ABSTRACT TURNER, TIMOTHY LAWRENCE. Modeling and Simulation of Reaction Kinetics for Biodiesel Production. (Under the direction of Richard R. Johnson and William L. Roberts) Biodiesel has emerged as a viable substitute for petroleum diesel. The fuel can be made easily from either virgin or waste vegetable oil. A common means of production is base-catalyzed transesterification. After twenty-five years of study, the description of the kinetics of transesterification for biodiesel remains controversial. There are conflicting findings as to the order of reaction, and estimates of reaction rate constants vary widely. After analyzing the prior work on kinetics, this thesis gives support to a kinetic model based on careful examination of chemical mechanisms and of competing reactions. It then describes the extensions to the model that are needed in order to build a predictive model for biodiesel reactions. A computer simulation of the rate equations, with extensions for the effect of temperature, is used to analyze the implications of the model. Modeling and Simulation of Reaction Kinetics for Biodiesel Production by Timothy Lawrence Turner A thesis submitted to the Graduate Faculty of North Carolina State University In partial fulfillment of the Requirements for the degree of Master of Science In Mechanical Engineering Raleigh, NC 2005 Approved by: _______________________________ _______________________________ Richard R. Johnson William L. Roberts Co-Chair of Advisory Committee Co-Chair of Advisory Committee _______________________________ Gregory D. Buckner DEDICATION This thesis is gratefully dedicated to my parents, Peter C. Turner and the late Betty Donovan Turner. ii PERSONAL BIOGRAPHY Timothy Lawrence Turner attended public schools in New York, New Jersey, and North Carolina. He has prior BS and MS degrees in Electrical Engineering from NC State. He worked on manual control of robotic manipulators at Jet Propulsion Laboratory from 1980 to 1982. He then worked at Research Triangle Institute from 1982 to 1993, designing and writing software for research in aircraft cockpit displays and decision support systems for rocket launch safety. He started a consulting company, Turner Engineering, in 1993, building software for scientific and research engineering applications. After taking undergraduate courses for a year in preparation, he entered the MS program in Mechanical Engineering at NC State in fall 2003, where he has focused on sustainable energy technologies. He received an NSF Graduate Research Fellowship in the spring of 2004. iii ACKNOWLEDGMENTS This work was funded by the Graduate Research Fellowship Program of the National Science Foundation. I would like to thank the members of my committee, Drs. Richard Johnson, William Roberts, and Gregory Buckner for their collaboration, guidance, and support. Other faculty members at NC State were also helpful. Dr. Philip Brown, a fellow biodiesel enthusiast, gave me the use of his organics lab, bought supplies, and talked me through the fine points of chemical analysis. I had many enlightening conversations with Drs. Laura Sremaniak, Charles Orji, and Ratna Sharma. Pedro Ordoñez of Universidad de San Carlos, Guatemala, worked with me for several months to analyze samples from ethanolysis of waste vegetable oils. At the outset of my graduate program, I worked at the NC Solar Center, where I first learned about biodiesel, and gained knowledge and experience in a variety of sustainable technologies. I am grateful to the director, Dr. Alex Hobbs, former Manager of the Alternative Fuel Vehicle Program, Kurt Creamer, and my co-workers John Garner and Morgan Crawford. I learned the nuts and bolts of fuel making from my North Carolina colleagues Rachel Burton and Leif Forer of Piedmont Biofuels in Pittsboro, and Eric Henry of T.S. Designs in Burlington Finally, I must thank my wife, Elizabeth Martinez, and our daughters Jessica, Adrienne, Amelia, and Sophia. Elizabeth had the selflessness and vision to iv recommend that I attend graduate school full time. And our daughters are an endless source of joy and discovery. v TABLE OF CONTENTS LIST OF TABLES..........................................................................................................vii LIST OF FIGURES....................................................................................................... viii LIST OF SYMBOLS, ABBREVIATIONS, AND NOMENCLATURE ..............................ix 1 INTRODUCTION................................................................................................. 1 1.1 General context ............................................................................................. 1 1.2 History of biodiesel ........................................................................................ 3 1.3 Biodiesel work at NC State............................................................................ 4 1.4 Chemical foundations of biodiesel-making.................................................... 6 1.4.1 Chemical building blocks...................................................................... 7 1.4.2 Principles of kinetics........................................................................... 12 2 KINETIC MODEL AND SIMULATION .............................................................. 17 2.1 Background.................................................................................................. 17 2.2 Komers’ kinetic model ................................................................................. 22 2.3 Analysis of prior work................................................................................... 30 3 PROPOSED EXTENSIONS TO KINETIC MODEL.......................................... 39 3.1 Re-cast state variables ................................................................................ 39 3.2 Generalization of the model......................................................................... 42 3.3 Effect of temperature ................................................................................... 45 3.4 Limiting factors............................................................................................. 53 4 POTENTIAL USES OF THE EXTENDED MODEL .......................................... 55 4.1 Predicting the product yield ......................................................................... 55 4.2 Optimizing the amount of alcohol and catalyst............................................ 55 4.3 Designing the production process ............................................................... 58 4.4 Predicting the effect of temperature ............................................................ 60 4.5 Predicting the effect of water ....................................................................... 60 4.6 Simulation of prior results ............................................................................ 61 5 SUMMARY AND CONCLUSIONS ................................................................... 63 6 REFERENCES.................................................................................................. 66 7 APPENDICES ................................................................................................... 68 7.1 PROGRAM “BODKIN”................................................................................. 68 vi LIST OF TABLES Table 1-1. Steps in base-catalyzed transesterification process.............................12 Table 2-1. Reaction rate constants by three different methods. ............................32 Table 2-2. Comparison of final concentrations by Komers and by simulation.......35 Table 2-3. Comparison of end concentrations using varying saponification rates 36 Table 3-1. Saponification rate constants form (Ishchuk 1992) ..............................46 Table 3-2. First attempt at estimating temperature dependencies ........................48 Table 3-3. Predicted temperatures at 50oC based on first attempt........................48 Table 3-4. Second attempt at estimating temperature dependencies...................51 Table 3-5. Predicted rate constants at 50oC based on second attempt ................51 Table 4-1. Comparison between two biodiesel-making recipes. ...........................56 vii LIST OF FIGURES Figure 1-1. Molecular structure of an idealized fatty acid .......................................7 Figure 1-2. Molecular structure of soap ...................................................................7 Figure 1-3. Molecular structure of glycerol...............................................................8 Figure 1-4. Molecular structure of methanol, ethanol, 1-propanol, and 1-butanol...8 Figure 1-5. Biodiesel molecules. Above is a methyl ester; below, an ethyl ester...9 Figure 1-6. Form of the ester compound. ................................................................9 Figure 1-7. Molecular structure of triglyceride........................................................10 Figure 1-8. Cetane molecule, above, versus ethyl ester, below............................10 Figure 2-1. Experimental results from (Komers 2002)...........................................32 Figure 2-2. Simulation of Komers’ experiment.......................................................34 Figure 3-1. Determination of activation energy of saponification...........................47 Figure 3-2. Temperature dependency from (Mittelbach 1990) ..............................50 Figure 3-3. Simulation of experiment from (MIttelbach 1990)................................52 Figure 4-1. Comparison of two biodiesel-making