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Microwave destruction of benzene in a packed bed reactor

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MICROWAVE DESTRUCTION OF BENZENE IN A PACKED BED REACTOR
A Thesis
Presented to
The Faculty o f the College o f Graduate Studies
Lamar University
In Partial Fulfillment
o f the Requirement for the Degree
Master o f Engineering Science
by
Hemal Navinchandra Rupani
December 2005
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UMI Number: 1438775
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MICROWAVE DESTRUCTION OF BENZENE IN A PACKED BED REACTOR
HEMAL NAVINCHANDRA RUPANI
Approved:
Thomas C. Ho
Supervising Professor
/&Tae Hoon Kim
Co-Supervising Professor
Che-Jen (Jepyj L in
Committee Member
KuyenUi
Chair, Department o f Chemical Engineering
k R. Hopper
ean, College x>f Engineering
&rry Bradley
associate Vice President for Research and
Dean o f Graduate Studies
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© 2005 by Hemal Navinchandra Rupani
No part o f this work can be reproduced without permission except as indicated by the
“Fair Use” clause o f the copyright law. Passages, images, or ideas taken from this work
must be properly credited in any written or published materials.
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ABSTRACT
Microwave Destruction o f Benzene in a Packed Bed Reactor
By
Hemal Navinchandra Rupani
This thesis reports an experimental study o f VOC destruction using microwave
energy. In the study, trace amount o f benzene vapor in air or nitrogen was sent through a
packed-bed reactor and was destroyed in the reactor by microwave heating. Granular
activated carbon (GAC) and silicon carbide (SiC) were used as the packed bed materials
as well as the absorption mediums for microwave energy. The inlet and outlet streams
were
analyzed
using
Gas
Chromatography/Mass
Spectrometry
(GC/MS).
The
Destruction and Removal Efficiency (DRE) was characterized under various microwave
power levels and flow rates.
The results show that silicon carbide is a better microwave absorption medium.
Silicon carbide in air is an effective combination for the destruction o f benzene with
more than 99% DRE observed. The microwave power needed is low in the range o f 200
to 400 watt. The reaction products have been found to be largely composed o f CO, CO 2
and H 2 O with trace amount o f other byproducts.
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ACKNOWLEDGEMENTS
I am grateful to my parents Mr. Navinchandra M. Rupani and Mrs. Dipika N.
Rupani for their unending support and great encouragement to pursue higher education
and to achieve excellence in my academic career. Also I am thankful to my younger sister
Ami for her augmenting motivation to emerge as a meaniful graduate.
I would like to express deep gratitude to my supervising professor Dr. Thomas C.
Ho for providing me with this opportunity to carry out the research under his guidance.
At every moment, I was encouraged to think beyond the boundaries and achieve the best.
I wish to thank my co-supervising professor Dr. Tae Hoon Kim for his active
involvement in our project and always added creative ideas to my work. I thank him for
being encouraging and improving my understanding o f the subject by instilling the
knowledge.
I would also like to thank committee member Dr. Jerry Lin for sharing his indepth knowledge and valuable suggestions to complete this work.
I am also thankful to the staff o f Chemical Engineering Department for their help.
1 appreciate help and cooperation given by Dan Rutman, Sammer Pallavkar and Suraj
Shetty. M y special thanks are to Balu Meka for his invariant support during this project
work.
Finally I gratefully acknowledge financial supports o f this study from the Gulf
Coast Hazardous Substance Research Center (Project No. 084LUB2856) and from the
National Science Foundation for a Major Research Instrumentation (MRI) award (Award
ID 0320818) on GC/MS.
iii
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TABLE OF CONTENTS
Page
List o f Tables
vii
List o f Figures
viii
Nomenclature
ix
Chapter
1. Introduction
1
1.1 Source o f VOCs
1
1.2 Cause o f Concern o f VOCs
2
1.3 Regulations to Control VOC Emissions
3
1.4 VOC Control Technologies
4
1.5 Microwave Technology
5
1.6 Objective o f the Study
5
2. Literature Review
7
2.1 Method for VOC Emission Control
7
2.2 Process and Equipment Modification
8
2.3 Add-On Control Techniques
9
2.3.1 Destruction o f VOCs
9
2.3.1.1 Thermal Oxidizer
9
2.3.1.1.1 Regenerative Thermal Oxidation
11
2.3.1.1.2 Recuperative Thermal Oxidation
11
2.3.1.2 Catalytic Oxidation
12
2.3.1.3 Reverse Flow Reactor
14
iv
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2.3.1.4 Bio-Filtration
15
2.3.2 Recovery o f VOC
18
2.3.2.1 Condensation
18
2.3.2.2 Absorption
19
2.3.2.3 Membrane Based Separation
20
2.3.2.4 Adsorption
22
2.4 Analysis o f Different Available Techniques
25
2.5 Microwave Technology
27
2.5.1 Microwave Radiation
28
2.5.2 Classification o f Material
29
2.5.3 Applications o f Microwave Technology
29
2.5.4 Advantages o f Microwave Heating
33
2.5.5 Safety Consideration
34
3. Experimental Aspects
36
3.1 Experimental SetU p
36
3.2 Microwave System
39
3.3 The Ultra Trace Toxic GC/MS System
42
3.3.1 System Configuration
43
3.3.2 Specifications
46
3.3.3 Concentrator Traps
47
3.3.4 Valve Operation for Toxics
49
3.3.5V alve Operation for Hydrocarbons
51
3.4 Experimental Procedure
52
v
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4. Results and Discussion
54
4.1 Experimental Parameters
54
4.2 General Results
57
4.3 Effect o f Absorption Medium
62
4.4 Effect o f Microwave Power
62
4.5 Effect o f Flow Rate
62
4.6 Reaction Products
63
5. Conclusions and Recommendations
64
5.1 Conclusions
64
5.2 Recommendations for Future Work
65
References
66
Biographical Note
71
vi
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LIST OF TABLES
Table
Page
3.1 Typical Configuration o f Ultra Trace Toxic
GC/MS System
44
4.1 Experimental Conditions
56
4.2 Experimental Results Associated with Test 1
57
4.3 Experimental Results Associated with Test 2
57
4.4 Experimental Results Associated with Test 3
57
4.5 Experimental Results Associated with Test 4
58
4.6 Experimental Results Associated with Test 5
58
4.7 Experimental Results Associated with Test 6
58
vii
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LIST OF FIGURES
Figures
Page
1.1
Major Sources o f VOCs
2.1
Electromagnetic Radiation
28
3.1
Experimental Set Up
37
3.2
Schematic Diagram o f Reactor Design
38
3.3
Picture o f Microwave Applicator
38
3.4
Microwave Test Unit
39
3.5
Microwave Assembly
41
3.6
The U ltra Trace Toxic GC/MS System
45
3.7
Traps and Valves
48
3.8
Valving Diagram for the Ultra Trace Toxic GC/MS System
50
4.1
Plot o f DRE vs Microwave Power for Test 1
59
4.2
Plot o f DRE vs Microwave Power for Test 2
59
4.3
Plot o f DRE vs Microwave Power for Test 3
60
4.4
Plot o f DRE vs Microwave Power for Test 4
60
4.5
Plot o f DRE vs Microwave Power for Test 5
61
4.6
Plot o f DRE vs Microwave Power for Test 6
61
2
viii
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NOMENCLATURE
Roman Symbols
cm
Centi Meter
CFM
Cubic Feet per Minute
CFR
Code o f Federal Regulations
CO
Carbon Monoxide
CO 2
Carbon Dioxide
DRE
Destruction and Removal Efficiency
EPA
Environmental Protection Agency
FRM
Federal Reference Method
GAC
Granular Activated Carbon
GC/MS
Gas Chromatography/Mass Spectrometry
H2 O
W ater
IR
Infrared
KW
Kilo Watt
LPM
Liters per Minute
MFC
Mass Flow Controller
mg
Milli Grams
NAAQS
National Ambient Air Quality Standards
NRC
National Research Council
PM
Particulate Matter
ppb
Parts Per Billion
ppm
Parts Per Million
ix
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ppt
Parts Per Trillion
TOC
Total Organic Carbon
RH
Relative Humidity
s
Seconds
Sic
Silicon Carbide
TOC
Total Organic Carbon
TSP
Total Suspended Particulates
VOC
Volatile organic compound
x
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Rupani 1
CHAPTER 1
Introduction
Volatile organic compounds (VOCs) are organic compounds that can readily
volatilize when gas stream is released to the ambient air (U.S.EPA 1997). VOCs are
among the most air pollutants from chemical, petrochemical, and allied industries. They
are one of the main sources o f photochemical reaction in the atmosphere leading to
various environmental hazards. Growing environmental awareness has put up stringent
regulations to control the VOC emissions. In such circumstances, it becomes mandatory
for each VOCS emitting industry or facility to opt for proper VOC control measures
(Khan and Ghosal 2000).
1.1 Source of VOCs
The majority o f anthropogenic VOCs released into the atmosphere are from
transportation sources and industrial processes as shown in Figure 1.1 (U.S.EPA 1997).
These VOCs include most solvent thinner, degreasers, cleaners, lubricants, and liquid
fuels. The minor sources include gas stations and dry cleaners, vehicles, and even trees.
Some common VOCs are benzene, tetrachloroethane, methyl chloride, and various
chlorohydrocarbons and perflurocarbons. Industrial emissions o f VOCs originate from
breathing and loading losses from storage tanks, venting o f process vessels, leak from
piping and equipment, wastewater streams and heat exchanger systems (Cooper and
Alley 2002).
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Rupani 2
51%
EH Industrial Processes
HU] Transportation
H Fuel Combustion
Data source; U.S. EPA 1
Figure 1.1 Major Sources of VOCs (U.S.EPA 1997)
1.2 Cause of Concern of VOCs
From environmental point o f view, it is necessary to limit and control vapor
emissions because they effect climate, the growth and decay o f plants, and health of
human beings. In addition, VOCs o f certain classes o f hydrocarbons are carcinogenic.
In the atmosphere, VOCs may combine with oxides o f nitrogen and sunlight to
form ozone and other photochemical oxidants, which is environmentally hazardous.
However, not all VOCs react the same. Formaldehyde and other industrial chemicals
such as ethylene, propylene and butadiene are very reactive and form ozone far more
rapidly than other VOCs. Most o f the VOCs emitted from vehicles, construction
equipment and vegetation do not produce ozone as rapidly as those from industry.
The emissions o f chlorofluro-methanes and chlorine containing compounds into
the atmosphere may increase absorption and emission o f infrared radiation and will
contribute to global warming (Khan and Ghosal 2000).
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Rupani 3
1.3 Regulations to Control VOCs Emissions
Growing environmental awareness has put up stringent regulations to control the
VOC emissions. In such circumstances, it becomes mandatory for each VOCs emitting
industry or facility to opt for proper VOC control measures. Regulations on controlling
organic vapor pollutants in air have been issued worldwide. In the Ambient Air Quality
Standards established by the US Environmental_Protection Agency, the maximum 3-hour
concentration o f hydrocarbon content is 1.6xl0‘4 kg/m3 (0.24 ppm), not to be exceeded
for more than a year (CFR 1997).
As mentioned earlier, VOCs may react with nitrogen oxides and other airborne
chem icals, in the presence o f sunlight (photo-chemically), to form ozone, which is a
primary component o f smog. Reduction o f VOCs emissions that exceeds the current
National Ambient Air Quality Standard for ozone o f 0.12 ppm is mandated under Title of
the US Clean Air Amendment o f 1990. In addition, Title III o f the Amendment requires
reduction o f em issions o f 189 hazardous air pollutants, most o f which are included under
definition o f VOCs as well (Ruddy and Carroll 1993). Thus, some sources o f VOCs may be
controlled under two separate sets o f regulations with potentially differing requirements.
The EPA has defined a Volatile Organic Compound (VOC) as any compound of
carbon that participates in atmospheric photochemical reactions excluding the few listed
as “Exempt VOC” by this section [40 CFR 51.100(s)(l)]. EPA Test Methods in 40 CFR
60, Appendix A are used for measuring VOCs when determining compliance with
emission limits. EPA VOC Emissions Report to Congress (U.S.EPA 1997) categorized
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Rupani 4
certain VOC as “not reportable” where the VOCs were solids, had a vapor pressure <0.1
mm Hg @ 20°C, or had more than 12 carbon atoms.
For gasoline emissions, the European Community stage emissions limit is 35 grams
-3
total organic compounds (TOC) per cubic meter gasoline loaded (35 g TOC/m ). Similarly,
the U nited States Environmental Protection Agency Standard 40 CFR Part 63 has
established an emission limit o f 10 g TOC/m3. The German TA-Luft Standard, the most
stringent known gasoline emission regulation, has set an emissions limit o f 150 mg TOC
(excluding methane) per cubic meter o f loaded product (0.15 g TOC/m3) (U.S.EPA 1991).
The Clean Air Act 1990 (Amendment) and the Factory Act 1986 (Amendment) limit the
release/emission of commonly hazardous chemicals that include most o f the VOCs.
1.4 VOC Control Techniques
VOC control techniques include thermal oxidation, catalytic oxidation, adsorption,
condensation and refrigeration, biological oxidation, and flaring. In a thermal oxidizer,
the VOC-laden air stream is heated to gas temperatures several hundred degrees
Fahrenheit above the auto ignition temperature o f the organic compounds that need to be
oxidized. They have the broadest applicability o f all the VOC control devices and can be
used for almost any VOC compounds. Catalytic oxidizers operate at substantially lower
temperatures than thermal oxidizers. Due to the presence o f the catalyst, oxidation
reactions can be performed at temperatures in the range o f 500 to 1000°F. Common types
o f catalysts include noble metals (i.e. platinum and palladium) and ceramic materials.
Condensation, refrigeration, and cryogenic systems remove organic vapor by making
them condense on cold surfaces. Condensation and refrigeration systems are usually used
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Rupani 5
on high concentration, low gas flow rate sources. In biological oxidation, VOCs can be
removed by forcing them to absorb into an aqueous liquid or moist media inoculated with
microorganisms that consume the dissolved and/or adsorbed organic compounds. Flaring
is used for gas streams having VOC concentrations exceeding approximately 25% o f the
lower explosive limit (LEL).
1.5 Microwave Technology
Microwave energy has unique heating properties for dielectric compounds and
can be used as the heating source for the destruction o f VOCs. Microwave energy
interacts with VOCs is a way different than all other thermal treatment processes. The
main advantage is energy saving for low flow rate or low concentrated VOC streams.
This technique does not require supplement fuel for bulk gas stream.
1.6 Objective of the Study
This thesis reports an experimental study o f VOC abatement using microwave
energy. Benzene was chosen for this study, which is an important industrial solvent and
precursor in the production o f drugs, plastic, gasoline, synthetic rubber, and dyes.
In the study, the benzene vapor were carried into a packed-bed reactor by air or
nitrogen and destroyed in the reactor via microwave heating. Granular activated carbon
(GAC) and silicon carbide (SiC) were used as packed bed materials as well as absorption
medium for microwave energy. The inlet and outlet streams were analyzed using Gas
Chromatography/Mass Spectrometry (GC/MS). The Destruction and Removal Efficiency
(DRE) was investigated by varying microwave power level and flow rate.
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Rupani 6
In this thesis, Chapter 1 is introduction; Chapter 2 reviews literature related to this
study; Chapter 3 describes experimental setup; Chapter 4 presents results and makes
discussion on the observed results; and Chapter 5 draws conclusions o f the study and
marks recommendations for future work.
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Rupani 7
CHAPTER 2
Literature Review
This chapter reviews literature related to this study. They include Methods for
VOC Emission Control, Process and Equipment Modification, Add-on-Control
Techniques, Analysis o f Different Available Techniques, Microwave Technology, and
Economic Analysis o f VOC Recovery Techniques.
2.1 Methods for VOC Emission Control
There are many techniques available to control VOCs emissions. These techniques
are basically classified into two different groups: (i) process and equipment modification
and (ii) add-on-control techniques. In the first group, control o f VOCs emissions is
achieved by modifying the process equipment, raw material, and/or change o f process,
while in the other group an additional control method has to be adopted to regulate
emissions. Elowever, the former is the most effective and efficient method, its applicability
is limited, as usually it is not possible to modify the process and/or the equipment. The
techniques in the second group are further classified into two sub-groups, namely the
destruction and the recovery o f VOCs (Khan and Ghosal 2000).
The first task in evaluating VOC emission control is to prepare a comprehensive
emissions inventory. The emissions inventory provides the basis for planning, determining
the applicability o f regulations permitting the selection o f control options for further
consideration. The inventory should cover the entire facility source-by-source, considering:
•
pollutants emitted,
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Rupani 8
•
the individual chem ical species w ithin each vent stream (to identify any
non-VOC materials that may have determined effects on particular types o f
control equipment),
•
hourly, annual, average, and w orst case emissions rates,
•
existence and condition o f certain pollution control equipment, and
•
regulatory status.
2.2 Process and Equipment Modification
Process and equipment modifications are usually the most preferred alternative for
reducing emissions. Modifications include the substitution o f raw materials to reduce
VOCs input to the process, changes in operating conditions to minimize the formation or
volatilization o f VOCs and the modification o f equipment to reduce opportunities for
escape of VOCs into the environment. The first two mechanisms vary with the process
being addressed and are not further discussed here. Equipment modification can take many
forms, but the objective is always to prevent the escape o f VOCs. VOCs can be emitted
through open vessel tops, vents, or leaks at flanges or valves, or they may be the result of
process conditions. The starting point is the vessel or structure in which the operation
takes place. Vessels can be
capped
or
equipped
w ith
ru p tu re
disks
or
pressure/vacuum vent caps to contain vapor emissions. Monitoring and repair programs can
be instituted to consistently reduce emissions due to leaks from valves, pumps, and
process piping connections in order to reduce emissions (Chadha and Parmele
1993).Similarly, process enclosures can be designed to reduce emissions. By enclosing the
source, a positive means of collecting the emissions can be provided. However, simply
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Rupani 9
providing an enclosure is not enough to reduce the emissions. If emissions are captured in
the enclosures but no additional measures are taken, the pollutants will eventually escape
into the environment. Typically, this situation is handled by end-of-pipe solutions. An
overall approach to develop collection and control strategies to reduce VOCs emissions is
via a stepwise analysis. The first step is to quantify air emissions by developing an
understanding o f the process steps and material balances, followed by sampling, measuring
or estimating vent flows. Then, significant sources can be identified and strategies and
designs are developed to address the issues (Khan and Ghosal 2000).
2.3 Add-on Control Techniques
Add-on Control Techniques are broadly classified into two types: destruction and
recovery, details o f which are presented in subsequent sections.
2.3.1 Destruction of VOCs
In this type o f technique, VOCs are destroyed by different types o f oxidation such
as thermal and catalytic, and digestion o f VOCs under aerobic conditions by microbes
(Bio-filtration).
2.3.1.1 Thermal Oxidizer
Thermal oxidation systems, also known as fume incinerators, are not simple flares or
afterburners. The modem thermal oxidizer is designed to accomplish from 95% to 99%
destruction o f virtually all VOCs. These systems can be designed to handle a capacity of
1,000 to 500,000 cfm (cubic feet per minute) and VOC concentration ranges from 100 to
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Rupani 10
2,000 ppm. Nominal residence time ranges from 0.5 to 1.0 s. Available with thermal energy
recovery options to reduce operating costs, thermal oxidizers are very popular.
Thermal oxidation systems combust VOCs at temperatures o f 1,300 - 1,800°F.
Actual operating temperature is a function o f the type and concentration o f material in
the vent stream and the desired DRE (Destruction and Removal Efficiency). Compounds
that are difficult to combust or that are present at low inlet concentrations will require
greater heat input and retention time in the combustion zone to ensure that the desired
DRE is accomplished. High DRE requirements will also require higher temperatures and
longer retention times. Inlet concentrations in excess o f 25% o f the LEL (Lower Explosive
Limit) are generally avoided by oxidizer manufacturers because o f potential explosion
hazards.
O perating tem peratures near 1800°F can produce elevated levels o f nitrogen
oxides (from nitrogen in air), a secondary pollutant that may, in turn require further
treatment. Halogenated compounds in the vent stream are converted to their acidic
counterparts. Sufficient quantities may necessitate the use o f expensive corrosion
resistant materials
o f construction
and the use
o f additional acid gas controls, such
as scrubbing, as follow-up treatment.
Two types of thermal energy recovery systems are in common use today,
regenerative and recuperative. Both use the heat content o f the combustion exhaust stream
to heat the incoming gas stream prior to entering the combustion zone (Khan and Ghosal
2000 ).
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Rupani 11
2.3.1.1.1 Regenerative Thermal Oxidation
System uses ceramic beds to capture heat from gases exiting the combustion zone.
As the bed approaches the combustion zone temperature, heat transfer becomes inefficient
and the combustion exhaust gas stream is switched to a lower temperature bed. The
incoming gas stream is then passed through the heated bed where it recovers the captured
heat prior to entering the combustion zone. By using multiple beds, regenerative systems
have achieved up to 95% recovery o f the thermal energy input to the system as fuel and the
heat content o f the com busted VOCs, W here the incoming gas stream contains
sufficient thermal energy potential from VOC combustion, regenerative systems can
operate without external fuel (excluding the need for a pilot light). The efficiency of the
thermal recovery system depends on the process operating characteristics. A process where
the flow rate and VOC content are relatively constant has a good potential for achieving
virtual no-fuel operation. Cyclic processes generally are not as compatible with
regenerative oxidation systems. The absorbed heat is lost to the environment during
periods of low activity. Operation with insufficient VOC content to supply thermal input
requirements necessitates the use o f external fuel sources.
2.3.1.1.2 Recuperative Thermal Oxidation
Systems recapture thermal energy with a simple metallic heat exchanger,
typically a shell-and-tube design. The maximum thermal energy recovery o f a
recuperative system is around 70% o f the fuel and VOC combustion energy input to the
system. The advantage over the regenerative system comes from the relatively short period
required for the heat exchanger to reach operating conditions. The larger mass o f the
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Rupani 12
regenerative heat recovery system requires time and relatively large initial fuel inputs to
reach operating conditions, while the recuperative heat exchanger reaches operating
conditions within several minutes o f start up. Recuperative systems are best suited to cyclic
operations where the versatility o f an oxidation system is required along with the ability to
respond to cyclic operating conditions.
The high concentrations o f organics in the regeneration stream, combined with the
short duration o f the desorption cycle, permits economical destruction of the VOCs in a
thermal oxidizer. Thermal oxidation is a costly disposal method for treating low
concentrations o f organics contained in the process exhaust. A properly designed thermal
oxidizer, which incorporates an effective heat exchanger and advanced refractory lining, is
able to utilize the calorific value o f the desorbed VOCs to generate the temperatures
required for destruction with minimum auxiliary fuel consumption. A destruction
efficiency o f more than 99 % can be achieved for most organics at temperatures ranging
from 1,400°F to 2,000°F with residence times o f 0.5 s to 2.0 s .
2.3.1.2 Catalytic Oxidation
Catalytic oxidation systems directly combust VOCs in a manner similar to thermal
oxidizers. The main difference is that the catalytic system operates at a lower tem­
perature—typically about 700-900°F. This is made possible by the use o f catalysts that
reduce the combustion energy requirements. The incoming gas stream is heated, most often
in a recuperative heat exchanger followed by additional input from a burner if needed, and
passed through a honeycomb or monolithic support structure coated with catalyst, Catalyst
systems can be designed to handle a capacity of 1,000 to 100,000 cfm and VOC
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Rupani 13
concentration ranges from 100 to 2,000 ppm. The catalytic system is well suited to low
concentration operations or those that operate in a cyclic manner. They are often used for
vent controls where flow rates and VOCs content are variable. Destruction efficiencies in
excess o f 90% are common with a maximum DRE o f 95% (Patkar and Laznow 1992;
Ruddy and Carroll 1993). High concentration vent streams can also be treated with catalytic
technology; however, as with thermal oxidation, it is not advisable for concentrations in
excess of 25% o f LEL. Lower operating temperatures, combined with a recuperative heat
exchanger, reduce the start up fuel requirement. Large catalytic systems have been
installed, but are not as popular as direct thermal oxidation systems, mainly due to the
high costs o f catalyst replacement. Catalyst systems, like thermal oxidizers, can produce
secondary combustion wastes. Halogens and sulphur compounds are converted to acidic
species by the catalytic combustion process; these are treated by using acid-gas scrubbers.
Also, the spent catalyst materials can require disposal as a hazardous waste if they are
not recyclable. Catalyst materials can be sensitive to poisoning by non-VOC materials such
as sulphur, chlorides and silicon. Many catalyst manufacturers have overcome sensitivity to
some o f these substances, but every catalyst has susceptibilities that must be considered
at the process selection stage. For example, some catalysts are sensitive to deactivation by
high molecular weight hydrocarbons or polymerizing materials. Also, the catalyst support
may become deformed at high temperatures and high concentrations. Researching these
issues should be part o f the process selection activity if catalytic oxidation is under
consideration (William and Lead 1997).
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Rupani 14
2.3.1.3 Reverse Flow Reactor (RFR)
A Reverse Flow Reactor (RFR) is an adiabatic packed bed reactor in which the
direction o f the feed flow is reversed periodically. Thus, the reactor is forced to operate
under transient conditions. RFR is becoming a strong alternative for the removal of
VOCs from polluted air because unsteady-state reactor operation can be profitable for the
chemical process (Matros, Noskov and Chumachenko 1993). The RFR, if operated on a
large scale, will behave close to adiabatically. For the RFR the dynamics o f the system
should be well defined and not influenced by its surroundings. This excludes the use of
insulation to obtain adiabatic conditions, because for a reasonable resistance against heat
losses the amount o f insulation needed is very large. The heat capacity o f the insulation
can easily be larger than that o f the packed bed and thus a significant additional heat buffer
is created. In addition, applying compensation heating has to be avoided, because in that
case, the dynamics o f the system are certainly influenced and no better defined. The best
way o f achieving adiabatic conditions and minimizing the influence o f surroundings is
making use o f an evacuated jacket with the provision o f a radiation shield at higher
operating temperatures (Van de Beld, Borman, Derkx, Van Woezil and Weserterp 1994).
Important parameters influencing the performance o f RFR are cycle period, gas
velocity, adiabatic temperature, different components and mixtures, and variations in
inlet concentration.
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Rupani 15
2.3.1.4 Bio-filtration
The bio-filtration process, which was originally developed for the odor
abatement o f waste gases, has proven recently to be an effective and inexpensive
method for the removal o f VOCs produced during various industrial activities (Ottenger
and van den Ocver 1983). This technique is based on the ability of microorganisms
(generally bacteria) to convert, under aerobic conditions, organic pollutants to water,
carbon dioxide and biomass. The bio-filter consists generally o f a simple structured packed
bed, intensively surrounded with an immobilized micro-flora .The contaminated gas is
directed through a bio-layer around the packing material. In practice, various types of
packing material are used, e.g. compost, soil, peat, etc. (Leson and Winer 1991; Jol and
Dragt 1995; Kiared, Bieau, Brzezinski, Viel, and Heitz 1996). The filter bed material
should have certain mechanical and physical properties (structure, void fraction, specific
area, flow resistance and water retention capacity), and biological properties (provision o f
inorganic nutrients and specific biological activity).
In the past decade increasing attention has been paid to bio-filtration as a waste
gas purification process due to some important advantages this technique has compared to
conventional purification methods. In addition to the mild operating conditions, a
biological decontamination process does not generally transfer the pollution problem to
another environmental compartment (gas to solid and/or gas to liquid), which is often the
case with many other purification methods. Moreover, biological treatment is especially
effective when the odors or toxic waste gas emission are in the lower concentration range,
i.e., at few ppm levels.
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Rupani 16
Bio-filtration is a process in which contaminated air is passed through a porous
packed medium that supports a thriving population o f micro-organisms. The contaminants
are first adsorbed from the air to the water bio-film phase o f the medium. The degree of
adsorption is a function o f the chemical characteristics o f the specific contaminant (water
solubility, Henry's constant, and molecular weight). Once the contaminants are adsorbed,
the micro organisms convert them to carbon dioxide, water, inorganic products and bio­
mass (Ottenger and van den Oever 1983; Liu 1994).
Bio-filter success is dependent upon the degradability o f the contaminants (Leson
and Winer 1991; Kiared, Bieau, Brzezinski, Viel, and Heitz 1996). Anthropogenic
compounds may contain complex bonding structures that resist microbial enzymatic reac­
tions. Oxidation may not be complete, and may even form degradation by-products more
toxic than the original compounds (Tahraoui 1994). For example, during the aerobic
transformation o f trichloroethylene, the highly toxic by-product vinyl chloride may be
formed (Webster, Torres and Basrai 1995). A successful bio-filter must also provide a
benign environment for micro-organisms. The moisture content o f the medium should be
maintained at optimum values to support microbial growth without clogging the pores.
Acceptable values o f the medium pH at which microorganisms can thrive must be
maintained.
Bio-filtration is effective in simultaneously removing hydrogen sulphide, VOCs
and toxic air emissions from discharges o f publicly owned treatment works (POTW). In
order o f degradability, data suggest that the order o f removal efficiency appears to be
hydrogen sulphide > aromatics > aldehydes and ketones >chlorinated hydrocarbons. The
preference for compounds with low molecular weights, higher solubility and less complex
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Rupani 17
structures is evident in the data (Webster, Dcvinny, Torres, and Basrai 1996). For both the
bench and pilot-scale reactors, TGNMO (Total Gaseous Non-Methane Organics) data
suggest that more than 65% removal is possible regardless o f reactor pH conditions.
Removal efficiencies for aro-matics ranged from 53 to 98%, aldehydes and ketones from
43 to 96% and chlorinated compounds from 0 to 98%. Complete oxidation o f hydrogen
sulfide occurred at both the bench and pilot-scale levels (Liu 1994). A recent laboratory
scale bio-filtration study was conducted by Kiared (1996) to remove vapors o f ethanol
and toluene present in air. Peat was chosen to serve as filter material. Under typical
operating conditions, the following conclusions have been drawn:
• The commercial peat offered a favorable life environment for micro-flora,
at moist conditions.
• The pH o f the medium, m easured regularly, was stable around the neutral
value (pH=7), hence, no acid intermediates were produced and consequently no
buffering agent was needed.
• The pressure drop was also minimal, i.e. less than 6 cm H 2 O per meter o f bio­
filter, for most o f the experimental periods.
• The RTD (Residence Time Distribution) measurement showed that the gaseous
phase was in plug flow affected by axial dispersion.
Good performance was obtained in terms o f removal efficiency and elimination
capacity. The micro-organism selected and used here maintained a good level o f activity
even under local dryness conditions (as has generally been the case in bio-filtration).
Overall, bio-filtration o f POTW waste air appears to be effective, while being resistant to
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Rupani 18
changing environmental conditions. This increases confidence in a technology, which must
be used under conditions which are not always optimal.
2.3.2 Recovery of VOC
Several techniques for recovery o f VOCs such as Condensation, Absorption,
Adsorption and Membrane separation are discussed here in detail.
2.3.2.1 Condensation
The driving force for condensation is over-saturation, which is achieved by chilling
or pressurization (or both) o f the waste gas stream. Condensation is most efficient for
VOCs with boiling points above 100CF at relatively high concentrations above 5,000 ppm.
Low-boiling VOCs can require extensive cooling or pressurization, which sharply increases
operating costs. Exceeding the 25% LEL threshold is more common with condensation
systems. In fact, some systems begin operation above UEL (Upper Explosive Limit).
This is dangerous, because the concentration will likely fall through the explosive range
during the condensation process. Therefore, inert gas blanketing o f the vessels or
unmanned process enclosures is advised to avoid the explosion hazard associated with high
VOC concentration. However, this causes additional operating costs. Polymerization
materials should also be avoided in the condensation system due to the potential for fouling
the heat-exchanger surface. Best suited to mono solvent systems, condensation produces a
liquid product that must be treated to remove condensed water and possibly to separate
various chemical species. Recovered VOCs can be reused within the process, used as wash
solvents during equipment cleanup, burned as an alternative boiler fuel, shipped off-site
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Rupani 19
for disposal or resold for reuse by others. Recovered water should be sent to a wastewater
treatment plant prior to discharge, if exposed to miscible VOCs (Spivey 1988).
2.3.2.2 Absorption
Absorption is used to remove VOCs from gas streams by contacting the
contaminated air with a liquid solvent. Any soluble VOCs will transfer to the liquid phase.
In effect, the air stream is scrubbed. This takes place in an absorber tower designed to
provide the liquid vapor contact area necessary to facilitate mass transfer. Using tower
packing and trays as well as liquid atomization can provide this contact. Packed bed and
mist scrubbing absorption systems are detailed here.
An absorption system can be designed to handle a capacity o f 2,000 to 100,000
cfm and VOCs concentration ranges from 500 to 5,000 ppm. An absorber can achieve
VOC rem oval efficiencies o f 95 to 98%. The design o f an absorption system for VOC
control is similar to the design o f an absorber for process application, using vapor liquid
equilibrium (VLE) data, liquid and vapor flux rates, liquid and vapor handling information,
and material balances.
Packed bed scrubbing uses packing material to improve vapor-liquid contact.
Packing can either be randomly dumped or stacked in the tower. Packing varies widely in
size, cost, contact surface area, pressure and material o f construction, and each packing
design has its own advantages under different conditions. Packed bed scrubbers can be
used well with a low solubility system due to the high liquid and vapor residence time (>10
s) associated with the entraining nature o f packing. Packed bed scrubbers should, however,
be used when liquid flow rates are low which causes inadequate wetting o f the packing
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Rupani 20
material. Also, the particulates on entering
the
air
stream
or
absorbent
create.
Absorb ate reaction products which can foul or plug the packing.
Mist scrubbers use spray nozzles to atomize the liquid stream into tiny droplets.
These droplets provide the surface area for liquid-vapor contact. M ist scrubbers require a
very low-pressure drop and must not be fouled by particulate in the incoming gas stream
(Ruhll993; Ruddy and Carroll 1993). The residence times o f liquid and vapor are low (110 s). Therefore, mist scrubbing should only be applied to highly soluble systems.
Absorption is not particularly suitable for cyclic operation due to start-up time constraints.
It is, however, good for a high humidity air stream (>50% RH).
2.3.2.3 Membrane Based Separation
Membrane based separation has been reported over a long period. This technique
initially was used back in 1960 for desalination purposes. Since then a number of
applications have been reported. According to various requirements, this technique has
been classified in many groups. Gas permeation and reverse osmosis are the techniques
used in the application o f solvent recovery (recovery o f VOCs) from air. This technique is
in the field o f experimental research and has yet to be commercialized, though a few pilot
plants have been developed and are continuously monitored for performance. A simplified
process diagram o f VOC recovery using the membrane technique is presented in Fig. 6.
A b rie f analysis o f this is presented below:
Baker, Yoshioka, Mohr and Khan (1987) conducted air and vapor permeation
experiments for various polymeric films, most o f the experimental work reported so far is
concentrated on composite silicon rubber membranes. Behling (1986) and Behling,
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Rupani 21
Ohlrogge, Peinemann and Kyburz (1988) have chosen poly (ether imides) as the
supporting material because it is much more stable to organic vapors than polysulfone.
Buys, M artens, Troos, Van Heuven and Tinnemans (1990) used polyhyd-antoine and
polyimide as the porous support to the silicon rubber-coating layer in their study. A
membrane system for the treatment o f low-volume, high vapour concentration gas streams
was tested, but information was disclosed concerning the membrane materials by Wijmans
and Helm (1989). The resistance o f silicon rubber to some organic vapours, for example
gasoline, is how ever poor. An attempt was made, therefore, by Deng, Sourirajan and
Matsuura (1996) to prepare membranes from a single polymeric material o f high organic
resistance. In their previous studies, asymmetric aromatic polyimide membranes were
investigated for die purpose. It was shown that controlling the conditions o f the membrane
preparation properly could produce membranes o f both high selectivity and reasonably high
permeability.
Deng, Sourirajan, and Matsuura. (1996) have conducted a thorough experimental
study on the recovery o f organic solvent from air with the help o f an aromatic poly
membrane. The study promises encouraging results. The important outcomes o f the study
are as follows:
Hydrocarbon mixtures can be effectively separated from air by an asym m etric
arom atic poly (ether imide) membrane without a silicone rubber coating. Water
permeability is higher than hydrocarbon permeability.
The presence o f water vapor in the feed does not affect the hydrocarbon
permeability. In other words, water and hydrocarbon molecules permeate through the
membrane independently. Gasoline vapor can be separated effectively by an asymmetric
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Rupani 22
aromatic poly (ether imide) membrane. Aromatic poly membranes are very good
commercial value for removing volatile organic compounds when the amount o f air to be
treated is relatively small.
2.3.2.4 Adsorption
The adsorption process is classified into two types, namely, physical adsorption and
chemisorptions based on the interaction between adsorbate and adsorbent. Physical
adsorption has been found to be more significant in the case of separation processes.
Physical adsorption is again classified into Thermal Swing Adsorption (TSA) and Pressure
Swing Adsorption (PSA), based on the operation o f the process. Both the processes have
their advantages and disadvantages. Physical adsorption occurs when organic molecules
are held on the surface and in the pores o f the adsorbent by the weak Van der Waals force
of attraction and is generally characterized by low heat o f adsorption, and by the fact that
the adsorption equilibrium is reversible and rapidly established. For details please refer to
Ruthven (1984). A very low VOC concentration in exhaust air is expensive to treat.
For m any low concentration situations it is possible to use adsorption to increase
the concentration.
Carbon adsorption is a very common method o f VOCs emission control. VOCs
are removed from (the inlet air by physical adsorption onto the surface o f the carbon.
The system is sized according to the maximum flow and concentrations expected, and
anything less usually improve efficiency. Carbon adsorption systems are flexible and
inexpensive to operate. Installation costs are often lower than those o f other systems
(Ruhl 1993; Stenzel 1993). The adsorption capacity of activated carbon for a given VOC
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Rupani 23
is often represented by an adsorption
isotherm
o f the
amount
o f VOC
adsorbed
(adsorbate) to the equilibrium pressure (concentration) at constant temperature.
VOC molecules are physically attracted and held to the surface o f the carbon.
Activated carbon is such a good adsorbent because o f its large surface area, which is a
result o f its vast infrastructure o f pores and micro-pores and micro-pores within micro­
pores. In a commercial activated-carbon solvent-recovery plant, solvent laden air passes
through a tank containing a bed o f activated carbon. The solvent is adsorbed on the carbon
surface and clean air is exhausted to atmosphere. W hen all o f the available surfaces o f the
carbon pores are occupied it w ill not capture any additional solvent. Now, to recover
the solvent for reuse, it must be released from the carbon surface. This is most commonly
done by heating the carbon with steam. The hotter the carbon, the less solvent it can
hold, so as the steam heats the carbon, solvent is released and flushed away by the steam.
The mixture o f steam and solvent is condensed by cooling and then separated in the
simplest case by gravity decanting. Factors affected the adsorption process are detailed
below.
•
Retentive ability: The ability o f absorbate to retain the absorbed VOC is
important characteristic. Retentivity as determined in laboratory studies,
represents the amount of adsorbate that, initially saturated with the VOCs, can
retain them when pure air is passed through the bed. The application of
adsorption in air pollution control is a dynamic process. As the contaminated
stream is passed through the adsorbate bed and adsorption occurs, the
saturation zone with the bed moves forward until the breakthrough point is
reached.
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Rupani 24
•
Pressure: The pressure needs to be maintained at the recovery plant inlet (both
adsorption and desorption processes) and whether the recovery plant
needs to provide suction to aid in exhausting must be known. Specifically,
PSA needs to m aintain precise pressure in the column, and this requires
sophisticated control accessories. This adds to the total capital cost o f the plant.
•
Concentration: It is desirable to operate solvent recovery plants at higher
concentrations. This minimizes the horsepower (by moving less air) and
steam consumption, decreasing the required frequency o f draining. In
general, VOC control system s that are designed always to operate below
25% o f the LEL do not require continuous monitoring or control. If continuous
monitoring o f the exhaust concentration is provided, continuous operation at
up to 40% LEL is allowable, with a provision for automatic shut down for
concentrations exceeding 40% o f LEL.
• Particulate concentration: The presence o f dust or other particulars could
clog the adsorbent. So it is prudent to install a filter upstream o f the
adsorbent. An estimate o f the particulate loading should be made to
determine how frequently the filter medium would have to be cleaned or
replaced. The particulate loading and type are the major factors in making the
selection o f the filter medium.
•
Type o f adsorbent: In the case o f activated carbon as adsorbent,
manufactured from coal, wood or coconut shells, it can be given either pellet
or granular shapes subject to their specific use. The most important
specification is the adsorption isotherm. So for a new application, pilot
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Rupani 25
testing (repeated) is recommended for verifying suitability and predicting
commercial performance o f each adsorb ate-adsorbent system.
•
D esorption methods: The m ost commonly used method for removing
adsorbed solvent from the adsorbent is to heat the adsorbent directly with
steam. The suitability o f this method depends on the solubility o f the solvent in
water. Hot air or nitrogen can also be used for desorption o f the solvent.
2.4 Analysis of Different Available Techniques
The available techniques are analyzed based on following important parameters
as discussed below.
•
Source characteristics'. Selection o f a point-source emission control technique is
made on the basis o f source characteristics (e.g. VOC concentration, flow rates,
etc.) and the desired control efficiency rather than on a particular source type.
•
Recycle potential'. Recycling presents the opportunity to partially recover costs
o f control equipment. Before the project has progressed significantly, the
viability o f this option must be assessed. If recycling is desirable and achievable,
further consideration o f direct destruction devices such as oxidation systems
m ay be unnecessary.
•
Variability o f loading: The nature of the process being controlled determines
the variability o f the loading to which the control device will be exposed. High
flow and/or concentration variability may exist with batch operations, and can
produce additional wear and tear on equipment, reduce thermal energy,
recovery efficiency, and possibly reduce the actual destruction and removal
efficiency (DRE) o f the device, The average loading determines the
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Rupani 26
applicability o f various processes.
Recovery options such
as direct
condensation require high inlet concentrations to operate successfully. Very low
average inlet concentrations req u ire som e p re-treatm en t (in creasin g the
concentration) prior lo actual treatment.
•
VOCs composition: The mixture or mixtures o f VOCs to be treated in the
control equipment has an effect on system applicability. The greater the
variability of substances to be controlled, the greater the limitations placed upon
the selection process. The DRE is dependent upon the "worst player”; (that is,
the m ost difficult com pound to remove or destroy) in the exhaust stream.
Recovery systems may require additional separation
equipment to recycle
materials.
•
Fire and explosion hazards: VOCs are fire-prone chemicals requiring extra
precautions and safety measures. This is particularly so in chemical process
industries involving bulk storage and operation at extreme conditions of
temperature and pressure. Khan and Abbasi (1997, 1998) have discussed these
in detail.
•
Presence o f non-VOCs: The presence o f non-VOCs contaminants in the vent
stream can produce problems for VOC control equipment. Particulate matter
in the exhaust stream can plug adsorption beds and heat recovery modules on
thermal oxidizers. Halogenated materials can be oxidized to form their acidic
counterparts, possibly requiring additional pollution control equipment and
costly corrosion resistant materials o f construction.
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Rupani 27
•
Maintenance: The level o f the maintenance facility available is also an
important consideration, If current maintenance is not very sophisticated (for
example, the source is neither large nor complete), the choice o f a control
system requiring continual monitoring and adjustment is not recommended,
•
Location: A central plant location may require certain technology
applications that include long ductwork runs to reach peripheral locations.
R oof mounted systems may require extensive reinforcement of the roof support
system as part o f the design.
•
High discharge temperature: Relatively high vent discharge temperature
precludes use o f condensation or adsorption options because o f the cost of
cooling the gas stream. However, thermal catalytic oxidation would benefit
from such a pretreated gas stream.
•
Cost: For further screening devices the total cost, consisting o f capital costs and
annual operating costs, should be estimated. Capital costs include equipment,
installation and site preparation costs. Annual costs include utilities, operating
and supervisory labor, maintenance labor, and materials.
•
Removal efficiencies: Expected emission reductions from the application of
control techniques on the basis o f the total VOC. Combustion devices are
capable o f providing higher removal (i.e. destruction) efficiencies.
2.5 Microwave Technology
Microwave energy have a unique heating properties for dielectric compounds and
which may be used as heating source for destruction o f VOC. Microwave energy
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Rupani 28
interacts with matter is a way different than all other thermal treatment processes. The
main advantage is energy saving for low flow rate or low concentrated voc stream since
no supplement fuel are required for this technique (Dauerman, Windgassse, Zhu and He
1992).
2.5.1 Microwave Radiation
Microwave radiation is an energy composed o f an electric field and a magnetic
field. It is a form o f electrom agnetic radiation, w ith the frequencies ranging from
100 M Hz to 300 GHz show n in Figure 2.1. Industrial heating application uses
m icrow ave w ith 2450M Hz. The FCC has reserved 915 MHz and 2450 MHz, among
other frequencies, for industrial applications. 2450 MHz is a wavelength o f 4.8" in air.
915 MHz is about 13" in air. Microwave power is usually measured in kilowatts. At room
temperature, and 1 atmosphere pressure, 1 kilowatt o f microwave energy will evaporate
approximately 2.5 pounds o f water in 1 hour.
F r e q u e n c y , Hz
103
I I I
I
! i!
A —c p o w e r i
! li
1010
!
3 X 107
10s
I'....
i
|
i
i
109
} J J i..r
1 !
li
S
jRadiOi . JM i c r o w a v e
i
3 X 104
i rI n .... r1
i<1l !
HTVli
j
30
i
!
1 0 ’5
1 0 12
.. t ...... ■ | .
i
|
!
nfra
i
Vi s i bl e
«
red
I
t
1
1
1
i
I
j
3 x 10’
2
1 0 18
i.......
t
i
1 X rays
i— — —
1
I
3 x 10~5
W a v e le n g th / c m
Figure 2.1 Electromagnetic Radiation
The electric field interacts with polar materials and the magnetic field reacts with
magnetic or charged materials. Polar molecules and free ions in receptive materials
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Rupani 29
respond to these fields by creating a molecular friction, which results in heat throughout
the mass o f the material (Metaxas and Meredith 1983).
2.5.2 Classification of Material
There are three classes o f materials in a microwave radiation processing system: 1)
conductors; 2) insulators; and 3) dielectrics. 1) Most good conductors are metallic
materials such as copper, brass, aluminum, silver, etc. These conductors reflect
microwaves except the ones with sharp edges. This reflective property is used to contain
and direct microwaves. For example, the wave-guide o f a microwave oven is usually
made o f brass or aluminum. 2) Insulators either reflect or transmit microwaves and
generally absorb only a little amount o f microwave energy. Teflon, polypropylene (PP),
etc. play the insulator part in a microwave system. 3) Dielectrics have properties in
between conductors and insulators. In the microwave radiation field, dielectrics and
materials to be treated with dielectric properties absorb microwave energy in varying
degrees.
Properties o f materials determine the absorption o f microwave radiation energy. It
is known that the dielectric constant o f a material plays an important role. On the contrary,
a material's dielectric constant is not a constant, but a property depends upon many
parameters. The dielectric constant depends on moisture content, temperature, and
geometric factors (Jones 2002).
2.5.3 Applications of Microwave Technology
Microwave energy represents a unique heating source with the potential of
providing a highly flexible means for (a) minimizing generation o f selected future wastes,
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Rupani 30
(b) reducing existing wastes and immobilizing hazardous components, and (c) reclaiming
or recycling reusable and sometimes valuable components in waste products (Oda 1992).
Microwave treatments include remediation o f discarded electronic circuitry and
reclamation o f the precious metals within, incinerator ashes, medical and infectious
wastes, industrial wastes/ sludge, rubber products including tires, asbestos, groundwater,
volatile organic compounds (VOC's), shipboard wastes, contaminated soils and sediments,
and radioactive wastes and sludge (high, low and intermediate level wastes, transuranic
and mixed wastes). The dem onstrated applications o f m icrow ave in environm ental
applications are discussed below (W icks and C lark 1999).
•
Soil Vapor R em ediation: This system captures and recovers volatile organic
compounds (VOCs). Overall, the process uses granular activated carbon
(GAC) to remove the VOCs from the air stream, continuously regenerates the
used GAC with microwaves, and recovers the VOCs, desorbed from the GAC,
by condensation. This project was funded by an SBIR granted by the National
Institute o f Environmental Health o f NIH. The process has been demonstrated
at McClellan Air Force Base in 2003.
The system can be used to recover
vapor discharged from Soil Vapor Extraction (SVE) treatment. It can also
recover vapor discharged from dry cleaners, paint booths and recover vapor
discharged from gasoline loading stations (Cha 2004).
•
Microwave Destruction o f Waste Rocket Fuels: This system was developed to
eliminate waste hydrazine-based rocket fuel and waste oxidizer, separately. In
rocket propulsion, fuel and oxidizer are combined to achieve self-
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Rupani 31
ignition. The hydrazine is passed through a microwave carbon-bed reactor
and then through a microwave oxidizing catalyst reactor.
There is no
adsorption and all the contaminants are destroyed. This project is directly
funded by the Air Force. W ater scrubber combined with high pressure - high
temperature UV oxidation - very expensive (Cha 2004).
•
Treatment o f PCB and secondary gases: A wide array o f electronic
components was successfully treated by a relatively simple and flexible, onestep, hybrid-heated microwave process. Significant volume reductions were
achieved in all studies. In this process, circuitry was completely converted
into two waste form products; a glass and a metal form, both o f which can be
reused and recycled. The metal form produced was subsequently processed to
effectively reclaim precious and valuable metals, including gold and silver.
The separation o f glass and metal waste products was very effective and
efficient. A new "tandem microwave" system successfully handled both
primary as well as secondary wastes in an integrated unit, i.e., the off-gases
produced were simultaneous treated along with the electronic circuitry. The
entire hybrid microwave process and specialized equipment (now being
patented) can also be mocked-up to a larger scale and made mobile, if desired
(W icks and C lark 1999).
•
VOC abatem ent at A ir fo rc e p a in t workshop: The U.S. Department o f Air
Force sponsored CHA Corporation under a Phase I and Phase II Small
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Rupani 32
Business Innovation Research program to investigate a feasibility of using a
novel microwave-based process for the removal and destruction of volatile
organic compounds in effluents from non-combustion sources such as paint
booth ventilation streams. The CHA microwave-based gas cleanup process is
designed to capture and destroy a wide variety o f both chlorinated and non­
chlorinated VOCs, many o f which are commonly found at industrial and
military sites. In addition, the CHA process also removes the pollutants S02
and NOx from flue gases. The technology has been successfully demonstrated
in March 1998 at McClellan Air Force Base where NOx and VOCs were
removed from diesel engine exhaust gas. This microwave process was
successfully demonstrated on a pilot-scale at McClellan Air force Base (Cha
2004).
•
Treatm ent o f Transuranic Wastes: The tandem microwave system, for
destruction and remediation o f radioactive transuranic (TRU) wastes is used.
Volume reductions o f 60-90% were generally achieved and main organic
constituents, sources for gas generation during storage, were eliminated by
this process. The products produced were either ashes or vitrified materials,
depending on the application (W icks and C lark 1999).
•
R ecycling o f rubber: Microwave energy is used to selectively break S-S and
S-C bonds in the rubber compound, while leaving the backbone o f the rubber
(C-C bonds) relatively intact, thus de-vulcanizing, but not de-polymerizing,
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Rupani 33
the rubber to be recycled. This also produces activated surfaces that when
combined with new rubber, yields a composite with excellent properties. This
produce new type o f "crumb rubber" that can be combined with "new rubber"
to produce high-quality tires, having 25% or more recycled rubber than
obtained previously (W icks and C lark 1999).
•
Treatment o f M edical Waste: The tandem microwave system provides a
unique means for treating both the primary and secondary medical waste, and
provides a system that is compact and can be made portable. It has been used
successfully, on a laboratory scale, for destruction o f a variety o f mixed
simulated infectious medical wastes, including plastics, clothing, sharps and
other metal components, etc. This technology has the potential o f a)
disinfecting b) sterilizing and c) destroying discarded medical waste products
(W icks and C lark 1999).
2.5.4 Advantages of Microwave Heating
Microwave energy interacts with matter is a way different from all other thermal
treatment processes. As a result o f these unique features, the advantages o f using
microwave energy for treating a vast array o f hazardous wastes can include many
potential advantages. The advantages ultimately realized will depend both on the type
and characteristics o f the wastes to be treated. The major advantages o f microwave
heating are listed below (Krause and Helt 1993).
•
R apid heating
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Rupani 34
•
S elective heating
•
Q uick startup and shutting
•
No need o f supplem ent fuel
•
Ease o f control
•
O verall reduction o f treatm ent costs
•
Portability o f equipment and process
•
Rapid and flexible process that can also be made remote
•
Ability to treat wastes in-situ
•
Treatment or immobilization o f hazardous components to meet regulatory
requirements for storage, transportation or disposal
•
Improved safety, including reductions in personnel exposure o f potentially
hazardous chemicals or materials for processing and disposition
2.5.5 Safety Consideration
Microwave heating equipment shall be designed, constructed and operated to
provide adequate protection against radiation hazards due to microwave leakage.
All equipment where accessibility to the applicator by a part o f the human body is
possible is to be provided with means o f access. Such means o f access may be omitted if
other types o f protective measures such as doors or barriers are arranged with required
interlocking.
The microwave leakage power density shall not exceed a power density o f 50
W/m2 (5 mW/cm2) at any accessible location 0.05 m from any portion o f the equipment
under conditions designated as "normal operation". In addition, the microwave leakage
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Rupani 35
shall not exceed a power density o f 100 W/m2 at any accessible location 0.05 m from any
portion o f the equipment under conditions designated as "abnormal operation". These
levels shall not be exceeded at any point located at distances greater than 0.05 m.
Electromagnetic Radiation Protection Guideline is mentioned under 29 CFR Ch.
XVII o f the Occupational Safety and Health Admin, Labor, § 1910.97 for non-ionizing
radiation. For normal environmental conditions and for incident electromagnetic energy
of frequencies from 10 MHz to 100 GHz, the radiation protection guideline is 10
mW/cm2 (milliwatts per square centimeter) as averaged over any possible 0.1 hour period.
This means the following:
Power density:
10 mW/cm2 for periods o f 0.1 hour or more.
Energy density:
1 mW-h/cm (milliwatt-hour per square centimeter) during any 0.1 -hour period.
This applies to all radiation organizing from radio stations, radar equipment and
other possible sources o f electromagnetic radiation such as used for communication,
radio navigation and industrial and scientific purposes. This section does not apply to the
deliberate exposure o f patients by, or under the direction of, practitioners o f the healing
arts. International Electro technical Commission (IEC) Standard publication 519-6, Part 6
discusses specifications for safety in industrial microwave heating equipment (Jou 1998).
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Rupani 36
CHAPTER 3
Experimental Aspects
The experimental aspects o f the study are described in this chapter. The Chapter is
divided into the following sections: Experimental Set Up, Analytical Apparatus, and
Experimental Procedure. These are detailed below.
3.1 Experimental Set Up
A schematic diagram o f the experimental set up is shown in Figure 3.1. It
consists o f a gas cylinder, a mass flow controller, a benzene vaporizer, a packed bed
reactor, a microwave applicator and a GC/MS for gas analysis.
The packed bed reactor was a 30.48 cm long and 11 mm I.D. quartz tube. It was
packed with microwave absorbing material (either activated carbon or silicon carbide
foam) supported on glass wool as shown in Figure 3.2. The packed bed height was 9.62
cm. The reactor was placed inside the applicator as shown in Figure 3.3.The bed
temperature was measured by an infrared thermometer. The flow rate was maintained by
a mass flow controller o f Brooks Instrument.
The sampling loop involving GC/MS had capability o f continuous analysis with
flexibility o f operation. The complete sampling loop uses SS-316 tubing and valves to
avoid any reaction o f process gases with tubing/ valve material. The valves are o f very
low dead volume. The compositions o f inlet and outlet streams were analyzed using the
ultra trace toxic GC/MS system. This system comprises o f GC/MS modified to have PPB
level detection capability. This will be discussed in details in the later part o f this chapter.
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Thermocouple
To Vent
Applicator
GC/MS
Reactor
Mass
Flow
Controller
Benzene
Vaporizer
Figure 3.1 Experimental Set Up
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GC/MS
Quartz Tube 30.48 cm X 11 mm
Infrared Thermometer
Microwave
Activated Carbon / Silicon Carbide
Nitrogen /
—
m
Benzene Vaporizer
Figure 3.2 Schematic Diagram of the Reactor Design
Figure 3.3 Picture of Microwave Applicator
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Rupani 39
3.2 Microwave System
The Microwave test unit has the following components as shown in Figure 3.4.
These components are described below.
W ater In
W ater Out
Dummy
Load
-> Vent
a
P ow er
Supply
M icrowave
G en erato r
3-Port
Dual Pow er
Monitor
C irculator
T uner
A pplicator
Sliding Short
Circuit
□r
w ater in __
w ater out <
Nitrogen
Monitor
Figure 3.4 Microwave Test Unit
•
Power Supply and Controls
Low ripple magnetron power supplies is used for microwave generation. The
power supply and microwave power head are compatible with each other. Controls
include complete local functionality and digital interfaces. This includes the adjustment
of supplied power to magnetron. Hence, desired watt o f power can be set. The power
supply up to 1.5 KW can be generated.
•
Microwave Power Head
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Rupani 40
The magnetron is separated from the power supply for convenience and to
conserve bench-top space. The maximum output power is selected as required by the
process.
•
Circulator 1
The magnetron must be protected from the damaging effects o f reverse power.
This is usually accomplished using a 3-port circulator and dummy load, often called an
isolator when used together. Reverse power is diverted to and absorbed by the dummy
load.
•
Dummy Load
The dummy load absorbs microwave power and, when used in an isolator
configuration, is rated for the maximum expected reverse power (usually maximum
forward power).
•
Circulator 2
A second circulator is often used in conjunction with another load and power
reflector to provide a means for variable power delivery at constant frequency. This
enhances operational stability when heating materials having low dielectric loss
characteristics.
•
Forward and Reverse Power Monitor
The forward power monitor indicates actual delivered microwave power while
the reverse power monitor indicates power reflected from the load.
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Rupani 41
•
Tuner
The impedance o f the process load must be matched to that o f the microwave source in
order to achieve optimal microwave power absorption. A multi-stub tuner is used and is
of manual versions. The reflected power monitor indicates tuning progress.
•
Applicator
Microwave power is coupled to the process material inside an applicator, also
called a chamber or cavity.
•
Termination
Waveguide applicators has attached with sliding a short circuit. This provides
termination to establish an internal standing wave at the material being heated. It can be
adjusted to provide maximum microwave absorption. The wave guide is made of
aluminum to ensure proper distribution o f microwave .
Figure 3.5 Microwave Assembly
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Rupani 42
3.3 The Ultra Trace Toxic GC/MS System
A conventional gas chromatograph does not have sufficient sensitivity to measure
low part-per-billion or part-per-trillion (ppt) levels o f hydrocarbons or other chemical
species in gas samples without some pre-concentration o f the sample.
To enhance
detection, large volumes must be injected, typically 10 to 1000 milliliters. However,
loading such large injection volumes into a capillary column can cause severe peak
broadening due to injection overload. By first trapping the sample into an absorbent trap
and then refocusing on a low-volume cryo-focus trap and then transferring the trapped
material to a capillary column, a large volume o f sample is concentrated sufficiently to
provide low ppb or ppt levels o f the analytes. This approach effectively injects a large
sample volume to the capillary column and yet still maintains the desired sharpness of
peaks.
The Ultra Trace Toxics GC/MS System from Lotus Consulting provides impressive
separations of trace volatile organics in ambient air samples.
The system features the
Varian 3800 Gas Chromatograph with built-in high performance sample concentrators
and the Varian Saturn 2200 Ion Trap Mass Spectrometer. This system handles both
pressurized canisters and Tedlar bags without hardware changes. And the system meets
the exacting requirements for the US EPA TO -14 and TO -15 implementation for
speciation o f toxic compounds.
Samples are loaded through a 16-position automated sampler and trapped onto a
low-volume adsorbent trap with a mass flow controller (MFC) setting the sample size.
The condensed sam ple is then transferred to a cryo-focus trap to reduce further
the effective volum e o f the sam ple to provide a sharp in jectio n volum e into the
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Rupani 43
capillary colum n. Identification and detection is facilitated with the extremely sensitive
Saturn 2200 MS. The system involves a cold trap, three automated valves with micro­
electric actuators, 16-position automated sampler, and one workstation.
All o f these
operations utilize nearly all o f the powerful and comprehensive capabilities o f the Varian
3800, Saturn 2200 and Star Workstation.
To extend the capabilities o f the system , tw o flam e ionization detectors,
an additional cold trap, appropriate valving w ith m icro -electric actuators, three
colum ns and a colum n-sw itching valve are added into the design for direct
m easurem ent o f all hydrocarbons from ethane (C 2 ) through tridecane (C 1 3 ).
T hrough program m ing o f valve actuations, either toxics can be m easured w ith
the m ass spectrom eter, or hydrocarbons can be detected w ith flam e ionization
detectors, or both m easurem ents can be m ade concurrently.
3.3.1 System Configuration
Modifications to a standard Varian 3800 Gas Chromatograph involve appropriate
valving to automate all operations. The system can accept any combination o f up to
sixteen Tedlar bags or stainless steel canisters.
Sample volumes are metered with an
included mass flow controller. All valve actuations and heated zones for the concentrator
are controlled through the standard keyboard o f the Varian 3800 or through Varian Star
Workstation. Some valving and the second cryo-focus trap are located on the separate
gas chromatograph to keep the physical distance between this trap and the analytical
column as short as possible.
All surfaces that contact the sample stream are nickel,
nitronic 60 or polyimide and are inert to most compounds, especially to hydrocarbons.
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Rupani 44
All interconnecting tubing is low-volume 0.16 cm (1/16 inch) nickel or stainless steel and
is kept as short as possible to minimize chromatographic peak broadening.
Since each o f the eight methods in the Varian 3800 can control all valve
actuations for the standard switching valves and set conditions for the concentrator traps,
eight independent operating conditions can be programmed into the Varian 3800.
Alternatively, with the Varian Star Workstation, many methods can be preprogrammed
for the Varian 3800. A typical system configuration is shown in Table 3.1.
Table 3.1 Typical Configuration of Ultra Trace GC/MS Toxics System
Valve/
Event
1
2
3
Function
Toxics Sample On/Off
+ Load Int.Std.1
Toxics Sample
to Front Adsorbent Trap
Toxics Sample to Middle
Trap Trap to Column
4
Hydrocarbon Sample On/Off
5
Hydrocarbon Sample
to Middle Cryogenic Trap
6
Middle Cryogenic Trap
Isolation
7
Column Selection
Action
O ff - Sample Flow O ff / Int.Std. Off
On - Sample Flow On / Int.Std. Load
O ff - Sample Bypass Trap
On - Sample to Front Trap
O ff - Sample to Middle Trap
On - Middle Trap to Column
O ff - Sample Flow O ff
On - Sample Flow On
O ff - Sample Bypass Trap
On - Sample to Middle Cryogenic
Trap
O ff - Middle Cryogenic Trap in Series
On - Middle Cryogenic Trap in Bypass
O ff - Alumina Column in line with
precolumn
On - CP5 Column in line with
precolumn
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Figure 3.6 The Ultra Trace Toxic GC/MS System
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3.3.2 Specifications
Concentrator Trap Volumes 0.16 cm (1/16 inch)
-
120 microliters
0.14 cm (1/18 inch)
-
700 microliters unpacked
Concentrator Trap temperature Range: from -196 °C to 450 °C
Maximum Heating Rate o f Traps - 250 °C/minute
Maximum Cooling Rate o f Traps - 400 °C/minute
Concentrator Trap Temperature Stability Concentrator Trap Temperature Overshoot -
± 2 °C (after 2 minute stabilization)
< 5 °C
Concentrator Trap Cryogen Usage - Typically, 4 liters per trap o f liquid nitrogen to cool
trap, stabilize for 2 minutes and sample for ten minutes.
Carrier Gas Flows - Capillary column carrier gases controlled by Electronic Flow
Controllers Type 3 and Type 5 and are correctable for atmospheric or vacuum at column
end. Pressure can be monitored on display o f Varian 3800 and with analog pressure
gauge on pneumatics panel.
Purge Flows - controlled by Digital Flow Controllers with companion analog pressure
gauges.
Valve Actuations - All valves under routine use are fully automated under timed control
of the Varian 3800 and use micro-electric actuators.
Temperature Probes - All temperature zones are monitored with self-calibrating platinum
probes and controlled with fully proportional heating/cooling.
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Gas Cylinder Requirements Helium (UHP) carrier - 551 kPa (80 psi)
Nitrogen (UHP) purge - 551 kPa (80 psi)
Hydrogen - 276 kPa (40 psi)
Air (ZERO) - 414 kPa (60 psi)
3.3.3 Concentrator Traps
The standard traps provided by Lotus Consulting are 0.32 cm (1/8 inch) nickel
tubing packed with multi-bed o f carbon adsorbents for the front adsorbent trap and 0.16
cm (1/16 inch) empty nickel tubing for the middle cryo-focus trap. Estimated volume for
the front trap is -7 0 0 micro liters and the volume for the middle trap is -120 micro liters.
The cryogenic trap for hydrocarbons is glass beads, 60/80 mesh.
Since the Varian 3800 has limited thermal zones for all o f the hardware installed
in the system, the heated zone for the cryo-focus trap for the Toxic analysis is combined
with the cryogenic trap for Hydrocarbons. To distinguish the two trap functions, the trap
employed for Toxics is referred to in this manual as “cryo-focus” and the trap for
Hydrocarbons is called “cryogenic”, even though they are the same heated zone in the
Varian 3800.
Standard traps in the Ultra Trace Toxics System require that the trap temperatures
be below the freezing points o f the desired components to effectively snare them.
Adsorbents, such as Tenax, can be used to selectively trap specific components above
their boiling points and allow undesirable components to pass untrapped. Advantages are
the more modest temperatures required for trapping and the special selectivity available
with the many adsorbents. However, their drawbacks are that they are often limited in
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Rupani 48
temperature range, have some irreversible adsorptions, introduce contributions to blank
backgrounds from breakdown products, and do not perform well as “universal” traps.
The materials used in the multi-bed absorbent trap are selected purposefully to effectively
trap the toxic compounds, provide little or no trap degradation products from repeated
temperature cycle, and are hydrophobic to avoid trapping o f water.
These materials
performed very well at modest temperatures above the freezing point o f water.
Front
Adsorbent
Trao
Middle
Cryofocus
Valve 6
Valve 3
Valve 5
Figure 3.7 Traps and Valves
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3.3.4 Valve Operation for Toxics
Sample containers are attached to the Valve AS. Valve 1, in the normally off
position, dead ends the sample flow and allows nitrogen purge gas top flow through to
the mass flow controller and sample flow meter on the side o f the instrument. When
Valve 1 is activated, sample flows to the mass flow controller, bypassing the front
adsorbent trap, to purge lines with the new sample; Valve 2 is normally off to divert
sample away from the adsorbent trap; the front adsorbent trap should be at its starting
temperature at this point.
When Valve 2 is activated, sample flow is directed to the adsorbent trap and the
sample loading time commences. The middle cryo-focus trap should start cooling during
this sample loading o f the front trap. During this step, the internal standard can be loaded
into its sample loop. The internal standard flow should be stopped and its loop allowed
to reach atmospheric pressure before the next step. By controlling the time between
activating Valve 2 and turning off Valve 1, the effective sample loading volume can be
varied. After Valve 1 is turned off, nitrogen then purges the interconnecting lines o f
sample and any sample components not trapped (such as oxygen, nitrogen, carbon
monoxide, methane, water and carbon dioxide, depending on the trap temperature.
Before the front trap is heated, Valve 2 is turned off to allow the trapped
components to be directed to the middle cryo-focus trap when the front trap is heating up.
After the analytes are transferred over to the middle trap, Valve 3 is then activated to
direct the effluent from the middle trap to the column. The middle trap is heated at this
point, and when components are volatilized in the trap. All valves are now in position for
the next cycle.
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F ro n t L a rg e
V alv e O v e r
Front
DFC -
V en t
0 .2 5 rr
\ Lot ip
Ven
V1 d e la y e d
tw h e n en ab led }
— ® ~
Int Std
Rear
DFC -
"Vacuum
V en t
Sample
Pressure
Reaulato
M FC
Middle MFC
Vacuum
N —
MFC
Nafion
Dryer
M iddle S m a l
V alv e O v en
■VMiddle FID
- Adsorbent Trap
Front 1079
Rear
EFC
Split
Vent
M iddle
-E F C
C olum n
-H e
Ven
Front
EFC
-H e
V7
V6
CP 5
Column
Middle
FID
Alumina
PLOT
CP 624
Column
Mass
Spec
Cryo-Focus
Trap
- Middle
Figure 3.8 Valving Diagram for the Ultra Trace Toxics GC/MS System
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3.3.5 Valve Operation for Hydrocarbons
Valve operations for hydrocarbons are similar to the toxics approach, except only
one trap is involved, and the sample passes through a nafion dryer to remove water prior
to the cryogenic focusing.
Sample containers are attached to the automated sampler (Valve AS). Valve 4, in
the normally off position, dead ends the sample flow and allows nitrogen purge gas top
flow through to the mass flow controller and sample flow meter on the side of the
instrument.
When Valve 4 is activated, sample flows to the mass flow controller,
bypassing the “middle” cryogenic trap, to purge lines with the new sample; Valve 5 is
normally off to divert sample away from the adsorbent trap; the front adsorbent trap
should be at its starting temperature at this point. When Valve 5 is activated, sample flow
is directed to the cryogenic trap and the sample loading time commences. The “middle
cryo-focus trap” must be cold during this sample loading o f the front trap. By controlling
the time between activating Valve 5 and turning off Valve 4, the effective sample loading
volume can be varied.
After Valve 4 is turned off, nitrogen then purges the interconnecting lines of
sample and any sample components not trapped (such as oxygen, nitrogen, carbon
monoxide and methane, depending on the trap temperature).
Before the front trap is
heated, Valve 6 is activated to isolate the cryogenic trap when the trap is heated. After
the trap reaches its maximum temperature and all analytes are volatilized, Valve 6 is
reopened to allow the analytes to be injected into the pre-column.
After the appropriate light hydrocarbons are allowed to pass through the pre­
column and onto the alumina PLOT column, the column-switching valve (Valve 7) is
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Rupani 52
actuated to then allow the heavier hydrocarbons to be passed over to the non-polar
capillary column. After Valve 7 is turned off, all valves are now in position for the next
cycle.
3.4 Experimental Procedure
The experimental procedure for each set o f experiment is described below.
Step 1: Purging with nitrogen
The ultra high purity (UHP) nitrogen is fed from cylinder. The flow rate is
controlled by mass flow controller. The gas flows through the entire system loop and the
same is being sucked out through the vent. The purging ensures removal o f any trapped
residuals.
Step 2: Warming up bed
At very beginning the water circulation, for cooling o f microwave generator as
well as for absorption, must be first started. Then the power supply is turned on. The
display will then indicate all displays lights on indicating proper operation. The
microwave generator is then turned on by changing the watt from power supply. The
desired watt o f power can be set. The tuning o f microwave has been done to have very
less reflected watt.
Step 3: Preparing inlet stream
During the above steps only the carrier gas (air or nitrogen) is flowing through
the applicator. Once the system reaches a steady state, a 3-way valve is open to allow the
carrier gas to pass over the liquid benzene. The gas sweeps the benzene vapor and the
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Rupani 53
resulted mixture passes through the reactor. After the reaction, the outlet stream is
directed towards the exhaust. The temperature o f the bed is measured using an infrared
thermometer.
Step 4: Analyzing gas using GC/MS
During the start up o f GC/MS, air and hydrogen cylinder valves must be turned
on. The valves o f the UHP helium and the UHP nitrogen cylinders must be kept open all
the time. The pressure is maintained as listed earlier under utility requirement. Then
liquid nitrogen tank valve is turned on.
From Saturn workstation Daily Check method needs to be loaded. Then the auto
timing is carried out to find air and water level in the system. This indicates leakages and
moisture level in the system. If the levels are within the specified limit, the TO 15 +
Hydrocarbon method must be loaded for analysis. The resultant mixture from the reactor
is analyzed using GC/MS.
Step 5: Purging of the system
The venting valve again is directed towards exhaust. The 3 way valve must be
directed towards the applicator. The gas flows through entire system loop and the same is
being sucked out through the vent. The purging ensures the removal o f any trapped
residuals.
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CHAPTER 4
Results and Discussion
This chapter reports results and makes discussion on the observed results. The
Chapter is divided into the following sub-sections: Experimental Parameters, General
Observations, Effect o f Absorption Medium, Effect o f Power Level, and Effect o f Flow
Rate. These are detailed below.
4.1 Experimental Parameters
Six experimental sets were conducted in this study. The experimental conditions
for each test set are summarized in Table 4.1. As indicated in the table, the experimental
parameters include absorption medium, mass o f absorption medium, medium bed height,
catalyst, carrier gas, flow rate, and inlet concentration.
Granular activated carbon (GAC) and silicon carbide (SiC) foam were used as
absorption medium for microwave energy. Both GAC 610 and GAC 1240 were acid
washed granular activated carbon o f high purity with apparent density o f 0.5 gm/ml.
GAC 610 represents mesh sizes greater than 6 and less than 10 while GAC 1240 is finer
and has mesh sizes greater than 12 and less than 40. For GAC 610 or GAC 1240, a
packed bed o f 7.62 cm (3 inch) in height was used and the mass o f GAC was about 3
grams. The silicon carbide foam used was 11 mm in diameter and 25 mm long and had a
mass o f 2.32 grams. For the silicon carbide experiments, three foams were involved with
packed bed height being 75 mm and mass being 6.96 grams. For Sets 2 and 3, platinum
gauze was used serving as a catalyst. As also indicated in Table 4.1, the carrier gases
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used in the experiments were air and nitrogen. The flow rates were from 0.3 to 0.5 liter
per minute (1pm). The inlet concentration o f benzene was from 1700 to 3900 ppm.
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Table 4.1 Experimental Conditions
Bed
Height
(cm)
7.62
Carrier
Gas
Flow
Rate
(1pm)
Inlet
Concentration
(ppm)
None
Nitrogen
0.5
1800
Catalyst
1
GAC 610
Mass of
Absorption
Medium
(grams)
3.1
2
GAC 1240
3.3
7.62
Platinum
Air
0.5
1700
3
GAC 1240
3.3
7.62
Platinum
Nitrogen
0.5
1800
4
GAC 1240
3.3
7.62
None
Nitrogen
0.3
3900
5
SiC
6.96
7.5
None
Air
0.4
2700
6
SiC
6.96
7.5
None
Air
0.5
1700
Test
No
Absorption
Medium
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4.2 General Results
Tables 4.2 through 4.7 summarize the experimental results for each test. The
microwave power levels o f 200 to 500 watt were used. The Destruction and Removal
Efficiency was found to be extremely high from 88% to 99.9%. Figures 4.1 through 4.6
display plots o f Destruction and Removal Efficiency (DRE) vs microwave power for
each test.
Table 4.2 Experimental Results Associated with Test 1
Power
(watt)
300
400
500
600
Exit
Concentration
(ppm)
48.1
25.2
3.8
2.5
Temperature
(°C)
Velocity
(m/sec)
850
980
1150
1250
0.33
0.37
0.42
0.45
Contact
Time
(sec)
0.23
0.21
0.18
0.17
DRE
97.33
98.60
99.79
99.86
Table 4.3 Experimental Results Associated with Test 2
Power
(watt)
350
400
450
Exit
Concentration
(ppm)
205.00
5.24
1.19
Temperature
(°C)
Velocity
(m/sec)
1080
1210
1450
0.40
0.44
0.51
Contact
Time
(sec)
0.19
0.17
0.15
%DRE
Residence
Time
(sec)
0.24
0.21
0.17
%DRE
87.94
99.69
99.93
Table 4.4 Experimental Results Associated with Test 3
Power
(watt)
350
400
450
Exit
Concentration
(ppm)
42.0
23.0
2.7
Temperature
(°C)
Velocity
(m/sec)
820
980
1250
0.32
0.37
0.45
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97.67
98.72
99.85
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Table 4.5 Experimental Results Associated with Test 4
Power
(watt)
300
400
450
Exit
Concentration
(ppm)
3.40
0.07
0.01
Temperature
(°C)
Velocity
(m/sec)
850
1070
1180
0.20
0.24
0.26
Contact
Time
(sec)
0.38
0.32
0.30
%DRE
99.912
99.998
99.999
Table 4.6 Experimental Results Associated with Test 5
Power
(watt)
200
250
300
Exit
Concentration
(PPm)
1.80
0.50
0.07
Temperature
(°C)
Velocity
(m/sec)
1042
1270
1350
0.31
0.36
0.38
Contact
Time
(sec)
0.24
0.21
0.20
%DRE
Contact
Time
(sec)
0.17
0.16
0.15
%DRE
99.93
99.98
99.99
Table 4.7 Experimental Results Associated with Test 6
Power
(watt)
250
300
400
Exit
Concentration
(ppm)
19.0
10.0
0.1
Temperature
(°C)
Velocity
(m/sec)
1250
1310
1390
0.45
0.47
0.49
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98.88
99.41
99.99
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100.00
99.86
■ 99.79
99.50
99.00
iu
DC
Q
98.60
98.50
98.00
97.50
97.33
97.00
200
300
400
600
500
700
M ic ro w a v e P o w e r (w a tt)
Figure 4.1 Plot of DRE vs Microwave Power for Test 1
102.00
99.93
100.00
99.69'
98.00
96.00
g
94.00
92.00
90.00
87.94
88.00
86.00
200
250
300
350
400
450
500
550
Microwave Power ( w a t t )
Figure 4.2 Plot of DRE vs Microwave Power for Test 2
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600
Rupani 60
100.00
♦ 99.85
99.50
99.00
♦ 98.72
98.50
98.00
♦ 97.67
97.50
300
320
340
360
380
400
420
440
460
M ic ro w a v e P o w e r ( w a t t )
Figure 4.3 Plot of DRE vs Microwave Power for Test 3
100
99.998
99.999
99.98
99.96
oc
a
99.94
99.92
♦ 99.912
99.9
200
250
300
350
400
450
M ic ro w a v e P o w e r (w a tt)
Figure 4.4 Plot of DRE vs Microwave Power for Test 4
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500
Rupani 61
100.00
99.99
99.99
99.98
99.98
99.97
99.96
99.95
99.94
99.93
99.93
99.92
150
170
190
210
230
250
270
290
310
P o w e r (w a tt)
Figure 4.5 Plot of DRE vs Microwave Power for Test 5
100
99.99
99.8
99.6
g 99.4
99.41
99.2
99
♦ 98.88
98.8
200
250
300
350
400
450
P o w e r (w a tt)
Figure 4.6 Plot of DRE vs Microwave Power for Test 6
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Rupani 62
4.3 Effect of Absorption Medium
Microwave requires an absorption medium for holding up the acquired heat
energy and in turn used it for benzene destruction. Both the granular activated carbon and
silicon carbide are good absorption mediums for microwave energy. However, as shown
in Figures 4.1 through 4.6, silicon carbide is found to be better microwave absorption
medium for benzene destruction with more than 99% DRE being observed at microwave
power levels in the range o f 200 to 400 watt.
4.4 Effect of Microwave Power
The results shown in Figures 4.1 through 4.6 have indicated that as the
microwave power increases, the DRE is also found increased. This is because more
energy is available for benzene destruction as seen in the increase in the reactor
temperature.
4.5 Effect of Flow Rate
Comparisons o f results between Tables 4.2 and 4.5 have indicated that as the flow
rate o f the carrier gas decreases, the DRE increases. This appears to suggest that the
contact time plays a role in the benzene destruction process. In addition, the slower flow
rate also results in a higher reactor temperature, which enhances the benzene destruction
process.
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Rupani 63
4.6 Reaction Products
Various reaction by-products have been observed in the experiments. With GAC
in nitrogen, these by-products include acetylene, carbon disulphide, toluene, cyclo
butene, ethyl benzene, o-xylene, and styrene. While with GAC in air, in addition to CO,
C 02 and water, the by-products include carbonyl sulphide and high molecular weight
compounds such as biphenyl and naphthalene. For silicon carbide in air, the majority o f
benzene is converted into CO, CO 2 and water with only trace amounts o f other
hydrocarbons (<1 ppm observed) including biphenyl and naphthalene.
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Rupani 64
Chapter 5
Conclusions and Recommendations
This thesis reports an experimental study o f benzene destruction using microwave
energy. The benzene vapor was carried into a packed-bed reactor by air or nitrogen and
destroyed in the reactor via microwave heating. Granular activated carbon (GAC) and
silicon carbide (SiC) were used as the packed bed materials as well as the absorption
mediums for microwave energy. The inlet and outlet streams were analyzed using Gas
Chromatography/Mass Spectrometry (GC/MS). The Destruction and Removal Efficiency
(DRE) was investigated by varying input power and flow rate.
5.1 Conclusions
The study has resulted in the following conclusions:
1. The proposed microwave VOC destruction technology has been found to be effective
with more than 99% DRE using granular activated carbon (GAC) or silicon carbide
(SiC) as the microwave absorption medium.
2. Silicon carbide in air is an effective combination for benzene destruction without
producing high molecular weight hydrocarbon compounds such as biphenyl and
naphthalene. More than 99% DRE has been observed with microwave power in the
range o f 200 to 400 watt. The reaction products have been found to be largely
comprises o f CO, CO 2 , and H 2 O with trace amount o f by-products.
3. As the microwave power increases, the DRE is also found increased.
4. A lower flow rates results in a higher DRE.
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Rupani 65
5.2 Recommendations for Future Work
The following are recommended for future study:
1. A scale-up test with a larger applicator should be used to provide data for potential
industrial applications. This will provide more residence time for the reaction. Hence,
a larger flow rate comparative to the industrial applications like emissions from tank
farm can be treated.
2. Additional tests involving other VOCs like toluene, xylene should be carried out. This
will demonstrate that this technology can be extended for the other VOC.
3. Catalysts like platinum or palladium impregnated on the SiC need to be tested to
increase the oxidation reaction rate and thus enabling the conversion or destruction at
lower reaction temperatures. This will lower the applied microwave power
requirement and in turn energy requirement.
4. A combined carbon and nitrogen analyzer can be employed. This will provide the
quantitative analysis o f NOX and carbon dioxide. This will ensure more
comprehensive analysis along with GC/MS analysis.
5. Quantitative calibration methods should be developed for quantifying reaction
products using mass spectroscopy. The standard like TO 15 standard should be used.
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Rupani 66
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Rupani 71
BIOGRAPHICAL NOTE
Hemal Navinchandra Rupani was bom in Bombay, India, on December 8th 1979.
He completed his Bachelor o f Engineering degree in Chemical Engineering in 2002 from
Laxminarayan Institute o f Technology (LIT), India. At present he is a candidate of
Master o f Engineering Science in Chemical Engineering at Lamar University, Beaumont,
Texas, USA.
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