# Evaluation of Algae-Based Biodiesel Production Topologies via Inherent Safety Index (ISI)
## 1 Introduction  
Increased energy needs worldwide have soared the consumption of fossil fuels such as coal, oil, and gas, leading to a series of environmental problems including atmospheric pollution and global warming [1]. Biomasses are received attention as a potential source of biofels in order to trade-off energy demands without affecting impacting the environment through viable and profitable ways [2, 3, 4]. Nowadays, biodiesel is an attractive biofel, which is a liquid fuel consisting of mono-alkyl esters (methyl or ethyl) of long-chain fatty acids [5]. Biodiesel has similar properties to petroleum-derived diesel and can be used directly in compression ignition engines without modification [6]. Biodiesel is derived from lipid sources such as oil crops, waste oils, microalgae, and animal fats [7]. Generally, oils used in biodiesel production are composed of triglycerides that can be converted into biofuels through three main methods: thermal cracking, microemulsion, and transesterification [8]. Microalgae are considered a promising option for biodiesel production due to their high lipid content [9], fast growth rate, and high oil production [10]. Other advantages of microalgae as the feedstock for third generation biofuel production include easy-to-operate cultivation conditions, worldwide distribution inhabiting freshwater and saltwater ecosystems, and the possibility of being harvestedthroughout the year [11; 12]. Over the last 20 years, several strains of algae and cyanobacteria have been isolated and studied for their capacity to produce biofuels; however, strains possess an inherent capacity for synthesizing and storing suitable levels of lipids and carbohydrates. Strains from the genera _Spirulina_ (Arthorspira) [13], _Botryococcus_ [14; 15], _Chlamydomonas_ [16; 17], _Chlorella_ [18; 19; 20; 21; 22; 23; 24; 25; 26; 27], _Nannochloropsis_ [28; 29; 30; 31; 32], _Scenedesmus_ [33; 34; 35; 28], and _Tetraselmis_ [36; 37; 38]. Biodiesel from microalgae requires the extraction and conversion of the lipid fraction into low atomic weight compounds, methyl esters of biodegradable fatty acids [39]. Different conversion techniques to obtain biofuels from microalgae have been developed including solvent extraction followed by transesterification, fermentation to alcohols, thermal conversion pathways, and hydrothermal liquefaction (HTL) [40]. Transesterification represents the most common and commercially used method to produce biodiesel [8]. Other studies concern strain selection and the development of cultivation methods [41]. Process technical aspects such as economic profitability, energy efficiency, and environmental performance have also been studied by several authors. [PERSON] and [PERSON] [42] performed the techno-economic analysis of microalgae-based liquid fuels production from wastewater; the cash flow analysis revealed that the minimum selling price of hydrocarbons is $4.3/GGE [PERSON] et al. [2] carried out the techno-economic analysis of five microalgae biorefinery scenarios for biodiesel production and glycerol valorization. The results showed that the biorefinery approach with glycerol valorization reached higher economic performance with an internal rate of return of 19.8%. On the other hand, [PERSON] et al. [43] evaluated various technologies for microalgae oil extraction from the exergetic viewpoint. The authors identified that hexane-based oil extraction is the most alternative route for large-scale biodiesel production with an exergy efficiency of 51%. Furthermore, [PERSON] et al. [44] analyzed the life cycle of three biodiesel production cases from microalgae. The results indicated a 156% reduction in greenhouse gas emissions for the hexane-based oil extraction process. In this work, the inherent safety analysis of three emerging topologies for biodiesel production from microalgae is proposed to measure process safety metrics, propose improvements for optimal performance, and select the safest route for industrial-scale biodiesel production.