Future and Emerging Technologies
One of the main objectives of the work performed in chemical engineering is the optimization of
processes. In the realm of distillation, there are several emerging processes and theories in place to
improve the degree of separation and the breadth of systems which can be separated. The most promising
of these are those pertaining to ultrafiltration, pervaporation and the use of various aspects of entrainers.
Ultrafiltration is emerging as a first post-distillation treatment for water-ethanol systems. Because of the
inherent azeotropic nature of a water-ethanol system, distillation becomes very expensive as the azeotrope
is approached. Separation in this range of purity is much more efficiently carried out with the use of
membranes. Because filtration does not involve a vapor-liquid equilibrium, the existence of an azeotrope
is of no consequence.
In order to selectively choose ethanol as the permeate in a water-ethanol system, many compounds and
filtration geometries are under current consideration. Of notable importance are cellulose esters, cellulose
acetates, polyacrylonitrile, polyethersulfone, [1] polydimethylsiloxane, and polysulfone [2]. These
materials, generally cast as a hollow-fiber membrane. The mixture of water, ethanol and (in a true
industrial system) salts and proteins passes through the tubular membrane at significantly raised
pressures. In a successful ultrafiltration, the ethanol is selectively permeated through the membrane
through diffusion. Permeability for some membranes tested even reached values of the range of 500 x 1015
m when conditioned correctly. [1]
Currently, studies are focusing on being able to maintain the integrity of the membranes for extended use
in ultrafiltration. Several factors contribute to the degradation of the membranes. Firstly, the elevated
pressure at which ultrafiltration must occur introduces a high level of mechanical stress on the fibers of
the membrane. Nanotechnology improvements are enabling researchers to strengthen the materials
without having to increase the size or thickness of the membranes [3]. Secondly, the somewhat corrosive
nature of a highly concentrated ethanol solution degrades the membranes. Because polymers with
different functional groups will interact with and transport molecules differently, a wide array of potential
membrane constructs is currently under consideration for use in ultrafiltration. Lastly, the biological
nature of the fermentation and subsequent need to separate out organic wastes and ionic molecules tends
to either corrode the membranes or else increase the amount of material that is deposited on the inside of
the membrane as the feed flows through. This is currently an issue which remains to be studied in depth.
Membranes designed and being researched presently are configured such that salts and other organic
materials pass through upon ultrafiltration, leaving the need for another method of separation to improve
purity [1].
Pervaporation is the combination of several different methods of separations in to one higher-efficiency
process. Processes included are partial vaporization and the use of membranes. The membranes make the
process especially efficient. The benefits to the use of membranes in the separation of mixtures include
the ability to “save in process costs…, raw materials can be recovered and reused, fermentation processes
can be carried out continuously and disposal problems can be reduced or eliminated”. These aspects
combined together apply well to improve upon the already-established separation involved in distillation.
The ethanol solution (generally already of a high purity after such previous separation processes as
distillation and ultrafiltration) is passed through another hollow-fiber membrane. The material which
passes through the membrane does so through diffusion. This is facilitated by the presence of a vacuum
on the permeate side of the membrane. Pressures of 13 Pa provide additional driving force, instigating a
selective separation. Selectivity, α, as high as 20 has been observed in some cases [1]. Many of the same
issues facing the membranes involved in ultrafiltration are present in pervaporation membranes. One
advantage that pervaporation membranes contain is that they are able to selectively permeate ethanol
while rejecting other organic materials and ionic compounds. This allows for as high as 99.5% pure
ethanol to be obtained through pervaporation.
One of the other fields in which much research is being conducted is that of the use of entrainers,
otherwise known as extractive distillation. While the addition of gasoline to a hydrous ethanol system
enables the water to be removed as the top product, the resultant bottoms is only usable as gasahol and
thus limits the possibilities for application [4]. By introducing other entrainers, better separation is
available, with the later recovery of high-purity ethanol possible. Among the leading species being
considered for use as an entrainer in an ethanol-water system are hyperbranched polyglycerol and various
ionic liquids. The goal of the addition of these entrainers to the feed is both to reduce the amount of heat
which must be used to perform distillation as well as to break current prohibitive azeotropes [5].
There are many benefits to the use of extractive distillation in comparison to traditional distillation
processes: continuous operation is facilitated as the solvent increases efficiency and reduces waste, purity
is improved, amount of solvent necessary is removed, and required theoretical stages can be greatly
reduced [4]. One of the current difficulties in the use of hyperbranched polymers is their nascent nature.
Because they are just beginning to find use in extractive distillation, not many different polymers have
been identified as optimal. Some polymers do not have the thermal stability required for distillative
action, and others simply degrade with time. As this field is expanded, the set of systems to which
polymers can be added as entrainers will be expanded. The benefit to the use of such polymers is that they
can be tailored for different applications including solubility, capacity, selectivity, viscosity, and thermal
stability [4].
While ethanol-water separation is one of the most long-standing and established VLE separations, there
are many emerging technologies which, when developed, will greatly improve the efficiency of the
separation and concurrently reduce the energy consumption required to perform that separation. The
greatest improvements will be a combination of various techniques such as extractive distillation followed
by ultrafiltration and pervaporation. When these techniques are combined, the selectivity and thus product
purity can be greatly enhanced.
[1] Shukla, Rishi and Munir Cheryan, “Performance of Ultrafiltration Membranes in Ethanol-Water
solutions: Effect of Membrane Conditioning.” Journal of Membrane Science, Volume 198, pp. 75-85,
2002.
[2]Mulder, MHV, Hendrikman, Hegeman, and Smolders, “Ethanol-Water Separation by Pervaporation”.
Journal of Membrane Science, Volume 16, pp. 269-284, 1983.
[3] Shukla, Rishi and Munir Cheryan, “Stability and Performance of Ultrafiltration Membranes in
Aqueous Ethanol”. Separation Science and Technology, Volume 38, No. 7, pp. 1533-1547, 2003.
[4] Gill I. D., A. M. Uyazán, J. L. Aguilar, G. Rodríguez and L. A. Caicedo, “Separation of Ethanol and
Water by Extractive Distillation with Salt and Solvent as Entrainer Process Simulation”. Brazilian
Journal of Chemical Engineering, Vol. 25, No. 01, pp. 207 - 215, January - March, 2008.
[5] Seiler, Matthias, Carsten Jork, Asimina Kavarnou, and Wolfgang Arlt, and Rolf Hirsch. “Separation of
Azeotropic Mixtures Using Hyperbranched Polymers or Ionic Liquids” AIChE Journal Vol. 50, No. 10
October 2004.