# Evaluation of Algae-Based Biodiesel Production Topologies via Inherent Safety Index (ISI)
## 3 Results and Discussion
However, a special alloy is needed in the transesterification and neutralization stages in topologies 1 and 2 due to the handling of sulfuric acid, which is a highly aggressive chemical for metals. Hastelloy c-200 was selected for these units since it is a nickel, chromium, and molybdenum-based alloy highly resistant to corrosion. Hastelloy c-200 was also required in the HTL, char separation, and bio-crude separation stages of topology 3, given the presence of highly corrosive substances and extreme conditions that affect the metals. Consequently, a score of two was assigned to Icor, max metric. Tables 4-6 presents the stages used in the biodiesel production from microalgae by the conventional method, in-situ transesterification, and HTL, respectively. the maximum temperature is reached in the hydrotreating stage (400 \({}^{\circ}\)C); hence, a score of three was assigned to the temperature subindex. The maximum pressure was observed in the hydrothermal liquefaction reactor (200 bar), and consequently, a score of four is assigned for this subindex. Another important parameter associated with inherent process safety is equipment safety. According to the equipment characteristics reported in Tables 2-4, it was indicated that the riskiest equipment in the three cases are the reactors where the transesterification, neutralization (topology 1 and 2), hydrothermal liquefaction, and hydrotreating (topology 3) reactions are carried out. Consequently, a score of three is assigned for the subindex \(I_{EQ,max}\) Finally, the safe structure subindex is established according to historical data, report heuristics, and engineering experience [58]. For the cases studied there is no historical information showing process safety aspects, therefore, a neutral position is assumed and a score of two is assigned for this subindex. This value refers to novel processes or emerging large-scale topologies such as the case of biodiesel production from microalgae. Figure 6 shows the results for the safety subindices evaluated for the three topologies of biodiesel production from microalgae. The chemical safety index for topology 1 and 3 was 22, values slightly higher than topology 1 with \(I_{CI}\) of 21. The contribution of the chemical reactivity, chemical interaction, and corrosiveness subindices is equal for the three topologies since the reactions performed are highly exothermic and the substances involved are corrosive and tend to form fire and explosion. However, the dangerous substance subindex contribution is lower for topology 2 since no flammable solvents are used compared to the conventional method and no highly flammable hydrocarbons are obtained as in topology 3. Regarding the process safety index, a score of 8 was achieved for topologies 1 and 2 and a score of 14 for the third topology. The findings revealed that inventory does not represent a risk or stress factor for the case studies since inventories under 50 t/h are handled. Temperature and pressure were no critical factors for the topology 1 and 2 safety because of the operation under relatively low temperatures and pressure at atmospheric conditions. For biodiesel production via HTL, the operating conditions of pressure and \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Unit** & **Type of Unit** & **Temperature (\({}^{\circ}\)C)** & **Pressure (bar)** & **Inventory (t)** & **Material** \\ \hline Culture & Tank & 25 & 1.013 & 10.00 & Stainless steel \\ Harvesting & Centrifuge & 25 & 1.013 & 10.00 & Stainless steel \\ HTL & Reactor & 350 & 200 & 4.76 & Hastelloy-c 200 \\ Char separation & Separator & 60 & 0.9 & 4.76 & Hastelloy-c 200 \\ Bio-crude separation & Separator & 87.22 & 0.9 & 4.75 & Hastelloy-c 200 \\ Hydrotreating & Reactor & 400 & 100 & 0.63 & Hastelloy-c 200 \\ Fractioning & Tank & 155.60 & 1.013 & 0.63 & Stainless steel \\ Biodiesel purification & Flash separator & 165.09 & 1.43 & 0.20 & Stainless steel \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Description of the main unit used for biodiesel production from microalgae via HTL.