20

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

24

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

28

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.

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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.