Iraqi Journal of Chemical and Petroleum Engineering 

 Vol.9 No.4 (December 2008) 1- 6  
ISSN: 1997-4884 

Separation of Hexane-Benzene Mixtures by Emulsion Liquid Membrane. 

Adil A. Al Hemiri
*
and Muhammad D. Al Zaidi. 

*
Chemical Engineering Department - College of Engineering - University of Baghdad – Iraq 

Abstract 

The effect of operating parameters on the batch scale separation of hydrocarbon mixture (benzene and hexane) using 

emulsion liquid membrane technique is reported. Sparkleen detergent was used as surfactant and heavy mineral oil as 

solvent to receive the permeates. 

From the experimental results, the parameters that influenced  the permeation are, composition of feed, contact time 

with solvent, ratio of volume of solvent to volume of hydrocarbon feed, ratio of volume of surfactant solution to volume 

of hydrocarbon feed, surfactant concentration, mixing intensity and glycerol as polar additive in the surfactant solution 

to eliminate drop breakup.   

The best conditions for the separation in this study were found to be: composition of feed (mole fraction of 
benzene=0.5245), contact time of 10min. , ratio of volumes of solvent to feed equal 3.5 , ratio of volumes of surfactant 

solution to feed of 0.4, surfactant concentration of 1wt%, mixing intensity equal 1000rpm and 70% by weight of polar           

additive. These conditions gave a separation factor of (8.0).  
 

Keywords: liquid membrane, hydrocarbon separation, hydrocarbon permeation, emulsion liquid membrane.   

Introduction 

Membranes have been developed which made separation 

process within range of being economically and 

technically feasible. Liquid membrane extraction was 

introduced as an alternative separation technique to the 

liquid –liquid extraction and to   the separation by means 

of solid polymeric membranes. On the other hand, liquid 

membrane was first suggested in 1968 by Li. Liquid 

membranes are generally formed by first     making an 

emulsion of two immiscible phases and then dispersing         

the emulsion in a third phase (continuous phase). 

Emulsion Liquid Membrane (ELM) is effective in 

separation hydrocarbon of different kinds, including 

these similar in their physical and chemical properties. It 

is a film composed of surfactants and their solvent. The 

film consists of water and one or more water soluble 

surfactant. This kind of film serves two purposes in a 

process of separating hydrocarbon. First, it permits 

selective permeation by one or more components of the 

feed. Second, it keeps the hydrocarbon feed from mixing 

with a solvent used to carry away the permeates.  

 

 

 

Furthermore liquid membrane is a film formed at an oil / 

water interface by a surfactant solution .Such films are 

formed by dispersing the solution to be separated in the 
form of droplets in a surfactant solution .The droplets 

covered with an organic solvent phase received the 

permeates .During this process one of the components of 

mixture transfers from the droplets through the liquid 

membrane and into the organic solvent at a faster rate 

than the other .The organic solvent becomes rich in the 

more permeable component, and the droplets become 

rich in the less permeable component , thus achieving a 

separation of the component (15). Where, the separation 

factor is a parameter to evaluate the process efficiency 

with respect to distillation process:-  

Separation factor =
hexaneoftcoefficienondistributi

benzeneoftcoefficienondistributi
 

 

 

University of Baghdad 

College of Engineering 
Iraqi Journal of Chemical 

and Petroleum Engineering 

 

phaseemulsionincontainedmixtureinhexaneoffractionmole

productpermeatedinhexaneoffractionmole

phaseemulsionincontainedmixtureinbenzeneoffractionmole

productpermeatedinbenzeneoffractionmole



Separation of Hexane-Benzene Mixtures by Emulsion Liquid Membrane. 

 

2 
 

Li (1971a) indicated that separation factor is a function 

of the solubility of permeate in water as well as the 

diffusivity of the permeate through the surfactant and 

water layers. Also found that the separation of 

hydrocarbons is independent of surfactant concentration 

in the ranges from (0.001-0.1) wt%.                                 

Li (1971b) studied the effect of glycerol on membrane 

life, permeation rate and separation factor. His results 

showed that 70 wt% of glycerol appear to be optimum 

concentration. Also found that separation factor depends 

on solubility of permeates and its diffusivity. 
Stelmaszek (1977) measured the equilibrium curves for 

benzene and hexane and studied the dependency of the  

separation coefficient of the mixture on the feed 

composition, flow rate, and the effect of the solvent flow 

rate on the volume fraction of hexane in the raffinate and 

permeate was determined. 

 

 

Experimental Work 

In the present study benzene and hexane mixture is 

separated with an aqueous solution of sparkleen 

detergent as the surfactant solution. It contains all sodium 

cations with 50 wt% sodium hexametaphosphate, 10 wt 

% sodium alkyl benzene sulphonate as anionic surfactant 

and 0.5 wt % as nonionic surfactant. It also contains 39.5 

wt % sodium carbonate and sodium bicarbonate as pH 

buffering agents. The heavy mineral oil as the organic 

phase to receive the permeates. In this study the 

effectiveness of the liquid membrane technique to 

separate the hydrocarbon mixture as compared to 

fractional distillation was investigated. 

 In addition, the investigation also included studying the 
effects on separation factor of several process variables, 

such as composition of feed, contact time with solvent, 

ratio of volume of solvent to volume of hydrocarbon 

feed, ratio of volume of surfactant solution to volume of 

hydrocarbon feed, concentration of surfactant, mixing 

intensity between solvent and emulsion and glycerol as 

polar additive to eliminate drop breakup. When the effect 

of one of variable was studied all the other variables were 

kept constant. 

 

Procedure 

Mixtures of benzene and hexane were prepared by 

mixing the required volumes of components by using a 

pipette to measure volumes. Then the mixture was 

emulsified in an aqueous phase composed of sparkleen 

detergent and water. The mixer for emulsification of this 

mixture is a cylindrical vessel equipped with four baffles. 

A variable steady speed impeller with four blades is used 

for mixing. 

 The mixture of benzene and hexane was stirred at low 
speed while the required quantity of surfactant solution is 

slowly poured into the mixer. After the emulsion is 

formed, the emulsion was mixed with the solvent, (heavy 

mineral oil), for the desired length of time. The mixer for 

contacting the emulsion with solvent is a jar test with two 

blade steel impeller with 50rpm to ensure uniform 

contact between the solvent and the emulsion, as shown 

in figure(1). 

As the system is agitated, permeation proceeds out of the 

droplets, through the aqueous film into the bulk solvent 

(heavy mineral oil). Since benzene permeates more 

rapidly through the aqueous liquid membrane than 
hexane, the residual mixture in the emulsion will be 

gradually depleted in hexane, while the heavy mineral oil 

becomes enriched in this constituent. 

 After contacting the emulsion with solvent, the solution 

was transferred to a separator funnel where it was 

allowed to separate. The mixture of solvent and emulsion 

separated into three layers in the separator funnel. The 

upper layer containing the emulsion composed of the 

surfactant solution and that portion of the mixture of 

benzene and hexane that did not permeate in the solvent. 

The middle layer composed of the solvent, heavy mineral 

oil, containing the permeate mixture of benzene and 

hexane. Finally the bottom layer consisted of aqueous 

surfactant solution which had not entered the emulsion 

phase or surfactant solution resulting from droplet 

breakup. 

In this study the effect of polar additive was studied by 

using glycerol to eliminate drop breakup in the solvent. 

The range for glycerol content was from (10-70) wt% in 

the emulsion. 
The recovery of mixture of benzene and hexane from the 

solvent was carried out by simple distillation. The boiling 

point of the solvent was greater than 110
0
C where the 

boiling point of benzene and hexane was less than 81
0
C; 

therefore, if the distillation is carried out at temperature 

between 110
0
C and 81

0
C, essentially all benzene and 

hexane will evaporate leaving the solvent. 

The recovery of mixture of benzene and hexane from the 

emulsion phase was carried out by physical method; i.e. 

by the addition of 0.3 M NaCl to the mixture will break 

the emulsion into two layers. The upper layer contains 

the mixture of benzene and hexane. The bottom layer 

contains the surfactant solution.   

The composition of the mixture of benzene and hexane is 

determined by measuring the refractive index of the 

mixture with the Abbe A60 refractometer. A calibration 

curve relating the refractive index and percent 

composition was prepared and used to determine the 

composition of unknown mixtures.   

 
 

 

 

 

 



Adil A. Al Hemiri and Muhammad D. Al Zaidi 

 

IJCPE Vol.9 No.4 (December 2008) 3 

 

Fig.1 Schematic diagram of operation. 

Results and Discussion 

Effect of Mole Fraction of Benzene in Feed on 

Separation Factor. 

Figure (2) shows the effect of mole fraction of 

benzene(0.1-0.9)  on separation factor. Also figure (3) 

relates the composition of benzene in permeated product 

and in the feed. It can be seen that both the separation 

factor and permeate composition vary with feed 

composition. This is owing to the dependency of 
membrane structure and thickness on the nature of feed. 

Li (1971) refers to the fact that the permeation of the 

molecule through the membrane depends upon the 

solubility of the molecule in the membrane and the 

diffusivity of this molecule through the membrane. 

From this fact, if we say that the only the solubility 

factor effect the separation, this leads us to that the 

separation factor is independent of feed composition 

and this is not reasonable with the results in the figures. 

On the other hand, if we consider only the diffusivity 

factor to effect the transfer and hence the separation, 

then the separation factor changes with feed 

composition to give constant permeate concentration. 

As a result from these facts and the data in figures (2) 

and (3) it can be said that both the solubility and 

diffusivity are important in this case. 

Benzene is more permeable than hexane in water about 

(187.5) times, therefore, it is more permeable than 

hexane through the liquid membrane due to the 

solubility. This results is analogous to the results that 

obtained by Stelmazak. 
Figure (4) shows the vapor liquid equilibrium for 

benzene and hexane. From figures (3) and (4), it can be 

seen that the emulsion liquid membrane separation is 

the best for benzene-hexane mixture separation. 

 

 

 

 

 

 

 

 

Fig. 2 Effect of mole fraction of benzene on separation 

factor. 

 

 

 

 

 

 

Fig. 3 Relationship between mole fraction of benzene in 

feed and solvent. 

 

 

 

 

 

 

Fig. 4 Vapor- liquid equilibrium curve 

(benzene/hexane) 
Y: Vapor phase mole fraction of benzene in hexane  

X: Liquid phase mole fraction of benzene in hexane  

 

mole fraction of benzene

s
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0

1

2

3

4

5

6

7

8

0.0 0.2 0.4 0.6 0.8 1.0

mole fraction of benzene in feed

m
o

le
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ra
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ti
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f 
b

e
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z
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n
t

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.0 0.2 0.4 0.6 0.8 1.0

X

Y

0.0

0.2

0.4

0.6

0.8

1.0

0.0 0.2 0.4 0.6 0.8 1.0



Separation of Hexane-Benzene Mixtures by Emulsion Liquid Membrane. 

 

4 
 

Effect of Contact Times of Emulsion with Solvent on 

Separation Factor.        

Figure (5) shows the effect of contact times on 

separation factor. From the figure we see that for short 

contact times the separation factor attains high values 

but decreases with increasing contact times until there is 

no separation. The maximum value obtained for 

separation factor occurs at contact time of about 10 

minutes. This result is analogous          to that of Li 

(1971). 

The low values of separation factor for short contact 

times are due to that the emulsion of the mixture of 

benzene and hexane with sparkleen surfactant under the 

conditions of the experiment such that some of the 

mixture of benzene and hexane is not emulsified. Thus 

when the emulsion is contacted with the solvent, the 

mixture that not emulsified pass immediately into the 

solvent without separation and leads to a low value of 

separation factor. 

As the contact time increases, the solvent phase 

becomes rich in benzene, therefore, the separation factor 
increases. The decrease in values of separation factor is 

due to the following reasons: the stability of the 

emulsion is limited then the emulsion will break 

allowing all of the benzene and hexane to go into the 

solvent, and the drops become rich in hexane and lean 

in benzene which result in an increase in the transfer 

rate for hexane because of the concentration gradient , 

which will reduce the separation factor. 

 

 

 

 

Conclusions 

         From the present study of using ANN in 
predicting the bubble size in the homogenous region in  

Fig. 5 Effect of contact times on separation factor. 

Effect of Ratio of Solvent to Volume of Hydrocarbon 

Feed 

 

Figure (6) shows the effect of ratio of solvent to feed on 

separation factor. It can be seen that the separation 

factor is low for relatively small volumes of solvent. As 

the relative volume of solvent increases, the separation 

factor increases and reaches a maximum at a ratio of 

(3.5).  

The low values of separation factor is because the 

droplets are close together in the solvent, therefore, the 

drops are more likely to coalesce. This process reduces 

the total surface area for mass transfer. For large 

volumes of solvent the separation factor decreases. This 

decrease is due to the fact that for both components the 

overall driving force (i.e. the difference of the 

concentration of the components between the drops and 

the solvent) increases where the droplets are more 

dispersed, resulting in nearly equal transfer rates for 

both benzene and hexane.  
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Fig. 6 Effect of ratio of volume of solvent to volume of 

hydrocarbon feed on separation factor. 

 

 

Effect of Ratio of Volume of Surfactant Solution to 

Volume of Hydrocarbon Feed 

 

Because the volume of surfactant solution is related 

directly to the droplet size and membrane thickness, the 

separation factor for separating benzene and hexane 

depends on the relative volumes of surfactant solution 

and feed. For a particular volume of feed, there is likely 

some minimum quantity of surfactant solution necessary 

to form a stable emulsion. If the amount of surfactant 

solution is less than the minimum, the separation factor 
is low because not all of the droplets of the mixture of 

benzene and hexane is covered with liquid membrane. It 

is expected that for large volumes of surfactant solution, 

the separation factor does not change (Li, 1968). 

Figure (7) shows experimental results for this ratio. It 

can be seen that separation factor is constant when the 

volume ratio         approximately 0.4. 

 

 

 

 

 

 

contact time(min.)

s
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p

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a
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to

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4.8

5.0

5.2

5.4

5.6

5.8

6.0

0 4 8 12 16 20 24 28

ratio of volume of solvent to volume of hydrocarbon feed

s
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p
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3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

7.5

8.0

1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0



Adil A. Al Hemiri and Muhammad D. Al Zaidi 

 

IJCPE Vol.9 No.4 (December 2008) 5 

 

 

 

 

 

 

 

 

 

 

 
 

 

 

Fig. 7 Effect of ratio of volume of surfactant solution to 

volume of hydrocarbon feed 

Effect of Surfactant Concentration 

Li (1971) found that the separation factor is independent 

of surfactant concentration from (0.001-0.1) wt%. 

Figure (8) shows the experimental work for the relation 

between separation factor and surfactant concentration. 
The results show that separation factor is low for low 

concentration of surfactant, and then increases as 

surfactant concentration is increased. At about 1 wt% 

surfactant concentration, the separation factor is 

maximum, then for higher concentration of surfactant, 

the separation factor decreases. At low surfactant 

concentration, there are related fewer molecules of 

surfactant in the membrane surrounding the droplets, 

therefore, in the absence of enough surfactant molecules 

in the membrane, there is greater chance for breaking of 

the membrane that result in low values of the separation 

factor. 

The decrease in separation factor at higher surfactant 

concentration because that at higher surfactant 

concentration there are enough molecules in the 

membrane around the droplets to make the membrane 

more stable, but when the surfactant concentration is 

high, there is less water in the membrane. Because the 

transfer of permeate through the membrane depends in 

part on the solubility of the material in the water, the 
transfer of these are decreased. And this leads to 

decreasing the separation factor at higher surfactant 

concentration. Also the resistant of surfactant molecules 

in the membrane increases with increased surfactant 

concentration at higher values this leads to a decrease in 

the transfer rates of material through the membrane thus 

decreasing the separation factor. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 
 

 

Fig. 8 Effect of surfactant concentration on separation 

factor. 

Effect of Mixing Intensity  

Figure (9) shows the effect of mixing intensity of 

emulsion on separation factor. It can be noticed that the 

separation factor increases with increasing mixing 

intensity up to 1000 rpm where the separation factor 

becomes constant. 
Depending on the surfactant used, some of the drops 

might breakup in the solvent phase due to rupture of 

weak membrane; therefore, the separation factor will 

decrease for low mixing intensity. But for increasing 

gradually the mixing intensity the emulsion becomes 

more stable than for low mixing intensity because drop 

diameters of emulsion decrease, that also improved the 

permeation rate due to the increasing of surface area for 

mass transfer. It can be seen that above 1000rpm the 

separation factor becomes almost constant. This is due 

to the fact that the droplets reach diameters that do not 

change at higher mixing intensity. The higher mixing 

intensity does stabilize the emulsion but does not 

eliminate drop breakup. 

 

 

 

 

 

 
 

 

 

 

 

 

 

 

Fig. 9 Effect of mixing intensity on separation factor. 

ratio of volume of surfactant solution to volume of hydrocarbon feed

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a

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to

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2

3

4

5

6

7

8

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

concentration of surfactant

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1

2

3

4

5

6

7

8

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4

mixing intensity (rpm)

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4.6

5.0

5.4

5.8

6.2

6.6

7.0

7.4

0 200 400 600 800 1000 1200 1400



Separation of Hexane-Benzene Mixtures by Emulsion Liquid Membrane. 

 

6 
 

Effect of Glycerol as Polar Additive on Separation 

Factor 

Figure (10) shows the effect of glycerol as polar 

additive to strengthen the surfactant membrane resulting 

in substantial elimination of drop breakup in the solvent 

phase and marked increase in separation factor. Adding 

glycerol to a membrane increase the water layer 

viscosity. 

Increasing gradually the percentage of glycerol will 

increase the values of separation factors. The reasons 

for this, is that for low values  of glycerol added the rate 

of drop breakup is more than for high values  of 

glycerol added up to 70% percent. After this percentage 

the surfactant layer becomes too viscous. The decrease 

for permeation rates of components can be controlled by 

increasing time to permeate. These results are analogous 

to that found by Li (1971) and for Li (1971) for that the 

optimum percentage for polar additives is 70wt%. 

 

 

 
 

 

 

 

 

 

 

 

 

 

 

 

Fig. 10 Effect of glycerol as polar additive on separation 

factor. 

 

 

Conclusions 

1. The increase in mole fraction of benzene in feed 
caused an increase in the separation factor up to mole 

fraction of benzene of 0.5245, and then the separation 

factor decreases with the increase of mole fraction. 

2. The separation factor increases with increasing 
contact time  up to contact time of about 10 min. and 

then decreased with increasing time of contact.  

3. The increase in ratio of volume of solvent to volume 

of feed caused an increase in separation factor up to 

ratio of 3.5, then the separation factor decreases with 

the increase of the ratio. 

4. The separation factor increases as the ratio of volume 
of surfactant solution to volume of feed increased up 

to 0.4, then the separation factor is essentially 

constant. 

5. As the surfactant concentration is increased the 

separation factor increases up to concentration of 

(1.0wt %), then the separation factor decreases with 

further increase in surfactant concentration. 

6. The increase in mixing intensity caused an increase in 

separation factor up to 1000 rpm then the separation 

factor is approximately constant. 

7. Glycerol is used as polar additive to eliminate drop 
breakup in solvent. The separation factor increased 

with the increase in weight percent of glycerol in 

surfactant solution up to 70%wt. Further increase in 

weight percent of glycerol make the liquid membrane 

is so viscous that no separation can be achieved. 

 

References 

1. Hala, E., Wichterle, I.  Polka. and Boublik, T., 
1968, "Vapor liquid equilibrium data at normal 

pressures", Britain, first edition.  
2. KralJ, D. and Brecevic, L., (1998), 

"Precipitation of some slightly soluble salts 

using emulsion liquid membrane ", Croatia 

chemical ACT, vol. 71, p.p. (1049). 

3. Li, N.N, Somerset and En's, (Nov., 12, 1968) 
"Separation of hydrocarbon with liquid 

membrane", U.S.Pat, 3 410 794. 

4. Li, N.N, (1971) "Permeation through liquid 
surfactant membranes", AICHE, vol. 17, p.p. 

(459). 

5. Li, N.N, (1971), "Separation of hydrocarbon by 
liquid membrane permeation", 

Ind.Eng.Chem.Process.Des.Develope, vol.12, 

p.p. (215). 

6. Mcauliffe, C., (1986) "Solubility in water of 
paraffin, cycloparraffin, olefin, acetylenes, 

cycloolefins and aromatic hydrocarbons", 

J.Phys.Chem. vol.70, P.P. (1267). 

7. Stelmaszek, J., (1977), "Separation of liquid 
mixtures by liquid membranes in a column 

apparatus", J.Memb.Sci,   vol.2, P.P. (197). 
8. Ward, W.J, (1970), "Analytical and 

experimental studies of facilitated transport ", 

AICHE vol. 16, p.p. (405). 

 

 

Glycerol wt%

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7

8

9

10

11

12

13

14

15

16

0 10 20 30 40 50 60 70 80