Study of the decomposition of Vacuum Gas Oils-Low Density Polyethylene blends: evolution of the gases

Abstract

The growth of the production and consumption of plastics has been increased drastically in this decade. An alternative approach of the tertiary recycling is based on the fact that it is possible to mix polymers of olefinic base (polyethylene, polypropylene) and of styrene base in the usual feed of the process of fluid catalytic cracking of hydrocarbons (FCC) in the refineries, and to turn them into products in the range of gasoline. Previous studies focused on the behaviour of different samples of vacuum gas oil (VGO), polyethylene (PE) and vacuum gas oil-polyethylene blends (VGO-PE) in the thermal and catalytic pyrolysis process, showed that the pyrolysis of the blends goes through two decomposition steps, being the first step related to the elimination of the VGO fraction, and the second to the pyrolysis of PE. Moreover, the presence of catalyst mainly affects the PE decomposition. In this work, the evolution with the temperature (or time) of the composition of the gases evolved in the thermal and catalytic pyrolysis of VGO, PE and a VGO-PE blend have been studied by GC/MS. The results obtained yield valuable information for designing processes of plastic recycling by this route, where the VGO-PE mixture can be treated in a unit similar to the industrial FCC unit, since the presence of polyethylene does not considerably affect the global composition of the gases with respect to the VGO, but adds an additional decomposition step at higher temperatures. Moreover, its presence favours the formation of iso-butane, propane, and butane, which form part of the liquefied petroleum gas (LP).