Low-pressure oligomerization of diluted ethylene on a HZSM-5 zeolite catalyst

Abstract

The oligomerization at low pressure of diluted ethylene coming from secondary streams is an attractive route for hydrocarbon production, by means of a low cost and energy efficient process. To evaluate the viability of this process, the effect of dilution with N2 or syngas on the low-pressure oligomerization of ethylene (1.5 bar) was studied on a catalyst prepared with a HZSM-5 zeolite agglomerated in a mesoporous matrix of α- and γ-Al2O3, aiming to produce C5+ hydrocarbons (gasoline). The experiments were performed in a fixed bed reactor at 325 °C and a space time of 10.6 gcatalyst h molC−1. For an ethylene partial pressure of 0.33 bar, conversion surpassed 80 % and high C5+ hydrocarbon yield was obtained: >40 % with N2 as diluent; and, >30 % with syngas. The greater effect of syngas dilution on suppressing the formation of aromatics is explained by the role of H2 in decreasing the extent of dehydrocyclization reactions. The dilution of ethylene limits the extent of the reaction stages, but it also attenuates the stages for coke formation, by facilitating the diffusion of soft coke in the mesoporous matrix and contributing to decrease the deposition of hard coke in the zeolite micropores. Consequently, a pseudo-steady state of the catalyst is reached with a notable remnant activity for the formation of higher hydrocarbons.