Published: April 12, 2011

r 2011 American Chemical Society 2347 dx.doi.org/10.1021/ef200279v | Energy Fuels 2011, 25, 2347–2355

ARTICLE

pubs.acs.org/EF

Energy Requirements for Butanol Recovery Using the FlashFermentation TechnologyAdriano P. Mariano,*,†,‡ Mohammad J. Keshtkar,† Daniel I. P. Atala,§ Francisco Maugeri Filho,||

Maria Regina Wolf Maciel,‡ Rubens Maciel Filho,‡ and Paul Stuart†

†NSERC Environmental Design Engineering Chair, Department of Chemical Engineering, �Ecole Polytechnique de Montr�eal,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: Acetone�butanol�ethanol (ABE) facilities have traditionally presented unattractive economics because of the largeenergy consumption during recovery of the products from a dilute fermentation broth (∼13 g/L butanol). This problem resultsfrom the high toxicity of butanol to microorganisms that catalyze its production. Flash fermentation is a continuous fermentationsystem with integrated product recovery. The bioreactor is operated at atmospheric pressure and the broth is circulated in a closedloop to a vacuum chamber where ABE is continuously boiled off at 37 �C and condensed afterward. With this technology the beerachieved a concentration of butanol as high as 30�37 g/L. This paper studies the energy requirements for butanol recovery using theflash fermentation technology and its effect on the energy consumption by the downstream distillation system. Compressors areused to remove the vapors from the flash tank, thus maintaining the desired vacuum. The heat recovery technique of vaporrecompression is used to reduce energy requirements. With this technique the heat generated by the compression and partialcondensation of the vapors provides the energy for boil up (heat of vaporization) in the flash tank. Thus the energy requirement forthe flash fermentation is essentially the electrical power demanded by compressors. Energy for recirculation pumps accounts forapproximately 0.5% of the total energy consumption. Small increments in butanol concentration in the beer can have importantpositive impacts on the energy consumption of the distillation unit. Nonetheless, the energy use of the recovery technology must beincluded in the energy balance. For a fermentation with a wild-type strain, the total energy requirement for butanol recovery (flashfermentationþ distillation) was 17.0 MJ/kg butanol, with 36% of this value demanded by the flash fermentation. This represents areduction of 39% in the energy for butanol recovery in relation to the conventional batch process. In the case of a fermentation with ahyper-butanol producing mutant strain, the use of the flash fermentation could reduce the energy consumption for butanol recoveryby 16.8% in relation to a batch fermentation with the same mutant strain.

1. INTRODUCTION

Butanol is highly toxic to microorganisms that catalyze itsproduction, and for this reason less than 13 g/L of butanol isproduced during batch fermentation. For the sake of comparison,in the ethanol fermentation the yeast cells tolerance to ethanol isapproximately ten times greater. Therefore, typical acetone�butanol�ethanol (ABE) fermentation has been plagued by theuse of dilute sugar solutions as substrates, large process volumes,high downstream process costs due to intensive energy require-ments for recovery of low concentrations of ABE in the beer, andlarge quantities of wastewater.

A solution to these problems can be addressed by using ge-netic engineering techniques to develop strains that couldtolerate higher concentrations of butanol and sugar.1,2 Anotheroption is the use of technologies designed to remove butanolcontinuously from the fermentation broth (integrated re-covery technologies). The product recovery reduces the effectof product inhibition and allows an increase in the substrateconcentration which results in a reduction in process streams,and higher productivity. With either approach (biological and

technological) the energy efficiency of the process is expected tobe enhanced.

Product-recovery technologies are designed to remove ABEfrom the bioreactor liquid as the fermentation is ongoing and theABE-depleted stream is returned to the bioreactor or, in the caseof an in situ products removal technology, the fermentationbroth never leaves the bioreactor. Product-removal techniquesinclude gas stripping, liquid�liquid extraction, membrane-basedmethods (pervaporation, perstraction), and adsorption. All thesetechniques have advantages and disadvantages in terms ofcapacity, selectivity, fouling, clogging, scale-up, operational sim-plicity, and energy requirement. A comparison among some ofthem can be found elsewhere.1,3�8

As a commodity, butanol production cost is mainly affected byfeedstock price. Economic modeling studies show that feedstockprice (sugar cane juice in Brazil) could account for 60% of the

Received: February 22, 2011Revised: April 12, 2011

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total variable operating cost. The remaining value is made up ofutilities costs.9 Thus the overall energy consumption duringrecovery is also an important operating cost factor. For thisreason, during the selection of an integrated recovery technology,the energy consumption must be a key decision factor and can beused as a ranking parameter.7

In our previous works we demonstrated the technical feasi-bility of the flash fermentation technology to recover ABE fromthe bioreactor.10,11 This integrated ABE fermentation involvesoperating the bioreactor at atmospheric pressure while the brothis circulated in a closed loop to a vacuum chamber where ABE iscontinuously boiled off. With this technology, a significant im-provement of productivity was observed, and the beer achieved aconcentration of butanol as high as approximately 30 g/L. Thesegood results prompted us to further investigate this technology asto its energy requirements and its effect on the energy consump-tion by the downstream distillation system. These studies arepresented in the next sections.

2. METHODOLOGY

The principles of the flash fermentation process are presentedin the scheme shown in Figure 1. The fermentation broth of acontinuous fermentor circulates through a vacuum flash tank.The partial vaporization of water and fermentation products thattakes place in the flash tank generates a vapor phase rich insolvents (ABE) and an ABE-depleted liquid stream. The vapor issubsequently condensed and combined with the beer stream andthen sent to distillation. The ABE-depleted liquid stream returnsto the fermentor, which is operated at atmospheric pressure.Temperature in the fermentor and in the flash tank is set to37 �C. Vacuum is regulated in order to keep constant the amountof broth vaporized.

In the flash tank, fermentation broth boils at low pressure(6.4�6.8 kPa) and an equilibrium mixture of fermentationproducts (ABE, organic acids, and CO2 and H2) and water istaken overhead. Compressors remove these vapors, thus main-taining the desired vacuum. The heat recovery technique ofvapor recompression is used to reduce energy requirements12

(Figure 2). In this integrated system, the first compressorcompresses the vapor to 0.3 atm. At this pressure the vaporcan be passed through the shell side of a shell-and-tube heatexchanger and heat is exchanged with the fermentation brothpresent in the flash tank. The flow rate of the fermentation broththrough the heat exchanger is set in order to allow temperature amaximum increase of 0.5 �C. This restriction is necessary due tothe strong sensibility of the microorganism activity to tempera-ture variations. Most of the vapor (∼80%, mass basis) condensesin the heat exchanger, providing the energy for boil up (heat ofvaporization) in the flash tank. The condensed fraction ispumped at low energy cost to atmospheric pressure and sentto distillation. The remaining vapor is compressed to 1.5 atm in asecond compressor and a subsequently partial (∼30%, massbasis) condensation suffices to meet the total reboiler duty of oneof the distillation columns (acetone column). Afterward, theremaining vapor stream is completely condensed by heating thebeer stream from the fermentor. Noncondensables (CO2 andH2) are sent to an absorber column in order to recover carriedABE. The pressure in the first compression stage (0.3 atm) is theminimum value demanded by the heat exchanger of the flash tankto meet its heat duty requirement. Pressure drop in each heatexchanger is assumed to be 0.14 atm (2 psia). Compressorsisentropic efficiency is considered to be 0.72.

Until the shutdown of the commercial butanol fermentationfacilities in the 1980s, the separation of the fermentation

Figure 1. Flash fermentation technology.

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products (ABE) was carried out in a series of five continuousdistillation columns, with the last two responsible for the sep-aration of butanol from water.13 Therefore, this design wasadopted in the present study (Figure 3). In addition to theproven utilization of this system in the past, distillation is amature 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) afterbeing heated to 112 �C. The stillage contains most of the brothwater content, acetic and butyric acid, cells, and remaining sugars.The overhead vapor from the beer column is separated in a seriesof four distillation columns operated under different pressures inorder to allow heat integration. In the top of the acetone column(30 stages; feed in stage 15), 99.5 wt % acetone is obtained andthe fermentation gases (CO2 and H2) are sent to an absorbercolumn (10 stages; feed in stage 10) in order to minimize thelosses of ABE in the distillation unit. The acetone column isoperated at 0.7 atm so that low pressure steam can be used in thereboiler (it should be noted that for a process with the flashfermentation, the total reboiler duty of the acetone column canbe met by partial condensation of the recompressed ABE-enriched vapor stream from the flash tank). The bottom streamof the acetone column is fed to the ethanol column (40 stages;feed in stage 10), which operates at 0.3 atm. Vacuum operationreduces the reflux needed to produce the 85 wt % ethanoloverhead product and allows the total reboiler duty to be metby condensing the overhead vapors from the beer column in theethanol column reboiler. A two-column distillation system in con-junction with a decanter is used to separate the heterogeneous

binary butanol/water azeotrope. The bottom stream of theethanol column is added to a decanter in conjunction with thetop streams from the water and butanol columns. In the decanter,butanol phase separates from the aqueous phase and rises to forman upper layer. The water-rich phase is refluxed to the watercolumn (10 stages; feed in stage 1), whose bottom (water)contains less than 0.05 wt % butanol. The butanol-rich phase isrefluxed to the butanol column (10 stages; feed in stage 1), whichproduces a 99.5 wt % butanol product. The water and butanolcolumns and the decanter are operated at atmospheric pressure.Solvent loss for the gas stream accounts for 1.7, 0.2, and 1.4% ofthe total amount of acetone, butanol, and ethanol, respectively.

By employing vapor recompression heating, the energy re-quirement for the flash fermentation is essentially the electricalpower demanded by the compressors. Energy for recirculationpumps accounts for approximately 0.5% of the total energyconsumption. The energy for vaporization is obtained from theheat generated during compression of the vapors. Thus, it is veryintuitive that the greater the amount of fermentation brothvaporized in the flash tank, the greater the energy consumptionnecessary to compress the vapors. Having this in mind, the firststep in the evaluation of the energy requirements was to determinethe effects of operating variables related to the flash tank (inlet flowrate of the flash tank and pressure) on the amount of fermentationbroth vaporized in the flash tank. In the same manner, the effectson the concentrations in the fermentor were also analyzed. Thesestudies were carried out using amathematicalmodel that simulatesthe fermentation and the vaporization in the flash tank.

In the computational simulation, fermentation starts up inbatch mode (500 m3 fermentation volume) with initial sugar

Figure 2. Vapor recompression heating system. Heat integration between the recompressed ABE-enriched vapor stream from the flash tank and thereboiler of the acetone column, and fermentation beer stream (stream conditions refers to scenario 2, Table 1).

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concentration of 60 g/L and after 17 h (the time butanolconcentration in the fermentor achieves 6 g/L) the continuousfeed of medium (50 m3/h) and the recirculation of the broththrough the flash tank are initiated (inlet flow rate of the flashtank varying between 200 and 350 m3/h). Equations 1�3 wereused to determine the concentrations of biomass, substrate, andproducts in the fermentor:

dXdt

¼ rX � FPUV

X þ FrVXr ð1Þ

dSdt

¼ rS � FPUV

Sþ FrVSr þ F0

VS0 ð2Þ

dPidt

¼ rPi �FPUV

Pi þ FrVPr;i ð3Þ

where i stands for butanol, acetone, ethanol, butyric acid, andacetic acid.

The kinetic models (rx, rs, and rpi) were experimentallydetermined by Mulchandani and Volesky14 based on the follow-ing assumptions: (1) carbon substrate (glucose) limitation only;(2) no nitrogen and nutrient limitation; (3) product inhibition;(4) acetic and butyric acid are intermediate metabolites andare reduced to acetone and butanol, respectively; (5) acetoneand butanol are also synthesized directly from carbon sub-strate; (6) ethanol is synthesized from carbon substrate only;(7) fermentation is performed at optimal temperature of37 �C and optimal pH of 4.5 under anaerobic conditions; (8)all the cells (Clostridium acetobutylicum ATCC824) aremetabolically active and viable. Integration of eq 1 to 3 wascarried out by the fourth order Runge�Kutta method using aFortran code.

The modeling of the flash tank was based on the isothermaland 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

Figure 3. Downstream distillation unit (stream conditions refers to scenario 2, Table 1).

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equation and activity coefficients were calculated by theUNIQUAC model.

Six scenarios were set in order to assess the energy require-ments for the flash fermentation and the subsequent distillationunit. They differ from each other in terms of substrate concen-tration, fermentation volume (dilution rate), and microbial strain(Table 1). For each scenario, the necessary vacuum in the flashtank able to balance the trade-off betweenminimum vaporizationof fermentation broth and high sugar conversion (98%) wasinitially determined using the above-described mathematicalmodel (eqs 1�3). In the simulated fermentations, the freshmedium fed to the fermentor had a flow rate of 50 m3/h and theinlet flow rate of the flash tank was 300 m3/h. For scenario VIanother microbial strain was considered.C. beijerinckii BA101 is ahyper-butanol producing mutant strain able to produce 25.3 g/LABE (19.7 g/L butanol, 5.0 g/L acetone, 0.6 g/L ethanol).16 Toobtain this same level of solvents production, the values of theparameters k7, k10, k15, and k11 of the kinetic model were changedto 0.65, 0.02, 1.9, and 0.045, respectively (the reader is referred toMulchandani and Volesky14 for identification of parameters).These alterations were necessary because the kinetic modeloriginally describes the typical ABE production level of a wild-type 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) werecalculated by process modeling in Aspen Plus 7.1. The completeprocess represented in Figures 1�3 was simulated in AspenPlus incorporating the results (operating conditions andsteady-state concentrations) obtained with the simulations ofthe fermentation in Fortran (eqs 1�3 and flash calculation).For this, a stoichiometric reactor model (RStoic) was used tosimulate the fermentor in Aspen Plus, considering the follow-ing reaction equations:

C6H12O6 þH2O f C3H6O ðacetoneÞ þ 3CO2 þ 4H2 ð4Þ

C6H12O6 f C4H10O ðbutanolÞ þ 2CO2 þH2O ð5Þ

C6H12O6 f 2C2H6O ðethanolÞ þ 2CO2 ð6Þ

C6H12O6 f C4H8O2 ðbutyric acidÞ þ 2CO2 þ 2H2 ð7Þ

C6H12O6 þ 2H2O f 2C2H4O2 ðacetc acidÞ þ 2CO2 þ 4H2

ð8Þ

C6H12O6 þ 1:1429NH3 f 5:7134CH1:8O0:5N0:2 ðbiomassÞþ 0:2857CO2 þ 2:5714H2O ð9Þ

For each reaction equation, fractional conversions of glucosewere assigned. In this manner, the same values of consumed

substrate and produced solvents and biomass previously deter-mined from eqs 1�3 were obtained in the Aspen Plus simula-tions. This procedure enabled the simplified reactor modelused in the Aspen simulator (eqs 4�9) to reproduce the steadystate concentrations values generated by a mathematical modelthat incorporates a sophisticated experimental kinetic model(eqs 1�3), including a nonlinear product inhibition model.The input data for the flash calculation in Aspen Plus weretemperature (37 �C) and vapor fraction. The latter was obtainedfrom the Fortran calculations.

Properties for biomass (CH1.8O0.5N0.2) were obtained fromthe NREL database.17 The energy demanded by the equipment(MJ/h) was divided by the mass flow rate (kg/h) of butanol atthe bottom of the butanol column to get the specific energyrequirement (MJ/kg butanol) for butanol purification.

Design specifications and operating conditions used in thesimulation of the distillation unit were based on the optimumconfigurations 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 interms of recovery or purity for a specific compound in either thetop or bottom sections. The design spec/vary feature of Aspen

Table 1. Scenarios Considered for the Energy ConsumptionStudies

scenarios

I II III IV V VI

S0 (g/L) 100 150 150 170 170 150

fermentation volume (m3) 500 500 750 500 750 750

strain a a a a a ba C. acetobutylicum ATCC824. b C. beijerinckii BA101.

Figure 4. Vapor�liquid equilibrium (VLE) of n-butanol/water binarymixture at atmospheric pressure (T-xy diagram) and at 50.07 �C (P-xydiagram). Data (lines) obtained from Aspen Plus simulator and validatedby experimental data (circles and squares). ASPEN 1 (black lines): defaultUNIQUAC parameters available in the simulator. ASPEN 2 (gray lines):UNIQUAC parameters reported by Fisher and Gmehling.20

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Plus was used to hold these specifications by varying the distillaterate and reflux ratio. Number of stages and feed location were fedto the simulator and manually optimized to obtain better valuesfor reboiler duty.18,19 Columns were simulated using the RadFracmodel available in the Aspen Plus simulator and it was assumedthat column trays have a uniform Murphree efficiency of 0.6.Thermodynamic properties of the system were determined bytheUNIQUAC activity coefficientmodel for the liquid phase andthe Hayden-O’Connell model for the vapor phase. Having inmind that the accuracy of the amount of butanol recovered in theflash tank is given by a good representation of the thermody-namics characteristics of the vapor�liquid equilibrium of then-butanol/water system, validation of the equilibrium calcula-tions was carried out using experimental data available in theliterature. Using the default UNIQUAC parameters available inthe Aspen Plus simulator, vapor�liquid equilibrium data at 1 atm(T-xy diagram) were accurately predicted. However, at vacuumconditions (P-xy diagram), the same precision was not observed,mainly for the liquid phase at the region of interest (dilutebutanol concentrations) (Figure 4). For this reason, in the sim-ulations the UNIQUAC parameters determined by Fisherand Gmehling20 were used. The values of the parameters areA12 = 506.31, A21 = 128.55, r1 = 0.92, r2 = 3.4543, q1 = 1.4,q2 = 3.052 (1 = H2O; 2 = butanol). With these parameters abetter representation of the vapor�liquid equilibrium data atvacuum conditions was observed.

3. RESULTS AND DISCUSSION

The effects of the inlet flow rate of the flash tank (Fc) onfermentation parameters were determined in a simulated fer-mentation with the following operating conditions: F0 = 50 m

3/h,S0 = 150 g/L, V = 500 m3, and amount of fermentation brothvaporized in the flash tank equal to 5% of Fc (vaporization ratemet by keeping Pflash between 6.51 and 6.56 kPa). As the amountof fermentation broth that circulates through the flash tankincreased (from 200 to 300 m3/h), more butanol was recovered.Consequently, the concentration of butanol in the fermentor

lowered, which represents a significant reduction in the productinhibitory effect. Biomass concentration increased, resulting inhigher conversion of substrate, elevation of productivity, andgreater concentration of butanol in the stream sent to distillation(Figure 5). The amount of fermentation broth vaporized alsoincreased. Thus, the vaporization of more water was necessary inorder to enhance fermentation performance. Butanol productiv-ity (up to 8 g/L 3 h) reported in our previous work11 wassignificantly greater than the values shown in Figure 5. This isdue to the fact that the cell retention systemwith amicrofiltrationmodule present in the previous design was here withdrawn. Thisdesign change was aimed at operational simplicity and costreduction by elimination of the use of membranes. Nevertheless,productivity values reported here (1.8�3.6 g/L 3 h) largelysurpass the usual value obtained from a conventional batchprocess (0.5�0.6 g/L 3 h).

The effects of the flash tank pressure (Pflash) on fermentationparameters were determined in a simulated fermentation withthe following operating conditions: F0 = 50 m3/h, S0 = 150 g/L,V = 500m3, and Fc = 300 m

3/h. The more vacuumwas applied inthe flash tank, the greater was the amount of broth vaporized(varied from 1.5 to 30 m3/h). This variation was more intensethan that observed with the manipulation of Fc (from 9.6 to14.5 m3/h). For this reason, fermentation parameters exhibitedgreater perturbations in the range of the evaluated pressures.And, as expected, the energy consumption for condensationwas positively correlated to the amount of broth vaporized(Figure 6). It is also important to note that the final butanolconcentration (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 alterationin the final butanol concentration was observed. Butanolselectivity,7 a measure of the preferential removal of butanol overother components present in the mixture such as water, was 20.

For the six scenarios shown in Table 1, theminimization of theenergy requirements for the flash fermentation was achieved bymanipulation of Fc and P

flash. These two operating variables wereregulated aiming at the minimization of the amount of fermenta-tion broth vaporized, respecting the chosen constraint of sugarconversion (equal to 98%). Such high substrate conversion wasonly obtained when Fc was set to 300 m

3/h and, for this reason,only the pressure in the flash tank was manipulated in order tominimize vaporization. It should be noted that the range of thesubstrate concentration chosen (100�170 g/L) was consider-ably higher than the typical maximum concentration found inconventional batch processes (60 g/L).

Performance parameters of the flash fermentation for the sixscenarios are shown in Table 2. When substrate concentration inthe feed was elevated from 100 to 150 g/L, more vaporizationwas necessary in the flash tank in order to meet the desiredconversion (comparison between scenarios I and II). On theother hand, vaporization could be reduced by increasing thefermentation volume (or decrease of dilution rate) (II comparedto III; and IV to V). In the fermentation with the hyper-butanolproducing strain (scenario VI), intensification of vaporizationwas also necessary (comparison between scenarios III and VI).

To determine which scenario provides the better energyefficiency to the process, the energy requirement for the distilla-tion unit must be included in the balance. As a previous analysis,the energy consumption by distillation was determined as afunction of butanol concentration in the fermentation beer (herethe reboiler duty of the acetone column was met by low pressure

Figure 5. Effects of the inlet flow rate of the flash tank (Fc) onfermentation parameters determined in a simulated fermentation withthe following operating conditions: F0 = 50 m3/h, S0 = 150 g/L,V = 500 m3, Pflash = 6.51�6.56 kPa, and amount of fermentation brothvaporized in the flash tank equal to 5% of Fc.

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steam). For the different concentrations of butanol considered inthis analysis, concentrations of solvents followed the proportion3:6:0.5 (A:B:E). Concentrations of acetic and butyric acid wereequal to 1.0 and 0.5 g/L, respectively. The amount of energyrequired per unit of butanol recovered decreases as the concen-tration of butanol in the beer stream increases (Figure 7).The energy consumption asymptotically approaches 9 MJ/kgbutanol as butanol concentration increases in the fermenta-tion beer. It should be noted that the energy is greatly affectedin the range of butanol concentrations (8�13 g/L) found inconventional fermentations. Thereby, a concentration incre-ment of as little as 1 g/L may have great effects on the energyconsumption.

Using the same wild-type strain and fermentation mediumconsidered in the present work, Votruba et al.21 obtained a beerwith 17.5 g/L ABE (11 g/L butanol, 5.0 g/L acetone, 1.5 g/Lethanol) in batch fermentation. The energy requirement fordistillation in this case would be 28 MJ/kg butanol. Thisseparation would have very low energy efficiency given that theenergy required for the complete production process could begreater than the energy content of the product itself (the heat ofcombustion of butanol is 36.2 MJ/kg). On the other hand, flashfermentation employing this same strain achieved a maximumbeer concentration of 37.1 g/L butanol (scenario V), resulting inan important reduction of 67% of the energy requirement fordistillation (9.3 MJ/kg butanol). It should be noted that not only

Figure 6. Effects of pressure in the flash tank (Pflash) on fermentation parameters determined in a simulated fermentation with the following operatingconditions: 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 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) 1.8 3.0 2.1 3.6 2.5 3.2

butanol yield (g/g) 0.18 0.20 0.21 0.22 0.22 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) 6.60 12.6 10.6 16.6 14.4 14.9

energy gain obtained from the heat integration scheme shown in Figure 2 (MJ/kg butanol) 4.1 5.5 4.1 5.5 4.6 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 21.0a Electrical power demanded by compressors and recirculation pumps. bReboilers.

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the butanol concentration was responsible for the decrease in theenergy consumption. The heat integration scheme shown inFigure 2 contributed with an energy reduction of 4.6 MJ/kgbutanol (Table 2). However, the energy use of the flash fermenta-tion (4.4�6.5 MJ/kg butanol) must be included in this balance.Thus, for the scenarios with the wild-type strain (I to V), the totalenergy requirement for butanol recovery (flash fermentation þdistillation) varied from 14.8 to 22.3 MJ/kg butanol. Thisrepresents a maximum reduction in energy consumption of47% in relation to the batch process.

Significant improvement in energy efficiency of the distillationunit and reduction of specific stillage volume (m3 stillage/m3

butanol) were observed when feed sugar concentration increasedfrom 100 to 150�170 g/L (Table 2). For the decision on whichfeed sugar concentration (150 or 170 g/L) would give flashfermentation the better performance, another important para-meter, the butanol concentration in the fermentor, was taken intoaccount. A problem found in long-time continuous ABE fermen-tation is the culture degeneration, which is the change of geneticcharacteristics over cell generations, due to butanol toxicity. Itresults in decline of solvent production over time, and aconcomitant increase in acid formation.22,23 For this reason, inmost of the studies that report stable continuous cultures thesolvent level did not exceed 10�13 g/L.24 Thus, scenario II(S0 = 150 g/L,V=500m

3/h) with a lowerABE concentration in thefermentor (6.9 g/L butanol, 3.0 g/L acetone, 0.6 g/L ethanol)was considered the better operation strategy for the flash fermenta-tion when using a wild-type strain. This means that the use offlash fermentation would result in a reduction of 39% on the energyfor butanol recovery in relation to the conventional batch process.

When the hyper-butanol producing mutant strain was used inthe flash fermentation (scenario VI), butanol concentration inthe stream sent to distillation was 48.6 g/L. It had a significantpositive impact on the energy requirement for distillation, and forthis reason, the energy efficiency of this scenario was better thanthose with the wild-type strain. The total energy requirement forscenario VI was 13.4 MJ/kg butanol. If this same mutant strain wasemployed in a conventional batch fermentation, producing 25.3 g/LABE (19.7 g/L butanol, 5.0 g/L acetone, 0.6 g/L ethanol),16 theenergy requirement for distillation in this case would be 16.1MJ/

kg butanol. Thus, in a fermentation with a mutant strain, the useof the flash fermentation could reduce the energy consumptionfor butanol recovery by 16.8%. The gain in this case was lowerthan that of the fermentation with the wild-type strain becausethe mutant strain is able to produce more butanol and for thisreason vaporization was intensified in order to recover a greateramount of butanol (Table 2).

When compared to the batch process, the use of the flashfermentation would result in reduction of process and waste-water streams and smaller fermentors with higher productivity.Moreover, less energy would be spent for sterilization of a moreconcentrated sugar solution. On the other hand, the operationcomplexity and the total project capital cost would increase inrelation to a batch process, as additional equipment such as theflash tank, compressors, heat exchangers, and recirculationpumps is required.

In relation to other recovery technology options, the energyrequirement for butanol recovery using flash fermentation anddistillation was lower than that reported for in situ gas stripping inconjunction with distillation (21.8 MJ/kg butanol).5 The sameauthors reported the energy consumption for other separationsystems, such as adsorption þ distillation (8.1 MJ/kg butanol)and pervaporation þ distillation (13.8 MJ/kg butanol). Matsu-mura et al.25 reported the energy requirement for a separationsystem combining pervaporation using a liquid membrane withdistillation (7.4 MJ/kg butanol to concentrate butanol from 0.5to 99.9 wt %) and a membrane separation system using both aliquid membrane and a hydrophilic membrane (6.5 MJ/kgbutanol to concentrate butanol from 0.5 to 95 wt %). Based onthese reports and on the studies presented in this paper, it isunquestionable that flash fermentation and gas stripping are lessenergy efficient than adsorption and membrane-based processes.However, it should be noted that the comparison of energyrequirements for different technologies must be looked at withcaution as process-specific details such as butanol productionbasis (substrate concentration and fermentation yield), butanolconcentration in the beer (microorganism strain), and heatintegrations are not entirely available for the other studies andare very likely to differ from those of the present study.

4. CONCLUSIONS

With the flash fermentation technology, high conversion ofconcentrated sugar solution into ABE can be obtained, resultingin high productivity and a more concentrated fermentation beer.For different fermentation conditions (substrate concentration,dilution rate, and microbial strain), two operating variables of theflash tank, inlet flow rate and pressure, must be regulated in orderto ensure the desired sugar conversion. These two variables re-gulate the amount of fermentation broth vaporized, minimiza-tion of which is crucial to enhance the energy efficiency of theflash fermentation.

Small increments in butanol concentration in the beer canhave important positive impacts on the energy consumption ofthe distillation unit. Nonetheless, the energy use of the recoverytechnology must be included in the energy balance. For a fer-mentation with a wild-type strain, the total energy require-ment for butanol recovery (flash fermentationþ distillation) was17.0MJ/kg butanol, with 36% of this value demanded by the flashfermentation. This represents a reduction of 39% in the energyfor butanol recovery in relation to the conventional batchprocess. In the case of a fermentation with a hyper-butanol

Figure 7. Energy requirement for the distillation unit to achievedehydration (99.5 wt %) of n-butanol as function of butanol concentra-tion in the fermentation beer. The heat integration scheme shown inFigure 2 was not considered here.

2355 dx.doi.org/10.1021/ef200279v |Energy Fuels 2011, 25, 2347–2355

Energy & Fuels ARTICLE

producing mutant strain, use of the flash fermentation couldreduce the energy consumption for butanol recovery by 16.8% inrelation to a batch fermentation with the same mutant strain.

’AUTHOR INFORMATION

Corresponding Author*E-mail: adrianomariano@yahoo.com.br. Mail: Av. AlbertEinstein 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~aoPaulo (FAPESP) (Contract grant 2007/00341-1) for the finan-cial support.

’NOMENCLATUREA = Heat-transfer area, m2

Aij = Parameter for GE (molar Gibbs energy) for the UNIQUAC

activity coefficient model (cal/mol)AA = Acetic acidABE = Acetone�butanol�ethanolBA = Butyric acidF0 = Fresh broth flow rate (continuous feed), m3/hFc = Inlet flow rate of the flash tank, m3/hFpu = Fermentor beer flow rate (beer), m3/hFr = Flash tank liquid outlet flow rate (liquid stream depleted in

ABE), m3/hNREL = National Renewable Energy Laboratory (USA)Pflash = Flash tank pressure, kPaPi = Fermentor product concentration, g/LPr = Product concentration in the flash tank liquid outlet flow, g/Lq = Molecular area parameter for the UNIQUAC activity

coefficient model, -Q = Heat, MWr = Molecular volume parameter for the UNIQUAC activity

coefficient model, -S = Fermentor substrate concentration, g/LSr = Substrate concentration in the flash tank liquid outlet flow, g/LS0 = Substrate concentration in fresh broth, g/LX = Fermentor biomass concentration, g/LXr = Biomass concentration in the flash tank liquid outlet flow, g/LU = Overall heat-transfer coefficient, W/m2 KV = Fermentation volume, m3

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