Catalytic partial oxidation of methane over nickel and ruthenium based catalysts for GTL applications
Abstract: The Gas to Liquids (GTL) process is an important alternative for monetizing natural gas through the production of long-chain liquid hydrocarbons, e.g. diesel fuel. The GTL process involves three main steps: synthesis gas production to obtain H2 and CO, Fischer-Tropsch synthesis to obtain a synthetic crude oil, and upgrading/refining to obtain final products. Since the synthesis gas production is the most expensive step, there is great interest in optimizing and exploring new routes for syngas production.This thesis focuses on the conversion of methane, the main component of natural gas, into synthesis gas by catalytic partial oxidation (CPO). Several aspects of the CPO reaction in the context of the GTL technology are discussed. The work contributes to an increased knowledge concerning utilizing a CPO reactor as pre-reformer in the synthesis gas production process as well as the influence of catalyst properties and composition on the catalytic behavior when using nickel and ruthenium-based catalysts in the CPO reaction.The thesis is a summary of five publications. The first two publications (Papers I and II) review the current status of both the GTL technology and the catalytic partial oxidation of methane. Paper III analyzes a process configuration comprising of a CPO pre-reformer followed by an autothermal reforming (ATR) reactor using a thermodynamic equilibrium approach. It was found that a proper manipulation of the process conditions is needed to obtain a suitable synthesis gas for GTL applications simultaneously of minimizing the risk of carbon formation in the CPO reactor; the operation of the CPO reactor demanded low O2/CH4 and H2O/CH4 feed molar ratios. Accordingly, in paper IV, the partial oxidation of methane at low O2/CH4 and H2O/CH4 ratios is investigated over nickel and ruthenium catalysts supported on MgO/MgAl2O4 and compared with a commercial nickel-based catalyst. The extent or impact of the combustion and reforming reactions along the catalytic bed are substantially influenced by catalyst properties and composition. Deactivation by carbon formation is also discussed; ruthenium-containing catalysts might positively overcome carbon formation. To gain greater insight concerning the influence of the catalyst composition and properties on carbon formation, a set of nickel and bimetallic nickel-ruthenium catalysts, supported on α-Al2O3, γ-Al2O3 and MgO/MgAl2O4, is tested in the CH4 decomposition reaction in Paper V. For these catalysts, the resistance towards carbon formation is mainly correlated with the nickel particle size.
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