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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	1.41	20/108	95.57	[49]
Kusum oil	Ba(OH)2	9:1	3%	323	80	20/250	96.8	[50]
Karanja oil	Ba(OH)2	9:1	5%	303	60	30/100	83.87	[51]
Palm oil	SrO/Al2O3	9.2:1	1.6%	333	30.2	20/200	80.2	[52]
Waste cooking oil	Ca-diglyceroxide	9:1	1%	333	30	22/120	93.5	[53]
Crude palm oil	Fly ash on CaO	12:1	4%	318	30	20/700	97.04	[54]
Jatropha oil	Na2SiO3@Fe3O4/C	7:1	5%	328	80	25/36	94.7	[55]
Soybean oil	KF/γ–Al2O3	12:1	2%	323	40	20/45	95	[56]
Waste cooking oil	Ba(OH)2	6:1	0.75%	333	2	25/200	83.5	[57]
Palm oil	Ostrich egg – CaO	9:1	8%	333	60	20/120	92.7	[58]
Milk thistle oil	TiO2/C4H4O6HK	16:1	5%	333	30	40/250	90.1	[59]
Waste cooking oil	MgO	5:1	1.5%	328	45	24/200	98.7	[60]
Jatropha oil	CaO	11:1	5.5%	337	60	35/35	96	[20]
Soybean oil	CaO	10.1:1	6%	335	60	35/35	90	[61]

Table 2.1(B) Ultrasound-assisted heterogeneously acid catalyzed biodiesel synthesis case studies.

Oil (source)	Catalyst	Reaction	Molar ratio (Methanol to oil)	Catalyst loading (wt% or w/w)	Reaction temperature (K)	Time (min)	Ultrasound frequency/power (kHz/W)	Yield (%)	Reference
Mixed oil feedstock	Sulfonated Carbon	Transesterification	12.8:1	8.18%	336	60	35/35	93.7	[9]
Erythrina mexicana oil	Cobalt (II) 3D MOF	Transesterification	10 mL:1gm	25 mg	333	720	40 kHz	80	[62]
Pistacia khinjuk seed oil	Sulphated tin oxide impregnated with silicon dioxide	Transesterification	13:1	3.5%	338	50	20 kHz	88	[63]
Oleic acid	PTA@MIL–53 (Fe) (hetero–polyacid on Fe(III)–based MOF)	Esterification	16:1	100 mg	333	15	37/50	96	[64]
Sunflower oil	Sono–sulfated zirconia on MCM–41	Transesterification	9:1	5%	333	30	20/90	96.9	[65]
Waste fish oil	Sulfonated activated carbon	Esterification	14.85:1	11.4%	328	60	20/296	56	[66]
Waste cooking oil	Sulfonated carbon catalyst from cyclodextrin	Transesterification	16:1	11.5%	390	8.8	25 kHz	90.8	[67, 68]
Palm fatty acid distillate	Sulfonated cellulose	Transesterification	6:1	3%	333	180	20/120	81.2	[69]
Waste cooking oil	Tri–potassium phosphate	Transesterification	6:1	3%	323	90	22/375	92	[70]
Crude Jatropha oil	carbon–supported heteropoly acid	Transesterification	20:1	4%	323	60	20/400	87.33	[71]
Crude Jatropha oil	Cesium doped heteropoly acid	Transesterification	25:1	3%	327	34	20/400	90.5	[72]

Table 2.1(C) Ultrasound-assisted immobilized enzyme catalysed biodiesel synthesis case studies.

Oil (source)	Enzyme	Support	Molar ratio (alcohol to oil)	Enzyme loading (w/w)	Reaction temperature (K)	Time (min)	Ultrasonic frequency/power (kHz/W)	Yield (%)	Reference
Mixed oil	Lipase from Thermomyces lanuginosus	Commercially immobilized	7.64:1	3.55%	309	120	35/35	90	[73]
Soybean and Waste frying oil	Lipozyme TL-IM, RM-IM Novozym 435	Commercially immobilized	9:1	15%	298	720	40/220	90	[74]
Macauba and Soybean oil	Lipase from Candida antarctica	Macroporous anionic resin	3:1	20%	343	120	20/40	88	[75]
Canola oil	Lipase from Candida rugosa	Commercially immobilized	5:1	0.23%	303	90	20/40-200	99	[76]
Waste lard	Lipase B from Candida antarctica	Commercially immobilized	4:1	6%	323	20	20/500	96.8	[77]
Waste tallow	Lipase B from Candida antarctica	Commercially immobilized	4:1	6%	323	20	20/500	85.6	[78]
Sunflower oil	Lipase from T. lanuginosu	Immobilized on silica granules	3:1	3%	313	240	40/120	96	[79]
Macauba coconut oil	Lipase B from Candida antarctica	Macroporous anionic resin	9:1	20%	338	30	40/132	70	[80]
Soybean oil	Lipase B from Candida antarctica	Macroporous anionic resin	3:1	20%	343	60	40/132	78	[81]
Waste cooking oil	Lipase from T. lanuginose	Silica-iron oxide nano- particles	4.34:1	43.6%	303	360	40/150	91	[82]
Soybean oil	Lipase from Rhizomucor miehei	Macroporous anion resin	3:1	5%	338	240	100 W	90	[83]
Jatropha curcas oil	Lipase from E. aerogenes	Activated silica	4:1	5%	298	30	24/200	84.5	[84]
Soybean oil	Lipase B from Candida antarctica	Immobilized on polyacrylic resin	6:1	6%	313	240	40/500	96	[85]

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 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.