INERT GAS DILUTION EFFECT ON FLAMMABILITY LIMITS OF HYDROCARBON MIXTURES A Dissertation by FUMAN ZHAO Submitted to the Office of Graduate Studies of Texas A&M University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY December 2011 Major Subject: Chemical Engineering INERT GAS DILUTION EFFECT ON FLAMMABILITY LIMITS OF HYDROCARBON MIXTURES A Dissertation by FUMAN ZHAO Submitted to the Office of Graduate Studies of Texas A&M University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Approved by: Chair of Committee, M. Sam Mannan Committee Members, Kenneth R. Hall Zhengdong Cheng Debjyoti Banerjee Head of Department, Charles Glover December 2011 Major Subject: Chemical Engineering iii ABSTRACT Inert Gas Dilution Effect on Flammability Limits of Hydrocarbon Mixtures. (December 2011) Fuman Zhao, B.S., University of Tianjin; M.S.; M.S., Texas A&M University Chair of Advisory Committee: Dr. M. Sam Mannan Flammability limit is a most significant property of substances to ensure safety of chemical processes and fuel application. Although there are numerous flammability literature data available for pure substances, for fuel mixtures these are not always available. Especially, for fuel mixture storage, operation, and transportation, inert gas inerting and blanketing have been widely applied in chemical process industries while the related date are even more scarce. Lower and upper flammability limits of hydrocarbon mixtures in air with and without additional nitrogen were measured in this research. Typically, the fuel mixture lower flammability limit almost keeps constant at different contents of added nitrogen. The fuel mixture upper flammability limit approximately linearly varies with the added nitrogen except mixtures containing ethylene. The minimum added nitrogen concentration at which lower flammability limit and upper flammability limit merge together is the minimum inerting concentration for nitrogen, roughly falling into the range of 45±10 vol % for all the tested hydrocarbon mixtures. iv Numerical analysis of inert gas dilution effect on lower flammability limit and upper flammability limit was conducted by introducing the parameter of inert gas dilution coefficient. Fuel mixture flammability limit can be quantitatively characterized using inert gas dilution coefficient plus the original Le chatelier’s law or modified Le Chatelier’s law. An extended application of calculated adiabatic flame temperature modeling was proposed to predict fuel mixture flammability limits at different inert gas loading. The modeling lower flammability limit results can represent experimental data well except the flammability nose zone close to minimum inerting concentration. Le Chatelier’s law is a well-recognized mixing rule for fuel mixture flammability limit estimation. Its application, unfortunately, is limited to lower flammability limit for accurate purpose. Here, firstly a detailed derivation was conducted on lower flammability limit to shed a light on the inherent principle residing in this rule, and then its application was evaluated at non-ambient conditions, as well as fuel mixture diluted with inert gases and varied oxygen concentrations. Results showed that this law can be extended to all these conditions. v ACKNOWLEDGEMENTS I would like to acknowledge and thank my dissertation advisor and committee chair, Dr. M. Sam Mannan, for his guidance, advice, and encouragement throughout this research project. Also, I appreciate the sharing of his experience in research and industries, which has been shedding me with numerous lights on engineering and life. I will always remember the scholarly and familial environment that he created in my graduate education in Mary Kay O’Connor Process Safety Center. I would like to express my appreciation to my committee members, Dr. Kenneth R. Hall, Dr. Zhengdong Cheng, and Dr. Debjyoti Banerjee for their supports, counsels, suggestions, availabilities and commitments. I thank Dr. Hans J. Pasman and Dr. William J. Rogers for their friendly, insightful, detailed directions and comments of my research and experiment conductions. A special thank you goes to my family for their love, support, encouragement, and patience. It is to you that I dedicate this dissertation and to whom I owe all that I have achieved. To the colleagues in Mary Kay O’Connor Process Safety Center and many friends who helped and encouraged me throughout my scholastic pursuits, your efforts and kindness will always be remembered and appreciated. vi TABLE OF CONTENTS Page ABSTRACT........................................................................................iii ACKNOWLEDGEMENTS………………………………………………………………v TABLE OF CONTENTS………………………………….…………………………….vi LIST OF FIGURES………………………………………..…………………………….xi LIST OF TABLES………....………………………….……….…………….…….…xvi CHAPTER I INTRODUCTION………………………………………………….………1 1.1 Motivations……………..…………………………………………..1 1.2 Objectives…………………………..………………………………3 1.3 Organization of dissertation…………………………………………4 II BACKGROUND.…………………………………………..…………….…6 2.1 Definition of flammability limits……………………….….…………6 2.2 Dependences of flammability limits …………………………………6 2.2.1 Flammability limit vs. temperature……………………..……7 2.2.2 Flammability limit vs. pressure………………………………8 2.2.3 Flammability limit vs. oxygen and inert gases………….…9 2.2.4 Flammability limit vs. apparatus size and shape………..…..10 2.2.5 Flammability limit vs. flame propagation direction………...11 vii CHAPTER Page 2.3 Flammability limit testing …...................…………………………12 2.3.1 Bureau of Mines method…………………...……………….13 2.3.2 ASTM methods……………………………….…………….14 2.3.3 ASHRAE method…………………….……………………..15 2.3.4 European methods……………………….………………….16 2.3.5 Counterflow burner method………………...………………16 2.4 Flammability test standardization and correction………..…….……17 2.5 Flammability limit estimation……………….……………………..18 2.5.1 Empirical correlations ……………………………………19 2.5.2 Calculated adiabatic flame temperature (CAFT) modeling………………..……………………………….….27 2.5.3 Structure group contribution (SGC) modeling…………..29 2.6 Fuel mixture flammability limit…..……………………………….34 2.6.1 Le Chatelier’s mixing rule………………………………….34 2.6.2 Calculated flame temperature modeling of fuel mixture LFL……………..………………………………...36 2.6.3 DIPPR SGC method for fuel mixture lower flammability limit…………………………….……………..38 III FLAMMABILITY APPARATUS AND EXPERIMENTAL METHOD………………………………………………………………… 39 3.1 Flammability apparatus overview….……..…………………...……39 3.1.1 Cylindrical reaction vessel………………………………...40 viii CHAPTER Page 3.1.2 Gas feeding system………………………………………....42 3.1.3 Gas mixer………………………….………………………..44 3.1.4 Gas mixture ignition system……………..…………………46 3.1.5 Data acquisition system…………………………………….47 3.2 Experiment procedures…………………...………………………....51 3.3 Combustion types in the reaction vessel…………………………..55 3.3.1 Non-propagation combustion……………………….………55 3.3.2 Flash combustion…………………………………….……..56 3.3.3 Discontinuous flame propagation……………………..……56 3.3.4 Temperate continuous flame propagation……………..……56 3.3.5 Violent continuous flame propagation…………...…………57 3.4 Flammability criterion and calibration………………………...….57 IV FUEL MIXTURE FLAMMABILITY IN AIR WITHOUT INERT GAS ADDITION………………………………………………….63 V FUEL MIXTURE FLAMMABILITY IN AIR WITH INERT GAS ADDITION …………………………………………………...…….65 5.1 Overview..……………….………………………………………….65 5.2 Experimental results..……………………………………….………67 5.3 Numerical data analysis……………...…………..……………….... 81 5.3.1 Hydrocarbon mixture LFL……………...…………………..81 5.3.2 Hydrocarbon mixture UFL……………...…………………..95 5.4 Fuel mixture MIC ……………………………...…..……………..111 ix CHAPTER Page 5.5 Conclusion……………………….………………………………114 VI CAFT MODELING ON BINARY HYDROCARBON FLAMMABILITY WITH INERT GAS DILUTION……………………116 6.1 Overview …………………….....……………………………….116 6.2 CAFT modeling on binary hydrocarbon mixture LFLs…………117 6.2.1 CAFT of pure hydrocarbon with additional nitrogen……..118 6.2.2 CAFT of binary hydrocarbon mixture with additional nitrogen …………………………………………………129 6.2.3 Binary hydrocarbon mixture LFL…….…..……………..130 6.3 CAFT modeling on binary hydrocarbon mixture UFLs…………141 VII LE CHATELIER’S LAW AND FUEL MIXTURE FLAMMABILITY……………………………………………………….149 7.1 Introduction……………………..……………...………………..149 7.2 Le Chatelier’s law on LFL…...………………………………..…..153 7.3 Le Chatelier”s law on UFL...……………………………….....…..169 7.4 Discussion………………………………………………………….171 7.5 Conclusion…………………….…………………………..…...…..172 VIII CONCLUSIONS AND FUTURE WORK…………………………...….174 8.1 Summary and conclusions………………………………….…….174 8.2 Future work………………...……………………………………..178 8.2.1 New flammability apparatus………………..……………..178 x Page 8.2.2 Combustion simulation at UFL using CHEMKIN-CFD…….181 REFERENCES………………………………………………………………………183 VITA ...…………………………………………………………………………….196 xi LIST OF FIGURES FIGURE Page 3.1 Schematic representation of experimental apparatus……..…………….…….39 3.2 Configuration of reaction vessel………………………….……………………..40 3.3 Gas feeding manifold……………………………………...…………………….44 3.4 External mixing vessel …………………………..…………………………..….45 3.5 Igniter system circuitry ………….….……………..…………..….…………….46 3.6 Igniter …………………………………………………………….……………47 3.7 Wheatstone bridge circuit used for flame detection ………………...………….49 3.8 LabVIEW data acquisition program (block diagram window)……….………50 3.9 LabVIEW data acquisition program (front panel)…………………..…………..51 3.10 Gas feeding manifold and marked controlling plug valves …………….…….52 3.11 Flame propagation temperature profiles with different methane concentrations in air: