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Experimental exergy analysis of transesterification in biodiesel production
Energy 196 (2020) 117092
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Experimental exergy analysis of transesterification in biodiesel production Golmohammad Khoobbakht a, Kamran Kheiralipour b, Hamed Rasouli c, Mojtaba Rafiee d, Mehrdad Hadipour e, f, Mahmoud Karimi d, * a
Department of Agricultural Engineering, Payame Noor University, Tehran, Iran Mechanical Engineering of Biosystems Department, Ilam University, Ilam, Iran Department of Agrotechnology, College of Abouraihan, University of Tehran, Tehran, Iran d Department of Biosystems Engineering, Arak University, Arak 38156-8-8349, Iran e Department of Environment, Faculty of Agriculture and Natural Resources, Arak University, Iran f Faculty of Biological Science, University of Kharazmi, Iran b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 June 2019 Received in revised form 26 January 2020 Accepted 3 February 2020 Available online 5 February 2020
Exergy flow analysis is applied to account wastes, determine the exergetic efficiency, compare substitutes and other types of energy sources and also define economic and environmental policies for resource use. The present study focuses on the exergy flow analysis of biodiesel production by esterification and transesterification of waste cooking canola oil to achieve benefits such as reducing material and energy consumptions and improving energy and exergy efficiencies. The input amounts of esterification for all the runs were kept constant, while the runs of transesterification were carried out in a variety of mass and energy input values as the experimental variables. The thermodynamics analysis was applied to determine exergy input and output of the system for the experimental runs. Impacts of experiment variables, including methanol:oil molar ratio, potassium hydroxide concentration and the reaction temperature of transesterification were evaluated on exergy efficiency and exergy loss in the transesterification. The maximum exergy efficiency (91.7%) and the minimum exergy loss (4320 kJ/kg biodiesel) were achieved at methanol:oil molar ratio of 8:1, potassium hydroxide concentration of 1 wt% and the reaction temperature of 55 C. Excessive use of methanol and catalyst reduced the yield and the exergy efficiency through increasing exergy loss by waste materials. © 2020 Elsevier Ltd. All rights reserved.
Keywords: Exergy loss Biodiesel Transesterification Thermodynamic analysis Conversion
1. Introduction Biodiesel derived from renewable resources has been considered as attractive alternative fossil fuels, because of its suitable chemical properties and also its environmental benefits such as non-toxicity, biodegradability, carbon neutrality [1e3] and its beneficent effects on diesel engine [4,5]. Because of some restrictions over the first generation of biofuels such as food vs. fuel issues, various researches have been switched on producing biodiesel from non-edible oils, waste cooking oils (WCO) and other feedstocks such as algae [6], microalgae [7], Jatropha [8], grease oil [9], mahua oil [10] and so on. Biodiesel is produced by esterification of fatty acids or
* Corresponding author. E-mail address: [email protected] (M. Karimi). https://doi.org/10.1016/j.energy.2020.117092 0360-5442/© 2020 Elsevier Ltd. All rights reserved.
transesterification of triglycerides with short chain alcohols like methanol and ethanol. Methanol is mostly used because of its lower cost compared with other alcohols, so biodiesel most commonly refers to fatty acid methyl esters (FAME). However, one of the major obstacles to wide application of biodiesel is its high cost in comparison of fossil diesel [11]. It has been reported that the cost of raw materials amounts to around 75% of the total biodiesel production cost [12]. Therefore, sustainability of biodiesel production depends on low cost feedstock such as waste cooking oil (WCO) to reduce the overall cost of biodiesel production [1,13]. Assessment of material and energy flows in each system is necessary in order to decrease the consumption of material and energy as well as promote the use of renewable resources and hence to reach production sustainability. Exergy flow assessment of the transesterification provides us critical information to manage wastes and optimize the process in terms of energy and material. Exergy efficiency as a useful tool measures the ratio of input to
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G. Khoobbakht et al. / Energy 196 (2020) 117092
output exergies of the transesterification that can be optimized through optimal consumption of inputs and reducing wastes in outputs. Sustainable biodiesel production needs to optimization of both yield efficiency and exergy destruction. Exergy-based criteria were found to give much better guidance for system improvement, as they account better for use of energy resources [14,15]. However, one of the most important reasons in this field is energy production through biodiesel. Therefor the system needs to be improved through both the reduction of exergy destruction and the enhancement of biodiesel yield, simultaneously [16]. Exergy is a thermodynamic property that quantifies the maximum possible work that could be extracted from a resource if that resource was to be fully equilibrated (thermally, mechanically and chemically) with the environment. Unlike energy, exergy is not a conserved quantity and is destroyed in any real process. As a nonconserved property, exergy is more akin to entropy, although it has the same units as energy [17]. A process that is more thermodynamically efficient will destroy less exergy, and a process that destroys large amounts of exergy should be examined for possible improvement [18]. Tracking and quantifying the exergy destroyed in a system is useful for two primary reasons. The first is the identification of a performance metric, the exergy efficiency, for a system. Depending on the desired output of the system (e.g., shaft work, or high-purity product), the exergy efficiency may be defined differently [19]. However, all definitions amount to comparing the exergy supplied to the system to the exergy of the desired output of the system. The second reason for tracking exergy destruction is to analyze the internal operation of a system [20]. This consists of finding not just the exergy efficiency of the system, but also identifying the locations in the system where the exergy destruction occurs [21]. Tracking exergy destruction and analyzing exergy efficiency assist us to improve the sustainability through optimization of the process. Apart from conversion efficiency, exergy efficiency in a system can be optimized to both reduce energy and resources consumption and increase production. In the literature, there are valuable works on exergy analysis of biodiesel production plants [22e25]. Till now, a gap is sensed in knowledge of the effects of the input energy and material in the transesterification of WCO on exergy destruction and exergy efficiency using experimental data. In the present work, biodiesel from waste cooking canola oil was produced and the experimental parameters, including methanol:oil molar ratio, catalyst (potassium hydroxide) concentration and reaction temperature, were evaluated on the exergy flow of the transesterification of WCO, which is the main reaction in the biodiesel production process. In the present work, thermodynamic analysis of transesterification has been based on the experimental data, which makes the analysis more accurate and reliable. In the experimental runs, yield of transesterification was measured using GC/MS, and along with the values of the input materials were applied for the thermodynamic analysis. An accurate knowledge of the effects of the variables in the transesterification on the exergy flow can be useful to improve the exergy efficiency and subsequently economic performance in commercial production of biodiesel through minimizing the exergy loss of the process. 2. Materials and methods 2.1. Materials Cooking canola oil used in frying of potato was collected and stored in a glass container at room temperature. The physicochemical properties of the WCO including density, viscosity, free
fatty acid (FFA) content and acid value were 0.906 g cm3, 51.3 cP, 5.68% and 11.16, respectively. The density and viscosity of oil were measured by a pycnometer and a viscometer, respectively, whereas the FFA content and acid values were determined by acid-base titration. Methanol, potassium hydroxide (97%), phosphoric acid, methyl salicylate, methyl palmitate, methyl stearate, methyl oleate, methyl linoleate and methyl linoleate were laboratory stock or purchased from Sigma-Aldrich.
2.2. Biodiesel production The first step of biodiesel production was acid catalyzed reaction, called esterification. The step was conducted in the present research to remove high water and FFA contents in WCO and prevent oil loss by saponification, as much as possible. For this, 500 g WCO, 0.8 g sulfuric acid and 100 ml methanol were mixed in a magnetic hot plate stirrer with temperature controller, for 1 h. The revolution of the stirrer and the temperature of reaction were kept constant at 400 rpm and 55 C, respectively. After completing the reaction, the mixture was allowed to be settled for 1 h, to make two distinct liquid phases. The excess methanol in the top phase was removed using separating funnel. The remaining phase containing methyl ester and unreacted triglyceride from transesterification was collected to then be used in the second step, an alkali catalyzed reaction called transesterification. Methanol:oil molar ratio in three levels of 4:1, 8:1 and 12:1, catalyst (potassium hydroxide) concentration in oil in three levels of 1, 2 and 3 wt% and the reaction temperature in three levels of 45, 50 and 55 C were considered as the experiment variables of transesterification. The experiments were carried out based on single factor experiment, in which the effects of the variables on the conversion of the reaction, exergy efficiency and exergy destruction were investigated. Each sample containing 100 g of the mixture prepared in the first step plus the determined values of methanol and potassium hydroxide were stirred for 1 h at the determined reaction temperature. Then the reaction mixture was retained and allowed to be settled and separated into two layers: crude biodiesel up and glycerol down. The remaining methanol was recovered under vacuum (10 ± 1 mm Hg) at 50 C with a rotational evaporator, and then crude biodiesel was washed by 10 wt% of water at 80 C to remove the soap which was formed by reaction between alkali and FFA. The wet crude biodiesel was then dried under a vacuum (5 ± 1 mmHg) at 90 C for an hour, using a rotational evaporator to prepare pure biodiesel.
2.3. FAME assay The FAME contents in the experiment samples were analyzed by gas chromatography (Shimadzu GC-2010), equipped with hydrogen flame ionization detector (FID). The separation was performed on a DB-1HT capillary column (30 m 0.25 mm). The GC was calibrated based on various concentrations of methyl salicylate, methyl palmitate, methyl stearate, methyl oleate, methyl linoleate, and methyl oleate. During the analysis, the temperature of sampling inlet was 370 C and the detector temperature was 375 C. The column temperature was raised regularly. The column temperature was initially maintained at 150 C for 2 min, then raised to 360 C at a rate of 10 C min1 and finally maintained at 360 C for 10 min. Nitrogen at a pre-column pressure of 100 kPa was used as carrier gas [26]. Methyl undecanoate was used as the internal standard. The FAME content of the samples was calculated using the following equation:
G. Khoobbakht et al. / Energy 196 (2020) 117092
Xð%Þ ¼
mFAME 100% mCB
(1)
where X is the FAME content, mFAME is the weight of FAME calculated with internal calibration method, and mCB is the weight of the crude biodiesel [26].
2.4. Exergy analysis Four balance equations must be applied in the process of the transesterification of WCO for a general steady state (no accumulation) in order to find the work and heat interactions. Mass input and output is always balanced according to the principle of mass conservation given by Eq. (2). Energy input and output is also balanced according to the first law of thermodynamics or the energy conservation principle given by Eq. (3). In real processes, entropy production increases according to the second law of thermodynamics as given by Eq. (4) and a part of the exergy input is always destroyed given by Eq. (5).
X ðm_ i Þ i
¼
X ðm_ i Þ i
in
X ðm_ i hi Þ i
(2)
out
¼
X ðm_ i hi Þ i
in
_ þ Q_ W
(3)
out
X Q_ i X X ðm_ i si Þout þ ðm_ i si Þin ¼ S_gen Ti i i i
(4)
_ _ _ _ _ Ex mass; in Exmass; out þ Exheat Exwork ¼ Exloss
(5)
_ mass ) is divided into four specific The mass exergy component (Ex components including chemical, physical, potential and kinetic exergy expressed in Eq. (6).
_ _ _ _ _ mass ¼ Ex Ex phy þ Exch þ Expot þ Exkin
(6)
The potential and kinetic terms of exergy are negligible and their contribution to the total exergy balance is minimal. The physical exergy is dependent on temperature, enthalpy and entropy as shown by Eq. (7).
_ _ _ Ex phy ¼ ðh h0 Þ T0 ðs_ s_0 Þ
(7)
The standard chemical exergy of many compounds can be found in Szargut et al. [27] and Ayres and Ayres [28]. When not available, the chemical exergy content of any pure substance can be computed by the approximate Eq. (8) which is a function of the chemical exergy of each elemental compound, the number of atoms of each element contained in the stream and the Gibbs free energy of formation for the compound [27e29].
_ ¼ D_G þ Ex ch f
X _ nelem Ex ch; i
3
X X X P T _ þ RT T T ni Ex ni Ln i þ ni C mean T Ln 0 0 0 i p;i T0 P0 i i i X n ni Ln P i þ RT0 i ni i
_ ¼ Ex
(9) _ is the overall exergy of a mixture of substances, n is the where: Ex i _ is the standard molar exergy number of the moles of substance i, Ex i of pure substance i, R is the molar gas constant, T0 is the temperature of environment, T is the temperature of substance i, Pi is the pressure that substance i expands itself reversibly at constant temperature T0 toward the state of volume V0 at pressure P0 in equilibrium with the atmosphere, P0 is the atmospheric pressure and C mean is the molar heat capacity of the substance i. The five p;i different terms of this equation indicates chemical exergy, pressure exergy for gaseous substances, thermal exergy due to temperature change and mixing exergy due to substances concentration change, respectively [30]. Mixing in the transesterification of WCO is an irreversible process in which all the work potential wastes. The total chemical exergy of a mixture is equal to the sum of the chemical exergy of all components of the mixture in addition to the exergy losses in mixing process. Based on exergy balance, the exergy transfer by _ heat flow (Ex heat ) at a temperature T and exergy by work flow _ (Ex ) were calculated by Eqs. (10) and (11), respectively [23,31]. work
T0 _ Q_ ¼ 1 Ex heat T
(10)
_ _ Ex work ¼ W
(11)
Usually an exergy conversion coefficient is defined to determine energy sources in terms of exergy unit (Joule). This coefficient can be used to estimate the chemical exergy content of fuels according to their heating value or enthalpy. The exergy coefficient of electricity is assumed to be 1.00, indicating that 1 kJ of electrical energy corresponds to an exergy flow of 1 kJ [28,32]. A global mass balance across the transesterification of WCO was performed, and the thermodynamic properties needed to develop the exergy balance were then obtained [22]. Chemical and physical exergies of input and output for the defined system of the transesterification of WCO were calculated. Exergy was determined for each compound, mixture and utility. Dead state conditions were taken as 25 C and 101.325 kPa with the exergy efficiency of each experiment related to the transesterification calculated using the following equation:
h¼1
Exloss Exinput
! (12)
3. Results and discussion
(8)
i
_ _ where: Ex ch is the chemical exergy of substance, DGf denotes the _ standard Gibbs free energy of formation of the substance and Ex ch; i is the chemical exergy of the ith pure element of the substance. The exergy of substances is determined by consisting of a physical part and a chemical part as explained above. By considering physical and chemical exergy, the overall exergy of a mixture _ is expressed as Eq. (9) [30]: of substances (Ex)
3.1. Product analysis The GC analysis on transesterification conversion products from a standard sample is shown in Fig. 1. The results indicated that the main components in the WCO-derived biodiesel were methyl salicylate, methyl palmitate, methyl stearate, methyl oleate, methyl linoleate and methyl oleate. The methyl esters were then quantified using the ratio of the areas under the peaks of each methyl ester to the internal standard and the known concentration of internal
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G. Khoobbakht et al. / Energy 196 (2020) 117092
Fig. 1. The GC chromatogram of WCO-derived biodiesel.
standard.
3.2. Identifying material wastes and energy loss The chemical exergy of the substances categorized into inputs, outputs and wastes in the transesterification of WCO under the reference condition (methanol:oil molar ratio of 8:1, potassium hydroxide concentration of 1 wt% and reaction temperature of 55 C) are listed in Table 1. The internal exergy destruction was calculated by deducting the total exergy output (Exout) from the total exergy input (Exin) for the operation [33,34]. The external exergy destruction is equal to the sum of the exergy of all waste streams in the production process, including 20% of the glycerides and methanol unreacted and remained in the reactor. The overall mass and exergy balance of transesterification in the reference condition is shown in Fig. 2. The major inputs in transesterification included WCO and methanol, whereas the major outputs included FAME (biodiesel), unreacted WCO, remaining methanol and glycerol. Exergy embodied in biodiesel (38820 kJ/kg) is more than that embodied in WCO (36700 kJ/kg). However, as can be seen in the flow diagram, about 1100 kJ exergy of WCO was consumed in transesterification for production of 1000 kJ exergy of biodiesel. The conversion efficiency (biodiesel yield) in the transesterification under the mentioned condition was determined to be 90.23%, with exergy efficiency of 91.73%, which can be further improved by process adjustments, such as optimization of the reaction variables, reduction of FFA content, water and existing impurities in WCO, resulting in incrementing the useful exergy (quality) of the inputs. After WCO, methanol was considered as the most exergy containing among the inputs. For 1000 kJ biodiesel production, 168 kJ
exergy embodied in methanol was used whereas 111 kJ of that was remained in the mixture of reaction. Most of the remained methanol was recovered (80%) and 20% of that was considered as the waste. Heat and electricity for stirring with consuming 131 kJ exergy per 1000 kJ biodiesel production was realized in the third place among the exergy inputs. While the chemical catalysts such as sulfuric acid and potassium hydroxide have significant environmental impacts, the exergy embodied in the chemical catalysts consumed in transesterification was nearly inconsiderable. In comparing with other researches, exergy efficiency of 90% for squez-Arredondo palm oil transesterification was reported by Vela et al. [35]; which prompts validity of our results (91.7%). The slight difference of exergy efficiencies can be attributed to the differences in the feedstock, concentrations of materials and the condition of the process. Also Karimi et al. [13] reported an exergy efficiency of 80% for the transesterification of WCO using immobilized lipase, with a considerable difference with the present work, which is attributed to the catalyst used in the transesterification. Immobilization of lipase on nano-materials caused considerable exergy destruction, resulting in reduction of exergy efficiency. The internal exergy destruction of the transesterification was found through setting up an exergy balance at constant parameters of the system environment, where it is required to consider the reaction inputs (including utilities), the main products, by-products and waste materials [28]. For 1000 kJ biodiesel production, the process of transesterification involved 91 kJ internal exergy losses with contribution of 6.7% of the total exergy input. The total exergy destruction including internal and external losses was determined to be 123 kJ with contribution of 9.1% of the total exergy input, attributed to heat loss and unreacted substances. In general, the exergy loss associated with heat loss can also be minimized through reusing in-process heat, resulting in reducing the energy supply and improving the exergy efficiency. Most of the unreacted substances can be recovered and stored for further application in the process to reduce the material consumption through enhancing the conversion efficiency and exergy loss through waste, causing an enhancement in the exergy efficiency. For example, the initial unreacted FFA can be used to acidulate the glycerol phase in the transesterification and then reused in the preesterification to increase the FAME conversion efficiency [32]. The technological choices can also considerably influence the exergy efficiency and exergy loss, and subsequently, effect on the renewability indicator of transesterification process. WCO conversion to biodiesel allows recycling and reusing an industrial and domestic waste with high exergy content. Reusing WCO as a fuel provides us a renewable resource of energy and also
Table 1 Chemical exergy of major substances in the transesterification. Substance Input WCO Sulfuric acid Potassium hydroxide Methanol Total Output Biodiesel Glycerol Methanol WCO Total Waste WCO Methanol Total
Standard chemical exergy (kJ/mol)
Mass (g)
Molecular mass (g/mol)
Chemical Exergy (kJ)
35785 163.4 107 718
100 0.8 1 26.26
974.5 98 56 32
3672.13 1.33 1.91 589.43 4264.81
12524 2114 718 35785
90.23 8.54 13.88 8.10
324 92.1 32 974.5
3487.77 196.24 311.58 297.64 4293.25
31785 718
1.01 3.47
974.5 32
33.04 77.89 110.94
G. Khoobbakht et al. / Energy 196 (2020) 117092
Heat & Electricity: 131.1 kJ
5
The internal exergy deruction WCO: 0.29 g, 9.5 kJ Methanol: 1.00 g, 22.4 kJ
WCO: 28.67 g, 1052.7 kJ Methanol: 7.45 g, 168.9 kJ KOH: 0.29 g, 0.6 kJ H2SO4: 0.23 g, 0.3 kJ
WCO: 2.32 g, 85.4 kJ Methanol: 3.99 g, 89.2 kJ Glycerol: 2.44 g, 56.2 kJ Biodiesel: 25.86 g, 1000.0 kJ Fig. 2. The simplified mass and exergy flow diagram for the transesterification of WCO.
prevents leaking a waste stream of materials and environmental impact in water currents and soil. The exergy content of a substance is a measure of the material quality. In the present work, the transesterification upgraded quality of WCO with exergy content of 36700 kJ/kg to biodiesel with exergy content of 38820 kJ/kg. Calculating the quality/exergy of substances can assist us to characterize by-products and wastes and design recycle networks, e.g. eco-industrial parks, to promote closed loop systems and minimize overall resource use [32]. 3.3. Effect of the experimental variables on the exergy flow 3.3.1. Methanol:oil molar ratio The stoichiometric ratio of methanol:triglyceride in transesterification is 3:1. However, excess methanol is required to drive the transesterification of triglyceride toward complete conversion, resulting in increasing biodiesel yield. It is recommended to be 6:1 for methanol:oil molar ratio to achieve as much as possible yield, if pure oil with low content of FFA and water is used as feedstock in an alkali catalyzed transesterification. High concentration of FFA and water in the oil leads the alkali catalyzed transesterification toward saponification, reducing the biodiesel yield and causing critical difficulties for separation and purification of the product in downstream. Acid catalyst is a commercially viable alternative for the conversion of triglyceride feedstock with high concentration of FFA and water, like WCO. The difference is that the reaction rate would be decreased and higher methanol:oil molar ratio, 30:1, and even 50:1 in the acid catalyzed one, would be required to impose the reaction toward FAME production [36,37]. The conversion in the present work involved both acid and alkali catalyzed reactions in separation steps. After reducing the FFA and water contents through acid catalyzed esterification of WCO in the first step, the alkali catalyzed transesterification for conversion of the major contribution of triglyceride was carried out with applying the experiment variables in the second step. For each run, the overall mass and exergy balance of transesterification in the reference condition were individually carried out. Except the factors of methanol:oil ratio, catalyst concentration and temperature, all other factors were same as the conditions in the control treatment explained in the previous section. Physical and chemical exergy of mass were calculated by Eq. (9). Exergy transfer in the reaction through heat and work were achieved by Eqs. (10) and (11). Internal exergy loss for each run was calculated
by deducting the total exergy output (Exout) from the total exergy input (Exin) for the operation through Eq. (5). The external exergy loss (which is avoidable through improving the technology or energy resources) is equal to the sum of the exergy of all waste streams in the production process including 20% of the glycerides and methanol unreacted and remained in the reactor. Three levels of 4:1, 8:1, 12:1 for methanol:oil molar ratio in the alkali catalyzed transesterification were applied and biodiesel yield along with the results of exergy analysis are listed in Table 2. Increasing the methanol:oil molar ratio from 4:1 to 8:1 caused an increment in both exergy efficiency and biodiesel yield and a reduction in the total exergy destruction of the transesterification (Fig. 3). The increment of biodiesel yield was more significant than that of exergy efficiency. As regards the mass input, and subsequently, the chemical exergy associated with methanol in the methanol:oil molar ratio of 4:1 was less than that in the ratio of 8:1, the low exergy input could partly compensate the reduction of exergy output attributed to the significant reduction of biodiesel yield in the ratio of 4:1. Moreover, the glycerides unreacted are recoverable and reusable, so only 20% of that was determined as waste and considered as the external exergy loss. Thus, so long as decreasing biodiesel yield is attributed to a reduction in the mass input, it is expected a slightly reduction in exergy efficiency in comparison with conversion efficiency (biodiesel yield). Both exergy efficiency and biodiesel yield decreased with increasing the methanol:oil molar ratio from 8:1 to 12:1. While the high concentration of methanol showed a negative effect on conversion efficiency that was not significant same as the low concentration of methanol (4:1). Unlike, the exergy efficiency of the transesterification decreased more in the high concentration of methanol (12:1) than the low concentration of that (4:1). Though biodiesel yield in the high concentration of methanol was more than that in the low concentration of methanol, the high mass (chemical exergy) input in the high concentration of methanol was considered more effective on exergy efficacy. In one hand the stoichiometric ratio of methanol:triglyceride in the transesterification is constant to be 3:1, and in the other hand, as mentioned above, 20% of the unreacted substances was considered as the waste of process, so it can be concluded 20% of the excess methanol of the stoichiometric ratio would be considered as the waste and the exergy loss (neglecting the exergy loss for recovering and purification of 80%).
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Table 2 Biodiesel yield and the results of exergy flow for unit biodiesel produced in the transesterification at the design points. Methanol:oil molar ratio
Catalyst (wt.%)
Temperature ( C)
Yield (%)
Input exergy (kJ)
Output exergy (kJ)
Internal exergy loss (kJ)
External exergy loss (kJ)
4 8 12 8 8 8 8
1 1 1 2 3 1 1
55 55 55 55 55 45 50
81.37 90.23 88.81 83.25 78.38 75.56 79.74
48887 52341 55670 52336 52337 51650 51994
42991 47603 48028 45559 43890 43480 44684
3488 2079 2920 3224 4155 3510 3280
2410 2671 4737 3564 4293 4664 4040
Exergy efficiency
11 10
Biodiesel yield
93
91.7 90.8
89.8
9 8 7 6 5 4
88.8
90.2
7.6 5.9
91 89 87 85 83 81
4.7
81.4
Biodiesel yield (%) and Exergy efficiency (%)
Total exergy loss (MJ/kg biodiesel)
Total exergy loss
79
3
77 4
8 Methanol:oil molar ratio
12
Fig. 3. Effect of methanol:oil molar ratio on biodiesel yield, exergy efficiency and total exergy loss of the transesterification.
3.3.2. KOH concentration Fig. 4 shows the effect of catalyst (potassium hydroxide) concentration on the biodiesel yield, the exergy efficiency and the total exergy loss at the methanol:oil molar ratio of 8:1 and the reaction temperature of 55 C. As can be seen in Fig. 4, the biodiesel yield and exergy efficiency decreased permanently with increasing the catalyst concentration from 1 to 3 wt%. The total exergy losses in the transesterification with the catalyst concentrations of 1 and 3 wt% were determined to be 4730 and 8420 kJ per one kg biodiesel production, respectively. The significant increment of the exergy destruction is attributed
11
3.3.3. Reaction temperature Transesterification can occur at different temperatures, depending on the properties of oils and catalysts. It could be at ambient temperature [38], or at a temperature close to the boiling
Exergy efficiency
Biodiesel yield
93
91.7
10 9
90.2
90.2
8
5
8.4
6.8
7 6
89.2
4.7
91 89 87 85 83
83.3
78.4
4 3
81 79
Biodiesel yield (%) and Exergy efficiency (%)
Total exergy loss (MJ/kg biodiesel)
Total exergy loss
to the sever reduction of the biodiesel yield in the high catalyst concentration. The mass exergy embodied in potassium hydroxide is not considerable, but the effect of that on the conversion process could significantly influence the exergy efficiency. The high concentration of KOH diverted the reaction toward saponification, resulting in a reduction in biodiesel yield.
77
2
75 1
2
3
KOH concentration (wt.%) Fig. 4. Effect of KOH concentration on biodiesel yield, exergy efficiency and total exergy loss of the transesterification.
G. Khoobbakht et al. / Energy 196 (2020) 117092
9
8.2 90.0
8 7
Exergy efficiency
89.6
7.3
Biodiesel yield
91.7
93
90.2
88
6 79.7
5 4
4.7
83 78
75.6
Biodiesel yield (%) and Exergy efficiency (%)
Total exergy loss (MJ/kg biodiesel)
Total exergy loss
7
73
3 45
50 Reaction temperature ( C)
55
Fig. 5. The effect of the reaction temperature on biodiesel yield, exergy efficiency and total exergy loss of the transesterification.
temperature of methanol [39]. Fig. 5 shows effect of the reaction temperature on biodiesel yield, exergy efficiency and total exergy loss at methanol:oil molar ratio of 8:1 and KOH concentration of 1 wt%. Both exergy efficiency and biodiesel yield increased with increasing the temperature from 45 to 55 C. The total exergy destructions in the transesterification of WCO in 45 and 55 C were determined to be 8210 and 4720 kJ per one kg biodiesel production, respectively. Increasing the reaction temperature from 45 to 55 C could significantly enhance the conversion efficiency thereby increasing the biodiesel yield from 75 to 90%. Though the exergy input increased with increment of the temperature, the more significant increment in the exergy output associated with the high biodiesel yield (90%) overcame the increment of exergy input caused by the increasing temperature, resulting in enhancing the exergy efficiency. 4. Conclusions Exergy analysis provides a tool for evaluating the environmental burdens associated with the products, processes or activity by identifying and quantifying energy and materials used and wastes released to the environment in the same comprehensive framework. Exergy provides an estimation of the resource requirement (energy and material) for the transesterification of WCO, so exergy efficiency analysis is useful for evaluating the production of biodiesel resources. The present work evaluated the exergy flow in the transesterification of WCO, with analyzing the effect of the experiment variables, including methanol:oil molar ratio, potassium hydroxide concentration and the reaction temperature, on biodiesel yield, exergy efficiency and exergy destruction. The data was collected through doing the experiments in the lab and directly delivered to be used in the exergy calculations. The maximum biodiesel yield and exergy efficiency were determined to be 90.2% and 91.7%, respectively, achieved at methanol:oil molar ratio of 8:1, potassium hydroxide concentration of 1 wt% and the reaction temperature of 55 C. At the same point, the total exergy loss in the transesterification was determined to be 4720 kJ per one kg biodiesel production. An excess methanol addition on the optimal value significantly reduced the exergy efficiency through the increasing material waste. Exergy loss in the transesterification also increased through an overdose potassium hydroxide concentration in the reaction, leading the reaction toward saponification. The lower temperature from the optimal point caused an increment in
the total exergy loss of transesterification through a significant reduction in the conversion efficiency. In biodiesel production, downstream purification is an important step in the overall process. The separation and recycling of methanol, unreacted glycerides and waste glycerol productions can significantly modify the overall process of biodiesel production in terms of exergy loss, economic efficiency and environmental sustainability. Refining technologies is considerable as an energyintensive process in the biodiesel production in the large scale. Reducing energy consumption and mass waste in the refining through new methods and technologies enhances exergy and economic efficiency of biodiesel production in the overall process [40e42]. Exergy analysis could also be applied to assess the environmental performance of the system under consideration and therefore to help other environmental assessment tools such as life cycle assessment and environmental and economic policies related to resource use.
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Experimental exergy analysis of transesterification in biodiesel production Golmohammad Khoobbakht a, Kamran Kheiralipour b, Hamed Rasouli c, Mojtaba Rafiee d, Mehrdad Hadipour e, f, Mahmoud Karimi d, * a
Department of Agricultural Engineering, Payame Noor University, Tehran, Iran Mechanical Engineering of Biosystems Department, Ilam University, Ilam, Iran Department of Agrotechnology, College of Abouraihan, University of Tehran, Tehran, Iran d Department of Biosystems Engineering, Arak University, Arak 38156-8-8349, Iran e Department of Environment, Faculty of Agriculture and Natural Resources, Arak University, Iran f Faculty of Biological Science, University of Kharazmi, Iran b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 June 2019 Received in revised form 26 January 2020 Accepted 3 February 2020 Available online 5 February 2020
Exergy flow analysis is applied to account wastes, determine the exergetic efficiency, compare substitutes and other types of energy sources and also define economic and environmental policies for resource use. The present study focuses on the exergy flow analysis of biodiesel production by esterification and transesterification of waste cooking canola oil to achieve benefits such as reducing material and energy consumptions and improving energy and exergy efficiencies. The input amounts of esterification for all the runs were kept constant, while the runs of transesterification were carried out in a variety of mass and energy input values as the experimental variables. The thermodynamics analysis was applied to determine exergy input and output of the system for the experimental runs. Impacts of experiment variables, including methanol:oil molar ratio, potassium hydroxide concentration and the reaction temperature of transesterification were evaluated on exergy efficiency and exergy loss in the transesterification. The maximum exergy efficiency (91.7%) and the minimum exergy loss (4320 kJ/kg biodiesel) were achieved at methanol:oil molar ratio of 8:1, potassium hydroxide concentration of 1 wt% and the reaction temperature of 55 C. Excessive use of methanol and catalyst reduced the yield and the exergy efficiency through increasing exergy loss by waste materials. © 2020 Elsevier Ltd. All rights reserved.
Keywords: Exergy loss Biodiesel Transesterification Thermodynamic analysis Conversion
1. Introduction Biodiesel derived from renewable resources has been considered as attractive alternative fossil fuels, because of its suitable chemical properties and also its environmental benefits such as non-toxicity, biodegradability, carbon neutrality [1e3] and its beneficent effects on diesel engine [4,5]. Because of some restrictions over the first generation of biofuels such as food vs. fuel issues, various researches have been switched on producing biodiesel from non-edible oils, waste cooking oils (WCO) and other feedstocks such as algae [6], microalgae [7], Jatropha [8], grease oil [9], mahua oil [10] and so on. Biodiesel is produced by esterification of fatty acids or
* Corresponding author. E-mail address: [email protected] (M. Karimi). https://doi.org/10.1016/j.energy.2020.117092 0360-5442/© 2020 Elsevier Ltd. All rights reserved.
transesterification of triglycerides with short chain alcohols like methanol and ethanol. Methanol is mostly used because of its lower cost compared with other alcohols, so biodiesel most commonly refers to fatty acid methyl esters (FAME). However, one of the major obstacles to wide application of biodiesel is its high cost in comparison of fossil diesel [11]. It has been reported that the cost of raw materials amounts to around 75% of the total biodiesel production cost [12]. Therefore, sustainability of biodiesel production depends on low cost feedstock such as waste cooking oil (WCO) to reduce the overall cost of biodiesel production [1,13]. Assessment of material and energy flows in each system is necessary in order to decrease the consumption of material and energy as well as promote the use of renewable resources and hence to reach production sustainability. Exergy flow assessment of the transesterification provides us critical information to manage wastes and optimize the process in terms of energy and material. Exergy efficiency as a useful tool measures the ratio of input to
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output exergies of the transesterification that can be optimized through optimal consumption of inputs and reducing wastes in outputs. Sustainable biodiesel production needs to optimization of both yield efficiency and exergy destruction. Exergy-based criteria were found to give much better guidance for system improvement, as they account better for use of energy resources [14,15]. However, one of the most important reasons in this field is energy production through biodiesel. Therefor the system needs to be improved through both the reduction of exergy destruction and the enhancement of biodiesel yield, simultaneously [16]. Exergy is a thermodynamic property that quantifies the maximum possible work that could be extracted from a resource if that resource was to be fully equilibrated (thermally, mechanically and chemically) with the environment. Unlike energy, exergy is not a conserved quantity and is destroyed in any real process. As a nonconserved property, exergy is more akin to entropy, although it has the same units as energy [17]. A process that is more thermodynamically efficient will destroy less exergy, and a process that destroys large amounts of exergy should be examined for possible improvement [18]. Tracking and quantifying the exergy destroyed in a system is useful for two primary reasons. The first is the identification of a performance metric, the exergy efficiency, for a system. Depending on the desired output of the system (e.g., shaft work, or high-purity product), the exergy efficiency may be defined differently [19]. However, all definitions amount to comparing the exergy supplied to the system to the exergy of the desired output of the system. The second reason for tracking exergy destruction is to analyze the internal operation of a system [20]. This consists of finding not just the exergy efficiency of the system, but also identifying the locations in the system where the exergy destruction occurs [21]. Tracking exergy destruction and analyzing exergy efficiency assist us to improve the sustainability through optimization of the process. Apart from conversion efficiency, exergy efficiency in a system can be optimized to both reduce energy and resources consumption and increase production. In the literature, there are valuable works on exergy analysis of biodiesel production plants [22e25]. Till now, a gap is sensed in knowledge of the effects of the input energy and material in the transesterification of WCO on exergy destruction and exergy efficiency using experimental data. In the present work, biodiesel from waste cooking canola oil was produced and the experimental parameters, including methanol:oil molar ratio, catalyst (potassium hydroxide) concentration and reaction temperature, were evaluated on the exergy flow of the transesterification of WCO, which is the main reaction in the biodiesel production process. In the present work, thermodynamic analysis of transesterification has been based on the experimental data, which makes the analysis more accurate and reliable. In the experimental runs, yield of transesterification was measured using GC/MS, and along with the values of the input materials were applied for the thermodynamic analysis. An accurate knowledge of the effects of the variables in the transesterification on the exergy flow can be useful to improve the exergy efficiency and subsequently economic performance in commercial production of biodiesel through minimizing the exergy loss of the process. 2. Materials and methods 2.1. Materials Cooking canola oil used in frying of potato was collected and stored in a glass container at room temperature. The physicochemical properties of the WCO including density, viscosity, free
fatty acid (FFA) content and acid value were 0.906 g cm3, 51.3 cP, 5.68% and 11.16, respectively. The density and viscosity of oil were measured by a pycnometer and a viscometer, respectively, whereas the FFA content and acid values were determined by acid-base titration. Methanol, potassium hydroxide (97%), phosphoric acid, methyl salicylate, methyl palmitate, methyl stearate, methyl oleate, methyl linoleate and methyl linoleate were laboratory stock or purchased from Sigma-Aldrich.
2.2. Biodiesel production The first step of biodiesel production was acid catalyzed reaction, called esterification. The step was conducted in the present research to remove high water and FFA contents in WCO and prevent oil loss by saponification, as much as possible. For this, 500 g WCO, 0.8 g sulfuric acid and 100 ml methanol were mixed in a magnetic hot plate stirrer with temperature controller, for 1 h. The revolution of the stirrer and the temperature of reaction were kept constant at 400 rpm and 55 C, respectively. After completing the reaction, the mixture was allowed to be settled for 1 h, to make two distinct liquid phases. The excess methanol in the top phase was removed using separating funnel. The remaining phase containing methyl ester and unreacted triglyceride from transesterification was collected to then be used in the second step, an alkali catalyzed reaction called transesterification. Methanol:oil molar ratio in three levels of 4:1, 8:1 and 12:1, catalyst (potassium hydroxide) concentration in oil in three levels of 1, 2 and 3 wt% and the reaction temperature in three levels of 45, 50 and 55 C were considered as the experiment variables of transesterification. The experiments were carried out based on single factor experiment, in which the effects of the variables on the conversion of the reaction, exergy efficiency and exergy destruction were investigated. Each sample containing 100 g of the mixture prepared in the first step plus the determined values of methanol and potassium hydroxide were stirred for 1 h at the determined reaction temperature. Then the reaction mixture was retained and allowed to be settled and separated into two layers: crude biodiesel up and glycerol down. The remaining methanol was recovered under vacuum (10 ± 1 mm Hg) at 50 C with a rotational evaporator, and then crude biodiesel was washed by 10 wt% of water at 80 C to remove the soap which was formed by reaction between alkali and FFA. The wet crude biodiesel was then dried under a vacuum (5 ± 1 mmHg) at 90 C for an hour, using a rotational evaporator to prepare pure biodiesel.
2.3. FAME assay The FAME contents in the experiment samples were analyzed by gas chromatography (Shimadzu GC-2010), equipped with hydrogen flame ionization detector (FID). The separation was performed on a DB-1HT capillary column (30 m 0.25 mm). The GC was calibrated based on various concentrations of methyl salicylate, methyl palmitate, methyl stearate, methyl oleate, methyl linoleate, and methyl oleate. During the analysis, the temperature of sampling inlet was 370 C and the detector temperature was 375 C. The column temperature was raised regularly. The column temperature was initially maintained at 150 C for 2 min, then raised to 360 C at a rate of 10 C min1 and finally maintained at 360 C for 10 min. Nitrogen at a pre-column pressure of 100 kPa was used as carrier gas [26]. Methyl undecanoate was used as the internal standard. The FAME content of the samples was calculated using the following equation:
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Xð%Þ ¼
mFAME 100% mCB
(1)
where X is the FAME content, mFAME is the weight of FAME calculated with internal calibration method, and mCB is the weight of the crude biodiesel [26].
2.4. Exergy analysis Four balance equations must be applied in the process of the transesterification of WCO for a general steady state (no accumulation) in order to find the work and heat interactions. Mass input and output is always balanced according to the principle of mass conservation given by Eq. (2). Energy input and output is also balanced according to the first law of thermodynamics or the energy conservation principle given by Eq. (3). In real processes, entropy production increases according to the second law of thermodynamics as given by Eq. (4) and a part of the exergy input is always destroyed given by Eq. (5).
X ðm_ i Þ i
¼
X ðm_ i Þ i
in
X ðm_ i hi Þ i
(2)
out
¼
X ðm_ i hi Þ i
in
_ þ Q_ W
(3)
out
X Q_ i X X ðm_ i si Þout þ ðm_ i si Þin ¼ S_gen Ti i i i
(4)
_ _ _ _ _ Ex mass; in Exmass; out þ Exheat Exwork ¼ Exloss
(5)
_ mass ) is divided into four specific The mass exergy component (Ex components including chemical, physical, potential and kinetic exergy expressed in Eq. (6).
_ _ _ _ _ mass ¼ Ex Ex phy þ Exch þ Expot þ Exkin
(6)
The potential and kinetic terms of exergy are negligible and their contribution to the total exergy balance is minimal. The physical exergy is dependent on temperature, enthalpy and entropy as shown by Eq. (7).
_ _ _ Ex phy ¼ ðh h0 Þ T0 ðs_ s_0 Þ
(7)
The standard chemical exergy of many compounds can be found in Szargut et al. [27] and Ayres and Ayres [28]. When not available, the chemical exergy content of any pure substance can be computed by the approximate Eq. (8) which is a function of the chemical exergy of each elemental compound, the number of atoms of each element contained in the stream and the Gibbs free energy of formation for the compound [27e29].
_ ¼ D_G þ Ex ch f
X _ nelem Ex ch; i
3
X X X P T _ þ RT T T ni Ex ni Ln i þ ni C mean T Ln 0 0 0 i p;i T0 P0 i i i X n ni Ln P i þ RT0 i ni i
_ ¼ Ex
(9) _ is the overall exergy of a mixture of substances, n is the where: Ex i _ is the standard molar exergy number of the moles of substance i, Ex i of pure substance i, R is the molar gas constant, T0 is the temperature of environment, T is the temperature of substance i, Pi is the pressure that substance i expands itself reversibly at constant temperature T0 toward the state of volume V0 at pressure P0 in equilibrium with the atmosphere, P0 is the atmospheric pressure and C mean is the molar heat capacity of the substance i. The five p;i different terms of this equation indicates chemical exergy, pressure exergy for gaseous substances, thermal exergy due to temperature change and mixing exergy due to substances concentration change, respectively [30]. Mixing in the transesterification of WCO is an irreversible process in which all the work potential wastes. The total chemical exergy of a mixture is equal to the sum of the chemical exergy of all components of the mixture in addition to the exergy losses in mixing process. Based on exergy balance, the exergy transfer by _ heat flow (Ex heat ) at a temperature T and exergy by work flow _ (Ex ) were calculated by Eqs. (10) and (11), respectively [23,31]. work
T0 _ Q_ ¼ 1 Ex heat T
(10)
_ _ Ex work ¼ W
(11)
Usually an exergy conversion coefficient is defined to determine energy sources in terms of exergy unit (Joule). This coefficient can be used to estimate the chemical exergy content of fuels according to their heating value or enthalpy. The exergy coefficient of electricity is assumed to be 1.00, indicating that 1 kJ of electrical energy corresponds to an exergy flow of 1 kJ [28,32]. A global mass balance across the transesterification of WCO was performed, and the thermodynamic properties needed to develop the exergy balance were then obtained [22]. Chemical and physical exergies of input and output for the defined system of the transesterification of WCO were calculated. Exergy was determined for each compound, mixture and utility. Dead state conditions were taken as 25 C and 101.325 kPa with the exergy efficiency of each experiment related to the transesterification calculated using the following equation:
h¼1
Exloss Exinput
! (12)
3. Results and discussion
(8)
i
_ _ where: Ex ch is the chemical exergy of substance, DGf denotes the _ standard Gibbs free energy of formation of the substance and Ex ch; i is the chemical exergy of the ith pure element of the substance. The exergy of substances is determined by consisting of a physical part and a chemical part as explained above. By considering physical and chemical exergy, the overall exergy of a mixture _ is expressed as Eq. (9) [30]: of substances (Ex)
3.1. Product analysis The GC analysis on transesterification conversion products from a standard sample is shown in Fig. 1. The results indicated that the main components in the WCO-derived biodiesel were methyl salicylate, methyl palmitate, methyl stearate, methyl oleate, methyl linoleate and methyl oleate. The methyl esters were then quantified using the ratio of the areas under the peaks of each methyl ester to the internal standard and the known concentration of internal
4
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Fig. 1. The GC chromatogram of WCO-derived biodiesel.
standard.
3.2. Identifying material wastes and energy loss The chemical exergy of the substances categorized into inputs, outputs and wastes in the transesterification of WCO under the reference condition (methanol:oil molar ratio of 8:1, potassium hydroxide concentration of 1 wt% and reaction temperature of 55 C) are listed in Table 1. The internal exergy destruction was calculated by deducting the total exergy output (Exout) from the total exergy input (Exin) for the operation [33,34]. The external exergy destruction is equal to the sum of the exergy of all waste streams in the production process, including 20% of the glycerides and methanol unreacted and remained in the reactor. The overall mass and exergy balance of transesterification in the reference condition is shown in Fig. 2. The major inputs in transesterification included WCO and methanol, whereas the major outputs included FAME (biodiesel), unreacted WCO, remaining methanol and glycerol. Exergy embodied in biodiesel (38820 kJ/kg) is more than that embodied in WCO (36700 kJ/kg). However, as can be seen in the flow diagram, about 1100 kJ exergy of WCO was consumed in transesterification for production of 1000 kJ exergy of biodiesel. The conversion efficiency (biodiesel yield) in the transesterification under the mentioned condition was determined to be 90.23%, with exergy efficiency of 91.73%, which can be further improved by process adjustments, such as optimization of the reaction variables, reduction of FFA content, water and existing impurities in WCO, resulting in incrementing the useful exergy (quality) of the inputs. After WCO, methanol was considered as the most exergy containing among the inputs. For 1000 kJ biodiesel production, 168 kJ
exergy embodied in methanol was used whereas 111 kJ of that was remained in the mixture of reaction. Most of the remained methanol was recovered (80%) and 20% of that was considered as the waste. Heat and electricity for stirring with consuming 131 kJ exergy per 1000 kJ biodiesel production was realized in the third place among the exergy inputs. While the chemical catalysts such as sulfuric acid and potassium hydroxide have significant environmental impacts, the exergy embodied in the chemical catalysts consumed in transesterification was nearly inconsiderable. In comparing with other researches, exergy efficiency of 90% for squez-Arredondo palm oil transesterification was reported by Vela et al. [35]; which prompts validity of our results (91.7%). The slight difference of exergy efficiencies can be attributed to the differences in the feedstock, concentrations of materials and the condition of the process. Also Karimi et al. [13] reported an exergy efficiency of 80% for the transesterification of WCO using immobilized lipase, with a considerable difference with the present work, which is attributed to the catalyst used in the transesterification. Immobilization of lipase on nano-materials caused considerable exergy destruction, resulting in reduction of exergy efficiency. The internal exergy destruction of the transesterification was found through setting up an exergy balance at constant parameters of the system environment, where it is required to consider the reaction inputs (including utilities), the main products, by-products and waste materials [28]. For 1000 kJ biodiesel production, the process of transesterification involved 91 kJ internal exergy losses with contribution of 6.7% of the total exergy input. The total exergy destruction including internal and external losses was determined to be 123 kJ with contribution of 9.1% of the total exergy input, attributed to heat loss and unreacted substances. In general, the exergy loss associated with heat loss can also be minimized through reusing in-process heat, resulting in reducing the energy supply and improving the exergy efficiency. Most of the unreacted substances can be recovered and stored for further application in the process to reduce the material consumption through enhancing the conversion efficiency and exergy loss through waste, causing an enhancement in the exergy efficiency. For example, the initial unreacted FFA can be used to acidulate the glycerol phase in the transesterification and then reused in the preesterification to increase the FAME conversion efficiency [32]. The technological choices can also considerably influence the exergy efficiency and exergy loss, and subsequently, effect on the renewability indicator of transesterification process. WCO conversion to biodiesel allows recycling and reusing an industrial and domestic waste with high exergy content. Reusing WCO as a fuel provides us a renewable resource of energy and also
Table 1 Chemical exergy of major substances in the transesterification. Substance Input WCO Sulfuric acid Potassium hydroxide Methanol Total Output Biodiesel Glycerol Methanol WCO Total Waste WCO Methanol Total
Standard chemical exergy (kJ/mol)
Mass (g)
Molecular mass (g/mol)
Chemical Exergy (kJ)
35785 163.4 107 718
100 0.8 1 26.26
974.5 98 56 32
3672.13 1.33 1.91 589.43 4264.81
12524 2114 718 35785
90.23 8.54 13.88 8.10
324 92.1 32 974.5
3487.77 196.24 311.58 297.64 4293.25
31785 718
1.01 3.47
974.5 32
33.04 77.89 110.94
G. Khoobbakht et al. / Energy 196 (2020) 117092
Heat & Electricity: 131.1 kJ
5
The internal exergy deruction WCO: 0.29 g, 9.5 kJ Methanol: 1.00 g, 22.4 kJ
WCO: 28.67 g, 1052.7 kJ Methanol: 7.45 g, 168.9 kJ KOH: 0.29 g, 0.6 kJ H2SO4: 0.23 g, 0.3 kJ
WCO: 2.32 g, 85.4 kJ Methanol: 3.99 g, 89.2 kJ Glycerol: 2.44 g, 56.2 kJ Biodiesel: 25.86 g, 1000.0 kJ Fig. 2. The simplified mass and exergy flow diagram for the transesterification of WCO.
prevents leaking a waste stream of materials and environmental impact in water currents and soil. The exergy content of a substance is a measure of the material quality. In the present work, the transesterification upgraded quality of WCO with exergy content of 36700 kJ/kg to biodiesel with exergy content of 38820 kJ/kg. Calculating the quality/exergy of substances can assist us to characterize by-products and wastes and design recycle networks, e.g. eco-industrial parks, to promote closed loop systems and minimize overall resource use [32]. 3.3. Effect of the experimental variables on the exergy flow 3.3.1. Methanol:oil molar ratio The stoichiometric ratio of methanol:triglyceride in transesterification is 3:1. However, excess methanol is required to drive the transesterification of triglyceride toward complete conversion, resulting in increasing biodiesel yield. It is recommended to be 6:1 for methanol:oil molar ratio to achieve as much as possible yield, if pure oil with low content of FFA and water is used as feedstock in an alkali catalyzed transesterification. High concentration of FFA and water in the oil leads the alkali catalyzed transesterification toward saponification, reducing the biodiesel yield and causing critical difficulties for separation and purification of the product in downstream. Acid catalyst is a commercially viable alternative for the conversion of triglyceride feedstock with high concentration of FFA and water, like WCO. The difference is that the reaction rate would be decreased and higher methanol:oil molar ratio, 30:1, and even 50:1 in the acid catalyzed one, would be required to impose the reaction toward FAME production [36,37]. The conversion in the present work involved both acid and alkali catalyzed reactions in separation steps. After reducing the FFA and water contents through acid catalyzed esterification of WCO in the first step, the alkali catalyzed transesterification for conversion of the major contribution of triglyceride was carried out with applying the experiment variables in the second step. For each run, the overall mass and exergy balance of transesterification in the reference condition were individually carried out. Except the factors of methanol:oil ratio, catalyst concentration and temperature, all other factors were same as the conditions in the control treatment explained in the previous section. Physical and chemical exergy of mass were calculated by Eq. (9). Exergy transfer in the reaction through heat and work were achieved by Eqs. (10) and (11). Internal exergy loss for each run was calculated
by deducting the total exergy output (Exout) from the total exergy input (Exin) for the operation through Eq. (5). The external exergy loss (which is avoidable through improving the technology or energy resources) is equal to the sum of the exergy of all waste streams in the production process including 20% of the glycerides and methanol unreacted and remained in the reactor. Three levels of 4:1, 8:1, 12:1 for methanol:oil molar ratio in the alkali catalyzed transesterification were applied and biodiesel yield along with the results of exergy analysis are listed in Table 2. Increasing the methanol:oil molar ratio from 4:1 to 8:1 caused an increment in both exergy efficiency and biodiesel yield and a reduction in the total exergy destruction of the transesterification (Fig. 3). The increment of biodiesel yield was more significant than that of exergy efficiency. As regards the mass input, and subsequently, the chemical exergy associated with methanol in the methanol:oil molar ratio of 4:1 was less than that in the ratio of 8:1, the low exergy input could partly compensate the reduction of exergy output attributed to the significant reduction of biodiesel yield in the ratio of 4:1. Moreover, the glycerides unreacted are recoverable and reusable, so only 20% of that was determined as waste and considered as the external exergy loss. Thus, so long as decreasing biodiesel yield is attributed to a reduction in the mass input, it is expected a slightly reduction in exergy efficiency in comparison with conversion efficiency (biodiesel yield). Both exergy efficiency and biodiesel yield decreased with increasing the methanol:oil molar ratio from 8:1 to 12:1. While the high concentration of methanol showed a negative effect on conversion efficiency that was not significant same as the low concentration of methanol (4:1). Unlike, the exergy efficiency of the transesterification decreased more in the high concentration of methanol (12:1) than the low concentration of that (4:1). Though biodiesel yield in the high concentration of methanol was more than that in the low concentration of methanol, the high mass (chemical exergy) input in the high concentration of methanol was considered more effective on exergy efficacy. In one hand the stoichiometric ratio of methanol:triglyceride in the transesterification is constant to be 3:1, and in the other hand, as mentioned above, 20% of the unreacted substances was considered as the waste of process, so it can be concluded 20% of the excess methanol of the stoichiometric ratio would be considered as the waste and the exergy loss (neglecting the exergy loss for recovering and purification of 80%).
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Table 2 Biodiesel yield and the results of exergy flow for unit biodiesel produced in the transesterification at the design points. Methanol:oil molar ratio
Catalyst (wt.%)
Temperature ( C)
Yield (%)
Input exergy (kJ)
Output exergy (kJ)
Internal exergy loss (kJ)
External exergy loss (kJ)
4 8 12 8 8 8 8
1 1 1 2 3 1 1
55 55 55 55 55 45 50
81.37 90.23 88.81 83.25 78.38 75.56 79.74
48887 52341 55670 52336 52337 51650 51994
42991 47603 48028 45559 43890 43480 44684
3488 2079 2920 3224 4155 3510 3280
2410 2671 4737 3564 4293 4664 4040
Exergy efficiency
11 10
Biodiesel yield
93
91.7 90.8
89.8
9 8 7 6 5 4
88.8
90.2
7.6 5.9
91 89 87 85 83 81
4.7
81.4
Biodiesel yield (%) and Exergy efficiency (%)
Total exergy loss (MJ/kg biodiesel)
Total exergy loss
79
3
77 4
8 Methanol:oil molar ratio
12
Fig. 3. Effect of methanol:oil molar ratio on biodiesel yield, exergy efficiency and total exergy loss of the transesterification.
3.3.2. KOH concentration Fig. 4 shows the effect of catalyst (potassium hydroxide) concentration on the biodiesel yield, the exergy efficiency and the total exergy loss at the methanol:oil molar ratio of 8:1 and the reaction temperature of 55 C. As can be seen in Fig. 4, the biodiesel yield and exergy efficiency decreased permanently with increasing the catalyst concentration from 1 to 3 wt%. The total exergy losses in the transesterification with the catalyst concentrations of 1 and 3 wt% were determined to be 4730 and 8420 kJ per one kg biodiesel production, respectively. The significant increment of the exergy destruction is attributed
11
3.3.3. Reaction temperature Transesterification can occur at different temperatures, depending on the properties of oils and catalysts. It could be at ambient temperature [38], or at a temperature close to the boiling
Exergy efficiency
Biodiesel yield
93
91.7
10 9
90.2
90.2
8
5
8.4
6.8
7 6
89.2
4.7
91 89 87 85 83
83.3
78.4
4 3
81 79
Biodiesel yield (%) and Exergy efficiency (%)
Total exergy loss (MJ/kg biodiesel)
Total exergy loss
to the sever reduction of the biodiesel yield in the high catalyst concentration. The mass exergy embodied in potassium hydroxide is not considerable, but the effect of that on the conversion process could significantly influence the exergy efficiency. The high concentration of KOH diverted the reaction toward saponification, resulting in a reduction in biodiesel yield.
77
2
75 1
2
3
KOH concentration (wt.%) Fig. 4. Effect of KOH concentration on biodiesel yield, exergy efficiency and total exergy loss of the transesterification.
G. Khoobbakht et al. / Energy 196 (2020) 117092
9
8.2 90.0
8 7
Exergy efficiency
89.6
7.3
Biodiesel yield
91.7
93
90.2
88
6 79.7
5 4
4.7
83 78
75.6
Biodiesel yield (%) and Exergy efficiency (%)
Total exergy loss (MJ/kg biodiesel)
Total exergy loss
7
73
3 45
50 Reaction temperature ( C)
55
Fig. 5. The effect of the reaction temperature on biodiesel yield, exergy efficiency and total exergy loss of the transesterification.
temperature of methanol [39]. Fig. 5 shows effect of the reaction temperature on biodiesel yield, exergy efficiency and total exergy loss at methanol:oil molar ratio of 8:1 and KOH concentration of 1 wt%. Both exergy efficiency and biodiesel yield increased with increasing the temperature from 45 to 55 C. The total exergy destructions in the transesterification of WCO in 45 and 55 C were determined to be 8210 and 4720 kJ per one kg biodiesel production, respectively. Increasing the reaction temperature from 45 to 55 C could significantly enhance the conversion efficiency thereby increasing the biodiesel yield from 75 to 90%. Though the exergy input increased with increment of the temperature, the more significant increment in the exergy output associated with the high biodiesel yield (90%) overcame the increment of exergy input caused by the increasing temperature, resulting in enhancing the exergy efficiency. 4. Conclusions Exergy analysis provides a tool for evaluating the environmental burdens associated with the products, processes or activity by identifying and quantifying energy and materials used and wastes released to the environment in the same comprehensive framework. Exergy provides an estimation of the resource requirement (energy and material) for the transesterification of WCO, so exergy efficiency analysis is useful for evaluating the production of biodiesel resources. The present work evaluated the exergy flow in the transesterification of WCO, with analyzing the effect of the experiment variables, including methanol:oil molar ratio, potassium hydroxide concentration and the reaction temperature, on biodiesel yield, exergy efficiency and exergy destruction. The data was collected through doing the experiments in the lab and directly delivered to be used in the exergy calculations. The maximum biodiesel yield and exergy efficiency were determined to be 90.2% and 91.7%, respectively, achieved at methanol:oil molar ratio of 8:1, potassium hydroxide concentration of 1 wt% and the reaction temperature of 55 C. At the same point, the total exergy loss in the transesterification was determined to be 4720 kJ per one kg biodiesel production. An excess methanol addition on the optimal value significantly reduced the exergy efficiency through the increasing material waste. Exergy loss in the transesterification also increased through an overdose potassium hydroxide concentration in the reaction, leading the reaction toward saponification. The lower temperature from the optimal point caused an increment in
the total exergy loss of transesterification through a significant reduction in the conversion efficiency. In biodiesel production, downstream purification is an important step in the overall process. The separation and recycling of methanol, unreacted glycerides and waste glycerol productions can significantly modify the overall process of biodiesel production in terms of exergy loss, economic efficiency and environmental sustainability. Refining technologies is considerable as an energyintensive process in the biodiesel production in the large scale. Reducing energy consumption and mass waste in the refining through new methods and technologies enhances exergy and economic efficiency of biodiesel production in the overall process [40e42]. Exergy analysis could also be applied to assess the environmental performance of the system under consideration and therefore to help other environmental assessment tools such as life cycle assessment and environmental and economic policies related to resource use.
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