Pyrolysis Reactions Of Some Hydrocarbons Relevant To Substitute Natural Gas Making Processes

The pyrolytic and carbon forming reactions of a number of petroleum based feedstocks have been studied. These fractions ranged from light distillates with a final boiling point (FBP) of 120°C to an atmospheric residual oil with an FBP of ~450°. The lighter feedstocks (naphtha to gas oil) were pyrolysed at 400 to 620°C and 40 bar alone and in atmospheres of steam and hydrogen. The heavier feedstocks were pyrolysed at 40 bar in an atmosphere of hydrogen at temperatures up to 800°C. Products were analysed by GC, MS, NMR and elemental analysis. At temperatures up to 600°C steam acts as an inert diluent and conditions likely to cause carbon formation can be detected by an increase in absorbance of the liquid product at about 220nm. Below 600°C only the alkane fraction of the feedstock reacts, mostly yielding gases, but some aromatisation takes place. These aromatic species appear to the carbon precursors. An activation energy (E.) of 210 -15kJ mol was obtained for long chain n-alkanes. Experiments on heavy residual oil confirmed that aromatic species were involved in carbon formation at temperatures below 650sC -2 A large proportion of the carbon formed by these oils came from asphaltenes. These asphaltenes lost their side chains as gas while the aromatic nuclei aggregated to yield carbon. Unsubstituted aromatics did not give carbon under these conditions, but partially reduced aromatics did. Carbon formation appears to be initiated by "activated" aromatic species formed by the loss of alkyl side chains from aromatic molecules. It was possible to predict the product compositions from the hydro pyrolysis of both an aromatic and a partially hydrogenated feedstock using a simple kinetic model.