algea biodiesel zzxxs

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Optimization of direct conversion of wet algae to biodiesel under

supercritical methanol conditions

Prafulla D. Patil a, Veera Gnaneswar Gude a, Aravind Mannarswamy a, Shuguang Deng a,*, Peter Cooke b,Stuart Munson-McGee a, Isaac Rhodes c, Pete Lammers c,1, Nagamany Nirmalakhandan d

a Chemical Engineering Department, New Mexico State University, Las Cruces, NM 88003, USAb Electron Microscopy Lab, New Mexico State University, Las Cruces, NM 88003, USAc Chemistry and Biochemistry Department, New Mexico State University, Las Cruces, NM 88003, USAd Civil and Environmental Engineering Department, New Mexico State University, Las Cruces, NM 88003, USA

a r t i c l e i n f o

Article history:

Received 27 March 2010

Received in revised form 26 May 2010

Accepted 7 June 2010

Available online 29 June 2010

Keywords:

Biodiesel

Wet algae

Supercritical methanol

Response surface methodology

a b s t r a c t

This study demonstrated a one-step process for direct liquefaction and conversion of wet algal biomass

containing about 90% of water to biodiesel under supercritical methanol conditions. This one-step pro-

cess enables simultaneous extraction and transesterification of wet algal biomass. The process conditions

are milder than those required for pyrolysis and prevent the formation of by-products. In the proposed

process, fatty acid methyl esters (FAMEs) can be produced from polar phospholipids, free fatty acids,

and triglycerides. A response surface methodology (RSM) was used to analyze the influence of the three

process variables, namely, the wet algae to methanol (wt./vol.) ratio, the reaction temperature, and the

reaction time, on the FAMEs conversion. Algal biodiesel samples were analyzed by ATR-FTIR and GC

MS. Based on the experimental analysis and RSM study, optimal conditions for this process are reported

as: wet algae to methanol (wt./vol.) ratio of around 1:9, reaction temperature and time of about 255 C,

and 25 min respectively. This single-step process can potentially be an energy efficient and economical

route for algal biodiesel production.

2010 Elsevier Ltd. All rights reserved.

1. Introduction

Biodiesel can be produced from algal biomass and oils by

extraction-transesterification, direct methanolysis and transesteri-

fication methods (Johnson and Wen, 2009; Belarbi et al., 2000).

Traditionally, algal biodiesel is produced from wet algal biomass

in a series of steps including preparation of dry algae powder,

extraction of algal oils with chemical solvents, and conversion of

the algal oil to biodiesel with a catalyst (Chisti, 2007). Drying the

biomass and extraction of algal oils by conventional methods are

both energy- and cost-intensive. An alternative to the conventional

extraction and transesterification methods is supercritical process.Using water in wet algae as a tunable co-solvent in supercritical

methanol process not only accelerates the conversion of fats and

algal oils to fatty acid methyl esters (FAMEs), but also increases

solubility and acidity.

In this work, a single-step supercritical process for simulta-

neous extraction and transesterification of wet algal biomass is

demonstrated. In the proposed process, FAMEs can be produced

from polar phospholipids, free fatty acids, and triglycerides by

increasing fluidity and volatility while reducing the polarity of

the high-energy molecules in algae at supercritical conditions.

Other advantages with the supercritical process are that they oper-

ate at modest temperatures and have lower energy requirements

compared to conventional extraction and transesterification meth-

ods due to single pot conversion of algal biomass to biodiesel. The

cost of biodiesel production from vegetable oils through supercrit-

ical process is estimated to be $0.26/gal which is about half of that

of the conventional transesterification methods, $0.51/gal (Anite-

scu et al., 2008). This may very well be applied to algal oils as

the algal biomass contains much higher levels of unsaturated fatty

acids, lipids and triglycerides. The objective of the present study isto demonstrate the direct liquefaction and conversion of wet algal

biomass into biodiesel via a single-step supercritical methanol pro-

cess in the presence of nitrogen, and to optimize the process

parameters that influence the super critical transesterification

reaction using response surface methodology (RSM).

2. Experimental

2.1. Materials and methods

Algal paste from outdoor open raceways (Inoculum Nannochlor-

opsis sp. (CCMP1776) provided by CEHMM Artesia, NM was used in

0960-8524/$ - see front matter 2010 Elsevier Ltd. All rights reserved.doi:10.1016/j.biortech.2010.06.031

* Corresponding author. Tel.: +1 575 646 4346; fax: +1 575 646 7706.

E-mail address: [email protected](S. Deng).1 Present address: Solix Biofuels, 430-B North College Ave., Fort Collins, Co 80524,

USA.

Bioresource Technology 102 (2011) 118122

Contents lists available at ScienceDirect

Bioresource Technology

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / b i o r t e c h
http://dx.doi.org/10.1016/j.biortech.2010.06.031mailto:[email protected]://dx.doi.org/10.1016/j.biortech.2010.06.031http://www.sciencedirect.com/science/journal/09608524http://www.elsevier.com/locate/biortechhttp://www.elsevier.com/locate/biortechhttp://www.sciencedirect.com/science/journal/09608524http://dx.doi.org/10.1016/j.biortech.2010.06.031mailto:[email protected]://dx.doi.org/10.1016/j.biortech.2010.06.031


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this study. For GCMS analysis, methyl heptadecanoate (C17),

standard for GC, was purchased from Fulka, Milwaukee, WI. Extra

pure (99%) methanol, hexane, acetic acid and sulfuric acid were

purchased from Acros Organics, New Jersey. For the purification

of crude algae FAME, SPE Silica columns were procured from Ther-

mo Scientific, Waltham, MA. The supercritical methanol process

was carried out in the PARR 4593 Micro-reactor with a 4843-con-

troller (Parr Instrument Company, Illinois, USA). Transmission

electron microscopy (TEM) of frozen and residue (after SCM) algal

biomass was examined with a model H-7650 electron microscope

(Hitachi High-Technologies, Pleasanton, CA) operated in the bright

field mode.

2.2. Characteristics of Nannochloropsis algal species

The ash-free dry weight of the algae sample and lipid yield on

dry weight basis were found to be 69.8% and 50%, respectively. Li-

pid extraction report for Nannochloropsis sp. has the following

composition: triglycerides: 37.74%; other non-polar hydrocarbons,

isoprenoids: 8.72% and polars, glycolipids, phospholipids: 3.54%.

The gross estimation of non-polar hydrocarbons and triglycerides

was done using thin layer chromatography (TLC) and densitometrytechnique. Previous work by other researchers (Damiani et al.,

2010; Pyle et al., 2008) showed that total fatty acids accounted

for 3050% of dry biomass, depending on different culture condi-

tions. Qualitative elemental analysis of crude algal biomass was

determined by scanning electron microscopy (SEM, HITACHI S-

3400 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%), magne-

sium (0.41%), silicon (0.93%), phosphorous (0.47%), chlorine

(1.52%), potassium (0.96%).

The FTIR spectra of the Nannochloropsis algal species show the

general features indicating: (i) the highly aliphatic character of

the residues revealed by the strong absorption at 720 cm1, (ii)

the presence of hydroxyl groups characterized by the absorption

centered at 3400 cm1, and (iii) the presence of carboxyl groups

characterized by the absorption band at 1710 cm1, (iv) The pres-

ence of carbonyl groups indicated by the absorption band at about

1735 cm1.

2.3. Biosynthesis of triglycerides in microalgae and SCM

transesterification mechanism

The biosynthesis route of triglycerides in microalgae may con-

sist of the following three steps: (a) the formation of acetyl coen-

zyme A (acetyl-coA) in the cytoplasm; (b) the elongation and

desaturation of the carbon chain of fatty acids; and (c) the biosyn-

thesis of triglycerides (Huang et al., 2010). Similar to other higher

plants and animals, microalgae are able to biosynthesize triglycer-

ides to store biomass and energy. In general, L-a-phosphoglycerol

and acetyl-coA are two major elements required for the biosynthe-

sis of triglycerides.

In the supercritical state, depending on pressure and tempera-

ture, the intermolecular hydrogen bonding in the methanol mole-

cule will be significantly decreased. As a result, the polarity and

dielectric constant of methanol are reduced allowing it to act as

a free monomer. Subsequently, methanol at supercritical condi-tions can solvate the non-polar triglycerides to form a single phase

of lipid/methanol mixture and yield fatty acid methyl esters and

diglycerides (Saka and Kusdiana, 2001; Kasim et al., 2009). In a

similar way, diglyceride is transesterified to form methyl ester

and monoglyceride, which is converted further to methyl ester

and glycerol in the last step.

2.4. Experimental design

The key variables in the proposed process affecting the FAME

content of the product are the wet algae to methanol (wt./vol.) ra-

tio, the reaction temperature, and the reaction time. A response

surface methodology (RSM) was used (Myers and Montgomery,

2002) to analyze the influence of these three process variables on

Table 1

Experimental design based on RSM for direct transesterification of wet algal biomass.

Run order Std order Temperature (oC) Methanol (wt./vol.) Reaction time (min) Observed FAME% Predicted FAME%

1 1 240 4 10 25.12 24.38

5 2 20.15

6 3 240 4 30 68.13 56.02

26 4 53.15

11 5 240 8 20 38.35 49.32

20 6 240 12 10 32.05 27.37

25 7 28.15

24 8 240 12 30 55.15 59.01

13 9 62.62

16 10 250 4 20 55.56 63.033 11 250 8 10 76.05 66.70

12 12 250 8 20 82.15 77.79

22 13 77.20

19 14 83.25

7 15 79.15

14 16 78.15

9 17 76.07

15 18 250 8 30 84.15 88.88

17 19 250 12 20 70.91 77.30

27 20 260 4 10 45.07 44.43

8 21 39.31

18 22 260 4 30 66.37 57.15

21 23 54.12

4 24 260 8 20 68.30 71.19

23 25 260 12 10 70.00 69.97

10 26 72.35

2 27 260 12 30 78.40 82.69

28 28 85.75

P.D. Patil et al. / Bioresource Technology 102 (2011) 118122 119


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the fatty acid methyl esters (FAMEs) content. Based on experience

and economic feasibility, a three factorial subset design proposed

byGilmour (2006)was employed. The design contains three levels

on three factors that could be represented by a cube with six rep-

lications at the center. The six replications at the center offer better

approximation of the true error which statistically helps in deter-

mining significance of the variables. Another advantage of this

method is its symmetry in design with regard to the center, which

offers equal importance to all levels of all parameters. The total

number of experimental runs was 28 with replications as shown

in Table 1. The wet algae to methanol ratios (wt./vol.), reaction

times, and reaction temperatures were varied in the ranges of

1:41:12, 1030 min, and 240260C, respectively. The lower

temperature limit, 240 C, was just above the critical temperature

of methanol and the upper temperature limit, 260 C, was deter-

mined by the decomposition temperature limit of algal biomass

(based on several trial runs).

A general second order linear model with the deconstructionist

approach was employed for its flexibility and ease of parametric

evaluation for the predicted response surface. Statistically insignif-

icant terms were excluded from the proposed design based on de-

sign hierarchy for the construction of the response surface. Also,

the interaction terms considered manifests a better estimation

on the combination effect of any two parameters considered. Lin-

ear least square method was used to predict the values of param-

eters involved. The confidence level of the statistical analysis

conducted was 95%.

2.5. Experimental procedure

The experimental protocol for one-step supercritical methanol

process is as follows: From the aliquots prepared previously (fro-

zen biomass in 50 mL falcon tubes at 80 C), 4 g of wet algae

paste (10% solids) was subjected to a non-catalytic supercritical

methanol (SCM) process in a 100 mL PARR micro-reactor 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. After the reaction was completed, the reactor contents

were transferred into a 50 mL round-bottom flask and freed of

methanol and volatiles at a reduced pressure in a rotary evapora-

tor. The remaining products were taken in hexane and then centri-

fuged (3200 rpm) for 5 min. The upper organic layer containing

non-polar lipids was extracted and run through a short column

of silica (Hyper SPE Silica). Neutral components were eluted with

the solvent. For qualitative analysis, an internal standard, methyl

heptadecanoate (C17:0) was added quantitatively to the eluted

neutral component-solvent solution and analyzed by gas chroma-

tography-mass spectroscopy (GCMS). 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 peakarea of the internal standard (methyl heptadecanoate, C17:0) ob-

tained from GCMS.

2.6. Statistical analysis

A general linear model which accounts for the single parame-

ters linear and quadratic effects with their interaction effects

was considered. The following is the general linear model for our

analysis:

l b0 X3

i1

bixi X3

i1

biix2

i

X2

i1

X3

ji1

bijxixj

where, x1, x2 and x3 are the levels of the factors and l is the pre-dicted response if the process were to follow the model. A decon-

structionist approach was followed which indicates the

consideration of a complete quadratic model and eliminating terms

which were not significant as the analysis continued. All further

analysis was carried out using both coded and uncoded variables.

Method of least squares was employed to ascertain the values of

the model parameters and ANOVA to establish their statistical sig-

nificance at a confidence level of 95% (in our case).

3. Results and discussion

3.1. Development of regression model

The central composite design (CCD) matrix and the response

obtained from the experimental runs are shown in Table 1. The

estimated regression coefficients for response and analysis of var-

iance (ANOVA) for response are shown in Tables 2.1 and 2.2,

respectively.

The regression analysis indicates that all the three parameters

had significant influence on the fatty acid methyl ester content,

which is confirmed by the P-values. The response surfaces were fit-

ted using process variables that were found to be significant after

the analysis. TheP-value of the lack of fit analysis is 0.133, whichis more than the 0.05 (confidence level is 95%). The regression

model provides accurate description of the experimental data indi-

cating successful correlation among the three transesterification

process parameters that affect the yield of algal biodiesel. This is

further supported by the correlation coefficient, R2 of 0.921.

3.2. Effects of process parameters and optimization

Fig. 1 showsthe response contours of FAME yield againstreaction

temperature and wet algae to methanol (wt./vol.) ratio at three dif-

ferentreaction time intervals andfixedreactionpressureof 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 notsuitable for transesterification reaction of the algal biomass at fixed

Table 2.1

Estimated regression coefficients for response (the analysis was done using coded

units).

Term Coef SE Coef T P Significant

Constant 77.012 1.938 39.73 0 Yes

Temp 10.933 1.337 8.179 0 Yes

Meth 7.133 1.337 5.336 0 Yes

Time 11.088 1.337 8.295 0 Yes

Temp Temp 20.213 3.38 5.981 0 Yes

Meth Meth 10.303 3.38 3.048 0.007 Yes

Time Time 6.562 3.38 1.942 0.068 No

Temp Meth 5.638 1.418 3.977 0.001 Yes

Temp Time 4.729 1.418 3.336 0.004 Yes

Meth Time 2.047 1.418 1.444 0.166 No

R2 = 94.46%, R-Sq (pred) = 84.35%,R-Sq (adj) = 91.69%.

Table 2.2

Analysis of variance for response.

Source DF Seq SS Adj SS Adj MS F P

Regression 9 9870.7 9870.7 1096.74 34.1 0

Linear 3 5280.7 5280.7 1760.23 54.73 0

Square 3 3656.5 3656.5 1218.83 37.9 0

Interaction 3 933.5 933.5 311.17 9.67 0.001

Residual error 18 578.9 578.9 32.16

Lack-of-fit 5 257.6 257.6 51.51 2.08 0.133

Pure error 13 321.4 321.4 24.72

Total 27 10449.6

120 P.D. Patil et al. / Bioresource Technology 102 (2011) 118122


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pressure of1200psi.Thismaybe due tothe fact thatthe oil/lipidand

the alkyl esters tend to decompose or become thermally unstable

above thespecified temperature owing to thehigh content of unsat-

uratedfatty acids (Gui etal., 2009). It was observed that athightem-

perature and pressure, unsaturated fatty acids tend to decompose

due to the isomerization of the double bond functional group from

cis-type carbon bonding (C@C) into trans-type carbon bonding

(C@C), which are naturally unstable fatty acids (Imahara et al.,

2008). The FT-IR spectrum (reaction temperature of 270 C) shows

a trans-type carbon bonding (C@C) group at the wavelength of

960 cm1 which is absent at this wavelength for the reaction tem-

perature of 250 C. The greater the percentage of unsaturation in

fatty acids or esters of algal biomass,the more it is susceptibleto oxi-

dation. The extent of unsaturationof in esters canbe reducedeasilyby partial catalytic hydrogenation of the oil (Dijkstra, 2006).

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.

Higher ratio of biomass to methanol could shift the reversible reac-

tion forward (as observed) perhaps due to increased contact area

between methanol and lipid, resulting in higher yield of FAME

and it also contributes to the lower critical temperature of the mix-

ture. However, its interaction with reaction temperature can, on

the other hand, cause a reduction in the yield of FAME due to either

the decomposition of FAME or the critical temperature of the reac-

tant/product mixture between methanol and FAMEs becomes

highly dependent on the concentration of methanol and may de-

crease the critical temperature of the reactant/product. When reac-

tant/product mixture is heated above critical temperature, it hasthe tendency to decompose (Hegel et al., 2008).

As expected, a longer reaction time allows the transesterifica-

tion reaction to proceed to completion and results in a higher yield

of FAMEs from algal biomass. According to Vieitez et al. (2009),

higher reaction time beyond particular limit in supercritical alco-

hol process for vegetable oil may lead to greater losses of unsatu-

rated FAME due to degradation reactions. Nevertheless, Fig. 1

shows that the effect of reaction time is more prominent at

wet algae to methanol (wt./vol.) ratio of 1:9 and reaction temper-

ature around 255 C at a fixed reaction pressure of 1200 psi. In

water added supercritical methanol reaction, the watermethanol

mixture has both strong hydrophilic and hydrophobic properties

that help speeding up the reaction significantly (Kusdiana and

Saka, 2004). Based on the experimental analysis and RSM study,

the optimal conditions for this process are reported as:

wet algae/methanol (wt./vol.) ratio of around 1:9, reaction temper-

ature and time of about 255 C, and 25 min respectively.

3.3. TEM of frozen algal biomass (raw material) and residual

Nannochloropsis algal sample after SCM process

For analysis of elemental composition, raw and residual sam-

ples were washed with distilled water and centrifuged pellets were

excised from centrifuge tubes, air-dried and glued to carbon adhe-

sive tabs (Electron Microscopy Sciences, Hatfield, PA) on aluminum

sample stubs. Elemental spectra were collected at 15 kV using a

model S-3400 N scanning electron microscope (Hitachi High-Tech-

nologies, Pleasanton, CA) equipped with a model Noran System Six

300 energy-dispersive spectrometer system (Thermo Electron

Corp., Madison, WI).

From TEM analysis report of frozen (raw) and residual algal bio-

mass, 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 in-

creased in residue) due to high content of unsaturated fatty acids

in lipid.

3.4. Analysis of algal biodiesel conversion

For the quantification of reaction product, the algal biodiesel

samples were analyzed by a gas chromatographymass spectrom-

etry (GCMS) system incorporated with an Agilent 5975 C MSD

and an Agilent 7890 A GC. The content of the fatty acid methyl es-

ter 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

GCMS. It is noted from GCMS results that algal biodiesel con-

tains a major proportion of mono and poly unsaturated fatty acid

methyl esters. The major fatty acids were palmitoleic acid

(C16:1, 3033%), oleic acid (C18:1, 3538%), eicosapentanoic acid

(EPA, C20:5n3, 58%), palmitic acid (C16:0, 510%,) and arachi-

donic acid (C20:4n6, 13%). From the GCMS peak and total ionchromatography (TIC) data, it was observed that the algal biodiesel

contains olefins, fatty alcohols and sterols in minor quantities

along with saturated and unsaturated FAMEs. The relative weight

compositions of organic compounds present in the algal biodiesel

were analyzed using GCMS, and summarized inTable 3.

ATR-FTIR spectra of petro-diesel, camelina biodiesel, and algal

biodiesel are shown in Fig. 2.The IR spectra were obtained using

a PerkinElmer Spectrum 400 FT-IR/FT-NIR spectrometer. The main

components of diesel are aliphatic hydrocarbons, whose chemical

structures are similar to long carbon chain of the main components

of biodiesel. The observation of an absorption Peaks around

1200 cm1 may be assigned to the antisymmetric axial stretching

vibrations of CC(@O)O bonds of the ester, while peaks around

1183 cm1

may be assigned to asymmetric axial stretching vibra-tions of OCC bonds (Silverstein and Webster, 1998). In addition,

Fig. 1. FAME yield against reaction temperature and wet algae wt/methanol

volume ratio at different reaction times using RSM.

P.D. Patil et al. / Bioresource Technology 102 (2011) 118122 121


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since biodiesel is mainly mono-alkyl ester, the intense C@O

stretching band of methyl ester appears at 1743 cm1 for algal

and camelina biodiesel which is absent in petro-diesel spectra.

4. Conclusions

The single-step process for wet algal biomass (Inoculum Nanno-

chloropsis sp., CCMP1776) may offer the benefits of shorter reactiontime, simple purification of products and maximum conversion of

triglycerides into their corresponding fatty acid methyl esters. The

single-step process favors the energy requirements for biodiesel

production by eliminating the needs for drying and extraction of

algal biomass. Process optimization using response surface meth-

odology (RSM) design proved to be a valuable tool for evaluating

the effects of the process variables on the FAMEs yield. The sin-

gle-step process has the potential to provide an energy efficient

and economical route to algal biodiesel production.

Acknowledgements

This project was partially supported by New Mexico State Uni-

versity Office of Vice President for Research and State of New Mex-

ico through a New Mexico Technology Research Collaborative

grant. The authors are thankful to CEHMM Artesia, NM for provid-

ing the wet algal biomass for biodiesel testing.

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Table 3

GCMS peak TIC data of crude biodiesel obtained from algal biomass.

Peak Retention time Area (%) Name

1 3.264 0.63 2H-pyran

2 3.802 0.03 Octanoic acid methyl ester

3 4.609 0.01 Nonanoic acid methyl ester

4 4.905 0.2 Dodecane

5 5.250 0.1 Benzenamine

6 5.547 0.1 Indole7 5.759 0.35 Decanoic acid methyl ester

8 6.354 0.18 Naphthalene,2,6-dimethyl

9 7.933 0.05 cis-5-Dodecenoic acid methyl ester

10 8.305 3.14 Undecanoic acid, 10-methyl ester

11 9.524 19.72 Tridecanoic acid methyl ester

12 10.245 0.02 8-Heptadecene

13 13.587 1.74 7-Hexadecenoic acid methyl ester

14 13.632 4.5 Cyclopropaneoctanal,2-octyl-9-Eicosyne

15 14.125 0.2 Cyclohexaneethanol

16 15.124 7.34 9-Hexadecenoic acid methyl ester

17 16.250 10.2 Tricyclo decane

18 16.923 9.92 Pentadecanoic acid, 13-methyl ester

19 17.089 33.28 9-Octadecenoic acid methyl ester

20 17.878 8.29 Eicosanoic acid methyl ester

Fig. 2. FTIR results of algal biodiesel, petro-diesel, and camelina biodiesel.

122 P.D. Patil et al. / Bioresource Technology 102 (2011) 118122

algea biodiesel zzxxs

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