<v

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.

4

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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UJ

<

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00 < : o

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cx:

CD < :

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to .^

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-M C 0* u i-<u Q-

'

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en

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.^ fO o (U z Ou

u> c o •r-•M •r-TJ C o o ••-> U) o; h-

•!-> U> O o; z 1—

Tim

e se

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S§: f — 1

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O l—i

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in

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r—

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n—

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in

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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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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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3. American Society for Testing and Materials: "Recommended Practice for General Gas Chromatography Procedures", Standard E260-69, Philadelphia, PA. (1969).

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58

>

59

14. Galetto, W.: Comments on Molecular Weight of Onion and Garlic Oils, Personal ommunications to C. L. McGowan (1975).

15. Green, W. and T. Elliott: "Control of Rendering Plant Odors in Philadelphia", paper no. 71-9, 64th annual meeting. Air Pollution Control Association, Atlantic City, NJ (1971).

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21. Mueller, F. X., Leob, L., and W. H. Mapes: "Decomposition Rates of Ozone in Living Areas", Environ. Sci. Technol. Ij. 342-346 (1973).

22 Nakano, T.: "The Example of Odor Control with Ozone", J. Pollut. Control Assoc. (Japan), 4: 359-363 (1968).

23. Okayama Prefecture Industrial Experiment Station: "Deodorization and Removal of Bad Smelling Gases", Experiment Station News, No. 10 (1971).

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25. Ostle, B.: Statistics in Research, 2nd ed., pp. 410-415. Iowa State University Press, Ames, IA (1972).

26. Sabersky, R. H., Sinema, D. A., and F. H. Shair: "Concentrations, Decay Rates, and Removal of Ozone and Their Relation to Estab­lishing Clean Indoor Air", Environ. Sci. Technol. ]_: 347-353 (1973)

60

27. Saltzman, B. E. and A. F. Wartburg: "Absorption Tube for Re­moval of Interfering Sulfur Dioxide in Analysis of Atmospheric Oxident", Anal. Chem. 37: 779-782 (1965).

28. Sax, N.I., ed.: Dangerous Properties of Industrial Materials, p. 989. Reinhold Publishing Corp., New York, NY (1968).

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30. Silberstein, 0.: Comments on the Formation of Odoriferous Disulfide Compounds, Personal communication to R. M. Bethea (1975).

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34. Yocom, J. E. and R. A. Duffee: "Controlling Industrial Odors", Chem. Eng. 77(13): 160-168 (1970).