(547f) Quaternary Phase Equilibrium Data of Quaternary Water + (Ethanol and 2-Propanol) + Isooctane System

Quaternary phase equilibrium data of quaternary
Water + (Ethanol  and
2-Propanol) + Isooctane system

                                             Danielle
L. de Klerk, Cara E. Schwarz¹*

¹Department
of Process Engineering, Stellenbosch University, Banghoekweg,
Stellenbosch, 7600, South Africa

*email: cschwarz@sun.ac.za,
Tel: +27 21 8084485

Introduction

Worldwide
there is a need for anhydrous low molecular mass alcohols, such as ethanol and 2-propanol.  These alcohols have many industrial
applications, such as in biofuel, high-purity cleaning solvents or as fuel
additives to enhance the octane rating of fuel. Very often the alcohols are
produced as dilute aqueous mixtures in large scale operations, such as
fermentation or by-products of the Fischer-Tropsch
process. However, ethanol and isopropanol both form binary azeotropes with
water and thus normal distillation is unable to produce anhydrous alcohols and
advanced separation techniques are required.

Heterogeneous
azeotropic distillation is a well-established technique capable of separating
alcohol and water mixtures. The addition of a third component, the entrainer,
overcomes the azeotrope by forming a ternary heterogeneous azeotrope with the
water-alcohol system. Industrially, benzene has been used as a very successful
entrainer in the separation of water and alcohols.  

Unfortunately,
benzene is carcinogenic and toxic. Thus, a strong incentive and drive exists to
replace benzene as an entrainer. Many alternatives have been proposed,
including isooctane. Several studies have been done to show that isooctane is a
possible suitable alternative as it forms a ternary heterogeneous azeotrope
with the water and ethanol or 2-propanol (Font,
et al., 2003; 2004). Furthermore, if the alcohols are to be used as fuel
additives, traces of isooctane are not detrimental to the process.

Existing
thermodynamic models struggle to accurately predict the phase behaviour of such
systems, and as such reliable phase equilibria data are required to understand
the system’s phase behaviour, for use in modelling correlation and subsequently
in process design. Phase equilibria data are available for the ternary water +
alcohol + entrainer systems, but very little, if any, research has been done
into the case where multiple alcohols are present. Investigation into such
systems is warranted as there is often more than one alcohol present in the
dilute aqueous stream undergoing purification, such as from Fischer Tropsch reaction water, and the presence of the second
alcohol may influence the existing ternary phase behaviour.

The
aim of this contribution is therefor to investigate the phase equilibria of the
quaternary water + ethanol + 2-propanol + isooctane system by presenting (1)
liquid-liquid equilibrium (LLE) data for a range of temperatures; (2)
vapour-liquid equilibrium (VLE); and (3) vapour-liquid-liquid equilibrium
(VLLE) data of the system at three 2-propanol: ethanol ratios at 101.3 kPa.

Methodology

LLE experimental methodology

The
LLE phase equilibrium data measurements were conducted in shakeflasks.
A known composition was loaded and allowed to reach thermal equilibrium, while
shaken regularly. The mixture was left for a minimum of fifteen hours to
separate, before samples were drawn from each phase. The ratio of the organic
components in the samples was determined via GC-FID analysis and the water
content via Karl Fischer titration. Further details are available in Swanepoel
and Schwarz (2017).

VLE and VLLE
experimental methodology

The VLE and VLLE data were
obtained using a modified PilotDist VLE100 dynamic
recirculating vapour liquid equilibrium still (Pienaar
et al., 2013). The unit was modified through the addition of an ultrasonic
homogenizer, which allows for the measurement of VLLE data. The same procedure
was followed for obtaining VLLE data as for VLE data, with the additional steps
of using the ultrasonic homogenizer to ensure the organic and aqueous phases
are sufficiently emulsified and allowing the liquid sample to separate for a
minimum of two hours before drawing samples from each phase for VLLE
measurements. The compositions of the samples were determined using GC analysis
with use of thermal conductivity detection.

Results and Discussion

The
results for the LLE data show that the presence of the second alcohol has a
noticeable effect of the LLE in the quaternary systems and needs to be
considered, in terms of both alcohol recovery to the organic phase and the size
of the immiscible region. The presence of ethanol reduced the concentration of 2-propanol
that reports to the organic phase while the presence of 2-propanol increases the concentration of ethanol that reports
to the organic phases. The interaction of ethanol with 2-propanol and vice
versa therefore significantly influences the phase behaviour of the system.

Figure 1. Quaternary diagram
representing the liquid-liquid phase equilibrium data for the quaternary system
of water + ethanol + 2-propanol + isooctane at 298K and atmospheric pressure,
for three 2-propanol: ethanol ratios

Likewise,
the effect of the presence of a second alcohol should also be considered for
the VLE and VLLE of the quaternary system. The effect of the presence of a
second alcohol can be studied in this quaternary water + ethanol + 2-propanol +
isooctane system but the insights gained can be applied to systems of other
alcohols and/or entrainers.

References

Font,
A., Asensi, J. C., Ruiz, F. & Gomis, V., 2003. Application of Isooctane to
the Dehydration of Ethanol. Design of a Column Sequence To Obtain Absolute
Ethanol by Heterogeneous Azeotropic Distillation. Industrial and Engineering
Chemistry Research, Volume 42, pp. 140-144.

Font,
A., Asensi, J. C., Ruiz, F. & Gomis, V., 2004. Isobaric Vapor-Liquid and
Vapor-Liquid-Liquid Equilibria Data for the System Water + Isopropanol +
Isooctane. Journal of Chemical and Engineering Data, Volume 49, pp.
765-767.

Pienaar,
C. Schwarz, C.E., Knoetze, J.H., Burger, A.J., 2013. Vapor−Liquid−Liquid
Equilibria Measurements for the Dehydration of Ethanol, Isopropanol, and n‑Propanol via Azeotropic Distillation Using DIPE and Isooctane
as Entrainers, Journal of Chemical and
Engineering Data, Volume 58, pp 537-550.

Swanepoel,
R. M. & Schwarz, C. E., 2017. Influence of Temperature on the Liqiud-Liquid
Phase Equilibria of Ternary (Water + Alcohol + Entrainer) Systems. Journal
of Chemical and Engineering Data, 62, pp. 2740-2754.