2.2 mostly deals with homogenously catalyzed transesterification process and rarely addressed the heterogeneously catalyzed system for biodiesel production.
Table 2.2 Hydrodynamic cavitation reactors assisted biodiesel synthesis case studies.
| Oil (source) | Catalyst | Molar ratio (Methanol to oil) | Catalyst loading (wt% or w/w) | Reaction temperature (K) | Time (min) | Design of HC reactor (Cavitation chamber details, pressure) | % FAME (yield) | Reference |
|---|---|---|---|---|---|---|---|---|
| Thumba oil | TiO2-Cu2O nanoparticles | 6:1 | 1.6% | 353 | 60 | Orifice – 2 mm and 20 holes; 2 bar pressure | 65 | [91] |
| Cannabis sativa L. oil | KOH | 6:1 | 1% | 333 | 20 | Orifice – 3 mm and 7 holes; ~ 15 bar pressure | 97.5 | [92] |
| Waste cooking oil | NaOH | 6.8:1 | 1% | 308 | 5 | Orifice – 0.3 mm and 100 holes; 7 bar pressure | 99 | [93] |
| Waste frying oil | KOH | 6:1 | 1.1% | 336 | 8 | Venturi apparatus; 3.27 bar pressure | 95.6 | [94] |
| Used frying oil | KOH | 4.5:1 | 0.55% | 318 | 20 | Orifice – 3 mm and 16 holes; 2 bar pressure | 93.86 | [95] |
| Rubber seed oil | 6:1 | 1% | 328 | 18 | Orifice – 1 mm and 21 holes; 3 bar pressure | 96.5 | [96] | |
| Waste cooking oil | KOH | 12:1 | 3% | 323 | 120 | High speed homogenizer (1200 – 3500 rpm) | 97 | [97] |
| Waste cooking oil | KOH | 6:1 | 1% | 333 | 15 | Orifice – 1 mm and 21 holes; 2 bar pressure | 98.1 | [98, 99] |
| Nagchampa oil | KOH | 6:1 | 1% | 333 | 20 | Orifice plate; 1.4 bar pressure | 91.8 | [100] |
| Used frying oil | KOH | 5:1 | 1% | 333 | 10 | Orifice – 2 mm and 25 holes; 2 bar pressure | 95 | [26] |
| Thumba oil | NaOH | 4.5:1 | 38 g | 323 | 30 | Orifice – 3 mm and 5 holes; 1.5 bar pressure | 80 | [101] |
The next section deals with the quantification of cavitational zonesbased analysis for the efficient design of hydrodynamic reactors.
2.4 Designing the Cavitation Reactors
As discussed in the previous sections, the chemical and physical effects of a cavitational phenomenon will be able to initiate as well as promote various chemical reactions. Thus, to design an efficient cavitational or sonochemical (either acoustic or hydrodynamic cavitation) reactor, one must be able to produce a cavitation zone, i.e., the area in the reactor in which formation, growth and the transient collapse of tiny bubbles should occur. Hence, the measurement of cavitation zone as well as cavitational intensity will ultimately impart the effectiveness of a selected cavitational reactor design. Along with cavitational intensity, analysis of pressure difference is also done, which leads to the development of a high number of cavitational bubbles and increases the impacts of physical effects of cavitational phenomena (especially in the case of a hetero-phasic reaction system). Moreover, application of either acoustic (either probe or ultrasound bath) or hydrodynamic (either orifice or venturi based) cavitational reactors is typically based on the size of reactants processed. In the following paragraphs, the basic methodology to determination of cavitational intensity in both cases is discussed elaborately.
As a standard procedure in chemical engineering practice, the lab-scale kinetics data is used to design a specific reactor followed by dimensional analysis; the design approach for a cavitational reactor will need an estimation of cavitational effects on the reaction mixture and variation of pressure amplitude throughout the reactor geometry. Gogate et al. [89] reported the methods commonly used to measure the variation in pressure amplitude and cavitational intensity. In this study, Gogate et al. [89] also reviewed the literature published in analyzing the cavitation intensity using an experimental- and mathematical-based approach. The authors reported the use of hydrophones to measure the variation of pressure amplitude experimentally. The obtained readings can be compared to the simulated (i.e., mathematically model-based) results to determine the efficiency and energy dispersion into the liquid under operating conditions. To estimate the chemical effects of cavitation, i.e., the formation of free radicals, authors suggested using degradation of potassium iodide as a model reaction. The quantity of iodine gas liberated can be measured using the UV/VIS spectrophotometer (measuring absorbance at a wavelength of 355 nm) [87, 89]. The cavitational yield for a given design and particular model reaction could be determined using the following equation [88]:
(2.1)
