(376bs) Ultrasound-Mediated Nonequilibrium Separation of Ethanol-Water Solutions, Including Avoidance of the Azeotropic Bottleneck
AIChE Annual Meeting
2018
2018 AIChE Annual Meeting
Separations Division
Poster Session: Separations Division
Tuesday, October 30, 2018 - 3:30pm to 5:00pm
Title: Ultrasound-mediated nonequilibrium
separation of ethanol-water solutions, including avoidance of the azeotropic bottleneck
Ozan Kahraman*
Department of Food Science and
Human Nutrition
University of Illinois at
Urbana-Champaign
Arne J. Pearlstein
Professor
Department of Mechanical Science
and Engineering
University of Illinois at
Urbana-Champaign
Hao Feng
Professor
Department of Food Science and
Human Nutrition
University of Illinois at
Urbana-Champaign
Introduction
Separation of
liquid mixtures, frequently by distillation, is ubiquitous in the chemical and
process industries (CPI). Distillation
accounts for ~95% of the energy used in liquid separations, ~25–40% of overall
energy used in CPI, and ~3% of global energy consumption.1-2 The low
efficiency of distillation is largely due to two issues. First, there are large irreversible losses
due to heat transfer.3 Second, a significant fraction of energy used
in liquid separations is used to separate azeotropic
mixtures in azeotrope-forming systems (e.g., ethanol/water). While a number of conventional distillation
technologies4-5 (e.g., pressure-swing, extractive distillation, and azeotropic distillation6) and new separation
approaches5 (e.g., dividing-wall columns, membranes, molecular
sieves, and bio-absorbance) have been developed for azeotropic
systems, these approaches largely rely on thermal separation via phase
equilibrium or involve large capital and/or operational costs.
Ultrasound
passing through a liquid layer with a free surface above produces droplets that
form a mist. Current thinking is that
this occurs at crests of capillary waves at the gas/liquid interface, by
bursting of cavitating micro-bubbles near the interface,
or possibly by both mechanisms.5 Regardless of the mechanism, it is
known that mist formation is accompanied by favorable partitioning of solute
between the mist and the mother solution,7 with studies in EtOH/water solutions showing significant EtOH enrichment in the mist8-10, including
enrichment of EtOH from 0.07 mass fraction in the
mother solution to nearly 1.0 in the mist.9 Current theories attribute enrichment to an
excess of ethanol in a thin layer on the liquid side of the interface10-11,
enhanced evaporation due to enlargement of surface area, or to EtOH-rich clusters in the bulk solution, directly brought
into the gas phase by bursting cavitation bubbles.
In
this study, we propose to develop, test, and demonstrate scalable nonequilibrium liquid separations (including for
azeotrope-forming systems), that avoid the heat transfer losses of distillation,
as well as the azeotropic bottleneck. Success will
result in significant savings in energy and capital costs, for a broad class of
systems.
Method
In the
first ultrasonic separation prototype, a 2.4 MHz ultrasonic transducer with a
diameter of 18 mm and rated power of 13.2 W was installed in the center of
the bottom of a custom-designed cylindrical separation unit. The unit (see Figure 1) was made from
transparent polyvinyl chloride resin (10 mm thickness), with a height of 305 mm
and an inside diameter of 100 mm. The temperature of the bulk liquid (at the
bottom of the unit) was kept nearly constant by circulating
controlled-temperature water through a volume laterally surrounding the bottom
section of the vessel. Mist generated by
ultrasonic actuation was carried out of the cylindrical volume by an air flow.
Figure 1. The
ultrasonic separation prototype.
In
experiments, the entire set-up was pre-conditioned to a target separation
temperature, e.g., 30oC, by water circulation, followed by adding
approximately 200 ml of EtOH solution with mass
fraction
into the separation unit,
to a liquid depth of about 25 mm. The
ultrasonic transducer was then turned on to produce a mist from the bulk liquid. Then the unit was opened to a flow of carrier
air, set at a flowrate between 10 to 30 L/min, to carry the mist to a
collector. The collected mist was filtered through a syringe filter into a 1.5
ml capped vial for HPLC analysis. Duplicate samples were analyzed using a
Waters e2695 HPLC series system (Waters Associates, Milford, MA, USA) equipped
with a Prevail Carbohydrate ES 5 µm column, automatic injector, and a Waters
2414 refractive index detector at 40oC. The mobile phase was 160 µL
H2SO4/L HPLC grade water, with a flow rate of 1
mL/min. Distilled water and 200 proof
ethanol (Decon Labs, King of Prussia, PA) were
used.
Results
Enrichment
of ethanol was observed in the mist for mother solutions at all initial compositions
(see Figure 2). For initial ethanol mass
fractions exceeding about winitial = 0.60, the ethanol mass fraction in the
collected mist exceeded the azeotropic vaalue of 0.956.
When winitial
exceeded about 0.65, the collected mist was nearly pure ethanol.
The
volumetric flow rate of the carrier air had a significant effect on enrichment.
For instance, when the air flowrate was
increased from 10 L/min to 30 L/min, the ethanol mass fraction in the mist
increased from 0.147 to 0.203 and 0.813 to 0.969, for winitial = 0.040 and
0.648, respectively (see Figure 3a-b). It can be seen that the air flowrate is
even more critical for mixtures with an initial mass fraction
. We found that the
optimal air flowrate for separation at 30oC was between 25 and 30
L/min. Another important parameter for
this separation is the collection time. At an air flowrate of 25 L/m, when the
sampling time increases, water droplets in the mist tend to reach equilibrium
with the surrounding air, which may result in a decrease in the efficiency of separation.
As shown in Figure 3c, increasing the sampling time from 2 min to 10 min for
samples with winitial
= 0.165, reduced the EtOH mass fraction in the mist from
0.326 to 0.193. For winitial = 0.542, the
same change in carrier air flow rate reduced the EtOH
mass fraction in the collected mist from 0.871 to 0.675.
Figure 2. Ethanol mass fraction in the bulk solution
and mist at different carrier air flow rates.
Figure 3. Effect of carrier air flowrate (a & b)
and collection time (c) on ethanol mass fraction in the collected mist.
Conclusion
Ultrasound-mediated
ethanol separation at low temperature is shown to be effective in enriching ethanol
in the ultrasound-generated mist compared to the bulk ethanol-water solution.
This nonthermal, nonequilibrium
separation method is able to break the ethanol-water azeotrope, showing promise
to reduce energy consumption in ethanol production.
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