Determination of Pure and Binary Volatile Organic Compounds (VOCs) Adsorption Equilibria
using the Dynamic Method
Laboratory Of Solid and Hazardous Waste Management
Phyllis Danford
Professors Toshiya Komatsu, Shoichi Fujita, Shuji Himeno
1. Introduction
Removal of volatile organic compounds (VOCs) is of significant interest in air
quality control. Activated carbon adsorption is widely used in the removal of VOCs. To
construct adsorption facilities, data on adsorption equilibrium and knowledge on the actual
adsorption behavioral patterns are needed. By using the dynamic method for adsorption
equilibria experiments these can be derived. Also when developing a mixed VOC component
adsorption equilibria model it is difficult to control the experimental conditions such as
keeping the total pressure constant and controlling the gas phase ratio especially for otherconventional techniques. To overcome these problems the dynamic method was used for theadsorption equilibrium experiments.
In this study an original apparatus for the dynamic method was constructed andexperimental studies were made on the adsorption of pure benzene and toluene and their binary
mixtures at various concentrations by activated carbon. Also experimental and theoreticalcorrelation of the adsorption equilibrium was also carried out using Toth equation andDubinin‑Radushkevich equation for pure component systems and for theoretical prediction of
the adsorption equilibria of binary component systems the Ideal Adsorbed Solution Theory
was used.
2. PureComponent
Adsorption
Equilibria Experiment
2.1 Method
An original apparatus used wasconstructed and is shown in Fig 1.
Diffusion and bubbling were used as
VOC evolution methods. The flow of
the VOC gas was controlled using mass
flow controllers. In the initialstages of the experiment a constantflow and concentration of the VOC gas
was confirmed by directing the flow
Fig.1 Apparatus used in the Dynamic Method
to the sample loop where automatic
sampling was carried out and analyzed by Gas Chromatograph equipped with Flame‑Ionization
Detector (Shimazu GC‑17A). At constant flow rate, constant concentration the VOC gas wasfed to the adsorption column. About 3 g of activated carbon was packed in a stainless steel
column of 1.60 cm ID. The VOC concentration of the outlet gas of the adsorption column wasautomatically analyzed at constant time intervals of 5 to 10 minutes. On completion of theexperiment the activated carbon in the adsorption column was regenerated using flow of purenitrogen at 403 K for 15 hours.
2.2 Results and Discussion
2.2.1 Comparison of Dynamic Method with Headspace Gas Chromatography
The amount adsorbed for the pure component (benzene, toluene) derived from the breakthroughcurves were compared to the amount adsorbed from Headspace Gas Chromatograph (HS‑GC) method 1)

A 2
W = W0 exp[ −()]
βε0

Amount Adsorbed (g/g-AC)

Amount Adsorbed （ｇ/1g AC ）1used in previously in my laboratory(Fig.2) and shows a slight deviation of–0.6〜22 % for benzene and ‑3〜10 % for
toluene, the reliability of theapparatus used in this dynamic method was0.1
confirmed.
Benzene HS-GC
Benzene Dynamic
2.2.2 Correlation using Toth andToluene HS-GC
Dubinin‑Radushkevich Equation
Toluene Dynamic2)
The Toth equation was used to correlate
Toth Equation
the experimental adsorption data for0.01
pure benzene and toluene.
0.00001 0.0001
0.001
0.01
0.1
N = mP/(b+Pt) 1/t
Relative Pressure (-)
N : amount adsorbed、m : saturated amount Fig. 2 Pure Benzene and Pure Toluene Isotherms foradsorbed [mol/g] 、 b : equilibrium
Dynamic and HS-GC Method
constant [(kPa)t ] 、 t :parameter1
indicating heterogeneity of adsorbent[‑]. The experimental adsorption
equilibrium data for pure toluene andbenzene were successfully correlatedwith Toth equation as shown in Fig.2.
0.1
Benzene Dynamic
Also correlation of experimental data
Benzene HS-GC3)
with Dubinin‑Radushkevich equation
Toluene Dynamic
shown below was carried out.
Toluene HS-GC
DR Equation

0.0100.20.4

Adsorption Potential2 (J/mol)2 ×109

0.6

C/Co

W：amount adsorbed [ml/g], W0 ：micropore
Fig. 3 Pure Benzene and Pure Toluene D-R Plot forvolume [mL], A： adsorption potential
Dynamic and HS-GC Method[J/mol], β：affinity coefficient [‑],
ε0：VOCs standard adsorption potential
2.5[J/mol]. Experimental data for pureBenzene
benzene and toluene showed a good
2.0Toluene
correlation with Dubinin‑Radushkevich
1.5
equation as shown Fig.3.
3. Binary Adsorption Equilibrium
1.0Experiment
3.1 Method
0.5
For the binary component adsorption
experiments the apparatus used was0.0
similar to that used in the pure0510
152025component adsorption experiment (Fig.
Time ( hr )1) except for a few alterations. In theFig. 4 Breakthrough Curve for Benzene andVOC solvent evaporator section, theToluene Mixture ( Bz 2 : To 8 )diffusion method was carried out using
the permeator. The two gas lines in thepermeator were used in the evolution of benzene and toluene, which were diluted to desiredconcentrations. The outlet flow of gases were monitored by mass flow controllers and then2.5Benzene
Toluene

C/Co

2.0
1.5
1.0
0.5
0.005Time ( hr )1015Fig. 5 Breakthrough Curve for Benzene andToluene Mixture ( Bz 4 : To 6 )2.5BenzeneToluene

C/Co

2.0
1.5
2

1.0
0.5
0.0
05

1015
Time ( hr )2025

Fig. 6 Breakthrough Curve for Benzene andToluene Mixture ( Bz 5 : To 5 )2.5Benzene
Toluene2.0C/Copassed to the mixing chamber. The binaryadsorption experiments of benzene andtoluene mixtures were carried out at theratios (0.2 〜 0.8) with the total
concentration fixed at 507ppm.
3.2 Results and Discussion
3.2.1 Binary Mixture Adsorption Equilibrium
Breakthrough Curves
The binary mixture adsorption experiments ofbenzene and toluene were carried out andbreakthrough curves are shown in Fig. 4 〜
Fig. 7 . From the breakthrough curves benzene
appears earlier and has a steeper slope
compared to toluene, regardless of the inlet
concentration. The competitive adsorption
pattern was observed, benzene was being
displaced by toluene, which was more thestrongly adsorbed compound causing thebenzene curve to display a “rollup phenomenon”.
The amounts adsorbed are shown in Table 1.
When comparing the amount adsorbed for binary
mixtures and the amount adsorbed for the same
concentration as a pure component calculatedusing the Toth Equation; for benzene theamount adsorbed is drastically decreased whenbeing adsorbed in the presence of toluene.
This is due to toluene displacing the adsorbed
benzene. However the amount of Toluene
adsorbed is slightly lower in the presence ofbenzene because benzene is not completely
displaced by toluene.
3.2.2 Correlation using Ideal Adsorbed
Solution (IAS) TheoryThebinary
adsorption
equilibria
experimental results were compared withresults derived from the Ideal Adsorbed
Solution Theory 4). The IAS theory model, which
is one of the most widely used multicomponent
systems models, is uses only pure component
isotherms for prediction. In this study, theToth equation was selected for calculation ofIAS theory. The basic equation of IAS theory
is similar to Raoult’s law for ideal
solutions:

1.5
1.0
0.5
0.00510
15Time ( hr )2025Fig. 7 Breakthrough Curve for Benzene andToluene Mixture ( Bz 8 : To 2 )Py i = x i P i o (π )Where P is the total pressure, yi is the gas phase mole fraction of component i, x i is theadsorbed phase mole fraction ofTable 1 Benzene and Toluene Mixture Experimental Resultscomponent i, and π? is theBz
ToBzToBz
ToBz
Tospreading pressure of the mixture.
0.2 0.8
0.40.6 0.5 0.5 0.8 0.2
The standard state pressureoAmountP i (π) is defined as the gasAdsorbedpressure when only component i is(g/g-AC) 0.008 0.258 0.012 0.245 0.016 0.201 0.042 0.170
present. This corresponds to the(Dynamicspreading pressure of the mixture.
Method)From the Gibbs isotherm, theAmountspreading pressure can be given Adsorbed
aswhen Pure 0.140 0.274 0.164 0.266 0.172 0.259 0.233 0.190( )oo
Pio n PπiAoii
π == Π i Pi = ∫dPioo0RTPi∗i( )(g/g-AC)(Toth)

Amount Adsorbed (mmol/g)

The comparison of experimental and theoretical4
results is shown in Fig. 8. It can be said that3
the prediction correlates well with theexperimental data for the benzene and toluene
3
mixed adsorption equilibria data. The good2
correlation is due to both benzene and toluene
2
Benzenehaving similar structures and both being
Toluene1
nonpolar compounds, thus the benzene andIAST
1toluene is considered to be an ideal mixture.
04. Conclusion
00.2
0.4
0.6
0.8
1
The original apparatus constructed and theBenzene Mole Fraction in Gas Phase
dynamic method w as successfully carried out for
both pure component and binary component Fig. 8 Comparison of Experimental and IAS
Theory Results
adsorption
equilibria
experiments.
Correlation of pure component experimental
data with Toth and Dubinin‑Radushkevich Equations were in good agreement. From the binary
component experiments, competitive adsorption was observed and the adsorption equilibria
data was successfully derived using the dynamic method. Prediction using the IAS theory forthe binary component was in good correlation with experimental data. It can be concluded
that prediction using the IAS theory for the binary component using the pure component
adsorption data was in good correlation with experimental data. Therefore the results andfindings from this study can be used for designing adsorption facilities.
References:
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pp.2587〜2587, 2002.
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