# Evaluation of Algae-Based Biodiesel Production Topologies via Inherent Safety Index (ISI)
## 3 Results and Discussion  
The inherent safety analysis for the three microalgae-based biodiesel production topologies included the assumption of the worst-case scenario for the subindices assessment. The first step was the evaluation of hazards regarding chemical reactions. For biodiesel production by conventional and in situ transesterification methods, the main reactions occur in the transesterification stage where triglycerides and fatty acids are converted into alkyl esters (biodiesel) and glycerol [57]. The conversion of palmitic acid and methanol to methyl palmitate and water (see Equation (3)) was identified as the most exothermic reaction, showing a heat of reaction equal to 10,298.62 J/g. Regarding the biodiesel production via HTL, the main reactions of depolymerization (fragmentation, hydrolysis, dehydration, deoxygenation, aromatization, and repolymerization) take place in the hydrothermal liquefaction stage, which showed an exothermic behavior. A score of four is assigned for the chemical reactivity (main reaction) subindex for the three cases. For the \(\mathrm{I_{IS,max}}\) metric, the side reactions for the first (conventional) and second (in situ transesterification) topologies were identified in the neutralization stage where the acid catalyst is neutralized by adding CaO (see Equation (13)). The reaction shows exothermic behavior releasing heat of \(-6768\) j/g; hence, a score of two is assigned for both topologies. The side reactions for the third topology (HTL) correspond to hydrotreating the HTL stream to produce hydrocarbon fuels, which also showed to be exothermic. The most dangerous substances are presented in Table 3 for each process. Properties related to toxicity, explosiveness, and flammability were checked in the safety data sheets. Hexane, methanol, piperidine, and carbon monoxide showed to be the most dangerous substances given that these are highly flammable and toxic, which led to a score of 8, 7, 8, 8, and 8, respectively. Other substances such as toluene, methylcyclohexane, heptane, methyl hexane, pentane, formic acid, acetone, ethanol, and ethylbenzene were also shown to be highly dangerous (score = 7) for topology 3. Moreover, the analysis of each property enabled the identification of the most dangerous undesired chemical interaction. For topology 1 and 2, the most dangerous interactions included the explosive mixture that vapors of substances such as hexane, methanol, and glycerol can form with air and the strongly violent reaction that occurs when sulfuric acid and water are mixed; therefore, a score of four was assigned for \(\mathrm{I_{int,max}}\). For topology 3 the possible heating of highly flammable substances (methanol, glycerol, 1-ethyl-2-pyrrolidinone, ethylbenzene, cresol, hexadecenoic acid, naphthalene, ethanol, acetone, formic acid, ammonium, acetic acid, methyl-butane, pentane, methyl pentane, hexane, methyl hexane, heptane, methyl hexane, heptane, methyl hexane, piperidine, toluene, methyl heptane, octane, xylene, and carbon monoxide), whose vapors form an explosive mixture with air is considered the most hazardous interaction, assigning a score of four also for this subindex. \begin{table}
\begin{tabular}{c c c} \hline \hline \(\mathrm{I_{PI}}\) & **Symbol** & **Score** \\ \hline Inventory & \(\mathrm{I_{I}}\) & 0–5 \\ Process temperature & \(\mathrm{I_{T}}\) & 0–4 \\ Process pressure & \(\mathrm{I_{P}}\) & 0–4 \\ Equipment safety & \(\mathrm{I_{EQ}}\) & 0–4 (ISBL); 0–3 (OSBL) \\ Safe process structure & \(\mathrm{I_{ST}}\) & 0–5 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The score for process safety subindices. Furthermore, the material of construction for the equipment was evaluated according to the substance requirements. It was found that the majority of the substances involved in the processes were corrosive; thus, stainless steel was required as the construction material.