ARTICLE pubs.acs.org/EF
Energy Requirements for Butanol Recovery Using the Flash Fermentation Technology Adriano P. Mariano,*,†,‡ Mohammad J. Keshtkar,† Daniel I. P. Atala,§ Francisco Maugeri Filho,|| Maria Regina Wolf Maciel,‡ Rubens Maciel Filho,‡ and Paul Stuart† cole Polytechnique de Montreal, NSERC Environmental Design Engineering Chair, Department of Chemical Engineering, E Montreal, QC, Canada ‡ Laboratory of Optimization, Design and Advanced Control (LOPCA), School of Chemical Engineering, University of Campinas (UNICAMP), Campinas, SP, Brazil § Center of Sugar Cane Technology (CTC), Piracicaba, SP, Brazil Laboratory of Bioprocess Engineering, School of Food Engineering, University of Campinas (UNICAMP), Campinas, SP, Brazil
)
†
ABSTRACT: Acetonebutanolethanol (ABE) facilities have traditionally presented unattractive economics because of the large energy consumption during recovery of the products from a dilute fermentation broth (∼13 g/L butanol). This problem results from the high toxicity of butanol to microorganisms that catalyze its production. Flash fermentation is a continuous fermentation system with integrated product recovery. The bioreactor is operated at atmospheric pressure and the broth is circulated in a closed loop to a vacuum chamber where ABE is continuously boiled off at 37 °C and condensed afterward. With this technology the beer achieved a concentration of butanol as high as 3037 g/L. This paper studies the energy requirements for butanol recovery using the flash fermentation technology and its effect on the energy consumption by the downstream distillation system. Compressors are used to remove the vapors from the flash tank, thus maintaining the desired vacuum. The heat recovery technique of vapor recompression is used to reduce energy requirements. With this technique the heat generated by the compression and partial condensation of the vapors provides the energy for boil up (heat of vaporization) in the flash tank. Thus the energy requirement for the flash fermentation is essentially the electrical power demanded by compressors. Energy for recirculation pumps accounts for approximately 0.5% of the total energy consumption. Small increments in butanol concentration in the beer can have important positive impacts on the energy consumption of the distillation unit. Nonetheless, the energy use of the recovery technology must be included in the energy balance. For a fermentation with a wild-type strain, the total energy requirement for butanol recovery (flash fermentation þ distillation) was 17.0 MJ/kg butanol, with 36% of this value demanded by the flash fermentation. This represents a reduction of 39% in the energy for butanol recovery in relation to the conventional batch process. In the case of a fermentation with a hyper-butanol producing mutant strain, the use of the flash fermentation could reduce the energy consumption for butanol recovery by 16.8% in relation to a batch fermentation with the same mutant strain.
1. INTRODUCTION Butanol is highly toxic to microorganisms that catalyze its production, and for this reason less than 13 g/L of butanol is produced during batch fermentation. For the sake of comparison, in the ethanol fermentation the yeast cells tolerance to ethanol is approximately ten times greater. Therefore, typical acetone butanolethanol (ABE) fermentation has been plagued by the use of dilute sugar solutions as substrates, large process volumes, high downstream process costs due to intensive energy requirements for recovery of low concentrations of ABE in the beer, and large quantities of wastewater. A solution to these problems can be addressed by using genetic engineering techniques to develop strains that could tolerate higher concentrations of butanol and sugar.1,2 Another option is the use of technologies designed to remove butanol continuously from the fermentation broth (integrated recovery technologies). The product recovery reduces the effect of product inhibition and allows an increase in the substrate concentration which results in a reduction in process streams, and higher productivity. With either approach (biological and r 2011 American Chemical Society
technological) the energy efficiency of the process is expected to be enhanced. Product-recovery technologies are designed to remove ABE from the bioreactor liquid as the fermentation is ongoing and the ABE-depleted stream is returned to the bioreactor or, in the case of an in situ products removal technology, the fermentation broth never leaves the bioreactor. Product-removal techniques include gas stripping, liquidliquid extraction, membrane-based methods (pervaporation, perstraction), and adsorption. All these techniques have advantages and disadvantages in terms of capacity, selectivity, fouling, clogging, scale-up, operational simplicity, and energy requirement. A comparison among some of them can be found elsewhere.1,38 As a commodity, butanol production cost is mainly affected by feedstock price. Economic modeling studies show that feedstock price (sugar cane juice in Brazil) could account for 60% of the Received: February 22, 2011 Revised: April 12, 2011 Published: April 12, 2011 2347
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Figure 1. Flash fermentation technology.
total variable operating cost. The remaining value is made up of utilities costs.9 Thus the overall energy consumption during recovery is also an important operating cost factor. For this reason, during the selection of an integrated recovery technology, the energy consumption must be a key decision factor and can be used as a ranking parameter.7 In our previous works we demonstrated the technical feasibility of the flash fermentation technology to recover ABE from the bioreactor.10,11 This integrated ABE fermentation involves operating the bioreactor at atmospheric pressure while the broth is circulated in a closed loop to a vacuum chamber where ABE is continuously boiled off. With this technology, a significant improvement of productivity was observed, and the beer achieved a concentration of butanol as high as approximately 30 g/L. These good results prompted us to further investigate this technology as to its energy requirements and its effect on the energy consumption by the downstream distillation system. These studies are presented in the next sections.
2. METHODOLOGY The principles of the flash fermentation process are presented in the scheme shown in Figure 1. The fermentation broth of a continuous fermentor circulates through a vacuum flash tank. The partial vaporization of water and fermentation products that takes place in the flash tank generates a vapor phase rich in solvents (ABE) and an ABE-depleted liquid stream. The vapor is subsequently condensed and combined with the beer stream and then sent to distillation. The ABE-depleted liquid stream returns to the fermentor, which is operated at atmospheric pressure. Temperature in the fermentor and in the flash tank is set to 37 °C. Vacuum is regulated in order to keep constant the amount of broth vaporized.
In the flash tank, fermentation broth boils at low pressure (6.46.8 kPa) and an equilibrium mixture of fermentation products (ABE, organic acids, and CO2 and H2) and water is taken overhead. Compressors remove these vapors, thus maintaining the desired vacuum. The heat recovery technique of vapor recompression is used to reduce energy requirements12 (Figure 2). In this integrated system, the first compressor compresses the vapor to 0.3 atm. At this pressure the vapor can be passed through the shell side of a shell-and-tube heat exchanger and heat is exchanged with the fermentation broth present in the flash tank. The flow rate of the fermentation broth through the heat exchanger is set in order to allow temperature a maximum increase of 0.5 °C. This restriction is necessary due to the strong sensibility of the microorganism activity to temperature variations. Most of the vapor (∼80%, mass basis) condenses in the heat exchanger, providing the energy for boil up (heat of vaporization) in the flash tank. The condensed fraction is pumped at low energy cost to atmospheric pressure and sent to distillation. The remaining vapor is compressed to 1.5 atm in a second compressor and a subsequently partial (∼30%, mass basis) condensation suffices to meet the total reboiler duty of one of the distillation columns (acetone column). Afterward, the remaining vapor stream is completely condensed by heating the beer stream from the fermentor. Noncondensables (CO2 and H2) are sent to an absorber column in order to recover carried ABE. The pressure in the first compression stage (0.3 atm) is the minimum value demanded by the heat exchanger of the flash tank to meet its heat duty requirement. Pressure drop in each heat exchanger is assumed to be 0.14 atm (2 psia). Compressors isentropic efficiency is considered to be 0.72. Until the shutdown of the commercial butanol fermentation facilities in the 1980s, the separation of the fermentation 2348
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Figure 2. Vapor recompression heating system. Heat integration between the recompressed ABE-enriched vapor stream from the flash tank and the reboiler of the acetone column, and fermentation beer stream (stream conditions refers to scenario 2, Table 1).
products (ABE) was carried out in a series of five continuous distillation columns, with the last two responsible for the separation of butanol from water.13 Therefore, this design was adopted in the present study (Figure 3). In addition to the proven utilization of this system in the past, distillation is a mature technology and widely applied in the biofuel industry, and accepted in terms of scale-up and operability.6 Solvents (ABE) are removed from the beer in the beer column (45 stages; feed in stage 1; stages are numbered from the top down) after being heated to 112 °C. The stillage contains most of the broth water content, acetic and butyric acid, cells, and remaining sugars. The overhead vapor from the beer column is separated in a series of four distillation columns operated under different pressures in order to allow heat integration. In the top of the acetone column (30 stages; feed in stage 15), 99.5 wt % acetone is obtained and the fermentation gases (CO2 and H2) are sent to an absorber column (10 stages; feed in stage 10) in order to minimize the losses of ABE in the distillation unit. The acetone column is operated at 0.7 atm so that low pressure steam can be used in the reboiler (it should be noted that for a process with the flash fermentation, the total reboiler duty of the acetone column can be met by partial condensation of the recompressed ABEenriched vapor stream from the flash tank). The bottom stream of the acetone column is fed to the ethanol column (40 stages; feed in stage 10), which operates at 0.3 atm. Vacuum operation reduces the reflux needed to produce the 85 wt % ethanol overhead product and allows the total reboiler duty to be met by condensing the overhead vapors from the beer column in the ethanol column reboiler. A two-column distillation system in conjunction with a decanter is used to separate the heterogeneous
binary butanol/water azeotrope. The bottom stream of the ethanol column is added to a decanter in conjunction with the top streams from the water and butanol columns. In the decanter, butanol phase separates from the aqueous phase and rises to form an upper layer. The water-rich phase is refluxed to the water column (10 stages; feed in stage 1), whose bottom (water) contains less than 0.05 wt % butanol. The butanol-rich phase is refluxed to the butanol column (10 stages; feed in stage 1), which produces a 99.5 wt % butanol product. The water and butanol columns and the decanter are operated at atmospheric pressure. Solvent loss for the gas stream accounts for 1.7, 0.2, and 1.4% of the total amount of acetone, butanol, and ethanol, respectively. By employing vapor recompression heating, the energy requirement for the flash fermentation is essentially the electrical power demanded by the compressors. Energy for recirculation pumps accounts for approximately 0.5% of the total energy consumption. The energy for vaporization is obtained from the heat generated during compression of the vapors. Thus, it is very intuitive that the greater the amount of fermentation broth vaporized in the flash tank, the greater the energy consumption necessary to compress the vapors. Having this in mind, the first step in the evaluation of the energy requirements was to determine the effects of operating variables related to the flash tank (inlet flow rate of the flash tank and pressure) on the amount of fermentation broth vaporized in the flash tank. In the same manner, the effects on the concentrations in the fermentor were also analyzed. These studies were carried out using a mathematical model that simulates the fermentation and the vaporization in the flash tank. In the computational simulation, fermentation starts up in batch mode (500 m3 fermentation volume) with initial sugar 2349
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Figure 3. Downstream distillation unit (stream conditions refers to scenario 2, Table 1).
concentration of 60 g/L and after 17 h (the time butanol concentration in the fermentor achieves 6 g/L) the continuous feed of medium (50 m3/h) and the recirculation of the broth through the flash tank are initiated (inlet flow rate of the flash tank varying between 200 and 350 m3/h). Equations 13 were used to determine the concentrations of biomass, substrate, and products in the fermentor: dX F PU Fr ¼ rX X þ Xr dt V V
ð1Þ
dS F PU Fr F0 ¼ rS S þ Sr þ S0 dt V V V
ð2Þ
dPi F PU Fr ¼ r Pi Pi þ Pr;i dt V V
ð3Þ
where i stands for butanol, acetone, ethanol, butyric acid, and acetic acid.
The kinetic models (rx, rs, and rpi) were experimentally determined by Mulchandani and Volesky14 based on the following assumptions: (1) carbon substrate (glucose) limitation only; (2) no nitrogen and nutrient limitation; (3) product inhibition; (4) acetic and butyric acid are intermediate metabolites and are reduced to acetone and butanol, respectively; (5) acetone and butanol are also synthesized directly from carbon substrate; (6) ethanol is synthesized from carbon substrate only; (7) fermentation is performed at optimal temperature of 37 °C and optimal pH of 4.5 under anaerobic conditions; (8) all the cells (Clostridium acetobutylicum ATCC824) are metabolically active and viable. Integration of eq 1 to 3 was carried out by the fourth order RungeKutta method using a Fortran code. The modeling of the flash tank was based on the isothermal and isobaric evaporation model15 and a multicomponent system (water, butanol, acetone, ethanol, acetic acid, and butyric acid) was considered. Saturation pressures were calculated by Antoine’s 2350
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Table 1. Scenarios Considered for the Energy Consumption Studies scenarios I
III
IV
V
VI
S0 (g/L)
100
150
150
170
170
150
fermentation volume (m3)
500
500
750
500
750
750
a
a
a
a
a
b
strain a
II
C. acetobutylicum ATCC824. b C. beijerinckii BA101.
equation and activity coefficients were calculated by the UNIQUAC model. Six scenarios were set in order to assess the energy requirements for the flash fermentation and the subsequent distillation unit. They differ from each other in terms of substrate concentration, fermentation volume (dilution rate), and microbial strain (Table 1). For each scenario, the necessary vacuum in the flash tank able to balance the trade-off between minimum vaporization of fermentation broth and high sugar conversion (98%) was initially determined using the above-described mathematical model (eqs 13). In the simulated fermentations, the fresh medium fed to the fermentor had a flow rate of 50 m3/h and the inlet flow rate of the flash tank was 300 m3/h. For scenario VI another microbial strain was considered. C. beijerinckii BA101 is a hyper-butanol producing mutant strain able to produce 25.3 g/L ABE (19.7 g/L butanol, 5.0 g/L acetone, 0.6 g/L ethanol).16 To obtain this same level of solvents production, the values of the parameters k7, k10, k15, and k11 of the kinetic model were changed to 0.65, 0.02, 1.9, and 0.045, respectively (the reader is referred to Mulchandani and Volesky14 for identification of parameters). These alterations were necessary because the kinetic model originally describes the typical ABE production level of a wildtype strain (11 g/L butanol, 5.0 g/L acetone, 1.5 g/L ethanol). For each scenario shown in Table 1, the energy requirements (energy for compressors and reboilers of distillation columns) were calculated by process modeling in Aspen Plus 7.1. The complete process represented in Figures 13 was simulated in Aspen Plus incorporating the results (operating conditions and steady-state concentrations) obtained with the simulations of the fermentation in Fortran (eqs 13 and flash calculation). For this, a stoichiometric reactor model (RStoic) was used to simulate the fermentor in Aspen Plus, considering the following reaction equations: C6 H12 O6 þ H2 O f C3 H6 O ðacetoneÞ þ 3CO2 þ 4H2 ð4Þ C6 H12 O6 f C4 H10 O ðbutanolÞ þ 2CO2 þ H2 O
ð5Þ
C6 H12 O6 f 2C2 H6 O ðethanolÞ þ 2CO2
ð6Þ
C6 H12 O6 f C4 H8 O2 ðbutyric acidÞ þ 2CO2 þ 2H2
ð7Þ
C6 H12 O6 þ 2H2 O f 2C2 H4 O2 ðacetc acidÞ þ 2CO2 þ 4H2 ð8Þ C6 H12 O6 þ 1:1429NH3 f 5:7134CH1:8 O0:5 N0:2 ðbiomassÞ þ 0:2857CO2 þ 2:5714H2 O ð9Þ For each reaction equation, fractional conversions of glucose were assigned. In this manner, the same values of consumed
Figure 4. Vaporliquid equilibrium (VLE) of n-butanol/water binary mixture at atmospheric pressure (T-xy diagram) and at 50.07 °C (P-xy diagram). Data (lines) obtained from Aspen Plus simulator and validated by experimental data (circles and squares). ASPEN 1 (black lines): default UNIQUAC parameters available in the simulator. ASPEN 2 (gray lines): UNIQUAC parameters reported by Fisher and Gmehling.20
substrate and produced solvents and biomass previously determined from eqs 13 were obtained in the Aspen Plus simulations. This procedure enabled the simplified reactor model used in the Aspen simulator (eqs 49) to reproduce the steady state concentrations values generated by a mathematical model that incorporates a sophisticated experimental kinetic model (eqs 13), including a nonlinear product inhibition model. The input data for the flash calculation in Aspen Plus were temperature (37 °C) and vapor fraction. The latter was obtained from the Fortran calculations. Properties for biomass (CH1.8O0.5N0.2) were obtained from the NREL database.17 The energy demanded by the equipment (MJ/h) was divided by the mass flow rate (kg/h) of butanol at the bottom of the butanol column to get the specific energy requirement (MJ/kg butanol) for butanol purification. Design specifications and operating conditions used in the simulation of the distillation unit were based on the optimum configurations determined by Luyben18 and van der Merwe.19 The final setup used here aimed the conciliation between capital (minimum number of trays) and energy (minimum reflux ratio) costs. For this, design specifications were fed to the simulator in terms of recovery or purity for a specific compound in either the top or bottom sections. The design spec/vary feature of Aspen 2351
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Figure 5. Effects of the inlet flow rate of the flash tank (Fc) on fermentation parameters determined in a simulated fermentation with the following operating conditions: F0 = 50 m3/h, S0 = 150 g/L, V = 500 m3, Pflash = 6.516.56 kPa, and amount of fermentation broth vaporized in the flash tank equal to 5% of Fc.
Plus was used to hold these specifications by varying the distillate rate and reflux ratio. Number of stages and feed location were fed to the simulator and manually optimized to obtain better values for reboiler duty.18,19 Columns were simulated using the RadFrac model available in the Aspen Plus simulator and it was assumed that column trays have a uniform Murphree efficiency of 0.6. Thermodynamic properties of the system were determined by the UNIQUAC activity coefficient model for the liquid phase and the Hayden-O’Connell model for the vapor phase. Having in mind that the accuracy of the amount of butanol recovered in the flash tank is given by a good representation of the thermodynamics characteristics of the vaporliquid equilibrium of the n-butanol/water system, validation of the equilibrium calculations was carried out using experimental data available in the literature. Using the default UNIQUAC parameters available in the Aspen Plus simulator, vaporliquid equilibrium data at 1 atm (T-xy diagram) were accurately predicted. However, at vacuum conditions (P-xy diagram), the same precision was not observed, mainly for the liquid phase at the region of interest (dilute butanol concentrations) (Figure 4). For this reason, in the simulations the UNIQUAC parameters determined by Fisher and Gmehling20 were used. The values of the parameters are A12 = 506.31, A21 = 128.55, r1 = 0.92, r2 = 3.4543, q1 = 1.4, q2 = 3.052 (1 = H2O; 2 = butanol). With these parameters a better representation of the vaporliquid equilibrium data at vacuum conditions was observed.
3. RESULTS AND DISCUSSION The effects of the inlet flow rate of the flash tank (Fc) on fermentation parameters were determined in a simulated fermentation with the following operating conditions: F0 = 50 m3/h, S0 = 150 g/L, V = 500 m3, and amount of fermentation broth vaporized in the flash tank equal to 5% of Fc (vaporization rate met by keeping Pflash between 6.51 and 6.56 kPa). As the amount of fermentation broth that circulates through the flash tank increased (from 200 to 300 m3/h), more butanol was recovered. Consequently, the concentration of butanol in the fermentor
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lowered, which represents a significant reduction in the product inhibitory effect. Biomass concentration increased, resulting in higher conversion of substrate, elevation of productivity, and greater concentration of butanol in the stream sent to distillation (Figure 5). The amount of fermentation broth vaporized also increased. Thus, the vaporization of more water was necessary in order to enhance fermentation performance. Butanol productivity (up to 8 g/L 3 h) reported in our previous work11 was significantly greater than the values shown in Figure 5. This is due to the fact that the cell retention system with a microfiltration module present in the previous design was here withdrawn. This design change was aimed at operational simplicity and cost reduction by elimination of the use of membranes. Nevertheless, productivity values reported here (1.83.6 g/L 3 h) largely surpass the usual value obtained from a conventional batch process (0.50.6 g/L 3 h). The effects of the flash tank pressure (Pflash) on fermentation parameters were determined in a simulated fermentation with the following operating conditions: F0 = 50 m3/h, S0 = 150 g/L, V = 500 m3, and Fc = 300 m3/h. The more vacuum was applied in the flash tank, the greater was the amount of broth vaporized (varied from 1.5 to 30 m3/h). This variation was more intense than that observed with the manipulation of Fc (from 9.6 to 14.5 m3/h). For this reason, fermentation parameters exhibited greater perturbations in the range of the evaluated pressures. And, as expected, the energy consumption for condensation was positively correlated to the amount of broth vaporized (Figure 6). It is also important to note that the final butanol concentration (concentration in the stream sent to distillation) reached a maximum (30 g/L) at a vaporization rate of 15 m3/h. By increasing vaporization rate above this value, no alteration in the final butanol concentration was observed. Butanol selectivity,7 a measure of the preferential removal of butanol over other components present in the mixture such as water, was 20. For the six scenarios shown in Table 1, the minimization of the energy requirements for the flash fermentation was achieved by manipulation of Fc and Pflash. These two operating variables were regulated aiming at the minimization of the amount of fermentation broth vaporized, respecting the chosen constraint of sugar conversion (equal to 98%). Such high substrate conversion was only obtained when Fc was set to 300 m3/h and, for this reason, only the pressure in the flash tank was manipulated in order to minimize vaporization. It should be noted that the range of the substrate concentration chosen (100170 g/L) was considerably higher than the typical maximum concentration found in conventional batch processes (60 g/L). Performance parameters of the flash fermentation for the six scenarios are shown in Table 2. When substrate concentration in the feed was elevated from 100 to 150 g/L, more vaporization was necessary in the flash tank in order to meet the desired conversion (comparison between scenarios I and II). On the other hand, vaporization could be reduced by increasing the fermentation volume (or decrease of dilution rate) (II compared to III; and IV to V). In the fermentation with the hyper-butanol producing strain (scenario VI), intensification of vaporization was also necessary (comparison between scenarios III and VI). To determine which scenario provides the better energy efficiency to the process, the energy requirement for the distillation unit must be included in the balance. As a previous analysis, the energy consumption by distillation was determined as a function of butanol concentration in the fermentation beer (here the reboiler duty of the acetone column was met by low pressure 2352
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Figure 6. Effects of pressure in the flash tank (Pflash) on fermentation parameters determined in a simulated fermentation with the following operating conditions: F0 = 50 m3/h, S0 = 150 g/L, V = 500 m3, Fc = 300 m3/h.
Table 2. Performance Parameters of the Flash Fermentation Considering the Scenarios Presented in Table 1 scenarios I
a
II
III
IV
V
VI
flash tank pressure (kPa)
6.49
6.47
6.53
6.47
6.50
6.45
volume of fermentation broth vaporized (m3/h)
8.20
18.5
13.9
24.1
20.9
26.7
butanol productivity (g/L.h) butanol yield (g/g)
1.8 0.18
3.0 0.20
2.1 0.21
3.6 0.22
2.5 0.22
3.2 0.33
ABE yield (g/g)
0.29
0.34
0.35
0.36
0.37
0.43
butanol concentration in the fermentor (g/L)
5.6
6.9
7.8
7.9
8.3
10.3
ABE concentration in the fermentor (g/L)
8.20
10.5
11.9
12.4
13.0
12.8
butanol concentration in the recovered (g/L)
81.8
69.4
91.6
67.0
77.3
82.1
fraction of produced butanol recovered by the flash (%)
75.3
84.9
81.2
88.0
86.2
90.0
final concentration of butanol (g/L)
17.9
30.0
31.0
36.3
37.1
48.6
biomass concentration (g/L) energy gain obtained from the heat integration scheme shown in Figure 2 (MJ/kg butanol)
6.60 4.1
12.6 5.5
10.6 4.1
16.6 5.5
14.4 4.6
14.9 4.5
energy requirement for flash fermentationa (MJ/kg butanol)
4.8
6.0
4.4
6.5
5.5
5.4
energy requirement for distillationb (MJ/kg butanol)
17.5
10.9
11.0
9.5
9.3
8.0
total energy requirement (MJ/kg butanol)
22.3
17.0
15.4
16.0
14.8
13.4
stillage to butanol ratio (m3/m3)
54.3
32.3
31.3
26.7
26.2
21.0
Electrical power demanded by compressors and recirculation pumps. b Reboilers.
steam). For the different concentrations of butanol considered in this analysis, concentrations of solvents followed the proportion 3:6:0.5 (A:B:E). Concentrations of acetic and butyric acid were equal to 1.0 and 0.5 g/L, respectively. The amount of energy required per unit of butanol recovered decreases as the concentration of butanol in the beer stream increases (Figure 7). The energy consumption asymptotically approaches 9 MJ/kg butanol as butanol concentration increases in the fermentation beer. It should be noted that the energy is greatly affected in the range of butanol concentrations (813 g/L) found in conventional fermentations. Thereby, a concentration increment of as little as 1 g/L may have great effects on the energy consumption.
Using the same wild-type strain and fermentation medium considered in the present work, Votruba et al.21 obtained a beer with 17.5 g/L ABE (11 g/L butanol, 5.0 g/L acetone, 1.5 g/L ethanol) in batch fermentation. The energy requirement for distillation in this case would be 28 MJ/kg butanol. This separation would have very low energy efficiency given that the energy required for the complete production process could be greater than the energy content of the product itself (the heat of combustion of butanol is 36.2 MJ/kg). On the other hand, flash fermentation employing this same strain achieved a maximum beer concentration of 37.1 g/L butanol (scenario V), resulting in an important reduction of 67% of the energy requirement for distillation (9.3 MJ/kg butanol). It should be noted that not only 2353
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Figure 7. Energy requirement for the distillation unit to achieve dehydration (99.5 wt %) of n-butanol as function of butanol concentration in the fermentation beer. The heat integration scheme shown in Figure 2 was not considered here.
the butanol concentration was responsible for the decrease in the energy consumption. The heat integration scheme shown in Figure 2 contributed with an energy reduction of 4.6 MJ/kg butanol (Table 2). However, the energy use of the flash fermentation (4.46.5 MJ/kg butanol) must be included in this balance. Thus, for the scenarios with the wild-type strain (I to V), the total energy requirement for butanol recovery (flash fermentation þ distillation) varied from 14.8 to 22.3 MJ/kg butanol. This represents a maximum reduction in energy consumption of 47% in relation to the batch process. Significant improvement in energy efficiency of the distillation unit and reduction of specific stillage volume (m3 stillage/m3 butanol) were observed when feed sugar concentration increased from 100 to 150170 g/L (Table 2). For the decision on which feed sugar concentration (150 or 170 g/L) would give flash fermentation the better performance, another important parameter, the butanol concentration in the fermentor, was taken into account. A problem found in long-time continuous ABE fermentation is the culture degeneration, which is the change of genetic characteristics over cell generations, due to butanol toxicity. It results in decline of solvent production over time, and a concomitant increase in acid formation.22,23 For this reason, in most of the studies that report stable continuous cultures the solvent level did not exceed 1013 g/L.24 Thus, scenario II (S0 = 150 g/L, V = 500 m3/h) with a lower ABE concentration in the fermentor (6.9 g/L butanol, 3.0 g/L acetone, 0.6 g/L ethanol) was considered the better operation strategy for the flash fermentation when using a wild-type strain. This means that the use of flash fermentation would result in a reduction of 39% on the energy for butanol recovery in relation to the conventional batch process. When the hyper-butanol producing mutant strain was used in the flash fermentation (scenario VI), butanol concentration in the stream sent to distillation was 48.6 g/L. It had a significant positive impact on the energy requirement for distillation, and for this reason, the energy efficiency of this scenario was better than those with the wild-type strain. The total energy requirement for scenario VI was 13.4 MJ/kg butanol. If this same mutant strain was employed in a conventional batch fermentation, producing 25.3 g/L ABE (19.7 g/L butanol, 5.0 g/L acetone, 0.6 g/L ethanol),16 the energy requirement for distillation in this case would be 16.1 MJ/
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kg butanol. Thus, in a fermentation with a mutant strain, the use of the flash fermentation could reduce the energy consumption for butanol recovery by 16.8%. The gain in this case was lower than that of the fermentation with the wild-type strain because the mutant strain is able to produce more butanol and for this reason vaporization was intensified in order to recover a greater amount of butanol (Table 2). When compared to the batch process, the use of the flash fermentation would result in reduction of process and wastewater streams and smaller fermentors with higher productivity. Moreover, less energy would be spent for sterilization of a more concentrated sugar solution. On the other hand, the operation complexity and the total project capital cost would increase in relation to a batch process, as additional equipment such as the flash tank, compressors, heat exchangers, and recirculation pumps is required. In relation to other recovery technology options, the energy requirement for butanol recovery using flash fermentation and distillation was lower than that reported for in situ gas stripping in conjunction with distillation (21.8 MJ/kg butanol).5 The same authors reported the energy consumption for other separation systems, such as adsorption þ distillation (8.1 MJ/kg butanol) and pervaporation þ distillation (13.8 MJ/kg butanol). Matsumura et al.25 reported the energy requirement for a separation system combining pervaporation using a liquid membrane with distillation (7.4 MJ/kg butanol to concentrate butanol from 0.5 to 99.9 wt %) and a membrane separation system using both a liquid membrane and a hydrophilic membrane (6.5 MJ/kg butanol to concentrate butanol from 0.5 to 95 wt %). Based on these reports and on the studies presented in this paper, it is unquestionable that flash fermentation and gas stripping are less energy efficient than adsorption and membrane-based processes. However, it should be noted that the comparison of energy requirements for different technologies must be looked at with caution as process-specific details such as butanol production basis (substrate concentration and fermentation yield), butanol concentration in the beer (microorganism strain), and heat integrations are not entirely available for the other studies and are very likely to differ from those of the present study.
4. CONCLUSIONS With the flash fermentation technology, high conversion of concentrated sugar solution into ABE can be obtained, resulting in high productivity and a more concentrated fermentation beer. For different fermentation conditions (substrate concentration, dilution rate, and microbial strain), two operating variables of the flash tank, inlet flow rate and pressure, must be regulated in order to ensure the desired sugar conversion. These two variables regulate the amount of fermentation broth vaporized, minimization of which is crucial to enhance the energy efficiency of the flash fermentation. Small increments in butanol concentration in the beer can have important positive impacts on the energy consumption of the distillation unit. Nonetheless, the energy use of the recovery technology must be included in the energy balance. For a fermentation with a wild-type strain, the total energy requirement for butanol recovery (flash fermentation þ distillation) was 17.0 MJ/kg butanol, with 36% of this value demanded by the flash fermentation. This represents a reduction of 39% in the energy for butanol recovery in relation to the conventional batch process. In the case of a fermentation with a hyper-butanol 2354
dx.doi.org/10.1021/ef200279v |Energy Fuels 2011, 25, 2347–2355
Energy & Fuels producing mutant strain, use of the flash fermentation could reduce the energy consumption for butanol recovery by 16.8% in relation to a batch fermentation with the same mutant strain.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Mail: Av. Albert Einstein 500, CEP 13083-852, Campinas, SP, Brazil. Telephone: þ5519-3521-3958. Fax: þ5519-35213965.
’ ACKNOWLEDGMENT We thank Fundac-~ao de Amparo a Pesquisa do Estado de S~ao Paulo (FAPESP) (Contract grant 2007/00341-1) for the financial support. ’ NOMENCLATURE A = Heat-transfer area, m2 Aij = Parameter for GE (molar Gibbs energy) for the UNIQUAC activity coefficient model (cal/mol) AA = Acetic acid ABE = Acetonebutanolethanol BA = Butyric acid F0 = Fresh broth flow rate (continuous feed), m3/h Fc = Inlet flow rate of the flash tank, m3/h Fpu = Fermentor beer flow rate (beer), m3/h Fr = Flash tank liquid outlet flow rate (liquid stream depleted in ABE), m3/h NREL = National Renewable Energy Laboratory (USA) Pflash = Flash tank pressure, kPa Pi = Fermentor product concentration, g/L Pr = Product concentration in the flash tank liquid outlet flow, g/L q = Molecular area parameter for the UNIQUAC activity coefficient model, Q = Heat, MW r = Molecular volume parameter for the UNIQUAC activity coefficient model, S = Fermentor substrate concentration, g/L Sr = Substrate concentration in the flash tank liquid outlet flow, g/L S0 = Substrate concentration in fresh broth, g/L X = Fermentor biomass concentration, g/L Xr = Biomass concentration in the flash tank liquid outlet flow, g/L U = Overall heat-transfer coefficient, W/m2 K V = Fermentation volume, m3
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