adsorptive desulfurization of kerosene and diesel oil by zn impregnated montmorollonite clay

ORIGINAL ARTICLEAdsorptive desulfurization of kerosene and diesel oil by Zn impregnated montmorollonite clay a Department of Chemistry,University of Malakand, Chakdarra, Lower Dir, KPK,

ORIGINAL ARTICLE

Adsorptive desulfurization of kerosene and diesel

oil by Zn impregnated montmorollonite clay

a

Department of Chemistry,University of Malakand, Chakdarra, Lower Dir, KPK, Pakistan

bInstitute of Chemical Sciences, University of Peshawar, 25120 KPK, Pakistan

Received 15 July 2012; accepted 24 December 2013

KEYWORDS

Adsorptive desulfurization;

Montmorollonite;

p-Complexation;

Impregnation;

Clay adsorbents

out by selective adsorption through metals impregnated montmorollonite clay (MMT) Different metals were impregnated on MMT by wet impregnation method which included Fe, Cr, Ni, Co,

Mn, Pb, Zn and Ag The adsorption study was carried out in batch operation initially for 1 h time

Zn-MMT In the case of kerosene highest desulfurization of 76% and in the case of diesel maximum desulfurization of 77% was achieved with adsorption through Zn-MMT Conditions were also opti-mized for the desulfurization process Highest yield of desulfurization was obtained at 1-h stirring

conditions the adsorbent was found to adsorb about 81% of DBT from the model oil containing

1000 ppm DBT dissolved in cyclohexane EDX, Surface characterization and SEM analysis of the adsorbents used in the study were conducted to evaluate their mineralogical nature and textural behavior Results show that the surface area, pore size and pore volume of the MMT has been found to be increased many fold with Zn impregnation Also the surface morphology of the MMT has also been improved with Zn impregnation

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1 Introduction

Desulfurization of liquid fuels is a challenging task for the

refiners, because of the undesirable effects of the sulfur

compounds in petroleum, which not only hampers the refining operations but also causes serious environmental degradation The current worldwide stringent environmental regulations intensify more to produce liquid fuels with ultralow levels of sulfur (Gang et al., 2011) At the present, catalytic hydrodesul-furization (HDS) is the sole process commercially used for the desulfurization of petroleum products HDS is however an expensive process in terms of utilizing expensive operating con-ditions i.e high temperature, high pressure of hydrogen gas and expensive catalyst, as well as it is inefficient to eliminate the sterically hindered sulfur compounds and thereby cannot achieve ultralow levels of sulfur in the product fuels ( Cam-pos-Martin et al., 2010) An alternative to HDS process is

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E-mail addresses: waqasaswati@gmail.com , waqasaswati@yahoo.

com (W Ahmad).

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the desulfurization through adsorption, wherein sulfur

com-pounds are selectively removed through adsorption on the

so-lid adsorbent leaving behind sulfur free fuel However the

process is in the juvenile stage and in order to be recognized

as a commercially acceptable process researchers are striving

to increase its efficiency Variety of adsorbents have been used

for this purpose such as modified composite oxides (Seredych

and Bandosz, 2010), activated carbon (Marin-Rosas et al.,

2010), mesopourous and microporous zeolites such as

SBA-15, MCM-41, Fujasite (McKinley and Angelici, 2003; Mingels

et al., 1988; Salem, 1994) 5-A, 13-X, ZSM-5 and Y-Zeolite, etc

(Salem and Hamid, 1997; Weitkamp et al., 1991; Velu et al.,

2003) We have also investigated the removal of sulfur

com-pounds from crude petroleum, kerosene and diesel oil by

reac-tive adsorption using metal oxide, it was found that PbO2and

MnO2were most effective in the desulfurization of all the three

fractions for 1 and 3 h reaction times in batch operation

adsorption experiments (Shakirullah et al., 2009)

Recent research studies revealed that reactive adsorption is

superior to ordinary physical adsorption, because it involves p

complexation between aromatic sulfur compounds and the

adsorbent, which is stronger than van der waals interaction

However p complexation can be broken easily by heating or

decreasing pressure, thereby it is easy to regenerate the

adsor-bent (Hernandez and Ralph, 2003) Different metal cations

supported on various supports have been used as reactive

adsorbents for the desulfurization of liquid fuels Yang et al

investigated the removal of thiophene from the simulated

feed-stock using Cu(I)-Y and Ag-Y zeolites (Hernandez and Ralph,

2003) McKinley et al employed Ag-I/SBA-15 and Ag-I/SiO2

as adsorbents for the selective removal of DBT and

4,6-DMDBT from Model oil (McKinley and Angelici, 2003)

Met-als halides, CuCl2 and PdCl2supported on activated carbon

(Wang et al., 2006; Wang and Yang, 2007) and PdCl2

sup-ported on SBA-15 and MCM-41 have also been found to be

effective for desulfurization of jet fuel (Wang et al., 2008a,b)

Desulfurization of jet fuel oil has also been studied with

Cu2O supported on SBA-15 and MCM-41, which have shown

that the adsorbent with MCM-41 was more effective than the

one with SBA-15 support, the adsorbent could be regenerated

by heating in air and reused (Wang et al., 2008a,b)

Mineral clays is a group of adsorbents which is enjoying

ra-pid popularity in the petroleum industry for various

separa-tions and adsorption processes e.g for the removal of

objectionable color from lube oil, separation of different

hydrocarbon groups and the removal of sulfur compounds

from petroleum products (Mikhail 1970; Occelli et al., 1984)

Mikhail et al investigated the selective adsorption of dimethyl

disulfide from cyclohaxane using acid activated kaolinite, acid

activated bentonite, charcoal, petroleum coke and cement kiln

dust (Occelli et al., 1984; Mikhail et al., 2002), they reported

that adsorption efficiency of acid activated bentonite and

char-coal was superior to all other adsorbents studied Li Shi et al

investigated the removal of mercaptans from model oil

through adsorptive desulfurization using bentonite modified

with Cu+2, Cu+1, Fe+3 and MnO4 , they concluded from

their findings that high desulfurization capacity of bentonite

modified with Fe+3and MnO4-1could be attributed to the

oxi-dation of marcaptans, and that of Cu impregnated bentonite

was because of p complexation (Tang et al., 2011)

In the present study we have investigated the selective

adsorption of sulfur compounds prevalent in commercial

kerosene and diesel over different metals loaded with acid modified montmorollonite clay The effect of various variables such as time, temperature and concentration on the efficiency

of adsorptive desulfurization has also been studied

2 Experimental Samples of Kerosene and Diesel oil were collected from Attock oil refinery, Rawalpindi The material was brought in the metal cans The samples were characterized by determining its various physico-chemical parameters including specific gravity, API gravity, kinematic viscosity, aniline point, flash point, fire point, ash contents, conradson carbon residue and total sulfur by employing the standard procedures of ASTM and IP A sample

of montmorollonite/bentonite clay was provided by the Material Research Laboratory (MRL), Department of Physics, Univer-sity of Peshawar All chemicals used were of Analytical grade 2.1 Acid modification of clay

Before the clay was used for adsorption, it was modified with acids in order to remove the organic materials and increase adsorption capacity The sample was cleaned, desilted and then modified with HCl solution 50 g of the clay sample was taken in the round bottomed flask and 250 ml of the 0.1 N HCl solution was added to it and refluxed for 2 h The clay slurry was then filtered through vacuum filtration and washed with excess of deionized water The sample was then dried in the oven at 120C for 6 h The dried clay was ground to fine powder and then screened through a 200 micron mesh sieve Finally the clay sample was activated by heating at 600C in the Muffle furnace for 5 h and the stored in a vacuum desiccator

2.2 Preparation of adsorbent

Adsorbent used for desulfurization was metal impregnated montmorollonite/bentonite clay, which was prepared by the wet impregnation method reported elsewhere (S Mikhail

et al., 2002) In a typical procedure stoichiometric amounts

of 0.2 M solution of different metals precursors i.e Ni(NO3)2, Ag(NO3)2, Fe(NO3)3, ZnCl2, MnCl2, Cr(NO3)2, Pb(NO3)2and Co(NO3)2was mixed with 3 g of modified clay The slurry was stirred via magnetic stirrers for 2 h at 60C and then dried in

an oven at 90C for 24 h The dried solid mass was ground to fine powder, which was screened via a 200 micron mesh sieve The adsorbent was calcined at 750C for 4–5 h and then stored in a vacuum dissicator

2.3 Desulfurization of kerosene and diesel oil

The kerosene and diesel oil (sulfur contents of 0.0542 and 1.041 wt.%, respectively) was provided by the Attock Oil Refin-ery for the adsorptive desulfurization study The Adsorbents used were various metals impregnated on montmorollonite and charcoal Adsorption was carried out in batch operation initially at room temperature and for one hour Later on the process conditions were also optimized In a typical procedure,

20 ml of the sample was taken in the Erlenmeyer flask and 1 g of clay or adsorbent was added to it The mixture was stirred with the help of a magnetic stirrer for about one hour at room

temperature After each time interval of 10 min the mixture was

given a rest of 2 min, the mixture was then filtered through

Wattman No 42 filter paper The filtrate was reserved for sulfur

analysis and the charged clay was kept for further examination

The same procedure was carried out with other adsorbents for

different time intervals i.e 1, 3 and 6 h at different temperatures

i.e room temperature (25C), 60 and 100 C, and also with

dif-ferent concentrations of adsorbent i.e 0.5, 1 and 1.5 g

2.4 Desulfurization of model oil

Desulfurization of model oil was carried out following the

same procedure as mentioned in section 2.3 The model oil

used consisted of dibenzothiophene (DBT) dissolved in

cyclo-hexane (1000 ppm DBT solution) Oil to adsorbent ratio was

10:0.5, adsorption was investigated at room temperature for

different time intervals i.e 10, 15, 30, 45 and 60 min

2.5 Sulfur analysis

Quantitative analysis of total Sulfur in the original sample and

treated oil samples was carried out with software controlled

CHNS analyzer (Leco SC-144DR carbon sulfur analyzer)

The concentration of DBT in model oil was determined by

UV–visible Spectrophotometer (Schimadzu, 2010, Japan) at

a wave length of 320 nm

2.6 Surface area, pore volume and pore diameter

The surface area, pore volume and pore diameter of the clay

samples was determined with surface area analyzer

(Quanta-chrome Nova station A), using BJH model and nitrogen gas

as adsorbent

2.7 Scanning electron microscopy (SEM)

The morphology of the clays used in the adsorption study was

examined by scanning electron microscope Model No

JEOL-Jsm-5910; Japan For this purpose, the powdered samples were

mounted on the sample stubs and placed in the sample carrier

of the machine The samples were then automatically analyzed

using computer software

2.8 Energy dispersive X-rays (EDX) analysis

The mineralogical composition of the clays used in the

adsorption study was examined by Energy Dispersive X-rays

Spectrometer (EDX Model Inea 200, UK Company Oxford)

3 Result and discussion

In the current study desulfurization of the kerosene and diesel was carried out using metals impregnated montmorollonite clay and activated charcoal The effect of time, temperature and concentration of adsorbent on desulfurization was also studied The results of desulfurization are discussed below 3.1 Characterization of the petroleum fractions

Before processing for desulfurization samples of kerosene and diesel oil were characterized physico-chemically Various phys-ico-chemical properties like specific gravity, kinematic viscos-ity, relative densviscos-ity, API gravviscos-ity, carbon residue, ash contents, flash point, and aniline point of the kerosene and die-sel oil, were determined The physico-chemical properties of the various fractions are summarized in theTable 1

Data in the table show that, values of specific gravity for kerosene and diesel, is 0.7879 and 0.8729 respectively, while API gravity is 48.0913 and 30.7706, respectively In the case

of kerosene and diesel the specific gravity increases gradually

as with an increase in their boiling points On the other hand, their API gravity decreases gradually, because the structural complexity of the molecules increases with increase in the boil-ing points of the fractions Kinematic viscosity is a function of the chemical nature of any fraction In the case of kerosene the kinematic viscosity is 2.1808 cst, while in the case of diesel it is 3.5136 cst The reason is that with the increase in the boiling point, the complexity of the molecular structures contained

in that fraction also increases

Aniline point, flash point and fire point have also showed

an increase in the same manner The values of aniline point, flash point and fire points for kerosene are, 58C, 42 C and

45C, respectively, while for diesel these are 62 C, 48 C and 50C respectively Thus in the case of diesel oil the given parameters increase due to complexity of prevailing hydrocar-bon molecules Conradson carhydrocar-bon residue is also related to the nature of the hydrocarbons For kerosene and diesel, the value

of Conradson carbon residue was 0.13% and 1.14%, respec-tively, i.e the value gradually increases The ash content in ker-osene and diesel oil was found to be 0.002 and 0.003 wt% respectively Sulfur contents also increase with the increase

of the boiling range of the fractions, as sulfur compounds exist

in different forms at different boiling ranges In the case of

Characteristics Method used Kerosene Diesel Specific gravity IP-160/87 0.7879 0.8720

Kinematic viscosity cSt @ 100 F ASTM-D 455-04 2.1808 3.5136 Aniline point (C) ASTM-D 611-04 58 62

Ash contents (wt%) ASTM-D 482-03 0.002 0.003 Conradson carbon residue (wt%) IP-13/92 0.13 0.14 Total sulfur (wt%) ASTM D 129-83 0.0542 1.041

kerosene sulfur content were 0.0542% while in diesel the sulfur

content were up to 1.04% by wt

3.2 Characterization of adsorbent

The adsorbents were characterized by determining its surface

area, pore diameter and pore volume, SEM and EDX in order

to know about the nature of the adsorbents Discussion on

these parameters is given as follows

3.2.1 Surface area and pore dimensions

The results of surface area and pore dimensions are given in

theTable 2 It is clear from the table that according to the

BET and BJH models, the surface area of the original clay

is 89.87 and 155.65 m2/g whereas that of Zn-impregnated

montmorollonite is 124.29 and 420.85 m2/g, respectively

The result shows that the surface area of the clay has been

increased due to Zn impregnation on the clay Similarly the

pore volume and pore diameter of the virgin clay is 0.46 cc/

g and 125.56 A˚, while that of Zn-impregnated

montmorollo-nite is 1.35 cc/g and 128.40 A˚, which indicates that during

impregnation treatment the pore dimensions of the clay were

raised significantly

3.2.2 Scanning electron microscopy

In order to examine the surface morphology of the clay adsorbents, scanning electron microscopic analysis of the samples was carried out, SEM images of the clay samples are displayed inFig 1 SEM micrographs of montmorollonite clay (Fig 1a) clearly show the porous and layered but non-uni-form textural surface of the clay The particles size is some-what non-uniform Major fissures and channels are evident The layered surface can be seen clearly The SEM micrographs

of Zn-impregnated montmorollonite clay (Fig 1b) indicate that fissures and channels on the surface are present Also the layered structure with larger pores can be seen The surface

is mainly comprised of irregularities and plateaus The textural non-uniformity is evident in both magnifications The particle size however seems of uniform size as compared to the original clay It shows that Zn cations are uniformly dispersed on the entire surface of the clay, and hence successfully impregnated 3.2.3 EDX analysis

The EDX analysis of the virgin and Zn-impregnated clays was carried out in order to know their mineralogical nature and chemical composition The EDX profile (Fig 2) of montmoroll-onite or bentmontmoroll-onite having chemical formula of (Na, Ca)0.33

Sample Surface area Pore volume (cc) Pore diameter (A o )

BET model (m 2 /g) BJH Model (m 2 /g) Montmorollonite 89.87 155.65 0.46 125.56

Zn-Montmorollonite 124.29 420.85 1.35 128.40

(a)1000 x magnification (b) 2000 x magnification (a)1000 x magnification (b) 2000 x magnification

(Al, Mg)2 Si4O10 (OH) n H2O, which belongs to subgroup

Smectite shows that the percentage of Al and Si in the sample

is 8.90% and 22.53%, respectively Whereas the percentages

of other metals like Fe, Ca, K, Mg and O is 3.19%, 4.05%,

2.74% and 54.11%, respectively The major constituents of

the clay are aluminum, silicon, magnesium, iron, oxygen and

calcium, which correspond to its chemical formula

The EDX analysis of the Zn-MMT shows that its

mineral-ogical composition is almost same as that of virgin, except the

%wt of Zn is 18.14%, which is close to the theoretical value of

20%

3.3 Desulfurization through adsorption with clays

Fig 2display the results of total desulfurization carried out by

adsorption in the case of kerosene oil and diesel oil through

charcoal activated and metals impregnated with

montmorollo-nite at 40C The adsorption process was carried out for one

hour The desulfurization efficiency and effect of time were

studied for each adsorbent

3.3.1 Desulfurization of kerosene

The desulfurization efficiency of variously metals impregnated

MMT in kerosene and diesel oil is displayed in theFig 2 In

the case of kerosene oil, untreated MMT and charcoal shows

desulfurization activity of about 16% and 21.98%,

respec-tively Out of metal impregnated clays, the highest

desulfuriza-tion has been shown by Zn-MMT, i.e 60%, followed by

Mn-MMT i.e 45.33%, Co-Mn-MMT i.e 40%, and Ni-Mn-MMT i.e 41%,

while for the other adsorbents desulfurization efficiency is

fairly low The results show that desulfurization efficiency of

the MMT clay has been increased with metals impregnation

3.3.2 Desulfurization of diesel

In the case of diesel oil the desulfurization trend is similar to

that of kerosene Desulfurization of diesel with

montmorollo-nite shows the value of 43.96% while that of charcoal is up to

27.80% However the in case of metal impregnated clays

desul-furization efficiency was enhanced Among these, the highest

desulfurization yield is obtained with Zn-MMT that is

62.48%, followed by Pb-MMT i.e 55.7%, Ni-MMT i.e

55.9%, whereas for the rest of adsorbent the desulfurization yield was not much appreciable Desulfurization of model and real oil has been investigated by many researchers using various transition metals exchanged/supported adsorbents, out of which adsorbents containing Ag, Ni, Cu etc have been used and found to be effective In the present case, it is clear from the results that the desulfurization yield of Zn based adsorbent is superior to others, which exhibit high desulfuriza-tion efficiency in the case of both kerosene and diesel oil 3.4 Optimization of conditions

Desulfurization of kerosene and diesel with Zn-MMT as adsorbent was carried out at different conditions of time, tem-perature and concentrations in order to find the optimum set

of conditions The effects of different parameters studied are given below

3.4.1 Effect of temperature Adsorptive desulfurization of kerosene and diesel was carried out with Zn-MMT at different temperatures i.e room temper-ature (25C), 40, 60 and 100 C Results for % desulfurization

of kerosene and diesel are given in theFig 3 The data show that in the case of kerosene the % desulfurization at room tem-perature was 62%, whereas at 40, 60 and 100C it was, 61%, 55% and 45% respectively Similarly for diesel at room tem-perature the% desulfurization was 63%, and at a temtem-perature

of 40, 60 and 100C it was 61%, 58% and 46%, respectively Hence the highest desulfurization is obtained at room temper-ature in the case of both kerosene and diesel From the results

it is concluded that with an increase in temperature the rate of desorption increases, that is why the decline in desulfurization has been observed, hence the optimum temperature for adsorptive desulfurization is room temperature i.e 25C Similar results are also reported by Majid et al., they used Ni-loaded Y type zeolite for adsorptive desulfurization of gas-oline, they found that with the increase in temperature from 25

to 60C, the adsorption capacity of the adsorbent decreased from 0.55 to 0.65 mg(S)/g (Majid and Seyedeyn-Azad, 2010)

It may be attributed to the exothermic nature of the process, which is hampered with the rise in temperature

0 10 20 30 40 50 60 70

Adsorbents

Kerosene Diesel

3.4.2 Effect of time

Fig 4shows the effect of time on desulfurization of kerosene

and diesel on Zn-MMT adsorbent In the case of both

frac-tions i.e kerosene and Diesel, the desulfurization increases

with increase in reaction time In kerosene, % desulfurization

increases from 62% to 69%, while in diesel the desulfurization

increases from 64% to 71% with an increase in reaction time

from 1 to 6 h Hence the highest desulfurization is attained

at 6 h It may be concluded that desulfurization occurs through

multilayer adsorption, hence as time passes multilayer

adsorp-tion progresses and completes at 6 h time Effect of time was

also studied by Majid et al using Ni/y zeolite, which showed

that desulfurization increases with time and completes at 4 h

(Majid and Seyedeyn-Azad, 2010), however Tang et al

re-ported that using Ga-Y zeolite, desulfurization of model

gaso-line completes at 6 h (Tang et al., 2008)

3.4.3 Effect of adsorbent quantity

The desulfurization of kerosene and diesel was also carried out

with different quantities of Zn-MMT i.e 0.25, 0.5, 1 and 1.5

Effect of concentration of adsorbent on desulfurization is

shown byFig 5 Increasing the oil to adsorbent ratio from

20:0.25 to 20:0.5, 20:1 and 20:1.5 the % desulfurization in

the case of kerosene increased from 57% to 71%, 73% and

76%, respectively, likewise in the case of diesel it was raised from 57% to 73%, 75% and 77%, respectively The increase

in the desulfurization yield with an increase in adsorbent con-centration may be attributed to the availability of a larger sur-face area and hence larger p complexation sites for the sulfur compounds Using high concentration of adsorbents provides more absorption sites for sulfur compounds, and hence the desulfurization yield is high

3.5 Desulfurization of model oil Desulfurization of model oil containing DBT as model sulfur compounds dissolved in cyclohexane (1000 ppm) was investi-gated through adsorption over Zn-MMT under room temper-ature at different adsorption times Results indicated inFig 6, show that at different adsorption times studied i.e 15, 30, 45, and 60 min, the % removal of DBT was 75%, 78%, 79% and 81%, respectively It can be seen from the data that the rate of DBT adsorption increases with an increase in adsorption time, however beyond 30 min adsorption there is very little increase

in the adsorption This indicates that unlike kerosene and die-sel oil, in the case of model oil, after 30 min the adsorbents be-comes saturated with the DBT In the case of kerosene and

0

10

20

30

40

50

60

70

Temperature o C

Kerosene Diesel

and diesel oil

56

58

60

62

64

66

68

70

72

Time (h)

Kerosene Diesel

oil

0 10 20 30 40 50 60 70 80 90

Concentration of Adorbents (g)

Kerosene Diesel

kerosene and diesel oil

50 55 60 65 70 75 80 85

Time (min)

adsorp-tion using Zn-MMT

diesel oil the sulfur removal is favored by increasing the

adsorption time It may be suggested that as in the case of

model oil the concentration of DBT is higher than kerosene

and diesel oil therefore adsorbent saturation occurs at less

adsorption time

4 Conclusion

The following conclusions can be drawn from the current

study,

 Montmorollonite clay, which is locally available, can be

efficiently used for adsorptive desulfurization

 Metals impregnation on MMT clay increases its adsorption

characteristics

 The surface area, pore size and pore volume of the MMT

has been found to be increased many fold with Zn

impregnation

 The surface morphology of the MMT has also been

improved with Zn impregnation

 Metal impregnated MMT has more high desulfurization

efficiency than the original MMT

 Zn impregnated MMT shows better adsorption efficiency

for sulfur compounds

 The selective adsorption of the sulfur compounds using

Zn-MMT is found to be higher at 1 h adsorption time, at 25C

(room temperature) and 1.5 g concentration of adsorbent

Acknowledgements

The authors acknowledge the cooperation of the Material

Research Laboratories (MRL), Department of Physics,

University of Peshawar for providing the Clay samples, the

Centralized Resources Laboratories (CRL) University of

Peshawar for facilitating the analytical work and the Attock

Oil Refinery, Rawalpindi Pakistan, for providing the oil

samples for this study The authors also acknowledge the

contribution of late Professor Dr M Shakirullah, ICS

University of Peshawar

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