distillation options

Communication

Analysis of Energy-Efficient ComplexDistillation Options to Purify Bioethanol

Three complex extractive distillation options were studied for the purification ofa dilute mixture of ethanol and water. The first option uses an extractive distilla-tion column and the other two options use thermally coupled extractive distilla-tion sequences. The results indicate that the fully thermally coupled extractive op-tion can reduce energy consumption by ca. 30 % compared to the scheme thatuses an extractive distillation column. This fully thermally coupled extractive dis-tillation sequence can produce ethanol as distillate with a mass fraction of 0.995,the entrainer as the bottoms product and a mixture of ethanol and water as thesidestream.

Keywords: Bioethanol, Energy, Ethanol, Extractive distillation, Thermally coupled distillationsequence

Received: December 4, 2007; revised: January 7, 2008; accepted: January 8, 2008

DOI: 10.1002/ceat.200700467

1 Introduction

Due to significant increases in the price of oil and environ-mental constraints, researchers in the area of process systemsengineering are interested in developing process systems capa-ble of efficient energy use and alternatives in the form of bio-fuels, including ethanol and biodiesel. In the case of bioetha-nol, it has been reported that its use as a gasoline oxygenateincreases oxygen content, enabling improved oxidation of hy-drocarbons, and consequently, a reduction in both hydrocar-bon and carbon dioxide emissions [1].

Bioethanol can be produced by fermentation of sugarcane,corn, sweet sorghum, etc. An important issue in the process ofbioethanol production is the purification of the ethanol from adilute solution, i.e., ca. 10 % ethanol in water. The key factorin the purification process is the formation of the binaryhomogeneous azeotrope of ethanol-water, and an additionalprocess is required to obtain high purity ethanol that can beused in motor vehicles. Two methods can be used: the first isdehydration using a salt, e.g., NaCl, KI, CaCl2, while the sec-ond method involves the use of ethylene glycol as an entrainer[2]. The main objective in these developments is to find a solu-tion that can be useful in terms of both total annual cost andoperational control properties. One alternative currently em-ployed in the chemical industry is the method using three ther-mally coupled distillation sequences (TCDS), as depicted in

Fig. 1. It is important to highlight that in 1949, Wright [3] pa-tented the first thermally coupled distillation sequence using adividing wall. However, no practical implementations were re-ported. This could be due to low oil prices (under US$ 5 perbarrel). In 1965, Petlyuk et al. [4] published a complete ther-modynamic study of the Petlyuk column and showed that, infact, thermally coupled distillation sequences could have lowerenergy requirements compared to conventional direct and in-direct distillation sequences, see Fig. 2. However, once again,no practical implementations were reported. Tedder and Rudd[5] presented a complete comparison of the total annual costsof eight distillation sequences, including conventional, side-stream and thermally coupled configurations, and they foundthat complex distillation sequences may offer significant ener-gy savings over conventional distillation sequences for the sep-aration of some ternary mixtures. Since their work [5], thesecomplex distillation sequences have been studied extensively interms of steady and dynamic behavior, and it has been foundthat the complex distillation sequences can achieve energy sav-ings of up to 40 % over conventional distillation sequences[6–12]. Furthermore, studies on dynamic properties haveshown that the energy savings predicted can be achieved in in-dustrial practice without introducing additional control prob-lems [13–16]. With regard to controllability analysis, undersome operational conditions, thermally coupled distillationschemes can exhibit better control properties than conven-tional distillation sequences.

Important developments in the field of thermally coupleddistillation have led to industrial implementation of the divid-ing wall distillation column [17, 18]. This practical implemen-tation has led to savings in both energy and capital costs [18].Moreover, no important control problems have been reported

© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim http://www.cet-journal.com

Salvador Hernández1

1 Universidad de Guanajuato,Facultad de Química,Guanajuato, México.

–Correspondence: Prof. S. Hernández ([email protected]), Uni-versidad de Guanajuato, Facultad de Química, Noria Alta s/n,Guanajuato, Gto., 36050 México.

Chem. Eng. Technol. 2008, 31, No. 4, 597–603 597

in the operation of the dividing walldistillation columns. At present, thenumber of thermally coupled distilla-tion columns is increasing due to high-er oil prices (around US$ 100 per bar-rel). In the early part of the 21stcentury, the reduction of energy con-sumption has become a top priority,and thermally coupled distillation se-quences show great promise.

In the case of ternary mixtures, it ispossible to use two conventional distil-lation sequences together, i.e., directand indirect distillation sequences, seeFig. 2. Several nonconventional distil-lation sequences can be used, includingthe three TCDS sequences depicted inFig. 1, the thermally coupled distilla-tion sequence using a side rectifier(TCDS-SR), see Fig. 1a), the thermallycoupled distillation sequence with aside stripper (TCDS-SS), see Fig. 1b),and the fully thermally coupled distil-lation sequence or Petlyuk column(FTCDC), see Fig. 1c). It has been re-ported that these complex distillationsequences can have lower total annualcosts than conventional distillationsequences for the separation of ternarymixtures (A + B + C), where the molefraction of the intermediate compo-nent B in the feed is lower than 10 %(dictates use of the thermally coupleddistillation sequence with side rectifieror side stripper) or above 60 %(dictates use of the Petlyuk column),


Figure 1. Thermally coupled distillation sequences for the separation of ternary mixtures: (a)TCDS-SR, (b) TCDS-SS and (c) FTCDS.

Figure 2. Conventional direct and indirect distillation sequences.

598 S. Hernández Chem. Eng. Technol. 2008, 31, No. 4, 597–603

and the purity of product B is lower than that of products Aand C.

When the product of the fermentation process is subjectedto conventional distillation, a binary homogeneous azeotropeis formed with 96 mass.-% of ethanol in water. When ethyleneglycol is added as an entrainer, a ternary mixture is formed.Taking the foregoing mixture into account, differences in theenergy consumption of the three options for the separation ofan ethanol-water mixture are obtained and compared. Thefirst option includes an extractive distillation column and theother two include thermally coupled extractive distillation col-umns.

2 Description of the Purification Options

According to Fig. 3, a dilute feed of ethanol in water is intro-duced to a conventional distillation column that removes thebinary homogeneous azeotrope as the distillate. The bottomsproduct of the first distillation column is almost pure water.

This conventional distillation column is required in the threedistillation options.

The first option uses an extractive distillation column withethylene glycol as the entrainer. The distillate of the column isethanol with a mass fraction of 0.995. The bottoms product ofthe extractive distillation column is a ternary mixture of etha-nol, water and ethylene glycol. This mixture is fed to a thirddistillation column in order to recover the entrainer as the bot-toms product, where the distillate is a mixture of ethanol andwater that can be returned to the first distillation columnwhere the azeotrope is formed.

The second distillation option uses an extractive TCDS-SR.As indicated in Fig. 3, this complex distillation option hasthree products. The distillate of the main column is ethanolwith the required purity and the bottoms product of this col-umn is the entrainer. The side rectifier column removes a mix-ture of ethanol and water that can be recycled to the first distil-lation column.

The last option includes an extractive FTCDS. This optionrecovers the ethanol in the distillate and the bottoms product


Figure 3. Complex distillation sequences implemented in Aspen PlusTM.

Chem. Eng. Technol. 2008, 31, No. 4, 597–603 Bioethanol 599

is the entrainer. The sidestream removes a mixture of ethanoland water that can be returned to the first conventional distil-lation column. This option is particularly important because itcan be implemented in industrial practice using a single distil-lation column divided by a wall [19].

3 Design and Optimization of theExtractive Systems

The design and optimization methods for thermally coupleddistillation described in Hernández and Jiménez [7, 8] for tern-ary mixtures can be extended to the study of extractive distilla-tion. These methodologies require an initial tray structure thatcan be obtained from the conventional distillation sequence,and subsequently, the recycle stream (VF) is varied until mini-mum energy consumption is achieved in the reboiler of the ex-tractive TCDS-SR. In the case of fully thermally coupled ex-tractive distillation, the two recycle streams (VF and LF) arevaried to detect the minimum energy required in the reboiler.

The optimization procedure requires three design specifica-tions for the composition of the three products that are in-cluded as constraints in the design and optimization proce-dures implemented in Aspen PlusTM. The thermodynamicproperties for the liquid and vapor phases were calculatedthrough the use of the NRTL model and the Redlich-Kwongequation, respectively. The proper modeling of the thermody-namic properties is very important since in the first stage ofthe separation procedure, a binary distillation column is re-quired to obtain the binary ethanol-water azeotrope. In this as-pect, the NRTL model can predict the formation of the binaryazeotrope. The addition of ethylene glycol as the entrainer inthe second stage of the separation sequence also causes interac-tion with the other components.

The optimization procedure requires a rigorous model foreach equilibrium stage in the distillation columns that can beobtained from a generic equilibrium stage [20]. Eqs. (1–5) de-scribe the equilibrium stage model1).– Total mass balance on stage j:

Lj–1 + Vj+1 + FjL + Fj

V – (Lj + Uj) – (Vj + Wj) = 0 (1)

– Component mass balances on stage j:

Lj–1Xi,j–1 + Vj+1Yi,j+1+FjLZL

i,j + FjVZV

i,j

– (Lj + Uj)Xi,j – (Vj + Wj)Yi,j = 0 (2)

– Equilibrium relationship on stage j:

Yi,j = Ki,jYi,j (3)

– Summation constraint on stage j:

�C

i�1

Ki�jXi�j � 1�0 � 0 (4)

– Energy balance on stage j:

Lj–1h̄j–1 + Vj+1H̄j+1+FjLh̄j

L + FjVH̄j

V

– (Lj + Uj)h̄j – (Vj + Wj)H̄j + Qj = 0 (5)

The flowsheet implemented in Aspen PlusTM is also depictedin Fig. 3.

4 Results and Discussion

The results presented here correspond to the separation of adilute feed of ethanol in water (10 mol.-% ethanol in water) at100 kg mol/h as a saturated liquid at 1 atm. The study focuseson the stage of separation of ethanol with a high mass fraction(0.995). The extractive columns have 20 stages and an entrai-ner flow of 20 kg mol/h is introduced in stage 3 (numberedfrom top to bottom). As indicated previously, the first optionuses an extractive distillation column with ethylene glycol asthe entrainer and a conventional distillation column to recoverthe entrainer. This option has an energy consumption of115.07 kW, see Tab. 1, and is considered as a basis for the anal-ysis.

In the case of the extractive TCDS-SR option, Fig. 4 presentsthe determination of the optimal interconnecting vapor flowvalue. As can be seen from Fig. 4, a minimum energy con-sumption of 124.13 kW is achieved at an interconnecting va-por flow value of 3.5 kg mol/h. This energy consumption is


Table 1. Minimum energy consumption of the three complex dis-tillation sequences.

Distillation Sequence Total EnergyConsumption(kW)

Percent of EnergySavings with Respectto the ExtractiveDistillation Column

Extractive Distillation Column 115.07 0

Extractive TCDS-SR 124.13 –7.87

Extractive FTCDS 87.19 24.22

Figure 4. Determination of the minimum energy consumptionof the extractive TCDS-SR.

–1) List of symbols at the end of the paper.


7.87 % above that which is required in the extractive distilla-tion column.

For the extractive FTCDS, the determination of minimumenergy consumption was conducted by varying the intercon-necting liquid flow and interconnecting vapor flow. Fig. 5 pre-sents three interconnecting vapor flow values. In this case, de-termination was made by varying the interconnecting liquid

flow for an assumed interconnecting vapor flow value. A mini-mum energy consumption of 87.19 kW for this complex distil-lation scheme was achieved for interconnecting vapor and liq-uid flow values of 2 and 8.5 kg mol/h, respectively. This systempresents energy savings of 24.22 % over the scheme that in-cludes an extractive distillation column. An important aspectto highlight with regard to the extractive FTCDS is that multi-ple solutions can be present for low interconnecting vapor flowvalues. For instance, Figs. 5a) and 5b) show two solutions forthe same interconnecting liquid flow value. One solution pre-sents higher energy consumption. If one analyzes Fig. 5a), itcan be seen that for an interconnecting liquid flow value of5 kg mol/h, there are two solutions with energy consumptionsof 88.6 and 184.3 kW, respectively (points A and B). Moreover,it was found in Fig. 5c, that when the interconnecting vaporflow value is increased, only one solution is present for eachinterconnecting liquid flow value. The presence of multiple so-


Figure 5. Determination of the minimum energy consumptionof the extractive FTCD.

Figure 6. Mass composition profiles in the liquid phase in theextractive distillation column.

Figure 7. Mass composition profiles in the liquid phase in theextractive TCDS-SR.


lutions in thermally coupled distillation sequences for the sep-aration of ternary mixtures was reported by Chávez et al. [21],in the case of separations using heat as the separating agent.Fig. 5 shows that multiple solutions can also be found for ther-mally coupled extractive distillation sequences.

In addition, it is important to analyze the composition pro-files in the extractive distillation systems. Figs. 6–8 present themass composition profiles of the extractive process. Fig. 6shows that the extractive column can produce ethanol with ahigh mass fraction (0.995) and the bottoms product is com-posed of ethylene glycol, water and ethanol. Figs. 7 and 8 showthat the thermally coupled extractive columns produce ethanolwith the same mass fraction, but it is important to highlightthat these complex columns separate the entrainer as the bot-toms product and the side rectifier or side stream removes amixture of ethanol and water.

According to the results, the extractive FTCDS is the mostenergy-efficient scheme to separate the binary homogeneousazeotrope of ethanol and water. In addition, this complex dis-tillation sequence can reduce capital costs since it can be im-plemented in a single distillation column using a dividing wall.

5 Conclusions

The separation of a typical mixture of ethanol and water froma fermentation process was studied using an extractive distilla-tion column and two thermally coupled extractive distillationsequences. The results show that the fully thermally coupledextractive distillation sequence can produce energy savings ofca. 30 % in comparison to an extractive distillation column.Furthermore, it was found that multiple solutions can exist atsome values for the interconnecting streams of the extractiveFTCDS. Finally, savings in capital costs can be expected sincethe extractive FTCDS can be implemented in a dividing walldistillation column.

Acknowledgements

The financial support provided by Universidad de Guanajuato,CONACyT and CONCyTEG (Mexico) is gratefully acknowl-edged.

Symbols used

F [–] feed streamH�

[kJ/mol] enthalpy of vaporh�

[kJ/mol] enthalpy of liquidK [–] vapor-liquid equilibrium

constantL [kg mol/h] liquid flow rateQ [kW] heat transferredU [kg mol/h] liquid sidestreamV [kg mol/h] vapor flow rateW [–] vapor sidestreamX [–] liquid mol fractionY [–] vapor mole fractionZ [–] composition of the feed stream

Superscripts

L liquidV vapor

Subscripts

i componentj stageL liquidV vapor

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