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Fuel 97 (2012) 822–831 Contents lists available at SciVerse ScienceDirect Fuel journal homepage: www.elsevier.com/loca...

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Fuel 97 (2012) 822–831

Contents lists available at SciVerse ScienceDirect

Fuel journal homepage: www.elsevier.com/locate/fuel

Comparison of direct transesterification of algal biomass under supercritical methanol and microwave irradiation conditions Prafulla D. Patil a, Veera Gnaneswar Gude b, Aravind Mannarswamy a, Peter Cooke c, Nagamany Nirmalakhandan d, Peter Lammers e, Shuguang Deng a,⇑ a

Chemical Engineering Department, New Mexico State University, Las Cruces, NM 88003, USA Civil and Environmental Engineering Department, Mississippi State University, Mississippi State, MS 39762, USA Electron Microscopy Laboratory, New Mexico State University, Las Cruces, NM 88003, USA d Civil and Environmental Engineering Department, New Mexico State University, Las Cruces, NM 88003, USA e Energy Research Laboratory, New Mexico State University, Las Cruces, NM 88003, USA b c

a r t i c l e

i n f o

Article history: Received 31 December 2011 Received in revised form 13 February 2012 Accepted 13 February 2012 Available online 10 March 2012 Keywords: Biodiesel Algal biomass Supercritical methanol Microwave-assisted transesterification Response surface methodology

a b s t r a c t In this comparative study, direct conversion of algal biomass into biodiesel using supercritical methanol (SCM) and microwave-assisted (MW) transesterification methods was investigated. Wet algal biomass was used as feedstock in the supercritical methanol process and dry algal biomass for the microwaveassisted transesterification. Experimental runs were designed using a response surface methodology and the process parameters such as wet/dry algae to methanol ratio, reaction temperature, reaction time and catalyst concentrations were optimized for both processes. The microwave-assisted approach improves extractions of algae significantly, with a higher efficiency, reduced extractive-transesterification time and increased yield. While the non-catalytic supercritical methanol method produces highly purified extracts (free of harmful solvents and catalyst residues), and reduces energy consumption in separation and purification steps. The algal biodiesel samples from SCM and MW processes were compared using FT-IR and TGA analysis methods to identify the functional group attributions and thermal stability of the biofuel samples, respectively. The transmission electron microscopy (TEM) analysis of algal biomass and lipid extracted algae (LEA) and energy requirements for the two processes are also presented. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Due to increasing concerns of energy security and global warming associated with the conventional energy resources, there is an impetus to investigate renewable and sustainable energy resources around the world. One of the many solutions to this global energy crisis is to produce and utilize biodiesel locally. Biodiesel is renewable and environmental-friendly, with calorific value equivalent to regular fossil fuel [1,2]. Biodiesel can be produced from a variety of feedstocks, including edible oils (corn, peanut, soybean named as first-generation biodiesel feedstock); non-edible oils (jatropha, karanja, animal fats and waste cooking oils: second generation); and algae (numerous species: third generation) [3–6]. Among these feedstocks, biodiesel production from algae has drawn special attention for the following reasons: (a) high lipid content (20–50%) and high growth rates (1–3 doublings per day); (b) tolerant to severe environmental conditions; (c) sequester carbon dioxide from the flue gases; (d) harvesting and transportation are economical compared to other crops (due to small size and the ⇑ Corresponding author. Tel.: +1 575 646 4346; fax: +1 575 646 7706. E-mail address: [email protected] (S. Deng). 0016-2361/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2012.02.037

use of arid and marginal lands); (e) very high actual photosynthetic yields (3–8%) compared to other terrestrial plants (0.5%) [7–11]. Algal biodiesel production consists, primarily of five steps. They are: (a) algae production; (b) algae harvesting; (c) oil extraction; (d) transesterification or chemical treatment; and (e) separation and purification [11]. All the steps involving from harvesting to purification are both energy and cost intensive. Currently, major hurdles for the algal biodiesel production are: dewatering the algae; oil extraction and transesterification [11,12]. The algal culture is usually concentrated to 15–20% from its original concentration of 0.02–0.05% through various techniques and contains major portion as water, which poses serious problems in the chemical treatment step, i.e., transesterification [12]. Apart from this, extraction of algal oil is not as simple as that would be from other crop seeds, which are usually done by mechanical pressing and solvent extraction methods, due to their rigid cell wall structure. As such, these three steps add significantly to the cost of the algal biodiesel product. Hence, it is critical to develop fast and easy methods that would reduce the chemical and energy consumption and processing time of the overall biodiesel production process. This research focused on developing two simple methods by which the raw algal biomass can be treated in single step

P.D. Patil et al. / Fuel 97 (2012) 822–831

extraction–transesterification processes. One method treats the wet algal biomass in supercritical methanol conditions, and the other method treats the dry algal biomass by direct microwave irradiation at atmospheric pressures. The premise behind the wet algal biomass treatment at supercritical methanol conditions is that the water present in the biomass can participate in the transesterification reaction as a co-solvent, thereby enhancing the extraction and conversion of the oil lipids into biodiesel and eliminates energy intensive dewatering step. At sub/supercritical conditions, water can serve as solvent, catalyst (or catalyst precursor), and reactant (e.g., in hydrolysis reactions) [13]. In water added supercritical methanol reaction, the water–methanol mixture has both strong hydrophilic and hydrophobic properties that help speed up the reaction significantly [14]. Levine et al. proposed a two-step, catalyst-free biodiesel production process for wet algae involving intracellular lipid hydrolysis coupled with supercritical in situ transesterification (SC-IST/E) to eliminate an organic solvent use during lipid extraction, and recover nutrients (e.g., N, P, and glycerol) for reuse [15]. Considering availability of dry algal biomass, microwaveassisted extraction and transesterification of oils seems to be an attractive solution. One benefit of using dried biomass is better percolation of the solvents in the biomass, leading to an increase in lipid extraction [16–18]. As mentioned earlier, mechanical pressing and solvent extraction are the methods implemented for the extraction of the crop and plant seeds. However, these methods are expensive and require long reaction times [19,20]. In addition, mechanical pressing generally requires drying the algae, which is more energy-intensive process and can account for up to 30% of the total production costs. Microwaves can easily penetrate through the cell wall structure to extract and transesterify the oils into biodiesel. The economics of this process is expected to be attractive due to short reaction time and efficient extraction of algal oils. It was also reported that biodiesel recovery from the reaction mixture is quicker (around 15–20 min) in a microwave assisted process compared to the conventional heating method (6 h) [21]. However, both supercritical methanol and microwave irradiation processes are probably limited by scaling up difficulties for an extractive-transesterification of microalgae. In this research, we have conducted several experiments to identify and optimize the effect of the process parameters involved in the direct extraction and transesterification of wet and dry algal biomass. A response surface methodology was applied to optimize the process parameters such as dry/wet algal biomass to methanol ratio, the amount of catalyst (for microwave process), reaction temperature and the reaction time. Results from the experimental studies are compared between the supercritical methanol process and microwave assisted transesterification process. TEM, Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA) analysis and energy requirements for the two different processes are presented. 2. Materials and methods The algal inoculum Nannochloropsis (CCMP1776) was obtained from the outdoor open raceway ponds, provided by CEHMM Artesia, NM. The standard reagent for GC–MS analysis, methyl heptadecanoate (C17), was purchased from Fluka, Milwaukee, WI. Extra pure (99%) methanol, hexane, acetic acid, diethyl ether, sulfuric acid and potassium hydroxide (KOH) flakes were purchased from Acros Organics, New Jersey. For the purification of crude algal fatty acid methyl esters (FAMEs), solid phase extraction (SPE) columns were procured from thermo scientific, Waltham, MA. PARR 4593 Micro-reactor coupled with a 4843-controller (Parr Instrument Company, Illinois, USA) was used to process the wet algae by a single-step supercritical methanol process. The Instrument can be

823

operated up to 350 °C and 120 bars. The microwave-assisted extractive-transesterification was performed using a modified domestic microwave oven (with an output power of 800 W). The microwave oven was modified and fitted with a temperature reader, an external agitator and a water-cooled reflux condenser. 2.1. Analytical methods For the quantification of reaction product, the algal biodiesel samples were analyzed by a GC–MS system incorporated with an Agilent 5975 C MSD (Triple-Axis Detector) and an Agilent 7890 A GC equipped with a capillary column (DB-23, 60 m  250 lm  0.15 lm nominal). Approximately, 1 lL sample was injected into the GC. Helium was used as the carrier gas. Sample injection was performed in split mode (20:1). The parameters of the oven temperature program consist of: start at 50 °C with 10 °C/min intervals up to 220 °C (1 min) and up to 250 °C with 5 °C/min intervals (2 min). The temperatures of the injector and detector were set at 250 °C. The Infrared (IR) spectra were obtained using a PerkinElmer Spectrum 400 FT-IR/FT-NIR spectrometer. Thermogravimetric analysis (TGA) of algal biomass and was performed using Perkin Elmer Pyris 1 TGA. The samples were heated from 25 to 950 °C at constant heating rate of 10 °C/min in an atmosphere of nitrogen at a constant purge rate of 20 mL/min at the pan. 2.2. Characteristics of nannochloropsis algal species The ash-free dry weight of the algae sample and total lipid yield on the dry weight basis were determined as 69.8% and 50%, respectively. The FT-IR spectrum of the Nannochloropsis algal species shows the following general characteristic features: (i) highly aliphatic character of the residues revealed by the strong absorption at 650–720 cm1; (ii) the presence of hydroxyl groups characterized by the absorption centered at 3200–3300 cm1; (iii) the presence of carboxyl groups characterized by the absorption band at 1600–1700 cm1; (iv) existence of phenols and alcohol groups are supported by the presence of the bands at 1300–1400 cm1. Qualitative elemental analysis of crude algal biomass was determined by scanning electron microscopy (SEM, HITACHI S3400 N) equipped with energy-dispersive X-ray spectroscopy (EDS). The major elements and their approximate composition (wt.%) were carbon (72%), oxygen (21%), sodium (1.5%), magnesium (0.41%), silicon (0.93%), phosphorous (0.47%), chlorine (1.52%), and potassium (0.96%). The major fatty acids (% as methyl esters) found in the algal lipid extracted and transesterified by SCM and microwave methods are hexadecanoic acid, C16:0; hexadecenoic acid, C16:1n7; cis-9-Octadecenoic acid, C18:1n9c and cis5,8,11,14,17-Eicosapentenoic acid, C20:5n3. The fatty acids, C16:0 and C16:1 contributes to about 65–70% of total fatty acids extracted and transesterified. The low ratios of the sums of unsaturated to saturated fatty acids indicate the suitability of Nannochloropsis sp. as a potential source for the production of biodiesel. It has been observed that MW process is superior to SCM process in terms of extraction of polyunsaturated fatty acids (PUFAs) and other high valuable bio-products (phycobiliproteins carotenoids, vitamins). The PUFAs obtained from SCM and MW processes are reported as 14.28% and 28.63% respectively. 2.3. Thermogravimteric analysis (TGA) of wet algal biomass decomposition The thermogravimetric plots in a linear and differential form for wet algal biomass are shown in Fig. 1. From the differential plot, the first region of weight loss occurred at about 100–150 °C. This indicates that dehydration of biomass occurred due to the percent loss of water (physical change) from algae. It was observed from

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P.D. Patil et al. / Fuel 97 (2012) 822–831

TGA Differential of TGA

Fig. 1. Thermogravimteric analysis of wet algal biomass decomposition.

the plot that decomposition of wet algal biomass started at 250 °C in the second region of weight loss observed within 250–350 °C. This result attributed to vaporization of organic matter and initiation of thermal degradation of sample at that temperature. The third region of weight loss (around 21% of the initial weight) was recorded in the range of 350–500 °C which attributed to the pyrolysis (rapid heating) of algae sample, where most of the thermal decomposition of algal biomass occurs. 2.4. Experimental procedures The experimental protocol for one-step supercritical methanol process of wet algal biomass is presented in Fig. 2a. From 50 mL aliquots, 4 g of wet algae paste were subjected to a non-catalytic supercritical methanol (SCM) process in a 100 mL PARR microreactor under a matrix of conditions: constant pressure of 1200 psi; reaction times of 10, 20, and 30 min; reaction temperatures of 240, 250, and 260 °C; and wet algae to methanol (wt./vol.) ratios of 1:4, 1:8, and 1:12. The experimental protocol for single-step microwave-assisted extraction and transesterification process for dry algal biomass is illustrated in Fig. 2b. Wet algal biomass was allowed to dry in a laboratory vacuum oven at 50–60 °C for 24 h. Dry algal powder was obtained by treating the algal biomass with liquid N2 and rupturing it in the laboratory grinder. TGA analysis revealed that there was still moisture (8–10%) left in the dry algal biomass sample. The process parameters were selected and optimized for FAME calculation based on these tolerable concentrations of the moisture content in the dry algae sample. Two grams of dry algae powder were added to the premixed homogeneous solution of methanol and KOH catalyst. The mixture was then subjected to the microwave irradiation with exiting power of 800 W (power dissipation level of 50% = 400 W), under a matrix of conditions: reaction times of 3, 6, and 9 min;

catalyst concentrations in the range 1–3 wt.% of dry biomass; and dry algae to methanol (wt./vol.) ratios of 1:9–1:15. After the reaction was completed, the reactor contents were transferred into a 50 mL round-bottom flask to remove methanol and volatile compounds at a reduced pressure in a rotary evaporator. The remaining products were taken in hexane-water mixture and then centrifuged (3200 rpm) for 5 min to induce biphasic layer. The upper organic layer containing non-polar lipids was extracted and run through a short column of silica (Hyper SPE silica) (Fig. 2c). Neutral components were eluted with the solvent. An internal standard, methyl heptadecanoate (C17:0) was added to the eluted neutral component-solvent solution and analyzed by gas chromatography-mass spectroscopy GC–MS and FT-IR. The downstream process in the SCM (non-catalytic) process is easier and quickly compared to the MW process, which reduces the additional energy required for separation and purification steps in the SCM process. 2.5. Statistical analysis and experimental design The effect of the three factors (for SCM and MW methods) and their interactions were studied using the response surface methodology [22] as shown in Fig. 3a and b. The total number of experimental runs was 28 including replications. Based on experience and economic feasibility, a three factorial subset design was employed [23]. Experimental designs based on RSM for direct transesterification of algal biomass are presented in Table 1. For SCM transesterification, the wet algae to methanol ratios (wt./vol.), reaction times and reaction temperature were varied in the ranges from 1:4 to 1:12, 10 to 30 min, and 240 to 260 °C, respectively. For microwave transesterification, the dry algae to methanol ratios (wt./vol.), catalyst concentration and reaction times were varied in the ranges from 1:9 to 1:15, 1% to 3%, and 3 to 9 min respectively. The

P.D. Patil et al. / Fuel 97 (2012) 822–831

825

Fig. 2. (a) The experimental protocol for one-step SCM for wet algae, (b) the experimental protocol for one-step microwave for dry algae, (c) separation and purification (downstream processing) of crude algal biodiesel.

following is the general linear model for the RSM analysis employed in this study:

l ¼ b0 þ

3 P i¼1

bi xi þ

3 P i¼1

bii x2i þ

2 P 3 P

bij xi xj

i¼1j¼iþ1

where x1, x2 and x3 are the levels of the factors and l is the predicted response if the process were to follow the model. Method of least squares was employed to ascertain the values of the model parameters and ANOVA to establish their statistical significance at a confidence level of 95% (in our case). 3. Results and discussion 3.1. Regression model The regression analysis indicates that all the three parameters investigated for both the methods had significant influence on the fatty acid methyl ester content, which was confirmed by the P-values of the analysis. The P-value of the lack of fit analysis was 0.133 and 0.061 for SCM and microwave method, respectively, which is more than 0.05 (confidence level is 95%). 3.2. Effects of process parameters and optimization 3.2.1. Supercritical methanol method Fig. 3a shows the response contours of FAME yield against reaction temperature and wet algae to methanol (wt./vol.) ratio at three different reaction time intervals and fixed reaction pressure of 1200 psi. The values and signs on the regression coefficients suggest that the reaction time affects the response positively for temperatures up to 255 °C; however, reaction temperatures above 255 °C were unsuitable for transesterification reaction of the algal biomass at a fixed pressure of 1200 psi. This may be because the oil/lipid and the alkyl esters tend to decompose or become thermally unstable above the specified temperature owing to the high content of unsaturated fatty acids [24]. The initiation of thermal decomposition of unsaturated fatty acids in algal lipid can be possible due to cis/trans isomerization of the CAC double bond at supercritical methanol (SCM) condition [25]. In addition, FAME

yield can be hampered due to the autoxidation of the reactive intermediates (organic substrates) and unsaturated fatty acids in algal biomass leads to the formation of hydroperoxides which are thermally unstable [26]. Wet algae to methanol (wt./vol.) ratios have a positive effect on the yield up to 1:9 but have a negative impact at higher levels. Under supercritical conditions, methanol behaves like both as a reactant and an acid catalyst [14]. Higher ratio of biomass to methanol could shift the reversible reaction forward (as observed) perhaps due to increased contact area between methanol and lipid, resulting in higher yield of FAME. As expected, a longer reaction time allows the transesterification reaction to proceed to completion and results in a higher yield of FAMEs from algal biomass. It has been reported that extended reaction times in the supercritical alcohol process for vegetable oil may result in significant loss of unsaturated fatty acid ethyl ester (FAEE) due to degradation reactions [27]. Nevertheless, Fig. 3a shows that the effect of reaction time is more prominent at wet algae to methanol (wt./vol.) ratio of 1:9 and reaction temperature around 255 °C at a fixed reaction pressure of 1200 psi. In supercritical methanol reaction, the reaction temperature is the most significant variable affecting the yield of biodiesel.

3.2.2. Microwave irradiation method The response contours for the effect of different process parameters namely algae to methanol (wt./vol.) ratio, catalyst concentration expressed as wt.% of dry algae, and reaction time (min) on the fatty acid methyl ester (FAME) contents are shown in Fig. 3b. The effect of methanol on the simultaneous extraction and transesterification reaction is significant with increasing dry algae to methanol ratios up to 1:12 (wt./vol.). In this reaction, methanol acts both as a solvent for extraction of the algal oils/lipids as well as the reactant for transesterification of esters. Methanol is a good microwave radiation absorption material (loss factor, tan d = 0.659 at 2.45 GHz) which absorbs most of the microwave effect in its entire spectrum to produce localized superheating in the reactants and assists the reaction to complete faster. However, higher volumes of methanol may also result in excess loss of the solvent or aggravated rates of solvent recovery. In addition, excessive methanol amounts may reduce the concentration of the catalyst in the

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P.D. Patil et al. / Fuel 97 (2012) 822–831

Contour for algae to methanol ratio(w/vol) = 12 9

250

255

70 69 68 67 66 65

74

76

4 3

260

1

1.5

Temp (oC)

9

250

255

72 71 70 76

3

260

76

4 9

10

o

4 240

78

245

82 8708 764 72 770 68

92

250

13

14

15

255

260

o

Temp ( C)

(a)

Contour for Time = 6 min

15

64

54 56 14 58 60 13

62 70

64 66 68

66

72

12 11 10 9

1

68 6 6 64

70

64

76

88 86 84

12

68

86 84

68 6 6

94

6

90

8

Algae to methanol ratio(w/vol)

92

88 90

10

11

Algae to methanol ratio(w/vol)

Contour for time = 30 min

82 0 8 74 72 70 68 66

Algae/Methanol ratio (wt/vol)

Temp ( C)

12

70 69 68 67

73

Time(min)

72

78

80

76

44

245

5

74

6 58

6

73 74

71 72

4 240

70 68 66 64 602 6 586 554 52

7

69

77

545

8

65 6 6

74 76

74 72 70 68 6 6 64 2 6 60

6

3

Contour for Catalyst concentration(wt%) = 2

78

52 50 468 444 42

Algae/Methanol ratio (wt/vol)

Contour for time = 20 min

8

2.5

Catalyst concentration(wt%)

12

10

2

75 74 73 72 71 7 0 69

71

5

75

6

75

245

7

76

54

72 73

75

4 240

52

8

77

464 5 80

70 68 66 64 62 60 58 55642 550

Time(min)

76

66 68

36

70 7 742

78

64 2 6 60 5856

8

6

74

10

44 402 348 3 6 3 4 32

Algae/Methanol ratio (wt/vol)

Contour for time = 10 min 12

1.5

2

2.5

3

Catalyst concentration(wt%)

(b)

Fig. 3. (a) FAME yield against reaction temperature and wet algae wt./methanol volume ratio at different reaction times using RSM in SCM process, (b) the effect of algae to methanol ratio (wt./vol.), catalyst concentration (wt.%), and reaction time (min) on the fatty acid methyl ester (FAME) content using RSM in MW process.

reactant mixture and retard the transesterification reaction [28].From the counter plot for catalyst concentration effect on FAME yields (Fig. 3b), catalyst concentrations up to 2% (wt.%) shows a positive effect on the transeseterification reaction. As this is two-phase reaction mixture, the oil/lipid concentration in the methanol phase is low at the start of the reaction leading to mass transfer limitations. As the reaction continues, the concentration of oil/lipid in the methanol phase increases, leading to higher transesterification rates with increased catalyst concentrations [29]. However, higher concentrations of catalyst above 2% (wt.%) did not show any positive effect on the biodiesel conversion. This may be due to the interaction of the other compounds resulting in byproducts. Other disadvantages of high basic catalyst concentrations, in general, are their corrosive nature and tendency to form soap which hinders the transesterification reaction [30]. The reac-

tion time has a significant effect on the FAME content. Generally, extended reaction times provide for enhanced exposure of microwaves to the reaction mixture which result in better yields of extraction and biodiesel conversion. Lower reaction times do not provide sufficient interaction of the reactant mixture to penetrate and dissolve the oils into the reaction mixture. The main advantage of using microwave accelerated organic synthesis is the shorter reaction time due to rate enhancement. The rate of reaction can be described by the Arrhenius equation as: K ¼ AeDG=RT , where ‘A’ is a pre-exponential factor, ‘DG’ is Gibbs free energy of activation. The rate of chemical reaction can be increased through the pre-exponential factor A, which is the molecular mobility that depends on the frequency of the vibrations of the molecules at the reaction interface [31] or the pre-exponential factor can be altered by affecting the free energy of activation [32].

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P.D. Patil et al. / Fuel 97 (2012) 822–831 Table 1 Experimental design based on RSM for direct transesterification of algal biomass. Supercritical method

Microwave method

Run order

Temp. (°C)

MeOH (wt./vol.)

Time (min)

Observed FAME (%)

Catalyst conc. (wt.%)

MeOH (wt./vol.)

Time (min)

Observed FAME (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

240 260 250 260 240 240 250 260 250 260 240 250 240 250 250 250 250 260 250 240 260 250 260 240 240 240 260 260

4 12 8 8 4 4 8 4 8 12 8 8 12 8 8 4 12 4 8 12 4 8 12 12 12 4 4 12

10 30 10 20 10 30 20 10 20 10 20 20 30 20 30 20 20 30 20 10 30 20 10 30 10 30 10 30

25.12 78.40 76.05 68.30 20.15 68.13 79.15 39.31 76.07 72.35 38.35 82.15 62.62 78.15 84.15 55.56 70.91 66.37 83.25 32.05 54.12 77.20 70.00 55.15 28.15 53.15 45.07 85.75

1 2 1 2 1 1 3 2 1 2 3 1 1 3 3 1 2 2 3 3 2 3 2 3 3 1 2 2

9 12 15 12 9 15 15 12 12 12 9 15 9 15 12 9 9 12 9 9 12 9 12 15 15 15 15 12

3 6 3 6 9 3 3 3 6 6 3 9 3 9 6 9 6 9 9 9 6 3 6 9 3 9 6 6

71.50 64.18 62.5 66.14 40.35 58.11 61.84 59.92 44.71 65.13 70.17 40.92 70.26 59 54 45.31 80.13 42 52.01 55.17 65.52 68.69 64.57 57.11 61.84 36.74 76 67.41

Table 2 Comparison between SCM and MW assisted extraction–transesterification methods. Species

Feed type

Process

Optimum parameters

FAME (%)

Feed amount (g)

Methanol amount (mL)

Nannochloropsis (CCMP1776)

Wet

SCM

250 °C 8 (wt/vol) 25 min

84.15

4

32

Dry

MW

Cat. 2 wt.% 9 (wt/vol) 6 min

80.13

2

18

Based on the experimental analysis and RSM study, the optimal conditions for SCM process are reported as: wet algae/methanol (wt./vol.) ratio of around 1:9, reaction temperature and time of about 255 °C, and 25 min respectively. For microwave assisted simultaneous extraction and transesterification reaction, optimal conditions are: dry algae to methanol ratio of 1:12 (wt./vol.), KOH concentration of 2% (wt.%) and the reaction time of 4–5 min at a reaction temperature around 60–64 °C (Table 1). The maximum FAME yield obtained from SCM and MW processes are 84.15% and 80.13% (based on total lipid content) respectively. Microwave irradiation is more effective in the destruction of the cells and accelerates better the transesterification reaction in a shorter reaction time. Whereas, the non-catalytic (‘‘green processing’’) supercritical methanol treatment reduces the energy cost of the biodiesel production due to simplified subsequent purification step and gives high quality and thermally stable biodiesel product that are free of harmful solvents residues. 3.3. Energy consumption A comparison of the energy requirements for the single step extraction and transesterification by the supercritical methanol process and microwave method is shown in Table 2. As it can be seen from Table 2, energy requirements for supercritical methanol

Energy consumption (kJ) Thermal Mechanical Total Thermal Mechanical Total

525 75 600 240.0 14.4 254.4

process are much larger than the microwave assisted method for many reasons. Supercritical methanol process operates at high temperatures and high pressures and with reaction times significantly longer than the microwave assisted method. The supercritical methanol method treats the wet algae along with excess quantities of methanol which is required for non-catalytic transesterification of the algal oil. On the other hand, the microwave assisted method is easy, fast and energy efficient with lower energy consumption. Microwave radiation passes through the transparent glass reactor and interacts directly with the reaction compounds at molecular levels to achieve high extraction and conversion of algal oils in a shorter period of time. Thermal and mechanical energy requirements for SCM and MW assisted methods are 600 and 255 kJ (considering 60% conversion efficiency from electrical energy to microwave energy) respectively for the test volumes used. Both methods resulted in comparable FAME’s extraction rates. Although, this analysis is very fundamental based on the experimental conditions applied at a laboratory scale, true energy and cost estimations need to be performed for a reference, large-scale biodiesel production plant considering all the steps involved in the biodiesel production. Further, regarding the energy consumption, the following analysis can be considered. Oil extraction and transesterification steps both involve energy consumption in the biodiesel production

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P.D. Patil et al. / Fuel 97 (2012) 822–831

Table 3 Energy requirement for algal biodiesel production. Biodiesel production step (basis: 1 kg of algal biodiesel)

Dry algal biomass (MJ)

Wet algal biomass (MJ)

Algae culture and harvesting Drying Extraction Oil transesterification Total

7.5 90.3 8.6 0.9 107.3

10.6 0 30.8 0.9 42.3

whether in the form of heat or electricity. Lardon et al. [11] reported that 1 kg of algal biodiesel production from wet algae requires 42.3 MJ and the same from dry algae requires 107.3 MJ. Energy consumptions involved in different steps are shown in Table 3. Biodiesel production from the wet algae eliminates the need for drying operations. However, extraction and transesterification steps account for almost 75% of the energy consumption in this process. For biodiesel production from dry algae, drying of the algae accounts for major portion of the total energy consumption which is 84% (Table 3). If the free, solar energy is utilized to dry the algal biomass, the energy requirements for dry algae biodiesel production can be reduced to as low as 17 MJ with 56% energy consumption for extraction and transesterification. In addition, solar drying when optimized is low cost and quality drying method. It preserves high-value bio-products and maintains bioavailability [33]. 3.4. TEM analysis of algal biomass and lipid extracted algae (LEA) From the TEM analysis report of frozen (raw) and lipid extracted algae (residue) (Fig. 4a and b), it was found that at SCM condition, algal cell wall structure was totally disturbed and fragmented while EDS report showed the evidence for thermal degradation of algal biomass (wt.% of ‘C’ increased in residue) due to high content of unsaturated fatty acids in lipid. The starting material

(frozen-thawed) contained a close-packed circular and pleomorphic profile of single cells ranging from 2.5 to 3.5 mm in diameter. The residual material contained only a few regular electron dense features but no structural components that could be identified with typical algal cytoplasm. For dry algae, comparable views of powder particles in thin sections of the lipid extracted algae residue (Fig. 4d) from microwave processing contained intact, very close-packed cell profiles with homogeneous and moderate electron-dense cytoplasmic contents but no large electron dense inclusions comparable to the unprocessed algal samples (Fig. 4c). We employed imidazole-buffered osmium tetroxide solution as a stain for visualizing lipids by electron microscopy as described by [34]. In rehydrated algal powder particles, many cells contained large electron-dense inclusions that were not found in cells of the residue following microwave heating. This probably indicates that the electron-dense inclusions contain algal cell lipids, and they were extracted during microwave heating. The further nutritional and toxicological analysis of LEA from SCM and MW processes needs to be performed to demonstrate the suitability of algae biomass as a valuable feed supplement or substitute for conventional protein sources. The exact composition of LEA depends on the algae species as well as the growth conditions and the lipid extractive-transesterification methods used. 3.5. Analysis of algal biodiesel It was observed from the GC chromatograms for both methods that algal biodiesel contains a major proportion of mono and poly unsaturated fatty acid methyl esters. The content of the fatty acid methyl ester in the final product was calculated quantitatively by comparing the peak areas of fatty acid methyl esters to the peak area of the internal standard (methyl heptadecanoate, C17:0) obtained from GC–MS. Fatty acid methyl esters content (%) in the algal biomass is calculated assuming total lipid content in the sample. From the GC–MS peak and total ion chromatography (TIC)

Fig. 4. TEM of Nannochloropsis algal sp.: (a) frozen (raw) wet algal biomass, (b) algal residue after SCM transesterification; (c) unprocessed dry algal biomass, (d) algal residue after MW transesterification.

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(a)

(b)

Fig. 5. (A) Thermogravimetric (TGA) analysis of algal biodiesel processed by SCM and MW processes; (B) FTIR analysis of FAME converted algal biomass from SCM and MW processes.

data, it was observed that the algal biodiesel contains olefins, fatty alcohols, sterols and vitamins in minor quantities along with saturated and unsaturated FAMEs. A low percentage of methyl esters with carbon chain of >18 carbons guarantees a low viscosity for the biodiesel. The percent purity of the FAME sample using peak areas in GC–MS revealed that SCM process was comparatively better than microwave (MW) process to extract high quality prod-

uct which in turn indicates the efficient separation and purification of the product in supercritical reaction. The thermogravimetric curves of methyl esters from SCM and MW processes are presented in Fig. 5A. The onset temperature for volatilization of algal biodiesel for SCM and MW processes was recorded around 140–145 °C. This temperature can be attributed to vaporization (starts from lower to higher boiling point

830

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Table 4 The various absorption peaks of algal biodiesel. SCM (wavenumber, cm1)

MW (wavenumber, cm1)

Group attribution

Vibration type

Absorption intensity

3474.27 3009.79 2926.11 2855.51 1744.11 1463.76 1361.57 1171.58 1017.90 723.61

3472.35 3009.19 2924.76 2854.34 1745.07 1464.81 1377.84 1163.90 1070.90 723.42

AOH @CH ACH2 ACH2 [email protected] ACH2 ACH3 CAOAC CAOAC ACH2

Stretching Stretching Asymmetric stretching vibration Symmetric stretching vibration Stretching Shear type vibration Bending vibration Symmetric stretching vibration Anti-symmetric stretching vibration Plane rocking vibration

Weak Strong Strong Strong Strong Middling Middling Middling Weak Weak

range) of the respective methyl esters. For SCM process, the sample weight loss (thermal degradation) of 10%, 50% and 90% to the initial weight was recorded at the temperature of 195 °C, 362 °C and 430 °C, respectively, while for MW process, the respective weight loss of the methyl ester was observed at the temperature of 150 °C, 362 °C and 415 °C, respectively for the same wt.% loss. These temperatures can be referred as ‘distillation temperature’ of the methyl ester samples. The difference in the decomposition pattern may be due to the variation in the quantity of unreacted methanol, moisture, fatty acids, volatile organic matter and other volatile impurities present in the methyl ester samples. Based on these results, it can be concluded that the SCM processed methyl ester has higher thermal stability compared to the MW transesterified product. In addition, low PUFA% in the SCM sample makes it reasonably more stable even at severe operating conditions. To check the oxidation stability of the biodiesel products from both the processes is also necessary as generally most of the methyl esters, due to its low viscous property and high percentage of unsaturated fatty acids, make them more susceptible to oxidation and resulting in a shorter shelf life. The possible structures obtained from FT-IR spectrums of methyl esters from both the processes are the alkyl group (general and long chain substituent), carboxylic acid ester (possibly aliphatic), long chain aliphatic esters (possibly unsaturated) and carbonyl compounds (Fig. 5B). The various absorption peaks of algal biodiesel transesterified by SCM and MW processes with their group attribution, vibration type and absorption intensity were listed in Table 4. Since biodiesel is mainly mono-alkyl ester, the intense [email protected] stretching band of methyl ester appears at 1744.11 cm1 and 1745.07 cm1 for SCM and MW processes, respectively. 4. Conclusions Two techniques for simultaneous extraction and transesterification of algal biomass into biodiesel are developed. The supercritical methanol process and the microwave-assisted methods were evaluated for processing the wet and dry algal biomasses, respectively. The experimental results show that the single-step methods can be attractive solutions to reduce chemical and energy consumption in the overall biodiesel production process. Excluding the energy requirements for drying, the microwave-assisted method for processing the dry algae seems to be an energy-efficient process for extracting and converting the algal oils in a single-step extractive-transesterification process. The non-catalytic supercritical methanol method produces comparatively high quality and thermally stable biodiesel product. Acknowledgments This project was partially supported by Department of Energy (DE-EE0003046), Air Force Research Laboratory (FA8650-11-C2127) and National Science Foundation (EEC-1028968). The

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