Feedstock recycling of polymers by catalytic cracking and hydrocracking using twin screw extrusion

Abstract

Plastic wastes are a pressing concern. Recent factors, such as European landfill bans, mean that old strategies for dealing with waste are no longer as practicable as they once were. Feedstock recycling provides a method of maintaining the chemical value of polymers by transforming them from waste into platform chemicals. The aim of this thesis has been the investigation of a method of feedstock recycling utilising a reactive twin screw extrusion process, with the target of producing a high-quality gasoline range product. Research into the feedstock recycling of polymers is well underway, and a wide range of reactors, feeds, conditions, and catalysts have been investigated. As such, literature was reviewed and research was focused onto important gaps. These were the use of reactive twin screw extrusion, development of an adhered heterogeneous catalyst (as opposed to using co-fed catalysts) and the recyclability of spent catalysts with analysis of the effects of polymer additives. A twin screw compounder was used as a reactor, with a gas sampling system developed in-house to allow for monitoring of product streams. Zeolites (zeolite Beta and USY), platinum loaded zeolites, and a commercial FCC catalyst were investigated as these catalysts have shown promise in earlier work. Variation in choice of catalyst and atmosphere (between N2 and 5% H2 in N2) were shown to provide some control over conversion and product slate, while outlet aperture size appeared to be a useful tool for increasing the hydrocracking characteristics of the process. Reducing the size of the aperture from 5 to 0.5mm produced increased conversions (from ~57% to ~96%) and gasoline yields (from ~38% to ~55%), at low intensity conditions (340 C, atmospheric pressure, 40 ml min-1 flowing 5% H2 in N2). When investigating the recyclability of catalysts used, acid and noble-metal modified catalysts appeared to lose some activity for polypropylene cracking within one reaction-regeneration cycle. However, they still showed good activity towards nC7 cracking, tentatively suggesting that active sites were still accessible to smaller molecules and that deactivation occurred by a loss of surface acidity. Investigation of using PEO to produce an adhered catalyst on an aluminium substrate yielded promising surfaces, with potential to further develop the catalyst by addition of a metal such as platinum. A TGA study of the activation energy of LDPE cracking over the adhered catalyst was inconclusive.

Date of Award	31 Aug 2021
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Arthur Garforth (Supervisor) & Laurence Stamford (Supervisor)