Microwave pyrolysis equipment

In summary, the apparatus comprised a conventional (domestic) microwave oven with a maximum power output of 1.2 kW (1). The reactor (2) was a fused silica crucible placed in a moulded microwave-transparent insulating brick (3) that was suspended from the 

The majority of the scientific literatnre devoted to pyrolysis of plastics is focused on the development of equipment or processes having recycling as their ultimate goal. Many of these have been introdnced in previous chapters and include studies using fluidized beds [61-77], cycled-sphere reactors [78, 79], fixed-bed reactors [80, 81], rotary kilns [82], screw reactors [83] and rotating cone reactors [84]. In all these studies the chemical analysis of the pyrolysis prodncts has been an important goal in order to asses the behavionr of the pyrolysis of plastics. 

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]. 

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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. 

The literature review of microwave-assisted or induced pyrolysis of plastics follows. In this section special attention is paid to the reactor configurations used, comparing them with the configurations found on more conventional pyrolysis equipment. The most important findings produced from this research are presented, including product yield, characteristics and composition. An analysis is presented to assess whether in any example there is evidence for nonthermal microwave effects promoting the pyrolytic reactions. 

Specihcally with regard to the pyrolysis of plastics, new patents have been filed recently containing variable degrees of process description and equipment detail. For example, a process is described for the microwave pyrolysis of polymers to their constituent monomers with particular emphasis on the decomposition of poly (methylmethacrylate) (PMMA). A comprehensive list is presented of possible microwave-absorbents, including carbon black, silicon carbide, ferrites, barium titanate and sodium oxide. Furthermore, detailed descriptions of apparatus to perform the process at different scales are presented [120]. Similarly, Patent US 6,184,427 presents a process for the microwave cracking of plastics with detailed descriptions of equipment. However, as with some earlier patents, this document claims that the process is initiated by the direct action of microwaves initiating free-radical reactions on the surface of catalysts or sensitizers (i.e. microwave-absorbents) [121]. Even though the catalytic pyrolysis of plastics does involve free-radical chain reaction on the surface of catalysts, it is unlikely that the microwaves on their own are responsible for their initiation. 

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]. 

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