Theses and Dissertations from UMD
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Item CARBON MOLECULAR SIEVE HOLLOW FIBER MEMBRANE REACTORS FOR PROPANE DEHYDROGENATION(2023) Liu, Lu; Zhang, Chen; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Propylene (C3H6) is a crucial petrochemical feedstock for a number of bulk chemicals and polymers. While steam cracking currently dominates C3H6 production, propane (C3H8) dehydrogenation (PDH) has been increasingly practiced addressing the gap between C3H6 demand and production. The C3H8 conversion of PDH reaction is challenged by thermodynamic limitation and catalyst deactivation at elevated reaction temperature. Membrane reactors can address both challenges and hence enhance the energy efficiency of PDH by achieving attractive and stable C3H8 conversion at low reaction temperature via selective removal of the hydrogen (H2) product to shift the reaction equilibrium. Large-scale practice of PDH membrane reactors has not occurred due to the lack of scalable membranes that can provide attractive H2/C3H8 separation performance at PDH conditions. Carbon molecular sieve (CMS) hollow fiber membranes are a class of tunable and scalable inorganic membranes that are stable under non-oxidative high-temperature conditions, and therefore are potentially promising for PDH membrane reactors.This PhD dissertation aims to investigate low-temperature propane dehydrogenation in novel membrane reactors comprising asymmetric CMS hollow fiber membranes. First, asymmetric polyimide-derived CMS hollow fiber membranes were fabricated and their high-temperature H2/C3H8 performance was assessed. The roles of membrane pyrolysis temperature, permeation temperature, and feed composition on high-temperature H2/C3H8 separation performance were systematically investigated. The effects of high-temperature H2 and C3H6 exposure on CMS pore structure and transport properties were also examined. Under a continuous permeation test (~130 hours) of H2/C3H8 feed mixture at 600 oC, the asymmetric CMS hollow fiber membranes showed stable separation performance with outstanding H2 permeance of 430 GPU and H2/C3H8 separation factor of 511 exceeding those of microporous oxide membranes. H2-permeable CMS hollow fiber membrane reactors were created using the asymmetric CMS hollow fiber membranes and platinum-based catalysts. The effects of reactor operating conditions (i.e., reaction temperature, feed space velocity, sweep flow rate, C3H8 partial pressure, number of CMS hollow fibers) on PDH performance of the CMS hollow fiber membrane reactor were studied. Due to selective removal of H2 product, the H2-permeable CMS hollow fiber membrane reactor showed up to 300% higher C3H8 conversion than equilibrium conversion. Stable performance with commercially attractive C3H8 conversion (above 30%) and high C3H6 selectivity (above 98%) were obtained in the CMS hollow fiber membrane reactor at 450 °C for over 110 hours. The CMS hollow fiber membrane reactors developed in this dissertation outperform PDH membrane reactors reported in literature by having higher conversion enhancement, lower reaction temperature, and the lowest deactivation rate. These experimental results demonstrated the attractiveness of the novel CMS hollow fiber membrane reactors for energy efficient C3H6 production. One-dimensional isothermal models were further developed by material balance to understand the cooperative reaction and separation in the CMS hollow fiber membrane reactors. The modeling results of H2-permeable CMS hollow fiber membrane reactor showed overall good agreement with experimental results. The models also demonstrated the viability of C3H6-permeable CMS hollow fiber membrane reactor and catalytic CMS hollow fiber membrane reactor, which provide valuable guidance to future development of CMS hollow fiber membrane reactors following this PhD research.Item WATER-ENERGY-CLIMATE NEXUS: INTERDEPENDANCIES AND TRADEOFFS, AND IMPLICATIONS FOR STRATEGIC RESOURCE PLANNING(2017) Liu, Lu; Forman, Barton; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The water-energy nexus has been an active area of research in recent decades and has been explored in many different directions pertaining to its core. It is imperative to manage water and energy in a holistic approach as there are critical interconnections between the two systems. Climate change is an intrinsic environmental variable that has vital implications for the study of water-energy nexus, and hence, the term water-energy-climate nexus is used throughout the dissertation in reference to the interdependencies and tradeoffs between these systems. This dissertation is composed of three research studies under the domain of the water-energy-climate nexus, and they are interconnected through the intrinsic linkages among the three systems. The first study deals with the vulnerability of U.S. thermoelectric power plants to climate change. Findings suggest that the impact of climate change is lower than in previous estimates due to the inclusion of a spatially-disaggregated representation of environmental regulations and provisional variances that temporarily relieve power plants from permit requirements. This study highlights the significance of accounting for legal constructs and underscores the effects of provisional variances along with environmental requirements. The second study demonstrates the adaptation measures taken by the U.S. energy system in the face of constraints on water availability. Results show that water availability constraints may cause substantial capital stock turnover and result in non-negligible economic costs for the western U.S. This work emphasizes the need to integrate water availability constraints into electricity capacity planning and highlights the state-level challenges to facilitate regional strategic resource planning. The last study assesses the potential of surface reservoir expansion for major river basins around the world as an adaption measure to secure a reliable water supply. Results suggest that conservation zones and future human migration will have a substantial, heterogeneous impact on the maximum amount of reservoir storage that can be expanded worldwide. Findings from this study highlight the importance of incorporating human development, land-use activities, and climate change drivers when quantifying available surface water yields and reservoir expansion potential. This dissertation takes an integrated holistic approach to examine water and energy system interrelationships, and assesses the role of climate change in reshaping the interconnectivity. The three studies are tied in to each other by identifying some of the challenges the society is facing in the water-energy-climate nexus (first study) and providing a few possible solutions in both energy supply (second study) and water supply (third study) sector. Novelty of this dissertation includes but not limited to 1) explicit representation of state-level environmental regulations pertaining to power plant operations in the U.S. 2) integrated approach that captures the interactions of energy system with other sectors of the economy; and 3) global assessment of reservoir capacity expansion potential with consideration of multiple constraints. General conclusions, along with further details, provide insights for sustainable resource planning and future research directions.Item NANO STRUCTURED MATERIALS FOR ENERGY APPLICATIONS(2017) Liu, Lu; Zachariah, Michael R; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation addressed the applications of nanostructured materials as oxygen carriers (OCs) and catalysts in poly lactic acid (PLA) thermal decomposition. In chapter 1~4, the stability and cyclibility of metal oxides and supported metal oxides as OCs were evaluated in an isothermal fixed bed reactor at different temperatures for 50 cycles with methane as fuel, up to 15h while their structural, physical and chemical properties were identified using XRD, SEM, TEM, BET, XPS and Ar/H2-TPR. In chapter 5, aerosol synthesized Bi2O3 was found to be a useful catalyst in thermal PLA decomposition, which could lower the on-set decomposition temperature by ~75 T. The developed study protocol could be applied to various metal oxides and polymers to study their catalytic thermal decompositions as well.