EVALUATION OF OZONATION FOR CONTROL OF ONION AND …
Transcript of EVALUATION OF OZONATION FOR CONTROL OF ONION AND …
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EVALUATION OF OZONATION FOR CONTROL OF
ONION AND GARLIC DEHYDRATION ODORS
by
CHARLES LARRY McGOWAN, B.S. in Ch.E.
A THESIS
IN
CHEMICAL ENGINEERING
Submitted to the Graduate Faculty of Texas Tech University in
Partial Fulfillment of the Requirements for
the Degree of
MASTER OF SCIENCE
IN
CHEMICAL ENGINEERING
Approved
December, 1975
PgT-^UK^ fit
' ^ ' ^ . /
ACKNOWLEDGEMENTS
The author wishes to express his deep appreciation to the members
of his committee for their assistance in the experimental work of this
thesis. A special thanks goes to Dr. R. M. Bethea for his invaluable
guidance and help, to Mr. David Ayers for assisting in building the
apparatus, to Mrs. Jayme Logan for typing, and to Gilroy Foods, Inc.
for funding the project.
n
TABLE OF CONTENTS
Page
ACKNOWLEDGEMENTS ii
LIST OF TABLES v
LIST OF FIGURES vi
CHAPTER I INTRODUCTION 1
CHAPTER II LITERATURE REVIEW 4
Use of Ozone as a Reactant 4
Ozone Measurement 9
CHAPTER III EXPERIMENTAL SYSTEM 10
Air Supply 10
Humidification 13
Temperature Control 15
Contaminant Addition and Mixing Duct 16
Mixing Duct 17
Reactor Design 18
Ozone Generation 19
Gas Chromatography 20
Mixing Studies 23
Safety Features 25
CHAPTER IV RESULTS AND DISCUSSION 27
Ozone-Decay Studies 27
Experimental Design 35
iii
Page
Results 37
CHAPTER V RECOMMENDATIONS 53
CHAPTER VI CONCLUSIONS 57
LIST OF REFERENCES 58
IV
LIST OF TABLES
Table Page
I Ozone Saturation of Stainless Steel Line 29
II Ozone Decay Results 34
III Percentage Reduction of Mean Peak Height - Onion. . 40
IV ANOVA for a 3x3 Latin Square Design 41
V Results of Extended Residence Times on Onion-Oil Peak Heights 44
VI Effect of Increased Residence Time on Peak Height Reduction 44
VII Reaction Conditions and Efficiencies 50
LIST OF FIGURES
figure Page
1 Atmosphere Preparation System 11
2 Reactor Flow Pattern 12
3 Latin Square Test Design 36
4 Onion Vapor at 2 ppm Before and After Ozonation at 20 ppm for 30 sees 46
5 Garlic Vapor at 2 ppm Before and After Ozonation at 20 ppm for 30 sees 47
VI
CHAPTER I
INTRODUCTION
The processing of raw onion and garlic into commercial products
creates a definite odor problem. The air used in the dehydrating of
the vegetables becomes contaminated with highly-odoriferous sulfur-
containing compounds to such a degree that the concentration is in
violation of common air-pollution control regulations. An example of
such a regulation is Regulation 2 of the Bay Area Air Pollution Control
District (8) which limits the total emission of mercaptans to 0.1 part
per million, ppm, expressed in terms of an equivalent amount of methyl
mercaptan. The purpose of this investigation was to determine if
chemical oxidation with ozone, 0^, could be technically feasible for
the control of the odoriferous emissions associated with the dehydration
of onions and garlic cloves.
The heart of the onion and garlic processing operation centers
around the dehydration of the produce. The dehydration is typically
accomplished in several Proctor-Schwartz dryers which are capable of
processing approximately 10,000 lb of raw product every hour which is
equivalent to over 40 x 10 BTU/hr capacity (1). Dehydration is
carried out in multiple stages with successively lower temperatures,
deeper product bed depths, and longer drying times as the residual
moisture content of the product decreases. The air used for drying
flows countercurrent to the product resulting in exhaust temperatures
from 90 - 130°F with relative humidities of 5 - 30r. Air-temperature
controllers and high-temperature limit switches monitor drying-air
temperatures to prevent overshooting and the resulting decrease in
product quality. Each of the dryer stages can have several exhausts.
The total dryer effluent can be as high as 10 cubic feet per mintue
(CFM) or greater. These exhaust streams contain significant quantities
of volatile organics liberated as a result of enzymatic action initiated
by cell wall collapse during slicing. The major odoriferous components
appearing in these exhaust emissions from the onion dehydrating were
identified by Belo (9) as dimethyl disulfide, methyl propyl disulfide,
dipropyl disulfide, methyl-1-propenyl disulfide, propyl-1-propenyl
disulfide and di-(1-propenyl) disulfide.
Although numerous odor and particulate emission sources are
associated with onion and garlic processing, this study was confined
to the odoriferous emissions from the dehydration step. Control of
these emissions can be potentially achieved by any of several dif
ferent methods including adsorption, scrubbing, or incineration.
The cost for adsorption or scrubbing would be excessive primarily
because of the pressure drop limitations involved with the high
volumetric flow rates in the dehydrating step (10). The use of in
cineration for control purposes was considered marginal due to the
costs of fuel and the capital cost of the afterburners for each dryer.
Therefore, the most feasible odor control route at the time this
study was initiated appeared to be U]_ situ gas-phase oxidation by
ozone.
The original intent was to evaluate the impact of temperature,
relative humidity, contaminant loading, ozone concentration, and
reaction-residence time on the effectiveness of ozone for odor control
In order to simulate the actual industrially-contaminated air stream,
an experimental atmospheric-preparation system was constructed to
simulate the industrial conditions. The mixing and ozone-decay char
acteristics of the experimental system were determined and then the
appropriate quantities of ozone and contaminant were added following
a Latin square experimental design. This design was used to evaluate
the effect of the primary variables (residence time, ozone concen
tration, and contaminant loading) on the observed amount of reaction
while also minimizing the total quantity of data that had to be taken.
The chief criterion for determination of the effectiveness of the
control technique was a decrease in sulfur-containing odoriferous
material as determined by gas-chromatographic analysis using a flame
photometric detector for the sulfur compounds.
CHAPTER II
LITERATURE REVIEW
Use of Ozone as a Reactant
Nakano (22) and Bauch and Burchard (7) have demonstrated the
capability of ozone to deodorize air. Ozonation has been used suc
cessfully for the treatment of sewage digestion odors in Nagoya,
Japan. The Japanese system handled 56,400 actual cubic feet per
minute, ACFM, and required 0.37 lb ozone per hour to maintain a
one-ppm ozone concentration at an annual operating cost of $290/year
in 1968. With a four-second residence time allowed for gas nixing
and reaction, ammonia concentration decreased 60 percent. None of
the normal principal components of sewage odors (hydrogen sulfide.
Indole, and skatole) could be detected in the treated air effluent.
Baba (5) has also discussed the application of ozone as the
main control technique for the odors of municipal sewage. Compar
ative cost data for ozonation, catalytic and thermal oxidation,
adsorption, and chlorination as odor control methods were presented.
Of these procedures, ozonation was least expensive, followed by
catalytic oxidation and then thermal oxidation.
Green and Elliott (16) proposed the use of 13-ppm ozone to
treat a rendering exhaust air stream of 1700 CFM. The odoriferous
components were primarily the C-i-C. oxygenated organics, amines, and
sul fur-contain ing compounds. The contact time required for e f fec t ive
control has not yet been determined.
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Ozone has been found to destroy fermentation odors from a
pharmaceutical plant (2). A small portion (75 - 100 CFM) of the gas
was passed through an ozonator and mixed with the bulk gas stream
(70,000 - 80,000 CFM). After a 10-second residence time, the ef
fluent containing 1 - 2 ppm ozone was vented to the atmosphere through
a stack 100 ft above grade. Ozonation was deemed to be an effective
and inexpensive odor-control technique.
Other reports (12, 34) of the successful use of ozone have been
made for rubber and phenol plants (3 - 10 ppm needed, contact time
of 3 - 6 sec) and fermentation (up to 50 ppm needed). These appli
cations were for ventilation air which has 12 changes per hour.
Summer (31) has described the use of ozone (produced by ultraviolet
lamps) for the control of odors produced by onion and meat frying in
restaurants. These odors were predominately produced by the over
heated oil (acrolein) and the release of onion volatiles (di-(l-
propenyl) disulfide and propyl-1-propenyl disulfide) during the
frying operation.
The effectiveness of ozonation for controlling hydrogen sulfide,
methyl mercaptan and other sulfides and disulfides produced by the
Kraft pulping process was demonstrated by Ishii (17) in 1971 in
England and the United States. The waste gases from black-liquor
furnace stacks were passed through a condenser where ozone was added
at both the entrance and exit at quantities below 10 ppm. A 2.2-
second contact time gave the best results. Other studies (23) have
shown that 1 0 - 5 0 ppm ozone are necessary for the elimination of
Kraft paper mill odors.
The use of ozone as a chemical oxidant for controlling phenol
odors from a munufacturing process using phenolic resins as a binder
(19) resulted in only 50 - 70% oxidation. The remaining unoxidized
portion was reportedly non-odoriferous because of a "combined neu
tralization effect" due to ozone. An explanation of this neutral
ization effect was not provided but was only said to be a formation
of a complex with the odor forming agents.
Some reports of the impracticability of the use of ozone as a
decontaminator of odorous air have been published. Huch, Beine and
Brocke (16) investigated ozone's effectiveness in treating concen
trated swine odors from stables. They reported that the contaminants,
with an input concentration based on a carbon content of 6 - 14 ppm,
were not significantly decreased at process periods of 0.1 to 1.0
min and ozone concentrations of 10 - 66 ppm. Their explanation for
the lack of effectiveness was that at the low concentrations char
acteristic of odorous emissions, even though very reactive partners
may be present, a small collision probability results in very little
reaction.
Maggiolo and Blair (18) have studied the ozonolysis of disulfides
and monosulfides. They found that these compounds were converted so
rapidly to the non-odoriferous sulfoxides and sulfones that conven
tional measurement of the reaction rates was impractical. The re
action between ozone and organic sulfides was found also to yield
minor amounts of the corresponding aldehydes and organic acids.
Disulfides were found to produce primarily sulfonic anhydrides and
some disulfones and thiosulfonates. Dimethyl sulfide was atypical
In that the measured products were sulfoxide and sulfonic anhydride.
The rate constant for the initial reaction between hydrogen
sulfide, H^S, and ozone in air has been determined by f^aggiolo and
Blair (18) as
dC^ /dt = -4.7 X 10^ exp (-8300/RT)C^ ' u moles/1iter-min
where C« = concentration of ozone, y moles/liter, ^3 R = universal gas constant, 1.98 cal/g mole-°K,
T = temperature, °K,
t = time, min.
The reaction is probably quasi-unimolecular in that the order is
zero in H^S and 1.5 in ozone. The above equation predicts a slow
reaction rate for the ozone concentrations used in the present study.
At 130°F (328°K) and 10 ppm ozone, the initial rate of reaction be
tween HpS and 0-. would be only 0.127 ppm 0-,/sec. Thus if the assump
tion is made that this initial rate of reaction does not change
significantly in an initial 10-second mixing and reaction time, then
after 10 seconds only 12.7". of the initial 10-ppm ozone would have
reacted. The kinetics of this reaction do not predict ozonation
to be a successful odor-control technique for H^S if residence time
is a limiting factor.
8
Okuno (24) has also discussed the mechanism of the destruction
of low molecular-weight sulfurous compounds, olefinic hydrocarbons,
and amines by ozone. He concluded that not all odorous components
can be removed by ozone oxidation due to the difficulty in reducing
1 ppm of malodorous components to 1 part per billion, ppb, (99.9.-
removal).
Mueller et al. have obtained data regarding ozone decomposition
(21) which was found to follow first-order kinetics in an aluminum
chamber and in living areas. They also reported that the nature of
the reactor surface and the effective surface-to-volume ratio affect
the rate of ozone decay. The rate of the decomposition was dramat
ically altered by variations in humidity or temperature. The first-
order decay rate was reported to be 0.054 ± 0.004 min' in an
aluminum chamber at 70 - 80°F and 26 - 50% relative humidity, RH.
The half-life of ozone at those conditions was 13 min. An approximate
value for the activation energy for ozone decomposition was given as
8.0 K cal/mole as determined by a coulometric analyzer.
Sabersky et al. (26) also investigated ozone decomposition.
Their results corroborated those of Mueller et al. Sabersky et al.
found that the rate of ozone decomposition depends directly on the
surface-to-volume ratio of the reactor. They reported rate constants
3 2 for several common surfaces in the range 0.001 to 0.1 ft /ft -min.
Materials such as rubber, fabrics and plastics had rate constants
higher than metals or glass. These results were i-.portant in deter
mining the appropriate material of construction for the reaction
chamber because of the desire to minimize ozone decomposition or
reaction with anything but the contaminant. It should be noted that
both research groups conducted their studies using a batch system
with clean indoor air as opposed to polluted air.
Ozone Measurement
The ozone concentration can be determined by use of the Vast
coulometric ozone detector. This method of analysis can be pre
judiced by the presence of SO^ formed as a reaction product of the
ozonolysis of onion and garlic dehydrator odors. If necessary,
simultaneous determinations in the effluent of the reaction chamber
can be made for SO2 using the pararosaniline syringe colori:retric
technique of Meador and Bethea (20). That method is not subject
to interference by ozone or NO^. If significant quantities of SO2
are present, the reaction chamber effluent samples can be first
passed through a chromium trioxide bubbler as recommended by Saltzman
and Wartburg (27) for the removal of this interference prior to
coulometric analysis for ozone. The calibration standard for any
ozone detector is the potassium iodide method (33).
CHAPTER III
EXPERIMENTAL SYSTEM
The experimental system designed to evaluate the effectiveness
of ozone for eliminating onion and garlic odors in air consists of
two basic sections. The first section is the atmosphere-preparation
section in which a controlled flow rate of air is heated and humid
ified to the desired conditions. As shown in Figure 1, the atmos
phere-preparation section also includes a mixing duct to ensure
complete mixing of the injected contaminant by the time the air
stream reaches the reactor. The reactor, shown in Figure 2, is the
second section of the system in which the actual contacting of the
ozone and the contaminated air occurs.
Air Supply
Air is supplied to the test apparatus from a single centrifugal
blower, model No. 23, from Buffalo Forge Co. of Buffalo, New York.
This prime mover has a three-phase, 220 volt, 100 amp motor and can
supply approximately 1,000 ACFM against no load and about 40 ACFM
through the test apparatus used for this research project. The
air flow rate was controlled by two hand-operated two inch gate
valves and is measured by a micro-differential manometer installed
across a standard 1-3/8 in orifice. This assembly was installed in
the 2 in schedule-40 line from the blower to the humidification
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chamber. A differential manometer (50 mm reservoirs, 2 mm tube
bore) was fabricated to convert the system pressure drop of 1.5 -
2.0 in water to an easily readable and reproducible range of 15.4
in at maximum air flow capacity.
Some problems were encountered in calibrating the manometer
readings at the low air flows involved. An Alnor velometer, series
6000-P, with the low-flow (30 - 300 ft/min) probe, type 6050-P, was
used. A standard 6-point traverse, with each point taken in triplicate,
was made across the exhaust duct for each manometer setting. The
average of the 18 points was taken as the corresponding air flow
rate. These overall results were later double-checked with a 25-
point traverse at each monometer setting. This precision was nec
essary so as to minimize the experimental error in measuring the
reaction residence time and the ozone and contaminant concentrations
after each had been mixed into the test atmosphere at the appropriate
location. The 90% confidence intervals for the average flow rate
showed that errors of 13.9"^ of the true value could occur over the
flow rate range studied.
Humidification
It was necessary to control and measure accurately the relative
humidity of the continuous air stream to ± 5:.' relative humidity (RH).
Three methods for producing this humidified air were considered be
fore an effective procedure was found. The first method, that of
a packed humidification column, was rejected because of the large
14
pressure drop involved. The second approach, an adiabatic spray
chamber, was rejected because It Involved pre-heatlng the ambient
air to a high temperature prior to humidification. The third method
involved the use of 80 pounds per square inch, gage (psig) super-heated
steam which was available at the experimental site. It was necessary
to reduce the steam pressure to 10 - 15 psig. At the lower pressure,
the steam rate could be controlled with two 1/2 in globe valves. One
of these valves was used to bleed excess steam and the other was used
to control the steam flow into the humidification chamber. The
humidification chamber (dimensions 34 in x 49 in x 37 in) was con
structed of 3/4 In marine plywood with Internal baffling that pro
vided six flow direction changes to promote adequate mixing. The
chamber was sealed and Internally coated with two coats of a water-
resistant wood preservative.
Measurement of the air humidity was obtained after addition of
the onion or garlic oil just before entering the reaction chamber.
A humldograph with a 24 hr strip chart (EPIC Weather Set, Model 250)
was used to measure and record the humidity. The humldograph was
calibrated against a sling psychrometer at approximately 20 and 80%
RH. A small chamber was constructed on top and at the end of the
mixing duct. Sample air was routed to the humldograph chamber by
cutting away a 3'1n x 6-1n section of the chamber floor, (mixing duct
roof), and installing a partial baffle to channel air Into the
chamber. Holes were drilled into the other end of the chamber floor
to allow for free flow of mixing-duct air through the humldograph
15
chamber. In addition, a small purge stream of less than 1% of the
total flow rate but still enought to exhaust the chamber volume com
pletely once every minute, was drawn out of the system through the
chamber roof by a vacuum pump. The entire chamber was well insulated,
except for the necessity of the transparent plastic front installed
for ease in taking data. Some heat loss was encountered, the humidity
measurement chamber being approximately 10 to 15°F below system
temperature. A correction was made for this temperature offset in
recording the humidity of the test system.
Temperature Control
The temperature of the humid air stream leaving the humidifi
cation chamber was adjusted by passing the air through a copper
counter-current flow heat exchanger. The humidified air was on the
tube side of the exchanger with a pneumatically-controlled steam
pressure on the shell side. Temperatures were measured just before
the entrance to the reactor. A Foxboro temperature controller
Model 41 and a Foxboro pneumatic valve model 3544 allowed control of
the humid air stream within ± 5°F of any desired set point. The
exchanger was a single pass type with 29, 3/4 in outside diameter,
2 20 in long tubes, equalling 9.49 ft of heat-exchange area. This
amount of heat-exchange capacity was found sufficient to heat
maximum flow rates (40 - 50 CFM) to the maximum temperature of
130°F. Because copper has an excellent thermal conductivity, it
was necessary to insulate the exchanger to prevent heat losses to
16
the ambient air and to prevent warping of the humidification chamber
wall at its junction with the heat exchanger.
Contaminant Addition and Mixing Duct
Onion or garlic oil was injected into the heated humidified air
stream leaving the heat exchanger. The concentration of the con
taminant was controlled in the range of 0.3 to 10.0 ppm by a Sage
model 355 syringe pump which could accurately meter the small amounts
required to obtain these low concentrations in the air stream. Each
syringe was calibrated separately so that the exact injection rate
would be known. Syringes used for this purpose included 2.5 ml and
10 ml Becton-Dickinson liquid syringes and a 1 ml Hamilton gas-tight
syringe. The concentrations used were only approximately known,
however, because exact molecular weights for onion and garlic oils
were unknown. A pseudo-molecular weight of 148 was adopted based
on propyl-2-propenyl disulfide being the major constituent of specific
interest as well as being intermediate in size for all compounds
known to be present in the oils (14). This estimate of the molecular
weight, coupled with the experimentally determined densities of 1.377
gm/ml for onion oil and 1.064 gm/ml for garlic oil, permitted the
calculation of the approximate concentrations on a molar basis.
The contaminants were added to the humidified air just downstrea-
of the junction of the air heat exchanger and the mixing duct. In
order to vaporize the volatile and most odoriforous components of the
oils uniformly, an uninsulated section of 1/2 in schedule 40 ster^m
17
line was routed through the mixing duct section. This steam line,
although uninsulated, was small enough so that it did not affect
the temperature of the passing air. The line had a cupped 2 in x
2 in steel plate welded on it to provide an adequate contaminant
vaporization area. It was hoped that the air temperature and flow
rate would be sufficient to cause complete vaporization without
steam heating. However, after some preliminary experiments with and
without steam heating, it was obvious that the use of steam heating
on the plate would lead to a more efficient and more rapid vapor
ization and hence provide better mixing. The steam-heated vapori
zation plate was used for all subsequent experiments.
Mixing Duct
A mixing duct with a cross-sectional area of 12 in x 12 in and
a length of 14.5 ft was constructed and connected between the heat
exchanger and the reactor. The duct was built with a 180° turn at
the 8-ft mark to conserve space, and to provide an additional im
pingement surface for mixing. The duct contained seven 12 in x 8 in
internal baffles that alternated from top to bottom, and thus pro
vided an effective cross-sectional flow channel of 12 in x 4 in.
The duct was constructed of 5/8 in marine plywood. The interior was
coated with two coats of a water-resistant wood preservative. To
keep heat losses at a minimum, the duct was insulated with a 2 in
blanket of glass wood backed with aluminum foil.
18
The purpose of the duct was to provide a mixing zone with a
sufficient residence time to ensure complete mixing of the air strear
and the contaminants. Such a homogeneous system was necessary
to test accurately the effectiveness of ozone for this odor-control
application. Therefore to measure the duct's mixing efficiency,
sample ports were installed 2 ft, 6 ft and 12 ft from the entrance
so that syringe samples for gas chromatographic (GC) analysis could
be obtained.
Reactor Design
The reactor was located at the end of the mixing duct and was
constructed of 3/4 in marine plywood with overall dimensions of 3
23 in X 23 in x 12 in. The total internal volume was 3.67 ft . The
reactor was constructed with a double-S flow path to promote rapid
and adequate mixing of the ozone with the contaminated air stream
through generation of turbulent flow conditions by impingement and
changes in flow direction. This double-S pattern was used to keep
the reactor surface-to-volume ratio, equal to 7.82, as low as
possible. It was thought that this would help keep any ozone-de
composition or adsorption reactions to a minimum. To reduce any
ozone losses by absorption on the reactor walls, all surfaces were
painted with two coats of an epoxy-based paint which is resistant
to ozone attack (31).
The calculated residence time in the reactor was approximately
4.5 seconds at an air blower rate of 42 ACFM. Four sample ports
19
were Installed and located at the end (flow impingement surface) of
each segment in the reactor. In this way calculated residence
times as short as 1.0 sec and as high as 9.6 sec at 20 ACFM could
be studied. All residence times are approximate due to the presence
of dead spaces which are inherent in any baffled or rectangularly-
shaped system. No attempt was made to correct the error in the
residence time because of the dead spaces.
The effluent from the reactor entered a galvanized sheet-metal
duct with a 12-in x 6-in cross section. This duct ran vertically
20 ft and was then joined into the main ventilation header for the
building. This header routed all flow to an outside stack, approx
imately 70 ft high, which adequately dispersed all effluents.
Ozone Generation
Ozone for this experimental work was generated by electrolysis
using a Welsbach T-816 Laboratory Ozonator. This model operated
on a 115-volt, 50/60-cycle, single-phase electric power supply.
With adjustable air flow rate, gas pressure, and voltage, a wide
variation in ozone production and concentration was possible. For
all experiments in which ozone was being used an as odor oxidant,
the voltage was set at 90v. For the ozone-decay studies, the voltage
was varied from 70 - 125v. The ozone concentrations from the
generator were quantitatively verified by the neutral-buffered
potassium iodide (KI) method for total oxidants (33). Increased
sensitivity and accuracy were achieved with the KI method when
the ozone feed was diluted with system air to the 5 - 4 0 ppm range.
20
The ozone was added to the system through a 1/4 in outside
diameter stainless-steel tube between the ozone generator and the
appropriate point in the mixing duct. The last 8 in section of
the ozone supply line was made into a sparger by plugging the end
and drilling along the tube several small holes to increase the
rate of mixing of the ozone and the contaminated air stream.
To check the ozone effluent concentration after reaction and
to aid in the ozone-decay studies, a Mast Coulometric Meter (model
724-2), Mast Development Co., was used. This instrument measured
ozone concentration in the 0 - 1 ppm range using neutral buffered-
KI solution and a coulometric detector.
Gas Chromatography
The analysis of the samples taken by syringe from the reactor
was performed on a Tracor Model 550 dual-column gas chromatograph
with a Tracor Westronics Model LSI IB recorder. The columns used
were packed with 5% OV-1 (stationary phase) on 80/100 mesh Chromo-
sorb W. These columns gave good resolution of the major peaks
along with convenient retention times.
The operating conditions used were a helium carrier gas with
a flow rate of 42 ml/min, a hydrogen flow rate of 155 ml/min, an
air flow rate of 40 ml/min, and an oxygen flow rate of 20 ml/min.
The column temperature was 100°C, the detector temperature was 175**C,
the inlet heater temperature was 216°C, and the effluent-splitter
temperature was 256°C. The chart speed for all runs was 1/2 in/min.
21
The gas chromatograph used in this study had two different
detection systems; one was a hydrogen flame ionization detector,
FID; the other was a flame photometric detector, FPD. The FID works
by burning the sample gases in a hydrogen flame. The burning pro
duces ions in the gas phase which are collected on a polarized ring
electrode. The collection of the ions results in a change in the
electric charge on this ring which generates a signal measured by
the reactor.
When the gases are burned around the polarized ring in the FID
detector, an optical lens capable of transmitting only one wavelength
of light can be used to detect the presence of a particular element
by use of a photomultiplier tube. Since in this study interest
centered around the presence of organic sulfides, a lens which trans
mitted only the wavelength light emitted when sulfur is burned, 3930 o
A, angstroms, was used. Thus all light was screened from the photo-
multiplier except the wavelengths resulting from suflur combustion.
The FPD was then specific for the detection of sulfur-containing
compounds.
The lower detection limit for this system was 0.2 - 0.3 ppm
based on the pseudo-molecular weight of 148 for both onion and
garlic oils. To sense concentrations this low an FPD attenuation
setting of 12,800 was best with a signal-to-noise ratio of approxi
mately 10%. Runs at higher concentrations were conducted with an
FPD attenuation of 32,000 or 25,600. It should be noted that the
FID system was much less sensitive than the FPD system. The small
22
peaks recorded from the FID detector were used only for qualitative
confirmation purposes. The FID attenuation for all runs was set at
the lowest recommended value, 16.
All samples were taken with model 1005 gas-tight Hamilton
syringes and were 5 ml except at the high contaminant concentration
studies were 3 ml samples were sufficient. The chromatograms showed
six sulfur-compound peaks for both onion and garlic samples. Based
on peak height, peaks three and five for the onion contaminant ac
counted for approximately 30'.' and 45'' of the total sample, respectively
Peak five on the onion oil sample was qualitatively identified as
dipropyl disulfide by the injection of known vapor standards im
mediately following the elution of an onion-oil vapor sample. All
attempts to identify the other peaks were unsuccessful. Peak five
was also the major peak in the garlic chromatogram accounting for
about 55% of the total sample. Peaks three, four and six were
roughly the same size each accounting for 10 - 15% of the total
sample.
The oxidation effectiveness of ozone for all experimental runs
was reported as the percentage of the peak-height reduction measured
on a day-to-day basis. Five to ten samples of non-ozonated contam
inant were taken until a reasonable consistency in peak height,
standard deviation of 10 - 20% of the mean peak height, was obtained.
After the system had come to steady state as determined by the re
peatability of the chromatograms, and an accurate measure of the
contaminant concentration had been made, ozonation was begun. Five-
23
to-ten additional samples were obtained over a two-to-four hour
period. The mean of the peak heights after the system had returned
to steady state was compared to the mean peak height before ozonation.
The difference in peak heights was reported as the oxidation efficiency
for the experimental conditions in use on that day. By reporting the
results on a day-to-day basis, the effects of minor fluctuations in
operating conditions were eliminated.
Mixing Studies
An evaluation of the effectiveness of the mixing duct was simu
lated for the oils by using a light volatile component, acetone
(boiling point 56.2°C) and a heavier component, iso-amyl alcohol
(boiling point 118.9°C). Acetone was metered into the mixing duct
through the syringe pump. The pump stroke was adjusted to achieve
about 10 ppm acetone at a flow rate of 31 CFM (at 130°F, = 15% RH).
The application of heat to the vaporization plate had no effect on
the results: the air stream was hot enough to vaporize the acetone
as fast as it was injected. 50 ml samples were taken through a
4 in needle inserted through the sample-port septum. This sample
was then used to flush a 2 ml syringe by inserting the needle past
the rubber plunger-tip into the barrel of the smaller syringe.
After the small syringe was purged, 2 ml were retained for injection
Into the chromatograph. This transfer technique can easily give
rise to systematic errors of t 10 ^ of the true value of the com
position. Thoroughness of mixing was demonstrated by the fact that.
24
within the limits of experimental error, identical chromatograms
were obtained at steady-state conditions when sequential samples
were taken from ports two through seven. By adjusting the speed
of the syringe pump and the size of the syringe used for contaminant
injection, constant concentrations of polluted air could be main
tained for 30 to 90 min.
The acetone-mixing studies simulated the mixing that can be
expected from any low molecular-weight aldehydes, ketones, or alcohols
in onion and garlic oils; repitition of the mixing studies with iso-
amyl alcohol simulated a high boiler in the range of some of the
disulfides. It also showed that heating the vaporization plate
would be necessary when using onion or garlic oils in the control
studies. The test air was at 126 - 131°F at about 10% RH during
these studies. The air flow rate was 31 CFM at test conditions.
The sample-transfer technique was used and constituted the bulk of
the experimental error. The peak heights were all measured at the
same chromatograph attenuation. A definite sorption effect was
noticed in the plastic syringes due to surface effects, but was
greatly eliminated when gas-tight glass syringes were used in later
tests.
As with acetone, air samples were also taken sequentially at
2 min intervals from all four reactor ports and from the second and
third mixing ports. At 6.9 ppm iso-amyl alcohol, the average peak
height from 80 samples was 2.05 in with a standard deviation of
18.2%. At 0.69 ppm, the average peak height from 82 samples was
25
1.10 In with a standard deviation of 24.1%. More important, there
was no difference in the GC response from those samples taken from
port two as compared to the samples taken from any other port from
three to seven.
The important conclusion from the iso-amyl tests was that
mixing was completed by the second sample port which corresponded to
a residence time of 11.2 sec at 32.1 CFM. Because heating of the
contaminant vaporization plate was necessary for reproducible vapor
ization of iso-amyl alcohol, the plate was heated for all runs
using onion and garlic oils.
Safety Features
Because ozone is a toxic and highly irritating gas, it was
necessary to take preventive measure against the possibility of
ozone leakage. Fortunately humans can smell the gas at 0.1 ppm or
less. The threshold limit value set by the American Conference of
Governmental Industrial Hygienists for ozone is 0.1 ppm which re
presents the conditions under which it is believed nearly all workers
may be repeatedly exposed day after day, without adverse effect (28).
A well-sealed system and a strong up-draft to the main header and
outside stack were the main preventive measures taken to minimize
worker exposure to ozone. A copper screen mesh was also placed in
the effluent duct to help reduce any residual ozone leaving the
reactor. The screen was ineffective. Finally all but one of the
main stack-header entry ports were blanked off with galvanized
26
sheet metal, gasketed and sealed with Silastic. The open port
(3 ft X 3 ft) was reduced to 6 in x 12 in to match our exhaust line
Sealing the entry ports helped prevent any hazard from ozone to
persons working in the pilot-plant area.
CHAPTER IV
RESULTS AND DISCUSSION
Ozone-Decay Studies
Before any experimental odor control was initiated, the ozone-
decay characteristics of the test chamber were determined. The
decay characteristics were necessary so that the ozone addition rate
could be properly adjusted for the anticipated contaminants addition
rate. This introductory study also provided data regarding the
expected ozone concentration leaving the reaction chamber. In ad
dition, the ozone-decay studies constituted a check on the effec
tiveness of the pollutant-ozone contact and provided a measure of
the amount of ozone consumed by natural decay and surface reactions
unassociated with the odor-control process.
The decay characteristics for this system were evaluated by
operation at various temperatures, flow rates, and relative humidities
without the presence of the contaminants in exactly the same manner
used when the contaminants were present. The decrease in the ozone
concentration through the system was to be used as a measure of
the decay characteristics of the entire apparatus. Thus it was
possible to estimate the net ozone concentration which would be
available for reaction with the onion and garlic oil components.
The concentration of the ozone was then adjusted for the experi-
27
28
mental odor-control studies to values up to 20 ppm actually entering
the reaction chamber.
Initially, ozone saturation was carried out in several runs to
eliminate (by reaction) the active sites in the reactor and the
ozone supply line. A purge of 2 £/min (20°C, 1 atmosphere, atm) con
taining about 5,000 ppm ozone was added to the reactor through the
stainless-steel sample line from the ozone generator. There was a
gradual increase in ozone concentration at the reactor entrance as
the active sites in the supply line became oxidized as shown in
Table I.
The next step in the ozone-decay studies was to test the dif
ference between the inlet and outlet ozone concentrations from the
reactor for residence times up to a maximum of eight seconds. The
ozone concentration was in the range 0.65 - 1.10 ppm at test con
ditions of 90°F (50 - 100% RH), 99 - 113°F (23 - 100 ^ RH) and 130°F
(10 - 65% RH). For all test conditions we typically experienced
from 20 - 40% difference in ozone concentration between the cal
culated inlet values and the coulometrically measured reactor
effluent. This difference was found to be constant and independent
of temperature, relative humitity, and residence time in the reactor
Because there were no trends in ppm ozone due to process changes,
we concluded that the ozone concentration difference was due to
some discrepancy in the method of measuring or calculating the
ozone concentration rather than to ozone decav.
29
TABLE I
OZONE SATURATION OF STAINLESS STEEL LINE
* Cumulative Deliverv Rate Test ppm 0, Saturation ., ^^^nnor ^ \ ^
3 time,hr / " t O ' , 1 atn)
1
2
3
4
1196
2740
4097
5240
0
3
8.5
10
0.261
0.205
0.205
0.205
This is the concentration of the ozone as measured by the potassium iodide method coming from the ozone generator at the effluent of a 5 ft x i/4 in outside-diameter stainless steel line
30
The random errors associated with the calculated ozone con
centrations were evaluated by differentiation of the equation used
to find these concentrations. The calculated ozone concentration,
P ppm, was found from:
where C = ppm ozone concentration before dilution,
V = ozone and air-flow rate from the ozone generator, CFM,
and
Q = air-flow rate, CFM.
Equation 2 was used to estimate (29) the uncertainty in P from the
variances in C, V, and Q.
2 where a p = variance of P,
2 Op = variance of C,
a 2 w = variance of V, and
2 a Q = variance of Q.
The working form of the equation after evaluation of the squared
partial derivatives Is:
2 _ /Vx2 2 ^ ,Cx2 2 ^ ,Cy^2 2
For the Initial ozone decay tests at ozone concentrations of approxi
mately 1 ppm, the values to be substituted in equation (4) are:
°-p = W°o * W'^' ^Tu «''
31
2 ^ r ' 214006.5 (based on six degrees of freedom),
2 -9 , a w = 6.7203 x 10 (based on four degrees of greedom),
2 a Q = 6.587 (based on two degrees of freedom),
C = 4072 (the mean value of seven KI tests) ,
V = 0.008040 (the mean value of five flow tests), and
Q = 31.05 ( the mean value of three flow tests.
Upon substitution, equation (4) becomes:
2 _ /0^008040v2/«T.^^c c\ J. fA9IL.\^tc 7OQ0 in-9x a p - ( 3 93 ) (214006.5) + (31 93) (6.7293^10 )
. [(4072)(0.0Q8040)^ g 587
(31.05)"^
a^p = 0.0213
The standard deviation, ap, is the square root of the variance and
is
o. / 2 = ± 0.146 ppm O3
At 10 ppm ozone a random error of ± 3.94 ppm at a 95% confidence
level was estimated. In the decay tests at approximately 40 ppm
ozone, a random error of - 27.0 ppm at a 95'c condifence level was
calculated. This high value was due to the high variance in C,
the ozone concentration in ppm before dilution.
32
The above procedure was used to estimate the random error for
all the calculated values of ozone concentration in the ozone-
decay studies. The error associated with the measured ozone con
centrations as given by the KI tests is 4.6% of the mean value
based on a 95% confidence interval (32). An estimation of the
systematic error was not possible.
Use of the error analysis showed that the calculated inlet
ozone concentration was 0.965 ± 0.375 ppm at a 95% confidence
level for 1.0 ppm (nominal) concentrations. In addition, several
tests revealed that for an inlet ozone concentration of 1.0 ppm,
the Mast meter read in the range 0.63 - 0.78 ppm with the most
commonly observed value being 0.66 ppm O3 with an average of 0.82
ppm 0- . The difference between the KI value and the calculated
value is within the experimental-error value of ± 0.375 ppm. Note
that at this concentration, the Mast meter was reading about 20%
lower than the corresponding value determined by the KI method.
Ozone-decay conditions at approximately 0.13 ppm O3 as cal
culated were also investigated for reactor residence times of up
to eight seconds. The results of these tests showed an average
ozone concentration of 0.142 ppm when samples were taken from the
first reactor port. The Mast readings from all ports were in the
range 0.06 - 0.09 ppm 0- with an average value of 0.075 ppm. This
constituted an error of approximately 50% by comparison to the
average KI values obtained using the reference method.
33
From these Initial ozone-decay tests we concluded that the
Mast meter was 20% low at ozone concentrations of 1.0 ppm and was
only about 50% accurate at concentrations near 0.1 ppm O3. The
constant nature of the readings, regardless of the process con
ditions, was adequate proof that we were experiencing negligible
ozone decay In the reactor for residence times up to eight seconds.
Later In the experimental study further ozone-decay studies
were necessitated as a result of using part of the mixing duct
for an extended contact reactor. The interior of that duct was
not painted with non-reactive epoxy paint as was the original re
actor. It was only sealed with a water-resistant lacquer. There
fore, 11 more decay tests were conducted for ozone decay over the
range 1 0 - 4 0 ppm. The results are shown In Table II. Column 2
Is the observed effluent concentration as determined by the
potassium Iodide method and the last column is the calculated In
let ozone concentration.
The results of these tests showed no trends with either changing
temperature or percent relative humidity. It was therefore con
cluded again that temperature and humidity level did not affect
the decay of ozone in the test system to any measurable extent.
Near the end of the experimental program, it becomes obvious
that total reaction times of up to 30 seconds should be examined.
Ozone-decomposition tests In the galvanized sheet-metal effluent
duct were therefore conducted before the residence time was ex-
34
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35
tended by using the duct to Increase the reaction zone. As shown
previously, ozone decay was not a function of temperature or humidity
In the mixing duct and reactor at residence times of approximately
25 seconds. The ozone-decomposition results for the metallic duct
showed that for an average inlet ozone concentration of 24.3 ppm
at 90°F and 45% RH, the effluent ozone concentration decreased to
23.8 ppm at 90°F and 92+% RH and to 21.3 ppm at 126°F and 70% RH.
These tests were conducted for 20 second residence time and are
accurate to within ± 3.21 ppm ozone at a 95% confidence level.
Because the amount of ozone decomposition observed in the mixing
duct, reactor, and effluent duct did not change with changes In
temperature or relative humidity, the decision was made to conduct
all further tests at 130°F and 15% RH. This temperature was chosen
as It Is the upper limit of the Industrial effluent temperatures.
If ozonation Is not effective at 130°F, It will not be effective
at any lower temperature.
Experimental Design
To determine the effectiveness of chemical oxidation of odors
with ozone, a Latin square experimental design was adopted (25).
The three variables to be Investigated using the Latin square
were the contaminant Inlet concentration (loading), the residence
time, and the ozone concentration.
Figure 3 Illustrated the Latin square used where
C , C , C = 2., 10., and 0.3 ppm onion or garlic oil I fc o
36
91
02
^3
1
5
I
8
K
2
J
2
6
J
9
I
3
K
S
7
K
1
J
4
I
Figure 3. Latin Square Test Design
37
vapor In the test gas, respectively.
® r ®2' ®3 " ^ •' ^" " 10. sec residence time, respectively,
for ozone reaction, and
I. J, K = 5., 10., and 20. ppm ozone, respectively, in the
bulk gas phase.
The row variable (contact time), the column variable (contam
inant concentration), and the starting point in the square were all
randomized to preclude bias. The numbers In each block represent
the order of testing the various sets of experimental conditions.
This experimental design was adopted to test, with the least amount
of data and In the most efficient manner, whether any of the three
variables had an effect on the odor-control efficiency of ozone.
Although the Latin square design assumes that no significant In
teractions exist among the three major variables, this assumption
did not constitute a limitation on the experimental work because
only major effects were of interest in the initial tests. It was
originally planned to use a factorial experiment to evaluate the
effect of the variables In more detail If any were shown to be
significant as a result of the statistical analysis of the Initial
Latin square data.
Results
Peak height was used as a quantitative measure of the con
taminant concentrations in the gas chromatography work. This
method of analysis was justified by the fact that for most chromato-
38
grams, the height was more than five times the base width (3).
The GC signal-to-noise ratio was not a factor except at the lowest
contaminant-loading concentration, 0.3 ppm. At that concentration.
It was necessary to operate at a wery sensitive photometric detec
tor signal attenuation of 12.800. A noise response of ± 10% of
full-scale deflection had to be tolerated at this setting.
Prior to ozonation studies two or more samples from the re
actor were obtained to test for residual contamination from pre
vious experiments. The first blank was a check of the GC response
for residuals In the sample syringe by Injection of a 5 ml sample
drawn from the ambient air. Care had to be taken that all onion
and garlic containers were sealed tightly at this time. Additional
blank samples were taken from the system to verify the absence of
residual contaminants. Purging the system for at least an hour
after cessation of experimentation each day ensured that the amount
of residual contaminant present on the next day would not be enough
to Interfere with that day's testing program.
After the blank tests, ozone was added to the system. The
ozone concentration was checked by the standard KI method. If
satisfactory results were obtained, the ozone purge was discon
tinued and the contaminant Injection was started. The system was
then allowed to run approximately 30 min to an hour before samples
were taken. Normally seven to twelve contaminant samples would be
taken with only the last five to eight of the total number of
39
samples being kept. The first few samples were generally not
acceptable due to the system's not being at steady state or to
Improper GC attentuatlon settings. After a constant contaminant
concentration was Indicated by repeatable peak heights, the ozone
purge was re-started. To reach steady state and to analyze the
first five to six samples by GC normally required about an hour.
Then a series of seven to twelve samples were taken with only the
last five to eight being used to evaluate ozone effectiveness.
The completed data for the Latin square analysis of the
effectiveness of ozone oxidation for the control of onion-vapor
constituents Is shown in Table III which lists test conditions as
well as the percentage reduction In peak height.
Peaks three and five are the major peaks In the chromatogram
accounting for probably 75% or more of the total composition. As
a result, these two peaks (along with peak six) were the ones
used to evaluate ozone effectiveness in terms of peak height re
duction. The Latin square analysis takes the hypothesis that
treatments (ozone concentration), rows (residence time) and columns
(onion vapor loading) In the analysis of variance (ANOVA) table
have no effect on ozone's odor-reducing potential. The ANOVA
table for peak five in given in Table IV as an example of these
analyses. On comparing the calculated F-values with the tabular
values the conclusion was that these variables did have an effect
within 80 - 85% confidence. Note that the experimental error sum
40
o I
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cx:
CD < :
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-M C 0* u i-<u Q-
'
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u> c o •r-•M •r-TJ C o o ••-> U) o; h-
•!-> U> O o; z 1—
Tim
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S§: f — 1
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41
TABLE IV
ANOVA FOR A 3x3 LATIN SQUARE DESIGN (Peak Five-Onion)
Source of Variation
Mean
Residence Time
Contaminant Load
Ozone Concentration
Experimental Error
Total
Degrees of Freedom
1
2
2
2
2
9
Sum of Squares
8,335.69
672.62
547.82
455.28
106.34
10,117.75
Mean Square
8,335.69
336.31
273.91
227.64
53.17
--
F Calc.
--
6.33
5.15
4.28
—
--
42
of squares was determined by the difference between the total sum
of squares and the sum of the squares due to the mean, the residence
time, the contaminant load, and the ozone concentration. The ex
perimental error was a result of various factors which included
the fact the contaminant oil studied was produced from two separate
batches. Also contributing were the inherent syringe sampling
errors, uncontrollable ozone generator fluctuations, small variations
in the potassium iodide analyses, and a 13.9 : variation in the air
flow rate.
The analysis of variance gave the following results for the
onion contaminant. For peak three, the second largest peak ac
counting for approximately 30% of the total sample (possibly a
monosulfide or a very low carbon-number disulfide, as methyl di
sulfide), there were indications that residence time, onion oil
concentration, and ozone concentration all had some effect on the
amount of reaction that took place. For peak five, the largest
peak accounting for about 45% of the total sample, dipropyl di
sulfide, there were strong indications that the ozone and onion
concentrations and the residence time had appreciable effects.
For peak six (a very short broad peak probably composed of a mix
ture of several components of di-and trisulfides that accounts
for about 10% of the total composition), there was about 90%
confidence of the three effects of these same three variables.
The greatest odor reduction (measured in terms of peak height
reductions) was obtained when the onion oil concentration was
43
approximately 2 ppm (based on a pseudo-molecular weight of 148
and a density of 1.077 gm/ml). This concentration was high enough
to be above the chromatograph's lower detection limit but yet was
not so high that the system was stoichiometrically overloaded.
It was also observed that the most effective ratio of ozone con
centration to onion oil concentration seemed to be around 10 to 1
as shown in test eight. ANOVA also showed that the amount of peak-
height reduction followed logically, increasing with increasing
ozone concentrations of 5, 10, and 20 ppm. However the effect of
increasing the residence tine from 5 to 10 to 20 sec was erratic.
The maximum peak-height reduction for peak five was only
67.3% with the average peak-height reduction over all tests 30.4%.
The chemical kinetics were expected to be slow, indicating a low
molecular collision frequency at low concentrations. Therefore
the residence time for reaction was increased to increase the
number of molecular collisions between any given species and the
amount of chemical reaction as reflected by the mean peak-height
reduction.
Test ten, which was an extension of test eight, was conducted.
The experimental conditions for test ten were exactly as those for
test eight (which was the most promising of the original nine ob
servations) except that the reaction time was increased from 5 to
30 seconds. The results show (see Tables V and VI that only 9 - 14%
more reduction in the various peak heights was obtained over the
test-eight conditions. Peak six was nearly elin.inated with an
44
TABLE V
RESULTS OF EXTENDED RESIDENCE TIMES ON ONION-OIL PEAK HEIGHTS
Peak
2
* 3
4
* 5
* 6
No. Peak Height Before O3
7.0
22.4
5.1
50.8
5.8
Peak Hei After C
6.0
8.2
1.75
10.3
0.75
ght
'3 A
1.0
14.2
3.35
40.5
5.05
% Reduction
14.2%
63.4^
65.7 =$
79. T:
87.1%
*
Peaks used in analysis of variance
EFFECT OF
TABLE
INCREASED RESIDENCE
VI
TIME ON PEAK-HEIGHT REDUCTION
Peak
2
3
4
5
6
No. Test 8 %
1.7
53.8
56.2
67.3
72.8
Red. Test 10 % Red.
14.2
63.4
65.7
79.7
87.1
Reduction
12.5
9.6
9.5
12.4
14.3
45
average of 87.1% reduction; peak five composing nearly half the
original sample on a peak-height basis, was reduced by nearly 80%.
Based on this result, slow kinetics due to the low concentrations of
the pollutants did seem to be a limiting factor. From the individual
test results and the Latin square data analysis it was observed and
shown statistically within an 80% confidence limit, that an in
creasing residence time will increase the reaction efficiency.
The majority of the total onion oil sample was contained within
chromatographic peaks three, five, and six.
Typical chromatograms for onion and garlic oils before and
after ozonation are shown in Figure 4 and 5. Although these peaks
decreased in size to some extent with increasing reaction time,
the Increase In reaction effectiveness will not be worth the ad
ditional time to acquire It. For example, the best results at test
eight conditions for onion oil (2 ppm contaminant, 20 ppm O3, 5 sec
residence time) achieved about 67.3% reduction In peak height (re
action efficiency). By increasing the residence time to 28 sec with
all other conditions the same (Test ten) only a 12.4% increase in
reaction efficiency to 79.7% was obtained. From this observation
It seems that most of the reaction occurs in a fairly short time,
probably upon Initial contact on mixing of ozone with the con
taminant. As the remaining contaminant molecules and ozone become
less concentrated, the probability for molecular collision becomes
smaller and there is a corresponding decrease in the reaction rates.
. CO
46
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TJ • C '—» to C
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O «o «•- c <D O
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48
An extension in residence time to achieve an additional 9 - 14%
reduction in contaminant concentration is not warranted. The size of
the contacting chamber required for the high volumetric flow rates
used in the commercial process would be infeasible.
In addition, it was observed that an increasing ozone con
centration had the effect of increasing the reaction efficiency.
A contaminant loading of 2 ppm and an ozone concentration of 20
ppm gave the most successful results.
The mechanism for ozone reactions was investigated in an
attempt to determine other possible factors that might be limiting
the observed amount of reaction. It was postulated that perhaps
ozone was not the true reactive species but was instead nascent
oxygen. However, a literature search for the correct reaction
mechanism disclosed no evidence regarding any tendency for ozone to
decompose and then react. Instead all references showed ozone it
self as the reactive species, acting in most cases as an electro-
philic (electron-seeking) agent (4, 6, 13).
An estimate of the amount of time required for 95% conversion
of the contaminant into non-odorous products was made. The cal
culations indicate that the time required will be 78 - 247 sec
depending on whether the reactor operates as a plug-flow or well-
stlrred unit, respectively. Assuming the reaction to be first
order with respect to both ozone and peak five (dipropyl disulfide)
49
for an overall second-order model, the rate constant is in the
vicinity of 0.001 (sec ppm)" . The most important concept dis
closed by the kinetic calculations is that the amount of con
version is not a linear function of residence time.
Four garlic vapor contaminant tests and two additional onion
tests were conducted with the results given in Table VII. As
mentioned earlier, extended residence-time experiments required
the use of the metallic effluent duct and wooden mixing chamber
for additional reactor volume. The corresponding ozone-decay studies
revealed that ozone decomposition in the concentration and residence-
time range of interest would not be significant. Note that although
the reaction appears to be less effective in the galvanized sheet-
metal duct than in the original epoxy-painted wooden reactor, the
flow geometries (turns, etc.) were different. On comparison of
tests 13 and 14, note that an increase in residence time from 5 sec
to 30 sec increased the observed amount of reaction for peak five
by only 14.2%. This agrees well with the results of the onion
tests showing a 9 - 14% increase in peak height reduction for the
same increase in residence time. Peak one for the garlic tests
appeared to become larger suggesting the possiblilty that it may
contain unresolved short retention-time compounds which are pro
ducts of the ozonation reaction. That ozonation occurred was indicated
by the corresponding reduction in the sizes of peaks five and six.
Peak one for onion oil and peak four for garlic oil did not generally
50
TABLE VII
REACTION CONDITIONS AND EFFICIENCIES
% Peak Height Reduction Test No. Test Conditions* Peak Number
1 2 3 4 5 6
11
12
13
14
15
16
onion O3 0
onion O3 0
garlic O3 0
garlic O3 0
garlic O3 0
garlic O3 0
II II
II
=
—
—
—
=
2ppm 20ppm 30 sec
2ppm 20ppm 5 sec
2ppm 20ppm 30 sec
2ppm 20ppm 5 sec
2ppm 20ppm 5 sec
2ppm 20ppm 5 sec
9.2%
0 %
0 % 43.5?^
+170 ?r*40.4%
+64.7?** 0 %
+10.9^*4.29^;
38.% 46.5%
34.1% 17.5'
54.8% -
46.2% -
0 % -
30.7% -
62.3% 47.5%
33.2". 28.3-"i
75.7% 80.1%
89.9% 95.3;;
42.5-' 42.8%
58.0% 52.5.
•*
Test 11, 12 on onion oil Test 13-16 on garlic oil Test 13, 14 in wood reaction zone only. Test 12, 15, 16 in metal reaction zone only.
A peak height increase was noted for peak one for garlic.
51
show significant levels above random noise. Some deviation in
results did arise in day-to-day testing. These four additional
tests with garlic contaminant were enough to show that the ef
fectiveness of ozone in eliminating garlic odors was no better
than for the onion oil components.
From the test data, under conditions of a 10/1 ozone-to-con
taminant ratio, only about 70 - 80% reduction for the major con
stituent and about 45% reduction for the minor components of the
sample can be achieved. At the low concentrations tested, ozone
did not effectively react with the onion and garlic vapors and
as a result, a considerable amount of residual ozone was observed
in the reactor effluent. The Mast Coulometric Meter, known to
read as much as 50% low, was off scale (> 1.0 ppm ozone) for all
tests except where the ozone concentration was in the 5 ppm
range. The Mast readings for these tests were 0.2 - 0.4 ppm ozone
which could be, with the 50% factor, 0.3 - 0.6 ppm ozone. This
effluent concentration was coupled with the fact that the ozone-
to-contaminant concentration ratio was only 0.5 which was much
too low to produce a significant reaction efficiency. With the
national standard limiting the emission of photochemical oxidants
to a maximum concentration (hourly basis) of 0.08 ppm not to be
exceeded more than once a year (32), and in addition having an
apparent optimum ozone-to-contaminant ratio of 10 to 1, it was
obvious that unless the residual ozone could be removed from the
system effluent the current problem would be traded for one that
was much worse.
CHAPTER V
RECOMMENDATIONS
From the experimental results obtained, it Is evident that
chemical oxidation of the components of onion and garlic oils by
ozonation is not feasible as an odor-control method. Some doubt
exists as to whether or not the compounds in the oils are repre
sentative of the typical industrially-contaminated air stream.
More specifically. It has been suggested that the di- and tri-
sulfides are not present in substantial amounts below the upper
sections of the exhaust stack (29). The presence of di- and tri-
sulfides was proven by Belo (9) through his analytical work to
Identify the vapor-space gases from freshly sliced onions. It is
thus theorized that if ozone could be introduced before the
larger molecular-weight compounds are formed or become concentrated,
then perhaps ozonation may be an effective control approach.
Future work should center around improving the simulation of
the contaminated air stream by passing the air stream over a given
weight and surface area of freshly sliced, chopped, or pureed
onions or garlic cloves. Quantification of the actual contaminant
loading thus produced could be accomplished by a comparison of the
resultant GC peak height against that obtained from the original
study using the onion and garlic oils. It would also be necessary
to measure the total volume of air passed over the vegetable pre-
53
54
paratlon. A minimum of approximately two hr will be necessary to
verify system steady state and to make chromatographic analyses
of the samples without intermediate storage. The introduction of
the chopped or pureed vegetable into the system air flow can be
accomplished by the use of trays with a constant exposed vegetable
surface area per unit time. An open screw conveyor could also work
by moving the ground vegetable at a constant rate across the air
flow path and then dumping the material into a solution of KMnO. or
NaOCl. Either method, trays or conveyor, will require special pre
cautions to prevent contaminant odors from entering the system
unintentionally.
Another possibility is the use of a pilot-size rotary or tunnel
dryer at the industrial site. This dryer can produce a well con
trolled contaminant-laden air stream at a much higher volume than
is possible at the laboratory site. If ozone is shown effective
when using the pilot-size dryer effluent or for a slip-stream from
one of the Proctor-Schwartz dryers, then a kinetic study using
laboratory and pilot data for the design of a full-size odor-
control system will be warranted.
Whichever of the methods is attempted, it is recommended that
both the gas chromatograph and a subjective odor test panel be used to
evaluate the effectiveness of ozone for odor reduction. Although
ozone has its own characteristic odor which can mask other odors,
it is felt that human response would be beneficial in this phase
55
of future work. Odor test panel procedures are currently being
evaluated In draft form by the American Society for Testing and
Materials, the Air Pollution Control Association, and the Environ
mental Protection Agency.
In the event that ozonation is found to be completely unsatis
factory, other odor-control routes are available. Adsorption is a
physical process In which the contaminant molecules are captured
on the surface of a solid such as activated carbon or silica gel.
This method can be effective for low-concentration odors in large
air volumes. The process Is favored by low temperatures with a
generally accepted upper limit of 120 - 130°F. The onion or garlic
dehydration process operates at atmospheric pressure at at temper
atures as high as 130°F, therefore the vapor pressure of the con
taminants may be a limiting factor. In addition, the presence of
particulates complicates matters and puts serious restraints on
adsorption as a control route unless the particulates are removed
from the gas stream ahead of the adsorber.
Although pressure drop Is a severe limitation in industrial
situations, reactive scrubbing may merit further investigation.
This approach is effective for controlling some odors when the
odoriferous air stream is contacted with a solution such as 1%
solium hypochlorite (90% efficient on dimethyl disulfide) as shown
by Doty (11). In addition, inertial impaction in a cross-flow
scrubber would eliminate any fine particulate emissions. Removal
56
of the larger particulate emissions will require the use of a
cyclone upstream from the scrubber.
For this application the major design problems will be the
prime movers and the type of scrubber itself. A cross-flow
scrubber may be the most feasible as it operates at a low pressure
drop, approximately 1 - 2 in water per foot of packing for Berl
saddles. Cross-flow scrubbers are difficult to design due to the
horizontal and vertical flow patterns, but should be applicable
to this problem because they can handle the gas and particulate
problems equally well.
Combustion by thermal oxidation is the surest way to eliminate
odoriferous compounds if the particulates can be first removed.
It is not the most economic due to the costs of an auxiliary fuel,
such as natural gas, usually required to maintain a stable flame.
Fuel costs can be offset somewhat by using the exhaust gases to
preheat the inlet air stream. To have an efficient thermal-
oxidation process without smoking, it is necessary to maintain the
temperature in the combustion zone in the range 1200 - 1600°F;
to maintain at least a 0.3-sec residence time in the combustion
zone; and to insure a high degree of turbulence. If these con
ditions can be achieved, then a highly effective method of elim
inating the odors without excessive pressure drop will result.
Before any of these control methods are evaluated experimentally,
they should be justified by an economic analysis.
CHAPTER VI
CONCLUSIONS
The conclusions reached from all tests in this study are as
follows:
1. Ozone decay did not vary significantly with temper
ature or humidity for the range of process conditions
studied.
2. Ozonation is not a satisfactory control approach for
the odors from onion and garlic oils.
57
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58
>
59
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60
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