Catalytic Upgrading of Biomass Fermentation Products and Bio-methane to Energy Dense Hydrocarbon Fuels
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Since the last few decades of 20th century, studies on renewable and alternative energy sources have drawn much attention both in academic and industrial research. The public and private sectors felt the obvious need of discovering alternative energy sources other than petroleum crude due to its limited supply compared to the increasing demand with fast pace of modern civilization and industrialization worldwide. New policies and acts are being enabled to encourage the research and application of renewable and alternative energy and fuels. In transportation sector, to use the alternative and renewable energy sources without massive change of the established infrastructure, only biomass and bio-resources can be chemically converted or upgraded to liquid transportation fuel which can be used as drop-in fuel or fuel blend in conventional automobile engines. Biomass conversion to liquid oil can be done in several ways like biochemical conversion i.e. fermentation, thermo-chemical conversion i.e. pyrolysis etc. In this study biomass fermentation products such as alcohol ketone and carboxylic acid mixtures and also bio-methane or shale gas are catalytically upgraded in a thermochemical conversion process to produce energy dense higher hydrocarbon molecules that can be used as liquid fuel or energy sources. Acetone-butanol-ethanol (ABE) mixture, containing 62.9 wt% n-butanol, 29.3 wt.% acetone and 7.8 wt.% ethanol, can be produced from biomass through the well-established ABE fermentation process using genetically-modified Clostridium acetobutylicum. In Chapter 2, the catalytic dehydration reactions of ABE mixture are studied to deoxygenate the mixture. Feed of ABE mixture was preheated and pumped through a catalytic packed bed tubular reactor in a continuous process at pressures of 3-6 bars. Experiments were run at different operating temperatures and feed flow rates to investigate the effect on the dehydration products, which are mixtures of three phases: (1) a gas phase consisting of light hydrocarbons and carbon dioxide, (2) an organic liquid phase consisting of heavy hydrocarbons, and (3) an aqueous phase with dissolved oxygenated hydrocarbons. The conversion was examined on two different catalysts: an alumina (γ-Al2O3) and a zeolite (ZSM-5). The dehydration products from the ABE mixture were mostly unsaturated hydrocarbon chains in the range of C2-C16. Based on the higher heating values (HHV) of the liquid products and infra-red spectra of the gas products, it can be concluded that the products from the ABE feedstock were different than those from the individual components, which suggests a cross reactivity of the components during the reaction. HHV of the liquid product increased with a decrease in the feed flow rate, and γ-Al2O3 catalyst was found to perform better than ZSM-5 for getting a good conversion of ABE in terms of liquid product energy content at a moderate reaction time. The gaseous product contained mostly 1-butene and its isomers and some other lighter unsaturated hydrocarbon gases. This gas stream was used as the extractant in Chapter 3 to study the separation of ABE components from dilute aqueous solution having the same concentration of the fermentation broth. Chemically pure 1-butene gas was liquefied in a pressure vessel where direct liquid-liquid extraction takes place and mole based distribution coefficient of 1.71 was attained for n-butanol single component extraction. For acetone, ethanol and butyric acid extraction separately from aqueous solvent the distribution coefficients were lower. To compare the separation efficiency a two-step approach of adsorption on activated charcoal followed by liquid-solid extraction using 1-butene and percent recovery of each of the components, mole and mass based distribution coefficients were calculated and it was observed that other than for n-butanol, the distribution coefficients for other components increased compared to that of the direct liquid-liquid extraction process. For extraction of ABE as a mixture, a preferential extraction of n-butanol was observed over the other components in the mixture due to its least polar characteristic among the components and thus higher solubility in the organic phase. Another significant and common product in biomass fermentation processes using Clostridium genus biocatalysts is butyrates or butyric acid. For example, it is produced as a by-product with acetone-n-butanol-ethanol (ABE) mixture in the well-known ABE fermentation process. Using genetically modified microbial strain in an advanced fermentation method with integrated separation, a butyric acid concentration of as high as 60 g/l can be achieved. Butyric acid can be further catalytically deoxygenated to produce ketones and long-chain hydrocarbons. In Chapter 4 of this study, conversional efficiencies of two commercial acid catalysts γ-Al2O3 and ZrO2 are examined. For in-situ aromatization of the deoxygenation products in a single step reactor, ZSM-5 catalyst was tested in a series bed followed by γ-Al2O3 and ZrO2. Due to the amphoteric properties of having both lewis acid and basic sites and also stronger aprotonic acid sites due to higher concentration of oxygen in the molecule, ZrO2 has much superior deoxygenation activity than γ-Al2O3, as former showed above 90% conversion of butyric acid to high-energy organic liquids; for example, the higher heating value (HHV) of the organic liquid product is 36 kJ/g for the deoxygenation at 400°C. The composition of the liquid product depends upon the temperature and weight-hourly-space-velocity (WHSV), and the heavy hydrocarbons can be produced in a single step though the yield decreases with increase in temperature. Almost equal amounts of n-heptanone and aromatic components are produced when a series packed bed of ZrO2 and ZSM-5 is used. An optimum condition for a series bed of ZrO2 and ZSM-5 catalysts has been determined to produce a mixture of energy-dense hydrocarbons and aromatics directly from butyric acid. In Chapter 5 of this study catalytic upgrading of methane from bio resources as well as from shale gas is discussed in a direct and more energy efficient route without using oxygen. In this work, a noble transition metal, ruthenium, is chosen as the catalyst with the objective of lowering the methane activation temperature, higher stability and also to have better conversion than other transition metal catalysts which are studied. The catalyst was prepared by 1.5 wt. % or 3 wt.% ruthenium loading on ZSM-5, zeolite support and on silica support separately to compare the effect of metal loading and metal support combination on the conversion process. The operating temperature was varied from 400° to 800°C. From online GC analysis and FT-IR analysis of the product gas it was observed that a sudden rise in methane conversion took place at 700°C operating temperature on 3 wt.% Ru/ZSM-5 catalyst bed and heavy hydrocarbon molecules of C4 to C10 range was produced but with a very low yield. For 1.5 wt.% Ru/ZSM-5 and 3 wt.% Ru/SiO2 catalyst beds , methane conversion were found to be low even at high temperature and no significant production of higher hydrocarbon molecules were observed. A catalyst bed of 3 wt.% Ru/SiO2 followed by pure ZSM-5 in series is also studied and the products are found to be comparable with that of 3 wt.% Ru/ZSM-5 catalyst bed with high methane conversion. The special framework structure in the ZSM-5 catalyst influenced the product molecular structure to produce cyclic higher hydrocarbon molecules after methane is activated on the surface with ruthenium metal catalyst and produced methyl radicals at above 700°C in a considerable amount. In this work as the catalysts are prepared in the lab, extensive catalyst characterization is done for both fresh and spent catalysts to determine the changes and stability. The probable future directions of continuation and improvement of the catalytic upgrading processes are discussed in a brief manner in Chapter 6 of this study.