Optimal Synthesis of Distillation Systems for Bioethanol Separation

Article pubs.acs.org/IECR

Optimal Synthesis of Distillation Systems for Bioethanol Separation. Part 1: Extractive Distillation with Simple Columns Massimiliano Errico,†,‡,* Ben-Guang Rong,† Giuseppe Tola,‡ and Maurizio Spano‡ †

University of Southern Denmark, Institute of Chemical Engineering, Biotechnology and Environmental Technology, Niels Bohrs Allé 1, DK-5230 Odense M, Denmark ‡ Dipartimento di Ingegneria Meccanica, Chimica e dei Materiali, Universitá degli Studi di Cagliari, Via Marengo 2, 09123 Cagliari, Italy S Supporting Information *

ABSTRACT: The article presents an analysis between different possible sequences for the separation of bioethanol from a typical fermentor’s stream. The preconcentrator column, necessary to approach the azeotropic composition was included as a fundamental part of the sequences. Starting from a recently proposed four-column configuration in the literature, a full set of alternatives was predicted and explored in detail. Different combinations of partial and total condensers were considered so as to possibly reduce the amount of equipment. It was proved that among all the simple configurations generated, two sequences with three columns are able to consistently reduce the energy demand and the capital costs. The results obtained represent the first step for the further generation of complex configuration sequences.

1. INTRODUCTION The definition of the best distillation sequence, in terms of energy consumption, remains one of the main open research issues. In a move away from the petrochemical applications, nowadays the development of energy efficient separation units is a fundamental step in the biofuel production. Considering the particular case of bioethanol purification, the presence of a homogeneous azeotrope between water and ethanol offers new challenges in the definition of alternative configurations for the production of anhydrous ethanol by distillation. As seen from an approach to the problem from a synthesis point of view, there are two main steps to be contemporaneously solved. The first is the method’s definition for the alternatives generation, and the second is what kind of sequences should be included. In the literature different methodologies have been proposed. One of the first attempts was done by Thompson and King.1 They created an algorithm initialized by the definition of a feasible product set and integrated it with different heuristic rules. The use of heuristic rules has been a popular tool for screening between different flowsheet alternatives2,3 even if their lack of trustworthiness was proved by different authors.4−6 More recently7,8 the problem of the generation of flowsheet alternatives was approached by the use of superstructures able to consider sequences with simple or complex columns. Agrawal9 proposed a network representation to generate both basic and thermally coupled configurations. The proposed methodology is complete up to four feed components and was recently extended to five or more.10 A different approach was presented by Rong during the last years.11−13 In his works the concept of individual split is used together with the definition of a step-by-step generation procedure starting from the simple column configurations through the intensified sequences14 or the divided wall columns.15 © 2013 American Chemical Society

Independently of the tool used to develop an as much complete as possible set of alternatives, it is clear that the definition of methods or criteria for the flowsheet generation has a paramount importance in searching the optimum solution for a specified separation. Unfortunately the same amount of research developed for the distillation of ideal mixtures is not available for azeotropic separations. This is because the development of the petrochemical and chemical industries deeply catalyzes the research efforts in the separation of ideal or near-ideal mixtures; moreover different separation techniques are available for the separation of azeotropic mixtures. By limiting our analysis only to separations by distillation, we may identify two groups depending if a mass separation agent is added or not to the system. If only heat is considered as the separation agent, pressure-swing distillation is effective in all the cases where the azeotrope composition changes with pressure.16 When a third substance is added as entrainer, depending on its affinity with the feed components, it is possible to distinguish between extractive distillation, heterogeneous azeotropic distillation, and homogeneous azeotropic distillation. Details about different separation methods for dehydrated bioethanol production can be found in more specialized literature.17−19 Considering the different separation opportunities, it appears more difficult to develop heuristic rules or design indications for azeotropic mixtures as was done for ideal or near-ideal mixtures. Extractive distillation, applied to the bioethanol production will be considered in detail, since its energy efficiency was already proved.20−22 The classical separation sequence, together with the alternative configurations proposed Received: Revised: Accepted: Published: 1612

July 10, 2012 September 24, 2012 January 1, 2013 January 1, 2013 dx.doi.org/10.1021/ie301828d | Ind. Eng. Chem. Res. 2013, 52, 1612−1619

Industrial & Engineering Chemistry Research

Article

According to Hohmann et al.31 a column section is defined as a column portion not interrupted by material or heat streams. Recently a new configuration for the ethanol−water separation by extractive distillation was presented by Li and Bai.32 As claimed by the authors the importance to define energy efficient sequences for the ethanol−water separation is nowadays related to the bioethanol production. For this reason, as reported in Figure 2, the preconcentrator column was

in the literature, is described, and by the definition of a systematic methodology, a connection between all the sequences will be defined. Process synthesis is regarded, for the most part, as an intuitive art,1 but an offhand flowsheet generation can lead the designer to consider only a small part among all the possible alternatives.

2. BIOETHANOL SEPARATION AND PURIFICATION BY DISTILLATION Bioethanol producing facilities are composed by different process sections arranged in different ways depending on the feed considered. Usually the ethanol recovery section is fed with an ethanol−water stream coming from the fermentation unit. The ethanol content of this stream is more dilute compared to the azeotropic composition; for this reason a preconcentration column is necessary to approach it.23,24 Flowsheets or case studies applied to the bioethanol separation plant based on the ethanol−water azeotropic feed composition cannot be considered truthful. Moreover the preconcentration could represent an opportunity to improve the flowsheet integration that otherwise will be neglected. All the configurations developed in this study are equipped with the preconcentration column. The base or classical separation sequence for the extractive distillation is reported in Figure 1. With respect to the

Figure 2. Configuration proposed by Li and Bai integrated with the preconcentrator column.

included to the original configuration. The principle used to develop this configuration is derived from the examination of the VLE curve for the ethanol−water system with or without the solvent. The authors noticed that when the concentration of ethanol in the liquid is below 21% mol the relative volatility of the system with the solvent is lower than the corresponding value without it. For this reason an amount of 0.21 mol of ethanol (free solvent basis) was left in the extractive bottom stream and then recovered in the last column. This sequence can be also derived applying first the already mentioned heuristic rule concerning the mass separation agent recovery, then the rule to perform first the separations with the higher relative volatility value.30 The same kind of sequence with a postdistillation column was patented by Lee et al.33 They develop the configuration considering that the energy requirement of the extractive distillation column can be improved limiting the ethanol recovery realized in that column. Compared to the base case reported in Figure 1, the configuration of Figure 2 employs one column more and a liquid recycle is sent back to the extractive column. Table 1 presents a summary of some of the most significant studies on the bioethanol purification. It is possible to notice that different approaches, different solvent, and different purity targets have been followed during the years. In the present work a set of alternative configurations was proposed. Their comprehensive analysis gives a complete overview on the sequences’ performance expressed in terms of energy utilization, capital costs, and solvent consumption. This approach differs from the fragmented generation procedure that has been observed in the literature until now.

Figure 1. Extractive distillation base configuration.

flowsheets reported in the literature, the preconcentrator column was added to consider the particular case of the bioethanol production process.25−28 The preconcentrator is followed by the extractive column and the recovery column. The azeotropic composition stream, from the first column’s distillate, is fed to the extractive column together with the solvent chosen for the extraction. The solvent does not form any azeotrope with the feed components. It should be noticed that it is possible to identify different types of solvents depending on how the liquid phase activity coefficients are altered. Figure 1 presents the case where the solvent is able to increase the relative volatility of the key components. Solvents that reverse the volatilities are not considered here.29 The distillate of the extractive column is composed of ethanol at the required purity, the bottom stream contains the solvent, water, and a trace of ethanol. Following the heuristic rules that require a removal of the mass separation agent in the separator immediately following the one into which it is introduced,30 the last column of the reference sequence is used to recover the solvent. On the whole, two water streams are obtained, one as bottom stream of the preconcentrator column and another as distillate of the recovery column. In Figure 1 and in the following paragraphs the column section notation is used.

3. GENERATION OF THE ALTERNATIVE CONFIGURATIONS The main limit in proposing a standalone configuration is that it is impossible to make any claims about its absolute convenience in terms of energy consumption or capital costs. This limitation appears because of the different approach used to develop the alternative configurations. By considering the 1613

dx.doi.org/10.1021/ie301828d | Ind. Eng. Chem. Res. 2013, 52, 1612−1619


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Table 1. Different Extractive Distillation Works for the Production of Fuel Grade Ethanol author

year

preconcentrator

no. of columns

solvent

study method

ethanol purity[mol %]

Knight and Doherty34 Chianese and Zinnamosca35 Laroche et al.36 Taylor and Wankat37 Garcia-Herreros et al.38 Li and Bai32 Present research

1989 1990 1992 2004 2011 2012 2012

yes no no yes no no yes

3 2 2 3 2 3 3

ethylene glycol benzene + N-octane methanol ethylene glycol glycerol ethylene glycol ethylene glycol

optimization analysis synthesis synthesis optimization synthesis synthesis

99.80 97.58 99.95 99.70 99.50 99.95 99.99

Figure 3. Configurations with one partial condenser.

the preconcentrator column. All the nonproduct streams associated with a condenser can be considered to be partially condensed. Figure 3 reports all the possibilities obtained from the configuration of Figure 2. 3.2. Configurations with Two Partial Condensers. The possibility to introduce a partial condenser in the separation sequence is not limited to one unit. To study the possible mutual influence, different possible pairs of partial condensers are considered. Figure 4a reports the case where a partial condenser is introduced in the preconcentrator and in the solvent recovery column, Figure 4b considers the case of the first and last column equipped with a partial condenser and in Figure 4c the partial condenser is associated with the last two columns. 3.3. Configuration with Three Partial Condensers. As a last option, the possibility to include all the partial condensers to the nonproduct streams is considered in Figure 5. A vapor stream is transferred from the preconcentrator to the extractive column and from the solvent recovery column’s distillate to the last column. The recycle back from the last column to the extractive column is in vapor phase. 3.4. Three-Column Configurations. All the sequences generated from Figures 2 through 5 have the common feature to include four columns, that is one more than the base case

ideal mixtures and limiting the analysis only to simple column sequences, we can predict all the possible alternatives using the formula derived by Thompson and King.1 This methodology cannot be applied to azeotropic mixtures since, in those cases, there are different variables like the effect of the solvent or the azeotrope type that make the generalization more complex or even not possible. To dispose of a more broad space of configuration alternatives, the Li and Baiś configuration32 was utilized as starting point to generate different sequences. With the use of this configuration, the sequences are compared in order to define the best solution with respect to the energy consumption, the capital cost investment, and the solvent consumption. 3.1. Configurations with a Single Partial Condenser. The possibility to introduce a partial condenser in the ethanol− water distillation sequence was initially proposed by Lynn and Hanson.39 It represents a slight modification of the base case since only a partial condenser, instead of a total one, was used to transfer the azeotropic stream between the first and the second column. This option was also reported by Seader et al.40 as an advantage to maintain a higher solvent concentration on the feed tray and on the trays immediately below. The possibility of introducing a partial condenser is not limited to 1614



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Figure 4. Configurations with two partial condensers.

column. In particular, the last column performs the separation between water and a mixture of water and ethanol close to the azeotropic composition. The same separation task is performed by the first column through sections 1 and 2. If the two columns are combined it is possible to directly recycle the distillate of the solvent recovery column to the preconcentrator. The recycle can be liquid or vapor, depending on if a partial or total condenser is employed. The corresponding sequences are reported in Figure 6 panels a and b, respectively. In this way one column less is used and at the same time only one water stream is obtained. The configurations reported in Figure 6 were also obtained by Taylor and Wankat.37 They developed the configurations considering the popular heuristic rules of removing the mass separation agent early in the sequence and avoiding the separation of the same compound in different units. It should be noted that starting from the configuration reported in Figure 2 it was possible to generate a subspace of

Figure 5. Configuration with three partial condensers.

reported in Figure 1. Moreover water is separated twice in the sequences; one water stream is obtained as bottom of the preconcentrator, and another one is obtained from the last

Figure 6. Three-column configurations: (a) liquid recycle, (b) vapor recycle. 1615



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be easily recovered in the process, for this reason the cost of that cooler is not considered. Finally, the capital cost was annualized considering a mean operational time of 10 years. Aspen Plus Economic Evaluator is not based on bare module factors but extensive data are used to estimate the costs of materials, labor and construction.

alternatives that includes sequences with different types of condensers and sequences obtained merging the units performing the same separation task. This approach differs from the other works32,37,41 where only a few configurations have been defined considering heuristic rules or analyzing the VLE curve. Disposing of a more broad subspace of alternatives offers to the designer a reliable searching space.

5. RESULTS This section includes the results for the configuration generated in section 5. The base case used as term of comparison is the configuration of Figure 2 recently proposed by Li and Bai.32 The total condenser and reboiler duty was used to compare the energy performance of the different configurations. 5.1. Base Configuration Results. The design parameters for all the columns, together with the energy requirements and the capital costs, are reported in Table 3. Concerning the water

4. CASE STUDY Some of the sequences discussed in the previous paragraphs were already reported in the literature by different authors in different years. This scattered prediction of some configurations makes difficult a rigorous comparison because different feed compositions were considered or because in some cases the preconcentrator column was not included in the sequence. It becomes of meaningful importance have a uniform comparison between all the configurations reported from Figure 2 to Figure 6. The main application for which the configurations are developed is bioethanol purification, for this reason a diluted ethanol−water solution produced by a fermentation process is considered as a feed for all the cases considered. The composition and the physical properties of the feed are reported in Table 2. All the simulations were performed by

Table 3. Design Parameters, Energy Requirement, and Capital Cost of the Configuration Reported in Figure 2 C1

Table 2. Feed Characterization temperature (K) vapor fraction enthalpy (GW hr−1) mole flow (kmol h−1) mole fraction ethanol mole fraction water mole fraction

C2

number of stages 44 reflux ratio (molar) 2.420 feed stage 30 solvent feed stage column diameter (m) 1.37 design pressure (kPa) 101.000 condenser duty (kW) 3736.356 reboiler duty (kW) 4084.769 total condenser duty (kW) total reboiler duty (kW) annualized capital cost (k$ yr−1)

363.050 0 −474.935 1694.240 0.050 0.950

28 0.185 24 5 0.78 101.000 1090.349 1516.456

C3

C4

17 0.390 6

19 3.035 17

0.51 0.27 101.000 101.000 296.097 147.736 552.729 153.115 5270.538 6307.069 161.3

obtained; the stream separated as bottom of the last column’s sequence represents only 1% of the total water produced, and its purity is slightly lower than the pure one produced from the preconcentrator (see Supporting Information). 5.2. Configurations with the Results of a Single Partial Condenser. Figure 3 includes the configurations belonging to this category. The introduction of a partial condenser in the connecting columns stream mainly affects, besides the heat duties, the feed locations and the column diameters. When necessary, the feed locations were redefined through a sensitivity analysis using the products purity as a controlled variable. The substitution of the total condenser associated with the recovery column with the partial condenser has the worst effect on the diameter distribution (see Supporting Information). Table 4 presents the total condenser and reboiler duties together with the capital cost evaluation. It is evident that the employment of a partial condenser in the last column of the sequence, as reported in Figure 3c, has a negligible influence on the total energy requirement and realizes only 2% reduction of the capital costs. By employing a partial condenser in the

means of the process simulator Aspen Plus V7.3. The NRTL method was utilized to evaluate the activity coefficients. Ethylene glycol was chosen as solvent considering its low volatility and the not excessively high boiling point.39 Moreover its suitability for large scale production was already proved.24 The ethanol minimum purity was set to 0.999 on a molar fraction basis to enable its use in oil-derived gasoline blends.42 All the simple columns in the sequences were first simulated utilizing the Winn−Underwood−Gilliland short-cut method.43−45 Then the number of stages, feed location, and reflux ratio were optimized utilizing the sensitivity analysis tool implemented in the RadFrac rigorous method.46 A solvent to feed ratio of 0.87 was employed in all the configurations simulated. The solvent recovery column was designed in order to minimize the makeup flow rate. The extractive column was simulated utilizing the Azeotropic convergence method based on Newton’s algorithm. Considering the similarity of the case study considered, the configuration parameters reported by Li and Bai32 were utilized to initialize the calculation, then they were optimized to get the minimum energy consumption for the defined purity target.47,48 The installation cost evaluation was performed with Aspen Plus Economic Analyzer. The distillation columns were considered equipped with sieve trays 0.6 m spaced; 2.5 m for the bottom sump height and 1.25 m for the vapor disengagement were also taken into account. For the auxiliary heat equipment, fixed tube condensers and floating head kettle reboilers were considered. In each sequence the solvent recovered is cooled from about 470 to 303 K. This heat can

Table 4. Total Heat Duties and Capital Cost Evaluation for the Sequences of Figure 3 configuration total condenser duty (kW) total reboiler duty (kW) annualized capital cost (k$ yr−1) 1616

Figure 3a

Figure 3b

Figure 3c

4174.477 5211.055 157.2

5137.854 6174.385 160.4

5227.593 6264.124 158.0



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are included in Table 5. The performances of this configuration do not differ in a significant way from the ones obtained for the sequences of Figure 4a,b. This because the duties of the last column are not so relevant compared to the preconcentrator or to the solvent recovery column. Anyway the vapor recycle stream to the extractive column has the benefit of increasing the solvent concentration in the liquid phase; for this reason it was possible to reduce the solvent makeup to 0.005 kmol h−1. On the whole the configuration with three partial condensers reduces the total condenser duty of 24%, the total reboiler duty of 20%, and the capital cost of 3% compared to the reference case of Figure 2. Additional reductions in the operative costs are expected by the decreased value of the fresh solvent makeup. 5.5. Results of the Three-Column Configurations. Analyzing the column composition profiles for the preconcentrator and for the last column of the sequence reported in Figure 2, it was evident that the two columns performed the same separation task (see Supporting Information). Bypassing the last column and recycling the solvent column’s distillate directly to the preconcentrator it is possible to obtain the configurations in Figure 6. Depending if a total or a partial condenser is used, the recycle stream can be liquid or vapor. The two alternatives are reported in Figure 6 panels a and b, respectively. In both cases the preconcentrator is equipped by a partial condenser since it was proved to be effective in the energy reduction of the configurations. In the configuration reported in Figure 6a the liquid recycle is fed together with the main feed, while when a vapor recycle is considered, its feed location was optimized by means of a sensitivity analysis. The design parameters and the heat duties for each column of the two configurations, together with the capital costs, are reported in Table 6. The best results were obtained for the configuration in Figure 6b when a vapor recycle is considered. For this case a 27% and 22% reduction of the condenser and reboiler duties was respectively achieved. Moreover differently from the other configurations examined in the previous paragraphs, 17% of capital costs saving were obtained. The configuration with the liquid recycle of Figure 6a can still be considered competitive compared to the base case and to those reported in Figures 4 and 5 because of the extra benefit of the capital cost reduction. For both the configurations the fresh solvent makeup was reduced to 0.004 kmol h−1 which represents the minimum value between all the alternatives considered.

solvent recovery column, as reported in Figure 3b, it is possible to save about 2% in both total condenser and reboiler duties without significant benefits for the capital costs. More interesting results come from the configuration of Figure 3a where the partial condenser is utilized in the preconcentrator column. For this case 21% and 17% reduction in the condenser and reboiler duties are respectively achieved. Moreover a 2.5% reduction of the capital costs is observed. Additional benefits can be highlighted, the ethanol purity increases from 0.999 to 1.000, and the solvent makeup flow rate was reduced by 25% (see Supporting Information). On the whole, the configuration of Figure 3a reduces the heat duties, reduces the capital investment, increases the solvent recovery, and improves the ethanol purity. 5.3. Configurations with Results of Two Partial Condensers. The configurations with different pairs of partial condensers are reported in Figure 4. The energy requirements and the capital costs are summarized in Table 5. The main Table 5. Total Heat Duties and Capital Cost Evaluation for the Sequences of Figures 4 and 5 configuration total condenser duty (kW) total reboiler duty (kW) annualized capital cost (k$ yr−1)

Figure 4a

Figure 4b

Figure 4c

Figure 5

4040.836

4148.886

5099.985

4021.409

5077.413

5185.484

6136.518

5057.006

156.3

156.1

160.4

156.3

benefits in terms of total duty reduction and capital cost savings are evident in the configurations of Figure 4a,b where the partial condenser of the preconcentrator column is utilized. A mean 20% reduction in both condenser and reboiler duties is observed. The saving in the capital costs is limited to 3%. The partial condensers associated with the solvent recovery column together with the one in the last column (see Figure 4c) have a very limited effect on the global energy performance of the sequence. It is possible to deduce that the partial condenser of the preconcentration column is the one that mainly affects the savings reached by the sequence. 5.4. Configuration with the Results of Three Partial Condensers. The possibility to introduce at the same time all the possible partial condensers is here considered. The corresponding sequence is shown in Figure 5 and the results

Table 6. Design Parameters, Energy Requirement, and Capital Costs for the Configurations Reported in Figure 6 Figure 6a C1 number of stages 44 reflux ratio (molar) 2.335 feed stage 30 liquid recycle feed stage 30 vapor recycle feed stage column diameter (m) 1.37 design pressure (kpa) 101.000 condenser duty (kw) 2638.673 reboiler duty (kW) 4122.202 total condenser duty (kW) total reboiler duty (kW) annualized capital cost (k$ yr−1)

Figure 6b

C2

C3

C1

C2

C3

28 0.246 25

17 0.390 7

44 2.337 30

28 0.246 25

17 0.397 6

0.79 101.000 1146.821 443.635 3871.042 4907.893 133.1

0.51 101.000 83.280 552.713

0.79 101.000 1146.848 443.638

1617

0.51 101.000 295.795 552.425 4081.316 5118.425 132.8

34 1.37 101.000 2640.941 3911.545



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Notes

It is clearly demonstrated that the three-column sequences represent, among the entire set of alternatives, the best solution in terms of energy utilization and capital cost investment.

The authors declare no competing financial interest.

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CONCLUSIONS It is of paramount importance that a reduction of the costs associated with alternative sequences for bioethanol production be included in the definition of the processes. A wide set of alternative sequences for bioethanol production was generated by the introduction of different combinations of partial condensers and by the possible merging of distillation columns utilized to perform the same separation task. Among all the sequences generated, some of them were already predicted in the literature, but a fair comparison was never done before because the sequences were predicted in different years and some of them were studied without the preconcentrator. A four-simple-column sequence, recently proposed in the literature, was utilized as starting point to initialize the generation procedure and as a reference to evaluate their performances. The alternatives include configurations with one, two, or three partial condensers and three-column sequences. Table 7 Table 7. Energy Utilization, Capital Cost, and Solvent Recovery Comparison configuration Figure 2 (reference) Figure 3a Figure 3b Figure 3c Figure 4a Figure 4b Figure 4c Figure 5 Figure 6a Figure 6b

number of columns

energy index (MJ/kg ethanol)

4

5.798

4 4 4 4 4 4 4 3 3

4.791 5.676 5.759 4.668 4.767 5.641 4.649 4.703 4.510

capital cost savings (% of the reference)

solvent makeup (kmol/h) 0.009

2.54 0.56 2.05 3.10 3.22 0.56 3.10 17.67 17.48

0.006 0.009 0.009 0.006 0.005 0.009 0.005 0.004 0.004

reports the energy use per kilogram of ethanol produced, the capital cost, and the solvent consumption for all the sequences considered. From the point of view of the energy demand, the sequences on Figures 3a, 4a, 5, 6a, and 6b are the most promising, but if also the possibility of a reduction in capital costs is considered, the performances of the sequences in Figure 6 overcome all the others. The tree-column sequences are able to substantially reduce the condenser and reboiler duty together with over 17% of capital cost saving and a minimum solvent makeup flow rate. They clearly represent the best options between all the simple column sequences considered.

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ASSOCIATED CONTENT

S Supporting Information *

Additional tables and column composition profiles. This information is available free of charge via the Internet at http://pubs.acs.org/.

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REFERENCES

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Optimal Synthesis of Distillation Systems for Bioethanol Separation

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