Catalytic Pyrolysis of Heavy Distillate from Plasticwaste into a Diesel Range Fuel

Abstract

Heavy distillate is a heavy fraction of plastic pyrolysis characterized by high viscosity, long-chain hydrocarbons which makes it low-graded biproduct. Catalytic pyrolysis is amongst the promising upgrading techniques which has greater comparative advantage

over thermal pyrolysis in cost control and lowering reaction temperature/time. Clay- based catalyst has not been adequately assessed in enhancing HD pyrolysis product

yield and there is limited knowledge on HD thermal characteristics. The main objective of this study was to upgrade HD from plastic waste into a diesel range fuel through catalytic pyrolysis. The specific objectives were to: characterize HD from plastic waste; produce diesel-like product from HD through catalytic pyrolysis; analyse effects of catalyst ratio and operating temperatures on product yield; and characterize the properties of fuel produced. The HD was obtained from Alterative Energy Systems Limited, Thika – Kenya. The thermal properties of HD were analyzed with simultaneous thermal analyzer at heating rates (5,10,15,20 oC/min) and ascertained using modified Coats Redfern method. A central composite design - response surface methodology was employed on a batch reactor with design matrix: reaction temperatures (350,375,400 oC), heating times (90,120,150 mins) during thermal pyrolysis; catalyst ratios (5,10,15%) added during catalytic pyrolysis. Kaolin was used as a catalyst to boost product yield. Design Expert and Minitab software were employed to analyze effects of process parameters on product yield. The HD and pyrolytic liquid oils (PLOs) were characterized based on Fourier transform infrared (FTIR), physical properties, elemental and gas chromatography-mass spectroscopy (GC-MS) analyses. At respective heating rates, derivative thermogravimetric curves indicate that HD structures break at 285.77, 290.63, 327.02, 343.46 oC, and fully decompose at 365.83, 391.38, 412.42, 424.40 oC; activation energy values during decomposition phase (63.04, 61.52, 70.16, 65.57 kJ/mol) were slightly lower than heavy crude oils (50-177 kJ/mol). The FTIR spectra of HD&PLOs showed presence of symmetric and asymmetric stretching modes for methyl/CH3, methylene/CH2, and methylidyne/C-H. The oil yields (73.28, 70.13, 20.50, 28.50 wt%) obtained with catalytic pyrolysis matrix (400 oC,5%,150 min; 400 oC,15%,150 min; 350 oC,5%,150 min; 350 oC,5%,150 min) were higher than those without catalyst (18.88, 63.63 wt%) from process matrix (350 oC,150 min; 400 oC,150 min). The Pareto, normal and surface plots divulged that oil yields largely depended on temperature as compared to catalyst ratio, heating time, and factor interactions. GC-MS results established heavy carbon range (>C23) noticeably decreased in area percentages from HD (41.82 wt%) to without catalyst (10.03 wt%) and with catalyst (8.88 wt%) where more diesel range organics (C6-C23) were produced. The density (864, 779, 788 kg/m3

), viscosity (14.00, 2.63, 2.88 cSt), and calorific value (44.52, 46.62, 47.23 MJ/kg) of HD, PLOs without and with catalyst, respectively compares favorably to diesel heating value (46 MJ/kg), elemental compositions increased in carbon contents (77.21, 83.24, 84.83 wt%) and decreased in hydrogen, nitrogen, sulphur and oxygen contents. In conclusion, the kaolin had a substantial role to enhance oil yields during pyrolysis than thermal pyrolysis and oil products are potential substitute for diesels. Further studies on desulfurization and dehalogenation of PLOs are suggested to obtain better diesel fuel.