DOI: 10.3303/CET2292105 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Paper Received: 27 November 2021; Revised: 29 March 2022; Accepted: 9 May 2022 
Please cite this article as: De Guido G., Moioli S., Gilardi M., Giudici F., Pellegrini L.A., 2022, Downstream Processing of Butanol Produced by 
Fermentation: Thermodynamic Measurements and Modelling, Chemical Engineering Transactions, 92, 625-630  DOI:10.3303/CET2292105 
  

 CHEMICAL ENGINEERING TRANSACTIONS  
 

VOL. 92, 2022 

A publication of 

 

The Italian Association 

of Chemical Engineering 

Online at www.cetjournal.it 

Guest Editors: Rubens Maciel Filho, Eliseo Ranzi, Leonardo Tognotti 

Copyright © 2022, AIDIC Servizi S.r.l. 

ISBN 978-88-95608-90-7; ISSN 2283-9216 

Downstream Processing of Butanol Produced by 

Fermentation: Thermodynamic Measurements and Modelling 

Giorgia De Guido, Stefania Moioli*, Matteo Gilardi, Fabio Giudici, Laura A. Pellegrini 

GASP – Group on Advanced Separation Processes & GAS Processing, Dipartimento di Chimica, Materiali e Ingegneria 

Chimica “G. Natta”, Politecnico di Milano, Piazza Leonardo da Vinci 32, I-20133 Milan, Italy 

stefania.moioli@polimi.it 

Several factors, including environmental concerns and the concept of sustainability, are leading to an increased 

interest in processes able to produce fuels and chemicals from alternative, renewable resources. In this context, 

the interest in the Acetone-Butanol-Ethanol (ABE) fermentation of lignocellulosic biomasses has been intensified 

in the last years as a route alternative to the petrochemical one for producing butanol both for the chemical 

industry and as a biofuel. This work focuses on the downstream separation process of the ABE fermentation, 

which represents a key challenge due to its high energy demand. The attention is devoted to the thermodynamic 

framework in order to properly describe the phase equilibrium conditions involved in the process. To this 

purpose, new experimental data, collected at the Process Thermodynamics laboratory (PT lab) of Politecnico 

di Milano, are presented as for Vapour-Liquid Equilibria (VLE) for the binary system ethanol+water and for 

Liquid-Liquid Equilibria (LLE) for the pair 1-butanol+water. The collected data show good agreement with the 

experimental data available in the literature. Then, the performances of the Non-Random Two-Liquid (NRTL) 

thermodynamic model, coupled with the Redlich-Kwong (RK) Equation of State for the calculation, respectively, 

of the activity coefficient in the liquid phase and of the fugacity coefficient in the vapor phase, are discussed to 

assess its reliability in the thermodynamic characterization of the systems of interest. The model can predict the 

vapour-liquid equilibrium temperature with a percent Average Absolute Deviation (AAD%) in the range   0.17 – 

0.26 %, the mole fraction of ethanol and water in the vapor phase with an AAD%, respectively, of    23.3 – 31.6 

% and 4.3 – 12.1 %. Concerning the LLE measurements for the system 1-butanol+water, the model predicts 

the mole fraction of water with an AAD% of 2.4 % and 0.17 %, respectively, in the organic and in the aqueous 

phases. 

1. Introduction 

The search for green chemicals and fuels is getting increasing interest towards a more sustainable future. In 

this scenario, the Acetone-Butanol-Ethanol (ABE) fermentation is a promising green route for the biological 

production of butanol, which finds application both as a biofuel and as a reactant. 

The ABE fermentation is a process that uses bacterial fermentation to produce acetone, 1-butanol and ethanol 

from starch-rich, sugar-rich or lignocellulosic materials (García et al., 2011). This process has been known since 

the beginning of the 20th century (Schiel-Bengelsdorf et al., 2013) and its industrial exploitation started in 1916 

during the First World War (Weizmann, 1919). However, the market penetration of biobutanol is still today limited 

due to its higher production costs in comparison with those for petrobutanol (Veza et al., 2021). In particular, 

the downstream separation of the three obtained products represents a major challenge to be addressed in the 

next years due to its high-energy demand. In this scenario, the collection of new thermodynamic equilibrium 

data under different operating conditions for the binary mixtures of interest for the ABE downstream separation 

process is essential to properly assess phase equilibrium conditions and to check the predictability of 

thermodynamic models in the characterization of the systems under investigation. 

 

 

625



This work presents new experimental data collected in the Process Thermodynamics laboratory (PT lab) of 

Politecnico di Milano for the systems ethanol+water (VLE data at 103.5, 150.0 and 200.0 kPa) and 1-

butanol+water (LLE data at atmospheric pressure in the temperature range 313.15 – 353.15 K), both of interest 

in the context of the ABE fermentation process (De Guido et al., 2019). 

2. Experimental Setup 

In this section, the apparatus and the procedure used for performing the VLE and LLE measurements are 

described. The experiments were carried out in the PT lab, which is located at Dipartimento di Chimica, Materiali 

e Ingegneria Chimica “Giulio Natta” and has been created within the “Ingegneria Chimica − Energia (ICE)” 

collaboration with the aim of collecting experimental data of phase equilibria (Vapour-Liquid-Equilibrium (VLE), 

Vapour-Liquid-Liquid-Equilibrium (VLLE) and Liquid-Liquid-Equilibrium (LLE)) of mixtures of interest for which 

there is a lack of data in the literature or in other existing databases. 

The PT lab comprises a unit (Fischer Labodest VLLE 602, Figure 1a) for these types of measurements that is 

a recirculating still in Borosilicate Glass 3.3, operating from vacuum to 400 kPa and from ambient temperature 

to 453.15 K. For the operation under vacuum, it is connected to a vacuum rotary pump (Edwards E2M5), 

provided with a trap upstream. For the operation with two liquid phases at atmospheric pressure, it is equipped 

with an ultrasound generator that enhances the mixing of the condensed phases to achieve the equilibrium with 

the vapor phase. Further details on the unit are reported in Moioli et al. (Moioli et al., 2021). The PT lab is also 

equipped with a gas chromatograph (GC) Agilent 7820A with a TCD detector, and has been recently improved 

with a thermostatic bath (Huber CC304) for the study of LLE at different temperatures. 

Ethanol (denatured with about 1% methyl ethyl ketone, with molecular weight of 46.07 g/mol and density of 0.79 

kg/l), 1-Butanol (with molecular weight of 74.12 g/mol and density of 0.81 kg/l) and demineralized water (ISO 

3696 Q3, ASTM D 1193 TYPE 4) were used for the measurements presented in this work. Table 1 summarizes 

information about the supplier and the purity of these chemicals. Nitrogen (> 99.99 mol.%) from Sapio, coming 

from a tank storing it in the liquid phase, is used for pressurizing the VLLE unit up to the desired pressure and 

as a carrier for the GC. The latter one was calibrated to determine the response factors for each component. 

Table 1: Purities and suppliers of the chemicals used in this work 

Chemical Supplier Purity 

Ethanol (CAS 64-17-5) Merck KGaA 99 % 

1-Butanol (CAS 71-36-3) Sigma-Aldrich 99.9 % 

Demineralized water Idrochimica ISO 3696 Q3, ASTM D 1193 TYPE 4 

2.1 VLE Measurements 

VLE measurements for the system ethanol+water were performed using the Fischer Labodest VLLE 602 unit 

(Figure 1a) and following the same experimental procedure described in a previous work (Moioli et al., 2021), 

to which the reader can refer for more details. In this work, a more detailed description of the LLE unit is given 

in the following section. For the system ethanol+water, measurements were performed at 103.5, 150.0 and 

200.0 kPa to cover the range of conditions in which data are missing in the literature (indeed, data at sub-

atmospheric pressures and at pressures above 200.0 kPa are already available). It was decided to start the 

measurements at 103.5 kPa (i.e., at a pressure close to the ambient one) to validate the apparatus and the 

experimental procedure for this system by comparison with literature data. The experiments started with pure 

ethanol and were continued by gradually adding water to the mixture to be charged into the unit so to cover the 

full range of composition from one pure component to the other one. As the pressure and the water content 

increased (> 80 mol% on a molar basis), it was necessary to set a higher duty value (40-50 %) for heating up 

the mixture while keeping the temperature stable. For this system, to deal with the more rapid evaporation of 

the feed inside the heater, the loading of the same feed mixture was more frequent to avoid leaving the 

resistance uncovered. 

2.2 LLE Measurements 

For LLE measurements for the system 1-butanol+water, the device used for charging the mixture (denoted by 

the term “equilibrium cell” (EC) in the following) to be analyzed was one of the two receivers (Figure 1b) the 

Fischer Labodest VLLE 602 unit is equipped with for sampling the liquid and vapor (in its condensed state) 

phases at equilibrium. This choice was made aiming at setting up a proper procedure for future VLLE 

measurements with that unit, which requires analyzing the two liquid phases in a separate equipment. At the 

beginning of each measurement, the thermostatic bath filled with silicone oil (SilOil P20.275.50) was switched 

on and the temperature at which the measurement had to be carried out was set. Then, in the EC a mixture of 

626



known composition (total volume of about 4 ml) of the two compounds was prepared under the hood. The EC 

was inserted in a flask to keep it firm during the weighting procedure. 

a) b)  c) 

Figure 1: Pictures of: a) the Fischer Labodest VLLE 602 unit; b) one of the two receivers of the Fischer Labodest 

VLLE 602 unit; c) the thermostatic bath used for LLE measurements and the support properly designed to 

submerge the EC in it 

A properly design support, made up of stainless steel, was used to submerge the EC in the thermostatic bath 

(Figure 1c). The temperature inside the EC was monitored by means of a digital thermometer (Hanna 

Instruments) until it reached a constant value (oscillations within 0.1 K). At this point, the cap was removed from 

the EC for stirring the mixture for about 5 min. The closed system was, then, left undisturbed for a period of time 

necessary for the two phases to settle. Such a time was evaluated for each temperature by performing some 

preliminary measurements aimed at determining the time interval required for obtaining a stable composition for 

the upper phase. It was found that 1 hour was enough for reaching equilibrium at the highest temperatures in 

the investigated range (i.e., 333.15 - 353.15 K), whereas a longer time (> 4 h) was necessary at the lowest 

temperatures (i.e., 313.15 - 323.15 K). 

Once stable conditions were reached, the sampling phase began, starting from the upper organic phase. The 

analyses for each phase were repeated at least three times to guarantee the repeatability of the measurements. 

All measurements were performed at atmospheric pressure. At the end of each set of measurements at a certain 

temperature, the thermostatic bath was switched off. Once the ambient temperature was reached, the EC was 

removed from it, using the support, and was carefully cleaned to make it ready for further measurements. 

3. Results 

3.1 VLE Measurements 

Figure 2 shows the VLE data measured in the PT lab at 103.5 kPa (Figure 2b), 150.0 kPa (Figure 2c) and       

200.0 kPa (Figure 2d) for the system ethanol+water. An analysis of errors was performed following the approach 

described by Taylor and Kuyatt (Taylor and Kuyatt, 2009), which resulted in uncertainties related to molar 

fractions within 5 %. 

In Figure 2a the most recent data available in the literature (Al-Rub et al., 1999; Alvarez et al., 2011; Arce et al., 

1996; Iwakabe and Kosuge, 2001; Kamihama et al., 2012; Kurihara et al., 1993; Lai et al., 2014; Li et al., 2012; 

Liu et al., 2012; Orjuela et al., 2010; Rojas et al., 2016; Tsanas et al., 2014; Wang and Yang, 2002; Xu et al., 

2011; Yang and Wang, 2002) at atmospheric pressure are also reported for comparison purposes. Experimental 

data presented in this work are slightly shifted to the right with respect to the other ones available in the literature. 

This is due to the fact that the system pressure (103.5 kPa) is a little higher than the atmospheric pressure at 

which the other literature data are available. By comparing Figure 2b-d it can be observed that, as the pressure 

increases, the amplitude of the VLE region decreases. 

Lines in Figure 2b-d refer to the results obtained using an indirect γ/φ method (NRTL-RK) with the default 

parameters available in Aspen Plus® V11 (AspenTech, 2019), since they have been reported (Lodi et al., 2017) 

to be suitable for satisfactorily reproducing experimental data. 

For a quantitative assessment, the percent Average Absolute Deviation (AAD%) has been taken into account, 

which has been calculated according to Eq(1), where “n” stands for the number of data, “vcalc” and “vexp” refer, 

respectively, to the calculated and experimental value of the generic variable (namely, the temperature or the 

mole fraction of each component in the vapor phase). 

627



, exp,

%

1 exp,

| |100 n calc i i

i i

v v
AAD

n v=

−
=   (1) 

a) b) 

c) d) 

Figure 2: VLE data for the ethanol+water system at: a), b) 103.5 kPa; c) 150.0 kPa; d) 200.0 kPa. Filled black 

symbols refer to the data measured in the PT lab; the other symbols in Figure 2a refer to data available in the 

literature, as specified in the legend; lines refer to predictions by the NRTL-RK model 

If a bubble-point calculation is performed, at given pressure and composition of the liquid phase equal to the 

experimental ones presented in this work, the model is able to predict the equilibrium temperature with an AAD% 

of 0.17 – 0.26 %, and the mole fraction of ethanol and water in the vapor phase with an AAD%, respectively, of 

23.3 – 31.6 % and 4.3 – 12.1 %, with the highest deviations obtained at the highest pressures, as shown in 

Figure 2. 

3.2 LLE Measurements 

Figure 3 is the temperature-composition diagram illustrating the LLE measurements carried out in this work for 

the system 1-butanol+water together with the data available in the literature. Of the many data that can be found 

in the NIST database, those which belong to a complete set (i.e., for which pressure, temperature and 

composition of both phases are all given) and consist of more than 3 points have been selected (Drouillon, 1925; 

Erichsen, 1952; Hill and Malisoff, 1926; Jones, 1929; Mueller et al., 1931; Ruiz et al., 1984; Stephenson and 

Stuart, 1986; Wannachod et al., 2016; Zhang et al., 1986), though some of them reach temperatures higher 

than the temperature of the heterogeneous azeotrope at atmospheric pressure, which occurs at 364.6 K with a 

composition of 76.33 mol. % water according to Luyben (Luyben, 2008). It is possible to observe that the new 

data collected in this work are in good agreement with the ones presented in previous literature works, to a 

lesser extent at the lowest investigated temperature. It is also interesting to notice, with reference to Figure 3b, 

that the data presented in this work exhibit the trend observed by Stephenson and Stuart (Stephenson and 

Stuart, 1986) as for the solubility curve of the alcohol that goes through a minimum at about 332.15 K. 

Lines in Figure 3 refer to the results obtained using an indirect γ/φ method (NRTL-RK) with the parameters for 

the NRTL model reported by Lodi et al. (Lodi et al., 2018), which were properly regressed to improve model 

350

355

360

365

370

375

0.00 0.20 0.40 0.60 0.80 1.00

T
e

m
p

e
ra

tu
re

 [
K

]

Ethanol mole fraction

 PT lab, this work
 Kurihara et al. (1993)
 Arce et al. (1996)
 Al-Rub et al. (1999)
 Iwakabe and Kosuge (2001)
 Wang and Yang (2002)
 Yang and Wang (2002)
 Orjuela et al. (2010)
 Alvarez et al. (2011)
 Xu et al. (2011)
 Kamihama et al. (2012)
 Li et al. (2012)
 Liu et al. (2012)
 Lai et al. (2014)
 Tsanas et al. (2014)
 Rojas et al. (2016)

350

355

360

365

370

375

0.00 0.20 0.40 0.60 0.80 1.00

T
e

m
p

e
ra

tu
re

 [
K

]

Ethanol mole fraction

360

365

370

375

380

385

390

0.00 0.20 0.40 0.60 0.80 1.00

T
e

m
p

e
ra

tu
re

 [
K

]

Ethanol mole fraction

365

370

375

380

385

390

395

0.00 0.20 0.40 0.60 0.80 1.00

T
e

m
p

e
ra

tu
re

 [
K

]

Ethanol mole fraction

628



predictions. By performing a flash calculation at given temperature and pressure, set equal to the experimental 

values, the model results to predict the mole fraction of water with an AAD% of 2.4 % and 0.17 %, respectively 

in the organic and in the aqueous phases. 

a) b) 

Figure 3: Temperature-composition diagram illustrating the LLE measurements carried out in this work (PT lab 

series, black symbols) and available in the literature (as specified in the legend, other symbols) for the system 

1-butanol+water: a) entire composition range; b) zoom on the composition of the water-rich phase 

4. Conclusions 

This work presents new VLE data for the system ethanol+water and new LLE data for the system                               

1-butanol+water, both of interest considering the renewed attention devoted to the ABE fermentation process. 

The VLE measurements fill the gap existing in the literature as for data between atmospheric pressure and 287 

kPa, whereas the LLE measurements have allowed to set-up the procedure in view of, then, using that unit for 

VLLE measurements. Comparison with literature experimental data suggests the new data are in good 

agreement with them. Satisfactorily results have been also obtained from the modelling point of view using the 

NRTL-RK property package, with parameters available by default in the Aspen Plus® V11 process simulator (for 

ethanol+water) or properly regressed in a previous literature work (for 1-butanol+water). As for the VLE 

measurements for the system ethanol+water, if a bubble-point temperature calculation is performed, the model 

is able to predict the equilibrium temperature with an AAD% of 0.17 – 0.26 %, and the mole fraction of ethanol 

and water in the vapor phase with an AAD%, respectively, of 23.3 – 31.6 % and 4.3 – 12.1 %, with the highest 

deviations obtained at the highest pressures. As for the LLE measurements for the system 1-butanol+water, by 

performing a flash calculation at given temperature and pressure the model predicts the water mole fraction with 

an AAD% of 2.4 % and 0.17 %, respectively in the organic and in the aqueous phases. 

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	Downstream Processing of Butanol Produced by Fermentation: Thermodynamic Measurements and Modelling