RHEOLOGICAL STUDIES OF FEEDSTOCK FOR THE HYDROCRACKING OF WASTE PLASTICS

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Abstract

Hydrocracking of plastic wastes offers the best value in terms of quality of its process oil product among other feedstock recycling methods capable of recycling mixed plastic waste; a paraffin-rich synthetic crude similar in composition to gasoline and diesel is produced. Additional benefits of the process include heteroatom removal, catalyst conservation as well as a lower process temperature. However PVC content in mixed plastics waste and the high viscosity of plastics are prominent issues in relation to subjecting plastics to petrochemical processes such as hydrocracking. A 5ppm chlorine limit and maximum feedstock viscosity of 0.5 Pas at 200oC is tolerable in the petrochemical industry. Although dechlorination of mixed plastic waste has been studied exhaustively, viscosity studies in relation to process improvement or efficiency in the pyrolysis or hydrocracking of plastics haven’t received as much attention. Viscosity has been identified as being inhibitive to heat and mass transfer, and transport into reactors, as well as being a major problem in relation to designing reactors for feedstock recycling.In this research, four of the main polymer types; high-density polyethylene (HDPE), linear low-density polyethylene (LLDPE), Polypropylene (PP), Polystyrene (PS) and Polyethylene Terephthalate (PET) were rheological characterised to establish the extent to which they exceed the recommended viscosity in the petroleum industry. Viscosities 400 – 1200 times the feedstock viscosity in the petrochemical industry at a shear rate of 500s-1, which is typical for pumping and atomisation operations, were obtained during the characterisation of the plastic samples in a conventional capillary rheometer. Saturated chain hydrocarbon solvents (iso-octane, decane, tetradecane, pentadecane and hexadecane) were investigated for treating HDPE, in a range of HDPE-solvent mixtures, in order to reduce its viscosity. Preliminary results of differential scanning calorimetry tests carried out on the solvent-treated HDPE revealed a 12 – 16% drop in the melting peak temperature of the pure HDPE (129 oC) using tetradecane (108 oC), pentadecane (110 oC) and hexadecane (113 oC) for the 20:80 PE-solvent mixtures. iso-octane and decane however only produced a viscosity drop of 3% and 4% respectively for the same 20:80 PE-solvent mixtures. Thermal stability of HDPE was largely unaffected by the solvent treatment except in the case of pentadecane which showed a reducing trend on the decomposition onset temperature as solvent concentration in the starting mixtures was increased, albeit marginal (from 441oC to 437oC). A custom built sealed-vessel impeller viscometer designed to facilitate the treatment of the HDPE via solvent refluxing and in situ viscosity measurement was calibrated by determining constants which enable the conversion of machine data to viscosity and shear rate using Newtonian and non-Newtonian calibration fluids. These constants, the shape factor and shear rate conversion factor, were determined to be 81.03 and 22.08, respectively, with corresponding 95% confidence limits of 79.21 and 86.26, and 21.47 and 24.00. Viscosity measurements of a 40:60 PE-nC15 mixture carried out in the sealed-vessel impeller viscometer at a shear rate of between 71s-1 and 80s-1 at 95% confidence level and 250oC was 7 Pas representing approximately 200 fold reduction from the virgin HDPE measured in the conventional capillary rheometer

Keyword(s)

Feedstock recycling; Hydrocracking; Plastics waste; Recycling; Rheology; Solvents

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