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In the early 1990s BP Chemicals first tested technology for feedstock recycling, using a fluid bed cracking process. Research on a laboratory scale was followed (1994) by demonstration at a continuous pilot plant scale (nominal 50 kg/hr) at BP's Grangemouth site, using mixed waste packaging plastics. The technology was further developed with some support from a Consortium of European companies (Elf Atochem, EniChem, DSM, CREED) and from APME. In 1998, BP Chemicals, VALPAK and Shanks & McEwan, set up a joint project (POLSCO), to study the feasibility of a 25 000 tonne/yr plant, including logistics infrastructure for supplying mixed plastics from Scotland. The project was seen as a remedy to the expected UK shortage in recycling capability, required to comply with EU and UK Packaging and Packaging Waste Directive.

Some preparation of the waste plastics feed is required before pyrolysis, including size reduction and removal of most nonplastics. This feed is charged into the heated fluidized bed reactor, operating at 500°C, in the absence of air. The plastics thermally crack to hydrocarbons, which leave the bed together with the fluidizing gas. Solid impurities and some coke either accumulate in the bed or are carried out as fine particles and captured by cyclones.

The decomposition of PVC leads to HCl formation, which is eventually neutralized by contacting the hot gas with solid lime, resulting in a CaCl2 fraction to be landfilled. The process shows very good results concerning the removal of chlorine. With an input of 1% Cl, the products contain ~10 ppm Cl, somewhat higher than the 5 ppm typical of refinery use. Also, metals like Pb, Cd and Sb can be removed to very low levels in the products. Tests have shown that all hydrocarbon products can be used for further treatment in refineries. The purified gas is cooled, condensing to a distillate feedstock, tested against agreed specifications before transfer to the downstream user plant. The light hydrocarbon pyrolysis gases are compressed, reheated and returned to the reactor as fluidizing gas. Part of this stream could be used as fuel gas for heating the cracking reactor, but as it is olefin-rich, recovery options were also considered. The flow scheme is illustrated in Figure 1.2.

The process flow diagram shows hydrocarbon recovery in two stages, since the heavy fraction becomes a wax at about 60°C. Once recovered, the light and heavy fractions (about 85% by weight) could be combined together for shipment to downstream refinery processing if the plastic feed is passed on as hydrocarbon liquid to the downstream plants. The balance is gaseous at ambient temperature and could be used to heat the process. In this way nearly all of the plastic is used with just solids separated as a waste product. The gas has a high content of monomers (ethylene and propylene) with only some 15% being methane.

Conceptually, the process can run in self-sufficient heating mode. In this case, overall gas calorific requirement may need a small net export or import as the product gas quality

Plastics waste i

Lime ,r

absorber

T

Fuel gas 1

Filter

  • M—». Hydrocarbon
  • Distillate feedstock

Figure 1.2 Schematic representation of the BP process. (Reproduced by permission of TNO)

varies with plastic feed specification and operating conditions. The other main utilities needed are:

  • electrical power ~60 kWh/tonne feed plastic;
  • cooling water 40 m3/tonne feed plastic;
  • steam 1.2 tonne/tonne feed.

All emissions are very low and should comply with local regulations. The cost of treatment depends on many factors such as scale, location, scope, required preparation stages, and economic parameters used. Hence, comparison of the processes is difficult. The investment costs of a plant of 25 000 tpa, located in Western Europe in 1998, were estimated as 15-20 M£. Under these conditions, a gate fee of some 250 €/tonne is necessary. For a 50000-tpa plant this gate fee could be some 150 €/tonne. These figures exclude the cost of collection and of preparation.

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