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New alternatives for the fermentation process in the ethanol production from sugarcane: Extractive and low temperature fermentation
Energy xxx (2014) 1e10
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New alternatives for the fermentation process in the ethanol production from sugarcane: Extractive and low temperature fermentation Reynaldo Palacios-Bereche a, *, Adriano Ensinas a, b, Marcelo Modesto a, Silvia A. Nebra a, c a
Centre of Engineering, Modelling and Social Sciences, Federal University of ABC (CECS/UFABC), Rua Santa Adélia 166, CEP 09210170 Santo André, SP, Brazil École Polytechnique Fédérale de Lausanne, Switzerland Interdisciplinary Centre of Energy Planning, University of Campinas (NIPE/UNICAMP), Rua Cora Coralina 330, PO Box 6166, CEP 13083896 Campinas, SP, Brazil
b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 January 2014 Received in revised form 8 March 2014 Accepted 14 April 2014 Available online xxx
Ethanol is produced in large scale from sugarcane in Brazil by fermentation of sugars and distillation. This is currently considered as an efficient biofuel technology, leading to significant reduction on greenhouse gases emissions. However, some improvements in the process can be introduced in order to improve the use of energy. In current distilleries, a significant fraction of the energy consumption occurs in the purification step e distillation and dehydration e since conventional fermentation systems employed in the industry require low substrate concentration, which must be distilled, consequently with high energy consumption. In this study, alternatives to the conventional fermentation processes are assessed, through computer simulation: low temperature fermentation and vacuum extractive fermentation. The aim of this study is to assess the incorporation of these alternative fermentation processes in ethanol production, energy consumption and electricity surplus produced in the cogeneration system. Several cases were evaluated. Thermal integration technique was applied. Results shown that the ethanol production increases between 3.3% and 4.8% and a reduction in steam consumption happens of up to 36%. About the electricity surplus, a value of 85 kWh/t of cane can be achieved when condensing e extracting steam turbines are used. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Sugarcane Ethanol Fermentation Energy Thermal integration
1. Introduction Nowadays, a lot of research is devoted to the production of second generation ethanol from sugarcane, but, there is room for improvements in the traditional production of first generation ethanol, where the attention is not focused. Only a few researchers are working in this direction [1]. A significant fraction of the energy consumption in ethanol production happens in the distillation step [2], since the conventional fermentation systems used in the sugarcane industry require a low concentration of substrate and hence produces a low content of ethanol in the wine, which must be distilled consequently with high energy consumption. Thus, to study more efficient alternatives for the fermentation process would be very useful, focussing in systems that allow a higher concentration of substrate, which * Corresponding author. Tel.: þ55 11 49968272. E-mail addresses: [email protected] (R. Palacios-Bereche), adriano. ensinas@epfl.ch (A. Ensinas), [email protected] (M. Modesto), silvia. [email protected], [email protected] (S.A. Nebra).
produces, consequently, a less energy consumption in the following stage: the distillation. Between these alternatives we have: low temperature fermentation and extractive fermentation. Both alternatives permit to work with a higher concentration of ethanol in the wine that goes in the distillation process. This fact has also another advantage: the production of stillage is reduced, diminishing its deposition costs and environmental impact.
1.1. Low temperature fermentation The fermentation under anaerobic conditions is an exothermic biochemical reaction producing 580 kJ/kg of TRS (Total Reducing Sugars) converted. Thus, cooling is required to remove the heat produced. The temperature should be controlled between 30 C and 34 C, since temperatures outside this range generally will result in a low yield. For higher temperatures, other products beyond ethanol may be formed, whilst for lower temperatures, the fermentation time is extended, which provides greater
http://dx.doi.org/10.1016/j.energy.2014.04.032 0360-5442/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Palacios-Bereche R, et al., New alternatives for the fermentation process in the ethanol production from sugarcane: Extractive and low temperature fermentation, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.04.032
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opportunities for the bacteria or other organisms to ferment sugars, producing unwanted products [3,4]. In the literature, several studies indicate that fermentation at low temperature, in the range of 25 Ce30 C improves the efficiency of the process ([5e7]), since it is possible to use higher concentrations of substrate, therefore achieving higher ethanol concentrations in the wine. Dias et al. [6] evaluated technological alternatives that aimed to improve the cooling system of the fermentation process. To provide cold water, this study evaluated the use of cooling towers during the coldest hours of the day, and the use of a secondary device during the hottest hours. The pieces of equipment proposed were an accumulator of cold water, a system driven by a steam ejector and an absorption refrigeration system. The authors indicate that the alcoholic fermentation conducted at 28 C enables an ethanol concentration of 13 GL (Gay Lussac degrees, percentage of ethanol by volume) in the wine, which decreases the production of stillage to 5.76 litres per litre of hydrous ethanol. Magazoni et al. [8] proposed the use of an absorption refrigeration system single-effect water/lithium bromide absorption chiller, for use in cooling the fermentation, powered by waste heat from the production processes. A leading Brazilian company, Dedini Basic Industries S. A. (Piracicaba, Brazil), installed a demonstration plant of 20,000 L of ethanol/day, with temperature control between 28 C and 32 C. An absorption refrigeration system, built by Thermax, was used for this purpose. The results indicate that temperatures in the range from 28 C to 30 C are more suitable for obtaining a high alcoholic content in wine [7]. Cardemil et al. [9] performed an economic evaluation of different alternatives for cooling the fermentation, which were: absorption refrigeration system of simple and double effect and conventional vapour compression. Refrigeration systems based on ejectors may also be used [6]; they have some advantages as the absence of moving parts, take up little space and have low cost [4]; but, they are not energy efficient [10], they have a low COP (Coefficient of Performance) and a lower cooling capacity.
1.2. Extractive fermentation The ethanol accumulated in the fermentation medium inhibits the metabolic activity of the yeast, presenting a significant effect on the rate of cell growth at concentrations above 15 g/L [11,12]. The maximum ethanol concentration at which cell growth ceases is 100 g/L while at a concentration of 105 g/L ethanol, the production ability of yeast Saccharomyces cerevisiae is completely inhibited. So, the use of techniques of extracting ethanol from the fermentation media improved the process performance; one of such techniques is the vacuum extractive fermentation, wherein the fermentation reactor is coupled to a vacuum flash tank. The removal of ethanol from fermentation media, as soon as it is produced, reduces its inhibitory effect on yeast cells. Furthermore, the vacuum and recycling of the liquid stream allow to maintain the reactor at relatively low temperatures (28 Ce32 C), without needing external heat exchangers. These low temperatures and the removal of ethanol from fermentation media allow the use of a substrate with higher concentration. Consequently, a smaller amount of stillage (vinasse) is produced and the energy consumption is diminished in the distillation stage. Maugeri and Atala performed the assembly and instrumentation of a continuous fermentation system coupled to a vacuum flash separator in laboratory scale using the yeast Saccharomyces cerevisiae and sugarcane molasses as a substrate. A result of this work
has been the patent PI-0500321-0 A “Vacuum extractive fermentation process for ethanol production” [13]. Besides this experimental work, there are studies of modelling and simulation in this area. Rivera et al. [14], using optimization techniques evaluated the kinetic parameters of the alcoholic fermentation process in batch. Costa et al. [15] conducted a modelling study based on factorial design to study the dynamic behaviour of vacuum extractive fermentation process. 1.3. Objective The ethanol production process has a unique characteristic: the sugarcane input of the process, provides at the same time the raw material (juice) and the fuel (bagasse), to obtain the two most important products: ethanol and electric energy (through cogeneration). So, the energy management is crucial to increase the total production. The authors previously mentioned have performed experimental studies and simulations of both types of systems: low temperature and extractive fermentation, but a deep study on the consequences of the introduction of these devices in the ethanol production system were not explored from the energy management point of view. Moreover, no one of them has worked with tools of thermal integration. Thus, the aim of this work is to perform a study focussing the energy management improvement through the introduction of alternative technologies in the fermentation process and in the refrigeration system, using the Pinch method to perform thermal integration in the entire production system. Simulations of the ethanol production, including these alternative technologies, using the software Aspen PlusÒ, were performed. In the fermentation at low temperature, different technologies of refrigeration have been evaluated, for instance: vapour compression and absorption refrigeration systems. Regarding the vacuum extractive fermentation, two proposals were studied: the first considers mechanical compressor for conveying the steam from the flash tank to the distillation columns; whereas in the second, flash vapour is condensed and then pumped to the distillation columns. The thermal energy consumption and the consequences in the electric energy generation were evaluated in all the cases. Furthermore, the thermal integration process was applied to all cases. The Pinch method was used in order to optimize the system and reduce the consumption of thermal energy. 2. Process description The production process of the current first generation ethanol from sugarcane can be briefly described according to the scheme shown in Fig. 1. 2.1. Sugarcane preparation and juice extraction Before entering the extraction system, a cleaning system removes excessive amounts of soil, rocks and trash coming with the sugarcane. After cleaning, sugarcane is prepared by means of rotary knives and shredders that cut it into small pieces, suitable for the subsequent extraction process. A juice extraction system separates bagasse and juice by compressing the cane. Bagasse is used as fuel for the cogeneration system, and the raw juice is sent to the processing system. 2.2. Juice treatment Some non-sugar impurities are separated by adding chemical reactants such as calcium oxide; heating is necessary for the
Please cite this article in press as: Palacios-Bereche R, et al., New alternatives for the fermentation process in the ethanol production from sugarcane: Extractive and low temperature fermentation, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.04.032
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Fig. 1. Scheme of the ethanol production process from sugarcane.
purification reactions. Following that, the juice goes through a flash tank before entering the clarifier (settler). The precipitate is separated from the clarified juice and sent to filters. The filtrate is returned to the process and mixed with the main juice stream, and the filter cake is rejected. The clarified juice is sent to the evaporation system.
2.6. Dehydration
2.3. Juice evaporation
It is constituted by boilers, back pressure turbines and/or extraction-condensing turbines, electric generators, deaerator, pumps. Bagasse is used as fuel.
Juice is concentrated in a multiple-effect evaporator. Exhaust steam from the cogeneration system is used as a thermal energy source in the first effect; water evaporated from the juice is used as heating source for the subsequent effect. The multiple-effect evaporator works with decreasing pressure due to a vacuum imposed on the last effect, producing the necessary temperature difference between consecutive effects. Vapour bleed can be used to cover heat requirements of other parts of the process, such as juice processing heaters and the sugar boiling system. Part of the juice for ethanol production goes through the evaporation system to reach the necessary concentration for the fermentation process. The remainder of the juice by-passes concentration and goes directly to the fermentation process, to be mixed with concentrated juice for mash preparation. 2.4. Fermentation The sugars of the must are converted in ethanol, with emission of carbon dioxide, in an exothermal reaction. After fermentation, the liquor, containing about 8% of ethanol (mass basis), is taken to the distillation system to remove the water. 2.5. Distillation Ethanol produced by fermentation is recovered by distillation. Fermented liquor is heated to a suitable temperature before entering the first distillation column. Hydrous ethanol is obtained by stripping and rectification stages. A large amount of vinasse is generated e 10e12 litres per litre of ethanol e which must be handled as an effluent.
In order to remove the remaining water and produce anhydrous ethanol, a dehydration stage is required at the end of the process. 2.7. Cogeneration
3. Modelling and simulation Referred to the mill capacity, the following values were adopted: milling of 2,000,000 t of cane/year; 4000 h/year of harvesting period, which results in 500 t of cane/h. These values correspond to a Brazilian standard size plant [16] (other possibilities of plant size were not considered, taking into account that other countries do not have an expressive production of ethanol from sugarcane). Each one of the control volume showed in Fig. 1 was simulated using Aspen Plus software. Adequate parameters were adopted for each one of the process; their values were taken from de references indicated. For the extraction process, a dry cleaning system and mills were adopted [4]. The bagasse composition (dry basis, wt.) was: cellulose 36.8%, hemicelluloses 35%, lignin 20.3%, and ashes 2.3%. This composition was calculated from the parameters of the cleaning, preparation and extraction system assuming the bagasse moisture content (wet basis, wt.) of 50%. The juice treatment included the operations of: heating, flashing, decantation, addition of flocculants and calcium hydroxide, filtration with filtrate recirculation [4,7,17]. For the juice concentration, an evaporation system of five effects, Roberts type, was adopted, with pressures from 1.69 to 0.16 bar, the first effect is heated with steam at 2.5 bar, coming from the cogeneration system, vapour bleedings are used to heat other plant streams [4,17]. The fermentation was simulated as a continuous process, the usual real process, MelleeBoinot type, is a fed-batch process. The
Please cite this article in press as: Palacios-Bereche R, et al., New alternatives for the fermentation process in the ethanol production from sugarcane: Extractive and low temperature fermentation, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.04.032
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Fig. 2. Vacuum extractive fermentation e proposal A [17,24].
main reactions are the hydrolysis of the sucrose to glucose and fructose, which are after converted to ethanol and carbon dioxide in a reaction catalysed by the Saccharomy cerevisiae yeast. In small quantities some other products are formed, as succinic acid, glycerol, acetic acid, etc., it happens also the yeast growth. The yeast is centrifuged and reused. In the conventional process, a temperature of 34 C is maintained, refrigerating the must with a cooling tower [7,17,18]. The distillation and rectification follow the pattern adopted in all the Brazilian distilleries [16]: a set of distiller column with three sections: stripping, rectifying, and head concentrator, the rectification of phlegm being done in a set stripping/rectifier (column B, B1) [17]. In the base case, the heating is made with steam at 2.5 bar. The extractive distillation with monoethylene glycol was adopted for the dehydration process; two columns are used, one for extraction and the other for the solvent recuperation; the column heating is performed with 6 bar steam, obtained from a bleeding in the steam turbines of the cogeneration system [17,19,20]. The boilers of the cogeneration system are fuelled by bagasse with an efficiency of 86%, LHV base (Lower Heating Value base); about the turbines, two situations were simulated: back pressure (Configuration I) and extraction-condensing turbines (Configuration II). An isentropic efficiency of 80% was considered for the turbines. The steam parameters were 90 bar and 530 C, which are currently used in this type of plants. Two products are obtained from this system: steam at 2.5 and 6 bar and electric energy [17,21,22]. 3.1. Low temperature fermentation A fermentation temperature of 28 C was adopted, lower than 34 C of the conventional process. A conversion of 92.7% of glucose to ethanol and 28% of TRS (Total Reducing Sugars) in the substrate, were adopted [3]. The must in the fermentation vat needs to be refrigerated and recirculated. Two technologies of refrigeration were simulated, a vapour compression system (COP ¼ 3.5) and a single-effect absorption system of Lithium e Bromide (LiBr/H2O) (COP ¼ 0.7), fuelled by heat from the evaporation bleedings.
3.2. Vacuum extractive fermentation In this process, a vacuum flash tank is linked to the fermentation process to remove the ethanol from the fermentative medium. Two proposals were evaluated for that system, one follow the proposal of Dias et al. [17,23] and Junqueira et al. [24] and the other, the proposal of Cohen et al. [25]. The proposal of Dias et al. and Junqueira et al. [17,23,24] is showed in Fig. 2. In this proposal, the wine produced in the fermentation reactor is sent to the centrifugal separator (SEPA-CEL), two phases are obtained. The heavy phase (CREAMLEV) with a high concentration of yeast cells is sent to the processing unit (TRATFER) while the light phase (wine without yeast) is sent to the vacuum flash tank (TFLASH). In the treatment unit yeast, water, sulphuric acid and nutrients are added. Yeast cells treated (LEV) are sent to the fermentation reactor. In vacuum flash tank, the vapour phase (VAP-W1) is compressed and sent to the distillation unit, while the liquid phase is divided into two streams. The stream R-W2 is recirculated to the fermentation reactor, while the stream RW4 is mixed with an alcohol solution recovered in the absorption column (SEPA2) and sent to the distillation column. The compression of vapour of the flash tank is represented by the block COMP-S in Fig. 2, a process of three stages of compression with inter-coolers and after-cooler was adopted. The proposal of Cohen et al. [25], is showed in Fig. 3. This proposal is very similar to the precedent; the main difference is the existence of a partial condenser (C-FL), where is condensed part of the steam separated in the flash tank (T-FLASH). The liquid phase obtained in the condenser is an alcoholic solution with a high content of ethanol, which is mixed with the wine purged (RW5) and alcoholic solution (ALCSOL) obtained in the absorption column (SEPA2). Finally the resulting wine (WINE2) is sent to the distillation stage. The vapour phase, remaining of the partial condenser, is compressed in the unit COMP-S and is sent to the absorption column (SEPA2) to recover the ethanol entrained in this stream. The temperature at the outlet of inter-coolers and after-cooler was specified at 30 C, 40 C and 80 C respectively.
Please cite this article in press as: Palacios-Bereche R, et al., New alternatives for the fermentation process in the ethanol production from sugarcane: Extractive and low temperature fermentation, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.04.032
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Fig. 3. Vacuum extractive fermentation e proposal B [25].
The partial condenser (Block CF-L in Fig. 3) must operate at temperatures below 0 C, therefore, the refrigerant should be at temperatures below this value. For this reason it was decided to use absorption refrigeration systems Ammonia e Water (NH3/ H2O), since LiBr/H2O systems have the limitation of the freezing point of the coolant (water) at 0 C. Thus, in this part of the study, several alternatives have been taken, which include a cooling system for vapour compression, an absorption refrigeration system NH3/H2O (COP 0.413) and an absorption refrigeration system NH3/H2O GAX type (exchange generatoreabsorber heat, COP 1.03) [26]. The energy consumption related to the operation of all types of refrigeration systems was considered in the evaluation. Table 1 presents a synopsis of the cases evaluated in this study. The proposals A and B are referred to the two types of fermentation system technology.
3.3. Thermal integration The Pinch method is used to perform the thermal integration [27e29]. The minimum temperature difference (DTmin) adopted Table 1 Description of the cases evaluated in reference to the fermentation system technology. Case
Refrigeration technology in the fermentation system
Description
BASE
Cooling tower for the fermentation vats Vapour compressiona
Conventional distillery
RE-SRC RE-SRA
Single effect absorption LiBr/H2Oa
EXT-A EXT-B-SRC EXT-B-SRA EXT-B-SRA/GAX a b
Vapour compressionb Single effect absorption LiBr/H2Ob Absorption NH3/H2O GAX typeb
Refrigeration system for the fermentation vats. Refrigeration system for the partial condenser.
Low temperature fermentation Low temperature fermentation Vacuum extractive fermentatione Prop. Vacuum extractive fermentatione Prop. Vacuum extractive fermentatione Prop. Vacuum extractive fermentatione Prop.
A B B B
was 10 C for the process streams and 4 C for the evaporation system streams. Since the vapour bleedings from the evaporation system are used to satisfy heating requirements of different processes, and their flow rates are a characteristic of them, the thermal integration is achieved by applying the following iterative steps: Step 1: Thermal integration of process streams available excluding evaporation system. Construction of the initial Grand Composite Curve (GCC).
Table 2 Streams adopted for thermal integration for each case. Ti ( C) Tf ( C) DH (MW)
Hot streams Sterilized juice Phlegmasse Vinasse Anhydrous ethanol Condensates of steama Condenser column D Condenser column B Condenser extractive column Alcoholic vapour INT-R1 Alcoholic vapour INT-R2 Alcoholic vapour P-R3 Cold streams Imbibition water Juice for treatment Juice pre-heating Juice for sterilization Final wine Reboiler column A Reboiler column B Reboiler extractive column Reboiler recuperative column Solution LiBr/H2O in the generatorb Solution NH3/H2O in the generatorc Solution NH3/H2O in the generatord
Base RE
EXT-A EXT-B
130 103.9 109.3 78.3 110 84.9 81.7 78.3 109.4 140.7 175.6
32 35 35 35 35 35 81.7 78.3 50 80 100
41 3 37.2 8.6 8.4 19.5 26.4 7.4
23.4 2.0 20.5 8.9 20.6 12.6 26.8 7.7
18.0 2.5 15.1 8.9 21.0 16.4 26.9 7.7 0.9 0.9 1.2
18.0 1.7 16.7 9.1 21.2 11.1 27.2 7.8
25 34.2 98.1 95.5 31.2 109.3 103.9 134.5 149.6 76.8 127.4 123.7
50 105 115 130 90 109.3 103.9 134.5 149.6 89.4 152.4 200
4.4 44 2.7 14.6 33.7 43.7 21.8 6.7 2.5
4.4 44 6.4 10.3 21.0 30.8 23.4 7.3 2.5 21.7
4.4 44 7.3 6.9 15.6 20.4 23.2 7.2 2.6
4.4 44 7.3 6.9 18.8 27.7 24.3 7.4 2.5 34.5 13.9
Ti ¼ inlet temperature, Tf ¼ outlet temperature, DH ¼ stream enthalpy variation in the heat exchanger. a The DH and temperature values change due to the iterative process. b For the case RE-SRA. c For the case EXT-B-SRA. d For the case EXT-B-SRA/GAX.
Please cite this article in press as: Palacios-Bereche R, et al., New alternatives for the fermentation process in the ethanol production from sugarcane: Extractive and low temperature fermentation, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.04.032
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Table 3 Anhydrous ethanol, vinasse and steam consumption in the distillation process, without thermal integration.
Anhydrous ethanol, (l/t cane) Vinasse (m3/m3 ethanol) Distillation steam consumption (kg/l ethanol) Ethanol production increasing (%)
Base
RE
EXT-A
EXT-B
80.4 11.4 2.74 e
83.1 6.0 2.19 3.37
83.4 4.2 1.76 3.76
84.3 4.8 2.08 4.87
Step 2: Calculation of appropriate vapour bleedings in each effect of the evaporation system, according to the procedure proposed in Refs. [30e32]. Step 3: Integration of the evaporation system with the appropriate demand including the vapour bleedings optimized in each effect. Step 4: Update of the mass rates of the evaporation system condensates. Return to Step 2 until convergence. Table 2 shows the streams adopted for the thermal integration process. In Table 2, supply and target temperatures are equal because only fermentation parameters were modified in the studied cases while the parameters of the other operations were kept equals. However, the heat flows are different in the operations related to: juice sterilization, distillation and evaporation.
4. Results and discussion Table 3 shows the results in terms of the ethanol produced, the vinasse quantity and the steam consumption in the distillation process, for each one of the cases, without thermal integration. These results show that the largest ethanol production corresponds to the vacuum extractive fermentation refrigerated with a vapour compression system (EXT-B). But, the less vinasse production and the lowest steam consumption correspond to the vacuum extractive fermentation (EXT-A).
The lower steam consumption in the distillation corresponds to the EXT-A case, with a reduction of 35.8%. The steam consumption in the EXT-B case results lower than the RE case, but higher than the EXT-A. This effect happens due to the flash steam is compressed and fed in the distillation columns directly in vapour phase. Moreover, in EXT-B case, part of the flash vapour is condensed and mixed with the liquid wine, being after heated and fed in the distillation column, this fact increases also the steam consumption in this stage. In the RE case, the final ethanol content in wine is 10.9% m.b. (mass base). The mass flow of water used in the absorption column is 17.1 m3/h (0.034 m3/t of cane), which permits to obtain an ethanol recuperation of 96.6%. In the EXT-A case, the steam obtained in the flash tank has an ethanol content of 44.1% (m.b.), and the liquid phase, 9.1% (m.b). The results obtained in the present study show that 90% of the liquid wine need to be recirculated, returning to the fermentation reactor, the other 10% is mixed with the alcoholic solution of the absorption column and sending to the distillation columns. This final wine has an ethanol content of 8.9% (m.b.). In this case, the mass flow of the absorption column is of 10 m3/h (0.02 m3/t of cane), reaching a value of 98.5% of ethanol recuperation. In the EXT-B case, the ethanol content in both phases, liquid and vapour in the flash tank are the same than in EXT-A case. In the partial condenser, the liquid phase obtained has an ethanol content of 47% (m.b.) and the vapour phase, 25.3% (m.b.), this flow is sent to the absorption column. As in EXT-A case, 90% of the liquid wine is recirculated to the fermentation reactor. The water used in the absorption column is of 20.1 m3/h (0.04 m3/t of cane), obtaining an ethanol recuperation of 98.5%. The ethanol content of the final wine is of 13.2% (m.b.). The Pinch method, applied according to the procedure described before, allowed determining the goals of minimum energy consumption. In this procedure, the first step corresponds to the construction and integration of the process streams without the bleedings of the evaporation system. In the second step, the vapour bleeding, optimized for each effect of the evaporation system, are determined. In the following step, the evaporation system, including
Fig. 4. GCCs for the base case and for both cases of fermentation at low temperature.
Please cite this article in press as: Palacios-Bereche R, et al., New alternatives for the fermentation process in the ethanol production from sugarcane: Extractive and low temperature fermentation, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.04.032
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Fig. 5. GCCs for the vacuum extractive fermentation cases.
the bleedings, is recalculated. Subsequently, the evaporation system streams are integrated into the process. Finally, the GCC is built, and the goals of minimum consumption of utilities are determined. Fig. 4 shows the final GCC for the base case and the case of fermentation at low temperature, while Fig. 5 shows the final GCC for the cases of vacuum extractive fermentation. Table 4 presents the proposed vapour bleedings in the evaporation system, determined from the thermal integration procedure. The big increase of the second effect bleeding in the case RE-SRA is due to that the absorption refrigeration system LiBr/H2O works near to this temperature range. From the GCC, the steam mass flow needed to satisfy the hot utility requirements can be calculated. To determine the steam consumption, a heat exchangers network was designed, and the steam consumption in each process unit was determined. Table 5 shows the steam consumption for the cases thermally not integrated and integrated. The network grid for the RESRA integrated case integrated case is presented in the Annex. Concerning the cases RE-SRC and RE-SRA (and all EXT-B) in Table 5, it can be observed the same values for steam consumption (except for evaporation system). It can be explained by the fact that operation conditions of stream processes are the same, being different only the energy input for cooling system, which for the cases SRC is mechanical power (electricity) and in the cases SRA is vapour or steam. Fig. 6 shows the results obtained in terms of the total steam consumption, for the systems without thermal integration. The juice heating is made using steam of the first evaporation effect (71.5 t/h at 1.69 bar). In the case RE-SRA the energy for the Table 4 Vapour bleedings in the evaporation system (kg/t of cane). Case
Base
First effect (115 C) Second effect (107.3 C)
88 0
RE
EXT-A
SRC
SRA
95.2 18.8
95 86
109.8 0
EXT-B SRC
SRA
SRA/GAX
98.4 19.8
98.4 20.4
98.4 19.8
Table 5 Steam consumption in each process operation (kg/t of cane). Base
RE SRC
EXT-A EXT-B SRA
SRC
SRA
SRA/GAX
Without thermal integration Steam at 6 bar Must sterilization 50.4 35.5 35.5 23.8 23.8 23.8 23.8 Ethanol dehydration: 23.0 25.1 25.1 25.0 25.4 25.4 25.4 extractive column Ethanol dehydration: 9.8 8.6 8.6 8.9 8.7 8.7 8.7 recuperative column Refrigeration absorption 119.2 system NH3/H2O Steam at 2.5 bar Evaporation system 166.6 233.0 291.2 248.6 248.6 248.6 248.6 Distillation: first column 144.2 101.6 101.6 67.4 91.5 91.5 91.5 Distillation: second column 71.9 77.2 77.2 76.6 80.1 80.1 80.1 Steam at 19 bar Refrigeration absorption 53.5 system NH3/H2O e GAX type. Total 465.9 481.0 539.2 450.2 478.1 597.4 531.7 With thermal integration Steam at 6 bar Must sterilization 15.1 8.8 8.8 4.9 3.8 3.8 3.8 Ethanol dehydration: 23.0 25.2 25.2 25.0 25.5 25.5 25.5 extractive column Ethanol dehydration: 9.8 8.6 8.6 9.0 8.6 8.6 8.6 recuperative column Refrigeration absorption 119.1 18.4 system NH3/H2O Steam at 2.5 bar Evaporation system 3.7 8.8 8.8 15.6 10.0 10.0 10.0 Distillation: first column 112.3 184.4 227.0 197.2 205.6 200.8 205.7 Distillation: second column 144.3 101.7 101.7 67.3 91.4 91.4 91.4 Steam at 19 bar Refrigeration absorption 32.4 system NH3/H2O e GAX type. Total 308.2 337.4 380.1 319.0 345.0 459.3 396.0
Please cite this article in press as: Palacios-Bereche R, et al., New alternatives for the fermentation process in the ethanol production from sugarcane: Extractive and low temperature fermentation, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.04.032
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Fig. 6. Steam consumption e without thermal integration (kg/t of cane).
Fig. 7. Steam consumption e thermal integrated systems (kg/t of cane).
refrigeration system is obtained also from the first evaporation effect (35.2 t/h). The minimum consumption was obtained for the system that includes vacuum extractive fermentation. The second lowest consumption corresponds to the system with refrigerated fermentation working with vapour compression (the refrigerated fermentation working with a single effect absorption system corresponds to the biggest steam consumption). Fig. 7 shows the results for the thermal integrated systems. The total steam consumption has decreased in all the cases, but the general tendency is the same like in Fig. 6. Table 6 reports the results in relation to the electricity and bagasse surplus. The results presented here must be analysed
together with that of Figs. 6 and 7, relative to the steam consumption, because the electricity generated is linked to the steam consumption in cogeneration systems with back pressure turbines (Configuration I), and also Table 3, relative to ethanol production. In the Configuration II, the bagasse surplus is zero because it was considered that all the bagasse is burnt in the boiler. The results in Tables 3 and 6 show that for the Configuration II, in the case that includes a distillation system based on vacuum extractive fermentation refrigerated with an absorption system NH3/H2O GAX type, we have an increase of 4.85% in the ethanol production but a diminution of 11.9% in the electricity surplus.
Table 6 Electricity and bagasse surplus, different configurations referred to the cogeneration system, with and without thermal integration.
Without thermal integration Configuration I: (T-CP) Electricity surplus (kWh/t of cane) Bagasse surplus (%)a Configuration II: (T-EC) Electricity surplus (kWh/t of cane) Bagasse surplus (%)a With thermal integration Configuration I: (T-CP) Electricity surplus (kWh/t of cane) Bagasse surplus (%)a Configuration II: (T-EC) Electricity surplus (kWh/t of cane) Bagasse surplus (%)a
Base
RE-SRC
RE-SRA
EXT-A
EXT-B e SRC
EXT-B e SRA
EXT-B e SRAeGAX
49.1 17.6
43.6 14.8
63.2 4.8
38.6 19.8
43.2 15.0
67.5 3.1
56.7 7.9
78.2 0
68.0 0
71.3 0
71.3 0
68.2 0
e e
67.5 0
19.6 44.0
16.5 38.7
33.1 31.5
13.5 41.8
17.8 37.4
41.1 20.1
31.5 30.1
92.4 0
80.7 0
85.2 0
82.8 0
79.7 0
74.4 0
81.4 0
T-CP: Back pressure turbine; T-EC: extraction condensation turbine. a Percentage in relation to the total bagasse produced.
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Comparing the results of the present study with others from the literature, Dias et al. [6] reported ethanol production increases of 4.8% and 3.8% for low temperature fermentation and vacuum extractive fermentation respectively, which are very close to the results obtained in the present work. Regarding the vinasse production, the results obtained by Ref. [6] are in the same range. On the other hand, considering the electricity surplus, the cases RESRA and EXT-A, without thermal integration, can only be compared, since these authors do not applied thermal integration and do not evaluate vapour compression refrigeration systems. Electricity surplus shown in the present study, for no integrated cases, resulted slightly higher than those presented by Ref. [6] due to the boiler steam parameters assumed. Other authors, as Magazoni et al. [8], reported a fermentation efficiency increase of 2.5%, in comparison with the conventional process; however, other results cannot be compared, since these authors simulated the fermentation process and its cooling system only.
5. Conclusions This study allowed us to assess the impacts on the production of ethanol and electricity due to the incorporation of alternative fermentation processes. Several cooling technologies were evaluated. As expected, conventional vapour compression systems are advantageous to reduce the steam consumption; in contrast, absorption refrigeration systems present advantages in the reduction of power consumption. Both types of fermentation systems showed a significant increase in ethanol production and a considerable reduction both in the production of vinasse and steam consumption in the distillation stage. The vacuum extractive fermentation, coupled to a partial condenser (EXT-B case) showed the highest production of ethanol, however, the power requirement of the refrigeration system has a significant effect on the energy balance. Moreover, the vacuum extractive fermentation (EXT-A case), had the lowest steam consumption in the distillation process, but the additional power consumption is significant.
9
The results showed that the steam consumption can be significantly reduced by the thermal integration process, but at the same time, electric energy surplus is reduced if back pressure turbines are adopted. No doubt that the cooled fermentation technologies conduct to an improved ethanol production process, with the production of more ethanol and less vinasse. So, from this point of view, their adoption is recommended for the factories. Other aspect is that the introduction of cooled fermentation technologies and/or vacuum extraction imply in an additional consumption of energy, either in the cooling system driving or the compression of the alcohol vapours. Thus, through the introduction of these technologies in the conventional process, the only significant gain is the increase in ethanol production due to the fermentation improvement, but an onus is paid relative to the electricity cogeneration. Other positive aspect of these systems is the reduction in the vinasse volume, whose disposal is a very important cost item for the factories. In fact, to perform a final decision in a particular case, a cost e benefit analysis needs to be performed, taking into account the cost of the new installations needed, the increase in steam consumption or the diminution in electricity selling and the benefits that can be obtained with the sale of more ethanol, and cost diminution with vinasse disposal.
Acknowledgements The authors wish to thank to CNPq (Process PQ 304820/2009-1) for the researcher fellowship and the Research Project Grant (Process 470481/2012-9), and FAPESP for the Post PhD fellowship (Process 2011/05718-1) and the Research Project Grant (Process 2011/51902-9).
Annex. Network grid for RE-SRA case (single effect absorption LiBr/H2O low temperature fermentation).
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References [1] Cortez LAB, editor. Sugarcane bioethanol e R&D for productivity and Sustainability. São Paulo, Brazil: E. Blucher; 2010. [2] Dias MOS, Junqueira TL, Jesus CDF, Rossell CEV, Maciel FR, Bonomi A. Improving second generation ethanol production through optimization of first generation production process from sugarcane. Energy 2012;43:246e52. [3] Dias MOS, Junqueira TL, Jesus CDF, Rossell CEV, Maciel FR, Bonomi A. Improving bioethanol production e comparison between extractive and low temperature fermentation. Appl Energy 2012;98:548e55. [4] Rein P. Cane sugar engineering. Berlin, Verlag: Dr. Albert Bartens K. G; 2010. [5] Torija MJ, Rozès N, Poblet M, Guillamon JM, Mas A. Effects of fermentation temperature on the strain population of Saccharomyces cerevisiae. Int J Food Microbiol 2003;80:47e53. [6] Dias MOS, Ensinas AV, Nebra SA, Maciel Filho R, Rossell CEV, Maciel MRW. Production of bioethanol and other bio-based materials from sugarcane bagasse: integration to conventional bioethanol production process. Chem Eng Res Des 2009;87:1206e16. [7] Olivério JL, Tamassia Barreira S, Boscariol FC, César ARP, Kiyomi Yamakawa C. Alcoholic fermentation with temperature controlled by ecological absorption chiller e EcoChill. Proc Int Soc Sugar Cane Technol 2010;27:1e9. Available from: http://codistil.com.br/index.php?option¼com_docman&task¼cat_ view&gid¼36&limit¼30&limitstart¼5&order¼name&dir¼ASC&Itemid¼ 40&lang¼en [accessed 27.02.14]. [8] Magazoni FP, Monteiro JB, Cardemil JM, Colle S. Cooling of ethanol fermentation process using absorption chillers. Int J Thermodyn 2010;13:111e8. [9] Cardemil JM, Colle S, Monteiro JB, Magazoni FC. Economic evaluation of refrigeration alternatives for alcoholic fermentation. In: Proceedings of 22nd International Conference on Efficiency, Cost, Optimisation, Simulation and Environmental Impact of Energy Systems. ECOS 2009, Foz de Iguaçú, Paraná, Brazil, August 20 e September 3, 2009. pp. 1455e64. Available from: http:// www.lepten.ufsc.br/publicacoes/solar/eventos/2009/ECOS/cardemil_colle.pdf. [accessed 27.02.14]. [10] Chunnanond K, Aphornratana S. An experimental investigation of a steam ejector refrigerator: the analysis of the pressure profile along the ejector. National Research Council of Thailand; 2003. Available from: http://www. energy-based.nrct.go.th/ [accessed 29.09.10]. [11] Luong JHT. Kinetics of ethanol inhibition in alcohol fermentation. Biotechnol Bioeng 1985;27(3):280e5. [12] Atala DIP. Assembly, instrumentation, control and experimental development of an extractive fermentation process for ethanol production. PhD Thesis. Campinas, SP, Brazil: Faculty of Food Engineering, University of Campinas; 2004 [in Portuguese]. [13] UNICAMP, Maugeri F, Atala DIP. Fermentative vacuum extraction process for the production of ethanol. Patent Int CI7.:, PI0500321-0; January 28th, 2005. Available from: http://www.patentesonline.com.br/processo-fermentativoextrativo-a-vacuo-para-producao-de-etanol-61210a.html. [accessed 29.10.10] [in Portuguese]. [14] Rivera EC, Costa AC, Atala DIP, Maugeri F, Wolf Maciel MR, Maciel Filho R. Evaluation of optimization techniques for parameter estimation: application to ethanol fermentation considering the effect of temperature. Process Biochem 2006;41:1682e7. [15] Costa AC, Atala DIP, Maugeri F, Maciel Filho R. Factorial design and simulation for the optimization and determination of control structures for an extractive alcoholic fermentation. Process Biochem 2001;37:125e37. [16] Leite RCC. Bioethanol fuel: an opportunity for Brazil. Brasilia, DF: Centro de Gestão e Estudos Estratégicos; 2009 [in Portuguese]. Available from: http://
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Energy journal homepage: www.elsevier.com/locate/energy
New alternatives for the fermentation process in the ethanol production from sugarcane: Extractive and low temperature fermentation Reynaldo Palacios-Bereche a, *, Adriano Ensinas a, b, Marcelo Modesto a, Silvia A. Nebra a, c a
Centre of Engineering, Modelling and Social Sciences, Federal University of ABC (CECS/UFABC), Rua Santa Adélia 166, CEP 09210170 Santo André, SP, Brazil École Polytechnique Fédérale de Lausanne, Switzerland Interdisciplinary Centre of Energy Planning, University of Campinas (NIPE/UNICAMP), Rua Cora Coralina 330, PO Box 6166, CEP 13083896 Campinas, SP, Brazil
b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 January 2014 Received in revised form 8 March 2014 Accepted 14 April 2014 Available online xxx
Ethanol is produced in large scale from sugarcane in Brazil by fermentation of sugars and distillation. This is currently considered as an efficient biofuel technology, leading to significant reduction on greenhouse gases emissions. However, some improvements in the process can be introduced in order to improve the use of energy. In current distilleries, a significant fraction of the energy consumption occurs in the purification step e distillation and dehydration e since conventional fermentation systems employed in the industry require low substrate concentration, which must be distilled, consequently with high energy consumption. In this study, alternatives to the conventional fermentation processes are assessed, through computer simulation: low temperature fermentation and vacuum extractive fermentation. The aim of this study is to assess the incorporation of these alternative fermentation processes in ethanol production, energy consumption and electricity surplus produced in the cogeneration system. Several cases were evaluated. Thermal integration technique was applied. Results shown that the ethanol production increases between 3.3% and 4.8% and a reduction in steam consumption happens of up to 36%. About the electricity surplus, a value of 85 kWh/t of cane can be achieved when condensing e extracting steam turbines are used. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Sugarcane Ethanol Fermentation Energy Thermal integration
1. Introduction Nowadays, a lot of research is devoted to the production of second generation ethanol from sugarcane, but, there is room for improvements in the traditional production of first generation ethanol, where the attention is not focused. Only a few researchers are working in this direction [1]. A significant fraction of the energy consumption in ethanol production happens in the distillation step [2], since the conventional fermentation systems used in the sugarcane industry require a low concentration of substrate and hence produces a low content of ethanol in the wine, which must be distilled consequently with high energy consumption. Thus, to study more efficient alternatives for the fermentation process would be very useful, focussing in systems that allow a higher concentration of substrate, which * Corresponding author. Tel.: þ55 11 49968272. E-mail addresses: [email protected] (R. Palacios-Bereche), adriano. ensinas@epfl.ch (A. Ensinas), [email protected] (M. Modesto), silvia. [email protected], [email protected] (S.A. Nebra).
produces, consequently, a less energy consumption in the following stage: the distillation. Between these alternatives we have: low temperature fermentation and extractive fermentation. Both alternatives permit to work with a higher concentration of ethanol in the wine that goes in the distillation process. This fact has also another advantage: the production of stillage is reduced, diminishing its deposition costs and environmental impact.
1.1. Low temperature fermentation The fermentation under anaerobic conditions is an exothermic biochemical reaction producing 580 kJ/kg of TRS (Total Reducing Sugars) converted. Thus, cooling is required to remove the heat produced. The temperature should be controlled between 30 C and 34 C, since temperatures outside this range generally will result in a low yield. For higher temperatures, other products beyond ethanol may be formed, whilst for lower temperatures, the fermentation time is extended, which provides greater
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opportunities for the bacteria or other organisms to ferment sugars, producing unwanted products [3,4]. In the literature, several studies indicate that fermentation at low temperature, in the range of 25 Ce30 C improves the efficiency of the process ([5e7]), since it is possible to use higher concentrations of substrate, therefore achieving higher ethanol concentrations in the wine. Dias et al. [6] evaluated technological alternatives that aimed to improve the cooling system of the fermentation process. To provide cold water, this study evaluated the use of cooling towers during the coldest hours of the day, and the use of a secondary device during the hottest hours. The pieces of equipment proposed were an accumulator of cold water, a system driven by a steam ejector and an absorption refrigeration system. The authors indicate that the alcoholic fermentation conducted at 28 C enables an ethanol concentration of 13 GL (Gay Lussac degrees, percentage of ethanol by volume) in the wine, which decreases the production of stillage to 5.76 litres per litre of hydrous ethanol. Magazoni et al. [8] proposed the use of an absorption refrigeration system single-effect water/lithium bromide absorption chiller, for use in cooling the fermentation, powered by waste heat from the production processes. A leading Brazilian company, Dedini Basic Industries S. A. (Piracicaba, Brazil), installed a demonstration plant of 20,000 L of ethanol/day, with temperature control between 28 C and 32 C. An absorption refrigeration system, built by Thermax, was used for this purpose. The results indicate that temperatures in the range from 28 C to 30 C are more suitable for obtaining a high alcoholic content in wine [7]. Cardemil et al. [9] performed an economic evaluation of different alternatives for cooling the fermentation, which were: absorption refrigeration system of simple and double effect and conventional vapour compression. Refrigeration systems based on ejectors may also be used [6]; they have some advantages as the absence of moving parts, take up little space and have low cost [4]; but, they are not energy efficient [10], they have a low COP (Coefficient of Performance) and a lower cooling capacity.
1.2. Extractive fermentation The ethanol accumulated in the fermentation medium inhibits the metabolic activity of the yeast, presenting a significant effect on the rate of cell growth at concentrations above 15 g/L [11,12]. The maximum ethanol concentration at which cell growth ceases is 100 g/L while at a concentration of 105 g/L ethanol, the production ability of yeast Saccharomyces cerevisiae is completely inhibited. So, the use of techniques of extracting ethanol from the fermentation media improved the process performance; one of such techniques is the vacuum extractive fermentation, wherein the fermentation reactor is coupled to a vacuum flash tank. The removal of ethanol from fermentation media, as soon as it is produced, reduces its inhibitory effect on yeast cells. Furthermore, the vacuum and recycling of the liquid stream allow to maintain the reactor at relatively low temperatures (28 Ce32 C), without needing external heat exchangers. These low temperatures and the removal of ethanol from fermentation media allow the use of a substrate with higher concentration. Consequently, a smaller amount of stillage (vinasse) is produced and the energy consumption is diminished in the distillation stage. Maugeri and Atala performed the assembly and instrumentation of a continuous fermentation system coupled to a vacuum flash separator in laboratory scale using the yeast Saccharomyces cerevisiae and sugarcane molasses as a substrate. A result of this work
has been the patent PI-0500321-0 A “Vacuum extractive fermentation process for ethanol production” [13]. Besides this experimental work, there are studies of modelling and simulation in this area. Rivera et al. [14], using optimization techniques evaluated the kinetic parameters of the alcoholic fermentation process in batch. Costa et al. [15] conducted a modelling study based on factorial design to study the dynamic behaviour of vacuum extractive fermentation process. 1.3. Objective The ethanol production process has a unique characteristic: the sugarcane input of the process, provides at the same time the raw material (juice) and the fuel (bagasse), to obtain the two most important products: ethanol and electric energy (through cogeneration). So, the energy management is crucial to increase the total production. The authors previously mentioned have performed experimental studies and simulations of both types of systems: low temperature and extractive fermentation, but a deep study on the consequences of the introduction of these devices in the ethanol production system were not explored from the energy management point of view. Moreover, no one of them has worked with tools of thermal integration. Thus, the aim of this work is to perform a study focussing the energy management improvement through the introduction of alternative technologies in the fermentation process and in the refrigeration system, using the Pinch method to perform thermal integration in the entire production system. Simulations of the ethanol production, including these alternative technologies, using the software Aspen PlusÒ, were performed. In the fermentation at low temperature, different technologies of refrigeration have been evaluated, for instance: vapour compression and absorption refrigeration systems. Regarding the vacuum extractive fermentation, two proposals were studied: the first considers mechanical compressor for conveying the steam from the flash tank to the distillation columns; whereas in the second, flash vapour is condensed and then pumped to the distillation columns. The thermal energy consumption and the consequences in the electric energy generation were evaluated in all the cases. Furthermore, the thermal integration process was applied to all cases. The Pinch method was used in order to optimize the system and reduce the consumption of thermal energy. 2. Process description The production process of the current first generation ethanol from sugarcane can be briefly described according to the scheme shown in Fig. 1. 2.1. Sugarcane preparation and juice extraction Before entering the extraction system, a cleaning system removes excessive amounts of soil, rocks and trash coming with the sugarcane. After cleaning, sugarcane is prepared by means of rotary knives and shredders that cut it into small pieces, suitable for the subsequent extraction process. A juice extraction system separates bagasse and juice by compressing the cane. Bagasse is used as fuel for the cogeneration system, and the raw juice is sent to the processing system. 2.2. Juice treatment Some non-sugar impurities are separated by adding chemical reactants such as calcium oxide; heating is necessary for the
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3
Fig. 1. Scheme of the ethanol production process from sugarcane.
purification reactions. Following that, the juice goes through a flash tank before entering the clarifier (settler). The precipitate is separated from the clarified juice and sent to filters. The filtrate is returned to the process and mixed with the main juice stream, and the filter cake is rejected. The clarified juice is sent to the evaporation system.
2.6. Dehydration
2.3. Juice evaporation
It is constituted by boilers, back pressure turbines and/or extraction-condensing turbines, electric generators, deaerator, pumps. Bagasse is used as fuel.
Juice is concentrated in a multiple-effect evaporator. Exhaust steam from the cogeneration system is used as a thermal energy source in the first effect; water evaporated from the juice is used as heating source for the subsequent effect. The multiple-effect evaporator works with decreasing pressure due to a vacuum imposed on the last effect, producing the necessary temperature difference between consecutive effects. Vapour bleed can be used to cover heat requirements of other parts of the process, such as juice processing heaters and the sugar boiling system. Part of the juice for ethanol production goes through the evaporation system to reach the necessary concentration for the fermentation process. The remainder of the juice by-passes concentration and goes directly to the fermentation process, to be mixed with concentrated juice for mash preparation. 2.4. Fermentation The sugars of the must are converted in ethanol, with emission of carbon dioxide, in an exothermal reaction. After fermentation, the liquor, containing about 8% of ethanol (mass basis), is taken to the distillation system to remove the water. 2.5. Distillation Ethanol produced by fermentation is recovered by distillation. Fermented liquor is heated to a suitable temperature before entering the first distillation column. Hydrous ethanol is obtained by stripping and rectification stages. A large amount of vinasse is generated e 10e12 litres per litre of ethanol e which must be handled as an effluent.
In order to remove the remaining water and produce anhydrous ethanol, a dehydration stage is required at the end of the process. 2.7. Cogeneration
3. Modelling and simulation Referred to the mill capacity, the following values were adopted: milling of 2,000,000 t of cane/year; 4000 h/year of harvesting period, which results in 500 t of cane/h. These values correspond to a Brazilian standard size plant [16] (other possibilities of plant size were not considered, taking into account that other countries do not have an expressive production of ethanol from sugarcane). Each one of the control volume showed in Fig. 1 was simulated using Aspen Plus software. Adequate parameters were adopted for each one of the process; their values were taken from de references indicated. For the extraction process, a dry cleaning system and mills were adopted [4]. The bagasse composition (dry basis, wt.) was: cellulose 36.8%, hemicelluloses 35%, lignin 20.3%, and ashes 2.3%. This composition was calculated from the parameters of the cleaning, preparation and extraction system assuming the bagasse moisture content (wet basis, wt.) of 50%. The juice treatment included the operations of: heating, flashing, decantation, addition of flocculants and calcium hydroxide, filtration with filtrate recirculation [4,7,17]. For the juice concentration, an evaporation system of five effects, Roberts type, was adopted, with pressures from 1.69 to 0.16 bar, the first effect is heated with steam at 2.5 bar, coming from the cogeneration system, vapour bleedings are used to heat other plant streams [4,17]. The fermentation was simulated as a continuous process, the usual real process, MelleeBoinot type, is a fed-batch process. The
Please cite this article in press as: Palacios-Bereche R, et al., New alternatives for the fermentation process in the ethanol production from sugarcane: Extractive and low temperature fermentation, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.04.032
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Fig. 2. Vacuum extractive fermentation e proposal A [17,24].
main reactions are the hydrolysis of the sucrose to glucose and fructose, which are after converted to ethanol and carbon dioxide in a reaction catalysed by the Saccharomy cerevisiae yeast. In small quantities some other products are formed, as succinic acid, glycerol, acetic acid, etc., it happens also the yeast growth. The yeast is centrifuged and reused. In the conventional process, a temperature of 34 C is maintained, refrigerating the must with a cooling tower [7,17,18]. The distillation and rectification follow the pattern adopted in all the Brazilian distilleries [16]: a set of distiller column with three sections: stripping, rectifying, and head concentrator, the rectification of phlegm being done in a set stripping/rectifier (column B, B1) [17]. In the base case, the heating is made with steam at 2.5 bar. The extractive distillation with monoethylene glycol was adopted for the dehydration process; two columns are used, one for extraction and the other for the solvent recuperation; the column heating is performed with 6 bar steam, obtained from a bleeding in the steam turbines of the cogeneration system [17,19,20]. The boilers of the cogeneration system are fuelled by bagasse with an efficiency of 86%, LHV base (Lower Heating Value base); about the turbines, two situations were simulated: back pressure (Configuration I) and extraction-condensing turbines (Configuration II). An isentropic efficiency of 80% was considered for the turbines. The steam parameters were 90 bar and 530 C, which are currently used in this type of plants. Two products are obtained from this system: steam at 2.5 and 6 bar and electric energy [17,21,22]. 3.1. Low temperature fermentation A fermentation temperature of 28 C was adopted, lower than 34 C of the conventional process. A conversion of 92.7% of glucose to ethanol and 28% of TRS (Total Reducing Sugars) in the substrate, were adopted [3]. The must in the fermentation vat needs to be refrigerated and recirculated. Two technologies of refrigeration were simulated, a vapour compression system (COP ¼ 3.5) and a single-effect absorption system of Lithium e Bromide (LiBr/H2O) (COP ¼ 0.7), fuelled by heat from the evaporation bleedings.
3.2. Vacuum extractive fermentation In this process, a vacuum flash tank is linked to the fermentation process to remove the ethanol from the fermentative medium. Two proposals were evaluated for that system, one follow the proposal of Dias et al. [17,23] and Junqueira et al. [24] and the other, the proposal of Cohen et al. [25]. The proposal of Dias et al. and Junqueira et al. [17,23,24] is showed in Fig. 2. In this proposal, the wine produced in the fermentation reactor is sent to the centrifugal separator (SEPA-CEL), two phases are obtained. The heavy phase (CREAMLEV) with a high concentration of yeast cells is sent to the processing unit (TRATFER) while the light phase (wine without yeast) is sent to the vacuum flash tank (TFLASH). In the treatment unit yeast, water, sulphuric acid and nutrients are added. Yeast cells treated (LEV) are sent to the fermentation reactor. In vacuum flash tank, the vapour phase (VAP-W1) is compressed and sent to the distillation unit, while the liquid phase is divided into two streams. The stream R-W2 is recirculated to the fermentation reactor, while the stream RW4 is mixed with an alcohol solution recovered in the absorption column (SEPA2) and sent to the distillation column. The compression of vapour of the flash tank is represented by the block COMP-S in Fig. 2, a process of three stages of compression with inter-coolers and after-cooler was adopted. The proposal of Cohen et al. [25], is showed in Fig. 3. This proposal is very similar to the precedent; the main difference is the existence of a partial condenser (C-FL), where is condensed part of the steam separated in the flash tank (T-FLASH). The liquid phase obtained in the condenser is an alcoholic solution with a high content of ethanol, which is mixed with the wine purged (RW5) and alcoholic solution (ALCSOL) obtained in the absorption column (SEPA2). Finally the resulting wine (WINE2) is sent to the distillation stage. The vapour phase, remaining of the partial condenser, is compressed in the unit COMP-S and is sent to the absorption column (SEPA2) to recover the ethanol entrained in this stream. The temperature at the outlet of inter-coolers and after-cooler was specified at 30 C, 40 C and 80 C respectively.
Please cite this article in press as: Palacios-Bereche R, et al., New alternatives for the fermentation process in the ethanol production from sugarcane: Extractive and low temperature fermentation, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.04.032
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5
Fig. 3. Vacuum extractive fermentation e proposal B [25].
The partial condenser (Block CF-L in Fig. 3) must operate at temperatures below 0 C, therefore, the refrigerant should be at temperatures below this value. For this reason it was decided to use absorption refrigeration systems Ammonia e Water (NH3/ H2O), since LiBr/H2O systems have the limitation of the freezing point of the coolant (water) at 0 C. Thus, in this part of the study, several alternatives have been taken, which include a cooling system for vapour compression, an absorption refrigeration system NH3/H2O (COP 0.413) and an absorption refrigeration system NH3/H2O GAX type (exchange generatoreabsorber heat, COP 1.03) [26]. The energy consumption related to the operation of all types of refrigeration systems was considered in the evaluation. Table 1 presents a synopsis of the cases evaluated in this study. The proposals A and B are referred to the two types of fermentation system technology.
3.3. Thermal integration The Pinch method is used to perform the thermal integration [27e29]. The minimum temperature difference (DTmin) adopted Table 1 Description of the cases evaluated in reference to the fermentation system technology. Case
Refrigeration technology in the fermentation system
Description
BASE
Cooling tower for the fermentation vats Vapour compressiona
Conventional distillery
RE-SRC RE-SRA
Single effect absorption LiBr/H2Oa
EXT-A EXT-B-SRC EXT-B-SRA EXT-B-SRA/GAX a b
Vapour compressionb Single effect absorption LiBr/H2Ob Absorption NH3/H2O GAX typeb
Refrigeration system for the fermentation vats. Refrigeration system for the partial condenser.
Low temperature fermentation Low temperature fermentation Vacuum extractive fermentatione Prop. Vacuum extractive fermentatione Prop. Vacuum extractive fermentatione Prop. Vacuum extractive fermentatione Prop.
A B B B
was 10 C for the process streams and 4 C for the evaporation system streams. Since the vapour bleedings from the evaporation system are used to satisfy heating requirements of different processes, and their flow rates are a characteristic of them, the thermal integration is achieved by applying the following iterative steps: Step 1: Thermal integration of process streams available excluding evaporation system. Construction of the initial Grand Composite Curve (GCC).
Table 2 Streams adopted for thermal integration for each case. Ti ( C) Tf ( C) DH (MW)
Hot streams Sterilized juice Phlegmasse Vinasse Anhydrous ethanol Condensates of steama Condenser column D Condenser column B Condenser extractive column Alcoholic vapour INT-R1 Alcoholic vapour INT-R2 Alcoholic vapour P-R3 Cold streams Imbibition water Juice for treatment Juice pre-heating Juice for sterilization Final wine Reboiler column A Reboiler column B Reboiler extractive column Reboiler recuperative column Solution LiBr/H2O in the generatorb Solution NH3/H2O in the generatorc Solution NH3/H2O in the generatord
Base RE
EXT-A EXT-B
130 103.9 109.3 78.3 110 84.9 81.7 78.3 109.4 140.7 175.6
32 35 35 35 35 35 81.7 78.3 50 80 100
41 3 37.2 8.6 8.4 19.5 26.4 7.4
23.4 2.0 20.5 8.9 20.6 12.6 26.8 7.7
18.0 2.5 15.1 8.9 21.0 16.4 26.9 7.7 0.9 0.9 1.2
18.0 1.7 16.7 9.1 21.2 11.1 27.2 7.8
25 34.2 98.1 95.5 31.2 109.3 103.9 134.5 149.6 76.8 127.4 123.7
50 105 115 130 90 109.3 103.9 134.5 149.6 89.4 152.4 200
4.4 44 2.7 14.6 33.7 43.7 21.8 6.7 2.5
4.4 44 6.4 10.3 21.0 30.8 23.4 7.3 2.5 21.7
4.4 44 7.3 6.9 15.6 20.4 23.2 7.2 2.6
4.4 44 7.3 6.9 18.8 27.7 24.3 7.4 2.5 34.5 13.9
Ti ¼ inlet temperature, Tf ¼ outlet temperature, DH ¼ stream enthalpy variation in the heat exchanger. a The DH and temperature values change due to the iterative process. b For the case RE-SRA. c For the case EXT-B-SRA. d For the case EXT-B-SRA/GAX.
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Table 3 Anhydrous ethanol, vinasse and steam consumption in the distillation process, without thermal integration.
Anhydrous ethanol, (l/t cane) Vinasse (m3/m3 ethanol) Distillation steam consumption (kg/l ethanol) Ethanol production increasing (%)
Base
RE
EXT-A
EXT-B
80.4 11.4 2.74 e
83.1 6.0 2.19 3.37
83.4 4.2 1.76 3.76
84.3 4.8 2.08 4.87
Step 2: Calculation of appropriate vapour bleedings in each effect of the evaporation system, according to the procedure proposed in Refs. [30e32]. Step 3: Integration of the evaporation system with the appropriate demand including the vapour bleedings optimized in each effect. Step 4: Update of the mass rates of the evaporation system condensates. Return to Step 2 until convergence. Table 2 shows the streams adopted for the thermal integration process. In Table 2, supply and target temperatures are equal because only fermentation parameters were modified in the studied cases while the parameters of the other operations were kept equals. However, the heat flows are different in the operations related to: juice sterilization, distillation and evaporation.
4. Results and discussion Table 3 shows the results in terms of the ethanol produced, the vinasse quantity and the steam consumption in the distillation process, for each one of the cases, without thermal integration. These results show that the largest ethanol production corresponds to the vacuum extractive fermentation refrigerated with a vapour compression system (EXT-B). But, the less vinasse production and the lowest steam consumption correspond to the vacuum extractive fermentation (EXT-A).
The lower steam consumption in the distillation corresponds to the EXT-A case, with a reduction of 35.8%. The steam consumption in the EXT-B case results lower than the RE case, but higher than the EXT-A. This effect happens due to the flash steam is compressed and fed in the distillation columns directly in vapour phase. Moreover, in EXT-B case, part of the flash vapour is condensed and mixed with the liquid wine, being after heated and fed in the distillation column, this fact increases also the steam consumption in this stage. In the RE case, the final ethanol content in wine is 10.9% m.b. (mass base). The mass flow of water used in the absorption column is 17.1 m3/h (0.034 m3/t of cane), which permits to obtain an ethanol recuperation of 96.6%. In the EXT-A case, the steam obtained in the flash tank has an ethanol content of 44.1% (m.b.), and the liquid phase, 9.1% (m.b). The results obtained in the present study show that 90% of the liquid wine need to be recirculated, returning to the fermentation reactor, the other 10% is mixed with the alcoholic solution of the absorption column and sending to the distillation columns. This final wine has an ethanol content of 8.9% (m.b.). In this case, the mass flow of the absorption column is of 10 m3/h (0.02 m3/t of cane), reaching a value of 98.5% of ethanol recuperation. In the EXT-B case, the ethanol content in both phases, liquid and vapour in the flash tank are the same than in EXT-A case. In the partial condenser, the liquid phase obtained has an ethanol content of 47% (m.b.) and the vapour phase, 25.3% (m.b.), this flow is sent to the absorption column. As in EXT-A case, 90% of the liquid wine is recirculated to the fermentation reactor. The water used in the absorption column is of 20.1 m3/h (0.04 m3/t of cane), obtaining an ethanol recuperation of 98.5%. The ethanol content of the final wine is of 13.2% (m.b.). The Pinch method, applied according to the procedure described before, allowed determining the goals of minimum energy consumption. In this procedure, the first step corresponds to the construction and integration of the process streams without the bleedings of the evaporation system. In the second step, the vapour bleeding, optimized for each effect of the evaporation system, are determined. In the following step, the evaporation system, including
Fig. 4. GCCs for the base case and for both cases of fermentation at low temperature.
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Fig. 5. GCCs for the vacuum extractive fermentation cases.
the bleedings, is recalculated. Subsequently, the evaporation system streams are integrated into the process. Finally, the GCC is built, and the goals of minimum consumption of utilities are determined. Fig. 4 shows the final GCC for the base case and the case of fermentation at low temperature, while Fig. 5 shows the final GCC for the cases of vacuum extractive fermentation. Table 4 presents the proposed vapour bleedings in the evaporation system, determined from the thermal integration procedure. The big increase of the second effect bleeding in the case RE-SRA is due to that the absorption refrigeration system LiBr/H2O works near to this temperature range. From the GCC, the steam mass flow needed to satisfy the hot utility requirements can be calculated. To determine the steam consumption, a heat exchangers network was designed, and the steam consumption in each process unit was determined. Table 5 shows the steam consumption for the cases thermally not integrated and integrated. The network grid for the RESRA integrated case integrated case is presented in the Annex. Concerning the cases RE-SRC and RE-SRA (and all EXT-B) in Table 5, it can be observed the same values for steam consumption (except for evaporation system). It can be explained by the fact that operation conditions of stream processes are the same, being different only the energy input for cooling system, which for the cases SRC is mechanical power (electricity) and in the cases SRA is vapour or steam. Fig. 6 shows the results obtained in terms of the total steam consumption, for the systems without thermal integration. The juice heating is made using steam of the first evaporation effect (71.5 t/h at 1.69 bar). In the case RE-SRA the energy for the Table 4 Vapour bleedings in the evaporation system (kg/t of cane). Case
Base
First effect (115 C) Second effect (107.3 C)
88 0
RE
EXT-A
SRC
SRA
95.2 18.8
95 86
109.8 0
EXT-B SRC
SRA
SRA/GAX
98.4 19.8
98.4 20.4
98.4 19.8
Table 5 Steam consumption in each process operation (kg/t of cane). Base
RE SRC
EXT-A EXT-B SRA
SRC
SRA
SRA/GAX
Without thermal integration Steam at 6 bar Must sterilization 50.4 35.5 35.5 23.8 23.8 23.8 23.8 Ethanol dehydration: 23.0 25.1 25.1 25.0 25.4 25.4 25.4 extractive column Ethanol dehydration: 9.8 8.6 8.6 8.9 8.7 8.7 8.7 recuperative column Refrigeration absorption 119.2 system NH3/H2O Steam at 2.5 bar Evaporation system 166.6 233.0 291.2 248.6 248.6 248.6 248.6 Distillation: first column 144.2 101.6 101.6 67.4 91.5 91.5 91.5 Distillation: second column 71.9 77.2 77.2 76.6 80.1 80.1 80.1 Steam at 19 bar Refrigeration absorption 53.5 system NH3/H2O e GAX type. Total 465.9 481.0 539.2 450.2 478.1 597.4 531.7 With thermal integration Steam at 6 bar Must sterilization 15.1 8.8 8.8 4.9 3.8 3.8 3.8 Ethanol dehydration: 23.0 25.2 25.2 25.0 25.5 25.5 25.5 extractive column Ethanol dehydration: 9.8 8.6 8.6 9.0 8.6 8.6 8.6 recuperative column Refrigeration absorption 119.1 18.4 system NH3/H2O Steam at 2.5 bar Evaporation system 3.7 8.8 8.8 15.6 10.0 10.0 10.0 Distillation: first column 112.3 184.4 227.0 197.2 205.6 200.8 205.7 Distillation: second column 144.3 101.7 101.7 67.3 91.4 91.4 91.4 Steam at 19 bar Refrigeration absorption 32.4 system NH3/H2O e GAX type. Total 308.2 337.4 380.1 319.0 345.0 459.3 396.0
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Fig. 6. Steam consumption e without thermal integration (kg/t of cane).
Fig. 7. Steam consumption e thermal integrated systems (kg/t of cane).
refrigeration system is obtained also from the first evaporation effect (35.2 t/h). The minimum consumption was obtained for the system that includes vacuum extractive fermentation. The second lowest consumption corresponds to the system with refrigerated fermentation working with vapour compression (the refrigerated fermentation working with a single effect absorption system corresponds to the biggest steam consumption). Fig. 7 shows the results for the thermal integrated systems. The total steam consumption has decreased in all the cases, but the general tendency is the same like in Fig. 6. Table 6 reports the results in relation to the electricity and bagasse surplus. The results presented here must be analysed
together with that of Figs. 6 and 7, relative to the steam consumption, because the electricity generated is linked to the steam consumption in cogeneration systems with back pressure turbines (Configuration I), and also Table 3, relative to ethanol production. In the Configuration II, the bagasse surplus is zero because it was considered that all the bagasse is burnt in the boiler. The results in Tables 3 and 6 show that for the Configuration II, in the case that includes a distillation system based on vacuum extractive fermentation refrigerated with an absorption system NH3/H2O GAX type, we have an increase of 4.85% in the ethanol production but a diminution of 11.9% in the electricity surplus.
Table 6 Electricity and bagasse surplus, different configurations referred to the cogeneration system, with and without thermal integration.
Without thermal integration Configuration I: (T-CP) Electricity surplus (kWh/t of cane) Bagasse surplus (%)a Configuration II: (T-EC) Electricity surplus (kWh/t of cane) Bagasse surplus (%)a With thermal integration Configuration I: (T-CP) Electricity surplus (kWh/t of cane) Bagasse surplus (%)a Configuration II: (T-EC) Electricity surplus (kWh/t of cane) Bagasse surplus (%)a
Base
RE-SRC
RE-SRA
EXT-A
EXT-B e SRC
EXT-B e SRA
EXT-B e SRAeGAX
49.1 17.6
43.6 14.8
63.2 4.8
38.6 19.8
43.2 15.0
67.5 3.1
56.7 7.9
78.2 0
68.0 0
71.3 0
71.3 0
68.2 0
e e
67.5 0
19.6 44.0
16.5 38.7
33.1 31.5
13.5 41.8
17.8 37.4
41.1 20.1
31.5 30.1
92.4 0
80.7 0
85.2 0
82.8 0
79.7 0
74.4 0
81.4 0
T-CP: Back pressure turbine; T-EC: extraction condensation turbine. a Percentage in relation to the total bagasse produced.
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Comparing the results of the present study with others from the literature, Dias et al. [6] reported ethanol production increases of 4.8% and 3.8% for low temperature fermentation and vacuum extractive fermentation respectively, which are very close to the results obtained in the present work. Regarding the vinasse production, the results obtained by Ref. [6] are in the same range. On the other hand, considering the electricity surplus, the cases RESRA and EXT-A, without thermal integration, can only be compared, since these authors do not applied thermal integration and do not evaluate vapour compression refrigeration systems. Electricity surplus shown in the present study, for no integrated cases, resulted slightly higher than those presented by Ref. [6] due to the boiler steam parameters assumed. Other authors, as Magazoni et al. [8], reported a fermentation efficiency increase of 2.5%, in comparison with the conventional process; however, other results cannot be compared, since these authors simulated the fermentation process and its cooling system only.
5. Conclusions This study allowed us to assess the impacts on the production of ethanol and electricity due to the incorporation of alternative fermentation processes. Several cooling technologies were evaluated. As expected, conventional vapour compression systems are advantageous to reduce the steam consumption; in contrast, absorption refrigeration systems present advantages in the reduction of power consumption. Both types of fermentation systems showed a significant increase in ethanol production and a considerable reduction both in the production of vinasse and steam consumption in the distillation stage. The vacuum extractive fermentation, coupled to a partial condenser (EXT-B case) showed the highest production of ethanol, however, the power requirement of the refrigeration system has a significant effect on the energy balance. Moreover, the vacuum extractive fermentation (EXT-A case), had the lowest steam consumption in the distillation process, but the additional power consumption is significant.
9
The results showed that the steam consumption can be significantly reduced by the thermal integration process, but at the same time, electric energy surplus is reduced if back pressure turbines are adopted. No doubt that the cooled fermentation technologies conduct to an improved ethanol production process, with the production of more ethanol and less vinasse. So, from this point of view, their adoption is recommended for the factories. Other aspect is that the introduction of cooled fermentation technologies and/or vacuum extraction imply in an additional consumption of energy, either in the cooling system driving or the compression of the alcohol vapours. Thus, through the introduction of these technologies in the conventional process, the only significant gain is the increase in ethanol production due to the fermentation improvement, but an onus is paid relative to the electricity cogeneration. Other positive aspect of these systems is the reduction in the vinasse volume, whose disposal is a very important cost item for the factories. In fact, to perform a final decision in a particular case, a cost e benefit analysis needs to be performed, taking into account the cost of the new installations needed, the increase in steam consumption or the diminution in electricity selling and the benefits that can be obtained with the sale of more ethanol, and cost diminution with vinasse disposal.
Acknowledgements The authors wish to thank to CNPq (Process PQ 304820/2009-1) for the researcher fellowship and the Research Project Grant (Process 470481/2012-9), and FAPESP for the Post PhD fellowship (Process 2011/05718-1) and the Research Project Grant (Process 2011/51902-9).
Annex. Network grid for RE-SRA case (single effect absorption LiBr/H2O low temperature fermentation).
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Please cite this article in press as: Palacios-Bereche R, et al., New alternatives for the fermentation process in the ethanol production from sugarcane: Extractive and low temperature fermentation, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.04.032