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Photodegradation of the Volatile Organic Compounds in the Gas Phase A Review.

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Dev. Chem. Eng. Mineral Process., 8(5/6),pp.405-439,2000.
Photodegradation of the Volatile Organic
Compounds in the Gas Phase: A Review
Madhumita Bhowmick Ray
Department of Chemical and Environmental Engineering
The National University of Singapore, 4 Engineering Drive 4
SINGAPORE 117576
~~
A review of photodegradation processes suitable for the treatment of Volatile Organic
Compounds (VOCs) present in the efluents from air-stripping and soil-vapor
extraction is presented. This paper outlines and summarizes the essential factors such
as kinetics and mechanism, intermediates of degradation processes, and reactor
modeling which lead to the successful application of this useful technology. In
addition, the effects of operating parameters such as humidity, temperature, additives
and types of VOCs are discussed. The review also identijies the areas where further
work is needed.
Introduction
Volatile organic compounds (VOCs) are ubiquitous chemicals used as industrial
cleaning and degreasing solvents. They are commonly found contaminants in gaseous
emissions from industrial processes, landfill, hazardous waste sites, groundwater and
soil all over the world. With the implementation of stringent regulation on the air
discharge of the VOCs, the two most commonly used methods to remove VOCs from
water and soil namely air stripping and soil vapor extraction processes, respectively,
require further treatment of the off-gas prior to the release to the atmosphere [ l ] .
Conventional methods for treating VOCs from gas streams, such as absorption,
adsorption, condensation and thermalkatalytic incineration all have inherent
limitations, and none are decisively cost effective [2-41. Therefore, great demand
405
M.B.Ray
exists for a more cost effective and environmentally benign process capable of
removing VOCs in relatively low concentrations (10 to 100 ppm) from gas streams.
One promising method of destroying wide spectrum of organic compounds is
photochemical degradation. This method has gained significant impetus over the
years and is considered to be a possible alternative for removal of refractory organics
in aqueous phase. Several variations of photochemical degradation are available: UV
photolysis, W photooxidation in presence of oxidants such as ozone, hydrogen
peroxide, hydroxyl radicals, and UV photocatalytic oxidation (PCO). Advanced
oxidation process (AOP) involves the formation of reactive hydroxyl radicals by the
decomposition of mostly ozone or hydrogen peroxide.
The major advantages of photochemical reactions over the thermal reactions are
selectivity, low reaction temperature, and complete degradation. Even the recalcitrant
compounds such as chlorophenols, chlorobenzenes, chlorinated biphenyl, chlorinated
dioxins, and DDT have been demonstrated to be photomineralized in water. Though,
the biphenyls, dioxins and furans are comparatively slow to photooxidize, the rate is
reasonably rapid as established from pilot plant treatment of the leachates from
landfill wastes [5]. While intermediates formed in the reactions are eliminated by
using extended treatment time, complete oxidation of the organics to carbon dioxide,
water and mineral anions can occur in a reasonably short period (typically in hours)
[6, 71. The first full-scale (126 Us) evaluation of AOP for the removal of
trichloroethylene (TCE) and tetracholroethylene (PCE) in the USA was completed by
the Department of Water and Power at Los Angeles (California, USA) [8]. However,
there are several issues still need to be resolved prior to the large scale application: (i)
treatability studies to provide data necessary for accurate design and cost estimate, (ii)
evaluation of the pre-treatment options for removal of the inorganics prior to the UV
oxidation, and (iii) evaluation of measures for ensuring that residual oxidants in the
effluent would not exceed toxicity limits for discharge to surface water [9]. The
estimated treatment cost depends on the type and concentration of the organics and
ranges from $0.68-$58.51/1000 gal of water [lo].
In recent years, some researchers have extended the application of photochemical
degradation to the treatment of VOCs in air streams [ l l , 121. Application of photo-
406
Photodegradationof Volatile Organic Compounds in Gas Phase: A Review
oxidation in the gas phase is more attractive than water due to the following reasons:
(i) lower UV absorption by air than water, (ii) higher mobility of dissociated species
which prevents the reverse process of recombination of the radicals, (iii) presence of
ample oxygen in the gas phase promoting oxidation by producing reactive species
such as ozone, and (iv) absence of scavengers such as bicarbonate and carbonate ions.
Consequently, complete oxidation of organics in the gas phase can occur in matter of
minutes if not in seconds [ 11, 131. The gas phase oxidation of TCE and benzene was
found to be five times faster than in water for the similar power of UV lamp [7, 111.
In addition, determination of the reaction mechanism and the identification of the
intermediates are relatively easier in the gas phase.
McGregor et al. [I41evaluated the use of an air-phase U V oxidation process to
treat the VOC-laden off-gas from an air stripper and found the process to be more cost
effective than granular activated carbon (GAC) adsorption for the control of VOC
emissions, however no detailed cost analysis was presented in that study. The use of
photoxidation to decompose VOCs in air was further extended by Bhowmick and
Semmens [15, 161 where VOCs present in the off-gas from an air stripper were
oxidized in presence of W light and the lean air was recycled back to the stripper in a
closed loop.
A preliminary cost comparison of closed loop air stripping (CLAS) with the
existing exhaust air treatment processes such as GAC adsorption, and incineration
identified that CLAS to be more expensive than air stripping in combination with
GAC adsorption, but more cost effective than incineration [ 17, 181. The cost of CLAS
can be dramatically reduced if the photooxidation rate constants of the compounds
considered were five times higher than that was obtained in the experiments of
Bhowmick and Semmens [13]. Another study on the cost analysis of a photocatalytic
oxidation (PCO) unit for indoor air treatment of the VOCs indicates that the annual
cost of PCO is 7 times greater than the granular activated carbon unit [19]. However,
a report published by the US A m y Toxic and Hazardous Material Agency
(USATHAMA) comparing economic viability of different control technologies for air
emissions from groundwater stripping and soil vapor extraction mentions that
UV/ozone and thermal incineration to be cost-effective at low flowrates [3].
407
M.B. Ray
Recently, UV oxidation was used effectively to regenerate granular activated carbon
used for controlling air-phase VOCs [20].
While literature on liquid-phase photochemical reaction of the organics with
potential application to treat pollutants has matured over the last three decades, the
information on gas-phase photochemical reactions is still in its nascent stage. The
initial studies were primarily directed at understanding atmospheric photochemistry of
the various chlorinated compounds [21, 221. A detailed account of the gas-phase
atmospheric photochemical transformation of over 100 alkanes, alkenes, aromatic
hydrocarbons, alcohols, and ethers is presented by W. Carter (1990) [22]. Although,
the matrix of reaction conditions of the organics in atmosphere is quite different from
that in the air stripping and soil-vapor extraction operations, these studies certainly
provide valuable information applicable to the treatment of air emission. During the
last several years, research has been consolidated to realize the potential of this
powerful technology in combating air pollution by degrading pollutants in the gas
phase. A review of photodegradation processes applied to treat harmful organics in
the gas phase with major emphasis to the VOCs has been presented in this paper.
Fundamentals
The absorption of light energy by organic compounds induces photophysical or
photochemical events to transpire. While photophysical processes include emission
of light or heat, photochemical changes produce new compounds by transformations
that include isomerization, bond cleavage, rearrangement, or intermolecular chemical
reactions. The radiation energy associated with a photon depends on the wavelength of
light according to the equation:
E(W /moZ) =12000/h(nm)
... (1)
One mole (Avogadro’s number) of photons is called an Einstein.
The extent of absorption of UV radiation and absorption spectra by any organic
compound is related to its molecularhond structure. The VOCs absorb UV and visible
wavelengths in the solar spectrum significantly due to the presence of one or more
double bonds involving carbon, nitrogen, and oxygen. However, saturated VOCs such
as chloroform and carbontetrachloride are usually transparent to solar U V . Electrons
Photodegradation of Volatile Organic Compounds in Gas Phase: A Review
that lie in
Q
orbitals, such as those of single covalent bonds, have absorption maxima
that lie far below 290 nm. On the other hand, the IC and n electrons (like the
nonbonding pairs associated with the oxygen atoms of carbonyl groups) often undergo
excitation processes. Table 1 shows the typical chromophoric values for some organic
groupings related to the VOCs.
Tablel. Wavelength of Maximum Absorption and Molar Absorptivity for
Bonds Common to the Standard VOCs [92].
C-H or C-C
180
1000
C-X
180
10,Ooo
c=c-c=c
220
20,000
Benzene
260
200
Naphthalene
310
200
Anthracene
350
10,000
Phenol
275
1500
280
20
320
50
220
20
c=o
c=c-c=o
C=N
The initial process of activation or light absorption in photochemical reaction is
known as the primary process. Subsequent to that, the desired reaction may occur,
although a series of deactivation reactions such as fluorescence, phosphorescence, any
physical quenching may also take place [23]. Primary quantum yield is the ratio of the
number of molecules undergoing energy absorption to the number of photons
absorbed [24]. Overall quantum yield is a better measure of the efficiency of the
overall process of decomposition of the organic compounds, which comprises the
primary process and all other secondary reactions that follow afterwards. However, it
does not have the quality of an intrinsic kinetic property and highly "processdependent" [23].
409
M.B. Ray
Photolysis
a) Direct
UV photolysis: In presence of W radiation in gaslair phase, organic
compounds can undergo photolysis, oxidation, and combination of oxidation and
photolyis. Extent of the individual reaction depends on the type and concentration of
the organics, the wavelength of radiation, and the presence of additional oxidants such
as ozone, hydrogen peroxide and hydroxyl radical. Many VOCs such as alkenes like
TCE, PCE [13, 251 and aromatics such as benzene, toluene, xylene undergo direct
photolysis by W irradiation [ 11, 261. Greater size and alkyl substitution increase the
sensitivity of the aromatic compounds to photolysis. Direct photolysis of the toxic
organics can only be brought about by supplying 4 eV to 7 eV or 300 nm to 175 nm
radiations, respectively [ 1 11. Direct photolysis often are kinetically simple and easily
modeled, especially if the absorption spectrum of the compound and its quantum yield
of disappearance are known or can be measured. The average photoreaction rate in
direct photolysis can be expressed as:
--dcA - @ I r n
dt
... (2a)
where cA = concentration of the organics.
@ = quantum yield of the reaction.
I, = average number of einsteins absorbed by the absorbing species per unit
volume and unit time.
b) Sensitized photolysis: In addition to direct photolysis, the possibility of indirect or
sensitised photolysis also exists, which occurs due to the transfer of energy from a
photochemically excited molecule to an acceptor. The acceptor, often oxygen, forms
a reactive transient form of oxygen such as semireduced-oxygen, singlet or triplet
oxygen. Singlet molecular oxygen [(O,('$)] mediated processes are of great potential
in natural water saturated with dissolved oxygen and sunlight. In aquatic systems,
photosensitizers include ferric and ferrous ions, humic materials, tetrapyrroles,
flavins, polycyclic aromatic hydrocarbons, and mineral surfaces. In some cases, these
sensitizers can increase the rate of decomposition by 20 to 60 times [27]. For
sensitized photolysis, the rate of degradation can be presented as:
410
Photodegradation of Volatile Organic Compounds in Gas Phase: A Review
rate = k [A]
... (2b)
where k is a constant that includes the light absorption rate and the concentration of
the sensitizer, as well as triplet quantum yield and triplet energy transfer terms (triplet
is the usually excited state involed in a sensitized reaction). [A] is the acceptor
concentration. If the sensitizer concentration changes during the experiment, the rate
expressions become much more complex.
In gas phase photooxidation of chlorinated VOCs, C1, initiatedsensitized oxidation
plays a significant role as rates of oxidation of isooctane and toluene can be increased
dramatically in presence of TCE, which serves as a source of chlorine radicals [28].
Similar augmentation in reaction rate also occurs for oxidation of toluene and benzene
in presence of 1, 2 dichloroethylene (DCE)[26]. The C1. sensitized reaction occurs in
the following way [29].
CCl2=CHCI + hv 9 HCIC=CCl. + C1'
... (3)
CC12=CHCl+ C1'
. .. (4)
. .. ( 5 )
+ CHC12CCIi
CHC12CC12' + 0 2 + CHC12CC1200.
2HClzCCI~OO'+2[CHClzCC120']+
0 2
... ( 6 )
The alkoxy radical then loses a C1 atom to form dichloroacetyl chloride (DCAC):
CHClZCC120'* CHC12CCl(O) + C1.
... (7)
or the C-C bond can rupture to form phosgene and a dichloromethyl radical:
CHC12CC120'+ CHCIi + CClzO
... (8)
Since C-Cl bond is weaker than C-C bond, possibility of step (7) is greater than (8).
Molecular chlorine formation is the chain termination step involved in the chlorine
sensitized reaction. C1-atom formation in the chlorinated alkene oxidation may also
occur due to the reaction with hydroxyl radical and oxidation reaction by atomic
oxygen species.
Oxidation
Oxidation of organics typically occurs either via addition or by substitution. Addition
is the inclusion of the oxidant into the chemical structure of the compound such as
addition of ozone or chlorine in the double bond of the olefins. Compounds with a
high electron density are active to the addition of electrophilic oxidants. Substitution
411
M.B.Ray
is the replacement of a reduced atom or group in the compound by the oxidizer such
as hydroxyl radical attack on a phenolic hydrogen atom.
a) Hydroxyl radical: Hydroxyl radical, the most powerful oxidant is capable of
oxidizing most organic compounds with rate constant values ranging from 10' to 10"
M's" [30]. As mentioned earlier, some of the AOP processes such as UV-ozone, UV-
H202produce hydroxyl radicals. Hydroxyl radicals are produced via the ultraviolet
photolysis of ozone to produce electronically excited singlet oxygen atoms [31]:
0 3
+ hv ( ~ 3 1 0+
) 0 (ID)+ 0 2
. .. (9)
A small fraction of 0 (ID)reacts with a water molecule to yield two OH radicals:
0 (ID)
+ H20
20H'
... (10)
Thus, in-situ hydroxyl radicals can be formed with low-pressure UV lamps with 185
nm radiation (ozone producing). Alkanes yield alkyl radicals directly by hydrogen
transfer when they are attacked by OH as shown below [32]:
R-H + OH'
+ R + H20
... (11)
For unsubtituted alkanes such as methane, ethane etc., the rate constant of oxidation
with OH' increases with increasing molecular weight of the organic compounds and it
varies from 8 . 7 1 ~10'' to 1.80 x 10" cm3 molecule-'s-' for the normal C-15
compounds [22,321. In contrast, OH attacks alkene molecule by abstracting the
reactive hydrogen or by adding to the double bonds. Alkenes register higher reaction
rate with OH. in the range of 7.0 x 10" to 9.0 x 1012cm3 molecule
s-' with bigger
molecules being more reactive. Thus, in case of saturated aliphatic VOCs such as
chloroform, hydroxyl radical abstracts the reactive hydrogen, whereas in case of
chloroethylenes, hydroxyl radical is predominantly added to the double bond. OH.
acts as an electrophilic reagent in the attack of an aromatic ring, and the presence of
electron donating groups such as OH on the ring activates the ring for electrophilic
substitution [33]. Benzene-OH adduct reacts with molecular oxygen to form benzene
oxide/oxepin as intermediate which upon further reaction with OH. leads to the ring
opening. Release of chlorine atom from the chlorinated aromatics only occurs after
the ring opening. Similar to alkanes and alkenes rate of oxidation of aromatics with
hydroxyl radical increases with the increase in the molecular weight of the aromatics
and ranges from 7.0 x 1012 to 9.0 x 10'" cm3 molecu1e"s~' [32].
412
Photodegradation of Volatile Organic Compounds in Gas Phase: A Review
Homogeneous gas phase reaction of OH' with TCE produces C1. through the series
of the following equations 1291, which may induce C1. sensitized reaction:
CCl2=CHCl+ OH'
+ 'CC12CHClOH
'CC12CHCIOH + 02
'
OOCClzCHClOH
2 ('00CC12CHClOH) 3 2 (.OCC12CHCIOH) + 0
'OCC12CHClOH
... (12)
... (13)
... (14)
... (15)
2
+ CHClOHCCl(0) + C1.
Loss of hydroxyl radicals from the active reaction scheme occurs by the following
reactions [31]:
+ H02'+02
O H + HOj + H20
... (16)
OH+ O3
0 +OH
+
OH +OH
... (17)
+02
0 2
... (18)
+H
+ H20 + 0
... (19)
b) Ozonation: Many of the commercial low-pressure UV lamps have a small
percentage (typically 5%) of radiation at 185 nm, which produces ozone in the
reactor. Hence, direct ozonation is also a mode of oxidation for the organics in UVphotooxidation. Direct ozonation of organics is much slower than the other processes
such as photolysis, and oxidation by hydroxyl radicals. Only a few percentage of
chloroform was oxidized in one hour in the study of Bhowmick and Semmens [ 131.
Ozonation studies conducted in water reveal the process to be first order related to the
concentration of both ozone and the organics [34]. The rate constants of ozonation of
n-alkanes in the gas phase are in the order of 1.4 x
7.9 x 10
cm3 molecule-'s',
while ozonation of the alkenes is much faster with rate constants in the range of 1.75
x lo''*- 2.0 x 10l6 cm3 molecule"s", yet lower than the oxidation by hydroxyl
radicals [32, 351.The ozonation rates observe the following order: alkene > alkane >
aromatic. Rate of ozonation also decreases with the degree of chlorine substitution in
the parent molecule. Thus, rate constant decreased by two orders of magnitude (0.1-
0.005 M-Is']) from methylene chloride to carbon tetrachloride [34]. This decrease is
primarily due to inductive and steric effects of the more substituted molecule.
However, W/03is found to be more effective oxidation process than UV only. The
dependence of W/03on UV intensity is slightly lower than the direct UV photolysis
and the length of the free radical chain reactions to decompose chloroethenes is
413
M.B.Ray
slightly shortened in presence of ozone [25]. However, external ozone can increase
the rate of reaction up to a certain concentration of ozone only (molar ratio of 03/TCE
= 7.45) [25]. The increased dosage of ozone can reduce the rate by scavenging of
hydroxyl radicals and absorption of active photons due to the high molar absorption
coefficient (3020 M-lcm-') of ozone at 254 nm.
The excess ozone acts as scavengers of hydroxyl radicals as shown by equation
(16). In addition, ozone undergoes the following reactions:
+ hV+ 0 + 0 2
c1.+ 0 3 + 0 2 + c10,
0 + c10+ c1,+ 0 2
0 3
... (20)
... (21)
... (22)
On a positive note, C1. radical can induce further chain reactions by C1, sensitized
reactions and increase the rate of degradation.
c) Oxygen: Photochemical reactions are often drastically altered by the presence of
molecular oxygen. Oxygen inserts itself into a substrate with the formation of
hydroxyhydroperoxides, which readily decompose to carbonyl compounds, peroxides,
hydroperoxides. In addition, forms of oxygen such as 0 ('D) are important oxidants
in gas phase photooxidations [36]. Oxygen atoms can also react with water to form
hydroxyl radicals.
0 + H20+ 20H'
... (23)
However, atomic oxygen reactions appeared to compete with chlorine atom reactions
and therefore inhibit the formation of chlorinated products at relatively high oxygen
concentrations.
d)
W/H202:
Advanced oxidation based on W/H202 is very efficient with a wide
range of applications in the treatment of the industrial effluents. Although there are
200 commercial installations treating contaminated water [37, 381, the application of
UVA-402 is yet to be found in the gas phase. Introduction of liquid phase Hz02 in
the gas phase reaction medium may cause some operational restrictions for this useful
and versatile oxidant, but it is an efficient method of producing hydroxyl radicals in
absence of 185 nm radiation. H202has a weak molar absorption coefficient, which
increases as the wavelength decreases in the 200-300 nm UV region. The photolysis
of H202produces hydroxyl radicals:
414
Photodegradation of Volatile Organic Compounds in Gas Phase: A Review
H202 + hv
+ 20H'
... (24)
Propagation:
OH'+ H202/H0; 3 HOi/O;
HO2'+ H202
+ OH' + H2O +
+ H2O
0 2
... (25)
... (26)
Termination:
HOi+ HOi
+ Hz02 +
0 2
... (27)
Quantum yield of generated OH. per photon absorbed is 1.O [37].
e) UV Photocatalysis: Photocatalytic oxidation (PCO) of the organics in gas phase
using catalyst possesses great potential as longer wavelength of UV can be used.
Similar to homogeneous photooxidation in the gas phase, PCO rates (normalized
oxidation rates using UV photon flux incident on the reactor) are also found to be
higher than the liquid phase K O . For example, PCO rate of TCE in contaminated air
streams is found to be at least one order of magnitude higher than the rates typically
reported for liquid-solid slurry reactors. PCO of 17 VOCs from different classes
including ketones, alcohols, chlorinated alkenes and alkanes, aromatics was tested by
Aberici and Jardim (1997)[39]. PCO at ambient conditions is capable of degrading
common indoor air contaminants such as acetone, benzene, toluene, formaldehyde
and acetaldehyde [40,41].
Typically Ti02 (crystal structure of primarily anatase) is used as the photocatalyst,
and coated as thin film (of several pm thickness) onto the internal glass surface of the
reactor. A comparison among several semiconductors such as TiO2, ZnO, ZnS, CdS,
Fe20-,, W03, indicated that Ti02 exhibited faster rates than others, and is extremely
stable [42]. Zinc oxide (ZnO) has a similar band gap (3.2 eV) of that of Ti02,
however ZnO may possess selectivity for complete mineralization of chlorinated
wastes [2]. Doping of the titania with phases such as platinum or vanadium oxide,
copper has demonstrated improvement in photocatalytic efficiency [43,44].
The summary of the PCO steps are given as:
Ti02 +hv+ h+"b+ e'CB
... (28)
h+"b+ H20 (ads)+ OH'+ H'
... (29)
h'*
+ O H (ads)+
OH. + H'
... (30)
415
M.B. Ray
These holes are very strong oxidizing agents, and the number of electron-hole pairs is
dependent on the intensity of the incident light and the electronic properties of the
material that prevent them from recombining and releasing the absorbed energy.
Presence of moisture exhibits an inhibition effect on the photocatalytic rate at high
concentrations of TCE (mol fraction greater than I@*) and limits the applicability of
the process for the treatment of the effluent air from air stripper which is saturated
with moisture. In low water concentration, water improves the oxidation rate whereas
at high concentration the moisture competes with the organics for active sites, the
effect is more pronounced for poorly adsorbing compounds such as acetone [45].
Although, some researchers indicate that in absence of water vapor permanent
deactivation of the catalyst occurs due to the irreversible consumption of surface
hydroxyl radicals [46]. Since surface hydroxyls are needed to oxidize the organic
compounds, complete dry condition is also not sought after. In order to sustain long
term photocatalytic activity, 23% RH found to be sufficient in a study [39]. Effect of
concentration of hydroxyl radical on PCO rate varies depending on the following four
types of interactions between photocatalytically formed OH,and the organic reactants
[47]:
1. The OH,radical and organic reactant both are adsorbed on the catalyst surface.
2. The OH, radical diffises to the reaction medium and reacts with the organic
reactant in the bulk medium.
3. The adsorbed 0H.radical reacts with the bulk solution organic reactant.
4. The free OH' radical reacts with the surface-adsorbed organic reactant.
Recent studies have shown the rate of photocatalytic degradation can be dramatically
increased by reduced pressure [48, 491. A relatively greater percentage enhancement
in performance is realized at low pressure range of 10-6 psia than that in the range of
21-10 psia. The performance was also better for low levels of VOC contamination
and high water vapor concentration, which is a deviation from the atmospheric
pressure reactions.
The homogeneous quantum yield concept is modified and an appropriate and
equivalent property for solid catalyzed systems is used in the literature.
Heterogeneous quantum yield depends on operating conditions such as temperature,
416
Photodegradation of Volatile Organic Compounds in Gas Phase: A Review
reactant and product concentration, pH, oxygen or air concentrations. The kinetics can
be expressed as one of Langmuir-Hinshelwood (LH) type, thus depending on both the
degradation rate and the adsorption rate constant. The mass transfer limitation
becomes important for PCO when the catalyst is used as thin film, and can be
overcome by using higher flow rates. If the reactor operates under the diffusion mass
transfer limited regime at atmospheric pressure, reducing the pressure will increase
the diffusivity of the reactants. For kinetically controlled reactors, a favorable shift in
adsorption competition between the VOC and water vapor occurs at reduced pressure
1481. Recent water phase PCO studies indicate that there exists an optimum catalyst
thickness beyond which no improvement in rate occurs due to the mass transfer
limitations [50]. Similar mass transfer limitations were found when PCO was used to
regenerate the spent adsorbents by mineralizng the adsorbed organics. Rate of
diffusion of adsorbates from the interior surface of the adsorbent to the exterior where
the reaction takes place limits the overall process [51].
The ground-level solar spectrum contains approximately 1% near-W photons of
sufficient energy to photoexcite TiOz. The use of sunlight in degrading gas phase
pollutant by using Ti02 may have some potential; however, it is anticipated that a
large reaction volume will be required. Although, there is limited information about
gas phase solar PCO of pollutants [52], study conducted in aqueous phase indicates
that the solution of halocarbon (concentration=50 ppm) can be completely
mineralized in two or three hours of illumination. It has been found the reaction rate
remained nearly constant over much of the day indicating very little variation of near-
W illumination [53].
Major challenges for the photocatalytic process are catalyst deactivation, slow
kinetics, low photoefficiency and unpredictable mechanism [52]. Deactivation of the
photo-catalyst can occur due to the deposition of gaseous HCI and the deactivated
catalyst can be regenerated by using flowing humid air in absence of W illumination,
or by illuminating the catalyst in presence of hydrogen peroxide [12, 391. Although,
photocatalyst can use longer UV wavelength, rate of catalytic degradation is
considerably lower than UV-photooxidation at 254 nm. For example, complete
degradation of TCE in gas phase by ZnO photocatalyst with a 200 W high pressure
417
M.B. Ray
mercury lamp is about 1500 minutes [54] compared to that of 10 minutes in U V photooxidation using a 14 W low pressure mercury lamp [13].
The selective
reactivity of the photocatalysts can also be a disadvantage for degradation of a
mixture of the VOCs, as Ti02 showed very poor reactivity to toluene and acetone
compared to TCE [43, 451. Only maximum conversion of 11.6 % was obtained for
acetone PCO in a flow reactor [45]. The hole-electron recombination rate is a
limitation for large-scale application of this technique and the modification of the
surface properties of photocatalyst is necessary to decrease the electron-hole
recombination [55]. The operating conditions needed to achieve the complete
mineralization of the VOC PCO are strongly dependent on the type of compound to
be removed.
Reaction Rate and Mechanism
This section summarizes the available information on gas phase photodegradation
kinetics of VOCs. Generally 1”‘ order kinetics was observed for chlorinated aliphatics
such as carbontetrachloride, and aromatics such as benzene, toluene and xylene [ 13,
36,261. Non-first order or pseudo-first order kinetics were observed for chloroform,
1,1,2 hichloromethane, dichloromethane, TCE, PCE, and 1,l DCE [13, 251. The
compounds, which exhibit non-fust order kinetics also show quantum yields greater
than one. These apparent higher quantum yields are due to the chlorine-sensitized
oxidation. It is possible for these compounds that they follow first order kinetics at
low concentration while a shift to second order may occur at higher concentration.
Heickelen and co-workers have found that chain induced photooxidation occurs for
all of the chlorinated ethenes except vinyl chloride [56]. In the liquid phase PCO of
chloroform, the rate of oxidation is first order with respect to chloroform
concentration, however, in the gas phase at high concentration, the rate is second
order with respect to chloroform concentration due to the C1-sensitized reaction [13].
Thus, chlorinated co,mpounds show one order of magnitude higher rates and global
quantum yield than the non-chlorinated compounds such as ethylene, propylene,
propanol, and ethanol. However, in order to sustain Cl’-sensitized reaction, the
organic compound should contain a reactive hydrogen atom, or a double bond. Hence,
compounds such as carbon tetrachloride may not undergo C1-sensitized reaction and
418
Photodegradationof Volatile Organic Compounds in Gas Phase: A Review
there is no shift in kinetics from lower to higher order with increasing concentration
for these compounds.
It is apparent from the studies of Glaze et al. (1992), and
Bhowmick and Semmens (1994), [36, 131 that the rate of carbon tetrachloride
oxidation did not increase with initial concentration. In a different study TCE, PCE
and trichloro-propene have been reported to achieve photo-efficiencies above 100%
in air due to the C1'-sensitized reactions, whereas for BTEX, only 10% efficiency is
reported [251.
The photooxidation rate constants for various VOCs and their intermediates
follow the following order: chloro-olefins > chloro-paraffin > chloro-acetic acids [53,
13, 261.
Generally, oxidation of nitrogen-containing compounds is slow when
compared to the compounds containing phosphorus, sulfur and chlorine [39].
Mineralization of chloro-ethylenes decreases with the increase in chlorine atom
substitution on the C=C bond. It is speculated that the photolysis of the compounds
with more substituted Cl atoms could increase the chain reactions involving chlorine
radicals and ozone, hinder the formation of OH radicals and restrain the
mineralization and dechlorination of organic intermediates [251. Table 2 summarizes
the recent research conducted on gas phase photodegradation of the VOCs.
Primary photochemical process is insensitive to the temperature range used in the
standard oxidation processes applied in the gas phase. However, organic molecules
absorb W radiation at higher efficiency and that the onset of absorption shifts to
longer wavelengths at higher temperature [57]. Sometimes, secondary processes may
follow thermal-catalytic mechanism and in that case increased temperature will
increase the rate and the effect of temperature can be anything from marginal to
significant [58]. For example in a PCO study, mineralization of converted TCE
improved by increasing temperature, whereas TCE degradation decreases very
quickly at temperatures above 125OC due to the adsorptionldesorption limitation [58].
Typically, photochemical reactions are conducted between 290 and 400 K, and
atmospheric pressure. The requirement of a transparent wall made of glass or quartz
to transmit the effective radiation between 185-380 nm limits the use of high-pressure
[59]. On the other hand, for PCO, reaction at low pressure may be beneficial to
remove diffusional mass transfer limitation.
419
M.B.Ray
Table 2. Summary of the gas phase photodegradation processes applied to treat VOCs.
moisture discussed. low quantum yicld.
no mechanism provided
420
Photodegradation of Volatile Organic Compounds in Gas Phase: A Review
The effect of moisture should be beneficial in presence of ozone or ozoneproducing radiation ( 185 nm), since hydroxyl radical can be generated. Increased
humidity increased the oxidation of chloroform and 1,1,2 tricholoroethane [ 131. In a
separate study, significant effect of humidity on the photooxidation rates of toluene
and benzene was found, while the effect was minimal for the photooxidation of 1,2
DCE [26].Effect of moisture has not been found to very effective in the gas phase
photooxidation of chloroethylenes [ 13, 603.The contribution to the decomposition of
TCE by direct photolysis was always higher than that by OH.reaction [25]. Similar
results were found in the reactions involving high-energy ionizing radiation, where
gas phase photochemistry of TCE did not change in presence of 90% RH [60]. PCO
of ethylene and chlorinated ethylenes such as TCE are mechanistically different as the
effect of experimental conditions such as initial concentration of oxygen, water vapor,
temperature, and initial concentration of the reactants produce different results [611.
Generally, homogeneous photooxidation rates increase with the increase in
intensity. Rate vs. intensity shows a square-root dependence for low concentration of
TCE, but at high concentration the relationship is linear [29, 251. The order of UV
light intensity for TCE oxidation was 1.81 indicating chain reaction occurring in the
process [62]. The order of intensity dependence of W-03system is 2.01 compared to
1.81 by direct photolysis which indicates that the length of the radical chain reaction
to decompose TCE could be promoted by adding ozone to the original photolytic
system. For PCO at high intensity mass transfer limitation starts to control. In
addition, at high intensity, recombination of hydroxyl radical occurs which reduces
the quantum yield of the process. Ohko et al. (1998) [63] indicated for gas phase
photocatalytic oxidation of 2-propanol, mass transport starts to control at the average
-
intensity of lo4 1O6pW/cm2,whereas at a high reactant concentration of 1-1000 ppm
light intensity controls the reaction.
Photooxidation in a mixture of organics is complicated as the rate of one
compound may be inhibited or enhanced by the presence of others. In addition, the
compositions of the intermediates and the by-products from photooxidation depend
on the types of the VOCs present initially in the reaction mixture. In a mixture of 40
ppm of DCE and 100 ppm of toluene, the average photooxidation rate of toluene
421
M.B.Ray
increased from 0.0024 (lh) to 0.0037 (l/s) (an increase of 54%). On the other hand,
the rate constant of DCE decreased from an average of 0.0071 (11s) to 0.0036 (11s) (a
drop of 50%) [26]. The Cl'radicals generated from DCE oxidation enhanced toluene
degradation by C1'-sensitized oxidation, whereas the degradation rate of DCE
decreased due to the competition with toluene for addition of photon, and C1' radical.
This feature of the halogenated compounds makes these compounds use as a
sensitizer to provide the radicals to break down otherwise refractory chemicals. For
example, carbon tetrachloride, which does not absorb at 254 nm radiation, can be
degraded in presence of chloroform at 254 nm [ 131. Haag and Johnson (1996)
reported increase in rate of oxidation of vinyl chloride and ethene when oxidation was
conducted in presence of TCE [64].
Intermediates and by-products
A major concern limiting the application of this W-photodegradation is the type of
intermediates and end products formed in the photooxidation of the VOCs. The
intermediates formed sometimes are more toxic than the parent compounds and
required to be decomposed completely. A field trial of a pilot-scale Ti02 photocatalytic reactor treating off-gases (5000 ppmv) from soil vapor extraction yielded
many undesirable byproducts such as phosgene, chloroform, carbon tetrachloride,
penta and hexa-chloroethane [65].
Carbonyl compounds are the most frequently found intermediates produced in the
photooxidation of the organic compounds in the atmosphere, which are also expected
to occur in the oxidation of the VOCs [32]. Carbonyl compounds, particularly
aldehydes are quite toxic, and some of the secondary compounds formed from
aldehydes, especially peroxyacylnitrates are much more dangerous than the parent
compounds. Any organic radical (R.) with the unpaired electron will undergo the
following reaction [32]:
R +0
2
+ ROj
... (31)
. .. (32)
... (33)
ROj+03+RO.+202
RO. + O2
+ carbonyl compound + H O j
+ R1O'+ R20.+ O2+ alcohol + aldehydes +
+ RI-0-0- R2 +
R1Oj + R202'
0 2
422
0 2
... (34)
Photodegradation of Volatile Organic Compounds in Gas Phase: A Review
where R1,R2are hydrogen or organic groups. The yield of the three possible reaction
paths (Eq. 34) depends on the nature of R 1and R2. The aldehyde thus formed can also
undergo photolysis, and the reactive hydrogen of aldehyde reacts with the other
reactive radicals in the following way:
+ RCO' + H2O
RCHO + 0 + RCO. + OH
RCHO + OH
... (35)
... (36)
However, in absence of hydroxyl radicals, aldehydes lose CO and form alkanes:
RCHO +hv
+ RH + CO
Organic peroxy radical
(R02) reactions
... (37)
are of significance since they represent an
important class of intermediates formed in the oxidation process of hydrocarbons
[66]. The rate constants of acetylperoxyl radical self-reaction and of its reaction with
H02are well characterized, with rate constant values of 1.4 x 1U" and I .3 x 10" cm3
molecule-' s-', respectively. Intermediates such as ethers and alcohols have enhanced
reactivity towards hydroxyl radical due to a ring-transition state between the attacking
OH radical, the ether oxygen atom and the -CH2- groups attached to the ether group
[67]. The rate constant of oxidation of these compounds is of similar order of
magnitude as of the alkanes.
In presence of C1. radical, acetyl radicals are formed which undergoes photolysis;
acetyl has peak absorption at 217 nm [68]:
+ CH3CHO + CH3CO' + HCI
CH3CO + C12 + CH$(O) C1+ C1
CH3CO' + CH3CO. 3 (CH3C0)2
C1
... (38)
... (39)
. .. (40)
Acetyl chloride is hydrolyzed to trichloroacetic acid [69].
Although many researchers have indicated that the degradation of the organic
compound is rather fast in the gas phase, the rate of degradation of intermediates such
as DCAC, phosgene was rarely monitored. Dichloroacetic acid (DCAC),
tricholoroacetaldehyde, trichloroacetic acids are the intermediates of oxidation of
TCE and PCE where DCAC is about 40 times more toxic than TCE and requires 100
times more exposure dose to reduce the effluent toxicity to acceptable levels [64, 691.
It is also found that the rate of dechlorination of the intermediates is slower than the
423
M.B. Ray
parent chloroethenes [2S]. DCAC formed is hydrolyzed in presence of water to
dichloroacetic acid (DCAA) [28, 131:
CHClzCOCl + Hz0
+ CHClzCOOH + HCI
CHC12COOH + 'OH
CC12COOH + 0 2 +
+ CClzCOOH + HzO
'00CClZCOOH
.OOCClZCOOH 3 COCl:!
+ CO2+ 'OH
... (41)
... (42)
.. . (43)
. .. (44)
Phosgene (COClz) and formylchloride are expecteL to be produced from the
photooxidation of many chlorinated alkanes and alkenes such as chloroform, methyl
chloride, methylene chloride, carbontetrachloride, trichloroethane, TCE and PCE [ 13,
211. However, these intermediates photolyze and hydrolyze easily to HCl and COz,
as shown below:
COClz + HzO 3 COz + 2HC1
. .. (45)
It is possible some higher molecular weight compounds than the parent compounds
are also formed in the photochemical reactions as seen in the study of Hung et al.
(1997) [28]. Higher molecular weight products such as hexa-chloroethane, pentachloroethane, 1, 1, 2, 2-tetrachloroethane and PCE were formed as the intermediates
along with chloroform, carbon tetrachloride, 1,2-dichloro-ethylene at different
concentration of oxygen [28]. Trichloro-methyl radicals are the prevalent radical
species in irradiated chloroform (following hydrogen abstraction by the primary
radicals) which form CzC16, while CZHC15 dominates the product spectrum [70]. The
formation of PCE from TCE can be explained by the following mechanism:
+ CHCkCCl. + C1'
CHCkCClZ + 'CCI=CClZ + H'
CHCkCClZ
' c c l = C c l ~+ c1.
+ cc12=cc1~
... (46)
... (47)
... (48)
PCO of TCE is found to produce chloroform and pentachloroethane to be the main
by-products.
Isomerization and the formation of addition products with alkenes are the most
noted reactions of benzene ring [7 13. Alkylbenzenes such as toluene form fairly stable
epoxides upon the addition of hydroxyl radical, which are potential toxic and
mutagenic compounds [72]. The subsequent reactions of the epoxide could lead to the
formation of epoxy carbonyls, which can react further with OH radicals or ozone until
424
Photodegradation of Volatile Organic Compounds in Gas Phase: A Review
smaller molecules are formed. Carbonyl products formed during the OH-initiated
oxidation of six alkylbenzenes such as toluene, p-xylene, m-xylene, o-xylene, 1,3,5trimethylbenzene and 1,2,4 trimethylbenzene were reported by Yu et al. 1997 [73].
Among the observed carbonyl products are aromatic aldehydes, quinones, diunsaturated 1,6-dicarbonyIs, unsaturated 1,6dicarbonyls, saturated dicarbonyIs,
glycolaldehyde, hydroxy acetone, and triones and epoxy carbonyls. Hexadienedials
(HCO-CH=CH-CHSH-CHO) have been postulated as primary ring opening
products of the OH-initiated degradation of homocyclic aromatics such as benzene,
toluene and xylenes [74], and hexadienedials react 1-2 orders of magnitude faster with
OH radicals than the parent aromatic compound and rapidly photolyzed by U V light.
In the photooxidation of toluene, benzaldehyde along with some benzene, benzyl
alcohol, and traces of benzoic acid are formed [52,75]:
OH'+C6H5 CH-j (ads) 3 CjHsCHi + H2O
... (49)
... (50)
... (51)
... (52)
... (53)
C6HsCHi + 0 2 (ads) 3 CaHsCH2 00
C&CH200
+ e- 3 C6H5CH0+ OH-
C&CHO (ads) + OH' 3 C6HsCO' + HzO
+ 0 2 (ads)
C~HSCO'
+ C6H5C 000'
C&sC 000.+ C&sCHO (ads)
+ C&sCO' + C&C
GH5COOOH (ads) + C~HSCHO
(ads)
c,jH~cOOH (ads)
*
OOOH (ads)
2C6H5C OOH (ads)
+ c02
c6&
... (54)
... (55)
... (56)
Photooxidation of ethylbenzene leads to intermediates such as 4-ethylphenol,
acetophenone, 2-methylbenzylalcoho1,2-ethylphenol and 3-ethylphenol [52]. H-atom
abstraction by OH, attack on the ethyl group is followed by the further ring cleavage.
Clearly, the fate and characteristics of these intermediates in UV radiation field need
to be determined extensively prior to the full-scale application.
Light source
Full-scale application of gas-phase photodegradation treating the effluents from air
stripping and soil vapor extraction is still premature. The low rates of direct
photolysis with the existing commercial lamps, difficulties of obtaining gas phase
hydrogen peroxide and ozone, and short lamp life limit the success of the process. In
425
M.B.Ray
what follows is a brief review of some of the W lamps used in the gas-phase
photodegradation studies of the VOCs.
As is apparent from the earlier discussions, a broad emission spectra which
encompasses a wavelength region from 175-380 nm is beneficial for the photodissociation of variety of VOCs and their intermediates. Such spectrum can be
generated by various pulsed devices [ll]. Typical W sources include low and
medium pressure mercury lamps having peak output at 254 nm with a smaller ( 4 5 % )
emission at 185 nm [5, 561. The rest of the light energy occurs in the visible and
infrared regions, which are not useful for organic photolysis. Most commonly used
germicidal lamps are narrow diameter tubes (1.5 to 2.0 cm diameter), generally 0.9
and 1.6 m long and the active or the arc portion is 0.75 and 1.47m long, respectively
[76].Low-pressure W lamp with very small output of about 5.3 watts is also
available and good for kinetic studies [lo].
As most VOCs absorb in the sub-250 nm region, mercury UV lamps rely
predominantly on hydroxyl radical processes for organic attack. Thus these lamps are
nearly always used in conjunction with the added oxidants in the water phase, as the
254 nm line will photolyze ozone or hydrogen peroxide, creating hydroxyl radicals.
However, these lamps will not be very effective in gas-phase in absence of ozone
producing 185 nm wavelength. Ozone producing lamps are made of fused silica
which is transparent to this line, although the power output of these lamps is limited
due to the self absorption of produced UV by Hg atoms [36].Recently manufacturers
of low pressure mercury lamps have strengthened the direct photolysis capability of
the UV lamps by augmenting the 185 nm line by diminishing the overall intensity to
some extent. Spangenburg et al. (1996)1691 reported a low pressure 20W-Hg lamp
(HNS Osram 2OWAJIOz) which produced a 113 of the U V output at 185 nm, which is
significantly higher than usual 5% obtained from the available commercial lamps.
185 nm radiation has its own limitations such as competitive absorption by oxygen
and the need for highly transmissive quartz lamps.
New xenon plasma flashlamps, which generate significant light intensity in the
deep UV region ( ~ 2 5 0nm) are better suited for direct photolysis than conventional
mercury-based UV lamps. Spectra of xenon flash lamps are different than those of
426
Photodegradation of Volatile Organic Compounds in Gas Phase: A Review
the mercury arc lamps and in the range of IR to the UV-C region (300-200 nm) [36].
The spectral emission of the xenon flash lamps depends on the current density and the
plasma temperature.
For K O ,longer wavelength lamps such as fluorescent W source with the output
wavelength spectrum ranging from 300 to 500 nm, with a maximum intensity near
390 nm is used. Argon ion laser with emission lines at roughly 330 and 360 nm is
also used for PCO [29]. The typical power used in UV-Ti02 is about 400-Watt and
higher than low pressure mercury lamps, and flash lamps.
One of the several commercial processes applying photolytic oxidation in soil and
ground water is offered from Thermatrix, Inc [77]. The process uses a xenon pulsedplasma flash lamp that emits short W at very high intensity. Field-testing of a fullscale prototype was used for a unit handling 500 cubic feet per minute (cfm) with
initial TCE concentrations in the air with 250 ppm by volume with a 99% treatment
goal. About 60 installations world wide are using this technology treating diverse
effluents from various industries such as petrochemical, pulp and paper,
pharmaceutical, cosmetics including waste water and waste treatment facilities [77].
Blystone et al. (1993) [56] observed the effective removal and decomposition of
several VOCs (TCE, PCE, 1, 1-dichloroethene, chloroform, and methyl chloride) by a
xenon flash lamp. Solarchem high power density lamp was used to treat toluene and
TCE in air streams [62]. In order to mimic solar radiation, Xe lamps with filters to
remove IR radiation are used [ 5 8 ] .
Reactor and light modeling
At present, systematic scaling up and detailed design procedures are lacking for the
photochemical reactors. The modeling of photoreactors is difficult due to complex
reaction kinetics, light intensity gradients created by dispersion, absorption, reflection
and shadowing, and hydraulically complex mixing profile [78]. In addition, there are
several factors related to the large scale application of the photoreactors remain to be
resolved or being resolved at present, and they are: (i) size limitations, (ii) lamp
operation and maintenance, and (iii) wall deposits affecting radiation entrance to the
reactor 1231.
427
M.B. Ray
Many designs of reactors are found in the literature: (i) annular reactors, (ii) flattray reactors, (iii)"merry-go-round" reactors, (iv) collimated beam reactors, (v) single
lamp multitubular reactor, and (vi) multi lamp tubular reactor [79, 59, 801. A critical
factor in photoreactor scale-up is the chain termination, which can occur due to the
both homogeneous and heterogeneous reactions. The importance of heterogeneous
reactions is considerable in scaling-up tubular flow photoreactors due to the variation
in surface-to-volume ratio with diameter [811.
Different photocatalytic reactor configurations are reported in literature including
annular reactor with a small thin film of catalyst coated on the inner surface, flat plate
fluidized beds, annular packed beds, and catalyst coated on honeycombed monoliths,
porous fibrous mesh, and optical fiber bundles [40, 821. Although titania catalyst
used in fluidized bed illustrates greater quantum efficiency than the fixed bed catalyst,
the reaction rate is far too low for a viable large-scale operation.
A significant uncertain aspect of photoreactor design is due to the existence of
non-uniform reaction rates in the reactor even if the reactor is well mixed. Such
variation occurs due to the absorption of radiation by the reactants, products, and the
medium. In addition, divergence of the light occurs due to the distance from the
source. In heterogeneous reactors, problem is even more complicated due to the
scattering of light by the solid particles. An efficient photoreactor should eliminate all
spatial variations of concentration, temperature and photon distribution within the
reactor. Reactor modeling should thus consider the mass, momentum, and energy
balance equations. In addition, an equation coupling mass and photon also should be
taken into consideration. The quantum yield is related to the local volumetric rate of
energy absorption (LVREA). The LVREA represents the amount of photons that are
absorbed per unit time and unit reaction volume, which in turn depends on the photon
distribution. LVREA is a non-uniform factor even in mixed reactor due to the
absorption, scattering and geometrical effects and it is essential that light intensity be
known precisely through the reactor [83].
A thorough review of intensity models depending on the lamp and reactor
configuration is presented by Alfano et al. (1986) [84]. Their work also indicates the
advantages/disadvantages of each model. There are two broad categories of models
428
Photodegradation of Volatile Organic Compounds in Gas Phase: A Review
available in the literature; incidence and emission models. Incidence models do not
make use of all operating variables such as power, diameter and length of the radiant
energy source and need one or more experimentally adjustable parameters that depend
upon the size of the reactor. Whereas line source emission models provide a
methodology for an a prion’ design of commercial reactors. Line source models lead
to simple LVREA formulations which allow uncomplicated numerical treatment.
The reactor scheme typically used in gas phase oxidation is annular type where the
lamp is inserted in the inner annulus made of a transparent material such as quartz.
This type of reactor can be used in both batch and continuous modes. Several light
models are successfully applied to the annular photoreactors [23, 841. However, the
model of Jacob and Dranoff (1970) [85] based on line source spherical emission
(LSSE) for cylindrical annular reactors with centrally mounted lamps, is one of the
most popular models. This was later adopted by many researchers with good success
[23, 59, 80, 86, 871. Finite lamp length is first approximated as a line source of the
same length. The resultant intensity is found by numerical integration of the point
source equation for sufficient number of points over the length of the line/lamp. The
model is based on Figure 1.
“ 4
.....................................................
b
Figure 1. The schematic of light model of Jacob and DranofS(I970).
The intensity of radiation at an arbitrary point A within the reactor due to a point
source at a distance z’ from the base of the lamp was described mathematically by:
429
M.B.Ray
where p2 = r2 + (z - z ' ) ~ .Sp,x is defined as the output power per point source. Sp,h can
L
I SL,J(-) = S,,nAz, where SPJhas units of energy per time
also be written as S ~ , =
n
and SLl has units of energy per time per length. The length of the each point source is
defined as Az, and as Az decreases, the approximation of this model becomes more
, ~coefficients accounting for the attenuation of radiation
accurate. pg, and p ~ are
intensity due to the absorbing species (i.e. VOC) and due to the quartz sheath with
thickness, 8,respectively. For the UV radiation >254 nm, the absorption of light by
quartz inner shell of lamp is negligible.
Ik,i is the contribution of each point source to the intensity. As the Az approaches
zero, the resultant radiation distribution of radiation within the reactor can be
established by integrating equation (56) over the entire length, L of the lamp. Since
integration cannot be performed analytically, summation of a finite number of point
sources each with finite length is used instead. The total intensity at a location in the
reactor can be calculated as follows:
1
The average intensity in multi-lamp reactors can also be calculated using the
above model of Jacob and Dranoff (1970) [85],which is typically performed for the
water disinfection reactors. The average intensity in the reactor is a function of
centerline spacing, and typically a centerline spacing of 75 mm is used [88].
Occasionally, the reactors are irradiated from outside by placing the reactor and the
lamp at the foci of an elliptical reflector [27]. The lamps placed outside the reactor
face less fouling on the lamps providing longer lives for the lamps.
The intensity profile for such a reactor is given by a radial incidence model [89]:
... (59)
430
Photodegradationof Volatile Organic Compounds in Gas Phase: A Review
where I,
= light intensity at the reactor wall (einsteins /cm2. sec)
= attenuation coefficient of reactant at the wavelength A (cm-I)
r
= radial distance from center of reactor tube, cm.
R, = reactor radius, cm
In the radial model, all incident rays intersect at the central axis of the reactor tube,
and the intensity approaches infinity as r 3 0. This model predicts nearly uniform
intensity if the radius of the reactor was less than 12 mm. However, for bigger
reactors a partial diffuse model which considers a parallel band of rays, wider than the
reactor diameter passes through the reactor cross section from all directions with
equal probability, works better. Thus, the average intensity over the whole reactor for
.. (60)
where, R2 = radius within which light is difksed and
8 , = S i n - 1R2 ; 8
Rl
=sin-I 5
r
... (61)
Radiation model involving multi-lamp reactors is provided by Yokota and Suzuki
[go]. Based on a diffused line source emission model, light absorption rate in any
geometrical photoreactor with multiple lamps was assessed, and the work reveals the
existence of optimum light arrangement. A more complex intensity calculation was
presented by Tymoschuk et al. (1993) [59] for a multi-tube photoreactor with a light
in the center, enclosed in a cylindrical reflector. It was concluded that about 25% of
the total radiation is due to reflection.
Radiation field inside a tubular multi-lamp reactor, and cylindrical photocatalytic
reactor was presented by Alfano et al. (1990) [80]and Romero et al. (1997) 1861,
respectively. It was indicated that in presence of titanium dioxide, highly non-uniform
radiation field in the radial direction occurs, and the scattering effects are very
significant [87]. Although, this group of researchers has presented radiation models
for many different reactor and lamp configurations, these models are fairly
complicated and need to be validated with experimental data. Hossain and Raupp
431
M.B. Ray
(1998, 1999) [40, 91J modeled radiation field in photocatalytic monolith reactors
irradiated externally by diffuse source. Monolith reactors experience low pressure
drop and easily adopted for the destruction of common air pollutants. For nonreflecting wall coatings, photon intensitylflux reduces to negligible values within
three to four channel widths, however, application of wall coatings with finite
reflectivity causes increased uniform radiation [40]. A relatively simple but useful
radiation field model for an annular packed-bed photocatalytic reactor was proposed
and tested by Raupp et al. (1997) [49]. The validated model can be used to predict the
optimum catalyst film thickness for a given reactor dimensions, packing size and
shape. However, the model requires an adjustable parameter, the mean free path of
interactions between photon and catalyst. A summary of different reactor models can
be seen at a glance in Table 3.
Commercial reactors such as Rayox (Calgon Carbon Oxidation Technologies,
USA) are cylindrical annular stainless steel reactors with 1 kW medium pressure Hg
lamp housed at the center [37]. Under actual operation, the rated average intensity
must be adjusted for the aging of the lamps and consequent reduction in W output,
and for the losses of energy as it passes through the quartz sleeves. An estimate of
the actual intensity under a given set of conditions is [76]:
I
x (F,) x (F,)
(Nominal I
. .. (62)
where F, is the ratio of the actual to the rated output of the lamps, and F, = ratio of the
actual to the rated transmittance (100%)of the quartz sleeves.
The efficiency in a photoreactor system can be calculated by following the
equation presented in Shen and Ku (1997) [62]:
EE -0,
$1
Pxl 000
Rx60x log[
...
where electrical energy per order (EE/O,) for gas phase systems was defined as the
number of kilowatt-hours required to reduce the concentration of a given pollutant in
air by one order of magnitude in 1000 standard cubic feet of air. P is the lamp power
in kW and R is the air flow rate in scfm. EE/O, is reported to have values of 1.4 at
low intensity to 0.7 at high intensity [62]. L o w efficiency at high intensity is
432
Yokota & Suzuki
Collimated-beam
reactors in continuous
flow systems
Photoreactor with
multiple lamps
Photoreactor with
multiple lamps
Tubular multilamp
General
Cassano et al.
1995 [23]
Blatcley
1997 [79J
1
&L Raupp
Alfano et al.
I986 [84]
Hossain
t""""L
-
-
Reactor
Reference
Extense source model with
Diffused line source
emission model
Spherical incidence model
Line source integration
model
Emission model
Type of Model
Remarks
Angular asymmetry comDlicates the result for attenuation coefficient.
25% of the total radiation
u.
Radial field inside the reactor is not uniform; reflected light is less than
Optimum lamp location in n-lamp symmetry is presented.
A review of theory and applications; Final equation for annular photoreactor provided.
Good comparison with experimental data; lamp output power can be
calibrated.
Table 3. Summary of the Intensity Calculations in Difeeren! I'hororenctors
M.B. Ray
attributed to the longer chain reactions involving hydroxyl radical. Over the next
decade researchers need to design reactors that increase the efficiency of photon
utilization and demonstrate economic feasibility.
Conclusions
During the last decade or so laboratory and field testing of the W-degradation
processes has proved to be useful for the detoxification of wide variety of harmful
chemicals in air. However, several challenges need to be met before the full potential
of this technology can be realized as shown: (i) complete mineralization of the parent
compound irrespective of different photoactivity, (ii) elimination of harmful
intermediates, (iii) enhancement of quantum yield, (iv) higher throughputs, (v) low
energy cost, (v) availability of more intense lamps at wavelenghts c254 nm, and (vi)
reliability of the process. It is the last point which requires serious attention since the
complexity of the process contributes to the controversy and contradictory results
even for frequently studied compounds like TCE. Low quantum yield except for
compounds where chlorine radical induces chain reaction ( 100% efficiency in those
cases) requires high power of U V lamps. While, many of the intermediates undergo
photodegradation along with the parent VOCs, intermediates such as DCAC, DCAA,
phosgene and acidic gases can be removed finally by using wet scrubbers.
Among the different processes, comparatively higher rates are obtainable for UVphotooxidation where hydroxyl radicals are generated. Although photocatalytic
oxidation occurs at longer wavelengths and thus saves energy and capital costs, the
rates are lower than the homogeneous photooxidation requiring much higher reactor
volume, or lower throughput. Water adsorption sometime competes with the
reactants, thus reducing the reaction rate of PCO. Weak adsorption of the reactant on
the catalyst surface also is a disadvantage of the process. On the other hand, presence
of water vapor increases the rate of oxidation of the homogeneous UV-photooxidation
making the process more useful than photocatalysis for treating the effluents from air
stripping and soil vapor extraction. Of the different operating parameters, intensity
and wavelength of radiation is of paramount interest followed by the concentrations
of the different oxidants such as hydroxyl and chlorine radical, ozone and reactive
species of oxygen.
434
Photodegradation of Volatile Organic Compounds in Gas Phase: A Review
The principal difficulty involved in full-scale operation of photochemical process
is the unavailability of technically consistent design approaches for photoreactors and
a detailed cost analysis of such a reactor.
The inherent uncertain nature of
photochemical processes makes the design process even more complicated. The
following steps are identified as necessary for the systematic design of a photoreactor:
i) to develop an approach to estimate the intensity of ultraviolet light in a multi-lamp
reactor accounting for reactor dimensions and geometry, ii) to develop an analytical
method to determine the relationship between the intensity of light in the reactor and
the reaction kinetics for the degradation of the VOCs. While, many studies are being
conducted at present, more methodical studies are required in the last two areas in
order to apply the process in full-scale.
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