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Design of an optimal process for enhanced production of bioethanol and biodiesel from algae oil via glycerol fermentation
Applied Energy 135 (2014) 108–114
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Design of an optimal process for enhanced production of bioethanol and biodiesel from algae oil via glycerol fermentation Mariano Martín a,⇑, Ignacio E. Grossmann b a b
Departamento de Ingeniería Química, Universidad de Salamanca, Pza. Caídos 1-5, 37008 Salamanca, Spain Chemical Engineering Department, Carnegie Mellon University, Pittsburgh, PA 15213, United States
h i g h l i g h t s Algae are used to produce simultaneously ethanol, biodiesel. Glycerol is a byproduct that can be further used to increase the yield to fuels. Glycerol fermentation to ethanol almost doubles its yield. The integrated facility is competitive with other uses of glycerol to produce methanol or ethers.
a r t i c l e
i n f o
Article history: Received 2 December 2013 Received in revised form 22 July 2014 Accepted 14 August 2014
Keywords: Biofuels Biodiesel Glycerol Ethanol Process integration
a b s t r a c t In this paper, we optimize a process that integrates the use of glycerol to produce ethanol via fermentation within the simultaneous production of biodiesel and bioethanol from algae. The process consists of growing the algae, determining the optimal fraction of oil vs. starch, followed by oil extraction, starch liquefaction and saccharification, to sugars, oil transesterification, for which we consider two transesterification technologies (enzymes and alkali) and the fermentation of sugars and glycerol. The advantage of this process is that the dehydration technologies are common for the products of the glucose and glycerol fermentation. Simultaneous optimization and heat integration is performed using Duran and Grossmann’s model. The fermentation of glycerol to ethanol increases the production of bioethanol by at least 50%. The energy and water consumptions are competitive with other processes that either sell the glycerol or use it to obtain methanol. However, the price for the biofuels is only competitive if glycerol cannot be sold to the market. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction The use of the byproducts from biorefineries has become very important for their profitability since first generation biofuels. In particular, the production cost of biodiesel is highly dependent on the price of glycerol. For some time, the wide range of uses of the glycerol and its limited production conferred it a reasonably high price. However, the increase in the production of biodiesel has created an excess saturating the market [1] and reducing the price of glycerol to values of $0.102/lb [2]. Based on the previous work by the authors [3,4], the production cost of biodiesel would increase $0.15/gal if the glycerol drops to this price. There are a number of alternative uses for the glycerol within the biorefinery. On the one hand we can generate syngas and later methanol so ⇑ Corresponding author. Tel.: +34 923294479. E-mail address: [email protected] (M. Martín). http://dx.doi.org/10.1016/j.apenergy.2014.08.054 0306-2619/Ó 2014 Elsevier Ltd. All rights reserved.
that we can reduce the dependency on fossil based raw materials of the current biodiesel production processes [5]. Another feasible alternative is the transformation of glycerol into fuel oxygenates by means of etherification and esterification reactions enhancing the production of diesels substitutes [6–15]. However, there is an even simpler integration option, the fermentation of glycerol to ethanol [16–19]. In this way we already have most of the technologies in place and we can use it for the production of biodiesel through ethanolysis. Furthermore, we enhance the production of bioethanol from algae [20]. Alternatively, we could use glycerol as a source of carbon for the algae growing [21]. The challenge is to integrate the fermentation of glycerol within the biodiesel production facilities. Even though we can produce ethanol out of the glycerol for the plants that involve methanolysis or ethanolysis, the residue of methanol in the glycerol may complicate the purification process. Therefore, we focus on the integration of the ethanol obtained from the fermentation of
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Nomenclature Ci
concentration of the chemical I in the reactor (mol/L) Cpi heat capacity fc (j,unit1, unit2) individual mass flow rate (kg/s) F (unit1,unit2) mass flow rate (kg/s) Pi partial pressure of component i (bar)
glycerol with the processes that simultaneously produces bioethanol and biodiesel from algae. The algae processing to bioethanol and biodiesel is based on previous papers by the authors [3,4,20]. The aim of this paper is to optimize a process that starting with algae, produces ethanol and biodiesel using the glycerol to enhance the production of ethanol. We compare the results of this study with the different integration alternatives that have been presented so far, namely, the use of glycerol to produce methanol, to produce fuel oxygenates or considering the production of glycerol as it is [5,15,20]. Therefore, the work also allows a discussion on the best use of glycerol. For the design of such an integrated plant we use mathematical programming techniques that allow us to account for the trade offs in the transesterification reaction related for instance to the excess of alcohol needed for the operation of the reactor. We divide the paper into 4 sections. First, we describe the process flowsheet. Next, we comment on the main modeling issues highlighting the glycerol fermentation to ethanol. Subsequently, we present the results and the comparison with different integration alternatives. Finally, we draw some conclusions.
2. Overall process description We divide the process into four sections, algae oil production, ethanol production from starch, biodiesel production from oil, and finally glycerol fermentation to ethanol, which will be recycled to the dehydration step. Interesting reviews on the different stages are available in the literature [22]. The first stage includes the production of biomass (oil, starch, protein), see Martín and Grossmann [3] for further details. Algae are grown by injecting CO2 into the water, which can be saline water so that the consumption of freshwater is reduced, together with air and fertilizers. The amount of water needed, the concentration of fertilizers is taken from the report by Pate [23], while the consumption of CO2 depends on the growth rate, typically 50 g/m2 d [24] and is given by the experimental results by Sazdanoff [25]. We assume that the dry algae biomass is composed of oil, up to a maximum of 60%w/w, starch and protein with a minimum of 10%w/w to be conservative [20,22,26]. Together with the algae, oxygen is produced and water is evaporated [23]. The energy consumed by the pond system is calculated based on the results by Sazdanoff [25]. Next, the algae are harvested from the pond. Recently, Univenture Inc. has presented an innovative technology capable of integrating harvesting and drying the algae with low energy consumption. It is based on the use of capillarity, membrane systems and paint drying to obtain 5% wet algae with a consumption of 40 W per 500 L/h. The biomass is mixed with cyclo-hexane and compressed so that oil is extracted and the biomass (starch and protein) is separated from the oil. On the other hand, the starch follows liquefaction (85 °C) and saccharification (65 °C) to break down the polymers into glucose. Next, the glucose is fermented into ethanol at 38 °C. The solid phase, mainly protein, is separated from the liquid phase and is sold. The liquid phase, mainly ethanol and water, but containing
T (unit1,unit2) temperature of the stream from unit 1 to unit 2 (°C) x (J,unit1,unit2) mass fraction of stream from unit 1 to unit 2 k vaporization heat (kJ/kg)
other products in small amounts such as glycerol, succinic acid, lactic acid, is distilled in a multi-effect distillation column to reduce the consumption of energy and cooling needs in the purification of ethanol. The last stage for the production of ethanol is the final dehydration using molecular sieves. This section is common for the ethanol produced from the starch, as well as for that obtained from the fermentation of glycerol. Part of this ethanol will be used in the transesterification of the oil and the rest can be sold as biofuel. Details on the process can be found in Martín and Grossmann [20]. The production of biodiesel via ethanolysis of algae oil was described in Severson et al. [4]. Two interesting catalysis were identified, enzymes and KOH. Surface response models for the reactors were developed to evaluate the trade-offs related to the operating variables at the reactor, namely, temperature, excess of ethanol, catalysis load and composition, and its effect on the yield. Next, the mixture of ethanol, glycerol and biodiesel is distilled to recover and recycle the excess of ethanol used. The polar phase containing glycerol is separated from the non polar phase containing the biodiesel, and while the biodiesel is purified in a distillation column to remove mainly the oil remaining, the glycerol is sent to etherification. The main process constraints can be seen in Table 1. Recently, it has been reported that glycerol can be fermented anaerobically to ethanol as main product using Escherichia coli [17–19,27]. Therefore, the byproduct of the synthesis of biodiesel from oil can be further converted to ethanol, increasing the liquid fuels production from algae following as main reaction Eq. (1)
C3 H8 O3 ! C2 H6 O þ H2 þ CO2
ð1Þ
The gas phase is recovered separately and the liquid phase containing the ethanol, biomass and traces of other organic chemicals is purified. This liquid phase is similar to the one that is obtained from the fermentation of glucose, and thus we can use the same purification and dehydration scheme. We mix the liquid phases from both fermentors, and after the recovery of the biomass and protein, we dehydrate the ethanol using a three effect multi-effect distillation column and molecular sieves. In Fig. 1 we present a general flowsheet for the entire superstructure of the process. 3. Mathematical modeling All the unit operations in the production process of liquid fuels and hydrogen from glycerol are modeled using surrogate models,
Table 1 Main operating constraints [3,4]. Equipment
Temperature limit
Alcohol separation column
Bottoms: < 150 °C Reflux ratio: 2–3 Top: <250 °C Bottoms: <350–375 °C Reflux ratio: 2–3 30–40 °C
Biodiesel purification column
Phase separation
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Fig. 1. Superestructure of the process.
design equations, mass and energy balances. Experimental data from the literature are used to predict the performance of the most important units such as fermentors, liquid–liquid separation, hydrolytic stages or ponds, whose operation cannot be easily modeled using first principles. The superstructure is written in terms of the total mass flows, component mass flows, component mass fractions, and temperatures of the streams in the network. The components in the system include those present in the algae, plus those produced during the process of ethanol production, and belong to the set J = {Water, EtOH, Glycerol, FAEE, FFA, Oil, Hexane, Starch, Glucose, Maltose, Protein, Succinic Acid, Acetic Acid, Lactic Acid, Urea, NH3, H2SO4, KOH, K2SO4, H3PO4, K3PO4, Algae, Biomass, CO2, O2, H2}. We describe the models below.
this figure we use supported enzymes as catalyst. We separate the glycerol phase from the biodiesel (FAEE) phase, we purify the biodiesel, and the glycerol is further fermented. This stream is mixed with that coming from the fermentation of glucose so that the ethanol obtained from the two sources is purified and dehydrated. Part of it will be used for the transesterification of the oil and thus the process does not depend on fossil fuel based raw materials (methanol), and the excess can be sold as bioethanol. The second alternative is the use of alkali catalysts. A similar flowsheet can be presented in the case of using an alkali catalyst. However, in this case the flowsheet differs in the separation of the polar and non polar phases since a washing step is required [4]. 3.3. Solution procedure
3.1. Biodiesel and ethanol production from algae For the sake of reducing the length of the paper, the models for the stages that lead to the production of biodiesel from algae oil using bioethanol are reported in previous papers [3,4,20]. The process involves, the algae growing and harvesting, the extraction of the oil using solvents, the recovery of the solvent so we can separate the biomass and the oil. At this point, the starch is liquefied, saccharified and the glucose obtained fermented into ethanol, while the oil is transesterified with the ethanol produced within the same plant. 3.2. Glycerol fermentation A solution of glycerol in water is fermented anaerobically at 38 °C with E. coli. The aim is that the concentration of ethanol in the liquid is at the most 10%. The reaction proceeds for 60 h [17–19,27]:
Glycerol ! CO2 þ H2 þ Ethanolð98%Þ
ð2Þ
CO2 þ Glycerol ! Succinic Acid þ H2 Oð1%Þ
ð3Þ
Glycerol ! 3:0075ðBiomassÞ þ H2 O ð1%Þ
ð4Þ
The liquid phase from this fermentor is mixed with that from the glucose fermentor, and after the separation of the solids and proteins, it is dehydrated using first a three effect distillation column followed by molecular sieves. In Fig. 2 we present a detailed flowsheet for the simultaneous production of bioethanol and biodiesel from algae. The starch is liquefied and saccharified to release the glucose that is fermented to ethanol. On the other hand, the oil extracted is transesterified. In
Surface response models are used for modeling the transesterification reactors, short-cut models based on experimental data from the literature, and/or models validated using process simulators are developed for the pretreatment stages, fermentations, liquid–liquid separations or the distillation columns as presented before. We have one binary variable related to the catalysts employed for the oil transesterification. In order to provide a proper comparison and not only report the optimal solution, we decompose the problem into two NLP’s, one per catalyst, each consisting of 4000 variables and 3500 equations. We simultaneously optimize and heat integrate each subproblem either for enzymatic or alkali catalyzed technologies using Duran and Grossmann’s model [28]. The objective function is a simplified production cost involving the income for the production of biodiesel ($1/kg), ethanol ($1/kg) and protein ($0.2/kg), and the energy costs and catalysts (enzymes and KOH) assuming short life cycles [3,4]. Next we design the optimal heat exchanger network using SYNHEAT [29]. A detailed cost analysis is performed involving raw material cost (oil production is part of the flowsheet), maintenance, cost of utilities and chemicals, labor, annualized equipment cost, and the cost for the management of the facility following Sinnot’s method [30]. Subsequently, we also design the water network based on the paper by Ahmetovic´ and Grossmann [31] to compare the results of water consumption with those presented in the integrated production of ethanol and biodiesel [20]. Finally, we compare the processes that involves the reuse of glycerol either for the production of methanol [5], diesel substitutes [15] and enhanced production of bioethanol presented in this paper. For the last two comparisons we focus on the most
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Fig. 2. Integrated flowsheet FAEE & enhanced ethanol.
promising catalysts, the supported enzymes, since their use avoids problems related to the presence of traces of salts in the glycerol.
Table 3 Operating conditions at the transesterification reactor: alkali catalyst (f fixed). Alone [4] (ethanol $1/gal)
Integrated Base case [20]
Enhanced ethanol (this work)
75 4f 5.7
75 4f 5.7
75 4f 5.7
0.5 1.5 –
0.5 1.5
0.5 1.5 –
4. Results and discussion The economic evaluation is carried out based on the factorial method [30] accounting for annualized equipment cost, management, labor, based on other plants, chemicals and utilities, which are updated from the literature (0.019 $/kg Steam, 0.057 $/ton cooling water [32], 0.06 $/kWh [33], 0.021 $/kg Oxygen [34], the cost of hydrogen is taken to be $1.6/kg based on DOE data, the cost of natural gas is $4 Million BTU [35]. Finally, the cost correlations for the different equipment can be found in the supplementary material of Martín and Grossmann [36] and updated to current prices. 4.1. Simultaneous production of bioethanol and biodiesel substitutes from algae: alkali catalyst In Table 2 we present the comparison between the algae composition, and the products when obtaining glycerol as byproduct [20] and when we reuse it to increase the production of ethanol. The algae growth conditions are similar, and ethanol production is increased by 70% to reach 13 MMgal/yr. Table 3 shows the main Table 2 Optimal algae growth for the simultaneous production of FAEE and enhanced ethanol using alkali catalyst. Base case [20] kg/s
This work w/w%
kg/s
EtOH Biodiesel Prot Glycerol
0.748 8.555 1.431 0.890
1.274 8.555 1.431
Algae comp Oil Star Prot
15 9 4.5 1.5
15 9 4.5 1.5
60 30 10
w/w%
Temperature (°C) Pressure (bar) ratio_et (mol ethanol/ mol oil) Time (h) Cat/lipase (%w/w) Water added (%w/w)
operating parameters of the transesterification reactor, with no difference whether biodiesel is produced alone [4], when there is simultaneous production of ethanol and biodiesel from algae, base case [20], or when we use the glycerol to increase the production of bioethanol. The reflux ratio for the ethanol recovery and biodiesel purification turns out to be 2. Finally, in Table 4 the main operating characteristics of the multieffect column used to dehydrate the ethanol are shown. These results are also similar to those presented when glycerol was a byproduct of the process. 4.2. Simultaneous production of bioethanol and biodiesel substitutes from algae. Enzymatic catalyzed For this second case study we optimize the simultaneous production of ethanol and FAEE using supported enzymes. Fig. 2 shows the process flowsheet. We optimize the operation of the transesterification reactor, the production of ethanol from the starch contained in the algae, including multieffect columns implemented in Table 4 Summary of the operating condition of the distillation multieffect columns. Alkali.
60 30 10
Column
a
b
P(LP) mmHg
IP/LP
HP/IP
Col5-7
0.084
0.238
193
2.14
2.04
Legend: LP: Low pressure, IP: Intermediate pressure, HP: High pressure, a: fraction of total feed to LP column, b: fraction of total feed to IP column.
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Table 5 Optimal algae growth for the simultaneous production of FAEE and enhanced ethanol using enzymatic catalyst. Base case [20] kg/s
This work w/w%
EtOH Biodiesel Prot Glycerol
0.866 8.353 1.430 0.869
Algae comp Oil Star Prot
15 9 4.5 1.5
kg/s
w/w%
1.296 8.350 1.445 15 9 4.5 1.5
60 30 10
60 30 10
Table 6 Operating conditions at the FAEE transesterification reactor. Enzyme catalyst (f fixed).
Temperature (°C) Pressure (bar) ratio_et (mol ethanol/ mol oil) Time (h) Cat/lipase (%) Water added
Alone [4] (ethanol $1/gal)
Integrated Base case [20]
Enhanced ethanol (this work)
45 4f 8.9
30 4f 4.1
30 4f 4.2
6.9 14.0 0.0
8.0 13.0 0.0
8.0 13.1 0.0
production of ethanol. As we can see, the optimal algae composition is the same based on the fact that the production of ethanol requires a large amount of energy in order to dehydrate the ethanol. On the one hand, part of this ethanol is needed for the transesterification of the oil. On the other hand, the solution does not suggest producing more oil, and the excess of ethanol is a good asset for the process. The ethanol production is increased by 50% for a total of around 13 MMgal/yr When it comes to the operating conditions of the main equipment such as transesterification reactor and multieffect distillation column, it turns out that the integrated process using glycerol to produce ethanol requires different operating conditions at the transesterification reactor from those required by the stand alone process. However, they are similar to those of the flowsheet that simultaneously produces bioethanol and biodiesel with glycerol as byproduct, see Tables 6 and 7. The reason is that with the reactor operating at 30 °C and being endothermic, it is easier to integrate the energy available within the system, while if the operating temperature increases, the heat integration is more difficult. The reflux ratio for the distillation columns used for ethanol recovery and FAEE purification is 2 in all cases.
4.3. Production costs
Table 7 Summary of the operating condition of the distillation multieffect columns. Enzymes as catalyst. Column
a
b
P(LP) mmHg
IP/LP
HP/IP
Col5-7
0.084
0.238
182
2.15
2.05
Legend: LP: Low pressure, IP: Intermediate pressure, HP: High pressure, a: fraction of total feed to LP column, b: fraction of total feed to IP column
In Fig. 3 we present the breakdown of the production cost of bioethanol and biodiesel from algae using glycerol to produce ethanol. Since the main raw material is the CO2, the chemicals do not play an important role compared to the annualized cost of equipment or utilities. In Table 8 we compare the results of this paper with the enhanced production of ethanol with the ones from Martín and Grossmann [20]. In that case, glycerol was considered to be sold (at $0.3/kg). We can see that the energy consumption per gallon of biofuel increases if we produce more ethanol. This is mainly due to the needs at the ethanol dehydration stage. However, freshwater consumption remains at the same level. Finally, the investment cost increases by 10–20% when we produce ethanol from glycerol due to the new equipment involved and the larger equipment needed for ethanol dehydration, resulting in $0.1/gal produc-
Fig. 3. Breakdown of the production cost of EtOH & FAEE.
the flowsheet, in contrast to the sequential approach by Karrupiah et al. [37]. Table 5 presents the optimal product distribution when glycerol is produced as byproduct [20] and when it is used for the enhanced
Fig. 4. Sensitivity analysis cost glycerol.
Table 8 Summary of results. Alkali cat
$/galbiofuel Energy (MJ/galbiofuel) Water (gal/galbiofuel) Investment (MM$) Capacity (Mgal/yr)
Enzymatic
Base Case [20]
(This work)
Base Case [20]
(This work)
0.32 6.72 0.77 175 91
0.38 7.65 0.84 198 96 (13 Mgal/yr bioethanol)
0.35 4.00 0.59 180 90
0.45 4.20 0.59 211 94(13 Mgal/yr bioethanol)
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$/galbiofuel Energy (MJ/galbiofuel) Water (gal/galbiofuel) Investment (MM$) Capacity (Mgal/yr)
Enzymatic base case [20]
Integrating MetOH [5]
Enzymatic EtOH + D &TTBG & FAEE [15]
Enhanced ethanol (this work)
0.35 4.00 0.59 180 90 (6 Bioet)
0.66 3.65 0.79 118 69
1.00 3.36 0.59 167 105 (96 biod/9bioet)
0.45 4.20 0.59 211 94 (81biod/13bioet)
tion cost higher than if we can sell the glycerol. However, as the biodiesel production increases, the market for glycerol gets saturated, and thus in the next section we evaluate the effect of the price of glycerol and its impact on the competitiveness of the process. Here we do not consider the gas from the glycerol fermentation containing hydrogen as a credit. 4.4. Sensitivity study 4.4.1. Effect of glycerol cost Fig. 4 compares the production cost of biofuels, bioethanol and biodiesel, obtained simultaneously from algae, whether we can sell the glycerol as byproduct, or if we use it to increase the production of bioethanol. As we can see, only if we cannot actually sell the glycerol, its use to produce ethanol is attractive 4.4.2. Comparison of integrating alternatives Recently the authors have evaluated the use of glycerol for the production of biodiesel and bioethanol with glycerol as byproduct [20], the production of methanol from glycerol to reduce the dependency on fossil fuels [5], the production of ethers from glycerol, enhancing the production of diesel substitutes [15], and finally the use of glycerol to increase the bioethanol production, which it is addressed in this paper. Table 9 shows the main results from all these processes. The advantage of using ethanol and not methanol is that it is produced within the biorefinery complex, reducing the dependency on fossil based raw materials since methanol is mainly produced from natural gas. Thus, the simultaneous production of ethanol and biodiesel is promising in terms of production cost and water consumption, but requires larger investment and energy consumption per gal of biofuel produced. The production capacity is interesting since we can increase it by around 10 MMgal/yr of biofuels with respect to the production of biodiesel alone. The use of glycerol to produce methanol, see column 2 in Table 9, reduces the dependency on fossil fuels up to 70%, but we cannot produce the amount of methanol that we need to run the plant. Furthermore, the production cost increases since the biofuel production capacity remains constant, while glycerol reforming, gas purification, synthesis and methanol purification stages are added, and glycerol is no longer sold. The largest biofuel capacity increment is found when we use the glycerol to produce ethers. The comparison is not that straightforward since we add i-butene as raw material. The result is that the production cost increases because i-butene is expensive. Recently, Martín and Grossmann [38] have shown that it is possible to produce i-butene from lignocellulosic raw materials at a competitive price. However, the process is still under development. The advantages of this process, see column 3 in Table 9, is that we maintain the energy and water consumption at low levels compared to any other due to the increased biofuel production capacity and the increased investment due to the separation stages for the glycerol ethers. Finally, the use of glycerol to produce ethanol, the process described in this paper whose results are summarized in column 4, Table 9, shows an increased production capacity of biofuels with the second lowest production price. The drawbacks of this process are related to
the increased investment and energy consumption, due to the dehydration steps of ethanol.
5. Conclusions In this paper, we have optimized the integrated production of bioethanol and biodiesel from algae by using the glycerol to enhance the production of bioethanol. This process shares the dehydration technologies and results in better process, heat and product integration. We consider two technologies for the transesterification of the oil, alkali and supported enzymes, and the fermentation of glycerol to ethanol. Out of the two, the cheapest alternative uses alkali. However, it does not correspond with the one that consumes less energy and water due to the catalysis cost. The flexibility to the oil composition and the slight difference in the production cost reveals the use of enzymes as the most promising out of the two. Finally, we compare different uses of glycerol to help in the economy by selling it of the biofuels or the increase the biofuels production capacity, the production of methanol and its use as transesterification alcohol. A number of interesting trade-offs have been identified in terms of production cost, energy and water consumption and biofuels total production capacity. Acknowledgments The authors gratefully acknowledge the NSF Grant CBET0966524 and the Center for Advanced Process Decision-making at Carnegie Mellon University. Dr. Mariano Martín acknowledges Salamanca Research for Software licenses. References [1] Pagliaro M, Rossi M. Future of glycerol. 2nd ed. Cambridge: The Royal Society of Chemistry; 2010. [2] Ahmed S, Papalias D. Hydrogen from glycerol: a feasibility study. In: Presented at the 2010 hydrogen program annual merit review meeting Washington DC, June 8, 2010.< http://www.hydrogen.energy.gov/pdfs/ progress10/ii_a_3_ahmed.pdf>. [3] Martín M, Grossmann IE. Process optimization biodiesel production from cooking oil and Algae. Ind Eng Chem Res 2012;51(23):7998–8014. [4] Severson K, Martín M, Grossmann IE. Optimal production of biodiesel using bioethanol. AICHE J 2012;59(3):834–44. [5] Martín M, Grossmann IE. Towards the optimal integrated production of biodiesel with internal recycling of methanol produced from glycerol. Environ Prog Sust Energy 2013;32(4):791–801. [6] Behr A, Obendorf L. Development of a process for the acid-catalyzed etherification of glycerine and isobutene forming glycerine tertiary butyl ethers. Eng Life Sci 2003;2(7):185–9. [7] Behr A, Obendorf L. Process development for acid-catalysed etherification of Glycerol with isobutene to form glycerol tertiary butyl ethers. Chem Ing Tech 2001;73:1463–7. [8] Cheng JK, Lee C-L, Jhuang Y-T, Ward JD, Chien L. Design and control of the glycerol tertiary butyl ethers process for the utilization of a renewable resource. Ind Eng Chem Res 2011;50:12706–16. [9] Noureddini H, Dailey WR, Hunt BA. Production of ethers of glycerol from crude glycerol-the by-product of biodiesel production Papers in Biomaterials. 1998 Paper 18. . [10] Vlad E, Bildea CS, Bozga G. Integrated design and control of glycerol etherification processes. Bull Inst Pol Iasi, LVI (LX) 2010;4:139–48.
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[23] Pate R. Biofuels and the Energy-Water Nexus AAAS/SWARM April 11, 2008 Albuquerque, NM; 2008. [24] Schenk PM, Thomas-Hall SR, Stephens E, Marx UC, Mussgnug JH, Posten C, et al. Second generation biofuels: high-efficiency microalgae for biodiesel production. Bioenergy Res 2008;1:20–43. [25] Sazdanoff N. Modeling and simulation of the algae to biodiesel fuel cycle. Undergraduate thesis The Ohio State University; 2006. [26] Faith Demirbas M. Biofuels from algae for sustainable development. Appl Energy 2011;88(10):3473–80. [27] Jarvis GN, Moore ERB, Thiele JH. Formate and ethanol are the major products of glycerol fermentation produced by a Klebsiella planticola strain isolated from red deer. J Appl Microbiol 1997;83:166–74. [28] Duran MA, Grossmann IE. Simultaneous optimization and heat integration of chemical processes. AIChE J 1986;32:123–38. [29] Yee TF, Grossmann IE. Simultaneous optimization models for heat integration – II. Heat exchanger networks synthesis. Comput Chem Eng 1990;28:1165–84. [30] Sinnott R, Towler G. Chemical engineering design. Oxford: Elsevier Ltd.; 2009. [31] Ahmetovic´ E, Grossmann IE. Global superstructure optimization for the design of integrated process water networks. AIChE J 2011;57(2):434–57. [32] Franceschin G, Zamboni A, Bezzo F, Bertucco A. Ethanol from corn: a technical and economical assessment based on different scenarios. Chem Eng Res Des 2008;86(5):488–98. [33] Balat M, Balat H, Öz C. Progress in bioethanol processing. Prog Energy Combust Sci 2008;34(5):551–73. [34] Forsberg CW, Gorensek MB. Relative economic incentives for hydrogen from nuclear, renewable, and fossil energy sources. In: American institute of chemical engineers annual meeting, Salt Lake City, November 4–9, 2007. [35]. Last accessed July 2014. [36] Martín M, Grossmann IE. Energy optimization of lignocellulosic bioethanol production via gasification. AIChE J 2011;57(12). 3408, 3428. [37] Karuppiah R, Peschel A, Grossmann IE, Martín M, Martinson W, Zullo L. Energy optimization of an Ethanol Plant. AICHE J 2008;54:1499–525. [38] Martín M, Grossmann IE. Optimization of ibutene production from Switchgrass. J Biomass Bioenergy 2014;61:93–103.
Contents lists available at ScienceDirect
Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Design of an optimal process for enhanced production of bioethanol and biodiesel from algae oil via glycerol fermentation Mariano Martín a,⇑, Ignacio E. Grossmann b a b
Departamento de Ingeniería Química, Universidad de Salamanca, Pza. Caídos 1-5, 37008 Salamanca, Spain Chemical Engineering Department, Carnegie Mellon University, Pittsburgh, PA 15213, United States
h i g h l i g h t s Algae are used to produce simultaneously ethanol, biodiesel. Glycerol is a byproduct that can be further used to increase the yield to fuels. Glycerol fermentation to ethanol almost doubles its yield. The integrated facility is competitive with other uses of glycerol to produce methanol or ethers.
a r t i c l e
i n f o
Article history: Received 2 December 2013 Received in revised form 22 July 2014 Accepted 14 August 2014
Keywords: Biofuels Biodiesel Glycerol Ethanol Process integration
a b s t r a c t In this paper, we optimize a process that integrates the use of glycerol to produce ethanol via fermentation within the simultaneous production of biodiesel and bioethanol from algae. The process consists of growing the algae, determining the optimal fraction of oil vs. starch, followed by oil extraction, starch liquefaction and saccharification, to sugars, oil transesterification, for which we consider two transesterification technologies (enzymes and alkali) and the fermentation of sugars and glycerol. The advantage of this process is that the dehydration technologies are common for the products of the glucose and glycerol fermentation. Simultaneous optimization and heat integration is performed using Duran and Grossmann’s model. The fermentation of glycerol to ethanol increases the production of bioethanol by at least 50%. The energy and water consumptions are competitive with other processes that either sell the glycerol or use it to obtain methanol. However, the price for the biofuels is only competitive if glycerol cannot be sold to the market. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction The use of the byproducts from biorefineries has become very important for their profitability since first generation biofuels. In particular, the production cost of biodiesel is highly dependent on the price of glycerol. For some time, the wide range of uses of the glycerol and its limited production conferred it a reasonably high price. However, the increase in the production of biodiesel has created an excess saturating the market [1] and reducing the price of glycerol to values of $0.102/lb [2]. Based on the previous work by the authors [3,4], the production cost of biodiesel would increase $0.15/gal if the glycerol drops to this price. There are a number of alternative uses for the glycerol within the biorefinery. On the one hand we can generate syngas and later methanol so ⇑ Corresponding author. Tel.: +34 923294479. E-mail address: [email protected] (M. Martín). http://dx.doi.org/10.1016/j.apenergy.2014.08.054 0306-2619/Ó 2014 Elsevier Ltd. All rights reserved.
that we can reduce the dependency on fossil based raw materials of the current biodiesel production processes [5]. Another feasible alternative is the transformation of glycerol into fuel oxygenates by means of etherification and esterification reactions enhancing the production of diesels substitutes [6–15]. However, there is an even simpler integration option, the fermentation of glycerol to ethanol [16–19]. In this way we already have most of the technologies in place and we can use it for the production of biodiesel through ethanolysis. Furthermore, we enhance the production of bioethanol from algae [20]. Alternatively, we could use glycerol as a source of carbon for the algae growing [21]. The challenge is to integrate the fermentation of glycerol within the biodiesel production facilities. Even though we can produce ethanol out of the glycerol for the plants that involve methanolysis or ethanolysis, the residue of methanol in the glycerol may complicate the purification process. Therefore, we focus on the integration of the ethanol obtained from the fermentation of
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Nomenclature Ci
concentration of the chemical I in the reactor (mol/L) Cpi heat capacity fc (j,unit1, unit2) individual mass flow rate (kg/s) F (unit1,unit2) mass flow rate (kg/s) Pi partial pressure of component i (bar)
glycerol with the processes that simultaneously produces bioethanol and biodiesel from algae. The algae processing to bioethanol and biodiesel is based on previous papers by the authors [3,4,20]. The aim of this paper is to optimize a process that starting with algae, produces ethanol and biodiesel using the glycerol to enhance the production of ethanol. We compare the results of this study with the different integration alternatives that have been presented so far, namely, the use of glycerol to produce methanol, to produce fuel oxygenates or considering the production of glycerol as it is [5,15,20]. Therefore, the work also allows a discussion on the best use of glycerol. For the design of such an integrated plant we use mathematical programming techniques that allow us to account for the trade offs in the transesterification reaction related for instance to the excess of alcohol needed for the operation of the reactor. We divide the paper into 4 sections. First, we describe the process flowsheet. Next, we comment on the main modeling issues highlighting the glycerol fermentation to ethanol. Subsequently, we present the results and the comparison with different integration alternatives. Finally, we draw some conclusions.
2. Overall process description We divide the process into four sections, algae oil production, ethanol production from starch, biodiesel production from oil, and finally glycerol fermentation to ethanol, which will be recycled to the dehydration step. Interesting reviews on the different stages are available in the literature [22]. The first stage includes the production of biomass (oil, starch, protein), see Martín and Grossmann [3] for further details. Algae are grown by injecting CO2 into the water, which can be saline water so that the consumption of freshwater is reduced, together with air and fertilizers. The amount of water needed, the concentration of fertilizers is taken from the report by Pate [23], while the consumption of CO2 depends on the growth rate, typically 50 g/m2 d [24] and is given by the experimental results by Sazdanoff [25]. We assume that the dry algae biomass is composed of oil, up to a maximum of 60%w/w, starch and protein with a minimum of 10%w/w to be conservative [20,22,26]. Together with the algae, oxygen is produced and water is evaporated [23]. The energy consumed by the pond system is calculated based on the results by Sazdanoff [25]. Next, the algae are harvested from the pond. Recently, Univenture Inc. has presented an innovative technology capable of integrating harvesting and drying the algae with low energy consumption. It is based on the use of capillarity, membrane systems and paint drying to obtain 5% wet algae with a consumption of 40 W per 500 L/h. The biomass is mixed with cyclo-hexane and compressed so that oil is extracted and the biomass (starch and protein) is separated from the oil. On the other hand, the starch follows liquefaction (85 °C) and saccharification (65 °C) to break down the polymers into glucose. Next, the glucose is fermented into ethanol at 38 °C. The solid phase, mainly protein, is separated from the liquid phase and is sold. The liquid phase, mainly ethanol and water, but containing
T (unit1,unit2) temperature of the stream from unit 1 to unit 2 (°C) x (J,unit1,unit2) mass fraction of stream from unit 1 to unit 2 k vaporization heat (kJ/kg)
other products in small amounts such as glycerol, succinic acid, lactic acid, is distilled in a multi-effect distillation column to reduce the consumption of energy and cooling needs in the purification of ethanol. The last stage for the production of ethanol is the final dehydration using molecular sieves. This section is common for the ethanol produced from the starch, as well as for that obtained from the fermentation of glycerol. Part of this ethanol will be used in the transesterification of the oil and the rest can be sold as biofuel. Details on the process can be found in Martín and Grossmann [20]. The production of biodiesel via ethanolysis of algae oil was described in Severson et al. [4]. Two interesting catalysis were identified, enzymes and KOH. Surface response models for the reactors were developed to evaluate the trade-offs related to the operating variables at the reactor, namely, temperature, excess of ethanol, catalysis load and composition, and its effect on the yield. Next, the mixture of ethanol, glycerol and biodiesel is distilled to recover and recycle the excess of ethanol used. The polar phase containing glycerol is separated from the non polar phase containing the biodiesel, and while the biodiesel is purified in a distillation column to remove mainly the oil remaining, the glycerol is sent to etherification. The main process constraints can be seen in Table 1. Recently, it has been reported that glycerol can be fermented anaerobically to ethanol as main product using Escherichia coli [17–19,27]. Therefore, the byproduct of the synthesis of biodiesel from oil can be further converted to ethanol, increasing the liquid fuels production from algae following as main reaction Eq. (1)
C3 H8 O3 ! C2 H6 O þ H2 þ CO2
ð1Þ
The gas phase is recovered separately and the liquid phase containing the ethanol, biomass and traces of other organic chemicals is purified. This liquid phase is similar to the one that is obtained from the fermentation of glucose, and thus we can use the same purification and dehydration scheme. We mix the liquid phases from both fermentors, and after the recovery of the biomass and protein, we dehydrate the ethanol using a three effect multi-effect distillation column and molecular sieves. In Fig. 1 we present a general flowsheet for the entire superstructure of the process. 3. Mathematical modeling All the unit operations in the production process of liquid fuels and hydrogen from glycerol are modeled using surrogate models,
Table 1 Main operating constraints [3,4]. Equipment
Temperature limit
Alcohol separation column
Bottoms: < 150 °C Reflux ratio: 2–3 Top: <250 °C Bottoms: <350–375 °C Reflux ratio: 2–3 30–40 °C
Biodiesel purification column
Phase separation
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Fig. 1. Superestructure of the process.
design equations, mass and energy balances. Experimental data from the literature are used to predict the performance of the most important units such as fermentors, liquid–liquid separation, hydrolytic stages or ponds, whose operation cannot be easily modeled using first principles. The superstructure is written in terms of the total mass flows, component mass flows, component mass fractions, and temperatures of the streams in the network. The components in the system include those present in the algae, plus those produced during the process of ethanol production, and belong to the set J = {Water, EtOH, Glycerol, FAEE, FFA, Oil, Hexane, Starch, Glucose, Maltose, Protein, Succinic Acid, Acetic Acid, Lactic Acid, Urea, NH3, H2SO4, KOH, K2SO4, H3PO4, K3PO4, Algae, Biomass, CO2, O2, H2}. We describe the models below.
this figure we use supported enzymes as catalyst. We separate the glycerol phase from the biodiesel (FAEE) phase, we purify the biodiesel, and the glycerol is further fermented. This stream is mixed with that coming from the fermentation of glucose so that the ethanol obtained from the two sources is purified and dehydrated. Part of it will be used for the transesterification of the oil and thus the process does not depend on fossil fuel based raw materials (methanol), and the excess can be sold as bioethanol. The second alternative is the use of alkali catalysts. A similar flowsheet can be presented in the case of using an alkali catalyst. However, in this case the flowsheet differs in the separation of the polar and non polar phases since a washing step is required [4]. 3.3. Solution procedure
3.1. Biodiesel and ethanol production from algae For the sake of reducing the length of the paper, the models for the stages that lead to the production of biodiesel from algae oil using bioethanol are reported in previous papers [3,4,20]. The process involves, the algae growing and harvesting, the extraction of the oil using solvents, the recovery of the solvent so we can separate the biomass and the oil. At this point, the starch is liquefied, saccharified and the glucose obtained fermented into ethanol, while the oil is transesterified with the ethanol produced within the same plant. 3.2. Glycerol fermentation A solution of glycerol in water is fermented anaerobically at 38 °C with E. coli. The aim is that the concentration of ethanol in the liquid is at the most 10%. The reaction proceeds for 60 h [17–19,27]:
Glycerol ! CO2 þ H2 þ Ethanolð98%Þ
ð2Þ
CO2 þ Glycerol ! Succinic Acid þ H2 Oð1%Þ
ð3Þ
Glycerol ! 3:0075ðBiomassÞ þ H2 O ð1%Þ
ð4Þ
The liquid phase from this fermentor is mixed with that from the glucose fermentor, and after the separation of the solids and proteins, it is dehydrated using first a three effect distillation column followed by molecular sieves. In Fig. 2 we present a detailed flowsheet for the simultaneous production of bioethanol and biodiesel from algae. The starch is liquefied and saccharified to release the glucose that is fermented to ethanol. On the other hand, the oil extracted is transesterified. In
Surface response models are used for modeling the transesterification reactors, short-cut models based on experimental data from the literature, and/or models validated using process simulators are developed for the pretreatment stages, fermentations, liquid–liquid separations or the distillation columns as presented before. We have one binary variable related to the catalysts employed for the oil transesterification. In order to provide a proper comparison and not only report the optimal solution, we decompose the problem into two NLP’s, one per catalyst, each consisting of 4000 variables and 3500 equations. We simultaneously optimize and heat integrate each subproblem either for enzymatic or alkali catalyzed technologies using Duran and Grossmann’s model [28]. The objective function is a simplified production cost involving the income for the production of biodiesel ($1/kg), ethanol ($1/kg) and protein ($0.2/kg), and the energy costs and catalysts (enzymes and KOH) assuming short life cycles [3,4]. Next we design the optimal heat exchanger network using SYNHEAT [29]. A detailed cost analysis is performed involving raw material cost (oil production is part of the flowsheet), maintenance, cost of utilities and chemicals, labor, annualized equipment cost, and the cost for the management of the facility following Sinnot’s method [30]. Subsequently, we also design the water network based on the paper by Ahmetovic´ and Grossmann [31] to compare the results of water consumption with those presented in the integrated production of ethanol and biodiesel [20]. Finally, we compare the processes that involves the reuse of glycerol either for the production of methanol [5], diesel substitutes [15] and enhanced production of bioethanol presented in this paper. For the last two comparisons we focus on the most
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Fig. 2. Integrated flowsheet FAEE & enhanced ethanol.
promising catalysts, the supported enzymes, since their use avoids problems related to the presence of traces of salts in the glycerol.
Table 3 Operating conditions at the transesterification reactor: alkali catalyst (f fixed). Alone [4] (ethanol $1/gal)
Integrated Base case [20]
Enhanced ethanol (this work)
75 4f 5.7
75 4f 5.7
75 4f 5.7
0.5 1.5 –
0.5 1.5
0.5 1.5 –
4. Results and discussion The economic evaluation is carried out based on the factorial method [30] accounting for annualized equipment cost, management, labor, based on other plants, chemicals and utilities, which are updated from the literature (0.019 $/kg Steam, 0.057 $/ton cooling water [32], 0.06 $/kWh [33], 0.021 $/kg Oxygen [34], the cost of hydrogen is taken to be $1.6/kg based on DOE data, the cost of natural gas is $4 Million BTU [35]. Finally, the cost correlations for the different equipment can be found in the supplementary material of Martín and Grossmann [36] and updated to current prices. 4.1. Simultaneous production of bioethanol and biodiesel substitutes from algae: alkali catalyst In Table 2 we present the comparison between the algae composition, and the products when obtaining glycerol as byproduct [20] and when we reuse it to increase the production of ethanol. The algae growth conditions are similar, and ethanol production is increased by 70% to reach 13 MMgal/yr. Table 3 shows the main Table 2 Optimal algae growth for the simultaneous production of FAEE and enhanced ethanol using alkali catalyst. Base case [20] kg/s
This work w/w%
kg/s
EtOH Biodiesel Prot Glycerol
0.748 8.555 1.431 0.890
1.274 8.555 1.431
Algae comp Oil Star Prot
15 9 4.5 1.5
15 9 4.5 1.5
60 30 10
w/w%
Temperature (°C) Pressure (bar) ratio_et (mol ethanol/ mol oil) Time (h) Cat/lipase (%w/w) Water added (%w/w)
operating parameters of the transesterification reactor, with no difference whether biodiesel is produced alone [4], when there is simultaneous production of ethanol and biodiesel from algae, base case [20], or when we use the glycerol to increase the production of bioethanol. The reflux ratio for the ethanol recovery and biodiesel purification turns out to be 2. Finally, in Table 4 the main operating characteristics of the multieffect column used to dehydrate the ethanol are shown. These results are also similar to those presented when glycerol was a byproduct of the process. 4.2. Simultaneous production of bioethanol and biodiesel substitutes from algae. Enzymatic catalyzed For this second case study we optimize the simultaneous production of ethanol and FAEE using supported enzymes. Fig. 2 shows the process flowsheet. We optimize the operation of the transesterification reactor, the production of ethanol from the starch contained in the algae, including multieffect columns implemented in Table 4 Summary of the operating condition of the distillation multieffect columns. Alkali.
60 30 10
Column
a
b
P(LP) mmHg
IP/LP
HP/IP
Col5-7
0.084
0.238
193
2.14
2.04
Legend: LP: Low pressure, IP: Intermediate pressure, HP: High pressure, a: fraction of total feed to LP column, b: fraction of total feed to IP column.
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Table 5 Optimal algae growth for the simultaneous production of FAEE and enhanced ethanol using enzymatic catalyst. Base case [20] kg/s
This work w/w%
EtOH Biodiesel Prot Glycerol
0.866 8.353 1.430 0.869
Algae comp Oil Star Prot
15 9 4.5 1.5
kg/s
w/w%
1.296 8.350 1.445 15 9 4.5 1.5
60 30 10
60 30 10
Table 6 Operating conditions at the FAEE transesterification reactor. Enzyme catalyst (f fixed).
Temperature (°C) Pressure (bar) ratio_et (mol ethanol/ mol oil) Time (h) Cat/lipase (%) Water added
Alone [4] (ethanol $1/gal)
Integrated Base case [20]
Enhanced ethanol (this work)
45 4f 8.9
30 4f 4.1
30 4f 4.2
6.9 14.0 0.0
8.0 13.0 0.0
8.0 13.1 0.0
production of ethanol. As we can see, the optimal algae composition is the same based on the fact that the production of ethanol requires a large amount of energy in order to dehydrate the ethanol. On the one hand, part of this ethanol is needed for the transesterification of the oil. On the other hand, the solution does not suggest producing more oil, and the excess of ethanol is a good asset for the process. The ethanol production is increased by 50% for a total of around 13 MMgal/yr When it comes to the operating conditions of the main equipment such as transesterification reactor and multieffect distillation column, it turns out that the integrated process using glycerol to produce ethanol requires different operating conditions at the transesterification reactor from those required by the stand alone process. However, they are similar to those of the flowsheet that simultaneously produces bioethanol and biodiesel with glycerol as byproduct, see Tables 6 and 7. The reason is that with the reactor operating at 30 °C and being endothermic, it is easier to integrate the energy available within the system, while if the operating temperature increases, the heat integration is more difficult. The reflux ratio for the distillation columns used for ethanol recovery and FAEE purification is 2 in all cases.
4.3. Production costs
Table 7 Summary of the operating condition of the distillation multieffect columns. Enzymes as catalyst. Column
a
b
P(LP) mmHg
IP/LP
HP/IP
Col5-7
0.084
0.238
182
2.15
2.05
Legend: LP: Low pressure, IP: Intermediate pressure, HP: High pressure, a: fraction of total feed to LP column, b: fraction of total feed to IP column
In Fig. 3 we present the breakdown of the production cost of bioethanol and biodiesel from algae using glycerol to produce ethanol. Since the main raw material is the CO2, the chemicals do not play an important role compared to the annualized cost of equipment or utilities. In Table 8 we compare the results of this paper with the enhanced production of ethanol with the ones from Martín and Grossmann [20]. In that case, glycerol was considered to be sold (at $0.3/kg). We can see that the energy consumption per gallon of biofuel increases if we produce more ethanol. This is mainly due to the needs at the ethanol dehydration stage. However, freshwater consumption remains at the same level. Finally, the investment cost increases by 10–20% when we produce ethanol from glycerol due to the new equipment involved and the larger equipment needed for ethanol dehydration, resulting in $0.1/gal produc-
Fig. 3. Breakdown of the production cost of EtOH & FAEE.
the flowsheet, in contrast to the sequential approach by Karrupiah et al. [37]. Table 5 presents the optimal product distribution when glycerol is produced as byproduct [20] and when it is used for the enhanced
Fig. 4. Sensitivity analysis cost glycerol.
Table 8 Summary of results. Alkali cat
$/galbiofuel Energy (MJ/galbiofuel) Water (gal/galbiofuel) Investment (MM$) Capacity (Mgal/yr)
Enzymatic
Base Case [20]
(This work)
Base Case [20]
(This work)
0.32 6.72 0.77 175 91
0.38 7.65 0.84 198 96 (13 Mgal/yr bioethanol)
0.35 4.00 0.59 180 90
0.45 4.20 0.59 211 94(13 Mgal/yr bioethanol)
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$/galbiofuel Energy (MJ/galbiofuel) Water (gal/galbiofuel) Investment (MM$) Capacity (Mgal/yr)
Enzymatic base case [20]
Integrating MetOH [5]
Enzymatic EtOH + D &TTBG & FAEE [15]
Enhanced ethanol (this work)
0.35 4.00 0.59 180 90 (6 Bioet)
0.66 3.65 0.79 118 69
1.00 3.36 0.59 167 105 (96 biod/9bioet)
0.45 4.20 0.59 211 94 (81biod/13bioet)
tion cost higher than if we can sell the glycerol. However, as the biodiesel production increases, the market for glycerol gets saturated, and thus in the next section we evaluate the effect of the price of glycerol and its impact on the competitiveness of the process. Here we do not consider the gas from the glycerol fermentation containing hydrogen as a credit. 4.4. Sensitivity study 4.4.1. Effect of glycerol cost Fig. 4 compares the production cost of biofuels, bioethanol and biodiesel, obtained simultaneously from algae, whether we can sell the glycerol as byproduct, or if we use it to increase the production of bioethanol. As we can see, only if we cannot actually sell the glycerol, its use to produce ethanol is attractive 4.4.2. Comparison of integrating alternatives Recently the authors have evaluated the use of glycerol for the production of biodiesel and bioethanol with glycerol as byproduct [20], the production of methanol from glycerol to reduce the dependency on fossil fuels [5], the production of ethers from glycerol, enhancing the production of diesel substitutes [15], and finally the use of glycerol to increase the bioethanol production, which it is addressed in this paper. Table 9 shows the main results from all these processes. The advantage of using ethanol and not methanol is that it is produced within the biorefinery complex, reducing the dependency on fossil based raw materials since methanol is mainly produced from natural gas. Thus, the simultaneous production of ethanol and biodiesel is promising in terms of production cost and water consumption, but requires larger investment and energy consumption per gal of biofuel produced. The production capacity is interesting since we can increase it by around 10 MMgal/yr of biofuels with respect to the production of biodiesel alone. The use of glycerol to produce methanol, see column 2 in Table 9, reduces the dependency on fossil fuels up to 70%, but we cannot produce the amount of methanol that we need to run the plant. Furthermore, the production cost increases since the biofuel production capacity remains constant, while glycerol reforming, gas purification, synthesis and methanol purification stages are added, and glycerol is no longer sold. The largest biofuel capacity increment is found when we use the glycerol to produce ethers. The comparison is not that straightforward since we add i-butene as raw material. The result is that the production cost increases because i-butene is expensive. Recently, Martín and Grossmann [38] have shown that it is possible to produce i-butene from lignocellulosic raw materials at a competitive price. However, the process is still under development. The advantages of this process, see column 3 in Table 9, is that we maintain the energy and water consumption at low levels compared to any other due to the increased biofuel production capacity and the increased investment due to the separation stages for the glycerol ethers. Finally, the use of glycerol to produce ethanol, the process described in this paper whose results are summarized in column 4, Table 9, shows an increased production capacity of biofuels with the second lowest production price. The drawbacks of this process are related to
the increased investment and energy consumption, due to the dehydration steps of ethanol.
5. Conclusions In this paper, we have optimized the integrated production of bioethanol and biodiesel from algae by using the glycerol to enhance the production of bioethanol. This process shares the dehydration technologies and results in better process, heat and product integration. We consider two technologies for the transesterification of the oil, alkali and supported enzymes, and the fermentation of glycerol to ethanol. Out of the two, the cheapest alternative uses alkali. However, it does not correspond with the one that consumes less energy and water due to the catalysis cost. The flexibility to the oil composition and the slight difference in the production cost reveals the use of enzymes as the most promising out of the two. Finally, we compare different uses of glycerol to help in the economy by selling it of the biofuels or the increase the biofuels production capacity, the production of methanol and its use as transesterification alcohol. A number of interesting trade-offs have been identified in terms of production cost, energy and water consumption and biofuels total production capacity. Acknowledgments The authors gratefully acknowledge the NSF Grant CBET0966524 and the Center for Advanced Process Decision-making at Carnegie Mellon University. Dr. Mariano Martín acknowledges Salamanca Research for Software licenses. References [1] Pagliaro M, Rossi M. Future of glycerol. 2nd ed. Cambridge: The Royal Society of Chemistry; 2010. [2] Ahmed S, Papalias D. Hydrogen from glycerol: a feasibility study. In: Presented at the 2010 hydrogen program annual merit review meeting Washington DC, June 8, 2010.
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