## 3 Results and Discussion  
### Lipid Extraction by Different Methods  
Choosing the right extraction method is crucial to obtain a high yield of crude oil. The efficiency of each extraction method is generally determined by the percentage of oil yield obtained after each extraction process. In this study, three different modes of microalgae biomass extraction were compared to determine the highest oil yield from two different species of microalgae viz. _Spirulina_ sp. and _Chlorella_ sp. The oil yields after 15 min of ultrasonication for _Chlorella_ sp. and _Spirulina_ sp. were 2.83 and 1.98 g, respectively. The oil yield after 8 h of Soxhlet extraction for those microalgae species was 1.05 and 1.25 g, respectively. The oil yield after 8 h of solvent extraction was 0.68 and 0.77 g, respectively. It was reported by [PERSON] et al. [52] that chemical extraction operating in dynamic mode, i.e., the Soxhlet extraction process, resulted in a roughly 280% increase in yield compared to static mode operation, i.e., the solvent extraction process. This significant improvement can be attributed to solvent refluxing, which exposed the cellular matrix to a fresh batch of hexane constantly and enabled continuous re-establishment of mass transfer equilibria [53]. Figure 3 shows the extracted product for the different lipid extraction methods.  
The percentage of the oil yield of three different methods for these two species of microalgae is depicted in Figure 4. As seen in the figure, for _Spirulina_ sp., the rate of oil yield for solvent extraction, Soxhlet extraction, and ultrasonication is 2.5%, 4.1%, and 6.6%, respectively. On the other hand, for _Chlorella_ sp., the percentage of oil yield is 2.3%, 3.5%, and 9.4%, respectively. Ultrasonication is the most effective among the three methods. The high efficacy of this method can be attributed to the disruption of the microalgae cells, as reported in other studies [44; 54; 55]. Sonic waves were passed to the microalgae culture while the ultrasound was done. They made a series of microbubble cavitations, which transferred kinetic energy to the cell surface, having broken up the cells [56]. Thus, ultrasonic waves help to reduce the size of dried algal fractions, which increases the surface area and the number of active sites available for reaction participation. [PERSON] et al. [20] stated that the utilization of microwave and autoclaves are the best technique for disrupting _Chlorella vulgaris_ cells, both of which produce a lipid yield of 10%. On the other hand, ultrasonication only resulted in a 5% lipid yield for a 5 min reaction time. In the present study, the 15-min sonication process resulted in a 9% lipid yield for _Chlorella_ sp., showing that the longer reaction time was conducive to disrupting the cell walls of _Chlorella_ sp., thereby increasing the lipid yield. However, it is to be noted that the effectiveness of cell disruption by ultrasonication will depend on the species of microalgae. For instance, [PERSON] et al. [57] reported that _Botryococcus_ species achieved a 16% lipid yield when ultrasonication was used. Thus, it is dependent on the microalgae cell wall thickness and diameter. Table 1 provides a study of oil production by the various methods of extraction. As shown in Figure 4, ultrasonication results in  
Figure 3: Extracted lipids using (**a**) Soxhlet extraction, (**b**) ultrasonication, and (**c**) solvent extraction.  
superior lipid extraction compared to other extraction processes. Structural analysis using SEM was performed to study the extraction methods in-depth. The analysis can be found in Appendix A.