PhD: Valorization of polymers as a secondary raw material

Karmina Miteva
Faculty of Technology and Metallurgy, Ss Cyril and Methodius Univeristy, Skopje, Macedonia
November, 2017
 

Abstract

The pyrolysis/thermolysis is an important alternative method for chemical recycling of waste polyolefines. To optimize the process of pyrolysis of polymers, the knowledge of thermal and catalytic degradation kinetics is needed and in this work it is studied by thermogravimetric analysis (TGA).The basic kinetic parameters such as activation energy, reaction order and pre-exponential factor are determined, under non-isothermal conditions at different heating rates (3-20 ºC min–1).The pyrolysis process occurred in a one-step decomposition between 400 ºC and 500 ºC. The values of kinetic parameters have been obtained, assuming a first order reaction kinetic. Coast-Redfern integral fitting kinetic model and five model-free methods, proposed by Flynn–Wall–Ozawa, Kissinger–Akahira–Sunose, Friedman, Dynamic and Kissinger were used to analyze non-isothermal solid-state kinetic data from TGA. The activation energy is in the range of 268-322 kJ for thermal and 195-302 kJ for catalytic degradation with 2 wt % ZSM-5 catalyst. Adding a ZSM-5 considerably decrease the Ea, so for 5 wt % the value is in the range of 177- 289 kJ and for 8 %wt it is 157-279 kJ.
In order produceing a high yield of liquid product, the investigation of thermal and catalytic pyrolysis of waste polyolefine mixture takes place in a stainless steel semi-batch reactor. Different types of catalysts, natural (opalized silicate tuff and SiO2) synthetic (Al2O3 and ZSM-5) and their mixtures, were used. The waste mixture was heated up to 550 ºC, at a heating rate of 10 ºC min–1. For process optimization, the ratio raw material/catalysts were varied. A high yield liquid fuel (87-92%) is the main product, accompanied by flammable gaseous and minor solid residue (<1%).
According to the obtained results the residence time, the quantity of catalysts and raw mixture have the great effect on the liquid product yield. The highest quantity of fuel oil was produced between 420-470 ºC. It was found that the greatest amount of condensed liquid products, around 92%, were formed during the catalytic degradation of plastic waste with mixture of Al2O3 and SiO2 as catalysts. The results obtained for physical properties have shown that obtained liquid fuels are gasoline and light fraction of diesel. The oil samples were characterized using FTIR and GC–MS, and the presence of different hydrocarbons, mostly alkanes and alkenes, in the pyrolytic fuel was confirmed. The exception was the fuel obtained by catalytic pyrolysis under ZSM catalyst, in which aromatic compounds such as toluene, xylene and derivate of benzene were the main components.
Also, the density and viscosity of blends, obtained by mixing commercially diesel and pyrolytic fuel, were investigated. The blends of pyrolysis oil with diesel fuel were at ratios of 1, 3, 6, 10, 20, 40, 50 and 75 %vol. Increasing the percent of pyrolytic fuel leads to density and viscosity decreasing. Based on the experimental data, an empirical first order, for density, and second-order equation, for viscosity calculation, as a function of the volume fraction of the fuel, was proposed. There is excellent agreement between the measured values of density and viscosity with estimated values proposed by empirical equations. The density and the kinematic viscosity of pyrolytic fuel and its mixtures are insignificantly lower than the minimum value of diesel fuel according to ASTM and EN 590 standards. The mixing of the fuels, produced by pyrolysis of the polyolefins under the investigated reaction conditions, with commercial diesel represents a promising method for valorisation of the solid plastic waste.