Risk Analysis Applied to Bioethanol Dehydration Processes: Azeotropic Distillation versus Extractive Distillation

Risk Analysis Applied to Bioethanol Dehydration Processes: Azeotropic Distillation versus Extractive Distillation

Krist V. Gernaey, Jakob K. Huusom and Rafiqul Gani (Eds.), 12th International Symposium on Process Systems Engineering and 25th European Symposium on ...

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Krist V. Gernaey, Jakob K. Huusom and Rafiqul Gani (Eds.), 12th International Symposium on Process Systems Engineering and 25th European Symposium on Computer Aided Process Engineering. 31 May – 4 June 2015, Copenhagen, Denmark © 2015 Elsevier B.V. All rights reserved.

Risk Analysis Applied to Bioethanol Dehydration Processes: Azeotropic Distillation versus Extractive Distillation Adriana Avilés-Martínez,b Nancy Medina-Herrera,a Arturo Jiménez-Gutiérrez,a* Medardo Serna-Gonzálezb and Agustín Jaime Castro-Montoyab a

Instituto Tecnológico de Celaya, Departamento de Ingeniería Química, Celaya, Gto., 38010, México b Universidad Michoacana de san Nicolás de Hidalgo, Facultad de Ingeniería Química, Morelia, Mich., 58000, México

Abstract Production of anhydrous ethanol in large scale has been made by extractive distillation using solvents such as ethylene-glycol or glycerol. In this work, azeotropic distillation is studied to dehydrate bioethanol using n-octane as entrainer. We use a procedure to account for risk and safety, in addition to economic evaluations, in the process design. A probabilistic methodology is applied for the evaluation of a distance likely to cause death as a risk index. The safety assessment combines a frequency and consequence analysis to calculate process total risk. The approach is applied to the design of ethanol dehydration processes, for which azeotropic and extractive distillation systems are compared. The properties of the solvents affect the inherent process safety. The comparison between the two alternatives is done in terms of individual risk, CO2 emissions and total annual cost. The results show that the azeotropic distillation using noctane as entrainer presents lower total energy consumption and risk compared to the purification process with extractive distillation using ethylene glycol. The product of the proposed separation scheme is a dehydrated mixture of ethanol and n-octane with 81 % mol (61% vol) ethanol, which can be used to produce gasohol. Keywords: Safety; Anhydrous ethanol; Azeotropic distillation; Bio-ethanol

1. Introduction Anhydrous ethanol demand has been increasing in response to environmental policies that promote the use of gasohol. Thus, bio-refineries have gained significant interest. The bioethanol production process has three main steps: pre-treatment, fermentation, and purification. Purification steps have been investigated especially because most of the energy requirements are spent at this point of the process. Ethanol and water form an azeotrope, which makes dehydration process costly, and complex configurations are needed to achieve dehydrated ethanol for fuel use. The selection of an adequate dehydration configuration requires a multi-criteria approach. Complex distillation systems have been tested for bioethanol dehydration (Luyben, 2013). Azeotropic and extractive distillations employ a separating agent to obtain anhydrous bioethanol. Both distillation schemes require the addition of an external component to the separation process, which implies additional risk and cost. Then, this added agent plays an important role in safety and economics of the dehydration process. In this work, we analyze azeotropic distillation and we compare the results with

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extractive distillation in order to select a proper configuration and entrainer considering safety, CO2 emissions and economics.

2. Methodology and case study After the fermentation step, ethanol is highly diluted in water, among other components. The first step of the purification process is the concentration of ethanol close to the azeotrope composition using simple distillation. After pre-concentration, a dehydration process is needed, which is the focus of this work. Extractive distillation and azeotropic distillation processes are considered. Heterogeneous azeotropic distillation is used when there is an immiscibility zone in the ternary diagram that can be used to attain the desired separation. Then, a decanter is used for the liquid-liquid separation. Figure 1 shows the proposed azeotropic distillation flowsheet, where C0 is the preconcentrator column, C1 is the azeotropic distillation column, C2 is the entrainer recovery column and T1 is a decanter. ORG

D1 D0 F0

T1

C1 AQ

C0

D2

SOL B1

C2

B0 B2

Figure 1. Azeotropic distillation flowsheet.

The dehydration processes are simulated using rigorous models with the ASPEN Plus process simulator. The feed stream for the azeotropic distillation process is 800 kmol/h with a composition of 90% mol water and 10% mol ethanol, which is a typical yield from fermentation of first generation raw materials for biofuels. UNIFAC was used as the thermodynamic model for the vapor-liquid and the liquid-liquid equilibrium. The conventional column C0 was initially designed using the Winn-Underwood-Gilliland method (DSTWU model) and manipulating the reflux ratio until a composition of about 80% mol ethanol was reached. Then, the RadFrac model was used to simulate C0 and C1. The initial design for C1 is assumed to recover the ethanol in F0, and initial guesses are needed for the composition of the organic phase recycle, which is a ternary mixture; the degrees of freedom (number of stages, reflux ratio, solvent to feed ratio, feed stages) are varied one by one aiming to minimize the energy requirements and to separate all of the water in the distillate stream. After the parameters for C1 were determined, C2 was designed applying first the DSTWU model to concentrate the ethanol-n-octane mixture to a composition near the binary azeotrope, and then the three columns with recycles were simulated with RadFrac. Once the simulation is done, risk, CO2 emissions and economic evaluations are carried out. The methodology proposed by Medina-Herrera et al. (2014) is used to evaluate risk. This probabilistic methodology combines a frequency and consequence analysis to calculate risk. A distance likely to cause death is used as a risk index. The total annual

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cost (TAC) is estimated using the Aspen Cost Estimator. A depreciation factor of 0.2 for equipment cost annualization was assumed. The utilities considered are saturated steam at 4.1x105 N/m2 (4.7/106 USD / KJ) and water with a cooling cost of 33.44 KJ/Kg (0.0251$/t). The carbon dioxide emission is reported considering the CO2 emission factor data source US-EPA-Rule-E9 5711. The analysis provides an understanding of safety, environmental and economics behavior. This configuration is then compared with extractive distillation. Gasohol is a mixture of gasoline and ethanol, which is used as a substitute for pure gasoline. The focus of this work is the dehydration of ethanol for its potential use in the production of gasohol. The main compound of gasoline is octane. Therefore we use noctane as an entrainer for the azeotropic configuration. The challenge found in this ternary system (see Figure 2) is that n-octane forms azeotropes with water and ethanol. Three binary azeotropes and one ternary heterogeneous azeotrope provide three distillation regions. Therefore, if the mixture to separate is located on the upper left side of the triangle, below the azeotrope ethanol-water (D0), there are two possible regions to explore in order to get dehydrated ethanol. Operating in the top region enables the total purification of ethanol, with the disadvantage that recycle streams with high ethanol composition and relatively high flow rates are required to achieve the purification. On the other hand, operating in the right distillation region presents lower flow rates and thus low energy requirements. Even though the product is not pure ethanol, it is a dehydrated mixture of ethanol and n-octane, which can then be used for gasohol production.

Figure 2. Ternary diagram with stream compositions (mol basis), obtained with the UNIFAC model at 1 atm.

3. Results and Discussion The stream from the fermentation step enters the column C0 (see Figures 1 and 2) to concentrate the diluted mixture to a composition near the azeotrope composition (D0). The distillate stream is the feed to the azeotropic column C1, where the dehydration step

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takes place and the top product (D1), containing mostly water, is located in the immiscibility zone of the ethanol-water-n-octane system. The product is condensed and sent to a liquid-liquid separator to obtain an n-octane rich phase (ORG) that is recirculated to the azeotropic column. The aqueous phase is recycled to the column C0 because it still contains ethanol. The bottom C1 product is an n-octane-ethanol mixture that enters the column C2 to recover the entrainer and obtain the final product, an ethanol-n-octane mixture (B2). The results of the design for the columns and mass flows are presented in Tables 1 and 2. Table 1. Design parameters of columns for the azeotropic distillation scheme

Number of stages Feed stages Design Pressure (N/m2) Reflux ratio Heat duty (kw)

C0 40 F0-20 A-21 101,325 2 4,906.00

C1 28 D0-10 SOL-2 ORG-3 101,325 3 1,457.65

C2 12 B1-6 101,325 0.0002 1,418.00

Table 2. Mass flowrates for streams

Flows (kg/s) F0 D0 B0 SOL D1 B1 ORG AQ D2 B2

Ethanol 1.02376 1.17127 0.151802 1.02376 0.004286 0.147516 1.02376 -

Water 3.60306 0.107584 3.60306 0.107712 0.000128 0.107584 -

n-octane 0.00572393 0.60284 0.17818 3.85052 0.172461 0.005724 0.60284 3.24768

T (‫)ܭ‬ 303.15 351.15 373.15 303.15 343.15 350.15 343.15 343.15 350.15 399.15

For risk calculations, five catastrophic scenarios are considered. There are two types of mass releases, instantaneous and continuous. In the case of an instantaneous release, the outcomes are boiling liquid expanding vapour explosion (BLEVE), unconfined vapour cloud explosion (UVCE), and flash fire due to instantaneous release (FFI). The other two scenarios correspond to a continuous release, jet fire and flash fire due to continuous release (FFC). The calculations were carried out considering only n-octane within the process. The estimated total distance likely to cause death (DD) was 0.1446371 m/y, which represents the total individual risk of the process considering the extractive and recovery columns. The corresponding risk of the five events can be seen in Figure 3 for columns C1 and C2. Table 3 shows the distances of impact obtained for all events. Although BLEVE scenarios have the greatest distances, we can see in Figure 3 that flash fire due to a continuous release represents the worst-case scenario for both columns. This is because FFC has a higher probability of occurrence. The probability of occurrence for FFC is 2.48 x 10-4/y in contrast to the BLEVE probability of occurrence of 5.75 x 10-6/y, a difference of two orders of magnitude.

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As mentioned above, the approach is based on a multi-criteria analysis. Therefore, Table 4 shows the cost of dehydrating the product, with a flow of 800 kmol/h and a composition of 90% mol water. The total purification cost is 0.0752 US$/kg ethanol. Carbon dioxide emissions were estimated assuming crude oil as fuel. The value reported in Table 4 shows CO2 emissions of 0.007639 Kg/s per kmol of ethanol dehydrated.

Total DD = 0.1446371 m/y

DD (m/year)

0.05 0.04 0.03 0.02

C1

0.01

C2

0 BLEVE

UVCE

FFI

JET FIRE

FFC

Scenarios Figure 3. Total distance likely to cause death for the azeotropic distillation scheme. Table 3. Fatal distances for the different events

BLEVE

Di (m) C1

Di (m) C2

2136.08

2182.66

UVCE

657.10

667.99

FF INS.

812.00

826.34

JET FIRE

29.06

29.05

FF CONT

189.46

192.72

Table 4. Azeotropic distillation scheme costs for an 80 kmol/h ethanol production

Cost Analysis Result Equipment Utilities Total Annual Cost

(USD / year) 1,116,920.00 1,309,450.00 2,426,370.00

CO2 Emissions

2200 Kg/hr

In order to compare the results with the extractive distillation process to dehydrate bioethanol, we considered the works reported by Avilés-Martinez et al. (2012) and Medina-Herrera et al. (2014), in which extractive distillation was used to obtain anhydrous bioethanol. Using the same ethanol production rate as in this work, MedinaHerrera et al. (2014) minimized the total individual risk in the extractive column and in the ethylene-glycol recovery column, and reported a distance likely to cause death of 0.2052 m/y, which is higher than the result obtained here of 0.1446 m/y. In the work by Avilés-Martinez et al. (2012), glycerol was considered as entrainer. Based on the design parameters reported in their work, we simulated their extractive distillation process for the diluted water-ethanol mixture considered here. The results include a higher TAC of 3,148,340 US$/y, equivalent to 0.0996 US$/kg ethanol, and a total heat duty of 0.010931 GJ/kg-ethanol. The CO2 emissions were estimated at 3096.23 kg/h, equivalent to 0.011 Kg/s for every kmol of ethanol purified. Figure 4 summarizes the results obtained for the economic, safety, energy and environmental terms analyzed for the

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separation schemes; it can be observed that all of these factors favor the use of azeotropic distillation over extractive distillation for the case study considered here. 0.2

0.012 0.01

0.15

0.008

P1

0.1

0.006

P1

0.004

P2

P2

0.05

0.002 0

0 USD/Kg Ethanol

DD (m/y)

QT (GJ/Kg Ethanol)

CO2 (Kg/s)

Figure 4. Comparison of total cost, total distances likely to cause death, energy requirements and CO2 emissions for the azeotropic (P1) and extractive distillation (P2) processes.

4. Conclusions Two alternatives for the ethanol dehydration process have been considered. The results for the azeotropic distillation option suggest that n-octane is a promising entrainer. Although the product is not highly purified ethanol, the product blend of ethanol and noctane is dehydrated (with composition near the binary azeotrope of 84% mol of ethanol), which can then be mixed directly with gasoline to produce gasohol. The comparison between extractive and heterogeneous azeotropic distillation has shown that the latter is a promising alternative to the conventional extractive distillation scheme in terms of both safety and economics. Both processes, however, are suitable for dehydration of ethanol because the energy requirements are lower than the ethanol heat of combustion.

References A. Avilés-Martínez, J. Saucedo-Luna, J.G. Segovia-Hernández, S. Hernández, F.I. Gómez-Castro, A.J. Castro-Montoya, 2012, Dehydration of bioethanol by hybrid process liquid-liquid extraction/extractive distillation, Ind. Eng. Chem. Res., 51, 58475855. W.L. Luyben, 2013, Comparison of extractive distillation and pressure-swing distillation for acetone/chloroform separation. Comput. Chem. Eng., 50, 1-7. N. Medina-Herrera, I.E. Grossmann, M.S. Mannan, A. Jiménez-Gutiérrez, 2014, An approach for solvent selection in extractive distillation systems including safety considerations, Ind. Eng. Chem. Res., 53, 12023-12031.