Oxidative dehydrogenation of ethane to ethylene with h2s and sulfur
Creators: name: Liu, Shunlan
Publisher: name: University of Calgary
Year: 2007

description: Bibliography: p. 135-140
description: Ethylene is a very important feedstock widely used for manufacture of polymers and solvents in modem chemical industry. Commercially ethylene is produced by steam cracking of ethane or naphtha. This process is highly endothermic and relies on the combustion of methane to satisfy its large energy demand. The consumption of methane is a growing concern. Environmentally, it is another source of CO2 emission. Economically, the rising prices of methane will result in loss of profitability of the plants. Consequently, production of ethylene by oxidative dehydrogenation (ODH) of ethane, an exothermic or mild endothermic reaction using either 02 or sulfur as oxidizing agent, has been widely investigated as an attractive alternative to the steam cracking process. The objective of this thesis is to study the chemistry of ODH of ethane with H2S/O2 combined or elemental sulfur alone as oxidant, and to establish a process that effectively produces ethylene at low energy cost. A series of studies were conducted with feed mixtures of C2HdO2/N2, C2HdH2S/O2/N2 and C2H6/S2/N2 using a tubular reactor in the temperature regime 700 - 850°C over residence time of 0.1 to 3.0 s. Quantitative product analyses were perfonned using GC and GC-MS technology with N2 as internal standard. For C2H6/O2/N2 system, ethylene yield was observed to be a function of the reaction conditions (temperature-residence time) and the feed composition. Feed composition had a great influence on ethylene formation with low C2H6/O2 ratio or high ethane partial pressure resulting in decreased ethylene selectivity and yield. The highest ethylene yield for gas phase ethane ODH was obtained using C2H6/O2 ratios from 2 to 4, at temperatures of 750 - 850°C with residence time from 3.0 s to 0.5 s for the low and high temperatures conditions respectively. However, the maximum ethylene selectivity obtained at given ethane conversions were lower than the value calculated for the steam cracking using empirically - based forn1ulae. The addition of H2S increased the selectivity to ethylene in the ODH of ethane. Overall, the maximum ethylene selectivity displayed an average ~20% increase at a given ethane conversion in the 40% to 80% range. Using a mixture of C2H6/H2 S/02/N2 at ratio of 40/10/10/30 over residence time of 0.5 s, selectivity of ethylene up to 90 % with conversion above 70% was obtained at 800°C. Examination of individual H2S/02 and S2/C2H6 reactions confirmed a two step mechanism consisting of initial oxidation ofH2S to sulfur species and oxidation of ethane to ethylene with re-formation of H2S. Elemental sulfur was observed to oxidize ethane to ethylene with high selectivity. Using a feed composition of C2H6/S2/H2S/N2 at ratio of 60/5/22/13, an ethylene selectivity up to 89% was obtained at ethane conversion of 84% at 850°C over 0.6 s. H2S appeared to be an effective dilutant for this sulfur/ethane reaction. The ethylene yield obtained with direct sulfur/ethane reaction exceeds that when obtained using a conventional steam cracking process. Reaction mechanisms are proposed to explain the results. The study on ethane/sulfur reaction presents the interesting possibility of designing an industrial process in which ethylene is co-generated in major sour gas processing facilities or refineries at a lower energy cost by utilizing sulfur and the waste heat from the Claus process.