Evaporative Drying of Cupric-Chloride Droplets in a Thermo-Chemical Cycle of Hydrogen Production by Gabriel Marin A Thesis Submitted in Partial Fulfillment of the Requirements for the Degree of Master of Applied Science in The Faculty of Engineering and Applied Science Mechanical Engineering University of Ontario Institute of Technology Oshawa April 2008 © Gabriel Marin, 2008 ISBN:978-0-494-49322-9 Abstract This thesis develops analytical and numerical solutions that predict the behavior of Cupric- Chloride droplets undergoing spraying and drying processes. Cupric-Chloride is a chemical compound with the formula CuCl2 and is present as a molten salt and slurry within the Copper-Chlorine thermo-chemical cycle for generation of hydrogen. Utilizing low-grade waste heat from nuclear or other industrial sources to assist in drying of Cupric-Chloride can result in increased efficiency of the overall process. Existing drying technologies normally operate at high temperatures that make their application to a thermo-chemical process for generation of hydrogen unattractive from the perspective of energy consumption. This thesis identifies parameters that can facilitate the utilization of low- temperature heat applied to a conventional drying process. Analytical correlations for heat and mass transfer are developed and applied to the analysis of a solution of Cupric- Chloride, subject to various drying conditions. The analysis takes a single droplet of CuCl2 solution in a continuum drying media. The validation of the model involves comparisons with experimental data from previous studies of different fluids on the basis of non- dimensional analysis. This thesis shows that it is possible to accomplish considerable drying through differentials of humidity alone. It also shows that the benefits of flashing the solution to enhance drying are relatively minor, and in some cases disadvantageous, when compared to the rate of evaporative drying in the spraying process. The study provides new information about the effects of different concentrations of water in the CuCl2 slurry drying at low air temperatures. iii TABLE OF CONTENTS Abstract ..................................................................................................................................................... iii List of Figures ............................................................................................................................................ v List of Tables ............................................................................................................................................. vi List of Symbols and Abbreviations .......................................................................................................... vii Acknowledgements ................................................................................................................................. xiii 1.0 Introduction.......................................................................................................................................... 2 1.1 Background .......................................................................................................................................... 5 1.2 Literature Review................................................................................................................................. 6 2.0 Physical Process Analysis ................................................................................................................... 20 2.1 FLASHING PROCESS .......................................................................................................................... 20 2.2 SPRAYING PROCESS .......................................................................................................................... 26 3.0 Formulation and Solution Procedure ................................................................................................ 34 3.1 INITIAL HEATING .............................................................................................................................. 34 3.2 TEMPERATURE DISTRIBUTION ........................................................................................................... 36 3.3 MASS TRANSFER PROCESS ................................................................................................................ 38 3.4 HEAT TRANSFER PROCESS ................................................................................................................ 47 3.5 KINETICS OF THE DROPLET ............................................................................................................... 48 4.0 Results and Discussion ....................................................................................................................... 52 5.0 Conclusions and Recommendations for Future Research ................................................................. 68 5.1 CONCLUSIONS .................................................................................................................................. 68 5.2 RECOMMENDATIONS FOR FUTURE RESEARCH .................................................................................... 70 References ................................................................................................................................................ 71 Appendix A .............................................................................................................................................. 75 iv List of Figures Figure 1. Simplified Schematic of a Thermo-Chemical Copper-Chlorine Cycle .................. 4 Figure 2. Pressure-Enthalpy Diagram of Flashing Process –ABCD- ................................. 21 Figure 3. Droplet Diameter vs. Fraction of Total Volume of Spray [8] .............................. 25 Figure 4. Schematic of a Co-Current Spray Dryer System ................................................ 27 Figure 5. Phase Diagram of a Spraying Process–ABCD- .................................................. 29 Figure 6. Stages of a Spray Drying Process ...................................................................... 32 Figure 7. Evaporation Rate at Critical Moisture Content .................................................. 46 Figure 8. Experimental (Nesic [24]) vs. Predicted (CCd) Drying History of Colloidal Silica in Air at 178oC .......................................................................................................... 54 o Figure 9. Droplet Evaporation Rate Vs. Changes in Relative Humidity of Air at Tdp = 75 C, V = 1.75 m/s, 3 mol H2O, Do = 1.0 mm .................................................................... 55 o Figure 10. Mass Ratio Evolution with Time Using Air at 75 C, Do = 1.0 mm, V = 1.75 m/s ................................................................................................................................. 58 o Figure 11. Change in Droplet Mass Vs. Time with Air at 75 C, RH = 50%, HR = 0.14, Tdp = 59oC, V = 1.75 m/s ................................................................................................ 59 Figure 12. Effects of Air Velocity on the Evaporation Rate with Air at 75oC, RH = 50%, 3 o mol H2O, Tdp = 59 C, HR = 0.14 kg/kg, Do = 1.0 mm ............................................... 60 Figure 13. Mass Ratio Evolution Vs. Time with Air at 75oC, RH = 50%, HR = 0.14 kg/kg, o Tdp = 59 C, Do = 1.0 mm, V = 1.75 m/s .................................................................... 62 v o Figure 14. Mass Ratio Evolution with Air at 95 C, RH = 50%, HR = 0.14 kg/kg, Tdp = o 77 C, Do = 1.0 mm, V = 1.75 m/s ............................................................................. 63 o o Figure 15. Mass Ratio Evolution at 105 C, RH = 50%, HR = 0.14 kg/kg, Tdp = 86 C, ...... 64 Figure 16. Mass Ratio Evolution Vs. Time with Air at 115oC, RH = 50%, HR = 0.14 kg/kg, o Tdp = 95 C, Do = 1.0 mm, V = 1.75 m/s .................................................................... 65 Figure 17. Droplet Mass Change vs. Ratio of Initial Droplet Temperature to Changing Air Temperature at RH = 50%, Do = 1 mm, V = 1.75 ..................................................... 66 Figure 18. Initial Water Content Vs. Total Drying Time with Air Temperatures of 75oC, 95oC, 105oC and 115oC ............................................................................................. 67 List of Tables Table 1 Characteristics of Silica, Cupric-Chloride and Air [27].……….……………….53 vi List of Symbols and Abbreviations A Surface area of droplet m2 a Calculation constant -- B Mass transfer number -- b Calculation Constant -- hLc k g Bi Biot Number = Nu k d 2k d c Concentration of solids in droplet kg CuCl2 / kg H2O CCd Cupric-Chloride Droplet -- CD Drag coefficient acting on droplet -- CDRC Characteristic Drying rate Curve -- CDRP Constant Drying Rate Period CFD Computational Fluid Dynamics -- 3 Cg Vapor concentration in air mol/m Cpg Specific heat of air J/kg . K Cp Specific heat droplet J/kg . K 3 Cs Vapor concentration on droplet surface mol/m CuCl2 Cupric-Chloride -- Dd Droplet diameter m dm Absolute mass derivative -- Do Initial droplet diameter m vii 2 Dv Diffusivity of vapor in air m /s Fb Buoyancy force on droplet N Fd Net force on droplet N FD Drag force on droplet N FDRP Falling Drying Rate Period -- Fg Gravity force on droplet N f(y) First derivative -- g Gravity acceleration m/s2 h Convective heat transfer coefficient W/m2. K H Enthalpy J/mol HR Humidity Ratio kg H2O / kg Air H2Ol Liquid water kg