Solvents superheating effect

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). 

Solvent Reaction conditions 40/38 + 39 ortho/para T(°C) Superheating effect (°C)... 

Hash devolatilization is a simple and effective method to remove the majority of solvent and unreacted monomers from the polymer solution. Product from the reactor is charged to a flash vessel and throttled to vacuum conditions whereby the volatile solvent and monomers are recovered and condensed. In the process, the polymer melt cools, sometimes considerably, due to the evaporation of volatiles. The polymer product is pumped from the bottom of the flash vessel with a gear pump or other suitable pump for viscous materials. Critical to operation of the flash devolatilization unit is prevention of air back into the unit that reduces stripping ability and potentially allows oxygen into the unit that can discolor products or pose a safety hazard if low autoignition temperature solvents are used. Often one flash devolatilization unit is insufficient to reduce the residual material to a sufficient level and thus additional units can be added in series [61]. In each vessel, the equilibrium concentration of volatile material in the polymer melt, is a function of the pressure and temperature the flash unit operates at, with consideration for the polymer solvent interaction effects described by the Hory-Huggins equation. Flash devolatilization units, while simple to operate, may be prone to foam development as the superheated volatiles rapidly escape from the polymer melt. Viscous polymers or polymers with mixed functionalities... 

In a subsequent paper [32], however, Berlan himself cast doubt on the existence of nonthermal effects, attributing the observed rate increases to localized hot-spots in the reaction mixture or to superheating of the solvent above its boiling point. He also mentioned the difficulty of measuring the temperature accurately in MW cavities. Furthermore, kinetic studies by Raner et al. [33], showed that the Diels-Alder reaction of 3 with 23 (Scheme 4.12) occurred at virtually the same rate under MW and conventional heating at the same temperature. 

It is interesting to note that when the same reaction was performed using a variable frequency MW system [49] with temperature control at 80 °C in the absence of a solvent, it occurred at the same rate as a similar reaction heated conventionally at the same temperature. The use of variable frequency provides very uniform heating, minimizing the possibility of hot spots. Thus it can be concluded that the modest rate enhancement observed in ethanol under reflux was because of hot spots or to a general superheating of the solvent. Again, it should be emphasized that these modest MW rate enhancements should not be taken as hard evidence for nonthermal MW effects. 

The average relaxation time is of course temperature dependent and maybe related to a rate constant k for the relaxation process of the molecules in solution. Table 1.3 gives some representative data for EtOH and illustrates the extent to which the relaxation time decreases with temperature. It is noteworthy that the relaxation time decreases from 270 to 49 ps as the temperature rises from 10 to 70°C, and therefore, as the temperature increases the alcohol couples more effectively with the microwave source at 2.45 GHz. Such a situation is ripe for superheating the solvent, since the extent of conversion increases as the temperature rises. It also follows that some organic solvents with very long relaxation times at room temperature may appear to be unsuitable candidates for dielectric heating, but since the match becomes more favourable with temperature then they may behave as effective couplers as the temperature rises, that is, after a slow start they may very well heat very rapidly. 

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