Process energy evaluation of fuel butanol production from sugarcane

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Biofuels and Biomass

Process energy evaluation of fuel butanol production from sugarcane-sweet sorghum juices by acetone-butanolethanol fermentation associated to a gas stripping system Eloísa Rochón, Mario Daniel Ferrari, and Claudia Lareo Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01660 • Publication Date (Web): 13 Aug 2018 Downloaded from http://pubs.acs.org on August 14, 2018

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Energy & Fuels

Process energy evaluation of fuel butanol production from sugarcanesweet sorghum juices by acetone-butanol-ethanol fermentation associated to a gas stripping system

Eloísa Rochón, Mario Daniel Ferrari, Claudia Lareo ∗ Departamento de Bioingeniería, Facultad de Ingeniería, Universidad de la República, Julio Herrera y Reissig 565, CP 11300, Montevideo, Uruguay

ABSTRACT Butanol production from sugarcane and sweet sorghum juice was studied using batch and fed batch acetone-butanol-ethanol (ABE) fermentation coupled to gas stripping process. The variations of the juice sugar content, butanol yield, and sugar conversion on the energy consumption of the industrial process were studied using experimental data with the Aspen Plus® software. Butanol yield was the most significant factor. Clarification and butanol recovery stages showed the major energy consumption (12.0 and 72.3% of total energy consumption, respectively). For a fed batch strategy where a concentration step is required, renewable energy sources are needed to have a favorable overall energy balance. Butanol production process by ABE fermentation showed higher energy consumption per kg product than an ethanol plant for a similar raw material which suggests that a substantial improvement in the upstream and downstream of the ABE process is needed.

Keywords: butanol, sugarcane, sweet sorghum, energy consumption, ABE fermentation, process simulation ∗

Corresponding author. Phone: +598 27142714; e-mail: [email protected]

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INTRODUCTION Butanol has been considered a promising compound to be used as an advanced biofuel and as a commodity chemical, which can be produced using Clostridium spp. A mixture of acetone-butanol-ethanol (ABE) is generally produced during fermentation. The ABE process has been considered one of the largest fermentation commercial industry in the last century.1 Butanol has clearly superior properties to ethanol since it has higher energy density, is less volatile and less hygroscopic.2-6 Raw material and energy consumption are the principal cost factors in the biobutanol production.3,4 Sugarcane and sweet sorghum are crops which contain large amount of readily soluble fermentable sugars. Sugarcane has been widely used to produce bioethanol in Brazil. Sweet sorghum has good agronomic characteristics, such as high-yields, drought tolerance, does not require many nutrients.7-11 Like sugarcane, a residue (bagasse) is produced during milling in the industrial process, which can be used to generate steam and meet the energy demand of the process. The surplus bagasse can be used to produce extra electrical energy that can be commercialized.12 A disadvantage of easily fermentable sugar-rich crops is that they are generally available seasonally and should be stored properly to avoid deterioration due to natural fermentation which reduces the sugar content. The juices extracted can be processed to be converted into storable syrups in dedicated conversion facilities.13,14 Butanol could be obtained from the same raw material as ethanol, using the same type of equipment. As a result, ethanol and butanol could be produced in the same industrial facility, which allows adjusting the production of both, according to market demand. Butanol is generally recovered by conventional distillation (D) which is a


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process of high energy consumption since in the ABE fermentation, low concentration of butanol is reached due to cell toxicity to butanol.15-20 In order to solve the toxicity problem, some approaches have been proposed, such as the search of new microorganisms that can tolerate high concentrations of butanol and the coupling of a butanol removal system in situ to the fermentation to remove the product as it is produced.21 Different methods have been evaluated, such as gas stripping, adsorption, pervaporation, liquid-liquid extraction, and vacuum fermentation.1,22-26 Furthermore, these techniques allow to obtain butanol in high concentration with low energy consumption.1 The combination of two methods further increases final product concentration and, therefore, reduces the energy used in its recovery. Xue et al.26 and Cai et al.1 have recently studied a gas stripping-pervaporation process which meets the advantages of simple operation by gas stripping (GS) and high butanol selectivity by pervaporation (PV). Process simulation softwares have been used to study the energy consumption for several configurations of industrial processes. Models for butanol production using Aspen Plus® software have been reported for raw materials such as sugarcane, sugarcane molasses and corn.27-34 Mansur et al.29 and Van der Merwe et al.33 simulated the production of butanol from an ABE process using sugarcane juice and molasses respectively. Mansur et al.29 separated the products in a stripper using N2 after the fermentation was finished. The organic phase of the condensate was sent to molecular sieves to eliminate traces of water and the azeotropes of butanol-water and ethanolwater, which simplifies the subsequent distillation step. They concluded that the main reason for the success on the positive energy and economic results was the use of a gas stripper and a molecular sieve, which concentrated the products before distillation. Van der Merwe et al.33 used a fed batch strategy with in situ product removal by gas


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stripping followed by liquid - liquid extraction and distillation. The main energy consumption was electricity, which was attributed to the use of compressors. Liu et al.29 simulated the butanol production by ABE fermentation from corn as a fed batch fermentation with in situ gas stripping and after that three stages of distillation and a molecular sieve for product recovery. The process included a concentration step of the culture medium. The fermentation and downstream process consumed 67% of the total steam and 30% of the total electricity. The authors reported that, although evaporation consumes energy, it decreases the volume of streams that must be treated downstream, which would reduce costs and energy. Therefore, eliminating the evaporation step would not necessarily lead to energy savings. Some researchers have particularly evaluated the use of energy of the butanol purification stages. Mariano et al.35 evaluated the flash fermentation technology and found that the energy consumption was essentially electrical demanded by compressors. The energy required for the flash fermentation represented 36% of the total energy for butanol recovery (flash fermentation plus distillation). This represented 39% reduction in energy for butanol recovery compared to the conventional batch process. Cai et al.1 found that the energy consumption of a GS-PV-D process was lower than that for a GSD process. Cai et al.36 found that the energy consumption of the distillation process after two stages of pervaporation was even lower than that of the GS-D and GS-PV-D processes. Although several authors worked on the evaluation of energy consumption in the butanol production process, the efficiencies of butanol recovery methods were not considered in those works, which makes their energy evaluation estimates more optimistic. None of them studied sweet sorghum as substrate which has been shown to be a complex material. In addition to simple sugars it contains organic acids that can


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inhibit fermentative processes.9,37 The energy consumption in the industrial butanol production by Clostridium acetobutylicum DSM 792 from an industrial mixture of sugarcane (75%) and sweet sorghum juices (25%) was studied in this work. Gas stripping assays were conducted to determine the gas stripping parameters (percentage of butanol recovery, time of gas stripping). A model for sugarcane-sweet sorghum juices based biobutanol production process was developed using a process simulation software (Aspen Plus®) and experimental data of raw material composition, kinetic parameters and process yields obtained experimentally, and values from expert consultations and literature38. Sensitivity studies were carried out to assess the variations in the sugar content in the industrial sugarcane and sorghum juice used as raw material, the butanol yield and the sugar conversion, on the energy consumption for butanol production. Also, the surplus energy generated marketable as electric energy was evaluated. A different strategy based on fed batch mode was also compared to the base case used (batch process).

MATERIALS AND METHODS

Experimental assays

Raw material A concentrated industrial juice composed of 75% of sugarcane and 25% of sweet sorghum was provided by Alur SA (Bella Unión, Uruguay). It was kept at 5°C. The sugarcane and sweet sorghum were harvested in May - June 2013 in the north of Uruguay (latitude: -30.249577, longitude: -57.579002). Its total sugar concentration was 87% (w/w) expressed as glucose equivalents (75% of sucrose, 5% glucose and 3%


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fructose). It also contained nitrogen (0.31% w/w) and potassium (0.76% w/w). The detailed composition was reported in a previous work.38

Gas stripping experiments Experiments were performed with 2 L of industrial juices fermented by Clostridium acetobutylicum DSM 792 in a 5-L bioreactor (Infors HT, Switzerland), as described previously38 with a content of 2.3 g/L of cells and 5.4, 7.1 and 1.8 g/L of acetone, butanol and ethanol, respectively. Gas stripping was carried out by recycling the headspace gasses through the solution at a flow rate of 1 L/min. The condenser was kept at 0 ºC, the bioreactor temperature 37°C and the agitation 150 rpm. Samples for analysis were aseptically withdrawn at different times during 43 h. A diagram of the system set up was described elsewhere.38

Analytical methods Acetone, butanol, ethanol and organic acids (acetic and butyric), were measured with a gas chromatograph (GC, Shimadzu GC-2010) equipped with a flame ionization detector and a fused silica column (RTX-Wax, 30 m long, 0.5 µm film thickness and 0.32 mm ID, Restek). The injector and detector temperatures were 250°C. The heating profile was 15°C per minute from 50°C to 100 °C, and 50°C per minute until 200°C.

Simulation methodology

Process description The facility processes 490000 t of sugarcane and sweet sorghum per year (annual production in Uruguay), works 180 days (24 h per day) per year since the crop


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is seasonally available. Butanol, acetone and ethanol purities were defined as 99.8% (w/w), 99.2% (w/w), and 99.2% (w/w), respectively. A block diagram for the industrial sugarcane and sweet sorghum juice conversion to butanol is shown in Fig. 1. The simulated process can be grouped into: pretreatment, fermentation with in situ gas stripping, butanol recovery and waste water treatment.

Figure 1 Pretreatment Both sugarcane and sweet sorghum are sent to the industrial plant in trucks. The transport energy consumption was estimated as 21 MJ t-1 from the work reported by Croce et al.39 for a sugarcane ethanol facility in Uruguay (average distance 20 km). The material is transported to the mill by a conveyor belt. Water is added in a water/raw material ratio of 2.5/1. The bagasse is separated from the juice (50% of moisture mass fraction) is sent to the boiler for steam generation. The pH is adjusted to 7 by adding lime.9 The juice is heated at 105 ºC by using two heat exchangers. Firstly, the juice is placed in contact with the hot pasteurized juice in order both to cool it and heat the cold, together with a second heating, up to 105 ºC. The juice is sent to a clarifier tank where flocculant is added. The clarified juice is sent to the fermenter for the fermentation process or is concentrated in a four-effect evaporator to be used as concentrated syrup in the fed batch strategy. The effects operate at 115, 106, 90 and 63 ºC, respectively.

Fermentation with in situ gas stripping The inoculum protocol is a direct transfer of a culture of Clostridium acetobutylicum DSM 792 to the fermenters. In order to provide 5% inoculum volume back to the production fermenters, 5% of the medium is split off to seed production.


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Each seed train consists of a number of bioreactors, which depends on the case considered, operating in batch mode for 24 h at 37 ºC. The inoculated cells (3x10-3 kg/L) are reutilized for a period of no more than 500 h (5 and 10 fermentations for the batch and fed batch configuration, respectively).40 Fermenters of 1700 m3 are used, a typical size of a Uruguayan facility. The fermenter temperature is kept constant at 37°C by pumping 2% of the culture medium (3100 kg/h) through an external heat exchanger.9 For both the batch and fed batch strategies the initial sugar concentration is fixed in 60 g/L to avoid substrate inhibition. The fermentation coupled with in situ gas stripping is done in three phases. The fermentation proceeds for 45 h until the butanol concentration is ~ 8 g/L, after which gas stripping is switched on. The off gasses (CO2 /H2) are then recycled and pass through the culture broth until the fermentation is completed (60 h). Gas stripping is continued after the fermentation is finished to recover the butanol remaining in the fermentation broth (see section Results and discussion, Post fermentation butanol recovery by gas stripping). The gasses and ABE vapors from fermentation are cooled in a condenser to 0 ºC and the remaining gas/vapor stream (mainly CO2) recycled. A CO2 fraction is removed, to obtain the CO2 flowrate needed for gas stripping. The gas/vapor stream is pressurized in a compressor and recycled to the bioreactor using a diffuser. The compressor is dimensioned to maintain a gas recycle rate equivalent to that optimum obtained in a previous work.38 The energy requirement of the gas stripping system was estimated by the vaporization enthalpy of the components in the fermentation broth and the energy requirement for the compressor. In the fed batch case, gas stripping started at the same time as the first medium addition, when the residual sugars decreased to 10 g/L. Concentrated juice solution


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(650 g/L total sugars) was added to reach a sugar concentration of 60 g/L. Ten additions of the concentrated media are made using the same culture. Table 1shows the kinetic parameters used to perform mass balances in the fermentation section. The kinetic parameters were obtained by Rochón et al.38 from the Eq. (1) to (3).

a

 µ S dX P  = m X 1 − − kd X dt K s + S  K p 

µ S dS µ X − = = m dt YX / S K s + S

(1)

a

  1 − P  X  K p  YX / S 

µ S dP µ X YP / S = = m dt YX / S Ks + S

(2)

a

  1 − P  X YP / S  K p  YX / S 

(3)

where X, S, and P are the dry cell weight, substrate and butanol concentrations (g/L), respectively; µ is the specific growth rate (h-1); µm is the maximum specific growth rate (h-1); kd is the specific cell death rate (h-1); YX/S (g/g) is the biomass yield; YP/S (g/g) is the butanol yield; Ks, Kp and a are kinetic parameters.

Table 1

Butanol recovery, waste water treatment The butanol final purification was similar to that adopted by Mansur et al.29. The recovery section consists of a holding tank to store the recovered condensate containing the ABE products, a decanter to further separate the products into a superior organic phase rich in butanol and an aqueous phase. The aqueous phase is then concentrated by


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a pervaporation stage. The remaining water is removed by molecular sieves and two distillation columns. The energy requirement for the distillation columns was estimated by the energy required for the reboilers using the Design Spec tool available in Aspen Plus®. When butanol concentration in the condensate is higher than its solubility in water (7.7% w/w at 20 ºC)41 an organic and an aqueous phase are obtained. The aqueous phase is sent to a pervaporation membrane, as studied by Xue et al.26, and then is mixed with the organic one to continue to dehydration and distillation columns to get the desired purity. When butanol concentration in the broth decreased to 6 g/L, the butanol concentration in the condensate does not allow phase separation and the stream is directly sent to the pervaporation membrane. The energy consumption of the pervaporation was calculated as reported by Vane.42 The fermented broth is centrifuged to separate bacterial cells. Cells are reused in the next batch. The liquid is treated in an anaerobic digestion wastewater treatment system. The biogas produced is sent to the boiler together with the bagasse. The liquid stream (mainly water) is sent to a reverse-osmosis system and recycled as water to the process. The heat generated is used to produce steam (80% efficiency) and electricity (44% efficiency from steam) supplying the plant.9 The surplus electricity is sold as coproduct. The H2 produced is low and is not recuperated as a product. Alternatively, it can be burned together with the biogas to obtain steam.

Process simulation The process was simulated using Aspen Plus® software (Aspen Technologies Inc., Cambridge, MA version V8.8). The non-random two-liquid activity coefficient model, using the Hayden-O'Connell model for the vapor phase, (NRTL-HOC) is used


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throughout most of the process for property estimations.33 A variable of the universal quasi-chemical (UNIQUAC) method (UNIQ-2) was used for liquid-liquid separation in the decanter (Help tool of Aspen Plus).43 The Aspen Plus model of the butanol plant (pretreatment, fermentation with in situ gas stripping, butanol recovery and waste water treatment) was developed in this work based on the results obtained in our laboratory (Table 1; experimental assays section)38, values from expert consultations and process configurations reported in the literature.1,29 Fig. 2 shows a simplified flow diagram of the process. Table 1 shows the main input parameters used for the fermentation coupled with in situ gas stripping. Sensitivity studies were carried out to evaluate the variations in: (1) sugar content in the extracted juice, (2) butanol yield, and (3) sugar conversion, on the energy consumption in the butanol production. Also, the surplus energy generated marketable as electric energy was evaluated. Table 2 shows the range of the process parameters and the base values studied, which were chosen after studying the production process in a Uruguayan facility and experimental data. A 33 factorial experimental design was done. A fed batch fermentation strategy was also compared to the base case in batch mode.

Figure 2 Table 2

Statistical analysis The data obtained were statistically evaluated by using the InfoStat software (student version 2013, http://www.infostat.com.ar). Analyses of variance (ANOVA) was used at significance level of p = 0.05. A multiple regression analysis was applied to the data from simulations to evaluate the effects of the sugar content in juice (S), the


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butanol yield (YP/S) and the sugar conversion (xs), respectively; on the overall energy consumption (E) and on the surplus of energy generated as electricity (G). The variables normalized are shown as Eq. (4) to (6): x1 =

S − 10 2

(4)

x2 =

Y P / S − 0 .20 0 . 05

(5)

x3 =

x s − 95 5

(6)

where S is expressed in % (w/w), YP/S in g/g, xs in %, and E and G in GJ/m3 butanol. The models used were:

E = a0 + a1 x1 + a2 x2 + a3 x3

(7)

G = b0 + b1 x1 + b2 x2 + b3 x3

(8)

RESULTS AND DISCUSSION

Post fermentation butanol recovery by gas stripping Data from gas stripping assays after the fermentation was completed, were analyzed using a polynomial function (Eq. (9)), which fitted well to the experimental values of mass fraction of butanol recovered from the medium and time of the gas stripping process.

R = −0.0293 t 2 + 2.4238 t − 0.1849

(r2 = 0.99)

(9)

where t is the time of the gas stripping (h) and R is the mass fraction of butanol recovered from the fermentation broth (%). This correlation together with data regarding the gas stripping process described previously38 was used in the simulations


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to determine the mass fraction of butanol recovered and the necessary time to have the gas stripping on. After 25 h of post fermentation gas stripping, 76% of butanol recovery was reached, obtaining a highly concentrated butanol condensate: 58% by gas stripping during fermentation and 42% by post fermentation gas stripping. A post fermentation gas stripping time higher than 25 h did not increase significantly the butanol recovery (Fig 3). The acetone and ethanol recovery efficiency were both 58%. No detectable amounts of organic acids were observed in condensates (< 0.02% w/v). In the fed batch strategy, gas stripping takes place most of the time together with the fermentation at a concentration of ~8 g/L of butanol. The last 25 h occurred in the same way as mentioned above for the batch case (total fed batch fermentation 490 h).

Figure 3

Evaluation of use of energy: batch fermentation strategy and sensitivity analysis The energy consumption reported for butanol production includes also the energy used for recovery and purification of acetone and ethanol. For the different conditions, that energy represented 0.4 - 0.6% of the total energy consumption. The energy generated by burning bagasse was enough to cover the energy required by the process for all studied cases (Fig. 4). The exceeding bagasse, and biogas from waste treatment in a minor contribution, allowed generating surplus of energy as electricity which can be commercialized. As the butanol yield, the sugar content in the industrial juice and the sugar conversion decreased, the energy consumption increased since less butanol was produced (Fig. 4). However, the surplus of electric energy generated increased because less butanol was produced for the same amount of annual


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raw material used, and therefore, more bagasse per m3 of butanol was available for this purpose.

Figure 4

The polynomial models, equations (10) and (11), describe the energy consumption (E) and surplus of energy generated as electricity (G) respectively, as a function of the normalized variables.

E = 36.71 – 1.26x1 – 8.85x2 – 1.82x3

(r2 = 0.98)

(10)

G = 45.85 – 13.87x1 – 11.65x2 – 2.87x3

(r2 = 0.96)

(11)

All studied variables were significant (p ≤ 0.05) as shown in Table 3. YP/S presented the most significant factor for energy consumption, since x2 presented the highest coefficient. The most significant factors were x1 and x2 for the surplus energy generated as electricity (S and YP/S respectively).

Table 3

The transport of the raw mat erial consumed less than 5% of the total energy in all cases studied (Table 4, Fig. 4). The clarification (at 105°C) and the butanol recovery stages showed the major energy consumption of the process (Table 4), representing 12% and 72% of the total energy consumption, respectively. The butanol recovery stages required 26.6 GJ/m3, which is slightly higher than the estimations done for gas stripping-distillation process (11-25 GJ/m3) by Qureshi et al.44 and Oudshoorn et al.45.


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Cai et al.1 found a total energy consumption of 20.1 GJ/m3 for a gas strippingpervaporation-distillation process. The higher values found in this work could be due to the fact that experimental values of butanol recovery efficiency were used for the gas stripping process in the simulations, which provide more conservative values. Nevertheless, according to our knowledge, no gas stripping recovery efficiencies have been considered in the literature for energy consumption estimations of the entirely butanol production by ABE fermentation using sugarcane and sweet sorghum. Furthermore, there are no energy evaluations works in the literature regarding sweet sorghum as raw matter for butanol production which has been shown to be a complex material.9,37,46

Table 4

The renewable net energy produced, calculated as the energy from the lower heating value of butanol, 26.9 GJ/m 3 butanol 41, plus the electric energy surplus generated, minus the energy consumed, was positive for all cases studied. However, if the surplus electric energy generated was not considered, the butanol produced net energy was positive only when the sugar content was 12%, butanol yield 0.25 g/g and sugar conversion 100%. This was because when the best values of the variables were evaluated, the amount of butanol produced was higher. The ethanol production from sweet sorghum juice, in a similar industrial plant having equivalent process configuration, was evaluated in a previous work using the same methodology9 (see Table 4). Based on the comparison with the sweet sorghum ethanol plant, the current butanol sugarcane/sweet sorghum plant consumes more energy to produce a unit of energy product. The energy recovered for butanol


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production including ethanol and acetone as coproducts and the surplus electric energy generated was 12.12 GJ/t fermentable sugars. If the use of the hydrogen generated during the ABE fermentation is considered as fuel for the generation of electricity, in addition to the use of biogas, the total energy recovered is 12.92 GJ/t fermentable sugars, assuming an efficiency of 35% (electricity output/energy input (LHV basis)). On the other hand, the energy recovered for ethanol production including the surplus electric energy generated was 24.48 GJ/t fermentable sugars (calculation from data reported by Larnaudie et al. 9). In order to be competitive, the current butanol plant must be accomplished with substantial enhancement of the ABE process such as development of high performance strains, improvements of the downstream processing and process integration.

Evaluation of use of energy: fed batch fermentation strategy The step of sugar concentration to obtain a syrup with 65% sugar content represented 42% of the total energy consumption. The total energy consumption was 69% higher for the fed batch strategy compared with the batch (Table 4). The total energy consumption in the evaporator stage was 26.3 GJ/m3. Chen and Chou47 and Eggleston et al.48 found that the sugar losses in the evaporators were less than 1%. In this work, a sugar loss of 1% was considered. Wu et al.34 obtained an energy consumption of 12.04 GJ/m3 in the evaporator to reach a sugar concentration of 430 g/L. It represented 70% of total energy consumption in the process from the evaporator to the recovery of butanol, while our results represent 49%. Larnaudie et al.9 obtained 10.79 GJ/ m3 ethanol to obtain a 25% sugar content in the syrup, which represented 48% of total energy consumption in the ethanol production process for a similar raw material.


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The utilization of concentrated substrates reduces the size of the fermentation equipment (or the number of fermenters needed) and the volume of the waste-disposal streams49, which would consequently decrease the process costs. However, from an energy perspective, as it found in our previous work for ethanol production9, the fed batch strategy with a concentrated syrup (650 g/L total sugars) can be attractive if juices, that were concentrated to avoid deterioration and save space during storage, are industrially available. Furthermore, a different fed batch configuration could be applied if a raw juice with sufficiently high sugar content is obtained, not requiring its concentration.

CONCLUSIONS The butanol yield was the factor that presented the greatest effect on the industrial energy consumption. This highlights the importance of having industrial strains with high performance. For a fed batch strategy where a concentration step is required, renewable energy sources are needed to have a favorable overall energy balance since this step consumes more than 40% of the total energy consumption in the process. Butanol production by ABE fermentation produces less butanol (energy basis) per kg of sugarcane/sweet sorghum than a conventional sweet sorghum ethanol production which suggests that a substantial improvement in the upstream and downstream of the ABE process is needed.

ACKNOWLEDGEMENTS The financial support was provided by CSIC-ANCAP (Uruguay). The authors thank Alur SA for providing industrial concentrated juices and to the Agencia Nacional


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de Investigación e Innovación and Comisión Sectorial de Investigación Científica for the posgraduate scholarship of Eloísa Rochón.

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Energy & Fuels

Figure Captions

Fig. 1. Simplified block flow diagram of the butanol production process.

Fig. 2. Simplified flow diagram of the butanol production from sugarcane and sweet sorghum in Aspen Plus®.

Fig. 3. Butanol recovered profile from post fermentation gas stripping assays. Experimental (symbols); simulated (lines).

Fig. 4. Variation of energy consumption and surplus electricity generation with: (a) sugar content in the juice (0.20 g/g butanol yield, and 95% sugar conversion), (b) butanol yield (10% sugar content in the juice, and 95% sugar conversion), c) sugar conversion (10% sugar content in the juice, 0.20 g/g butanol yield).


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Tables Table 1 Main input parameters for fermentation and gas stripping simulations. Parameter

Unit

Value

µm

h-1

0.22

Ks

g/L

0.20

Kp

g/L

11

-1

kd

h

0.04

a

-

1.7

Acetone

g acetone/g glucose

0.05

Ethanol

g ethanol/g glucose

0.03

g butanol/glucose

0.20

Acetic acid

g acetic acid/g glucose

0.018

Butyric acid

g butyric acid/g glucose

0.011

g hydrogen/g glucose

0.021

g carbon dioxide/g glucose

0.69

g biomass/g glucose

0.11

Fermentation temperature

ºC

37

Fermenter operating pressure

Pa

101325

Sugar conversion

%

Butanol (YP/S)

Mass fraction yield

Hydrogena CO2a Cells (YX/S)

Gas recycle rate

vvm

Gas stripping efficiency Condenser temperature

100 b

0.4

%

76

ºC

0

a

28

Data reported by Liu et al.

b

vvm: volume of gas per volume of medium per min


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Energy & Fuels

Table 2 Process parameters evaluated in sensitivity analysis. Value Parameter

Minimum

Sugar content in juice (% w/w) a Butanol yield (g/g) Sugar conversion (%) a

Baseline scenarios

Maximum

8

10

12

0.15

0.20

0.25

90

95

100

Total sugars expressed as glucose equivalents.


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Table 3 Analysis of variance (ANOVA) for energy consumption.

Source Regression Sugar in juice (x1) Butanol yield (x2) Sugar conversion (x3) Error Total

Degree of freedom 3 1 1 1 23 26

Sum of squares adjusted 1496.11 28.53 1408.21 59.37 32.34 1528.44

Mean square adjusted 498.70 28.53 1408.21 59.37 1.41

F-Value

p-Value

354.71 20.29 1001.62 42.23

Process energy evaluation of fuel butanol production from sugarcane

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