In an effort to understand the efficiency of supercritical extraction and also for a comparative evaluation of the physicochemical properties of the extracted oil, traditional solvent extraction of the oil was also performed. Sd fine Chem. Mumbai, India and were of high purity. The dried Simarouba gluaca seeds were ground to powder and the oil was extracted in a Soxhlet apparatus using hexane.
The unit is comprised of a chiller, CO 2 pump, co-solvent pump, heat exchanger, extraction vessel, ABPR automatic back pressure regulator , fraction collector, etc. The volume of the extraction vessel is mL and the system is controlled by PLC. Initially, g of seed sample were taken in a 0. The CO 2 filled cylinder was connected to the unit. The CO 2 gas was pre-liquefied by passing through the shell-side heat exchanger and then pumped into extraction vessel where the temperature and pressure were maintained at above critical conditions.
The supercritical state of CO 2 was achieved at this stage. The oil extraction was carried out at different operating conditions of pressure and temperature. Each experiment was run for obtaining the maximum possible oil. The extraction time was set at 8 hours. Oil was then collected in a sample bottle from the collection vessel after each run and weighed to get the oil yield.
The sample bottle was tightly sealed and kept in a refrigerator for further analysis. Three different oil samples were obtained for each extraction condition and the parameters were measured for each one. The physico-chemical characteristics of the extracted oils were measured and analyzed according to the well-established standard methods. The analysis was performed on an Agilent series HPLC unit equipped with a fluorescence detector.
The excitation and emission wave length of the detector were set at and nm, respectively. The isocratic mobile phase consisting of hexane and isopropyl alcohol The fatty acid composition of the extracted oil was analyzed by gas chromatography GC after derivatization of the oil to its methyl ester. The flow rates of air and hydrogen were mL min -1 and 30 mL min -1 , respectively. Injection was performed in triplicate for each sample and average values are reported. In the current investigation, soxhlet extraction of Simarouba gluaca seeds was carried out by employing hexane and the oil content in the seeds was found to be The physicochemical properties of the extracted oil were thoroughly analyzed and are given in Table 1.
The composition of different fatty acids in the extracted oil was also analyzed by GC after derivatization and is shown in Table 2. The extraction conditions for the supercritical CO 2 extraction of oil from Simarouba gluaca seeds were applied in the present study for the maximum possible extraction of oil.
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The run time of extraction was fixed at 8 hours, matching the run time of conventional solvent extraction. The oil yields were found to be 4. Thus there exists an increasing trend in extracted oil yield with an increase in extraction temperature and pressure. However, a considerable increase in extracted oil yield 16, The percentages of extracted oil were found to be Thus the maximum possible extraction of oil At the above set conditions of temperature, pressure and flow rate, another set of extraction was conducted in order to optimize the extraction time 2, 4, 6 and 8 hours.
The results obtained indicate that the maximum oil yields of However, this study could not be extended beyond the studied temperature, pressure and flow rate because of the limitations of the system. Figure 1.
Figure 2. Figure 3. Table 1 depicts the physico-chemical properties of Simarouba gluaca oil extracted with supercritical CO 2 as well as using hexane. However, peroxide values showed a slight difference of 0. This indicates that the quality of supercritical CO 2 extracted oil was slightly better when compared to solvent extracted oil. The content of tocols of the supercritical CO 2 extracted oil was more when compared to the solvent extracted oil. Earlier reports have shown very few physical properties of the oil Anil et al.
A major and important observation was the non-extractability of phospholipids in the oil extracted by supercritical CO 2 when compared to the solvent extracted oil. During supercritical CO 2 extraction, the phospholipids do not get extracted into the oil. This is because of the non polar nature of the CO 2 which does not dissolve the phospholipids contained in the seeds which are otherwise polar in nature.
The values were 97 and 8. Due to this reason, the color of supercritical CO 2 extracted oil was light yellow and was better in appearance when compared to the solvent extracted oil. Lesser amount of phosphorous indicates that de-gumming, which is an important processing operation in vegetable oil processing, can be avoided when the oil is extracted using supercritical CO 2.
The fatty acid composition of both the extracted oils is almost similar and is shown in Table 2 and the respective chromatograms are shown in figures 4 and 5. The oil contained Table 3 gives the solid fat content of solvent and supercritical CO 2 extracted oils.
Figure 4. GC chromatogram of fatty acid methyl ester of supercritical fluid extracted Simarouba gluaca seed oil. Figure 5. GC chromatogram of fatty acid methyl ester of solvent extracted Simarouba gluaca seed oil. Solvent extracted and supercritical CO 2 extracted oils were thoroughly characterized for physico-chemical properties. It was found that the phosphorous content of the supercritical CO 2 extracted oil had a much smaller amount of phosphorous compared to the solvent extracted oil.
Total tocol contents were also higher in supercritical CO 2 extracted oil. The other parameters almost remained the same for both supercritical CO 2 and solvent extracted oil.
Green solvents and technologies for oil extraction from oilseeds
Materials 2. According to Chisti Y , energy production, goods and services are necessary, but they must be socially, economically and environmentally sustainable. Microalgae is an energy source that offers considerable amounts of fuel from small crop areas and lower production costs, which further helps in the mitigation of global warming; its culturing tolerates high concentrations of CO 2 and decreases the amount of nitrogen oxides released into the atmosphere.
The most conventional biodiesel-from-microalgae production chain until now is composed by the stages of cultivation, harvesting of biomass, drying, lipid extraction and oil transesterification . Despite of continuous and positive advances in algal research, biodiesel-from-microalgae production chain is not sustainable yet, in energy terms, comparison of energy demands for microalgal biodiesel production shows that energy required in all stages of production process is more than energy produced by third generation biodiesel , In this sense, results of studies related to bioprospecting, exploration and production of microalgae biomass made by research centers as the NREL In United States, the CISOT and CIEMAT in Spain , the CIDES and ICP in Colombia , among others, concludes that production of biodiesel from microalgae can be economically viable if total biomass components are used for obtaining biofuels and high value products and the concept of biorefinery is incorporated.
Biorefining is processing biomass in a sustainable way within a spectrum of marketable products and energy, this concept can be extended, according to Cherubini , to a laboratory or a set of laboratories that integrates biomass transformation processes and equipment for the production of fuels for transportation, energy and chemicals.
The biorefinery concept has been identified as the most promising for the creation of an industry based on biomass. However, this concept has not been applied so far to the biomass of microalgae define a path-oriented technology for the production of biofuels and high added value products based on the physicochemical characterization of a promising species, a microalgae based biorefinery must take into account several issues for its sustainability as water requirements, production costs, environmental impacts and process efficiency .
The extraction of carbohydrates, lipids, pigments, proteins and special substances from microalgae biomass is under research for obtaining several bioproducts  focusing on the use of multifunctional processes for simultaneous extraction separation and transformation of two or more desired products , or in optimization of operating conditions and routes for obtaining a desired specific metabolite, pigments extraction can be made by cell breaking, solvent extraction and centrifugation, and purification is made using microfiltration, drying or lyophilization , reducing sugars can be obtained by hydrolysis reaction with simultaneous cell wall disruption for oil extraction , proteins are extracted for use as fertilizer , animal feed supplement  and substrate for fermentation .
Several methodologies are under study in lab-scale for extracting and separating lipids from microalgae biomass, most methods are composed by the stages of cell wall disruption and lipid separation from biomass. For cell wall disruption, various thermal, chemical and physical methods have been evaluated. More advanced methods are also been evaluated as enzymatic extraction , supercritical fluid extraction , wet extraction , Osmotic shock  and in-situ transesterification .
One of the goals pursued by researchers in this area, is to find a method for microalgae oil extraction which can be at the same time efficient, cheap, selective to lipids desired, reproducible and scalable, for achieve this goal, several studies must be developed in order to find the process that allows an effective oil extraction in terms of efficiency, purity of product desired, energy requirements, costs and environmental impacts.
Although is well known by the authors the availability of robust methodologies for evaluation of each one of parameters discussed in this study as energy, exergy, and emergy analysis from the energetic point of view , techno-economic analysis with scenarios comparison and sensitivity analysis for evaluation of technologies from the economic point of view , and optimization of biorefineries taking into account economic and safety objectives , the scope of this research is to provide a big picture of the behavior of several oil extraction methods used on several microalgae strains in lab-scale under several criteria in order to provide some lights for further deeper study of techniques.
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As secondary contribution, morphological response of bioprospected strains used for evaluation of oil extraction methods is also discussed such as some issues to consider for integration of technologies developed with other methods for extraction and separation of additional microalgae metabolites according to biorefinery concept. As is mentioned in abstract, microalgae strains used for this study were Nannochloropsis sp. Oil extraction in lab-scale Solvent-based oil extraction methods evaluated hexane and cyclohexane based methods, solvent extraction with high speed homogenization, continuous reflux solvent extraction and ethanol-hexane method were designed and adjusted by authors in previous works , finding the best operating conditions as the first stage of cell wall disruption as second stage of solvent oil extraction and lipid purification, for all methods cell disruption is intended to destroy the microalgae cell wall to facilitate the recovery of intracellular products and obtain greater amounts of lipids, all oil extraction experiments were made by triplicate, methods were performed as follows:.
Improved Solvent extraction assisted with high speed homogenization SHE. Solvents are recovered by evaporation and condensation using a roto-evaporator. Finally, the lipid extract was allowed to volatilize to constant weight for its measurement, cell disruption in this method is achieved by mechanical action in homogenization stage . Ethanol is used in the first stage to recover the lipid content of microalgae; the crude oil obtained with ethanol contains unsaponifiable lipids, such as pigments, proteins, amino acids and other lipid and non-lipid contaminants.
As a second step, the addition of water and hexane to the crude extract, obtained above, generates the formation of a biphasic system, in which lipids are transferred to the hexane phase, and the impurities are retained in the hydroalcoholic phase. This phase separation occurs due to the difference in solubility between solvents.
It is performed by decanting and is repeated five times by adding more water and hexane to the hydroalcoholic phase. The proportion water content has been optimized to displace the equilibrium distributions of lipids to the hexane phase, for cell disruption a solution with 5g of biomass and 0. Improved Continuous reflux solvent extraction CSE. This is a multiple-extraction procedure that consists in a first cell disruption stage in which 5g of biomass are mixed with water, methanol and sulphuric acid in a After extraction, extract-solvent mixture was filtered, distilled and the remnant solvent was evaporated.
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Total lipids were also quantified by gravimetric methods . In the first stage of cell disruption, 5g of microalgae biomass are mixed with hydrochloric acid 0. Parameters for comparison of oil extraction methods Lipid yield and lipid extraction efficiency. It was estimated the yields and efficiencies for each of the methods based on the gravimetric analysis done to each, oil yield in every test was calculated using the Equation 1 , from amount of biomass used and oil obtained.
To calculate lipid extraction effectiveness, the term Relative Extraction Ratio is introduced; this ratio is defined as the lipid yield reached using any extraction method evaluated respect to lipid yield reached performing SHE method, which is used for total lipid determination, Equation 2 was used for calculation of Relative Extraction Ratio. Statistical comparison of lipid yield. Cost of extraction. Excepting the CSE method, cost decrease by solvent reutilization was not taken into account. As all microalgae oil extraction methods evaluated in this study are solvent-based, toxicity is considered as a very important aspect due to the implications of the use of these substances; toxicity was used as safety gross evaluation criteria.
In order to obtain a better data analysis, values were normalized to the same biomass amount 1g of dry biomass and extraction time 1h. Energy requirements. Values were estimated according to the electric power of the equipment used in each stage homogenization, drying, vacuum separation, solvent recovery etc. Morphological response. Observation in optical microscope is performed to the biomass of the five strains at objective x before and after every procedure in order to see its influence in the cell and its damage on the morphology of the same.
Characterization of microalgae strains According to the characterization of studied microalgae strains shown in Table 1 , Amphiprora sp. Profile more suitable for the development of a topology of biorefinery corresponds to Amphiprora sp. Multicriteria comparison of oil extraction methods in lab-scale Extraction Efficiency.
As is shown in Table 2 , extraction efficiency depends as extraction method performed as microalgae strain used, according to extraction results is clear that microalgae strain Amphiprora sp.
Guinardia sp. After solvent extraction with high speed homogenization SHE method , Continuous reflux solvent extraction method CSE presents the highest average relative extraction ratio, being potentially used for effective lipid extraction in lab scale, however, the scaling-up of this method can represent a process design challenge, owing to equipment, energy and solvent requirements. Batch methods as hexane and cyclohexane based extraction HBE and CBE respectively presents good extraction ratios in comparison to CSE method, with the advantage of an easier scaling-up, and lower solvent requirements, HBE extraction can be more attractive for a large scale microalgae processing owing to solvent cost, oil extraction using the ethanol-hexane mixture presents the lowest average standard deviation of methods evaluated which could be positive for ensure reproducibility of the oil extraction, however relative extraction ratio of this method does not overcome relative extraction ratio of any other method evaluated for the same strain.
Costs of extraction. If extraction costs in lab-scale are compared, lowest value belongs to EHE method and followed by EHE method, these values are due to low solvents amount needed to perform these methods and low cost of ethanol and hexane in comparison to other organic solvents, while higher extraction costs belongs to CBE method, which is drastically increased by the costs of cyclohexane which is near to 13 times more expensive than hexane in local market. Statistical comparison of methods.
Table 3 shows the results of statistical comparison of oil extraction methods taking into account the extraction efficiency, results shows that although behaviour of oil extraction methods is affected by the strain evaluated which is coherent with the analysis made in previous section, however, it can be seen that in most of cases strains there is no significant differences between performing HBE and CBE methods, showing that not worth it to continue using both methods in lab-scale for future work, nevertheless, is also clear that selection criteria between HBE and CBE cannot be efficiency, for selecting the more convenient method, must be compared using additional criteria discussed in further sections of this work.
Oil extraction and analysis: critical issues and comparative studies.
It also can be seen that there is no significant differences between CSE and HBE for most of strains evaluated, so, other criteria must be taken into account for a more robust comparison of these two methods. On the other hand, EHE method presents significant differences in comparison to other C6-based extraction methods in all cases.
Values of solvents used shows that SHE method is the most harmful of methods evaluated, owing to the use of highly toxic solvents as methanol and chloroform which is disadvantageous for a large-scale processing without appropriate safety-based process design, extraction methods which uses hexane as solvent CSE and HBE presents the lowest toxicity. If is analyzed the toxicity parameter together with solvent recovery for studied methods, can be seen a disadvantage of performing this method frequently in lab-scale, by the release of high amounts of highly toxic solvents, requiring adequate facilities and protection, can be convenient to use SHE method once for an estimation of total lipid content of feedstock and used as reference.
However, using an adequate large-scale process design which takes into account all safety aspects or appropriate assumptions, can be interesting the evaluation of this method. CSE presents higher solvent loses in comparison to HBE, however, in SCE case solvent is lost by continuous evaporation and condensation and for HBE, bulk of the solvent non-recovered is in mixture with algae meal after extraction, for this reason is recommendable a further drying of algae meal and condensation of vapours released for a more effective hexane recovery.
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Energy Requirements. When solvent recovery is considered for evaluation of oil extraction methods, is understandable that energy requirements increases, because an additional energy input is necessary for condensing the solvent separated from the lipid extract, and for separating solvent mixtures in methods where is required, in this scenario, method with higher energy requirements is EHE, for efficient first-step extraction with ethanol, recovered solvent must be separated from water added for phase separation, and hexane must be condensed after lipid extraction and separation.
Taking into account results obtained in Table 4 , can be established that for a lab-scale microalgae oil extraction, method most convenient to perform is HBE, because its low energy consumption compared to other methods, low extraction costs and relatively low toxicity of solvent used, on the other hand, CBE method becomes non-convenient for oil extraction from microalgae due to its high cost of cyclohexane and high toxicity, in addition, lipid yield obtained with this method is similar to yields of HBE method.
For EHE method, algal meal contains only ethanol, because there is no contact between hexane and biomass, which allows higher possibilities of further processing of algae meal without significant co-product purification, if is desired to convert meal carbohydrates into reducing sugars, can be used a organosolv pretreatment which includes ethanol with an acid for hydrolysis reaction, in this sense, is more convenient the EHE method in comparison to SHE method, hexane is also easily recovered from hydrophobic phase and can be used again for extraction decreasing processing costs.
In CSE method, as the solvent is continuously evaporated and condensed during extraction for effective lipid recovery, this continuous reflux increases solvent loses during extraction process, and is more significant at long extraction times, issue that is characteristic of this method. On the other hand, if the extraction process is stopped when the amount of solvent in contact with biomass is minimum, cost of processing will decrease by more solvent recovery and further processing of algal meal for obtaining other products will be chapter.
By the nature of the process, solvent separation from lipid extract can be performed in the same extraction system, which is a benefit in lab-scale, but difficult to achieve in large scale without additional equipment. Morphological response by strain to oil extraction methods Guinardia sp.
Morphological comparison of a microalgae strain to all oil extraction methods performed was made using the strain Guinardia sp. Amphiprora sp. After observation of cells before extraction process can be seen that Amphiprora sp. After performing SHE extraction using this biomass Figure 3b , can be observed significant changes in the morphology of the cell as the presence of chloroplast outside of the cell and changes in shape and colour of the cell, this changes are promoted by two main factors, mechanical destruction by high speed homogenization and effectiveness of solvents mixture used for microalgae compounds removal, however, degree of cell destruction confirms the low selectivity of SHE method for extraction of lipids usable in biodiesel production.
When biomass is submitted to CSE method can be seen that microalgae cell wall is still present although is drastically deformed and damaged, is also shown that most of intracellular content including lipids was released, hexane could break through the degraded cell wall dissolving neutral lipids and other non-polar components Figure 3c. Navicula sp. For Navicula sp. After oil extraction using EHE method Figure 4b , can be still found cells without damage and other with most of metabolites present within the cell, this morphological response helps to explain the low efficiency of EHE method in comparison to other microalgae oil extraction methods evaluated, Figure 4c shows microalgae biomass after performing HBE method where can be seen a higher percentage of broken cell walls in comparison to EHE method, can be observed several chloroplast outside of the cell which means that metabolites were released, but were not dragged by the solvent, behaviour of microalgae biomass after CBE method performing was very similar Figure 4d , this observation confirms the selectivity of non-polar solvent based extraction methods to microalgae lipids.
On the other hand, Nannochloropsis sp. Taking into account biomass composition, morphologic response and oil yield, microalgae genera Amphiprora sp. SHE method shows the highest yield as result of combination of polar and non-polar solvents, as disadvantage presents the extraction of non-desirable lipids for biodiesel production, as sterols, pigments and other non-lipid metabolites, taking into account that, in lab-scale is convenient the utilization of this method for total lipid determination in non-characterized strains, however, overestimation of lipid percentage derived of extraction of other microalgae metabolites must be taken into account, in addition, SHE method presents the highest toxicity and lowest percentage of solvent recovery of methods evaluated, which makes expensive and risky the continuous utilization of this method even with solvent recovery strategies.