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Evaporation of the Ethanol and Water Components Comprising a Binary Liquid Mixture.

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Evaporation of the Ethanol and Water
Components Comprising a Binary
Liquid Mixture
K.D. O'Hare', P.L. Spedding2' and J. Grimshaw3
' British Gas plc, Midlands Research Station, Solihull, West
Midland, 691 2JW, UK
* Department of Chemical Engineering, The Queen's
University of Belfast, Belfast BT9 5AH, UK
Department of Applied Chemistry, The Queen's University of
Belfast, BT9 5AH, UK
Evaporation studies were conducted on the ethanol-water system. The evaporation
rates of the ethanol and water components were determined experimentally, over a
range of compositions. The rate of change in the composition of the evaporating
mixture was monitored, and the effect of concentration on the ethanol component
evaporation rate (per unit mass of ethanol remaining) was investigated. Preferential
adsorption of alcohol on the interjace in low ethanol concentration solutions allowed
ethanol to evaporate more rapidly in these solutions.
The evaporation of pure liquids has been well documented in the literature.'-" By
comparison very little research has been conducted on the evaporation of binary
liquid mixtures. This can be attributed to the difficulties involved with the
experimental measurement of the component evaporation rates, and the
calculation of mass transfer rates considering that the mass transfer resistance can
be in both the gas and liquid phases.
The aim of this work was to investigate experimentally the evaporation of
ethanol and water from an ethanol-water mixture (from a plane surface of a mixed
liquid, under conditions of turbulent air flow) and to determine whether: (i) the
composition of the evaporating mixture changed as evaporation proceeded;
(ii) the component evaporation rates were concentration dependant; (iii) the
component evaporation rates were greater than for the pure liquids alone.
Literature Review for Binary-Liquid Evaporation
HoffmanI2 conducted one of the earliest investigations into the evaporation of
binary mixtures. He concluded that binary mixtures could be divided into two
* To whom all correspondence should be addressed.
Developments in Chemical Engineering and Mineral Processing, Vol. 1, No 2/3,page 118
Evaporation of the Ethanol and Water Components
groups: (i) mixtures that showed ideal behaviour; and (ii) constant evaporating
mixtures. He stated that the first group consisted of mixtures from the same
homologous series, like toluene and xylene, where both solvents evaporated
simultaneously at different rates and had no effect upon each other. When the
faster-evaporating component was gone, the remainder of the other component
evaporated at a constant rate. The second group consisted of mixtures of two
dissimilar compounds such as an alcohol and an aliphatic hydrocarbon. HoffmanI2
stated that these evaporated at a constant rate without a change in composition,
the evaporation rate being faster than for either component alone.
Lewis et al. l 3 studied the evaporation of mixed solvents in relation to their use
for protective coatings, such as lacquers. They concluded that the evaporation
process was more complicated than was suggested by Hoffman.12They found that
for the evaporation of a methanol-benzene mixture, the composition of the liquid
mixture changed continuously from the initial value to a mixture containing no
detectable methanol, which was the more volatile component.
S a r n ~ t s k y ’investigated
the evaporation rate of mixed solvents in relation to
paint films. He found that the evaporation of alcohol from alcohol-hydrocarbon
mixtures was concentration dependant and that the alcohol component evaporated
faster than was calculated assuming ideal behaviour. He also found that the
evaporation rate of a toluene-isopropanol mixture was greater than for either of
the two components alone.
Richardson” studied the evaporation of binary mixtures using the
Winkelmann16 method, which involved the passing of air above an evaporation
tube containing the liquid mixture. He studied the evaporation of (i) acetone and
dibutyl phthalate; and (ii) carbon tetrachloride and dibutyl phthalate. Dibutyl
phthalate was virtually a non-volatile liquid. Richardson” found that in the first
case, the more volatile component evaporated faster leaving a residual liquid,
which being of greater density, sank into the bulk of the liquid causing
considerable mixing. However, in the second case, because the dibutyl phthalate
was less dense than the more volatile component, the residue tended to
concentrate in the upper layers of the mixture and therefore the volatile liquid had
to diffuse through this region before it could evaporate. Clark and Judson King17
measured the evaporation rates of n-pentane, cyclopentane and ethyl ether from
n-tridecane into flowing nitrogen. They found that cellular convection, which was
concentration-gradient induced and surface tension driven, served to increase
liquid-phase mass transfer coefficients substantially.
Mackay and MatsuguI8 studied the evaporation rates of liquid hydrocarbon
spills on land and water, and stated that two limiting conditions existed for the
evaporation of binary mixtures. First, assuming that there was perfect mixing in
the liquid, the vapour leaving the surface would be in equilibrium with the bulk
liquid and generally had a composition different from that of the liquid. The liquid
composition was constant with depth, but changed with time. The more volatile
material evaporated first, and as a result the rate of evaporation fell as mass
transfer to the gas proceeded. The second limiting condition existed when there
was assumed to be no mixing in the liquid phase. The vapour composition was
equal to that of the liquid, and the liquid composition remained constant with time
and depth providing the evaporation rate remained constant. Physically this
process was envisaged as successive layers of liquid ‘peeling off’ the surface to
form vapour. In practice a situation intermediate between these two conditions
existed, and was controlled by the magnitudes of the evaporation rate and the
liquid diffusion rate.
K.D.O'Hare, P.L. Spea'ding and J. Grimshaw
Experimental Details
In order to measure the evaporation rates of the ethanol and water components
from a mixture thereof, it was necessary to determine both the total amount of
liquid evaporated and the change in composition with time. With a contained body
of liquid, evaporation causes the level of the evaporating surface to decline.
Hinchley and Himus' and Powell and Griffiths4 observed that the presence of a
'rim' affected the evaporation rate of liquid. Therefore, in this work it was
considered desirable to maintain the evaporating liquid surface flush with the top
of the container. For pure liquids, a satisfactory procedure would be to add
make-up liquid during an experimental run, but for a binary liquid mixture this
would alter the liquid composition. The best solution appeared to be to incorporate
a positive-displacement device in the base of the container which could
compensate for the change in volume on evaporation, and thus maintain the
position of the liquid surface.
The experimental system is shown schematically in Figure 1. The main
features of the system were: (i) a wind tunnel; (ii) a novel evaporation cell; (iii) a
density meter.
The evaporation studies were conducted in a horizontal Perspex wind tunnel,
2 m long and 88 mm by 98 mm cross-section. Air was blown through the wind
tunnel by a positive-displacement blower, the air flow rate being set by use of a
bypass system. For flow rates below 80 m3 h-', measurements were taken using
rotameters, while for higher flow rates, a calibrated orifice plate was used. Air
flow rates up to 100 m3 h-' were used in the experiments. The air temperature and
relative humidity entering the wind tunnel were monitored using a digital
thermohygrometer (Solomat 355). All other temperature measurements were
obtained using E-type thermocouples in conjunction with a digital voltmeter. The
air pressure was monitored using a digital micrometer (Air Ltd., model
MP3KDU), while the atmospheric pressure was measured using a Fartin-type
An evaporation cell was machined from a cylinder of solid nylon as detailed
in Figure 2. The evaporating liquid within the cell was moved in a circular pattern
by pumping externally through a peristaltic pump to a density meter
(Stanton-Redcroft, model DMA 602 HT). The bulk temperature of the evaporating
liquid was maintained at approximately the same temperature as the air flowing
through the wind tunnel (+l"C), by using an air bypass system to heat the
evaporation cell and the peristaltic pump. Temperature, pressure and velocity
profiles were recorded to check that the conditions at the gas-liquid interface were
constant, in terms of hydrodynamics and temperature. The ideal experimental
situation was to have a plane liquid surface at constant temperature over the
duration of a run, above which existed a turbulent sublayer, bounded by a
turbulent stream of flowing gas. The bulk liquid was mixed so as to ensure
constant boundary layer conditions at the upper liquid surface.
A fixed reference level was installed at the air-liquid interface in the form of
a stainless steel hypodermic needle. The required liquid level was achieved by
raising the floor of the cell until the liquid interface was just pierced by the needle
point. The movable cell floor was sealed using rubber O-rings. Upward movement
of the floor was obtained and measured using a micrometer screw gauge. In any
time interval, the total amount of liquid evaporated was measured by the
micrometer screw gauge, while the change i n mixture composition was
determined by continuous measurement of the liquid mixture density using the
v -
I P T -
H -
G -
D E P -
c -
B -
A -
Warm Air Enclosure
Peristaltic Pump
Density Meter
Water Bath
Orifice Plate
Pressure Measurement
Temperature Measurement
Point Velocity Measurement
Relative Humidity Measurement
Wind tunnel
K.D. O’Hare, P.L. Spedding and J. Grimshaw
Figure 2 The evaporation cell.
Evaporation of the Ethanol and Water Components
density meter. When the vertical movement was translated into an amount of
liquid evaporated, the overall accuracy was within f0.5%. However, the
composition was obtained to f0.05%.
The evaporation cell contained approximately 200 cm3 of liquid at the
beginning of a run and this diminished by approximately 50% over about 18 h.
The bulk liquid temperature of the evaporating mixture was maintained at a
constant value within f2"C during an experiment, while the average temperature
varied for different experimental conditions.
Further experimental details are given in O'Hare and Spedding."
Results and Discussion
Figure 3 shows the concentration profiles for three typical runs. While the
temperature varied less than 1°C during a run, there was a temperature variation
of approximately 3°C between the three runs; and the atmospheric pressure varied
between runs but remained constant during any given run. Atmospheric pressure
had its largest variation between the first and subsequent runs, resulting in the
curves not coinciding between the conclusion of run 1 and the start of run 2.
However, the profiles in Figure 3 show that the average bulk composition of an
evaporating mixture changed continuously as evaporation proceeded. As the bulk
ethanol concentration decreased during a run, the rate of evaporation of the
ethanol component also decreased. For example, the ethanol evaporation rate
decreased from 1.82 to 1.26 g h-' during run 1. Figure 4 illustrates that the
average component evaporation rates were concentration dependant; the ethanol
10 -
TIME (hours)
Figure 3 Average li uid ethanol concentration (% w h ) against time at an
air-jlow rate of 40
h-'. Ethanol component evaporation rate (start to pnish):
Run I : 1.82-1.26 g h-'; Run 2: 1.09-0.70g h-'; Run 3: 0.50-0.33 g h-I.
K.D. 0 ’Hare, P.L. Spedding and J. Grimshaw
Figure 4 Average evaporation rate (g h-I) against ethanol concentration (% w h )
for both the ethanol and water components of the solution, at an airflow rate
through the wind tunnel of 40 m3 h-I.
component evaporation rate increased markedly with increasing ethanol
concentration. However, the individual evaporation rates of both the ethanol and
water components were less than for the pure liquids alone. The trends shown in
Figure 4 were explained in terms of vapour pressure driving forces as discussed
in O’Hare and Spedding.’’
Figure 5 shows the evaporation rate of the ethanol component per unit mass of
ethanol remaining in the evaporating mixture as a function of ethanol
concentration. Again there was a slight temperature effect and a more pronounced
effect of total pressure particularly affecting run 1. However the data clearly
showed that for a contained body of liquid, the ethanol component evaporation
rate (glh per g ethanol remaining) increased significantly as the ethanol
concentration decreased below about 4% wlw. On Figure 5 , the average
evaporation rate for pure ethanol (obtained for nine experimental runs) is shown
as a reference line in order to illustrate the magnitude of the increased relative
ethanol evaporation. It is important to note that at concentrations less than
approximately 4% wlw ethanol, the ethanol component evaporation rate was
significantly greater than for the pure ethanol alone. The results obtained were
also experimentally significant being well within the experimental accuracy range
of the apparatus.
Detailed calculations of activity coefficients using the method suggested by
Reid et aLZogave a +150% variation in ethanol mass transfer coefficient for run 3,
suggesting that the data obtained in Figure 5 cannot be attributed to the effect of
activity coefficients alone but must be caused by some other effect.
Sanatsky14 studied the evaporation of alcohols from dilute hydrocarbon
solutions and found that the alcohol component evaporation rate was as much as
50 times greater than expected. He postulated that the evaporation of a pure
Evaporation of the Ethanol and Water Components
Pure Ethanol ( 3 1 ° C )
Figure 5 Ethanol component evaporation rate (g h-' per g of ethanol) against
ethanol concentration (% w h ) at an airjlow rate through the wind tunnel of 40
m3 h-'. The horizontal line indicates the corresponding evaporation rate for pure
ethanol at 100% w h for a comparison only.
alcohol was hindered by the molecular attraction due to hydrogen bonding.
However, when the alcohol was mixed with a non-hydrogen bonding compound,
for example toluene, hydrogen bonding of the alcohol molecules was diminished
due to their physical separation, causing the alcohol component to evaporate more
quickly. It follows that the greater the fraction of the alcohol molecules that were
separated through increasing dilution, the faster the effective evaporation rate of
the alcohol became.
In an ethanol-water mixture, there are three types of hydrogen bonding;
ethanol-ethanol, ethanol-water and water-water, listed in order of increasing
strength. In ethanol-water mixtures of low ethanol concentration the results
obtained would be explained if the water-water hydrogen bond became dominant
such that the ethanol-water hydrogen bonds were weakened in comparison, thus
allowing the ethanol component to evaporate more quickly. Infrared spectroscopy
studies failed to identify any valid spectral changes over the concentration range
of interest, even under the most favourable experimental conditions. This is in
agreement with the work of Franks and Ives21 who suggested that the complex
interactions occurring in the alcohol-water system covered a wide continuous
range of energies, and resulted in broad adsorption bands that were virtually
unresolveable. Further, SymonsZ2 concluded from studies on bonding in this
system that IR and NMR studies were unlikely to lead (by themselves) to the
identification of valid mechanisms to explain any unusual experimental
Various data for the system (detailed as follows) have shown that
abnormalities occurred in the low alcohol concentration ran e. The density and
partial molar volume;23 sound velocity and compressibility;2Fheat capacity, heats
of mixing and temperature of maximum density;21 all showed evidence of the
K.D. O'Hare, P.L. Spedding and J. Grimshaw
alcohol molecule being accommodated in existing cavities in the
three-dimensional hydrogen-bonded low-density arrangement of water. The
hydroxyl ion is hydrogen bonded to the enclosing water molecules such that the
hydrophobic alkyl group of the alcohol is masked in a fluctuating clathrate
17H20), thus reinforcing t h e over all
hydrate structure ( C~HSOH.
a ~ ~ a n g e m e n tThe
. ~ ~effect
* ~ ~is?observed
to reach a maximum around 13% wlw
alcohol where presumably the cavity sites in the water are saturated with alcohol.
Such structural arrangements, while being interesting in their own right, act at
bulk concentrations well removed from the enhanced evaporation region reported
in this work, and would in general be expected to minimise rather than enhance
evaporative mass transfer.
Butler and Wightman26 showed that while the surface tension data for low
ethanol-water concentrations superficially appeared to be regular,27 the Gibbs
adsorption equation predicted that the excess alcohol surface concentration rose
significantly in the low alcohol bulk concentration region. Gurney,28and Snavely
and Schmidt29 justified the validity of the equation theoretically and
experimentally, respectively. Later Breitenbach and Edelha~ser,~'
and Scatchard31
confirmed the earlier work of Butler and Wightman,26 while Vergara and
L e ~ p i n a s s eextended
the approach to enable the excess surface alcohol
concentration to be calculated at infinite dilution. These latter workers concluded
that the alcohol molecule was oriented perpendicularly to the interface and that
surface conditions extended only a few molecules depth into the liquid. These
conclusions were in agreement with earlier suggestions made by
upon extensive experimental work, and were later reinforced by theoretical
work.34-37 For example, Gubbins and Thompson34 concluded that with the
perpendicular orientation of the alcohol molecule at the liquid surface, the larger
more strongly interacting atoms were upwards towards the gas phase. There was
also some evidence that on the gas side the molecules lay parallel to the interface.
Such theoretical studies have been used to accurately predict the surface tension
for simple fluids,35 and therefore can be extended with confidence to provide
insight into surface orientation.
It is postulated, therefore, that the increased evaporation reported in this work
for <4% wlw alcohol solutions is caused by excess adsorption of alcohol at the
interface. The authors calculated values of r for alcohol in the surface layer
(where r is Gibbs excess surface adsorption, usually in mol m2, but for constant
volume conditions in A, see ref.31), using the Gibbs adsorption equation, and
showed that the excess alcohol surface concentration rose initially in a consistent
manner up to an inflection point at about 4% wlw alcohol, at which stage about
40% of the surface was occupied by alcohol molecules. Thus the alcohol
preferentially passed to the surface layer, while in the bulk liquid it was being
incorporated into the cavities within the water structure so that very little change
occurred in the molar volumes. In such a structural arrangement, the alcohol at the
interface would be readily available for evaporative mass transfer. Above 4% wlw
alcohol, the rate of increase of r decreased and the evaporation of alcohol became
constant. The calculated excess alcohol concentration in the lower layers of the
bulk liquid under the surface did not become positive until the water cavities in
the bulk phase were completely saturated with alcohol, occurring at about 1,3%
wlw concentration.
The partial molar volume measurements of Franks and Johnson23 for the
aqueous ethanol system, indicated an inflection in the data at about 4-5% wlw
alcohol. This corresponds to the calculated decrease in the rate of change of r
Evaporation of the Ethanol and Water Components
with bulk concentration mentioned above. These changes indicated some
mechanistic alteration of the orientation of the molecules at the liquid surface. It
is possible that the alcohol molecules began to interact with the surface orientation
if there were insufficient water molecules present to provide adequate separation.
For the ethanol-water system, it was experimentally determined that:
(1) the component evaporation rates were concentration dependant;
(2) the composition of the evaporating mixtures changed continuously as
evaporation proceeded;
(3) the rate of change of composition decreased as ethanol concentration
(4) the ethanol component evaporation rate (per unit mass of ethanol remaining)
increased as the ethanol concentration decreased;
( 5 ) at low ethanol concentrations ( ~ 4 %w/w), the ethanol component evaporation
rate (per unit mass of ethanol remaining) was significantly greater than the pure
ethanol evaporation;
(6) the enhanced evaporation was caused by the preferential adsorption of alcohol
% The course of the
on the interface at these lower ethanol concentrations ~ 4 w/w.
adsorption can be predicted using the Gibbs adsorption equation.
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Received: 2 December 1991; accepted: 10 September 1992.
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