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 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. UMI Number: 1438775 INFORMATION TO USERS The quality of this reproduction is dependent upon the quality of the copy submitted. Broken or indistinct print, colored or poor quality illustrations and photographs, print bleed-through, substandard margins, and improper alignment can adversely affect reproduction. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if unauthorized copyright material had to be removed, a note will indicate the deletion. ® UMI UMI Microform 1438775 Copyright 2007 by ProQuest Information and Learning Company. All rights reserved. This microform edition is protected against unauthorized copying under Title 17, United States Code. ProQuest Information and Learning Company 300 North Zeeb Road P.O. Box 1346 Ann Arbor, Ml 48106-1346 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. © 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. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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 22.214.171.124 Thermal Oxidizer 9 126.96.36.199.1 Regenerative Thermal Oxidation 11 188.8.131.52.2 Recuperative Thermal Oxidation 11 184.108.40.206 Catalytic Oxidation 12 220.127.116.11 Reverse Flow Reactor 14 iv Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 18.104.22.168 Bio-Filtration 15 2.3.2 Recovery o f VOC 18 22.214.171.124 Condensation 18 126.96.36.199 Absorption 19 188.8.131.52 Membrane Based Separation 20 184.108.40.206 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 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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, Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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). 220.127.116.11 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 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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 ). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Rupani 11 18.104.22.168.1 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. 22.214.171.124.2 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 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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 . 126.96.36.199 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 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Rupani 14 188.8.131.52 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. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Rupani 15 184.108.40.206 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. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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. 220.127.116.11 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 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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). 18.104.22.168 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 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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). 22.214.171.124 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, Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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. 126.96.36.199 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 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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, Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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- Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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, Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Rupani Thermocouple To Vent Applicator GC/MS Reactor Mass Flow Controller Benzene Vaporizer Figure 3.1 Experimental Set Up Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Rupani 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 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Rupani 45 Figure 3.6 The Ultra Trace Toxic GC/MS System Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Rupani 46 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. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Rupani 47 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 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Rupani 49 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. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Rupani 50 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 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Rupani 51 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 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Rupani 54 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 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Rupani 55 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. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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 Rupani 56 Rupani 57 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 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 97.67 98.72 99.85 Rupani 58 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 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 98.88 99.41 99.99 Rupani 59 100.00 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 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Rupani 66 REFERENCES Baker, R. W., N. Yoshioka, J. M. Mohr, and A. J. Khan. 1987. Separation o f organic vapors from air. Journal o f Membrane Science 31:259. Behling, R. D., K. Ohlrogge, K. V. Peinemann, and E. Kyburz. 1988. The separation of hydrocarbons from waste vapor streams. AIChE Symposium Series 48 (272): 68. Buys, H. C. W. M., H. F. Martens, L. M. Troos, J. W. Van Heuven, and A. H. A. Tinnemans. 1990. New intrinsic separation characteristics o f poly membranes of organic vapour/nitrogen mixtures. In Proceedings o f the International Conference on membrane and membrane processes, Chicago, 83. Cha, C. Y. 2004. Microwave induced oxidation. http://chacorporation.com/water (accessed August, 2004). Chadha, N., and C. S. Parmele.1993. Minimize emissions o f air toxics via process change. Chemical Engineering Progress 89 (1), 37. Code o f Federal Regulations (CFR). 1997. National Primary and Secondary Ambient Air Quality Standards, Final Rules, Title 40, Parts 5 0- 53 and 58. Cooper, D.C. and F.C. Alley. 2002. Particulate matter. Air pollution control: A design approach, 3rd ed. Illinois: Waveland Press, Inc., 99-102. Dauerman, L., G. Windgassse, N. Zhu, and Y. He. 1992. Microwave treatment of hazardous wastes: Physical chemical mechanisms. Materials Research Society 269: 465-469 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Rupani 67 Deng, S., A. Sourirajan, and T. Matsuura.1996. Study o f volatile hydrocarbon emission control by an aromatic poly (ether imide) membrane. Indian Engineering and Chemical Research 34: 4494. Jol, A., and A. J. Dragt. 1995. Biotechnological elimination o f volatile organic compounds in waste gases. In Dechema Biotechnology Conference, Proceedings Bioreactor Downstream Process 2\ 373. Jones, D. A., T. P. Lelyveld, S. D. Mavrofidis, S. W. Kingman, and N. J. Miles. 2002. Microwave heating applications in environmental engineering: A review. Resources, Conversation and Recycling 34: 75-90. Jou, G. C. J. 1998. Application o f activated carbon in a microwave radiation field to treat trichloroethylene. Carbon 36(11): 1643-1648. Khan. F. I., and S. A. Abbasi. 1998. Accident simulation as a tool for assessing and calculation environmental risk in CPE: A case study. Korean Journal o f Chemical Engineering II (2): 12. Khan, F. I., and A. K. Ghoshal. 2000. Review: Removal o f volatile organic compounds from polluted air. Journal o f Loss Prevention on the Process Industries 13: 527545. Kiared, K., L. Bieau, R. Brzezinski, G. Viel, and M. Heitz. 1996. Biological elimination of VOCs in bio-filter. Environmental Progress 15 (3): 148. Krause, R.T. and J.E. Helt. 1993. Applications o f microwave radiation in environmental remediation technologies. In Microwaves: Theory and application in materials processing II, Ceramic Transactions. Edited by D.E. Clark, W.R. Tinga and J.R. Laia. American Ceramic Society 36: 53-59. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Rupani 68 Leson, G., and A. M. Winer. 1991. Bio-filtration: An innovative air pollution control technology for VOC emissions. Journal o f the Air and Waste Management Association 41 (8): 1045. Liu, P. K. T. 1994. Engineered bio-filter for removing organic contaminants in air. Journal o f Air and Waste Management Association 48: 299. Matros, Yu Sh., A. S. Noskov, and V. A. Chumachenko. 1993. Progress in reverse-process application to catalytic incineration problem. Chemical Engineering Progress 32: 89. Metaxas, A.C. and R.J. Meredith. 1983. Industrial microwave heating, 1st ed. London: IEE Press Inc., 2-9. Oda, S J. 1992. Microwave remediation o f hazardous waste: A review. Microwave processing o f materials III. Edited by R.L. Beatty, W.H. Sutton and M.F. Iskander. Materials Research Society 269: 453-464. Ottenger. S. P. P., and A. H. C. van den Ocvcr. 1983. Kinetics o f organic compound removal from waste gases with a biological filter. Biotechnology and Bioengineering 12 (25): 3089. Patkar, A. N., and J. Laznow. 1992. Hazardous air pollutant control technologies. Hazmat World 2: 78. Ruddy, E. N., and L. A. Carroll. 1993. Select the best VOC control strategy. Chemical Engineering Progress 7: 28. Ruhl, M. J. 1993. Recover VOCs via adsorption on activated carbon. Chemical Engineering Progress July: 37-41. Ruthven, D. M.1984. Principles o f adsorption processes. New York: John Wiley. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Rupani 69 Spivey, J. J. 1987. Complete catalytic oxidation o f VOCs. Indian Engineering and Chemical Research 26: 2165. Spivey, J. J. 1988. Recovery o f volatile organics from small industrial sources. Environmental Progress 7 (I):31. Stenzel, M H. 1993. Remove organics by activated carbon adsorption. Chemical Engineering Progress 89 (4):36. Tahraoui, K.1994. Bio-degradation o f BTX from waste gases in a bio-filler reactor. Journal o f Air and Waste Management Association, 87th Annual Meeting and Exhibition, Cincinnati, Ohio, p. 2. U.S. Environmental Protection Agency (EPA). 1991. 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Basrai. 1996. Bio-filtration of odours, toxics and volatile organic compounds from publicly owned treatment works. Environmental Progress 15 (3): 141. Webster, T. S., E. M.Torres, and S. Basrai. 1995. Study o f bio-filtration for control of odours, VOC and toxic emissions from waste-water treatment plants—phase II. Bench and pilot scale experiments. In Proceedings o f the 1995 Conference on Bio-filtration, Los Angles, California, October. Wicks, C. G., D. E. Clark, R.L. Schulz, and D. C. Folz. 1999. Microwave Technology for waste management applications including disposition o f electronic circuitry. Ceramic Transactions 59: 79-89. Wijmans, J. G., and V. D. Helm. 1989. A membrane system for separation and recovery of organic vapours from gas stream. AlChE Symposium Series 272. AlChE, New York. 85:74. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.