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 biofuels 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 biofuel, 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 harvested throughout 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 [
28,
33,
34,
35], 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. Ranganathan and Savithri [
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 Tejada Carbajal 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, Peralta-Ruiz 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, Pardo-Cárdenas 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. The topologies considered include the conventional method of lipid extraction and transesterification, in-situ transesterification (extraction and transesterification in one step), and hydrothermal liquefaction. The novelty of this project lies in the extension of a laboratory-scale process and the safety evaluation for large-scale applications.
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 IRS,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 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, 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.
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. 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 I
cor,
max metric.
Table 4,
Table 5 and
Table 6 presents the stages used in the biodiesel production from microalgae by the conventional method, in-situ transesterification, and HTL, respectively.
Regarding the inherent safety analysis for process attributed the total inventory estimated at 41.08 t for topology 1, 28.63 t for topology 2, and 35.73 t for topology 3. It was found that the process by the conventional method handles a higher amount of mass, however, the score assigned for the inventory sub-index was 2 for all three cases. The maximum temperature for topologies 1 and 2 were registered in the biodiesel purification stage (138.6 °C and 134.87 °C); therefore, a score of one is assigned for It,max for both processes. The pressure for cases 1 and 2 were kept at safe conditions (0.5–5 bar). For topology 3, the maximum temperature is reached in the hydrotreating stage (400 °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
Table 2,
Table 3 and
Table 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
IEQ,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
ICI 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 temperature represent a high safety risk. The high temperatures and pressures required in the hydrothermal liquefaction and hydrotreating stages represent critical variables for the process.
The most unsafe equipment for the three processes were the reactors owing to their exothermic reactions and operating conditions reached in these stages. Therefore, constant monitoring of the reactors is fundamental to avoid incidents. The safe process structure subindex contribution on the safe process index was two for the three cases since there is no information related to the behavior of the set of equipment and operations that integrate these systems.
As shown in
Figure 7, was calculated a total inherent safety index of 30, 29, and 36 for the three processes. According to Heikkilä [
55], inherently safe processes show an below 24 and inherently unsafe processes reach an
ITI above 24. The results reflect that the three topologies exhibit an inherently risky performance. The main risks are associated with the chemical factor due to the dangerous substances and the reaction type taking place. The factor associated with operating conditions represented no risk for topology 1 and 2; however, the high pressures and temperatures required for processing stages in the third topology strongly affect the process safety. It was also found that among the three pathways studied, the biodiesel production by in situ transesterification method presents an inherently safer performance compared to the other two methods. This result is attributed to the lipid extraction and transesterification carried out in one step to avoid the use of solvents or flammable substances. In addition, this method does not require critical temperatures or pressures to reach the reaction conditions. Topology 3 showed to be the most unsafe routes for biodiesel production from microalgae evaluated in this research.
Similar studies related to biodiesel production from residual biomass confirm the low performance of these processes from a safety perspective. The inherent safety assessment of biodiesel production from waste oil revealed that the process is unsafe, indicating that the amount of chemicals and their flammability characteristics represent the main hazard along the route [
59]. The emerging risk analysis for a bio-diesel production process by transesterification of virgin and renewable oils concluded that the topology is highly unsafe and identified that the main risk is associated with methanol fire and explosion [
60]. Furthermore, processes for other types of biofuels production showed inherently unsafe performance, such as a biorefinery for ethanol, butanol, succinic acid, among others, from lignocellulosic biomass that exhibited an
ITI = 36 [
61].
The results provide information about the unsafe performance of biodiesel production from biomasses due to the use of highly dangerous substances. To reduce the inherent risks, some intensification options are proposed including the elimination of dangerous substances, such as methanol and sulfuric acid, by the incorporation of supercritical transesterification without using catalysts [
62] or with enzymatic catalysts such as Tert Butanol which is equally efficient as acidic and basic catalysts [
59]. Furthermore, it is proposed to replace methanol with other alcohols including ethanol, propanol, or n-butanol to reduce the toxicity and flammability hazard [
63]. Heterogeneous catalysts are also proposed as an option to overcome the inherent safety issues of biodiesel production from microalgae; these are easily removed from the reaction and increasing the control over the neutralization process [
60].