Comparison of the main ethanol dehydration technologies through process simulation

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1 20 th European Symposium on Computer Aided Process Engineering ESCAPE20 S. Pierucci and G. Buzzi Ferraris (Editors) 2010 Elsevier B.V. All rights reserved. Comparison of the main ethanol dehydration technologies through process simulation Paola A. Bastidas, a Iván D. Gil, a Gerardo Rodríguez a a Grupo de Procesos Químicos y Bioquímicos Departamento de Ingeniería Química y Ambiental, Universidad Nacional de Colombia Sede Bogotá, Carrera , Bogotá, Colombia, idgilc@unal.edu.co Abstract Anhydrous ethanol production has become to be one of the most important issues for many countries in the world due to the great efforts directed to use biofuels and diminishment in pollution and environmental effects of fossil fuels. The process of anhydrous alcohol production comprises three main important steps: fermentation, distillation and dehydration. In the final dehydration step the quality of ethanol is determined by the operating conditions, the technology used and its benefits related to the quality and costs of ethanol. In Brazil and United States, the two largest producers of ethanol in the world, azeotropic distillation with cyclohexane, extractive distillation with ethyleneglycol and adsorption with molecular sieves are used. In this work, an investigation and comparative analysis of the three main ethanol dehydration technologies was made. Aspen Plus process simulator was used to simulate azeotropic, extractive and adsorption processes and to determine the main operating conditions taking a case base of 300 cubic meters per day of anhydrous ethanol. Additionally, a preliminary costs analysis was implemented taking into account total investment and operating costs of each technology. The results showed that extractive distillation process is the most promising technology from operating and economical points of view and that is necessary to investigate for new solvents that improve the efficiency and sustainability of the alcohol production. Keywords: ethanol dehydration, extractive distillation, azeotropic distillation, adsorption 1. Introduction Process simulation tools have become a very useful way in the design, analysis and retrofit of processes of particular interest from energetic and economical point of view, opening the possibility to make different sensitivity analyses and to combine optimization studies, cost estimation, detailed design and controllability analysis. Biofuels industry and in particular the bioethanol production process are demanding from process engineering fast and easy answers about the technologies and optimal conditions with they can be used. Ethanol is one of the most used biofuels that contributes diminishing environmental effects of fossil fuels. Their properties and its renewable origin ensure environmental sustainability and the process economy. In the final dehydration step the quality of ethanol is determined by the operating conditions, the technology used and its benefits related to the quality and costs of ethanol. In Brazil and United States, the two largest producers of ethanol in the world, azeotropic distillation with cyclohexane, extractive distillation with ethyleneglycol and adsorption with molecular sieves are used.

2 P. Bastidas et al. Heterogeneous azeotropic distillation has been widely studied in many papers and textbooks and widely applied in alcohol industry to dehydrate ethanol (e.g. 60% of dehydration plants in Brazil are azeotropic distillation based). However, heterogeneous azeotropic distillation reports some disadvantages associated with the high degree of nonlinearity, multiple steady states, distillation boundaries, long transients, and heterogeneous liquid-liquid equilibrium, limiting the operating range of the system under different feed disturbances [1-5]. Extractive distillation is based on the introduction of a selective solvent that interacts differently with each of the components of the mixture and mainly shows affinity with one of the key components [3, 6]. The principle driving extractive distillation is based on the introduction of a selective solvent that interacts differently with each of the components of the original mixture and which generally shows a strong affinity with one of the key components [4, 7, 8]. Adsorption on molecular sieves takes advantage of the difference of molecular size of ethanol and water molecules to adsorb in a selective way water molecules and allowing ethanol separation. Molecular sieves are materials composed by microporous substances that are characterized by their excellent ability to retain on its surface defined types of chemical species. These materials packed into a vessel make possible to separate ethanol from ethanol-water mixtures by adsorption mechanisms at high pressure. In this work the three main ethanol dehydration technologies will be studied in order to establish the main operating conditions required to obtain high purity ethanol. Rigorous simulations in Aspen Plus for a plant producing 300 cubic meters per day of anhydrous ethanol will be carried out and some economical considerations are included in the comparison of the technologies available. 2. Process Simulation Ethanol-water mixture at atmospheric pressure has a minimum-boiling homogeneous azeotrope at 78.1 C of composition 89 mol% ethanol. The NRTL physical property model is used to describe the nonideality of the liquid phase and the vapor is assumed to be ideal. All NRTL model binary parameters are taken from Aspen Plus database. For all of the three processes simulated azeotropic ethanol was the feed and anhydrous ethanol with purity higher than 99.5 mole % was fixed as the main product. On the next subsections are described briefly each one of the processes and the main operating conditions established are reported Azeotropic distillation with cyclohexane Azeotropic distillation uses a solvent with an intermediate boiling point to introduce new azeotropes to the mixture and at the same time to generate two liquid phases that allow, in a combined way, separating ethanol from water. This technique although is widely used has lost acceptance due to its poor stability and high energy consumption. The process flowsheet of azeotropic distillation is shown on Fig. 1. The process has two columns and one decanter. The first heterogeneous azeotropic distillation column is designed to obtain high-purity ethanol product at the column bottom while obtaining minimum boiling ethanol-water-cyclohexane azeotrope at the top of the column. The azeotrope obtained at the top is heterogeneous and the top vapor stream is then condensed to form two liquid phases in the decanter [7, 9]. The organic phase containing mainly cyclohexane is refluxed back to the heterogeneous azeotropic distillation column. The aqueous phase is drawn out from the decanter to be sent to the entrainer recovery column where at the bottoms stream is obtained water essentially pure and at the top is removed cyclohexane to be recycled to the first column.

3 Comparison of the main ethanol dehydration technologies through process simulation Makeup Cyclohexane kmol/h Recycle 65 ºC kw kw P = 2 atm P = 1 atm S1 F kmol/h EtOH = Water = R1-ORG kmol/h EtOH = Water = Cyclohex = NS = 31 ID = 2.26 m D kmol/h EtOH = Water = Cyclohex = RR = 0.3 NS = 22 ID = 1.74 m D kmol/h EtOH = Water = Cyclohex = kw B kmol/h EtOH = Water = Cyclohex = kw B kmol/h EtOH = Water = Cyclohex = 1.0 e-20 Figure 1. Flowsheet for azeotropic distillation with cyclohexane The results obtained show that is possible to produce anhydrous ethanol using cyclohexane as entrainer with high mole recovery of ethanol. As the top vapor concentration approaches to the ternary heterogeneous azeotrope, the separation is achieved is improved in the dehydration column. As the organic reflux flow rate and the recycle flow rate increase the ethanol concentration at the bottoms of the dehydration column also increase improving the separation performance but also increasing the heat duties. The operating conditions are used to calculate the hydraulic performance and to estimate the column diameters, information useful to calculate the capital costs Extractive distillation with ethyleneglycol Extractive distillation is a partial vaporization process in the presence of a non-volatile and high boiling point entrainer which does not form any azeotropes with the original components of the azeotropic mixture. The process flowsheet of extractive distillation system is presented on Fig. 2. The process has two columns: the extractive distillation column and the entrainer recovery column. The entrainer is continuously fed in one of the top stages of the extractive column while the azeotropic feed is entered in a middle stage lower down the column. At the top of the extractive distillation column is obtained anhydrous ethanol and at the bottoms stream is removed a mixture of waterethyleneglycol which is send to the second entrainer recovery column. In the recovery column at the top water is withdrawn with some traces of ethanol and at the bottom high-purity ethyleneglycol is recycled back to the extractive distillation column [9]. Extractive distillation process with ethyleneglycol show some important advantages respect to azeotropic ones. The makeup entrainer is much lower than azeotropic case and additionally the quantity of entrainer is lower which affect the diameter of the columns. It can be observed that the column diameters are smaller in the extractive distillation systems and also the energy consumption in the columns. On the other hand,

4 P. Bastidas et al. the most important variables used to achieve the desired ethanol concentration are the entrainer to feed molar ratio and the reflux ratio. The former has a little effect over the energy consumption compared with the reflux ratio impact on the reboiler duty, for this reason the reflux ratio in extractive distillation column is fixed at the best low value. Makeup EthGlycol kmol/h Ethylene-Glycol Recycle 80 ºC kw kw P = 1 atm P = 0.26 atm S1 F kmol/h EtOH = Water = RR = 0.5 NS = 22 ID = 1.48 m D kmol/h EtOH = Water = EthGlyc = 8.3 e-04 B kmol/h EtOH = 1.0 e-06 Water = EthGlyc = RR = 1.0 NS = 12 ID = 0.85 m D kmol/h EtOH = 8.06 e-06 Water = EthGlyc = 1.0 e-06 Cooler Duty -715 kw B kmol/h EtOH = 3.17 e-14 Water = 4.32 e-04 EthGlyc = ºC kw kw Figure 2. Flowsheet for extractive distillation with ethyleneglycol 2.3. Adsorption with molecular sieves Figure 3. Flowsheet for adsorption with molecular sieves

5 Comparison of the main ethanol dehydration technologies through process simulation Dehydration by molecular sieves operates by dehydration/regeneration cycles; while one bed is in a dehydrating cycle the other one is being regenerated. In the first bed is passed azeotropic ethanol vapor from rectifying column that has been heated in a vaporizer previously, in order to increase the pressure to 25 psig. Regeneration is made by recirculating 15% of superheated anhydrous ethanol vapors to the second bed, in order to remove accumulated moisture in the previous dehydration cycle. The process flowsheet is shown on Fig. 3. The net flowrate of the anhydrous ethanol produced is lower than the obtained in the distillation based operations. This is due to the high ethanol recycle required to regenerate the second bed. This affects in an important way the efficiency of the process and increases the total energy consumption required to produce one kilogram of ethanol. Also, it is important to take into account the energy involved in the vacuum pump used in the regeneration cycle and the energy used to redistillate the dilute ethanol solution obtained in the regeneration step. 3. Costs analysis The capital costs of the columns and adsorption beds are affected seriously by reflux ratios, recycle flow rates and entrainer usages in the distillation cases. Additionally these parameters affect directly the heat duties of the process and the quality of the final ethanol product. In order to evaluate the costs associated to each technology, empirical correlations were used, and are briefly described below. Table 1. Results of cost calculations for each technology Azeotropic Extractive Adsorption Equipment Cost (U$) Equipment Cost (U$) Equipment Cost (U$) C C T C C T Decanter Cond Heater Cooler Reb Cooler Reb Cond Cond Reb Reb Cooler Total Total Total For heat exchangers, condensers and reboilers of the distillation columns, the correlations are based on the heat-exchange surface area; all heat exchangers were simulated as shell and tube type, so this area is referred to the outside surface area of the tubes. The correlations also have taken into account the corrections for the length tube, the materials of the shell and tubes, the pressure drop in the shell side and the type of equipment (kettle vaporizer, U-Tube, floating head, etc.) [10]. On the other hand, correlations by Mulet, Corripio and Evans [11] were used to estimate distillation columns and decanter costs. The vessels or towers could be horizontally (decanter) or vertically (distillation columns) arranged; they also operate at pressure higher than atmospheric pressure or at vacuum, and the correlations used differ according to these parameters. The base cost is corrected by the weight of the empty shell including nozzles, manholes and supports, and the cost of platforms and ladders. For the case of

6 P. Bastidas et al. adsorption with molecular sieves, the cost was estimated in two steps: the first, estimating the cost of the vessel as described above, and the second, estimating the cost of the molecular sieve by the volume required of this material. Table 1 summarizes the results of costs estimation. 4. Conclusions The process simulation allowed identifying extractive distillation with ethyleneglycol as the best option to dehydrate ethanol and to be implemented to the fuel ethanol production process. The current trend in process design demands energy efficiency in all unit operations like one of the prerequisites to be considered. Naturally, ethanol dehydration processes not escape to this trend and, hence, energy consumption in the production of one kilogram of anhydrous ethanol is one of the main parameters in choosing technology. Also, another important factor in selecting the best technological alternative is the utilities consumption, as well as investment costs incurred during initial deployment of technology. Then, taking into account these last two factors, extractive distillation with ethyleneglycol represents the most interesting alternative because the energy consumptions and capital investment costs are competitive and represent important savings in final cost of ethanol produced. 5. Acknowledgements This work is supported by the Departamento Administrativo de Ciencia, Tecnología e Innovación - Colciencias under grant research project code References [1] D. Barba, V. Brandani, G. Di Giacomo, 1985, Hyperazeotropic ethanol salted-out by extractive distillation. theorical evaluation and experimental check, Chem. Eng. Sci., 40, 12, [2] C. Black, 1980, Distillation modeling of ethanol recovery and dehydration processes for ethanol and gasohol, Chem. Eng. Prog, 76, [3] A. Meirelles, S. Weiss, H. Herfurth, 1992, Ethanol dehydration by extractive distillation, J. Chem. Tech. Biotechnol, 53, [4] A. Chianese, F. Zinnamosca, 1990, Ethanol dehydration by azeotropic distillation with mixed solvent entrainer, The Chem. Eng. J., 43, [5] V. Gomis, R. Pedraza, O. Francés, A. Font, J. Asensi, 2007, Dehydration of ethanol using azeotropic distillation with isooctane, Ind. Eng. Chem. Res., 46, 13, [6] N. Hanson, F. Lynn, D. Scott, 1988, Multi-effect extractive distillation for separating aqueous azeotropes, Ind. Eng. Chem. Process Des. Dev., 25, [7] S. Widagdo, W. Seider, 1996, Azeotropic Distillation, AIChE J, 42, [8] C. Black, D. Distler, 1972, Dehydration of Aqueous Ethanol Mixtures by Extractive Distillation. Extractive and Azeotropic Distillation, Advances in Chemistry Series, 115, [9] M. Doherty, M. Malone, 2001, Conceptual Design of Distillation Systems, McGraw Hill: New York. [10] W.D. Seider, J.D. Seader, D.R. Lewin, 2003, Product and Process Design Principles, Synthesis, Analysis, and Evaluation, John Wiley and Sons, Chap. 16 [11] A. Mulet, A. B. Corripio, L.B. Evans, 1981, Estimate Costs of Pressure Vessels via Correlations, p. 145.