Pyrolysis/pyrolytic degradation

This book provides an overview of the science and technology of pyrolysis of waste plastics. The book will describe the types of plastics that are suitable for pyrolysis recycling, the mechanism of pyrolytic degradation of various plastics, characterization of the pyrolysis products and details of commercially mature pyrolysis technologies. 

A study of the cumulative yields of volatile products at various temperatures showed that, after pyrolysis for 18 hours at 156 and 188°, although water was the main product from the starches, carbon dioxide and carbon monoxide were also formed. Limited, pyrolytic degradation must, therefore, have occurred at these temperatures. There was a large increase in all three products at 218.6°, indicating that major decomposition occurs near this temperature. In contrast, cellulose did not form comparable quantities of carbon dioxide and carbon monoxide until temperatures of 260-270° were reached. 

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. 

Pyrolysis kinetics have been carried out on poly-L-lactone salts using TGA linked to various methods such as NMR spectroscopy, gas chromatography and pyrolysis-gas chromatography-mass spectrometry to identify volatile decomposition products. The effect of the end structures on pyrolysis kinetics was examined and the mechanisms of pyrolytic degradation for both polymers identified. 

Investigation of the pyrolytic degradation of ion exchange resins by means of foil pulse pyrolysis coupled with gas chromatography/mass spectrometry. Sep. Sci. TechnoL, 28, 653 —673. 

Figure 13.22 GC total ion chromatogram (TIC) of the pyrolysis products from a 0.04 mg sample of a 54.4 KDa monomodal crosslinked PDMS elastomer. The products of pyrolytic degradation are labeled D3 to D15 (cyclic siloxanes) and i-vii (misc. small molecule, branched and linear species).

The performance of a novel microwave-induced pyrolysis process was evaluated by studying the degradation of HDPE and aluminiutn/polymer laminates in a semibatch bench-scale apparatus. The relationship between temperature, residence time of the pyrolytic products in the reactor, and the chemical composition of the hydrocarbon fraction produced was investigated. 28 refs. 

The experiments illustrated in Figure 21.4 however, were carried out with 4 g of material because, as was mentioned before, the aim was not to elucidate the reaction pathway or the kinetics parameters of the pyrolytic reaction, but to provide know how about the microwave pyrolysis process. Therefore as can be seen in the figure, the fastest degradation was achieved with the laminate because of its smaller thickness (plastic layer 90-150 p,m) in comparison with the average diameter of the HOPE powder (150 p,m) and pellets (3 mm diameter, 1 mm high). 

The aromatization of streptamine by pyrolysis supported the evidence for the cyclic carbon skeleton, although it is to be noted that structural conclusions drawn from pyrolytic data are ordinarily open to question. Since the aromatization was achieved without elimination of nitrogen, the degradation also yielded information regarding the orientation of the nitrogen atoms. It was found that when hexaacetylstreptamine was heated in a sealed tube at 350 for one hour, 3.5 moles of acetic acid were produced and two crystalline products were obtained. One of the products was shown to be identical with 2,4-diacetamidophenol (X). ... 

In wood pyrolysis, it is known that several parameters influence the yield of pyrolytic oil and its composition. Among these parameters, wood composition, heating rate, pressure, moisture content, presence of catalyst, particle size and combined effects of these variables are known to be important. The thermal degradation of wood starts with free water evaporation. This endothermic process takes place at 120 to 150 C, followed by several exothermic reactions at 200 to 250°C, 280 to 320 C, and around 400 C, corresponding to the thermal degradation of hemicelluloses, cellulose, and lignin respectively. In addition to the extractives, the biomass pyrolytic liquid product represents a proportional combination of pyrolysates from cellulose, hemicelluloses. 

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