2.5.2.2 Microwave Irradiation
The dipole rotation and ionic conduction achieved by means of microwave irradiation is useful for rapid heating in a system since heat is generated from inside out, thus generated heat and mass transfer gradients travel in the same direction, compared with being opposite during conventional heating. In a microwave, the electricity is converted to heat according to Eq. (2.3), where P = power dispersion per unit volume of reaction mixture, K = constant, f = applied frequency, ε = dielectric constant of compound (average for mixtures), E = electric field strength, and tan δ = dielectric loss tangent [2]. From this, the power dispersion is visible dependent on reactant polarity and E as selected from (irradiation strength) during the conversion process.
Compared with conventional processes, the dispersion of generated heat is thus rapid, facilitating very high conversion rates and drastically reducing required time and energy [49]. However, slight disadvantages exist since presence of solids greatly disrupts the microwave penetration, resulting in irregular diffusion and reactivity. Also volatile compounds pose greater risks when exposed to microwaves compared to uniform heating due to rapid heat generation [50]. This process has been successfully applied in both fed‐batch and continuous processes, as listed in Table 2.3.
2.5.2.3 Ultrasonication
Vibrations ranging between 18 kHz and 100 MHz can be classified as ultrasound, and the process of ultrasonication involves the use of sound waves generated from a probe or in a bath for enhancement of chemical conversions such as transesterification [51]. In the bulk liquid, ultrasound generates acoustic pressure (P a), which adds to the already existing hydrostatic pressure (P h). P a depends on exposure time t, amplitude pressure of wave P A, and acoustic cycle sin 2π as depicted in