Microwave penetration depth effect

Fig. 14 The effect of moisture on microwave penetration depth. (From Ref...

Buschmuller et have demonstrated that microwave resonance can be used effectively as means to monitor the moisture levels in a fluidized-bed dryer during the granulation process. The penetration depth of microwave resonance may be limited to a few microns, and hence this technique may not have any real advantages over NIR which has also been used for monitoring moisture in dryers, and has the advantage of providing chemical information such as solvent levels in addition to water, and other important properties such as polymorphic form, and particle size. 

The more absorptive a material, i.e., the higher the loss factor e", the less deep microwave energy will penetrate into that material. A parameter, penetration depth, dp, has been defined which measurers this penetration, dp is a function of both e and e" and serves as a guideline to the heating effectivity of a material. 

The pH change could have a significant effect on dielectric properties of a material. In the case of SPl, it was observed that the e value increased at pH 4.5 and 10. However, there were not any significant differences in e at pH 4.5 and 10 as compared to the value at the neutral pH. The penetration depth varied significantly with pH, temperature, and frequency. As microwave treatment time progresses, an increase in binding and redistribution of water around protein molecules was observed. This effect could be attributed to the transformation of macromolecules due to heating (Ahmed et al., 2008). 

Mirin is a condiment with almost 40%-50% sugar and is widely used in Japanese cuisine. Dielectric loss factor of mirin is affected by both the dipolar loss component and the ionic conductivity. Ionic conductivity is lower at higher microwave frequencies. The combined effect of temperature, microwave frequency, and sugar content is complex and hard to describe. Nevertheless, the e" increases with frequency and temperature. The penetration depth decreases as the processing temperature increases. The effect of temperature on the is significant at lower processing frequencies at higher frequencies, temperature has only moderate effect on the d. Tanaka et al. (2005) reported similar results for soy sauce. It was noted by Liao et al. (2003) that this trend is distinctive for thick or complex solutions. 

The key limiting factor is the penetration depth of microwave irradiation, which is only a few centimeters in most solvents at 2.45 GHz. An issue therefore arises in getting sufficient microwave power into the reaction mixture to achieve the desired heating effect. The core of a large reactor vessel will not receive any microwave radiation as it will all have been absorbed by the outer layers. As a result, the center is effectively conductively or convectively heated, and the potential benefits of microwave heating will be lost. Penetration depth does, however, vary with frequency. Only a limited number of Industrial, Scientific, and Medical (ISM) frequencies are allowed so as not to interfere with military and civil aviation frequencies and telecommunications. Alternative frequencies are used for other large-scale applications and thus may provide an alternative solution to the scale-up of micro-wave chemistry. ... 

A potential solution to the issue of limited penetration depth could be to add a microwave reactor externally to a large batch reactor and cycle the reaction mixture through a loop continuously. However, this is effectively using the conventional reactor as a reservoir while processing small quantities of reaction mixture in the microwave field and, overall, it offers no advantage. Another solution is to either process smaller volumes in batch mode or else use a continuous-flow micro-wave reactor. These are the options that most investigators of microwave scale-up have used. 

The effective penetration depth decreases with increase in frequency which in turn causes less heating. Hence, a suitable combination of parameters in Eqs. (2) and (4) is required for achieving optimum coupling. It can be inferred from this discussion that low dielectric loss materials take longer time and high dielectric loss materials take shorter duration in the microwave sintering. 

Because microwaves can heat substances uniformly to their maximum depth of penetration, an individual who is irradiated could sustain severe deep tissue damage. The consequences of microwave injury are quite varied. The list below details some of the effects of exposure to microwaves. 

An alternative to either the use of susceptors or additives is to increase the frequency of the incident microwaves. The higher the frequency the greater the power deposited in the sample and thus even low-loss materials can be made to couple. The disadvantage of high frequencies is a decreased depth of penetration making heating a more surfeKe effect. 

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