ArticlePDF Available

Performance of Air Cleaners for Removing Multi-Volatile Organic Compounds in Indoor Air

Authors:

Abstract and Figures

Fifteen air cleaners, representing different technologies and types of devices, were tested with a mixture of 16 representative VOCs (17 VOCs in tests for products associated with ozone generation) in a full-scale stainless steel chamber by using a "pull-down" test method. Their initial performance was evaluated in terms of single-pass efficiency (η) and the clean air delivery rate (CADR). Technologies evaluated include sorption filtration, ultraviolet-photocatalytic oxidation (UV-PCO), ozone oxidation, air ionization (plasma decomposition), and botanical air cleaning. Based on test results, the relative effectiveness of the available technologies tested and the effect of product configuration and VOC properties on the VOC removal efficiencies are analyzed. The implication of test results on the development of a standard test method for performance evaluation of gas-phase air cleaners is also discussed briefly in this paper.
Content may be subject to copyright.
©2005 ASHRAE. 1101
ABSTRACT
Fifteen air cleaners, representing different technologies
and types of devices, were tested with a mixture of 16 repre-
sentative VOCs (17 VOCs in tests for products associated with
ozone generation) in a full-scale stainless steel chamber by
using a “pull-down” test method. Their initial performance
was evaluated in terms of single-pass efficiency (η) and the
clean air delivery rate (CADR). Technologies evaluated
include sorption filtration, ultraviolet-photocatalytic oxida-
tion (UV-PCO), ozone oxidation, air ionization (plasma
decomposition), and botanical air cleaning. Based on test
results, the relative effectiveness of the available technologies
tested and the effect of product configuration and VOC prop-
erties on the VOC removal efficiencies are analyzed. The impli-
cation of test results on the development of a standard test
method for performance evaluation of gas-phase air cleaners
is also discussed briefly in this paper.
INTRODUCTION
Poor indoor air quality (IAQ) can significantly affect
people’s health, comfort, satisfaction, and productivity. Air
cleaning/purification devices, which can be an effective strat-
egy for improving IAQ in conjunction with source control and
ventilation, have held a substantial market for use in resi-
dences and offices for removing various contaminants (US
EPA 1996). Volatile organic compounds (VOCs) represent a
major class of indoor pollutants. Typical sources include new
building materials and furnishings, consumer products, main-
tenance materials, tobacco smoke, and polluted outdoor air.
VOC pollutants may cause offensive odors, skin and
membrane irritations, allergic reactions, and chronic effects
including cancer. In recent years, more and more air cleaning
devices are advertised in the market for removing chemical
pollutants such as VOCs and for odor control. However, there
is limited information available regarding their performance
beyond the general claims of the manufacturers, and there are
no standard methods for testing the removal of gaseous
contaminants by air cleaning devices.
This paper briefly reviews the available technologies for
removing indoor VOCs and presents test results for initial
VOC removal efficiencies of 15 air cleaners (12 portable air
cleaners and 3 in-duct devices for typical residential applica-
tions), representing different technologies. It discusses the
relative effectiveness of the available technologies tested and
the effect of product configuration and VOC properties on
VOC removal efficiencies.
OVERVIEW OF TECHNOLOGIES
FOR REMOVING INDOOR VOCS
Technologies for removing indoor VOC contaminants
mainly include sorption filtration, ultraviolet-photocatalytic
oxidation (UV-PCO), ozone oxidation, air ionization (plasma
decomposition), and botanical air cleaning. Here, “removing”
generally refers to the concentration decrease of target VOC
pollutants in indoor air. They can either be physically removed
from air by adherence to, and retention on, the solid sorbents
(at least temporarily) or be chemically changed to other
substances such as CO2 and water as the desired final products.
The former is a reversible process, while the latter is an irre-
versible one.
Sorption filtration removes gaseous contaminants from
indoor air by adsorption on solid adsorbents. It is a traditional
and most commonly used technology. Most off-the-shelf
commercial products are based on this technology. The effec-
Performance of Air Cleaners for
Removing Multiple Volatile
Organic Compounds in Indoor Air
Wenhao Chen Jianshun S. Zhang, PhD Zhibin Zhang, PhD
Student Member ASHRAE Member ASHRAE
Wen hao Che n is a graduate student, Jianshun S. Zhang is an associate professor, and Zhibin Zhang is a research scientist in the Department
of Mechanical, Aerospace, and Manufacturing Engineering, Syracuse University, Syracuse, NY.
OR-05-17-2
1102 ASHRAE Transactions: Symposia
tiveness of cleaners based on adsorption technology depends
on the properties and amount of sorbents, the packing (or coat-
ing) density of the sorbent layer, the velocity and flow rate of
air passing through the sorbent media, the properties of the
VOCs, and environmental conditions such as relative humid-
ity and temperature. Depending on specific application
requirements, adsorbents such as activated carbon, zeolite,
and activated alumina with various packing density can be
used as filtration media. In some cases where specific contam-
inant(s) is targeted, the adsorbents can also be impregnated
with selected chemicals that will react with target substances
(chemisorption) (ASHRAE 1999). Activated carbon, espe-
cially granular activated carbon (GAC), is the most common
media for general indoor gaseous pollutant removal purposes
(Henschel 1998; VanOsdell and Sparks 1995). Due to the satu-
ration effect of adsorbents after long-term use, the complete
evaluation of a sorption type device should include evaluation
of both the initial performance (e.g., by initial removal effi-
ciencies) and the long-term performance (e.g., by break-
through time). In addition, possible reemission of the adsorbed
VOCs is a concern.
UV-PCO removes gaseous contaminants via chemical
reactions on semiconductor catalyst surface under UV irradi-
ation. More specifically, when a semiconductor material is
irradiated by photons with energy that matches or exceeds the
band gap energy (Eg) of the semiconductor, an electron is
promoted from the valence band (VB) to the conduction band
(CB), leaving a hole behind. These photogenerated holes and
electrons diffuse to the surface and react with adsorbed water
molecules. The resultant hydroxyl radicals are highly reactive
species that can oxidize VOCs adsorbed on the catalyst surface
(Hager and Bauer 1999; Jacoby et al. 1996). Hoffmann et al.
(1995) reported that the application of illuminated semicon-
ductors for the remediation of contaminants has been success-
fully used for a wide variety of compounds (alkanes, simple
aromatics, etc.). However, there is lack of widespread
commercialization of this technology, and only a few products
are available in the US market. The effectiveness of cleaners
based on UV-PCO technology depends on the photoactivity of
the catalyst, the UV light intensity on the catalyst surface,
contact time between the contaminated airflow and catalyst
surface, the properties of VOCs, and environmental conditions
such as relative humidity and temperature. The most widely
used photocatalyst for air purification today is TiO2 with Eg =
3.2 eV. Depending on the type and concentration level of the
contaminants treated, generation of harmful intermediates and
by-products may be a concern.
Ozone is a strong oxidizer. Theoretically, it can react with
many VOCs found indoors. In today’s market, some ozone-
oxidation-based air purifiers are advertised for regular use in
homes and offices for removing chemicals and odors.
However, under low VOC and ozone concentration levels, the
reaction rate might be too low to be effective for most indoor
VOCs. In case of those VOCs that do react with ozone fast
enough (e.g., a subset of VOCs with unsaturated carbon-
carbon bonds), reaction with ozone may produce other
contaminants, e.g., aldehydes and organic aerosols (Weschler
2000). In addition, lack of adequate control on the ozone
generation level can be a concern of safety because ozone
itself is a potent lung irritant and is harmful to people at
elevated levels.
Air ionizers create charged air molecules upon the appli-
cation of an energy source (Daniels 2002). Theoretically, air
ionization forms “nonthermal” plasmas—cluster ions (radi-
cals)—which decompose VOCs by a complex series of oxida-
tion reactions with eventual products of CO2 and water.
Destruction efficiency depends on ion density, treatment time,
and chemical structures of VOCs (Yan et al. 1998). Two modes
of ionization have often been employed: photon ionization and
electronic ionization. However, since indoor air chemistry
triggered by the ionization process mainly relies on the natural
tendency of the ions and the chemicals that exist in the
surrounding air, harmful intermediates and by-products may
be generated during the process. In addition, many ionizers
may produce ozone. Related fundamental studies are limited
and experimental data are lacking for the demonstration of this
technology for indoor environmental applications.
Botanical air cleaning refers to the removal of gaseous
contaminants from indoor air by plants and their soils through
biological processes. It is a relatively novel idea. There are no
commercial products in the US market advertised for this tech-
nology, although significant reductions in formaldehyde and
other VOCs were reported in several initial static chamber
studies (Godish 2001).
DESCRIPTION OF AIR CLEANERS
SELECTED FOR EVALUATION
For residential houses, two types of devices are readily
available: stand-alone (portable) room air cleaners (including
desktop units) and in-duct devices (filters). Portable air clean-
ers can be easily operated in a room with flexible time sched-
ules but only clean the air in a limited area (e.g., up to several
connected rooms without obstructions to airflow). In-duct
devices (filters) work for the whole house but need to be
installed in the air-handling system and function only when
the air-handling system is operating. Table 1 presents descrip-
tions of air cleaners tested in this study. In Table 1, the air
cleaners are grouped based on the product type first, then the
primary technology for VOC removal, and finally are sorted
according to the purchase price within the same technology
group.
EXPERIMENTAL METHODS
Test Facilities
All of the tests for characterizing the VOC removal effi-
ciencies were carried out in a full-scale environmental cham-
ber (Figure 1), which has an interior volume of 1920 ft3 (16 ft
by 12 ft by 10 ft). The chamber and all its components are
made of stainless steel to minimize the adsorption/desorption
of contaminants by the chamber itself. It has a dedicated
ASHRAE Transactions: Symposia 1103
HVAC system for the control of the airflow rates and environ-
mental conditions in the chamber. A detailed description of
this chamber facility and its performance evaluation can be
found in Herrmann et al. (2003).
Test Procedures
ASHRAE has sponsored several research projects to
investigate feasible test methods for determining the effective-
ness and capacity of gas-phase air filtration equipment for
indoor applications (Ostojic 1985; VanOsdell 1994). A draft
standard (ASHRAE Standard 145.1P) has been proposed. It
mainly addresses the media performance and therefore cannot
be used for rating the overall performance of an air cleaner. To
our best knowledge, no standard test methods have been estab-
lished for testing the effectiveness of gaseous portable or resi-
dential in-duct air cleaners. AHAM has developed an ANSI-
approved standard (AHAM 2002) for testing the particulate
removal efficiencies of portable air cleaners. Under this stan-
dard, the effectiveness of the room air cleaner is quantified by
the clean air delivery rate (CADR). This concept has been
applied to testing the initial gaseous contaminant removal effi-
Table 1. Summary of Tested Products
Product
Type
Primary Technology
for VOC Removal Device No.
Purchase
Price
Type of Air Cleaning Technologies
(Stated by Manufacturer)
Flow
Rate*
(CFM)
Portable Sorption filtration P1 $120 (1) Special high grade of activated carbon filter and (2)
allergy relief filter
300
P2 $158 Activated carbon pre-filter and (2) HEPA filter 335
P3 $299 HEPA filter, (2) plasma deodorization unit, and (3) activated
charcoal filter
240
P4 $300 Pre-filter, (2) cotton retaining filter, (3) 6.5 lbs. of carbon-
zeolite mixture impregnated with potassium iodide, and (4)
HEPA filter media
160
P5 $315 Aluminum mesh pre-filter, (2) HEPA filter, (3) polyester
fiber filters treated with an anti-microbial solution, and (4)
activated charcoal filter
250
P6 $360 Two layers of nonwoven, polyester filter media impregnated
with activated carbon and (2) HEPA filter
200
P7 $470 Pre-filter, (2) electronic cell, and (3) activated carbon post-
filter
320
UV-PCO P8 $399 High-intensity UV lamp and photo-catalytic semi-conductor 110
UV-PCO
+ Air ionization
P9 $699 Photo-ionization module, including UV lamp and tri-metallic
catalyst, and (2) electron generator
14
Air ionization P10 $150 Needlepoint ionization – use 16 stainless steel, ion-producing
electrodes to produce a high intensity of negative ions and
generate ozone as a by-product
-
Ozone oxidation P11 $200 Photoplasma/photochemistry 8
Botanical air cleaning P12-Biofiltration – plant and a proprietary lightweight soil 22**
In-duct Sorption filtration P13 $33 Granular activated carbon and activated alumina impregnated
with potassium permanganate
N/A††
UV-PCO + Sorption fil-
tration
P14 $485 (1) High-intensity UV lamps and photo-catalytic semi-con-
ductor and (2) pleated activated carbon filter
N/A
UV-PCO P15-Two honeycomb monoliths coated with titanium dioxide cat-
alyst and an array of 3 germicidal UV lamps in between
N/A
*Measurement methods can be found later in section of flow rate measurements. Results shown here are for maximum speed level for each air cleaner.
Product P10 is an ionizer with no fan unit but generates tiny ionic breeze. Accurate measurement of the flow rate, although possible, was difficult using our current experi-
mental setup and therefore was not conducted.
Product P12 and P15 are prototypes of innovative products.
** Product P12 is an indoor plant system but has a small fan to deliver air movement through the soil and the plant root system.
†† “N/A” means “Not Applicable” because product P13, P14, and P15 are in-duct devices and their flow rates are controlled by the air-handling system. The measured pressure
drop under test conditions was 0.022 in. w.g., 0.05 in. w.g., and < 0.01 in. w.g. for product P13, P14, and P15, respectively. The initial resistance from manufacturer is only
available for product P13 and the given initial resistance is 0.18 in. w.g. at 300 ft/min.
1104 ASHRAE Transactions: Symposia
ciencies of portable air cleaners by previous researchers
(Daisey and Hodgson 1989; Niu et al. 1998).
Since most of the selected products were portable air
cleaners, a “pull-down” test method, similar to that used by
Daisey and Hodgson (1989) and Niu et al. (1998), was used in
this study. It consisted of three test periods under full-recircu-
lation condition: injection period (1 h), static period (1 h), and
dynamic period (12 h) (Figure 2). The injection of a known
amount of contaminants into the experimental system,
followed by a static period, resulted in stable initial high
concentration levels. The time when the air cleaner was turned
on was defined as time zero, at which the dynamic period
began. Using the measured concentration decay rate from the
dynamic period, the clean air delivery rate (CADR) as well as
the removal efficiency of the cleaner could then be calculated
for each VOC tested. It should be noted that only initial perfor-
mance of air cleaners was tested under this procedure.
For all of the tests, air temperature in the chamber was
maintained at 73.4±1°F, and relative humidity was maintained
at 50±5%. Tracer gas (CO2 or SF6) was injected and monitored
during the tests to account for any mass loss due to the cham-
ber leakage and air sampling.
For portable air cleaners, the flow rate through the device
was controlled by its own operation level setting. A 160 CFM
(5 ACH) air recirculation rate was provided through a square
type diffuser by the HVAC system to achieve good air mixing
in the chamber (i.e., mixing level > 94%). For in-duct air
cleaners, airflow through the device was controlled by the flow
rate of the chamber’s HVAC system. While it is desirable to
have a flow rate that results in a face velocity of 300-600 ft/min
as in real applications, the current chamber HVAC could only
provide a maximum of 800 CFM recirculation flow, which
was used and resulted in a 200 ft/min face velocity for the 2 ft
by 2 ft cross section of the air cleaning devices tested.
Test VOCs
Over 300 VOCs have been found indoors, and all of these
compounds may not be of equal importance in a single envi-
ronment. A mixture of 16 VOCs (Table 2) was chosen in this
study, covering major chemical categories and a wide range of
molecular weight and boiling points. For air cleaning devices
associated with ozone generation, d-Limonene was added
because it belongs to the subset of VOCs that contains unsat-
urated carbon-carbon bond(s) and can react with ozone much
faster according to theory (Weschler 2000). Choice of the
initial concentrations for testing also proved to be a problem.
The initial concentration levels should be reasonable for
indoor applications and yet still high enough so that the
concentration decay curve can be accurately measured before
it goes below the instrument’s detection limit. In this research,
initial concentrations of 1 mg m-3 were targeted for all VOCs
except for formaldehyde and acetaldehyde that had a target
initial concentration of 2 mg m-3. Formaldehyde was gener-
ated by directly heating solid paraformaldehyde inside the
chamber, and other VOCs were introduced into the chamber
by vaporizing a known amount of VOC liquid mixture during
the injection period.
Test Specimen
The portable air cleaners were tested as received without
modification. Each air cleaner was positioned at the same
place inside the chamber (3 ft away from the corner) and oper-
ated at its maximum speed setting except for P11, which was
run at “low” operation level according to the product’s user
guide due to the concern on the ozone emission.
For the in-duct devices, necessary modifications were
made so that the device can be installed and sealed reasonably
well in the test box on the return duct of the chamber’s HVAC
system. These modifications did not alter any VOC removal
component(s) of the devices.
Figure 1 Full-scale chamber. Figure 2 Schematic of “pull-down” test method.
ASHRAE Transactions: Symposia 1105
Instrumentation for Sampling and Analysis
A precalibrated photoacoustic multi-gas monitor was
used for real-time measurements of the concentration of total
hydrocarbon as toluene equivalent (TVOCtoluene), the concen-
tration of total formaldehyde and acetaldehyde as formalde-
hyde equivalent (Cformal+acetal), and the concentration of tracer
gas (CO2 or SF6). For TVOCtoluene and Cformal+acetal, since the
sensitivity and response factor of the instrument for different
compounds are different, the readings from the gas monitor
were only used as semi-quantitative measures to characterize
the change of TVOC concentrations over time and how they
differ for different air cleaning devices. For quantitation of
individual VOC, air samples were collected on the return duct
of the chamber using sorbent tubes (Tenax TA, 0.2 mg). At
least eight samples were taken during the 12 h dynamic period
for each test. The tube sampling periods varied from 0.7 L to
5 L, depending on the expected highest concentration level of
individual VOC. These sample tubes were then analyzed by an
ATD-GC/MS (automated thermal desorber-gas chromato-
graph/mass spectrometer) system to determine the concentra-
tion of each individual compound, excluding formaldehyde
and acetaldehyde. The detection limit was estimated to be
0.004 mg m–3 for a 0.75 L sample. For formaldehyde and
acetaldehyde, DNPH-Silica cartridges were used to collect
samples and then analyzed by an HPLC system. The detection
limit was estimated to be 0.025 mg m–3 for a 10 L sample. The
measurement uncertainty for individual VOC was estimated to
be ±15% based on the 95% confidence interval. A chemilu-
minescene ozone analyzer was used to measure ozone concen-
tration, which has a precision of 0.5% of reading and a
detection limit of 0.6 ppb.
Flow Rate Measurement for Portable Air Cleaners
Airflow rate measurements were made at each speed
setting for each portable air cleaner. Since flow rates of tested
devices varied from a few CFM to hundreds of CFM, two
different experimental setups were used. For flow rates above
50 CFM, a flow hood system was used to directly measure the
flow rate. For flow rates below 50 CFM, the velocities and
cross-sectional area at air intake (or outlet) were measured and
the flow rate was then calculated. The measurement uncer-
tainty was estimated to be within ±15%.
Empty Chamber Characterization
An empty chamber test was performed first to investigate
the possible effects of chamber characteristics (mainly sink
effect) on test results for the air cleaners. Figure 3a shows the
measured results of TVOCtoluene, Cformal+acetal, and tracer gas
SF6 from the gas monitor. Figure 3b shows the measured
results for several individual VOCs. Normalized concentra-
tions (i.e., concentration divided by the initial concentration at
time t = 0) were used to facilitate the comparison.
Table 2. Components of Challenge VOC Mixture and Their Properties
Chemical Category Chemical Name
Molecular
Formula MW BP (°C)
VP at 23°C
(mm Hg)
ASHRAE Std
145.1P*
Alkane n-Hexane C6H14 86.2 69 139.88 X
n-Octane C8H18 114.2 126 12.56
n-Decane C10H22 142.3 174 1.25
n-Undecane C11H24 156.3 196 0.35
n-Dodecane C12H26 170.3 216 0.12
Aromatic Tol u e ne C7H892.1 111 25.64 X
Ethylbenzene C8H10 106.2 136 8.58
Chlorocarbon Dichloromethane CH2Cl284.9 40 399.27 X
Tetrachloroethylene C2Cl4165.8 121 16.66 X
1,2-Dichlorobezene C6H4Cl2147.0 180 1.19
Aldehyde Formaldehyde CH2O30.0 –19 3652.58 X
Acetaldehyde C2H4O44.1 20 835.7 X
n-Hexanal C6H12O100.2 128 10.00
Ketone 2-Butanone C4H8O72.1 80 86.95 X
Cyclohexanone C6H10O98.2 156 3.82
Alcohol 2-Butanol C4H10O74.1 100 16.01
Terpene HC d-Limonene*C10H16 136.2 177 1.74
*Compounds included in VOC challenge group of ASHRAE Standard 145.1P were included (indicated by “X”).
Compound with “*” was used only in tests for products P9, P10, and P11.
1106 ASHRAE Transactions: Symposia
Results indicate that the decay rate of SF6 was very small
(0.003 ACH) under full-recirculation mode, which was negli-
gible and indicated good airtightness of the chamber. As for
the chamber sink effect, the decays of TVOCtoluene and Cfor-
mal+acetal under full-recirculation mode (5 ACH recirculating
flow rate) were 0.005 ACH, which was only a little higher than
the decay rate of SF6 and indicated a small sink effect. Indi-
vidual VOC analysis results show that the decay rate varied
from compound to compound and the maximum decay rate
was 0.023 ACH for dodecane (the heaviest compound in the
test VOC mixture). In addition, during the flush period
(0.5 ACH clean makeup air) following the full recirculation
period, the VOC concentration decayed at almost the same
rates as SF6 until the concentrations reached the detection
limit of the measurement instrument, which further verifies
that the reversible sink effect of chamber was small under the
experimental conditions. As a result, the maximum error
caused in estimating Ncl is less than 0.023 ACH if the chamber
sink effect is neglected. Since the CADR correspondent to
0.023 ACH is only 0.74 CFM, we neglect the chamber sink
effect in this research and regard the effectiveness of air
cleaner with Ncl 0.03 ACH (CADR 1 CFM) as insignifi-
cant. The initial VOC concentrations did not exactly match the
expected values, which were possibly caused by the irrevers-
ible sink effect, the injection method uncertainty, and concen-
tration measurement uncertainty. However, this did not affect
the performance evaluation tests because the effectiveness of
an air cleaner was measured by the concentration decay from
its initial measured value.
Data Analysis Procedure
For each air cleaner tested, the single-pass efficiency (η)
and the clean air delivery rate (CADR) were calculated for
each VOC, respectively.
Single-pass efficiency (η) represents the fraction of
pollutants removed from the airstream as it passes through the
air cleaner. It is defined as
(1)
where
Cin = contaminant concentration at the inlet of air cleaner,
mg m–3,
Cout = contaminant concentration at the outlet of air cleaner,
mg m–3, and
Qcl = airflow rate through the air cleaner, m3 h–1 (CFM).
CADR reflects the “overall” effectiveness of the air
cleaner. It is defined as
(2)
where
Ed= short-circuiting factor of the air cleaner, Ed = Cin /
C, where C is the average VOC concentration in
the test chamber (Ed = 1 under well-mixed
condition).
The data analysis procedure was similar to that used by
Daisey and Hodgson (1989) and Niu et al. (1998), which was
based on the well-mixed single-zone model. Assuming perfect
mixing in the chamber and neglecting sink effect, the mass
balance for a test VOC under full-recirculation mode during
the dynamic period can be written as
(3a)
or
(3b)
Figure 3 Measurement results of TVOCtoluene, Cformal+acetal, and tracer gas (a) and some selected individual VOCs (b) for
empty chamber test.
ηQcl Cin Cout
()
QclCin
-------------------------------------- Cin Cout
Cin
------------------------- ,==
CADR ηQcl Ed ,⋅⋅=
VdC
dt
-------Qleak CADR+()Ct()=CC
0
= at t0=()
dC
dt
-------Nleak Ncl
+()Ct()=CC
0
= at t0=() ,
ASHRAE Transactions: Symposia 1107
where
V= volume of chamber system, m3 (ft3);
Qleak = leakage flow rate of chamber system, m3 h-1 (CFM);
t= time from beginning of dynamic period, h;
Nleak = air change rate due to leakage (Nleak = Qleak / V); and
Ncl = equivalent clean air change rate of the air cleaner
(Ncl = CADR / V).
If CADR does not change (i.e., η is constant) during the
test period, an analytical solution can be obtained from Equa-
tion 3a as
(4)
Ncl can be determined by linear regression of ln (C/C0) versus
t from measured concentration decay curve. The CADR and η
can then be estimated.
However, the obvious decrease of CADR was observed
during the later part of the 12 h dynamic period for some sorp-
tion-based air cleaners. In addition, for devices using air clean-
ing technologies other than sorption filtration (e.g., P15), the
concentration decay of some test VOCs was faster during the
later part than the initial part of dynamic period. These made
the direct fitting of all experimental data to Equation 4 inap-
propriate. To account for these effects, the CADR1h and
CADR12h were defined, respectively, for the purpose of
comparing different air cleaners and calculated as follows:
1. Calculate a 12-h average equivalent clean air change rate
(Ncl-12h) based on the time-averaged VOC concentration
during the test period
(5)
where, T is the length of the dynamic test period (T = 12
h).
Then obtain CADR12h by CADR12h = Ncl-12h · V and
calculate the removal efficiency η12h according to Equa-
tion 2. Ncl-12h reflects the average effect of the air cleaner
during the entire test period.
2. For sorption type devices, calculate Ncl-1h by fitting the first
hour experimental data to Equation 4. Obtain CADR1h by
CADR1h = Ncl-1h · V and calculate the removal efficiency
η1h according to Equation 2. If Ncl-12h 0.03ACH, the
effectiveness of the air cleaner is considered “insignificant”
for the given compound, and CADR1h is not reported.
It can be easily proved that CADR1h is equal to CADR12h
if CADR remains constant during the test period.
TEST RESULTS
TVOCtoluene and Cformal+acetal
from Gas Monitor Measurement
Figure 4 and Figure 5 present the TVOCtoluene and Cfor-
mal+acetal measured by gas monitor for all the air cleaners
tested, respectively. Normalized concentrations were used to
facilitate the comparison. For product P14, the test was
extended to 24 h since it consists of both a “UV-PCO” compo-
nent and a pleated media filter. During the first 12 h, only the
UV-PCO component was installed (P14[A]). During the
second 12 h, both the UV-PCO component and the pleated
media filter were installed (P14[B]).
A significant decrease of TVOC level was observed
during the test for all sorption-related products except for P6.
However, the performance varied a lot from product to prod-
uct. For example, product P1 and P14(B) worked best. They
reduced TVOC level most quickly and followed the single
exponential decay well. However, for product P7, the decrease
of TVOC level was significant and approximately followed
the single exponential decay initially but became very slow
during the latter part of test period, suggesting the decrease of
removal efficiency. For product P6, the ATD-GC/MS analysis
of tube samples showed the presence of a significant amount
of a new compound. This new compound was tentatively iden-
tified as (S)-(+)-3-Bromo-2-Methyl-1-Propanol by MS scan
analysis with NIST library (matching quality = 81%) and most
likely came from the new product itself. As a result, an
increase of TVOC level was observed during the experiment.
However, it did reduce concentration of most of the injected
VOCs very significantly. Product P15, which is a prototype
UV-PCO device, also significantly reduced the TVOC level
until the detection limit of the instrument, although not as
quickly as the best sorption-based products. The reduction of
TVOC level by other tested products (P8 to P12 and P14[A])
was similar to or only slightly larger than that during the empty
chamber test, meaning that these products had negligible
removal effects.
For the removal of formaldehyde and acetaldehyde,
results indicate that product P15 worked best. The Cfor-
mal+acetal level at the end of test was lowest for product P15,
although the initial decrease of Cformal+acetal level for product
P15 was slower than that for product P14(B), P13, and P4. For
sorption-related products, product P4 and P13, which contain
either impregnated carbon/zeolite mixture or impregnated
activated carbon/activated alumina mixture, significantly
reduced the Cformal+acetal level. However, products with acti-
vated carbon only could not remove or removed only a small
amount of formaldehyde and acetaldehyde, except for product
P14(B), which has a thick (approximately 3.5 in.) pleated
filter. Product P12 also significantly reduced the Cformal+acetal
level. The effectiveness of other products was either insignif-
icant or marginal.
CADR and Single-Pass
Removal Efficiency for Individual VOC
Figures 6a and 6b present the measured concentration
decay (normalized by initial concentrations) of several VOCs
(e.g., alkane group, formaldehyde, and acetaldehyde) during
the dynamic period for products P1 and P15, respectively.
Similar concentration decay curves were obtained for each air
cleaner tested, from which the CADR and the single pass
Ct() C0e
Nleak Ncl
+()t
=t0() .
Ctd
0
T
T
------------
C0e
Nleak Ncl 12h
+()t
td
0
T
T
-------------------------------------------------------------- ,=
1108 ASHRAE Transactions: Symposia
Figure 4 TVOCtoluene for air cleaners based on sorption filtration (a) and other technologies (b).
Figure 5 Cformal+acetal for air cleaners based on sorption filtration (a) and other technologies (b).
Figure 6 Measurement results of some selected individual VOCs for Product P1 (a) and Product P15 (b).
ASHRAE Transactions: Symposia 1109
removal efficiency were then calculated. Results are summa-
rized in Table 3. They clearly indicate that the VOC removal
characteristics varied a lot from product to product and were
in good agreement with the decay trend of TVOCtoluene and
Cformal+acetal measured by the gas monitor. In addition, results
indicate that the removal efficiencies varied from compound to
compound even for the same air cleaner. None of the tested
products could equally remove all the test VOCs at the same
rate. The characteristics of the concentration decay curve
depended on the properties of the compound, the technology
used, as well as the product design.
Ozone Emission and Its Effect on VOC Removal
Ozone concentrations much higher than the safety limit
set by OSHA (100 ppb) were observed during the dynamic
test period for products P9 and P11 (Figure 7). Product P10
also had significant ozone generation. Test results indicate
that these products only significantly reduced the concentra-
tion of d-Limonene. Figure 8 compares the measured d-
Limonene concentration and ozone concentration for prod-
ucts P9, P10, and P11. It clearly shows the effect of ozone
concentration on the decay of d-Limonene concentration.
The decay of d-Limonene concentration was quickest for
product P11 and slowest for product P10, which had the larg-
est and smallest ozone emissions, respectively. Stable prod-
ucts (e.g., organic acids and organic aerosols) might be
generated from ozone/d-Limonene reaction but were not
detected due to the limitation of experimental design as well
as the sampling and analyzing techniques used in this study.
Small ozone emissions were detected during the test for P7
(< 20 ppb) and P3 (< 4 ppb) due to the use of electronic cell
(P7) and plasma unit (P3). Ozone generations from other
devices were negligible (< 1 ppb).
DISCUSSION
Comparison with Previous Studies
Test results obtained in this study were compared with
some of the previous researchers’ work (Daisey and Hodgson
1989; Niu et al. 1998; Reed et al. 2002). Daisey tested four
portable air cleaners (all contained a certain amount of acti-
vated carbon) for their initial effectiveness for removing NO2
and six representative VOCs (dichloromethane, 2-butanone,
n-heptane, toluene, tetrachloroethylene, and hexanal). Niu
tested 27 portable air cleaners for their initial effectiveness of
gaseous contaminant removal by using toluene as the test
compound. Reed tested both a portable and an in-duct gaseous
air cleaner using a different test procedure. Only toluene was
used as test VOC and it was continuously injected to the house
at a constant rate. Once a steady-state concentration had been
reached, the air cleaner was turned on and concentration decay
was measured until a new steady-state concentration had been
achieved. Both CADR and single-pass efficiency were calcu-
lated. Their test results indicate that (1) none of the tested air
cleaners could significantly remove dichloromethane, (2) no
effects were observed for air cleaners only employing ioniza-
tion technologies, and (3) the removal efficiency for toluene
varied a lot from product to product for sorption-based air
cleaners, ranging from 0.3% to 53%. Our test results agree
with their observations. The present study provides results for
more VOCs and more types of air cleaning technologies.
Correlation between Removal Efficiency
and Properties of VOC
It is generally acknowledged that the adsorbability of
gases and vapors on the adsorbent material depends on their
molecular size (weight), boiling point, and vapor pressure. For
sorption-based products, the general trend observed in this
study was that the efficiency increased with the increase of
molecular weight and boiling point of the compound and
decreased as the vapor pressure of the compound increased.
However, the relationship was not linear but a stepwise one
(Figure 9).
Results indicate that for the very volatile organic
compounds, such as formaldehyde, acetaldehyde, and dichlo-
romethane, their removal characteristics were different from
other tested compounds and they were difficult to remove. Use
of other sorbent materials with special impregnants besides
activated carbon may improve the removal efficiencies for
these compounds, but the improvement varied from product to
product. For the tested compounds with vapor pressure less
than 2 mmHg at 23°C, the removal efficiencies by the same air
cleaner were similar (Table 4). This suggests that the number
of test compounds may be reduced and decane can be selected
to represent heavier compounds (e.g., VP < 2 mmHg at 23°C)
for the purpose of testing. Since toluene (VP = 25.64 mmHg
at 23°C) is often selected as the reference compound for quan-
tifying TVOC, the removal efficiency of toluene and the aver-
age removal efficiency of test compounds with vapor pressure
less than toluene (including toluene) but larger than decane
was also compared (Table 5). Results indicate that we could
have a reasonably good estimation for the removal efficiency
of VOCs with middle range vapor pressure (e.g., 2 < VP < 26
mmHg at 23°C) if toluene was used as the representative
compound for the purpose of testing.
As for the effective UV-PCO device (i.e., P15), the high-
est single-pass removal efficiencies were observed for n-hexa-
nal, cyclohexanone, and 2-butanol (7.0%, 7.1%, and 5.9%,
respectively). The lowest removal rates were observed for
dichloromethane, hexane, and tetrachloroethylene (0.3%,
0.6%, and 0.7%, respectively). For some compounds, such as
dichloromethane, hexane, and tetrachloroethylene, the
removal rates were very low at the beginning and became
larger later during the test period, which was quite different
from the removal characteristics for sorption-based products.
It seems that the reactions of these compounds on the catalyst
surface were prohibited by the coexistence of other VOCs at
high concentration levels at the beginning, possibly due to the
competition of the available adsorption sites. Once the other,
more reactive VOCs have been decomposed, the reactions for
these compounds became faster. In the previous studies by
1110 ASHRAE Transactions: Symposia
Table 3. Summary of CADR and Single-Pass Efficiency for Individual VOC for Tested Air Cleaners
VOC Name*
CADR12h (CFM) ±15%
P1 P2 P3 P4 P5 P6 P7 P12 P13 P14(B) P15
Formaldehyde 1.7 0.8 2.8 6.4 12.9 0.8 4.6 11 24 13
Acetaldehyde 0.8 0.1 1.3 4.4 0.9 0.1 04.3 5.8 14 11
Dichloromethane 0.4 0.3 0.6 12 0.8 0.6 0.4 0 4 97 2
n-Hexane 53 12 8.9 49 10 19 2.4 023 328 4.5
2-Butanone 20 8.5 5.2 40 7.1 12 2 0 18 329 11
Tol u ene 96 26 16 31 14 52 6.4 028 337 14
Tetrachloroethylene 94 20 14 53 13 45 4.7 0.1 26 373 5.4
2-Butanol 53 17 11 27 11 33 3.7 329 345 47
n-Octane 107 33 20 51 15 89 11 028 344 9.1
Hexanal 114 35 23 50 15 93 11 4.9 43 370 56
Ethylbenzene 117 35 22 55 16 98 11 0.3 30 355 21
Cyclohexanone 117 32 22 29 16 69 8.8 2.4 36 364 57
n-Decane 120 42 23 50 15 124 26 027 358 20
1,2-Dichlorobenzene 129 43 24 42 14 104 22 0.8 32 356 21
n-Undecane 121 44 23 47 14 122 36 0.6 27 345 24
n-Dodecane 117 47 22 43 13 110 44 0.9 28 333 32
VOC Name a
η12h (%) ±21%
P1 P2 P3 P4 P5 P6 P7 P12 P13 P14(B) P15
Formaldehyde 0.6 0.2 1.2 4.0 0.4 1.5 0.3 20.9 1.4 3.0 1.6
Acetaldehyde 0.3 0.0 0.5 2.8 0.4 0.1 0.0 19.5 0.7 1.8 1.4
Dichloromethane 0.1 0.1 0.3 7.5 0.3 0.3 0.1 0.0 0.5 12.1 0.3
n-Hexane 17.7 3.6 3.7 30.6 4.0 9.5 0.8 0.0 2.9 41.0 0.6
2-Butanone 6.7 2.5 2.2 25.0 2.8 6.0 0.6 0.0 2.3 41.1 1.4
Tol u ene 32.0 7.8 6.7 19.4 5.6 26.0 2.0 0.0 3.5 42.1 1.8
Tetrachloroethylene 31.3 6.0 5.8 33.1 5.2 22.5 1.5 0.5 3.3 46.6 0.7
2-Butanol 17.7 5.1 4.6 16.9 4.4 16.5 1.2 13.6 3.6 43.1 5.9
n-Octane 35.7 9.9 8.3 31.9 6.0 44.5 3.4 0.0 3.5 43.0 1.1
Hexanal 38.0 10.4 9.6 31.3 6.0 46.5 3.4 22.3 5.4 46.3 7.0
Ethylbenzene 39.0 10.4 9.2 34.4 6.4 49.0 3.4 1.4 3.8 44.4 2.6
Cyclohexanone 39.0 9.6 9.2 18.1 6.4 34.5 2.8 10.9 4.5 45.5 7.1
n-Decane 40.0 12.5 9.6 31.3 6.0 62.0 8.1 0.0 3.4 44.8 2.5
1,2-Dichlorobenzene 43.0 12.8 10.0 26.3 5.6 52.0 6.9 3.6 4.0 44.5 2.6
n-Undecane 40.3 13.1 9.6 29.4 5.6 61.0 11.3 2.7 3.4 43.1 3.0
n-Dodecane 39.0 14.0 9.2 26.9 5.2 55.0 13.8 4.1 3.5 41.6 4.0
*The test VOCs are listed in the order of decreasing vapor pressure
Note: For product P8 - P11 and P14(A), none of the injected VOC except d-Limonene was significantly removed (Ncl 0.05ACH). Therefore, they are not included here.
ASHRAE Transactions: Symposia 1111
Hall et al. (1998) and Hossain et al. (1999), the contaminants
were often treated on a non-interacting basis when developing
the design model for the UV-PCO device with honeycomb
configuration. However, the above test results suggest that the
interference effect among different VOCs should be consid-
ered even under indoor contaminant concentration levels.
Currently, there is no such model available. In addition, the
increase of reaction rate as the increase of molecular weight
and hydroxyl radical reaction rate constant was reported for
the single-compound test for the alkane group (Obee and Hay
1999; Sattler 1996). A similar trend was observed for the
alkane group in our test, suggesting that it is possible to predict
the removal rate of each VOC in a multi-component system
using the removal rate obtained from a single VOC test. More
fundamental studies are needed in this research area to verify
this hypothesis and establish the appropriate correlations.
Effect of Design Configuration
on the Performance of Air Cleaners
As seen from our test results and previous researchers’
work, the measured CADRs and single pass efficiencies varied
a lot from product to product for sorption-based cleaners.
Table 3 and Figure 9 indicate that the 12-h average efficiencies
for all test compounds were less than 15% for products P2, P3,
P5, P7, and P13, and ranged from 25% to 55% for most of the
test compounds for products P1, P4, P6, and P14 (B). To have
a better understanding about the range of single-pass effi-
ciency a sorption type device may provide, we further discuss
the relationship between single-pass efficiency and the filter
design. For products P1 and P14, pleated media filter are used
(1 in. thick for P1 and 3.5 in. thick for P14). Since the pleated
media filters provide more surface area and better contact
between contaminated airflow and the adsorbent granular,
both of them showed high efficiencies. For product P4, it has
6.5 lbs of sorbent pellets packed in a cylindrical design
(approximate 1 in. thick). As we know from fundamentals of
the sorption process, for the VOC molecule to be adsorbed, it
must first be transported from air across the boundary layer
surrounding the adsorbent granule (or pellet) and then diffuse
into the pores to reach the internal surface of the adsorbent. In
this product configuration, it means that the VOC molecules
have to travel for a longer path due to the porosity of the packed
bed and the larger radius of pellets. Therefore, the single-pass
efficiencies for P4 were a little smaller than P1 and P14,
although the amount of sorbents is large. The design of sorp-
tion filter is similar for product P3, P5, and P13. All have gran-
ular adsorbents loosely packed in a frame or between other
filter media. The packed adsorbent layer is thin and void space
between the adsorbent granule can be easily seen by the naked
eye, indicating that a significant amount of flow may bypass
the adsorbent granule each time. As a result, the single-pass
efficiencies for these products were low. As for products P2,
Figure 7 Measured ozone concentration. Figure 8 Ozone effect on removal of d-Limonene.
Figure 9 η vs. VP of test VOCs for sorption-related air
cleaners.
1112 ASHRAE Transactions: Symposia
P6, and P7, the activated carbon is finely impregnated in the
fibrous filter and cannot be separated from the fibers. For prod-
uct P6, it has two layers of filter media impregnated with acti-
vated carbon (total 7/16 in. thick). Therefore, it had relatively
high removal efficiencies. For product P7, the filter media
impregnated with activated carbon is very thin (less than 1/8
in. thick). Therefore, its single-pass efficiencies were low. In
addition, the removal efficiencies for this type of filter design
seemed to vary most from compound to compound. Since the
adsorption of VOCs by filtration media is a complex process,
a more quantitative analysis will involve detailed consider-
ations of flow arrangement, external mass transfer of VOC
molecules through the boundary layer, diffusion of VOC
molecules inside the micro-pores, and adsorption equilibrium
on the internal surface of adsorbent granule, which is a subject
for future studies.
As for the UV-PCO devices, test results indicate that a
properly designed UV-PCO device (P15) was effective for all
test compounds, although the removal efficiency was different
from compound to compound. However, poorly designed UV-
PCO devices (e.g., P8 and P14[A]) could not significantly
remove any of the test VOCs. Experiments confirmed that
only UV irradiation could not effectively remove VOCs. An
effective UV-PCO device must have properly selected cata-
lyst, a sufficiently large surface area with catalyst coating, and
a good common interface for UV light, catalyst, and contam-
inated airflow. Longer contact (residence) time could increase
the single-pass removal efficiency. A comprehensive model,
simultaneously describing the UV light distribution, surface
reaction kinetics, and contaminant mass transfer, is needed for
designing an efficient UV-PCO device.
Technology Evaluation and Comparison
Five types of technologies have been used in the tested
products: sorption filtration, UV-PCO, ozone oxidation, air
ionization (plasma decomposition), and botanical air cleaning
(plant and its special soil).
For all sorption-related products (P1 to P7, P13, and P14
[B]), significant removal efficiencies have been observed for
most test VOCs. The light and very volatile gases, such as
dichloromethane, formaldehyde, and acetaldehyde, were
difficult to remove by activated carbon only. However, the
removal efficiencies for these gases could be improved if
specific sorption media (e.g., activated alumina impregnated
with potassium permanganate) were added. Therefore, the
sorption filtration should be regarded as an effective and prac-
tical technology for general indoor VOC control purpose if
sorbent materials are properly selected. However, it must be
noted that only the initial performance characteristic for each
air cleaner was tested under the current “pull-down” test
method. Since the adsorbent media only have limited lifetime
and must be replaced on a regular basis (typically three to six
months according to manufacturer’s recommendations) to
maintain good performance, more research is needed to accu-
rately measure or predict the useful lifetime of sorption type
devices.
The UV-PCO technology has been successfully demon-
strated in lab reactors and prototype devices (Hall et al. 1998)
and has been regarded as a promising technology for air puri-
fication (Hoffmann et al. 1995). Our test results show that a
properly designed UV-PCO device (P15) had removal effi-
ciencies competitive to sorption type devices. Only one by-
product with significant amount (positively identified as acetic
Table 4. Summary of Single-Pass Efficiency for Compounds with VP < 2 mm Hg at 23°C
η 12h
Device No.
P1 P2 P3 P4 P5 P6 P7 P13 P14(B)
Decane 40.0 12.5 9.6 31.3 6.0 62.0 8.1 3.4 44.8
Average*40.6 13.1 9.6 28.4 5.6 57.5 10.0 3.6 43.5
S.D.1.7 0.6 0.3 2.3 0.3 4.8 3.1 0.3 1.4
*Average was calculated for decane, 1,2-dichlorobenzene, undecane, and dodecane.
S.D. means “standard deviation.”
Table 5. Summary of Single-Pass Efficiency for Compounds with 2 < VP < 26 mm Hg at 23°C
η 12h
Device No.
P1 P2 P3 P4 P5 P6 P7 P13 P14(B)
Tol u ene 32.0 7.8 6.7 19.4 5.6 26.0 2.0 3.5 42.1
Average*33.2 8.4 7.6 26.4 5.7 34.2 2.5 3.9 44.4
S.D. 7.6 2.2 1.9 7.9 0.7 12.9 1.0 0.7 1.7
*Average was calculated for toluene, tetrachloroethylene, 2-butanol, octane, n-hexanal, ethylbenzene, and cyclohexanone.
S.D. means “standard deviation.”
ASHRAE Transactions: Symposia 1113
acid [CH3COOH] by comparing its retention time with acetic
acid standard injection) was detected by ATD-GC/MS analy-
sis under the test conditions. However, there are only a few
commercial products readily available in the US market and
some of them (e.g., P8), although advertised as UV-PCO
based, were not good implementation of this technology and
could not effectively remove any of the test VOCs. It is clear
that the UV-PCO technology lacks widespread and mature
commercialization for indoor applications, which may be
caused by two reasons: (1) relatively large power consumption
requirement and low quantum efficiency and (2) difficulty of
efficiently bringing UV light, catalyst, and contaminated
airflow together. According to Raissi et al. (2003), even when
a low-pressure mercury lamp (LPML) is used as the source of
UV light, the electricity to UV energy conversion is less than
40%, and a considerable amount of wasted thermal radiation
is given off. After UV photons are adsorbed by TiO2 and elec-
tron/hole pairs are generated, lots of them are recombined
instead of participating in further chemical reactions, resulting
in the further decrease of quantum efficiency. Henschel (1998)
estimated the overall quantum efficiency was only about 0.1%
at the low VOC concentrations. In addition, since the under-
lying reaction mechanisms and kinetics of photocalytic oxida-
tion for many VOCs, especially in the multiple-component
systems, are not very well understood, there is lack of univer-
sal model for UV-PCO device design optimization.
For portable air cleaners that use air ionization and ozone
oxidation (P9 to P11), results show that they did not signifi-
cantly remove any of test VOCs except d-Limonene. Instead,
all of them, especially P11 and P9, generated significant
amounts of ozone. Therefore, such products are not recom-
mended for use in office or residential rooms for VOC control
purpose. For ozone oxidation, reaction rate data summarized
by Weschler (2000) indicated that the majority of indoor
VOCs, except for the small subset with unsaturated carbon-
carbon bonds (e.g., d-Limonene), cannot react with ozone
(below 50 ppb) fast enough to compete with typical ventilation
rates. The current test results are in agreement with the theory.
As for air ionization, the technology is theoretically feasible
and has been demonstrated in some lab reactors (Yan et al.
1998). However, products tested did not show significant
removal effectiveness for the majority of test VOCs under the
current test procedure. Further study is needed to examine the
design and performance of this technology.
Finally, for botanical air cleaning (plant and its special
soil), only one prototype product (P12) was tested. Results
show that it significantly removed n-hexanal, formaldehyde,
and acetaldehyde. The single-pass removal efficiencies were
approximately 20%, although the CADR numbers were not
very large. For other contaminants, the device showed either
insignificant or marginal removal characteristics. In addition,
an obvious increase of 2-butanone concentration was
observed during the test and reasons were not very clear.
Currently, it is not a mainstream technology for indoor air
purification, but potential exists due to its performance for
removing aldehydes and its attractive appearance.
CONCLUSIONS
The major findings from this study are:
1. Sorption filtration is still the most effective off-the-shelf
commercial technology, at least in the initial period, for
general removal of indoor VOC pollutants. For all sorption-
based products tested, significant removal efficiencies were
observed for most test VOCs. The light and very volatile
gases, such as dichloromethane, formaldehyde, and acetal-
dehyde, could not be efficiently removed by activated
carbon only. However, the removal efficiencies for these
gases could be improved if specific sorption media (e.g.,
activated alumina impregnated with potassium permanga-
nate) were added.
2. For sorption-based products, the removal efficiencies and
clean air delivery rates (CADRs) varied a lot from product
to product. The sorption filter design plays an important
role. In general, the filters, which could provide more
surface area and better contact between contaminated
airflow and the adsorbent granular, had higher efficiencies.
In addition, the removal efficiencies depend on the proper-
ties of VOCs. A reasonably good estimation of the removal
efficiency can be made for compounds with VP < 26 mmHg
(at 23°C) by using toluene and decane as the test VOCs.
3. UV-PCO is an attractive technology because it appears to
convert most VOCs into CO2 and water under typical
indoor concentration levels. Results show that a properly
designed UV-PCO device (P15) had removal efficiencies
competitive to sorption-based devices with only one
modest by-product detected under the specific experimen-
tal and measurement conditions used in this study.
However, the commercialization of this technology as room
air cleaners is still in the beginning stage. The off-the-shelf
UV-PCO-based air cleaners did not perform well due to
poor product designs. The key issues for successful
commercialization seem to be the improvement of overall
quantum efficiency and system design optimization. Test
results also suggest that the interference effect among
different VOCs should be considered even under indoor
contaminant concentration levels.
4. It is not recommended to use room air cleaners (such as
ozone generators and ionizers) that either intentionally
generate ozone or produce ozone as a by-product for indoor
VOC control purpose. Although this type of product may be
very quiet and use less power, their removal efficiencies for
most indoor VOCs cannot compete with even moderate
ventilation (e.g., 0.1 ACH), and they are likely to lead to
unsafe ozone concentration levels.
More studies are needed to investigate the long-term
performance of sorption type devices, develop the design opti-
mization model for the UV-PCO device, and integrate perfor-
mance data of air cleaners into building system design in
conjunction with source control and ventilation strategies for
better IAQ.
1114 ASHRAE Transactions: Symposia
ACKNOWLEDGMENTS
The authors are grateful for the financial support from
Niagara Mohawk—a National Grid Company, New York
State Energy Research and Development Authority
(NYSERDA), CASE Center and EQS-STAR Center at Syra-
cuse University, and the assistance of NYIEQ Center, Inc., in
selecting the test products. We thank Mr. Jim F. Smith, Ms.
Bing Guo, and Ms. Mary Sarich for their help on chamber
operation and sample analysis.
REFERENCES
AHAM. 2002. ANSI/AHAM Standard AC-1-2002, Method
for Measuring Performance of Portable Household
Electric Cord-connected Room Air Cleaners. Associa-
tion of Home Appliance Manufacturers.
ASHRAE 1999. 1999 ASHRAE Handbook—Applications.
Chapter 44, Control of gaseous indoor air contaminants.
Atlanta: American Society of Heating, Refrigerating
and Air-Conditioning Engineers, Inc.
ASHRAE. 2000. ASHRAE Std. 145.1P. Laboratory Test
Method of Assessing the Performance of Gas-Phase Air
Cleaning Media (draft). Atlanta: American Society of
Heating, Refrigerating and Air-Conditioning Engineers,
Inc.
Daisey, J.M., and A.T. Hodgson. 1989. Initial efficiencies of
air cleaners for the removal of nitrogen dioxide and vol-
atile organic compounds. Atmospheric Environment
23(9):1885-1892.
Daniels, S.L. 2002. On the ionization of air for removal of
noxious effluvia (Air ionization of indoor environments
for control of volatile and particulate contaminants with
non-thermal plasmas generated by dielectric-barrier dis-
charge). IEEE Transaction on Plasma Science
30(4):1471-1481.
Godish T. 2001. Indoor Environmental Quality. Lewis Pub-
lishers.
Hager, G., and R. Bauer. 1999. Heterogeneous photocatalytic
oxidation of organics for air purification by near UV
irradiated titanium dioxide. Chemosphere 38(7):1549-
1559.
Hall, R.J., P. Bendfeldt, T.N. Obee, and J.J. Sangiovanni.
1998. Computational and experimental studies of UV/
Titania photocatalytic oxidation of VOCs in honeycomb
monoliths. J. of Adv. Oxid. Technol. 3(3):243-252.
Henschel, D.B. 1998. Cost analysis of activated carbon ver-
sus photocatalytic oxidation for removing organic com-
pounds from indoor air. Journal of the Air and Waste
Management Association, Vol. 48, pp. 985-994.
Herrmann T.J., J.S. Zhang, Z. Zhang, J. Smith, X. Gao, H.
Li, W. Chen, and S. Wang. 2003. Performance test
results for an innovative large coupled indoor/outdoor
environmental simulator (C-I/O-ES). ASHRAE Transac-
tions 109(2):503-516.
Hoffmann, M.R., S.T. Marin, W. Choi, and D.W. Bahne-
mann. 1995. Environmental applications of semicon-
ductor photocatalysis. Chemical Reviews 95(1):69-96.
Hossain, Md. M., G.B. Raupp, S.O. Hay, and T.N. Obee.
1999. Three-dimensional developing flow model for
photocatalytic monolith reactors. AIChE Journal
45(6):1309-1321.
Jacoby, W.A., D.M. Blake, J.A. Fennell, J.E. Boulter, L.M.
Vargo, M.C. George, and S.K. Dolberg. 1996. Heteroge-
neous photocatalysis for control of volatile organic com-
pounds in indoor air. Journal of the Air & Waste
Management Association, Vol. 46, pp. 891-898.
Niu J., T.C.W. Tung, and V.W.Y. Chui. 1998. Using large
environmental chamber technique for gaseous contami-
nants removal equipment test. ASHRAE Transactions
104(2):1289-1996.
Obee, T.N., and S.O. Hay. 1999. The estimation of photocat-
alytic rate constants based on molecular structure:
extending to multi-component systems. J. of Adv. Oxid.
Technol. 4(2):147-152.
Ostojic, N. 1985. Test method for gaseous contaminant
removal devices. ASHRAE Transactions 91(2):594-614.
Raissi, A.T., N.Z. Muradov, and E. Martin. 2003. Apparatus
and method for low flux photocatalytic pollution con-
trol. US patent, Patent No. 6,531,035 B2.
Reed, C.H., S.J. Nabinger, and S.J. Emmerich. 2002. Mea-
surement and simulation of the indoor air quality impact
of gaseous air cleaners in a test house. Proceedings of
Indoor Air 2002, pp 652-657.
Sattler, M.L. 1996. Rates of photocatalytic oxidation of
organic compounds in air over titanium dioxide. Ph.D.
thesis, University of Texas.
U.S. EPA. 1996. Residential Air Cleaning Devices: A Sum-
mary of Available Information. EPA Document No. EPA
402-K-96-001. U.S. Environmental Protection Agency.
VanOsdell, D.W. 1994. Evaluation of test methods for deter-
mining the effectiveness and capacity of gas-phase air
filtration equipment for indoor air applications – Phase
I: Literature review and test recommendations. ASHRAE
Transactions 100(2):511- 523.
VanOsdell, D.W., and L.E. Sparks. 1995. Carbon adsorption
for indoor air cleaning. ASHRAE Journal 95(2):34-40.
Weschler, C.J. 2000. Ozone in indoor environments: concen-
tration and chemistry. Indoor Air 2000, pp. 269-288.
Yan K., H. Hui, M. Cui, J. Miao, X. Wu, C. Bao, and R. Li.
1998. Corona induced non-thermal plasmas: fundamen-
tal study and industrial applications. Journal of Electro-
statics, Vol. 44, pp. 17-39.
... A study conducted by Chen et al., (2005) reviewed 15 different air cleaning technologies, including a botanical biofilter. The filters were subject to testing in a full-scale steel chamber (~54 m 3 ) to calculate the single pass removal efficiency (SPRE) of 17 VOCs and the clean air delivery rate (CADR) of each filtration type. ...
... The study noted that the botanical biofilter prototype demonstrated significant removal of formaldehyde, acetaldehyde and n-hexanal, with a SPRE of approximately 20 %, however it was concluded that the CADR was not competitive with the other filters tested. Despite the prototype status of this technology, Chen et al., (2005) concluded the botanical biofilter showed significant promise for further development. Liddy et al., (2005) explored efficiency improvements through determining the effect of plant density and nutrient load on the phytoremediation of methyethylketone as a test VOC, within a partially sealed 65 m 3 room. ...
Conference Paper
Full-text available
Increasing urban populations and behaviour associated with urban lifestyles has led to an increase in awareness of indoor environmental quality, especially maintenance and control of indoor air quality (IAQ). Potted plants, and more recently green walls, within indoor spaces, have increased in popularity in recent decades, not only for their aesthetic value but more recently for their proposed IAQ improvement potential. However, limitations are associated with the rate at which potted plants remove pollutants, rendering their practical efficiency negligible. Recent developments have fused the removal mechanisms of the plant foliage with biofiltration technology to create active green walls (botanical biofilters), which have proven to be an efficient means for the removal of gaseous pollutants. This article will provide; real-world examples of current in situ implementations of phytosystem technology, including laboratory prototypes, that demonstrate significant pollutant removal. Further, the synergistic plant-microbe interactions for air phytoremediation, with considerations related to the release of harmful microorganisms associated with indoor plants will be addressed. Finally, opportunities for enhanced phytoremediation by stimulation of plant-microbe interactions, from simple and practical steps to advanced genetic manipulations will be explored.
... 17,39 Some emerging air treatments involve the deliberate removal of gases from indoor air via chemical transformation to more oxidized compounds. 4,40,41 In concept, chemical transformation of air pollutants is often intended to yield CO 2 ("mineralization") and H 2 O. 42 In practice, production and release of reactive species by ionizers, hydroxyl radical generators, photocatalytic oxidation devices, plasma devices, or O 3 generators result in the formation of oxidized VOCs, generation of PM via secondary chemistry, and/or collateral damage to indoor materials. 4,17,32,39,40,43 At present, the California Air Resources Board (CARB) enforces a regulation Figure 1. ...
... 4,40,41 In concept, chemical transformation of air pollutants is often intended to yield CO 2 ("mineralization") and H 2 O. 42 In practice, production and release of reactive species by ionizers, hydroxyl radical generators, photocatalytic oxidation devices, plasma devices, or O 3 generators result in the formation of oxidized VOCs, generation of PM via secondary chemistry, and/or collateral damage to indoor materials. 4,17,32,39,40,43 At present, the California Air Resources Board (CARB) enforces a regulation Figure 1. Qualitative summary of chemical effects of select air cleaning and disinfection technologies. ...
Article
Full-text available
Amplified interest in maintaining clean indoor air associated with the airborne transmission risks of SARS-CoV-2 have led to an expansion in the market for commercially available air cleaning systems. While the optimal way to mitigate indoor air pollutants or contaminants is to control (remove) the source, air cleaners are a tool for use when absolute source control is not possible. Interventions for indoor air quality management include physical removal of pollutants through ventilation or collection on filters and sorbent materials, along with chemically reactive processes that transform pollutants or seek to deactivate biological entities. This perspective intends to highlight the perhaps unintended consequences of various air cleaning approaches via indoor air chemistry. Introduction of new chemical agents or reactive processes can initiate complex chemistry that results in the release of reactive intermediates and/or byproducts into the indoor environment. Since air cleaning systems are often continuously running to maximize their effectiveness and most people spend a vast majority of their time indoors, human exposure to both primary and secondary products from air cleaners may represent significant exposure risk. This Perspective highlights the need for further study of chemically reactive air cleaning and disinfection methods before broader adoption.
... The active green wall system has a great approach by means of a vertical alignment, increased plant density within a small area, and high efficacy to remediate indoor air quality by which the contaminated air can go through the foliage as well as substrate aiding mechanical ventilation (Soreanu, 2016). Unlike the standard ventilation technique, there is a possibility that active green wall technology has the potential to maintain standard IAQ, utilising the unique approach of biofiltration of recirculating air (Chen et al., 2005). However, development and alteration of conventional strategies is needed in order to achieve the final goal. ...
... Moreover, most manufacturers do not provide data on VOC removal rates; the limited previous study of oxidation-based, consumer-grade air cleaners has found their efficacy to be widely variable. 10 Further, fully characterizing the impact of a given air cleaner on indoor VOCsboth its effectiveness at removing VOCs and its generation of byproductscan be challenging, due to the large number and diversity of organic compounds present in indoor air. ...
Article
Full-text available
Levels of volatile organic compounds (VOCs) in the indoor environment can be decreased by the use of “air cleaners”, devices that remove VOCs by sorption and/or oxidative degradation. However, efficacies of these technologies for removing VOCs tend to be poorly constrained, as does the formation of oxidation byproducts. Here, we examine the influence of several oxidation-based air cleaners, specifically ones marketed as consumer-grade products, on the amounts and composition of VOCs. Experiments were conducted in an environmental chamber, with a suite of real-time analytical instruments to measure direct emissions, VOC removal efficacies (by the addition of either limonene and toluene), and byproduct formation. We find that the air cleaners themselves can be a source of organic gases, that removal efficacy can be exceedingly variable, and that VOC loss is primarily driven by physical removal in some cleaners. When oxidative degradation of VOCs was observed, it was accompanied by the formation of a range of oxidation byproducts, including formaldehyde and other oxygenates. These results indicate that some consumer-grade portable air cleaners can be ineffective in removing VOCs and that the air delivered may contain a range of organic compounds, due to direct emission and/or byproduct formation.
... Air filtration methods are divided into in-duct devices (filters) and portable (stand-alone) air cleaners. In-duct devices work for the entire house but function only during the operation of an air-handling system where they are installed (Chen, Zhang, & Zhang, 2005). Portable air cleaners (PACs) can be operated and positioned in a room with the flexibility to target the problem areas and have received increasing attention in recent years. ...
Article
The global spread of coronavirus disease 2019 poses a significant threat to human health. In this study, recent research on the characteristics of expiratory particles and flow is reviewed, with a special focus on different respiratory activities, to provide guidance for reducing the viral infection risk in the built environment. Furthermore, environmental influence on particle evaporation, dispersion, and virus viability after exhalation and the current methods for infection risk assessment are reviewed. Finally, we summarize promising control strategies against infectious expiratory particles. The results show that airborne transmission is a significant viral transmission route, both in short and long ranges, from infected individuals. Relative humidity affects the evaporation of middle-sized droplets most, and temperature accelerates the inactivation of SARS-CoV-2 both on surfaces and in aerosols. Future research is needed to improve infection risk models to better predict the infection potential of different transmission routes. Moreover, further quantitative studies on the expiratory flow features after wearing a mask are needed. Systematic investigations and the design of advanced air distribution methods, portable air cleaners, and ultraviolet germicidal irradiation systems, which have shown high efficacy in removing contaminants, are required to better control indoor viral infection.
... In addition, health issues may arise from potentially toxic indoor levels of O 3, which has a typical exposure limit of only 0.1 ppm v for 8 h (Luengas et al. 2015;Hubbard et al. 2005;Chen et al. 2005). This paper presents the recent research findings on biological indoor air purification methods as a 'green' alternative to physical-chemical methods for improving indoor air quality, with emphasis on the recent advances, current challenges, and opportunities for further development. ...
Article
Full-text available
Studies on human exposure to indoor air pollution reveal that indoor environments could be at least twice as polluted as outdoor environments. Indoor air pollution has not received as much attention than outdoor air pollution, despite an adult spending now most of the time indoors as a result of the global shift in the economy from the manufacturing sector towards the service and knowledge-based sectors, which operate in indoor office environments. Additionally, the health threats caused by a long-term exposure to indoor air pollution have become more apparent over the last decades as buildings are progressively sealed against the outside climate conditions to obtain heating and cooling energy cost savings and in response to stricter safety guidelines. Currently there is not a single technology that can efficiently provide a complete and satisfactory purification of indoor air. Biological systems for improving indoor air quality are promising, but challenges need to be examined to properly address the bioavailability of low pollutant concentrations, guarantee microbial safety, and incorporate CO2-removal. This study presents the recent research advances in biological indoor air purification methods as a ‘green’ alternative to physical–chemical methods, with emphasis on current challenges and opportunities it can provide for improving Indoor Environment Quality, building energy cost savings and improvements on indoor comfort and well-being. Graphical abstract
Article
A computational model was developed to predict the photocatalytic oxidation of organic compounds in an ultraviolet light-emitting diode (UV-LED) reactor. To simulate the overall reactor performance, the fluid flow, mass transfer, radiation, and kinetics were modeled and integrated by solving the governing equations of mass, momentum, species, and radiant energy conservation, using computational fluid dynamics (CFD) software. The kinetic parameters were experimentally measured and were defined in the model. The model could reliably predict the reactor’s performance and was in close agreement with the experiments within the range of studied photocatalyst orientation, flowrates (Re = 1125 – 36000), and ultraviolet (UV) irradiances (Iavg = 1.4 – 13.8 mW/cm2). Our results revealed that toluene, as the model organic compound, can effectively diffuse to the catalyst surface resulting in minimal mass transfer limitation (less than 4%) even at the lowest examined flow rate (Re = 1,125). The experimental data and simulation results both showed close-to-zero mass transfer limitation at the turbulent flow regime (Re ≥ 4,600). The modeling results identified the areas of the highest mass transfer limitation using the local values of velocity and concentration. The developed model can be applied to the virtual prototyping as well as design and optimization of UV-LED air purification reactors.
Article
Activated carbon is one of the most commonly used porous materials for capturing gaseous pollutants from indoor air. To investigate the adsorption behavior of activated carbon in an indoor environment, the benzene adsorption capacities of 11 commercial activated carbons were measured in the concentration range of 0.05–6 ppmv (parts per million by volume). The method of purging using clean air was used to measure the amount of benzene desorbed from the saturated activated carbons when the concentration decreased to zero. During the experiment, the temperature was 24 ± 2 °C and relative humidity was 45 ± 5%. The specific pore volumes and specific surface areas of the activated carbons were characterized, and their influence on the adsorption and reversible adsorption capacities was analyzed. The results showed that the benzene adsorption capacity has the strongest correlation with the pore volume in the pore width range of 0.6–0.9 nm, while the reversible adsorption capacity has the strongest correlation with the surface area for pore widths larger than 1 nm. Considering the strong correlation between the pore characteristics of the activated carbons and their adsorption behavior for benzene, it may be feasible to predict the adsorption equilibrium of activated carbons at low concentrations using the pore characteristics and empirical correlations.
Article
Herein, we investigate the interplay between a photocatalyst (TiO2) and a catalyst (Pt/TiO2 and Pt/CeO2) for the oxidation of formaldehyde and toluene at room temperature. A luminous textile is used as support and as light source for the photocatalyst. Our results indicate that the presence of the catalyst and the photocatalyst increases the catalytic performance for the oxidation of formaldehyde, while the photocatalytic performance for toluene oxidation decreases. The overall performance (toluene and formaldehyde degradation) of the system can be optimized with respect to the choice of support for the catalyst (e.g. TiO2 or CeO2), the quantity of Pt used, and the ratio between the catalyst and photocatalyst. In addition, different configurations of the photocatalyst and the catalyst on the textile are studied: under leaching and flow-through gas streams, catalyst and photocatalyst deposition on the same and opposite site of the textile are tested. The performance of the system can be optimized by adapting a configuration where the gas stream goes through the textile, while the deposition side of the catalyst and/or photocatalyst with respect to the gas stream is of minor importance.
Article
Full-text available
The adsorption filters used for cleaning indoor air are often made as a thin honeycomb skeleton filled with granular adsorbents. The resistance of honeycomb adsorption filters is one of the main parts for sizing the blowers. But currently no method is available to predict it accurately. Therefore, the pressure drop of honeycomb adsorption filters with different structures and geometric parameters was experimentally studied. The particles in the adsorption filters had mesh number of 2.5 to 20 and shapes of sphere, column and irregular fragment. The void ratio ranged from 0.293 to 0.919, and the Reynolds number was 13 to 862. The effects of the vertical placement when partially filled were also investigated. According to the measured data, an improved friction factor correlation was obtained. And its prediction accuracy is acceptable for the commonly used honeycomb adsorption filters.
Article
Full-text available
Residential and commercial gaseous air cleaning technologies have not gained wide acceptance in the marketplace, in part due to the lack of performance data from field tests or simulation studies. This paper describes a field study of the performance of two gaseous air cleaners in a single room test house and simulations based on these tests with an indoor air quality model (CONTAMW). Air cleaner effective cleaning rates (ECR) were experimentally measured for toluene and ranged from 93 m3/h to 202 m3/h compared to the average toluene loss rates of 18 m3/h due to infiltration and 7.7 m3/h due to sorption on surfaces in the house. As a result, the effectiveness of the air cleaners for toluene in these tests ranged from 82 % to 94 % for different experimental conditions. CONTAMW proved to be an effective tool for predicting the impact of air cleaner performance in the single zone environment. INDEX TERMS Air cleaners, Indoor air, VOC transport, Field study, Model validation
Article
Full-text available
The civilian, commercial, and defense sectors of most advanced industrialized nations are faced with a tremendous set of environmental problems related to the remediation of hazardous wastes, contaminated groundwaters, and the control of toxic air contaminants. Problems with hazardous wastes at military installations are related in part to the disposal of chemical wastes in lagoons, underground storage tanks, and dump sites. Typical wastes of concern include heavy metals, aviation fuel, military-vehicle fuel, solvents and degreasing agents, and chemical byproducts from weapons manufacturing. In the civilian sector, the elimination of toxic and hazardous chemical substances such as the halogenated hydrocarbons from waste effluents and previously contaminated sites has become a major concern. General classes of compounds of concern include: solvents, volatile organics, chlorinated volatile organics, dioxins, dibenzofurans, pesticides, PCB's, chlorophenols, asbestos, heavy metals, and arsenic compounds. Advanced physicochemical processes such as semiconductor photocatalysis are intended to be both supplementary and complementary to some of the more conventional approaches to the destruction or transformation of hazardous chemical wastes such as high-temperature incineration, amended activated sludge digestion, anaerobic digestion, and conventional physicochemical treatment. 441 refs.
Article
The National Status and Trends (NS+T) Program for Marine Environmental Quality was initiated by NOAA in 1984. The purpose of this program is to determine the status and detect changes in the environmental quality of our Nation's estuarine and coastal waters. The NS+T Quality Assurance Project is one of the seven components of the Program and applies to the two monitoring projects: the National Benthic Surveillance Project (NBSP) and the Mussel Watch Project (MWP). The NBSP collected and analyzed benthic fish and sediments from sites around the coastal and estuarine United States, including Alaska. The NS+T Mussel Watch Project is designed to represent chemical distributions over national and regional scales; there are 300 sites currently sampled as part of this project.
Article
The U.S. Association of Home Appliance Manufacturers (AHAM) has set a voluntary standard for testing the initial dust-removal capacity of portable air cleaners. In our test of portable air cleaners for the local consumer council, the AHAM method was extended to test the initial removal capacity for gaseous phase pollutants. Also, carbon filters' efficiency change over time in toluene removal on a number of air cleaners was tested. In using a large chamber to carry out these tests, the chamber wall adsorption and re-emission effects were experimentally quantified. These tests indicated that a large chamber, with its wall surface adsorption controlled, is simple and robust to use to quantify the initial cleaning capacity for gaseous phase pollutants. Based on these test results, a large chamber method is proposed to test the performance lifetimes of portable air cleaners. The system advantages of the method over the in-duct performance life test methods are that no continuous air-cleaning system is required and that the chamber's humidity and temperature can be maintained at the desired values more easily with the combination of a unitary dehumidifier and a bubbler system. This paper will present our trial results with portable air cleaner tests and discuss the large environmental chamber techniques.
Article
The use of honeycombs as monolith supports for UV/Titania photocatalysis of volatile organic compounds (VOCs) has been investigated theoretically and experimentally. A theoretical model based on analysis of the UV intensity distribution, transport of gaseous contaminants to the catalyzed surface, and surface photocatalytic destruction has been developed. Both diffuse and specular reflection models have been developed for the UV illumination calculations. The model uses measured intrinsic photocatalytic destruction rates determined in a research reactor which gives the concentration-, humidity-, and UV intensity-dependences of the rates. Model predictions have been compared to measurements in a high-flow, staged monolith, demonstration purifier for several gaseous contaminants of interest. Agreement between modeling and experiment is generally quite good. The sensitivity of purifier efficiency to monolith pore size and illuminated contact time is discussed. The performance obtained with honeycombs is comparable to that with porous foams, but with much reduced weight, pressure drop, and cost. The conclusion of this research is that honeycomb monoliths are attractive for use in practical UV/Titania photocatalytic devices, particularly at very high flow rates where pressure drop is a concern, and for those situations where monolith weight is an important design constraint.
Article
Gas-phase air filtration equipment (GPAFE) has been applied for many years to control industrial gaseous contaminants. Interest in cleaning recirculation air to provide ventilation without the need to condition excessive outdoor air has promoted increased interest in GPAFE as indoor air control devices. The removal of volatile organic compounds (VOCs) using granular activated carbon (GAC) is the focus of this article. First, the authors present performance measurements for GAC at low challenge VOC concentrations that might be encountered indoors. Unlike previously reported tests, these were continued long enough to directly determine the GAC`s expected lifetime. The results suggest that test results obtained at high challenge concentrations may be extrapolated to low, indoor concentrations. Further study is needed, but these data are encouraging. Second, they will discuss the implications of these performance measurements for the use of GAC to remove VOCs and improve indoor air quality (IAQ) using an indoor air building simulation model.
Article
It has been demonstrated previously that the estimation of titania promoted photocatalytic oxidation rates based on the physical adsorption characteristics of the precursor can be a powerful tool. To a zeroth order approximation, the rate constant depends upon the binding strength of the precursor to the titania surface, e.g. hydrogen bonding bonded precursors react faster than dipole-dipole interaction bonded precursors, etc. This "rule of thumb" can be quite useful in predicting the oxidation rate of species whose rate has not been measured. However, most applications involve the oxidation of more than one precursor simultaneously. In order to apply these considerations to a multiple component system one must study such systems, beginning in pairs. We have chosen to look initially at a titania/water/butanol/propanal system. One expects that the reaction rate for a given precursor will decrease commensurate with the decrease in surface coverage. In other words, the winner of the competition for sites will have the reaction rate affected the least, and the size of the effect will be determined by the magnitude of the victory in this site competition. Furthermore, one would expect the limiting cases to be that of a strongly attracted molecule paired with a weakly attracted molecule and a strongly attracted molecule paired with another strongly attracted molecule. In the case of the former, the expectation is that the reaction rate of the strongly bound species will be relatively unaffected, while the reaction rate of the weakly bound species will approach zero. We observe that this is indeed the case, although the effect of butanol on propanal is stronger than we predicted, 2 ppm of butanol effectively shuts down the propanal reaction, while 0.5 ppm butanol reduces the photocatalytic oxidation rate of propanal by only 20%.
Article
A cost comparison has been conducted of 1 m/sec indoor air cleaners using granular activated carbon (GAC) versus photocatalytic oxidation (PCO) for treating a steady-state inlet volatile organic compound (VOC) concentration of 0.27 mg/m. The commercial GAC unit was costed assuming that the inlet VOCs had a reasonable carbon sorption affinity, representative of compounds having four or more atoms (exclusive of hydrogen). A representative model PCO unit for indoor air application was designed and costed, using VOC oxidation rate data reported in the literature for the low inlet concentration assumed here, and using a typical illumination intensity. The analysis shows that, for the assumptions used here, the PCO unit would have an installed cost more than 10 times greater, and an annual cost almost seven times greater, than the GAC unit. It also suggests that PCO costs cannot likely be reduced by a factor greater than 2–4, solely by improvements in the PCO system configuration and reductions in unit component costs. Rather, an improved catalyst having a higher quantum efficiency would be needed, increasing reaction rates and reducing illumination requirements relative to the catalysts reported in the literature. GAC costs would increase significantly if the VOCs to be removed were lighter and more poorly sorbed than assumed in this analysis.
Article
A first-principles mathematical model describes performance of a titania-coated honeycomb monolith photocatalytic oxidation (PCO) reactor for air purification. The single-channel, 3-D convection–diffusion–reaction model assumes steady-state operation, negligible axial dispersion, and negligible homogeneous reaction. The reactor model accounts rigorously for entrance effects arising from the developing fluid-flow field and uses a previously developed first-principles radiation-field submodel for the UV flux profile down the monolith length. The model requires specification of an intrinsic photocatalytic reaction rate dependent on local UV light intensity and local reactant concentration, and uses reaction-rate expressions and kinetic parameters determined independently using a flat-plate reactor. Model predictions matched experimental pilot-scale formaldehyde conversion measurements for a range of inlet formaldehyde concentrations, air humidity levels, monolith lengths, and for various monolith/lamp-bank configurations. This agreement was realized without benefit of any adjustable photocatalytic reactor model parameters, radiation-field submodel parameters, or kinetic submodel parameters. The model tends to systematically overpredict toluene conversion data by about 33%, which falls within the accepted limits of experimental kinetic parameter accuracy. With further validation, the model could be used in PCO reactor design and to develop quantitative energy utilization metrics.