Recent Advances in Lipid Extraction for Biodiesel Production

Chapter 10

Recent Advances in Lipid Extraction for Biodiesel Production Narges Moradi-kheibari1, Hossein Ahmadzadeh1, Ahmad Farhad Talebi2, Majid Hosseini3 and Marcia A. Murry4 1

Department of Chemistry, Ferdowsi University of Mashhad, Mashhad, Iran, 2Microbial Biotechnology Department, Semnan University, Semnan, Iran, 3Manufacturing and Industrial Engineering Department, The University of Texas Rio Grande Valley, Edinburg, TX, United States, 4Department of Biological Sciences, California State Polytechnic University, Pomona, CA, United States

10.1 INTRODUCTION Over the last decade, global population growth increased by 10%, while oil consumption grew by 13%, a consequence of the industrialization of emerging countries [1,2]. Thus the search for the renewable and sustainable biofuels has recently received much attention. Among many alternative transportation fuels, biodiesel is gaining ground because normal compression ignition engines are able to consume it without adverse effects to operating performance [3]. Biodiesel can be produced from the transesterification of plant or animal oils, vegetable oil, waste cooking oil, algae oil, and fats [4 6]. Glycerides, or acylglycerols (AGs), are the energy storage form of lipids within algae and plant cells. Generally, algae lipids are classified as either nonpolar or polar [4]. Nonpolar or neutral lipids (NL) are composed of free fatty acids (FFAs) and AGs [4]. Polar lipids are mainly comprised of two subcategories, phospholipids (PLs) and glycolipids [7]. Although the lipid members of both categories can be converted to biofuels, nonpolar lipids and, especially AGs, are ideal lipid fractions for biodiesel production, because these fractions are easily transesterifiable [8]. Furthermore, the unsaturation degree of AGs is lower than other lipid types found in algae, which produces fatty acid methyl esters (FAMEs) with higher oxidation stability [7]. The chain-lengths of microalgae fatty acids (FAs) range from 12 up to 22 [9]. Biodiesel fuel quality is impacted by the specific types of lipids Advances in Feedstock Conversion Technologies for Alternative Fuels and Bioproducts. DOI: https://doi.org/10.1016/B978-0-12-817937-6.00010-2 © 2019 Elsevier Inc. All rights reserved.

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extracted from the algae biomass [4,10,11]. Therefore, extraction of lipids represents a major step in successful biodiesel production from microalgae feedstock. Both novel and traditional means of oil extraction have been explored at industrial and laboratory scales. For example, mechanical extraction, “accelerated solvent extraction (ASE),” “ultrasound-assisted extraction (UAE)” [12], “microwave-assisted extraction (MAE)” [13], “electrochemical extraction (ECE)” [14], “supercritical fluid extraction (SCFE)” [15], and “subcritical water extraction (SCWE)” [16] have attracted considerable research attention in the last decade, with the goal being to develop technologies for reliable oil extraction from microalgae [17]. In marine systems, members of Pyrrophyta and Chrysophyta are the main microalgae divisions represented [18]. High photosynthetic efficiencies coupled with rapid growth lead to rapid biomass production and lipid accumulation rates [7]. Microalgae lipid content is dictated by the species and environmental conditions [7,19]. Although biodiesel has many advantages in comparison with petrodiesel, its production cost is a major barrier for largescale applications of this technology [20]. New technologies for cultivation, harvesting, and oil extraction must be developed for microalgae biofuels to be cost-effective and economically feasible. As an example, cultivation of microalgae for CO2 mitigation and wastewater bioremediation can potentially reduce overall biofuels production cost [21,22]. Due to the influences of extraction methods on biodiesel quality, the extraction method should be properly selected. This chapter will provide a broad examination of currently available microalgae lipid extraction techniques along with their advantages and shortcomings. In addition, a comprehensive discussion of the recent advances in lipid extraction methods is provided.

10.2 LIPID EXTRACTION METHODS 10.2.1 Expeller Pressing First used in the 1900s to extract oils from plant seeds, expeller or screw pressing uses high mechanical pressure to crush the often tough cell wall of algae biomass to compress the lipid out from cells. One of the advantages of this method is that no chemicals are added during the expeller pressing, and the device used is not complicated. However, this method suffers from some disadvantages such as the need for a large amount of biomass [9], lower oil recovery relative to chemical extraction methods, not being selective for triacylglycerols (TAGs) [23], and the extraction of pigments along with oil. Furthermore, because of the high pressure imposed on the cell wall, mechanical friction causes the biomass to heat up. As such, the higher temperatures inherent in expeller processing negatively affect the extracted lipid’s quality [24]. There are few laboratory studies of expeller extraction of microalgae

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oils in the literature. A 75% lipid extraction efficiency was achieved from 70 kg of seaweed (macroalgae) using screw expeller press [25].

10.2.2 Solvent Extraction Method Solvent extraction (SE) is well established in the separation of lipids from other cellular components based on their relative solubility in organic solvents. Hexane, chloroform, and dichloromethane (i.e., organic solvents) easily dissolve NL and are extensively used in traditional lipid extraction from algae [17]. Combinations of solvents with different polarities have been applied successfully [4]. When the feedstock is wet due to the hydrophilic nature and hydrated surface of algae cell walls, a combined nonpolar/polar solvent system was reported to achieve higher yields of lipid extraction [26]. When the feed is wet and nonpolar solvents are used individually, the penetration of solvent into the cell wall does not occur efficiently, resulting in lower extraction efficiency. On the other hand, polar solvents can operate as a dispersive solvent which increases mass transfer and surface contact between the algae lipid and nonpolar organic solvents, reducing the extraction time and enhancing extraction efficiency [27]. Due to their lower toxicity and cost, methanol and ethanol have widely been used as polar solvents [28]. The polar solvent should be soluble in the nonpolar solvent, and addition of both polar and nonpolar solvent is preferred to be simultaneous [29]. Separation of solvents is achievable through the addition of water. Water molecules form hydrogen bonds with polar solvents and eject the nonpolar solvent containing lipids from the aqueous phase [30]. Chloroform is a commonly used solvent in batch extraction procedures [29]. Even in laboratory-scale experiments, less toxic solvents are preferred due to health concerns. When recovery of a specific lipid product is desired, the selectivity of the solvent is important. As an example, for biodiesel production, the separation of TAGs is favored [4,7] and the coextraction of pigments lower the quality of biodiesel [4,31]. A selective solvent solubilizes the target component much more efficiently than other components of the sample, is based on solvent polarity, and must be matched to that of the TAGs’ polarity [4,32]. Long-chain AGs are more soluble in solvents with low polarity. SE methods are performed in both continuous and batch process [9]. Continuous organic SE was originally designed for the extraction of solutes with low solubility from a solid material. Lipid extraction via this methodology allows for the processing of a large amount of biomass, while recycling a small amount of solvent. A Soxhlet extractor is a laboratory apparatus used for continuous extraction in which a specialized piece of glassware is placed between a flask and a condenser [33]. Continuous extraction methods have some disadvantages including that the methods cannot be applied to small amounts of biomass, the thermolabile components are at risk of being

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degraded, and high extraction efficiency is time consuming [34]. Batch extraction procedure is a commonly used approach for lipid extraction at small scales (e.g., in laboratory) due to its relative speed and that it requires only minute amounts of sample [7]. However, it is worth noting that the yield of batch SE processes for single-step extraction is typically not very high. SE is generally limited to laboratory practice and shows limited ability of extraction from intact cells. Therefore, the SE method, when combined with an effective cell disruption techniques, can be used to increase the yield of lipid extraction.

10.2.3 Supercritical Fluid Extraction Method SCFE is a methodology that employs solvents in the supercritical condition to extract lipids from biomass [7]. Supercritical fluids (SCFs) are the extracting solvent in a state that can separate lipid content from the matrix of biomass. SCF refers to a compound at ambient conditions above the critical point in which distinct liquid and gas phases of solvent are nonexistent (Fig. 10.1) [35]. Due to the compressibility of a SCF, it behaves as a gas. SCFs have the density of a liquid (between 0.1 and 1.0 g mL21) and therefore have the dissolving power of a liquid. Furthermore, supercritical viscosity values and diffusion coefficients lie between those of liquids and gases, which result in low surface tension and high mass transfer capability [36]. The intermediate characteristics of these two extremes of state grant the solvent power of liquids coupled with the transport properties common to gases.

FIGURE 10.1 Schematic of a typical SCFE system. SCFE, Supercritical fluid extraction.

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10.2.3.1 Supercritical Carbon Dioxide Extraction Method Among many solvents, supercritical carbon dioxide (SCCD) has been a frequently utilized [37]. The most common advantages of SCCD extractions are that CO2 is nonflammable, relatively inexpensive, and nontoxic [38]. Moreover, due to the gaseous state at ambient pressure, no separation step is necessary in order to remove the solvent from the extracts. In addition, the lower critical temperature of CO2 extraction does not degrade thermolabile compounds [39]. Furthermore, when carbon dioxide used is in the extraction, it can be condensed and liquefied by utilizing a compressor and eventually returned to the extraction cycle [40]. Due to the low-processing temperatures and solvent-free technique, the residual biomass maybe utilized as fertilizer and animal feed [41], which is not possible with solvent-based processing. Accordingly, the use of supercritical carbon dioxide extraction (SCCDE) has been the subject of many investigations recently [7,42]. SCCD in the extraction process represents unique features. First of all, due to the low polarity of SCCD, nonpolar or partially polar compounds are more efficiently extracted. SCCD also has a higher extraction efficiency of low molecular weight solutes. In addition, as SCCD pressure increases, the solubility of solutes with high molecular weight is enhanced. SCCD has high solubility of oxygenated organic compounds of medium molecular weight. Finally, varying critical conditions (i.e., pressure/temperature combination) could change the dissolving power and selectively of SCCD, allowing for the sequential extraction of different lipids [43]. Optimization of extraction parameters, including temperature, pressure, flow rate, time, and sample size, improves the extraction efficiency of specific compounds [44]. Generally, increasing pressure up to about 30 50 MPa enhances the extraction efficiency while raising the temperature above 50 C exhibits a reduction in oil extraction form some green algae [15]. Furthermore, the physiochemical properties of SCCD can be altered by using modifiers (i.e., cosolvents). The addition of a modifier can cause enhancement in the SCFE efficiency through enhanced separation of more polar compounds [43]. The results reported by Tang et al. [45] showed considerable improvement in the lipid yield obtained using ethanol as cosolvent. Moreover, addition of EtOH or CH2Cl2 as cosolvents in SCCDE improved the extraction of TAGs during lipid extraction from Nannochloropsis oculata [46]. A typical SCFE apparatus is depicted in Fig. 10.1. The essential features include a CO2 source, a means to pressurize the gas (i.e., pump), an extraction vessel within an oven, an extraction line restrictor in order to maintain high pressure, as well as an analyte-collection device [47]. The material of construction is usually stainless steel because it is chemically inert and resists high pressures [41]. The extraction process starts with flowing the liquid CO2 toward the pump. The pressure rises above the critical pressure, and by using a heater, the temperature of the fluid reaches the critical

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temperature. The SCCD passes through the extraction vessel’s loaded sample and, upon exit, enters a collection vessel. Here, the fluid is quickly depressurized by a micrometering valve (restrictor). Before the extraction vessel, a modifier can be added to the SCCD via an external pump, directly added to the extraction cell’s sample [47]. To prevent the SCCD flow from carrying away the sample biomass, a set of frits are situated at both the extraction vessel’s ends. Upon complete depressurization, the SCCD becomes a gas, precipitating the extracted crude lipids in the collection device [7]. Pressure-reducing valves are usually equipped with heaters to prevent the line blockage due to freezing water as well as the formation of dry ice and/or deposition of heavy molecules inside the pipes [35]. SCFE can be accomplished in dynamic, static, or coupled mode (i.e., static/dynamic). By combining an initial static period with a subsequent dynamic extraction where SCCD flow is maintained through the extraction chamber, an optimized quantitative approach can be obtained [36]. SCCDE is an environment-friendly method which has no negative effects on the residual biomass and extracted lipid. The main advantages over SE methods include SCCDE’s nonexplosive, nontoxic, and nonflammable properties as well as its ability to be reclaimed. CO2 can easily evaporate, facilitating separation of the analyte from the solvent, and allows for the CO2 to be recycled using a condenser. Although SCCD was first discovered in the early 1800s, new practical applications of this discovery have only recently appeared and have been embraced by industrial communities in the last few years. However, instrumentation necessities for SCCDE contribute to the high capital cost of commercially available equipment when evaluated against typical extraction processes.

10.2.4 Subcritical Water Extraction SCWE is another example of an extraction process that uses pressurized liquid water at temperatures above its boiling point while remaining below its critical point [48]. At these conditions (i.e., subcritical, below 374 C/647 K, 22.1 MPa), water is superheated rather than supercritical [49]. At high temperature and pressure, water exhibits very different behavior than water at ambient conditions because water’s hydrogen-bonded lattice is disrupted. Liquid water at subcritical conditions dissolves compounds with low polarity [50]. Applying pressure has a minimum impact on the characteristics of subcritical water, and it is utilized for the purpose of maintaining water in its liquid form [51]. However, when temperatures exceed 300 C, the physical properties of SCWE (e.g., density) change significantly with high pressure [49]. The increased extraction rate with increasing pressure at temperatures below 300 C may be due to effects on the substrate, particularly in plant and algae feedstocks, rather than changing physical properties of water. Varying temperature also imposes greater changes to water behavior than would be

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expected. Properties such as viscosity, dielectric constant, density, dissociation constant, and surface tension as well as diffusivity of water, all vary with temperature which can modify solubility of less polar compounds in SCWE [48]. At 275 C, liquid water’s dielectric constant and saturation pressure are nearly equal to that of CH3OH and C2H5OH at ambient conditions, which increases solubility for less polar compounds [52,53]. Water’s dissociation constant (Kw) increases with temperature and causes pH decreases to about 5.5. Since hydronium ions act as a catalyst, decreased pH increases the risk of hydrolysis [50]. Increasing the temperature also causes lipid degradation and causes oxidation reactions to occur in the system if oxygen is not purged from the water before the extraction [54]. Other parameters that can influence extraction efficiency include extraction time, flow rate, addition of modifiers (e.g., surfactants, light alcohols), as well as the biomass’ particle size and moisture content [49]. Several studies have examined microalgae lipid extraction using SCWE [55 57]. Reddy et al. [16] used static SCWE to extract lipids from Nannochloropsis salina and optimized extraction temperature (i.e., 215 C), time (i.e., 25 minutes), and biomass loading. A significant enhancement was observed in the total lipid yield using SCWE compared to the Folch method, while TAGs content did not show significant differences. However, diacylglycerols (DAGs) and monoacylglycerols (MAGs) showed an enrichment over the lipid content of extracts produced using the Folch method, indicating glycerol lipid acyl chains have been partially hydrolyzed at subcritical conditions [16]. In a reported study, a comparison between the total lipid yield and composition extracted from Scenedesmus sp. using SCWE and Bligh and Dyer methods [58] indicated that while the methods delivered identical total lipid yields, analysis of the lipid composition showed lipid extracted using the SCWE method contained more TAGs and DAGs relative to the Bligh and Dyer method [58]. The equipment needed for SCWE are similar to those used in SCFE, and both techniques can be run in dynamic, static, or a combination of the two modes [49]. Dynamic SCWE also requires an extraction vessel, a collection vessel, a heater, a pump, and a pressure restrictor. The extraction vessel and the tubing are typically composed of stainless steel [54]. Frits are typically installed at both ends of the extraction vessel to hinder sample losses and line plugging. A pressure restrictor before the collection vessel is necessary to avoid pressure drop and boiling of water [51]. Decrease in temperature and pressure in the outlet pipe can cause deposition of the analyte, blocking the system, which is avoided by using a second pump to inject a solvent into the tube lines after the extraction and by keeping the vessel warm using a heating tape [49]. In static SCWE mode, pump and pressure restrictor is not obligatory. Manual water addition to the extraction vessel is accomplished via an

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apparatus and mechanism resembling an autoclave. As the vessel is closed and the temperature increases, the pressure rises and subcritical conditions are created [59]. A major disadvantage to the static mode of extraction is that the residence time of analytes is increased when compared to that of the dynamic mode, causing degradation of thermally labile analytes [60].

10.2.5 Electrochemical Extraction ECE of lipid from microalgae cells is simple, clean, cheap, and quick [61]. The method disrupts cells using a pulsed electric field (PEF) providing a single-step extraction and, as such, is gaining attention. A schematic of ECE is depicted in Fig. 10.2. The use of an external electric field, above the critical threshold of biological cells, can structurally alter the cell membrane along with the PL layers. The cell membrane loses its gatekeeping function and becomes permeable [62]. Based upon the electric field’s intensity, pulse width, and a variety of taxa-specific algae cell characteristics, reversible or irreversible permeabilization occurs [63,64]. Irreversible destruction of cell membranes allows the cell contents to release into the medium [62,65]. Teissie et al. [66] have reviewed the theory of cell membrane electropermeabilization. A typical PEF system includes a “pulse generator” that allows for steady and continual pulse treatment, electrode filled flow chambers, and a fluidhandling system [61]. The extraction conditions for maximum extraction efficiency are species specific and vary based on the algae cells characterization and compounds of interest [63]. Several parameters may influence the extraction and electrochemical performance efficiency including the distance between electrodes, surface area of the electrodes, pulse duration, number of pulses, field strength [62], the type of materials used for anode electrodes, and the conductivity between the anode and cathode electrodes, with increased salt levels enhancing cell disruption [67]. Furthermore, electroporation efficiency needs to be optimized while considering the algae cell’s geometry and size. Smaller microorganisms need greater electric field strength [64,68]. For example, an electric field strength of 10 15 kV cm21 may be optimal for the electroporation of microbial cells that are 1 10 µm in diameter [64].

FIGURE 10.2 Schematic of ECE method: (1) applying pulsed electric filed, (2) rupturing the cell wall by applying electric field, and (3) phase separation. ECE, Electrochemical extraction.

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Although PEF has been typically used as a pretreatment to enhance the extracted lipid in the laboratory scale, its application as a lipid extraction method is relatively new [62]. The major issue is the separation of the lipid layer after extraction from the aqueous surface layer. Flisar et al. [61] evaluated a continuous flow PEF system for extraction of lipids from Chlorella vulgaris, in which 22% lipids from dry mass were extracted. The PEF approach is promising for commercial operations to extract microalgae lipids since it is a nonthermal and environment-friendly method which neither inserts additional impurities into the products nor induces any adverse changes in the target analyte [62,67]. Furthermore, the PEF methods are applicable for wet biomass and dewatering steps, a major expense in algae feedstock is avoided [68].

10.2.6 Modifications in Lipid Extraction Methods 10.2.6.1 Microwave-Assisted Extraction MAE is an extraction technique that speeds up the process of extraction. The kinetics of extraction will be faster when it is performed under microwave exposure. Microwave’s energy enables the solvent penetration into layers of the samples and, therefore, increases the rate of SE [9]. An oscillating electric field causes “vibrations of polar molecules” along with inter- and intramolecular friction [69]. The friction of all charged ions in the sample causes a very rapid heating of the whole sample (volumetric heating) [69]. Intracellular water evaporation increases pressure resulting in cell disruption. Subsequently, the maximal partition of lipids into the solvent phase is observed at the same solvent ratio, increasing the efficiency of extraction [24,69]. The mechanism of microwave extraction has been discussed comprehensively [70]. Recent studies of MAE in lipid extraction from algae biomass have demonstrated the enhancement in the extraction yield relative to conventional methods [69,71,72]. Balasubramanian et al. [69] and Iqbal and Theegala [71] demonstrated a 40% and 8% improvement in lipid yield, respectively, in comparison to Soxhlet extraction. It is worth noting that over longer periods of time, raising the vessel’s temperature can cause more conversion of TAGs into MAGs and DAGs. However, in a MAE, the recovery of biodiesel from the reaction mixture was reported to take about 15 20 minutes, which is faster than that of conventional heating method (6 hours) [24,73]. MAE techniques benefit from several advantages over traditional methods including feasibility of lipid extraction from wet biomass, eliminating of dewatering steps, and shortened extraction time due to the rapid cell warming reducing energy consumption [24]. Therefore, MAE was proposed as a rapid and economical approach for lipid extraction [13,74]. The factors affecting the efficiency of MAE techniques include time, temperature,

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dielectric properties of the sample mixture, and solvent type [69]. The need for special equipment, low selectivity, and unavoidable reaction in high temperature is regarded as MAE drawbacks [75]. Oxidation and hydrolysis of AGs during MAE affect biodiesel production yield and quality [76].

10.2.6.2 Ultrasonic-Assisted Extraction UAE uses ultrasonic waves throughout the extraction procedure, thus improving the efficiency of extraction. Ultrasound involves the generation of mechanical waves which can be spread in an elastic medium at frequencies above the human threshold of hearing while also at submicrowave frequencies (20 kHz 10 MHz) [77,78]. Pulsing ultrasonic waves with high frequencies ranging from 20 to 1000 kHz through a liquid induces consecutive compression/decompression cycles [79]. The rapid variation in pressure creates cavitation through the liquid [80]. Each cavitation bubble creates a high temperature spot (around 4000 6000 K) with high pressures (roughly 100 200 MPa) [79]. The cavitation effect can disrupt cell membranes, aiding in the release of extractable compounds and enhancing mass transfer for lipid extraction [77]. UAE has been used to accelerate both SE [81] and SCFE [82] processes enhancing extraction efficiency. UAE can take place in an ultrasonic bath or through the use of an ultrasound instrument/probe [80]. In SCFE, an ultrasound probe is practical as it also agitates the sample in the extraction vessel to improve the yield [82]. UAE improves lipid extraction from intact cells by providing a greater contact surface between the extracted compounds and solvent that reduces the extraction times and does not require addition of chemicals [17]. Furthermore, UAE provides enough energy to break firmly bound lipids [77]. Nevertheless, extending the duration of ultrasonication may result in free radical formation, thus reducing lipid quality [24]. In some cases, UAE has no effect on the cell wall disruption and shows no improvement in the lipid yield. Widjaja et al. [83] used UAE on C. vulgaris which showed no improvement in efficiency of the lipid extraction and time [83]. Therefore, the effect of ultrasound on extraction yield is species specific and is influenced by the cell wall composition. It was reported that using UAE resulted in an improvement in both extraction efficiency and extraction time when compared with Soxhlet extraction [84,85]. Moreover, Bermu´dez Mene´ndez et al. [86] demonstrated the beneficial aspect of an ultrasound-assisted approach on the efficiency of lipid extraction in the microalga Nannochloropsis gaditana and its operating cost. 10.2.6.3 Accelerated Solvent Extraction In ASE method, organic solvents in subcritical conditions are utilized. ASE is quite similar to the SCWE method [34]. There are several advantages of

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using hot organic solvents compared to the low-temperature extractions. High temperature of solvent increases the lipid solubility [9], and a lower amount of solvent is needed [87]. Temperature increases the diffusion rates of solvent, which reduces the viscosity of solvents and lipid and the surface tension, increases the mass transfer, and lowers extraction time [88,89]. Furthermore, disruption of the strong noncovalent bonds increases lipid extraction efficiency [88]. However, this method suffers from some disadvantages including high energy requirements to supply the subcritical condition. Moreover, the probability of lipid decomposition increases with temperature [88]. A comparison of ASE and SE in algae lipid extraction showed changes in the FA composition, especially those with longer carbon chains, in comparison with conventional SE method [87]. The result reported by Pieber et al. [90] showed a high lipid extraction yield from Nannochloropsis oculata by using pressurized hot ethanol, which was attributed to a decrease in selectivity and extraction of nonesterifiable lipids. The apparatus used in ASE is similar to that used in SCWE and SCFE as it can be operated under dynamic, static, or a combination of both modes. The parameters affecting the extraction yield include temperature, sample size and matrix, solvent type, and flow rate in the case of dynamic mode [34].

10.3 INFLUENCE OF EXTRACTION METHODS ON BIODIESEL PROPERTIES The FA composition of extracted lipid influences the produced biodiesel’s physicochemical properties [4,91]. Carbon chain length influences the physical features of FAME molecules and the number of double bonds [92]. These features influence kinematic viscosity, cold filter plugging point, higher heating value, cetane number, and density, all determining factors for biodiesel quality [91]. The quality criteria are specified by international standards, including for example, ASTM 6751-3 or EN 14214, and widely used to establish the standard quality of biodiesel. These quality specifications are monitored experimentally and can be predicted using empirical equations based on the FAME profile of the biodiesel product [18,93,94]. Recently, in silico approaches have been developed to accelerate this evaluation process [95]. The extraction method may also influence FAME profiles, quantity, quality of extracted lipid fractions, and the cost of the produced biodiesel. The specific algae strains and the cultivation conditions dictate FAME profiles and the quantity of derived biodiesel [4,18,96]. For example, polar lipids are relatively proportional to cell numbers as opposed to storage of TAGs which depend on physiochemical conditions of cultivation. These conditions include but not limited to light levels, carbon availability, and essential nutrient contents especially nitrogen and phosphate [19]. Since high concentrations of phosphorus

TABLE 10.1 Methods and Results Summary of Some Microalgae Lipids Extraction Techniques Microalgae

Extraction Method

Solvents

Pretreatment

Extraction Condition

Lipid (%)

References

Chlorococcum sp.

Soxhlet

n-Hexane

Oven Drying (85 C, 16 h)

4 g Algae, 300 mL solvent, 330 min

3.2

[99]

Pavlova sp.

Soxhlet

n-Hexane

Bead-beating

2 g Algae, 450 mL, 900 min

15.5

[38]

Chlorella vulgaris

Soxhlet

Acetone

Oven drying (60 C, 24 h)

5 g Algae, 110 mL solvent, 480 min

1.8

[84]

Nannochloropsis oculata

Soxhlet

Ethanol

Freeze drying

10 g Algae, 300 mL solvent, 960 min

40.9

[46]

Nannochloropsis sp.

SE

Chloroform/methanol (1/1 v/v)

Freeze drying

0.1 g Algae, 8 mL solvent, vortexing

30.2

[104]

N. salina

SE

Chloroform/methanol (5.7/3 v/v)

Freeze drying

1 g Algae, 15 mL solvent, 25 C, 120 min vortexing

47.7

[105]

N. salina

SE

Chloroform/methanol (1/2 v/v)

Drying

0.1 g Algae, 5 mL solvent, 65 C, 60 min vortexing

35

[58]

Scenedesmus sp.

SE

Chloroform/methanol (2/1 v/v)

Drying

0.1 g Algae, 2 mL solvent, 25 C, 30 min vortexing

20

[16]

Pavlova sp.

UAE

Ethyl acetate/ methanol (2/1 v/v)

Bead-beating

10 g Algae,72 mL, 180 min

44.7

[38]

C. vulgaris

UAE

Chloroform/methanol (1/2 v/v)

Oven drying (60 C, 24 h)

5 g Algae,37.5 mL, 20 min sonication

52.5

[84]

N. oculata

UAE

Petroleum ether

Oven drying (105 C, 48 h)

Frequency 40 kHz, 60 min

3.3

[106]

Nannochloropsis gaditana

UAE

Methanol

Drying

Frequency 40 kHz, 50 60 C, 20 min

38.1

[86]

N. gaditana

MAE

Methanol

Drying

Frequency 2.45 GHz, 60 C, 20 min

39.6

[86]

Chlorella sorokiniana

MAE

Ionic liquid [BMIM] [HSO4]

Vacuum drying oven

1 g Algae, 5 g solvent, 120 C, 60 min

23

[13]

C. sorokiniana

MAE

Chloroform/methanol (1/1 v/v)

Vacuum oven drying

1 g Algae, 10 mL solvent, 120 C, 60 min

9

[13]

N. salina

MAE

Ionic liquid [BMIM] [HSO4]

Vacuum oven drying

1 g Algae, 5 g solvent, 120 C, 60 min

10

[13]

Chlorococcum sp.

SCCDE

CO2

Oven drying (85 C, 16 h)

60 C, 30 MPa, 80 min

5.8

[99]

Pavlova sp.

SCCDE

CO2

Bead-beating

60 C, 30 MPa, 360 min

17.9

[38]

40 C, 35 MPa, 30 min

33.9

[45]



Shizochytrium limacinum

SCCDE

CO2 1 Ethanol

Nannochloropsis sp.

SCCDE

CO2

Freeze drying

55 C, 55 MPa, 100 min

25.2

[107]

Scenedesmus sp.

SCWE

Water/[HNEt3] [HSO4] (100/1 v/v)

Drying

110 C, 1 MPa, 60 min

35.7

[58]

N. salina

SCWE

Water

Drying

Batch, 220 C, 25 min

30

[16]

MAE, Microwave-assisted extraction; SCWE, subcritical water extraction; SE, solvent extraction; SCCDE, supercritical carbon dioxide extraction; UAE, ultrasoundassisted extraction.

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and sulfur are present in polar (membrane) lipids, lipids insulated from strains with few storage TAGs will have relatively higher concentrations of these elements which reduce the quality of final biodiesel [97,98]. Again, implementation of an approach with high selectivity and solubility for TAGs and low solubility for polar lipid is more appropriate in production of biodiesel with higher quality. Applying high temperature during the extraction methods (e.g., SCWE, MAE, and ASE) may increase the risk of TAGs’ decomposition [50,76,88], which decreases the biodiesel production efficiency. In a similar trend, raising the temperature in SCCDE resulted in the reduction of lipid extraction yield [15,99]. Heating up the wet feedstock can cause hydrolysis of TAGs and producing FFAs [50,76]. As a matter of fact, high FFA content influences the transesterification process for biodiesel production [5,6,100]. The extraction method could also affect the FAME content and its profile. The FA profiles containing a high amount of oleic and palmitic acids fall within the optimal parameters set by international standards [10,92]. The effect of the lipid extraction process on the FAMEs profile has to be evaluated early when developing production methodologies. For instance, a comparison of the Bligh and Dyer method and SCCDE showed a lower total lipid yield and less extractability of longer chain FAs by the SCCDE method [101]. However, when oleic and palmitic acid are present in large quantities in the FAME profile of a lipid extracted by SCCDE, this method resulted in a better quality biodiesel than the former [101]. When the SCWE method was used for lipid extraction from macroalgae feedstock, raising the temperature resulted increased the total lipid extraction yield; however, the amount of the FAME fraction was reduced significantly [102]. According to a reported study, despite the lower total lipid yield for SCCDE in comparison with SE, SCCDE extracted a greater fraction of FAMEs [103]. A summary of some commonly used microalgae lipid extraction techniques utilized for biodiesel production are presented in Table 10.1.

10.4 CONCLUSION Lipid extraction is a crucial step which will affect the biodiesel quality, quantity, and cost efficiency of the operation in algae-based biodiesel. An ideal extraction method needs to be solvent free, low cost, selective, and efficient. Currently, the search to find novel lipid extraction techniques, which are more environment friendly, less energy and labor intensive, timely, costeffective, scalable, and sustainable, is still in progress. The methods such as SE and SCCDE have exhibited excellent results in terms of extraction yields. But the challenges in large-scale application should be addressed. The SCCDE method may be considered as a scalable and environment-friendly method where CO2 is used as the solvent. Since the CO2 can be recycled during extraction, SCCDE method may be considered as a technique that inflicts less damage on the environment. Furthermore, the residual biomass

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is solvent free and is suitable for agricultural use (e.g., fertilizer, feed). The production of coproducts may be essential, at least for the short term to allow for the economic viability of algae biodiesel. The ECE approach has low efficiency, but due to the low cost of required facilities and the resulting solvent free residual biomass, lipid extraction by this method is gaining more attention. SCWE and ECE methods omit the harvesting and drying of microalgae biomass and may be tied directly to the microalgae culture facilities. Many studies have evaluated extraction methods as a downstream process for biodiesel production at the laboratory scales, but industrial-scale production of algae biodiesel continues to be hindered by inadequate scale up of extraction procedures.

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