Materials Sciences and Applicatio ns, 2011, 2, 1499-1506
doi:10.4236/msa.2011.210202 Published Online October 2011 (http://www.SciRP.org/journal/msa)
1499
Concentration of Bio-Ethanol through Cellulose
Ester Membranes during Temperature-Difference
Controlled Evapomeation
Tadashi Uragami
Department of Chemistry and Materials Engineering, Kansai University, Osaka, Japan; Development of Innovative Science and
Technology, Kansai University, Osaka, Japan.
Received July 4th, 2011; revised July 29th, 2011; accepted August 25th, 2011.
ABSTRACT
To evaluate the high-performance of membrane materials in the concentration of an aqueous solution of dilute bio-
ethanol under temperature-difference controlled evapomeation (TDEV), asymmetric porous cellulose nitrate (CN) and
cellulose acetate (CA) membranes were prepared by a phase inversion method. In the concentration of dilute ethanol
under TDEV, these membranes showed a high permeation rate and high ethanol/water selectivity. In membranes with
almost the similar pore size, the ethanol/water selectivity was considerably higher for the CN membrane than the
corresponding CA membrane. This result suggested that the affinity between the membrane material and the permeant
is an important factor in the separation selectivity.
Keywords: Bio-Ethanol, Concentration, Membrane, Cellulose Ester, Temperature-Difference Controlled Evapomeation
1. Introduction
Ethanol that can be produced by biomass fermentation is
a clean energy source, but typically its concentration is
about 10 wt% in aqueous solution. To use such an
aqueous ethanol solution as an energy source, we need to
concentrate the dilute solution by distillation. However,
distillation requires a high energy input. If the dilute
ethanol solution could be concentrated by ethanol/water
selective membranes with high permeation rate and high
ethanol selectivity, this would offer a significant energy
saving.
We have been studying the permeation and separation
characteristics of aqueous ethanol solutions passed
through various hydrophilic and hydrophobic polymer
membranes. Hydrophilic membranes can selectively re-
move water from aqueous solutions of high ethanol con-
centration [1-6], whereas hydrophobic membranes allow
ethanol permeation from aqueous solutions of dilute
ethanol [7-9].
We have developed evapomeation (EV) [10,11], as
shown in Figure 1(b), as a new membrane separation
technique which utilizes the advantages of pervaporation
(PV) (see Figure 1(a)) while limiting its disadvantages.
We have also proposed temperature-difference controlled
evapomeation (TDEV) [12-21] in which a temperature
difference is established between the feed solution and
the membrane surroundings as shown in Figure 1(c).
Poly(dimethylsiloxane) [7,8] and modified poly(dime-
thylsiloxane) [9] membranes in TDEV showed high
ethanol/water selectivity for aqueous solutions of dilute
ethanol.
In this study, to evaluate membrane materials in the
concentration of bio-ethanol in TDEV, we selected cel-
lulose nitrate (CN) and cellulose acetate (CA) as mem-
brane materials, which have different affinities for etha-
Feed solution
Vacuum
Feed vapor
Membrane
Vacuum
Membrane
Feed solution
(a) PV(b) EV(c) TDEV
Feed solution
Vacuum
Feed vapor
Membrane
(A)
(B)
Figure 1. Schematics showing fundamental principles of PV,
EV and TDEV.

Concentration of Bio-Ethanol through Cellulose Ester Membranes during Temperature-Difference 1500
Controlled Evapomeation
nol. Specifically, we describe in detail the permeation
and separation characteristics of asymmetric porous CN
and CA membranes during TDEV of an aqueous solution
of dilute ethanol, and discuss the results in terms of the
physical and chemical structures of the cellulose ester
membranes.
2. Experimental Section
2.1. Materials
As membrane materials, cellulose nitrate (CN) having a
nitro content of 11.5% was obtained from Daicel Che-
mical Industry and recrystallized from methanol/water,
and cellulose acetate (CA) having an acetyl content of
40.1% was obtained from Wako Chemical Indus- tries
and recrystallized from acetone/water. Commercial me-
thanol, 1,4-dioxane, and formamide from Wako Chemi-
cal Industries were used as solvents for casting solutions.
Poly(ethylene glycol) (PEG) samples of 1000, 4000, and
6000 in average Mw were obtained by Sanyo Chemical
Industries for use as feed solutes in ultrafiltration ex-
periments.
2.2. Preparation of Asymmetric Porous CN and
CA Membranes
The asymmetric porous membranes were prepared by a
phase-inversion method [22,23]. The casting solutions of
the CN and CA membranes were consisted of CN (13
wt%) and a mixture of methanol and 1,4-dioxane (as a
non-solvent additive), and CA (13 wt%) and a mixture of
1,4-dioxane and formamide (as a non-solvent additive),
respectively. The non-solvent additives in the casting
solutions are selected as poor solvents for the membrane
materials. Asymmetric porous CN and CA membranes
were made by pouring the casting solutions onto a glass
plate, drawing a blade across the plate, allowing the sol-
vent to evaporate for 10 sec at 25˚C, and immersing the
glass plate together with the membrane into cold water
(6˚C - 7˚C) as a gelation medium.
2.3. Permeation Measurements
The apparatus for the TDEV experiments is described in
a previous paper [7,8]. Figure 2 shows the permeation
cell for the TDEV experiments. The TDEV experiments
were performed by maintaining the temperature of the
feed solution at 40˚C and changing the temperature of the
membrane surroundings while maintaining a pressure of
665 Pa at the downstream side. The ethanol concentra-
tions in the feed solution and permeate were measured
using gas chromatography (Shimadzu GC-9A).
2.4. Scanning Electron Microscopy (SEM)
Asymmetric porous CN and CA membranes were coated
1
2
3
5
6
7
4
4
4
4
5
6
To vacuum system
1. feedsolution, 2.membrane, 3.poroussupport,4.coolingjacket,
5. Heatingjacket,6. concentrationcontrollingsystem ,7. stirrer
Figure 2. Permeation cell for TDEV.
with a layer of evaporated gold and the lyophilized
membranes were observed through a scanning electron
microscope (Japan Electron Optics, JEOL 100CX).
2.5. Determination of Average Pore Size of
Asymmetric Porous Membranes
Porous membranes are known [24-26] to be subject to
complex hydro-dynamic flow through pores of various
sizes, and often a general mathematical analysis is used.
In general, when a viscous liquid flows through a narrow
pore, the flux of liquid, u, is represented by the Hagen-
Poiseuille equation [24]:
232
c
uQpgr l

  (1)
where r is the pore diameter, l is the length of the pore
(skin layer thickness), η is the viscosity of the liquid, Δp
is the pressure difference between the upstream solution
and the downstream side of the membrane, ρ is the liquid
density, gc is the gravitational conversion factor, and Q is
the flux.
The permeation of liquid through the skin layer of
asymmetric porous CN and CA membranes may be due
to both diffusive and viscous flows. Therefore, in this
case, the permeation rate (PR) of the liquid does not cor-
respond exactly to the flux (Q) given in Equation (1).
However, if we assume that the flow through the skin
layer of asymmetric porous CA and CN membranes fol-
lows viscous flow, Equation (3) can be obtained from

Concentration of Bio-Ethanol through Cellulose Ester Membranes during Temperature-Difference 1501
Controlled Evapomeation
Equations (1) and (2), as
2
1PR

 (2)
2
PR Q (3)
By substituting Equation (3) into Equation (1), the av-
erage apparent pore diameter in CA and CN membranes
is given by

12
32 c rlrQpg


(4)
where Q' is PR1/2, and γ is η/ρ (the kinetic viscosity).
The average apparent pore diameter in the skin layer
of asymmetric porous CA and CN membranes can be
estimated using Equation (4) and the permeation flux for
water at various temperatures.
2.6. Measurement of Contact Angle
The contact angles of water and methylene iodide on the
surface of asymmetric porous CA and CN membranes
were measured by a contact angle meter (Erma, model
G-1) at 25˚C. The contact angles, θ, were determined
from the advancing contact angle (θa) and receding con-
tact angle (θr) by Equation (5) [27].
 
1
cos coscos2ar 

 (5)
In this study, the surface free energy was obtained
from the contact angles of water and methylene iodide
using Equation (6), which was proposed by Owens et al.,
[28] as follows:
 

12 12
1cos2 dd pp
lslsl
dp
ssl
 



(6)
where d
s

and
p
l

are the surface free energies of the
solid and liquid, respectively, and d
s

,
p
s

, d
l

and
p
l

are the dispersion force components and polar force
components of the surface free energies of the solid and
liquid, respectively. The values for these components
have been reported by Forkes [29,30].
2.7. Determination of Amount of Bound Water
in Membranes
The amount of bound water in the asymmetric CA and
CN membranes was measured using the water content of
the membrane and the heat of fusion obtained from fro-
zen water in the membrane, ΔH, which was determined
by differential scanning calorimetry (DSC) (Rigaku,
TAS-200) based on reports by Frommer [31].
2.8. Ultrafiltration
Ultrafiltration experiments were performed by the me-
thod reported in a previous paper [32]. The effective
membrane area active in the cell was 12.7 cm2. All ex-
periments were for the short-run type. The rejection per-
centage (R) is defined as


1 100
tf
RCC (7)
where Cf is the starting feed concentration and Ct is the
concentration in the permeate after t hours.
3. Results and Discussion
3.1. Structure of CN and CA Membranes
Figure 3 shows SEM images of cross sections of the CN
and CA membranes formed by different amounts of the
non-solvent additive in the casting solution. As can be
seen from these images, the membrane structures are
significantly influenced by the amount of non-solvent
and the resulting membranes obtained from the wet
method consist of a skin layer on the air side and a sponge
layer on the glass-plate side. The examples shown are
typical of asymmetric membranes.
3.2. Average Pore Diameter in Skin Layer of
Asymmetric Porous CN and CA Membranes
Figure 4 shows the effects of the non-solvent additive in
the casting solution on the average pore diameter of
asymmetric cellulose ether membranes. As can be seen,
as the amount of non-solvent is increased, the average
pore diameter of the asymmetric CA membrane increases
remarkably, whereas that of the asymmetric CN mem-
brane hardly changes. These results suggest that the for-
mamide added to the casting solution in preparation of
the CA membrane is effective as a non-solvent in relation
to CA. Consequently, more sponge like asymmetric CA
membranes could be formed. In contrast, the 1,4-dioxane
used in the preparation of the CN membrane does not
10wt%20wt% 30wt%40wt% 50wt%
0wt%10wt%20wt% 30wt%40wt%
CN membranes
CA membranes
100mm
100μm
Figure 3. Effect of the amount of non-solvent additive in the
casting solution on the cross-section structure of asymmet-
ric CN and CA membranes as observed by SEM.

Concentration of Bio-Ethanol through Cellulose Ester Membranes during Temperature-Difference 1502
Controlled Evapomeation
0
100
200
300
400
500
600
0 102030405
Average pore diameter (Å)
Additional solvent content (wt%)
0
Figure 4. Effect of the amount of non-solvent additive in the
casting solution on the average pore diameter in the skin
layer of asymmetric CN () and CA () membranes.
appear to be effective as a non-solvent in relation to CN.
3.3. Surface Free Energy and Bound Water of
CA and CN Membranes
Table 1 shows the contact angles for water and methyl-
ene iodide on the surface skin layer of the asymmetric
porous CN and CA membranes, along with the surface
free energies calculated from these contact angles. The
surface free energy for the CN membrane was lower than
that for the CA membrane. This indicates that the CN
membrane is more hydrophobic and consequently has a
higher affinity for ethanol over water in an aqueous
ethanol solution. Table 2 summarizes the amounts of
bound water in the asymmetric porous CN and CA
membranes as determined by DSC measurements. The
amount of bound water in the CN membrane is found be
considerably lower than that in the CA membrane. The
results of the surface free energy and amount of bound
water suggest that the CN membrane is more hydropho-
bic than the CA membrane.
3.4. Effect of Temperature of Membrane
Surroundings in TDEV
Figure 5 shows the permeation and separation character-
istics for an aqueous solution of 10 wt% ethanol passed
through asymmetric CN and CA membranes during
TDEV, in which the temperature of the feed was kept at
40˚C and the temperature of the membrane surroundings
was varied. As can be seen in the figure, when the tem-
perature of the membrane surroundings is decreased, the
ethanol concentration in the permeate increases but the
permeation rate decreases in both the CN and CA mem-
branes. Also both the ethanol concentration in the per-
meate and the permeation rate were higher in the CN
membrane than in the CA membrane. The decrease in
Table 1. Contact angles and surface free energies of asym-
metric CN and CA membranes.
Contant angle (deg) Surface free energy (erg/cm2)
membrane
H2O CH3O γd γp γh
CA 56.2 29.8 35.2 16.4 51.6
CN 63.4 36.3 33.7 12.8 46.5
Table 2. Amount of bound water in CN and CA mem-
branes.
Membrane Amount of bound water
(mol water/mol repeated unit in polymer)
CA 2.58
CN 0.67
0
20
40
60
80
100







EtOH in permeate (wt%)
Permeation rate (kg/m2hr)
(a)
0
20
40
60
80
100
0.0
0.4
0.8
1.2
1.6
2.0
20 2530 3540 45
Temperature of membrane surroundings(oC)
EtOHin permeate (wt%)
Permeation rate (kg/m2hr)
(b)
Figure 5. Effects of the temperature of the membrane sur-
roundings on the permeation rate and ethanol concentra-
tion in the permeate for an aqueous solution of 10 wt%
ethanol passed through asymmetric porous CN (a) and CA
(b) membranes during TDEV. The temperature of the feed
was kept at 40˚C and that of the membrane surroundings
was varied. The reduced pressure on the downstream side
was 650 Pa. The average pore diameters of asymmetric CN
and CA membranes were 280 and 284 Å, respectively.

Concentration of Bio-Ethanol through Cellulose Ester Membranes during Temperature-Difference 1503
Controlled Evapomeation
permeation rate that occurs with a decrease in tempera-
ture of the membrane surroundings could be due to the
fact that the increase in temperature difference between
the feed and the feed vapor would lead to a decrease in
the pressure difference between the feed side and the
permeate side.
The increase in ethanol/water selectivity that occurs
with decreasing temperature of the membrane surround-
ings in TDEV can be attributed to the tentative mecha-
nism shown in Figure 6. That is, when water and ethanol
molecules, vaporized from the feed solution, approach
the lower-temperature membrane surroundings, the water
vapor aggregates more readily than the ethanol vapor
because the freezing point of water is much higher than
that of ethanol. As such, the aggregated water molecules
tend to liquefy as the temperature of the membrane sur-
roundings becomes lower. On the other hand, because
the membrane has a relatively high affinity to the ethanol
molecules, they are absorbed within the pores of the
membrane, and an absorbed layer of ethanol is formed in
the initial stage of permeation. Consequently, the vapor-
ized ethanol molecules may be able to permeate across
the membrane by surface diffusion on the absorbed layer
of the ethanol molecules within the pores.
Both mechanisms, aggregation of the water molecules
and surface diffusion of the ethanol molecules in the
pores, are considered to be responsible for the increase in
the ethanol/water selectivity in TDEV. The level of in-
crease in ethanol/water selectivity would then be related
to the degree of water aggregation on the membrane sur-
roundings and the thickness of the absorbed layer of
ethanol within the pores, which are significantly gov-
：H2O：EtOH
Membran e
Feed solution
Lower
temperature
Higher
temperature
Figure 6. Tentative mechanism for the concentration of
aqueous ethanol solutions through asymmetric porous CN
and CA membranes in TDEV.
erned by the temperature of the membrane surroundings.
When the temperature of the membrane surroundings
becomes lower, the degree of water aggregation and the
thickness of the adsorbed layer of ethanol are increased,
corresponding to an increase in the ethanol/water selec-
tivity for aqueous ethanol solutions. The observation that
both the ethanol concentration in the permeate and the
permeation rate were higher in the CN membrane than in
the CA membrane can be easily understood based on the
results of the surface free energy and amount of bound
water obtained in the CN and CA membranes as shown
in Tables 1 and 2.
3.5. Effect of Average Pore Diameter on
Permeation and Concentration
Characteristics in TDEV
Figure 7 shows the permeation and concentration char-
0
20
40
60
80
100
0.0
0.4
0.8
1.2
1.6
2.0
260 280300 320
Average pore diameter(Å)
EtOH in permeate(wt%)
Permeation rate (kg/m
2
hr)
(a)
0
20
40
60
80
100
0.0
0.4
0.8
1.2
1.6
2.0
300
4
00
500
600
Average pore diameter(Å)
EtOH in permeate(wt%)
Permeation rate (kg/m
2
hr)
(b)
Figure 7. Effects of the average pore diameter in the skin
layer of asymmetric CN (a) and CA (b) membranes on the
permeation rate and ethanol concentration in the permeate
for an aqueous solution of 10 wt% ethanol during TDEV, in
which the temperatures of the feed and the membrane sur-
roundings were 40˚C and 25˚C, respectively. The reduced
pressure on the downstream side was 650 Pa.

Concentration of Bio-Ethanol through Cellulose Ester Membranes during Temperature-Difference 1504
Controlled Evapomeation
acteristics for an aqueous solution of 10 wt% ethanol
passed through asymmetric CN and CA membranes dur-
ing TDEV as a function of the average pore diameter in
the skin layer of asymmetric cellulose ester membranes.
In the experiments, the temperature of the feed was 40˚C
and that of the membrane surroundings was 20˚C. As can
be seen in both the CN and CA membranes, when the
average pore diameter decreases, the ethanol concentra-
tion in the permeate increases and the permeation rate
decreases. The decrease in permeation rate with decreas-
ing average pore diameter is likely due to a decrease in
the pore flow through these membranes.
3.6. Chemical and Physical Structure of
Asymmetric Porous Membranes
Table 3 shows the effects of the chemical and physical
structure on the permeation rate and ethanol concentra-
tion in the permeate through asymmetric porous CN and
CA membranes by TDEV for membranes with almost
similar pore size and similar rejection values for some
PEGs in ultrafiltration. These results suggest the physical
structures of the CN and CA membranes are the same.
As can be seen from the ethanol concentrations in the
permeate in Table 3, it is found that the CN membrane
has a higher ethanol/water selectivity than the CA mem-
brane. This result can be attributed to the fact that the CN
membrane is more hydrophilic than the CA membrane as
mentioned above.
4. Conclusions
When asymmetric porous CN and CA membranes were
applied to TDEV in the concentration of an aqueous
ethanol solution, a high permeation rate and high etha-
nol/water selectivity were obtained. The characteristics
of permeation and separation through the CN and CA
membranes were significantly influenced by the tem-
perature of the membrane surroundings. In the CN and
Table 3. Characteristics of asymmetric porous CN and CA
membranes.
Membrane
CN CA
Average pore diameter (Ǻ) 280 284
Rejection
PEG 1000 13 11
PEG 4000 24 23
PEG 6000 46 46
EtOH in permeate 61 48
CA membranes, the ethanol/water selectivity was attrib-
uted to both aggregation of water molecules on the
lower-temperature membrane surroundings and surface
diffusion of the ethanol molecule via an absorbed layer
of ethanol formed within the pores. The permeation rate
and ethanol/water selectivity of the asymmetric porous
membranes were significantly influenced by the degree
of water aggregation, the thickness of the absorbed layer
of ethanol and the pore size of the membrane. In this
study, we have demonstrated that a high permeation rate
and high ethanol/water selectivity can be achieved by
applying porous membranes to TDEV. CN membranes,
in particular, offer potential for selective separation of
ethanol/water.
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Controlled Evapomeation
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