Selection of oil extraction process from Chlorella species of microalgae by using multi-criteria decision analysis technique for biodiesel production

3.1 Preliminary technology selection

Many technologies have been explored for microalgal oil extraction on the laboratory scale, yet no well-established industrial methods have been selected [6]. However, many pieces of research have been finished to optimize extraction residence time, enhance extraction yield, and improve free fatty acids in the extracted product. In addition, solvents play a crucial role in this process to be used directly or assisted by chemical, biological, or mechanical cell disruption for improving extraction efficiency [14].

Chemical-microalgal oil extraction technology includes several solvents: organic solvent, acid-based solvent, nanoparticle, supercritical fluids, and ionic fluids. The organic solvent used in microalgal oil extraction is a well-matured technology in other industrial sectors (i.e., pharmaceuticals). Many publications revealed it with several combinations and other variables optimization. The supercritical fluid model has advantages over organic solvent extraction techniques. It does not need to use polluting solvents or uses polluting solvents but only at the minimum level. Besides, it can extract lipid easily [15]. Based on preliminary studies, organic solvent and supercritical fluid solvent have been selected for further detailed studies. Acid-base, nanoparticle, and ionic solvents are advantageous due to their higher lipid recovery than conventional organic solvents. However, using these technologies in large-scale processes is still further away as it is still in recent development or optimization in the laboratory scale.

Methods using mechanical-assisted solvents for microalgal oil extraction include conventional mechanical pressing, microwave-assisted oil extraction (MAE), ultrasound-assisted oil extraction (UAE), electric pulse-assisted oil extraction (EPE), and cell homogenization. Conventional mechanical pressing causes high biomass losses and low lipid selectivity, thus not feasible for industrial application [16]. On the other hand, cell homogenization can reduce cost intensive in the drying process and can be scaled up in large volumes [17]. However, for Chlorella vulgaris, Santos et al. [18] found that the yield in the homogenization process was more insufficient than, and even similar to, the conventional extraction under similar solvent conditions. Thus, it could be concluded that cell homogenization is not suitable for Chlorella vulgaris. Therefore, mechanical pressing and cell homogenization were not chosen in this preliminary selection step.

The biological-assisted oil extraction method, which is generally enzyme-based cell lysis, is more efficient in energy use and environmentally friendly. But considering its high operational cost, longer reaction time, and enzymes specificity, its implementation on an industrial-scale needs to be addressed [15].

The recent development in microalgal oil extraction is thermochemical-assisted technology, namely hydrothermal liquefaction (HTL) technology. It is a potential method for converting wet algal biomass into a crude bio-oil [19]. Hot compressed water at around 325°C is used to break down the bio-macromolecules in algae [20]. As a result, biomass is converted into high-water contents, thus minimizing the high cost and energy consumption in the drying steps. Hence, HTL is selected to be one of the technologies considered in this study.

Thus, the technologies selected from the preliminary study were conventional organic solvent extraction (COE), UAE, MAE, EPE, supercritical fluid extraction (SFE), and HTL.

3.2 COE

Conventional extraction of lipid using an organic solvent is done based on the “like dissolves like” chemical concept. The polar solvent dissolves the polar lipid, and, at the same time, the non-polar solvent dissolves the non-polar lipid [21]. Commercially, this process is well-established as studies have been conducted to obtain the most suitable solvent. Ideally, the solvent must have high selectivity to lipid fraction and low boiling point to ensure the minimum distillation energy and the simplicity of product separation [22]. However, organic solvents have a significant drawback in the environmental aspect due to their adverse effects and toxicity [6].

Chloroform, hexane, and methanol have been considered the most potential solvents [14]. Choi et al. [23] discovered the effect of different solvents; chloroform–methanol solvent (1:1 v/v) achieved the most excellent yield with 378 mg/g of cells of lipid extracted. As for hexane and hexane methanol solvents (7:3 v/v), the extracted lipid amounts were 184.5 and 185.4 mg/g, respectively. Thus, solvents, including their ratios, are the main factor in organic solvent technology. For detailed information, the summary of several studies is presented in Table 1.

3.3 UAE

Mechanical pretreatment approach of algal biomass aids in releasing intracellular lipids into the solvent, followed by enhancement of its recovery [15]. One of the most promising mechanical-assisted extraction is ultrasonic extraction that utilizes a sound wave frequency of >20 kHz to propagate the pressure fluctuations that induce cavitation [22,29], which will disrupt the cytoplasm and open the cell bodies, leading the solvent extraction yield to increase. Studies [28,30] on microalgae Chlorella sp. reported that ultrasonic pretreatment can increase the yield of extraction and reduce the extraction time up to more than 15 times, compared to conventional organic extraction.

Additionally, Natarajan et al. [30] concluded that rigid walled microalgae such as Chlorella sp. is suitable to be processed with UAE. However, it is inapplicable on microalgae with flexible walls such as Nannochloropsis sp. and T. suecica. However, the ultrasonic pretreatment process may lead to intracellular denaturation and escalate the energy cost due to its high temperature [31]. For detailed information, the summary of several studies is shown in Table 2.

3.4 MAE

Microwave-assisted technology utilizing frequencies of electromagnetic radiation ranging from 0.3 to 300 GHz has been used widely for intracellular components’ drying and extraction in food and pharmaceutical industries [15]. Microwaves interact with polar compounds and damage the cell membrane by inducing localized heat and pressure, leading to intracellular lipid extraction. Like ultrasonic-assisted technology, the microwave-assisted method offers a minimal amount of solvents and shorter extraction residence time.

Studies [28,32,34] on microalgae Chlorella sp. reported that microwave-based pretreatment increases the yield of extraction and reduces the extraction time by more than 15 times, compared to conventional organic extraction. Microwaves offer various advantages related to their suitability for the large-scale process. However, there are significant concerns related to energy requirements [15], polar solvents issues for volatile compounds [36], and microwave radiation effect on carbon chain bonds [35]. For detailed information, the summary of several studies is presented in Table 3.

3.5 EPE

Electric pulse can affect the properties of a biological cellular membrane by producing about 36 kV/cm of pulsed electric fields. An increase in trans-membrane voltage will increase both the membrane conductivity and permeability [37]. Garoma and Janda [28] reported lower lipid yields in EPE than ultrasonic and microwave-assisted technology. However, the ratio of energy gain per energy input in this technology was highest, compared to that using microwave and ultrasonic methods.

Considering the simplicity and lower energy consumption, this method is suitable for large-scale applications [15]. Furthermore, selective extraction of lipids, carbohydrates, pigments, and protein compounds in microalgae can be done in this technology, making it more suitable for biorefinery applications [38]. However, the electric pulse sensitivity to the salty medium becomes a significant drawback for seawater algae without prior washing [39]. For detailed information, the summary of several studies is presented in Table 4.

3.6 SFE

Supercritical fluids (SCFs) are alternative solvents for oil extraction. Their characteristics are high diffusivity, high dissolving power, low viscosity, and ease of separation [40]. A fluid presenting above the critical point at which different liquid and gaseous phases do not exist is supercritical. The most widely used SCF for the extraction of bio-compounds is supercritical CO₂ [41].

Several studies [42,43,44] have reported the yield of supercritical CO₂-based extraction as at par with conventional organic extraction, but it took a shorter time, thus increasing extraction productivity. However, this technology has more advantages than the COE techniques, including having better yield without using any polluting solvent or using polluting solvent but only at the minimum level. It, moreover, offers more flexibility in controlling criteria and easiness in lipid separation [6].

Unfortunately, studies on Chlorella sp. based microalgae are still fewer than those on the COE method. Therefore, the effect of such variables in supercritical oil extraction from Chlorella sp. must be optimized more. Furthermore, the high temperature and pressure requirement will be a significant concern in industrial applications. For detailed information, the summary of several studies is displayed in Table 5.

3.7 HTL

HTL is considered one of the promising alternative method in converting wet algal biomass into a crude bio-oil [19]. HTL involves the reaction between biomasses in the water at high temperature and pressure with or without any catalyst. With this process, HTL bypasses the drying process, thus eliminating significant cost and energy consumption [45].

HTL is applied in a closed reactor at 200–350°C and 5–15 MPa, usually assisted by water [46]. Studies [20,45,47] reported that HTL shows the highest bio-oil yield compared to the other five technologies. However, the reaction time is still longer than that of mechanical-assisted oil extraction. The major disadvantages of this technology are high temperature and pressure. For detailed information, the summary of several studies can be seen in Table 6.

3.8 Criteria and weighting factor

The criteria ranking and weighting factor using the ROC method are presented in Table 7.

3.9 Reference selection

The selected technologies from the preliminary study were COE, UAE, MAE, EPE, SFE, and HTL. The detailed information of the selected references is shown in Table 8.

3.10 Technology scoring

The scoring result of the selected technology for each criterion is presented in Tables 9–13. The score summary of each parameter in all technologies is shown in Table 14.

4.1 Criteria and weighting factor

Extraction efficiency, reactivity with lipids, duration, process safety, capital, operational costs, and waste generated are categorized as crucial variables for implementing microalgal oil extraction technology on an extensive scale application [48]. In this study, the criteria to evaluate the six most promising technologies in industrial-scale oil extraction are easy scalability, extraction productivity, energy input, additional compound, and environmental impact. These criteria need to be weighted based on their importance using equation (1).

Easy scalability is the measure of the easiness of the technology to be implemented on an industrial-scale. This criterion considers TM, difficulty in OM, and safety impact considering the use of the hazardous compound or extreme operational condition. Scalability is an essential criterion since the objective is to select microalgae extraction technology in industries. Extraction productivity represents the amount of obtained lipid per time. Since those technologies have different yields and times of extraction, this criterion provides an objective comparison. Extraction productivity is the second important factor because the high productivity will lead to cost reduction due to the minimum additional compound or energy input requirement.

Energy input corroborates the required energy per mass lipid extracted from microalgae. This criterion is placed in the same rank as extraction productivity due to its impact on production cost. On the other hand, the additional compound is considered the least important criterion. It depicts the price of the added compound to the extraction system. Although it impacts production cost, most solvents used in selected technology can be reused. Hence, this criterion will not affect production cost as high as the energy input. Environmental impact is placed in the same rank as an additional compound because, with the proper treatment, environmental impact can be minimized. Although it leads to increased production cost, the effect will not be as high as the energy input. The criteria ranking and its weighting using the ROC method can be checked in Table 7.

4.2 Reference selection

Several pieces of research have been conducted and reported for each kind of technology. Therefore, a preliminary selection of those references is needed. In this step, the main concern in the criterion extraction productivity due to yield and extraction time data is mainly provided by the reference quantitatively. For example, in extraction technology, Choi et al. [23] reported that several different solvents such as hexane, hexane–methanol (7:3 v/v), and chloroform–methanol (2:1 v/v), with a 1 h extraction time, show yields of 184.5, 185.4, and 378 mg/g, respectively. Lam et al. [24] reported a yield of lipid of 327 mg/g in a 12 h extraction time with a solvent of chloroform and methanol (1:2 v/v). Garoma and Janda [28] reported a lipid yield of 0.246 mg/g using chloroform, water, and methanol (2:2:1 v/v). The comparative study showed that the solvent mixture of chloroform and methanol (2:1 v/v) gave the highest oil extraction productivity [23].

For UAE, Lee et al. [32] reported a yield of lipid of 80 mg/g using chloroform and methanol solvent. Drira et al. [33] reported a yield of lipid of 2.4 mg/g after 30 min of total sonification and extraction process using chloroform and methanol solvent. Natarajan et al. [30] reported that fatty acid yield was 215 mg/g. Unfortunately, the extraction time and solvent were not directly stated. Garoma and Janda [28] published that the oil yield was 311 mg/g after 21 min of residence time using chloroform and water solvent in the ultrasound-assisted extractor. Based on the highest result, the study by Garoma and Janda [28] was used as a reference for MAE technology.

Lee et al. [30] reported a yield of lipid of 10 mg/g using chloroform and methanol solvent and microwave pretreatment. Cheng et al. [34] reported a yield of lipid of 251.92 for 20 min of the whole process with chloroform and methanol solvent. Garoma and Janda [28] reported an oil yield of 317 mg/g after 21 min extraction and 50 s microwave pretreatment using chloroform and water solvent. Garoma and Janda [28] reported the highest extraction productivity. Therefore, the study by Garoma and Janda [28] was referred to MAE technology.

For electric pulse oil extraction technology, Lee et al. [32] reported a yield of lipid of 128 mg/g after 30 h of the complete process, using chloroform and methanol solvent. Luengo et al. [38] reported that the oil yield was 0.4 pigment/g cell using ethanol solvent. Garoma and Janda [28] reported that the oil yield was 259 mg/g after 21 min extraction and 500 s electric pulse treatment using chloroform and water solvent. The article by Garoma and Janda [28] was then referred to EPE technology.

Obeid et al. [42] reported a yield of lipid of 40.3% from a total lipid of 244 mg/g using supercritical CO₂ and ethanol as co-solvent in a 4 h extraction time. Solana et al. [43] reported a yield of lipid of 181.5 mg/g using a similar solvent mixture after a 1 h 30 min extraction time. Dejoye et al. [44] reported a lipid yield of 1.81% after a 9 h extraction time from 150 g cell using supercritical carbon solvent. Thus, the study by Solana et al. [43] was referred to SFE technology.

Yang et al. [20] reported a bio-oil yield of 320 mg/g after a 1 h HTL process. Biller and Ross [45] reported a 20–40% bio-oil yield after a 1 h HTL process. Li et al. [47] reported a bio-oil yield of 82.9% after 90 min HTL treatment. Finally, Li et al. [47] showed the highest extraction productivity and was then referred to HTL technology. For detailed information, the summary of several studies is shown in Table 8.

4.3 Technology scoring: easy scalability

Easy scalability is the measure of the easiness of the technology to be implemented on an industrial-scale. This criterion considers TM, difficulty in OM, and safety (S) aspects considering the use of hazardous compounds or extreme operational conditions. TM, OM, and S were weighted by 40, 30, and 30% for the easy scalability score. The justification of each technology is shown in Table 9.

4.4 Technology scoring: extraction productivity

Extraction productivity defines the amount of extracted oil per time. The highest extraction productivity was obtained by MAE technology. Therefore, a score of 10 was given for this technology. The lowest extraction productivity was reported in SFE process. It was then scored by 1. The other technologies were scored by linearization. Detailed data and justification are shown in Table 10.

4.5 Technology scoring: energy input

Energy input defines the required energy per mass of extracted oil. The lowest energy input was achieved by the conventional organic extraction with a score of 10. The highest energy input was required by SFE with a score of 1. The other technologies were scored by linearization. Detailed data and justification are shown in Table 11.

4.6 Technology scoring: additional compound

Additional compound defines the price of compound added to the extraction system. Although it impacts production costs, most of the solvents used in selected technology can be reused. Therefore, this criterion will not affect production cost as high as the energy input. The score of this criterion was measured by the price of solvent per extraction cycle. The lowest additional compound price was HTL technology, so it was given a score of 10. The other technologies are scored by linearization. Detailed data and justification are shown in Table 12.

4.7 Technology scoring: environmental impact

Environmental impact is placed in the same rank as an additional compound because, with the proper treatment, environmental impact can be minimized. Although it will increase production costs, the effect will not be as high as energy input. This aspect was evaluated by comparing the toxicity of each solvent used in the particular technology. Toxicity was scaled based on MSDS provided by the NFPA. The lowest toxicity was obtained by HTL technology, so it was given a score of 10. The other technologies are scored by linearization and several justification adjustments. Detailed data and justification are shown in Table 13.

4.8 SMARTER selection

The score summary of each criterion in all technologies is shown in Table 14. The obtained scores were then multiplied by the weighting factor of each measure. The results of the weighted scores are shown in Table 15. The final scores of the evaluation based on SMARTER-MCDA are shown in Table 15.

4.9 Economic feasibility and future prospect

The study mainly focused on reviewing and selecting microalgal oil extraction technology using the SMARTER-MCDA technique. The criteria involved in the evaluation are: (1) easy scalability, (2) extraction productivity, (3) energy input, (4) additional compound, and (5) environmental impact. The preliminary economic feasibility in this study is measured indirectly by criteria numbers 1–4, which will affect the capital investment cost and operational cost. Therefore, further study on capital investment and the operating cost of selected extraction technology is required on the biodiesel production feasibility study.

With many potential prospects, the development of microalga-based biodiesel faces numerous challenges. They are a wide variation in optimum culture growth, high capital investment, and operational cost [50]. Based on the study by Kovacevic and Wesseler [51], microalgae-based biofuel could cost 51.60 euros/GJ by 2020, compared to crude oil which is around $10–20/GJ. It indicates that microalgae-based biodiesel requires further development to select the optimum conditions of each stage from cultivation, harvesting, extraction, conversion, and refinery.