Pyrolysis microwave

Another area where microwave heating is nsed to heat carbon is in the regeneration of activated carbons. When spent activated carbon is snbject to a microwave field, the heat generated within the particles prodnces rapid temperatnre rises and the release of other componnds adsorbed on the carbon [19-23], In a similar process, carbon nsed as adsorbent to remove NO and SO from gas streams can be regenerated with microwaves prodncing CO2 and N2 as gases and elemental snlphnr [24], 

Considerable attention has been paid over the last few years to the use of microwave pyrolysis for the processing of scrap tyres. Approximately 2.5 million tonnes in North America, 2.0 million tonnes in the European Union and 0.5 million tonnes in Japan, of scrap tyres are discarded per year. As much as 50% of this waste is landfilled which is clearly causing an increasing unsustainable and unacceptable situation. Other recycling 

Examples exist of other processes, in which microwave heating of microwave-absorbents is used as a way to transfer energy to a microwave-transparent material in order to accomplish the pyrolysis of the latter. For example, the pyrolysis of chlorodifluoromethane has been carried out in a microwave-heated fluidized bed with a performance comparable to that of tubular reactors, the best traditional equipment for the pyrolysis of this compound [45]. 

The simultaneous decomposition of pentachlorophenol and regeneration of activated carbon, using microwaves was reported [46], claiming that the quality of the carbon was maintained or actually increased after several adsorption/microwave-regeneration cycles. Carbon, in graphite form, has also been used as a microwave absorbent for the microwave pyrolysis of urea [47]. 

Similarly, microwaves have been used for pyrolysis of coal, which is known to have very poor microwave absorption, by mixing it with inorganic oxides (very good microwave receptors) or with carbon. After the initial stages of pyrolysis the coal undergoes some graphitization, turning into carbon black that further absorbs microwaves [48, 49]. 

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Microwave pyrolysis of plastic (or plastic-containing) wastes is a relatively new area that has been studied only in the last decade or so. Because of this, there is considerably less information in the scientific literature compared with other approaches to the pyrolysis of plastics. Also, there is a substantial amount of information contained in patents, which suggests that the development of microwave pyrolytic processes has been more intuitive rather than strictly scientific. This chapter will summarize the developments in this area that are described both in scientific and commercial literature. 

The chapter begins by introducing the concepts behind microwave heating and the properties that make plastics transparent to this kind of radiation. This is followed by a definition of microwave pyrolysis of plastics and the ways that microwave transparency of plastics can be overcome in order to use this microwave energy as a source of heat for pyrolysis. A number of microwave pyrolytic processes for materials other than plastics are also introduced. 

Microwave pyrolysis has also been tested with materials that, even though they are dielectric, contain some molecules responsive to microwave fields and can therefore absorb microwaves, heat up and pyrolyse. Example are wood blocks [50-52] and oil shales [53]. 

Another material treated with microwave pyrolysis has been sewage sludge. Disposal of this material, which is a by-product in wastewater treatment processes, is a considerable problem and currently accounts for up to 60% of the operational cost of wastewater treatment plants. Microwave pyrolysis of sludge provides a rapid and efficient process with reduced process time and energy requirements compared with conventional pyrolysis [54]. 

Furthermore, the condensables from microwave pyrolysis contain less carcinogenic compounds than those produced in conventional pyrolysis [55] and the noncondensables have a higher concentration of CO and H2 (synthesis gas) after microwave pyrolysis than after conventional pyrolysis [56],... 

At the same time, a similar system was developed with the specific aim of performing thermogravimetric (TG) experiments during the microwave pyrolysis of plastics and plastic/aluminium laminates [26]. The schematic diagram of the apparatus developed is illustrated in Figure 21.2. 

Figure 21.2 Schematic diagram of the microwave pyrolysis thermogravimetric apparatus. First published at the 6th World Congress of Chemical Engineering...

As mentioned above, the main difference between microwave and conventional pyrolysis is the initial sonrce of thermal energy and the way this is transferred to the plastic. Nonetheless, there are other differences, particularly when microwave pyrolysis is compared with flnidized-bed pyrolysis equipment in the latter, the primary reaction prodncts are carried ont of the reactor by a hot gas stream which enables these products to take part in secondary and tertiary reactions. On the other hand, in microwave pyrolysis, once the pyrolytic prodncts leave the carbon bed, they stop receiving heat by conduction from the hot carbon and come in contact with a relatively cold carrier gas. This has an important effect in the nnmber of consecntive reactions occnrring and therefore, on the natnre of the prodncts, as is shown in Section 3.2.2. 

Due to the novelty of the microwave pyrolysis process, there are no other reports in the scientific literature, with details of equipment for the degradation of plastics. However, for the degradation of other materials, details of the apparatus utilized for the microwave pyrolysis of wood have been presented [50, 51]. 

Figure 21.4 Pyrolysis of (A) HOPE pellets, (B) HOPE powder and (C) toothpaste laminate at 550 C using a microwave pyrolysis thermogravimetric apparatus [26]. First published at the eth World Congress of Chemical Engineering...

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 results shown in Table 21.1 do not imply that microwave pyrolysis is slower than conventional pyrolysis, but confirm the need to consider heat and/or mass transfer limitations because of the particle and size samples used in the experiments [26]. 

Figure 21.5 Cumulative yield of condensable products for the microwave pyrolysis of HOPE pellets pyrolysis at (A) 500°C and (B) 600°C [85]. (Reproduced by permission of the American Chemical Society)...

Table 21.2 shows various results for product (phases) yields for the degradation of PE at 500 and 600°C along with the results obtained using microwave pyrolysis. As can be seen in the table, in the latter case the increase in temperature caused little difference in the yields of the products. These results, which seem to contradict most previous findings, may be explained by the configuration of the microwave pyrolysis equipment. 

As mentioned in Section 2.2.1, the recovery of clean aluminium from real plastic-containing wastes has been one of the main focuses of the research into microwave pyrolysis. With the semi-batch apparatus shown in Figure 21.3, experiments were performed using toothpaste tube laminate and depulped drink carton laminate (a Tetra Pak ... 

The advantage of microwave pyrolysis over conventional pyrolysis methods do not rely on changes in chemical pathways, but in the advantages that have been mentioned previously. 

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