Comparison of Virgin LDPE and an Industrial Multilayer Plastic Film in a Continuous Pilot-Scale Fluidized-Bed Pyrolysis Reactor

Abstract

Abstract Thermal pyrolysis of industrial multilayer plastic film was experimentally investigated in a pilot-scale fluidized-bed reactor with steam as the carrier gas. Experimental design was constructed by using two different feeds: virgin-quality LDPE and multilayer plastic (MLP) film from industry at two pyrolysis temperatures: 500 and 550 °C. The MLP consisted of LDPE, PP, PA-6, and EVOH polymers, where the polyolefin (LDPE + PP) concentration was 65 wt % and the PA-6 share was 30 wt %. Results showed that the condensable yield ranged between 53 and 84 wt %, where the maximum was achieved at T = 500 °C with MLP as the feed. The highest yield of products boiling below 350 °C was obtained at T = 550 °C with LDPE as the feed, where around 60 wt % of the raw material was converted into the light-to-middle distillate fraction. When the feed was shifted from LDPE to MLP, the condensable product distribution shifted toward heavier hydrocarbon fractions, which was hypothesized to be related to partial repolymerization, cyclization, or reforming reactions involving PA-6. Moreover, the gaseous stream showed components such as CO, CO2, HCN, and NH3, suggesting secondary reactions of PA-6. Interestingly, CO was also found with pure LDPE, which was most likely associated with the additives present in the feed. Analysis of both the organic and condensed water fractions revealed that the majority (90 wt %) of formed caprolactam from PA-6 ended up in the condensed water fraction from the steam carrier gas. Furthermore, other polar components such as carboxylic acids, acetone, and HCN were found in the water phase. Based on elemental analysis, the majority (∼60 wt %) of the analyzed nitrogen in the condensables was found in the water phase, and only 15–23 wt % was found in the organic phase. The use of steam as a carrier gas was shown to scrub the organic pyrolysis product, providing an integrated purification step by removing part of the heteroatom components. This combined with the use of a scalable fluidized-bed reactor could offer a viable chemical recycling route for hard-to-recycle plastic wastes such as multilayer films, which are not currently efficiently utilized.