The studies summarized in Table 2.1 (A), (B) and (C) are aimed at addressing the problems associated with the application of heterogeneous catalyst for biodiesel production. In contrast, Table 2.2 summarizes the use of HC reactors in process intensification, mostly for homogeneous catalysed processes. As stated previously, the application of heterogeneous catalysts for transesterification reaction makes the reaction mixture become a 3–phase heterogeneous system (solid–liquid–liquid), which has high mass transfer constraints. The conventional mixing and heating method includes hot plates (laboratory scale), oil, or sand baths, and water heated jacketed reactors combined with mechanical mixing are not efficient for improving intermixing of reactants and catalysts, and thus, usually takes longer times to complete the reaction with uneven heat distribution [39, 57]. With utilization of sonication (either in the form acoustic or hydrodynamic), the processes were intensified and resulted in lower requirement of catalyst and solvents with higher conversion in short reaction time [27, 88]. However, the research published in this area revealed that acoustic cavitation was investigated comprehensively compared to HC processes. The bottleneck on the application of HC in heterogeneous catalyzed process is in its working principle.
As the liquid passes through the venturi or orifice or throttled value, the flow regimes of the liquid changes, resulting in loss of pressure across the section. To generate the cavities or tiny bubbles from flowing liquid, the pressure should drop below the vapor pressure of the fluid. Thus, to achieve this, small diameter holes are more preferable, ranging in the zone 0.1 to 3 mm [90, 102]. The application of heterogeneous catalysts in the reaction mixture requires a high flow velocity to suspend the catalyst particles in the reaction mixture. On the other hand, to avoid chocking of catalyst particle over cavitating equipment, either particle size of catalyst should be significantly lower (at least 10 – 50 times) than the hole opening diameter or the hole opening diameter should be enlarged. The former solution will increase the production cost due to an increase in catalyst production cost [11, 40, 91, 103]; on the other hand, the latter solution will affect the working of the reactor. With a given inlet pressure, only mixing can occur in the HC reactor as the pressure drop is insufficient to vaporize the liquid for production of tiny bubbles [88, 104]. Thus, the literature revealed in Table
