Characterisation and Process Development Studies on Waxes Derived from Plastic Pyrolysis

Robinson, Josh and Sweeney, Joe and Doulcet, Julien (2021) Characterisation and Process Development Studies on Waxes Derived from Plastic Pyrolysis. Masters thesis, Lancaster University.

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Abstract

Pyrolysis enables plastic waste to be utilised as a resource to produce useful raw materials for further downstream processing. Light olefin gases, oils and waxes are major products of plastic pyrolysis that have applications in the manufacture of petroleum products (e.g. gasoline, diesel and paraffin wax) and polymers. Pyrolysis-derived waxes possess undesirable colour and odour properties that must be improved to produce a useful raw material. It has been the aim of this work to remove colour and odour inducing impurities from pyrolysis-derived waxes using suitable refining methods. Liquid chromatography, solvent deoiling using ethyl acetate and urea clathrate formation were investigated as methods to separate impurities from crude waxes. Crude waxes, refined waxes and extracts/fractions containing impurities were comprehensively analysed to determine: chemical functionality by FTIR and NMR, molecular weight distribution by GPC, heavy metals content by ICP and the identification of volatiles by GC-MS. Headspace GC-MS was used to analyse volatile components to identify possible odour inducing contaminants emitted by crude waxes. UVvis spectroscopy was introduced in this research as a method to assess the bulk colour characteristics of wax to identify wavelengths in which colour-contaminated wax absorbs and refined wax does not. Two suitable wax refining procedures have been proposed based on methods of separation known to be effective: solvent de-oiling and hydrogenation. Silica adsorption was used as a pre-treatment for both refining procedures to remove polar components which may be harder to remove by solvent de-oiling or hydrogenation alone. Separation of impurities was most effective using liquid chromatography and afforded a wax with the most desirable colour and odour characteristics at high mass recoveries, while solvent de-oiling and urea clathrate extraction afforded a hard wax and an oil. The hard waxes obtained by solvent de-oiling exhibited the least favourable colour characteristics and had a high oil content. Urea clathrate extraction of waxes was not possible for wax A, however, waxes B and C exhibited desirable colour characteristics after the procedure had been carried out. The main disadvantage of solvent de-oiling and urea clathrate extraction processes is the low recovery of wax due to the high oil content of pyrolysis derived waxes. The combination of solvent de-oiling using ethyl acetate and silica adsorption demonstrated that the removal of polar components by adsorption prior to solvent de-oiling improves the colour properties significantly compared to solvent de-oiling alone. Similarly, the combination of silica adsorption and hydrogenation was highly effective at decolourising the waxes. Hydrogenation successfully converted the yellow/orange oil extract (obtained by solvent deoiling of wax B) into colourless soft wax. This is advantageous compared to solvent de-oiling, however, the requirement of hydrogen gas, catalyst and catalyst regeneration/recycling process are bound to lead to increased monetary expenditure.