A Quantitative Study of the Chlorine Atom Concentration in Plasma. A thesis presented to. the faculty of

Transcription

1 A Quantitative Study of the Chlorine Atom Concentration in Plasma A thesis presented to the faculty of the Russ Engineering College Engineering and Technology of Ohio University In partial fulfillment of the requirements for the degree Master of Science Sreerupa Basu March 2008

2 2 This thesis titled A Quantitative Study of the Chlorine Atom Concentration in Plasma by SREERUPA BASU has been approved for the Department of Chemical and Biomolecular Engineering and the Russ College of Engineering and Technology by David J. Bayless Loehr Professor of Mechanical Engineering Dennis Irwin Dean, Russ College of Engineering and Technology

3 3 ABSTRACT BASU, SREERUPA, M.S., March 2008, Chemical and Biomolecular Engineering A Quantitative Study of the Chlorine Atom Concentration in Plasma (83 pp.) Director of Thesis: David J. Bayless Chlorine can oxidize elemental mercury to a species soluble in water. The addition of chlorine gas into an electrostatic precipitator, used to remove particulates from flue gas may increase the mercury removal efficiency in the precipitator. The determination of the chlorine atom concentration, formed inside the precipitator, is a key to the evaluation of this efficiency. A series of experiments were performed to dissociate the chlorine gas in a corona-discharge field formed inside a 10 cm x 3 cm Pyrex tube at one atmosphere. Chlorine atoms formed were measured by reacting them with hydrogen and chlorine concentration was determined as a function of voltage supplied, chlorine injected and distance between the electrodes. This data could help in optimizing the amount of chlorine reagent gas needed to obtain enhanced mercury removal efficiency in an electrostatic precipitator. Approved: David J. Bayless Loehr Professor of Mechanical Engineering

4 4 ACKNOWLEDGMENTS I acknowledge the support and advice I had received from my graduate committee members and above all my advisor, Dr. David J. Bayless whose guidance have helped me to complete this project work. I would definitely like to thank the staff and expertise of Ohio Coal Research Centre, especially Micah McCreery and Shyler Switzer, for helping me to build the apparatus for the experiments conducted for this research work. I also appreciate the valuable thoughts I had received from Dr. Mark Stoy during the course of this work. It has been a wonderfully enlightening hands-on experience for me, which is an integral part of my education for earning a masters degree.

5 5 TABLE OF CONTENTS Page ABSTRACT...3 ACKNOWLEDGMENTS...4 LIST OF TABLES...7 LIST OF FIGURES...8 CHAPTER 1: INTRODUCTION Forms of Mercury in the Environment Mercury Speciation in Coal-Combustion-Formed Flue Gas Electrostatic Precipitation as a Mercury Emission Control Technology Uniqueness of this Research Objectives of this Research Thesis Contents...15 CHAPTER 2: LITERATURE REVIEW AND BACKGROUND Oxidation of Hg 0 with Chlorine and Chlorine Containing Species Study of Reaction(s) in Chlorine Plasma Various Methods for Determining Plasma Properties The Reaction of Chlorine Atoms with Hydrogen...24 CHAPTER 3: EXPERIMENTAL SETUP AND METHODOLOGY Experimental Setup Drager Tube Analysis Optimizing the Reaction Environment Parameters...34

6 6 3.4 Test Procedure Calculating the Chlorine Atom Concentration...38 CHAPTER 4: TEST MATRIX AND RESULTS Test Matrix Results...45 CHAPTER 5: CONCLUSIONS AND RECOMMENDATIONS Conclusions Recommendations...52 REFERENCES...55 APPENDIX...59 A1. Estimating the Voltage Required and the Number of Chlorine Molecules Dissociated (Peek 1929)...59 A2. Optimizing the Amount of Chlorine in the Plasma Chamber...61 A3. Estimating Time Required to Fill Up the Sampling Chamber...64 A4. Kinetic Scheme of Formation and Decay of Active Species in Chlorine Plasma A5. Designing the Cathode to Obtain Higher Field Strength for the Experiments..68 A6. Designing the Timer Circuit for the Solenoid Valve Operation...74 A7. Making Up a Hydrogen Gas Mixture in Nitrogen...76 A8. Drager Tube Analysis...80 A9. Result Analysis...81

7 7 LIST OF TABLES Table 4.1 Test Matrix...42 Table Results of testing for a residence time t=40ms...45 Table Results of testing for a residence time t=75ms...45 Table Results of testing for a residence time t=150ms...46 Table Results of testing for a residence time t=200ms...46 Table Cl concentration data plotted in Figure Table A2.1 Setup Parameters...63 Table A5.1 First Cathode Design Results...71 Table A5.2 Second Cathode Design Results...72 Table A5.3 Third Cathode Design Results...73 Table A8.1 Testing the Accuracy of the Drager Tubes (09/07/2007)...80 Table A8.2 Repeatability Test for Drager Tube Analysis (09/10/2007)...81 Table A9.1 Regression Data...82

8 8 LIST OF FIGURES Figure Speciated fractions of 1999 aggregate coal-fired emissions (Brown et al. 1999) Figure Schematic representation of a wet ESP function (Altman et al. 2000) Figure Top view of the experimental apparatus Figure The cathode tube Figure The anode tube...28 Figure The two chambers with the solenoid valve Figure Flow control setup Figure Layout of the plasma chamber and the reaction/sampling chamber...32 Figure Drager tube with yellow showing the presence of HCl gas in test sample..34 Figure Arbitary Represntation of the Hypothetical Inverted-V Curve Figure Trend of Cl in active plasma region Figure A5.1. Design Figure A5.2. Design Figure A6.1. Square wave form of 555 timer circuit Figure A6.2. Timer Circuit Diagram Figure A7.1. Sketch of the Vacuum Manifold System Figure A7.2. Vacuum Manifold System Setup Figure A9.1. Excel Solver to obtain the alpha value...83

9 9 CHAPTER 1: INTRODUCTION The aim of this research is to quantify the formation of chlorine atoms by behavior which could be advantageous in chlorine-based oxidation of elemental mercury present in flue gas, using only an electrostatic precipitator (ESP). It is anticipated that the results of this study could help assess the feasibility of using chlorine as an additive reagent gas to a wet ESP to increase its mercury removal efficiency. It is also hoped that the data could help evaluate the mechanism for the reaction between the elemental mercury and chlorine in the plasma-enhanced electrostatic precipitator-based removal of mercury from flue gas. 1.1 Forms of Mercury in the Environment Mercury is present in the atmosphere in three forms. Inorganic mercury, generally as a compound with chlorine, such as HgCl 2, is often referred to as oxidized mercury. Organic mercury, generally found as methyl mercury with properties of bioaccumulation, is known to cause neurological damage to humans (ASTDR 1999). The third form with the greatest potential for public concern (Senior et al. 2000) is elemental mercury (Hg 0 ), which exists in vapor form in the atmosphere. The elemental form can be easily transported within the food chain, where it can form dangerous organic compounds within living organisms (ATSDR 1999), causing neurological disorders. Mercuric compounds within living organisms are known to cause respiratory, gastrointestinal, and cardiovascular effects even gene mutation and death. The 1990

10 10 Clean Air Act categorizes mercury as one of 189 hazardous air pollutants (HAP) required to be controlled. A U.S. Environmental Protection Agency report (EPA-452/R ) states that 25% of the total mercury in the environment is derived from anthropogenic sources, and 90% of that number is attributable to coal-fired utilities in the U.S. EPRI Report no. TR indicates that on an average 0.02 to 0.25 ppm of mercury is present in every form of mined coal. 1.2 Mercury Speciation in Coal-Combustion-Formed Flue Gas Mercury is present in coal flue gas in three forms: elemental mercury (Hg 0 ), particle bound mercury (Hg p ), and oxidized mercury (Hg 2+ ). Figure indicates the average fractions of mercury species in coal flue gas. Figure Speciated fractions of 1999 aggregate coal-fired emissions (Brown et al. 1999). There are various models aimed at explaining the different Hg speciation pathways possible in the post-combustion stream from coal burning (PCA R&D Serial No. 2578). One of the most accepted speciation pathways is heating mercury within the

11 11 temperature range of 400 to C forming its oxide, which decomposes at temperatures above C (Heble 2001). The oxide is changed to the chloride form via homogeneous gas-phase reaction with hydrogen chloride or chlorine present in the flue gas. Also, as Senior et al. (2000) indicate, a heterogeneous gas-solid reaction between the oxidized mercury and chlorine-containing compounds is also possible, forming the particulate-bound mercury species. Another speciation pathway postulates a direct gasphase oxidation of elemental mercury by active chlorine atoms (Horne 1968; Ariya 2002). The oxidized form (Hg 2+ ) may be highly soluble in water and hence can be removed in wet scrubbers used for desulphurization. The problem arises when most of the mercury is not oxidized and is released in the stack gas as elemental mercury. Hg 0 is very stable at the high temperatures typical of coal-fired boilers. The formation of the oxides of mercury is controlled by the amount of oxidizing agents present and quench rate of the flue gas. Several areas of research and technology are being worked on today to minimize the amount of the elemental mercury released with the exhaust gases from coal combustion. Efforts are being put in the direction of the optimized yield of mercuric chloride from the elemental form via reaction with chlorine atoms derived from different chlorine species, such as the chlorine and hydrogen chloride gases present in the flue gas stream (Sliger et al. 2000; Fuller et al. 2001; Montgomery et al. 2005). In order to optimize the production of mercuric chloride, a better understanding of the reaction between Hg 0 and Cl has to be achieved.

12 Electrostatic Precipitation as a Mercury Emission Control Technology Electrostatic precipitation has been efficiently used by industry to remove small particles such as fly ash of sizes less than 1 µm. A schematic representation of how a wet electrostatic precipitator (ESP) works is shown in Figure The discharge electrodes charge the fly ash particles; these charged particles are drawn toward oppositely charged collecting electrodes where the fly ash particles are collected. Water is used to trap the collected particles and the resulting slurry is removed. Figure Schematic representation of a wet ESP function (Altman et al. 2000). However, for removal of submicron gaseous pollutants like mercury and NOx, the plasma-enhanced electrostatic precipitation (PEESP) technique works more efficiently (Montgomery et al. 2005). PEESP uses the physics of plasma with the basic working principles of a wet ESP. An oxidizing gas or reagent gas is passed through the discharge

13 13 electrodes to form a plasma of the oxidizing particles, which can more effectively oxidize the elemental mercury present in the background gas in the precipitator. Highenergetic electrons generated in the corona discharge react with the molecules in the gas phase, producing reactive species such as radicals and excited molecules that undergo further reactions. Voltage is applied to the precipitator such that the potential near to the discharge electrode is high and it ionizes the air around it to form plasma (a stage before it can become conductive). The current, which is generated by the ions in this plasma region, forms the visible corona around the tips of the discharge electrodes. The ions near the discharge electrodes will eventually pass the charge to nearby areas of lower potential and recombine to form neutral particles. Thus the environment in the precipitator, besides the region near the discharge electrodes, is kept neutral (Peek 1929). Ohio University has a wet membrane PEESP in Stocker 045. Two separate experiments have been performed previously by injecting steam and hydrochloric acid (HCl) as the reagent gases to study the increased efficiency of mercury removal using the electrostatic precipitation technique. (Jayaram 2005, Liang 2005) 1.4 Uniqueness of this Research The idea of using hydrogen chloride gas for a previous experiment (Jayaram 2005) and the present choice of chlorine as the reagent gas are based on the fact that the amount of chlorine-containing species in coal has been reported to have a positive affect on the amount of mercury oxidized in the flue gas (EPRI TR ). The attack of the Cl atom on the elemental mercury is believed to be the primary path for forming mercuric

14 14 chloride. The chlorine atoms are created in the chlorine plasma formed when the chlorine gas or hydrogen chloride gas is injected through the discharge electrodes. To have a better understanding of this reaction pathway, measurement of chlorine dissociation inside the plasma region is required. Various studies have been made to understand the chlorine plasma chemistry in inductively coupled chlorine plasma, or photoluminescence induced chlorine dissociation. The present research aims to quantify chlorine atom concentration over time in the plasma region formed near the discharge electrode in a simulated ESP condition, minus the presence of other radical groups in typical flue gas. 1.5 Objectives of this Research The objectives of this research work are to: 1. Set up a reaction chamber to generate chlorine plasma at standard temperature and pressure conditions, with chlorine atoms generated in the plasma collected in a sampling/reacting bulb containing hydrogen for analysis. 2. Vary the residence time of the chlorine gas in the active plasma region, then collect the chlorine atoms from the plasma region in the reaction chamber for different residence time conditions. 3. Study the amount of chlorine atom formation for the different sets of residence time under constant voltage readings and attempt to determine a trend for chlorine atom behavior.

15 Thesis Contents The mercury-chlorine reactions and the chlorine plasma chemistry have been widely investigated in literature. Chapter 2 of this thesis covers a discussion of the literature review and background of the present project. Chapter 3 details the setting up of the apparatus and the methods used for conducting the experiments. Chapter 4 has the explanation of the test matrix used which contains the variables used in the experiment and the results obtained for the different observation points. A discussion on the analysis of the result, the conclusions obtained from the observations and the future endeavor recommendations are presented in chapter 5. The appendix at the end have the auxiliary explanations for the different steps encountered in the process of this project completion; like designing the dimensions for the experimental set-up including the glass apparatus, electrodes, the timer circuit for controlling the solenoid valve functioning, the process of making up the hydrogen gas mixture which is used for reacting with the chlorine atoms, and finally choosing the observation points for this process.

16 16 CHAPTER 2: LITERATURE REVIEW AND BACKGROUND 2.1 Oxidation of Hg 0 with Chlorine and Chlorine Containing Species Understanding the pathway for the reaction between mercury and chlorine has been the objective of various research works for many years (Sliger et al. 2000; Ariya et al. 2002; Wilcox et al. 2004; Jayaram 2005). This pathway has atmospheric implications because the halogen (chlorine) atoms react with abundantly available elemental mercury in the surrounding air to form mercuric compounds that bioaccumulate (Ariya et al. 2002). According to Sliger et al. (2000), understanding the reaction kinetics is important for optimizing the efficiency of a plasma-enhanced electrostatic precipitator (PEESP). Liu et al. (2001) studied the effect of the chlorine content in coal on the amount of elemental mercury emitted in coal flue gas. Their study shows that fluidized bed combustion of a high-chlorine-containing coal converted 45% of the initial elemental mercury present to its oxidized form, and a very small amount (only 4.5%) remained in elemental form. Their analysis, based on data available from the literature, indicates the most likely reaction pathway is the decomposition of chlorine molecules or the chlorinecontaining species to form chlorine atoms, which oxidizes the elemental mercury. The study of the combination of mercury and chlorine atoms dates back to 1968, when Horne et. al. did the photolysis dissociation of CF 3 Cl molecules to form Cl atoms and then reacted them with elemental mercury vapor to study the formation of mercuric chloride (HgCl 2 ) and dimercuric chloride, Hg 2 Cl 2 (the dimerized form), from the HgCl radicals. Using the absorption spectrum data of HgCl radicals at 720 torr pressure and temperatures ranging from 110ºC to 170ºC, the kinetics were quantified. It was concluded

17 17 that the dimerization of HgCl to form Hg 2 Cl 2 is in the second order of the concentration of the HgCl radicals [HgCl]. Ariya et al. (2002) further studied the same reaction products and kinetics at standard atmospheric pressure and temperature, but along with chlorine, different halogen atoms and molecules and other natural oxidants such as HO also were examined. The rate constant for the reaction Hg 0 + Cl products was determined as (1.0 ± 0.4) x10-11 cm 3 molecule -1 sec -1. Also, the determined values of the rate constants of the reaction of mercury vapor with the molecular halogens other than chlorine gas inferred that these reactions are not very important for mercury oxidations at atmospheric conditions. Li et al. (2003) theoretically determined the rate constant of the reaction yielding mercuric chlorides from elemental mercury by reacting with chlorine species, using the Ab Initio MP2 method (from Wilcox et al. 2004) based on transition state theory for the following reaction system: Hg + HOCl HgCl + OH (2.1.1) Hg + HCl HgCl + H (2.1.2) HgCl + Cl2 HgCl2 + Cl (2.1.3) HgCl + HCl HgCl2 + H (2.1.4) HgO + HCl HgCl + OH (2.1.5) HgO + HOCl HgCl + HO 2 (2.1.6) Hg + HgCl (2.1.7) 2 Hg 2Cl2 The Ab Initio MP2 is a quantum chemistry method used to determine electronic structures of molecules and thermodynamic properties of chemical systems. It is based on approximating the changes made in a molecular system such as bond breaking or loss

18 18 of electrons to an assumed unperturbed condition. The Schrodinger energy equation for this unperturbed system is then solved iteratively to accommodate the different perturbations in each step, which can be both time dependent or independent, and the convergent solution obtained gives an idea of the changes made in the molecular system. As in the above reactions, it was inferred that the oxychloride is formed from the oxidation of chlorine species such as HCl in the presence of oxygen or ozone in the flue gas stream. The reaction rate constants determined for these reactions according to this mechanism agrees with their experimental conclusions, and also do not vary more than 0.4% from the rate constant value(s) mentioned in earlier literature. The possibility of a reaction step involving the oxy-radicals that may have formed was an addition to the predicted reaction steps as found in Sliger et al. (2004). They developed a kinetic model for studying the oxidation of mercury with chlorine-containing species in coal combustion gas. The model clearly indicates that the chlorine atoms formed at slightly higher temperatures of around ºC favors the oxidation of elemental mercury. Referring to Hranisaveljevic (1997), the energy barrier for the reaction between HCl and elemental mercury is very high and hence is likely to occur at atmospheric temperature and pressure conditions. The kinetics of the elementary combination of Hg 0 and Cl has been well established with the known reaction rate constant. The reaction steps following the production of the HgCl can be theorized as: HgCl + HCl HgCl2 + HCl (2.1.8) HgCl + Cl2 HgCl2 + Cl (2.1.9) HgCl + Cl HgCl 2 (2.1.10)

19 19 Or an abstraction path may be considered for the last reaction step: HgCl + Cl Hg + Cl 2 (2.1.11) The density functional theory (DFT) predicts that the reaction is the most probable step, it having no energy barrier. Reaction is less likely to occur, considering that a lower number of chlorine molecules exists in the present plasma condition. In highertemperature conditions, reaction is expected to compete with reaction for the chlorine atoms. Hence Sliger et al. (2000) concluded that to predict the behavior of the oxidation of elemental mercury in coal combustion, the chlorine atom behavior needs to be predicted first. 2.2 Study of Reaction(s) in Chlorine Plasma Malyshev et al. (1999) studied the dynamics of the pulsed chlorine discharges. It was found that increasing the chlorine pressure in a plasma system increases the concentration of chlorine negative ions, and these negative ions tended to build up in number especially when the plasma is off, which was attributed to the dissosciative attachment of a chlorine molecule with an electron (reaction 2.2.2). Neuilly et al. (2001) designed a global model to explain the chlorine dissociation rate as a function of the source power and the gas pressure, in inductively coupled chlorine plasma measured by UV spectroscopy. The possible chlorine reactions in chlorine plasma stated by them are: Cl + e 2Cl + e 2 (2.2.1) Cl 2 + e Cl + Cl (2.2.2)

20 20 Cl + e Cl + 2e (2.2.3) 2Cl + wall Cl 2 (2.2.4) According to the kinetics of these reactions, the rate of conversion of chlorine molecules into atoms is given as k Cl2 ] = ( k + k k )[ Cl ] (2.2.5) tot [ diss ion + dat 2 where k diss is the dissociation reaction rate constant (Equation 2.2.1); k dat is the dissosciative attachment rate constant (Equation 2.2.2); k ion is the ionization reaction rate constant (Equation 2.2.3); and This model, however, cannot be directly applied to the present research, mainly due to the differences in the plasma conditions. The inductively coupled chlorine plasma of Neuilly et al. (2001) was formed at pressures of the order of millitorrs. Effremov et al. (2003) studied active species, or the chlorine atoms in chlorine plasma and its chemistry. Again, inductive chlorine plasma was generated in a quartz discharge cell with chlorine molecules excited with DC and RF discharges. A set of 19 reactions consisting of electron impact reactions, charged volume reactions, neutral volume reactions, and heterogeneous reactions occurred. These reactions are described in Appendix A4. It was concluded that electron impact dissociation and ionization were the two main steps to yield the chlorine atoms and that radicals were the active species in the chlorine plasma. The main difference between the above-mentioned works and this research is the mode and condition of yielding the chlorine plasma. The present interest is the chlorine

21 21 plasma formed in a PEESP, where the plasma is formed by electric discharge around the electrode through which the chlorine gas is injected and hence charged to form the cluster of charged and neutral chlorine species. The temperature and pressure conditions are maintained at atmospheric (room) conditions. Battleson et al., (2003) in collaboration of MSE Technology, CRCAT and EPRI performed a set of experiments to study the mercury removal efficiency of a PEESP using chlorine as the reagent gas using different sets of simulated flue gases of varying constituents. Their work demonstrated higher mercury removal efficiency when different chlorine species were added to the plasma region inside the precipitator. This further validates the importance of the oxidation of elemental mercury by the chlorine species. In the PEESP, the chlorine atoms are formed by the charged electron impact on the chlorine molecules near the cathode region. Since the number density of the chlorine atoms within the plasma region is an important factor affecting the rate of oxidization of the elemental mercury, steps should be taken to minimize the recombination of the chlorine atoms. The recombination of chlorine atoms to chlorine molecules has previously been determine to be a wall-catalyzed reaction (Ogryzlo 1961) At atmospheric pressure, chlorine atoms do not survive more than 20 collisions with the walls, irrespective of the wall materials used (Ogryzlo 1961). The quartz discharge tube, in which chlorine atoms were being formed from dissociation of chlorine molecules, was initially coated with silver to detect the presence of chlorine atoms by the discoloration of the silver surface due to the white solid silver chloride formation. It was observed that the chlorine atoms do not exist more than 1 cm away from the site of the

22 22 discharge. Various wall coatings or poisons were then tested for the experimental conditions, to generate chlorine atoms in a flow system. Use of Kel-F grease (a chlorofluorocarbon grease) coating on the walls of the discharge tube helped in sustaining the chlorine atoms formed in the chamber for a longer duration. An elaborate process of cleaning the vessel with concentrated KOH solution, washing it with distilled water and 10% solution of the Kel-F in water, and then evacuating the whole system for 2 hours was followed to set the poison coating on the walls of the discharge tube to reduce the rate of recombination of the chlorine atoms. Kirillov et al. (2004) used relaxation techniques to study the rate coefficient for recombination reaction of chlorine atoms to produce chlorine molecules. The method was based on the principle of tracking the rate of the chemical system in plasma by measuring the change in concentration of the chlorine in the plasma region at different times corresponding to the intervals between current pulses supplied to the discharge producing the chlorine plasma. The rate coefficient for the wall catalyzed recombination reaction on (quartz) glass walls was determined to be 9.6 ± 1.6 s -1 (Kirillov et al. 2004); on Pyrex walls the rate coefficient was calculated as 0.8 s -1 (Ogryzlo 1961). 2.3 Various Methods for Determining Plasma Properties There are various techniques for determining plasma properties such as ionic and atomic number densities, field strength, temperature, pressure, and other physical properties. The Langmuir probe isotherm method, ultraviolet absorption spectroscopy,

23 23 molecular beam mass spectrometry, and actinometry principles are a few plasma diagnostics techniques currently available. The Langmuir probe isotherm method determines physical properties such as electron temperature, electron density, and plasma potential of the plasma. It is based on the principle of introducing electrodes into the plasma region. A variable field is created and the measure of the variation of current and potential helps track the physical properties of the plasma. This isotherm method is applicable for low-pressure plasmas (Hopkins et al. 1986). In UV and microwave optical techniques, UV or microwave radiation is made incident on the plasma region. Depending on the absorption spectrum obtained, or certain changes in the molecular structures of the constituents recorded via change in current readings or other output methods, the composition of the plasma region is then determined (Neuilly et al. 2001). This is a well-established analytical method used by chemists to help determine the composition of any mass. Another spectroscopic method used is mass spectrometry. Molecular beam mass spectrometry uses a high-tech device to extract and stream single molecules from the plasma region and measure their mass to determine composition. Ideally, this method can detect and track the chlorine atom concentration in the plasma region (Hiden Analytical Inc. product no. HPR60). Another indirect method of determining chlorine atom concentration is gas-phase titration of the chlorine atoms using hydrogen gas. The absolute rate of the reaction is ( ± 0.11) 10 cm molecule s (Eberhard, et al. 1997) at 298 K, which is explained by the Arrhenius expression 3.7x10-11 [cm 3 /molecule s] e [kj/mole]/rt.

24 The Reaction of Chlorine Atoms with Hydrogen K eq Cl + H 2 H + HCl (2.4.1) Reaction has been studied and well documented in literature since In the 60 s some discrepancies were noted about this reaction s equilibrium constant deviating from the microscopic reversibility principle. According to Westenberg (1968) as mentioned by Kita and Stedman (1982), the equilibrium rate constant K eq of Equation is 2-3 times larger than the calculated k f k r values; where k f is the forward reaction rate constant and k r is the reverse reaction rate constant. However, later studies have disproved any presence of such discrepancy in the reaction rate constant value. Spencer and Glass (1975) reexamined the above reaction and have eliminated the discrepancy hypothesis on the ground of nonidentical reaction conditions for measuring the k f and k r values. They have, however, mentioned the recombination reaction of chlorine atoms catalyzed by the reactor wall as a possibility of any deviations if noted. This principle was discussed in Section 2.2. The side wall reaction (referred to in Equation 2.2.4), is again represented as in Equation below: wall Cl Cl 2 (2.4.2) (2.4.2) H + Cl2 HCl + Cl (2.4.3) If the above reaction is not taken into consideration while calculating the rate constants, the result will be erroneous. Later studies (Lee et al. 1977; Miller and Gordon 1981) have

25 25 taken the wall effect into consideration and have devised the following equations for calculating the rate constants: ln[ Cl] = kobserved t + ln[ Cl (2.4.4) ] 0 k = k [ H 2 ] + k (2.4.5) observed f d where kd is the diffusion correction term that accounts for the Cl atoms lost due to wall reactions or other termination reaction due to recombination, and t is the time for the reaction. The reaction has been studied at pseudo-first order kinetic conditions, that is, [ H >> Cl. Lee et al. (1977) have observed the value of 2 ] 0 [ ] 0 k f = (2.40 ± 0.32) 10 exp( 2200 ± 40/T)cm molecule s (2.4.6) and k d 20% of k f at 227K (2.4.7) This value agrees with that measured by Kita and Stedman (1982) in Eq k r = (6.00 ± 0.5) 10 exp( 2470 ± 100/T)cm molecule s (2.4.8) According to Kita and Stedman (1982), the rate constant value is better related as the ln[ Cl] slope obtained from the graph between ( )[ H ] and [ H 2 ], instead of the 2 t conventional first-order differential of these two terms. According to Lee et al. (1977) and Miller and Gordon (1981), the equilibrium rate constant for the reaction between hydrogen and chlorine atoms after considering the diffusion corrections and recombination reaction corrections is

26 26 k eq = (2.66 ± 0.42) 10 exp( 2230 ± 60/T)cm molecule s (2.4.9) The equilibrium rate constant value for T = 298K from Equation yields a value of molecule s. This indicates that for 1 mole of hydrogen-chlorine gas mixture in equilibrium, = collisions are expected per second, which demonstrates that the rate of production of HCl can be very explosive. To keep the reaction within control, steps were taken to choose chlorine as the limiting reactant here. This is further discussed in Section 3.5. Validating the equilibrium constant value (Equation 2.4.9) for the reaction is beyond the scope of this research objective.

27 27 CHAPTER 3: EXPERIMENTAL SETUP AND METHODOLOGY 3.1 Experimental Setup As described in Section 1.5, the main objectives of this research work are: 1. Set up a reaction chamber to generate chlorine plasma at standard temperature and pressure conditions, with chlorine atoms generated in the plasma collected in the sampling/reacting bulb containing hydrogen for analysis. 2. Vary the residence time of the chlorine gas in the active plasma region, then collect the chlorine atoms from the plasma region in the reaction chamber for different residence time conditions. 3. Study the amount of chlorine atom formation for the different sets of residence time under constant voltage readings and attempt to determine a trend for chlorine atom behavior. The main part of the experimental setup is divided into a plasma chamber and a sampling/reaction chamber, as shown in Figure Figure Top view of the experimental apparatus.

28 28 The plasma chamber is the 15 cm x 3 cm Pyrex cylinder, which houses the cathode and the anode (Figure 3.1.4). Figure 3.1.2, below shows the cathode tube. It is 2.5 cm long and ¼" outer diameter. It has two holes and four projected spikes along the sides. A transformer rectifier (TR) set was used to supply voltage to the cathode. Figure The cathode tube. The anode is a stainless steel (100 mesh) plate. It is grounded. It is shown in Figure Steel mesh Figure The anode tube

29 29 The plasma chamber is connected to the sampling/reaction chamber aligned at a perpendicular axis. The sampling chamber has a volume of 1 liter. A solenoid valve opens and closes the path in between these two chambers to allow the flow of the chlorine atoms into the reaction chamber. A timer circuit was designed to facilitate the opening of the solenoid valve for a definite period of time to pull out a fixed amount of gas from the plasma chamber into the reaction chamber. The timer circuit has a rheostat, set to 453 Ώ such that the valve opens for 35 ms, and it pulls out chlorine gas mixture into the reaction chamber which causes a 20 torrs pressure difference. The design of the timer circuit is illustrated in Appendix A6. The flow through the solenoid valve was feasible due to the pressure difference between the two chambers. The plasma chamber was at atmospheric pressure. The pressure in the sampling chamber was measured using a capacitance manometer. In the present experiment, this chamber was filled with 10 torrs of 1% hydrogen gas mixture to start the reaction with the chlorine atoms. Figure shows the plasma chamber and the reaction chamber with the solenoid valve between them. Chlorine gas was injected into the plasma chamber through the cathode holes. A bulk nitrogen gas flow also was maintained in the plasma chamber to control the total velocity of the gas flowing along its length. Pressure gauges and orifices were used along the supply lines for precise regulation of the flow rates. A filter with a hole diameter lesser than the smallest orifice diameter used also was installed in the flow-line to prevent clogging the orifice holes with unwanted debris. To obtain the desired residence time of

30 30 the reagent gas molecules in the active region, the linear flow velocity of the gas was varied between 14 and 70 cm/s (Appendix A2.1) for the electrodes placed at 2.8 cm apart from each other. Figure shows the orifices and pressure gauge mounted along the flow lines to control flow rates. Cathode Anode Plasma chamber Sampling chamber Solenoid valve Exhaust KF flange for manometer Figure The two chambers with the solenoid valve. Chlorine gas was carried to the plasma chamber via 1/8" Teflon tubing, chosen for its nonreactive and flexible properties. The same material also was used to connect the sampling port on the reaction chamber and the inlet port on the analysis set. This

31 connection tubing was kept short to prevent buildup of dead volume and also to minimize the chance of hydrochloric acid condensing in the line. 31 1/8" Teflon tubing Orifice test fixture Filter Digital pressure gauge Figure Flow control setup. The reaction chamber has two outlet ports. One port was used to connect it to the vacuum manifold system, which evacuated the reaction chamber and then supplied the hydrogen gas in nitrogen mixture into it. Appendix A7 documents the steps to make the 1% hydrogen gas mixture used. The hydrogen gas was used in this experiment to react with the chlorine atoms. The reaction product was analyzed for hydrochloric acid (HCl) gas. The concentration of the HCl gas was then used to determine the

32 32 concentration of chlorine atoms initially reacted. This observation was made for four different residence time conditions. The second port on the reaction chamber was used to pull the HCl gas (product gas) sample into the Drager tubes used for analysis. Figure shows the layout of the experimental setup. Chlorine gas supply Sampling/ reacting chamber Chlorine cylinder Plasma chamber Electrode grounding Sample outlet Nitrogen supply Solenoid Valve Timer circuit box Vacuum line Figure Layout of the plasma chamber and the reaction/sampling chamber

33 Drager Tube Analysis A Drager tube for detecting HCl was used in this research for sample analysis. A Drager detector tube is a glass vial and this one used for HCl analysis contains a chemical reagent, Bromophenol Blue, to detect HCl. The sample is pulled into the tube using a hand-operated pump (Drager Accuro 2000). The reagent changes color from blue to gray if any HCl is present in the sample. This system can detect HCl gas in concentrations as low as 1 ppm in the sample. The accuracy of the device varies between ± 5% to 15% as per the manufacturer. For determining the accuracy of the device and to estimate the repeatability, tests were conducted. As shown in Appendix A8, values of HCl read from the Drager tubes have a standard deviation of 0.97 ppm. The variation of the accuracy with tubes from different packs was determined to be negligible. Figure shows the discoloration of the Drager tube that had detected HCl gas in the sample.

34 34 Figure Drager tube with yellow showing the presence of HCl gas in test sample. 3.3 Optimizing the Reaction Environment Parameters The main parameters to be considered for the experiment were: 1. The voltage and current supplied, which determines the electric field strength in the plasma chamber, producing the corona discharge. 2. The flow rates of the chlorine and the background gas, which determine the residence time of the chlorine gas inside the plasma chamber. 3. The temperature and pressure conditions. The maximum voltage-current (V-I) conditions for which a stable corona discharge was observed in the plasma chamber was noted as 10KV for a corresponding current

35 35 value of 2mA. The actual experiments were carried out with this V-I combination. This had a power input of 20W, which is enough to dissociate about 4 millimoles of chlorine gas (Appendix A2.2). In an effort to accommodate a field strength as high as achievable inside the plasma chamber, the cathode dimensions were changed three times, which resulted in an 81.43% increase in the field strength, from 2.6 W to 20 W (Appendix A5 has details of the three designs). Higher field strength was predicted to increase chlorine dissociation, irrespective of the total gas pressure in the plasma chamber attributed to the increased rate of electron-impact-induced dissociation of the chlorine molecules (Malyshev et al. 1999; Fuller et al. 2001; Neuilly et al. 2001). This was attained by either reducing the distance between the electrodes or increasing the voltage applied. An alternative step that can be taken is to replace the ¼" OD discharge electrode with a larger-diameter electrode. It had been observed that electrodes of a bigger radius generally produce more voltage for a given current (Theodore et al. 1982). However, this step was not taken at present, as this would have called for a redesign of the dimensions of the plasma chamber. As shown in Appendix A1, the minimum value of the voltage to create a corona between electrodes spaced 2.8 cm apart has been calculated to be 20 KV. A currentvoltage characterization test for the plasma chamber was performed to determine the working voltage for the experiments. For the same electrode spacing, a minimum voltage of 10 KV was selected to start the optimization experiments and gradually increased until a corona was observed; the voltage was increased until an arc discharge was observed and the transformer rectifier (TR) set tripped. In the following trial, the voltage was

36 36 increased to a value lower than the point when the TR set tripped. The maximum voltage at which the corona discharge was observed as stable for at least 15 minutes (more than the 10-minute approximate time required for one test run) was chosen as the working voltage condition. For the same distance between electrodes, the velocity of the gas inside the plasma chamber had been observed to have a significant effect on the maximum stable voltage achievable. Four points (40 ms, 75 ms, 150 ms, 200 ms) of residence times were chosen for the present experiment. The flow rates of the chlorine gas were correspondingly set at 2.96 l/min, 2.18 l/min, 1.14 l/min, and 1.13 l/min for the four sets of readings. The corresponding flow rates of the nitrogen gas were maintained at 4.42 l/min, 5.01 l/min, l/min, and l/min. The flow rates were controlled using orifices. The flow rates were determined by controlling the pressure drop across the orifices. The equation governing the flow rate through the orifice by adjusting the pressure drop across the orifices is given as P Flow = Factor#3 d1 (3.3.1) 29.7 M. WGas Temp R where P 1 is the inlet pressure, M.WGas is the molecular weight of the gas flowing through the orifice, d 1 is the diameter of the orifice, and Factor #3 is calculated as the ratio of the pressure drop across the orifice to P 1. A deviation from the desired flow rate values were attributed to an inaccuracy of ± 1% of the pressure gauge reading at the inlet pressure across the orifice, whose diameter has a manufacturer inaccuracy of ± 0.1µm. A small deviation in the flow rate control thus

37 37 affected the actual residence time of the gas in the plasma region. However, tables in Appendix A2.1 demonstrate that the deviations were very well within the allowed experimental error limits. The temperature and pressure conditions inside the plasma chamber were maintained at atmospheric levels. 3.4 Test Procedure A detailed description of the test operation procedures and the failure and safety evaluation methods analyses undertaken for the present experiments is found in Document no: SOP_Micro-ESP system_ on the Ohio Coal Research Center (OCRC) server. A brief description of the test procedure is given below: 1. The glassware was washed in 1M phosphoric acid solution. This special arrangement is required to ward off the wall-effects on the chlorine dissociation reaction, which will otherwise affect the results from the experiment. 2. The electrodes were set up in the plasma chamber and the sampling chamber was assembled on the assembly board. 3. The sampling chamber was filled with the 1% hydrogen-nitrogen mixture until the pressure gauge reads 10 torrs. 4. DC voltage was applied to the electrodes. This charges the chlorine gas and the spikes on the cathode concentrate the charge in their vicinity to form the corona. The plasma chamber now consisted of chlorine molecules and dissociated chlorine atoms. After 2 minutes, the voltage supply was switched off.

38 38 5. A sample volume consisting of chlorine molecules and chlorine atoms was then pulled from the plasma region into the sampling chamber by the set pressure difference between the two chambers. This reacted with the hydrogen to yield hydrogen chloride gas. 6. The products of the reaction between hydrogen and chlorine atoms in the reaction chamber constituted the sample sent for analysis. 7. After the analysis was done, the whole setup was purged with nitrogen gas. 3.5 Calculating the Chlorine Atom Concentration A sample was collected from the plasma region and reacted immediately with a reducing gas, which yielded products that were analyzed to determine the chlorine atom concentration in the original sample. NOCl, or nitrosyl chloride, as the reducing gas was explored in the literature in the initial stages of this research work, but it was abandoned because NOCl is a poisonous green gas and it is hard to obtain the product to calibrate equipment. Instead, hydrogen was reacted with the sample to yield hydrochloric acid as a gaseous product. The amount of the products formed for a particular sample was analyzed with Drager tubes. An elementary molecular balance related the amount of HCl formed times the multiplication factor of the reaction, directly to the actual amount of chlorine atoms present in the original sample collected from the plasma region, which reacted with hydrogen. The overall reaction expression is given by Equation 3.5.1: k H 2 + Cl HCl + H (3.5.1)

39 Because it was assumed that the reaction was first order and assuming steady state, it could be concluded from the reaction stoichiometry that d[hcl] d[cl] = at equilibrium conditions (3.5.2) dt dt Thus the rate of formation of HCl can be set equal to the rate of decrease of chlorine atoms that started the reaction. It was assumed in this study that the amount of power supplied to the test was enough to dissociate all the chlorine molecules in the active plasma region. Part of the chlorine atoms formed recombined back to form chlorine molecules, and the rest were available for the reaction. Let [Cl] denote the number of the chlorine atoms formed. [Cl] recombined denote the number of chlorine atoms that recombined to form Cl 2 molecules. [Cl] 0 denote the initial number of chlorine atoms that are available for reaction. It was also assumed that the recombination reaction had a zero-order rate. This assumption was made based on the conclusions of Ogryzlo (1971). The k 1 is a constant representing that rate of recombination. d[cl] recombined rate of recombination of [Cl] = = f1(t) (3.5.3) dt f 1(t) = k 1 (3.5.4) whereas the H 2 -Cl 2 reaction that made the HCl molecules has an exponential form based on the conclusions of the study by Kita and Stedman (1982), as: 39 k2t [ HCl] = A e [ Cl ] 0 (3.5.5)

40 40 2 f t) 2 ( k t = A e (3.5.6) f ( ) also could be referred to as the multiplication factor of the H 2-Cl 2 reaction. 2 t The initial number of chlorine atoms formed from 0.5 ppm of chlorine molecules was assumed to represent a 100%. Thus the equation for the initial total number of chlorine atoms could be written as: [ Cl] = [ Cl] recombined + [ Cl (3.5.7) ] 0 [ HCl] = + (3.5.8) A e 1 k1t k2t The experimental results yielded the [HCl] values with respect to change in the residence times. A statistical analysis of all the collected data points was used to evaluate the values of the three constants A, k 1, and k 2. Equation may be rewritten as: k2t [ HCl] = (1 k1 t) A e (3.5.9) such that k t k2t [ HCl] = A e 2 k A e t (3.5.10) 1 α 2t α 2t [ HCl] = α e α e t (3.5.11) 1 1 k1 A = α 3; 3 A = α ; k = α (3.5.12) 2 2 Regression analysis of the Equation was used to evaluate the unknown constant values. Consequently, the initial concentration of active chlorine atoms was determined [ HCl] by the value of the term in respect to the 100% value assumed in Equation k2t A e

41 41 CHAPTER 4: TEST MATRIX AND RESULTS As discussed in Section 1.5, the objective of this research was to build an apparatus to house chlorine plasma at atmospheric conditions and then determine the chlorine atom concentration ([Cl]) in the active plasma region as a function of the time the chlorine molecules (Cl 2 ) are inside the active region. At Cl 2 residence times between 40 ms and 200 ms, reactive chlorine atoms (Cl) were pulled from the active region into the reaction chamber, reacted with hydrogen (H 2 ), and analyzed for the hydrogen chloride (HCl) formed. The HCl concentration was used to calculate the number of Cl atoms initially reacted. During the experimentation, steady state or a quasi-steady condition was established in the plasma chamber before initiating the sampling. Sufficient time (at least 60 seconds) was allowed to pass to attain a quasi-steady condition. This time is referred here as the sitting time. It is the time gap between starting the voltage supply and flipping the timer circuit switch to pull out the chlorine atoms into the reaction chamber. Observations were made at three different sitting times to determine the effect this time might have on the amount of the [Cl] calculated for a particular residence time. Other parameters of experimentation were noted in Section 3.3. Because the reaction chamber is initially at a lower pressure (50 torrs, containing 1% hydrogen in nitrogen mixture) compared to the atmospheric pressure inside the plasma chamber, a gas sample containing Cl and undissociated Cl 2 are pulled into the reaction chamber. The solenoid valve opened for 35 ms to pull the sample to react with H 2 to form HCl. The reaction products were allowed to sit for about 10

42 42 minutes before the sampling into Drager tubes for analysis. For ease of sampling with Drager tubes, the sampling chamber was diluted with nitrogen gas from 80 torrs to about 800 torrs for each observation. The [Cl] is calculated according to the methods described in Section 3.3. For every instance of the gas flow rate to be inspected, a sample is collected from the plasma chamber, reacted with H 2, and analyzed for the corresponding chlorine atom concentrations. This process was repeated randomly at least four times to provide a measure of data integrity in the results. 4.1 Test Matrix The test matrix is shown in Table 4.1. Flow rate Q(Cl 2 ) Flow rate Q(N 2 ) 3 10 Table 4.1 Test Matrix Distance D Residence time t Power supplied W Sitting time ts (ml/min) (ml/min) (cm) (ms) (W) (min) ±0.44 1,2, ±0.80 2, ±0.37 2, ± The flow rates were set by controlling the inlet and outlet pressures across orifices of appropriate diameter sizes installed along the gas flow lines. Considering the instrumental errors of the pressure gauge and the orifices, the flow rate data did have 0.05% error. Table 4.2 shows the pressure values for the corresponding diameters of the orifices used

43 for setting the desired flow rates of the two gas streams inside the plasma chamber. The distance between the two electrodes was 2.8 cm. Table 4.2 Pressure Drop across an Orifice to Maintain the Flow Rate of Gas Element Orifice diameter (µm) Inlet pressure (psig) Outlet pressure (psig) Flow rate (ml/min) Chlorine Nitrogen Chlorine Nitrogen Chlorine Nitrogen Chlorine Nitrogen The flow rates of the gas streams were calculated based on this distance and the targeted residence time of the chlorine gas in between the two electrodes. The cross sectional area of flow was calculated to be 5.23 cm 2. The range of the residence time of the gas in the active plasma region was chosen as 40 ms to 200 ms. Two factors were taken into consideration when deciding the mentioned range. First, the characteristic residence time in the plasma chamber was varied by changing the distance between the electrodes and the flow rate of the bulk gas between them. Thus, in order to study the Cl atom behavior in the time range below 40 ms, either the bulk gas velocity had to be very large, which required a flow rate of thousands of liters per minute, or the distance between the electrodes had to be decreased. A higher value of the flow rate increased safety concerns with small-scale laboratory experiment. Decreasing the distance between the two electrodes was observed to cause spark discharges for the voltage supplied, which may have not been sufficient to

44 44 dissociate a large enough number of chlorine molecules (Theodore et. al. 1982). It was therefore decided to study the behavior of the chlorine atoms in the time gap between 40 ms, when steady state had been presumably achieved in the plasma system, and 1 second, which is the characteristic residence time inside an industrial precipitator. The residence times for sampling and determining [Cl] in this experiment were 40ms, 75ms, 150ms, and 200ms. The gas velocity inside the chamber was the cumulative sum of the individual gas velocities of the chlorine stream and the carrier gas stream, obtained by dividing the distance traveled by the gas by the velocity at which the gas was flowing. The power supplied was the product of the amount of voltage supplied and the corresponding current measured in the ammeter on the transformer rectifier (TR) set. The operable voltage current values for the four sets of flow rates had been noted before the actual experimentations. The total power supplied for each of the cases is kept almost constant at a value of 5.74 ± 0.14W (Appendix A5). It was observed that for all four sets, the optimal voltage supplied for which the corona was stable without sparking decreased slightly in the later runs (from 19KV to 14KV). The layer of corrosion on the cathode formed due to the chlorine gas flowing through it could have accounted for the observation. The sitting time value was changed for all four sets of experiments to determine its effect on the obtained results. However, an analysis of the test results actually show that this time has no effect on the amount of HCl formed. Thus, it was confirmed that the chlorine atom concentration in the active plasma region was steady between 1 and 3 minutes after the plasma is formed.

45 Results Tables 4.2.1, 4.2.2, 4.2.3, and tabulate the results obtained for HCl concentration in the product samples. The four tables are for the four sets of experiments, performed over a time period of nine days. Date Table Results of Testing for a Residence Time t=40ms Voltage (KV) Current (ma) Power (W) Sitting time (min) Dilution Reading,R1 (ppm) Actual result,rf (ppm) 9/19/ /20/ /20/ /21/ /24/ /24/ /25/ /21/ /19/ Date Table Results of Testing for a Residence Time t=75ms Voltage (KV) Current (ma) Power (W) Sitting time (min) Dilution Reading,R1 (ppm) Actual result,rf (ppm) 9/19/ /20/ /20/ /24/ /24/ /25/ /21/

46 46 Date Table Results of Testing for a Residence Time t=150ms Voltage (KV) Current (ma) Power (W) Sitting time (min) Dilution Reading,R1 (ppm) Actual result,rf (ppm) 9/19/ /20/ /20/ /24/ /24/ Date Table Results of Testing for a Residence Time t=200ms Voltage (KV) Current (ma) Power (W) Sitting time (min) Dilution Reading,R1 (ppm) Actual result,rf (ppm) 9/19/ /20/ /20/ /23/ The actual HCl concentration (Rf) was obtained by multiplying the Drager tube reading (R1) for a particular observation by the dilution factor (Dilution). The reaction product in the sampling chamber was diluted with pure nitrogen gas to raise the total pressure inside this chamber above the atmospheric pressure to facilitate the Drager tube analysis. For all the observation points, the pressure was raised to 800 torrs. The Dilution is calculated as the quotient of the final diluted pressure over the initial reaction pressure in the reaction chamber. From the above individual tables it was observed that varying the sitting time value did not make any significant change in the HCl concentrations measured. An approximate inverted-v curve similar to that shown in Figure was expected from the experimental readings. It was hypothesized that for a very small

47 47 residence time value, not enough chlorine atoms were formed. But as the time value increase, more chlorine molecules dissociate and the chlorine atom concentration gradually increases. This trend peaks or becomes constant over a certain time period range. Further increasing the residence time gives the chlorine atoms enough time to recombine back to form chlorine molecules. The curve for the chlorine atom concentration versus the residence time then gradually decreases. A downward curve implies only that the behavior of the chlorine atoms for residence time below 40 ms needs further investigation. An upward curve implies only that more observations for time values near and above 200 ms should be done to understand the behavior better. chlorine atom concentration vs residence time of the gas cm E E E E E E E E msec Figure Arbitary Represntation of the Hypothetical Inverted-V Curve. In the conclusion of the present study, a decreasing curve was observed, as shown in Figure A nonlinear regression of Equation using the combined data from the tables yielded the following equation (see Appendix A9).

48 48 [HCl] = + 7 (4.2.1) (1.57s ) t e The [Cl] 0 values corresponding to the four residence time values were calculated from the second term in Equation and are shown in Table The + values are calculated from the maximum-average, and the - value is calculated from average-minimum. Table Cl Concentration Data Plotted in Figure Residence time (ms) [Cl] (ppm) t Range Figure shows the behavior of the chlorine atoms with changes in the residence time, is based on the data from Table The graph shows that the active chlorine atom concentration in the plasma region (Y-value) decreases with increase in the residence time of the gas in the active region (X-value).

49 49 [Cl] vs t [Cl]0, ppm t, ms Figure Trend of Cl in active plasma region.

50 50 CHAPTER 5: CONCLUSIONS AND RECOMMENDATIONS 5.1 Conclusions An experimental set-up was devised, as discussed in Chapter 3. A chlorine plasma was set up in it at standard atmospheric conditions. The chlorine atoms generated in the plasma region were captured in the reaction chamber and reacted with hydrogen gas. HCl gas, which was the reaction product was analyzed for using Drager tubes. Details are referred to in Section 3.1 and Section 3.2. The calculated recombination rate for the chlorine atoms was found to be 1.57s -1, which is comparable to the 0.8 s -1 value concluded by Orgzylo (1971). Calculations leading to this value for the recombination rate are in Appendix A9. The data obtained from all the observation points were regressed using literature to calculate the constantvalues in Equation The previous study by Orgzylo was conducted at lower total pressure conditions, and a higher rate of recombination of the chlorine atoms was expected at atmospheric conditions as described by Le Chatelier s Principle. It was observed from Figure that the chlorine atom concentration [Cl] decreases from an average of ppm to ppm with an increase in the residence time value from 40 ms to 150 ms. The most likely reason for the decrease in [Cl] was an increase of Cl recombination reactions as residence time increased. Over the time period between 150 ms and 200 ms, [Cl] was observed to have attained a steady state, most likely because the chlorine recombination rate equals out the chlorine dissociation rate. Initially a hypothesis had been made about the expected results from this experiment. It was expected that the chlorine recombination behavior would follow a

51 51 gradually increasing path until it reached a peak and then decreased with an increase in the residence time of the chlorine gas in the active plasma region. Until a certain amount of time, an increase in the residence time of the chlorine gas will allow more chlorine atoms to be formed, which explained the increasing path of the curve. Over this path, more chlorine atoms were formed due to dissociation when compared to the number of chlorine atoms recombining. This behavior reached a peak, after which point the residence time of the chlorine gas is so great that the number of chlorine atoms recombining to form back the chlorine molecules gradually becomes greater than the number of chlorine atoms formed due to dissociation. The decreasing path of the curve continued until the dissociation and recombination processes reached equilibrium and the number of chlorine atoms in the active plasma region became steady. Based on this initial hypothesis, the peak in [Cl] may have been obtained in the chlorine plasma at a residence time less than 40 ms. However, in the current setup, a residence time lower than 40 ms was not achievable (see Appendix A5) and hence the peak point could not be validated at this point. Besides, residence times greater than 150 ms were considered as having achieved steady state concentration of chlorine atoms. That consideration may be useful for further experiments to explore the design parameters of using chlorine as a reagent gas to increase elemental mercury removal efficiency using an ESP. In terms of practical applications, ESPs usually have gas residence times exceeding one second. The results from this study indicate that if 0.5 ppm of chlorine gas was injected into the plasma region of the discharge electrodes in an ESP and residence time of the gas in this region is greater than 150 ms, no more than 0.19 ppm of chlorine atoms

52 52 would be available to oxidize elemental mercury in the flue gas stream. This conclusion holds true only if the present observations can be extrapolated for residence times greater than 200 ms. The extrapolation is recommended to be experimentally verified in future projects. This could significantly increase the fraction of oxidized mercury in the flue gas, which is easier to remove than elemental mercury vapor. 5.2 Recommendations To successfully optimize the process of determining the desired efficiency for the working of the ESP, other factors such as the presence of NO, O 2, steam, and fly ash particles in the flue gas, along with the temperature of the gas stream, require consideration. To mimic actual industrial precipitator conditions, the same set of experiments as presented in this work should be repeated with different reaction conditions, including: 1. Higher temperatures. At higher temperatures, chlorine atom recombination reaction rate is not expected to change (Ogryzlo 1961). The chlorine atom recombination behavior is not temperature dependent. Also Montgomery (2005) states that the efficiency of removing mercury in a plasma enhanced electrostatic precipitator is temperature dependent only within 1%. Still, it is important to perform the experiment at higher temperatures between 200 F and 300 F, which is the operating temperature of industrial electrostatic precipitators.

53 53 2. Presence of impurities, such as nitrogen oxides and sulphur compounds. The presence of sulphur compounds has been shown to increase overall ESP efficiency by reducing the resistance of the fly ash (IEA Report 2004). This conclusion is concurrent with the results from Montgomery et al., 2005 who observed a 87% mercury removal efficiency with sulphur compound present in the flue gas compared to 75% efficiency without the presence of sulphur compound. But their observation of a negative effect of the presence of nitrogen compound on the total amount of mercury oxidized in the flue gas differs from the analysis presented in the IEA report. The IEA report stated that simultaneous presence of nitrogen oxides will increase Hg deposition on the fly ash particles and consequently improve mercury removal efficiency of the ESP. However, the presence of any of these compounds affecting the concentration of the active chlorine atoms in the active plasma region have not been studied in literature till date. Along the guidelines of the experiment conducted by Montgomery et al., 2005 and the data portrayed in the IEA Report, 2004; the following experiment can be conducted with 70ppmv NO and 100 ppmv SO Testing with elemental Hg vapor. By replacing the hydrogen gas in the reaction chamber with elemental mercury, the resultant product, which would be predominantly expected to be mercuric chloride, could then be analyzed using Hydra AA analysis. The trend of formation of the HgCl 2 could be related to the trend of chlorine atom formation as obtained in the present

54 54 experiments. This correlation can validate one postulated reaction mechanism of the Hg-Cl reaction. Average amount of elemental mercury present in the coal flue gas varies between 0.02 and 0.25 ppm (EPRI 1996). For further experimentation, a range of 0.02 to 0.25 ppm of elemental mercury can be introduced in the sampling chamber with the 0.5 ppm of chlorine gas in the plasma chamber (same as the present conditions). If the postulated reaction pathway with the chlorine atom acting as the main oxidizing agent of the elemental mercury in coal flue gas is true, we expect to see the oxidized mercury amount in the resultant media behaving just opposite to the trend observed in Figure Testing for higher residence time values. As concluded in Section 5.1, more observations are required for residence time of chlorine gas having a value more than 200 ms. This was beyond the ability of the present research. However, in order to have a more definite say about the fact that the chlorine atom concentration reaches an equillibrium value when the residence time of the chlorine gas in plasma is more than 200ms, experimental validation at residence time values between 200 ms and 1 s is necessary. 5. Direct measurements of chlorine atom concentrations in the active plasma region. By measuring actual [Cl], such data could validate the conclusions from these experimental calculations. Obtaining real-time chlorine atom concentration values for different residence times could be achieved by using the expensive molecular beam spectroscopy method discussed in Section 2.3.

55 55 REFERENCES Altman, Ralph, Buckley, Wayne, Ray, Issac, Application of wet electrostatic precipitation technology in the utility industry for multiple pollutant control including mercury. Altman, Ralph, Ray, Issac, Buckley, Wayne, Reynolds, James, Montgomery, John, Pilot scale test results on plasma enhanced electrostatic precipitation technology for mercury removal. Ariya, Parisa A., Khazilov, A.F., Gidas, A Reactions of gaseous mercury with atomic and molecular halogens, J Phys Chem A 106: ASTDR Toxicological profile of mercury. CAS# Brown, Thomas D., Smith, Dennis N., Hargis, Richard A., Mercury measurement and its control: What we know, have learned, and need to further investigate. J Air and Waste Mngmnt Assn 6: Effremov, A.M., Kim, D.P., Kim, C.L., Volume and heterogeneous chemistry of active species in chlorine plasma. Thin Solid Films 435: EPRI Protocol for estimating atmospheric mercury deposition. Palo Alto, CA: Electric Power Research Institute Report No. TR :55. Fuller, N. C. M., Herman, I.P., Donnelly, V.M., Optical actinometry of Cl 2, Cl, Cl + and Ar + densities in inductively coupled Cl 2 -Ar plasmas. J App Phy 90(7):

56 56 Hiden Analytical Inc.. Product name HPR Hopkins, M. B., Graham, W.G., Langmuir probe technique for plasma parameter measurement in a medium density discharge. Rev Sci Instrum 57(9): Horne, D. G., Gosavi, R., Strausz, O.P., Reactions of metal atoms: Combination of mercury and chlorine atoms and the dimerization of HgCl. J Chem Phys 48(10): (accessed April 30, 2007). IEA Clean Coal Center. November Air pollution control technologies and their interactions. Report no. PF Jayaram, Varalakshmi, Capture of mercury in wet membrane PEESP using hydrochloric acid as the reagent gas. Masters Thesis, Ohio University, Nov Kirillov, Yu. V., Sitanov, D.V., Determination of the rate coefficient for the heterogeneous recombination of chlorine atoms in a plasma reactor by the relaxation technique. High Energy Chemistry 38: Li, Lai-Cai, Deng, P., Tian, A.M., Xu, M.H., Zheng, C.G., Wong, N.B., A study on the reaction mechanism and kinetic of mercury oxidation by chlorine species. J Mol Structure 625: Liu, K., Gao, Y., Riley, J.T., Pan, W.P., An investigation of mercury emissions from FBC systems fired with high chlorine coals. Energy and Fuels 15:

57 57 Theodore, L., Buonicore, A.J. Book Title - air pollution control equipment, 1982 by Prentice Hall Inc., Englewood Cliffs, N.J, p.p Malyshev, M. V., Donnelly, V. M., Collonel, J.I., Samukawa, S., Dynamics of pulsed power chlorine plasmas. J App Phy 86(9): Montgomery, John L., Battleson, D.M., Whitworth, C.G., Altman, Ralph, Ray, Issac, Buckley, Wayne, Reynolds, James, Latest developments of the plasmaenhanced electrostatic precipitator for mercury removal in coal fired boiler flue gas. Env Eng Sci 22(2): Neuilly, F., Booth, J.P., Vallier, L., Chlorine dissociation fraction in an inductively coupled plasma measured by ultraviolet absorption spectroscopy. J Vac Sci Technol A 20(1): Ogryzlo, E.A Halogen atom reactions: Electrical discharge as a source of halogen atoms. Can J Chem 39: PCA, Vagn. C Johansen Mercury speciation in other combustion sources: A literature review. PCA R&D Serial No Peek, F.W.,1929. Dielectric phenomena in high voltage engineering. Schenectady, New York: McGraw-Hill Senior, Constance L., Sarofim, A., Zeng, T., Helbe, J.J., Mamanipaco, R., Gas phase transformations of mercury in coal fired power plants. Fuel Proc Tech 63(2):

58 58 Sliger, Rebecca N., Kramlicha, J.C., Marinov, N.M., Towards the development of a chemical kinetic model for the oxidation of mercury by chlorine species. Fuel Proc Tech 65-66: U.S. Environmental Protection Agency. Office of Air Quality Planning & Standards and Office of Research & Development Mercury study report to congress, volume 1: Executive summary. EPA-452/R Washington, DC: USEPA Wilcox, Jennifer, Marsden, David, C.J., Blowers, P., Evaluation of basis sets and theoretical methods for estimating rate constants of mercury oxidation reactions involving chlorine. Fuel Processing Technology 85: Yan, Liang, Mercury Precipitation Control by Aqueous Ozone in the Wet Scrubbing System. Masters Thesis, Ohio University, Nov 2005.

59 59 APPENDIX A1. Estimating the Voltage Required and the Number of Chlorine Molecules Dissociated (Peek 1929) An estimation of the voltage required to cause a corona discharge for the present electrode spacing is computed using certain established mathematical relationships. There are two types of voltages associated with a corona. The voltage in which a corona is visible is termed as the visible voltage. The corona remains visible until the voltage reaches the disruptive critical voltage. The minimum voltage required to be supplied to the electrodes to form a visual corona is calculated as V (min) = mgδ r( / r )ln( S / r) (A1.1) where S is the distance between the two electrodes = 2 cm r is the radius of the electrode wires = 1.5 mm (for two wires in a parallel orientation, a corona does not form when S r < ) m implies the roughness of the wire = 0.98 for roughened wires g is the disruptive critical voltage gradient = 29.8 KV/cm δ is the air density factor = 1 (at atmospheric temperature and pressure) Putting the values for each factor in the Equation A1.1, we get V (min) = 20 KV Optimization of the current-voltage characteristics will be done after the setup is built.

60 60 Capacitance in the wire is calculated as *10 12 Cw = = 0.17 *10 Farads / cm (A1.2) 1 S cosh [ ] r The number of chlorine molecules that can be dissociated under the present conditions can be estimated as energy stored in the wire energy required to dissociate 1 Cl molecule 2 (A1.3) where energy stored in the wire = 0.5CwV 2 = 3.38*10 5 Joules energy required for dissociating 1 molecule of chlorine Joules From Equation A1.3, the number of chlorine molecules dissociated can be related as *10 Joules Joules = = 3.38*10 13 Cl 2 molecules (A1.4) Volume of the spherical tip of the electrode = 4 π r( tip) 3 3 = 5.2*10 4 cm 3 Concentration of chlorine that, when injected through the cathode, can be dissociated *10 = 2700 ppm = 0.27% *10 * 2.4*10

61 61 A2. Optimizing the Amount of Chlorine in the Plasma Chamber Assuming a basis of a minimum of 100 ppm of HCl can be detected by the analysis technique used: Volume of sample pulled into the Drager tube = 100 ml 100 ppm of HCl in the sample = 0.01μl of HCl 6 1atm l 7 = = mol of HCl = 0.41μmol of HCl latmK mol 298K Moles of chlorine atoms required to form the above number of moles of HCl: = μmol = 0. 28μmol (assuming every two moles of Cl atoms form one mole of HCl) Assuming the chlorine is only 20% dissociated to form the chlorine atoms, the number of moles of chlorine molecules required in the plasma chamber is: = = 0.7μmol Volume of the plasma chamber V = = (1.5) (15) cm The moles of gas in the plasma chamber is calculated as 3 ( 1atm) ( l) 3 = = moles (0.082l atm / K mol) (298K) The ppm of chlorine gas required can be estimated as = 10 = ppm = 0.02% of Cl 3 2 in N 2 gas

62 62 This indicates that a minimum of 0.02% of chlorine gas inside the plasma chamber has to be maintained in order to obtain enough amount of HCl gas to be analyzed by the chromatographic technique. For the actual experiments, 0.5 ppm of chlorine gas in the gas matrix is maintained to increase the chances of obtaining more chlorine atoms via the dissociation in the plasma. While deciding the ppm value of chlorine gas inside the plasma chamber the safety factor has also been calculated. A2.1 Controlling the Flow Rate of Chlorine Gas in the Plasma Chamber Table A2.1 tabulates the flow rates of the chlorine gas stream and the carrier gas stream to obtain the desired residence time of the gas between the electrodes. Total velocity of gas in plasma chamber distance between the electrodes V g = resdience time Area of cross section of the plasma chamber A = = *(1.5) 7.065cm Total volumetric flow rate of the gas flow Q = V g g A 0.5% chlorine in nitrogen gas mixture is obtained from Praxair for the chlorine gas supply to the experiment setup. When this mixture is flowing in through the plasma chamber along with the pure nitrogen bulk flow, the ppm of Cl 2 in the plasma chamber is determined as: Q + 0 Q 2 10 Q + Q mix bulk 6 ppm of Cl = (A2.2) mix bulk

63 Referring to Equation A2.1, the ppm of Cl 2 in the plasma chamber is to be maintained at a minimum value of Also, Q = Q + Q (A2.3) g mix bulk In the above equation, mixture, and Q mix represents the flow rate of the chlorine gas in nitrogen gas Q bulk represents the bulk flow rate of the nitrogen gas. The ratio of Qmix to theq bulk is fixed at a value x. Qmix and Qbulk are calculated using Equations A2.2, A2.3. Chlorine gas supply: 0.5% Cl 2 in N 2 (represented by the n value in the table) ppm of chlorine in the gas matrix = 0.5 Distance between the electrodes = 2.8 cm t,ms Table A2.1 Setup Parameters d, cm 3 V g, cm / s Q g, cm / s n Q bulk,ml / min,ml / min Q mix The flow rate of the carrier gas stream is changed from 4.42 l/min to a maximum value of l/min. The flow rate of the carrier gas is maintained at a value that is 1,000 times more than the flow rate of the chlorine gas stream through the cathode. This

64 arrangement is to obtain the residence time of the gas in the plasma chamber varying from a minimum of 40 ms to a maximum of 200 ms. 64 A2.2 Power Required to Dissociate Enough Chlorine Atoms Based on the above calculations, Moles of chlorine required to make up at least 100 ppm of HCl gas in the reaction chamber = μ mol = molecules = Cl2 molecules Energy required to dissociate 1 Cl 2 molecule = Joules To dissociate Cl 2 molecules = Joules = Joules of energy is required. Power supplied in the experiments = 10 KV 2mA = 20W Considering that the power is supplied for two minutes, energy supplied = 20 W 2 60s = 2400Joules This is sufficient to dissociate chlorine molecules to collect enough chlorine atoms to make up 100 ppm of HCl by reacting it with hydrogen gas. A3. Estimating Time Required to Fill Up the Sampling Chamber Assumptions made for the calculation: 1. The density of the gas remains constant. 2. The gas present in the whole system is ideal. 3. The process is isothermal. Initial plasma chamber conditions:

65 65 Pressure = P 1 (known value) = 750 torr 2 Volume = V 1 (known value) = π ( 1.5) 10 = 70.68cm Initial sampling chamber conditions: Pressure = P 2 (known value) = 600 torr Volume = V 2 (known value) = 1 l = 1000 cm 3 Because the difference in the pressure of the two vessels forces the (chlorine atoms + chlorine gas) to rush into the sampling chamber from the plasma chamber, the energy balance equation can be written as: m v = ( P P ) L n (A.3.1) In the above equation: m = mass of the fluid in the nozzle = ρ Vn v = velocity gained by the fluid in the nozzle L n = length of the nozzle = 2.5cm V n = volume of the nozzle = π *0.5 2 * 2.5 = cm 3 A n = cross sectional area of the nozzle = 2* π * 2.5*0.5 = 7.85cm 2 Taking the value of v from Equation A.3.1, the molar flow rate can be derived as: P P L 2( 1 2 ) n & = ρ An v = ρ An (A.3.2) ρ Vn m Density of the fluid can be assumed as a constant as the nozzle path is very short compared to the volumes V 1 and V 2. ρ + ρ 2 ( ) ρ = = = mol / cm (A.3.3)

66 Now, considering the gas flow in rate of the sampling chamber, the ideal gas law equation is represented as: 66 dp V2 = m& R T 2 (A.3.4) dt ( P P ) 1 2 f dp An P1 P = dt V ρ R T ( P1 P2 ) i 2 n 0 2 V t = 1.4sec Thus the sampling chamber requires an estimated time of about 1.4 sec to fill up with the required amount of chlorine atoms. If the time for the reaction between hydrogen and chlorine atoms is allowed until completion (as in A2), which is a fraction of a second, and the time required to flush the reaction products into the GC which is again done by pressure difference, a sample can be collected for analysis every 30 seconds. 2 t dt

67 A4. Kinetic Scheme of Formation and Decay of Active Species in Chlorine Plasma 67 Figure A4.1. Source: Effremov et al. (2003).

68 68 A5. Designing the Cathode to Obtain Higher Field Strength for the Experiments The optimum voltage at which the corona is formed in the plasma chamber is obtained by adjusting the distance between the electrodes and gradually increasing the voltage supplied to the cathode and measuring the current recorded in the ammeter. At first, the electrodes are kept 1.8 cm away from each other. A minimum voltage of 5 KV is applied to the cathode. Then carefully the voltage supplied is increased until a corona is observed. The voltage for which a stable corona with a minimum fluctuation on the ammeter current is read is accepted as the working voltage. If there is a spark between the electrodes and the TR set trips off, the next observation is conducted at a voltage lower than the previous observations for the same distance value between the two electrodes. When the working voltage for this setting is determined, the distance between the electrodes is increased to 2 cm and in the same way, the observations are repeated. The distance is increased to accommodate a higher working voltage and hence a corresponding higher power supplied. However, in compliance with the present experimental conditions, a voltage greater than 25 KV cannot be supplied. This is the deciding factor for setting the optimum distance between the electrodes for these experiments. When the working conditions are set, the VI characteristics for the four different flow rate conditions are observed repeatedly five times for each. The dimension of the cathode has been changed three times during the parameter optimization period of the experiment setup. Figures A5.1 and A5.2 show the first two designs. The actual experiments have been conducted using the third design.

69 69 Figure A5.1. Design 1 Figure A5.2. Design 2 The repeatability test for establishing the flow rate control of each gas stream and the corresponding VI characteristic of the plasma chamber for both the designs are tabulated below. In Tables 5.1, 5.2, and 5.3: d (orifice) represents the diameter of the orifice used in the line. P (X) represents the inlet pressure in the X line. Q (X) represents the flow rate of the X gas stream. V represents the velocity of the bulk gas flow in the plasma chamber. d represents the distance between the two electrodes. t represents the residence time of the gas flowing in the plasma chamber. V represents the voltage supplied.

70 70 I represents the amount of current read on the TR set ammeter. W represents the amount of power supplied in each case. SD represents the standard deviation of the values in the above rows of corresponding column. The values in the last column (W) also demonstrate that the second cathode design helped in achieving a higher total power input to the plasma chamber. The Design 3 has much thinner wires made of galvanized steel. Thinner wires of 0.25" diameter increased the total optimum power input by 18% over the Design 1. With Design 3 which has thinner spikes and a distance of 3 cm between the electrodes total power supplied has been increased from a 2.60 ± 0.30 W to 5.75 ± 0.20 W, compared to the Design 1.

71 71 Table A5.1 First Cathode Design Results Chlorine gas line Nitrogen gas line d (orifice) P (Cl 2 ) Q (Cl 2 ) d (orifice) P (N 2 ) Q (N 2 ) Q (total) V (cm/sec) d t V (KV) I (ma) W (W) SD SD SD SD

72 72 Table A5.2 Second Cathode Design Results d (orifice) Chlorine gas line Nitrogen gas line P Q d P (Cl 2 ) (Cl 2 ) (orifice) (N 2 ) Q (N 2 ) Q (total) V (cm/sec) d t V (KV) I (ma) W (W) SD SD SD SD

73 73 Table A5.3 Third Cathode Design Results Chlorine gas line Nitrogen gas line P Q d P (Cl 2 ) (Cl 2 ) (orifice) (N 2 ) d (orifice) Q (N 2 ) Q (total) V (cm/sec) d t V (KV) I (ma) W (W) SD SD SD SD

74 74 The uncertainties in the flow rate values for the chlorine gas are due to the uncertainty in the pressure gauge readings and the preciseness of the orifice diameter values controlling the flow rate of the gas inside the plasma chamber. This reflects in the uncertainty in the velocity calculation. The uncertainties in the current readings are based on the standard deviations of five readings for each of the sets, and this is reflected in the power calculations. A6. Designing the Timer Circuit for the Solenoid Valve Operation The circuit layout is based on the 555 astable timer circuit ( It needs a 12 V input voltage. The circuit diagram is shown in Figure A6.1. There are two resistors, R 1 and R 2, through which the current flows and the capacitor C 1 is charged up. When the voltage reaches 2/3 of its threshold value, the capacitor starts discharging through R 2. The values of R 1 and R 2 determine the mark time (Tm) or the charging part of the cycle and the space time (Ts) or when the cycle is not charged. The astable circuit has a square wave form that continuously changes between high (Tm) and low (Ts) as shown in Figure A6.1. Figure A6.1. Square wave form of 555 timer circuit.

75 75 The time period T is the time for a complete cycle. A duty cycle is defined as the ratio between Tm and T. A smaller duty cycle implies a smaller Tm, which results in a smaller time period when the solenoid valve is open. For a duty cycle to be less than 50%, an extra diode is added in parallel with the R 2. During the charging part of the cycle R 2 is bypassed, so that Tm depends only on R 1 and C 1. The timer circuit designed for this experiment is shown in Figure A6.2. Figure A6.2. Timer circuit diagram. The values chosen for the capacitor and resistor are: C 1 = 100 µf R 1 = 47 Ω + variable resistance which varies between 400 Ω and 1400 Ω R 2 = 30 KΩ The space time and the mark time are defined as: T S =.7 R C 2. 1s =

76 76 T M = 0.7 R C 1 1 The R 1 is adjusted to obtain a T M such that the solenoid valve opens for a definite time to pull in only enough gas into the sampling chamber. The response time of the valve is 30ms. By trial and error, the R 1 is set to 453 Ω for a Tm = 35 ms. It has been determined that when the valve opens for the 35 ms, the pressure difference caused in the sampling chamber by pulling in chlorine gas sample into it is 20 torrs. A7. Making Up a Hydrogen Gas Mixture in Nitrogen Figure A7.1 shows a schematic of the setup used to make the 1% hydrogen gas mixture in nitrogen. This mixture is used in the reaction chamber to react with the chlorine atom containing gas sample.

77 Figure A7.1. Sketch of the vacuum manifold system. 77

78 78 A lecture bottle of research-grade pure hydrogen gas is hooked up to Tube 1 using valve V1. The valves V5 and V2 are kept open and the valve VM is closed to read the pressure in Tube 1 plus makeup bulb, using the manometer. Valve V1 is opened to let hydrogen gas in to the Tube 1. Pressure in this tube plus makeup bulb is adjusted to 1 torr. Valve V2 is then closed to trap 1 torr of pure hydrogen gas. The rest of the gas is evacuated from the Tube 1. Valves V4 and V5 supply nitrogen gas to this vacuum manifold system. Valve V5 is opened to let nitrogen gas into Tube 1 until the pressure reads 1000 torrs. V2 is opened to pull in nitrogen gas in one stroke (due to the high pressure difference between the two vessels). The valve V2 is closed when the manometer reads 1000 torrs. Thus a nitrogen gas mixture is obtained in the makeup bulb that has ppm = 1000 ppm of hydrogen gas in nitrogen gas 1000 Tube 1 is evacuated again to read almost absolute 0 torrs on the manometer. VM is then opened. The valve connecting the four sample bulbs and the Tube regions is then opened. Opening V2 pulls out the 1000 ppm makeup gas into this region plus the bulb B1. The gas inside the available region is adjusted so that the pressure reads 1 torr. The valve on B1 is closed, and the rest of the region is evacuated. Again nitrogen gas is pulled into the system. When the pressure on the manometer reads something near 1000 torrs, the valve on the B1 is opened to pull in the nitrogen gas. This process is continued until total pressure reads 1000 torrs. The valve on B1 is closed and the rest of the region is evacuated again. Now the percentage of hydrogen gas volume in the bulb B1 is:

79 = 1% This 1% hydrogen in nitrogen gas mixture is used in the experiment. The setup is shown in Figure A7.2. Vacuum manifold system Capacitance manometer Vacuum hose Cylinder with liquid nitrogen Hydrogen cylinder Figure A7.2. Vacuum manifold system setup.

80 80 A8. Drager Tube Analysis Samples of different ppm values of HCl are made up in the vacuum manifold system in the Stocker 045 lab. HCl is obtained by heating sodium chloride (NaCl) and sodium bisulfate (NaHSO 4 ) in the Shlenk tube in the system (Figure A7.1). The reaction is: Δ NaCl ( s) + NaHSO4 ( s) HCl( g) + Na 2SO4 ( s) The HCl gas is captured in Tubes 1 and 2. Mixing 1 torr of HCl gas with a makeup of 1000 torrs with nitrogen yields a 1 ppm HCl gas mixture in the make up bulb. torr ppm in make up bulb = ppm 1000torr = To make a p ppm of HCl gas in one of the bulbs (B1-4), p torrs of the gas is pulled from the makeup bulb into B1. mole fraction of HCl in make up bulb, X ppm in B1 = p 10 6 = p HCl = Table A8.1 Testing the Accuracy of the Drager Tubes (09/07/2007) Bulb HCl tested Tube reading F factor Actual reading Error % (ppm) The F factor in Table 8.1 is the ratio between 1013 hpa and the actual barometric pressure when the sample is collected in hpa. The readings from the Drager tubes are well within the

81 81 experimental error limits. The errors in the readings are attributed to the manual error in reading the color change level of the tube. The following tests were done to see if the observations are reproducible. Four 6 ppm standards are prepared and tested for separately. Standard deviation of the readings is ± 0.97 ppm. Table A8.2 Repeatability Test for Drager Tube Analysis (09/10/2007) Bulb HCl tested (ppm) Tube reading F factor Actual reading A9. Result Analysis The postulated equation is: = k t + [ HCl] 1 1 k2t A e The above equation is rearranged to get a form to be able to relate the known values, [HCl] and t. [ 1 2 HCl] = (1 k t) A e k t Denoting the unknown variables in the above equation as: A = α ; α k = α 1 k1 A = 3; 2 2 Using all the observation points for the 4 sets of readings (Table ), an Excel equation solver is used to optimize the problem. The error equation is represented as: f α t α 2 2t = α1 e α 3 e t [ HCl]

82 82 The sum of the error squares is minimized to obtain solutions for the three α values. The solver options are set at iterations = 10000, tolerance = 5%, and convergence = Figure A9.1 shows a screenshot of the solution from the Excel sheet. The data is tabulated in the Table A9.1 Table A9.1 Regression Data t, sec [HCl], ppm [Cl] 0 ppm average max min average max min average max min average max min

83 Figure A9.1. Excel solver to obtain the alpha value. 83