Exergy-based sustainability assessment of ethanol production via Mucor indicus from fructose, glucose, sucrose, and molasses

Energy 98 (2016) 240e252

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Exergy-based sustainability assessment of ethanol production via Mucor indicus from fructose, glucose, sucrose, and molasses Mortaza Aghbashlo a, *, Meisam Tabatabaei b, c, **, Keikhosro Karimi d, e, *** a

Department of Mechanical Engineering of Agricultural Machinery, Faculty of Agricultural Engineering and Technology, College of Agriculture and Natural Resources, University of Tehran, Karaj, Iran b Microbial Biotechnology Department, Agricultural Biotechnology Research Institute of Iran (ABRII), P.O. Box: 31535-1897, AREEO, Karaj, Iran c Biofuel Research Team (BRTeam), Karaj, Iran d Department of Chemical Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran e Industrial Biotechnology Group, Institute of Biotechnology and Bioengineering, Isfahan University of Technology, Isfahan 84156-83111, Iran

a r t i c l e i n f o

a b s t r a c t

Article history: Received 31 October 2015 Received in revised form 20 December 2015 Accepted 12 January 2016 Available online xxx

This paper presents an in-depth exergy analysis of the ethanol fermentation process with various forms of fungus Mucor indicus under aerobic and anaerobic conditions to select the most productive and sustainable conditions. Various carbon sources including fructose, glucose, and sucrose as well as the whole and inverted sugar beet and sugarcanes molasses were used during the fermentation. The rational and process exergetic efficiencies were found to be in the range of 65.21%e88.54% and 0.00%e44.31%, respectively. Overall, the exergy-based parameter based on the process outputs could provide useful information about the sustainability and productivity of the fermentation process compared to the rational analysis. More specifically, the inverted sugar beet molasses with MF (mostly filamentous) form of M. indicus under anaerobic cultivation was shown to be the best option for industrial production phase with respect to the productivity and sustainability issues. The results obtained confirmed that the process yield alone cannot perfectly reflect the exact sustainability parameters of the renewable ethanol production systems. Finally, the developed exergetic framework could help engineers to couple biochemical and physical concepts more robustly for achieving the most cost-effective and eco-friendly pathways for bioethanol production. © 2016 Elsevier Ltd. All rights reserved.

Keywords: Ethanol Exergy Mucor indicus Morphology Sugar Sustainability

1. Introduction Despite the unique advantages of biofuels from the environmental point of view, their production from food/feed resources has triggered a controversial competition over the use of land and water [1e4]. Therefore, there is a vital need to explore viable, costeffective, and eco-friendly alternative energy options produced from non-edible resources which pose no threats to food security either [5]. Among various solutions proposed to achieve that, bioethanol production from organic wastes such as sugar industry's

* Corresponding author. Tel.: þ98 2632801011; fax: þ98 263 2808138. ** Corresponding author. Biofuel Research Team (BRTeam), Karaj, Iran. Tel.: þ98 2632703536; fax: þ98 263 2701067. *** Corresponding author. Department of Chemical Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran. E-mail addresses: [email protected] (M. Aghbashlo), meisam_tab@yahoo. com (M. Tabatabaei), [email protected] (K. Karimi). http://dx.doi.org/10.1016/j.energy.2016.01.029 0360-5442/© 2016 Elsevier Ltd. All rights reserved.

wastes (i.e., molasses) is considered very promising due their relative abundance and carbon neutral character [6]. In fact, these invaluable waste feedstocks can be upgraded to more useful forms of energy and organic material by employing various ethanolproducing organisms through the fermentation process. Despite the propitious nature of such processes, it is still essential to strive to find the most eco-friendly and cost-effective routes by applying advanced engineering tools. Nowadays, exergy analysis based on the second law of thermodynamic has appeared to be an effective means of developing, analyzing, optimizing, and retrofitting various energy conversion systems from the sustainability and productivity viewpoints [7]. Simply speaking, exergy refer to the maximum quantity of obtainable work from an energy system when it is brought to complete equilibrium with its reference environment through reversible processes [8e14]. This unique thermodynamic analysis has an undeniable superiority over the other sustainability indictors for environmentally-conscious decision making due to its

M. Aghbashlo et al. / Energy 98 (2016) 240e252

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Table 2 Standard chemical exergy of the gas media components [34].

Fig. 1. Schematic illustration of ethanol fermentation via M. indicus in a batch bioreactor.

strong conceptual features in detecting the sites, types, sources, and magnitudes of thermodynamic imperfection of a system under consideration [15]. In recent years, a large number of research efforts have been devoted to applying exergy analysis for various bioethanol production systems [16e18]. For instance, Yang et al. [15] proposed a modified exergy-based indicator in order to quantify the renewability of the total corn-ethanol production process in China. In the same year, Ensinas et al. [16] applied exergetic performance assessment for an integrated sugar and ethanol production plant designed for minimizing the irreversibility generation. Later, Ojeda et al. [19] employed computer-aided process engineering tools together with exergetic and life cycle analyses to evaluate various biofuels production pathways from lignocellulosic biomass. In another survey, Sohel and Jack [20] also used exergy analysis for biofuel production from lignocelluosic biomass using a novel pathway. Furthermore, Lythcke-Jørgensen et al. [18] applied exergy analysis for a CHP (combined heat and power) plant with an integrated lignocellulosic ethanol production system. Recently, Aghbashlo et al. [21,22] developed an exergy-based framework to

Component

Standard chemical exergy (kJ/mol)

Nitrogen (N2) Oxygen (O2) Water vapor (H2O) Carbon dioxide (CO2) Argon (Ar) Neon (Ne) Helium (He) Krypton (Kr)

0.72 3.97 9.5 19.87 11.69 27.19 30.37 34.36

assess continuous ethanol and acetate production from syngas through the WoodeLjungdahl pathway. According to the outcomes of the above-mentioned surveys, various bioethanol production processes and systems could be successfully designed and optimized by means of the exergy analysis. Although a number of investigations has been performed and published to date on ethanol production from sugar industry's wastes using various fungi [23e25], the goal of those studies were to only investigate the feasibility and modeling features of the studied processes. Therefore, the main purpose of the current research was to apply exergy analysis to ethanol production through the assimilation of glucose, fructose, sucrose as well as the whole and inverted sugarcane and sugar beet molasses by various forms of Mucor indicus under both aerobic and anaerobic conditions. This safe fungus can produce high yields of ethanol from different hexoses, pentoses, and lignocellulosic hydrolysates, by all its morphologies, i.e., filamentous and yeast-like forms or a mixture of these forms [26e28]. In the yeast-like form, the fungus grows similar to the typical yeasts by living in the form of separate cells and multiplying by budding. Alternatively, in the filamentous-form, it grows by forming long, cylindrical, branched cells, similar to the typical molds [28]. Moreover, the M. indicus biomass is a worthwhile byproduct, since it contains considerable amounts of chitin and chitosan which have numerous applications in agriculture, food, and pharmaceutical industries. The exergy-based sustainability assessment applied herein would be applicable to scrutinize the environmental impacts and economic benefits of various bioethanol production pathways for achieving the most eco-friendly and cost-effective route.

Table 1 The chemical formula and standard chemical exergy of materials applied for culture media preparation. Name

Chemical formula

Standard chemical exergy (kJ/mol)

Water Glucose Glucose monohydrate Fructose Sucrose Yeast extract Ammonium sulphate Magnesium sulphate heptahydrate Dipotassium hydrogen phosphate Calcium chloride Protein Raffinose Acetic acid Ethanol Glycerol

H2O C6H12O6 C6H12O6$H2O C6H12O6 C12H22O11 C19H14O2 (NH4)2SO4 MgSO4$7H2O K2HPO4 CaCl2.7H2O CH1.62N0.31O0.3S0.005 C18H32O16 C2H4O2 C2H6O C3H8O3

0.9a 3091.57b 3092.47b,c 3091.57b 6007.80a 9535.46b 660.60a 87.00a,c 78.92d 89.70a 1668124.00b,e 33545.06b 919.00a 1357.70a 1762.62b

a

Obtained from Wall [32]. Calculated based on equations (4) and (5). c Standard chemical exergy of these materials can be calculated for an equimolar mixture of their component. This is done because thermodynamic data for them are not available. d Obtained from http://www.exergoecology.com/. e Computed based on sugar beet and sugarcane molasses typical protein formula. b

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Table 3 Specific chemical exergy of three types of fungus M. indicus.

Table 5 The overall uncertainties of the computed exergetic parameters.

Morphology

Typical chemical formula

Chemical exergy (kJ/kg)

Exergetic parameters

Unit

Total uncertainty (%)

Nominal value

PY (purely yeast like) MY (mostly yeast like) MF (mostly filamentous)

CH1.79O0.52N0.18 CH1.82O0.51N0.17 CH1.84O0.50N0.15

22705.51 23144.36 23693.18

Exergy of liquid media Exergy of gas media Exergy of biomass Exergy of mechanical work Exergy of product Exergy destruction Rational exergetic efficiency Process exergetic efficiency

kJ kJ kJ kJ kJ kJ % %

±2.12 ±1.34 ±1.16 ±0.98 ±1.67 ±3.71 ±3.86 ±3.32

145.57 0.40 34.19 42.81 65.47 58.11 69.16 34.75

2. Materials and methods 2.1. Microorganism, cultivation, and analyses

" UF ¼

vF u vz1 1

2



vF þ u vz2 2

2



vF þ…þ un vzn

2 # 1 2 =

The detailed information on the ethanol production via M. indicus CCUG 22424 (Culture Collection University of Gothenburg, Sweden) can be found in our previous report [26]. The growth medium used for aerobic or anaerobic cultivation of M. indicus was based on our previous study [29]. Moreover, aerobic cultivations were carried out in 500 ml cotton plugged Erlenmeyer flasks, while a loop trap was used for anaerobic cultivation similar to the system used by Taherzadeh et al. [30]. Both aerobic and anaerobic cultivations were performed in a shaker incubator at 32  C for 3 d. Invertase enzyme was also used to break down sucrose dimers into fructose and glucose as comprehensively explained by Sharifia et al. [26]. More comprehensive and detailed information on liquid sampling procedure used as well as ethanol, glycerol, biomass, and liquid media analyses could be found in our previous report [26]. Finally, morphological development of the fungal cells was monitored periodically using light microscopy. All data reported in this paper are the average of the two replications.

(1)

where UF represents the uncertainty in the result, u1 , u2 , …, un indicate the uncertainty in the independent variables, z1 , z2 , …, zn are the independent variables, and F shows the function of the independent variables.

2.3. Theoretical considerations Fig. 1 represents a schematic illustration of the bioreactor as a control mass with input and output terms for ethanol fermentation. According to Fig. 1, the exergy balance equation for the bioreactor can be written as follows:

ExLM;in þ ExGM;in þ ExBM;in þ ExW ¼ ExLM;out þ ExGM;out þ ExBM;out þ Exdes

2.2. Uncertainty/error analysis In order to demonstrate the repeatability and accuracy of the computed exergetic values, error/uncertainty analysis was performed using the methodology previously presented by Holman [31] as follow:

(2)

where ExLM , ExGM , and ExBM stand for the exergetic values of liquid media, gas media over the culture, and biomass, respectively, at the beginning and end of the experiments, ExW denotes the exergy of mechanical work delivered to culture media from orbital shaker,

Table 4 Material balance for aerobic and anaerobic cultivations of M. indicus with different morphologies. Carbon source

Morphology Fermentation Glucose Fructose Sucrose Ethanol Glycerol Biomass Carbon dioxide condition consumption (g) consumption (g) consumption (g) production (g) production (g) production (g) evolution (g)

Glucose

MYa MF PY MF

Aerobic Aerobic Anaerobic Anaerobic

4.73 4.95 4.73 4.95

e e e e

e e e e

1.05 1.21 1.63 1.71

0.02 0.05 0.09 0.19

1.46 0.99 0.90 0.50

1.00 1.16 1.56 1.64

Fructose

MY MF PY MF

Aerobic Aerobic Anaerobic Anaerobic

e e e e

4.65 4.65 4.68 4.68

e e e e

1.11 1.11 1.61 1.85

0.11 0.09 0.21 0.29

1.35 1.12 0.84 0.47

1.06 1.06 1.54 1.77

Sugar beet molasses

MY MF PY MF

Aerobic Aerobic Anaerobic Anaerobic

0.26 0.26 0.26 0.26

0.24 0.24 0.24 0.24

0.00 0.00 0.00 0.00

0.00 0.00 0.00 0.00

0.00 0.00 0.00 0.00

0.00 0.00 0.00 0.00

0.00 0.00 0.00 0.00

Inverted sugar beet molasses

MY MF PY MF

Aerobic Aerobic Anaerobic Anaerobic

2.10 2.61 2.07 2.59

2.03 2.34 2.00 2.35

0.00 0.00 0.00 0.00

1.09 1.40 1.61 1.90

0.11 0.06 0.13 0.22

1.28 0.64 0.81 1.73

1.04 1.34 1.54 0.21

Sugarcane molasses

MY MF PY MF

Aerobic Aerobic Anaerobic Anaerobic

1.13 1.13 1.13 1.20

1.26 1.26 1.26 1.64

0.00 0.00 0.00 0.00

0.52 0.59 0.64 0.83

0.07 0.04 0.08 0.05

0.88 1.00 0.74 0.54

0.50 0.56 0.61 0.79

Inverted sugarcane MY molasses MF PY MF

Aerobic Aerobic Anaerobic Anaerobic

2.06 2.18 2.12 2.18

2.13 2.66 2.11 2.66

0.00 0.00 0.00 0.00

1.19 1.43 1.51 1.81

0.08 0.09 0.10 0.18

1.46 1.06 1.02 0.44

1.14 1.37 1.44 1.73

a

MY: mostly-yeast like, MF: mostly-filamentous, and PY: purely-yeast like.

Table 6 Exergy balance for aerobic and anaerobic cultivations of M. indicus with different morphologies. Morphology Fermentation Exergy of the condition culture media at the beginning of experiment (kJ)

Exergy of the culture media including produced ethanol and glycerol at the end of experiment (kJ)

Exergy of the gas Exergy of the gas Exergy of the Exergy of the Exergy of the Exergy destruction produced produced produced media at the end media at the (kJ) glycerol (kJ) of experiment (kJ) biomass (kJ) ethanol (kJ) beginning of experiment (kJ)

Rational exergetic efficiency (%)

Process exergetic efficiency (%)

Glucose

MYa MF PY MF

Aerobic Aerobic Anaerobic Anaerobic

145.57 149.40 145.57 149.40

95.67 101.21 114.10 118.38

0.00 0.00 0.01 0.01

0.40 0.47 0.65 0.68

34.19 23.65 20.56 11.83

30.94 35.82 48.26 50.63

0.33 0.97 1.57 3.47

58.11 66.87 53.07 61.32

69.16 65.21 71.83 68.10

34.75 31.45 37.37 34.30

Fructose

MY MF PY MF

Aerobic Aerobic Anaerobic Anaerobic

144.27 144.27 144.69 144.69

99.37 99.03 115.84 124.30

0.00 0.00 0.01 0.01

0.43 0.43 0.64 0.74

31.48 26.66 19.27 11.17

32.94 32.94 47.74 54.77

2.06 1.68 3.85 5.32

55.80 60.95 51.75 51.29

70.18 67.42 72.40 72.65

35.54 32.76 37.79 38.01

Sugar beet molasses MY MF PY MF

Aerobic Aerobic Anaerobic Anaerobic

181.08 181.08 181.08 181.08

172.61 172.61 172.61 172.61

0.00 0.00 0.01 0.01

0.00 0.00 0.01 0.01

0.00 0.00 0.00 0.00

0.00 0.00 0.00 0.00

0.00 0.00 0.00 0.00

51.27 51.27 51.27 51.27

77.10 77.10 77.10 77.10

0.00 0.00 0.00 0.00

Inverted sugar beet molasses

MY MF PY MF

Aerobic Aerobic Anaerobic Anaerobic

172.74 186.70 171.80 186.59

136.04 144.43 151.76 161.66

0.00 0.00 0.01 0.01

0.42 0.55 0.64 0.08

29.85 15.36 18.64 41.39

32.12 41.41 47.65 56.14

2.08 1.18 2.39 4.12

49.24 69.17 43.57 26.28

77.16 69.86 79.70 88.55

29.72 25.25 32.01 44.31

Sugarcane molasses

MY MF PY MF

Aerobic Aerobic Anaerobic Anaerobic

170.81 171.06 169.27 176.03

146.43 147.08 148.27 152.64

0.00 0.00 0.01 0.01

0.19 0.21 0.24 0.32

20.63 23.94 16.96 12.90

15.38 17.35 18.93 24.49

1.25 0.66 1.53 0.98

46.37 42.64 46.61 52.99

78.30 80.06 78.02 75.79

17.44 19.62 17.64 17.54

Inverted sugarcane molasses

MY MF PY MF

Aerobic Aerobic Anaerobic Anaerobic

161.85 173.02 162.68 173.02

126.59 134.01 136.41 146.73

0.00 0.00 0.01 0.01

0.46 0.56 0.60 0.72

34.17 25.43 23.26 10.40

35.13 42.39 44.71 53.46

1.55 1.73 1.84 3.42

43.45 55.83 45.23 57.98

78.78 74.14 78.00 73.14

34.62 32.23 33.97 31.18

a

M. Aghbashlo et al. / Energy 98 (2016) 240e252

Carbon source

MY: mostly-yeast like, MF: mostly-filamentous, and PY: purely-yeast like.

243

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Exdes represents the exergy destruction within the bioreactor due to thermodynamics irreversibilities. The exergetic value of liquid media in the bioreactor at the beginning and end of the experiments can be determined as:

ExLM ¼ nLM

X

xi εi þ RT0

X

i

! xi lnðxi Þ

(3)

i

where nLM denotes the mole number of culture media, xi is the molar fraction of each component, εi stands for the standard chemical exergy of i th component (kJ/mol), R stands for the gas constant (8.314 J/mol. K), and T0 is the reference state temperature considered to be 32  C, i.e., temperature of the shaker incubator. The standard chemical exergy of the inorganic materials used in the current survey study were obtained from the published literature [32] (Table 1). Moreover, the following equation proposed by Song et al. [33] was used to determine the standard chemical exergy of organic substances, which was not available in the literature.

exOM ¼ 363:439C þ 1075:633H  86:308O þ 4:14N

(4)

þ 190:798S  21:1A

Exergy of products (kJ)

120

εOM ¼ MOM exOM

(5)

where exOM represents the specific chemical exergy (kJ/kg), C, H, O, N, S and A stand for the percentage of carbon, hydrogen, oxygen, nitrogen, sulphur and ash quantity of each organic material, respectively, MOM is the molecular weight of an organic substance (kg/mol). It was postulated that the sugar beet and sugarcane molasses used consisted of seven main elements including sucrose, fructose, glucose, protein, raffinose, acetic acid, and water. Table 1 tabulates the chemical formula and standard chemical exergy of the various components used in the culture media preparation as well as the components produced during the fermentation. The chemical exergy of the gas media over the culture media during aerobic and anaerobic cultivations was determined using the following equation:

0 ExGM ¼ nGM @

X

xj εj þ RT0

X

j

1   xj ln xj A

j

Aerobic fermentation with mostly yeast (MY) and mostly filamentous (MF) forms

Exergy of ethanol Exergy of glycerol

100

Exergy of biomass

80 MY 60

MF

MY

MY

MF

MY MF MF MY

40

MF

20 MY MF

0

Carbon source

Exergy of products (kJ)

120 100 80

Exergy of ethanol

Anaerobic fermentation with purely yeast (PY) and mostly filamentous (MF) forms MF

Exergy of glycerol Exergy of biomass

PY

MF

PY

MF

PY

PY

MF

60 PY

40

MF

20 0

PY

MF

Carbon source Fig. 2. Effect of M. indicus morphology, carbon source, and aerobicity on the exergy of products.

(6)

M. Aghbashlo et al. / Energy 98 (2016) 240e252

where nGM is the mole number of gas media, xj denotes the molar fraction of each component, and εj represents the standard chemical exergy of j th component (kJ/mol). For calculating the chemical exergy of the gas media over the culture during aerobic cultivation, the standard molar percentage of different species in the dead state was as: (N2) ¼ 75.67, (O2) ¼ 20.34, H2O(g) ¼ 3.03, (CO2) ¼ 0.03, (Ar) ¼ 0.92, (He) ¼ 0.00052, (Ne) ¼ 0.0018, and (Kr) ¼ 0.000076 [34]. Moreover, the standard chemical exergy of the gas media components during both aerobic and anaerobic cultivations is summarized in Table 2. Furthermore, the specific chemical exergies of three forms of fungus M. indicus i.e., PY (purely yeast like), MY (mostly yeast like), and MF (mostly filamentous) forms were determined using Eq. (4) (Table 3). Accordingly, the exergetic quantity of biomass was found using the following equation:

ExBM ¼ mBM exBM

(7)

245

The exergetic value of mechanical work delivered to the mass of the culture media from the orbital shaker can be determined as follows:

ExW ¼ mLM adDt

(8)

where mLM denotes the mass of the culture media, a stands for the acceleration of the shaker (m/s2), d represents the amplitude of shaking, and Dt is the fermentation time. The amount of exergy supplied to the culture media from the orbital shaker was computed to be 42.81 kJ during the 72 h of the fermentation process. The total exergy of the products (ExP ) was computed by summing the exergies of ethanol, glycerol, and biomass. The biomass of M. indicus was considered as a product due to the fact that it includes considerable quantities of chitin and chitosan which have enormous industrial applications in agriculture, food, and pharmaceutical industries [26].

ExP ¼ ExBM þ ExEthOH þ ExGly

(9)

where mBM is the weight of biomass in the bioreactor.

Exergy destruction (kJ)

80

Aerobic fermentation

70

Mostly yeast like Mostly filamentous

60 50 40 30 20 10 0 Glucose

Fructose

Sugar beet

Inverted sugar beet

Sugarcane

Inverted sugarcane

Carbon source

Exergy destruction (kJ)

80

Anaerobic fermentation

70

Purely yeast like Mostly filamentous

60 50 40 30 20 10 0 Glucose

Fructose

Sugar beet

Inverted sugar beet

Sugarcane

Inverted sugarcane

Carbon source Fig. 3. Effect of M. indicus morphology, carbon source, and aerobicity on the exergy destruction.

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M. Aghbashlo et al. / Energy 98 (2016) 240e252

where ExEthOH and ExGly are the specific exergies of the produced ethanol and glycerol, respectively. The rational exergetic efficiency of the fermentation process was determined as follows:

j¼

ExLM;out þ ExGM;out þ ExMB;out Exout ¼ Exin ExLM;in þ ExGM;in þ ExBM;in þ ExW

(10)

J¼

ExBM þ ExEthOH þ ExGly Exdes

(12)

The rational exergetic sustainability index was determined using the following equation:

SIj ¼

1 1j

(13)

Furthermore, the process exergetic efficiency was computed using the following equation:

Furthermore, the process exergetic sustainability index was obtained as follows:

ExBM þ ExEthOH þ ExGly f¼ ExLM;in þ ExGM;in þ ExBM;in þ ExW

SIf ¼

(11)

The exergetic productivity index was also defined in this study for decision making on the M. indicus morphology, sugar type, and cultivation condition (i.e., aerobic or anaerobic) by considering the exergy of the products formed and the amount of exergy destructed during the fermentation as follows:

Rational exergy efficiency (%)

100

1 1f

(14)

3. Results and discussions Table 4 summarizes the material balance for the aerobic and anaerobic cultivations of M. indicus with different morphologies.

Aerobic fermentation

90

Mostly yeast like Mostly filamentous

80 70 60 50 40 30 20 10 0 Glucose

Fructose

Sugar beet

Inverted sugar beet

Sugarcane

Inverted sugarcane

Carbon source Rational exergy efficiency (%)

100

Anaerobic fermentation

90

Purely yeast like Mostly filamentous

80 70 60 50 40 30 20 10 0 Glucose

Fructose

Sugar beet

Inverted sugar beet

Sugarcane

Inverted sugarcane

Carbon source Fig. 4. Effect of M. indicus morphology, carbon source, and aerobicity on the rational exergy efficiency.

M. Aghbashlo et al. / Energy 98 (2016) 240e252

Process exergetic efficiency (%)

Obviously, M. indicus could not assimilate sucrose, while it successfully consumed glucose and fructose. The maximum amount of biomass and ethanol were produced by MF form of M. indicus under anaerobic condition using inverted sugar beet molasses. Moreover, the highest amount of glycerol was obtained from fructose by MF form of M. indicus under anaerobic condition. Table 5 tabulates the overall uncertainties of the computed exergetic values caused by measurement errors. The results showed that all uncertainties were below an acceptable error level (<5%). Exergy analysis of ethanol production was conducted for 24 different fermentation trials in a batch fermenter by applying the experimental data. The exergy analysis conducted offered a unified and effective strategy for assessing the sustainability and productivity of the investigated ethanol production process by M. indicus from various sugars. Table 6 tabulates the outcomes of the exergy analysis for aerobic and anaerobic cultivations of M. indicus with different morphologies. Fig. 2 illustrates the effect of M. indicus morphology, carbon source, and aerobicity on the exergy of the products. In general, the

50

247

exergy of the products under anaerobic condition was higher than what obtained during the aerobic cultivation. This occurred due to the fact that a portion of the fermented ethanol and glycerol with higher chemical exergy contents were gradually utilized by M. indicus for biomass production under aerobic condition after complete consumption of the sugars. Moreover, the morphology of M. indicus did not significantly affect the exergy of the products. The maximum exergy of the products was determined at 101.65 kJ for inverted sugar beet molasses with MF morphology of M. indicus under anaerobic condition. Interestingly, the exergy of the products for the whole sugar beet molasses under both aerobic and anaerobic cultivations with various morphologies of M. indicus was found to be zero. This could be ascribed to the fact that fungus M. indicus was unable to assimilate sucrose during fermentation of the sugar beet molasses. Furthermore, fructose fermentation by MF and PY forms of M. indicus under anaerobic cultivation stood in the second and third ranks according to the exergy of the products, respectively. This could be ascribed to the fact that the fungus M. indicus was able to assimilate fructose anaerobically and convert it into desired

Aerobic fermentation

45

Mostly yeast like Mostly filamentous

40 35 30 25 20 15 10 5 0 Glucose

Fructose

Sugar beet

Inverted sugar beet

Sugarcane

Inverted sugarcane

Process exergetic efficiency (%)

Carbon source 50

Anaerobic fermentation

45

Purely yeast like Mostly filamentous

40 35 30 25 20 15 10 5 0 Glucose

Fructose

Sugar beet

Inverted sugar beet

Sugarcane

Inverted sugarcane

Carbon source Fig. 5. Effect of M. indicus morphology, carbon source, and aerobicity on the process exergetic efficiency.

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M. Aghbashlo et al. / Energy 98 (2016) 240e252

Exergetic productivity index (-)

materials. This in turn indicated the need for invertase enzyme to break down the sucrose dimers available in the sugar beet molasses into fructose and glucose. It is very controversial to judge about the productivity and sustainability of an energy conversion process without incorporating the resource destroyed for production of a given amount of a product. Therefore, further analyses along with introducing conceptual exergetic indicators were also needed for decision making on the process sustainability and renewability. The variations in the exergy destruction of the fermentation process as a function of the M. indicus morphology, carbon source, and aerobicity are shown in Fig. 3. The quantity of exergy destruction during the aerobic fermentation was remarkably higher than what determined for the anaerobic condition. In general, intensive chemical and biochemical reactions, biomass growth, and mechanical work dissipation were the main reasons for exergy destruction during the fermentation of the sugars by M. indicus. Therefore, the consumption of a portion of the fermented ethanol and glycerol during the aerobic cultivation by microorganisms lengthened the chain of the biochemical reactions, which in turn augmented the quantity of the

4.5

exergy destruction compared to the anaerobic condition. Furthermore, the highest and lowest exergy destruction values belonged to the MF form of M. indicus during aerobic and anaerobic cultivations, respectively. The minimum exergy destruction was found to be 26.28 kJ for the inverted sugar beet molasses with MF microorganisms under anaerobic condition. Interestingly, the maximum exergy destruction was measured at 69.17 kJ for the inverted sugar beet molasses with MF microorganisms under aerobic condition due to the massive utilization of the produced ethanol and glycerol by the microorganisms, as previously elucidated. However, it is worth quoting that the exergy destruction alone cannot be a perfect performance metric for decision making on the sustainability of ethanol fermentation via M. indicus. Fig. 4 expresses the effect of M. indicus morphology, carbon source, and aerobicity on the rational exergy efficiency of the fermentation process. Generally, the rational exergy efficiency of the fermentation process during the anaerobic condition was slightly better than that computed for the aerobic cultivation mainly due to the consumption of the produced ethanol and glycerol by the microorganisms. However, the exergy efficiency did

Aerobic fermentation

4.0

Mostly yeast like Mostly filamentous

3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 Glucose

Fructose

Sugar beet

Inverted sugar beet

Sugarcane

Inverted sugarcane

Exergetic productivity index (-)

Carbon source 4.5

Anaerobic fermentation

4.0

Purely yeast like Mostly filamentous

3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 Glucose

Fructose

Sugar beet

Inverted sugar beet

Sugarcane

Inverted sugarcane

Carbon source Fig. 6. Effect of M. indicus morphology, carbon source, and aerobicity on the exergetic productivity index.

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Rational sustainability index (-)

not show any tendency with M. indicus morphology. The highest rational exergy efficiency was computed as 88.55% for fermentation of the inverted sugar beet molasses with MF M. indicus under anaerobic condition. This could be attributed to the effective utilization of fructose and glucose available in the culture media for ethanol fermentation. The lowest rational exergy was found to be 65.21% for glucose fermentation with MF form of microorganisms under aerobic condition possibly due to the rapid consumption of the fermented ethanol and glycerol for biomass production. Like the exergy destruction, the rational exergy efficiency cannot be employed as a sole reliable indicator because of overlooking the exergy of the formed products in the computations. The effect of M. indicus morphology, carbon source, and aerobicity on the process exergetic efficiency of the ethanol fermentation is presented in Fig. 5. Obviously, the process exergetic efficiency of the anaerobic cultivation was remarkably higher than what determined under aerobic condition because of the utilization of the fermented ethanol and glycerol by the microorganisms, as explained previously. This phenomenon was very obvious in the case of the inverted sugar beet molasses. The highest process

10

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exergetic efficiency was calculated as 44.31% during the anaerobic fermentation of the inverted sugar beet molasses with MF form of M. indicus. Taking in consideration the results obtained from the process exergetic efficiency, this fermentation trial was recommended for the industrial application phase. Moreover, the process exergetic efficiency of the ethanol fermentation of the whole sugar beet molasses under both aerobic and anaerobic cultivations was zero due to debility of M. indicus in fermentation of sucrose, as illustrated previously. The results of this study showed that the fungus M. indicus was not a good candidate for assimilation of sugarcane molasses from sustainability and productivity point of views. Therefore, other pathways must be explored for sustainable and effective utilization of sugarcane molasses. Overall, this thermodynamic indicator can provide more conceptual and practical insights compared to the rational exergy efficiency. The exergetic productivity index of the fermentation process at various M. indicus morphologies, carbon source, and aerobicities can be seen in Fig. 6. Clearly, the exergetic productivity index of the anaerobic cultivation was significantly higher than that computed for the aerobic condition. Like the process exergetic

Aerobic fermentation

Mostly yeast like Mostly filamentous

8 6 4 2 0 Glucose

Fructose

Sugar beet

Inverted sugar beet

Sugarcane

Inverted sugarcane

Carbon source Rational sustainability index (-)

10

Anaerobic fermentation

Purely yeast like Mostly filamentous

8 6 4 2 0 Glucose

Fructose

Sugar beet

Inverted sugar beet

Sugarcane

Inverted sugarcane

Carbon source Fig. 7. Effect of M. indicus morphology, carbon source, and aerobicity on the rational sustainability index.

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Process sustainability index (-)

efficiency, the highest exergetic productivity index was found for the fermentation of the inverted sugar beet molasses under anaerobic condition with MF form of M. indicus. Moreover, the lowest exergetic productivity index was found to be zero for the whole sugar beet molasses. Generally, this exergy-based indicator can be satisfactorily employed as a decision making index to optimize the operating conditions of industrial fermenters, since it can fruitfully minimize the resource destruction to produce a given quantity of a product. Fig. 7 presents the effect of different M. indicus morphologies, carbon source, and aerobicities on the rational sustainability index of the fermentation process. The anaerobic cultivation showed higher rational sustainability index compared to the aerobic condition. The effects of experimental variables on the rational sustainability index of the ethanol fermentation could be similarly interpreted by their effects on the rational exergy efficiency. This could be ascribed to the fact that the sustainability index of the fermentation process is directly proportional to the exergetic efficiency of the system according to Eq. (13). It must be noted that the rational sustainability index of the process was considerably higher

2.0

for the inverted sugar beet molasses with MF morphology of M. indicus under anaerobic condition. Thus, this approach is a promising option for producing the fermentative ethanol in sugar industries to meet a portion of the world's growing energy demands. Moreover, the produced biomass can be used as an animal feed and for the production of the other valuable products such as chitin and chitosan. Fig. 8 depicts the effect of various M. indicus morphologies, carbon source, and aerobicities on the process sustainability index. The highest process sustainability index was found to be 1.79 for the inverted sugar beet molasses fermentation with MF form of microorganisms under anaerobic cultivation. The lowest process sustainability index was determined at 1.00 for the whole sugar beet molasses under various fermentation conditions. A comparison between Figs. 7 and 8 revealed that the process sustainability index was profoundly lower than that of the rational sustainability index. This could be attributed to the lower process exergetic efficiency compared to the rational exergy efficiency (compare Figs. 5 and 6). However, the process sustainability index could serve as a much more effective conceptual framework than the rational

Aerobic fermentation

1.8

Mostly yeast like Mostly filamentous

1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 Glucose

Fructose

Sugar beet

Inverted sugar beet

Sugarcane

Inverted sugarcane

Process sustainability index (-)

Carbon source 2.0

Anaerobic fermentation

1.8

Purely yeast like Mostly filamentous

1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 Glucose

Fructose

Sugar beet

Inverted sugar beet

Sugarcane

Inverted sugarcane

Carbon source Fig. 8. Effect of M. indicus morphology, carbon source, and aerobicity on the process sustainability index.

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sustainability index by taking into account the exergetic values of the products in its computations. Finally, the exergy flow diagrams of three of the most exergetically-sustainable conditions of ethanol fermentation by M. indicus on the basis of the process exergetic efficiency are depicted in Fig. 9. This in turn further demonstrated the suitability of the inverted sugar beet molasses for ethanol and biomass production

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with MF form of microorganisms under anaerobic condition. Moreover, fructose assimilation under anaerobic condition with MF and PY morphologies stood in the second and third ranks, respectively, from the exergetic point of view. Overall, the exergy-based analyses are strongly recommended for optimizing commercial bioreactors used for the production various biofuels with an aim to achieve the most cost-effective and eco-friendly pathways and conditions.

Fig. 9. The exergy flow diagram of the three of the most exergetically-sustainable conditions of ethanol fermentation by M. indicus. (A) Anaerobic assimilation of inverted sugar beet molasses with MF form of M. indicus; (B) Anaerobic assimilation of fructose with MF form of M. indicus; (C) Anaerobic assimilation of fructose with PY form of M. indicus.

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4. Conclusions Exergy-based sustainability assessment of batch fermentation of various sugars for simultaneous ethanol, glycerol, and biomass production via various forms of M. indicus under aerobic and anaerobic cultivations was carried using the experimental data for the first time. The fermentation of the inverted sugar beet molasses by filamentous M. indicus under anaerobic condition was found to be the best option for large-scale industrial production. This study demonstrated that the exergy-based indicators could be fruitfully employed for mitigating the devastative environmental impacts associated with renewable ethanol production. Moreover, this unique approach could be satisfactorily applied for securitizing, developing, optimizing, and retrofitting industrial-scale fermenters for bioethanol production according to the sustainability and productivity viewpoints. Furthermore, future research works should be directed towards the optimization of ethanol fermentation process via advanced optimization techniques together with emerging exergy extensions like exergoeconomic and exergoenvironmental approaches to find the most cost-effective and eco-friendly pathways/conditions. Acknowledgment The author would like to acknowledge the support of University of Tehran and Biofuel Research Team (BRTeam). References [1] De S, Luque R. Upgrading of waste oils into transportation fuels using hydrotreating technologies. Biofuel Res J 2014;1(4):107e9. [2] Barchyn D, Cenkowski S. Process analysis of superheated steam pre-treatment of wheat straw and its relative effect on ethanol selling price. Biofuel Res J 2014;1(4):123e8. [3] Sharma YC, Singh B, Madhu D, Liu Y, Yaakob Z. Fast synthesis of high quality biodiesel from ‘waste fish oil’ by single step transesterification. Biofuel Res J 2014;1(3):78e80. [4] Shirazi MJA, Bazgir S, Shirazi MMA. Edible oil mill effluent; a low-cost source for economizing biodiesel production: electrospun nanofibrous coalescing filtration approach. Biofuel Res J 2014;1(1):39e42. [5] Aghbashlo M, Tabatabaei M, Mohammadi P, Pourvosoughi N, Nikbakht AM, Goli SAH. Improving exergetic and sustainability parameters of a DI diesel engine using polymer waste dissolved in biodiesel as a novel diesel additive. Energ Convers Manage 2015;105:328e37. [6] Edama NA, Sulaiman A, Rahim SNA. Enzymatic saccharification of Tapioca processing wastes into biosugars through immobilization technology. Biofuel Res J 2014;1(1):2e6. [7] Aghbashlo M, Mobli H, Madadlou A, Rafiee S. Influence of spray dryer parameters on exergetic performance of microencapsulation process. Int J Exergy 2012;10(3):267e89. [8] Aghbashlo M, Mobli H, Rafiee S, Madadlou A. A review on exergy analysis of drying processes and systems. Renew Sust Energ Rev 2013;22:1e22. € [9] Genc S, Sorguven E, Ozilgen M, Kurnaz IA. Unsteady exergy destruction of the neuron under dynamic stress conditions. Energy 2013;59:422e31. € [10] Sorgüven E, Ozilgen M. Thermodynamic efficiency of synthesis, storage and breakdown of the high-energy metabolites by photosynthetic microalgae. Energy 2013;58:679e87. [11] Aghbashlo M. Exergetic simulation of a combined infrared-convective drying process. Heat Mass Transf 2015. http://dx.doi.org/10.1007/s00231-015-1594-3.

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