Introduction
Ethanol is one of the most widely used chemicals on the planet. There are very few industries and
processes, especially those which relate to products that come into direct contact with humans, which
do not involve the use of ethanol at some stage or another. Ethanol is used as a solvent due to its
polarity and small size, it is used as a raw or intermediate material in the synthesis of everything from
medicines to esters to paints1, and perhaps most importantly, it is used as a fuel or a fuel additive. The
use as a fuel has become increasingly important in the last few years as a pending oil crisis looms since
ethanol for use as fuel can be created from renewable resources. Due to the fact that the use of ethanol
is so widespread, making its production process more efficient could potentially save untold amounts of
resources. Thus, it is of the utmost importance to understand and explore the manufacture and
purification processes that create ethanol. This paper will focus explicitly on the process of purifying
ethanol following its manufacture. Even more specifically, this paper will explore the process of
separating ethanol from water. Most ethanol is created by either the fermentation of biomass or by the
hydration of ethylene as a result of the petroleum refining process, both of which result in a liquid
ethanol/water mixture, making this separation especially meaningful.
There are various ways in which two chemicals can be separated. In order to determine the best way to
do this, the physical properties of each of these chemicals must first be explored. Figure 1 shows a
comparison of various physical properties of ethanol and water. The separation method needs to take
advantage of large disparities in any of these physical properties.
Figure 1.
1
2
Physical Properties of Ethanol and Water2
Gil et al., Separation of Ethanol and Water by Extractive Distillation…
Nielsen, Dept. of ChE, ASU
For ethanol and water, some of the most significant differences in properties are for the boiling point,
heat of vaporization, and vapor pressure. Coincidentally, the process that is used almost exclusively to
separate ethanol and water is fractional distillation. All of these properties contribute to a difference in
volatilities. If the two chemicals are heated up to a temperature between their respective boiling points,
the creation of two phases will occur. Most of the more volatile component, in this case, ethanol, will
vaporize, creating the vapor phase. Meanwhile, the less volatile component will remain in the liquid
feed phase. These two phases can then be easily separated by density as vapors are significantly less
dense than any solution, resulting in one product with more of the volatile component and one with
more of the less volatile component. The relative ease or difficulty of a separation in this manner can be
characterized with a ratio known as the relative volatility of the two substances:
𝛼𝑖𝑗 ≝
𝐾𝑖
𝐾𝑗
(Equation 1)3
Where K represents the vapor-liquid equilibrium ratio:
𝐾𝑖 =
𝑦𝑖
𝑥𝑖
(Equation 2)4
Where y represents the fraction of the substance in the vapor phase at equilibrium and x the fraction in
the liquid phase at equilibrium. Essentially, the K-value represents the volatility of the material at
equilibrium, and the relative volatility represents the how more likely one of the materials is likely to
vaporize in comparison to the other. When the relative volatility has a value around one, separation by
distillation is nearly impossible. As might be expected, relative volatility is also therefore dependent on
the amount of each material in the mixture. For an ethanol/water mixture of 80/20wt%, the relative
volatility is about 1.83.5
3
Seader et al., pg. 39
Seader et al., pg. 39
5
Need reference. Can be calculated using NRTL model
4
The basic literal operation of a distillation tower system is not overly complicated. Figure 2 shows a basic
schematic of the system. The distillation column consists of a series of trays, each representing a
different flash stage. The phases flow countercurrent to one another, with the liquid phase always
moving down and the vapor phase always moving up. The reflux drum and reboiler simply increase the
efficiency of the system by recycling certain portions back into the column to be separated again. The
two main products are the bottoms, which contains the liquid phase consisting mainly of the less volatile
component in the initial mixture, and the distillate, which contains the vapor phase consisting mainly of
the less volatile component. Generally, the distillate contains the desired product that the process is
trying to purify. This is the case in an ethanol/water distillation.
Figure 2.
6
Seader et al., pg. 261
Fractional Distillation Column6
The main problem that occurs with this separation method is that conventional fractional distillation
cannot create an ethanol/water mixture with a purity of greater than about 95 wt% ethanol. At this
composition, an azeotrope forms, which means that the mixture cannot be separated further by
conventional distillation. The mixture will only boil together and this is a serious obstacle if a greater
purity is desired. It is generally desired, especially for use as fuel, that the final ethanol product is as
close to anhydrous as possible. Figure 3 shows a T-x-y diagram for the ethanol/water system at standard
conditions. As can be seen, past 95 wt%, the mixture follows the dashed line representing equal vapor
and liquid phase compositions.
Figure 3.
Txy Diagram for an Ethanol/Water System7
There are several methods which can overcome this obstacle or break this azeotrope. Several of the
more interesting methods currently under development are the use of entrainers, pervaporation, and
supercritical fluid extraction. Entrainers involve adding a third chemical to the mixture. For this specific
system, popular choices are benzene and cyclohexane. The additional chemical changes the composition
and chemistry of the mixture, essentially breaking the azeotrope. This allows for almost complete
purification of the ethanol. The major problem with this method is the residual additive chemical that
inherently remains in the ethanol. While it may only be a small percentage on the order of thousandths
or less, chemicals such as benzene are carcinogens, making ethanol purified in this manner unfit for use
7
Kvaalen et al., Alcohol Distillation…
in products that come into contact with humans. Pervaporation involves using a membrane to separate
the compounds in the mixture then vaporizing the permeate, which would be ethanol in this instance.
This is beneficial in that it can be run as a continuous process, as shown in Figure 4. Supercritical fluid
extraction involves using a supercritical fluid as a solvent to extract the desired product from the
mixture. Currently, using supercritical carbon dioxide to separate ethanol from water is being studied.
This process is obviously limited by the difficulty of creating, maintaining, and working with supercritical
fluids. Further discussion of these methods will be developed later on in the course of this work.
Sugar Feed
Fermenter
Ultrafiltration
Permeate
5% EtOH
Salts
Sugars
E-1
Pervaporation
Permeate
40% EtOH
Pervaporation
95...99.5% EtOH
Permeate
10% EtOH
Figure 4.
8
Pervaporation Process8
Mulder et al., Ethanol-Water Separation by Pervaporation
References
Gil, I.D., Uyazan, A.M., Aguilar, J.L., Rodriguez, G., and Caicedo, L.A. Separation of Ethanol and Water by
Extractive Distillation with Salt and Solvent as Entrainer: Process Simulation. Brazilian J. of ChE.
Vol 25, No. 1 (2008) 207-215.
Kvaalen, E., Wankat, P.C., McKenzie, B.A. Alcohol Distillation: Basic Principles, Equipment, Performance
Relationships, and Safety. Cooperative Extension Service, Purdue University. (1984).
Mulder, M.H.V., Oude Hendrikman, J., Hegeman, H., and Smolders, C.A. Ethanol-Water Separation by
Pervaporation. J. of Membrane Science. 16 (1983) 268-294.
Seader, J.D., Henley, E.J., Roper, D.K. Separation Process Principles: Chemical and Biochemical
Operations. 3rd Ed. John Wiley & Sons, Inc. United States: 2011.