ADSORPTION OF SUPERCRITICAL CARBON DIOXIDE ON MICROPOROUS ADSORBENTS: EXPERIMENT AND SIMULATION DISSERTATION Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University by Weihong Gao, M.S. The Ohio State University 2005 Dissertation Committee Approved by Dr. David L. Tomasko, Adviser Dr. Isamu Kusaka --------------------------- Dr. James F. Rathman Adviser Graduate Program in Chemical Engineering ABSTRACT Supercritical carbon dioxide is an efficient solvent for adsorption separations because it can potentially be used as both the carrier solvent for adsorption and the desorbent for regeneration. Recent results have demonstrated an anomalous peak or “hump” in the adsorption isotherm near the bulk critical point when adsorption isotherm is plotted as a function of bulk density. This work presents new data for adsorption and desorption of carbon dioxide on NaY zeolite over a wide range of pressures (vaccum- 2800psia) at temperatures near the critical point of carbon dioxide (32.0 to 50.0°C). The results indicate a strong affinity for CO2 as well as a significant “hump” near the critical point. The lattice model previously developed by Aranovich and Donohue is applied to correlate adsorption isotherms. The model successfully predicted the adsorption isotherms at the whole pressure range but failed to predict the adsorption “hump” near the critical point with physically meaningful parameters. To investigate this behavior in more detail, molecular simulation is executed to explore adsorption of CO2 on activated carbon and Na Y zeolite at 32.0°C. We checked the effect of pore width on the adsorption, and compared simulation with experiment data. The excess adsorption by simulation is larger than experiment data, and simulation did not catch adsorption “hump” near the critical point. ii This dissertation is dedicated to my parents. iii ACKNOWLEDGMENTS I wish to thank my adviser, Dr. Tomasko, for his patience and guidance throughout the past four years. His enthusiasm, encouragement and good humor made earning this degree an enjoyable experience. I would like to thank Dr. Kusaka for his instruction of molecular simulation. I am grateful to Derrick Butler for his contribution to modeling part of this work. I would like to thank Dr. Xueqing Wang for measuring of adsorbent surface area, Paul Matter for pretreatment of zeolite, Alissa Park for measuring of particle size distribution. I also wish to thank all SCF group members, Hongbo Li, Xiangmin Han, Yong Yang, Dehua Liu, Max Wingert, Kemi Ayodeji, and Zhihua Guo, for the happy life in this group. I am indebted to my parents, my sister, my brother, and my wife. Their support and sacrifices make this achievement possible. iv VITA September 1974……… Born - Zhejiang, China June 1995 …………… B.S., Chemical Engineering, East China University of Science and Technology, Shanghai, China. April 1998…………… M.S., Chemical Engineering, East China University of Science and Technology, Shanghai, China. 1998-2000…………… Process Engineer, Shanghai Chemical Institute, Shanghai, China 2000-2005…………… Research Assistant, The Ohio State University, Columbus, Ohio. PUBLICATIONS 1. Gao, W.; Butler, D.; Tomasko, D. L.; “High Pressure Adsorption of CO2 on Na-Y zeolite and Model Prediction of Adsorption Isotherms.” Langmuir. (2004), 20(19), 8083-8089. 2. Zhang, X.; Zhang, Ch.; Xu, G.; Gao, W.; Wu, Y.; “An Experimental Apparatus to Mimic CO2 Removal and Optimum Concentration of MDEA Aqueous Solution.” Ind. & Eng. Chem. Res (2001), 40(3), 898-901 FIELDS OF STUDY Major Field: Chemical Engineering v TABLE OF CONTENTS Page Abstract ………………………………………………………………………………….ii Dedication………………………………………………………………………..………iii Acknowledgments……………..…………………………………..…………………….iv Vita………………………………………………………...…………………….………..v List of Tables………………………………………………..…………….…….…..….viii List of Figures…………………………………………………………..……….…..…..ix Chapters: 1. Introduction………………………….…………………..…….…..……………..1 1.1 Motivation……………………………………………………………………..1 1.2 Research outline……………………………………………………………….8 1.3 Thesis outline………………………………………………………………….9 2. High pressure adsorption of CO2 on NaY zeolite and model prediction of adsorption isotherms……………………..…………………………………….10 2.1 Introduction……………………….……………………………….…...…….10 vi 2.2 Experimental………………………..…………………...…………….……..14 2.2.1 Materials……………..……………………..……….…..…….……..14 2.2.2 Pretreatment and loading……………..………...………..…………..15 2.2.3 Gravimetric adsorption apparatus……………………….…………..16 2.3 Results and discussion…………………………...………………………..…17 2.3.1 Buoyancy correction………………….………………………...……17 2.3.2 Adsorption isotherms………………………….…….…….…………19 2.4 Thermodynamic analysis and modeling……………………………………..22 2.4.1 Heat of adsorption……………………………………...…………….23 2.4.2 Ono-Kondo lattice model…………………..……………….………..27 2.5 Conclusion…….……………………………………...……………….……..39 3. Molecular simulation of supercritical CO2 on activated carbon and NaY zeolite……………………………………………..………...………..…….…….40 3.1 Introduction………………………………………….…...…..……..………..40 3.2 Algorithm and modeling……………………………..……………....………43 3.3 Results and discussion…………………………………..………..….………47 3.4 Conclusion………………………………………………..………….………76 vii 4. Summary and recommendations……………………………………..………..77 4.1 Summary……………………………………………….…………..…….…..77 4.2 Recommendations……………………………………….………..….………78 Appendix A……………………………………………………………..……………….81 Appendix B…………………………...…………………………...………….…………98 References……………………………………………………………..………..…….107 viii LIST OF TABLES Table Page 2.1 NaY Zeolite Properties………………..………..….…….………………...…….14 2.2 Adsorbed Phase Properties Calculated from High Pressure Adsorption Data.….27 2.3 Result of fitting all 4 Parameters in the Adsorption Model……..…….…………30 2.4 Results of fitting only εA and am in the Adsorption Model (2.5 parameters)….…32 3.1 Potential parameters for NaY framework…………………..………………..…..47 ix LIST OF FIGURES Figure Page 2.1 Zeolite Pore Size Distribution……………………………………………………15 2.2. Calculated Volume Difference from Helium Adsorption as a Function of Pressure…………………………………………………………………………..18 2.3. Adsorption-Desorption Isotherms of CO2 on NaY Zeolite, CO2 Density (line) is Plotted on the Right Axis………………………………………………………...20 2.4 Excess Adsorption as a Function of CO2 Density showing Anomalous “Hump”. (Open symbols represent data from Hocker et al)…………………………...…..21 2.5 Isosteric Heat of Adsorption Calculated directly from Adsorption Data……..…25 2.6 Total adsorption as a Function of CO2 Density (Open symbols represent data from Hocker et al)…………………………………………………………...…...……26 2.7 Bimodal micropore model of adsorbent…………………………………...…….31 2.8 Comparison of the lattice model (2 and 3 layers in S1 and S2 respectively) with experimental data. (Globally fitted)…………………………….….…………….34 x 2.9 Comparison of the lattice model (2 and7 layers in S1 and S2 respectively) with experimental data. (Globally fitted)…………………………………….……….35 2.10 Comparison of the lattice model (2% external surface) with experimental data. (Globally fitted)……………………………………………………....………….37 2.11 Comparison of the lattice model (10% external surface) with experimental data. (Globally fitted)………………………………………………………….………38 3.1 Framework of NaY zeolite (Jaramillo, Grey et al. 2001)…………….………….46 3.2 Gas-liquid coexistence curve…………………………………………………….49 3.3 Density of CO2 fluid at 32.0ºC ……………………………………………...…..50 3.4 Compressibility of CO2 fluid at 32.0ºC ………………………...……………….51 3.5 Pore size distribution of activated carbon……………………………..…………53 3.6 Adsorption simulation on carbon wall with different size (P=933.6 psia, H=14.73σ, t=32.0 ºC)………………………………………………..…….…….54 3.7 Adsorbate density inside carbon slit……………………………………………..55 3.8 Excess density inside carbon slit..………………………………………………..57 3.9 Excess adsorption inside carbon slit (T*=1.0028)……………………………….59 3.10 Excess adsorption inside carbon slit (T*=1.029)…………………...……………60 3.11 Excess adsorption inside carbon slit (T*=1.093)..……………………………….61 3.12 Density probability inside carbon slit……………………………………………63 3.13 Simulation system of pore fluid……………....…………………………….……64 3.14 Compressibility of pore fluid (T*=1.0028)………………………………………65 3.15 Density of pore fluid (T*=1.0028)……………………………………………….66 xi 3.16 Compressibility of pore fluid (T*=1.093)………………………………………..68 3.17 Density of pore fluid (T*=1.093)………………………………………………...69 3.18 Excess adsorption of CO2 in carbon slit pore with width of 60σ………………...70 3.19 Excess adsorption of CO2 on activated carbon with 2% mesopores (32.0ºC)…...71 3.20 Excess adsorption of CO2 on activated carbon with 5% mesopores (32.0ºC)…...73 3.21 Excess adsorption of CO2 on activated carbon (40.0ºC)………………………...74 3.22 Excess adsorption of CO2 on NaY zeolite (32.0ºC)……………………………..75 xii CHAPTER 1 INTRODUCTION 1.1 Motivation When the temperature and pressure of a fluid is above its critical temperature and critical pressure, the fluid is at supercritical state. Supercritical fluids have both liquid- like densities and gas-like transport properties, and minor adjustments of pressure and temperature may bring big changes in those properties. The viscosity of supercritical fluid is as low as that of gas and the solute diffusion coefficient is usually an order of magnitude higher than that in liquid. Also, liquid-like density makes the supercritical fluid a strong solvent for some compounds. Since the density near critical point is adjustable, the solubility can be changed significantly. With those properties, supercritical fluids are widely used in research and industry (McHugh and Krukonis 1993; Sihvonen 1999).