Article pubs.acs.org/IECR
Integrated Synthesis of Biodiesel, Bioethanol, Isobutene, and Glycerol Ethers from Algae Verónica de la Cruz,† Sara Hernández,† Mariano Martín,*,† and Ignacio E. Grossmann‡ †
Department of Chemical Engineering, University of Salamanca, Pza. Caídos 1-5, 37008 Salamanca, Spain Department of Chemical Engineering, Carnegie Mellon University. Pittsburgh, Pennsylvania 15213, United States
‡
ABSTRACT: In this paper, we design an integrated process for the production of diesel substitutes, biodiesel and glycerol ethers, from algae with internal production of the intermediates ethanol and isobutene. The starch from the algae is converted into glucose. Part of it is fermented to ethanol so that we produce the alcohol needed to transesterify the algae oil while the rest is fermented to isobutene, which is needed in the production of glycerol ethers to enhance biofuels production capacity. We use CHEMCAD coupled with MATLAB for the rigorous simulation of the integrated biorefinery and evaluate the algae composition for such integrated facility. This is followed by energy integration using SYNHEAT and a detailed economic analysis. The integrated facility has promising production cost for liquid fuels, 0.46 $/gal, and an investment costs of $205M, almost half the production cost and only and investment $40M above the plant that buys the isobutene from the market.
1. INTRODUCTION For quite some time biodiesel facilities have been designed for the production of this particular biofuel from cooking oil, algae, or vegetable oil. Even though they are regarded as biorefineries, they actually are focused on the production of one main product, biodiesel, and some byproducts (i.e., fertilizers, glycerol) while some raw materials came from fossil resources, for instance, methanol. Even though the use of methanol was traditionally supported by its lower price and faster reaction times, Severson et al.1 proved that it is economically competitive to produce biodiesel using ethanol. Furthermore, Martı ́n and Grossmann2 integrated the production of bioethanol and biodiesel from algae oil resulting in competitive production costs compared to those resulting from the use of methanol. With the current need for sustainable liquid fuels, a number of alternatives have been presented to increase the production of biofuels using the byproducts of the current biorefineries. For instance, the production of biodiesel generates glycerol. The high price as a result of its use in the cosmetic and food industries has been an asset for the production of biodiesel and it has improved the economics of such plants. However, the increase in the production of biodiesel is saturating the market, and new uses for glycerol are being developed to help reduce our dependence on fossil fuels. Recently, Cheng et al.3 developed a process for the use of glycerol to produce oxygenated derivates, glycerol ethers. The advantage is that they can also be used as diesel substitutes increasing the yield to biofuels.4 The drawback is that, apart from glycerol, the second raw material is isobutene, an expensive C4 chemical typically obtained in the fractionation of crude. However, van Leeuwen5 proved that it is possible to produce isobutene from sugars and recently Martı ́n and Grossmann6 found that its production from biomass containing glucose and xylose is economically promising. Algae are a particularly rich raw material whose composition consists mainly of lipids, starch, and protein. Therefore, from algae we have all the components to design an integrated facility that produces ethanol and isobutene from starch as inter© XXXX American Chemical Society
mediates, oil from the lipids that is transesterified with ethanol producing biodiesel (Fatty acid ethyl ester, FAEE), while the glycerol, byproduct of the transesterification reaction, reacts with isobutene producing glycerol ethers which are diesel substitutes. Since isobutene and bioethanol are produced from the starchy biomass within the algae, the composition of the algae may need to be adjusted so that we can self-sufficiently produce the intermediates that are needed for the production of biodiesel and glycerol ethers avoiding the need for fossil based raw materials. Thus, in this paper, we present the conceptual design of an integrated biorefinery from algae, which produces diesel substitutes, biodiesel (FAEE), and glycerol ethers, but which can also produce isobutene and ethanol, for internal use. We use a hybrid approach employing modular process simulators such as CHEMCAD coupled with equation based software, MATLAB. After the rigorous simulation, we design a heat exchanger network and perform an economic evaluation of the operation of such facility. The paper is organized as follows. Section 2 presents the description of the different sections of the process and the main operating conditions and constraints. Section 3 describes the modeling approach using CHEMCAD and MATLAB. Section 4 presents results from the material and energy balances, water consumption, and the economic evaluation. Finally, in section 5, we draw some conclusions.
2. OVERALL PROCESS DESCRIPTION As seen in Figure 1, we divide the process in five sections: algae oil production, ethanol production from starch, isobutene production from starch, biodiesel production from oil, and finally high glycerol ethers (di- and tri-tert-butyl glycerol: DTBG + TTBG = hTBG) production from glycerol. Received: June 5, 2014 Revised: August 13, 2014 Accepted: August 13, 2014
A
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Figure 1. Flowsheet for the self-sufficient biorefinery out of algae.
Figure 2. Algae growth.
2.1. Algae Oil Production. As see in Figure 2, the production of oil and starch from algae (see also the supplementary material of Martı ń and Grossmann7) is performed by injecting CO2 into saline or wastewater, so that the consumption of freshwater is reduced, together with air and fertilizers. The amount of water needed, 0.006 kg lost by evaporation per kg of biomass, and the concentration of fertilizers, 0.14 kg per kg of dry biomass, are taken from the report by Pate,8 while the consumption of CO2 depends on the growth rate, typically 50g/m2 d,9 and it is given by the experimental results by Sazdanoff.10 We assume that the dry algae biomass is composed of lipids, starch and protein. Together with the algae, oxygen is produced and water is evaporated.8 The energy consumed by the pond system is calculated based on the results by Sazdanoff, 0.1 kW/1000 m2 pond.10 Next, the algae are harvested from the pond. This stage has typically been the most energy intensive. However, 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 based membrane systems and paint drying to obtain 5% wet algae with a consumption of 40 W for 500 L/h of flow. The biomass is mixed with cyclo-hexane on a 1:1 molar
ratio and compressed so that oil is extracted and the biomass is separated from the oil. The biomass can be used to obtain energy for the system,7 or it can also be further treated to obtain ethanol and isobutene from the glucose that can be produced from the starch, as seen below. The oil is used for the production of biodiesel. Algae composition is a key variable. We can control the amount of carbohydrates and lipids by modifying the algae growth. The starch can reach up to 45% depending on the growing conditions and the strain and the oil up to 75%.2,11,12 We consider that the algae consists of the following species:13,14 Protein: C3H5NO
Carbohydrates:
C6H10O5 Lipids: C57H104O6
We model the algae growth, for 49% lipids, 41% starch, and 10% protein as given by eq 1 for the self-production of B
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intermediates, bioethanol and isobutene, and maximum yield to biofuels. Later in section 4, we present some sensitivity studies on the yield to different products as a function of the algae composition. The conversion is 100% using 20% excess of CO2. We assume that the sunlight provides the energy for such a reaction
1. Glucose to ethanol: C6H12O6 ⎯⎯⎯⎯→ 2C2H6O + 2CO2 ΔH = − 84.394 kJ/mol 2. Glucose to glycerol:
3. Glucose to succinic acid: yeast C6H12O6 + 2CO2 ⎯⎯⎯⎯→ 2C4 H6O4 + O2
0.76380539CO2 + 0.64215947H 2O + 0.02110343NH3 → 0.008291138C57H104O6 + 0.037983375C6H10O5 + 0.02110343C3H5NO + 0.95450156O2
(1)
C6H12O6 ⎯⎯⎯⎯→ 3C2H4O2
5. Glucose to lactic acid:
C6H12O6 ⎯⎯⎯⎯→ 2C3H6O3
yeast
(4)
The solid phase, mainly protein, is separated from the liquid phase and is sold. The liquid phase, mainly ethanol and water but containing other products in small amounts such as glycerol, succinic acid, lactic acid, is distilled in a three effect distillation column to reduce the consumption of heating and cooling in the purification of ethanol. We need to determine the fraction of the feed fed to each of the columns and the operating temperatures so that the lower pressure column reboiler acts as intermediate pressure column condenser and the same for the intermediate and higher pressure columns. The last stage for the production of ethanol is the final dehydration using molecular sieves where air with 70% humidity is used to regenerate the zeolite. We allow the hot air at 95 °C leaving the zeolites to have 70% relative humidity. Part of this ethanol is used in the transesterification of the oil and the rest can be sold as biofuel. 2.3. Biodiesel Production. According to Severson et al.,1 one can competitively use ethanol to transesterify the oil extracted from the algae. In Figure 5, we present the detailed flowsheet for this section of the process. The results presented in that paper showed that the use of enzymes as catalysts is promising in the sense that the process consumes less energy and water than the one using KOH as catalysts. However, its disadvantage is the high cost of the enzymes. The operating conditions are taken from Martı ́n and Grossmann,2 see Table 2, to achieve a conversion of around 96% in the reaction given by eq 5.
α‐amylase
(2)
glucoamylase
C12H 22O11 + H 2O ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ 2C6H12O6
yeast
4. Glucose to acetic acid:
6. Glucose to cell mass: yeast C6H12O6 + 1.2NH3 ⎯⎯⎯⎯→ 6CH1.8O0.5N0.2 + 2.4H 20 + 0.3O2
2.2. Ethanol Production from Starch. As presented in previous papers,2,6 it is possible to obtain bioethanol and isobutene from the starch obtained from the algae. For that, the starch has to be processed similarly as in the production of bioethanol from corn. The starch is first liquefied at 90 °C followed by saccharification at 65 °C with residence times of 30 min each so that the polymers are broken into glucose. The amount of α-amylase to be added is 0.005% w/w of the biomass, while the required glucoamylase is 0.1% w/w of the incoming mash.15 We assume 99% conversion at each stage. See Figure 3. 2(C6H10O5)n + nH 2O ⎯⎯⎯⎯⎯⎯⎯⎯⎯→ nC12H 22O11
yeast
C6H12O6 + 2H 20 ⎯⎯⎯⎯→ 2C3H8O3 + O2
(3)
Figure 3. Glucose production.
Next, as seen in Figure 4, part of the glucose is fermented into ethanol at 38 °C. However, there are a number of secondary reactions taking place, as given by eq 4. Table 1 shows the corresponding conversions.
C57H104O6 + 3C2H5OH → 3C20H38O2 + C3H8O3
(5)
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 nonpolar phase
Figure 4. Ethanol Production. C
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Table 1. Chemical Reactions in Fermentor16 reaction
conversion
yeast
0.92
glucose ⎯⎯⎯⎯→ 2 ethanol + 2 carbon dioxide yeast
0.034
yeast
0.01
C6H12O6 + 2H 20 ⎯⎯⎯⎯→ 2C3H8O3 + O2 C6H12O6 + 2CO2 ⎯⎯⎯⎯→ 2C4 H6O4 + O2 yeast
0.002
yeast
0.0024
glucose ⎯⎯⎯⎯→ 2 lactic acid glucose ⎯⎯⎯⎯→ 3 acetic acid yeast
0.0316
glucose + 1.2 ammonia ⎯⎯⎯⎯→ 6 cell mass + 2.4 water + 0.3 oxygen
Figure 5. Biodiesel production.
Table 2. Operating Conditions for Biodiesel Production2
solubility in water is lost. The main reaction is given by eq 6 where we assume a conversion of 85%, 25 g/g vs the theoretical 31 g/g.5
enzymatic temp. (°C) pressure (bar) ratio_et time (h) cat/lipase (%) water added
30 4 4.1 8.0 13.0 0.0
yeast
C6H12O6 ⎯⎯⎯⎯→ C4 H8 + 2CO2 + 2H 2O + 2ATP ΔH = − 41.9 kJ/mol
The gas phase consists of isobutene together with CO2 and moisture. First, we condense the water accompanying the gas phase, and then, a membrane separation separates the CO2 from the isobutene.6 2.5. Biodiesel Complements: Ethers from Glycerol. For the production of the ethers of glycerol we adopt part of the solution reported by Cheng et al.3 The flowsheet can be seen in Figure 7. The etherification consists of three equilibrium reactions between the mono-, di-, and tri-tert-butyl glycerol (MTBG, DTBG, and TTBG), given by eq 7. We select the operating conditions at the reactor, 90 °C and 20 bar, based on the fact that the conversion to di- and tri-glycerol ethers is highest at 90 °C, taking into account that we cool down the feed to the reactor.
containing the biodiesel. The biodiesel is purified in a distillation column to remove mainly the remaining oil, the glycerol is sent to etherification. The main process constraints to achieve good separation of the liquid−liquid mixture and product recovery and to avoid species decomposition can be seen in Table 3. Table 3. Main Operating Constraints1,2,14,17 equipment alcohol separation column biodiesel purification column phase separation
(6)
temp. limit bottoms ≤ 175 °C; reflux ratio ≤ 2 top ≤ 250 °C; bottoms ≤ 350−375 °C; reflux ratio ≤ 2 30−40 °C
2.4. Isobutene Production. The rest of the glucose is fermented to produce isobutene using S. cerevisiae. See Figure 6 for the detailed flowsheet of this section. To be on the safe side, we assume that the fermentation process is performed with 100 g·L−1 fermentable carbohydrate.5 This value affects the water consumption of the plant, even though for the production of isobutene is not crucial since the main product is recovered from the gas phase. Most of the isobutene is recovered and only its
k1
glycerol + i‐butene XooY MTBG k −1
k2
MTBG + i‐butene XooY DTBG k −2 k3
DTBG + i‐butene XooY TTBG k −3
D
(7)
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Figure 6. Isobutene production.
Figure 7. Ethers production.
(pTS). The kinetic model parameters with Arrhenius form are given in Table 4.
In order to compute the outlet concentrations, we use the kinetics of the reactions as given by eq 8 dCGlycerol
Table 4. Behr and Obendorf Kinetic Data17,18
= −k1CGlycerolC ibutene + k −1CMTBG
dt dCMTBG = k1CGlycerolC ibutene − k −1CMTBG − k 2CMTBG dt C ibutene + k −2C DTBG dC DTBG = k 2CMTBGC ibutene − k −2C DTBG − k 3C DTGB dt C ibutene + k −3C TTBG
collision factor
(min−1 mol−1)
activation energies
(kJ/mol)
k1 k−1 k2 k−2 k3 k−3
3.04 × 108 3.69 × 1013 1.70 × 1011 8.54 × 1014 2.26 × 1010 6.35 × 1015
E1 E−1 E2 E−2 E3 E−3
74.04 111.78 92.80 118.06 92.56 125.13
After the etherification reaction, a liquid−liquid separation stage is used at around 90 °C that uses glycerol as solvent. The phase containing mainly glycerol and mono ether is recycled back to the reactor, while the other one containing the di- and triethers together with the isobutylene is separated.19 The isobutene is separated in a stripping column and the bottoms of the column are the desired product.
dC TTBG = k 3C DTGBC ibutene − k −3C TTBG dt dC ibutene = −k1CGlycerolC ibutene + k −1CMTBG dt − k 2CMTBGC ibutene + k −2C DTBG − k 3C DTGBC ibutene + k −3C TTBG
3. PROCESS MODELING We use CHEMCAD as framework for modeling the process. However, there are a number of challenges related to the limitations of the package, such as the absence or certain species
(8)
The model parameters are taken from Behr and Obendorf17,18 for glycerol etherification catalyzed by p-toluenesulfonic acid E
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Table 5. Definition of Main Compounds bp (°C)
groups
correlation
FAEE
species
C20H38O2
formula
MW (g/mol) 310
330.86
(CH3)2 (>CH2)15 (CH)2 COO
group contribution− UNIFAC
starch protein maltose cell mass MTBG
(C6H10O5)n (C3H5NO)m C12H22O11 CH1.8O0.5 C7H16O3
162.1436 71.0785 342.3 24.625 148.2
150 200 102 200 264.65
DTBG
C11H24O3
204.31
240.45
TTBG
C15H32O3
260.41
253.05
pseudocomponent pseudocomponent pseudocomponent pseudocomponent group contribution− Joback
(−CH3)3 (>CH2)2 CH >C< (OH)2 O (CH3)6 (>CH2)2 CH (>CCH2)2 CH (>C
