feedstocks. Potential alternate feedstocks for biodiesel production are non-edible oils, animal fats, micro-and macroalgal biomass, waste cooking oil, etc. [9, 10]. Comprehensive research is being performed for successful production of biodiesels from the different feedstock. Factors such as catalyst type, reactor geometries, mixing modes, etc., positively affect the biodiesel production [2, 11]. The research in the area of biodiesel is driven by its increasing demand globally. The global production of biodiesel reached over 34 billion liters in 2018, with more than 200 industries commercially producing it [12]. As per the report of the International Energy Agency (IEA) in 2019, the production of biodiesel is expected to grow in different parts of the world except for the European Union, where the solar-driven and fuel cell driven vehicles are getting more attention in the period of the next 5 years [13]. Regional distribution of global biodiesel production (current and projected) is depicted in Figures 2.1 and 2.2.
The report of the Renewable Energy Policy Network (REN 21), published in 2014, has highlighted the attention gained by biodiesel with an annual growth rate of ~ 11.5% as an alternative fuel [14]. Different aspects of viable and economical production of biodiesel include the development of different catalysts (homogeneous, heterogeneous, acid, alkali, resins, enzymes, etc.), reactor designs (batch, continuous, packed bed, CSTR, microchannel, etc.), mode of mixing (co-solvent, microwave, ultrasound, etc.) [15–17]. Briefly, the R&D efforts in biodiesel production can be categorized in three sections: (i) development of efficient and cost-effective heterogeneous catalyst; (ii) utilization of cheaper feedstocks like non-edible oils for biodiesel production; and (iii) efficient reactor designs with novel methods with energy inputs. Each of these aspects significantly contributes to the sustainability, viability, and economy of biodiesel production.
In recent years, with a compulsory mandate from various governments, for blending 5-20% biodiesel in conventional diesel, the demand for biodiesel has increased drastically. To fulfil the demand of biodiesel, without affecting the edible oil sources for its production, alternative non-edible sources are being explored now. However, non-edible feedstocks contain high levels of free fatty acids and require pre-processing for the removal of the fatty acids. Alternatively, heterogeneous catalysts that are capable of performing simultaneous esterification and transesterification can also be used [9, 18, 19]. Among these, the use of heterogeneous catalysts to improve the overall economy of the process (lowering the downstream processing and minimizing the contamination of by-product glycerol) is more appealing. However, the use of heterogeneous catalysts makes the reaction mixture 3-phase, with substantial mass transfer limitation resulting in slower kinetics and lower yields [20–22]. Many researchers have adopted the application of novel mixing methods to address this issue, and one such efficient technique is sonication [23–26]. Sonication of liquid-liquid heterogeneous mixture results in the formation of a fine emulsion which largely overcomes the mass transfer barriers and improves the kinetics of the transesterification process. Many authors have reported the successful application of sonication or cavitation phenomenon for biodiesel production. This chapter deals with the basic principles of sonication or ultrasound irradiation in the context of process intensification, and important aspects of cavitational reactor designs (either acoustic or hydrodynamic type) for biodiesel production. Moreover, the recent advances in reactor design for biodiesel production have also been reviewed.
Figure 2.1 Cumulative production of biodiesel among various regions over the period of 2013-18
[Source: IEA report [13]].
Figure 2.2 Biodiesel production data over the period 2013-2018 and expected production of biodiesel for the period 2019-2024
[Source: IEA report [13]].
2.2 Principles of Ultrasound and Cavitation
As stated in the previous section, sonication or ultrasound irradiation is a relatively new technique of introducing energy into the reaction system. Ultrasound and its secondary effect cavitation make energies available on time and spatial scales that are not available from any other source. Oscillatory motion of liquid elements induced by passage of ultrasound waves generates intense micro-mixing in the system, while transient collapse of cavitation bubbles generates radicals that can induce/accelerate chemical reactions [27, 28]. Some basic principles of ultrasound and cavitation are briefly explained in this section.
Ultrasound essentially represents the sound waves with frequency higher than the upper limit of human hearing (i.e., between 20 kHz – 500 MHz). Cavitation is a secondary effect of ultrasound [29]. Cavitation occurs through the formation of tiny bubbles known as nucleation, followed by the growth of these bubbles driven by bulk pressure variation due to passage of ultrasound, and the final implosive transient collapse of the bubbles. The transient collapse of cavitation bubbles creates extreme temperatures and pressure on extremely narrow spatial and temporal scale resulting in high heating and cooling rates, which induces or favours numerous chemical reactions [30].
Apart from the ultrasound wave, the bulk pressure in a reaction system or especially flow reaction system can also vary due to changes in flow geometry. This phenomenon is known as hydrodynamic cavitation. In recent years, hydrodynamic cavitation has emerged as an alternative to acoustic cavitation [29].
Physical properties of the liquid medium, as well as physical aspects of the ultrasound wave, influence the nature and characteristics of the cavitation process, which in turn is manifested in terms of kinetics and yield of the process. Some of these physical parameters and the nature of their influences are listed below.
1 Temperature: With an increase in the bulk liquid temperature, the net intensity of cavitation decreases which occurs mostly because due to the rise in liquid temperature, the equilibrium vapor pressure of the system rises and results in a greater fraction of solvent vapor entering the bubble in the expansion cycle. This entrapped vapor in the bubble lowers the energy concentration during the transient collapse. Thus, the resulting temperature and pressure attained in the bubble at transient collapse is reduced, and correspondingly the production of radicals [29, 31].
2 Ultrasound wave frequency: The frequency of the ultrasound controls the nature of bubble dynamics and the intensity of the transient cavitation. With increase in ultrasound frequency, the duration of the rarefaction half cycle gets reduced and is not adequate for the growth of the bubbles. This results in the lesser intensity of bubble collapse. Generally, frequencies in the range of 20–30 kHz are usually preferred for effective sonochemical applications [29].
3 Acoustic power: The cavitation intensity shows maxima with respect to the acoustic power. Although the intensity of transient collapse of a single bubble reduces with increasing acoustic power, at very high power inputs, there is an abrupt and large formation of cavitation bubble clouds at the ultrasound source that obstructs the transmission of energy after certain point from the probe to the fluid [29, 30].
4 Physical properties of liquid medium: The most relevant physical properties of liquid in the context of cavitation phenomena are surface tension, viscosity, boiling point, and vapor pressure. The liquid medium with high surface tension, low viscosity, and low vapor pressure are favourable for getting high intensity cavitation phenomena [29].
The main physical effect of ultrasound
