*4.4. Characterization of Biodiesel*

#### 4.4.1. Differential Scanning Calorimetry

DSC is a device used to determine the amount of energy required to achieve a zerotemperature differential between a component and an inert reference substance by subjecting the two specimens to comparable temperature regimes in a contained manner [23]. The heat capacity or enthalpy of a sample of known mass is measured as changes in heat transfer. This analysis is suitable for glycerol and biodiesel, which are highly viscous melts [11].

Calibration of trials is used to record the temperature change and correlate it to the enthalpy change in the sample. The crude biodiesel sample is brought into equilibrium between −<sup>15</sup> ◦C and 30 ◦C, at the rate of 5 ◦C min−<sup>1</sup> for DSC measurement. In this research, it is vital to determine the crystallization point of the biodiesel sample to determine the lowest point of cooling temperature for the SAC process [11]. The crystallinity of materials is linked to the change in enthalpy by the energy required from the melting transition to proceed [23].

In this study, in DSC analysis, the sample was equilibrated, at 30 ◦C, and cooled immediately, at −<sup>15</sup> ◦C, at a rate of 5 ◦C min−1. Afterwards, the sample was maintained for 1 min and heated to 30 ◦C at a rate of 5 ◦C min−1. Therefore, the procedure for transesterification was now complete. The remaining crude biodiesel was further used for gravity settling for 24 h before proceeding with the DSC analysis.

## 4.4.2. Gas Chromatography–Mass Spectroscopy

GC-MS is an analytic technology which combined gas–liquid chromatography separation features with mass spectrometry detection techniques to identify distinct compounds inside a test sample [24]. The mass of the analyte fragments is being used to identify these compounds. In academic research, this device facilitates the characterization and detection of newly synthesized or derivatized compounds by studying the new components [24]. Retention time (RT) is the time required for the compound to pass through the injection port to reach the detector [25].

In this study, the GC-MS device used in this experiment is PerkinElmer Clarus 600 Gas Chromatograph (GC). A flame ionization detector (FID) and an Elite 5-MS column with a dimension of 30 m × 250 μm × 0.25 μm of film thickness were installed in the GC. This device is used twice in this experiment, once after the reaction of transesterification (initial content of biodiesel) and lastly after conducting SAC (final content of biodiesel). During GCMS analysis, the oven temperature was set at 150 ◦C and held for 1 min. Afterwards, the temperature was raised to 240 ◦C, at 5 ◦C min–1 ramping speed, and was maintained for 5 min.

To determine the biodiesel purity, the percentage composition of individual FAME was computed using the following Equation (1). The biodiesel purity computation was then performed for all prominent peaks. For each SAC trial run as well, GC-MS would be used to determine the yield of FAME over all purified biodiesel samples. The purified biodiesel was left to melt after being treated to SAC and was collected for GC-MS analysis to determine the percentage of FAME composition to define its purity using the same mentioned formula.

$$\text{Percentage composition of FAME } (\%) = \frac{\text{Peak area of individual component}}{\text{Sumulation of correction area}} \quad (1)$$

### **5. Conclusions**

Biodiesel is a non-toxic and biodegradable diesel alternative that is synthesized by the process of transesterification. 2-Methyltetrahydrofuran (2-MeTHF) can be used in chemical synthesis as an alternative to organic solvents for this project. In this context, it is used to replace solvent 1-butanol from a conducted previous study. The process parameters for SAC, which are different coolant temperatures, cooling time and stirring speed, are studied and analyzed for optimization. The chemical composition of biodiesel was taken into account when purified by the SAC process. The optimization process is considered successful once the optimum parameter value produces the highest purity of biodiesel, thus indicating that the biodiesel is free of contaminants. The optimum value to yield the highest purity of FAME for parameters coolant temperature, cooling time and stirring speed is −4 ◦C, 10 min and 210 rpm, respectively. Hence, by the proposed optimize parameter, it can be taken into account that SAC is effective in the purification of biodiesel. In conclusion, experimental research on the SAC method can assist in improving biodiesel purification.

For future study, a techno-economic feasibility study (TEFS) and cost benefit analysis will be carried out in order to estimate the cost as well as energy for this SAC system for implementation in industrial applications. The energy cost of the process can vary depending on factors such as the source of the feedstock, the type of equipment used (refrigerated, stirrer), and the efficiency of the process. Additionally, the benefits of the biodiesel purification strategy, such as reducing greenhouse gas emissions and decreasing dependence on fossil fuels, may outweigh the energy costs. The analysis would be needed to determine whether the biodiesel purification strategy is a worthwhile endeavor. In addition, process simulation software will be used for the determination of the scale-up process, including the equipment's size, design, operation, and process parameters optimization.

**Author Contributions:** Conceptualization, W.N.A.W.O. and S.S.; methodology, W.N.A.W.O. and S.S.; validation, N.A.I.B., W.N.A.W.O. and S.S.; formal analysis, N.A.I.B.; investigation, N.A.I.B.; resources, N.A.I.B. and S.S.; data curation, N.A.I.B., W.N.A.W.O. and S.S.; writing—original draft preparation, N.A.I.B. and W.N.A.W.O.; writing—review and editing, W.N.A.W.O. and S.S.; visualization, W.N.A.W.O. and S.S.; supervision, S.S.; project administration, S.S.; funding acquisition, S.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Ministry of Education Malaysia via FRGS (Cost Centre: 015MA0-094, Reference Code: FRGS/1/2019/TK10/UTP/03/3) and Universiti Teknologi PETRONAS via YUTP-FRG (Cost Centre: 015LC0-378), and facilities support from HICoE–Centre for Biofuel and Biochemical Research (CBBR).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors would like to acknowledge the assistance from the Ministry of Education Malaysia via FRGS (Cost Centre: 015MA0-094, Reference Code: FRGS/1/2019/TK10/UTP/03/3) and facilities support from HICoE–Centre for Biofuel and Biochemical Research (CBBR) and Chemical Engineering Department. Support from the Ministry of Education Malaysia through the HICoE award to CBBR is duly acknowledged. The support from Universiti Teknologi PETRONAS through YUTP-FRG is also acknowledged.

**Conflicts of Interest:** The authors declare no conflict of interest.

**Sample Availability:** Samples of the compounds, if available, are available from the authors.
