is the generation of intense micro-convection in the system. This microconvection is generated due to several distinct phenomena. These are (i) microstreaming, (ii) microturbulence, (iii) shock waves, and (iv) microjets [29, 32]. All these physical effects occur on extremely small spatial scales (nanoscale) and results in energy concentration. For a heterogeneous reaction system, these effects generate fine emulsion of the reaction mixture, and reduce the mass transfer barrier. Moreover, the microjets and shock waves generated in the reaction media due to cavitation also helps in uniform distribution of solid particles (catalyst) in the reaction system, which tend to settle down.
The chemical effect of cavitation, commonly known as the sonochemical effect, is essentially the generation of highly reactive free radicals such as ·O, ·OH, and HO2· during the transient collapse of the cavitation bubbles. The radicals are generated through the thermal dissociation of the vapor molecules entrapped in the cavitation bubbles due to the extreme temperature and pressure level reached inside the bubbles at the moment of transient collapse. The extent of free radicals production mainly depends on the physical properties of the medium and thus, liquids having lower viscosity with high vapor pressure will lead to the generation of relatively large number of free radicals [29–32]. For greater details, readers are advised to refer the previous literature [1, 24, 25, 29]. Several mathematical models have been developed in the past few decades, which can predict the intensity of cavitation phenomena.
2.3 Intensification of Biodiesel Production Processes Through Cavitational Reactors
As discussed in the preceding sections, the process intensification can occur in the cavitational reactors through the process of cavitation phenomena, i.e., transient collapse of tiny bubbles due to pressure variation in the liquid media. The various studies published over the last few years have demonstrated the successful application of ultrasound irritation or sonication in process intensification for different chemical processes including biodiesel production. In this section, some critical studies related to process intensification of biodiesel synthesis, especially with the utilization of a heterogeneous catalyst, are presented briefly. Moreover, state-of-the-art reviews focusing on sonication as a tool for process intensification of biodiesel synthesis are discussed as follows. Lam et al. [33] published a comprehensive review on the homogeneous, heterogeneous, and enzymatic catalysts for biodiesel production. The authors discussed various methods for intensifying the transesterification reaction and lowering the heterogeneity of the system through the application of co–solvent method, oscillatory flow reactor system, microwave mixing, and ultrasound-assisted system. Ramachandran et al. [34] reviewed the developments in the heterogeneously catalysed biodiesel the production in presence of ultrasound irradiation. The review concludes that ultrasonic energy emulsifies the reactants to reduce the catalyst requirement, methanol–oil ratio, reaction time, and reaction temperature compared to conventional mechanically agitated systems. Lerin et al. [35] overviewed the ultrasound-assisted enzymatic esterification and transesterification reactions for biodiesel production. The review figured out the advantages of sono-chemical reactors for the production of biodiesel at a commercial scale. Islam et al. [36] had compared various advancements in catalytic and non-catalytic reactions for biodiesel production with the application of ultrasound as a tool for process intensification. The review published by Lourinho and Brito [37] analysed the novel developments in biodiesel production in terms of feedstock selection and process intensification. They discussed different operational aspects of process intensification technologies among ultrasound irradiation, microwave heating, co-solvents, and membrane reactors for economic biodiesel production. Ho et al. [38] summarized the advances in ultrasound-assisted transesterification reaction. The authors critically appraised current technology’s status on the application of ultrasound energy in conjunction with heterogeneous catalysts for biodiesel production. Chuah et al. [3] discussed issues of cleaner intensification technologies in biodiesel production and emphasized application of hydrodynamic cavitational reactors for biodiesel production at large-scale level, over ultrasonic cavitation and conventional mechanical agitation. Gude and Martinez-Guerra [39] assessed the process of intensification in sustainable biodiesel production using a green chemistry approach. They compared the reaction efficiency between the conventional mechanical agitation, microwave, and ultrasound-enhanced biodiesel synthesis.
The literature published on cavitation-assisted biodiesel synthesis can be categorized based on the type of cavitation employed, viz., acoustic cavitation and hydrodynamic cavitation.
2.3.1 Acoustic Cavitation (or Ultrasound Irradiation) Assisted Processes
These are essentially lab-scale studies that have reported the role of ultrasound for intensifying the yield of the reaction. The ultrasound has been applied either through direct mode, i.e., with the ultrasonic probe or indirect mode, i.e., ultrasonic bath. The comparative analysis of some important studies using different feedstocks and catalyst in the presence of acoustic cavitation is summarized in Table 2.1.
2.3.2 Acoustic or Ultrasonic Cavitation Assisted Processes
Hydrodynamic cavitation is a sub-type of cavitation technology with a wide range of applications that include biodiesel production. The studies utilized the hydrodynamic cavitation (HC) reactor for biodiesel synthesis with reactor capacity varying from 5 L/h to 100 L/h and hence, also considered as pilot-scale studies [86]. The application of HC reactors for biodiesel production at commercial scale is preferably sensible compared to acoustic cavitation-based reactors. The major advantage of HC reactors’ usage over acoustic cavitation reactors is the lower energy and solvent consumption to process a large quantity of feedstock [87]. The design-based approach of HC reactors is discussed explicitly in the next section. At present, the basic principle of HC is briefly explained. When a liquid flow is altered by passing the liquid through either a venturi or an orifice plate, it results in enhanced fluid velocity across the region of vena contracta by losing the local pressure. If this pressure fall is achieved below the threshold pressure value, it results in the formation of cavitational phenomena, i.e., growth of tiny bubbles. The consequent transient collapse of these tiny bubbles, which occurs in the expansion phase of liquid, resulted in the completion of the cavitation process [88]. A typical configuration of HC reactor mainly consists of a feed tank, a pump (preferably reciprocating pump), pressure gauges, and the cavitation chamber – either in the form of venturi or an orifice plate or throttling valve. Among these three, the orifice plates are used more often for the efficient performance of the HC reactor as the pressure drop is much higher, and extended cavitation zone can be achieved with orifice plates with different geometry of holes (orifices) [28, 87, 89, 90]. In this section, the literature available for biodiesel production using the HC reactor is summarized in Table 2.2. This will give a comprehensive analysis of various studies and reaction parameters used to optimize the biodiesel yield in each case.
Table 2.1(A) Ultrasound-assisted heterogeneously base catalyzed biodiesel synthesis case studies.
| Oil (source) | Catalyst | Molar ratio (Methanol to oil) | Catalyst loading (wt% or w/w) | Reaction temperature (K) | Time (min) | Ultrasonic frequency/power (kHz/W) | % FAME (yield) | Reference |
|---|---|---|---|---|---|---|---|---|
| Mixed oil | KI/ZnO | 11.68:1 | 7% | 332 | 60 | 35/35 | 92.35 | [22] |
