4 Ab Initio Calculations of Organophosphorus Combustion Chemistry
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Oxidation Kinetics of Methylphosphonic Acid in Supercritical Water: Experimental Measurements and Model Development by Patricia A. Sullivan B.S., Chemical Engineering University of Notre Dame, 1999 M.S., Chemical Engineering Practice Massachusetts Institute of Technology, 2002 Submitted to the Department of Chemical Engineering in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY IN CHEMICAL ENGINEERING at the MASSACHUSETTS INSTITUTE OF TECHNOLOGY November, 2003 ( Massachusetts Institute of Technology 2003 All Rights Reserved Signature of Author: Department of Chemical Engineering November 10, 2003 Certified by: t/' Professor Jefferson W. Tester &(>1~ ThesisSupervisor Accepted by: Professor Daniel Blankschtein --1-----_- rroIessor t01 nmlcal rnglneenrng OFTECHNOLOGY Chairman, Committee for Graduate Students I NOV 1 2 2003 I _·Y ARCHIVES -. LIBRARIES 2 Oxidation Kinetics of Methylphosphonic Acid in Supercritical Water: Experimental Measurements and Model Development by Patricia A. Sullivan Submitted to the Department of Chemical Engineering on November 10, 2003 in partial filfillment of the requirements for the degree of Doctor of Philosophy in Chemical Engineering ABSTRACT Above its critical point (Tc=374 °C, Pc=221 bar), the physical properties of pure water change drastically from liquid-like to dense gas-like behavior. Supercritical water is a nonpolar solvent with moderate densities (approximately 0.1 g/mL) and gas-like diffusivities and viscosities. Above 450 °C, radical pathways dominate due to the higher temperatures and decreased ionic reaction rates when the ion-dissociation constant of water is less than 10-19. Supercritical water is employed as an oxidation medium for the destruction of dilute organic aqueous waste streams because organic compounds and gases are both soluble in supercritical water. Oxidation proceeds quickly and completely without interphase mass transfer limitations, with characteristic reaction times of one minute needed for total conversion of C/H/N/O organic compounds to water, carbon dioxide, and molecular nitrogen. With this as a motivation, the supercritical water oxidation kinetics of the model organophosphorus compound, methylphosphonic acid (MPA or PO(OH)2CH3), was the primary focus of this thesis. Organophosphorus oxidation in supercritical water is being considered as a destruction method for stockpiled organophosphorus chemical warfare agents. MPA is a refractory intermediate and its reaction kinetics are important for the complete oxidation of larger organophosphorus compounds. Previous experimental MPA oxidation studies focused on determining the conditions necessary to achieve high destruction efficiencies at excess oxygen and long residence times. The primary goal of our research was to improve the mechanistic understanding of MPA oxidation kinetics in supercritical water. Our approach was to experimentally measure MPA oxidation rates and product yields at well-defined operating conditions and to develop.both microscopic and macroscopic models, ranging from regressed global models to an elementary reaction mechanism, to quantify MPA oxidation kinetics in supercritical water. MPA hydrolysis and oxidation rates were experimentally measured in a laboratory-scale plug flow reactor. The effects of MPA concentration (0.5 to 1.0 mM), oxygen concentration (1.0 to 3.8 mM), temperature (478 to 5720 C) and pressure (138 to 277 bar) on oxidation rates were determined for residence times ranging from 3.0 to 9.5 s. Conversion due to hydrolysis was less than 6% after t=7 s at all temperatures studied. For t=7 s at stoichiometric conditions and P=246 bar, low conversion (X<30% at r=10 s) was observed at T<503 °C, while almost complete conversion (X=99%) occurred at T=571 °C. The only phosphorus-containing product was phosphoric acid, while the carbon-containing intermediates, carbon monoxide and methane, were present in varying concentrations in addition to final carbon-containing product, carbon 3 dioxide. Methane was only a minor product, with a carbon yield less than 20% at all experimental conditions. MPA oxidation rates varied with oxygen concentration and pressure (or water density), but were relatively independent of initial MPA concentration. A global MPA oxidation rate law was regressed from the data with its dependence on temperature, MPA concentration, oxygen concentration, and water concentration quantified. The - 1 1 regressed parameters were a pre-exponential factor of 1014 0:16 (S M- 47), an activation energy of 228±22 kJ/mol, a first-order MPA dependence, an oxygen order of 0.30±0.18, and a water order of 1.17±0.30 (all parameters to 95% confidence). A four-pathway model was also developed where MPA reacted to form the carbon-containing intermediates, CO and CH4, with subsequent reactions of CH4 to form CO and CO to form CO2. From these results, the reaction rate and oxygen dependence for each pathway was estimated. This macroscopic model can be used to predict the fate of all carbon at different operating conditions during MPA oxidation. The first step in the development of an MPA SCWO elementary reaction rate model was to accurately estimate organophosphorus thermochemistry and transition state theory reaction rates by employing ab initio calculations using the CBS-Q method in Gaussian 98. Current organophosphorus combustion mechanisms contain few MPA reaction pathways and all rate constants are estimated with high uncertainty (Korobeinichev et al., 2000; Glaude et al., 2000). Through the ab initio calculations, rate constants were estimated for newly identified reaction pathways, such as MPA hydrogen abstraction reactions and P'O(OH)2 reactions. As part of this study, calculated transition state theory rate constants were compared to estimated organophosphorus rate constants from literature combustion mechanisms to determine their accuracy. Reactions involving P-O bonds were typically overestimated by several orders of magnitude in the combustion mechanisms because the P-O bond is much stronger than similar C- O or N-O bonds. An MPA SCWO elementary reaction rate model was developed with 94 new organophosphorus reactions and 14 new organophosphorus intermediates. This model improved on previous organophosphorus combustion models by adding hydrogen abstraction reaction channels that were necessary to properly predict the experimentally measured MPA product yields. The MPA SCWO model correctly predicts the concentration profiles of MPA and its carbon-containing products at temperatures between 478 and 572 °C at stoichiometric conditions and 246 bar. The model also qualitatively predicts that MPA oxidation rates increase with increasing oxygen concentration, as found experimentally, but was unable to predict that MPA oxidation rates increase with increasing water concentration. This limitation could be due to an incomplete representation of water's role as both a solvent and a reactant in supercritical water. This model introduced new organophosphorus reaction pathways and intermediates that could also be important in other organophosphorus combustion mechanisms. Thesis Supervisor: Jefferson W. Tester Herman P. Meissner Professor of Chemical Engineering 4 Acknowledgements The best part of my time here at MIT has been the people that I've met. And to think that I almost didn't visit this school because I was convinced that I wouldn't like it here until my dad insisted that I give it a try. I was pleasantly surprised by all the nice and interesting people that I met then and I contine to be impressed by the amazing friends that I have met here and I look forward to the great things that they will all do one day. I must first thank my advisor, Jeff Tester, who provided the right mixture of guidance and freedom so that I could define my project and pursue interesting avenues. Jeff treats all of his students like we're family and he creates a friendly group atmosphere that makes working much easier and more enjoyable. I must also thank my thesis committee for all of their helpful insight to my project. The members always brought up interesting ideas and questions during our meetings which really helped me along the way. I need to especially thank Bill Green for patiently answering all my modeling questions and concerns. I would not have been able to develop the model without his expertise and guidance. In addition, I also need to thank Sumathy Raman for all the help and time that she put into the ab initio study with me. She is a wonderfully kind and helpful teacher who is always willing to answer the silliest questions. I also need to thank the past and present members of the Tester group with whom I spent countless hours in the basement, including Josh, Mike, Paul, Murray, Jason, Heather, Brian, Chad, Russ, and Anish (fellow basement dweller). We always had the right mix of discussing football teams one second and someone's research problems the next second. I must wish the best of luck to Jason and Russ, who are carrying on the SCWO torch in the laboratory. Along with all of my friends in the basement, I must also thank my friends at MIT, Kim, April, Kevin, Paul, Ian, and sometimes Ley, who came and rescued me from the reactor or Chemkin to make sure that I ate lunch most days. It will be hard to leave here and no longer have the "lunch knock" anymore. Finally, I need to thank my family (Pat, Judy, Sean, and Meghan) who have always been there to support me through all of life's adventures. As I finish up grad school here, I only-hope that Sean has