Author: Yu-Yen Chen (Ph. D. in chemical engineering)

Publisher:

Published: 2021

Total Pages: 0

ISBN-13:

Chemical looping is an effeicient technology for chemical processing. The core concept of chemical looping is to decompose a given chemical reaction into multiple sub-reactions by looping recyclable chemical intermediates. Such a scheme inherently prevents the undesired mixing between the feedstock and products so that highly concentrated products can be directly collected, yielding an economically viable process that minimizes the energy intensive penalty required for downstram gas separations and product purifications. By looping different types of intermediates, such as oxides or sulfides, chemical looping can be a versatile platform for transforming different types of feedstock into heat and electricity, syngas and hydrogen, or even value-add chemicals. The aim of this dissertation is to develop general methodologies for the design of materials and reactors in chemical looping systems through kinetic modelling under different scales. Ab initio quantum chemical calculations, specifically, density functional theory (DFT) calculations, were performed to explore the reaction pathways and energetics in three different chemical looping processes, including CH4 and CO oxidation on iron oxides, H2S splitting for H2 production using iron sulfides, and CH4 partial oxidation for syngas generation by a highly coking-resistant hybird oxide material. Detailed electronic-structure analyses were performed to elucidate the underlying physics and to identify the key reactivity descriptors, thus providing rationales at the molecular level for the future looping material design and optimization. These DFT predictions were validated by temperature-programmed reductions (TPR) and cyclic redox experiments in thermogravimetric analysis (TGA) apparatus and fix-beds. Once the optimized chemical formula is determined, the materials would be synthesized into particles with size of desired hydrodynamic properties for upscale operations. Therefore, gas-solid kinetic models in the continuum scale that simultaneously consider the chemical reactions and the gaseous reactan/product diffusion through the external gas-film and the interior of the paricle are of great importance for the reactor design and overall process optimization. Based on the experimental observations, we developed generalized and physically significant kinetic models for the redox of 1.5 mm iron-titanium composite metal oxide sperical paritcles without presuming any rate-limiting mechanisms. Our results revealed the importance of the topochemical pattern in the multistep gas-solid reactions. Methods to improve the redox kinetics based on pyhsical modifications without changing the chemical formula were proposed. To achieve a comprehensive reactor modelling, coupling of kinetics and transport properties is essential. We performed experiments in a three-phase cold-flow apparatus to simulate the hydrodynamics of solid fuels in a packed moving-bed reactor. Empirical models that describe the momentum exchange between the pneumatically transported fine powders and the packed dense-particles in movind-bed reactors was developed.