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Synthesis of new separation processes for bioethanol production by extractive distillation
Separation and Purification Technology 96 (2012) 58–67
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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Synthesis of new separation processes for bioethanol production by extractive distillation Massimiliano Errico ⇑, Ben-Guang Rong University of Southern Denmark, Institute of Chemical Engineering, Biotechnology and Environmental Technology, Niels Bohrs Allé 1, DK-5230 Odense M, Denmark
a r t i c l e
i n f o
Article history: Received 20 January 2012 Received in revised form 10 May 2012 Accepted 11 May 2012 Available online 19 May 2012 Keywords: Bioethanol separation Extractive distillation Process synthesis Process intensification Energy saving
a b s t r a c t Biofuels and bioethanol are catalyzing the attention of researchers due to the big potential in reducing the dependence on crude oil together with the possible reduction in the pollution associated with the combustion processes. The bioethanol separation process is significant in terms of its production cost. In this paper, the availability of new distillation sequences for the separation of pure ethanol from the fermentation broth is considered. The new sequences are generated following a step-by-step procedure. Extending the concept of thermally coupled structures and column sections recombination, already successfully applied to ideal mixtures, it was possible to generate new distillation sequences for azeotropic mixtures. The new arrangements are proved to have a lower energy consumption together with a reduced capital cost compared to the classical sequence proposed in the literature. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Nowadays most of the liquid fuels used in the sector of transportation are derived from crude oil refining and the consequent exhaust from vehicles can cause serious harm to the population health. Biofuels are considered as renewable energy source able to improve the security of fuel supply and to reduce the pollution due to engine exhaust [1]. Moreover they are considered carbon–neutral since the carbon dioxide released when they are burned has been recently fixed from the atmosphere. In this work, bioethanol has been considered as a possible alternative to conventional petroleum-derived fuels. Different processes and different feedstocks for the biomass to ethanol conversion are examined in the literature, so as different processes for producing anhydrous ethanol [2]. The lignocellulosic conversion process (second generation biofuels) is recognized as the most promising option for bioethanol production because of its low cost and it has the benefit to avoid the food competition [3,4]. As reported by Quintero and Cardona [5], the general process sequence comprises five stages: pretreatment, cellulose hydrolysis, concentration/detoxification, fermentation and separation. Depending on the raw feed considered, this sequence can be modified including more steps or changing the technology, in any case the separation unit is always included. The function of this unit is the high purity ethanol recovery from the raw fermentation beer. The beer stream is widely composed ⇑ Corresponding author. Tel.: +45 65507482; fax: +45 65507354. E-mail address: [email protected] (M. Errico). 1383-5866/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.seppur.2012.05.022
of ethanol and water, but for the usage of ethanol in gasohol, only a limited amount of water is allowed in order to prevent engine malfunctioning. The formation of a homogeneous minimum-boiling azeotrope at 95.6% by weight content of ethanol at 78.15 °C and 101.3 kPa, limits the maximum purity achievable with the traditional separation methods. The dehydration possibilities can be grouped depending on whether an additional mass separation agent is added or not to the azeotropic mixture [6]. In the latter case, separation by membranes is considered the category frontrunner. In this method, ethanol passes though the selective membrane without the limitation of the azeotropic composition [7]. Another possibility is the pressure swing distillation based on the variation of the azeotrope composition with pressure that makes possible the feed components’ recovery [8]. Finally, water adsorption in both liquid or vapor phase can also be an efficient process for high purity ethanol production [9]. But, if a solvent is added to the azeotropic mixture, different separation techniques are possible, depending on its affinity with the feed components. If the solvent forms new azeotropes with the initial feed component and it is possible to generate two liquid phases, one rich in ethanol and the other in the remaining components, the corresponding separation method is called heterogeneous azeotropic distillation. In the extractive distillation instead, a high-boiling point solvent which does not form any azeotrope with the feed components, but can alter the liquid phase activity coefficients and the relative volatility of the key components, is added [10]. This last mentioned method is considered here. Different issues to the subject have been studied during the time, like the selection of the suitable solvents and the VLE data
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[11], as well as the study of alternative process configurations [12– 15]. In order to decrease the production cost of bioethanol and extend its utilization, two strategies can be adopted. The former is improving the technology of the pre-treatment and fermentation section. The latter, considered here, is the increase of the energy efficiency of the separation section. The European Union Member States are required to meet a minimum binding target of 10% renewable energy share in the transport section by 2020 [16]. This target increases even more the necessity to define the best process solution in terms of energy and capital cost. Although different separation techniques are available, distillation, or distillation combined with other units operation, remains the main opponent for alternative technologies [17]. Extractive distillation was already identified as a promising separation opportunity [18–19] and the aim of this paper is to present a systematic method to generate new distillation sequences for the bioethanol separation. The new distillation configurations were obtained following the principles of Process Intensification to save both energy and capital costs. 2. Analysis of the existing configurations The classical configuration for the separation of an ethanol– water mixture by extractive distillation is reported in Fig. 1. The first column is the pre-concentrator and its function is to separate water as bottom stream and a near-azeotropic composition mixture as distillate. The distillate from the pre-concentrator column and the solvent chosen for the separation are fed to the second column, usually referred as extractive column. For minimum boiling azeotrope, like the ethanol–water case, the solvent with volatility lower than the key components is added a few trays above the feed location. The solvent flows down in the liquid phase with minimum losses in the vapor phase. Ethanol, with the desired purity, is obtained as distillate and the solvent/water mixture obtained as bottom stream is transferred to the solvent recovery column. The solvent is recovered as bottom stream, cooled and recycled back to the extractive column. Moreover an additional water stream is obtained as distillate. It should be pointed out that the order of separation shown in Fig. 1 is not the only possible one. Indeed some solvents are able to reverse the ethanol–water volatility; obtaining the water as top product of the extractive column and the ethanol-solvent mixture as bottom stream [20]. These kinds of solvents, mainly composed by refinery cuts in the range from carbon five to carbon eight, are not considered here. Starting from the classical extractive configuration, Taylor and Wankat [15], developed the improved sequence reported in Fig. 2. According to that, the fermentation broth is fed to the first column where water is obtained as bottom stream. A partial condenser is utilized to transfer the near-azeotropic mixture to the extractive column. The solvent and the ethanol rich stream interact each other and ethanol is obtained as distillate. The bottom stream is sent to the third column for the solvent recovery. By means of a partial
59
Fig. 2. Reference extractive configuration.
condenser the overhead vapor, mainly composed by ethanol and water, is recycled to the first column to increase the ethanol and water recoveries. It is possible to notice that only one water stream is obtained instead of two like in the classical design. It was proved that, compare to the design reported in Fig. 1, this configuration can reduce both heating and cooling requirements of over 20% [15]. Another interesting alternative to the configuration of Fig. 2 was obtained combining the second and the third column in a single one where a single vapor withdrawal below the feed was introduced for the water recovery [21]. From this brief review it appears that the generation of alternative sequences for the ethanol–water separation was done following general design indications or heuristic rules. The main point of this study is to introduce a general methodology to map the subspace of alternatives derived from the flowsheet reported in Fig. 2 and here used as a reference. 3. Synthesis methodology In this section, the systematic methodology to synthesize the modified extractive distillation configurations for the separation of bioethanol from the fermentation broth, is presented. The systematic method consists of four steps. Each step will introduce the specific intended modifications to the reference configuration. 3.1. Step 1: identification of the corresponding simple column sequence The sequence reported in Fig. 2 was considered as the reference for the generation of the alternative structures. Three distillation columns are utilized; in the first one the heaviest feed component is separated, while in the second the lightest one is obtained as distillate. In the third column the remaining components are recovered. It is easy to recognize that the configuration of Fig. 2, except for the presence of the recycle streams, resembles the indirect–direct simple column configuration for the separation of four-component mixtures. As can be noted from the figure, the column section was designated with corresponding number. To this aspect, according to Hohmann et al. [22], a column section is traditionally defined as a portion of distillation column not interrupted by entering or exiting streams or heat flows. The presence of the recycle streams modifies the sections 1 and 3 of the configuration reported in Fig. 2 and for this reason the star notation was introduced. 3.2. Step 2: definition of the modified thermally coupled sequences
Fig. 1. Classical extractive distillation configuration.
Utilizing the similarity highlighted in the previous step, it is possible to generate the subspace of the modified thermally
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coupled sequences by substitution of the heat exchanger/s associated to no-product streams with a liquid–vapor interconnection. The sequence showed in Fig. 3a was obtained from the reference configuration by substitution of the partial condenser of the first column with a bidirectional liquid and vapor connection. Fig. 3b reports the configuration obtained from the reference distillation sequence by substitution of the second column’s reboiler with the thermal coupling. If both the condenser and the reboiler are removed at the same time, the configuration reported in Fig. 3c is obtained. Whereas for ideal mixtures the convenience to utilize a distillation sequence is mainly related to the feed composition, in the case of azeotropic separation, the presence of the solvent stream and the recycles could give preliminary indication about the applicability of one particular configuration. Considering Fig. 3a and c, the liquid stream from the second to the first column is supposed to be mostly composed by the high boiling point solvent. This aspect limits the maximum purity obtainable for the water stream. Not positive improvements should be expected from these configurations. Even if it appears that the subspace of distillation sequences included in Fig. 3 is not completely promising to improve the separation efficiency for the bioethanol production, it is a fundamental step of the design procedure to reach the final configurations in the following steps. 3.3. Step 3: definition of the modified thermodynamic equivalent configurations If ideal mixtures are considered, from the thermally coupled configurations it is possible to generate the corresponding thermodynamic equivalent configurations by moving a column section that provides the common reflux ratio or the vapor boil up between two consecutive columns. The procedure for the azeotropic mixture considered cannot be as direct as for the ideal mixture. Fig. 4 reports the sequences obtained by section recombination of the modified thermally coupled configuration reported in Fig. 3. If section 3⁄ of the configuration reported in Fig. 3a is moved up to section 1⁄, the sequence of Fig. 4a is obtained. The sequence of Fig. 3b has the section 6 movable; the corresponding modified thermodynamically equivalent sequence is reported on Fig. 4b. The presence of two thermal couplings between the first and the second, and the second and the third column in the configuration reported in Fig. 3c, makes both sections 3⁄ and 6 movable. Three possible sequences can be obtained moving the single sections independently or at the same time. These possibilities are reported in Fig. 4c–e. It is possible to notice that in the sequences of Fig. 4a, c, and e the section 3⁄, associated with the solvent stream, is included in the same column where the water is separated. Considering that the solvent has the highest boiling point between all the
(a)
(b)
components, its distribution on the liquid phase could limit the maximum purity achievable for the water stream. 3.4. Step 4: generation of the side stream sequences Following the procedure introduced by Errico et al. [23] for ideal mixtures, it is possible to substitute the single column sections of the thermodynamically equivalent sequences with a liquid or vapor side-draw or even with a thermal coupling. The authors proved that the sequences generated can be cost effective for certain feed composition cases. Appling this procedure to the azeotropic mixture considered it is possible to generate the five configurations reported in Fig. 5. A strict relationship exists between the sequences included in the different subspaces, for this reason, the same considerations for the configurations reported in Fig. 4a, c, and e are still valid for those reported on Fig. 5a, c, and e in terms of the maximum purity achievable for the water stream. 4. Case study To compare the performances of the new sequences proposed, the reference configuration reported in Fig. 2 was simulated using an adapted version of the case study proposed by Bastidas et al. [24]. A flowrate of 4313 kmol h 1 of saturate liquid mixture containing 11.9 wt.% ethanol and water was considered as the feed stream to the separation plant. Ethylene glycol was selected as suitable solvent since it was demonstrated to have the potential to increase the relative volatility of both ethanol and water, to have a low volatility and to not have an excessively high boiling point [25]. The ethanol purity was fixed according to the EN 15376 standard to 99.8%. The column configuration parameters reported by Bastidas et al. [24] were used to initialize the simulation, then the number of stages, feed and solvent flowrate and location, reflux ratios for all the columns have been optimized. In particular, the extractive column parameters were optimized following the optimization procedure proposed by Hernandez [26] and Avilez-Martinez et al. [27], then the recycle streams are optimized as showed by Taylor and Wankat [15]. The NRTL was selected as thermodynamic model for the liquid activity coefficients evaluation. All the simulations were performed by means of Aspen Plus V7.3. The total heat duty and the annualized capital cost were utilized to compare the performances of the different sequences. The total heat duty was minimized considering the constraint of the ethanol purity [13]. The annualized capital cost includes the installation cost for the heat exchangers (reboilers and condensers) and for the columns (shell and trays) and it was estimated by means of Aspen Economic Evaluator. Fixed tube
(c)
Fig. 3. Modified thermally coupled configurations for extractive distillation.
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(a)
(c)
(b)
(d)
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(e) Fig. 4. Modified thermodynamically equivalent sequences for extractive distillation.
(a)
(d)
(b)
(c)
(e) Fig. 5. Intensified sequence for extractive distillation.
heat exchangers for the condensers, kettle for the reboilers and sieve trays, 0.6 m tray spacing, 2.5 m for the bottom sump height and 1.25 m for the vapor disengagement for the distillation
columns, were considered. Should be pointed out that for each sequence the solvent is cooled from about 197 °C to 30 °C. This heat can be partially transferred in the reboiler associated to the
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water stream, or used for the feed pre-heating. For this reason the cooler was not considered in the capital cost estimation. Finally, the capital cost was annualized considering a mean operational time of 10 years. 5. Simulation results and discussion The design parameters, the energy performance and the capital cost evaluation, for the reference configuration of Fig. 2, are reported in Table 1. For the same sequence, the complete mass and energy balance is reported in Table 2. The liquid composition profiles for the extraction column and the temperature profile are reported in Fig. 6a and b, respectively. In the following sub-sections the results for each subspace of distillation sequences, generated applying the proposed synthesis methodology, are discussed. 5.1. Modified thermally coupled configurations As discussed in Section 3, the configurations reported in Fig. 3a and c are not promising to increase the separation efficiency, due to the liquid stream flowing from the second to the first column. Their significance is mainly served as an intermediate step in the derivation of the configurations showed in Fig. 4a, c–e. To show the positive effect of the modification of the reference case, the configuration reported in Fig. 3b was simulated keeping the same configuration parameter of the reference configuration. The vapor thermal coupling flowrate was defined minimizing the configuration’s energy requirement. To avoid convergence problems related to the presence to the recycles, they were initially disconnected. The vapor thermal coupling flowrate was manipulated until the purity target of ethanol was reached, then the recycle streams were reconnected again one by one and the optimum interconnecting stream verified. The mass and energy balances, the energy consumption and the capital cost evaluation are reported in Table 3. The liquid composition profiles together with the temperature profiles are reported in Fig. 7a and b, respectively. It is possible to notice that 2% reduction in both condenser and reboiler’s duty, together with 5% reduction of the capital costs, was achieved compare to the reference case. 5.2. Modified thermodynamically equivalent configurations This section includes the results of the sequences reported in Fig. 4. It is possible to consider two different groups of configurations depending if the pre-concentration column is combined or not with the extractive column. 5.2.1. Group 1: pre-concentrator combined with the extractive column Configurations of Fig. 4a, c, and e belong to this group. The preconcentration column is combined with the section 3⁄ of the Table 1 Design parameters, energy requirement and capital cost of the configuration reported in Fig. 2.
Number of stages Reflux ratio (molar) Feed stage Solvent feed stage Vapor recycle feed stage Column diameter (m) Design pressure (kPa) Condenser duty (MW) Reboiler duty (MW) Total condenser duty (MW) Total reboiler duty (MW) Annualized capital cost (k$ yr
1
)
C1
C2
C3
48 5.65 45 – 24 3.10 101.35 15.07 18.13 19.51 21.48 186
19 0.80 16 3 – 1.45 101.35 4.06 2.05
10 0.71 4 – – 0.78 101.35 0.38 1.30
extractive column, in this way the feed broth and the solvent are fed to the same column and ethanol and water are obtained as distillate and residue, respectively. Among all the configurations, the results for the one reported in Fig. 4e are showed in Tables 4 and 5, while the composition and temperature profiles in Fig. 8. It is possible to notice that, compared with the reference case reported in Fig. 2, this sequence realizes 35% reduction of both condenser and reboiler duties and 11% reduction of the capital costs. Anyway for all the configurations of this group the maximum molar purity achievable for the water stream was 0.972. This result was theoretically expected since ethylene glycol has the highest boiling point among all the components and its concentration naturally increases in the bottom stream. The loss of the solvent in the water stream should be carefully considered. In the case presented, ethylene glycol is used and its loss must be replenished increasing the solvent make-up flowrate. As can be noticed from Table 5 the amount of solvent recovered is only one third of the total flowrate already defined in the base case. The extra cost of the solvent can reduce the benefits of the energy and capital cost savings. But this result cannot be generalized since new solvents like bio-glycol are cheap, renewable and biodegradable and their cost does not affect the economy of the process [28,29]. The bio-glycerol produced as a by-product in biodiesel plants can be integrated with the bioethanol purification section. A more accurate economic analysis can define the trade-off between energy saving, solvent cost and water purification. It is possible to conclude that the importance of the configurations of Fig. 4a, c, and e is mainly related to the solvent cost and its availability and they represent a valid alternative for energy and capital cost reduction. 5.2.2. Pre-concentrator stand-separate from the extractive column This group includes the configurations reported in Fig. 4b and d. The pre-concentration column saves its original function and the connection with the extractive column can be realized with a single stream like in Fig. 4b otherwise with a thermal coupling so as reported in Fig. 4d. The configuration reported in Fig. 4b has the same pre-fractionator column design as the reference case, but the extractive column performs the separation between the most volatile component (ethanol) and the higher boiling point one (solvent). The external section 5 is used to recover the intermediate boiling component (water) minimizing, at the same time, the loss of solvent. Considering that the distillate flowrate of the extractive column cannot be changed since it is specified by the ethanol production, the vapor flowrate from the extractive column to section 5 and its stage location are the outstanding parameters in the analysis of the whole configuration. The thermal coupling was located at the 19th stage in agreement with the composition and temperature profiles reported in Fig. 9. This also match the initial design parameters obtained considering that the sequence of Fig. 4b is generated by column section recombination of the reference configuration of Fig. 2. The vapor flowrate is mainly composed of water and must be fixed taking into account the quantity of water introduced in the column with the feed and according to the overhead vapor recycled to the first column. In this way it is possible to recover almost pure solvent as bottoms stream without any purity penalty for the ethanol. The design parameters of the configuration, together with the heat duties and the capital costs are reported in Table 6, the mass and energy balance in Table 7. The total reboiler and condenser duty are 7% and 2%, respectively lower than the corresponding reference sequence. Also the capital costs of the sequence are reduced by 9%. This sequence represents a useful configuration to achieve a reduction of the separation cost related to the bioethanol production keeping the same purity target for all the streams. Substituting the partial condenser of the first column with a thermal coupling like showed in Fig. 4d it was not possible to reach the purity targets required. This is mainly due to the composition
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M. Errico, B.-G. Rong / Separation and Purification Technology 96 (2012) 58–67 Table 2 Mass and energy balance for the reference design of Fig. 2.
Temperature (K) Pressure (kPa) Vapor frac Enthalpy (GW h 1) Mole flow (kmol h 1) Mole frac Ethanol Water Glycol
Fermentation broth
Az. feed
Water
Ethanol
Solvent recycle
Solvent make up
Overhead vapor
363.050 101.350 0 1209.034 4313.000
351.010 101.350 1 56.829 244.600
371.900 101.350 0 1150.024 4105.400
351.150 101.350 0 56.218 208.000
469.810 101.350 0 62.782 146.140
303.150 101.350 0 0.184 0.400
378.200 101.350 1 8.828 37.040
0.051 0.949 0
0.879 0.121 Trace
0.003 0.997 77 ppm
0.998 0.002 407 ppm
Trace 4 ppm 1.000
0 0 1.000
0.199 0.792 0.009
Fig. 6. Base-case configuration: (a) Liquid-phase concentration profiles: ethanol (), glycol (N), water (j) and (b) temperature profile.
Table 3 Mass balance, energy requirement and capital cost for the configuration of Fig. 3b.
Temperature (K) Pressure (kPa) Vapor frac Enthalpy (GW h 1) Mole flow (kmol h 1) Mole frac Ethanol Water Glycol Total condenser duty (MW) Total reboiler duty (MW) Annualized capital cost (k$ yr
1
)
Ethanol
Water
Solvent recycle
Overhead vapor
Thermal coupling
351.150 101.350 0 56.222 208.000
371.850 101.350 0 1150.200 4105.800
469.750 101.350 0 62.781 146.140
385.650 101.350 1 8.966 37.440
435.350 101.350 1 31.273 109.800
0.998 0.002 488 ppm 19.170 21.140 176
0.003 0.997 73 ppm
Trace 195 ppb 1.000
0.202 0.780 0.008
0.122 0.495 0.383
Fig. 7. Modify thermally coupled configuration 3(b): (a) Liquid-phase concentration profiles: ethanol (), glycol (N), water (j) and (b) temperature profile.
of the liquid used to reflux the first column that includes also part of the high-boiling solvent. 5.3. Intensified sequences As pointed out in Section 3, there is a correspondence between the sequences generated through the step-by-step procedure.
Therefore the configurations reported in Fig. 5 can also be classified into two groups as done for the ones in Fig. 4. The first group includes the configurations where the fermentation broth and the solvent are fed in the same column. As discussed before for these configurations it is possible to achieve considerable energy and capital cost reductions but the cost of the solvent loss dictates the convenience of their application. In the second group the
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Table 4 Design parameters, energy consumption and capital cost of the configuration reported in Fig. 4e.
Number of stages Reflux ratio (molar) Feed stage Solvent feed stage Vapor recycle feed stage Thermal coupling stage Column diameter (m) Design pressure (kPa) Condenser duty (MW) Reboiler duty (MW) Total condenser duty (MW) Total reboiler duty (MW) Annualized capital cost (k$ yr
1
)
C1
C2
C3
63 4.57 60 3 39 16 2.63 101.35 12.54 10.73 12.61 14.11 165
8 – – – – 4 1.25 101.35 0 3.38
3 – – – – – 1.25 101.35 0.07 0
Table 6 Design parameters, energy consumption and capital cost of the configuration reported in Fig. 4b.
Number of stages Reflux ratio (molar) Feed stage Solvent feed stage Vapor recycle feed stage Thermal coupling stage Column diameter (m) Design pressure (kPa) Condenser duty (MW) Reboiler duty (MW) Total condenser duty (MW) Total reboiler duty (MW) Annualized capital cost (k$ yr
1
)
C1
C2
C3
48 5.65 45 – 24 – 3.10 101.35 15.06 17.99 19.17 19.88 170
24 0.8 16 3 – 19 1.43 101.35 4.08 1.89
3 – – – – – 0.93 101.35 0.03 0
Table 5 Mass balance, energy requirement and capital cost for the configuration of Fig. 4e.
Temperature (K) Pressure (kPa) Vapor frac Enthalpy (GW h 1) Mole Flow (kmol h Mole frac Ethanol Water Glycol
1
)
Ethanol
Water
Solvent recycle
Overhead vapor
TCL
TCV
315.150 101.350 0 56.205 208.000
372.550 101.350 0 1196.309 4209.140
469.750 101.350 0 18.385 42.800
353.050 101.350 1 12.750 55.000
353.450 101.350 0 90.836 304.000
384.250 101.350 1 13.345 57.000
0.998 0.002 134 ppm
0.003 0.972 0.025
25 ppb 2 ppm 1.000
0.950 0.047 0.002
0.834 0.023 0.143
0.921 0.047 0.032
Fig. 8. Modify thermodynamically equivalent configuration 4(e): (a) Liquid-phase concentration profiles: ethanol (), glycol (N), water (j) and (b) temperature profile.
Fig. 9. Modify thermodynamically equivalent configuration 4(b): (a) Liquid-phase concentration profiles: ethanol (), glycol (N), water (j) and (b) temperature profile.
configurations reported in Fig. 5b and d are considered. As for the sequence in Fig. 4d, also for the sequence in Fig. 5d it was not possible to reach the purity target. The configuration of Fig. 5b was
generated from the one reported in Fig. 4b by substitution of the column section 5 with a vapor side-stream. The main function of section 5 is to recover part of the ethylene glycol from the vapor
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M. Errico, B.-G. Rong / Separation and Purification Technology 96 (2012) 58–67 Table 7 Mass balance, energy requirement and capital cost for the configuration of Fig. 4b.
Temperature (K) Pressure (kPa) Vapor frac Enthalpy (GW h 1) Mole Flow (kmol h Mole frac Ethanol Water Glycol
1
)
Ethanol
Water
Solvent recycle
Overhead vapor
Thermal coupling
350.950 101.350 0 60.498 208.000
371.850 101.350 0 1150.024 4105.400
469.750 101.350 0 62.735 146.14
378.350 101.350 1 275.259 37.040
389.35 101.350 0 0.465 1.26
0.998 0.002 407 ppm
0.003 0.997 78 ppm
Trace 41 ppm 1.000
0.200 0.791 0.009
0.014 0.449 0.536
Fig. 10. Intensified sequence 5(b): (a) Liquid-phase concentration profiles: ethanol (), glycol (N), water (j) and (b) temperature profile.
Table 8 Design parameters, energy requirement and capital cost of the configuration reported in Fig. 5b.
Number of stages Reflux ratio (molar) Feed stage Solvent feed stage Vapor recycle feed stage Vapor withdrawal Column diameter (m) Design pressure (kPa) Condenser duty (MW) Reboiler duty (MW) Total condenser duty (MW) Total reboiler duty (MW) Annualized capital cost (k$ yr
1
)
C1
C2
48 5.65 45 – 24 – 3.10 101.35 15.07 18.13 19.13 21.10 157
28 0.8 16 3 – 20 1.45 101.35 4.06 2.97
phase in order to avoid its loss in the water stream recycled to the first column. Removing this section the vapor is directly withdrawn from the second column and fed to the first one, a small amount of solvent could be inevitably lost. To avoid this loss the
number of stages of section 6 were modified and the vapor sidestream location was defined accordingly. The vapor flowrate sets the purity of the solvent stream and its choice is related to avoid the increase of the solvent make-up flowrate. The composition profiles for the extractive column and the corresponding temperature profile are showed in Fig. 10. The design parameters of the configuration, together with the condenser and reboiler heat duties and capital costs are reported in Table 8. The mass and heat balance are reported in Table 9. It is possible to notice that the energy consumption is 2% lower than the reference case, more significantly, the reduction of capital cost by 16% is achieved. For the studied case the configurations of Fig. 4b and b represented the best solutions in terms of both energy and capital costs keeping the same purity target as the reference case. It should be pointed out that this article is focused on the systematic synthesis of alternative sequences for the bioethanol purification from fermentation broth. The new sequences are then explored in details considering a fixed feed composition and a specified solvent. We do not exclude the other derived configurations as potential optimal configurations when different mixture compositions and different solvents are considered in the separation process. This is because different raw materials from different
Table 9 Mass and energy balance for the configuration of Fig. 5b.
Temperature (K) Pressure (kPa) Vapor frac Enthalpy (GW h 1) Mole Flow (kmol h Mole frac Ethanol Water Glycol
1
)
Ethanol
Water
Solvent recycle
Solvent make up
Overhead vapor
351.15 101.35 0 56.218 208.00
371.85 101.35 0 1150.062 4105.40
469.15 101.35 0 62.764 146.14
303.15 101.35 0 0.184 0.40
382.96 101.35 1 8.853 37.04
0.998 0.002 407 ppm
0.003 0.997 131 ppm
Trace 0.001 0.999
0 0 1.000
0.200 0.785 0.015
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flowsheet definition subject to the production specifications imposed by the specific process.
Table 10 Energy performance and capital cost comparison.
Reference case Modified thermally couples sequence, Fig. 3b Modified thermodynamic equivalent sequence, Fig. 4b Intensified sequence, Fig. 5b
Energy index (MJ/kg ethanol)
Capital cost savings (% on the base case)
8.08 7.95
– 5
7.47
9
7.93
16
biomass resources and different manufacturing processes are employed for bioethanol productions. Another observation regards the order of energy saving realized by the new configurations. It is true that 7% or 2% cannot be defined a huge saving, but must be considered that the configuration used as a starting point for the generation procedure is an already optimized sequence able to reduce the total energy requirement of the condensers and reboilers of 20% compare to the classical design for extractive distillation [15]. Anyway applying the synthesis methodology described it was possible to generate a more complete subspace and some configurations are valid alternative to save energy and/ or capital costs depending if some solvent loss in the water stream is admitted or not. A final observation regards the controllability of the new configurations. It was already proved that thermally coupled configurations have, in some case, a better dynamic performance compared to the classical simple columns [30,31]. A detailed analysis is out of the scope of the present work, but can be an independent future work to complete the study of the best configurations selected. 6. Conclusions A systematic method for the generation of new distillation sequences is presented and applied for the case of extractive distillation for bioethanol production. The method extends the thermal coupling technique already successfully applied for ideal mixtures to bioethanol separation. A specific mixture and a specific solvent are used for the evaluation of the new sequences. The results showed that the methodology can produce improved configurations than the base case configuration for the studied case. It is understood that the relative advantages of the derived configurations will be different when different solvents and different feeds are considered for bioethanol productions. For different solvents and different mixtures the exploration of the whole space can lead to the best configuration. It was possible to identify different promising configurations with different performance indicators. The results for all the configurations considered are summarized in Table 10, where the specific energy use per kg of ethanol produced was used to compare the energy performances of the different alternatives [31]. Moreover the capital cost saving respect to the reference case is reported. It is possible to notice that all the proposed configurations have a lower energy index compared to the traditional distillation configuration considered as base case. The energy reduction is not so significant compared to the savings in the capital cost. The limited value of the energy performance of the new sequences can be explained considering that the reference configuration used in the synthesis methodology is an already optimized configuration able to reduce the duty of the condensers and reboilers of above 20%. Anyway for the configuration of Fig. 5b a capital cost reduction of 16% can be considered significant. This work represents one step more in the definition of a more complete design space with new derived alternatives. Each new configuration has benefits only in certain cases which the designer should consider during the
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Turunen, A method for the systematic synthesis of multicomponent distillation systems with less than N-1 columns, Chemical Engineering Process 48 (2009) 907–920. [24] P.A. Bastidas, I.D. Gil, G. Rodriguez, Comparison of the main ethanol dehydration technologies though process simulation, in: Paper presented at 20th European Symposium on Computer Aided Process Engineering – ESCAPE20, Ischia, Italy, 2010. [25] S. Lynn, D.N. Hanson, Multieffect extractive distillation for separating aqueous azeotropes, Industrial Engineering and Chemical Process Desired Devices 25 (1986) 936–941. [26] S. Hernandez, Analysis of energy-efficient complex distillation options to purify bioethanol, Chemical Engineering Technology 31 (2008) 597–603. [27] A. Alives Martinez, J. Saucendo-Luna, J.G. Segovia-Hernandez, S. Hernandez, F.I. Gomez-Castro, A.J. 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Contents lists available at SciVerse ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Synthesis of new separation processes for bioethanol production by extractive distillation Massimiliano Errico ⇑, Ben-Guang Rong University of Southern Denmark, Institute of Chemical Engineering, Biotechnology and Environmental Technology, Niels Bohrs Allé 1, DK-5230 Odense M, Denmark
a r t i c l e
i n f o
Article history: Received 20 January 2012 Received in revised form 10 May 2012 Accepted 11 May 2012 Available online 19 May 2012 Keywords: Bioethanol separation Extractive distillation Process synthesis Process intensification Energy saving
a b s t r a c t Biofuels and bioethanol are catalyzing the attention of researchers due to the big potential in reducing the dependence on crude oil together with the possible reduction in the pollution associated with the combustion processes. The bioethanol separation process is significant in terms of its production cost. In this paper, the availability of new distillation sequences for the separation of pure ethanol from the fermentation broth is considered. The new sequences are generated following a step-by-step procedure. Extending the concept of thermally coupled structures and column sections recombination, already successfully applied to ideal mixtures, it was possible to generate new distillation sequences for azeotropic mixtures. The new arrangements are proved to have a lower energy consumption together with a reduced capital cost compared to the classical sequence proposed in the literature. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Nowadays most of the liquid fuels used in the sector of transportation are derived from crude oil refining and the consequent exhaust from vehicles can cause serious harm to the population health. Biofuels are considered as renewable energy source able to improve the security of fuel supply and to reduce the pollution due to engine exhaust [1]. Moreover they are considered carbon–neutral since the carbon dioxide released when they are burned has been recently fixed from the atmosphere. In this work, bioethanol has been considered as a possible alternative to conventional petroleum-derived fuels. Different processes and different feedstocks for the biomass to ethanol conversion are examined in the literature, so as different processes for producing anhydrous ethanol [2]. The lignocellulosic conversion process (second generation biofuels) is recognized as the most promising option for bioethanol production because of its low cost and it has the benefit to avoid the food competition [3,4]. As reported by Quintero and Cardona [5], the general process sequence comprises five stages: pretreatment, cellulose hydrolysis, concentration/detoxification, fermentation and separation. Depending on the raw feed considered, this sequence can be modified including more steps or changing the technology, in any case the separation unit is always included. The function of this unit is the high purity ethanol recovery from the raw fermentation beer. The beer stream is widely composed ⇑ Corresponding author. Tel.: +45 65507482; fax: +45 65507354. E-mail address: [email protected] (M. Errico). 1383-5866/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.seppur.2012.05.022
of ethanol and water, but for the usage of ethanol in gasohol, only a limited amount of water is allowed in order to prevent engine malfunctioning. The formation of a homogeneous minimum-boiling azeotrope at 95.6% by weight content of ethanol at 78.15 °C and 101.3 kPa, limits the maximum purity achievable with the traditional separation methods. The dehydration possibilities can be grouped depending on whether an additional mass separation agent is added or not to the azeotropic mixture [6]. In the latter case, separation by membranes is considered the category frontrunner. In this method, ethanol passes though the selective membrane without the limitation of the azeotropic composition [7]. Another possibility is the pressure swing distillation based on the variation of the azeotrope composition with pressure that makes possible the feed components’ recovery [8]. Finally, water adsorption in both liquid or vapor phase can also be an efficient process for high purity ethanol production [9]. But, if a solvent is added to the azeotropic mixture, different separation techniques are possible, depending on its affinity with the feed components. If the solvent forms new azeotropes with the initial feed component and it is possible to generate two liquid phases, one rich in ethanol and the other in the remaining components, the corresponding separation method is called heterogeneous azeotropic distillation. In the extractive distillation instead, a high-boiling point solvent which does not form any azeotrope with the feed components, but can alter the liquid phase activity coefficients and the relative volatility of the key components, is added [10]. This last mentioned method is considered here. Different issues to the subject have been studied during the time, like the selection of the suitable solvents and the VLE data
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[11], as well as the study of alternative process configurations [12– 15]. In order to decrease the production cost of bioethanol and extend its utilization, two strategies can be adopted. The former is improving the technology of the pre-treatment and fermentation section. The latter, considered here, is the increase of the energy efficiency of the separation section. The European Union Member States are required to meet a minimum binding target of 10% renewable energy share in the transport section by 2020 [16]. This target increases even more the necessity to define the best process solution in terms of energy and capital cost. Although different separation techniques are available, distillation, or distillation combined with other units operation, remains the main opponent for alternative technologies [17]. Extractive distillation was already identified as a promising separation opportunity [18–19] and the aim of this paper is to present a systematic method to generate new distillation sequences for the bioethanol separation. The new distillation configurations were obtained following the principles of Process Intensification to save both energy and capital costs. 2. Analysis of the existing configurations The classical configuration for the separation of an ethanol– water mixture by extractive distillation is reported in Fig. 1. The first column is the pre-concentrator and its function is to separate water as bottom stream and a near-azeotropic composition mixture as distillate. The distillate from the pre-concentrator column and the solvent chosen for the separation are fed to the second column, usually referred as extractive column. For minimum boiling azeotrope, like the ethanol–water case, the solvent with volatility lower than the key components is added a few trays above the feed location. The solvent flows down in the liquid phase with minimum losses in the vapor phase. Ethanol, with the desired purity, is obtained as distillate and the solvent/water mixture obtained as bottom stream is transferred to the solvent recovery column. The solvent is recovered as bottom stream, cooled and recycled back to the extractive column. Moreover an additional water stream is obtained as distillate. It should be pointed out that the order of separation shown in Fig. 1 is not the only possible one. Indeed some solvents are able to reverse the ethanol–water volatility; obtaining the water as top product of the extractive column and the ethanol-solvent mixture as bottom stream [20]. These kinds of solvents, mainly composed by refinery cuts in the range from carbon five to carbon eight, are not considered here. Starting from the classical extractive configuration, Taylor and Wankat [15], developed the improved sequence reported in Fig. 2. According to that, the fermentation broth is fed to the first column where water is obtained as bottom stream. A partial condenser is utilized to transfer the near-azeotropic mixture to the extractive column. The solvent and the ethanol rich stream interact each other and ethanol is obtained as distillate. The bottom stream is sent to the third column for the solvent recovery. By means of a partial
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Fig. 2. Reference extractive configuration.
condenser the overhead vapor, mainly composed by ethanol and water, is recycled to the first column to increase the ethanol and water recoveries. It is possible to notice that only one water stream is obtained instead of two like in the classical design. It was proved that, compare to the design reported in Fig. 1, this configuration can reduce both heating and cooling requirements of over 20% [15]. Another interesting alternative to the configuration of Fig. 2 was obtained combining the second and the third column in a single one where a single vapor withdrawal below the feed was introduced for the water recovery [21]. From this brief review it appears that the generation of alternative sequences for the ethanol–water separation was done following general design indications or heuristic rules. The main point of this study is to introduce a general methodology to map the subspace of alternatives derived from the flowsheet reported in Fig. 2 and here used as a reference. 3. Synthesis methodology In this section, the systematic methodology to synthesize the modified extractive distillation configurations for the separation of bioethanol from the fermentation broth, is presented. The systematic method consists of four steps. Each step will introduce the specific intended modifications to the reference configuration. 3.1. Step 1: identification of the corresponding simple column sequence The sequence reported in Fig. 2 was considered as the reference for the generation of the alternative structures. Three distillation columns are utilized; in the first one the heaviest feed component is separated, while in the second the lightest one is obtained as distillate. In the third column the remaining components are recovered. It is easy to recognize that the configuration of Fig. 2, except for the presence of the recycle streams, resembles the indirect–direct simple column configuration for the separation of four-component mixtures. As can be noted from the figure, the column section was designated with corresponding number. To this aspect, according to Hohmann et al. [22], a column section is traditionally defined as a portion of distillation column not interrupted by entering or exiting streams or heat flows. The presence of the recycle streams modifies the sections 1 and 3 of the configuration reported in Fig. 2 and for this reason the star notation was introduced. 3.2. Step 2: definition of the modified thermally coupled sequences
Fig. 1. Classical extractive distillation configuration.
Utilizing the similarity highlighted in the previous step, it is possible to generate the subspace of the modified thermally
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coupled sequences by substitution of the heat exchanger/s associated to no-product streams with a liquid–vapor interconnection. The sequence showed in Fig. 3a was obtained from the reference configuration by substitution of the partial condenser of the first column with a bidirectional liquid and vapor connection. Fig. 3b reports the configuration obtained from the reference distillation sequence by substitution of the second column’s reboiler with the thermal coupling. If both the condenser and the reboiler are removed at the same time, the configuration reported in Fig. 3c is obtained. Whereas for ideal mixtures the convenience to utilize a distillation sequence is mainly related to the feed composition, in the case of azeotropic separation, the presence of the solvent stream and the recycles could give preliminary indication about the applicability of one particular configuration. Considering Fig. 3a and c, the liquid stream from the second to the first column is supposed to be mostly composed by the high boiling point solvent. This aspect limits the maximum purity obtainable for the water stream. Not positive improvements should be expected from these configurations. Even if it appears that the subspace of distillation sequences included in Fig. 3 is not completely promising to improve the separation efficiency for the bioethanol production, it is a fundamental step of the design procedure to reach the final configurations in the following steps. 3.3. Step 3: definition of the modified thermodynamic equivalent configurations If ideal mixtures are considered, from the thermally coupled configurations it is possible to generate the corresponding thermodynamic equivalent configurations by moving a column section that provides the common reflux ratio or the vapor boil up between two consecutive columns. The procedure for the azeotropic mixture considered cannot be as direct as for the ideal mixture. Fig. 4 reports the sequences obtained by section recombination of the modified thermally coupled configuration reported in Fig. 3. If section 3⁄ of the configuration reported in Fig. 3a is moved up to section 1⁄, the sequence of Fig. 4a is obtained. The sequence of Fig. 3b has the section 6 movable; the corresponding modified thermodynamically equivalent sequence is reported on Fig. 4b. The presence of two thermal couplings between the first and the second, and the second and the third column in the configuration reported in Fig. 3c, makes both sections 3⁄ and 6 movable. Three possible sequences can be obtained moving the single sections independently or at the same time. These possibilities are reported in Fig. 4c–e. It is possible to notice that in the sequences of Fig. 4a, c, and e the section 3⁄, associated with the solvent stream, is included in the same column where the water is separated. Considering that the solvent has the highest boiling point between all the
(a)
(b)
components, its distribution on the liquid phase could limit the maximum purity achievable for the water stream. 3.4. Step 4: generation of the side stream sequences Following the procedure introduced by Errico et al. [23] for ideal mixtures, it is possible to substitute the single column sections of the thermodynamically equivalent sequences with a liquid or vapor side-draw or even with a thermal coupling. The authors proved that the sequences generated can be cost effective for certain feed composition cases. Appling this procedure to the azeotropic mixture considered it is possible to generate the five configurations reported in Fig. 5. A strict relationship exists between the sequences included in the different subspaces, for this reason, the same considerations for the configurations reported in Fig. 4a, c, and e are still valid for those reported on Fig. 5a, c, and e in terms of the maximum purity achievable for the water stream. 4. Case study To compare the performances of the new sequences proposed, the reference configuration reported in Fig. 2 was simulated using an adapted version of the case study proposed by Bastidas et al. [24]. A flowrate of 4313 kmol h 1 of saturate liquid mixture containing 11.9 wt.% ethanol and water was considered as the feed stream to the separation plant. Ethylene glycol was selected as suitable solvent since it was demonstrated to have the potential to increase the relative volatility of both ethanol and water, to have a low volatility and to not have an excessively high boiling point [25]. The ethanol purity was fixed according to the EN 15376 standard to 99.8%. The column configuration parameters reported by Bastidas et al. [24] were used to initialize the simulation, then the number of stages, feed and solvent flowrate and location, reflux ratios for all the columns have been optimized. In particular, the extractive column parameters were optimized following the optimization procedure proposed by Hernandez [26] and Avilez-Martinez et al. [27], then the recycle streams are optimized as showed by Taylor and Wankat [15]. The NRTL was selected as thermodynamic model for the liquid activity coefficients evaluation. All the simulations were performed by means of Aspen Plus V7.3. The total heat duty and the annualized capital cost were utilized to compare the performances of the different sequences. The total heat duty was minimized considering the constraint of the ethanol purity [13]. The annualized capital cost includes the installation cost for the heat exchangers (reboilers and condensers) and for the columns (shell and trays) and it was estimated by means of Aspen Economic Evaluator. Fixed tube
(c)
Fig. 3. Modified thermally coupled configurations for extractive distillation.
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(a)
(c)
(b)
(d)
61
(e) Fig. 4. Modified thermodynamically equivalent sequences for extractive distillation.
(a)
(d)
(b)
(c)
(e) Fig. 5. Intensified sequence for extractive distillation.
heat exchangers for the condensers, kettle for the reboilers and sieve trays, 0.6 m tray spacing, 2.5 m for the bottom sump height and 1.25 m for the vapor disengagement for the distillation
columns, were considered. Should be pointed out that for each sequence the solvent is cooled from about 197 °C to 30 °C. This heat can be partially transferred in the reboiler associated to the
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water stream, or used for the feed pre-heating. For this reason the cooler was not considered in the capital cost estimation. Finally, the capital cost was annualized considering a mean operational time of 10 years. 5. Simulation results and discussion The design parameters, the energy performance and the capital cost evaluation, for the reference configuration of Fig. 2, are reported in Table 1. For the same sequence, the complete mass and energy balance is reported in Table 2. The liquid composition profiles for the extraction column and the temperature profile are reported in Fig. 6a and b, respectively. In the following sub-sections the results for each subspace of distillation sequences, generated applying the proposed synthesis methodology, are discussed. 5.1. Modified thermally coupled configurations As discussed in Section 3, the configurations reported in Fig. 3a and c are not promising to increase the separation efficiency, due to the liquid stream flowing from the second to the first column. Their significance is mainly served as an intermediate step in the derivation of the configurations showed in Fig. 4a, c–e. To show the positive effect of the modification of the reference case, the configuration reported in Fig. 3b was simulated keeping the same configuration parameter of the reference configuration. The vapor thermal coupling flowrate was defined minimizing the configuration’s energy requirement. To avoid convergence problems related to the presence to the recycles, they were initially disconnected. The vapor thermal coupling flowrate was manipulated until the purity target of ethanol was reached, then the recycle streams were reconnected again one by one and the optimum interconnecting stream verified. The mass and energy balances, the energy consumption and the capital cost evaluation are reported in Table 3. The liquid composition profiles together with the temperature profiles are reported in Fig. 7a and b, respectively. It is possible to notice that 2% reduction in both condenser and reboiler’s duty, together with 5% reduction of the capital costs, was achieved compare to the reference case. 5.2. Modified thermodynamically equivalent configurations This section includes the results of the sequences reported in Fig. 4. It is possible to consider two different groups of configurations depending if the pre-concentration column is combined or not with the extractive column. 5.2.1. Group 1: pre-concentrator combined with the extractive column Configurations of Fig. 4a, c, and e belong to this group. The preconcentration column is combined with the section 3⁄ of the Table 1 Design parameters, energy requirement and capital cost of the configuration reported in Fig. 2.
Number of stages Reflux ratio (molar) Feed stage Solvent feed stage Vapor recycle feed stage Column diameter (m) Design pressure (kPa) Condenser duty (MW) Reboiler duty (MW) Total condenser duty (MW) Total reboiler duty (MW) Annualized capital cost (k$ yr
1
)
C1
C2
C3
48 5.65 45 – 24 3.10 101.35 15.07 18.13 19.51 21.48 186
19 0.80 16 3 – 1.45 101.35 4.06 2.05
10 0.71 4 – – 0.78 101.35 0.38 1.30
extractive column, in this way the feed broth and the solvent are fed to the same column and ethanol and water are obtained as distillate and residue, respectively. Among all the configurations, the results for the one reported in Fig. 4e are showed in Tables 4 and 5, while the composition and temperature profiles in Fig. 8. It is possible to notice that, compared with the reference case reported in Fig. 2, this sequence realizes 35% reduction of both condenser and reboiler duties and 11% reduction of the capital costs. Anyway for all the configurations of this group the maximum molar purity achievable for the water stream was 0.972. This result was theoretically expected since ethylene glycol has the highest boiling point among all the components and its concentration naturally increases in the bottom stream. The loss of the solvent in the water stream should be carefully considered. In the case presented, ethylene glycol is used and its loss must be replenished increasing the solvent make-up flowrate. As can be noticed from Table 5 the amount of solvent recovered is only one third of the total flowrate already defined in the base case. The extra cost of the solvent can reduce the benefits of the energy and capital cost savings. But this result cannot be generalized since new solvents like bio-glycol are cheap, renewable and biodegradable and their cost does not affect the economy of the process [28,29]. The bio-glycerol produced as a by-product in biodiesel plants can be integrated with the bioethanol purification section. A more accurate economic analysis can define the trade-off between energy saving, solvent cost and water purification. It is possible to conclude that the importance of the configurations of Fig. 4a, c, and e is mainly related to the solvent cost and its availability and they represent a valid alternative for energy and capital cost reduction. 5.2.2. Pre-concentrator stand-separate from the extractive column This group includes the configurations reported in Fig. 4b and d. The pre-concentration column saves its original function and the connection with the extractive column can be realized with a single stream like in Fig. 4b otherwise with a thermal coupling so as reported in Fig. 4d. The configuration reported in Fig. 4b has the same pre-fractionator column design as the reference case, but the extractive column performs the separation between the most volatile component (ethanol) and the higher boiling point one (solvent). The external section 5 is used to recover the intermediate boiling component (water) minimizing, at the same time, the loss of solvent. Considering that the distillate flowrate of the extractive column cannot be changed since it is specified by the ethanol production, the vapor flowrate from the extractive column to section 5 and its stage location are the outstanding parameters in the analysis of the whole configuration. The thermal coupling was located at the 19th stage in agreement with the composition and temperature profiles reported in Fig. 9. This also match the initial design parameters obtained considering that the sequence of Fig. 4b is generated by column section recombination of the reference configuration of Fig. 2. The vapor flowrate is mainly composed of water and must be fixed taking into account the quantity of water introduced in the column with the feed and according to the overhead vapor recycled to the first column. In this way it is possible to recover almost pure solvent as bottoms stream without any purity penalty for the ethanol. The design parameters of the configuration, together with the heat duties and the capital costs are reported in Table 6, the mass and energy balance in Table 7. The total reboiler and condenser duty are 7% and 2%, respectively lower than the corresponding reference sequence. Also the capital costs of the sequence are reduced by 9%. This sequence represents a useful configuration to achieve a reduction of the separation cost related to the bioethanol production keeping the same purity target for all the streams. Substituting the partial condenser of the first column with a thermal coupling like showed in Fig. 4d it was not possible to reach the purity targets required. This is mainly due to the composition
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M. Errico, B.-G. Rong / Separation and Purification Technology 96 (2012) 58–67 Table 2 Mass and energy balance for the reference design of Fig. 2.
Temperature (K) Pressure (kPa) Vapor frac Enthalpy (GW h 1) Mole flow (kmol h 1) Mole frac Ethanol Water Glycol
Fermentation broth
Az. feed
Water
Ethanol
Solvent recycle
Solvent make up
Overhead vapor
363.050 101.350 0 1209.034 4313.000
351.010 101.350 1 56.829 244.600
371.900 101.350 0 1150.024 4105.400
351.150 101.350 0 56.218 208.000
469.810 101.350 0 62.782 146.140
303.150 101.350 0 0.184 0.400
378.200 101.350 1 8.828 37.040
0.051 0.949 0
0.879 0.121 Trace
0.003 0.997 77 ppm
0.998 0.002 407 ppm
Trace 4 ppm 1.000
0 0 1.000
0.199 0.792 0.009
Fig. 6. Base-case configuration: (a) Liquid-phase concentration profiles: ethanol (), glycol (N), water (j) and (b) temperature profile.
Table 3 Mass balance, energy requirement and capital cost for the configuration of Fig. 3b.
Temperature (K) Pressure (kPa) Vapor frac Enthalpy (GW h 1) Mole flow (kmol h 1) Mole frac Ethanol Water Glycol Total condenser duty (MW) Total reboiler duty (MW) Annualized capital cost (k$ yr
1
)
Ethanol
Water
Solvent recycle
Overhead vapor
Thermal coupling
351.150 101.350 0 56.222 208.000
371.850 101.350 0 1150.200 4105.800
469.750 101.350 0 62.781 146.140
385.650 101.350 1 8.966 37.440
435.350 101.350 1 31.273 109.800
0.998 0.002 488 ppm 19.170 21.140 176
0.003 0.997 73 ppm
Trace 195 ppb 1.000
0.202 0.780 0.008
0.122 0.495 0.383
Fig. 7. Modify thermally coupled configuration 3(b): (a) Liquid-phase concentration profiles: ethanol (), glycol (N), water (j) and (b) temperature profile.
of the liquid used to reflux the first column that includes also part of the high-boiling solvent. 5.3. Intensified sequences As pointed out in Section 3, there is a correspondence between the sequences generated through the step-by-step procedure.
Therefore the configurations reported in Fig. 5 can also be classified into two groups as done for the ones in Fig. 4. The first group includes the configurations where the fermentation broth and the solvent are fed in the same column. As discussed before for these configurations it is possible to achieve considerable energy and capital cost reductions but the cost of the solvent loss dictates the convenience of their application. In the second group the
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Table 4 Design parameters, energy consumption and capital cost of the configuration reported in Fig. 4e.
Number of stages Reflux ratio (molar) Feed stage Solvent feed stage Vapor recycle feed stage Thermal coupling stage Column diameter (m) Design pressure (kPa) Condenser duty (MW) Reboiler duty (MW) Total condenser duty (MW) Total reboiler duty (MW) Annualized capital cost (k$ yr
1
)
C1
C2
C3
63 4.57 60 3 39 16 2.63 101.35 12.54 10.73 12.61 14.11 165
8 – – – – 4 1.25 101.35 0 3.38
3 – – – – – 1.25 101.35 0.07 0
Table 6 Design parameters, energy consumption and capital cost of the configuration reported in Fig. 4b.
Number of stages Reflux ratio (molar) Feed stage Solvent feed stage Vapor recycle feed stage Thermal coupling stage Column diameter (m) Design pressure (kPa) Condenser duty (MW) Reboiler duty (MW) Total condenser duty (MW) Total reboiler duty (MW) Annualized capital cost (k$ yr
1
)
C1
C2
C3
48 5.65 45 – 24 – 3.10 101.35 15.06 17.99 19.17 19.88 170
24 0.8 16 3 – 19 1.43 101.35 4.08 1.89
3 – – – – – 0.93 101.35 0.03 0
Table 5 Mass balance, energy requirement and capital cost for the configuration of Fig. 4e.
Temperature (K) Pressure (kPa) Vapor frac Enthalpy (GW h 1) Mole Flow (kmol h Mole frac Ethanol Water Glycol
1
)
Ethanol
Water
Solvent recycle
Overhead vapor
TCL
TCV
315.150 101.350 0 56.205 208.000
372.550 101.350 0 1196.309 4209.140
469.750 101.350 0 18.385 42.800
353.050 101.350 1 12.750 55.000
353.450 101.350 0 90.836 304.000
384.250 101.350 1 13.345 57.000
0.998 0.002 134 ppm
0.003 0.972 0.025
25 ppb 2 ppm 1.000
0.950 0.047 0.002
0.834 0.023 0.143
0.921 0.047 0.032
Fig. 8. Modify thermodynamically equivalent configuration 4(e): (a) Liquid-phase concentration profiles: ethanol (), glycol (N), water (j) and (b) temperature profile.
Fig. 9. Modify thermodynamically equivalent configuration 4(b): (a) Liquid-phase concentration profiles: ethanol (), glycol (N), water (j) and (b) temperature profile.
configurations reported in Fig. 5b and d are considered. As for the sequence in Fig. 4d, also for the sequence in Fig. 5d it was not possible to reach the purity target. The configuration of Fig. 5b was
generated from the one reported in Fig. 4b by substitution of the column section 5 with a vapor side-stream. The main function of section 5 is to recover part of the ethylene glycol from the vapor
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M. Errico, B.-G. Rong / Separation and Purification Technology 96 (2012) 58–67 Table 7 Mass balance, energy requirement and capital cost for the configuration of Fig. 4b.
Temperature (K) Pressure (kPa) Vapor frac Enthalpy (GW h 1) Mole Flow (kmol h Mole frac Ethanol Water Glycol
1
)
Ethanol
Water
Solvent recycle
Overhead vapor
Thermal coupling
350.950 101.350 0 60.498 208.000
371.850 101.350 0 1150.024 4105.400
469.750 101.350 0 62.735 146.14
378.350 101.350 1 275.259 37.040
389.35 101.350 0 0.465 1.26
0.998 0.002 407 ppm
0.003 0.997 78 ppm
Trace 41 ppm 1.000
0.200 0.791 0.009
0.014 0.449 0.536
Fig. 10. Intensified sequence 5(b): (a) Liquid-phase concentration profiles: ethanol (), glycol (N), water (j) and (b) temperature profile.
Table 8 Design parameters, energy requirement and capital cost of the configuration reported in Fig. 5b.
Number of stages Reflux ratio (molar) Feed stage Solvent feed stage Vapor recycle feed stage Vapor withdrawal Column diameter (m) Design pressure (kPa) Condenser duty (MW) Reboiler duty (MW) Total condenser duty (MW) Total reboiler duty (MW) Annualized capital cost (k$ yr
1
)
C1
C2
48 5.65 45 – 24 – 3.10 101.35 15.07 18.13 19.13 21.10 157
28 0.8 16 3 – 20 1.45 101.35 4.06 2.97
phase in order to avoid its loss in the water stream recycled to the first column. Removing this section the vapor is directly withdrawn from the second column and fed to the first one, a small amount of solvent could be inevitably lost. To avoid this loss the
number of stages of section 6 were modified and the vapor sidestream location was defined accordingly. The vapor flowrate sets the purity of the solvent stream and its choice is related to avoid the increase of the solvent make-up flowrate. The composition profiles for the extractive column and the corresponding temperature profile are showed in Fig. 10. The design parameters of the configuration, together with the condenser and reboiler heat duties and capital costs are reported in Table 8. The mass and heat balance are reported in Table 9. It is possible to notice that the energy consumption is 2% lower than the reference case, more significantly, the reduction of capital cost by 16% is achieved. For the studied case the configurations of Fig. 4b and b represented the best solutions in terms of both energy and capital costs keeping the same purity target as the reference case. It should be pointed out that this article is focused on the systematic synthesis of alternative sequences for the bioethanol purification from fermentation broth. The new sequences are then explored in details considering a fixed feed composition and a specified solvent. We do not exclude the other derived configurations as potential optimal configurations when different mixture compositions and different solvents are considered in the separation process. This is because different raw materials from different
Table 9 Mass and energy balance for the configuration of Fig. 5b.
Temperature (K) Pressure (kPa) Vapor frac Enthalpy (GW h 1) Mole Flow (kmol h Mole frac Ethanol Water Glycol
1
)
Ethanol
Water
Solvent recycle
Solvent make up
Overhead vapor
351.15 101.35 0 56.218 208.00
371.85 101.35 0 1150.062 4105.40
469.15 101.35 0 62.764 146.14
303.15 101.35 0 0.184 0.40
382.96 101.35 1 8.853 37.04
0.998 0.002 407 ppm
0.003 0.997 131 ppm
Trace 0.001 0.999
0 0 1.000
0.200 0.785 0.015
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flowsheet definition subject to the production specifications imposed by the specific process.
Table 10 Energy performance and capital cost comparison.
Reference case Modified thermally couples sequence, Fig. 3b Modified thermodynamic equivalent sequence, Fig. 4b Intensified sequence, Fig. 5b
Energy index (MJ/kg ethanol)
Capital cost savings (% on the base case)
8.08 7.95
– 5
7.47
9
7.93
16
biomass resources and different manufacturing processes are employed for bioethanol productions. Another observation regards the order of energy saving realized by the new configurations. It is true that 7% or 2% cannot be defined a huge saving, but must be considered that the configuration used as a starting point for the generation procedure is an already optimized sequence able to reduce the total energy requirement of the condensers and reboilers of 20% compare to the classical design for extractive distillation [15]. Anyway applying the synthesis methodology described it was possible to generate a more complete subspace and some configurations are valid alternative to save energy and/ or capital costs depending if some solvent loss in the water stream is admitted or not. A final observation regards the controllability of the new configurations. It was already proved that thermally coupled configurations have, in some case, a better dynamic performance compared to the classical simple columns [30,31]. A detailed analysis is out of the scope of the present work, but can be an independent future work to complete the study of the best configurations selected. 6. Conclusions A systematic method for the generation of new distillation sequences is presented and applied for the case of extractive distillation for bioethanol production. The method extends the thermal coupling technique already successfully applied for ideal mixtures to bioethanol separation. A specific mixture and a specific solvent are used for the evaluation of the new sequences. The results showed that the methodology can produce improved configurations than the base case configuration for the studied case. It is understood that the relative advantages of the derived configurations will be different when different solvents and different feeds are considered for bioethanol productions. For different solvents and different mixtures the exploration of the whole space can lead to the best configuration. It was possible to identify different promising configurations with different performance indicators. The results for all the configurations considered are summarized in Table 10, where the specific energy use per kg of ethanol produced was used to compare the energy performances of the different alternatives [31]. Moreover the capital cost saving respect to the reference case is reported. It is possible to notice that all the proposed configurations have a lower energy index compared to the traditional distillation configuration considered as base case. The energy reduction is not so significant compared to the savings in the capital cost. The limited value of the energy performance of the new sequences can be explained considering that the reference configuration used in the synthesis methodology is an already optimized configuration able to reduce the duty of the condensers and reboilers of above 20%. Anyway for the configuration of Fig. 5b a capital cost reduction of 16% can be considered significant. This work represents one step more in the definition of a more complete design space with new derived alternatives. Each new configuration has benefits only in certain cases which the designer should consider during the
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