Continuous Flow of Supercritical Phase Fluids for Nanomaterial Synthesis

Abstract

For fluids in the supercritical phase, slight variations in temperature or pressure can led to large changes in thermophysical properties. This tunability of these fluid properties provide a unique opportunity to enhance the heat transfer and mixing environment for nanomaterial synthesis. Optimizing the synthesis environment is critical to the eventual scale-up of processes that are currently performed only at laboratory scale. A core challenge in the use of supercritical fluids as reaction media arises from these large variations in thermodynamic and transport properties with small changes in temperature and pressure. Direct measurement of these properties under these conditions is difficult with the property values holding a high uncertainty. Because of this, the design and scale-up of synthesis reactors has relied on modeling. Fortunately, the improved availability of computer resources provides an important tool to aid accurate design decisions. The work presented here focuses on two aspects of the use of modeling in the supercritical reactor design problem. The first aspect involves the selection of appropriate equations of state to characterize the challenging behavior near the critical point. The second involves the use of this information to design and operate a continuous flow supercritical reactor for the synthesis of nanopartical metal organic framework materials. An important goal of the latter is the demonstration of scale-up methodologies and performance. This dissertation begins with a theoretical investigation on the impact of equation of state (EoS) selection on computational cost and accuracy for modeling the behavior of supercritical carbon dioxide (scCO2). Four EoS’s of varying accuracy ranging from the simple Ideal Gas Law to the multiparameter Helmholtz-energy EoS were studied in a robust 99-case simulation matrix for scCO2. This theoretical EoS study developed a set of guidelines for producing simulations that accurately capture heat and mass transfer. In the second part of this dissertation, these guidelines are used to design a custom counter-current mixer, leading to the first report in the literature of a continuous flow scCO2 synthesis reactor. This novel continuous flow scCO2 reactor synthesized the zirconium-based MOF UiO-66 at a production rate ten times higher than previous state-of-the-art methods. Expanding on the continuous flow scCO2 reactor work, a 25-experiment mechanistic study was completed that examined the effect of varying scCO2 temperature, reactor temperature, reactor pressure, and mole fraction of CO2 on the properties of the copper-based MOF, HKUST-1. The robustness of the simulation, design, and execution methodology was then repeated with similar success for the tunable synthesis of vanadium (IV) oxide nanoparticles in a continuous flow supercritical water (H2O) reactor. All of these approaches demonstrated the synthesis of high surface area nanoparticles in short reactor residence times.