Assessment of Sustainability Indicators for Biodiesel Production

applied sciences

Article Assessment of Sustainability Indicators for Biodiesel Production

Edith Martinez-Guerra 1,2 and Veera Gnaneswar Gude 1,*

1 Department of Civil and Environmental Engineering, Mississippi State University, Starkvill, MS 39762, USA; [email protected] 2 Environmental Laboratory, Engineer Research and Development Center, Vicksburg, MS 39180, USA * Correspondence: [email protected] or [email protected]; Tel.: +1-662-325-0345

Received: 31 July 2017; Accepted: 22 August 2017; Published: 25 August 2017

Featured Application: Sustainability Indicator Concepts used for biodiesel production in this study can be applied to various biofuels and other chemical reaction designs, synthesis and process development.

Abstract: Biodiesel production may provide a sustainable route to reduce environmental pollution caused by fossil fuel consumption. In order to minimize environmental impacts of biodiesel production, the chemical process should be optimized to minimize waste generation and energy consumption. Therefore, it is important to design biodiesel chemical reactions and processes using green chemistry and green engineering principles to develop sustainable chemical processes. This study provides the results of the synergistic effect of microwave and ultrasound irradiations to produce biodiesel using ethanol or methanol as the alcohol donor. The biodiesel yields are presented along with their respective green metrics, such as atom economy, environmental factor (E-factor), atom economy (utilization) or atomic efﬁciency, mass intensity, reaction mass efﬁciency, atom utilization, and stoichiometric factor. These green metrics are crucial to determine the sustainability and environmental impact of biodiesel production. Evaluation of these green metrics indicates that methanol is a better alternative for biodiesel production provided it is derived from renewable sources. Sustainability indicator concepts used for biodiesel production in this study can be applied to various biofuels and other chemical reaction designs, synthesis and process development.

Keywords: biodiesel; green chemistry; microwave; ultrasound; green metrics; atom economy; environmental factor (E-factor); mass intensity; reaction mass efﬁciency; stoichiometric factor

1. Introduction In recent years, there has been a concerted effort across the scientiﬁc community to produce biofuels in an environmentally friendly and sustainable manner [1–3]. Green chemistry and Green Engineering principles have been adopted to achieve this goal. These two sets of principles are primarily oriented towards reaction design and optimization with respect to materials and energy usage, waste reduction and economic feasibility. These principles also focus on minimal use of toxic and hazardous chemicals and maximum safety practices. These principles are commonly applied to evaluate the performances of individual chemical reactions, chemical processes and chemical synthesis design and plans in various chemistry, chemical and process engineering disciplines. The fundamental concept for both green chemistry and green engineering is that ongoing efforts to improve reactions are directly connected to the optimization of key parameters that govern their performances. It is obvious that some of these parameters will need to be maximized and others minimized [3]. This research article focuses on the use of green chemistry metrics for evaluating a biodiesel production process.

Appl. Sci. 2017, 7, 869; doi:10.3390/app7090869 www.mdpi.com/journal/applsci Appl. Sci. 2017, 7, 869 2 of 14

In order to be able to control chemical processes, Anastas and Warner introduced the concept of green chemistry along with the twelve green chemistry principles: (1) Prevention; (2) atom economy; (3) less hazardous chemical syntheses; (4) design safer chemicals; (5) safer solvents and auxiliaries; (6) design for energy efficiency; (7) use renewable feedstocks; (8) reduce derivatives; (9) catalysis; (10) design for degradation; (11) real-time analysis for pollution prevention; (12) inherently safer chemistry for accident prevention [4]. Green chemistry, in general, incorporates a new approach to the synthesis, processing and application of chemical substances in such a manner as to reduce threats to health and the environment. This new approach is also known as environmentally benign chemistry; clean chemistry; atom economy; and benign-by-design chemistry [5]. The concept of green chemistry refers to the design of chemical products and processes that minimize or eliminate the generation of hazardous substances and that seek to prevent or reduce the risk to human health and the environment [1]. Thus, green chemistry evaluates the greenness of a chemical process and allows to compare the greenness of existing solutions with newly developed ones [6]. Correspondingly, with green chemistry, the green chemistry metrics were introduced as metrics that measure aspects of chemical processes based on the twelve aforementioned green chemistry principles. Green metrics have proved to be a simple and versatile method for an immediate evaluation of the sustainability of a chemical process of any kind. Different components must be taken into account when seeking the sustainability of competing chemical reactions; these should include the toxicity of a product, reaction conditions, reagents, and reaction types. The most common metrics used to determine the sustainability of a chemical process are atom economy [7], and the E-factor [7–11]. These are simple calculations that mainly focus on waste, except for effective mass yields that focus on the toxicity of reagents. In addition to the aforementioned metrics, several other green chemistry metrics will be discussed within this study including atom efficiency [7], mass intensity [8,12], mass productivity [8], reaction mass efficiency, atom utilization, and stoichiometric factor [3]. This study attempts to use green metrics calculations to determine the greenness of biodiesel production using waste cooking oil. Each individual metric is explained in detail within the materials and methods sections. Green chemistry metrics to calculate the sustainability of a product or a chemical process have been used mostly in the pharmaceutical industry [13,14]. Additionally, green metrics have been used in several other processes such as the production of certain electronic devices [12]. No previous or current experimental data has been reported for the application of green chemistry metrics in biodiesel production. Therefore, the main focus of this study is to report the findings of the application of green chemistry metrics to the synergistic effect of microwave and ultrasound irradiations in the conversion of waste cooking oil into biodiesel. These metrics can be used in any chemical process implemented for biofuel production.

2. Materials and Methods

2.1. Materials The waste cooking oil (WCO) was obtained from the Mississippi State University cafeteria. The acid value of WCO was found to be 3.5 mg KOH/g, corresponding to a free fatty acid (FFA) level of 1.7%. From the Gas Chromatograph-ﬂame ionization detector (GC-FID), the molecular weight for the waste cooking oil was 840 g/mol. The alcohols (ethanol and methanol) and the catalyst are of analytical grade and were purchased from Fisher Scientiﬁc (Hampton, NH, USA). A microwave/ultrasound reactor unit and three-neck custom-fabricated reaction vessels made of borosilicate glass were manufactured by Columbia International Technologies® (Calgary, AB, Canada).

2.2. Methods Various conditions involved in transesterification reaction were evaluated through a mechanistic study. These include reaction time, catalyst amount, methanol to oil molar ratio and oil volume ratios. The tested conditions include reaction time for 1–4 min with increments of 1 min; a methanol to oil Appl. Sci. 2017, 7, 869 3 of 14 molar ratio of 4.5:1 to 12:1; catalyst concentrations ranging from 0.50–1.25%; and an equal power output for microwave and ultrasound (100/100 W/W). The oil volume was fixed to 20 mL and was added to a homogenous mixture of different weight percentages of NaOH and different alcohol volumes. For example, a 6:1 alcohol to oil ratio was equivalent to 6.15 mL of methanol or 8.35 mL of ethanol for 20 mL of oil; all calculations are based on molecular weight and the specific gravity of each alcohol and the oil. However, when studying different oil volumes, the alcohol to oil ratio was fixed to 6:1, catalyst amount was 0.75% (wt/wt), and the combined power output was 400 W (200 W for min reaction time). Two layers of product were derived after each test. The two layers—biodiesel and the glycerol—were separated and were both measured to determine green metrics. The benefits of combining microwave and ultrasound effects in biofuel production have been discussed elsewhere [15]. In addition, details of process parametric evaluation studies focusing on microwave and ultrasound effects, alcohol and catalyst effect on transesterification of waste cooking oils can be found in our previous studies [16–18]. Experiments conducted in this study were intended to evaluate the green metrics of the microwave and ultrasound-enhanced biodiesel production.

2.3. Evaluation of Sustainability Indicators

2.3.1. Environmental Factor (E-factor) The E-factor is a simple and fast metric, and is the most frequently used metric to measure the potential environmental acceptability of a chemical process. First introduced by Roger Sheldon [11], the E-factor is expressed as the ratio (equation) of the actual total amount of waste to the desired product (Equation (1)), excluding water because it will lead to high E-factors which are not useful for comparing processes [7–11]. A higher E-factor implies that an excess amount of waste is generated, and therefore, the risk of negative environmental impact is higher. Hence, an ideal E-factor is zero meaning that no waste was generated during the process.

Total waste (kg) E − f actor = (1) Product (kg)

2.3.2. Atom Economy (AE) and Atom Efficiency Atom economy has been considered as a measure of environmental sustainability in minimizing the amount of theoretical waste [19] as shown in Equation (2). This concept was introduced by Barry M. Trost at Stanford University in 1991, and it depends on the molecular weights of the reactants and the final product. It was initially proposed to avoid relying only on the yield of a product to measure its greenness. Unfortunately, the usefulness of AE is limited since it only considers the stoichiometry of the reaction and does not take into account the yield of the desired product [20]. The ideal atom economy for a chemical transformation is taken as the process where all reactant atoms are found in the desired product [19]. Since the catalyst concentration for biodiesel production was insignificant and consumed during the reaction, it is excluded from the AE calculations. Similarly, atom efficiency (Equation (3)) is the product of atom economy and the obtained percentages yield from the reaction. Atom economy provides the actual efficiency and utilization of atoms to produce useful products [1].

MW of productC AE = × 100 (2) MW o f A + MW o f B

On the other hand, atom efﬁciency provides a more concise solution because it also takes into account the production yield percentage.

Atom ef ficiency = AE × % yield (3) Appl. Sci. 2017, 7, 869 4 of 14

2.3.3. Mass Intensity (MI) and Mass Productivity (MP) Also known as process mass intensity (PMI), mass intensity (MI) was proposed by Constable et al. [8]. MI, as shown in Equation (4), is deﬁned as the total mass used in a process divided by the mass of the product. Mass used in the process includes reagents, solvents, and catalyst. MI can also be calculated as the E-factor plus one; therefore, the ideal MI is 1 (kg/kg) with zero for the E-factor. Equation (5) is used to calculate mass productivity, which is just the reciprocal of the MI; thus, MP depends merely on MI. Also, MI is the preferred metric to drive greater efﬁciencies in pharmaceutical syntheses [14].

Total mass used in a process (kg) MI = (4) Mass of final product (kg)

1 MP = × 100 (5) MI

2.3.4. Reaction Mass Efficiency or Material Efficiency (RME) RME has been emphasized as a realistic metric for describing the greenness of a process [21]. It offers a better and easy way of identification of the best or worst reactions that have influence on the whole industrial process or synthesis [6]. RME takes into account yield, stoichiometry, and atom economy, and it is related to the E-factor by an inverse expression. According to Jimenez-Gonzalez et al. [14], PM and RME are an indispensable intermediate step to estimate life cycle impacts and footprints. There are several RME equations, which depend on the conditions of the reaction process; for this study, Equations (7) and (8) have been used to calculate the RME of WVO transesterification. Ideally, RME for a reaction involving reactants of A and B to yield products C and D, as shown in Equation (6), can be presented as Equations (7) and (8).

A + B → C + D (6)

Mass of product (C + D)(kg) RME = (7) Mass o f A (kg) + mass o f B (kg) or 1 RME = (8) 1 + Ef

2.3.5. Atom Utilization AU provides a fast and simple evaluation reaction in terms of produced waste; however, solvents are excluded from calculations [7].

Mass of the final product AU% = (9) Total mass of all substances produced

2.3.6. Solvent and Catalyst Environmental Impact Parameter (f ) This parameter has a value of zero only if all materials used in the process are recycled, recovered, or eliminated; otherwise, f > 0 [3,22,23]

∑ mass of reactants and postreaction solvents and materials + mass of catalyst used f = (10) Mass of final product

2.3.7. Stoichiometric Factor The stoichiometric factor (SF) is a green metric that accounts for chemical reactions run under nonstoichiometric conditions, with one or more reagents in excess [22]. According to Andraos [22], SF Appl. Sci. 2017, 7, 869 5 of 14

equalAppl. toSci. unity 2017, 7 means, 869 that the reaction is operated under stoichiometric conditions; otherwise,5 of SF14 is greater than 1 [6]. reagents (AE) ∑ Mass of excess ()∑ chemicals (11) SF = 1 + ℎ (11) = 1 + Expected product mass at 100% yield 100% The appendix shows sample calculations for evaluating these metrics using the results obtained The appendix shows sample calculations for evaluating these metrics using the results obtained from our experimental studies. from our experimental studies. 3. Results and Discussions 3. Results and Discussions This section discusses the results from the evaluation of green chemistry metrics for biodiesel This section discusses the results from the evaluation of green chemistry metrics for biodiesel production under the synergistic effect of microwave and ultrasound using ethanol and methanol as production under the synergistic effect of microwave and ultrasound using ethanol and methanol as solvents. The most common method of biodiesel production is by transesteriﬁcation (alcoholysis) of solvents. The most common method of biodiesel production is by transesterification (alcoholysis) of oils (triglycerides) with methanol in the presence of a catalyst [24,25] which is shown in Figure1[ 26]. oils (triglycerides) with methanol in the presence of a catalyst [24,25] which is shown in Figure 1 [26]. TheThe stoichiometric stoichiometric reaction reaction requires requires 1 mole 1 mole of triglyceride of triglyceride and 3and moles 3 moles of an alcoholof an alcohol solvent. solvent. However, excessHowever, alcohol excess is used alcohol to driveis used the to drive reversible the reversible reaction reaction forward forward in order in toorder increase to increase the yields the yields of the alkylof the esters alkyl and esters to assist and to phase assist separation phase separation from the from glycerol. the glycerol.

Figure 1. Reaction scheme for transesteriﬁcation of triglycerides, modiﬁed from [26]. Figure 1. Reaction scheme for transesterification of triglycerides, modified from [26].

AccordingAccording to to the the twelve twelve principlesprinciples ofof green ch chemistry,emistry, a a green green solvent solvent should should meet meet numerous numerous criteriacriteria such such as as low low toxicity, toxicity, non-flammability,non-flammability, non-mutagenicity,non-mutagenicity, non-volatility non-volatility and and widespread widespread availabilityavailability among among others. others. Moreover,Moreover, thesethese green solv solventsents have have to to be be cheap cheap and and easy easy to tohandle handle and and recyclerecycle [27 [27].]. Here, Here, we we considered considered ethanol ethanol asas a solventsolvent due due to to its its renewable renewable ch characteristics.aracteristics. Producing Producing biodieselbiodiesel from from low low quality quality feedstockfeedstock suchsuch as wast wastee cooking cooking oils oils is is a achallenging challenging task task due due to tothe the presencepresence of of high high free free fatty fatty acid acid (FFAs) (FFAs) content.content. In addition, use use of of homogeneous homogeneous catalysts catalysts can can cause cause seriousserious separation separation issues issues [ 28[28].]. However,However, useuse ofof heterogeneousheterogeneous catalysts catalysts requires requires high high temperatures temperatures and possibly high pressures with extended reaction times. Production of heterogeneous catalysts is and possibly high pressures with extended reaction times. Production of heterogeneous catalysts is also not a simple task. Cheap materials such as egg shells and other carbon-based catalysts can be also not a simple task. Cheap materials such as egg shells and other carbon-based catalysts can be used used for this purpose [29] but this is not the focus of this research. For these reasons, we have for this purpose [29] but this is not the focus of this research. For these reasons, we have considered considered the process intensification effects of microwave and ultrasound irradiations with the the process intensification effects of microwave and ultrasound irradiations with the homogeneous homogeneous catalyst-mediated transesterification reaction. catalyst-mediated transesterification reaction. 3.1. Environmental Factor (E-factor) 3.1. Environmental Factor (E-factor) The E-factor was calculated assuming glycerol was a waste; however, a lower E-factor can be obtainedThe E-factor if bothwas biodiesel calculated and glycerol assuming are glyceroltaken as wasproducts. a waste; Hence, however, all the acalculations lower E-factor for eachcan be obtainedmetric were if both done biodiesel making and the glycerol same assumption are taken as and products. taking biodiesel Hence, all as the the calculations only main product. for each The metric wereE-factor done was making calculated the same for each assumption condition and when taking producing biodiesel biodiesel as the only using main waste product. cooking The oilE-factor and wasmethanol calculated or ethanol for each as conditionthe reactant. when For each producing condition, biodiesel it was usingnoticed waste (Figure cooking 2) that oilmethanol and methanol has a orlower ethanol E-factor as the, which reactant. means For that each less condition, waste was it was produced noticed when (Figure using2) that methanol methanol as a has reactant. a lower E-factorHowever,, which it is means known that that less ethanol waste is wasan “environmental produced when friendly” using methanol alcohol because as a reactant. it can be However, produced it is knownfrom thatrenewable ethanol material. is an “environmental However, it was friendly” noticed alcohol that higher because amounts it can of be soap produced were present from renewable in the material.reaction, However, which was it was higher noticed when that highercompared amounts to the of soapuse of were methanol. present inThis the reaction,implies that which the was environmental factor is able to calculate or to predict which process (or material) will create more Appl. Sci. 2017, 7, 869 6 of 14

Appl. Sci. 2017, 7, 869 6 of 14 higher when compared to the use of methanol. This implies that the environmental factor is able towaste. calculate Additionally, or to predict it can which be observed process (orthat material) the E-factor will is create almost more zero waste. for a higher Additionally, methanol it can to oil be observedmolar ratio, that but the itE-factor is higheris almostwhen zeroincreasing for a higher the ca methanoltalyst concentration. to oil molar This ratio, can but be it isattributed higher when to a increasinghigh content the of catalyst free fatty concentration. acids present This in canthe bewaste attributed cooking to oil, a high which content increases of free the fatty soap acids formation present inas thethe wastecatalyst cooking concentration oil, which increases. increases The the E-factor soap formation for different as the oil catalystvolumes concentration was influenced increases. by the TheprocessE-factor intensificationfor different effect oil volumes of microwave was influenced and ultrasound by the process at a fixed intensification power supply effect which of microwave is defined andas power ultrasound density. at aUnder fixed this power condition, supply whichthe E-factor is defined for ethanol as power reactions density. was Under lower this because condition, ethanol the E-factorwas ablefor to ethanol act as reactionsa solvent wasin the lower reaction because [18]. ethanol Methanol was abledoes to not act have as a solventa similar in theability reaction to act [18 as]. Methanolsolvent and does therefore not have has a lower similar reaction ability toyields act as and solvent higher and waste therefore production. has lower This reaction suggests yields that andthe higherdiffusive waste process production. can be used This to suggests reduce the that energy the diffusive requirements process for can biodiesel be used production to reduce thebut energyexcess requirementssolvents will be for required. biodiesel production but excess solvents will be required.

Figure 2.2. EnvironmentalEnvironmental factor factor results for the simultaneous effect of microwave and ultrasoundultrasound (MW/US)(MW/US) in in the the waste waste cooking cooking oil oil (WCO) (WCO) transesterification. transesteriﬁcation. Effect Effect of ( ofa) (differenta) different reaction reaction time; time; (b) (alcoholb) alcohol to oil to molar oil molar ratio; ratio; (c) catalyst (c) catalyst concentration concentration and and (d) (sampled) sample (oil) (oil) volume. volume.

3.2. Atom Economy (AE) or Atom EfficiencyEfficiency The molecular weights of biodiesel product, oil, catalyst, and alcohol were used to calculate the AE forfor ethanolethanol and methanol, which were 86% and 88% respectively. EvenEven thoughthough atom economy is one of the most commonly used green metrics [[21],21], a co conclusionnclusion cannot be made from this result since it is only based onon molecularmolecular weight.weight. Figure3 3 shows shows thethe atom efficiencyefficiency resultsresults basedbased onon biodieselbiodiesel yields, and it was observedobserved that methanol is more atom efficientefficient than ethanol. This could be due to the fact that ethanol has a higher molecular weightweight and and higher higher boiling boiling point, point, and itand requires it requires higher temperaturehigher temperature (energy) than(energy) methanol than tomethanol react. The to efficiencyreact. The increases efficiency as theincreases reaction as time the increasesreaction duetime to increases higher reaction due to temperatures higher reaction and reactiontemperatures exposure. and However,reaction itexposure. is not favorable However, when it usingis not low favorable catalyst concentrationswhen using low due tocatalyst lower yieldconcentrations and lower due atom to economy.lower yield and lower atom economy. Appl. Sci. 2017, 7, 869 7 of 14 Appl. Sci. 2017, 7, 869 7 of 14

Appl. Sci. 2017, 7, 869 7 of 14

FigureFigure 3. Atom 3. Atom efﬁciency efficiency results results for for the the simultaneous simultaneous effect of of MW/US MW/US in WCO in WCO transesterification. transesteriﬁcation.

EffectEffect of (a )of different (a) different reaction reaction time; time; ((bb) alcohol alcohol to to oil oil molar molar ratio; ratio; (c) catalyst (c) catalyst concentration concentration and (d) and sample (oil) volume. (d) sampleFigure (oil) 3. Atom volume. efficiency results for the simultaneous effect of MW/US in WCO transesterification. Effect of (a) different reaction time; (b) alcohol to oil molar ratio; (c) catalyst concentration and (d) 3.3. Masssample Intensity (oil) volume. (MI) and Mass Productivity (MP) 3.3. Mass Intensity (MI) and Mass Productivity (MP) The results for MI and MP are shown in Figures 4 and 5, respectively. The process mass intensity Theis3.3. muchresults Mass higherIntensity for MI when (MI)and usingandMP Massare ethanol, shown Productivity which in Figures (MP)means 4 andthat 5the, respectively. total mass used The processin the process mass intensityis not is much higherrecovered,The when results and using the for massMI ethanol, and of MPthe which arefinal shown product means in Figures is that low. the 4Despite and total 5, resp massthis,ectively. the used results inThe the revealed process process mass that is nottheintensity ideal recovered, and theMIis massmuch (1 kg/kg) ofhigher the was ﬁnalwhen almost product using achieved ethanol, islow. by methanol; which Despite means the this, MI, that the when the results totalusingrevealed mass ethanol, used thatwas in also the idealprocessclose toMI unity.is (1not kg/kg) Therefore, both of these alcohols are green solutions or solvents to produce biodiesel, with methanol was almostrecovered, achieved and the by mass methanol; of the thefinalMI, productwhen is usinglow. Despite ethanol, this, was the also results close revealed to unity. that Therefore, the ideal both beingMI (1 kg/kg)better thanwas almostethanol. achieved It should by be methanol; noted that the with MI, whenhigher using alcohol ethanol, ratios was the alsomass close productivity to unity. of these alcohols are green solutions or solvents to produce biodiesel, with methanol being better than decreasesTherefore, for both ethanol of these which alcohols mainly are depends green solution on thes experimental or solvents to yields produce achieved biodiesel, in this with study. methanol ethanol.being It should better than be noted ethanol. that It withshould higher be noted alcohol that with ratios higher the mass alcohol productivity ratios the mass decreases productivity for ethanol whichdecreases mainly depends for ethanol on which the experimental mainly dependsyields on theachieved experimental in this yields study. achieved in this study.

Figure 4. Mass Intensity results for the simultaneous effect of MW/US in WCO transesterification. Effect of (a) different reaction time; (b) alcohol to oil molar ratio; (c) catalyst concentration and (d) sample (oil) volume. FigureFigure 4. Mass 4. Mass Intensity Intensity results results for for the the simultaneous simultaneous effect of of MW/US MW/US in WCO in WCO transesterification. transesteriﬁcation. Effect of (a) different reaction time; (b) alcohol to oil molar ratio; (c) catalyst concentration and (d) Effect of (a) different reaction time; (b) alcohol to oil molar ratio; (c) catalyst concentration and sample (oil) volume. (d) sample (oil) volume. Appl. Sci. 2017, 7, 869 8 of 14 Appl. Sci. 2017, 7, 869 8 of 14

Appl. Sci. 2017, 7, 869 8 of 14

Figure 5. Mass Productivity for the simultaneous effect of MW/US in WCO transesterification. Effect Figure 5. Mass Productivity for the simultaneous effect of MW/US in WCO transesterification. Effect of (a) differentof (a) different reaction reaction time; time; (b) (b alcohol) alcohol to to oil oil molar molar ratio; (c (c) )catalyst catalyst concentration concentration and and(d) sample (d) sample Figure(oil) volume. 5. Mass Productivity for the simultaneous effect of MW/US in WCO transesterification. Effect (oil) volume. of (a) different reaction time; (b) alcohol to oil molar ratio; (c) catalyst concentration and (d) sample 3.4. Reaction(oil) volume. Mass Efficiency (RME) or Material Efficiency (ME) 3.4. Reaction Mass Efficiency (RME) or Material Efficiency (ME) The results shown in Figures 6 and 7 were calculated using Equations (7) and (8), respectively. 3.4. Reaction Mass Efficiency (RME) or Material Efficiency (ME) TheThe results results shown in in Figure Figures 7 are6 andbased7 wereon the calculated E-factor, but using a similar Equations trend was (7) followed and (8), using respectively. both The resultsequations.The shown results As shown in shown Figure in in Figure7 Figuresare based7, the6 and reaction on 7 thewere maE-factor calculss efficiencyated, but using a percentages similar Equations trend were(7) wasand higher (8), followed respectively. when based using both The results shown in Figure 7 are based on the E-factor, but a similar trend was followed using both equations.on the As E-factor shown, which in Figure could7 be, the due reaction to the fact mass the efficiencyE-factor is solely percentages based on were the amount higher of when waste based equations.produced. Therefore,As shown morein Figure than 7, one the al reactionternative ma is ssavailable efficiency to calculatepercentages RME were. The higher results when favored based the on the E-factor, which could be due to the fact the E-factor is solely based on the amount of waste ontransesterification the E-factor, which reaction could using be due methanol to the forfact both the expressions.E-factor is solely In addition, based on the the differences amount ofbetween waste produced. Therefore, more than one alternative is available to calculate RME. The results favored the produced.MI and MP Therefore, values are more due tothan the one differences alternative in environmis availableental to calculate factors calculated RME. The in results these favoredreactions. the transesterificationtransesterification reaction reaction using using methanol methanol for for bothboth expressions. In Inaddition, addition, the differences the differences between between MI andMIMP andvalues MP values are dueare due to theto the differences differences in inenvironmental environmental factors factors calculated calculated in these in these reactions. reactions.

Figure 6. Reaction mass efficiency for the simultaneous effect of MW/US in WCO transesterification. Effect of (a) different reaction time; (b) alcohol to oil molar ratio; (c) catalyst concentration and (d) FigureFiguresample 6. Reaction 6. (oil) Reaction volume. mass mass efﬁciency efficiency for for the the simultaneous simultaneous effect effect of of MW/US MW/US in WCO in WCO transesterification. transesteriﬁcation. Effect of (a) different reaction time; (b) alcohol to oil molar ratio; (c) catalyst concentration and (d) Effect of (a) different reaction time; (b) alcohol to oil molar ratio; (c) catalyst concentration and sample (oil) volume. (d) sample (oil) volume. Appl. Sci. 2017, 7, 869 9 of 14 Appl. Sci. 2017, 7, 869 9 of 14 Appl. Sci. 2017, 7, 869 9 of 14

FigureFigure 7. 7.Reaction Reaction mass mass efficiency efficiencyor orMaterial Material efficiencyefficiency for the simultaneous effect effect of of MW/US MW/US in in WCO transesterification. Effect of (a) different reaction time; (b) alcohol to oil molar ratio; (c) catalyst WCOFigure transesterification. 7. Reaction mass Effectefficiency of (a or) different Material reaction efficiency time; for ( bthe) alcohol simultaneous to oil molar effect ratio; of MW/US(c) catalyst in concentration and (d) sample (oil) volume. WCOconcentration transesterification. and (d) sample Effect (oil) of (volume.a) different reaction time; (b) alcohol to oil molar ratio; (c) catalyst concentration and (d) sample (oil) volume. 3.5.3.5. Atom Atom Utilization Utilization 3.5. Atom Utilization AtomAtom utilization utilization was was calculated calculated usingusing EquationEquation (9) (see (see Figure Figure 8).8). Since Since the the atom atom utilization utilization is is basedbased onAtom on the the utilization waste waste generated generated was calculated in in reactions, reactions, using thethe Equation obtainedobtained (9) results (see Figure from from 8).the the Since WCO WCO the conversion conversion atom utilization appear appear tois to bebasedbe low; low; however,on however, the waste as as previouslygenerated previously in stated, stated,reactions, thisthis the is dueobtained to the the results fact fact that thatfrom glycerol glycerol the WCO was was conversion considered considered appear a waste. a waste. to Therefore,beTherefore, low; however, these these results results as previously can can be be improved improved stated, this ifif aais properproper due to use the is isfact given given that toglycerol to glycerol glycerol was or or itconsidered itis isconsidered considered a waste. as asa a beneficialTherefore,beneficial product. product. these results can be improved if a proper use is given to glycerol or it is considered as a beneficial product.

Figure 8. Atom utilization for the simultaneous effect of MW/US in WCO transesterification. Effect of FigureFigure(a) different 8. 8.Atom Atom reaction utilization utilization time; for for(b) thethealcohol simultaneoussimultaneous to oil molar effect effect ratio; of of (MW/USc MW/US) catalyst in concentrationWCO in WCO transesterification. transesteriﬁcation. and (d) sample Effect (oil) Effect of of (volume.a) different different reaction reaction time; time; (b ()b alcohol) alcohol to oil to oilmolar molar ratio; ratio; (c) catalyst (c) catalyst concentration concentration and (d and) sample (d) sample (oil) (oil)volume. volume.

Appl. Sci. 2017, 7, 869 10 of 14 Appl. Sci. 2017, 7, 869 10 of 14

3.6. Solvent and Catalyst Envi Environmentalronmental Impact Parameter (f) As shown in Figure9 9,, thethe solventsolvent andand catalystcatalyst environmentalenvironmental impactimpact parameterparameter waswas higherhigher forfor ethanol duedue to to its its higher higher molecular molecular weight weight of (46.07 of (46.07 g/mol) g/mol) when when compared compared to methanol to methanol (32.04 g/mol). (32.04 Ing/mol). addition, In addition, mass of themass ﬁnal of the product final usingproduct ethanol using was ethanol lower was because lower there because was there more was waste more produced waste inproduced the process. in the process.

Figure 9. SolventSolvent and catalyst environmentalenvironmental impact impact parameter parameter (f) for(f) for the the simultaneous simultaneous effect effect of MW/USof MW/US in in WCO WCO transesteriﬁcation. transesterification. Effect Effect of (aof) different(a) different reaction reaction time; time; (b) alcohol (b) alcohol to oil to molar oil molar ratio; (ratio;c) catalyst (c) catalyst concentration concentration and (d and) sample (d) sample (oil) volume. (oil) volume.

3.7. Stoichiometric Factor As mentioned earlier, completion of the transesteriﬁcationtransesterification reaction requires excess quantities of solvents overover the the stoichiometric stoichiometric ratios. ratios. The ethanolThe ethanol and methanol and methanol to oil ratios to usedoil ratios in the experimentalused in the studiesexperimental were signiﬁcantlystudies were higher significantly (1.5 to 4 higher times) (1.5 than to the 4 times) stoichiometric than the ratios stoichiometric which contributed ratios which to a highercontributed stoichiometric to a higher factor stoichiometric for both solvents factor for (see both Table solvents1). Of these, (see itTable should 1). beOf notedthese, thatit should ethanol be hasnoted slightly that ethanol higher has values slightly due tohigher its higher values molecular due to its weight. higher molecular weight.

Table 1. Stoichiometric factor. Process Condition Stoichiometric Factor Ethanol Methanol Process Condition Stoichiometric Factor Ethanol Methanol 4.5:1 1.07 1.05 4.5:1 1.07 1.05 6:1 1.14 1.12 6:1 1.14 1.12 9:19:1 1.281.28 1.22 1.22 12:112:1 1.431.43 1.32 1.32

4. Conclusion Conclusions and and Future Future Directions Directions The applicationapplication ofof green green chemistry chemistry metrics metrics or or sustainability sustainability indicators indicators to biodieselto biodiesel production production has beenhas been investigated investigated and quantiﬁedand quantified based based on the on experimental the experimental results results derived derived in this study.in this Evenstudy. though Even thesethough metrics these metrics consist consist of simple of simple calculations, calculations, they canthey provide can provide a clear a clear insight insight into into the the greenness greenness of biodieselof biodiesel production. production. From From the the results, results, it wasit was noted noted that that the the reaction reaction is moreis more favorable favorable or or “green” “green” or sustainableor sustainable when when using using methanol methanol as as a a reactant. reactant. However, However, ethanol ethanol has has beenbeen referredreferred toto asas a “green solvent” in many applications including biodiesel production due to its environmentallyenvironmentally friendly characteristics. From the green metrics evaluations conducted in this study, it can be concluded that even though ethanol can be produced from renewable materials, its use does not necessarily make Appl. Sci. 2017, 7, 869 11 of 14 characteristics. From the green metrics evaluations conducted in this study, it can be concluded that even though ethanol can be produced from renewable materials, its use does not necessarily make biodiesel production greener. Methanol as a reactant is not entirely desirable due to its nonrenewable nature and environmental impacts associated with its production. Therefore, it is suggested to further investigate the use of renewable feedstock to produce a methanol or microbial biomethanol production route. The production of biomethanol would not only be a contribution to sustainable biodiesel production, but also a viable alternative for other biofuel, biochemical, industrial and pharmaceutical processes that utilize methanol as a reactant or solvent.

Acknowledgments: The authors would like to acknowledge the partial funding support by The United States Environmental Protection Agency (USEPA) under the Grant number SU835519. No funds were received for covering the costs to publish in open access. Author Contributions: E.M.G. and V.G.G. conceived and designed the experiments; E.M.G. performed the experiments; E.M.G. and V.G.G. analyzed the data; E.M.G. and V.G.G. wrote the paper. Conﬂicts of Interest: The authors declare no conﬂict of interest. The funding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.

Appendix A

Sample calculations for each metric: Assuming: 6:1 ethanol to oil ratio, 1% NaOH, 80 mL of oil, 20 mL of glycerol (1.26 g/mL density), 71 mL of biodiesel (0.9 g/mL density) Find: each green metric for the sample Solution: Weight = volume × density Oil weight = 80 mL × 0.92 g/mL = 73.6 g of oil Ethanol weight = 33.39 mL × 0.79 g/mL = 26.4 g of ethanol Catalyst weight = 73.6 g of oil × 1% NaOH= 0.736 g of NaOH (wt/wt) Glycerol weight = 20 mL × 1.26 g/mL = 25.2 g of glycerol Biodiesel weight = 71 mL × 0.9 g/mL = 63.9 g of biodiesel E-Factor: Mass in the process = Oil + ethanol + catalyst = 73.6 g + 26.4 g + 0.736 g = 100.736 g Products = Glycerol + biodiesel = 25.2 g + 63.9 g = 89.1 g Waste= 100.736 g − 89.1 g = 11.636 g − = Total waste (kg) E f actor Product (kg) = 0.011636 kg/0.0891 kg = 0.13 E-factor = 0.13 Atom Economy: MW of productC AE = × 100 MW o f A + MW o f B Molecular weight: biodiesel (856 g/mol), ethanol (46.07 g/mol), catalyst (39.997 g/mol), oil (922 g/mol)

856 g/mol AE = = 84.9% ( g + g + g ) 922 mol 39.997 mol 46.07 mol Atom Economy = 85% Atom Efﬁciency: Atom ef ficiency = AE × % yield Assuming: yield = 71 mL biodiesel/80 mL oil = 88.75 %

Atom ef ficiency = 84.9% × 88.75% = 75.3%

Atom efﬁciency = 75% Appl. Sci. 2017, 7, 869 12 of 14

Mass Intensity (MI) and Mass Productivity (MP):

Total mass used in a process (kg) MI = Mass of final product (kg)

Mass used in the process (kg) = 0.100736 (from E-factor calculation) Mass of ﬁnal product (kg) = 0.0891

0.100736 kg MI = = 1.13 kg/kg 0.0891 kg

1 MP = × 100 MI 1 MP = × 100 = 88.5% 1.13 MI = 1.13 kg/kg and MP = 88.5 % Reaction Mass Efﬁciency: Mass of product (C + D)(kg) RME = Mass o f A (kg) + mass o f B (kg) or 1 RME = 1 + Ef 0.0891 kg RME = = 88% 0.100736 kg or 1 1 RME = = = 88% 1 + Ef 1 + 0.13 RME = 88% Atom Utilization: Mass of the final product AU% = Total mass of all substances produced 63.9 g AU% = = 72% 89.1 g AU = 72% Solvent and Catalyst Environmental Impact Parameter:

∑ mass of reactants and postreaction solvents and materials + mass of catalyst used f = Mass of final product

73.6 + 26.4 + 0.736 + 25.2 f = = 1.97 63.9 f = 1.97 kg/kg Stoichiometric Factor: reagents (AE) ∑ Mass of excess SF = 1 + chemicals Expected product mass at 100% yield The ideal stoichiometry is a 3:1 alcohol to oil molar ratio. In these sample calculations, the used molar ratio is 6:1. Therefore, the ideal ethanol amount should be 16.7 mL (13.2 g), but 33.39 mL (26.4 g) was used instead. Expected product mass at 100% yield= 13.2 + 73.6 + 0.736 = 87.536 g

(0.849)(13.2) SF = 1 + = 1.12 87.536 SF = 1.12 kg/kg

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