Thermal reactions of aromatic hydrocarbons in the pyrolysis of ethane and propane

Roman Dimitriu,Flavian Cuiban,Mikael Begtrup

doi:10.3998/ark.5550190.0003.211

Abstract

The influence of aromatics on the pyrolysis of ethane and propane was studied using benzene, toluene, α-methylnaphthalene and anthracene as model compounds. The experiments were performed in a tubular flow reactor at ordinary pressure, temperatures of 700-850°C, residence times between 0.1-1s and low concentration of aromatics (1-6%mole). Aromatic hydrocarbons inhibited the pyrolysis rate of ethane and propane and the following order of inhibitory effect - which decreases with the increase of temperature - was found: toluene > α-methylnaphthalene > benzene > anthracene. The C1-C4 composition of the effluent was only slightly influenced by the presence of aromatics, which were found to suffer an appreciable transformation to alkylated derivatives, depending on their structure and despite their thermal stability when highly diluted with N2 at the same temperatures. The transformation of the aromatic hydrocarbons was explained by the interference of the main chain propagators (·H, ·CH3, ·C2H3, ·C2H5) with the aromatic and benzyl type radicals. Benzene has only a small contribution to soot formation, a phenomenon which however was found to increase when alkylaromatic and polycyclic hydrocarbons are present in the feed.

Highlights

Industrial pyrolysis of alkanes in order to obtain low olefins is always performed in the presence of variable quantities of aromatic hydrocarbons, which can originate from the feed and/or from secondary pyrolytic reactions
Our preliminary attempts revealed that the aromatic hydrocarbons we took as a model do not suffer perceptible change by heating them in nitrogen atmosphere (95%) at the temperature used for alkane pyrolysis; only a slight toluene dealkylation (0.5%) was observed for high temperature and long reaction time, as opposed to the transformation of benzene and toluene which take place without using an inert gas dilution, when diphenyl and diphenylethane were formed
The ethane yield was slightly increased in the case of propane pyrolysis with benzene or toluene, as it can be observed from figure 1

Summary

Introduction

Industrial pyrolysis of alkanes in order to obtain low olefins is always performed in the presence of variable quantities of aromatic hydrocarbons, which can originate from the feed and/or from secondary pyrolytic reactions. The yield of higher aromatic hydrocarbons in case of the propane-toluene mixture is much greater than for the ethane-toluene pyrolysis, as shown in table 2.

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Results

Conclusion