METHANOL PRODUCTION BY DIRECT OXIDATION OF METHANE IN A PLASMA REACTOR by RICK MOODAY, B.S. A DISSERTATION IN CHEMICAL ENGINEERING Submitted to the Graduate Faculty of Texas Tech University in Partial Fulfillment of the Requirements for the Degree of DOCTOR OF PHILOSOPHY Approved Accepted August, 1998 ACKNOWLEDGEMENTS I would like to thank Phillips Petroleum Company for their constant support during the course of my work. This research project was made possible by their generous financial assistance. I would like to extend my gratitude to Dr. Uzi Mann, for his guidance, creative ideas, encouragement, and unlimited patience. Dr. Mann ensured that the project never strayed off the correct path and often contributed well beyond the call of duty as my committee chairman. Sincere thanks are also extended to Dr. Richard W. Tock, Dr. Dominick J. Casadonte, Jr., and Dr. Lynn L. Hatfield, for their active participation in the project and technical guidance while serving on my committee. Dr. Raghu S. Narayan was extremely kind and made me feel more than welcome in the Chemical Engineering Department at Texas Tech. I would like to extend special thanks to Dr. Robert M. Bethea. My experience working for Dr. Bethea in the Unit Operations Lab re-introduced me to education and chemical engineering after being away for a time. His example and guidance have been invaluable and will continue to serve me well throughout my career. Many people contributed to making this time a success for me. Dr. James Riggs is a highly respected chemical engineer and faculty member at Texas Tech, but I acknowledge him here for his ability to play the sport of golf I must admit that he taught me much about course etiquette, scoring, foreign golf traditions ("Aussie Rules"), and temperament. I will miss our morning outings but I will not forget the significance of a "tainted par" or a "fully legitimate birdie." 11 Thanks must go out to a few of the people who helped me through the difficult times during my pursuh of this degree. My good friends Ravishankar Sethuraman, Mahesh Rege, Aashish Ahuja, Robert Ellis, Steve Tsai, Johnson Fung, Siva Natarajan, Scott Hurowitz, Coby Crawford, and Mike Barham deserve special mention. They are responsible for many good times at the office, on the golf course/football field, or at some other establishment. Everyone should be so lucky to have friends like these. Many thanks go out to Bob Spruill, Marybeth Abemathy, Tammy Low, and Kathy Womble for their unfailing support. It was a pleasure to work with such a group of professionals who always got the job done well, and on time. I must express my deepest thanks to my family and friends for their unconditional love and support throughout my life. Distance, no matter how great, has never cracked the foimdation that their love gives me. When I am reunited wdth them, it is always as if we never parted. Lastly, and mostly, I would like to thank my parents. Loving, selfless, and capable parents are and have always been my greatest earthly gift. They taught by example and never discouraged my dreams or desires. They made my education their priority and selflessly gave of themselves to that end. Only after I became a man did I begin to understand the sacrifices that they made for me and my sisters. I hope to be as strong as they one day, and I will consider myself to be successful only when I have given to my children what they have given to me. Ill TABLE OF CONTENTS ACKNOWLEDGEMENTS ii ABSTRACT viii LIST OF TABLES x LIST OF FIGURES xi CHAPTER I. INTRODUCTION 1 n. TECHNICAL BACKGROUND .... 6 2.1 Background of Natural Gas and Methanol 6 2.2 ABrief History of Methanol Synthesis 7 2.3 Commercial Methanol Synthesis ... 8 2.3.1 Synthesis Gas Preparation Techniques 8 2.3.2 Methanol Synthesis from Synthesis Gas 14 2.4 Potential Advances in Methanol Use . .20 2.4.1 Methanol as Transportation Fuel 20 2.4.2 Methanol to Facilitate the use of Methane 22 HI. LITERATURE REVIEW 24 3.1 Homogeneous Partial Oxidation ... 24 3.1.1 Effect of Reactor Walls, Additives, and Promoters 2 5 3.1.2 Kinetics and Kinetic Modeling 27 IV 3.2 Heterogeneous Catalytic Partial Oxidation 29 3.3 Methane Oxidation in the Liquid Phase 32 3.4 Methane Oxidation in Plasma Reactors 34 IV. PLASMA 36 4.1 Background ...... 36 4.1.1 Plasma in Nature .... 37 4.1.2 Potential Applications for Plasma 38 4.2 Plasma Characteristics, Generation and Uses. 39 4.2.1 Plasma States ..... 39 4.2.2 Plasma Generation .... 41 4.3 ABrief Description of the Physics of Plasmas 45 4.3.1 Criteria for Plasma Occurrence 47 V. EXPERIMENTAL SYSTEM AND PROCEDURES 50 5.1 Experimental Approach 50 5.2 Experimental Apparatus .... 52 5.2.1 Feed System ..... 52 5.2.2 Plasma Generation System ... 56 5.2.3 Reactor System . 61 5.2.4 Product Collection System ... 62 5.2.5 Product Analysis .... 63 5.3 Experimental Procedures .... 65 5.3.1 System Preparation and Warm-up 66 5.3.2 Experimental Run Procedures and Checklist. 68 5.3.3 Hazards and Emergency Procedures . 70 VI. RESULTS AND DISCUSSION 73 6.1 Preliminary Phase Experiments ... 73 6.1.1 Injection Distance .... 77 6.1.2 Water Concentration .... 80 6.1.3 Methanol Selectivity Dependence on Methane Conversion .... 80 6.2 Phase I Experiments ..... 82 6.2.1 Injection Distance 83 6.2.2 Water Concentration .... 83 6.2.3 Oxygen Concentration ... 85 6.2.4 Mixing Effects .... 88 6.2.5 Overall Performance of Phase I Experiments. 92 6.3 Phase II Experiments ..... 92 6.3.1 Injection Distance .... 94 6.3.2 OveraH Performance of Phase II Experiments 94 6.4 Phase III Experiments..... 97 6.4.1 Oxygen with Methane Stream 99 6.4.2 Oxygen Divided into Plasma and Methane Streams 100 6.5 Overall Performance Evaluation . 102 VI 6.5.1 Mixing and Flow Analysis 104 6.5.2 Material Balances 104 6.6 Sources of Error . 105 Vn. CONCLUSIONS AND RECOMMENDATIONS . 108 BIBLIOGRAPHY Ill APPENDICIES A. CALIBRATION OF EXPERIMENTAL EQUIPMENT AND INFORMATION ON SYSTEM HARDWARE 117 B. SAMPLE CALCULATIONS 123 C. RAW EXPERIMENTAL DATA 128 VII ABSTRACT Methanol is one of the most widely-produced chemicals in the world. It is a key raw material in the production of many chemicals in the petrochemical industry. Methanol also has vast potential for expanded applications as a fuel. It is currently produced by an energy intensive and expensive two step process. An economically feasible one step process could significantly reduce methanol production cost, saving millions of dollars. A methane-to-methanol process, built at remotely located methane reserves, would convert methane into a different energy form that is much easier to transport. This would make methane a much more attractive and valuable energy source. The purpose of this investigation was to evaluate the feasibility of producing methanol by direct oxidation of methane using a plasma reactor. The chemistry of methane oxidation is well understood and free radicals play a central role in methane oxidation reactions. Low pressure experiments by other researchers indicated that methanol can be produced by direct oxidation of methane in plasma reactors. However, the viability of a plasma-based methanol production process depends on its ability to convert large quantities of methane. This work was directed at plasma reactor operation near atmospheric pressure to increase the amount of material processed. The focus of this mvestigation was the design and construction of an experimental apparatus which could achieve methanol synthesis in a plasma reactor by direct oxidation of methane at atmospheric pressure. A microwave source provided the energy to generate the plasma. The system was designed to study the effects of reactant viii concentration and flow configurations on methanol production. Since high levels of methanol selectivity are the primary consideration in direct synthesis of methanol from methane, improvements in methanol selectivity were desired. The objective of the four experimental phases was to investigate reactor operating conditions and improve methanol production and selectivity. Methanol production at atmospheric pressure was demonstrated in this plasma system and steady improvements in methanol selectivity were achieved as the investigation proceeded. Experiments showed that high concentrations of water and low concentrations of oxygen improved methanol selectivity. In the last experimental phase, oxygen was divided into both reactant streams, but this approach did not improve methanol production. It was observed that higher methanol selectivities were obtained only at low methane conversions. As in other plasma studies, methanol production did not approach what would be required for commercial feasibility. IX LIST OF TABLES 5.1 Technical Characteristics of the Microwave Generation System 57 6.1 Preliminary Phase Experimental Parameters . ... 74 6.2 Phase 1 Experimental Parameters ..... 82 6.3 Phase II Experimental Parameters ..... 94 6.4 Phase III Experimental Parameters ..... 98 6.5 Reynolds Number Calculation Results (run 0213) 104 C.l Preliminary Phase and Phase I Raw Experimental Data 129 C.2 Phase II and Phase III Raw Experimental Data 133 LIST OF FIGURES 2.1 M. W. Kellogg Methanol Synthesis Loop 19 4.1 Electron Avalanche ....... 43 5.1 Schematic of General Approach 51 5.2 Schematic Diagram of Reactor Configuration and Methane Injection 53 5.3 Schematic Diagram of Experimental Apparatus ... 54 5.4 Schematic of Waveguide and Quartz Reactor Tube Configuration . 60 6.1 Schematic of Injection Distance ..... 75 6.2 Preliminary Phase-Methane Conversion Versus Injection Distance . 78 6.3 Preliminary Phase-Methanol Selectivity Versus Methane Conversion 81 6.4 Phase I-Methane Conversion Versus Injection Distance 84 6.5 Phase I-Methane Conversion Versus Oxygen Concentration.