Using renewable ethanol and isopropanol for lipid transesterification in wet microalgae cells to produce biodiesel with low crystallization temperature

Energy Conversion and Management 105 (2015) 791–797

Contents lists available at ScienceDirect

Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman

Using renewable ethanol and isopropanol for lipid transesterification in wet microalgae cells to produce biodiesel with low crystallization temperature Rui Huang, Jun Cheng ⇑, Yi Qiu, Tao Li, Junhu Zhou, Kefa Cen State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China

a r t i c l e

i n f o

Article history: Received 6 July 2015 Accepted 13 August 2015

Keywords: Biodiesel Ethanol Isopropanol Wet microalgae cell Microwave-assisted transesterification Crystallization temperature

a b s t r a c t Renewable ethanol and isopropanol were employed for lipid transesterification in wet microalgae cells to produce biodiesel with low crystallization temperature and reduce the alcohol volume needed for biodiesel production. Decreased droplet size and lipid polarity were observed after transesterification with alcohol in microalgae cells. Such decrease was beneficial in extracting lipid from microalgae with apolar hexane. The effects of reaction temperature, reaction time, and alcohol volume on microwave-assisted transesterification with ethanol and isopropanol were investigated, and results were compared with those with methanol. Microwave-assisted transesterification with ethanol and isopropanol, which were more miscible with lipid in cells, resulted in higher fatty acid alkyl ester (FAAE) yields than that with methanol when the reaction temperature was lower than 90 °C. The ethanol and isopropanol volumes in the transesterification with 95% FAAE yield were only 75% of the methanol volume. The crystallization temperatures (0.19 °C and
3.15 °C) of biodiesels produced from wet microalgae through lipid transesterification in cells with ethanol and isopropanol were lower than that with methanol (2.08 °C), which was favorable for biodiesel flow in cold districts and winter. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Biodiesel production from microalgae is a promising approach to solve the global problems of energy shortage and air pollution [1–3]. This approach involves different processes, including microalgae cultivation, microalgae separation and harvesting, oil extraction, and biodiesel conversion [4]. Among these processes, a cost-effective oil extraction and biodiesel conversion process is critical for the industrial-scale production of microalgal biodiesel [5,6]. Microalgae drying consumes high energy. Thus, various processes have been developed to produce biodiesel from wet microalgae, such as lipid extraction from wet microalgae after cell disruption, direct transesterification of wet microalgae with chloroform, and conversion of wet microalgae in supercritical alcohols [7–10]. However, most reported processes that obtain high biodiesel yields from wet microalgae involved either stringent reaction conditions (high temperature and pressure) or the use of toxic chloroform, both of which posed challenges for the efficient scale-up of production. Microwave irradiation can efficiently disrupt microalgae cell walls and promote biodiesel production from wet microalgae [11]. However, the lipid extraction process is performed before the ⇑ Corresponding author. Tel.: +86 571 87952889; fax: +86 571 87951616. E-mail address: [email protected] (J. Cheng). http://dx.doi.org/10.1016/j.enconman.2015.08.036 0196-8904/Ó 2015 Elsevier Ltd. All rights reserved.

transesterification process. No prior transesterification was performed to promote the lipids extraction with hexane. Thus, the effects of transesterification on lipid extraction have not been investigated. To further increase the biodiesel yield, a previous study produced crude biodiesel through extraction with hexane after microwave-assisted lipid transesterification in wet microalgae under mild reaction conditions without using toxic chloroform [12]. The hexane-extracting fatty acid methyl ester from wet microalgae was increased sixfold after lipid transesterification, a yield that is comparable with that obtained through direct lipid transesterification of dried microalgae biomass with chloroform. However, the effect of transesterification on the lipid droplets was not investigated; only methanol was investigated for transesterification. Although the lipids in wet algae were effectively converted into biodiesel in the previous study, methanol, which is used in microwave-assisted lipid transesterification in wet microalgae biomass, is a toxic and nonrenewable commodity [13]. Compared with methanol, longer-chain alcohols such as ethanol, isopropanol, and butanol are nontoxic and are successfully produced from renewable resources through fermentation [14,15]. Moreover, the transesterification of lipid in wet microalgae cells can only be catalyzed by acid because of the presence of substantial water and free fatty acid [16,17]. Previous studies reported that acidcatalyzed transesterification conducted with longer-chain alcohols

792

R. Huang et al. / Energy Conversion and Management 105 (2015) 791–797

produced 30% higher yields than methanol because of their increased miscibility with triglyceride [18,19]. Using longer-chain alcohols also increased the flow properties of biodiesel at low temperature, which is critical for commercial year-round biodiesel production from microalgae [20]. The poor cold-flow property of biodiesel significantly limits its utilization in cold districts and during winters [21,22]. The use of alcohols with long and branching carbon chains for transesterification was determined to be an effective measure to improve the low-temperature properties of biodiesel [23]. The oxidation stability of biodiesel, which was improved with the increase in carbon number of alcohols [25], was important for fuel storage and transportation [24]. When the straight-chain carbon length of alcohol moiety was increased, NOx emission was also reduced [15]. Co-solvents, including benzene, carbon tetrachloride, chloroform, n-hexane, and toluene, were employed to promote the acid-catalyzed transesterification of wet microalgae with methanol. Chloroform resulted in the highest biodiesel yield among the used co-solvents [26]. To avoid the use of toxic chloroform, the transesterification of wet Nannochloropsis salina with methanol and ethanol via conversional heating was recently investigated by Kim et al. [27]. However, the changes of the lipid droplets in wet microalgae cells after transesterification were not characterized. The effects of using various alcohols for microwave-assisted transesterification on the subsequent hexane extraction, as well as the low-temperature properties of crude biodiesel, were also not investigated. Given the relatively low ability of longer-chain alcohols to absorb microwave, it is also unknown whether the microwave-assisted transesterification efficiency of lipid with longer-chain alcohols in wet microalgae cells is still higher than that with methanol. Renewable ethanol and isopropanol were employed for lipid transesterification in wet microalgae cells to produce biodiesel with low crystallization temperature and reduce the alcohol volume needed for biodiesel production. The changes in size and polarity of lipid droplets in wet microalgae cells after transesterification were also characterized. The effects of reaction temperature, reaction time, and alcohol volume on microwave-assisted transesterification with ethanol and isopropanol were investigated and compared with methanol. The properties of crude biodiesel produced with various alcohols for microwave-assisted transesterification were characterized and compared. 2. Methods

microwave digestion system (2.45 GHz; Shanghai Yiyao Microwave Chemistry Company, Shanghai, China) [12]. The stored C. pyrenoidosa biomass (2 g) was thawed and then transferred into the 60 mL digestion reactor of the microwave digestion system. Ethanol and isopropanol with 3 vol.% of concentrated sulfuric acid were added into the reactor and mixed with the thawed C. pyrenoidosa biomass. The reactor was sealed, and the mixture in the digestion reactor was heated to a preset reaction temperature via microwave irradiation with a power output of 600 W. The power output of microwave irradiation was set at 500 W to maintain the mixture at the reaction temperature for the preset reaction time. After the microwave-assisted lipid transesterification, the mixture was air-cooled to room temperature. The cooled mixture was then poured into a centrifuge tube. The reaction temperature (70 °C–110 °C), reaction time (2–30 min), and alcohol volume (ratio of alcohol volume to wet microalgae weight varied from 8:1 to 2:1) were varied to investigate the microwave-assisted lipid transesterification with ethanol and isopropanol in wet microalgae cells. To extract the resulted biodiesel after the microwave-assisted transesterification, hexane (16 mL) and water (16 mL) were added into the centrifuge tube and mixed with the cooled reactant through vigorous shaking of the centrifuge tube for 5 min. After the mixture was centrifuged, the resulted upper hexane layer was transferred into a pre-weighed glass vial and heated in a baking oven at 65 °C to regenerate the hexane. Crude biodiesel was then obtained and gravimetrically determined after the hexane regeneration. 2.3. Characterization of the lipid droplets in wet microalgae cells after transesterification The microalgae cells before and after transesterification were dyed using Nile Red (9-diethylamino-5H-benzo[a]phenoxazine-5 -one; Sigma-Aldrich, St. Louis, MO, USA). The fluorescence of the lipid droplets was observed through an inverted microscope (TI-S; Nikon, Tokyo, Japan) equipped with a B-2A light filter (EX 450–490 nm) and an analysis software (NIS-ELEMENTS D) to evaluate the size changes of lipid droplets after transesterification [30]. The contact angle of hexane against fatty acid and fatty acid alkyl ester (FAAE) was measured using a contact angle tester (JC2000D; Powereach, Shanghai, China) to evaluate the polarity changes of lipid droplets after transesterification.

2.1. Materials Bristol’s medium-cultivated Chlorella pyrenoidosa were used for the microwave-assisted lipid transesterification in microalgae cells [28]. A bioreactor with a volume of 2 L was pumped with air to cultivate C. pyrenoidosa for 15 days with illumination from an incandescent lamp (2500 lx, dark/light cycle: 12/12 h). The cultivated C. pyrenoidosa was dewatered by centrifugation (Beckman Avanti J26-XP, USA) at 8500 r/min for 10 min in individual centrifuge tubes to obtain microalgae paste (2 g) with a water content of 77 wt.%. The resulting microalgae paste was then stored at
20 °C until use. Commercial alcohol was used in this experiment because renewable alcohol produced through fermentation can be easily upgraded to analytical purity [29]. All reagents (i.e., hexane, ethanol, isopropanol, and sulfuric acid) were purchased from Sinopharm Chemical Reagent (Shanghai, China). 2.2. Microwave-assisted lipid transesterification with various alcohols in wet microalgae cells for biodiesel production Microwave-assisted lipid transesterification with ethanol and isopropanol in wet microalgae cells was conducted in a WX-4000

2.4. Determination of FAAE yield of lipid transesterification in wet microalgae and characterization of crude biodiesel Wet microalgae biomass (2 g) was lyophilized to determine the FAAE yield of lipid transesterification in wet microalgae. The lyophilized microalgae biomass was reacted with 16 mL of alcohol with 3 vol.% of concentrated sulfuric acid in the microwave digestion system at a temperature of 90 °C for 30 min [31]. The obtained FAAE was subsequently extracted by hexane, as described in Section 2.2. The obtained crude biodiesel samples were mixed with an internal standard (C19:0) and dissolved in the hexane for gas chromatograph analysis. A gas chromatograph (7890A GC; Agilent, Santa Clara, CA, USA) equipped with a flame ionization detector and an HP-INNOWax column (30 m  320 lm  0.25 lm; Agilent, Santa Clara, CA, USA) was employed to analyze the crude biodiesel samples [32]. The carrier gas for gas chromatograph analysis was He. The injection temperature and detector temperature of the gas chromatograph were set at 250 °C. The oven of the gas chromatograph was initially maintained at 150 °C for 1 min, and then heated to 200 °C at a heating rate of 15 °C/min. Subsequently, it

793

R. Huang et al. / Energy Conversion and Management 105 (2015) 791–797

was heated to 250 °C at a heating rate of 2 °C/min. Finally, the oven temperature was maintained at 250 °C for 5 min. A comparison of retention times and peak areas between the crude biodiesel samples and standards then gave the components and weight of FAAE contained in crude biodiesel. The FAAE yield of microwave-assisted lipid transesterification in wet microalgae was then calculated according to the following equation:

weight of FAAEs produced through transesterification in wet microalgae : weight of corresponding FAAEs produced through transesterification in lyophilized microalgae

The FAAE content in crude biodiesel was calculated according to the following equation:

FAAE content ¼

weight of FAAE contained in crude biodiesel : weight of crude biodiesel ð2Þ

A computer-controlled differential scanning calorimeter (model TA Q200, TA Instruments, New Castle, DE, USA) was used to determine the melting phase transitions of crude biodiesel. An elemental analyzer (Flash EA 1112, Thermo Scientific, USA) was employed to measure the C, H, and N contents of crude biodiesel. All measurements were repeated in triplicate, and the mean values were reported. 3. Results and discussions 3.1. Characterization of lipids contained in microalgae cells after microwave-assisted transesterification As reported in [12], the yield of lipid extracted with hexane from microwave-treated wet microalgae was only 3.4% (based on the weight of microalgae biomass). However, the yield of lipid extracted with hexane from wet microalgae after microwaveassisted transesterification was increased sixfold to 20.8%. Martinez–Guerra et al. also reported that high biodiesel yield was obtained from microalgae with hexane after microwaveassisted transesterification [33,34]. Thus, the transesterification of lipids in microalgae significantly benefits the hexane extraction process. However, the mechanism by which the transesterification of lipids promote the subsequent hexane extraction process is still unknown. The lipid droplets contained in microalgae cells were dyed with Nile Red before and after transesterification and then observed through a microscope to reveal the promotion mechanism of transesterification. Lipid droplets in the microalgae cells appeared as yellow dots before transesterification. However, no yellow dots were observed after microwave-assisted transesterification, and the entire field of view changed from red to yellow. This finding indicated that the lipid droplets were split into small pieces and dispersed in the mixture after transesterification because of the conversion of triglyceride into FAAE. As the molecular weight of FAAE is only one-third of triglyceride, the molecular size of lipid was subsequently decreased after transesterification. Meanwhile, the structure of the lipid droplet was destroyed during microwave-assisted transesterification. The split and dispersion of lipid droplets significantly facilitated the subsequent hexane extraction. Given that substantial free fatty acid was contained in microalgae cells [16,17], the contact angle of hexane against fatty acid and

ð1Þ

transesterification. The contact angle of hexane against FAAE was determined to be 12.76°, which is lower than that of hexane against fatty acid. This finding indicated that the polarity difference between hexane and lipids decreased after esterification because the hydrophilic carboxyl group of fatty acid was transformed into the hydrophobic ester group of FAAE. The decreased polarity difference between hexane and lipids enhanced the solubility of lipids in hexane and facilitated the subsequent hexane extraction. The characterization of lipids contained in microalgae cells validated that the droplet size and polarity of algal lipids decreased after microwave-assisted transesterification. The decrease in droplet size and polarity of the algal lipids significantly facilitated the extraction with apolar hexane. 3.2. Effects of reaction temperature on microwave-assisted lipid transesterification in wet microalgae cells with various alcohols for biodiesel production The initial lipid content of C. pyrenoidosa was determined to be 21.26% through direct transesterification of lyophilized microalgae. The effects of temperature on microwave-assisted lipid transesterification in wet microalgae cells with various alcohols are shown and compared in Fig. 1. When the reaction temperature was 70 °C, the FAAE yield of ethanol and isopropanol were 80.6% and 85.7% respectively. These FAAE yields were higher than the 50.5% yield reported in the case of methanol [12]. This result indicated

100

Fatty acid alkyl ester yield (%)

FAAE yield ¼

FAAE were also measured to evaluate the polarity change of algal fatty acid after esterification. Based on the measurement, the contact angle of hexane against fatty acid was 18.3°. As the lipid droplet of microalgae contained considerable fatty acid, the contact angle of hexane against fatty acid represent the contact angle of hexane against original lipids content in the microalgae biomass. Meanwhile, the FAAE was the main product of original lipids after

90 80 70 60 a

Methanol Ethanol Isopropanol

50 40 70

80

90

100

110

Reaction temperature (°C) Fig. 1. Effects of reaction temperature on microwave-assisted lipid transesterification in wet microalgae cells with various alcohols for biodiesel production. Notes: a The data for transesterification with methanol was calculated based on Ref. [12]. b The reaction time and alcohol volume for transesterification with ethanol and isopropanol was 30 min and 6 mL, respectively.

794

R. Huang et al. / Energy Conversion and Management 105 (2015) 791–797

that the FAAE yield increased with the increase in alcohol carbon length, which is consistent with the data from a previous report on acid-catalyzed transesterification reactions [35]. This increase in FAAE yield is caused by the decreased polarity of alcohol with the increase in carbon length. As the polarity of alcohol decreased with the increase in carbon length, the miscibility of alcohol with apolar triglyceride was increased. Thus, mass transfer was induced during transesterification and resulted in enhanced FAAE yields. The FAAE yields of all types of alcohols increased with the increase in temperature and reached 95% when the reaction temperature was increased to 90 °C. This temperature is lower than that for the transesterification of microalgal biomass via conversional heating (105 °C) [27], indicating that the transesterification was promoted by microwave. The difference of the FAAE yields of alcohols decreased with the increase in reaction temperature because molecular motion was induced by the increase in reaction temperature. As the molecular motion was induced, alcohol molecules were more likely to move across the phase boundary and react with lipids. Thus, the effect of miscibility on mass transfer was reduced. The FAAE yields of lipid transesterification in wet microalgae cells were significantly higher than the reported FAAE yields of transesterification conducted with soybean oil [35] because mass transfer was enhanced in wet microalgae cells. Given that transesterification occurred in wet microalgae cells, the organic compounds contained in microalgae cells may act as intermediaries to induce lipid contact with alcohols [36]. Meanwhile, the lipid droplets were significantly smaller in wet microalgae cells compared with those in soybean oil. However, the FAAE yield decreased when the reaction temperatures were higher than 90 °C and 100 °C for ethanol and isopropanol respectively. This outcome was caused by the increased amount of alcohol evaporated into steam along with the increased reaction temperatures. The inflexion point of isopropanol was slightly higher than that of ethanol because the boiling point of isopropanol is higher than that of ethanol. 3.3. Effects of reaction time on microwave-assisted lipid transesterification in wet microalgae cells with various alcohols for biodiesel production The reaction behavior of microwave-assisted lipid transesterification in wet microalgae cells with various alcohols was compared

Fatty acid alkyl ester yield (%)

100 90 80 70 60

Methanol a Ethanol Isopropanol

50 40 0

5

10

15

20

25

30

Reaction time (min) Fig. 2. Effects of reaction time on microwave-assisted lipid transesterification in wet microalgae cells with various alcohols for biodiesel production. Notes: aThe data for transesterification with methanol was calculated based on Ref. [12]. bThe reaction temperature and alcohol volume for transesterification with ethanol and isopropanol was 90 °C and 6 mL, respectively.

by examining the effects of reaction time when the reaction temperature was set at 90 °C (Fig. 2). The FAAE yield increased to 58.61% and 48.9% in the case of ethanol and isopropanol respectively. These FAAE yields were slightly lower than the yield reported in the case of methanol [12]. Thus, the FAAE yield decreased with the increase in alcohol carbon length. The reaction times needed for an FAAE yield of nearly 90% were 10 min for methanol and 20 min for ethanol. These reaction times are only one-third of the time needed to achieve 90% FAAE yield for the conversional heated transesterification with methanol and ethanol respectively [27]. The difference of the FAAE yields of alcohols decreased with the increase in reaction time. The FAAE yields of all types of alcohols increased with the increase in reaction time and reached 95% when the reaction time was increased to 30 min. To quantify the reaction rate of transesterification with various alcohols, the reaction-time-dependent FAAE yields were fitted to the first-order exponential equation using the program Origin 8.0 (Fig. 2) [37,38]. As shown in Fig. 2, the correlation coefficient R2 for all fitted curves ranged from 0.9939 to 0.9992. This finding indicated that the first-order exponential equation gave a robust fit of the simulation for microwave-assisted lipid transesterification in wet microalgae. Based on the fitted curves, the rate constant calculated for lipid transesterification in wet microalgae with methanol (0.14 min
1) was determined to be the highest, followed by the rate constants calculated for ethanol (0.1 min
1) and isopropanol (0.075 min
1). This finding indicated that the reaction rate decreased with the increase in alcohol carbon length, a decreases that can be explained by several reasons. One, as microwave irradiation selectively heats the solvents with high polarity, the alcohol with a short carbon length tends to absorb more microwave energy because of its relatively high polarity [39]. Meanwhile, the oxygen atom from small alcohol molecules more readily attacks the carbon atom of the carbonyl functional group from triglycerides and results in an intermediate compound via transfer of a methoxide moiety for FAAE generation [40]. This steric effect was determined to be stronger than the effect of miscibility at elevated temperature. 3.4. Effects of alcohol volume on microwave-assisted lipid transesterification in wet microalgae cells with various alcohols for biodiesel production The effects of alcohol volume on microwave-assisted lipid transesterification in wet microalgae cells with various alcohols are shown in Fig. 3. The figure reveals that the FAAE yield of 79.5% for isopropanol was significantly higher than that of 47.6% for ethanol when the alcohol volume was 2 mL for 1 g of wet microalgae. When the alcohol volume was increased to 4 mL, the FAAE yields of isopropanol and ethanol increased and were significantly higher than the 60.5% yield for methanol [12]. This result can be explained by the following reasons. As discussed in Section 3.2, the miscibility of alcohol with apolar triglyceride increased with the increase in carbon length. With increased miscibility, the opportunity for lipid molecules to react with alcohol was significantly enhanced. The boiling point of alcohol also increased from 64.7 °C to 82.45 °C with the increase in carbon length. As the reaction temperature of 90 °C was higher than the boiling point of alcohol, a considerable amount of alcohol was evaporated during treatment. However, the amount of alcohol that evaporated decreased with the increase in boiling point. Thus, the fraction of alcohol molecules that remained in the liquid state increased and increased the FAAE yield. The difference of the FAAE yields of isopropanol and ethanol decreased when the alcohol volume was increased to 5 mL because the excess amount of alcohol reduced the effects of miscibility on transesterification. The ethanol volume needed to obtain an FAAE

795

R. Huang et al. / Energy Conversion and Management 105 (2015) 791–797

alcohol volume. The ‘‘hot spots” generated during microwave heating significantly reduced with the decrease in power density because of their relatively low ability to absorb microwave energy for alcohol with long carbon length. As ‘‘microwave hot spots” are essential to induce transesterification, the FAAE yield decreased with the increase in carbon length of alcohol [41,42].

Fatty acid alkyl ester yield (%)

100 90 80 70 60 Methanol a Ethanol Isopropanol

50 40 2

3

4

5

6

7

8

9

10

Ratio of alcohol volume to wet microalgae weight (mL/g) Fig. 3. Effects of alcohol volume on microwave-assisted lipid transesterification in wet microalgae cells with various alcohols for biodiesel production. Notes: aThe data for transesterification with methanol was calculated based on Ref. [12]. bThe reaction temperature and reaction time for transesterification with ethanol and isopropanol was 90 °C and 30 min, respectively.

yield of 91.23% was 5 mL for 1 g of wet biomass, a yield that is comparable to the data reported by Kim et al. [27]. In contrast, the volume needed to obtain an FAAE yield of 90.5% for isopropanol was only 4 mL. Thus, isopropanol volumes in transesterification reactions with 90.5% FAAE yield were only 80% of the ethanol volume. The FAAE yields of isopropanol and ethanol increased to 95% when the alcohol volume was increased to 6 mL. This FAAE yield was higher than that of 81.9% for methanol when the alcohol volume was 6 mL. Thus, the ethanol and isopropanol volumes in transesterification reactions with 95% FAAE yield were only 75% of the methanol volumes [12]. The alcohol volume needed for the transesterification was reduced by 25% with the use of ethanol and isopropanol. Meanwhile, the latent heat of vaporization of isopropanol (159 cal/g) and ethanol (204 cal/g) were lower than that of methanol (265 cal/g), which means using isopropanol and ethanol as solvent needs less heat or energy to recover the solvent. The reduced solvent consumption and latent vaporization heat of solvent significantly benefited the economic feasibility of in situ transesterification of wet microalgae. However, the FAAE yields of isopropanol and ethanol decreased to 87.5% and 91.2% respectively when the alcohol volume was further increased to 8 mL. These FAAE yields were lower than that of 95.9% of methanol. The FAAE yield of methanol also decreased when the alcohol volume was increased to 10 mL. As the microwave power was set as constant, the microwave field strength and power density in the reactant decreased with the increase in

3.5. Characterization of crude biodiesel obtained through microwaveassisted lipid transesterification in wet microalgae cells with various alcohols The characterization of crude biodiesel obtained through microwave-assisted lipid transesterification in wet microalgae cells with various alcohols is shown and compared in Table 1. As shown in the table, the crystallization temperatures were determined to be 0.19 °C and
3.15 °C for ethanol and isopropanol respectively. These crystallization temperatures were lower than that reported in the case of methanol (2.18 °C). With the decrease in crystallization temperatures, the flowability of biodiesel at low temperature subsequently improved. The crystallization temperature of isopropanol decreased to 5.33 °C, which was comparable with the 7 °C achieved by the addition of a large amount of cold flow improver [21]. This finding indicated that the use of renewable ethanol and isopropanol for microwave-assisted lipid transesterification in wet microalgae cells was an effective method to improve the low-temperature flowability of biodiesel derived from wet microalgal biomass. The FAAE content in crude biodiesel was 83.42% and 81.59% respectively in the case of ethanol and isopropanol (Table 1). These FAAE contents in crude biodiesel were slightly lower than that of 86.74% for methanol [12]. This result indicated that the FAAE content in crude biodiesel decreased with the increase in carbon chain length of alcohol, which can be explained by the decreased polarity of alcohol. As the alcohol reacted with lipids in microalgae cells, the pigment contained in microalgae cells was extracted by the alcohol. The amount of extracted pigment increased with the decrease in polarity of alcohol [43]. This increased amount of extracted pigment resulted in decreased FAAE content in crude biodiesel. Along with the decreased FAAE content in crude biodiesel, the higher heating value slightly decreased from 41.18 MJ/kg to 40.12 MJ/kg. The element composition of crude biodiesel is also shown in Table 1. The nitrogen content slightly increased from 0.32 to 0.38 as the carbon chain length of alcohol increased because of the increased pigment content in crude biodiesel. The carbon content slightly decreased from 77.16 to 75.94 with the increase in the carbon chain length of alcohol because FAAE content slightly decreased in crude biodiesel. Likewise, the hydrogen content slightly decreased from 11.9 to 11.53 with the increase in the carbon chain length of alcohol. The oxygen content slightly increased from 10.62 to 12.05 with the increase in the carbon chain

Table 1 Characterization of crude biodiesel obtained through microwave-assisted lipid transesterification in wet microalgae cells with various alcohols. Microalgal biodiesel samples

Carbon content (%)

Hydrogen content (%)

Nitrogen content (%)

Oxygen contentb (%)

Higher heating valuec (MJ/kg)

FAAE content in crude biodiesel (%)

Crude biodiesel obtained through transesterification with methanola Crude biodiesel obtained through transesterification with ethanol Crude biodiesel obtained through transesterification with isopropanol

77.16

11.9

0.32

10.62

41.18

86.74

2.08

76.99

11.7

0.36

10.94

40.78

83.42

0.19

75.94

11.53

0.38

12.05

40.12

81.59


3.15

Crystallization temperature (°C)

Notes: a The data for crude biodiesel obtained through transesterification with methanol was obtained from the Ref. [12]. b Oxygen content was calculated by subtraction of carbon, hydrogen and nitrogen from 100%. c The higher heating value was estimated with the Dulong formula [44]: HHV(MJ/kg) = 0.338C + 1.428(H–O/8) + 0.095S, where C, H, O, and S were weight percentages of elemental compositions in materials.

796

R. Huang et al. / Energy Conversion and Management 105 (2015) 791–797

Table 2 Compositions of biodiesel obtained through microwave-assisted lipid transesterification in wet microalgae cells with various alcohols. Biodiesel compositions

Weight percentages of biodiesel compositions (%) Biodiesel obtained through transesterification with methanol

Biodiesel obtained through transesterification with ethanol

Biodiesel obtained through transesterification with isopropanol

C16:0 C16:1 C16:2 C16:3 C17:0 C18:0 C18:1 C18:2 C18:3

27.91 0.23 1.54 5.32 0.31 4.27 14.32 16.13 12.19

27.09 0.27 1.57 5.49 0.28 3.96 14.22 16.46 13.24

26.84 0.24 1.49 5.97 0.29 3.01 14.63 16.83 13.66

length of alcohol. As the FAAE contents were lower than 96.5% in all the cases, a simple purification step such as vacuum distillation was mandatory to meet the EN 14214 biodiesel standards. After the purification, the mentioned differences of higher heating value, oxygen content, and nitrogen contents are simply minimized. However, the significantly reduced crystallization temperature will remain after the purification and benefit the final biodiesel product. Compositions of biodiesel obtained through microwaveassisted lipid transesterification in wet microalgae cells with various alcohols are shown in Table 2. The content of saturated FAAEs, such as C16:0 and C18:0, slightly decreased with the increase in carbon length of alcohol. However, unsaturated FAAEs, such as C16:3 and C18:3, slightly increased with the increase in carbon length of alcohol. This outcome can be explained by the following reasons. As ‘‘hot spots” were generated during microwave heating, the high temperature decomposed the unsaturated FAAEs [45]. The decomposition of unsaturated FAAEs were reduced for alcohol with long carbon length because of their relatively low ability to absorb microwave energy and generate ‘‘hot spots”. 4. Conclusion The crystallization temperatures of biodiesels produced from microalgae through lipid transesterification in cells with renewable ethanol and isopropanol (0.19 °C and
3.15 °C respectively) were lower than that with methanol (2.08 °C), which improved biodiesel flowability in low temperature. Decreased droplet size and polarity of lipid were observed after the transesterification in cells, a result that was beneficial in extracting lipid from microalgae using hexane. Microwave-assisted transesterification with ethanol and isopropanol resulted in higher biodiesel yields than that with methanol when the reaction temperature was lower than 90 °C. The ethanol and isopropanol volumes in transesterification with 95% FAAE yield were only 75% of the methanol volume. Acknowledgements This study was supported by the National Natural Science Foundation-China (51176163, 51476141), Zhejiang Provincial Natural Science Foundation-China (LR14E060002). References [1] Asikainen M, Munter T, Linnekoski J. Conversion of polar and non-polar algae oil lipids to fatty acid methyl esters with solid acid catalysts–A model compound study. Bioresour Technol 2015;191:300–5.

[2] Speranza LG, Ingram A, Leeke GA. Assessment of algae biodiesel viability based on the area requirement in the European Union, United States and Brazil. Renewable Energy 2015;78:406–17. [3] Kiran B, Kumar R, Deshmukh D. Perspectives of microalgal biofuels as a renewable source of energy. Energy Convers Manage 2014;88:1228–44. [4] Hidalgo P, Toro C, Ciudad G, Schober S, Mittelbach M, Navia R. Evaluation of different operational strategies for biodiesel production by direct transesterification of microalgal biomass. Energy Fuels 2014;28:3814–20. [5] Islam MA, Brown RJ, O’Hara I, Kent M, Heimann K. Effect of temperature and moisture on high pressure lipid/oil extraction from microalgae. Energy Convers Manage 2014;88:307–16. [6] Park J-Y, Park MS, Lee Y-C, Yang J-W. Advances in direct transesterification of algal oils from wet biomass. Bioresour Technol 2015;184:267–75. [7] Kim B, Im H, Lee JW. In situ transesterification of highly wet microalgae using hydrochloric acid. Bioresour Technol 2015;185:421–5. [8] Lee J-Y, Yoo C, Jun S-Y, Ahn C-Y, Oh H-M. Comparison of several methods for effective lipid extraction from microalgae. Bioresour Technol 2010;101: S75–7. [9] Reddy HK, Muppaneni T, Patil PD, Ponnusamy S, Cooke P, Schaub T, et al. Direct conversion of wet algae to crude biodiesel under supercritical ethanol conditions. Fuel 2014;115:720–6. [10] Sitthithanaboon W, Reddy HK, Muppaneni T, Ponnusamy S, Punsuvon V, Holguim F, et al. Single-step conversion of wet Nannochloropsis gaditana to biodiesel under subcritical methanol conditions. Fuel 2015;147:253–9. [11] Wahidin S, Idris A, Shaleh SRM. Rapid biodiesel production using wet microalgae via microwave irradiation. Energy Convers Manage 2014;84:227–33. [12] Cheng J, Huang R, Li T, Zhou J, Cen K. Biodiesel from wet microalgae: extraction with hexane after the microwave-assisted transesterification of lipids. Bioresour Technol 2014;170:69–75. [13] Bejan CCC, Celante VG, Vinicius Ribeiro de Castro E, Pasa VMD. Effect of different alcohols and palm and palm kernel (palmist) oils on biofuel properties for special uses. Energy Fuels 2014;28:5128–35. [14] Bankar SB, Jurgens G, Survase SA, Ojamo H, Granström T. Enhanced isopropanol–butanol–ethanol (IBE) production in immobilized column reactor using modified Clostridium acetobutylicum DSM792. Fuel 2014;136:226–32. [15] Hellier P, Ladommatos N, Allan R, Rogerson J. The influence of fatty acid ester alcohol moiety molecular structure on diesel combustion and emissions. Energy Fuels 2012;26:1912–27. [16] Mathimani T, Uma L, Prabaharan D. Homogeneous acid catalysed transesterification of marine microalga Chlorella sp. BDUG 91771 lipid–An efficient biodiesel yield and its characterization. Renewable Energy 2015;81:523–33. [17] Ashokkumar V, Agila E, Sivakumar P, Salam Z, Rengasamy R, Ani FN. Optimization and characterization of biodiesel production from microalgae Botryococcus grown at semi-continuous system. Energy Convers Manage 2014;88:936–46. [18] Ataya F, Dubé MA, Ternan M. Variables affecting the induction period during acid-catalyzed transesterification of canola oil to FAME. Energy Fuels 2007;22:679–85. [19] Bynes AN, Eide I, Jørgensen KB. Optimization of acid catalyzed transesterification of jatropha and rapeseed oil with 1-butanol. Fuel 2014;137:94–9. [20] Laamanen CA, Shang H, Ross GM, Scott JA. A model for utilizing industrial offgas to support microalgae cultivation for biodiesel in cold climates. Energy Convers Manage 2014;88:476–83. [21] Lv P, Cheng Y, Yang L, Yuan Z, Li H, Luo W. Improving the low temperature flow properties of palm oil biodiesel: addition of cold flow improver. Fuel Process Technol 2013;110:61–4. [22] Moser BR. Impact of fatty ester composition on low temperature properties of biodiesel–petroleum diesel blends. Fuel 2014;115:500–6. [23] Wang M, Nie K, Yun F, Cao H, Deng L, Wang F, et al. Biodiesel with low temperature properties: Enzymatic synthesis of fusel alcohol fatty acid ester in a solvent free system. Renewable Energy 2015;83:1020–5. [24] Islam MA, Brown RJ, Brooks P, Jahirul MI, Bockhorn H, Heimann K. Investigation of the effects of the fatty acid profile on fuel properties using a multi-criteria decision analysis. Energy Convers Manage 2015;98:340–7. [25] Hong IK, Park JW, Kim H, Lee SB. Alcoholysis of canola oil using a short-chain (C1–C3) alcohols. J Ind Eng Chem 2014;20:3689–94. [26] Im H, Lee H, Park MS, Yang J-W, Lee JW. Concurrent extraction and reaction for the production of biodiesel from wet microalgae. Bioresour Technol 2014;152:534–7. [27] Kim T-H, Suh WI, Yoo G, Mishra SK, Farooq W, Moon M, et al. Development of direct conversion method for microalgal biodiesel production using wet biomass of Nannochloropsis salina. Bioresour Technol 2015;191:438–44. [28] Cheng J, Huang Y, Feng J, Sun J, Zhou J, Cen K. Mutate Chlorella sp. by nuclear irradiation to fix high concentrations of CO2. Bioresour Technol 2013;136:496–501. [29] Loy Y, Lee X, Rangaiah G. Bioethanol recovery and purification using extractive dividing-wall column and pressure swing adsorption: an economic comparison after heat integration and optimization. Sep Purif Technol 2015;149:413–27. [30] Cheng J, Feng J, Sun J, Huang Y, Zhou J, Cen K. Enhancing the lipid content of the diatom Nitzschia sp. by 60 Co-c irradiation mutation and high-salinity domestication. Energy 2014;78:9–15.

R. Huang et al. / Energy Conversion and Management 105 (2015) 791–797 [31] Wahlen BD, Willis RM, Seefeldt LC. Biodiesel production by simultaneous extraction and conversion of total lipids from microalgae, cyanobacteria, and wild mixed-cultures. Bioresour Technol 2011;102:2724–30. [32] Sathish A, Marlar T, Sims RC. Optimization of a wet microalgal lipid extraction procedure for improved lipid recovery for biofuel and bioproduct production. Bioresour Technol 2015;193:15–24. [33] Martinez-Guerra E, Gude VG, Mondala A, Holmes W, Hernandez R. Extractivetransesterification of algal lipids under microwave irradiation with hexane as solvent. Bioresour Technol 2014;156:240–7. [34] Martinez-Guerra E, Gude VG, Mondala A, Holmes W, Hernandez R. Microwave and ultrasound enhanced extractive-transesterification of algal lipids. Appl Energy 2014;129:354–63. [35] Wahlen BD, Barney BM, Seefeldt LC. Synthesis of biodiesel from mixed feedstocks and longer chain alcohols using an acid-catalyzed method. Energy Fuels 2008;22:4223–8. [36] Lam MK, Lee KT. Catalytic transesterification of high viscosity crude microalgae lipid to biodiesel: effect of co-solvent. Fuel Process Technol 2013;110:242–8. [37] Niu S-L, Huo M-J, Lu C-M, Liu M-Q, Li H. An investigation on the catalytic capacity of dolomite in transesterification and the calculation of kinetic parameters. Bioresour Technol 2014;158:74–80.

797

[38] Zheng S, Kates M, Dube M, McLean D. Acid-catalyzed production of biodiesel from waste frying oil. Biomass Bioenergy 2006;30:267–72. [39] Rodríguez AM, Prieto P, de la Hoz A, Díaz-Ortiz Á, Martín DR, García JI. Influence of polarity and activation energy in Microwave-Assisted Organic Synthesis (MAOS). ChemistryOpen 2015;2014(00):1–11. [40] Farobie O, Matsumura Y. A comparative study of biodiesel production using methanol, ethanol, and tert-butyl methyl ether (MTBE) under supercritical conditions. Bioresour Technol 2015;191:306–11. [41] Xia A, Cheng J, Lin R, Lu H, Zhou J, Cen K. Comparison in dark hydrogen fermentation followed by photo hydrogen fermentation and methanogenesis between protein and carbohydrate compositions in Nannochloropsis oceanica biomass. Bioresour Technol 2013;138:204–13. [42] Yuan H, Yang B, Zhu G. Synthesis of biodiesel using microwave absorption catalysts. Energy Fuels 2008;23:548–52. [43] Yao L, Gerde JA, Wang T. Oil extraction from microalga Nannochloropsis sp. with isopropyl alcohol. J Am Oil Chem Soc 2012;89:2279–87. [44] Biller P, Riley R, Ross A. Catalytic hydrothermal processing of microalgae: decomposition and upgrading of lipids. Bioresour Technol 2011;102:4841–8. [45] Cheng J, Sun J, Huang Y, Feng J, Zhou J, Cen K. Dynamic microstructures and fractal characterization of cell wall disruption for microwave irradiationassisted lipid extraction from wet microalgae. Bioresour Technol 2013;150:67–72.