Chemical Recycling of Plastic Waste  

Project Title: Chemical Recycling of Plastic Waste

Applicant : Hong Kong University of Science and Technology

Total Approved Grant: $148,500 (Actual Expenditure: $85,552.97)

Duration: 1.5.2001 - 28.2.2002

Project Status/Remarks: completed

Scope:
To carry out a detailed evaluation of a chemical process - accelerated pyrolysis - for converting plastic waste into useful products such as fuels, solvents and lubricants.

Summary of the Findings/Outcomes:

The following are the main findings of the project:

  • at atmospheric pressure, the rates of conversion of individual plastics and mixtures increase when carbon dioxide is used as the pyrolysis gas instead of nitrogen;

  • the use of air further increases the rate of conversion but, while the resulting exothermic oxidation reactions reduce the process' energy requirements, they also increase the complexity of the products and losses to total oxidation products (water and carbon dioxide);

  • the presence of a common cracking catalyst (sodium Y-zeolite) increases the rate of degradation for three out of the four plastics (polypropylene, high- and low-density polyethylene, but not polystyrene) and both plastic mixtures examined;

  • pressure has a noticeable effect on the molecular weight distribution of the hydrocarbon products and extent of losses to coke formation;

  • typically 70% (by weight) of the plastics are converted to liquid products suitable for use as fuel, over 20% to gaseous products and less than 10% remain as residue and/or coke;

  • cleaned waste plastics and as-discarded waste plastics can both be treated by this process, the rate of conversion appears to follow the slowest of the individual components' rates and increased contamination further reduces conversion rates.
Overall:

The project successfully achieved its main aim of demonstrating that the technology is capable of converting as-discarded mixed waste plastics into more useful forms. The condensable products formed invariably consisted of complex mixtures of branched chain and aromatic hydrocarbons with a relatively wide molecular weight distribution, with actual products varying according to: waste plastic feed composition, contaminant level, operating pressure, residence time and catalyst type.

Typical Results:

For example, a predominantly polyolefinic feed, using sodium Y zeolite as the catalyst (in a mass ratio of 1:5, catalyst to plastic waste), operating at atmospheric pressure and 410°C under an atmosphere of carbon dioxide nominally results in over 70 weight percent of the original plastics being recovered as a liquid hydrocarbon mixture and approximately 20% as a gaseous hydrocarbon mixture. In such an experiment the liquid products identified were mostly branched chain paraffins and olefins (with some aromatics) in the C6 to C15 range (extending in some cases up to C20), with the most significant contribution in the C8 to C10 range. The liquid product could thus be considered as a mixture of heavy naphtha (C7 to C10), gasoline (C8 to C10) and light gas oil (C10 to C20) fractions. The gaseous products typically contain C1 to C4 parrafinic hydrocarbons with some olefins. As such, most of the liquid products formed would be ideally suited to further processing in a petrochemical refinery, with the gases used directly as fuels. Any solid or tarry residues produced have no beneficial applications other than again being burned to provide energy and regenerate the catalyst.

Effect of Contaminants:

In almost all cases, contamination of the products from contaminants in the waste plastic feed (below 2 to approximately 5 weight percent) was quite limited. Common salts remained in the reactor with the residues, indicating that corrosion could be a problem in longer-term operation. Water (both as a contaminant and produced from the breakdown of contaminants) did condense and mix with the liquid products. Although water is relatively easy to separate from hydrocarbons when it is the only polar compound present, small quantities of acids in the liquid product resulting from the breakdown of greases and food-related contaminants resulted in much greater miscibility of water-hydrocarbon mixtures, increasing the difficulty of separation and likely product losses.

Objectives achieved.

Baseline data on the effect of temperature, catalyst and pyrolysis carrier gas on the conversion of the four most common plastics in municipal solid waste - high and low density polyethylene (HDPE and LDPE), polypropylene (PP) and polystyrene (PS) - were obtained using a thermogravimetric analyser (TGA). Follow-up experiments were carried out using a bench-scale stirred batch reactor to provide data on product distributions and recoveries, as well as allowing the effect of pressure to be examined. The resulting data were used to determine appropriate conditions for the pyrolytic conversion of mixtures of these plastics in three forms: virgin plastics, cleaned waste plastics and as-discarded waste plastics. The results from this final stage indicate that the process was able to maintain operation, albeit at a reduced rate, in the presence of reasonable levels of contamination.

Objectives that cannot be fully achieved and reasons.

The economic evaluation of the potential of the technology for use in Hong Kong was not fully completed for several reasons:

  • the high cost of the land had a significant adverse impact on the profitability of constructing a plant locally - given the need to construct a relatively large plant to gain any appreciable economy of scale, use of a small plot of land under a short-term lease (such as those currently available for recycling) would not suffice;

  • based purely on the sales price for the products, the estimated process operating costs and the likely cost of construction (ignoring land costs), the technology in its existing form would find it difficult to compete against an estimated sales price of HK$ 1,300 per tonne for recycled plastics[1];

  • the size of the reactor would strongly depend on the extent of contamination, which in turn depends on how easily the plastic component of MSW can be segregated;

  • the inclusion of costs for waste collection and basic sorting or segregation would result in need to introduce a waste collection fee - both would depend on government policy.
    In view of the above, the resulting economic evaluation would be subject to a potential error margin well in excess of 100% - and thus, would be less than meaningful.

1 "Monitoring of Solid Waste in Hong Kong - Waste Statistics for 2000", HK EPD.

Remarks

Domestic waste producers currently need more incentives to help encourage them to recycle plastic and other wastes. Improving the accessibility of waste collection points is one issue that needs to be addressed, implementation of the polluter pays principle for household waste is another. In the absence of concrete steps to tackle these two issues, the low returns generated may not attract sufficient investment to sustain any substantial recycling activities in the long term.

N.B. This does not imply that market-distorting subsidies or direct government involvement are necessary.