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Pentane, Hexane, and Heptane - in a Supersonic Nozzle

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Pentane, Hexane, and Heptane - in a Supersonic Nozzle Investigating the Phase Transitions of lower n-alkanes – pentane, hexane, and heptane - in a supersonic nozzle DISSERTATION Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University By Kehinde Emeka Ogunronbi, M.S. Graduate Program in Chemical Engineering The Ohio State University 2019 Dissertation Committee: Barbara E. Wyslouzil, Advisor Isamu Kusaka Nicholas Brunelli Copyright by Kehinde Emeka Ogunronbi 2019 Abstract n-Alkanes play important roles in our everyday lives, and they are basic constituents of many complex lipid molecules. A knowledge of the phase behavior of these aliphatic hydrocarbons provides insight into the behavior of any given lipid class, as there are interesting similarities between chain packing in normal alkanes and other aliphatic lipids. The importance of saturated aliphatic hydrocarbons is not limited to terrestrial applications, as interplanetary studies have shown that they are both major and minor components of the giant planet atmospheres and Saturn’s moon -Titan. Thus, in these situations, short chain alkanes can play a role similar to that of water on earth. Nucleation is a phenomenon that initiates many phase transitions, and supersonic nozzles are characterized by a large temperature gradient that results in high supersaturations and nucleation rates. Therefore, the goal of this work is to study the phase transitions of lower n-alkanes in a supersonic nozzle. First, we investigate and expand the vapor to liquid nucleation and condensation database for lower n-alkanes – pentane, hexane, and heptane - over a broad range of temperatures and partial pressures. Secondly, we apply the first and second nucleation theorems to determine the properties of the critical clusters of these chain molecules and advance our understanding of the nucleation physics. Thirdly, we study the freezing behavior of n-pentane, n-hexane, and n-heptane droplets and advance our understanding of the surface-templating effect in these short chain n-alkanes. The experimental vapor to liquid nucleation rates, at temperatures between 109 K and 168 K, for n-pentane, n-hexane and n-heptane were obtained by combining data from pressure trace measurements and small angle x-ray scattering experiments. For all n- ii alkanes, the nucleation rates increase with increase in supersaturation and decrease in temperature. Using two nozzles, the critical cluster sizes of n-heptane ranged from ~ 8 to ~12 and increased with temperature. Overall, the molecular contents of the critical clusters determined from experiments are higher than predictions from classical nucleation theory. Motivated by the success of the first nucleation rates experiments, we determined the properties of the critical clusters of short chain n-alkanes. Additional nucleation measurements (pressure trace measurements and small angle x-ray scattering) were made in another nozzle with higher expansion rates. Again, for all n-alkanes, the molecular content of the critical clusters was found to be from ~ 2 to ~ 9. Remarkable consistency was found between the critical cluster sizes determined using two and three supersonic nozzles. Furthermore, we delved into answering the question of when the surface-templating effect in the crystallization of a homologous series of n-alkanes slows down or vanishes. The freezing behavior of n-hexane and n-heptane are consistent with those of n-octane through n-decane. However, complementary results from pressure trace measurements, small angle x-ray scattering experiments, and Fourier transform infrared ray experiments suggested that freezing did not occur in n-pentane nanodroplets despite being supercooled by ~ 60 K. We resorted to molecular dynamics to rationalize these results. Thus, molecular dynamics simulations, using a slab geometry, supported our hypothesis that if the rearrangement of molecules at the surface becomes less pronounced as chain length decreases then freezing of the whole droplet may not occur on the timescale of our experiment. iii To my father, late mother, hungry kids on the planet, homeless humans, and the universe. iv Acknowledgments My first utmost sincere and unparalleled appreciation goes to my academic advisor, Dr. Barbara Ellen Wyslouzil, who has been very supportive and kind. Working with her for the past five years has shaped my approach to scientific inquiries and honed my skills in the field of chemical engineering. I have been imparted with the virtue of doggedness via working with her as she always offered help in making sure I achieved my goals. I would also like to thank my committee members - Dr. Isamu Kusaka and Dr. Nicholas Brunelli - for their constructive criticism of my research. Words cannot express how much I am happy to have them on my committee. I am also forever grateful for the inestimable assistance I received on molecular dynamics simulations from Dr. Sherwin Singer. He was always patient to explain to me the nitty-gritty of molecular dynamics simulations. The completeness of my work has been possible with the help of Dr. Soenke Seifert and Randall Winans at the Argonne National Laboratory in Illinois. Contributions of Dr. Judith Wolk in running small angle x-ray scattering (SAXS) experiments are also appreciated. My gratitude also goes to my former colleagues, Dr. Viraj Modak and Dr. Andrew Amaya, for getting me started with pressure trace measurements and Fourier transform infrared ray experiments in the aerosols research laboratory. They were always there to answer all my questions, both the smart and dumb ones. I would also like to appreciate my v former colleague (Dr. Yensil Park) and current colleagues – Kayane Dingilian, Sun Tong, and Jiaqi Luo – for making the office environment a non-toxic and conducive one for me. My appreciation also goes to my Nigerian friends at the Ohio state university – Dr. Olumuyiwa Adesoye, Ayo Olugbuyiro, Kayode Odumboni, and Sylvester Odonnell. It was fun spending some Christmas and new year holidays together with them. Finally, I want to deeply appreciate my father, my late mother of blessed memory, my siblings, my aunt, uncle, and first cousins (The Oyebodes) for their support throughout my academic journey in three continents on this planet. Knowing that I have family members who would have sleepless nights over me makes me think life might have a meaning and the universe might have a purpose. vi Vita May 2003 - Nov 2008 ………………… Bachelor of Chemical Engineering, Obafemi Awolowo University, Ile-ife, Nigeria. September 2011 – June 2013 …………… Master of Science in Chemical Engineering King Fahd University of Petroleum and Minerals, Dhahran, Kingdom of Saudi Arabia. August 2013 – June 2014 ………………. Graduate Research Associate University of South Carolina August 2014 – May 2017 ………………… Master of Science in Chemical Engineering The Ohio State University August 2014 – present ……………………. Graduate Research Associate The Ohio State University vii Publications K.E. Ogunronbi, A. Sepehri, B. Chen, and B.E. Wyslouzil “Vapor phase nucleation of the short- chain n-alkanes (n-pentane, n-hexane and n-heptane): Experiments and Monte Carlo simulations”. Journal of Chemical Physics, Volume 148, pp. 144312 (2018). Fields of Study Major Field: Chemical Engineering viii Table of Contents Abstract ............................................................................................................................ii Dedication ........................................................................................................................iv Acknowledgement ...........................................................................................................v Vita ...................................................................................................................................vii List of Tables ...................................................................................................................xiii List of Figures ..................................................................................................................xvi Chapter 1: Introduction ....................................................................................................1 1.1 Overview and Motivation ........................................................................................1 1.2 Research Objectives .................................................................................................7 1.3 Thesis Outline ..........................................................................................................8 Chapter 2: Nucleation Theory ..........................................................................................14 2.1 Vapor-Liquid Nucleation Theory ............................................................................14 2.2 Liquid-Solid Nucleation Theory ..............................................................................20 Chapter 3: Nucleation of highly Supersaturated Vapors of lower n-alkanes ...................25 3.1 Introduction ..............................................................................................................26 3.2 Experiments and Data Analysis ...............................................................................29 3.2.1 Chemicals and Physical Properties .....................................................................29 ix 3.2.2 Flow Apparatus and Supersonic nozzle ..............................................................30 3.2.3 Pressure Trace Measurements
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