Hydrocracking of mixed polymer waste, NovaCrack

Since plastics are extremely stable, decomposition in a landfill occurs over extended periods, and with the introduction of more stringent environmental regulation and rising landfill costs there is an increasing need to redirect plastic waste from landfill towards recycling options, enhancing recovery of raw materials.There are two major routes for the recycling of plastic waste; mechanical and feedstock. The most widespread approach to feedstock recycling is the pyrolysis (or cracking) of the plastic waste. However, this process requires high operating temperatures (typically 500°C – 900°C) with a subsequent large adiabatic temperature drop across the reactor (fixed bed or fluidised) which combined with catalyst deactivation results in significantprocessing issues[1,2]. Work at Manchester[3] from 1994 focussed on a fluidised bed reactor where there are advantages in terms of heat and mass transfer. HDPE cracking was carried out using pure zeolites and fresh,steam deactivated and “equilibrium” catalysts (E-Cats) with different rare earth oxides and Ni and V loadings (Table 1).The effect on product distribution of zeolite type and the influence in the formulated FCC catalyst can be seen in the changing yields of paraffins and olefins (Fig. 2 and Table 2). The anticipated loss in reactivity is also seen from fresh to steam deactivated FCC catalysts and rare earth stabilisation in Cat 7/7S activities compared to Cat 1/1S. There appeared little effect in the significant increase in Ni and V loading in the ECats tested suggesting a “waste catalyst for waste recycling” strategy might be appropriate as the cost of disposal increases significantly.Before design predictions could be made, an understanding of the interface between the polymer and the catalyst must be developed. The mechanism of interaction is highly complex, with three phases (liquid polymer, solid catalyst and gaseous products), mass transfer by diffusion, convection and bulk flow as well as cracking type reactions with a large number of products. Fig. 3 shows a scanning electron micrograph (SEM) of a finelyblended mixture of high density polyethylene (HDPE) and ZSM-5 after heating from ambient to 573 K and the melted polymer can be seen to have completely “wetted” the zeolite particles (Fig. 3).A more energy neutral option to catalytic cracking of plastics is that of hydrocracking, which in the presence of a suitable catalyst not only offers the potential for the selective recovery of useful chemical fractions, but is also is tolerant of the presence of heteroatoms such as chlorine or fluorine in the plastic. The hydrocracking process offers the opportunity to produce medium chain hydrocarbons such as naphtha and diesel fuel.The focus of work on hydrocracking has been batch reactor studies on polymers or blends of polymers withcoal or vacuum gas oil (Table 3). Since 2005, work at Manchester has demonstrated that the mildly exothermic process can be carried out at much reduced temperatures (200°C–350°C) whilst maintaining production/conversion yields comparable to the cited literature values. Most importantly, significantly shorter reaction times (typically 5 mins) now make continuous processing of polymer waste a possibility (Table 3).