained
from our previous results (SO2 and NO sorptions) [11] was fitted to
deactivation kinetic model. In the present work,kinetic parameters
such as deactivation rate constant, initial sorption rate constant,
activation energy and frequency (pre-exponential) factor of the
SO2
an NO sorptions were estimated from the breakthrough data
through nonlinear regression analysis. In chemical engineering,the
rate of reaction is a prerequisite to the design and evaluation of
fixed-bed reactor performance especially under dry-type gas–solid
reaction–sorption processes.
2. Experimental
2.1. Preparation of sorbent
RHA-based sorbents (RHA/CaO/CeO2) were prepared from
rice husk ash (RHA), CaO (BDH Laboratories, England) and
Ce(NO3)3·6H2O (Fluka, 98%). The raw RHA was collected directly
without any pretreatment from Kilang Beras & Minyak Sin Guan
Hup Sdn. Bhd., Nibong Tebal, Malaysia. Prior to use, the RHA wassievedto produce less than 200m particle size. Thechemicalcom-
position of raw RHA was 68.0% SiO2, 2.30% K2O, 1.20% P2O5, 0.71%
MgO, 0.59% CaO, 0.32% SO3, 0.32% Cl2O, 0.16% Al2O3, 0.40% others
and 26.0% LOI. The preparation method was based on the optimum
hydration conditions reported in our previous studies [12].
2.2. Activity test
The sorption/activity of the prepared sorbents was tested in a
laboratory-scale stainlesssteel fixed-bed reactor (Swagelok,10 mm
ID, 50cm length) which was vertically fitted in a tube furnace (Lin-
berg/Blue M). The schematic diagram and details of the activity
study is presented elsewhere [7]. The experiments were conducted
at various reactor temperature range of 70–170
◦
C while maintain-ing the simulated flue gas under the fixed condition of 2000 ppm
SO2, 500 ppm NO, 10% O2, 10% RH, balance N2 with total gas flow
rate of 150ml/min. Other operating conditions are given in our
previous study [11].
2.3. Kinetic parameters estimation of RHA/CaO/CeO 2 sorbent
using deactivation kinetic model.
The analysis of kinetic parameters was carried out using
breakthrough data of single component gases of SO2 and NO,
respectively. The deactivation kinetic parameters such as initial
sorption rate constants (k0) and deactivation rate constants (kd)
were calculated from breakthrough curve analysis. The outline of
theanalysisusing deactivationkinetic modeling is given as follows.As in a typical gas–solid reaction, pore structure, active surface
area and activity per unit area of the solid reactant have significant
effects on the reaction rate. In the deactivation model, the effects
of all these factors are combined in an activity term (a) introduced
into the sorption rate expression and is written in Eq. (1) [16].
−
da
dt = kdC
man (1)
where kd is the deactivation rate constants(min−1), C is theconcen-
tration of the reactant gas (kmol/m3), t is the reaction time (min),
and m and n areexponential coefficients. Assuming thatthe concen-
tration of the reactant gas is independent along the reactor ( m = 0)
and the deactivation of the sorbent is first-order with respect to
the solid active site (n = 1), integration of Eq. (1) gives the following
expression.
a = a0 exp(−kdt ) (2)
Furthermore, the following basic assumptions were made in the
derivation of the deactivation model, such as isothermal and
pseudo-steady state conditions, and axial dispersion in the fixed
bedreactor andany mass transfer resistances were neglected. Con-
sidering these assumptions, and the initial activity (a0) of the solid
as unity, the pseudo-steady state species conservation equation forgases in the fixed bed reactor is given by Eq. (3) [16–18].
−Q dC
dW = k0Ca (3)
where Q is thevolumetric flowrate(m3/min), W is thesorbentmass
(kg) and k0 is the initial sorption rate constant (m3 kg−1 min−1).
Combining Eqs. (2) and (3) and solving these equations will yield
Eq. (4)
C
C 0= exp[−k0B exp(−kdt )] (4)
whereby B is equal to W /Q and this kinetic model is known as the
zeroth solution of deactivation model, which predicts the behav-
ior of breakthrough curves for a gas–solid non-catalytic reaction.
This solution assumes a fluid phase concentration that is indepen-
dent of deactivation process along the reactor. However, it would
be reasonable to expect the deactivation rate to be concentration
dependent and axial position dependent in the fixed bed reactor.
In order to find analytical solutions of Eqs. (1) and (2) by con-
sidering concentration and axial position dependents in the fixed
bedreactor(m = n = 1), iterativeprocedurewas applied. The method
used was similar to the method for the estimated solution of non-
linear equations proposed by Dogu [19]. In this procedure, Eq. (4)
was substituted into Eq. (1) with m = n = 1 and the first estimated
value forthe activity (a) term wasobtained byintegrating theequa-
tion. Then, the estimated value for the activity (a) term expression
was substituted into Eq. (3), and integration of this equation gave
the following corrected solution for the breakthrough curve.
C
C 0= exp
1 − exp(k0B[1 − exp(−kdt )])
1 − exp(−kdt ) exp(−kdt )
(5)
This Eq. (5) is also known as the solution of two-parameter deac-
tivation kinetic model. Deactivation rate constant (kd) and initial
sorption rateconstant (k0) wasthen calculated by using a nonlinear
regression technique.
A commercial software, MATHEMATICA ver. 5.2 (Wolfram
Research Inc.), was used for nonlinear regression analysis together
with the experimental/breakthrough datato find the rate constants
for the model. In order to obtain the best fitting results, an error
minimization technique was also applied and included after run-
ning the main program code of MATHEMATICA. MATHEMATICA
software was run under Microsoft Windows XP Professional (ver.
2002) environment.
Based on the analysis of the experimental breakthrough data at
different temperatures, the initial sorption rate constants (k0) can
be obtained by fitting Eq. (5) using nonlinear regression technique.
Then, Arrhenius equation [16] was used for the determination of
activation energy and frequency (pre-exponential) factor for SO2
and NO sorptions at different temperatures, and is given in Eq. (6).
k0 = A exp
−E a
RT
(6)
where A is a frequency (pre-exponential) factor, E a is the activation
energy, R is the gas constant (8.314J/(mol K)) and T is the temper-
ature (K).
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Table 1
Rate parameters obtained from the breakthrough data at different temperature.
Temp. (◦C) k0 (W/Q) k0 (cm3 /(gmin)) kd (min−1) R2
SO2 sorption NO sorption SO2 sorption NO sorption SO2 sorption NO sorption SO2 sorption NO sorption
70 4.06 10.22 1.22E+03 3.06E+03 0.12 0.11 0.987 0.989
87 5.84 11.00 1.75E+03 3.30E+03 0.15 0.12 0.975 0.990
100 8.57 11.45 2.57E+03 3.43E+03 0.20 0.126 0.972 0.954
120 10.85 13.38 3.25E+03 4.01E+03 0.21 0.128 0.983 0.976
150 13.12 14.56 3.93E+03 4.36E+03 0.23 0.130 0.991 0.964
170 11.69 15.78 3.50E+03 4.73E+03 0.24 0.135 0.965 0.957
3. Results and discussion
Fig. 1a and b shows the experimental SO2 and NO breakthrough
curves obtained under various operating temperatures, respec-
tively. The initial sorption rate constants (k0) and deactivation rate
constants (kd) values were estimated by nonlinear fitting of Eq. (5)
to the experimental SO2 and NO breakthrough curves at different
temperatures. The results of rate parameters from the regression
analysis of the dataobtained withRHA/CaO/CeO2 sorbents at differ-
enttemperatures aregiven in Table 1. The accuracy of theproposed
deactivation kinetic model was assessed from the coefficient of
determination (R
2
) which was found to be 0.95 or higher. Otherkind of regression results (including statistical analysis) could be
obtained from the nonlinear regression analysis after running the
main program code of MATHEMATICA.
The initial sorption rate constants and deactivation rate
constants, as expected, increased with increasing temperature
(Table 1). However, at operating temperature of 170 ◦C, the ini-
tial sorption rate constant for SO2 was decreased. The decrease in
the rate of SO2 sorption at higher temperatures might be due to
Fig. 1. Effect of operating temperature on the (a) SO 2 and (b) NO sorptions.
water that accumulated and gas dissolving on the RHA/CaO/CeO2
sorbent surface was reduced [11]. The predictions of the break-
through curves from Eq. (5) at different temperatures using these
rate constants are also shown in Fig. 1, whereby the deactivation
kinetic model shows good agreement with the experimental data
at different temperatures. As predicted for SO2 sorption at high
temperature (170 ◦C), the breakthrough curves shifted to shorter
time (Fig. 1(a)). For NO sorption, the initial sorption rate con-
stants still increasedat high temperature (170 ◦C) andthe resulting
breakthrough curves shifted to longer time (Fig. 1(b)). This might
be attributed to a lesser amount of water accumulated on the
RHA/CaO/CeO2 sorbent surface thus allowing the metal species(CeO2) present in the sorbent to become more active [11].
Based on the data obtained in Table 1, Arrhenius equation (Eq.
(6)) was used for the estimation of activation energy (E a) and fre-
quency (pre-exponential) factor ( A) for SO2 and NO sorptions at
different temperatures. Fig.2(a)and(b)showsln k0 versus1/T plots
for SO2 and NO sorptions, respectively at different temperatures.
The plots were found to yield a straight line indicating that the
SO2 sorption
y = -2165.3x + 13.51
R 2 = 0.9428
7.0
7.2
7.4
7.6
7.8
8.0
8.2
8.4
8.6
b
a
3.0E-032.8E-032.6E-032.4E-032.2E-032.0E-03
1/T (K -1
)
l n ( k
o )
NO sorption
y = -678.48x + 9.9921
R 2 = 0.9844
7.9
8.0
8.1
8.2
8.3
8.4
8.5
3.0E-032.8E-032.6E-032.4E-032.2E-032.0E-03
1/T (K -1
)
l n ( k
o )
Fig. 2. Arrhenius plot of sorption rate constant versus reciprocal of operating tem-
perature for (a) SO2 and (b) NO sorptions.
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Fig. 3. Comparison between predicted and experimental breakthrough curves at
two different experimental conditions.
sorption rate constant obtained from deactivation kinetic model do
follow the Arrhenius law as in Eq. (6). Accordingly, the slope of the
plot equal to E a/R and intercept equivalent to ln A, from which acti-
vation energy(E a) andfrequencyfactor ( A)forSO2 andNOsorptionscan be obtained, respectively.
The value of frequency factor ( A) for SO2 and NO sorptions were
calculated to be 7.37 ×105 cm3/(g min) and 2.19×104 cm3/gmin,
respectively. Whereas the activationenergy (E a) values determined
for the SO2 and NO sorptions were 18.0 kJ/mol and 5.64kJ/mol,
respectively. The activation energy of the SO2 sorption at low
temperature using the RHA/CaO/CeO2 sorbent was found to be
slightly higher as compared to sorbent prepared from coal fly
ash/Ca(OH)2 (14.94–15.47 kJ/mol) [20], activated carbon from oil
palm shell with KOH impregnation (13.2 kJ/mol) [21] and acti-
vated carbon from oil palm shell (12.6kJ/mol) [22]. However,
the activation energy obtained in this study was lower than
the SO2 sorption when Ca(OH)2 (32 kJ/mol) [23] and coal fly
ash/CaO/CaSO4 (22.9 kJ/mol) [24] were used as the sorbent, andalso much lower than the reported value by Irabien et al. [25]
and Renedo and Fernandez [26] using Ca(OH)2 (75kJ/mol) and coal
fly ash/Ca(OH)2/CaSO4 (57.7 kJ/mol), respectively. Apart from that,
this activation energyfor SO2 sorption at lowtemperature wasalso
found to be similar as compared to sorbents prepared from vari-
ous type of CaCO3 (15.2–19.5 kJ/mol) [27]. For the case of the NO
sorption, the value of activation energy was also much lower than
previously reported in the literature which include the sorbent pre-
pared from V2O5/NH4Br/TiO2/SiO2 (30.1 kJ/mol) [28], V2O5–Al2O3
(53.56 kJ/mol) [29] and Fe-ZSM-5 (54 kJ/mol) [30]. However, most
ofthe reportedstudiesfor NOsorptionwerecarriedoutat high tem-
perature processes. The relatively small activation energy obtained
in this study suggested an easy sorption process of SO2 and NO by
this kind of sorbent. In other word, the sorption between SO2/NOand the reference sorbent synthesized from RHA/CaO/CeO2 is eas-
ier to occur due to the easier access of SO2 and NO molecules to the
active species in the sorbent.
In order to verify the proposed deactivation kinetic model,
additional independent experiments were carried out at different
conditions using 0.5g RHA/CaO/CeO2 sorbent. The first experiment
was conducted at initial condition of 1500 ppm SO2, 1200ppmNO,
10% O2, 60% RH, balance N2 and 150 ml/min of total flow rate at
a reactor temperature of 80 ◦C. While the second experiment was
conducted at the following conditions of 1800 ppm SO2, 800ppm
NO, 10% O2, 40% RH, balance N2 and 150 ml/min of total flow rate
at a reactor temperature of 110 ◦C. Fig. 3 shows the experimental
versus predicted breakthrough curves of SO2 and NO sorptions at
two different experimental conditions. It was shown that the deac-
SO2 sorption
0.0
0.1
0.20.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0a
b
1.00.90.80.70.60.50.40.30.20.10.0
Experimental C/Co
P r e d i c t e d C / C o
NO sorption
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.00.90.80.70.60.50.40.30.20.10.0
Experimental C/Co
P r e
d i c t e d C / C o
Fig. 4. Plot of all experimental C/C0 vs predicted C/C0 under various operating con-
ditions for (a) SO2 and (b) NO sorptions.
tivation kinetic model provided a very accurate description of the
experimental data.
For further confirmation, the breakthrough curvesdata fromour
previous results [11] was fitted to the proposeddeactivation kinetic
model. The comparison between predicted breakthrough curves
(obtained with deactivation kinetic model) with the experimen-
tal results was performed for all the SO2/NO sorption experiments
under various operating conditions. Fig. 4(a) and (b) shows the
comparison between the experimental C /C 0 versus predicted C /C 0ofSO2 and NO sorptionsin all experiments, respectively. The results
indicated that the proposed model prediction agrees reasonably
well with the experimental data of the SO2/NO sorptions within
the range of 10% experimental error.
4. Conclusions
The deactivation model wasapplied successfullyto describe the
experimental breakthrough curves for the sorption of SO2
and NO
from simulated flue gas in a fixed-bed reactor over RHA/CaO/CeO2
sorbent. Thebreakthrough data obtained forboth SO2 and NO sorp-
tions was fitted to the proposed deactivation kinetic model. Both
the initial sorption rate constants and deactivation rate constants
increased with increasing temperature, except at operating tem-
perature of 170◦C whereby theinitialsorptionrateconstantfor SO2
decreased. The breakthrough curves obtained by using the devel-
oped deactivationkinetic model were found to fit theexperimental
breakthrough curves very well.
Acknowledgements
Theauthors wish to acknowledge thefinancialsupport from the
Ministry of Science, Technology and Innovation (MOSTI) Malaysia,
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I. Dahlan et al. / Journal of Hazardous Materials 185 (2011) 1609–1613 1613
Yayasan Felda andUniversiti Sains Malaysia (ShortTerm Grant A/C.
6035278 and RU Golden Goose Project Grant A/C. 814004).
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