Iraqi Journal of Chemical and Petroleum Engineering 

 Vol.15 No.2 (June 2014) 27- 37  

ISSN: 1997-4884 

 
 
 
 

Research Octane Number Improvement of Iraqi Gasoline by 

Adsorption of n-Paraffins Using Zeolite Molecular Sieves 

  
Nada Sadoon Ahmedzeki and Arowa Salah Alddin Mahdi 

Chemical Engineering Department, College of Engineering, University of Baghdad, Baghdad, 

Iraq 

 
Abstract 

   This study focused on the improvement of the quality of gasoline and enhancing its 

octane number by the reduction of n-paraffins using zeolite 5A. This study was made 

using batch and continuous mode. The parameters which affected the n-paraffin 

removal efficiency for each mode were studied. Temperature (30 and 40 ˚C) and 

mixing time up to 120 min for different amounts of zeolite ranging (10-60 g) were 

investigated in a batch mode. A maximum removal efficiency of 64% was obtained 

using 60 g of zeolite at 30 ˚C after a mixing time 120 min. The effect of feed flow rate 

(0.3-0.8 l/hr) and bed height (10-20 cm) were also studied in a continuous mode. The 

equilibrium isotherm study was made using different amounts of zeolite (2-20 g) and 

then simulated for n-hexane and n- heptane using Langmuir, Freundlich isotherms. 

Kinetic of the adsorption of n-hexane and n-heptane was investigated using pseudo 

first order, pseudo second order and intra particle model. The Research octane number 

(RON) for some selected samples was measured to show the effect of the removal on 

the quality improvement of gasoline. Results showed an increase of 5.5 units in RON 

using 40 g zeolite at 40 ˚C and after 120 ˚C min. also an increase in 7 units was 

obtained in the continuous mode after 30 min using flow rate of o.3 l/hr and bed 

height of 10cm. Experimental data for n-hexane and n-heptane was found to fit 

Langmuir isotherm with correlation coefficient of 92.7% and 88.8% for n-C7 and     

n-C6, respectively.  Adsorption of n-hexane and n-heptane was found to follow 

second order kinetics with correlation coefficient of 99.9% and 99.8% for n-C7and n-

C6 respectively.  This study revealed a favorable adsorption of n-paraffins on zeolite 

5A as indicated by the separating factor of 0.6 and 0.44 for n-C7and n-C6, 

respectively.  
 

Keywords: Octane number upgrading, Adsorption, Zeolite 5A. 
 

Introduction 

   Separation and purification of 

mixtures by selective adsorption of one 

or more of its components on a micro- 

or meso-porous solid adsorbent is a 

major unit operation in the chemical, 

petrochemical, environmental, 

medical, and electronic gas industries. 

The main reasons are as follows: 

Commercial availability of a spectrum 

of porous adsorbents (zeolites, 

activated carbons, silica and alumina 

gels, polymeric sorbents, etc.) which 

offer many different choices of core 
adsorptive properties (equilibria, 

kinetics, and heats) for a given gas 

Iraqi Journal of Chemical and 
Petroleum Engineering 

 

University of Baghdad 
College of Engineering 



Research Octane Number Improvement of Iraqi Gasoline by Adsorption of n -Paraffins Using 

Zeolite Molecular Sieves  

28                                        IJCPE Vol.15 No.2 (June 2014)               -Available online at: www.iasj.net 
 

separation or purification application. 

Zeolite 5A is used in two major 

separation processes. The first is the 

separation of linear and iso- paraffins 

by allowing the former to pass through 

the channels and the latter is not. The 

second process is air separation where 

the equilibrium loading of Nitrogen on 

the 5A zeolite is much greater than that 

of Oxygen and Argon [1]. 

   In spite of the potential advantages of 

adsorptive separation, it did not 

achieve wide commercial acceptance 

until the introduction of the UOP 

Sorbex process in the early 1960s 

Invented by UOP in the 1960s, the 

Sorbex technique was the first large-

scale commercial application of 

continuous adsorptive separation. The 

first commercial Sorbex unit, a Molex 

unit for the separation of linear 

paraffins, came on-stream in 1964. 

Most applications of the Sorbex 

process deliver high-purity products 

that can be sold or used in downstream 

technologies. This process offered a 

truly continuous adsorptive separation 

process that produced products with 

essentially constant compositions [2]. 

Octane rating or Octane number is a 

standard measure of the performance 

of a motor or aviation fuel. The higher 

the Octane number, the more 

compression the fuel can withstand 

before detonating. In broad terms, fuels 

with a higher octane rating are used in 

high-compression engines that 

generally have higher performance. In 

contrast, fuels with low octane 

numbers are ideal for diesel engines. 

Use of gasoline with low octane 

numbers may lead to the problem of 

engine knocking [3]. 

The most common type of octane 

rating worldwide is the Research 

Octane Number (RON). RON is 

determined by running the fuel in a test 

engine with a variable compression 

ratio under controlled conditions, and 

comparing the results with those for 

mixtures of iso-octane and n-heptane. 

Modern automobile engines require 

high octane gasoline for efficient 

operation. Previously lead had been 

added to gasoline to increase the 

octane number. However, with the 

removal of lead from the gasoline pool 

due to environmental concerns, other 

methods for increasing the octane 

number are needed. However, MTBE, 

while generally less toxic than lead, 

has been linked to ground water 

contamination. At the same time, some 

of the high octane components 

normally present in gasoline, such as 

benzene, aromatics, and olefins, must 

also be reduced. The coking gasoline is 

usually used as the blending 

components of automobile gasoline [4]
 

or the feed of the catalytic reforming 

process after being hydrogenated. 

Obviously, a process which will 

increase the Octane of gasoline without 

the addition of toxic or 

environmentally adverse substances 

will be preferable [5-7].  

   Some versions of UOP’s 

hydroisomerization processes use 

highly active zeolite-based, Pt 

containing hydroisomerization 

catalysts, such as UOP I-7, which 

contains modified synthetic (large-

port) mordenite. In the presence of 

hydrogen at moderate conditions, such 

catalysts optimize isomerization and 

minimize hydro cracking [8]. To 

further increase octane level, products 

from a hydroisomerization unit can be 

sent to the Molex process, where the 

remaining lower-octane, linear 

paraffins are separated from the other 

compounds by using a zeolite 

adsorbent and a liquid desorbent; the 

Molex process is an example of UOP’s 

Sorbex simulated- moving-bed 

technology [9]. The extracted linear 

paraffins are recycled to the hydro 

isomerization unit, and the remaining 

higher-octane fraction is recovered for 

gasoline blending. The combination of 

http://en.wikipedia.org/wiki/Engine_knocking
http://en.wikipedia.org/wiki/Engine
http://en.wikipedia.org/wiki/Compression_ratio
http://en.wikipedia.org/wiki/Compression_ratio


Nada Sadoon Ahmedzeki and Arowa Salah Alddin Mahdi 

 

-Available online at: www.iasj.net                      IJCPE Vol.15 No.2 (June 2014)                                 29 
 

the hydroisomerization and Molex 

processes boosts the research octane 

number of a typical feed from 68–70 to 

89–92. Shell and Union Carbide are 

commercially using zeolite CaA 

(¼4.6A°, also referred to as zeolite 5A) 

in the separation of linear paraffins 

from iso-paraffins in their TIP process 

(Total Isomerization Process). In this 

process, the linear C5 and C6 paraffins 

were first isomerized to the 

thermodynamic equilibrium where 

after the linear paraffins are selectively 

adsorbed on zeolite CaA and recycled 

to the isomerization reactor [10]. The 

isomerization catalyst of the TIP 

process is zeolite Pt-H-Mordenite, 

showing the catalytic potential of 

Zeolites. In this case, the zeolite not 

only serves as a support for the 

platinum particles but also as a solid-

acid catalyst [11]. The TIP process 

(>100 plants worldwide) is an 

excellent example of the versatile 

nature of zeolites in industrial 

processes. The demand for new zeolite 

is, however, modest because of the 

long service life of the separating and 

the isomerizing zeolites. To achieve 

higher octane levels, UOP’s TIP total 

isomerization process uses the once-

through isomerization process 

combined with UOP’s Iso Siv process, 

which uses size-selective zeolite 

adsorption of the unreacted linear 

paraffins so that they can  be  recycled 

and converted  to extinction. Both the 

TIP and Iso Siv processes originally 

were developed at Union Carbide’s 

Molecular Sieve Department, now part 

of UOP. [12].  

This study represents a first unique 

study of commercial light naphtha 

obtained from Al Dora refinery as an a 

attempt to upgrade the quality of 

gasoline by improving its research 

octane number. The objective of the 

present work is to study the removal of 

n-paraffins from Iraqi light naphtha 

using zeolite 5A in batch mode and 

continuous mode adsorption 

experiments. In the batch mode, the 

influence of   temperature, zeolite 

dosage and agitation time were studied 

while in the continuous mode, the 

influence of bed height and feed flow 

rate on the removal of n-alkanes were 

studied in a fixed bed adsorber. For 

both modes, the effect of removal 

efficiency on RON was tested for 

selected samples. The equilibrium 

adsorption isotherms using the most 

common models and the kinetics for 

the adsorption of n-hexane and n-

heptane using pseudo-first- order, 

pseudo- second-order and intra-particle 

diffusion were also investigated. 
 

Experimental Work 

   Hydrogenated light naphtha, 

consisting 35.134 wt. % of total n-

paraffins, was supplied by Al-Durra 

Refinery. Zeolite 5A was supplied by 

Arab company for detergent chemicals 

(0.833 mm white spherical, pore 

volume 11.1% (V)). The zeolite 

samples were activated by heating for 

2 hours in a stepwise manner from 

room temperature to 200 ˚C. 
 

Equilibrium Study 

   The procedure involved filling five 
flasks with 50 ml of hydrogenated light 

naphtha. Amounts of zeolite 5A (2, 5, 

10, 15, and 20 gm) were added into the 

flasks, sealed and shaken for 24 hours 

at speed 200 rpm. The supernatant 

after filtration by Buchner funnel was 

analyzed for the remaining amount of 

n-paraffins (pentane, hexane and 

heptane) by gas chromatography. The 

equilibrium amounts of n-paraffins 

sorbed by zeolite qe (g/g zeolite.) in 

the sorption system were calculated 

using the mass balance: 
 

qe = V (Co-Ce) /m                         …(1) 
 

   Where V is the solution volume (l), 

m is the amount of sorbent (g), and Co 

and Ce (g/l) are the initial and 



Research Octane Number Improvement of Iraqi Gasoline by Adsorption of n -Paraffins Using 

Zeolite Molecular Sieves  

30                                        IJCPE Vol.15 No.2 (June 2014)               -Available online at: www.iasj.net 
 

equilibrium concentration, 

respectively. 

 

Adsorption Kinetic Experiments  

Batch Mode 

   A specified amounts of the zeolite 

(10, 40, 50 and 60 g) were added in a 

500 ml conical flask containing a 300 

ml of light naphtha and temperature 

was set as 30 or 40 ˚C. Samples were 

withdrawn at different agitation time 

(15, 30, 60, 90, 120 and180 min). Each 

samples was filtered using Whatman-

31 filter paper and analyzed for total n-

paraffins concentration. A schematic 

representation of the batch adsorption 

unit is shown in Fig. 1. 
 

 
Fig. 1: Schematic diagram of laboratory batch 

adsorption unit 

 

Continuous Mode 

   A 2.54 cm quartz column and 35cm 

long, packed with zeolite 5A was used 

as the adsorption column.  About 41.2 

and 82.4 g of the 5A zeolite were 

packed in the vertical column having a 

bed length of 10 and 20 cm, 

respectively. Down flow conditions 

were used in the experiment. The feed 

of light naphtha was pumped into the 

fixed-bed reactor through a distributer 

at a flow rate of 0.3 and 0.8 liter hr
−1

  

as measured by a rotameter N-paraffin 

content of the effluent was sampled at 

different operating intervals up to 

120min and analyzed by GC. Fig .2 

shows a diagram of the fixed bed 

adsorption unit. 

 
Fig. 2: Schematic diagram of   laboratory fixed 

bed adsorption unit 

 

Results and Discussions 

Batch Mode 

 

Effect of Contact Time 

   The total removal efficiency of n-

paraffins is presented in Figs. 3 to 6. It 

can be seen that the removal by 

adsorption process proceeded in two 

stages. An initial rapid increase in the 

first 15 minutes of contact time is very 

sharp, and becomes slower and the 

uptake rate gradually decreases with 

the increase of contact time. This is 

due to the continuous filling of the 

adsorption sites as the time proceeds 

and to continuous uptake of these 

materials until reaching equilibrium. 

Some fluctuations are seen around 

equilibrium due to the adsorption 

desorption phenomena. Moller et al. 

[13] had obtained such results by 

studying the sorption uptake of n-

butane and n-hexane on zeolite 5A and 

ZSM-5.  

 

 
Fig. 3: The relation between the removal 

efficiency of total n-paraffins with temperature 

using 10 g of zeolite in 300ml of Light naphtha 

 

0

0.2

0.4

0.6

0.8

1

0 50 100 150 200

R
to

t 

Time(min) 

30  ̊ C 

40  ̊ C 



Nada Sadoon Ahmedzeki and Arowa Salah Alddin Mahdi 

 

-Available online at: www.iasj.net                      IJCPE Vol.15 No.2 (June 2014)                                 31 
 

 
Fig. 4: The relation between the removal 

efficiency of total n-paraffins with temperature 

using 40g of zeolite in 300ml of Light naphtha 
 

 
Fig. 5: The relation between the removal 

efficiency of total n-paraffins with temperature 

using 50g of zeolite in 300ml of Light naphtha 
 

 
Fig. 6: The relation between the removal 

efficiency of total n-paraffins with temperature 

using 60g of zeolite in 300ml of Light naphtha 
 

Effect of Temperature  
The effect of temperature on reduction 

of n- paraffins was studied at 30 ˚C and 

40 ˚C and at different amounts of 

zeolite of 10, 40, 50 and 60 g. As a 

general trend, increasing the 

temperature had a negative effect on 

the reduction of n- paraffins; i.e. 

increasing the temperature decreases 

the adsorption of the solute; therefor, 

desorption or regeneration of the 

adsorbent is normally achieved by 

raising the bed temperature [1]. This 

result is in agreement with Wender et 

al [14]
 
as they found a decrease in the 

adsorption capacity for individual 

system of n-hexane up to n-dodecane 

with increasing temperature. In our 

study on the cut of light naphtha which 

is very heat sensitive for it contains 

different compounds of aromatics, 

naphthenes, iso-paraffins and n-

paraffins with different vapor 

pressures, so it is expected that high 

interference on the uptake of adsorbent 

to linear paraffin and a continuous 

adsorption/desorption process 

occurred.  

This result agrees well with the result 

of Nahdhir et al [15] who studied the 

adsorption of methane in the presence 

of ethane or ethylene and concluded 

that the model was better fit for 

paraffin system and not for others and 

a separation can be achieved easily at 

any conditions but was the best at low 

temperature and pressure. 
 

Effect of Amount of Zeolite 
Fig. 7 shows the results of adsorption 

study carried out with different 

adsorbent dose. It was found that with 

increasing dosage of adsorbent the rate 

of removal of adsorbate (n-C7) 

increases. The data obtained reveals 

that there is a substantial increase in 

adsorption when amount of adsorbent 

is increased from 2 to 20 g. Increasing 

the amount of the adsorbent offers 

more adsorption sites available and 

ready for higher uptake [16].  
 

 
Fig. 7: Effect of amount of adsorbent for the 

removal of n-C6,n-C7and n-CT using 5A 

zeolite:ambient temperature; time = 24 h 

0
0.2
0.4
0.6
0.8

1

0 50 100 150 200

R
to

t 

Time(min) 

30 ̊ C 
40 ̊ C 

0

0.2

0.4

0.6

0.8

1

0 50 100 150 200

R
to

t 

Time(min) 

30  ̊ C 

40  ̊ C 

0

0.2

0.4

0.6

0.8

1

0 50 100 150 200

R
to

t 

Time,min 

30  ̊ C 

40  ̊ C 

0

0.2

0.4

0.6

0.8

1

0 10 20 30

R
 

wt.(g) 

C6

C7

C5

CT



Research Octane Number Improvement of Iraqi Gasoline by Adsorption of n -Paraffins Using 

Zeolite Molecular Sieves  

32                                        IJCPE Vol.15 No.2 (June 2014)               -Available online at: www.iasj.net 
 

Competitive Adsorption of n-

Paraffins 
   By observing Fig. 8, it can be seen 

that the uptake of n-C7 on 5A zeolite 

was higher than n-pentane and n-

hexane. This is due to the fact that long 

Chain hydrocarbons are favorably 

adsorbed on 5A zeolite [1]. The 

removal efficiency of n-C7was 53% 

after 30 minutes while 45% of n-C6 
and 27% of n-C5 at the same time. 

After 180 minutes, the adsorption of n-

C7 reached a maximum value where 

61% was removed. The removal 

efficiency of n-C6 was 46% while for 

n-C5 a continuous decrease of the 

removal efficiency until reaching 14%. 

The decrease for n-C5 can be attributed 

to the desorption occurring in the 

zeolite structure and replacement by 

other compounds of higher affinity; i.e. 

n-C6 and n-C7. 

  

 
Fig. 8: The removal of n-hexane, n-heptane 

and n-pentane using 5A zeolite: temperature = 

40  C; wt. = 40g 

 

Effect Removal of n-Paraffins on 

RON 

   From Table 1, it can be seen that the 

removal of n- paraffins and specifically 

n-heptane improves the RON for cut of 

light naphtha which is studied in the 

present work.  The RON of n-heptane 

is zero so the reduction in this 

constituent greatly enhance the fuel 

property. Removing about 56% of n- 

heptane increases the RON of light 

naphtha .This table indicates that the 

amount of n-C7 adsorbed is higher than 

n-C6  using 40 g of zeolite so that the 

RON increases 5.5 units  from (64.5 

to70). 
 

Table 1: Research octane number for light 

naphtha in the batch mode 

Wt %Rtot %RC7 RON 
Increase in 

RON 

10 31. 56.26 66 1.5 

40 30.1 61.49 70 5.5 

 

Continuous Mode 

Effect of Flow Rate 

The effect of flow rate 0.8 and 0.3 l/h 

on the adsorption of n-paraffins is 

shown by breakthrough curves drawn 

in Figure 9.The adsorbent bed height 

was fixed at 10 cm and inlet 

concentration of n-paraffins of 

35.134%. It can be seen that the 

saturationtime decreases for n-paraffin 

with flow increase.This can be 

explained by that incearsing the  rate of 

adsorption and the saturation of bed 

occurs at a shorter time. Besides, the 

effluent concentration for higher flow 

rate is higher as compared to low flow 

rate at constant time because the 

adsorbate will not have enough time to 

diffuse into the pores of adsorbent. 
 

 
Fig. 9: breakthrough curves for adsorption     

n- paraffins at Z=10 cm 

 

Effect of Bed Height 

   Two bed heights had been studied as 

10 and 20 cm which represent a weight 

of 41.2 and 82.4 g, respectively. The 

flow rate was fixed on 0.3 l/hr. Fig. 10 

shows the breakthrough curves for the 

two bed heights at constant flow rate of 

0.3 l/hr.  

0

0.2

0.4

0.6

0.8

1

0 100 200

%
R

 

time,min 

C5

C6

C7



Nada Sadoon Ahmedzeki and Arowa Salah Alddin Mahdi 

 

-Available online at: www.iasj.net                      IJCPE Vol.15 No.2 (June 2014)                                 33 
 

It can be observed that, as the bed 

height increased the time needed for 

breakpoint increases. This shows that 

for a value of Ce/C0=0.6, the time is 

(5min) and (17min) for 10 and 20 cm 

respectively. The increase of bed 

height means an increase in the mass 

of the adsorbent packed in the column 

which offers more available adsorption 

sites for adsorbate to be adsorbed. 

Furthermore, the bed would be 

saturated in less time for smaller bed 

height. 
 

 
Fig. 10: Breakthrough curves for adsorption n- 

Paraffins at Q=0.3l/h 

 

Effect of Removal of n-Paraffins on 

RON 

The effect of the reduction of n-

Paraffins is clarified by the 

measurement of RON for selected 

samples. Records are shown in   Table 

2. The time for a specified volume of 

(300 ml) of the effluent from the start 

of operation is shown along with feed 

flow rate and bed height. In these 

experiments, the feed RON was 66.2. 

Values of the increase can be attributed 

to the reduction of n-Paraffins present 

in the fuel. For bed height of 10 cm 

and 0.3 l/h, an increase of 7 units is 

obtained, and the quality of the fuel is 

enhanced. 
 

Table 2: RON for the continuous mode 

Time, 

min 

Feed 

flow 

rate 

l/h 

Bed 

height 

(Z) cm 

RON Increase 

20 0.8 10 67.1 0.9 

30 0.3 10 73.2 7 

35 0.3 20 71.3 5.1 

Adsorption Isotherm 
The profile obtained from the study of 

concentration at room temperature was 

used to obtain Langmuir and 

Freundlich adsorption isotherms by 

using well-known adsorption isotherm 

equations [17]. In both the cases, linear 

plots were obtained, which reveal the 

applicability of these isotherms on the 

ongoing adsorption process. Figs. 11 

and 12 exhibit Langmuir and 

Freundlich plots respectively for the 

adsorption n-hexane and n-heptane on 

feathers and different Freundlich and 

Langmuir constants derived from these 

plots are presented in Table 3. 

To examine the progression of 

adsorption dimensionless constant, 

separation factor r was calculated by 

the following equation: 

 

  
 

     
                                      …(2) 

 

Where b and Co values were derived 

from Langmuir isotherm. The 

parameters indicate the shape of 

isotherm accordingly. 

 
Table 3: Relationship between separation 

factor and type of isotherm 

r value Type of isotherm 

r > 1 

r = 1 

0<r < 1 

Unfavorable Linear 

Favorable 

 

The separation factor, as calculated 

from Equation 2, indicates a favorable 

adsorption. Values given in Table 4 

confirm the higher affinity of zeolite 

5A to n-heptane as compared to n-

hexane. 

   The adsorption data of n-hexane and 

n-heptane were described equally well 

by Langmuir and by Freundlich. The 

exponent (n) in the Freundlich 

isotherm indicates favorable 

adsorption. These isotherms are shown 

in Fig. 12. 
 

 

 

0
0.2
0.4
0.6
0.8

1

0 50 100 150

C
e
/C

o
 

Time (min) 

Z=20cm

Z=10cm



Research Octane Number Improvement of Iraqi Gasoline by Adsorption of n -Paraffins Using 

Zeolite Molecular Sieves  

34                                        IJCPE Vol.15 No.2 (June 2014)               -Available online at: www.iasj.net 
 

 
Fig. 11: Langmuir isotherm at room 

temperature for n-hexane and n-heptane 

adsorbing to zeolite 5A 
 

Results show that the adsorption 

capacity for n-heptane on 5A zeolite 

was higher than that of n-hexane, 

which can be attributed to the higher 

molecular weight or higher carbon 

atoms. Liu and shan [18] concluded 

also that the low-carbon-number 

normal paraffins occupying the 

microchannels of 5A zeolite can be 

partly replaced by normal paraffins 

with more carbon numbers. This is a 

basic phenomenon for adsorption that 

was reported by the earlier studies [1]. 

Silva et al. [19] also concluded that 

zeolite 5A has more favorable nature 

of n-hexane compared to n-pentane. 
 

 
Fig. 12: Freundlich isotherm at room 

temperature for n-hexane and n-heptane 

adsorbing to zeolite 5A 

 
Table 4: The Separation factor 

Component r 
Type of 

isotherm 

n-C6 0.441939 Favourable 

n-C7 0.601999 Favourable 

 

Table 5: Constants for the isotherm models 

 Langmuir Freundlich 

Q
0 

(g/gzeolite) 
b (l/g) 

Q
0 

(g/gzeolite) 

b 

(l/g) 

Q
0 

(g/gzeolite) 
b (l/g) 

n-C6 29.762 0.327 0.888 2.228 0.558 0.8770 

n-C7 53.47 0.123 0.927 1.547 0.457 0.9087 
 

Kinetic Study  

   The Pseudo first-order and second-

order kinetic models are the most 

popular models. In order to investigate 

the mechanism of sorption and 

potential rate controlling steps, the 

pseudo first and second order models 

were used. 

The first-order rate expression is: 
 

Ln (qe −qt)=lnqe−K1t                   …(3) 
 

To investigate the fitness of pseudo 

first-order equation for n-hexane and 

n-heptane adsorption, ln(qe −qt) versus 

t is plotted as shown in Fig. 13. The 

low R
2
 value gives poor correlation of 

this model. 

The pseudo second order kinetic model 

is expressed as: 

t/qt = 1/K2qe
2
 + t/qe                       …(4) 

 

If the pseudo second-order kinetics is 

applicable, the plot of t/qt versus t 

should give a linear relationship. The 

linearized form of this model is 

presented in Fig. 14. 
 

 
Fig. 13: Pseudo first-order equation for n-

hexane and n-heptane adsorption 

0
0.02
0.04
0.06
0.08

0.1
0.12
0.14
0.16

0 0.5 1

1
/q

e
 

1/Ce 

Langmuir
C6

0

0.5

1

1.5

2

2.5

3

3.5

0 1 2 3

ln
 q

e
 

lnCe 

Freundlich C6

Freundlich C7



Nada Sadoon Ahmedzeki and Arowa Salah Alddin Mahdi 

 

-Available online at: www.iasj.net                      IJCPE Vol.15 No.2 (June 2014)                                 35 
 

 
Fig. 14: Pseudo-second-order equation for n-

hexane and n-heptane adsorption, at 

temp.=40  C  t.=40g andvolume=0.3liter 

 

The intra particle diffusion model is 

also investigated. Then- paraffin 

uptake is assumed to vary almost 

proportionally with t
1/2 

rather than with 

the contact time. The intra partical 

diffusion model is expressed in 

Equation 5.  

 

qe =K t
1/2 

+C                                 …(5) 

 

Where K is the intra particle diffusion 

rate constant (g/g min
1/2

), C is the 

intercept (g/g) and a plot of qe versus 

t
1/2 

gives a linear relationship as shown 

in Fig. 15. 

 
Fig. 15: Intra-particle diffusion model for n-

hexane and n-heptane adsorption 

 

   Comparison of the three kinetic 

models revealed that the pseudo 

second order fits the experimental data 

quite well as high correlation 

coefficients of 0.999 and 0.998 are 

obtained. Values of the kinetic models 

parameters are listed in Table 6. Silva 

and Rodrigues [20] proposed that a 

macropore diffusion is controlling 

when they studied the sorption and 

diffusion of n-pentane on pellets of 5A 

(8mm). Most of the previous literature 

performed this study in gas phase 

using a pressure swing adsorption 

(PSA) which is highly recommended 

for further study. 

 

Table 6: kinetic parameters for n-hexane and n-heptane adsorption 

Kinetic 

models 
Parameters 

Compounds 

n-Hexane n-Heptane 

Pseudo-

1storder 

K1 (min-1) 0.013 0.015 

qe (g/gzeolite) 2.0422 3.2772 

R2 0.555 0.895 

Pseudo-2nd 

order 

K2 (min-1) 0.01281 0.00822 

qe (g/gzeolite) 13.6986 25.641 

R2 0.998 0.999 

Intra-particle 

diffusion 

K (g/g.min0.5) 0.155 0.386 

C (g/gzeolite) 11.00 19.86 

R2 0.306 0.932 

 

Conclusions 

 Increasing the agitation time and the 
amount of zeolite increased the 

removal efficiency of n-paraffins. 

 As a general trend, increasing the 
temperature had a negative effect on 

the reduction of n- paraffins but the 

effect of temperature needed further 

evaluation at different temperatures. 

 The uptake of n-C7 on 5A zeolite 
was higher than n-pentane and n-

hexane favorably adsorbed on 5A 

zeolite. A desorption of n-

C5occursfrom the zeolite structure 

and replacement by other 

compounds of higher affinity; i.e. n-

C6 and n-C7. 

0

5

10

15

20

25

30

0 5 10 15

q
e

 

t^0.5 



Research Octane Number Improvement of Iraqi Gasoline by Adsorption of n -Paraffins Using 

Zeolite Molecular Sieves  

36                                        IJCPE Vol.15 No.2 (June 2014)               -Available online at: www.iasj.net 
 

 The removal efficiency increased 
with increasing bed height and 

decreasing feed flow rate using 

continuous mode investigations. 

Furthermore, the bed would be 

saturated in less time for smaller 

bed height. 

 The results of this study indicated 
that the adsorption of n-C6and n-

C7from light naphtha followed the 

pseudo 2
nd

 order kinetic. 

 An increase in RON of 5.5 units 
was obtained using 40 g of zeolite 

in 300 ml light naphtha in a batch 

mode operation. On the other hand, 

an increase of 7 units was obtained 

after 30 min of a continuous 

operation using abed of 10 cm and 

0.3 l/hr. 

 A favorable type of the adsorption 
for the present case study was as 

presented by a Langmuir isotherm 

model for both n-hexane and n-

heptane adsorption on zeolite 5A. 

The calculated value of the 

separation factor confirmed the 

great tendency of zeolite 5A to n-

paraffins. 
 

Acknowledgment 

   The authors express their thanks to 

the Petroleum and Research 

Development Center for the financial 

support. Further, the authors thank the 

Arab Company for Detergents and 

Chemicals for assistance in conducting 

the analysis. Many thanks to Mr. Salah 

Mahdi for his continuous help and 

guidance. 
 

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