Superheating effect

Closely related to the superheating effect under atmospheric pressure are wall effects, more specifically the elimination of wall effects caused by inverted temperature gradients (Fig. 2.6). With microwave heating, the surface of the wall is generally not heated since the energy is dissipated inside the bulk liquid. Therefore, the temperature at the inner surface of the reactor wall is lower than that of the bulk liquid. It can be assumed that while in a conventional oil-bath experiment (hot vessel surface, Fig. 2.6) temperature-sensitive species, for example catalysts, may decompose at the hot reactor surface (wall effects), the elimination of such a hot surface will increase the lifetime of the catalyst and therefore will lead to better conversions in a microwave-heated as compared to a conventionally heated process. 

For liquid products (solvents), only polar molecules selectively absorb microwaves, because nonpolar molecules are inert to microwave dielectric loss. In this context of efficient microwave absorption it has also been shown that boiling points can be higher when solvents are subjected to microwave irradiation rather than conventional heating. This effect, called the superheating effect [13, 14] has been attributed to retardation of nucleation during microwave heating (Tab. 3.1). 

As described above, however, some rather small differences could be observed, taking into account the superheating effect of the solvent under the action of micro-waves in the absence of any stirring. This probably occurs in the isomerization of sa-frole and eugenol in ethanol under reflux [31] (MW 1 h, A 5 h to obtain equivalent yields). 

Baghurst, D.R. and Mingos, D.M.P., Superheating effects associated with microwave dielectric heating, /. Chem. Soc., Chem. Commun., 1992, 674. 

H. Richterova and M. Hajek. Localized superheating effects in heterogeneous reactions. Proceedings from the International Conference on Microwave Chemistry, Prague, Czech Republic 1998. 

By means of synchrotron radiation it is also possible to follow melting under isothermal conditions. Such experiments were performed on Polyethyleneterephtha-late. It was shown that with oriented samples melting occured more slowly and at higher temperatures than with unoriented samples. This is in agreement with the explanation of superheating effects by means of entropy considerations. 

Walker D., Norby L., and Jones J. H. (1993) Superheating effects on metal/siUcate partitioning of siderophile elements. Science 262, 1858-1861. 

It is worth noting further that, under certain conditions, polymers will also display superheating effects, in which case the mesophase may only appear on heating this can be regarded as "monotropic" behavior in the reversed sense. An example of this in connection with polyethylene will be quoted further below. 

With the exception of D, Tm estimates using the sample temperature are generally closer to the true value. The closest estimates of Tm were achieved by use of the sample temperature at point C or the reference temperature at point A. The Tb estimated for toluene at points A, B, and D were higher than at point C, and this is probably due to superheating effects. 

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