Anhydrous bioethanol production using bioglycerol – simulation of extractive distillation processes

Anhydrous bioethanol production using bioglycerol – simulation of extractive distillation processes

19th European Symposium on Computer Aided Process Engineering – ESCAPE19 J. Jeżowski and J. Thullie (Editors) © 2009 Elsevier B.V. All rights reserved...

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19th European Symposium on Computer Aided Process Engineering – ESCAPE19 J. Jeżowski and J. Thullie (Editors) © 2009 Elsevier B.V. All rights reserved.

519

Anhydrous bioethanol production using bioglycerol – simulation of extractive distillation processes Marina O.S. Dias,a Tassia L. Junqueira, a Rubens Maciel Filho, a Maria R.W. Maciel, a Carlos Eduardo Vaz Rossell b a

School of Chemical Engineering, State University of Campinas, UNICAMP, P.O. Box 6066, 13083-970, Campinas – SP, Brazil, [email protected] b Interdisciplinary Center for Energy Planning, State University of Campinas, UNICAMP, P.O. Box 6192, 13400-970, Campinas – SP, Brazil

Abstract Bioethanol has been increasingly used as fuel in the anhydrous form, mixed with gasoline. In this work, two configurations of the extractive distillation process using bioglycerol as a solvent for anhydrous bioethanol production were investigated. Simulations results show that bioglycerol is a suitable agent for the separation of ethanol-water mixtures, with low energy consumption on the column reboilers and the production of high quality anhydrous bioethanol. Keywords: bioethanol, extractive distillation, bioglycerol, simulation

1. Introduction Increase in oil prices and global concern about climate change have motivated the use of alternative forms of energy all over the world. In the transportation sector, more specifically, bioethanol and biodiesel have been increasingly used as substitutes of gasoline and diesel, respectively. Bioethanol is mainly produced from fermentation of sugars. Wine obtained after fermentation contains about 7-12 wt% ethanol. In industry, wine undergoes conventional distillation, where hydrous ethanol containing around 93 wt% ethanol is produced. In order to be used as a gasoline additive, wine must be concentrated at least to 99.3 wt% ethanol. Since water and ethanol form an azeotrope with 95.6 wt% ethanol at 1 atm, conventional distillation does not achieve the separation that meets product specification. Common separation methods used in the industry are azeotropic distillation with cyclohexane as entrainer, extractive distillation with monoethyleneglycol as solvent and adsorption onto molecular sieves. In the last years bioglycerol availability has increased, since it is obtained as a byproduct of biodiesel production process, making glycerol prices fall. Since it is not harmful to humans or the environment, and it is a suitable agent for the separation of ethanol – water mixtures [1], it can be safely used to produce anhydrous ethanol for use in food or pharmaceutical industries. In this work UniSim Design was used to simulate and investigate the performance of extractive distillation processes with glycerol for anhydrous bioethanol production. Two different configurations were studied, aiming the lowest energy consumption in the production of 1000 m³/day of anhydrous bioethanol.

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2. Extractive distillation processes In extractive distillation, also known as homogeneous azeotropic distillation, a separating agent, called solvent or entrainer, is added to the azeotropic mixture in order to alter the relative volatility of the components in the original mixture. Among possible criteria for solvent selection, the solvent used must not form a second liquid phase with the components of the mixture and should have a high boiling point. In the conventional extractive distillation process, solvent is added to the first column (extractive column), above the azeotropic feed. On the top of the extractive column, anhydrous ethanol is produced, while in the bottom a mixture containing solvent and water is obtained. The solvent is recovered in a second column (recovery column), cooled and recycled to the extractive column [2]. An alternative configuration for this process makes use of a single column to perform both removal of water and solvent recovery, by means of a side draw of water in vapor phase located a few stages above the bottom of the extractive column [3]. Thus, there is no need to use a second column to recover solvent, since anhydrous ethanol is obtained on the top of the extractive column, pure water vapor obtained as a side draw and pure solvent is produced on the bottom. Solvent is cooled down and recycled back to the column. Conventional extractive distillation processes employed in the industry for the separation of ethanol – water mixtures use monoethyleneglycol (MEG), which is a fossil and toxic solvent. Glycerol can be used as solvent in extractive distillation processes to produce anhydrous ethanol, since it eliminates the azeotrope by modifying the mixture vapor-liquid equilibrium, increasing the volatility difference between the compounds. Glycerol decomposes into acrolein when heated above 280°C [4], which is below its boiling point at atmospheric pressure. In order to avoid decomposition, extractive distillation process must be performed at sub-atmospheric pressures when needed. This is an important process condition that has to be considered.

3. Simulation of the extractive distillation processes Simulations were carried out using software UniSim Design. Hydrous ethanol in vapor phase containing 93 wt% ethanol was used as raw material. In order to produce 1000 m³/day of anhydrous ethanol (99.3 wt% ethanol), approximately 1045 m³/day of hydrous ethanol (847 kmol/h) must be fed to the extractive distillation system. UNIQUAC was the model used to calculate the activity coefficient on liquid phase and equation of state SRK was used as the vapor model. Binary coefficients for ethanolglycerol pairs are not available at UniSim database and were estimated using UNIFACVLE. Extensive studies, not shown in this paper, were carried out to characterize the possible mixtures found in the process and to find out which is the most suitable thermodynamic package to represent the interactions between components. 3.1. Conventional extractive distillation process Configuration of the conventional extractive distillation process for anhydrous bioethanol production using glycerol as a solvent is depicted in Figure 1. Firstly, a base case was considered. Optimization of the conventional process was carried out using factorial planning (26-4); the following parameters were evaluated: number of stages (N) and reflux ratio (RR) of both extractive column (EC) and recovery column (RC), solvent to feed ratio (S/F) and solvent inlet temperature (T). Sixteen simulations were carried out. Anhydrous ethanol purity and energy consumption on column reboilers were analyzed. Parameters considered on the factorial planning are shown in Table 1.

Anhydrous Bioethanol Production Using Bioglycerol – Simulation of Extractive Distillation Processes

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Figure 1. Configuration of conventional extractive distillation process. Table 1. Values for the variables considered on the factorial planning - conventional process. Level

+1 -1

N-EC

38 32

N-RC

14 11

RR-EC

1.00 0.94

RR-RC

0.012 0.008

S/F

0.32 0.29

T (ºC)

150 110

Software STATISTICA 7.0 was used to generate and analyze the results. It was verified that the number of stages (N-EC and N-RC) and recovery column reflux ratio (RR-RC) are not significant in a 95 % confidence interval. Variation of energy consumption (Q) as a function of the most significant parameters (RR-EC, S/F and T) can be seen in Figure 2.

(a)

(b)

Figure 2. Energy consumption on column reboilers (Q, kJ/kg AE) on the conventional process, as a function of RR-EC and S/F (a) and T (b).

Further optimization was carried out based on the case that presented the lowest consumption of energy and met product specification, which had the following specifications: S/F=0.32, RR-EC=0.94 and T=150 ºC. Firstly, the optimization considered the reduction on energy consumption based on feed stages (solvent, hydrous ethanol and solution). After the lowest energy consumption situation was achieved,

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glycerol flow was reduced to the minimum possible value that met product specification, thus reducing even more process energy consumption. Anhydrous ethanol produced does not contain traces of solvent, so it can be safely used in different industries and as raw material. Simulation parameters and results for the optimized simulation are shown on Table 2. Table 2. Parameters of the optimized configuration - conventional extractive distillation.

(a) (b)

Process Parameter

Value

Process Parameter

Value

Solvent to feed ratio (S/F) Glycerol inlet temperature (ºC)

0.316 150

99.98 9.15x10-6

Solvent losses (%)

0.0099

Extractive column Number of stages Solvent inlet stage(b) Hydrous ethanol inlet stage(b) Reflux ratio(a) AE flow rate (kmol/h)(a) Reboiler duty (kW)

38 36 13 0.94 723.0 6073

Water stream purity (% mole) Ethanol losses (%) Energy consumption on reboilers (kJ/kg AE) Recovery column Number of stages Feed inlet stage(b) Glycerol recovery (%)(a) Reflux ratio(a) Reboiler duty (kW) Pressure (kPa)

1057 11 4 99.99 0.012 3599 50

Column specifications Stage numbering increases towards the top of the column

3.2. Alternative configuration of the extractive distillation process Alternative configuration of extractive distillation is presented in Figure 3. A base case was simulated and optimized through factorial planning (24). Parameters studied for the factorial design in this case included number of stages (N), reflux ratio (RR), solvent to feed ratio (S/F) and solvent temperature (T). Sixteen simulations were carried out; energy consumption and mass fraction of ethanol on the anhydrous ethanol were analyzed. All variables were considered significant on a 95 % confidence interval. Parameters are given in Table 3.

Figure 3. Configuration of the alternative extractive distillation process.

Anhydrous Bioethanol Production Using Bioglycerol – Simulation of Extractive Distillation Processes

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Table 3. Values for the variables considered on the factorial planning – alternative process. Level

N

RR

S/F

T (ºC)

+1 -1

35 30

1.0 0.9

0.420 0.365

140 120

Optimized parameters values in the simulation that presented the lowest energy consumption were: N=30, RR=0.9, S/F=0.365 and T=140 ºC. Further optimization was carried out considering ethanol feed and solvent inlet stages, as well as side stream draw stage. It was verified that changes on these parameters did not decrease significantly the energy consumption of the process. It was possible to achieve lower energy consumption on reboilers by varying glycerol flow, but this variable must be studied with caution, since lower solvent flows increase ethanol losses on the side stream. Parameters for the optimized simulation are shown on Table 4. Table 4. Parameters of the optimized configuration - alternative extractive distillation. Process Parameter

Value

Process Parameter

Value

Solvent to feed ratio (S/F) Glycerol inlet temperature (ºC)

0.355 140

99.95 6.84x10-5

Solvent losses (%)

0.021

Water stream purity (% mole) (a) Ethanol losses (%) Energy consumption on reboilers (kJ/kg AE) Side stream stage(b) Hydrous ethanol inlet stage(b) Reboiler duty (kW) Pressure (kPa)

Number of stages 30 Solvent inlet stage(b) 29 Reflux ratio(a) 0.90 AE flow rate (kmol/h)(a) 723.0 (a) Column specifications (b) Stage numbering increases towards the top of the column

1085 2 13 9932 60

4. Discussions Energy consumption on the alternative configuration of the extractive distillation process is only 2.7 % larger than that of the conventional two-column configuration. The main disadvantage of the alternative configuration is the higher temperature of the extractive column: around 266 ºC, as opposed to the 153 ºC in the extractive column of the conventional configuration, on which the highest temperature (266 ºC) is only achieved on the recovery column. Thus, more steam (of high temperature and pressure) or another heat source must be used on the alternative column reboilers in order to maintain column at that high temperature. Ethanol losses are smaller than 0.0001 % on both studied cases; even though the alternative configuration of the extractive distillation process promotes higher glycerol losses, this should not be a problem, since glycerol is a cheap, renewable and biodegradable solvent, and on both cases losses are smaller than 0.025 %. Values obtained for the energy consumption of different processes for the separation of water – ethanol mixtures are displayed in Table 5. It can be verified that the processes studied in this work present relatively low energy consumption on column reboilers. Thus, the extractive distillation process with glycerol seems to be a competitive process and deserves further investigation.

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Table 5. Energy consumption of ethanol – water separation processes. Process

Conventional extractive distillation with glycerol Alternative extractive distillation with glycerol Extractive distillation with MEG and CaCl2 [5] Extractive distillation with MEG [6] Azeotropic distillation with pentane [7] Azeotropic distillation with benzene [7] Extractive distillation with gasoline [7]

Energy consumption (kJ/kg ethanol) 1057 1085 1425 1760 3348 4683 2695

5. Conclusions Both alternative and conventional configurations of the extractive distillation process with glycerol are suitable for anhydrous ethanol production. Glycerol, the solvent used, is derived from renewable materials and it is not harmful to humans or the environment, as opposed to the conventional solvent employed on extractive distillation process for anhydrous bioethanol production (MEG). Energy consumption on both cases are similar and relatively low, when compared to that of different processes reported in the literature, as well as ethanol and solvent losses. Anhydrous ethanol produced is not contaminated by solvent, which is of great importance when considering process sustainability and the possible use of bioethanol as raw material.

6. Acknowledgements The authors acknowledge CNPq and FAPESP for financial support.

7. References [1] F. Lee and R. Pahl, Solvent screening study and conceptual extractive distillation process to produce anhydrous ethanol from fermentation broth, Ind. Eng. Chem. Process Des. Dev., 24 (1985) 168 - 172 [2] H. Huang et al., A review of separation technologies in current and future biorefineries. Separation and Purification Technology, 62 (2008) 1 - 21 [3] R. P. Brito, M.R.W. Maciel, A.J.A. Meirelles, New extractive distillation configuration for separating binary azeotropic mixtures. In: The First European Congress of Chemical Engineering, Italy, 1 (1997) 1333 - 1336 [4] J. A. Young. CLIP: Glycerol, Journal of Chemical Education, 80 (2003) 25 [5] I. Gil et al., Separation of ethanol and water by extractive distillation with salt and solvent as entrainer: process simulation. Brazilian Journal of Chemical Engineering, 25 (2008) 207 - 215 [6] A. Meirelles, S. Weiss, H. Herfurth, Ethanol dehydration by extractive distillation. Journal of Chemical Technology and Biotechnology, 53 (1992) 181 – 188 [7] C. Black, Distillation modeling of ethanol recovery and dehydration processes for ethanol and gasohol. Chem. Eng. Prog., 76 (1980) 78-85