dc.description.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. |
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