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(2.4):

(2.4)

The generated sound waves have regions of compression during one half of the acoustic cycle where the liquid is compacted, creating a high pressure (P _a + P _h) along with heat generation, and regions of rarefaction during the other half when the liquid experiences a local vacuity (P _a − P _h), experiencing sudden cooling owing to decreasing local pressure [52]. The wave intensity I is related to P _A as shown in Eq. (2.5), where ρ is liquid density, while c is the speed of sound in the liquid:

(2.5)

The intensity of the wave is distributed as energy (generating heat) and thus gradually decreases over increasing distance as shown in Eq. (2.6), where I ₀ = sound intensity at probe, α = absorption coefficient, and d = distance of molecule from probe:

(2.6)

      Regions in the rarefaction region often experience a fall in pressure exceed the critical distance when P _a − P _h is quite large, and this results in molecules being stretched beyond the critical distance R, which causes cavitation bubbles to form, subsequently when the compression wave hits this region. Pressure increase causes temperature and pressure inside the cavitation bubble to increase abnormally (up to 4000 K and 90 MPa) before it implodes, spreading the heat afterward, which greatly enhance chemical conversion such as transesterification. The investment costs in this process are low, and the process itself is very energy efficient; however, exposing the oil to such extreme changes in temperature and pressure at a molecular level may have damaging effects on the glycerides and FFAs, hampering fuel yield. Nevertheless, it is still one of the most sought PI approaches in commercial biodiesel production [46], as seen in Table 2.3.

2.6 Economics and Environmental Impact

      Despite extensive research data reported every year and the successful utilization of biodiesel blends in a few countries, the feasibility of utilizing biodiesel globally is still debated since the process is complex, involving costly equipment and high reactant losses, while the feedstock used are not sufficiently available. However, the cultivation and/or utilization of these nonedible and waste feedstock for commercial fuel synthesis provides monetary incentives to farmers, which can bolster economy [53]. The distribution of potential sources of WCO collection also plays an important function in determining the best site for establishing a production plant, since transportation must be minimized. Highly populated areas remain a lucrative choice due to increased number of sources; however, the plant must be strategically placed to avoid local contamination from industrial discharge and emissions. Unfortunately, no extensive studies have been reported so far on this aspect.

Life cycle assessment (LCA) studies provide an insight into the impact of biodiesel synthesis on the market and environment [54]. Lee et al. reported an estimated cost reduction by 10–20% on manufacturing and 42% feedstock cost when using WCO in supercritical systems [55]. A notable point is that waste oil collected from cafes, eateries, and restaurants generally consists of WCO mixed with grease trap waste, which cannot be utilized for fuel synthesis without incurring heavy energy and other purification losses during intensive pretreatment. This also adds to CO₂ emissions (estimated at 92%) during biodiesel synthesis life cycle, with 25% of it generated from transportation of this unusable waste. Anaerobic digester employing methanogens, for example, can be used to solve this [55]. Thus WCO is physically separated before transport and use. Another aspect that can generate more revenue is the processing and sale of glycerol or other products generated from glycerol‐free processes, since they boast widespread use [53]. Carbon footprint remains uninfluenced in a worst‐case scenario when using WCO, since WCO generation can be equated to our dietary activities, and its utilization expectedly resulted in negligible changes for global CO₂ uptake, while nonedible oil usage showed positive impact.

2.7 Conclusion and Perspectives

      Not without challenges, WCO is remarkably lucrative for biodiesel synthesis. The sourcing and utilization of WCO benefits both economy and the environment, especially due to its easy availability. The removal of impurities and pretreatment prior to production is a hassle, which is eclipsed by its applicability to various production approaches with polar or nonpolar alcohols. While catalyzed processes boast smoother operation and low capital costs at the cost of increased time and labor, continuous PI approaches boast rapid conversion and operation ease while being high on investments, maintenance, and alcohol consumption. However, with high variance in composition each time it is procured, the efficiency under any given set of reaction conditions is still unpredictable. The separation, collection, and purification of by‐products for sale can further help bolster competitiveness of biodiesel with petrodiesel, making the inevitable transition easier.

References

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