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Sulfur dioxide adsorptionisotherms and breakthroughanalysis on molecular sieve 5AzeoliteTürkan KopaÇ a & Sefa Kocabaş aa Department of Chemistry, Zonguldak KaraelmasUniversity, Zonguldak, TurkeyPublished online: 09 Sep 2010.
To cite this article: Türkan KopaÇ & Sefa Kocabaş (2003) Sulfur dioxide adsorptionisotherms and breakthrough analysis on molecular sieve 5A zeolite, ChemicalEngineering Communications, 190:5-8, 1041-1054, DOI: 10.1080/00986440302103
To link to this article: http://dx.doi.org/10.1080/00986440302103
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SULFURDIOXIDE ADSORPTION ISOTHERMSANDBREAKTHROUGHANALYSISONMOLECULARSIEVE 5A ZEOLITE
TU« RKANKOPACSEFAKOCABAS
Zonguldak Karaelmas University,Department of Chemistry,Zonguldak,Turkey
In this study sulfur dioxide adsorption and rate were investigated on mole-
cular sieve 5A zeolite in a packed bed system at 473K. The experimental
adsorption isotherms were compared with Langmuir, Freundlich, and Du-
binin-Radushkevitch-Kaganer (DRK) adsorption isotherm models. The
simple Langmuir and the Freundlich models provided a better description of
sulfur dioxide sorption on molecular sieve 5A zeolite, so that a selective
surface adsorption mechanism for adsorption of sulfur dioxide on molecular
sieve 5A zeolite can be suggested. The breakthrough behavior for sulfur di-
oxide adsorption on molecular sieve 5A was also investigated in this study.
The experimental breakthrough curves were compared with deactivation
models proposed by Orbey et al., and the recent models of Suyadal et al. and
Yas� yerli et al. The deactivation models were found to give good agreement
with the experimental results.
Keywords: Sulfur dioxide adsorption; Molecular sieve 5A; Adsorption iso-
therms; Deactivation models
INTRODUCTION
Sulfur dioxide emissions from combustion gases cause importantenvironmental problems. For this reason, extensive reasearch workhas already been done for sulfur dioxide emission control. Flue gas
Received 10 May 2001; in final form 14 March 2002.
Address correspondence to T. Kopac, Zonguldak Karaelmas University, Department
of Chemistry, 67100 Zonguldak, Turkey. E-mail: [email protected]
Chem. Eng. Comm.,190: 1041�1054, 2003Copyright# 2003 Taylor & Francis
0098-6445/03 $12.00+ .00
DOI: 10.1080/00986440390207594
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desulfurization is a successful technology developed to reduce sulfurdioxide emissions. Wet scrubbing, spray dry scrubbing, sorbent injec-tion prosesses, regenerable processes, and combined SO2/NOx removalprocesses are some of the several methods of flue gas desulfurization.Sulfur dioxide desulfurization processes using carboneous adsorbents[1�4] and pure or mixed metallic oxide sorbents [1, 5, 6] have beenextensively studied. Many of the literature related with sulfur dioxideadsorption deals mainly with the search or development of an appro-priate adsorbent that possesses the ideal properties for the selectiveadsorption of SO2.
In previous studies on sulfur dioxide adsorption, Kopac et al. [7] usednonisobaric pulse chromatography for the investigation of the adsorptionproperties of sulfur dioxide on molecular sieve 4A, 5A, and AW300-typezeolites in a temperature range 523�718K. Kopac [8] investigatedadsorption equilibrium parameters of sulfur dioxide on molecular sieve13X and activated carbon. In those studies adsorption equilibrium con-stants were calculated using the moment analysis of the chromatographicpeaks, and heats of adsorption of sulfur dioxide were determined on theseadsorbents [7, 8]. Kocabas [9] obtained adsorption isotherms of sulfurdioxide on silica gel and molecular sieve 3A and 4A zeolites at 473K.Kopac and Kocabas [10] studied adsorption equilibrium and break-through analysis for sulfur dioxide adsorption on silica gel. In that studythe recent deactivation model proposed by Suyadal et al. [11] was used forsulfur dioxide breakthrough analysis. It was found that the adsorption ratedata fitted well with this model. Observed adsorption rate constants andthe first-order deactivation rate constants were obtained from the model.
In this work the adsorption equilibrium and breakthrough analysisof sulfur dioxide were investigated on molecular sieve 5A zeolite.
Equilibrium studies on adsorption provide information on thecapacity of the adsorbent. Usually adsorption equilibrium data areexpressed by adsorption isotherms. Adsorption isotherms are char-acterized by certain constants, the values of which express the surfaceproperties and affinity of the adsorbent and can be used to compare theadsorptive capacities of the adsorbent for different pollutants. Theadsorption isotherm models used in this study are the Freundlich,Langmuir, and DRK models [12, 13] as shown in Table II.
There have been various models in the literature for breakthroughanalysis of gas-solid systems [11, 14�19]. Some of the important factorsconsidered in modeling of gas-solid reactions are pore and product layerdiffusion resistances, changes in pore structure, and variation in surfacearea during the reaction [6]. Changes in pore structure, active surfacearea, and active site distribution were expected to cause significantdeactivation of the solid. Deactivation models proposed in the literaturefor gas solid reactions with significant changes of activity of the solid due
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to textural changes and due to product layer diffusion resistance duringthe reaction were quite successful in predicting breakthrough curves.
Orbey et al. [15] used a model that accounts for structural changesfor the breakthrough analysis of solid-gas noncatalytic reactions takingplace in a packed bed reactor. Analytic expressions derived for thebreakthrough curves were successfully used to analyze the kinetics of thereaction between precalcined limestone and sulfur dioxide. The effects offilm mass transfer, pore diffusion, and structural changes on the observedrate were investigated, and it has been shown that the experimentalresults and the predictions from the model agree well. Yas� yerli et al. [17]applied the deactivation model for char gasification with CO2 successfullyand indicated that the deactivation model describes gas-solid noncatalyticreactions more accurately than the unreacted core and volume reactionmodels. Suyadal et al. [11] proposed a deactivation model for theadsorption of trichloroethylene vapor on activated carbon bed and foundthat the deactivation model described the experimental breakthroughcurves more accurately compared to the adsorption isotherms given inliterature. In the model proposed by Suyadal et al. it was assumed thatthe packed bed adsorber could be approximated to a batch-solids reactorwith a plug constant flow of fluid. The deactivation of the adsorbent wasassumed to be first order with respect to the solid surface, and anexponential decrease was assumed with time in its available surface. Theother assumptions made were negligible external mass-transfer limita-tions, and isothermal system, and a pseudo-steady-state throughout theadsorption column. Kopac et al. [18] proposed a unit cell model for thereaction of SO2 with activated soda. The authors considered the changesin pore length and radius with reaction extent and variations in productlayer diffusion resistance.
Yasyerli et al. [19] proposed a deactivation model for the predictionof breakthrough curves in packed adsorption colums. They showed thatthis model gave excellent predictions for hydrogen sulfide breakthroughcurves. In the models of Orbey et al. [15] and Suyadal et al. [11], thedeactivation of the solid was assumed to be independent of position inthe column and independent of concentration of the adsorbing gas. In therecent model of Yasyerli et al. [19] the deactivation model was modifiedby including the concentration dependence of the deactivation term. Withthis modification, position dependence of the deactivation was implicitlyintroduced into the model.
Aim ofThis Study
The aim of this study is to investigate the sulfur dioxide adsorptionisotherms and breakthrough analysis on molecular sieve 5A zeolite. Theexperimental adsorption equilibrium data were compared with various
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adsorption isotherm models existing in the literature such as Freundlich,Langmuir, and Dubinin-Rudukovic-Kaganer isotherm models. For thebreakthrough analysis, deactivation models proposed by Orbey et al. [15],and the recent models of Suyadal et al. [11] and Yasyerli et al. [19], wereused for sulfur dioxide adsorption on molecular sieve 5A.
EXPERIMENTALWORK
Materials
In this work molecular sieve 5A samples of Fluka were used as adsorbentsfor sulfur dioxide adsorption, and 1/16 in. pellets were used in theexperiments. The physical properties of molecular sieve 5A samples aregiven in Table I. In each adsorption experimental run the amount ofadsorbent samples used was 4.22 g of molecular sieve 5A zeolite. Nitrogenwas the carrier gas.
Apparatus
A schematic diagram of the experimental setup is presented in Figure 1.A stainless steel tube (0.95 cm i.d� 10 cm) loaded with about 4.22 g ofmolecular sieve 5A zeolite was used for adsorption experiments. Beforepacking, adsorbent samples were dried at 373K under vacuum for 2 hrand then weighed and packed to columns. The columns were then placedin a tube furnace. A TESTO 350 sulfur dioxide gas analyzer was used.The physical properties of the molecular sieve 5A zeolite samples weredetermined using a 30,000 psi Micromeritics Model 9310 MercuryIntrusion Porosimeter and a Micromeritics ASAP 2000 surface areaanalyzer and are tabulated in Table I.
Procedure
The packed columns were purged with nitrogen for 2 hr at 473K.The experimental procedure consisted of introducing sulfur dioxide-
Table I Physical Properties of Molecular Sieve 5A Zeolite Samples
Porosity 0.78
BET surface area (m2/g) 172.85
Solid density (g/cm3) 1.97
Apparent density (g/cm3) 0.71
Micropore area (m2/g) 143.76
Average pore diameter (A) 20.04
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containing carrier gas on the inlet of the adsorbent packed column andmeasuring the concentration change with time leaving the other end ofthe column by a sulfur dioxide gas analyzer. Experimental runs werecarried out at variable inlet SO2 concentration at a constant temperatureof 473K in the range 0.0618�0.5462 moles/m3 in inert carrier gas.Breakthrough curves were obtained for each different sulfur dioxide inletconcentration. These concentration versus time curves were analyzed foradsorption rates and adsorption isotherm models.
RESULTSANDDISCUSSION
In this study sulfur dioxide adsorption equilibrium and rate on molecularsieve 5A zeolite have been investigated.
Adsorption Isotherms
The SO2 concentration in the gaseous phase measured at the exit of theadsorbent bed with respect to time at 473K temperature for different SO2
inlet concentrations is shown in Figure 2. The concentration of sulfurdioxide in the solid phase (q) was determined according to
q ffiQRt0
ðC0 � CÞdt
mads; ð1Þ
Figure 1. Figure of the experimental system.
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where Q is the volumetric flow rate and mads is the mass of the adsorbent.In this equation the accumulation of sorbing species in the interparticlespace of the bed was neglected. This is an admissible neglection becausethis amount of sorbing species accumulated is usually negligible ascompared with that sorbed in the solid phase. The equilibrium values ofsulfur dioxide concentration on the solid ðqeÞ is given by
qe ffiQR10 ðC0 � CÞdt
mads: ð2Þ
In Figure 3 change in sulfur dioxide concentration adsorbed on mole-cular sieve 5A ðqÞ with time is shown. By using the data given in Figures 2and 3 and Equation (2), the experimental adsorption isotherms have beenobtained and plotted as qe versus Ce as shown in Figure 4. Ce is theequilibrium value of the sulfur dioxide concentration in the fluid phasewhen no change is observed with time, that is, the concentration atinfinite time.
The experimental adsorption isotherms were compared with theadsorption isotherm models existing in literature (see Table II), and theconstants appearing in those models were estimated by the nonlinear least
Figure 2. Variation of sulfur dioxide concentration in the gas phase with time for adsorption
on molecular sieve 5A zeolite at 473K for different inlet sulfur dioxide concentrations in a
nitrogen carrier.
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Figure 3. Variation of sulfur dioxide concentration adsorbed on molecular sieve 5A zeolite
with time for different sulfur dioxide inlet concentrations in a nitrogen carrier at 473K.
Figure 4. Comparison of experimental adsorption isotherms of sulfur dioxide with adsorp-
tion isotherm models on molecular sieve 5A zeolite at 473K.
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squares estimate method. The adsorption isotherm models used are theFreundlich, Langmuir, and DRK models. The values of the constants ofeach model with the square of the correlation coefficients ðr2Þ are tabu-lated in Table II. The Langmuir isotherm gave a fit with the experimentaldata with a correlation coefficient of around 0.811, and the Freundlichisotherm gave a fit with a correlation coefficient of 0.794. The Langmuirand Freundlich equation models are widely used because of theirsimplicity and ability to describe experimental results in a wide range ofconcentrations. The Langmuir model assumes (1) homogeneous surfacesin which the affinity of each binding site for gas molecules is the same and(2) that the adsorbed gas molecules are localized. The Freundlichmodel is an empirical equation based on sorption on a heterogeneoussurface, suggesting that binding sites are not equivalent and/or inde-pendent [12, 13]. The agreement of the experimental data with the pre-dictions of the DRK model, which suggests a pore-filling mechanism foradsorption, were similar with the other two models investigated, whichgave a correlation coefficient of 0.736. However, this model gave a nega-tive model parameter, which is unexpected. Figure 4 shows the compar-ison of the experimental adsorption isotherms of sulfur dioxide withadsorption isotherm models. It can be concluded that the experimentaldata can be represented by both the simple Langmuir and the Freundlichmodels and that a selective surface adsorption mechanism for adsorptionof sulfur dioxide on molecular sieve 5A zeolite can be suggested.
Breakthrough Analysis
The experimental breakthrough curves were compared with the deacti-vation models proposed by Orbey et al. [15], Suyadal et al. [11], and
Table II Adsorption Isotherm Models and Constants for SO2 on Molecular Sieve 5A
Adsorption isotherms Constants and correlation
coefficients
Freundlich isotherm
qe ¼ KFC1=ne KF ¼ 0:000687
n ¼ 1:6088
r2 ¼ 0:7940
Langmuir isotherm
qe ¼ QobCe
1þbCe
Qo ¼ 0:000826
b ¼ 2:3256
r2 ¼ 0:8115
DRK isotherm
qe ¼ a� exp½�b� ln2ðCeÞ�a ¼ 0:000498
b ¼ �0:2366
r2 ¼ 0:7363
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Yas� yerli et al. [19] for sulfur dioxide adsorption rate data on molecularsieve 5A zeolite. Figures 5, 6, and 7 show the experimental and modelresults for different initial sulfur dioxide concentrations in the range0.0618�0.5462 moles/m3 at 473K. A nonlinear least squares estimate wasused to determine the parameters of each model.
Table III shows the parameters of the deactivation model of Orbeyet al. [15] for sulfur dioxide adsorption on molecular sieve 5A zeolite at473K for different sulfur dioxide inlet concentrations. The parameters ofthis model are f0, N, d, and n. f0 is the initial Thiele modulus whenstructural changes are negligible. The values of f0 gives informationabout the relative importance of reaction rate to diffusion rate. Thevalues of d provide information about the ratio of time required to stopthe reaction to the characteristic diffusion time of sulfur dioxide in theparticle, and N is expressed by ðbt=2Þ [15]. t is the time after which nosignificant conversion of the solid reactant occurs, and b is an empiricalparameter. n is a dimensionless parameter, which is the ratio of retentiontime within the bed to the time required for the reaction to stop given byLeb=U0t [15]. The parameters of the model determined by nonlinearregression are f0, N, and d=N. f0 values change in the range0.0389�0.1045 for sulfur dioxide inlet concentrations in the range
Figure 5. Comparison of the experimental results with the deactivation model of Orbey et al.
[15] for sulfur dioxide adsorption on molecular sieve 5A zeolite at 473K for different sulfur
dioxide inlet concentrations.
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0.0618�0.2834 moles/m3. This result shows that the diffusion resistance issmall in the pores. The values of the model parameters at 0.5462 moles/m3 concentration are rather different from the results of the values ofother concentrations. d values determined for sulfur dioxide adsorptionin this work are in the order of magnitude 103�104. In the theory of thismodel the pseudo-steady-state approximation was made [15]. These highvalues of d indicate this approximation to be justifiable. The values of Nlie in the range of 1.181�1.885 for 0.0618�0.2834 moles/m3 sulfurdioxide concentration range. It can be seen from Table III that the fit ofthe model predictions with the experimental data are very good, with r2
values greater than 0.99 at all concentrations. Figure 5 shows theexperimental and model results for different inlet sulfur dioxide con-centrations at 473K.
Table IV shows the parameters of the deactivation model ofSuyadal et al. [11]. Parameters of this model are kst and kd. ks is theadsorption rate constant, t the surface time, and kd is the first-orderdeactivation rate constant. The values of the dimensionless parameterðkstÞ are 2.70�5.1, and kd values are 0.0598�0.20081, respectively, for0.0618�0.5462 moles/m3 concentrations. As seen from Table IV, r2
Figure 6. Comparison of the experimental results with the deactivation model of Suyadal
et al. [11] for sulfur dioxide adsorption on molecular sieve 5A zeolite at 473K for different
sulfur dioxide inlet concentrations.
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values are greater than 0.99 for all sulfur dioxide inlet concentrations.However, we observe changes in the values of the parameters withchange in sulfur dioxide inlet concentration, which is an unexpected
Figure 7. Comparison of the experimental results with the deactivation model of Yasyerli et
al. [19] for sulfur dioxide adsorption on molecular sieve 5A zeolite at 473K for different sul-
fur dioxide inlet concentrations.
TABLE III Parameters of the Deactivation Model of Orbey et al. [15] for Sulfur
Dioxide Adsorption on Molecular Sieve 5A Zeolite at 473 K for Different Sulfur
Dioxide Inlet Concentrations
C
C0¼ eNn sinh fo expð�NyÞ½ �
sinh fo expNðn� yÞ½ �
� �d=N
C0, SO2(mol/m3) N fo d/N r2
0.0618 1.1810 0.0560 4756.7 0.9989
0.0928 1.1950 0.1045 1796.9 0.9961
0.1494 1.7795 0.0504 2493.4 0.9948
0.1752 1.4552 0.0618 3878.7 0.9967
0.2164 1.8845 0.0476 2668.5 0.9947
0.2834 1.7275 0.0389 5339.7 0.9972
0.5462 3.9922 10.821 0.0996 0.9989
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result. Comparison of the experimental results with this model is shownin Figure 6.
In Table V parameters of the deactivation model of Yas� yerli et al.[19] are shown. Parameters of this model determined by nonlinearregression analysis are k0W=Q and kd. k0 is the initial sorption rateconstant, W the adsorbent mass, Q the volumetric flow rate, and kd isdeactivation rate constant. k0W=Q values are in the order of magnitudeof 2.1�2.6, and kd values are 0.0806�0.2782. The values of r2 are
Table V Parameters of the Deactivation Model of Yasyerli et al. [19] for Sulfur Dioxide
Adsorption on Molecular Sieve 5A Zeolite at 473 K for Different Sulfur Dioxide
Inlet Concentrations
C
C0¼ exp
1� exp koWQ 1� exp �kdtð Þð Þ
� �h i1� exp �kdtð Þ½ � exp �kdtð Þ
24
35
C0, SO2(mol/m3) koW/Q kd (min�1) r2
0.0618 2.2370 0.0806 0.9817
0.0928 2.5771 0.1702 0.9881
0.1494 2.1767 0.2279 0.9714
0.1752 2.6114 0.1714 0.9840
0.2164 2.2303 0.1972 0.9720
0.2834 2.3072 0.1216 0.9789
0.5462 2.5780 0.2782 0.9850
Table IV Parameters of the Deactivation Model of Suyadal et al. [11] for Sulfur Dioxide
Adsorption on Molecular Sieve 5A Zeolite at 473 K for Different Sulfur Dioxide
Inlet Concentrations
C
C0¼ exp �kSt expð�kdtÞ½ �
C0, SO2(mol/m3) kSt kd (min7 1) r2
0.0618 3.6100 0.05980 0.9993
0.0928 4.9206 0.12531 0.9940
0.1494 2.7007 0.14949 0.9976
0.1752 5.0989 0.12614 0.9951
0.2164 2.9798 0.13177 0.9943
0.2834 3.6510 0.08705 0.9966
0.5462 4.5766 0.20081 0.9964
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0.972�0.988. Figure 7 shows the comparison of the experimental resultswith the deactivation model of Yasyerli et al.
As seen from the results, the deactivation models tested all gavereasonable fits as far as the r2 values are concerned for sulfur dioxideadsorption rate data at the sulfur dioxide inlet concentration rangeinvestigated. The good fit of the experimental data with the deactivationmodel predictions suggests a significant decrease of activity of the sorbentwith time with respect to probable changes in pore structure, in the activesurface area, and in the active site distribution of the sorbent. In themodels of Orbey et al. [15] and Suyadal et al. [11] the deactivation of thesolid was assumed to be independent of position in the column andindependent of concentration of the adsorbing gas. As far as the r2 valuesare concerned, the results of both models gave very good fits with theexperimental data. However, we do not expect any changes in the modelparameters with change in sulfur dioxide inlet concentration. We observesignificant changes in the values of the parameters of the deactivationmodel of Suyadal et al. [11]. Parameters of the deactivation model ofYasyerli et al. [19], in which the concentration dependence of the deac-tivation term was introduced, gave more reasonable values, in whichchanges that were not very significant were observed with concentration.
ACKNOWLEDGMENTS
This work was funded by Research Fund AFP99-13-02-08 of ZonguldakKaraelmas University. The authors would like to acknowledge ProfessorDr. Guls� en Dogu, Chemical Engineering Department of Gazi Universityand Professor Dr. Timur Dogu, Chemical Engineering Department ofMiddle East Technical University for useful comments and helpful sug-gestions and for the characterization of the adsorbent samples.
NOMENCLATURE
a Constants in adsorption isotherm models
b Constants in adsorption isotherm models
C Outlet sulfur dioxide concentration, mol=m3
C0 Inlet sulfur dioxide concentration, mol=m3
Ce Equilibrium gas-phase concentration, mol=m3
k0 Initial sorption rate constant of Yasyerli et al. [19] model
kd First-order deactivation rate constant, min�1
ks Observed adsorption rate constant, m min�1
Kf Freundlich isotherm constant
mads Mass of adsorbent, g
n Freundlich exponent
N Parameter of Orbey et al. [15] model
Q Volumetric flow rate
Q0 Langmuir equilibrium constant
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q Sulfur dioxide concentration adsorbed on the solid per gram of adsorbent, mol/g
qe Equilibrium solid phase-concentration of sulfur dioxide, mol/g
r Coefficient of correlation
t Time, min
T Temperature, K
U0 Superficial velocity
W Catalyst mass
Greek letters
b Empirical parameter of Orbey et al. [15] model
eb Void fraction of the bed
d Parameter of Orbey et al. [15] model
y Dimensionless time of Orbey et al. [15] model, t/tn Parameter of Orbey et al. [15] model
t Surface-time ( min m�1) of Suyadal et al. [11] model
t Time (of Orbey et al. [15] model) after which no significant conversion occurs
f0 Initial Thiele modulus
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