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CHAPTER 2
LITERATURE REVIEW
Adsorption is defined as a selective concentration or retention of one or more
components from a gaseous mixture on a solid surface. The solid that adsorbs a
component is called the adsorbent, and the component adsorbed is called the adsorbate.
The adsorption phenomenon provides an excellent method of separation particularly at
low concentrations and hence it is recognized as an important mass transfer operation.
There are various adsorption isotherms available which relates the amount adsorbed per
unit mass of the adsorbent to the partial pressure of the adsorbate in the gas phase at
equilibrium, depending upon the nature of adsorbent and adsorbate. Fixed bed adsorption
studies are very important in adsorption which basically tells us the performance of the
adsorbent under various operating conditions such as bed height, flow rate, inlet adsorbate
concentration etc by drawing a breakthrough curve which can be found out
experimentally or can be predicted through mathematical modeling.
This chapter presents a critical review of various works done on the adsorption of
VOCs, together with different theoretical models developed to understand the mechanism
of the removal process. Various adsorbents are used for the adsorption of VOCs having
different characteristics such as particle diameter, pore diameter, surface area and
adsorption capacities. A review of various studies on the VOC adsorption reveals that
most of the works may be categorized mainly in three groups: (1) investigation of various
types of adsorbents like zeolites, clays etc used in the VOC adsorption in which their
adsorption capacities are found out with the help of adsorption isotherms. (2) use of
activated carbon in different forms for VOC adsorption and to find out their performance
and adsorption capacities with the help of breakthrough curves and by various adsorption
isotherms and (3) development of various mathematical models and their solution
techniques used in predicting the behavior of breakthrough curves.
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2.1 Different Types of Adsorbents Used in the VOC Adsorption Anfruns et al. (2011) prepared adsorbents from pyrolysed sewage-sludge
following two different methodologies, namely acid washing and activation with alkaline
hydroxides. Air streams loaded with low concentrations (<100 ppm, v/v) of three VOCs
linked to odor episodes in wastewater treatment facilities (toluene, methylethylketone and
limonene) were used in dynamic adsorption / desorption experiments. Adsorption
capacities as high as 350, 220 and 640 mg of toluene, methylethylketone and limonene
were obtained, respectively, per g of alkaline activated sludges. Desorption experiments
under similar conditions (293 K, 250 ml min-1 of air with 20% relative humidity) showed
that a significant part of the pre-adsorbed VOCs are irreversibly adsorbed. It was also
found that adsorbents prepared by a simple acid-washing of the pyrolysed sludges make
them comparable to highly porous materials including the commercial ACs.
Sone et al. (2008) demonstrated carbon nanotubes with a highly crystalline
structure to be capable of selectively adsorbing aromatic VOCs. Air containing 23 added
VOCs (1,1-dichloroethylene, dichloromethane, trans-1,2-dichloroethylene, cis-1,2-
dichlooethylene, chloroform, 1,1,1-trichloroethane, carbon tetrachloride, 1,2-
dichloroethane, benzene, tichlorothylene, 1,2-dichloropropane, bromodichloromentane,
cis-1,3- dichloropropene, toluene, trans-1,3-dichloropropene, 1,1,2-trichloroethane,
tetrachloroethylene, dibromochloromethane, m-xylene, p-xylene, o-xylene, bromoform,
and p-dichlorobenzene) was studied for model samples. Adsorptive experiments were
carried out by passing the air samples through a cartridge packed with HC- MWCNTs.
The affinity of a VOCs compound for MWCNTs was determined both by its LUMO and
HOMO values. From the molecular orbital point of view, the adsorption of the aromatic
VOCs by HC- MWCNTs can be considered a kind of “soft-chemical bonding”
interaction.
Yang et al. (2011) investigated the adsorption of gaseous VOCs on metal-organic
frameworks MIL-101, a novel porous adsorbent with extremely large Langmuir surface
area of 5870 m2/g and pore volume of 1.85 cm3/g. It was observed that MIL-101 is a
potential superior adsorbent for the sorptive removal of VOCs including polar acetone
and nonpolar benzene, toluene, ethylbenzene, and xylenes. MIL-101 was of higher
adsorption capacities for all selected VOCs than zeolite, activated carbon and other
reported adsorbents. Adsorption of VOCs by MIL-101 is captured by a pore filling
mechanism, showing the size and shape selectivity of VOC molecules.
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Hodar et al. (2007) used thirteen monolithic aerogels with different pore textures
for toluene adsorption. Adsorption was carried out under both static and dynamic
conditions. Under static conditions at 25°C and at saturation, both meso- and micropores
were involved in toluene adsorption. Under these conditions, an adsorption capacity as
high as 1.36 cm3/g or 1180 mg/g was achieved. Toluene adsorption was a reversible
process in all carbon aerogels, and the adsorbed toluene was completely recovered by
heating them at 400°C. Regenerated adsorbents showed larger surface area and micropore
width than the original samples, indicating that no pore blockage was produced.
Adsorption under dynamic conditions at 100°C was also completely reversible after at
least three consecutive adsorption-desorption cycles. The ability of these carbon aerogels
to reversibly adsorb toluene could be useful for their application in thermal swing
adsorption or pressure swing adsorption equipments.
Qu et al. (2009) found Porous clay heterostructures (PCHs) are capable of
adsorbing volatile organic compounds (VOCs). PCH was first synthesized by modifying
bentonite (Bent) with cetyltrimethylammonium bromide (CTMAB) and dodecylamine
(DDA). Adsorption of six volatile organic compounds (VOCs) including acetone,
toluene, ethylbenzene, o-xylene, m-xylene and p-xylene by PCH was investigated.
Adsorption isotherms of these VOCs on PCH, determined at ambient temperature by
gravimetric method, demonstrated different aspects of their adsorption mechanisms.
Based on the qualitative and quantitative results from the proposed multiple linear
regression (MLR) analysis model, enthalpy of vaporization and critical volume were the
most important parameters influencing adsorption capacities of VOCs on PCH. In
general, if only the total adsorption capacities were considered, HOMO energy values
were to be preferred. Thus, PCH had much higher adsorption affinity for aliphatic
hydrocarbon compound such as acetone than that for aromatic compounds due to HOMO
energy levels of VOCs.
Hodar (2011) prepared two series of Pt-catalysts by impregnation or doping of
carbon aerogels and different porous textures and Pt-dispersion were obtained. The
performance of the samples in the elimination of organic compounds (VOCs) by
adsorption and catalytic combustion was studied and compared with the characteristics of
both the VOCs and the catalysts and the interactions between them. Toluene, xylenes and
acetone were selected as representative aromatic or oxygenated VOCs. The adsorption of
aromatic compounds on carbon aerogels was favored by the increasing carbonization
temperature, because this increases the micropore volume and carbon surface becomes
23
more hydrophobic. Oxygenated compounds however were preferably adsorbed on carbon
aerogels obtained at low carbonization temperature. The adsorption capacity was favored
at room temperature by the presence of mesopores while the macroporosity only
contributes to micropore feeding. In dynamic conditions, the adsorption rate was quite
similar in all cases because the micropore size was also similar, as adsorption capacity
was determined by the micropore volume as a function of the adsorption temperature.
Long et al. (2010) studied the removal characteristics of benzene and
chlorobenzene vapor using a microporous hypercrosslinked polystyrene adsorbent (HP
sorbent). The HP sorbent had the similar equilibrium adsorption capacities for benzene
and chlorobenzene vapors with commercial granular activated carbon (GAC). The
breakthrough adsorption capacities of benzene and chlorobenzene in the single and binary
vapor streams were also investigated and they were found to be lower in binary vapor
stream in comparison to single vapor stream. However there was co-adsorption of
benzene and chlorobenzene, and benzene-chlorobenzene mixture removal efficiency of
column adsorption was above 99% before the breakthrough of first component occurred.
Also, a pilot-scale experiment was carried out to investigate the effectiveness of using HP
sorbent to remove benzene-chlorobenzene vapors mixture from industrial byproduct
hydrogen chloride gas. The results showed that the hydrogen chloride gas did not have an
adverse effect on the adsorption of benzene and chlorobenzene. In sum, HP sorbent
should be potentially an effective adsorbent for removal of benzene-chlorobenzene vapors
not only from air stream but also from hydrogen chloride gas.
Zhao et al. (2011) investigated the adsorption and diffusion properties of p-xylene
on the chromium-terphthalate-based crystals (MIL-101) and the surface energy of MIL-
101 by using gravimetric and inverse gas chromatography (IGC) techniques. Adsorption
isotherms and kinetic curves of p-xylene on the MIL-101 were measured at temperatures
of (288, 298, 308 and 318) K and vapor pressures up to 6 mbar, respectively. Results
showed that the amount adsorbed of p-xylene on the synthesized MIL-101 was up to 10.9
mmol/g at 2880K and 6 mbar. The isotherms of xylene on the MIL-101 can be described
favorably by Langmuir-Freundlich adsorption model. The isosteric heat of adsorption (-
s) for p-xylene on the MIL-101 was about 24.8-44.2 kJ/mol.
Zaitan et al. (2008) investigated the adsorptive performance of natural clay of
bentonite type as a potential VOC adsorbent on the basis of its promising physical-
chemical an -Al2O3 solid. The
vapor-solid adsorption isotherms were measured at different temperatures ranging from
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300 to 373 K using dynamical method and the obtained data confronted to classical
models such as Langmuir and Freundlich. It was concluded that the Langmuir model lead
to a good correlation of the data. The capacity of bentonite to adsorb xylene was
-80% of the xylene was
regenerated (desorption) from the adsorbent under nitrogen flow. Test of adsorption of o-
xylene indicated that the bentonite here have good possibilities to be used as adsorbent of
VOCs regarding its performance and lower cost.
Shim et al. (2010) studied the heterogeneous adsorption and catalytic oxidation of
benzene, toluene and o-xylene (BTX) over the spent platinum catalyst supported on
activated carbon (Pt/AC) as well as the chemically treated spent catalysts to understand
their catalytic and adsorption activities. Sulfuric aqueous acid solution (0.1 N, H2SO4)
was used to regenerate the spent Pt/AC catalyst. The experimental results indicated that
the spent Pt/AC catalyst treated with the H2SO4 aqueous solution had higher toluene
adsorption and conversion ability than that of the spent Pt/AC catalysts. The catalytic
activity of H2SO4 treated Pt/AC catalyst for BTX decreased in the order benzene >
toluene > o-xylene. An analysis of the adsorption affinity clearly revealed that aromatic
hydrocarbons were more strongly adsorbed on platinum than that of the activated carbon.
Jarraya et al. (2010) used porous texture of clays for a possible use in the
elimination of environment contamination by the adsorption of several pollutants. The
material used in the study was taken from the Douiret formation in Tataouine in the south
of Tunisia. The adsorption isotherms were of type II of the B.E.T. classification therefore
the adsorption was multi-layered. These isotherms proved that the raw and the Na-clay
materials have a little affinity for the studied VOC adsorption. The adsorption isotherms
of these VOCs on the organoclay showed that the intercalation of didodecyldimethyl
ammonium bromide can significantly improve the toluene, cyclohexane and
chlorobenzene adsorption to reach 2, 2.5 and 3.5 mg g 1 respectively, indicating that this
clay material was more effective than raw and Na-clays. Besides, the comparison of the
chlorobenzene, toluene and cyclohexane adsorption isotherms on zeolyst with those of
organoclay shows that the organoclay adsorption capacities are close to those of zeolyst.
Furthermore, tests of adsorption of chlorobenzene indicated that the clay used in this
study has good possibilities to be used as adsorbent of VOCs regarding its performances
and lower cost.
Benmaamar and Bengueddach (2007) studied the ability of NaY, KY, BaY, BaX
and NaX zeolites to adsorb m-xylene and toluene experimentally. Thus, the adsorption
25
isotherms of two volatile organic compounds, toluene and m-xylene, on NaY, KY, BaY,
BaX and NaX were measured at 298, 308, 318, and 333 K using a vacuum microbalance
system. The toluene and the m-xylene were chosen because they belong to the same
chemical family. The experimental data obtained were correlated with different existing
adsorption isotherm models such as the Langmuir model, the Freundlich model, the
Fowler-Guggenheim model, the Hill-De Boer model and the Sips model. The Langmuir
model was well adapted to the description of m-xylene and toluene adsorption on NaY,
KY, BaY, BaX and NaX zeolites at all four temperature. The Sips model was also found
to be well adapted to describe the adsorption of toluene on to NaY, KY, BaY, BaX and
NaX zeolites at all four temperature. The Freundlich model, the Fowler-Guggenheim
model, and the Hill-De Boer model were not satisfactory. The adsorption affinity of m-
xylene on NaY, KY, BaY, BaX and NaX zeolites was sufficiently greater than the affinity
of toluene. The adsorption affinity of m-xylene and toluene decreased in the following
order NaY>NaX>BaX>KY>BaY. These results demonstrated the high capacity of NaY,
KY, BaY, BaX and NaX zeolites to remove vapors of m-xylene and toluene at very low
concentrations.
Singh et al. (2002) recognized adsorption by an activated carbon fiber as one of
the feasible regenerative control processes to separate and recover VOCs for reuse. The
adsorption behavior of hexane and benzene in a single-component and in a mixture
system onto activated carbon fabric cloth was studied. The data were correlated with
Langmuir and Freundlich adsorption isotherms. The adsorption of n-hexane was found to
be more in comparison to benzene. The activated carbon cloth also worked well for the
mixture of benzene and hexane. Kinetics studies were undertaken to determine various
rate constants. Depending upon the design of the experiment, it was concluded that
internal diffusion control the adsorption of benzene and n-hexane on activated carbon
cloth.
Fuertes et al. (2003) prepared activated carbon fibre monoliths (ACFMs) from the
rejects of polymeric fibres (Nomex). These were carbonized, agglomerated with a
phenolic resin and steam activated at burnoff degrees between 0 and 40%. Adsorption
experiments with n-butane at 30°C showed that, at high adsorbate concentrations, the
amount adsorbed is a function of pore volume, but at low concentrations this mainly
depends on pore size distribution. The porosity of Nomex-based ACFMs is formed by
narrow micropores, which permit higher amounts of vapour to be adsorbed in low
concentrations compared to monoliths prepared from different commercial activated
26
fibres and a commercial granular activated carbon, which exhibits wider pores. The
agglomeration of Nomex-fibres to form ACFMs does not cause any loss in adsorption
properties with respect to non-agglomerated activated fibres. From the adsorption
experiments of different vapours on a Nomex-based ACFM (40% burnoff) it was found
that at high concentrations the adsorbed volume was independent of the nature of the
adsorbate and depended only on pore volume. However, at low vapor concentrations the
amount of adsorbed depended on the adsorbate being well correlated to the molecular
parachor and the polarizability of the adsorbates.
Marban and Fuertes (2004) performed water–n-butane co-adsorption experiments
on Activated Carbon Fibre based Monoliths (ACFMs) under equilibrium and dynamic
conditions at 30°C. ACFMs proved to be better adsorbents than active carbon particles for
the dry adsorption of n-butane at low concentration levels. The presence of water vapor in
the gas stream has a negligible influence on n-butane adsorption for values of relative
humidity (RH) equal to or below 25%. Higher water pressures cause a significant loss of
adsorption capacity in ACFMs.
Solis et al. (2004) studied carbon-coated ceramic monoliths for the dynamic
adsorption of low-concentration n-butane. They exhibit a very sharp breakthrough
performance, especially in the low-concentration regime, that illustrates the better
breakthrough performance of monoliths with respect to shallow carbon packed beds. The
low pressure drop of the monolithic system and its excellent performance under
discontinuous flow conditions make it an attractive option for gas mask canister
applications. A simulation model is presented, including terms of adsorption, diffusion
and mass transfer. A parametric analysis had been carried out to study the influence of
different variables on the breakthrough profile. The breakthrough performance was best
described by considering a gas velocity distribution over the monolith cross-section.
Stacking of the monolithic pieces, increasing cell density, and gas redistribution between
pieces reduced this distribution and improved the breakthrough performance.
Cal et al. (1996) investigated the effects of relative humidity (RH) on the
adsorption of soluble (acetone) and insoluble (benzene) volatile organic compounds
(VOCs) with activated carbon cloths (ACC). A gravimetric balance was used in
conjunction with a gas chromatograph/mass spectrophotometer to determine the
individual amounts of water and VOC adsorbed on an ACC sample. RH values from 0 to
90% and organic concentrations from 350 to 1000 ppmv were examined. The presence of
water vapor in the gas-stream along with acetone (350 and 500 ppmv) had little effect on
27
the adsorption capacity of acetone even at 90% RH. Water vapor in the gas stream had
little effect on the adsorption capacity of benzene (500 ppmv) until about 65% RH, when
a rapid decrease resulted in the adsorption capacity of benzene with increasing RH. This
RH was also about where capillary condensation of water vapor occurs within ACC
pores. Water vapor condenses within the ACC pores, making them unavailable for
benzene adsorption. Increasing benzene concentration can have a significant effect on the
amount of water vapor adsorbed. At 86% RH and 500 ppmv, 284 mg/g water was
adsorbed, while at 86% RH and 1000 ppmv, only 165 mg/g water was adsorbed. Water
vapor was inhibitorier for benzene adsorption as benzene concentration in the gas stream
decreased.
Demeestere et al. (2003) focused on the adsorption of gaseous trichloroethylene,
toluene and chlorobenzene on the photocatalyst TiO2 Degussa P25. An optimized EPICS
(Equilibrium Partitioning in Closed Systems) methodology was used to study equilibrium
partitioning. For the three compounds investigated, equilibrium adsorption was reached
within 60 min of incubation. Adsorption isotherms, determined at a temperature (T) of
298.2 K and relative humidities (RH) of 0.0% and57.8% were found to be linear
indicating that no monolayer surface coverage was reached in the concentration interval
studied (0.02 mg/l g .45 mg/l). Within the linear part of the isotherm, the influence
of both relative humidity and temperature was investigated in a systematic way and
discussed from a thermodynamic point of view.
Mombello et al. (2009) employed a multi-step anodization and leaching process to
produce three-dimensional nanometer scale structured alumina plates, used to adsorb
volatile organic compounds (VOCs) dissolved in liquids and present in a gas phase.
Nanostructured porous anodic alumina (PAA) plates were observed by means of atomic
force microscopy (AFM) and scanning electron microscopy (SEM). After exposure to
VOCs, PAA was analyzed by gas chromatography–mass spectrometry after cryo-
desorption through a thermal desorption unit. A direct comparison between PAA and
other VOC adsorbing/sorpting systems, such as solid phase microextraction (SPME) and
stir bar sorptive extraction (SBSE), was performed. PAA proved to be a suitable and
inexpensive material for the adsorption of VOCs with adsorbing properties comparable to
the more expensive SPME and SBSE.
Bhatia et al. (2009) studied the adsorption behaviors of butyl acetate in air over
silver-loaded Y (Si/Al = 40) and ZSM-5 (Si/Al = 140) zeolites. The silver metal was
loaded into the zeolites by ion exchange (IE) and impregnation (IM) methods. The
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adsorption study was mainly conducted at a gas hourly space velocity (GHSV) of 13000
h 1 with the organic concentration of 1000 ppm while the desorption step was carried out
at a GHSV of 5000h 1. The impregnated silver-loaded adsorbents showed lower uptake
capacity and shorter breakthrough time by about 10 min, attributed to changes in the pore
characteristics and available surface for adsorption. Silver exchanged Y (AgY(IE)) with
lower hydrophobicity showed higher uptake capacity of up to 35%, longer adsorbent
service time and easier desorption compared to AgZSM-5(IE). The presence of water
vapor in the feed suppressed the butyl acetate adsorption of AgY(IE) by 42% due to the
competitive adsorption of water on the surface and the effect was more pronounced at
lower GHSV. Conversely, the adsorption capacity of AgZSM-5(IE) was minimally
affected, attributed to the higher hydrophobicity of the material. A mathematical model
was proposed to simulate the adsorption behavior of butyl acetate over AgY(IE) and
AgZSM-5(IE). The model parameters were successfully evaluated and used to accurately
predict the breakthrough curves under various process conditions with root square mean
errors of between 0.05 and 0.07.
Liu et al. (2009) investigated the adsorption equilibria of trichloroethylene (TCE)
and benzene vapors onto hypercrosslinked polymeric resin (NDA201) by the column
adsorption method in the temperature range from 303 to 333K and pressures up to 8 kPa
for TCE, 12 kPa for benzene. The Toth and Dubinin–Astakov (D–A) equations were
tested to correlate experimental isotherms, and the experimental data were found to fit
well by them. The good fits and characteristic curves of D–A equation provided evidence
that a pore-filling phenomenon was involved during the adsorption of TCE and benzene
onto NDA-201. The characteristic curve and its prediction of TCE and benzene vapors
adsorption calculated by Polanyi-based isotherm modeling have a potential applicability
for field applications. In addition, Yoon and Nelson model could predict the whole
breakthrough curve and provided good correlation of the effects of TCE and benzene
concentration on breakthrough curves of adsorption through a NDA-201 column. The
calculated theoretical breakthrough curves were in agreement with the corresponding
experimental data. This study was found to be very useful for adsorption process design
and hypercrosslinked polymeric resin is a promising adsorbent for removing and
recovering VOCs from polluted vapor streams in the chemical process industries.
Hu et al. (2009) synthesized organofunctionalized SBA-15 materials, including
methyl-SBA-15 and phenyl-SBA-15, by co-condensation method of tetraethyl
orthosilicate (TEOS) and organosilanes such as methyltriethoxysilane (MTES) and
29
phenyltriethoxysilane (PTES) under acidic conditions, and used as adsorbents for the
VOCs abatement. These adsorbents were characterized by powder X-ray diffraction, N2
adsorption/desorption isotherms and FT-IR spectroscopy techniques. The results
indicated that all samples showed a highly ordered two dimensional hexagonal
mesostructure and the organic groups were chemically incorporated into the pore surface
of SBA-15 substrate. The dynamic adsorption behaviors of single VOC component
(benzene) on methyl-SBA-15 and phenyl-SBA-15 adsorbents were evaluated via
breakthrough curves. It was found that the adsorbent with the PTES/TEOS molar ratio at
1:10 had the largest capacity (0.650 mmol/gadsorbent) in the single component dynamic
adsorption and the phenyl groups had stronger attractive interaction with benzene than the
methyl groups. A mathematical model for the organofunctionalized SBA-15 adsorbents
was successfully employed to describe the adsorption breakthrough curves of benzene.
For binary component adsorption, the phenyl-SBA-15 exhibited the -electrons effect
between the adsorbate and the adsorbent, as the phenyl-SBA-15 materials preferred to
adsorb benzene to cyclohexane. The larger dynamic VOCs capacity of the
organofunctionalized SBA-15 materialswas attributed to the synergetic effect between the
amount of organic groups and the total pore volume.
Kim et al. (2008) prepared an inorganic–organic hybrid nanoporous materials by
the co-condensation of 1,2-bis(triethoxysilyl)ethane (BTSE) with 1,3-
bis(triethoxysilyl)benzene (BTSB) or 4,40-bis(triethoxysilyl)-1,10-biphenyl (BTSP). The
nanoporous materials had broad pore-size distribution in the range of mesopore and
macropore. The nanoporous materials prepared by co-condensation of BTSE with more
than 30 wt% of BTSP showed an enhanced removal capability of volatile organic
compounds (VOCs) from air compared to that prepared with BTSE only, probably due to
the affirmative interaction between VOCs and aromatic ring in the nanoporous materials.
The VOC generation from polypropylene/talc composite (PPF) was reduced to around
half when three part of these inorganic–organic hybrid nanoporous material was added
per 100 part of PPF during the melt compounding process as an additive, which suggested
a new application of nanoporous material.
Pires et al. (2001) studied the adsorption of four common volatile organic
compounds (VOCs), namely 1,1,1-trichloroethane, trichloroethylene, methanol and
propanone by the gravimetric method in different cationic forms of Y zeolite and in
pillared interlayered clays (PILCs). The later were pillared with aluminum or zirconium
oxide pillars using as starting materials two types of clays: a natural smectite and a
30
synthetic laponite. In this way the adsorption of VOCs was studied in solids with different
types of porosity, since zeolites and pillared smectites were mainly microporous
materials, but the pillared laponites had a high proportion of mesoporous volume.
Therefore, the adsorption isotherms of the VOCs in the zeolites and PILCs were mainly
of Type I, according to the IUPAC classification, but in the case of the pillared laponites
type II isotherms were found. The absolute values of the amounts of VOCs adsorbed in
the zeolites were about three times higher than that for the pillared smectites. In the case
of the pillared laponites the amounts adsorbed approached those of the pillared smectites
in the region of the low relative pressures and, at higher relative pressures, approached or
in some cases exceeded the amounts adsorbed in Y zeolites. The adsorption isotherms
were analyzed by the Langmuir and the Dubinin-Astakhov equations. The later was able
to reproduce the experimental data.
Table 2.1 lists the variety of adsorbents used for the adsorption of VOCs by
various authors. From the above literature review it is found that variety of adsorbents
such as zeolites, clays, polymeric resins, activated carbon fiber (ACF) are used for the
adsorption of various VOCs such as toluene, xylene, benzene, methyethylketone, ethyl
benzene, trichloroethylene etc. The vapor-solid adsorption isotherms are measured at
different temperatures by using gravimetric method or dynamic method. Then the data
obtained are fitted in different isotherms like Langmuir isotherm, Freundlich isotherm and
Sipps isotherm etc from where the adsorption capacities of different adsorbents are found
out.
Table 2.1: Use of different adsorbents for VOC adsorption Author Name of VOC Adsorbent Anfruns et al. (2011) Toluene, methylethylketone,
limonene Pyrolysed sewage-sludge
Sone et al. (2008) 23 different VOCs Carbon nanotubes Yang et al. (2011) Acetone, Benzene, Toluene MIL-101 Hodar et al. (2007) Toluene Monolithic Aerogels Qu et al. (2009) Acetone, toluene,o-xylene PCHs Hodar et al. (2011) Toluene, Xylene, Acetone Pt-catalysts Long et al. (2010) Benzene, Chlorobenzene HP sorbent Zhao et al. (2011) p-xylene MIL-101 Zaitan et al. (2008) Xylene Natural clay Shim et al. (2010) Benzene, Toluene, Xylene Platinum catalyst supported
on activated carbon Jarraya et al. (2010) Toluene, cyclohexane,
chlorobenzene Clays
31
Benmaamar and Bengueddach (2007)
m-xylene, toluene Zeolites
Singh et al. (2002) Hexane, Benzene ACF Fuertes et al (2003) n-Butane ACFMs Cal et al. (1996) Acetone and Benzene ACC Demeestere et al. (2003)
Trichloroethylene, Toluene, Chlorobenzene
P25
Bhatia et al. (2009) Butyl acetate Zeolites Liu et al. (2009) Trichloroehylene and Benzene Hypercrosslinked
polymeric resin Hu et al. (2009) Benzene SBA-15 Pires et al. (2001) 1,1,1-trichloroethane,
trichloroethylene, methanol and propanone
Zeolites
2.2 Use of Activated Carbon in the VOC Adsorption Chiang et al. (2002b) found that ozonation can modify the surface property of an
activated carbon such as specific surface area, pore volume, and functional group. Results
indicated that specific surface area of an activated carbon increased from 783 to 851 m2/g
due in part to increasing micropores (those below 15 A°). The effect of ozone treatment
on the adsorption of volatile organic compounds was exemplified by methylethylketone
(MEK) and benzene. The adsorption density of MEK and benzene by ozone treated
activated carbon were greater than that by the untreated activated carbon, with MEK
being more adsorbable than benzene. Results of the factorial analysis indicated that
physical characteristics, namely, micropore, BET surface area, pore diameter, micropore
volume play an important role on benzene and MEK adsorption.
Chaisarn et al. (2008) used activated charcoal produced from Para-rubber sawdust
as an adsorbent for adsorbing four different VOCs including toluene, ethylbenzene, p-
xylene and o-xylene present in furniture manufacturing. Activated charcoal was produced
from para-rubber based sawdust. Phosphoric acid was used as activating agent with 2:1
impregnation ratio. BET surface area of activated charcoal produced in this condition was
1635.65 m2/g. Adsorption isotherms were then drawn for these 4 VOCs on activated
charcoal and data was found to be best fitted with Freundlich isotherm.
Cheng (2008) studied series of adsorption experiments (temperature = 27°C) with
granular activated carbon which was used as the adsorbent in an internal circulation
cabinet for storing organic solvents in the laboratories of universities, colleges and
hospitals in Taiwan. It was found that the toluene adsorption capacity, Q (g toluene / g
GAC), can be simulated as the natural logarithm of adsorption time for average toluene
32
concentrations ranging from 102 to 2652 ppm. Additionally, the SPME fiber installed in
the outlet air stream for adsorbing toluene exhausted through GAC effectively indicated
the breakthrough of VOCs.
Pre et al. (2002) investigated quantitative relationships to predict the energetic
interactions resulting from either adsorption or desorption of VOCs onto granular
activated carbon. An experimental database was first built. Heats of adsorption and
desorption were determined onto one activated carbon material for a 40 VOCs panel. The
measurements were performed using differential scanning calorimetry coupled to
thermogravitmetry analysis. Adsorption energies were found to range between 40 and 80
kJ/mol, whereas the desorption energies appear to be about 16% higher.
Mohan et al. (2009) determined the toluene removal efficiency and breakthrough
time using commercially available coconut shell-based granular activated carbon in
packed bed reactor. To study the effect of toluene removal and breakpoint time of the
GAC, the parameters studied were bed lengths (2, 3 and 4 cm), concentrations (5, 10 and
15 mg/l) and flow rates (20, 40, and 60 ml/min). The percentage of adsorption was found
to be maximum at lower flow rates. At higher flow rates the percentage adsorption was
less and breakpoint was reached earlier. At higher concentration the adsorption was lesser
than that at the lower concentration. But the total amount of loading or adsorption
capacity increased with the increase in concentration. Increase in length of the bed
provides a better adsorption percentage and higher breakpoint time.
Rodenas et al. (2006) analyzed adsorption of mixtures of benzene and toluene at
low concentrations on a wide variety of activated carbon with different porosities to
determine the effect of porosity on the adsorption of mixtures. Performance of chemically
activated carbons, physically activated carbon with steam and commercial samples were
studied. The study showed that chemically activated carbons have very high adsorption
capacities for the benzene-toluene mixture. The breakthrough time also varied markedly
for the different activated carbons studied; with values from 19 to 205 min. Porosity was
found to be key factor and those activated carbons with higher volumes of micro pores
exhibit higher adsorption capacities and breakthrough times. Thus, the volume of narrow
micropores is a very important parameter when selecting an activated carbon, both for
adsorption of single VOCs and benzene-toluene mixtures at low concentrations.
Li et al. (2011) treated coconut shells based carbons chemically by ammonia,
sodium hydroxide, nitric acid, sulfuric acid, and phosphoric acid to determine suitable
modification for improving adsorption ability of hydrophobic VOCs on granular activated
33
carbons (GAC). The saturated adsorption capacities of o-xylene, a hydrophobic volatile
organic compound, were measured and adsorption effects of the original and modified
activated carbons were compared. Results showed that GAC modified alkalies had better
o-xylene adsorption capacity. Uptake amount was enhance by 26.5% and reduced by
21.6% after modification by NH3, H2O and H2SO4, respectively. The texture and surface
chemistry of tested GACs varied dramatically after modification by alkalis and acids. The
surface area and pore volume increased and total oxygen containing functional groups
were diminished when treated by alkalis. The opposite was observed for the acid
treatment.
Horng et al. (2008) investigated the adsorption characteristics of chloroform,
acetone, and acetonitrile on commercial activated carbon (C1), two types of activated
carbon fibers (F1 and F2), and sludge adsorbent (S1). The chloroform influent
concentration ranged from 90 to 7800 ppm and the acetone concentration from 80 to 6900
ppm; the sequence of the adsorption capacity of chloroform and acetone on adsorbents
was F2 > F1~ C1 ~ S1. The adsorption capacity of acetonitrile ranged from 4 to 100
mg/g, corresponding to the influent range from 43 to 2700 ppm for C1, S1, and F1. The
acetonitrile adsorption capacity of F2 was ~20% higher than that of the other adsorbents
at temperatures <30°C. The adsorption rate of activated carbon fiber was higher than that
of activated carbon and sludge due to the smaller fiber diameter. In addition, the surface
area of the fibers was higher than those of the others. Results also indicated that sludge
adsorbent could be used for VOC adsorption, especially in more polar adsorbates and at
lower temperatures.
Albero et al. (2009) prepared activated carbons with increasing porosity by
chemical activation of olive stones using ZnCl2 followed by physical activation with CO2.
Ethanol adsorption at 298 K was studied using a series of activated carbons with
increasing burn-off. Breakthrough column experiments showed that the amount of
ethanol adsorbed (g/100 g) increases with the activation degree upto a maximum on
sample AC40 (7.4 g / 100 g AC), the amount adsorbed decresing thereafter. Apparently,
the adsorption of ethanol on activated carbons exhibits a critical micropore size which
favors an optimum packing of adsorbed ethanol molecules, i.e. probably a pore size able
to accommodate two adsorbed layers of ethanol.
Kim et al. (2006) investigated adsorption capacity and desorption characteristics
of impregnated activated carbon (IAC) prepared with various acids and bases in order to
apply to adsorption part of adsorption-desorption and catalytic oxidation hybrid system.
34
Among the prepared IACs, PA/AC showed the greatest adsorption capacity for benzene,
toluene, p-xylene, methanol, ethanol and iso-propanol due to chemical modification of its
surface despite the decreased specific surface area. Also, the maximum BET surface area
with impregnated content of PA/AC showed 1 wt% PA/AC. The amount of VOC
adsorbed on 1 wt% PA/AC was larger than that on purified AC excepting that of o-
xylene, m-xylene, and MEK. The adsorbed toluene and MEK were easily desorbed by
heat treatment to 300°C, suggested the possibility for repeated use. The findings
confirmed the potential of 1 wt% PA/AC as a promising adsorbent for the hybrid system
in controlling VOC emissions with very low concentrations.
Carvalho et al. (2006) prepared granular forms of powdered activated carbons
using clays (a natural montmorillonite and a synthetic laponite) as binders. Two carbons
were essayed: a commercial sample and a carbon obtained by chemical activation of cork
with K2CO3. The later clay was more favorable since it permitted the preparation of more
plastic dough. The extrudates obtained with the commercial carbon, and laponite as
binder, were thermally stable in inert atmosphere up to 600°C, but under air they kept
their textural characteristics only up to 400°C. The granular form obtained with laponite
and the activated carbon obtained from cork, CL-0.25, preserved its textural properties
when calcined under N2 at 400°C, but under air flow, at the same temperature, the
carbonaceous matrix was totally consumed. Extrudates CL-0.25 calcined under N2 at
400°C, having an apparent specific surface area of 808 m2/g, a microporous volume of
0.36 cm3/g were tested in the adsorption of various VOCs. The total amounts adsorbed,
compared favorably with commercial extrudates of activated carbon, and allowed the
conclusion that the extrudates prepared with chemically activated wastes of the cork
industry and laponite have adequate adsorption characteristics to be used in the abatement
and re-use of VOCs.
Shiue et al. (2010) used chemical filters in the cleanrooms of the semiconductor
factories to remove airborne molecular contamination (AMC). Adsorption by activated
carbon as media within the chemical filter was one of the practical methods for removal
of gaseous contamination in a cleanroom. The objective of the study was to evaluate
coconut shell activated carbon adsorbent-loaded nonwoven fabric media performance by
determining the breakthrough curves, the linear driving force (LDF), the intra-particle
diffusion characteristics, the empty bed contact time (EBCT) and the bed depth service
time (BDST), the mass-transfer zone (MTZ), and pressure drop. The testing conditions
were maintained at 28 ± 1°C, and relative humidity at 40 ± 2% with face velocities of
35
0.076, 0.114 and 0.152 m/s for removal efficiency and capacity determination. The
challenge gas concentrations of toluene were fixed at 10, 31, 42 and 70 ppm to accelerate
the breakthrough of media adsorption. Results showed that in the single vapor of toluene
adsorption experiments, the breakthrough time decreased with increasing inlet
concentration of toluene and face velocity. The saturated adsorption ratios rose with
increase in testing concentrations and were also raised with decreased face velocity.
Pei et al. (2011) developed a new “dynamic-volumetric” adsorption test method
combining the advantages of static volumetric method and dynamic breakthrough
method. The adsorption capacity of toluene on activated carbon at low concentration
levels (0.1 – 100 ppm) was obtained by this method at three relative humidity conditions
(20%, 50%, and 80%). The measured adsorption isotherm was analyzed by least-square
regression with Langmuir model, Freundlich model, and D-R model, respectively.
Langmuir model provided the best fit followed by D-R model and Freundlich model. A
linear adsorption isotherm was valid when the concentration was below 1.5 ppm. The
measured data was then used to determine the model parameters (partition coefficient,
mass transfer coefficient and diffusion coefficient) by least-square fitting to a mechanistic
numerical model. The fitted partition coefficients matched with measured values well.
The diffusion coefficients were strongly concentration dependent. For toluene adsorption
on activated carbon, it was in the order of 10-10 – 10-8 m2/s at the concentration range of
0.1 – 100 ppm.
Liu et al. (2011) prepared stainless steel microfibrous entrapped activated carbon
composites by wet layup paper-making and sintering process. The composite beds were
filled with granular activated carbons and micro fibrous composites in the inlet and outlet
of the fixed bed, respectively. The adsorption breakthrough curves of toluene in the
composite bed were measured, and compared with that in the fixed bed with granular
activated carbons (GAC) alone. The effects of different operation parameters such as flow
rate, bed height and inlet concentration on the breakthrough curves of toluene in the
composite bed were investigated. The length of unused bed (LUB) was determined by
analysis of breakthrough curves. The composite bed adsorption system was found to
perform better compared with the individual GAC bed. The breakthrough time of the
composite bed clearly increased and the breakthrough curves were relatively sharper
compared with that of the individual GAC bed at the same volume. The adsorption of
toluene in the composite bed was strongly dependent on the flow rate, bed height and
inlet concentration. The breakthrough time increased with increase in bed height but
36
decreased with increase in both flow rate and inlet concentration. The LUB value of the
composite bed was found to obviously decrease compared with that of the individual
GAC bed at the same volume. The LUB values of composite bed increased with increase
in both flow rate and inlet concentration.
Ramos et al. (2010) prepared activated carbon cloth (ACC) from lyocell, a novel
regenerated cellulose nanofibre fabric, by phosphoric acid activation in inert atmosphere
at two different final thermal treatment temperatures (864 and 963°C). Benzene, toluene
and n-hexane isotherms at 298 and 273 K were then measured. The isotherms of the three
VOCs exhibited a classical type-I shape, characteristic of microporous adsorbents. The
textural parameters calculated from the hydrocarbon isotherms by applying the Dubinin-
Radushkevich model were in good agreement with those evaluated from nitrogen
isotherms for the ACC with the wider microporosity. The adsorption capacity of the ACC
obtained at 964°C was 1.5 times higher than those developed at 864°C. This result was
attributed to the higher specific surface area, average pore diameter and large micropore
volume shown by the former. More-over, the electrical behavior of the ACC indicated
that they have greater potential for electrical applications and, particularly, for in situ
regeneration of the spent adsorbents after being used for VOC removal by the Joule
effect.
Nouri et al. (2004) carried out the adsorption of p-nitrophenol in one untreated
activated carbon (F100) and three treated activated carbons (H2, H2SO4 and Urea treated
F100) at undissociated and dissociated conditions. To characterize the carbon, N2 and
CO2 adsorption were used. X-ray Photoelectron Spectroscopy (XPS) was used to analyze
the surface of the activated carbon. The experimental isotherms were fitted via the
Langmuir homogenous model and Langmuir binary model. The fitted parameters
obtained from Langmuir Equation (homogenous model & binary Langmuir model)
showed that Qmax and adsorption affinity of carbon (K1) depends on the electron density
of the solute and carbon. Different treatment changes the adsorption capacity of the
carbon. In low pH, where the molecular species are dominant, Kl Km and when the
solute is highly ionized Kl Ki .
Chaiya and Boonamnuayvitaya (2003) produced the activated carbon from coffee
residue activated by ZnCl2 impregnation and CO2. The experimental parameters were the
weight ratio of ZnCl2 to coffee residue (2.5, 3.0 and 3.5), CO2 soaking times (2, 3 and 4 h)
and activation temperatures (600, 700 and 800°C). The activated carbon was
37
characterized for mesopore volume by nitrogen adsorption isotherm at 77 K. It was that
the ratio of ZnCl2 to coffee of 3.0:1, CO2 soaking time of 4 hours and activation
temperature of 600° C were the suitable conditions. At these conditions the BET surface
area of the coffee activated carbons were 900 m2/g; total pore volume was 1.01 cc/g and
mesopore content (ratio of mesopore volume to total pore volume) was 92%. The effect
of pore diameter size was tested by the adsorption of various adsorbates (phenol,
methylene blue and erythrosine red). Toluene with concentration range 100 to 740 ppm
was adsorbed on coffee activated carbon. The adsorption capacity of toluene on the coffee
derived activated carbon was superior to that of commercial activated carbon. The
adsorption capacity of toluene vapor increased with increasing of adsorbing time and
toluene concentration. However, the adsorption capacity decreased with increasing
temperature, due to their exothermic heat of adsorption (= -3.49 kcal/mol). Since FT-IR
results demonstrated that the main functional group of coffee activated carbons was C-H
group, therefore the hydrophobicity of the coffee activated carbon could be considered as
main cause.
Yun et al. (1997) did experimental studies on the isothermal fixed-bed adsorption
of benzene, toluene and p-xylene vapors, and their binary and ternary mixtures on
activated carbon. From the breakthrough curve analysis, the equilibrium relationships for
pure-, binary- and ternary- adsorption on activated carbon were obtained at 30°C. It was
found that the adsorption equilibria can be expressed by the Freundlich equation and the
adsorbed solution theory. Also, the experimental breakthrough curves can be predicted by
the mathematical model with the linear driving force (LDF) approximation for
intraparticle diffusion.
Wang et al. (1999) measured the adsorption kinetics of benzene, toluene and their
binary vapor mixtures on Ajax-activated carbon with a differential adsorber bed (DAB)
rig and analyzed using the heterogeneous finite kinetics model. The size distribution of
the slit-shaped micropore (MPSD) and the Lennard– Jones potential theory were
employed to account for the adsorption energetic heterogeneity of the system. This
MPSD was compared with the pore size distribution (PSD) derived from high-pressure
methane adsorption data with the Grand Canonical Monte Carlo (GCMC) technique. It
was found that, with three mass transfer mechanisms being used to describe the uptake in
activated carbon and the Maxwell–Stefan equation being used to describe the bulk phase
diffusion, the finite kinetics model can fit the pure component adsorption kinetics of
38
benzene and toluene and has the capability to simulate the multicomponent adsorption
kinetics of their mixtures on Ajax-activated carbon.
Wen et al. (2011) evaluated the adsorption performances of activated carbon
derived from sewage sludge (ACSS) for gaseous formaldehyde removal compared with
three commercial activated carbons (CACs) using self-designing adsorption and
distillation system. Formaldehyde desorption of the activated carbons for regeneration
was also studied using thermogravimetric (TG) analysis. The porous structure and surface
characteristics were studied using N2 adsorption and desorption isotherms, scanning
electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). At
formaldehyde concentration of 498 mg/m3 and 0.41 mg/m3, the adsorption capacity could
achieve 74.27 mg/g and 7.62 mg/g and the initial removal efficiency was 83.72% and
89.56%, respectively. Compared with another three commercial activated carbons, ACSS
exhibited excellent adsorption performances at both concentrations of 498 mg/m3 and
0.41 mg/m3 owing to its higher surface area and percentage of micropores combined with
hydrophilic functional groups of -OH, -NH2, NO2 and CO. The results showed that ACSS
has excellent adsorption performance, which was overall superior to the CACs.
Adsorption theory indicates that the ACSS outperforms the CACs due to its appropriate
porous structure and surface chemistry characteristics for formaldehyde adsorption. The
TG analysis of desorption showed that the optimum temperature to regenerate ACSS is
75°C, which was affordable and economical for recycling.
Yates et al. (2011) determined the dynamic adsorption capacity for a series of
ceramic composites towards toluene at 30°C. Often activated carbons (ACs) are
employed owing to their large specific surface areas, high micropore volumes, rapid
adsorption capabilities and selectivity towards organic molecules compared to water
vapour or air. However, when large volumes of gas are to be treated pressure drop
limitations may arise from the use of conventional powder adsorption beds. For these
applications conformation of the activated carbon as open channel honeycomb monoliths
can take advantage of the almost null pressure drop caused by these structures. Similarly,
conformation as extrudates or tubes although increasing the pressure drop due to the
turbulent gas flow can improve any diffusion limitations that the open channel monoliths
can suffer. Conformation of the AC as a ceramic composite also improved the handling
characteristics. By the use of a silicate clay binder a commercially available AC, was
conformed in three different monolithic geometries; changing the channel width and the
wall thicknesses and as solid extrudates and tubes. The textural and mechanical properties
39
of these conformed composite structures were also determined. The results showed that
under sever conditions of a 0.25 s contact time the most important feature in limiting the
dynamic adsorption capacity of open channel monolithic adsorption beds was the width
of the open channel. Reduction in the channel width from 2.5 mm to 2.0 mm led to a four
fold increase in the amount adsorbed. However, reduction in the wall thickness had a
negligible effect on the amount adsorbed, indicating that the largest difficulty towards
achieving high adsorption capacities under dynamic conditions was the diffusion of the
gas to be treated into the composite. For solid extrudates and hollow tubes of the same
external diameter the greater geometric area of the latter led to a doubling of the
adsorption capacity.
Chiang et al. (2002a) investigated the adsorption of volatile organic compounds
(VOCs), exemplified by benzene and methylethylketone (MEK), onto seven different
types of activated carbon. Thermodynamic parameters of the adsorption of benzene and
MEK onto seven activated carbon samples were studied. Results indicated that the
adsorption energy of benzene were greater than that of MEK for the same adsorbent.
Based on the energy distribution of benzene adsorption, it was concluded that the lower
the adsorption energy the sharper was the energy distribution. Hence benzene was easily
adsorbed by activated carbon than MEK.
Chiang et al. (2001) investigated the pore structure of three activated carbons and
determine the temperature dependence of the adsorption of VOCs onto activated carbon.
Three kinds of activated carbon made of different raw materials and four VOC species
were chosen. The microporosity of activated carbon was assessed by the pore size
distribution. The adsorption of VOCs showed that only C6H6 exhibited the activated entry
effect. The VOC adsorption C6H6 capacity of peat-derived carbon was less dependent on
temperature. A characteristic curve was observed for the peat-derived carbon. Benzene
adsorption was the most preferable compared to other three VOCs because of higher
heats of adsorption and lower entropy change. Results indicated physical adsorption
played a critical role during adsorption processes in this study system.
Oh et al. (2010) adsorbed volatile Organic Compounds (VOCs) such as methanol,
ethanol, methyl ethyl ketone, benzene, n-propanol, toluene, and o-xylene were adsorbed
in a laboratory-scale packed-bed adsorber using granular activated carbon (GAC) at 101.3
kPa. The adsorber was operated batchwise to obtain the breakthrough curves of VOCs
under the adsorption conditions such as adsorption temperatures (298-323 K), flow rates
of nitrogen (60×10-6- 150 × 10-6 m3/min), GAC amount of 0.002 kg, and concentration of
40
VOCs (3,000-6,000 ppmv). The adsorption kinetics was obtained by fitting the
experimental breakthrough data to the deactivation model, combining the adsorption of
VOCs and the deactivation of GAC. The adsorption isotherm, and adsorbed amount and
adsorption heat of VOCs were obtained using the breakthrough curve: the former for
comparison with the conventional isotherm models, the latter for correlation with the
physical properties of VOCs.
Table 2.2 lists the use of activated carbon in different forms for VOC adsorption.
From the above literature review it is found that activated carbon obtained from various
sources such as para-rubber saw dust, coffee residue, and coconut shell, commercial and
in various forms such as granular, powdered, impregnate are extensively used as an
adsorbent for VOC adsorption. Adsorption capacity of activated carbon derived from
various sources are found to be high due to their larger surface area which makes them
suitable for VOC adsorption.
Table 2.2: Use of different forms of activated carbon for VOC adsorption Author Name of VOC Adsorbent Chiang et al. (2002b) Methylethylketone and
benzene Activated Carbon
Chaisarn et al. (2008) Toluene, ethylbenzene, p-xylene and o-xylene
Activated charcoal produced from Para-rubber sawdust
Cheng (2008) Toluene Granular activated carbon Pre et al. (2002) 40 VOCs Granular activated carbon Mohan et al. (2009) Toluene Coconut shell-based granular
activated carbon Rodenas et al. (2006) Mixture of benzene and
toluene Activated carbon
Li et al. (2010) o-xylene Granular activated carbon Horng et al. (2008) Chloroform, acetone and
acetonitrile Activated carbon
Albero et al. (2009) Ethanol Activated carbon Kim et al. (2006) Benzene, toluene, p-xylene,
methanol, ethanol and iso-propanol
Impregnated activated carbon
Carvalho et al. (2006) Variety of VOCs Granular forms of powdered activated carbons using clays
Shiue et al. (2010) Toluene Coconut shell activated carbon Pei et al. (2011) Toluene Activated carbon Liu et al. (2011) Toluene Stainless steel micro fibrous
entrapped activated carbon Ramos et al. (2010) Benzene, toluene and
xylene ACC
Nouri et al. (2004) p-nitrophenol Activated carbon
41
Chaiya and Boonamnuayvitaya (2003)
Toluene Activated carbon from coffee residue
Yun et al. (1997) Benzene, toluene and xylene
Activated carbon
Wang et al. (1999) Benzene and toluene Ajax-activated carbon Wen et al. (2011) Formaldehyde Activated carbon derived from
sewage sludge Yates et al. (2011) Toluene Activated carbon Chiang et al. (2002a) Benzene and
Methylethylketone Activated carbon
Oh et al. (2010) Methanol, ethanol, methylethyl ketone, benzene, n-propanol, toluene and o-xylene
Granular activated carbon
2.3 Various Mathematical Models Used in the Adsorption Zhang et al. (2011) considered carbon fixed beds as an inexpensive and highly
effective way for controlling chlorofluorocarbon (CFCs) emissions. In this work, a
dynamic model under constant-pattern wave conditions was developed to predict the
breakthrough behavior of trichlorofluoromethane (CFC-11) adsorption in a fixed bed
packed with activated carbon fibers (ACFs). With the Langmuir isotherm, the analytical
solution of a dynamic model based on constant-pattern wave approach had been derived
with the dynamics obtained by fitting the breakthrough curves. The effects of the packed
bed height, gas flow rate and initial CFC-11 concentration on the breakthrough curves
were investigated experimentally. The results showed that, in a deep bed (> 120 mm), the
analytical model based on the external mass transfer with the Langmuir isotherm could
describe the adsorption dynamics well. The characteristic breakthrough time, t0, and
extraparticle transfer coefficient, kf, were determined by curve-fitting of the model to the
experimental breakthrough data. It was found that the mass transfer from fluid phase to
the fiber surface dominated the CFC-11 sorption onto ACFs in fixed beds because CFC-
11 molecules need not pass through the macropores and mesopores to reach the
adsorption sites in the micropores. The breakthrough behavior showed that t0 decreased
with increasing the gas flow rate and feed concentration and increased with increasing the
bed height.
Chuang et al. (2003) used a BDR model to assess the effect of temperature on the
adsorption and desorption of three VOCs. Compared with Langmuir isotherm and LDF
(with Langmuir) model, the BDR model showed the same trend with both of them, such
42
as CCl4 had a larger ka/kd (BDR model) and KL (Langmuir isotherm) and k (LDF with
Langmuir) than C6H6 and CHCl3, and the ka/kd values of CCl4 and C6H6. Based on these
parameters adsorption isotherms and breakthrough curves were predicted under various
operational conditions. Both experimental and model-predicted data indicated that both
adsorption and desorption rate constants increased whereas equilibrium adsorption
constants decreased under high reaction temperatures.
Gironi et al. (2011) reported original experimental data from the adsorption of
vapors mixtures of MTBE, cyclohexane and air onto commercial activated carbon.
Equilibrium adsorption data for MTBE + air and cyclohexane + air systems had been
obtained from breakthrough curves and then correlated by means of the Langmuir
isotherm. Furthermore, the pseudoternary system MTBE + cyclohexane + air had been
studied and the experimental competitive equilibrium data had been successfully
compared with those obtained from theory of adsorption for an ideal solution. The results
showed that MTBE adsorption capacity of activated carbon was strongly in the presence
of cyclohexane.
Yaneva et al. (2008) studied the adsorption of two substituted nitrophenols,
namely 4-nitrophenol (4-NP) and 2,4- dinitrophenol (2,4- DNP), from aqueous solutions
onto perfil using a fixed bed column. The theoretical solid diffusion control (SDC) model
describing single solute adsorption in a fixed bed based on the Linear Driving Force
(LDF) kinetic model was successfully applied to the investigated systems. The model
parameters of solid diffusion coefficient, Ds, axial dispersion coefficient, DL, and external
mass transfer coefficient, kf, for the investigated systems were estimated by the means of
a best fit approach. Some deviations were found between the predicted and the
experimental data which reflected the fact that the assumptions of the model were not
quite fulfilled for these experiments. It was thus necessary to adjust the values of the solid
diffusion coefficient, the axial dispersion coefficient and the external mass transfer
coefficient to obtain satisfactory agreement between the simulated and the experimental
breakthrough curves.
Aribike and Olafadehan (2008) investigated mathematical modeling of liquid
phase adsorption of phenols in fixed beds of granular activated carbon. The model
considered the effects of axial diffusion in the fluid, the external film and internal
diffusional mass transfer resistances of the particles, and the nonlinear adsorption
isotherm of Freundlich. It was shown that the analysis of a complex multicomponent
adsorption system could be simplified by converting it into a pseudo single-component
43
adsorption system. This was achieved by lumping the concentrations of the components
together as one single parameter, chemical oxygen demand. The resulting model
equations were solved using the orthogonal collocation method and third-order semi-
implicit Runge–Kutta method combined with a step-size adjustment strategy. Excellent
agreement between simulated results and pilot plant data was obtained. Also, the
breakthrough profiles revealed the formation of a primary monomolecular layer on the
adsorbent surface.
Xiang et al. (2008) investigated the adsorption of dibenzofuran (DBF) on three
commercial granular activated carbons (GAC) to correlate the adsorption equilibrium and
kinetics with the morphological characteristics of activated carbons. Breakthrough
experiment was conducted to determine the isotherm and kinetics of dibenzofuran on the
activated carbons. All the experiment runs were performed in a fixed bed with a process
temperature of 368 K. The effects of adsorbent morphological properties on the kinetics
of the adsorption process were studied. The equilibrium data were found satisfactory
fitted to the Langmuir isotherm. An intraparticle diffusion model based on the obtained
Langmuir isotherm was developed for predicting the fixed bed adsorption of
dibenzofuran. The results indicated that this model fit all the breakthrough curves well.
The surface diffusion coefficients of dibenzofuran on the activated carbon were
calculated, and a relationship with the microporosity was found. As it was expected, the
dibenzofuran molecule found more kinetic restrictions for the diffusion in those carbons
with narrower pore diameter.
Shim et al. (2006) investigated structural characteristics of pelletized MCM-48
using X-ray diffraction, nitrogen adsorption and desorption. The adsorption equilibrium
data were also obtained for seven pure vapors (acetone, benzene, cyclohexane, hexane,
methanol, MEK and toluene) at 300.15 K for different pelletized MCM-48 using a
gravimetric method. From experimental and theoretical works on adsorption of nitrogen
and VOCs vapor, the following conclusions were reached. The BET surface area and the
pore volume were highly dependent on the pelletizing pressure, while the average pore
diameter remains unaffected with increasing pelletizing pressure up to 400 kg/cm2. The
single species adsorption isotherm data showed typical type IV of IUPAC classifications.
The adsorption amount of VOC was reduced by around 70% from the parent sample to
the maximum sample pressed at 500 kg/cm2. Adsorption equilibrium data measured at a
wide range of pressures were well fitted to two hybrid isotherm model equations
(Langmuir–Sips and inhomogeneous DA). In accord with the column dynamic
44
experiments, the patterns of adsorption breakthrough curves were highly influenced by
the influent concentration and pelletizing pressures that were closely related with the
adsorption isotherm shape. The determined mass transfer coefficients of VOCs on
pelletized MCM-48 showed that the minimum values correspond to the capillary
condensation region. Furthermore, the simple dynamic model employed successfully
simulated adsorption breakthrough behavior of VOCs under various operating conditions.
Silva et al. (2005) presented the modeling of an adsorption column of fixed bed
for the n-pentane separation of a mixture of n-pentane, iso-pentane and nitrogen,
considering the system non-isothermal and non-adiabatic. The mathematical model
equations of the mass and heat transfer in the adsorption column were presented, as well
as the boundary and initial conditions. The volume finite method was used in the
discretization of the equations to get the system of algebraic equations and posterior
development of the computational algorithm. The partial pressure influence of the n-
pentane (0.19, 0.09 and 0.04) was analyzed in the breakthrough line and in the
temperature profile along adsorption column. For each analyzed case, the maximum error
obtained in the numerical temperature values for 12 cm of the inlet comparatively at the
experimental data was of 0.52%, while that for the outlet temperature in the column was
of 1.86%. The developed methodology predicted with good precision the concentration
profile of the temperature and chemical species of interest inside of the adsorption
column of fixed bed.
Bautista et al. (2003) carried out kinetic studies on the adsorption of -amylase
from Aspergillus oryzae on the anion exchanger, Duolite A-568, and the hydrophobic
resin, Duolite XAD-761 in a fixed bed. The efficiency with respect to the adsorbate and
adsorbent was determined to estimate the performance of the process of adsorption. The
effect of flow rate, -amylase inlet concentration, temperature, and particle size of the
packing was analyzed. In addition, the transient response, in the form of experimental
breakthrough curves, was fitted using a phenomenological mathematical model
accounting for the external-film and pore-diffusion mass-transfer mechanisms as well as
axial dispersion along the column. Also, a Langmuir isotherm was included in the model
to account for the extent of the equilibrium of adsorption. The developed mathematical
model was solved by using orthogonal collocation technique. The model reproduced
adequately the experimental results once the best-fitting parameters were attained. The
model described satisfactorily the experimental breakthrough curves and it proved to
predict successfully the behavior of a scaled-up bed using the kinetic and equilibrium
45
parameters estimated previously. The shape of the breakthrough curves was sensitive to
changes in both the pore diffusion coefficient and the external-film mass-transfer
coefficient. This showed the control of both mass-transfer mechanisms. Thus, the
mathematical model can be considered a reliable tool for process design and scale-up of
similar systems.
Joly and Perrard (2009) considered the models for the dynamic adsorption of
volatile organic compound (VOC) traces in air. They were based on the linear driving
force approximation associated with various adsorption isotherms characteristic of the
couple VOC-adsorbent (Langmuir, Freundlich, Type V and Dubinin–Astakhov (D–A)).
The occurrence of constant pattern breakthrough curves made easier the prediction of
breakthrough time, i.e. time necessary for the pollutant concentration at the column outlet
to reach a given fraction of its inlet value (e.g. 2%). A necessary and sufficient condition
has been given for the existence of such a constant pattern and has been applied to various
models associated to the considered adsorption isotherms. Theoretical results have been
illustrated by numerical simulation based on a finite element method, which provided a
set of typical behaviours of breakthrough curves with the increase of bed length. For
practical applications, the aim was to achieve breakthrough times as long as possible for a
given pollutant. This goal can be reached by choosing an adsorbent with a large
adsorption capacity and able to yield the steepest possible constant pattern breakthrough
curves.
Hwang et al. (1997) did an experimental and theoretical study of the adsorption
and regeneration of methylene chloride vapor in a fixed bed of activated carbon, using
nitrogen carrier gas. A non equilibrium, non adiabatic mathematical model was developed
to calculate concentration and temperature curves for both adsorption and regeneration
runs. In this mathematical model, there were five partial differential equations (PDEs) for
mass and energy balance within the packed bed. The PDE representing the packed bed
dynamics were solved by numerical method of lines. The PDEs were reduced to ODEs
and then solved by using Gear’s method. A linear driving force mass transfer model was
found to be an acceptable fit to the experimental data. Experimental and modeling results
were used to study the effects of operation variables such as purge temperature, initial bed
temperature, and feed concentration of adsorption step. The adsorption and regeneration
processes of the methylene chloride vapor and activated carbon system were found to be
intra-particle mass transfer controlled, and the surface diffusion was the dominant intra-
particle mass transfer mechanism.
46
Serbezov and Sotirchos (1997) developed a general dynamic model describing the
adsorptive separation of mulicomponent gaseous mixtures. The dusty gas model and
D’Arcy’s law were used to describe the diffusive and viscous mass transport in the
adsorbing bed, respectively, and the local equilibrium assumption or the linear driving
force approximation were used for the uptake rate. The relative importance of the
diffusive (bulk and Knudsen) and viscous mass transport and the effects of the different
uptake rate representations were also investigated. For the PSA process, it was found that
viscous transport dominates in the adsorbing bed and the inclusion of other modes of
transport in the model equations has practically no effect on the solution. However, for
dynamic processes occurring in porous media of smaller pore sizes both the viscous and
the diffusive modes of transport must be included in the overall model to predict the
system behavior correctly.
Rivero et al. (2002) focused on the analysis and modeling of styrene drying, raw
material in the manufacture of synthetic rubber, by means of adsorption onto activated
alumina. Equilibrium experiments, carried out under isothermal conditions at 10°C,
correlated to the equation q (kg/kg) = 2.659×10 4 C (mg/kg). Experimental breakthrough
curves were determined at different flow rates in the range 0.50–1.98 l/h and for different
bed depths (62.5–250 g of alumina). A mathematical model including film and pore
resistances to mass transfer was developed and correlated to the experimental data
obtaining the best fitting based on the minimum value of the weighted standard deviation
when the design parameters took the values Dp= 6.101×10-9 m2/s and kf was calculated
from the correlation of Wilson–Geankoplis Sh=1.09 eRe0.33Sc0.33. The parameters
obtained for the design of a styrene drying led to the conclusion that the main controlling
resistance to mass transfer lies within the particle.
Borba et al. (2006) investigated the nickel(II) ions biosorption process by marine
algae Sargassum filipendula in a fixed bed column for the following experimental
conditions: temperature = 30°C and pH = 3.0. The experimental breakthrough curves
were obtained for the following chosen flow rates 0.002, 0.004, 0.006, and 0.008 L/min.
A mathematical model was developed to describe the nickel ion sorption in a fixed bed
column. The model of three partial differential equations (PDE) had considered the
hydrodynamics throughout the fixed bed column as well as the sorption process in the
liquid and solid phases. The internal and external mass transfer limitations were
considered, as well. The nickel ion sorption kinetics had been studied utilizing the
Langmuir isotherm. The PDE of the system were discretized in the form of ordinary
47
differential equations (ODE) and were solved for the given initial and boundary
conditions using the finite volume method. A new correlation for external mass transfer
coefficient was developed. Some of the model parameters were experimentally
determined ( , dp) where the others such as (KF, KS) were evaluated on the base of
experimental data parameters. The identification procedure was based on the least square
statistical method. The robustness and flexibility of the developed model was checked out
using four sets of experimental data and the predictive power of the model was evaluated
to be good enough for the all studied cases. The developed model can be useful tool for
nickel ion removal process optimization and design of fixed bed columns using biomass
of S. filipendula as a sorbent.
Grande et al. (2006) measured adsorption equilibrium of propane and propylene in
a honeycomb monolith where zeolite 4A crystals were mixed with an inert material by
extrusion. The data were collected at 423 and 473K and in a pressure range from 0 to 100
kPa. Adsorption equilibrium was measured using a gravimetric Rubotherm unit, and
breakthrough curves were also determined for comparison. Two different mathematical
models were developed. The first one was a complete bidisperse model (axial dispersion,
macro and micropore resistances) retaining the 3D description while the second one is a
more simplified description of the bidisperse model, considering variations only in the
axial direction of the fluid and the radial direction in the solid. In this second model, also
the effects of non-linear isotherm and energy balances (gas and solid) were incorporated.
Both models were solved using gPROMS. The orthogonal collocation method on finite
elements was used in all cases with two interior collocation points per element. A
comparison between these models was performed indicating that for propylene adsorption
in zeolite 4A honeycomb monolith, the simplified model could be used without losing
accuracy but decreasing drastically the computing time.
Park (2002) analyzed the fixed bed adsorption kinetics to test the validity of the
simplified model based on the linear driving force approximation by comparison with the
exact model by using the orthogonal collocation method. The axial dispersion, the
external film diffusion, and the intraparticle diffusion were considered to be the major
mass transfer phenomena involved with the fixed bed adsorption kinetics in this study. It
was assumed that a local equilibrium was attained at the fluid-solid interface and the
equilibrium can be described by the Langmuir isotherm. A homogeneous particle
diffusion model was employed to describe the intraparticle diffusion. Among four LDF
models cited in the present study, the model which was based on the parabolic
48
concentration profile in the particle, showed to be best agreement with the exact model.
The LDF model based on the parabolic concentration profile deviated to some extent
from the exact model in shorter bed. However, the deviation became negligible in a
sufficiently long bed, in which the intraparticle concentration profile approaches a
symmetric form. As the intraparticle diffusion resistance becomes relatively less
important compared to both the axial diffusion resistance and the external diffusion
resistance, the LDF model based on the parabolic concentration profile approximates the
exact model more closely.
Sridhar (1996) achieved affinity separation by batch and fixed bed modes.
Mathematical models including film mass transfer, intraparticle diffusion, and reversible
reaction were formulated for both fixed bed and batch adsorption processes. Orthogonal
collocation method was used to numerically evaluate the performance of batch and fixed
bed adsorption. Nine collocation points in the column (0, 0.02545, 0.12923, 0.29708, 0.5,
0.70292, 0.87077, 0.97455, 1.0) and six collocation points (0.24929, 0.48291, 0.68619,
0.84635, 0.95331, 1.0) for the particle were used for fixed bed simulation studies. The
efficiencies were evaluated in terms of both the solute recovery and adsorbent utilization.
The effect of the following parameters was simulated for the comparison purpose: solute
concentration, reaction kinetics, ligand content, and particle size. Fixed bed model was
found to be efficient.
Cheng et al. (2004) proposed a new model to describe the removal of volatile
organic compounds (VOCs) from a gas stream passing through a bed packed with
activated carbon fibers (ACFs). Toluene was used as the test compound. Both pore
diffusion and surface diffusion were considered in the model. The equilibrium behavior
was shown to fit the Dubinin–Radushkevich isotherm with the values of parameters K
and W0 of 1.101 × 10 9 and 57.73 kg/m3, respectively. Mathematical model was also
developed which was numerically solved by using finite difference technique. The
experimental results showed that this model can predict VOC breakthrough curve very
well.
Gupta et al. (2004) carried out the adsorption experiments under dynamic
conditions for the removal of trace sulfur-dioxide (SO2) in nitrogen by 5A zeolites. The
experiments were conducted to characterize the breakthrough characteristics of SO2 in a
fixed bed under different operating conditions including temperature, pellet size,
concentration levels, and gas flow rate. At a reaction temperature of 70.8°C, the
breakthrough time was found to be maximum. The adsorption isotherm was found to be
49
linear over the gas concentration range from 1000 to 10000 ppm. The exothermic heat of
adsorption assuming Arrhenius type of temperature dependence of the equilibrium
constant was determined to be 9.8 kcal/mol. The mathematical model was developed to
predict the breakthrough profiles of SO2 during adsorption over the biporous zeolites
(containing both macro and micro-pores). The model incorporates all resistances to mass
transfer, namely: diffusion in the gas film around pellets in the bed, diffusion in the
binder-phase of zeolites and within the crystals, and adsorption/desorption at the interface
of binder-phase and crystals. The model was solved by converting the partial differential
equations into set of ordinary differential equations by finite difference method. The
model was successfully validated with the observed experimental breakthrough data. The
study showed potential application of 5A zeolites in controlling SO2 emissions at trace
levels.
Marban et al. (2006) proposed a model to predict the breakthrough profile of the
adsorption n-butane by microporous fibrous adsorbents (ACFM). The model included
non-instantaneous adsorption at the external surface and surface diffusion inside the pores
of the fibers. It was proved that the external surface resistance to adsorption for the lower
concentrations cannot be neglected. The selection of the appropriate expression for the
isotherm to be employed in the model was a key issue to obtain a suitable fit between the
experimental data and the model, especially for the low-concentration experiments. The
model solved for non-instantaneous surface adsorption and D–R-based surface diffusivity
in the pore system adequately simulated the breakthrough profiles for adsorption of n-
butane in diluted streams, since a good agreement between the experimental and the
simulated data is obtained for the whole profiles at different values of n-butane
concentration in the inlet gases.
San et al. (1998) performed a simulation of the performance of an activated
carbon packed-bed system for adsorption of toluene from air. For a non-switched
operation the time-varying exit toluene concentration of a 30 mm-depth bed was
measured. The simulation result was compared with the measured data. The computer
analysis was based on a modified solid-side resistance model which was originally
proposed by Pesaran and Mills. The effects of cycle operating time, regeneration
temperature and Ntu on the adsorption performance were investigated. The maximum
removal and the corresponding optimum cycle time were obtained. The influence of the
number of grid points on the accuracy of the numerical scheme was discussed. The
50
effects of tortuosity factor and transfer coefficients on the adsorption were also
investigated.
Siahpoosh et al. (2009) prepared an adsorption package to simulate adsorption /
desorption operation for both single and multi-component systems in an isothermal
condition by different mechanisms such as; local adsorption theory and mass transfer
resistance (rigorous and approximated methods). Different mass transfer resistance
mechanisms of pore, solid and bidispersed diffusion, together with nonlinear isotherms
(Langmuir, Frendlich, Sips and Toth) were taken into account in modeling the fixed bed
adsorbers. The Extended Langmuir isotherm was found to explain properly the binary and
ternary mixtures in adsorption/desorption process. Almost all the mass transfer
approximations were explained by the linear driving force, LDF, although the alternative
driving force, ADF, approximation was examined in some cases. The numerical solution
was the Implicit Method of Lines which converted the partial differential equations to the
ODEs then solving them by the Runge-Kutta method. Validation of the models was
performed by the experimental data derived from the literature for different types of
adsorbents and adsorbates. The sensitivity analyses was carried out to find out variation
of the breakthrough curves against some physical and operational parameters such as;
temperature, flow rate, initial and inlet concentration and particle adsorbent size. The
results revealed excellent agreement of simulated and previously published experimental
data.
Table 2.3 shows the use of different techniques used by different authors in
solving partial differential equations. Form the above literature review it is found that
various mathematical models are developed incorporating external film and pore
resistance as a main mass transfer phenomenon for predicting the breakthrough curves.
Then the resulting partial differential equations are discretized by different authors by
using various techniques such as finite difference method, finite element method and
orthogonal collocation method. Then the resulting ordinary differential equations are
either solved by Runge-Kutta method or by Gears method. Finally the predicted
breakthrough curves are compared with the experimental breakthrough curves and then
the effect of various operating parameters such as bed height, flow rate, inlet adsorbate
concentration etc on breakthrough curves are found out. Also the rate-controlling step is
identified which is generally found to be either external mass transfer or intraparticle
diffusion step.
51
Table 2.3: Different solution techniques used in solving partial differential equations Author Solution Technique Used Aribike and Olafadehan (2008)
Orthogonal collocation method and third order semi-implicit Runge-kutta method
Silva et al. (2005) Volume finite method Joly and Perrard (2009) Finite element method Grande et al. (2006) Using gPROMS Sridhar et al. (1996) Orthogonal collocation method Gupta et al. (2004) Finite difference technique Siahpoosh et al. (2009) Implicit method of lines
Hence from the literature review it is evident that granular activated carbon is one
of the most promising adsorbent which is extensively used for VOC adsorption. In this
thesis, both experimental and theoretical studies have been carried out to determine the
breakthrough characteristics granular activated carbon during adsorption of VOCs under
varying operating conditions. The data on systems using toluene and xylene is relatively
scanty. The data is essential for design of such adsorption systems both for single
components and binary. While in the present studies individual components have been
taken to validate the proposed model. A detailed description of the theoretical analysis
and mathematical model, to understand the mechanism of process is discussed in the next
chapter.
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