Conventional thermal processing

Durable changes of the catalytic properties of supported platinum induced by microwave irradiation have been also recorded [29]. A drastic reduction of the time of activation (from 9 h to 10 min) was observed in the activation of NaY zeolite catalyst by microwave dehydration in comparison with conventional thermal activation [30]. The very efficient activation and regeneration of zeolites by microwave heating can be explained by the direct desorption of water molecules from zeolite by the electromagnetic field this process is independent of the temperature of the solid [31]. Interaction between the adsorbed molecules and the microwave field does not result simply in heating of the system. Desorption is much faster than in the conventional thermal process, because transport of water molecules from the inside of the zeolite pores is much faster than the usual diffusion process. 

The remote nature ofthe interaction between microwaves and the sample means that when the power is switched off the sample rapidly cools. Consequently, it is possible to use the microwave applicator in a more flexible fashion than a normal heating furnace. This becomes an important consideration in the economics of ceramics processing, since the ability to use the furnace flexibly for several processes combined with the shorter reaction times can lead to a more economic use of plant than the conventional thermal processing. 

Research conducted in the past has clearly demonstrated the effectiveness of PEF in food processing applications, especially for microbial inactivation. PEF can replace many conventional thermal processing methods to produce higher quality food products, which today consumers expect from food processors. 

In comparison with conventional thermal processes and/or other membrane desalination processes, such as evaporation and RO for example, the MD process offers the following potential benefits (El-Zanati and El-Khatib, 2007 Khayet et al, 2003) ... 

The microwave equipment consisted of a microwave generator (85 W) and a tunable cavity operating in the TE mode, while temperature was monitored using a fiber-optic temperature sensor. The samples were maintained in a Teflon vessel of 1.5 cm diameter hole and 1.5 cm deep. During a typical run, 25 W of micro-wave power was required to heat the sample to the desired temperature over 80-200 s. It was demonstrated that microwave irradiation increased the rate of solution imidization over that obtained for conventional treatment by a factor of 20-34, depending on the reaction temperature. The apparent activation energy for this imidization, determined from an Arrhenius analysis, was reduced from 105 to 55 kj/mol when microwave activation was utilized rather than conventional thermal processing. 

In the early works, it was found that the pulse method could lead to the fastest heating of epoxy resins [98] and improve their mechanical properties [99]. For example, it was shown that a computer controlled pulsed microwave processing of epoxy systems that consisted of diglycidyl ether of bisphenol A (DER 332) and 4,4 -diaminodiphenyl sulfone (DOS) in a cavity operated in TM012 mode could be successfully applied to eliminate the exothermic temperature peak and maintain the same cure temperature at the end of the reaction [100]. The epoxy systems under pulsed microwave irradiation were cured faster, and it was possible to cure them at higher temperatures when compared with a continuous microwave or conventional thermal processing. 

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