Propylene and other light olefins are an important class of compounds in the petrochemical industry. Currently, the majority of the world’s propylene production comes from the cracking of hydrocarbons, where propylene is a co-product of the process. With increasing demand for propylene, interest in developing on-demand processes geared specifically towards propylene production has increased. Oxidative dehydrogenation is a process that has the potential to overcome many of the limitations of catalytic dehydrogenation.
The objective of this work is to study the reaction between propane, butane or mixtures of the two alkanes with O2 and H2S to produce propylene with high selectivity and conversion and to obtain an understanding of all aspects of the process. The experiments were carried out using various feeds containing either N2/HC, N2/HC/O2, N2/HC/H2S, N2/HC/O2/H2S (Where HC = C3H8 or C4H10) through a tubular reactor at 5-200 ms residence/contact times in the 823-1023 K temperature range. Analysis of the gases was carried out by gas chromatography.
The addition of ~ 5% H2S to a stream of C3H8 (~61% N2/36% C3H8) caused an increase in the conversion of C3H8 at 1023 K at a contact time of 35 ms. An increase in C3H6 selectivity by 7% and a decrease in C2H4 selectivity by 6% were also observed. The overall yield of C3H6 more than doubled. Addition of a catalyst enhanced the conversion of C3H8 and selectivity to C3H6 at 923 K; however, conversions at this temperature range were too low to be of industrial use. At 1023 K, thermal contributions took over and the results obtained were very similar to those obtained for the gas-phase reaction.
Addition of H2S to the reaction between C3H8 and O2 caused a significant enhancement in the selectivity towards C3H6 and a decreased selectivity towards C2H4. The presence of
propylene suggested that this reaction was not operating in a thermodynamic regime, as propylene is a partial oxidation product and in a purely thermodynamic regime, solid carbon or carbon oxides would be the most favored carbon products.
Finally, the effect of H2S on the reactions between C4H10 and O2 was studied in the gas-phase and over a vanadium catalyst, respectively. Surprisingly, the results showed a significant enhancement in the selectivity to C3H6 in the presence of H2S. In addition, increased conversion of C4H10 was also observed due to the addition of H2S to the reactant feed gas, and combined with the enhanced selectivity to C3H6, it resulted in an increased yield of C4-C3 olefins.
The reaction between C3-C4 alkanes, O2 and H2S has the potential to overcome some of the limitations of ODH by O2 alone, as reduced selectivity to carbon oxides, increased conversion level of the alkane and improved selectivity to propylene were observed. The gas-phase reactions involving H2S were observed to be efficient at higher temperatures hence removing the need for the vanadia catalyst from this system. Since all reactant gases are readily available at a Claus plant, this research opens the door to the idea of a small-scale on-demand process for propylene production using cheap raw materials that are already available on-site.