Concentration of Bio-Ethanol through Cellulose Ester Membranes during Temperature-Difference Controlled Evapomeation

doi:10.4236/msa.2011.210202

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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.

Email: [email protected]

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

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

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

1PR



 (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



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].





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

ssl

 







(6)

where d



and



are the surface free energies of the

solid and liquid, respectively, and d



, d



and



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

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

100

200

300

400

500

600

0 102030405

Average pore diameter (Å)

Additional solvent content (wt%)

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

100















EtOH in permeate (wt%)

Permeation rate (kg/m2hr)

(a)

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-

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

hr)

(a)

100

0.0

0.4

0.8

1.2

1.6

2.0

300

500

600

Average pore diameter(Å)

EtOH in permeate(wt%)

Permeation rate (kg/m

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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