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] |
| Soybean oil | Barium polymer | 12:1 | 6% | 338 | 150 | 37/100 | 99 | [40] |
| Waste cooking oil | ZnO | 6:1 | 1.5% | 333 | 15 | 32 kHz | 96 | [41] |
| Canola oil | CaO, Ca-diglyceroxide | 7.48:1 | 5.35% | 333 | 150 | 20/40 | 99.4 | [42] |
| Karbi oil | CaO | 12:1 | 5% | 333 | 120 | 20-30/50 | 94.1 | [43] |
| Soybean oil | Sodium Zincronate | 6:1 | 3% | 328 | 480 | 25/360 | 80 | [44] |
| Mixed oil | Cu2O | 10.6:1 | 7.25% | 335.5 | 40 | 35/35 | 98.33 | [21] |
| Refined Palm oil | CaO | 9:1 | 8% | 323 | 37 | 28/200 | 95 | [45] |
| Canola oil | Dolomite | 9:1 | 5% | 333 | 90 | 20/45 | 97.4 | [46] |
| Palm oil | CaO | 9:1 | 2% | 333 | 3.5 | 20-50/800 | 80 | [47] |
| Waste cooking oil | Hydrotalcite | 15:1 | 0.08 g/g oil | 330 | 60 | 20/11 | 76.45 | [48] |
| Waste cooking oil | Coal fly ash | 10.71:1 | 4.97% | 333 |
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