Biodiesel Production Using Supercritical Methanol with Carbon Dioxide and Acetic Acid

8/11/2019 Biodiesel Production Using Supercritical Methanol with Carbon Dioxide and Acetic Acid

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Hindawi Publishing CorporationJournal o ChemistryVolume , Article ID ,pageshttp://dx.doi.org/.//

Research ArticleBiodiesel Production Using Supercritical Methanol withCarbon Dioxide and Acetic Acid

Chao-Yi Wei, Tzou-Chi Huang, and Ho-Hsien Chen

Department of Food Science, National Pingtung University of Science and Technology, Neipu, Pingtung , Taiwan

Correspondence should be addressed to Ho-Hsien Chen; [email protected]

Received September ; Revised December ; Accepted December

Academic Editor: Ahmed A. Mohamed

Copyright Chao-Yi Wei et al. Tis is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

ransesterication o oils and lipids in supercritical methanol is commonly carried out in the absence o a catalyst. In this work,supercritical methanol, carbon dioxide, and acetic acid were used to produce biodiesel rom soybean oil. Supercritical carbondioxide was added to reduce the reaction temperature and increase the ats dissolved in the reaction medium. Acetic acid wasadded to reduce the glycerol byproduct and increase the hydrolysis o atty acids. Te aguchi method was used to identiy optimalconditions in the biodiesel production process. With an optimal reaction temperature o C, a methanol-to-oil ratio o , andan acetic acid-to-oil ratio o , a .% yield o atty acid methyl esters (FAMEs) was observed afer min at a reaction pressureo MPa. While the common approach to biodiesel production results in a glycerol byproduct o about % o the yield, the practicesreported in this research can reduce the glycerol byproduct by .% and thereby meet international standards requiring a FAME

content o>%.

1. Introduction

Due to higher energy demands, more problems are associatedwith the widespread use o ossil uels, recent rises in pe-troleum prices, and other energy concerns; it is increasinglynecessary to develop renewable energy sources with smallerenvironmental impacts. In recent years, there have beenincreased demands or biodiesel, which is used as uel indiesel engine systems. Generally, biodieselis a mixture o atty

acid methyl esters (FAMEs), which may be derived rom avariety o oils, ats, and waste oils and has similar physic-ochemical properties to conventional diesel []. Hence,biodiesel is compatible with existing diesel engines and canbe utilized without major engine modications. In addition,biodiesel combustion decreases emissions o CO2, SO, andunburned hydrocarbons. Biodiesel obtained rom energycrops avorably affects the environment and can help developnew industries, such as the agroenergy industry, which createemployment and boost regional development. For thesereasons, this renewable and environmentally riendly biouelhasthe potentialto ensurethe sustainability o energy sourcesin the uture by replacing exhaustible ossil uels as the main

energy supply. Despite the enormous benets o biodiesel,high processing costs and expensive eedstock have acted asbarriers to its development [].

ransesterication o oils and lipids into biodiesel con-sists o replacing the glycerol o triglycerides with a short-chain alcohol, which can be achieved using various processes.ransesterication reactions can be catalyzed using alkaline,acidic, enzymatic, or other kinds o catalysts []. Mostcommonly, biodiesel production utilizes an alkaline catalyst,

but it is difficult to adapt this process or use with somewaste oils and ats. Enzymatic catalysis takes a long time tocompletely convert oils and ats into FAMEs. Biodiesel uelscan also be processed rom oils via noncatalytic transester-ication with supercritical alcohol [], a process developedto resolve various problems. Such supercritical treatment cansignicantly reduce the reaction time, and the propertieso the product mixture were ound to ulll internationalstandards as well []. Te raw material o oils or ats with highcontents o ree atty acids can also be converted to FAMEsby an esterication reaction in supercritical methanol [,].With this process, a high yield o FAMEs can be obtained, andthere are no alkaline soaps generated. Moreover, separation


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Journal o Chemistry

and purication o the products are easy [,]. Unlike thealkali-catalyzed method, this method can also be applied torelatively long-chain alcohols.

Te production o glycerol as a byproduct, however,has not been avoided (), and with the increased produc-tion o biodiesel in years to come, a glut o glycerol may

result. A method or producing biodiesel without produc-ing glycerol, thereore, may prove efficacious. Ilham andSaka [] proposed that dimethyl carbonate be used ornoncatalytic, supercritical treatment in biodiesel production.Subsequently, Saka and Isayama [] carried out a processutilizing noncatalytic, supercritical methyl acetate to producea mixture o FAMEs and triacetylglycerol, commonly knownas triacetin, without producing glycerol [,]. Hence, pro-ducing a mixture o FAMEs and triacetin has proven easible,at least in cold climates and at high production costs. Suchprocesses, which convert the glycerol byproduct into useulbiodiesel additives, not only solve the problem o a glycerolglut in the market but also have the potential to improve the

properties o the biodiesel produced. For instance, methylacetate was proven to enhance biodiesel properties such asviscosity, ash point, and oxidation stability. riacetin wasalso shown to be a valuable antiknock additive [].

Saka et al. [] also proposed a two-step process orconverting oils and ats to atty acids and triacetin usingsubcritical acetic acid ollowed by the conversion o attyacids to FAMEs using supercritical methanol. We believethat methyl acetate can be produced rom methanol andacetic acid (). Methanol and CO2may be catalyzed to ormdimethyl carbonate () []. Hence, rom an economic pointo view, the simultaneous production o FAMEs and triacetinin a one-step reaction is a promising prospect [].

In short, this ideal scenario improves the quality obiodiesel andminimizesthe cost o processing biodiesel addi-tives. Tis reaction is made possible by the transestericationo triglycerides and methyl acetate to produce FAMEs withtriacetin as the byproduct instead o glycerol.

In this work, glycerol was subjected to estericationwith acetic acid to produce a mixture o acetylglycerols(monoacetin, diacetin, and triacetin) as part o the produc-tion o biodiesel (). We hope, thereore, to reduce the by-product glycerol produced in biodiesel production and toimprove the easibility o biodiesel production in light othe processing technology and availability o raw materials,thus contributing to the development o this valuable and

renewable energy resource

+ +3CH3OH

Methanol

R1-COOCH3

R2-COOCH3

R3-COOCH3

Fame

CH2-OH

CH-OH

CH2-OH

Glycerol

CH2-OOC-R1

CH-OOC-R2

CH2-OOC-R3

Triglyceride

()

+ +

Methanol

CH3OH

Acetic acid

CH3COOH

Methyl acetate

CH3COOCH3

Water

H2O

()

+

Methanol Dimethyl carbonate

+

Water

H2O2CH3OH CH3OCO2CH3CO2

()

+ 3CH3COOH

Acetic acid Acetylglycerols

CH2-OH

CH-OH

CH2-OH

Glycerol

+

Water

2O3H

CH2-OCOCH3

CH2-OCOCH3

CH-OCOCH3

()

2. Materials and Methods

.. Experimental Planning. Tis experimental research planwas carried out using the aguchi design method andan analysis o variance (ANOVA). Te aguchi method[] systematically applies design and analysis principlesto experiments, allowing the use o smaller amounts oexperimental data to effectively identiy improvements in theproduction process. Tis method can be employed in mostkinds o industries to design processes and improve productquality with decreased costs, thereby economically optimiz-ing manuacturing processes which may involve multipleactors at different levels. In the aguchi method, orthogonalarrays are used to reduce the parameter numbers o theexperimental trials to a practical yet effective level. Wherea great number o independent variables are involved inthe design o an experiment, the aguchi method allows ormore-sophisticated analyses o such variables with a smallernumber o experiments.

Te signal-to-noise ratio (S/N ratio,) is a ratio o theaverage and standard deviation calculated rom the experi-mental data. Te S/N ratio is the way the aguchi methodstatistically measures the quality o production. Te S/Nratio equation depends on the criterion o the three qualitycharacteristics to be optimized: higher-the-better, normal-the-better, and smaller-the-better. In this research on theproduction o biodiesel with higher FAMEs and less glycerol,the S/N ratio o the methyl ester concentration in biodiesel(the higher-the-better unction) was used as ollows ():

= 10 log1=1

12

() , ()where

is the

th quality parameter and

is the number o

trials.A number o independent variables are involved in the

production o biodiesel rom soybean oil, including the reac-tion temperature, reaction time, molar ratio o methanol/oil,and molar ratio o acetic acid/oil. aguchi L designs and

aguchi 4 partial ractional actorial designs were used,including standard aguchi L orthogonal arrays requiringnine experiments (able ). In this study, the inuences oour variables (reaction time, reaction temperature, molarratio o methanol/oil, and molar ratio o acetic acid/oil) onglycerol contents were investigated. Te experimental errorwas determined by conducting two replicate experimentswith detailed statistical analyses o the results. Te levels


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Journal o Chemistry

: Experimental plan by the L aguchi design applied tobiodiesel production.

L Factors

Run emp. ime Methanol/oil Acetic acid/oil

+ + + + + + + + + + + +

: Levels o variables in biodiesel production.

Factors Parameter Levels and values

Low () Medium () High () emperature (C) ime (min) Methanol/oil Acetic acid/oil (: temperature;: time;: molar ratio(methanol/oil);: molar ratio(aceticacid/oil)).

and values o all variables applied in the experiments arepresented inable .

.. System Set-Up. Soybean oil bought rom a market(aisugar, Kaohsiung, aiwan) was used in this research.Samples were converted to methyl esters by noncatalytictransesterication in a supercritical uid system (Lian-Sheng,aichung, aiwan). All runs o the experiment were per-ormed in a mL cylindrical autoclave made o stainlesssteel (no. ) as shown inFigure .

Te sample was loaded into the autoclave by a high-pressure pump or each run. In a typical run, the auto-clave was charged with a given amount o soybean oil( mL) and liquid methanol ( mL) (Nihon ShiyakuIndustries, Osaka, Japan) with variable molar ratios. Tereaction tank was heated with an electrical heater, and

power was adjusted to give reaction temperatures o C and reaction times o min. Te system waspressurized with carbon dioxide ( MPa) (Jing-Shang Gas,Kaoushiung, aiwan). Afer completion o the reaction, thehigh-pressure valve was opened to stop the reaction andseparate the products rom the reactor. Solvents o controlgroups used to replace supercritical Methanol were methylacetate (Panreac, Barcelona, Spain) mL and dimethyl car-bonate (Ala Aesar, MA, USA) mL, respectively. Withoutadding acetic acid, only mL o soybean oil was added.It was pressurized to MPa at temperature C withcarbon dioxide, and reaction time was maintained up to min.

V5 TG PG

V3

V1

V2V4

E

T P

H

P1

P2

F : Schematic diagram o the supercritical reaction system:E, reaction vessel; H, heater and jacket; P, high-pressure liquid

pump; P, super-critical CO2 pump; V, high-pressure valves;V, back-pressure valve; V, release valve; PG, pressure gauge; G,temperature gauge.

.. Conversion of FAME. FAME concentrations o each sam-ple were analyzed (modied rom ISO) by a gas chro-matographic (GC) system (Hewlett-Packard PackardSeries II, Ramsey, MN, USA), equipped with a DB-waxcapillary vessel column ( m . mm ID . m lmthickness o polyethylene glycol). Te detector and injectortemperatures were both set to C. Te oven temperaturewas maintained at a constant C. Methyl heptadecanoate

was obtained rom Sigma-Aldrich (St. Louis, MO, USA) as aninternal standard. All o the analytical assays were perormedin duplicate, and mean values are presented. According to theGC integral data, the contraction o FAMEs was calculated by()

=() 100%, ()

where is the peak integral area o C to C: FAMEs,is the peak area o the internal standard (heptadecanoicacid methyl ester; HAME),is the concentration o HAMEin the solution (mg/mL),is the volume o HAME in thesolution (mL), and

is the sample weight (mg).

3. Results and Discussion

.. FAME Content. Tis study compared the supercriticalmethanol method o biodiesel production with two glycerol-ree methods: the supercritical methyl acetate and supercriti-cal dimethyl carbonate methods.able shows critical prop-erties o various solvents. In this work, temperature (C)and pressure ( MPa) were used, which were higher thanthose critical points inable , to ensure that a supercriticalmedium was generated. Beore the reaction, the methanol,soybean oil, and acetic acid divided clearly into three layers.Te dimethyl carbonate and methyl acetate mixed very well


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Journal o Chemistry

: Te critical properties o various solvents.

Solvent Critical temperature

(C)Critical pressure

(MPa)

Carbon dioxide . .

Methanol . .

Methyl acetate . .Dimethyl carbonate . .

with the soybean oil. We predicted that the contact area oreaction substrates would affect the reaction rate and yield.Afer completing the reaction and collecting samples, theglycerol and FAME contents were calculated individually.Figure shows the peaks o the FAME analysis with aninterstandard analysis by GC. Samples were obtained usingdifferent supercritical uids in the same reaction situation assupercritical methanol (Run no. in able ), using a runtemperature o C, a reaction time o min, a pressure o

MPa, and a solvent-to-oil ratio o . Te results showedthat using supercritical methanol with acetic acid producedbiodiesel with a much better FAME content (.%) thanmethods using supercritical methyl acetate (.%) or super-critical dimethyl carbonate (.%). Tis may have beendue to an application o the international standard FAMEanalytical method to C to C: FAMEs in the biodieselsamples. Producing biodiesel using the supercritical methylacetate and supercritical dimethyl carbonate methods alsoresults in the production o monoglycerol acetate, diglycerolacetate, and triglycerol acetate.

Te FAME content, which was calculated with a knownconcentration o HAME ( mg/mL) using (), o each run

o the experiment is shown in able . We studied theeffects o individual experimental actors on the methyl estercontent based on good quality (higher-the-better, ())andthecalculation o the S/Nratio o each o the experimental actorsto determine a set o optimalconditions, as shown in Figure .An optimal biodiesel yield resulted with a temperature oC, a reaction time o min, a methanol-to-oil molarratio o , and an acetic acid-to-oil molar ratio o . Aferrunning a conrmation experiment, we obtained an optimalmean FAME content o .%.

ANOVA is a way to statistically authenticate how con-trol actors and parameters affect results. able showsthe ANOVA values or biodiesel production, demonstrating

that each reaction actor signicantly affected the glyceroland FAME concentrations in the biodiesel reaction system.Biodiesel samples produced were also tested by using themin a diesel generator, which ran smoothly and showed nodifferences in perormance rom regular diesel.

.. Effect of Reaction Temperature. We investigated theeffects o the reaction temperature on the conversion oFAMEs. Results showed that the reaction temperature wasthe most signicant actor affecting both the FAME andglycerol contents. Te higher the temperature, the aster thereaction was completed and the more FAMEs that wereproduced. Higher reaction temperatures resulted in better

solubility o the substrates and better molecular activity,which increased the relative probability o collisions andthe reaction rate. An increase in the reaction temperaturealso allowed easier breaking o the bonds o triglyceridesand methyl combinations into FAMEs. oo high a reactiontemperature or too long a period, however, will break down

the FAMEs that have ormed and will increase heating costs.Higher reaction temperatures, however, also producedhigher glycerol contents. Tus, a balance between meetinginternational biodiesel standards and lowering the outputo byproducts must be struck in the biodiesel productionprocess.

.. Effect of Reaction Time. Increasing reaction times result-ed in no signicant increase in FAMEcontent, suggesting thatthe biodiesel conversion reaction had reached homeostasisafer a period o time, a result which was consistent withthe reaction kinetics curve. In other words, the biodieselconversion was already completed in a short time. An

increased reaction time, however, did increase the glycerolcontent.

.. Effect of the Methanol-to-Oil Ratio. Results showed thata higher proportion o methanol to oil achieved a betterconcentration o FAMEs in the samples produced; as the pro-portion o methanol increased, the concentration o FAMEsgenerated also increased, because methanol acts as a reactantand also provides the main medium or the reaction system.Using more methanol achieved a higher reactant solubilityin the reaction system, but samples collected contained ahigher ratio o methanol. An additional process was requiredto recover it.

.. Effect of the Acetic Acid-to-Oil Ratio. Te molar ratioo acetic acid to oil affected the FAME content; addingmore acetic acid to the system produced a higher FAMEcontent. Adding acetic acid also promoted the hydrolysis otriglycerides into atty acids, which improved the conversionyield. At the same time, methanol and acetic acid reacted inthe system to orm methyl acetate, which then reacted withthe glycerol byproduct to orm triacetin and thereby loweredthe glycerol content. Methyl acetate is also a cosolvent, whichincreased the solubility o substrates in the supercriticalmethanol system.

4. Conclusions

A method utilizing supercritical methanol to manuacturebiodiesel resulted in higher FAME contents while avoidinghigher glycerol contents. Tis method may assist in meetinginternational standards or biodiesel production while alsolowering the production o byproducts. In this research,we added acetic acid to the FAME conversion system, andat the same time, methanol and acetic acid reacted toproduce methyl acetate. Te acetic acid promoted triglyc-eride hydrolysis into atty acids and thereby enhanced thebiodiesel conversion in supercritical methanol. Te glycerolbyproduct then reacted with the methyl acetate to orm


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Journal o Chemistry

: Results o L () orthogonal array experiments on atty acid methyl ester (FAME) and glycerol contents o biodiesel productionsamples.

Run FAME content Glycerol content

est (%) est (%) AVG SD S/N ratio (db) est (%) est (%) AVG

. . . . . . . .

. . . . . . . . . . . . . . . .

. . . . . . . .

. . . . . . .

. . . . . . . .

. . . . . . . .

. . . . . . . .

. . . . . . . .

C . . . . . . . .

AVG: average; SD: standard deviation; S/N: signal-to-noise ratio; C: conrmation test.

C17 methylheptadecanoate

S. C. methanol (run no.8 in Table 4)

S. C. dimethyl carbonate

S. C. methyl acetate

FAMEs of C14 to C21:1

Counts

10000

8000

6000

4000

2000

0

2000

0

2000

0

0 5 10 15 20 25(min)

F : GC diagram o atty acid methyl esters (FAMEs) with an internal standard (methyl heptadecanoate) prepared by supercritical uidmethods.

38.5

37.5

36.5

35.5

34.5

33.5Factors and its levels

S/Nra

tio

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

F : Response graph o higher-the-better signal-to-noise ratiosor atty acid methyl aster (FAME) contents at different conditions(A, temperature;B , time;C, molar ratio (methanol/oil); D , molarratio (acetic acid/oil)).

: Analysis o variance summary table o atty acid methylester (FAME) concentrations.

SS DF MS CF . . . .

. . . .

. . . . . . . .Error . .

otal .

(: temperature;: time;: molar ratio(methanol/oil);: molar ratio(aceticacid/oil)).SS:sum o squares; DF: degrees o reedom; MS:meansquare;: ratio, CF:condence.

triacetin, decreasing the glycerol content. Methyl acetate isalso a co-solvent, which provides substrate solubility in thesupercritical methanol system. In this research, an optimal


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Journal o Chemistry

reaction temperature o C, a methanol-to-oil ratio o, and an acetic acid-to-oil ratio o yielded a FAMEcontent o .% and a glycerol content o .% afer min at a reaction pressure o MPa. Te current standardmethod or converting biodiesel using supercritical methanolproduced about % glycerol. Te method used in this study,

thereore, decreased the production o the glycerol byproductby about % andmet the international standard orbiodieselrequiring a FAME content exceeding %.

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