Mixed plastics waste pyrolysis

The thermal cracking of a light ffaction of mixed plastics waste was carried out in a fluidised bed reactor and the fractions obtained were analysed by elemental analysis, gas chromatography and ashing. The main components of the waste were PE and PP with a small amount of PS and the bed was fluidised by pyrolysis gas, nitrogen or preheated steam. Experiments conducted at different temperatures and residence times were compared by calculating the crack severity for each experiment. The results obtained revealed that the amounts of ethene and propene obtained by pyrolysis with steam were comparable with those obtained using a commercial steam cracker. 

ROLE OF PVC IN THE RESOURCE RECOVERY OF HYDROCARBONS FROM MIXED PLASTIC WASTES BY PYROLYSIS... 

Feedstock recycling is examined as a method of plastics recovery. The range of techno logics currently employed are described, and include pyrolysis, hydrogenation, gasification, and chemolysis. Methods for the recycling of mixed plastics wastes are discussed, which include work by BP Chemicals, VEBA Oil, Shell Chemicals and Leunawerke. 

This paper diseusses in detail the option of fluidised-bed reaetors to eraek mixed plastics waste into valuable raw materials, under the headings thermal cracking for feedstocks, pyrolysis of polyolefins, and other options. 7 refs. 

The BP-led feedstock recycling consortium recently unveiled its new larger-scale fluidised bed pyrolysis pilot plant, located on the BP refinery site at Grangemouth. The 2 tonne/day plant will take mixed plastics waste from a variety of sources to provide more extensive trial results, to be used in the conceptual design of a 25,000 t/y semicommercial demonstration plant. The consortium envisages a series of plants, of around 25,000-50,000 t/y, scattered across Europe. 

Current methods of feedstock recovery are reviewed. Brief details are given of pyrolysis, hydrogenation, gasification, and chemolysis. Activities of some European companies are briefly discussed in the areas of recycling mixed plastics waste and closed-loop recycling. 

Two types of approach exist to study the behaviour of plastic mixtures during pyrolysis. On one hand, some authors [46] work on plastic waste mixtures while other authors [47] work on simulated mixed plastic waste prepared with a specific composition. The proportions of the different polymers are based on mean values related to real waste mixtures. 

The detailed analysis of the derived oil/wax and gas products from the pyrolysis of plastics in relation to process conditions and different types of plastic is essential in providing data for the assessment of the feedstock recycling process. In addition, the yields and composition of gases and oils from the pyrolysis of mixed plastic waste are important in assessment of the process and to determine the possibility of any interactions between the plastics during pyrolysis. 

M. F. Ali, and M. N. Siddiqui, Thermal and catalytic decomposition behavior of PVC mixed plastic waste with petroleum residue. Journal of Analytical and Applied Pyrolysis, 74, 282-289, (2005). 

Thermal cracking often yields a low-value mixture of hydrocarbons with a very broad volatility range that can extend from hydrogen to coke. It is therefore important to determine the optimal pyrolysis conditions and/or the most advantageous catalyst to obtain marketable products (e.g. diesel fuel) from mixed plastic wastes. 

J. Menzel, H. Perkow and H. Sinn, Chemistry and Industry, 570-573, June (1973). W. Kaminsky and H. Rossler, Olefins from waste, ChemTech, 22, 108-113 (1992). W. Kaminsky, B. Schiesselmann and C. M. Simon, Olefins from polyolefins and mixed plastics by pyrolysis, J. Anal. App. Pyrolysis, 32, 19-27 (1995). 

J. Kim, W. Kaminsky and B. Schlesselmann, Pyrolysis of a fraction of mixed plastics wastes depleted in PVC, Journal of Analytical and Applied Pyrolysis, 40-41, 365-372 (1997). 

Only a few studies have appeared with the aim of determining the possible presence of interactions and synergistic effects during the degradation of polymer mixtures and real mixed plastic wastes. In some cases the results and conclusions of these studies are contradictory. Thus, while Wu et al.94 did not observe any interaction between the components during the pyrolysis of a mixture of HDPE, LDPE, PP, PS, ABS and PVC, other authors have observed significantly different results when degrading mixed plastics compared to the conversion of the individual polymers. Several of these works are commented on below. 

The pyrolysis is complicated by the fact that plastics show poor thermal conductivity, while the degradation of macromolecules requires considerable amounts of energy. The pyrolysis of mixed plastic wastes and used tires has been studied in melting vessels, blast furnaces, autoclaves, tube reactors, rotary kilns, cooking chambers, and fluidized bed reactors [17,18]. 

From mixed plastics, the formation of ethene, methane and hydrogen increase with temperature but the formation of propene and higher hydrocarbons decreases. Whether the isolation of olefins can compete economically with the oil cracking process remains to be seen, but the total hydrocarbon pyrolysate is similar to naphtha, the feedstock for petrochemicals production. It should be noted that because pyrolysis is primarily a non-oxidative process carried out under carefully controlled conditions, the possibility of dioxin formation is much reduced. In 1995 about 100 kT of mixed plastics wastes were converted into chemical feedstocks in Europe by pyrolysis, and this is expected to increase at the expense of incineration over the next ten years. 

TABLE 9.3 Yield of Products from Pyrolysis of Mixed Plastic Waste at 440°C (dehydrochlorination Step was at 300°C for 30Min)... 

Mechanism of pyrolytic degradation is complex involving a large set of reactions, even for a single class of plastic. Generic degradation reactions such as chain scission, H-transfer, unzipping, disproportionation, and combination occur in the process. Typical mix of products from mixed plastic waste streams with different pyrolysis conditions are shown in Table 9.3. 

Considering the preservation or saving of the material, the material recycling is the most effective procedure. The usable fractions are about 70 to 80 % [6]. Also raw-material recycling processes achieve an output of 70 up to 80 %, exceptional the pyrolysis of mixed plastics waste. Hydrogenation of a clean... 

Pyrolysis of mixed plastics waste to produce oil fractions is unfavourable because in the resulting pyrolysis oil pollutants are abducted. Especially the organic bonded chlorine is a big problem. Practically the products are not to commercialize. 

This is very similar to pyrolysis, but in this process the mixed plastic waste (MPW) is heated with hydrogen. As the molecules are cracked (the process is often termed hydrocracking), they are saturated with the hydrogen molecules to produce a saturated liquid and gaseous hydrocarbons. The synthetic crude oil produced is of a very high quality. It is necessary to keep the pressure of the hydrogen sufficient to suppress repolymerisation or the generation of undesirable by-products. 

The most suitable disposal systems for polymers in collected mixed plastics waste are pyrolysis with recovery of useful chemicals or incineration with heat energy. Degradable plastics will not interfere with these processes. 

One of the most expensive and time-consuming aspects of polymer recycling is the separation process. It would therefore make economic sense if mixed plastics waste could be taken and recycled back to suitable feedstocks. The essential feature of feedstock recycling processes is the use of heat (thermolysis) to break bonds, similar to crude oil refining. The process may be carried out by heat alone (pyrolysis), in a hydrogen atmosphere, or in the presence of oxygen (gasification). In the latter case... 

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