Influence of residence time and catalyst regeneration on the pyrolysis–zeolite catalysis of oil shale

Oil shale from the Kark region of Pakistan has been pyrolysed in a fixed bed batch reactor and the properties of the derived shale oil determined. The reactor system was then modified to incorporate a second reactor where the derived vapours from oil shale pyrolysis were passed directly to the second reactor containing zeolite ZSM-5 catalyst. The influence of the process parameters of vapour residence time (VRT) over the catalyst and the regeneration of the catalyst were examined. The yield and composition of the derived gases before and after catalysis were determined. In addition, the yield and composition of the derived oil in terms of total nitrogen and sulphur content and the content of aromatic hydrocarbons in the oils was investigated. The results showed that the yield of oil after catalysis was reduced with a consequent higher yield of gases and formation of coke on the catalyst. The main gases from the pyrolysis of oil shales were CO₂, CO, H₂, CH₄, C₂H₄, C₂H₆ and C₃H₆, C₃H₈ and minor concentrations of other hydrocarbon gases. The main role of catalysis was to convert the long chain alkanes and alkenes in the oil to lower molecular weight, short chain, alkyl substituted and iso species and high concentrations of aromatic hydrocarbons. Total nitrogen and sulphur contents in the oils were markedly reduced after catalysis. This reduction was reflected in the reduced concentration of nitrogen and sulphur containing aromatic hydrocarbons. The influence of longer VRTs was to increase the formation of aromatic hydrocarbons, reduce the nitrogen, and sulphur compounds in the oils. The influence of catalyst regeneration, involving five regenerations was not significant on the yield and composition of the derived catalytically upgraded oils.